A multitechnique approach to spin-flips for Cp₂Cr(II) chemistry in confined state

Paramagnetic solid-state NMR, extended X-ray absorption fine structure (EXAFS), and Raman spectroscopies, along with detailed quantum mechanical calculations performed with different density functional theory (DFT) functionals, are successfully applied to investigate the magnetic, structural, and vibrational properties of molecularly isolated chromocene (Cp₂Cr, where Cp = C₅H₅) and of its Cp₂Cr(CO) adduct. Paramagnetic solid-state NMR unequivocally demonstrates that a spin flip occurs by coming from the paramagnetic Cp₂Cr (triplet state) to the diamagnetic Cp₂Cr(CO) adduct (singlet state), thus clarifying the theoretical dilemma of the disagreement among different functionals in predicting the most stable spin state. EXAFS and Raman spectroscopies are able to experimentally discriminate between singlet and triplet states, because a different spin state corresponds to a different geometry of the molecule, and therefore to different vibrational features. The here reported multitechnique approach could have great relevance in establishing the occurrence of spin flip in the chemical reactivity of transition metal complexes in both homo- and heterogeneous catalysis.