Self-assembly and solvent-mediated structural transformation of one-dimensional cluster-based coordination polymer

Abstract

Reactions between [Nb₆Cl₁₂(CN)₆]⁴⁻ and [Mn(salen)]⁺ in MeOH–H₂O solvent mixture led to the formation of a new compound: {[(Mn₂(salen)₂)][[Mn(salen)(H₂O)]_0.67[Mn(salen)(MeOH)]_1.33(Nb₆Cl₁₂(CN)₆)]}·1.7H₂O·2MeOH (1). Single crystal X-ray analysis has shown that 1 possesses 1D framework built of heterotrimeric supramolecular anions, {[Mn(salen)(s)]₂[Nb₆Cl₁₂(CN)₆]}²⁻ and [Mn₂(salen)₂]²⁺ phenoxo bridged dimers linked to each other through two cyanide ligands located in axial positions. When 1 was left in a methanolic solution of (Me₄N)⁺, its 1D framework was transformed into a 2D framework of the previously reported compound [Me₄N]₂[(Mn(salen))₂(Nb₆Cl₁₂(CN)₆))] (2) which has a 2D anionic 4,4-network built of [Nb₆Cl₁₂(CN)₆]⁴⁻ and [Mn(salen)]⁺ nodes linked by CN ligands (Zhou, Day and Lachgar, Chem. Mater., 2004, 16, 4870).¹Magnetic susceptibility measurements indicate that 1 exhibits weak antiferromagnetic interactions due to the phenolate-bridged out-of-plane Mn(III) dimers with J = −1.50(5) cm⁻¹. Thermal stability of 1 was studied.

---

Introduction

Edge-bridged octahedral metal clusters M₆Lⁱ₁₂L^a₆ (M = Nb, Ta; L = halide) are characterized by their various electronic and magnetic properties arisen from the presence of metal–metal bond.² When equipped with suitable terminal ligands, these clusters can self-assemble with different metal ions or metal complexes to form nanomolecular assemblies or hybrid inorganic–organic materials with different topologies and interesting properties.^3–6 Compared with mononuclear species, the use of M₆clusters as nodes is characterized by their large size (≧1 nm), availability of multiple coordination sites, as well as the potential to create materials with structural features and chemical–physical properties that can be affected by the use of different metals and/or different ligands.^3–6

Hybrid materials and coordination polymers have received much attention in the past decade due primarily to their potential applications and attractive structures.⁷ Initial investigations focused on rigid frameworks and recently there has been growing interest in new generation of flexible frameworks bearing pore size adjustment or even giant structural changes in response to external stimuli.⁸ This interesting dynamic structural transformations can be used to prepare a new class of materials that possess new functions such as switching or sensing.⁹ Most transformations have been suggested to have a solid state (crystal-to-crystal or amorphous-to-crystal) mechanism, but solvent-mediated mechanism has been proposed for other transformations.¹⁰

Previous studies by our group showed that different supramolecular assemblies and multi-dimensional coordination polymers can be prepared through the combination of the niobium cyanochloride clusters [Nb₆Cl₁₂(CN)₆]⁴⁻ and Mn(III) complexes of the tetradentate salen-type ligands.^1,6Solvent-mediated structural transformation behavior (2D → 2D or 2D → 0D) has been demonstrated for some of these compounds.⁶ Herein we report (1) the synthesis and characterization of a new 1D compound through the reaction of [Nb₆Cl₁₂(CN)₆]⁴⁻ and [Mn(salen)]⁺ in MeOH–H₂O; (2) solvent-mediated structural transformation of 1 into a previously reported 2D compound (1D → 2D).¹

Experimental

General

(Me₄N)₄[Nb₆Cl₁₂(CN)₆]·2MeOH and [Mn(salen)]ClO₄·2H₂O were synthesized according to literature methods.^1,11 LiCl (99%), NbCl₅ (99%, metal basis), Nb powder (99.8%, metal basis), KCN (96%) and Me₄NCl (98%) were purchased from Alfa Aesar. Mn(OAc)₃·2H₂O (98%) and ethylenediamine (99%) were purchased from ACROS. Salicylicaldehyde was purchased from Fisher. All chemicals were used as received. Solvents EtOH, THF, MeCN and MeOH were used as received without further purification.

