Crystal structure of the unusual coordination polymer catena-poly[[gold(I)-μ-1,2-bis­(di­phenyl­phosphino­thio­yl)ethane-κ2S:S′] di­bromido­aurate(I)]

Taouss, C.; Calvo, M.; Jones, P.G.

doi:10.1107/S2056989020013675

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ISSN: 2056-9890

Volume 76| Part 11| November 2020| Pages 1768-1770

https://doi.org/10.1107/S2056989020013675

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Crystal structure of the unusual coordination polymer catena-poly[[gold(I)-μ-1,2-bis(diphenylphosphinothioyl)ethane-κ²S:S′] dibromidoaurate(I)]¹

Christina Taouss,^a Marina Calvo ^a ‡ and Peter G. Jones ^a ^*

^aInstitut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany
^*Correspondence e-mail: p.jones@tu-bs.de

Edited by J. Ellena, Universidade de Sâo Paulo, Brazil (Received 30 September 2020; accepted 13 October 2020; online 20 October 2020)

In the title compound, {[Au(C₂₆H₂₄P₂S₂)][AuBr₂]}_n, the gold(I) centres of the cation are coordinated by the P=S groups of the disulfide ligands to form a chain polymer parallel to the c axis. Both independent gold atoms lie on the same twofold axis, and the midpoint of the H₂C—CH₂ bond lies on an inversion centre. The anions flank the polymeric chain; they are connected to it by short aurophilic interactions and C—H⋯Br contacts, and to each other by Br⋯Br contacts.

Keywords: crystal structure; polymer; phosphine sulfide; gold.

CCDC reference: 2036798

1. Chemical context

Although phosphane sulfides are known to act as ligands towards gold(I) centres, not many complexes have been structurally characterized in which two such ligands coordinate to gold(I). A search of the Cambridge Database (2019 Version, ConQuest 2.0.5) revealed only three structures involving the cation [(Ph₃P=S)₂Au]⁺; the PO₂F₂⁻ salt (LeBlank et al., 1997 ), the nitrate (Jones & Geissler, 2016a ) and a bis(methylsulfonyl)amide salt (Jones & Geissler, 2016b ). Cationic 1:1 complexes of gold(I) with diphosphane disulfides can only be achieved if the ligand geometry allows for linear coordination at the gold atom, which is not generally the case unless suitable spacers, such as ferrocene units or other metal centres, are present (Gimeno et al., 2000 , and references therein; Parkanyi & Besenyei, 2017 ; Wang & Fackler, 1990 ).

In the course of our studies of phosphane chalcogenide complexes of gold (Upmann et al., 2019 , and references therein) we planned to study complexes of the diphosphane disulfides 1,2-bis(diphenylphosphinothioyl)ethane [previously known as 1,2-bis(diphenylphosphino)ethane disulfide; dppeS₂] and bis(diphenylthiophosphinoyl)methane [previously known as bis(diphenylphosphino)methane disulfide; dppmS₂] with gold(I) halide fragments AuBr and AuCl, with particular attention to the mononuclear complexes. This succeeded to some extent; we were able to isolate and determine the structure of dppmS₂AuCl, the isotypic dppmS₂AuBr and its oxidation product with bromine [(dppmS₂)AuBr₂]⁺ [AuBr₄]⁻ (Jones et al., 2020a ,b ,c , respectively), but yields were poor and it was clear that scrambling reactions were a problem. With dppeS₂ even less was achieved, but a few thin needles, isolated from the attempted synthesis of dppeS₂AuBr, proved to be an unusual coordination polymer [(dppeS₂)Au]_nⁿ⁺·n[AuBr₂]⁻, the structure of which we report here.

