Dynamic Exchange of Axial Ligation in the Columnar Liquid

Fabio D. Cukiernik, Mohammed Ibn-Elhaj, Zulema D. Chaia, Jean-Claude Marchon, Anne-Marie Giroud-Godquin, Daniel Guillon, Antoine Skoulios, and Pascale...
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Langmuir 1995,11, 2306-2311

Dynamic Exchange of Axial Ligation in the Columnar Liquid-Crystalline Phase of Dirhodium Tetracarboxylates Michel Bardet, Pascale Maldivi, Anne-Marie Giroud-Godquin, and Jean-Claude Marchon*,+ CEAIDBpartement de Recherche Fondamentale s u r la MatiBre CondensBe JSESAM, a n d CNRS JLaboratoire de Chimie de Coordination (URA 11941, Centre d'Etudes NuclBaires de Grenoble, 38054 Grenoble Cedex 9, France Received February 28, 1995. I n Final Form: March 24, 1995

Introduction Phase transitions from a lamellar crystalline solid to a columnar liquid-crystalline phase have been observed recently for a number of binuclear transition metal carboxylates of general formula Mz(OzCR)4,l such as the fatty acid complexes of Rh(II),2 C U ( I I ) , ~R u ( I I ) , ~Cr(I1) and M O ( I I ) . ~In the solid state, these metallomesogens exhibit a loosely polymeric, laddered structure wherein each Mz(OzCR)4 dinuclear unit is b o u n d t o its two neighbors by weak axial M. * SObonds ( F i g u r e 1). When heated above a transition temperature of about 100-120 "C, they undergo a transition to a liquid-crystalline phase of the hexagonal columnar type ( F i g u r e 2). The p r e s e n c e of heavy atoms, andor unpaired d electron density and metal-metal bonds, in these m e s d g e n s allows a variety of spectroscopic techniques t o be used for the characterization of their transitions into and through the mesophases, in a d d i t i o n t o the classical methods of hot-stage t e-mail: [email protected].

(1)For reviews, see: (a)Marchon,J.C.; Maldivi,P.; Giroud-Godquin, A. M.; Guillon, D.; Skoulios,A,; Strommen, D. P. Philos. Trans. R. Soc. London, Ser. A 1990, 330,109-116. (b) Marchon, J. C.; Maldivi, P.; Giroud,A. M.; Guillon,D.;Ibn-Elhaj, M.; Skoulios,A. InNanostructures Based on Molecular Materials; Gopel, W., Ziegler, C., Eds.; VCH: Germany, Weinheim, 1992; pp 285-291. (2) (a)Giroud-Godquin,A. M.; Marchon, J. C.; Guillon, D.; Skoulios, A. J . Phys. Chem. 1986, 90, 5502-5503. (b) Maldivi, P. These de Doctorat,UniversiteJoseph Fourier, Grenoble, France, 1989. (c) Poizat, 0.;Strommen, D. P.; Maldivi, P.; Giroud-Godquin,A. M.; Marchon, J. C. Inore. Chem. 1990.29.4851-4853. (d)Barbera. J.: Esteruelas. M. A,; Lev&, A. M.; Oro, L.'A.; Serrano, J. L.; Sola, E. inorg. Chem. 1992, 31, 732-737. (e) Ibn-Elhaj, M.; Guillon, D.; Skoulios, A.; Maldivi, P.; Giroud-Godquin,A. M.; Marchon, J. C. J . Phys. 2 Fr. 1992,2, 223799.52

