d–d Dative Bonding Between Iron and the Alkaline‐Earth Metals Calcium, Strontium, and Barium

Abstract Double deprotonation of the diamine 1,1′‐(tBuCH2NH)‐ferrocene (1‐H2) by alkaline‐earth (Ae) or EuII metal reagents gave the complexes 1‐Ae (Ae=Mg, Ca, Sr, Ba) and 1‐Eu. 1‐Mg crystallized as a monomer while the heavier complexes crystallized as dimers. The Fe⋅⋅⋅Mg distance in 1‐Mg is too long for a bonding interaction, but short Fe⋅⋅⋅Ae distances in 1‐Ca, 1‐Sr, and 1‐Ba clearly support intramolecular Fe⋅⋅⋅Ae bonding. Further evidence for interactions is provided by a tilting of the Cp rings and the related 1H NMR chemical‐shift difference between the Cp α and β protons. While electrochemical studies are complicated by complex decomposition, UV/Vis spectral features of the complexes support Fe→Ae dative bonding. A comprehensive bonding analysis of all 1‐Ae complexes shows that the heavier species 1‐Ca, 1‐Sr, and 1‐Ba possess genuine Fe→Ae bonds which involve vacant d‐orbitals of the alkaline‐earth atoms and partially filled d‐orbitals on Fe. In 1‐Mg, a weak Fe→Mg donation into vacant p‐orbitals of the Mg atom is observed.


Synthesis of 1-Sr:
A 10 mL glass ampoule was charged with a solution of Sr[N(SiMe3)2]2 (100 mg, 0.245 mmol) in THF (4 mL) and a solution of 1-H2 (70 mg, 0.196 mmol) in THF (4 mL) was added. Subsequently the ampoule was frozen in liquid nitrogen and vacuum sealed. After warming to room temperature the ampoule was placed in a pre-heated oil bath at 125°C for 1 day which led to formation of a red solution in which small red crystals started to form. Subsequently the oil bath was slowly cooled to room temperature and large bright red crystals formed. To complete crystallization the ampoule was stored at room temperature for one week. The crystals were isolated by decantation, washed with THF (3x 0.5 mL) and dried under vacuum. 60 mg of bright red crystals suitable for X-ray crystallography were obtained. The mother liquor was dried under reduced pressure and the resulting powder was transferred in a 10 mL glass ampoule, suspended in THF (8 mL) and placed into a pre heated oil bath at 125°C until all solid had dissolved. Subsequently the oil bath was slowly cooled to room temperature and the ampoule was stored at room temperature for prolonged time (about 2 weeks) forming a second crop of crystals.

Synthesis of 1-Ba:
A 10 mL glass ampoule was charged with a solution of Ba[N(SiMe3)2]2(THF)2 (100 mg, 0.166 mmol) in THF (4 mL) and a solution of 1-H2 (50 mg, 0.140 mmol) in THF (4 mL) was added. Subsequently the ampoule was frozen in liquid nitrogen and vacuum sealed. After warming to room temperature the ampoule was placed in a pre-heated oil bath at 125°C for 2 days which led to formation of a red solution in which small red crystals started to form. Subsequently the oil bath was slowly cooled to room temperature and large bright red crystals formed. To complete crystallization the ampoule was stored at room temperature for one week. The crystals were isolated by decantation, washed with THF (3x 0.5 mL) and dried under vacuum. 47 mg of bright red crystals suitable for X-ray crystallography were obtained. The mother liquor was dried under reduced pressure and the resulting powder was transferred in a 10 mL glass ampule, suspended in THF (8 mL) and placed into a pre heated oil bath at 125°C until all solid had dissolved. Subsequently the oil bath was slowly cooled to room temperature and the ampoule was stored at room temperature for prolonged time (about 2 months) forming a second crop of crystals.
Subsequently the crystals were isolated by decantation, washed with THF (3x 0.5 mL) and dried under vacuum. An additional 11 mg of bright red crystals were obtained. Yield:  ,52.89;H,7.29;N,4.41;Found: C,52.58;H,7.19,N,4.46. Note: NMR spectra were recorded in pyridine at 100°C due to the poor solubility of the S5 compound. THF-d8 was observed in the 13 C-NMR, which was found to be an impurity in the batch of pyridine-d5 used for the 13 C-NMR sample.

