51V NMR Spectroscopy and Ring-Size Effects on Catalytic Oxidative Alcohol Dehydrogenation in {V(O)(μ2-O)(pinF)}n
John E Cerritelli1, Douglas G Fraser1, Grant L Spencer1
1Boston University, Chemistry Department, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States.
Abstract:
Previously published (NMe4)2[V2(O)2(μ2-O)2(pinF)2], (NMe4)2[2], has been shown to aerobically catalyze the oxidation of benzylic and allylic alcohols under mild conditions. Herein, we report syntheses of [V3(O)3(μ2-O)3(pinF)3]3- trimers, which are also active in OAD catalysis. Trimer formation requires an ammonium cation with at least two hydrogen atoms per cation (e.g., H2NR2+ or H3NR+ but not HNR3+). The trimers (H2pip)3[V3(O)3(μ2-O)3(pinF)3], (H2pip)3[3], and (H3NAd)3[V3(O)3(μ2-O)3(pinF)3], (H3NAd)3[3], are reported herein along with the dimer (HNEt3)2[V2(O)2(μ2-O)2(pinF)2], (HNEt3)2[2]. Trimer formation is nearly quantitative with NH4+, H3NR+, and H2NR2+ salts, whereas HNR3+ and NR4+ salts did not result in trimer formation, as studied by 51V NMR. Monomer (NMe4)[V(O)2(pinF)(DMAP)], (NMe4)[1-DMAP], was also prepared to understand solution speciation. OAD catalysis of benzyl alcohol to benzaldehyde by trimers (H3NAd)3[3] and (NH4)3[3], formed in situ, is slower than (NMe4)2[2]; however, (H3NAd)3[3] is more robust to degradation than (NMe4)2[2] and (NH4)3[3]. The difference may correspond to the geometric differences in the {V(O)(μ2-O)}n rings that give rise to distinct reaction thermodynamics and kinetics. The stoichiometric reaction of (H3NAd)3[3] with 9-fluorenol results in net HAT and the formation of mixed-valent (H3NAd)3[V3(O)3(μ2-OH)(μ2-O)2(pinF)3], (H3NAd)3[5a] containing one V(4+) and two V(5+) centers. SCXRD characterization for trimers (H2pip)3[3], (H3NAd)3[V3(O)3(μ2-O)3(pinF)3], (H3NAd)3[3], (H3NAd)3[V3(O)3(μ2-OH)(μ2-O)2(pinF)3], (H3NAd)3[5a], dimer (HNEt3)2[V2(O)2(μ2-O)2(pinF)2], (HNEt3)2[2], and monomer (NMe4)[V(O)2(pinF)(DMAP)], (NMe4)[1-DMAP], are reported herein.
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