Digold(I) Thianthrenyl Complexes. Effect of Diphosphine Ligands on Molecular Structures in the Solid State and in Solution.

A series of digold complexes possessing two thianthrenyl ligands, Au2(Thi)2(Ph2P(CH2) n PPh2) (Thi: 1-thianthrenyl; 1: n = 1, 2: n = 2, 3: n = 3, 4: n = 4), were prepared and characterized by crystallographic and spectroscopic measurements. X-ray crystallography of complexes 1 and 3 revealed U-shaped structures with short Au-Au distances [3.2171(3) Å and 3.0735(2) Å]. Complex 2 and three of the four structure-determined molecules of complex 4 showed structures without Au-Au contacts. UV-vis spectroscopic measurements of 1-4 and TD-DFT calculations of the two conformers of 1 revealed that complexes 1 and 3 in the solution phase contained conformers with Au(I)-Au(I) interactions in a much higher proportion than complexes 2 and 4. As a result, complexes with diphosphine ligands containing an odd number of methylene groups preferred structures with Au-Au interactions in the solid state and in solution. Oxidation of 1 with 2 equiv of PhICl2 yielded a mixture of monomeric and dimeric thianthrenes and its dimer via ligand elimination and C-C coupling, respectively.

Introduction

Digold(I) complexes with diphosphine ligands, formulated as Au2X2(R2P–Y–PR2) (X = halogeno, thiolato, acetylide; Y = (CH2), C6H4, or 1,1′-ferrocenylene), have been widely investigated because of their antitumor activity,[1] ability to serve as precursors of multinuclear metal complexes,[2−4] fluorescence switching behavior under an external stimulus,[5] and catalytic activity for the synthesis of polymers and organic compounds.[6,7] Crystallographic analysis of Au complexes bearing a dppm ligand (dppm = 1,1-diphenylphospinomethane) revealed U–shaped molecular structures with two proximal Au atoms showing d10–d10 interactions.[8,9] However, the structure of the Au2X2(R2P–CH2–PR2) complexes in solution has not been fully explored. U-shaped structures have been observed for several supramolecules in the solution phase. In some cases, the digold(I) complexes in the solution may adopt other conformations than the U-shaped structure because of the low energy of the d10–d10 interaction.

Thus, we attempted the synthesis of digold(I) complexes with polyaromatic ligands in order to stabilize the U-shaped structure by the π–π interaction of the two aromatic ligands. Thianthrene has been reported to form complexes with Ag(I), Au(I), and Pd(II) as neutral S-ligands.[10−12] Pd complexes containing a phosphine ligand bearing a thianthrenyl substituent have been reported to form a cyclic compound with a Pd–S bond.[13] An Fe complex with η6-coordinated thianthrene has also been prepared and reported to show electrochemical responses from the ligand and Fe centers.[14] However, studies on transition-metal complexes with C-bonded thianthrenyl ligands are scarce. Here, we report the synthesis of a series of the digold complexes with two thianthrenyl ligands, which prefer the structures with Au–Au interactions when the supporting diphosphine ligand contains an odd number of methylene groups in the spacer.

Results and Discussion

Following the discovery of the transmetalation of arylboronic acids or esters with Au(I) complexes, including Au2X2(dppm) (X = Cl, Br),[15−20] an increasing number of arylgold(I) complexes have been reported over the past 2 decades. In this study, we conducted the reaction of 1-thianthrenylboronic acid with dichlorogold(I) complexes bearing bis(diphenylphosphino)alkane ligands, Au2Cl2(Ph2P(CH2)PPh2) (n = 1–4), in the presence of Cs2CO3. The reaction produced bis(thianthrenyl)gold(I) complexes, Au2(thi)2(Ph2P(CH2)PPh2) (thi: 1-thianthrenyl; 1: n = 1, 2:n = 2, 3:n = 3, 4:n = 4), in 60–91% yield, as shown in Scheme .

Scheme 1

Synthesis of Digold(I) Complexes 1–4 and Bis(thianthrenyl)naphthalene 5

A naphthalene derivative with two thianthrenyl groups at the 1,8-positions (5) was also prepared using the Suzuki–Miyaura reaction of 1-thianthrenylboronic acid with dibromonaphthalene and used as a reference compound for the Au complexes.

