Uranium versus Thorium: Synthesis and Reactivity of [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ].

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U=P-2,4,6-tBu3 C6 H2 (1) yields the stable uranium metallacyclopropene, [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U-(η2 -C=C) moiety increases significantly compared to the related ThIV compound [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 Th[η2 -C2 Ph2 ], which also results in more covalent bonds between the [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U2+ and [η2 -C2 Ph2 ]2- fragments. Although the thorium and uranium complexes are structurally closely related, different reaction patterns are therefore observed. For example, 2 reacts as a masked synthon for the low-valent uranium(II) metallocene [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 UII when reacted with Ph2 E2 (E=S, Se), alkynes and a variety of hetero-unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five-membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.

Introduction

class="Chemical">Metallacyclopropenes, esclass="Chemical">pecially class="Chemical">pan class="Chemical">those of d‐transition metals, have been extensively studied for the last three decades. Within this class of compounds group 4 metallacyclopropenes bearing a Cp’2M fragment (where Cp’=substituted or unsubstituted η 5‐cyclopentadienyl) are probably the most thoroughly investigated class. In the presence of a suitable unsaturated substrate, the coordinated alkyne is readily displaced releasing a Cp’2MII fragment which reacts with the provided substrate to yield highly functionalized organic molecules or heterocyclic main group element compounds.[ , ] The reactivity of these group 4 metallacyclopropenes varies with the steric and electronic properties exerted by the Cp’ and alkyne ligands.[ , ] In contrast to this well‐established chemistry of group 4 metals, metallacycles of the lanthanides and actinides have only recently attracted renewed attention after many years of inactivity. These studies should be considered in the context of current developments in the actinide field focusing on small molecule activation and the impact of 5f orbital contributions on bonding and the reactivity.

We have been interested in class="Chemical">thorium and class="Chemical">pan class="Chemical">uranium metallacycles for some time, which we recently documented with the synthesis of two stable actinide metallacyclopropenes [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) and (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2]. The alkyne in the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) reacts as a nucleophile towards hetero‐unsaturated molecules such as aldehydes, ketones, CS2, carbodiimides, nitriles, isothiocyanates, organic azides, and diazoalkane derivatives or as a strong base inducing intermolecular C−H bond activation.[ , ] In contrast, the uranium metallacyclopropene (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2] acts as a masked synthon for the (η 5‐C5Me5)2U(II) fragment when reacted with unsaturated molecules.[ , ] Unfortunately, at the time we could not directly compare [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) to its uranium analogue [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2), so that some of the differences observed for [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) and (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2] may also be traced to the different steric requirements of the coordinated ligands. Only recently, we could serendipitously isolate the missing uranium counterpart [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2), while studying the reactivity of [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1). This now allowed us to directly evaluate both actinide metallacyclopropenes and to establish differences and similarities in the reactivity of these compounds. These results are described in this manuscript.

Results and Discussion

Synthesis of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2)

Heating a mixture of class="Chemical">the class="Chemical">pan class="Chemical">uranium phosphinidene metallocene [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1) with PhC≡CPh in toluene at 50 °C forms the air and moisture sensitive metallacyclopropene, [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2), which can be isolated as brown crystals in 80 % yield, while the phosphaindane 3,3‐Me2‐5,7‐tBu2C8H5P is formed as the side‐product (Scheme 1). Complex 2 is very soluble in and readily recrystallized from an n‐hexane solution. The molecular structure of 2 is shown in Figure 1, and selected bond lengths and angles are listed in Table 1. The relevant C(41)−C(42) distance of 1.33(2) Å agrees with the value found for a typical double bond (1.331 Å) and is essentially identical to those found in the uranium metallacyclopropene (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2] (1.338(11) Å) and the thorium metallacyclopropenes [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (1.343(4) Å) and [(η 5‐C5Me5)2Th(η 2‐C2Ph(SiMe3))(Cl)][Li{MeO(CH2CH2O)2Me}2] (1.360(7) Å), indicating a doubly reduced alkyne ligand, [η 2‐C2Ph2]2−. The angle (33.2(6)°) of C(41)‐U(1)‐C(42) also parallels that in the uranium metallacyclopropene (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2] (33.3(3)°) and the C‐Th‐C angle (32.6(1)°) in the related thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2). Furthermore, the angles of C(41)‐C(42)‐C(43) (127(2)°) and C(40)‐C(41)‐C(42) (130(2)°) approach a value of 120°, which is the expectation value for sp2‐hybridized carbon atoms. The U−C distances are 2.35(2) Å for C(41) and 2.298 (19) Å for C(42), which are similar but slightly more asymmetric than those in (η 5‐C5Me5)2U[η 2‐C2(SiMe3)2] (2.315(9) and 2.350(9) Å). For comparison the Th−C distance in the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) is 2.395 (2) Å, which is longer than expected based on the different ionic radii of ThIV (1.05 Å) and UIV (1.00 Å) (with a coordination number of 8).

Scheme 1

Synthesis of complex 2.

Figure 1

Molecular structure of 2 (thermal ellipsoids drawn at the 35 % probability level).

Table 1

Selected distances (Å) and angles (deg) for compounds 2, 5, 9, 10, 12, 14, 16–18 and 20–23.[a]

Compound	C(Cp)−U[b]	C(Cp)−U[c]	Cp(cent)−U[b]	U−X	Cp(cent)‐U‐Cp(cent)	X‐U‐X/Y
2	2.79(2)	2.71(2) to 2.890(18)	2.51(2)	C(41) 2.35(2), C(42) 2.298(19)	141.2(6)	33.2(6)
2’ (Th) [6a]	2.861(2)	2.798(2) to 2.950(2)	2.592(2)	Th‐C 2.395(2), 2.395	138.7(2)	32.6(1)
5	2.803(5)	2.746(5) to 2.855(5)	2.528(5)	C(41) 2.475(5), C(42) 2.449(5) C(43) 2.448(5), C(44) 2.463(5)	139.4(2)	92.4(2)[d]
9	2.798(3)	2.737(3) to 2.873(3)	2.523(3)	N(1) 2.222(2), N(2) 2.214(2)	149.2(1)	72.3(1)
10	2.817(5)	2.731(6) to 2.927(6)	2.544(6)	N(1) 2.252(5), N(1A) 2.252(5)	144.3(2)	70.9(3)
12	2.818(5)	2.748(5) to 2.914(5)	2.546(5)	N(1) 2.208(4), C(35) 2.482(6)	136.4(2)	69.3(2)
14	2.806(3)	2.749(3) to 2.888(3)	2.532(2)	N(1) 1.977(3), N(2) 1.974(3)	139.2(1)	98.7(1)
16	2.841(5)	2.729(5) to 2.976(5)	2.570(5)	N(1) 1.968(4)	134.1(3)
17	2.787(5)	2.725(5) to 2.846(5)	2.511(5)	N(1) 2.227(4), N(2) 2.418(4) C(39) 2.517(6)	140.4(2)	33.4(2)[e]
18	2.804(5)	2.709(5) to 2.931(4)	2.530(5)	S(1) 2.659(1), S(2) 2.628(1)	142.5(1)	100.4(1)
20	2.795(10)	2.699(10) to 2.895(10)	2.522(10)	S(1) 2.649(2), C(37) 2.480(10)	140.9(2)	76.0(2)
21	2.828(3)	2.731(3) to 2.962(3)	2.557(3)	O(1) 2.062(2), C(37) 2.581(3)	133.6(1)	67.6(1)
22	2.817(4)	2.726(3) to 2.953(4)	2.546(4)	O(1) 2.069(2), C(43) 2.577(4)	133.8(1)	67.5(1)
23	2.833(5)	2.735(5) to 2.916(5)	2.562(5)	O(1) 2.076(4), C(35) 2.572(6)	126.7(2)	67.1(2)

[a] Cp=cyclopentadienyl ring. [b] Average value. [c] Range. [d] The angle of C(41)‐U(1)‐C(44). [e] The angle of N(1)‐U(1)‐N(2).

Synpan class="Chemical">thesis of comclass="Chemical">plex 2.

Molecular structure of 2 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

pan class="Chemical">Selected distances (Å) and angles (deg) for comclass="Chemical">pounds 2, 5, 9, 10, 12, 14, 16–18 and 20–23.[a]

2’ (pan class="Chemical">Th)

pan class="Chemical">Th‐C 2.395(2), 2.395

[a] Cp=cyclopentadienyl ring. [b] Average value. [c] Range. [d] pan class="Chemical">The angle of C(41)‐U(1)‐C(44). [e] class="Chemical">pan class="Chemical">The angle of N(1)‐U(1)‐N(2).

