As with peripheral coordination, axial coordination also permits construction of linear, cyclic, branched and polymeric porphyrin arrays. The axial ligand may vary in complexity from a single atom to a peripheral ligating group of another porphyrin molecule.
In the structurally simplest cases a non-porphyrinic ligand bridges between two porphyrin centres. The bridging ligand can be a single atom and indeed perhaps the most well known case is the Fe(III) porphyrin oxo bridged dimer80 formed by oxidation of Fe(II) porphyrins by dioxygen. The mechanism of this oxidation is believed to involve a peroxo bridged dimer of the form Fe(III)-O-O-Fe(III).81
Yamamoto and Akiba have reported a series of heterodinuclear main group element oxo bridged porphyrin dimers, 48 - 50, prepared by reaction of (OEP)M(CH3)(OH) (M = P, As, Sb) with (OEP)AlCH3 using DBU as a base to deprotonate the hydroxyl group.82 The crystallographically determined distance between the porphyrin planes of (OEP)Al-O-As(OEP)(CH3) is 3.71 Å.
Larger metal ions such as Zr(IV),83,84 Hf(IV),84,85 and Y(III)86 have been found to form porphyrin dimers with up to four bridging oxo or hydroxy ligands. In these complexes the metal ions are displaced out of the porphyrin plane towards the bridging ligands. An interesting difference exists between the structures of the TPP dimers of Mo(V) and Nb(V) with an M2O34+ unit, 51 and 52.87 The Mo is six coordinate with a single oxo bridge and terminal Mo=O groups, whereas the Nb(V) is seven coordinate with three bridging oxo ligands. In a theoretical study using extended Hückel theory the difference between the structures was explained in terms of the d0 versus d1 configuration of the metals and the tendency to maximize M - O p bonding.88 More recently it was found that by reaction with (TPP)MoO(OClO3), the molybdenum dimer may be extended to a trimeric species, 53.89 The equilibrium between the trimer and its constituents was studied by spectrophotometry and the trimer was demonstrated to be the major species in DCM solution.90 These experiments did not provide evidence for higher oligomers although their presence could not be ruled out.
Other examples of oxo or hydroxy singly bridged metalloporphyrin dimers reported to date include Ru(IV),91,92 Mn(III),93 Sc(III),94,95and Ti(IV).96Bridging atoms are not limited to oxygen, and analogues with nitrido,97-99 sulfido100 and selenido100 bridges are known.
Many non-polymeric axial coordination dimers and higher oligomers have been prepared from non-porphyrinic multidentate nitrogen donor ligands. Kadish et al. electrochemically characterized a series of (TPP)RhCl dimeric complexes with bidentate pyridine ligands, 54 - 57, in which the linker between the pyridine groups was varied.101 It was found that complexes of non-conjugated ligands 54 and 55 underwent a single 2e irreversible reduction corresponding to a pair of single electron reductions at non-interacting metal centres. However the complexes of conjugated ligands 56 and 57 displayed two separate 1e reductions due to electronic communication between the metal centres via the bridging ligand. In both cases the ultimate product of the reaction was the metal-metal bonded dimer [(TPP)Rh]2 and free ligand. The 1H NMR spectra of related dimeric and trimeric complexes of Ru(II) porphyrins have also been reported by Anderson102 and Darling.103
Ladder-like porphyrin complexes were reported to form when butadiyne linked Zn porphyrin oligomers were titrated with DABCO in dichloromethane.104,105 The porphyrin dimer 58 existed as a dimeric p stacked aggregate in the absence of DABCO. Addition of one equivalent of DABCO formed a highly stable ladder complex 59, the NMR and UV spectra of which indicated that the covalently linked porphyrin components are not held coplanar but can still twist about the butadiyne linker.
Crossley et al. developed a covalently linked zinc porphyrin dimer with a cleft between the faces of the porphyrins.106 Spectrophotometric titrations with a,w-diaminoalkanes revealed formation of 1:1 complexes for alkyl chain lengths of seven methylene units or over. This is the shortest chain that can bridge across the cleft between the Zn centres. Complexation of the dimer with a branched tetramine afforded a capsular structure, 60, as evidenced by spectrophotometric titration and by the observation of downfield shifts of the porphyrin b-pyrrole protons in the 1H NMR spectrum at -50 °C. Molecular modelling suggested this structure to be more stable than the alternative in which nine atoms of the amine ligand span the cleft of each dimeric unit.
