Side‐On Bridging Coordination of N2: Spectroscopic Characterization of the Planar Zr2N2 Core and Theoretical Investigation of Its Butterfly Distortion

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Abstract
The vibrational and electronic structure of the side‐on N2‐bridged Zr complex [{(P2N2)Zr}2(μ‐η2:η2‐N2)] (P2N2=PhP(CH2SiMe2NSiMe2CH2)2PPh) were analyzed. The vibrational characterization of the planar Zr2N2 core was based on resonance Raman and infrared spectroscopy. In the Raman spectrum, the NN stretching band is found at 775 cm−1 with an isotope shift of 22 cm−1. Due to its appearance in many overtones and combination modes, the metal–metal stretch is assigned to the peak at 295 cm−1. The two ungerade modes of the Zr2N2 core were identified in the infrared spectrum. Based on these four vibrations of the Zr2N2 unit, a quantum chemical assisted normal coordinate analysis (QCA‐NCA) was performed. The force constants for the NN and ZrN bonds were calculated to be 1.53 and 2.58 mdyn Å−1, respectively. The butterfly distortion of the Zr2N2 unit obtained in DFT geometry optimizations of planar side‐on N2‐bridged Zr complexes was analyzed in more detail. It was found that on bending of the Zr2N2 core, the lone pairs of the axial amide ligands are rotated by 90°. The bent Zr2N2 unit is 11 kcal mol−1 lower in energy than the planar core due to a more uniform distribution of electron density between the metal atoms and N2 and delocalization of electron density from the amide ligands to the Zr2N2 unit. The spectroscopic implications of this distortion are analyzed.

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