Experimental and Theoretical Studies on Magnetic Exchange in Silole-Bridged Diradicals.

International audienceFive bis(tert-butylnitroxide) diradicals connected by a silole [2,5-R2-3,4-diphenylsilole; R = Me3CN(®O.bul.)Z; Z = p-C6H4 (7a), p-C6H4C6H4-p (7b), 1,4-naphthalenediyl (7c), m-C6H4 (7d)] or a thiophene [2,5-R2-thiophene; R = p-Me3CN(®O.bul.)C6H4 (12)] ring as a coupler were studied. Compd. 12 crystallizes in the orthorhombic space group Pna21 with a 20.752(5), b 5.826(5), and c 34.309(5) .ANG.. X-ray crystal structure detn., electronic spectroscopy, variable-temp. EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the mol. conformations and electronic spin coupling in this series of mols. Whereas compds. 7b, 7c, and 7d are quite stable both in soln. and in the solid state, 7a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoid form and a monoradical species owing to the fact that they correspond to the open form of a p-conjugated Kekule structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topol. These interactions are best reproduced by a Bieaney-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 temp. dependence of the EPR half-field transition in frozen CH2Cl2 solns. is consistent with singlet ground states and thermally accessible triplet states for diradicals 7b, 7c, and 7d with DET-S values of 3.48, 2.09, and 8 cm-1, resp. No evidence of a populated triplet state was found for diradicals 7a and 12. Similarities between the DET-S and J values (DET-S = -2 J) clearly show the intramol. origin of the obsd. antiferromagnetic interaction. Analyses of the data with a Karplus-Conroy-type equation enabled the authors 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 non-aromaticity of the silole which thus permits a better magnetic interaction through it. DFT calcns. also support the exptl. results, indicating that the magnetic exchange pathway preferentially involves the C p system of the silole