The electrochemistry of nine monomeric molybdenum(V)-oxo complexes in dimethylformamide has been investigated by cyclic voltammetry and controlled-potential coulometry at a platinum electrode. MoOC13L (L = o-phenanthroline, ol,a'-bipyridyl), MoOClL2 (L = 8-hydroxyquinoline, 8-mercaptoquinoline), and MoOClL (L = disalicylaldehyde ophenylenediimine, N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine, N,N'-bis(2-mercapto-2-methylpropyl)-ethylenediamine) are facilely reduced by one-electron reductions to Mo(1V) species. (C2H5)4NMoOC12(salicylaldehyde o-hydroxyanil) is reduced in a two-electron step to an Mo(1II) species. None of the complexes are oxidizable in the voltage range used (+OS0 to -2.50 V vs. SCE) to Mo(V1) complexes. Comparison with reduction peaks for molybdenum(V1)-dioxo complexes indicates the Mo(V) monomers are not obtainable by electrochemical reduction of the Mo(V1) complexes, and are, in some cases, thermodynamically unstable to disproportionation into Mo(1V) and Mo(V1). Implications for redox states in molybdenum enzymes are discussed. IntroductionMolybdenum is now well established as a necessary cofactor for a number of redox and there is considerable current interest in the properties and reactions of its complexes as possible models for these During catalysis, Mo(V) has been identified by electron spin resonance (ESR) spectrometry for the molybdenum enzymes xanthine oxidase, aldehyde oxidase, sulfite oxidase, and nitrate redu~tase.',~*~ The ESR signal appears to arise from a monomeric Mo(V) center as a result of electron transfer to or from the ~u b s t r a t e . '~~~~ Clearly, a knowledge of the structures, properties, and reactions of monomeric Mo(V) complexes is of importance for an understanding of these enzymes.The aqueous chemistry of Mo(V) is dominated by ESR

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The pH dependence of the acylation of (I) and the deacylation of (III) shows a plateau region in the intermediate range.

All three possible monomeric macrocyclic produds and one dimeric product have now been isolated and spectrally characterized from the recently described3 macrocyclization reaction which leads to imidazole-containing coronands.Alkali metal ion promoted reactions provide a small selectivity for the largest of the three monomeric macrocycles, coronand 1, and leads to improved yields (57%). Regimelective internal vs external N-*lation of the imidazole derivatives of 1 is controlled by the counter cation employed. Small cations lead to external N-alkylation, while large cations lead to internal N-alkylation. This is evidence of selective cation binding within the cavity for the lithium and sodium imidazolates, but for potassium and larger ions, binding is at the external peripheral nitrogen atoms. Larger ions are likely excluded from the cavity because of the higher conformational energy costa involved.The X-ray structure of the symmetrically protected derivative of 1 has been obtained and confirms previous structural assignments. (1) (a) Di Vaira, M.; Mani, F.; Stoppioni, P. J. Chem. SOC., Chem. Commun. 1989,127. (b) Knapp, S.; Keenan, T. P.; Zhang, X.; Fikar, R.; Potenza, J. A.; Schugar, H. J. and references cited within. (c) van Veggel, F. C. J. M.; Bos, M.; Harkema, S.; van de Bovenkamp, H.; Verboom, W.; Reedijk, J.; Reinhoudt, D. N. J. Org. Chem. 1991,56,225 and references cited within. (d) Traylor, T. G.; Hill, K. W.; Tain, Z.-Q.; Reingold, A. L.; Peisach, J. McCracken, J. Am. Chem. SOC. 1988, 110, 5571-5573 and references cited within. (3) Street, J. P. Tetrahedron Lett. 1991, 32, 3333. J.