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Brian M. Hoffman

Publications and source records attributed to Brian M. Hoffman.

11 recordsLinked to original sources
PubMed1999

High-Frequency and -Field Electron Paramagnetic Resonance of High-Spin Manganese(III) in Porphyrinic Complexes.

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study two complexes of high-spin manganese(III), d(4), S = 2. The complexes studied were (tetraphenylporphyrinato)manganese(III) chloride and (phthalocyanato)manganese(III) chloride. Our previous HFEPR study (Goldberg, D. P.; Telser, J.; Krzystek, J.; Montalban, A. G.; Brunel, L.-C.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 1997, 119, 8722-8723) included results on the porphyrin complex; however, we were unable to obtain true powder pattern HFEPR spectra, as the crystallites oriented in the intense external magnetic field. In this work we are now able to immobilize the powder, either in an n-eicosane mull or KBr pellet and obtain true powder pattern spectra. These spectra have been fully analyzed using spectral simulation software, and a complete set of spin Hamiltonian parameters has been determined for each complex. Both complexes are rigorously axial systems, with relatively low magnitude zero-field splitting: D approximately -2.3 cm(-)(1) and g values quite close to 2.00. Prior to this work, no experimental nor theoretical data exist for the metal-based electronic energy levels in Mn(III) complexes of porphyrinic ligands. This lack of information is in contrast to other transition metal complexes and is likely due to the dominance of ligand-based transitions in the absorption spectra of Mn(III) complexes of this type. We have therefore made use of theoretical values for the electronic energy levels of (phthalocyanato)copper(II), which electronically resembles these Mn(III) complexes. This analogy works surprisingly well in terms of the agreement between the calculated and experimentally determined EPR parameters. These results show a significant mixing of the triplet (S = 1) excited state with the quintet (S = 2) ground state in Mn(III) complexes with porphyrinic ligands. This is in agreement with the experimental observation of lower spin ground states in other metalloporphyrinic complexes, such as those of Fe(II) with S = 1.

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PubMed1999

Super-Charged Porphyrazines: Synthesis and Physical Properties of Octacationic Tetraazaporphyrins.

We report the synthesis of octacationic macrocycles based on the porphyrazine (tetraazaporphyrin) core. The first stage is the synthesis of Mg(II) 2,3,7,8,12,13,17,18-octakis(4-pyridyl)porphyrazine, [Mgpz(pyr)(8)], and the parent [H(2)pz(pyr)(8)]. Both compounds are freely soluble in aqueous acid due to protonation of the pyridyl groups but with concomitant loss of Mg(2+) by the latter. Methylation of [H(2)pz(pyr)(8)] gives the octacation, 2,3,7,8,12,13,17,18-octakis(N-methyl-4-pyridiniumyl)porphyrazine, [H(2)pz(Me-pyr)(8)](8+), which is freely soluble in water as the chloride salt. The peripheral charges greatly lower the pK(a) values of the pyrrole protons of [H(2)pz(Me-pyr)(8)](8+) in aqueous solution. Global fitting of optical pH titration curves gives pK(a) values for the first and second deprotonations of 6.3 and 8.8 in 2.0 M NaCl and 4.1 and 5.6 at 0.1 M NaCl. The analysis further gives the UV-visible spectra of the [H(2)pz(Me-pyr)(8)](8+), [Hpz(Me-pyr)(8)](7+), and [pz(Me-pyr)(8)](6+) species. The free-base compound [H(2)pz(Me-pyr)(8)](8+) rapidly incorporates a variety of metal ions (Ba(II), Cd(II), Co(II), Cu(II), Mn(III), Ni(II), Pd(II), and Zn(II)) to form [Mpz(Me-pyr)(8)](8+). We present the complete synthesis and characterization of the [Nipz(Me-pyr)(8)](8+) and [Cupz(Me-pyr)(8)](8+) derivatives.

