Pentacyanocobaltate II

In order to gain further insight into the role of superoxocobalt(III) complexes in the oxygenation of hindered phenols, 

Oxidation of 1 by the superoxocobalt(III) complex was observed under N. When 0 was bubbled through the DMF solutions at 0 - 25 °C, 

O -insertion into the phenol occurred. Excellent regioselectivity was 

H-atom from 1, which is in contrast with the reported behavior of the superoxocobalt(III) Schiff-base complexes [27]. Coordinated superoxide 

Copper(II) acetate in MeOH containing morpholine gives o-benzoquinone as the primary product [13,65]. Aryloxy radicals are formed in a side-reaction only. Copper(I)-amine systems in non-polar solvents afford aryloxy radicals from phenols, and the corresponding coupling products [66-69]. Copper(II) complexes of aliphatic and heterocyclic amines are known to catalyze the oxidation of substituted phenols [70-72]. The basicity and steric properties of the ligand influence the nature of oxidation products. 

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A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

The [Co(CN)5]3 complex is an effective catalyst for some reactions, particularly the isomerization of alkenes. Newer and more efficient catalysts have been developed for some of the processes, but the catalytic behavior of the pentacyanocobalt(II) ion is also significant from a historical perspective. In reactions such as that shown in Eq. (22.10), two Co2+ ions increase one unit in oxidation state, instead of the more common situation in which one metal ion increases by two units in oxidation state. The cobalt complex also reacts with CIT3I, Cl2, and H202, which are indicated as X-Y in the equation... 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

The interconversion of a- and 7r-allyl complexes has been observed by Kwiatek, Mador, and Seyler in the interesting homogeneous catalytic system of potassium pentacyanocobaltate(II), K3Co(CN)5 (46). This solution absorbs molecular hydrogen to form the active hydride species, Kj[Co (CN)jH]. Addition of butadiene to the hydride results in the formation of a [Pg.36]

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

CN-stretching frequencies, 12 387 kinetic data for, 12 413 ligand substitution reactions, 12 406 Pentacyanocobalt(II) exchange reactions of, 10 201, 202 kinetic data for, 10 202... 

Carl H. Brubaker, Jr. I agree with Dr. Yalman that this represents a very complete piece of work, and I think, the majority of the conclusions are fairly clear cut. There is not much that can be added aside from speculation. I would hope that a little later Prof. Wilmarth or others will speculate about the structures of this transition state species, or several species of the pentacyanocobaltate(II) that are supposed to be the transition state complex, or an intermediate. 

Jack Halpem I would like to comment further on the question that Dr. Anbar raised about the possibility of generating the pentacyanocobalt(III) by electron transfer from pentacyanocobalt(II). 

Dr. Brubaker I have been interested personally in the work that has come from Australia in the last year by Betts and Winfield and others on the oxidation also of the pentacyano with oxygen. Here they find the oxygen binds a proposed intermediate species which may be a monoperoxo monomer, which this group scavenges very well for the pentacyanocobalt(II) and forms the familiar decacyano-/x-peroxyldicobalt(III) complex. So in this case too, the oxidant sticks, not water. 

The pentacyanocobalt(II) ion reacts with acetylene to form a yellow crystalline salt K6[Co2(CN)io(C2H2)HH20 108). The nuclear magnetic resonance spectrum of the ion shows a single proton resonance line in the... 

Organic nitroxides (136a, 5), nitroparafins, and nitrolic acids (107) react with aqueous solutions of pentacyanocobaltate(II) salts to give alkyl pentacyanocobalt(III) nitroxide anion radicals, spin traps involving the formation of an N—Co bond (5). 

It is possible that a small portion of the hydroxo complex is also formed by the reaction of pentacyanocobaltate(II) with hydrogen peroxide, which is known to be almost quantitative (4). No cyanocobaltate(III) species is known to activate hydrogen, and we have observed that the addition of hexacyanocobaltate(III) to CoH (H2 atmosphere) does not result in absorption of hydrogen. 

