Tris(4-tert-butyl­phen­yl)phosphine oxide

Hao, Y.-G.; Yao, J.-C.; Li, J.-X.; He, Y.-X.; Zhang, Y.-Q.

doi:10.1107/S1600536809052155

organic compounds

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ISSN: 2056-9890

Volume 66| Part 1| January 2010| Page o211

doi:10.1107/S1600536809052155

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Tris(4-tert-butylphenyl)phosphine oxide

Yin-Ge Hao,^a Jin-Cai Yao,^b Jun-Xian Li,^a Yu-Xin He ^a and Yu-Qing Zhang ^a ^*

^aKey Laboratory of Polymer Science and Nanotechnology, Henan University of Science and Technology, Luoyang 471003, People's Republic of China, and ^bCollege of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, People's Republic of China
^*Correspondence e-mail: zhangyq8@126.com

(Received 21 November 2009; accepted 4 December 2009; online 19 December 2009)

In the title compound, C₃₀H₃₉OP, the P=O bond length is 1.4866 (12) Å and the P—C bond lengths range from 1.804 (2) to 1.808 (13) Å. The molecle is located on a crystallographic mirror plane. The methyl groups of one tert-butyl group are disordered over two sites in a 0.776 (4):0.224 (4) ratio.

Related literature

For applications of phosphine ligands in palladium-catalysed syntheses, see: Buchwald et al. (2006 ); Surry & Buchwald (2008 ); Xu et al. (2009 ). For related structures, see: Baures & Silverton (1990 ); Shawkataly et al. (2009 ). For the synthesis, see: Issleib & Brack (1954 ).

Experimental

Crystal data

C₃₀H₃₉OP
M_r = 446.58
Orthorhombic, P n m a
a = 11.7986 (10) Å
b = 20.9246 (18) Å
c = 10.5204 (9) Å
V = 2597.3 (4) Å³
Z = 4
Mo Kα radiation
μ = 0.13 mm⁻¹
T = 294 K
0.45 × 0.43 × 0.42 mm

Data collection

Bruker SMART APEX CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.946, T_max = 0.949
17327 measured reflections
2485 independent reflections
2143 reflections with I > 2σ(I)
R_int = 0.027

Refinement

R[F² > 2σ(F²)] = 0.042
wR(F²) = 0.117
S = 1.03
2485 reflections
161 parameters
18 restraints
H-atom parameters constrained
Δρ_max = 0.38 e Å⁻³
Δρ_min = −0.30 e Å⁻³

Data collection: SMART (Bruker, 2004 ); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536809052155/si2225sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536809052155/si2225Isup2.hkl

checkCIF report

Comment top

Arylphosphines are the most frequently used as ligands in transition metal catalysis (Buchwald et al., 2006; Surry & Buchwald 2008; Xu et al., 2009). While preparing tris(4-tert-butylphenyl) phosphines, we have obtained the title compound as a side product.

The title compound, C₃₀H₃₉OP, has a P=O bond length of 1.4866 (12) Å. The P—C bond lengths range from 1.804 (2) to 1.808 (13) Å. It is located on a crystallographic mirror plane. All the bond distances and angles in the structure are within normal ranges, similar to those found in the related compounds (Baures & Silverton 1990; Shawkataly et al., 2009). The methyl groups of one tert-butyl group are disordered over two sites in a 0.776 (4):0.224 (4) ratio.

Related literature top

For applications of phosphine ligands in palladium-catalysed syntheses, see: Buchwald et al. (2006); Surry & Buchwald (2008); Xu et al. (2009). For related structures, see: Baures & Silverton (1990); Shawkataly et al. (2009). For the synthesis, see: Issleib & Brack (1954).

Experimental top

The title compound was obtained as a side product from the reaction of PCl₃ and 4-C(CH₃)₃-C₆H₄—MgBr as described in the literature (Issleib & Brack 1954) and recrystallized from ethanol at room temperature to give the desired crystals suitable for single-crystal X-ray diffraction.

Computing details top

Data collection: SMART (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures top

Fig. 1. The molecular structure of the title compound with displacement ellipsoids at the 30% probability level (Symmetry code A: x, -y + 1/2, z).

