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A bimetallic oxide hybrid material constructed from a coordination complex polymer and molybdenum oxide subunits, [Ni(3,4′-bipyridine)2MoO4]·3H2O
Inorganica Chimica Acta 332 (2002) 79 – 86 www.elsevier.com/locate/ica
A bimetallic oxide hybrid material constructed from a coordination complex polymer and molybdenum oxide subunits, [Ni(3,4%-bipyridine)2MoO4]·3H2O Robert L. LaDuca Jr. a,*, Matthew Desiak a, Randy S. Rarig Jr. b, Jon Zubieta b,* b
a Department of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711, USA Department of Chemistry, Syracuse Uni6ersity, Room 1 -014, Syracuse, NY 13244 -4100, USA
Received 19 October 2001
Abstract The hydrothermal reaction of NiCl2·6H2O, MoO3, 3,4%-bipyridine (3,4%-bpy) and H2O in the mole ratio 1.0:1.0:2.1:1500 yields [Ni(3,4%-bpy)2MoO4]·3H2O (1·3H2O) in 80% yield. The structure of 1·3H2O consists of a three-dimensional coordination polymer + {Ni(3,4%-bpy)2}2n with entrained {MoO4}2 − tetrahedra and with water molecules of crystallization occupying channels within n the bimetallic oxide–ligand framework. Crystal data: C20H16N4O4NiMo·3H2O (1·3H2O), tetragonal P41212, a=13.1866(5) A, , c=29.458(2) A, , V = 5122.3(4) A, 3, Z=8, Dcalc =1.532 g cm − 3. © 2002 Published by Elsevier Science B.V. Keywords: Hybrid material; Coordination polymer; Bimetallic oxide; Nickel molybdate
The significant contemporary interest in solid state inorganic oxides derives from their range of physical properties which give rise to applications in catalysis, sorption, clathration, electrical conductivity, magnetism and photochemistry [1 – 5]. The discovery of new materials of this class is driven by synthesis. Although the rational design of solid state oxides remains an elusive goal [6], one useful strategy for the modification of the oxide microstructure exploits organic components as structure-directing subunits of a hybrid material [7]. One manifestation of this general approach borrows from the field of supramolecular chemistry and ‘crystal engineering’ [8–10]. As we have recently demonstrated [11 – 27], a building block approach to oxide materials combines an anionic oxomolybdate or oxovanadate component with a secondary metal – ligand coordination polymer cation. The appropriate secondary metal centers are linked through suitable polydentate ligands into extended architectures through self-assembly. Polyfunctional rod-like ligands have been demonstrated to
* Corresponding authors. Tel.: + 1-315-443 2547; fax: + 1-315-443 4070. E-mail address: [email protected] (J. Zubieta).
act as effective tethers for the construction of coordination complex polymers with diverse topologies, which reflect both the ligand geometry, as manifested in tether length, steric constraints and relative orientations of donor groups, and the coordination preferences of the metal component. The synergistic interactions of the polymeric cation substructure and the oxometalate anionic component are expressed in often structurally unique bimetallic oxide hybrid materials. As part of our investigations of organic –inorganic oxide materials, we have elaborated aspects of the structural chemistry of the copper-molybdate and nickel-molybdate families with organodiimine ligands. As an extension of these synthetic and structural studies, the consequences of ligand geometry, specifically structural modifications, which reflect changes in donor group orientations, have come under scrutiny. In this regard, the structural chemistry of 3,4%-bipyridine with Ni(II) molybdates has been investigated as a companion study to our previous reports of 4,4%-bipyridine and 3,3%-bipyridine nickel molybdates [12,28]. In this contribution, the synthesis and structure of [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) are reported and contrasted to those of the previously described [{Ni(3,3%bpy)2}2Mo4O14] and [{Ni(H2O)2(4,4%-bpy)2}2Mo8O26].
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Table 1 Crystal data and structure refinement parameters for [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) Empirical formula Formula weight Temperature (K) u (A, ) Crystal system Space group Unit cell dimension a (A, ) c (A, ) V (A, 3) Dcalc (g cm−3) v (cm−1) Reflections R1 a wR2 b a b
C20H22N4O7NiMo 585.06 293(2) 0.71073 tetragonal P41212 13.1866(5) 29.458(2) 5122.3(4) 1.532 12.71 3593 0.0608 0.1283
1. Experimental Reagents were purchased from Aldrich Chemical Co. and used without further purification. All syntheses were carried out in 23 ml polytetrafluoroethylene-lined stainless steel containers under autogenous pressure. The reactants were stirred briefly before heating. Water was distilled above 3.0 V in-housing using a Barnstead Model 525 Biopure Distilled Water Center.
1.1. Synthesis of [Ni(3,4 %-bpy)2MoO4] ·3H2O (1 ·3H2O) A solution of NiCl2·6H2O (0.10 g, 0.412 mmol), MoO3 (0.059 g, 0.409 mmol), 3,4%-dipyridine (0.133 g, 0.850 mmol) and water (10 g, 0.556 mol) was heated for 48 h at 120 °C to give green crystals of 1·3H2O in 80% yield based on molybdenum. Anal. Calc. for C20H22N4O7MoNi: C, 41.0; H, 3.76; N, 9.57. Found: C, 41.3; H, 3.52; N, 9.35%. IR (KBr pellet, cm − 1): 1625 (m), 1425 (m), 1375 (m), 937 (s), 860 (m), 810 (s), 760 (m).
