Acylchalcogenourea complexes of silver(I)

Acylthioor acylselenoureas react with silver oxide to form tetranuclear silver(I) complexes containing the deprotonated acylchalcogenourea ligands binding to the silver atoms via the chalcogen and oxygen atoms. These tetrasilver(I) species react with either four or eight equivalents of a phosphine giving either dinuclear silver(I) phosphine complexes or tetrahedral silver diphosphine complexes. In these compounds too, the acylchalcogenourea ligands form six-membered rings coordinating to the metal through the chalcogen and oxygen atoms. In one case we observed a very rare example of an acylthiourea ligand coordinating through the nitrogen and sulfur atoms, resulting in a four-membered ring. A silver(I) complex containing a deprotonated acylselenourea and an N-heterocyclic carbene ligands is also reported. This was prepared from the corresponding acetate derivative and the acylselenourea; the acetate in this case acts as the base. All compounds reported were characterised by spectroscopic techniques and many also by X-ray diffraction.


Introduction
Deprotonated acylthio-or selenoureas of the type [ArC(O)NC(E)NR2] -(E = S, Se) are monoanionic O,Sor O,Sechelating ligands, which readily form complexes with many transition metals and also maingroup-metals. Deprotonation of an acylthio/selenourea can either be accomplished using an external base or a basic metal precursor. In some cases, simply heating the acylthio/selenourea with a metal chloride forms the metal chelate complex accompanied by elimination of HCl. A recent review summarizes the structures, chemistry and biological properties of acylthiourea derivatives. [1] Silver(I) complexes of acylthioureas have to date not been studied systematically. A report from 1985 describes the isolation of colorless crystals from a black tar formed in the reaction of PhC(O)NHC(S)NEt2 with AgNO3 and sodium acetate in aqueous ethanol. [2] The crystals were found to consist of a silver(I)hydrosulfide complex containing three, neutral Scoordinated acylthioureas bound to the metal. Using slightly different reaction conditions (AgNO3 in EtOH without base), another group later obtained the same silver(I)hydrosulfide species with PhC(O)NHC(S)N i Bu2. [3] The source of the hydrosulfide ligand in these complexes was never explained, however, it probably originates from decomposition of the sulfur ligands during the reaction. Richter was the first to report a silver complex containing a deprotonated acylthiourea ligand, formed by the reaction of PhC(O)NHC(S)NEt2 with AgNO3 and KOH. [4] In this case, the isolated compound was the tetranuclear silver(I) cluster [Ag4{PhC(O)NC(S)NEt2}4], in which the deprotonated acylthiourea forms O,S-chelates with the silver atoms. It was later shown, that an analogous Ag4-cluster can also be formed when sodium acetate is used as base. [5] Yet another structural motif that has been observed in acylthiourea silver(I) chemistry is the dinuclear silver(I) species [Ag2{RC(O)NC(S)NEt2}2{RC(O)NHC(S)NEt2}2] (R = camphanyl) containing one deprotonated, chelating acylthioureato ligand and one neutral S-coordinating acylthiourea ligand. [6] Although the coordination chemistry of acylselenourea ligands has also been studied with a number of transition-and main-group-metals, there are no reports of any silver compounds containing such selenium ligands. Given our interest in the chemistry and applications of metal complexes with acylthio/selenourea ligands, [7] we report here results of a systematic study of acylselenoureas and their sulfur counterparts with silver(I) ions as well as their reactivity with various phosphines.
The proton NMR spectra of 1 and 2 confirm the absence of the signal due to the NH-proton but provide no further insight into the exact molecular structure of the compounds. Fortunately, we obtained single crystals, which allowed us to determine the solid-state structure of both complexes ( Figures  1 and 2). The compounds consist of four deprotonated acylchalcogenourea ligands bound to four silver atoms through the sulfur/selenium and oxygen atoms. In addition, the sulfur/selenium atoms bridge two neighboring silver atoms. The metal cluster core thus consists of four silver atoms arranged in a butterflyshape with Ag-Ag distances of 2.9956(3) Å and 3.0499(5) Å (complex 1) and 2.9342(5) Å in complex 2. These silver-silver distances are considerably shorter than the 3.44 Å van der Waals radius of Ag and can therefore be considered argentophilic contacts. [9] [4][5] The Ag-Se bond lengths of 2.5171(5) Å and 2.5341(6) Å in 2 are somewhat shorter than those observed in related tetranuclear silver clusters containing selenium ligands. In the cation [Ag4{Ph2P(Se)NP(Se)Ph2}3] + containing an anionic Se,Sechelating ligand, the average Ag-Se distance is about 2.6 Å [10] and, similarly, the same 2.6 Å Ag-Se distance is observed in the silver cluster containing isopropyl selenolato ligands [Ag4(dppe)2(Se i Pr)4]. [11]  These results thus confirmed our initial idea, that Ag2O could indeed be used as a precursor for defined silver compounds of acyl-chalcogenoureas. The reaction is fast and clean with water being the only side-product. Our subsequent investigation focused on the reactivity of these tetranuclear silver compounds.