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(3) (a)Godquin-Giroud,A. M.; Marchon, J. C.; Guillon, D.; Skoulios, A. J . Phys. Lett. 1984, 45, L681-L684. (b) Giroud-Godquin, A. M.; Latour, J. M.; Marchon, J. C. Inorg. Chem. 1985,24, 4452-4454. ( c ) Abied, H.; Guillon, D.; Skoulios,A,; Weber, P.; Giroud-Godquin,A. M.; Marchon, J. C. Liq. Cryst. 1987, 2, 269-279. (d) Strommen, D. P.; Giroud-Godquin,A. M.; Maldivi, P.; Marchon, J. C.; Marchon, B. Liq. Cryst. 1987,2,689-699. (e)Abied, H.;Guillon, D.; Skoulios,A.;Dexpert, H.; Giroud-Godquin,A. M.; Marchon, J. C. J . Phys. Fr. 1988,49,345352. (0 Abied, H.; Guillon, D.; Skoulios, A.; Giroud-Godquin,A. M.; Maldivi, P.; Marchon, J. C. Colloid Polym. Sci. 1988,266,579-582. (g) Maldivi, P.;Guillon, D.; Giroud-Godquin,A. M.; Marchon, J. C.; Abied, H.; Dexpert, H.; Skoulios, A. J . Chim. Phys. 1989,86,1651-1664. (h) Giroud-Godquin,A. M.; Maldivi,P.;Marchon, J. C.; Aldebert,P.; Peguy, A,; Guillon, D.; Skoulios,A. J . Phys. Fr. 1989,50,513-519. (i) Attard, G. S.; Cullum, P. R. Liq. Cryst. 1990, 8, 299-309. (j) Ibn-Elhaj, M.; Guillon, D.; Skoulios, A,; Giroud-Godquin, A. M.; Marchon, J. C. J . Phys. 2Fr. 1992,2,2197-2206. (k)Ibn-Elhaj, M.; Guillon,D.; Skoulios, A.; Giroud-Godquin,A. M.; Maldivi, P. Liq. Cryst. 1992,11, 731-744. (1) Maldivi,P.; Bonnet, L.; Giroud-Godquin,A. M.;Ibn-Elhaj,M.;Guillon, D.; Skoulios, A. Adu. Mater. 1993, 5, 909-912. (4) (a) Maldivi, P.; Giroud-Godquin,A. M.; Marchon, J. C.; Guillon, D.; Skoulios, A. Chem. Phys. Lett. 1989,157,552-554. (b) Cukiernik, F. D.; Maldivi, P.; Giroud-Godquin,A. M.; Marchon, J. C.; Guillon, D.; Skoulios, A. Liq. Cryst. 1991, 9, 903-906. ( c ) Bonnet, L.; Cukiernik, F. D.; Maldivi, P.; Giroud-Godquin,A. M.; Marchon, J. C.; Ibn-Elhaj, M.; Guillon, D.; Skoulios, A. Chem. Mater. 1994, 6 , 31-38. (5) (a) Cayton, R. H.; Chisholm, M. H.; Darrington, F. D. Angew. Chem., Znt. Ed. Engl. 1990,29,1481-1483. (b) Baxter, D. V.; Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Putilina, E. F.; Tagg, S. L.; Wesemann, J. L.; Zwanziger,J. W.; Darrington, F. D. J . A m . Chem. SOC. 1994,116, 4551-4566.

0743-746319512411-2306$09.00/0

polarizing microscopy, differential scanning calorimetry, and low-angle X-ray scattering. Thus, we have demonstrated that E X A F S , ~ magnetic ~ P ~ ~ , ~~ u s c e p t o m e t r y , ~ ~ , ~ ~ , ~ and Raman spectroscopy2c are sensitive probes of the transition from the solid phase to the columnar mesophase of various Cu, Ru, and Rh binuclear metal carboxylates. On the basis of 95Moand 13C NMR studies of Moz(0zC(CH&CH3)4, Baxter et al. have concluded recently that in the liquid crystal phase the Mo-Mo quadruple bond is aligned perpendicularly to the applied magnetic field.5b In this p a p e r , we r e p o r t 13C high-resolution NMR spectroscopic studies of dirhodium tetracarboxylates in their crystal and liquid crystal phases. We show that 13Chighresolution NMR spectroscopy provides a direct probe of the local symmetry and dynamics of the columnar mesophase of these complexes.

