Stabilization of the labile metal configuration in halfsandwich complexes [CpRh(PN)Hal]X

PN ligands 3 and 4, derived from 2-diphenylphosphanylmethylpyridine 2a, were synthesized, to which in the backbone a tether to a cyclopentadiene system and for comparison an iPr substituent were attached. The chiral compounds were resolved by introduction of a menthoxy substituent into the 2-position of the pyridine system and/or palladium complexes with enantiomerically pure co-ligands. The tripod ligand 3b contains three different binding sites (Cp, P, N) connected by a resolved chiral carbon atom. (SC)-configuration of this tripod ligand enforces (RRh)-configuration at the metal atom in the halfsandwich rhodium complex (LMent,SC,RRh)-7b. The opposite metal configuration is inaccessible. Substitution of the chloro ligand in (LMent,SC,RRh)-7b by halide (Br, I) or pseudohalide (N3, CN, SCN) ligands occurs with retention of configuration to give complexes 8b–11b. However, in the reaction of (LMent,SC,RRh)-7b with PPh3 the pyridine arm of the tripod ligand in compound 13b becomes detached from the metal atom. In the Cp*Rh and CpRh compounds of the bidentate PN ligands 4a and 4b both metal configurations are accessible and in complexes 14a–17a and 14b–17b they equilibrate fast. The stereochemical assignments are corroborated by 9 X-ray analyses.