The literature contains a miscellany of other porphyrin dimers with a bridging ligand such as the organo Rh compounds 61 and 62,107 and related Ru species,108 a dithiol bridged Ru nitrosyl complex 63,109 and assorted sulfate bridged dimers.110-112
With properties such as conductivity, magnetism and non-linear optics in mind, chemists have prepared axial coordination polymers of porphyrins with bidentate ligands.
Short ‘wheel and axle’ oligomers, 64, of P(V) porphyrin were prepared by refluxing [(TPP)PCl2]+ with ethylene glycol in butyronitrile for several days.113 The different oligomers, up to a tetramer, were separated by column chromatography and identified by 1H and 31P NMR spectroscopy.
So called ‘shish-kebab’ polymers of Ru(II) porphyrin were prepared either by photolysis of Ru(II)CO porphyrin in the presence of bidentate ligands,114 or by reaction of the metal-metal bonded dimer [Ru(OEP)]2 with the ligand dissolved in toluene.115,116 The insoluble polymers 65 could be partially oxidized by reaction in a toluene slurry with I2 overnight. This rendered them electrically conductive, and the conductivity was found to vary with the bridging ligand in the sequence pyrazine > 4,4'-bipyridine > DABCO suggesting that conduction occurs most readily through the conjugated ligands.115
A possible solution to the problem of poor processability of porphyrin polymers is to electrochemically polymerize a soluble precursor to form a film on an electrode substrate. This method of film preparation avoids the need to handle the polymer in solution or as a melt. To this end, P(V) porphyrin 66 with oligothiophene alkoxy ligands was synthesized and electrochemically polymerized on an ITO electrode.117
Porphyrin polymers with unsymmetrical bridging ligands were investigated as potentially these could possess non-linear optical properties if all of the ligands were aligned in the same direction in the solid state. The polymer 67 was found to crystallize in a non-centrosymmetric space group, although this was an exception as more commonly ligands were disordered over both orientations or crystallized such that ordered polymer chains ran in opposite directions leading to an overall centrosymmetric structure.118
When co-crystallized with electron acceptors, Mn(II) porphyrins form salts in which an electron is transferred from the Mn centre to the electron acceptor group. Such ‘electron transfer salts’ containing tetracyano-p-diquinodimethane119 and tetrachloro-1,4-benzoquinone120 adopted polymeric structures, 68 and 69, in which the acceptor was reduced to a radical anion which coordinated to the Mn(III) porphyrin. Magnetic measurements demonstrated one dimensional antiferromagnetic coupling along the polymeric chains. Another unusual Mn(III) porphyrin polymer, 70, was generated serendipitously by washing a dichloromethane solution of (TPP)MnCl with aqueous sodium hydroxide solution.121 Solvent decomposition apparently led to the formation of formate which bridged between the Mn(III) porphyrins in the solid state.
To generate larger supramolecular assemblies porphyrin axial ligand coordination may be combined with hydrogen bonding recognition sites. If these recognition elements are not to interfere with each other then the hydrogen bond donors and acceptors must not compete as axial ligands for the porphyrin, and the ligand group should not compete for hydrogen bonds.
Based on these principles Kuroda and coworkers reported the assembly of a heterometallic porphyrin trimer.122 The dialkyl tartrate derivative 71 was found to bind to porphyrin 72 by formation of four hydrogen bonds to the amide protons of 72. The NMR spectrum of a mixture of 71 and 72 in CDCl3 displayed resonances at -3.81 and -4.13 ppm which were attributed to the hydroxyl protons of 71, shielded due to their close approach to the porphyrin, and split into two peaks as a result of the unsymmetrical aryl substitution pattern of 72. The association constant was estimated as > 105 M-1 by spectrophotometric titration. The pyridyl groups of 71 each bound (OEP)RhCl as evidenced by upfield shifts of the pyridyl resonances by up to 8.5 ppm whereas the tartrate moiety was considerably less shifted. The 1H NMR spectrum of a mixture of 71 (1 mM), 72 (1 mM) and (OEP)RhCl (2 mM) in CDCl3 displayed upfield shifted pyridyl and tartrate resonances suggesting formation of a quaternary assembly. The association of the porphyrin and pyridyl components appeared quantitative at this concentration, although by integration the association constant of 71 and 72·2(OEP)RhCl was estimated as 3000 M-1. The decrease in association constant was ascribed to steric hindrance between the terminal Rh porphyrins and 72.
