Syntheses and Structural Characterizations of a Series of Capped Keggin Derivatives①

JIANG Qi-Fei YANG Wen-BinWU Xiao-Yuan LU Can-Zhong

(State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

1 INTRODUCTION

Polyoxometalates have been the subject of intense research for a long time because of their unusual topological properties and economically important applications in many fields such as catalysis, biology, medicine, materials science and magnetochemistry[1-3]. Although various preparation methods have been proposed to obtain such highnuclear clusters, the driving force for the formation of these species is not yet well understood, and commonly described as self-assembly. This makes it rather difficult to straightforward prepare such compounds, even in some very simple cases. Therefore,a rational synthetic method is still one of the most challenges for chemists in the POMs field. It has been demonstrated that the one-pot hydrothermal method is well suitable for the preparation and crystal growth of isopoly/heteropoly oxomealates[4,5]and inorganic-organic hybrid metal-oxo materials[6-8].

In recent years, we have been also interested in the self-assembly processes including hydrothermal synthesis of polyoxomolybdates, and isolated a series of nano-sized polyoxoanions with fascinating structural topologies[9-11]. Here we report the application of the hydrothermal technique to the Mo/V heteropoly system, which resulted in the formation of a series of capped Keggin derivatives 1～4. X-ray analyses revealed that the anions in 1～3 are two-capped Keggin derivatives with charac-teristic trans-vanadium caps, whereas the heteropoly anion in 4 is a tetra-capped α-Keggin derivative.[N(CH3)4]2Na3(NH4)2[(VVO4)MoVI8VIV4O36(VIVO)2]·13H2O (1) [NH4]7[(VVO4)MoVI8VIV4O36(VIVO)2]·7H2O (2)[HN(CH2CH2)3NH][(PO4)MoV3MoVI9O36(VIVO)2]·3[N(CH2CH2)3N]·(en)·4.5H2O (3) and [HNH2OH]-[NH4]2[(VVO4)MoVI8VIV4O36(VIVO)4]·24H2O (4)

2 EXPERIMENTAL

2.1 Materials and general procedures

All chemicals were used as purchased. All syntheses were carried out in polytetrafluoroethylene-lined stainless containers under autogenous pressure.Elemental analyses of C, H and N were performed with an EA1110 CHNS-0 CE elemental analyzer,and the contents of Na, Mo and V were determined by atomic absorption method. All IR (KBr pellet)spectra were recorded on a Nicolet Magna 750FT-IR spectrometer.

2.2 Synthesis of the compounds

[N(CH3)4]2Na3(NH4)2-[(VVO4)MoVI8VIV4O36(VIVO)2]·13H2O (1)A mixture of Na2MoO4·2H2O (0.48 g, 1.98 mmol), NH4VO3(0.12 g, 1.03 mmol), Na2HPO2·H2O (0.25 g, 1.95 mmol), (CH3)4NCl (0.34 g, 3.10 mmol) and H2O (9 mL) was sealed in a Teflon-lined stainless reactor and heated at 190 ℃ for three days.After the reaction system was slowly cooled down to room temperature, black octahedral crystals of 1 were obtained in 80% yield based on Mo. Anal.Calc. for 1: C, 4.21; H, 2.56; N, 2.45; Na, 3.03, Mo,33.61, V, 15.62%. Found: C, 4.17; H, 2.52; N, 2.56;Na, 3.01, Mo, 33.69, V, 15.55%. Characteristic IR bands for 1 (KBr pellet, ν(cm-1)): 3351(b), 3035(m),1626(w), 1481(s), 1446(w), 1284(w), 1063(m),957(vs), 863(s), 796(vs), 580(m), 51(m), 467(w).

[NH4]7[(VVO4)MoVI8VIV4O36(VIVO)2]·7H2O (2)Hydrothermal treatment of a mixture of Na2MoO4·2H2O (1.0 g, 4.13 mmol), NH4VO3(0.2 g, 1.71 mmol), Mo(CO)6(0.26 g, 0.98 mmol), CH3COONH4(0.6 g, 7.78 mmol), HCl (1.25 mL, 3.5%) and H2O (10 mL) at 160 ℃ for 4 days gives a large number of black block crystals of 2 (84% based on Mo). Anal. Calc. for 2: H, 2.07; N, 4.79; Mo, 37.47;V, 17.41%. Found: H, 2.05; N, 4.81; Mo, 37.51; V,17.36%. Characteristic IR bands for 2 (KBr pellet,ν(cm-1)): 3495(bs), 3142(s), 1618(m), 1402(vs),958(vs), 901(vs), 692(vs), 569(vs).

