Synthesis and Characterization of Hydrido Carbonyl Molybdenum and Tungsten PNP Pincer Complexes.

In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

Introduction

Tridentate PNP ligands in which the central pyridine-based ring donor contains −CH2PR2 substituents in the two ortho positions are widely utilized ligands in transition-metal chemistry (e.g., Fe, Ru, Rh, Ir, Pd, Pt).[1−10] As part of our effort to create tridentate PNP pincer-type ligands in which the steric, electronic, and stereochemical properties can be easily varied, we have recently described the synthesis of a series of modularly designed PNP ligands based on N-heterocyclic diamines and R2PCl which contain both bulky and electron-rich dialkylphosphines as well as various P–O bond containing achiral and chiral phosphine units.[11] In these PNP ligands the central pyridine ring contains −NR′PR2 (R′ = H, alkyl, R = alkyl, aryl) substituents in the two ortho positions. This methodology was first developed for the synthesis of N,N′-bis(diphenylphosphino)-2,6-diaminopyridine (PNP-Ph).[12]

With these types of PNP ligands, we have thus far studied their reactivity toward different transition-metal fragments, which resulted in the preparation of a range of new pincer transition-metal complexes, including several new square-planar Ni(II), Pd(II), and Pt(II) PNP complexes,[13] various iron complexes acting as CO sensors[14] and catalysts for the coupling of aromatic aldehydes with ethyldiazoacetate,[15] and several pentacoordinated nickel complexes.[16] Surprisingly, as far as group 6 PNP complexes are concerned, only a few examples have been described in the literature. A few years ago Haupt and co-workers reported the synthesis of [M(PNP-Ph)(CO)3] (M = Cr, Mo, W),[12] while Walton and co-workers described the synthesis of the dinuclear molybdenum complex [Mo(PNP)Mo(HPCy2)Cl3] (PNP = 2,6-bis(dicyclohexylphosphinomethyl)pyridine).[17] Most recently, dinuclear molybdenum and tungsten dinitrogen complexes bearing bulky PNP pincer ligands were found to work as effective catalysts for the formation of ammonia from dinitrogen.[18] Finally, Templeton and co-workers described the synthesis of a series of hydrido carbonyl and halo carbonyl tungsten pincer complexes featuring the silazane-based PNP pincer-type ligand HN(SiMe2CH2PPh2)2.[19] In a preliminary study we have prepared cationic seven-coordinate halo carbonyl molybdenum pincer complexes of the types [Mo(PNP-iPr)(CO)3I]+ and [Mo(PNP-iPr)(CO)2(CH3CN)I]+.[13] Here we report on the synthesis, characterization, and reactivity of a series of new hydrido carbonyl molybdenum(II) and tungsten(II) PNP pincer complexes.[20]

Results and Discussion

Molybdenum(0) and Tungsten(0) Complexes

We have recently reported the synthesis of molybdenum tricarbonyl complexes of the type [Mo(PNP)(CO)3] (2a–c) by reacting [Mo(CO)3(CH3CN)3], prepared in situ by refluxing a solution of [Mo(CO)6] in CH3CN for 4 h, with the PNP ligands PNP-Ph (1a), PNP-iPr (1b), and PNP-tBu (1c) in 74–90% isolated yields.[13] The same procedure was followed with the N-methylated PNP ligand PNPMe-iPr (1d), affording [Mo(PNPMe-iPr)(CO)3] (2d) in 80% yield (Scheme 1). The new PNP ligand 1d was prepared in a three-step procedure involving borane protection of the phosphine, a deprotonation/alkylation step, and deprotection of the phosphine, as shown in Scheme 2. The analogous tungsten complexes [W(PNP)(CO)3] can be prepared in a similar fashion but require much longer reaction times (several days to prepare the intermediate [W(CO)3(CH3CN)3]); moreover, the yields turned out to be significantly lower (10–25%). It has to be noted that already a few years ago Haupt and co-workers reported the synthesis of [M(PNP-Ph)(CO3)] (M = Mo, W) in 34 and 22% yields.[12] We thus developed an alternative method to obtain tungsten(0) complexes [W(PNP)(CO)3] via the intermediacy of the known dinuclear complex [W(CO)4(μ-Br)Br]2,[21] prepared in situ from W(CO)6 and stoichiometric amounts of Br2 in CH2Cl2 at −70 °C. Treatment of a solution of [W(CO)4(μ-Br)Br]2 in CH2Cl2 at room temperature with the PNP ligands 1a–c afforded on workup the seven-coordinate tungsten(II) complexes [W(PNP)(CO)3Br]Br (3a–c) in 60–80% yields (Scheme 3).

Scheme 1

Scheme 2

Scheme 3

It has to be noted that the reaction of PNP ligands with [Mo(CO)4(μ-Br)Br]2,[21] prepared in situ from Mo(CO)6 and Br2 in CH2Cl2 at −70 °C, affords the analogous seven-coordinate molybdenum complexes [Mo(PNP)(CO)3Br]Br. This has been demonstrated for the synthesis of [Mo(PNP-Ph)(CO)3Br]Br (4a) and [Mo(PNP-iPr)(CO)3Br]Br (4b), as illustrated in Scheme 3.

The solid-state structures of 3a and 4b were determined by single-crystal X-ray diffraction. Molecular views of 3a and 4b are depicted in Figures 1 and 2, respectively, with selected bond distances and angles reported in the captions. While the Mo-bonded bromide in 4b was clearly in an axial position, the bromide in the tungsten complex 3a adopted an axial position at about 86% occupancy (Br1 in Figure 1), while the remaining 14% exchanged places with the carbonyl group C32–O3.

Figure 1

Structural view of [W(PNP-Ph)(CO)3Br]Br·CH3OH (3a·CH3OH) showing 20% thermal ellipsoids (H atoms, Br– counterion, solvent molecule and subordinate Br/CO positions omitted for clarity). Selected bond lengths (Å) and bond angles (deg): W–C(31) = 2.017(5), W–C(30) = 2.020(6), W–C(32) = 2.024(8), W–N(1) = 2.237(3), W–P(1) = 2.4955(10), W–P(2) = 2.4895(11), W–Br(1) = 2.6015(5); P(1)–W–P(2) = 152.34(4), N(1)–W–P(1) = 77.44(9), N(1)–W–P(2) = 75.92(9), N(1)–W–C(30) = 137.76(16), N(1)–W–C(31) = 82.35(16), N(1)–W–C(32) = 150.87(8), N(1)–W–Br(1) = 82.28(9).

Figure 2

Structural view of [Mo(PNP-iPr)(CO)3Br]Br (4b) showing 50% thermal ellipsoids (Br– counterion omitted for clarity). Selected bond lengths (Å) and bond angles (deg): Mo–C(18) = 2.037(2), Mo–C(19) = 1.979(2), Mo–C(20) = 2.006(2), Mo–N(1) = 2.236(2), Mo–P(1) = 2.5242(5), Mo–P(2) = 2.5172(5), Mo–Br(1) = 2.6713(3); P(1)–Mo–P(2) = 150.80(2), N(1)–Mo–P(1) = 75.67(4), N(1)–Mo–P(2) = 75.35(4), N(1)–Mo–C(18) = 85.95(7), N(1)–Mo–C(19) = 77.73(6), N(1)–Mo–C(20) = 127.23(6), N(1)–Mo–Br(1) = 84.22(4).

Structural view of [Mo(PNP-iPr)(n class="Chemical">CO)3Br]Br (4b) showing 50% thermal ellipsoids (Br– counterion omitted for clarity). Selected bond lengths (Å) and bond angles (deg): Mo–C(18) = 2.037(2), Mo–C(19) = 1.979(2), Mo–C(20) = 2.006(2), Mo–N(1) = 2.236(2), Mo–P(1) = 2.5242(5), Mo–P(2) = 2.5172(5), Mo–Br(1) = 2.6713(3); P(1)–Mo–P(2) = 150.80(2), N(1)–Mo–P(1) = 75.67(4), N(1)–Mo–P(2) = 75.35(4), N(1)–Mo–C(18) = 85.95(7), N(1)–Mo–C(19) = 77.73(6), N(1)–Mo–C(20) = 127.23(6), N(1)–Mo–Br(1) = 84.22(4).

