Synthesis, crystal structure, magnetic, and electron paramagnetic resonance properties of a spiroconjugated biradical. Evidence for spiroconjugation exchange pathway

A spiroconjugated nitronyl nitroxide biradical, 6,6'-(4,4,5,5-tetramethylim idazolidine-3-oxide-1-oxyl)-3,3,3',3'-tetramethyl- 1,1'-spirobisindane (1), has been prepared by functionalization of a 3,3,3',3'-tetramethy-,11'-spir obisindane framework followed by Ullman condensation and subsequent oxidati on. The biradical crystallizes in the monoclinic space group C2/e with four molecules in the unit cell of dimensions a = 24.861 (10) Angstrom, b = 12. 129(3) Angstrom, and c = 12.258(6) Angstrom. X-ray analysis of a blue-plate single crystal has revealed dihedral angles of 28 degrees between the nitr onyl nitroxide moiety and aromatic ring with:intramolecular through space r adical-radical distances of 8.25 and 10.11 Angstrom. In the solid state, th e temperature dependence of the molar magnetic susceptibility reveals antif erromagnetic interactions. These interactions are best fit using a pair mod el, affording the value J = -4.0 cm(-1) where J is the interaction paramete r appearing in the spin Hamiltonian H = -JS(1).S-2. The field dependence of the magnetization measured at 2 K is consistent with a pair model. Frozen matrix EPR spectra of biradical 1 in CH2Cl2 at 100 K shows a half field tra nsition at 1700 G. Temperature dependence of the half field transition inte nsity has been found to be consistent with a ground singlet state and therm ally accessible triplet state. The magnetic interaction observed in the sol id state is also observed in solution. Thus, room-temperature solution spec tra display a nine-line pattern, with hyperfine coupling to four "equivalen t" nitrogen atoms and a hyperfine coupling constant a(N) = 3.8 G. Temperatu re dependence of the solution EPR spectra of biradical 1 displays alternati ng line width effects caused by conformational dynamics in solution. This b ehavior has been attributed to modulation of exchange and hyperfine interac tions most likely caused by rotational motion about the nitronyl nitroxide- phenyl bond. Biradical 1 therefore exists as a ground-state singlet with a thermally accessible triplet at ca. 4 cm-l higher in energy with a conforma tional dependence of intramolecular exchange in solution. This coupling may present evidence for spiroconjugation as an exchange pathway. Density func tional calculations (B3/6-311G(D)) have been performed to investigate this possibility.