Reactions of Mesityl Azide with Ferrocene-Based N-Heterocyclic Germylenes, Stannylenes and Plumbylenes, Including PPh2 -Functionalised Congeners.

The reactivity of ferrocene-based N-heterocyclic tetrylenes [{Fe(η5 -C5 H4 -NSitBuMe2 )2 }E] (E=Ge, Sn, Pb) towards mesityl azide (MesN3 ) is compared with that of PPh2 -functionalised congeners exhibiting two possible reaction sites, namely the EII and PIII atom. For E=Ge and Sn the reaction occurs at the EII atom, leading to the formation of N2 and an EIV =NMes unit. The germanimines are sufficiently stable for isolation. The stannanimines furnish follow-up products, either by [2+3] cycloaddition with MesN3 or, in the PPh2 -substituted case, by NMes transfer from the SnIV to the PIII atom. Whereas [{Fe(η5 -C5 H4 -NSitBuMe2 )2 }Pb] and other diaminoplumbylenes studied are inert even under forcing conditions, the PPh2 -substituted congener forms an addition product with MesN3 , thus showing a behaviour similar to that of frustrated Lewis pairs. The germylenes of this study afford copper(I) complexes with CuCl, including the first structurally characterised linear dicoordinate halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Introduction

Since more than a century, the Staudinger reaction of organic azides RN3 with phosphanes R’3P (R, R’=alkyl or aryl) has provided access to iminophosphoranes RN=PR’3 via intermediate phosphazides RN=N−N=PR’3, which are generally unstable, but could be isolated in certain cases. A significant kinetic stabilisation of phosphazides by intramolecular coordination of the P‐bonded nitrogen atom to a Lewis acid was suggested by Grützmacher in 1999. In view of the well‐known analogy between phosphanes and N‐heterocyclic carbenes (NHCs), it is not surprising that organic azides can react with NHCs in a similar manner to afford cyclic guanidine derivatives NHC=NR, as was shown by Bielawski in 2005. The intermediate triazenes NHC=N−N=NR are thermally much more robust towards N2 extrusion than the analogous phosphazides RN=N−N=PR’3. We note in this context that the synthesis of imines of the type RN=CCl2 (i. e. isocyanide dichlorides) by reaction of organic azides with the transient singlet carbene Cl2C had been reported by Baldwin already in 1968; N2 extrusion occurs even below room temperature in this case. The heavier carbene analogues can react with organic azides in a similar fashion, giving rise to imine analogues with formal E=N (E=Si–Pb) double bonds. This was first shown by Satgé in 1978, who reported the formation of the transient germanimine (Me2N)2Ge=NPh together with N2 in the reaction of the diaminogermylene (Me2N)2Ge with PhN3. In 1991 Meller described the first structurally characterised stable germanimines [(Me3Si)ArN]2Ge=NAr [obtained from the reaction of ArN3 with [(Me3Si)ArN]2Ge, Ar=mesityl (Mes) or 2,6‐diisopropylphenyl (Dipp)]. This was followed in 1993 by the first structurally characterised stable stannanimine [(Me3Si)2N]2Sn=NDipp (obtained from DippN3 and [(Me3Si)2N]2Sn at −30 °C). The only other stable stannanimine known to date, [(Me3Si)2CH]2Sn=N[SitBu2(N3)] [obtained from [(Me3Si)2CH]2Sn and SitBu2(N3)2], was reported one year later by Ando. The paucity of isolable stannanimines is due to the fact that, owing to their reactive polar Sn=N bond, stannanimines readily form their head‐to‐tail dimers (viz. 1,3,2,4‐diazadistannetidines) or give [2+3] cycloadducts with the organic azide to furnish stannatetrazoles, as was shown by Pinchuk and by Neumann already in the 1980s. The publication of the N‐heterocyclic silylene (CHNtBu)2Si, the first stable compound containing dicoordinate SiII, by Denk and West in 1994 was followed in the same year by their report of its reaction with trityl azide, which furnished the THF complex of the silanimine (CHNtBu)2Si=NCPh3. Since then, numerous reactions of organic azides with free or base‐stabilised silylenes furnishing free or base‐stabilised silanimines have been described.[ , , ] Among the reactions of organic azides with heavier carbene analogues, plumbylenes have apparently been investigated least extensively. We are aware of only three studies in this context.

The first study was published in 2002 by Klinkhammer, who described the reaction of [(Me3Si)3Si]2Pb (1) with 1‐adamantyl azide in toluene at −30 °C, which furnished the triazenido PbII complex 2 with a four‐membered PbN3 heterocycle due to migration of a (Me3Si)3Si group from Pb to N (Scheme 1, top). In 2017 Song reported the reaction of the N‐heterocyclic plumbylene o‐C6H4(NDipp)2Pb (3) with mesityl azide under harsh conditions (110 °C, toluene). N2 was liberated, but no plumbanimine was observed. The reaction resulted in a donor‐functionalised plumbylene 4 with tricoordinate PbII due to a chelating secondary amino group, which was plausibly formed by insertion of an NMes unit into a benzylic C−H bond, thus affording a Me2C−NH−Mes moiety (Scheme 1, bottom).

Scheme 1

Reactions of organic azides with plumbylenes described by Klinkhammer (top) and Song (bottom); Ad=1‐adamantyl, Mes=mesityl.

Of particular interest for the present work is Wesemann's study of the reaction of organic azides with intramolecular Lewis pairs composed of a tetrylene and a phosphane unit, which included also the plumbylene Ar*Pb[CHPh(PPh2)] [5, Ar*=2,6‐(2,4,6‐iPr3C6H2)2C6H3] (Scheme 2). The PbII atom of 5 is dicoordinate. In contrast, the corresponding stannylene 6 and germylene 9 exhibit tricoordinate tetrel atoms due to intramolecular coordination of the P atom, thus resembling our recently reported ferrocene‐based N‐heterocyclic plumbylene 12, stannylene 13 and germylene 14 functionalised with a PPh2 group (Figure 1, top).

Scheme 2

Reactions of 1‐adamantyl azide with PPh2‐functionalised tetrylenes described by Wesemann; Ar*=2,6‐(2,4,6‐iPr3C6H2)2C6H3, Ad=1‐adamantyl.

Figure 1

PPh2‐functionalised ferrocene‐based N‐heterocyclic tetrylenes (top) and unfunctionalised congeners (bottom) investigated in this study.

Compounds 5, 6, 9 and 12–14 contain two different sites suitable for reaction with an organic azide, viz. the P atom and the tetrel atom. Wesemann observed that plumbylene 5 and stannylene 6 react with AdN3 in the same fashion, affording addition products 7 and 8 featuring a four‐membered ENPC heterocycle (E=Sn, Pb; N denotes the terminal AdN3 nitrogen atom; Scheme 2, top). An analogous reaction had previously been reported by Ionkin for the stannylene [tBu2PCH2C(CF3)2O]2Sn, which contains a tetracoordinate SnII atom due to intramolecular P coordination; only one equivalent of AdN3 was consumed even under forcing conditions. From a formal point of view, the products of these reactions can be described as phosphazides which are engaged in an intramolecular coordination of their P‐bonded nitrogen atom to the divalent tetrel atom. A completely different behaviour was found for germylene 9, where formation of N2 and of the corresponding germanimine 10 (kinetic product] was observed, followed by slow isomerisation of the latter to the iminophosphorane‐functionalised germylene 11 (thermodynamic product; Scheme 2, bottom). The reactions of the Lewis pairs 5 and 6 with 1‐adamantyl azide are reminiscent of reactions reported for several borane‐phosphane (B/P) frustrated Lewis pairs (FLPs) with organic azides, where FLP addition to the terminal nitrogen atom of the azide (N) occurred, resulting in four‐ or five‐membered heterocycles with a BNP subunit. Wesemann's results inspired us to investigate the behaviour of plumbylene 12, stannylene 13 and germylene 14 towards an organic azide. We selected MesN3, which is a readily available aryl azide considered particularly safe for use (C/N atom number ratio not lower than 3). We were interested in the influence of the Lewis pair nature of the donor‐functionalised N‐heterocyclic tetrylenes 12–14 in this context and therefore included the unfunctionalised congeners [{Fe(η5−C5H4−NSitBuMe2)2}E] [E=Pb (15), Sn (16), Ge (17); Figure 1, bottom] in our study. The SitBuMe2 substituent was chosen instead of the SiMe3 substituent present in 12–14, because all three unfunctionalised tetrylenes are stable and can be isolated in pure form, whereas the trimethylsilyl‐substituted plumbylene [{Fe(η5−C5H4−NSiMe3)2}Pb] is unstable. Furthermore, the bulkier nature of SitBuMe2 in comparison with SiMe3 is expected to compensate for part of the reduced steric encumbrance of the tetrel atom due to the absence of the PPh2 substituent.