A similar explanation is employed as for benzoquinone, except that there is no evidence of interaction of pentacyanocobaltate(II) itself with benzaldehyde, the normal quantity of hydrogen being absorbed when the catalyst is formed in the presence of an excess of the substrate. 

Initial complexes formed from the interaction of CoH and hydrogen peroxide, nitrobenzene, or anthiaquinone either react further with the excess CoH present to form pentacyanocobaltate(II) or are spontaneously hydrolyzed to yield hydroxypentacyanocobaltate(III). The latter species may then undergo the reverse aging process with CoH, forming pentacyanocobaltate(II), thereby effecting catalytic hydrogenation. 

Since the pentacyanocobaltate(II) system is simply prepared and hydrogenations are rapid, we now have an additional hydrogenation method suitable... 

The pentacyanocobaltate(II) ion has long been known to catalyze alkene hydrogenation, mainly of conjugated dienes. A review of the early work is available.45 The catalyst system shows negligible activity for the hydrogenation of non-activated monoenes. A major disadvantage is that the system is inhibited by excess substrate, and the turnover numbers obtained are generally less than 2. 

The overall course of reaction of pentacyanocobaltate(II) with methyl and benzyl halides is depicted by the scheme of Equations 5, 6, and 7, in which the initial halogen abstraction step is rate determining. 

Ohkubo K, Kanaeda H, Tsuchihashi K. An MO-theoretical interpretation of the reductive cleavage of organic halides by pentacyanocobaltate(II). Bull Chem Soc Jpn 1973 46(10) 3095-3098. 

A yellow form of the pentacyanocobaltate(II) ion has been observed in DMF solution. Various monomeric (NR4)3[Co(CN)5] complexes have been crystallized26 and a structural study on one shows the anion to possess a truly five-coordinate square-pyramidal geometry.27 Electron irradiation of solid K3[Co(CN)6] gives a presumed pentacyanocobaltate(II) ion,28 the visible spectrum of which is virtually identical with that of [Co(CN)5]3a ). The structural parallels between the green and yellow forms of [Co(CN)5]3 and isoelectronic [Co(CNR)5]2+ ions indicate that all the green forms observed both in the solid state and in solution are weakly coordinated in the sixth axial position. 

Potassium pentacyanocobaltate(II), derived form cobalt(II) chloride and KCN, catalyzes the hydrogenation of 1,3-dienes to monoalkenes. 1,3-Butadiene (22), isoprene and 1-phenyl-1,3-butadiene (23), have been converted to mixtures of the corresponding 1-butenes, fra s-2-butenes and cw-2-butenes. The product distribution depends highly on reaction conditions such as the cyaniderCo ratio, the concentra-... 

Although does not form inner-orbital 6-co-ordinate complexes, it does produce a spin-paired 5-co-ordinate pentacyanocobaltate(II) ion of bipyra-midal form (pp. 105 and 495) ... 

The hydrido complex may also arise from sources other than molecular hydrogen. The homolytic cleavage of water by pentacyanocobaltate (II) (9, 16) (Reaction 2) permits various substrates to be reduced stoichiometrically in the absence of molecular hydrogen (20). 

Figure 2. The effect of acrylonitrile on propylene and propane yields from isopropyl iodide and pentacyanocobaltate(II) (0.08M)...

Figure 3. Amounts of propylene formed at various ratios of isopropyl iodide to pentacyanocobaltate(II) (0.08M) in the presence of...

Table I. Effect of Additives on Relative Amounts of Propylene and Propane Formed from Equimolar Quantities of Isopropyl Iodide and Pentacyanocobaltate(II)...

Alkyl Halides. Reaction with Co (CN) 5. Manya-bondedorgano-pentacyanocobaltate(III) complexes have been prepared recently by the reaction of organic halides with pentacyanocobaltate(II) (i3, 14, 21, 22) (Reaction 5). The second-order kinetics observed for this reaction (14)... 

These observations may be interpreted as being derived from a pentacyanocobaltate( II)-catalyzed disproportionation of an isopropyl radical intermediate (Radical Mechanism I) (Reactions 9-11). 

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