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Marilyn M Olmstead

Publications and source records attributed to Marilyn M Olmstead.

At least 19 recordsLinked to original sources
PubMed2003

Germanium and tin analogues of alkynes and their reduction products.

The reduction of terphenylgermanium(II) or terphenyltin(II) chlorides with alkali metals was investigated. Treatment of Ar'GeCl or ArGeCl (Ar' = C(6)H(3)-2,6-Dipp(2), Dipp = C(6)H(3)-2,6-Pr(i)(2); Ar = C(6)H(3)-2,6-Trip(2), Trip = C(6)H(2)-2,4,6-Pr(i)(3)) with lithium, sodium, or potassium afforded the neutral alkyne analogues Ar'GeGeAr', 1, ArGeGeAr, 2, the singly reduced radical species NaArGeGeAr, 3, or KAr'GeGeAr', 4, or the doubly reduced compounds Li(2)Ar'GeGeAr', 5, Na(2)ArGeGeAr, 6, or K(2)ArGeGeAr, 7. Similarly, reduction of Ar'SnCl or ArSnCl afforded the neutral Ar'SnSnAr', 8, or ArSnSnAr, 9, the radical anions [(THF)(3)Na[rSnSnAr]], 10, [K(THF)(6)][Ar'SnSnAr'], 11, [K(THF)(6)][ArSnSnAr], 12, [K(18-crown-6)(THF)(2)] [ArSnSnAr], 13, or the doubly reduced Na(2)ArSnSnAr, 14, K(2)Ar'SnSnAr', 15, or K(2)ArSnSnAr, 16. The compounds were characterized by UV-vis, (1)H and (13)C NMR or EPR spectroscopy. The X-ray crystal structures of all compounds were determined except those of 2 and 9. The neutral 1 and 8 displayed planar, trans-bent CMMC (M = Ge and Sn) cores with M-M-C angles of 128.67(8) and 125.24(7) degrees, respectively. The M-M bond lengths, 2.2850(6) and 2.6675(4)A, indicated considerable multiple character and a bond order approaching two. Single and double reduction of the neutral species resulted in the narrowing of the M-M-C angles by ca. 12-32 degrees and changes in the Ge-Ge and Sn-Sn bond lengths. One-electron reduction afforded a slight (ca. 0.03-0.05A) lengthening of the Ge-Ge bonds in the case of germanium species 3 and 4 and a greater lengthening (ca. 0.13-0.15A) for the Sn-Sn bonds in the tin compounds 10-13. The addition of another electron yielded salts of the formal dianions [Ar'MMAr'](2)(-) and [ArMMAr](2)(-) which are isoelectronic to the corresponding doubly bonded, neutral arsenic and antimony derivatives. All the dianion salts were obtained as contact ion triples with two alkali metal cations complexed between aryl rings. The Ge-Ge bonds in the dianions of 5-7 were longer, whereas the Sn-Sn distances in the dianions in 14, 15, and 16 were shorter than those in the monoanions. Unusually, the Li(2)Ar'GeGeAr' salt, 5, displayed a longer Ge-Ge bond (by ca. 0.06A) than those of its Na(+) or K(+) analogue salts which was attributed to the greater polarizing power of Li(+). It was concluded that the M-M bond lengths in 3-7 and 10-16 are dependent on several factors that include M-M-C angle, Coulombic repulsion, alkali metal cation size, and the character of the molecular energy levels. The M-M bonding in the neutral compounds was accounted for in terms of a second-order Jahn-Teller mixing of sigma- and a pi-orbital which afforded bond orders near two for the neutral compounds, 1, 2, 8, and 9. Calculations on MeMMMe (M = Ge or Sn) model species showed that the LUMO corresponded to an orbital that had n(+) lone pair character. The slight Ge-Ge bond length increase upon one-electron reduction is consistent with these results, and the further bond lengthening upon double reduction is consistent with increased Coulombic repulsion. The greater Sn-Sn bond length increase seen for one-electron reduction of the tin species is probably due to the increased p-character of orbitals comprising the Sn-Sn sigma-bond when the Sn-Sn-C angle is decreased by ca. 30 degrees. Upon further reduction, the slight decrease in the Sn-Sn bond is probably a result of the reduced importance of Coulombic repulsion due to the larger size of tin and a widening of the Sn-Sn-C angles which may shorten the Sn-Sn sigma-bond.

Journal Article↗
PubMed2003

Modulation of the pK(a) of metal-bound water via oxidation of thiolato sulfur in model complexes of Co(III) containing nitrile hydratase: insight into possible effect of cysteine oxidation in Co-nitrile hydratase.

