Mixed Valency in Hydrogen Bonded ‘Dimers of Dimers’

Abstract

Dimolybdenum quadruply bonded compounds containing a pendant lactam functional group form self-complementary hydrogen bonded 'dimers of dimers' in the solid-state and CH(2)Cl(2) solutions. Electrochemical studies in CH(2)Cl(2) show two consecutive one-electron redox processes corresponding to oxidation of the Mo(2)(4+) cores. Spectroelectrochemical studies on the 'dimers of dimers' show no evidence of intervalence charge transfer bands in the mixed valence radical cations formed by one-electron oxidation, indicating that they are examples of proton-coupled mixed valency.

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

“…The chemical potentials ( E 1/2 ) fall in the range of −0.3 to 0.1 V (vs. Fc +/0 ) and the potential separations (Δ E 1/2 ) are estimated to be ∼100 mV from Richard–Taube methods (Supplementary Figure 31) 23 , as listed in Table 2. In DMF, these complexes show only one redox couple, as expected, for the corresponding monomers (Supplementary Figure 3) 15 . For 1a , 2a , and 3a , the N(CH 3 ) 2 groups on the DArF ancillary ligands are redox active ( E 1/2 = 0.1–0.5 V vs. Fc +/0 ), which makes the redox waves of the Mo 2 centers weak and irreversible.…”

“…Mixed-valence (MV) D–B–A molecules, which have identical D and A sites differing only in formal oxidation states, are valuable experimental models for study of electron self-exchange reaction with Marcus–Hush theory 11,12 , which has been successfully used to evaluate intramolecular ET through covalently bonded bridges 13,14 . Recent efforts have produced various examples of hydrogen bonded MV complexes, in which self-complementary HB interactions are used to bridge the electron donor and acceptor 15–17 , with the aim to evaluate the D–A EC optically upon analysis of the intervalence charge transfer (IVCT) absorbance. However, this optical feature has only been observed in few examples of hydrogen bonded MV compounds, and lack of a test-bed series of compounds has hindered the kinetic study of thermal ET in electron self-exchange reactions with zero driving force (−Δ G ° = 0) 5,16,17 .…”

Small anion-assisted electrochemical potential splitting in a new series of bistriarylamine derivatives: organic mixed valency across a urea bridge and zwitterionization

“…Redox stimuli are a promising trigger to directly change the charge distributions of molecules and assemblies, potentially allowing tuning of the strength of non-covalent interactions, including hydrogen bonds (H-bonds) [23–26]. In this context, a number of redox-active compounds bearing H-bond donors and acceptors were investigated to realize electrochemically controlled H-bonding [23–36]. Especially, oxidation-active ureas are an important class of such compounds [25–26].…”

Mixed Valency in Quadruple Hydrogen-Bonded Dimers of Bis(biimidazolate)dirhodium Complexes

“…K c values for the oxidized and reduced species of [Re III Cl 2 (PBu 3 ) 2 (Hbim)] 2 are 5.41 × 10 4 and 1.14 × 10 4 , respectively, as calculated from the redox potentials 4. The values for the dimers of dimolybdenum complexes are 111−487 5. 6 [1′(PPh 3 ) 2 ] 2 + has a similar value, K c = 224, to the dimolybdenum species and this means that [1′(PPh 3 ) 2 ] 2 + has comparable stability to the doubly H-bonded dimer of dimolybdenum complexes; nevertheless, H bonds between the complexes are not coplanar.…”

“…Patmore and co-workers also showed the stability of the proton-coupled mixed-valence states for the hydrogen-bonded dimers of dimetal complexes [M 2 (TiPB) 3 (HDON)] 2 (M = Mo, W; TiPB = 2,4,6-triisopropylbenzoate; H 2 DON = 2,7-dihydroxy-1,8-naphthyridine) and related compounds. 5,6 All of the reported mixed-valence dimer complexes are doubly or quadruply hydrogen-bonded dimers in which the selfcomplementary hydrogen bonds lie in the same plane.…”