Rhodium and Iridium Complexes Bearing “Capping Arene” Ligands: Synthesis and Characterization

A series of olefin-coordinated Rh^I and Ir^I complexes bearing “capping arene” ligands (5-^XFP and 6-^XFP, see below) of the general formulas (FP)­M­(olefin)­X, [(FP)­M­(olefin)₂]­[M­(olefin)₂X₂], and [(FP)­M­(olefin)₂]­BF₄ (FP = “capping arene” ligands, X = halide or pseudohalide, olefin = ethylene, cyclooctene, (olefin)₂ = (C₂H₄)₂ or cyclooctadiene) were synthesized and characterized. Single-crystal X-ray diffraction studies revealed structural differences that are a function of the identity of the capping arene ligand and the metal. For 5-^XFP ligands (5-^XFP = 1,2-bis­(N-7-azaindolyl)-benzene and derivatives with substituents on the arene moiety), the coordination to both Rh and Ir gives rise to complexes that are best described as 16-electron and square planar. For 6-^XFP ligands (6-^XFP = 8,8′-(1,2-phenylene)­diquinoline and derivatives with substituents on the arene moiety), the structures of Rh and Ir complexes are better considered as 18-electron and trigonal bipyramidal due to an η²-C,C interaction between the metal center and the arene group of the capping arene ligand. Variable-temperature ¹H NMR spectroscopy studies of ethylene rotation demonstrated that the Ir complexes possess higher activation barriers to rotation in comparison to Rh complexes and the 6-^XFP complexes tend to give ethylene higher rotational barriers in comparison to 5-^XFP complexes for complexes of the type (FP)­Rh­(η²-C₂H₄)­Cl. DFT calculations are consistent with enhanced Rh to ethylene π-back-donation for Rh complexes ligated by the 6-^XFP ligands in comparison to the 5-^XFP ligands.