R1 = Fo − Fc / Fo . wR2 = {(w(F o2−F c2)2/w(F o2)2)}1/2.
Table 2 Selected bond lengths (A, ) and bond angles (°) for 1·3H2O Bond lengths Mo(1)O(1) Mo(1)O(4) Mo(1)O(3) Mo(1)O(2) Ni(1)O(2)c 1 Ni(1)O(4) Ni(1)N(2) Ni(1)N(1) Ni(1)N(4) Ni(1)N(3)
1.731(8) 1.766(8) 1.765(8) 1.786(8) 2.019(8) 2.029(8) 2.084(10) 2.083(10) 2.116(9) 2.116(10)
Bond angles O(1)Mo(1)O(4) O(1)Mo(1)O(3) O(4)Mo(1)O(3) O(1)Mo(1)O(2) O(4)Mo(1)O(2) O(3)Mo(1)O(2) O(2)c 1Ni(1)O(4) O(2) c 1Ni(1)N(2) O(4)Ni(1)N(2) O(2) c 1Ni(1)N(1) O(4)Ni(1)N(1) N(2)Ni(1)N(1) O(2)c 1Ni(1)N(4) O(4)Ni(1)N(4) N(2)Ni(1)N(4) N(1)Ni(1)N(4) O(2) c 1Ni(1)N(3) O(4)Ni(1)N(3) N(2)Ni(1)N(3) N(1)Ni(1)N(3) N(4)Ni(1)N(3) Mo(1)O(2)Ni(1)c2 Mo(1)O(4)Ni(1)
107.0(4) 111.1(4) 111.1(4) 110.1(4) 108.4(4) 109.1(4) 176.6(3) 89.3(3) 88.6(3) 86.9(4) 90.7(4) 95.0(4) 90.4(3) 91.9(4) 178.0(4) 87.0(3) 90.6(3) 92.1(4) 92.8(4) 171.8(4) 85.3(4) 159.6(5) 158.0(5)
Symmetry transformations used to generate equivalent atoms: c 1, −x+1/2, y+1/2, −z+5/4; c 2, −x+1/2, y−1/2, −z+5/4.
1.2. X-ray crystallography Structural measurements for 1·3H2O were performed in a Bruker SMART-CCD diffractometer at a temperature of 1509 1 K, using graphite monochromated Mo Ka radiation (u(Mo Ka) =0.71073 A, ). The data were corrected for Lorentz and polarization effects and absorption using SADABS. The structures were solved by direct methods [29]. After location of the {Ni(3,4%-bpy)2MoO4} framework, the difference Fourier map revealed the presence of multiple water sites. These were modeled as partially occupied positions O5–O11, with a total population for the waters of recrystallization of approximately 3.0. This value was confirmed by thermal gravimetric analysis. In all cases, all non-hydrogen atoms were refined anisotropically. Neutral atom scattering coefficients and anomalous dispersion corrections were taken from the International Tables, Volume C [30]. All calculations were performed using the SHELXTL [31] crystallographic software packages. Crystallographic details for the structure of 1·3H2O are summarized in Table 1. Selected bond lengths and angles for 1·3H2O are given in Table 2.
2. Results and discussion Although well established for the preparation of aluminosilicates, hydrothermal techniques [32] have only recently been adopted for the preparation of a wide variety of metastable materials, including transition metal phosphates, metal organophosphonates, and complex polyoxoalkoxometalates [33]. Hydrothermal
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ligands providing the spatial extension. The structure is + framework, constructed from a 3-D {Ni(3,4%-bpy)2}2n n shown in Fig. 2(a), with molybdate units embedded in the cavities. The 3-D Ni(II)-organodiimine scaffolding is a consequence of the relative dispositions of donor groups on the ligand, which produce a tetrahedral arrangement of four neighboring Ni(II) sites around a given {Ni(3,4%-bpy)4} center, shown in Fig. 2(b). The {MoO4} tetrahedra are embedded in this framework so as to adopt an O,O% bridging mode between Ni(II) sites. As shown in Fig. 3(b), this results in a one-dimensional (1-D) nickel-molybdate chain of corner-sharing {NiO2N4} octahedra and {MoO4} tetrahedra. The coordination geometry at the Mo(VI) sites is defined by two bridging oxo-groups and two terminal
Fig. 1. A polyhedral representation of the structure of [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) viewed parallel to the crystallographic c-axis.
