Greg 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.

SPIN CROSSOVER COMPLEXES OF IRON(III) WITH AN N4 TETRADENTATE SCHIFF BASE LIGAND

Abstract Iron(III) complexes of the Schiff base formed from the condensation of 2-aminobenzaldehyde and ethylenediamine, H2amben, were prepared by the reaction of H2amben with iron(III) halides in acetone. Fe(amben)Cl and Fe(amben)Br·H2O exhibit a spin crossover between the 2T and 6A states as shown by magnetic susceptibility measurements, variable temperature Mossbauer spectra, and electron spin resonance at 77 K. Structural investigation of this ligand system was carried out on the copper(II) and nickel(II) analogues. Cu(amben) and Ni(amben) are both orthorhombic with space group P212121 (No. 19) and unit cell parameters and volume of a = 7.744(2), b = 26.486(7), c = 6.9706(8) A , V = 1429.7(8) A 3 for Cu(amben) and a = 7.651(1), b = 25.993(5), c = 7.040(1) A , V = 1400.1(4) A 3 for Ni(amben). The final R values for Cu(amben) (a total of 1515 reflections of which 1246 were classified as observed (I>3.00σ(I)) and Ni(amben) (a total of 1483 reflections of which 979 were classified as observed) are R = 0.059, Rw = 0.90 and R = 0.065, Rw = 0.085, respectively. Both Cu(amben) and Ni(amben) are monomeric and approximately square planar. The large dihedral angles between the phenyl rings of Cu(amben) (26.7°) and Ni(amben) (27.4°) may be related to the observation of spin crossover behavior in the iron(III) complexes.

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 METAL COMPLEXES OF AN UNSYMMETRIC N3O TETRADENTATE SCHIFF BASE LIGAND

Abstract The nickel(II) and copper(II) complexes of the unsymmetrical N 3 O tetradentate Schiff base ligand, H 2 ambprsal, which is the 1:1:1 condensation product of 2-aminobenzaldehyde (amb), 1,3-diaminopropane (pr) and salicylaldehyde (sal), were prepared by the reaction of HsalprNH 2 (HsalprNH 2 is the 1:1 monocondensate of salicylaldehyde with 1,3-diaminopropane) with 2-aminobenzaldehyde, metal nitrate and triethylamine in methanol. Replacement of the nickel in Ni(ambprsal) with FeCl was accomplished by reaction of Ni(ambprsal) with dimethylglyoxime in benzene followed by reaction with iron(III) chloride. Structural investigation was carried out on the nickel(II) and copper(II) complexes. Both Ni(ambprsal) and Cu(ambprsal) are monomeric and approximately square planar. The six-membered ring containing the metal, the azomethine nitrogen atoms and the three carbins of the 1,3 diaminopropane bridge adopt the symmetrical boat conformation with the metal and the central carbon atoms occupying the bow and stern positions. The conformation of the ligand and the N 3 O donor set may both contribute to the high spin state of the iron atom in Fe(ambpral)Cl.

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.

Synthesis of imidazolate bridged dinuclear Cu(II)Fe(III) porphyrins

Abstract Reaction of octaethylporphyriniron(III) perchlorate, Fe(oep)ClO 4 , or an iron capped porphyrin perchlorate, Fe(C 2 cap)ClO 4 , with a copper(II) imidazolate complex, CuIm, gives the imidazolate bridged dinuclear monoadducts, [Fe(oep)CuIm]ClO 4 or [Fe(C 2 cap)CuIm]ClO 4 , which are characterized by variable temperature magnetic susceptibility as spin coupled intermediate spin and admixed intermediate spin systems, respectively.

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.