Gas-Phase Assembling of Dirhodium Units into a Novel

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CRYSTAL GROWTH & DESIGN

Gas-Phase Assembling of Dirhodium Units into a Novel Organometallic Ladder: Structural and DFT Study

2006 VOL. 6, NO. 6 1479-1484

Alexander S. Filatov, Andrey Yu. Rogachev, and Marina A. Petrukhina* Department of Chemistry, UniVersity at Albany, State UniVersity of New York, Albany, New York 12222 ReceiVed March 3, 2006; ReVised Manuscript ReceiVed April 3, 2006

ABSTRACT: A hydrocarbon that has three separate aromatic rings, namely 1,4-bis(p-tolylethyl)benzene (C24H26), has been structurally characterized for the first time and tested as an elongated aromatic π-donor in assembling reactions under solvent-free conditions. Its gas-phase interaction with the electrophilic dimetal complex, [Rh2(O2CCF3)4], affords single crystals of a novel organometallic complex, [Rh2(O2CCF3)4]2‚(C24H26). The product has a ladder type structure built on weak but directional rhodium(II)-π-arene interactions. In the above framework, each C24H26 exhibits a tetradentate-bridging coordination having four rhodium(II) centers attached to the opposite sides of the two peripheral benzene rings. The underlying reasons for such coordination defining the formation of the title product have been discussed based on the results of density functional theory, namely the orbital interaction analysis of the reacting partners. Introduction Crystal engineering of organometallic supramolecular architectures is currently less developed compared to the construction of inorganic hybrid materials built on various metal-ligand coordination bonds.1 Generally, the expectations for organometallic assembling are the same; the geometry of building blocks should predetermine the structural organization of a supramolecular system, while the properties of individual units should define the properties of a hybrid product. However, effective utilization of intermolecular metal-π-arene interactions for self-assembling in solutions may be thwarted by lability of complexes, by solvent and ion-template effects, or by solvent competition for coordination.2 In this context, the use of new crystal engineering tools that exclude solvents3 is very important in order to achieve the controlled assembling geometry, the adequate packing of the molecules, and, finally, the desired properties of hybrid materials. Therefore, in contrast to solution self-assembling methods, we develop a microscale gas-phase technique based on codeposition of volatile complementary building units.4 We have recently demonstrated that deposition is very effective in utilizing directional metal-π-arene interactions to form multidimensional organometallic networks based on open geodesic polyarenes.5 Herein we expand this study to the gas-phase reactions of an elongated hydrocarbon having three separate aromatic rings, C24H26 (1). On one hand, various organic π-systems are interesting and useful spacers in assembling reactions since they can impose a specific network topology, for example, of ladder types6 or multidimensional nets with hollows.7 The degree of structural control may be achieved through several parameters such as the dimensions of the ligand chain, the controlled positioning of binding sites along the chain, the change of possible coordination mode, as well as the introduction of functional capping groups at the ends. On the other hand, the rich redox chemistry and interesting optical and magnetic properties associated with multiply bonded dimetal complexes coupled with their high affinity for axial coordination render them ideal building blocks for the construction of new types of molecular architectures.8 Recently, we have used the electrophilic dirhod* To whom correspondence should be addressed. E-mail: marina@ albany.edu.

