Crystal structure of an unknown tetrahydrofuran solvate of tetrakis(μ 3-cyanato-κ3 N:N:N)tetrakis[(triphenylphosphane-κP)silver(I)]

In the title compound a distorted Ag4N4-heterocubane core is set up by AgI cations and N atoms of cyanate anions. The core is decorated by four triphenylphosphine ligands bonded to the AgI cations. Ag⋯Ag distances as short as 3.133 (9) Å suggest the presence of argentophilic (d 10⋯d 10) interactions.

In the title compound, [{[(C 6 H 5 ) 3 P]Ag} 4 {NCO} 4 ], a distorted Ag 4 N 4 -heterocubane core is set up by four Ag I ions being coordinated by the N atoms of the cyanato anions in a 3 -bridging mode. In addition, a triphenylphosphine ligand is datively bonded to each of the Ag I ions. Intramolecular AgÁ Á ÁAg distances as short as 3.133 (9) Å suggest the presence of argentophilic (d 10 Á Á Ád 10 ) interactions. Five moderate-to-weak C-HÁ Á ÁO hydrogen-bonding interactions are observed in the crystal structure, spanning a three-dimensional network. A region of electron density was treated with the SQUEEZE procedure in PLATON [Spek (2015). Acta Cryst. C71, [9][10][11][12][13][14][15][16][17][18] following unsuccessful attempts to model it as being part of disordered tetrahydrofuran solvent molecules. The given chemical formula and other crystal data do not take into account these solvent molecules.
In contrast, hardly any research has been done on compounds such as metal alkyl allophanates. Despite the interesting features of this type of compounds, only few research groups have so far been involved in the synthesis (Clusius & Endtinger, 1960;Becker & Eisenschmidt, 1973;Dains & Wertheim, 1920) and further modification of this family of compounds (Kawakubo et al., 2015;Potts et al., 1990;Bachmann & Maxwell, 1950;Murray & Dains, 1934;Biltz & ISSN 2056-9890 Jeltsch, 1923. To the best of our knowledge, two synthetic approaches for the preparation of potassium and silver salts of ethyl allophanate have been described in the literature (Blair, 1926;Dains et al., 1919). The identity of metal allophanates has been confirmed by elemental analysis. For the application of these precursors, full characterization and the investigation of their thermal behaviour is required. In the context of precursor design for MOD (metal organic deposition) inks, we are interested in the synthesis, characterization and application of such complexes for inkjet printing.
To get access to a large range of metal allophanates (e.g. Cu, Ni or Zn), a modified synthetic procedure with respect to the method reported by Dains et al. (1919) was applied for the synthesis of silver allophanates among others. The initial step involved conversion of ethyl allophanate with sodium ethanolate for use of the resulting solid in a further reaction to form the respective silver complex. To analyse the sparingly soluble compound, IR spectroscopy has been applied. A comparison of the measured spectrum with that of ethyl allophanate showed the absence of the carbonyl band at 1701 cm À1 and the appearance of a new band at 2170 cm À1 of high intensity, indicating the formation of silver isocyanate (Ellestad et al., 1972). To confirm the assumption of the formation of silver isocyanate, the respective solid was treated with triphenylphosphine (PPh 3 ) in tetrahydrofuran (THF) and subsequently crystallized. The characterization of the crystals obtained by X-ray diffraction, NMR and IR spectroscopy is in accordance with the formation of the title compound, [{((C 6 H 5 ) 3 P)Ag} 4 {NCO} 4 ], (I).

Structural commentary
The title compound consists of a Ag 4 N 4 -heterocubane core formed by N-coordination of four cyanate anions towards four Ag I cations in a 3 -bridging mode (Fig. 1). Each Ag I cation is additionally coordinated by a PPh 3 ligand. Disorder is observed in the crystal structure of (I) affecting the Ag3 and Ag4 sites, together with their bonded PPh 3 moieties. However, the respective components of both disordered Ag(PPh 3 ) units share one phenyl ring (C41-C46 and C59-C64). The Ag 4 N 4heterocubane is distorted which is reflected by the variation of the Ag-N distances in the range 2.273 (13)-2.605 (12) Å . Likewise, the Ag-N-Ag [78.7 (3) -98.5 (3) ] and N-Ag-N [80.9 (3) -98.5 (3) ] angles significantly deviate from 90 . The Ag 2 N 2 -faces of the Ag 4 N 4 -core are not planar [r.m.s. deviations in the range 0.0293 (Ag1, Ag4, N2, N3) to 0.1947 Å (Ag3, Ag4 0 , N3, N4)], however, the opposing least-squares planes are almost parallel [angles between planes: 0.40 (3) and 3.2 (3) ]. Opposing planes are twisted by some degrees relative to each other, which is reflected by the Ag-N-Ag-N and N-Ag-N-Ag torsion angles ranging from 2.8 (3)-19.4 (3) . As a result of the distortion of the Ag 4 N 4 -core, the AgÁ Á ÁAg and NÁ Á ÁN separations differ significantly. The shortest distances are observed between Ag1 and Ag2 as well as Ag3/Ag3 0 and Ag4/Ag4 0 (Table 1). Considering the contact radius of silver (1.72 Å ; Bondi, 1964) a weak argentophilic interaction between these pairs of atoms is most likely (Schmidbaur & Schier, 2015). The Ag-P separations [2.336 (15)-2.39 (2) Å ] are characteristic for an Ag I (PPh 3 ) fragment. The scattering contributions of two severely disordered THF solvent molecules were treated with the SQUEEZE procedure in PLATON (Spek, 2015). The calculated electron count of 350 electrons per unit cell is in good agreement with the composition of (I)Á2THF. In contrast, NMR analysis of the crystals after decantation of the supernatant solvent and drying in vacuo reveals a composition of (I)Á0.25THF. This discrepancy may be due to a facile evaporation of the co-crystallized solvent under reduced pressure.

Supramolecular features
Five moderate-to-weak C-HÁ Á ÁO hydrogen bonds (Steiner, 2002) are observed in the crystal structure of (I) ( Table 2). Four of those participate in the formation of a three-dimensional network. No obvious --stacking interactions between the phenyl rings are present. The molecular structure of (I) with displacement ellipsoids drawn at the 30% probability level. Hydrogen atoms and the minor parts of the disordered atoms are omitted for clarity.  (Bowmaker et al., 1998;Partyka & Deligonul, 2009). These include the tricyclohexylarsine analogue of (I) as well as its pyridine solvate (Bowmaker et al., 1998). All reported Ag 4 N 4 -heterocubanes are less distorted than (I), which is reflected in a much less pronounced deviation of the AgÁ Á ÁAg distances in the heterocubane. A 3 -N coordination of the cyanate anions towards Ag I has been described for five compounds only (Bowmaker et al., 1998;Di Nicola et al., 2005, 2006. The average Ag-N distance in these compounds (2.433 Å ) is in good agreement with the corresponding value of 2.408 Å in (I).

Figure 2
Packing diagram of (I) viewed along [001]. Voids in the structure are represented by red spheres (drawn using the CAVITYPLOT routine in PLATON; Spek, 2009). Hydrogen atoms were omitted for clarity. Dashed lines represent coordinative bonds. Colour code: black (C), red (O), yellow (P), green (Ag).
calculated a void volume of approximately 2494 Å 3 occupied by 350 electrons per unit cell which points to the presence of two THF molecules per formula unit. Fig. 2 shows the positions of the voids within the unit cell.