Organometallics 1995, 14, 1992-2001
1992
Intramolecular and Intermolecular Bonding in Rus(C0)12, R u s ( C O ) ~ @ ~ : ~ ~ : ~ ~ and :~~-C~H~), RU3(c0)6@-Co)S(/l3-S3C3H6) Dario Braga* and Fabrizia Grepioni Dipartimento di Chimica G. Ciamician, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy
Maria Jose Calhorda” Instituto de Tecnologia Quimica e Biolbgica, R. da Quinta Grande 6, 2780 Oeiras, Portugal and IST, Lisboa, Portugal
Luis F. Veiros Instituto Superior Ticnico, Departamento de Engenharia Quimica, 1096 Lisboa Codex, Portugal Received October 24, 1994@ The relationship between molecular and crystal structures of Ru3(C0)12 (11, of the benzene (21, and of the 1,3,5-trithiacyclohexanederivative RUT derivative Ru3(C0)9013-r2:r2:r2-C6H6) (CO)601-C0)3(~3-S3C3Hs) (3)has been investigated by a combined use of extended Huckel molecular orbital calculations, empirical atom-atom pairwise packing potential energy calculations and computer graphics. The presence of CO bridges has been related to the capabilities of the “facial” ligand (C6H6 and S3C3H6) as a n-acceptor or a n-donor. Bridging carbonyl groups in the equatorial plane are more effective in competing with the facial ligand for back-donation, and thus they stabilize n-donors. In R u ~ ( C O ) ~three Z , carbonyls replace C6H6 as x-acceptors. Hydrogen-bonding networks of the C-H---0 type involving the carbonyl oxygen atoms have been detected in crystalline 2 and 3. Bridging CO’s are observed to form preferential interactions with respect to terminal ligands.
Introduction There is an increasing interest in the chemistry of transition metal clusters as chemical materials.‘ The properties of the solids formed by organometallic complexes and clusters are a bridge between those of typical molecular solids formed by organic molecules and those of inorganic materialsU2These properties all depend on the relationship between the characteristics of the individual molecular entity (intramolecular bonds and steric interactions, size, shape, charge, etc.) and those resulting from the collection of identical molecules present in the crystal (intermolecular bonds, interactions between ions, inclusion, clathration, segregation, phase transitions, conductive and magnetic properties, etc.L3 The relationship between the molecule and crystal becomes especially intriguing when dealing with flexible molecules whose molecular structure can be conditioned @Abstract published in Advance ACS Abstracts, March 15, 1995. (1)(a) Mingos, D. M. P.; Wales, D. J. In Introduction to Cluster Chemistry; Grimes, Russell N., Ed.; Prentice-Hall International Editions: Englewood Cliffs, NJ, 1990. (b)Physics and Chemistry ofFinite Systems: From Clusters to Crystals; Jena, P., Khanna, S. N., Rao, B. K., Eds.; NATO AS1 Series; Kluwer: Dordrecht, The Netherlands, 1989. (c) Metal-Metal Bonds and Clusters in Chemistry and Catalysis; Fackler, P., Jr., Ed.; Plenum Press, NY,1989. (d) The Chemistry of Metal Cluster Complexes; Shriver, D. F., Kaesz, H. D., Adams, R. D., Eds.; VCH: NY,1990. (2) (a) Desiraju, G. R. Crystal Engineering, The Design of Organic Solids; Elsevier: Amsterdam, 1989; p 47. (b) Inorganic Materials; Bruce, D. W., O’Hare, Dermot, Eds.; John Wiley & Sons: Chichester, U.K., 1992. (3) Braga, D.; Grepioni, F. Acc. Chem. Res. 1994,27, 51.
by the need for a simultaneous optimization of intramolecular and intermolecular bonding. We have recently investigated several such cases in the organometallic chemistry field. The intrdintermolecular dualism has been tackled essentially via a combination of extended Huckel theoretical calculations (to address specific bonding problems) and packing potential energy calculations and packing analysis (to study the intermolecular bonding). With the aid of these tools we have recently reported on the relationship between the structure of penta- and hexaruthenium cluster isomers containing benzene ligands bound in facial and/or apical bonding modes and the respective crystal^,^ as well as on the relationship between the presence of intermolecular hydrogen bonds and the conformation and the bending of C-H bonds in bidarene) complexes of the type [(arene)2Ru2Cl3lf(arene = benzene, t ~ l u e n e ) . ~ In this paper we focus our attention on the relationship between molecular and crystal structures of three closely related complexes, namely Ru3(C0)12 (1),6the benzene derivative R~3(C0)9~3:r~:r~:~~-c6H6) (21,’ and ~~
~
(4) Braga, D.; Dyson, P. J.;Grepioni, F.; Johnson, B. F. G.; Calhorda, M. J. Znorg. Chem. 1994,33, 3218.
