Pyrazolylborate-zinc-nucleobase-complexes, 3:(1) base pairing studies

In solution, the pyrazolylborate-zinc-nucleobase complexes show self-association and base pairing with external nucleobases. The self-association was studied quantitatively for Tp(Cum,Me)Zn-hypoxanthinate and Tp(Cum,Me)Zn-thyminate; the dimerization constants K(D) are 63 +/- 8 and 0.2 +/- 0.1 M(-1), respectively. Of the external nucleobases, 9-ethyladenine forms stable base pairs with the thyminate, uracilate, and xanthinate complexes, 9-isobutylguanine only with the cytosinate complex, 1-methylthymine with the adeninate and diaminopurinate complexes, and 1-methyluracil with the diaminopurinate complex. The association constant for the base pair Tp(Cum,Me)Zn-thyminate:9-ethyladenine was determined by NMR methods as K = 66 +/- 10 M(-1). Structure determinations of the crystalline adducts have confirmed the base pairing for Tp(Cum,Me)Zn-thyminate:9-ethyladenine, Tp(Cum,Me)Zn-cytosinate:9-isobutylguanine, and Tp(Cum,Me)Zn-xanthinate:9-ethyladenine. Both Watson-Crick and Hoogsteen base pairs have been observed. In the solid state, extended base pairing leads to quartet and polymer arrangements.     

Synthesis and structural comparison of a series of divalent Ln(Tp(R,R)')2 (Ln = Sm, Eu, Yb) and trivalent Sm(Tp(Me2))2X (X = F, Cl, I, BPh4) complexes

Reaction of LnI2 (Ln = Sm, Yb) with two equivalents of NaTp(Me2) or reduction of Eu(Tp(Me2))2OTf gives good yields of the highly insoluble homoleptic Ln(II) complexes, Ln(Tp(Me2))2 (Ln = Sm (1a), Yb (2a), Eu (3a)). Use of the additionally 4-ethyl substituted Tp(Me2,4Et) ligand produces the analogous, but soluble Ln(Tp(Me2,4Et))2 (1-3b) complexes. Soluble compounds are also obtained with the Tp(Ph) and Tp(Tn) ligands (Tn = thienyl), Ln(Tp(Ph))2 (Ln = Sm, 1c; Yb, 2c) and Ln(Tp(Tn))2 (Ln = Sm, 1d; Yb, 2d). To provide benchmark parameters for structural comparison the series of Sm(Tp(Me2))2X complexes (X = F, 1e; Cl, 1f; Br, 1g; I, 1h; BPh4, 1j) were prepared either via oxidation of the Sm(Tp(Me2))2 or salt metathesis from SmX3 (X = Cl, Br, I). The solid-state structures of 1-3a, 1b, 1-2c and 1e, 1f, 1h, and 1j were determined by single-crystal X-ray diffraction. The homoleptic bis-Tp complexes are all six-coordinate with trigonal antiprismatic geometries, planes of the kappa(3)-Tp ligands are parallel to one another. In the series of Sm(Tp(Me2))2X complexes the structure changes from seven-coordinate molecular compounds, with intact Sm-X bonds, for X = F, Cl, to six-coordinate ionic structures [Sm(Tp(Me2))2]X (X = I, BPh4), suitable crystals of the bromide compound could not be obtained. The dependence of the structures on the size of X is understandable in terms of the interplay between the size of the cleft that the [Sm(Tp(Me2))2](+) fragment can make available and the donor ability of the anionic group toward the hard Sm(III) center.     

Small Molecule Chemistry of the Pyrazolylborate-Zinc Unit Tp(Cum,Me)Zn

The synthesis of the highly encapsulating pyrazolylborate ligand hydrotris(3-p-cumenyl-5-methylpyrazolyl)borate (L = Tp(Cum,Me)) and of its zinc hydroxide complex L.Zn-OH (1) are described. 1 is converted by H(2)S into the hydrosulfide complex L.Zn-SH (2). Both 1 and 2 seem to be contaminated with traces of the isomeric species 1' and 2' containing L' with one 3-methyl-5-p-cumenyl substituent. Thermal condensations of 1' and 2 yield the molecular zinc oxide and sulfide complexes L'.Zn-O-Zn.L' (3') and L.Zn-S-Zn.L (4). The hydroxide complex 1 has been found to react readily with cumulated double-bonded species: CO(2) is incorporated in alcoholic solutions to form the alkylcarbonate complexes L.Zn-OCOOR (5). Similarly, CS(2) in ethanol forms the O-ethyl dithiocarbonate complex L.Zn-SC(S)OEt (6). SO(2) is converted to a bridging sulfito ligand in L.Zn-O-SO-O-Zn.L (7), and phenyl isothiocyanate is bound as a thiocarbamidato ligand in L.Zn-SC(O)NHPh (8). Complexes 1, 2, 2', 3', 4, 5, and 6 have been confirmed by structure determinations and complexes 7 and 8 by spectral data.     

Protonation and reactivity towards carbon dioxide of the mononuclear tetrahedral zinc and cobalt hydroxide complexes, [Tp(Bu)t(,Me)]ZnOH and [Tp(Bu)t(,Me)]CoOH: comparison of the reactivity of the metal hydroxide function in synthetic analogues of carbonic anhydrase

The tris(3-tert-butyl-5-methylpyrazolyl)hydroborato zinc hydroxide complex [Tp(Bu)t(,Me)]ZnOH is protonated by (C(6)F(5))(3)B(OH(2)) to yield the aqua derivative [[Tp(Bu)t(,Me)]Zn(OH(2))][HOB(C(6)F(5))(3)], which has been structurally characterized by X-ray diffraction, thereby demonstrating that protonation results in a lengthening of the Zn-O bond by ca. 0.1 A. The protonation is reversible, and treatment of [[Tp(Bu)t(,Me)]Zn(OH(2))](+) with Et(3)N regenerates [Tp(Bu)t(,Me)]ZnOH. Consistent with the notion that the catalytic hydration of CO(2) by carbonic anhydrase requires deprotonation of the coordinated water molecule, [[Tp(Bu)t(,Me)]Zn(OH(2))](+) is inert towards CO(2), whereas [Tp(Bu)t(,Me)]ZnOH is in rapid equilibrium with the bicarbonate complex [Tp(Bu)t(,Me)]ZnOC(O)OH under comparable conditions. The cobalt hydroxide complex [Tp(Bu)t(,Me)]CoOH is likewise protonated by (C(6)F(5))(3)B(OH(2)) to yield the aqua derivative [[Tp(Bu)t(,Me)]Co(OH(2))][HOB(C(6)F(5))(3)], which is isostructural with the zinc complex. The aqua complexes [[Tp(Bu)t(,Me)]M(OH(2))][HOB(C(6)F(5))(3)] (M = Zn, Co) exhibit a hydrogen bonding interaction between the metal aqua and boron hydroxide moieties. This hydrogen bonding interaction may be viewed as analogous to that between the aqua ligand and Thr-199 at the active site of carbonic anhydrase. In addition to the structural similarities between the zinc and cobalt complexes, [Tp(Bu)t(,Me)ZnOH] and [Tp(Bu)()t(,Me)]CoOH, and between [[Tp(Bu)t(,Me)]Zn(OH(2))](+) and [[Tp(Bu)t(,Me)]Co(OH(2))](+), DFT (B3LYP) calculations demonstrate that the pK(a) value of [[Tp]Zn(OH(2))](+) is similar to that of [[Tp]Co(OH(2))](+). These similarities are in accord with the observation that Co(II) is a successful substitute for Zn(II) in carbonic anhydrase. The cobalt hydroxide [Tp(Bu)()t(,Me)]CoOH reacts with CO(2) to give the bridging carbonate complex [[Tp(Bu)t(,Me)]Co](2)(mu-eta(1),eta(2)-CO(3)). The coordination mode of the carbonate ligand in this complex, which is bidentate to one cobalt center and unidentate to the other, is in contrast to that in the zinc counterpart [[Tp(Bu)t(,Me)]Zn](2)(mu-eta(1),eta(1)-CO(3)), which bridges in a unidentate manner to both zinc centers. This difference in coordination modes concurs with the suggestion that a possible reason for the lower activity of Co(II)-carbonic anhydrase is associated with enhanced bidentate coordination of bicarbonate inhibiting its displacement.     

