Identification of a Two‐Coordinate Iron(I)–Oxalate Complex

Abstract Exotic oxidation states of the first‐row transition metals have recently attracted much interest. In order to investigate the oxidation states of a series of iron–oxalate complexes, an aqueous solution of iron(III) nitrate and oxalic acid was studied by infrared free liquid matrix‐assisted laser desorption/ionization as well as ionspray mass spectrometry. Here, we show that iron is not only detected in its common oxidation states +II and +III, but also in its unusual oxidation state +I, detectable in both positive‐ion and in negative‐ion modes, respectively. Vibrational spectra of the gas phase anionic iron oxalate complexes [FeIII(C2O4)2]−, [FeII(C2O4)CO2]−, and [FeI(C2O4)]− were measured by means of infrared photodissociation spectroscopy and their structures were assigned by comparison to anharmonic vibrational spectra based on second‐order perturbation theory.

S8. Natural population analysis (NPA) for the iron-oxalate complexes (energetically lowest-lying isomers) 12 S9. Optimized Cartesian coordinates in Å of the energetically lowest-lying isomers of the studied iron-oxalate complexes. 13

S1.1 IR-FL-MALDI
For IR-FL-MALDI measurements, an aqueous solution of iron(III) nitrate (c = 1.6 mmol L -1 ) and oxalic acid (c = 3.2 mmol L -1 ) dissolved in water/methanol (2:1, v/v) was used. The main components of the setup consist of a thin liquid µ-beam, an IR-laser (Photonic Industries, DP20-OPO) and a reflectron-type mass spectrometer. The liquid beam is injected into high vacuum via a 20 μm quartz nozzle generated by a HPLC pump. [1] For injection of the samples into the liquid beam line a high-pressure valve is used. The IR-laser emits nanosecond pulses at a pulse repetition rate of 1 kHz with a pulse energy of 1mJ at a wavelength of 2900 nm. The light is focused onto the liquid using a CaF2 lens with a focal length of 50 mm, generating a focus diameter of 56 μm and peak energy densities up to GW/cm 2 . The IR-light excites the OH-stretch of the water molecules, heats up and then disperses the liquid beam, and in turn desolvates the ions. It displays a linear response between liquid phase ion concentration and gas phase ion signal over several orders of magnitude. This soft and sensitive MS approach allowed us to detect ionic species in different oxidation states. [2] More details on the experimental setup and the method are given in Ref. [1,3] .

S1.2 Leipzig cyrogenic ion trap triple mass spectrometer
Nano-ionspray mass spectra and IRPD spectra were recorded using the Leipzig 6 K ion trap triple mass spectrometer described in detail elsewhere. [4,5] In brief, ions were transferred from a solution of iron(III) nitrate (c = 1.6 mmol L -1 ) and oxalic acid (c = 3.2 mmol L -1 ), dissolved in water/methanol (2:1, v/v) into the gas phase using a nanospray ion source. The ion beam was collimated and compressed in phase space in a He-buffer-gas filled radio frequency (RF) ion-guide, held at room temperature. Subsequently, the ions with m/z = 144, 188 and 232 were mass-selectedthey using a quadrupole mass filter and focused in a RF ring-electrode ion-trap. The trap was filled with deuterium (D2) buffer gas at a temperature of 13 K. The trapped ions were thermalized and weakly bound ion-messenger complexes with D2 are formed by many-body collisions. [6] Messenger-tagging ensures measuring an IRPD spectrum in the linear absorption regime and facilitates structural assignment based on a comparison to calculated vibrational spectra. Every 100 ms, all ions were extracted from the ion trap and focused into the center of the extraction region of an orthogonally mounted reflectron tandem mass spectrometer. IRPD spectra were obtained by irradiating the ions in the center of the extraction region with an IR laser pulse from a Nd:YAG laser (Continuum Surelight EX1) pumped OPO/OPA/AgGaSe2 laser system (Laser Vision) [7] operated at 10 Hz and with a bandwidth of approximately 3.5 cm -1 . IRPD spectra were recorded by continuously scanning the laser wavelength with a scan speed such that a TOF mass spectrum averaged over 20 laser shots was obtained every 2 cm -1 .
Typically, three to four scans were measured and averaged to obtain an IRPD spectrum. The photodissociation cross-section σIRPD was determined as described previously. [4,5]

