Part I. Spin-state isomerism in crystalline (trifluoromethanesulfonato)(meso-tetraphenylporphinato)iron(III). Part II. Toward refined model compounds for the cytochrome c oxidase active site: A new picket-fence porphyrin with short imidazole pickets

Part I. The unsolvated porphyrin compound (Fe$\sp{\rm III}$(TPP)(OSO$\sb2$CF$\sb3$)) has been structurally characterized by single-crystal X-ray diffraction in a monoclinic phase at 293 K and in a triclinic phase at 293, 188 and 103 K. While only one type of molecular site is found in the monoclinic phase, the temperature-dependent structural parameters and magnetic susceptibility data (5.82 $\mu\sb{\rm B}$ at 293 K; 4.86 $\mu\sb{\rm B}$ at 20 K) together indicate the existence of two crystallographically and magnetically distinct spin-admixed crystal lattice sites in the triclinic phase. One site (molecule 1) is unique in that its structure is temperature dependent, whereas the second site (molecule 2) has a structure which is essentially independent of temperature. This distinct site assignment has been further investigated by Mossbauer and EPR spectroscopies which suggest different spin ground states for molecules 1 and 2. This triclinic phase is also the first report of molecule pairs of the same porphyrin complex interacting in very different ways in the same crystal lattice in that molecule 1 pairs form $\pi$-$\pi$ dimers, whereas molecule 2 pairs do not. Part II. A new binucleating picket-fence porphyrin ligand, 5,10,15,20-Tetrakis (o-4-methylimidazole-5-ethylcarboxyl-2-oxy)phenyl) porphyrin, N$\sb4$-PH$\sb2$, has been synthesized and characterized to be used eventually as a vehicle to study the structure, function and reactivity patterns of the active site of cytochrome c oxidase. The enzyme itself contains an (Fe(porphyrin)$\cdots$Cu) binuclear active site of unknown structure. The properties which set N$\sb4$-PH$\sb2$ apart from other binucleating picket-fence porphyrin ligands previously prepared as active site ligating molecules are the four authentic imidazole nitrogen donor atoms in the potential binding site for the Cu center and a separation between potential metal binding sites of only 3.5 to 4.0 A, made possible by the ligand's exceptionally "short" picket-fence arms derived from imidazole-ether linkages