Abstract
The bonding between gold and main-group metallic elements (M) featuring Auδ−−Mδ+ polarity, has been studied recently. The gold in the bonds is expected to have the oxidation number of −1, and hence, nucleophilic. However, the knowledge of the reactivity of the gold-metal bonds remains limited. Here, we report digold-substituted germanes of the form of R’2Ge(AuPR3)(AuGeR’2) (3a; R = Me, 3b; R = Et), featuring two Au-Ge(IV) and one Au-Ge(II) bonds. DFT calculations of 3a revealed the existence of high-lying σ(Ge-Au) type HOMO and low-lying LUMO with germylene pπ nature. A pendular motion of AuPR3 group between Ge(IV) and Ge(II) of 3 occurs in the NMR time scale, suggesting that the Ge(II) center has an enhanced electrophilicity to be attacked by the nucleophilic gold (−I) atom. 3a reacts with nucleophilic Cl− and electrophilic MeOTf reagents at Ge(II) and Ge(IV) centers, respectively.
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Introduction
Among transition metal elements, gold is unique with the large electronegativity (χ = 2.54) and high electron affinity (2.30 eV) owing to the large relativistic effects1. Auride ion Au− has been known as a halogen-like anion with the electronic configuration of 5d106s2 since the discovery of caesium auride by Sommer in 19432.
The chemistry of gold compounds bonded to main-group elements has been developed extensively in recent years3,4,5,6,7. Goicoechea, Aldridge and coworkers have reported that novel gold-aluminum complex I (Fig. 1) has a nucleophilic auride character due to its Alδ+–Auδ‒ bond polarity and reacts with CO2 and a carbodiimide giving the corresponding insertion products6. Borylgold complexes II and III (Fig. 1) have been synthesized by Nozaki et al. and Kinjo et al., respectively, using the reactions of the corresponding boryllithiums with Ph3PAuCl4,5.
Gold complexes bonded to metal and metalloid elements I–IV possessing the nucleophilic auride character.
During our study of unique cyclic (R2SnAu)3 complex8, we have discovered that digoldstannane IV is obtained by the reduction of the corresponding gold-substituted tin chloride V with KC8 followed by the addition of excess PEt3 (Fig. 1). Compounds with a gold–germanium bond may be interesting because the bond is less polarized than a gold-tin bond due to larger electronegativity of germanium (χ = 2.01) than silicon (χ = 1.90) and tin (χ = 1.96)1,9. Although a variety of compounds with gold–germanium bonds have been studied since the first report of Glockling and Hooton in 196210, knowledge of the chemistry including the bonding characteristics and reactivities is still limited in gold-monosubstituted germanes11,12,13,14,15,16,17,18 and germylene gold complexes19,20,21,22,23,24,25,26,27,28. While the preparation of isolable digoldgermane with Au(‒I)–Ge bonds remains challenging, a number of single-atom bridged polygold complexes including those bridged with hydride29,30,31,32, carbon33,34,35,36,37,38,39, nitrogen40,41,42,43,44,45, oxygen46,47, and halogen33,48,49,50,51,52 are known and widely utilized in the homogenous catalysis53 and materials science29,54.
In this work, we report the synthesis and properties of digoldgermanes 3 (3a: R = Me; 3b: R = Et) that feature two gold atoms coordinated by dialkylgermylene 1 and trialkylphosphine respectively (Fig. 2a). The structural characteristics of 3 were elucidated using NMR spectroscopy, X-ray crystallography and density-functional theory (DFT) calculations. The discussion is focused mainly on structural characteristics of the two different types of Au–Ge bonding of 3, fluctuation of the AuPR3 group between 1Ge and 2Ge atoms of 3 in solution, and their distinctive reactions and catalysis.
a The synthesis of digoldgermanes 3a and 3b. b Molecular structures of 3a and 3b; hydrogen atoms are omitted for clarity. Trimethylsilyl, ethyl, and methyl groups are depicted in a wireframe model.
Results
Synthesis and structural elucidation of digoldgermanes 3a (R = Me) and 3b (R = Et)
Digoldgermanes 3a and 3b are synthesized by applying the reaction route shown in Fig. 2a; see the SI for the experimental details. The reactions of an isolable dialkylgermylene 1 with R3PAuCl (R = Me and Et) in tetrahydrofuran (THF) at ambient temperature give 2a (R = Me) and 2b (R = Et), respectively, as white solids in almost quantitative yields; many similar insertion reactions of tetrylenes into Au–X bonds have been reported13,27. When 2a and 2b are treated with potassium graphite (KC8) in THF at room temperature, the corresponding digoldgermanes 3a and 3b are obtained as dark-green solids in 49% and 54% yields, respectively. Compounds 3a and 3b are isolated as pure materials by recrystallization from a cooled hexane solution, which are stable in the solid-state under argon and can be stored at ambient temperatures for a few months without decomposition.
