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Bruce C Noll

Publications and source records attributed to Bruce C Noll.

At least 19 recordsLinked to original sources
PubMed2006

Mononucleating bis(beta-diketonate) ligands and their titanium(IV) complexes.

Novel bis(beta-diketones) linked by 2,2'-biphenyldiyl, 2,2'-tolandiyl, and 2,2'-bis(methylene)biphenyl moieties have been prepared. All are metalated readily by titanium(IV) isopropoxide, but the nature of the complexes formed depends on the linker structure. The biphenyl-bridged ligand gives only traces of a mononuclear complex, which is thermodynamically unstable with respect to oligomerization. The tolan-bridged ligand does form mononuclear complexes, but only as a mixture of geometric isomers. In contrast, the substituted 2,2'-bis-(2,4-dioxobutyl)biphenyl ligands, R2BobH2 (R = tBu, p-Tol), react with Ti(OiPr)4 to give, initially, a mixture of monomer and oligomers, which is converted quantitatively to monomer upon heating in the presence of excess Ti(OiPr)4. Only a single relative configuration of the biphenyl and bis(chelate) titanium moieties, established by crystallography of (tBu2Bob)Ti(O-2,6-iPr2C6H3)2 to be the (R)-/(S)- diastereomer, is observed. The kinetic and thermodynamic robustness of the (R2Bob)Ti framework is confirmed by reactions with Lewis acids. For example, (Tol2Bob)Ti(OiPr)2 reacts with trimethylsilyl triflate or triflic acid to substitute one or both of the isopropoxide groups with triflates without any redistribution or loss of the diketonate ligands. Cationic complexes can be prepared by abstraction of triflate from (Tol2Bob)Ti(OiPr)(OTf) with Na[B(C6H3(CF3)2)4]. For example, in the presence of diethyl ether, the crystallographically characterized [(Tol2Bob)Ti(OiPr)(OEt2)][B(C6H3(CF3)2)4], containing a rapidly dissociating ether ligand, is formed.

Journal Article↗
PubMed2006

Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)(2)]Cl): a consequence of hydrogen bonding?

The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important role in the differences in the two structures. Mössbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data: monoclinic, space group P2(1)/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, beta = 109.67 degrees , and Z = 4.

Crystallography, X-Ray↗
PubMed2006

Kinetics and mechanism of ketone enolization mediated by magnesium bis(hexamethyldisilazide).

Magnesium bis(hexamethyldisilazide), Mg(HMDS)(2), reacts with substoichiometric amounts of propiophenone in toluene solution at ambient temperature to form a 74:26 mixture of the enolates (E)- and (Z)-[(HMDS)(2)Mg(2)(mu-HMDS){mu-OC(Ph)=CHCH(3)}], (E)-1 and (Z)-1, which contain a pair of three-coordinate metal centers bridged by an amide and an enolate group. The compositions of (E)-1 and (Z)-1 were confirmed by solution NMR studies and also by crystallographic characterization in the solid state. Rate studies using UV-vis spectroscopy reveal the rapid and complete formation of a reaction intermediate, 2, between the ketone and magnesium, which undergoes first-order decay with rate constants independent of the concentration of excess Mg(HMDS)(2) (DeltaH++ = 17.2 +/- 0.8 kcal/mol, DeltaS++ = -11 +/- 3 cal/mol.K). The intermediate 2 has been characterized by low-temperature (1)H NMR, diffusion-ordered NMR, and IR spectroscopy and investigated by computational studies, all of which are consistent with the formulation of 2 as a three-coordinate monomer, (HMDS)(2)Mg{eta(1)-O=C(Ph)CH(2)CH(3)}. Further support for this structure is provided by the synthesis and structural characterization of two model ketone complexes, (HMDS)(2)Mg(eta(1)-O=C(t)Bu(2)) (3) and (HMDS)(2)Mg{eta(1)-O=C((t)Bu)Ph} (4). A large primary deuterium isotope effect (k(H)/k(D) = 18.9 at 295 K) indicates that proton transfer is the rate-limiting step of the reaction. The isotope effect displays a strong temperature dependence, indicative of tunneling. In combination, these data support the mechanism of enolization proceeding through the single intermediate 2 via intramolecular proton transfer from the alpha carbon of the bound ketone to the nitrogen of a bound hexamethyldisilazide.

