A triclinic polymorph of bis­(μ2-ethane­thiol­ato)-1:2κ2S:S;3:4κ2S:S-(μ4-disulfido-1:2:3:4κ4S:S:S′:S′)tetra­kis­[tricarbonyl­iron(II)](2 Fe—Fe)

Si, Y.; Chen, H.; Chen, C.N.

doi:10.1107/S1600536811017405

metal-organic compounds

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Volume 67| Part 6| June 2011| Pages m752-m753

doi:10.1107/S1600536811017405

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A triclinic polymorph of bis(μ₂-ethanethiolato)-1:2κ²S:S;3:4κ²S:S-(μ₄-disulfido-1:2:3:4κ⁴S:S:S′:S′)tetrakis[tricarbonyliron(II)](2 Fe—Fe)

Youtao Si,^a ^* Hui Chen ^b and Chang Neng Chen ^b

^aKey Laboratory of Humid Subtropical Eco-Geographical Processes, Ministry of Education, Fujian Normal University, Fuzhou 350007, People's Republic of China, and ^bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Fuzhou, Fujian 350002, People's Republic of China
^*Correspondence e-mail: siyoutao@gmail.com

(Received 27 April 2011; accepted 9 May 2011; online 14 May 2011)

Next to the monoclinic polymorph [Cheng et al. (2005 ). Acta Cryst. E61, m892–m894], the triclinic title compound, [Fe₄(C₂H₅S)₂(S₂)(CO)₁₂], is the second known form of this composition. The structure is composed of an [Fe₂(C₂H₅S)(S)(CO)₆] subcluster, which is linked to its counterpart by an inversion centre located at the mid-point of the central disulfide bond. The Fe₂S₂ core of each subcluster exhibits a butterfly-like shape, with two S atoms bridging two Fe atoms. In the subcluster, each Fe atom is coordinated in a distorted octahedral coordination by three terminal carbonyl C atoms, two S atoms and one Fe atom. The crystal packing is accomplished through van der Waals interactions.

Related literature

For more details about hydrogenases, including Fe—Fe hydrogenases, see: Darensbourg et al. (2000 ). Two procedures are mainly used for the synthesis of model compounds containing the Fe₂S₂ subcluster of Fe—Fe hydrogenases, see: Lawrence et al. (2001 ); Li & Rauchfuss (2002 ). The monoclinic polymorph (space group P2₁/c) of the title compound has been reported by Cheng et al. (2005).

Experimental

Crystal data

[Fe₄(C₂H₅S)₂(S₂)(CO)₁₂]
M_r = 745.88
Triclinic, $[P \overline 1]$
a = 8.365 (4) Å
b = 9.296 (5) Å
c = 10.209 (5) Å
α = 87.57 (2)°
β = 70.082 (17)°
γ = 66.103 (17)°
V = 678.2 (6) Å³
Z = 1
Mo Kα radiation
μ = 2.46 mm⁻¹
T = 293 K
0.15 × 0.12 × 0.03 mm

Data collection

Rigaku Mercury70 CCD diffractometer
Absorption correction: multi-scan (CrystalClear; Rigaku, 2002 ) T_min = 0.771, T_max = 1.000
5323 measured reflections
3060 independent reflections
1939 reflections with I > 2σ(I)
R_int = 0.036

Refinement

R[F² > 2σ(F²)] = 0.048
wR(F²) = 0.100
S = 0.99
3060 reflections
163 parameters
H-atom parameters constrained
Δρ_max = 0.39 e Å⁻³
Δρ_min = −0.42 e Å⁻³

Table 1
Selected bond lengths (Å)

Fe1—C1	1.786 (5)
Fe1—C2	1.793 (5)
Fe1—C3	1.824 (5)
Fe1—S1	2.2393 (15)
Fe1—S2	2.2688 (16)
Fe1—Fe2	2.5183 (15)
Fe2—C5	1.789 (5)
Fe2—C6	1.794 (5)
Fe2—C4	1.806 (5)
Fe2—S1	2.2457 (15)
Fe2—S2	2.2711 (18)
S1—S1ⁱ	2.113 (2)
Symmetry code: (i) -x, -y+2, -z+2.