Synthesis

{[(Mn₂(salen)₂)][[Mn(salen)(H₂O)]_0.67[Mn(salen)(MeOH)]_1.33(Nb₆Cl₁₂(CN)₆)]}·1.7H₂O·2MeOH (1). To 10.0 ml of 4.0 mM aqueous solution of (Me₄N)₄[Nb₆Cl₁₂(CN)₆]·2MeOH was added 10.0 ml of 12.0 mM solution of [Mn(salen)]ClO₄·2H₂O in methanol. A brown precipitate formed immediately and the mixture was left undisturbed, dark brown crystals began to form after approximately one hour. After 1 day the solid product was filtered and the crystals were easily separated by hand from the remaining powder. The crystals were washed in methanol and dried in air (Yield: 20.0 mg, 25.9% based on cluster). Anal. Calc. For C_73.33H_74.07Cl₁₂Mn₄N₁₄Nb₆O_13.70: C, 34.23; H, 2.90; N, 7.62%. Found: C, 33.18; H, 2.86; N, 7.51%. ν_CN = 2134 cm⁻¹.

Solvent-mediated structural transformation

(Me₄N)Cl (100 mg, 0.9 mmol) was added to a suspension of crystals of 1 (26 mg, 0.01 mmol) in methanol (10 ml) at room temperature. The reaction mixture was left undisturbed for 2 days after which a brown microcrystalline solid formed and was separated by centrifugation, washed with water and methanol, then dried in vacuum to get 2. Anal. Calcd. for 2C₄₆H₅₂Cl₁₂Mn₂N₁₂Nb₆O₄: C, 28.63; H, 2.72; N, 8.71%. Found: C, 28.79; H, 2.87; N, 8.23%. The phase identity and purity was determined by comparing the observed powder X-ray diffraction to that calculated from the crystal structure determined from single crystal X-ray diffraction data.

Structural determination

Intensity data from a single crystal of (1) were measured at 193(2) K on a Bruker SMART APEX CCD area detector system. Data were corrected for absorption effects using the multi-scan technique (SADABS). Structure was solved and refined using the Bruker SHELXTL (Version 6.1) Software Package. The integration of the data using a triclinic cell yielded a total of 17 609 reflections to a maximum θ angle of 25.00°, of which 8362 were independent (R_int = 5.05%), and 6033 (72.15%) were greater than 2σ(F²). The structure was solved and refined in the space group P1̄ (No. 2), with Z = 1. All non-hydrogen atoms were refined anisotropically. O5 is fully occupied but present as coordinated water (1/3 of the time) and coordinated MeOH (2/3 of the time). Since the disordered lattice solvent could not be unambiguously modeled, the Platon Squeeze option was utilized based on the model that included the coordinated MeOH/H₂O atoms only.¹² Squeeze indicates 4 solvent regions in the cell corresponding to about 53 electrons/cell or approximately 2 methanol and 1.7 water molecules per cell.

The final anisotropic full-matrix least-squares refinement on F² with 532 variables converged to R₁ = 4.70% for observed data and wR₂ = 11.19% for all data. The maximum residual peak and hole on the final difference electron density map was found to be 1.152 e⁻/ų (1.25 Å from H51B) and −0.715 e⁻/ų (0.89 Å from Nb2), respectively. A summary of the most important crystal and structure refinement data is given in Table 1.

Table 1 Crystal data and structure refinement for 1.^a

	1
a *R1 = ∑(|Fo| − |Fc|)/∑|Fo|, **wR2 = {∑w[(F^2o − F^2c)]/∑w[(F^2o)^2]}^0.5, w = [σ^2(F^2o) + (0.0490P)^2 + 0.0000P] ^−1, where P = (F^2o + 2F^2c)/3].
formula	C73.33H74.07Cl12Mn4N14Nb6O13.70
Mr/g	2573.32
Crystal system	Triclinic
Space group	P1̄
a/Å	12.606(3)
b/Å	13.815(3)
c/Å	14.013(3)
α/°	78.919(2)
β/°	86.321(2)
γ/°	87.030(2)
V/Å^3	2388.0(8)
Z	1
d calcd/g cm^−3	1.789
F(000)	1272
θ range°	3.80–25.00
Reflections collected/unique	17 609/8362
R(int)	0.0505
R1,* wR2*, I > 2σ(I)	R1 = 0.0470, wR2 = 0.1038
R1,* wR2*, all data	R1 = 0.0715, wR2 = 0.1119
Goodness-of-fit on F^2	0.972
Max/mean shift in final cycle	0.001/0.000