Experimental Section Synthesis. Rh2(02C(CH2)3CH3)4,Rhz(OzC(CHz)&H3)4,and Rh2(02C(CH2)&H3)4 were prepared by heating rhodium acetate i n the corresponding carboxylic acid (valeric, pelargonic, and arachidic, respectively). Ligand exchange was followed by extraction of unreacted rhodium acetate in water. Slow recrystallization from the neat fatty acid (valerate and pelargonate complexes) or from n-heptane (arachidate) gave an analytically pure sample i n 50-60% yield.2b Solid StateNMR Spectroscopy. High-resolution solid-state 13C NMR spectra were obtained on a Bruker MSL 200 spectrometer operating at 50.3 MHz with magic angle spinning(MAS1. The sample (300-350 mg) was contained in a double-bearing rotor made of zirconia. The spinning speed was set in the range 3300-3500 Hz; faster spinning did not improve the quality of the spectra. Except for the spectra recorded in the mesophase, all spectra were obtained by using cross-polarization (CP) with 600ps contact time and 2 s recycle delay. The IH radiofrequency field strength was set to give a 90" pulse duration of the order of 5ps; the same value was used for the dipolar decouplingprocess. The I3C radiofrequency field strength was set by matching the Hartman-Hahn condition. For each spectrum, 800-1600 transients were collected. Interrupted decoupling experiments6 were performed by intervalling i n the CP experiment a 40 ps delay with zero field strength before the acquisition period. Direct polarization spectra were obtained with MAS by applying a single pulse (SP) with high power decoupling during the acquisition period. The full sequence is referred to as a SP-MASexperiment. The 13C and lH field strengths were identical to those used for the CPexperiment, but the recycle delay was set to 14s. Chemical shifts were calibrated via the carbonyl signal of glycine, which was set at 176 ppm relative to tetramethylsilane (TMS).

Results and Discussion SpectralAssignment (Solid State). The dirhodium tetracarboxylates investigated in this study are long-chain homologs of dirhodium tetrabutyrate Rhz(OzC(CHz)zCH3)4, the structure of which is illustrated in Figure 3 with the numbering system used for s p e c t r a l assignment. The 13C high resolution solid-state spectra of rhodium valerate Rh2(02C(CHz)&H3)4 measured at room temperature by means of CP-MAS and interrupted decoupling experiments are shown in Figure 4. The first remarkable feature of the CP-MAS s p e c t r u m is its high resolution, with five groups ofwell-separated singlets o r p a i r s of singlets, which can be attributed to the five c a r b o n a t o m s of the valerate ligands. This striking resolution is indicative of the high crystalline q u a l i t y o f t h e s a m p l e ; in c o n t r a s t , longer-chain homologs, which yield a m o r p h o u s solids, exhibit poorly resolved spectra (see below). Attribution of the pair of peaks at 193.9 and 195.9 p p m t o carboxylate resonances ((2-1) is obvious. The large shift (ca. 20 p p m ) to higher frequencies relative to the corresponding isotropic reso(6) Opella, S. J.; Frey, M. H. J . Am. Chem. SOC.1979, 101, 58545856.

0 1995 American Chemical Society

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Langmuir, Vol. 11, No. 6, 1995 2307

Figure 1. View of the laddered structure of CU~(O~C(CH~)~CH& in the lamellar crystalline state. Color code: black, copper; gray, oxygen; white, carbon. Atomic coordinates of copper(I1)decanoate [Lomer, T. R.; Perera, K. Acta Crystallogr., Sect. B 1974,30, 2912-29131 were input into the Sybyl molecular modeling software (Tripos),and the dinuclear unit was expanded by translations along b and c. The resulting image was saved as a mol file, and drawn with MolDraw [Cense, J. M. Tetrahedron Comput.Method. 1989,2, 65-71].

Figure 2. Schematic view of the ordered hexagonal columnar mesophase Dh, of a dirhodium tetracarboxylate complex.

nance (170- 176 ppm) of silver carboxylates7 is probably related to the deshielding effect of the Rh-Rh single bond; it is however larger than that (ca. 10 ppm) induced by the quadruple Mo-Mo bond of molybdenum octanoate M o ~ ( O ~ C ( C H ~ ) &(6H184 ~ ) ~~ p m ) Assignment .~~ of the alkyl resonances was facilitated by an interrupted decoupling experiment (Figure 4b) which left the resonance of the terminal methyl carbon (C-5) unaffected. The remaining carbon resonances ((2-2 to (3-4) were then assigned by comparisonwith the chemical shifts of valeric acid. A similar procedure was used to assign the resonances of rhodium pelargonate Rha(OsC(CH2)7CH3)4; in that case, assignments of the solid-state spectrum were facilitated by examination of the much simpler spectrum of the columnar mesophase (see below). A list of observed chemical shifts is presented in Table 1 for the two complexes and the corresponding acids. The 13C solid(7)Veeman, W.s. Prog. NMR Spectrosc. 1984,16, 193-235.