Neverclass="Chemical">theless, in contrast to class="Chemical">pan class="Chemical">the formation of the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2), the reduction of [η 5‐1,2,4‐(Me3C)3C5H2]2UCl2 (3) in the presence of an excess of potassium graphite (KC8) and diphenylacetylene (PhC≡CPh) does not cleanly yield the desired uranium metallacyclopropene 2, but a mixture of the uranium metallacyclopropene 2 and the uranium(III) chloride species [η 5‐1,2,4‐(Me3C)3C5H2]2UCl (4) is formed (Scheme 2), which can be explained by the moderate reduction potential of UIV/UIII (E °=−0.63 V). The ratio of 2 and 4 is roughly 1:3 (as confirmed by 1H NMR spectroscopy). Unfortunately, this mixture cannot be converted to pure 2 upon prolonged reduction with excess potassium graphite (KC8) in the presence of diphenylacetylene. Furthermore, attributed to a remarkably similar solubility the mixture of complexes 2 and 4 could not be separated by recrystallization.

Scheme 2

Synthesis of complexes 2 and 4.

Synpan class="Chemical">thesis of comclass="Chemical">plexes 2 and 4.

Bonding studies

Density functional class="Chemical">theory (DFT) comclass="Chemical">putations at class="Chemical">pan class="Chemical">the B3PW91 level of theory were performed to probe the interaction between the [η 5‐1,2,4‐(Me3C)3C5H2]2U2+ and the [η 2‐C2Ph2]2− fragments, which also allows the bonding in 2 to be compared to its thorium analogue [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (2’). Computed and experimentally determined molecular structure of 2 are in good agreement and reproduce the asymmetry within the An[η 2‐C2Ph2] metallacyclopropene moiety with two in‐plane An−C σ‐bonds and one out‐of‐plane π‐bond interacting with the metal center, as illustrated in Figure 2. The natural localized molecular orbital (NLMO) analysis (Table 2) suggests that σ1(U−C) bond combines a carbon hybrid orbital (72.9.0 %; 25.7 % s and 74.3 % p) and a uranium hybrid orbital (22.0 %; 41.1 % 5f and 54.0 % 6d), whereas σ2(U−C) bond is formed by a carbon hybrid orbital (72.9 %; 25.7 % s and 74.3 % p) and a uranium hybrid orbital (22.2 %; 40.9 % 5f and 54.2 %). In addition, two bonding orbitals are identified for the C−C bond: the σ‐bond (σ(C=C)) composes of two carbon hybrid orbitals (47.7 %; 29.0 % s and 71.0 % p; and 47.7 %; 28.9 % s and 71.1 % p), whereas the π‐bond (π[U(C=C)]) is made up by 84.6 % carbon occupancy consisting of only p orbitals and a 11.7 % contribution from a uranium hybrid orbital (48.5 % 5f and 50.6 % 6d). These results implicate that electron density is also shifted from the alkyne π‐orbital to the electron deficient metal uranium atom.

Figure 2

Plots of HOMOs for 2 (hydrogen atoms have been omitted for clarity).

Table 2

Natural localized molecular orbital (NLMO) analysis of An−(C2Ph2) bonds,[a] bond order, and the natural charges for the [η 5‐1,2,4‐(Me3C)3C5H2]2An and [η 2‐C2Ph2] units.

		2 (U)	2’ (Th)
σ 1 An−C	%An	22.0	16.0
	%s	3.6	5.1
	%p	1.3	1.9
	%d	54.0	76.3
	%f	41.1	16.7
	%C	72.9	79.0
	%s	25.7	25.6
	%p	74.3	74.6
σ 2 An−C	%An	22.2	16.1
	%s	3.6	5.0
	%p	1.3	1.8
	%d	54.2	76.6
	%f	40.9	16.6
	%C	72.9	79.0
	%s	25.7	25.4
	%p	74.3	74.6
σ C=C	%An	3.0	2.9
	%s	1.8	2.7
	%p	3.3	3.5
	%d	44.0	51.1
	%f	50.9	42.7
	%C	47.7	47.9
	%s	29.0	31.6
	%p	71.0	68.4
	%C	47.7	47.9
	%s	28.9	31.5
	%p	71.1	68.5
π An(C=C)	%An	11.7	8.4
	%p	0.9	2.1
	%d	50.6	66.7
	%f	48.5	31.2
	%C	42.3	44.0
	%p	100	100
	%C	42.3	44.1
	%p	100	100
Wiberg bond order (An‐C2Ph2)		0.801 0.804	0.678 0.681
NBO charge (An)		1.31	1.60
NBO charge (Cp2An)		0.78	1.06
NBO charge (C2Ph2)		−0.78	−1.06

[a] The contributions by atom and orbital are averaged over all the ligands of the same type (complexes of U and Th) and over alpha and beta orbital contributions (complex of U).

Plots of HOMOs for 2 (pan class="Chemical">hydrogen atoms have been omitted for clarity).

Natural localized molecular orbital (NLMO) analysis of An−(class="Chemical">C2Ph2) bonds,[a] bond order, and class="Chemical">pan class="Chemical">the natural charges for the [η 5‐1,2,4‐(Me3C)3C5H2]2An and [η 2‐C2Ph2] units.

2’ (pan class="Chemical">Th)

Wiberg bond order (An‐pan class="Chemical">C2Ph2)

NBO charge (Apan class="Chemical">n)

NBO charge (Cp2Apan class="Chemical">n)

NBO charge (pan class="Chemical">C2Ph2)

[a] class="Chemical">The contributions by atom and orbital are averaged over all class="Chemical">pan class="Chemical">the ligands of the same type (complexes of U and Th) and over alpha and beta orbital contributions (complex of U).

However, in class="Chemical">the class="Chemical">pan class="Chemical">thorium counterpart [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (2’), the metal contribution to the bonding of the Th(η 2‐C2Ph2) moiety is significantly reduced (16.0 % and 16.1 %% Th for Th−C σ1 and σ2 bond, respectively, and 8.4 % Th for Th‐(C=C) π bond) (Table 2). An increased charge separation result, which increases the electrostatic interaction between the individual [η 5‐1,2,4‐(Me3C)3C5H2]2An2+ and [η 2‐C2Ph2]2− fragments, that is, 1.56 for U (2) and 2.12 for Th (2’) (Table 2). Furthermore, the Wiberg bond order of the An‐C2Ph2 is reduced from 0.801 and 0.804 (for 2) to 0.678 and 0.681 (for 2’)) (Table 2). Both observations reflect the increased polarization and ionicity within the bonding between the metallocene [η 5‐1,2,4‐(Me3C)3C5H2]2Th2+ and the alkyne [η 2‐C2Ph2]2− fragments. Also the π‐donation from the π‐MO of the coordinated alkyne to the metal atom is significantly less efficient, which is due to an increase in the 5f orbital energy of the thorium atom relative to that of the uranium atom.[ , ] The evaluation of the 5f orbital contribution to the U‐C σ (41.1 % and 40.9 %% for σ1 and σ2 bond, respectively) and U‐(C=C) π (48.5 %) bonds in 2 reveals it to be substantially larger than that of the 5f orbitals in 2’ (16.7 % and 16.6 % for Th−C σ1 and σ2 bond, respectively, and 31.2 % for Th‐(C=C) π bond), which is in line with the previously investigated systems.[ , , , , ] Overall, this difference should also manifest itself in divergent reactivities of the uranium complex 2 relative to that of the thorium metallacyclopropenes.[ , , ]

Reactivity studies

We class="Chemical">then investigated class="Chemical">pan class="Chemical">the reactivity of 2 towards a series of organic substrates and compared the reaction outcomes to those obtained for the thorium metallacyclopropene complex [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (2’). Figure 3 summarizes the products obtained for 2’.

Figure 3

Selected reactivity of [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (2’).

class="Chemical">Selected reactivity of [η 5‐1,2,4‐(Me3C)3C5H2]2class="Chemical">pan class="Chemical">Th(η 2‐C2Ph2) (2’).

In accordance wiclass="Chemical">th class="Chemical">pan class="Chemical">the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2), no alkyne dissociation could be detected by NMR spectroscopy within the temperature range of 20–100 °C. However, contrary to the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2), the coordinated diphenylacetylene ligand in 2 is labile enough to be exchanged by internal alkynes. For example, addition of 1,4‐diphenylbutadiyne (PhC≡CC≡CPh) at 40 °C gives the uranium metallacyclopentatriene complex [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 4‐C4Ph2) (5) and diphenylacetylene (PhC≡CPh) (Scheme 3). To account for this transformation, it is proposed that diphenylbutadiyne replaces diphenylacetylene to give a metallacyclopropene complex, which converts by a [1,3]‐U migration to yield complex 5 (Scheme 3). The molecular structure of 5 is provided in Figure 4, and selected bond distances and angles are listed in Table 1. The C−C distances of C(41)−C(42), C(42)−C(43) and C(43)−C(44) are 1.307(7), 1.305(8) and 1.300(7) Å, respectively, which suggest a delocalized cumulene moiety. The angles of C(35)‐C(41)‐C(42) and C(45)‐C(44)‐C(43) are 127.5(5) and 128.3(5)°, respectively, approach the value of 120°, consistent with sp2‐hybridization at the carbon atoms. Nevertheless, the cumulene fragment itself remains rather strained with C(41)‐C(42)‐C(43) and C(44)‐C(43)‐C(42) angles of 150.2(5)° and 149.9(5)°, respectively. Similar structural parameters were also found for the previously reported actinide metallacyclopentatrienes (η 5‐C5Me5)2An(η 4‐C4Ph2) (An=Th, U ), (η 5‐C5Me5)2U[η 4‐C4(SiMe3)2], [η 5‐1,3‐(Me3C)2C5H3]2U(η 4‐C4Ph2) and (η 5‐C5Me5)2Th[η 4‐C4(SiMe3)2].

