Alkyl-substituted Schlenk hydrocarbon diradicals with triplet and singlet ground states in frozen solutions

Ground states for alkyl-substituted Schlenk hydrocarbon diradicals 1-4 in frozen 2-MeTHF or THF are determined at low temperatures by EPR spectroscopy (4-80 K) or SQUID magnetometry (2-80 K). Diradicals 1, 2, and 3 are triplet ground states. Diradical 4 is the singlet ground state with the singlet-triplet gap, ΔE_ST ≈ -0.2 kcal mol^-1. Another minor diradical present in the samples of 4 possesses ΔE_ST ≈ -0.02 kcal mol^-1; most likely, this diradical corresponds to another conformational isomer of 4. Compound 4 is the first hydrocarbon diradical with antiferromagnetic coupling through 1,3-phenylene, a ubiquitous mediator of a ferromagnetic coupling.