ium units, [Rh2(O2CCF3)4], to test coordination properties of the cyclic analogue of 1, namely [2.2.2]paracyclophane (C24H24). The gas-phase codeposition of the above building blocks resulted in the organometallic network having infinite open channels with aromatically functionalized “walls” comprised of the benzene rings of [2.2.2]paracyclophane.9 In this work we use the advantages of the gas-phase coordination and crystallization method to explore the assembling reactions in the [Rh2(O2CCF3)4]-C24H26 system. Experimental Section General Procedures. All manipulations were carried out in a dry, oxygen-free, dinitrogen (HP 99.998) atmosphere by employing standard Schlenk techniques. Elemental analysis was performed by Chemisar Laboratories Inc., Ontario, Canada. IR spectra were recorded on a Nicolet Magna 550 FTIR spectrometer using KBr pellets. The 1H NMR spectra were recorded at 300 MHz, and chemical shifts are reported relative to internal TMS δ ) 0.0 ppm. The 19F NMR spectra were recorded at 282 MHz, and chemical shifts are reported relative to internal CFCl3 δ ) 0.0 ppm. Mass spectra were obtained on a HewlettPackard HP 5989/5970 GC-MS instrument at 70 eV. Sublimationdeposition procedures were performed in small glass ampules (ca. 7 cm long with an o.d. of 1.1 cm), which were placed in electric furnaces having a small temperature gradient along the length of the tube. [Rh2(O2CCF3)4] was prepared according to the literature procedure.10 Synthesis of 1,4-Bis(p-tolylethyl)benzene, C24H26 (1). A suspension of 1,4-bis(p-tolylethynyl)benzene (0.100 g, 0.33 mmol) and Pd/C (0.010 g) in DMF (20 mL) was agitated with hydrogen at 70 °C and 1 atm for 5 h and then for an additional 12 h at ambient temperature. After all this, the mixture was filtered through a pad of silica gel (2-3 cm) and the resulting colorless solution was concentrated under reduced pressure to ca. 5 mL. Then, 10 mL of EtOH (95%) was added and the solution was allowed to stay at -10 °C for 7 h to yield a white precipitate in essentially quantitative yield. This precipitate was filtered and dried overnight under vacuum at ambient temperature. It was then sublimed in a small glass ampule at 105 °C to give colorless plateshaped crystals of 1. MP: 141-142 °C. IR (KBr, cm-1): 3050w, 3022w, 3003w, 2944w, 2918m, 2856w, 1514s, 1452m, 1142w, 1092w, 1024w, 818s, 770w, 710w, 546m, 526s, 470w. 1H NMR (CDCl3, ppm): 7.09 (d, J ) 6.3 Hz, 12H, Ph), 2.85 (s, 8H, -CH2-), 2.30 (s, 6H, Me). EI-MS, m/z (%): 314 (32) [M+], 209 (47), 105 (100). Synthesis of [Rh2(O2CCF3)4]2 (C24H26) (2). A mixture of Rh2(O2CCF3)4 (0.030 g, 0.046 mmol) and 1 (0.007 g, 0.022 mmol) was sealed under vacuum in a glass ampule and placed in an electric tube furnace at 155 °C. Green plates of 2 were obtained in the middle part of the ampule after 12 h. Yield: 45-50%. Anal. Calcd for C40H26F24O16Rh4:

10.1021/cg060118c CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006

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Table 1. Crystallographic Data and Structural Refinement Parameters for C24H26 (1) and [Rh2(O2CCF3)4]2‚(C24H26) (2) empirical formula fw cryst system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Å) Dcalc (g‚cm-3) µ (mm-1) no. of reflns collected no. of independent reflns data/restr/parameters largest diff peak R1,a wR2b [I > 2σ(I)] R1,a wR2b (all data) quality-of-fitcon F2 c

1

2

C24H26 314.45 monoclinic P21/c 16.5263(16) 6.0730(6) 9.0693(9) 90 100.973(2) 90 893.59(15) 2 173(2) 0.71073 1.169 0.065 7395 2079 2079/0/150 0.293 0.0527, 0.1691 0.0658, 0.1795 1.050

C40H26F24O16Rh4 815.21 triclinic P1h 8.3981(4) 8.6539(5) 17.9261(9) 98.8370(10) 97.2080(10) 103.3120(10) 1235.10(11) 1 173(2) 0.71073 2.192 1.473 10874 5548 5548/36/437 0.978 0.0321, 0.0770 0.0401, 0.0811 1.042

a R1 ) ∑||F | - |F ||/∑|F |. b wR2 ) [∑[w(F 2 - F 2)2]/∑[w(F 2)2]]1/2. o c o o c o Quality-of-fit ) [∑[w(Fo2 - Fc2)2]/(Nobs - Nparams)]1/2.

