A Dinuclear Ni(I) System Having a Diradical Ni2N2 Diamond Core Resting State: Synthetic, Structural, Spectroscopic Elucidation, and Reductive Bond Splitting Reactions

One-electron reduction of the square-planar nickel precursor (PNP)NiCl (1) (PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)) with KC8 effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(mu(2)-PNP)](2) (2), containing a Ni2N2 core structure, as inferred by its solid-state X-ray structure. Solution magnetization measurements are consistent with a paramagnetic Ni(l) system likely undergoing a monomer <-> dimer equilibrium. The room-temperature and 4 K solid-state X-band electron paramagnetic resonance (EPR) spectra display anisotropic signals. Lowtemperature solid-state X-band EPR data at 4 K reveal rhombic values g(z) = 1.980(4), g(x) = 2. 380(4), and g(y) = 2.225(4), as well as a forbidden signal at g = 4.24 for the Delta M-S = 2 half field transition, in accord with 2 having two weakly interacting metal centers. Utilizing an S = 1 model, full spin Hamiltonian simulation of the low-temperature EPR spectrum on the solid sample was achieved by applying a nonzero zero-field-splitting parameter (D = 0.001 cm(-1)), which is consistent with an S = 0 ground state with a very closely lying S = 1 state. Solid-state magnetization data also corroborate well with our solid-state EPR data and reveal weak antiferromagnetic behavior (J = -1.52(5) cm(-1)) over a 2-300 K temperature range at a field of 1 Tesla. Evidence for 2 being a masked "(PNP)Ni" scaffold originates from its reaction with N2CPh2, which traps the Ni(l) monomer in the form of a T-shaped species, Ni(PNP=NNCPh2), a system that has been structurally characterized. The radical nature of complex 2, or its monomer component, is well manifested through the plethora of cooperative H-X-type bond cleavage reactions, providing the nickel(II) hydride (PNP)NiH and the corresponding rare functionalities -OH, -OCH3, -PHPh, and -B(catechol) integrated into the (PNP)Ni moiety in equal molar amounts. In addition to splitting H-2, compound 2 can also engage in homolytic X-X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X = S or Se).