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PubMed1999

Porphyrazinehexamines and Dinitroporphyrazines: Synthesis, Characterization, and Complementary Electrochemistry.

Unsymmetrical porphyrazines, M[pz(H)(2)(R)(6)], with three substituted pyrroles (substituents, R = -NMe(2), propyl, or 4-tert-butylphenyl) at the periphery and one unsubstituted pyrrole have been synthesized from inexpensive and readily available materials. Linstead crossover macrocyclization of 8 with 9, 11, and 12 gave pigments 1, 3, and 5, respectively. Subsequent demetalation by treatment with TFA gave free-base porphyrazines 2, 4, and 6. Generally, pyrrolines 11 and 12 are better co-cyclization partners and give higher yields for the desired unsymmetrical porphyrazines compared to maleonitriles 9 and 10 when co-cyclized with 8. Nickel porphyrazine 7 was obtained by reaction of 6 with Ni(OAc)(2) in chlorobenzene and dimethylformamide. Both free-base and nickel porphyrazines (6 and 7) were readily nitrated by nitrogen dioxide in CH(2)Cl(2), yielding dinitroporphyrazines 18 and 19, respectively. Electrochemical studies have shown that porphyrazinehexamine 2 is easy to oxidize, having the first oxidation at E(1/2) = - 0.18 V, which is 0.91 and 1.04 V lower than that of 4 and 6, respectively. Dinitroporphyrazine 18, however, displays a 0.6 V of cathodic shift for the two reversible reduction waves compared to those observed for the parent porphyrazine 6.

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PubMed1999

Investigation of the Unusual Electronic Structure of Pyrococcus furiosus 4Fe Ferredoxin by EPR Spectroscopy of Protein Reduced at Ambient and Cryogenic Temperatures.

The hyperthermophilic archaeon Pyrococcus furiosus contains a novel ferredoxin (Pf-Fd) in which, in the native 4Fe form, three of the Fe ions are coordinated to the protein by cysteinyl thiolato ligands, but the fourth Fe is coordinated by an aspartyl carboxylato ligand ([Fe(4)S(4)(cys)(3)(asp)](2)(-)(,3)(-)). Chemical reduction at ambient temperature of the oxidized 4Fe form (Pf-Fd 4Fe-ox, S = 0 ground state, with the cluster core indicated by [Fe(4)S(4)](2+)(ox)) produces a reduced 4Fe form (Pf-Fd 4Fe-red, with the cluster core indicated by [Fe(4)S(4)](+)(red)). Pf-Fd 4Fe-red, [Fe(4)S(4)](+)(red) core, in frozen solution exhibits S = (1)/(2) and (3)/(2) electronic states that are not in thermal equilibrium. The two spin states thus represent alternate ground states of the reduced cluster (cluster cores indicated by [Fe(4)S(4)](+)(red1) and [Fe(4)S(4)](+)(red2), respectively), rather than a ground and excited spin state. Low-temperature (77 K) reduction of 4Fe-ox in frozen solution by gamma-irradiation produces in high yield the reduced state of the cluster that is trapped in the structure of the oxidized parent cluster, and thus has a cluster core denoted by [Fe(4)S(4)](+)(ox). The [Fe(4)S(4)](+)(ox) form also exhibits non thermally converting S = (3)/(2) and (1)/(2) components in the same proportion as seen for [Fe(4)S(4)](+)(red). The EPR signal of the S = (3)/(2) component that results from cryoreduction ([Fe(4)S(4)](+)(ox2)) is indistinguishable, within experimental variability, from that seen in the ambient-temperature, chemically reduced protein ([Fe(4)S(4)](+)(red2)), and the signals of the two S = (1)/(2) components ([Fe(4)S(4)](+)(ox1) and [Fe(4)S(4)](+)(red1), respectively) closely resemble each other, although they are not identical. Previous NMR studies at ambient temperature showed evidence for only one species in fluid solution for both Pf-Fd 4Fe-ox and 4Fe-red. Taken together, the NMR and EPR results indicate that fluid solutions of either oxidized or reduced Pf-Fd contain only one conformer, but that frozen solutions of each contain two distinct conformers, with each one of the pair of oxidized protein forms having a corresponding reduced form. A shift in the coordination mode of the aspartyl carboxylato ligand is proposed to account for this conformational flexibility.

