Carbon Dioxide Reduction by Terminal Tantalum Hydrides: Formation and Isolation of Bridging Methylene Diolate Complexes

Abstract

The preparation of tantalaziridine-hydride complex (Ar[(t)BuCH(2)]N)(2)(eta(2)-(t)Bu(H)CNAr)TaH (1) is reported (Ar = 3,5-Me(2)C(6)H(3)). While stable for months in the solid state at -35 degrees C, in solution this complex undergoes partial conversion to isomeric hydride (Ar[(t)BuCH(2)]N)(2)(kappa(2)-CH(2)C(Me)(2)CH(2)NAr)TaH (2). Although 1 and 2 exist in equilibrium in benzene solution, complex 2 can be isolated cleanly from 1 by selective precipitation using cold n-pentane; solid-state structures for both 1 and 2 are presented. Exposure of 1 to ca. 1 atm of CO(2) results in the production of methylene diolate complex {(Ar[(t)BuCH(2)]N)(2)(eta(2)-(t)Bu(H)CNAr)Ta}(2)(mu-OCH(2)O) as a mixture of rac and meso diastereomers (3r,m). Similar reactivity for the Nb congener of 1 is reported herein. Further-more, methylene diolate complex {(Ar[(t)BuCH(2)]N)(2)(kappa(2)-CH(2)C(Me)(2)CH(2)NAr)Ta}(2)(mu-OCH(2)O) (4) is produced from 2 upon treatment with CO(2) and was characterized crystallographically. Complexes 3r,m (and the Nb analogues) as well as 4 establish the feasibility of the formation of methylene diolate products from CO(2) and two terminal transition-metal hydrides. Reaction of formate (Ar[(t)Bu]N)(3)TiOC(O)H with 1 generates the related, structurally characterized heterobimetallic complex (Ar[(t)BuCH(2)]N)(2)(eta(2)-(t)Bu(H)CNAr)TaOCH(2)OTi(N[(t)Bu]Ar)(3) (5), which further contributes to the class of complexes reported herein that effectively stabilizes an unusual H(2)CO(2)(2-) ligand between two metal centers.