Donor Atom Dependent Geometric Isomers in Mononuclear Oxo−Molybdenum(V) Complexes: Implications for Coordinated Endogenous Ligation in Molybdoenzymes

We have previously demonstrated that the complex [(L1O)MoOCl2], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.;
Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281−1291). Here, the cis isomer
is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as
the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the
thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this
report, we show that for complexes of [(LO)MoOCl2], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate
containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be
observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare
example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the
origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they
are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl2] (1) include that it crystallizes in
the triclinic space group P1̄with cell dimensions a = 8.9607 (12) Å, b = 10.596 (4) Å, c = 13.2998 (13) Å, α =
98.03 (2)°, β = 103.21 (2)°, γ = 110.05(2)°, and Z = 2. [(L1O)MoO(OPh)Cl]·2CH2Cl2 (2·2CH2Cl2) crystallizes in
the triclinic space group P1̄, with cell dimensions a = 12.2740 (5) Å, b = 13.0403 (5) Å, c = 13.6141 (6) Å, α
= 65.799 (2)°, β = 64.487 (2)°, γ = 65.750 (2)°, and Z = 2. [(L3S)Mo(O)Cl2] (3) crystallizes in the orthorhombic
space group Pna21, with cell dimensions a = 13.2213 (13) Å, b = 8.817 (2) Å, c = 15.649 (4) Å, and Z = 4. The
implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the
DMSO reductase, are discussed.