Naked five-coordinate Fe(III)(NO) porphyrin complexes: vibrational and reactivity features

Model ferric heme nitrosyl complexes, [Fe(TPP)(NO)](+) and [Fe(TPFPP)(NO)](+), where TPP is the dianion of 5,10,15,20-tetrakis-phenyl-porphyrin and TPFPP is the dianion of 5,10,15,20-tetrakis-pentafluorophenyl-porphyrin, have been obtained as isolated species by the gas phase reaction of NO with [Fe(III)(TPP)](+) and [Fe(III) (TPFPP)](+) ions delivered in the gas phase by electrospray ionization, respectively. The so-formed nitrosyl complexes have been characterized by vibrational spectroscopy also exploiting (15)N-isotope substitution in the NO ligand. The characteristic NO stretching frequency is observed at 1825 and 1859 cm(-1) for [Fe(III)(TPP)(NO)](+) and [Fe(III)(TPFPP)(NO)](+) ions, respectively, providing reference values for genuine five-coordinate Fe(III)(NO) porphyrin complexes differing only for the presence of either phenyl or pentafluorophenyl substituents on the meso positions of the porphyrin ligand. The vibrational assignment is aided by hybrid density functional theory (DFT) calculations of geometry and electronic structure and frequency analysis which clearly support a singlet spin electronic state for both [Fe(TPP)(NO)](+) and [Fe(TPFPP)(NO)](+) complexes. Both TD-DFT and CASSCF calculations suggest that the singlet ground state is best described as Fe(II)(NO(+)) and that the open-shell AFC bonding scheme contribute for a high-energy excited state. The kinetics of the NO addition reaction in the gas phase are faster for [Fe(III)(TPFPP)](+) ions by a relatively small factor, though highly reliable because of a direct comparative evaluation. The study was aimed at gaining vibrational and reactivity data on five-coordinate Fe(III)(NO) porphyrin complexes, typically transient species in solution, ultimately to provide insights into the nature of the Fe(NO) interaction in heme proteins.     

The influence of ligand conformation on the thermal spin transitions in iron(III) saltrien complexes

We report the syntheses of six new salts of the iron(III) complexes of four hexadentate Schiff bases of the saltrien-type derived from 1,8-diamino-3,6-diazaoctane and 2 equiv. of the appropriate salicylaldehyde derivative. Three of these new compounds undergo gradual spin-transitions centred between 155-204 K in the solid state, that proceed to 35-50% completeness. One of the other compounds remains high-spin on cooling, another is low-spin, while the last undergoes spin-crossover centred above room temperature. This spin-state variability is typical of solid compounds of this type. As an aid to understanding this behaviour, the crystal structures and magnetochemistry of the known spin-crossover salts [Fe(saltrien)]PF6, [Fe(saltrien)]ClO4 and [Fe(saltrien)]BPh4 have also been reexamined. The structural chemistry of all three salts was found to differ significantly from what has been previously reported. Correlation of the crystal structures and magnetic behaviour of the compounds in this and previous work suggests that the disposition of the two ligand phenoxy groups plays an important role in determining whether high-spin [Fe(saltrien)]+ complexes can undergo thermal spin-transitions.     

Formation and reactivity of Ir(III) hydroxycarbonyl complexes

Kinetic studies show that the reaction of [TpIr(CO)2] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO2H)(CO)H] (2) is second order (k = 1.65 x 10(-4) dm(3) mol(-1) s(-1), 25 degrees C, MeCN) with activation parameters DeltaH++= 46+/-2 kJ mol(-1) and DeltaS++ = -162+/-5 J K(-1) mol(-1). A kinetic isotope effect of k(H2O)/k(D2O) = 1.40 at 20 degrees C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp*Ir(CO)2] (1*, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp*Ir(CO2H)(CO)H] (2*). A proposed mechanism consistent with the relative reactivity of 1 and 1* involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2* shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O...O) 2.65 A). Complex 2* is thermally stable but (like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp*Ir(CO)2H]+ (3*) and decarboxylation with OH- to give [TpIr(CO)H2] (4*). Complex 2 undergoes thermal decarboxylation above ca. 50 degrees C to give [TpIr(CO)H2] (4) in a first-order process with activation parameters DeltaH++ = 115+/-4 kJ mol(-1) and DeltaS++ = 60+/-10 J K(-1) mol(-1).     