I

Figure 3. View of the crystal structure of dirhodium tetrabutyrate showing the two pairs of symmetry-related bridging carboxylateligands,and the numberingsystem used for spectral assignment. Color code: black, rhodium; gray, carbon; white, oxygen. The drawing procedure was the same as for Figure 1.

2308 Langmuir, Vol. 11, No. 6, 1995

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Figure 4. I3C high resolution solid-statespectra of R~z(OZC(CHZ)~CH& measured at room temperature by means of (a) CP-MAS and (b) interrupted decoupling experiments. Table 1. lSC Chemical Shifts for Dirhodium Tetracarboxylates in the Solid and Liquid Crystal States c-1 c-2 c-3 c-4 c-5 C-6 c-7 C-8 HOZC(CHZ)&H~~ 179.4 33.8 26.7 21.7 13.2 Rhz(OzC(CHz)&H3)4 (solid state)*

HOZC(CH&CH~~ Rhz(OzC(CHz)7CH3)4(solid state)b Rhz(OzC(CHd7CHd4 (mesophase)d a

193.9 195.9 180.6 193.7 195.5 194.5

37.1 34.2 37.3 37.6

27.7 30.8 24.8 26.4 29.2 26.7

22.9 29.3 31.1 32.3 29.5

14.5 15.3 29.2 31.1 32.3 29.5

29.2 31.1 32.3 29.5

31.9 33.2 34.2 32.1

22.7 23.2 24.0 22.7

c-9

14.1 14.2 14.8 13.8

Water solution, see ref 13. b 25 "C (this work). c Chloroform solution; see ref 14. d 105 "C (this work).

state NMR spectra obtained for Rh2(02C(CH~)&H3)4 and Rh2(02C(CH2)&H3)4 (Figure 5) show that the peak resolution decreases, and that the peak multiplicity increases for the latter. This is particularly clear for the carbonyl resonances which appear as a pair of singlets for the former and as a broad, ill-defined multiplet for the latter, and it suggests that sample crystallinity decreases with increasing chain length. Examination of peak splittings in the CP-MAS spectra of RhdOzC(CHd3CH3)4 and Rhz(OzC(CH2)&H& complexes reveals some common features. The resonances of the carboxylate ((3-1) and of the terminal methyl ((3-5 and C-9, respectively) carbon atoms are split in a pair of singlets, as is also the resonance of C-3;the largest splitting is exhibited by the latter (ca. 3 ppm) in both compounds. The resonance of C-2 is a singlet in both cases. These features can be explained by considering the crystal

structure of dirhodium tetrabutyrate.8 We assume that the tetrapentanoate and the tetranonanoate congeners, which have the same dimetal tetracarboxylate core and differ from the former only by the length of the four peripheral alkyl chains, exhibit analogous structures and similar crystallographic symmetries. This assumption is reasonable since rhodium(I1) butyrate is isostructural to copper(I1)butyrategon one hand, and because crystalline copper alkanoates of different chain lengths all display the same type of triclinic lamellar lattice3con the other hand. The molecule of dirhodium tetrabutyrate shows the typical paddle-wheel type core with weak intermolecular Rh..*O bonds shown in Figure 3. There is a crystallographically imposed inversion center a t the midpoint of the Rh-Rh bond. The dirhodium tetracarboxylate core has approximate D4h symmetry, but the (8) Cotton, F. A.; Shiu, K. B.Rev. Chim. Miner. 1986,23,14-19. (9) Campbell, G. C.; Haw, J. F. Inorg.Chem. 1988,27,3706-3709.

