Boron complexes porphyrins

The lack of adequate experimental data on chelates of germanium, boron, titanium, and vanadium prevents a similar comparison for these elements however, their high ionic potentials do indicate that they should form stable chelates. Vanadium does occur naturally in the very stable vanadium porphyrin complex. For similar reasons, molybdenum and tin are not discussed further. 

Conceptually, this was first demonstrated using a metallated porphyrin that was functionalized with 2 diols (zinc dicatechol porphyrin) (Fig. 5). After addition of 3-pyridylboronic acid, absorption spectroscopy indicated that the pyridyl moiety was coordinated to the Zn-porphyrin. The observed affinity constant for this interaction, however, was more than 30 times what was expected for a simple pyridyl-Zn-porphyrin complex. It was, therefore, reasoned that the diols reacted with the boronic acids to afford the esters that would result in a cyclic structure (Fig. 5). Indeed, vapor phase osmometry (VPO) confirmed the 2 2 dimeric nature of the complex. Here the ester served as the key covalent linkage, though associated coordination was required between the metal and pyridine to create the cyclic structure. 

Several reports of carboranylated porphyrins have appeared for utilization in the BNCT of cancer.150 The examples include both nido- (126) (Fig. 73) and closo-carborane cluster-containing (124 and 125) (Fig. 73) materials. Metal-free nido-carboranylporphyrins were able to deliver a higher amount of boron to cells than the corresponding zinc complexes. 

In spite of its unusual structural outcome, the N-fusion reaction is a very general process. It was observed in a number of other N-confused macrocycles. For instance, an N-fused intermediate forms in the synthesis of trans doubly N-confused porphyrin 96 [238], N-fusion was also induced by the reaction with PhBCl2, in which case the reaction was promoted by the small radius of the coordinating boron(III) center [251], The resulting boron complexes were also aromatic, even though in some cases further chemical modification of the macrocycle took place (as in 110). The fusion process, which appears to be a nucleophilic addition-elimination, is reversible, and N-fused macrocycles can often be reopened when treated with nucleophiles such as alkoxides [248, 249],... 

Shinkai s group applied another method for the chiral spatial fixation of two porphyrins via peripheral substitution at the corresponding meso-position with boronic acid residue [67,68]. Thus, anionic 25 and cationic 26 formed an optically active 1 1 complex only in the presence of glucose and xylose in aqueous solution as a result of the boronic acid binding to the sac-... 

The reaction of the porphyrin ligand, TTP (tetra-p-tolylporphyrin) with BF3-OEt2 leads to the oxide fluoride complex, B2OF2(TTP) and the structure has been established using H, 13C, nB and 19F NMR spectroscopy and FAB mass spectrometry [9], The structure contains a B—O—B bridge in which each boron is bonded to fluorine and to a nitrogen of TTP. The structure of the diboron complex was confirmed by an X-ray crystal structure determination of the tetrakis-(p-chlorophenyl)porphyrin (TpCiPP) derivative. 

Covalent attachment of electronically active groups such as N,W-dimethyl-aminophenyl (8) [12] or ferrocene (9) [10] as a second electron donor further improves the systems performance (Fig. 7). After photoirradiation of the Donor-Por-Ceo system, a charge-separated state Donor" -Por-Cgo is generated with high quantum efficiency and moderate charge stabihzation. In the BDP-Por-Ceo complex, the boron dipyrrin unit acts as an antenna unit in that it transfers its excited state energy to the porphyrin, which is followed by electron transfer to the fuUerene to give the complex BDP-Por" -Ceo", reminiscent of the combined antenna-reaction center events in natural photosynthesis. 

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