# Variations on the Honeycomb Topology: From Triangular- and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes

Pierre Thuéry, Jack Harrowfield

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Pierre Thuéry, Jack Harrowfield. Variations on the Honeycomb Topology: From Triangular- and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes. Crystal Growth & Design, 2017, 17, pp.963-966. 10.1021/acs.cgd.7b00126. cea-01475689

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# Variations on the Honeycomb Topology: from Triangular- and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes

Pierre Thuéry*,<sup>†</sup> and Jack Harrowfield‡

<sup>†</sup>NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France

<sup>‡</sup>ISIS, Université de Strasbourg, 8 allée Gaspard Monge, 67083 Strasbourg, France

## Supporting Information Placeholder

**ABSTRACT:** Depending on the counterion, uranyl tricarballylate, UO₂(tca)⁻, is shown to crystallize either as two-dimensional nets or as one-dimensional tubules, all with honeycomb topology and united into higher dimensional assemblies when additional metal cations (Ag¹, Pb¹¹) are present. A regular geometric progression involving ligands reorientation is apparent in the series, with triangular furrows in [H₂NMe₂][UO₂(tca)]·H₂O (1) followed by deepening square grooves in [UO₂Ag(tca)(H₂O)]·0.5H₂O (2) and full closure of square tubules in [NH₄][(UO₂)₂Pb(tca)₂(NO₃)(bipy)].

Since the first reports of uranyl-based nanotubular structures in phosphonate complexes by Clearfield et al. more than 20 years ago,¹⁻⁵ the number of such species has steadily increased and, although they are still uncommon, as expected for this linear cation, they appear to be accessible from a variety of ligand classes. While most examples are still found among phosphonates,¹⁻⁸ remarkable inorganic tubular arrangements have been found in uranyl selenates,⁹⁻¹¹ and polycarboxylates, prevalent in the design of uranyl-organic coordination polymers,¹²⁻¹⁴ have also provided several cases in recent years.¹⁵⁻¹⁹ Among the latter, phthalate (1,2-benzenedicarboxylate) gives tubular species through connection of hexanuclear rings¹⁶ (an analogous complex with neptunyl has also been reported²⁰); Kemp's tricarboxylate (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylate) yields uranyl-nickel(II) heterometallic tubules or uranyl octanuclear cages depending on whether Ni¹¹-bound 2,2'-bipyridine molecules are present or not,¹⁷ and the related cis,cis-1,3,5-cyclohexanetricarboxylate gives either two-dimensional (2D) honeycomb networks or a tubular assembly retaining the same topology, depending on the choice of counterions.¹⁸ Apart from coordination bonds, supramolecular interactions may also be summoned to create such species, and tubular channels are formed through hydrogen bonding of corrugated hexanuclear uranyl-iminodiacetate rings.¹⁹ Due to its curved shape, tricarballylic

acid (1,2,3-propanetricarboxylic acid, H₃tca) is of potential interest to generate uranyl-containing polymers with curved architectures, but the only complex reported so far, [UO₂Na(tca)(H₂O)₄], crystallizes as a 2D network with honeycomb topology.²¹ By varying the experimental conditions, we have now obtained three novel uranyl tricarballylate complexes, two of which are heterometallic and include Ag¹ or Pb¹¹ cations, that were synthesized solvohydrothermally and have been characterized by their crystal structure.²² One of these complexes provides a new example of a tubular arrangement, deriving from a regular progression of geometric features in the series.

The asymmetric unit in [H₂NMe₂][UO₂(tca)]·H₂O (1) contains a unique uranyl ion located on a mirror plane and chelated by three carboxylate groups from three tca³⁻ ligands (Fig. 1). The U–O(oxo) bond lengths of 1.757(8) and 1.758(7) Å, and the U–O(carboxylate) bond lengths of 2.462(5)–2.468(5) Å are unexceptional. Both metal and ligand are threefold nodes and they generate a 2D network parallel to (1 0 0), with the point (Schläfli) symbol {6³} (honeycomb topology). This assembly is similar to that in the previously reported complex [UO₂Na(tca)(H₂O)₄] (4),²¹ both displaying triangular grooves. In both cases, grooves from adjacent layers are facing one another so as to form channels that are occupied by carboxylate- and oxo-bound Na(H₂O)₄⁺ cations in complex 4, and by disordered, hydrogen bonded H₂NMe₂⁺ cations in 1, the latter cations being formed in situ from DMF hydrolysis; the Kitaigorodski packing index (KPI) for 1 is ~0.69. This peculiar geometry of the sheets results from the 1,3-carboxylate groups being in a common plane approximately perpendicular to that of the 2-carboxylate group [dihedral angle θca = 86.8(8)°], the equatorial planes of the uranyl ions bound to these groups retaining approximately the same geometrical relationship.