C, 29.57; H, 1.71. Found: C, 29.69; H, 1.80. IR (KBr, cm-1): 2965w, 2925w, 2863w, 1662s, 1561m, 1508m, 1455w, 1193s, 865m, 824w, 789w, 739w, 669w, 549w, 532w, 476w. 1H NMR (CDCl3, ppm): 7.08 (d, J ) 5.7 Hz, 12H, Ph), 2.85 (s, 8H, -CH2-), 2.30 (s, 6H, Me). 19F NMR (CDCl3, ppm): -75.04. X-ray Crystallographic Procedures. X-ray data sets for 1 and 2 were collected on a Bruker APEX CCD X-ray diffractometer equipped with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The frames were integrated with the Bruker SAINT software package,11a and the data were corrected for absorption effects using SADABS.11b The structures were solved by direct methods and refined against F2 by the full-matrix least-squares technique as implemented in the Bruker SHELXTL software (Version 6.1).11c All non-hydrogen atoms were refined anisotropically, except the disordered fluorine atoms of the two CF3 groups in 2, for which disorder was modeled over three rotational orientations. The hydrogen atoms of the hydrocarbons in 1 and 2 were found in the difference Fourier map and refined independently except for those of methyl groups. The latter were placed in geometric positions using a riding model but were allowed to rotate freely. Crystallographic data and X-ray experimental conditions for 1 and 2 are listed in Table 1. Selected distances and angles are given in Tables 2-4. Computational Details. Full geometry optimizations of molecular structures were performed at the DFT level of theory using the hybrid exchange-correlation functional Becke-Lee-Yang-Parr (B3LYP) as it is implemented in the Gaussian 03 (Revision C.02) program package.12a A Hay and Wadt effective core potential (ECP2)12b and the LANL2DZ basis set were placed on Rh atoms, while the 3-21G(d) basis sets were used for all other atoms. The C2h and C4 point group symmetry constraints were imposed for the geometry optimizations of C24H26 and [Rh2(O2CCF3)4], respectively. Real harmonic frequencies indicated that the optimized structures corresponded to the true minima on the potential energy surfaces. When optimizations were achieved, single-point calculations along with the NPA/NBO population analyses12c were performed using the same basis set for Rh atoms and the 6-311G(d,p) basis sets for all other atoms. The bond orders quoted in the text are those from the Wiberg formulation12d incorporated in the NPA population analysis. Molecular orbital maps (isosurface 0.035 au) are drawn with the help of the ChemCraft program.12e

Results and Discussion The hydrocarbon spacer, C24H26 (1), has been synthesized by hydrogenation of 1,4-bis(p-tolylethynyl)benzene, C24H18, which was prepared according to the literature procedure (Scheme 1).13

Scheme 1

Crystals of 1 suitable for the single-crystal diffraction study have been obtained by deposition from the gas phase at 105 °C. Compound 1 crystallizes in the monoclinic space group P21/c with the centroid of the middle benzene ring residing on an inversion center (Figure 1). The C-C bond lengths (Table 2) within the aromatic rings (ca. 1.39 Å) are all very similar and consistent with values typical for the para-substituted arenes. The C24H26 molecules display a planar conformation of the three benzene rings and aggregate into a two-dimensional network through a number of hydrogen bonds. Each molecule in this network acts as a donor of two C-H hydrogen bonds from the two methyl groups (CH3) and four C-H hydrogen bonds from the four methylene groups (CH2). It is also acts as a π-arene acceptor of six hydrogen bonds, thus adopting twelve hydrogen bonds overall (Figure 2, Table 3). It is worth mentioning that the packing motif for 1 is somewhat similar to that of the dehydrogenated hydrocarbon, C24H18. However, the latter forms a pseudo-two-dimensional network through the CH3‚‚‚π(arene) hydrogen bonds only.13b Hydrogenation of C24H18 to C24H26 provided more H atoms along the backbone of the hydrocarbon to be included into the hydrogen bonded network in the structure of 1. To maximize the hydrogen bonding, every other layer (along the c direction) in the network of 1 is rotated to provide all benzene rings for acceptance of additional hydrogen bonding interactions. The gas-phase codeposition reaction of complementary units, [Rh2(O2CCF3)4] and C24H26, was performed at 155 °C to afford green plates of the title organometallic product 2 in good yield. Crystals of 2 are stable in dry air but moisture sensitive. The elemental analysis data were consistent with the [Rh2(O2CCF3)4]:(C24H26) ) 2:1 composition. The IR spectrum confirmed the presence of aromatic and carboxylate functions. The significant shift of the aromatic C-H stretches in the IR spectrum of 2 compared to 1 showed relatively strong metal coordination to the benzene rings of C24H26 in the solid state. Contrarily, weak interactions between metal and ligand exist in solution (chloroform), as the coupling constant of the benzene protons slightly changes from 5.7 to 6.3 Hz, while the chemical shifts of all protons in the 1H NMR spectrum remain the same as in the free ligand. The X-ray structural characterization of 2 revealed a very interesting type of structure. The building units of the composition [Rh2(O2CCF3)4]2‚(C24H26) (Figure 3) are further assembled through the open Rh(2)-ends and the peripheral benzene rings (through the C6 and C7 atoms) of hydrocarbons to form a ladder-like type of framework (Figure 4). The ladder structure of 2 can be described as comprised of parallelograms with the edge lengths being approximately 8.4 and 13.3 Å and the angles being 97.2 and 82.8° (Scheme 2). Each C24H26 ligand exhibits a tetradentate-bridging coordination having four rhodium atoms attached to the opposite sides of the two peripheral benzene rings. The ligand bridges the dirhodium units in a way that the benzene ring planes are almost perpendicular to the dimetal axis (79.1°). Both rhodium atoms of the dimetal unit are bound to two carbon atoms or to one C-C bond of the benzene ring of the ligand so that the coordination mode is of the η2 type with respect to each metal center. However, the Rh-C linkages of

