Alain Tabard

New Synthesis of trans‐Disubstituted Cyclam Macrocycles – Elucidation of the Disubstitution Mechanism on the Basis of X‐ray Data and Molecular Modeling

A new way to synthesize trans-disubstituted cyclam tetraazamacrocycles 1 is reported. The synthesis proceeds in three steps via the tricyclic 1,4,8,11-tetraazatricyclo[9.3.1.14,8]hexadecane system 2, which can be selectively dialkylated and hydrolyzed under basic conditions to give the final product 1. An understanding of the reactivity, based on the X-ray experimental electrostatic potential and molecular modeling of the 1,4,8,11-tetraazatricyclo[9.3.1.14,8]hexadecane macrotricycle, has permitted the elucidation of a new reaction pathway leading to the trans-disubstituted cyclam.

The 5/2,3/2 Spin Admixture in the Chloroiron(III) Derivative of the Sterically Crowded 2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphyrin.

Despite similar ring deformations in solution and in the solid state, the chloroiron(III) derivative of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin ([FeCl(oetpp)], shown schematically) prepared in this study exhibits only a very weak quantum-mechanical admixture of spin S=3/2 (only 4-10 %) with spin S=5/2. In contrast, for the variety of [FeCl(oetpp)] studied earlier by other researchers a 40 % contribution of the S=3/2 state was found.

Electrooxidation of porphyrin free bases: fate of the π-cation radical

In contrast to metalloporphyrins with non-electroactive metal centres, the π-cation radicals of porphyrin free bases (H2OEP, H2TPP, H2CdiE) electrogenerated in strictly anhydrous solvents are not stable and give rise to a quantitative chemical reaction. Conjunction of electrochemical and spectroscopic data (UV/VIS, EPR and NMR) demonstrates unambiguously that the porphyrin skeleton is not modified during the chemical reaction. The reaction product is the protonated free base, and thus the free base can be regenerated by reduction of the protons.

Synthesis, characterization and X-ray crystal structures of cyclam derivatives. Part VI. Proton binding studies of a pyridine -strapped 5,12-dioxocyclam based macrobicycle

The 14-membered cyclic diamide 1,4,8,11-tetraazacyclotetradecane-5,12-dione (5,12-dioxocyclam) can be considered as a trans-autodiprotected tetraazamacrocycle and provides a convenient starting material for the preparation of macrobicyclic receptors. As an example, the secondary amine nitrogen atoms located at the 1 and 8 positions were cross-bridged with a 1,3-pyridyl strap, affording the constrained ansa-dioxocyclam ligand 1,9,12,18,22-pentaazatricyclo[7.6.6.13,7]docosa-3,5,7(22)-triene-13,19-dione (L1). The proton binding properties of this cage-type compound, which possesses a hemispherical cavity, were fully investigated by spectroscopic (IR, NMR, UV, MALDI-TOF MS), quantum chemical, and potentiometric methods. While both bridgehead tertiary amines have their free lone pairs oriented inside the cavity, intramolecular hydrogen bonding was found to play a key role in determining the structural features of the free base and its protonated forms. L1 behaves as a diprotic base in water with log K011 = 8.94(1) and log K012 = 2.32(9), but most interestingly shows slow proton-transfer rates on the NMR timescale.

Copper(II) and nickel(II) complexes of pyridylamido hexadentate ligands: chemical speciation and spectroscopic studies

Abstract Two novel potentially hexadentate ligands, 1,10-(2-bis picolinamide)-4,7-diazadecane (pycdpnen) and 1,8-bis(2-picolinamide)-3,6-dioxaoctane (pycdado) have been synthesised as their hydrochloride salt; its protonation constants and the stability constants of the copper(II) and nickel(II) chelates have been determined by potentiometry. Amide groups deprotonation permits the formation of [MLH −1 ] + species in all cases, while only pycdado gives [MLH −2 ] species. The solid complexes of copper and nickel with the neutral and the deprotonated ligands have been synthesised and characterised by IR, UV–Vis and ESR spectroscopy. The amidic groups are coordinated through the oxygen atoms in all solid complexes [ML](ClO 4 ) 2 (M=Cu 2+ and Ni 2+ ). The complexes obtained with the deprotonated forms of the ligands imply the coordination through the nitrogen atoms of the amidic groups.

