F. Gruber and M. Jansen
Intercluster compounds (ICCs) that consist of different, charged building blocks with sizes ex-ceeding 1 nm in diameter are a new fascinat-ing class of compounds. They constitute well-defined materials with respect to structural or-der, which allow the study of particle-size-dependent (nanoscopic) properties without in-homogeneous signal broadening effects. In the past, we have realized such ICCs with gold and silver clusters as cationic building units and var-ious polyoxometalates and fullerides as anionic units [1]. The structures of these crystalline compounds were analyzed by single-crystal X-ray diffraction showing that in most cases the pre-fabricated building blocks remained struc-turally unchanged. The respective structure of an ICC is controlled by a sensitive competi-tion between the long-range Coulomb forces and the diverse family of short-range bond-ing interactions. Recently, we have extended this work in two directions: For one, we uti-lized the well-known diverseness of polyox-ometalates and metal clusters, to include larger
cluster units. These are either pre-synthesized or formed in situ from dissolved feedstocks, with crystallization serving as the selection pro-cess. The second objective is the removal of the ligands that separate the cationic and anionic building blocks, in order to make the materials more responsive to external electronic or mag-netic stimuli.
Polymeric silver alkynyl compounds turned out to be well-suited feedstocks for cationic sil-ver cluster units, since these compounds de-compose to oligomeric units of different sizes while dissolving. Combining such polymeric silver compounds with different polyoxometa-lates new classes of ICCs were obtained. The first is an intercluster sandwich compound [2]
[Ag42(CO3)(C≡CtBu)27(CH3CN)2][CoW12O40]2– [BF4] 1 consisting of a large silver toroid lo-cated between two Keggin anions with direct Ag–O–W bonds between them (Fig. 16(a)).
While the Keggin anion [CoW12O40]6− en-tered the intercluster compound with virtually no change in its composition and structure, the
silver alkynyl cluster with 42 silver ions was formedin situ during cystallization. As shown in Figs. 16(b) and 16(c), the silver cage is built up from a circular arrangement of nine six-membered rings of silver ions that are joined by theirtransedges. The geometry of the rings is not regular; in particular, the hexagons are dis-torted towards a boat conformation. The Ag–Ag bond lengths are in the range of 2.859(1) – 3.207(1) ˚A. Such distances clearly indicate the presence of d10–d10 interactions. At the mid-point height of the cylinder, there are six addi-tional silver ions that link the central carbonate group to the surface silver atoms. The C–O bond lengths range from 1.25(1) to 1.26(3) ˚A, which is in good agreement with the distances found in other carbonates.
Figure 16: (a) Polyhedral/ball-and-stick represen-tation of compound 1. (b) representation of the [Ag42(CO3)] cluster, and (c) a top-down view in-cluding ligands.
Each of the oxygen atoms coordinates two sil-ver ions at distances of 2.038(14) – 2.111(13) ˚A, while each silver ion of the inner ring is in contact with four silver ions of the outer cage.
The whole silver cage is stabilized by a lig-and sphere of 27tert-butylethynyl and two ace-tonitrile molecules. The ligands are arranged in three rows around the cluster capping triangles (µ3) and squares (µ4) of silver atoms. The two additional acetonitrile molecules coordinate to only one silver atom each. The ‘naked’ silver cage has a diameter of about 1.1 nm; if one in-cludes the ligand sphere, the diameter amounts to 2 nm. The polyoxometalates are situated on top and beneath the silver toroid, each with the first layer of three linked[WO6] octahedra com-pletely surrounded by the silver cage. The dis-tances between the silver atoms of the cluster and the oxygen atoms of the polyoxometalates range from 2.517(1) ˚A to 2.778(9) ˚A. Thus they are comparable to the Ag–O contacts found, e.g., in Ag2SO4, AgVO3or AgMnO4.
Two further new ICCs are chain-like interclus-ter compounds, which were obtained by com-bining polymeric silver compounds with the Wells-Dawson polyoxometalate [P2W18O62]6−. Compound 2 {[Ag14(C≡CtBu)8(C3H7NO)10]–
[Ag12(C≡CtBu)6Cl(C3H7NO)10]H[P2W18O62]2}n
and compound3{[Ag16(C≡CtBu)11(CH3CN)6]–
[P2W18O62]2}n–[Ag14(C≡CtBu)12(CH3CN)2] constitute the first two chain-like intercluster compounds with nanometer-sized silver alkynyl clusters and the polyoxometalate arranged in an alternating, linear sequence. In both com-pounds, the linkage of the polyoxometalate and the silver cluster building blocks is me-diated by direct Ag–O–W bridges, i.e., these interfaces are free of ligands. Compound 2 exclusively consists of the intercluster chains shown in Fig. 17 (upper part). Remarkably, the Wells-Dawson anions alternate with two differ-ent kinds of silver cluster units, which on their part are also alternating on their positions. Both silver clusters, shown in Figs. 18(a) and 18(b), had not been encountered previously.
Figure 17: The chain-like intercluster units of2and3. The Wells-Dawson anions are shown as polyhedra, the silver clusters in ball-and-stick representation.
They consist of a cage of 12 and 14 silver ions, respectively, shielded by a ligand sphere of tert-butylethynyl anions and dimethylformamid molecules. The Ag+–Ag+ bonds are in the ex-pected range of 2.841(7) – 3.288(7) ˚A. Both sil-ver building units form eight direct bonds to the neighboring polyoxometalates with distances from 2.46(4) ˚A to 2.73(3) ˚A that correspond to the sums of the effective radii and thus are com-parable to the Ag–O distances found in1.
The crystal structure of 3 contains two dif-ferent structure motives. One is the silver alkynyl cluster [Ag14(C≡CtBu)12(CH3CN)2]2+, whose composition and structure is equal to the Ag14 clusters found in previous ICC.
The second one is the intercluster chain {[Ag16(C≡CtBu)11(CH3CN)6]–[P2W18O62]2}n
(structure shown in Fig. 17 lower part). While the polyoxometalate entered the chain with vir-tually no change of composition and structure,
Figure 18: Structure of the silver cluster units of the intercluster compounds2((a) and (b)) and3(c) (Ag pink, O red, Cl green, N blue, C gray, H dark gray).
a new silver alkynyl cluster with 16 silver ions, shown in Fig. 18(c), was formed. The sil-ver cage has a quite uncommon structure, with Ag+–Ag+ bond lengths of 2.870(2) ˚A – 3.211(2) ˚A. It is coordinated by 11 tert-butyl-ethynyl ligands and 6 acetonitrile molecules.
In addition to the ligand sphere there are seven direct Ag–O bonds to the neighboring polyoxometalates, ranging from 2.43(1) ˚A to 2.78(1) ˚A.
These successful syntheses of new ICCs demonstrate that, with the introduction of polymeric silver compounds as feedstocks,
we can obtain the desired ligand-free inter-faces between the building blocks. The direct Ag–O–W bridges connect the building units ei-ther to macromolecules or to one-dimensional, supramolecular aggregates. Furthermore, these new compounds comprise as yet unknown, large silver alkynyl clusters.
References:
[1] Gruber, F., M. Schulz-Dobrick and M. Jansen.
Chemistry – A European Journal16, 1464-1469 (2010);Gruber, F. and M. Jansen.Inorganica Chimica Acta363, 4282–4286 (2010).
[2] Gruber, F. and M. Jansen.Angewandte Chemie International Edition49, 4924–4926 (2010).