Inferences from the ¹H-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 effects 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 ), and [(F 8 -TPP)Fe III −OH−Cu II (TMPA)] + ( 2 ) (F 8 -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 1 H-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)(CH 3 CN)] 2+ ( S = 3/2) (X-ray structure reported) and [Cu(TMPA)(CH 3 CN)] 2+ ( S = 1/2) demonstrate downfield-shifted peaks consistent with a σ contact shift mechanism. Assignments for all complexes were achieved via 1 H- and 2 H-NMR spectroscopy of appropriately synthesized methylated and deuteriated derivatives. In [Cu(TMPA)(CH 3 CN)] 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 −CH 2 −, 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 δ 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 .