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A hexadecameric copper(II) phosphonate†

Vadapalli Chandrasekhar* and Loganathan Nagarajan

Received 19th March 2009, Accepted 7th July 2009First published as an Advance Article on the web 15th July 2009DOI: 10.1039/b905456a

The first example of a hexadecanuclear copper(II) phospho-nate containing pyrazole, hydroxide and acetate ancillaryligands has been assembled in a reaction involving copper(II)nitrate, pyrazole, t-butylphosphonic acid and triethylamine.

Transition metal phosphonates possessing extended structures arequite well-known.1 In contrast, molecular metal phosphonateshave been attracting interest only in recent years.2 One of thedifficulties in this area has been the lack of suitable preparativeprocedures that allow isolation of discrete molecular entities incontrast to the more preferred extended structures. To a largeextent the synthesis problem can be addressed by a choice oflipophilic phosphonic acids in combination with other ancillaryligands.2 Our efforts in this area allowed us to assemble copper(II)phosphonates with varying nuclearity.2c,2d,3 However, in spite ofseveral attempts we have been unable to increase the size ofthese Cu(II) ensembles beyond twelve. By adopting a multi-component synthesis protocol we have now been able to isolatethe first example of a hexadecameric copper(II) phosphonatecage. The synthesis of such large-sized metal aggregates is also ofinterest from the view point of assembling molecular compoundspossessing nano dimensions.4

The reaction of Cu(NO3)2·3H2O with pyrazole (PzH) andt-butylphosphonic acid (t-BuPO3H2) in the presence of triethy-lamine afforded a blue solid which upon recrystallization fromacetonitrile/ethylacetate afforded blue crystals of {[Cu8(Pz)4(m3-OH)2(m-OH)2(t-BuPO3)3(CH3COO)2(CH3CN)]2·(EtOAc)2} (1)(Scheme 1).‡ Compound 1 crystallizes in a triclinic P-1 spacegroup§ whose asymmetric unit contains half the molecule.5a

Additionally two ethylacetate molecules are also present in thecrystal lattice. The acetate ligands present in 1 are generated in situat room temperature presumably by a metal-assisted hydrolysisof ethylacetate. Although such a hydrolysis is quite unusual it isnot unprecedented. Recently, Mukherjee and co-workers havedemonstrated the hydrolysis of ethylacetate using copper(II)complexes.5b

Compound 1 is a hexadecameric ellipsoid cage (Fig. 1). Thecage structure of 1 results from the fusion of two distinct subunits:a central decameric part whose top and bottom are capped bytwo identical trimeric units. The central decamer itself contains aCu2 unit (Cu7 and Cu7*) connected to two tetrameric Cu4 unitson either end (Cu1, Cu5*, Cu6*, Cu8 and Cu1*, Cu5, Cu6, Cu8*).The two central copper atoms are connected to each other by two

Department of Chemistry, Indian Institute of Technology, Kanpur, 208 016,India. E-mail: vc@iitk.ac.in; Fax: (+91) 512 259 0007/7436; Tel: (+91)512 259 7259† Electronic supplementary information (ESI) available: Experimental de-tails, additional schemes and figures (Fig. S1–S14, Tables S1–S3, SchemesS1–S3. CCDC reference number 720270. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/b905456a

Scheme 1 (a) Synthesis of 1. (b) Different binding modes of ligandsin 1.5c

[t-BuPO3]2- ligands from the front and back in a staggered manner.These phosphonate ligands also assist in fusing the central Cu2

part with the two tetrameric units on either side. The latter alsocontain bridging hydroxide ligands for structural support whiletwo pyrazole ligands assist in stitching a pair of copper centerseach.

The central decameric part of 1 is connected on top and bottomby two trimeric units, each of which contain a spirocyclic copperatom (Cu3 and Cu3*) connected on either side to two copper(II)centers (Cu2, Cu4 and Cu2*, Cu4*) by bridging pyrazolate andhydroxide ligands (Scheme 1, Fig. 1). The entire trimeric unit isfused with the central decamer through phosphonate as well asbridging acetate ligands. The latter connect Cu1 (Cu1*) and Cu5(Cu5*). Interestingly, the solvent acetonitrile acts as a bridgingligand between Cu4 (Cu4*) and Cu5 (Cu5*).6 The moleculardimensions of the hexadecameric cage as estimated by measuring

6712 | Dalton Trans., 2009, 6712–6714 This journal is © The Royal Society of Chemistry 2009

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Fig. 1 Molecular structure of 1. Carbon atoms of pyrazole, acetonitrile,carboxylates and phosphonic acid ligands have been removed for clarity.Dashed lines show the presence of O–H–O hydrogen bonding between thehydroxide and phosphonate ligands.

the distance between end-end surface hydrogen atoms are 20 ¥ 15 ¥10 A3. The inter-copper distances in 1 are: Cu3-Cu3*, 12.587(3);Cu2-Cu2* 12.541(3) and Cu4-Cu4* 12.678(3) A. Even if oneconsiders the core structure of 1 only, it can be seen that it containsseveral symmetrically related ring systems whose size varies fromsixteen to four.5a The overall core structure of 1 also reveals that itpossesses an approximate chair shape (Fig. 2). The sixteen copperatoms present in 1 can be classified into five types dependingon their coordination environment and geometry.5a Three typesof copper atoms possess a coordination number of five (Cu4,Cu5, Cu4* and Cu5* (2 N, 3O); Cu6, Cu8, Cu6* and Cu8*(1 N, 4O); Cu7 and Cu7* (5O)) in a square-pyramidal geometry.The other two types of copper centers (Cu3 and Cu3* (2 N, 2O);Cu1, Cu2, Cu1* and Cu2* (1 N, 3O)) are four-coordinate in asquare-planar geometry. The coordination modes of the various

Fig. 2 Chair-shaped core present in cage 1.

ligands involved in assembling 1 are summarized in Scheme 1.The phosphonate ligand is the most diverse and binds to amaximum of four (4.221 and 4.211 mode)5c copper centers. Aninteresting aspect of the structure of 1 is the presence of intricateintramolecular hydrogen bonding between the hydroxide andphosphonate ligands which appears to be a stabilizing influence onthe hexadecameric assembly (Fig. 1; see also Fig. S8 in the ESI†).Additionally the presence of intramolecular p–p interactionsbetween two pairs of pyrazole ligands in the molecular cage(Fig. S9, ESI†).