Scheme 3

Synthesis of complexes 5 and 6.

Figure 4

Molecular structure of 5 (thermal ellipsoids drawn at the 35 % probability level).

Synpan class="Chemical">thesis of comclass="Chemical">plexes 5 and 6.

Molecular structure of 5 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

class="Chemical">Diphenylacetylene disclass="Chemical">placement in 2 is also encountered in class="Chemical">pan class="Chemical">the presence of hetero‐unsaturated organic molecules. For example, complex 2 reacts with the aldimine PhCH=NPh to yield the metallaaziridine [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐CHPhNPh) (6) (Scheme 3). Nevertheless, treatment of 2 with the hydrazine derivative (Ph2C=N)2 yields the bisiminato complex [η 5‐1,2,4‐(Me3C)3C5H2]2U(N=CPh2)2 (7) and diphenylacetylene (Scheme 4). Like in the reaction with PhCH=NPh, presumably diphenylacetylene replacement with (Ph2C=N)2 furnishes a metallaaziridine, which converts by N−N bond cleavage to 7 (Scheme 4). Moreover, it is of note that the uranium bipy complex [η 5‐1,2,4‐(Me3C)3C5H2]2U(bipy) (8) can also be accessed by the addition of 2,2’‐bipyridine (bipy) to 2 (Scheme 4).

Scheme 4

Synthesis of complexes 7 and 8.

Synpan class="Chemical">thesis of comclass="Chemical">plexes 7 and 8.

class="Chemical">Diphenylacetylene substitution is also encountered in class="Chemical">pan class="Chemical">the reaction of 2 with the nitriles RCN (R=C6H11, Ph2CH), in which five‐membered metallaheterocycles [η 5‐1,2,4‐(Me3C)3C5H2]2U[(N=CR)2] (R=C6H11 (9), Ph2CH (10)) are formed (Scheme 5). Again, in analogy to the reaction with PhCH=NPh, RCN may initially replace the diphenylacetylene ligand to give a η 2‐coordinated nitrile intermediate, which spontaneously incorporates a second molecule of RCN to give the five‐membered heterometallacycles 9–10 (Scheme 5). The molecular structure of 9 is shown in Figure 5, whereas the structure of 10 is provided in the Supporting Information. The U−N distances are 2.222(2) and 2.214(2) Å for 9, and 2.252(5) Å for 10, and the N‐U‐N angles are 72.3(1)° for 9 and 70.9(3)° for 10. Nevertheless, when the slightly less sterically encumbered PhCN is used, only the insertion of 1 equiv of PhCN into the uranium metallacyclopropene moiety of 2 occurs at room temperature to yield the five‐membered heterocyclic complex [η 5‐1,2,4‐(Me3C)3C5H2]2U[N=C(Ph)(C2Ph2)] (11) in quantitative conversion (Scheme 5). A similar reaction was also observed for the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) with PhCN (Figure 3). However, when benzyl nitrile PhCH2CN is added to 2 the five‐membered heterocyclic complex [η 5‐1,2,4‐(Me3C)3C5H2]2U[NHC(=CHPh)(C2Ph2)] (12) formed in quantitative conversion (Scheme 5). We assume that 2 initially reacts with PhCH2CN to give a five‐membered heterocyclic intermediate (analogous to compound 11), which converts by [1,3]‐H migration to yield the final product 12 (Scheme 5). Figure 6 illustrates the molecular structure of 12 and selected bond lengths and angles are collected in Table 1. The C(37)−C(50) distance is 1.379(8) Å, and C(37)−N(1) distance is 1.384(7) Å. The U−N distance is 2.208(4) Å, whereas U−C(35) distance is 2.482(6) Å, and the angle of N(1)‐U‐C(35) is 69.3(2)°.

Scheme 5

Synthesis of complexes 9–12.

Figure 5

Molecular structure of 9 (thermal ellipsoids drawn at the 35 % probability level).

Figure 6

Molecular structure of 12 (thermal ellipsoids drawn at the 35 % probability level).

Synpan class="Chemical">thesis of comclass="Chemical">plexes 9–12.

Molecular structure of 9 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Molecular structure of 12 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

However, complex 2 yields wiclass="Chemical">th class="Chemical">pan class="Chemical">the diazenes RN=NR (R=Ph, p‐tolyl) the bisimido uranium(VI) complexes [η 5‐1,2,4‐(Me3C)3C5H2]2U(=NR)2 (R=Ph (13), p‐tolyl (14)) in quantitative conversion (Scheme 6). Analogously to the reaction with PhCH=NPh, RN=NR replaces the diphenylacetylene fragment to form a three‐membered metallacyclic intermediate, which transforms by electron transfer and NN bond cleavage to yield the bisimido products 13–14 (Scheme 6). The molecular structure of 14 is shown in Figure 7, and the selected bond distances and angles are listed in Table 1. The short U−N distances (1.977(3) Å for N(1) and 1.974(3) Å for N(2)) and the angles of U‐N(1)‐C(35) (168.4(2) and U‐N(2)‐C(42) (173.0(3)°) are consistent with a U=N double bond. These structural parameters may be compared to those found in [η 5‐1,2,4‐(Me3C)3C5H2]2U(=NPh)2 (13) with the U−N distances of 1.985(4) and 1.981(4) Å and the U‐N‐C angles of 171.4(4) and 172.8(4)°, (η 5‐C5Me5)2U(=N‐p‐tolyl)2 with the U−N distances of 1.971(4) and 1.975(3) Å and the U‐N‐C angles of 178.8(3) and 179.1(3)°, and (η 5‐C5Me5)2U(=NPh)2 with the U−N distance of 1.952(7) Å and the U‐N‐C angle of 177.8(6)°. It is of note that the uranium metallocenes such as [η 5‐1,2,4‐(Me3C)3C5H2]2U(bipy), (η 5‐C5Me5)U(bipy), [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1), (η 5‐C5Me5)2U{η 2‐C2(SiMe3)2},[ , ] [(C5Me5)2UH]2, (η 5‐C5Me5)U[(μ‐Ph)2BPh2],[ , ] [(C5Me5)2U]2(μ‐η 6:η 6‐C6H6), and (η 5‐C5Me5)U[P(SiMe3)(2,4,6‐Me3Ph)](THF) may also act as Cp2UII synthons forming bisimido uranium(VI) complexes.

Scheme 6

Synthesis of complexes 13 and 14.

Figure 7

Molecular structure of 14 (thermal ellipsoids drawn at the 35 % probability level).

Synpan class="Chemical">thesis of comclass="Chemical">plexes 13 and 14.

Molecular structure of 14 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Moreover, complex 14 may also be formed from class="Chemical">the reaction of 2 wiclass="Chemical">pan class="Chemical">th p‐tolylN3 (Scheme 7). This contrasts the transformation of the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) with organic azides, in which insertion or isomerization products were isolated. Instead the reactivity of 2 more closely resembles that observed for the bipy complexes [η 5‐1,2,4‐(Me3C)3C5H2]2An(bipy) (An=Th, U) towards p‐tolylN3.[ , ] p‐TolylN3 displaces the diphenylacetylene in 2 and releases N2 to give the imido complex [η 5‐1,2,4‐(Me3C)3C5H2]2U=N(p‐tolyl) (15), which reacts with a second molecule of p‐tolylN3 to yield the bisimido uranium(VI) compound 14 concomitant with N2 evolution (Scheme 7).

Scheme 7

Synthesis of complexes 14–16.

Synpan class="Chemical">thesis of comclass="Chemical">plexes 14–16.

Moreover, in analogy to class="Chemical">the reactivity of class="Chemical">pan class="Chemical">the bipy complex [η 5‐1,2,4‐(Me3C)3C5H2]2Th(bipy) towards Ph3CN3, the bulky trityl azide Ph3CN3 displaces the diphenylacetylene in 2 and releases N2 to give the uranium(IV) imido complex [η 5‐1,2,4‐(Me3C)3C5H2]2U=NCPh3 (16) in quantitative conversion (Scheme 7). The molecular structure of 16 is provided in Figure 8, while selected bond distances and angles are presented in Table 1. The short U−N distance (1.968(4) Å)) and the angle of U‐N(1)‐C(35) (169.3(3)°) are consistent with a U=N double bond. These structural parameters may be compared to those found in [η 5‐1,2,4‐(Me3C)3C5H2]2U=N(p‐tolyl) (15) with the U−N distance of 1.988(5) Å and the U‐N‐C angle of 172.3(5)°.

Figure 8

Molecular structure of 16 (thermal ellipsoids drawn at the 35 % probability level).

