Reduction Pathway of End-On Terminally Coordinated Dinitrogen. IV. Geometric, Electronic, and Vibrational Structure of a W(IV) Dialkylhydrazido Complex and Its Two-Electron-Reduced Derivative Undergoing N−N Cleavage upon Protonation

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Abstract
The molybdenum and tungsten dialkylhydrazido complexes [M(dppe)(2)(NNC5H10)](2+) (M = Mo, W; compounds A(Mo) and A(W)) and their two-electron-reduced counterparts [M(dppe)(2)(NNC5H10)] (compounds B-Mo and B-W) are characterized structurally and spectroscopically. The crystal structure of B-W indicates a geometry between square pyramidal and trigonal bipyramidal with the NNC5H10 group in the apical position and in the trigonal plane of the complex, respectively. Temperature-dependent P-31 NMR spectra of B-Mo show that this geometry is present in solution as well. At room temperature, rapid Berry pseudorotation between the "axial" and "equatorial" ligand positions gives rise to a singlet in the P-31 NMR spectrum. This exchange process is slowed at low temperature, leading to a doublet. The N-N distance of B-W is 1.388 angstrom, and the W-N distance is 1.781 angstrom. Infrared and Raman spectroscopy applied to A(W), B-W, and their N-15 isotopomers reveals extensive mixing between the N-N and W-N vibrations of the metal-N-N core with the modes of the piperidine ring. The N-N force constant of A(W) is determined to be 6.95 mdyn/A, which is close to the values of the Mo and W NNH2 complexes. In B-W, the N-N force constant decreases to 6.4 mdyn/angstrom, which is between the values found for the Mo/W NNH3 and NNH2 complexes, This allows us to attribute N-N double bond character to A(W) and intermediate character between the double and single bonds for the N-N bond of B-W. These findings are supported by DFT calculations. More importantly, the HOMO of B-W corresponds to a linear combination of the metal d(sigma) orbital with a ligand orbital having N-N sigma* character, inducing a weakening of the N-N bond. This contributes to the cleavage of the N-N bond taking place upon protonation of B-W at the N-beta atom of the NNC5H10 group.

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