The Co(III) complexes of N,N'-bis(2-mercaptophenyl)pyridine-2,6-dicarboxamide (PyPSH(4)), a designed pentadentate ligand with built-in carboxamide and thiolate groups, have been synthesized and studied to gain insight into the role of Cys-S oxidation in Co-containing nitrile hydratase (Co-NHase). Reaction of [Co(NH(3))(5)Cl]Cl(2) with PyPS(4)(-) in DMF affords the thiolato-bridged dimeric Co(III) complex (Et(4)N)(2)[Co(2)(PyPS)(2)] (1). Although the bridged structure is quite robust, reaction of (Et(4)N)(CN) with 1 in acetonitrile affords the monomeric species (Et(4)N)(2)[Co(PyPS)(CN)] (2). Oxidation of 2 with H(2)O(2) in acetonitrile gives rise to a mixture which, upon chromatographic purification, yields K(2)[Co(PyPSO(2)(OSO(2))(CN] (3), a species containing asymmetrically oxidized thiolates. The Co(III) metal center in 3 is coordinated to a S-bound sulfinate and an O-bound sulfonate (OSO(2)) group. Upon oxidation with H(2)O(2), 1 affords an asymmetrically oxidized dimer (Et(4)N)(2)[Co(2)(PyPS(SO(2)))(2)] (4) in which only the terminal thiolates are oxidized to form S-bound sulfinate groups while the bridging thiolates remain unchanged. The thiolato-bridge in 4 is also cleaved upon reaction with (Et(4)N)(CN) in acetonitrile, and one obtains (Et(4)N)(2)[Co(PyPS(SO(2)))(CN)] (5), a species that contains both coordinated thiolate and S-bound sulfinate around Co(III). The structures of 1-4 have been determined. The spectroscopic properties and reactivity of all the complexes have been studied to understand the behavior of the Co(III) site in Co-NHase. Unlike typical Co(III) complexes with bound CN(-) ligands, the Co(III) centers in 2 and 5 are labile and rapidly lose CN(-) in aqueous solutions. Since 3 does not show this lability, it appears that at least one thiolato sulfur donor is required in the first coordination sphere for the Co(III) center in such species to exhibit lability. Both 2 and 5 are converted to the aqua complexes [Co(PyPS)(H(2)O)](-) and [Co(PyPS(SO(2))(H(2)O)](-) in aqueous solutions. The pK(a) values of the bound water in these two species, determined by spectrophotometry, are 8.3 +/- 0.03 and 7.2 +/- 0.06, respectively. Oxidation of the thiolato sulfur (to sulfinate) therefore increases the acidity of the bound water. Since 2 and 5 promote hydrolysis of acetonitrile at pH values above their corresponding pK(a) values, it is also evident that a metal-bound hydroxide is a key player in the mechanism of hydrolysis by these model complexes of Co-NHase. The required presence of a Cys-sulfinic residue and one water molecule at the Co(III) site of Co-NHase as well as the optimal pH of the enzyme near 7 suggests that (i) modulation of the pK(a) of the bound water molecule at the active site of the enzyme could be one role of the oxidized Cys-S residue(s) and (ii) a cobalt-bound hydroxide could be responsible for the hydrolysis of nitriles by Co-NHase.

Chemical Phenomena↗
PubMed2003

Eu10Mn6Sb13: a new ternary rare-Earth transition-metal Zintl phase.

A new transition-metal-containing Zintl compound, Eu(10)Mn(6)Sb(13), was prepared by a high-temperature Sn-flux synthesis. The structure was determined by single-crystal X-ray diffraction. Eu(10)Mn(6)Sb(13) crystallizes in the monoclinic space group C2/m with a = 15.1791(6) A, b = 19.1919(7) A, c = 12.2679(4) A, beta = 108.078(1)*, Z = 4 (R1 = 0.0410, wR2 = 0.0920), and T = 90(2) K. The structure of Eu(10)Mn(6)Sb(13) is composed of double layers of Mn-centered tetrahedra separated by Eu(2+) cations. The double layers are composed of edge- and corner-sharing Mn-centered tetrahedra which form cavities occupied by Eu(2+) cations and [Sb(2)](4-) dumbbells. Linear [Sb(3)](5-) trimers bridging two tetrahedra across the cavity are also present. Bulk susceptibility data indicate paramagnetic behavior with a ferromagnetic component present below 60 K. Temperature-dependent electrical resistivity measurements show semiconducting behavior above 60 K (E(a)() = 0.115(2) eV), a large and unusually sharp maximum in the resistivity at approximately 40 K, and metallic behavior below 40 K. (151)Eu Mössbauer spectra confirm that the europium is divalent with an average isomer shift of -11.2(1) mm/s at 100 K; the spectra obtained below 40 K reveal magnetic ordering of six of the seven europium sublattices and, at 4.2 K, complete ordering of the seven europium sublattices.