---

Other physical measurements

Elemental analyses were carried out by Atlantic Microlab, Inc. Thermogravimetric analyses were performed with 14 mg sample under a flow of air (40 mL/min) at a heating rate of 5 °C/min, using a Perkin-Elmer Pyris 1 TGA system. Infrared spectra were recorded as KBr pellets on a Mattson Infinity System FTIR spectrometer. The magnetic susceptibility data were collected using a SQUID (superconducting quantum interference device, MPMS, Quantum Design) magnetometer. Data were corrected for the diamagnetic moment of the sample holder, the diamagnetic contribution of the octahedral {Nb₆} cluster¹³ and of the ligands using Pascal's constants.¹⁴X-Ray powder diffraction data were collected at room temperature using a BRUKER P4 general-purpose four-circle X-ray diffractometer equipped with a GADDS/Hi-Star detector positioned 20 cm from the sample. The goniometer was controlled using the GADDS software suite.¹⁵ The sample was mounted on tape and data were collected in transmission mode. The data ere reduced by area integration methods to produce a single powder diffraction pattern for each frame. Individual powder diffraction patterns were merged and analyzed with the program EVA to produce a single one-dimensional pattern.¹⁶

Results and discussion

Crystal structure of 1

The structure of 1 is 1D coordination polymer that consists of neutral zigzag chains built of {[Mn(salen)(s)]₂[Nb₆Cl₁₂(CN)₆]}²⁻ supramolecular assemblies and [Mn₂(salen)₂]²⁺ dimers linked by cyanide ligands (Fig. 1(a)). Selected bond lengths and angles are shown in Table 2. The {[Mn(salen)(s)]₂[Nb₆Cl₁₂(CN)₆]}²⁻ anions are built of [Nb₆Cl₁₂(CN)₆]⁴⁻cluster that connects to two [Mn(salen)(s)]⁺ monomers by the N end of two CN ligands that are trans to each other. The [Nb₆Cl₁₂(CN)₆]⁴⁻cluster unit has an octahedral Nb₆ metal core equipped with 12 edge-bridging Cl ligands and six terminal CN⁻ ligands connected to the cluster through their carbon end. The average bond lengths and angles of Nb–Nb (2.925(2) Å), Nb–Cl (2.466(7) Å), Nb–C(2.278(9) Å), C≡N (1.14(1) Å) and Nb–C≡N (176(1)°) are close to the corresponding values reported for other compounds containing the octahedral cyanochloride niobium clusters with 16 valence electrons available for metal-metal bonding.^1,6 The IR vibration corresponding to C≡N stretching band is observed near 2134 cm⁻¹ close to that found for the starting material (Me₄N)₄[Nb₆Cl₁₂(CN)₆] (2129 cm⁻¹).¹

Fig. 1 Structure of compound 1. (a) The structure of {[Mn(salen)(s)]₂[Nb₆Cl₁₂(CN)₆]}²⁻heterotrimer; (b) the structure of [Mn₂(salen)₂]²⁺ dimer; (c) the neutral coordination chain; (d) the stacking of the chains viewed along the b direction. Hydrogen bonding holding the chains together is represented as dotted black lines. The salen ligand, chlorine atoms and free solvent molecules are omitted for clarity.

Table 2 Selected bond lengths (Å) and bond angles (°) for 1.^a

a Symmetry transformations used to generate equivalent atoms: #1 − x, −y − 1, −z + 2. b The three Nb atoms are inside an equatorial plane. c The three Nb atoms make up one face of the octahedron. d There are two kinds of [Mn(salen)]^+ units. Mn(1) forms the [Mn2(salen)2]^2+ dimer, and Mn(2) belongs to the heterotrimer. e O(5) is belongs to the solvent ligand.
[Nb6Cl12(CN)6]^4^−
Average Nb–Nb	2.925(2)	Average Nb–Nb–Nb^b	90.00(4)
Average Nb–Cl	2.466(7)	Average Nb–Nb–Nb^c	60.00(6)
Average Nb–C	2.278(9)	Average Cl–Nb–Cl	88.7(7)
Average C≡N	1.14(1)	Average Nb–C≡N	176(1)
[Mn(salen)]^+ unit ^d
Mn(1)–O(2)	1.894(4)	Mn(2)–O(3)	1.864(4)
Mn(1)–O(1)	1.907(4)	Mn(2)–O(4)	1.891(4)
Mn(1)–N(5)	1.985(5)	Mn(2)–N(8)	1.978(5)
Mn(1)–N(4)	1.987(5)	Mn(2)–N(7)	1.997(5)
Mn(1)–N(2)	2.206(5)	Mn(2)–N(1)	2.258(5)
Mn(1)–O(1) (phenoxo bridge)	2.405(4)	Mn(2)–O(5)#1^e	2.272(5)
Average C≡N–Mn	158(1)	Mn(1)–O(1)–Mn(1)#1	102.03(16)

---

In the [Mn(salen)(s)]⁺ monomer, each Mn(III) ion is chelated by a tetradentate salen ligand with average Mn–O = 1.88(2) Å and Mn–N = 1.98(1) Å, compared with Mn–O = 1.882 (1) Å and Mn–N = 1.983(1) Å in [Mn(salen)Cl(H₂O)]).¹⁷ The distorted octahedral coordination environment of Mn(III) is completed by the coordination of one cyanide ligand with Mn–N_CN = 2.206(5) Å and Mn–N≡C = 158(1)°, and one solvent molecule with Mn–O = 2.272(5) Å.

The [Mn₂(salen)₂]²⁺ dimers are formed through phenoxo bonding between two Mn(salen) complexes with Mn⋯Mn = 3.366(1) Å, Mn–O = 2.405(4) Å and ∠Mn–O–Mn = 102.1(2)°, within the range reported for other Mn(salen)-type dimers (3.19–4.38 Å and 91.1–102.4° respectively).¹⁸ Each dimer uses its remaining two coordination sites to link to two cyano ligands from the cluster with Mn–N_CN distance of 2.206(5) Å and C≡N–Mn angle of 159.2(5)°.

The neutral chains are stacked parallel to each other along the crystallographic [1 0 −1] axis. Hydrogen bonds between the remaining non-bridging cyanides and the coordinated solvent molecules (O5–H51⋯N3 = 2.742(7) Å, symmetry code: x, y + 1, z) connect the chains into layers that stack perpendicular to the c direction (Fig. 1(d)). Adjacent layers are related to each other by an inversion center and are separated by disordered solvent molecules.

Solvent-mediated structural transformation

When samples of 1 consisting of large crystals were suspended in a methanol solution containing (Me₄N)Cl, an interesting structural transformation took place in which 1 was transformed into compound 2 which has a 2D anionic 4,4′-network built of [Nb₆Cl₁₂(CN)₆]⁴⁻ and [Mn(salen)]⁺ nodes linked by CN ligands.¹ The layers in 2 stack on top of each other in a staggered fashion and generate channels where (Me₄N)⁺ are located. The transformation is complete as indicated by PXRD (Fig. 2), elemental analysis and IR. However, 2 was found to remain stable for weeks when suspended in neat methanol or in a methanolic solution containing [Mn(salen)]⁺, which indicates that the transformation is irreversible.

Fig. 2 (a) Powder X-ray diffraction patterns of 1: pattern simulated from crystal structure in black; observed pattern at room temperature in red; observed pattern after being heated to 950 °C in green. (b) A comparison of the powder patterns of 2 (simulated), 1 and after the reaction of 1 with (Me₄N)Cl.