Tris(4-tert-butylphenyl)phosphine oxide top

Crystal data top

C₃₀H₃₉OP	D_x = 1.142 Mg m⁻³
M_r = 446.58	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 5668 reflections
a = 11.7986 (10) Å	θ = 2.6–28.1°
b = 20.9246 (18) Å	µ = 0.13 mm⁻¹
c = 10.5204 (9) Å	T = 294 K
V = 2597.3 (4) Å³	Block, colourless
Z = 4	0.45 × 0.43 × 0.42 mm
F(000) = 968

Data collection top

Bruker SMART APEX CCD area-detector diffractometer	2485 independent reflections
Radiation source: fine-focus sealed tube	2143 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.027
phi and ω scans	θ_max = 25.5°, θ_min = 2.6°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −14→14
T_min = 0.946, T_max = 0.949	k = −25→25
17327 measured reflections	l = −12→12

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.042	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.117	H-atom parameters constrained
S = 1.03	w = 1/[σ²(F_o²) + (0.0575P)² + 1.3437P] where P = (F_o² + 2F_c²)/3
2485 reflections	(Δ/σ)_max < 0.001
161 parameters	Δρ_max = 0.38 e Å⁻³
18 restraints	Δρ_min = −0.30 e Å⁻³

Crystal data top

C₃₀H₃₉OP	V = 2597.3 (4) Å³
M_r = 446.58	Z = 4
Orthorhombic, Pnma	Mo Kα radiation
a = 11.7986 (10) Å	µ = 0.13 mm⁻¹
b = 20.9246 (18) Å	T = 294 K
c = 10.5204 (9) Å	0.45 × 0.43 × 0.42 mm

Data collection top

Bruker SMART APEX CCD area-detector diffractometer	2485 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	2143 reflections with I > 2σ(I)
T_min = 0.946, T_max = 0.949	R_int = 0.027
17327 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.042	18 restraints
wR(F²) = 0.117	H-atom parameters constrained
S = 1.03	Δρ_max = 0.38 e Å⁻³
2485 reflections	Δρ_min = −0.30 e Å⁻³
161 parameters

