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PubMed2000

First success of catalytic epoxidation of olefins by an electron-rich iron(III) porphyrin complex and H2O2: imidazole effect on the activation of H2O2 by iron porphyrin complexes in aprotic solvent.

An electron-rich iron(III) porphyrin complex (meso-tetramesitylporphinato)iron(III) chloride [Fe(TMP)Cl], was found to catalyze the epoxidation of olefins by aqueous 30% H2O2 when the reaction was carried out in the presence of 5-chloro-1-methylimidazole (5-Cl-1-Melm) in aprotic solvent. Epoxides were the predominant products with trace amounts of allylic oxidation products, indicating that Fenton-type oxidation reactions were not involved in the olefin epoxidation reactions. cis-Stilbene was stereospecifically oxidized to cis-stilbene oxide without giving isomerized trans-stilbene oxide product, demonstrating that neither hydroperoxy radical (HOO*) nor oxoiron(IV) porphyrin [(TMP)FeIV=O] was responsible for the olefin epoxidations. We also found that the reactivities of other iron(III) porphyrin complexes such as (meso-tetrakis(2,6-dichlorophenyl)porphinato)iron(III) chloride [Fe(TDCPP)Cl], (meso-tetrakis(2,6-difluorophenyl)porphinato)iron(III) chloride [Fe(TDFPP)Cl], and (meso-tetrakis(pentafluorophenyl)porphinato)iron(III) chloride [Fe(TPFPP)CI] were significantly affected by the presence of the imidazole in the epoxidation of olefins by H2O2. These iron porphyrin complexes did not yield cyclohexene oxide in the epoxidation of cyclohexene by H2O2 in the absence of 5-Cl-1-MeIm in aprotic solvent; however, addition of 5-Cl-1-MeIm to the reaction solutions gave high yields of cyclohexene oxide with the formation of trace amounts of allylic oxidation products. We proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O-O bond cleavage of an iron(III) hydroperoxide porphyrin (or H2O2-iron(II) porphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O-O bond cleavage.

Alkenes↗
PubMed2000

Generation of oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes by m-CPBA oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C. Evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes FeIV = O(tmp*)X (X = Cl-, Br-), by Mössbauer and X-ray absorption spectroscopy.

The generation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-caption radical complexes by m-CPBA (meta-chloroperbenzoic acid) oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C was investigated. UV-Vis and EPR spectroscopies indicate that the axial ligand present in the ferric starting derivatives is retained in the high-valent iron complexes. Indirect evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin complexes FeIV = O(tmp*)X (X=Cl-, Br-) by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) in butyronitrile at - 78 degrees C was also obtained by Mössbauer spectroscopy. Direct confirmation of the presence of a halide ion as second axial ligand of iron in these high-valent iron species was obtained by X-ray absorption spectroscopy. The EXAFS spectra of the samples obtained by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) were refined using two different coordination models including both four porphyrinato-nitrogens and the axial oxo group. The two models include (model I) or exclude (model II) the axial halogen. The statistical tests indicate the presence of a halide ion as second axial ligand of iron in both derivatives. The refinements led to the following bond distances: FeIV=O(tmp*)Cl(3):Fe-O=1.66(1),Fe-Cl=2.39(2) and Fe-Np=1.99(1) A;FeIV=O(tmp*)Br(4):Fe-O=1.65(1),Fe-Br=2.93(2), Fe-Np=2.02(1) A. The lengthening of the Fe-X(X=Cl-, Br-) distances relative to those occurring in the ferric precursor porphyrins is, most probably, related to the strong trans influence of the oxoiron(IV) fragment present in 3 or 4.

Chlorobenzoates↗
PubMed1995

Carney complex: the complex of myxomas, spotty pigmentation, endocrine overactivity, and schwannomas.

