Detour‐saturated graphs

A graph is said to be detour‐saturated if the addition of any edge results in an increased greatest path length. In this paper, we add to the relatively small amount that is known about detour‐saturated graphs. Our main result is a determination of all connected detour‐saturated graphs with exactly one cycle. (The family of detour‐saturated trees was found by Kászonyi and Tuza 7 .) We also show that the smallest detour‐saturated graph of girth 5 is the graph obtainable from the Petersen graph by splitting one of its vertices into three, each of degree 1. © 2005 Wiley Periodicals, Inc. J Graph Theory     

A highly anisotropic cobalt(II)-based single-chain magnet: exploration of spin canting in an antiferromagnetic array

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.     

Intercalation is not required for DNA light-switch behavior

The DNA light-switch complex [Ru(bpy)2(tpphz)]2+ (1, bpy = 2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:3',2'-h:2',3'-j]phenazine) is luminescent when bound to DNA and in organic solvents and weakly emissive in water. To date, light-switch behavior by transition metal complexes has generally been regarded as confirmation of DNA intercalation. In contrast, the present work demonstrates that the nonintercalating bimetallic complex [(bpy)2Ru(tpphz)Ru(bpy)2]4+ (2) behaves as a DNA light-switch. Weak emission from the 3MLCT excited state of 2 is observed in water with lambda(em) = 623 nm (phi(em) = 1.4 x 10(-4)), and a red shift (lambda(em) = 702 nm) and 40-fold increase in intensity are observed upon addition of 100 microM calf thymus DNA (ct-DNA). Addition of increasing concentrations of 2 to 1 mM herring sperm DNA does not result in an increase in the viscosity of the solution, indicating that the complex is not an intercalator. Additionally, experiments were conducted to ensure that the emission enhancement did not arise from threading intercalation of the complex. The in situ generation of 2 intercalated between the base pairs of ct-DNA in a threading fashion, however, exhibits emission maximum at 685 nm, which is blue-shifted from that of surface-bound 2. DFT calculations show low-lying orbitals in 2 that are expected to exhibit nonemissive character when contributing to the MLCT state, in accord with the lower emission intensity observed for 2 relative to that for 1. To our knowledge, the present work is the first example of a nonintercalating light-switch metal complex, thus showing that light-switch behavior cannot be used exclusively as confirmation of intercalation.     

Role of axial donors in the ligand isomerization processes of quadruply bonded dimolybdenum(II) compounds

Quadruply bonded dimolybdenum(II) complexes with NP-R (2-(2-R)-1,8-naphthyridine; R = thiazolyl (NP-tz), furyl (NP-fu), thienyl (NP-th)) and 2,3-dimethyl-1,8-naphthyridine (NP-Me 2) have been synthesized by reactions of cis-[Mo2(OAc)2(CH3CN)6][BF4]2 with the corresponding ligands. The products cis-[Mo2(NP-tz)2(OAc)2][BF4]2 (1), trans-[Mo2(NP-fu)2(OAc)2][BF4]2 (2), trans-[Mo2(NP-th)2(OAc)2][BF4]2 (3), and trans-[Mo2(NP-Me2)2(OAc)2][BF4]2 (4) were isolated and characterized. The NP-R ligands with stronger R = pyridyl and thiazolyl donors result in cis isomers whereas the weaker furyl and thienyl appendages lead to compounds having a trans orientation of the ligands. The use of NP-Me2 leads to a trans structure with a tetrafluoroborate anion occupying one of the axial sites. Complete replacement of two acetate groups by acetonitrile in 1 and 2 resulted in the cis isomers [Mo2(NP-tz)2(CH3CN)4][OTf]4 (5) and [Mo2(NP-fu)2(CH3CN)4][OTf]4 (6) respectively. The combination of one acetate and two acetonitriles as ancillary ligands, however, yields trans-[Mo2(NP-tz)2(OAc)(CH3CN)2][BF4]3 (7) in the solid state as determined by X-ray crystallography. (1)H NMR spectra of the products are diagnostic of the cis and trans dispositions of the ligands. Solution studies reveal that the ligand arrangements observed in the solid state are mostly retained in the acetonitrile medium. The only exception is 7, for which a mixture of cis and trans isomers are detected on the NMR time scale. The isolation of trans compounds 2- 4 from the cis precursor [Mo2(OAc)2(CH3CN)6][BF4]2 indicates that an isomerization process occurs during the reactions. The mechanism involving acetate migration through axial coordination has been invoked to rationalize the product formation. Compounds 1- 7 were structurally characterized by single-crystal X-ray methods.     

Switching surface chemistry with supramolecular machines

Tethered supramolecular machines represent a new class of active self-assembled monolayers in which molecular configurations can be reversibly programmed using electrochemical stimuli. We are using these machines to address the chemistry of substrate surfaces for integrated microfluidic systems. Interactions between the tethered tetracationic cyclophane host cyclobis(paraquat-p-phenylene) and dissolved pi-electron-rich guest molecules, such as tetrathiafulvalene, have been reversibly switched by oxidative electrochemistry. The results demonstrate that surface-bound supramolecular machines can be programmed to adsorb or release appropriately designed solution species for manipulating surface chemistry.     