[HN(CH2CH2)3NH][(PO4)MoV3MoVI9O36(VIVO)2]·3[N(CH2CH2)3N]·(en)·4.5H2O (3) A mixture of Mo(CO)6(0.13 g, 0.49 mmol), V2O5(0.28 g, 1.54 mmol), N(CH2CH2)3N (0.44 g, 3.92 mmol),H2NCH2CH2NH2(0.15 mL), H3PO4(0.86 g, 85%wt) and H2O (8 mL) was sealed in a Teflon-lined stainless-steel reactor and heated at 195 ℃ for 4 days, then slowly cooled to room temperature to give deep-violet prism crystals of 3 in high yield(91% based on Mo). Anal. Calc. for 3: C, 12.26; H,2.65; N, 5.50; Mo, 45.18, V, 4.00%. Found: C,12.31; H, 2.69; N, 5.58; Mo, 45.21; V, 3.98%.Characteristic IR bands for 3 (KBr pellet, ν(cm-1)):3510(bm), 3128(m), 3070(m), 2980(m), 2802(m),2667(m), 2495(m), 1626(w), 1473(m), 1454(m),1396(m), 1362(w), 1288(w), 1182(w), 1157(w),1061(w), 1030(w), 949(vs), 843(m), 793(s), 714(m),656(m), 598)m), 536(m), 507(m), 449(w).

[HNH2OH][NH4]2[(VVO4)MoVI8VIV4O36(VIVO)4]·24H2O (4) A mixture of Na2MoO4·2H2O (1.57 g,6.49 mmol), V2O5(0.18 g, 0.99 mmol), NH2OH·HCl (1.3 g, 18.71 mmol), CH3COONH4(1.5 g,19.45 mmol), HCl (3.5%, 2 mL) and H2O (16 mL)was sealed in a Teflon-lined stainless-steel reactor and heated at 160 ℃ for 24 h, then slowly cooled to room temperature to afford black tetragonal crystals of 4 in medium yield (76% based on Mo). Anal.Calc. for 4: H, 2.49; N, 1.73; Mo, 31.55; V, 18.85%.Found: H, 2.45; N, 1.81; Mo, 31.62; V, 18.81%.Characteristic IR bands for 4 (KBr pellet, ν(cm-1)):3566(s), 3190(bs), 2819(w), 1619(m), 1402(vs),970(s), 949(vs), 850(s), 808(m), 750(m), 604(m),515(m), 453(m).

2.3 X-ray crystallography analysis

The unit cell determination and data collection for compounds 1～4 were performed on a Siemens SMART CCD diffractometer equipped with gra-phite-monochromated Mo-Kα radiation (λ =0.71073?) at room temperature. In all cases, an empirical absorption correction by SADABS[12]was applied to the intensity data. All structures were solved with direct methods by locating the molybdenum and vanadium atoms, and the remaining non-hydrogen atoms were located from subsequent Fourier syntheses, using the SHELXL-97 software package[13]. In all cases, hydrogen atoms on the carbon and nitrogen atoms were generated geometrically, whereas the hydrogen atoms of H2O, NH4+or OH-were not located. For 1, all non-hydrogen atoms, except sodium atoms and oxygen ones of water molecules, were refined by the full-matrix least-squares minimization of (∑w(Fo– Fc)2with anisotropic thermal parameters. For 2, parts of the skeleton oxygen atoms were slipped into two sites,each with partial occupancy of 0.5. For 3 and 4,only skeleton non-hydrogen atoms were refined anisotropically. Crystal parameters and other experimental details of the data collected for 1～4 are summarized in Table 1. Selected bond lengths and angles are given in Tables 2～5.

Table 1. Summary Crystallographic Data for Compounds 1～4

3 RESULTS AND DISCUSSION

3.1 Synthesis

It is well-known that under hydrothermal conditions many reagents (such as organic amines and organic solvents) can provide effective reducing electrons to reduce the metal centers[14,15]. Herein,hydrothermal reactions of Na2MoO4·2H2O and NH4VO3/V2O5with different reducing reagents (e. g.Na2HPO2·2H2O, Mo(CO)6, NH2OH·HCl and organic amines) gave rise to the formation of a series of capped-Keggin derivatives. In compounds 1, 2 and 4, all Mo atoms and the central tetrahedrallycoordinated V atom remain their highest valence states, while octahedral and square-pyramidal V atoms are reduced to be VIV. In 3, part of Mo and all V centers are reduced to lower valence states (MoVand VIV).

