Investigation of the Stability of the M‐H‐B Bond in Borane σ Complexes [M(CO)₅(η¹‐BH₂R⋅L)] and [CpMn(CO)₂(η¹‐BH₂R⋅L)] (M=Cr, W; L=Tertiary Amine or Phosphine): Substituent and Lewis Base Effects

Abstract

We investigated the influence of a substituent and a Lewis base on boron upon the thermodynamic stability of metal complexes of borane-Lewis base adducts, [M(CO)5(eta1-BH(2)R.L)] (M=Cr, W) and [CpMn(CO)2(eta1-BH2R.L)], where R=Cl, I, m-C6H4F, Ph, H, Me, Et; L=PMe3, PPh3, NMe3, quinuclidine. In these compounds, the stability of the metal-borane linkage was enhanced by increasing the electron-releasing ability of the substituent on boron. A stronger base L additionally stabilized the complexes. The strength of the borane-metal interaction is thus mainly ascribed to the electron donation from the BH sigma orbital to metal rather than the back-donation into the BH sigma* orbital. This result supports the bonding model for the B-H-M linkage in the borane complexes suggested by MO calculations, where the borane-to-metal electron donation is predominant while the metal back-donation into the BH sigma* orbital is negligible. Such a stability trend of the borane complexes makes a sharp contrast to that of many silane and dihydrogen complexes.

Photolysis of arene chromium tricarbonyl complexes in presence of amine–boranes: Observation of σ-borane complexes in solution

“…Table 4 summarizes the T c and DG # values of all the three r-borane complexes. Shimoi and co-workers estimated the free energy of activation (DG # ) for the H-atom scrambling process for the carbonyl rich r-borane complex 3 and found to be 34 kJ/mol at 213 K. Replacement of three Cr-H-B 3c-2e bond [14]. The value is about 28% less than that of the free Me 3 NÁBH 3 , indicating weakening of the B-H r-bond to a certain extent.…”

“…It has been previously established that, in r-borane complexes the bound B-H hydrogen atom and the terminal BH 2 hydrogen atoms are involved in fast exchange with each other at ambient temperatures [14]. At room temperature, the time scale of this dynamic process is short so as to observe the signals due to the bound and the terminal B-H hydrogen atoms separately in the NMR spectra.…”

“…Despite the difficulties, the typical g 2 -binding mode of tri-coordinated boranes to the metal center [7][8][9][10], where back donation of filled metal d-orbital to empty p-orbitals of borane stabilizes a complicated 4c-4e bond has been established. Shimoi and co-workers synthesized and structurally characterized r-borane complexes, in which a tetra-coordinated boron is coordinated to the metal in an g 1 -fashion [11][12][13][14][15]. In this type of binding mode, r ⁄ orbitals of boranes of high energy rule out any back donation.…”

Gallane-coordinated transition metal complexes and related compounds

“…(9)) [9,10,18]. Crystal structure analysis of (OC) 5 Cr(H 3 B·PMe 3 ) (8) revealed that the borane-Lewis base adduct interacts with metal center via a B-H bond to give a M-H-B 3c2e bonding.…”

“…Manganese complexes Cp(OC) 2 Mn(H 3 B·L) (9; L = PMe 3 , 10; L = NMe 3 , 11; L = quinuclidine) were also prepared by photochemical reaction of CpMn(CO) 3 and H 3 B·L, however, these were isolated in only low yield due to the high lability of borane ligands (Eq. (10)) [18,23]. The interatomic Mn-B distance (9; 2.682(3) Å and 10; 2,573(2) Å) is again significantly longer than those observed in usual Mn-B single bonds (2.06-2.16 Å) [44], indicating negligible interaction between Mn and B.…”