The molecular structures of compounds 1–5, as confirmed by X-ray crystallography, are shown in Figures and S22–S27. Complex 1 with the dppm ligand exhibits a cyclic Au2P2C core (Au–Au distance 3.2171(3) Å) and two P–Au–C bonds with the torsion angle of 23.3°. The two phenylyne rings bonded to the Au centers face each other in a parallel orientation. Complex 2 in the crystalline state demonstrates P–Au–C bonds oriented in the opposite directions with apparent C2 symmetry around the midpoint of the zigzag P(CH2)2P chain. Crystallographic measurements of complex 3 revealed a folded Ph2P(CH2)3PPh2 ligand and two close Au(I) centers. The short Au(I)–Au(I) distance [3.0735(2) Å] suggested Au(I)–Au(I) interactions. Complex 4 obtained by recrystallization from a C2H4Cl2–hexane solution shows P–Au–C bonds oriented in the opposite directions with apparent C2 symmetry. Crystals of complex 4 obtained from acetone contained two crystallographically independent molecules. One molecule had a structure similar to that of the crystals obtained from C2H4Cl2–hexane, while the other had a short Au–Au distance (3.0707(4) Å). All in all, three of the four obtained crystal structures for 4 did not exhibit Au–Au interactions.

Figure 1

Crystal structures of (a) 1, (b) 2, (c) 3, (d) 4 (recrystallized from C2H2Cl4/hexane), and (e) 5.

Figure shows the optimized structure of complex 1, obtained by DFT calculations.[21] Two close aromatic rings of the thianthrenyl ligands are in a parallel position with a short contact of the centroids (3.73 Å). The HOMO of the molecule was spread over the thianthrenyl ligands, while the LUMO was distributed over two gold atoms.

Figure 2

HOMO (a, −7.89 eV) and LUMO (b, 0.732 eV) of complex 1 by DFT analysis.

Thus, complexes 1 and 3 with dppm and dppp adopted the structure with Au(I)–Au(I) interactions, while complexes 2 and 4 with dppe and dppb ligands revealed an anti-structure with the two Au(I)–C bonds in the opposite direction. A similar tendency was observed for the previously reported digold(I) complexes with the diphosphine ligands, X–Au–R2P–(CH2)–PR2–X (X: anionic ligand, R = Ph, Cy, etc., n = 1–4). The crystal conformation of digold(I) complexes bearing dppm ligands and an Au2P2C ring varies depending on the anionic ligands.[8,9] Dichloro and diiodo complexes, (XAu)2(dppm) (X = Cl, I), had a twisted Au2P2C ring with large torsion angles between the two Au–P bonds (>60°) and Au(I)–Au(I) distances of 3.418–3.575 Å.[22] The Au2P2C ring in the structure of aryl complexes, (ArAu)2(dppm), exhibited a less-twisted conformation (torsion angle <55°), and these complexes contained shorter Au(I)–Au(I) distances (3.012–3.154 Å).[6d,6d,9c−9e,22a,22a] Complexes of 4-fluorophenyl and anthryl ligands demonstrated two parallel aryl groups bonded to the Au2(dppm) unit,[9c,22a] whereas two perfluorophenyl ligands showed no overlap, suggesting the lack of ligand π–π interactions between the ligands.[6d] An Au–Au distance of 1 (3.2171(3) Å) was longer than that of other diaryl–digold(I) complexes, suggesting that the π–π interaction of 1 is not a major factor to contribute to the conformation of the Au2P2C ring and the Au–Au distance.

Digold(I)–dppe complexes with anionic ligands have been characterized by X-ray crystallography.[15,23−26] On the one hand, 11 complexes showed the structures with anti-conformation, similar to complex 2.[23,26] On the other hand, four complexes, (bpy–C≡C)2Au2(dppe), (4-O2NC6H4N=C(OMe)S)2Au2(dppe), [(μ-Ag){Au2(μ-mes)2(dppe)}]+, and Ph2Au2(dppe), adopted a syn-conformation with the Au(I)–Au(I) distances of 3.021, 3.117, 3.189, and 2.923 Å, respectively.[15,24−26] As for the reported digold(I) complexes with the dppp ligand, structures of the nine molecules with the syn-conformation (Au(I)–Au(I) distance, 3.029–3.372 Å) and those of the three molecules with the anti-conformation have been reported.[24,25,27−30] The complexes with dppb ligands preferred the anti-conformation or a conformation without the Au(I)–Au(I) interaction.[30−32] Thus, the conformation of complexes 1–4 in the crystal form is aligned with the structure of previously reported complexes.

