709040-66-4Relevant articles and documents
Strain in organometallics II: Controlling the properties of tetra-coordinated iridium complexes using diastereomers of a bis(tropp) ligand system
Laporte, Cècile,Büttner, Torsten,Rüegger, Heinz,Geier, Jens,Sch?nberg, Hartmut,Grützmacher, Hansj?rg
, p. 1931 - 1947 (2004)
The tetra-chelating ligands 1,2-bis[(5H-dibenzo[a,d]cyclohepten-5-yl) phenylphosphanyl]-ethane, bis(troppPh)ethane, and 1,3-bis[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphanyl]-propane, bis(troppPh)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(troppPh)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(troppPh)ethane) (cod)]+, where the bis(troppPh)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(troppPh)ethane)(cod)]+: k-1; rac-[Ir(bis(troppPh)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(troppPh)propane)]+. The diastereomers were separated by fractional crystallisation. The complex rac-[Ir(bis(troppPh)propane)]+ is reduced at relatively low potentials (E11/2=-0.95 V, E2 1/2=-1.33 V versus Ag/AgCl) to give the neutral 17-electron complex [Ir(bis(troppPh)propane)]0 and the 18-electron anionic iridate [Ir(bis(troppPh)propane)]-, respectively. With acetonitrile, [Ir(bis(troppPh)propane)]+ reacts to give the penta-coordinated complex rac-[Ir(MeCN)(bis(troppPh)propane)] + (K=45 M-1, kf=6×103 M -1 s-1, kd=1×102 s -1) and with chloride to yield the relatively stable complex rac-[Ir(Cl)(bis(troppPh)propane)] (kd-1). Compared to the rac-isomer, the meso-[Ir(bis(tropp Ph)propane)]+ shows significantly cathodically shifted reduction potentials (E11/2=-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(troppPh)propane)], is much more labile (kd≈20′000 s-1). meso-[Ir(bis(troppPh)propane)]+ reacts with one equivalent H2 to give the trans-dihydride complex-cation, meso-[Ir(H)2(bis(troppPh)propane)]+, while the rac-isomer, rac-[Ir(bis(troppPh)propane)]+, reacts with two equivalents H2 to give rac-{Ir(H)2(OTf)[(tropp Ph)(H2troppPh)propane]}, a cis-dihydride complex containing a hydrogenated 10,11-dihydro-5H-dibenzo[a,d]cycloheptene unit, H2troppPh. 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(troppPh)propane) ]+, enforces a planar coordination sphere at the metal. On the contrary, already in the tetra-coordinated rac-[Ir(bis(troppPh) propane)]+, the metal has a tetrahedrally distorted coordination sphere which does not impede the reduction to the d9-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.
