Synthesis of Bent-Form Dinuclear Complexes and Cooperative Interaction of the Two Metal Centers with a Substrate

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A bent form of edge-sharing square planar bimetallic complexes is of interest, particularly because axial orbitals of the two metals are in close proximity under the hinged planes to interact cooperatively with a substrate. Such an interaction is expected to play a significant role in binding a substrate to a bimetallic complex.In this study, three types of bisphosphido ligands, MPE, PPE, and PPP (MPE = MePCH_2CH_2PMe^<2->, PPE = PhPCH_2CH_2PPh^<2->, and PPP = PhPCH_2CH_2CH_2PPh^<2->), were used as a bridging unit for the preparation of the edge-sharing bis-squareplanar complexes. X-ray crystal structure analyses were performed for the dipalladium(II) and diplatinum(II) complexes with a general formula (μ-bridge)-[M(DPPE)]_2X_2 (M = Pd, Pt; X = Cl^-, I^-, B(3,5-(CF_3)_2-Ph)_4^-), in which the two metal centers were bridged with the linked-bisphosphido group to form a rare butterfly structure. The dihedral angles between the two "wings" (formed by two metal centers and two bridging P atoms) were around 120°for the ethylene-linked MPE and PPE complexes, whereas the propylene-linked PPP complex adopted a larger value, 131°. It is noteworthy that, in the PPE- and PPP-Pd_2 complexes, a Cl^- or I^- counter anion was situated so as to coordinate to the two Pd(II) centers simultaneously under the hinged planes.Since Rh(I) phosphine complexes are known to be a useful catalyst for numbers of organic reactions, a Rh(I) analog of the above-mentioned butterfly-type dinuclear complex is of interest from the point of view of its catalytic activity. Thus, a dinuclear Rh(I) complex (μ-PPE)-[Rh(COD)]_2 was prepared according to a method similar to that reported for the preparation of (μ-PPh_2)_2-[Rh(COD)]_2. These phosphido-bridged dirhodium(I) complexes were used as a catalyst for polymerization of phenylacetylene to give a cis-transoidal polyphenylacetylene with high regularity.