Data collection: CrystalClear (Rigaku, 2002); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: WinGX (Farrugia, 1999 ); software used to prepare material for publication: publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536811017405/wm2485sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536811017405/wm2485Isup2.hkl

checkCIF report

Comment top

Fe—Fe hydrogenases are enzymes capable of efficiently catalysing the reversible transformation between H⁺ and H₂ (Darensbourg et al., 2000). Chemists have been trying to achieve H₂ production technologies of practical use by studying the catalytic process by such kind of hydrogenases, aiming at solving the current energy problem. The well known active site of Fe—Fe hydrogenases, established by X-ray crystallographic and spectroscopic techniques, has an Fe₂S₂ cluster linked to a Fe₄S₄ cuboidal unit by a cysteine-S atom. While the Fe₄S₄ unit is assumed to be reponsible for transferring eletrons, the Fe₂S₂ cluster plays an important role in the catalysis process. Thus, many works concentrate on compounds containing the Fe₂S₂ cluster.

Two kinds of procedures are frequently used to synthesize model substances of the Fe₂S₂ cluster, e.g. Fe₂(SCH₂)₂NR(CO)₆. The first procedure is a condensation of (ClCH₂)₂NR and Li₂[Fe₂S₂(CO)₆], and the second is a condensation of Fe₂(SH)₂(CO)₆ with formaldehyde in the presence of primary amines (Lawrence et al., 2001; Li & Rauchfuss, 2002). In both cases, LiEt₃BH are used to cleave the S—S bond of the starting material Fe₂S₂(CO)₆. When trying to get some new complexes using the first procedure, we found some by-products which reflect the diversity of the reactivity of (FeS)_n clusters. Here we report a triclinic polymorph, (I), of [Fe₄(C₂H₅S)₂(S₂)(CO)₁₂]. Another monoclinic polymorph (space group P2₁/c) has been reported previously (Cheng et al., 2005).

As can be seen in Fig. 1, the crystallographically imposed center of inversion is located at the mid-point of the S1—S1A bond, and thus the asymmetric unit contains one half of the [Fe₄(C₂H₅S)₂(S₂)(CO)₁₂] formula unit. The two Fe atoms of the asymmetric unit (Fe1, Fe2) are linked through an Fe—Fe single bond and are bridged by two S atoms (S1, S2). Thus a butterfly-like arrangement is formed, with a dihedral angle between the two Fe₂S planes being 100.53 (6)°. The average Fe—S bond length is 2.256 (16) Å, and the average Fe—S—Fe angle is 67.9 (6)°. The octahedral coordination geometry around each Fe atom is completed by three carbonyl C atoms [average Fe—C distance 1.799 (14) Å, average C—Fe—C angle 97 (4)°].

The packing diagram is shown in Fig. 2. There is only one molecule in each unit cell, and neighbouring molecules pack along the a axis; the crystal is stabilized by van der Waals interactions.

In comparison with the monoclinic polymorph (Cheng et al., 2005), the configuration of the [Fe₄(C₂H₅S)₂(S₂)(CO)₁₂] molecules is different, just like the packing in the crystal.

Related literature top

For more details about hydrogenases, including Fe—Fe hydrogenases, see: Darensbourg et al. (2000). Two procedures are mainly used for the synthesis of model compounds containing the Fe₂S₂ subcluster of Fe—Fe hydrogenases, see: Lawrence et al. (2001); Li & Rauchfuss (2002). The monoclinic polymorph (space group P2₁/c) of the title compound has been reported by Cheng et al. (2005).

Experimental top

All experiments were carried out under an atmosphere of purified, oxygen-free and dry nitrogen using standard Schlenk techniques. THF and hexane were dried and freshly distilled prior to use according to standard methods. The commercially available products paraformaldehyde, [Fe(CO)₅], LiBEt₃H, F₃CCOOH and C₅H₉NH₂ were of reagent grade and were used as received. The starting material [Fe₂S₂(CO)₆] was prepared according to the literature.

[Fe₂S₂(CO)₆] (1 mmol, 0.344 g) was dissolved in dry THF (40 ml) under a nitrogen atmosphere and then cooled to 195 K with acetone and liquid nitrogen. After the solution was stirred for 30 minutes, LiBEt₃H (2 mmol) was added dropwise very slowly. At the midpoint of the addition, the color of the reaction mixture turned from red to dark green; for the rest of addition it remained green. After another 30 minutes, F₃CCOOH (2 mmol, 0.149 ml) was added. The new mixture was stirred for an additional hour. The cool solution was added to a mixture of paraformaldehyde (40 mmol, 1.2 g) and C₅H₉NH₂ (1 mmol, 1.98 ml) in THF which had been stirred for 10 h and cooled to 273 K. The last mixture was stirred for 24 h and the majority of the solvent was evaporated under vacuum. The remaining residual was filtered through silica gel. A red fraction was collected by elution with hexane. Recrystallization of the crude product from fresh distilled pentane in a fridge at 253 K for several days gave the title complex as a by-product in low and varing yields (<5%).

