Cynthia T. Brewer

Synthesis and characterization of a neutral, low spin iron(III) complex of a hexadentate tripodal ligand containing three imidazolate arms. Use as a dinucleating agent

Abstract The iron(III) complex of the Schiff base formed from the condensation of 1 equiv. of trist(2-aminoethyl)amine (tren) and 3 equiv. of 4-methyl-5-imidazolecar☐aldehyde has been synthesized by the reaction of iron(III) methoxide, tren and 4-methyl-5-imidazolecar☐aldehyde in methanol. Fetren(meim)3 is low spin as indicated by magnetic moment, Mo¨ssbauer and ESR measurements. The crystal structure of Fetren(meim)3 was determined (space group Pna21, R1 = 0.0331 and wR2 = 0.0857). The complex is six-coordinate although the ligand is potentially heptadentate. Basicity of the three ‘backside’ imidazolate nitrogens is demonstrated in the crystal structure by H-bonding interactions and in solution by reactions with acidic metal complexes. ZnTPP and M(hfa)2 (M = Cu(II), Ni(II)), to give imidazolate bridged adducts. Products of the reaction of Fetren(meim)3 with M(hfa)2 were isolated and identified as the 1:1 adducts.

Synthesis and characterization of manganese(II) and iron(III) d5 tripodal imidazole complexes. Effect of oxidation state, protonation state and ligand conformation on coordination number and spin state

The 1 : 3 Schiff base condensates of tris(2-aminoethyl)amine (tren) or tris(3-aminopropyl)amine (trpn) with 4-methyl-5-imidazolecarboxaldehyde, H3L1 and H3L2, respectively, were generated in situ and used to prepare complexes with manganese(II) and iron(III). The resultant complexes, [MnH3L1](ClO4)2, [MnH3L1](ClO4)2.EtOH.H2O, [MnH3L2](ClO4)2, [FeH3L1](ClO4)3.1.5(EtOH) and [FeHL1](I3) (0.525)(I)(0.475).2.625H2O, have been characterized by EA, IR, ES MS, variable temperature magnetic susceptibility, X-ray crystallography, and Mössbauer spectroscopy for the iron complexes. The three manganese(II) complexes are high spin with [MnH3L2](ClO4)2 exhibiting coordination number seven while the others are six coordinate. [FeH3L1](ClO4)3.1.5(EtOH) has two iron sites, a seven coordinate and a pseudo seven coordinate site. The complex is high spin at room temperature but exhibits a magnetic moment that decreases with temperature corresponding to conversion of one of the sites to low spin. [FeHL1](I3) (0.525)(I)(0.475).2.625H2O is low spin even at room temperature. In the present complexes the apical nitrogen atom, N(ap), of the tripodal ligand is pyramidal and directed toward the metal atom. The data show that the M-N(ap) distance decreases as the oxidation state of the metal increases, as the number of bound imidazole protons on the ligand increases, and as the number of carbon atoms in the backbone of the ligand (tren vs. trpn) increases. In a limiting sense, short M-N(ap) distances result in high spin seven coordinate mono capped octahedral complexes and long M-N(ap) distances result in low spin six coordinate octahedral complexes.

Synthesis and structural and magnetic characterization of discrete phenolato and imidazolate bridged Gd(III)–M(II) [M=Cu, Ni] dinuclear complexes

Three dinuclear complexes formed by the reaction of Gd(hfa)3 (hfa is hexafluoroacetylacetonate) with Schiff base complexes of Cu(II) and Ni(II) have been synthesized and characterized. The crystal structures of the complexes [Gd(hfa)3M(prpen)] {MCu(II (1)), Ni(II) (2)} are reported. (H2prpen is the Schiff base derived from the condensation of 2 equiv. of 2-hydroxypropiophenone and 1 equiv. of ethylenediamine.) Both 1 and 2 are discrete dinuclear complexes consisting of an eight coordinate Gd atom which is bridged to four coordinate M(II) via both phenolate oxygen atoms of the prpen ligand. The crystal structure shows there is no tendency toward dimerization between adjacent Cu(II) Schiff base units in 1. Cryomagnetic measurements show a ferromagnetic interaction between Gd(III) and Cu(II) in 1 as predicted by theory with J 1.91 cm 1 . The reaction of Gd(hfa)3 with Ni(L) (H2L is the Schiff base derived from the condensation of 1 equiv. each of 5-chlorobenzophenone, 1,2-diaminobenzene, and 5-methyl-4-imidazolecarboxaldehyde) produced Gd(hfa)3Ni(L) (3) in which imidazolate is available to bridge Gd(III) and Ni(II).

