Tetrahedral [Sb(AuMe)4]3− Occurring in Multimetallic Cluster Syntheses: About the Structure‐Directing Role of Methyl Groups

Abstract The anion of [K(crypt‐222)]3[Sb(AuMe)4]⋅py (1; crypt‐222=4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane; py=pyridine) represents a rare example of a homoleptic heavy p‐block metal atom being surrounded by four free‐standing transition metal complex fragments, and the third example for a corresponding Sb compound. In contrast to all reported complexes of this type, the transition metal atoms possess twofold coordination only, hence the complex as a whole does not exhibit significant steric shielding or further linkage of the metal atoms. This is reflected in a high flexibility, as confirmed by slight deviations from a tetrahedral coordination of the Sb atom in the crystal and soft vibrational modes. An alternative pyramidal conformer, observed for a related arsenic compound with terminal phosphine ligands, is apparently disfavored owing to electron correlation effects. The compound is formed in a reaction that in another solvent or at other reactant concentrations yields salts of ternary cluster anions. By a combined experimental and theoretical study of different reaction conditions and previously unidentified side‐products, we provide insight into multimetallic cluster synthesis reactions.

. Crystal data and details of the structure determination of 1.  Figure S2. Unit cell view of compound 1. Atoms are shown as thermal ellipsoids with 50% probability.  Figure S3. Molecular structure of the anion in compound A·py (left) with labelling scheme (Symmetry code: i = 1-x, -y, 1-z), and unit cell view of A·py (right). Atoms are shown as thermal ellipsoids with 50% probability. The two-colored octants indicate that the two atom types are not distinguishable by means of normal X-ray diffraction, yet the atom types were assigned in accordance with quantum chemical calculations of the most stable isomer.

Energy-Dispersive X-Ray Spectroscopy (EDX)
EDX analysis of single crystals of compound 1 was carried out using an EDX-device Voyager 4.0 of Noran Instruments coupled with an electron microscope CamScan CS 4DV. Data acquisition was performed with an acceleration voltage of 15 kV and 100 s accumulation time.
Results are summarized in Table S4. Note that the deviation of the K content is observed very frequently in EDX analyses of these very air-sensitive Zintl cluster compounds. The REM image of the investigated sample is shown along with the spectrum in Figure S4.  Figure S4. EDX analysis of 1. REM image of the sample (top) and spectrum (bottom).

Methods
All mass spectra were recorded with a Thermo Fischer Scientific Finnigan LTQ-FT spectrometer in negative ion mode. We prepared a fresh solution of single crystals of the compound 1 in freshly distilled DMF inside a glovebox. The solutions were injected into the spectrometer with gastight 250 µL Hamilton syringes by syringe pump infusion. All capillaries within the system were washed with dry DMF for 2 hours before and at least 10 minutes in between measurements to avoid decomposition reactions and consequent clogging.

ESI(−) Mass Spectrum of 1
The following ESI parameters were used: Spray Voltage: 3.6 kV, Capillary Temp: 290 °C, Capillary Voltage: −20, Tube lens Voltage: −121.75, Sheath Gas: 45, Sweep Gas: 0, Auxiliary Gas: 40. The overview mass spectrum in the range 150-2000 m/z is provided in Figure S5, assignable high-resolution mass peaks are shown in Figures S6-S10. Note that all species were detected as monoanions, which does not necessarily correspond to their original charge in solution. Figure S5. Overview ESI(−) mass spectrum recorded immediately upon injection of a fresh solution of 1 in DMF.  Figure S6. High-resolution ESI mass spectrum in negative ion mode of Au -, recorded immediately upon injection of a fresh solution of 1 in DMF. Top: measured, bottom: simulated. Figure S7. High-resolution ESI mass spectrum in negative ion mode of (AuCH3) -, recorded immediately upon injection of a fresh solution of 1 in DMF. Top: measured, bottom: simulated.  Figure

EDX analysis of the Solid Residue upon Filtration of the Reaction Mixture to Form Compound 1
The EDX analysis was carried out using an EDX-device Voyager 4.0 of Noran Instruments coupled with an electron microscope CamScan CS 4DV. Data acquisition was performed with an acceleration voltage of 15 kV and 100 s accumulation time. Results are summarized in Table  S5. The REM image of the investigated sample is shown along with the spectrum in Figure S13.
There are considerable overlaps of K/Sn and Sn/Sb peaks, respectively, resulting in large abs. error. values. Thus, the element ratios of the residue was not calculated. However, the data clearly indicates the presence of potassium, tin, and antimony in the residue.  Figure S13. EDX analysis of the solid residue after filtration of the reaction mixture. REM image of the sample (top) and spectrum (bottom). The presence of Si in the sample is due to abrasion of glass from the glass-coated stirring bar and the glass tube while vigorously stirring the reaction mixture. S14

