Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes
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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(CO)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.
15电子平面正方形配合物（square-planar complexes）[Co(PCPMe-iPr)Cl]（2a）与[Co(PCP-tBu)Cl]（2b）可分别与硼氢化钠（NaBH4）快速反应，以较高收率得到配合物[Co(PCPMe-iPr)(η²-BH4)]（4a）与[Co(PCP-tBu)(η²-BH4)]（4b），该结果经红外光谱（IR spectroscopy）、X射线晶体衍射（X-ray crystallography）及元素分析（elemental analysis）验证。硼氢配体（borohydride ligand）以η²配位模式与钴中心对称结合。上述化合物均为顺磁性（paramagnetic）物质，有效磁矩（effective magnetic moments）分别为2.0(1)和2.1(1) μB，与对应1个未成对电子（unpaired electron）的d⁷低自旋体系（d7 low-spin system）相符。上述两类配合物均无法与二氧化碳（CO₂）反应生成甲酸盐配合物（formate complexes）。为进行结构与反应性对比，我们通过两条不同合成路径（synthetic routes）制备了类似的Ni(II)硼氢配合物[Ni(PCPMe-iPr)(η²-BH4)]（5）：其一以[Ni(PCPMe-iPr)Cl]（3）与硼氢化钠为原料，其二则使用氢化物配合物（hydride complex）[Ni(PCPMe-iPr)H]（6）与硼烷四氢呋喃络合物（BH3·THF）。两条路径均能以高收率得到产物5。与4a和4b不同，配合物5中的硼氢配体以η²模式与镍中心结合时呈不对称配位。在三乙胺（NEt3）存在下升温时，[Ni(PCPMe-iPr)(η²-BH4)]（5）可轻易脱除BH3，生成6。配合物5与6均为反磁性（diamagnetic）物质，通过¹H核磁共振谱（¹H NMR）、¹³C{¹H}核磁共振谱（¹³C{¹H} NMR）、³¹P{¹H}核磁共振谱（³¹P{¹H} NMR）、红外光谱及元素分析进行了表征，其晶体结构通过X射线晶体衍射确定。配合物5与6均可与CO₂反应生成甲酸盐配合物[Ni(PCPMe-iPr)(OC(=O)H)]（7）。我们通过密度泛函理论计算（DFT calculations）研究了NH₃辅助[Co(PCPMe-iPr)(η²-BH4)]（4a）与[Ni(PCPMe-iPr)(η²-BH4)]（5）脱除BH3，分别生成对应氢化物配合物[Co(PCPMe-iPr)H]与[Ni(PCPMe-iPr)H]（6）及BH3NH3的过程，结果显示镍氢化物的生成在热力学上更为有利，而钴(II)氢化物的生成则与实验结果一致，热力学上不利。通过密度泛函理论计算阐明了[Co(PCPMe-iPr)(η²-BH4)]（4a）与[Ni(PCPMe-iPr)(η²-BH4)]（5）中硼氢配体的电子结构（electronic structures）与成键模式。
