Cobaltocenes

Cobaltocene, [Co ()7 -C5H5)2], is a dark-purple air-sensitive material, prepared by the reactions of sodium cyclopentadiene and anhydrous C0CI2... 

Analog laBt sich bei -2 V Kohlendioxid addieren (l-Carboxy-1-hydro-cobaltocen). Beim Dimethyl-cobaltocen werden Isomerengemische erhalten (Ausbeute jeweils 96% d.Th.)1. Zur Reduktion von Rhodocen s. Lit.2. 

A special type of oxidative addition takes place between cobaltocene and boron halides (MeBBr2, PhBCl2, BCI3, BBr3) . Treatment of cobaltocene in hexane or... 

Cobaltocene is partially oxidized and in part undergoes insertion of a borylene group, RB. The borinato ligands derive from the unknown borabenzene ( 6.5.3.4). Some porphyrinatoindium and thallium complexes ( 6.5.2.2) can also be synthesized via oxidative addition reactions (TPP)InCl is added oxidatively to Co2(CO)g and Mn2(CO) to give (TPP)In—Co(CO)4 and (TPP)ln — Mn(CO)j, respectively, and (oep)InCl is added to CojfCOg to yield (oep)ln— 0(00)4. 

Cobaltocene and nickelocene were studied by Wheeler and McClin . Retention of Ni in nickelocene was about 65%, and increased to 92% on annealing at 100 °C for one hour. Nickelocinium ion was formed to... 

Further work on nickelocene and cobaltocene was done by Ross , who synthesized the respective compounds using Ni, Ni and " Co, which decay be E.C., jS and a fully converted isomeric transition, respectively, all producing radioactive cobalt isotopes. The results showed retentions, after sublimation, of 84%, 83% and 80%, respectively. The composition of the unsublimable residue was largely CoCp2, except for the highly converted "Co, where only 30% CoCpj could be detected. This was interpreted as showing that by internal conversion the molecules are totally destroyed, by the same sort of argument as was used by Riedel and Merz . 

To simplify the catalytic system further, Kodadek and Woo investigated the activity of [Fe(F2o-TPP)Cl] for alkene cyclopropanation with EDA in the absence of cobaltocene. These workers proposed that electron-deficient porphyrin would render the Fe(III) porphyrin more easily reduced by EDA. Indeed, [Fe(F2o-TPP)Cl] efficiently catalyzes alkene cyclopropanation with EDA with high catalyst turnover... 

In 1970, the synthesis of the orange-red sandwich cation 1 from cobaltocene and PhBCl2 (1) marked a further starting point in the chemistry of boron metal compounds. The presence of a planar benzenoid C5H5B ligand moiety in 1 was deduced from XH and UB NMR data (1). This was made ironclad by two X-ray structure determinations which revealed typical centrosymmetric sandwich structures for the 19-e complexes Co(C5H5BOMe)2 (6) (21,22) and Co(C5H5BMe)2 (7) (22) as shown in Fig. 1. 

The reaction of cobaltocene with organoboron dihalides RBX2 (R = Me, Ph and X = Cl, Br mainly) and boron trihalides (BC13, BBr3) leads essentially to three types of (boratabenzene) cobalt complexes, 19,20, and 21 (7,57). CoCp2 plays a dual role in part it acts as a reductant, in part it... 

This unique formation of a borabenzene ring skeleton is thought to proceed as delineated in Scheme 1. A more detailed explanation together with comments on stoichiometry, reaction conditions, and by-products may be found elsewhere (2,7). However, it should be noted that Scheme 1 is based on the well-known reducing power of cobaltocene and its ability to add radicals efficiently (80), both properties being intimately connected with the uncommon 19-e configuration of CoCp2. 

In principle other highly reducing 19-e complexes should exhibit reactivity similar to cobaltocene. This challenging hypothesis has apparently been tested in only one case. The metallacarbaborane 22 can be converted to the boratabenzene analog 23 by reduction and subsequent treatment with PhBCl2 (Scheme 2) (70). 

Bora-2,5-cyclohexadienes have a much greater synthetic potential than is apparent from the examples given so far. This may be exemplified by two recent reactions. Reductive complex formation in the system Co(acac)3/COD/25/Mg/THF affords complex 51 via the organotin route (77) while an earlier synthesis used the cobaltocene route (60). Ni(COD)2 very cleanly forms the (Tj3-l,4,5-cyclooctenyl)nickel complex 52 (29). 

Borabenzene metal complexes, like their cyclopentadienyl counterparts, are not readily amenable to breaking of the metal-ligand bond. Such bond breaking will occur more easily in complexes with antibonding electrons. By fortuitous chance the cobaltocene route to borabenzene chemistry (Section V,A) provided 19-e complexes with the useful property of inherently weakened metal-ligand bonds. Thus most of this section centers on the reactivity of Co(C5H5BMe)2 (7) and Co(CsH5BPh)2 (13). 

In general this is borne out by the experimental results. Thus, in the spectrum of crystalline cobaltocene, between 1200 and 350 mju, Ammeter and Swalen (88) found only... 

To illustrate the problem the approach of Ammeter and Swalen(88) for the d1 configuration (represented by cobaltocene) is now followed, writing the a, ir, and 5 metal-ligand combinations as... 

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