The structures of 2 and 3 were determined by multi-nuclear NMR spectroscopy and single crystal X-ray diffraction analysis; see also the SI for the details. The solid-state structures of 3a and 3b (Fig. 2b) show that their skeletal structures are very similar to each other. The 1Au–1Ge and 2Au–1Ge bond distances are 2.4475(4) and 2.4460(5) Å for 3a and 2.4371(7) and 2.4307(8) Å for 3b. On the other hand, the distance between the divalent germanium (2Ge) and 1Au [2.4146(4) and 2.4089(8) Å for 3a and 3b, respectively] is somewhat shorter than those of 1Au–1Ge and 2Au–1Ge bonds, suggesting a similar bonding nature between Ge(IV)–Au and Ge(II)–Au bonds. The bond angles 1Ge–1Au–2Ge and 1Ge–2Au–P of 3a are 168.042(5) and 175.25(3)° and those of 3b are 171.73(3) and 174.02(7)°, indicating linear arrangement of the sets of the three atoms in accord with the theoretical calculations (see below). Two germacyclopentane rings of 3a and 3b are almost perpendicular to each other with dihedral angles between the averaged ring planes of 78.915° and 84.588°, respectively. The X-ray analysis of 3a and 3b shows long 1Au-2Au distances of 3.913(5) and 3.9172(5)Å respectively, suggesting any aurophilic interactions to be weak at best. Significant aurophilic bonding is regarded to occur when the distance is in the range of 2.8–3.5 Å55,56,57,58,59,60. The 1Au–1Ge–2Au angle of 3a (and 3b) is 106.909(16)° [and 107.61(3)°], indicating the tetrahedral geometry around 1Ge. The sum of the bond angles around 2Ge atom of 3a (and 3b) is 359.999° (and 359.994°), which manifests the trigonal planar geometry around 2Ge. The unique bonding features of 3 will be discussed later on the basis of the theoretical calculations.
The 1H, 13C, 29Si, and 31P NMR spectra of 3a and 3b at ambient temperatures are consistent with the structures determined by X-ray crystallography, though the spectra are complex due to the fluxionality of the molecules, whose dynamic behavior was analyzed using VT-NMR; see the SI for their detailed NMR data and spectra. In the 1H NMR at room temperature in THF-d8, the signals of ring and trimethylsilyl (TMS) protons of 1GeC4 and 2GeC4 rings of 3a (and 3b) appear at 2.28 and 0.29 ppm as broad singlets (Supplementary Fig. 1). However, at −30 °C in THF-d8, three sharp singlets are observed at 0.35, 0.28, and 0.26 ppm for the TMS protons of 3b with the ratio of 2:1:1, being in accordance with the asymmetric structure with respect to the 1GeC4 ring (Supplementary Fig. 27). A similar but a little more broadened spectral pattern is observed in the 1H NMR spectrum of 3a at −30 °C (Supplementary Fig. 22).
Broadening of the signals of TMS and ring methylene protons shown in the 1H NMR spectra of 3a and 3b suggests the fluxionality of the molecules occurring on the NMR time scale. As the methyl proton signals of PMe3 and methyl and methylene proton signals of 3b remain sharp even at room temperature, the dynamic process is suggested to be a pendular motion of the AuPR3 group or the isomerization between the two equivalent structures shown in Supplementary Fig. 3. The variable temperature 1H NMR spectra of 3b in the TMS proton resonance region are shown in Supplementary Fig. 2. The TMS proton signals on 1Ge and 2Ge atoms coalesce at around −10 °C. The isomerization rate kC at the coalescence temperature (TC = 263 K) is estimated as ca. 90 s-1 using the equation of \({k}_{C}=\pi ({v}_{1}-{v}_{2}){/}{\sqrt{2}}\), where ν1 is the resonance frequency of TMS protons on 2Ge (0.35 ppm, 175 Hz) and ν2 as the average of two TMS resonances on 2Ge (0.27 ppm, 135 Hz). The activation free-energy (ΔGC‡) at the Tc is estimated as 13.0 kcal mol−1. While the mechanism of the pendular motion remains open, a plausible transition state (TS) is suggested to be 3T as shown in Supplementary Fig. 3; at the transition state, the aurophilic stabilization is supposed to be important to lower the activation energy.
The UV–Vis spectrum of 3a in hexane shows the maximum absorption wavelength at 590 nm with the absorptivity ε/(M–1 cm–1) of 3560 (see Supplementary Fig. 11). It is worth mentioning that the band is broad but more red-shifted than the n → 4p band of germylene 1 (λmax = 450 nm, ε/M–1 cm–1 = 320)61.
Theoretical studies of 3a
A plausible mechanism for the formation of 3 by the reduction of 2 could be proposed as shown in Supplementary Fig. 4; the initial reduction of 2 with KC8 affords A as an intermediate, and then A attacks another molecule of 2 in a nucleophilic manner giving 3. Although the intermediary product A was not detected, the DFT calculations (at B3PW91-GD3 level in the gas phase with the basis sets of SDD level for Au) suggest that A would be better described as a trigonal pyramidal germyl anion as shown in Supplementary Fig. 59. The NBO analysis of A shows that the lone-pair electrons are largely localized on 4 s orbital of Ge with hybridization of sp0.25 but developed to the 6p and other vacant orbitals of Au; the natural charges on Au and Ge are –0.330 and 0.502, respectively. The Au atom of A may serve as the nucleophilic center to attack the germanium atom of 2.