(5) Braga, D.; Calhorda, M. J.; Dyson, P.; Grepioni, F.; Johnson, B. F. G.; Veiros, L. Organometallics, in press. (6) Churchill, M. R.; Hollander, F. J.; Hutchinson, J. P. h o g . Chem. 1977, 16, 2655. (7) Braga, D.; Grepioni, F.; Johnson, B. F. G.; Lewis, J.; Housecroft, C. E.; Martinelli, M. Organometallics 1991, 10, 1260. (8)(a) Hoferkamp, L.;Rheinwald, G.; Stoeckli-Evans, H.; Sass-Fink, G. Helu. Chim. Acta 1992, 75, 2227. (b) Rossi, S.;Kallinen, JS.; Pursiainen, J.;Pakkanen, T. T.; Pakkanen, T. A. J.Organomet. Chem. 1992,440, 367.
0276-733319512314-1992$09.00/0 0 1995 American Chemical Society
Bonding in R u Complexes ~
Organometallics, Vol. 14,No. 4,1995 1993
the 1,3,5-trithiacyclohexane(TTC, hereafter) derivative
(ah8
8
RU3(C0)6~-C0)3(~3-S3C3H6)
The reasons for this choice are manifold and reside in the structural differences a t the molecular level and in the differences in intermolecular interactions that the three complexes present in their respective crystals. Upon substitution of benzene for three axial COS the distribution of CO ligands present in Ru~(C0)1pis preserved in 2, uiz. each Ru atom bears two radial and one axial CO’s in the terminal bonding mode. In 3, on the contrary, substitution of the TTC ligand for the three axial CO’s leads to a new distribution of CO ligands, uncommon in Ru3 structural chemistry, with three edge-bridging CO’s spanning the Ru-Ru bonds and six ligands terminally bound on each Ru atom. This leads to the following questions: why a bridged structure for 3, and an all-terminal structure for 2? What is the relative stability of the bridged and unbridged isomers for these molecules? In terms of intermolecular interactions we also face an interesting situation. The TTC ligand in 3 has been shown to participate in the formation of hydrogen bonds of the C-Ha .O(CO)type. Interactions of this type were detected for example in crystals of the two isomers 11-4(C0)6(~-C0)3~3-S3C3H6) and Ir4(C0)g(~3-S3C3Hs).g It was shown that, in the case of the bridged isomer, the bridging CO ligands are preferentially involved in short C-Ha -0 interactions. This observation is in keeping with the higher Lewis basicity usually attributed to bridging CO’s with respect t o terminal ones.1° Such a behavior has been recently substantiated through a systematic study of C-H.. .O interactions in organometallic crystals containing metal-bound CO 1igands.ll As a result of this study, it has become apparent that also arene hydrogens are involved in C-H* -0 interactions with CO ligands and that CO cannot compete in hydrogen bond formation when stronger hydrogenacceptor groups are present in the same crystal structure.12 On this premise it should be of interest to study molecular assembly and intermolecular interactions in the crystals of benzene and TTC derivatives 2 and 3. This study brings about new questions: is the presence of bridging ligands in the structure of 3 related to the formation of a stronger intermolecular hydrogen bonding network? Secondly, what is the role of C-H---0 interactions in the stabilization of crystalline 2? We will approach these problems from two different sides: the molecular structures of 1-3 and the relative stability of their possible isomeric forms will be investigated by extended Huckel molecular orbital calculations. The crystal structures will be studied with the aid of empirical atom-atom pairwise packing potential energy calculations and computer graphics. When used together, the two tools afford complementary information, providing valuable insights into the structural and solid state chemistry of organometallic complexes and clusters. (9) Braga, D.; Grepioni, F. J. Chem. SOC.,Dalton Trans. 1993,1223. (10)Horwitz, C. P.; Shriver, D. F. Adu. Organomet. Chem. 1984, 23, 218. (11)Braga, D.; Biradha, K.; Grepioni, F.; Pedireddi, V. R.; Desiraju, G. R. J.Am. Chem. SOC.,in press. For a more general reference on C-H.. -0interactions see also: Desiraju, G. R. Acc. Chem. Res. 1991, 24, 290. (12)Braga, D.; Grepioni, F.; Sabatino, P.; Desiraju. G. R. Organometallics 1994,13, 3532.
P
C
Figure 1. The solid state structures of (a) Ru~(CO)IP (11, (b)RUQ(CO)S (/43-)12:)12:?72-C6H6) (2), and (C) RU~(CO)~(/~-CO)S(p&C&6)
(3).
Molecular Structures in the Solid State The most relevant structural features of RudC0)lz (l), (21, and Rus(CO)S~-CO)S@3-&C3H6) (3) as determined from X-ray diffraction studies will now be briefly summarized. Sketches of the three molecules are given in Figure 1; relevant structural parameters, as obtained from the original papers, are reported in Table 1.
1994 Organometallics, Vol. 14, No. 4, 1995
Braga et al.