Xenophilic complexes bearing a Tp(R) Ligand, [Tp(R)M--M'Ln] [Tp(R) = Tp(iPr2), Tp(#) (Tp(Me2,4-Br)); M=Ni, Co, Fe, Mn; M'Ln = Co(CO)4, Co(CO)3(PPh3), RuCp(CO)2]: the two metal centers are held together not by covalent interaction but by electrostatic attraction

A series of dinuclear complexes, [Tp(R)M--M'L(n)] [Tp(iPr(2) )M--Co(CO)(4) (1; M=Ni, Co, Fe, Mn); Tp(#)M--Co(CO)(4) (1'; M=Ni, Co); Tp(#)Ni--RuCp(CO)(2) (3')] (Tp(iPr(2) )=hydrotris(3,5-diisopropylpyrazolyl)borato; Tp(#) (Tp(Me(2),4-Br))=hydrotris(3,5-dimethyl-4-bromopyrazolyl)borato), has been prepared by treatment of the cationic complexes [Tp(iPr(2) )M(NCMe)(3)]PF(6) or the halo complexes [Tp(#)M--X] with the appropriate metalates. Spectroscopic and crystallographic characterization of 1-3' reveals that the tetrahedral, high-spin Tp(R)M fragment and the coordinatively saturated carbonyl-metal fragment (M'L(n)) are connected only by a metal-metal interaction and, thus, the dinuclear complexes belong to a unique class of xenophilic complexes. The metal-metal interaction in the xenophilic complexes is polarized, as revealed by their nu(CO) vibrations and structural features, which fall between those of reference complexes: covalently bonded species [R--M'L(n)] and ionic species [M'L(n)](-). Unrestricted DFT calculations for the model complexes [Tp(H(2) )Ni--Co(CO)(4)], [Tp(H(2) )Ni--Co(CO)(3)(PH(3))], and [Tp(H(2) )Ni--RuCp(CO)(2)] prove that the two metal centers are held together not by covalent interactions, but by electrostatic attractions. In other words, the obtained xenophilic complexes can be regarded as carbonylmetalates, in which the cationic counterpart interacts with the metal center rather than the oxygen atom of the carbonyl ligand. The xenophilic complexes show divergent reactivity dependent on the properties of donor molecules. Hard (N and O donors) and soft donors (P and C donors) attack the Tp(R)M part and the ML(n) moiety, respectively. The selectivity has been interpreted in terms of the hard-soft theory, and the reactions of the high-spin species 1-3' with singlet donor molecules should involve a spin-crossover process.     

A series of dinuclear homo- and heterometallic complexes with two or three bridging sulfido ligands derived from the tungsten tris(sulfido) complex [Et(4)N][(Me(2)Tp)WS(3)] (Me(2)Tp = hydridotris(3,5-dimethylpyrazol-1-yl)borate)

The generation of heterobimetallic complexes with two or three bridging sulfido ligands from mononuclear tris(sulfido) complex of tungsten [Et(4)N][(Me(2)Tp)WS(3)] (1; Me(2)Tp = hydridotris(3,5-dimethylpyrazol-1-yl)borate) and organometallic precursors is reported. Treatment of 1 with stoichiometric amounts of metal complexes such as [M(PPh(3))(4)] (M = Pt, Pd), [(PtMe(3))(4)(micro(3)-I)(4)], [M(cod)(PPh(3))(2)][PF(6)] (M = Ir, Rh; cod = 1,5-cyclooctadiene), [Rh(cod)(dppe)][PF(6)] (dppe = Ph(2)PCH(2)CH(2)PPh(2)), [CpIr(MeCN)(3)][PF(6)](2) (Cp = eta(5)-C(5)Me(5)), [CpRu(MeCN)(3)][PF(6)], and [M(CO)(3)(MeCN)(3)] (M = Mo, W) in MeCN or MeCN-THF at room temperature afforded either the doubly bridged complexes [Et(4)N][(Me(2)Tp)W(=S)(micro-S)(2)M(PPh(3))] (M = Pt (3), Pd (4)), [(Me(2)Tp)W(=S)(micro-S)(2)M(cod)] (M = Ir, Rh (7)), [(Me(2)Tp)W(=S)(micro-S)(2)Rh(dppe)], [(Me(2)Tp)W(=S)(micro-S)(2)RuCp] (10), and [Et(4)N][(Me(2)Tp)W(=S)(micro-S)(2)W(CO)(3)] (12) or the triply bridged complexes including [(Me(2)Tp)W(micro-S)(3)PtMe(3)] (5), [(Me(2)Tp)W(micro-S)(3)IrCp][PF(6)] (9), and [Et(4)N][(Me(2)Tp)W(micro-S)(3)Mo(CO)(3)] (11), depending on the nature of the incorporated metal fragment. The X-ray analyses have been undertaken to clarify the detailed structures of 3-5, 7, and 9-12.     