Results and Discussion

The N‐heterocyclic germylene [{Fe(η5−C5H4−NSitBuMe2)2}Ge] (17) was conveniently prepared analogous to the stannylene [{Fe(η5−C5H4−NSitBuMe2)2}Sn] (16) by reacting LiN(SiMe3)2, [GeCl2(1,4‐dioxane)] and [Fe(η5−C5H4−NHSitBuMe2)2] in a 2 : 1 : 1 molar ratio in THF. The product was obtained in 92 % yield and was structurally characterised by single‐crystal X‐ray diffraction (XRD). Pertinent metric parameters of germylene 17 and of the products of the reactions with MesN3 obtained in this study are collected in Table 1. Data for known N‐heterocyclic tetrylenes which served as starting materials for the new compounds contained in Table 1 have been included in this Table for comparison. The molecular structure of 17 is shown in Figure 2. The molecule exhibits approximate C 2 symmetry about the Fe−Ge axis. The germanium bond lengths and angle are very similar to the values published for the N‐trimethylsilyl homologue. The germanium bond angle of 107.22(7)° determined for 17 is essentially identical to the value reported by Lappert for the emblematic acyclic diaminogermylene [(Me3Si)2N]2Ge, viz. 107.1(2)°. This is in line with previous observations that the 1,1’‐ferrocenylene backbone of N‐heterocyclic carbenes and their heavier analogues [{Fe(η5−C5H4−NR)2}E] (E=C–Pb) gives rise to large bond angles at the divalent tetrel atom close to those of corresponding acyclic congeners.[ , , , , ] The Sn−N bonds of stannylene 16 are longer than the Ge−N bonds of germylene 17 by ca. 0.2 Å, which is in accord with the difference of the covalent radii of tin (1.39 Å) and germanium (1.20 Å). In turn, the GeII bond angle of 17 is slightly wider (by 2°) than the SnII bond angle of 16, which is in agreement with Bent's rule.[ , ]

Table 1

Pertinent metric parameters for the N‐heterocyclic tetrylenes of this study and the products of their reactions with mesityl azide.

	E−N1, E−N2 [Å]	E−N/P [Å]	N1−E−N2 [°]	Tilt angle [°][a]	N−Cipso−Cipso−N [°]
12 (E=Pb)	2.260(2),	2.8624(8)	95.32(9)	6.3	26.2	ref. [23]
	2.213(2)
13 (E=Sn)	2.126(3),	2.7526(8)	98.11(10)	6.1	23.1	ref. [23]
	2.084(3)
14 (E=Ge)	1.9530(18),	2.6497(6)	100.69(7)	8.9	22.2	ref. [23]
	1.9160(18)
16 (E=Sn)	2.058(2),		105.09(10)	2.8	8.2	ref. [28]
	2.066(2)
17 (E=Ge)	1.8669(17),		107.22(7)	7.8	12.0	this work
	1.8685(18)
18 (E=Sn)[b]	2.035(5),	2.063(5)	115.60(19)	1.0	0.9	this work
	2.027(5)	2.072(5)
19 (E=Ge)[c]	1.830(5),	1.714(5)	114.6(2)	5.7	4.5	this work
	1.827(5)
20 (E=Pb)	2.308(3),	2.386(3)	91.19(11)	3.4	0.7	this work
	2.241(3)
21 (E=Sn)	2.174(3),	2.285(2)	94.05(10)	5.1	10.0	this work
	2.136(3)
22 (E=Ge)	1.8909(16),	1.7268(17)	108.06(7)	9.8	26.4	this work
	1.8625(16)	2.4535(5)
23 (E= Ge)	1.976(3),	2.110(3)	98.24(13)	7.7	11.1	this work
	1.943(3)

[a] Dihedral angle formed by the best planes of the cyclopentadienyl rings. [b] Two independent molecules with very similar bond parameters; data arbitrarily given for molecule 1. [c] Two independent molecules, one of them showing disorder; data given for the non‐disordered molecule.

Figure 2

Molecular structure of germylene 17 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl groups drawn as capped sticks).

With the complete series of donor‐functionalised heavier N‐heterocyclic tetrylenes 12–14 and the unfunctionalised congeners 15–17 in hand, we next studied the reactivity of these six compounds towards mesityl azide (Scheme 3). Reactions were observed for all compounds except [{Fe(η5−C5H4−NSitBuMe2)2}Pb] (15). Since plumbylene 15 was inert even under rather forcing conditions (105 °C, toluene) almost identical to those used by Song for o‐C6H4(NDipp)2Pb (3; Scheme 1, bottom), we tested three additional diaminoplumbylenes in this context, viz. the acyclic congener [(Me3Si)2N]2Pb as well as our recently reported five‐ and six‐membered N‐heterocyclic plumbylenes o‐C6H4(NSiMe3)2Pb and nap(NSiMe3)2Pb (nap=naphthalene‐1,8‐diyl), all containing Me3Si instead of SitBuMe2 substituents to decrease steric congestion. However, these compounds proved to be equally inert. The stannylene [{Fe(η5−C5H4−NSitBuMe2)2}Sn] (16) showed a smooth and swift reaction with MesN3 (2 equiv.) at room temperature, furnishing the stannatetrazole 18 (structurally characterised by XRD, Figure 3) in 83 % yield. The use of 1 equiv. of the azide resulted in an equimolar mixture of 18 and unreacted 16. This is in line with previous reports that an initially formed stannanimine is prone to form a [2+3] cycloadduct with an organic azide.[ , , ]

Scheme 3

The ferrocene‐based N‐heterocyclic tetrylenes 15–17 and the Ph2P‐functionalised congeners 12–14 (used as racemic compounds, only one enantiomer is shown) and their reactions with mesityl azide (isolated yield given in brackets).

Figure 3

Molecular structure of stannatetrazole 18 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl groups drawn as capped sticks). Selected bond lengths [Å] and angle [°]: N3−N4 1.390(7), N4−N5 1.267(8), N5−N6 1.396(7); N3−Sn1−N6 106.9(2).

Stannatetrazole 18 is a spiro compound with a five‐ and a six‐membered heterocycle connected by the Sn atom. Schulz recently confirmed computationally that closely related compounds obtained from the acyclic diaminostannylene [(Me3Si)2N]2Sn and aryl azides contain a tin(IV) atom with four highly polar Sn−N single bonds instead of a tin(II) atom chelated by a tetraazabutadiene ligand. A comparison of the Sn−N bond lengths of stannylene 16 (average value 2.06 Å, dicoordinate SnII) and stannatetrazole 18 (average value 2.05 Å, tetracoordinate SnIV) shows that obviously the increase in coordination number from two to four is compensated by the decrease in the covalent radius on going from SnII to SnIV. This behaviour is not unusual. For example, a comparison of the cyclic diaminostannylene Me2Si(NDipp)2Sn and the corresponding tin(IV) spiro compound [Me2Si(NDipp)2]2Sn reveals essentially identical Sn−N bond lengths of 2.06 Å for both compounds. In agreement with the few structurally characterised stannatetrazoles known to date,[ , ] the central N−N bond of the N4 unit of 18 is considerably shorter than the other two N−N bonds (1.27 vs. 1.39 Å), which is compatible with a double bond and adjacent single bonds, thus supporting the N−N=N−N Lewis structure advocated by Schulz.