Organometallic Ladder: Structural and DFT Study

Crystal Growth & Design, Vol. 6, No. 6, 2006 1481

Figure 1. Molecular structure of 1. ORTEP plot with the 50% displacement ellipsoids showing the numbering scheme. Symmetry operation used to generate equivalent atoms: -x + 1, -y + 1, -z + 2. Table 2. Selected Distances (Å) and Angles (deg) for 1 from the X-ray Crystallographic Analysis and DFT (B3LYP/3-21G(d)) Calculations

C(1)-C(2) C(2)-C(3) C(2)-C(7) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(5)-C(8) C(6)-C(7) C(8)-C(9) C(9)-C(10) C(10)-C(11) C(10)-C(12) C(11)-C(12a) C(1)-C(2)-C(3) C(2)-C(3)-C(4) C(3)-C(2)-C(7) C(3)-C(4)-C(5) C(4)-C(5)-C(8) C(5)-C(8)-C(9) C(8)-C(9)-C(10) C(9)-C(10)-C(11) C(10)-C(11)-C(12a)

X-ray dataa

DFT

1.5086(17) 1.3921(18) 1.3904(19) 1.3897(17) 1.3910(19) 1.3879(18) 1.5129(16) 1.3920(17) 1.5319(19) 1.5127(16) 1.3922(17) 1.3931(19) 1.3916(17) 120.41(12) 120.96(12) 117.80(11) 121.26(12) 120.10(12) 112.71(11) 112.64(10) 120.84(12) 121.06(12)

1.52 1.40 1.40 1.39 1.40 1.40 1.52 1.39 1.56 1.52 1.40 1.40 1.39 120.9 120.9 118.2 120.9 121.1 111.4 111.3 120.9 120.9

a Symmetry operation used to generate equivalent atoms: (a) -x + 1, -y + 1, -z + 2.

Figure 2. Fragment of the crystal structure of 1 showing the formation of a two-dimensional network in the bc plane through the C-H‚‚‚ p(arene) hydrogen bonds.

2.673(3) and 2.720(3) Å for Rh(1) and of 2.653(3) and 2.717(3) Å for Rh(2) are inequivalent (Table 4). The Rh-Rh distance within the dimetal unit of 2.4181(3) Å is 0.04 Å longer than that in the unligated dirhodium(II,II) tetratrifluoroacetate.10 The Rh-Rh-C angles in 2 range from 161.06(7)° to 169.15(8)°. A comparison of the geometrical parameters of C24H26 in 1 and 2 confirms that coordination of dirhodium units does not perturb the structure of the aromatic ligand since all distances lie inside

Table 3. Selected Hydrogen-Bonding Interactions (Å) for 1a D-H‚‚‚A

d(D-H)

d(H‚‚‚A)

d(D‚‚‚A)

∠(DHA)

C(1)-H(1B)‚‚‚Cg1a

0.98 0.994(16) 0.998(15)

2.76 2.771(16) 2.875(14)

3.7157(17) 3.7143(15) 3.8492(14)

164 158.5(13) 165.2(10)

C(8)-H(8A)‚‚‚Cg1b C(9)-H(9B)‚‚‚Cg2c

a Symmetry operations for equivalent atoms: (a) x, -y - 1/ , z - 1/ . 2 2 (b) x, -y + 1/2, z + 1/2. (c) x, -y + 1/2, z - 1/2.

