Structural Diversity of Alkaline Earth Centered Gold(I) Metalla- coronates

One-pot reactions of the catechol-scaffolding aroylbis(N,N-diethylthiourea) H2L with mixtures of alkaline earth nitrates M(NO3)2 (M2+ = Ca2+, Sr2+ or Ba2+) and (NEt4)[AuCl4] or [Au(tht)Cl] (tht = tetrahydrothiophene) in methanol in the presence of Et3N as supporting base give rise to neutral trinuclear gold(I) {2}-metallacoronates with the composition of {M ⊂ [Au2(L)2]} (1). Similar reactions with the pyridine-centered aroylbis(N,N-diethylthiourea) H2L, however, produce complexes with the same metal-to-ligand ratio but with higher nu-


Introduction
The coordination chemistry of aroyl-N,N-dialkylthioureas is pioneered by the work of L. Beyer et al. [1] and attracted the interests of many other chemists during the last four decades. Most contributions in this field focus on the aroyl mono-HL ben and bis-thioureas H 2 L pth (Scheme 1), which are versatile chelators for most transition metal ions. [2][3][4][5][6][7][8][9][10][11] The structural chemistry of metal complexes with aroyl-N,N-dialkylthioureas is dominated by the monoanionic S,O-chelating fashion of aroylthiourea moieties (see Scheme 1). It is interesting that with the two S,Ochelating moieties in the symmetrical bipodal phthaloylbis(N,Ndialkylthioureas) m-/p-H 2 L pth and divalent metal ions such as Ni 2+ , Pt 2+ or Cu 2+ , which prefer square-planar or pseudoplanar coordination spheres, multinuclear complexes are formed. The structures of the resulting multinuclear systems strongly de-clearity {2M ⊂ [Au 4 (L py ) 4 ]} (2). In both 1 and 2, Au(I) ions are exclusively S-bonded with the organic ligands and adopt a virtually linear coordination fashion. Such metal-ligand binding is responsible for the formation of metallacoronands, which accommodate alkaline earth metal ions in their molecular voids, thereby resulting in host-guest coordination assemblies. The level of metal-ligand aggregation in the resulting assemblies is dependent on the denticity, size and flexibility of the centered building block of the aroylbis(N,N-diethylthiourea) ligands.
Recently, the modification of m-H 2 L pth by the replacement of the phenylene ring by other spacers possessing potential donor atom(s) such as a disubstituted pyridine ring (H 2 L py ) or a catechol building block (H 2 L cat ) brought about a new generation of ligands with bifunctional coordination sites each of which favorably binds to a particular type of metal ions. Such interesting structural features gave access to the syntheses of a large variety of trinuclear bimetallic complexes from simple one-pot reactions of the ligands and mixtures of two metal ions with different Pearson's acidity. [18][19][20] In such self-assembling processes, the "soft" metal ions prefer bonding to the satellite aroylthiourea moieties, while the harder ions such as alkali, alkaline earth metal or lanthanide ions are directed to the central binding sites. As a result of this selective coordination, trinuclear, bimetallic systems could be recognized as host-guest compounds, for example the Zn(II) {2}-metallacoronate III or the Fe(III) {2}-metallacryptate IV (see Scheme 2), [19,21] where the "hard" guest ions are encapsulated in metallamacrocycles consisting of the ligands and the softer metal ions.
A number of studies show that such inclusion compounds with diverse compositions, structures and physicochemical properties can be rationally designed by self-assembly from a mixture of metal ions and the ligands H 2 L cat or H 2 L py . [18][19][20]22,23] To get deeper insight in the control of the self-assembling process, in this report, the utilization of the ligands H 2 L cat and H 2 L py as subunits for the construction of host-guest assemblies is continued.