To gain more insight into the structural characteristics of 3a and related compounds, DFT calculations were performed at B3PW91-GD3 level (see the Methods for calculation details). As shown in Supplementary Table 2, the structural parameters of R’2Ge(AuP)(AuGe) skeleton of 3a determined by X-ray analysis are well reproduced by the calculations. Frontier molecular orbitals (FMOs) of 3a are shown in Fig. 3a. HOMO and HOMO‒1 are assigned as the symmetric and antisymmetric combinations of two 1Ge–Au σ orbitals, respectively, and LUMO has the nature of originally vacant 2Ge pπ orbital. The HOMO and LUMO energy levels of 3a are −4.46 and −2.31 eV, respectively, and they are significantly higher and lower than those of germylene 1 (−5.55 and −1.77 eV at the same calculation level), suggesting higher reactivity of 3a than 1. The narrower HOMO-LUMO gap of 3a (2.15 eV) than that of 1 (3.70 eV) is also in good agreement with the absorption maximum of 3a (λmax = 590 nm) observed at longer wavelength than that of 1 (λmax = 450 nm).
a FMOs of 3a calculated at B3PW91-GD3 level. Hydrogen atoms are omitted in the wireframe structure of 3a. b Calculated effective atomic charges of Ge and Au atoms in 3a (the effective atomic charges of all atoms in 3a were calculated using the AIM method). Topological analysis for 3a: c Plot of the Laplacian of the electron density on the GeAu2 plane, with bond paths (light green lines), BCPs (blue dots). d ELF plot on the GeAu2 plane.
Natural bond orbital (NBO) analysis of the theoretical structure of 3a shows that it is comprised of three molecular units of R’2GeAu2, PMe3, and GeR2’, where the latter two ligands coordinate to each of the two Au atoms. The 1Ge–1Au [and 1Ge–2Au] bonds are both covalent, formed by the overlap between a 4sp3.13 hybrid orbital of 1Ge and 6s orbitals of 1Au and 2Au; their Wiberg bond indices are 0.6251 and 0.7122 with the occupancy of 1.8740 and 1.8909, respectively. The occupancies of the lone-pair and a vacant orbital on 2Ge are 1.6123 and 0.2173, showing the existence of significant dative bond between 1Au and 2Ge and small back bonding from 1Au to 2Ge. The second-order perturbation theory analysis shows the strong dative bonds from phosphine and germylene to 2Au and 1Au, respectively; the largest perturbation energy between the phosphine lone-pair orbital and 2Au–1Ge antibonding orbital amounts to 127.5 kcal mol−1 and that between the germylene lone-pair orbital and 1Au–1Ge antibonding orbital is 249.8 kcal mol−1. To study the charge distribution in the Ge–Au bond of complexes 3, the effective atomic charges of all atoms in 3a were calculated using AIM62. The effective charges of the gold atoms in 3a are −0.516 and −0.549, while that of germanium atoms are +1.088 and +0.699 respectively (Fig. 3b). The balancing negative charge in 3a is mostly localized at two gold atoms with the −1 oxidation state, in which gold atom demonstrates halogen-like behavior featuring two conspicuously polarized Auδ−–Geδ+ bonds.
The quantum theory of atoms in molecules shows that there are two bond critical points (BCPs) between the 1Ge and two Au atoms. In addition, the other two BCPs are found between 2Au and P and between 1Au and 2Ge (Fig. 3c). The plot of the Laplacian of the electron density, ∇2ρ(r), shows regions of electron density concentration on the two Au atoms (Fig. 3c). The electron localization function (ELF) plot of 3a exhibits the inverted “V” region with the highly localized electron density at the 1Ge–1Au and 1Ge–2Au bonds (Fig. 3d). No BCP and electron density concentration are found between 1Au and 2Au atoms.
The reactivity of 3a
Based on the DFT results, 3 could be regarded as amphoteric molecules, which involve a nucleophilic gold atom with the −1 oxidation state and a dialkylgermylene ligand bearing enhanced electrophilicity. We may expect their distinctive types of reactions. The treatment of 3a with two moles of PMe3 at room temperature (Fig. 4a) gives the corresponding digoldgermane 4, which is coordinated by two phosphines and is isolated and characterized by X-ray (Supplementary Fig. 9) and NMR analysis (Supplementary Figs. 32–35). The HOMO of 4 holds almost the same character with that of 3a featuring the antisymmetric combination of the two Ge–Au σ orbitals with the energy level of −4.52 eV. On the other hand, the LUMO mainly possesses the nature of the vacant Au 6p orbitals and its energy level (−0.54 eV) is much higher than that of 3a (Supplementary Fig. 58). No reaction occurs when excess dialkylgermylene 1 is added to the solution of 4. Neither N-heterocyclic carbene (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) nor carbon monoxide (CO) reacts with 3a.
a Synthesis of 4 from digoldgermane 3a. b Synthesis of 5 from digoldgermane 3a. c DFT-calculated free-energy profile of a plausible mechanism for the reaction between 3a and MeOTf in the gas phase, as determined at the B3PW91/def2-SVP level of theory.
