Bis[O-propan-2-yl (4-eth­oxy­phen­yl)dithio­phospho­nato-κ2S,S′]nickel(II)

Sewpersad, S.; Van Zyl, W.E.

doi:10.1107/S1600536812045114

metal-organic compounds

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ISSN: 2056-9890

Volume 68| Part 12| December 2012| Page m1457

doi:10.1107/S1600536812045114

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Bis[O-propan-2-yl (4-ethoxyphenyl)dithiophosphonato-κ²S,S′]nickel(II)

Shirveen Sewpersad ^a and Werner E. Van Zyl ^a ^*

^aSchool of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa
^*Correspondence e-mail: vanzylw@ukzn.ac.za

(Received 19 October 2012; accepted 31 October 2012; online 3 November 2012)

The title compound, [Ni(C₁₁H₁₆O₂PS₂)₂], is a neutral four-coordinate mononuclear complex with a square-planar geometry. The complex lies on an inversion center. The metal atom is surrounded by two chelating isobidentate O-propan-2-yl (4-ethoxyphenyl)dithiophosphonate ligands in a trans configuration binding through the S-donor atoms. The Ni—S bond lengths are 2.2328 (5) and 2.2369 (5) Å, an insignificant difference to be considered anisobidentate. The Ni⋯P separation is 2.8224 (5) Å and the S—P bond lengths are 2.0035 (7) and 2.0053 (7) Å. The S—Ni—S (chelating) and S—Ni—S (trans) bond angles are 88.321 (18) and 180°. The Ni—S—P bond angles are 83.26 (2) and 83.33 (2)°, indicating a very minor distortion from ideal square-planar geometry for the Ni atom. The P atom, however, is distorted quite significantly from an ideal tetrahedral geometry, as reflected by the S—P—S and O—P—C bond angles of 101.93 (3) and 100.70 (7)°, respectively.

Related literature

For information on dithiophosphonate compounds, see: Van Zyl & Fackler (2000 ); Van Zyl (2010 ). For examples of nickel(II) dithiophosphonate complexes, see: Liu et al. (2004 ); Gray et al. (2004 ); Aragoni et al. (2007 ); Arca et al. (1997 ); Malatesta & Pizzotti (1945 ); Hartung (1967 ).

Experimental

Crystal data

[Ni(C₁₁H₁₆O₂PS₂)₂]
M_r = 609.37
Triclinic, $[P \overline 1]$
a = 7.8893 (6) Å
b = 8.4178 (7) Å
c = 11.4825 (10) Å
α = 109.530 (4)°
β = 101.959 (4)°
γ = 93.913 (5)°
V = 695.22 (10) Å³
Z = 1
Mo Kα radiation
μ = 1.14 mm⁻¹
T = 173 K
0.39 × 0.26 × 0.14 mm

Data collection

Nonius KappaCCD diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.665, T_max = 0.857
14410 measured reflections
3062 independent reflections
2381 reflections with I > 2σ(I)
R_int = 0.035

Refinement

R[F² > 2σ(F²)] = 0.030
wR(F²) = 0.065
S = 1.02
3062 reflections
154 parameters
H-atom parameters constrained
Δρ_max = 0.27 e Å⁻³
Δρ_min = −0.31 e Å⁻³

Data collection: COLLECT (Nonius, 2000 ); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997 ); data reduction: DENZO-SMN; 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: SHELXL97.

