Unique Digoldgermanes: Structural Characteristics, Dynamic Behaviour and Distinctive Reactions

doi:10.21203/rs.3.rs-929227/v1

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Unique Digoldgermanes: Structural Characteristics, Dynamic Behaviour and Distinctive Reactions

https://doi.org/10.21203/rs.3.rs-929227/v1

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published 04 Apr, 2022

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Digoldgermanes with a gold coordinated by a dialkylgermylene ligand, R’₂Ge(AuPR₃)(AuGeR’₂) (3a; R = Me, 3b; R = Et), were synthesized as green solids through the reactions of stable dialkylgermylene 1 with R₃PAuCl followed by the reduction with KC₈ at ambient temperatures. The structural characteristics of 3a and 3b were elucidated using NMR spectroscopy, X-ray crystallography, and DFT calculations. An intense absorption maximum was observed at 590 nm in the UV-vis spectrum of 3a in hexane. A pendular motion of AuPR₃ group between Ge(IV) and Ge(II) of 3a and 3b occurring in the NMR time scale was found by the dynamic ¹H NMR analysis, suggesting that the Ge(II) ligand has an enhanced electrophilicity to be attacked by the nucleophilic gold atom which closes to ‒1 oxidation state. DFT calculations of 3a revealed the existence of high-lying σ(Ge-Au) type HOMO and low-lying LUMO with germylene pπ nature. We show the bond formation and activation alternatively at Au or Ge atom, a methylation of digoldgermane 3a with MeOTf affords methylgermane 5. Moreover, the digoldgermane 3a reacts with Cl⁻ ion of Ph₄PCl and CH₃C(O)Cl smoothly to form the corresponding chloride-addition product 7 and chlorogoldgermane 9, respectively. Cyclic trimerization reactions of aromatic isocyanates were high-efficiently catalyzed by 3a giving the corresponding 1,3,5-triaryl isocyanurates.

Materials Chemistry

Organic Chemistry

Digoldgermanes

gold

gold-germanium bonds

With the advent of new interests in catalysis, microelectronics, materials, and pharmacology, gold and its complexes have become the focus of intense research both in theoretical and experimental chemistry for decades.^1-15 Among transition metal elements, gold is unique with the large electronegativity (ꭓ = 2.54)¹⁶ and high electron affinity (2.30 eV) owing to the large relativistic effects.¹ Auride ion Au^-serving as a halogen-like anion with the electronic configuration of 5d¹⁰6s² has been known since the discovery of caesium auride by Sommer in 1943.¹⁷ Related gold compounds bonded to main group metal and metalloidal atoms have long attracted much attention since then.

The chemistry of gold compounds bonded to main group elements has been developed extensively.^7,18-21 Very recently, a nucleophilic gold complex I released by Goicoechea, Aldridge and colleagues (Chart 1), was employed firstly in the activation of CO₂ and carbodiimide unfolding an unprecedented nucleophilicity of auride complex.²⁰ The umpolung reactivity of gold was ascribed to its high polarization of Au-Al bond. The halogen-like behaviour of gold has been applied to furnish these species containing B-Au bond due to the electronegativity values of boron are also lower than that of gold. Two borylgold complexes of Ph₃PAu(boryl) II and III (Fig. 1) were reported by Nozaki, and Kinjo, respectively, utilizing boryl anions as strong σ-donating ligands in combination with Ph₃PAuCl.^18,19

In a previous paper, we have found that reducing a stannylgold by KC₈ at a low temperature followed by the addition of an excess PEt₃gives diphosphine-coordinated digoldstannane IV as shown in Fig. 1,²² and intermediacy of V was suggested during the reactions. Compounds with a gold-germanium bond may be particularly interesting because the bond is polarized due to the difference of electronegativity between germanium (ꭓ = 2.01) and gold.^1,16 Although a variety of compounds with gold-germanium bonds have been studied, since the first report of Glockling and Hooton in 1962,²³ knowledge of the chemistry including the bonding characteristics and reactivities is still limited in gold-monosubstituted germanes^24-31 and germylenes gold complexes.^32-41 To the best of our knowledge, the isolable digoldgermane is still unknown to date, where two gold atoms are ascribed as halogen-like atom with an oxidation state of ‒1.