As the negative charge in 3 is localized largely at two gold atoms, they may work as electrophiles for the addition or substitution reactions. The treatment of 3a with methyl triflate (MeOTf), a powerful electrophilic methylation agent, at ambient temperature unexpectedly gives rise to the methylation on a germanium atom giving methylgermane 5 together with the elimination of Me3PAu+ moiety (Fig. 4b). The formation of Me3PAuOTf is evidenced by 31P NMR (δ = 13.13 ppm).
Product 5 was identified by NMR spectroscopy and single crystal X-ray diffraction analysis (Supplementary Fig. 10). The 1Ge–1Au distance [2.4330(7) Å] of 5 is similar to those of Ge–Au covalent bonds in 2 and 3 but a little longer than the bond length of 1Au–2Ge [2.3997(6) Å] in 5. The 1Au–1Ge–17C and 1Ge–1Au–2Ge bond angles in 5 are 105.02(18)° and 168.47(2)°, respectively. The dihedral angle between the two five-membered rings is 68.98°, smaller than that of 3a (78.91°).
While MeOTf as an electrophile may prefer to attack an Au(‒I) in 3a, in reality, a Ge–Me bond is formed during the reaction. A pathway via the direct attack of Me+ on a Ge atom giving Int2 (Fig. 4c) as an intermediate is supported by the DFT calculations for two possible intermediates, Int1 and Int2 (Fig. 4c), which are formed by the attack of Me+ on Au and Ge, respectively, at the B3PW91/def2-SVP level. While both intermediates are located as minima, Int1 is found to be 19.5 kcal/mol higher in energy than Int2 with pentacoordinate germanium atom (Fig. 4c); Int2 is even lower in energy than that of the starting reagents 3a + MeOTf.
Since 3a has a low-lying LUMO, which is even lower than that of germylene 1, the 2Ge center of 3a should be highly electrophilic. Facile isomerization between 3 and 3’ as shown in Supplementary Fig. 3 suggests the high electrophilicity at the 2Ge to be attacked by an intramolecular gold nucleophile. To our anticipation, 3a reacts readily with external nucleophiles at the 2Ge center. The reaction of 3a with a stoichiometric amount of tetraphenylphosphonium chloride 6 (Ph4P+Cl–) at ambient temperature giving the corresponding chlorogermane 7 featuring a Ge–Cl bond (Fig. 5a).
a The reactions of 3a with tetraphenylphosphonium chloride (Ph4PCl) 6, and acetyl chloride. b Molecular structure of 7 (Hydrogen atoms are omitted for clarity. Trimethylsilyl, ethyl, phenyl and methyl groups are depicted in a wireframe model). c Molecular structure of 9 (Hydrogen atoms are omitted for clarity. Trimethylsilyl, ethyl, phenyl and methyl groups are depicted in a wireframe model).
The structure of 7 was determined by NMR spectroscopy and single crystal X-ray diffraction analysis (Fig. 5b). In 1H NMR spectra of 7 in benzene-d6, trimethylsilyl proton resonances appear as four sharp singlets at 0.31, 0.25, 0.24 and 0.20 ppm, respectively, with 1:1:1:1 intensity ratio (Supplementary Fig. 39), showing that the solid-state structure is maintained in solution. A doublet 1H NMR signal observed at 1.34 ppm is assigned to the methyl protons of a PMe3 group. In the 31P NMR spectrum, two signals at 26.51 and 21.20 ppm are assigned to those of a PMe3 ligand and a Ph4P cation, respectively (Supplementary Fig. 42). Three Ge–Au distances of 7 [2.4325(5), 2.4284(5), and 2.4458(5)Å] are similar to those of Ge–Au covalent bonds found for 2 and 3 but a little longer than the 1Au-2Ge distance of 3a. The 1Ge–2Au–P and 1Ge–1Au–2Ge bond angles of 7 are 173.81(4)° and 163.649(17)°, respectively. The dihedral angle between the two five-membered rings is 78.915°, which is similar with that found in 3a.
The reaction of 3a with acetyl chloride 8 at ambient temperature afforded chlorogoldgermane 9 in 67%, while an expected by-product, acetyl(trimethylphosphine)gold, is not detected (Fig. 5a). The reaction of 3a with 8 may produce 7 anion with an acetyl counterion, but in reality gives the corresponding chlorogermane 9. As acetyl cation is more electrophilic than Ph4P+, the 1Ge–2Au bond of 7 once formed would be cleaved by the acetyl counter cation to generate 9; the formation of Ge–Cl bond and the cleavage of 1Ge–2Au bond may occur concertedly as shown in Supplementary Fig. 6. The structure of 9 was determined by NMR spectroscopy and single crystal X-ray diffraction analysis (Fig. 5c). The 1Au–1Ge–Cl and 1Ge–1Au–2Ge bond angles of 9 are 97.614(65)° and 167.659(34)°, respectively. Two germacyclopentane rings of 9 are almost in-plane with the dihedral angle of 166.99°, which is very different from those of 3, 5, and 7 (69–85°).