the “six σ limit” and can be considered statistically equivalent.14 The F‚‚‚H intermolecular contacts in 2 are all longer than 2.9 Å. It is noteworthy that although a great number of various infinite coordination ladder-like frameworks with several different structural motifs are known to date,6 the organometallic ladders based on transition metal centers are still very rare.15 Often 1D molecular ladders further assemble into higher organized structural systems via interpenetration to form polycatenated frameworks. This process is driven by packing forces to compactly fill empty spaces in crystal structures. However, if pores are clogged with solvent or guest molecules, ladders usually show no interpenetration.16 In the solid-state structure of 2, the ladder frameworks run parallel to each other along the b axis with each single ladder extending along the a axis (Figure 5). Despite the absence of any guest molecules in the structure, there is no interpenetration between individual infinite ladder frameworks, and the dihedral angle between the neighboring planes is 0°. There are no π-π-stacking interactions found in 2. DFT Analysis. (a) Bond Order Approach. We have previously shown for both planar15d and nonplanar5 polyaromatic π-donor systems that the electrophilic dirhodium(II,II) tetratrifluoroacetate units tend to coordinate to carbon-carbon bonds of an arene having the highest π-bond orders. Herein we estimated charge distribution in the benzene rings of C24H26 and the [Rh2(O2CCF3)4‚(C24H26)] unit and found the greatest negative charges to be located at the carbon atoms that are in the ortho-positions to the points of alkyl substitution (Supporting Information, Tables 7 and 9). Calculations of the bond orders using the DFT/B3LYP method showed that the bond orders between these atoms are essentially the same for the peripheral and for the central rings of C24H26 (Table 5). Lack of conjugation between the benzene rings in C24H26 is the most probable reason for the failure of the bond order approach to explain the observed coordination of the dirhodium units to the peripheral sites of the aromatic ligand in 2. Therefore, we decided to test a different strategy, namely the orbital interaction analysis (see Supporting Information for details), to understand the formation of the title organometallic complex. (b) Orbital Interaction Analysis. It is well accepted that donation of electrons from aromatic systems to metal complexes and back-donation from metal complexes to aromatic systems are both equally important for the formation of organometallic compounds.17 Taking this into account, we based our approach on the orbital analysis of interacting molecular orbitals of both

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Figure 3. Perspective drawing of the building unit in 2. Atoms are represented by thermal ellipsoids at the 50% probability level. H atoms are shown as spheres of arbitrary radii. Disordered F atoms are omitted for clarity. Table 4. Selected Distances (Å) and Angles (deg) for 2a Rh(1)-Rh(2) Rh(1)-O(eq, av) Rh(2)-O(eq, av) Rh(1)-C(3) Rh(1)-C(4) Rh(2b)-C(6) Rh(2b)-C(7) C(1)-C(2) C(2)-C(7) C(2)-C(3) C(3)-C(4) Rh(2)-Rh(1)-C(3) Rh(2)-Rh(1)-C(4) Rh(1b)-Rh(2b)-C(6) Rh(1b)-Rh(2b)-C(7) C(7)-C(2)-C(3) C(4)-C(3)-C(2)

Figure 4. Fragment of the organometallic ladder in 2: Rh, blue; O, red; F, green; C, dark gray; H, light gray.

2.4181(3) 2.029(2) 2.032(2) 2.673(3) 2.720(3) 2.717(3) 2.653(3) 1.495(5) 1.398(5) 1.399(5) 1.384(5) 169.15(8) 161.06(7) 164.54(7) 165.35(7) 117.0(3) 121.3(3)

C(6)-C(7) C(4)-C(5) C(5)-C(6) C(5)-C(8) C(8)-C(9) C(9)-C(10) C(10)-C(11) C(10)-C(12) C(11)-C(12a) C(12)-C(11a) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(6)-C(7) C(2)-C(7)-C(6) C(10)-C(11)-C(12a) C(11)-C(10)-C(12)

1.380(5) 1.394(5) 1.404(5) 1.506(5) 1.519(5) 1.507(5) 1.388(5) 1.390(5) 1.381(5) 1.381(5) 121.7(3) 117.0(3) 121.3(3) 121.7(3) 122.0(3) 116.9(3)

a Symmetry operations used to generate equivalent atoms: (a) -x, -y - 1, -z; (b) x - 1, y, z.