Whitesides attempted to combine hydrogen bonded assembly based on the association of isocyanuric acid and melamine with imidazole axial coordination to (TPP)Zn.123 Evidence for the formation of a five component bis-rosette aggregate, 75, from 73 and 74 in CD2Cl2 was provided by the observation of resonances in the 1H NMR spectrum over the range 13 - 16 ppm assigned to the hydrogen bonded protons of 74. Titration of 75 with (TPP)Zn lead to broadening and upfield shifts of the peripheral protons of the aggregate consistent with porphyrin binding to the pendant imidazole groups. However spectrophotometric titration suggested binding of only four porphyrins to the aggregate, instead of six, at the concentration employed. The stability of 75 was studied by its behaviour on a gel permeation column. Dissociation of aggregates to smaller constituent components which have a longer retention time leads to tailing of peaks, whereas an aggregate which does not dissociate elutes as a single sharp peak. The imidazole functionalized aggregate 75 was found to be less stable than an analogue in which the imidazole groups were replaced with tertiary butyl groups. This was attributed to competition of the imidazole groups for hydrogen bonds, thus destabilizing 75. In agreement with this conclusion was the observation that addition of (TPP)Zn resulted in an increase in the stability of 75, presumably by coordination to imidazole removing its ability to compete for hydrogen bonds.
The Reinhoudt group have recently reported a similar system, except the rosette forming components were themselves functionalized with zinc porphyrins.124 Hydrogen bond mediated assembly afforded a double rosette structure with six pendant porphyrins in the correct geometry for binding two molecules of tripyridine 76. Mixing the porphyrin appended building block with an analogue lacking the porphyrin afforded an equilibrating mixture of rosettes, which could be displaced towards the six porphyrin assembly by addition of 76 which is likely to bind most strongly to this component of the mixture.
A broad range of structures of nanometer dimensions is available by a combination of the self assembly strategies described in sections 1.5 and 1.6.1 namely axial coordination of a peripheral group of a porphyrin to the metal centre of another porphyrin. The full range of topologies may be generated, from discrete acyclic entities, through cyclic oligomers to infinite three dimensional coordination polymers. A selection of examples of each category will be reviewed in this section.
Ru(II) and Os(II) porphyrins have been a favourite choice for supramolecular chemists as they possess the advantage of comparatively tight binding and slow exchange of axial ligands such as pyridines, amines and phosphines. This ensures that the ligand is predominantly in the complexed form in solution, and gives rise to NMR spectra which are typically in the slow exchange limit with sharp resonances.
Reaction of one equivalent of a phosphine substituted Zn porphyrin with a Ru(II)CO porphyrin afforded the heterometallic dimer 77.125 However if two equivalents of phosphine porphyrin were used in boiling chloroform then a trimeric array was produced by displacement of the CO ligand from the Ru porphyrin. The UV spectra of these complexes could be calculated by summing the spectra of the components demonstrating no electronic interaction between the porphyrins in the ground state.
Multitudinous arrays from dimers to pentamers have been prepared by coordination of Ru(II)51,126-130 or Os(II)131 porphyrins to pyridyl porphyrin. In all these arrays the NMR spectra show upfield shifted pyridyl resonances at approximately 6 and 2 ppm. Ordinarily pyridyl ligands do not displace the CO ligand from Ru(II) and Os(II) porphyrins so trimeric arrays such as 84 were synthesized by photolytic removal of the CO in the presence of two equivalents of pyridyl porphyrin.
