Stable Mixed-Valent Complexes Formed by Electron Delocalization Across Hydrogen Bonds of Pyrimidinone-Linked Metal Clusters

Abstract

Electron transfer across a mixed-valent hydrogen-bonded self-dimer of oxo-centered triruthenium clusters bridged by a pair of 4(3H)-pyrimidinones is reported. Spectroelectrochemical studies in methylene chloride reveal that 1 rapidly self-dimerizes upon oneelectron reduction, forming the strongly coupled mixedvalent hydrogen-bonded dimer (1 2 ) − . In the mixed-valent state, significantly broadened, partially coalesced ν(CO) bands are observed, allowing estimation of the electron transfer rate (k ET ) by an optical Bloch line shape analysis. Simulation of the FTIR line shapes provides an estimate of k ET on the order of 10 11 s −1 , indicating a highly delocalized electronic structure across the hydrogen bonds. These findings are supported by the determination of the formation constant (K MV ) for (1 2 ) − , which is found to be on the order of 10 6 M −1 , or nearly 4 orders of magnitude higher than that for the neutral isovalent dimer (1 2 ). This represents a stabilization of approximately 5.7 kcal/mol (1980 cm −1 ) arising from electron exchange across the hydrogen bonds in the mixed-valent state. Significantly, an enormous intensity enhancement of the amide ν(NH) band (3300 cm −1 ) of (1 2 ) − is observed, supporting strong mixing of the bridging ligand vibrational modes with the electronic wave function of the mixedvalent state. These findings demonstrate strong donor− bridge−acceptor coupling and that highly delocalized electronic structures can be attained in hydrogen-bonded systems, which are often considered to be too weakly bound to support strong electronic communication.

Steric and electronic control of an ultrafast isomerization

“…4, S2 and S3). 1,[10][11][12][13][14][15][16][17][18][19] Owing to the similar line shape observed for complexes 1-3 ( Fig. S2 †) phosphine electronic effects appear to have little impact on the isomerization reaction.…”

Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways

“…The electrochemical behaviors demonstrate that these Mo 2 complexes exist as HB dimers in both neutral and the oxidized states in DCM solution. This is contrary to the Ru 3 O D–HB–A system in which the HB bridged dimer is formed upon reduction of one of the metal cluster to yield the mixed-valent species 24 . The cyclohexylene bridged complexes 3a – d present the smallest Δ E 1/2 values (∼70 mV) (Supplementary Figures 5 and 33), showing very weak coupling due to the electrostatic effect.…”

“…For the D–HB–A systems, we are able to measure the ET rate constants by analysis of ν (NH) band broadening in the IR spectra, a method used for determination of ET rate constants in Ru 3 O–Ru 3 O MV systems 24,40 . This methodology has also been employed to evaluate thermally induced ET reactions in MV D–B–A systems with covalent bridges using NMR 41 and EPR 13,35,42 spectroscopy techniques.…”

“…Therefore, for the MV species ( 1 + ), the coalesced ν (NH) C band profile is reflective of the dynamics of the free N–H bonds caused by ET across the HB interface (Fig. 2a–d), and thus can be exploited to simulate the ET kinetics 24,40 . For each of the four adducts band simulations were performed using the published software Zoerbex 44 , as shown in Fig.…”

“…In the series, the largest rate constant, k obs = 6.0 × 10 10 s −1 , is obtained for 1a + and the smallest rate, k obs = 2.0 × 10 10 s −1 for 1b + , essentially following the same trend in the calculated rate constants. Recently, Kubiak’s group reported a rate constant of 3.8 × 10 11 s −1 derived by broadening analysis of the ν (CO) band for an HB bridged Ru 3 O–Ru 3 O MV complex, which has shorter ET distance 24 . Comparison of ET rates for these two distinct systems verifies the magnitudes of k ET in the Mo 2 –Mo 2 system.…”