Synthesis, Structural, Magnetic, and Redox Properties of Asymmetric Diiron Complexes with a Single Terminally Bound Phenolate Ligand. Relevance to the Purple Acid Phosphatase Enzymes

Abstract

New asymmetrical ligands (H 2 L) have been synthesized to provide both a bridging and a terminal phenolate to a pair of iron ions in order to mimic the binding of a single terminal tyrosinate at the diiron center of the purple acid phosphatases. H 2 L1 is 2-[(bis(2-pyridylmethyl)amino)methyl]-6-[((2-pyridylmethyl)(2-phenol)amino)methyl]-4-methylphenol and H 2 L′1 and H 2 L2 are obtained by replacing the 2-phenol group by the 5-nitro-2-phenol and the 6-methyl-2-phenol residues, respectively. A series of mixed valence diiron complexes [Fe II Fe III L(X) 2 ](Y) have been obtained where (X) 2 is the dianion of m-phenylenedipropionate or (H 2 PO 4 ) 2 and Y )Diferric complexes have been obtained also either by direct synthesis or by iodine oxidation of the mixed valence precursor (L ) L1, 3a (X) 2 ) mpdp, Y ) BPh 4 , 3d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 ; L ) L2, 4d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 . Complex 1a [Fe II Fe III L(mpdp)](BPh 4 ) has been characterized by X-ray diffraction techniques. 1a crystallizes in the monoclinic space group P21/a with the following unit cell parameters: a ) 22.038 (9) Å, b ) 16.195 (8) Å, c ) 16.536 (7) Å, β ) 97.26 (1)°, Z ) 4. The significant differences in the Fe-O bond lengths indicate that the metal centers are ordered. The complexes have been studied by electronic spectral, resonance Raman, magnetic susceptibility, Mo ¨ssbauer, NMR, and electrochemical techniques. Mo ¨ssbauer and NMR spectroscopies concur to probe that the valences of the mixed valence compounds are trapped in solution as well as in the solid state at room temperature. The electronic spectrum of the mixed-valence compounds are dominated by a charge transfer transition in the 400-600 nm domain which moves to the 550-660 nm range upon oxidation to the diferric state. In addition they exhibit a weak and broad intervalence transition close to 1100 nm. Electrochemical studies show that the systems exist in the three redox states Fe

A new heterobinuclear FeIIICuII complex with a single terminal FeIII–O(phenolate) bond. Relevance to purple acid phosphatases and nucleases

“…Finally, pK a3 is ascribed to the deprotonation of the Cu-H 2 O [33,34,36]. The observed lability of the metal-carboxylate bonds in related complexes [57], and the need for a nucleophile to initiate catalysis are consistent with this proposal. However, from the pH dependence of its kinetics parameters (see later) it appears that 1 has only two protonation equilibria relevant to catalysis.…”

New dinuclear Co(II) and Mn(II) complexes of the phenol-based compartmental ligand containing formyl and amine functions: structural, spectroscopic and magnetic properties

“…The second peak, located at 1051 nm for the perchlorate complex and 1042 nm for the tetrafluoroborate complex corresponds to the 4 T 2g 4 T 1g (F) d-d transition. In addition, in the spectra of both complexes another peak is observed at approximately 520 nm, as a shoulder of the LMCT transition band, which corresponds to the 4 T 1g (P) 4 T 1g (F) transition. The latter two bands are typical for d-d transitions in octahedrally surrounded Co(II) ions [21].…”

“…Since then, a very large number of such ligands were designed, and their coordination compounds were thoroughly investigated. The possible applications of the complexes with this type of ligands vary from modeling the active sites of many metalloenzymes [2][3][4], to hosting and carrying small molecules [5][6][7] or catalysis [8,9].…”

The Catalytic Mechanisms of Binuclear Metallohydrolases

“…Often, synthetic models are binuclear complexes containing symmetrical ligands, since the preparation of the ligands is greatly simplified. [105][106][107] However, unsymmetrical ligands presenting two potentially different coordination environments allow the generation of heterobinuclear Fe(III)-M(II) complexes. 108,109 Such systems include a single terminal Fe-O phenolate bond which is crucial in mimicking structural, spectroscopic, electrochemical, and catalytic properties of PAPs.…”