The Reduction Pathway of End-on Coordinated Dinitrogen. II. Electronic Structure and Reactivity of Mo/W−N₂, −NNH, and −NNH₂Complexes

Abstract

DFT calculations (B3LYP/LanL2DZ) of simplified models of [Mo(N(2))(2)(dppe)(2)] and the two protonated derivatives [MoF(NNH)(dppe)(2)] and [MoF(NNH(2))(dppe)(2)](+) (dppe = 1,2-bis(diphenylphosphino)ethane) provide quantitative insight into the reduction and protonation of dinitrogen bound end-on terminally to transition metals. This "asymmetric" reduction pathway is characterized by a stepwise increase of covalency and a concomitant charge donation from the metal center during each protonation reaction. The major part of metal-to-ligand charge transfer occurs after the first protonation leading to coordinated diazenido(-). In contrast, addition of the second proton is accompanied by a minor change of covalency leading to a NNH(2) species which is neutral and hence corresponds to coordinated isodiazene. UV-vis data of Mo and corresponding W complexes support the calculated energy level schemes. Moreover, calculated vibrational frequencies and force constants show good agreement with experimental values determined in Part I of this series (Lehnert, N.; Tuczek, F. Inorg. Chem. 1999, 38, 1659-1670). The implications of the electronic structure description obtained for the above model complexes with respect to the reduction and protonation of dinitrogen in small-molecule systems and nitrogenase are discussed.

Energetics and Mechanism of Ammonia Synthesis through the Chatt Cycle: Conditions for a Catalytic Mode and Comparison with the Schrock Cycle

“…This is the counterpiece of our earlier theoretical investigations of the Schrock cycle, applying the same assumptions and quantum-chemical methodology (functional, basis sets, solvent correction etc.). Moreover, the present DFT investigation complements our spectroscopic and theoretical studies on important intermediates of the Chatt cycle, that is, Mo and W bis(dinitrogen complexes), diazenido(À) and hydrazido(2À) complexes, [18,23] hydrazidium complexes [24] and Mo nitrido/imido complexes, [21] all with diphosphine (dppe or depe) ligands. In view of the fact that the electronic structures, spectroscopic properties and chemical reactivities of these (and other) intermediates of the Chatt cycle have been defined and detailed insight into many elemental steps of the Chatt cycle has been achieved, [28] quantum chemistry can now be employed to establish an overall mechanistic picture of this chemistry and help to solve the remaining obstacles to a working catalytic system.…”

“…On the basis of this finding, the Mo center is assigned a + 2 valence state (as opposed to the usual Mo IV hydrazido(2À) formula). [23] One-electron reduction of 3 a then leads to the Mo ). The resulting hydrazidium complex 4 a exhibits a highly elongated N À N bond (2.25 ), much longer than in the Mo…”

“…[8] In contrast to this scenario, we anticipate that the fluoro ligand stays coordinated once it has been introduced in the complex, such that the actual catalyst corresponds to a [MoF- 2À formulation of the latter systems, a description as Mo II -(NNH 2 ) 0 is more appropriate due to the double-bond character of the NNH 2 group. [23] Further protonation of 3 a generates the Mo…”

Activation of Molecular Nitrogen: Coordination, Cleavage and Functionalization of N2 Mediated By Metal Complexes

“…[96,97] While many cis derivatives of molybdenum and tungsten dinitrogen complexes have shown higher activity than their trans analogues, the trans nitrile derivatives retain the trans ligand upon protonation to the corresponding NNH 2 complex and improve activity. The bis(dinitrogen) complexes exchange the second N 2 ligand with the counter-ion of the acid used for protonation.…”

Molybdenum(0)‐N2 Complexes Supported by the Tripod Ligand 1,1,1‐Tris(Diphenyl‐Phosphinomethyl)Ethane: Steric Influence on the Coordination of Mono‐ and Diphosphine Coligands

“…We also investigated the use of the bulkier monophosphines PMePh 2 2 ], as inferred from the respective IR and 31 P NMR spectra. [21,22] The formation of mono(dinitrogen) complexes supported by the tdppme and diphosphine ligands thus appears to be restricted to diphosphines exhibiting bite angles smaller than 70° ( Table 2). Evidently, the reactivity of [MoBr 3 (tdppme)] towards monoand diphosphine coligands is mainly determined by the steric demand of the ligand tdppme, and mono(dinitrogen) complexes are only accessible by using the comparatively small ligands dppm, dmpm and PMe 3 .…”