Stereochemical Nonrigidity of a Chiral Rhodium Boryl Hydride Complex: A -Borane Complex as Transition State for Isomerization
详细信息 查看全文
- 作者：Marius V. Câ ; mpian ; Eric Clot ; Odile Eisenstein ; Ulrike Helmstedt ; Naseralla Jasim ; Robin N. Perutz ; Adrian C. Whitwood ; David Williamson
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：April 2, 2008
- 年：2008
- 卷：130
- 期：13
- 页码：4375 - 4385
- 全文大小：339K
- 年卷期：v.130,no.13(April 2, 2008)
- ISSN：1520-5126
文摘
Experimental and computational studies are reported on half-sandwich rhodium complexes thatundergo B-H bond activation with pinacolborane (HBpin = HB(OCMe2CMe2O)). The photochemical reactionof [Rh(
5-C5H5)(R,R-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpingenerates the boryl hydride in two distinguishable isomers [(SRh)-Rh(5-C5H5)(Bpin)(H)(R,R-phospholane)]5a and [(RRh)-Rh(5-C5H5)(Bpin)(H)(R,R-phospholane)] 5b that undergo intramolecular exchange. Thepresence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by1D 1H EXSY spectroscopy in C6D6 solution yielding 
H
= 83.4 ± 1.8 kJ mol-1 for conversion of 5a to 5band 79.1 ± 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJmol-1 determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for thefull system place the transition state 76.8 kJ mol-1 above the average energy of the two isomers. Thecalculations indicate that the exchange proceeds via a transition state with a 
-B-H-bonded borane. TheB-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an 2-B-Hligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(5-C5H5)Br2(R,R-phospholane)] (characterized crystallographically) and [Rh(5-C5H5)(H)2(R,R-phospholane)].The latter exhibits two inequivalent hydride resonances that undergo exchange with H = 101 ± 2 kJmol-1. DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than thedihydride complex because of steric hindrance between the phospholane and Bpin ligands in the borylhydride.
5-C5H5)(R,R-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpingenerates the boryl hydride in two distinguishable isomers [(SRh)-Rh(5-C5H5)(Bpin)(H)(R,R-phospholane)]5a and [(RRh)-Rh(5-C5H5)(Bpin)(H)(R,R-phospholane)] 5b that undergo intramolecular exchange. Thepresence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by1D 1H EXSY spectroscopy in C6D6 solution yielding H = 83.4 ± 1.8 kJ mol-1 for conversion of 5a to 5band 79.1 ± 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJmol-1 determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for thefull system place the transition state 76.8 kJ mol-1 above the average energy of the two isomers. Thecalculations indicate that the exchange proceeds via a transition state with a -B-H-bonded borane. TheB-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an 2-B-Hligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(5-C5H5)Br2(R,R-phospholane)] (characterized crystallographically) and [Rh(5-C5H5)(H)2(R,R-phospholane)].The latter exhibits two inequivalent hydride resonances that undergo exchange with H = 101 ± 2 kJmol-1. DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than thedihydride complex because of steric hindrance between the phospholane and Bpin ligands in the borylhydride.