reactions are typically carried out in the temperature range 100–260 °C under autogenous pressure, so as to exploit the self-assembly of the product from soluble precursors. The reduced viscosity of water under these conditions enhances diffusion processes so that solvent extraction of solids and crystal growth from solution are favored. Since differential solubility problems are minimized, a variety of simple precursors as well as a number of organic and/or inorganic structure-directing agents may be introduced, from which those of appropriate size and shape may be selected for efficient crystal packing during the crystallization process. Under such nonequilibrium crystallization conditions, metastable kinetic phases rather than the thermodynamic phase are most likely isolated. While several pathways, including that resulting in the most stable phase, are available in such nonequilibrium mixtures, the kinetically favored structural evolution results from the smallest perturbations of atomic positions. Consequently, nucleation of a metastable phase may be favored. In this fashion, the hydrothermal reaction of NiCl2·6H2O, MoO3 and 3,4%-dipyridine in water afforded [Ni(3,4%-bpy)2MoO4]·3H2O (1·3H2O) as green plates in 80% yield. The infrared spectrum exhibited prominent peaks at 920 and 790 cm − 1, assigned to w(MoO) and w(NiOMo), respectively. The pattern of medium intensity peaks in the 1000– 1500 cm − 1 region was assigned to the ligand stretching frequencies. As shown in Fig. 1, the structure of 1·3H2O consists of a three-dimensional (3-D) framework of {NiN4O2} octahedra and {MoO4} tetrahedra, with 3,4%-bipyridyl
+ Fig. 2. (a) A view of the {Ni(3,4%-bpy)2}2n coordination polymer n framework. (b) The connectivity of one Ni(II) site to the four adjacent Ni(II) centers through the 3,4%-bpy ligands.
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Fig. 3. (a) A view of the structure of 1·3H2O parallel to the crystallographic a-axis, showing the cavities occupied by the water molecules of crystallization and the 1-D nickel-molybdate chains running parallel to the a- and b-axes. (b) A polyhedral representation of a {NiMoO4} chain.
oxo-groups. The six coordination at the Ni(II) positions results from four nitrogen donors from four 3,4%-bpy ligands in the equatorial plane and trans bridging oxogroups occupying the axial positions. As shown in Fig. 3, the nickel-molybdate chains run parallel to the a- and b-axes, such that alternate groups of parallel chains lie in the plane and normal to the plane of the projection, respectively. The view of Fig. 3 also reveals the channels occupied by the water molecules of crystallization. It is instructive to compare the structure of 1·3H2O to previously reported examples of nickel molybdates incorporating dipodal bipyridyl ligands to assess the structural influences of simple donor group orientations on the oxide architecture. Thus, the structure of [{Ni(3,3%-bpy)2}2Mo4O14] (2), shown in Fig. 4, consists of a layered covalent network, constructed from a + grid with two-dimensional (2-D) {Ni(3,3%-bpy)2}2n n 4− clusters occupying intralamellar cavities. {Mo4O14} The geometry about a {Ni(3,3%-bpy)4} unit of 2, shown in Fig. 4(b), may contrasted to that of 1·3H2O, illustrated in Fig. 2(b). The 3,3%-bipyridyl ligands in 2 curve above and below the layer so as to direct the nitrogen donors in a disposition which places the tethered grouping of nickel sites in a planar orientation. As noted above, in the case of 1·3H2O, the 3,4%-bpy ligands fold about a Ni(II) site so as to project a pseudotetrahedral disposition of distal donor groups. This geometry is a consequence of the bonding at each Ni(II) site by
two bipyridyl ligands employing the N-3 position and two attached through the N-4 loci. It is noteworthy that the bimetallic oxide substructure of 2 is a 2-D sheet of molybdate clusters linked through corner-sharing {NiN4O2} octahedra, while the oxide component of 1·3H2O is a 1-D chain. The structure of [{Ni(H2O)2(4,4%-bpy)2}2Mo8O26] (3) demonstrates the remarkable variability of the nickelmolybdate/organodiimine family of materials. As shown in Fig. 5, 3 possesses a 2-D covalent network with rather unexpected constituent building blocks. The nickel/ligand component, shown in Fig. 5(b), consists of a 1-D chain. Each Ni(II) site bonds to three nitrogen donors from three 4,4%-bipyridine ligands in a facial arrangement, two aqua ligands, and a bridging oxogroup from the molybdate cluster. A curious feature of the chain is the presence of a pendant pyridyl group on one of the 4,4%-bpy ligands of each Ni(II) center, a feature which prevents extension of the nickel/ligand substructure in two dimensions. The molybdate is present as the unusual o-{Mo8O26}4 − cluster, an ellipsoidal array of two octahedra and six square pyramids. Each cluster bonds to nickel sites on two adjacent chains (Fig. 5(c)) to produce a bimetallic oxide/ligand network, shown in Fig. 5(d). Not unexpectedly, 2,2%-bipyridine functions as a chelating ligand to the Ni(II) sites in [Ni(2,2%bpy)2Mo4O13] (4) [14]. As shown in Fig. 6, the structure of 4 consists of {Ni(bpy)2}2 + units bridging b-
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Fig. 4. (a) A view of the 2-D structure of [{Ni(3,3%-bpy)2}2Mo4O14] (2). (b) A polyhedral representation of the ligand bridging geometry from a central Ni site of 2 to the four adjacent in-plane Ni centers.
{Mo8O26}4 − clusters into a 1-D chain. Thus, there is no extended Ni/bipyridine substructure in this instance. The thermal decomposition of 1·3H2O exhibits a weight loss of 10% (theoretical: 9.2%) at 140– 160 °C, corresponding to the loss of the three water molecules of crystallization. A second weight loss of 54.5% (theoretical: 53.4%), corresponding to the loss of ligand, occurs at 450–490 °C to produce an amorphous gray powder. The mechanism of dehydration is dependent on topological and energetic considerations, such that the accessibility of water molecules to the crystal tunnel structure in 1·3H2O should favor a relatively low temperature for the release of water [34].
