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Contributions to the Chemistry of Boron, 210. η-Transition Metal Complexes of the Ligand 9-Fluorenylidene(2,2,6,6-tetramethylpiperidino)borane
Helm, Scott W.,Linti, Gerald,Noeth, Heinrich,Channareddy, Sreelatha,Hofmann, Peter
, p. 73 - 86 (2007/10/02)
The boraolefin 9-fluorenylidene(tetramethylpiperidino)borane L forms complexes 1 - 4 with the isolobal fragments Fe(CO)4, CpCo(CO), Cp'Mn(CO)2, and C6H6Cr(CO)2, respectively.Compounds FeL(CO)3PR3 (PR = PMe3, PhPMe2, Ph2PMe, PCl3, PhPCl2, 1a-e) are obtained photochemically from Fe(CO)5, L, and PR3 in toluene solution.An X-ray structure analysis 1 and 1a reveals a distorted trigonal-bipyramidal geometry, with the boron atom of 1 in an equatorial position. 2 contains two independent molecules in the asymmetric unit.The two molecules are enantiomers with Co atoms in a pseudotetrahedral environment.MO calculations demonstrate that the distortion found in 1 is due to electronic rather than steric effects as exemplified by the model compound Fe(CO)4(H2N-B=CH2) (6). ?-Back bonding from the metal-centred 2a' orbital to the boron-centred ?BC* orbital contributes significantly to the stabilization of the energy-minimized structure 8.Experimental evidence for this back bonding is provided by the shorter M-B bond in 1 and 2 as compared with the M-C bond to the boraolefin unit.However, the reverse is true for the manganese complex 3. Key Words: 9-Fluorenylidene(tetramethylpiperidino)borane ligand / Iron complexes / Cyclopentadienyl ligands / Calcylations, MO
