6447
J. Am. Chem. SOC.1995,117, 6447-6455
Unusual Icosahedral Cluster Compounds: Open-Shell N ~ A 6 T 1 1 3(A = K, Rb, Cs) and the Metallic Zintl Phase N a s K g T 1 1 3 (How Does Chemistry Work in Solids?) Zhen-Chao Dong and John D. Corbett* Contribution from the Ames Laboratory-DOE' University, Ames, Iowa 50011
and Department of Chemistry, Iowa State
Received December 28, 1994@
Abstract: The line phases in the title are obtained in high yields after fusion of the elements in stoichiometric proportions in Ta containers followed by slow cooling or annealing. The structures were established by singlecrystal X-ray means [Na&T113, A = K, Rb, Cs: Zm3, Z = 2, a = 11.5075(7) A, 11.6648(5) A, 11.8332(4) A, R,(F) = 2.9, 4.9, 4.2%, respectively; Na&T113: Rgm, Z = 3, a = 11.0890(6) A, c = 23.154(2) A, R, = 4.9%]. The structure of cubic Na&T113 is practically identical with that reported for NaK9T113, a composition that cannot be achieved in this structure. The two structure types contain isolated, centered icosahedra of T113l0- (Th) and T113Il(D3d) in bcc and ccp arrangements, respectively. All cations bridge between cluster faces or vertices, and their mixed sizes appear critical to the formation of the two structures containing these clusters, which do not occur in the binary systems. The phases exhibit Curie-Weiss (1.7, 1.9 p ~ (cubic ) Rb, Cs) and temperature-independent kh.1 1.9 x emdmol) (rhombohedral K) paramagnetism and poor metal-like resistivities (e293 = 310,625,69pQcm), respectively. The novel one-electron-deficient T113l0- is a result of cation packing limitations, while T1I3l1- fulfills (Wade's rule) predictions for a centered icosahedron. Centering of the clusters by thallium contributes markedly to the bonding in both. Theoretical calculations at the extended Huckel and relativistic Huckel (REX) levels for the clusters and at the LMTO (self-consistent band) level for NaRb6T113are all consistent with the experimental finding. Mixing of 6~112and 6~312levels in the clusters reduces relativistic separations substantially (to -0.2 eV) relative to that in the free T1 atom. Electron localization within the clusters appears dominant.
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Introduction A variety of isolated thallium cluster anions have been synthesized and characterizedin alkali metal-thallium system^.^-^ The rich cluster chemistry is related to an approximate 1:l atom: cluster charge ratio which allows (1) electronically for an appropriate number of electrons for the formation of deltahedral clusters with significant Tl-Tl bond strength and (2) structurally for a sufficient number of alkali metal cations embodied into the structure to efficiently separate the clusters. Such observations are also consistent with the strong relativistic effects for this heavy element.7 Spin-orbit splitting stabilizes the T1 (6~112)~ shell, causes an anomalously large electron affinity for the element, and makes the formal T1- state favorable. Parallel to electronic requirements, size factors also appear to play a crucial role in determining what cluster structures are formed. A number of novel thallium clusters not