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The iron(II1) "picket-fence" porphyrin complex [ Fe1ii(TPpivP)(OS0,CF3)(H20)] (1) was synthesized and characterized by its UV-visible, 'H NMR, EPR, magnetic, and Mossbauer properti'es. The X-ray structure of 1 was determined at -100 OC. Crystal data: [Fe(TPpivP)(OS0,CF3)(HzO)] (C65H6sN80sF3sFe); monoclinic; a = 13.161 (3), b = 19.196 (6), c = 26.212 (6) A; 0 = 103.34 (2)"; Z = 4; d,,, = 1.270 g cm-); space group P2i/c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-N, = 2.021 (16) A), to an oxygen atom of the triflate ion (Fe-O(triflate) = 2.188 (5) A), placed inside the molecular cavity of the picket-fence porphyrin, and to a water molecule (Fe-O(water) = 2.133 (5) A). The susceptibility measurementsin an external field of 1.5 T show that the effective magnetic moment varies from 4.0 to 5.7 pB in the temperature range 2-300 K. The EPR data yield g, = 5.7, which corresponds to a mixture of 85% spin sextet $Ai and 15% spin quartet 'A2 in the lowest Kramers doublet. This mixture is somewhat stronger than in typical high-spin iron porphyrins. Mossbauer spectra were recorded at temperatures varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature-independent quadrupole splitting, AEQ = 2.2 mm s-l, which lies between the AEQ values characteristic for high-spin (S = 5/2) and intermediate-spin (S = 3/z) porphyrins. The magnetic hyperfine patterns of the measured Mossbauer spectra, in the slow and fast relaxation limit, are successfully simulated within the IO-state model for the spin mixture between 6A, and 4A2 by using parameters that have been derived from susceptibility and EPR data within the same model. The applicability of the usual spin (S = 5/2) Hamiltonian analysis and its relation to the IGstate model are discussed. Spinspin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime within the framework of the (S = 5/z) spin Hamiltonian. The degree of spin mixture together with the axial and porphyrinato-nitrogen coordination of iron in 1 and related complexes is discussed on the basis of a putative spin-state/stereochemical relationship.

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The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)↔(2)T(2) transition curve is sigmoidal with T(½)∼280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.