Supporting information

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536812045114/bh2460sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812045114/bh2460Isup2.hkl

checkCIF report

Comment top

The phosphor-1,1,-dithiolate class of compounds is the heavier and softer congener of the more popular phosphonate derivatives. It contains the S₂P functionality as a common feature and several sub-categories are known which include the dithiophosphato [S₂P(OR')₂]¯, (typically, R' = alkyl), dithiophosphinato [S₂PR₂]¯ (R = alkyl or aryl), and dithiophosphonato [S₂PR(OR')]¯, (typically, R = aryl or ferrocenyl, R' = alkyl) monoanionic ligands. The latter may be described as a hybrid of the former two, and are also much less developed. Amongst all metals involved in the coordination chemistry of dithiophosphonato ligands, however, nickel(II) is by far the best represented (Aragoni et al., 2007; Arca et al., 1997; Liu et al., 2004; Gray et al., 2004), with the first example dating back to 1945 (Malatesta & Pizzotti, 1945) whilst the first X-ray structural report of a nickel(II) dithiophosphonate complex reported more than 2 decades later (Hartung, 1967). The complex in the present study was formed from the reaction between NiCl₂.6H₂0 and the ammonium salt of [S₂P(OⁱPr)(4-C₆H₄OEt)] (molar ratio 1:2) in an aqueous/methanolic solution, the NH₄Cl by-product was dissolved and the precipitated product filtered off and washed with water. General and convenient methods to prepare dithiophosphonate salt derivatives have been reported (Van Zyl & Fackler, 2000; Van Zyl, 2010).

Related literature top

For information on dithiophosphonate compounds, see: Van Zyl & Fackler (2000); Van Zyl (2010). For examples of nickel(II) dithiophosphonate complexes, see: Liu et al. (2004); Gray et al. (2004); Aragoni et al. (2007); Arca et al. (1997); Malatesta & Pizzotti (1945); Hartung (1967).

Experimental top

A colorless methanol (40 ml) solution of NH₄[S₂P(OⁱPr)(4-C₆H₄OEt)] (982 mg, 3.347 mmol) was prepared. A second green solution of NiCl₂.6H₂0 (399 mg, 1.679 mmol) in deionized water (20 ml) was prepared, and added to the colorless solution with stirring over a period of 5 min. This resulted in a purple precipitate indicating the formation of the title complex. The precipitate was collected by vacuum filtration, washed with water (3 × 10 ml) and allowed to dry under vacuum for a period of 3 hrs, yielding a dry, free-flowing purple powder. Purple crystals suitable for X-ray analysis were grown by the slow diffusion of hexane into a dichloromethane solution of the title complex. Yield: 761 mg, 75%. M.p. 168–169°C.

³¹P NMR (CDCl₃): δ (p.p.m.): 97.96. ¹H NMR (CDCl₃): δ (p.p.m.): 7.96 (2H, dd, o-ArH), 6.95 (2H, dd, m-ArH), 5.19 (1H, d quart, OCH), 4.06 (2H, quart, ArOCH₂), 1.42 (3H, t, ArOCH₂CH₃), 1.38 (6H, d, CH₃). ¹³C NMR (CDCl₃): δ (p.p.m.): 162.22 (p-ArC), 131.71 (m-ArC), 128.04 (Ar—C₁), 114.44 (o-ArC), 72.10 (CH), 63.76(ArOCH₂), 24.30 (CH₃), 14.67 (ArOCH₂CH₃).

Computing details top

Data collection: COLLECT (Nonius, 2000); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997); 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: SHELXL97 (Sheldrick, 2008).

Figures top

Fig. 1. The molecular structure of the title complex, shown with 50% probability displacement ellipsoids.

Bis[O-propan-2-yl (4-ethoxyphenyl)dithiophosphonato-κ²S,S']nickel(II) top

Crystal data top

[Ni(C₁₁H₁₆O₂PS₂)₂]	Z = 1
M_r = 609.37	F(000) = 318
Triclinic, P1	D_x = 1.455 Mg m⁻³
Hall symbol: -P 1	Melting point: 441 K
a = 7.8893 (6) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.4178 (7) Å	Cell parameters from 14410 reflections
c = 11.4825 (10) Å	θ = 2.6–27.5°
α = 109.530 (4)°	µ = 1.14 mm⁻¹
β = 101.959 (4)°	T = 173 K
γ = 93.913 (5)°	Block, purple
V = 695.22 (10) Å³	0.39 × 0.26 × 0.14 mm

