The ruthenium-mediated transfer hydrogenation (TH) of ketones under basic conditions employing iPrOH or other H 2 -donor solvents has emerged as an atom-economical methodology for the synthesis of secondary alcohols.[1] Although the catalytic abilities of Ru complexes supported by a range of structurally diverse ancillary ligands have been evaluated, precatalysts featuring a RuÀNH linkage commonly offer the highest levels of activity and selectivity in the TH of ketones. [1,2] In the case of the landmark {(h 6 -arene)Ru} catalysts featuring chiral diamido ligands developed by Noyori and co-workers, [3] this now well-established N À H effect has been rationalized in terms of an outer-sphere TH mechanism involving the concerted transfer of H 2 from a {(H)RuÀNH 2 R} intermediate to a ketone substrate. [1c, 3] More recently, Baratta and co-workers have reported an alternative class of precatalysts supported by 2-(aminomethyl)pyridine (ampy) coligands that give rise to the most active catalysts known for the TH of ketones.[4] For both of these prominent Ru-based TH catalyst systems, the inferior performance exhibited by the analogous {RuÀNMe 2 R} species underscores the crucial role that the ancillary ligand NH 2 terminus plays in enhancing catalyst activity and selectivity. [3c, 4] Notwithstanding the advancements in Ru-mediated TH that have been enabled through the development of ancillary ligands featuring NÀH donors, such a structural prerequisite limits the future design of alternative classes of Ru-based TH catalysts for use in mediating new and increasingly challenging substrate transformations. In this context, the identification of novel ligation strategies that do not rely on the NÀH effect and which give rise to highly active and selective Ru-based TH catalysts represents an important goal in modern catalysis research. [1,2] Encouraged by the catalytic abilities of Noyoris [(h 6 -arene)Ru(diamido)] complexes, [3] and in light of the observation that the pairing of P-and N-donor ligands is a common feature in several highly effective Ru-based TH catalysts, [1c, 2b, 4] we became interested in developing new non-NÀ H [(h 6 -arene)Ru(k 2 -P, N)(Cl)] precatalysts for use in the TH of ketones. In the context of our research comparing the reactivity properties of late-transition-metal cations and zwitterions derived from 1-H, [5] we identified the cationic complexes 2 + X À and the zwitterion 3 as being worthwhile candidates for catalytic studies (Scheme 1). Herein, we report the synthesis and characterization of 2 + X À and 3 as well as their application as precatalysts in the TH of ketones. Despite lacking an ancillary ligand NÀH functionality, complex 3 is among the most active ketone TH precatalysts known, providing near-quantitative conversions and exhibiting consistently high turnover frequencies (TOFs; up to 220 000 h À1 ) for a diversity of alkyl and aryl ketone substrates at low catalyst loadings. Furthermore, while the anticipated zwitterionic hydrido complex 4 is formed quantitatively upon treatment...

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A masked variant of the first coordinatively unsaturated [Cp*Ru(κ2‐P,N)]+ cation (1) is reported, which has proven capable of single intramolecular CH activation. In contrast, the isostructural zwitterionic complex 2 apparently rearranges to a hydridocarbene by way of a remarkably facile, ligand‐assisted double CH bond activation process (see scheme).

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.