Crystal structure of cyclo-bis(μ4-2,2-diallylmalonato-κ6 O 1,O 3:O 3:O 1′,O 3′:O 1′)tetrakis(triphenylphosphane-κP)tetrasilver(I)

In the title compound, the silver(I) ions are coordinated by four triphenylphosphane ligands and two 2,2-diallylmalonate anions in a μ4-(κ6 O 1,O 3:O 3:O 1′,O 3′:O 1′) mode, setting up an Ag4O8P4 core.

In the tetranuclear molecule of the title compound, [Ag 4 (C 9 H 10 O 4 ) 2 (C 18 H 15 P) 4 ], the Ag I ion is coordinated by one P and three O atoms in a considerably distorted tetrahedral environment. The two 2,2-diallylmalonate anions bridge four Ag I ions in a 4 -( 6 O 1 ,O 3 :O 3 :O 1 0 ,O 3 0 :O 1 0 ) mode, setting up an Ag 4 O 8 P 4 core (point group symmetry 4..) of corner-sharing tetrahedra. The shortest intramolecular AgÁ Á ÁAg distance of 3.9510 (3) Å reveals that no direct d 10 Á Á Ád 10 interactions are present. Four weak intramolecular C-HÁ Á ÁO hydrogen bonds are observed in the crystal structure of the title compound, which most likely stabilize the tetranuclear silver core.

Chemical context
Silver(I) carboxylates of general type [AgO 2 CR] n (n is the degree of aggregation) are of interest due to their versatile structures in the solid state and in solution, their synthetic methodologies and their manifold reaction behavior (see, for example: Schliebe et al., 2013;Jahn et al., 2010;Wang et al., 2008;Ferná ndez et al., 2007;Olson et al., 2006;Szymań ska et al., 2007). These metal-organic complexes are of importance not only in the field of basic research but also in multipurpose applications including, for example, metallization processes for micro-and nano-structured new materials in electronic systems and devices (e.g. using chemical vapour deposition, CVD), since silver possesses the highest electrical conductivity of any element Lang & Dietrich, 2013), catalytic processes (Steffan et al., 2009) and their use in biological studies (Djokić, 2008;Zhu et al., 2003).
The CVD process requires metal precursors possessing high vapour pressures. On a molecular level this is typically achieved by designing low aggregated metal compounds. In the case of silver, this can be realized by the use of phosphanes as a Lewis base; however, the concomitant increase of the molecular weight of the transition metal complex may decrease its vapour pressure. Circumventing this difficulty, we ISSN 1600-5368 have investigated the use of olefines as ligands for silver(I) carboxylates, in which the olefin is covalently bonded to the carboxylate. In the context of this approach, the title compound [{(Ph 3 P)Ag} 4 {(O 2 C) 2 C(CH 2 CH CH 2 ) 2 } 2 ], (I), was obtained by the reaction of the silver salt of 2,2-diallylmalonic acid with triphenylphosphane.

Structural commentary
The asymmetric unit of (I) contains one quarter of the molecule which is completed by application of a fourfold screw axis as the symmetry element. The resulting tetranuclear silver core is decorated by four triphenylphosphane ligands, whereby the metal ions are bridged by two 2,2-diallylmalonate anions in a 4 - Fig. 1). There is no example in the literature of a transition metal malonate displaying this type of coordination. The environment around silver, set up by one phosphorus and three oxygen atoms, is best described as distorted tetrahedral. Ag1 is oriented slightly above the plane of O1, P1 and O2 ii [distance 0.2911 (10) Å ], which is supported by the respective bond angles around Ag1 (Table 1) summing up to 354.3 . The O-Ag1-P1 angles are substantially larger than the O-Ag1-O angles, which may be attributed to the chelating coordination of the malonate ligands and the bulkiness of the triphenylphosphane ligand. The Ag-O bond lengths are more than 0.2 Å shorter for the two oxygen atoms of the aforementioned plane than for the third apical oxygen atom (Table 1). However, the values are in the expected range for Ag-O bonds in silver carboxylates.
The cyclic corner-sharing arrangement of the described O 3 P tetrahedra gives the tetranuclear structure of (I) (Fig. 2). The four silver ions are oriented in a butterfly-like arrangement, which delimits the title compound from Ag 4 O 4 heterocubanes (Jakob et al., 2011;Zhang et al., 2008, Kü hnert et al., 2007 in which the four silver ions form a tetrahedron. In contrast, there are some similarities with [bis(1,8-naphthalenedicarboxylato)][tetrakis(triphenylphosphane)silver(I)] (van der Ploeg et al., 1979); however, in the structure of this compound one silver ion is pentacoordinated.

Supramolecular features
Four weak intramolecular C-HÁ Á ÁO hydrogen bonds (Steiner, 2002) are observed in the crystal structure of (I) ( Table 2), which most likely stabilize the silver core.
In contrast to iridium and platinum complexes of 2,2diallylmalonic acid and derivatives thereof, the C C double bond does not coordinate the transition metal in (I). Furthermore, no obviousstacking interactions are observed between the allyl and the phenyl substituents. Therefore, the packing seems to be dominated by dispersion forces (Fig. 3).

Figure 1
The Ag 4 O 8 P 4 core of the title compound with surrounding atoms. Displacement ellipsoids are displayed at the 50% probability level. The carbon atoms of the phenyl substituents except the ipso-carbon atoms and all hydrogen atoms have been omitted for clarity. [Symmetry codes: Table 2 Hydrogen-bond geometry (Å , ).

Figure 2
Structure fragment showing the cyclic corner-sharing arrangement of the AgO 3 P polyhedra giving the tetranuclear silver core of composition Ag 4 O 8 P 4 .

Table 1
Selected bond lengths (Å ) and bond angles ( ).   Jung et al., 1999;Lee et al., 1999). To the best of our knowledge, no diallylmalonate silver(I) compounds have been described in the literature so far.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )