Tailoring the Photophysical Properties of a Homoleptic Iron(II) Tetra N-Heterocyclic Carbene Complex by Attaching an Imidazolium Group to the (C∧N∧C) Pincer Ligand—A Comparative Study

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)2](PF6)4 (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)2](PF6)2 (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)2](PF6)2 (cpbmi = 1,1′-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)2](PF6)4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)2](PF6)4 a promising pH-insensitive analogue of [Fe(cpbmi)2](PF6)2. Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)2](PF6)4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.


S1. Synthesis and structure identifications
All commercial reagents and solvents were used as received unless otherwise stated. 1 H and 13 C NMR spectra were recorded on a Bruker Avance II 400 MHz NMR spectrometer.Chemical shifts (δ) are reported to the shift-scale calibrated with the residual NMR solvent; CD3CN (1.94 ppm for 1 H NMR spectra).Electrospray ionization-high resolution mass spectrometry (ESI-HRMS) and atmospheric pressure chemical ionization (APCI) for mass spectrometry were recorded on a Waters Micromass Q-Tof micro mass spectrometer.Infrared spectra were recorded as the neat compound on a Bruker Alpha-P FTIR spectrometer.Melting points of the compounds were measured on a Stuart Scientific Melting Point Apparatus-SMP3 and were corrected using standard substances.Elemental analyses were performed by Mikroanalytisches Laboratorium KOLBE (Mülheim an der Ruhr, Germany).Size of the bio-beads column is 120 cm in length and 4.5 cm in width.
After cooling at room temperature, a saturated solution of KPF6 was added to precipitate the product.The resulting precipitate was washed with distilled water and dried under vacuum to afford the white solid as a product (1.39 g, Yield: 92%); Mp: 273

S2. Single Crystal X-ray diffraction
All SC-XRD measurements were performed using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) using the Agilent Xcalibur Sapphire3 diffractometer high-brilliance IμS radiation source.Data collections were performed at 295 K for all the structures.The structure was solved by direct methods and refined by full-matrix least-squares techniques against F2 using all data (SHELXT, SHELXS).S1,S2 All non-hydrogen atoms were refined with anisotropic displacement parameters if not stated otherwise.Absorption was corrected using multi-scan empirical absorption correction with spherical harmonics as implemented in the SCALE3 ABSPACK scaling algorithm.S3 Hydrogen atoms were constrained in geometric positions to their parent atoms using OLEX2 software.

S3. Mößbauer spectroscopy
Mössbauer measurements were carried out in an Oxford Instrument flow cryostat at 295 K and 85 K, using a 57CoRh source held at room temperature.The studied powder materials were mixed with inert BN, pressed and formed as disc absorbers with a concentration of about 52 mg/cm2 of studied substances.Calibration spectra were recorded from a natural iron metal foil held at 295 K.The resulting spectra were analysed using a least square Mössbauer fitting program.

S4. Electrochemistry and spectroelectrochemsitry
Electrochemical and spectro-electrochemical measurements were carried out in a standard three electrode setup consisting of a working (1 mm dia., glassy carbon, CH Instruments), counter (platinum rod in a separate compartment) and reference electrode (0.01 M Ag + /Ag).

S5. Steady State Absorption Spectroscopy
Steady-state absorption measurements were performed in a Perkin Elmer Lambda 1050 Spectrophotometer.The complex was weighed and dissolved in filtered acetonitrile collected from a dry solvent dispenser (Innovative technology, PS-micro).In volumetric flasks, a dilution series was prepared by taking variable amounts of the stock solution and transferring it with a graded pipette, see Figure S14.Absorbance of all prepared concentrations was measured in a standard quartz-glass cuvette of path length 1 mm (Hellma -Optical Special Glass).For reference, the same cuvette with pure solvent was measured.The extinction coefficient was evaluated where appropriate by performing a linear fit to the absorbance as a function of concentration for each wavelength (see Figure S15) after the background had been corrected.