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq	Occ. (<1)
C1	0.77025 (14)	0.31883 (8)	0.30317 (15)	0.0298 (4)
C2	0.66436 (14)	0.33374 (8)	0.35431 (16)	0.0340 (4)
H2	0.6014	0.3093	0.3328	0.041*
C3	0.65284 (15)	0.38473 (8)	0.43698 (17)	0.0370 (4)
H3	0.5814	0.3944	0.4689	0.044*
C4	0.74507 (15)	0.42213 (8)	0.47393 (16)	0.0342 (4)
C5	0.84902 (15)	0.40734 (9)	0.42064 (17)	0.0400 (4)
H5	0.9119	0.4319	0.4419	0.048*
C6	0.86199 (14)	0.35678 (9)	0.33627 (17)	0.0381 (4)
H6	0.9329	0.3483	0.3016	0.046*
C7	0.72901 (17)	0.47686 (9)	0.56942 (18)	0.0422 (4)
C8	0.8417 (2)	0.50323 (12)	0.6179 (2)	0.0656 (7)
H8A	0.8839	0.4696	0.6579	0.098*
H8B	0.8276	0.5366	0.6784	0.098*
H8C	0.8844	0.5201	0.5478	0.098*
C9	0.6599 (2)	0.45282 (11)	0.6836 (2)	0.0654 (7)
H9A	0.5866	0.4390	0.6553	0.098*
H9B	0.6513	0.4868	0.7443	0.098*
H9C	0.6988	0.4176	0.7228	0.098*
C10	0.6641 (2)	0.53167 (10)	0.5049 (2)	0.0592 (6)
H10A	0.7100	0.5498	0.4387	0.089*
H10B	0.6467	0.5640	0.5667	0.089*
H10C	0.5950	0.5155	0.4691	0.089*
C11	0.6881 (2)	0.2500	0.0829 (2)	0.0299 (5)
C12	0.64869 (17)	0.30649 (9)	0.03060 (18)	0.0432 (5)
H12	0.6743	0.3453	0.0627	0.052*
C13	0.57171 (17)	0.30618 (9)	−0.06884 (18)	0.0453 (5)
H13	0.5472	0.3450	−0.1021	0.054*
C14	0.5301 (2)	0.2500	−0.1204 (2)	0.0342 (5)
C15	0.4417 (2)	0.2500	−0.2271 (2)	0.0433 (6)
C16	0.3243 (4)	0.2500	−0.1637 (5)	0.0934 (17)	0.776 (4)
H16A	0.3112	0.2906	−0.1240	0.140*	0.776 (4)
H16B	0.2671	0.2500	−0.2269	0.140*	0.776 (4)
C17	0.4487 (4)	0.3089 (2)	−0.3094 (4)	0.1069 (16)	0.776 (4)
H17A	0.3956	0.3054	−0.3782	0.160*	0.776 (4)
H17B	0.5240	0.3129	−0.3429	0.160*	0.776 (4)
H17C	0.4309	0.3460	−0.2594	0.160*	0.776 (4)
C18	0.5182 (14)	0.2500	−0.3521 (17)	0.0934 (17)	0.224 (4)
H18A	0.4698	0.2500	−0.4229	0.140*	0.224 (4)
H18B	0.5516	0.2086	−0.3543	0.140*	0.224 (4)
C19	0.3708 (5)	0.3102 (7)	−0.2313 (5)	0.1069 (16)	0.224 (4)
H19A	0.3178	0.3074	−0.3003	0.160*	0.224 (4)
H19B	0.4193	0.3465	−0.2436	0.160*	0.224 (4)
H19C	0.3303	0.3148	−0.1526	0.160*	0.224 (4)
O1	0.91419 (9)	0.2500 (7)	0.15255 (12)	0.0397 (4)
P1	0.79669 (5)	0.2500 (7)	0.20357 (5)	0.02885 (19)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0308 (8)	0.0304 (8)	0.0281 (8)	−0.0013 (7)	−0.0028 (6)	0.0019 (7)
C2	0.0283 (8)	0.0345 (9)	0.0393 (9)	−0.0042 (7)	−0.0036 (7)	−0.0025 (7)
C3	0.0323 (9)	0.0385 (9)	0.0401 (9)	0.0005 (7)	0.0026 (7)	−0.0037 (8)
C4	0.0393 (9)	0.0304 (9)	0.0327 (9)	−0.0023 (7)	−0.0031 (7)	0.0006 (7)
C5	0.0341 (9)	0.0405 (10)	0.0455 (10)	−0.0099 (8)	−0.0034 (8)	−0.0057 (8)
C6	0.0299 (9)	0.0429 (10)	0.0414 (9)	−0.0036 (7)	0.0022 (7)	−0.0047 (8)
C7	0.0487 (11)	0.0362 (9)	0.0418 (10)	−0.0038 (8)	0.0006 (8)	−0.0080 (8)
C8	0.0646 (15)	0.0595 (14)	0.0728 (15)	−0.0078 (12)	−0.0120 (12)	−0.0304 (12)
C9	0.0945 (19)	0.0592 (14)	0.0425 (11)	−0.0148 (13)	0.0137 (12)	−0.0147 (10)
C10	0.0685 (14)	0.0424 (12)	0.0665 (14)	0.0090 (10)	0.0018 (12)	−0.0086 (11)
C11	0.0328 (12)	0.0310 (12)	0.0260 (11)	0.000	0.0004 (9)	0.000
C12	0.0580 (12)	0.0298 (9)	0.0418 (10)	−0.0022 (8)	−0.0149 (9)	−0.0015 (7)
C13	0.0597 (12)	0.0328 (10)	0.0434 (10)	0.0061 (9)	−0.0153 (9)	0.0027 (8)
C14	0.0334 (13)	0.0424 (13)	0.0270 (11)	0.000	0.0005 (10)	0.000
C15	0.0486 (16)	0.0461 (15)	0.0352 (14)	0.000	−0.0122 (12)	0.000
C16	0.055 (3)	0.157 (5)	0.068 (3)	0.000	−0.026 (2)	0.000
C17	0.121 (3)	0.117 (3)	0.083 (3)	−0.045 (3)	−0.065 (2)	0.053 (2)
C18	0.055 (3)	0.157 (5)	0.068 (3)	0.000	−0.026 (2)	0.000
C19	0.121 (3)	0.117 (3)	0.083 (3)	−0.045 (3)	−0.065 (2)	0.053 (2)
O1	0.0300 (9)	0.0473 (10)	0.0419 (10)	0.000	0.0059 (7)	0.000
P1	0.0270 (3)	0.0314 (3)	0.0282 (3)	0.000	0.0000 (2)	0.000

