There is emerging interest in cyclic conjugated molecules that happen to have triplet spin states and which might be expected to follow a 4n rule for aromaticity.[cite]10.1002/anie.201705228[/cite] The simplest such system would be the triplet state of cyclobutadiene, for which a non or anti-aromatic singlet state is always found to be lower in energy. Here I explore some crystal structures containing this motif for possible insights.
AbstractTetrakis(trimethylsilyl)cyclobuta‐1,3‐diene (1) was subjected to a temperature‐dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene (2). From the temperature dependent EPR absorption area we derive a singlet→triplet (1→2) energy gap, EST, of 13.9 kcal mol−1, in agreement with calculated values. The zero‐field splitting parameters D=0.171 cm−1, E=0 cm−1 are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.
Abstract The cis-diphenyl-substituted cyclobutadiene cobalt complex (1), prepared from the reaction of 1,8-diphenyl-3,6-disila-1,7-octadiyne with CpCo(CO)2, reacted with lithium metal in tetrahydrofuran (THF) to yield pale orange crystals of the dilithium salt of the cis-diphenylcyclobutadiene dianion (2). The structure was determined by spectroscopic methods and X-ray crystallography to reveal the aromatic character of the doubly-charged four-membered ring system.