22045-50-7Relevant academic research and scientific papers
Oxidation of azidopentaamminecobalt(III) ion by hydroxyl radicals
Boucher, Heather,Sargeson, Alan M.,Sangster, David F.,Sullivan, James C.
, p. 3719 - 3721 (1981)
Oxidation of [Co(NH3)5(N3)]2+ with hydroxyl radical (k = 1.1 × 109 M-1 s-1) has been observed to yield a surprisingly stable coordinated radical, presumed to be [Co(NH3)5NNNOH·]2+ which decays by a first-order intramolecular redox process (k = 60 ± 10 s-1 at 25°C) to give Co(II), N2, and N2O as products. N2O appears to arise from dimerization of NOH to H2N2O2 which decomposes in acidic solution to N2O and H2O.
Synthesis, solution structure, and reactivity of oxygen-bound amides on cobalt(III)
Angus, Patricia M.,Fairlie, David P.,Jackson, W. Gregory
, p. 450 - 459 (2008/10/08)
A series of pentaamminecobalt(III) amide complexes containing oxygen-bonded amides are reported. The electronic structure of the amide ligand remains delocalized on coordination to the metal ion, and there is evidence for increased polarization of the amide upon coordination. There is restricted rotation about the carbon-nitrogen bond as shown by separate NMR signals for the amide nitrogen substituents. Also the unsymmetrically substituted formamides (HCONHCH3 and HCONHC6H5) are in both the Z and E configurations for both the free and oxygen-coordinated species. In aqueous acid solution and in Me2SO the complexes slowly producing free amide; the rates of solvolysis have been measured. Complexes of formamides (HCONR1R2) solvolyze more slowly than those of carbon-substituted amides (R3CONR1R2); e.g., 106kH = 5.25 s-1 and 118 s-1 for formamide-O and acetamide-O. In both categories electron-releasing substituents (-CH3, -C2H5) on the amide nitrogen retard solvolysis compared with the primary amide complexes, while electron-withdrawing substituents on the amide nitrogen accelerate it. Complexes with an electron-withdrawing substituent on the amide carbon solvolyze fastest; e.g. for fluoroacetamide-O, 106kH = 1300 s-1. In basic solution at 22°C (0.1 M NaOH, 1.0 M NaClO4) all formamide complexes undergo ligand hydrolysis (N,N-diethylformamide-O, 46%; N,N-diphenylformamide-O, 90%) producing (formato)pentaamminecobalt and free amine; the balance is hydroxopentaamminecobalt(III) complex. Rates of reaction have been measured and are correlated with the nature of the substituent on the formamide nitrogen; kOH = 24.2 M-1 s-1 for formamide-O and 0.32 M-1 s-1 for N,N-diethylformamide-O, I = 1.1 M (NaClO4), 25°C. The pKa of the formamide-O complex is 11.9, and that for the formanilide-O complex, 12.0. In contrast the carbon-substituted amide complexes are less reactive coward ligand hydrolysis, the major product being the hydroxopentaamminecobalt(III) ion. The acetamide-O complex yields only 1 % (acetato)pentaamminecobalt(III), and the chloroacetamide- and fluoroacetamide-Ocomplexes yield 7% and 8% of the relevant carboxylato ions, respectively. However complexes of benzamide, acrylamide, acetanilide, and Nmethyl-, N,N-dimethyl-, and N,N-diethylacetamide yield only the hydroxopentaamminecobalt(III) complex under the same conditions. The rates of these reactions have been measured and acidity constants for some primary and secondary amide complexes have been determined: kOH = 30.2 M-1 s-1, pKa 11.6, acetamide-O; kOH = 33 M-1 s-1, pKa 10.6, benzamide-O; kOH = 70 M-1 s-1, pKa 9.7, acetanilide-O; kOH = 150 M-1 s-1, pKa 9.4, chloroacetamide-O; kOH = 55 M-1 s-1, pKa 9.7, fluoroacetamide-O [I = 1.00 M, NaClO4, 25°C]. The base hydrolysis of the dimer [(NH3)5CoOCHNHCo(NH3)5]5+ has been investigated. The reaction is slow and proceeds largely by cobalt-oxygen cleavage but with detectable Co-N cleavage. No amide O- to N-bonded linkage isomerization was detected for any of these complexes, and the reactivity of coordinated amides is compared with that of coordinated ureas and cyclic amides.