Digoldgermanes 3 were synthesized by the reduction of 2 obtained by the insertion of stable dialkylgermylene 1^42-45 into Cl-Au bond of R₃PAuCl (Scheme 1). Compounds 3 are unique with the two gold atoms heterolytically coordinated by dialkylgermylene 1 and a trialkylphosphine. Characteristics of the two different types of Au-Ge bonding, fluctuation of the AuPR₃ group between ¹Ge and ²Ge atoms of 3 in solution, and their distinctive reactions are also discussed in this paper.

Digoldgermanes 3a (R = Me) and 3b (R = Et) are synthesized by applying a reaction route shown in Scheme 1; see the Supplementary Information 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 a white solid in almost quantitative yields; many similar insertion reactions of terylenes into Au-X bonds have been reported until now.^26,40 When 2a and 2b are treated with potassium graphite (KC₈) in THF at room temperature, the corresponding digoldgermanes 3a and 3b are obtained as a dark-green solid in 49 and 54% yields, respectively. Compounds 3 are isolated as pure materials by recrystallization from a cooled hexane solution, stable in the solid state under argon and stored at ambient temperatures for a few months.

The structures of 2 and 3 were determined by multi-nuclear NMR spectroscopy and single crystal X-ray diffraction analysis; see also the Supplementary Information (SI) for the details. The solid-state structures of 3a and 3b (Fig. 2) show that their skeletal structures are like 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 a divalent germanium (²Ge) and ¹Au [2.4146(4) and 2.4089(8) Å for 3a and 3b, respectively] is slightly shorter than those of ¹Au-¹Ge and ²Au-¹Ge bonds, suggesting significant difference in the bonding nature between Ge(IV)-Au and Ge(II)-Au bonds. The latter bond would be strengthened by the significant back-bonding interaction between a gold lone-pair orbital with a vacant germylene pp orbital. 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 they are almost linear and these five atoms are almost in the same plane, being in accord with the VSEPR theory. The dihedral angle between the averaged planes of the two germacyclopentane rings is 78.915 º for 3a and 84.588 º for 3b, suggesting these two planes are intrinsically perpendicular to each other. The ¹Au-¹Ge-²Au angle of 3a (and 3b) is 106.909(16) ° [and 107.61(3) °], indicating the tetrahedral geometry around ¹Ge. Trigonal planar geometry around ²Ge is evidenced by the sum of the bond angles around ²Ge atom of 3a (and 3b), which is 359.999 º (and 359.994 º). The unique bonding feature of 3 is discussed later based on 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 a little confused by the fluxionality of the molecules, whose dynamic behaviour was analyzed using DNMR; 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 (Fig. S1). 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 (Fig. S22). A similar but a little more broadened spectral pattern is observed in the ¹H NMR spectrum of 3a at -30 °C (Fig. S17).

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 in the NMR time scale. Because the methyl proton signals of PMe₃ and methyl and methylene proton signals of 3b remains sharp even at room temperature, the dynamic process is suggested to be the pendular motion of the AuPR₃ group or the isomerization between the two equivalent structures shown in Scheme 2. The variable temperature ¹H NMR spectra of 3b in the TMS proton resonance region are shown in Fig. S1. The TMS proton signals on ¹Ge and ²Ge atoms coalesce at around -10 °C. The isomerization rate k_C at the coalescence temperature (T_C = 263 K) is estimated as ca. 90 s^-1 using the equation of , where n₁ is the resonance frequency of TMS protons on ²Ge (0.35 ppm, 175 Hz) and n₂ as the average of two TMS resonances on ²Ge (0.27 ppm, 135 Hz). The activation free energy (ΔG_C^‡)at the T_c is evaluated as 13.0 kcal mol^‒¹. While the transition state of the interconversion remains open, a plausible transition state is suggested to be 3a^T due to the aurophilicity^46-48 as shown in Scheme 2.