Synthesis, structures, and reactivity of yttrium alkyl and alkynyl complexes with mixed Tp(Me2)/Cp ligands

The structurally characterized Tp(Me2)-supported rare earth metal monoalkyl complex (Tp(Me2))CpYCH(2)Ph(THF) (1) was synthesized via the salt-metathesis reaction of (Tp(Me2))CpYCl(THF) with KCH(2)Ph in THF at room temperature. Treatment of 1 with 1 equiv of PhC≡CH under the same conditions afforded the corresponding alkynyl complex (Tp(Me2))CpYC≡CPh(THF) (2). Complex 1 exhibits high activity toward carbodiimides, isocyanate, isothiocyanate, and CS(2); treatment of 1 with such substrates led to the formation of a series of the corresponding Y-C(benzyl) σ-bond insertion products (Tp(Me2))CpY[(RN)(2)CCH(2)Ph] (R = (i)Pr(3a), Cy(3b), 2,6-(i)Pr-C(6)H(3)(3c)), (Tp(Me2))CpY[SC(CH(2)Ph)NPh] (4), (Tp(Me2))CpY[OC(CH(2)Ph)NPh] (5), and (Tp(Me2))CpY(S(2)CCH(2)Ph) (6) in 40-70% isolated yields. Carbodiimides and isothiocyanate can also insert into the Y-C(alkynyl) σ bond of 2 to yield complexes (Tp(Me2))CpY[(RN)(2)CC≡CPh] (R = (i)Pr(7a), Cy(7b)) and (Tp(Me2))CpY[SC(C≡CPh)NPh] (9). Further investigation results indicated that 1 can effectively catalyze the cross-coupling reactions of phenylacetylene with carbodiimides. However, treatment of o-allylaniline with a catalytic amount of 1 gave only the benzyl abstraction product (Tp(Me2))CpY(NHC(6)H(4)CH(2)CH═CH(2)-o)(THF) (10), without observation of the expected organic hydroamination/cyclization product. All of these new complexes were characterized by elemental analysis and spectroscopic properties, and their solid-state structures were also confirmed by single-crystal X-ray diffraction analysis.     

Zinc complexes of Ttz(R,Me) with O and S donors reveal differences between Tp and Ttz ligands: acid stability and binding to H or an additional metal (Ttz(R,Me) = tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)

Alkylzinc complexes, (Ttz(R,Me))ZnR' (R = tBu, Ph; R' = Me, Et), show interesting reactivity with acids, bases and water. With acids (e.g. fluorinated alcohols, phenols, thiophenol, acetylacetone, acetic acid, HCl and triflic acid) zinc complexes of the conjugate base (CB), (Ttz(R,Me))ZnCB, are generated. Thus the B-N bonds in Ttz ligands are acid stable. (Ttz(R,Me))ZnCB complexes were characterized by (1)H, (13)C-NMR, IR, MS, elemental analysis, and, in most cases, single crystal X-ray diffraction. The four coordinate crystal structures included (Ttz(R,Me))Zn(CB) [where R = Ph, CB (conjugate base) = OCH(2)CF(3) (2), OPh (6), SPh (8), p-OC(6)H(4)(NO(2)) (10); R = tBu, CB = OCH(CF(3))(2) (3), OPh (5), SPh (7)*, p-OC(6)H(4)(NO(2)) (9) (* indicates a rearranged Ttz ligand)]. The use of bidentate ligands resulted in structures [(Ttz(Ph,Me))Zn(CB) (CB = acac (12), OAc (14))] in which the coordination geometries are five, and intermediate between four and five, respectively. Interestingly, three forms of (Ttz(Ph,Me))Zn(p-OC(6)H(4)(NO(2))) (10) were analyzed crystallographically including a Zn coordinated water molecule in 10(H(2)O), a coordination polymer in 10(CP), and a p-nitrophenol molecule hydrogen bonded to a triazole ring in 10(Nit). Ttz ligands are flexible since they are capable of providing κ(3) or κ(2) metal binding and intermolecular interactions with either a metal center or H through the four position nitrogen (e.g. in 10(CP) and HTtz(tBu,Me)·H(2)O, respectively). Preliminary kinetic studies on the protonolysis of LZnEt (L = Ttz(tBu,Me), Tp(tBu,Me)) with p-nitrophenol in toluene at 95 °C show that these reactions are zero order in acid and first order in the LZnEt.     

