Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex

Abstract

A comprehensive spectroscopic and structural investigation of [Co^II (l-N₄ tBu₂ )(dbsq)][B(p-C₆ H₄ Cl)₄ ] (1, l-N₄ tBu₂ =N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq^1- =3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) Co^II semiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) Co^II semiquinonate state instead of the frequently observed valence tautomerism from ls Co^III catecholate to hs Co^II semiquinonate. DFT calculations demonstrate that the (closed shell) Co^III catecholate suffers from a triplet instability leading to the ls Co^II semiquinonate ground state. The thorough temperature-dependent spectroscopic study of the SCO enables a photophysical investigation. Thus, by selective photoexcitation of the ls fraction of 1 in solution at room temperature, ultrafast conversion to the hs state is observed using femtosecond electronic and IR-vibrational (infrared) transient absorption spectroscopy. The kinetics of the photocycle is described by a stretched exponential with τ=3.3-3.6 ps and β=0.52-0.54, representing an upper limit for the hs-ls relaxation time. This is, to our knowledge, the fastest interconversion ever determined for a SCO complex, and is attributed to the special situation that in 1 a Co^II complex is coordinated to a π-radical ligand allowing very efficient coupling between the ls and hs spin states.

Keywords: cobalt dioxolene complexes; kinetics; metal radical complexes; spin crossover; time resolved spectroscopy.