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

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• 作者：Fabian Rupp ; Dr. Katharina Chevalier ; Dr. Michè ; le Graf ; Markus Schmitz ; Dr. Harald Kelm ; Anneken Gr&uuml ; n ; Manuel Zimmer ; Prof. Dr. Markus Gerhards ; Prof. Dr. Christoph van W&uuml ; llen ; Prof. Dr. Hans-J&ouml ; rg Kr&uuml ; ger and Prof. Dr. Rolf Diller
• 刊名：Chemistry - A European Journal
• 出版年：2017
• 出版时间：February 10, 2017
• 年：2017
• 卷：23
• 期：9
• 页码：2119-2132
• 全文大小：2764K
• ISSN：1521-3765

文摘

A comprehensive spectroscopic and structural investigation of [Co^II(lass="smallCaps">l-N₄ tBu₂)(dbsq)][B(p-C₆H₄Cl)₄] (1, lass="smallCaps">l-N₄ tBu₂=N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq¹⁻=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.