Table 1. Comparison of Relevant Structural Parameters for 1-3 RU1(CO)s~i:g2:g2:g2-CbH6)
RU~(CO)I~ M-Mrange M-Mm M-CO,, M-CO,,
c-oe, c-oax
2.851-2.859(1) 2.853 1) 1.92(1) 1.94(1) 1.13(1) 1.13(1)
M-X M-COb,
193 K
Ru~(C0)12possesses the well-known “all terminal” structure with each ruthenium atom carrying four CO ligands (see Figure la). Six ligands are radially bound and lie roughly in the plane of the triruthenium cluster, whereas the other six ligands are axially bound three above and three below the cluster plane. The overall idealized molecular symmetry is D3h. The Ru-Ru bonds, however, are not equivalent, with one “long”and two “short”bonds [2.8595(4)uersus 2.8521(4)and 2.8518(4)A, respectively]. This small but significant difference has been shown to have an intermolecular origin. The most relevant (in terms of intermolecular cohesion) packing motif in crystalline Ru~(C0)12is based on the insertion of one axial CO of one molecule into the tetracarbonyl unit generated by two axial and two radial CO’s on a next neighboring molecule (the so-called “key-keyhole” intera~tion).’~The same feature is present in the isomorphous crystals of the isostructural species O S ~ ( C O ) ~ ~ . ~ ~ The structure of Ru3(C0)9CU3:r2:r2:r2-c6H6) (2) has been determined at room temperature and 193 K (see Figure lb). The species is almost isostructural with the osmium analogue Os3(C0)9~3:r2:~2:r2-C6H~).15 In the ruthenium species the H atoms bend out-of-plane with respect t o the ruthenium atoms. The average bending with respect to the c6 plane is 21.1 and 21.5” at room temperature and at 193 K, respectively. The benzene and RUBplanes are almost parallel. This is a common feature in pus-arene clusters carrying facial ligands.16 The solid state molecular structure of the trithiane derivative R~3(C0)6CU-C0)3(p3-s3c3H6)(3) has been determined in the same crystalline form independently by two groups of scientists8 and is very much related to that of the benzene cluster. The trithiane ligand binds the three Ru atoms via the interaction of one lone pair on each sulfur atom (see Figure IC),as has been observed, for example, for the two isomers of Ir4(Co)g(p&3C3H6) mentioned above. The most remarkable feature about the structure of 3 is the presence of three bridging CO ligands coplanar with the RUBtriangle and spanning the three edges. The Ru-Ru bonds are slightly shorter than in 1 and present the same pattern, uiz. two “short”[2.8378(9)and 2.8306(13)Braga, D.;Grepioni, F. Organometallics 1991, 10,1254. (14)Churchill, M. R.;DeBoer, B. G. Inorg. Chem. 1977,16,878. (15)Gomez-Sal, M. P.;Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Wright, A. H. J. Chem. SOC.,Chem. Commun. 1985, 1682. (16)Braga, D.; Grepioni, F.; Dyson, P. J.; Johnson, B. F. G. Chem. Rev. 1994,94, 1585. (17)(a) Schilling, B. E. R.; Hoffmann, R. J . Am. Chem. SOC.1979, 101,3456.(b) Pinhas, A. R.; Albright, T. A.; Hofmann, P.; Hoffmann, R. Helu. Chim. Acta 1980,63,29.(c) Delley, B.;Manning, M. C . ; Ellis, D. E.; Berkowitz, J.; Trogler, W. C. Inorg. Chem. 1982,21,2247.(d) Chesky, P.T.; Hall, M. B. Inorg. Chem. 1983,22,2998.(e) Halet, J.F.; Saillard, J.-Y.; Lissillour, R.; McGlinchey, M. J.; Jaouen, G. Inorg. Chem. 1985,24,218.
RU~(C0)60c-CO)~Oc~-S~C~Hs)
2.827-2.855( 1) 2.837( 1) 1.91(1) 1.88(1) 1 . w1) 1.15(1) C 2.331(4)
2.838-2.868(1) 1.90(1) 1.88(1) S 2.428(2)
2.14(1)
(13) AI and one “long” bond [2.8687(8) AI. All p-CO groups are nearly symmetric.
