Dinuclear Nickel(II) Complexes of an Unsymmetric “End-Off” Compartmental Ligand:  Conversion of Urea into Cyanate at a Dinuclear Nickel Core

Abstract

A phenol-based “end-off” compartmental ligand, 2-[N,N-di(2-pyridylmethyl)aminomethyl]-6-{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenol (HL), having an iminic bidentate and an aminic tridentate chelating arms on the 2- and 6-positions of the phenolic ring, respectively, forms dinuclear nickel complexes [Ni2(L)(AcO)(NCS)2] (1), [Ni2(L)(AcO)2(MeOH)]PF6 (2), and [{Ni2(L)(OH)(MeOH)}2(CO3)](PF6)2 (3). Complex 1 crystallizes in the monoclinic space group P21/c, a = 14.165(5) Å, b=15.198(4) Å, c = 17.395(8) Å, β = 100.62(4)°, V = 3680(2) Å3, and Z = 4. The pair of Ni ions present are bridged by the phenolic oxygen of L- and an acetate group in syn − syn mode (Ni−Ni: 3.373(3) Å). An isothiocyanate nitrogen atom coordinates to each Ni providing an asymmetric dinuclear core with a mixed {5/6} coordination number set. Complex 2 crystallizes in the monoclinic space group P21/c, a = 13.505(5) Å, b = 12.028(4) Å, c = 22.774(9) Å, β = 103.78(3)°, V = 3592(2) Å3, and Z = 4. It has a dinuclear core bridged by the phenolic oxygen of L- and two acetate groups in syn − syn mode, providing a μ-phenoxo-bis(μ-carboxylato)dinickel(II) core (Ni−Ni: 3.396(6) Å). A methanol molecule coordinates to the Ni bound to the bidentate arm, forming a dinuclear core having a {6/6} coordination number set and an asymmetric donor atom environment. Complex 3 crystallizes in the orthorhombic space group Pbcn, a = 19.056(5) Å, b = 18.997(4) Å, c = 19.919(6) Å, α = β = γ = 90°, V = 7211(5) Å3, and Z = 8. In each dinuclear unit a pair of Ni ions are bridged by the phenolic oxygen of L- and a hydroxo oxygen (Ni−Ni: 3.087(2) Å). A carbonate further bridges two of the dinuclear units to present a composite dimer. The Ni bound to the bidentate arm attains six-coordinate geometry by further interaction with two oxygens of the bridging carbonato group. The Ni bound to the tridentate arm assumes six-coordinate geometry by further coordination of a methanol oxygen. Complexes 1−3 react with urea in ethanol to form the isocyanate complexes [Ni2(L)(AcO)(NCS)(NCO)] (1 ‘), [Ni2(L)(AcO)(NCO)(EtOH)]PF6 (2 ‘), and [{Ni2(L)(NCO)(EtOH)}2(CO3)](PF6)2 (3 ‘), respectively. Complex 3 ‘ crystallizes in the triclinic space group P1̄, a = 20.072(7) Å, b = 21.145(6) Å, c = 18.688(6) Å, α = 106.20(2)°, β = 90.01(3)°, γ = 88.73(3)°, V = 7614(4) Å3, and Z = 4. It has a dimeric structure very similar to that of 3, except for the replacement of the hydroxy bridge and the methanol ligand in 3 by isocyanate bridge and ethanol ligand, respectively, in 3‘.

The assembly of oligonuclear nickel(II) complexes from asymmetric compartmental ligands in the presence of alkali metal salts

“…The proligand 2-propyliminomethyl-4-methyl-6-{[methyl-bis-(2-pyridin-2-yl-methyl)-amino]-methyl}-phenol HL 2 , having available adjacent bidentate {NO} and tetradentate {N 3 O} donor sets, was prepared from 2-hydroxy-5-methyl-3-{[methyl-bis-(2-pyridin-2-yl-methyl)-amino]-methyl}-benzaldehyde [6] by reaction with n-propylamine according to the methodology of [7]. The complexes were prepared by the following general procedure.…”

“…The complexes were prepared by the following general procedure. Nickel acetate tetrahydrate (0.57 mmol) was added to a solution of HL 2 , (0.285 mmol) in methanol (10 cm 3 ), together with MX (0.285 mmol) (MX ¼ NaClO 4 , NaPF 6 , KPF 6 cool to r.t. giving crystals suitable for X-ray structural determination. 1 Solution of the structures showed that the compounds formed from the sodium salts were the heteropentanuclear nickel(II) complexes, [NaNi 4 (L 2 ) 2 (OAc) 6 6 ] 2 , 4, were formed from the reaction in the presence of potassium and ammonium salts.…”

Synthetic Models for the Active Sites of Nickel‐Containing Enzymes

“…This has been taken as indication that a cyanate intermediate may also be relevant for the enzyme, and a high-level computational study by Estiu and Merz on urea decomposition promoted by Lippard's phtalazine-based dinickel model complexes led the authors to propose that both the elimination and hydrolytic pathways may indeed compete in the urease active site [187]. The fi rst experimental evidence for the transformation of urea to cyanate at dinickel(II) sites came from the groups of Okawa [162] and Meyer [167] [167]. However, subsequent hydrolysis of the formed cyanate could not be observed with those systems, even when the more stable pyrazolate complex with triazacyclononane compartments was used that allows investigations in aqueous solution.…”

“…Compartmental ligands based on bridging alkoxide [155][156][157][158][159], phenolate [160][161][162][163][164], pyrazolate [165][166][167][168][169][170][171] or phtalazine units [172][173][174] are the most prominent ones. Selected examples are 69 [160] and 70 [158], in which the two nickel ions are additionally spanned by carboxylate bridges.…”

“…A series of dinuclear Ni II complexes derived from an unsymmetric phenolate ligand (such as 95) react with urea in refl uxing ethanol solution to give dinickel products 96 with N-bridging cyanate in low yields (17-23% after heating for 24 h)[162]. Heating of 92 gives rise to ammonia extrusion from the N,O-bridging ureate and formation of the cyanate-bridged complex 97…”