Combined Mossbauer and EPR Studies of the S = 3 State of an Exchange-Coupled Fe"'Curl Complex: Test for Quantitative EPR Analysis of Integer Spin Systems

Abstract: We have studied acetonitrile solutions of the bimetallic complex [ Fe"'Cu"( BPMP)C12] (BPh,),, where BPMP is the anion 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylph~nol, with Mossbauer and EPR spectroscopy. Both spectroscopic techniques show that the complex is ferromagnetically coupled ( H = JSI.S2, J < 0) to yield a ground state spin S = 3. Analysis of the Mossbauer spectra taken in applied fields up to 6.0 T yielded for the FeII'site the zero-field splitting parameters D, = +1.2 cm-I and E l = 0.1 1 cm-I, the magnetic hyperfine coupling constant A, = -28.8 MHz, quadrupole spitting AEQ = 0.67 mm/s, and isomer shift 6 = 0.48 mm/s. The zero-field splitting term of the ferric ion mixes the excited S = 2 multiplet with the ground manifold, the mixing being proportional to D,/J; by analyzing this mixing we have determined that -2 cm-' > J > -5 cm-I. The Fe"'Cu" complex gives an X-band resonance at g = 12 which we have studied with a bimodal cavity in the temperature range from 2 to 50 K. We have analyzed the position and line shape of the g = 12 signal with a computer program in the framework of an S = 3 spin Hamiltonian, allowing a Gaussian distribution of the zero-field splitting parameters D and E. Finally, we have determined the S = 3 spin concentration from spectral simulations relative to a known standard; the value obtained agrees with the optically determined concentration. Our result shows that it is possible to determine spin concentrations of integer spin complexes with a precision that approaches that obtained for systems with half-integral spin. The availability of such techniques is highly desirable for the study of an ever increasing number of metalloproteins. We describe a method that allows one to determine the spin of the coupled system directly from the Mossbauer spectrum. Using this metho