Bimetallic, Spirocyclic, Methylene-Bridged Carbene Complexes of Rhodium and Palladium Derived by Stepwise Metalations of Lithiated Bis(diphenylphosphoranotrimethylsilylimido)methandiide

摘要

The Li-C-Rh, bimetallic, phosphinimine spirocyclic, bridged carbene complex I was readily prepared front dilithiobis(diphenylphosphoranosilylimido)methandiide, [Li2L](2), and an equimolar (Rh/Li, 1:1) amount of [Rh(cod)Cl](2). Subsequent reaction with a different metal precursor, in the present cases, [Pd(allyl)Cl](2), yielded the spirocyclic Rh-Pd heterobimetallic 2 or, if a second equivalent of [Rh(cod)Cl](2) was used, the spirocyclic Rh-Rh homobimetallic 3 was obtained; thus the system presents an effective synthetic route for the preparation of homo- or heterobimetallic complexes. All of the complexes have constrained symmetry, which creates in some cases isomeric and enantiomeric structures. The Li-Rh complex 1 thermally rearranged to the chelated, orthometalated, Rh complex 6, wherein the Rh(I) center is bound to the ipso carbon of a phenyl ring and a methine carbon formed from protonation of the spiro carbon. Remarkably, controlled hydrolysis of 6 reversed the orthometalation and provided an imino-phosphorane-methine chelate complex of Rh(I), 7. Reactions of the cod-substituted complexes 2, 3, and 7 with CO resulted in the replacement of cod substituents to form parallel Rh carbonyl complexes (4, 5, and 8). The allyl substituents in the system were not affected. The carbonyl complexes 5 and 8 could be made independently from [Rh(CO)(2)Cl](2) and either the dilithium methandiide or the methine form of the ligand as appropriate. Complexes 2-7 have been structurally characterized herein. Complexes 3 and 5 are chiral (because they lack rotation-reflection axes), but the products here are unresolved racemic mixtures according their NMR spectra. The complex 2 and its derivative 4 exist as unequal diastereomeric pairs according to the NMR. Fluxional NMR behavior is observed for 1, wherein the cod appears to partly dissociate and invert, for 2 and 3 where the cod exchange appears to involve a different mechanism being assisted by dissociation of one imine substituent, for 2 and 4 wherein allyl syn-syn, anti-anti exchange occurs between the diastereomers, and for 7 wherein the coordinated and uncoordinated imine groups exchange.