A new series of Rh2(II,II) complexes with formula cis-[Rh2(DTolF)2(bpnp)(L)]2+, where bpnp = 2,7-bis(2-pyridyl)-1,8-naphthyridine, DTolF = N,N'-di(p-tolyl)formamidinate, and L = pdz (pyridazine; 2), cinn (cinnoline; 3), and bncn (benzo[c]cinnoline; 4), were synthesized from the precursor cis[Rh2(DTolF)2(bpnp)(CH3CN)2]2+ (1). Complexes 2 – 4 were characterized in solution and through single crystal x-ray diffraction, and their ground state and photophysical properties were evaluated. In addition, the potential for 4 to act as a single-molecule photocatalyst for H2 production with low energy red light was investigated. The first reduction couple in 2 – 4 is localized on the bpnp ligand at approximately –0.52 V vs Ag/AgCl, followed by the reduction of the diazine ligand at −1.22 V (pdz in 2), −0.85 V (cinn in 3), and −0.76 V (bncn in 4) vs Ag/AgCl in CH3CN (0.1 M TBAPF6). The reversible oxidation of 2 – 4 observed at ~+1.0 vs Ag/AgCl in the same solvent is independent of the diazine ligand and is assigned as metal centered with contributions from the DTolF ligand. Complex 1 exhibits a Rh2(δ*)/DTolF→bpnp(π*) metal/ligand-to-ligand charge transfer (1ML-LCT) absorption with maximum at 757 nm (ε = 5,800 M–1cm–1). This transition is also present in the spectra of 2 – 4, overlayed with the Rh2(δ*)/DTolF→L(π*) 1ML-LCT bands at 516 nm in 2 (L = pdz), 640 nm in 3 (L = cinn), and 721 nm in 4 (L = bncn). Complexes 2 and 3 exhibit Rh2(δ*)/DTolF→bpnp
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3ML-LCT excited states with lifetimes, τ, of 3 ns and 5 ns, respectively, measured using transient absorption spectroscopy in CH3CN, whereas the lowest energy 3ML-LCT state in 4 is Rh2(δ*)/DTolF→bncn in nature with τ = 1 ns. Irradiation of 4 with 670 nm light in DMF under a N2 atmosphere in the presence of 0.1 M TsOH (p-toluene sulfonic acid) and 30 mM BNAH (1-benzyl-1,4-dihydronicotinamide) results in the production of H2 with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh2(II,II) bimetallic core with the bpnp ligand prevents the formation of a Rh-H hydride intermediate, such that the photocatalytic H2 production does not require open coordination sites. These results show that the observed photocatalytic reactivity is localized on the bncn ligand, representing the first example of ligand-centered H2 production.
Photomagnetism has been observed in various transition metal-containing complexes and clusters, where steady state irradiation with light results in a change of the magnetic property. Photomagnets have great potential for applications that include sensors, displays, high density information storage, quantum computing, and spintronics. . Trigonal bipyramidal polynuclear complexes of the type {[Ma(L))2]3[Mb(CN)6]2}, where L = tmphen (3,4,7,8-tetramethyl-phen), Ma = Fe, Co, and Mb = Fe, Co have been documented to exhibit photomagnetic behavior. The Co3Fe2 cluster in particular was found to undergo thermally-activated and photo-induced charge transfer, together with a change in the spin state. The electronic absorption spectrum of the cluster Co3Fe2 exhibits two distinct features, one at 475 nm (ε = 0.7 × 103 M–1cm–1) and the other at 689 nm (ε = 1.8 × 103 M–1cm–1); the latter is ascribed to the metal to metal charge transfer (MMCT) transition from FeII → CoIII. Two oxidation couples were observed for Co3Fe2 at +0.70 V and +0.89 V vs Ag/AgCl in methanol for CoIII/II and FeIII/II, respectively. Ultrafast time-resolved IR (TRIR) and transient absorption (fsTA) studies have provided insight into the
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excited states of the Co3Fe2 cluster. The fsTA for the Co3Fe2 cluster following 800 nm excitation in acetonitrile exhibits a positive feature at 385 nm attributed to the formation of a FeIII-CN, consistent with FeIICoIII MMCT. This MMCT signal decays with τ = 0.5 ns, however, the bleach signal can be fitted to a biexponential function which, in addition to this short component, persists into the nanosecond range. Nanosecond transient absorption (nsTA) measurements are consistent with initial MMCT followed by spin crossover, thus delaying the observed regeneration of the ground state. TRIR measurements of Co3Fe2 following 780 nm excitation in DMF show a loss of the ground state ν(CN) stretch assigned to bridging CoIII-CN-FeII at 2104 cm–1 and the growth of a band at 2055 cm–1 is likely associated with hot ν(CN) vibrations previously observed in the excited state TRIR spectra of ferro- and ferricynide. Broad positive signal is also observed in the 2120–2180 cm–1 range that corresponds to the stretch (CN)FeIII. With better understanding of the dynamics of the spin crossover at room temperature in Co3Fe2, it is now possible to tune the ligation sphere to improve the photomagnetic properties of this and other clusters.
Co3(dpa)4Cl2 (dpa = 2,2'-dipyridylamine) and [Co3(dpa)4CH3CN)2]2+ trinuclear complexes were prepared to investigate their potential use in photocatalytic hydrogen evolution. These complexes were found to be photoactive, with short lived excited states with lifetimes of 1-4 ps attributed to intramolecular MMCT. The ability for these complexes to absorb into the red, 449 nm (ε = 3100 M−1cm−1), 561 nm (ε = 1600 M−1cm−1) for Co3(dpa)4(CH3CN)2 and 449 nm (ε= 5800 M−1cm−1), 573 nm (ε = 1800 M−1cm−1), for Co3(dpa)4(Cl)2, is a promising attribute for first row photosensitizers. The [Co3(dpa)4(CH3CN)2]2+ complex itself exhibits electrocatalytic
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activity upon the addition of acetic acid, with modest electrochemical hydrogen production, and a faradaic efficiency of 70%.