The mechanism for the formation of 3 remains open but may be proposed to proceed as shown in Scheme 3. The initial reduction of 2 with KC₈ affords A as an intermediate whose nucleophilic attack to another molecule of 2 gives 3 undergoing a substitution reaction. While no experimental evidence for the intermediacy of A has been obtained, 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 Fig. S54. The NBO analysis shows that the lone pair electrons are largely localized on 4s orbital of Ge with hybridization of sp^0.25 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 center of A would be allowed to nucleophilic attack the germanium atom of 2.

The UV-vis spectrum of 3a in THF shows the maximum absorption wavelength at 590 nm with the absorptivity ε/(M^–1 cm^–1) of 3,560 (see Fig. S5). It is worth mentioning, the band is broad but more red-shifted than the n→4p band of germylene 1 (λ_max = 450 nm, ε/M^–1 cm^–1= 320).⁴²

To gain more insight into the structural characteristics of 3a and related compounds, the DFT calculations were performed at B3PW91-GD3 level (see the SI for calculation details). As shown in Table S2, 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. 3. HOMO and HOMO‒1 are assigned as the symmetric and antisymmetric combinations of two ¹Ge-Au s orbitals, respectively, and LUMO has the nature of originally vacant ²Ge pp 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, respectively 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 (l_max = 590 nm) observed at longer wavelength than that of 1 (l_max = 450 nm).

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^.13hybrid orbital of ¹Ge and a 6s orbitals of ¹Au and ²Au; their Wiberg bond index is 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 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 AIM.⁴⁹ 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. 3). The balancing negative charge in 3a is mostly localized at two gold atoms with the ‒1 oxidation state, in which gold atom demonstrates firstly the halogen-like behaviour featuring two conspicuously polarized Au^δ−-Ge^δ+bonds.

Upon the DFT results, 3 could be regarded as amphoteric molecules, which involves a nucleophilic gold atom with the ‒1 oxidation state and a dialkylgermylene ligand having an enhanced electrophilicity. We may expect their distinctive types of reactions. The treatment of 3a with two moles of PMe₃ at room temperature (Eq. 1) gives the corresponding digoldgermane coordinated by two phosphines, 4, which is isolated and characterized by X-ray (Fig. S5) and NMR analysis (Fig. S27-30). HOMO of 4 holds almost the same shape and the energy level with that of 3a; the HOMO of 4 features the anti-symmetric combination of the two Ge-Au s orbitals with the energy of –4.52 eV. On the other hand, the LUMO has mainly the nature of the vacant Au 6p orbitals and its energy level (–0.54 eV) is much higher than that of 3a (Fig. S53). No reaction occurs when excess stannylene is added in the solution of 3a. Neither N-Heterocyclic carbene (NHC) nor carbon monoxide (CO) does react with 3a.

Because the negative charge in 3 is mostly localized equally at tow gold atoms, they should react with electrophiles via addition or substitution reactions The treatment of 3a with a powerful electrophilic methylation agent, methyl triflate (MeOTf), was performed subsequently at ambient temperature (Eq. 2). Unexpectedly, the methylation occurs on germanium atom instead of gold atom accompanying the elimination of Me₃PAu⁺ moiety.

The corresponding product 5 was identified by NMR spectroscopy and single crystal X-ray diffraction analysis (Fig. 4). Three Ge1-Au1 distance [2.4330(7) Å] of 5 is similar to those of Ge-Au covalent bonds found for 2 and 3 but a little longer than the Au1-Ge2 distance [2.3997(6) Å] of 5. The Ge1-Au1-C17 and Ge1-Au1-Ge2 bond angles of 5 are 105.02(18) º and 168.47(2) º, respectively. The dihedral angle between the two five-membered rings is 68.98 º, which is slightly acute than that (78.91 º) of 3a.

