Strain in organometallics II: controlling the properties of tetra-coordinated iridium complexes using diastereomers of a bis(tropp) ligand system

Abstract

The tetra-chelating ligands 1,2-bis[(5H-dibenzo[ a, d]cyclohepten-5-yl)phenylphosphanyl]-ethane, bis(tropp Ph)ethane, and 1,3-bis[(5H-dibenzo[ a, d]cyclohepten-5-yl)phenylphosphanyl]-propane, bis(tropp Ph)propane, were synthesised. For the binding of transition metals, these ligands offer two olefin moieties and two phosphorus centres and form mixtures of diastereomers with a R, S-configuration at the phosphorus centres ( meso), or a R, R( S, S)-configuration ( rac), respectively. meso/ rac-bis(tropp Ph)ethane was separated by fractional crystallisation and reacted with [Ir(cod) 2]OTf (cod=cylcooctadiene, OTf −=CF 3SO 3 − ) to give the penta-coordinated complex-cations meso/ rac-[Ir(bis(tropp Ph)ethane)(cod)] +, where the bis(tropp Ph)ethane serves as tridentate ligand merely. One olefin unit remains non-bonded, however, a slow intra-molecular exchange between this olefin and the coordinated olefin unit was established ( meso-[Ir(bis(tropp Ph)ethane)(cod)] +: k<0.5 s −1; rac-[Ir(bis(tropp Ph)ethane)(cod)] +: k≈35 s −1). The ligand meso/ rac-bis(tropp Ph)propane reacts with [Ir(cod) 2]OTf to give the corresponding complexes containing the tetra-coordinated 16-electron complex-cations meso/ rac-[Ir(bis(tropp Ph)propane)] +. The diastereomers were separated by fractional crystallisation. The complex rac-[Ir(bis(tropp Ph)propane)] + is reduced at relatively low potentials ( E 1 1/2=−0.95 V, E 2 1/2=−1.33 V versus Ag/AgCl) to give the neutral 17-electron complex [Ir(bis(tropp Ph)propane)] 0 and the 18-electron anionic iridate [Ir(bis(tropp Ph)propane)] −, respectively. With acetonitrile, [Ir(bis(tropp Ph)propane)] + reacts to give the penta-coordinated complex rac-[Ir(MeCN)(bis(tropp Ph)propane)] + ( K=45 M −1, k f=6×10 3 M −1 s −1, k d=1×10 2 s −1) and with chloride to yield the relatively stable complex rac-[Ir(Cl)(bis(tropp Ph)propane)] ( k d<0.5 s −1). Compared to the rac-isomer, the meso-[Ir(bis(tropp Ph)propane)] + shows significantly cathodically shifted reduction potentials ( E 1 1/2=−1.25 V, E 2 1/2=−1.64 V versus Ag/AgCl), an acetonitrile complex could not be detected, and the chloro-complex, meso-[Ir(Cl)(bis(tropp Ph)propane)], is much more labile ( k d≈20′000 s −1). meso-[Ir(bis(tropp Ph)propane)] + reacts with one equivalent H 2 to give the trans-dihydride complex-cation, meso-[Ir(H) 2(bis(tropp Ph)propane)] +, while the rac-isomer, rac-[Ir(bis(tropp Ph)propane)]+, reacts with two equivalents H 2 to give rac-{Ir(H) 2(OTf)[(tropp Ph)(H 2tropp Ph)propane]}, a cis-dihydride complex containing a hydrogenated 10,11-dihydro-5H-dibenzo[ a, d]cycloheptene unit, H 2tropp Ph. The triflate anion in this complex is rather firmly bound and dissociates only slowly ( k=29 s −1). All differences between the different stereoisomers are attributed to the fact that the ligand backbone in the meso-isomer, meso-[Ir(bis(tropp Ph)propane)] +, enforces a planar coordination sphere at the metal. On the contrary, already in the tetra-coordinated rac-[Ir(bis(tropp Ph)propane)] +, the metal has a tetrahedrally distorted coordination sphere which does not impede the reduction to the d 9-Ir(0) and d 10-Ir(−1) complexes and allows more easily a distortion towards a trigonal bipyramidal (tbp) or octahedral structure for penta- or hexa-coordinated complexes, respectively. A comparison of the NMR data for iridium bonded olefins in equatorial or axial positions in tbp structures shows that the latter experience only modest metal-to-ligand back-donation, while the olefins in the equatorial positions have a high degree of metallacyclopropane character.