A Straightforward Electrochemical Approach to Imine‐ and Amine‐bisphenolate Metal Complexes with Facile Control Over Metal Oxidation State

Abstract Synthetic methods to prepare organometallic and coordination compounds such as Schiff‐base complexes are diverse, with the route chosen being dependent upon many factors such as metal–ligand combination and metal oxidation state. In this work we have shown that electrochemical methodology can be employed to synthesize a variety of metal–salen/salan complexes which comprise diverse metal–ligand combinations and oxidation states. Broad application has been demonstrated through the preparation of 34 complexes under mild and ambient conditions. Unprecedented control over metal oxidation state (MII/III/IV where M=Fe, Mn) is presented by simple modification of reaction conditions. Along this route, a general protocol‐switch is described which allows access to analytically pure FeII/III–salen complexes. Tuning electrochemical potential, selective metalation of a Mn/Ni alloy is also presented which exclusively delivers MnII/IV–salen complexes in high yield.


General considerations
Where stated, manipulations were performed under an atmosphere of dry nitrogen by means of standard Schlenk line or Glovebox techniques. Anhydrous solvents were prepared by passing the solvent over activated alumina to remove water, copper catalyst to remove oxygen and molecular sieves to remove any remaining water, via the Dow-Grubbs solvent system. Deuterated chloroform and acetonitrile were dried over CaH 2 , cannula filtered or distilled, and then freeze-pump-thaw degassed prior to use. All other reagents and solvents were used as supplied. 1 H and 13 C NMR spectra were recorded on a Bruker DPX300 spectrometer or a Bruker AV500 Commercially available AAS standard solutions were obtained as 1000 mgmL -1 stock and diluted as required using ultra-pure water.
A PSD 30/3B high performance digital power supply was used in constant voltage mode (CV), with current measurements made via a 15XP-B Amprobe digital multimeter, at a milliampere scale.
to 0 ˚C, upon which a bright orange precipitate formed which was collected via vacuum filtration, washed with cold ethanol (10 mL) followed by diethyl ether (3 × 30 mL) and dried in vacuo to deliver the title compound as a microcrystalline orange solid. Yield: 5.76 g, 18.2 mmol, 89 %. 1

Synthesis of Zn II -salen complexes, 4a-e
General procedure

Synthesis of Mn IV -salen complexes, 7a-e
General procedure

Synthesis of Mn II -salen complexes, 8a, b, e
A flame-dried three-necked round bottomed flask equipped with stirrer bar was charged with salen precursor (1.0 mmol), tetrabutylammonium tetrafluoroborate (0.03 mmol) and further dried in vacuo.

Cyclic voltammetry measurements
Electrochemical measurements were conducted using an Autolab PGSTAT20 voltammetric analyser under an argon atmosphere, solvated in pre-dried/degassed CH 3

Atomic absorption spectroscopy
Atomic absorption spectroscopy (AAS) was used to determine the presence (or absence) of elements (Mn and Ni) within bulk samples of 7a-c, which were dissolved in 30 % nitric/10 % hydrochloric acid solution.

X-ray fluorescence analysis
All samples were run as solids on a Horiba XGT7000 X-ray analytical microscope instrument using a partial vacuum (X-ray voltage = 30 kV; preset time = 60 s; current set to auto; process time = 6; XGT beam size = 1.2 mm with no filter (F4) or Nb filter (F3). Solid samples were mounted on a glass slide and sealed using sellotape.
The first reference sample was benzylmanganese(I) pentacarbonyl ( Figure S14)data was collected at three separate positions to ensure that the sample was homogeneous. Sample 7a was analysed under the same conditions as for the reference samples ( Figure S16). In total, six independent measurements were made to examine for sample homogeneity (with F3 applied). In  independent measurements were made to examine the sample homogeneity. It was found that Ca and Cl (K lines at 2.816/2.621 KeV) were present in trace amounts when no filter (F4) was used ( Figure   S17). The Ca and Cl signals were lost using the F3 filter, with only Mn being visible ( Figure S18). Ni was not found within this sample, to the limits of detection. Figure S17. XRF analysis of sample 7b (F4).

Sample MRC261
The analysis would indicate that trace CaCl is present in the sample. For quantification, it would be necessary to calibrate the analysis sample using authentic CaCl samples.
Sample 7c was analysed under the same conditions as for the reference samples. In total, five independent measurements were made to examine the sample homogeneity. For two of the five samples, a trace signal that may be indicative of Ca, was observed when no filter (F4) was used in the measurement ( Figure S19), which was lost when the Nb filter (F3) was used ( Figure S20). In all cases, the Mn signal was dominant. Ni was not found within this sample, to the limits of detection.

Magnetic susceptibility measurements
The solution magnetic susceptibility measurements were made by NMR methods on a Bruker AV500 spectrometer at 298 K using a modified Evans method. 6 A flame-sealed coaxial insert containing a deuterated reference solvent (C 6 D 6 /C 6 H 6 , 50:50) was placed in a Young's NMR tube under an inert atmosphere. An anhydrous (anoxic) C 6 D 6 (100 %) solution of the paramagnetic complex was added to the tube and 1 H{ 13 C} NMR spectra recorded. The volume magnetic susceptibility (χ) of the paramagnetic compound was calculated from the following equation:

Hydrous procedure:
A pair of metal electrodes are prepared as above. A three-necked flask is charged with salen precursor, dissolved with HPLC-grade acetonitrile and both electrodes are introduced to the solution.
Whilst under an atmosphere of air, a potential is applied to the cell (see above).

Crystallographic details
X-Ray diffraction data were collected on an Agilent SuperNova diffractometer fitted with an Atlas CCD detector with Mo Kα radiation (λ = 0.7107 Å) or Cu Kα radiation (λ = 1.5418 Å). Crystals were mounted under oil on nylon fibres. Data sets were corrected for absorption using a multiscan method, and the structures were solved by direct methods using SHELXS-97/SHELXT and refined by fullmatrix least squares on F 2 using ShelXL-97, interfaced through the program Olex2. 8 Molecular graphics for all structures were generated using POV-RAY in the X-Seed program.
Crystallographic details for 2g: