The complex (6,13-bis(ethoxycarbonyl)-5,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,6,12,14-tetraenato(2−))iron(III) iodide, 3 (chemical formula C18H26O4N4FeI·0.5 C7H8), has been characterized by X-ray crystallography (a = 16.063(3) Å, b = 15.895(3) Å, c = 19.703(4) Å, β = 107.11(3)°; monocline; I2/a; Z = 8), EPR and Mössbauer spectroscopy, and temperature-dependent magnetic susceptibility as well as by MO calculations. The pentacoordinated iron atom is 0.34 Å outside of the [N4] plane. The equatorial Fe−N distances (1.92 Å for the N's in neighborhood of the methylated C-atoms and 1.88 Å for the others) are not significantly different from those found in octahedral low-spin iron(III) complexes with the same macrocyclic [N4 2-] ligand. The magnetic moment (μ eff = 4.19 μB up to 20 K; ϑ = −7.37 K), the EPR measurements (g y eff = 5.38, g x eff = 2.70, and g z eff = 1.74), and the Mössbauer data (δ = 0.18 mm/s, Δ E Q = 3.56 mm/s, T = 120 K) indicate a pure intermediate-spin state of S = 3/2. The MO calculations yield a complete picture of bonding modes, intramolecular interactions, and the electronic structure around the iron center, as indicated by the correct determination of the ground state and by the agreement between the measured and calculated quadrupole splitting.

Metal Complexes of Unsymmetrically Bridged IN4] Macrocycles: Synthesis, Structure and Redox BehaviourMetal complexes M-5a (M = Cu, Ni, CO, Fe, and I-Fe") of 6,13-Di(ethoxycarbonyl)-7,12-dimethyl-benzo[ b]-l,4,8,11tetraaza-cyclotetradeca-2,5,7,11,13-pentaene and the free ligand H2-5a have been synthesized. The crystals of Cu-5a . H20 and Ni-5a . H 2 0 consist of planar complex molecules which are linked by H bridges between the water molecules and the peripheric ester groups to give 1D chains. The average M-N-distances are equal for both complexes (Cu 1.928, Ni 1.924 A), being unexpectedly large for the nickel complex. 'The redox properties of all the new compounds as well as some acetyl derivatives M-5b have been examined by cyclic voltammetry and compared with those of the previously re-ported symmetrically phenylene (M-4) and ethylene (M-6) bridged derivatives. All complexes give a reversible single electron reduction which clearly shows the increasing electron density around the central atom in the order M-4 < M-5 < M-6. Like the carbonyl substituted tetraazaannulenes M-4 the copper complexes Cu-5 give two, the cobalt complexes CO-5 three, and the iron complex Fe-5a probably four reversible single electron oxidations. They can be attributed to the species [M"'L-I']+, (M"L0J2+, [M"1Lo]3f, and [Fe1VLo]4+ with Lo being a "non innocent" neutral 1,2-quinodiimine ligand. Very surprisingly, since in contrast to Ni-4 and Ni-6, the nickel complexes Ni-5 gave no reversible oxidation. Makrocyclisch [Ni-1-koordinierte Chelatkomplexe der 3d-Elemente wirken als Aktivzentren vieler Biokatalysatoren. Beispiele sind das allen Hamenzymen zugrunde liegende Eisenporphyrin 1, das Corrin 2 der B12-Enzyme und das Dihydrocorphingeriist 3 des Cofaktors F 430[']. Obwohl sie fur unterschiedlichste Prozesse der biologischen Stoff-und Energiewandlung funktionsoptimiert sind, zeigen sie eine Reihe ubereinstimmender Strukturmotive, die offenbar gunstige Voraussetzungen fur eine Steuerung der katalytischen Eigenschaften biecen: Das Zentraiion bildet mit dem Bquatorial koordinierten, anionischen [N4]-Liganden mindestens zwei konjugiert-ungesattigte Chelatsechsringe aus. Hinsichtlich des x-Elektronensystems und der Ringgliederzahl m des inneren Makrocyclus sowie der Ladung z des Ligandanions bestehen charakteristische Unterschiede (1: 18 n; m = 16, z = -2; 2: 14 x; m = 15, z = -1; 3: 12 x; m = 16, z = -1). Die fur das Design biomimetischer Katalysatoren wichtige Frage, wie derartige Strukturmerkmale das katalytische Leistungsvermogen und die dafur entscheidenden Eigenschaften solcher Komplexe beeinflussen, 1al3t sich bisher kaum beantworten. Vor 30 Jahren erhielten wir uber eine Templatsynthese erstmals Komplexe des ,,Tetraaza[14]annulen6'-Typs 4, die sich von cyclischen 2:2-Kondensationsprodukten aus p-Dicarbonylverbindungen und 1,2-Phenylendiamin ableitenL'1. Sie unterscheiden sich vom inneren Makrocyclus des Por-1 4 4-8 I R' R2 R3 2 R2 5 X=C2H, p 3 R3&R1 / N \ A X M X ' 6 phyrinsystems nur durch das Fehlen zweier Methingruppen (16 n; m = 14...

COMMUNICATIONSMW but a measure of the hydrodynamic volume. Thus, by SEC using randomly coiled polystyrene standards, the number average molecular weights (M,) of rigid-rod polymers are usually greatly overestimated relative the actual molecular weights. Accordingly, the SEC-determined M , values of the octamer 20 ( M , = 2800, actual MW =1981) and the 16-mer 23 ( M , = 6650, actual MW = 3789) are much greater than the actual MWs. These results further confirm the formation of the 16-mer 23. Also, as predicted, the monomer 2, dimer 8, and tetramer 14 have M , values that are reasonably close to the actual MWs (slope = 1 .O in Fig. 3) because they are in the low M W region, in which the polystyrene standards are not coiled. In all cases, the SEC-determined values of M,/M, are 1.04-1.07. Thus Figure 3 can serve as a useful calibration chart for very rigid linear oligomers as they compare to polystyrene standards by SEC. 0 . I. Paynter, D.

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SummaryTo obtain more insight into catalytic mechanisms of metallo enzymes and specific metal complexation by proteins we use linear and cyclic pseudopeptides as mimetics. Knowledge about tendencies of complex formation of different ligands with selected transition metal ions is an indispensable prerequisite for the development of homoand hetero-dinuclear metallo enzyme mimetics. Three pseudotripeptide ligands were investigated with respect to formation tendency and properties of complexes with the transition metal ions Cu 2+, Zn 2+, Ni 2+, Co 2+ and Mn 2+. To study complexation tendencies we applied different methods. One of the important prerequisites for the application in a screening of series of peptide ligands is the necessity for a minimal amount of substance. We used and compared certain masspectrometric methods for the estimation of a rank order of complexation of certain transition metal ions. We also applied spectrophotometric titration, circular dichroism measurements, capillary electrophoresis and pH-rate profile of catalytic activity in the attempt to evaluate complex formation tendencies. Except for the spectrophotometric pH-titration and the pH-profile of catalytic activity all methods were applicable, but each method has its advantages and disadvantages depending on the separation effect of the ligand from the metal complex, and depending on the spectroscopic properties of ligand and complex. The results regarding complex formation are compared to each other. Comparison of pairs by MALDI-TOF-and ESI-MS allows an estimation of the rank order of complexation tendency of one ligand with different metal ions and requires the least amount of substance. The other investigated methods provided additional information on structural properties of the formed complexes; however either they required too much pseudopeptide ligand or were not applicable for all transition metal ions used in this study.