The fact that large and well shaped crystals of compound 1 are transformed into microcrystalline powder of 2 and that the transformation is accompanied by siginficant structural changes, seems to indicate that the transformation is solvent-mediated and cation-induced. The mechanism of such transformation can be described as follows (Scheme 1): the 1D framework of 1 slowly dissociates to form the dication [Mn₂(salen)₂]²⁺ and the anionic [Mn(salen)₂(Nb₆Cl₁₂(CN)₆)]²⁻heterotrimer built of [Nb₆Cl₁₂(CN)₆]⁴⁻ linked to two [Mn(salen)]⁺ complexes through two transcyanide ligands. In a subsequent step the heterotrimer assembles in the presence of (Me₄N)⁺ cations to form the 2D 4,4′-net in which the layers pack in a staggered way. As 2 slowly crystallizes (precipitates), the solubility equilibrium is shifted toward the product and more of 1 is dissolved to form more 2 until complete transformation of 1. Similar solvent-mediated processes have been observed in the structural transformations of cluster-based 2D frameworks⁶ and anion exchange in Ag-based coordination polymers.¹⁰

Scheme 1 Possible mechanism for the solvent-mediated structural transformation of 1.

Thermal stability

The thermal stability of 1 was studied by thermal analysis under air flow using polycrystalline samples. The result showed that the compound gradually decomposes upon heating from RT to 700 °C. PXRD showed that a mixture of MnNb₂O₆ and Mn₃O₄ (% weight loss (obs.) = 42.31%; (calc.) = 42.22%) is formed after 700 °C (Fig. S1, ESI†).¹⁹

Magnetic properties

The magnetic susceptibility of 1 was measured between 300 and 5 K on microcrystalline powders. Fig. 3 shows the data in the forms of μ_effvs T and 1/χ_Mvs. T. At room temperature the μ_eff values for 1 is 4.90 μ_B, compared with the spin-only value (4.90 μ_B) for four Mn^III ions with g = 2.0 and S = 2. As the temperature decreases μ_eff slowly decreases until ca. 70 K, then sharply decreases to reach 3.67 μ_B at 5 K, indicating weak antiferromagnetic coupling between Mn^III ions. The χ_m⁻¹vs. T plot follows Curie–Weiss law with C = 0.62 emu mol⁻¹ K⁻¹ and θ = −4.91 K also indicating antiferromagnetic coupling, which can be attributed to intradimer coupling. The magnetic properties were modeled using the Heisenberg dimer model with the Hamiltonian Ĥ = −2JŜ₁Ŝ₂ giving the van Vleck equation:

Fig. 3 Temperature dependence of magnetic susceptibility of 1 (5–300 K at 1000 G).

The best fit gave: g = 2.011(5), J = −1.50(5) cm⁻¹ as shown in Fig. 3. The result is comparable with that observed in compounds containing Mn(salen)-type dimers as building blocks.¹⁸

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. DMR-0446763. We are grateful to Dr Gordon Yee from the chemistry Department at Virginia Tech for magnetic susceptibility data collection.