Journal Article↗
PubMed2003

New structural features of unsupported chains of metal ions in luminescent [(NH3)4Pt][Au(CN)2]2 x 1.5(H2O) and related salts.

Luminescent [(NH(3))(4)Pt][Au(CN)(2)](2).1.5(H(2)O), which forms from aqueous solutions of [(NH(3))(4)Pt]Cl(2) and K[Au(CN)(2)], crystallizes with extended chains of the two ions with multiple close Pt...Au (3.2804(4) and 3.2794(4) A) and Au...Au (3.2902(5), 3.3312(5), and 3.1902(4) A) contacts. Nonluminescent [(NH(3))(4)Pt][Ag(CN)(2)](2).1.4(H(2)O) is isostructural with [(NH(3))(4)Pt][Au(CN)(2)](2).1.5(H(2)O). Treatment of [(NH(3))(6)Ni]Cl(2) with K[Au(CN)(2)] forms [(NH(3))(2)Ni][Au(CN)(2)](2) in which the [Au(CN)(2)](-) ions function as nitrile ligands toward nickel, which assumes a six-coordinate structure with trans NH(3) ligands. The [Au(CN)(2)](-) ions self-associate into linear columns with close Au...Au contacts of 3.0830(5) A, and pairs of gold ions in these chains make additional but longer (3.4246(5) A) contacts with other gold ions.

Journal Article↗
PubMed2003

The neonicotinoid electronegative pharmacophore plays the crucial role in the high affinity and selectivity for the Drosophila nicotinic receptor: an anomaly for the nicotinoid cation--pi interaction model.

Cation-pi interaction, a prominent feature in agonist recognition by neurotransmitter-gated ion channels, does not apply to the anomalous action of neonicotinoids at the insect nicotinic acetylcholine receptor (nAChR). Insect-selective neonicotinoids have an electronegative pharmacophore (tip) in place of the ammonium or iminium cation of the vertebrate-selective nicotinoids, suggesting topological divergence of the agonist-binding sites in insect and vertebrate nAChRs. This study defines the molecular and electronic basis for the potent and selective interaction of the neonicotinoid electronegative pharmacophore with a unique subsite of the Drosophila but not of the vertebrate alpha4beta2 nAChR. Target site potency and selectivity are retained when the usual neonicotinoid N-nitroimine (=NNO(2)) electronegative tip is replaced with N-nitrosoimine (=NNO) or N-(trifluoroacetyl)imine (=NCOCF(3)) in combination with an imidazolidine, imidazoline, thiazolidine, or thiazoline heterocycle. X-ray crystallography establishes coplanarity between the heterocyclic and imine planes, including the electronegative substituent in the trans configuration. The functional tip is the coplanar oxygen atom of the N-nitrosoimine or the equivalent oxygen of the N-nitroimine. Quantum mechanics in the gas and aqueous phases fully support the conserved coplanarity and projection of the strongly electronegative tip. Further, a bicyclic analogue with a nitro tip in the cis configuration but retaining coplanarity has a high potency, whereas the N-trifluoromethanesulfonylimine (=NSO(2)CF(3)) moiety lacking coplanarity confers very low activity. The coplanar system between the electronegative tip and guanidine-amidine moiety extends the conjugation and facilitates negative charge (delta(-)) flow toward the tip, thereby enhancing interaction with the proposed cationic subsite such as lysine or arginine in the Drosophila nAChR.

Animals↗
PubMed2003

An optically-active subphthalocyanine dimer.

A subphthalocyanine (SubPc) dimer linked by racemic 1,1'-binaphth-2,2'-dioxy (BINOL), 1, has been synthesized and characterized by 1- and 2-D 1H NMR, high resolution mass spectrometry (FAB), X-ray crystallography and electronic absorption spectroscopy. Additionally, the dimer derived from optically-active (R)-(+)-BINOL, 2, has been made and studied using CD and MCD spectroscopies.

Journal Article↗
PubMed2003

Heme cleavage with remarkable ease: paramagnetic intermediates formed by aerobic oxidation of a meso-amino-substituted iron porphyrin.