Powder X-Ray Diffraction (PXRD) of the Solid Residue upon Filtration of the Reaction Mixture to Form Compound 1
Powder X-ray diffraction (PXRD) data were collected on a Stoe StadiMP diffractometer system equipped with a Mythen 1 K silicon strip detector and Cu-Kα-radiation (λ = 1.54056 Å). A sample of the solid residue upon filtration of the reaction mixture after 3h reaction time, and washing of the precipitate with py for 3 times, was filled into a glass capillary (0.3 mm diameter), which was sealed air-tightly with soft wax. The tube was then mounted onto the goniometer head using wax (horizontal setup) and rotated throughout the measurement. The diffraction pattern is shown in Figure S14. Figure S15 shows the diffraction pattern of the starting material [K(crypt-222)]2(Sn2Sb2)·en for comparison. The diffraction pattern in Figure S14 shows the elements Sn and Sb to be present in crystalline form, while the starting material is not observed. Figure S14. PXRD pattern of the solid residue upon filtration of the reaction mixture for the formation of compound 1, after 3h reaction time, and washing of the precipitate with py for 3 times. Reflections that are indicative for the respective elements are assigned. Figure S15. PXRD pattern of [K(crypt-222)]2(Sn2Sb2)·en for comparison as measured (black line) and simulated from the single-crystal CIF deposited as CCDC 742876.
Figure S17. 31 P NMR spectrum of PPh3 in py. S16 Figure S18. 31 P-NMR spectrum of [AuMePPh3] in en. The low peak intensity is due to the low solubility of [AuMePPh3] in en at room temperature.

Electrospray Ionization Mass Spectrometry (ESI-MS) of the Reaction Solution to Form Compounds 1/B (1:2 in py)
The mass spectra was recorded with a Thermo Fischer Scientific Finnigan LTQ-FT spectrometer in negative and positive ion mode. The following ESI parameters were used for ESI(-/+): Spray Voltage: 3.6 kV, Capillary Temp: 290 °C, Capillary Voltage: −20, Tube lens Voltage: −121.75, Sheath Gas: 45, Sweep Gas: 0, Auxiliary Gas: 40. We prepared a fresh solution of [K(crypt-222)]2(Sn2Sb2)·en and [AuMePPh3] in a 1:2 ratio in py. After 3h reaction time, the solution was diluted with freshly distilled DMF inside a glovebox. The solution was injected into the spectrometer with a gastight 250 µL Hamilton syringe by syringe pump infusion. All capillaries within the system were washed with dry DMF for 2 hours before and at least 10 minutes in between measurements to avoid decomposition reactions and consequent clogging. The overview mass spectra in the range 50-500 m/z are provided in Figure  S22 for ESI(-) and in Figure S26 for ESI(+), assignable high-resolution mass peaks are shown in Figures

CH 4 Detection by Gas Chromatography-Mass Spectrometry (GC-MS) Analysis of the Gas-Phase above a Reaction Solution to Form Compounds 1/B (1:2 in py)
To validate the CH4 release that was proposed by quantum chemical studies for a reaction mixture of (Sn2Sb2) 2and [AuMePPh3] to form compounds 1/B (1:2 in py, see Scheme 2 in the main document and equation D1 in Table S18), gas chromatography-mass spectrometry (GC-MS) analyses were performed on a 5973N/6890N Single-Quadrupol GC-MS-System from Agilent. The gas above py solutions comprising either only [K(crypt-222)]2(Sn2Sb2)·en, or [AuMePPh3], or a 1:2 mixture of both (after 5 min or 1.5h reaction time, respectively), was loaded into a syringe upon piercing the rubber septum (that tightly closed the respective Schlenk tube) with a hollow needle, and immediately injected into the instrument. The results are illustrated in Figures S28 -S31. They prove the formation of CH4 already after 5 min reaction time, in contrast to no CH4 in solutions of the separated reactants.

Hydrogen Detection in the Reaction Solution to Form Compounds A (2:1 in en) or 1/B (1:2 in py)
To validate the H2 release that was proposed by quantum chemical studies for reaction mixtures of (Sn2Sb2) 2and [AuMePPh3] to form compound A (2:1 in en, see Scheme 2 in the main document and equation B3 in Table S13) or compounds 1/B (1:2 in py, see Scheme 2 in the main document and equation D1 in Table S13) were prepared prior to the test. After 3h reaction time, the release of hydrogen was probed according to a previously reported method, [7] yet by using metallic palladium powder instead of palladium plates for the absorption of H2.
(1) Preparation of the indicator: Molybdenum trioxide (1 g) was dissolved in 5 mL of NaOH solution (20%). 7 mL of an HCl solution (20%) was added, and the mixture was diluted by water to adopt a total volume of 200 mL.
(2) Palladium powder (0.2 g) was placed in a 15 mL Schlenk flask, which was connected to a bubbler. The metal powder was heated to 200 degree for 15 minutes under dynamic vacuum and washed with argon two times. The atmosphere over the reaction solution was transferred by applying slight vacuum to the flask containing palladium. The connecters were closed, whereupon the flask was heated up to 200 degree for ten minutes. The H2-loaded palladium powder was poured into the indicator solution.
As shown in Figures S32 and S33, the resulting colorless solution for the test of the reaction to form compound A indicates no H2 release, while the light blue color observed during the corresponding analysis of the reaction to form compounds 1/B indicates H2 release.

