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.