3.2 Structure description

[N(CH3)4]2Na3(NH4)2[(VVO4)MoVI8VIV4O36(VIVO)2] ·13H2O (1) As shown in Fig. 1, the structure of the polyoxoanion of 1, which is very similar to the units encountered in the discrete [MoVI16VIV12VV2O84]14-anion[16], can be described as follows: four MoVIcenters of an α-Keggin type [MoVI12O36(VVO4)]3-species are replaced by VIVatoms so as to produce a“l(fā)ayered” type structure in which the vanadium atoms form a central {V5} belt, and the molybdenum centers form two {Mo4} rings bonded above and below this {V5} belt and separated by 4.76 ?. It is interesting to note that each {Mo4} ring is capped by a terminal V=O group, generating a “transvanadium” capped α-Keggin type heteropoly structure (Fig. 1a). The MoVIand VIVcenters of the Keggin part have rather regular octahedral coordination environments, while the two capping V(4)atoms reside in a square pyra- midal coordination environ- ment. All Mo–O and V–O bond lengths(Table 2) are comparable with those observed in Keggin derivatives. Interestingly, heteropoly anions are linked into infinite chains (Fig. 1b) along the b axis via {Na4} clusters consisting of two NaO-(H2O)5octahedra and two Na(H2O)5square pyramids (Na–O: 2.18～2.81 ?).

Table 2. Selected Bond Lengths (?) and Bond Angles (o) for Complex 1

[NH4]7[(VVO4)MoVI8VIV4O36(VIVO)2]·7H2O (2)Similar to the heteropoly compound Na0.5K6.5[(VVO4)MoVI8VIV4O36(VIVO)2]·12.5H2O reported by Prof. Müller and co-workers[17], the crystal structure of 2 also shows the presence of polyoxoanionic chains (Fig. 2) built by “trans-vanadium” capped α-Keggin type {Mo8V7O42} units.However, all bridging oxygen (O(2), O(6)) and capping oxygen (O(4)), μ4-O atoms (O(1)) and the oxygen atom (O(8)) of the capping {VO2+} groups in 2 are statistically located on two sites, each with occupancy ratio of 0.5. The Keggin fragments in 2 are linked by V(3)–O(8)4–V(3) bridges, forming chains along the [100] direction (V(3)–O(8) 1.846 ?; separation of adjacent V(3) centers 2.842 ?).

Fig. 1. (a) Ball and stick representation of the two-capped α-Keggin type anion with stoichiometry of [(VVO4)MoVI8VIV4O36(VIVO)2]7-. Mo, olivaceous, V, purple, Na, blue, O, red. (b) Ball and stick representation of {Na4} clusters with stoichiometry of O2Na4(μ2-OH2)2(μ3-OH2)2(H2O)6; (c) Polyhedral representation of the polymeric chain formed by [(VVO4)MoVI8VIV4O36(VIVO)2]7- and {Na4} clusters(Mo and MoO6 octahedra, cyan, V and VOx polyhedra, purple, Na and NaOx polyhedra, blue, O, red)

Fig. 2. Structure of the polymeric anion in the crystals of 2: (a) Ball-and-stick representation of the two-capped Keggin type unit with stoichiometry [(VIVO4)MoVI8VIV4O36(VIVO)2]8- (Mo atoms parallel lines, V atoms cross hatched). Bridging oxygen atoms (O(2), O(6)), capping oxygen atom (O(4)), μ4-O atom (O(1)) and the oxygen(O(8)) of the capping {VO2+} group are statically located on two sites with partial occupancy of 0.5. (b) Polyhedral representation (projection down the [011] direction, V–O polyhedral pink, Mo–O polyhedra cyan)

[HN(CH2CH2)3NH][(PO4)MoV3MoVI9O36(VIVO)2]·3[N(CH2CH2)3N]·(en)·4.5H2O (3) The structure analysis performed on a crystal of compound 3 shows discrete [(PO4)MoV3MoVI9O36(VIVO)2]2-anions, [HN(CH2CH2)3NH]2+cations (one per cluster anion), lattice N(CH2CH2)3N, en and H2O molecules. As shown in Fig. 3, the structure of the cluster anion may be viewed as a Keggin isomer[(PO4)MoV3MoVI9O36]6-capped by two V=O groups in trans positions, where the Keggin fragment can be described as a shell of {Mo12O36} encapsulating a PO4tetrahedron at its center. Four oxygen atoms of the central phosphate moiety are covalently bonded in a μ4-bridging mode (P-μ4-O = 1.530～1.564 ?) to three different molybdenums of the shell. Thus, twelve Mo centers in the Keggin fragment display essentially distorted octahedral environments defined by one terminal oxo-group with short Mo–O bond lengths (1.666～1.706 ?),four doubly bridging oxo-groups with intermediate bond lengths (1.826～2.072 ?) and one μ4-briding oxygen atom with long Mo-μ4-O distances (2.437～2.549 ?). No signal was found in the EPR spectra,the lack of signals for Mo5+and V4+indicating that five electrons of the heteropolyoxoanion are delocalized. The assignment of oxidation states for vanadium and molybdenum is based on valence sum calculations performed using Brown equation[18],which give the values for Mo(1)–Mo(12) of 6.212,6.155, 5.864, 5.562, 5.726, 5.690, 5.660, 5.584,5.848, 5.809, 5.813 and 5.616, while the calculated valence sum for V(1) and V(2) is 4.271 and 4.066,respectively. The average valences for the calculated oxidation states of Mo and V are 5.78 (expected value for MoV3MoVI9is 5.75) and 4.169, respectively. The calculated results are consistent with the formula of heteropoly compound 3, and supported by the manganometric titration of MoVand VIVsites(5 reduced electrons per formula unit).