In contrast to stannylene 16, the corresponding germylene [{Fe(η5−C5H4−NSitBuMe2)2}Ge] (17) afforded the germanimine [{Fe(η5−C5H4−NSitBuMe2)2}Ge=NMes] (19; structurally characterised by XRD, Figure 4) in 72 % yield.[ , ] In comparison to stannanimines, the tendency of germanimines to undergo [2+3] cycloadditions with organic azides is less pronounced. For example, Meller found [2+3] cycloadduct (i. e. stannatetrazole) formation from the stannylene [(Me3Si)2N]2Sn and 2,6‐diethylphenyl azide at −50 °C, whereas Fulton observed germanimine formation from the corresponding germylene [(Me3Si)2N]2Ge and the significantly less bulky phenyl azide at −30 °C and with mesityl azide even at room temperature. Germanimine 19 contains a trigonal‐planar GeIV atom (sum of angles 360°) exhibiting a short (1.71 Å) and two long Ge−N bonds (1.83 Å), in good agreement with germanimines derived from other diaminogermylenes.[ , ] In comparison to germylene 17, the Ge−N single bonds of 19 are slightly shorter (by 0.04 Å). Similar to what was noted above for tin compounds 16 and 18, the increase in coordination number from two to four is obviously outbalanced by the decrease in the covalent radius on going from GeII to GeIV, in accord with findings of our recent systematic study addressing oxidation reactions of several homologues of 17.

Figure 4

Molecular structure of the non‐disordered molecule of germanimine 19 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl groups drawn as capped sticks).

In contrast to [{Fe(η5−C5H4−NSitBuMe2)2}Pb] (15), which was inert towards MesN3 even under forcing conditions (see above), the donor‐functionalised N‐heterocyclic plumbylene 12 reacted with this azide already under fairly mild conditions (60 °C, toluene), affording the addition product 20 (structurally characterised by XRD, Figure 5) with a six‐membered PbNPCCN heterocycle (N denotes the terminal MesN3 nitrogen atom). 20 may be viewed as a phosphazide stabilised by intramolecular coordination of the P‐bonded nitrogen atom to the Lewis acidic PbII atom, which is in a trigonal pyramidal bonding environment (sum of angles 274°).

Figure 5

Molecular structure of phosphazide 20⋅Et2O in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms and solvent molecule omitted for clarity, alkyl and aryl groups drawn as capped sticks). Selected bond lengths [Å]: P1−N3 1.643(3), N3−N4 1.375(4), N4−N5 1.274(4).

The Pb−NP bond is considerably longer (2.39 Å) than the two other Pb−N bonds (average value 2.27 Å). These values may be compared with the Pb−N distances of the 4‐dimethylaminopyridine (DMAP) adduct of plumbylene 15, which exhibits two similarly short Pb−N bonds to the amino substituents (average value 2.27 Å), while the coordinative Pb−Npyridine bond (2.50 Å) of [15(DMAP)] is ca. 11 Å longer than the Pb−NP bond of 20, pointing to a significant ylidic character of the phosphazide unit (R’3P+−N−−N=NR) present in the latter. The Pb−NP bond length of Wesemann's plumbylene‐azide addition compound 7 has a value of 2.43 Å and is thus slightly longer (by 4 Å) than the corresponding bond of 20, while their P−N bond lengths are essentially identical (1.64 Å).

Whereas the reaction of [{Fe(η5−C5H4−NSitBuMe2)2}Sn] (16) with MesN3 afforded a [2+3] cycloadduct (stannatetrazole 18, see above), the donor‐functionalised congener 13 furnished the iminophosphorane‐functionalised stannylene 21 under the same mild conditions (structurally characterised by XRD, Figure 6).

Figure 6

Molecular structure of the iminophosphorane‐functionalised stannylene 21 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl groups drawn as capped sticks).

The SnII atom of 21 is in a trigonal pyramidal bonding environment (sum of angles 289°). The Sn−NP bond is considerably longer (2.29 Å) than the two other Sn−N bonds (average value 2.16 Å) and essentially identical with the Sn−NP bond of the iminophosphorane‐functionalised diarylstannylene Ar*Sn{o‐C6H4[P(NAd)Ph2)]} reported by Wesemann; in the same vein, the P−N bond lengths of both compounds are indistinguishable within experimental error, viz. 1.619(3) Å for 21 and 1.613(5) Å for Wesemann's compound. A comparison of the tetrel‐nitrogen bonds present in 20 and 21 reveals that the bonds of stannylene 21 are shorter by ca. 0.1 Å than the corresponding bonds of plumbylene 20, which is in line with the difference of the covalent radii of Sn (1.39 Å) and Pb (1.46 Å).

In comparison to 12 and 13, which contain tricoordinate P,N,N‐bonded tetrel atoms, the 207Pb NMR signal of 20 and the 119Sn NMR signal of 21 are moderately high‐field shifted in C6D6 solution [δ(207Pb)=2493 vs. 3050 ppm for 20 and 12, respectively; δ(119Sn)=−1 vs. 187 ppm for 21 and 13, respectively], in line with a tricoordinate N,N,N‐bonded nature of the respective tetrel atom. For comparison, a substantially low‐field shifted signal indicative of dicoordinate PbII and SnII, respectively, was observed for [{Fe(η5−C5H4−NSiMe3)2}Pb] [δ(207Pb)=4333 ppm] and [{Fe(η5−C5H4−NSiMe3)2}Sn] [δ(119Sn)=589 ppm], which do not contain a donor substituent.

Finally, the reaction of mesityl azide with the donor‐functionalised germylene 14 at room temperature afforded the germanimine 22, which was found to undergo a slow isomerisation to the corresponding iminophosphorane‐functionalised germylene 23. Both isomers were structurally characterised by XRD.

The PhP2‐functionalised germanimine 22 contains a tetracoordinate GeIV atom due to an intramolecular coordinative Ge−P bond (Figure 7), which is ca. 0.2 Å shorter than that of germylene 14. Analogous to germanimine 19, the GeIV atom is involved in a short (1.73 Å) and two long Ge−N bonds (average value 1.88 Å). These bonds are slightly elongated with respect to 19 (1.71 and 1.83 Å, see above) due to the higher coordination number of the GeIV atom of 22, viz. four vs. three in 19. Similar to 19, Wesemann's germanimine 10, which was obtained as the kinetic product from germylene 9 and AdN3, also contains a tricoordinate GeIV atom and exhibits a Ge−N bond length of 1.71 Å; however, no coordination of the PPh2 unit was observed in this case.

Figure 7

Molecular structure of germanimine 22 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl groups drawn as capped sticks).

The iminophosphorane‐functionalised germylene 23 (Figure 8), which is formed as thermodynamic product from the PPh2‐functionalised germanimine 22 by rearrangement, contains a GeII atom in a trigonal pyramidal bonding environment (sum of angles 300°). The molecular structure is analogous to that of the corresponding stannylene 21. The Ge−NP bond is considerably longer (2.11 Å) than the two other Ge−N bonds (average value 1.96 Å), in accord with the corresponding bond lengths determined for 21, when the difference of the covalent radii of Sn (1.39 Å) and Ge (1.20 Å) is taken into account. The P−N bond lengths of both compounds are essentially identical, viz. 1.625(3) Å for 23 and 1.619(3) Å for 21; they also compare well with the value of 1.608(2) Å reported by Wesemann for the iminophosphorane‐functionalised germylene 11.

Figure 8

Molecular structure of the iminophosphorane‐functionalised germylene 23 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl substituents drawn as capped sticks).

When compared with the reactions of Wesemann's plumbylene 5 and germylene 9 with AdN3 (Scheme 2), the respective behaviour of plumbylene 12 and germylene 14 towards MesN3 is completely analogous. However, while Wesemann's PPh2‐functionalised stannylene 6 afforded an addition product (8), an iminophosphorane (21) was obtained from our PPh2‐functionalised stannylene 13. The primary interaction of organic azides with main‐group element Lewis acids has been shown to involve the C‐bonded N atom. In contrast, the Staudinger reaction begins with a nucleophilic attack of the phosphane PR’3 on the terminal nitrogen atom (N) of the organic azide RN3.[ , ] Slootweg recently addressed the mechanism of the reaction of RN3 with the B/P FLP tBu2PCH2BPh2 and found that the initial nucleophilic attack typical of a Staudinger reaction is kinetically less favourable than adduct formation of the Lewis acidic B atom with the Lewis basic C‐bonded N atom. With MesN3 and tBuN3, the reaction afforded the respective addition product containing a four‐membered BNPC heterocycle via a six‐membered ring (BNNNPC) intermediate. The inertness of the unfunctionalised plumbylenes of our study towards MesN3 strongly indicates that the reaction of the azide occurs at the PPh2 unit of the donor‐functionalised plumbylenes 5 and 12. The fact that 5 reacts already at room temperature, while elevated temperatures are needed in the case of 12 is perfectly plausible in view of the fact that the P atom of 12 is engaged in an intramolecular coordinative bond to the Pb atom, while no such bond is present in 5, whose P atom is therefore readily available for reaction with the azide. In both cases, however, the resulting phosphazide is efficiently stabilised in this scenario by intramolecular adduct formation with the respective Lewis acidic PbII atom. Our results obtained with the lighter congeners, viz. stannylene pair 13 and 16 and germylene pair 14 and 17, suggest that in these cases the reaction with the azide occurs at the divalent tetrel atom, leading to an E=N double bond, which is highly reactive in the case of E=Sn so that only follow‐up products (18, 21) were observed.