The catalysis reactivity of 3a
Digoldgermane 3a has been found to exhibit effective catalytic ability for the cyclic trimerization of aryl isocyanates (Supplementary Fig. 7). In the presence of 0.01 mol% of 3a, the trimerization of various phenyl isocyanates 10a-e takes place smoothly giving triaryl isocyanurates 11a–e in 78–98% isolated yields (Supplementary Fig. 7). Neither related goldgermanes 2a, 4a nor dialkylgermylene 1 show the catalytic activity for the trimerization of aryl isocyanates. There have been many catalysts discovered and diverse mechanisms have been proposed for the trimerization63,64,65. It is inferred that the high electrophilicity of the 2Ge atom in 3a is essential for the catalytic activity; see Fig. S3 for a proposed catalytic cycle.
Discussion
In conclusion, digoldgermanes 3 with a germylene ligand have been synthesized through the reaction of stable dialkylgermylene 1 with (R3P)AuCl followed by the KC8 reduction. The DFT calculations of 3a show that the HOMO is high-lying with the nature of Ge–Au σ bonding orbital and the LUMO has largely germylene vacant 4pπ nature with significantly lower energy level than that of 1. Digoldgermanes 3 feature amphiphilic reactivity with the electrophilic Ge(II) atom and the nucleophilic Au(‒I)–Ge(IV) bond. Digoldgermanes 3 show: (1) the pendular motion of AuPR3 ligand between two germanium atoms of 3 occurring on the NMR time scale; (2) the electrophilic methylation of 3a with MeOTf occurring at the Ge(IV) atom rather than the Au(‒I) atom; (3) the facile reactions with the nucleophiles, Ph4P+Cl− and acetyl chloride, giving the chlorination products, while the latter is accompanied by the Ge(IV)–Au bond cleavage; and (4) the catalytic activity towards the cyclic trimerization of aryl isocyanates giving the corresponding triaryl isocyanurates.
Methods
General synthetic procedure
All reactions were performed under an atmosphere of argon by using standard Schlenk or dry box techniques; solvents were dried over Na metal or CaH2 under nitrogen atmosphere. (R3P)AuCl (R = Me, Et) were synthesized using literature procedures66,67,68. 1H, 13C, 29Si, and 31P NMR spectra were obtained with a Bruker AV 400 instrument at 400 MHz (1H NMR), 101 MHz (13C NMR) and 162 MHz (31P NMR), as well as Bruker AV 500 instrument at 500 MHz (1H NMR), 126 MHz (13C NMR), 99 MHz (29Si NMR), 202 MHz (31P NMR) at 298 K. Unless otherwise noted, the NMR spectra were recorded in benzene-d6 at ambient temperature. The 1H and 13C NMR chemical shifts were referenced to residual 1H and 13C signals of the solvents. NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, dt = doublet of triplets, m = multiplet, and brs = broad singlet. Coupling constants J are given in Hz. Electrospray ionization (ESI) mass spectra were obtained at the Mass Spectrometry Laboratory at Hangzhou Normal University with a Bruker Daltonics MicroQtof spectrometer. Melting points were measured with a BUCHI Melting Point M-560. Sampling of air-sensitive compounds was carried out using a MBRAUN’s MB-10-G glove box. UV–Vis spectra were recorded on a Shimadzu UV-1800 spectrophotometer.
Synthesis of gold(I) complexes 2a and 2b
In a glove box, (R3P)AuCl (R = Me or Et, 1.0 mmol) was added into a THF (10 mL) solution of germylene 1 (350 mg, 1.02 mmol) and the mixture was stirred at room temperature for 5 min. The solvent was removed under vacuum to afford the residue, which was washed with hexane (5 mL) for 3 times. The residual solvents were evaporated in vacuo affording gold(I) complex 2a or 2b, which were stable under argon for a few months. 2a (547 mg, 90%): white powder. M.p.: 153 °C (dec.); 1H NMR (400 MHz, C6D6, 25 °C) δ 2.46–2.38 (m, ring-CH2, 2H), 2.20–2.12 (m, ring-CH2, 2H), 0.60 (s, SiCH3, 18H), 0.51 (s, SiCH3, 18H), 0.46 (d, 2JH-P = 9.20 Hz, PCH3, 9H); 13C NMR (101 MHz, C6D6, 25 °C) δ 35.28 (d, 4JC-P = 2.93 Hz, ring-CH2), 26.60 (d, 3JC-P = 14.04 Hz, ring-Cq), 14.87 (d, 1JC-P = 28.58 Hz, PCH3), 5.23 (SiCH3), 5.13 (SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 4.17, 2.02; 31P NMR (162 MHz, C6D6, 25 °C) δ 26.01; HRMS (ESI): m/z calcd. for C19H49AuGeClPSi4 (M+): 725.9677, m/z calcd. for [M–Cl]+, 691.1538, found: 691.1524. 2b (580 mg, 92%): white powder. M.p.: 172 °C (dec.); 1H NMR (400 MHz, C6D6, 25 °C) δ 2.45–2.39 (m, ring-CH2, 2H), 2.18–2.13 (m, ring-CH2, 2H), 0.96–0.88 (m, PCH2, 6H), 0.71 (dt, PCH2CH3, 3JH-P = 17.60 Hz, 3JH-H = 7.60 Hz, 9H), 0.61 (s, SiCH3, 18H), 0.51 (s, SiCH3, 18H); 13C NMR (101 MHz, C6D6, 25 °C) δ 35.29 (d, 4JC-P = 2.52 Hz, ring-CH2), 26.56 (d, 3JC-P = 13.13 Hz, ring-Cq), 17.97 (d, 1JC-P = 26.06 Hz, PCH2), 8.76 (s, PCH2CH3), 5.25 (s, SiCH3), 5.06 (s, SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 4.10, 2.14; 31P NMR (162 MHz, C6D6, 25 °C) δ 60.48. HRMS (ESI): m/z calcd. for C22H55AuGeClPSi4(M+) 768.0474, m/z calcd. for [M–Cl]+ 733.2023, found: 733.1994.