Data collection top

Nonius KappaCCD diffractometer	3062 independent reflections
Radiation source: fine-focus sealed tube	2381 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.035
1.2° ϕ scans and ω scans	θ_max = 27.5°, θ_min = 2.6°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = 0→10
T_min = 0.665, T_max = 0.857	k = −10→10
14410 measured reflections	l = −14→14

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)] = 0.030	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.065	H-atom parameters constrained
S = 1.02	w = 1/[σ²(F_o²) + (0.028P)² + 0.2462P] where P = (F_o² + 2F_c²)/3
3062 reflections	(Δ/σ)_max < 0.001
154 parameters	Δρ_max = 0.27 e Å⁻³
0 restraints	Δρ_min = −0.31 e Å⁻³
0 constraints

Crystal data top

[Ni(C₁₁H₁₆O₂PS₂)₂]	γ = 93.913 (5)°
M_r = 609.37	V = 695.22 (10) Å³
Triclinic, P1	Z = 1
a = 7.8893 (6) Å	Mo Kα radiation
b = 8.4178 (7) Å	µ = 1.14 mm⁻¹
c = 11.4825 (10) Å	T = 173 K
α = 109.530 (4)°	0.39 × 0.26 × 0.14 mm
β = 101.959 (4)°

Data collection top

Nonius KappaCCD diffractometer	3062 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	2381 reflections with I > 2σ(I)
T_min = 0.665, T_max = 0.857	R_int = 0.035
14410 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.030	0 restraints
wR(F²) = 0.065	H-atom parameters constrained
S = 1.02	Δρ_max = 0.27 e Å⁻³
3062 reflections	Δρ_min = −0.31 e Å⁻³
154 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Ni1	0.5000	0.5000	0.0000	0.02229 (10)
S1	0.64099 (6)	0.75272 (6)	0.02356 (5)	0.02729 (13)
S2	0.58144 (7)	0.38847 (6)	−0.18193 (5)	0.02842 (13)
P1	0.64598 (6)	0.63370 (6)	−0.15849 (5)	0.02271 (12)
O1	0.19743 (19)	0.8621 (2)	−0.53641 (14)	0.0416 (4)
O2	0.82744 (15)	0.67472 (17)	−0.18857 (12)	0.0266 (3)
C1	0.5013 (2)	0.7025 (2)	−0.26799 (17)	0.0231 (4)
C2	0.4982 (3)	0.8752 (3)	−0.2422 (2)	0.0317 (5)
H2	0.5664	0.9551	−0.1634	0.038*
C3	0.3977 (3)	0.9335 (3)	−0.3289 (2)	0.0336 (5)
H3	0.3976	1.0523	−0.3099	0.040*
C4	0.2974 (2)	0.8179 (3)	−0.44330 (19)	0.0308 (5)
C5	0.2947 (3)	0.6447 (3)	−0.4682 (2)	0.0405 (6)
H5	0.2227	0.5647	−0.5455	0.049*
C6	0.3960 (3)	0.5880 (3)	−0.3815 (2)	0.0360 (5)
H6	0.3937	0.4690	−0.3996	0.043*
C7	0.2176 (3)	1.0381 (3)	−0.5253 (2)	0.0463 (6)
H7A	0.3430	1.0834	−0.5079	0.056*
H7B	0.1696	1.1073	−0.4548	0.056*
C8	0.1178 (3)	1.0441 (4)	−0.6510 (3)	0.0654 (8)
H8A	0.1653	0.9734	−0.7200	0.098*
H8B	0.1299	1.1621	−0.6482	0.098*
