Abstract
Hydride transfer from transition metal hydrides (MH) to Ph3C+BF4 - gives M-FBF3 and Ph3CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH2Cl2 solution, 25°C). For hydride transfer from Cp(*)(CO)3MoH (Cp(*) = η5-C5Me5) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Ph(n)(p-MeOC6H4)3- (n)C+BF4 -; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k(MoH)/k(MoD) = 1.7-1.9 as the rate constant decreases from k(H) = 6.5 x 103 to 1.4 M-1 s-1. For hydride transfer to Ph3C+BF4 - from five metal hydrides [Cp(CO)3MoH, Cp(*)(CO)3WH, (indenyl)(CO)3WH, Cp(*)(CO)3MoH, and trans-Cp(CO)2(PCy3)MoH; Cp = η5-C5H5] with second- order rate constants k(H)- ≥ 3.8 x 102 M-1 s-1, the kinetic isotope effects are also k(MH)/k(MD) = 1.7-1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k(WH)k(WD) = 1.8 to 0.47 as the rate constant decreases (from k(H)- = 2.0 x 103 to 0.72 M-1 s-1). The steadily decreasing values of k(MH)/k(MD) with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows clown in progressing from more electron-donating Cp ligands to less electron- rich Cp ligands. The inverse isotope effect (k(WH)/k(WD) = 0.47) found for the slowest tungsten hydride, (C5H4CO2Me)(CO)3WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.
| Original language | English |
|---|---|
| Pages (from-to) | 3150-3155 |
| Number of pages | 6 |
| Journal | Journal of the American Chemical Society |
| Volume | 121 |
| Issue number | 13 |
| DOIs | |
| Publication status | Published - Apr 7 1999 |
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- Chemistry(all)
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Isotope effects on hydride transfer reactions from transition metal hydrides to trityl cation. An inverse isotope effect for a hydride transfer. / Cheng, Tan Yun; Bullock, R Morris.
In: Journal of the American Chemical Society, Vol. 121, No. 13, 07.04.1999, p. 3150-3155.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Isotope effects on hydride transfer reactions from transition metal hydrides to trityl cation. An inverse isotope effect for a hydride transfer
AU - Cheng, Tan Yun
AU - Bullock, R Morris
PY - 1999/4/7
Y1 - 1999/4/7
N2 - Hydride transfer from transition metal hydrides (MH) to Ph3C+BF4 - gives M-FBF3 and Ph3CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH2Cl2 solution, 25°C). For hydride transfer from Cp(*)(CO)3MoH (Cp(*) = η5-C5Me5) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Ph(n)(p-MeOC6H4)3- (n)C+BF4 -; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k(MoH)/k(MoD) = 1.7-1.9 as the rate constant decreases from k(H) = 6.5 x 103 to 1.4 M-1 s-1. For hydride transfer to Ph3C+BF4 - from five metal hydrides [Cp(CO)3MoH, Cp(*)(CO)3WH, (indenyl)(CO)3WH, Cp(*)(CO)3MoH, and trans-Cp(CO)2(PCy3)MoH; Cp = η5-C5H5] with second- order rate constants k(H)- ≥ 3.8 x 102 M-1 s-1, the kinetic isotope effects are also k(MH)/k(MD) = 1.7-1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k(WH)k(WD) = 1.8 to 0.47 as the rate constant decreases (from k(H)- = 2.0 x 103 to 0.72 M-1 s-1). The steadily decreasing values of k(MH)/k(MD) with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows clown in progressing from more electron-donating Cp ligands to less electron- rich Cp ligands. The inverse isotope effect (k(WH)/k(WD) = 0.47) found for the slowest tungsten hydride, (C5H4CO2Me)(CO)3WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.
AB - Hydride transfer from transition metal hydrides (MH) to Ph3C+BF4 - gives M-FBF3 and Ph3CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH2Cl2 solution, 25°C). For hydride transfer from Cp(*)(CO)3MoH (Cp(*) = η5-C5Me5) to substituted trityl cations containing zero, one, two, or three p-MeO groups [Ph(n)(p-MeOC6H4)3- (n)C+BF4 -; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k(MoH)/k(MoD) = 1.7-1.9 as the rate constant decreases from k(H) = 6.5 x 103 to 1.4 M-1 s-1. For hydride transfer to Ph3C+BF4 - from five metal hydrides [Cp(CO)3MoH, Cp(*)(CO)3WH, (indenyl)(CO)3WH, Cp(*)(CO)3MoH, and trans-Cp(CO)2(PCy3)MoH; Cp = η5-C5H5] with second- order rate constants k(H)- ≥ 3.8 x 102 M-1 s-1, the kinetic isotope effects are also k(MH)/k(MD) = 1.7-1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k(WH)k(WD) = 1.8 to 0.47 as the rate constant decreases (from k(H)- = 2.0 x 103 to 0.72 M-1 s-1). The steadily decreasing values of k(MH)/k(MD) with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows clown in progressing from more electron-donating Cp ligands to less electron- rich Cp ligands. The inverse isotope effect (k(WH)/k(WD) = 0.47) found for the slowest tungsten hydride, (C5H4CO2Me)(CO)3WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.
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UR - http://www.scopus.com/inward/citedby.url?scp=0033531692&partnerID=8YFLogxK
U2 - 10.1021/ja983448x
DO - 10.1021/ja983448x
M3 - Article
AN - SCOPUS:0033531692
VL - 121
SP - 3150
EP - 3155
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 13
ER -