The asymmetric unit in [UO₂Ag(tca)(H₂O)]·0.5H₂O (2) contains two uranyl and two silver(I) cations, and two tca³⁻ ligands (Figs. 2 and S1, Supporting Information). Both uranyl cations are in similar environments with three chelating carboxylate groups from three ligands [U–O bond
<figure><img src="image_2.png" /></figure>

**Figure 1.** Top: View of compound 1. Displacement ellipsoids are drawn at the 30% probability level. The solvent molecule and carbon-bound hydrogen atoms are omitted, and only one position of the disordered counterion is shown. The hydrogen bond is shown as a dashed line. Symmetry codes: i = x, 3/2 - y, z; j = 1/2 - x, y + 1/2, z - 1/2; k = 1/2 - x, 1 - y, z - 1/2; l = x, 1/2 - y, z; m = x, y - 1, z; n = 1/2 - x, 1 - y, z + 1/2. Middle: View of the 2D assembly. Bottom: View of the packing. Uranium coordination polyhedra are colored yellow and solvent molecules and hydrogen atoms are omitted in the last two views.

lengths of 2.418(8)–2.499(8) Å, average 2.46(2) Å. In contrast, the two silver(I) cations are in different environments. Ag1 is chelated by atoms O14 and O15 from two carboxylate groups of the same ligand [bond lengths of 2.516(10) and 2.455(9) Å, respectively] and bound to a water molecule (O17) at 2.228(10) Å; three longer contacts with two carboxylate oxygen atoms from two more ligands [2.733(11) and 2.895(9) Å] and with the oxo atom O1¹ [2.739(9) Å] make for a very irregular environment, with the oxo atom in axial position with respect to the average plane defined by the five other donors. Atom Ag2 is bound to the two carboxylate oxygen atoms O7 and O11¹, at 2.518(8) and 2.327(8) Å, respectively, and to a bridging water molecule (O18) and its image by inversion [2.414(10) and 2.493(9) Å]; it is also involved in two longer contacts with the carboxylate atom O10 at 2.700(9) Å, and the oxo atom O1m at 2.779(10) Å. The environment of Ag2 has also an irregular geometry, but it can be viewed as a very distorted octahedron. The oxo atom O1 is thus involved in a bifurcated bond with the two silver cations, which is however weak since no significant lengthening of the U1–O1 bond is measured [1.792(8) Å, versus 1.780(8) Å for U1–O2; the bond lengths for U2 are 1.766(8) and 1.770(8) Å]. The dihedral angles $\theta_{ca}$ are 85.7(12) and 78.5(11)° for the two independent anions, the latter significantly reduced with respect to the value in 1. Each tca³⁻ ligand is bound to three

<figure><img src="image_3.png" /></figure>

**Figure 2.** Top: View of compound 2. Displacement ellipsoids are drawn at the 50% probability level. The solvent molecule and carbon-bound hydrogen atoms are omitted. Symmetry codes: i = x - 1, y, z; j = 1 - x, y + 1/2, 1/2 - z; k = -x, y + 1/2, 1/2 - z; l = -x, y - 1/2, 1/2 - z; m = x + 1, y, z; n = 2 - x, 1 - y, 1 - z; o = 1 - x, y - 1/2, 1/2 - z. Bottom: View of the 3D framework with the uranium coordination polyhedra colored yellow and silver atoms shown as blue spheres; the 2D honeycomb subunits are viewed edge-on; solvent molecules and hydrogen atoms are omitted.