Results and Discussion
Reactions of H 2 L cat (4 equiv.) with methanolic solutions containing mixtures of alkaline earth nitrates (1 equiv.) and the common Au(III) starting material (Et 4 N)[AuCl 4 ] (2 equiv.) give rise to colorless solids with the chemical composition {M ⊂ [Au 2 (L cat ) 2 ]} (1) (M 2+ = Ca 2+ (1a), Sr 2+ (1b) and Ba 2+ (1c)). The color of the products gives a good hint for the formation of Au(I) complexes from the reduction of (Et 4 N)[AuCl 4 ] by H 2 L cat as reducing agent. the smaller ions Ca 2+ and Sr 2+ are eight-coordinate and adopt snub-diphenoidal coordination polyhedra, [49] while one additional coordinating acetonitrile molecule is responsible for the coordination number nine of the Ba 2+ ion with a "hulla-hop" coordination geometry (Figure 1b). [50,51] Furthermore, the deviation of the C(S)-NEt 2 moieties in opposite directions from the mean plan of the ligands causes a twisted conformation, which in turn produces the helical Ca 2+ -or Sr 2+ -binding complexes with dihedral angles between two mean plans of the ligands of 78.01(2)°and 76.32(2)°respectively ( Figure S2.2 and S2.4). A similar deviation but in the same direction brings about the untwisted conformation, which induces a larger void, thereby resulting in a Ba 2+ inclusion compound with a narrow dihedral  (6) [a] Symmetry operations used to generate equivalent atoms: i -x + 1, y, -z + 3/2.
angle of 4.91(8)°( Figure S2.6), hence, without helicity. The partial double bond character of C-S, C-O and C-N bonds indicates the well-known π-electron delocalization in deprotonated aroylthioureas. However, this delocalization of electron density is smaller than in the previously reported compounds with chelating aroylthioureas. [19] The longer C-S and the shorter C-O bonds in the compounds of the present study indicate a higher degree of electron density on sulfur atoms of the coordinated aroylthioureas.
In addition to the X-ray structural analyses, the bonding situation of the host-guest assemblies was characterized by spectroscopic methods. In the IR spectra of the complexes, strong bands in the region between 1570 and 1590 cm -1 can be assigned to the ν C=O stretches. This corresponds to bathochromic shifts in the range of 70 cm -1 to 90 cm -1 with regard to the uncoordinated ligand. In comparison with the common values of approximately 150 cm -1 for chelating aroylthioureas, [18,19,52] the shift is modest and confirms that the C-O bonds possess more double bond character in the anionic S-bonded aroylthioureato ligands. The existence of the ligands in their deprotonated form {L cat } 2is confirmed by the disappearance of the ν NH stretches in the region above 3100 cm -1 in the IR spectra as well as by the absence of the signal corresponding to the NH protons in their 1 H NMR spectra. It is interesting that the splitting patterns of the methylene protons of the OCH 2 and NCH 2 groups illustrate the varying rigidity of the organic framework in the resulting complexes. In the 1 H NMR spectra of 1b and 1c (Figure S1.9 and S1.12), the signals assigned to OCH 2 and NCH 2 protons resemble the corresponding resonances in the 1 H NMR spectrum of the ligand. Particularly, the signal belonging to the OCH 2 protons appears as a broad singlet at about 4.9 ppm and the resonances of the NCH 2 protons are detected in the range 3.0-4.0 ppm as two broad signals or two quartets for 1b and 1c respectively. In contrast to the simple pattern described above, the 1 H NMR spectrum of 1a reveals two singlets around 4.8 ppm with the typical geminal spin-spin coupling constants of 13.0-13.5 Hz for OCH 2 protons and three sextets in the range 3.2-3.7 ppm with ABX 3 splitting patterns, where J AB (ca. 14.0 Hz) is approximately twice of J AX (ca. 7.0 Hz), for NCH 2 protons (Figure S1.6). The more delicate fine structures of the signals associated with the methylene protons in 1a is a strong evidence for the significant increase of the rotation barrier around the O-CH 2 and C(S)-NEt 2 bonds, in other words of the rigidity of the organic backbones, due to accommodating the smaller Ca 2+ guest ion. Despite of the complication of the 1 H NMR spectra, the corresponding 13 C NMR spectra ( Figure  S1.7, S1.10 and S1.13) are straightforward because of an only small influence of the hindered rotation around the C(S)-NEt 2 bonds. Therefore, the resonances of the CH 2 and CH 3 carbon atoms of the NEt 2 groups appear as two separate signals in the upfield region from 10 ppm to 50 ppm. The signals belonging to the OCH 2 carbon atoms are found around 72 ppm, while those of the aromatic carbon atoms are in the range of 110 ppm to 150 ppm. The weak signals at approximately 180 ppm and 170 ppm are attributed to the resonances of C=O and C=S carbon atoms, respectively.
