The NMR spectra of the porphyrins 16—zinc(II) meso‐tetraphenylporphyrin (Zn TPP) as a diamagnetic shift reagent. A quantitative ring current model

Abstract

The facile preparation of zinc(I1) meso-tetraphenylporphyrin (Zn TPP) and derivatives from substituted benzaldehydes and pyrrole, combined with the calculation of ring current shifts in these molecules, provides a useful series of selective diamagnetic shift reagents. The porphyrin-ligand equilibrium is examined for some nitrogenous bases and the complexation shifts (AP values) are obtained in precisely the same manner as LIS.The parameterization of the double-dipole model of the porphyrin ring current is given, with the inclusion of the phenyl ring currents and free rotation about the substrate-Zn bond. Precise agreement with the porphyrin proton chemical shifts and the complexation shifts of the geometrically rigid substrates of pyridine, 4-picoline and quinuclidine is obtained. In contrast, the 13C complexation shifts in these molecules are subject to additional effects other than the ring current shift. INTRODUC~IONLanthanide shift reagents (LSR) have proved to be extremely useful reagents in NMR spectroscopy, both in spectral simplification and in structural and conformational studies.'" However, despite recent advances in interpretati~n,~" their general use as geometrical probes of molecules in solution has not fulfilled their original promise.A major fundamental deficiency of LSR is their lack of specificity. Because of this, the analysis of the lanthanide induced shifts (LIS) obtained with multifunctional ligands in which more than one group complexes with the LSR becomes prohibitively complex. Indeed, often, due to exchange and chelation processes, the spectra obtained on adding LSR to multifunctional ligands are too broad and ill-resolved to permit any analysis.Another considerable disadvantage with many LSR is the large contact contribution to the LIS which precludes their use as geometrical probes via the pseudocontact term. Indeed, most 13C LIS fall into this category. Although this problem can be overcome by using specific l a n t h a n i d e~,~.~ this produces further limitations, in particular line broadening due to paramagnetic relaxation can increase to unacceptable levels.The use of lanthanide porphyrins as shift reagents has recently been proposed by Horrocks, who has synthesized suitable derivatives for organic6 and aque0us7 solutions. However, the analysis and interpretation of the observed shifts is complex due to the influence of both the metal paramagnetism and por- * Author to whom correspondence should be addressed phyrin diamagnetism, and to date these reagents have not been used as LSR.In a pioneering investigation Storm and Corwin' observed large shifts of the ligands on diamagnetic metallo-porphyrins and showed how they could be used to obtain geometric information, but they were handicapped at that time by the lack of an accurate model of the porphyrin ring current.Since then the aromatic ring current of the porphyrin ring has been used as a conformational probe in biological investigations' but, with one exception, has not been considered as a general conformational probe. Gaudeme...

Bis(aryloxo) derivatives of tin(IV) porphyrins: synthesis, spectroscopy and redox activity

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Complexation-Induced Changes in1H NMR Chemical Shift for Supramolecular Structure Determination

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A thermochemical study of 5,10,15,20-tetraphenylporphine zinc(II) by rotating bomb combustion calorimetry and by Knudsen effusion experiments

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