Cobalt-induced facile degradation of phenylurea to ammonia, carbon dioxide, and anilinium ion and other reactions of linkage isomeric cobalt(III) complexes of phenylurea

Abstract

A search for metal ion promoted hydrolysis of urea and the factors influencing linkage isomerization on a metal has led us to prepare and examine reactivities of N- and O-bound phenylurea complexes of pentaamminecobalt(III). The N-bonded isomer reacts in aqueous acid to yield principally [Co(NH3)6]3+, CO2, and anilinium ion (ca. 70%) but also via parallel paths leading to [(NH3)5CoOC(NH2)NHC6H5]3+ (linkage isomerization) and [(NH3)5CoOH2]3+ (aquation). The major path involves an elimination reaction of [(NH3)5CoNH2CONHC6H5]3+ to give anilinium ion and [(NH3)5CoNCO]2+. The latter complex, which has been isolated and characterized, is known to undergo rapid hydration (pH l 2) to [(NH3)5CoNH2CO2H]3+, which subsequently decomposes along three parallel paths to [Co(NH3)6]3+ and traces of [(NH3)5CoOH2]3+ and [(NH3)5CoOCONH2]3+. Kinetic data for the three parallel reactions of [(NH3)5CoNH2CONHC6H5]3+ in aqueous HCIO4 (1.0 M, 25.0 dC) are kelim= 1.04 X 10-2 s-1 (elimination), kNo = 2.39 X 10-3 s-1 (isomerization), and kaq = 1.13 X 10-3 s-1 (aquation). A single-crystal structure reveals that the urea exo C-N bond is 0.11 A longer than the endo C-N bond and, as in uncoordinated phenylurea, the electron-with-drawing phenyl ring is neither planar with nor conjugated to the urea moiety. These features are likely retained in the protonated form and account for the facility of the elimination path. By contrast, neither elimination nor hydrolysis of the O-bonded phenylurea complex was detected; instead it slowly undergoes parallel aquation and O- to N-linkage isomerization. Both processes are base-catalyzed, and each obeys a rate law of the form k(obsd) = ks + Koh[Oh-]. The equilibrium constant (K'No= Kno/Kon= 166) reflects the strong thermodynamic preference for the O rather than the N terminus of the neutral phenylurea molecule. However, the observed equilibrium is pH-dependent (K'(obsd) = K'NO[H+]/(Ka + [H+])); for pH g 3, the N-bonded isomer is more stable due to its selective deprotonation (K'NO(obsd) a 10n5, pH 6.2). The single-crystal structure determination of [(NH3)5CoNHCONHC6H5](C104)2-H20 shows the cobalt in an approximately octahedral environment with phenylurea bonded as its anion through the indicated nitrogen center. As observed for the free ligand, the phenyl substituent is twisted (37d) with respect to the approximately planar urea moiety. The compound crystallizes in the monoclinic system (b = 91.64d), with V = 1887.0 A3, space group C2/c, Z = 8, a = 15.751 (6) A, b = 12.277 (5) A, c = 9.762 A, and R = 0.046 and RW = 0.043 for 1381 diffractometer data (244 variables) with 1 g 3s(I), m = 105.47 cm-1, and F(000) = 1023.96.