2. Structural commentary

The title compound is shown in Fig. 1. The cation has the stoichiometry [dppeS₂Au]⁺, and forms a chain polymer (⋯Au—S=PCH₂CH₂P=S⋯)_n parallel to the c axis; the anion is [AuBr₂]⁻. Both gold atoms lie on twofold axes $[1\over2]$ , y, $[1\over4]$ and show the linear coordination geometry expected for Au^I; the midpoint of the central H₂C—CH₂ bond lies on the inversion centre $[1\over2]$ , $[1\over2]$ , $[1\over2]$ . Bond lengths and angles may be considered normal; for a selection, see Table 1. Coordination polymers are scarce for diphosphane disulfide ligands (see below).

Table 1
Selected geometric parameters (Å, °)

Au1—S1	2.3125 (9)	Br1⋯Br1ⁱ	3.7424 (8)
Au1⋯Au2	2.9622 (3)	P1—S1	2.0097 (12)
Au2—Br1	2.3746 (5)	C1—C1ⁱⁱ	1.528 (6)

S1—Au1—S1ⁱⁱⁱ	178.43 (4)	Br1—Au2⋯Au1	93.201 (11)
S1—Au1⋯Au2	90.78 (2)	Au2—Br1⋯Br1ⁱ	126.21 (2)
Br1—Au2—Br1ⁱⁱⁱ	173.60 (2)	P1—S1—Au1	98.34 (4)
Symmetry codes: (i) -x+1, -y+2, -z+1; (ii) -x+1, -y+1, -z+1; (iii) $[-x+1, y, -z+{\script{1\over 2}}]$ .

Figure 1
Structure of the title complex in the crystal; the asymmetric unit is numbered. The aurophilic contact and the S—Au connections to the next gold atoms in the polymer are indicated by filled and open dashed bonds, respectively.

3. Supramolecular features

The gold atoms of the cation and anion are connected via a short aurophilic contact of 2.9622 (3) Å, and the anions thus flank the cation polymer (Fig. 2). Neighbouring anions are connected by short Br⋯Br contacts of 3.7424 (8) Å (operator 1 − x, 2 − y, 1 − z), and also provide links to adjacent polymers (not shown in Fig. 2). We have previously noted an example of short contacts between dibromoaurate(I) anions (Döring & Jones, 2013 ); for a further example, see Beno et al. (1990 ). We have also described Br⋯Br and Cl⋯Cl contacts in a series of tetrabromidoaurate(III) and tetrachloridoaurate(III) salts (Döring & Jones, 2016 ).

Figure 2
The polymeric structure of the title compound, viewed perpendicular to the (101) plane. Aurophilic contacts are shown as thick bonds and Br⋯Br contacts as thin dashed bonds. Hydrogen atoms are omitted for clarity.

Two C—H⋯Br contacts between cation and anions are sufficiently short and linear to be considered `weak' hydrogen bonds (Table 2), and thus to contribute further cohesion to the structure, but are omitted from Fig. 2 for clarity.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C16—H16⋯Br1ⁱⁱ	0.95	2.81	3.712 (4)	159
C26—H26⋯Br1ⁱⁱ	0.95	2.89	3.775 (4)	155
Symmetry code: (ii) -x+1, -y+1, -z+1.

4. Database survey

A database search (CSD 2019 Version, ConQuest 2.0.5) found 11 hits for systems involving two P=S units bonded to Au^I. The P=S bond lengths range from 1.985–2.039, av. 2.018 Å, and the S—Au bond lengths from 2.275–2.317, av. 2.296 Å. The only other coordination polymer found for a diphosphane disulfide was [(CuCN)₂(dppeS₂)]_n (Zhou et al., 2006 ), a two-dimensional polymer involving four-coordinate Cu centres.

