Lars Preuss Nielsen

Synthesis, structural characterisation and electronic behaviour of iron(II) and nickel(II) complexes of N,N-bis(2-pyridylmethyl)aniline

Abstract Ni(II) and Fe(II) complexes of the new ligand N,N-bis(2-pyridylmethyl)aniline (phdpa) of the general formula [M(phdpa)2](ClO4)2 have been prepared and characterised. [Ni(phdpa)2](ClO4)2·0.5 H2O was characterised by X-ray crystallography. For comparison, the X-ray structure of [Ni(medpa)2](ClO4)2 (where medpa is bis(2-pyridylmethyl)methylamine) has been determined. The ligand in both [Ni(phdpa)2](ClO4)2 and [Ni(medpa)2](ClO4)2 exhibit cis–facial geometry.

Syntheses, Structures, and Properties of Copper(II) Complexes of Bis(2-pyridylmethyl) Derivatives of o-, m-, and p-Phenylenediamine and Aniline

Copper(II) complexes of the ligand 1,n-bis[bis(2-pyridylmethyl)amino]benzene with n = 2-4 (1,n-tpbd) and its mononuclear derivative bis(2-pyridylmethyl)aniline (phbpa) were synthesized and structurally characterized. Magnetic measurements and DFT calculations were performed on [CuCl2(phbpa)], [Cu2Cl4(1,3-tpbd)], [(Cu2Cl2(ClO4)(1,3-tpbd))Cl(Cu2Cl2(OH2)(1,3-tpbd))](ClO4)2, and [Cu2(OH2)2(S2O6)(1,3-tpbd)]S2O6, and the exchange-polarization mechanism was successfully demonstrated.

DEPROTONATION OF LOW-SPIN MONONUCLEAR IRON(III)-HYDROPEROXIDE COMPLEXES GIVE TRANSIENT BLUE SPECIES ASSIGNED TO HIGH-SPIN IRON(III)-PEROXIDE COMPLEXES

Purple iron(III)–hydroperoxide complex ions [Fe(Rtpen)(η1-OOH)]2+ can be reversibly deprotonated to give transient blue species showing spectroscopic properties consistent with iron(III)–peroxide complexes; a novel ferryl species is produced in MS/MS experiments with the iron(III) hydroperoxide ion.

Solid and solution state structures of mono- and di-nuclear iron(III) complexes of related hexadentate and pentadentate aminopyridyl ligands

Two mononuclear iron(III) complexes formed with the related ligands N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) and N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethylenediamine (bztpen) have been studied. X-Ray crystallography reveals for the complex [Fe(tpen)][ClO4]3 that tpen acts as a hexadentate ligand in the solid state. In methanol or water containing solutions it was shown by EPR and UV-Vis spectroscopy that one pyridyl arm is exchanged by a solvent molecule. In dmf solution only partial exchange of a pyridyl arm was observed. The complex with the related pentadentate ligand bztpen [Fe(bztpen)Cl][ClO4]2 showed exchange of the coordinated chloride with methanol and water, but not with dmf. In water containing solutions both complexes are slowly converted into the dimeric μ-oxo complexes [(tpen)FeIII(Cl)OFeIII(Cl)(tpen)]2+ or [(bztpen)FeIII(Cl)OFeIII(Cl)(bztpen)]2+. This reaction is accelerated by addition of an excess of chloride.

Mononuclear non-heme iron(III) peroxide complexes: syntheses, characterisation, mass spectrometric and kinetic studies