Stirring complexes 3a–c with an excess of 10% sodium amalgam in THF gave the desired W(0) complexes [W(PNP-Ph)(CO)3] (5a), [W(PNP-iPr)(CO)3] (5b), and [W(PNP-tBu)(CO)3] (5c) as yellow solids in 70–80% isolated yields (Scheme 3). This methodology also yields the analogous Mo(0) complexesl thus being an alternative method to that described previously.[13] The use of Zn as reducing reagent turned out to be problematic, due to the formation of highly soluble and, thus, difficult to remove bromozincate anions, e.g., [ZnBr3·solvent]− (solvent = acetone, THF) and ZnBr42–. Complexes 5a–c were fully characterized by a combination of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy, and elemental analysis. Characteristic features of 5a–c comprise, in the 13C{1H} NMR spectrum, two low-field triplet resonances (1/2 ratio) in the ranges of 206–221 and 196–210 ppm assignable to the carbonyl carbon atoms trans and cis to the pyridine nitrogen, respectively. The 31P{1H} NMR spectra exhibit singlet resonances at 85.2, 106.6.1, and 131.6 ppm with 1JWP coupling constants of 315–329 Hz. The tungsten–phosphorus coupling was observed as a doublet satellite due to 183W, 14% abundance with I = 1/2, superimposed over the dominant singlet. The IR spectra show the typical three strong to medium absorption bands of a mer CO arrangement in the range of 2017–1760 cm–1 assignable to one weaker symmetric and two strong asymmetric νCO stretching modes. The νCO frequencies, in particular the symmetric CO stretch, is indicative of the increasing electron donor strengths of the PNP ligands and follow the order PNP-Ph < PNP-iPr ≈ PNPMe-iPr < PNP-tBu (Table 1). The CO stretching frequencies are 1964 (2a, PNP-Ph), 1936 (2b, PNP-iPr), 1936 (2d, PNPMe-iPr), and 1922 cm–1 (2c, PNP-tBu). The same order is found for the respective tungsten complexes. In all complexes the PNP pincer ligand adopts the typical mer coordination mode with no evidence for any fac isomers.[22]

Table 1

Selected IR, 31P{1H} NMR, and 13C{1H} NMR Data for [M(PNP)(CO)3] and [M(PNP)(CO)3H]+ (M = Mo, W)

complex	νCO, cm–1	δP, ppm	δCO, ppm	δH, ppm
2a	1964, 1858, 1765	116.2	228.4, 211.2
2b	1936, 1809, 1790	143.6	231.4, 216.9
2c	1922, 1808, 1771	161.9	233.1, 224.0
2d	1936, 1810, 1795	171.0	230.8, 217.9
5a	1955, 1847, 1759	100.2	206.0, 196.6
5b	1929, 1805, 1784	128.5	221.1, 210.6
5c	1914, 1799, 1759	147.2	224.7, 219.4
7a	2042, 1940, 1937	111.5, 97.8	212.7, 203.2	–3.78
7b	2035, 1923, 1920	142.3, 121.4	212.3, 205.8	–4.98
7c	2019, 1937, 1916	158.8, 142.8	213.7, 209.5	–4.34
7d	2028, 1928, 1910	166.1, 147.7	210.8, 205.4	–5.49
8a	2038, 1963, 1918	95.5, 84.8	205.9, 196.6	–3.43
8b	2027, 1910, 1906	125.9, 108.6	205.5, 198.0	–4.83
8c	2021, 1934, 1897	141.1, 126.8		–4.16

In addition to the spectroscopic characterization, the solid-state structures of 2d and 5b,c were determined by single-crystal X-ray diffraction. Structural views are depicted in Figures 3–5, respectively, with selected bond distances and angles given in the captions. The coordination geometry around the tungsten center of 5b,c corresponds to a distorted octahedron with P1–W–P2 and trans-CCO–W–CCO bond angles 154.43(4) and 165.7(2)° (5b), and 151.42(1) and 156.46(9)° (5c), respectively. For comparison, in the analogous [Mo(PNP)(CO)3] complexes 2a–d the P1–Mo–P2 angles are hardly affected by the size of the substituents of the phosphorus atoms, being 155.0(2), 155.62(1), 155.3(1), and 151.73(1)°, respectively.[12,13] The carbonyl–Mo–carbonyl angles of the CO ligands trans to one another, on the other hand, vary strongly with the bulkiness of the PR2 moiety (PNP-Ph2 < PNP-iPr2 < PNPMe-iPr < PNP-tBu2) and decrease from 171.1(8)° in [Mo(PNP-Ph)(CO)3], to 166.03(5)° in [Mo(PNP-iPr)(CO)3], to 162.93(7)° in [Mo(PNPMe-iPr)(CO)3], and finally to 156.53(4)° in [Mo(PNP-tBu)(CO)3].

Figure 3

Structural view of [Mo(PNPMe-iPr)(CO)3] (2d) showing 50% thermal ellipsoids (H atoms are omitted for clarity; the complex is mirror symmetric; symmetry code i for x, 1/2 – y, z). Selected bond lengths (Å) and bond angles (deg): Mo–C(14) = 1.956(2), Mo–C(15) = 2.0153(13), Mo–N(1) = 2.2589(15), Mo–P(1) = 2.3977(5), Mo–P(2) = 2.4070(5); P(1)–Mo–P(2) = 155.25(2), N(1)–Mo–P(1) = 77.74(4), N(1)–Mo–P(2) = 77.51(4), N(1)–Mo–C(15) = 98.52(4), C(15)–Mo–C(15i) = 162.93(7).

Figure 5

Structural view of [W(PNP-tBu)(CO)3]·THF (5c·THF) showing 50% thermal ellipsoids (H atoms and solvent molecule omitted for clarity). Selected bond lengths (Å) and bond angles (deg): W–C(22) = 1.941(3), W–C(23) = 1.997(2), W–C(24) = 2.001(2), W–N(1) = 2.277(2), W–P(1) = 2.4583(5), W–P(2) = 2.4656(5); P(1)–W–P(2) = 151.42(2), N(1)–W–P(1) = 76.52(4), N(1)–W–P(2) = 76.00(4), N(1)–W–C(22) = 169.53(8), N(1)–W–C(23) = 113.16(8), N(1)–W–C(24) = 90.27(7), C(23)–W–C(24) = 156.46(9).

Structural view of [Mo(PNPMe-iPr)(n class="Chemical">CO)3] (2d) showing 50% thermal ellipsoids (H atoms are omitted for clarity; the complex is mirror symmetric; symmetry code i for x, 1/2 – y, z). Selected bond lengths (Å) and bond angles (deg): Mo–C(14) = 1.956(2), Mo–C(15) = 2.0153(13), Mo–N(1) = 2.2589(15), Mo–P(1) = 2.3977(5), Mo–P(2) = 2.4070(5); P(1)–Mo–P(2) = 155.25(2), N(1)–Mo–P(1) = 77.74(4), N(1)–Mo–P(2) = 77.51(4), N(1)–Mo–C(15) = 98.52(4), C(15)–Mo–C(15i) = 162.93(7).

Structural view of [W(PNP-iPr)(n class="Chemical">CO)3]·THF·1/2C6H14 (5b·THF·1/2C6H14) showing 30% thermal ellipsoids (H atoms, solvent molecules, and alternative orientation of iPr group C(16a)–C(15a)–C(17a) omitted for clarity). Selected bond lengths (Å) and bond angles (deg): W–C(18) = 2.014(5), W–C(19) = 2.015(5), W–C(20) = 1.934(4), W–N(1) = 2.257(3), W–P(1) = 2.4080(12), W–P(2) = 2.4013(12); P(1)–W–P(2) = 154.43(4), N(1)–W–P(1) = 77.30(9), N(1)–W–P(2) = 77.18(9), N(1)–W–C(18) = 100.8(2), N(1)–W–C(19) = 93.3(2), N(1)–W–C(20) = 175.8(2), C(18)–W–C(19) = 165.7(2).

Structural view of [W(PNP-n class="Chemical">tBu)(CO)3]·THF (5c·THF) showing 50% thermal ellipsoids (H atoms and solvent molecule omitted for clarity). Selected bond lengths (Å) and bond angles (deg): W–C(22) = 1.941(3), W–C(23) = 1.997(2), W–C(24) = 2.001(2), W–N(1) = 2.277(2), W–P(1) = 2.4583(5), W–P(2) = 2.4656(5); P(1)–W–P(2) = 151.42(2), N(1)–W–P(1) = 76.52(4), N(1)–W–P(2) = 76.00(4), N(1)–W–C(22) = 169.53(8), N(1)–W–C(23) = 113.16(8), N(1)–W–C(24) = 90.27(7), C(23)–W–C(24) = 156.46(9).

