Crystal Structure and Thermal Stability Study on Tetrabutylammonium Hexamolybdate [n-Bu₄N]₂[Mo₆O₁₉](TBAM)

Abstract

The crystal structure of $[n-Bu_4N]_2[Mo_6O_{19}]$(TBAM) (n-Bu4N=tetrabutylammonium) has been determined by X-ray crystallography. It crystallizes in the monoclinic system, space group C2/c, with lattice parameters ${\alpha}$=16.314(5), b=17.288(5), c=17.776(4)${\AA}$ ${\beta}$=101.47(3), and Z=4. In $[Mo_6O{19}]^{2-}$ anion, Mo atoms occupy six vertices of octahedron and each Mo atom is coordinated by six oxygen atoms to adopt distorted octahedral coordination geometry. The average bond distance of Mo-Ot (terminal), Mo-Ob (bridged) and Mo-Oc (central) are 1.680 ${\AA}$, 1.931 ${\AA}$ and 2.325 ${\AA}$ respectively. In $[n-Bu_4N]^+$ cation, the N atom possesses a slightly distorted tetrahedral geometry. There are some potential extensive C-H ${\cdots}$ O hydrogen bonds in the lattice, by which connecte molecules and stabilize the crystal structure. Thermogravimetric analysis suggests that thermal decomposition of the title compound includes two transitions and it loses weight at 356.0 and 803.5 $^{\circ}$, respectively, and the residue presumable be $Mo_2O_2$. Accordingly, the title compound has high thermal stability.

$[n-Bu_4N]_2[Mo_6O_{19}]$(TBAM)의 결정구조는 X-ray 분석기로 결정되었다. 그 결정은 monoclinic 계이며 ${\alpha}$=16.314(5), b=17.288(5), c=17.776(4) ${\AA}$, ${\beta}$=101.47(3) 그리고 z=4의 결정파라미터를 갖는 sapce group 이 C2/c인 결정이다 $[Mo_6O{19}]^{2-}$ 음이온에서, Mo 원자는 팔면체의 여섯 개 모서리를 차지하며 각 Mo 원자는 여석개의 산소원자에 배위되어 찌그러진 팔면체 배위 기하학을 지닌다. Mo-Ot(말단기), Mo-Ob(연결된), 그리고Mo-Oc(중앙)의 평균거리는 각각 1.680 ${\AA}$, 1.931 ${\AA}$ 및 2.325 ${\AA}$ 이다. $[n-Bu_4N]^+$ 양이온에서 N원자는 약간 찌그러진 사면체 모형을 갖는다. 격자안에서 폭 넓은 C-H ${\cdots}$ O 수소결합이 있으며, 그것에 의하여 분자들을 연결하고 결정구조를 안정화 한다. 열분석에 의하여 제목의 열적분핸ㄴ 두개의 전이가 일어나며 356.0와 803.5 $^{\circ}$에서 각각 무게를 잃는다. 그리고 분해된 생성물은 $Mo_2O_2$로 추정된다. 따라서, 제목의 화합물은 높은 열적 안정성을 갖는다.

INTRODUCTION

It is well known that polyoxoanions conjugated with organic molecules have the abilitiy for photo-chromism, and that photochromism involving these compounds is truly reversible.1-7 In order to prepare rversible photochromism materials, photochromism materials, photosensitive organic molecules as donor and Type I polyoxometalate anions having easily reduced properties8 as accptor have been adopted. Such naions are still of interest because of their high electron acceptor capability.9 Xu et al has reported reversible photochromism charge-transfer salt using photosensitive tetrabutylammonium (TBA) as donor and the hex-amolybdate dianion as acceptor under ultraviolet irrdiation.10 However. the crystal structure of tetrabutylammonium hexamolybdate[n-Bu4N]2[Mo6O19](TBAM) hasnever been reported. Inthis paper, we report the crystal structure of TBAM, and the thermal stability of it.

 

EXPERAMENT SECTION

Hydrothermal synthesis of TBAM. An acetonitrile solution of the tetrabutylammonium bromide(TBABr) and isopolyoxomolybdate are mixed with stirring and its pH value was adjusted to 6 with dilution HCI, then the mixture was sealed in a 25mL stainless-steel reactor with Teflon liner at 100℃ for 72h, resulting in the formation of the light blue crystals of the title complex. Yield: 85%. Calc. for C32H72Mo6N2O19:C, 28.16%; H, 5.31%; N, 2.05%. Anal. Found: C, 28.01%; H, 4.98%; N, 1.97%

X-ray structure determinatio. The selected crytal of [n-Bu4N]2[Mo6O19] was mounted on an Rigaku Raxis-Ⅳ diffractometer. Reflection data were measured at 293K, wsing graphite monochromated M0-Kα(λ=0.71073 Å) radiation ω scan mode. Intensities were corrected for Lorentz and polarization effects and empirical absorption, and te data reduction using SADABS program.11 The structure were solved by direct methods using SHELX-97.12 All the nonhydrogen atoms were refined on F2 anistropically by full-matrix least squares method. The hydroge atom positions were fixed geometrically at calculated distances and alowed to ride on the parent carbon atoms. The contributions of these hydrogen atoms were included instructure-factor calculations. The final least-square cycle gave R=0.0334, Rw=0.0563 for 2572 reflections with I>2σ(I); the weighting scheme, w= 1/[σ2(F02)+(0.0259P)2+0.0000P], where P=(F02)+2F02)/3. Atomic scatteringfactors and anomalous dispersion corrections were taken from International Table for X-ray Crystallography.13 A summary of the key crystallographic information is given in Table 1. The final position parameters of nonhydrogen atoms are given in Table 2. Selected bond lengths(Å), possible hydrogen bonds (Å) and bond angles(°) are presented in Table 3, respectively.

