The tripodal ligand N[CH2CH2NHC(O)NHC(CH3)3]3 ([H61]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [Co(II/III)H31(OH)](2-/1-), [Fe(II/III)H31(OH)](2-/1-), and [Zn(II)H31(OH)](2-) have been isolated and characterized. The source of the hydroxo ligand in these complexes is water, which was confirmed with an isotopic labeling study for [Co(III)H31(OH)](1-). The synthesis of [M(II)H31(OH)](2-) complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of < or = 40% for [Co(II)H31(OH)](2-). When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the M(II)H31(OH)](2-) complexes. This improvement is attributed to the presence of an intramolecular basic site within the cavity, which scavenges protons produced during formation of the M(II)-OH complexes. The molecular structures of [Zn(II)H31(OH)](2-), [Fe(II)H31(OH)](2-), [Co(II)H31(OH)](2-), and [Co(III)H31(OH)](1-) were examined by X-ray diffraction methods. The complexes have trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M(II)-OH complexes crystallized with nearly identical lattice parameters, and each contains two independent anions in the asymmetric unit. The complexes have intramolecular H-bonds from the urea cavity of [H31](3-) to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (>2.00 A) compared to those of the few previously characterized synthetic examples. The longer bond distances in [M(II)H31(OH)](2-) reflect the intramolecular H-bonds in the complexes. The five-coordinate [Zn(II)H31(OH)](2-) has an average Zn-O(H) distance of 2.024(2) A, which is similar to that found for the zinc site in carbonic anhydrase II (2.05(2) A). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [Co(II)H31(OH)](2-) and [Fe(II)H31(OH)](2-) have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yield their corresponding M(III)-OH complexes. [Co(III)H31(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co(III) complex.

Nine new pseudotetrahedral Zn(II) complexes of the heteroscorpionate ligands (3-tert-butyl-2-hydroxy(or thio)-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, L1OH or L2SH, have been prepared and characterized (in most cases crystallographically). Complexes isolated include [(L1O)ZnCl], [(L1O)ZnI], [(L1OH)ZnI(2)], [(L1O)ZnCH(3)], [(L1O)ZnOAc], [(L1O)ZnSPh], [(L1O)ZnSBz], [(L2S)ZnCH(3)], [(L2S)ZnSPh], and [(L2S)(2)Zn]. In conjunction with the widely studied tris(pyrazolyl)borates this new series of heteroscorpionates provides a set of isolobal and isoelectronic ligands differing in donor set, i.e. N(3), N(2)O, and N(2)S. Preliminary reactivity studies with HX species, MeI, or trimethyl phosphate suggest differences between the three sets of ligands.

Manganese-oxo complexes have been widely studied because of their importance in biological processes and their utility as synthetic reagents. 1 In biological systems, these species are proposed as intermediates in certain catalases 2 and peroxidases, 3 and in the oxidation of water to O 2 in the oxygen-evolving complex of photosynthesis. 4 Synthetic manganese-oxo complexes, such as those with imines 5 and porphyrin ligands, 6 have been postulated to be the reactive species in catalytic oxidations of various organic compounds. These oxo complexes are believed to have high-valent manganese centers in either 4+ or 5+ oxidation states. The isolation and structural analysis of highvalent, mononuclear manganese-oxo complexes is limited to only Mn(V)dO complexes of tetraanionic chelating ligands. 7 In contrast, low-valent manganese complexes (e.g., 3+ valence) with terminal oxo ligands are not known. Low-valent manganese complexes contain µ-oxo bridges, where Mn(III)-(O) n -Mn(III) dimers (n ) 1, 2) are the norm. 8 This report describes the preparation and properties of a monomeric Mn(III)-oxo complex and its Mn(III)-hydroxo analogue. The isolation of these complexes is accomplished by using the chelating ligand [H 3 1] 3-, which forms a protective hydrogen-bonding cavity around the Mn(III)-O(H) units.The importance of H-bonding in regulating metal ion reactivity is exemplified by metalloproteins. Several metalloproteins have active sites that contain either H-bond donors or acceptors that interact with external ligands that are covalently bonded to a metal center. 9,10 Efforts to mimic this multimode binding in synthetic complexes have involved both heme and non-heme systems. 11

Nature uses hydrogen bonds to regulate a variety of metal-based reactions. These effects are emulated in the stabilization of trigonal-bipyramidal, paramagnetic Co-OH complexes such as the mono- or dianionic redox-active complex 1 by use of a new hydrogen-bonding ligand.