Theoretical Studies on Metal-Metal Interaction, Excited States, and Spectroscopic Properties of Binuclear Au-Au, Au-Rh, and Rh-Rh Complexes with Diphosphine Ligands: Buildup of Complexity from Monomers to Dimers

To understand their photocatalytic activity and application in luminescent materials, a series of gold and rhodium phosphine complexes (mononuclear [Au-I(PH3)(2)](+) (1) and [Rh-I(CNH)(2)(PH3)(2)](+) (2); homobinuclear [Au-2(I)(PH2CH2PH2)(2)](2+) (3)and[Rh-2(I)(CNH)(4)(PH2CH2PH2)(2)](2+) (4);heterobinuclear[(AuRhI)-Rh-I(CNH)(2)(PH2CH2PH2)(2)](2+) (5),[(AuRhI)-Rh-I(CNH)(2)(PH2NHPH2)(2)Cl-2] (6), and [(AuRhI)-Rh-I(CNH)(2)(PH2NHPH2)(2)](2+) (7); and oxidized derivatives [(AuRhII)-Rh-II(CNH)(2)(PH2CH2PH2)(2)](4+) (8), [(AuRhII)-Rh-II(CNH)(2)(PH2NHPH2)(2)Cl-3](+) (9), and [(AuRhII)-Rh-II(CNH)(2)(PH2NHPH2)(2)](4+) (10)) were investigated using ab initio methods and density functional theory. With the use of the MP2 method, the M-M' distances in 3-7 were estimated to be in the range of 2.76-3.02 angstrom, implying the existence of weak metal-metal interaction. This is further evident in the stretching frequencies and bond orders of M-M'. The two-electron oxidation from 5-7 to their respective partners 8-10 was shown to mainly occur in the gold-rhodium centers. Experimental absorption spectra were well reproduced by our time-dependent density functional theory calculations. The metal-metal interaction results in a large shift of d(z2) -> p(z) transition absorptions in binuclear complexes relative to mononuclear analogues and concomitantly produces a low-lying excited state that is responsible for increasing visible-light photocatalytic activities. Upon excitation, the metal-centered transition and the metal-to-metal charge transfer strengthen the metal-metal interaction in triplet excited states for 3-6, while the promotion of electrons into the sigma*(d(z2)) orbital weakens the interaction in 9.