Reduction Pathway of End-On Coordinated Dinitrogen. 3. Electronic Structure and Spectroscopic Properties of Molybdenum/Tungsten Hydrazidium Complexes

Abstract

The spectroscopic properties and electronic structure of the hydrazidium complexes [MF(NNH(3))(depe)(2)](BF(4))(2), M = Mo and W, are investigated (depe = 1,2-bis(diethylphosphino)ethane). Vibrational spectroscopic data for both compounds are evaluated with a quantum-chemistry-assisted normal coordinate analysis, giving an N-N force constant of 6.03 mdyn/A and metal-N force constants of 8.01 (Mo-N) and 7.31 mdyn/A (W-N), respectively. On the basis of these results and DFT calculations on a [MoF(NNH(3))(PH(3))(4)](2+) model system, the N-N bond order in these systems is 1 (single sigma bond) and metal-N bonding corresponds to a triple bond. The metal centers are assigned a +IV oxidation state (d(2) configuration) and the NNH(3) ligand is assigned a -1 formal charge which by sigma- and pi-donation to the metal is reduced to +0.48. The two metal-d electrons are located in the nonbonding (n) d(xy)() orbital. This bonding description is supported by the results of optical absorption spectroscopy showing the n --> (metal-ligand)pi transition at 536 nm (not observed in the tungsten compound) and the (metal-ligand)pi --> (metal-ligand)pi transition at 251 nm for the MoNNH(3) and at 237 nm for the WNNH(3) complex. The activation enthalpy for splitting of the N-N bond in these systems to generate NH(4)(+) is estimated to be larger than 40 kcal/mol. Hydrazidium complexes with diphosphine coligands are therefore inert with respect to N-N cleavage and thus represent the ultimate stage of N(2) reduction at six-coordinate d(6) metal centers in the absence of external reductants.

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.…”

“…[24] Instead, complex 3 a is reduced to Mo I -NNH 2 complex 3 b exhibiting a bent isodiazene ligand. b-protonation of 3 b leads to the hydrazidium complex 4 a which exhibits a linear structure, a very short Mo À N bond and an extremely elongated N À N bond.…”

Theoretical, spectroscopic, and mechanistic studies on transition‐metal dinitrogen complexes: Implications to reactivity and relevance to the nitrogenase problem

“…One of the N 2 ligands is thereby exchanged against the conjugate base of the employed acid. We have characterized these systems using infrared, Raman, and UV/Vis absorption spectroscopies coupled to DFT calculations 69–75. Based on N–N stretching frequencies, the parent Mo(0) and W(0) dinitrogen complexes were characterized as moderately activated .…”

(Dinitrogen)molybdenum Complexes Supported by Asymmetric Silicon‐Centered Tripod Ligands: Steric and Electronic Influences on the Coordination of Mono‐ and Diphosphine Coligands

“…So far, a large number of N 2 complexes have been synthesized, only a few of which, however, allow the reduction of N 2 to NH 3 in a cyclic or even catalytic fashion. [11][12][13][14][15][16][17][18] More recently we focused our attention on the design of new ligand environments. On the basis of the latter systems, the first complete mechanistic scenario of N 2 reduction has been established by Chatt and co-workers.…”

“…[3][4][5][6][7][8][9] In this regard, complexes that contain low-valent iron and molybdenum centers play a prominent role. [19][20][21][22][23][24] The goal of this research is to overcome drawbacks of the classic Chatt complexes, namely, (1) a breaking of metal-phosphine bonds at higher oxidation states of the central atoms, [12,25] and (2) an intrinsic loss of 50 % of catalyst per reduction cycle, which occurs when intermediate Mo I complexes undergo disproportionation. [10] Our group has been involved in spectroscopic and theoretical investigations of key steps of the Chatt cycle.…”