Molecular Orbital Calculations The M3 triangular clusters have been the subject of many theoretical studies, done a t several levels.17J8In cluster was a recent one, the OS3(C0)gCU3-r2:r2:r2-c6H~) addressed and the bonding between the benzene molecule and the Os3(C0)9fragment discussed in detail.lg The benzene-M3 interaction can be described according to the general scheme through which electrons are donated from benzene (the low lying a2u and l e n orbitals), while the metals back-donate to the high-lying benzene ~t orbitals, especially to the ezU set. The bonding is favored by the bending back of the hydrogens, and this distortion is found wherever benzene coordinates a M3 metallic cluster. Also typical of such a coordination is the distortion of benzene toward cyclohexatriene upon coordination, the difference between short and long C-C bonds in benzene depending, for instance, upon the nature of the metal. In the pair of similar M ~ ( C O ) & Q - ~ 2-C6H6) ~ : ~ ~molecules, : C-C bonds range from 1.41 to 1.56 when M = Os and from 1.37 to 1.45 A when M = Ru. It is induced by the mixing of the elg and ezUlevels of benzene taking place in this coordination mode. Theoretical studies of M3(C0)12clusters are extremely abundant for M = Fe, but not so much for the heavier metal derivative^.^^^^^,^^ The cluster containing the sulfur donor ligand 1,3,54rithiacyclohexane,TTC, has not yet been theoretically studied, as far as we know. We will now investigate the reason for the presence of three bridging carbonyls in the Ru3 plane for RUB(C0)6CU-C0)3CU3-S3C3H6)while in the other clusters all carbonyls are terminal. The study of the bonding between the TTC ligand and the cluster, using the extended Hiickel method,21will be the starting point. In the observed geometry, TTC binds to the Ru3(CO)6(p2-COI3 fragment using three linear combinations of sulfur lone pairs of a1 and e symmetry pointing toward the metal atoms. Electrons are donated t o three empty z , z2 ruthenium orbitals, forming the three Ru-S 0 bonds. The other sulfur lone pairs are approximately parallel to the Ru3 triangle and their interaction with
x
(18)(a) Mealli, C. J . Am. Chem. SOC.1986,107,2245.(b) Barreto, R. D.; Fehlner, T. P.; Hsu, L.-Y.; Shore, S. G. Inorg. Chem. 1986,25, 3572. (c) Griewe, G. L.; Hall, M. B. Inorg. Chem. 1988,27,2250.(d) Cotton, F.A.; Feng, X.Inorg. Chem. 1991,30,3666.(e) Gallop, M.A.; Gomez-Sal, M. P.; Housecroft, C . E.; Johnson, B. F. G.; Lewis, J.; Owen, S. M.; Raithby, P. R.; Wright, A. H. J . Am. Chem. SOC.1992,114,2502. (19)Riehl, J.-F.; Koga, N.; Morokuma, K.Organometallics 1993,12, 4788. . .. (20)(a) Baerends, E. J.; Rosa, A. New J . Chem. 1991,15,815.(b) Li, J.;Jug, K. Inorg. Chim. Acta 1992,196,89. (21)(a)Hoffmann, R.J . Chem. Phys. 1983,39,1397. (b)Hoffmann, R.;Lipscomb, W.N. J . Chem. Phys. 1962,36,2179,3489.
Bonding in RUBComplexes
Organometallics, Vol. 14, No. 4, 1995 1995
-I
-7
-81
-9 -
EnergyIeV -10-11-12-13-
le
-14-
s'hs'
/JRY Ru-RU
rs7
s-
Figure 2. Bonding between 'M'C and the fragment Ru3(CO)&-C0)3 in 3. Table 2. Relative Energies, Interaction Energies, and Overlap Populations between Fragments for the Four Clusters Containing Benzene or TTC and Ru~(CO)&-CO)~ or Ru3(C0)9
energy" AEb
OP
0.17 -2.52 0.98
Relative energy (ev).
0 -3.04 1.05
0 -8.72 1.41
1.05 -8.02 1.41
AE = Emoiecule - (Eilgand-t E R ~fragment). ,
metal orbitals is negligible. This is shown schematically in Figure 2. Knowing the bonding modes of the facial ligands, benzene and TTC, to the rest of the molecules, the effect of the bridging carbonyls can be analyzed. This problem has been briefly addressed for the binuclear system M2(CO)s, where the interaction of two bridging or two terminal carbonyls with Mz(CO)~was studied and compared.22 Using for the two R u ~ ( C Ofragments )~ the geometries they exhibit in each complex, the energies of the four possible clusters were determined. They agree with the molecular structures observed in the crystals, as can be seen from their values in Table 2. The relative energies indicate which geometry should be more stable: benzene prefers to bind to the allterminal Ru3(CO)gfragment, while the TTC cluster is stabilized by the fragment containing bridging carbonyls. The interaction energy, AE,given by the difference between the energy of a molecule and the sum of the energies of its fragments when not interacting, makes allowance for the different energies of the two Ru3 groups [Ru~(CO)~(U-CO)~ has a lower energy than Ru3(CO)91. Finally, the overlap population between fragments, on the last row, reflects the strength of the bond (22) Thorn, D.L.;Hoffmann,R.Inorg. Chem. 1978,17, 126.
