Reductive Formation of Straight Linear Metal−Metal Bonded Tetranuclear Complexes X−M−Mo−Mo−M−X from X2M···Mo−Mo···MX2Supported by Four Tridentate 6-Diphenylphosphino-2-pyridonate Ligands (M = Pd, Pt; X = Cl, Br, I)

Abstract

Straight linear tetranuclear complexes have been prepared by using pyphos ligand (pyphos = 6-diphenylphosphino-2-pyridonate), which has unique coordinating sites comprised of three kinds of elements, i.e., phosphine, nitrogen, and oxygen atoms, in almost linear fashion. The starting complex is the quadruply bonded dinuclear molybdenum(II) complex, Mo2(pyphos)4 (1), which was prepared by the reaction of Mo2(OAc)4 with sodium salt of pyphos. Single crystal X-ray analysis revealed that 1 consists of quadruply-bonded Mo2 and trans-arranged diphosphine at both axial position of Mo2 core. Thus, transition metal can be expected to be placed at both axial positions of Mo2 core. Thus, treatment of 1 with Pd(II) and Pt(II) complexes gave rise to tetranuclear complexes of the general formula, Mo2Pd2X4(pyphos)4 (2) and Mo2Pt2X4(pyphos)4 (4) (a: X = Cl; b: X = Br; c: X = I). Single crystal X-ray analysis of 2a, 2b, and 4a revealed the structure of complexes 2 and 4 that consist of M(II)···MoMo···M(II) skeleton supported by four pyphos ligands. They have quadruply-bonded Mo2 core and no bonding between a M(II) atom and a Mo atom. Treatment of 2 and 4 with reducing reagents, photoirradiation, or thermal reduction resulted in the formation of metal−metal bonded tetranuclear complexes Mo2Pd2X2(pyphos)4 (6) and Mo2Pt2X2(pyphos)4 (7) (a: X = Cl; b: X = Br; c: X = I), respectively. All of them were characterized by single crystal X-ray analysis, which revealed that they consist of M(I)-MoMo-M(I) skeleton supported by pyphos ligands. The bond lengths between M(I) and Mo are short enough to be formally regarded as a single M(I)-Mo bond, while the longer Mo−Mo distance compared to that of 1, 2, and 4 indicated that the Mo−Mo bond of 6 and 7 are formally triple (π2δ component). The electrochemical study indicates that all the atoms in the M−Mo−Mo−M system are electronically strongly coupled to allow an effective 2-electron redox process, and thus the chemical reduction of 2 and 4 readily gives 6 and 7, respectively.