Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Five bis(tert-butylnitroxide) diradicals connected by a silole (7a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy SQUID measurements and DFT computations (UB3LYP/6-31 + G*) wore used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7b, 7c, and 7d are quite stable both in solution and in the solid state, 7a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekule structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a Bleaney-Bowers model that gives values of J = -142.0 cm(-1) for 7a, -1.8 cm(-1) for 7b, -1.3 cm(-1) for 7c, -4.2 cm(-1) for 7d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7b, 7c, and 7d with Delta ET-S values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7a and 12. Similarities between the AE(T-S) and J values (Delta ET-S = -2J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a Karplus-Conroy-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.