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

Abstract

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 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*) 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 pi-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 DeltaE(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 DeltaE(T-S) and J values (DeltaE(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 pi system of the silole.

Conversion of a Singlet Silylene to a stable Biradical

“…A diluted solution of polymorph I in C 6 D 6 shows six hyperfine lines (see the Supporting Information). The signal broadens on cooling to 200 K. [12] The EPR intensities of both the solution and the solid phase plotted against T À1 follow straight lines where the signal intensity increases with decreasing temperature, thus indicating very weak interactions between two spins in polymorph I of 2. Since polymorph II is diamagnetic in the temperature range of 300-2 K, it is EPR silent.…”

“…Power [8] and Lee et al reviewed persistent stable free radicals of Group 14. [9] Grützmacher et al [10] reported on radicals of main-group elements, Rajca et al [11] described singlet-triplet bistability and Roques et al [12] investigated the exchange in silole-bridged biradicals. Neese et al [13] studied the interaction of radical pairs through bonds and through space.…”

“…Polymorph II of 2 melts in the range of 179-181 8C and decomposes at 185-186 8C. The UV absorption at 569 nm may be compared with that of the tert-butylnitroxides (450 nm) [12] or that of the bissilyl phenyl biradical (555 nm). [7] The powder sample enriched with polymorph I melts at 167-168 8C and decomposes at 172-173 8C.…”

“…However, antiferromagnetic interactions in polymorph II of 2 are stronger than those found in the biradical bis(tert-butylnitroxide). [12] The radical centers in the latter are connected with each other by a molecular bridge.…”

Umwandlung eines Singulett‐Silylens in ein stabiles Biradikal

“…Grützmacher et al [10] berichteten über Radikale von Hauptgruppenelementen. Rajca et al [11] beschrieben Singulett-Triplett-Bistabilitäten und Roques et al [12] untersuchten den Austausch in silolverbrückten Biradikalen. Studien über die Bindungs-und Raumwechselwirkungen zwischen Radikalpaaren wurden von Neese et al [13] verçffentlicht.…”