REACTIVITY AND COORDINATION CHEMISTRY OF AROMATIC CARBOXAMIDE RC(O)NH2 AND CARBOXYLATE LIGANDS - PROPERTIES OF PENTAAMMINERUTHENIUM(II) AND PENTAAMMINERUTHENIUM(III) COMPLEXES

The pH dependences of the spectral and electrochemical properties of mononuclear carboxamido (NH3)5RuNHC-(O)R (R = Ph, 4-py-N-Me+, 4-py-N-H+) and carboxylato (NH3)5RuOC(O)R (R = 4-py-N-Me+) complexes of Ru(II) and Ru(III) in aqueous solution have been examined. In contrast to the carboxylate complex (E1/2=-0.053 V vs NHE), the deprotonated (-NHC(O)R-) Ru(III/II) couples have rather negative reduction potentials, -0.25 (R = Ph), -0.23 (R = 4-py), and -0.13 (R = N-Me-4-py) V vs NHE, which are pH independent above the pK(a) of the Ru(II) complex (pH 4-8 depending upon R). In contrast, the carboxamido-Ru(III) complexes are weak bases, being protonated only in strongly acidic solutions (e.g. 5 M HClO4). From the structural work (d(Ru(III)-amido N) for R = N-Me-4-py is 1.998(9) angstrom) and the behavior of the ligand-to-metal charge-transfer bands in carboxamido-Ru(III) complexes, considerable oxygen pip=ruthenium(III) pid bonding is inferred. The electronic absorption spectra of carboxamido ruthenium(II) complexes, produced either by reduction of the corresponding Ru(III) complex at high pH or by direct reaction of (NH3)5Ru(OH2)2+ with the amide in 0.01 M NaOH, exhibit intense (epsilon (2.5-7) x 10(3) M-1 cm-1) bands in the visible region arising from metal-to-ligand (aromatic ring) charge-transfer transitions. Rate and equilibrium constants for formation of the protonated Ru(II) amide complexes with R = Ph and R = 4-py-N-Me+ at 25-degrees-C and 0.1 M ionic strength are (7 +/- 1) X 10(-2) M-1 s-1 and 2 X 10(-3) M-1 for the first R and 1.2 x 1 10(-2) M-1 s-1 and 2.0 x 10(-3) M-1 for the second, respectively. For the carboxylate under similar conditions, the values 0.6 M-1 s-I and 0.6 M-1 are obtained. The free amide ligand N-methylisonicotinamide triflate undergoes rapid alkaline hydrolysis, yielding N-methylisonicotinate and ammonia under exceptionally mild conditions (2-h half-life in 0.01 M NaOH, 15-degrees-C). At 15, 25, and 35-degrees-C and 1 M ionic strength the rate of hydrolysis (monitored by UV-vis spectroscopy) is first order in the amide concentration. The dependence on hydroxide ion is between first and second order. Thus at 25-degrees-C, the second-order rate constant increases from 1.43 X 10(-2) M-1 s-1 at 0.01 M OH- to 3.08 x 10(-2) M-1 s-1 at 0.05 M OH-. The structures of the Ru(III) carboxamide [(NH3)5Ru(C7N2H8O)]-(ClO4)3, R = 4-py-N-Me+, and of the N-pyridyl-bonded Ru(II) complex of isonicotinamide (NH4)[Ru(NH3)5(C6N2H6O)](PF6)3 are reported.