between the ligand, benzene or "C, and the rest of the cluster. It suggests that the local bond to TTC does not depend much on the nature of the metallic moiety, while benzene can form stronger bonds to RUQ(CO)S when all CO groups are terminal. The interaction diagram for the bonding of benzene to both metal fragments is shown in Figure 3, with the benzene n orbitals in the center, those of RU3(CO)6(UCO)3 on the left, and those of Ru3(C0)9 on the right, while the orbitals of the possible clusters occupy the areas in between. The orbitals of a triangular Ms(C0)g cluster with terminal carbonlys have been described earlier17a and can be derived from those of the wellknown M(C0h half-~ctahedron.~~ There are three cluster orbitals which are mainly responsible for the energy difference between the two structures, namely the 2a1 orbital (HOMO of the cluster with terminal carbonyls; at lower energy in the other) and the l e set. The other levels have comparable energies. The l e and 2e sets of benzene n orbitals are allowed to mix under C3" symmetry. In the free undistorted molecule they were the elg(occupied)and the e%(empty) levels. On the other hand, each of the Ru3-based fragments has two sets of e symmetry at an appropriate energy, one full, le, one empty, 2e. They seem t o fulfill the conditions for strong interaction. What can be seen, however, is that the complete interaction is only taking place when there are no bridging CO ligands, as both levels are pushed down by it (right side). Conversely, in the case of Ru~(CO)&-CO)~, the energy of the lower l e set remains the same. It does not interact with the corresponding empty n orbital of benzene and, because of this, there is practically no back-donation. These (23) Albright, T. A.; Whangbo, M.-H.; Burdett, J. K. Orbital Znteractions in Chemistry; John Wiley & Sons, New York, 1985.
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1996 Organometallics, vel. 14, No. 4, 1995
n
-5-
-7-
-9 -
-1 1-
-1 3-
-1 5CO -Ru-
CO \ / \ / RU RU \ /
-
co
0
Ru / \ Ru-RU
~ U 3 ( ~ 6 ~ 6 ) ( ~ ~ ) 6RW(C6H6)(CO)g ( ~ - ~ ~ ) 3 Figure 3. Interaction diagram for the bonding between the benzene fragment and the two fragments, Ru3(C0)6(112-C0)3 on the leR (3) and RuQ(CO)~ on the right (2). The more important orbitals of the metallic frgaments are shown in Figures 4 and 5. interactions only are shown in Figure 4 for a better comparison. The main reason why there is practically no backdonation from Ru~(CO)~(U-CO)~ is that the lower energy set, le, of this fragment in involved in an interaction with carbonyl n* orbitals. It is therefore much less localized in the ruthenium atoms than the corresponding l e set in the nonbridged fragment, and overlap with the benzene empty 2e orbitals is smaller. It can be said, in a different way, that there is a competition between back-donation from Rua(CO)e to benzene and to the bridging carbonyls. As carbonyls are strong n acceptors, back-donation to benzene becomes less important and the bonding of benzene weaker, as mentioned earlier. We can see that back-donation has indeed decreased, when there are CO bridges, from the smaller population of the e2(e2,) level (0.158 vs. 0.179). Notice that mixing of all e levels, allowed by C3u symmetry, slightly obscures this picture. The extra stabilization due to back-donation is partly lost from the destabilization of one a1 level. The lowest n orbital of benzene is a1 under CsUsymmetry and can therefore interact with a1 Ru3 orbitals. Their overlap will be better when the 2, z2 (out-of-plane) character
increases (compared with in-plane x2 - y2,for instance). For the R U ~ ( C O ) ~ ( ~ -fragment, CO)~ such an orbital is empty, the low-energy la1 orbital being the one mainly in the plane. On the other hand, for Ru3(C0)9, la1 has z2 character and is occupied, giving rise to a fourelectron destabilizing interaction. The resulting antibonding orbital has this character decreased by mixing in a bonding way of the high-energy empty 2a1, essentially z , but it has two electrons. This interaction is schematically depicted in Figure 5. The previous effect is obviously less important than the one discussed earlier concerning back-donation to benzene 2e orbitals. In order to check for the reliability of this explanation, we tried to find other examples of clusters. The only one available seems to be Ru3(C0)12,whose structure was discussed above. Indeed, the three carbonyl groups on one face of the ruthenium triangle can be considered as replacing benzene and they are also n acceptor ligands. The structure has no bridges. As RudC0)6(pU-CO)3 and Rua(CO)e fragments have the geometry found in the benzene and 'M'C clusters, and no attempt at optimizing them in this case was done, the energies are not good indicators. But, if one takes the overlap
Bonding in R u Complexes ~
Organometallics, Vol. 14, No. 4, 1995 1997
le Ru /' Ru-Ru RU3(C6H6)(C0)6(I.r-CO)3 RU3(c6H6)(co)9 Figure 4. Comparison between the interactions of benzene 2e orbitals with the two Rus(C0)gfragments.
Ru Ru-RU /'
~ ~ 3 ( ~ 6 ~ 6 ) ( ~ ~ )Ru3(C6H6)(CO)9 6 ( ~ ~ 0 ) 3 Figure 5. Comparison between the interactions of the benzene la1 orbital with the two Rus(C0)gfragments. 2.32 to 2.24 with introduction of three equatorial population betwen fragments, one fragment now being bridges. The pattern is the same. The bridging carbothe group of three facial carbonyls, it decreases from
Braga et al.