We initially expected that the MeOTf as an electrophile is attacked by an Au atom in 3a, but in reality, Me⁺ formed a bond with the Ge atom in 3a. To elucidate the pathways of the reaction of 3a with MeOTF giving 5, DFT calculations were performed at the B3PW91/def2-SVP level in the gas phase. While two intermediates Int1 and Int2 were located by the calculations, Int1 is 19.5 kcal/mol higher in energy than Int2 and 3a, indicating Int1 will not be an intermediate of the reaction, as shown in Fig. 6. Instead, the reaction would proceed directly via Int2, which is slightly more stable than the starting materials ∆G = 1.0 kcal‧mol^‒¹. The formation of Me₃PAuOTf was determined by ³¹P NMR (δ = 13.13 ppm); see the Supplementary Information for the details.

Because 3a has a low-lying LUMO that is even lower than that of germylene 1, 3a should be highly electrophilic at the ²Ge center. Facile isomerization between 3 and 3’ as shown in Scheme 3 suggests the high electrophilicity at the ²Ge to be attacked by an intramolecular gold nucleophile. Expectedly, 3a readily reacts with external nuclephiles at the ²Ge center of 3a. It reacts with stoichiometric amounts of tetraphenylphosphonium chloride 6 (Ph₄P⁺Cl^–) at ambient temperature giving the corresponding chlorogermane 7 featuring a Ge-Cl bond (Fig. 7a). The structure of 7 was determined by NMR spectroscopy and single crystal X-ray diffraction analysis (Fig. 7b). In ¹H NMR spectra, trimethylsilyl proton resonances of 7 appear as four sharp singlets at 0.31, 0.25, 0.24 and 0.20 ppm, respectively, with 1:1:1:1 intensity ratio (Fig. S34), showing that the solid-state structure is maintained in solution. A doublet ¹H NMR signal appeared at 1.34 ppm is assigned to the methyl protons of a PMe₃group. Two signals in the ³¹P NMR appearing at 26.51 and 21.20 ppm are assignable to a PMe₃ ligand and a Ph₄P cation in 7, respectively (Fig. S37). 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)º and 163.649(17)º, respectively. The dihedral angle between the two five-membered rings is 78.915º, which is similar with that found in 3a.

Acyl chlorides were also used as a chloride source to react with germylene and stannylenes, in which the corresponding acylgermanes and acylstannanes have been obtained undergoing an insertion of acyl-Cl bond.^50-52 While the reaction of 3a with acetyl chloride 8 was carried out at ambient temperature giving the chlorogoldgermane 9 in 67% together with acetyl(triemethylphosphine)gold (Fig. 7a). The formation of 9 may occur concertedly from an acetylchloride-3a complex, from which the cleavage of the C–Cl bond and the recombination of Ge-Cl bond afford an acetyl derivate of 7 as the intermediate. Subsequently, acetyl cation possessing a stronger Lewis acidity than that of tetraphenylphosphonium (Ph₄P⁺) would considerably promote the dissociation of Ge-Au bond through the electrostatic and steric effect giving the final product 9 (Scheme S2). The structure of 9 was determined by NMR spectroscopy and single crystal X-ray diffraction analysis (Fig. 7c). The Au1-Ge1-Cl1 and Ge1-Au1-Ge2 bond angles of 9 are 97.614(65) º and 167.659(34) º, respectively. It is worth noting that the dihedral angle between the two five-membered rings is 166.99 º, two planes almost parallel with each other, which is quite different with those of 3, 5, and 7.

Digoldgermane 3a has been found to exhibit effective catalytic ability for the cyclic trimerization of aryl isocyanates (Eq. 3). 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 (Scheme S3). The high electrophilicity of the ²Ge atom in 3a may be essential to the catalytic activity. Related goldgermanes without germylene coordination like 2a or 4a showed no catalytic activity for the trimerization of aryl isocyanates. There have been many catalysts discovered and diverse mechanisms have been proposed for the trimerization.^53-55 Though no evidence has been obtained, the Lewis acidity of 3 may be essential for the present catalytic mechanism; see Fig. S3 for a proposed catalytic cycle.

