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首页> 外文期刊>Chemistry: A European journal >Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for Alkane Hydroxylation by Using m-CPBA as Oxidant: A Combined Experimental and Computational Study**
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Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for Alkane Hydroxylation by Using m-CPBA as Oxidant: A Combined Experimental and Computational Study**

机译：以m-CPBA为氧化剂的五齿N5配体镍（II）配合物作为烷烃羟化反应的催化剂：结合实验和计算研究**

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摘要

A new family of nickel(II) complexes of the type [Ni(L)(CH_3CN)](BPh_4)_2, where L=N-methyl-N,N',N'-tris(pyrid-2- ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N',N'-tris(pyrid- 2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N'-bis(pyrid- 2-ylmethyl)-N'-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(quinolin-2-ylmethyl)- ethylenediamine (L4, 4), and N-methyl-N,N'-bis(pyrid- 2-ylmethyl)-N'-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L3)- (CH_3CN)](BPh_4)_2 reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni~(II)/Ni~(III) redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K= ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)- (CH_3CN)](BPh_4)_2 by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH_3CN)](BPh_4)_2 or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)- (CH_3CN)](BPh_4)_2 and the quinolylmethyl arms ([Ni(L4)- (CH_3CN)](BPh_4)_2, both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni~(II)-O~-] intermediate (ts1_(hs), ts2_(doublet)), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni~(II)(L5)(OOCOC_6H_4Cl)]~+ (5a). Adamantane is oxidized to 1- adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2- propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the NiII leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.
机译：[Ni（L）（CH_3CN）]（BPh_4）_2类型的新的镍（II）配合物家族，其中L = N-甲基-N，N'，N'-三（吡啶-2-基甲基）-乙二胺（L1，1），N-苄基-N，N'，N'-三（吡啶-2-基-甲基）-乙二胺（L2，2），N-甲基-N，N'-双（吡啶- 2-基甲基）-N'-（6-甲基-吡啶-2-基-甲基）-乙二胺（L3，3），N-甲基-N，N'-双（吡啶-2-基甲基）-N'- （喹啉-2-基甲基）-乙二胺（L4，4）和N-甲基-N，N'-双（吡啶-2-基甲基）-N'-咪唑-2-基甲基）-乙二胺（L5，5）已通过元素分析，质谱，UV / Vis光谱和电化学方法进行了分离和表征。 [Ni（L3）-（CH_3CN）]（BPh_4）_2的单晶X射线结构表明，镍（II）中心位于由五齿配体的所有五个氮原子构成的扭曲的八面体配位几何结构中和乙腈分子。在二氯甲烷/乙腈溶剂混合物中，所有络合物在八面体配位几何结构的可见区域均显示出配体场带。它们表现出对应于Ni〜（II）/ Ni〜（III）氧化还原对的单电子氧化，其电势取决于配体供体的功能性。新的络合物在存在作为氧化剂的m-CPBA的情况下催化环己烷的氧化，其周转数高达530，且具有良好的醇选择性（A / K，7.1-10.6，A =醇，K =酮）。用[Ni（L5）（CH_3CN）]（BPh_4）_2中的强σ键但弱π键的咪唑基甲基臂代替[Ni（L1）-（CH_3CN）] [BPh_4）_2中的吡啶基甲基臂后，在空间上需要6-甲基吡啶基甲基（[Ni（L3）-（CH_3CN）]（BPh_4）_2和喹啉基甲基臂[[Ni（L4）-（CH_3CN）]（BPh_4）_2，催化活性和选择性均降低。配合物1和5对环己烷氧化的研究表明，高自旋[（N5）Ni〜（II）-O〜-]中间体（ts1_（hs），ts2_（doublet））的两种自旋态反应性具有低自旋态和高自旋态能量，配合物5的较低催化活性主要是由于形成了热力学上较难获得的[Ni〜（II）（L5）的m-CPBA配位前体。 （OOCOC_6H_4Cl）] +（5a）。将金刚烷氧化为1-金刚烷醇，2-金刚烷醇和2-金刚烷酮（3°/ 2°，10.6-11.5），并将枯烯选择性氧化为2-苯基-2-丙醇。空间阻碍素的引入NiII周围的g吡啶基甲基和喹啉基甲基供体配体导致金刚烷氧化的3°/ 2°键选择性高，这与配合物的较低环己烷氧化活性相反。

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