cis-trans Isomers of PtX₄L₂ (X = halogen and L = neutral ligand): trans-bis­(dimethyl sulfide)tetra­iodidoplatinum(IV)

The octa­hedral title complex, [PtI₄(C₂H₆S)₂] or trans-PtI₄(dms)₂ (dms is dimethyl sulfide), crystallizes in the monoclinic space group P2₁/n (Z = 2), with mol­ecular symmetry C_i, which is the most frequently occurring point group for trans-PtX₄L₂ {56%, 28 structures in the Cambridge Structural Database (CSD) [Allen (2002). Acta Cryst. B58, 380-388]}, followed by C₁ (22%, 11 structures). The complexes form a puckered pseudo-hexa­gonal layer in the (10) plane, and the layers are stacked with an inter­planar distance of 7.10 Å. Density functional theory (DFT) calculations on an isolated complex with the observed parameters as a starting structure converged to C_2h. Constraints to C_i on the observed geometry give 3-4 kJ mol^-1 higher energy compared with C_2h. DFT calculations on [PtCl₄(PzH)₂] (PzH is pyrazole), reported in the CSD in both the cis and trans forms, show an energy difference of 21 kJ mol^-1 in favour of the trans complex. A CSD search for PtX₄L₂-type complexes, where X is a halogen and L is a ligand with a donor atom from group 14, 15 or 16, indicated a preferred trans geometrical arrangement, with a total fraction of 68%. The dominating crystal packing operators for the trans complexes are an inversion centre combined with a screw axis/glide plane (48%), followed by an inversion centre alone (28%).

S1. Comment

The principal nonzero oxidation states of Pt are II and IV. cis/trans- Pt^IIX₂L₂ complexes, where X is a halogen and L a ligand with a donor atom from groups 14, 15 or 16, have been extensively studied in the solid state, with 316 entries in the Cambridge Structural Database (Allen, 2002). Data for Pt^IVX₄L₂ are scarce, and here we report the structure of the title compound together with data mining for this class of compounds in the CSD (2006 release), with emphasis on the following: (i) Is there a preference for cis- or trans-complexes in the solid state? (ii) What molecular symmetries are retained in the solid state? (iii) Which are the dominating crystal packing operators?

The title compound, trans-PtI₄(dms)₂ (dms is dimethyl sulfide), crystallizes in the monoclinic space group P2₁/n (Z = 2), with molecular symmetry C_i (Fig. 1). The Pt—I distances (Table 1) are slightly larger than those observed in cis-[PtI₄(net)₂] [net is ethylamine, NC₂H₅; Pt—I = 2.65 Å for I atoms in trans positions and 2.61 Å for I atoms trans to N (Thiele et al., 1999)]. Geometry optimization of the title compound, using density functional theory (DFT) calculations with the observed parameters as a starting structure, converged to C_2h. Constraints to C_i on the observed geometry give 3–4 kJ mol^-1 higher energy compared with C_2h, which would require a space group that hampers close packing, indicating that intermolecular forces determine the point group for the complex. There is good agreement between the calculated and observed geometry (Table 2), and the deviation from 90° of the I—Pt—S angle observed in the crystal structure is even larger in the calculated one, suggesting that it is the result of an intramolecular effect.

The geometries of isostructural trans-PtX₄(dms)₂ (X= Cl, Br or I) are given in Table 2. The Pt—S distances are about the same for X = Cl and Br (2.36 Å) but significantly larger for X = I [2.390 (2) Å]. The X—Pt—X angles are close to 90° for all complexes and the X—Pt—S angles are 5–7° off the ideal 90°, which further supports the idea that this is an intramolecular effect. The intermolecular X···X contact of 3.92 (6) Å is on the short side for X = I and on the long side for X = Cl and Br. However I···I contacts as short as 3.48 Å have been observed in cis-[PtI₄(net)₂].

trans-PtX₄(dms)₂ forms a puckered pseudo-hexagonal close-packed layer in the (101) plane, with four I···I contacts [3.92 (1) Å] and two S···S contacts [3.69 (1) Å; Fig. 2]. Halogen–halogen interactions are known for stabilizing supramolecular structures (Desiraju, 1995). The stacking layers are parallel-displaced, resulting in a pseudo-close packing with an inter-layer distance of 7.10 Å, with notable inter-layer contacts [H···I = 3.23 Å and C—H···I = 153.12°] and also some soft H···H contacts (2.86 Å). For the isostructural compounds, the distortion from an ideal close packing is about the same for the bromide but much more severe for the chloride, as manifested by the Pt···Pt···Pt angles within a layer, which are close to 120° for both the iodide and bromide but only about 100° for the chloride.

The CSD was searched using CONQUEST (Bruno et al., 2002) for cis/trans-PtX₄L₂ complexes, where X is a halogen and L a ligand containing donor atoms from groups 14, 15 or 16. Simple solvates are included, but no chelates or structures with no coordinates, obscure connectivity or disorder. The majority of these 50 complexes (68%) preferred the trans configuration (Tables A and B, deposited material), compared with 50% in the system trans-PtX₂L₂ (Hansson et al., 2006). The complex [PtCl₄(PzH)₂] (PzH is pyrazole, N₂C₃H₄) is reported in the CSD as both the cis and the trans isomers (Khripun et al., 2006). DFT calculations on these complexes show a difference between the cis and trans isomers of 21 kJ mol^-1 in favour of the trans complex in the gas phase, also indicating a preference for the trans configuration.

