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Journal of Chemical Crystallography, Vol. 32, No. 11, November 2002 (C© 2002)

The crystal and molecular structure of one of the possibleconformers of “the smallest piece of ice”—A solid-stateisolation of a hydrogen bonded water hexamer trappedwithin channels of a crystal of a dirhodium molecule

Ivan Bernal(1)∗ and John L. Bear(1)

Received April 24, 2001

Reexamination of our study of [Rh2(HNOCCH3)4(2H2O)] · 3H2O (Ahsan, M.Q.; Bernal,I.; Bear, J.L.Inorg. Chem.1986, 26, 260) showed it to be interesting not just because thedirhodium molecule is an antineoplastic but because it contains a hexameric cluster ofwaters trapped in RhRh lattice cavities. It may well provide an interesting model for the“smallest piece of ice” (Nauta, K.; Miller, R.E.Science2000, 287,293).

KEY WORDS: dirhodium molecule; antineoplastic; hexameric cluster.

Sometime ago, the reaction of dirhodiumtetraacetate with molten acetamide was ex-plored, and the six compounds produced (threewere geometrical isomers with composition[Rh2(O2CCH3)2(HNOCCH3)2) were isolated andcharacterized.1 At that time, the emphaseis ofthese studies were on (a) the development of effi-cient synthetic routes for the desired compounds,(b) the isolation and physical characterization ofthese species, (c) their antineoplastic properties,and (d) their crystal and molecular structures inorder to determine their conformations and con-figurations and, hopefully, correlate them withtheir physiological properties and spectral char-acteristics. This approach followed the classicalapproach of drug syntheses aimed at obtainingmaximum pharmacological potency with mini-mum side effects. For a detailed description of

(1) Chemistry Department, University of Houston, Houston, Texas77204-5641.

∗ To whom correspondence should be addressed. E-mail:ibernal@uh.edu.

the earliest studies of the chemotherapeutic prop-erties of dirhodium compounds carried out at ourdepartment, see Ref. 2.

Recently, at a symposium sponsored by theAmerican Chemical Society’s Physical ChemistryDivision,3 on the novel properties of moleculesstudied a very low temperatures, the work of K.Nauta and R. E. Miller on the nature of water clus-ters created inside a helium nanodroplet was dis-cussed. Under the varying conditions selected bythese scientists, water clusters of different sizeswere recorded. One of them was a cluster of sixwaters which was described as a hexagonal sub-unit of common ice. It was dubbed “the smallestpiece of ice.”3

In the paper given as Ref. 1, we did not ex-amined thoroughly the “waters of crystallization”present in [Rh2(HNOCCH3)4(2H2O)]·3H2O,1 ex-cept for a brief mention in the last paragraph ofthat report. The stereochemical details and graph-ics were relegated to the Supplementary material.1

As a result of an inquiry concerning the fractional

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Fig. 1. The water hexameric cluster with the numberingsystem employed in the original report, (see Ref. 1). It istrapped in cavities created by dirhodium molecules, as shownin Fig. 4.

coordinates of one of the oxygen atoms of a waterof crystallization, we examined our original datafiles and discovered that they coordinate of O6(see coordinates in Ref. 1; in the CSD File theREFCODE for this molecule is DOKJAD) shouldbe−0.3625(2), not−0.2625(2) as listed in theoriginal paper.1 Consequently, the waters of crys-tallization (O4 and O5 at a mirror plane; O6 andO7 in general positions, plus their mirror images,O6′ and O7′) form hexameric clusters that havethe shape described in Fig. 1, as well as in thecartoon for the Index.

Those waters at mirror planes form two hy-drogen bonds, of which only one is indepen-dent. O5 hydrogen bonds to another two watermolecules of the cluster (to O7 and O7′) whileone hydrogen on each of O6 and O6′ form hy-drogen bonds to another water of crystallization,O4. O5 forms two hydrogen bonds to two coordi-nated waters on adjacent RhRh dimers, O5 be-ing the electron donor. Finally, O4 and O5 arethe only water oxygens surrounded by four hy-drogens in a tetrahedral (idealized) arrangement,as found in Ice II. The difference being that, inboth, the cubic and hexagonal forms, Ice II con-tains six-membered rings in whichall oxygensare surrounded by four hydrogens (H4O clus-ters) having tetrahedral shapes. The clusters in

Fig. 2.

Ice II resemble cyclohexane rings, whereas theoxygens of our cluster are nearly planar (see be-low, Fig. 2) and its deviation from planarity canbe described as that of a distorted twist-boat.For details of the shape, bond lengths, angles,and torsional angles, see Figs. 1–3. The devia-tions of the oxygens from the best least-squaresplane are

O4= 0,O5= 0,O6= 0.158,O6′ = −0.158,

O7= −0.104, and O7′ = 0.104.