Results of NMR and UV–vis spectroscopy results revealed the relative stability of the conformations of 1–4. Figure shows the 1H NMR spectra of complexes 1–4. The signals were assigned using 1H–1H COSY NMR spectroscopy. The spectrum of 1 showed signals of the three hydrogens of the phenylyne group bonded to the Au center, He, Hf, and Hg, at 7.04, 6.72, and 6.97 ppm, respectively. The signals were at a higher magnetic field strength than those of 2–4 (He = 7.25–7.40, Hf = 7.00–7.23, and Hg = 7.15–7.25 ppm). Compound 5 with thianthrenyl groups (centroid distance by X-ray crystallography: 3.43 Å) showed the corresponding 1H NMR signals at even lower ppm (6.77, 5.44, and 6.73 ppm, respectively). The NMR results suggested that the structure of complex 1 in solution is similar to its crystal structure, and in both solution and crystal phases, the two thianthrenyl ligands are in close proximity to each other. Some NMR signals of the aromatic hydrogens in diplatinum(II) complexes with 2-phenylpyridine and dppm ligands have been reported to appear at 5.9–6.1 ppm, which was ascribed to the close stacking of the aromatic ligands at close positions.[33] At room temperature, complex 3 exhibited Hf and Hg signals at slightly higher magnetic field positions than those of complexes 2 and 4. The conformation of 3 in the solution was determined by UV–vis spectroscopy (vide infra).

Figure 3

1H NMR spectra (500 MHz, CDCl3, rt). (a) 1, (b) 2, (c) 3, and (d) 4.

The low-temperature 1H NMR spectrum of 1 showed the Hf signal at 6.58 ppm (−60 °C) and 6.52 ppm (−90 °C). At 25 °C, and the signal of Hf appears at 6.72 ppm. The significant shift of this signal as a function of temperature suggested that the solution contained an equilibrium between the conformational isomers. It also highlights that the structural change is faster than the NMR time scale. No significant change was observed for the Hf signal of 2–4 (within 0.07 ppm) at different NMR measurement temperatures (25 and −90 °C).

Figure shows the UV–vis spectra of complexes 1–4 in a CHCl3 solution (1.0 × 10–3 mM). Absorption peaks were observed at similar positions due to π–π* transitions. The spectra of complexes 1 and 3 showed a shoulder at 310–330 nm, although such shoulder absorption was weak for complexes 2 and 4. The appearance of the shoulder at a lower energy region than the common π–π* transition is attributed to the overlap of the aromatic ligands in the structures of complexes 1 and 3. The fluorescence and excitation spectra of 1 in CHCl3 showed peaks at 440 and 320 nm, respectively (Figure S16). Although the intensity of the fluorescence peak is small, the results are consistent with the presence of the peak shoulder in the UV–vis spectra and π–π interactions between the two thianthrenyl ligands in the syn structure.

Figure 4

UV–vis spectra of complexes 1–4 in a 1.0 × 10–6 mol L–1CHCl3 solution.

We conducted TD-DFT calculations of complex 1 to obtain further insights into the relationship between the molecular structure and the optical properties of the complexes. The results are shown below. Figure shows the calculated absorption spectra for the two structures of complex 1.[34] The molecule with syn-oriented ligands showed a high intensity π–π* absorption at 270–310 nm and a weaker absorption at 330 and 337.5 nm (Figure a). The theoretical results are in good agreement with the experimental UV–vis spectrum (Figure ), which showed a shoulder absorption at wavelengths higher than 300 nm.

Figure 5

TD-DFT-calculated absorption spectra of complex 1 with different molecular conformations. (a) Structure with syn-oriented ligands and with Au–Au interactions. (b) Structure with anti-oriented ligands and without Au–Au interactions.

The calculated absorptions at 330 and 337.5 nm for the former conformer were assigned to the HOMO–1 to LUMO and HOMO to LUMO transitions, respectively. The smaller transition energy compared to the latter conformer is ascribed to the higher energy levels of HOMO and HOMO–1 due to π–π interactions and the lower energy level of LUMO caused by the Au–Au interaction. Polymers, containing cyclophane units[35] and those composed of electron-donating and electron-withdrawing aromatic groups,[36] have been reported to show a shift in the peak to low energies or appearance of the shoulder at long wavelengths due to π–π interactions.