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PubMed1998

Peripheral Palladium(II) and Platinum(II) Complexes of Bis(dimethylamino)porphyrazine.

The unsymmetrical porphyrazine (tetraazaporphyrin) bearing a single peripheral bis(dimethylamino) functionality, Mg[pz(NMe(2))(2)(Pr)(6)], was prepared by base-catalyzed cross condensation of dipropyl maleonitrile (in excess) with dimethylamino maleonitrile. The freebase (2H[pz(NMe(2))(2)(Pr)(6)]) and centrally metalated forms (M[pz(NMe(2))(2)(Pr)(6)]; M = Ni(II), Cu(II), Mn(III)) were prepared by treatment of Mg[pz(NMe(2))(2)(Pr)(6)] with trifluoroacetic acid and then the appropriate metal salt. PdCl(2) and PtCl(2) were coordinated to the peripheral bis(dimethylamino) chelates, yielding the bimetallic complexes, M[pz(NMe(2))(2)(Pr)(6)]M'Cl(2) (M = Ni, Cu; M' = Pd, Pt). The heteroleptic [N(2)-Pd-S(2)]-capped porphyrazines were prepared readily by substituting the chloride ions of M[pz(NMe(2))(2)(Pr)(6)]PdCl(2) with the dithiolene chelates, maleonitriledithiolate (mnt(2)(-)), benzenedithiolate (bdt(2)(-)), and 1,3-dithiole-2-one-4,5-dithiolate (dmid(2)(-)). The [N(2)-Pt-S(2)] complexes were prepared by reaction of M[pz(NMe(2))(2)(Pr)(6)]PtCl(2) with the dialkyltin-protected dithiolates dibutyltin(toluene-3,4-dithiolate) and dibutyltin(dmit). The peripheral heteroleptic [N(2)-M'-S(2)] core was found to be electroactive for electron-rich dithiolene ligands (bdt(2)(-), E(1/2)(Pd(III)/Pd(II)) = 0.22 V; tdt(2)(-), E(1/2)(Pt(III)/Pt(II)) = 0.20 V; dmid(2)(-), E(1/2)(Pd(III)/Pd(II)) = 0.19 V; dmit(2)(-), E(1/2)(Pt(III)/Pt(II)) = 0.19 V) but not for the relatively electron-poor dithiolene, maleonitriledithiolate (mnt(2)(-)). The X-ray structure of Ni[pz(NMe(2))(2)(Pr)(6)]Pd(mnt) (13) was determined. Crystal data for C(85)H(109)Cl(3)N(24)Ni(2)Pd(2)S(4): space group P2(1)/n; a = 17.435(4) Å, b = 17.982(2) Å, c = 30.577(2) Å; beta = 104.27(2) degrees; Z = 4.

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PubMed1998

Porphyrazines and Norphthalocyanines Bearing Nitrogen Donor Pockets: Metal Sensor Properties.

