The high-spin dichloro Mn 2+ and Fe 2+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1) and 4, 10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2) provide durable new compounds of these elements for important fundamental studies and applications. The compounds are especially noteable for their exceptional kinetic stabilities and redox activity. The X-ray crystal structures of all four complexes demonstrate that the ligands enforce a distorted octahedral geometry on the metals with two cis sites occupied by labile chloride ligands. Magnetic measurements reveal that all are high spin with typical magnetic moments. Cyclic voltammetry of the complexes shows reversible redox processes at +0.110 and +0.038 V (versus SHE) for the Fe 3+ /Fe 2+ couples of Fe(1)Cl 2 and Fe(2)Cl 2 , respectively, while the Mn 3+ /Mn 2+ and Mn 4+ /Mn 3+ couples were observed at +0.585 and +1.343 V, and +0.466 and +1.232 V for the complexes Mn(1)Cl 2 and Mn(2)Cl 2 , respectively. Mn 2+ (1) was found to react with H 2 O 2 and other oxidizing agents to produce the Mn 4+ (1) complex. The catalytic efficacy of Mn 4+ (1) in aqueous solution has been assessed in the epoxidation reaction of carbamazepine and hydrogen abstraction reaction with 1,4-cyclohexadiene. The complex has been found to be a selective catalyst, exhibiting moderate catalytic activity in oxygen transfer, but significantly more effective catalytic activity in hydrogen abstraction reactions.

A novel monomeric tetravalent manganese complex with the cross-bridged cyclam ligand 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (Me2EBC), [Mn(IV)(Me2EBC)(OH)2](PF6)2, was synthesized by oxidation of Mn(II)(Me2EBC)Cl2 with H2O2 in the presence of NH4PF6)in aqueous solution. The X-ray crystal structure determination of this manganese(IV) compound revealed that it contains two rare terminal hydroxo ligands. EPR studies in dry acetonitrile at 77 K show two broad resonances at g = 1.96 and 3.41, indicating that the manganese(IV) exists as a high-spin d3 species. Resonance Raman (rR) spectra of this manganese(IV) species reveal that the dihydroxy moiety, Mn(IV)(OH)2, is also the dominant species in aqueous solution (pH < 7). pH titration provides two pK(a) values, 6.86(4) and 10.0(1), associated with stepwise removal of the last two oxygen-bound protons from [Mn(IV)(Me2EBC)(OH)2](2+). The cyclic voltammetry of this manganese(IV) complex in dry acetonitrile at 298 K demonstrates two reversible redox processes at +0.756 and -0.696 V (versus SHE) for the Mn4+/Mn3+ and Mn3+/Mn2+ couples, respectively. This manganese(IV) complex is relatively stable in weak acidic aqueous solution but easily degrades in basic solution to manganese(III) derivatives with an 88 +/- 1% yield.

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A family of Mn3+ and Fe3+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1) and 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (2) has been prepared by the chemical oxidation of the divalent manganese and iron analogues. The ligands are ethylene cross-bridged tetraazamacrocycles derived from cylam and cyclen, respectively. The synthesis and characterization of these complexes, including X-ray crystal structure determinations, are described. The structural evidence demonstrates that the tetradentate ligands enforce distorted octahedral geometries on the metal ions, with two cis sites occupied by labile ligands. Magnetic measurements reveal that the complexes are high spin with typical magnetic moments. Cyclic voltammetry shows reversible redox processes for the Fe3+/Fe2+ couples of the iron(III) complexes, while Mn3+/Mn2+ and Mn4+/Mn3+ couples were observed for the complexes with manganese(III). The manganese chemistry of 1 was studied in depth. The dichloro manganese(III) cation of 1 undergoes facile ligand substitution reactions at the labile, monodentate sites, for example substituting azide for chloride ligands. Air oxidation of the dichloro complex of Mn (1)2+ in basic solution does not give the expected mu-oxo dimeric product common to manganese. Instead, an unusual manganese(III)-OH complex has been isolated from this reaction and structurally characterized. A similar reaction under slightly different conditions gives a putative MnIII(OH)2 complex that metathesizes to MnIII(OMe)2 upon recrystallization from methanol.