Fig. 3. (a) Keggin fragment found in the heteropoly anion with stoichiometry [(PO4)MoV3MoVI9O36(VIVO)2]2-in the crystals of 3. (b) Polyhedral representation of the heteropoly anion (Mo–O octahedra,cyan, V–O square pyramids, pink, P–O tetrahedron, blue)

[HNH2OH][NH4]2[(VVO4)MoVI8VIV4O36(VIVO)4]·24H2O (4) Similar to the Mo/V tetra-capped Keggin structure heteropoly compounds reported by Prof. Xu and co-workers[19,20], heteropoly compound 4, which crystallizes in the tetragonal space group I4/m, contains a discrete tetra-capped Keggin structural polyoxoanion [(VVO4)MoVI8VIV4O36(VIVO)4]3-based on the well-known α-Keggin structure of[XM12O40]n-with four additional square-pyramidal terminal VO2+units. The α-Keggin part in 4 may be viewed as a shell of {Mo8V4O36} encapsulating a central {VO4} moiety, in which each trimetallic group contains one V and two Mo atoms, and the central V(1) atom is surrounded by eight O oxygen atoms, each with partial occupancy of 0.5. As shown in Fig. 4, an important unusual feature of compound 4 is that the V=O groups cap the four square windows on the equatorial plane so as to produce a tetra-capped structure, in which the vanadium atoms form a central belt, and the Mo4rings are bonded above and below this belt. To our knowledge, only few tetra-capped Keggin structural hederopolyanions have been synthesized and structurally characterized[19,20]. The Mo–O bond lengths in MoO6are Mo–Ot, 1.650(8); Mo–Ob,1.874(9) ～ 1.876(9); Mo– O(μ3), 1.985(8) ～1.996(8); and Mo–O(μ4), 2.451(13)～2.455(13) ?.Three vanadiums display different coordination environments: VV(1)O tetrahedron with V–O(μ4),1.676(13) ?; VIV(2)O6octahedron with V–Ot,1.621(9), V–O(μ3), 1.957(8)～1.968(8), and V–O(μ4), 2.421(12) ?; and VIV(3) square pyramid with V–Ot, 1.663(12), V–O(μ3), 1.946(8)～1.952(8) ?.

Fig. 4. Structure of [(VIVO4)MoVI8VIV4O36(VIVO)4]4-, (a) ball-and-stick representation; (b) and(c) polyhedral representation (MoO6 octahedra, cyan, VO6 octahedra, pink, VO5 square pyramids, green, and the central VO4 tetrahedron, blue) , viewed along a and c axes, respectively

Table 3. Selected Bond Lengths (?) and Bond Angles (o) for Complex 2

Table 4. Selected Bond Lengths (?) and Bond Angles (o) for Complex 3

In contrast to the extensive researches on Keggin and transition-metal-substituted Keggin species,capped Keggin derivatives are relatively limited[16-20]. The successful synthesis and structural characterization of 1～4 expands the family member of Keggin derivatives. For compounds 1, 2 and 4, an important structural feature is that all capping vanadium atoms and the octahedral V atoms in parent Keggin part ([VVMoVI12-xVIVxO40]n-) are in reduced oxidation (VIV), while other metals (Mo and the central tetrahedral V) remain their highest oxidation states. The assignment of oxidation states for the molybdenum and vanadium atoms are in agreement with their coordination geometries and confirmed by valence bond sum (BVS) calculations.Surely, the central vanadium atom with tetrahedral coordination in these heteropoly com- pounds is of type VV, and BVS calculations give a value in the range of 5.987～6.409 for each Mo, and 3.540～4.444 for each octahedral or square- pyramidal V.

Table 5. Selected Bond Lengths (?) and Bond Angles (o) for Complex 4

In summary, we have synthesized single crystals of a series of capped Keggin derivatives under different hydrothermal conditions. X-ray analyses revealed that the anions in 1～3 are two-capped Keggin derivatives with characteristic trans vanadium-oxygen caps, whereas that in 4 is a tetracapped α-Keggin derivative with characteristic vanadium-oxygen caps located on the equatorial plane.

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