Carbon Monoxide, Isocyanide, and Nitrile Complexes of Cationic, d0 Vanadium Bisimides: π‑Back Bonding Derived from the π Symmetry, Bonding Metal Bisimido Ligand Orbitals

A series of carbon monoxide, isocyanide, and nitrile complexes of [V­(PR3)2(NtBu)2]­[Al­(PFTB)4], (R = Me, Et) were prepared. [V­(PMe3)3(NtBu)2]­[Al­(PFTB)4], (PFTB = perfluoro-tert-butoxide) reacts with 2,6-xylylisocyanide (CNXyl) or acetonitrile to afford complexes [V­(PMe)2(NtBu)2(CNXyl)]­[Al­(PFTB)4] (2) and [V­(PMe)2(NtBu)2(MeCN)]­[Al­(PFTB)4] (3). Complex 2 was crystallographically characterized, revealing a C–N bond length of (1.152(4) Å); the C–N stretching frequency occurs at 2164 cm–1 in the IR. To access the analogous carbon monoxide complex, the larger cone angle phosphine, triethylphosphine, was employed, starting from the chloride VCl­(PEt3)2(NtBu)2, 4, then to the lower coordinate C2v symmetrical complex, [V­(PEt3)2(NtBu)2]­[Al­(PFTB)4], 5. Derivatization of 5 with DMAP (4-dimethylaminopyridine) afforded complex 6, [V­(DMAP)­(PEt3)2(NtBu)2]­[Al­(PFTB)4]. Treatment of 5 with CNXyl yielded [V­(CNXyl)­(PEt3)2(NtBu)2]­[Al­(PFTB)4] (7) in 60% yield (υC–N = 2156 cm–1). The d0 vanadium bisimido, carbonyl complex, [V­(η1-CO)­(PEt3)2(NtBu)2]­[Al­(PFTB)4] (8), was prepared via the exposure of 5 to 1 atm of CO. Complex 8 has a C–O stretching frequency of 2015 cm–1. Isotopic labeling with 99% 13CO reveals a stretching frequency of 1970 cm–1, which confirms the assignment of the complex as a terminal η1-CO complex. This is also implied by its NMR data in comparison to the other crystallographically characterized compounds presented here. The 13C­{31P}­{1H} NMR spectrum of 8-13C reveals a broad singlet at 228.36 ppm implying deshielding of the carbonyl carbon. This datum, in conjunction with the shielded vanadium NMR shift of −843.71 ppm, suggests π back-bonding is operative in the bond between carbon monoxide and the d0 vanadium center in 8. This model was further confirmed by density functional theory (DFT) analysis of the model complex [V­(η1-CO)­(PMe3)2(NtBu)2]+, 9, which reveals that the basis of the reduced stretching frequency in 8 is π back-bonding from the 2b1 and 1b2 orbitals of 8.