X-RAY STRUCTURE AND PHYSICAL-PROPERTIES OF THE ORE-BRIDGED COMPLEX [(F-8-TPP)FE-O-CU(TMPA)](-8-TPP EQUALS TETRAKIS(2,6-DIFLUOROPHENYL)PORPHYRINATE(2-), TMPA EQUALS TRIS(2-PYRIDYLMETHYL)AMINE - MODELING THE CYTOCHROME-C-OXIDASE FE-CU HETERODINUCLEAR ACTIVE-SITE(), F)

The iron/copper heterodinuclear center in cytochrome c oxidases has at tracted the interest of inorganic chemists since this is the site of d ioxygen (O-2) four-electron four-proton reduction to water by the redu ced enzyme, while the oxidized (''resting'') isolated protein dinuclea r center exhibits unusual physical, spectroscopic, and ligand-binding properties. We describe here a model compound, [(F-8-TPP)Fe-III-O-Cu-I I(TMPA)](+) (3), generated directly from O-2-reduction by 1:1 mixtures of the reduced copper(I) and iron(II) complexes [(TMPA)Cu-I(CH3CN)]() (1) (TMPA = tris(2-pyridylmethyl)amine) and (F-8-TPP)Fe-II(pip)(2) ( 2) (F-8-TPP = tetrakis(2,6-difluorophenyl)porpyrinate-(2-), pip = pipe ridine) in O-2-saturated CH2Cl2 at -80 degrees C; warming to 0 degrees C, followed by precipitation with diethyl ether, affords 3 in similar to 70% yield, An X-ray diffraction study on [(F-8-TPP)Fe-O-Cu(TMPA)]( ClO4).3C(7)H(8).2CH(3)CN (3-ClO4) as well as the mu-oxo iron dimer [(F -8-TPP)Fe]O-2.3C(7)H(8) allows for direct structural cc,mparisons, as well as with other relevant compounds. Compound 3 also possesses a lin ear mu-oxo-bridging ligand, where angle Fe-O-Cu = 178.2-(4)degrees; un usually short metal-oxygen distances of Fe-O = 1.740(5) Angstrom and C u-O = 1.856(5) Angstrom are observed, and Fe...Cu = 3.596(2) Angstrom. The coordination geometry around Cu is distorted from the usual (i.e. , for TMPA-Cu(II) complexes) trigonal-bipyramidal configuration toward a square-based pyramidal arrangement, and the three pyridine rings ar e situated between the four F-8-TPP phenyl rings, thus leaving one ''s lot'' free. H-1-NMR spectra of 3-ClO4 in CD2Cl2 exhibit a pyrrole sign al at 65 ppm (corroborated by H-2-NMR) which is upfield shifted relati ve to ol her axially symmetric high-spin (tetraphenylporphyrinate)iron (III) complexes; this is consistent with electronic/maegnetic coupling of the Fe(III) and Cu(II) centers. Additional upfield-shifted signals are tentatively assigned to the TMPA ligand on copper, thus implicati ng a bridged structural arrangement which is maintained in solution. A detailed magnetic Mossbauer spectroscopic study (4.2 K at 0, 0.5, and 8 T) has been carried out. The data unambiguously indicate that 3 pos sesses a high-spin ferric (S-Fe = 5/2) ion and exhibits characteristic parameters: D = 6.0 +/- 0.5 cm(-1) E/D = 0.03 +/- 0.01, Delta E(Q) = -1.26 +/- 0.02 mm/s, eta = 0, delta = 0.46 +/- 0.01 mm/s, and A/g(n) b eta(n) = -23.7 +/- 0.3 T. Furthermore, the magnetic properties observe d, i.e., in the presence or absence of an applied field, demonstrate t hat the ferric ion is spin-coupled to the cupric ion to form an overal l integer spin system in [(F-8-TPP)(FeO)-O-III-Cu-II(TMPA)](ClO4) (3-C lO4). The Mossbauer isomer shift and quadrupole splitting parameters c ompare favorably with those previously obtained for data on enzyme res ting-state preparations, although they differ in certain details. The conclusion that 3-ClO4 is an S = 2 system is determined by multifield saturation magnetization measurements, carried out both in solution (2 -200 K) and the solid state (30-300 K). An exact magnetic coupling con stant J (for exchange coupling according to (-2JS(1).S-2)) could not b e determined from the solution data, due to the inherent ''noise'' fou nd in these measurements, even for similar to 1 mM solutions. These fi ndings carl into question previous evaluations for the lower limit of \J\ greater than or equal to 200 cm(-1) for the dinuclear Fe(III)-X-Cu (II) site in as-isolated ''resting'' oxidized cytochrome c oxidase enz yme; a lower limit of \J\ greater than 50 cm(-1) is suggested as a bet ter estimate. Solid-state measurements on 3-ClO4 give a value of -J = 87 cm(-1), i.e., moderately strong antiferromagnetic coupling between high-spin iron(III) and copper(II) ions, consistent with the NMR and M ossbauer spectroscopic data. The possible biological relevance of the oxo-bridged complex [(F-8-TPP)Fe-III-O-Cu-II(TMPA)]ClO4) (3-ClO4) and its observed magnetic/electronic properties are discussed.