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Acta Cryst. (2008). E64, m553-m554    [ doi:10.1107/S1600536808006661 ]

Tetrakis([mu]-2-methylbenzoato)bis[(2-methylbenzoic acid)copper(II)]

A. C. Sunil, B. C. B. Bezuidenhoudt and J. M. Janse van Rensburg

Abstract top

In the title centrosymmetric dinuclear compound, [Cu2(C8H7O2)4(C8H8O2)2], four o-toluate anions form a cage around two Cu atoms in a syn-syn configuration. Two more o-toluic acid molecules are apically bonded to the Cu atoms, which show a square-pyramidal coordination geometry. The acid H atoms are hydrogen bonded to the cage carboxyl O atoms [O...O = 2.660 (2) Å]. The molecular packing forms a puckered pseudo-hexagonal close-packed layer in the (h00) plane, with soft intermolecular H...H contacts (2.46-2.58 Å).

Comment top

In our endeavours to produce phenols from aromatic carboxylic acids, we came across work by Kaeding (1967). In order to verify the reaction sequence as proposed by Kaeding, we prepared the copper salt of o-toluic acid and obtained single crystals.

The title compound, (I), [Cu2(C8H7O2)4(C8H8O2)2] crystallized with molecular symmetry -1 (Fig.1). The title compound exhibits a cage-like structure. Four o-toluic anionic ligands form a cage around two Cu-atoms in a syn-syn configuration. Two more carboxylic acid are apically bonded to the Cu-atoms. The acid protons are hydrogen bonded to the cage carboxylate O atoms, O32—H32···O22 = 167.6° and O32···O22 = 2.6604 (18) Å. Another intra-molecular H···H short contact is present at C16···O21 with C16—H16···O21 = 100.9° and C16···O21 = 2.721 (2) Å.

Comparing the Van der Waals radii of copper, 2.32 Å, and the metallic Cu—Cu bond length, 2.55 Å, to the Cu—Cu separation in (I), 2.5780 (9) Å, one would expect the presence of weak orbital interaction. The Cu—O bond lengths of the cage carboxylates vary between 1.9402 (12) - 1.9900 (13) Å. The Cu—O bond distances to the adducted acid molecules show a ca 0.2 Å increase. Each Cu atom is displaced from the basal plane of the four caged carboxylates by 0.171 Å, towards the apical O atom. Compound (I) forms a puckered pseudo-hexagonal close packed layer in the (h 0 0) plane, with soft inter-molecular H···H contacts, 2.457–2.580 Å (Fig.2).

Related literature top

For the synthesis of aromatic carboxylic acids, see: Kaeding (1967). For tetrakis(µ2-2-fluorobenzoato)-bis(2-fluorobenzoic acid)-di-copper(II), see: Valach et al. (2000), For tetrakis(µ2benzoato)-bis(2-fluorobenzoic acid)-di-copper(II), see: Kawata et al. (1992).

Experimental top

The complex [Cu2(C8H7O2)4(C8H8O2)2] was prepared by heating o-toluic acid (2.52 g, 18.5 mmol), copper carbonate (0.73 g, 3.3 mmol) and magnesium oxide (0.19 g, 4.68 mmol) under reflux, in toluene (30 ml) for 24 h. The product was extacted and crystallized from diethyl ether to yield a blue crystalline solid. (Yield: 84%)

Refinement top

Hydrogen atoms were placed in calculated positions, and they ride on the parent C-atoms, with U set to 1.2 to 1.5 times Ueq(C).

Computing details top

Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT-Plus (Bruker, 2004); data reduction: SAINT-Plus and XPREP (Bruker, 2004); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg & Putz, 2005); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. A view of (I) showing the atom-numbering scheme with displacement ellipsoids at the 30% probability level, non labelled atoms are symmetric equivalents. For the phenyl C-atoms, the first digit indicates ring number and the second digit the position of the atom in the ring.
[Figure 2] Fig. 2. Indication of pseudo-hexagonal close packing along the b axis.
Tetrakis(µ-2-methylbenzoato)bis[(2-methylbenzoic acid)copper(II)] top
Crystal data top
[Cu2(C8H7O2)4(C8H8O2)2]Z = 1
Mr = 939.96F000 = 486
Triclinic, P1Dx = 1.455 Mg m−3
Hall symbol: -P 1Mo Kα radiation
λ = 0.71069 Å
a = 10.530 (3) ÅCell parameters from 8974 reflections
b = 10.579 (3) Åθ = 2.6–28.3º
c = 10.773 (4) ŵ = 1.06 mm−1
α = 109.248 (2)ºT = 100 (2) K
β = 93.156 (2)ºNeedle, blue
γ = 106.287 (2)º0.25 × 0.08 × 0.06 mm
V = 1073.0 (6) Å3
Data collection top
Bruker X8 APEXII 4K KappaCCD
diffractometer		4668 reflections with I > 2σ(I)
Monochromator: graphiteRint = 0.026
T = 100(2) Kθmax = 28º
ω and φ scansθmin = 2.2º
Absorption correction: multi-scan
(SADABS; Bruker, 2004b)		h = −13→13
Tmin = 0.778, Tmax = 0.939k = −13→13
17497 measured reflectionsl = −14→14
5138 independent reflections
Refinement top
Refinement on F2H-atom parameters constrained
Least-squares matrix: full  w = 1/[σ2(Fo2) + (0.024P)2 + 0.7431P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.026(Δ/σ)max = 0.001
wR(F2) = 0.065Δρmax = 0.41 e Å−3
S = 1.05Δρmin = −0.33 e Å−3
5138 reflectionsExtinction correction: none
284 parameters
Crystal data top
[Cu2(C8H7O2)4(C8H8O2)2]γ = 106.287 (2)º
Mr = 939.96V = 1073.0 (6) Å3
Triclinic, P1Z = 1
a = 10.530 (3) ÅMo Kα
b = 10.579 (3) ŵ = 1.06 mm−1
c = 10.773 (4) ÅT = 100 (2) K
α = 109.248 (2)º0.25 × 0.08 × 0.06 mm
β = 93.156 (2)º
Data collection top
Bruker X8 APEXII 4K KappaCCD
diffractometer		5138 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2004b)		4668 reflections with I > 2σ(I)
Tmin = 0.778, Tmax = 0.939Rint = 0.026
17497 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.026284 parameters
wR(F2) = 0.065H-atom parameters constrained
S = 1.05Δρmax = 0.41 e Å−3
5138 reflectionsΔρmin = −0.33 e Å−3
Special details top

