NO Reactions Inside Crystals

Reactions of chemisorbed reagents inside the crystalline molecular solid state are rare but offer unexploited methods for selective solvent-free chemical synthesis. Here we show that the greenhouse gas precursor, nitric oxide (NO) is chemisorbed by crystals of the hexafluorophosphate salts of complexes containing dicobalt sites. On NO sorption a cascade of reactions results in the in-crystal synthesis of nitrite and other gaseous NO_x. Recrystallization enabled structural elucidation of the mixed valent {[(bpbp)Co₂(μ-(η¹-O : η¹-N)-ONO)]₂(bdc)}⁴⁺ (bpbp=2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato, bdc=1,4-benzenedicarboxylato) cation. Overlapping signals in the solid-state EPR spectra confirm the Co^IICo^III oxidation state and the presence of NO₂ trapped inside the unrecrystallised solid products (br. g=4, triplet g=2 (340 mT), A(N)=73 MHz), despite three cycles of vacuum and N₂ flushing. Consistently, ν_N−O bands appear in the Raman and IR spectra that are due to the coordinated nitrate and the trapped NO₂ that were synthesized in-crystal. The latter is expelled by heating the solid to 160 °C or by recrystallization. Dimetallic cooperativity is proposed for the NO transformations in these rare examples of selective, chemisorptive substrate reactions in the solid-state.