Synthesis of gold complexes 3a and 3b
2a or 2b (1.00 mmol) was mixed with KC8 (1.05 mmol, 177 mg) in THF (10 mL). The mixture was stirred at ambient temperature for 12 h. Then the mixture was concentrated under vacuum. The reside was washed with hexane (4 mL) for three times and extracted with toluene (20 mL). The extracts were concentrated under vacuum to afford 3a or 3b as blue-green solids. 3a (347 mg, 49%): M.p.: 150 °C (dec.); 1H NMR (400 MHz, C6D6, 25 °C) δ 2.33 (brs, ring-CH2, 8H), 0.89 (d, 2JH-P = 7.60 Hz, PCH3, 9H), 0.49 (brs, SiCH3, 72H); 1H NMR (600 MHz, THF-d8, –30 °C) δ 2.47 (brs, ring-CH2, 4H), 2.07 (brs, ring-CH2, 4H), 1.43 (d, 2JH-P = 8.20 Hz, PCH3, 9H), 0.33 (brs, SiCH3, 36H), 0.25 (s, SiCH3, 18H), 0.22 (s, SiCH3, 18H); 13C NMR (101 MHz, C6D6, 25 °C) δ 36.99 (ring-CH2), 30.22 (ring-Cq), 16.20 (d, 1JC-P = 21.61 Hz, PCH3), 4.36 (brs, SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 0.14 (brs); 31P NMR (202 MHz, C6D6, 25 °C) δ 39.10; UV/Vis: λmax 590 nm; HRMS (ESI): m/z calcd. for [C32H80AuGe2Si8]–: 1032.2535, found: 1032.2587. 3b (380 mg, 54%): M.p.: 162 °C (dec.); 1H NMR (500 MHz, THF-d8, 25 °C) δ 2.28 (brs, ring-CH2, 8H), 1.81 (m, PCH2, 6H), 1.22 (m, PCH2CH3, 9H), 0.29 (brs, SiCH3, 72H); 1H NMR (600 MHz, THF-d8, –30 °C) δ 2.47 (brs, ring-CH2, 4H), 2.07 (brs, ring-CH2, 4H), 1.82 (p, 2JH-P = 7.7 Hz, 6H), 1.21 (dt, 3JH-P = 16.9, 3JH-H = 7.6 Hz, 9H), 0.32 (brs, SiCH3, 36H), 0.25 (s, SiCH3, 18H), 0.23 (s, SiCH3, 18H); 13C NMR (126 MHz, THF-d8, 25 °C) δ 37.20 (s, ring-CH2), 19.17 (d, 1JC-P = 20.50 Hz, PCH2), 8.84 (d, 2JC-P = 1.52 Hz, PCH2CH3), 4.26 (s, SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 0.13 (brs); 31P NMR (202 MHz, THF-d8, 25 °C) δ 55.10; UV/Vis: λmax 596 nm; HRMS (ESI): m/z calcd. for [C32H80AuGe2Si8]–: 1032.2535, found: 1032.2597.
Synthesis of complex 4
PMe3 (12 mg, 0.16 mmol) was added into a THF (2 mL) solution of 3a (100 mg, 0.08 mmol) and the mixture was stirred at room temperature for 20 min. The solution color changed from blue to light green. The reside was washed with cooled hexane (2 mL) for three times and extracted with toluene (2 mL). The extracts were concentrated under vacuum to give 4 as light green crystals in 92% yield (71 mg): M.p.: 148 °C (dec.); 1H NMR (400 MHz, C6D6, 25 °C) δ 2.45 (s, ring-CH2,4H), 0.7 (s, SiCH3, 36H), 0.68 (d, PCH3, JH-P = 7.6 Hz, 18H); 13C NMR (126 MHz, C6D6, 25 °C) δ 37.42 (ring-CH2), 19.74 (t, 3JC-P = 6.0 Hz, ring-Cq), 15.98 (dd, 1JC-P = 21.4 Hz, 5JC-P = 3.8 Hz, PCH3), 5.86 (SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 1.93; 31P NMR (162 MHz, C6D6, 25 °C) δ 40.95; HRMS (ESI): m/z calcd. for [C22H58Au2GeP2Si4]+: 964.1624, found: 964.1632.