present in the binary systems have been successfully isolated by size tuning through mixing of alkali metals. These include the Tls7- and T1g9clusters in Na2K21T1lg5 and the centered icosahedral polyanions in the title compounds. The icosahedral unit of T1, complete or defect, has been recurrently observed in our recent investigations. It functions either as a building block in the condensed binary cluster phases Abstract published in Advance ACS Abstracts, May 15, 1995. (1) This research was supported by the Office of the Basic Energy Sciences, Materials Sciences Division, U.S. Department of Energy. Ames Laboratory is operated for the DOE by Iowa State University under Contract No. W-7405-Eng-82. (2) Hansen, D. A.; Smith, J. F. Acta Crystullogr. 1967, 22, 836. (3) Cordier, G.; Muller, V. Z . Kristullogr. 1992, 198, 281. (4) Dong, Z.-C.; Corbett, J. D. J . Am. Chem. Soc. 1993, 115, 11299. (5) Dong, Z.-C.; Corbett, J. D. J . Am. Chem. Soc. 1994, 116, 3429. (6) Dong, Z.-C.; Corbett, J. D. J . Cluster Sci. 1995, 6, 187. (7) Pyykko, P.; Desclaux, J. P. Acc. Chem. Res. 1979, 12, 276. @
(e.g., & T l l 3 (A = Rb, Cs) and K23Tl5& or as a primary packing fragment in the temary isolated cluster compounds. The latter are represented by (1) T1g9-, a defect centered icosahedron missing four adjacent T1 atoms,5 (2) TlloM'O- (M = Ni, Pd, Pt), which can be viewed as defect M-centered icosahedra missing two adjoining vertices,8 and (3) Tl1310-'11- in the present examples and in the reported NaKgT113: a structure that we cannot reproduce at that composition. While icosahedral fragments are widely present in elemental boron, metallic borides,I0 gallides," aluminides,I2 and i n d i d e ~ , ' ~ .isolated '~ icosahedral clusters of other elements have only been reported for [ B I ~ H I ~ ] its ~ - derivatives,I6 ,'~ and (Al-iBu)122- l 7 as well as some centered transition-metal clusters (e.g., [RhlzSb(C0)27l3- and [Au&12(PMe2Ph)lo13+ 19). The icosahedron is the most stable geometry for 12-atom clusters because it maximizes the number of nearest neighbors on the spherical surface. Two other possible arrangements are the cuboctahedron and anticuboctahedron. Although isolated units of the latter (8) Dong, Z.-C.; Corbett, J. D. Unpublished research. (9) Cordier, G.; Muller, V . Z. Nuturforsch. 1994, 49b, 935. (10) Matkovich, V. I., Ed. Boron and Refractory Borides; SpringerVerlag: Berlin, 1977. (11) Belin, C.; Tillard-Charbonnel, M. Prog. Solid State Chem. 1993, 22, 59. (12) Kreiner, G.; Franzen, H. F . J . Alloys Compds. 1994, In press. (13) Sevov, S. C.; Corbett, J. D. Inorg. Chem. 1992, 31, 1895. (14) Sevov, S. C.; Corbett, J. D. Inorg. Chem. 1993, 32, 1612. (15) Wunderlich, J. A.; Lipscomb, W. N. J . Am. Chem. Soc. 1960, 82, 4427. (16) Beall, H., In Boron Hydride Chemistry; Muetterties, E. L., Ed.: Academic Press: New York, 1975; pp 302-347. (17) Hiller, W.; Klinkhammer, K.-W.; Uhl, W.; Wagner, J. Angew. Chem., In?. Ed. Engl. 1991, 30, 179. (18) Vidal, J . L.; Troup, J. M. J . Orgunomet. Chem. 1981, 20, 238. (19) Briant, C. E.; Theobald, B. R. C.; White, J. W.; Bell, L. K.; Mingos, D. M. P. J . Chem. Soc., Chem. Commun. 1981, 201.