Inspired in part by Breher's study of the ligand properties of the N‐mesityl homologue of 17 in transition metal chemistry as well as the recent report by Jambor and Herres‐Pawlis on copper(I) germylene complexes in the context of lactide polymerisation, we also addressed the coordination behaviour of the unfunctionalised germylene 17 and the donor‐functionalised congeners 14 and 23 towards CuCl (Scheme 4). Me2Si(NtBu)2Ge appears to be the only cyclic diaminogermylene investigated in terms of CuCl coordination to date. The complex obtained resulted from the reaction of six equivalents of this germylene with four equivalents of CuCl, thus exhibiting a 3 : 2 stoichiometric ratio of these components. In contrast to this, the reaction of 17 with CuCl furnished a product (24) with a composition corresponding to a 1 : 1 complex [(17)CuCl]. Although an NMR spectroscopic analysis of a C6D6 solution of this product revealed no significant coordination‐induced signal shifts in comparison to free 17, an XRD study clearly showed that formation of a germylene‐copper(I) complex had taken place and confirmed the 1 : 1 ratio already inferred from microanalytical data. While the composition of product 24 corresponds to a simple 1 : 1 complex [(17)CuCl], the solid state structure is not that simple. The Lewis structure of 24 given in Scheme 4 corresponds to the molecular structure in the crystal, which is shown in Figure 9. Pertinent metric parameters of 24 and of the other copper(I) complexes of this study are collected in Table 2.

Scheme 4

Reactions of the N‐heterocyclic germylenes 17, 14 and 23 with CuCl (isolated yields given in brackets). The Lewis structures of the copper(I) complexes 24, 25 and 26 reflect the results obtained for the solid state by XRD.

Figure 9

Molecular structure of copper(I) complex 24 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl substituents drawn as capped sticks). Selected bond length [Å] and angle [°]: Ge3−Cl4 2.5991(14); Cl4−Ge3−Cu3 93.98(4).

Table 2

Pertinent metric parameters for the copper complexes of this study.

	Ge−N[a] [Å]	Ge−N/P [Å]	N−Ge−N[a] [°]	Cu−Ge [Å]	Cu−Cl [Å]
24	1.824(5),		111.2(2)	2.2561(9)	2.3459(16),[b]
	1.832(5);				2.2417(15);
	1.829(5),		112.0(2)	2.2504(9)	2.3838(15),[b]
	1.834(5);				2.2253(17);
	1.852(5),		107.4(2)	2.2881(8)	2.3078(14),[b]
	1.861(4);				2.3000(16)
	1.839(5),		110.7(2)	2.2983(9)	2.2972(14),
	1.836(5)				2.2609(15)
25	1.911(10),		102.0(4)	2.3817(19)
	1.945(10);
	1.946(8),	2.429(3)	100.8(4)	2.3530(19)
	1.849(11)
26 [c]	1.889(3),	1.979(3)	96.55(13)	2.2522(6)	2.1026(11)
	1.878(3)

[a] Cyclopentadienyl‐bonded N atoms. [b] Bond to tricoordinate Cl1. [c] Two independent molecules; data refer to the non‐disordered one.

The crystal structure of 24 exhibits four CuI‐bonded germylene moieties in the asymmetric unit. Two of them form a chlorido‐bridged dimeric complex [(17)Cu(μ‐Cl)]2 containing two tricoordinate GeII atoms (Ge1 and Ge2) as part of a diamond‐shaped Cu2Cl2 core. The other two moieties form a less symmetric dimer, which contains one tricoordinate (Ge4) and one tetracoordinate GeII atom (Ge3). Only one of the two Cl atoms (Cl3) adopts a bridging position between the two CuI atoms of this dimer. The second Cl atom (Cl4) is in a bridging position between the tetracoordinate GeII atom Ge3 and the CuI atom Cu4 bonded to the tricoordinate GeII atom Ge4 of this less symmetric dimer. Instead of the diamond‐shaped Cu2Cl2 core of the other dimer, a five‐membered heterocyclic GeCu2Cl2 core is present in the less symmetric dimer. All four CuI atoms are in a trigonal planar coordination environment with two chlorine atoms and one germanium atom as bonding partners. The CuI atom Cu3 bonded to the tetracoordinate GeII atom Ge3 in the less symmetric dimer is connected to one of the Cl atoms (Cl1) of the Cu2Cl2 core of the symmetric dimer, thus joining the two dimeric units together and making this particular Cl atom μ3‐tricoordinate. The Cu−Cl distances of this tricoordinate Cl atom range from ca. 2.31 to 2.38 Å, while the other three Cl atoms exhibit shorter Cu−Cl bonds (2.23–2.30 Å) due to their dicoordinate nature. The Cu−Ge bond lengths of 24 lie in the small range from 2.25 to 2.30 Å, which compares well with other germylene complexes of tricoordinate CuI.[ , , ] The fact that Cl4 is bridging a CuI atom and a GeII atom, instead of two CuI atoms, suggests that the Cu−Cl and the Ge−Cl interactions in our system are of similar strength. Note that a chloride transfer to the germanium atom upon coordination of transition metal chlorides (MCl2, M=Fe, Co, Ni, Zn; CuCl), corresponding to the formation of a chlorogermyl ligand containing tetravalent germanium by GeII insertion into the M−Cl bond, was recently described by Cabeza for a donor‐stabilised N‐heterocyclic germylene with tricoordinate GeII due to intramolecular coordination of a PiPr2 unit.[ , ] The germanium atom of Cabeza's chlorogermyl copper complex is in a distorted pseudotetrahedral bonding environment, showing a distance of 0.75 Å to the plane formed by its two nitrogen atoms and the copper atom (sum of angles with respect to these three atoms: 321°). The situation is quite different in the present case. Ge3 has a distance of only 0.23 Å from its CuN2 plane, close to a trigonal planar arrangement (sum of angles: 356°). The bond vector formed with its additional bonding partner, Cl4, is almost perpendicular to the CuN2 plane, in accord with a donor‐acceptor interaction of the chlorido ligand with the vacant p‐type orbital at the GeII atom. This notion is further supported by the Ge3−Cl4 distance of 2.60 Å. This bond is much longer than the Ge−Cl bonds of CuI complexes obtained from chlorogermylenes with tricoordinate GeII due to chelating β‐diketiminato or aminotropiminato units,[ , ] which are typically 2.30 Å and thus close to the sum of the covalent radii of Ge (1.20 Å) and Cl (1.02 Å). In the same vein, the Cl−Ge−Cu angles in these chlorogermylene complexes are ca. 120°, while the Cl4−Ge3−Cu3 angle of 24 is 94°, reflecting the approximately perpendicular orientation of the Cl4−Ge3 bond vector with respect to the CuN2 plane as described above.

