Some interesting structural chemistry of lithium

www.elsevier.nl/locate/ica Inorganica Chimica Acta 298 (2000) 216 220 Some interesting structural chemistry of lithium F. Albert Cotton *, Hong-Cai Zhou Department of Chemistry, Laboratory for Molecular Structure and Bonding, Texas A&M Uni ersity, PO Box 30012, College Station, TX 77842-3012, USA Received 7 July 1999; accepted 14 September 1999 Abstract The lithium salt of -carboline, Li carb, (for which the systematic name is 1H-pyrido[2,3-b]indole), has been prepared and crystallized from both tetrahydrofuran and toluene. In each case the molecule is tetranuclear, with a distorted tetrahedron of lithium atoms, and in each case, the ligands are arranged in a Li 4 (carb) 4 assembly having S 4 point symmetry. In the compound crystallized from tetrahydrofuran, each lithium atom is coordinated by three nitrogen atoms and an oxygen atom, whereas in the compound obtained from toluene each lithium atom is only three-coordinate. The structures are topologically the same, but differ in the spatial orientations of the carboline planes. 2000 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Lithium compounds 1. Introduction The existence of the element lithium has been known since 1817 and the metal was isolated in 1855. Its chemistry [1] has been relatively slow to develop, especially the structural aspect, but in recent decades, X-ray crystallography has made a huge contribution. Nevertheless, there is still a great deal to be learned about the basic principles and possibilities in the structural chemistry of lithium. We present here two compounds that have very remarkable structures; moreover, these two structures differ from each other for reasons that relate directly to the different methods of preparation. 2. Experimental 2.1. General procedures * Corresponding author. Tel.: +1-409-845 4432; fax: +1-409-845 9351. E-mail address: cotton@tamu.edu (F.A. Cotton) All manipulations were carried out under nitrogen using standard Schlenk and glove box techniques. Solvents were purified using conventional methods, and were freshly distilled under nitrogen prior to use. The compound -carboline (Hcarb) was synthesized following a literature method [2], and sublimed before use; MeLi (1.0 M in THF cumene) and BuLi (1.6 M in hexanes) were purchased from Aldrich and used as received. 2.2. Preparation of Li 4 (carb) 4 (THF) 4 (1) The compound Hcarb (0.34 g, 2.0 mmol) was dispersed in THF (10 ml), and cooled to 78 C. Methyl lithium in THF cumene (1.0 M, 2.0 ml) was added dropwise. There was immediate effervescence, and a pale yellow solution was obtained. The solution was then transferred to a Schlenk tube, and layered with hexanes (80 ml). Colorless, block-shaped crystals were obtained in quantitative yield. 2.3. Preparation of Li 4 (carb) 4 toluene (2 toluene) The preparative procedure is essentially the same as that of compound 1, but the reaction was carried out in toluene instead of in THF, and BuLi in hexanes was used in order to avoid introducing coordinating solvent molecules. 0020-1693/00/$ - see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0020-1693(99)00453-3