1998 Organometallics, Vol. 14, No. 4, 1995
Table 3. Hydrogen Bonding Interactions in Crystalline 2 and 3 donor-H.*.acceptor
D-.A
(A)
H**.A (A)
R~~(C0)90C?-r2:r2:r2-c6H6) (2) at C(ll)-H(2).*.0(1) 3.363 C( 12)-H(3). * Q(5) 3.306 C( 14)-H(5). ..0(2) 3.207 C(lS)-H(6)...0(9) 3.463
R U ? ( C 0 ) 9 ~ ~ ? - t l ~ : t l ~ : r ’ - C g(2) H g )at
C(ll)-H(2).*.0(1) C(12)-H(3).. . 0 ( 5 ) C(14)-H(5).*.0(2) C(15)-H(6).. G(9) C( 10)-H( 1). *.0(4)
3.318 3.263 3.156 3.435 3.501
D-H.*.A (deg)
Room Temperature 2.573 129.3 2.416 138.9 2.372 133.0 2.597 136.7 193 K
2.582 2.287 2.334 2.470 2.549
R u ~ ( C O ) ~ @ ~ - S ~(3) C ~atHRoom ~ ) Temperature C( 120)-H( 120). *a( 13) 3.328 2.369 C(120)-H(121).*.0(23) 3.146 2.330 C( 120)-H(121) Q(5) 3.307 2.547 C(230)-H(230). * a(13) 3.246 2.262 C(230)-H(230). * O(1) 3.141 2.559 C(230)-H(231). *Q(12) 3.340 2.538
Figure 6. (a) Anti-cuboctahedral ligand polyhedron of Ru,(CO)12possessing only triangular and rectangular faces formed by sets of three and four terminal CO ligands. (b) Two-dimensional crystal of R u ~ ( C Omolecules. )~~ Note how the outermost packing motif is made up of groups of three and four terminal COS. nyls compete more efficiently for back-donation from the metals than terminal ones.
Crystal Structures We will now proceed by investigating the way molecules assemble in crystals of the three species with a focus on the intermolecular interactions established by the CO ligands in terminal and bridging bonding modes with the surrounding atoms. In terms of molecular shape, R u ~ ( C Ocan ) ~ ~be described as an anti-cuboctahedron. This polyhedron is delimited by triangular and are rectangular faces, which, in the case of Ru~(CO)~Z, formed by groups of three and four terminal CO’s, respectively (see Figure 6a). The overall idealized molecular symmetry is D3h. As discussed previous1y,l4 the most relevant packing motif in crystalline Ru3(CO)12 is the “key-keyhole’’interaction attained via insertion of one axial CO of one molecule into the tetracarbonyl unit generated by two axial and two radial CO’s on a next neighboring molecule. Molecules linked in such a way form a molecular row of Ru3(CO)12molecules. Once the packing pattern has been decoded, one can reconstruct the experimental crystal structure in the follow-
124.7 149.3 131.6 148.2 146.6 148.3 131.7 127.3 152.1 113.3 131.0
ing (hypothetical) process of molecule deposition and crystal nucleation. A molecular layer is obtained by placing molecular rows side by side, leaving groups of three and four terminal CO’s protruding from the surface of the layer (see Figure 6b). Such a surface pattern affords “Velcro-like” units for incoming molecules to cling to in order t o generate the stacking of molecular layers and the tridimensional structure. Although R~3(CO)gCU3-)7~:lj1~:v~-CgHg) and Os3(CO)g(u3q2:v2:v2-C6H6) are almost isostructural in the solid state, their crystals differ substantially in terms of intermocryslecular organization. R~3(C0)9CU3-)7~:v~:r~-c6H6) tallizes in the space group P21 with 2 = 2. The “enclosure shell” (uiz.the molecules in the immediate neighborhood of the one chosen for reference) consists of 12 molecules distributed in anti-cuboctahedral arrangement (A/B/A sequence of layers). In spite of the structural similarity, the osmium analogue Os3(CO)g@3-v2:v2:v2-C6H6) possesses a different crystal structure.15 Because of the presence of two independent molecules, the “enclosure shell” is formed by two groups of 12 molecules both organized in cuboctahedral fashion (A/B/C sequence of layers).l3 We have discussed previously that the differences in structure and packing between Ru3(C0)9@3-v2:v2:v2C6Hd and Os3(C0)9CU3-r2:r2:r2-c6H6) can be explained by assuming that the two crystal structures represent two alternative solutions to the minimization of the “global” energy (i.e. inter- and intramolecular energy) of the molecule-crystal system. The crystals of the two benzene clusters can thus be regarded as a sort of polymorphic modification, whose existence might be due to the difference in intermolecular cohesion caused by substitution of Ru for Os. Our findings on the participation of C-H---O(CO) interactions in the stabilization of organometallic crystalsl’ have prompted us to re-investigate the crystal structure of 2. We have indeed discovered the existence in crystalline 2 of an intricate network of intermolecular bonds between the facially bound benzene ligands and the CO ligands. Donor-acceptor separations, as well as H---0 distances, and C-H---0 angles for the hydrogen bonds are given in Table 3. The comparison (24) (a)Green, R. D. Hydrogen Bonding by C-H Groups; Wiley: New
York,1974. (b) Taylor, R.; Kennard, 0. J.Am. Chem. SOC.1982,104, 5063.
