Geometry and electronic structure of a heterometallic cluster Mo2Mg2 in different oxidation states of Mo: a DFT study

Abstract

Reduction of tetranuclear heterometallic complex Mo 2 Mg 2 was simulated using the B3LYP and PBE density functional methods. The results of geometry calculations of the initial complex [Mo VI O 2 Mg(MeOH) 2 (OMe) 4 ] 2 and a partially reduced Mo V complex are in good agreement with experimental data. The reduced Mo III complex is characterized by a decrease in the binding energy of aqua ligands. Structural rearrangement of the complex with release of a coordination position at the Мо atoms requires small energy expenditure. One can assume that the reduction of the polynuclear complex causes overcrowding of its coordination sphere, which favors formation of dinitrogen complexes.Key words: quantum chemical calculations, DFT methods B3LYP and PBE, catalysts of dinitrogen fixation, binding energy, complex structure.Catalytic systems for dinitrogen reduction are usually based on transition metal ions with d 2 and d 3 electronic configurations. 1 Among them, of particular interest are molybdenum compounds (this element is a constituent of nitrogenase, a natural dinitrogen fixation enzyme). Based on a biomimetic approach, a unique catalytic system containing polynuclear Mo III complexes for dinitrogen fixation was developed (see Ref.2), which catalyzes the reduction of nitrogen under conditions similar to biolog ical ones. Recently, 3 it has been established that the ac tive site of nitrogenase (an Fe Mo cofactor) only can reversibly coordinate N 2 under identical conditions. Therefore, information on the structure of catalytically active polynuclear complex (Mo III ) 8 Mg 2 is of great im portance to gain a better insight into the specific role of the protein environment of the Fe Mo cofactor. Due to the experimental difficulties in the determination of the structure of the complex, it is of interest to elaborate a theoretical approach to the structure elucidation using modern quantum chemical methods that have become highly accurate recently. In the present study, theore tical methods of investigation of polynuclear Мо comp lexes were evaluated using tetranuclear comp lexes [Mo VI O 2 Mg(MeOH) 2 (OMe) 4 ] 2 (1) and [Mo V O 2 (OMe)Mg(MeOH) 2 (OMe) 4 ] 2 (2) as examples; these systems differ in the oxidation state of Mo and their crystal structures are known. 4 The oxidation state of Mo changes with conservation of electroneutrality of the sys