S1.3 Computational details
Electronic structure calculations and vibrational frequency analysis were carried out with the Gaussian 16, rev C.01 program package [8] using the MP2 method [9] in combination with the def2-SVPD basis sets.
[10] The simulated vibrational spectra were obtained from unscaled anharmonic vibrational frequencies and intensities, and convoluting the corresponding stick spectra with a Gaussian line shape function (12 cm -1 full-widt-at-half-maximum). QTAIM calculations [11] for determination of the Bader charge on Fe were performed at the DFT (B3LYP) level for the MP2-optimized structures by means of the ADF2019 software package. [12,13] S2. IR-FL-MALDI (-) mass spectrum assignment

S3. Comparison of harmonic MP2 vs. B3LYP spectra
The harmonic vibrational spectra calculated at the DFT (B3LYP) and the MP2 levels of theory agree well for the iron-oxalate complexes [Fe (III) (C2O4)2] -( Figure S5a) and [Fe (I) (C2O4)] -( Figure S5c). However, for [Fe (II) (C2O4)CO2]the IR spectra of several isomers exhibit substantial differences (B3LYP vs. MP2). On example is shown in Figure  S1. The comparison reveals significant differences in the absorption frequencies of the antisymmetric and the symmetric CO stretching vibrations of the single CO2 moiety. As the comparison with the experimental IRPD spectrum indicates, the MP2 calculation overestimates the absorption frequencies of these bands. This overestimation is attributed to the slight difference in the geometry of the complexes optimized with the two different methods. As Figure S1 illustrates, the MP2 calculation predicts a planar geometry, with a dihedral angle between the intersecting planes of the oxalate and the CO2 moieties of 0°. In contrast, in the B3LYP-optimized structure, the oxalate and CO2 moieties are not co-planar, the dihedral angle being 28°. Figure S1. Comparison between the unscaled harmonic vibrational spectra of [Fe (III) (C2O4)2]as calculated at the DFT (B3LYP) and at the MP2 levels of theory (the def2-SVPD basis sets were used in both cases). The minimumenergy structures of [Fe (II) (C2O4)CO2]with M = 4, as identified at the two levels of theory, and the respective dihedral angles between the intersecting planes of the oxalate and CO2 moieties are also shown. Figure S2. Comparison of the IRPD spectra (red trace) of D2-tagged [Fe (III) (C2O4)2]to harmonic B3LYP (left) and MP2 (rigth) calculated spectra (scaled by 0.9928, the def2-SVPD basis sets were used in both cases) of the three energetically low-lying isomers with spin multiplicities 6, 4 and 2.  Figure S2.   Figure S4. Comparison of the IRPD spectra (green trace) of D2-tagged [Fe (I) (C2O4)]to harmonic B3LYP (left) and MP2 (rigth) calculated spectra (scaled by 0.9928, the def2-SVPD basis sets were used in both cases) of the energetically low-lying isomers with spin multiplicities 2, 4 and 6. Two distinct binding motifs of the oxalate ligand are distinguished and the corresponding types of structures are labeled "end-on" and "side-on". Table S4. ZPE-corrected relative energies (in kJ mol -1 ) of the energetically low-lying isomers of [Fe (I) (C2O4)]with various spin multiplicities (M). The corresponding structures are shown in Figure S4.   (C2O4)] -, all with spin multiplicity of M = 6, as discussed in the main text. NPA were calculated via the NBO 6.0 software [14] at the B3LYP/Zora scalar/TZ2P level of theory with the ADF2019 software package. [12,13]