Bonding in R u Complexes ~
Organometallics, Vol. 14, No. 4, 1995 1999
Figure 7. C-H-a-0 hydrogen bonds between a reference molecule and the first neighboring molecules in crystalline 2. between the data at room temperature and at 193 K allows one to see the effect of the temperature on these interactions. The H-bond network is shown in Figure 7. Four of the six benzene hydrogens participate in interactions that are shorter than 2.6 A (i.e. shorter than the sum of the van der Waals radii: 1.20 and 1.50 A, respectively). Two of these are short [H(3)---0(5)2.416, H(5)---0(2)2.372 AI and fall toward the lower limits for C-H---0 interactions in organic and organometallic crystals.ll It is important to recall in this context that C-H---0 interactions, being mainly electrostatic in nature, decrease much more slowly with distance and contribute to crystal cohesion at distances which can be even longer than those of the van der Waals interact i o n ~ .Another ~~ important criterion is the C-H---0 angle, since it has been observed that the shortest H---0 separations involving CO ligands are associated with C-H---0 angles usually in the range 120-140'.11a Indeed, the C-H---0 angles in crystalline 2 span the narrow range 129.3-138.9' at room temperature, and 124.7-149.3' a t 193 K (see Table 3). On the basis of these observations it seems possible to conclude that C-H---0 interactions in crystalline 2 participate in the stabilization of the crystalline edifice and should be taken into account in evaluating the effect of crystal structure optimization on the molecular features. We had previously pointed out that the deviation from idealized C3" symmetry (caused by the tilt of the benzene ligand away from exact eclipsing of the C=C bond midpoints over the Ru atom and by the rotation of the tricarbonyl units around the 3-fold axes) appears to have an essential intermolecular origin. Since (C0)s torsion accompanied by benzene tilting costs little to the bonding in 2, the interpenetration of the molecular units at the expense of small deformations of the molecular structure allows a high efficiency of packing. We can now add that the benzene H atoms in 2 appear to be sufficiently acidic so as to establish hydrogen bonds with the carbonyl oxygens.
Let us now discuss the molecular arrangement in crystalline 3, which not only possesses both terminal and bridging CO ligands but also carries the TTC ligand. We have previously observed, in the study of the crystals of the two isomers I ~ ~ ( C O ) ~ ( L L - C O ) ~ ( , U ~ S&H6) and Ir4(C0)g(,~3-S3C3H6),~ that the H atoms of the CH2 units are sufficiently acidic to participate in C-H---0 hydrogen-bondingnetworks and that bridging CO ligands are preferentially involved in short C-H---0 interactions. The first neighboring molecules are distributed in cuboctahedral fashion in crystalline 3. A network of C-HMOinteractions is clearly detectable in this crystal. Relevant geometrical features are listed in Table 3. The shortest interactions are those established by the bridging CO ligands. Carbonyl CO(131, in particular, appears to be involved in a bifurcated interaction [0(13).*. H(120) 2.369, O(13).**H(230)2.262Al; CO(23) and CO(12) are instead in close proximity of only one H atom [0(23>-.H(121)2.330,0(12>-.H(231) 2.538 AI, but these two interactions are present twice in the same molecule because of symmetry relationships with the first neighboring molecules (see Figure 8a). C-H---0 bonds are not confined to these ligands, though. Two interactions involve two terminal CO's trans to the S atoms, as shown in Figure 8b. These interactions, however, are appreciably longer than those involving the bridging ligands (see Table 3). A further point of interest arises from the participation of the lone pairs of a S atom in an interaction with an H atom. The separation S(2). H(131)is 2.924 A, uiz. appreciably shorter than the sum of the van der Waals radii of hydrogen and sulfur (1.20 and 1.85 A, r e s p e c t i ~ e l y ) . ~ ~
Conclusions With this paper we have addressed the dualism between molecular structure in the solid state and crystal structure for three closely related cluster com(25) (a) Bondi, A. J . Phys. Chem. 1964, 68, 441. (b)Gavezzotti, A.
Nouu. J . C h i n . 1982, 6,443.
2000 Organometallics, Vol. 14, NO.4, 1995
Braga et al.
Figure 8. (a) C-H...O hydrogen bonds involving bridging CO ligands in crystalline 3.The interaction between a H(CH) atom and a S atom is also shown. (b) C-H.*.O interactions involving two terminal CO’s trans to the S atoms. plexes of ruthenium. Our observations can be summarized as follows: (i) Extended Huckel calculations have shown that, since bridging CO’s are more efficient n-acceptors than terminal CO’s, their presence in 3 is required in order to compensate for the substitution of the o-donor ligand TTC for three axial ligands in the structure of RUB(C0112. (ii) This is not so in 2 where the facial C6Hs ligand
takes the place of the axial COS as n-acceptor and stabilizes the all-terminal structure. (iii)Whereas 1 forms a van der Waals solid based on the interlocking of CO ligands, hydrogen-bonding networks of the C-H***O type involving the carbonyl oxygen atoms stabilize the crystal structures of 2 and 3.