Mössbauer investigations of the hexachlorantimonate salt of the phenyliron 2,3,7,8,l2,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrinate, [Fe(oetpp)Ph]SbCl6 and X-ray structure of the phenyliron(III) precursor Fe(III)(oetpp)Ph

The phenyliron derivative, [Fe(oetpp)C 6 H 5 )]SbCl 6 ( 2 ) generated in dichloromethane by oxidation of the phenyliron(III) complex, Fe(III) (oetpp)C 6 H 5 ( 1 ) of the 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin with 1 equiv. of phenoxathiinylium hexachloroantimonate, [C 12 H 8 OS]SbCl 6 , was studied by Mossbauer spectroscopy. This compound exhibits an isomer shift δ of 0.13 mm s −1 and a quadrupole splitting Δ E Q of 3.23 mm s −1 . The measured magnetic hyperfine pattern obtained in the temperature range 4.2–40 K in a field of 3.5 and 7 T, applied perpendicular to the γ-beam, has been consistently analyzed in the spin Hamiltonian approximation assuming an iron(IV) S =1 electronic configuration. This result indicates that oxidation of the ferric precursor 1 causes oxidation of the metal and not from the macrocycle. This finding corresponds to that of the corrole complex Fe(me 2 et 6 c)C 6 H 5 (me 2 et 6 c=trianion of 7,13-dimethyl-2,3,8,l2,l7,l8-hexaethylcorrole) which recently was also characterized as an iron(IV) S =1 derivative. The molecular structure of the five-coordinate, low-spin, ferric starting derivative, Fe(oetpp)C 6 H 5 ( 1 ) is reported. A comparison of the porphyrin ring conformations in the phenyliron(III) species, Fe(tpp)C 6 H 5 ( 3 ) and Fe(oetpp)C 6 H 5 ( 1 ) indicates that the more favorable oxidation potential of the oetpp complex 1 relative to that of the non-peripherally crowded tpp ring derivative 3 is related to the non-planar distortion of the oetpp ring, distortion which is, due to the steric crowding, maintained in solution.

Electrochemical behavior of [octaethylporphyrin iron(III)-σ-bonded pyrrole] and its electrocatalytic activity for the reduction of dioxygen

Abstract The electrochemistry of [octaethylporphyrin iron(III)-σ-bonded pyrrole] (abbreviated as (OEP)Fe III (Pyr)) was investigated in CH 2 Cl 2 + TBAP solution by cyclic voltammetry (CV) and in situ UV-visible electronic absorption spectrometry. It was found that (OEP)Fe III (Pyr) can undergo a quasi-reversible one-electron reduction step to form (OEP)Fe II (Pyr), and two sorts of irreversible one-electron oxidation step: one was the formation of [(OEP)Fe III (Pyr)] + firstly, followed by loss of the σ-bonded pyrrole to produce [(OEP)Fe III ] + ; the other was probably the direct formation of [(OEP)Fe III ] + with simultaneous loss of the σ-bonded pyrrole. It was also found that (OEP)Fe III (Pyr) adsorbed on the surface of glassy cardon (GC) exhibited high activity for the electrocatalytic reduction of dioxygen, but this activity was not stable. However, this catalyst can be buried in polypyrrole (PPyr) film, resulting in more stable activity. Rotation ring-disk experiments (RRDE) demonstrated that (OEP)Fe III (Pyr) whether adsorbed on a GC surface or buried in PPyr film could catalyze the reduction of dioxygen to give water predominantly. In these two cases, the catalytic mechanisms were found to be the same and the water production efficiencies were nearly 90% in the potential region −0.30 to 0.35 V (vs. SCE) in O 2 saturated 0.05 M H 2 SO 4 solution. The slope of the Tafel curve suggested that the one-electron transfer from [(OEP)Fe II (Pyr)O 2 ] to [(OEP)Fe III (Pyr)O 2 − ] was the rate-controlling step before the limiting disk current was reached. 1. (i) The electrochemistry and in situ UV-visible absorption spectra suggested that the electrochemical oxidation of (OEP)Fe III (Pyr) might produce the porphyrin ring cation with the loss of its σ-bonded pyrrole, resulting in irreversible oxidation of (OEP)Fe III (Pyr) to form [(OEP)Fe III ] + species. The electrochemical reduction of (OEP)Fe III (Pyr) seems to be a reversible process in which the porphyrin ring anion is formed first, followed by electron transfer from the porphyrin ring to the central iron to yield (OEP)Fe II (Pyr). 2. (ii) There may be two different forms of (OEP) Fe III (Pyr) adsorbed on the GC surface. The so-called “different adsorption form” is resulted probably from the different distance between the active sites on which (OEP)Fe III (Pyr) could adsorb. This might lead to a different interaction pattern of bonding with dioxygen. For type I sites (as described in Section 3.3.4) perhaps the side-on and/or end-on interactions occurred favorably, whereas for type II sites perhaps the bridge interaction was predominant [36,37]. 3. (iii) Burying (OEP)Fe III (Pyr) in PPyr film might proceed only at the beginning of the electrochemical polymerization of pyrrole monomers. The concentration of (OEP)Fe III (Pyr) buried in the PPyr film appeared to be independent of the thickness of the film. (OEP)Fe III (Pyr), whether buried in PPyr film or adsorbed on the GC surface, exhibited a similar electrocatalytic activity for dioxygen reduction. The catalytic mechanisms were found to be the same. Furthermore, the burying method clearly could increase the stability of this activity. 4. (iv) In the low polarization region where the potential is higher than −0.120 V, the catalytic reduction of dioxygen by (OEP)Fe(Pyr)] ad /GC underwent successive 2e − reduction steps to form water, whereas in the higher polarization region, the direct 4e − reduction occurred to give water predominantly. The Tafel slope in the kinetic controlled region suggested that the reaction rate was controlled by the first one-electron transfer step. In the more negative potential region, the reaction rate was controlled by the diffusion of dioxygen in solution phase. Thus, (OEP)Fe III (Pyr) has a high activity for the electrocatalytic reduction of dioxygen with an efficiency of water production of 75%–90%, which is close to the activity observed on a Pt electrode. This activity appeared very unstable, probably resulting from the loss of (OEP)Fe III (Pyr) from the GC surface. However, it was satisfactory to find that the burying method, for example burying (OEP)Fe III (Pyr) in a PPyr film, could increase the catalytic activity and its stability markedly. Furthermore, it was interesting to find that this kind of σ-bonded metalloporphyrin could be used as the cathodic catalyst in a full cell if further research into increasing its stability were accomplished, according to the conditions for practical use.

