Nuclear Magnetic Resonance Studies of Alkynes as Four-Electron Donor Ligands in Monomeric Tungsten(II) Complexes

Article abstract of DOI:10.1021/ja00525a011

Tungsten alkyne complexes of the type W(CO)(R¹C<*>CR²)(S2CNR2)2 have been prepared for R¹=R²=H, Me, Et, Ph and R¹=H, R²=Ph by allowing tricarbonylbis(dithiocarbamato)tungsten(II) to react with the appropriate alkyne in toluene at ambient temperature.Ethylene does not react with the W(CO)3(S2CNR2)2 reagent under the same conditions.In contrast to the alkynes, phosphorus ligands such as PPh3, PEt3, P(n-Bu)3, and P(OEt)3 displace only one carbon monoxide from the tungsten tricarbonyldithiocarbamate complex to form W(CO)2L(S2CNR2)2 products.Reaction with bis(diphenyl phosphine)acetylene initially forms the phosphorus bound dicarbonyl product analogous to the complexes resulting from substitution of one carbon monoxide with a simple phosphorus donor ligand.The orange solid which is isolated undergoes a ligand rearrangement when dissolved in acetone and loses another carbonyl ligand to form the green monocarbonyl derivative W(CO)(η²-Ph2PC<*>CPPh2)(S2CNR2)2 which is analogous to the other alkyne comlexes isolated.The symmetrically substituted alkyne ligands undergo a fluxional process which averages the two ends of the bound alkyne at room temperature while the four independent alkyl groups of the two dithiocarbamate ligands remain distinct.Activation energies of 11-12 Kcal mol^-1 characterize the alkyne rotations investigated.The NMR properties of W(CO)(PhC<*>CH)(S2CNR2)2 are consistent with the existence of only one isomer in solution, and the relative orientation of the acetylene sublstituents in solution has been determined by employing the heteronuclear Overhauser effect by selectively saturating protons while monitoring the 13C NMR of the cis carbon monoxide ligand.Comparison of the NMR data obtained for PhC<*>CH as a ligand with the NMR properties of W(CO)(HC<*>CH)(S2CNR2)2 leads to the conclusion that the proximal acetylenic proton couples to the carbon-13 of the carbonyl ligand much more effectively than does the distal acetylenic proton.Hybridization of the acetylenic carbons has been probed by measuring ¹J_C-H=210 Hz.The low-field 13C chemical shifts of acetylenic carbons in these tungsten comlexes (in the range of 200-210 ppm below Me4Si) suggest that the mode of alkyne-metal bonding may be correlated with 13C NMR data.We believe that the chemical reactivity patterns and spectroscopic data reported provide support for the four-electron donor role of alkynes in these monomeric tungsten(II) complexes.