Rigid Molecular Rods with Cumulene-Bridged Polyphosphines: Synthesis, Electronic Communication, Molecular Photophysics, Mixed-Valence State, and X-ray Photoelectron Spectroscopic Study.

Abstract

The synthesis, molecular photophysics, redox characteristics, and electronic interactions, as well as an X-ray photoelectron spectroscopic (XPS) study of a series of Ru(II) and Os(II) complexes with a polyphosphine/cumulene spacer, namely, 1,1',4,4'-tetrakis(diphenylphosphino)cumulene (C(4)P(4)), are studied and compared with the corresponding systems containing spacers with shorter sp carbon chain (C(n)()) lengths. Characterizations of all mono-, homo-, and heterobimetallic complexes with PF(6)(-) counteranions are accomplished using (1)H, (13)C, and (31)P{(1)H} NMR, fast atom bombardment (FAB/MS), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF/MS) mass spectroscopy and elemental analysis. From the electrochemical study it is observed that the length of the C(n)() bridges has a profound influence on redox potentials and the electronic interaction between the two metal-based termini. XPS studies reveal that a simple change in carbon chain length affects the electron donation of the phosphine spacer to the metal-based termini. As a result, the redox potential of the Ru(II) or Os(II) center is shifted significantly. The comproportionation constant, K(c), is calculated as 1.3 x 10(7) (M = Ru(II)) or 4.5 x 10(10) (M = Os(II)) for homobimetallic [(bpy)(2)M(C(4)P(4))M(bpy)(2)](4+), suggesting a strong electronic communication across the C(4)P(4) spacer. However, the K(c) value is estimated to be ca. 4 for the corresponding complexes [(bpy)(2)M(C(3)P(4))M(bpy)(2)](4+) (M = Ru, Os; C(3)P(4) = 1,1',3,3'-tetrakis(diphenylphosphino)allene), indicative of a system with electronic isolation between the two termini. In heterobimetallic [(bpy)(2)Ru(C(n)()P(4))Os(bpy)(2)](4+) (n = 3, 4), the energy transfer from Ru(II) to Os(II) is found to be very efficient, with rate constants k(en) of ca. 3 x 10(9) s(-)(1) (n = 3) and 1 x 10(11) s(-)(1) (n = 4). The increased value of k(en) upon the change from C(3) to C(4) can be explained by the increase in the electronic communication across spacers. Detailed studies and calculations have revealed a Dexter-type of mechanism for the triplet energy transfer in the system.