With the aim of constructing the similar Au(I)-metallacoronates 2 derived from the pyridine-centered ligand H 2 L py , the same synthetic route was applied for reactions between H 2 L py and mixtures of [Au(tht)Cl] and alkaline earth nitrates M(NO 3 ) 2 (M 2+ = Ca 2+ , Sr 2+ or Ba 2+ ) (Scheme 4). Such reactions result in analytically pure, neutral products, which deposit from the methanolic reaction mixtures in good yields. Assuming that H 2 L py could perform the same coordination mode as H 2 L cat , the obtained products should have the compositions of {M ⊂ [Au 2 (L py ) 2 ]}, which would resemble those of the preceding compounds. The assumption was (preliminarily) supported by the elemental analyses as well as mass spectroscopic studies with the appearance of signals matching the expected fragments {M ⊂ [Au 2 (L py ) 2 ] + Na} + in the ESI + mass spectra of the complexes. However, the same mass spectra show signals with higher m/z values, which could be explained by a cluster ion formation in the matrix, but may also indicate that the inclusion compounds formed with the pyridine-based thiourea derivative have a higher nuclearity.
The question could be answered by the determination of the crystal structures of the compounds 2. The structures clearly confirm the formation of larger aggregates with the general composition of {2M ⊂ [Au 4 (L py -κS) 4 ]}. The products possess the same metal-to-ligand ratio, but a higher nuclearity than the inclusion compounds of type 1. Figure 2 presents the structures of the Ca 2+ -(2a), Sr 2+ -(2b) and Ba 2+ -(2c) containing complexes. Selected bonding parameters are listed in Table 2.    (Table  S2.1 to S2.3). The pyridinedicarboxamide moieties serve as planar tridentate ligand systems and coordinate the guest M 2+ ions through their ONO donor sets. Furthermore, small coordinating solvent molecules saturate the coordination spheres of such divalent ions. In the isostructural compounds 2a and 2b, each alkaline earth metal ion is coordinated by two pyridinedicarboxamide moieties, methanol and/or water molecules. This leads to the coordination number of eight with a biaugmented trigonal prismatic coordination geometry around the guest metal ions. [49] In contrast to the coordination environments observed in 2a and 2b, a coordination number of nine is found for the Ba 2+ ions in compound 2c. The Ba 2+ ions adopt a "muffin-shape" coordination polyhedron by directional interactions with two pyridinedicarboxamide groups, one bridging carbonyl oxygen atom, one bridging water molecule and one sulfur atom with a Ba-S(40) distance of 3.536(3) Å. [50,51] Spectroscopic studies on 2 are in a good agreement with the conclusion drawn from the solid-state structures. The IR spectra of the resulting {4}-metallacoronates 2 are quite similar to those of the previously discussed {2}-metallacoronates 1. The absence of ν NH bands in the region above 3100 cm -1 indicates the double deprotonation of H 2 L py during the complex formation. In addition, the modest bathochromic shifts of the ν C=O bands by only approximately 85 cm -1 to the area around 1550 cm -1 proves the coordination of the oxygen atoms of the carboxamide groups with the metal ions and confirms the only moderate delocalization of π-electron density within the aroylthiourea moieties as being concluded from the crystallographic A comparison of the compositions and structures of the selfassembled inclusion compounds 1 and 2 demonstrates that due to its lower denticity, size and flexibility, the ligand H 2 L py forms larger metallamacrocycles than the corresponding catechol-based ligand. Particularly, the higher flexibility of H 2 L cat due to its aliphatic backbone may enable this compound to provide the lone pairs of the donor atoms in positions for a EurJIC European Journal of Inorganic Chemistry more efficient coordination of the large alkaline earth ions, which in return gives the cations more influence to act as templates during the self-assembly of the multi-metallic compounds. More experiments with other metal ions are in preparation to shed more light to this points.