The profound influence of a single metal-carbon bond on the reactivity of bio-relevant cobalt(III) complexes

This Perspective reports the development of mechanistic insight over the past 6 years, on the substitution behaviour of cobalamins that contain a single Co-C bond. The effect of the alkyl group, located in the trans position, on the thermodynamic, kinetic and ground state trans effect, was studied in detail. The substitution reactions of different alkylcobalamins with CN- were investigated, the apparent mechanistic discrepancy reported for the co-enzyme B12 was resolved and a logical explanation could be offered. In addition, a complete picture of the effect of pressure on the UV-Vis spectra of different base-on and base-off cobalamins is presented, which clearly shows the role of the alkyl group in controlling the equilibrium between five- and six-coordinate species, and the possible participation of such species in the studied ligand substitution reactions. The kinetics of the base-on/base-off equilibration was studied for the first time using a pH-jump technique. All in all the novel mechanistic information adds to the understanding of the profound effect that a single metal-carbon bond can have on the reactivity of such Co(III) complexes.     

Electronic structures of five-coordinate iron(III) porphyrin complexes with highly ruffled porphyrin ring

The spin states of the iron(III) complexes with a highly ruffled porphyrin ring, [Fe(TEtPrP)X] where X = F-, Cl-, Br-, I-, and ClO4(-), have been examined by 1H NMR, 13C NMR, EPR, and M?ssbauer spectroscopy. While the F-, Cl-, and Br- complexes adopt a high-spin (S = 5/2) state, the I- complex exhibits an admixed intermediate-spin (S = 5/2, 3/2) state in CD2Cl2 solution. The I- complex shows, however, a quite pure high-spin state in toluene solution as well as in the solid. The results contrast those of highly saddled [Fe(OETPP)X] where the I- complex exhibits an essentially pure intermediate-spin state both in solution and in the solid. In contrast to the halide-ligated complexes, the ClO4(-) complex shows a quite pure intermediate-spin state. The 13C NMR spectra of [Fe(TEtPrP)ClO4] are characterized by the downfield and upfield shifts of the meso and pyrrole-alpha carbon signals, respectively: delta(meso) = +342 and delta(alpha-py) = -287 ppm at 298 K. The data indicate that the meso carbon atoms of [Fe(TEtPrP)ClO4] have considerable amounts of positive spin, which in turn indicate that the iron has an unpaired electron in the d(xy) orbital; the unpaired electron in the d(xy) orbital is delocalized to the meso positions due to the iron(d(xy))-porphyrin(a(2u)) interaction. Similar results have been obtained in analogous [Fe(TiPrP)X] though the intermediate-spin character of [Fe(TiPrP)X] is much larger than that of the corresponding [Fe(TEtPrP)X]. On the basis of these results, we have concluded that the highly ruffled intermediate-spin complexes such as [Fe(TEtPrP)ClO4] and [Fe(TiPrP)ClO4] adopt a novel (d(xz), d(yz))3(d(xy))1(d(z)(2)1 electron configuration; the electron configuration of the intermediate-spin complexes reported previously is believed to be (d(xy))2(d(xz)), d(yz))2(d(z)(2))1.     

Color tuning associated with heteroleptic cyclometalated Ir(III) complexes: influence of the ancillary ligand

We report the preparation of a series of new heteroleptic Ir(III) metal complexes chelated by two cyclometalated 1-(2,4-difluorophenyl)pyrazole ligands (dfpz)H and a third ancillary bidentate ligand (L=X). Such an intricate design lies in a core concept that the cyclometalated dfpz ligands always warrant a greater pi pi* gap in these series of iridium complexes. Accordingly, the lowest one-electron excitation would accommodate the pi* orbital of the ancillary L=X ligands, the functionalization of which is then exploited to fine-tune the phosphorescent emission wavelengths. Amongst the L=X ligands designed, three classes (series 1-3) can be categorized, and remarkable bathochromic shifts of phosphorescence were observed by (i) replacing the 2-benzoxazol-2-yl substituent (1a) with the 2-benzothiazol-2-yl group (1b) in the phenolate complexes, (ii) converting the pyridyl group (2a) to the pyrazolyl group (2b) and even to the isoquinolyl group (2c) in the pyrazolate complexes and (iii) extending the pi-conjugation of the benzimidazolate ligand from 3a to 3b. Single-crystal X-ray diffraction study on complex [(dfpz)Ir(bzpz)] (2b) was conducted to confirm their general molecular architectures. Complex 2b was also used as a representative example for fabrication of multilayered, green-emitting phosphorescent OLEDs using the direct thermal evaporation technique.     