Molecular structure of 16 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Moreover, for class="Chemical">the reaction of class="Chemical">pan class="Chemical">the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) with 9‐diazofluorene (C12H8)CN2 insertion or isomerization products are isolated. This contrasts the uranium(V) imido cyanido [η 5‐1,2,4‐(Me3C)3C5H2]2U(=NN=CHSiMe3)(CN) (17) isolated from the reaction of 2 with Me3SiCHN2 (Scheme 8). To rationalize this product formation it is proposed that 2 initially reacts with 2 equiv of Me3SiCHN2 resulting in diphenylacetylene replacement followed by electron transfer to yield a uranium(VI) bisimido complex. In the next step, this bisimido complex forms a four‐membered intermediate, which converts via [1,3]‐Si migration to yield a uranium(VI) isonitrile complex, in which the N−N bond is homolytically cleaved to yield 17 and the amine radical Me3SiNH⋅. The latter further dimerizes to the hydrazine derivative (Me3SiNH)2 (Scheme 8). The molecular structure of 17 is presented in Figure 9, whereas relevant bond distances and angles are compiled in Table 1. The U−N distances are 2.227(4) Å for N(1) and 2.418(4) Å for N(2), whereas the U−C(39) distance is 2.517(6) Å. The C(39)−N(3) distance is 1.145(8) Å, whereas the C(35)−N(2) distance is 1.293(7) Å. The angle of N(1)‐U‐N(2) is 33.4(2)°, whereas the linear angle of U‐C(39)‐N(3) is 176.6(5)°. The N(1)−N(2) distance is 1.348(7) Å, and the N(2)−C(35) distance is 1.293(7) Å.

Scheme 8

Synthesis of complex 17.

Figure 9

Molecular structure of 17 (thermal ellipsoids drawn at the 35 % probability level).

Synpan class="Chemical">thesis of comclass="Chemical">plex 17.

Molecular structure of 17 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Furclass="Chemical">thermore, reclass="Chemical">placement of class="Chemical">pan class="Chemical">the coordinated diphenylacetylene with S−S and Se−Se bond cleavage are observed in the reaction of 2 with Ph2S2 or Ph2Se2, in which the disulfido complex [η 5‐1,2,4‐(Me3C)3C5H2]2U(SPh)2 (18) and the diselenido complex [η 5‐1,2,4‐(Me3C)3C5H2]2U(SePh)2 (19) are formed, respectively (Scheme 9). Figure 10 shows the molecular structure of 18 and selected bond distances and angles are compiled in Table 1. The U−S distances are 2.659(1) Å for S(1) and 2.628(1) Å for S(2), and the angle of S(1)‐U‐S(2) is 100.4(1)°.

Scheme 9

Synthesis of complexes 18–24.

Figure 10

Molecular structure of 18 (thermal ellipsoids drawn at the 35 % probability level).

Synpan class="Chemical">thesis of comclass="Chemical">plexes 18–24.

Molecular structure of 18 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Neverclass="Chemical">theless, in class="Chemical">pan class="Chemical">the presence of suitable substrates, the reactivity of the uranium metallacyclopropene 2 may also parallel its thorium analogue [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2) (Figure 3).[ , ] For example, insertion of 1 equiv of PhNCS into the uranium metallacyclopropene moiety of 2 is observed at room temperature to yield the five‐membered heterocyclic complex [η 5‐1,2,4‐(Me3C)3C5H2]2U[SC(=NPh)(C2Ph2)] (20) (Scheme 9). The molecular structure of 20 can be found in Figure 11 and selected bond distances and angles are compiled in Table 1. The U−S distance is 2.649(2) Å, whereas U−C(37) distance is 2.480(10) Å, and the angle of S(1)‐U‐C(37) is 76.0(2)°.

Figure 11

Molecular structure of 20 (thermal ellipsoids drawn at the 35 % probability level).

Molecular structure of 20 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Moreover, treatment of 2 wiclass="Chemical">th 1 eqclass="Chemical">pan class="Chemical">uiv of aldehydes RCHO (R=p‐tolyl, p‐ClPh) or ketone (CH2)5CO also gives the five‐membered heterocyclic compounds [η 5‐1,2,4‐(Me3C)3C5H2]2U[OCR(R’)(C2Ph2)] (R=H, R’=p‐tolyl (21), p‐ClPh (22); R=R’=(CH2)5 (23)) (Scheme 9). The molecular structure of 23 is shown in Figure 12, whereas the structures of 21 and 22 are provided in the Supporting Information. The U−O distances are 2.062(2) Å for 21, 2.069(2) Å for 22 and 2.076(4) Å for 23, whereas the U−C distances are 2.581(3) Å for 21 (C37), 2.577(4) Å for 22 (C43) and 2.572(6) Å for 23 (C35), and the angles of O‐U‐C are 67.6(1)° for 21 (C37), 67.5(1)° for 22 (C43) and 67.1(2)° for 23 (C35). However, when the bulky ketone Ph2CO is used as substrate, the diphenylacetylene moiety is replaced to form the uranium pinacolate [η 5‐1,2,4‐(Me3C)3C5H2]2U[(OCPh2)2] (24) (Scheme 9), irrespectively of the quantity of added Ph2CO. Product formation may be explained by diphenylacetylene substitution to form a uranium η 2‐ketone intermediate, which immediately reacts with a second molecule of Ph2CO to furnish 24 (Scheme 9).

Figure 12

Molecular structure of 23 (thermal ellipsoids drawn at the 35 % probability level).

Molecular structure of 23 (pan class="Chemical">thermal elliclass="Chemical">psoids drawn at class="Chemical">pan class="Chemical">the 35 % probability level).

Conclusions

class="Chemical">The intrinsic reactivity of a stable class="Chemical">pan class="Chemical">uranium metallacyclopropene complex, η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2), was evaluated and compared to that of the other uranium and thorium metallacyclopropenes. In analogy to the uranium metallacyclopropene derivative (η 5‐C5Me5)2U{η 2‐C2(SiMe3)2}, density functional theory (DFT) suggests that the 5f orbitals contribution to the σ and π‐bonds of the U‐(η 2‐C=C) moiety increases substantially compared to the related thorium metallacyclopropene complex, which also renders the bonds between the [η 5‐1,2,4‐(Me3C)3C5H2]2U2+ and [η 2‐C2Ph2]2− fragments more covalent than those found in the related thorium metallacyclopropene. Whereas the coordinated alkyne in the thorium metallacyclopropenes is inert to ligand substitution, it reacts as a nucleophile towards hetero‐unsaturated molecules or as a strong base inducing the inter‐ or intramolecular C−H bond activations.[ , , ] However, in analogy to the uranium metallacyclopropene (η 5‐C5Me5)2U{η 2‐C2(SiMe3)2},[ , ] the reactivity patterns of the uranium complex 2 change considerably, that is, the uranium complex 2 serves as a synthetically useful [η 5‐1,2,4‐(Me3C)3C5H2]2U(II) synthon in the reaction with Ph2E2 (E=S, Se) and unsaturated molecules such as alkynes, imines, ketazine, bipy, nitriles, organic azides, and azo derivatives, in which the coordinated diphenylacetylene was readily replaced during the reaction.

Neverclass="Chemical">theless, class="Chemical">pan class="Chemical">thorium and uranium metallacyclopropenes also exhibit similar reactivity patterns, e.g., when exposed to isothiocyanates, aldehydes and ketones, for which mono insertion of these substrates into the actinide metallacyclopropene moieties occurs to yield the five‐membered heterometallacycles.[ , , , ] However, like the thorium metallacyclopropene [η 5‐1,2,4‐(Me3C)3C5H2]2Th(η 2‐C2Ph2),[ , ] the coordinate PhCCPh in 2 is readily displaced, when the sterically encumbered Ph2CO is used as substrate, but the metallaoxirane intermediate [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐Ph2CO) is too reactive to be observed and a second molecule of Ph2CO inserts to yield the uranium pinacolate [η 5‐1,2,4‐(Me3C)3C5H2]2U[(OCPh2)2] (24). Further investigations concerning the intrinsic reactivity of actinide metallacycles are ongoing and will be reported in due course.