In conclusion, unprecedented digoldgermanes having a germylene ligand, 3a and 3b, are synthesized through the reaction of stable dialkylgermylene 1 with (R₃P)AuCl followed by the KC₈reduction. The DFT calculations of 3a show that the HOMO has the nature of Ge-Au s bonding orbital, the LUMO has largely germylene vacant 4pp nature with significantly lower energy level than that of 1. These support that the heterolytically coordinated digoldgermanes 3 feature amphipathic behaviour in theory involving nucleophilic gold atoms with the ‒1 oxidation state and stronger electrophilicity than the germylene itself. In accord with these characteristics, 3 shows (1) the pendular motion of AuPR₃ ligand between two germanium atoms of 3 occurring in an NMR time scale; (2) a methylation of 3a with MeOTf exhibiting the nucleophilic behaviour of gold atom with the ‒1 oxidation state; (3) facile reactions with Ph₄P⁺Cl^- and acetylchloride to give the corresponding chloride adducts; and (4) catalytic activity towards the cyclic trimerization of aryl isocyanates giving the corresponding triaryl isocyanurates.

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 procedures.^56,57 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 of the solvents. NMR multiplicities are abbreviated as follows: s = singlet, brs = broad singlet, d = doublet, dt = doublet of triplets, t = triplet, and m = multiplet. 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 spectrum was 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 white gold(I) complex 2a or 2b, where both of gold(I) complexes 2 are stable under argon for few months. 2a (547 mg, 90%): a white powder; mp. 153 ºC (dec.); ¹H NMR (400 MHz, C₆D₆) 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, ²J_H-P = 9.20 Hz, PCH₃, 9H); ¹³C NMR (101 MHz, C₆D₆) δ 35.28 (d, ⁴J_C-P= 2.93 Hz, ring-CH₂), 26.60 (d, ³J_C-P= 14.04 Hz, ring-C^q), 14.87 (d, ¹J_C-P= 28.58 Hz, PCH₃), 5.23 (SiCH₃), 5.13 (SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) δ 4.17, 2.02; ³¹P NMR (162 MHz, C₆D₆) 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%): a white powder; mp. 172 ºC (dec.); ¹H NMR (400 MHz, C₆D₆) 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, ³J_H-P = 17.60 Hz, ³J_H-H = 7.60 Hz, 9H), 0.61 (s, SiCH₃, 18H), 0.51 (s, SiCH₃, 18H); ¹³C NMR (101 MHz, C₆D₆) δ 35.29 (d, ⁴J_C-P= 2.52 Hz, ring-CH₂), 26.56 (d,³J_C-P= 13.13 Hz, ring-C^q), 17.97 (d, ¹J_C-P= 26.06 Hz, PCH₂), 8.76 (s, PCH₂CH₃), 5.25 (s, SiCH₃), 5.06 (s, SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) δ 4.10, 2.14; ³¹P NMR (162 MHz, C₆D₆) 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) and KC₈ (1.05 mmol, 177 mg) was mixed in THF (10 mL). The mixture was stirred at ambient temperature for 12 h. Then the mixture was concentrated under vacuum. Washing the reside with hexane (4 mL) for 3 times and extracting from the residue with toluene (20 mL) followed by concentration under vacuum gave 3a or 3b as a blue-green solid. 3a (347 mg, 49%): mp. 150 ºC (dec.); ¹H NMR (400 MHz, C₆D₆) δ 2.33 (brs, ring-CH₂, 8H), 0.89 (d, ²J_H-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, ²J_H-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) δ 36.99 (ring-CH₂), 30.22 (ring-C^q), 16.20 (d, ¹J_C-P= 21.61 Hz, PCH₃), 4.36 (brs, SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) δ 0.14 (brs); ³¹P NMR (202 MHz, C₆D₆) δ 39.10; HRMS (ESI): m/z calcd for [C₃₂H₈₀AuGe₂Si₈]⁻: 1032.2535, found: 1032.2587. 