Geometric parameters (Å, º) top

C1—C6	1.387 (2)	C11—P1	1.804 (2)
C1—C2	1.396 (2)	C12—C13	1.385 (3)
C1—P1	1.808 (13)	C12—H12	0.9300
C2—C3	1.383 (2)	C13—C14	1.384 (2)
C2—H2	0.9300	C13—H13	0.9300
C3—C4	1.396 (2)	C14—C13ⁱ	1.384 (2)
C3—H3	0.9300	C14—C15	1.533 (3)
C4—C5	1.384 (3)	C15—C17ⁱ	1.509 (4)
C4—C7	1.535 (2)	C15—C17	1.509 (4)
C5—C6	1.389 (2)	C15—C19ⁱ	1.512 (13)
C5—H5	0.9300	C15—C19	1.512 (13)
C6—H6	0.9300	C15—C16	1.537 (6)
C7—C8	1.527 (3)	C15—C18	1.596 (19)
C7—C9	1.537 (3)	C16—H16A	0.9600
C7—C10	1.537 (3)	C16—H16B	0.9471
C8—H8A	0.9600	C17—H17A	0.9600
C8—H8B	0.9600	C17—H17B	0.9600
C8—H8C	0.9600	C17—H17C	0.9600
C9—H9A	0.9600	C18—H18A	0.9378
C9—H9B	0.9600	C18—H18B	0.9517
C9—H9C	0.9600	C19—H19A	0.9600
C10—H10A	0.9600	C19—H19B	0.9600
C10—H10B	0.9600	C19—H19C	0.9600
C10—H10C	0.9600	O1—P1	1.4866 (12)
C11—C12ⁱ	1.384 (2)	P1—C1ⁱ	1.808 (13)
C11—C12	1.384 (2)

C6—C1—C2	118.28 (15)	C14—C13—H13	118.9
C6—C1—P1	117.83 (17)	C12—C13—H13	118.9
C2—C1—P1	123.78 (18)	C13—C14—C13ⁱ	116.2 (2)
C3—C2—C1	120.19 (15)	C13—C14—C15	121.88 (11)
C3—C2—H2	119.9	C13ⁱ—C14—C15	121.88 (11)
C1—C2—H2	119.9	C17ⁱ—C15—C17	109.6 (4)
C2—C3—C4	122.08 (16)	C17ⁱ—C15—C19ⁱ	48.2 (3)
C2—C3—H3	119.0	C17—C15—C19ⁱ	133.9 (3)
C4—C3—H3	119.0	C17ⁱ—C15—C19	133.9 (3)
C5—C4—C3	116.92 (16)	C17—C15—C19	48.2 (3)
C5—C4—C7	122.78 (16)	C19ⁱ—C15—C19	112.7 (7)
C3—C4—C7	120.30 (16)	C17ⁱ—C15—C14	112.50 (18)
C4—C5—C6	121.79 (16)	C17—C15—C14	112.50 (18)
C4—C5—H5	119.1	C19ⁱ—C15—C14	113.5 (2)
C6—C5—H5	119.1	C19—C15—C14	113.5 (2)
C1—C6—C5	120.69 (16)	C17ⁱ—C15—C16	107.4 (3)
C1—C6—H6	119.7	C17—C15—C16	107.4 (3)
C5—C6—H6	119.7	C19ⁱ—C15—C16	60.9 (4)
C8—C7—C4	112.37 (16)	C19—C15—C16	60.9 (4)
C8—C7—C9	108.61 (18)	C14—C15—C16	107.2 (2)
C4—C7—C9	109.45 (15)	C17ⁱ—C15—C18	59.7 (3)
C8—C7—C10	108.16 (17)	C17—C15—C18	59.7 (3)
C4—C7—C10	109.22 (15)	C19ⁱ—C15—C18	106.8 (4)
C9—C7—C10	108.98 (19)	C19—C15—C18	106.8 (4)
C7—C8—H8A	109.5	C14—C15—C18	102.6 (6)
C7—C8—H8B	109.5	C16—C15—C18	150.1 (6)
H8A—C8—H8B	109.5	C15—C16—H16A	109.5
C7—C8—H8C	109.5	C15—C16—H16B	109.7
H8A—C8—H8C	109.5	H16A—C16—H16B	101.0
H8B—C8—H8C	109.5	C15—C17—H17A	109.5
C7—C9—H9A	109.5	C15—C17—H17B	109.5
C7—C9—H9B	109.5	C15—C17—H17C	109.5
H9A—C9—H9B	109.5	C15—C18—H18A	108.1
C7—C9—H9C	109.5	C15—C18—H18B	104.7
H9A—C9—H9C	109.5	H18A—C18—H18B	103.5
H9B—C9—H9C	109.5	C15—C19—H16A	90.7
C7—C10—H10A	109.5	C15—C19—H19A	109.5
C7—C10—H10B	109.5	C15—C19—H19B	109.5
H10A—C10—H10B	109.5	H19A—C19—H19B	109.5
C7—C10—H10C	109.5	C15—C19—H19C	109.5
H10A—C10—H10C	109.5	H19A—C19—H19C	109.5
H10B—C10—H10C	109.5	H19B—C19—H19C	109.5
C12ⁱ—C11—C12	117.3 (2)	O1—P1—C1	111.72 (19)
C12ⁱ—C11—P1	121.2 (5)	O1—P1—C11	114.10 (9)
C12—C11—P1	121.2 (5)	C1—P1—C11	106.6 (5)
C11—C12—C13	121.07 (17)	O1—P1—C1ⁱ	111.72 (16)
C11—C12—H12	119.5	C1—P1—C1ⁱ	105.59 (10)
C13—C12—H12	119.5	C11—P1—C1ⁱ	106.6 (5)
C14—C13—C12	122.14 (17)

