Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex.

Abstract

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)CoIV(O)]2- (1; H4TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)CoIII]·3(H2O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λexc = 393 nm) resulted in generation of the excited state S2* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S2* converted to a lower-lying excited state, S1* (λmax = 580 nm), with rate constant kc = 7(2) × 1011 s-1 (S2* → S1*). S1* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant kd = 1.7(3) × 109 s-1 (S1* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)2 (OTf- = triflate anion), [Fe(bpy)3]3+ (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA•+). The electron-transfer reactivity of S1* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( Eox vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S1* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the Eox values of electron donors allowed the estimation of the one-electron reduction potential of S1* ( Ered*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( Ered = 0.86 V vs SCE).