Cobalt borabenzene complexes

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

One of the most intensely studied boracychc compound is the borabenzene ligand synthesized by Her-berich and coworkers. Bis- and monoborabenzene cobalt sandwich complexes were first obtained by treating cobaltocene with boron trihalides BX3 (R = Cl, Br) or alkyl/arylboron dihalides (R = alkyl, aryl) as outlined in... 

All borabenzene-metal complexes investigated structurally so far show very similar patterns for the ligand geometry (Table I) and for the metal-ligand bonding (Table II) only the cobalt complex 6 deserves separate consideration (see below). 

Redox properties of (borabenzene)cobalt complexes have been investigated electrochemically. The reduction potentials are displaced uniformly to more positive values by about 0.5 V per borabenzene ligand. The existence of the... 

Cyclic voltammetry studies of borabenzene cobalt complexes, (borinato)-cyclopentadienyliron, and bis(borinato) complexes of iron, chromium, and vanadium have appeared. The electrochemical behaviour and e.s.r. spectroscopy of heterocyclic organoboron compounds (60) have been studied. ... 

U. Koelle, J. Organometallic Chem., 1978, 152, 225. Cyclic voltammetry of borabenzene-cobalt complexes. 

Herberich, G.E., Raabe, E. Syntheses with l-phenylbora-2,5-cyclohexadiene. Complexes of Iron, Cobalt and Nickel with Partially Hydrogenated Borabenzene Ligands . J. Organomet. Chem. 1986, 309, 143-156. 

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