A series of transient interconvertible protonated and deprotonated mononuclear Fe(III) peroxo species are derived from the pH dependent reaction of dihydrogen peroxide with mononuclear iron(II) or iron(III) complexes of general formulation [Fe(Rtpen)X](A)n, n = 1, 2; X = Cl, Br; Rtpen = N-alkyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine, alkyl = R = CH3CH2, CH3CH2CH2, HOCH2CH2, (CH3)2CH, C6H5, and C6H5CH2; A = ClO4, PF6. The low-spin iron(III) hydroperoxide complex ions [Fe(Rtpen)(η1-OOH)]2+ are purple chromophores and the high-spin iron(III) peroxide complexes, [Fe(Rtpen)(η2-OO)]+ are blue chromophores. The spectroscopic observation (ESR, UV-vis, ESI MS) of a low-spin iron(III) precursor species [Fe(Rtpen)(η1-OCH3)]2+ and kinetic studies show that formation of [Fe(Rtpen)(η1-OOH)]2+ from iron(II) solution species is a two step process. The first step, the oxidation of the iron(II) complex to [Fe(Rtpen)(OCH3)]2+, is faster than the subsequent ligand substitution during which [Fe(Rtpen)(η1-OOH)]2+ is formed. The kinetic data are consistent with an interchange associative mechanism for the ligand substitution, and a role for the proton bound to the uncoordinated hydroperoxide oxygen atom is suggested. The stability of [Fe(Rtpen)(η1-OOH)]2+ R = HOCH2CH2, is significantly lower than for the peroxide complexes generated from the other alkyl substituted ligands (t1/2ca. 10 min vs. several hours). Tandem MS/MS experiments with the [Fe(Rtpen)(η1-OOH)]2+ ions show fragmentation via O–O cleavage to give the novel ferryl species [Fe(Rtpen)(O)]2+. By contrast the [Fe(Rtpen)(η2-OO)]+ ions are stable under the same gas phase conditions. This indicates a weaker O–O bond in the Fe(III) hydroperoxide complex ions, and that [FeIIIOOH]2+ rather than [FeIIIOO]+ species are the precursors to, at least, the ferryl FeIVO species. Crystal structures of four starting iron(II) compounds, [Fe(Rtpen)Cl]PF6, R = HOCH2CH2, CH3CH2CH2, C6H5CH2, and [Fe(bztpen)Br]PF6 show the iron atoms in distorted octahedral geometries with pentadentate Rtpen coordination with the halide ion as the sixth ligand. The structure of [Fe(etOHtpen)Cl]PF6 shows an intermolecular H-bonding interaction between the dangling hydroxyethyl group and the chloride of a neighbouring molecule with O–H⋯Cl, 3.219(2) A.

Copper complexes of a p-phenylenediamine-based bis(tridentate)ligand

A new bis(tridentate) compound, N,N,N′,N′-tetrakis(2- pyridylmethyl)benzene-1,4-diamine (tpbd), its diprotonated derivative, [H 2 tpbd] 2+ , and a dicopper complex [Cu 2 (tpbd)(H 2 O) 4 ][S 2 O 6 ] 2 have been prepared and structurally characterized. The pyridyl nitrogen atoms are strongly hydrogen bonded in the crystal structure of the yellow diprotonated salt tpbd·2HClO 4 ·2Me 2 CO. The bis(picolyl)amine ends of the ligand show a meridional-type co-ordination to the copper ions in the complex. Magnetic susceptibility measurements on [Cu 2 (tpbd)(H 2 O) 4 ][S 2 O] 2 indicate that the p-phenylenediamine bridge commutes a weak antiferromagnetic coupling [J = -15.56(6) cm -1 ]. Two related dicopper complexes, Cu 2 (tpbd)Cl 4 and Cu 2 (tpbd)(NO 3 ) 4 , were also isolated. The ESR spectra of the chloride and nitrate complexes indicated negligible magnetic exchange coupling. Reaction of tpbd with one-electron oxidants generated a purple radical cation which has been characterized by ESR and UV/VIS spectroscopy.

Biologically relevant mono- and di-nuclear manganese II/III/IV complexes of mononegative pentadentate ligands