5. Synthesis and crystallization

The compound arose from an attempt to synthesize dppeS₂AuBr. A solution of thtAuBr (tht = tetrahydrothiophene; 0.775 g, 2.12 mmol) in CH₂Cl₂ (50 ml) was added to dppeS₂ (0.981 g, 2.12 mmol) dissolved in CH₂Cl₂ (50 ml). After stirring for 1 h, the solvent was removed, and the solid thus obtained was dried under vacuum and recrystallized from dichloromethane/n-pentane. The elemental analysis was approximately correct for the expected stoichiometry: calculated, C 42.23%, H 3.27%, S 8.67%; found, C 43.22%, H 3.87%, S 8.19%. However, attempts to obtain crystals suitable for X-ray structure analysis (by evaporation from a solution in CH₂Cl₂) led only to a few very thin needles of the title compound, with overall stoichiometry dppeS₂(AuBr)₂.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were included using a riding model starting from calculated positions (C—H_aromatic = 0.95, C—H_methylene = 0.99 Å). The U_iso(H) values were fixed at 1.2 times the equivalent U_iso value of the parent carbon atoms.

Table 3
Experimental details

Crystal data
Chemical formula	[Au(C₂₆H₂₄P₂S₂)][AuBr₂]
M_r	1016.26
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	21.4112 (8), 11.9708 (2), 13.7726 (4)
β (°)	128.316 (7)
V (Å³)	2769.7 (3)
Z	4
Radiation type	Cu Kα
μ (mm⁻¹)	25.63
Crystal size (mm)	0.12 × 0.005 × 0.005

Data collection
Diffractometer	Oxford Diffraction Xcalibur, Atlas, Nova
Absorption correction	Multi-scan (CrysAlis PRO; Oxford Diffraction, 2010 $[Oxford Diffraction (2010). CrysAlis PRO. Oxford Diffraction Ltd, Yarnton, England.]$ )
T_min, T_max	0.387, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	27632, 2873, 2630
R_int	0.037
(sin θ/λ)_max (Å⁻¹)	0.629

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.021, 0.053, 1.06
No. of reflections	2873
No. of parameters	155
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	1.42, −0.99
Computer programs: CrysAlis PRO (Oxford Diffraction, 2010 $[Oxford Diffraction (2010). CrysAlis PRO. Oxford Diffraction Ltd, Yarnton, England.]$ ), SHELXS97 (Sheldrick, 2008 ), SHELXL2018/3 (Sheldrick, 2015 ) and XP (Siemens, 1994 ).

Supporting information

CCDC reference: 2036798

Crystal structure: contains datablocks I, global. DOI: https://doi.org/10.1107/S2056989020013675/ex2037sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989020013675/ex2037Isup2.hkl

checkCIF report

Computing details top

Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell refinement: CrysAlis PRO (Oxford Diffraction, 2010); data reduction: CrysAlis PRO (Oxford Diffraction, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015).

catena-Poly[[gold(I)-µ-1,2-bis(diphenylphosphinothioyl)ethane-κ²S:S'] dibromidoaurate(I)] top

Crystal data top

[Au(C₂₆H₂₄P₂S₂)][AuBr₂]	F(000) = 1880
M_r = 1016.26	D_x = 2.437 Mg m⁻³
Monoclinic, C2/c	Cu Kα radiation, λ = 1.54184 Å
a = 21.4112 (8) Å	Cell parameters from 17272 reflections
b = 11.9708 (2) Å	θ = 4.1–75.8°
c = 13.7726 (4) Å	µ = 25.63 mm⁻¹
β = 128.316 (7)°	T = 100 K
V = 2769.7 (3) Å³	Needle, colourless
Z = 4	0.12 × 0.01 × 0.01 mm

Data collection top

Oxford Diffraction Xcalibur, Atlas, Nova diffractometer	2873 independent reflections
Radiation source: Nova (Cu) X-ray Source	2630 reflections with I > 2σ(I)
Mirror monochromator	R_int = 0.037
Detector resolution: 10.3543 pixels mm^-1	θ_max = 76.0°, θ_min = 4.5°
ω scans	h = −26→26
Absorption correction: multi-scan (CrysAlisPro; Oxford Diffraction, 2010)	k = −14→15
T_min = 0.387, T_max = 1.000	l = −17→16
27632 measured reflections