Elucidating drug-metalloprotein interactions with tris(pyrazolyl)borate model complexes

The tetrahedral zinc complex [(Tp(Me,Ph))ZnOH] (Tp(Me,Ph) = hydrotris(5,3-methylphenylpyrazolyl)borate) was combined with acetohydroxamic acid, 3-mercapto-2-butanone, N-(methyl)mercaptoacetamide, beta-mercaptoethanol, 3-mercapto-2-propanol, and 3-mercapto-2-butanol to generate the complexes [(Tp(Me,Ph))Zn(ZBG)] (ZBG = zinc-binding group). These complexes were prepared to determine the mode of binding for three different types of thiol-derived matrix metalloproteinase (MMP) inhibitors. The solid-state structures of all six metal complexes were determined by X-ray crystallography. The structures reveal that while beta-mercaptoketones and beta-mercaptoamides bind the zinc ion in a bidentate fashion, the three beta-mercaptoalcohol compounds only demonstrate monodentate coordination via the sulfur atom. Prior to this work, no experimental data were available for the binding conformation of these types of inhibitors to the zinc active site of MMPs. The results of these model studies reveal different binding modes for these ZBGs and are useful for explaining the results of inhibition assays and in second-generation drug design. This work demonstrates the utility of model complexes as a tool for revealing drug-metalloprotein interactions.     

67Zn solid-state NMR spectroscopy of ([Tp But,Me]Zn(OH2))[HOB(C6F5)3]. the importance of the anion [HOB(C6F5)3]-

One of the paradigms of Zn2+ metallobiochemistry is that coordination of water to Zn2+ provides a mechanism of activation that involves lowering the pKa by approximately 7 pH units. This idea has become central to the development of mechanisms of action for zinc metalloproteins. However, the direct measurement of the pKa of water bound to Zn2+ in a metalloprotein has yet to be accomplished. Developing models for Zn2+-OH2 species has been a significant challenge, but we have utilized solid-state 67Zn NMR spectroscopy as a means to characterize one of the few examples of water bound to mononuclear tetrahedral Zn2+: ([Tp(But,Me)]Zn(OH2))[HOB(C6F5)3]. The measured quadrupole coupling (Cq) constant is 4.3 MHz with an asymmetry parameter of etaq of 0.6. Likewise, due to the small value of Cq, anisotropic shielding also contributed to the observed 67Zn NMR lineshape. As expected, the computed values of the magnetic resonance parameters depend critically on the nature of the anion. The predicted value of Cq for ([Tp(But,Me)]Zn(OH2))[HOB(C6F5)3] is -4.88 MHz. We discuss the results of these calculations in terms of the nature of the anion, the local electrostatics, and its subsequent hydrogen bonding to [Tp(But,Me)]Zn(OH2)+.     

Nature of the Rh-H(2) Bond in a Dihydrogen Complex Stabilized Only by Nitrogen Donors. Inelastic Neutron Scattering Study of Tp(Me)2RhH(2)(eta(2)-H(2)) (Tp(Me)2 = Hydrotris(3,5-dimethylpyrazolyl)borate)

The tunnel splitting of the librational ground state and the torsional frequencies of the dihydrogen ligand in Tp(Me)()2RhH(2)(eta(2)-H(2)) (Tp(Me)()2 = hydrotris(3,5-dimethylpyrazolyl)borate) were measured using inelastic neutron scattering spectroscopy. The barrier for the rotation of the H(2) ligand and its H-H separation, calculated from these data, are 0.56(2) kcal/mol and 0.94 ?, respectively. These values indicate that pi-back-donation from the Tp(Me)()2RhH(2) fragment to H(2) is relatively weak and/or the interaction between the coordinated dihydrogen molecule and the two cis-hydride ligands significantly lowers the barrier for H(2) rotation.     

Syntheses, structures, and magnetic properties of low-dimensional heterometallic complexes based on the versatile building block [(Tp)Cr(CN)3]-