The reaction of 14 with CuCl afforded a product (25) with a composition corresponding to a 2 : 1 complex [(14)2CuCl] according to microanalytical data, which, however, were not in accord with an analytically pure sample of such composition. The product gave rise to rather complicated NMR spectra, which were not suitable to provide conclusive evidence for the nature of the species in solution. A structural investigation by XRD revealed a trigonal‐planar coordination environment of the CuI atom, which is bonded to a P atom and two different Ge atoms, one of them carrying the Cl atom. The Lewis structure of 25 given in Scheme 4 corresponds to the molecular structure in the crystal (Figure 10). The NMR spectra obtained for 25 are compatible with such a structure also in solution. Note that the tetravalent Ge atom Ge1 is a centre of chirality. Consequently, in combination with the two different planar‐chiral ferrocene moieties, four diastereomers may result. We have isolated only a single diastereomer, which was obtained as a racemic compound. Figure 10 arbitrarily shows the (R p,R,S p) enantiomer. The NMR spectra of the crude product do not indicate the presence of other diastereomers. In particular, the 31P{1H} NMR spectrum (Figure S26 in the Supporting Information) exhibits only two signals, as expected for 25 with its two different phosphorus atoms. 25 was obtained in only 26 % yield. We cannot exclude, therefore, that other diastereomers were also formed in the reaction of 14 with CuCl, but remained unnoticed due to low solubility.

Figure 10

Molecular structure of copper(I) complex 25 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl substituents drawn as capped sticks). Only one of the two enantiomers is shown. Selected bond lengths [Å]: Ge1−Cl1 2.261(3), Cu1−P1 2.269(3).

The quality of the crystals obtained was poor (very small crystal size and weak scattering ability), thus compromising the result of the XRD analysis performed for 25. Nevertheless, a meaningful discussion of metric parameters is possible at least for the heavy atoms. The CuI atom is in a trigonal planar bonding environment, being coordinated by two germanium atoms and one phosphorus atom. The Cu−P distance of 2.27 Å lies in the region typical for tricoordinate CuI triarylphosphane complexes, comparing well with, for example, [CuX2(PPh3)][NR4] (2.21 Å for X=Cl, R=Et; 2.24 Å for X=Br, R=nBu; 2.23 Å for X=I, R=nPr), [CuX(PPh3)2] (2.27, 2.28 and 2.27 Å for X=Cl, Br, I, respectively), [Cu(PPh3)3][BPh4] (2.26–2.29 Å), and [Cu{Ge(C6F5)3}(PPh3)2] (2.27 Å), the latter germyl complex apparently being the only structurally characterised tricoordinate CuI phosphane complex with a copper‐germanium bond (Cu−Ge 2.38 Å). Both germanium atoms of 25 are tetracoordinate and reside in a distorted pseudotetrahedral bonding environment. Their shared bonding partner is the CuI atom, the Cu−Ge bond length being 2.38 and 2.35 Å for Ge1 and Ge2, respectively. The intramolecular coordination of the P atom present in germylene 14 has remained intact for Ge2, but not for Ge1. Ge1 is tetravalent. The Cu−Ge1 (2.38 Å), Ge1−Cl (2.26 Å) and Ge1−N distances (average value 1.93 Å) are similar to those reported for Cabeza's chlorogermyl copper complex (see above; Cu−Ge 2.36 Å, Ge−Cl 2.26 Å, Ge−N 1.93 Å). The same holds true for the distance of the Ge atom to the CuN2 plane, which is 0.61 Å for Ge1 in 25 and 0.75 Å for Cabeza's compound (see above). However, the corresponding distance of Ge2 is only 0.29 Å. Ge2 is part of still intact germylene 14 acting as a ligand for the chlorogermyl‐bonded CuI atom. The loss of electron density at Ge2 by copper(I) complexation obviously leads to a stronger coordination of the PPh2 moiety, as is reflected by a Ge2−P2 distance of 2.43 Å as opposed to 2.65 Å determined for the Ge−P bond of germylene F; a much smaller, but still significant, contraction is observed for the corresponding Ge−N bonds (average value 1.93 Å in 14 vs. 1.90 Å for Ge2 in 25). A similar effect, albeit less pronounced, has been observed by Baceiredo for a donor‐stabilised germylene with tricoordinate GeII due to intramolecular coordination of a PPh2 unit, whose Ge−P distance shortens from 2.43 to 2.39 Å upon complexation by a {RhCl(COD)} fragment. To summarise, compound 25 contains a tetravalent germanium atom (Ge1, chlorogermyl ligand) and a divalent germanium atom (Ge2, donor‐stabilised germylene ligand). To a first approximation, bonding in this complex may be rationalised, and symbolised, in the following simplified way: P:→Ge:→Cu−Ge.

Finally, the reaction of the iminophosphorane‐functionalised germylene 23 with CuCl cleanly afforded the germylene‐copper(I) complex [(23)CuCl] (26, Scheme 4). In view of the substantial number of CuI iminophosphorane complexes, it has not been obvious a priori that this reaction leads to a CuI germylene complex. Solution NMR spectra are in accord with the structure found in the solid state, which is shown in Figure 11. The coordination environment of the CuI atom of 26 is linear dicoordinate (bond angle 176°). The Cu−Cl bond length of 2.10 Å is typical for this arrangement and several structurally characterised examples of complexes [CuCl(L)] with L=carbene (CAAC, NHC) or donor‐stabilised silylene, but not with L=germylene or heavier analogues, have been reported. The Cu−Ge distance of 2.25 Å corresponds to the shortest Cu−Ge bond lengths determined for 24 (see above), where we have a combination of tricoordinate GeII and tricoordinate CuI. In the case of 26, we have a combination of dicoordinate CuI and tetracoordinate GeII, since the intramolecular coordination of the iminophosphorane N atom found for germylene 23 is also present in its copper complex 26. CuI coordination leads to a substantial shortening of all three Ge−N bonds. Analogous to 25 (see above), the effect is largest (0.13 Å) for the coordinative bond, which is contracted from 2.11 Å in 23 to 1.98 Å in 26, while the two other Ge−N bonds experience a less pronounced contraction (0.08 Å on average).

Figure 11

Molecular structure of copper(I) complex 26 in the crystal (ORTEP with ellipsoids drawn at the 50 % probability level, hydrogen atoms omitted for clarity, alkyl and aryl substituents drawn as capped sticks). Selected bond angle [°]: Ge1−Cu1−Cl1 176.24(4).

Conclusion

We have compared the reactivity of the ferrocene‐based N‐heterocyclic tetrylenes [{Fe(η5−C5H4−NSitBuMe2)2}E] [E=Pb (15), Sn (16),Ge (17)] towards mesityl azide with that of the PPh2‐functionalised congeners 12–14, whose phosphorus(III) atom constitutes a second possible reaction site in addition to the respective tetrel(II) atom. Our results indicate that the reaction of this azide with the stannylenes 13 and 16 and germylenes 14 and 17 invariably occurs at the divalent tetrel atom, leading to an E=N double bond. The resulting germanimines 19 and 22 could be isolated. However, the latter isomerised readily to iminophosphorane 23 by NMes transfer from the GeIV to the PIII atom due the rather reactive Ge=N bond. In line with previous observations (see above), the reactivity of the Sn=N bond is even higher, so that only follow‐up products were observed in the reactions of the stannylenes 13 and 16, namely, iminophosphorane 21 (most likely formed by NMes transfer from the SnIV to the PIII atom of the transient stannanimine) and stannatetrazol 18 (formed by [2+3] cycloaddition of the transient stannanimine with mesityl azide). All four unfunctionalised diaminoplumbylenes of our study, viz. the N‐heterocyclic compounds o‐C6H4(NSiMe3)2Pb, nap(NSiMe3)2Pb and [{Fe(η5−C5H4−NSitBuMe2)2}Pb] (15) as well as the acyclic congener [(Me3Si)2N]2Pb, proved to be inert towards MesN3 even under forcing conditions. In contrast, the behaviour of the PPh2‐functionalised diaminoplumbylene 12 towards mesityl azide is analogous to that of Wesemann's PPh2‐functionalised (alkyl)(aryl)plumbylene 5, being strongly reminiscent of B/P FLPs in both cases. While 5 reacts already at room temperature, moderately higher temperatures are needed in the case of 12. This may be ascribed to the fact that the P atom of 12, in contrast to that of 5, is engaged in an intramolecular coordinative bond to the Pb atom. Consequently, plumbylene 12 exhibits a reduced frustration in comparison to 5 and thus belongs to the so‐called active Lewis pairs (ALPs), which can show FLP‐like behaviour due to the weakness of their coordinative bond. Investigations addressing the activation of fundamentally important small molecules with 12 and closely related ALPs are underway in our laboratory. In addition, our study has demonstrated the ability of the unfunctionalised N‐heterocyclic germylene 17 and its donor‐functionalised relatives 14 and 23 to act as ligands for copper(I), thus underlining the potential of such ferrocene‐based, and hence redox‐functionalised, N‐heterocyclic germylenes in coordination chemistry. Notably, compound 26 is the first structurally characterised linear dicoordinate copper(I) halogenido complex [CuX(L)] with a heavier tetrylene ligand L.