The chemical shift of the pyrrole NH protons of the central free-base pyridyl porphyrin of 78 - 83 was found to be sensitive to the number of porphyrins coordinated to the peripheral pyridyl groups, each coordinated porphyrin resulting in an upfield shift of ~0.5 ppm.131 The effects of porphyrin tautomerism were visible in the NMR spectra of 78 and 79 at 193 K. At this temperature the tautomerism of the pyrrolic protons was sufficiently slow that 78 exhibited two resonances between -3 and -4 ppm due to the inequivalence of the pyrrole protons in the two degenerate tautomers. 79 displayed three pyrrole resonances in a 1:2:1 intensity ratio. In this case there are two non-degenerate tautomers, one of which has two equivalent pyrrole protons but in the other these protons are inequivalent. The integrations indicated equal population of the two tautomers.131
In the solid state 83 was found to adopt a tilted geometry in which the angle between the best fit porphyrin and pyridyl planes was 63° instead of the expected 90°. This tilting was a combination of a deviation of the pyridyl N - Ru bond from the perpendicular to the porphyrin and from the plane of the pyridyl group. The tilted structure was ascribed to crystal packing effects, and was also observed in the X-ray structure of a similar Zn porphyrin coordination dimer.132
Ru porphyrin coordination chemistry was used to assemble dendritic multiporphyrin structures using two approaches. In one approach a covalently linked porphyrin pentamer, 85, with Ru at its core was coordinated to tetrapyridylporphyrin to generate a 21 porphyrin array, as evidenced by the diagnostic change in chemical shift of the pyrrole protons at the centre of the array.133 A complementary approach used a Ru porphyrin as a core around which to assemble two porphyrin dendritic wedges by photolytic removal of CO to afford the heptamer 86.103,134
As an alternative to pyridyl ligands, others have used combinations of phenolate and oxophilic P(V), Sn(IV) and Ge(IV) porphyrin,135-137 or carboxylate and Sn(IV) porphyrin138 to create dimers and trimers such as 87 and 88.
Cyclic porphyrin dimers in which the porphyrins are held close together in a well defined geometry have received interest as models for a variety of biological systems, and for fundamental studies on electronic interactions between metal centres.
The oxidative degradation pathway of heme to biliverdin occurs via an Fe(III) oxophlorin. Oxophlorins are porphyrins in which a meso position is substituted with a hydroxyl group and potentially these compounds can exist in keto- and enol-like tautomers. As a model of this intermediate, Fe(III) octaethyloxophlorin was prepared and found to be a cyclic dimer, 89, in which the hydroxyl group is deprotonated and coordinates to an Fe centre.139,140 The crystal structure of In(III) octaethyloxophlorins reveals that the cyclic structure is maintained in the solid state.141
Ethanolysis of 2-benzyloxy-tetraphenylporphyrin Ga(III)Cl afforded a cyclic trimer, 90, in which the oxygen coordinated to the Ga centre.142 Evidence for the nature of the product came from a broadened blue shifted Soret band unlike that of other five coordinate (TPP)Ga(III) complexes and from the observation of a molecular ion in the mass spectrum. The 1H NMR spectrum displayed resonances at 2.82, 2.18 and 1.82 ppm which were assigned to the protons of the 3 position of the pyrrole groups, which make a close approach to the plane of an adjacent porphyrin and hence are upfield shifted. Clearly the three porphyrin units are not identical and this was ascribed to the existence of R and S enantiomers of the constituent monomer units. The 1H NMR spectrum of the trimer was consistent only with a composition of RRS or SSR, which renders all of the protons of each of the porphyrins inequivalent. A complete assignment was possible using COSY, NOESY and ROESY techniques. Fe(III) and Mn(III) analogues were found to have an identical structure.143,144
A range of porphyrins with 2-pyridyl,145,146 2-phenol,147,148 or imidazolyl149 substituents have been found to dimerize in solution and the solid state. Hunter demonstrated that the compound 91 dimerized in a highly cooperative manner as was intended from the design of the linker group which disposes the pyridyl ligand at almost 90° to the porphyrin such that the geometry is self complementary (scheme 1.7).150 The association constant of the dimer was estimated as greater than 108 M-1 in DCM compared to 5.6 ´ 103 M-1 for the association of reference compounds with a single Zn-pyridine interaction. Furthermore, by variation of the linker group between the porphyrin and pyridine and consequently their relative orientation, Hunter was able to obtain cyclic trimers and tetramers.151
A simple Ru(II)CO pyridyl porphyrin associated in solution to form a tetramer 92 in which each porphyrin unit was equivalent according to 1H NMR spectroscopy.152,153 Four b-pyrrole resonances were observed as required by a cyclic structure. A peak at m/z 2973 in the FAB mass spectrum was consistent with the tetrameric nature of the product, with other fragment peaks assigned to trimeric and monomeric species.
