MODELS FOR INTRAMOLECULAR EXCHANGE IN ORGANIC PI-CONJUGATED OPEN-SHELL SYSTEMS - A COMPARISON OF 1,1-ETHENEDIYL AND CARBONYL LINKED BIS(ARYLNITRENES)

Linkage of two phenylnitrene electron-spin-bearing units by exchange coupling linker groups leads to model open-shell pi-conjugated systems of the general structure:N-Ph-X-Ph-N: of various possible connectivity types. Use of variable-temperature electron spin resonance (ESR) spectroscopy of the dinitrenes with cross-conjugating linker groups -X- allowed study of the ground-state spin multiplicities of open-shell systems that are related in a connectivity sense to the theoretically important diradicals trimethylenemethane (-X- = >C=CH2, 4,4'-connectivity), oxyallyl (-X- = >C=O, 4,4'-connectivity), and 1,1,2,3,3-pentamethylenepropane (-X- = >C=CH2 or >C=O, 3,3'-connectivities). For 1,1-bis(4-nitrenophenyl)ethene, a quintet ground state was found with zero-field splitting parameters \D/hc\ = 0.151 cm-1 and \E/hc\ = 0.0029 cm-1. For the connectivity isomer (E)-3,4'-stilbenedinitrene, a quintet ground state with zfs parameters \D/hc\ = 0.146 cm-1, \E/hc\ = 0.0024 cm-1 was found. For 4,4'-dinitrenobenzophenone, a quintet ground state with zfs parameters \D/hc\ = 0.156 cm-1, \E/hc\ = 0.0046 cm-1, was found. For 1,1-bis(3-nitrenophenyl)ethene, a quintet state ESR spectrum was observed, with temperature-dependent intensity behavior that could be fit to a singlet < triplet < quintet states equilibrated model with a singlet ground-state, triplet at -28 cal/mol (-117 J/mol, a negative energy here signifying a low-spin singlet ground state), and quintet at -84 cal/mol (-351 J/mol). For 3,3'-benzophenonedinitrene, a quintet state spectrum was also observed, with temperature-dependent intensity behavior that could be fit by the same model with an ordering of states as follows: singlet ground state, triplet at -16 cal/mol (-67 J/mol), and quintet at -48 cal/mol (-201 J/mol). Overall, the results support the idea that minor heteroatom substitution (such as replacement of a 1,1-ethenediyl unit with a carbonyl unit) need not reverse qualitative connectivity-based exchange coupling effects. The dinitrenes that are disjoint are experimentally found to have low-spin ground states as predicted by the Borden-Davidson model (Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587), but in qualitative disagreement with some simple pi-connectivity models (e.g.: Ovchinnikov, A. A. Theor. Chim. Acta 1978, 47, 297).