Donor Properties of the Vanadyl Ion: Reactions of Vanadyl Salicylaldimine β-Ketimine and Acetylacetonato Complexes with Groups 14 and 15 Lewis Acids

Reactions of organosilicon, -germanium, -tin, -lead, -antimony, and -tin tetrahalide Lewis acids with VO(salen)
[H2salen = N,N‘-bis(salicylidene)ethane-1,2-diamine], related vanadyl salicylaldimines, VO(acacen) [H2acacen =
N,N‘-bis(acetylacetonato)ethane-1,2-diamine], and VO(acac)2 (acac = acetylacetonato) have been investigated,
revealing VO(salen) and VO(acacen) to be significantly stronger vanadyl donors than VO(acac)2. The vanadyl
donor strength of VO(salen) significantly diminishes with the introduction of electron-withdrawing substituents on
the salicylaldimine ligand, and the introduction of methyl substituents on the imine carbon atoms can result in a
preference for phenolic over vanadyl oxygen donation. Vanadyl donation results in an increase in the vanadyl bond
length, while it leaves the distance of vanadium from the basal plane relatively unaffected. Coordination of water
trans to a vanadyl oxygen that is involved in a donor bond to tin or lead has little or no effect on the vanadyl bond
length but results in a marked movement of vanadium toward the basal plane and a decrease of the VOD (D
= Sn or Pb) bond angle by as much as 13°, the latter reflecting a loss of multiple bond character of the vanadyl
bond. Formation of a vanadyl donor bond results in a decrease in both the vanadyl stretching frequency (infrared
spectrum) and energy of the eπ* ← b2 transition (electronic spectrum), the latter being intimately related to the
strength of the vanadyl donor bond, while the shift of the b1* ← b2 transition to higher or lower energy is relatively
small for vanadyl salicylaldimine and β-ketimine complexes. Donation through the phenolic oxygen atoms results
in an increase in the vanadyl stretching frequency and energy of the eπ* ← b2 transition, which can result in eπ*
← b2/b1* ← b2 energy crossover.