Unique Digoldgermanes: Structural Characteristics, Dynamic Behaviour and Distinctive Reactions

Digoldgermanes with a gold coordinated by a dialkylgermylene ligand, R’ 2 Ge(AuPR 3 )(AuGeR’ 2 ) (3a; R = Me, 3b; R = Et), were synthesized as green solids through the reactions of stable dialkylgermylene 1 with R 3 PAuCl followed by the reduction with KC 8 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 AuPR 3 group between Ge(IV) and Ge(II) of 3a and 3b occurring in the NMR time scale was found by the dynamic 1 H 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 Ph 4 PCl and CH 3 C(O)Cl smoothly to form the corresponding chloride-addition product 7 and chlorogoldgermane 9, respectively. Cyclic trimerization reactions of aromatic isocyanates were high-eciently catalyzed by 3a giving the corresponding 1,3,5-triaryl isocyanurates.

may be particularly interesting because the bond is polarized due to the difference of electronegativity between germanium ( = 2.01) and gold. 1,16 Although a variety of compounds with gold-germanium bonds have been studied, since the rst 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][25][26][27][28][29][30][31] and germylenes gold complexes. [32][33][34][35][36][37][38][39][40][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. 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 1 Au-1 Ge and 2 Au-1 Ge 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 ( 2 Ge) and 1 Au [2.4146(4) and 2.4089(8) Å for 3a and 3b, respectively] is slightly shorter than those of 1 Au-1 Ge and 2 Au-1 Ge bonds, suggesting signi cant difference in the bonding nature between Ge(IV)-Au and Ge(II)-Au bonds. The latter bond would be strengthened by the signi cant back-bonding interaction between a gold lone-pair orbital with a vacant germylene pp orbital. The bond angles 1 Ge-1 Au-2 Ge and 1 Ge-2 Au-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 ve 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 1 Au-1 Ge-2 Au angle of 3a (and 3b) is 106.909(16) ° [and 107.61(3) °], indicating the tetrahedral geometry around 1 Ge. Trigonal planar geometry around 2 Ge is evidenced by the sum of the bond angles around 2 Ge 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 1 H, 13 C, 29 Si and 31 P 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 uxionality of the molecules, whose dynamic behaviour was analyzed using DNMR; see the SI for their detailed NMR data and spectra. In the 1 H NMR at room temperature in THF-d 8 , the signals of ring and trimethylsilyl (TMS) protons of 1 GeC 4 and 2 GeC 4 rings of 3a (and 3b) appear at 2.28 and 0.29 ppm as broad singlets (Fig. S1). However, at -30 °C in THF-d 8 , 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 1 GeC 4 ring (Fig. S22). A similar but a little more broadened spectral pattern is observed in the 1 H NMR spectrum of 3a at -30 °C (Fig. S17).
Broadening of the signals of TMS and ring methylene protons shown in the 1 H NMR spectra of 3a and 3b suggests the uxionality of the molecules occurring in the NMR time scale. Because the methyl proton signals of PMe 3 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 AuPR 3 group or the isomerization between the two equivalent structures shown in Scheme 2. The variable temperature 1 H NMR spectra of 3b in the TMS proton resonance region are shown in Fig. S1. The TMS proton signals on 1 Ge and 2 Ge atoms coalesce at around -10 °C. The isomerization rate k C at the coalescence temperature (T C = 263 K) is estimated as ca. 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 KC 8 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 redshifted 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' 2 Ge(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 1 Ge-Au s orbitals, respectively, and LUMO has the nature of originally vacant 2 Ge pp orbital. The HOMO and LUMO energy levels of 3a are -4.46 and -2.31 eV, respectively, and they are signi cantly 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 shows the strong dative bonds from phosphine and germylene to 2 Au and 1 Au, respectively; the largest perturbation energy between the phosphine lone-pair orbital and 2 Au-1 Ge antibonding orbital amounts 127.5 kcal mol -1 and that between the germylene lone-pair orbital and 1 Au-1 Ge 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 rstly 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 PMe 3 at room temperature (Eq. 1) gives the corresponding digoldgermane coordinated by two phosphines, 4, which is isolated and characterized by X-ray ( 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 tri ate (MeOTf), was performed subsequently at ambient temperature (Eq. 2).
Unexpectedly, the methylation occurs on germanium atom instead of gold atom accompanying the elimination of Me 3 PAu + moiety.
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 Me 3 PAuOTf was determined by 31 P 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 2 Ge center. Facile isomerization between 3 and 3' as shown in Scheme 3 suggests the high electrophilicity at the 2 Ge to be attacked by an intramolecular gold nucleophile. Expectedly, 3a readily reacts with external nuclephiles at the 2 Ge center of 3a. It reacts with stoichiometric amounts of tetraphenylphosphonium chloride 6 (Ph 4 P + 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) are similar to those of Ge-Au covalent bonds found for 2 and 3 but a little longer than the 1 Au-2 Ge distance of 3a. The 1 Ge-2 Au-P and 1 Ge-1 Au-2 Ge bond angles of 7 are 173.81(4)º and 163.649(17)º, respectively. The dihedral angle between the two ve-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][51][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 (Ph 4 P + ) would considerably promote the dissociation of Ge-Au bond through the electrostatic and steric effect giving the nal 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 ve-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 2 Ge 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][54][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 (R 3 P)AuCl followed by the KC 8 reduction. 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 signi cantly 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) Figure 1 Literature-known motifs of gold complexes I-V possessing the halogen-like behaviour.  FMOs of 3a calculated at B3PW91-GD3 level. Hydrogen atoms are omitted in the wire frame structure of 3a.

Figure 4
Calculated effective atomic charges for the Ge and Au centres in 3a. The effective atomic charges of all atoms in 3a were calculated using the AIM method.

Figure 5
Molecular structures of 5. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Trimethylsilyl, ethyl, and methyl groups are depicted in a wireframe model. DFT-calculated free-energy pro le 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.   Scheme 2. Facile isomerization between 3 and its equivalent structure 3'.