Monoadducts of imido alkylidene complexes, syn and anti rotamers, and alkylidene ligand rotation

Article abstract of DOI:10.1021/om00052a033

Complexes of the type M(CH-t-Bu)(NAr)(OR)₂ (M = Mo, W; Ar = 2,6-C₆H₃-i-Pr₂; OR = OCMe(CF₃)₂, OCMe₂(CF₃)) form five-coordinate adducts upon addition of PMe₃ or quinuclidine. PMe₃ attacks the C/N/O face of the pseudotetrahedral complexes to give chiral TBP species in which the phosphine is bound in an axial position and the imido and alkylidene ligands lie in the equatorial plane. Two isomers containing syn and anti rotamers of the alkylidene ligand are observed. The syn rotamer forms first; the anti rotamer is the final product. PMe₃ binds weakly when OR = O-t-Bu and is lost readily in vacuo. Quinuclidine adds to either the C/O/O face or N/O/O face to give an achiral syn isomer and to the C/N/O face to give an anti chiral TBP species analogous to that formed for the PMe₃ adduct. An equilibrium mixture of syn and anti forms is observed with time. An X-ray structure of syn-Mo(CH-t-Bu)(NAr)[OCMe-(CF₃)₂]₂(PMe ₃) shows that the t-Bu group points toward the imido ligand and the phenyl ring of the imido ligand lies approximately in the equatorial plane in a relatively crowded coordination environment (a = 10.979 (4) ?, b = 17.945 (7) ?, c = 18.375 (8) ?, β = 106.34 (3)°, Z = 4, V = 3474 (4) ?³, ρ = 1.490 g/cm³, R = 0.037, R_w = 0.045). Pyridine adducts of Mo complexes containing the 2,6-dichlorophenoxide ligand also have been characterized. Three isomers of five-coordinate molybdenum or tungsten complexes containing a cis- or trans-2-butenylidene ligand and quinuclidine are found at equilibrium, syn and anti rotamers of the chiral core previously described and a syn rotamer with an achiral core. An X-ray structure of anti-W(trans-CHCH=CHMe)(NAr)[OCMe(CF₃)₂]₂(quin) showed the expected trigonal-bipyramidal core with alkylidene and imido ligands occupying equatorial sites and OCMe(CF₃)₂ ligands occupying one axial and one equatorial site (a = 12.972 (9) ?, b = 18.049 (7) ?, c = 15.038 (9) ?, β = 92.07 (3)°, Z = 4, V = 3518 (6) ?³, ρ = 1.673 g/cm³, R₁ = 0.038, R_w = 0.040). The only significant difference between the structure of this anti adduct and the syn adduct described above is that the anti adduct is markedly less crowded in the equatorial plane. Syn and anti rotamers in five-coordinate adducts have been shown to interconvert after losing the base in several cases. The barrier to rotation of the alkylidene ligand has been measured in several four-coordinate species and shown to lie in the range ΔG?₂₉₈ = 15-18 kcal mol^-1. These findings are discussed in relation to the proposed mechanism of olefin metathesis by pseudotetrahedral complexes of the type M(CHR′)(NAr)(OR)₂.