Experimental Section

General procedures

All reactions and product manipulations were carried out under an atmosphere of dry class="Chemical">dinitrogen wiclass="Chemical">pan class="Chemical">th rigid exclusion of air and moisture using standard Schlenk or cannula techniques, or in a glove box. All organic solvents were freshly distilled from sodium benzophenone ketyl immediately prior to use. Diphenylacetylene was purified by sublimation. [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1) and [η 5‐1,2,4‐(Me3C)3C5H2]2UCl2 (3) were prepared according to literature procedures. All other chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co. and used as received unless otherwise noted. Infrared spectra were recorded in KBr pellets on an Avatar 360 Fourier transform spectrometer. 1H and 13C{1H} NMR spectra were recorded on a Bruker AV 400 spectrometer at 400 and 100 MHz, respectively. All chemical shifts are reported in δ units with reference to the residual protons of the deuterated solvents, which served as internal standards, for proton and carbon chemical shifts. Melting points were measured on an X‐6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2)

Meclass="Chemical">thod A: A class="Chemical">pan class="Chemical">toluene (10 mL) solution of PhC≡CPh (178 mg, 1.0 mmol) was added to a toluene (10 mL) solution of [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1; 981 mg, 1.0 mmol) with stirring at room temperature. After the solution was stirred at 50 °C overnight, the solvent was removed. The residue was extracted with n‐hexane (10 mL×3) and filtered. The volume of the filtrate was reduced to 10 mL, brown crystals of 2 were isolated when this solution was kept at −20 °C for two days. Yield: 706 mg (80 %). M.p.: 178–180 °C (dec.). 1H NMR (400 MHz, C6D6): δ=26.59 (s, 4 H, phenyl), 16.62 (s, 4 H, phenyl), 10.79 (d, J=5.6 Hz, 2 H, phenyl), 9.30 (br s, 18 H, C(CH 3)3), −15.00 (br s, 18 H, C(CH 3)3), −32.03 (s, 18 H, C(CH 3)3) ppm; ring C−H atoms were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=202.7 (UC), 201.8 (phenyl C), 201.0 (phenyl C), 151.4 (phenyl C), 138.4 (phenyl C), 137.9 (C(CH3)3), 137.3 (C(CH3)3), 136.7 (C(CH3)3), 85.8 (C(CH3)3), −50.1 (ring C), −51.1 (ring C) ppm; one ring C overlapped. IR (KBr): =2960 (s), 1460 (m), 1384 (m), 1259 (s), 1093 (s), 1020 (s), 800 (s) cm−1. Anal. Calcd for C48H68U: C, 65.28; H, 7.76. Found: C, 65.35; H, 7.73.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhC≡CPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U=P‐2,4,6‐tBu3C6H2 (1; 20 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 2 along with those of 3,3‐Me2‐5,7‐tBu2C8H5P (1H NMR (400 MHz, C6D6): δ=7.46 (dd, J=3.8, 1.5 Hz, 2 H, phenyl), 4.39 (ddd, J=181.6, 11.9, 7.9 Hz, 1 H, PH), 1.59 (d, J=3.6 Hz, 1 H, CH 2), 1.56 (s, 9 H, (CH 3)3C), 1.34 (s, 3 H, CH 3), 1.31 (s, 9 H, (CH 3)3C), 1.29 (d, J=3.6 Hz, 1 H, CH 2), 1.11 (s, 3 H, CH 3) ppm) were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 50 °C overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2) and [η 5‐1,2,4‐(Me3C)3C5H2]2UCl (4)

class="Chemical">KC8 (1.20 g, 8.80 mmol) was added to a class="Chemical">pan class="Chemical">toluene (20 mL) solution of [η 5‐1,2,4‐(Me3C)3C5H2]2UCl2 (3; 1.94 g, 2.5 mmol) and diphenylacetylene (0.45 g, 2.5 mmol) with stirring at room temperature. After this solution was stirred one day at 40 °C, the solvent was removed. The residue was extracted with n‐hexane (20 mL×3) and filtered. The volume of the filtrate was reduced to 15 mL, green microcrystals were isolated when this solution was kept at −20 °C for 2 days. The 1H NMR spectrum recorded in C6D6 showed the present of 2 and 4 (1H NMR (400 MHz, C6D6): δ=−7.95 (s, 36 H, C(CH 3)3), −25.40 (s, 18 H, C(CH 3)3) ppm; protons of the rings were not observed) in a 1:3 ratio. Unfortunately, this mixture could not be converted to exclusively yield 2 upon prolonged reduction in the presence of diphenylacetylene with an excess of potassium graphite (KC8). Under these conditions, some other yet unidentified species were formed. In addition, on a synthetic scale the mixture of complex 2 and 4 could not be separated to yield pure materials because of their similar solubilities. However, a few green crystals of 4 suitable for X‐ray diffraction analysis were selected from those microcrystals that recrystallized from an n‐hexane at −20 °C, and the molecular structure of 4 was further verified by X‐ray diffraction analysis (see Supporting Information for details).

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C4Ph2) (5)

Meclass="Chemical">thod A: A class="Chemical">pan class="Chemical">toluene (10 mL) solution of PhC≡CC≡CPh (51 mg, 0.25 mmol) was added to a toluene (10 mL) solution of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) with stirring at room temperature. After the solution was stirred at 40 °C for one week, the solvent was removed. The residue was extracted with n‐hexane (10 mL×3) and filtered. The volume of the filtrate was reduced to 10 mL, brown crystals of 5 were isolated when this solution was kept at −20 °C for two days. Yield: 186 mg (82 %). M.p.: 117–119 °C (dec.). 1H NMR (400 MHz, C6D6): δ=14.35 (s, 4 H, phenyl), 10.23 (s, 18 H, C(CH 3)3), 8.70 (s, 4 H, phenyl), 8.30 (s, 2 H, phenyl), −1.28 (br s, 18 H, C(CH 3)3), −15.18 (br s, 18 H, C(CH 3)3) ppm; ring C−H atoms were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=294.5 (UCPh), 207.7 (UC), 179.1 (ring C), 175.5 (ring C), 139.2 (phenyl C), 132.7 (phenyl C), 129.2 (phenyl C), 128.3 (phenyl C), 49.4 (C(CH3)3), 32.3 (C(CH3)3), 31.9 (C(CH3)3), 29.8 (C(CH3)3) ppm; other carbon atoms overlapped. IR (KBr): =2958 (s), 1952 (w, C=C=C=C), 1460 (s), 1361 (s), 1238 (s), 1097 (s), 1070 (s), 1024 (s), 825 (s) cm−1. Anal. Calcd for C50H68U: C, 66.20; H, 7.56. Found: C, 66.25; H, 7.53.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhC≡CC≡CPh (4.0 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 5 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 40 °C for one week.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐CHPhNPh) (6)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and PhCH=NPh (46 mg, 0.25 mmol) in toluene (15 mL) at 100 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 5. Yield: 177 mg (80 %). 1H NMR (400 MHz, C6D6): δ=129.18 (s, 1 H, CHPh), 34.03 (s, 1 H, phenyl), 26.49 (s, 2 H, phenyl), 23.78 (s, 1 H, phenyl), 13.63 (s, 9 H, C(CH 3)3), 13.36 (s, 2 H, phenyl), 12.24 (s, 9 H, C(CH 3)3), 7.42 (s, 1 H, phenyl), −0.60 (s, 1 H, phenyl), −2.57 (s, 1 H, phenyl), −10.04 (s, 9 H, C(CH 3)3), −17.56 (s, 9 H, C(CH 3)3), −35.03 (s, 9 H, C(CH 3)3), −42.50 (s, 9 H, C(CH 3)3), −68.53 (s, 1 H, phenyl) ppm; ring C−H atoms were not observed. These spectroscopic data agreed with those reported in the literature.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhCH=NPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 6 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 100 °C for 5 days.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(N=CPh2)2 (7)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and (Ph2C=N)2 (90 mg, 0.25 mmol) in toluene (15 mL) at 100 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 221 mg (83 %). 1H NMR (400 MHz, C6D6): δ=33.06 (br s, 2 H, ring CH), 14.59 (br s, 6 H, C(CH 3)3), 12.45 (br s, 18 H, C(CH 3)3), 7.70 (s, 1 H, phenyl), 7.41 (s, 2 H, phenyl), 7.37 (s, 1 H, phenyl), 7.04 (s, 2 H, phenyl), 2.29 (s, 18 H, C(CH 3)3), 1.45 (s, 9 H, phenyl), 1.28 (s, 5 H, phenyl), −23.34 (br s, 12 H, C(CH 3)3), −75.71 (br s, 2 H, ring CH) ppm. These spectroscopic data were in line with those reported in the literature.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of (Ph2C=N)2 (7.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 7 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 100 °C for 5 days.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(bipy) (8)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as green microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and bipy (39 mg, 0.25 mmol) in toluene (15 mL) at 100 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 181 mg (84 %). 1H NMR (400 MHz, C6D6): δ=1.26 (s, 4 H, ring CH), 1.17 (s, 36 H, C(CH 3)3), −7.47 (d, J=4.9 Hz, 2 H, bipy), −9.01 (s, 18 H, C(CH 3)3), −58.93 (s, 2 H, bipy), −99.40 (s, 2 H, bipy), −125.80 (s, 2 H, bipy) ppm. These spectroscopic data agreed with those reported in the literature.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of bipy (3.1 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 8 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 100 °C for 3 days.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[(N=C(C6H11))2] (9)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and C6H11CN (55 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 188 mg (78 %). M.p.: 165–167 °C (dec.). 1H NMR (400 MHz, C6D6): δ=19.82 (br s, 2 H, Cy), 18.03 (s, 2 H, CH), 15.00 (br s, 2 H, Cy), 13.53 (br s, 2 H, Cy), 12.61 (br s, 2 H, Cy), 11.47 (s, 18 H, C(CH 3)3), 9.55 (br s, 2 H, Cy), 8.89 (br s, 2 H, Cy), 6.30 (br s, 4 H, Cy), 5.32 (d, J=15.2 Hz, 2 H, Cy), 4.93 (d, J=12.8 Hz, 2 H, Cy), −10.01 (s, 36 H, C(CH 3)3) ppm; protons of CpH were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=216.1 (N=C), 77.0 (CH), 41.6 (C(CH3)3), 32.5 (C(CH3)3), 31.7 (C(CH3)3), 16.7 (CH2), 5.7 (CH2), −18.9 (ring C), −38.8 (ring C) ppm; other carbons overlapped. IR (KBr): =2928 (s), 1450 (s), 1359 (s), 1240 (s), 1180 (m), 964 (s), 763 (s) cm−1. Anal. Calcd for C48H80N2U: C, 62.45; H, 8.73; N, 3.03. Found: C, 62.41; H, 8.76; N, 3.02. Brown crystals of 9⋅0.5C6H14 suitable for X‐ray structural analysis were grown from an n‐hexane solution.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of C6H11CN (4.4 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 9 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2) with C6H11CN

