The conformational behaviour of a series of monohydroxamic acids, p-RC,H,CONR'OH (R = Me, R' = H, Me; R = MeO, R' = H, Me; R = NO2, R' = H), and a series of dihydroxamic acids, (CH,),(CONR'OH), (n = 3-8, 10, R' = H and n = 7, R' = Me), in methanol, DMSO and chloroform and in the solid state has been examined using IR and NMR spectroscopy. X-Ray crystal structure determinations of p-MeC,H,CoNMeOH and the monohydrate of glutarodihydroxamic acid ( n = 3) together with ab initio molecular orbital calculations for several hydrated and unhydrated hydroxamic acids have been performed. Hydrogen bonding effects are shown to be important in both the solid state and solution. The cis(Z) conformation of the hydroxamate group(s) (CONHOH) is preferentially stabilized by hydrogen bonding with water molecules.

The first example of cis-truns isomerism in monoalkylhydroxamic acids, detected by 'H, I3C and "N NMR spectroscopy, is reported. The "N NMR spectrum of CH,CO"NHOH gave a clear assignment of the OH and NH protons of both 2 and E isomers. The assignment was confirmed by 'H and "C NMR spectroscopy. A rotational barrier of isomerization (AC,') of 17.8 kcal mol-' was calculated from the variabletemperature 'H NMR spectra using the method of Shanan-Atidi and Bar-Eli.

'H and "C NMR studies of N-methyl-substituted hydroxamic acids, RCON(CH,)OH (R = CH,, C,H, and C6Hs), show that the series exhibits &-trans isomerism about the C-N bond. The Z/E ratio increases in the series CH, < C,H, < n-CsH,, < n-C,H,, for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH, group. In DMSO-d, the 2 isomer is preferentially stabilized by solvation, whereas in C,D6 and toluene-d,, the unsolvated E form is thermodynamically more stable, as predicted by ab initio molecular orbital calculations. (12) At high concentrations (in CDCI, , CD,CI, , C6D6 and toluend,,) the E isomer may also be stabilized by intermolecular association. In the 0-methyl-substituted series, RCONHOCH, (R = CH,, C6H,), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.

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The cobalt(II) urease model complex [Co(2)(mu-OAc)(3)(urea)(tmen)(2)][OTf] (2) prepared from the cobalt model hydrolase [Co(2)(mu-H(2)O)(mu-OAc)(2)(OAc)(2)(tmen)(2)] (1) undergoes facile reaction with acetohydroxamic acid (AHA) to give the monobridged hydroxamate complex [Co(2)(mu-OAc)(2)(mu-AA)(urea)(tmen)(2)][OTf]( )()(3) while 1 gives the dibridged hydroxamate complex [Co(2)(mu-OAc)(mu-AA)(2)(tmen)(2)][OTf] (4). The structures and Co-Co distances of the hydroxamate derivatives of 1 and 2 are very close to those of their nickel analogues and suggest that hydroxamic acids can also inhibit cobalt-based hydrolases as well as inhibiting urease. 1 also reacts with glutarodihydroxamic acid (gluH(2)A(2)) to eliminate hydroxylamine with formation of [Co(2)(mu-OAc)(2)[mu-O(N) (OC)(2)(CH(2))(3)](tmen)(2)][OTf] (5), the structure of which is very close to that of its nickel analogue. Both 1 and 3 show weak antiferromagnetic coupling. Oxidation of 1 with H(2)O(2) gives three dicobalt(III) hydroxy complexes (7-9), the first of which [Co(2)(mu-OAc)(2)(OAc)(2)(mu-OH)(tmen)(2)][OTf] (7) contains a bridging hydroxyl and the second [Co(2)(mu-OAc)(2)(OAc)(mu-OH)(OH)(tmen)(2)][OTf] (8) containing both a bridging and terminal hydroxyl, while the third [Co(2)(mu-OAc) (OAc)(2)(mu-OH)(2)(tmen)(2)][OTf] (9) contains two bridging OH groups with mixed-valence Co(II)/(Co(III) intermediates.