Serguei Prikhodovski

Synthetic and structural studies on metal complexes of tripyrrin

A general two-step procedure for the synthesis of metallotripyrrinates TrpyMOAcf with M = Co(II), Cu(II), Zn(II) and Pd(II), and OAcf = trifluoroacetate, is described, starting from well-known monopyrrolic precursors and simple transition metal acetates. X-ray structural investigations were undertaken on four different complexes, and the results reveal, that the nature of the metal ion, rather than the ligand, determines the coordination geometry of these porphyrin fragment complexes. The finding of pseudotetrahedral and strained pseudoplanar coordination polyhedra at the metal centres makes a clear distinction between metalloporphyrins and metallotripyrrins and shows the latter to be related to some recently discovered metal chelates of macrocyclic porphyrin analogues.

Monomeric and polymeric copper and zinc tripyrrins

Neutral transition metal complexes of different alpha,omega-dimethyltripyrrins TrpyMX with M = Cu(II) and Zn(II) have been prepared with a variety of anionic halogeno and pseudohalogeno ligands X, and have been studied with respect to coordination modes and structural distortion. Only four- and five-coordinate species have been observed throughout the series. All four-coordinate species display unstrained, but distorted tetrahedral or strained and distorted square-planar coordination environments for zinc(II) and copper(II) species, respectively, thus following the expectations from simple ligand field arguments. Five-coordinate species do not form easily and were observed either in donor solvents or in the solid as 1D coordination polymers with distorted trigonal-bipyramidal coordination and different topologies.

Free-base tripyrrins

The hitherto unavailable free-base tripyrrins HTrpy have been prepared by cyanide promoted demetallation of a nickel(ii) tripyrrin and by acid-induced condensation of a diformylpyrrole and an alkylpyrrole with a sterically shielded alpha-position.

The first (tripyrrinato)nickel(ii) complexes, TrpyNiX with X = Cl, Br, I: synthesis, structures and solvent coordinationDedicated to Prof. Waldemar Adam on the occasion of his 65th birthday.

A first series of nickel(II)complexes TrpyNi(II)X of the new tripyrrolic ligand 2,15-dimethyl-3,4,8,9,13,14-hexaethyltripyrrin with X = Cl, Br and I was prepared and characterized by spectroscopic and structural means. The coordination geometry found for the four-coordinate, paramagnetic bromo- and iodo-derivatives in the solid state can best be described as distorted trigonal-bipyramidal with one ligand missing in the trigonal plane. For the chloro derivative, this empty site is occupied in the crystal by a water ligand. As proton NMR studies on the paramagnetic TrpyNiCl reveal, an equilibrium exists between the four- and five-, but not a six-coordinate form, and for pyridine-N-oxide as the fifth ligand thermodynamic data of the ligand association could be obtained by a temperature dependent NMR titration study.

Electrochemical investigations of tripyrrin complexes

Electrochemical investigations on divalent transition metal complexes with a conjugated linear tripyrrole ligand, namely 3,4,8,9,13,14-hexaethyl-2,15-dimethyltripyrrin (HTrpy) are reported. This tripyrrin ligand behaves as a tridentate monoanionic ligand and forms a series of neutral metal complexes of the type TrpyMX, where M = Zn(II), Cu(II), Ni(II) Co(II) or Pd(II) and X is a chloride anion (Cl-). The studied nickel, cobalt and zinc complexes undergo respectively three and two ligand-centered reversible one-electron reductions and a reversible ligand-centered one-electron oxidation. Only irreversible electron transfers are observed for TrpyPdCl, due to the instability of the reduced complex. TrpyCoCl does not undergo any metal-centered electron transfer which is unexpected compared to related complexes. The copper(II) complex shows an additional reduction step occurring on the metal, generating a stable copper(I) complex. A comparison of the redox behavior of tripyrrin complexes with porphyrins and linear tetrapyrrolic complexes, namely bidipyrrins, clearly indicates that the observed redox behavior of tripyrrin complexes is mainly metal dependent. The observed HOMO-LUMO gap in the studied series of tripyrrins is rather small (about 1.70 V) compared to porphyrins (2.25 V), but it is close to bidipyrrins (1.60 V).