The polyoxometallate [(CH₃)₄N]₂[Mo₄O₁₀(OCH₃)₄Cl₂]

The crystal and molecular structure of bis(tetra­methyl­ammonium) di­chloro­tetra-[mu]₂-methoxo-di-[mu]₂-oxo-octo­oxo­tetra­molybdate(VI), (C₄H₁₂N)₂[Mo₄O₁₀(OCH₃)₄Cl₂], has been determined from X-ray diffraction data. The crystallographically centrosymmetric anion is built up of four edge-sharing octahedra, two MoO₆ and two MoO₅Cl.

S1. Comment

During a continuing investigation of the structure–mechanism–function relationship regarding the role of molybdenum as a catalyst in the esterification reactions of 2-mercaptonicotinic acid (Cindrić et al., 1998.), we obtained the tetramethylammonium salt of dichloro-di-µ₂-oxo-tetra-µ₂-methoxo-octaoxotetramolybdate(VI), [(CH₃)₄N]₂[Mo₄O₁₀(OCH₃)₄Cl₂]. The same anion has already been observed in the complex (ⁿBu₄N)₂[Mo₄O₁₀(OCH₃)₄Cl₂] (Liu et al., 1987; Kang et al., 1989). The tetranuclear unit is by far the most common compositional motif in the coordination chemistry of polyoxomolybdates, as adopted by [Mo₄O₁₀(OMe)₆]²⁻ (Liu et al., 1987; Kang et al., 1989), with four edge-sharing octahedra in the compact cluster. Such structures illustrate a common feature of the chemistry of polymolybdates in alcoholic solvents, i.e. the incorporation of alkoxy groups into the cluster. Formation of the underivatized polyoxomolybdate parent structure, [Mo₄O₁₆]⁸⁻, is most likely precluded by the high negative charge. Replacement of bridging oxo groups by alkoxy ligands serves to reduce the overall cluster charge and hence to stabilize the unit in alcoholic solvents. Thus, the same core structure with replacement of peripheral and/or bridging alkoxy groups is common to the structures of [Mo₄O₁₀(OCH₃)₄Cl₂]²⁻, [Mo₄O₁₀(OCH₃)₂(OC₆H₄O)₂]²⁻ (Kang et al., 1989) and [Mo₄O₈(OC₂H₅)₂{RC(CH₂O)₃}₂] (Wilson et al., 1983).

The crystal structure of the title complex, (I), is built up of tetranuclear [Mo₄O₁₀(OCH₃)₄Cl₂]²⁻ anions and tetramethylammonium cations. The centrosymmetric anion consists of four edge-sharing octahedra, two MoO₆ and two MoO₅Cl. As a result of displacement of metal ions towards the polyanion surface, all four octahedra are distorted. The Mo2 site displays an [MoO₆] geometry through ligation by two terminal and one bridging oxo groups and two triply bridging and one doubly bridging methoxo group, while the Mo1 centre displays [MoO₅Cl] coordination by one doubly bridging and one triply bridging methoxy group, one bridging and two terminal oxo groups, and the terminal chloride ligand. All bond lengths and angles are comparable with those observed in previously mentioned complexes (Table 1).

S2. Experimental

In an attempt to prepare a molybdenum(VI) complex with methyl ester of 2-hydroxynicotinic acid, a mixture of MoO₂Cl₂ (0.4 g), [(CH₃)₄N]Cl (0.22 g) and methanol suspension of 2-hydroxynicotinic acid (0.31 g in 10 ml MeOH) was dissolved in methanol (20 ml) and heated under reflux for 4 h. The resulting colourless solution was left at room temperature. After 2 d, transparent colourless crystals (0.17 g; 38.6% yield) were isolated. Elemental analysis (%) found (calculated) for [(CH₃)₄N]₂[Mo₄O₁₀(OCH₃)₄Cl₂]: Mo 43.6 (43.3), C 16.6 (16.3), H 4.2% (4.1%).

S3. Refinement

All H atoms were fixed geometrically.

