An Unusual Exchange between Alkylidyne Alkyl and Bis(alkylidene) Tungsten Complexes Promoted by Phosphine Coordination:  Kinetic, Thermodynamic, and Theoretical Studies

Abstract

d0 Tungsten alkylidyne alkyl complex (Me3SiCH2)3W(CSiMe3)(PMe3) (4a) was found to undergo a rare, PMe3-promoted exchange with its bis(alkylidene) tautomer (Me3SiCH2)2W(=CHSiMe3)2(PMe3) (4b). Thermodynamic studies of the exchange showed that 4b is favored and gave Keq and the enthalpy and entropy of the equilibrium: DeltaH degrees = -1.8(0.5) kcal/mol and DeltaS degrees = -1.5(1.7) eu. Kinetic studies of the alpha-H migration between 4a and 4b by variable-temperature NMR gave rate constants k1 and k-1 for the reversible reactions and activation enthalpies and entropies: DeltaH1 = 16.2(1.2) kcal/mol and DeltaS1 = -22.3(4.0) eu for the forward reaction (4a --> 4b); DeltaH2 = 18.0(1.3) kcal/mol and DeltaS2 = -20.9(4.3) eu for the reverse reaction (4b --> 4a). Ab initio calculations at the B3LYP level revealed that PMe3 binds with the bis(alkylidene) tautomer relatively more strongly than with the alkylidyne tautomer and thus stabilizes the bis(alkylidene) tautomer.

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

“…The calculated high H-transfer energy barriers are consistent with those computed for the isolobal tetrahedral alkylidyne-alkylidene rhenium complexes [69], which at the same time is in agreement with the rarity of a-H transfer processes in d 0 complexes [72][73][74]. Therefore, calculations suggest that intramolecular H-transfer steps are less likely to take place during the reaction of 2 with hydrogen than heterolytic cleavage and reverse H 2 -abstraction discussed above.…”

Alkane metathesis by a tungsten carbyne complex grafted on gamma alumina: Is there a direct chemical role of the support?

“…3b ( E = 24 kJ mol −1 and E ‡ = 56 kJ mol −1 ), a well known process for analogous complexes [29,30]. Thus, every element is present to produce a metallacyclobutane: propene and carbene 8.…”

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…The equilibrium constant could be determined at various temperatures (e.g., at 278 K, K T = 12.3) [143]. The alkylidene carbon 13 C signals are characteristically different: 76a: δ( 13 C) = 358.8 ppm ( 2 J P,C = 14.5 Hz) and in 76b: δ( 13 C) = 256.4 ppm ( 1 J C,H = 123.5, 2 J P,C = 11.8 Hz) and 254.7 ppm ( 1 J C,H = 102.6 Hz, 2 J P,C = 12.6 Hz), respectively, at −50 • C in toluene-d 8 [143].…”

“…The equilibrium constant could be determined at various temperatures (e.g., at 278 K, K T = 12.3) [143]. The alkylidene carbon 13 C signals are characteristically different: 76a: δ( 13 C) = 358.8 ppm ( 2 J P,C = 14.5 Hz) and in 76b: δ( 13 C) = 256.4 ppm ( 1 J C,H = 123.5, 2 J P,C = 11.8 Hz) and 254.7 ppm ( 1 J C,H = 102.6 Hz, 2 J P,C = 12.6 Hz), respectively, at −50 • C in toluene-d 8 [143]. Quantum chemical calculation of chemical shifts of the quadrupolar nuclei 14 N, 17 O, and 33 S and comparison with experimental δ-values allowed reliable prediction of the deprotonation sites of hydantoin, urazile, and N-hydroxy urea, as well as the effect of solvation, especially hydration, on the relative stability of the thus generated ions [144].…”