NMR scale: A class="Chemical">C6D6 (0.3 mL) solution of class="Chemical">pan class="Chemical">C6H11CN (2.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 9 along with those of unreacted 2 and PhC≡CPh were observed by 1H NMR spectroscopy (50 % conversion based on 2) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[(N=C(CHPh2))2] (10)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Ph2CHCN (97 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 5. Yield: 218 mg (80 %). M.p.: 104–106 °C (dec.). 1H NMR (400 MHz, C6D6): δ=39.49 (s, 1 H, phenyl), 29.51 (s, 1 H, phenyl), 23.77 (s, 3 H, phenyl), 16.10 (s, 3 H, phenyl), 13.97 (s, 18 H, C(CH 3)3), 11.00 (s, 3 H, phenyl), 10.45 (s, 1 H, phenyl), −1.31 (s, 2 H, CH), −13.12 (s, 18 H, C(CH 3)3), −16.00 (s, 18 H, C(CH 3)3), −21.16 (s, 7 H, phenyl), −62.00 (s, 1 H, phenyl) ppm; protons of the rings were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=235.1 (C=N), 160.1 (phenyl C), 158.5 (phenyl C), 145.8 (phenyl C), 141.0 (phenyl C), 137.8 (phenyl C), 135.7 (phenyl C), 131.5 (phenyl C), 123.7 (phenyl C), 120.6 (phenyl C), 52.9 (C(CH3)3), 49.9 (C(CH3)3), 35.1 (C(CH3)3), 17.0 (C(CH3)3), −3.4 (ring C), −42.8 (ring C), −50.3 (ring C) ppm; other carbons overlapped. IR (KBr): =2957 (s), 1595 (s), 1554 (s), 1492 (s), 1452 (s), 1359 (s), 1238 (s), 964 (s), 812 (s) cm−1. Anal. Calcd for C62H80N2U: C, 68.23; H, 7.39; N, 2.57. Found: C, 68.21; H, 7.36; N, 2.60.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Ph2CHCN (7.7 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 10 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2) with Ph2CHCN

NMR scale: A class="Chemical">C6D6 (0.3 mL) solution of class="Chemical">pan class="Chemical">Ph2CHCN (3.9 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 10 along with those of unreacted 2 and PhC≡CPh were observed by 1H NMR spectroscopy (50 % conversion based on 2) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[N=C(Ph)(C2Ph2)] (11)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and PhCN (26 mg, 0.25 mmol) in toluene (15 mL) and recrystallization from an n‐hexane solution by a similar procedure as in the synthesis of 5. Yield: 212 mg (86 %). M.p.: 107–109 °C. 1H NMR (400 MHz, C6D6): δ=36.09 (s, 2 H, ring CH), 18.02 (s, 18 H, C(CH 3)3), 17.73 (s, 2 H, phenyl), 14.80 (s, 3 H, phenyl), 10.61 (s, 2 H, phenyl), 9.98 (s, 1 H, phenyl), 3.67 (s, 18 H, C(CH 3)3), −2.35 (s, 2 H, phenyl), −3.66 (s, 1 H, phenyl), −9.08 (s, 1 H, phenyl), −12.03 (s, 1 H, phenyl), −22.14 (s, 2 H, phenyl), −22.84 (s, 18 H, C(CH 3)3), −33.17 (s, 2 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=266.2 (UCPh), 230.6 (CPh), 181.4 (C=N), 155.4 (phenyl C), 134.2 (phenyl C), 134.0 (phenyl C), 130.4 (phenyl C), 126.9 (phenyl C), 126.3 (phenyl C), 119.7 (phenyl C), 116.0 (phenyl C), 114.8 (phenyl C), 108.3 (phenyl C), 107.4 (phenyl C), 101.7 (phenyl C), 86.7 (C(CH3)3), 84.9 (C(CH3)3), 52.2 (C(CH3)3), 47.4 (C(CH3)3), 44.1 (C(CH3)3), −1.3 (ring C), −1.8 (ring C), −2.4 (ring C), −4.9 (ring C), −57.2 (ring C) ppm; one C resonance of Me3 C‐groups overlapped. IR (KBr): =2958 (s), 1458 (s), 1361 (s), 1261 (s), 1238 (s), 1095 (s), 1072 (s), 1022 (s), 806 (s) cm−1. Anal. Calcd for C55H73NU: C, 66.98; H, 7.46; N, 1.42. Found: C, 67.02; H, 7.43; N, 1.41.

Meclass="Chemical">thod B, NMR Scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhCN (2.1 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 11 were observed by 1H NMR spectroscopy (100 % conversion in 10 min).

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[NHC(=CHPh)(C2Ph2)] (12)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and PhCH2CN (30 mg, 0.25 mmol) in toluene (15 mL) and recrystallization from an n‐hexane solution by a similar procedure as in the synthesis of 5. Yield: 205 mg (82 %). M.p.: 123–125 °C. 1H NMR (400 MHz, C6D6): δ=16.49 (s, 18 H, C(CH 3)3), 14.85 (s, 1 H, NH), 8.72 (s, 2 H, ring CH), 6.00 (s, 2 H, phenyl), 4.51 (t, J=6.5 Hz, 1 H, phenyl), 3.90 (t, J=6.8 Hz, 2 H, phenyl), 3.62 (s, 1 H, phenyl), 3.09 (d, J=6.2 Hz, 1 H, phenyl), 2.24 (d, J=7.2 Hz, 2 H, phenyl), 0.68 (d, J=7.8 Hz, 2 H, phenyl), −0.50 (s, 1 H, PhCH), −1.68 (s, 1 H, phenyl), −2.08 (s, 18 H, C(CH 3)3), −6.64 (t, J=6.4 Hz, 1 H, phenyl), −8.30 (s, 2 H, ring CH), −19.26 (s, 18 H, C(CH 3)3), −38.49 (d, J=4.6 Hz, 2 H, phenyl) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=322.1 (UCPh), 318.5 (CPh), 302.8 (CNH), 214.5 (phenyl C), 155.8 (phenyl C), 142.0 (phenyl C), 123.6 (phenyl C), 118.8 (phenyl C), 115.0 (phenyl C), 107.8 (ring C), 98.6 (ring C), 97.3 (ring C), 97.1 (ring C), 96.9 (ring C), 78.4 (CHPh), 48.8 (C(CH3)3), 43.2 (C(CH3)3) ppm; other carbons overlapped. IR (KBr): =2958 (s), 1591 (m), 1456 (m), 1361 (s), 1240 (s), 1072 (s), 1028 (s), 808 (s) cm−1. Anal. Calcd for C56H75NU: C, 67.24; H, 7.56; N, 1.40. Found: C, 67.26; H, 7.53; N, 1.41.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhCH2CN (2.4 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 12 were observed by 1H NMR spectroscopy (100 % conversion in 10 min).

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(=NPh)2 (13)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and PhN=NPh (46 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 186 mg (84 %). 1H NMR (400 MHz, C6D6): δ=9.48 (t, J=7.5 Hz, 4 H, phenyl), 4.99 (s, 4 H, ring CH), 3.07 (d, J=6.6 Hz, 4 H, phenyl), 1.65 (s, 36 H, C(CH 3)3), 1.62 (s, 18 H, C(CH 3)3), 0.17 ppm (t, J=7.2 Hz, 2 H, phenyl). These spectroscopic data agreed with those reported in the literature.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhN=NPh (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 13 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 50 °C for 5 days.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[=N(p‐tolyl)]2 (14)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and bis(p‐tolyl)diazene (53 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 183 mg (80 %). M.p.: 185–187 °C. 1H NMR (400 MHz, C6D6): δ=9.34 (d, J=7.6 Hz, 4 H, phenyl), 8.08 (s, 6 H, CH 3), 5.00 (s, 4 H, ring CH), 2.83 (d, J=4.8 Hz, 4 H, phenyl), 1.67 (s, 36 H, C(CH 3)3), 1.63 (s, 18 H, C(CH 3)3) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=187.2 (phenyl C), 166.7 (phenyl C), 142.8 (phenyl C), 140.3 (phenyl C), 118.8 (ring C), 105.9 (ring C), 104.6 (ring C), 38.1 (C(CH3)3), 37.9 (C(CH3)3), 35.8 (C(CH3)3), 31.5 (C(CH3)3), 23.6 (CH3) ppm. IR (KBr): =2958 (s), 1460 (s), 1361 (s), 1240 (s), 1099 (s), 821 (s) cm−1. Anal. Calcd for C48H72N2U: C, 63.00; H, 7.93; N, 3.06. Found: C, 63.04; H, 7.93; N, 3.04.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of bis(p‐tolyl)diazene (4.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 14 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 50 °C for 5 days.

