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.