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 [RC<sub>2</sub>Cr(μ-N<sub>2</sub>)CrC<sub>2</sub>R]<sup><i>n</i>+</sup> (R = Ph−, <sup><i>i</i></sup>Pr<sub>3</sub>Si−; <i>n</i> = 0, 1, 2) backbone: [(dmpe)<sub>4</sub>Cr<sub>2</sub>(C<sub>2</sub>Ph)<sub>2</sub>(μ-N<sub>2</sub>)] (<b>1</b>), [(dmpe)<sub>4</sub>Cr<sub>2</sub>(C<sub>2</sub>Si<sup><i>i</i></sup>Pr<sub>3</sub>)<sub>2</sub>(μ-N<sub>2</sub>)] (<b>2</b>), [(dmpe)<sub>4</sub>Cr<sub>2</sub>(C<sub>2</sub>Si<sup><i>i</i></sup>Pr<sub>3</sub>)<sub>2</sub>(μ-N<sub>2</sub>)]BAr<sup>F</sup><sub>4</sub> (<b>3</b>), and [(dmpe)<sub>4</sub>Cr<sub>2</sub>(C<sub>2</sub>Si<sup><i>i</i></sup>Pr<sub>3</sub>)<sub>2</sub>(μ-N<sub>2</sub>)](BAr<sup>F</sup><sub>4</sub>)<sub>2</sub> (<b>4</b>). Compounds <b>3</b> and <b>4</b> are synthesized via chemical oxidation of <b>2</b> with [Cp<sub>2</sub>Co]<sup>+</sup> and [Cp*<sub>2</sub>Fe]<sup>+</sup>, 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 <b>2</b> and <b>4</b> are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex <b>3</b> exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in <b>2</b> and <b>4</b> 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 <b>3</b> is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of <b>3</b> reproduces the observed Hund’s rule violation for the <i>S</i> = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.