3b (380 mg, 54%): mp. 162 ºC (dec.) ¹H NMR (500 MHz, THF-d₈) 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, ²J_H-P = 7.7 Hz, 6H), 1.21 (dt, ³J_H-P = 16.9, ³J_H-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) δ 37.20 (s, ring-CH₂), 19.17 (d, ¹J_C-P= 20.50 Hz, PCH₂), 8.84 (d,²J_C-P= 1.52 Hz, PCH₂CH₃), 4.26 (s, SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) δ 0.13 (brs); ³¹P NMR (202 MHz, THF-d₈) δ 55.10; 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 reacted at room temperature for 20 min. The solution color changed from blue to light green. Washing the reside with cooled hexane (2 mL) for 3 times and extracting from the residue with toluene (2 mL) followed by concentration under vacuum gave 4 as a light green crystal in 92% yield (71 mg): mp. 148 ºC (dec.); ¹H NMR (400 MHz, C₆D₆) δ 2.45 (s, ring-CH₂,4H), 0.7 (s, SiCH₃, 36H), 0.68 (d, PCH₃, J_H-P = 7.6 Hz, 18H); ¹³C NMR (126 MHz, C₆D₆) δ 37.42 (ring-CH₂), 19.74 (t, ³J_C-P = 6.0 Hz, ring-C^q), 15.98 (dd, ¹J_C-P = 21.4 Hz, ⁵J_C-P = 3.8 Hz, PCH₃), 5.86 (SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) δ 1.93; ³¹P NMR (162 MHz, C₆D₆) 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 that was extracted with hexane (20 mL). The residual solvents were evaporated in vacuo and recrystallized in hexane to separate 5 (45.9 mg, 57%): a red solid; mp. 188 ºC; ¹H NMR (400 MHz, C₆D₆) 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) δ 62.09 (ring-C^q), 36.58 (ring-CH₂), 35.92 (ring-CH₂), 18.90 (ring-C^q), 11.31 (CH₃), 5.55 (SiCH₃), 5.21(SiCH₃), 2.69 (SiCH₃); ²⁹Si NMR (99 MHz, C₆D₆) 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 3 times. Finally, a light-red solid powder compound 7 was obtained. 7 (125 mg, 97%): mp. 172 ºC (dec.); ¹H NMR (400 MHz, THF-d₈) 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, ²J_P-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) δ 136.28 (Ar-C), 135.48 (d, Ar-C,³J_P-C = 11.0 Hz), 131.20 (d, Ar-C, ²J_P-C = 11.1 Hz), 118.94 (d, ¹J_P-C = 89.7 Hz), 37.36 (ring-CH₂), 36.21(ring-CH₂), 28.82 (ring-C^q), 18.96 (d, PCH₃, J = 5.4 Hz), 16.82 (d, ring-C^q, J = 18.9 Hz), 5.77 (SiCH₃), 5.46 (SiCH₃); ²⁹Si NMR (99 MHz, THF-d₈) δ 1.81, 1.58, 1.28, ‒0.04;³¹P NMR (162 MHz, THF-d₈) δ 26.47 (PMe₃), 21.21 (PPh₄); HRMS (ESI): m/z calcd for [C₃₅H₈₉Au₂ClGe₂PSi₈]^–: 1341.2299, found: 1341.2312.

Reaction of dialkylgermylene 1 with Ph₄PCl. In a glove box, PPh₄Cl (29.4 mg, 0.078 mmol) was added into a THF (10 mL) solution of 1 (29.2 mg, 0.070 mmol) and the mixture was stirred at room temperature for 48 h. The solvent was removed under vacuum to afford the residue that was extracted with hexane (5 mL) for 3 times. Finally, a colorless solid powder compound 1•HCl was obtained (28.9 mg, 91%): mp. 82 ºC; ¹H NMR (400 MHz, C₆D₆) δ 6.64 (s, Ge-H, 1H), 1.94 (m, ring-CH₂, 2H), 1.77 (m, ring-CH₂, 2H), 0.31 (s, SiCH₃, 18H), 0.15 (s, SiCH₃, 18H); ¹³C NMR (101 MHz, C₆D₆) δ 33.32 (ring-CH₂), 16.41 (ring-C^q), 3.50 (SiCH₃), 2.79 (SiCH₃).

