Charge and Spin States in Schiff Base Metal Complexes with a Disiloxane Unit Exhibiting a Strong Noninnocent Ligand Character: Synthesis, Structure, Spectroelectrochemistry, and Theoretical Calculations

Abstract

Mononuclear nickel(II), copper(II), and manganese(III) complexes with a noninnocent tetradentate Schiff base ligand containing a disiloxane unit were prepared in situ by reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane followed by addition of the appropriate metal(II) salt. The ligand H2L resulting from these reactions is a 2:1 condensation product of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane. The resulting metal complexes, NiL·0.5CH2Cl2, CuL·1.5H2O, and MnL(OAc)·0.15H2O, were characterized by elemental analysis, spectroscopic methods (IR, UV-vis, X-band EPR, HFEPR, (1)H NMR), ESI mass spectrometry, and single crystal X-ray diffraction. Taking into account the well-known strong stabilizing effects of tert-butyl groups in positions 3 and 5 of the aromatic ring on phenoxyl radicals, we studied the one-electron and two-electron oxidation of the compounds using both experimental (chiefly spectroelectrochemistry) and computational (DFT) techniques. The calculated spin-density distribution and localized orbitals analysis revealed the oxidation locus and the effect of the electrochemical electron transfer on the molecular structure of the complexes, while time-dependent DFT calculations helped to explain the absorption spectra of the electrochemically generated species. Hyperfine coupling constants, g-tensors, and zero-field splitting parameters have been calculated at the DFT level of theory. Finally, the CASSCF approach has been employed to theoretically explore the zero-field splitting of the S = 2 MnL(OAc) complex for comparison purposes with the DFT and experimental HFEPR results. It is found that the D parameter sign strongly depends on the metal coordination geometry.

A five-coordinate manganese(iii) complex of a salen type ligand with a positive axial anisotropy parameter D

“…The lack of resonances in the high-field region of the spectrum indicated a positive axial zero-field-splitting (ZFS) parameter D. By using tunable-frequency EPR, 18 the spin Hamiltonian parameters were estimated. 16 However, we were unable to explain the low quality of the measured spectra, whether in the solid state or in solution. Being intrigued by these not very favourable results we decided to prepare closely related systems, in which the acetato ligand was replaced by exclusively monodentate ligands, such as NCS − or N 3…”

“…We recently reported on salen-type Schiff bases derived from 1 : 2 condensation reactions of 1,3-bis (3-aminopropyl) tetramethyldisiloxane and substituted 2-hydroxybenzaldehydes. 15,16 Copper(II) and manganese(III) complexes with these unusual Schiff bases were prepared and investigated by spectroscopic techniques and X-ray crystallography. A feature of note is the formation of a 12-membered central chelate ring, in which the tetramethyldisiloxane unit separates the aliphatic chains, reducing markedly the mechanical strain in the chelate ring via a "shoulder yoke effect".…”

“…These complexes were found to act as catalysts or catalyst precursors for the oxidation of some secondary alcohols to ketones using tert-butyl hydroperoxide as the oxidant in conjunction with low-power microwave irradiation. 17 The axially compressed octahedral complex MnL(OAc), where H 2 L is a Schiff base resulting from 2 : 1 condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,3-bis(3-aminopropyl)tetramethyldisiloxane, 16 appeared to be a suitable candidate for HFEPR investigation. However, we obtained a low quality spectrum of the polycrystalline solid at 10 K that did not permit a spectral analysis.…”

Hydrophobic, amorphous metal–organic network readily prepared by complexing the aluminum ion with a siloxane spaced dicarboxylic acid in aqueous medium

“…The addition of the sodium dicarboxylate solution over the solution containing an excess of aluminum salt creates the premises for coordinating the two carboxyl groups of the same molecule to different metal ions so as to form an extended rather than a molecular structure. The high flexibility of the internal siloxane bond would allow binding to the same metal center with the closure of a cycle as happens in the case of bis‐azomethines having the same siloxane‐based spacer . However, in all the structures, we have built up to this time with this acid, it binds to metal, either with both carboxyl groups at two different metal centers, or with only one group at one metal center, the other being protonated.…”

A Bis(µ‐chlorido)‐Bridged Cobalt(II) Complex with Silyl‐Containing Schiff Base as a Catalyst Precursor in the Solvent‐Free Oxidation of Cyclohexane

“…Being interested in metal complexes with ligands based on both silane and siloxane (either open‐chain or macrocyclic) we reported recently on metal complexes with Schiff bases resulting from reactions of 1,3‐bis(3‐aminopropyl)tetramethyldisiloxane with substituted 2‐hydroxybenzaldehydes,, pyrrole‐2‐carbaldehyde and 2,6‐diformyl‐4‐methylphenol (DFF), or trimethylsilylmethyl p ‐aminobenzoate with DFF . The tetranuclear copper(II) complexes [Cu 4 (µ 4 ‐O)(L 1 )Cl 4 ] and [Cu 4 (µ 4 ‐O)(L 2 ) 2 Cl 4 ], where H 2 L 1 is a macrocyclic ligand resulting from [2+2] condensation of 1,3‐bis(3‐aminopropyl)tetramethyldisiloxane and DFF and HL 2 is a 1:2 condensation product of trimethylsilylmethyl p ‐aminobenzoate and DFF, are effective catalyst precursors for hydrocarboxylation of a series of linear and cyclic alkanes.…”