The complex of myxomas, spotty pigmentation, endocrine overactivity, and schwannomas (the Carney complex) is a multisystem tumorous disorder that is transmitted as a mendelian autosomal dominant trait. Approximately 150 affected patients are known worldwide. The myxomas, which tend to be multiple in the involved organ, affect the heart, skin and breast. Typical sites for the skin myxomas are the eyelids, external ear canal, and nipples. The lesions commonly recur after excision. The spotty skin pigmentation includes lentigines and blue nevi, but ephelides and junctional and compound nevi also occur. The lentigines are widespread and typically involve the centrofacial area, including the vermilion border of the lips, and the conjunctiva, especially the lacrimal caruncle and the conjunctival semilunar fold. One or more intraoral pigmented spots are seen occasionally. The blue nevi occur on the face, trunk, and limbs, but not the hands and feet. Endocrine overactivity includes Cushing's syndrome (caused by primary pigmented nodular adrenocortical disease), acromegaly (caused by growth hormone-producing pituitary adenoma), and sexual precocity (caused by large-cell calcifying Sertoli cell tumor). The schwannomas are a special histological type, featuring psammoma bodies and melanin. Most commonly, they affect the upper gastrointestinal tract and sympathetic nerve chains, but a few have occurred in the skin. The most serious component of the Carney complex is cardiac myxoma. Patients suspected of having the syndrome (and their primary relatives) should be examined for this neoplasm.

Endocrine System Diseases↗
PubMed2001

Spectro- and electrochemical studies of some ruthenium and osmium complexes of 2-(2'-pyridyl)benzimidazole; complexes with intra-molecular charge transfer.

Reaction of Ru3(CO)12, with 2-(2'-pyridyl)benzimidazole (HPBI) resulted in the formation of Ru(CO)3(HPBI) (I) complex. In presence of pyridine or dipyridine, the two derivatives [Ru(CO)3(HPBI)].Py (II) and [Ru(CO)3(HPBI)].dpy (III) were isolated. The corresponding reactions of Os3(CO)12 yielded only one single product; Os(CO)2(HPBI)2 (IV). Spectroscopic studies of these complexes revealed intramolecular metal to ligand CT interactions. Reactions of RuCl3 with HPBI gave three distinct products; [Ru(HPBI)2Cl2]Cl (V), [Ru(HPBI)(dipy)Cl2]C1 (VI) and [Ru(PBI)2(py)2]Cl (VII). The UV-vis studies indicated the presence of intramolecular ligand to metal CT interactions. Electrochemical investigation of the complexes showed some irreversible, reversible and quasi-reversible redox reactions due to tautomeric interconversions through electron transfer.

Benzimidazoles↗
PubMed1992

Passage of RNA polymerase from open complex to elongation mode at the Escherichia coli lacUV5 promoter: nucleolytic hypersensitivity as a probe for complex conformational changes.

In transcriptionally active complexes between RNA polymerase and promoters, the center of the melted region is hyperreactive to the nucleolytic activity of the cuprous complex of 1,10-phenanthroline (OP-Cu). In the first part of this work, using synthetic oligonucleotides and exploiting gel retardation assays, I demonstrate that DNA unpairing is not the only determinant of this hyperreactivity. Polymerase binding is directly implicated, presumably participating in the stabilization of an intermediate required for the cutting. In the second part of the work, I show that, from fine analysis of the nucleolytic pattern of lacUV5 promoter DNA towards OP-Cu and Phe OP-Cu, it is possible to locate polymerase and to characterize its contacts at any time during the early stages of transcription. This analysis provides a description of the passage from the "open complex" to the elongation mode in terms of, first, release of the upstream contacts, and second, loss of sigma subunit. Occupancy of the overlapping promoter, P2, has a positive effect on the escape of polymerase from abortive cycling. The involvement of sigma and beta subunits in the reactivity pattern is discussed with respect to previous cross-linking studies.

Base Sequence↗
PubMed1991

Conformational transition of fructose-1,6-bisphosphatase: structure comparison between the AMP complex (T form) and the fructose 6-phosphate complex (R form).

A structure of the neutral form of fructose-1,6-bisphosphatase complexed with AMP has been determined by the molecular replacement method and refined at a 2.5-A resolution to a crystallographic R factor of 0.169. The root-mean-square errors of the structure from standard geometry are 0.013 A for bond lengths and 2.99 degrees for bond angles. Comparison of the AMP complex with the F6P complex shows that dimer C3-C4 twists about 19 degrees about a molecular 2-fold axis when dimers C1-C2 of the R and T forms of the enzyme are superimposed one another and that a slight shift of about 1 A of the AMP domain partially compensates this twist. The R to T transition of the enzyme does not significantly change the conformation of the F6P-binding site. However, residues at the divalent metal site and the AMP site show significant positional shifts. If these results can be extended to substrate in place of F6P, they suggest that regulation of the enzyme by AMP may occur partly through effects on metal-ion affinity or position. AMP binds to the same sites of the T and R forms, but only half-occupancy was observed in the alkaline R form. Sequential binding of AMP, at least in pairs, is suggested as the unligated R form is converted to the T form. Two possible pathways are suggested for allosteric communication over about 28 A between the AMP site and the active site: one via helices H1, H2, and H3 and another via the eight-stranded beta-sheet.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Monophosphate↗
PubMed1985

Complex photophysics of the single tryptophan of porcine pancreatic phospholipase A2, its zymogen, and an enzyme/micelle complex.

The fluorescence emission of the single tryptophan in porcine pancreatic phospholipase A2, its zymogen, and a micellar complex of the enzyme with the nonhydrolyzable substrate analogue n-hexadecylphosphocholine has been studied by both steady-state and time-resolved techniques. Stern-Volmer quenching studies with acrylamide indicate that, both in the enzyme and in the zymogen, the tryptophan is exposed to solvent. Similar studies with ionic quenchers show that there is appreciable ionic character to the tryptophan environment. Single photon counting fluorescence measurements were performed using a high repetition rate synchronously pumped dye laser as a light source. When tryptophan fluorescence is collected with a broad-band (80-nm) emission filter, the decay kinetics in the enzyme and the zymogen require at least three, and often four, exponential terms for a proper description. The decay kinetics can be adequately described by three exponential terms when the fluorescence is collected at specific wavelengths by using narrow (10-nm) band-pass filters. The lifetimes are approximately constant across the emission band, but the amplitudes vary with the fraction of the long lifetime increasing at longer emission wavelengths. Formation of a complex between phospholipase A2 and micelles of n-hexadecylphospohocholine produces large changes in the tryptophan emission that are associated with transfer to a hydrophobic environment. The decay kinetics of tryptophan in the enzyme/micelle complex appears to require only two exponential terms. This is the first reported instance of fluorescence data from a single tryptophan protein requiring more than double-exponential decay kinetics. The results are discussed in terms of the range of environments sampled by the tryptophan residue and the resulting distribution of lifetimes.

Animals↗
PubMed1990

The 56-59-kilodalton protein identified in untransformed steroid receptor complexes is a unique protein that exists in cytosol in a complex with both the 70- and 90-kilodalton heat shock proteins.

It has previously been shown that 9S, untransformed progestin, estrogen, androgen, and glucocorticoid receptor complexes in rabbit uterine and liver cytosols contain a 59-kDa protein [Tai, P. K., Maeda, Y., Nakao, K., Wakim, N. G., Duhring, J. L., & Faber, L. E. (1986) Biochemistry 25, 5269-5275]. In this work we show that the monoclonal antibody KN 382/EC1 raised against the rabbit 59-kDa protein reacts with 9S, untransformed glucocorticoid receptor complexes in cytosol prepared from human IM-9 lymphocytes but not with 4S salt-transformed receptors. The human protein recognized by the EC1 antibody is a 56-kDa protein (p56) of moderate abundance located predominantly in the cytoplasm by indirect immunofluorescence. There are at least six isomorphs of p56 by two-dimensional gel analysis. N-Terminal sequencing (20 amino acids) shows that p56 is a unique human protein. When p56 is immunoadsorbed from IM-9 cell cytosol, both the 70- and 90-kDa heat shock proteins are coadsorbed in an immune-specific manner. Neither heat shock protein reacts directly with the EC1 antibody. We conclude that p56 exists in cytosol in a higher order complex containing hsp70 and hsp90, both of which in turn have been found to be associated with untransformed steroid receptors.

Antibodies, Monoclonal↗
PubMed2001

SmtB-DNA and protein-protein interactions in the formation of the cyanobacterial metallothionein repression complex: Zn2+ does not dissociate the protein-DNA complex in vitro.

The synechococcal metallothionein locus smt consists of two divergent genes: smtA coding for the metallothionein SmtA, and smtB coding for the trans-acting regulator SmtB. The latter binds at two inverted repeats, designated S1/S2 and S3/S4, in the overlapping promoter/operator sites between the two genes. We have determined the binding stoichiometries to the entire operator/promoter DNA and to the separate S1/S2 and S3/S4 half-operator oligonucleotides using sedimentation equilibrium and sedimentation velocity measurements. The full promoter/operator DNA binds two SmtB dimers. The hydrodynamic behavior of this complex supports a compact nucleoprotein structure. Each separate S1/S2 and S3/S4 operator sequence also binds two dimers. An equal molar mixture of separate S1/S2 and S3/S4 operator sequences, in excess SmtB, forms a S1/S2-SmtB:SmtB-S3/S4 bridge complex. Combining these results with previously published binding interference data, which showed consecutive S1/S2 and S3/S4 SmtB occupancy on the operator/promoter DNA, we have developed a model for the establishment of the repression complex that appears to involve significant DNA compaction, presumably DNA bending, stabilized by SmtB-SmtB bridge interactions. DNase I footprinting titrations also showed consecutive S1/S2 and S3/S4 SmtB occupancy. The footprints expand considerably in the presence of Zn2+. Hence, SmtB remains bound to the operator sites when Zn2+ ions are present. This result is further supported by gel retardation assay. Failure of the metal ions to dissociate SmtB from the DNA points to a hitherto unknown function of SmtB in the regulation of the smt locus.

Amino Acid Sequence↗
PubMed2002

The 2.1 A structure of Torpedo californica creatine kinase complexed with the ADP-Mg(2+)-NO(3)(-)-creatine transition-state analogue complex.

Creatine kinase (CK) catalyzes the reversible conversion of creatine and ATP to phosphocreatine and ADP, thereby helping maintain energy homeostasis in the cell. Here we report the first X-ray structure of CK bound to a transition-state analogue complex (CK-TSAC). Cocrystallization of the enzyme from Torpedo californica (TcCK) with ADP-Mg(2+), nitrate, and creatine yielded a homodimer, one monomer of which was liganded to a TSAC complex while the second monomer was bound to ADP-Mg(2+) alone. The structures of both monomers were determined to 2.1 A resolution. The creatine is located with the guanidino nitrogen cis to the methyl group positioned to perform in-line attack at the gamma-phosphate of ATP-Mg(2+), while the ADP-Mg(2+) is in a conformation similar to that found in the TSAC-bound structure of the homologue arginine kinase (AK). Three ligands to Mg(2+) are contributed by ADP and nitrate and three by ordered water molecules. The most striking difference between the substrate-bound and TSAC-bound structures is the movement of two loops, comprising residues 60-70 and residues 323-332. In the TSAC-bound structure, both loops move into the active site, resulting in the positioning of two hydrophobic residues (one from each loop), Ile69 and Val325, near the methyl group of creatine. This apparently provides a specificity pocket for optimal creatine binding as this interaction is missing in the AK structure. In addition, the active site of the transition-state analogue complex is completely occluded from solvent, unlike the ADP-Mg(2+)-bound monomer and the unliganded structures reported previously.

Adenosine Diphosphate↗
PubMed1996

Peptide interaction with a class I major histocompatibility complex-encoded molecule: allosteric control of the ternary complex stability.

Thermodynamics and kinetics of interaction between a soluble class I MHC heterodimer composed of the H-2Kd heavy chain (H) and human beta 2microglobulin (beta 2m) with a dansylated peptide series based on residues 147-155 of influenza virus nucleoprotein sequence were studied by means of real-time fluorescence measurements. Peptide-heterodimer binding is a second-order process with specific rates practically independent of peptide structure (3-5 x 10(6) M-1 s-1). The ternary complex assembly involves a rate-limiting step of beta 2m association with H to yield an unstable heterodimer (tau < or = 5 s, 37 degrees C). Peptide binding provides a positive feedback enhancing H's affinity for beta 2m, thus stabilizing the ternary complex. The latter decays by either peptide or beta 2m dissociation. The first-order rate constants of peptide dissociation (0.5 x 10(-2))-(0.4 x 10(-3)) s-1, 37 degrees C) depend on their structures and are faster than that of beta 2m dissociation. The former process decreases the H affinity for beta 2m and induces their dissociation. This dissociation, in turn, drastically lowers H affinity for peptide. Thus, these three components produce a system which is stable as a trimer. This behavior is rationalized by the functional requirements of class I molecules: Peptide structure determines the ternary complex's lifetime, and peptide rebinding on the cell surface is rendered unlikely by the limited stability of the empty heterodimers and the very low peptide affinity of the heavy chains.