Molybdenum(II) and Tungsten(II) Hydride Complexes

Addition of HBF4 to a CH2Cl2 solution of [Mo(PNP)(CO)3] (2a–d) and [W(PNP)(CO)3] (5a–c) resulted in an immediate color change from yellow to pale yellow consistent with protonation at the tungsten and molybdenum centers to generate tungsten(II) and molybdenum(II) hydride complexes [Mo(PNP)(CO)3H]BF4 (7a–d) and [W(PNP)(CO)3H]BF4 (8a–c), respectively (Scheme 4). The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are re-formed quantitatively (Scheme 4). All hydride complexes are thermally robust pale yellow solids that are air stable in the solid state but slowly decompose in solution. Characterization was accomplished by elemental analysis and by 1H, 13C{1H}, and 31P{1H} NMR and IR spectroscopy (Table 1). The recording of a 13C{1H} NMR spectrum of 8c was precluded due to the poor solubility of this complex in most common solvents.

Scheme 4

Seven-coordinate complexes are well-known for their fluxional behavior in solution,[23,24] since typically none of the idealized geometries such as capped prism, capped octahedron, and pentagonal bipyramid or any of the less symmetrical arrangements are typically characterized by a markedly lower total energy.[25] Hence, interconversions between these various structures are quite facile. The fluxional behavior of complexes 7a–d and 8a–c was evident in variable-temperature 1H and 31P{1H} NMR spectra. At room temperature, the 1H NMR spectrum of complexes 7 and 8 confirmed the presence of one hydride ligand, which appeared in the range of −3.89 to −5.36 ppm either as a well-resolved doublet of doublets (8b,c) or as triplets (7a–d, 8a).

At −60 °C, all hydride resonances appear as a well-resolved doublet of doublets with one large and one small coupling constant of about 21–36 and 47–53 Hz, respectively. As an example, the variable-temperature 300 MHz 1H NMR spectra of the hydride region of [Mo(PNP-Ph)(CO)3H]BF4 (7a) are shown in Figure 6. At low temperatures the hydride signal constitutes the X part of an AMX spin system, giving rise to a doublet of doublets which, at elevated temperatures in the fast exchange regime, becomes a simple A2X spin system where the X part exhibits a triplet resonance.

Figure 6

Variable-temperature 300 MHz 1H NMR spectra of the hydride region of [Mo(PNP-Ph)(CO)3H]BF4 (7a) in CD2Cl2.

Variable-temperature 300 MHz 1H n class="Chemical">NMR spectra of the hydride region of [Mo(PNP-Ph)(CO)3H]BF4 (7a) in CD2Cl2.

In the 13C{1H} NMR spectrum of complexes 7a–d and 8a–d the most noticeable resonances are two low-field resonances of the carbonyl carbon atoms trans and cis to the pyridine nitrogen observed as two triplets in a 1:2 ratio. At room temperature, no 31P{1H} NMR signals could be detected for molybdenum complexes 7a–d and the tungsten complex 8a due to their fluxional behavior. At −60 °C, however, the 31P{1H} NMR spectra of all complexes 8 give rise to two doublets with a large geminal coupling constant of about 80 Hz, which is indicative of the phosphorus atoms being in mutually trans positions. The IR spectra of 7 and 8 show three strong to medium absorption νCO bands of the one symmetric and the two asymmetric vibration modes (Table 1), which again are typical for a mer CO arrangement.

In addition to the spectroscopic characterization, the solid-state structures of 7a,d and 8a were determined by single-crystal X-ray diffraction. Structural diagrams are depicted in Figures 7–9, respectively, with selected bond distances given in the captions. The coordination geometry around the molybdenum center corresponds to a distorted capped octahedron, in which a hydride ligand occupies the capping position of an octahedral face. The crystal structure showed the tridentate PNP ligand to be bound meridionally with three carbonyl ligands filling the remaining three sites. The carbonyl trans to nitrogen was pushed toward one of the phosphine ligands to accommodate the hydride ligand. The metal–hydride bond length in the three complexes averages 1.65 Å (1.64–1.70 Å), the mean bond angle H–M–P to the nearest P atom is 67° (65–69°), and the mean bond angle H–M–C between hydride and the equatorial carbonyl group is 53° (50–55°). The hydride to carbonyl C atom distance (1.62 Å in 7d) and the almost linear attachment of the equatorial carbonyl group (Mo1–C22–O3 = 179° in 7d) do not indicate a significant bonding interaction between hydride and the adjacent carbonyl C atom in the three structurally characterized hydrido carbonyl complexes.

Figure 7

Structural view of [Mo(PNP-Ph)(CO)3H]BF4 (7a) showing 50% thermal ellipsoids (BF4– counterion and alternative orientation of C(32)–O(3) and H(1) omitted for clarity). Selected bond lengths (Å) and bond angles (deg): Mo–H(1) = 1.67(4), Mo–C(30) = 2.0368(15), Mo–C(31) = 2.0465(15), Mo–C(32) = 2.017(3), Mo–N(1) = 2.2357(11), Mo–P(1) = 2.4409(4), Mo–P(2) = 2.4489(4); P(1)–Mo–P(2) = 154.64(1), N(1)–Mo–P(1) = 77.39(3), N(1)–Mo–P(2) = 77.39(3), N(1)–Mo–C(30) = 87.62(5), N(1)–Mo–C(31) = 86.97(5), N(1)–Mo–C(32) = 163.73(9), N(1)–Mo–H(1) = 146.0(15), C(32)–Mo–H(1) = 50.2(15).

Figure 9

Structural view of [W(PNP-Ph)(CO)3H]BF4 (8a) showing 50% thermal ellipsoids (BF4– counterion and alternative orientation of C(32)–O(3) and H(1) omitted for clarity). Selected bond lengths (Å) and bond angles (deg): W–H(1) = 1.65(5), W–C(30) = 2.033(2), W–C(31) = 2.039(2), W–C(32) = 2.022(4), W–N(1) = 2.2316(15), W–P(1) = 2.4444(5), W–P(2) = 2.4459(5); P(1)–W–P(2) = 154.32(2), N(1)–W–P(1) = 77.17(4), N(1)–W–P(2) = 77.29(4), N(1)–W–C(30) = 87.78(7), N(1)–W–C(31) = 86.88(7), N(1)–W–C(32) = 162.35(11), N(1)–W–H(1) = 143.5(17), C(32)–W–H(1) = 53.9(17).

Structural view of [Mo(PNPMe-iPr)(CO)3H]BF4·n class="Chemical">CH2Cl2 (7d·CH2Cl2) showing 50% thermal ellipsoids (BF4– counterion and CH2Cl2 omitted for clarity). Selected bond lengths (Å) and bond angles (deg): Mo–H(1) = 1.63(2), Mo–C(20) = 2.0440(13), Mo–C(21) = 2.0239(13), Mo–C(22) = 1.9856(13), Mo–N(1) = 2.2462(10), Mo–P(1) = 2.4425(3), Mo–P(2) = 2.4776(3); P(1)–Mo–P(2) = 154.61(1), N(1)–Mo–P(1) = 77.29(3), N(1)–Mo–P(2) = 77.31(3), N(1)–Mo–C(20) = 94.27(4), N(1)–Mo–C(21) = 89.76(4), N(1)–Mo–C(22) = 162.26(4), N(1)–Mo–H(1) = 145.7(8), C(22)–Mo–H(1) = 52.0(8).

