J. Am. Chem. Soc. 1997, 119, 2869-2876
2869
Synthesis, Characterization, Electronic Structure, and Bonding of Heteroatomic Deltahedral Clusters: Na49Cd58.5Sn37.5, A Network Structure Containing the First Empty Icosahedron without a Group 13 Element and the Largest closo-Deltahedron Evgeny Todorov and Slavi C. Sevov* Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed NoVember 25, 1996. ReVised Manuscript ReceiVed January 30, 1997X
Abstract: The intermetallic compound Na49Cd58.5Sn37.5 was obtained in nearly 100% yield after fusion of the elements in stoichiometric proportions in a Nb container and quenching of the mixture. The structure was determined by single-crystal X-ray diffraction (rhombohedral, R3hm, Z ) 3, a ) 16.034(1) Å, c ) 50.64(1) Å; R, Rw(F) ) 3.3%, 4.2%). The covalently-bonded part of the structure is a network of cadmium and tin that contains the following: (a) the first empty icosahedra built without atoms of an element of group 13; (b) 12-bonded 18-atom closo-deltahedra, the largest closo-deltahedron ever made; (c) 20-atom spacers. The sodium cations are located between the clusters and also center the 18-atom cluster. Band calculations on the network and molecular orbital calculations on the 12-bonded 18-atom cluster show that the latter is hyperelectronic with 2n + 4 skeletal electrons, and also that its six lone-pair orbitals are not occupied. Magnetic measurements show small temperature-independent paramagnetism.
Introduction The fascination with the high symmetry of three-dimensional bodies is not new and dates back to the age of the five Platonic solids and even before that. Today, this fascination is still very much alive, and we find it in the shapes of the fullerenes, the “metcars” of Ti8C12, the octahedral transition metal clusters, etc. It is also in the high-symmetry Platonic and Platonic-like main-group deltahedral clusters found in intermetallic and Zintl solid-state compounds. These latter clusters come mainly from group 13, although there are also tetrahedral clusters of groups 14 and 15, and a recently synthesized monocapped square antiprism of germanium, the first larger cluster of this group in the solid state.1 A variety of these deltahedral clusters of Ga, In, and Tl was synthesized and characterized in the last few years.2,3 The most reoccurring shape among them is the icosahedron, and the pronounced tendency of the elements of this group to form icosahedra has led to suggestions for naming them “icosogens” (similarly to pnictogens, chalcogens, and halogens).4 Furthermore, all known icosahedral species, M12, contain one or more atoms of group 13, i.e., it seemed that the presence of an icosogen was crucial for the formation of icosahedra, the trademark of the boron group. Some examples of phases with homonuclear icosahedra are β-boron, RbGa7,2 Li2Ga7,5 Na3K8Tl13,6 and Rb3Na26In48.7 Compounds with heteroatomic icosahedra in the structures are, for example, Mg11Zn11Al6,8 Na35Cd24Ga56,9 Na23In38.4Zn4.6, Na9In16.8Zn2.3, X Abstract published in AdVance ACS Abstracts, March 15, 1997. (1) Queneau, V.; Sevov, S. C. Submitted. (2) Belin, C.; Tillard-Charbonnel, M. Prog. Solid State Chem. 1993, 22, 59. (3) Corbett, J. D. In Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S., Ed.; VCH Publishers: New York, 1996. (4) King, R. B. Inorg. Chem. 1986, 28, 2796. (5) Tillard-Charbonnel, M.; Belin, C.; Soubeyroux, J. L. Eur. J. Solid State Inorg. Chem. 1990, 27, 759. (6) Dong, Z. C.; Corbett, J. D. J. Am. Chem. Soc. 1995, 117, 6447. (7) Sevov, S. C.; Corbett, J. D. Inorg. Chem. 1993, 32, 1612. (8) Bergman, C.; Waugh, J. L. T.; Pauling, L. Acta Crystallogr. 1957, 10, 254.
