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

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• 作者：Wesley A. Hoffert ; Anthony K. Rapp ; Matthew P. Shores
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：October 18, 2010
• 年：2010
• 卷：49
• 期：20
• 页码：9497-9507
• 全文大小：1049K
• 年卷期：v.49,no.20(October 18, 2010)
• ISSN：1520-510X

文摘

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)]BAr^F₄ (3), and [(dmpe)₄Cr₂(C₂SiⁱPr₃)₂(μ-N₂)](BAr^F₄)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.