We first examined reactivity of the tetrasilver complexes with phosphines. Addition of one equivalent (per silver atom) of Ph3P or 1,3,5-triaza-7-phosphaadamantane (PTA) to the in situ formed tetranuclear compounds afforded colorless or yellow solids (3-10) after work-up (Scheme 1), which were characterized spectroscopically and, in some cases, by X-ray diffraction. The same products can also be obtained from the reaction of the isolated tetranuclear complexes 1 or 2 with four equivalents of the respective phosphine. Apart from the disappearance of the NH-proton resonance, the 1 H NMR spectra of the compounds are of little diagnostic value, since the chemical shifts and multiplicity of the observed resonances remain virtually unchanged. However, the 31 P NMR spectra confirm that the phosphines are indeed coordinated to the metal center, since the chemical shifts of the singlet resonances are considerably shifted compared to those of the phosphines themselves. The resonances for the Ph3P complexes are observed at around 10-15 ppm, whilst those of the PTA derivatives appear at around -86 ppm. Unfortunately, we were unable to observe any P-Ag coupling in the 31 P-NMR spectra. Nevertheless, single crystal X-ray diffraction allowed us to determine the solid-state structures of complexes 3, 4 and 7; their molecular structures are shown in Figures 3, 4, and 5, respectively. Because the structural features of these compounds are quite similar, they will be discussed here together. In each case, the molecule consists of a deprotonated acylchalcogenourea chelating a silver(I) center through both oxygen and sulfur or selenium atoms. In addition, the sulfur or selenium atoms bridge two silver atoms forming a planar, four-membered Ag2S2 or Ag2Se2 rings. The two silver atoms are held together by argentophilic contacts ranging from 3.06 to 3.15 Å. The coordination geometry about each silver atom is completed by the respective phosphine ligand. A similar Ag2S2 structural motif is found in a few other dinuclear silver phosphine  [12] or thienyl [13] ), [Ag2(Ph3P)2(2-SC6H4NO)2] [14] and [Ag2(Ph3P)2(tu)2(NO3)2] (tu = thiourea). [15] In these complexes the Ag-Ag distances range from ca. 2.9 to 3.2 Å. The only known selenium derivatives containing an Ag2Se2 core are [Ag2(Ph3P)3{SeC(O)Ph}2] [16] and [Ag2(Ph3P)3(SePh)2]. [17] In these two complexes the Ag-Ag distances range from 2.9 to 3.0 Å. In the molecular structures of complexes 3, 4 and 7, the two chalcogen-silver distances are not identical: In each case, one can observe one longer and one shorter Ag-S/Se bond. The difference between these Ag-S/Se distances varies from as little as 0.07 to 0.29 Å Whilst the singlet resonances for the dinuclear complexes in the 31 P NMR spectra are observed at around 10-15 ppm, those of the monomeric complexes 11-13 appear in the range  = 5-8 ppm. The 1 H NMR spectra of these compounds confirm the deprotonation of the NH-group and also indicate 2:1 phosphine/acylthiourea ratios. The X-ray crystal structure of complex 12 ( Figure 6) shows the typical O,S-chelating mode of a deprotonated acylthiourea forming a six-membered ring.
The tetrahedral coordination environment about the silver center is completed by the two phosphine ligands.
However, in the case of the p-tolyl derivative [Ag{4-MeC6H4C(O)NC(S)NEt2}(PPh3)2] (11), the coordination mode of the deprotonated acylthiourea ligand is very different (Figure 7). Here, the sulfur-and the nitrogen atoms of the deprotonated acylthiourea are bound to the silver atom forming a four-membered chelate ring. The four-coordinate geometry about the silver is completed by the two phosphine ligands. This N,Scoordination mode is extremely rare for acylthiourea derivatives. The reported by Warsink is the only other example of a metal complex with this coordination mode. [18]  To try and explain why this unusual coordination mode is observed, we carried out some computational studies. Energy differences between the four-and six-membered ring silver complexes for both the nitro-and methyl-substituted ligands were calculated. The results indicate that the energy differences (G298) between the six-and four-membered ring isomers are +7.6 kJ/mol for the nitro-derivative and +6.8 kJ/mol for the methyl congener. Given the similarity of the stabilities of the six-and four-membered ring isomers, we conclude that the observed crystallization of the fourmembered ring complex is most probably due to solubility and not a consequence of any systematic energy difference. Silver(I) complexes containing NHCs as C-donor ligands are nowadays common synthons for transmetallation reactions in organometallic chemistry. In addition, they have been shown to be biologically active molecules and also useful in homogeneous catalysis. [19]   The compound was spectroscopically and also structurally characterized. The 1 H NMR data is consistent with a 1:1 ratio of IPr and deprotonated acylselenourea. Due to poor solubility of the compound and inherent low intensity of the signal, we were unable to directly observe the 13 C resonance of the Agcarbene carbon atom. However, through a 2D HMBC experiment, we could identify this resonance at 188 ppm, a typical value for a silver-bound carbene carbon atom. We were able to obtain X-ray quality crystals, which allowed us to unambiguously determine the solid-state structure of the compound (Figure 8 [21] In the diketonato-compounds the O-Ag-O angles are considerably smaller (80-82.5°) than the corresponding O-Ag-Se angle in (14). The other bond distances and angles in (14) are as expected for this class of compounds. Whilst the Ag-O bond lengths in (14) are very similar to those in the dimeric complex (7), the Ag-Se bond length in (14) (ca. 2.49 Å) is significantly shorter than that in (7). Uniquely, the sixmembered ring formed by the coordinating acylselenourea is significantly distorted from planarity. In the phosphine complexes described above, the ring is virtually planar, whereas in (14) it adopts almost a saddle-shaped conformation. The silver, oxygen, selenium and nitrogen atoms lie in a plane whilst the two carbon atoms are found above and below this plane, respectively.