Allosteric Regulation↗
PubMed1997

Cytochrome c/cytochrome c peroxidase complex: effect of binding-site mutations on the thermodynamics of complex formation.

The cytochrome c/cytochrome c peroxidase system has been extensively investigated as a model for long-range electron transfer in biology. Two models for the structure of the one-to-one cytochrome c/cytochrome c peroxidase complex in solution exist: one is based upon computer docking of the two proteins and the second is based upon the structure of the complex in the crystalline state. Titration calorimetry is used to investigate the interaction of horse ferricytochrome c with baker's yeast cytochrome c peroxidase and with six cytochrome c peroxidase mutants. Five of the six peroxidase mutants eliminate a negative charge in the cytochrome c binding site by replacing a side-chain carboxylate with an amide. The sixth mutation replaces a surface alanine residue with phenylalanine. The binding affinity between cytochrome c and the cytochrome c peroxidase mutants varies from no significant change in comparison to the wild-type enzyme to a 4-fold decrease in the equilibrium association constant. The pattern of decreasing cytochrome c binding affinity for the cytochrome c peroxidase mutants is consistent with the cytochrome c binding domain defined by X-ray crystallography [Pelletier, H., & Kraut, J. (1992) Science 258, 1748-1755]. For those mutants which have lower affinity for cytochrome c, the lower affinity is due to a decrease in the entropy change upon complex formation, consistent with the difference in hydration of carboxylate and amide groups.

Animals↗
PubMed1999

Crystal structures of the Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase-GMP and -IMP complexes: comparison of purine binding interactions with the XMP complex.

The crystal structures of the guanosine 5'-monophosphate (GMP) and inosine 5'-monophosphate (IMP) complexes of Toxoplasma gondii hypoxanthine-guanine phosphoribosyltransferase (HGPRT) have been determined at 1.65 and 1.90 A resolution. These complexes, which crystallize in space groups P2(1) (a = 65.45 A, b = 90.84 A, c = 80. 26 A, and beta = 92.53 degrees ) and P2(1)2(1)2(1) (a = 84.54 A, b = 102.44 A, and c = 108.83 A), each comprise a tetramer in the crystallographic asymmetric unit. All active sites in the tetramers are fully occupied by the nucleotide. Comparison of these structures with that of the xanthosine 5'-monophosphate (XMP)-pyrophosphate-Mg(2+) ternary complex reported in the following article [Héroux, A., et al. (1999) Biochemistry 38, 14495-14506] shows how T. gondii HGPRT is able to recognize guanine, hypoxanthine, and xanthine as substrates, and suggests why the human enzyme cannot use xanthine efficiently. Comparison with the apoenzyme reveals the structural changes that occur upon binding of purines and ribose 5'-phosphate to HGPRT. Two structural features important to the HGPRT mechanism, a previously unrecognized active site loop (loop III', residues 180-184) and an active site peptide bond (Leu78-Lys79) that adopts both the cis and the trans configurations, are presented.

Amino Acid Sequence↗
PubMed2001

Rhenium(I) carbonyl complexes of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT). Rhenium(I)-promoted methoxylation of the triazine ring carbon atom in dinuclear rhenium complexes.

