Stereochemical nonrigidity of a chiral rhodium boryl hydride complex: A sigma-borane complex as transition state for isomerization

TY - JOUR

T1 - Stereochemical nonrigidity of a chiral rhodium boryl hydride complex: A sigma-borane complex as transition state for isomerization

AU - Campian, Marius V.

AU - Clot, Eric

AU - Eisenstein, Odile

AU - Helmstedt, Ulrike

AU - Jasim, Naseralla

AU - Perutz, Robin N.

AU - Whitwood, Adrian C.

AU - Williamson, David

PY - 2008/4/2

Y1 - 2008/4/2

N2 - Experimental and computational studies are reported on half-sandwich rhodium complexes that undergo B-H bond activation with pinacolborane (HBpin = HB(OCMe2CMe2O)). The photochemical reaction of [Rh(eta(5)-C5H5)(RR-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpin generates the boryl hydride in two distinguishable isomers [(S-Rh)-Rh(eta(5)-C5H5)(Bpin)(H)(R,R-phospholane)] 5a and [(R-Rh)-Rh(eta(5)-C5H5)(Bpin)(H)(RR-phospholane)] 5b that undergo intramolecular exchange. The presence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by 1D H-1 EXSY spectroscopy in C6D6 solution yielding Delta H-double dagger = 83.4 +/- 1.8 W mol(-1) for conversion of 5a to 5b and 79.1 +/- 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJ mol(-1) determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2-CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for the full system place the transition state 76.8 kJ mol(-1) above the average energy of the two isomers. The calculations indicate that the exchange proceeds via a transition state with a a-B-H-bonded borane. The B-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an)72-B-H ligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(eta(2)-B-H) C5H5)Br-2(R,R-phospholane)] (characterized crystallographically) and [Rh(eta(5)-C5H5)(H)2(RR-phospholane)]. The latter exhibits two inequivalent hydride resonances that undergo exchange with Delta H-double dagger = 101 2 kJ mol(-1). DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than the dihydride complex because of steric hindrance between the phospholane and Bpin ligands in the boryl hydride.

AB - Experimental and computational studies are reported on half-sandwich rhodium complexes that undergo B-H bond activation with pinacolborane (HBpin = HB(OCMe2CMe2O)). The photochemical reaction of [Rh(eta(5)-C5H5)(RR-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpin generates the boryl hydride in two distinguishable isomers [(S-Rh)-Rh(eta(5)-C5H5)(Bpin)(H)(R,R-phospholane)] 5a and [(R-Rh)-Rh(eta(5)-C5H5)(Bpin)(H)(RR-phospholane)] 5b that undergo intramolecular exchange. The presence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by 1D H-1 EXSY spectroscopy in C6D6 solution yielding Delta H-double dagger = 83.4 +/- 1.8 W mol(-1) for conversion of 5a to 5b and 79.1 +/- 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJ mol(-1) determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2-CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for the full system place the transition state 76.8 kJ mol(-1) above the average energy of the two isomers. The calculations indicate that the exchange proceeds via a transition state with a a-B-H-bonded borane. The B-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an)72-B-H ligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(eta(2)-B-H) C5H5)Br-2(R,R-phospholane)] (characterized crystallographically) and [Rh(eta(5)-C5H5)(H)2(RR-phospholane)]. The latter exhibits two inequivalent hydride resonances that undergo exchange with Delta H-double dagger = 101 2 kJ mol(-1). DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than the dihydride complex because of steric hindrance between the phospholane and Bpin ligands in the boryl hydride.

KW - C-H BONDS

KW - CATALYZED DEHYDROGENATIVE BORYLATION

KW - POTENTIAL BASIS-SETS

KW - METAL-COMPLEXES

KW - DIHYDROGEN COMPLEXES

KW - OXIDATIVE ADDITION

KW - POLARIZATION FUNCTIONS

KW - ROOM-TEMPERATURE

KW - B-H

KW - REGIOSPECIFIC FUNCTIONALIZATION

UR - http://www.scopus.com/inward/record.url?scp=41549137857&partnerID=8YFLogxK

U2 - 10.1021/ja077357o

DO - 10.1021/ja077357o

M3 - Article

SN - 0002-7863

VL - 130

SP - 4375

EP - 4385

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

IS - 13

ER -