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)] = 0.021	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.053	H-atom parameters constrained
S = 1.06	w = 1/[σ²(F_o²) + (0.0283P)² + 12.9976P] where P = (F_o² + 2F_c²)/3
2873 reflections	(Δ/σ)_max = 0.001
155 parameters	Δρ_max = 1.42 e Å⁻³
0 restraints	Δρ_min = −0.99 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	U_iso*/U_eq
Au1	0.500000	0.61954 (2)	0.250000	0.01687 (7)
Au2	0.500000	0.86699 (2)	0.250000	0.01950 (7)
Br1	0.47685 (3)	0.87807 (3)	0.39751 (4)	0.03124 (11)
P1	0.60133 (5)	0.46450 (7)	0.49985 (8)	0.01384 (16)
S1	0.61453 (5)	0.61690 (7)	0.45351 (8)	0.01915 (17)
C1	0.50616 (19)	0.4520 (3)	0.4701 (3)	0.0162 (7)
H1A	0.462644	0.452353	0.379701	0.019*
H1B	0.504140	0.380043	0.503474	0.019*
C11	0.6769 (2)	0.4432 (3)	0.6634 (3)	0.0167 (7)
C12	0.7460 (2)	0.5083 (3)	0.7329 (4)	0.0214 (8)
H12	0.752518	0.568996	0.695455	0.026*
C13	0.8047 (2)	0.4839 (4)	0.8566 (4)	0.0290 (9)
H13	0.851649	0.528125	0.903677	0.035*
C14	0.7962 (2)	0.3962 (4)	0.9128 (4)	0.0267 (8)
H14	0.836922	0.380547	0.997887	0.032*
C15	0.7280 (2)	0.3310 (3)	0.8446 (3)	0.0227 (7)
H15	0.721865	0.270674	0.882930	0.027*
C16	0.6684 (2)	0.3542 (3)	0.7197 (3)	0.0198 (7)
H16	0.621780	0.309224	0.672798	0.024*
C21	0.61000 (19)	0.3529 (3)	0.4211 (3)	0.0154 (6)
C22	0.6509 (2)	0.3700 (3)	0.3737 (4)	0.0220 (7)
H22	0.670919	0.441911	0.377148	0.026*
C23	0.6622 (3)	0.2805 (4)	0.3212 (4)	0.0287 (8)
H23	0.689530	0.291899	0.287999	0.034*
C24	0.6338 (2)	0.1754 (3)	0.3171 (4)	0.0274 (8)
H24	0.641465	0.114817	0.280842	0.033*
C25	0.5943 (2)	0.1586 (3)	0.3657 (4)	0.0264 (8)
H25	0.575405	0.086098	0.363559	0.032*
C26	0.5819 (3)	0.2461 (3)	0.4176 (4)	0.0237 (8)
H26	0.554451	0.233937	0.450538	0.028*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Au1	0.01921 (11)	0.01291 (10)	0.01950 (11)	0.000	0.01250 (9)	0.000
Au2	0.01812 (11)	0.01358 (10)	0.02586 (12)	0.000	0.01316 (9)	0.000
Br1	0.0371 (2)	0.0274 (2)	0.0408 (3)	−0.00694 (16)	0.0299 (2)	−0.00832 (17)
P1	0.0127 (4)	0.0131 (4)	0.0155 (4)	−0.0004 (3)	0.0086 (3)	−0.0009 (3)
S1	0.0192 (4)	0.0151 (4)	0.0211 (4)	−0.0024 (3)	0.0115 (4)	−0.0003 (3)
C1	0.0129 (15)	0.0177 (16)	0.0170 (16)	−0.0012 (12)	0.0088 (13)	−0.0015 (13)
C11	0.0153 (15)	0.0178 (16)	0.0192 (17)	0.0005 (12)	0.0117 (14)	−0.0041 (13)
C12	0.0180 (17)	0.0258 (19)	0.0236 (19)	−0.0060 (14)	0.0144 (16)	−0.0051 (15)
C13	0.0168 (17)	0.041 (2)	0.022 (2)	−0.0063 (16)	0.0087 (16)	−0.0067 (17)
C14	0.0164 (17)	0.040 (2)	0.0161 (18)	0.0064 (15)	0.0063 (15)	−0.0014 (16)
C15	0.0236 (18)	0.0245 (18)	0.0183 (18)	0.0060 (14)	0.0121 (15)	0.0027 (14)
C16	0.0170 (16)	0.0182 (16)	0.0197 (18)	0.0009 (13)	0.0091 (14)	−0.0008 (13)
C21	0.0139 (15)	0.0147 (15)	0.0160 (16)	0.0032 (12)	0.0085 (14)	0.0022 (12)
C22	0.0235 (18)	0.0204 (17)	0.0269 (19)	0.0039 (13)	0.0181 (16)	0.0025 (14)
C23	0.033 (2)	0.032 (2)	0.035 (2)	0.0043 (17)	0.0276 (19)	0.0005 (18)
C24	0.032 (2)	0.0236 (19)	0.026 (2)	0.0078 (16)	0.0173 (17)	−0.0010 (15)
C25	0.034 (2)	0.0164 (17)	0.028 (2)	−0.0033 (15)	0.0186 (18)	−0.0053 (15)
C26	0.028 (2)	0.0203 (19)	0.026 (2)	−0.0037 (13)	0.0187 (18)	−0.0027 (14)

