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

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

doi:10.1107/S1600536812047368

metal-organic compounds

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ISSN: 2056-9890

Volume 68| Part 12| December 2012| Page m1534

doi:10.1107/S1600536812047368

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

Shirveen Sewpersad,^a Bernard Omondi ^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 4 November 2012; accepted 18 November 2012; online 24 November 2012)

The title compound, [Ni(C₁₁H₁₆O₂PS₂)₂], contains a four-coordinate Ni^II cation with an idealized square-planar geometry. The metal atom is surrounded by two chelating isobidentate dithiophosphonate ligands in a trans or anti configuration, binding through the S-donor atoms.

Related literature

For information on the first structure of an Ni^II–dithiophosphonate complex, see: Hartung (1967 ). For general preparative procedures for dithiophosphonates, see: Van Zyl (2010 ); Van Zyl & Fackler (2000 ). For a comprehensive review on dithiophosphonates, see: Van Zyl & Woollins (2012 ). For reports on the synthesis and structures of different types of Ni^II–dithiophosphonate complexes, see: Liu et al. (2004 ); Gray et al. (2004 ); Aragoni et al. (2007 ); Arca et al. (1997 ).

Experimental

Crystal data

[Ni(C₁₁H₁₆O₂PS₂)₂]
M_r = 609.37
Monoclinic, P 2₁ /c
a = 9.4227 (2) Å
b = 15.6479 (3) Å
c = 9.5281 (2) Å
β = 102.878 (1)°
V = 1369.54 (5) Å³
Z = 2
Mo Kα radiation
μ = 1.16 mm⁻¹
T = 173 K
0.40 × 0.34 × 0.11 mm

Data collection

Bruker APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2008 ) T_min = 0.655, T_max = 0.883
32798 measured reflections
3446 independent reflections
3335 reflections with I > 2σ(I)
R_int = 0.020

Refinement

R[F² > 2σ(F²)] = 0.019
wR(F²) = 0.053
S = 1.09
3446 reflections
153 parameters
H-atom parameters constrained
Δρ_max = 0.35 e Å⁻³
Δρ_min = −0.39 e Å⁻³

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT-Plus (Bruker, 2008); data reduction: SAINT-Plus and XPREP (Bruker, 2008); 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/S1600536812047368/fj2606sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536812047368/fj2606Isup2.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 (Van Zyl, 2010). It contains the S₂P functionality as a common feature and several sub-categories are known which include the dithiophosphato [S₂P(OR)₂]¯, (R = typically alkyl), dithiophosphinato [S₂PR₂]¯ (R = alkyl or aryl), and dithiophosphonato [S₂PR(OR')]¯, (R = typically 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 [Liu et al. (2004); Gray et al. (2004); Aragoni et al. (2007); Arca et al. (1997); Van Zyl & Woollins, (2012)] with the first X-ray structural report of a nickel(II) dithiophosphonate complex reported more than 4 decades ago (Hartung, 1967). The structure of the title complex does not differ significantly from related Ni^II complexes previously reported (see related literature). The Ni—S bond length is 2.2254 (2) and 2.2264 (2) Å, which is an insignificantly small difference to be considered anisobidentate. The Ni—P bond length is 2.8310 (3) Å, and the S—P bond length is 2.0081 (3) and 2.0026 (4) Å, respectively.

The complex in the present study was formed from the reaction between NiCl₂.6H₂0 and the ammonium salt of [S₂P(OPr)(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).

Related literature top

For information on dithiophosphonate compounds, see: Hartung (1967); Van Zyl (2010); Van Zyl & Fackler (2000); Van Zyl & Woollins (2012); Liu et al. (2004); Gray et al. (2004); Aragoni et al. (2007); Arca et al. (1997). [Please give more information on what type of information is available from the different references, e.g. 'For the synthesis of dithiophosphonate salt derivatives, see: Van Zyl & Fackler (2000)']

Experimental top

A colorless methanol (40 ml) solution of NH₄[S₂P(OPr)(4-C₆H₄OEt)] (997 mg, 3.398 mmol) was prepared. A second green solution of NiCl₂.6H₂0 (424 mg, 1.699 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 x 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: 740 mg, 30%. M.p. 122°C.

