Accurate molecular structures of 16-electron rhodium hydrido boryl complexes: Low-temperature single-crystal X-ray and neutron diffraction and computational studies of [(PR3)2RhHCl(boryl)] (boryl = Bpin, Bcat)

Abstract

Rhodium hydrido boryl complexes of the form [(PR3)2RhHCl{B(OR′)2}] are key first intermediates in several Rh-catalyzed borylation processes. Previous theoretical studies have examined model compounds, e.g with PH3 and BH2 or B(OH)2 groups, and there were little structural data available for this class of compounds to compare with the calculated structures. This paper reports the results of single-crystal X-ray (at 120 K) and neutron (at 20 K) diffraction studies on two such complexes, namely [(PiPr3)2RhHCl(Bpin)] (3Bpin) and [(PiPr3)2RhHCl(Bcat)] (3Bcat) (pin = OCMe2CMe2O; cat = 1,2-O2C6H4), providing the first accurate location of the hydride ligands in a hydrido boryl complex. The orientations of the boryl ligands with respect to the equatorial plane of a distorted-trigonal-bipyramidal (DTBP) structure differs for the two compounds (Bpin lies nearly perpendicular to this plane, whereas Bcat is roughly coplanar with it), and the Cl-Rh-B angles are also very different, being 117.73(4) and 137.87(5)° (X-ray data), respectively, for 3Bpin and 3Bcat. The Rh-H distances are 1.571(5) and 1.531(11) Å, and the B-Rh-H angles are only 67.8(2) and 68.5(4)°, leading to B⋯H separations of 2.013(5) and 2.004(10) Å (neutron data) in the two compounds. Thus, these are best described as RhIII hydrido boryl rather than RhI σ-borane (σ-HB(OR)2) complexes, although there is a modest residual B⋯H interaction in both compounds. DFT calculations on model compounds employing PH3 and PMe3, as well as on 3Bpin and 3Bcat, incorporating the actual PiPr3, Bpin, and Bcat ligands allowed a detailed examination of the factors influencing structure and bonding in these compounds as well as a direct comparison of theory and experiment. The perpendicular arrangement of the boryl ligand is calculated to be favored for all cases except for 3Bcat (i.e. PiPr3; Bcat), for which steric constraints appear to be responsible for a small preference for the coplanar form. This and the calculated geometric parameters are in good agreement with the experimental structures. The theoretical study confirms the residual B⋯H interaction in both 3Bpin and 3Bcat, indicating that B-H interaction can occur through both the "empty" BO2 p(π*) and BO2 σ* orbitals, which are mutually perpendicular. The latter interaction has not been addressed in previous studies. The degree of oxidative addition of the B-H bond depends on the electron-donating properties of the phosphine ligand and also on the nature and orientation of the boryl ligand. Thus, the balance of factors involved in the determination of the structure of these species is quite subtle, and care needs to be taken in using models in the analysis of these systems.