Reactions of (Silyl)(silylene)tungsten and -molybdenum Complexes with Sulfur Reagents

Synthesis, structure, and reactivity of a pyridine-stabilized silanonetungsten complex

A pyridine-stabilized silanonetungsten complex Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(py)}(SiMe₃) (1b, Cp* = η⁵-C₅Me₅, Mes = 2,4,6-Me₃C₆H₂, py = C₅H₅N) was obtained by the reaction of a silyl(silylene) complex Cp*(OC)₂W([double bond, length as m-dash]SiMes₂)(SiMe₃) (3) with pyridine-N-oxide in pyridine. X-ray crystal structure determination revealed that complex 1b shows a similar geometry to that observed for a previously synthesized DMAP-stabilized analogue, Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(DMAP)}(SiMe₃) (1a, DMAP = 4-NMe₂C₆H₄N). The Si[double bond, length as m-dash]O and W-O bond distances in 1b are comparable to those observed in 1a, but the nitrogen to silicon coordination bond of 1b is slightly longer (ca. 0.05 Å) than that of 1a, indicating the weaker coordination of pyridine than that of DMAP. The reaction of 1b with excess PMe₃ in C₆D₆ at r. t. proceeded via elimination of pyridine to afford a five-membered metallacyclic carbene complex, Cp*(OC)W([double bond, length as m-dash]C(SiMe₃)OSiMes₂O)(PMe₃) (5), but that of 1a with PMe₃ did not proceed at all. Complex 5 was further transformed in C₇D₈ at 100 °C for 4 h to give a four-membered W-O-Si-O metallacyclic complex with carbyne and PMe₃ ligands, Cp*W(OSiMes₂O)([triple bond, length as m-dash]CSiMe₃)(PMe₃) (7). The structural features of complexes 1b, 5, and 7 are comparable to those suggested theoretically as intermediates in the reaction of 3 with a sulfuration reagent to afford a six-membered metallacyclic carbene complex, Cp*W(S){[double bond, length as m-dash]C(SiMe₃)C([double bond, length as m-dash]O)OSiMes₂S} (6), indicating that complex 1b and the theoretically proposed silanethione complex are transformed via a similar reaction pathway.

Substitution effects on the formation of T-shaped palladium carbene and thioketone complexes from Li/Cl carbenoids

The preparation of palladium thioketone and T-shaped carbene complexes by treatment of thiophosphoryl substituted Li/Cl carbenoids with a Pd(0) precursor is reported. Depending on the steric demand, the anion-stabilizing ability of the silyl moiety (by negative hyperconjugation effects) and the remaining negative charge at the carbenic carbon atom, isolation of a three-coordinate, T-shaped palladium carbene complex is possible. In contrast, insufficient charge stabilization results in the transfer of the sulfur of the thiophosphoryl moiety and thus in the formation of a thioketone complex. While the thioketones are stable compounds the carbene complexes are revealed to be highly reactive and decompose under elimination of Pd metal. Computational studies revealed that both complexes are formed by a substitution mechanism. While the ketone turned out to be the thermodynamically favored product, the carbene is kinetically favored and thus preferentially formed at low reaction temperatures.