It's all about Me: methyl-induced control of coordination stereochemistry by a flexible tridentate N,C,N′ ligand

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A chiral, tridentate, pyridyl-functionalised NHC pro-ligand, S-L(Me)-H[PF₆], has been prepared diastereoselectively via a five step synthesis starting from 1R,3S-diamino-1,2,2-trimethylcyclopentane. The S prefix refers to the stereochemistry of a methyl substituted stereogenic carbon in one of the pyridyl arms which is generated by a stereoselective BH4(-) reduction of an imine precursor. The ligand has been coordinated to Rh(I) and Ir(I) to give trigonal bipyramidal complexes of the type [M(κ(3)-N,C,N'-S-L(Me))(1,5-COD)]PF6 (M = Rh, Ir) as single diastereomers. A combination of spectroscopic and X-ray techniques confirm the stereoselective formation of the thermodynamically preferred endo,endo isomer. Similar reactions with R,S-L(Me)-H[PF₆] gave a mixture of endo,endo-[M(κ(3)-N,C,N'-S-L(Me))(1,5-COD)](+) and exo,exo-[M(κ(3)-N,C,N'-R-L(Me))(1,5-COD)](+). The absolute configuration at the metal is, therefore, solely dictated by the stereochemistry of the single methylpyridyl carbon. The observation of stereoselection extends to the square planar Ni(II) complex [Ni(δ-κ(3)-N,C,N'-S-L(Me))Cl](+) which is isolated as one (δ) of the two possible conformational isomers. DFT studies have been employed to explain the observed stereoselectivity with the configurations observed in the solid state being confirmed as those of lowest energy.