3. Conclusions A useful strategy for the modification of inorganic oxide structure combines hydrothermal techniques with the introduction of organic compounds as structure-directing subunits in the construction of organic–inorganic composite materials. In the molybdenum oxides, the organic component may be introduced so as to adopt a variety of structural roles: (a) as a charge-compensating organonitrogen cation; (b) as a ligand bound directly to the molybdenum oxide substructure; and (c) as a ligand to a secondary metal cation. In exploring organodiimines in this latter role as ligands to Ni(II) in molybdenum-based oxides, the dramatic structural differences afforded by varying the relative dispositions of
the donor nitrogen atoms are revealed. As summarized in Table 3, the ligand identity influences not only the overall covalent connectivity of the bimetallic oxide, but also the molybdate substructure and, consequently, the nickel molybdenum oxide subunit. Thus, the Ni/ molybdate/bipyridine family supports 1-, 2- and 3-D phases. The nickel/ligand substructure may be isolated polyhedra, chains, networks (2-D) or frameworks (3D), depending on the disposition of donor groups in the ligand. The nickel-molybdate substructures include clusters, chains and 2-D networks. It has now been conclusively demonstrated that ligand components can profoundly influence inorganic oxide architectures. The synthetic approach does, however, have significant limitations. The fundamental principle underlying these syntheses is that the information necessary for the self-assembly of the extended composite architectures is inherent at the molecular level of the component building blocks. While this concept, borrowed from supramolecular chemistry and ’crystal engineering’, provides a general design blueprint, structural predictability is not intrinsic to this synthetic strategy. The structures described in this work are primitive examples of hierarchically ordered materials, a characteristic, which sets natural limits on the degree of predictability. The chemistry will require continued empirical development, both to generate a structural and compositional database for these complex materials and to effect the fine-turning of structure so as to control desired properties.
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Fig. 5. (a) A polyhedral representation of the structure of [{Ni(4,4%-bpy)2(H2O)2}2Mo8O26] (3). The ligand carbon atoms have been omitted for + clarity. (b) The 1-D {Ni(4,4%-bpy)2(H2O)2}2n substructure of 3. (c) The {Ni2Mo8O26} cluster, which represents the bimetallic oxide component n of 3. (d) The 2-D network, which results from the covalent connectivity of 3. The pendant 4,4%-bpy arms have been omitted for clarity.
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Fig. 5. (Continued)
Fig. 6. The 1-D structure of [Ni(2,2%-bpy)2Mo4O13] (4).
Table 3 Comparison of structural features for nickel molybdates incorporating dipodal organodiimine ligands Compound
Overall covalent connectivity
Ni/ligand substructure
Molybdate substructure
Ni/molybdate substructure
[{Ni(H2O)2(4,4%-bpy)2}2Mo8O26] [{Ni(3,3%-bpy)2}2Mo4O14] [Ni(3,4%-bpy)2MoO4] [Ni(2,2%-bpy)2Mo4O13]
2-D 2-D 3-D 1-D
chain layer 3-D framework {NiN4O2} octahedron
o-{Mo8O26}4− cluster {Mo4O14}4− cluster {MoO4}2− tetrahedron b-{Mo8O26}4− cluster
{NiMo8O26} cluster {Ni2Mo4O14} network {NiMoO4} chains {Ni2Mo4O13} chains
4. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 164856 for compound 1·3H2O. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2, 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk). See http://www.rsc.org/?data/dt/b0/ for crystallographic data in CIF format.
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Atomic positional parameters, full tables of bond lengths and angles and anisotropic temperature factors are available from the authors on request.
Acknowledgements This work was supported by a grant from the National Science Foundation (CHE9987471).