Electrochemical reduction of Fe(III), Cr(III), and Mn(III) porphyrin complexes in DMSO

The half-wave potentials are given for the electrochemical reduction of Fe(III), Cr(III), and Mn(III) porphyrin complexes. Cyclic voltamperometry was used to study the reversibility of these reactions. An effect was demonstrated for the structure of the porphyrin ligand on the half-wave potentials for the reduction of the Mn(III) complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1665–1668, July, 1991.     

Carbon-nitrogen bond activation of amines by rhodium(III) porphyrin complexes

Carbon-nitrogen bond activation of amines by rhodium porphyrin chloride has been achieved to give rhodium porphyrin alkyl complexes. Rhodium porphyrin hydride and rhodium porphyrin dimer were proposed as the intermediates in cleaving the C-N bond.     

DNA-binding studies of mixed ligand cobalt(III) complexes

The DNA binding characteristics of mixed ligand complexes of the type [Co(en)₂(L)]Br₃ where en = N,N′-ethylenediamine and L = 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), 1,10-phenanthroline-5,6-dione (phendione), dipyrido[3,2-a:2′,3′-c]phenazine (dppz) have been investigated by absorption titration, competitive binding fluorescence spectroscopy and viscosity measurements. The order of intercalative ability of the coordinated ligands is dppz > phen > phendione > bpy in this series of complexes.     

Chelate polypyridine ligand rearrangement in Au(III) complexes

The reaction of gold(III) neutral complexes AuBr(CN)₂(N–N) {N–N = 2,2′-bipyridine (bpy), 5,5′-dimethyl-2,2′-bipyridine (^Me2bpy), 1,10-phenanthroline (phen)} with a stoichiometric amount of K[AuCl₄] · 2H₂O in nitromethane at room temperature led to the formation of 1:1 electrolytes which were characterized by NMR and IR spectroscopy, conductivity measurements, elemental analyses and X-ray diffraction. Both the anions and the cations of these salts are singly charged square-planar Au(III) complexes and the cations have general formula [AuCl₂(N–N)]⁺. A hypothesis on the possible reaction mechanisms is presented to give an explanation for the formation of the reaction products.     

Synthesis, reactivity, and properties of N-fused porphyrin rhenium(I) tricarbonyl complexes

The thermal reactions of N-fused tetraarylporphyrins or N-confused tetraarylporphyrins with Re2(CO)10 gave the rhenium(I) tricarbonyl complexes bearing N-fused porphyrinato ligands (4) in moderate to good yields. The rhenium complexes 4 are characterized by mass, IR, 1H, and 13C NMR spectroscopy, and the structures of tetraphenylporphynato complex 4a and its nitro derivative 15 are determined by X-ray single crystal analysis. The rhenium complexes 4 show excellent stability against heat, light, acids, bases, and oxidants. The aromatic substitution reactions of 4 proceed without a loss of the center metal to give the nitro (15), formyl (16), benzoyl (17), and cyano derivatives (19), regioselectively. In the electrochemical measurements for 4, one reversible oxidation wave and two reversible reduction waves are observed. Their redox potentials imply narrow HOMO-LUMO band gaps of 4 and are consistent with their electronic absorption spectra, in which the absorption edges exceed 1000 nm. Theoretical study reveals that the HOMO and LUMO of the rhenium complexes are exclusively composed of the N-fused porphyrin skeleton. Protonation of 4 takes place at the 21-position regioselectively, reflecting the high coefficient of the C21 atom in the HOMO orbital. The skeletal rearrangement reaction from N-confused porphyrin Re(I) complex (8) to N-fused porphyrin Re(I) complex (4) is suggested from the mechanistic study as well as DFT calculations.     

Synthesis, reactivity, and properties of N-fused porphyrin manganese(I) tricarbonyl complexes

The reactions of N-fused tetraphenylporphyrin (NFTPP, 1a) and its 21-substituted derivatives, 21-Br-NFTPP (1b), 21-NO(2)-NFTPP (1c), and 21-Bz-NFTPP (1d), with Mn(CO)(5)Br gave the manganese(I) tricarbonyl complexes bearing N-fused tetraphenylporphyrinato ligands (2a-d), respectively, in 46-99% yields. The complexes were characterized by mass, IR, (1)H and (13)C NMR spectroscopy, and the final structural proof was evident from the X-ray crystallographic analysis for 2a. The crystals of 2a·CH(2)Cl(2) belong to the monoclinic space group P2(1)/n (#14), with a = 15.007(2) ?, b = 12.5455(19) ?, c = 21.150(3) ?, β = 102.227(4)°, and Z = 4. The lengths (?) of three manganese-nitrogen and three manganese-carbon bonds are inequivalent respectively [Mn-N(2), 2.007(2); Mn-N(23), 2.033(2); Mn-N(24), 1.988(3); and Mn-CO, 1.798(4), 1.804(4), 1.841(3)], reflecting the asymmetric structure of the NFp ligand. The aromatic substitution reactions of 2a, such as nitration, formylation, and chlorination, proceeded without a loss of center metal to give the corresponding 21-nitro (2c), 21-formyl (2e), and 21-chloro (2f) derivatives, regioselectively. In the electrochemical measurements of 2, one reversible oxidation and two reversible reduction waves were observed. The redox potentials of 2 indicate the narrow energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) being consistent with the electronic absorption spectra that display the absorption edges over 1000 nm. Protonation occurred at the inner core nitrogen of 2a upon the addition of acids, which is inferred from the (1)H NMR spectra as well as theoretical calculations. By a treatment with amine N-oxides, demetalation of 2 proceeded to afford the corresponding NFP free-bases (1).     