Table 1.Summary of Crystallographic Results for [n-Bu4N]2[Mo6O19]

Table 2.Atomic coordinates(×104) and equivalent isotropic displacement parameters(Å2×103). U(eq) is defined as one thied of the trace of the orthogonalizad Uij tensor.

Table 3.Symmetry transformations used to generate

 

RESULT AND DISSCUSSION

Structure. The crystal structure of the complex [n-Bu4N]2[Mo6O19] consists of symmetry [Mo6O19]2- anion and two [n-Bu4N]+ cations. Fig. 1 shows a perspective view of the title compound with atomic numbeing scheme, and Fig. 2 shows a perspective view of the crystal packing in the unit cell.

Fig. 1.Molecular structure for [n-Bu4N]2[Mo6O19] with the atomic numbering scheme.

Fig. 2.A view of the crystal packing down the a axis for [n-Bu4N]2[Mo6O19]

In [Mo6O19]2- anion, six Mo atoms locate at the six vertexes of slightly distorted octahedron and nineteen oxygen atoms are divided into three categories, with one oxygen atom lying in the central of above octahedron(Oc), six oxygen atoms occupying the terminal positions of above octahedron(Ot) and twelve oxygen as birdged atoms(Ob) birdging six Moatoms, respetively. All the angels of Mo-Oc-Mo are nearly 90° or 180°. Each Mo atom possesses a distorted octahedral coordination geometry, which is coordinated by six oxygen atoms with the central atom and one terminal oxygen atom in axial positon, and four bridged oxygen atoms occupying equatorial position. As seen from the Table 3, the trans bond angles forming by terminal oxygen atom, Mo atom and central oxygen atom are close to 180° and the cis angles of O-Mo-O are nearly to 90°. Because of the existence of birdged oxygen atoms, the angles of opposite Ob-Mo-Ob are all smaller than 180° with average Ob-Mo-Ob about 153°. The average bond distance of Mo-Ot 1.680Å equates to that in TPPM[TPPM=bis(2,4,6-triphenylpyryllium) hexamolybdate], the distance Mo-Ob 1.931Å and Mo-Oc 2.325Å are longer than thse in TPPM[Mo-Ob 1.917Å and Mo-Oc 2.312 Å]10. but all these values are in the range of with those reported previously.14-15 In the slight distorted octahedral geometry of [Mo6O19]2- anion, three σh symmertrical planes were occupied by thre molecular planes, each of which contained thirteen atoms(four Mo atoms, four Ob atoms, four Ot atoms and one Oc atom) and the largest deviations of which are 0.012, 0.045 and 0.016Å, respectively. The above three molecular planes are almost vertical each other, with the dihedral 89.82, 89.90 and 89.92°, respectively.

In the [n-Bu4N]+ cation, the N atom adopts a slightly dstorted terahedral geometry with the C-N-C bond angles ranging from 108.1° to 111.5°. The C-N and C-Cbond lengths fall within the normal rang.

There are some potentially weak(C-H⋯Y hydrogen bond, Y=O) interactions in the lattice.16-17 The O(3) atom with C(6) atom in [n-Bu4N]+ cation forms potentially weak C-H⋯O intramolecular interaction, the donor and acceptor distance being 3.2600Å for C(8)-H(8B)⋯O(1)(symmetry code: 1/2x, 1/2-y,-z). The bond angles of C(6)-H(6A)⋯O(3) and C(8)-H(8B)⋯O(1) are 164.86° and 131.18°, respectively. Inthe solid state, these interactions together with electrostatic forces connected molecules and stabilize the crystal structures

Thermogravimetric analysis. The curves of the thermogravimetric(TG) analysis and differential thermal gravimetric(DTG) analysis for the title compound are shown in the Fig. 3. It can be seen that the thermal decomposition of the compound includes two transitions There are two edothermic peaks, one intense heat-absorbing peak at 356.0℃ and the other at 803.5℃. It shows no decomposition before 356.0℃; but at 356.0℃, decomposition occurs. On the base of weight changes in the TG curve, the first process of the weight loss (40.42%) corresponds to the loss of two [n-Bu4N]+ groups and four oxygen atoms of [Mo6O19]2- anions (found 40.42%, calc. 40.16%) (356.0-600.0 ℃), with an intense endothermic phenomenon; the second process of the weight loss (42.86%) is attributed to the further decomposition of the [Mo6O15] group and the residue may be Mo2O2 (found 16.41%, clac.16.72%)(600.0-803.5℃). The title compound also has high thermal stability.

Fig. 3.TG/DTG curves of [n-Bu4N]2[Mo6O19].