Do sitting-atop metalloporphyrin complexes exist? Observation of N-H- - -pi bonding in arene solvates of a diprotonated porphyrin dication.
The existence of sitting-atop metalloporphyrin complexes, the proposed intermediates in the metalation of free-base porphyrins, has been explored via the microscopic reverse reaction, i.e., protonation of metalloporphyrins with a strong acid. The reaction of M(Tp-TP) (M = Zn, Cu, Ni; Tp-TP = dianion of tetra-p-tolylporphyrin) with mesitylenium carborane salts produced only the demetalated porphyrin in its diprotonated form, [H4Tp-TP][carborane]2 (carborane = CHB11H5Cl6-, CHB11H5Br6-, CHB11Cl11-). In arene solvents, the H4Tp-TP2+ dication shows an unusually upfield shifted 1H NMR resonance at ca. -6 ppm, which X-ray crystallography reveals to arise from N-H- - -pi hydrogen bonding of the acidic protons to arene solvent molecules.
Garate-Morales
J Luis
JL
Center for S and P Block Chemistry, Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
Tham
Fook S
FS
Reed
Christopher A
CA
D047250:Calixarenes / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008665:Metalloporphyrins / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D011522:Protons; D012680:Sensitivity and Specificity
D002412:Cations; D008665:Metalloporphyrins; D011166:Porphyrins; D011522:Protons; D047250:Calixarenes