Synthesis of complex 5
In a glove box, MeOTf (25.1 mg, 0.153 mmol) was added into a THF (10 mL) solution of 3a (100 mg, 0.0766 mmol) and the mixture was stirred at room temperature for 24 h. The solvent was removed under vacuum to afford the residue, which was extracted with hexane (20 mL). After evaporation of the solvents in vacuo, recrystallization from hexane gave pure 5 (45.9 mg, 57%): red solids; M.p.: 188 °C; 1H NMR (400 MHz, C6D6, 25 °C) δ 2.22 (m, ring-CH2, 4H), 2.16 (s, ring-CH2, 4H), 1.11 (s, CH3, 3H), 0.56 (s, SiCH3, 18H), 0.43 (s, SiCH3, 18H), 0.23 (s, SiCH3, 36H); 13C NMR (101 MHz, C6D6, 25 °C) δ 62.09 (ring-Cq), 36.58 (ring-CH2), 35.92 (ring-CH2), 18.90 (ring-Cq), 11.31 (CH3), 5.55 (SiCH3), 5.21(SiCH3), 2.69 (SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 3.41, 2.43, 1.32. HRMS (ESI): m/z calcd. for [M + Cl]–: [C33H83AuGe2Si8Cl]– 1083.2416, found: 1083.2439.
Synthesis of complex 7
In a glove box, PPh4Cl (29.4 mg, 0.078 mmol) was added into a THF (10 mL) solution of 3a (100 mg, 0.0766 mmol) and the mixture was stirred at room temperature for 24 h. The solvent was removed under vacuum to afford the residue that was washed with ether (5 mL) for three times. Finally, compound 7 was obtained as light-red powder. 7 (125 mg, 97%): M.p.: 172 °C (dec.); 1H NMR (400 MHz, THF-d8, 25 °C) δ 7.97-7.93 (t, Ar-H, J = 7.2 Hz, 4H), 7.79-7.75 (m, Ar-H, 16H), 2.25-2.23 (t, ring-CH2, J = 5.60 Hz, 2H), 2.09-2.07 (t, ring-CH2, J = 6.4 Hz, 2H), 2.00 (s, ring-CH2, 4H), 1.35-1.33 (d, PCH3, 2JP-H = 7.6 Hz, 9H), 0.31 (s, SiCH3,18H), 0.25 (s, SiCH3, 18H), 0.23 (s, SiCH3, 18H), 0.2 (s, SiCH3, 18H); 13C NMR (126 MHz, THF-d8, 25 °C) δ 136.28 (Ar-C), 135.48 (d, Ar-C, 3JP-C = 11.0 Hz), 131.20 (d, Ar-C, 2JP-C = 11.1 Hz), 118.94 (d, 1JP-C = 89.7 Hz), 37.36 (ring-CH2), 36.21(ring-CH2), 28.82 (ring-Cq), 18.96 (d, PCH3, J = 5.4 Hz), 16.82 (d, ring-Cq, J = 18.9 Hz), 5.77 (SiCH3), 5.46 (SiCH3); 29Si NMR (99 MHz, THF-d8, 25 °C) δ 1.81, 1.58, 1.28, −0.04; 31P NMR (162 MHz, THF-d8, 25 °C) δ 26.47 (PMe3), 21.21 (PPh4); HRMS (ESI): m/z calcd. for [C35H89Au2ClGe2PSi8]–: 1341.2299, found: 1341.2312.
Synthesis of complex 9
In a glove box, CH3COCl (18.0 mg, 0.23 mmol) was added into a benzene (10 mL) solution of 3a (150 mg, 0.115 mmol) and the mixture was stirred at room temperature for 10 h. The solvent was removed under vacuum to afford the residue that was extracted with hexane (5 mL) for three times. Finally, compound 9 was obtained as rufous solid. 9 (125 mg, 67%): M.p.: 172 °C (dec.); 1H NMR (400 MHz, C6D6, 25 °C) δ 2.49-2.45 (m, ring-CH2, 2H), 2.26-2.22 (m, ring-CH2, 2H), 2.13 (s, ring-CH2, 4H), 0.56 (s, SiCH3, 18H), 0.49 (s, SiCH3, 18H), 0.20 (s, SiCH3, 36H); 13C NMR (126 MHz, C6D6, 25 °C) δ 36.44 (ring-CH2), 35.21 (ring-CH2), 27.26 (ring-CH2), 27.19 (ring-CH2), 4.80 (SiCH3), 4.78 (SiCH3), 4.43 (SiCH3), 4.40 (SiCH3), 2.66 (SiCH3), 2.62 (SiCH3); 29Si NMR (99 MHz, C6D6, 25 °C) δ 3.55, 3.45, 0.67, 0.57, −1.02; HRMS (ESI): m/z calcd. for [M–Cl]+: [C32H80AuGe2Si8]+ 1083.2416, found: 1083.2439.