F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216 220 217 2.4. Crystallographic studies Data collection for compound 1 was performed on a Nonius FAST area detector diffractometer at 60 C. The data set for 2 toluene was obtained using an Enraf Nonius CAD4 diffractometer at room temperature. Crystal and structure refinement data for 1 and 2 toluene are listed in Table 1. respectively. In 2 toluene, the disordered interstitial molecule of toluene was modeled in two orientations, and refined at 64% occupancy for the major orientation. 3. Results and discussion Our interest in the anion of -carboline (I), began several years ago [3] for the purpose of having a ligand very much like the anion of 7-azaindole (II), but modified so that the tendency of II to be disordered in crystals of M 2 (azindolate) 4 (III), would be thwarted. Utilization of I ordinarily requires the treatment of -carboline with lithium to give the reagent lithium carbolinate, Li carb. It was in the course of examining this reagent that we obtained the crystals whose structures are reported here. The crystals obtained from THF and toluene are designated as 1 and 2 toluene, Both molecules 1 and 2 have a distorted tetrahedral array of lithium atoms, but in each case the arrangement of the carbolinate ligands reduces the symmetry to S 4. In both molecules the carbolinate ligands form bridges across tetrahedral faces in the way shown schematically as A. Table 1 Crystal data and structure refinement Compound 1 2 toluene Empirical formula C 60 H 60 Li 4 N 8 O 4 C 51 H 36 Li 4 N 8 Formula weight 984.92 788.64 Space group P1 Pna2 1 a (A ) 12.823(3) 21.569(4) b (A ) 13.939(2) 13.975(3) c (A ) 18.221(6) 13.519(3) ( ) 69.82(4) 90 ( ) 69.81(3) 90 ( ) 64.15(2) 90 V (A 3 ) 2677(1) 4075(2) Z 2 4 Temperature (K) 213(2) 293(2) Radiation, (A ) 0.71073 0.71073 D calc (g cm 3 ) 1.222 1.285 Absorption coefficient (mm 1 ) 0.076 0.076 Data collection instrument Reflection collected Nonius FAST 13702 Enraf Nonius CAD4 3057 Independent reflections 6776 3050 [R int =0.0600] [R int =0.0122] Data/restrains/parameters 6771/0/685 3050/17/526 Goodness-of-fit on F 2 1.095 1.207 Largest peak (e A 3 ) 0.31(5) 0.64(7) a R c d 1 /R 1 0.075/0.104 0.075/0.147 b wr c d 2 /wr 2 0.163/0.189 0.193/0.227 a R 1 = ( F o F c )/ F o. b wr 2 = w[(f 2 o F 2 c ) 2 / [w(f 2 o ) 2 ]] 1/2 ; w=1/[ 2 (F 2 o )+(a P) 2 +b P], P=[max(F 2 o or 0)+2(F 2 c )]/3. c Denotes value of the residual considering only the reflections with I 2 (I). d Denotes value of the residual considering all the reflections. The major difference between 1 and 2 arises because of the solvent from which each is crystallized. From THF, one obtains molecule 1, with each lithium atom coordinated by a THF molecule to complete a very distorted tetrahedral arrangement, LiN 3 O, about it. For 2, which is crystallized from toluene, with no additional potential ligand available, the lithium atoms remain three-coordinate. However, 2 can be recrystallized from THF to give 1. Thus, 1 could be formulated as 2 (THF) 4. Presumably, 2 could be recrystallized from many other coordinating solvents, S, to give 2 (S) 4 products with structures akin to that of 1. We now examine and compare these molecules in more detail. For 1 the molecule is depicted in Fig. 1, and the principal dimensions are listed in Table 2. The molecule of 2 is shown in Fig. 2 and its principal dimensions are listed in Table 3. Fig. 3 shows a molecule of 1 stripped of its four THF ligands side by side with a molecule of 2, each viewed from the same direction. A numbering scheme common to both molecules has been used in all figures. The main features of the two molecules may now be compared quantitatively.