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPT) bridged dinuclear rhenium(I) tricarbonyl halide complexes with the composition (mu-TPT)[ReX(CO)(3)](2) (3, X = Cl; 4, X = Br) can be made either by one-pot reaction of TPT with 2 equiv of [ReX(CO)(5)] (X = Cl and Br) in chloroform or by reacting mononuclear [ReX(CO)(3)(TPT)] (2) (1, X = Cl; 2, X = Br) with an excess amount of [ReX(CO)(5)]. Crystal data are as follows. 1: monoclinic, P2(1)/c, a = 11.751(1) A, b = 11.376(1) A, c = 15.562(2) A, beta = 103.584(2) degrees, V = 2022.0(4) A(3), Z = 4. 2: monoclinic, P2(1)/c, a = 11.896(1) A, b = 11.396(1) A, c = 15.655(1) A, beta = 104.474(2) degrees, V = 2054.9(3) A(3), Z = 4. 3: triclinic, P1, a = 11.541(2) A, b = 12.119(2) A, c = 13.199(2) A, alpha = 80.377(2) degrees, beta = 76.204(3) degrees, gamma = 66.826(2) degrees, V = 1642.5(4) A(3), Z = 2. Crystals of 4 crystallized from acetone: triclinic, P1, a = 11.586(5) A, b = 12.144(5) A, c = 13.364(6) A, alpha = 80.599(7) degrees, beta = 76.271(8) degrees, gamma = 67.158(8) degrees, V = 1678.0(12) A(3), Z = 2. Crystals of 4' are obtained from CH(2)Cl(2)-pentane solution: monoclinic, C2/c, a = 17.555(4) A, b = 15.277(3) A, c = 13.093(3) A, beta = 111.179(3) degrees, V = 3274.0(12) A(3), Z = 4. By contrast, similar reactions in the presence of methanol yielded complexes with the composition [mu-C(3)N(3)(OMe)(py)(2)(pyH)][ReX(CO)(3)](2) (5, X = Cl; 6, X = Br). Crystal data for 5: monoclinic, C2/c, a = 26.952(2) A, b = 16.602(1) A, c = 14.641(1) A, beta = 116.147(1) degrees, V = 5880.5(8) A(3), Z = 8. 6: monoclinic, C2/c, a = 27.513(3) A, b = 16.740(2) A, c = 14.837(2) A, beta = 116.925(2) degrees, V = 6092.8(10) A(3), Z = 8. An unusual metal-induced methoxylation at the carbon atom of the triazine ring of the bridging TPT ligand was observed. The nucleophilic attack of MeO(-) on C(3) results in a tetrahedral geometry around the carbon atom. Concomitantly, the uncoordinated pyridyl ring is protonated and rotated into a perpendicular orientation relative to the central C(3)N(3) ring. Reaction of TPT with [NEt(4)](2)[ReBr(3)(CO)(3)] in benzene-methanol resulted in an unexpected dinuclear complex 7, with formulation [mu-C(3)N(3)(OMe)(py)(3)][Re(CO)(3)][ReBr(CO)(3)]. The methoxylated TPT ligand functions simultaneously as a tridentate and bidentate ligand with two fac-Re(CO)(3)(+) cores. Crystal data for 7: monoclinic, P2(1)/n, a = 12.114(1) A, b = 14.878(1) A, c = 15.807(1) A, beta = 104.601(1) degrees, V = 2756.9(3) A(3), Z = 4.

Journal Article↗
PubMed2001

Charge distribution in bis-dioxolene radical metal complexes. synthesis and DFT characterization of dinuclear Co(III) and Cr(III) complexes with a mixed-valent, S = 1/2 semiquinone-catecholate ligand.

Bis-dioxolene bridged dinuclear metal complexes of general formula M2(CTH)2(diox-diox)(PF6)n (n = 2, 3; M = Co(III), Cr(III); CTH = tetraazamacrocycle) have been synthesized using the bis-bidentate ligand 5,5'-di-tert-butyl-3,3',4,4'-tetrahydroxybiphenyl. These complexes were characterized by means of ESR, UV-vis, temperature dependent magnetic susceptibility, and cyclic voltammetry. Our results unambiguously suggest that the tripositive dimetal cations can be described as containing a fully delocalized bis-dioxolene trinegative radical ligand (Cat-Sq) bridging two tripositive metal cations. In this frame the sextet electronic ground state characterizes the Cr2(CTH)2(Cat-SQ)3+ as a result of the antiferromagnetic coupling of the radical bridging ligand with the two equivalent paramagnetic metal centers. The electronic and geometrical structure and the magnetic properties of Cat-Sq and of its complexes have been studied with density functional theory.

Journal Article↗
PubMed2001

Preparation and X-ray structures of Cu(I), Ni(II), and Pd(II) (N,S) complexes of the monoanion [(tBuN)(S)P(mu-N(t)Bu)2P(S)(NH(t)Bu)]- and a Pt(II) (S,S') complex of the dianion [((t)BuN)(S)P(mu-N(t)Bu)2P(S)(N(t)Bu)]2-.

