Linearly Aligned Metal Clusters: Versatile Reactivity and Bonding Nature of Tetrametal M–Mo–Mo–M Complexes (M = Pt, Pd, Ir, and Rh) Supported by 6-Diphenylphosphino-2-pyridonato Ligand

A tridentate ligand, pyphos (6-diphenylphosphino-2-pyridonato), was utilized to prepare tetrametal complexes since this ligand has unique coordinating sites comprised of three different elements, i.e., phosphorus, nitrogen, and oxygen, in almost linear fashion. By using pyphos ligand, linearly aligned tetrametal complexes of group 10 metals [Mo2M2(pyphos)4X2n] (M = Pt and Pd; n = 0, 1, and 2) were prepared, and for group 9 metals, [Mo2M2(pyphos)4(RNC)4X2n]2+ (M = Ir and Rh; n = 0 and 1). Fully metal-to-metal bonded complexes were obtained by reduction of MII to MI for group 10 metals and by oxidation of MI to MII for group 9 metals. Both reactions afforded complexes having unique M–Mo≡Mo–M skeletons, i.e., metalla-2-butyne. Structural and chemical properties were systematically investigated for M0 (M = Pt and Pd) and MI (M = Ir and Rh). Thus, oxidative reactions of Pd0 complexes [Mo2Pd2(pyphos)4] and IrI complexes [Mo2Ir2(pyphos)4(RNC)4]2+ with RX or X2 were studied and unique 1,4-addition reaction was demonstrated. Dichromium complexes analogous to dimolybdenum complexes were prepared and axial donation of PtMe2 moiety significantly elongated the Cr–Cr bond, due to the dative bonding interaction between CrII and PtII units.