A Dinuclear Ni(I) System Having a Diradical Ni₂N₂Diamond Core Resting State: Synthetic, Structural, Spectroscopic Elucidation, and Reductive Bond Splitting Reactions

Abstract

One-electron reduction of the square-planar nickel precursor (PNP)NiCl ( 1) (PNP (-) = N[2-P(CHMe 2) 2-4-methylphenyl] 2) with KC 8 effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(mu 2-PNP)] 2 ( 2), containing a Ni 2N 2 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 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 N 2CPh 2, which traps the Ni(I) monomer in the form of a T-shaped species, Ni(PNPNNCPh 2), 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, -OCH 3, -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).

Terminal NiII−OH/−OH2 complexes in trigonal bipyramidal geometries derived from H2O

“…Using steric effects in this manner is also effective in synthetic systems, as most synthetic terminal nickel hydroxido and aqua complexes use sterically encumbered ligand frameworks around the metal center to prevent bridging. Cámpora prepared the earliest examples of monomeric square planar terminal Ni II –OH moieties [16,17], and a number of similar 4-coordinate terminal Ni II –OH complexes have been developed for catalysis [18–21]. To the best of our knowledge, there are only two examples of crystallographically characterized monometallic 5-coordinate Ni II –OH complexes.…”

Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

“…One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,20 and biphenyldiyl 15,21,22 ligands, or in the form of unsupported Ni-Ni bonds. One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,…”

“…[23][24][25] Antiferromagnetic coupling through a ligand and direct Ni-Ni bonding results in diamagnetic behaviour in most instances, albeit triplet ground states have been proposed for the Ni 2 (μ-Br) 2 core bound to the neutral form of a redox active 1,8-naphthyridine diimine ligand. 2,5,9 The reversible formation of reactive Ni(I) species appears to represent one of the two major modes of reactivity displayed by binuclear Ni(I) complexes. In the biradical state the Ni-Ni interaction is non-bonding and dissociation is a favourable and facile process.…”

Selective hydration of nitriles to amides catalysed by PCP pincer supported nickel(ii) complexes

“…In addition to ligand resonances, isolated hydroxo compounds 2 R exhibit characteristic signals at 4.73-4.84 ppm for the benzylic proton on the PCP ligand and triplets at -2.94 to -3.22 ppm ( 3 J HP ≈ 5-6 Hz) for the O-H protons in the 1 H NMR spectra. It is also conceivable that direct dissociation of the OH ligand occurs; the Ni-O bond distance of 1.978(2) Å found for 2 iPr 11 is longer than any reported in related Ni-OH compounds [12][13][14][15][16][17] and the C sp3 anchoring group of the PCP pincer ligand should have a strong trans influence. However, when 17 O enriched 2 iPr was treated with H 2 O, rapid depletion of label from the Ni-17 OH moiety was observed (Fig.…”