Self-assembly of a high-nuclearity chloride-centered copper(II) cluster. Structure and magnetic properties of [Au(PPh3)2][trans-Cu6(micro-OH)6[micro-(3,5-CF3)2pz]6Cl

A chloride-centered hexanuclear copper(II) pyrazolate [Au(PPh3)2][trans-Cu6(micro-OH)6[micro-(3,5-CF3)2pz]6Cl] is isolated from the reaction of the trinuclear copper(I) pyrazolate [Cu3[micro-(CF3)2pz]3] with PPh3AuCl and Ph3P in moist air. The six copper atoms are bridged by pyrazolate and hydroxyl ligands, above and below the copper plane. The chloride anion exists at the center of the planar cavity formed by the copper atoms with Cu-Cl distances of 3.02-3.13 A. The magnetic susceptibility measurements show a strong antiferromagnetic coupling between the copper centers with an estimated exchange constant of J approximately 650 cm-1.     

Synthesis and characterization of four metal-organophosphonates with one-, two-, and three-dimensional structures

A series of metal-organic hybrid compounds were synthesized using two new phosphonic acids, pyridyl-4-phosphonic acid and p-xylylenediphosphonic acid (H(2)O(3)PCH(2)C(6)H(4)CH(2)PO(3)H(2)). The phosphonic acid ligands have been synthesized from their corresponding bromides following two different types of reactions. The reaction of pyridyl-4-phosphonic acid with three different divalent metal salts results in the formation of molecular structures of different dimensionality. The reaction of Cu(II) with the phosphonic acid under hydrothermal conditions yields a three-dimensional (3D) open framework structure having the molecular formula [Cu(4)(NC(5)H(4)-PO(3))(4)(H(2)O)(10)] (1). The reactions with Mn(II) and Zn(II) salts with the same phosphonic acid resulted in a two-dimensional layered and a dinuclear compound with molecular formulas [Mn(3)(NC(5)H(4)-PO(3))(4)(H(2)O)(6)(ClO(4))(2)] (2) and [Zn(2)(NHC(5)H(4)-PO(3)H)(2)Cl(4)] (3), respectively. Compound 1 crystallizes in the triclinic crystal system having space group P with structural parameters a = 7.4564(15) Angstrom, b = 9.1845(19) Angstrom, c = 11.582(2) Angstrom, alpha = 100.842(3) degrees, beta = 104.303(3) degrees, gamma = 94.774(3) degrees, and Z = 1. Compound 2 crystallizes in the triclinic crystal system, space group P, with structural parameters a = 7.6871(14) Angstrom, b = 10.576(2) Angstrom, c = 14.470(3)Angstrom, alpha = 81.340(3) degrees, beta = 81.561(3) degrees, gamma = 68.757(3) degrees, and Z = 2, whereas compound 3 crystallizes in a monoclinic crystal system with space group P2(1)/n. The structural parameters are as follows: a = 8.4969 (5) Angstrom, b = 9.3911 (5) Angstrom, c = 12.3779 (6) Angstrom, beta = 90.860(17) degrees, and Z = 4. The pyridylphosphonate ligand shows different ligation behavior toward the three divalent metal ions. On the other hand, p-xylylenediphosphonic acid on reaction with Co(II) formed a 3D compound [Co(2)(O(3)PCH(2)C(6)H(4)CH(2)PO(3))(2)(H(2)O)(2)] (4) with a layered and pillared structure. Compound 4 crystallizes in an orthorhombic crystal system with space group Pnma. The structural parameters are a = 21.744(4) Angstrom, b = 5.6744(10) Angstrom, c = 4.7927(9) Angstrom, and Z = 4.     

Hydrothermal synthesis, structural chemistry, and magnetic properties of materials of the MII/triazolate/anion family, where MII = Mn, Fe, and Ni

Hydrothermal chemistry has been exploited in the preparation of a series of manganese(II), iron(II), and nickel(II) triazolate frameworks, [Mn7(trz)8(CH3CO2)4(OH)2].2.5H2O (1.2.5H2O), [Mn5(Htrz)2(SO4)4(OH)2] (2), [Fe5(Htrz)2(SO4)4(OH)2] (3), [Fe3(Htrz)3(HSO4)(SO4)2(OH)].H2O (4.H2O), [Ni3(trz)3(OH)3(H2O)4].5H2O (5.5H2O), and [Ni3(trz)5(OH)].2.5H2O (6.2.5H2O). The materials all exhibit three-dimensional structures, reflecting the tendency of triazole/triazolate ligands to bridge multiple metal sites. A prominent characteristic of the structures is the presence of embedded metal clusters as building blocks: heptanuclear MnII units in 1, pentanuclear MII sites in 2 and 3, and trinuclear MII clusters in 4 and 5. The presence of the pentanuclear and trinuclear clusters of magnetic metal cations in 2-5 is reflected in the unusual magnetic characteristics of these materials, all of which exhibit spin frustration. The compound 5.5H2O reversibly desorbs/sorbs solvent. However, the dehydrated phase does not adsorb methanol, N2, O2, or H2, presumably as a consequence of the highly polar void volume and the narrow channels connecting the larger cavities of the void structure.     

Infrared spectroscopy of arginine cation complexes: direct observation of gas-phase zwitterions

The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.