Redox rich dicobalt macrocycles as templates for multi-electron transformationsw

The reversible transformation of substrates by several electron equivalents is a key goal of chemical energy storage. Toward this aim, extensive effort has been devoted to study catalysts that mediate the multi-electron reduction of substrates including protons, CO2 and N2. 2 In order to achieve such transformations with minimal energy input, catalysts that operate at low overpotentials are desired. Toward this end, we and others have been investigating cobalt tetra-imine macrocyclic complexes that are able to mediate the reduction of weak acids to H2 at comparatively low overpotentials. Using a Co(dmgBF2)2(CH3CN)2 (dmgBF2 = difluoroboryldimethylglyoxime) precatalyst, near-quantitative Faradaic yields can be obtained for the electrocatalytic reduction of H using CF3COOH as the proton source, with promising kinetic efficiency at an overpotential of less than 100 mV. While the interconversion of proton and electron equivalents does not necessarily require more than one metal site (to effect twoelectron reduction), the reduction of more highly-oxidized substrates such as CO2 or NO3 may be facilitated by multimetallic scaffolds. Such molecular constructs featuring more than one metal site can potentially transfer multi-electron equivalents, as well as facilitate the binding of weak donor ligands through cooperative pathways. Within this context, our group has begun to target bimetallic analogues of various Co(dmgBF2)2L2 complexes that have the ability to undergo several (44) reversible redox events with the ultimate goal of transferring the multiple redox equivalents to substrates of interest. Herein, we describe a series of dicobalt macrocycles that support up to five oxidation states and are also active electrocatalysts for proton reduction at comparatively low overpotentials. A bimetallic macrocycle that incorporates two Co-centers within a planar imine environment can be realized using pyridazine-based building blocks (Fig. 1). The pyridazine dioxime precursors are accessible from the appropriate pyridazine dicarbonyl following condensation with hydroxylamine. An anionic borate linkage has been included in the design presented here to decrease coulombic repulsions between H and positively-charged Co-species. A series (R = H, Me, Ph; Fig. 1) of new bimetallic CoCo complexes bearing this ligand architecture was prepared from the respective dioxime precursors by reaction with Co(OAc)2 and BF3 Et2O (see the ESIw). These complexes were characterized structurally, spectroscopically and electrochemically. The solidstate structures of the set of [LCo2] 2+ complexes reveal tetragonally distorted 6-coordinate geometries around each cobalt center with solvent ligands occupying the axial sites (Fig. 2). The electrochemistry of the set of [LCo2] 2+ complexes was examined by cyclic voltammetry (Table 1). For each complex, the cathodic scan revealed at least two electrochemicallyreversible 1e reduction events (ascribed to CoCo/CoCo and CoCo/CoCo couples; Table 1). The Co-based reduction events are sufficiently well separated (Kc = 2.6 10–9.5 10 for [LCo2] ; L = L, L, L) to suggest that the mixed-valence species should be reasonably stable. Following the metal-based reduction events, other presumed ligandcentered reduction events are noted at more negative potentials based on comparison with a Zn2-analogue that has been independently prepared and characterized, (see the ESIw) [LZn2] . Scanning in the anodic direction for [LCo2] 2+ revealed the presence of two 1e reversible or quasi-reversible oxidation events. Because well-defined bimetallic CoCo complexes are still unknown, the isolation of mixed-valence CoCo complexes was pursued. The CoCo complexes were generated via controlled potential electrolyses and were isolated following the removal of the [Bu4N][ClO4] electrolyte by washing with DME several times. The UV-visible spectrum of each complex exhibits broad low-energy IVCT bands at 874, 947 and 875 nm for [LCoCo], [LCoCo], and [LCoCo], respectively. The X-Band EPR spectrum in DMF glass for [LCoCo] at 77 K (Fig. 3) reveals an axial pattern with g tensors of g>=2.26 (A>=22G) and gJ=2.02 (AJ=76G), consistent with low-spin Co(II) in a tetragonally-distorted coordination environment. Similar EPR spectra have been reported for mononuclear CoL2(dmgBF2)2 systems (L = MeCN, MeOH). The solid-state structure of [LCoCo], obtained by diffusion of diethyl ether into an acetonitrile solution (Fig. 2d), shows that the coordination environments of the Fig. 1 Cobalt complexes and abbreviations used in this study.