Note that this isolated ring, in theory, couldhave grown into an infinite string of hexamersstitched together like the beads of a necklace;however, the cavity created by the packing of

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Model for the “smallest piece of ice” 487

Fig. 3. Three views of the conformation of the water hexamer.

the four Rh Rh dimers (shown in Fig. 4) pro-vides hydrogen bonds which limit the size ofthe water cluster to a single ring. If the ringhad been oriented along thec-axis, rather thanalong theb-axis, the cluster could have becomean infinite string. Obviously, the best hydrogen-bonded interactions are those shown in Fig. 4,which is a packing diagram viewed along thea-direction.

The conformation of the six oxygens of thewater hexamer is not the chair or boat of cyclo-hexane. Instead, it is a distorted ring closer in con-formation to a twist boat, but certainly not that ofa classical one. Its geometry can be understood ifwe use the following model: take a chair cyclo-hexane and exert a pull in opposite directions ofcarbons 1 and 4. This causes a partial flatteningof the ring; but, it also causes a torsional motionof the single bonds linking the carbons. This is

Fig. 4. The packing of the molecules in the unit cell. Note thehydrogen bonds formed by waters O4 and O5 with hydrogensof the dirhodium cluster.

what happens to the hexameric water ring as a re-sult of the hydrogen bonds formed by O4 and O5(see Fig. 4, packing). In an effort to maximize thestrength of these bonds, the hexamer is stretched.Note that one of the hydrogens of O6, O6′, O7,and O7′ are used in forming the cluster. The otherone is not significantly bonded to additional donoratoms which could balance the forces pulling onO4 and O5; consequently, the hexamer acquiresthe conformation shown in Figs. 1–4. The mid-dle view on Fig. 3 is specially revealing in thatrespect.

The hexameric cluster described by Nautaand Miller4 was observed in an isotropic, non-bonding, fluid medium at a very low tempera-ture. Consequently that cluster is expected to beat its lowest energy conformation, which shouldbe a chair, both by virtue of its environmentas well as the temperature at which it was ob-served. Therefore, although different in environ-ment and conformation, the isolation of our hex-americ cluster lends strength to the notion ofisolated, hexameric water clusters but it is nota good model for that of Nauta and Miller.3 Itis, however, a higher energy form of theirs byvirtue of both, the directional forces causing its

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distortion and the temperature at which it was ob-served, 21◦C.

A calculation of the energy of our hexamer,forced to keep its conformation, would be veryinteresting, specially when contrasted with the re-sults of a calcuation in which energy is minimizedand geometry is allowed to acquire whatever shapeis inherent to the energy-minimized molecule.

Acknowledgments

We thank the Robert A. Welch Foundation forsupport of this study and Hyungphil Chun for help withthe graphics.

References

1. Ahsan, M.Q.; Bernal, I.; Bear, J.L.Inorg. Chem.1986, 26,260.2. Hughes, R.G.; Bear, J.L.; Kimball, A.P.Proc. Amer. Assoc. Can-

cer Res.1972, 13, 120; Erck, A.; Rainen, L. ; Whileyman, J.;Chang, I.M.; Kimball, A.P.; Bear, J.L.Proc. Soc. Exp. Biol. Med.1974,145,1278; Bear, J.L.; Gray, H.B.; Rainen, L.; Chang, I.M.;Howard, R.; Serio, G.; Kimball, A.P.Cancer Chemother Rep.1975, 59,611; Kitchens, J.; Bear, J.L.J. Inorg. Nuclear Chem.1969, 31,2415; Kitchens, J.; Bear, J.L.J. Inorg. Nuclear Chem.1971,33,3479; Kitchens, J.; Bear, J.L.Thermochim. Acta,1970,1, 537; Kitchens, J.; Bear, J.L.J. Inorg. Nuclear Chem.1970,32,49; Rainen, L.; Howard, R.A.; Kimball, A.P.; Bear, J.L.In-org. Chem.1975, 14, 2752; Das, K.; Bear, J.L.Inorg. Chem.1976, 15,2093; Das, K.; Kadish, K.M.; Bear, J.L.Inorg. Chem.1978, 17,930; Das, K., Simmons, E.L.; Bear, J.L.Inorg. Chem.1977, 16, 1268; Erck, A.; Sherwood, E.; Bear, J.L.; Kimbal1,A.P.Cancer Res.1976, 36,2204.

3. Chem. Eng. News2000, 78,50.4. Nauta, K.; Miller, R.E.Science2000, 287,293.