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Synthesis of Various (Arylimido)vanadium(V)−Methyl Complexes Containing Ketimide Ligands and Reactions with Alcohols, Thiols, and Borates:  Implications for Unique Reactivity toward Alcohols

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posted on 03.04.2020, 17:27 by Junji Yamada, Michiya Fujiki, Kotohiro Nomura
The reactions of (ArN)VMe(NCtBu2)2 (1, Ar = 2,6-Me2C6H3) with various alcohols, thiols, and borates (Brönsted and/or Lewis acids) were investigated. Treatment of complex 1 with 1.0 equiv of various alcohols cleanly afforded other methyl complexes of the type (ArN)VMe(NCtBu2)(OR) [OR = O-2,6-Me2C6H3 (2a), O-4-tBu-2,6-iPr2C6H2 (2b), OPh (2c), OC6F6 (2d), OiPr (2e), OCH2CH2CHCH2 (2f), OCH2(CH2)3CHCH2 (2g)], and a reaction with the methyl group did not occur in all cases. In contrast, protonolysis of the methyl group took place in the reaction of 1 with 2,6-Me2C6H3SH or 1-hexanethiol. The reaction of 1 with [PhN(H)Me2][B(C6F5)4] and [Ph3C][B(C6F5)4] in THF gave the cationic complex [(ArN)V(NCtBu2)2(THF)2][B(C6F5)4] (4a), exclusively through abstraction or protonolysis of the methyl group. The reaction of 2a (or 2b) with 2,6-Me2C6H3OH (or 4-tBu-2,6-iPr2C6H2OH) gave the bis(aryloxo) complex (ArN)VMe(OAr‘)2 [Ar‘ = O-2,6-Me2C6H3 (5a), O-4-tBu-2,6-iPr2C6H2 (5b)], and the reaction with a methyl group did not occur even in the presence of an additional equivalent of phenol. The reaction of 2a with 4-tBu-2,6-iPr2C6H2OH at 25 °C afforded the aryloxo scrambled mixture of 2a and 2b and then gave three bis(aryloxo) analogues upon heating to 60 °C for 12 h. The results clearly indicate that the reactions with phenols proceed via pentacoordinated trigonal bipyramidal intermediates formed by coordination of the oxygen atom in the phenol trans to the methyl group.