Conclusions
Two series of Au(I) metallacoronates encapsulating alkaline earth metal ions have been prepared by one-pot reactions using catechol-and pyridine-scaffolding aroylbis(N,N-diethylthioureas). The almost linear coordination of S-bonded Au(I) centers with such ligands produces two types of products: (i) the {2}metallacorands [Au 2 (L cat ) 2 ] 2and (ii) the {4}-metallacorands [Au 4 (L py ) 4 ] 4-. The central voids of the obtained metallacycles capture one and two alkaline earth metal ions, respectively. The denticity, size and flexibility of the central spacers exert considerable influence on the level of metal-ligand aggregation, and, thus, the compositions and structures of the resulting hostguest coordination assemblies.

Experimental Section
Materials. All chemicals used in this study were reagent grade and used without further purification. Solvents were dried and used freshly distilled unless otherwise stated. [Au(tht)Cl] was prepared by the standard procedure. [53] The ligands were synthesized according to the procedures recently reported. [19] Physical measurements. IR spectra were measured as KBr pellets on a Shimadzu FTIR-spectrometer between 400 and 4000 cm -1 . 1 H and 13 C NMR spectra were taken with an Ascend 500 MHz multinuclear spectrometer. ESI mass spectra were measured with an Agilent 6210 ESI-TOF (Agilent Technology) mass spectrometer. All MS results are given in the form: m/z, assignment. Elemental analysis of carbon, hydrogen, nitrogen, and sulfur were determined using a Heraeus Vario EL elemental analyzer. Reproductions of the IR, NMR and MS spectra are given as Supporting Information.  (2 mL) and a few drops of water were added. The ligand dissolved rapidly and the color of the solution changed from pale-yellow to colorless. The mixtures were stirred at room temperature for 30 min before 3 drops of Et 3 N were added and the temperature was increased to 50°C and kept for 1 h. The complexes deposited from the reaction mixtures as colorless solids, which were filtered off, washed with a small amount of MeOH and dried in vacuo. The complexes were synthesized following method 2 described above. Single crystals suitable for X-ray diffraction were obtained from slow diffusion of MeOH into the solutions of complexes in CH 2 Cl 2 .

Syntheses of the complexes
X-ray Crystallography. The intensities for the X-ray determinations of {Ca ⊂ [Au 2 (L cat -κS) 2 ]} and {(MeCN)Ba ⊂ [Au 2 (L cat -κS) 2 ]}·MeCN were collected on a Bruker D8 QUEST CMOS instrument at 100 K with Mo K α radiation (λ = 0.71073 Å) using a TRIUMPH monochromator. The intensities for the X-ray determinations of {(MeOH) 3  collected on a STOE IPDS 2T instrument at 200 K using Mo K α radiation with a graphite monochromator. Standard procedures were applied for data reduction and absorption correction. Structure solutions and refinements were performed with the SHELXT and SHELXL 2014/7 programs included in the WinGX program package. [54][55][56] The structure of {(MeOH) 2 (H 2 O) 2 Sr 2 ⊂ [Au 4 (L py -κS) 4 ]}· MeOH·1.5H 2 O was refined as a two-component twin. The final refinement was performed using HKLF 5 with reflection data prepared using TwinRotMat of PLATON program. [57] Hydrogen atoms were calculated for idealized positions and treated with the "riding model" option of SHELXL. More details on data collections and structure calculations are contained in Table 3. The representation of molecular structures was done using the program DIAMOND. [58] Since ball and stick presentations of the molecules are used in all of the Figures of this paper for reason of clarity, ellipsoid representations of all compounds are contained in the Supporting Information. Stereochemical analysis of the coordination spheres of the guest alkaline metal ions are performed by the program SHAPE 2.1. [59] More details about the analyses are contained in the Supporting Information.
Deposition Numbers 1950399, 1950400, 2021266, 2021267 and 2021268 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.