Synthesis, Characterization, and Reactivity of Urea Derivatives Coordinated to Cobalt(III). Possible Relevance to Urease

The syntheses of a cobalt(III) complex, 2, containing N-(2-pyridylmethyl)urea and of six complexes, 3, containing phenyl-substituted N-2-pyridylmethyl-N‘-(X)phenylureas (where X = 4-H, 4-CH3, 4-Br, 3-Cl, 4-CF3, and 4-NO2), have been accomplished by reaction of [(en)2Co(OSO2CF3)2](CF3SO3) with the urea ligands in tetramethylene sulfone. The complexes have been characterized by UV−vis, FTIR, 1H NMR, and 13C NMR spectra along with elemental analysis. Also, X-ray crystallographic analysis of 2 confirms that the urea ligand chelates as a bidentate through the pyridyl nitrogen atom and the endo deprotonated, urea nitrogen atom to form a stable five-membered ring. Crystals of the perchlorate salt of 2 were monoclinic, space group P21/c with a = 9.743(1) Å, b = 13.924(3) Å, c = 15.006(4) Å, β = 97.07(1)°, and Z = 4. Reflection data (3454) with I = 3σ(I) were refined to conventional R factors of 0.037 and 0.051. In acidic solution (0.05−1.00 M HCl at 55 °C), the phenyl-substituted complexes undergo hydrolysis to form the bis(ethylenediamine)(2-picolylamine-N,N‘)cobalt(III) ion, 4, aniline, and CO2. The hydrolysis kinetics of the phenyl-substituted complexes were studied by UV−vis spectroscopy (I = 1.00 M HCl/LiCl). At 55 °C the observed rate constants fit the rate law k obsd = kK[H+]/(1 + K[H+]). It is proposed that the protonated urea eliminates aniline to give a coordinated isocyanate intermediate that hydrolyzes rapidly to the pyridyl methylamine complex and CO2 via the carbamate complex. Since all of the studies of this kind to date appear to involve the NCO intermediate, it raises the prospect that urease also functions by a similar path and that urease should be tested with NCO- as a substrate.