A family of dioxo-molybdenum(VI) complexes of N2X heteroscorpionate ligands of relevance to molybdoenzymes.

Four new Mo(VI)-dioxo complexes of a family of N2X heteroscorpionate ligands are reported which, together with data already available for (TpR)-, provide a unique example of a comprehensive set of isostructural, isoelectronic complexes differing only in one biologically relevant donor atom. A study of these complexes allows for a direct comparison of structural, spectroscopic, and oxygen atom transfer reactivity properties of the Mo(VI)-dioxo center (of relevance to various families of molybdoenzymes) as a function of donor atom identity.

Donor atom dependent geometric isomers in mononuclear oxo-molybdenum(V) complexes: implications for coordinated endogenous ligation in molybdoenzymes.

We have previously demonstrated that the complex [(L1O)MoOCl(2)], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.; Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281-1291). Here, the cis isomer is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this report, we show that for complexes of [(LO)MoOCl(2)], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl(2)] (1) include that it crystallizes in the triclinic space group P(-)1 with cell dimensions a = 8.9607 (12) A, b = 10.596 (4) A, c = 13.2998 (13) A, alpha = 98.03 (2) degrees, beta = 103.21 (2) degrees, gamma = 110.05(2) degrees, and Z = 2. [(L1O)MoO(OPh)Cl].2CH(2)Cl(2) (2.2CH(2)Cl(2)) crystallizes in the triclinic space group P(-)1 with cell dimensions a = 12.2740 (5) A, b = 13.0403 (5) A, c = 13.6141 (6) A, alpha = 65.799 (2) degrees, beta = 64.487 (2) degrees, gamma = 65.750 (2) degrees, and Z = 2. [(L3S)Mo(O)Cl(2)] (3) crystallizes in the orthorhombic space group Pna2(1), with cell dimensions a = 13.2213 (13) A, b = 8.817 (2) A, c = 15.649 (4) A, and Z = 4. The implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the DMSO reductase, are discussed.

Modulating the reduction potential of mononuclear cobalt(II) complexes via selective deprotonation of tris[(2-benzimidazolyl)methyl]amine.

Remote site deprotonation of the coordinated tripodal ligand, tris((2-benzimidazolyl)methyl)amine, was examined using electronic spectroscopy and electrochemistry techniques. The solid-state structures [CoH(3)1(tba)(NCS)]+ and [CoH(2)1(tba)(NCS)] are reported. These complexes crystallized in the triclinic space group P1 [a = 13.3043(2) A, b = 13.8019(2) A, c = 14.1322(2) A, alpha = 63.6670(10) degrees, beta = 68.0590(10) degrees, gamma = 81.8960 degrees; Z = 2] and the monoclinic space group P2(1)/n [a = 15.3530(9) A, b = 11.0645(6) A, c = 19.1319(10) A, beta = 105.6750(10) degrees; Z = 4], respectively. Preliminary results suggest that selective and reversible deprotonation of coordinated benzimidazolyl ligands can tune the reduction potential of several isostructural cobalt(II) complexes.

Hydrogen-Bonding Cavities about Metal Ions: Synthesis, Structure, and Physical Properties for a Series of Monomeric M-OH Complexes Derived from Water.

The tripodal ligand N[CH2CH2NHC(O)NHC(CH3)3]3 ([H6 1]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [CoII/IIIH3 1(OH)]2-/1-, [FeII/IIIH3 1(OH)]2-/1- , and [ZnIIH3 1(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 [CoIIIH3 1(OH)]1-. The synthesis of [MIIH3 1(OH)]2- complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of ≤40% for [CoIIH3 1(OH)]2-. When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the MIIH3 1(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 MII-OH complexes. The molecular structures of [ZnIIH3 1(OH)]2-, [FeIIH3 1(OH)]2-, [CoIIH3 1(OH)]2-, and [CoIIIH3 1(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 MII-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 [H3 1]3- to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (>2.00 Å) compared to those of the few previously characterized synthetic examples. The longer bond distances in [MIIH3 1(OH)]2- reflect the intramolecular H-bonds in the complexes. The five-coordinate [ZnIIH3 1(OH)]2- has an average Zn-O(H) distance of 2.024(2) Å, which is similar to that found for the zinc site in carbonic anhydrase II (2.05(2) Å). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [CoIIH3 1(OH)]2- and [FeIIH3 1(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 MIII-OH complexes. [CoIIIH3 1(OH)]1-, with an S = 1 ground state, is a rare example of a paramagnetic CoIII complex.

The Synthesis and Characterization of 4, 5, and 6 Coordinate Ni(II) Complexes of the "Heteroscorpionate" Ligand (3-tert-Butyl-2-hydroxy-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane.

Six new Ni(II) complexes of the "heteroscorpionate" ligand (3-tert-butyl-2-hydroxy-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, L1OH, have been prepared and characterized (in most cases crystallographically). These complexes include: tetracoordinate [Ni(L1O)Cl]; pentacoordinate [Ni(L1O)(acac)] and [Ni(L1O)(OAc)]; and hexacoordinate [Ni(L1O)(acac)(pz)], [Ni(L1O)(OAc)(MeOH)], and [Ni(L1O)(2)]. This second generation of heteroscorpionate ligand supports a tetrahedral environment for Ni(II) but is not a tetrahedral enforcer. Thus the tetra- and pentacoordinate species readily add additional ligands to produce 5- and 6-coordinate complexes. The formation of the undesirable, coordinatively saturated, [Ni(L1O)(2)], "sandwich" complex is only a minor interference in most of these reactions unlike the situation seen with the previous generation of ligand. This family of heteroscorpionates shows promise as platforms for biomimetic studies which are already underway.

Synthesis and Characterization of Pseudotetrahedral N(2)O and N(2)S Zinc(II) Complexes of Two Heteroscorpionate Ligands: Models for the Binding Sites of Several Zinc Metalloproteins.

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.

Hydrogen-Bonding Cavities about Metal Ions: A Redox Pair of Coordinatively Unsaturated Paramagnetic Co-OH Complexes.

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.