Notes

Langmuir, Vol. 11, No. 6, 1995 2309

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Figure 5. I3C high resolution CP-MAS solid-state spectra of (a) Rhz(OzC(CHz),CH3)4and (b)Rh2(02C(CHZ)l&H& measured at room temperature.

molecular symmetry is lower due to (i) the presence of interdimer bonding, which leads to two types of carboxylate groups, and (ii) the two different conformations of the propyl chains, which define two pairs of symmetry-related bridging carboxylate ligands. One can note that those bridging carboxylates which are involved in interdimer bonding have a n all-trans alkyl chain (nonprimed atoms), while those of the other pair have a gauche bond between C-2' and C-3'. These crystallographic features provide a n obvious explanation for the 13C NMR splittings described above. In Rh2(02C(CH2)7CH3)4, each of the carbon resonances is split in a pair of singlets in agreement with the 2-fold molecular symmetry, except the signal of C-2 and C-2' which appears as a singlet. This anomalous behavior of C-2 and C-2' is accounted for by their common location in each of the two Rh204 planes which exhibit very similar magnetic environments. A similar reasoning would predict that the carboxylate carbons C-1 and C-1' should give a degenerate singlet resonance, contrary to the experimentally observed pair of singlets. In that case, however, the interdimer bonding which involves only one pair of carboxylate oxygen atoms adequately accounts for the splitting. Finally, the two expected resonances of the penultimate alkyl carbon atom accidentally overlap in the spectrum of Rh2(02C(CH2)3CH3)4(C-4),while they are separated for rhodium Rhz(O2C(CHz),CHsh(C-8). At this stage, it is also worth noting that similar splittings of all (or almost all) the carbon atom resonances in the crystalline phase could be expected for other dimetal

tetracarboxylate complexes which are structurally analogous to rhodium butyrate, such as molybdenum ~ c t a n o a t e .Recent ~~ NMR data for this complex in the crystalline state have been analyzed on the implicit assumption that a single 13C chemical shift tensor is observed for the carboxylate carbon atom. The present work suggests that the signal observed by Baxter et ~ 1 . may in fact result from the convolution of two distinct 13C chemical shift tensors corresponding to the two structurally distinct carboxylate carbon atoms of the complex. Columnar Mesophase. We have demonstrated earlie9 that dirhodium tetracarboxylate complexes exhibit a thermotropic hexagonal columnar mesophase in which each column is made of stacked dirhodium tetracarboxylate cores with a period of ea.4.6 A. Increasing to 105 "C the temperature of a sample of Rh2(02C(CH2)7CH3)4 resulted in the 13Chigh resolution NMR spectra illustrated in Figure 6. The transition from the solid to the liquid crystalline phase takes place a t 100 "C for that complex,2b therefore the sample is in the columnar mesophase at that temperature. The resonances of the terminal carbon atoms (C-7 to C-9) are absent in the CP-MAS spectrum measured at 105 "C (Figure 6a), indicating considerable motion of the chain ends in the mesophase,1° and facilitating the peak assignments (see above). Increasing the contact time to 3 ms resulted in efficient polarization (10) Carpentier, L.; BBe, M.; Giroud-Godquin, A. M.; Maldivi, P.; Marchon, J. C. Mol. Phys. 1989,68,1367-1378.

~

~

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2310 Langmuir, Vol. 11, No. 6, 1995

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C +&+4++

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Figure 6. 13C high resolution solid-state spectra of Rhz(OzC(CHZ)&H& in the columnar mesophase at 105 "C by means of (a) a CP-MAS experiment with a 600 p s contact time, (b) a CP-MAS sequence with a 3 ms contact time, and (c) a SP-MAS sequence. of C-7 and C-8 (Figure 6b). Finally, all the expected resonances were present in the spectrum obtained at 105 "C with a SP-MAS sequence (Figure 6c). In contrast to the solid-state CP-MAS spectrum (Figure 5a), each carbon resonance appears as a single peak in the SP-MAS spectrum of the mesophase. This is seen most clearly in the isolated, low-field carboxylate resonances; the chemical shift of the singlet observed for the mesophase (194.5 ppm) ii3 the average of those of the corresponding pair obtained a t room temperature (193.7195.5). The changes are perfectly reversible when the sample is cooled to room temperature. This observation, which is typical of a two-site exchange process, shows that motional averaging takes place in the columnar mesophase. The single resonances of the alkyl carbon atoms are somewhat shifted (0.5-2.0 ppm) in the mesophase relative to the middle of the pairs of the solid.ll Averaging Motions in the Columnar Mesophase. Our observations that the splittings of the 13Cresonances which are observed in the solid state disappear in the columnar mesophase, and that the new carboxylate resonance of the mesophase appears midway in the roomtemperature pair of singlets, provide a unique insight into the dynamic processes which occur in the mesophase. In the closely related series of copper(I1) carboxylates, a random disorder model has been proposed by Guillon and (11)Kentgens, A. P. M.; Markies, B. A.; van der Pol, J. F.; Nolte, R. J. M. J.Am. Chem. SOC.1990,112,8800-8806.