S0002-7863(96)04077-2 CCC: $14.00
Na49In90-xSnx, K37In69-xCdx,10 etc. We have embarked on exploring the possibilities of building icosahedral and larger deltahedral clusters without the “help” of an icosogen. Our approach in the search for such clusters is to use combinations of post-transition elements that will be isoelectronic with an icosogen, and study eventual Zintl (or Zintl-like) phases in the corresponding alkali metal-containing ternary systems. The Zintl phases form a large and important class in inorganic chemistry but only a very small part of their enormous combinatorial potential has been investigated to date. Usually the structures are very complex, difficult to understand, and have even more complex chemical bonding. Phases containing heteroatomic clusters of the kind mentioned above are expected to be even more complicated in that respect. In addition, a general observation can be made that the more “electron-poor” the elements in these heterometallic compounds (groups 11 and 12), the more unusual the structural motifs they create. This usually leads to novel bonding since the electronic requirements and the electronic structure as a whole will be quite different in these compounds. We report here on the synthesis and structure of the first compound, Na49Cd58.5Sn37.5, made by following the strategy of combinations of elements that are isoelectronic with an icosogen. This compound contains the first empty icosahedron made without group 13 elements. Also, an 18-atom closo-deltahedral cluster, the largest closo-deltahedron ever made, is present in the structure. In support of the elimination of the exclusive rights of the boron group to form icosahedra, we have also obtained a second compound, Na13Cd20Pb7, with icosahedra of Cd and Pb only.11 The 12-bonded 18-atom deltahedron has unusual electronic structure in what appears to be the first example of a closo-deltahedron with empty lone-pair orbitals. This apparently is due to the size of the cluster and the resulting curvature of its surface. Band calculations on the three(9) Tillard-Charbonnel, M.; Belin, C. Mater. Res. Bull. 1992, 27, 1277. (10) Sevov, S. C. Ph.D. Thesis, Iowa State University, 1993. (11) Todorov, E.; Sevov, S. C. Unpublished research.
© 1997 American Chemical Society
2870 J. Am. Chem. Soc., Vol. 119, No. 12, 1997 dimensional anionic network show a narrow band in the density of states that is low-lying above the Fermi level. The band is due to the same lone-pair orbitals and is empty. The cluster is also unusual in that it requires 2n + 4 electrons for bonding rather than the prescribed by the Wade’s rules 2n + 2. Experimental Section Synthesis. Series of mixtures of Na (Alfa, 99.9%, sealed under Ar), Cd (Alfa, 99.999%), and Sn (Alfa, 99.9%) were prepared in a N2filled glovebox ( 2σI) R indices (all data)
Na49Cd58.34(3)Sn37.69(2) 12157.75 a ) 16.034(1) Å c ) 50.64(1) Å V ) 11275(2) Å3 3 R3hm (No. 166) 143.2 cm-1 0.3653-0.9985 15 672 5.372 g/cm3 R1b ) 3.29%, wR2c ) 6.82% R1b ) 4.23%, wR2c ) 7.21%
a Room temperature Guinier data with Si as an internal standard (λ ) 1.540562 Å). b R ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑[w(Fo2 - Fc2)2/ ∑[w(Fo2)2]]1/2; w ) 1/[σ2Fo2 + (0.0875P)2 + 4.7318P], P ) (Fo2 + 2Fc2)/3.
sites did not bring recognizable differences either. Nevertheless, it was clear that two sites, M12 and M13 (M represents either Cd or Sn), were partially occupied. This was determined by varying the multiplicities of all anionic sites while keeping the sodium multiplicities fixed. All, except for M12 and M13, were within 3.5σ of full occupancy, independent of whether Cd or Sn was placed at a particular position. Therefore, these sites were kept with full occupancies for the subsequent procedures. The occupancies of the sodium sites were varied next while keeping the occupancies of the anionic sites fixed. All were within 3σ of full occupancy. For the final refinement all multiplicities except those of M12 and M13 were fixed. The stoichiometry of the compound was determined by other means (see below). The formula was found to be exactly or very close to Na49Cd58Sn38 based on the loaded stoichiometry of a reaction with 100% yield of the compound. Since the occupancies of the anionic sites by either Cd or Sn did not lead to significant differences in the final results, these sites were more or less arbitrarily assigned to the two elements but in such a way that (a) would reproduce most closely the determined stoichiometry and (b) would place the Sn atoms at sites with the smallest thermal parameters when the structure is refined with Cd only. Thus, there are six different Sn atoms (Sn3, 5, 8, 11, 12, 14) and eight different Cd atoms (Cd1, 2, 4, 6, 7, 9, 10, 13) in the structure. The partially occupied site M12 is Sn12 while M13 is Cd13. Of course, this assignment of different atoms for the different positions does not mean that some or all of the sites could not be with mixed Cd-Sn occupancies. The final refined formula is Na49Cd58.34(3)Sn37.69(2). Stoichiometry. The atomic ratio of the alkali metal to Cd-Sn was easily determined from the refinement as 49:96.03(3) but the Cd to Sn ratio was unclear. Since it is next to impossible to distinguishing Cd from Sn by X-rays, we looked for other ways to determine that ratio. We used molecular orbital and band calculations to estimate a meaningful value of x in Na49CdxSn(96-x). The result was rather a range of possible numbers, 52.5 e x e 58.5, due to the presence of nonbonding states that could be either populated or empty (see below). Subsequently reactions with stoichiometries within this range were loaded in order to determine the correct stoichiometry. These were heated at 680 °C for 1 day and then quenched in cold water in order to avoid any possible peritectic disproportionation. This was followed by annealing at 350 °C for 2 weeks. The X-ray powder patterns taken from the samples showed that all but the one with x ) 58 had another unknown phase in addition to the expected one. (Work is in progress to characterize this other unknown compound.) The powder pattern of the reaction loaded as Na49Cd58Sn38 showed only lines of the refined structure, and no lines of other phase could be seen. (For the Guinier camera, we expect ca. 3 wt % detection limit assuming similar scattering powers of the phases.14 ) The 100((3)% yield defines the stoichiometry of the compound (the (14) West, A. R. Solid State Chemistry and its Applications; Wiley: New York, 1987; p 50. (It is stated that “the lower limit of detection of impurity phases in routine work is usually in the range of 1-5 percent” but “under favorable conditions such as when looking for a specific impurity, the detection limit may be decreased considerably ... by increasing the time of exposure”. These limits are given for a maximum of 1 h exposure time, whereas we had 2 h of exposure for these particular samples.)
