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

The preparation of tantalaziridine−hydride complex (Ar[tBuCH2]N)2(η2-tBu(H)CNAr)TaH (1) is reported (Ar = 3,5-Me2C6H3). While stable for months in the solid state at −35 °C, in solution this complex undergoes partial conversion to isomeric hydride (Ar[tBuCH2]N)2(κ2-CH2C(Me)2CH2NAr)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 CO2 results in the production of methylene diolate complex {(Ar[tBuCH2]N)2(η2-tBu(H)CNAr)Ta}2(μ-OCH2O) 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[tBuCH2]N)2(κ2-CH2C(Me)2CH2NAr)Ta}2(μ-OCH2O) (4) is produced from 2 upon treatment with CO2 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 CO2 and two terminal transition-metal hydrides. Reaction of formate (Ar[tBu]N)3TiOC(O)H with 1 generates the related, structurally characterized heterobimetallic complex (Ar[tBuCH2]N)2(η2-tBu(H)CNAr)TaOCH2OTi(N[tBu]Ar)3 (5), which further contributes to the class of complexes reported herein that effectively stabilizes an unusual H2CO22− ligand between two metal centers.