Experimental. The intensity data was collected on a Bruker X8 Apex II 4 K Kappa CCD diffractometer using an exposure time of 30 s/frame. The 1757 frames were collected with a frame width of 0.5° covering up to θ = 28° with 99.3% completeness accomplished.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Cu10.444832 (17)0.389154 (17)0.395103 (17)0.01027 (6)
O320.51055 (11)0.07974 (12)0.26534 (12)0.0217 (2)
H320.55250.15940.31750.033*
O310.36065 (11)0.18596 (11)0.23619 (10)0.0154 (2)
O210.47477 (11)0.50089 (11)0.70973 (10)0.0175 (2)
O220.38457 (10)0.65099 (11)0.57571 (11)0.0168 (2)
O110.38005 (11)0.30802 (11)0.52758 (10)0.0157 (2)
O120.29312 (10)0.46128 (11)0.39073 (10)0.0150 (2)
C160.36586 (15)0.36861 (16)0.87485 (15)0.0161 (3)
H160.39990.46660.90490.019*
C310.31192 (15)−0.05882 (15)0.11267 (14)0.0142 (3)
C110.35899 (14)0.29021 (15)0.74017 (14)0.0126 (3)
C360.37677 (17)−0.14773 (16)0.03537 (16)0.0189 (3)
H360.4698−0.12260.05170.023*
C250.00903 (16)0.72093 (17)0.56202 (18)0.0235 (3)
H25−0.03260.74790.63580.028*
C3210.09555 (16)−0.00753 (18)0.17572 (17)0.0229 (3)
H32A0.09540.06740.14430.034*
H32B0.1380.03180.26720.034*
H32C0.0049−0.0650.16840.034*
C10.40786 (14)0.37152 (15)0.65233 (14)0.0127 (3)
C120.30930 (15)0.14154 (15)0.69355 (15)0.0143 (3)
C150.32315 (16)0.30349 (17)0.96407 (16)0.0183 (3)
H150.32850.35691.05330.022*
C20.29274 (14)0.57566 (15)0.47500 (14)0.0119 (3)
C130.26600 (15)0.07881 (16)0.78583 (16)0.0166 (3)
H130.2318−0.01910.75710.02*
C320.17142 (16)−0.09665 (16)0.09292 (15)0.0168 (3)
C210.17807 (14)0.62904 (14)0.45946 (15)0.0131 (3)
C330.10027 (17)−0.22517 (17)−0.00704 (16)0.0229 (3)
H330.0071−0.2536−0.02130.027*
C220.13237 (16)0.63473 (16)0.33772 (16)0.0182 (3)
C1210.30291 (18)0.04573 (16)0.55228 (16)0.0211 (3)
H12A0.278−0.05050.54750.032*
H12B0.38920.07070.52560.032*
H12C0.23750.05610.49390.032*
C260.11778 (16)0.67358 (16)0.57076 (16)0.0175 (3)
H260.1510.67140.65160.021*
C340.16490 (19)−0.31154 (17)−0.08566 (16)0.0254 (4)
H340.1149−0.396−0.15250.03*
C30.39545 (15)0.08020 (15)0.21105 (14)0.0137 (3)
C350.30350 (19)−0.27297 (17)−0.06539 (16)0.0238 (4)
H350.3469−0.3305−0.11890.029*
C140.27235 (15)0.15762 (17)0.91873 (16)0.0177 (3)
H140.24250.11250.97750.021*
C24−0.03711 (16)0.72774 (17)0.44262 (19)0.0261 (4)
H24−0.110.75980.4360.031*
C2210.1966 (2)0.5912 (2)0.21626 (17)0.0314 (4)
H22A0.29170.63780.23880.047*
H22B0.16030.61730.14810.047*
H22C0.17890.49090.18420.047*
C230.02467 (17)0.68697 (17)0.33293 (18)0.0250 (4)