218 F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216 220 Table 2 Selected bond lengths (A ) and angles ( ) for 1 Fig. 1. The Li 4 (carb) 4 (THF) 4 molecule. Atoms in the Li 4 N 8 O 4 core are represented by thermal displacement ellipsoids at the 50% probability level. Carbon atoms are drawn at an arbitrary scale, and hydrogen atoms are omitted for clarity. 3.1. Li Li distances In each case the Li(1) Li(3) and Li(2) Li(4) distances are significantly longer than the other four, but the difference is more exaggerated in the case of 2, as shown by the following ranges: Li Li 1 2 1 3, 2 4: 3.460 3.464 3.43 3.60 Other four: 3.147 3.242 2.93 3.06 Bond lengths Li(1) N(101) 2.122(7) Li(1) N(201) 2.101(7) Li(1) N(402) 2.096(7) Li(1) O(1) 2.021(7) Li(2) N(102) 2.059(8) Li(2) N(201) 2.110(8) Li(2) N(301) 2.091(8) Li(2) O(2) 2.003(7) Li(3) N(202) 2.075(7) Li(3) N(301) 2.106(7) Li(3) N(401) 2.122(7) Li(3) O(3) 2.020(7) Li(4) N(101) 2.088(7) Li(4) N(302) 2.058(7) Li(4) N(401) 2.110(7) Li(4) O(4) 2.005(7) Bond angles O(1) Li(1) N(402) 101.2(3) O(1) Li(1) N(201) 100.2(3) N(402) Li(1) N(201) 128.3(3) O(1) Li(1) N(101) 114.5(3) N(402) Li(1) N(101) 103.8(3) N(201) Li(1) N(101) 109.0(3) O(2) Li(2) N(102) 102.8(3) O(2) Li(2) N(301) 103.1(3) N(102) Li(2) N(301) 122.6(3) O(2) Li(2) N(201) 111.7(3) N(102) Li(2) N(201) 102.8(3) N(301) Li(2) N(201) 113.4(3) O(3) Li(3) N(202) 99.4(3) O(3) Li(3) N(301) 113.9(3) N(202) Li(3) N(301) 104.3(3) O(3) Li(3) N(401) 103.0(3) N(202) Li(3) N(401) 126.0(3) N(301) Li(3) N(401) 110.0(3) O(4) Li(4) N(302) 100.9(3) O(4) Li(4) N(101) 99.7(3) N(302) Li(4) N(101) 124.6(3) O(4) Li(4) N(401) 112.4(3) N(302) Li(4) N(401) 103.8(3) N(101) Li(4)N(401) 114.5(3) each lithium atom has only a small effect upon the Li N bonds. 3.3. Dihedral angles between ligand planes As can be seen in Fig. 3, the only major difference between the two structures is in the orientations of the planar ligands. The situation is reminiscent of the way the pitch of an airplane propeller can be varied. In 2, each ligand plane is approximately perpendicular to the On the whole, the Li tetrahedron is a little more distorted but is also more compact in 2 than in 1. 3.2. N Li distances and N Li N angles It might be expected that for 1, where the coordination number of each lithium atom is four, the Li N distances might be longer than in 2, where each Li atom is only three-coordinate. This is true, as the following figures show: 1 2 Average Li N (A ) 2.10 2.04 Range of Li N (A ) 2.06 2.12 1.95 2.17 As for the N Li N angles, there is no significant difference, with the average values and ranges being 114 and 102 125 in 1 compared to 114 and 103 128 in 2. On the whole, the presence or absence of a fourth bond to Fig. 2. Drawing of the Li 4 (carb) 4 molecule in 2 toluene, with displacement ellipsoids drawn at the 50% probability level. All hydrogen atoms are omitted for clarity.

F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216 220 219 Table 3 Selected bond lengths (A ) and angles ( ) for 2 Bond lengths Li(1) N(101) 1.95(3) Li(1) N(201) 2.09(4) Li(1) N(402) 2.05(4) Li(1) C(210) 2.68(4) Li(2) N(102) 2.02(3) Li(2) N(201) 1.99(3) Li(2) N(301) 2.17(3) Li(2) C(310) 2.62(4) Li(3) N(202) 1.98(3) Li(3) N(301) 1.97(3) Li(3) N(401) 2.10(4) Li(3) C(410) 2.65(4) Li(4) N(101) 2.17(4) Li(4) N(302) 2.03(4) Li(4) N(401) Bond angles 1.97(3) Li(4) C(110) 2.64(4) N(402) Li(1) N(201) 109(2) N(402) Li(1) N(101) 118(2) N(201) Li(1) N(101) 117(2) N(102) Li(2) N(301) 102(2) N(102) Li(2) N(201) 125(2) N(301) Li(2) N(201) 112(2) N(202) Li(3) N(301) 119(2) N(202) Li(3) N(401) 108(2) N(301) Li(3) N(401) 115(2) N(302) Li(4) N(101) 107(2) N(302) Li(4) N(401) 119(2) N(101) Li(4) N(401) 114(2) planes of those adjacent, these angles being: 91.2, 91.5, 95.0, and 89.3. However, in 1 the ligands are raked back