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

Abstract

Heterometallic triangular palladium–cobalt clusters stabilized by three bridging diphosphine ligands such as Ph 2PNHPPh 2 (dppa), (Ph 2P) 2N(CH 3) (dppam), (Ph 2P) 2N(CH 2) 3Si(OEt) 3 (dppaSi), or mixed ligand sets Ph 2PCH 2PPh 2 (dppm)/dppa, dppm/dppam or dppm/dppaSi have been prepared with the objectives 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 determination of [CoPd 2( μ 3-CO) 2( μ-dppam) 3]PF 6 confirms the triangular arrangement of the metal core, with each edge bridged by a dppam ligand, although disorder problems prevent a detailed discussion of the bonding parameters. Different approaches are given to functionalize the heterometallic clusters: alkylation of the nitrogen atom of co-ordinated dppa ligands or introduction of a third bridging diphosphine in a precursor tetranuclear cluster containing only two bridging diphosphine ligands. In the latter case, it was found that their nature critically determined whether or not the reaction occurred. The diversity of bridging ligands allowed an investigation of their influence on the electrochemical properties of the clusters. By comparison with [CoPd 2( μ 3-CO) 2(CO) 2( μ-dppm) 2] + which contains only two assembling ligands, it is generally observed that trinuclear cationic CoPd 2 clusters containing three (identical or different) edge-bridging bidentate diphosphine ligands show increased redox flexibility. A notable stabilisation of the metal core is observed when three dppm ligands bridge the metal–metal bonds and [CoPd 2( μ 3-CO) 2( μ-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 diphosphine: a lower redox aptitude is exhibited by the dppa and dppam derivatives [CoPd 2( μ 3-CO) 2( μ-dppa) 3] + and [CoPd 2( μ 3-CO) 2( μ-dppam) 3] +.