σ-Borane Complexes of Manganese and Rhenium

Abstract

The late transition metal borane complexes (MeCp)Mn(CO) 2 (HBR 2 ) (R ) alkyl, alkoxy) and Cp*Re-(CO) 2 (HBpin) (HBpin ) pinacolborane, Cp* ) pentamethylcyclopentadienyl) have been synthesized and isolated. All complexes were characterized spectroscopically, and X-ray crystal structure analyses were performed for (MeCp)Mn(CO) 2 (HBpin), (MeCp)Mn(CO) 2 (HBcat) (HBcat ) catecholborane) and (MeCp)Mn(CO) 2 (HBCy 2 ) (Cy ) cyclohexyl). These data show that the complexes contain a borane ligand with a weakened but intact B-H bond. The borane ligand in (MeCp)Mn(CO) 2 (HBcat) is replaced by PhCCPh, Ph 3 SnH, Ph 2 MeSiH, CO, and excess HBpin in ligand substitution reactions. The mechanism of substitution by PhCCPh was investigated. Kinetic studies showed that the reaction is first-order in complex and that borane and PhCCPh react competitively with the 16-electron intermediate, (MeCp)Mn(CO) 2 . The ∆H q value for borane dissociation was 24 ( 3 kcal/ mol for (MeCp)Mn(CO) 2 (HBcat) and 21 ( 1 kcal/mol for (MeCp)Mn(CO) 2 (HBCy 2 ), which provided an upper limit on the metal-borane binding energies. Dynamic NMR spectroscopy of (MeCp)Mn(CO) 2 (HBcat) revealed two types of rotations of the borane ligand.

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…A major advance was made recently when he obtained NMR spectra of labelled pentane complexes of {(h 5 -C 5 H 4 iPr)Re(CO) 2 } with a slight preference for binding of methylene sites (Scheme 6). [5] The structure appears to be very close to those of the corresponding Mn(h 2 -Si-H) and [36] and [RuH(m-H 2 Bpin)(h 2 -HBpin)(PCy 3 ) 2 ] (bottom; HBpin = pinacolborane; the H atoms bound to carbon are omitted and only the C a atoms of the phosphine ligands are shown). [38] R. N. Perutz and S. Sabo-Etienne Reviews 2582 www.angewandte.org Mn(h 2 -B-H) complexes characterized both by NMR and Xray diffraction.…”

“…[36] The s-borane bond is characterized by a B À H length between 1.24(2) and 1.31(2) and a 1 J B,H coupling constant of 95 Hz. A recent structure shows an h 2 -HBEt 2 group coordinated to a {Ni(dcpe)} fragment (dcpe = bis(dicyclohexylphosphanyl)ethane) with a BÀH bond length of 1.23 (5) .…”

Chelating Phosphane–Boranes as Hemilabile Ligands – Synthesis of[Mn(CO)3(η2‐H3B·dppm)][BArF4] and [Mn(CO)4(η1‐H3B·dppm)][BArF4]

“…[23] It is significantly shorter than in the fourcoordinate boryl complex Mn(CO) 4 + bonding description is suggested. [19] A similar short distance is observed in [Cr(CO) 4 …”

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

“…[38] These compounds undergo substitution of the borane ligand by diphenylacetylene through a dissociative mechanism. Kinetic studies were undertaken to obtain the activation parameters for the borane dissociation.…”