Synthesis and characterization of a spin crossover iron(II)–iron(III) mixed valence supramolecular pseudo-dimer exhibiting chiral recognition, hydrogen bonding, and π–π interactions

Reaction of iron(II) and the 3 : 1 Schiff base condensate of 5-methylpyrazole-3-carboxaldehyde and tris(2-aminoethyl)amine in air gives a pseudo-dimer complex with a triple helix structure made of Δ–Δ and Λ–Λ pairings of spin crossover iron(II) and low spin iron(III) cations that are held together by three π–π and hydrogen bonding interactions.

Supramolecular assemblies prepared from an iron(II) tripodal complex, tetrafluoroborate, and alkali metal cations. The effect of cation size on coordination number, anion disorder and hydrogen bonding

The iron(II) complex cation, 12+, of the 1 : 3 Schiff base condensate of tris(2-aminoethyl)amine (tren) with imidazole-2-carboxaldehyde (H3L1) was reacted with MBF4 (M = Na, K, Rb, Cs and NH4). The products were double salts of the formula {[FeH3L1](BF4)2}·MBF4, 1[M(BF4)3]. The complexes have been characterized by EA, IR, X-ray crystallography, and Mossbauer spectroscopy. The resulting complexes crystallize in P-3 and the cations are located at the origin (0, 0, 0) and c/2 (0, 0, 0.5) on the c axis. Six tetrafluoroborate anions surround the cation sites and each binds to the sodium in a monodentate fashion and to the other cations in a bidentate fashion resulting in distorted octahedral and icosahedral complexes, respectively. Disorder is observed in the tetrafluoroborate anion of both 1[Na(BF4)3] and 1[Cs(BF4)3], resulting in two sets of fluorine atom positions (F set A and F set B), but is absent in the ammonium, potassium, and rubidium double salts. The disorder is attributed to coordination number preference and poor size match, respectively. The tetrafluoroborate anions are involved in extensive hydrogen bonding with the imidazole NH and imine CH of the iron complex. The hydrogen bonding arrangements observed with potassium, rubidium, cesium (F set A) and ammonium are almost identical with one another, but are altered in 1[Na(BF4)3] and 1[Cs(BF4)3] (F set B) complexes. These effects are explained on the basis of cation size. The formation of the double salt results in a shift in the spin equilibrium of the parent iron(II) complex from 62% high spin: 38% low spin at 295 K to pure low spin in the double salts.

Synthesis and structural characterization of copper complexes of N4 tetradentate Schiff bases derived from N-alkylated 2-aminobenzaldehyde. Effects of cyclization, oxidation, and dinucleation

Abstract The structures of the copper complexes of 7,8,15,16,17,18-hexahydro-dibenzo[e,m][1,4,8,11]tetraazacyclotetradecine (Cu(2,2 mac)) 7,8,15,18-tetrahydro-dibenzo[e,m][1,4,8,11]tetraazacyclotetradecine-16,17-dionato (Cu(2,2 oxomac)) and Cu(NCH3 amben) and Cu(2,2 oxomac)Ni(hfa)2 are reported. The three copper complexes containing a 14-membered macrocycle are essentially square planar complexes and exhibit no short axial interactions. The analogous acyclic complex, Cu(NCH3 amben), exhibits an average tetrahedral distortion of 34.35° which appears to be due to steric repulsion between the two methyl groups. The dinuclear complex, which exhibits a planar exchange coupled oxamido core, was prepared by reaction of Cu(2,2 oxomac) with Ni(hfa)2. The analogous complex, Cu(2,2 oxomac)Cu(hfa)2, can be prepared by the same procedure or by reaction of Cu(2,2 mac) with Cu(hfa)2. The former method is a general synthetic route while the latter is specific for Cu(hfa)2 relying on its ability to catalyze the oxidization of the ethylene unit between the anilino nitrogen atoms to the oxamide (NCH2CH2N to NCOCON) under ambient conditions.