uranyl ions and to either two or three Ag¹ ions, the coordination modes of the carboxylate groups being κ²O,O', µ₂-κ¹O:κ²O,O' or µ₃-κ¹O:κ²O,O':κ¹O'. If only the uranyl cations and tca³⁻ ligands are considered, a 2D network parallel to (0 0 1) is generated, with the same honeycomb topology as that in **1** and **4**; however, instead of displaying a triangular furrowing, this assembly is deeply corrugated and the grooves are square-shaped, a difference apparent in the nodal representations shown in Fig. 3. The difference between the geometries observed in **1/4** and **3** arises from the different number of ligand units defining each groove, two in the former case and three in the latter. Ag1 and its symmetry equivalents are located inside these layers and do not lead to a dimensionality increase, while Ag2 and its equivalents are located at the periphery of the sheets and, through formation of doubly water-bridged silver(I) dimers, unite the 2D subunits into a 3D framework. Roughly elliptical channels run along the *a* axis, due to the grooves facing each other as in **1**, that do not exceed ~3 Å in their narrowest parts and are thus too small for any practical use. The framework is compact, as shown by its KPI of 0.77. The coordinated and free water molecules form an intricate network of rather weak simple, bifurcate or even trifurcate hydrogen bonds, the acceptors being uranyl oxo, carboxylate and water oxygen atoms [O···O 2.716(15)–3.298(14) Å, H···O 2.14–2.52 Å, O–H···O 108–164°].

Replacement of Ag¹ by the divalent Pb¹¹ results in a different stoichiometry in the complex [NH₄][(UO₂)₂Pb(tca)₂(NO₃)(bipy)] (3) (bipy = 2,2'-bipyridine), in which the ammonium cations appear to have been generated in situ from acetonitrile hydrolysis possibly catalyzed by UVI, a similar phenomenon having previously
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Nodal representation of the triangular-grooved network in 1/4 (a), the square-grooved network in 2 (b), and the tubular chains in 3 (c), and schematic representation of the transformation. Yellow: uranium, red: oxygen, blue: tricarballylate ligand.</figcaption></figure>

been encountered during the synthesis of a uranyl pimelate complex.²³ The two crystallographically independent uranyl ions are both chelated by three carboxylate groups, with unexceptional U–O(oxo) [1.758(4)–1.779(5) Å, average 1.771(8) Å] and U–O(carboxylate) [2.422(4)–2.498(5) Å, average 2.46(2) Å] bond lengths (Figs. 4 and S2, Supporting Information). The unique lead(II) cation is chelated by the bipy molecule [Pb–N bond lengths 2.422(6) and 2.452(5) Å] and is bound to the carboxylate atom O6 at 2.722(4) Å and three oxygen atoms pertaining to two nitrate anions, two of them (O17 and O18) being chelating [2.515(5) and 2.723(6) Å] and the third (O19') monodentate [2.847(6) Å]. The nitrate ion is thus bound in a µ₂-κ¹O:κ²O',O'' coordination mode. A longer contact exists between Pb1 and O14k, at 3.193(4) Å. Depending on whether this last contact is considered as a true bond or not, the coordination number is six or seven, but the environment is in both cases more hemi-than holodirected.²⁴ The tca³⁻ ligand is here also in its right-angled shape, with dihedral angles θca of 87.8(5) and 88.2(7)°, but, instead of generating a 2D lattice, the UO₂(tca)⁻ assembly is folded so as to create a tubular 1D polymer directed along the *a* axis, while retaining the {6³} topology. The cross-section of the tubules is square, with an edge length of ~6 Å, the uranium atoms on the four edges being offset along the *a* axis, so that sufficient inner space is available for inclusion of the NH₄⁺ counterions. The latter are hydrogen bonded to two uranyl oxo and two carboxylate groups from opposite sides of the cavity [N···O 2.971(8)–3.031(9) Å, H···O 2.16–2.35 Å, N–H···O 134–175°], so that it may be surmised that they exert a structure-directing role. The tubules are further assembled into a 2D lattice parallel to (0 1 0) by centrosymmetric nitrate-bridged PbII dimers (KPI 0.74), an arrangement reminiscent of the bridging of uranyl phosphonate tubules by CuII cations.⁸

A regular geometric progression is thus apparent in this
series, from triangular and square grooves defined by two
and three ligands, respectively, to tubular shapes lined by
four ligands. From a purely geometric point of view, this