H23−0.00620.69450.25390.03*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cu10.01089 (9)0.00896 (9)0.01017 (9)0.00293 (6)0.00121 (6)0.00270 (6)
O320.0162 (5)0.0158 (5)0.0258 (6)0.0068 (4)−0.0053 (5)−0.0019 (5)
O310.0165 (5)0.0124 (5)0.0150 (5)0.0054 (4)−0.0001 (4)0.0019 (4)
O210.0211 (6)0.0135 (5)0.0143 (5)0.0004 (4)0.0038 (4)0.0047 (4)
O220.0126 (5)0.0159 (5)0.0178 (5)0.0056 (4)−0.0018 (4)0.0006 (4)
O110.0205 (5)0.0135 (5)0.0122 (5)0.0031 (4)0.0019 (4)0.0053 (4)
O120.0145 (5)0.0132 (5)0.0157 (5)0.0058 (4)−0.0003 (4)0.0025 (4)
C160.0170 (7)0.0144 (7)0.0160 (7)0.0042 (6)0.0020 (6)0.0052 (6)
C310.0177 (7)0.0117 (6)0.0126 (7)0.0041 (6)0.0018 (6)0.0043 (5)
C110.0099 (6)0.0153 (7)0.0139 (7)0.0045 (5)0.0023 (5)0.0066 (6)
C360.0235 (8)0.0156 (7)0.0187 (8)0.0083 (6)0.0057 (6)0.0057 (6)
C250.0163 (8)0.0180 (8)0.0319 (9)0.0058 (6)0.0088 (7)0.0026 (7)
C3210.0146 (7)0.0262 (8)0.0265 (9)0.0060 (6)0.0035 (6)0.0082 (7)
C10.0115 (7)0.0141 (7)0.0142 (7)0.0061 (5)0.0027 (5)0.0055 (5)
C120.0121 (7)0.0154 (7)0.0158 (7)0.0048 (6)0.0011 (5)0.0057 (6)
C150.0197 (8)0.0227 (8)0.0139 (7)0.0081 (6)0.0042 (6)0.0067 (6)
C20.0121 (7)0.0116 (6)0.0131 (7)0.0026 (5)0.0035 (5)0.0066 (5)
C130.0134 (7)0.0149 (7)0.0216 (8)0.0028 (6)0.0021 (6)0.0083 (6)
C320.0190 (7)0.0156 (7)0.0152 (7)0.0028 (6)0.0008 (6)0.0073 (6)
C210.0109 (6)0.0093 (6)0.0169 (7)0.0015 (5)0.0013 (5)0.0038 (5)
C330.0232 (8)0.0182 (8)0.0211 (8)−0.0025 (6)−0.0041 (7)0.0082 (6)
C220.0196 (8)0.0158 (7)0.0181 (8)0.0072 (6)−0.0005 (6)0.0038 (6)
C1210.0297 (9)0.0130 (7)0.0189 (8)0.0054 (6)0.0039 (7)0.0048 (6)
C260.0165 (7)0.0151 (7)0.0191 (7)0.0040 (6)0.0038 (6)0.0044 (6)
C340.0394 (10)0.0137 (7)0.0149 (8)−0.0004 (7)−0.0029 (7)0.0034 (6)
C30.0138 (7)0.0146 (7)0.0120 (7)0.0041 (6)0.0036 (5)0.0043 (5)
C350.0401 (10)0.0146 (7)0.0167 (8)0.0110 (7)0.0069 (7)0.0034 (6)
C140.0147 (7)0.0242 (8)0.0194 (8)0.0063 (6)0.0046 (6)0.0141 (6)
C240.0141 (7)0.0183 (8)0.0399 (10)0.0076 (6)−0.0033 (7)0.0022 (7)
C2210.0442 (11)0.0427 (11)0.0175 (8)0.0258 (9)0.0062 (8)0.0134 (8)
C230.0246 (9)0.0205 (8)0.0264 (9)0.0094 (7)−0.0085 (7)0.0041 (7)
Geometric parameters (Å, °) top
Cu1—O21i1.9402 (12)C321—H32B0.96
Cu1—O111.9559 (12)C321—H32C0.96
Cu1—O121.9585 (13)C12—C131.397 (2)
Cu1—O22i1.9900 (13)C12—C1211.510 (2)
Cu1—O312.1622 (13)C15—C141.385 (2)
Cu1—Cu1i2.5780 (9)C15—H150.93
O32—C31.3184 (19)C2—C211.492 (2)
O32—H320.82C13—C141.386 (2)
O31—C31.2250 (18)C13—H130.93
O21—C11.2670 (18)C32—C331.394 (2)
O21—Cu1i1.9402 (12)C21—C261.394 (2)
O22—C21.2764 (18)C21—C221.398 (2)
O22—Cu1i1.9900 (13)C33—C341.385 (3)
O11—C11.2626 (18)C33—H330.93
