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Paul V. Bernhardt

Publications and source records attributed to Paul V. Bernhardt.

7 recordsLinked to original sources
PubMed1999

Electrochemistry of Macrocyclic Cobalt(III/II) Hexaamines: Electrocatalytic Hydrogen Evolution in Aqueous Solution.

The macrocyclic cobalt hexaamines [Co(trans-diammac)](3+) and [Co(cis-diammac)](3+) (diammac = 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) are capable of reducing the overpotential for hydrogen evolution on a mercury cathode in aqueous solution. Protons are reduced in a catalytic process involving reoxidation of the Co(II) species to its parent Co(III) complex. The cycle is robust at neutral pH with no decomposition of catalyst. The stability of the [Co(trans-diammac)](2+) and [Co(cis-diammac)](2+) complexes depends on the pH of the solution and the coordinating properties of the supporting electrolyte. Electrochemical studies indicate that the adsorbed Co(II) complex on the surface of mercury is the active catalyst for the reduction of protons to dihydrogen.

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PubMed1999

A Supramolecular Synthon for H-Bonded Transition Metal Arrays.

The heterocycle melamine appended to a 14-membered polyazamacrocyclic copper(II) complex ring exhibits a strong propensity for ordered arrays driven by H-bonding forces. In this work, the X-ray crystal structures of three distinctly different arrays based on the same macrocyclic complex are reported, comprising cocrystals with barbituric acid (a sheet structure) and cyanuric acid (a linear chain) and the parent complex (a linear tape).

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PubMed1998

Aminotriazines as Locking Fragments in Macrocyclic Synthesis.

The macrocyclic compounds (6-(4',6'-diamino-1',3',5'-triazinyl)-1,4,6,8,11-pentaazacyclotetradecane)copper(II) triperchlorate dihydrate, [Cu(HL(2))](ClO(4))(3).2H(2)O, (6-(6'-amino-4'-oxo-1'H-1',3',5'-triazinyl)-1,4,6,8,11-pentaazacyclotetradecane)copper(II) diperchlorate hydrate, [CuL(3)](ClO(4))(2).H(2)O, and [(6-(4',6'-dioxo-1'H-1',3',5'-triazinyl)-1,4,6,8,11-pentaazacyclotetradecane)copper(II)] diperchlorate, [CuL(4)](ClO(4))(2), have been synthesized. The macrocycles synthesized contain respectively pendant melamine, ammeline, and ammelide rings. The X-ray cyrstallographic analyses of [Cu(HL(2))](ClO(4))(3).2H(2)O, triclinic, space group P&onemacr;, a = 9.489(10) Å, b = 12.340(2) Å, c = 24.496(4) Å, alpha = 87.74(10) degrees, beta = 85.51(10) degrees, gamma = 70.95(10) degrees, and Z = 4, and {[CuL(3)](ClO(4))(2).H(2)O}(2), monoclinic, space group C2/c, a = 18.624(8) Å, b = 17.160(2) Å, c = 15.998(6) Å, beta = 117.82(2) degrees, and Z = 4, are reported. The structure of [Cu(HL(2))](ClO(4))(3).2H(2)O shows the formation of linear tapes, formed by a combination of hydrogen bonds and pi-pi stacking interactions. The structure of [CuL(3)](ClO(4))(2).H(2)O displays formation of dimers, formed by a coordinate bond from the oxygen in one molecule to the copper atom of another. The tautomeric forms of the ammeline and ammelide moieties have been determined. The potential of these compounds as subunits for cocrystallization has been investigated.

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PubMed1998

N-Mesityl-C-acylketenimines: 1,5-Sigmatropic Shifts and Electrocyclization to Quinolines.

Flash vacuum thermolysis (FVT) of triazoles 6a-c generates alpha-oxoketenimines 10, the ester 10a being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a. Ketenes 9 and ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups under mild FVT conditions (ca. 350-400 degrees C). Both 9 and 10 are directly observable by IR spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca. 400 degrees C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal mol(-)(1) (161 kJ mol(-)(1)) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal mol(-)(1).

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PubMed1998

Thermochromism and Structure of Piperazinium Tetrachlorocuprate(II) Complexes.

The tetrachlorocuprate(II) ion can crystallize in two different structures with the piperazinium dication (pipzH(2)). Both structures contain discrete CuCl(4)(2-) species. A yellow compound (pipzH(2))[CuCl(4)].2H(2)O (1) is monoclinic (C2/c, Z = 4, a = 10.538(3) Å, b = 7.4312(5) Å, c = 17.281(4) Å, beta = 111.900(10) degrees ) and contains the CuCl(4)(2-) ion as a distorted tetrahedron. A green compound (pipzH(2))(2)[CuCl(4)].Cl(2).3H(2)O (2) is triclinic (P&onemacr;, Z = 2, a = 9.264(3) Å, b = 10.447(2) Å, c = 11.366(2) Å, alpha = 68.38 degrees, beta = 82.86(2) degrees, gamma = 83.05(2) degrees ) and contains the CuCl(4)(2-) ion with a square planar geometry. This latter compound shows thermo/photochromism, changing from green to yellow upon heating or laser irradiation.

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PubMed1997

Structural and Electron Self-Exchange Rate Variations in Isomeric (Hexaamine)cobalt(III/II) Complexes.

The syntheses and characterisation of the new macrocyclic hexaamine trans-(5(S),7(S),12(R),14(R)-tetramethyl)-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L(6)) and its Co(III) complex are reported. The X-ray crystal structural analyses of [CoL(6)]Cl(2)(ClO(4)) [monoclinic, space group C2/c, a = 16.468(3) Å, b = 9.7156(7) Å, c = 15.070(3) Å, beta = 119.431(8) degrees, Z = 4] and the closely related cis-diamino-substituted macrocyclic complex [CoL(2)](ClO(4))(3). 2H(2)O (L(2) = cis-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine) [orthorhombic, space group Pna2(1), a = 16.8220(8) Å, b = 10.416(2) Å, c = 14.219(3) Å, Z = 4] reveal significant variations in the observed Co-N bond lengths and coordination geometries, which may be attributed to the trans or cis disposition of the pendent primary amines. The Co(III/II) self-exchange electron transfer rate constants for these and other closely related hexaamines have been determined, and variations of some 2 orders of magnitude are found between pairs of trans and cis isomeric Co(III) complexes.

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