H8C	−0.0063	1.0008	−0.6662	0.098*
C9	0.9927 (2)	0.6387 (3)	−0.11992 (19)	0.0302 (5)
H9	0.9693	0.6009	−0.0502	0.036*
C10	1.1185 (3)	0.8017 (3)	−0.0629 (3)	0.0646 (8)
H10A	1.1334	0.8447	−0.1302	0.097*
H10B	1.2319	0.7811	−0.0211	0.097*
H10C	1.0725	0.8863	0.0001	0.097*
C11	1.0541 (3)	0.4983 (3)	−0.2128 (2)	0.0567 (7)
H11A	0.9642	0.3971	−0.2481	0.085*
H11B	1.1628	0.4711	−0.1695	0.085*
H11C	1.0758	0.5343	−0.2819	0.085*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Ni1	0.02098 (18)	0.0252 (2)	0.0246 (2)	0.00292 (14)	0.00633 (14)	0.01367 (16)
S1	0.0299 (3)	0.0276 (3)	0.0257 (3)	0.0000 (2)	0.0069 (2)	0.0119 (2)
S2	0.0343 (3)	0.0258 (3)	0.0305 (3)	0.0050 (2)	0.0128 (2)	0.0138 (2)
P1	0.0197 (2)	0.0268 (3)	0.0260 (3)	0.00378 (19)	0.0065 (2)	0.0145 (2)
O1	0.0440 (9)	0.0502 (10)	0.0364 (9)	0.0170 (7)	0.0024 (7)	0.0253 (8)
O2	0.0170 (6)	0.0397 (8)	0.0333 (8)	0.0077 (6)	0.0074 (6)	0.0248 (7)
C1	0.0184 (9)	0.0282 (11)	0.0257 (10)	0.0047 (8)	0.0065 (8)	0.0125 (9)
C2	0.0288 (11)	0.0299 (12)	0.0315 (12)	0.0026 (9)	−0.0009 (9)	0.0100 (10)
C3	0.0329 (11)	0.0284 (11)	0.0413 (13)	0.0073 (9)	0.0041 (10)	0.0170 (10)
C4	0.0259 (10)	0.0406 (13)	0.0323 (12)	0.0111 (9)	0.0076 (9)	0.0198 (10)
C5	0.0466 (14)	0.0352 (13)	0.0292 (12)	0.0069 (10)	−0.0062 (10)	0.0073 (10)
C6	0.0407 (12)	0.0283 (11)	0.0355 (12)	0.0090 (9)	0.0008 (10)	0.0112 (10)
C7	0.0392 (13)	0.0601 (16)	0.0630 (16)	0.0159 (11)	0.0154 (12)	0.0483 (14)
C8	0.0461 (15)	0.107 (2)	0.082 (2)	0.0262 (15)	0.0198 (14)	0.0779 (19)
C9	0.0194 (10)	0.0415 (12)	0.0363 (12)	0.0080 (9)	0.0029 (8)	0.0240 (10)
C10	0.0313 (13)	0.0465 (15)	0.097 (2)	0.0041 (11)	−0.0166 (14)	0.0217 (16)
C11	0.0352 (13)	0.0701 (18)	0.0548 (17)	0.0292 (12)	−0.0002 (12)	0.0123 (14)

Geometric parameters (Å, º) top

Ni1—S2	2.2328 (5)	C4—C5	1.387 (3)
Ni1—S2ⁱ	2.2328 (5)	C5—C6	1.378 (3)
Ni1—S1ⁱ	2.2369 (5)	C5—H5	0.9500
Ni1—S1	2.2369 (5)	C6—H6	0.9500
Ni1—P1	2.8224 (5)	C7—C8	1.513 (3)
Ni1—P1ⁱ	2.8224 (5)	C7—H7A	0.9900
S1—P1	2.0035 (7)	C7—H7B	0.9900
S2—P1	2.0053 (7)	C8—H8A	0.9800
P1—O2	1.5828 (12)	C8—H8B	0.9800
P1—C1	1.7894 (18)	C8—H8C	0.9800
O1—C4	1.364 (2)	C9—C11	1.489 (3)
O1—C7	1.438 (3)	C9—C10	1.497 (3)
O2—C9	1.484 (2)	C9—H9	1.0000
C1—C6	1.385 (3)	C10—H10A	0.9800
C1—C2	1.386 (3)	C10—H10B	0.9800
C2—C3	1.383 (3)	C10—H10C	0.9800
C2—H2	0.9500	C11—H11A	0.9800
C3—C4	1.381 (3)	C11—H11B	0.9800
C3—H3	0.9500	C11—H11C	0.9800