Using the tricyano precursor (Bu(4)N)[(Tp)Cr(CN)(3)] (Bu(4)N(+) = tetrabutylammonium cation; Tp = tris(pyrazolyl)hydroborate), a pentanuclear heterometallic cluster [(Tp)(2)Cr(2)(CN)(6)Cu(3)(Me(3)tacn)(3)][(Tp)Cr(CN)(3)](ClO(4))(3)·5H(2)O (1, Me(3)tacn = N,N',N'-trimethyl-1,4,7-triazacyclononane), three tetranuclear heterometallic clusters [(Tp)(2)Cr(2)(CN)(6)Cu(2)(L(OEt))(2)]·2.5CH(3)CN (2, L(OEt) = [(Cp)Co(P(O)(OEt)(2))(3)], Cp = cyclopentadiene), [(Tp)(2)Cr(2)(CN)(6)Mn(2)(L(OEt))(2)]·4H(2)O (3), and [(Tp)(2)Cr(2)(CN)(6)Mn(2)(phen)(4)](ClO(4))(2) (4, phen = phenanthroline), and a one-dimensional (1D) chain polymer [(Tp)(2)Cr(2)(CN)(6)Mn(bpy)](n) (5, bpy = 2,2'-bipyridine) have been synthesized and structurally characterized. Complex 1 shows a trigonal bipyramidal geometry in which [(Tp)Cr(CN)(3)](-) units occupy the apical positions and are linked through cyanide to [Cu(Me(3)tacn)](2+) units situated in the equatorial plane. Complexes 2-4 show similar square structures, where Cr(III) and M(II) (M = Cu(II) or Mn(II)) ions are alternatively located on the rectangle corners. Complex 5 consists of a 4,2-ribbon-like bimetallic chain. Ferromagnetic interactions between Cr(III) and Cu(II) ions bridged by cyanides are observed in complexes 1 and 2. Antiferromagnetic interactions are presented between Cr(III) and Mn(II) ions bridged by cyanides in complexes 3-5. Complex 5 shows metamagnetic behavior with a critical field of about 22.5 kOe at 1.8 K.     

Structure and bonding in a cyclobutyl tris(pyrazolyl)boratoniobium complex and the variation in agostic behaviour with ring size in the series Tp(Me2)NbCl(c-C(n)H(2n-1))(MeC[triple bond]CMe), n = 3-6

The synthesis and characterisation of the cyclobutyl complex Tp(Me2)NbCl(c-C4H7)(MeC[triple bond]CMe) completes the family of cycloalkyl complexes Tp(Me2)NbCl(c-C(n)H(2n-1)), n = 3-6. The properties of the cyclobutyl complex are qualitatively similar to those of its cyclopentyl and cyclohexyl analogues, and dramatically different from those of the cyclopropyl derivative. Most conspicuously, the cyclobutyl system has an alpha-C-H agostic interaction in the dominant isomer, with no evidence for the alpha-C-C agostic character found for the smaller ring. C-C agostic character therefore seems to be unique to the cyclopropyl complex, where the acute C-C-C angles destabilise the C-C bonding orbitals.     

Cisplatin--DNA cross-link models with an unusual type of chirality-neutral chelate amine carrier ligand, N,N dimethylpiperazine (Me2ppz): Me2ppzPt(guanosine monophosphate)2 adducts that exhibit novel properties

Most simple cis-PtA2G2 complexes that model the G-G cross-link DNA lesions caused by the clinically used anticancer drug cis-PtCl2(NH3)2 undergo large fluxional motions at a rapid rate (A2 = two amines or a diamine; G = guanine derivative). The carrier amine ligands in active compounds have NH groups, but the fundamental role of the NH groups has been obscured by the dynamic motion. To assess carrier ligand effects, we examine retro models, cis-PtA2G2 complexes, in which dynamic motion has been reduced by the incorporation of steric bulk into the carrier ligands. In this study we introduce a new approach employing the chirality-neutral chelate (CNC) ligand, Me2ppz (N,N'-dimethylpiperazine). Because they lie in the Pt coordination plane, the methyl groups of Me2ppz do not clash with the 06 of the base of G ligands in the ground state, but such clashes sterically hinder dynamic motion. NMR spectroscopy provided conclusive evidence that Me2ppzPt(GMP)2 complexes (GMP = 5'- and 3'-GMP) exist as a slowly interconverting mixture of two dominant head-to-tail (HT) conformers and a head-to-head (HH) conformer. Since the absence of carrier ligand chirality precluded using NMR methods to determine the absolute conformation of the two HT conformers, we used our recently developed CD pH jump method to establish chirality. The most abundant HT Me2ppzPt(5'-GMP)2 form had A chirality. Previously this chirality was shown to be favored by phosphate-cis G NIH hydrogen-bonding interligand interactions; such interactions also favor the HT conformers over the HH conformer. For typical carrier ligands, G O6 and phosphate interactions with the carrier ligand NH groups also favor the HT forms. These latter interactions are absent in Me2ppzPt(GMP)2 complexes, but the HT forms are still dominant. Nevertheless, we do find the first evidence for an HH form of a simple cis-PtA2G2 model with A2 lacking any NH groups. In previous studies, the absence of the HH conformer in cis-PtA2G2 complexes lacking carrier NH groups may be due to the presence of out-of-plane carrier ligand bulk. Such bulk forces both G O6-G O6 and G O6-carrier ligand clashes, thereby disfavoring the HH form. The major DNA cross-link adduct has the HH conformation. Thus, for anticancer activity, the small bulk of the NH group may be more important than the H-bonding interaction.     