Hemes must be oxidatively stable to carry out their functions as biological oxidants, but introduction of a single amino group at a meso position of octaethylheme renders it extremely sensitive to ring opening by dioxygen. Exposure of a red pyridine (py) solution of diamagnetic (py)(2)Fe(II)(H(2)N-OEP) (1) (H(2)N-OEP is the dianion of meso-amino-octaethylporphyrin) to air results in the immediate formation of a green intermediate which is subsequently converted into a second species that has been crystallized and characterized by X-ray diffraction. This process is distinct from coupled oxidation, a model for biological heme cleavage, because it does not require a sacrificial reducing agent to initiate the process.

Heme↗
PubMed2003

Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins.

Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.

Crystallography, X-Ray↗
PubMed2003

The crystal structure and magnetic properties of a new ferrimagnetic semiconductor: Ca21Mn4Sb18.

Single crystals of the new transition metal Zintl phase, Ca(21)Mn(4)Sb(18), were prepared by high temperature melt synthesis. The crystal structure was determined by single crystal X-ray diffraction to be monoclinic in the space group C2/c. Crystal information was obtained at 90 K, and unit cell parameters were determined (a = 17.100(2) A, b = 17.073(2) A, c = 16.857(2) A, beta = 92.999(2) degrees, Z = 2, R1 = 0.0540, wR2 = 0.1437). The structure can be described as containing 4 discreet units per formula unit: 1 linear [Mn(4)Sb(10)](22-) anion, 2 dumbbell-shaped [Sb(2)](4-) anions, 4 individual Sb(3-) anions, and 21 Ca(2+) cations. The [Mn(4)Sb(10)](22-) anion contains four edge-shared MnSb(4) tetrahedra with distances between Mn ions of 3.388(4) A, 2.782(4) A, and 2.760(4) A. Electron counting suggests that the Mn are 2+. Temperature dependent magnetization shows a ferromagnetic-like transition temperature at approximately 52 K which is suppressed with increasing magnetic field. The paramagnetic regime is best fit to a ferrimagnetic model, providing a total effective moment of 4.04(2) mu(B), significantly less than that expected for 4 Mn(2+) ions (11.8 mu(B)). Temperature dependent resistivity shows that this compound is a semiconductor with an activation energy of 0.159(2) eV (100-300 K).

Journal Article↗
PubMed2003

Reaction of (mu-oxo)diiron(III) core with CO2 in N-methylimidazole: formation of mono(mu-carboxylato)(mu-oxo)diiron(III) complexes with N-methylimidazole as ligands.

Several iron(III) complexes with N-methylimidazole (N-MeIm) as the ligand have been synthesized by using N-MeIm as the solvent. Under anaerobic conditions, [Fe(N-MeIm)(6)](ClO(4))(3) (1) reacts with stoichiometric amounts of water in N-MeIm to afford the (mu-oxo)diiron(III) complex, [Fe(2)(mu-O)(N-MeIm)(10)](ClO(4))(4) (3). Exposure of a solution of 3 in N-MeIm to stoichiometric and excess CO(2) gives rise to the (mu-oxo)(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-HCO(2))(N-MeIm)(8)](ClO(4))(3) (4) and the methyl carbonate complex [Fe(2)(mu-O)(mu-CH(3)OCO(2))(N-MeIm)(8)](ClO(4))(3) (5), respectively. Formation of the formato-bridged complex 4 upon fixation of CO(2) by 3 in N-MeIm is unprecedentated. Methyl transfer from N-MeIm to a bicarbonato-bridged (mu-oxo)diiron(III) intermediate appears to give rise to 5. Complex 3 is a good starting material for the synthesis of (mu-oxo)mono(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-RCO(2))(N-MeIm)(8)](ClO(4))(3) (where R = H (4), CH(3) (6), or C(6)H(5) (7)); addition of the respective carboxylate ligand in stoichiometric amount to a solution of 3 in N-MeIm affords these complexes in high yields. Attempts to add a third bridge to complexes 4, 6, and 7 to form the (mu-oxo)bis(mu-carboxylato)diiron(III) species result in the isolation of the previously known triiron(III) mu-eta(3)-oxo clusters [[Fe(mu-RCO(2))(2)(N-MeIm)](3)O](ClO(4)) (8). The structures of 3, 4, 6, and 7 allow one, for the first time, to inspect the various features of the [Fe(2)(mu-O)(mu-RCO(2))](3+) moiety with no strain from the ligand framework.

Carbon Dioxide↗
PubMed2003

Aurophilic interactions in cationic gold complexes with two isocyanide ligands. Polymorphic yellow and colorless forms of [(cyclohexyl isocyanide)2AuI](PF6) with distinct luminescence.

