Chemistry of tetrairidium carbonyl clusters. Part 1. Synthesis, chemical characterization, and nuclear magnetic resonance study of mono- and di-substituted phosphine derivatives. X-Ray crystal structure determination of the diaxial isomer of [Ir4(CO)7(µ-CO)3(Me2PCH2CH2PMe2)

Abstract

The reactions of the anionic cluster [NEt4][Ir4(CO)11Br] with uni- and bi-dentate phosphines and arsines have been investigated. The bromide ligand is quantitatively displaced by 1 mol equivalent of phosphine or arsine at low temperature, the only complexes being formed under these mild conditions being the monosubstituted products [Ir4(CO)11L](L = PPh3, PPh2Me, PPhMe2, or AsPh3), [Ir4(CO)11(L–L)](L–L =trans-Ph2PCHCHPPh2), and [(OC)11Ir4(L–L)Ir4(CO)11][L–L =trans-Ph2PCHCHPPh2 or Ph2P(CH2)nPPh2(n= 3 or 4)]. Similar reactions with higher than stoicheiometric amounts of phosphine (L = PPh3, PPh2Me, or PPhMe2) or diphosphine [L–L = Ph2P(CH2)nPPh2(n= 1–4), Me2P(CH2)2PMe2, cis-Ph2PCHCHPPh2, or o-Ph2PCH2C6H4CH2PPh2] give in good yields the disubstituted compounds [Ir4(CO)10(L–L)] respectively. The stereochemical arrangements of the phosphine ligands and the dynamic processes occurring in solutions of these complexes are discussed on the basis of i.r. and n.m.r, data. The structure of the diaxial isomer of [Ir4(CO)7(µ-CO)3(Me2PCH2CH2PMe2)] has been determined by X-ray diffraction. The complex crystallizes in the monoclinic space group P21/c, with cell constants a= 15.694(2), b= 10.403(2), c= 15.706(2)A, β= 92.63(2)°, and Z= 4. The structure has been solved from 2 289 diffraction intensities collected by counter methods, and refined by least-squares calculations to R= 0.087 (R′= 0.091). The four iridium atoms define a tetrahedron with three bridging CO ligands around a triangular face. All remaining carbonyls are terminally bonded and the two P atoms of the Me2PCH2CH2PMe2 ligand are found in axial positions, generating a six-membered ring.