Cobalt I Complexes

Submitted by PASCUAL ROYO, NATIVIDAD ESPA.NA, and AMELIO VAZQUEZ 

Both organocobalt(I) and (II) carbonyl complexes can be synthesized from te-trahydrofuran (THF) solutions of bis(pentafluorophenyl)cobalt(II), and this is an 

Anhydrous cobalt(II) bromide (21.9 g, 100.0 mmoles), freshly distilled THF (150 mL), and a magnetic, Teflon-coated stirring bar are placed in a 1-L round-bottomed, two-necked flask, fitted with a rubber stopper and a condenser topped with a stopcock that is connected to nitrogen and to a vacuum line. The flask is alternatively evacuated and filled with dry nitrogen three times. 

One liter of distilled dry hexane is added (with stirring) to obtain a blue 

The yellow solution (cooled to - 10°) is evaporated to dryness to give crude Co(C6F5)(CO)4 (18.2 g, 53.8 mmoles. Yield 82%. This compound can be purified by sublimation at 20-25° under reduced pressure. Anal. Calcd. for Co(C6F5)(CO)4 C, 35.53 Co, 17.43. Found C, 35.89 Co, 17.06. 

The complex is a yellow, crystalline solid soluble in organic solvents, including hexane. The solid is unstable and decomposes slowly at room temperature and rapidly in solution. It can be stored indefinitely at - 20° under nitrogen. Melting point 36°. The infrared spectrum shows v(CO) at 2125(s), 206(Xvs), and 2040(vs) (hexane). The complex serves as a useful reagent for the preparation of carbonyl phosphine cobalt(I) complexes, as one or two CO groups can be readily displaced at room temperature by different ligands. 

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Introduction of the cobalt atom into the corrin ring is preceeded by conversion of hydrogenobyrinic acid to the diamide (34). The resultant cobalt(II) complex (35) is reduced to the cobalt(I) complex (36) prior to adenosylation to adenosylcobyrinic acid i7,i -diamide (37). Four of the six remaining carboxyhc acids are converted to primary amides (adenosylcobyric acid) (38) and the other amidated with (R)-l-amino-2-propanol to provide adenosylcobinamide (39). Completion of the nucleotide loop involves conversion to the monophosphate followed by reaction with guanosyl triphosphate to give diphosphate (40). Reaction with a-ribazole 5 -phosphate, derived biosyntheticaHy in several steps from riboflavin, and dephosphorylation completes the synthesis. 

More recently Schrauzer, Weber, and Beckham (159) showed the existence of equilibria involving the loss of a proton from the (r-alkylcobalt(III) complex to give a Tr-olefin-cobalt(I) complex, i.e.. 

The cobalt(I) complex CoBr(PPh3)3 as a boron trifluoride etherate selectively hydrogenates conjugated dienes to monoenes via an unusual 1,2-hydrogen addition at the more-substituted double bond (186). 

Although the preparative chemistry of (vinylketene)cobalt(I) complexes is relatively limited in the literature, the methods used include all the major procedures that have been more widely exploited in the analogous chromium and iron systems. There are many similarities between the intermediates involved in the synthesis of vinylketene complexes of iron, chromium, and cobalt, but as the metal is varied the complexes containing analogous ligands often exhibit significant differences in stability and reactivity (see Sections II and VI). Comparison of such species has often been an important aim of the research in this area. The (vinylketene)cobalt(I) complexes have also been shown to be synthetically useful precursors to a variety of naphthols, 2-furanones, ce-pyrones, phenols,6,22,95 >8, y-unsaturated esters,51 and furans.51,96a... 

The cobalt(I) complexes also react with organic aryl azides to form the terminal cobalt(III) imido complexes [(TIMEN" OCo(NAr )]Cl (Ar = xyl, mes, R — p-PhMe, p-PhOMe) at —35°C (Fig. 17). These deep-green complexes are fully characterized, including NMR, IR, UV-Vis spectroscopy, and combustion analysis 10). These are diamagnetic (d low-spin, S — 0), and the NMR spectra suggest a Ca-symmetry of these molecules in solution. 

Cobalt catalyst precursors are cobalt(III) or cobalt(II) salts ligated to nitrogen and eventually oxygen-containing polydentate molecules like B12, salen, C2(DO)(DOH)p . The III/II electroreduction occurs at around OV vs SCE. Further reduction at ca. — 1 V vs SCE corresponds to the formation of cobalt(I) complexes which are the reactive species involved in the reactions mentioned below. 

In a classic study, Hume and KolthofF[13] obtained polarographic evidence that, in a 1 M aqueous solution of potassium cyanide, Co(H20)(CN)s is irreversibly reduced at a dropping mercury electrode to a cobalt(I) species, the composition of which was not elucidated. furthermore, the cobalt(I) complex was reported to undergo neither oxidation nor reduction. In addition, the cobalt(III) complex, Co(H20)(CN)5 , was seen to be reducible at the dropping mercury electrode, whereas Co(CN)6 is not electroactive. In earlier work [14], cobalt(II) cyanide complexes were reduced electrolytically to cobalt(I) cyanide species. 

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

A number of publications have appeared on the synthesis of cationic cobalt(I) complexes by known routes. The cations are of the well-established type [Co(CNR)5]+ (R = aryl) (119-122). Reactions of aromatic isocyanides with Co2(CO)g in refluxing toluene have given the fully substituted cobalt(O) dimer Co CNR) (R = xylyl, C6H2Me3-2,4,6, C6H2Br-4-Me2-2,6) (25,123). 

Cobalt(i) complexes [Co(chel)], where chel = NN -ethylene(acetylacetonedi-imi-nato), iViV -ethylenebis(iViV -dimethylsalicylideneiminato), NN -ethylenebis(salicyli-... 

Peone and Vaska (58, 59) have reported the synthesis of a five-coordinate cobalt(I) complex, Co(CO)2(Ph3P)20C103, by treating the corresponding chloro complex with AgC104 in benzene. In a tetrahe-... 

In contrast to the thermal reactions of 1-alkynylboronic esters, the cycloaddition smoothly takes place under very mild conditions in the presence of metal catalysts (Scheme 28). A cobalt(i) complex catalyzed the [4+2]-cycloaddition of alkynyl boronates with 1,3-dienes to give cycloalkenyl boronates 266431 and with ct,cu-diynes giving arylboronates... 

Scheme 7.8 Preparation of the half-sandwich type cyclopentadienylcobalt(I) complexes 32 and 33 from cobaltocene and their degradation with alkali metals M (Li or K) in the presence of THF or tmeda to give the anionic olefin cobalt(-I) complexes 34-37 (the coordination of THF and tmeda to the cations M+ is omitted for clarity)...

Finally, phosphole sulfides can behave as 77 4-electron or Tj -CjTj -Y-b-electron donors toward transition metals. For example, derivative 106 reacts with CpCo(CO)2 to afford the [7] -(phosphole sulphide)]cobalt(I) complex 107 (Scheme 31), which was characterized by X-ray diffraction <2006HAC344>. The chromium complex 109 was obtained in high yield from thioxophosphole 108 and its molecular structure determined crystallographically (Scheme 31). 

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