Experimental and Theoretical Studies of Magnetic Exchange in Silole‐Bridged Diradicals

Five bis( tert‐ butylnitroxide) diradicals connected by a silole ( 7 a – d ) or a thiophene ( 12 ) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna 2 1 with a = 20.752(5), b = 5.826(5), and c = 34.309(5) Å. X‐ray crystal structure determination, electronic spectroscopy, variable‐temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6‐31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b , 7 c , and 7 d are quite stable both in solution and in the solid state, 7 a 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 π‐conjugated Kekulé 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 7 a , −1.8 cm −1 for 7 b , −1.3 cm −1 for 7 c , −4.2 cm −1 for 7 d , 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 7 b , 7 c , and 7 d with Δ E T–S values of 3.48, 2.09, and 8 cm −1 , respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12 . Similarities between the Δ E T–S and J values (Δ E T–S = −2 J ) 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 π system of the silole.