Zintl Anions

The chemistry of nine-atom deltahedral Zintl anions of Group 14 has advanced greatly in recent years. Redox reactions of these clusters have led to dimers, trimers, tetramers, and infinite chains of germanium clusters such as [Ge9-Ge9] 6 , [Ge9=Ge9=Ge9] 6 , [Ge9=Ge9=Ge9=Ge9] 8 , 1[-(Ge9) 2 -], and 1[-(HgGe9) 2 -]. Similar redox reactions combined with nucleophilic addition of various anions have produced, for the first time, substituted clusters such as [Ge9SnPh3] 3 , [Ph3Sn-Ge9-SnPh3] 2 , [Ph2Sb-Ge9-SbPh2] 2 , and [Ph3Sn-Ge9-Ge9-SnPh3] 3 . Also, it has been known for quite some time that ligand-exchange reactions with organometallic compounds can add a transition-metal vertex to the cluster and form species such as [Sn9M(CO)3] 4 for M=Cr, Mo, W. More recent experiments with [Pt(PPh3)4], [Ni(CO)2(PPh3)2], and [Ni(cod)2] (cod= 1,5-cyclooctadiene) have shown that the nine-atom cluster can accommodate a central transition-metal atom to form [Ni@(Ge9Ni-PPh3)] 2 , [Ni@(Sn9Ni-CO)] 2 , and [Pt@(Sn9Pt-PPh3)] 2 . Note that these species also contain a ligated transition-metal atom in the form of {Ni-PPh3}, {Ni-CO}, or {Pt-PPh3} fragments as a vertex of the cluster and are thus ten-atom clusters. Similar reactions involving Pb9 clusters resulted in cluster enlargement and formation of the first ligand-free, transition-metal centered deltahedral clusters [Ni@Pb10] 2 and [Pt@Pb12] 2 with the shapes of a bicapped square antiprism and an icosahedron, respectively. Also, a reaction of the As7 3 ions with [Ni(cod)2] leads to a large cluster of 20 arsenic atoms around an arsenic-centered {Ni12} core. [18] Herein we report the synthesis and characterization of a ligand-free anion that contains a linear trimer of nickel enclosed inside two nine-atom germanium clusters, [(Ni-NiNi)@(Ge9)2] 4 (1; Figure 1). This compound is the first ligandfree deltahedral formation of Group 14 with more than one transition-metal atom inside. The anion was structurally characterized in the species [K(2,2,2-crypt)]4[Ni3@(Ge9)2]· 2 tol (2,2,2-crypt= 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, tol= toluene) made in a high yield by a reaction of an ethylenediamine(en) solution of K4Ge9 with a threefold excess of [Ni(cod)2]. The two end atoms of the nickel trimer center the two nine-atom germanium clusters while the central nickel atom is positioned at an inversion center and plays the role of a tenth (shared) vertex for each cluster. A closer look at the nineatom germanium clusters reveals that, similar to many empty clusters, they can be described as distorted tricapped trigonal prisms (Figure 1). Typically, the distortion in empty clusters is expressed in elongation of one, two, or all three of the vertical prismatic edges 1-7, 2-8, and 3-9. The nickel-centered clusters in 1 show the same distortion, all three edges being significantly longer, 3.061(2), 3.052(2), and 3.016(2) , than the remaining Ge–Ge distances, 2.604(1)–2.749(2) . However, the clusters in 1 are additionally distorted by the opening of one of the triangular bases of the prisms, the base made of atoms 7-8-9. This distortion is clearly caused by the insertion of the additional nickel atom, Ni2, as a vertex of the two nineatom germanium clusters (Figure 1) and results in two nickelcentered ten-atom clusters made of one nickel and nine germanium atoms, {Ni@Ge9Ni}. The two clusters form a dimer by sharing the nickel vertex, and the formula of 1 can be Figure 1. The anion 1. Ge: green. The Ni Ni bond length is 2.395(1) . Ni2 is at an inversion center and is octahedrally coordinated by germanium with distances [ ] from Ni2 to: Ge7 2.529(1), Ge8 2.540(1), Ge9 2.505(1) , and angles in the range 86.09(4)– 93.91(4)8. Each Ge9 cluster is a tricapped (atoms 4, 5, 6) trigonal prism (triangular bases of atoms 1-2-3 and 7-8-9) in which one of the triangular bases (7-8-9) has been opened by insertion of the nickel trimer. Distances [ ] from Ni1 to: Ge1 2.475, Ge2 2.487, Ge3 2.510, Ge4 2.446, Ge5 2.456, Ge6 2.453, Ge7 2.338, Ge8 2.340, Ge9 2.362. Other distances [ ]: Ge1-Ge7 3.016, Ge2-Ge8 3.052, Ge3-Ge9 3.061, Ge1-Ge2 2.654, Ge1-Ge3 2.634, Ge1-Ge4 2.726, Ge1-Ge6 2.749, Ge2Ge3 2.640, Ge2-Ge4 2.712, Ge2-Ge5 2.713, Ge3-Ge5 2.724, Ge3-Ge6 2.738, Ge4-Ge7 2.662, Ge4-Ge8 2.621, Ge5-Ge8 2.653, Ge5-Ge9 2.604, Ge6-Ge7 2.621, Ge6-Ge9 2.612. (The standard deviations for all distances are 1 or 2 10 3 .).