The metathetical reactions of the lithium derivative of the monoanion [((t)BuN)(S)P(mu-N(t)Bu)(2)P(S)(NH(t)Bu)](-) (L) with CuCl/PPh(3), NiCl(2)(PEt(3))(2), PdCl(2)L'(2) (L' = PhCN, PPh(3)), and PtCl(2)(PEt(3))(2) produced the complexes (PPh(3))CuL (5), NiL(2) (6), PdCl(L)(PPh(3)) (7), PdL(2) (8), and Pt(PEt(3))(2)[((t)BuN)(S)P(mu-N(t)Bu)(2)P(S)(N(t)Bu)] (9). The X-ray structures of 5, 6, and 8 reveal a N,S-coordination for the chelating monoanion L with the metal centers in trigonal planar, tetrahedral, and square planar environments, respectively. By contrast, the dianionic ligand in the square planar Pt(II) complex 9 is S,S'-chelated to the metal center. (31)P NMR spectra readily distinguish between the N,S and S,S' bonding modes, and, on that basis, N,S chelation is inferred for the Pd(II) complex 7. Crystal data: 5, monoclinic, P2(1)/c, a = 19.175(4) A, b = 20.331(4) A, c = 10.017(6) A, beta = 91.79(3) degrees, V = 3903(2) A(3), and Z = 4; 6, orthorhombic, Pbcn, a = 14.298(5) A, b = 15.333(5) A, c = 24.378(5) A, beta = 90.000(5) degrees, V = 5344(3) A(3), and Z = 4; 8, monoclinic, P2(1)/n, a = 13.975(3) A, b = 14.283(3) A, c = 15.255(4) A, beta = 116.565(18) degrees, V = 2723.5(11) A(3), and Z = 2; 9, monoclinic, P2(1)/n, a = 12.479(6) A, b = 21.782(7) A, c = 17.048(5) A, beta = 100.30(3) degrees, V = 4559(3) A(3), and Z = 4.

Journal Article↗
PubMed2001

Dicopper(I) complexes with reduced states of 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine: crystal structures and spectroscopic properties of the free ligand, a radical species, and a complex of the 1,4-dihydro form.

The complexes [(mu-bmtz(*-))[Cu(PPh(3))(2)](2)](BF(4)) (1) and [(mu-H(2)bmtz)[Cu(PPh(3))(2)](2)](BF(4))(2) (2) (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine and H(2)bmtz = 1,4-dihydro-3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine) were obtained as stable materials that could be crystallized for structure determination. 1.2 CH(2)Cl(2): C(84)H(70)BCl(4)Cu(2)F(4)N(8)P(4); monoclinic, C2/c; a = 26.215(7) A, b = 22.122(6) A, c = 18.114(5) A, beta = 133.51(1) degrees; Z = 4. 2.CH(2)Cl(2): C(83)H(70)B(2)Cl(2)Cu(2)F(8)N(8)P(4); triclinic, P1; a = 10.948(2) A, b = 12.067(2) A, c = 30.287(6) A, alpha = 93.82(3) degrees, beta = 94.46(3) degrees, gamma = 101.60(3) degrees; Z = 2. Bmtz itself was also structurally characterized (C(10)H(6)N(8); monoclinic, P2(1)/c; a = 3.8234(8) A, b = 10.147(2) A, c = 13.195(3) A, beta = 94.92(3) degrees; Z = 2). Whereas the radical complex ion contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex of H(2)bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine ring.

Journal Article↗
PubMed2001

A general high-yield route to bis(salicylaldimine) zinc(II) complexes: application to the synthesis of pyridine-modified salen-type zinc(II) complexes.

A general, direct, and high-yield synthesis of bis(salicylaldimine) zinc complexes from the ligands and Et(2)Zn is reported. This synthetic method is particularly valuable, not only because it allows the efficient preparation of salen-type complexes of zinc but also because it can be used to prepare bifunctional pyridine-modified zinc(II) bis(salicylidene) complexes, which are potentially useful compounds for applications in asymmetric catalysis and materials chemistry. The synthesis and complete structural characterization of a new series of pyridine-modified zinc(II) bis(salicylidene) ligands is discussed.

Journal Article↗