The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations[26] for the molybdenum and tungsten complexes [Mo(PNP)(CO)3H]+ (7a–c) and [W(PNP)(CO)3H]+ (8a–c). The free energy profile for the “pseudorotation” is depicted in Figure 10. The optimized structures of 8a,c and the corresponding transition states TS are shown in Figures 11 and 12. In the fluxional process, the CO and the hydride ligands in the PNP plane exchange positions in a single-step path. During that process the rest of the molecule has to change in order to accommodate the overall transformations associated with the pseudorotation. The main geometry change that happens along the path is the hydride ligand moving from the PNP plane to the perpendicular plane, i.e., the plane of the three CO ligands, in the transition state (TS) and then back to the PNP plane again but on the other side of the CO ligand. Thus, in TS, the two trans CO ligands have to create enough space to allow the presence of an extra ligand in the NCCC plane, opening the corresponding OC–M–CO angle and bending away from the hydride ligand. Accordingly, In the case of the molybdenum and tungsten PNP complexes bearing the less bulky Ph and iPr substituents (7a,b and 8a,b), the C30–W1–C31 angle changes from about 177° in the ground-state structure to 166° in TS, where both trans CO ligands are bent toward the PNP ligand (Figure 11). On the other hand, in the case of the molybdenum and tungsten PNP complexes bearing the bulky tBu substituents (7c and 8c) the situation is somewhat different. The C30–W1–C31 angle increases from 169° in the minima to 178° in TS, while one of the two trans CO ligands is severely bent away from the PNP ligand and the other one is bent toward the PNP ligand (Figure 12). In the course of all these transformations also the H1–W1–C32 angles change from about 55° in 7a–c and 8a–c to roughly 43° in the TS, respectively. Bond distances are hardly affected by the interconversion. It is interesting to note that, in agreement with the X-ray structures of 7a, 7d, and 8a (Figures 6–8), the distance between H1 and C32 is rather short, being in the range of 1.80–1.66 Å. A Wiberg index[27] of 0.20, in 8c, seems to indicate an attractive interaction between the hydride and the neighboring CCO atom (C32). Most importantly, the free energy barriers ΔG⧧ are 18.6 and 21.3 kcal/mol for 7c and 8c, respectively, containing the bulky tBu substituents. In the case of all other complexes the free energy barrier is lower, being in the range of 12.5–16.1 kcal/mol, as shown in Figure 9. These results corroborate a process that can be stopped in the temperature range employed in the NMR studies and a more facile process in the case of the Mo species, as observed.

Figure 10

Free energy profile (in kcal/mol) for the “pseudorotation” of CO and hydride ligands in the complexes [M(PNP)(CO)3H]+ (M = W, Mo).

Figure 11

Front (left) and side views (right) of the optimized structures (DFT/B3LYP) of the tungsten complex [W(PNP-Ph)(CO)3H]+ (8a; top) and the transition state TS (bottom) with most phenyl carbon atoms and hydrogen atoms omitted for clarity.

Figure 12

Front (left) and side views (right) of the optimized structures (DFT/B3LYP) of the tungsten complex [W(PNP-tBu)(CO)3H]+ (8c; top) and the transition state TS (bottom) with most tBu carbon atoms and hydrogen atoms omitted for clarity.

Figure 8

Structural view of [Mo(PNPMe-iPr)(CO)3H]BF4·CH2Cl2 (7d·CH2Cl2) showing 50% thermal ellipsoids (BF4– counterion and CH2Cl2 omitted for clarity). Selected bond lengths (Å) and bond angles (deg): Mo–H(1) = 1.63(2), Mo–C(20) = 2.0440(13), Mo–C(21) = 2.0239(13), Mo–C(22) = 1.9856(13), Mo–N(1) = 2.2462(10), Mo–P(1) = 2.4425(3), Mo–P(2) = 2.4776(3); P(1)–Mo–P(2) = 154.61(1), N(1)–Mo–P(1) = 77.29(3), N(1)–Mo–P(2) = 77.31(3), N(1)–Mo–C(20) = 94.27(4), N(1)–Mo–C(21) = 89.76(4), N(1)–Mo–C(22) = 162.26(4), N(1)–Mo–H(1) = 145.7(8), C(22)–Mo–H(1) = 52.0(8).

Free energy profile (in kcal/mol) for the “pseudorotation” of CO and n class="Chemical">hydride ligands in the complexes [M(PNP)(CO)3H]+ (M = W, Mo).

Front (left) and side views (right) of the optimized structures (DFT/B3LYP) of the tungsten complex [W(n class="Chemical">PNP-Ph)(CO)3H]+ (8a; top) and the transition state TS (bottom) with most phenyl carbon atoms and hydrogen atoms omitted for clarity.

Front (left) and side views (right) of the optimized structures (DFT/B3LYP) of the tungsten complex n class="Chemical">[W(PNP-tBu)(CO)3H]+ (8c; top) and the transition state TS (bottom) with most tBu carbon atoms and hydrogen atoms omitted for clarity.

Conclusion

In the present study the Mo(0) and W(0) complexes [M(PNP)CO)3] as well as seven-coordinate cationic hydrido carbonyl and halo carbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3Br]+ and [M(PNP)(CO)3H]+ featuring PNP pincer ligands based on 2,6-diaminopyridine were prepared and fully characterized. The synthesis of the Mo(0) complexes [Mo(PNP)CO)3] was accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromo carbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M(PNP)(CO)3] resulted in protonation at the tungsten and molybdenum centers to formally generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are re-formed quantitatively. All seven-coordinate complexes exhibit fluxional behavior in solution, since none of the idealized geometries (capped prism, capped octahedron, and pentagonal bipyramid) or any of the less symmetrical arrangements are typically characterized by a markedly lower total energy. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. Thereby the CO and the hydride ligands which are situated in the PNP plane are interconverted. The structures of representative complexes were determined by X-ray single-crystal analyses.

Experimental Section

General Considerations

All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques. The solvents were purified according to standard procedures.[28] The ligands and complexes N,N′-bis(diphenylphosphino)-2,6-diaminopyridine (PNP-Ph; 1a), N,N′-bis(diisopropylphosphino)-2,6-diaminopyridine (PNP-iPr; 1b), N,N′-bis(di-tert-butylphosphino)-2,6-diaminopyridine (PNP-tBu; 1c), [Mo(PNP-Ph)(CO)3] (2a), [Mo(PNP-iPr)(CO)3] (2b), and [Mo(PNP-tBu)(CO)3] (2c) were prepared according to the literature.[13] The deuterated solvents were purchased from Aldrich and dried over 4 Å molecular sieves. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker AVANCE-250 and AVANCE-300 DPX spectrometers and were referenced to SiMe4 and H3PO4 (85%), respectively.

N,N′-Bis(diisopropylphosphino-borane)-2,6-diaminopyridine (PNP-iPr; 1b·BH)

BH3·THF (43.1 mL, 1.0 M, 43.06 mmol) was added slowly to a solution of 1b (7.00 g, 20.50 mmol) in 100 mL of dry THF. After the mixture was stirred for 30 min at room temperature, the solvent was evaporated under reduced pressure. The white solid was further dried under vacuum for 2 h to give the product in quantitative yield. Anal. Calcd for C17H39B2N3P2: C, 55.32; H, 10.65; N, 11.39. Found: C, 55.28; H, 10.70; N, 11.43. 1H NMR (δ, CDCl3, 20 °C): 7.32 (t, J = 7.9 Hz, 1H, py4), 6.26 (d, J = 7.9, 2H, py3,5), 4.58 (d, J = 8.4 Hz, 2H, NH), 2.62 (septd, J = 6.9 Hz, J = 13.7 Hz, 4H, CH(CH3)2), 1.16 (m, 24H, CH(CH3)2), 0.60 to −0.15 (bs, 6H, BH). 13C{1H} NMR (δ, CDCl3, 20 °C): 154.4 (s, py2,6), 140.1 (s, py4), 103.1 (s, py3,5), 24.5 (d, J = 36.3 Hz, CH(CH3)2), 17.2 (d, J = 4.3 Hz, CH(CH3)2), 17.0 (s, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20 °C): 88.5 (br, m).

N,N′-Bis(diisopropylphosphino-borane)-N,N′-methyl-2,6-diaminopyridine (PNPMe-iPr; 1d·BH)

To a solution of 1b·BH (7.45 g, 20.19 mmol) in THF (50 mL) at −20 °C was slowly added n-BuLi (17.0 mL, 2.5 M, 41.39 mmol). The reaction mixture was allowed to reach room temperature and was stirred for 2 h. Methyl iodide (3.15 mL, 50.46 mmol) was then added slowly via syringe. After the mixture was stirred for 12 h at room temperature, the reaction was quenched with a saturated NH4Cl solution (100 mL) and 5 mL of concentrated NH3. The aqueous phase was extracted twice with CH2Cl2, and the combined organic phases were washed with 25 mL of brine and dried over Na2SO4. The solvent was removed under reduced pressure to afford 1d·BH as a yellow oil. The crude product was purified via flash chromatography using silica gel and THF to give the product as a white solid. Anal. Calcd for C19H43B2N3P2: C, 57.56; H, 10.91; N, 10.58. Found: C, 57.62; H, 10.89; N, 10.61. Yield: 5.05 g (63%). 1H NMR (δ, CDCl3, 20 °C): 7.48 (t, J = 8.0 Hz, 1H, py4), 6.49 (d, J = 8.0, 2H, py3,5), 3.17 (d, J = 7.9 Hz, 6H, NCH3), 2.80 (septd, J = 7.0 Hz, J = 21.2 Hz, 4H, CH(CH3)2), 1.22 (dd, J = 6.9 Hz, J = 16.5 Hz, 12H, CH(CH3)2), 1.03 (dd, J = 7.0 Hz, J = 15.11 Hz, 12H, CH(CH3)2), 0.70 to −0.30 (bs, 6H, BH). 13C{1H} NMR (δ, CDCl3, 20 °C): 156.9 (s, py2,6), 139.1 (s, py4), 105.8 (s, py3,5), 37.6 (s, NCH3), 25.6 (d, J = 36.2 Hz, CH(CH3)2), 17.8 (s, CH(CH3)2), 17.2 (s, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20 °C): 105.9 (br, m).