Electronic and solvent effects in the reaction of zinc porphyrins with a metal imidazolate complex or 1-methylimidazole

Abstract The reactions of Zn(p-XTPP) derivatives (X = CF3, Cl, F, H, CH3, OCH3, N(Et)2) with a copper imidazolate complex to yield imidazolate bridged binuclear axial adducts to the porphyrin were followed in the visible region between 700–450 nm. The α and β bands of the porphyrin shift to lower energy and the ϵβ/ϵα ratio decreases upon adduct formation. Equilibrium constants for the reaction were measured in toluene at 25.3 °C using the β band maximum of the parent zinc porphyrin. A plot of log Keq with the Hammett constant is linear with a slope of 0.208, which indicates that the formation of the binuclear adducts is aided by electron with drawing substituents on the porphyrin. Replacement of the copper atom in the imidazolate complex with nickel significantly reduces the value of Keq for the reaction with Zn(TPP) while the free ligand fails to react at all. The reduced reactivity with the nickel imidazolate complex is attributed to entropy effects and that of the non-reacting metal free complex to conformational effects. Equilibrium constants for the reaction of N-CH3Im with Zn(TPP) conducted in dichloromethane, toluene, dimethoxyethane, and dimethylsulfoxide reveal a linear dependence of log Keq on the position of the α or β in a given solvent. The relative ordering of Keq in various solvents is given by the position of the α or β band maximum of the parent zinc porphyrin. The lower the energy of the bands the smaller the value of Keq.

Synthesis and characterization of seven-coordinate tripodal imidazole complexes of iron(II) and manganese(II)

The iron(II) and manganese(II) complexes of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 1-methyl-2-imidazolecarbaldehyde and the manganese(II) complex of the N(7) Schiff-base condensate of tris(3-aminopropyl)amine with 4-imidazolecarbaldehyde are high-spin mono capped octahedral seven-coordinate complexes with a short, approximately 2.44 è, metal to apical nitrogen bond.

Effect of molecular complexation of the porphyrin ligand on the axial ligation of FeTPPCl

Abstract The formation of a molecular complex between tetraphenylporphyrin or a metallotetraphenylporphyrin and electron acceptors in toluene and in dimethylsulfoxide is evidenced by a red shift and loss of intensity in the Soret region. Complexation between a donor or acceptor and the porphyrin periphery in a metalloporphyrin is expected to effect reactions at the metal center. The effect of molecular complexation on the axial ligation of 5,10,15,20- tetraphenyl-21H,23H-porphine iron(III) chloride (FeTPPCl) was investigated by a determination of the equilibrium constant and thermodynamic data for the reaction between FeTPPCl and 1-methylimidazole (MeIm) in dimethylsulfoxide in the presence of an electron donor and an electron acceptor. ΔH values in the presence of the donor 1,10-phenanthroline and the acceptor II are −58.3 and −32.8 kJ mol−1, respectively, compared to −47.8 in the absence of either a donor or an acceptor. ΔS values for the reaction in the presence of a donor, in the presence of an acceptor, and in the absence of either are −0.12, −0.036 and −0.082 kJ (Kmol)−1, respectively. The enthalpies indicate that an electron donor favors the low spin bisadduct of FeTPPCl and MeIm while an acceptor favors the high spin non-ligated form. The entropy effects are in opposition to the enthalpy effects.

INTERMEDIATE-SPIN IRON(III) OCTAETHYLTETRAAZAPORPHYRINATES WITH WEAKLY CO-ORDINATING ANIONS : SYNTHESIS, CHARACTERIZATION AND CRYSTAL STRUCTURE

The metathesis reaction of AgZ(Z = ClO4–, PF6–, SbF6– or CF3CO2–) with chloroiron(III) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraazaporphyrinate chloride, Fe(oetap)Cl, in tetrahydrofuran or toluene gave the Fe(oetap)Z derivatives in approximately 70% yield. The complexes are air-stable solids but decompose in solution to [Fe(oetap)]2 O (Z = ClO4– or CF3CO2–) or Fe(oetap)F (Z = PF6– or SbF6–). They have been characterized by variable-temperature magnetic susceptibility measurements and Mossbauer, UV/VIS, IR and ESR spectroscopies. In addition, Fe(oetap)ClO4 has been structurally characterized by single-crystal X-ray diffraction. All of the complexes are intermediate-spin (S=) based on their magnetic properties and Mossbauer and ESR spectra.