Synthesis and characterisation of second-generation metallodithiolene complexes of the type [Tp*ME(dithiolene)](M=Mo, W; E=O, S) and a novel 'organoscorpionate' complex of tungsten

Paramagnetic, chalcogenido-M(v) dithiolene complexes, [Tp*ME{S2C2(CO2Me)2}][M=Mo, E=O, S; M=W, E=O, S; Tp*=hydrotris(3,5-dimethylpyrazol-1-yl)borate] are generated in the reactions of dimethyl acetylenedicarboxylate (DMAC) and the sulfur-rich complexes NEt4[Tp*MoS(S4)] and NEt4[Tp*WS3]; the oxo complexes result from hydrolysis of the initial sulfido products. As well, a novel 'organoscorpionate' complex, [W{S2C2(CO2Me)2}{SC2(CO2Me)2-Tp*}], has been isolated from the reactions of NEt4[Tp*WS3] with excess DMAC. Complexes , and have been isolated and characterised by microanalytical, mass spectrometric, spectroscopic and (for and) X-ray crystallographic techniques. Complexes and have been partially characterised by mass spectrometry and IR and EPR spectroscopy. Six-coordinate, distorted-octahedral contains a terminal sulfido ligand (W=S=2.108(3)A), a bidentate dithiolene ligand (S-Cav=1.758 A, C=C=1.332(10)A) and a fac-tridentate Tp* ligand. Seven-coordinate contains a planar, bidentate dithiolene ligand (S-Cav=1.746 A, C=C=1.359(5)A) and a novel pentadentate 'organoscorpionate' ligand formed by the melding of DMAC, sulfido and trispyrazolylborate units. The latter is coordinated through two pyrazolyl N atoms (kappa2-N,N') and a tridentate kappa3-S,C,C' unit appended to N-beta of the third (uncoordinated) pyrazolyl group. The second-generation [Tp*ME(dithiolene)] complexes represent a refinement on first-generation [Tp*ME(arene-1,2-dithiolate)] complexes and their synthesis affords an opportunity to compare and contrast the electronic structures of true vs. pseudo-dithiolene ligands in otherwise analogous complexes.     

U(Tp(Me2))2(bipy)]+: a cationic uranium(III) complex with single-molecule-magnet behavior

The addition of 2,2'-bipyridine to [U(Tp(Me2))(2)I] (1) results in the displacement of the iodide and the formation of the cationic uranium(III) complex [U(Tp(Me2))(2)(bipy)]I (2). This compound was isolated as a dark-green solid in good yield and characterized by IR and NMR spectroscopies, and its molecular structure was determined by single-crystal X-ray diffraction. Studies of its magnetic properties revealed a frequency dependence of magnetization with a blocking temperature of 4.5 K and, at lower temperatures, a slow relaxation of magnetization with an energy barrier of 18.2 cm(-1), characteristic of single-molecule-magnet behavior.     

Synthesis, crystal structures, and magnetic properties of cyano-bridged heterobimetallic chains based on [(Tp)Fe(CN)3]-

