Maxwell J. Gunter

Metalloporphyrins as models for the cytochromes p-450

Characterization of high-valent complexes formed in manganese porphyrin-catalysed hydrocarbon oxidations and oxidation mechanisms suggested by the iron and manganese porphyrin P-450 model systems

The role of the axial ligand in meso-tetraarylmetalloporphyrin models of the P-450 cytochromes

Abstract With iodosobenzene as an oxygen source, MnTPP(Cl), [MnTPP(H 2 O) 2 ]ClO 4 , MnTPPNO 3 , FeTPP(Cl), [FeTPP(H 2 O) 2 ]ClO 4 and (FeTPP) 2 O were used as model P-450 catalysts for the oxidation of cyclohexene in methylene chloride, benzene, benzonitrile and acetonitrile. The effect of added methanol and 4-methylpyridine was also investigated. A surprisingly high (80%) yield of cyclohexene oxide returned by [MnTPP(H 2 O) 2 ]ClO 4 in benzonitrile required a reconsideration of the accepted role of the trans axial ligand in model P-450 reactions. Performance differences between the MnTPP + and FeTPP + derivatives are attributed to differences in metal—porphyrin orbital mixing. It is proposed that D 4 h metal e g (d xz , d yz ) and porphyrin e g (π*) orbital mixing in six-coordinate manganese porphyrins prevents electron loss from the porphyrin a 2 u orbital. Porphyrin ligand oxidation in pentacoordinate manganese complexes is attributed to an unsymmetrical metal—porphyrin interaction. The metal e g (d xz , d yz ) orbitals of six-coordinate iron porphyrin complexes are thought to lie below the porphyrin e g (π*) orbitals, thus preventing a protective orbital interaction. The effects of σ and π charge donation from the trans axial ligand of the model P-450 catalysts are differentiated. It is proposed that σ donation can accelerate oxene transfer from the catalyst to the substrate, while π charge donation can additionally alter the energy of the acceptor orbital on the active catalyst that is responsible for initiating substrate oxidation.

Neutral π-associated porphyrin [2]catenanes

A series of neutral porphyrin-containing catenanes has been synthesised, consisting of a zinc porphyrin strapped by a polyethylene glycol chain containing four or six ethylenoxy-units and incorporating a central naphthoquinol unit, interlinked with a naphthalene diimide macrocycle. The napthalene diimide precursor units exhibit only weak binding with the strapped porphyrins (Ka between 8 and 0.02 M−1), but good yields of the catenanes were obtained by Glaser coupling of the alkynyl napthalene diimide precursors in the presence of the porphyrins. Structures and solution conformations were determined by mass spectral and detailed 1H NMR studies. For the longer strapped porphyrins, the diimide macrocycle rotates around the central naphthoquinol unit at 420–450 times per second, while rotation is virtually prevented in the tighter strapped derivatives. A second dynamic process occurring in both sets of catenanes and described as ‘yawing’ leads to inequivalence in the naphthalene moieties. UV-Visible spectra indicate charge transfer interactions and electronic communication between the two components of the catenane.

Amide-appended porphyrins as scaffolds for catenanes, rotaxanes and anion receptors

The synthesis of a porphyrin with an over-arching strap incorporating an isophthalamide unit produced both a porphyrin monomer with a potentially H-bonding receptor site, and a [2]catenane. Although both compounds exhibit fluxional behaviour, 1H NMR and MS data was used to distinguish them, and was interpreted in terms of the dynamics of each system. The self-complementarity of the isophthalamide unit provides an ideal building block for assembling [2]pseudorotaxanes with similarly functionalised threads. This was illustrated by the self-assembly of a ruthenium–porphyrin stoppered rotaxane under thermodynamically controlled conditions, by simple mixing of the strapped porphyrin, a complementary thread unit containing an isophthalamide central unit and pyridine-attached ends, and a ruthenium carbonyl porphyrin. The strapped porphyrin was also shown to act as a receptor for chloride ion, and its binding ability with other H-bonding isophthalamide guests was found to be modulated by the presence of chloride ion. Chloride complexation was observed in both metallated and free base porphyrin monomers as well as the rotaxanes counterparts, but no anion binding was observed for the catenane.

Flexible self-assembling porphyrin supramolecules

The design and chemical synthesis of a series of hybrid flexible self-assembling supramolecules utilising both crown ether-naphthalene diimide host-guest chemistry and metalloporphyrin-pyridyl coordination is discussed. The resulting compound structures and dynamics are probed using a variety of techniques, including diffusion ordered NMR spectroscopy (DOSY) and cold-spray ionisation mass spectrometry (CSI-MS).

Porphyrin [2]catenanes—dynamic control through protonation

Protonation of the previously reported porphyrin [2]catenanes in strongly acidic solutions results in electrostatic repulsion of their two components, and conformational reorientations and changes in rotation rates, which are dependent on the structural flexibility of the molecules.

Towards molecular scale mechano–electronic devices: porphyrin catenanes

Using self-assembly strategies, catenanes consisting of zinc and free base porphyrins with hydroquinol-containing polyether straps of different lengths interlinked with a tetracationic bipyridinium cyclophane have been synthesised.

5,15-diaryl substituted benzochlorins : synthesis and structure

Abstract Nickel 5,15-Bis(p-substituted aryl)porphyrins having p-substituents CH3, NO2, N(CH3)2, OTosyl, were treated with 3-(dimethylamino)acrolein to produce the nickel meso-(2″-formylvinyl)porphyrins (1) – (4). These undergo cyclisation to the corresponding nickel diaryl benzochlorins (5) to (8) and thence to the corresponding free bases. All these derivatives show unusual electronic spectra with strong absorptions in the visible region around 700 nm. A crystal structure of the p-dimethylamino benzochlorin (7) reveals a structure highly distorted from planarity, with the macrocycle having a distinct saddle shape. The p-dimethylamino benzochlorin (7a) readily protonates in solution to a dimethylaminoquinomethene moiety. An alternative route to the benzochlorins via decarboxylation of meso-acrylate derivatives and formylation of the formed vinyl compounds, produced dimeric species (15) and (16) which have been characterised by spectroscopic methods; the dimers are composed of a meso-vinyl porphyrin linked to a porphodimethene unit via an ethylenic bridge. (1) – (4) under basic conditions cyclise to the corresponding 5,15-diaryl formyl purpurins.

A building BLOCK approach to bis-porphyrin cavity systems with convergent and divergent wall orientations

Porphyrin containing molecular building blocks have been linked together using s-tetrazine or 1,3-dipolar coupling protocols to yield bis-porphyrin cavities of defined shape and size which were evaluated by molecular modelling.

Photoinduced electron transfer in supramolecular assemblies of transition metal complexes

Abstract Photoinduced electron transfer in supramolecular assemblies consisting of π -donor dialkoxyarene-functionalized photosensitizers and bipyridinium electron acceptors is examined. The photosensitizers include Ru(II)-tris-bipyridine complexetethered by multi-branch one-shell and two-shell dialkoxybenzene π -donor sites or a Zn(II)-porphyrin capped by a dialkoxybenzene receptor site. The photosensitizer/electron-acceptor supramolecular complexes behave as non-covalent diads and polyads. Effective internal electron transfer quenching within the supramolecular assemblies proceeds. A quantitative model that accounts for the photoinduced electron transfer in the systems is formulated.