Within this laboratory a combination of the complementary coordination chemistries of Zn(II), Sn(IV) and Ru(II) porphyrins was used to assemble a thermodynamically stable trimetallic assembly.154 Sn(IV) porphyrins are six coordinate and selectively bind carboxylates in preference to pyridine, with comparatively low ligand exchange rates. Zinc porphyrins selectively bind pyridyl ligands, in a labile complex. Reaction of dicarboxylic acid 94 with pyridyl porphyrin 93 afforded the dimeric assembly 95. Two types of cooperative processes are responsible for the assembly of 95 in preference to a polymer - the coordination of a single carboxylate group to the Sn(IV) centre, and the pyridyl - Zn interaction. The pyridyl porphyrin component was found to undergo a rotational motion around the O-Sn-O bonds, and separate 1H resonances for free and bound pyridyl groups were only observed as the sample was cooled. Addition of one equivalent of Zn porphyrin slowed down the rotation by coordination to the free pyridyl group, whilst addition of a more substitutionally inert Ru(II) porphyrin conformationally locked the structure. The assembly 96 appeared to be the thermodynamic product, and was also obtained simply by mixing the three components in solution.
Other noteworthy arrays assembled using porphyrin peripheral - porphyrin axial coordination include Warrener’s pentameric ‘molecular universal joint’155 and Hunter’s porphyrin - naphthalenediimide trimer.156
The nature of the self assembly of zinc amino porphyrins, 97, in solution was found to depend on the position of the amino substituent.157 Complexation induced shifts measured for the ortho and meta isomers agreed with values calculated from ring current theory using the geometries of dimeric structures. Association constants calculated from 1H NMR dilution experiments indicated dimerization to be a cooperative process. However the para isomer displayed broadened NMR spectra which sharpened on dilution, suggesting formation of higher order aggregates. On steric grounds a dimeric structure in which both amino groups are coordinated to a zinc centre appears implausible. Dilution studies, fitted to either a dimer or polymer model, revealed an association constant almost identical to simple aniline - Zn porphyrin binding, suggesting non-cooperative self assembly. No dimeric geometry could be found which satisfied the observed complexation induced shifts. These results combined indicate formation of coordination oligomers in solution as the porphyrin concentration increases.
A system which could be ‘switched’ between a polymeric and dimeric form by alkene isomerization was reported.158 Cis and trans isomers, 98 and 99, could be separated chromatographically, and the cis form isomerized to the trans by irradiation with UV light. The 1H NMR spectra of both isomers showed upfield shifted pyridyl resonances due to coordination to Zn, and crystallizations afforded a dimeric structure for 98 but a coordination polymer of 99.
Fleischer and Shachter reported the polymeric crystal structure of Zn pyridyl porphyrin 100, which featured a pyridyl ligand tilted from the porphyrin perpendicular, and claimed that a similar structure existed in solution on the basis of 1H chemical shifts.159,160 However Hunter reinterpreted these results in terms of formation of a cyclic tetramer analogous to 92 in dilute solution and formation of polymer only at the high concentrations required for crystallization.161
Genuine ladder-like polymers (figure 1.14) have been prepared by the use of a six coordinate metal and a porphyrin with two coordinating substituents. Examples reported to date include the combination of Mg(II) with imidazolyl ligands,162 and Co(II) with pyridyl ligands.163
The three dimensional isostructural coordination polymers of Mn(II) and Co(II) tetrapyridylporphyrin were recently described,164 although an almost identical structure of Zn tetrapyridylporphyrin with an unusual six coordinate Zn ion had been reported a number of years earlier.165 These crystals possess an open structure with solvent filled hexagonal channels into which uncoordinated pyridyl groups point. Thermogravimetric analysis and powder X-ray diffraction of the Mn and Co derivatives revealed that the structure was maintained after the solvent molecules had been removed by heating above 200 °C. Two dimensional coordination polymers of 101 were formed on crystallizing from a variety of solvents.79 These structures too feature six coordinate Zn demonstrating how the solid state can lead to coordination modes not normally encountered in solution. The layer packing was found to be preserved between several different structures which included different solvent molecules trapped between the layers.
A mixed coordination polymer, 102, was prepared by co-crystallization of (TPP)Mn(III) ClO4 with free-base tetrapyridylporphyrin from chloroform.166 Voids in the structure were occupied by ClO4- counterions and nitrobenzene solvent. The diffraction data located approximately twenty nitrobenzene molecules per unit cell, occupying 46 % of the crystal volume. Occupation of void space by solvent molecules is a recurrent feature of porphyrin crystals.
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