Iron complexes acting as nitric oxide carriers

Abstract A new strategy for NO-carrier synthesis depending on the strength of the ligand field, the configuration of the complex and the oxidation state of the metal is developed. The preparation of the trans- and cis -[Fe( 1 )(NO)(Cl)]Cl and trans -[Fe( 2 )(NO)(Cl)]Cl complexes, where 1 is 1,4,8,11-tetraaza-cyclotetradecane or cyclam and 2 is 6,6,13,13-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane, is reported. On the basis of spectroscopic data and PM3(tm) molecular modeling calculations, both species are characterized as complexes of the [FeNO] 7 type possessing an S =1/2 and S =3/2 ground state for the trans - and cis -[Fe( 1 )(NO)(Cl)]Cl, respectively. The cis isomer can act as a NO carrier.

Metalloporphyrins with metalmetal σ-bonds. Synthesis, spectroscopic characterization, and electrochemistry of (P)MRe(CO)5 where P is the dianion of octaethylphorphyrin (OEP) or tetraphenylporphyrin (TPP) and M = Al, Ga, In or Tl

Abstract The synthesis, physicochemical properties, and electrochemistry of a new series of metal metal σ-bonded metalloporphyrins are reported. The investigated compounds are represented as (P)MRe(CO)5 where P is the dianion of octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP) and M = Al, Ga, In, or Tl. These compounds provide the first examples of Al and Ga metalloporphyrins with covalent metalmetal bonds as well as give the first series of compounds in which the same metalate anion is covalently bonded to four different group 13 metalloporphyrins. Each synthesized complex was characterized by 1H NMR, IR, and UV-visible spectroscopy as well as by electrochemistry. Graham Δσ and Δπ parameters, and calculated residual charges on the rhenium atom of (P)MRe(CO)5 demonstrate that the covalency of the metalmetal bond decreases in the order: Tl > In > Ga > Al. The thallium derivative is stable upon electrooxidation by either one or two electrons. However, the In, Ga, and Al complexes undergo rapid cleavage of the metalrhenium bond after formation of a [(P)MRe(CO)5]+· cation radical. The chemical reactivity and physical properties, as well as the electrochemistry, of each (P)MRe(CO)5 species were analyzed as a function of the central metal ion or porphyrin ring basicity. The spectroscopic properties and electrochemistry of these metalmetal σ-bonded species are compared with data for other metal metal bonded metalloporphyrins of the type (P)MM′(L) as well as with data for related metalcarbon bonded porphyrins of the type (P)M(R).

Physicochemical study of σ-bonded heteronuclear metalloporphyrin complexes of the type (P)InM(L) and (P)TIM(L) where M(L) = Mn(CO)5, Co(CO)4, Cr(CO)3Cp, Mo(CO)3Cp, or W(CO)3Cp

Abstract A physicochemical study of twenty metal-metal indium and thallium porphyrins is reported. The axial metalate ligands were Mn(CO) 5 , Co(CO) 4 , Cr(CO) 3 Cp, Mo(CO) 3 Cp, and W(CO) 3 Cp. UV-visible, 1 H NMR, and IR spectroscopic studies demonstrated the presence of a single metal-metal covalent bond. Infrared spectra in the carbonyl stretching region have been assigned, and values of the Cotton-Kraihanzel stretching and interaction force constants calculated, as well as the Graham σ and π parameters; these data showed that the covalent character of the metal-metal bond is strongly dependent on the nature of metals. Calculation of residual charges has shown that the thallium-manganese bond is typically covalent.