SYNTHESIS, SOLUTION STRUCTURE, AND REACTIVITY OF OXYGEN-BOUND AMIDES ON COBALT(III)

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., 10(6)k(H) = 5.25 s-1 and 118 s-1 for formamide-O and acetamide-0. 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-0, 10(6)k(H) = 1300 s-1. In basic solution at 22-degrees-C (0.1 M NaOH, 1.0 M NaClO4) all formamide complexes undergo ligand hydrolysis (N,N-diethylformamide-0, 46%; NN-diphenylformamide-0, 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; k(OH) = 24.2 M-1 s-1 for formamide-0 and 0.32 M-1 s-1 for NN-diethylformamide-0, I = 1. 1 M (NaClO4), 25-degrees-C. The pK(a) of the formamide-O complex is 11.9, and that for the formanilide-0 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-O complexes yield 7% and 8% of the relevant carboxylato ions, respectively. However complexes of benzamide, acrylamide, acetanilide, and N-methyl-, N,N-dimethyl-, and NN-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: k(OH) = 30.2 M-1 s-1, pK(a) 11.6, acetamide-O; k(OH) = 33 M-1 s-1, pK(a) 10.6, benzamide-0; k(OH) = 70 M-1 s-1, pK(a) 9.7, acetanilide-O; k(OH) = 150 M-1 s-1, pK(a) 9.4, chloroacetamide-0, k(OH) = 55 M-1 s-1, pK(a) 9.7, fluoroacetamide-0 [I = 1.00 M, NaClO4,25-degrees-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 0- 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.