N,N′-Bis(diisopropylphosphino)-N,N′-methyl-2,6-diaminopyridine (PNPMe-iPr; 1d)

1d·BH (5.00 g, 12.59 mmol) was refluxed for 72 h in 100 mL of Et2NH. After removal of the solvent under reduced pressure the remaining oil was dissolved in THF, filtered through Celite, and obtained as a yellow oil after evaportaion of the solvent under reduced pressure. The crude product was purified by recrystallization from acetonitrile to afford 2d as a white solid. Anal. Calcd for C19H37N3P2: C, 61.77; H, 10.09; N, 11.37. Found: C, 61.69; H, 10.16; N, 11.44. Yield: 3.25 g (70%). 1H NMR (δ, CD2Cl2, 20 °C): 7.22 (t, J = 8.0 Hz, 1H, py4), 6.64 (bs, py3,5), 3.04 (d, J = 2.3 Hz, 6H, NCH3), 2.22 (bs, 4H, CH(CH3)2), 1.10 (dd, J = 6.9 Hz, J = 16.9 Hz, 12H, CH(CH3)2), 0.98 (dd, J = 7.0 Hz, J = 12.1 Hz, 12H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 160.4 (s, py2,6), 136.9 (s, py4), 99.1 (d, J = 22.3 Hz, py3,5), 33.7 (bs, NCH3), 26.2 (d, J = 15.3 Hz, CH(CH3)2), 19.4 (s, CH(CH3)2), 19.2 (s, CH(CH3)2), 19.0 (s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 81.3 (bs).

[Mo(PNPMe-iPr)(CO)3] (2d)

A suspension of Mo(CO)6 (714 mg 2.7 mmol) in n class="Chemical">acetonitrile (10 mL) was refluxed for 4 h. After that PNPMe-iPr (1d; 1.00 g, 2.7 mmol) was added and the mixture was refluxed for an additional 12 h. The solvent was then removed under reduced pressure, and the product was washed twice with diethyl ether and dried under vacuum. Yield: 1.19 g (80%). Anal. Calcd for C22H37MoN3O3P2: C, 48.09; H, 6.79; N, 7.65. Found: C, 48.15; H, 6.82; N, 7.61. 1H NMR (δ, CD2Cl2, 20 °C): 7.40 (t, J = 7.6 Hz, 1H, py), 6.30 (d, J = 7.6 Hz, 2H, py), 3.01 (s, 6H, NCH), 2.47 (m, 4H, CH(CH3)2), 1.30 (m, 12H, CH(CH3)2), 1.09 (m, 12H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 230.8 (t, J = 6.0 Hz, CO), 217.9 (t, J = 10.8 Hz, CO), 174.9 (t, J = 2.6 Hz, py), 137.8 (s, py), 96.7 (t, J = 2.2 Hz, py), 33.9 (t, J = 1.8 Hz, N(CH3)2), 32.9 (t, J = 9.0 Hz, CH(CH3)2), 29.6 (s, N(CH3)2), 19.2 (t, J = 7.5 Hz, CH(CH3)2), 18.1 (s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 171.0 (s). IR (ATR, cm–1): 1936 (νCO), 1810 (νCO), 1795 (νCO).

[W(PNP-Ph)(CO)3Br]Br (3a)

To a suspension of W(CO)6 (2.0 g, 5.68 mmol) in CH2Cl2 (30 mL) was added Br2 (292 μL, 5.68 mmol) at −70 °C, and the mixture was stirred for 1 h at that temperature and for an additional 1 h at room temperature. After that, PNP-Ph (1a; 2.72 g, 5.68 mmol) was added and the mixture was stirred for 5 h at room temperature. After removal of the solvent under reduced pressure, a yellow solid was obtained, which was washed with a 1/9 methanol/diethyl ether mixture and dried under vacuum. Yield: 4.11 g (80%). Anal. Calcd for C32H25Br2N3O3P2W: C, 42.46; H, 2.78; N, 4.64. Found: C, 42.29; H, 2.79; N, 4.55. 1H NMR (δ, acetone-d6, 20 °C): 10.85 (bs, 2H, NH), 7.78 (bs, 11H, py, Ph), 7.60 (bs, 10H, Ph), 7.43 (d, J = 7.6 Hz, 2H, py). 13C{1H} NMR (δ, CDCl3, 20 °C): 225.4 (t, J = 10.3 Hz, CO), 210.3 (t, J = 8.6 Hz, CO), 159.7 (d, J = 7.5 Hz, py), 159.6 (d, J = 7.5 Hz, py), 133.1 (s, py), 131.9 (s, Ph), 131.9 (s, Ph), 129.0 (d, J = 5.0 Hz, Ph), 128.2 (d, J = 5.4 Hz, Ph), 103.1 (s, py). 31P{1H} NMR (δ, acetone-d6, 20 °C): 85.2. IR (ATR, cm–1): 2030 (νCO), 1958 (νCO), 1933 (νCO).

[W(PNP-iPr)(CO)3Br]Br (3b)

This complex was prepared analogously to 3a with n class="Chemical">W(CO)6 (2.0 g, 5.68 mmol) and PNP-iPr (1b; 1.95 g, 5.69 mmol) as starting materials. Yield: 3.06 g (70%). Anal. Calcd for C20H33Br2N3O3P2W: C, 31.23; H, 4.32; N, 5.46. Found: C, 31.13; H, 4.42; N, 5.50. 1H NMR (δ, CD2Cl2, 20 °C): 9.32 (bs, 2H, NH), 7.66 (t, J = 7.2 Hz, 1H, py), 6.67 (d, J = 8.1 Hz, 2H, py), 3.54 (m, 2H, CH(CH3)2), 2.88 (m, 2H, CH(CH3)2), 1.43 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.2 (t, J = 10.1 Hz, CO), 191.3 (t, J = 7.6 Hz, CO), 161.0 (s, py), 142.2 (s, py), 101.9 (s, py), 30.6 (t, J = 15.0 Hz, CH(CH3)2), 29.3 (t, J = 15.3 Hz, CH(CH3)2), 19.2 (s, CH(CH3)2), 19.1 (s, CH(CH3)2), 18.9 (s, CH(CH3)2), 17.7 (s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 106.6. IR (ATR, cm–1): 2023 (νCO), 1953 (νCO), 1922 (νCO).

[W(PNP-tBu)(CO)3Br]Br (3c)

This complex was prepared analogously to 3a with W(CO)6 (2.0 g, 5.68 mmol) and PNP-tBu (1c) (2.26 g, 5.69 mmol) as starting materials. Yield: 2.51 g (60%). Anal. Calcd for C24H41Br2N3O3P2W: C, 34.93; H, 5.01; N, 5.09. Found: C, 34.81; H, 5.02; N, 5.20. 1H NMR (δ, CD2Cl2, 20 °C): 9.11 (s, 2H, NH), 7.74 (d, J = 7.8 Hz, 2H, py), 7.23 (t, J = 8.7 Hz, 1H, py), 1.70 (d, J = 8.2 Hz, 9H, C(CH3)3), 1.68 (d, J = 7.3 Hz, 9H, C(CH3)3), 1.54 (d, J = 7.5 Hz, 9H, C(CH3)3), 1.48 (d, J = 7.1 Hz, 9H, C(CH3)3). 13C{1H} NMR (δ, CDCl3, 20 °C): 229.9 (t, J = 14.8 Hz, CO), 212.9 (t, J = 7.6 Hz, CO), 162.6 (t, J = 4.2 Hz, py), 153.3 (d, J = 5.8 Hz, py), 149.9 (s, py), 103.8 (s, py), 46.0 (t, J = 7.2 Hz, C(CH3)3), 44.6 (t, J = 7.2 Hz, C(CH3)3), 31.9 (s, C(CH3)3), 30.4 (s, C(CH3)3), 26.9 (s, C(CH3)3). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 131.6. IR (ATR, cm–1): 2019 (νCO), 1941 (νCO), 1909 (νCO).

