Inferences from the ¹H-NMR spectroscopic study of an antiferromagnetically coupled heterobinuclear Fe(III)-(X)-Cu(II) S = 2 spin system (X = O^2-, OH^-)

This study considers how the electronic relaxation rate enhancement effects of strong antiferromagnetic coupling in the Fe(III)-X-Cu(III) unit (X = O2-, OH-) of complexes [(F8-TPP)Fe(III)-O-Cu(II)(TMPA)]+ (1), and [(F8-TPP)Fe(III)-OH-Cu(II)(TMPA)]+ (2) (F8-TPP = tetrakis(2,6- difluorophenyl)porphyrinate(2-); TMPA = tris(2-pyridylmethyl)amine) are manifested as observable upfield-shifted resonances of the TMPA moiety in the 1H-NMR spectra. The pyrrole resonances appear at 65 and 69 ppm, respectively, for 1 and 2, consistent with an S = 2 ground state derived from antiferromagnetic coupling of high-spin Fe(III) (S = 5/2) and Cu(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 peaks consistent with a σ contact shift mechanism. Assignments for all complexes were achieved via 1H- and 2H-NMR spectroscopy of appropriately synthesized methylated and deuteriated derivatives. In [Cu(TMPA)(CH3CN)]2+, the observed ligand peaks (298 K) are broad; in 1, however, 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 and aminomethyl -CH2-, respectively, of TMPA. The observance of these upfield peaks is a consequence of enhancement of the electronic relaxation rate for Cu(II) due to antiferromagnetic coupling with the faster relaxing Fe(III). This observation represents the prototype of a Fe(III)-X-Cu(II) S = 2 spin state hitherto only theoretically predicted. The attenuation of 6 for a particular hydrogen as its distance (through bonds) from Cu(II) increases correlates with a σ contact shift mechanism. 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 temperature dependence manifested as augmentation of δ 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 spin state in each case; i.e., the extent of antiferromagnetic coupling is strong.