The amino-functionalized porphyrazines 1, 2, and 4 and norphthalocyanines 3 and 5, which were prepared via Linstead macrocyclization reactions of dinitrile precusors, were subject to UV-vis titrations in the presence of inorganic ions. These polydentate macrocycles are able to bind metal ions by the peripheral ligating diazacrown or vicinal diamine entities in addition to metal ion binding within the porphyrazine (norphthalocyanine) cavity. Although 1 only weakly bound Ag(I) and Hg(II), 2 to 5 were superior polydentate ligands. Surprisingly, porphyrazine 4 was a superior ligand for binding 4 equiv of Co(II), Cu(II), Zn(II), and Cd(II) than the corresponding tetracrowned porphyrazine 2. Both 2 and 4 were excellent ligands for peripherally complexing 4 equiv of Ag(I) and Hg(II) and only for one metal, Pb(II), was the crowned system 2 superior to 4. The crowned norphthalocyanine 3 was generally inefficient in the peripheral complexation of inorganic cations, whereas 5 was superior for the peripheral binding of Co(II), Cu(II), Zn(II), Cd(II), and Hg(II) (1 equiv). Again, only with Pb(II) was the crowned system 3 a superior ligand to 5. X-ray crystallographic characterization of diazacrown 12a, porphyrazine 1 (M = Mg), and the complexes of crown 12b with Cu(BF(4))(2) and with HClO(4) are reported.

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PubMed1998

Pulsed ENDOR and ESEEM Study of [Bis(maleonitriledithiolato)nickel](-): An Investigation into the Ligand Electronic Structure.

By a combination of Q-band pulsed ENDOR (electron nuclear double resonance) and X-band ESEEM (electron stimulated echo envelope modulation) techniques, we have determined the hyperfine tensors for ethylene (C1) and cyano (C2) carbons and N, of [Ni(mnt)(2)](-), along with the quadrupole tensor for nitrogen. These measurements give pi electron spin densities of rho(C1) approximately 0.03 in the C1 2p(z)() orbital, rho(C2) < 0.003, rho(N) approximately 0.01, such that in total, approximately 0.15 of the spin resides on the ligand atoms C and N, while the rest resides in the NiS(4) core, giving rho(NiS(4)(-)) = 0.85. These results are compared with extended Hückel and density functional (BLYP) MO calculations, as well as with Xalpha calculations reported earlier.

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PubMed1997

Structural, Electrical, Magnetic, and Spectroscopic Properties of Ring-Oxidized Molecular Metals Produced by Iodination of Metal-Free and Nickel Tetrabenzporphyrins.

Detailed studies of the structure, conductivitity, magnetoresistance, optical spectra, and magnetic properties (susceptibility, EPR) for the new molecular metal tetrabenzporphyrin iodide (H(2)(tbp)I) and the electrical, spectral, and magnetic properties of Ni(tbp)I are reported. Paramagnetic transition-ion impurities were carefully excluded during the synthesis of H(2)(tbp)I and Ni(tbp)I, and both materials show much higher, metal-like conductivites than previously seen for less-pure Ni(tbp)I. Comparison of the specular reflectance data for Ni(tbp)I and H(2)(tbp)I allows a distinction between purely ring pi-transitions and metal-involved charge-transfer transitions, and the spectra fix the energy levels of the pi orbitals involved in conduction. Transport, magnetic, and optical properties show that both H(2)(tbp)I and Ni(tbp)I are ring-based conductors that have metal-like conductivities, varying as approximately 1/T, down to ca. 30-40 K. However, the remaining level of defects is higher in the tbp conductors than in H(2)(pc)I, and whereas the latter is metallic down to the mK temperature range, the defects in the (tbp) compounds localize the conduction electrons at approximately 10 K (Ni(tbp)I) and approximately 30 K (H(2)(tbp)I), leading to transport through one-dimensional variable-range hopping. EPR g-values for H(2)(tbp)I and Ni(tbp)I are close to that for the free electron and are nearly temperature-independent. The line widths for both samples are extremely narrow and also are nearly temperature-independent. These results show that Ni(tbp)I does not display doubly-mixed valence, as thought earlier: Paramagnetic impurities significantly altered the EPR signals of the prior samples. H(2)(tbp)I crystallizes in the space group P4/mcc with cell constants of a = 14.173(10) Å and c = 6.463(4) Å. Full-matrix least-squares refinement of 63 variables gave an R index of 0.061 on F(o)(2).

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