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^-)

Abstract

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(2pyridylmethyl)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.

Synthesis, Structure, and Solution NMR Studies of Cyanide−Copper(II) and Cyanide-Bridged Iron(III)−Copper(II) Complexes

“…17 The coefficients for Cu in the ground state (S ) 2) are negative (-1 / 6 ) and in agreement with the observed upfield-shifted TMPA (Cu-ligand) proton signals. The present case shows the other extreme of magnetically induced isotropic shift possibilities for an iron-copper coupled system.…”

“…Compound 2-(PF 6 ) is a mononuclear pentacoordinate copper(II) species, with ligation to 4 N's from the TMPA ligand and a carbon from the cyanide ligand. The geometry around Cu(II) is best described as slightly distorted trigonal bipyramidal (TBP) with the pyridyl nitrogen atoms N(2), N(3), N(4) forming the equatorial plane and amine 17 The Cu(II) ion is displaced from the basel plane toward C (19), by 0.32 Å. Selected bond distance and angle parameters for 2 are given in Table 2.…”

“…17 It was shown that the 3-and 5-pyridyl protons resonate at approximately the same position in the spectrum (as a broad peak at 30 ppm) while the most distant pyridyl 4-H is found to give a peak at 10.5 ppm. 17 When comparing the spectrum of [Cu II (TMPA)(CH 3 CN)] 2+ with [Cu II (TMPA)(CN)] + (2) (Figure 4), once again we see the pyridyl-3 and -5 proton signals together at 35 ppm and the pyridyl 4-H at 10.1 ppm. As was noted for [Cu II (TMPA)(CH 3 CN)] 2+ , this reduction of the chemical shift as a function of the relative distance from the paramagnetic metal center is indicative of a σ contact shift mechanism.…”

Models of Copper Enzymes and Heme-Copper Oxidases

“…131 The unusual behavior wherein the Cu"(TMPA) ligands proton resonances shift upfield, whereas the pyrrole hydrogens on the iron porphyrin shifts downfield, is ascribed to the S = 2 ground state with strong antiferromagnetic coupling. 131 This behavior has also been observed in protein systems where two metals with different spins are antiferromagnetically coupled, i.e. Fe m -X-Fe" in ferredoxins or Cu'^imidazolate-Co 11 in cobalt-substituted Cu-Zn superoxide dismutase.…”

Superoxo, μ-peroxo, and μ-oxo complexes from heme/O 2 and heme-Cu/O 2 reactivity: Copper ligand influences in cytochrome c oxidase models

“…To unambiguously identify pyrrole resonances, which aid iron oxidation and spin-state assignments (27), complementary 2 H NMR spectroscopic investi-**A minor 2͞O2 reaction is observed in CH2Cl2͞6% EtCN, but it does not affect the 1͞2͞O2 kinetics described in this report. ) S ϭ 2 systems, as previously described (14,15,17,28).…”