Unusual reactivity of a nickel N-heterocyclic carbene complex:: tert-butyl group cleavage and silicone grease activation
Unusual reactivity of a nickel N-heterocyclic carbene complex:: tert-butyl group cleavage and silicone grease activation
复制标题
DOI：
10.1002/anie.200460955
复制
发表时间：
2004-01-01
影响因子：
16.6
通讯作者：
Lewis, AKD
中科院分区：
文献类型：
--
作者：
Caddick, S;Cloke, FGN;Lewis, AKD
在过去的十年里，人们对N-杂环卡宾配体(NHCs)非常感兴趣，特别是作为钯[1]和Ru介导的[2]催化一系列有机转化的膦的替代品。最近，也描述了NiII/咪唑盐/碱或Ni0/NHC组合用于催化胺化反应，其中活性催化剂被认为是Ni0-NHC络合物。[3]到目前为止，我们在这一领域的兴趣主要集中在使用分离的双(NHC)钯(0)络合物进行催化胺化[4]和机理研究；在后一种情况下，我们最近报道了一项关于4-氯甲苯与双(1，3-二叔丁基咪唑-2-亚甲基)钯的氧化加成反应的详细机理研究。[5]因此，我们有兴趣将这种研究扩展到镍类似物，在此，我们报道了与尝试常规合成双(1，3-二叔丁基咪唑-2-亚甲基)镍相关的非常不寻常的反应活性。我们已经报道了通过金属气相合成(MVS)制备[Ni{C(NT-BUCH)2}2](1)，尽管当时还没有对该化合物进行结构表征。[6]我们现在确定了1的结构(图1)。Ardeungo等人。已报道了由[Ni(1，5-cod)2](COD值=环辛二烯)与游离基反应合成的1，3-二甲基咪唑-2-亚甲基类似物的分子结构。[8]与前者相比，这两种结构都在镍中心附近呈线形几何构型，尽管镍-卡宾键长略长于1(1.874(2)比1.827(6))。然而，它们的不同之处在于，1中配体的平面从共面扭曲758，而在Arduengo络合物中的扭曲角为538。这类化合物中的键本质上主要是σ键，π键很少，因此扭曲角几乎肯定是由固态中微妙的空间相互作用和/或晶体堆积力决定的。在溶液中，不对称取代的双(NHC)-第10基团M0的核磁共振波谱研究表明，在核磁共振时间刻度上围绕金属-卡宾键快速旋转到±908。[10]由于我们希望开发一种传统的溶液相路线到1，[Ni(1，5-Cod)2]，在四氢呋喃中用过量的1，3-二叔丁基咪唑-2-亚甲基处理，在Schlenk管中用润滑塞密封。经过两周的反应时间，正己烷的加工和结晶得到了低产率的紫色晶体[{Ni[C(NtBuCH)2][O(Me2SiOSiMe2)-μ-O]}2](2；图2)。结构显示了一个关于镍的扭曲的方形平面构型的双核镍(II)络合物，其中两个金属中心由来自硅油的二硅氧烷单元连接(见方案1)。中心Ni2(μ-O)2核心围绕O-O轴被218个平面外折叠，非对称μ-O桥的键长为Ni-O(1)1.958(4)，Ni-O(1‘)1.896(4)，与μ-O单元中[{Ni[C(SiMe3)(2-SiMe2C5H4N)(SiMe2O)]}2](1.918(4)，1.891(45))中的键长相当；[11]2(Ni-O(3)1.839(4))的非桥氧镍键长与[Ni(η2-OCHMeCH2NMe2)2](1.829(3))非常接近。[12]2中的镍-卡宾Ni(1)-C(1)键长(1.883(6))与1的相同(在估计标准差范围内)，
During the past decade there has been considerable interest in N-heterocyclic carbene ligands (NHCs), particularly as alternative to phosphanes for both palladium-[1] and ruthenium-mediated [2] catalysis of a range of organic transformations. Recently, the use of NiII/imidazolium salt/base or Ni0/NHC combinations for catalytic aminations, in which the active catalyst is believed to be a Ni0–NHC complex, has also been described.[3] Our interest in this area to date has focused on the use of isolated, two-coordinate bis (NHC) palladium (0) complexes for catalytic amination [4] and mechanistic studies; in the latter context we have recently reported a detailed mechanistic study on the oxidative addition of 4-chlorotoluene to bis (1, 3-bis-tert-butylimidazol-2-ylidene) palladium.[5] Thus we were interested in extending such studies to the nickel analogue, and herein we report the highly unusual reactivity associated with the attempted conventional synthesis of bis (1, 3-bis-tert-butylimidazol-2-ylidene) nickel. We have already reported the preparation of [Ni {C (Nt-BuCH) 2} 2](1) by metal vapor synthesis (MVS), although the compound was not structurally characterized at that time.[6] We have now determined the structure of 1 (Figure 1). Ardeungo et al. have reported the molecular structure of an analogue of 1 derived from 1, 3-bis-mesitylimidazol-2-ylidene, synthesized by the treatment of [Ni (1, 5-cod) 2](COD= cyclooctadiene) with the free NHC.[8] Comparison of 1 with the former shows that both structures display linear geometry around the nickel center, although the nickel–carbene bond length is slightly longer in 1 (1.874 (2) for 1, compared to 1.827 (6)). However, they differ in that the planes of the ligands in 1 are twisted 758 from coplanarity whereas in the Arduengo complex the twist angle is 538. The bonding in such compounds is predominantly σ in nature, with minimal π bonding,[9] so the twist angle is almost certainly governed by subtle steric interactions and/or crystal packing forces in the solid state. In solution, NMR spectroscopy studies on asymmetrically substituted bis (NHC)-Group 10 M0 indicate rapid rotation about the metal–carbene bond on the NMR timescale down to À908.[10] Since we wished to develop a conventional, solutionphase route to 1,[Ni (1, 5-cod) 2] was treated with an excess of 1, 3-bis-tert-butylimidazol-2-ylidene in THF in a Schlenk tube sealed with a greased stopper. After a two-week reaction time, work-up and crystallization from hexane afforded purple crystals of [{Ni [C (NtBuCH) 2][O (Me2SiOSiMe2)-μ-O]} 2](2; Figure 2) in low yield.The structure reveals a dinuclear nickel (ii) complex, with distorted square-planar geometry about nickel, in which the two metal centers are bridged by disiloxane units derived from silicone grease (see Scheme 1). The central Ni2 (μ-O) 2 core is folded by 218 out-of-plane about the O–O axis, with the bond lengths of the asymmetric μ-O bridges, Ni–O (1) 1.958 (4), Ni–O (1’) 1.896 (4), comparable to those found in the μ-O unit in [{Ni [C (SiMe3)(2-SiMe2C5H4N)(SiMe2O)]} 2](1.918 (4), 1.891 (45));[11] the non-bridging oxygen–nickel bond length in 2 (Ni–O (3) 1.839 (4)) is very close to that in [Ni (η2-OCHMeCH2NMe2) 2](1.829 (3)).[12] The nickel–carbene Ni (1)–C (1) bond length (1.883 (6)) in 2 is identical to that in 1 (within estimated standard deviations (esds)),