³¹P NMR (CDCl₃): δ (p.p.m.): 101.27. ¹H NMR (CDCl₃): δ (p.p.m.): 7.95 (2H, dd, J(³¹P-¹H) = 13.96 Hz, J(¹H -¹H) = 8.80 Hz, o-ArH), 6.94 (2H, dd, J(³¹P-¹H) = 8.82 Hz, J(¹H -¹H) =3.06 Hz, m-ArH), 4.26 (2H, dt, J(¹H -¹H) = 7.89 Hz, POCH₂), 4.06 (2H, q, J(¹H-¹H) = 6.97 Hz, ArOCH₂), 1.74 (2H, m, J(¹H -¹H) = 7.16 Hz, POCH₂CH₂), 1.41 (3H, t, J(¹H -¹H) = 6.98 Hz, ArOCH₂CH₃), 0.96 (3H, t, J(¹H -¹H) = 7.38 Hz, POCH₂CH₂CH₃).

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT-Plus (Bruker, 2008); data reduction: SAINT-Plus and XPREP (Bruker, 2008); 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 ORTEP molecular structure of the title complex, shown with 50% probability.

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

Crystal data top

[Ni(C₁₁H₁₆O₂PS₂)₂]	F(000) = 636
M_r = 609.37	D_x = 1.478 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 32798 reflections
a = 9.4227 (2) Å	θ = 2.2–28.8°
b = 15.6479 (3) Å	µ = 1.16 mm⁻¹
c = 9.5281 (2) Å	T = 173 K
β = 102.878 (1)°	Block, purple
V = 1369.54 (5) Å³	0.40 × 0.34 × 0.11 mm
Z = 2

Data collection top

Bruker APEXII CCD diffractometer	3446 independent reflections
Radiation source: fine-focus sealed tube	3335 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.020
ϕ and ω scans	θ_max = 28.5°, θ_min = 2.2°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −12→12
T_min = 0.655, T_max = 0.883	k = −20→20
32798 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)] = 0.019	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.053	H-atom parameters constrained
S = 1.09	w = 1/[σ²(F_o²) + (0.0288P)² + 0.5042P] where P = (F_o² + 2F_c²)/3
3446 reflections	(Δ/σ)_max = 0.001
153 parameters	Δρ_max = 0.35 e Å⁻³
0 restraints	Δρ_min = −0.39 e Å⁻³

Crystal data top

[Ni(C₁₁H₁₆O₂PS₂)₂]	V = 1369.54 (5) Å³
M_r = 609.37	Z = 2
Monoclinic, P2₁/c	Mo Kα radiation
a = 9.4227 (2) Å	µ = 1.16 mm⁻¹
b = 15.6479 (3) Å	T = 173 K
c = 9.5281 (2) Å	0.40 × 0.34 × 0.11 mm
β = 102.878 (1)°

