Dataset Information

Roles of Hydrogen Bonding in Proton Transfer to ?P,?N,?P-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes.

ABSTRACT: The nickel PNP pincer complex ( i PrPNP)NiPh ( i PrPNP = ?P,?N,?P-N(CH2CH2P i Pr2)2) was prepared by reacting ( i PrPNP)NiBr with PhMgCl or deprotonating [( i PrPNHP)NiPh]Y ( i PrPNHP = ?P,?N,?P-HN(CH2CH2P i Pr2)2; Y = Br, PF6) with KO t Bu. The byproducts of the PhMgCl reaction were identified as [( i PrPNHP)NiPh]Br and ( i PrPNP')NiPh ( i PrPNP' = ?P,?N,?P-N(CH=CHP i Pr2)(CH2CH2P i Pr2)). The methyl analog ( i PrPNP)NiMe was synthesized from the reaction of ( i PrPNP)NiBr with MeLi, although it was contaminated with ( i PrPNP')NiMe due to ligand oxidation. Protonation of ( i PrPNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( i PrPNHP)NiX]+ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl-, HO-, or MeO-). Protonation of the Ni-C bond was observed at room temperature with ( i PrPNP)NiMe, whereas at 70 °C with ( i PrPNP)NiPh, both resulting in [( i PrPNHP)NiCl]Cl as the final product. Protonation of ( i PrPNP)NiBr was complicated by site exchange between Br- and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( i PrPNP)NiPh and ( i PrPNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( i PrPNP')NiBr, ( i PrPNP)NiPh, ( i PrPNP')NiPh, ( i PrPNP)NiMe, [( i PrPNHP)NiPh]Y (Y = Br, PF6, BPh4), [( i PrPNHP)NiPh]2[NiCl4], [( i PrPNHP)NiMe]Y (Y = Cl, Br, BPh4), [( i PrPNHP)NiBr]Br, and [( i PrPNHP)NiCl]Cl, were characterized by X-ray crystallography.

SUBMITTER: Wellala NPN

PROVIDER: S-EPMC6045406 | biostudies-literature | 2018 May

REPOSITORIES: biostudies-literature

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Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes.

Wellala Nadeesha P N NPN Luebking John D JD Krause Jeanette A JA Guan Hairong H

ACS omega 20180508 5

The nickel PNP pincer complex ( i PrPNP)NiPh ( i PrPNP = κP,κN,κP-N(CH2CH2P i Pr2)2) was prepared by reacting ( i PrPNP)NiBr with PhMgCl or deprotonating [( i PrPNHP)NiPh]Y ( i PrPNHP = κP,κN,κP-HN(CH ...[more]

PMID: 30023906

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Project description:The dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( i Pr-PNHP)} fragment [ i Pr-PNHP = κ3-(CH2CH2P i Pr2)2NH] is reported. Neutral Ir( i Pr-PNHP)H3 or Ir( i Pr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4 solution. In contrast, addition of [NMeH3][BArF 4] (ArF = 3,5-(CF3)2C6H3) to Ir( i Pr-PNHP)H3 immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with M n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir( i Pr-PNHP)(H)2(H2)][BArF 4]; σ-amine-borane [Ir( i Pr-PNHP)(H)2(H3B·NMe3)][BArF 4]; and [Ir( i Pr-PNHP)(H)2(NMeH2)][BArF 4]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal-ligand cooperativity, and this is supported experimentally by N-Me substituted [Ir( i Pr-PNMeP)(H)2(H3B·NMe3)][BArF 4] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H3B·NMeH2 with the N-H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir( i Pr-NHP)(H)2(H2)][BArF 4], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH2 (via B-N bond cleavage), results in the formation of essentially inactive Ir( i Pr-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir( i Pr-PNHP)H3 results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH3]+/[H2B(NMeH2)2]+/catalyst in insoluble polyaminoborane. The role of [NMeH3]+/[H2B(NMeH2)]+ as chain control agents is also noted.

Project description:Interest in O2-dependent aliphatic carbon-carbon (C-C) bond cleavage reactions of first row divalent metal diketonate complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract information to develop applications in organic synthesis. A recent report of O2-dependent aliphatic C-C bond cleavage at ambient temperature in Ni(ii) diketonate complexes supported by a tridentate nitrogen donor ligand [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-Cl; MBBP = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) in the presence of NEt3 spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni(ii) diketonate complexes of the general formula [(TERPY)Ni(R2-1,3-diketonate)]ClO4 (1: R = CH3; 2: R = C(CH3)3; 3: R = Ph) was prepared under air and characterized using single crystal X-ray crystallography, elemental analysis, 1H NMR, ESI-MS, FTIR, and UV-vis. Analysis of the reaction mixtures in which these complexes were generated using 1H NMR and ESI-MS revealed the presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY)2Ni](ClO4)2 (4). Through selective crystallization 1-3 were isolated in analytically pure form. Analysis of reaction mixtures leading to the formation of the MBBP analogs [(MBBP)Ni(R2-1,3-diketonate)]X (X = ClO4: 5: R = CH3; 6: R = C(CH3)3; 7-ClO4: R = Ph; X = Cl: 7-Cl: R = Ph) using 1H NMR and ESI-MS revealed the presence of [(MBBP)2Ni](ClO4)2 (8). Analysis of aerobic acetonitrile solutions of analytically pure 1-3, 5 and 6 containing NEt3 and in some cases H2O using 1H NMR and UV-vis revealed evidence for the formation of additional bis-ligand complexes (4 and 8) but suggested no oxidative diketonate cleavage reactivity. Analysis of the organic products generated from 3, 7-ClO4 and 7-Cl revealed unaltered dibenzoylmethane. Our results therefore indicate that N3-ligated Ni(ii) complexes of unsubstituted diketonate ligands do not exhibit O2-dependent aliphatic C-C bond clevage at room temperature, including in the presence of NEt3 and/or H2O.

Dataset Information

Roles of Hydrogen Bonding in Proton Transfer to ?P,?N,?P-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes.

Publications

Roles of Hydrogen Bonding in Proton Transfer to κ<sup>P</sup>,κ<sup>N</sup>,κ<sup>P</sup>-N(CH<sub>2</sub>CH<sub>2</sub>P <i><sup>i</sup></i> Pr<sub>2</sub>)<sub>2</sub>-Ligated Nickel Pincer Complexes.

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