Geometric parameters (Å, º) top

Au1—S1	2.3125 (9)	C13—H13	0.9500
Au1—S1ⁱ	2.3125 (9)	C14—C15	1.387 (6)
Au1—Au2	2.9622 (3)	C14—H14	0.9500
Au2—Br1	2.3746 (5)	C15—C16	1.393 (5)
Au2—Br1ⁱ	2.3746 (5)	C15—H15	0.9500
Br1—Br1ⁱⁱ	3.7424 (8)	C16—H16	0.9500
P1—C11	1.800 (4)	C21—C22	1.394 (5)
P1—C21	1.801 (4)	C21—C26	1.401 (5)
P1—C1	1.819 (3)	C22—C23	1.394 (5)
P1—S1	2.0097 (12)	C22—H22	0.9500
C1—C1ⁱⁱⁱ	1.528 (6)	C23—C24	1.384 (6)
C1—H1A	0.9900	C23—H23	0.9500
C1—H1B	0.9900	C24—C25	1.381 (6)
C11—C16	1.395 (5)	C24—H24	0.9500
C11—C12	1.399 (5)	C25—C26	1.384 (5)
C12—C13	1.382 (6)	C25—H25	0.9500
C12—H12	0.9500	C26—H26	0.9500
C13—C14	1.383 (6)

S1—Au1—S1ⁱ	178.43 (4)	C14—C13—H13	119.5
S1—Au1—Au2	90.78 (2)	C13—C14—C15	119.9 (4)
S1ⁱ—Au1—Au2	90.78 (2)	C13—C14—H14	120.1
Br1—Au2—Br1ⁱ	173.60 (2)	C15—C14—H14	120.1
Br1—Au2—Au1	93.201 (11)	C14—C15—C16	119.9 (4)
Br1ⁱ—Au2—Au1	93.201 (11)	C14—C15—H15	120.1
Au2—Br1—Br1ⁱⁱ	126.21 (2)	C16—C15—H15	120.1
C11—P1—C21	107.45 (15)	C15—C16—C11	120.2 (3)
C11—P1—C1	106.44 (16)	C15—C16—H16	119.9
C21—P1—C1	108.84 (15)	C11—C16—H16	119.9
C11—P1—S1	109.38 (12)	C22—C21—C26	119.9 (3)
C21—P1—S1	113.24 (12)	C22—C21—P1	120.2 (3)
C1—P1—S1	111.20 (12)	C26—C21—P1	119.7 (3)
P1—S1—Au1	98.34 (4)	C21—C22—C23	119.4 (4)
C1ⁱⁱⁱ—C1—P1	111.1 (3)	C21—C22—H22	120.3
C1ⁱⁱⁱ—C1—H1A	109.4	C23—C22—H22	120.3
P1—C1—H1A	109.4	C24—C23—C22	120.5 (4)
C1ⁱⁱⁱ—C1—H1B	109.4	C24—C23—H23	119.8
P1—C1—H1B	109.4	C22—C23—H23	119.8
H1A—C1—H1B	108.0	C25—C24—C23	119.8 (4)
C16—C11—C12	119.5 (3)	C25—C24—H24	120.1
C16—C11—P1	118.5 (3)	C23—C24—H24	120.1
C12—C11—P1	121.8 (3)	C24—C25—C26	120.8 (4)
C13—C12—C11	119.6 (4)	C24—C25—H25	119.6
C13—C12—H12	120.2	C26—C25—H25	119.6
C11—C12—H12	120.2	C25—C26—C21	119.5 (4)
C12—C13—C14	121.0 (4)	C25—C26—H26	120.2
C12—C13—H13	119.5	C21—C26—H26	120.2