(iv) Although the terminal ligands participate in C-He -0 bonding in both 2 and 3,the shortest interactions are observed between the TTC hydrogen atoms and the bridging CO’s in 3. This is in keeping with the charge distribution obtained from the extended Hiickel calculations, which assigns a slightly more positive charge to the TTC hydrogens with respect to those of benzene (+0.03us +0.02)and a slightly more negative charge to the oxygen atoms of the bridging ligands (-0.70 us -0.63 and -0.62 for the terminal ligands in 2 and in 3, respectively). (v) As the calculations show, each cluster is present in its solid state structure as the isomeric form which optimizes the bonding, Le. with only terminal COS for 1 and 2 and with bridging CO’s for 3.
Bonding in Rug Complexes
Methodology All the molecular orbital calculations were done using the extended Huckel method2Iwith modified H I ~ , sThe .~~ basis set for the metal atom consisted of ns, np, and ( n - l)d orbitals. Only 3s and 3p orbitals were considered for sulfur. The s and p orbitals were described by single Slater-type wave functions, and the d orbitals were taken as contracted linear combinations of two Slatertype wave functions. Standard parameters were used for H, C, 0, and S, while those for Ru were the following (HiileV, 5): 59, -10.40, 2.078; 5p, -6.89, 2.043; 4d, -14.90, 5.378, 2.303 (521, 0.5340 (Ci), 0.6365 (Cz). Three-dimensional representations of orbitals were drawn using the program Idealized models having C3" symmetry were used for the clusters studied. The two metallic fragments Ru~(CO)~@-CO)~ and Ru~(C0)g are based on the geometries they exhibit on the respective benzene and TTC cluster^.^^^ The following distances (A) were used: RuRu 2.85, Ru-C(termina1) 1.90, Ru-Ubridging) 2.14, C-0 1.15, Ru-Ubenzene) 2.33, Ru-S 2.42, C-C 1.40, C-S 1.80, C-H 1.09. The benzene hydrogen atoms were considered as bent back from the metal atoms by 20". Crystal structure analysis was carried out with the aid of the computer program OPEC,29awhich allows the calculation of packing potential energies as well as of molecular volumes and packing coefficients. Partition(26)Ammeter, J. H.; Biirgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J . Am. Chem. SOC.1978,100,3686. (27)Mealli, C.; F'roserpio, D. M. J . Chem. Educ. 1990,67, 39. (28)(a)Filippini, G.; Gavezzotti, A. Acta Crystallogr. 1993,B49,868. (b)Gavezzotti, A.;Filippini, G. J. Phys. Chem. 1994,98, 4831. (29)(a) Gavezzotti, A. OPEC, Organic Packing Potential Energy Calculations, University of Milano, Italy. See also: Gavezzotti, A. J. Am. Chem. SOC.1983,195,5220.(b)Keller, E.SCHAKAL92,University of Freiburg, Germany, 1992. (c) Spek, A. L. PLATON. Acta Crystallogr., Sect A 1990,46,C31.
Organometallics, Vol. 14,No. 4,1995 2001
ing of the packing potential energy among the molecules forming the enclosure shell allows recognizion of most relevant packing motifs and crystal structure decoding. In particular, when atom-atom potential parameters for purely van der Waals interactions are used,28 the interaction between the oxygen acceptor and the hydrogen atom participating in a hydrogen bond gives rise to a substantial repulsion because the distance between the atoms is below that of a van der Waals contact. This repulsion in the crystal is overcome by the electrostatic attraction between the two atoms involved in the hydrogen bond. Therefore, strongly repulsive 0---H interactions in the list of intermolecular interactions can be taken as diagnostically indicative of the presence of a hydrogen bond. In the present study no attempt was made to evaluate the energy contribution to packing cohesion of the C-H---0 interactions. The geometric features of the intermolecular hydrogen-bonding networks were investigated by using the graphic program SCHAKAL9229band the suite of programs PLATON.29c Atomic coordinates and crystal data were obtained from the Cambridge Crystallographic Database.30 The available coordinates for the hydrogen atoms were normalized by extending the C-H distances along the C-H vectors to the typical neutron-derived value of 1.08 A.31
Acknowledgment. D.B., F.G., M.J.C., and L.F.V. acknowledge CNR (Italy) and JNICT (Portugal) for joint financial support. OM940815E (30)Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, 0.; Kennard, 0.;Macrae, C. F.; Watson, D. G. J. Chem. In6 Comput. Sci. 1991,31, 204. (31)Murray-Rust, P.; Glusker, J. P. J.Am. Chem. SOC.1984,106, 1018.