Manganese(II) complexes of mononegative pentadentate N4O ligands [Mn2(mgbpen)2(H2O)2](ClO4)2 (1), (mgbpen− = N-methyl-N′-glycyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) and [Mn2(bzgbpen)2(H2O)2](ClO4)2 (2), (bzgbpen− = N-benzyl-N′-glycyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) have been prepared. The crystal structure of the Mn(II)–aqua complex of 1, shows it to be dimeric via (μ-κO)-bridging through one carboxylate oxygen atom of each of the two ligands. The non-coordinated carboxylate oxygen atoms are H-bonded to the water ligands on the adjacent Mn ion. The magnetic coupling interaction is weak and antiferromagnetic, J = −1.3(1) cm−1. The dimeric structures of 1 and 2 are retained in solution and can exist in the gas phase. Complexes 1 and 2 are air stable but can be oxidised by tBuOOH to give unstable mononuclear Mn(III) complexes, or oxo-bridged dimanganese(III) and di-μ-oxo-dimanganese(IV) complexes, depending on solvent. The [Mn(III)–OR]+, R = H or CH3 complexes are generated in water or methanol, respectively, and are potentially useful spectroscopic models for active Mn–lipoxygenases. In acetonitrile, di-μ-oxo-dimanganese(IV) complexes are the highest oxidation state products detected, and these are formed via shorter-lived intermediate μ-oxo-dimanganese(III) compounds. The rate of formation of the various oxidized products is slower in the case of the bzgbpen− systems which contains a bulkier non-coordinating arm. The oxidised complexes were characterised by UV-visible spectroscopy, ESI mass spectrometry and cyclic voltammetry. In addition, III–IV and II–III species were electrochemically generated. Thus the new mononegative pentadentate ligand systems display significant flexibility in the range of Mn oxidation states and species of biological relevance that are accessible: A series of dinuclear compounds with different structures in the five oxidation levels between II–II and IV–IV has been identified. No solid state structures were obtained for high oxidation state species, however it is assumed that in the oxo-bridged compounds the carboxylate groups are terminally ligated in contrast to the starting Mn(II) complexes. Thus the system represents examples of limiting structures in the “carboxylate shift” mechanism proposed to be important in non-heme H2O and O2 activation processes.

Stepwise construction of mono-, di- and tri-nuclear 2 ∶ 1, 1 ∶ 2, 2 ∶ 3 ligand ∶ mixed-metal complexes using a bis-tridentate bridging ligand

Extended molecular structures have been constructed with the bis-tridentate ligand N,N,N′,N′-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (tpbd), and the bis-tridentate, metal ‘complex’ ligand, [Ru(tpbd)2](PF6)2 (1) as building blocks. The ligand tpbd and complex 1 react analogously with Cu(II) salts to yield the corresponding di- and tri-nuclear aqua complexes, [Cu2(tpbd)(H2O)4](ClO4)4 (2a) and [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·2H2O (3a). The labile water ligands in these complexes have been replaced by other solvents or 2,2′-bipyridine (bipy). In the latter case the mixed ligand complexes [Cu2(tpbd)(bipy)2](PF6)4·H2O (2b) and [Cu2Ru(BF4)2(tpbd)2(bipy)2](PF6)4·2H2O (3b) respectively are obtained. These reactions represent prototype substitutions for the controlled stepwise evolution of even larger molecular entities. Recrystallisation of 3a in 2% NaBF4 methanol/water solution, resulted in crystals of [Cu2Ru(tpbd)2(H2O)4](BF4)4(PF6)2·8H2O (3a′). Recrystallisation of 3b in 2% NaBF4 CH3CN/water solution gave [Cu2Ru(BF4)2(tpbd)2(bipy)2](BF4)2(PF6)2·H2O (3b′). The X-ray structures of the mononuclear and trinuclear ruthenium-containing complexes (1, 3a′, 3b′) show cis–fac six-coordinate geometries around the Ru atoms. As a consequence the phenylene groups of the tpbd ligands are approximately perpendicular to each other. In the trinuclear Ru–Cu2 complexes the copper atoms are located anti to each other about the [(tpbd)Ru(tpbd)]2+ building block. The flexibility of tpbd is demonstrated by the geometries of the copper coordinated ends of tpbd, which are coordinated meridionally in 3a′, but facially in 3b′. Solution spectroscopy, and the detection of appropriate ions by mass spectrometry, show that the dicopper and trinuclear copper–ruthenium complexes are present in solution. UV-Visible spectroscopy, EPR and cyclic voltammetry indicate that there is insignificant electronic communication between the metal centers in the di- and tri-nuclear complexes since salient metal-based features are additive.