Synthesis of 1-Eu:
1-H2 (50 mg, 0.140 mmol) and KN(SiMe3)2 (56 mg, 0.281 mmol) were dissolved in THF (2 mL) and stirred over night at room temperature. The solvent was removed under reduced pressure and the resulting residue was dried until a free flowing red powder was obtained. Subsequently the crude 1-K2 was redissolved in THF (10 mL), EuI2 (68.2 mg, 0.168 mmol) was added and the reaction was stirred for 3 days. The precipitated KI was removed by centrifugation and filtration. The Eu complex is clearly better soluble than the corresponding Sr complex. Dark red crystals suitable for X-ray diffraction were grown from the resulting clear solution by slowly evaporating of the solvent. The crystals were isolated by decantation, washed with n-hexane (3x 0.5 mL) and dried in vacuum yielding 40 mg of red-orange crystals. Yield: 40 mg; 0.069 mmol 49 %. Anal. Calcd. for C48H76Eu2Fe2N4O2 (MW = 1156.78 g/mol): C,49.84;H,6.62;N,4.84;Found: C,49.54;H,7.08;N,4.52; Note: NMR spectra could not be recorded due to the paramagnetism of this compound. Figure S1: 1 H NMR spectrum of 1-Mg in C6D6 + THF-d8 at 25°C.

General experimental information
Crystals were embedded in inert perfluoropolyalkyl ether (viscosity 1800 cSt; ABCR GmbH) and mounted using a Hampton Research CryoLoop. The crystal under investigation was then flash cooled to 100 K in a nitrogen gas stream and kept at this temperature during the experiment. The crystal structures were measured on a SuperNova Dual source diffractometer (Cu at home/near) with Atlas S2 detector using either a CuKα microfocus source (1-Mg, 1-Ca, 1-Sr) or a MoKα microfocus source (1-H2, 1-Eu, 1-Ba). The measured data was processed with the CrysAlisPro software package. [S4] Using Olex2, [S5] the structure was solved with the ShelXT [S6] structure solution program using Intrinsic Phasing and refined with the ShelXL [S7] refinement package using Least Squares minimization. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms (if not otherwise stated below) were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters.
Crystals of compound 1-H2 were found to be twinned (racemic twinning) and in addition, the compound is heavily disordered. The fractional contributions of the two twin domains were refined to 0.51(2) and 0.49(2). The disorder was modeled with the help of similarity restraints (SADI) and rigid bond restraints (RIGU). [S8] . The relative occupancies of the two alternative orientations of the molecule were refined to 0.521(5) and 0.479(5). The positions of the amine hydrogen atoms were observed from difference Fourier maps and refined with restraints (SADI).
In case of 1-Ca, the hydrogen atoms H16A and H16B of the CH2 group adjacent to N2 were observed from difference Fourier maps and refined. The same applies to 1-Sr.

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Crystallographic Data has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1996024 for 1-H2, CCDC 1996025 for 1-Mg, CCDC 1996026 for 1-Ca, CCDC 1996027 for 1-Sr, CCDC 1996028 for 1-Eu and CCDC 1996029 for 1-Ba. This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Table S1: Crystal data and structure refinement for 1-H2.

Identification code hasj190902a
Empirical formula C20H32FeN2    ORTEP plot). Hydrogen atoms are omitted for clarity.
S28 Table S3: Crystal data and structure refinement for 1-Ca.