Computing details top

Data collection: CrystalClear (Rigaku, 2002); cell refinement: CrystalClear (Rigaku, 2002); data reduction: CrystalClear (Rigaku, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: WinGX (Farrugia, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

	Fig. 1. The molecular structure of (I), with atom labels and 20% probability displacement ellipsoids for all non-H atoms. [Symmetry operator A: -x, -y + 2, -z + 2.]
	Fig. 2. The packing diagram of (I), viewed down the b axis.

bis(µ₂-ethanethiolato)-1:2κ²S:S;3:4κ²S:S- (µ₄-disulfido-1:2:3:4κ⁴S:S:S':S')tetrakis [tricarbonyliron(II)](2 Fe—Fe) top

Crystal data top

[Fe₄(C₂H₅S)₂(S₂)(CO)₁₂]	Z = 1
M_r = 745.88	F(000) = 370
Triclinic, P1	D_x = 1.826 Mg m⁻³
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 8.365 (4) Å	Cell parameters from 1200 reflections
b = 9.296 (5) Å	θ = 2.1–27.5°
c = 10.209 (5) Å	µ = 2.46 mm⁻¹
α = 87.57 (2)°	T = 293 K
β = 70.082 (17)°	Prism, orange
γ = 66.103 (17)°	0.15 × 0.12 × 0.03 mm
V = 678.2 (6) Å³

Data collection top

Rigaku Mercury70 CCD diffractometer	3060 independent reflections
Radiation source: fine-focus sealed tube	1939 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.036
CCD_Profile_fitting scans	θ_max = 27.5°, θ_min = 2.4°
Absorption correction: multi-scan (CrystalClear; Rigaku, 2002)	h = −10→10
T_min = 0.771, T_max = 1.000	k = −11→11
5323 measured reflections	l = −12→13

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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.100	H-atom parameters constrained
S = 0.99	w = 1/[σ²(F_o²) + (0.0419P)²] where P = (F_o² + 2F_c²)/3
3060 reflections	(Δ/σ)_max < 0.001
163 parameters	Δρ_max = 0.39 e Å⁻³
0 restraints	Δρ_min = −0.42 e Å⁻³

Crystal data top

[Fe₄(C₂H₅S)₂(S₂)(CO)₁₂]	γ = 66.103 (17)°
M_r = 745.88	V = 678.2 (6) Å³
Triclinic, P1	Z = 1
a = 8.365 (4) Å	Mo Kα radiation
b = 9.296 (5) Å	µ = 2.46 mm⁻¹
c = 10.209 (5) Å	T = 293 K
α = 87.57 (2)°	0.15 × 0.12 × 0.03 mm
β = 70.082 (17)°