[Mo(PNP-Ph)(CO)3Br]Br (4a)

This complex was prepared analogously to 3a with n class="Chemical">Mo(CO)6 (300 mg, 1.14 mmol) and PNP-Ph (1a; 570 mg, 1.20 mmol) as starting materials. Yield: 775 mg (83%). Anal. Calcd for C32H25Br2MoN3O3P2: C, 47.03; H, 3.08; N, 5.14. Found: C, 46.95; H, 3.12; N, 5.01. 1H NMR (δ, acetone-d6, 20 °C): 8.70 (s, 2H, NH), 7.59 (m, 5H, Ph), 7.47 (m, 5H, Ph), 7.16 (t, J = 7.2 Hz, 1H, py), 7.01 (m, 5H, Ph), 6.85 (m, 5H, Ph), 6.56 (d, J = 7.8 Hz, 2H, py). 31P{1H} NMR (δ, acetone-d6, 20 °C): 125.3. IR (ATR, cm–1): 2040 (νCO), 1975 (νCO), 1875 (νCO).

[Mo(PNP-iPr)(CO)3Br]Br (4b)

This complex was prepared analogously to 3a with n class="Chemical">Mo(CO)6 (500 mg, 1.89 mmol) and PNP-iPr (1b; 678 mg, 1.98 mmol) as starting materials. Yield: 965 mg (75%). Anal. Calcd for C20H33Br2MoN3O3P2: C, 35.26; H, 4.88; N, 6.17. Found: C, 35.13; H, 4.92; N, 6.20. 1H NMR (δ, CDCl3, 20 °C): 9.08 (s, 2H, NH), 7.24 (d, J = 9.9 Hz, 2H, py), 7.10 (t, J = 7.4 Hz, 1H, py), 3.55 (m, 2H, CH(CH3)2), 2.96 (m, 2H, CH(CH3)2). 1.44 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CDCl3, 20 °C): 234.6 (t, J = 7.2 Hz, CO), 218.3 (t, J = 11.5 Hz, CO), 175.1 (s, py), 175.0 (s, py), 160.0 (t, J = 4.9 Hz, py), 142.1 (s, py), 102.7 (s, py), 30.8 (d, J = 12.8 Hz, CH(CH3)2), 30.6 (d, J = 14.9 Hz, CH(CH3)2), 30.3 (d, J = 12.4 Hz, CH(CH3)2), 30.1 (d, J = 11.6 Hz, CH(CH3)2), 19.4 (s, CH(CH3)2), 19.3 (s, CH(CH3)2), 19.1 (s, CH(CH3)2), 18.1 (s, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20 °C): 126.7. IR (ATR, cm–1): 2030 (νCO), 1967 (νCO), 1936 (νCO).

[W(PNP-Ph)(CO)3] (5a)

A solution of 3a (200 mg, 0.22 mmol) in THF (15 mL) was stirred in the presence of NaHg (10%) (16 mg, 0.66 mmol) for 8 h at room temperature. The solvent was then removed under reduced pressure. The residue was redissolved in acetone (10 mL), and the solution was filtered through Celite. After removal of the solvent under reduced pressure, a yellow solid was obtained, which was washed twice with diethyl ether (10 mL) and dried under vacuum. Yield: 140 mg (85%). Anal. Calcd for C32H25N3O3P2W: C, 51.57; H, 3.38; N, 5.64. Found: C, 51.63; H, 3.41; N, 5.50. 1H NMR (δ, acetone-d6, 20 °C): 7.64 (t, J = 8.6 Hz, 1H, py), 7.44 (bs, 8H, Ph), 7.30 (m, 12H, Ph), 6.43 (d, J = 7.6 Hz, 2H, py), 5.93 (d, J = 8.9 Hz, 2H, NH). 13C{1H} NMR (δ, acetone-d6, 20 °C): 206.0 (t, J = 4.6 Hz, CO), 196.6 (t, J = 9.1 Hz, CO), 159.6 (d, J = 4.7 Hz, py), 159.3 (d, J = 5.5 Hz, py), 142.1 (s, py), 132.0 (s, Ph), 131.0 (d, J = 12.4 Hz, Ph), 129.1 (d, J = 11.5 Hz, Ph), 127.9 (d, J = 6.6 Hz, Ph), 102.0 (d, J = 10.7 Hz, py). 31P{1H} NMR (δ, acetone-d6, 20 °C): 100.2 (s, JW–P = 327 Hz). IR (ATR, cm–1): 1955 (νCO), 1847 (νCO), 1759 (νCO).

[W(PNP-iPr)(CO)3] (5b)

This complex was prepared analogously to 5a with 3b (500 mg, 0.65 mmol) and n class="Chemical">NaHg (45 mg, 1.95 mmol) as starting materials. Yield: 315 mg (80%). Anal. Calcd for C20H33N3O3P2W: C, 39.43; H, 5.46; N, 6.90. Found: C, 39.51; H, 5.42; N, 6.84. 1H NMR (δ, CD2Cl2, 20 °C): 7.15 (t, J = 8.2 Hz, 1H, py), 6.15 (d, J = 7.9 Hz, 2H, py), 5.50 (bs, 2H, NH), 2.40 (m, 4H, CH(CH3)2), 1.24 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 221.1 (t, J = 2.0 Hz, CO), 210.6 (t, J = 7.1 Hz, CO), 162.5 (t, J = 8.5 Hz, py), 137.1 (s, py), 96.6 (t, J = 3.1 Hz, py), 32.60 (t, J = 12.8 Hz, CH(CH3)2), 18.5 (t, J = 1.7 Hz, CH(CH3)2), 18.1 (t, J = 2.9 Hz, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 128.5 (s, JW–P = 315 Hz). IR (ATR, cm–1): 1929 (νCO), 1805 (νCO), 1784 (νCO).

[W(PNP-tBu)(CO)3] (5c)

This complex was prepared analogously to 5a with n class="Chemical">3c (350 mg, 0.42 mmol) and NaHg (30 mg, 1.27 mmol) as starting materials. Yield: 215 mg (77%). Anal. Calcd for C24H41N3O3P2W: C, 43.32; H, 6.21; N, 6.32. Found: C, 43.23; H, 6.42; N, 6.48. 1H NMR (δ, CD2Cl2, 20 °C): 7.37 (bs, 2H, NH), 7.18 (t, J = 8.0 Hz, 1H, py), 6.29 (d, J = 7.5 Hz, 2H, py), 1.41 (t, J = 5.6 Hz, 36H, C(CH3)3). 13C{1H} NMR (δ, acetone-d6, 20 °C): 224.7 (t, J = 6.4 Hz,CO), 219.4 (t, J = 7.0 Hz, CO), 162.6 (d, J = 6.7 Hz, py), 137.8 (s, py), 97.1 (s, py), 40.7 (t, J = 7.5 Hz, C(CH3)3), 25.8 (d, J = 7.3 Hz, C(CH3)3). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 147.2 (s, JW–P = 329 Hz). IR (ATR, cm–1): 1914 (νCO), 1799 (νCO), 1759 (νCO).

Alternative Synthesis of [Mo(PNP-iPr)(CO)3] (2b)

This complex was prepared analogously to 5a with n class="Chemical">4b (88 mg, 0.13 mmol) and NaHg (9 mg, 0.39 mmol) as starting materials. Yield: 61 mg (90%). All spectral data for 2a are identical with those of the authentic sample reported previously.[13]

[Mo(PNP-Ph)(CO)3H]BF4 (7a)

To a solution of 2a (200 mg, 0.30 mmol) in CH2Cl2 (10 mL) was added HBF4 ((46 μL, 0.45 mmol, 54% solution in Et2O) at room temperature. After the solution was stirred overnight, a pale yellow precipitate was formed, which was collected on a glass frit, washed with diethyl ether, and dried under vacuum. Yield: 193 mg (85%). Anal. Calcd for C32H26BF4MoN3O3P2: C, 51.57; H, 3.52; N, 5.64. Found: C, 51.66; H, 3.59; N, 5.70. 1H NMR (δ, acetone-d6, −60 °C): 9.25 (s, 2H, NH), 8.23 (m, 13H, Ph, py), 7.93 (m, 8H, Ph), 6.99 (d, J = 8.0 Hz, 2H, py), −3.78 (dd, 2JHP = 21.6 Hz, 2JHP = 48.5 Hz, 1H, MoH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.7 (t, J = 11.2 Hz, CO), 203.2 (t, J = 8.4 Hz, CO), 158.4 (d, J = 6.1 Hz, py), 158.1 (d, J = 8.3 Hz, py), 141.6 (s, py), 135.3 (s, py), 134.5 (d, J = 55.6 Hz, Ph), 131.8 (s, Ph), 130.7 (d, J = 13.7 Hz, Ph), 129.0 (d, J = 11.1 Hz, Ph), 102.2 (d, J = 8.3 Hz, py). 31P{1H} NMR (δ, acetone-d6, −60 °C): 111.5 (d, 2JPP = 88.4 Hz), 97.8 (d, 2J = 88.4 Hz). IR (ATR, cm–1): 2042 (νCO), 1940 (νCO), 1937 (νCO).

