Strain in organometallics: synthesis of rhodium and iridium complexes with a novel rigid tetrachelating phosphine olefin ligand and their redox properties

The new tetrachelating diphosphanes 1,2-[(5H-dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R,S-11 [R,S-bis(troppPh)], and the corresponding enantiomers R,R-bis(troppPh), R,R-12, and S,S-bis(troppPh), S,S-12, were synthesised from 1,2-bis(phenylphosphano)ethane (9), and 5-chloro-5H-dibenzo[a,d]cycloheptene (10). With the meso form R,S-11, the rhodium(I) complex [Rh{R,S-bis(troppPh)}]PF6 (15), and the iridium(I) complex [Ir(cod){R,S-bis(troppPh)}]O3SCF3 (16), were prepared. The cation of the 16-electron rhodium complex 15 has a square planar structure, which is markedly distorted towards a square pyramid with the rhodium centre in the apex. The structure of the cation of the 18-electron complex of 16 is trigonal bipyramidal and unprecedented a phosphorus and an olefin unit occupy the axial positions. Cyclic voltammetry data of 15 indicate, that considerable strain energy (ca. 30 kJ mol−1) is build up when the [Rh{R,S-bis(troppPh)}]+ cation is reversibly reduced in THF by one electron to the paramagnetic neutral complex [Rh{R,S-bis(troppPh)}]0.