Journal Article↗
PubMed2006

Coordination of diatomic ligands to heme: simply CO.

The synthesis and molecular structures of three iron(II) porphyrinates with only CO as the axial ligand(s) are reported. Two five-coordinate [Fe(OEP)(CO)] derivatives have Fe-C = 1.7077(13) and 1.7140(10) A, much shorter than those of six-coordinate [Fe(OEP)(Im)(CO)], although nu(C-O) is 1944-1948 cm(-1). The six-coordinate species [Fe(OEP)(CO)2] has also been studied. The competition for pi-back-bonding of two CO ligands leads to Fe-C distance of 1.8558(10) A and nu(C-O) being increased to 2021 cm(-1). The Mössbauer spectrum has a quadrupole splitting constant of 0 mm/s at 4.2 K, indicating high electronic symmetry.

Carbon Monoxide↗
PubMed2006

Synthesis, structural characterization, gas sorption and guest-exchange studies of the lightweight, porous metal-organic framework alpha-[Mg3(O2CH)6].

Unsolvated magnesium formate crystallizes upon reaction of the metal nitrate with formic acid in DMF at elevated temperatures. Single-crystal XRD studies reveal the formation of [Mg3(O2CH)6 [symbol: see text] DMF], 1, a metal-organic framework with DMF molecules filling the channels of an extended diamondoid lattice. The DMF molecules in 1 can be entirely removed without disruption to the framework, giving the guest-free material alpha-[Mg3(O2CH)6], 2. Compound 2 has been characterized by both powder and single-crystal XRD studies. Thermogravimetric analyses of 1 show guest loss from 120 to 190 degrees C, with decomposition of the sample at approximately 417 degrees C. Gas sorption studies using both N2 and H2 indicate that the framework displays permanent porosity. The porosity of the framework is further demonstrated by the ability of 2 to uptake a variety of small molecules upon soaking. Single-crystal XRD studies have been completed on the six inclusion compounds [Mg3(O2CH)6 [symbol: see text] THF], 3; [Mg3(O2CH)6 [symbol: see text] Et2O], 4; [Mg3(O2CH)6 [symbol: see text] Me2CO], 5; [Mg3(O2CH)6 [symbol: see text] C6H6], 6; [Mg3(O2CH)6 [symbol: see text] EtOH], 7; and [Mg3(O2CH)(6) [symbol: see text] MeOH], 8. Analyses of the metrical parameters of 1-8 indicate that the framework has the ability to contract or expand depending on the nature of the guest present.

Journal Article↗
PubMed2006

Electronic configuration of high-spin imidazole-ligated iron(II) octaethylporphyrinates.

The preparation and characterization of two new five-coordinate, imidazole-ligated, high-spin iron(II) octaethylporphyrinates is described. [Fe(OEP)(1,2-Me2Im)] and [Fe(OEP)(2-MeHIm)] have been characterized by X-ray structure determinations and temperature-dependent Mössbauer spectroscopy in zero and applied magnetic fields. The distinction between imidazole-ligated and other ligands in high-spin iron(II) porphyrinates, noted for a series of tetraarylporphyrinate derivatives (Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schiltz, C. E.; Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675), is seen here as well. The sign of the quadrupole-splitting constant is again negative, which is unique to the imidazole-ligated derivatives and suggests a distinct electronic structure. The derivatives again display a remarkable temperature dependence in the quadrupole splitting, which is also seen for deoxymyoglobin and -hemoglobin. Structural features for the two new derivatives are similar to those seen earlier, although the core conformations show somewhat more doming character.

