Adduct formation of Dichloridodioxidoniolybdenum(vi) and Methyltrioxidorhenium(vii) with a series of Bidentate Nitrogen donor ligands

The stability of a variety of bidentate N-base adducts of MoO ₂Cl₂ and (CH₃)ReO₃ (MTO) was investigated in thf and CH₂C1₂ as solvents. The formation constants were determined from the spectrophotometric data based on 1:1 adduct formation. The adduct formation constants for [MoO₂Cl ₂L₂](L₂ = bidentate nitrogen ligand) are 10⁴-10⁶ times higher than those for [(CH ₃)ReO₃L₂] with the same ligands under the same conditions. The adduct stability of both systems is very sensitive to the electronic nature of the ligands and increases with their donor ability. Hammett correlations of the formation constants against σgive relatively large negative values for the reaction constants (p_Re = -5.9, p _Mo = -6.6). The stability is also governed by steric and strain factors. Thus, sterically hindered 6,6'-disubstituted 2,2' -bipyridines do not form ad ducts with MTO, and only 6,6'-dimethyl- and 6,6'-diphenyl- 2,2'-bipyridines form adducts with MoO₂Cl₂. However, these adducts are much less stable than other methyl derivatives of 2,2'-bipridine adducts. The steric strain between the two methyl groups in 3,3-dimethy 1-2,2'-bipyridine influences the bipyridine planarity upon complexation and reduces the adduct stability. The thermodynamic parameters (enthalpy and entropy) were determined from temperature-dependence studies. The adduct stability is mainly due to the strongly exothermic binding of the nitrogen-bidentate ligand. The entropy change is small and has little effect on adduct stability.