Electronic coupling in mixed-valence binuclear ruthenium ammine complexes as probed by an electrochemical method and an extension of Mulliken's theory of donor-acceptor interactions

Abstract

An electrochemical approach to assessing the degree of electronic coupling in mixed-valence binuclear complexes is outlined. The method relies on the comparison of electrochemical potential shifts induced at both the directly and indirectly perturbed metal sites when a ligand substitution process is carried out at one site, e.g., [symmetric] ( N H~)~R U -L~~R U ( N H~)~~+ /~+ /~+ -[asymmetric] L(NH~)~RU-LI,,-R~(NH~)~~+/~+/~+, where the bridging ligandLbr is either pyrazine or 4-cyanopyridine and the perturbing ligand L is a substituted pyridine. It is found that the degree of electronic coupling in these systems is at least three times that which would be predicted based solely on spectroscopic measurements. The stabilization energy due to electron delocalization in these complexes can be accounted for with near-quantitative accuracy. It is also shown how the Mulliken analysis of the data allows for an estimation of the Wolfsberg-Helmholz constant K, which can be used in the calculation of off-diagonal matrix elements for molecular donor-acceptor interactions. (6) De la Rosa, R.; Chang, P. J.; Salaymeh, F.; Curtis, J. C. Inorg. Chem. 1985,24,4229-423 1. (7) Theuseofsubstitutbdpyridinesto'tune"thegroundandexcitcd electronic propcrticsof ruthenium amminecomplexes has been extensively employed by Ford and co-workers: (a) Malouf, G.; Ford, P. C.

Driving the Localized-to-Delocalized Transition in Unsymmetrical Dinuclear Ruthenium Mixed-Valence Complexes

“…The validity of Eqn (3) is expected to become increasingly invalid as the degree of electronic coupling increases. [7][8][9][10][11] Application of the electrochemical analysis to the series of mixed-valence complexes trans-[{Ru(NH 3 ) 5 }(µ-pyz){Ru(NH 3 ) 4 (L)}] 5+ [7] yielded a value of ρ = 0.74 for the Creutz-Taube ion (L = NH 3 ), consistent with a delocalized classification and the conclusions of an extensive number of experimental, theoretical, and computational studies. [5,12,[15][16][17][18][19] Recent studies in our laboratory have demonstrated that the diastereoisomers of symmetrical dinuclear mixed-valence complexes such as meso-and rac-[{Ru(bpy) 2 } 2 (µ-BL)] 5+ (bpy = 2,2 -bipyridine, BL = dpb (2,3-bis(2-pyridyl)-1,4benzoquinoxaline), dpb (dipyrido(2,3-a;3 ,2 -c)benzophenazine); Fig.…”

“…On the basis of the electrochemical method reported by Curtis and co-workers, [8][9][10][11] the extent of coupling between the metal centres is related to the ratio of the redox perturbation directly induced by ligand substitution at one end of the dimer to that indirectly induced at the other end. For both series of complexes, [{Ru(bpy) 2 }(µ-BL){Ru(pp) 2 }] 4+ (BL = dpb, dpb ) linearity was observed in the plots of E ox1 versus E ox2 for pp = bpy, Me 2 bpy, and Me 4 bpy, and the slopes were identical (within experimental error) for the meso and rac diastereoisomers of each series.…”

“…Curtis and co-workers [7][8][9][10][11] have proposed a semiquantitative interpretation for E 0 effects, based on a classical two-state perturbation treatment and electrochemical potential measurements. Central to this two-state classical analysis is the assumption that a E 0 contribution is manifested spectroscopically as an equivalent variation in ν max at constant H ab and λ.…”

“…The introduction of redox asymmetry into the dinuclear complexes [{Ru(bpy) 2 } 2 (µ-dpb )] 5+ and [{Ru(bpy) 2 } 2 -(µ-dpb)] 5+ by the substitution of the terminal bpy ligands at one end by Me 2 bpy (4,4 -dimethyl-2,2 -bipyridine) and Me 4 bpy (4,4 ,5,5 -tetramethyl-2,2 -bipyridine) diminishes the degree of delocalization, and provides a strategy for examining the transition between the delocalized and localized regimes within a closely related series of complexes. In the present study, the electrochemical and spectral analysis of Curtis and co-workers [9][10][11]23] is applied to investigate the onset of localization in the diastereoisomeric forms of the series of 'unsymmetrical' complexes [{Ru(bpy) 2 }(µ-BL)-{Ru(pp) 2 }] 5+ (BL = dpb , dpb; pp = Me 2 bpy, Me 4 bpy). The work also addresses the limited experimental data which exist to probe the influence of the inherent stereochemical complexities on the physical properties of polymetallic assemblies.…”

Oxidized Styrylruthenium–Ferrocene Conjugates: From Valence Localization to Valence Tautomerism

“…Although ligand substitution at the ruthenium center will affect the positioning of the minima of both diabatic hypersurfaces, its effects will be stronger on the alkenylruthenium site. Thus, the intrinsic energy difference Δ G 0 between the local minima of the diabatic hypersurfaces can be varied . This opens the possibility to realize, within the same basic system, the bordering situations of valence delocalization, of predominant valence localization at one site or the other, or to achieve electronic bistability, that is, the phenomenon of valence tautomerism.…”