Role of Axial Base Coordination in Isonitrile Binding and Chalcogen Atom Transfer to Vanadium(III) Complexes

The enthalpy of oxygen atom transfer (OAT) to V­[(Me₃SiNCH₂CH₂)₃N], 1, forming OV­[(Me₃SiNCH₂CH₂)₃N], 1–O, and the enthalpies of sulfur atom transfer (SAT) to 1 and V­(N­[t-Bu]­Ar)₃, 2 (Ar = 3,5-C₆H₃Me₂), forming the corresponding sulfides SV­[(Me₃SiNCH₂CH₂)₃N], 1–S, and SV­(N­[t-Bu]­Ar)₃, 2–S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene) and Ph₃SbS as chalcogen atom transfer reagents. The V–O BDE in 1–O is 6.3 ± 3.2 kcal·mol^–1 lower than the previously reported value for 2–O and the V–S BDE in 1–S is 3.3 ± 3.1 kcal·mol^–1 lower than that in 2–S. These differences are attributed primarily to a weakening of the V–N_axial bond present in complexes of 1 upon oxidation. The rate of reaction of 1 with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for 2. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both 1 and 2 forming high spin adducts of V­(III). The enthalpies of ligand addition to 1 and 2 in toluene solution are −19.9 ± 0.6 and −17.1 ± 0.7 kcal·mol^–1, respectively. The more exothermic ligand addition to 1 as compared to 2 is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V–N_axial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of 1, 1–O, 1–S, 1–CNAd, and 2–CNAd have been determined by X-ray crystallography and are reported.