A series of olefin-coordinated Rh I and Ir I complexes bearing "capping arene" ligands (5-X FP and 6-X FP, see below) of the general formulas (FP)M(olefin)X, [(FP)M(olefin) 2 ][M-(olefin) 2 X 2 ], and [(FP)M(olefin) 2 ]BF 4 (FP = "capping arene" ligands, X = halide or pseudohalide, olefin = ethylene, cyclooctene, (olefin) 2 = (C 2 H 4 ) 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-X FP ligands (5-X FP = 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-X FP ligands (6-X FP = 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 η 2 -C,C interaction between the metal center and the arene group of the capping arene ligand. Variable-temperature 1 H 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-X FP complexes tend to give ethylene higher rotational barriers in comparison to 5-X FP complexes for complexes of the type (FP)Rh(η 2 -C 2 H 4 )Cl. DFT calculations are consistent with enhanced Rh to ethylene π-back-donation for Rh complexes ligated by the 6-X FP ligands in comparison to the 5-X FP ligands.

The ligand influence on olefin hydrogenation using four capping arene ligated Rh(I) catalyst precursors (FP)Rh(η 2 -C 2 H 4 )Cl {FP = capping arene ligands, including 6-FP (8,8′-(1,2-phenylene)diquinoline), 6-NP FP (8,8′-(2,3naphthalene)diquinoline), 5-FP (1,2-bis(N-7-azaindolyl)benzene) and 5-NP FP [2,3-bis(N-7-azaindolyl)naphthalene]} has been studied. Our studies indicate that relative observed rates of catalytic olefin hydrogenation follow the trend (6-FP)Rh(η 2 -C 2 H 4 )Cl > (5-FP)Rh(η 2 -C 2 H 4 )Cl. Based on combined experimental and density functional theory modeling studies, we propose that the observed differences in the rate of (6-FP)Rh(η 2 -C 2 H 4 )Cl and (5-FP)Rh(η 2 -C 2 H 4 )Clcatalyzed olefin hydrogenation are most likely attributed to the difference in the activation energies for the dihydrogen oxidative addition step. We are unable to directly compare the rates of olefin hydrogenation using (6-NP FP)Rh(η 2 -C 2 H 4 )Cl and (5-NP FP)Rh(η 2 -C 2 H 4 )Cl as the catalyst precursor since (5-NP FP)Rh(η 2 -C 2 H 4 )Cl undergoes relatively rapid formation of an active catalyst that does not coordinate 5-NP FP.