Some ideas about the assembly of 1 can emerge by examiningstructurally analogous motifs in other copper(II) cages. We havebeen able to prepare a decanuclear copper(II) cage {[Cu5(m3-OH)2(t-BuPO3)3(2-PyPz)2(MeOH)]2·(MeOH)10·(H2O)2} whosecore structure is similar to the central decameric portion of1.3b,5a Similarly the trimeric motif present in 1 is also presentin a dodecanuclear copper(II) cage {[Cu12(m-DMPz)8(h1-DMPzH)2(m4-O)2(m3-OH)4(m3-t-BuPO3)4]·3MeOH}.3d,5a

ESI-MS analysis of 1 in acetonitrile/formic acid (80:20% and0.1% water) reveals dominant fragments due to the decameric andtrimeric units.5a It is therefore not unreasonable to suggest that 1 isformed as a result of the combination of these two structural units.We have also found evidence for other simpler building blocks inthe ESI-MS of 1.5a

Magnetic studies carried out on 1 between 350–5.0 K on aSQUID magnetometer revealed a paramagnetic behavior in thewhole range of temperature studied.5a The room temperature cmTvalue is around 2.57 cm3 K mol-1 which decreases to finally reach avalue of 1.05 cm3 K/mol at 5 K. Detailed magnetic analysis of thedata has not been attempted in view of the complex topology ofthe multi-copper core and the multiple interaction pathways thatexist between the various copper (II) centers in 1.

In conclusion, we have assembled the first example of ahexadecameric copper(II) phosphonate cage. The cage structureis supported by other ligands such as hydroxides, acetates, andpyrazolates. We are currently investigating the applicability ofsuch a multi-component approach for building molecular cagescontaining other transition metal ions.

Acknowledgements

We are thankful to DST, India, for financial support includingsupport for a CCD X-ray Diffractometer facility at IIT-Kanpur.LN thanks CSIR, New Dehli, India and IIT-Kanpur for a researchfellowship. VC is thankful to the Department of Science andTechnology for a J. C. Bose fellowship. VC is a Lalit Kapoor ChairProfessor at the Indian Institute of Technology Kanpur, India.

Notes and references

‡ Synthesis: Cu(NO3)2·3H2O (0.185 g, 0.764 mmol) was dissolved inacetonitrile (30 mL) to which was added a solution of pyrazole (0.026 g,0.382 mmol), t-BuPO3H2 (0.053 g, 0.382 mmol) and triethylamine(0.25 mL, 0.790 mmol) in acetonitrile (20 mL). The reaction mixturewas stirred at room temperature for 16 h. The resulting deep-blue clearsolution was filtered and evaporated to obtain a blue-colored solid whichupon crystallization from the mixture of acetonitrile/ethylacetate affordedblue-colored crystals of 1. Yield 0.034 g (25%). IR (KBr, n, cm-1): 3472.3(s, br), 2965.39 (m) 2948.03 (m), 2904.4 (w), 2866.7 (s), 2484.8 (w, br),1739.8 (w), 1705.0 (w), 1567.0 (vs), 1480.9 (m), 1436.1 (s), 1382.1 (s),1284.1 (s), 1175.3 (s), 1124.6 (s), 1054.23 (vs), 964.2 (m), 947.3 (m),

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831.5 (s), 757.3 (s), 677.0 (s), 624.2 (m), 527.1 (s). ESI-MS: 1452.09,C30H58Cu8N8O20P3 {(M/2)-CH3CN + CH3COOH + H2O + 1}+. TGA:weight loss, % (@ T, ◦C) 42% (@ 281 ◦C, loss of four acetate, twoacetonitrile, two pyrazole and two [Cu3(OH)2(Pz)2] molecules); 17% (@423 ◦C) and 10% of residue remains until 650 ◦C.§ Formula = C60H104Cu16N18O34P6, Mol. weight = 2824.07, T = 153(2) K,l = 0.71069 A, triclinic, P-1, V = 2685.6(16) A3, a = 13.714(5), b =13.929(5), c = 15.394(5) A, a = 76.035(5), b = 70.235(5), g = 85.334(5)◦,Z =1, r = 1.746 mg m-3, m = 3.260 mm-1, F(000) = 1416, q = 2.09to 25.25, limiting indices = -11 � h � 16, -16 � k � 16, -17 � l � 18,reflections collected/unique = 14442/9584 [R(int) = 0.0381], completeness(to q = 25.25) = 98.5%, refinement method = full-matrix least-squares onF 2, data/restraints/parameters = 9584/4/632, GOOF = 1.017, final Rindices [I > 2s(I)] R1 = 0.0595, wR2 = 0.1298, R indices (all data) R1 =0.0827, wR2 = 0.1394. Largest diff. peak and hole 1.452 (near Cu atom)and -0.633 e A-3.

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