Data collection top

Bruker APEXII CCD diffractometer	3446 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2008)	3335 reflections with I > 2σ(I)
T_min = 0.655, T_max = 0.883	R_int = 0.020
32798 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.019	0 restraints
wR(F²) = 0.053	H-atom parameters constrained
S = 1.09	Δρ_max = 0.35 e Å⁻³
3446 reflections	Δρ_min = −0.39 e Å⁻³
153 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
C1	0.29971 (11)	−0.07749 (6)	0.10196 (10)	0.01471 (18)
C2	0.31929 (11)	−0.15981 (7)	0.15941 (11)	0.01713 (19)
H2	0.3857	−0.1688	0.2490	0.021*
C3	0.24319 (11)	−0.22889 (7)	0.08761 (11)	0.01787 (19)
H3	0.2576	−0.2847	0.1275	0.021*
C4	0.14523 (11)	−0.21525 (7)	−0.04391 (11)	0.01670 (19)
C5	0.12225 (11)	−0.13257 (7)	−0.10066 (11)	0.0193 (2)
H5	0.0537	−0.1233	−0.1889	0.023*
C6	0.19899 (11)	−0.06433 (7)	−0.02877 (11)	0.01805 (19)
H6	0.1835	−0.0084	−0.0680	0.022*
C7	0.08205 (13)	−0.36349 (7)	−0.07133 (13)	0.0237 (2)
H7A	0.0514	−0.3672	0.0214	0.028*
H7B	0.1845	−0.3826	−0.0560	0.028*
C8	−0.01499 (13)	−0.41831 (8)	−0.18383 (15)	0.0293 (3)
H8A	−0.1153	−0.3973	−0.2004	0.044*
H8B	−0.0113	−0.4776	−0.1501	0.044*
H8C	0.0187	−0.4156	−0.2739	0.044*
C9	0.45560 (11)	0.16125 (6)	0.09836 (11)	0.01833 (19)
H9A	0.5607	0.1608	0.0983	0.022*
H9B	0.4439	0.1817	0.1935	0.022*
C10	0.37431 (12)	0.21896 (7)	−0.01933 (13)	0.0217 (2)
H10A	0.3759	0.1934	−0.1140	0.026*
H10B	0.4247	0.2748	−0.0132	0.026*
C11	0.21705 (14)	0.23324 (9)	−0.00963 (17)	0.0339 (3)
H11A	0.1684	0.1779	−0.0093	0.051*
H11B	0.1666	0.2668	−0.0927	0.051*
H11C	0.2149	0.2642	0.0793	0.051*
O1	0.06717 (8)	−0.27749 (5)	−0.12533 (8)	0.02031 (15)
O2	0.39444 (8)	0.07533 (5)	0.06998 (8)	0.01738 (15)
P1	0.39744 (3)	0.010268 (16)	0.19771 (3)	0.01352 (6)
S2	0.30614 (3)	0.057093 (16)	0.35296 (3)	0.01596 (6)
S4	0.40325 (3)	0.020730 (18)	0.69008 (3)	0.01702 (6)
Ni1	0.5000	0.0000	0.5000	0.01246 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
C1	0.0179 (4)	0.0127 (4)	0.0136 (4)	−0.0006 (3)	0.0037 (3)	−0.0009 (3)
C2	0.0203 (4)	0.0160 (5)	0.0145 (4)	0.0006 (4)	0.0025 (3)	0.0016 (4)
C3	0.0214 (5)	0.0135 (4)	0.0188 (5)	−0.0003 (4)	0.0049 (4)	0.0014 (4)
C4	0.0166 (4)	0.0159 (5)	0.0185 (4)	−0.0024 (4)	0.0058 (4)	−0.0034 (4)
C5	0.0198 (5)	0.0195 (5)	0.0167 (5)	−0.0009 (4)	−0.0003 (4)	0.0002 (4)
C6	0.0218 (5)	0.0142 (4)	0.0169 (5)	0.0003 (4)	0.0016 (4)	0.0017 (3)
C7	0.0246 (5)	0.0148 (5)	0.0315 (6)	−0.0027 (4)	0.0060 (4)	−0.0044 (4)
C8	0.0228 (5)	0.0204 (5)	0.0436 (7)	−0.0046 (4)	0.0050 (5)	−0.0106 (5)
C9	0.0231 (5)	0.0132 (4)	0.0191 (5)	−0.0042 (4)	0.0054 (4)	−0.0005 (4)
C10	0.0220 (5)	0.0158 (5)	0.0283 (5)	0.0011 (4)	0.0079 (4)	0.0049 (4)
C11	0.0239 (6)	0.0256 (6)	0.0537 (8)	0.0051 (5)	0.0119 (5)	0.0057 (6)
O1	0.0216 (4)	0.0159 (3)	0.0225 (4)	−0.0040 (3)	0.0029 (3)	−0.0041 (3)
O2	0.0258 (4)	0.0121 (3)	0.0136 (3)	−0.0032 (3)	0.0030 (3)	0.0007 (3)
P1	0.01689 (12)	0.01224 (12)	0.01123 (12)	−0.00026 (8)	0.00272 (9)	0.00008 (8)
S2	0.01740 (12)	0.01656 (12)	0.01382 (11)	0.00237 (8)	0.00325 (9)	−0.00129 (8)
S4	0.01565 (11)	0.02265 (13)	0.01304 (12)	0.00131 (9)	0.00382 (9)	−0.00118 (9)
Ni1	0.01398 (9)	0.01286 (9)	0.01053 (9)	−0.00043 (6)	0.00272 (7)	−0.00086 (6)

