Octalkoxy-substituted phosphorus(V) triazatetrabenzcorroles via ring contraction of phthalocyanine precursors

As part of efforts toward developing the synthesis of novel corrole analogues, the new triazatetrabenzcorrole (TBC) phosphorus(V) compounds (BuO)8(TBC)P(OCH3)2 (3), [(BuO)8(TBC)P(OH)]+OH- (4) ((BuO)8TBC=3,6,10,13,17,20,24,27-octabutoxytriazatetrabenzcorrolate), and [(BuO)8Cl8(TBC)P(OH)]+OH- (7) ((BuO)8Cl8TBC=3,6,10,13,17,20,24,27-octabutoxy-4,5,11,12,18,19,25,26-octachlorotriazatetrabenzcorrolate) were prepared. These TBCs were synthesized via a ring-contraction reaction mediated by PBr3 in pyridine in which a meso-nitrogen atom is extruded from an appropriate phthalocyanine precursor. Two of the compounds prepared, 3 and 4, are contracted analogues of the parent phthalocyanine (BuO8)PcH2 (1) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine, which has been shown for the first time to be susceptible to ring-contraction despite the potential steric crowding imposed by the butoxy substituents. Likewise, the octachloro-substituted (BuO8)Cl8PcH2 (6), 1,4,8,11,15,18,22,25-octabutoxy-2,3,9,10,16,17,23,24-octachlorophthalocyanine, has also been shown to smoothly afford 7 via the same ring-contraction method. In addition, a rare example of a bona fide phosphorus(V) phthalocyanine, [(BuO)8(Pc)P(OCH3)2]+OH- (2), has been prepared for spectroscopic comparisons with the TBC compounds. These molecules are all extremely soluble in common organic solvents because of the octabutoxy substituents and have been characterized in detail by 1H NMR, 31P NMR, UV-vis, MALDI-MS, elemental analysis, and electrochemical studies. A clear trend in the phosphorus chemical shifts for 5 versus 6 coordination has been delineated: 31P NMR for 2, -179.8; 3, -186.1; 4, -105.1; and 7, -105.1. These data are compared to the 31P chemical shifts for related porphyrinoid(P(V)) molecules. The MALDI-MS data reveal the tendency of the TBC macrocycles to ionize as the radical cations (M(+*)) and has been useful in determining the axial ligands at phosphorus. A consequence of ring-contraction is reflected in the dramatic red-shifts (approximately 200 nm) observed for the Soret bands of the TBC compounds relative to the parent phthalocyanines. The magnitude of the red-shift is much greater than that reported for other TBCs. In addition, insertion of phosphorus causes a large red-shift in the Q-band of 2 found at 889 nm compared to 760 nm for 1. Cyclic voltammetry of the compounds in this study reveals multiple oxidation and reduction waves for each compound, and some interesting trends in redox potentials have been observed. The CV data for the octachloro-substituted compounds 6 and 7 show that the Cl substituents have an expected strong electron-withdrawing effect on the macrocycles. In general, the TBC compounds are significantly easier to oxidize and harder to reduce than the Pc counterparts, supporting the notion that corrole-type macrocycles favor higher oxidation states.