[Mo(PNP-iPr)(CO)3H]BF4 (7b)

This complex was prepared analogously n class="Species">to 7a with 2b (200 mg, 0.38 mmol) and HBF4 (78 μL, 0.58 mmol) as starting materials. Yield: 200 mg (87%). Anal. Calcd for C20H34BF4MoN3O3P2: C, 39.43; H, 5.63; N, 6.90. Found: C, 39.53; H, 5.71; N, 6.99. 1H NMR (δ, acetone-d6, −60 °C): 8.86 (d, J = 41.2 Hz, 2H, NH), 8.00 (t, J = 8.2 Hz, 1H, py), 6.95 (d, J = 2.91 Hz, 1H, py), 6.92 (d, J = 2.91 Hz, 1H, py), 3.27 (m, 4H, CH(CH3)2), 1.83 (m, 24H, CH(CH3)2), −4.98 (dd, 2JHP = 36.7 Hz, 2JHP = 51.0 Hz, 1H, MoH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.3 (t, J = 11.3 Hz, CO), 205.8 (t, J = 9.0 Hz, CO), 159.8 (d, J = 3.9 Hz, py), 159.7 (d, J = 3.9 Hz, py), 141.5 (s, py), 100.5 (d, J = 7.5 Hz, py), 31.5 (d, J = 29.5 Hz, CH(CH3)2), 17.8 (d, J = 2.3 Hz, CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, −60 °C): 142.3 (d, 2JPP = 90.4 Hz), 121.4 (d, 2JPP = 90.4 Hz). IR (ATR, cm–1): 2035 (νCO), 1923 (νCO), 1920 (νCO).

[Mo(PNP-tBu)(CO)3H]BF4 (7c)

This complex was prepared analogously n class="Species">to 7a with 2c (240 mg, 0.42 mmol) and HBF4 (85 μL, 0.62 mmol) as starting materials. Yield: 243 mg (87%). Anal. Calcd for C24H42BF4MoN3O3P2: C, 43.33; H, 6.36; N, 6.32. Found: C, 43.43; H, 6.42; N, 6.28. 1H NMR (δ, acetone-d6, −60 °C): 8.06 (bs, 2H, NH), 7.62 (t, J = 8.3 Hz, 1H, py), 6.62 (d, J = 7.9 Hz, 2H, py), 1.82 (d, J = 5.8 Hz, 18H, C(CH3)3), 1.80 (d, J = 6.5 Hz, 18H, C(CH3)3), −4.34 (dd, 2JHP = 20.7 Hz, 2JHP = 47.8 Hz, 1H, MoH). 13C{1H} NMR (δ, acetone-d6, 20 °C): 213.7 (t, J = 12.1 Hz, CO), 209.5 (t, J = 9.4 Hz, CO), 160 (d, J = 7.0 Hz, py), 141.8 (s, py), 137.9 (s, py), 101.5 (d, J = 6.5 Hz, py), 97.3 (s, py), 41.2 (d, J = 15.8 Hz, C(CH3)3), 24.8 (d, J = 7.1 Hz, C(CH3)3). 31P{1H} NMR (δ, acetone-d6, −60 °C): 158.8 (d, 2JPP = 81.5 Hz), 142.8 (d, 2JPP = 81.5 Hz). IR (ATR, cm–1): 2019 (νCO), 1937 (νCO), 1916 (νCO).

[Mo(PNPMe-iPr)(CO)3H]BF4 (7d)

This complex was prepared analogously n class="Species">to 7a with 2d (67 mg, 0.11 mmol) and HBF4 (23 μL, 0.17 mmol) as starting materials. Yield: 63 mg (90%). Anal. Calcd for C22H38BF4MoN3O3P2: C, 41.46; H, 6.01; N, 6.59. Found: C, 41.55; H, 6.12; N, 6.47. 1H NMR (δ, acetone-d6, −60 °C): 7.93 (t, J = 8.2 Hz, 1H, py), 6.65 (d, J = 8.2 Hz, 2H, py), 3.41 (s, 6H, N(CH3)), 3.25 (m, 4H, CH(CH3)2), 1.24 (d, J = 7.00 Hz, 6H, CH(CH3)2), 1.22 (d, J = 7.80 Hz, 12H, CH(CH3)2), 1.16 (d, J = 7.00 Hz, 6H, CH(CH3)2), −5.49 (dd, 2JHP = 20.9 Hz, 2JHP = 49.3 Hz, 1H, MoH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 210.8 (t, J = 10.8 Hz, CO), 205.4 (t, J = 9.5 Hz, CO), 161 (t, J = 5.2 Hz, py), 141.9 (s, py), 100.7 (s, py), 35.3 (s, NCH3), 32.3 (d, J = 24.9 Hz, CH(CH3)2), 19.3 (d, J = 7.6 Hz, CH(CH3)2), 17.9 (s, CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, −60 °C): 166.1 (d, 2JPP = 85.4 Hz), 147.7 (d, 2JPP = 85.4 Hz). IR (ATR, cm–1): 2028 (νCO), 1928 (νCO), 1910 (νCO).

[W(PNP-Ph)(CO)3H]BF4 (8a)

This complex was prepared analogously n class="Species">to 7a with 5a (210 mg, 0.28 mmol) and HBF4 (58 μL, 0.42 mmol) as starting materials. Yield: 186 mg (80%). Anal. Calcd for C32H26BF4N3O3P2W: C, 46.13; H, 3.15; N, 5.04. Found: C, 46.23; H, 3.12; N, 5.20. 1H NMR (δ, acetone-d6, −60 °C): 9.74 (bs, 2H, NH), 8.15 (s, 8H, Ph), 8.25 (t, J = 8.4 Hz, 1H, py), 8.15 (m, 8H, Ph), 7.94 (m, 12H, Ph), 7.04 (d, J = 8.0 Hz, 2H, py), −3.43 (dd, 2J = 22.4 Hz, 2J = 53.8 Hz 1H, WH). 13C{1H} NMR (δ, acetone-d6, 20 °C): 205.9 (t, J = 9.7 Hz, CO), 196.6 (t, J = 8.7 Hz, CO), 159.5 (d, J = 5.9 Hz, py), 159.3 (d, J = 5.9 Hz, py), 142.1 (s, py), 132.0 (s, py), 130.9 (d, J = 13.0 Hz, Ph), 129.5 (d, J = 13.7 Hz, Ph), 129.1 (d, J = 12.1 Hz, Ph), 127.9 (d, J = 4.6 Hz, Ph), 102.0 (d, J = 9.4 Hz, py). 31P{1H} NMR (δ,acetone-d6, −60 °C): 95.5 (d, 2J = 85.9 Hz), 84.8 (d, 2J = 85.9 Hz). IR (ATR, cm–1): 2038 (νCO), 1963 (νCO), 1918 (νCO).

[W(PNP-iPr)(CO)3H]BF4 (8b)

This complex was prepared analogously to 8a with 5b (200 mg, 0.33 mmol) and n class="Chemical">HBF4 (20 μL, 0.49 mmol) as starting materials. Yield: 207 mg (90%). Anal. Calcd for C20H34BF4N3O3P2W: C, 34.46; H, 4.92; N, 6.03. Found: C, 34.55; H, 5.02; N, 6.10. 1H NMR (δ, CD2Cl2, 20 °C): 8.47 (bs, 2H, NH), 7.50 (t, J = 8.5 Hz, 1H, py), 6.67 (d, J = 7.7 Hz, 2H, py), 2.41 (m, 4H, CH(CH3)2), 1.30 (m, 28H, CH(CH3)2), −4.83 (dd, 2JHP = 23.3 Hz, 2JHP = 54.7 Hz, 1H, WH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 205.5 (t, J = 8.5 Hz, CO), 198.0 (t, J = 8.1 Hz, CO), 160.1 (d, J = 3.9 Hz, py), 141.6 (s, py), 100.4 (d, J = 6.4 Hz, py), 31.9 (d, J = 25.5 Hz, CH(CH3)2), 18.0 (d, J = 4.2 Hz, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 125.9 (d, 2JPP = 80.0 Hz), 108.6 (d, 2JPP = 80.0 Hz). IR (ATR, cm–1): 2027 (νCO), 1910 (νCO), 1906 (νCO).