UV/Vis Spectra General Information
All UV/Vis spectra were recorded under rigorous exclusion of moisture and oxygen in screw sealed, gas tight, glass cuvettes, using an Agilent Technologies Cary 60 UV-vis spectrometer. All samples were prepared in a glovebox. The cuvettes were treated with t-BuLi (1.7M in pentane) and washed with n-pentane prior to usage. All compounds were measured in freshly distilled and thoroughly dried pyridine. (Sigma Aldrich, ACS reagent grade, distillation over freshly ground CaH2, additional drying over activated MS 3Å). All complexes are intensely colored. Due to the very high air and moisture sensitivity of the compounds and unavoidable traces of water in the pyridine (< 10 ppm) a dilution to the level necessary for applying Lambert-Beer-Law (absorbance below 1.0 a.u. with respect to the first peak in the UV-region) could not be achieved without considerable complex hydrolysis. The UV/vis spectra were recorded at the concentrations of given in Table S7.
Since we could not dilute these solutions any further due to hydrolysis, only a rough estimation of the extinction coefficients is given. The characteristic absorption for complexes with a  3 -bis(donor)ferrocenyl-metal bonding mode [S9] is located around 500 nm. All spectra were recorded at room temperature and in all cases a background spectrum was measured first and subtracted from the spectrum of the compound of interest. The measured extinction coefficients are summarized in table S7.
S37 Figure S23: UV/Vis of 1-H2.    Due to the very high air and water sensitivity the dilution of these highly colored solutions is limited by trace amounts of water (< 10 ppm) in the pyridine.

Cyclic Voltammetry General Information
The A three-electrode setup was used with a glassy carbon working electrode, a coiled platinum wire as counter electrode, and a coiled silver wire as a pseudo-reference electrode. The ferrocene/ferrocenium couple was used as internal reference. The ferrocene was sublimated prior to its use. All determined values are depicted in table S8.

Computational Details
The geometry optimization followed by the harmonic vibrational frequencies calculations of 1-Ae (Ae = Mg, Ca, Sr, Ba) complexes were performed at the BP86-D3(BJ)/def2-SVP level. [S10] Effective core potential was used for the 28 and 46 core electrons of Sr and Ba atoms, respectively, to take care of the relativistic effects. Superfine integration grid was used for all computations. All these calculations were carried out with Gaussian 16 program package. [S11] The natural bond orbital (NBO) analysis [S12] was done with the NBO 6.0 program. [S13] Quantum theory of atoms in molecules (QTAIM) analysis [S14] was performed with wave functions generated at the BP86-D3(BJ)/def2-SVP/x2c-SVPall//BP86-D3(BJ)/def2-SVP level, where an all-electron x2c-SVPall basis set was used for Sr and Ba atoms to avoid the use of ECP in the program, by using AIMALL program. [S15] The bonding situations were analysed by means of an energy decomposition analysis (EDA) [S16] together with the natural orbitals for chemical valence (NOCV) [S17] method by using the ADF 2018.105 program package. [S18] The EDA-NOCV calculations were carried out at the BP86-D3(BJ)/TZ2P+-ZORA level [S19] using the BP86-D3(BJ)/def2-SVP optimized geometries. TZ2P+ is a triple-ζ quality basis set augmented by two sets of polarization functions. In this analysis, the intrinsic interaction energy (ΔΕint) between two fragments is divided into four energy components as follows: The electrostatic ΔEelstat term is originated from the quasiclassical electrostatic interaction between the unperturbed charge distributions of the prepared fragments, whereas the Pauli repulsion ΔEPauli corresponds to the energy change associated with the transformation from the superposition of the unperturbed electron densities of the isolated fragments to the wavefunction, which properly obeys the Pauli principle through explicit antisymmetrization and renormalization of the production wavefunction. Since D3(BJ) is coupled with the functional, it gives additional dispersion contribution between two interacting fragments. The orbital term ΔEorb is originated from the mixing of orbitals, charge transfer and polarization between the isolated fragments, which can be further decomposed into contributions from each irreducible representation of the point group of the interacting system as follows:

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(2) The combination of the EDA with NOCV enables the partition of the total orbital interactions into pairwise contributions of the orbital interactions which is very vital to get a complete picture of the bonding. The charge deformation Δρk(r), resulting from the mixing of the orbital pairs k(r) and -k(r) of the interacting fragments presents the amount and the shape of the charge flow due to the orbital interactions (Equation 3), and the associated energy term ΔEorb provides with the size of stabilizing orbital energy originated from such interaction (Equation 4).
(3) (4) More details about the EDA-NOCV method and its application are given in recent reviews articles. [S20]       [a] The values in parentheses give the percentage contribution to the total attractive interactions ΔEelstat + ΔEorb + ΔEdisp [b] The values in parentheses give the percentage contribution to the total orbital interactions ΔEorb.