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

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posted on 17.11.2008, 00:00 by Debashis Adhikari, Susanne Mossin, Falguni Basuli, Benjamin R. Dible, Mircea Chipara, Hongjun Fan, John C. Huffman, Karsten Meyer, Daniel J. Mindiola
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(μ2-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(I) 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. Low-temperature solid-state X-band EPR data at 4 K reveal rhombic values gz = 1.980(4), gx = 2. 380(4), and gy = 2.225(4), as well as a forbidden signal at g = 4.24 for the ΔMS = 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(I) 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 H2, compound 2 can also engage in homolytic X−X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X = S or Se).