us in 1988 to account for the structural features of the columnar mesophase observed by X-ray diffraction and by EXAFS.3e7gAccording to this model, adjacent dicopper tetracarboxylate cores along a column axis are randomly shifted along the four possible metal-oxygen directions, perpendicular to that axis which allows interdimer bonding. The dynamic nature of this random disorder was suggested by Baxter et al. on the basis of static solid 95Moand 13CNMR studies of the columnar mesophase of molybdenum ~ c t a n o a t e .The ~ ~ present observations unambiguously indicate that the dynamic process which averages the 13C resonances or rhodium pelargonate statistically results in a D4h local symmetry for the dinuclear unit in a column. The two structural features which lower the local symmetry in the solid state (see above) are therefore absent in the columnar mesophase. Thus a t high temperature, (i) each pair of opposite carboxylate ligands in a dinuclear unit is involved in interdimer bonding on the NMR time scale, and (ii) the conformations of the alkyl chains are statistically averaged. Among the two types of molecular motions (horizontal shift and C4rotation) which can result in statistical interdimer bonding (Figure 7), the latter has been singled out by Baxter et al. as the most consistent with the static I3C NMR spectrum of the mesophase of Moz(OzC(CHzI6CH3)4 and with the kinetic lability of axial Moy..O interaction^.^^ A similar conclusion was also reached by

Notes

Langmuir,Vol. 11, No. 6,1995 2311

Figure 7. Static view of a dynamically disordered column of Rh2(02C(CH2)2CH& in the liquid crystal phase. This model was generated from the atomic coordinates of dirhodium tetrabutyrate in the crystalline state by random rotations (goo, M O O , -90') around successive +a1 Rh* 0 bonds, using Sybyl. Note that the average interdimer distance in the disordered model (4.6 A)is identical to the 4.6 A reflectioq seen by X-ray diffraction,2aand much shorter than the dimer repeat distance along the ladder direction in the crystal (5.231 A).

tetracarboxylates in both their crystalline and liquid B6e et aZ.12on the basis of neutron scattering studies of the columnar mesophase of C U ~ ( O & ( C H ~ ) ~ ~ C onHthe ~ ) ~ : crystalline states. It provides clear-cut information on 4x s time scale, anisotropic motions of the methylene the dynamic nature of the statistical processes which groups take place around the column axes. The present increase the local symmetry of the mesogenic units in the 13Chigh resolution NMR data do not allow a discrimination mesophase. This 13C high-resolution solid state NMR to be made between horizontal shift and C4 rotation, but investigation provides the first strong evidence of dynamic such a distinction may be artificial since a C4 rotation can behavior about the M-M bond in the mesophase, and it be the product of two perpendicular horizontal shifts.Thus, firmly supports our conclusion that the dimetallic cores the actual nature of the pathways which lead to statistical undergo fast axial bond exchange in this type of liquid interdimer bonding along the columns of the mesophase crystalline material. The potential of 13Chigh-resolution remains an open question. solid-state NMR experiments for the understanding of Conclusions the structure and dynamics of columnar metallome13Chigh-resolution solid-state NMR spectroscopy is an sogens15J6would deserve further investigation. invaluable tool for investigatingthe structure of dirhodium LA950161U (12) BBe, M.; Giroud-Godquin, A. M.; Maldivi, P.; Williams, J. Mol. Phys. 1994,81,57-68. (13) Hagen, R.; Roberts, J. D. J . Am. Chem. SOC.1969,91,45044506. (14) Johnson, L. F.;Jankowski, W. C. Carbon-13 NMR Spectra; Wiley: New York, 1969; p 362.

(15) Serrette, A. G.; Swager, T. M. Angew. Chem., Int. Ed. Engl. 1994,33,2342-2945. (16)Atencio, R.; Barbera, J.; Cativiela, C.; Lahoz, F. J.; Serrano, J. L.; Zurbano, M. M. J . A m . Chem. SOC.1994,116,11558-11559.