## Abstract

We have studied acetonitrile solutions of the bimetallic complex [Fe^{III}Cu^{II}(BPMP)Cl_{2}](BPh_{4})_{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 = JS_{1}·S_{2} 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 D_{1} = +1.2 cm^{−1} and E_{1} = 0.11 cm^{−1}, the magnetic hyperfine coupling constant A_{0} = −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 D_{1}/J; by analyzing this mixing we have determined that −2 cm^{−1} > J > −5 cm^{−1}. The Fe^{III}Cu^{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^{III}Cu^{II}(BPMP)Cl_{2}](BPh_{4})_{2} and [Fe^{III},Cu^{II}BPMP(OAc)_{2}](BPh_{4})_{2} have S = 3 and 2, respectively. At T = 93 K the zero-field Mössbauer spectrum of the integer spin complex [Fe^{1II}Cu“(BPMP)Cl_{2}](BPh_{4})_{2} exhibits unusual line broadening, similar to that observed for Thermus thermophilus cytochrome c_{1}aa_{3}.

Original language | English (US) |
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Pages (from-to) | 518-525 |

Number of pages | 8 |

Journal | Journal of the American Chemical Society |

Volume | 113 |

Issue number | 2 |

DOIs | |

State | Published - Jan 1 1991 |