Meclass="Chemical">thod C, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of p‐tolylN3 (5.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 14 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U=NCPh3 (16)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Ph3CN3 (72 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 209 mg (83 %). M.p.: 173–175 °C. 1H NMR (400 MHz, C6D6): δ=85.85 (s, 2 H, ring CH), 37.79 (s, 6 H, phenyl), 18.84 (s, 18 H, C(CH 3)3), 12.51 (s, 6 H, phenyl), 10.19 (s, 3 H, phenyl), −18.22 (s, 18 H, C(CH 3)3), −45.33 (s, 18 H, C(CH 3)3), −47.73 (s, 2 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=172.6 (phenyl C), 171.9 (phenyl C), 170.8 (phenyl C), 159.5 (phenyl C), 143.6 (ring C), 141.0 (ring C), 139.5 (ring C), 100.3 (CPh3), 58.2 (C(CH3)3), 57.8 (C(CH3)3), 31.9 (C(CH3)3), 31.8 (C(CH3)3) ppm; other carbons overlapped. IR (KBr): =2957 (s), 1485 (m), 1357 (m), 1236 (m), 1089 (s), 1066 (s), 1030 (s), 806 (s) cm−1. Anal. Calcd for C53H73NU: C, 66.16; H, 7.65; N, 1.46. Found: C, 66.14; H, 7.69; N, 1.44. Brown crystals of 16⋅0.5C6H14 suitable for X‐ray structural analysis were grown from an n‐hexane solution.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Ph3CN3 (5.7 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 16 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(=NN=CHSiMe3)(CN) (17)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as orange crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Me3SiCHN2 (58 mg, 0.50 mmol) in toluene (15 mL) at room temperature and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 5. Yield: 152 mg (72 %). M.p.: 149–151 °C. 1H NMR (400 MHz, C6D6): δ=4.40 (s, 18 H, C(CH 3)3), −1.65 (s, 18 H, C(CH 3)3), −1.81 (s, 9 H, Si(CH 3)3), −3.46 (s, 18 H, C(CH 3)3) ppm; protons of CpH and CHSi were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=198.2 (CN), 90.1 (CHSi), 38.1 (C(CH3)3), 35.0 (C(CH3)3), 34.9 (C(CH3)3), 33.3 (C(CH3)3), 30.1 (C(CH3)3), 29.6 (C(CH3)3), −2.6 (ring C), −3.2 (ring C), −7.0 (ring C), −7.2 (ring C), −8.9 (SiCH3), −10.4 (ring C) ppm. 29Si{1H} NMR (C6D6): δ=−1.1 ppm. IR (KBr): =2957 (s), 2069 (s), 1944 (w, CN), 1599 (m), 1564 (s), 1492 (m), 1458 (m), 1357 (s), 1242 (s), 1166 (m), 839 (s) cm−1. Anal. Calcd for C39H68N3SiU: C, 55.43; H, 8.11; N, 4.97. Found: C, 55.44; H, 8.09; N, 5.00.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Me3SiCHN2 (4.6 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 17 along with those of PhC≡CPh and (Me3SiNH)2 (1H NMR (400 MHz, C6D6): δ=2.43 (s, 2 H, NH), 0.28 (s, 18 H, Si(CH 3)3) ppm) were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(SPh)2 (18)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Ph2S2 (55 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 5. Yield: 201 mg (87 %). M.p.: 168–170 °C (dec.). 1H NMR (400 MHz, C6D6): δ=6.08 (s, 36 H, C(CH 3)3), −0.28 (d, J=5.8 Hz, 2 H, phenyl), −0.35 (s, 4 H, phenyl), −9.47 (s, 18 H, C(CH 3)3), −24.22 (s, 4 H, phenyl) ppm; ring C−H atoms were not observed. 13C{1H} NMR (100 MHz, C6D6): δ=165.4 (ring C), 164.6 (ring C), 163.7 (ring C), 133.4 (phenyl C), 133.2 (phenyl C), 103.0 (phenyl C), 102.8 (phenyl C), 67.4 (C(CH3)3), 46.0 (C(CH3)3), 39.2 (C(CH3)3) ppm; other carbons overlapped. IR (KBr): =2958 (s), 1577 (s), 1473 (s), 1361 (s), 1238 (s), 1080 (s), 1024 (s), 831 (s) cm−1. Anal. Calcd for C46H68S2U: C, 59.85; H, 7.42. Found: C, 59.82; H, 7.43.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Ph2S2 (4.4 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 18 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 50 °C overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U(SePh)2 (19)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Ph2Se2 (78 mg, 0.25 mmol) in toluene (15 mL) at 50 °C and recrystallization from an n‐hexane solution by a similar procedure as that in the synthesis of 4. Yield: 193 mg (82 %). M.p.: 134–136 °C (dec.). 1H NMR (400 MHz, C6D6): δ=5.76 (br s, 36 H, C(CH 3)3), 0.99 (s, 2 H, phenyl), −8.42 (s, 18 H, C(CH 3)3), −20.14 (s, 4 H, phenyl), −21.06 (s, 4 H, phenyl), −22.64 (s, 4 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=174.1 (ring C), 173.4 (ring C), 172.6 (ring C), 134.0 (phenyl C), 129.2 (phenyl C), 118.6 (phenyl C), 103.9 (phenyl C), 69.1 (C(CH3)3), 48.4 (C(CH3)3), 32.6 (C(CH3)3), 31.5 (C(CH3)3) ppm. IR (KBr): =2958 (s), 1575 (s), 1471 (s), 1361 (s), 1238 (s), 1020 (s), 831 (m), 732 (s) cm−1. Anal. Calcd for C40H63Se2U: C, 51.12; H, 6.76. Found: C, 51.14; H, 6.73.