General procedure for cyclic trimerization of aryl isocyanates catalyzed by 3a
A THF solution of complex 3a (7.66 × 10–4 M in THF, 130 μL, 0.01 mol%) was introduced in a thick-walled tube, which contained THF (2.0 mL) and arylisocyanate 10 (1 mmol). The reaction mixture was heated at 80 °C for 14 h. After being washed with n-hexane, the corresponding triarylisocyanurate 11 was obtained. 11a (Ar = Ph, 116 mg, 97%): 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.53-7.45 (m, Ar-H, 6H), 7.43-7.40 (m, Ar-H, 6H); 13C NMR (101 MHz, CDCl3, 25 °C) δ 148.84 (O = C), 133.73 (Ar-C), 129.51 (Ar-C), 128.55 (Ar-C). 11b (Ar = p-ClC6H4, 132 mg, 86%): 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.49-7.47 (d, J = 8.8 Hz, Ar-H, 6H), 7.33-7.31 (d, J = 8.8 Hz, Ar-H, 6H); 13C NMR (101 MHz, CDCl3, 25 °C) δ 148.27 (O = C), 135.73 (Ar-C), 131.86 (Ar-C), 129.86 (Ar-C), 129.86 (Ar-C). 11c (Ar = p-CF3C6H4, 135 mg, 72%): 1H NMR (400 MHz, C6D6, 25 °C) δ 7.81-7.79 (d, J = 8.4 Hz, Ar-H, 6H), 7.56-7.54 (d, J = 8.4 Hz, Ar-H, 6H); 13C NMR (101 MHz, C6D6, 25 °C) δ 147.92 (O = C), 136.27 (Ar-C), 132.00 (q, 2JC-F = 33.13, Ar-C), 129.23 (Ar-C), 126.83 (q, 3JC-F = 3.63, Ar-C), 122.27 (C-F). 11d (Ar = p-CH3C6H4, 131 mg, 98%): 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.26 (d, J = 2.4 Hz, Ar-H, 12H), 2.38 (s, CH3, 9H); 13C NMR (101 MHz, CDCl3, 25 °C) δ 148.98 (O = C), 139.35 (Ar-C), 131.20 (Ar-C), 130.07 (Ar-C), 128.18 (Ar-C), 21.32 (CH3). 11e (Ar = p-MeOC6H4, 147 mg, 98%): 1H NMR (400 MHz, CDCl3, 25 °C) δ 7.30-7.28 (d, J = 8.8 Hz, Ar-H, 6H), 6.99-6.97 (d, J = 8.8 Hz, Ar-H, 6H), 3.82 (s, OCH3, 9H); 13C NMR (101 MHz, CDCl3, 25 °C) δ 159.99 (Ar-C), 149.24(O = C), 129.52 (Ar-C), 126.39 (Ar-C), 114.69 (Ar-C), 55.59 (OCH3).
Theoretical calculations
Theoretical calculations were performed for the compounds 1, 3a, 4, and anion A, using the Gaussian 16 program package69 at TianHe-2 located in Shanxi Supercomputing Center. The structures phase were optimized using a dispersion-corrected DFT method at the B3PW9170-GD3 level71,72 with the basis sets of 6-31 + G(d,p) for C, H, Si, P, and Ge atoms + SSD for Au. As shown in Table S2, the structural parameters of R’2Ge(AuP)(AuGe) moiety of 3a determined by X-ray analysis were well reproduced by the calculations. The structures related to the reaction of 3a with MeOTf were optimized at the B3PW91/def2-SVP73 level in the gas phase. All of the structures obtained herein were verified by examination of their Hessian matrix as minima (all frequencies real). The solvent effects on the relative stability of the compounds were not evaluated. The AIM charges of the atoms were calculated using the basin analysis module of Multiwfn3.862.
Data availability
Metrical data for the solid-state structures of 2b, 3a, 3b, 4, 5, 7, and 9 in this paper have been deposited at the Cambridge Crystallographic Data Centre under reference numbers CCDC: 2040233, 2040232, 2040235, 2040234, 2093574, 2040236, and 2093575, respectively. Copies of the data can be obtained free of charges from www.ccdc.cam.ac.uk/structures/. All other data supporting the findings of this study are available within the article and its Supplementary Information.
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Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 22071039, and 22101068), and the Natural Science Foundation of Zhejiang Province (Grant No. LY19B020007 and LQ21B020007). We are extremely grateful to Profs. Liejin Zhou and Lichun Kong of Zhejiang Normal University for the contributions of variable temperature (VT) NMR experiments.
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L.W. performed preliminary experiments on the system. G.Z., Y.L., L.Y., L.H., and X.C. carried out the synthetic work and analytical characterization. L.W. and M.K. performed DFT calculations. X.C. acquired the XRD data. L.W., X.C., M.K., and Z.L. wrote the paper, all authors discussed and commented on the manuscript. M.K. and Z.L. directed and coordinated the research.
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Wang, L., Zhen, G., Li, Y. et al. Structure and reactivity of germylene-bridged digold complexes. Nat Commun 13, 1785 (2022). https://doi.org/10.1038/s41467-022-29476-1
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DOI: https://doi.org/10.1038/s41467-022-29476-1
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