Crystallography, X-Ray↗
PubMed2006

Side reaction of significance in preparation of peptide- or peptidomimetic-based hydroxamate enzyme inhibitors.

Peptidic and peptidomimetic hydroxamates are increasingly being developed as potential pharmaceutical agents in targeting metalloenzymes. For a number of practical considerations, the hydroxamate moiety is often introduced in the synthesis onto valuable advanced synthetic precursors. The approach entails activation of the carboxylate of the synthetic precursor by any number of methods for the preparation of the hydroxamate. We report herein that this widely used approach in preparation of an entire class of enzyme inhibitors is problematic as documented by the formation of an undesirable cyclization product (characterized in this report), which could at times be the exclusive outcome of the carboxylate activation process.

Crystallography, X-Ray↗
PubMed2006

Methyl 4-O-beta-L-galactopyranosyl-beta-D-glucopyranoside (methyl beta-L-lactoside).

Methyl beta-L-lactoside, C13H24O11, (II), is described by glycosidic torsion angles phi (O5Gal-C1Gal-O4Glc-C4Glc) and psi (C1Gal-O1Gal-C4Glc-C5Glc) of 93.89 (13) and -127.43 (13) degrees , respectively, where the ring atom numbering conforms to the convention in which C1 is the anomeric C atom and C6 is the exocyclic hydroxymethyl (CH2OH) C atom in both residues (Gal is galactose and Glc is glucose). Substitution of L-Gal for D-Gal in the biologically relevant disaccharide, methyl beta-lactoside [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719-1721], (I), significantly alters the glycosidic linkage interface. In the crystal structure of (I), one inter-residue (intramolecular) hydrogen bond is observed between atoms H3OGlc and O5Gal. In contrast, in the crystal structure of (II), inter-residue hydrogen bonds are observed between atoms H6OGlc and O5Gal, H6OGlc and O6Gal, and H3OGlc and O2Gal, with H6OGlc serving as a donor with two intramolecular acceptors.

Carbohydrate Conformation↗
PubMed2006

4-Methoxy- and 4-cyano-substituted lithium aryloxides: electronic effects of substituents on aggregation.

The para-substituted lithium aryloxides [{4-NC-C6H4OLi.(Pyr)2}2.Pyr] 1a, [{4-NC-C6H4OLi.(THF)2}2] 1b, [{4-MeO-C6H4OLi.Pyr}4] 2a, [4-MeO-C6H4OLi.(THF)n] 2b, [{4-NC-2,6-(t-Bu)2-C6H2OLi.(Pyr)2}infinity] 3a, [{4-NC-2,6-(t-Bu)2-C6H2OLi.(THF)2}infinity] 3b, [{4-MeO-2,6-(t-Bu)2-C6H2OLi.Pyr}2.(Pyr)2] 4a, and [4-MeO-2,6-(t-Bu)2-C6H2OLi.(THF)n] 4b were prepared by the direct deprotonation of the corresponding phenol with an alkyllithium base (BuLi or MeLi) in the appropriate solvent, either pyridine or THF. All compounds were characterized by 1H and 13C NMR spectroscopy, and the crystal structures of 1a, 1b, 2a, 3a, 3b and 4a were elucidated. The cyano derivatives 1a and 1b adopt discrete tetrasolvated Li2O2 ring dimers whereas the methoxy analogue 2a crystallizes as a tetrasolvated molecular tetramer with a pseudo cubic Li4O4 core. The sterically encumbered cyano derivatives 3a and 3b form isostructural 1D polymeric chains of monomers via bridging of the phenolate ligands through Li...NC and Li-O contacts. In comparison, the crystal structure of the methoxy counterpart 4a is a disolvated molecular Li2O2 ring dimer. Solution NMR spectroscopic studies of 1-4 in d5-pyridine and d8-THF indicate that the methoxy complexes are more highly aggregated than the cyano derivatives, consistent with the solid-state studies. Ab initio molecular orbital calculations at the HF/6-31G* level of theory indicate that the origin of the aggregation state variations between the cyano and methoxy complexes is due to electronic effects.