Complex 1, with the molecular conformation consisting of two ligands with anti-orientation did not exhibit absorption at wavelengths higher than 300 nm, as shown in Figure b. This result suggests that the solution of 1 adopts the former conformation with Au–Au interactions. The UV–vis spectrum of complex 3 with the dppp ligand also showed a clear shoulder absorption at 310–330 nm, whereas such shoulder peaks of complexes 2 and 4 were much weaker (Figure ). These observations suggest that complexes 2 and 4 in the solution prefer the conformers with the two P–Au–C bonds in opposite directions.

We attempted to obtain further spectroscopic evidence for the Au(I)–Au(I) interaction between the thianthrene ligands and determine the precise ratios of the conformers in the solution. However, NMR measurements using the 2D rotating frame Overhauser effect spectroscopy (ROESY) technique did not provide any cross-peaks derived from the close contact of the two thianthrenyl ligands. In addition, 1H NMR measurements at different concentrations did not reveal alterations in the spectra caused by the aggregation of the thianthrenyl ligands (Supporting Information, Figures S14 and S15).

Recently, the oxidation of diaryldigold(I)–dppm complexes by PhICl2 has been reported to induce facile 1,2-reductive elimination of biaryl as the coupling product.[6d] This reaction has been employed to synthesize an aromatic macrocycle.[20] The reaction involved oxidation of an intermediate digold(I) complex with the bis(dicyclohexylphosphino)methane ligand.[37] Complex 1 is thus expected to undergo both the oxidation at the two Au(I) centers and the thianthrenyl ligands.

Complex 1 is oxidized by NOBF4 to form thianthrene, as shown in Scheme (a). The thianthrenyl ligand underwent one-electron oxidation, generating a cation radical species to weaken the Au–C bond, resulting in hydrogenerative liberation of the ligand. The reaction of PhICl2 with complex 1 at below room temperature yielded a mixture of thianthrene and its dimer[38] in 26 and 34% yields, respectively (Scheme (b)). Complex 4 also reacted with PhICl2 to form the two products in similar yields (26 and 28%). These two oxidation products are considered to have originated from two independent mechanisms. One mechanism involves the oxidation of the thianthrenyl ligands to generate a radical cation intermediate, similar to the reaction using NOBF4. Another oxidation reaction may occur at the two Au(I) centers, similar to those reported for the reaction of PhICl2 with diaryldigold(I)–dppm complexes.[6d]

Scheme 2

(a) Oxidation of Complex 1 by NOBF4; (b) Oxidation of Complexes 1 and 4 by PhICl2

Figure shows the results of electrochemical measurements for 1, 4, and 5. Compound 5 showed two reversible oxidation and reduction peaks with E1/2 = 0.78 and 0.92 V. The peaks correspond to a complex oxidation at the thianthrenyl groups to form the cation radical and dication species, as previously reported for other thianthrene derivatives.[39]

Figure 6

Cyclic voltammograms of 1, 4, and 5. Conditions: sample 1.0 mM, solv. 0.10 mM Bu4NPF6–CH2Cl2, WE GC, CE Pt wire, and RE 0.010 mM AgNO3 in MeCN/Ag.

Complexes 1–4 showed one irreversible oxidation and two quasi-reversible oxidation-reduction processes. We tentatively assigned the latter oxidation-reduction processes at higher potentials to the two-step redox reaction of the thianthrenyl ligands and the irreversible oxidation at a low potential to that of the digold(I) center. The two oxidation processes correspond to the formation of the two products in the PhICl2 oxidation.

Conclusions

Bis(thianthrenyl)digold(I) complexes with bridging diphosphine ligands adopt different conformations depending on the diphosphine ligands and the number of methylene groups in the spacer. The dppm complex 1 revealed a structure with Au–Au interactions both in the solid state and in the solution, as confirmed by X–ray crystallography, DFT, TD-DFT, 1H NMR, and UV–vis spectroscopy. Complex 3 also preferred a structure with Au–Au interactions. However, X-ray structure characterizations and UV–vis spectra revealed that the structures without Au–Au interactions are more common for complexes 2 and 4. Both π–π interactions of the polyaromatic ligands and the d10–d10 interactions of the metal centers stabilize the conformers with Au–Au interactions. In addition, the structures of the complexes are influenced by the structure of the bridging diphosphine ligands, particularly by the number of the methylene groups in the diphosphine ligands.