S1—Au1—Au2—Br1	−65.75 (3)	C11—C12—C13—C14	−0.2 (6)
S1ⁱ—Au1—Au2—Br1	114.25 (2)	C12—C13—C14—C15	0.2 (6)
S1—Au1—Au2—Br1ⁱ	114.24 (3)	C13—C14—C15—C16	0.1 (6)
S1ⁱ—Au1—Au2—Br1ⁱ	−65.75 (3)	C14—C15—C16—C11	−0.4 (6)
Au1—Au2—Br1—Br1ⁱⁱ	158.03 (3)	C12—C11—C16—C15	0.5 (5)
C11—P1—S1—Au1	−172.78 (12)	P1—C11—C16—C15	176.2 (3)
C21—P1—S1—Au1	67.42 (12)	C11—P1—C21—C22	−98.2 (3)
C1—P1—S1—Au1	−55.51 (12)	C1—P1—C21—C22	146.9 (3)
Au2—Au1—S1—P1	156.28 (4)	S1—P1—C21—C22	22.7 (3)
C11—P1—C1—C1ⁱⁱⁱ	67.1 (4)	C11—P1—C21—C26	76.0 (3)
C21—P1—C1—C1ⁱⁱⁱ	−177.3 (3)	C1—P1—C21—C26	−38.9 (3)
S1—P1—C1—C1ⁱⁱⁱ	−51.9 (4)	S1—P1—C21—C26	−163.1 (3)
C21—P1—C11—C16	−70.7 (3)	C26—C21—C22—C23	1.1 (6)
C1—P1—C11—C16	45.7 (3)	P1—C21—C22—C23	175.3 (3)
S1—P1—C11—C16	166.0 (2)	C21—C22—C23—C24	−0.6 (6)
C21—P1—C11—C12	104.9 (3)	C22—C23—C24—C25	−0.3 (6)
C1—P1—C11—C12	−138.7 (3)	C23—C24—C25—C26	0.7 (6)
S1—P1—C11—C12	−18.4 (3)	C24—C25—C26—C21	−0.2 (6)
C16—C11—C12—C13	−0.2 (5)	C22—C21—C26—C25	−0.7 (6)
P1—C11—C12—C13	−175.7 (3)	P1—C21—C26—C25	−174.9 (3)

Symmetry codes: (i) −x+1, y, −z+1/2; (ii) −x+1, −y+2, −z+1; (iii) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C16—H16···Br1ⁱⁱⁱ	0.95	2.81	3.712 (4)	159
C26—H26···Br1ⁱⁱⁱ	0.95	2.89	3.775 (4)	155

Symmetry code: (iii) −x+1, −y+1, −z+1.

Footnotes

¹Phosphane chalcogenides and their metal complexes, Part VI. Part V: Upmann et al. [(2019). Z. Naturforsch. Teil B, 74, 389–404].

‡Advanced practical student from the University of Valencia, Spain. Current address: Hikma Pharmaceuticals, Schiffgraben 23, 38690 Goslar, Germany.

Acknowledgements

M. Calvo was supported by the Erasmus scheme.

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