Hydroxo complexes cobalt

The complexes [LCo(p-02)(p-OH)CoL] [L = en, trien, dien, tetra-ethylenepentamine, or tris-(2-aminoethyl)amine] have been studied, and the new complexes [[Co(imidazole)(gly)2 202],4H20 [ Co2(imidazole)2-(gly)402 0H],3H20, and [Co(imidazole)(gly)2(02)H20] have been prepared The spectroscopic properties of various p-peroxo- and p-superoxo-cobalt(iii) complexes have been examined. The singly-bridged p-peroxo-compounds have a strong band at 300 nm, whereas this falls at 350 nm for p-peroxo-p-hydroxo-complexes and two peaks at 480 and 700 nm are observed for p-superoxo-species. The i.r. spectra of p-peroxo-bridged complexes of cobalt(iii)-cyclam have been reported. ... 

All the examples so far have been homonuclear. Complexes (14) provide examples of hetero-nuclear di- -OH cations, with M = Cu, Mn, Co, Ni or Zn, in solution.73 NiI+/Al3+ dimers with di- -OH have been characterized in the solid state, and may well also exist in solution.74 Although cocrystallization of cobalt and chromium hydroxo complexes normally gives the statistically expected mixture of Co2, Cr2 and CoCr di-/i-OH species (difficult to separate), if optically active starting materials are used a very high yield of the mixed dinuclear species [(en)2Cr(jt-OH)2Co(en)2]4+ can be obtained. 

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

The inertness of the dinuclear complexes is greatest in slightly acidic solutions, which therefore have been employed for the reprecipitation reactions. Apparently the chromium systems are much more labile toward bridge breaking than are the cobalt systems. In aqueous solution the meso-[(en)2Cr(OH)2Ci(en)2] cation (I) enters into a rapidly established (t 1 min. at room temperature) equilibrium with the mono-ju-hydroxo complex [(OHXen)2Cr(OH)Cr(en)2-(HaO)] (n) The equilibrium constant K = [II]/[I] is 0.83 in 1 Af NaC104 at 0°. The salts (dithionate, bromide, chloride, and perchlorate) of the di-p-hydroxo cation are less soluble than the respective salts of the mono-/i-hydroxo cation. It is therefore possible to precipitate the pure salts of the di-/i-hydroxo cation from the equilibrium mixture following the procedure given above. 

Cobalt oxide and soluble aquo or hydroxo complexes have been known as water oxidation catalysts since the early 1980s, under heterogeneous and homogeneous conditions [52,53]. However, cobalt ions in aqueous solution present poor stability, showing detrimental separation as insoluble oxides, these latter being characterized by very low activity. 

These metal-alkynyl complexes can be protonated to afford the free alkynes and parent cobalt hydroxo complex (comparable reactivity to their alkyl and aryl congeners), but have proven inert toward oxygenation and carbonylation. They are also thermally stable up to 100 °C. Attempts to explore the reactions of these compounds with unsaturated hydrocarbons were typically fruitless. The one exception is the reaction between 53 and its parent alkyne (HC = C02Me, Scheme 6), which under benzene reflux effects catalytic, stereospecific, linear trimerisation of the alkyne to afford ( , )-buta-l,3-dien-5-yne. The reaction was, however, slow (4.5 turnovers in 20 h) and suffered from catalytic deactivation due to hydrolysis of 53, which subsequently reacted with adventitious CO2 to irreversibly form an inert /x-carbonato complex. The catalytic cycle was concluded to involve initially a double coordination-insertion of the C = C bond of methylpropiolate into the Co-Caikyne linkage. Subsequent hydrolysis of the Co-C bond by a third equivalent of HC = CC02Me would then afford the observed product and regenerate 53. However, a definitive explanation for the stereospecificity of the process was not established. 

Acid aquation of the bisoxalato-di-/x-hydroxo-complex (15) involves bridge fission to give dioxalatodiaquocobaltate(m) previous to electron transfer, en route to the final cobalt(n) product. Another reaction involving substitution and redox is that of (16) with nitrous acid. The active... 

Iron(II) complex of tris(N -tert-butylurea-ylato)-N-ethylene]aminato activates dioxygen at room temperature to afford an iron(III) complex containing a single terminal oxo ligand. X-ray structures show that the three urea molecules act as a tridentate N,N,N-hgand [52]. The tripodal ligand was also used to synthesise complexes of cobalt, iron or zinc with terminal hydroxo ligands (Scheme 8) [53]. 

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