Ligand- and Metal-Based Reactivity of a Neutral Ruthenium Diolefin Diazadiene Complex; The Innocent, the Guilty, and the Suspicious.

Coordination of the diazadiene diolefin ligand (trop(2)dad) to ruthenium leads to various complexes of composition [Ru(trop(2)dad)(L)]. DFT studies indicate that the closed-shell singlet (CSS), open-shell singlet (OSS), and triplet electronic structures of this species are close in energy, with the OSS spin configuration being the lowest in energy for all tested functionals. Singlet-state CASSCF calculations revealed a significant multireference character for these complexes. The closed-shell singlet wavefunction dominates, but these complexes have a significant (approximate to 8-16%) open-shell singlet [d(7)-Ru-I(L)(trop(2)dad(center dot-))] contribution mixed into the ground state. In agreement with their ambivalent electronic structure, these complexes reveal both metal- and ligand-centered reactivity. Most notable are the reactions with AdN(3), diazomethane, and a phosphaalkyne leading to scission of the C-C bond of the diazadiene (dad) moiety of the trop(2)dad ligand, resulting in net (formal) nitrene, carbene, or P C insertion in the dad C-C bond, respectively. Supporting DFT studies revealed that several of the ligand-based reactions proceed via low-barrier radical-type pathways, involving the dad(center dot-) ligand radical character of the OSS or triplet species.