[W(PNP-tBu)(CO)3H]BF4 (8c)

This complex was prepared analogously to 8a with n class="Chemical">5c (200 mg, 0.30 mmol) and HBF4 (18 μL, 0.45 mmol) as starting materials. Yield: 207 mg (90%). Anal. Calcd for C24H42BF4N3O3P2W: C, 38.27; H, 5.62; N, 5.58. Found: C, 38.29; H, 5.52; N, 5.52. 1H NMR (δ, acetone-d6, 20 °C): 8.00 (bs, 2H, NH), 7.60 (t, J = 6.7 Hz, 1H, py), 6.75 (d, J = 8.3 Hz, 2H, py), 1.59 (s, 18H, C(CH3)3), 1.53 (s, 18H, C(CH3)3). −4.16 (dd, 2JHP = 37.8 Hz, 2JHP = 50.8 Hz, 1H, WH). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 141.1 (d, 2JPP = 83.6 Hz), 126.8 (d, 2JPP = 83.6 Hz). IR (ATR, cm–1): 2021 (νCO), 1934 (νCO), 1897 (νCO).

X-ray Structure Determination

Single crystals of the complexes 3a, 4b, 2d, 5b,c, 7a,d, and 8a suitable for X-ray diffraction were mainly obtained by the solvent/antisolvent liquid–liquid diffusion method at room temperature using CH2Cl2/diethyl ether (4b, 2d, 7a,d, 8a) or THF/n-hexane (5b,c), while 3a was crystallized from methanol at −20 °C. The crystals of 3a, 5b,c, and 7d were solvates (3a·CH3OH, 5b·THF·1/2hexane, 5c·THF, 7d·CH2Cl2). X-ray diffraction data were collected at T = 100 K on a Bruker Kappa APEX-2 CCD area detector diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and φ- and ω-scan frames covering complete spheres of the reciprocal space with θmax = 30°. Corrections for absorption and λ/2 effects were applied using the program SADABS.[29] After structure solution with the program SHELXS97 refinement on F2 was carried out with the program SHELXL97.[30] Non-hydrogen atoms were refined anisotropically. Most hydrogen atoms were placed in calculated positions and thereafter treated as riding. Crystal data are reported in Table 2, and detailed structural data are given in CIF format in the Supporting Information. Variata are as follows. The solid-state structure of 3a (3a·CH3OH) contained methanol and eventually some water disordered in a large oval infinite channel along the a axis embodying also the free bromide anions. The contribution of this solvent to the structure factors was removed with the procedure SQUEEZE of the program PLATON.[31] This structure shows also a Br by CO and a complementary CO by Br substitution disorder (equatorial C32–O3 by Br1′ and axial Br1 by C32′ and O3′ in 86:14 proportion). The crystal structure of 5b contains an ordered THF and a disordered n-hexane solvent molecule, the latter across a center of inversion; the solid is therefore 5b·THF·1/2C6H14. The contribution of the n-hexane solvent molecule to the structure factors was removed with the procedure SQUEEZE of the program PLATON. 5b has also an orientation-disordered isopropyl group. The hydride complexes 7a and 8a form an isostructural pair, [Mo/W(PNP-Ph)(CO)3H]BF4, space group Pbca, with very similar unit cell dimensions, bond lengths, and bond angles. Both structures show a disorder of the equatorial CO group (C32–O3) and the hydride H atom (H1), which appear in two approximately equivalent positions left and right of the plane bisecting the complexes perpendicular to the pyridine ring. Due to the very good quality of the diffraction data of both complexes and due to good separations of the respective atom positions, it was possible to refine the approximately half-occupied positions of the hydride H atoms without restraints. The left/right ratio of the population parameter of CO and H in the molybdenum complex 7a is 0.540(3)/0.460(3) and in the tungsten complex 8a is 0.586(4)/0.414(4) (for C32, O3, H1/C32′, O3′, H1′; cf. Figures 7 and 9, which depict only the dominant nonprimed part). In contrast, the analogous molybdenum hydrido carbonyl complex 7d·CH2Cl2 was perfectly ordered and gave on refinement with high-quality diffraction data a hydride position in very good agreement with complexes 7a and 8a, fully supporting the split atom refinements of these two crystal structures.

Table 2

Details for the Crystal Structure Determinations of Compounds 3a·CH3OH, 4b, 2d, 5b·THF·1/2C6H14, 5c·THF, 7a, 7d·CH2Cl2, and 8a

	3a·CH3OH	4b	2d	5b·THF·1/2C6H14
formula	C33H29Br2N3O4P2W	C20H33Br2MoN3O3P2	C22H37MoN3O3P2	C27H48N3O4P2W
fw	937.20	681.19	549.43	724.47
cryst size, mm	0.16 × 0.06 × 0.05	0.59 × 0.20 × 0.18	0.24 × 0.14 × 0.10	0.50 × 0.40 × 0.20
color, shape	orange prism	yellow plate	yellow prism	yellow plate
cryst syst	triclinic	monoclinic	orthorhombic	monoclinic
space group	P1̅ (No. 2)	C2/c (No. 15)	Pnma (No. 62)	P21/c (No. 14)
a, Å	10.1741(6)	29.6913(15)	18.6552(5)	10.1399(10)
b, Å	13.1580(7)	10.7919(5)	12.1772(3)	15.5053(18)
c, Å	14.1040(8)	16.7764(8)	10.9035(2)	20.036(2)
α, deg	99.456(3)	90	90	90
β, deg	95.412(3)	97.954(1)	90	94.485(5)
γ, deg	104.306(3)	90	90	90
V, Å3	1786.63(17)	5323.9(4)	2476.93(10)	3140.5(6)
T, K	296(2)	173(2)	100(2)	100(2)
Z	2	8	4	4
ρcalcd, g cm–3	1.742	1.700	1.473	1.532
μ(Mo Kα), mm–1	5.598	3.640	0.687	3.815
F(000)	908	2720	1144	1468
abs corr	multiscan, 0.77–0.56	multiscan, 0.50–0.32	multiscan, 0.93–0.83	multiscan, 0.75–0.46
θ range, deg	1.96–30.00	2.30–30.00	2.18–30.07	1.66–30.00
no. of rflns measd	46633	36362	20758	77629
Rint	0.060	0.022	0.033	0.042
no. of unique rflns	10399	7732	3770	9124
no. of rflns with I > 2σ(I)	7880	6562	3328	6686
no. of params/restraints	393/6	294/96	167/0	335/75
R1 (I > 2σ(I))a	0.0385	0.0273	0.0217	0.0398
R1 (all data)	0.0625	0.0351	0.0276	0.0689
wR2 (I > 2σ(I))	0.0822	0.0693	0.0507	0.0805
wR2 (all data)	0.0884	0.0734	0.0534	0.0907
min/max diff Fourier peaks, e Å–3	–1.39/1.56	–1.01/0.97	–0.32/0.43	–1.80/1.85

R1 = ∑||Fo| – |Fc||/∑|Fo|; wR2 = {∑[w(Fo2 – Fc2)2]/∑[w(Fo2)2]}1/2.

Computational Details

All calculations were performed using the GAUSSIAN 09 software package[32] on the Phoenix Linux Cluster of the Vienna University of Technology and the B3LYP functional[33] without symmetry constraints. The optimized geometries were obtained with the Stuttgart/Dresden ECP (SDD) basis set[34] to describe the electrons of the tungsten and molybdenum atoms. For all other atoms a standard 6-31g** basis set was employed.[35] All geometries were optimized without symmetry constraints. Frequency calculations were performed to confirm the nature of the stationary points, yielding one imaginary frequency for the transition states and none for the minima. Each transition state was further confirmed by following its vibrational mode downhill on both sides and obtaining the minima presented on the energy profiles. All energies reported are Gibbs free energies and thus contain zero-point, thermal, and entropy effects at 298 K and 1 atm pressure. A natural population analysis (NPA)[36] and the resulting Wiberg indices[27] were used to study the electronic structure and bonding of the optimized species. The NPA analysis was performed with the NBO 5.0 program.[37]