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–Naxial 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–Naxial 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.