Unusual Electronic Effects Imparted by Bridging Dinitrogen: an Experimental and Theoretical Investigation

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC2Cr(μ-N2)CrC2R]n+ (R = Ph−, iPr3Si−; n = 0, 1, 2) backbone: [(dmpe)4Cr2(C2Ph)2(μ-N2)] (1), [(dmpe)4Cr2(C2SiiPr3)2(μ-N2)] (2), [(dmpe)4Cr2(C2SiiPr3)2(μ-N2)]BArF4 (3), and [(dmpe)4Cr2(C2SiiPr3)2(μ-N2)](BArF4)2 (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp2Co]+ and [Cp*2Fe]+, respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr−N−N−Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ orbitals. Significant N−N and Cr−N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hund’s rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.

本研究针对一类含[RC₂Cr(μ-N₂)CrC₂R]ⁿ⁺（其中R为苯负离子(Ph⁻)、三异丙基硅基负离子(iPr₃Si⁻)；n取值为0、1、2）骨架的、与氧化还原相关的二氮桥联乙炔铬配合物家族，详细介绍其制备方法、结构与磁学表征手段，以及电子结构计算工作，涉及的配合物依次为[(dmpe)₄Cr₂(C₂Ph)₂(μ-N₂)]（1）、[(dmpe)₄Cr₂(C₂SiiPr₃)₂(μ-N₂)]（2）、[(dmpe)₄Cr₂(C₂SiiPr₃)₂(μ-N₂)]BArF₄（3）与[(dmpe)₄Cr₂(C₂SiiPr₃)₂(μ-N₂)](BArF₄)₂（4），其中dmpe为1,2-双(二甲基膦基)乙烷，BArF₄为四(3,5-二三氟甲基苯基)硼酸盐。配合物3和4分别通过以二茂钴阳离子([Cp₂Co]⁺)和五甲基二茂铁阳离子([Cp*₂Fe]⁺)对配合物2进行化学氧化制得。X射线结构分析结果表明，铬的形式氧化态改变并未显著改变Cr−N−N−Cr骨架结构。针对配合物2和4采集的磁学数据分别对应高自旋三重态和五重态基态。混合价态配合物3表现出温度依赖性磁学行为，符合四重态⇌二重态双中心自旋平衡的特征。对完整的配合物2和4开展的电子结构计算（采用B3LYP泛函）结果显示，高自旋态源于单占据正交π*轨道与可变占据dδ轨道的耦合作用。显著的N−N与Cr−N π键作用锁定了π轨道集的电子占据情况，进而导致dδ轨道的电子占据数可变。与之相反，B3LYP杂化密度泛函模型无法很好地描述混合价态配合物3。针对配合物3的简化模型开展的[9,11]多参考二级受限活性空间组态相互作用(CAS-SORCI)研究，复现了实验观测到的S=1/2基态违反洪特规则的现象，并计算得到最低四重态较二重态基态高1.45 kcal/mol。