Experimental Section

All reactions involving air‐sensitive compounds were performed in an inert atmosphere (argon or dinitrogen) by using standard Schlenk techniques or a conventional glovebox. Starting materials were procured from standard commercial sources and used as received. [Fe(η5−C5H4−NHSitBuMe2)2], [GeCl2(1,4‐dioxane)], 12–14, 15, 16 and MesN3 were synthesised by adapted versions of the published procedures. NMR spectra were recorded at ambient temperature with Varian NMRS‐500 and MR‐400 spectrometers operating at 500 and 400 MHz, respectively, for 1H. Elemental analyses were carried out with a HEKAtech Euro EA‐CHNS elemental analyser at the Institute of Chemistry, University of Kassel, Germany.

Synthesis of 17: LiN(SiMe3)2 (395 mg, 2.36 mmol) was added to a stirred solution of [Fe(η5−C5H4−NHSitBuMe2)2] (500 mg, 1.12 mmol) in THF (8 mL). After 30 minutes [GeCl2(1,4‐dioxane)] (260 mg, 1.12 mmol) was added and stirring was continued for 2 h. Volatile components were removed under reduced pressure. n‐Hexane (6 mL) was added to the residue. Insoluble material was removed by filtration through a short pad of celite. The solvent was removed from the filtrate under reduced pressure, leaving the product as a brownish‐yellow viscous oil. Yield 534 mg (92 %). Crystallisation of the product was initialised in a concentrated n‐hexane solution by scratching with a glass rod. After the first crystals appeared, the flask was stored at −40 °C. The mother liquor was separated from the orange product, which was finally dried under reduced pressure. 1H NMR (400 MHz, C6D6): δ=3.87, 3.78 (2 m, 2×4 H, cyclopentadienyl H), 1.00 (s, 18 H, CMe3), 0.29 (s, 12 H, SiMe3) ppm. 13C{1H} NMR (101 MHz, C6D6): δ=110.1 (Cipso), 68.9, 66.7 (2×CH), 27.6 (CMe 3), 18.9 (CMe3), −1.0 ppm (SiMe2). Anal. calcd for C22H38N2FeGeSi2 (515.20): C 51.29, H 7.43, N 5.44 %; found C 50.74, H 7.56, N 5.19 %.

Synthesis of 18: Mesityl azide (77 mg, 0.24 mmol) was added to a stirred solution of 16 (132 mg, 0.24 mmol) in n‐hexane (3 mL). Stirring was discontinued after 30 min. The solution was stored at −40 °C for crystallisation. The mother liquor was separated from the yellow crystals, which were subsequently dried under reduced pressure. Yield 168 mg (83 %). 1H NMR (400 MHz, C6D6): δ=6.93 (s, 4 H, Mes CH), 3.82, 3.73 (2 m, 2×4 H, cyclopentadienyl H), 2.83 (s, 12 H, Mes o‐CH3), 2.23 (s, 6 H, Mes p‐CH3), 0.62 (s, 18 H, CMe3), 0.09 ppm (s, 12 H, SiMe2). 13C{1H} NMR (101 MHz, C6D6): δ=142.7 (Mes Cipso), 131.9 (Mes CH), 128.8, 127.4 (2×Mes CCH3), 100.2 (cyclopentadienyl Cipso), 70.3, 67.7 (2×cyclopentadienyl CH), 27.2 (CMe 3), 24.5 (Mes o‐CH3), 20.6 (Mes p‐CH3), 20.5 (CMe3), −2.6 ppm (SiMe2). 119Sn{1H} NMR (186 MHz, C6D6): δ=−217.7 ppm. Anal. calcd for C40H60N6FeSi2Sn (855.67): C 56.15, H 7.07, N 9.82 %; found C 56.34, H 7.01, N 8.88 %.

Synthesis of 19: A solution of mesityl azide (52 mg, 0.32 mmol) in n‐hexane (1 mL) was added to a stirred solution of 17 (165 mg, 0.32 mmol) in n‐hexane (5 mL). After 10 minutes the volume of the solution was reduced to ca. 3 mL. The solution was stored at −40 °C for crystallisation. The orange product was separated from the mother liquor and subsequently dried under reduced pressure. Yield 149 mg (72 %). 1H NMR (400 MHz, C6D6): δ=7.01 (s, 2 H, Mes CH), 3.91, 3.82 (2 m, 2×4 H, cyclopentadienyl H), 2.55 (s, 6 H, Mes o‐CH3), 2.30 (s, 3 H, Mes p‐CH3), 0.94 (s, 18 H, CMe3), 0.26 ppm (s, 12 H, SiMe2). 13C{1H} NMR (101 MHz, C6D6): δ=149.7 (Mes Cipso), 129.1 (Mes CH), 94.7 (cyclopentadienyl Cipso), 69.3, 68.5 (2×cyclopentadienyl CH), 27.5 (CMe 3), 21.4 (Mes o‐CH3), 21.1 (CMe3), 20.1 (Mes p‐CH3), −2.8 ppm (SiMe2); Mes CCH3 not detected. Anal. calcd for C31H49N3FeGeSi2 (648.39): C 57.42, H 7.62, N 6.48 %; found C 58.40, H 6.86, N 6.44 %.

Synthesis of 20: A solution of mesityl azide (22 mg, 0.13 mmol) in toluene (2 mL) was added to a solution of 12 (100 mg, 0.13 mmol) in toluene (3 mL). The stirred mixture was heated to 60 °C for 10 h. Subsequently, volatile components were removed under reduced pressure. The crude product was dissolved in diethyl ether/n‐hexane (1 : 1, 2 mL) and the solution was stored at −40 °C for crystallisation. This afforded the product as red diethyl ether solvate, which was separated from the mother liquor and subsequently dried under reduced pressure. Yield 68 mg (52 %). 1H NMR (400 MHz, C6D6): δ=7.72–7.68, 7.67–7.60 (2 m, 2×2 H, Ph o‐H), 7.05–6.98 (m, 2 H, Ph p‐H), 6.98–6.87 (m, 4 H, Ph m‐H), 6.72 (s, 2 H, Mes CH), 4.29, 3.97 (2 m, 2×2 H, cyclopentadienyl H), 3.93, 3.65, 3.59 (3 m, 3×1 H, cyclopentadienyl CH), 2.25 (s, 6 H, Mes o‐CH3), 2.09 (s, 3 H, Mes p‐CH3), 0.48, 0.25 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=145.9 (Mes Cipso), 135.5 (Mes p‐CCH3), 133.6, 133.0 (2 d, J=10.2 Hz, Ph o‐CH), 132.9, 132.4 (2 d, J=2.9 Hz, Ph p‐CH), 131.0, 130.0 (Mes CH), 129.1 (second line of doublet concealed by solvent signal, Ph Cipso), 128.7 (Mes o‐CCH3), 128.6 (two very closely spaced signals, 2×Ph CH), 126.4 (d, 1 J=89.9 Hz, Ph Cipso), 122.3 (d, J=7.7 Hz), 115.9 (2×cyclopentadienyl CipsoN), 72.7 (d, J=10.9 Hz), 72.1, 69.4, 67.9 (d, J=16.7 Hz), 66.5 (d, J=14.3 Hz), 65.6, 63.0 (7×cyclopentadienyl CH), 55.4 (d, 1 J=130.3 Hz, cyclopentadienyl CipsoP), 20.9 (Mes p‐CH3), 20.2 (Mes o‐CH3), 3.3, 3.3 ppm (2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=38.4 ppm (2 J PbP=96 Hz). 207Pb NMR (105 MHz, C6D6): δ=2493 ppm. Anal. calcd for C37H46N5FePPbSi2⋅C4H10O (985.12): C 49.99, H 5.73, N 7.11 %; found C 50.64, H 5.43, N 7.41 %.