O12—C21.2518 (18)C22—C231.399 (2)
C16—C151.382 (2)C22—C2211.502 (2)
C16—C111.400 (2)C121—H12A0.96
C16—H160.93C121—H12B0.96
C31—C361.397 (2)C121—H12C0.96
C31—C321.404 (2)C26—H260.93
C31—C31.485 (2)C34—C351.384 (3)
C11—C121.410 (2)C34—H340.93
C11—C11.497 (2)C35—H350.93
C36—C351.384 (2)C14—H140.93
C36—H360.93C24—C231.383 (3)
C25—C241.381 (3)C24—H240.93
C25—C261.383 (2)C221—H22A0.96
C25—H250.93C221—H22B0.96
C321—C321.506 (2)C221—H22C0.96
C321—H32A0.96C23—H230.93
O21i—Cu1—O11170.01 (4)O12—C2—O22124.23 (14)
O21i—Cu1—O1287.99 (5)O12—C2—C21118.96 (13)
O11—Cu1—O1291.79 (5)O22—C2—C21116.81 (13)
O21i—Cu1—O22i89.93 (5)C14—C13—C12122.17 (14)
O11—Cu1—O22i88.55 (5)C14—C13—H13118.9
O12—Cu1—O22i169.88 (4)C12—C13—H13118.9
O21i—Cu1—O3198.74 (6)C33—C32—C31117.35 (15)
O11—Cu1—O3191.16 (6)C33—C32—C321119.26 (15)
O12—Cu1—O3199.14 (5)C31—C32—C321123.37 (14)
O22i—Cu1—O3190.97 (5)C26—C21—C22120.83 (14)
O21i—Cu1—Cu1i87.75 (5)C26—C21—C2117.76 (13)
O11—Cu1—Cu1i82.27 (5)C22—C21—C2121.41 (13)
O12—Cu1—Cu1i86.61 (4)C34—C33—C32121.67 (16)
O22i—Cu1—Cu1i83.41 (4)C34—C33—H33119.2
O31—Cu1—Cu1i171.44 (3)C32—C33—H33119.2
C3—O32—H32109.5C21—C22—C23117.30 (15)
C3—O31—Cu1129.50 (10)C21—C22—C221122.57 (15)
C1—O21—Cu1i119.83 (10)C23—C22—C221120.12 (15)
C2—O22—Cu1i123.41 (10)C12—C121—H12A109.5
C1—O11—Cu1125.45 (10)C12—C121—H12B109.5
C2—O12—Cu1122.02 (10)H12A—C121—H12B109.5
C15—C16—C11121.39 (14)C12—C121—H12C109.5
C15—C16—H16119.3H12A—C121—H12C109.5
C11—C16—H16119.3H12B—C121—H12C109.5
C36—C31—C32120.86 (14)C25—C26—C21120.55 (15)
C36—C31—C3118.27 (14)C25—C26—H26119.7
C32—C31—C3120.78 (13)C21—C26—H26119.7
C16—C11—C12119.90 (13)C35—C34—C33120.44 (15)
C16—C11—C1116.94 (13)C35—C34—H34119.8
C12—C11—C1123.16 (13)C33—C34—H34119.8
C35—C36—C31120.42 (16)O31—C3—O32123.59 (13)
C35—C36—H36119.8O31—C3—C31122.29 (14)
C31—C36—H36119.8O32—C3—C31114.10 (13)
C24—C25—C26119.35 (16)C36—C35—C34119.22 (16)
C24—C25—H25120.3C36—C35—H35120.4
C26—C25—H25120.3C34—C35—H35120.4
C32—C321—H32A109.5C15—C14—C13120.05 (14)
C32—C321—H32B109.5C15—C14—H14120
H32A—C321—H32B109.5C13—C14—H14120
C32—C321—H32C109.5C25—C24—C23120.20 (16)
H32A—C321—H32C109.5C25—C24—H24119.9
H32B—C321—H32C109.5C23—C24—H24119.9
O11—C1—O21124.51 (13)C22—C221—H22A109.5
O11—C1—C11118.61 (13)C22—C221—H22B109.5
O21—C1—C11116.88 (13)H22A—C221—H22B109.5
C13—C12—C11117.39 (14)C22—C221—H22C109.5
C13—C12—C121117.89 (14)H22A—C221—H22C109.5
C11—C12—C121124.70 (13)H22B—C221—H22C109.5
C16—C15—C14119.09 (15)C24—C23—C22121.72 (16)
C16—C15—H15120.5C24—C23—H23119.1
C14—C15—H15120.5C22—C23—H23119.1
O21i—Cu1—O31—C3108.89 (13)C11—C12—C13—C14−0.8 (2)