Heteroatomic Deltahedral Clusters
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2871
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters of Na49Cd58.34(3)Sn37.69(2) atom
N
x
y
z
Ueq (Å2)a
Cd1 Cd2 Sn3 Cd4 Sn5 Cd6 Cd7 Sn8 Cd9 Cd10 Sn11 Sn12b Cd13c Sn14 Na1 Na2 Na3 Na4 Na5 Na6 Na7 Na8 Na9 Na10
18h 18h 18h 18h 18h 18h 36i 18h 36i 18h 18h 6c 18h 36i 6c 18h 18h 18h 18h 36i 6c 6c 3a 18f
0.2718(1) 0.6050(1) 0.2268(1) 0.2711(1) 0.5399(1) 0.5615(1) 0.6674(1) 0.2169(1) 0.6668(1) 0.5602(1) 0.2182(1) 0 0.1111(1) 0.4913(1) 0 0.5406(2) 0.1270(2) 0.2170(2) 0.5404(2) 0.2898(3) 0 0 0 0.3757(5)
-x -x -x -x -x -x 0.1479(1) -x 0.9930(1) -x -x 0 -x 0.3374(1) 0 -x -x -x -x 0.3328(3) 0 0 0 0
0.9269(1) 0.9880(1) 0.9793(1) 0.7362(1) 0.9430(1) 0.0418(1) 0.8935(1) 0.8807(1) 0.8642(1) 0.2927(1) 0.7841(1) 0.4713(1) 0.9828(1) 0.1672(1) 0.4045(2) 0.1072(1) 0.1053(1) 0.0478(1) 0.8340(1) 0.9312(1) 0.2016(2) 0.1287(2) 0 0
0.017(1) 0.021(1) 0.021(1) 0.019(1) 0.019(1) 0.023(1) 0.020(1) 0.020(1) 0.017(1) 0.020(1) 0.019(1) 0.041(1) 0.035(1) 0.018(1) 0.029(2) 0.026(1) 0.029(1) 0.028(1) 0.033(1) 0.032(1) 0.023(2) 0.023(2) 0.044(4) 0.051(2)
a U is defined as one-third of the trace of the orthogonalized U eq ij tensor. b Occupancy 84.6(8)%. c Occupancy 72.4(5)%.
Cd to Sn ratio to be more specific). The 3% detection limit translates into the following uncertainties in the formula Na49Cd58(2)Sn96.03(5)-58(2). For the calculation, we used the two extreme cases of pure Sn or pure Cd as the second phase below the detection limit. An X-ray powder diffraction pattern was calculated based on the refined structure, and the lines on the observed powder patterns were indexed. A least-squares refinement of their 2θ values together with those of the standard Si resulted in a small interval of fluctuation for the a parameter of the compound, a ) 16.039(3) and 16.068(2) Å (ca. 7σ difference) for the end members with x ) 58 and 43, respectively, but no change for the c parameter, c ) 50.690(10) and 50.680(8) Å for the same stoichiometries. Since Cd and Sn have very similar sizes, large changes are not expected even if compounds with different CdSn ratios exist. On the other hand, different temperature treatments may result in differences in the lattice parameters. Thus, the parameters of the refined structure are somewhat closer to the parameters for composition with x ) 58, although the loaded stoichiometry is closer to that of x ) 43 (the actual x is 43.6). The difference between these reactions is in the cooling rates. Magnetic Susceptibility. The magnetization of 25 mg of the sample with 100% yield (loaded as Na49Cd58Sn38) was measured at fields of 3 T and of 100 Oe over the ranges of 6-295 K and 2-10 K, respectively. Quantum Design MPMS SQUID magnetometer was used for that purpose. A sample holder made of 3 × 5 × 160 mm (i.d. × o.d. × length) tubing and two half-length pieces of tightly fitting rods, all made of fused-silica, was used. One piece of rod is inserted all the way into the tubing and the ends of tubing, and rod are sealed together. The sample is then loaded from the other end of the tube (inside the glovebox), the other piece of rod is inserted, and the second end is sealed as well. The raw data were corrected for the susceptibility of the container.
Results Structure Description. The final positional and equivalent isotropic displacement parameters and the important distances (