Crystallographic studies of yellow and colorless forms of [(C(6)H(11)NC)(2)Au(I)](PF(6)) show that they are polymorphs with differing, but close, contacts between the gold atoms which form extended chains. In the colorless polymorph the gold cations form linear chains with a short Au...Au contact (3.1822(3) A) indicative of an aurophilic attraction. The structure of the yellow polymorph is more complicated with four independent cations forming kinked, slightly helical chains with very short Au...Au contacts of 2.9803(6), 2.9790(6), 2.9651(6), and 2.9643(6) A. However, in the related compound, [(CH(3)NC)(2)Au(I)](PF(6)), each cation is surrounded by six hexafluorophosphate ions and there is no close Au...Au contact despite the fact that the isocyanide ligand has less steric bulk. The crystalline colorless and yellow polymorphs are both luminescent at 298 K, lambda(max): 424 nm (colorless) or 480 nm (yellow). Colorless solutions of the two polymorphs have identical absorption spectra and are nonluminescent at room temperature. Freezing solutions of [(C(6)H(11)NC)(2)Au(I)](PF(6)) produces intense luminescence which varies depending upon the solvent involved. Each polymorph melts to give a colorless but luminescent liquid which reverts to the yellow polymorph upon cooling.

Journal Article↗
PubMed2002

Use of ab initio calculations to predict the biological potency of carboxylesterase inhibitors.

Carboxylesterases are important enzymes responsible for the hydrolysis and metabolism of numerous pharmaceuticals and xenobiotics. These enzymes are potently inhibited by trifluoromethyl ketone containing (TFK) inhibitors. We demonstrated that the ketone hydration state was affected by the surrounding chemical moieties and was related to inhibitor potency, with inhibitors that favored the gem-diol conformation exhibiting greater potency. Ab initio calculations were performed to determine the energy of hydration of the ketone, and the values were correlated with esterase inhibition data for a series of carboxylesterase inhibitors. This system was examined in three different mammalian models (human liver microsomes, murine liver microsomes, and commercial porcine liver esterase) and in an insect enzyme preparation (juvenile hormone esterase). In all cases, the extent of ketone hydration was strongly correlated with biological potency. Our results showed a very strong correlation with the extent of hydration, accounting for 94% of activity for human liver microsome esterase inhibition (p < 0.01). The atomic charge on the carbon atom of the carbonyl group in the TFK also strongly correlated with inhibitor potency, accounting for 94% of inhibition activity in human liver microsomes (p < 0.01). In addition, we provide crystallographic evidence of intramolecular hydrogen bonding in sulfur-containing inhibitors and relate these data to gem-diol formation. This study provides insight into the mechanism of carboxylesterase inhibition and raises the possibility that inhibitors that too strongly favor the gem-diol configuration have decreased potency due to low rate of ketone formation.

Animals↗
PubMed2002

Spontaneous reduction of a low-spin Fe(III) complex of a neutral pentadentate N(5) Schiff base ligand to the corresponding Fe(II) species in acetonitrile.

The iron complexes of a designed pentadentate Schiff base ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine (SBPy(3)) have been synthesized. The low-spin mononuclear Fe(III) complex [(SBPy(3))Fe(DMF)](ClO(4))(3) (2), though stable in the solid state, is spontaneously reduced to the corresponding Fe(II) species [(SBPy(3))Fe(MeCN)](2+) in MeCN. Fe(II) complex [(SBPy(3))Fe(MeCN)](BF(4))(2) (3) has been isolated independently and characterized by crystallography. Electrochemical studies indicate that SBPy(3), like other pentadentate polypyridine ligands, stabilizes the Fe(II) center to a great extent (E(1/2) = 1.01 V vs SCE in MeCN). This fact is responsible for the ready reduction of 2. It is evident that such reactivity has brought complications in the syntheses of iron complexes of polypyridine ligands reported in previous accounts. Very low solubility of 2 in MeOH has allowed isolation of analytically pure 2 in the present work. Storage of dilute methanolic solution of 2 results in the formation of the mu-oxo Fe(III) dimer [(SBPy(3))FeOFe(SBPy(3))](ClO(4))(4) (5), the structure of which has also been determined. Fe(II) complex 3 reacts with CN(-) to afford cyanide adduct [(SBPy(3))Fe(CN)](BF(4)) (4) but does not exhibit any reactivity toward NO. The azomethine moiety (CH=N-py) of 2 is rapidly oxidized by H(2)O(2) to a pyridine-2-carboxamido (C(=O)-N-py) unit and affords [(PaPy(3))Fe(MeCN)](ClO(4))(2) (1), a complex previously reported by us.

Acetonitriles↗