Synthesis and reactivity of the pentaamminecobalt(III) linkage isomers of succinimide
Angus, Patricia M.,Jackson, W. Gregory
, p. 4806 - 4813 (2008/10/08)
The linkage isomers of (succinimido)pentaamminecobalt(III) have been selectively synthesized and characterized by 1H and 13C NMR, IR, and UV-visible spectroscopies. The deprotonated imide ligand bonds to the metal through oxygen or nitrogen. The oxygen-bonded isomer is the less stable form. In water (ksON = 1.7 × 10-4 s-1, 25°C) and Me2SO (ksON = 5.1 × 10-5 s-1) it spontaneously isomerizes to the nitrogen-bonded form; in aqueous acid (pKa = 2.7, I = 0.1 M, LiClO4) and acidified Me2SO it protonates and rapidly solvolyzes; the protonated species in water has reactivity comparable (kH = 2.3 × 10-2 s-1, I = 0.1 M, LiClO4, 25°C) to the most reactive isolable [(NH3)5CoX]n+ species known. In aqueous base three competing reactions have been detected, namely solvolysis (40%, 25°C), base-catalyzed O- to N-bonded linkage isomerization (30%), and nucleophilic attack on the coordinated carbonyl group by hydroxide ion leading to the formation of the carboxylate-bonded isomer of (succinamato)pentaamminecobalt(III) (30%) (kOH(obsd) = 9.0 × 10-2 M-1 s-1, I = 0.1 M, KF, 25°C). The individual rates and rate laws for all these reactions have been determined. In acid and base the nitrogen-bonded imido complex is less reactive than the O-bonded form. It is base hydrolyzed relatively slowly, and a term second order with respect to hydroxide ion is dominant in the rate law (kN = 6.1 × 10-3 M-2 s-1, I = 1.0 M, NaClO4, 25°C); 18O studies establish the reversible addition of OH- in the first step. The product is the nitrogen-bonded succinamato complex, which has been characterized through crystallization in its basic and acidic forms (pKa = 1.8 (amide) and 3.55 (carboxylic acid), I = 1.0 M, NaCl, 25°C). The succinimido-N complex is protonated in water and Me2SO only in very strong acid. The protonated species has been crystallized and characterized; it is a strong acid (pKa 1/2 = days, 25°C). A N- to O-bonded isomerization reaction has not been detected. The structure and reactivity of these imide complexes are compared with those of the related amide and urea complexes.
Reactivity of coordinated phosphate esters: Pentaamminecobalt(III) complexes
Hendry, Philip,Sargeson, Alan M.
, p. 92 - 97 (2008/10/08)
The reactivity of two phosphate ester complexes designed to test the efficacy of different modes of activation of phosphate esters by metal ions has been investigated. Ethyl 4-nitrophenyl phosphate coordinated to the pentaamminecobalt(III) moiety liberates nitrophenolate in basic solution 106-fold faster than the free phosphodiester. The reaction proceeds via attack of coordinated amido ion to yield a four-membered N,O chelate phosphoramidate ethyl ester. The four-membered chelate does not display the enhanced reactivity of the five-membered-ring cyclic ethylene phosphates but decays with cobalt-ligand bond rupture and yields finally free ethyl phosphoramidate. The aminolysis is accompanied by some loss of ethyl 4-nitrophenyl phosphate by the SN1(CB) mechanism. The binuclear complex (μ-nitrophenyl phosphato)decaamminedicobalt(4+) undergoes aminolysis in basic aqueous media, also by intramolecular attack of coordinated amido ion. The reaction proceeds some 102-fold faster than the analogous aminolysis of the mononuclear complex, (4-nitrophenyl phosphato)pentaamminecobalt(1+). The reaction is also accompanied by some SN1(CB) loss of the intact ligand; in this case, the ligand is the mononuclear complex. This study illuminates some of the modes by which metal ions can enhance the reactivity of phosphate esters. In agreement with other studies, the electrostatic and inductive effects are estimated to contribute ～102-fold to the rate enhancement, while the intramolecularity of the reaction is responsible for the remainder of the observed rate enhancement.