Meclass="Chemical">thod B. NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Ph2Se2 (6.2 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 19 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 50 °C overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[SC(=NPh)(C2Ph2)] (20)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as brown crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and PhNCS (34 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as in the synthesis of 5. Yield: 209 mg (82 %). M.p.: 155–157 °C. 1H NMR (400 MHz, C6D6): δ=53.36 (s, 2 H, ring CH), 21.16 (s, 18 H, C(CH 3)3), 4.29 (s, 1 H, phenyl), 4.04 (s, 2 H, phenyl), 3.52 (s, 2 H, phenyl), 3.03 (d, J=5.8 Hz, 2 H, phenyl), 2.52 (s, 1 H, phenyl), 1.88 (d, J=3.6 Hz, 2 H, phenyl), 1.22 (s, 1 H, phenyl), −5.65 (s, 2 H, phenyl), −7.25 (s, 18 H, C(CH 3)3), −7.87 (s, 2 H, phenyl), −18.18 (s, 18 H, C(CH 3)3), −33.80 (s, 2 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=204.7 (UCPh), 143.6 (phenyl C), 125.1 (phenyl C), 123.4 (phenyl C), 122.4 (phenyl C), 120.2 (phenyl C), 118.9 (phenyl C), 110.5 (ring C), 110.3 (ring C), 103.7 (ring C), 97.9 (ring C), 97.1 (ring C), 96.3 (CPh), 92.2 (C=N), 60.3 (C(CH3)3), 41.7 (C(CH3)3), 41.6 (C(CH3)3) ppm; other carbons overlapped. IR (KBr): =2957 (s), 1593 (m), 1491 (s), 1384 (s), 1361 (s), 1217 (s), 1112 (s), 823 (s) cm−1. Anal. Calcd for C55H73NSU: C, 64.87; H, 7.23, N, 1.38. Found: C, 64.85; H, 7.24, N, 1.35.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of PhNCS (2.7 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 20 were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[OCH(p‐tolyl)(C2Ph2)]⋅0.5C6H6 (21⋅0.5C6H6)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as orange crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and p‐tolylCHO (30 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as in the synthesis of 5. Yield: 219 mg (84 %). M.p.: 139–141 °C. 1H NMR (400 MHz, C6D6): δ=126.96 (s, 1 H, ring CH), 77.14 (s, 1 H, ring CH), 41.94 (s, 2 H, phenyl), 19.77 (s, 2 H, phenyl), 17.39 (br s, 20 H, phenyl, OCH and C(CH 3)3), 15.38 (s, 2 H, phenyl), 9.44 (s, 1 H, phenyl), 9.32 (s, 2 H, phenyl), 9.11 (s, 3 H, CH 3), 7.15 (s, 3 H, C6 H 6), −4.06 (s, 9 H, C(CH 3)3), −4.51 (s, 2 H, phenyl), −5.80 (s, 2 H, phenyl), −11.94 (s, 9 H, C(CH 3)3), −17.51 (s, 9 H, C(CH 3)3), −19.11 (s, 9 H, C(CH 3)3), −31.77 (s, 1 H, ring CH), −58.53 ppm (s, 1 H, ring CH). 13C{1H} NMR (100 MHz, C6D6): δ=247.6 (UCPh), 171.1 (CPh), 154.5 (phenyl C), 149.5 (phenyl C), 134.5 (phenyl C), 128.5 (C 6H6), 123.3 (ring C), 119.1 (ring C), 112.4 (ring C), 100.1 (ring C), 66.0 (CHO), 45.6 (C(CH3)3), 29.1 (C(CH3)3), 27.1 (C(CH3)3), 16.6 (CH3) ppm; other carbons overlapped. IR (KBr): =2958 (s), 1384 (m), 1359 (s), 1240 (s), 1060 (s), 1003 (s), 821 (s) cm−1. Anal. Calcd for C59H79OU: C, 67.99; H, 7.64. Found: C, 67.97; H, 7.62.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of p‐tolylCHO (2.4 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 21 were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[OCH(p‐ClPh)(C2Ph2)]⋅1.5C6H6 (22⋅1.5C6H6)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as orange crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and p‐ClPhCHO (35 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as in the synthesis of 5. Yield: 245 mg (86 %). M.p.: 143–145 °C. 1H NMR (400 MHz, C6D6): δ=122.11 (s, 1 H, ring CH), 74.37 (s, 1 H, ring CH), 40.61 (s, 2 H, phenyl), 19.46 (s, 2 H, phenyl), 15.39 (br s, 20 H, phenyl, OCH and C(CH 3)3), 14.95 (s, 2 H, phenyl), 9.36 (s, 1 H, phenyl), 9.19 (s, 2 H, phenyl), 7.15 (s, 9 H, C6 H 6), −3.62 (s, 9 H, C(CH 3)3), −4.31 (s, 2 H, phenyl), −5.50 (s, 2 H, phenyl), −11.38 (s, 9 H, C(CH 3)3), −16.82 (s, 9 H, C(CH 3)3), −18.65 (s, 9 H, C(CH 3)3), −30.75 (s, 1 H, ring CH), −53.49 (s, 1 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=245.5 (UCPh), 171.1 (CPh), 150.6 (phenyl C), 149.6 (phenyl C), 148.7 (phenyl C), 147.8 (phenyl C), 134.5 (phenyl C), 129.3 (phenyl C), 128.5 (C 6H6), 125.6 (phenyl C), 123.3 (ring C), 119.4 (ring C), 112.2 (ring C), 112.0 (ring C), 100.6 (ring C), 65.1 (CHO), 46.2 (C(CH3)3), 25.5 (C(CH3)3), 17.8 (C(CH3)3), 7.9 (C(CH3)3) ppm. IR (KBr): =2958 (s), 1487 (s), 1361 (s), 1240 (s), 1087 (s), 1070 (s), 1004 (s), 821 (s) cm−1. Anal. Calcd for C64H82ClOU: C, 67.38; H, 7.24. Found: C, 67.36; H, 7.22.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of p‐ClPhCHO (2.8 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 22 were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U{OC[(CH2)5](C2Ph2)} (23)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as orange microcrystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and (CH2)5CO (25 mg, 0.25 mmol) in toluene (15 mL) at room temperature and recrystallization from a benzene solution by a similar procedure as in the synthesis of 5. Yield: 195 mg (76 %). M.p.: 161–163 °C. 1H NMR (400 MHz, C6D6): δ=55.40 (s, 2 H, ring CH), 28.55 (br s, 4 H, phenyl), 22.65 (s, 2 H, phenyl), 19.72 (s, 4 H, phenyl), 12.95 (s, 1 H, Cy), 10.86 (s, 4 H, Cy), 9.88 (s, 2 H, Cy), −5.09 (s, 3 H, Cy), −6.57 (s, 9 H, C(CH 3)3), −15.85 (br s, 45 H, C(CH 3)3), −32.54 (s, 2 H, ring CH) ppm. 13C{1H} NMR (100 MHz, C6D6): δ=170.1 (CPh), 134.0 (phenyl C), 132.3 (phenyl C), 129.4 (phenyl C), 127.9 (phenyl C), 122.4 (ring C), 120.2 (ring C), 117.9 (ring C), 99.0 (ring C), 61.3 (CO), 47.0 (C(CH3)3), 44.1 (C(CH3)3), 35.7 (CH2), 30.5 (CH2), 29.7 (CH2) ppm; other carbons were not observed. IR (KBr): =2960 (s), 1384 (s), 1259 (s), 1089 (s), 1022 (s), 798 (s) cm−1. Anal. Calcd for C54H78OU: C, 66.10; H, 8.01. Found: C, 66.08; H, 8.02. Brown crystals of 23⋅0.5C6H14 suitable for X‐ray structural analysis were grown from an n‐hexane solution.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of (CH2)5CO (2.0 mg; 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 23 were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at room temperature overnight.

Preparation of [η 5‐1,2,4‐(Me3C)3C5H2]2U[(OCPh2)2] (24)

Meclass="Chemical">thod A: class="Chemical">pan class="Chemical">This compound was prepared as orange crystals from the reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 221 mg, 0.25 mmol) and Ph2CO (91 mg, 0.50 mmol) in toluene (15 mL) at 60 °C and recrystallization from a benzene solution by a similar procedure as that in the synthesis of 5. Yield: 224 mg (84 %). 1H NMR (400 MHz, C6D6): δ=61.95 (s, 2 H, ring CH), 27.41 (s, 1 H, phenyl), 23.01 (s, 1 H, phenyl), 16.08 (s, 1 H, phenyl), 13.84 (s, 18 H, C(CH 3)3), 13.42 (s, 1 H, phenyl), 9.72 (s, 2 H, phenyl), 8.43 (s, 2 H, phenyl), 7.66 (s, 4 H, phenyl), 7.01 (s, 5 H, phenyl), 4.61 (s, 1 H, phenyl), 2.90 (s, 1 H, phenyl), −1.30 (s, 1 H, phenyl), −5.29 (s, 18 H, C(CH 3)3), −23.24 (s, 2 H, ring CH), −43.97 (s, 18 H, C(CH 3)3) ppm. These spectroscopic data agreed with those reported in the literature.

Meclass="Chemical">thod B, NMR scale: A class="Chemical">pan class="Chemical">C6D6 (0.3 mL) solution of Ph2CO (7.3 mg, 0.04 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.2 mL). Resonances of 24 along with those of PhC≡CPh were observed by 1H NMR spectroscopy (100 % conversion) after the sample was kept at 60 °C for 5 days.

Reaction of [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2) with Ph2CO

NMR scale: A class="Chemical">C6D6 (0.2 mL) solution of class="Chemical">pan class="Chemical">Ph2CO (3.6 mg, 0.02 mmol) was slowly added to a J. Young NMR tube charged with [η 5‐1,2,4‐(Me3C)3C5H2]2U(η 2‐C2Ph2) (2; 18 mg, 0.02 mmol) and C6D6 (0.3 mL). Resonances of 24 along with those of unreacted 2 and PhC≡CPh were observed by 1H NMR spectroscopy (50 % conversion based on 2) after the sample was kept at 60 °C for 5 days.

X‐ray crystallography

Single‐crystal X‐ray diffraction measurements were carried out on a Rigaku Saturn CCD diffractometer at 100(2) K using class="Chemical">graphite monochromated CuKα radiation (λ=1.54184 Å). An emclass="Chemical">pirical absorclass="Chemical">ption correction was aclass="Chemical">pclass="Chemical">plied using class="Chemical">pan class="Chemical">the SADABS program. All structures were solved by direct methods and refined by full‐matrix least squares on F 2 using the SHELXL program package. All the hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 2, 5, 9, 10, 12, 14, 16–18 and 20–23 are summarized in the Supporting Information. Selected bond lengths and angles are listed in Table 1. It is of note that the structural data of 2 were relatively poor due to crystal twinning, which led to a large positive residual density (9.77 e A−3) close to the uranium atom (0.99 Å) and also to low bond precision within the C−C distances (0.02776 Å). These B level alerts in the checkCIF could not be removed on refinement.

Deposition numbers 2054372 ( contain class="Chemical">the suclass="Chemical">pclass="Chemical">plementary crystallograclass="Chemical">phic data for class="Chemical">pan class="Chemical">this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Computational methods

All calculations were carried out wiclass="Chemical">th class="Chemical">pan class="Chemical">the Gaussian 09 program (G09), employing the B3PW91 functional, plus a polarizable continuum model (PCM) (denoted as B3PW91‐PCM), with standard 6‐31G(d) basis set for C and H and a quasi‐relativistic 5f‐in‐valence effective‐core potential (ECP60MWB) treatment with 60 electrons in the core region for U and the corresponding optimized segmented ((14s13p10d8f6g)/[10s9p5d4f3g]) basis set for the valence shells of U, to fully optimize the geometries of the complexes.

Conflict of interest

pan class="Chemical">The auclass="Chemical">pan class="Chemical">thors declare no conflict of interest.

As a class="Chemical">service to our auclass="Chemical">pan class="Chemical">thors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

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