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

The tetra-chelating ligands 1,2-bis(5H-dibenzoa,d]cyclohepten-5-yl)phenylphosphanyl]-ethane, bis(tropp^Ph)ethane, and 1,3-bis(5H-dibenzoa,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⁻=CF₃SO₃ ⁻) 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 (E¹_1/2=−0.95 V, E²_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=6×10³ M⁻¹ s⁻¹, k_d=1×10² 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 (E¹_1/2=−1.25 V, E²_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 H₂ 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 H₂ to give rac-{Ir(H)₂(OTf)(tropp^Ph)(H₂tropp^Ph)propane]}, a cis-dihydride complex containing a hydrogenated 10,11-dihydro-5H-dibenzoa,d]cycloheptene unit, H₂tropp^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 d⁹-Ir(0) and d¹⁰-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.