0002-7863/95/1517-6447$09.00/00 1995 American Chemical Society
6448 J. Am. Chem. SOC., Vol. 117, No. 24, 1995
Dong and Corbett
Table 1. Selected Data Collection and Refinement Parameters for NaA6T113(A = K, Rb, Cs) and Na3KgT113 empirical formula space group, Z lattice parameters," A a
N++&Tl I 3 Im3,2
N@b6T113 Im3,2
N&S~T~I~ Im3,2
NgsKsT113 R3m, 3
11.0575(7)
11.6648(5)
11.8332(4)
196, 15 700.2 0.75-1.62 0.024,0.029
194, 15 753.3 0.54-1.32 0.045,0.049
211, 15 700.9 0.56- 1.27 0.033,0.042
11.0890(6) 23.154(2) 435,29 651.5 0.67-1.24 0.038,0.049
c
no. of indep obsd refns ( I 2 30(I)), variables p , cm-' (Mo Ka)
transmission range residuals: R, RWb a
Guinier data, 1 = 1.540 562
A, 23 "C. b R = 211Fol - ~ F c ~ ~ R,/ ~=~[2w(lF0l F o ~ ;- ~ F c 1 ) z / ~ ~ ( F o ) 2w] 1=' 2 1/02, ;
are rare in solids (Rh13(C0)24Hsand [Rh1zPt(CO)241~20), they are more frequently found in condensed intermetallic phases.21 However, except for T113,the vertices in all of the polyatomic ions mentioned above are instead bonded to either ligands or like atoms in neighboring units. In this article, we report further examples of how tuning with cation sizes can allow the isolation of remarkable cluster units not known in the binary alloys, the centered icosahedral polyanions of T1131°- in NaA6T113 (A = K, Rb, Cs) (hereafter denoted as 4-6-13) and T113"- in NasKsT113(hereafter denoted 3-8-13). These are striking polyanions not just because they are "naked" and isolated but also because both open-shell and closed-shell examples of basically the same cluster exist. The important effects of centering and cation size on bonding and stability are discussed. We also correlate the structures with the observed electrical and magnetic properties, for which those of the closed-shell NasKsTl13 compound also provide further support for the concept of a "metallic Zintl phase" developed p r e v i o u ~ l y .Efforts ~ ~ ~ ~ are ~ also made to show, through these magnificent examples, how chemistry is involved in the synthesis, structure, bonding, and properties of the solids.
Experimental Section Syntheses. All the syntheses were performed by reaction of the constituent elements. The surfaces of the Na chunks (99.9%. Alpha) and T1 metal bar (99.998%. Johnson-Mathey) were cleaned with a scape1 before use, while the K (99.9%. Baker), Rb (99.9%, Alpha), and Cs (99.98%, Johnson-Mathey) were used as received sealed under Ar. The general reaction techniques in welded Nb or Ta tubing have been described e l ~ e w h e r e . ~Because .~ both reagents and products are very sensitive to air and moisture, all operations were carried out in Nz- or He-filled gloveboxes with typical humidity levels less than 0.1 ppm (~01). No temary phase diagrams are available for mixed alkali metalthallium systems. Following both the binary phase diagrams of Thummel and KlemmZ4and our experience, reaction mixtures were usually melted at -500 "C and then slowly cooled to room temperature. The 4-6-13 phases were first discovered following reactions of compositions Na3&Tll1 (A = Rb, Cs) in welded Nb tubing that were homogenized at 400 OC for 1 week, and then slowly cooled to room temperature at 3 "Ch. Such reactions provided N&n13 in high yields ('90%) and &T1138as minor products. The Na3KgT113 phase was first discovered as the major product (>70%) (together with an unidentified and probably Na-richer temary phase) after fusion of a NaKTlz composition in Ta tubing at 500 "C for 2 days, annealing at 250 "C for 30 days, and slow cooling (3 " C h ) to room temperature. Once the stoichiometries had been established by crystallography, single phase NaA6T113 (A = Rb, Cs) and Na3KsT113 were obtained through stoichiometric syntheses from the constituent elements in Ta, fusion of the mixture at 500 "C for 1 week being followed by slow cooling (3 (20) Albano, V. G.; Ceriotti, A,; Chini, P.; Martinengo, S.; Anker, W. M. J . Chem. Soc., Chem. Commun. 1975, 859. (21) Chabot, B.; Cenzual, K.; Parthi, E. Acru Clystullogr. 1981, A37, 6. (22) Nesper, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 789. Corbett, J. D. Inorg. Chem. 1995, in press. (23) Zhao, J.-T.; (24)ThiimmeL R.; Klemm, W. Z . Anorg. A&. Chem. 1970, 376, 44. (25) Shinar, J.; Dehner, B.; Beaudry, B. J.; Peterson, D. T. Phys. Rev. 1988, 37E, 2066.