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Top: View of compound 3. Displacement ellipsoids are drawn at the 40% probability level. Carbon-bound hydrogen atoms are omitted and hydrogen bonds are shown as dashed lines. Symmetry codes: *i* = *x* + 1, *y*, *z*; *j* = 1 − *x*, 1 − *y*, 1 − *z*; *k* = 2 − *x*, 1 − *y*, 1 − *z*; *l* = 2 − *x*, 1 − *y*, 2 − *z*; *m* = *x* − 1, *y*, *z*. Middle: View of one tubular subunit. Bottom: Two views of the packing parallel and perpendicular to the tubules axis, with the uranium coordination polyhedra colored yellow, lead atoms shown as green spheres, and hydrogen atoms omitted, except for those of the NH₄⁺ counterions.</figcaption></figure>

transformation involves reorientation of otherwise
conformationally almost invariant ligands so that the
direction of connection changes by 180°, the topology being
unmodified (Fig. 3). A similar conversion of an undulated
honeycomb 2D network into a tubular assembly has
previously been encountered in uranyl ion complexes with
cis,cis-1,3,5-cyclohexanetricarboxylate,¹⁸ but no
intermediate step was found in this case. Of course, the
mechanism of formation of the tubules in **3** is unknown and,
although formation of nanotubes through folding of the
corresponding sheets is a known mechanism,⁹,¹⁰ it most
probably does not involve 2D intermediates analogous to
those in **1/4** and **2** since ligand reorientation would require
the breaking of bonds. The nature of the counterions
obviously plays a key-role in the choice of a particular
outcome. In particular, the ammonium counterions may be
essential in organizing the ligands, during polymer growth,
in the best position for hydrogen bonding, thus favoring the
formation of a closed assembly encircling the $NH_4^+$
divergent, fourfold hydrogen bond donor. The aptitude of
honeycomb nets to accommodate folding is particularly
evident in carbon nanotubes.²⁵ With respect to those, the size
of the tubular species in **3** is limited by the fact that only four
metal/ligand units are sufficient to ensure closure, which is
related to one uranium node being markedly displaced from
the mean plane of the five other nodes of the hexagonal
honeycomb ring, thus giving the latter a pronounced angular
shape well suited to define a square cross-section. Larger
tubules are obtained when six uranyl/ligand units subtend
the tube perimeter, a situation common with other
polycarboxylates¹⁵⁻¹⁹ as well as phosphonates,¹ although
narrower tubes with four-membered cross-sections were
also found in the latter case.²,⁴

The emission spectra of complexes **1** and **3** in the solid state were measured at room temperature under excitation at 420 nm (Figure S3, Supporting Information; no sufficient amount of pure **2** could be obtained). The usual vibronic fine structure<sup>26–28</sup> is observed in both cases, with the maxima positions being redshifted by 3 nm in complex **3** with respect to those in **1**. These positions (463, 480, 500, 522 and 545 nm in **1**) are in good agreement with those for other complexes with uranyl ions chelated by three carboxylate groups.<sup>18,23,29</sup>

In summary, it appears that, depending on the choice of
counterion, uranyl tricarballylate crystallizes as 2D
networks with variable degrees and shapes of corrugation, or
as a tubular species in which closure is fully achieved. All
these forms having a common honeycomb topology, a
frequently encountered geometry in uranyl coordination
polymers, this points to the possibility of generating other
tubular structures, with a diameter depending on the ligand
curvature and geometrical flexibility.

ASSOCIATED CONTENT

**Accession Codes**

CCDC 1529441–1529443 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

**Supporting Information**

The Supporting Information is available free of charge on
the ACS Publications website at DOI:.. Experimental details,
additional figures, emission and excitation spectra of
compounds **1** and **3**. (PDF)

**AUTHOR INFORMATION**

**Corresponding Authors**

*E-mail: pierre.thuery@cea.fr (P. T.)

**Notes**

The authors declare no competing financial interest.

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# Variations on the Honeycomb Topology: from Triangular- and Square-Grooved Networks to Tubular Assemblies in Uranyl Tricarballylate Complexes

Pierre Thuéry and Jack Harrowfield

<figure><img src="image_6.png" /></figure>

The right-angled curvature of the tricarballylate ligand gives a corrugated shape to the two-dimensional honeycomb assemblies it forms with the uranyl cation. The shape of the furrows can be monitored through the choice of counterions, resulting in triangular or square geometries, and finally closure of the grooves to form tubules with square cross-section.