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(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄], (R = Me, Et) were prepared. [V­(PMe₃)₃(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄], (PFTB = perfluoro-<i>tert</i>-butoxide) reacts with 2,6-xylylisocyanide (CNXyl) or acetonitrile to afford complexes [V­(PMe)₂(N^<i>t</i>Bu)₂(CNXyl)]­[Al­(PFTB)₄] (<b>2</b>) and [V­(PMe)₂(N^<i>t</i>Bu)₂(MeCN)]­[Al­(PFTB)₄] (<b>3</b>). Complex <b>2</b> 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(N^<i>t</i>Bu)₂, <b>4</b>, then to the lower coordinate <i>C</i>_2<i>v</i> symmetrical complex, [V­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄], <b>5</b>. Derivatization of <b>5</b> with DMAP (4-dimethylaminopyridine) afforded complex <b>6</b>, [V­(DMAP)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄]. Treatment of <b>5</b> with CNXyl yielded [V­(CNXyl)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] (<b>7</b>) in 60% yield (υ_C–N = 2156 cm^–1). The d⁰ vanadium bisimido, carbonyl complex, [V­(η¹-CO)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] (<b>8</b>), was prepared via the exposure of <b>5</b> to 1 atm of CO. Complex <b>8</b> 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 <b>8-</b>^<b>13</b><b>C</b> 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 <b>8</b>. This model was further confirmed by density functional theory (DFT) analysis of the model complex [V­(η¹-CO)­(PMe₃)₂(N^<i>t</i>Bu)₂]⁺, <b>9</b>, which reveals that the basis of the reduced stretching frequency in <b>8</b> is π back-bonding from the 2b₁ and 1b₂ orbitals of <b>8