Geometric parameters (Å, º) top

C1—C2	1.3957 (14)	C9—C10	1.5089 (14)
C1—C6	1.4028 (13)	C9—H9A	0.9900
C1—P1	1.7865 (10)	C9—H9B	0.9900
C2—C3	1.3899 (14)	C10—C11	1.5216 (16)
C2—H2	0.9500	C10—H10A	0.9900
C3—C4	1.3973 (14)	C10—H10B	0.9900
C3—H3	0.9500	C11—H11A	0.9800
C4—O1	1.3554 (12)	C11—H11B	0.9800
C4—C5	1.4004 (14)	C11—H11C	0.9800
C5—C6	1.3825 (14)	O2—P1	1.5822 (7)
C5—H5	0.9500	P1—S4ⁱ	2.0026 (4)
C6—H6	0.9500	P1—S2	2.0081 (3)
C7—O1	1.4364 (13)	P1—Ni1	2.8310 (3)
C7—C8	1.5113 (16)	S2—Ni1	2.2264 (2)
C7—H7A	0.9900	S4—P1ⁱ	2.0026 (4)
C7—H7B	0.9900	S4—Ni1	2.2254 (2)
C8—H8A	0.9800	Ni1—S4ⁱ	2.2254 (2)
C8—H8B	0.9800	Ni1—S2ⁱ	2.2264 (2)
C8—H8C	0.9800	Ni1—P1ⁱ	2.8310 (3)
C9—O2	1.4640 (12)

C2—C1—C6	119.20 (9)	C9—C10—H10B	109.1
C2—C1—P1	120.07 (7)	C11—C10—H10B	109.1
C6—C1—P1	120.69 (8)	H10A—C10—H10B	107.8
C3—C2—C1	121.07 (9)	C10—C11—H11A	109.5
C3—C2—H2	119.5	C10—C11—H11B	109.5
C1—C2—H2	119.5	H11A—C11—H11B	109.5
C2—C3—C4	119.20 (9)	C10—C11—H11C	109.5
C2—C3—H3	120.4	H11A—C11—H11C	109.5
C4—C3—H3	120.4	H11B—C11—H11C	109.5
O1—C4—C3	124.71 (9)	C4—O1—C7	118.10 (8)
O1—C4—C5	115.18 (9)	C9—O2—P1	120.67 (6)
C3—C4—C5	120.12 (9)	O2—P1—C1	100.55 (4)
C6—C5—C4	120.23 (9)	O2—P1—S4ⁱ	114.86 (3)
C6—C5—H5	119.9	C1—P1—S4ⁱ	113.69 (3)
C4—C5—H5	119.9	O2—P1—S2	113.25 (3)
C5—C6—C1	120.15 (9)	C1—P1—S2	113.57 (3)
C5—C6—H6	119.9	S4ⁱ—P1—S2	101.530 (15)
C1—C6—H6	119.9	O2—P1—Ni1	139.70 (3)
O1—C7—C8	106.40 (10)	C1—P1—Ni1	119.74 (3)
O1—C7—H7A	110.4	S4ⁱ—P1—Ni1	51.409 (9)
C8—C7—H7A	110.4	S2—P1—Ni1	51.421 (9)
O1—C7—H7B	110.4	P1—S2—Ni1	83.742 (11)
C8—C7—H7B	110.4	P1ⁱ—S4—Ni1	83.894 (11)
H7A—C7—H7B	108.6	S4ⁱ—Ni1—S4	180.0
C7—C8—H8A	109.5	S4ⁱ—Ni1—S2ⁱ	91.498 (9)
C7—C8—H8B	109.5	S4—Ni1—S2ⁱ	88.502 (9)
H8A—C8—H8B	109.5	S4ⁱ—Ni1—S2	88.502 (9)
C7—C8—H8C	109.5	S4—Ni1—S2	91.498 (9)
H8A—C8—H8C	109.5	S2ⁱ—Ni1—S2	180.0
H8B—C8—H8C	109.5	S4ⁱ—Ni1—P1ⁱ	135.304 (8)
O2—C9—C10	107.37 (8)	S4—Ni1—P1ⁱ	44.696 (8)
O2—C9—H9A	110.2	S2ⁱ—Ni1—P1ⁱ	44.837 (8)
C10—C9—H9A	110.2	S2—Ni1—P1ⁱ	135.163 (8)
O2—C9—H9B	110.2	S4ⁱ—Ni1—P1	44.696 (8)
C10—C9—H9B	110.2	S4—Ni1—P1	135.304 (8)
H9A—C9—H9B	108.5	S2ⁱ—Ni1—P1	135.163 (8)
C9—C10—C11	112.50 (10)	S2—Ni1—P1	44.837 (8)
C9—C10—H10A	109.1	P1ⁱ—Ni1—P1	180.0
C11—C10—H10A	109.1