C6—C1—C2—C3	−0.8 (2)	C13—C14—C15—C17	28.3 (4)
P1—C1—C2—C3	175.4 (4)	C13ⁱ—C14—C15—C17	−152.7 (3)
C1—C2—C3—C4	−1.1 (3)	C13—C14—C15—C19ⁱ	−154.7 (4)
C2—C3—C4—C5	2.1 (3)	C13ⁱ—C14—C15—C19ⁱ	24.3 (5)
C2—C3—C4—C7	−177.92 (16)	C13—C14—C15—C19	−24.3 (5)
C3—C4—C5—C6	−1.4 (3)	C13ⁱ—C14—C15—C19	154.7 (4)
C7—C4—C5—C6	178.71 (16)	C13—C14—C15—C16	−89.5 (2)
C2—C1—C6—C5	1.5 (3)	C13ⁱ—C14—C15—C16	89.5 (2)
P1—C1—C6—C5	−174.9 (3)	C13—C14—C15—C18	90.5 (2)
C4—C5—C6—C1	−0.5 (3)	C13ⁱ—C14—C15—C18	−90.5 (2)
C5—C4—C7—C8	−11.6 (3)	C6—C1—P1—O1	−9.6 (3)
C3—C4—C7—C8	168.51 (18)	C2—C1—P1—O1	174.21 (16)
C5—C4—C7—C9	−132.3 (2)	C6—C1—P1—C11	−134.9 (3)
C3—C4—C7—C9	47.8 (2)	C2—C1—P1—C11	48.9 (6)
C5—C4—C7—C10	108.5 (2)	C6—C1—P1—C1ⁱ	112.0 (2)
C3—C4—C7—C10	−71.5 (2)	C2—C1—P1—C1ⁱ	−64.1 (2)
C12ⁱ—C11—C12—C13	0.9 (4)	C12ⁱ—C11—P1—O1	86.9 (3)
P1—C11—C12—C13	174.97 (17)	C12—C11—P1—O1	−86.9 (3)
C11—C12—C13—C14	0.3 (3)	C12ⁱ—C11—P1—C1	−149.3 (4)
C12—C13—C14—C13ⁱ	−1.4 (4)	C12—C11—P1—C1	36.9 (4)
C12—C13—C14—C15	177.7 (2)	C12ⁱ—C11—P1—C1ⁱ	−36.9 (4)
C13—C14—C15—C17ⁱ	152.7 (3)	C12—C11—P1—C1ⁱ	149.3 (4)
C13ⁱ—C14—C15—C17ⁱ	−28.3 (4)

Symmetry code: (i) x, −y+1/2, z.

Experimental details

Crystal data
Chemical formula	C₃₀H₃₉OP
M_r	446.58
Crystal system, space group	Orthorhombic, Pnma
Temperature (K)	294
a, b, c (Å)	11.7986 (10), 20.9246 (18), 10.5204 (9)
V (Å³)	2597.3 (4)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.13
Crystal size (mm)	0.45 × 0.43 × 0.42

Data collection
Diffractometer	Bruker SMART APEX CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.946, 0.949
No. of measured, independent and observed [I > 2σ(I)] reflections	17327, 2485, 2143
R_int	0.027
(sin θ/λ)_max (Å⁻¹)	0.606

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.042, 0.117, 1.03
No. of reflections	2485
No. of parameters	161
No. of restraints	18
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.38, −0.30

Computer programs: SMART (Bruker, 2004), SAINT (Bruker, 2004), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Acknowledgements

We are grateful to the Fund for Distinguished Talents of Henan Province (No. 074200510019) for financial support of this work.

References

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