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[14] P.J. Zapf, C.J. Warren, R.C. Haushalter, J. Zubieta, Chem. Commun. (1997) 1543. [15] D. Hagrman, R.C. Haushalter, J. Zubieta, Chem. Mater. 10 (1998) 361. [16] P.J. Zapf, R.P. Hammond, R.C. Haushalter, J. Zubieta, Chem. Mater. 10 (1998) 1366. [17] D. Hagrman, C.J. Warren, R.C. Haushalter, R.S. Rarig Jr., K.M. Johnson III, R.L. LaDuca Jr., J. Zubieta, Inorg. Chem. 37 (1998) 3411. [18] D. Hagrman, P.J. Zapf, J. Zubieta, Chem. Commun. (1998) 12830. [19] D. Hagrman, R.P. Hammond, R.C. Haushalter, J. Zubieta, Chem. Mater. 10 (1998) 3294. [20] D. Hagrman, P.J. Hagrman, J. Zubieta, Angew. Chem., Int. Ed. Engl. 38 (1999) 3165. [21] D. Hagrman, J. Zubieta, C. R. Acad. Sci. Paris Serie IIc —Chimie 3 (2000) 231. [22] R.L. LaDuca Jr., C. Brodkin, R.C. Finn, J. Zubieta, Inorg. Chem. Commun. 3 (2000) 248. [23] P.J. Hagrman, J. Zubieta, Inorg. Chem. 39 (2000) 5218. [24] D.E. Hagrman, J. Zubieta, J. Solid State Chem. 152 (2000) 141. [25] D. Hagrman, P.J. Hagrman, J. Zubieta, Comments Inorg. Chem. 21 (1999) 225 (and references therein). [26] D.J. Chesnut, D. Hagrman, P.J. Zapf, R.P. Hammond, R.L. LaDuca Jr., R.C. Haushalter, J. Zubieta, Coord. Chem. Rev. 190-192 (1999) 737 (and references therein). [27] D. Hagrman, J. Zubieta, Trans. ACA 33 (1998) 109 (and references therein). [28] R.L. LaDuca Jr., M. Desciak, M. Laskoski, R.S. Rarig Jr., J. Zubieta, J. Chem. Soc., Dalton Trans. (2000) 2255. [29] SHELXTL, PC™ Siemens Analytical X-Ray Instruments, Madison, WI, 1993. [30] International Tables for Crystallography, vol. C, Tables 4.2.6.8 and 6.1.1.4. [31] G.M. Sheldrick, SHELXTL, Structure Determination Programs, Version 5.0, PC™ Siemens Analytical Systems, Madison, WI, 1994. [32] J. Gopalakrishnan, Chem. Mater. 7 (1995) 1265. [33] M.I. Khan, J. Zubieta, Prog. Inorg. Chem. 43 (1995) 1. [34] S. Petit, G. Coquerel, Chem. Mater. 8 (1996) 2247.
A bimetallic oxide hybrid material constructed from a coordination complex polymer and molybdenum oxide subunits, [Ni(3,4%-bipyridine)2MoO4]·3H2O Robert L. LaDuca Jr. a,*, Matthew Desiak a, Randy S. Rarig Jr. b, Jon Zubieta b,* b
a Department of Chemistry and Physics, King’s College, Wilkes-Barre, PA 18711, USA Department of Chemistry, Syracuse Uni6ersity, Room 1 -014, Syracuse, NY 13244 -4100, USA
Received 19 October 2001
Abstract The hydrothermal reaction of NiCl2·6H2O, MoO3, 3,4%-bipyridine (3,4%-bpy) and H2O in the mole ratio 1.0:1.0:2.1:1500 yields [Ni(3,4%-bpy)2MoO4]·3H2O (1·3H2O) in 80% yield. The structure of 1·3H2O consists of a three-dimensional coordination polymer + {Ni(3,4%-bpy)2}2n with entrained {MoO4}2 − tetrahedra and with water molecules of crystallization occupying channels within n the bimetallic oxide–ligand framework. Crystal data: C20H16N4O4NiMo·3H2O (1·3H2O), tetragonal P41212, a=13.1866(5) A, , c=29.458(2) A, , V = 5122.3(4) A, 3, Z=8, Dcalc =1.532 g cm − 3. © 2002 Published by Elsevier Science B.V. Keywords: Hybrid material; Coordination polymer; Bimetallic oxide; Nickel molybdate
The significant contemporary interest in solid state inorganic oxides derives from their range of physical properties which give rise to applications in catalysis, sorption, clathration, electrical conductivity, magnetism and photochemistry [1 – 5]. The discovery of new materials of this class is driven by synthesis. Although the rational design of solid state oxides remains an elusive goal [6], one useful strategy for the modification of the oxide microstructure exploits organic components as structure-directing subunits of a hybrid material [7]. One manifestation of this general approach borrows from the field of supramolecular chemistry and ‘crystal engineering’ [8–10]. As we have recently demonstrated [11 – 27], a building block approach to oxide materials combines an anionic oxomolybdate or oxovanadate component with a secondary metal – ligand coordination polymer cation. The appropriate secondary metal centers are linked through suitable polydentate ligands into extended architectures through self-assembly. Polyfunctional rod-like ligands have been demonstrated to
* Corresponding authors. Tel.: + 1-315-443 2547; fax: + 1-315-443 4070. E-mail address: [email protected] (J. Zubieta).
act as effective tethers for the construction of coordination complex polymers with diverse topologies, which reflect both the ligand geometry, as manifested in tether length, steric constraints and relative orientations of donor groups, and the coordination preferences of the metal component. The synergistic interactions of the polymeric cation substructure and the oxometalate anionic component are expressed in often structurally unique bimetallic oxide hybrid materials. As part of our investigations of organic –inorganic oxide materials, we have elaborated aspects of the structural chemistry of the copper-molybdate and nickel-molybdate families with organodiimine ligands. As an extension of these synthetic and structural studies, the consequences of ligand geometry, specifically structural modifications, which reflect changes in donor group orientations, have come under scrutiny. In this regard, the structural chemistry of 3,4%-bipyridine with Ni(II) molybdates has been investigated as a companion study to our previous reports of 4,4%-bipyridine and 3,3%-bipyridine nickel molybdates [12,28]. In this contribution, the synthesis and structure of [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) are reported and contrasted to those of the previously described [{Ni(3,3%bpy)2}2Mo4O14] and [{Ni(H2O)2(4,4%-bpy)2}2Mo8O26].