An anomalous spin-polarization mechanism in high-spin manganese(III) porphyrin complexes

Confluence of NMR for paramagnetic molecules and the complementary density functional theory calculations reveals an anomalous spin-polarization mechanism that is maximized in high-spin d(4) complexes. It is critical to realize this mechanism to correctly rationalize the spin-density distribution around the porphyrin macrocycle.     

Cytotoxic half-sandwich rhodium(III) complexes: Polypyridyl ligand influence on their DNA binding properties and cellular uptake

The DNA binding of polypyridyl (pp) (η⁵-C₅Me₅)Rh^III complexes of the types [(η⁵-C₅Me₅)RhCl(pp)](CF₃SO₃) (2-6) (pp = bpy, phen, dpq, dppz, dppn), [(η⁵-C₅Me₅)Rh{(Me₂N)₂CS}(pp)](CF₃SO₃)₂ (7-9) (pp = dpq, dppz, dppn) and [(η⁵-C₅Me₅)Rh(L)(pp)](CF₃SO₃) (10) (L = C₆H₅S⁻) and (11) (L = C₁₀H₇S⁻) has been studied by UV/Vis spectroscopy, circular dichroismus and viscosity measurements. Complexes 3-11 are cytotoxic towards the human MCF-7 breast and HT-29 colon cancer cell lines and exhibit IC₅₀ values in the range 0.56-10.7 μM. Stable intercalative binding into CT-DNA is indicated for the dpq and dppz complexes by large increases ΔT_m of 6-12 °C in the DNA thermal denaturation temperature for r = [complex]/[DNA] = 0.1 and by induced CD bands and large viscosity increases. In contrast, significant DNA lengthening is not observed after incubation of the biopolymer with the dppn complexes 2 and 9 at molar ratios of r < 0.08. Pronounced hypochromic shifts for the π-π^∗ transitions of the dppn ligands in the range 320-425 nm indicate the possible presence of surface stacking. The in vitro cytotoxicities of the chloro complexes 4-6 and the (Me₂N)₂CS complexes 7-9 are dependent on the size of the polypyridyl ligand with IC₅₀ values decreasing in the order dpq > dppz > dppn. For instance, IC₅₀ values of 5.3, 1.5 and 0.91 μM were determined for 7-9 against MCF-7 cells. Rapid Cl⁻/H₂O exchange leads the formation of aqua dications for 4-6, whose levels of cellular uptake and cytotoxicities are similar to those established for 7-9. Intramolecular interactions between the aromatic thiolate and dppz ligands of 10 and 11 prevent significant DNA intercalation. X-ray structural determinations have been performed for 2-7 and 11.     

Axial Ligand exchange in chiral macrocyclic ytterbium(III) complexes

We investigate the role of axial ligands on the near-IR-optical and paramagnetic NMR spectra of the complex [YbL](+3) where L is the stereodefined enantiopure chiral macrocycle (L = hexaazapentacyclo[25.3.1.1(12,24).0(4,9).0(19,24)]dotriaconta-1(31),2,10,12,14,16(32),17,25,27,29-decaene). The conformation in solution of the lanthanide complex is characterized by analyzing the pseudocontact 1H NMR shifts and is consistent with X-ray data of single crystal of analogue systems. The macrocycle is confined within a thin equatorial disk, leaving the cation open to at least two axial sites, on the opposite hemispheres. We recorded, assigned, and analyzed the 1H NMR spectra of several species upon changing the anion in solution, calculating the magnetic susceptibility anisotropy tensor for each. Near-IR circular dichroism is used to investigate the solution equilibria involving the competing ligands and to derive a spectroscopic series for Yb.     