and the corresponding set of angles is: 115, 113, 112, and 124. As a consequence, the closest nonbonded Li C contacts are somewhat different in the two cases, viz., an average of 2.99 A in 1 and 2.65 A in 2. To our knowledge, structures of precisely this kind have not been previously reported for lithium compounds. We are aware of several compounds in which there is a tetrahedron of lithium atoms with each face bridged by a 3 -carbon atom and an appended donor atom is also coordinated. In several of these, the molecular symmetry is S 4 and in all cases the lithium atoms are four-coordinate without the presence of any further ligands [4]. In these compounds the Li Li distances are much shorter (ca. 2.5 A ) than those in 1 and 2 and the key feature is the presence of 3 -carbon atoms. We have found no previous example in which there are three-coordinate lithium atoms as in 2. There are also analogous structures for certain compounds of copper(i) and silver(i), where the bridging ligands are dithiocarbamates or dithiophosphates [5]. A final observation on these structures is that they have the somewhat unusual point symmetry S 4. This symmetry is of interest because it provides the only known exception among real molecules (1,3,5,7- tetramethylcyclooctatetraene being another S 4 case) to the commonly used (but false) criterion for chirality, namely that a molecule having neither a plane nor a center of symmetry will be chiral. It is evident that 1 and 2 fulfill that (false) criterion, but they are superimposable on their mirror images. The correct criterion [6] is, of course, that if a molecule lacks any improper axis of rotation ( and i being S 1 and S 2, respectively) it will be chiral. In practice the only point group of Fig. 3. Views of 1 and 2 along the same direction, with the THF molecules removed from the former. Atoms in the Li 4 N 8 cores are represented by thermal displacement ellipsoids at the 50% probability level. Carbon atoms are shown as arbitrarily scaled small circles, and hydrogen atoms are omitted. S n type and lacking an inversion center ever observed is S 4. Note that with an S 6 axis (which is known to occur [7,8]), there is necessarily a center of inversion (S 63 =i ). 4. Supplementary material Full listings of atomic coordinates, bond distances, bond angles, anisotropic displacement parameters, hydrogen atom coordinates (27 pages) are available from author F.A. Cotton.

220 F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216 220 Acknowledgements We thank the National Science Foundation for support. We also thank Dr X. Wang for crystallographic assistance. References [1] A.-M. Sapre, P. von R. Schleyer (Eds.), Lithium Chemistry, Wiley, New York, 1995. [2] W. Lawson, W.H. Perkin Jr., R. Robinson, J. Chem. Soc. 125 (1924) 626. [3] T.J. Mueller, Ph.D. Dissertation, Texas A&M University, 1990. [4] (a) J.T.B.H. Jastrzebski, G. van Koten, M. Konijn, C.H. Stam, J. Am. Chem. Soc. 104 (1982) 5490. (b) G.W. Klumpp, P.J.A. Geurink, A.L. Spek, A.J.M. Duisenberg, J. Chem. Soc., Chem. Commun. (1983) 814. (c) A.L. Spek, A.J.M. Duisenberg, G.W. Klumpp, P.J.A. Geurink, Acta Crystallogr., Sect. C (Cr. Str. Comm.) 40 (1984) 372. (d) S. Harder, J. Boersma, L. Brandsma, G.P.M. van Mier, J.A. Kanters, J. Organomet. Chem. 364 (1989) 1. (e) A. Muller, B. Neumuller, K. Dehnicke, Chem. Ber. 129 (1996) 253. [5] J.P. Fackler Jr., R.J. Staples, C.W. Liu, R.T. Stubbs, C. Lopez, J.T. Pitts, Pure Appl. Chem. 70 (1998) 839. [6] F.A. Cotton, Chemical Applications of Group Theory, third ed., Wiley, New York, 1990, pp. 34 39. [7] (a) C.W. Liu, R.J. Staples, J.P. Fackler Jr., Coord. Chem. Rev. 174 (1998) 147. (b) C.W. Liu, J.T. Pitts, J.P. Fackler Jr., Polyhedron 16 (1997) 3899. [8] F.A. Cotton, L.M. Daniels, C.A. Murillo, H.-C. Zhou, C.R. Acad. Sci. (1999) in press..