Data collection top

Rigaku Mercury70 CCD diffractometer	3060 independent reflections
Absorption correction: multi-scan (CrystalClear; Rigaku, 2002)	1939 reflections with I > 2σ(I)
T_min = 0.771, T_max = 1.000	R_int = 0.036
5323 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.048	0 restraints
wR(F²) = 0.100	H-atom parameters constrained
S = 0.99	Δρ_max = 0.39 e Å⁻³
3060 reflections	Δρ_min = −0.42 e Å⁻³
163 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
Fe1	0.31028 (8)	1.00320 (7)	0.78114 (6)	0.03542 (19)
Fe2	0.33627 (9)	0.73745 (7)	0.86174 (7)	0.03887 (19)
S1	0.06448 (15)	0.94806 (12)	0.89480 (11)	0.0364 (3)
S2	0.37128 (16)	0.79317 (13)	0.63756 (12)	0.0425 (3)
O2	0.2904 (5)	1.1642 (4)	1.0304 (4)	0.0659 (10)
O1	0.7064 (5)	0.9407 (5)	0.6498 (4)	0.0765 (12)
O3	0.1430 (6)	1.2810 (4)	0.6441 (4)	0.0801 (13)
O5	0.3256 (6)	0.7693 (4)	1.1491 (4)	0.0767 (12)
C1	0.5507 (7)	0.9675 (5)	0.6996 (5)	0.0485 (12)
O6	0.7439 (5)	0.5643 (5)	0.7670 (5)	0.0864 (14)
C3	0.2065 (7)	1.1746 (6)	0.6958 (5)	0.0477 (12)
C6	0.5855 (8)	0.6296 (6)	0.8033 (5)	0.0559 (14)
C2	0.2944 (6)	1.1039 (5)	0.9341 (5)	0.0444 (11)
C5	0.3261 (7)	0.7589 (5)	1.0381 (6)	0.0527 (13)
C4	0.2657 (8)	0.5763 (6)	0.8735 (6)	0.0601 (14)
O4	0.2224 (8)	0.4741 (5)	0.8805 (6)	0.1078 (17)
C7	0.1744 (7)	0.8070 (6)	0.5873 (5)	0.0546 (13)
H7A	0.0604	0.8465	0.6691	0.065*
H7B	0.1589	0.8820	0.5186	0.065*
C8	0.2037 (11)	0.6549 (7)	0.5290 (9)	0.124 (3)
H8A	0.0984	0.6666	0.5044	0.187*
H8B	0.2173	0.5809	0.5972	0.187*
H8C	0.3149	0.6166	0.4468	0.187*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Fe1	0.0322 (4)	0.0369 (4)	0.0375 (4)	−0.0163 (3)	−0.0103 (3)	0.0028 (3)
Fe2	0.0377 (4)	0.0317 (3)	0.0461 (4)	−0.0122 (3)	−0.0159 (3)	0.0021 (3)
S1	0.0297 (6)	0.0386 (6)	0.0380 (6)	−0.0142 (5)	−0.0079 (5)	−0.0008 (5)
S2	0.0354 (6)	0.0472 (7)	0.0405 (6)	−0.0142 (5)	−0.0110 (5)	−0.0050 (5)
O2	0.078 (3)	0.079 (2)	0.058 (2)	−0.048 (2)	−0.024 (2)	−0.0046 (19)
O1	0.044 (2)	0.109 (3)	0.076 (3)	−0.041 (2)	−0.008 (2)	0.001 (2)
O3	0.086 (3)	0.063 (2)	0.086 (3)	−0.018 (2)	−0.042 (3)	0.027 (2)
O5	0.092 (3)	0.077 (3)	0.057 (2)	−0.020 (2)	−0.040 (2)	0.009 (2)
C1	0.045 (3)	0.055 (3)	0.046 (3)	−0.025 (3)	−0.011 (2)	0.005 (2)
O6	0.045 (2)	0.079 (3)	0.110 (3)	0.004 (2)	−0.029 (2)	−0.025 (2)
C3	0.052 (3)	0.048 (3)	0.045 (3)	−0.022 (2)	−0.019 (2)	0.008 (2)
C6	0.049 (3)	0.049 (3)	0.066 (4)	−0.010 (3)	−0.026 (3)	−0.005 (3)
C2	0.037 (3)	0.049 (3)	0.053 (3)	−0.024 (2)	−0.015 (2)	0.007 (2)
C5	0.053 (3)	0.043 (3)	0.056 (3)	−0.010 (2)	−0.023 (3)	0.008 (2)
C4	0.075 (4)	0.047 (3)	0.071 (4)	−0.030 (3)	−0.036 (3)	0.015 (3)
O4	0.150 (5)	0.074 (3)	0.149 (4)	−0.078 (3)	−0.076 (4)	0.033 (3)
C7	0.050 (3)	0.062 (3)	0.058 (3)	−0.021 (3)	−0.027 (3)	−0.001 (3)
C8	0.131 (7)	0.086 (5)	0.194 (9)	−0.029 (5)	−0.116 (7)	−0.027 (5)

Geometric parameters (Å, º) top

Fe1—C1	1.786 (5)	O2—C2	1.137 (5)
Fe1—C2	1.793 (5)	O1—C1	1.145 (5)
Fe1—C3	1.824 (5)	O3—C3	1.123 (5)
Fe1—S1	2.2393 (15)	O5—C5	1.140 (6)
Fe1—S2	2.2688 (16)	O6—C6	1.138 (6)
Fe1—Fe2	2.5183 (15)	C4—O4	1.138 (6)
Fe2—C5	1.789 (5)	C7—C8	1.451 (7)
Fe2—C6	1.794 (5)	C7—H7A	0.9700
Fe2—C4	1.806 (5)	C7—H7B	0.9700
Fe2—S1	2.2457 (15)	C8—H8A	0.9600
Fe2—S2	2.2711 (18)	C8—H8B	0.9600
S1—S1ⁱ	2.113 (2)	C8—H8C	0.9600
S2—C7	1.842 (5)

