Phosphorus-centered (or P-centered) porphyrins are conjugated polycyclic ring systems consisting of either four pyrroles with inward-facing nitrogens and a phosphorus atom at their core or porphyrins with one of the four pyrroles substituted for a phosphole. Unmodified porphyrins are composed of pyrroles and linked by unsaturated hydrocarbon bridges often acting as multidentate ligands centered around a transition metal like Cu II, Zn II, Co II, Fe III. Being highly conjugated molecules with many accessible energy levels, porphyrins are used in biological systems to perform light-energy conversion and modified synthetically to perform similar functions as a photoswitch or catalytic electron carriers. Phosphorus III and V ions are much smaller than the typical metal centers and bestow distinct photochemical properties unto the porphyrin. Similar compounds with other pnictogen cores (As, Sb, Bi) or different polycyclic rings coordinated to phosphorus result in other changes to the porphyrin’s chemistry.
Synthesis
Phosphorus core Early experiments with group 15 elements at the porphyrin core by Barbour et al. in 1992 included syntheses of P-centered porphyrin compounds from meso-Tetra-p-tolylporphyrin (TTP). Phosphorus oxychloride (POCl3) added to H2TTP forms the P-centered porphyrin molecule [P(TTP)Cl2]+Cl−. Several chemical variants were synthesized by refluxing in solutions of pyridine solvent and alcohols to produce [P(TTP)OCH2CH3]+Cl−, [P(TTP)(O-p-C6H4OH)2]+OH−, and other similar compounds. Other researchers including Poddutoori in 2015 and 2022 have used such synthetic methods to yield hypervalent phosphorus (V) bonded to porphyrin as well as axial alcohols substituents.
Later syntheses have been performed with other phosphorus precursors, including PhPCl2 and octaethylporphyrin (OEP) in DCM to yield [POEP(Cl)2]+Cl−. PCl3/POCl3 and KPF6 yield similar porphyrin products with a PF6− counterion. Other halogens like bromine have been used successfully in place of chlorine for this synthesis method of P-centered rings. More syntheses with complex alcohols have been reported. Porphyrins functionalized with axial carbazolylvinylnaphthalimides are synthesized using similar methods to the previously described process for binding alcohols to phosphorus cores. In a similar synthetic process, Susumu et al. linked several modified porphyrins in a center-to-edge bonding scheme. The chlorines on a P-centered porphyrin are first substituted by an external hydroxyphenyl group on another porphyrin. The substituent porphyrins are then refluxed with POCl3 to synthesize the final center-to-edge porphyrin array. The resulting P-centered complex consists of three porphyrins with phosphorus atoms bound at each core.
Phosphaporphyrins Synthesis of a phosphole-substituted porphyrin or phosphaporphyrin involves a more complex chemical route. Phosphaporphyrins are not created using an unmodified porphyrin ring as a synthetic reagent. As reported by Matano and Imahori in 2008, a phosphaporphyrin is constructed with a phosphole linked to two pyrrole functional groups which is then bound to another pyrrole molecule. Specifically, addition of 2,5-bis(hydroxymethyl)-1-phenyl-1-thiophosphole to excess pyrrole in the presence of BF3·OEt2 results in the phosphatripyrrane precursor. A phosphaporphyrinogen ring is formed through dehydration condensation of the phosphatripyrrane with a 2,5-difunctionalized pyrrole. Dichloro-dicyanobenzoquinone (DDQ) oxidation of the product yields the highly conjugated 18π-system phosphaporphyrin product.
Metals can be coordinated in the core of the phosphaporphyrin by introducing metal salts. Rhodium metal is very easily inserted into the core of a phosphaporphyrin without the presence of a stabilizing thiophene-substituted pyrrole.
Properties Several varieties of the P-centered porphyrin exist. The porphyrin with a core phosphorus (V) ion can be tuned with additional substituents added to either the outside of the polycyclic ring system or axially to the core phosphorus. Meso-substituted porphyrins like meso-tetra-p-tolylporphyrin (TTP) and octaethylporphyrin (OEP) are often used in synthesis of the core phosphorus porphyrin. Substituents on the hypervalent phosphorus also result in the existence of a diverse array of molecules with varying properties. Axial substituents on the phosphorus include a wide variety of alkyl (-CH3, -CH2CH3), alkoxy (-OCH3, -OCH2CH3), aryl (-C6H5), and halide functional groups. Crystallographic experiments reveal that substituents affect the structure properties of these complexes. The porphyrin P-N bond distances decrease as the electronegativity of the axial substituents increase. Porphyrins bound to unnaturally small ions at the core result in ruffling, a deviation in position of the carbon atoms from the median plane of the aromatic conjugated system. This phenomenon, like saddling and doming, has been observed as well with small transition metal ions like nickel II. The ruffling effect of phosphorus (V) in a porphyrin is apparent because of the small size of the ion. More electronegative axial groups result in greater ruffling while large, sterically bulky groups have a similar effect. The degree to which various substituents cause ruffling was determined by Akiba et al. in 2001. Phosphaporphyrins possess phospholes usually bound to a phenyl group resting above the porphyrin plane in a trigonal pyramidal molecular geometry as is typical of phosphorus centers. In addition to the bound phenyl group, these molecules may also possess a metal ion core that coordinates to the three pyrroles and phosphole and distorts the naturally planar molecule. Very negative nucleus independent chemical shift (NCIS) values used to quantify aromaticity indicate the aromatic character of the phosphaporphyrins. These values however are more positive than NICS values for the undistorted four-pyrrole structure, which is a result of the less planar π-system in phosphaporphyrins.
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