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Table 1 Crystal data and structure refinement parameters for [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) Empirical formula Formula weight Temperature (K) u (A, ) Crystal system Space group Unit cell dimension a (A, ) c (A, ) V (A, 3) Dcalc (g cm−3) v (cm−1) Reflections R1 a wR2 b a b
C20H22N4O7NiMo 585.06 293(2) 0.71073 tetragonal P41212 13.1866(5) 29.458(2) 5122.3(4) 1.532 12.71 3593 0.0608 0.1283
1. Experimental Reagents were purchased from Aldrich Chemical Co. and used without further purification. All syntheses were carried out in 23 ml polytetrafluoroethylene-lined stainless steel containers under autogenous pressure. The reactants were stirred briefly before heating. Water was distilled above 3.0 V in-housing using a Barnstead Model 525 Biopure Distilled Water Center.
1.1. Synthesis of [Ni(3,4 %-bpy)2MoO4] ·3H2O (1 ·3H2O) A solution of NiCl2·6H2O (0.10 g, 0.412 mmol), MoO3 (0.059 g, 0.409 mmol), 3,4%-dipyridine (0.133 g, 0.850 mmol) and water (10 g, 0.556 mol) was heated for 48 h at 120 °C to give green crystals of 1·3H2O in 80% yield based on molybdenum. Anal. Calc. for C20H22N4O7MoNi: C, 41.0; H, 3.76; N, 9.57. Found: C, 41.3; H, 3.52; N, 9.35%. IR (KBr pellet, cm − 1): 1625 (m), 1425 (m), 1375 (m), 937 (s), 860 (m), 810 (s), 760 (m).
R1 = Fo − Fc / Fo . wR2 = {(w(F o2−F c2)2/w(F o2)2)}1/2.
Table 2 Selected bond lengths (A, ) and bond angles (°) for 1·3H2O Bond lengths Mo(1)O(1) Mo(1)O(4) Mo(1)O(3) Mo(1)O(2) Ni(1)O(2)c 1 Ni(1)O(4) Ni(1)N(2) Ni(1)N(1) Ni(1)N(4) Ni(1)N(3)
1.731(8) 1.766(8) 1.765(8) 1.786(8) 2.019(8) 2.029(8) 2.084(10) 2.083(10) 2.116(9) 2.116(10)
Bond angles O(1)Mo(1)O(4) O(1)Mo(1)O(3) O(4)Mo(1)O(3) O(1)Mo(1)O(2) O(4)Mo(1)O(2) O(3)Mo(1)O(2) O(2)c 1Ni(1)O(4) O(2) c 1Ni(1)N(2) O(4)Ni(1)N(2) O(2) c 1Ni(1)N(1) O(4)Ni(1)N(1) N(2)Ni(1)N(1) O(2)c 1Ni(1)N(4) O(4)Ni(1)N(4) N(2)Ni(1)N(4) N(1)Ni(1)N(4) O(2) c 1Ni(1)N(3) O(4)Ni(1)N(3) N(2)Ni(1)N(3) N(1)Ni(1)N(3) N(4)Ni(1)N(3) Mo(1)O(2)Ni(1)c2 Mo(1)O(4)Ni(1)
107.0(4) 111.1(4) 111.1(4) 110.1(4) 108.4(4) 109.1(4) 176.6(3) 89.3(3) 88.6(3) 86.9(4) 90.7(4) 95.0(4) 90.4(3) 91.9(4) 178.0(4) 87.0(3) 90.6(3) 92.1(4) 92.8(4) 171.8(4) 85.3(4) 159.6(5) 158.0(5)
Symmetry transformations used to generate equivalent atoms: c 1, −x+1/2, y+1/2, −z+5/4; c 2, −x+1/2, y−1/2, −z+5/4.
1.2. X-ray crystallography Structural measurements for 1·3H2O were performed in a Bruker SMART-CCD diffractometer at a temperature of 1509 1 K, using graphite monochromated Mo Ka radiation (u(Mo Ka) =0.71073 A, ). The data were corrected for Lorentz and polarization effects and absorption using SADABS. The structures were solved by direct methods [29]. After location of the {Ni(3,4%-bpy)2MoO4} framework, the difference Fourier map revealed the presence of multiple water sites. These were modeled as partially occupied positions O5–O11, with a total population for the waters of recrystallization of approximately 3.0. This value was confirmed by thermal gravimetric analysis. In all cases, all non-hydrogen atoms were refined anisotropically. Neutral atom scattering coefficients and anomalous dispersion corrections were taken from the International Tables, Volume C [30]. All calculations were performed using the SHELXTL [31] crystallographic software packages. Crystallographic details for the structure of 1·3H2O are summarized in Table 1. Selected bond lengths and angles for 1·3H2O are given in Table 2.
2. Results and discussion Although well established for the preparation of aluminosilicates, hydrothermal techniques [32] have only recently been adopted for the preparation of a wide variety of metastable materials, including transition metal phosphates, metal organophosphonates, and complex polyoxoalkoxometalates [33]. Hydrothermal
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ligands providing the spatial extension. The structure is + framework, constructed from a 3-D {Ni(3,4%-bpy)2}2n n shown in Fig. 2(a), with molybdate units embedded in the cavities. The 3-D Ni(II)-organodiimine scaffolding is a consequence of the relative dispositions of donor groups on the ligand, which produce a tetrahedral arrangement of four neighboring Ni(II) sites around a given {Ni(3,4%-bpy)4} center, shown in Fig. 2(b). The {MoO4} tetrahedra are embedded in this framework so as to adopt an O,O% bridging mode between Ni(II) sites. As shown in Fig. 3(b), this results in a one-dimensional (1-D) nickel-molybdate chain of corner-sharing {NiO2N4} octahedra and {MoO4} tetrahedra. The coordination geometry at the Mo(VI) sites is defined by two bridging oxo-groups and two terminal
Fig. 1. A polyhedral representation of the structure of [Ni(3,4%bpy)2MoO4]·3H2O (1·3H2O) viewed parallel to the crystallographic c-axis.
reactions are typically carried out in the temperature range 100–260 °C under autogenous pressure, so as to exploit the self-assembly of the product from soluble precursors. The reduced viscosity of water under these conditions enhances diffusion processes so that solvent extraction of solids and crystal growth from solution are favored. Since differential solubility problems are minimized, a variety of simple precursors as well as a number of organic and/or inorganic structure-directing agents may be introduced, from which those of appropriate size and shape may be selected for efficient crystal packing during the crystallization process. Under such nonequilibrium crystallization conditions, metastable kinetic phases rather than the thermodynamic phase are most likely isolated. While several pathways, including that resulting in the most stable phase, are available in such nonequilibrium mixtures, the kinetically favored structural evolution results from the smallest perturbations of atomic positions. Consequently, nucleation of a metastable phase may be favored. In this fashion, the hydrothermal reaction of NiCl2·6H2O, MoO3 and 3,4%-dipyridine in water afforded [Ni(3,4%-bpy)2MoO4]·3H2O (1·3H2O) as green plates in 80% yield. The infrared spectrum exhibited prominent peaks at 920 and 790 cm − 1, assigned to w(MoO) and w(NiOMo), respectively. The pattern of medium intensity peaks in the 1000– 1500 cm − 1 region was assigned to the ligand stretching frequencies. As shown in Fig. 1, the structure of 1·3H2O consists of a three-dimensional (3-D) framework of {NiN4O2} octahedra and {MoO4} tetrahedra, with 3,4%-bipyridyl
+ Fig. 2. (a) A view of the {Ni(3,4%-bpy)2}2n coordination polymer n framework. (b) The connectivity of one Ni(II) site to the four adjacent Ni(II) centers through the 3,4%-bpy ligands.
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Fig. 3. (a) A view of the structure of 1·3H2O parallel to the crystallographic a-axis, showing the cavities occupied by the water molecules of crystallization and the 1-D nickel-molybdate chains running parallel to the a- and b-axes. (b) A polyhedral representation of a {NiMoO4} chain.
oxo-groups. The six coordination at the Ni(II) positions results from four nitrogen donors from four 3,4%-bpy ligands in the equatorial plane and trans bridging oxogroups occupying the axial positions. As shown in Fig. 3, the nickel-molybdate chains run parallel to the a- and b-axes, such that alternate groups of parallel chains lie in the plane and normal to the plane of the projection, respectively. The view of Fig. 3 also reveals the channels occupied by the water molecules of crystallization. It is instructive to compare the structure of 1·3H2O to previously reported examples of nickel molybdates incorporating dipodal bipyridyl ligands to assess the structural influences of simple donor group orientations on the oxide architecture. Thus, the structure of [{Ni(3,3%-bpy)2}2Mo4O14] (2), shown in Fig. 4, consists of a layered covalent network, constructed from a + grid with two-dimensional (2-D) {Ni(3,3%-bpy)2}2n n 4− clusters occupying intralamellar cavities. {Mo4O14} The geometry about a {Ni(3,3%-bpy)4} unit of 2, shown in Fig. 4(b), may contrasted to that of 1·3H2O, illustrated in Fig. 2(b). The 3,3%-bipyridyl ligands in 2 curve above and below the layer so as to direct the nitrogen donors in a disposition which places the tethered grouping of nickel sites in a planar orientation. As noted above, in the case of 1·3H2O, the 3,4%-bpy ligands fold about a Ni(II) site so as to project a pseudotetrahedral disposition of distal donor groups. This geometry is a consequence of the bonding at each Ni(II) site by
two bipyridyl ligands employing the N-3 position and two attached through the N-4 loci. It is noteworthy that the bimetallic oxide substructure of 2 is a 2-D sheet of molybdate clusters linked through corner-sharing {NiN4O2} octahedra, while the oxide component of 1·3H2O is a 1-D chain. The structure of [{Ni(H2O)2(4,4%-bpy)2}2Mo8O26] (3) demonstrates the remarkable variability of the nickelmolybdate/organodiimine family of materials. As shown in Fig. 5, 3 possesses a 2-D covalent network with rather unexpected constituent building blocks. The nickel/ligand component, shown in Fig. 5(b), consists of a 1-D chain. Each Ni(II) site bonds to three nitrogen donors from three 4,4%-bipyridine ligands in a facial arrangement, two aqua ligands, and a bridging oxogroup from the molybdate cluster. A curious feature of the chain is the presence of a pendant pyridyl group on one of the 4,4%-bpy ligands of each Ni(II) center, a feature which prevents extension of the nickel/ligand substructure in two dimensions. The molybdate is present as the unusual o-{Mo8O26}4 − cluster, an ellipsoidal array of two octahedra and six square pyramids. Each cluster bonds to nickel sites on two adjacent chains (Fig. 5(c)) to produce a bimetallic oxide/ligand network, shown in Fig. 5(d). Not unexpectedly, 2,2%-bipyridine functions as a chelating ligand to the Ni(II) sites in [Ni(2,2%bpy)2Mo4O13] (4) [14]. As shown in Fig. 6, the structure of 4 consists of {Ni(bpy)2}2 + units bridging b-
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Fig. 4. (a) A view of the 2-D structure of [{Ni(3,3%-bpy)2}2Mo4O14] (2). (b) A polyhedral representation of the ligand bridging geometry from a central Ni site of 2 to the four adjacent in-plane Ni centers.
{Mo8O26}4 − clusters into a 1-D chain. Thus, there is no extended Ni/bipyridine substructure in this instance. The thermal decomposition of 1·3H2O exhibits a weight loss of 10% (theoretical: 9.2%) at 140– 160 °C, corresponding to the loss of the three water molecules of crystallization. A second weight loss of 54.5% (theoretical: 53.4%), corresponding to the loss of ligand, occurs at 450–490 °C to produce an amorphous gray powder. The mechanism of dehydration is dependent on topological and energetic considerations, such that the accessibility of water molecules to the crystal tunnel structure in 1·3H2O should favor a relatively low temperature for the release of water [34].
3. Conclusions A useful strategy for the modification of inorganic oxide structure combines hydrothermal techniques with the introduction of organic compounds as structure-directing subunits in the construction of organic–inorganic composite materials. In the molybdenum oxides, the organic component may be introduced so as to adopt a variety of structural roles: (a) as a charge-compensating organonitrogen cation; (b) as a ligand bound directly to the molybdenum oxide substructure; and (c) as a ligand to a secondary metal cation. In exploring organodiimines in this latter role as ligands to Ni(II) in molybdenum-based oxides, the dramatic structural differences afforded by varying the relative dispositions of
the donor nitrogen atoms are revealed. As summarized in Table 3, the ligand identity influences not only the overall covalent connectivity of the bimetallic oxide, but also the molybdate substructure and, consequently, the nickel molybdenum oxide subunit. Thus, the Ni/ molybdate/bipyridine family supports 1-, 2- and 3-D phases. The nickel/ligand substructure may be isolated polyhedra, chains, networks (2-D) or frameworks (3D), depending on the disposition of donor groups in the ligand. The nickel-molybdate substructures include clusters, chains and 2-D networks. It has now been conclusively demonstrated that ligand components can profoundly influence inorganic oxide architectures. The synthetic approach does, however, have significant limitations. The fundamental principle underlying these syntheses is that the information necessary for the self-assembly of the extended composite architectures is inherent at the molecular level of the component building blocks. While this concept, borrowed from supramolecular chemistry and ’crystal engineering’, provides a general design blueprint, structural predictability is not intrinsic to this synthetic strategy. The structures described in this work are primitive examples of hierarchically ordered materials, a characteristic, which sets natural limits on the degree of predictability. The chemistry will require continued empirical development, both to generate a structural and compositional database for these complex materials and to effect the fine-turning of structure so as to control desired properties.
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Fig. 5. (a) A polyhedral representation of the structure of [{Ni(4,4%-bpy)2(H2O)2}2Mo8O26] (3). The ligand carbon atoms have been omitted for + clarity. (b) The 1-D {Ni(4,4%-bpy)2(H2O)2}2n substructure of 3. (c) The {Ni2Mo8O26} cluster, which represents the bimetallic oxide component n of 3. (d) The 2-D network, which results from the covalent connectivity of 3. The pendant 4,4%-bpy arms have been omitted for clarity.
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Fig. 5. (Continued)
Fig. 6. The 1-D structure of [Ni(2,2%-bpy)2Mo4O13] (4).
Table 3 Comparison of structural features for nickel molybdates incorporating dipodal organodiimine ligands Compound
Overall covalent connectivity
Ni/ligand substructure
Molybdate substructure
Ni/molybdate substructure
[{Ni(H2O)2(4,4%-bpy)2}2Mo8O26] [{Ni(3,3%-bpy)2}2Mo4O14] [Ni(3,4%-bpy)2MoO4] [Ni(2,2%-bpy)2Mo4O13]
2-D 2-D 3-D 1-D
chain layer 3-D framework {NiN4O2} octahedron
o-{Mo8O26}4− cluster {Mo4O14}4− cluster {MoO4}2− tetrahedron b-{Mo8O26}4− cluster
{NiMo8O26} cluster {Ni2Mo4O14} network {NiMoO4} chains {Ni2Mo4O13} chains
4. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 164856 for compound 1·3H2O. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2, 1EZ, UK (fax: + 44-1223-336033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk). See http://www.rsc.org/?data/dt/b0/ for crystallographic data in CIF format.
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Atomic positional parameters, full tables of bond lengths and angles and anisotropic temperature factors are available from the authors on request.
Acknowledgements This work was supported by a grant from the National Science Foundation (CHE9987471).
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