A THERMALLY ACCESSIBLE TRIPLET DIRADICAL STATE OF A SILYL SUBSTITUTED CYCLOBUTADIENE

Kostenko Arseni ¹ Boris Tumanskii ¹ Yuzuru Kobayashi ² Masaaki Nakamoto ² Akira Sekiguchi ² Yitzhak Apeloig ¹

¹Schulich Faculty of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology
²Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba

Ever since Kekulé first attempted the preparation of cyclobutadiene in 1872, this fascinating molecule presented a challenge to experimental and theoretical chemists. It was not until the mid-1970ies that cyclobutadiene derivatives could be synthesized and characterized. Herein, we report the observation by EPR spectroscopy of a thermally accessible triplet state of 1,2,3,4-tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1). To our best knowledge, this is the first example in which a triplet diradical state of a cyclobutadiene derivative was observed spectroscopically. An EPR study of 1, at 300K–395K, shows a typical spectrum for triplet diradicals with zero-field splitting (ZFS) |D(2)|_exp=0.17cm^-1. From the temperature dependent EPR absorption area we derive a singlet-triplet energy gap, E_ST, of 14.2 kcal·mol^-1. This value is supported by quantum chemical calculations of model 1 and 2 (in which the SiMe₃ substituents were replaced by SiH₃) at the CCSD(T)/def2-TZVPP//B3LYP/6-311+G(d,p) level, that gives E_ST=12.4 kcal·mol^-1. To determine D theoretically, D values of other related delocalized organic diradicals were calculated at the B3LYP/TZVP level giving a linear experimental-theoretical correlation (D_exp=1.565·D_calc, R²=0.99). Using this correlation we determine |D(2)|_calc=0.167 cm^-1, in excellent agreement with |D(2)|_exp=0.17cm^-1. The mechanism of the retro-cycloaddition of 1 (Scheme 1) and the role of 2 in this process will be discussed.