Computer Simulation↗
PubMed2005

Synthesis and characterization of manganese(II) and iron(III) d5 tripodal imidazole complexes. Effect of oxidation state, protonation state and ligand conformation on coordination number and spin state.

The 1 : 3 Schiff base condensates of tris(2-aminoethyl)amine (tren) or tris(3-aminopropyl)amine (trpn) with 4-methyl-5-imidazolecarboxaldehyde, H3L1 and H3L2, respectively, were generated in situ and used to prepare complexes with manganese(II) and iron(III). The resultant complexes, [MnH3L1](ClO4)2, [MnH3L1](ClO4)2.EtOH.H2O, [MnH3L2](ClO4)2, [FeH3L1](ClO4)3.1.5(EtOH) and [FeHL1](I3) (0.525)(I)(0.475).2.625H2O, have been characterized by EA, IR, ES MS, variable temperature magnetic susceptibility, X-ray crystallography, and Mössbauer spectroscopy for the iron complexes. The three manganese(II) complexes are high spin with [MnH3L2](ClO4)2 exhibiting coordination number seven while the others are six coordinate. [FeH3L1](ClO4)3.1.5(EtOH) has two iron sites, a seven coordinate and a pseudo seven coordinate site. The complex is high spin at room temperature but exhibits a magnetic moment that decreases with temperature corresponding to conversion of one of the sites to low spin. [FeHL1](I3) (0.525)(I)(0.475).2.625H2O is low spin even at room temperature. In the present complexes the apical nitrogen atom, N(ap), of the tripodal ligand is pyramidal and directed toward the metal atom. The data show that the M-N(ap) distance decreases as the oxidation state of the metal increases, as the number of bound imidazole protons on the ligand increases, and as the number of carbon atoms in the backbone of the ligand (tren vs. trpn) increases. In a limiting sense, short M-N(ap) distances result in high spin seven coordinate mono capped octahedral complexes and long M-N(ap) distances result in low spin six coordinate octahedral complexes.

Crystallography, X-Ray↗
PubMed2005

Electronic dissymmetry in chiral recognition.

The pi-bonding of the beta-diketonate ligands in C2-symmetric (dike)2TiX2 complexes engenders a significant electronic dissymmetry, with the LUMO of the complex resembling a "tipped" dz2 orbital whose tilt with respect to the plane containing the metal and the two X groups is determined by the configuration at titanium. The ability of this electronic dissymmetry to promote chiral recognition has been probed in 1,1'-bi-2-naphtholate complexes (RCOCHCOR)2Ti(BINOL) (R = CH3, Ph, or tBu). The complexes show exclusively one diastereomer by NMR, which has been established as the one predicted on electronic grounds by crystallography of the dibenzoylmethane and 2,2,6,6-tetramethylheptanedione complexes. Confirmation that the selectivity is due to electronic rather than steric factors is given by the behavior of the tin analogues, which are structurally similar but show much smaller diastereoselectivities.

Journal Article↗
PubMed2005

A chelating beta-diketonate/phenoxide ligand and its coordination behavior toward titanium and scandium.

Dibenzoylmethane derivatives with one (L1H2) or both (L2H3, L3H3) benzenes linked at their ortho positions to 4,6-di-tert-butylphenol moieties by two-carbon linkers have been synthesized. The mono-beta-diketone-monophenol ligand L1H2 is metalated by titanium alkoxides to form the homoleptic complex (L1)2Ti and heteroleptic complexes (L1)Ti([OCH2CH2]2NR) (R = H, CH3), and reacts with Cp3Sc to form CpSc(L1). These are the first examples of complexes of a beta-diketonate ligand which is further chelating to a single metal center. Crystallographic analysis of (L1)2Ti indicates that the 10-membered ring allows chelation of the phenoxide with little strain, and both fac and mer geometries are accessible in solution. Protonolysis of the second cyclopentadienyl ring of Cp3Sc appears to take place by an indirect, Cp3Sc-catalyzed pathway.

Journal Article↗
PubMed2005

Methyl 4-O-beta-D-galactopyranosyl alpha-D-glucopyranoside (methyl alpha-lactoside).

Methyl alpha-lactoside, C13H24O11, (I), is described by glycosidic torsion angles varphi (O5gal-C1gal-O1gal-C4glc) and psi (C1gal-O1gal-C4glc-C5glc), which have values of -93.52 (13) and -144.83 (11) degrees, respectively, where the ring atom numbering conforms to the convention in which C1 is the anomeric C atom and C6 is the exocyclic hydroxymethyl (-CH2OH) C atom in both residues. The linkage geometry is similar to that observed in methyl beta-lactoside methanol solvate, (II), in which varphi is -88.4 (4) degrees and psi is -161.3 (4) degrees. As in (II), an intermolecular O3glc-H...O5gal hydrogen bond is observed in (I). The hydroxymethyl group conformation in both residues is gauche-trans, with torsion angles omegagal (O5gal-C5gal-C6gal-O6gal) and omega(glc) (O5glc-C5glc-C6glc-O6glc) of 69.15 (13) and 72.55 (14) degrees, respectively. The latter torsion angle differs substantially from that found for (II) [-54.6 (2) degrees; gauche-gauche]. Cocrystallization of methanol, which is hydrogen bonded to O6glc in the crystal structure of (II), presumably affects the hydroxymethyl conformation in the Glc residue in (II).

Carbohydrate Conformation↗
PubMed2005

Structural control of the photodynamics of boron-dipyrrin complexes.

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.

Boron↗
PubMed2005

Dependence of field switched ordered arrays of dinuclear mixed-valence complexes on the distance between the redox centers and the size of the counterions.

trans-[(H(2)NCH(2)CH(2)C triple bond N)(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)(N triple bond CCH(2)CH(2)NH(2))][PF(6)](2), 2[PF(6)](2), a derivative of trans-[Cl(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)Cl] functionalized for binding to a silicon substrate, has been prepared and characterized spectroscopically, electrochemically, and with a solid state, single-crystal structure determination. Covalent binding via reaction of one amine group to a boron-doped, smooth Si-Cl substrate is verified by XPS measurements and surface electrochemistry. Vertical orientation is demonstrated by film thickness measurements. Synthesis of the 2[PF(6)](3) mixed-valence complex on the surface is established by electrochemical techniques. Measurement of the ac capacitance of the film at 1 MHz as a function of voltage across the film with a pulse-counter pulse technique demonstrates controlled electric field generation of the two stable mixed-valence forms differing in the spatial location of one electron, that is, switching. As compared to [trans-Ru(dppm)(2)(C triple bond CFc)(NCCH(2)CH(2)NH(2))][PF(6)][Cl], 1[PF(6)][Cl], the magnitude of the capacitance signal per complex observed on switching is shown to increase with increasing distance between the metal centers. Additional experiments on 1[X][Cl] show that the potential for switching 1[X][Cl] increases in the order [X](-) = [SO(3)CF(3)](-) < [PF(6)](-) < [Cl](-). A simple electrostatic model suggests that the smaller is the counterion, the greater is the perturbation of the metal sites and the larger is the barrier for switching.

Journal Article↗
PubMed2005

Proton-mediated electron configuration change in high-spin iron(II) porphyrinates.

The synthesis, molecular structure, and electronic structure characterization of two five-coordinate high-spin imidazolate-ligated iron(II) porphyrinates are reported. Their electronic structure, as deduced from Mössbauer spectra obtained in strong magnetic fields, is distinctly different from that of the analogous imidazole-ligated species. The resulting electronic structure models are consistent with all observed differing features in the two classes.

Electron Spin Resonance Spectroscopy↗