S6. Transient absorption spectroscopy
Transient absorption (TA) spectroscopy was performed using an in-house build setup.The basis of this setup is a Spitfire Pro XP (Spectra Physics) laser amplifier system that produces ~80 fs pulses at a central wavelength of 796 nm at 1 kHz repetition rate.The amplifier output is divided into two parts that each pump a collinear optical parametric amplifier (TOPAS-C, Light Conversion).One of the TOPAS generates the pump beam (wavelength roughly set to the absorption maximum of each sample, here ~500 nm), while the other one generates a NIR beam (1350 nm) that is focused onto a 5 mm CaF2 crystal to generate a supercontinuum probe beam.The delay between the pump and probe beams is introduced by a computer-controlled delay stage (Aerotech) placed in the probe beam's path.After supercontinuum generation the probe pulses are split into two parts: the former being focused to ~100 µm spot size and overlapping with the pump pulse in the sample volume, and the latter serving as a reference.
After passing the sample the probe beam is collimated again and relayed onto the entrance slit of a prism spectrograph.The reference beam is directly relayed on the said spectrograph.Both beams are then dispersed onto a double photodiode array, each holding 512 elements (Pascher Instruments).The intensity of excitation pulses was set to roughly 1 mW.Mutual polarization between pump and probe beams was set to the magic angle (54.7°) by placing a Berek compensator in the pump beam.
A solution of [Fe(miHpbmi)2](PF6)4 in filtered acetonitrile was filled in a 1 mm optical path length cuvette (Hellma -Optical Special Glass) and measurements were performed at room temperature.The measured samples were translated after each scan to avoid photodegradation.
To check for the stability of each sample steady-state absorption spectra were measured before and after TA experiments.Before analysis, the measured data were corrected for group velocity dispersion (GVD -"chirp") using the KiMoPack software.see Figure S23, with only slight variations in the fitted oscillatory parameters, see Table S5.       2 At 15°C.

S7. Quantum Chemistry
Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations were performed by using the program Gaussian 09 S7 using the B3LYP* S8 level of theory with the basis set 6-311G(d) S9,S10 , modelled in an empirical solvent model of acetonitrile.Kohn-Sham orbitals and spin density contours were visualized in the program GaussView.
48 h over 3 Å activated molecular sieves was used as solvent, together with 0.1 M tetrabutylammonium hexafluorophosphate (electrochemical grade, Sigma) dried for 24 h under vacuum at 80 o C as supporting electrolyte.Sample solutions were deaerated by purging with solvent saturated Ar.Cyclic voltammograms were record at 0.05 V/s, and differential pulse voltammograms with step potential: 5 mV, modulation amplitude: 25 mV, modulation time: 0.05 s, interval time: 0.1 s.UV-Vis spectroelectrochemistry was carried out during controlled potential electrolysis in the same cell by switching the working electrode to a platinum mesh electrode placed in the 1 mm optical path.An Autolab potentiostat (PGSTAT302) was used to control the three-electrode setup using the GPES 4.9 software, and an Agilent 8453 diode array spectrophotometer was used to record the spectral traces.

Figure S14 .
Figure S14.Absorption spectra of the dilution series of five different samples of

Figure S15 .
Figure S15.Linear fit of absorbance versus concentration of [Fe(miHpbmi)2](PF6)4 at 495nm S5 Data were fitted by using the KiMoPack global analysis software, not assuming any model only fitting a sum of exponential decay components convoluted with a rise component determined by the instrument response function ~90 fs.

Figure S16 .
Figure S16.Decay associated spectra resulting from a global fit of the transient absorption

Figure S17 .
Figure S17.Transient absorption kinetic at 550 nm, fitted with a damped cosine function added

Figure S22 .
Figure S22.Decay associated spectra resulting from a global fits of the transient absorption data measured in a) H2O, b) DMSO and c) 50/50% mixture of THF and MeCN, cut at 0.5 ps, corrected for background, chirp and cut to avoid excitation scatter.

Table S5 .
Summary of the oscillations fitted for the transient absorption kinetic at 560 nm of [Fe(miHpbmi)2](PF6)4 in different solvents.The solvent polarity is also included in the table.

Table S6 .
Mulliken spin density on Fe, average Fe-C/N bond length and state energy relative to the 1 GS state of the optimized [Fe(miHpbmi)2](PF6)4 states shown in FigureS24.

Table S7 .
Single point energy at different optimized geometries for singlet, triplet and quintet surfaces.The potential energy surface landscape is visualized in the main manuscript, Figure8a.The optimized 1 GS is put to 0 eV.

Table S8 .
The first 40 allowed singlet-singlet transitions from the 1 GS to higher singlet states of [Fe(miHpbmi)2](PF6)4, calculated by TD-DFT.The nature of each transition is deemed by the involved Kohn-Sham molecular orbitals.