Rhodium and Iridium Complexes Bearing "capping Arene" Ligands: Synthesis and Characterization

A series of olefin-coordinated RhI and IrI 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.