Synthesis of complex 9. In a glove box, CH₃CClO (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 3 times. Finally, a rufous solid compound 9 was obtained: (125 mg, 67%): mp. 172 ºC (dec.); ¹H NMR (400 MHz, C₆D₆) 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) 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) 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 (7.66x10–4 M in THF, 130 μL, 0.01 mol%) was introduced in a thick-walled tube which contained the solvent THF (2.0 mL) and the desired aryl isocyanate 10 (1 mmol). The reaction mixture was heated at 80 ºC for 14 h. Upon completion, the products were purified by washing with n-hexane. 11a (125 mg, 97%): ¹H NMR (400 MHz, CDCl₃) δ 7.53-7.45 (m, Ar-H, 6H), 7.43-7.40 (m, Ar-H, 6H); ¹³C NMR (101 MHz, CDCl₃) δ 148.84 (O=C), 133.73 (Ar-C), 129.51 (Ar-C), 128.55 (Ar-C). 11b (86 mg , 86%): ¹H NMR (400 MHz, CDCl₃) 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) δ 148.27 (O=C), 135.73 (Ar-C), 131.86 (Ar-C), 129.86 (Ar-C), 129.86 (Ar-C). 11c (72 mg, 72%): ¹H NMR (400 MHz, C₆D₆) 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) δ 147.92 (O=C), 136.27 (Ar-C), 132.00 (q, ²J_C-F = 33.13, Ar-C), 129.23 (Ar-C), 126.83 (q, ³J_C-F = 3.63, Ar-C), 122.27 (C-F). 11d (98 mg, 98%): ¹H NMR (400 MHz, CDCl₃) δ 7.26 (d, J = 2.4 Hz, Ar-H, 12H), 2.38 (s, CH₃, 9H); ¹³C NMR (101 MHz, CDCl₃) 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 (98 mg, 98%): ¹H NMR (400 MHz, CDCl₃) 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) δ 159.99 (Ar-C), 149.24(O=C), 129.52 (Ar-C), 126.39 (Ar-C), 114.69 (Ar-C), 55.59 (OCH₃).

Dialkylgermylene 1 catalyzed cyclic trimerization of 1-isocyanato-4-methylbenzene 10d. A THF-d₈ solution of 1 (10 mg, 5 mol%) was introduced in a Young-NMR tube which contained 1-isocyanato-4-methylbenzene 10d (67 mg, 0.5 mmol). The reaction mixture was heated at 80 ºC for 14 h. Upon completion, the product 10d was detected in 92% yields by ¹H NMR spectrum.

Theoretical calculations. Theoretical calculations were performed for the following compounds 1, 3a, 4, and anion A, using the Gaussian 16 program package⁵⁸ at Southern University of Science and Technology. The structures phase were optimized using a dispersion-corrected DFT method at the B3PW91⁵⁹-GD3 level^60,61 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 are well reproduced by the calculations. The structures related the reaction of 3a with MeOTf were optimized at the B3PW91/def2-SVP⁶² 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 Multiwfn3.8.⁴⁹

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.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY19B020007 and LQ21B020007), the National Natural Science Foundation of China (Grant No. 22071039, and 22101068). We are extremely grateful to Prof. Liejin Zhou and Lichun Kong of Zhejiang Normal University for the contributions of variable temperature (VT) NMR experiments.

Author contributions

L.W. performed preliminary experiments on the system. G.Z., Y.L., 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.

Competing interests

The authors declare no competing financial interest.

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published 04 Apr, 2022

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Unique Digoldgermanes: Structural Characteristics, Dynamic Behaviour and Distinctive Reactions

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