C1—Fe1—C2	90.8 (2)	C4—Fe2—Fe1	149.28 (16)
C1—Fe1—C3	99.4 (2)	S1—Fe2—Fe1	55.72 (4)
C2—Fe1—C3	99.1 (2)	S2—Fe2—Fe1	56.27 (4)
C1—Fe1—S1	157.40 (15)	S1ⁱ—S1—Fe1	111.12 (8)
C2—Fe1—S1	94.02 (15)	S1ⁱ—S1—Fe2	111.36 (8)
C3—Fe1—S1	101.64 (16)	Fe1—S1—Fe2	68.32 (5)
C1—Fe1—S2	87.52 (16)	C7—S2—Fe1	114.87 (16)
C2—Fe1—S2	156.70 (15)	C7—S2—Fe2	113.05 (18)
C3—Fe1—S2	104.06 (16)	Fe1—S2—Fe2	67.38 (5)
S1—Fe1—S2	79.39 (5)	O1—C1—Fe1	177.8 (5)
C1—Fe1—Fe2	101.44 (15)	O3—C3—Fe1	179.3 (5)
C2—Fe1—Fe2	101.42 (15)	O6—C6—Fe2	178.4 (5)
C3—Fe1—Fe2	150.37 (16)	O2—C2—Fe1	177.7 (4)
S1—Fe1—Fe2	55.96 (4)	O5—C5—Fe2	177.8 (5)
S2—Fe1—Fe2	56.35 (5)	O4—C4—Fe2	179.6 (6)
C5—Fe2—C6	90.6 (2)	C8—C7—S2	111.8 (4)
C5—Fe2—C4	99.0 (2)	C8—C7—H7A	109.3
C6—Fe2—C4	100.4 (2)	S2—C7—H7A	109.3
C5—Fe2—S1	94.44 (15)	C8—C7—H7B	109.3
C6—Fe2—S1	156.35 (18)	S2—C7—H7B	109.3
C4—Fe2—S1	101.55 (18)	H7A—C7—H7B	107.9
C5—Fe2—S2	158.11 (17)	C7—C8—H8A	109.5
C6—Fe2—S2	87.54 (17)	C7—C8—H8B	109.5
C4—Fe2—S2	102.75 (18)	H8A—C8—H8B	109.5
S1—Fe2—S2	79.21 (5)	C7—C8—H8C	109.5
C5—Fe2—Fe1	102.83 (16)	H8A—C8—H8C	109.5
C6—Fe2—Fe1	100.63 (17)	H8B—C8—H8C	109.5

Symmetry code: (i) −x, −y+2, −z+2.

Experimental details

Crystal data
Chemical formula	[Fe₄(C₂H₅S)₂(S₂)(CO)₁₂]
M_r	745.88
Crystal system, space group	Triclinic, P1
Temperature (K)	293
a, b, c (Å)	8.365 (4), 9.296 (5), 10.209 (5)
α, β, γ (°)	87.57 (2), 70.082 (17), 66.103 (17)
V (Å³)	678.2 (6)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	2.46
Crystal size (mm)	0.15 × 0.12 × 0.03

Data collection
Diffractometer	Rigaku Mercury70 CCD diffractometer
Absorption correction	Multi-scan (CrystalClear; Rigaku, 2002)
T_min, T_max	0.771, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	5323, 3060, 1939
R_int	0.036
(sin θ/λ)_max (Å⁻¹)	0.649

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.048, 0.100, 0.99
No. of reflections	3060
No. of parameters	163
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.39, −0.42

Computer programs: CrystalClear (Rigaku, 2002), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), WinGX (Farrugia, 1999), publCIF (Westrip, 2010).

Selected bond lengths (Å) top

Fe1—C1	1.786 (5)	Fe2—C5	1.789 (5)
Fe1—C2	1.793 (5)	Fe2—C6	1.794 (5)
Fe1—C3	1.824 (5)	Fe2—C4	1.806 (5)
Fe1—S1	2.2393 (15)	Fe2—S1	2.2457 (15)
Fe1—S2	2.2688 (16)	Fe2—S2	2.2711 (18)
Fe1—Fe2	2.5183 (15)	S1—S1ⁱ	2.113 (2)

Symmetry code: (i) −x, −y+2, −z+2.

Acknowledgements

We thank the National Natural Science Foundation of China (21071145) for financial support.

References

Cheng, L.-X., Ma, C.-B., Hu, M.-Q. & Chen, C.-N. (2005). Acta Cryst. E61, m892–m894. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Darensbourg, M. Y., Lyon, E. J. & Smee, J. J. (2000). Coord. Chem. Rev. 206, 533–561. Web of Science CrossRef Google Scholar
Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837–838. CrossRef CAS IUCr Journals Google Scholar
Lawrence, J. D., Li, H. X., Rauchfuss, T. B., Benard, M. & Rohmer, M. M. (2001). Angew. Chem. Int. Eng. Ed. 40, 1768–1771. CrossRef CAS Google Scholar
Li, H. & Rauchfuss, T. B. (2002). J. Am. Chem. Soc. 124, 726–727. Web of Science CSD CrossRef PubMed CAS Google Scholar
Rigaku (2002). CrystalClear. Rigaku Corporation, Tokyo, Japan. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar

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