Notes and references

1. H. J. Zhou, C. S. Day and A. Lachgar, Chem. Mater., 2004, 16, 4870.
2. F. A. Cotton and R. A. Walton, Multiple bonds between metal atoms, second ed., Oxford University Press, 1993; E. J. Welch and J. R. Long, Prog. Inorg. Chem., 2005, 54, 1; J.-C. P. Gabriel, K. Boubekeur, S. Uriel and P. Batail, Chem. Rev., 2001, 101, 2037; H. D. Selby, B. K. Roland and Z. P. Zheng, Acc. Chem. Res., 2003, 36, 933; H. J. Zhou and A. Lachgar, Eur. J. Inorg. Chem., 2007, 1053.
3. E. G. Tulsky, N. R. M. Crawford, S. A. Baudron, P. Batail and J. R. Long, J. Am. Chem. Soc., 2003, 125, 15543; D. H. Johnston, C. L. Stern and D. F. Shriver, Inorg. Chem., 1993, 32, 5170; Y. V. Mironov, V. E. Fedorov, I. Ijjaali and J. A. Ibers, Inorg. Chem., 2001, 40, 6320.
4. N. G. Naumov, A. V. Virovets, M. N. Sokolov, S. B. Artemkina and V. E. Federov, Angew. Chem., Int. Ed., 1998, 37, 1943; M. V. Bennett, M. P. Shores, L. G. Beauvais and J. R. Long, J. Am. Chem. Soc., 2000, 122, 6664; Y. Kim, S. M. Park, W. Nam and S. J. Kim, Chem. Commun., 2001, 1470.
5. B. B. Yan, H. J. Zhou and A. Lachgar, Inorg. Chem., 2003, 42, 8818; N. G. Naumov, D. V. Soldatov, J. A. Ripmeester, S. B. Artemkina and V. E. Federov, Chem. Commun., 2001, 571.
6. H. J. Zhou, C. S. Day and A. Lachgar, Cryst. Growth Des., 2006, 6, 2384; H. J. Zhou, K. C. Strates, M. A. Munoz, K. J. Little, D. M. Pajerowski, M. W. Meisel, T. A. Talham and A. Lachgar, Chem. Mater., 2007, 19, 2238; J. J. Zhang and A. Lachgar, J. Am. Chem. Soc., 2007, 129, 250; J. J. Zhang, H. J. Zhou and A. Lachgar, Angew. Chem., Int. Ed., 2007, 46, 4995; J. J. Zhang, Y. Zhao, S. A. Gamboa and A. Lachgar, Cryst. Growth Des., 2008, 8, 172; J. J. Zhang, Y. Zhao, S. A. Gamboa, M. Muñoz and A. Lachgar, Eur. J. Inorg. Chem., 2008, 2982.
7. R. Robson in Comprehensive Supramolecular Chemistry, Pergamon, New York, 1996, ch22, p733; M. Eddaoudi, B. Moler, H. Li, B. Chen, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319.
8. S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334; S. Kitagawa and K. Uemura, Chem. Soc. Rev., 2005, 34, 109.
9. D. Bradshaw, J. B. Claridge, E. J. Cussen, T. J. Prior and M. J. Rosseinsky, Acc. Chem. Res., 2005, 38, 273.
10. A. N. Khlobystov, N. R. Champness, C. J. Roberts, S. J. B. Tendler, C. Thompson and M. Schröder, CrystEngComm, 2002, 4, 426; R. Custelcean and B. A. Moyer, Eur. J. Inorg. Chem., 2007, 1321.
11. H. Li, J. Z. Zhong, C. Y. Duan, X. Z. You, T. C. W. Mak and B. Wu, J. Coord. Chem., 1997, 41, 183.
12. SQUEEZE (handling of disordered solvents in structure refinement) A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
13. J. G. Converse and R. E. McCarley, Inorg. Chem., 1970, 9, 1361.
14. O. Kahn, Molecular Magnetism, Wiley-VCH, New York, 1993, P.3.
15. GADDS V4.1.14 “General Area Detector Diffraction System Program for Instrument Control and zData Collection” BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711–5373 USA.
16. EVA V8.0 “Graphics Program for 2-dimensional Data evaluation and Presentation” BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711–5373 USA.
17. A. Panja, N. Shaikh, M. Ali, P. Vojtisek and P. Banerjee, Polyhedron, 2003, 22, 1191.
18. S. Saha, D. Mal, S. Koner, A. Bhattacherjee, P. Gutlich, S. Mondal, M. Mukherjee and K.-I. Okamoto, Polyhedron, 2004, 23, 1811; H. Miyasaka, R. Clérac, T. Ishii, H. C. Chang, S. Kitagawa and M. Yamashita, J. Chem. Soc., Dalton Trans., 2002, 1528 and references therein Z. L. Lu, M. Yuan, F. Pan, S. Gao, D. Q. Zhang and D. B. Zhu, Inorg. Chem., 2006, 45, 3538.
19. H. Weitzel, Z. Kristallogr., 1976, 144, 238 (ICSD code for MnNb2O6: 31944) L. M. Kuznetsov, A. N. Tsvigunov and K. P. Burdina, Geokhimiya, 1979, 254 (ICSD code for Mn3O4: 30005).

---

Footnote

† Electronic supplementary information (ESI) available: Plots of TGA and XRD of the phase after being heated to 950 °C. CCDC reference number 762244. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c004119j

---