S2—Ni1—S2ⁱ	180.0	O1—C4—C5	116.20 (19)
S2—Ni1—S1ⁱ	91.679 (18)	C3—C4—C5	119.59 (18)
S2ⁱ—Ni1—S1ⁱ	88.321 (18)	C6—C5—C4	120.3 (2)
S2—Ni1—S1	88.321 (18)	C6—C5—H5	119.8
S2ⁱ—Ni1—S1	91.679 (18)	C4—C5—H5	119.8
S1ⁱ—Ni1—S1	180.0	C5—C6—C1	120.7 (2)
S2—Ni1—P1	44.885 (16)	C5—C6—H6	119.6
S2ⁱ—Ni1—P1	135.115 (17)	C1—C6—H6	119.6
S1ⁱ—Ni1—P1	135.174 (16)	O1—C7—C8	106.7 (2)
S1—Ni1—P1	44.827 (16)	O1—C7—H7A	110.4
S2—Ni1—P1ⁱ	135.115 (16)	C8—C7—H7A	110.4
S2ⁱ—Ni1—P1ⁱ	44.885 (16)	O1—C7—H7B	110.4
S1ⁱ—Ni1—P1ⁱ	44.827 (16)	C8—C7—H7B	110.4
S1—Ni1—P1ⁱ	135.173 (16)	H7A—C7—H7B	108.6
P1—Ni1—P1ⁱ	180.0	C7—C8—H8A	109.5
P1—S1—Ni1	83.26 (2)	C7—C8—H8B	109.5
P1—S2—Ni1	83.33 (2)	H8A—C8—H8B	109.5
O2—P1—C1	100.70 (7)	C7—C8—H8C	109.5
O2—P1—S1	113.76 (6)	H8A—C8—H8C	109.5
C1—P1—S1	113.38 (7)	H8B—C8—H8C	109.5
O2—P1—S2	114.16 (6)	O2—C9—C11	107.84 (16)
C1—P1—S2	113.47 (7)	O2—C9—C10	107.22 (17)
S1—P1—S2	101.93 (3)	C11—C9—C10	113.9 (2)
O2—P1—Ni1	141.40 (5)	O2—C9—H9	109.3
C1—P1—Ni1	117.90 (6)	C11—C9—H9	109.3
S1—P1—Ni1	51.913 (17)	C10—C9—H9	109.3
S2—P1—Ni1	51.790 (18)	C9—C10—H10A	109.5
C4—O1—C7	118.46 (17)	C9—C10—H10B	109.5
C9—O2—P1	121.36 (11)	H10A—C10—H10B	109.5
C6—C1—C2	118.40 (18)	C9—C10—H10C	109.5
C6—C1—P1	121.83 (15)	H10A—C10—H10C	109.5
C2—C1—P1	119.72 (15)	H10B—C10—H10C	109.5
C3—C2—C1	121.31 (19)	C9—C11—H11A	109.5
C3—C2—H2	119.3	C9—C11—H11B	109.5
C1—C2—H2	119.3	H11A—C11—H11B	109.5
C4—C3—C2	119.60 (19)	C9—C11—H11C	109.5
C4—C3—H3	120.2	H11A—C11—H11C	109.5
C2—C3—H3	120.2	H11B—C11—H11C	109.5
O1—C4—C3	124.21 (19)

