Synthesis, structure, and properties of molybdenum octahedral d6 bis(alkyne) complexes

Article abstract of DOI:10.1016/0022-328X(93)80467-P

The complex Mo(CO)3(CH3CN)3 reacts with two equiv of alkyne to give Mo(CO)2(CH3CN)2(alkyne)2.Addition of 1 equiv of NN to the latter compound yields the desired product Mo(CO)2(NN)(alkyne)2 (alkyne = DMAC (dimethylacetylenedicarboxylate), NN = en (1), pn (2), o-phenylenediamine (3), and o-NH2CH2py (4); alkyne = DEAC (diethylacetylenedicarboxylate), NN = en (5), and o-NH2CH2py (6); alkyne = PhC<*>CCO2Et, NN = bipy (7), and phen (*)).Compounds 1 and 2 may also be synthesized from the reaction of Mo(CO)3(NN)2 with excess DMAC.The structure of Mo(CO)2(en)(DMAC)2 (1) hasbeen determined by X-ray crystallography and refined to R = 0.0235 (R_w = 0.0242) by using 3063 independent reflections with intensity <*> 3?(I).The crystals were triclinic, space group P<*>; a = 8.508(2), b = 9.587(2), c = 13.214(3) Angstroem; α = 78.35(2), β = 87.98(2), and γ = 78.99(2); Z = 2.The molecule adopts an octahedral geometry with the two CO groups cis to each other, the two DMAC ligands in trans positions and the remaining two sites being occupied by the nitrogen atoms of the ethylenediamine group.The orientations of the two trans alkyne ligands are mutually perpendicular with each DMAC ligand eclipsing a N-Mo-CO vector.All the present complexes 1-8 exhibit fluxional behavior on the NMR timescale due to the rotation of alkyne ligands around the axis through the metal center and perpendicular to the two alkyne ligands.At low temperatures, the NMR spectra of 4 and 6 reveal the presence of four different signals for each type of nuclei on the alkyne ligands.Each set of the four signals averages to only one at the fast-exchange limit.For Mo(CO)2(phen)(PhC<*>CCO2Et)2 (8), two conformational isomers A and B exist insolution arising from the different orientations of the PhC<*>CCO2Et ligands relative to the N-Mo-CO vectors.The possible mechanisms for the observed fluxional behavior of these complexes are discussed.