Investigations of the [Sb(AuMe) 4 ] 3− anion in 1
Density functional theory (DFT) calculations were carried out to simultaneously optimize the geometric and electronic structure of the [Sb(AuMe)4] 3− anion in compound 1. The calculations were done with the program system TURBOMOLE, [8] using the functional TPSS [9] and basis sets dhf-TZVP with corresponding auxiliary bases. [10] Effective core potentials were applied to model the Bi atoms. [11] To compensate for the negative charge of the molecule, COSMO [12] was employed using standard settings and an infinite dielectric constant. Force constants and the vibrational spectrum were calculated by application of the NumForce program. The absence of any imaginary frequencies proves the structure to be a local minimum. The optimized structure of the calculated anion are shown in Figure S34. Selected MOs are provided in Figure S35. Localized molecular orbitals (LMOs) of the anion in 1 were calculated using the method of Boys and Foster. [13] Selected LMOs are provided in Figure S36. MO and LMO plots were created with gOpenMol. [14] The calculated vibrational spectrum is shown in Figure S37.  Table S6.
Natural charges were calculated by means of natural population analyses (NPA). [17] For Pn = Sb, Mulliken contributions of atomic orbitals to high occupied MOs are listed in Tables S7 to S10 for the optimized tetrahedral and pyramidal structures (level TPSS/dhf-TZVP), and for pyramidal [Pn(AuPH3)4] + also at level HF/dhf-TZVP in Table S11.    * Reaction energies for reactions indicated with an asterisk cannot be calculated, as the atomic anion "Sb 3-" cannot be reasonably modeled by means of quantum chemistry. a Note that the formation of the energetically preferred 4,4'-isomers of (bipy) •and (bipy) 2are considered only. b Values were corrected by consideration of atomization energies to account for the instant precipitation of the metal "atoms" in condensed phase (uncorrected values are given in parentheses).

Investigations of hypothetical formation reactions of the [Sb(AuMe) 4 ] 3− anion in 1, the[ (Sn 2 Sb 2 ) 2 Au] 3− anion in A, and the [(Sn 5 Sb 3 Au) 2 ] 4− anion in B
The correction of data by experimentally observed atomization energies [18] for all reactions that produce neutral atoms is needed (although representing a mixture of methods) to account for the fact that in condensed phases, the atoms would never stay isolated, but would instantly form metal precipitates; this is in accordance with our observations. Furthermore, in most of the directly compared reactions, the amount of metal atoms that form according to the reaction schemes is the same, so the errors are systematically the same, too, in turn enabling the direct comparison.
We note that the indicated side-products, PPh3, CH4, H2, [AuMe2] -, [19] and the metals Sn and Sb were experimentally proven, thus generally rationalizing the reaction schemes (see Section 5 for details).
The detection of (H2NCH2CH2NH)in en solution has not been possible to date, as the signals are covered by those of neutral en and crypt-222, and ESI mass spectrometry would not serve to distinguish the anion's abundance in solution from its formation during the electrosprayionization process. However, the formation of this anion was suggested in several formation reactions of Zintl clusters in previous works, as the best possible suggestion. [20] Bipyridine anions, (C10H8N2) •and (C10H8N2) 2-, which were suggested by Sevov and coauthors to form during the formation of the heterometallic cluster anion [Bi4Ni4(CO)6] 2− from [Bi3Ni4(CO)6] 3− , and by our group in the context of the formation of Bi11 3− from (GaBi3) 2− in pyridine, [21] cannot be reasonably detected in the reaction mixture, as 1 H-NMR is not indicative for it besides py and PPh3, as EPR fails in Zintl chemistry reaction mixtures (in contrast to solutions of isolated products), and as ESI MS would not work for the same reasons as mentioned for the (H2NCH2CH2NH)anion. However, its formation was previously proven by corresponding crystal structures. [22] Also in the context of Zintl chemistry, the Sevov group reported that, upon dissolving K5Bi4 in pyridine in the presence of crypt-222, a dark-brown solution is formed. After stirring for several hours (4 h and more), the color turned to dark purple, indicative of the presence of (C10H8N2) •-. [23]