Conclusions
Overall, we have shown that silver(I)oxide is suitable starting material for silver complexes of thio-and selenourea ligands. The initially formed tetrameric compounds react with phosphines resulting in different products depending on the stoichiometry.

Experimental Section
General Reactions were carried out under ambient conditions in reaction vessels shielded from light. Solvents (HPLC quality) and reagents were sourced commercially and were used as received. The acylthio/selenoureas, [22] [Ag(OAc)(IPr)] [23] as well as PTA [24] were prepared following literature procedures. NMR spectra were recorded on Bruker Avance 400 or Bruker Avance III 600 instruments. Elemental analyses were carried out in-house using an Elementar Vario EL instrument. Samples for elemental analysis were dried in vacuum at 60 °C overnight.

Preparation of the silver(I) tetramers
To a solution of the acylthio-or selenourea (0.36 mmol) in CH2Cl2 (10 mL) was added solid Ag2O (0.55 equivalents, 0.19 mmol). After 30-40 minutes most of the silver oxide had dissolved. The mixture was filtered and left to stand in the dark. Within a few days X-ray quality crystals deposited, which were isolated by filtration and dried. To isolate the bulk material, the filtrate was evaporated to dryness in vacuum and the resulting solid was washed with Et2O and dried in air.

Preparation of the dinuclear silver(I) phosphine complexess
To a solution of the acylthio-or selenourea (0.36 mmol) in a mixture of MeOH and CH2Cl2 (5 mL each) was added solid Ag2O (0.55 equivalents, 0.19 mmol). After 30-40 minutes the mixture was filtered and subsequently one equivalent (0.36 mmol) of the appropriate phosphine was added to the filtrate. After a further 30 minutes at room temperature, the product was isolated by evaporation of the solvent and washing the residue with Et2O. X-ray quality crystals were obtained by slow evaporation of CH2Cl2 solutions of the compounds.

Preparation of the mononuclear silver(I) phosphine complexes
These were prepared as described above except that after filtration two equivalents (0.72 mmol) of the appropriate phosphine were added to the filtrate. After 30 minutes at room temperature, the products were subsequently isolated by evaporation of the solvent and washing the residue with Et2O. X-ray quality crystals were obtained by slow evaporation of CH2Cl2 solutions of the compounds.

X-ray crystallography
Data were collected at 150 K on a Rigaku Oxford Diffraction Gemini Ultra diffractometer or at 100 K on a Bruker AXS Apex II with an Incoatec IS micro focus source (Mo-K radiation  = 0.71073 Å).
Calculations were carried out using Olex2 incorporating the SHELXL programs. [25] Important crystallographic and refinement details are collected in Table 1. In the structure of complex (2) the NEt2 group is disordered over two positions. In addition, the asymmetric unit contains one quarter each of methanol and dichloromethane. The oxygen atom of the MeOH and the carbon atom of CH2Cl2 have identical coordinates. In the structure model of complex (11) there remained a solvent accessible void of 110 Å 3 , containing 32 electrons.
No satisfactory model could be found to account for this electron density, the data was therefore treated using the Squeeze procedure implemented in Platon. [26] CCDC 1513650 (for 1), 1513651 (for 2), 1513652 (for 3), 1513653 (for 4), 1513654 (for 7), 1406811 (for 11), 1513655 (for 12), and 1513654 (for 14) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallogaphic Data Centre.

Computational details
Structures were optimized with a revPBE GGA [27] functional and def2-TZVPP [28] basis sets as implemented in the ORCA [29] program; resolution of identity (RI) approximation [30] was employed to speed the calculations and Grimme's empirical corrections [31] to treat the dispersion forces. The fundamental frequencies were calculated to assess the nature of stationary points and to estimate the zero-point energy (ZPE) corrections and Gibbs energies.