Synthesis of 21: A solution of mesityl azide (27 mg, 0.21 mmol) in n‐hexane (1 mL) was added to a stirred solution of 13 (110 mg, 0.17 mmol) in n‐hexane (3 mL). Stirring was discontinued after 10 min. The solution was stored at −40 °C for crystallisation. The mother liquor was separated from the orange crystals, which were subsequently dried under reduced pressure. Yield 91 mg (69 %). 1H NMR (500 MHz, C6D6): δ=7.97, 7.40 (2 m, 2×2 H, Ph o‐H), 7.06 (m, 1 H, Ph p‐H), 6.97 (m, 2 H, Ph m‐H), 6.92 (m, 1 H, Ph p‐H), 6.84 (m, 2 H, Ph m‐H), 6.73, 6.63 (2 s, 2×1 H, Mes CH), 4.61, 4.42, 4.05, 3.95, 3.90, 3.70, 3.47 (7 m, 7×1 H, cyclopentadienyl H), 2.43 (d, J=1.8 Hz, 3 H, Mes CH3), 2.09 (d, J=2.4 Hz, 3 H, Mes CH3), 1.83 (d, J=1.9 Hz, 3 H, Mes CH3), 0.53, 0.43 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=139.3 (d, J=6.6 Hz, Mes Cipso), 137.6 (d, J=5.2 Hz, Mes CCH3), 137.1 (d, J=5.9 Hz, Mes CCH3), 133.9 (d, J=9.8 Hz, Ph CH), 133.6 (d, J=9.3 Hz, Ph CH), 133.5 (d, J=3.8 Hz, Ar CH), 132.2 (d, J=2.9 Hz, Ar CH), 132.0 (d, 1 J=89.8 Hz, Ph Cipso), 131.5 (d, J=2.8 Hz, Ar CH), 130.2 (d, J=3.4 Hz, Ar CH), 130.0 (d, J=3.1 Hz, Ar CH), 129.8 (d, 1 J=90.8 Hz, Ph Cipso), 128.0, 127.7 (2×Ar CH), 119.9 (d, J=6.6 Hz, cyclopentadienyl CipsoN), 111.6 (cyclopentadienyl CipsoN), 72.4, 70.4 (d, J=9.8 Hz), 69.9, 68.1 (d, J=13.6 Hz), 67.8 (d, J=16.5 Hz), 67.5, 63.8 (7×cyclopentadienyl CH), 57.0 (d, 1 J=135.5 Hz, cyclopentadienyl CipsoP), 21.7 (d, J=1.6 Hz, Mes CH3), 21.1 (d, J=1.3 Hz, Mes CH3), 20.8 (d, J=1.5 Hz, Mes CH3), 3.3 ppm (two very closely spaced signals, 2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=23.3 ppm. 119Sn{1H} NMR (186 MHz, C6D6): δ=−1.3 ppm. Anal. calcd for C37H46N3FePSi2Sn (794.48): C 55.94, H 5.84, N 5.29 %; found C 55.97, H 6.16, N 5.03 %.

Synthesis of 22: A solution of mesityl azide (29 mg, 0.18 mmol) in n‐hexane (1 mL) was added to stirred solution of 14 (110 mg, 0.18 mmol) in n‐hexane (5 mL). Stirring was discontinued after 10 min. The solution was stored at −40 °C for crystallisation. The mother liquor was separated from the orange crystals, which were subsequently dried under reduced pressure. Yield 81 mg (67 %). 1H NMR (400 MHz, C6D6): δ=7.87–7.82, 7.40–7.35 (2 m, 2×2 H, Ph), 7.06 (m, 5 H, Ph and Mes CH), 6.88–6.82 (m, 3 H, Ph and Mes CH), 4.53, 4.13, 3.83, 3.81, 3.68, 3.65, 3.24 (7 m, 7×1 H, cyclopentadienyl H), 2.59 (s, 6 H, Mes o‐CH3), 2.36 (s, 3 H, Mes p‐CH3), 0.48, 0.33 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=153.5 (d, 2 J=6.6 Hz, Mes Cipso), 135.3 (d, J=10.7 Hz, Ar CH), 132.6 (d, J=10.3 Hz, Ar CH), 132.3 (d, 4 J=2.8 Hz, Ph p‐CH), 131.2 (d, 1 J=35.9 Hz, Ph Cipso), 131.0 (d, 4 J=2.8 Hz, Ph p‐CH), 129.3 (d, J=4.1 Hz, Ar CH), 129.3 (d, J=5.3 Hz, Ar CH), 129.0 (d, J=10.2 Hz, Ar CH), 127.1 (d, 3 J=9.7 Hz, Mes o‐CCH3), 126.6 (d, 1 J=43.6 Hz, phenyl Cipso), 122.0 (Mes p‐CCH3), 112.8 (d, J=20.7 Hz, cyclopentadienyl CipsoN), 102.0 (d, J=1.4 Hz, cyclopentadienyl CipsoN), 74.7 (d, J=2.3 Hz, cyclopentadienyl CH), 71.1 (cyclopentadienyl CH), 71.0 (d, J=7.0 Hz, cyclopentadienyl CH), 69.8 (cyclopentadienyl CH), 69.0 (d, J=4.2 Hz, cyclopentadienyl CH), 67.5 (cyclopentadienyl CH), 63.8 (d, J=1.4 Hz, cyclopentadienyl CH), 53.7 (d, 1 J=61.3 Hz, cyclopentadienyl CipsoP), 23.7 (d, 4 J=1.9 Hz, Mes o‐CH3), 23.1 (Mes p‐CH3), 2.7, 2.1 ppm (2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=−28.2 ppm. Anal. calcd for C37H46N3FeGePSi2 (748.40): C 59.38, H 6.20, N 5.61 %; found C 59.70, H 6.53, N 5.36 %.

Synthesis of 23: A solution of mesityl azide (29 mg, 0.18 mmol) in toluene (1 mL) was added to stirred solution of 14 (110 mg, 0.18 mmol) in toluene (5 mL). After 10 minutes the stirred solution was heated to 60 °C for 4 h. Volatile components were removed under reduced pressure. The orange residue was washed with a minimal amount of n‐hexane and was subsequently dried under reduced pressure. Yield 110 mg (83 %). 1H NMR (500 MHz, C6D6): δ=7.96–7.92 (m, 2 H, Ph o‐H), 7.64–7.59 (m, 2 H, Ph o‐H), 7.06–7.03 (m, 1 H, Ph p‐H), 6.96–6.85 (m, 5 H, Ph), 6.75, 6.67 (2 s, 2×1 H, Mes CH), 4.55, 4.37, 4.12, 4.10, 3.98, 3.75, 3.73 (7 m, 7×1 H, cyclopentadienyl H), 2.21 (s, 3 H, Mes CH3), 2.11 (d, J=2.3 Hz, 3 H, Mes CH3), 1.96 (d, J=1.8 Hz, 3 H, Mes CH3), 0.60, 0.38 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=139.0 (d, J=5.3 Hz, Mes Cipso), 138.2 (d, J=5.0 Hz, Mes CCH3), 138.1 (d, J=5.7 Hz, Mes CCH3), 133.9 (d, J=3.6 Hz, Mes CCH3), 133.7 (d, 1 J=90.3 Hz, Ph Cipso), 133.6 (d, J=9.8 Hz, Ph o‐CH), 133.0 (d, J=9.3 Hz, Ph o‐CH), 132.1 (d, J=2.9 Hz, Ph p‐CH), 131.5 (d, J=2.7 Hz, Ph p‐CH), 130.4 (d, J=3.1 Hz, Mes CH), 129.7 (d, J=2.8 Hz, Mes CH), 128.8 (second line of doublet concealed by solvent signal, Ph Cipso), 128.2, 127.9 (2×Ph m‐CH), 118.3 (d, J=5.7 Hz, cyclopentadienyl CipsoN), 109.6 (cyclopentadienyl CipsoN), 71.9 (cyclopentadienyl CH), 69.8 (d, J=9.0 Hz, cyclopentadienyl CH), 69.5 (cyclopentadienyl CH), 68.7 (d, J=13.5 Hz, cyclopentadienyl CH), 68.4 (cyclopentadienyl CH), 67.5 (d, J=14.9 Hz, cyclopentadienyl CH), 64.2 (cyclopentadienyl CH), 56.0 (d, 1 J=134.4 Hz, cyclopentadienyl CipsoP), 21.4 (d, J=1.2 Hz, Mes CH3), 21.3, 20.9 (2 d, J=1.5 Hz, 2×Mes CH3), 3.1, 3.0 ppm (2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=18.7 ppm. Anal. calcd for C37H46FeGeN3PSi2 (748.40): C 59.38, H 6.20, N 5.61 %; found C 60.31, H 6.58, N 5.52 %.