" C h ) to room temperature. Although fusion of Na&Tl13 composition followed by slow cooling yielded only -20% cubic Na&T113 plus -80% of the close-by Na3KsT113,the yield of the target was raised to 95% (plus 3-8-13) by fast quenching of the melt from 500 "C followed by annealing at 300 "C for 1 week. We have been unable to achieve the synthesis of the reported NaK9T113phase;9 instead, reactions at the designated composition carried out as above yield only K8Tlll (-408) and Na3KgT113 (-60%). The size flexibilities of the two structures with respect to alkalimetal contents were also checked with the aid of powder diffraction data. Reactions starting with either A3A'gT113 or &A16T113 composition yielded (a) 4-6-13 but no 3-8-13 phases when A = Na and A' = Rb or Cs and (b) A~T111-type phases as major products6 (but no 3-8-13 or 4-6-13 products) when A = K or Rb and A' = Rb or Cs. Sodium is evidently always required in the A positions while A' = K distinguishes A3A'8T113 from A4A'6T113. Mixing of K, Rb, and Cs in the A' positions was also tested, with answers of "no" for the 3-8-13 phase but "yes" for the 4-6-13 phases, for which the cubic lattice constants vary over the whole range for A' = K-Cs from 11.51 (K) to 11.84 (Cs) A. X-ray powder patterns for samples mounted between pieces of cellophane were collected with the aid of an Enraf-Nonius Guinier camera, Cu K a radition (A = 1.540 562 A), and NIST (NBS) silicon as an intemal standard. Least-squares refinements of 38 to 64 lines indexed on the basis of the refined structural models resulted in the lattice constants given in Table 1. As is usually the case for compounds with isolated clusters, both NaA6T113(A = K, Rb, Cs) and Na3KgT113 appear to be line compounds (except of course when K, Rb, and Cs are mixed in the larger A' site in the former); mixtures with different overall compositions gave the same cell parameters with deviations of less than 0.007 A (loa) for each. These materials are very reactive reducing agents, and traces of elemental Tl are sometimes seen as broad lines in the Guinier patterns whenever adventitious impurities, probably traces of H20, are encountered during exposure to the X-rays. Property Measurements. The resistivities of pure phases were ~ ,sieved ~ ~ samples examined by the electrodeless "Q' meter m e t h ~ dfor of 93, 89, and 100 mg respectively of NaRb6T113, Na&s6T113, and Na3K8Tl13. These had average grain sizes of -200pm and were diluted with chromatographic A1203. The "Q'-meter method is based on the change of the quality factor "Q' of an LC circuit caused by skin absorption in selectively sized grains when the sample is placed in the cavity of a coil. The resistivity values have been found to be correct within a factor of 3, while the temperature coefficients are more reliable. Measurements were made at 34 MHz over the range of 100-293 K. Magnetic susceptibility measurements on powdered Na&bsT113 (23 mg), Na4Cs6Th (24 mg), and Na3KgT113 (58 mg) at a field of 3 T were obtained over the range of 6-300 K with the aid of a Quantum Design MPMS SQUID magnetometer. Samples were held between two fused silica rods, which were in tum fixed inside a silica tube that was sealed at both ends under He. The raw data were corrected for the susceptibilities of the containers and the diamagnetism of the coresz6 as well as for the Larmor precession contribution of the delocalized valence electrons in the cluster orbital^.^.'^ The same samples for magnetic measurements were also used for ESR experiments, which were recorded with a Bruker ER-200D spectrometer (X-band) at 295 K. Structure Determination. Black block-like crystals were picked up and sealed in thin-walled capillaries and then checked by Laue or
(26) Selwood, P. W. Mugnetochemistry, 2nd ed.; Interscience Publishers: New York, 1956; p 70.
J. Am, Chem. SOC., Vol. 117, No. 24, 1995 6449
Unusual Icosahedral Cluster Compounds Table 2. Positional Coordinates and Isotropic Thermal Parameters for NaA6T113(A = K, Rb, Cs) and Na3K8Tl13 site atom symmetry
X
Y
z
B,(A2Y
T11 T12 K Na
m?. m.. mm2 .3.