S2—Ni1—S1—P1	12.58 (2)	S1—P1—O2—C9	58.46 (15)
S2ⁱ—Ni1—S1—P1	−167.42 (2)	S2—P1—O2—C9	−57.97 (15)
P1ⁱ—Ni1—S1—P1	180.0	Ni1—P1—O2—C9	0.38 (19)
S1ⁱ—Ni1—S2—P1	167.43 (2)	O2—P1—C1—C6	101.89 (17)
S1—Ni1—S2—P1	−12.57 (2)	S1—P1—C1—C6	−136.21 (15)
P1ⁱ—Ni1—S2—P1	180.0	S2—P1—C1—C6	−20.53 (18)
Ni1—S1—P1—O2	−137.71 (6)	Ni1—P1—C1—C6	−78.31 (17)
Ni1—S1—P1—C1	107.99 (7)	O2—P1—C1—C2	−75.50 (16)
Ni1—S1—P1—S2	−14.35 (3)	S1—P1—C1—C2	46.41 (17)
Ni1—S2—P1—O2	137.47 (6)	S2—P1—C1—C2	162.08 (14)
Ni1—S2—P1—C1	−107.90 (7)	Ni1—P1—C1—C2	104.30 (15)
Ni1—S2—P1—S1	14.37 (3)	C6—C1—C2—C3	−2.3 (3)
S2—Ni1—P1—O2	−81.30 (9)	P1—C1—C2—C3	175.20 (15)
S2ⁱ—Ni1—P1—O2	98.70 (9)	C1—C2—C3—C4	0.4 (3)
S1ⁱ—Ni1—P1—O2	−99.27 (9)	C7—O1—C4—C3	10.4 (3)
S1—Ni1—P1—O2	80.73 (9)	C7—O1—C4—C5	−169.75 (19)
S2—Ni1—P1—C1	99.02 (8)	C2—C3—C4—O1	−178.27 (18)
S2ⁱ—Ni1—P1—C1	−80.98 (8)	C2—C3—C4—C5	1.9 (3)
S1ⁱ—Ni1—P1—C1	81.05 (8)	O1—C4—C5—C6	177.9 (2)
S1—Ni1—P1—C1	−98.95 (8)	C3—C4—C5—C6	−2.2 (3)
S2—Ni1—P1—S1	−162.03 (3)	C4—C5—C6—C1	0.3 (3)
S2ⁱ—Ni1—P1—S1	17.97 (3)	C2—C1—C6—C5	1.9 (3)
S1ⁱ—Ni1—P1—S1	180.000 (1)	P1—C1—C6—C5	−175.49 (17)
S2ⁱ—Ni1—P1—S2	180.0	C4—O1—C7—C8	169.83 (18)
S1ⁱ—Ni1—P1—S2	−17.97 (3)	P1—O2—C9—C11	112.06 (18)
S1—Ni1—P1—S2	162.03 (3)	P1—O2—C9—C10	−124.87 (18)
C1—P1—O2—C9	−179.91 (14)

Symmetry code: (i) −x+1, −y+1, −z.

Experimental details

Crystal data
Chemical formula	[Ni(C₁₁H₁₆O₂PS₂)₂]
M_r	609.37
Crystal system, space group	Triclinic, P1
Temperature (K)	173
a, b, c (Å)	7.8893 (6), 8.4178 (7), 11.4825 (10)
α, β, γ (°)	109.530 (4), 101.959 (4), 93.913 (5)
V (Å³)	695.22 (10)
Z	1
Radiation type	Mo Kα
µ (mm⁻¹)	1.14
Crystal size (mm)	0.39 × 0.26 × 0.14

Data collection
Diffractometer	Nonius KappaCCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.665, 0.857
No. of measured, independent and observed [I > 2σ(I)] reflections	14410, 3062, 2381
R_int	0.035
(sin θ/λ)_max (Å⁻¹)	0.649

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.030, 0.065, 1.02
No. of reflections	3062
No. of parameters	154
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.27, −0.31

Computer programs: COLLECT (Nonius, 2000), DENZO-SMN (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Acknowledgements

The authors thank the National Research Foundation (NRF) and UKZN for financial support.

References

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