C6—C1—C2—C3	1.47 (15)	C6—C1—P1—Ni1	153.56 (7)
C1—C2—C3—C4	−0.24 (15)	O2—P1—S2—Ni1	−135.98 (3)
C2—C3—C4—C5	−1.32 (15)	C1—P1—S2—Ni1	110.14 (4)
C3—C4—C5—C6	1.64 (16)	S4ⁱ—P1—S2—Ni1	−12.297 (14)
C4—C5—C6—C1	−0.40 (16)	P1ⁱ—S4—Ni1—S2ⁱ	10.854 (12)
C2—C1—C6—C5	−1.14 (15)	P1ⁱ—S4—Ni1—S2	−169.146 (12)
O2—C9—C10—C11	68.95 (12)	P1—S2—Ni1—S4ⁱ	10.827 (12)
C3—C4—O1—C7	2.04 (15)	P1—S2—Ni1—S4	−169.173 (12)
C10—C9—O2—P1	−151.82 (7)	O2—P1—Ni1—S4ⁱ	−83.72 (5)
C9—O2—P1—C1	176.20 (7)	C1—P1—Ni1—S4ⁱ	97.82 (4)
C9—O2—P1—S4ⁱ	−61.34 (8)	S2—P1—Ni1—S4ⁱ	−164.515 (17)
C9—O2—P1—S2	54.69 (8)	O2—P1—Ni1—S4	96.28 (5)
C9—O2—P1—Ni1	−2.44 (10)	C1—P1—Ni1—S4	−82.18 (4)
C2—C1—P1—O2	156.81 (8)	S2—P1—Ni1—S4	15.485 (17)
C6—C1—P1—O2	−25.43 (9)	O2—P1—Ni1—S2ⁱ	−99.21 (5)
C2—C1—P1—S4ⁱ	33.54 (9)	C1—P1—Ni1—S2ⁱ	82.33 (4)
C6—C1—P1—S4ⁱ	−148.71 (7)	S4ⁱ—P1—Ni1—S2ⁱ	−15.485 (17)
C2—C1—P1—S2	−81.91 (8)	O2—P1—Ni1—S2	80.79 (5)
C6—C1—P1—S2	95.85 (8)	C1—P1—Ni1—S2	−97.67 (4)
C2—C1—P1—Ni1	−24.20 (10)	S4ⁱ—P1—Ni1—S2	164.515 (17)

Symmetry code: (i) −x+1, −y, −z+1.

Experimental details

Crystal data
Chemical formula	[Ni(C₁₁H₁₆O₂PS₂)₂]
M_r	609.37
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	173
a, b, c (Å)	9.4227 (2), 15.6479 (3), 9.5281 (2)
β (°)	102.878 (1)
V (Å³)	1369.54 (5)
Z	2
Radiation type	Mo Kα
µ (mm⁻¹)	1.16
Crystal size (mm)	0.40 × 0.34 × 0.11

Data collection
Diffractometer	Bruker APEXII CCD diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2008)
T_min, T_max	0.655, 0.883
No. of measured, independent and observed [I > 2σ(I)] reflections	32798, 3446, 3335
R_int	0.020
(sin θ/λ)_max (Å⁻¹)	0.671

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.019, 0.053, 1.09
No. of reflections	3446
No. of parameters	153
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.35, −0.39

Computer programs: APEX2 (Bruker, 2008), SAINT-Plus (Bruker, 2008), SAINT-Plus and XPREP (Bruker, 2008), 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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