Combined Mössbauer and EPR Studies of the S = 3 State of an Exchange-Coupled Fe^IIICu^II Complex: Test for Quantitative EPR Analysis of Integer Spin Systems

We have studied acetonitrile solutions of the bimetallic complex [Fe^IIICu^II(BPMP)Cl₂](BPh₄)₂, where BPMP is the anion 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol, with Mössbauer and EPR spectroscopy. Both spectroscopic techniques show that the complex is ferromagnetically coupled (Ĥ = JS₁·S₂ J < 0) to yield a ground state spin S = 3. Analysis of the Mössbauer spectra taken in applied fields up to 6.0 T yielded for the Fe^III site the zero-field splitting parameters D₁ = +1.2 cm⁻¹ and E₁ = 0.11 cm⁻¹, the magnetic hyperfine coupling constant A₀ = −28.8 MHz, quadrupole spitting ΔE_Q = 0.67 mm/s, and isomer shift δ = 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 D₁/J; by analyzing this mixing we have determined that −2 cm⁻¹ > J > −5 cm⁻¹. The Fe^IIICu^II 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 Mössbauer spectrum. Using this method we show here that the ground manifolds of [Fe^IIICu^II(BPMP)Cl₂](BPh₄)₂ and [Fe^III,Cu^IIBPMP(OAc)₂](BPh₄)₂ have S = 3 and 2, respectively. At T = 93 K the zero-field Mössbauer spectrum of the integer spin complex [Fe^1IICu“(BPMP)Cl₂](BPh₄)₂ exhibits unusual line broadening, similar to that observed for Thermus thermophilus cytochrome c₁aa₃.