Synthesis of 24: CuCl (30 mg, 0.30 mmol) was added to a stirred solution of 17 (120 mg, 0.23 mmol) in toluene (5 mL). After 18 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1.5 mL). Volatile components were removed from the combined filtrate and washing solution. The product was extracted from the residue with n‐hexane (3×2 mL). After filtration of the extract to remove trace amounts of insoluble material, the volume was reduced to ca. 0.5 mL. Yellow crystals were obtained after several days, which were separated from the mother liquor and subsequently dried under reduced pressure. Yield 61 mg (42 %). 1H NMR (400 MHz, C6D6): δ=3.84, 3.75 (2 m, 2×4 H, cyclopentadienyl H), 1.10 (s, 18 H, tBu), 0.54 ppm (s, 12 H, SiMe2). 13C{1H} NMR (101 MHz, C6D6): δ=106.9 (Cipso), 69.3, 67.3 (2×cyclopentadienyl CH), 27.9 (CMe 3), 19.2 (CMe3), −0.1 ppm (SiMe2). Anal. calcd for C22H38N2ClCuFeGeSi2 (614.19): C 43.02; H 6.24; N 4.56 %; found C 42.11; H 6.41; N 4.72 %.

Synthesis of 25: CuCl (25 mg, 0.21 mmol) was added to a stirred solution of 14 (90 mg, 0.15 mmol) in toluene (5 mL). After 72 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1 mL). The volume of the combined filtrate and washing solution was reduced to ca. 0.5 mL. n‐Hexane (0.5 mL) was added. The solvent was slowly evaporated under ambient conditions, affording the crude product as a dark brownish red microcrystalline solid, which was washed with n‐hexane (2×0.2 mL) and subsequently dried under reduced pressure. Yield 25 mg (26 %). 1H NMR (400 MHz, C6D6): δ=7.95, 7.47 (2 br., 2×2 H, Ph), 7.31–7.29 (m, 5 H, Ph), 7.20–7.18 (m, 1 H, Ph), 7.10–7.05 (m, 4 H, Ph), 6.99–6.95 (m, 5 H, Ph), 6.87 (m, 1 H, Ph), 4.60, 4.22, 4.11, 3.98 (4 m, 4×1 H, cyclopentadienyl H), 3.89, 3.74 (2 m, 2×2 H, cyclopentadienyl H), 3.66–3.62 (m, 4 H, cyclopentadienyl H), 3.30, 3.02 (2 m, 2×1 H, cyclopentadienyl H), 0.53 (s, 18 H, SiMe3), 0.50, 0.24 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=135.7 (d, J=10.8 Hz), 134.4 (d, J=14.5 Hz), 134.2, 133.9, 133.7 (d, J=14.3 Hz), 132.7, 132.2, 132.1, 130.9, 129.9 (d, J=11.0 Hz), 129.7, 129.7, 129.4 (d, J=9.6 Hz), 128.6, 128.5, 128.2 (16×Ph), 109.3 (d, J=17.5 Hz), 105.1, 104.0 (3×cyclopentadienyl CipsoN, one of the expected four CipsoN signals and CipsoP signals not detected), 76.4 (d, J=6.6 Hz), 74.5, 72.4, 71.9 (d, J=5.7 Hz), 70.4, 69.3, 69.3, 68.3 (d, J=5.4 Hz), 67.5, 67.4, 66.7 (d, J=4.2 Hz), 66.3, 66.1, 65.1 (14×cyclopentadienyl CH), 5.1 (two isochronous signals, SiMe3), 3.6, 3.1 ppm (2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=−7.5, −17.5 ppm. Satisfactory microanalytical data could not be obtained.

Synthesis of 26: CuCl (10 mg, 0.10 mmol) was added to a stirred solution of 23 (67 mg, 0.09 mmol) in toluene (1 mL). After 24 h insoluble material was removed by filtration through a short pad of celite, followed by washing with toluene (1 mL). The volume of the combined filtrate and washing solution was reduced to ca. 0.5 mL. The solution was placed in a 5 mm NMR tube and was subsequently layered with n‐hexane (ca. 3 mL). After two weeks yellow crystals had formed, which were separated from the mother liquor and subsequently dried under reduced pressure. Yield 51 mg (67 %). 1H NMR (400 MHz, C6D6): δ=7.76–7.72, 7.54–7.49 (2 m, 2×2 H, Ph), 7.10–7.06 (m, 1 H, Ph), 6.93–6.89 (m, 3 H, Ph), 6.84 (s, 1 H, Mes CH), 6.83–6.79 (m, 2 H, Ph CH), 6.70 (s, 1 H, Mes CH), 4.43, 4.20, 4.05, 4.03, 3.82, 3.68, 3.65 (7 m, 7×1 H, cyclopentadienyl H), 2.11, 2.06, 1.97 (3 s, 3×3 H, Mes CH3), 0.62, 0.41 ppm (2 s, 2×9 H, SiMe3). 13C{1H} NMR (101 MHz, C6D6): δ=138.4 (d, J=4.0 Hz), 138.0 (d, J=4.7 Hz), 136.7 (d, J=3.0 Hz), 136.1 (d, J=4.8 Hz, Mes Cipso), 133.6 (d, J=10.2 Hz, Ph m‐CH), 133.1 (d, J=2.8 Hz), 132.8 (d, J=9.4 Hz, Ph m‐CH), 132.6 (d, J=2.8 Hz), 131.8 (d, J=2.6 Hz), 131.5 (d, 1 J=93.8 Hz, Ph Cipso), 130.3 (d, J=2.5 Hz), 128.5, 128.4 (2×Ph o‐CH), 125.3 (d, 1 J=96.1 Hz, Ph Cipso), 113.5 (d, J=6.0 Hz, cyclopentadienyl CipsoN), 105.6 (cyclopentadienyl CipsoN), 73.0 (cyclopentadienyl CH), 71.1 (d, J=9.5 Hz, cyclopentadienyl CH), 70.3 (d, J=12.6 Hz, cyclopentadienyl CH), 69.9, 69.6 (2×cyclopentadienyl CH), 68.0 (d, J=14.3 Hz, cyclopentadienyl CH), 65.8 (cyclopentadienyl CH), 54.7 (d, 1 J=126.6 Hz, cyclopentadienyl CipsoP), 22.2 (d, J=1.1 Hz, Mes CH3), 21.2 (d, J=1.3 Hz, Mes CH3), 20.9 (d, J=1.2 Hz, Mes CH3), 3.9, 3.6 ppm (2×SiMe3). 31P{1H} NMR (202 MHz, C6D6): δ=26.0 ppm. Anal. calcd for C37H46ClCuFeGeN3PSi2 (847.40): C 52.44, H 5.47, N 4.96 %; found C 53.28, H 4.47, N 5.54 %.

X‐ray crystallography: For each data collection a single crystal was mounted on a micro‐mount and all geometric and intensity data were taken from this sample at 100(2) K, except for 23, which was measured at 253(2) K due to a phase transition below this temperature. Data collections were carried out either on a Stoe IPDS2 diffractometer equipped with a 2‐circle goniometer and an area detector on a Stoe StadiVari diffractometer equipped with a 4‐circle goniometer and a DECTRIS Pilatus 200 K detector. The data sets were corrected for absorption, Lorentz and polarisation effects. The structures were solved by direct methods (SHELXT) and refined using alternating cycles of least‐squares refinements against F 2 (SHELXL2014/7). C‐bonded H atoms were included in the models in calculated positions, heteroatom‐bonded H atoms have been found in the difference Fourier lists. All H atoms were treated with the 1.2‐fold or 1.5‐fold isotropic displacement parameter of their bonding partner. Experimental details for each diffraction experiment are given in Table S1 in the Supporting Information.

CCDC 2159886 (for contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors 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.

Supporting Information

Click here for additional data file.