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A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water.

ABSTRACT: The nickel(ii) chemistry with the tridentate ligands bis[(N'-R-ureido)-N-ethyl]-N-methylamine (H(4)(R), R = isopropyl, tert-butyl) is described. The Ni(ii)-OH complexes, [Ni(II)H(2)(R)(OH)](-) were generated using water as the source of the hydroxo ligand. These complexes are pseudo-square planar, in which the primary coordination sphere contains three nitrogen donors from [H(2)(R)](2-) and the oxygen atom from the hydroxide (Ni-O(H), 1.857(1) A). The Ni(ii)-OH unit also is involved in two intramolecular hydrogen bonds between the urea groups of the [H(2)(R)](2-) and the hydroxo oxygen atom. Attempts to deprotonate the Ni(ii)-OH unit to produce Ni(ii)-oxo complexes were unsuccessful. A variety of bases with pK(a) of less than 15 (in DMSO) were unable to deprotonate the hydroxo ligand. Treating the Ni(ii)-OH complexes with KOBu(t) (pK(a) approximately 29) afforded the ligand substitution product, [Ni(II)H(2)(R)(OBu(t))](-). Ni(ii)-siloxide complexes were isolated when the [Ni(II)H(2)(R)(OH)](-) complexes were allowed to react with K[N(TMS)(2)].

SUBMITTER: Powell-Jia D

PROVIDER: S-EPMC3863642 | biostudies-literature | 2009 Apr

REPOSITORIES: biostudies-literature

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A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water.

Powell-Jia Darla D Ziller Joseph W JW Dipasquale Antonio G AG Rheingold Arnold L AL Borovik A S AS

Dalton transactions (Cambridge, England : 2003) 20090227 16

The nickel(ii) chemistry with the tridentate ligands bis[(N'-R-ureido)-N-ethyl]-N-methylamine (H(4)(R), R = isopropyl, tert-butyl) is described. The Ni(ii)-OH complexes, [Ni(II)H(2)(R)(OH)](-) were generated using water as the source of the hydroxo ligand. These complexes are pseudo-square planar, in which the primary coordination sphere contains three nitrogen donors from [H(2)(R)](2-) and the oxygen atom from the hydroxide (Ni-O(H), 1.857(1) A). The Ni(ii)-OH unit also is involved in two intra ...[more]

PMID: 19352526

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Project description:The 15e square-planar complexes [Co(PCPMe-iPr)Cl] (2a) and [Co(PCP-tBu)Cl] (2b), respectively, react readily with NaBH4 to afford complexes [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Co(PCP-tBu)(η2-BH4)] (4b) in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η2-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μB consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCPMe-iPr)(η2-BH4)] (5) via two different synthetic routes. One utilizes [Ni(PCPMe-iPr)Cl] (3) and NaBH4, the second one makes use of the hydride complex [Ni(PCPMe-iPr)H] (6) and BH3·THF. In both cases, 5 is obtained in high yields. In contrast to 4a and 4b, the borohydride ligand is asymmetrically bound to the nickel center but still in an η2-mode. [Ni(PCPMe-iPr)(η2-BH4)] (5) loses readily BH3 at elevated temperatures in the presence of NEt3 to form 6. Complexes 5 and 6 are both diamagnetic and were characterized by a combination of 1H, 13C{1H}, and 31P{1H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes 5 and 6 react with CO2 to give the formate complex [Ni(PCPMe-iPr)(OC(C=O)H] (7). The extrusion of BH3 from [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) with the aid of NH3 to yield the respective hydride complexes [Co(PCPMe-iPr)H] and [Ni(PCPMe-iPr)H] (6) and BH3NH3 was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co(PCPMe-iPr)(η2-BH4)] (4a) and [Ni(PCPMe-iPr)(η2-BH4)] (5) were established by DFT computations.

Project description:Two new monomeric Cu(II) alkoxide complexes were prepared and fully characterized as models for intermediates in copper/radical mediated alcohol oxidation catalysis: Tp(tBuR)Cu(II)OCH2CF3 with Tp(tBu) = hydro-tris(3-tert-butyl-pyrazol-1-yl)borate 1 or Tp(tBuMe) = hydro-tris(3-tert-butyl-5-methyl-pyrazol-1-yl)borate 2. These complexes were made as models for potential intermediates in enzymatic and synthetic catalytic cycles for alcohol oxidation. However, the alkoxide ligands are not readily oxidized by loss of H; instead, these complexes were found to be hydrogen atom acceptors. They oxidize the hydroxylamine TEMPOH, 2,4,6-tri-t-butylphenol, and 1,4-cyclohexadiene to the nitroxyl radical, phenoxyl radical, and benzene, with formation of HOCH2CF3 (TFE) and the Cu(I) complexes Tp(tBuR)Cu(I)-MeCN in dichloromethane/1% MeCN or 1/2 [Tp(tBuR)Cu(I)]2 in toluene. On the basis of thermodynamics and kinetics arguments, these reactions likely proceed through concerted proton-electron transfer mechanisms. Thermochemical analyses give lower limits for the "effective bond dissociation free energies (BDFE)" of the O-H bonds in 1/2[Tp(tBuR)Cu(I)]2 + TFE and upper limits for the free energies associated with alkoxide oxidations via hydrogen atom transfer (effective alkoxide ?-C-H BDFEs). These values are summations of the free energies of multiple chemical steps, which include the energetically favorable formation of 1/2[Tp(tBuR)Cu(I)]2. The effective alkoxide ?-C-H bonds are very weak, BDFE ? 38 ± 4 kcal mol(-1) for 1 and ?44 ± 5 kcal mol(-1) for 2 (gas-phase estimates), because C-H homolysis is thermodynamically coupled to one electron transfer to Cu(II) as well as the favorable formation of the 1/2[Tp(tBuR)Cu(I)]2 dimer. Treating 1 with the H atom acceptor (t)Bu3ArO(•) did not result in the expected alkoxide oxidation to an aldehyde, but rather net 2,2,2-trifluoroethoxyl radical transfer occurred to generate an unusual 2-substituted dienone-ether product. Treating 2 with (t)Bu3ArO(•) gives no reaction, despite evidence that overall ligand oxidation and formation of 1/2[Tp(tBuMe)Cu(I)]2 is significantly exoergic. The origin of this lack of reactivity may be due to insufficient weakening of the alcohol ?-C-H bond upon complexation to copper.

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A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water.

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A structure and reactivity analysis of monomeric Ni(II)-hydroxo complexes prepared from water.

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