Andrew M. Danby

Oxidative Reactivity Difference among the Metal Oxo and Metal Hydroxo Moieties: pH Dependent Hydrogen Abstraction by a Manganese(IV) Complex Having Two Hydroxide Ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of natures choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

Fluoride Ion Receptors: A Comparison of a Polyammonium MonocycleVersusits Bicyclic Corollary

Abstract Two polyammonium macrocyclic receptors, the monocyclic 3,6,9,17,20,23-hexaazatricyclo [23.3.1.111,15]triaconta-1(29), 11,13,15(30),25,27-hexaene (L1) and its bicyclic analog 1,4,12,15,18,26,31,39-octaazapentacyclo[13.13.13.16,10. 120,24.133,37]-tetratetraconta-6,7,9,20(43),21,23,33(42),34,36-nonaene (L2), have been synthesized as their hexatosylate salts. The propensity for binding fluoride ion was examined using both NMR and potentiometric techniques. The fluoride salts of both receptors have been characterized by X-ray crystallographic methods. For the monocycle, the complex crystallized as the mixed fluoride-bifluoride salt, [H6L1]6+ ·4F− ·2FHF− ·4H2O, and the bicycle crystallized as a complex salt, [H6L2]6+ ·F− ·2FHF− ·1.5SiF6 2− ·7H2O.

Padlock macrocyclic complexes. The synthesis of a range of nickel(II) complexes of N-alkyl azacyclams and the crystal structure of (3-ethyl-1,3,5,8,12-penta-azacyclotetradecane) nickel(II) perchlorate

Abstract A range of (3-alkyl-1,3,5,8,12-penta-azacyclotetradecane)nickel(II) complexes [NiL] (ClO 4 ) 2 (R = Me, Et, n -Pr, n -Bu, n -octyl, n -tridecyl, n -octadecyl) has been prepared and characterised. The planar octahedral equilibrium in solution has been studied in detail for R = Et. For this equilibrium Δ H 0 = −19.2±0.5 kJ mol −1 and Δ S 0 = −76.4±1.5 JK −1 mol −1 , with K = 0.24 at 25°. The crystal structure of 3-ethyl-1,3,5,8,12-penta-azacyclotetradecane)nickel(II) perchlorate has been determined. The complex is square planar and the NiN bond lengths are 1.959(5) and 1.906(5) A. Both six-membered chelate rings adopt a chair conformation and the five-membered rings are gauche with the sec -NH centres having the RSRS configuration. The NEt group is axial.

Accordion porphyrins: Hybrid models for heme and binuclear monooxygenases

Abstract The synthesis, characterization, and examination of the monooxygenase activity of tetrapyrrolic macrocyclic complexes with an accordion porphyrin-like ligand framework are described. Cyanovinyl protection techniques were used to obtain a diformyl dipyrromethane, which was condensed with 1,3-diaminopropane in the presence of metal acetate template to achieve manganese(II), nickel(II), and copper(II) complexes. The macrocycle was found to be in the dipyrromethene form for both the manganese(II) and copper(II) complexes, and in the dipyrromethane form for the nickel(II) complex. The crystal structure of the acetate salt of the manganese(II) complex was obtained. The manganese(II) complex was found to catalyze the epoxidation of cyclohexene, yielding primarily the epoxide (40%) with only minor side products, while the nickel(II) and copper(II) complexes showed little catalytic activity. A lack of stereoselectivity was observed in the epoxidation of cis - and trans -stilbene by the manganese(II) complex, which yielded only trans -epoxide in the reaction with trans -stilbene, but both cis and trans products in the reaction with cis -stilbene.

Similarities and differences in properties and behavior of two H2O2-activated manganese catalysts having structures differing only by methyl and ethyl substituents

The complex [Mn(IV)(Me2EBC)(OH)2](PF6)2, in which Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, is a remarkably selective H2O2 oxidation catalyst that has been shown to be useful in removing stains from fabrics without affecting their colors. Mn(IV) is the highest oxidation state detected and the dihydroxo complex forms a peroxyhydroxy derivative that is responsible for catalytic oxidations. Study of the diethyl homolog of this catalyst has revealed surprising differences in chemical behavior. Oxidation of this new manganese complex, Mn(Et2EBC)Cl2, using aqueous H2O2, at −30°C following removal of chloride ion, yields [Mn(Et2EBC)(OH)2](PF6)2. Above 0°C, H2O2 oxidation of Mn(Et2EBC)Cl2 oxidizes the ethyl substituents. X-ray structure determinations of Et2EBC complexes with Mn(II), Mn(III), and Mn(IV) are reported. The complex [Mn(Et2EBC)(OH)2](PF6)2 displays a surprisingly mild oxidizing potential of +0.556 V for the Mn4+/Mn3+ couple; however, its hydrogen abstraction ability for selected substrates is limited by the BDECH value of 82 kcal mol−1, the same as reported for [Mn(Me2EBC)(OH)2](PF6)2. However, unlike the methyl derivative, electrochemical results indicate a 5+/4+ couple, in addition to the expected 4+/3+ and 3+/2+ couples. The significance of these differences in behavior is discussed. Mass spectral studies have identified some products of ethyl group oxidations.