With the use of the tailored cyanometalate precursor, (Bu4N)[(Tp)Fe(CN)3] (Tp = Tris(pyrazolyl)hydroborate) as the building block to react with fully solvated Cu(II), Co(II), and Ni(II) cations, four one-dimensional (1D) heterobimetallic cyano-bridged chain complexes of squares, [(Tp)2Fe(III)2(CN)6Cu(CH3OH).2CH3OH]n (1), [(Tp)2Fe(III)2(CN)6Cu(DMF).DMF]n (2), [(Tp)2Fe(III)2(CN)6M(CH3OH)2.2CH3OH]n (M = Co (3) and Ni (4)), have been prepared. In complexes 1 and 2, the Cu(II) ions are pentacoordinated in the form of a slightly distorted square-based pyramid, and they are linked by distorted octahedrons of [(Tp)Fe(CN)3]- to form 1D chains of squares. In complexes 3 and 4, both the central Co(II) and Ni(II) ions have a slightly distorted octahedral coordination geometry, and they are bridged by [(Tp)Fe(CN)3]- to form similar 1D chains of squares. There are weak interchain pi-pi stacking interactions through the pyrazolyl groups of the Tp ligands for complexes 3 and 4. The crystal structures and magnetic studies demonstrate that complexes 1 and 2 exhibit intrachain ferromagnetic coupling and single-chain magnets behavior, and the blocking temperature is ca. 6 K for complex 1 and ca. 3 K for complex 2. Complexes 3 and 4 show significant metamagnetic behavior, where the cyanides mediate the intrachain ferromagnetic coupling between Fe(III) and Co(II) or Ni(II) ions and the interchain pi-pi stacking interactions lead to antiferromagnetic couplings. The field dependence of the magnetization measurements shows that the critical field is around 1 kOe for complex 3 and 0.8 kOe for complex 4 at 1.8 K.     

Examination of novel zinc-binding groups for use in matrix metalloproteinase inhibitors

The tetrahedral zinc complex [(Tp(Ph,Me))ZnOH] (Tp(Ph,Me) = hydrotris(3,5-phenylmethylpyrazolyl)borate) was combined with 1-hydroxy-2(1H)-pyridinone, 3-hydroxy-2(1H)-pyridinone, 3-hydroxy-1-methyl-2(1H)-pyridinone, 3-hydroxy-1,2-dimethyl-4(1H)-pyridinone, 1-hydroxy-2(1H)-pyridinethione, and 3-hydroxy-2-methyl-4-pyrone to generate the complexes [(Tp(Ph,Me))Zn(ZBG)] (ZBG = zinc-binding group). These complexes were synthesized to explore the coordination geometry of potential novel zinc-binding groups for use in matrix metalloproteinase (MMP) inhibitors. The solid-state structures of all six metal complexes were determined by X-ray crystallography. These structures combined with IR and (1)H NMR data demonstrate that these ZBGs bind in a strong, bidentate fashion to the zinc(II) ion. Modeling studies indicate that these ZBGs can easily fit into the MMP active site. In an effort to develop more effective inhibitors of MMPs, this work has revealed molecular-level interactions for six potential new ZBGs.     

Preparation of chalcogenolato-bridged dinuclear Tp*Rh-Cp*Ru complexes (Tp*= hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp*=eta(5)-pentamethylcyclopentadienyl) and binding of dioxygen to their Ru sites

Reactions of [Tp*Rh(coe)(MeCN)](1; Tp*= hydrotris(3,5-dimethylpyrazol-1-yl); coe = cyclooctene) with one equiv of diphenyl dichalcogenides PhEEPh (E = Se, Te) afforded the mononuclear Rh(III) complexes [Tp*Rh(EPh)(2)(MeCN)](2b: E = Se; 2c: E = Te), as reported previously for the formation of [Tp*Rh(SPh)(2)(MeCN)](2a) from the reaction of 1 and PhSSPh. Complexes 2a-2c were treated with the Ru(II) complex [(Cp*Ru)(4)(mu(3)-Cl)(4)](Cp*=eta(5)-C(5)Me(5)) in THF at room temperature, yielding the chalcogenolato-bridged dinuclear complexes [Tp*RhCl(mu-EPh)(2)RuCp*(MeCN)](3). Complex 3a (E = S) in solution was converted slowly into a mixture of 3a and the sterically less encumbered dinuclear complex [Tp*RhCl(SPh)(mu-eta(1)-S-eta(6)-Ph)RuCp*](4a) at room temperature. In 4a, one SPh group binds only to the Rh center as a terminal ligand, while the other SPh group bridges the Rh and Ru atoms by coordinating to the former at the S atom and to the latter with the Ph group in a pi fashion. The Se analogue 3b also underwent a similar transformation under more forcing conditions, e.g. in benzene at reflux, whereas formation of the mu-eta(1)-Te-eta(6)-Ph complex was not observed for the Te analogue 3c even under these forcing conditions. When complexes 3 was dissolved in THF exposed to air, the MeCN ligand bound to Ru was substituted by dioxygen to give the peroxo complexes [Tp*RhCl(mu-EPh)(2)RuCp*(eta(2)-O(2))](5a: E = S; 5b: E = Se; 5c: E = Te). X-Ray analyses have been undertaken to determine the detailed structures for 2c, 3a, 3b, 4a, 5a, 5b, and 5c.