Computing details

Data collection: COLLECT (Hooft, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); software used to prepare material for publication: SHELXL97.

tetramethylammonium di-chloro-di-µ₂-oxo-tetra-µ₂-methoxotetradioxo -tetramolybdate(VI)

Crystal data

(C4H12N)2[Mo4O10(OCH3)4Cl2]	F(000) = 872
Mr = 887.09	Dx = 2.030 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 8.810 (2) Å	Cell parameters from 6079 reflections
b = 11.047 (2) Å	θ = 2.7–28.2°
c = 15.085 (3) Å	µ = 1.93 mm^−1
β = 98.67 (3)°	T = 293 K
V = 1451.4 (5) Å^3	Prism, colourless
Z = 2	0.38 × 0.36 × 0.35 mm

Data collection

Nonius KappaCCD diffractometer	3048 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.016
Graphite monochromator	θmax = 28.2°, θmin = 2.7°
2° ϕ and ω scans	h = −11→11
6079 measured reflections	k = −13→14
3568 independent reflections	l = −20→20

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.031	H-atom parameters constrained
wR(F^2) = 0.079	w = 1/[σ^2(Fo^2) + (0.0358P)^2 + 0.9758P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max = 0.034
3568 reflections	Δρmax = 0.43 e Å^−3
161 parameters	Δρmin = −0.68 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0035 (4)

Special details

Experimental. Lattice constants were determined from all reliable data. Crystal to detector distance was 28 mm, with exposure time of 50 s per frame.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mo1	0.85020 (3)	0.28376 (2)	0.047340 (17)	0.03755 (10)
Mo2	1.07432 (3)	0.50911 (2)	0.122172 (16)	0.03626 (10)
Cl	0.61200 (9)	0.27307 (8)	−0.05835 (6)	0.0496 (2)
N	0.6067 (3)	0.5861 (3)	0.33865 (19)	0.0502 (7)
O11	0.8838 (3)	0.1330 (2)	0.04660 (17)	0.0549 (6)
O12	0.7500 (3)	0.3075 (2)	0.13316 (16)	0.0531 (6)
O21	1.2545 (3)	0.5083 (2)	0.17934 (17)	0.0560 (7)
O22	0.9586 (3)	0.5359 (2)	0.20005 (16)	0.0554 (6)
O1	1.0483 (2)	0.3400 (2)	0.10084 (14)	0.0419 (5)
O2	0.8523 (2)	0.48359 (18)	0.01202 (13)	0.0326 (4)
O3	0.9444 (2)	0.3170 (2)	−0.07902 (14)	0.0394 (5)
C1	0.7212 (3)	0.5590 (3)	0.0216 (2)	0.0435 (7)
H11	0.7503	0.6427	0.0207	0.065*
H12	0.6865	0.5410	0.0775	0.065*
H13	0.6399	0.5432	−0.0270	0.065*
C2	0.9790 (6)	0.2199 (4)	−0.1373 (3)	0.0677 (12)
H21	0.8991	0.2145	−0.1879	0.101*
H22	0.9858	0.1448	−0.1048	0.101*
H23	1.0752	0.2361	−0.1576	0.101*
C3	0.5916 (8)	0.7075 (5)	0.2978 (4)	0.121 (3)
H31	0.5782	0.7663	0.3429	0.181*
H32	0.5043	0.7091	0.2514	0.181*
H33	0.6827	0.7263	0.2727	0.181*
C4	0.4626 (5)	0.5539 (5)	0.3760 (3)	0.0824 (15)
H41	0.4707	0.4727	0.3989	0.124*
H42	0.3758	0.5596	0.3293	0.124*
H43	0.4493	0.6089	0.4235	0.124*
C5	0.7404 (6)	−0.0817 (4)	−0.0893 (4)	0.102 (2)
H51	0.7515	−0.0013	−0.0651	0.152*
H52	0.8316	−0.1035	−0.1134	0.152*
H53	0.7252	−0.1376	−0.0427	0.152*
C6	0.6249 (5)	0.4915 (4)	0.2705 (3)	0.0653 (11)
H61	0.7199	0.5040	0.2480	0.098*
H62	0.5410	0.4967	0.2220	0.098*
H63	0.6254	0.4129	0.2977	0.098*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mo1	0.03280 (15)	0.03919 (17)	0.04078 (16)	−0.00940 (10)	0.00589 (11)	−0.00099 (11)
Mo2	0.02949 (15)	0.04784 (18)	0.03062 (15)	−0.00999 (11)	0.00182 (10)	−0.00211 (10)
Cl	0.0359 (4)	0.0569 (5)	0.0541 (5)	−0.0137 (4)	0.0010 (3)	−0.0036 (4)
N	0.0528 (16)	0.0411 (15)	0.0499 (15)	0.0088 (13)	−0.0143 (13)	−0.0040 (13)
O11	0.0548 (14)	0.0420 (13)	0.0655 (16)	−0.0067 (11)	0.0014 (12)	0.0004 (11)
O12	0.0524 (14)	0.0603 (15)	0.0497 (13)	−0.0177 (12)	0.0172 (11)	−0.0042 (11)
O21	0.0416 (13)	0.0720 (18)	0.0493 (14)	−0.0125 (11)	−0.0097 (11)	−0.0001 (12)
O22	0.0569 (15)	0.0677 (17)	0.0445 (13)	−0.0148 (13)	0.0175 (11)	−0.0081 (12)
O1	0.0349 (11)	0.0447 (13)	0.0438 (11)	−0.0046 (9)	−0.0013 (9)	0.0044 (10)
O2	0.0243 (9)	0.0385 (11)	0.0354 (10)	−0.0041 (8)	0.0058 (8)	−0.0022 (8)
O3	0.0350 (11)	0.0428 (12)	0.0416 (11)	−0.0083 (9)	0.0094 (9)	−0.0099 (9)
C1	0.0304 (15)	0.0484 (19)	0.0527 (18)	0.0023 (13)	0.0097 (13)	−0.0030 (15)
C2	0.085 (3)	0.054 (2)	0.071 (3)	−0.012 (2)	0.035 (2)	−0.022 (2)
C3	0.165 (6)	0.056 (3)	0.121 (5)	0.010 (3)	−0.043 (5)	0.033 (3)
C4	0.075 (3)	0.121 (4)	0.055 (2)	0.036 (3)	0.022 (2)	−0.008 (3)
C5	0.103 (4)	0.064 (3)	0.113 (4)	0.004 (3)	−0.067 (3)	−0.009 (3)
C6	0.054 (2)	0.080 (3)	0.065 (3)	0.006 (2)	0.023 (2)	−0.017 (2)

