Synthesis, fluxional behavior, and endo, exo-stereoisomers of allyl molybdenum complexes with O-ethyldithiocarbonate, EtOCS −2, containing ligand: Crystal structures of [Mo(CH 3CN)(η 3-C 3H 5)(CO) 2(S 2COEt)], exo-[Mo(dppm)(η 3-C 3H 5)(CO)(S 2COEt)], and endo-, exo-[Mo(dppa)(η 3-C 3H 5)(CO)(S 2COEt)

Abstract

The very air-sensitive η 3-allyldicarbonylethoxydithiocarbonate molybdenum(II) compound [Mo(η 3-C 3H 5)(CO) 2(S 2COEt)(CH 3CN)] ( 1) are accessible by the reaction of [Mo(η 3-C 3H 5)(CO) 2(Br)(CH 3CN) 2] with KS 2COEt in methanol at ambient temperature. The rotational behavior of 1 in solution state was detected by variable-temperature 1H NMR spectroscopy. The mechanism can be described as a trigonal twist, in which the rotation of the triangular face formed by the nitrogen ligand and the two sulfur atoms relative to the face formed by the allyl and the two carbonyl groups. The reactions of 1 with piperidine, 1-piperidinecarbodithioate, and bipyridine ligands give the replacement of the acetonitrile complex [Mo(η 3-C 3H 5)(CO) 2(S 2COEt)(C 5H 10NH)] ( 2), 16-electron complex [Mo(η 3-C 3H 5)(CO) 2(S 2CNC 5H 10)] ( 3), and η 1- O-ethyldithiocarbonate complex [Mo(η 3-C 3H 5)(CO) 2(S 2COEt)(bipy)] ( 4). Treatment of 1 with various diphos ligands form a mixtures of endo- and exo-complexes [Mo(η 3-C 3H 5)(S 2COEt)(CO)(diphos)] (diphos: dppm = {bis(diphenylphosphino)methane} ( endo-, exo- 5); dppe = {1,2-bis(diphenylphosphino)ethane} ( endo-, exo- 6); dppa = {bis(diphenylphosphino)amine} ( endo-, exo- 8)) with ratios of 1:5, 1:2, and 4:5 according to the integration of 31P{ 1H} NMR spectra, respectively. The orientations of endo and exo are defined for the open face of the allyl group and carbonyl group in the same direction in the former and opposite directions in the latter. The activation barriers of interconversion were determined to be 13.9 ± 0.2 kcal mol −1 for ( 1) and 14.6 ± 0.2 kcal mol −1 for ( 2). The X-ray crystal structures of [Mo(η 3-C 3H 5)(CO) 2(S 2COEt)(CH 3CN)] ( 1b), [Mo(η 3-C 3H 5)(S 2COEt)(CO)(dppm)] ( exo- 5), and [Mo(η 3-C 3H 5)(S 2COEt)(CO)(dppa)] ( endo-, exo- 8) are employed to elucidate the coordination mode of the dithiocarbonate ligand and the endo-, exo-orientations. One crystallographically independent molecule of 8 is contained in the cell, in which the open face of the allyl group is disordered toward two directions, i.e. endo and exo. In addition, the endo: exo ratio in the solid state is 62:38.