The electronic transitions of the binuclear ferric active site of methemerythrin and oxyhemerythrin (oxyHr) are assigned using variable temperature circular dichroism and polarized single crystal absorbance spectroscopies. These transitions are correlated with crystallographic information to obtain insight into the electronic structure of the oxo-bridged binuclear high spin ferric active site. Ligand field bands are assigned to each iron, and the energy shifts of these bands with variation of exogenous ligand are used to evaluate the ligand field strength of these ligands, particularly the peroxide of oxyHr. Peroxide is found to bind as a very strong field ligand, with both strong u-donor and very weak n-donor character. The exogenous ligand to Fe(II1) charge-transfer spectrum shows that both azide and peroxide have a strong bonding interaction at the active site in addition to the bond to the iron. This is consistent with protonation of the peroxide as suggested from earlier Raman data and with the ligand field strength of peroxide in oxyHr. The intense UV transitions are assigned as oxo-Fe charge transfer and are used to generate a model of the electronic structure of the oxo-bridged iron dimers and in particular the change in the Fe-oxo bond with variation in Fe-0-Fe angle. These results provide insight into the reversible binding of dioxygen by the Hr active site. (1) (a) Okamura, M. Y.; Klotz, I. M. Inorganic Biochemistry; Eichhorn, G. L., Ed.; Elsevier: 1973; pp 320-343. (b) Kurtz, D. M., Jr.; Shriver, D. F.; Klotz, I. M. Coord. Chem. Gray, H. B.; Hoenig, H. E.; Rossman, G. R.; Schredder, J. M.; Wang, R.-H. Biochemistry 1972, 11, 461-465. (c) Okamura, M. y.; Klotz, 1. M.; Johnson, C. E.; Winter, M. R. C.; Williams, R. (7) Stenkamp, R. E.; Sieker, L. C.; Jensen, L. H. J. Am. Chem. SOC. 1984, 106, 618-622. (8) Stenkamp, R. E.; Sieker, L. C.; Jensen, L. H.; McCallum, J. D.; (9) The terminology for met'H20" or met-aquo" Hr is from early work (10) Note. however. a weak C1--Fe(II1) CT transition does, in fact, Sanders-Loehr, J. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 713-716. on the protein" which indicated H20 was bound to the metal site. contribute to the 300-400-nm region (vide infra). Biochem. Biophys. 1969, 135, 419-434. 0 1989 American Chemical Societv (11) Garbett, K.; Darnall, D. W.; Klotz, I. M.; Williams, R. J. P. Arch. , , I -2004

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The parallel syntheses of two new cross-bridged tetraazamacrocyclic complexes whose ligands are derived from 1,4,8,11-tetraazacyclotetradecane (cyclam = 14N4) and rac-1,4,8,11-tetraaza-5,5,7,12,12,14-hexamethylcyclotetradecane (tetB = 14N4Me(6)) have been characterized through the crystal structure determination of every stepwise intermediate ligand in the multistep ligand syntheses. These structures show that although the final ligand skeletons are nearly identical, the immediate precursors differ greatly because of the six additional methyl groups of the 14N4Me(6) macrocycle. The inversion from one diastereomer to another of the tetracycle derived from rac-14N4Me(6) has been chemically induced through the successive addition of methyl groups to the reactive tertiary nitrogens, and the novel heterocycles produced have been crystallographically characterized with one showing a conformation not previously known for these systems. The structures of the two copper(II) complexes have significant geometrical differences, and accordingly, their electrochemical and spectroscopic properties are compared. The complexes exhibit remarkable kinetic stability under harsh conditions.

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