Geometric parameters (Å, º)

Mo1—O11	1.692 (2)	Mo2—O2^i	2.217 (2)
Mo1—O12	1.694 (2)	Mo2—O2	2.384 (2)
Mo1—O1	1.913 (2)	N—C3	1.473 (5)
Mo1—O3	2.221 (2)	N—C5^ii	1.478 (5)
Mo1—O2	2.272 (2)	N—C4	1.507 (5)
Mo1—Cl	2.4395 (12)	N—C6	1.491 (5)
Mo2—O21	1.689 (2)	O2—C1	1.449 (3)
Mo2—O22	1.694 (2)	O3—C2	1.448 (4)
Mo2—O1	1.904 (2)	C5—N^iii	1.478 (5)
Mo2—O3^i	2.027 (2)
O11—Mo1—O12	105.62 (12)	O1—Mo2—O2^i	85.73 (8)
O11—Mo1—O1	99.99 (11)	O3^i—Mo2—O2^i	72.13 (8)
O12—Mo1—O1	100.07 (11)	O21—Mo2—O2	164.68 (11)
O11—Mo1—O3	93.94 (10)	O22—Mo2—O2	89.30 (10)
O12—Mo1—O3	159.31 (11)	O1—Mo2—O2	72.60 (8)
O1—Mo1—O3	82.72 (9)	O3^i—Mo2—O2	82.53 (8)
O11—Mo1—O2	161.34 (10)	O2^i—Mo2—O2	71.91 (8)
O12—Mo1—O2	93.00 (10)	C3—N—C5^ii	110.4 (4)
O1—Mo1—O2	75.17 (8)	C3—N—C4	110.0 (4)
O3—Mo1—O2	67.72 (7)	C5^ii—N—C4	109.9 (4)
O11—Mo1—Cl	94.70 (9)	C3—N—C6	111.2 (4)
O12—Mo1—Cl	90.62 (9)	C5^ii—N—C6	109.1 (3)
O1—Mo1—Cl	158.71 (7)	C4—N—C6	106.0 (3)
O3—Mo1—Cl	80.96 (6)	Mo2—O1—Mo1	117.98 (11)
O2—Mo1—Cl	86.01 (5)	C1—O2—Mo2^i	114.64 (18)
O21—Mo2—O22	105.35 (13)	C1—O2—Mo1	120.01 (17)
O21—Mo2—O1	99.27 (11)	Mo2^i—O2—Mo1	105.27 (8)
O22—Mo2—O1	102.65 (11)	C1—O2—Mo2	116.41 (17)
O21—Mo2—O3^i	101.17 (11)	Mo2^i—O2—Mo2	108.10 (8)
O22—Mo2—O3^i	91.65 (11)	Mo1—O2—Mo2	89.22 (7)
O1—Mo2—O3^i	150.91 (9)	C2—O3—Mo2^i	121.4 (2)
O21—Mo2—O2^i	94.90 (11)	C2—O3—Mo1	122.6 (2)
O22—Mo2—O2^i	156.30 (11)	Mo2^i—O3—Mo1	114.16 (9)

Symmetry codes: (i) −x+2, −y+1, −z; (ii) x, −y+1/2, z+1/2; (iii) x, −y+1/2, z−1/2.