The non-formation of macrocyclic tetraamides by coordinated ligand reactions

Abstract A recent paper has described the synthesis of macrocyclic tetraamide complexes by the reaction of a diamine (ethylenediamine or propylenediamine) with malonic, succinic or glutaric acids in methanol solution in the presence of a metal(II) salt at room temperature. We present evidence that macrocycle formation does not occur under these conditions and only mixed-ligand complexes are formed. The blue crystalline complex obtained with malonic acid, 1,3-diaminopropane and copper(II) chloride is shown by X-ray crystallography to be [Cu(mal)(pn)Cl2]2− pnH22+ (mal = malonate; pn = 1,3-diaminopropane). The structure comprises discrete [Cu(mal)(pn)Cl2]2− anions and pnH22+ cations. Within the anionic complex the geometry at each copper(II) is tetragonal with two long axial bonds to the chloride ligands, Cu  Cl (1) = 2.862(2) and Cu  Cl 2 = 3.042(2) A . The malanato ring has a boat conformation with Cu  O = 1.966(3) A and the 1,3-diaminopropane ring a chair conformation with Cu  N = 1.993(4) A . A crystallographic mirror plane bisects the two rings through the copper and chloride ligands.

Novel structural determination of a bilayer network formed by a tripodal lipophilic amide in the presence of anions

The crystal structure of a protonated tripodal lipophilic amide, N-{2-[bis(2-octanoylaminoethyl)amino]ethyl}octanamide, identified an elegantly architected bilayer hydrogen bonded structure consisting of ladder-like cascades of the amide, with the rungs comprised of the lipophilic tails pointing inward and the poles made up of polar quaternary ammonium heads lining channels filled with nitrate ions.

The reaction of acetone with [Cu(en)2]2+: the crystal structure of the copper(II)-β-aminoketone complex initially formed and the kinetics of its base-catalysed conversion to the copper(II) complex of trans[14]dieneN4

The copper(II) complex of the β-aminoketone 14-amino-4,4,9,11,11-pentamethyl-5,8,12-triazatetradec-8-en-2-one (L), [CuL](ClO4)2·MeCN has been prepared and its structure established by X-ray crystallography. C18H37N5O9Cl2Cu, triclinic, space group P1 a = I0.752(9), β = 15.980(10), c = 8.112(7), α = 100.24(7), β = 95.43(7), γ = 93.54(6), V = 1361(1) A3, Z = 2. In basic solution the complex undergoes base-catalysed ring closure to give the copper(II) complex of the macrocycle trans[14]dieneN4 The kinetics of the ring closure reaction have been studied in detail. Ring closure occurs by an intramolecular reaction involving the hydroxocomplex [CuLOH]+.

Preparation of the copper(II) complex of the binucleating hexaaza macrocycle 2,5,8,17,20,23-hexaaza(9.9)paracyclophane and its solution chemistry, hydrolytic activity and acid dissociation kinetics ‡

The binucleating hexaaza macrocycle 2,5,8,17,20,23-hexaaza[9.9]paracyclophane (PEA) and its N-permethylated derivative 2,5,8,17,20,23-hexamethyl-2,5,8,17,20,23-hexaaza[9.9]paracyclophane (Me6PEA) have been prepared. The copper(II) complex of PEA has been prepared and the stability constants of the copper complexes determined in aqueous solution by potentiometric and spectrophotometric methods. The protonation sites of PEA have been identified by NMR titration. The crystal structure of the complex [Cu2(PEA)Cl3]+Cl−·2.6MeCN·2.9H2O has been determined. The copper centres have an N3Cl2 donor set with a square pyramidal geometry. The Cu ⋯ Cu distance is 7.02 A. The copper complex catalyses hydrolysis of the phosphotriester 2,4-dinitrophenyl diethyl phosphate (DNPDEP) and the [Cu2(PEA)(OH)2]2+ and [Cu2(PEA)(OH)]3+ complexes have been identified as the active species. The acid catalysed dissociation of the copper complex has been studied by stopped-flow methods. The reaction is monophasic and displays saturation kinetics at high acidities.

Synthesis, metal-exchange kinetics and X-ray structure of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexa-azatricyclo [13.3.1.16,10]eicosane, [NiL] (ClO4)2

The preparation of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexaazatricyclo [13.3.1.16,10]eicosane (2) by the reaction of the nickel(II) complex of N-(2-aminoethyl)-1,3-diaminopropane with formaldehyde in MeOH solution is described. The crystal structure of [NiL](ClO4)2 has been determined. The nickel atom is four coordinate and planar with Ni-N bond lengths of 1.969(4) and 1.928(3)Å in a centrosymmetric structure. The basic diazacyclam ring system has a trans III configuration with the two additional six-membered rings fused in a chair conformation.The kinetics of the metal exchange:for the nickel complexes (1) and (2) have been studied in detail. Under the experimental conditions employed, with copper(II) in at least a tenfold excess, the reaction is independent of the copper(II) concentration. The copper(II) effectively scavenges the free ligand as the nickel(II) complex dissociates. For the nickel complex (1) k = 2 × 10−4 s−1 at 60°C and Δ H ‡ = 126 ± 5 kJmol−1 and Δ S298‡ = 61 ± 15 JK−1mol−1. For the complex (2), k = 1.8 × 10−4 s−1 at 60°C and Δ H‡ = 99 ± 6 kJmol−1 and Δ S298‡ = −21 ± 10 JK−1 mol−1.