NaK6T1I 3 0 0 0 0.24157(7) 0.3284(7) 0 114 1I4
0 0.14142(8) 112 114
0.84 15(7) 1.50(5) 2.6(4) 1.378(3)
T11 T12 Rb Na
m? . m.. mm2 .3.
NaRbsT113 0 0 0 0.1406(1) 0.1707(5) 0 114 1I4
0 0.2378(1) 112 114
1.141(1) 1.78(6) 2.7(2) 0.538(3)
T11 T12 cs Na
m? . m.. "2 .3.
NaCs6T113 0 0 0 0.1388(1) 0 0.1701(2) 114 114
0 0.2333(1) 112 1I4
1.276(1) 1.92(5) 2.7(1) 0.478(3)
T11 T12 T13 K1 K2 Na
3m .m .m 3m .m .2/m
0 0.10579(5) 0.02376(4) 0.2413(5) 0.0824(3) 112
0.86(6) 1.33(2) 1.27(2) 2.1(3) 2.57(9) 0.7(6)
a
Na&Tl13 0 0 0.10755(6) -0.10755 -0.16797(5) 0.16797 0 0 0.4656(4) -0.4656 112 0
Be, = ( 8 ~ c ~ 1 3 ) ~ . , ~ ~ U , a , * a , * ~ ~ ~ ~ .
oscillation photography for their singularity. Na&T113 data were collected on Rigaku AFC6R and the rest on an Enraf-Nonius CAD-4 diffractometer with graphite-monochromated Mo K a radiation at 23 O C . Programmed indexing of 25 reflections from a random search over 13 5 2 8 5 30" yielded body-centered cubic cells for NaA6T113and a rhombohedral cell (hexagonal setting) for Na3KgT113. Two octants of data were measured in each case, with no absences imposed for N a Rb6T113, I-centering for Na&T113 and NaCs6T113,and R-centering for Na3KgT113 after the last condition was confirmed for the first 500 reflections. Assignments of the space groups Im3 for NaA6T113 and R3m for Na3KgT113 were made on the basis of the systematic absences, statistical analyses of intensity distributions, and the subsequent successful refinements. The structures were solved by direct method^.^' Peak assignments were easily made in terms of both bond distances and peak heights. The serious absorption effects, with linear absorption coefficients around 700 cm-I, were in each case corrected empirically according to, first, the average of the ly-scan curves for five to eight strong reflections at different 0 values and, subsequently, by DIFABS (as recommended)2s after the isotropic refinement converged. Multiplicity refinements of T1 atoms (B's varying) with the alkali-metal atoms fixed indicated that the former sites are fully occupied with deviations of less than 3 a from unity. The inverse procedure gave the same answer for alkali-metal atoms in all cases except for Na, which gave seemingly over-occupied sites but with deviation of those from unity of less than 4a. Fully occupied models were used in the final anisotropic refinements with secondary extinction corrections (at a 4-6a significance level) included. All data reduction and structure refinements were performed using the TEXSAN packagez9 on a VAX station. Some details of the data collections and refinements are listed in Table 1, the atomic positional and isotropic equivalent displacement parameters are in Table 2, and the important distances are given in Table 3. (No twinning or cells with different orientations were encountered for each of the cubic phases studied although such behavior is common for space group Im3.) The generally more accurate Guinier-based lattice parameters were employed in all distance calculations. More details as well as anisotropic displacement parameters are contained in the supplementary material, and these as well as structure factor data are also available from J.D.C. After the report of a NaK9T113 composition9 with substantially the same cubic structure as had been refined for NaA6T113(A = Rb, Cs), (27) Sheldrick, G. M. SHELXS-86; Universitat Gljttingen, BRD, 1986. (28) Walker, N.; Stuart, D. Acta Crystallogr. 1986, A39, 158. (29) TEXSAN, Version 6.0 package; Molecular Structure Corp.; The Woodlands, Texas, 1990.
we also refined the structure of the virtually identical Na&T113, with the results summarized in Tables 1-3. Among the positional parameters, our coordinate of T12 deviates from that reported by 1 . 8 while ~ the other two differ by
