Regan David Hartnell

Peripherally-metallated porphyrins: meso-η1-porphyrinyl-platinum(II) complexes of 5,15-diaryl- and 5,10,15-triarylporphyrins

Attempted metathesis reactions of peripherally-metallated meso-η1-porphyrinylplatinum(II) complexes such as trans-[PtBr(NiDPP)(PPh3)2](H2DPP = 5,15-diphenylporphyrin) with organolithium reagents fail due to competitive addition at the porphyrin ring carbon opposite to the metal substituent. This reaction can be prevented by using 5,10,15-triarylporphyrins, e.g. 5,10,15-triphenylporphyrin (H2TrPP) and 5-phenyl-10,20-bis(3′,5′-di-t-butylphenyl)porphyrin (H2DAPP) as substrates. These triarylporphyrins are readily prepared using the method of Senge and co-workers by addition of phenyllithium to the appropriate 5,15-diarylporphyrins, followed by aqueous protolysis and oxidation. They are convenient, soluble building blocks for selective substitutions and subsequent transformations at the remaining free meso carbon. The sequence of bromination, optional central metallation and oxidative addition of Pt(0) tris(phosphine) complexes generates the organoplatinum porphyrins in high overall yields. The bromo ligand on the Pt(II) centre can be substituted by alkynyl nucleophiles, including 5-ethynyl NiDPP, to form the first examples of meso-η1-porphyrinylplatinum(II) complexes with a second Pt-C bond. The range of porphyrinylplatinum(II) bis(tertiary phosphine) complexes was extended to the triethylphosphine analogues, by oxidative addition of H2TrPPBr to Pt(PEt3)3, and the initially-formed cis adduct is only slowly thermally transformed to trans-[PtBr(H2TrPP)(PEt3)2]16. The molecular structures of NiDAPP9b, trans-[Pt(NiDPP)(C2NiDPP)(PPh3)2]14 and 16 were determined by X-ray crystallography.

Butadiyne-linked bis(chlorin) and chlorin-porphyrin dyads and an improved synthesis of bis[octaethylporphyrinatonickel(II)-5-yl]butadiyne using the Takai iodoalkenation

Abstract meso -EthynylNiOEC (OEC=dianion of octaethylchlorin) was obtained by dehydrohalogenation of the novel meso -(2′-bromovinyl)NiOEC, prepared from NiOEC CHO by low-temperature Wittig bromoalkenation. An improved synthesis of the known conjugated bis(porphyrin) NiOEP C 4 NiOEP (OEP=dianion of octaethylporphyrin) was achieved via the previously unreported meso -(2′-iodovinyl)NiOEP, which was prepared by Takai iodoalkenation (CHI 3 , CrCl 2 ) of meso -formylNiOEP. The Takai chemistry was also applied to the NiOEC aldehyde, but the yield was much lower than for the Wittig bromoalkenation. The reaction was unsuccessful for H 2 OEP CHO, ZnOEP CHO and CuOEP CHO. The ethynyl chlorin was homocoupled to give the first example of a conjugated alkyne-linked bis(chlorin), and heterocoupled with 5-ethynylNiDPP (DPP=dianion of 5,15-diphenylporphyrin) to give the first chlorin-alkyne–porphyrin dyad. The first meso , meso -butadiyne-linked heteroporphyrin dyad NiOEP C 4 NiDPP was also prepared.

Selective meso-monobromination of 5,15-diarylporphyrins via organopalladium porphyrins

The selective meso-monobromination of 5,15-diarylporphyrins is difficult to achieve and extensive chromatography is required to obtain pure products. A sequence of (i) dibromination, (ii) selective monoinsertion of Pd(dppe) (dppe = 1,2-bis(diphenylphosphino)ethane) and (iii) hydrodepalladation using methanolic base affords pure monobromoporphyrins in typically ≥ 60% overall yield without isolation of the organopalladium porphyrin. Monobromo derivatives of even highly lipophilic 5,15-diarylporphyrins are thus readily available without tedious, expensive and environmentally undesirable chromatography.

The Marriage of Peripherally Metallated and Directly Linked Porphyrins: Bromidobis(phosphine)platinum(II) as a Cation-Stabilizing Substituent on Directly Linked and Fused Triply Linked Diporphyrins

A meso-bromidoplatiniobis(triphenylphosphine) η(1)-organometallic porphyrin monomer was prepared by the oxidative addition of meso-bromoZnDPP (DPP=dianion of 5,15-diphenylporphyrin) to a platinum(0) species. The meso-meso directly linked dimeric porphyrin (5) was prepared from this monomer by silver(I)-promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer (8). Acidic demetallation of 8 afforded the bis(free base) 9. Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10, the X-ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square-planar trans-[Pt(PPh3)2Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low-energy, near-IR electronic absorption band near 1100 nm. Electrochemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one-electron oxidation potential (-0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc(+))) and a narrow HOMO-LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt(PPh3)2Br] electron-donating substituents is sufficient to make dimer too aerobically unstable to allow further elaboration.

Remarkable homology in the electronic spectra of the mixed-valence cation and anion radicals of a conjugated bis(porphyrinyl)butadiyne

The pi-radical cation and anion of the dizinc complex of a bis(triarylporphyrinyl)butadiyne, 1+ and 1-, respectively, display remarkably similar near-IR signatures, with intense bands near 1000 and 2500 nm, as predicted by the appropriate frontier-orbital model for inter-porphyrin coupling across the conjugated bridge.