O11—Cu1—O31—C3−69.75 (13)C121—C12—C13—C14177.70 (14)
O12—Cu1—O31—C3−161.75 (13)C36—C31—C32—C33−0.5 (2)
O22i—Cu1—O31—C318.81 (13)C3—C31—C32—C33176.08 (14)
O12—Cu1—O11—C1−88.41 (12)C36—C31—C32—C321178.06 (14)
O22i—Cu1—O11—C181.47 (12)C3—C31—C32—C321−5.4 (2)
O31—Cu1—O11—C1172.41 (12)O12—C2—C21—C26131.59 (15)
Cu1i—Cu1—O11—C1−2.08 (11)O22—C2—C21—C26−47.95 (19)
O21i—Cu1—O12—C2−87.01 (12)O12—C2—C21—C22−48.5 (2)
O11—Cu1—O12—C283.00 (12)O22—C2—C21—C22131.91 (15)
O22i—Cu1—O12—C2−8.8 (3)C31—C32—C33—C34−1.1 (2)
O31—Cu1—O12—C2174.46 (11)C321—C32—C33—C34−179.70 (15)
Cu1i—Cu1—O12—C20.85 (11)C26—C21—C22—C230.1 (2)
C15—C16—C11—C12−0.8 (2)C2—C21—C22—C23−179.71 (14)
C15—C16—C11—C1179.95 (14)C26—C21—C22—C221178.43 (16)
C32—C31—C36—C352.2 (2)C2—C21—C22—C221−1.4 (2)
C3—C31—C36—C35−174.48 (14)C24—C25—C26—C21−1.9 (2)
Cu1—O11—C1—O215.1 (2)C22—C21—C26—C251.7 (2)
Cu1—O11—C1—C11−175.02 (9)C2—C21—C26—C25−178.42 (14)
Cu1i—O21—C1—O11−5.4 (2)C32—C33—C34—C351.0 (2)
Cu1i—O21—C1—C11174.78 (9)Cu1—O31—C3—O32−13.1 (2)
C16—C11—C1—O11−166.58 (13)Cu1—O31—C3—C31168.70 (10)
C12—C11—C1—O1114.2 (2)C36—C31—C3—O31144.89 (15)
C16—C11—C1—O2113.27 (19)C32—C31—C3—O31−31.8 (2)
C12—C11—C1—O21−165.91 (14)C36—C31—C3—O32−33.43 (19)
C16—C11—C12—C131.3 (2)C32—C31—C3—O32149.91 (14)
C1—C11—C12—C13−179.57 (13)C31—C36—C35—C34−2.2 (2)
C16—C11—C12—C121−177.11 (14)C33—C34—C35—C360.7 (2)
C1—C11—C12—C1212.1 (2)C16—C15—C14—C130.6 (2)
C11—C16—C15—C14−0.1 (2)C12—C13—C14—C15−0.2 (2)
Cu1—O12—C2—O22−5.0 (2)C26—C25—C24—C230.3 (2)
Cu1—O12—C2—C21175.52 (9)C25—C24—C23—C221.6 (3)
Cu1i—O22—C2—O127.4 (2)C21—C22—C23—C24−1.8 (2)
Cu1i—O22—C2—C21−173.12 (9)C221—C22—C23—C24179.87 (17)
Symmetry codes: (i) −x+1, −y+1, −z+1.
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O32—H32···O22i0.821.852.6604 (18)168
C16—H16···O210.932.392.721 (2)101
Symmetry codes: (i) −x+1, −y+1, −z+1.
Table 1
Selected geometric parameters (Å) top
Cu1—O21i1.9402 (12)Cu1—Cu1i2.5780 (9)
Cu1—O111.9559 (12)C31—C31.485 (2)
Cu1—O121.9585 (13)C11—C11.497 (2)
Cu1—O22i1.9900 (13)C2—C211.492 (2)
Cu1—O312.1622 (13)
Symmetry codes: (i) −x+1, −y+1, −z+1.
Table 2
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O32—H32···O22i0.821.852.6604 (18)168
C16—H16···O210.932.392.721 (2)101
Symmetry codes: (i) −x+1, −y+1, −z+1.
Acknowledgements top

Financial assistance from the University of the Free State and SASOL to ACS is gratefully acknowledged. Opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of SASOL.

references
References top

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