Scheme 2

the dirhodium(II,II) tetratrifluoroacetate unit and the C24H26 hydrocarbon. Calculations of the orbital model of the dirhodium tetratrifluoroacetate complex (using the more advanced hybrid DFT technique than that reported previously10) coupled with symmetry considerations showed that the unoccupied σ* orbital of the dimetal unit is an acceptor of electrons and one of the occupied π* orbitals is a donor of electrons with respect to the ligand. Then, suitable interacting molecular orbitals of the hydrocarbon were found using the back-donation principle as a prerequisite (Scheme 3; Supporting Information, Schemes 2 and 3).

Figure 5. Crystal packing of 2 showing parallel stacking of the individual ladder frameworks.

It was found that coordination of the first dirhodium unit to the peripheral benzene ring of C24H26 highly perturbs the electronic structure of the aromatic ligand. As a result, the following coordination of the second dirhodium unit becomes only possible to the C3a-C4a or C6a-C7a bonds of the other peripheral benzene ring (Figure 6). Taking into account the bond orders for these bonds (Table 5), the C6a-C7a bond seems a more preferable site for the next

Organometallic Ladder: Structural and DFT Study Table 5. Bond Orders in C24H26 (DFT/B3LYP/6-311G(d,p)) and [Rh2(O2CCF3)4‚(C24H26)] (DFT/B3LYP/LANL2DZ/6-311G(d,p))a C(3)-C(4) C(6)-C(7) C(11)-C(12a) C(11a)-C(12) C(3a)-C(4a) C(6a)-C(7a)

C24H26

[Rh2(O2CCF3)4‚(C24H26)]

1.441 1.441 1.440 1.440 1.441 1.441

1.381 1.378 1.446 1.404 1.435 1.446

a The C-C bond order in the benzene molecule is 1.436 (DFT/B3LYP/ 6-311G(d,p)).

Crystal Growth & Design, Vol. 6, No. 6, 2006 1483

plane of the peripheral rings upon coordination of dirhodium units. To probe the rotational freedom of the central ring in C24H26, a potential energy curve was modeled at the ab initio Hartree-Fock level with the 6-31G(d,p) basis sets. An interesting feature of this curve (Figure 7) is the existence of the intermediate with the central ring being perpendicular to the peripheral rings, which is confined by a very small energy barrier (ca. 0.1 kcal/mol). The single-point geometry calculation with the atomic coordinates for the organic ligand being extracted from the X-ray data of 2 showed only small energy loss due to rotation. Conclusions

Figure 6. Donor orbital of the [Rh2(O2CCF3)4‚(C24H26)] complex (DFT/ B3LYP/LANL2DZ/6-311G(d,p)).

We have demonstrated that the deposition approach is an effective tool for supramolecular assembling of organometallic frameworks from the gas phase. The solvent-free conditions provide a unique environment to use intermolecular metal-π interactions without interfering effects of solvents. The topology of a supramolecular framework then depends only on the geometry and electronic properties of complementary building units. Thus, the use of a linear dimetal unit and an elongated organic spacer in this work resulted in the preparation of a novel organometallic ladder, {[Rh2(O2CCF3)4]2‚(C24H26)}. The formation of the title product was successfully explained based on orbital interaction analysis of the reacting moieties. The latter approach can be generally useful for prediction of coordination of electrophilic metal centers to nonconjugated aromatic systems. Acknowledgment. M.A.P. is very grateful to the Dr. Nuala McGann Drescher Leave Program, to the donors of the Petroleum Research Fund, administered by the American Chemical Society (PRF # 42910-AC3), and for a National Science Foundation Career Award (NSF-0546945) for the support of this work. The authors also thank Dr. E. V. Dikarev at the University at Albany for assistance with the X-ray experiments and the National Science Foundation (NSF01300985) for the provision of the CCD X-ray diffractometer.

Figure 7. Potential energy curve for the rotation of the central ring in C24H26 calculated at the HF level of theory with the 6-31G(d,p) basis sets.

Scheme 3. Schematic Representation of the Interacting Molecular Orbitals

dirhodium unit over the C3a-C4a site. After coordination of the second dimetal complex, the building units of composition [Rh2(O2CCF3)4]2‚(C24H26) are formed, and those can further interact through the open Rh-ends to assemble a ladder-like type of framework. (c) Flexibility of a Molecular Skeleton in C24H26. A comparison of structures 1 and 2 indicates that the central benzene ring of the hydrocarbon spacer is rotated out of the

Supporting Information Available: X-ray crystallographic files, in CIF format; computational details and tabular materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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