Synthesis, structure and electrochemical studies of the first mixed-metal clusters with the P-N-P assembling ligands (Ph2P)(2)NH (dppa), (Ph2P)(2)N(CH3) (dppam) and (Ph2P)(2)N(CH2)(3)Si(OEt)(3) (dppaSi)

Heterometallic triangular palladium-cobalt clusters stabilized by three bri dging diphosphine ligands such as Ph2PNHPPh2 (dppa), (Ph2P)(2)N(CH3) (dppam ), (Ph2P)(2)N(CH2)(3)Si(OEt)(3) (dppaSi), or mixed ligand sets Ph2PCH2PPh2 (dppm)/dppa, dppm/dppam or dppm/dppaSi have been prepared with the objectiv es of comparing the stability and properties of the clusters as a function of the short-bite diphosphine ligand used and of making possible their use in the sol-gel process (case of dppaSi). The crystal structure determinatio n of [CoPd2(mu(3)-CO)(2)(mu-dppam)(3)]PF6 confirms the triangular arrangeme nt of the metal core, with each edge bridged by a dppam ligand, although di sorder problems prevent a detailed discussion of the bonding parameters. Di fferent approaches are given to functionalize the heterometallic clusters: alkylation of the nitrogen atom of co-ordinated dppa ligands or introductio n of a third bridging diphosphine in a precursor tetranuclear cluster conta ining only two bridging diphosphine ligands. In the latter case, it was fou nd that their nature critically determined whether or not the reaction occu rred. The diversity of bridging ligands allowed an investigation of their i nfluence on the electrochemical properties of the clusters. By comparison w ith [CoPd2(mu(3)-CO)(2)(CO)(2)(mu-dppm)(2)](+) which contains only two asse mbling ligands, it is generally observed that trinuclear cationic CoPd2 clu sters containing three (identical or different) edge-bridging bidentate dip hosphine ligands show increased redox flexibility. A notable stabilisation of the metal core is observed when three dppm ligands bridge the metal-meta l bonds and [CoPd2(mu(3)-CO)(2)(mu-dppm)(3)] reversibly undergoes either a single-step two-electron oxidation or two distinct one-electron reductions. Complexes with the other diphosphines exhibit similar redox behaviour, but the stability of their redox congeners depends upon the nature of the diph osphine: a lower redox aptitude is exhibited by the dppa and dppam derivati ves [CoPd2(mu(3)-CO)(2)(mu-dppa)(3)](+) and [CoPd2(mu(3)-CO)(2)(mu-dppam)(3 )](+). (C) 1999 Elsevier Science S.A. All rights reserved.