INFERENCES FROM THE H-1-NMR SPECTROSCOPIC STUDY OF AN ANTIFERROMAGNETICALLY COUPLED HETEROBINUCLEAR FE(III)-(X)-CU(II)S=2 SPIN SYSTEM (X=O2-, OH-)

This study considers how the electronic relaxation rate enhancement ef fects of strong antiferromagnetic coupling in the Fe-III-X-Cu-II unit (X = O-2, OH-) of complexes [(F-8-TPP)Fe-III-O-Cu-II(TMPA)](+) (1), an d [(F-8-TPP)Fe-III-OH-Cu-II(TMPA)](+) (2) (F-8-TPP = tetrakis(2,6-difl uorophenyl)porphyrinate(2-) TMPA = tris(2-pyridylmethyl)amine) are man ifested as observable upfield-shifted resonances of the TMPA moiety in the H-1-NMR spectra. The pyrrole resonances appear at 65 and 69 ppm, respectively, for 1 and 2, consistent with an S = 2 ground state deriv ed from antiferromagnetic coupling of high-spin Fe-III (S = 5/2) and C u-II (S = 1/2) through the bridging ligand X. Paramagnetic mononuclear complexes [Co(TMPA)(CH3CN)](2+) (S = 3/2) (X-ray structure reported) and [Cu(TMPA)(CH3CN)](2+) (S = 1/2) demonstrate downfield-shifted peak s consistent with a sigma contact shift mechanism. Assignments for all complexes were achieved via H-1- and H-2-NMR spectroscopy of appropri ately synthesized methylated and deuteriated derivatives. In [Cu(TMPA) (CH3CN)](2+), the observed ligand peaks (298 K) are broad; in 1, howev er, they are considerably sharper and upfield-shifted to -61, -7, 4.5, -21, and -104 ppm, corresponding to pyridyl 6-H, 5-H, 4-H, and 3-H an d aminomethyl -CH2-, respectively, of TMPA. The observance of these up field peaks is a consequence of enhancement of the electronic relaxati on rate for Cu-II due to antiferromagnetic coupling with the faster re laxing Fe-III. This observation represents the prototype of a Fe-III-X -Cu-II S = 2 spin state hitherto only theoretically predicted. The att enuation of delta for a particular hydrogen as its distance (through b onds) from Cu-II increases correlates with a sigma contact shift mecha nism. Only one peak per type of pyridyl H is observed (from 220 to 300 K for 1 and from 220 to 270 K for 2), indicating dynamic behavior of the Cu-TMPA moiety in solution. Both 1 and 2 exhibit pseudo-Curie temp erature dependence manifested as augmentation of delta in both upfield and downfield directions as temperature is lowered. Linear Curie and anti-Curie plots of the pyrrole and TMPA chemical shifts (from 220 to 300 K for 1 and from 220 to 270 K for 2) imply a predominantly S = 2 s pin state in each case; i.e., the extent of antiferromagnetic coupling is strong.