Synthesis and reactivity of substituted cyclopentadienyl rhodium(I) and (III) complexes

New cyclopentadienyl derivatives of rhodium COD complexes [Cp*=C₅H₄COOCH₂CHCH₂ (1); C₅H₄CH₂CH₂CHCH₂ (2); C₅H(i-C₃H₇)₄ (3)] and carbonyl complex [Cp*=C₅H(i-C₃H₇)₄ (4)] were synthesized from [RhCl(COD)]₂ and [RhCl(CO)₂]₂. 1, 2 and 3 oxidized by iodine gave iodine bridged dimers 5, 6 and 7, respectively. Triphenyl phosphine, carbon monoxide and carbon disulfide molecules broke down the iodine bridged structure easily and produced monomer products Cp*RhI₂L [Cp*=C₅H₄COOCH₂CHCH₂, L=CS₂ (8); L=PPh₃ (9). Cp*=C₅H(i-C₃H₇)₄, L=CO (10)]. All of these new compounds were characterized by elemental analysis, ¹H NMR, IR, UV-Vis and mass spectroscopy. The crystal structure of 1 was solved in the triclinic space group with one molecule in the unit cell, the dimensions of which are a=7.082(9) Å, b=8.392(3) Å, c=13.889(5) Å, α=101.19(3)°, β=99.06(6)°, γ=105.11(5)°, and V=763(1) ų. The crystal structure of 3 was solved in the orthorhombic space group Pn21a with four molecules in the unit cell, the dimensions of which are a=9.748(3) Å, b=16.054(5) Å, and V=2319(1) ų. Least squares refinement leads to values for the conventional R₁ of 0.0251 for 1 and 0.0558 for 3, respectively. Compared to that in 1, a shorter metal-ligand bond length in 3 was observed and this is attributed to the rich electron density on Rh(I) metal center piled up by the C₅H(i-C₃H₇)₄ ligand.     

Trans ligand influence in cobalt(III) Schiff base complexes: Electronic and steric effects on the kinetics of hydrolysis

Rate constants for the hydrolysis (k_h) of six different amines in trans‐[Co((BA)₂en)(amine)₂]ClO₄ complexes (amine = aniline 1a , para‐toluidine 1b , benzylamine 1c (primary amines), pyrrolidine 2a , piperidine 2b , morpholine 2c (secondary amines), and (BA)₂en = Bisbenzoylacetoneethylenediiminato) in mixed methanol/water (1:1) solvent have been determined between 30 and 55°C. The hydrolysis product of 2c , trans‐[Co((BA)₂en)(morpholine)(H₂O)]ClO₄, has been separately prepared and characterized by UV–vis and ¹H NMR spectroscopy. Depending on the nature of the axial amine ligand the limiting first‐order rate constants for the amine hydrolysis at 40°C range from (3.42 ± 0.10) × 10^?5 to (5.32 ± 0.13) × 10^?5 s^?1. At the first glance, a reasonable trend cannot be established between k_h and the basicity or the inductive trans effect of the amine ligands. However, when the complexes are classified into two groups, based on the type of the amine (primary and secondary), the values of k_h correlate well with the basicity or inductive effect of the amine in each group. The observed trend in k_h values for the complexes with primary amines is 1a (5.32 ± 0.13) × 10^?5 s^?1 > 1b (3.51 ± 0.14) × 10^?5 > 1c (1.72 ± 0.03) × 10^?5 (40°C), which is opposite to the amine basicity strength. In the case of the complexes with secondary amines, the observed trend in k_h values is in accord with amine basicity (or inductive trans effect), i.e. 2a (5.02 ± 0.22) × 10^?5 > 2b (4.18 ± 0.10) × 10^?5 > 2c (3.42 ± 0.10) × 10^?5 s^?1 (40°C). © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 387–393, 2002     

Axial Coordination of Pyridine- and Imidazole-Based Drug Molecules to Co(III)-Tetra(4-Carboxyphenyl)porphyrin

The substitutions of water molecules in the bisaqua axial complex of carboxy-substituted Co(III)-tetraphenylporphyrin by drug molecules based on amines and nitrogen-containing heterocycles are studied. The strongest bonded Co(III)-porphyrin complexes are formed with imidazole and pyridine derivatives. Depending on the nature and positions of functional groups in the heterocyclic moiety, imidazole- and pyridine- containing compounds can form either mono- or bis-axial complexes with Co(III) porphyrinate, these complexes having various stabilities. Aniline and quinuclidine drugs were found to react with Co(III) porphyrinate in aqueous media to produce only unstable monoaxial complexes.