Complexes trans-PtX₄L₂ may adopt the molecular point groups C₁, C_i, C_s, C₂, C_2v and C_2h. Table C (deposited material) shows that C_i is retained in 56% of cases, followed by C₁ (22%), C_s (12%), C₂ (8%) and C_{2 h} (2%), with no representatives for C_2v; the order of preference is C_i (78%), C₁ (16%), C₂ (11%) and C_2h (2%) for trans-PtX₂L₂ (Hansson et al., 2006). C_2v is not represented in any of the systems and C_2h has very low frequency, in accordance with Kitaigorodsky's rule (Kitaigorodsky, 1973) that C_2v requires space groups that hamper close packing and C_2h may adopt maximal density in space group C2/m, which is actually observed in both systems.

The dominating crystal-packing operator is an inversion centre as the sole packing operator in 28% of cases (neglecting pure translations which are always present), an inversion centre together with a screw axis/glide plane in 48% and an inversion centre in combination with other operators in 6% (Table C, deposited). It is interesting to note that P2₁ and P2₁2₁2₁ only represent 6% os cases, in contrast to the situation for organic molecules, where these space groups are relatively frequent (Brock & Dunitz, 1994; Oskarsson, 2007).

Related literature

For related literature, see: Allen (2002); Alrichs et al. (1989); Brock & Dunitz (1994); Bruno et al. (2002); Constable et al. (2006); Desiraju (1995); Hansson et al. (2006); Khripun et al. (2006); Kitaigorodsky (1973); Olsson (1986); Roulet & Barbey (1973); Thiele et al. (1999).

S2. Experimental

The title compound was obtained during an attempted synthesis of trans-PtI₂(diphenylsulfide)₂, by recrystallization of trans-PtI₂(dms)₂ in diphenylsulfide. It is known that triodide and iodine formed by air oxidation of iodide may oxidize Pt^II to Pt^IV (Olsson, 1986). The synthesis of trans-PtI₂(dms)₂ was performed according to a literature procedure, with a few modifications (Roulet & Barbey, 1973). [K₂PtCl₄] (100 mg, 0.241 mmol) was dissolved in water (20 ml). To this, KI (1.5 equivalents, 60 mg, 0.361 mmol) was added. The solution was stirred for 30 min, an excess of dimethyl sulfide (0.45 ml, 6.03 mmol) was added and the mixture was left to stir. After 2 h, the complex trans-PtI₂(dms)₂ was filtered off (yield 93 mg, 71%). trans-PtI₂(dms)₂ (50 mg, 0.087 mmol) was dissolved in diphenyl sulfide (8 ml) and left in a freezer. From this, trans-PtI₄(dms)₂ crystallized out as red cuboids.

DFT calculations were performed at the s-VWN level with the basis sets def-TZVPP for Pt and I, TZVPP for Cl, and 6–31G* for C, H and N atoms, using the software TURBOMOLE 5.5 (Alrichs et al., 1989).

S3. Refinement

The H atoms were positioned geometrically with fixed C—H distances of 0.96 Å [U_iso(H) = 1.5U_eq(C)]. The highest residual peak is 1.65 ų, 0.68 Å from I2 and the deepest hole -1.22 ų, 0.69 Å from Pt [not in accordance with values above]. A Hirshfeld test failure appeared in the structure validation, with the U values of two iodine sites being large in relation to the Pt centre. Suspecting compositional disorder, from Cl in the starting material, a disordered model was introduced using the PART instruction in SHELXL97 (Sheldrick, 1997), thus refining the U values of a possible Cl atom at an occupancy value of 0.02. Although the Hirshfeld test was passed, given that the occupancy is so small the values were considered unreliable and the disorder model was not adopted. A similar problem was previously overcome [encountered? reported?] in the crystal structure of [PtI₂(C₉H₂₁P)₂] (Constable et al., 2006).

Computing details

Data collection: SMART-NT (Bruker, 1998); 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, 1997); molecular graphics: DIAMOND (Brandenburg & Berndt, 2001); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. A view of (I) with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry code: (i) 2 - x, -y, 2 - z.]
	Fig. 2. A view of the pseudo-hexagonal close-packed layer, (101); the b axis is vertical. Dashed lines represent the positions of short contacts. Displacement ellipsoids are drawn at the 30% probability level.

trans-bis(dimethyl sulfide)tetraiodidoplatinum(IV)

Crystal data

[PtI4(C2H6S)2]	Z = 2
Mr = 826.95	F(000) = 716
Monoclinic, P21/n	Dx = 3.71 Mg m^−3
Hall symbol: -P 2yn	Mo Kα radiation, λ = 0.71073 Å
a = 7.5635 (15) Å	µ = 18.06 mm^−1
b = 7.4984 (15) Å	T = 293 K
c = 13.346 (3) Å	Rod, red
β = 102.06 (3)°	0.17 × 0.05 × 0.03 mm
V = 740.2 (3) Å^3

Data collection

Bruker SMART 1K CCD diffractometer	1479 reflections with I > 2σ(I)
Radiation source: rotating anode	Rint = 0.040
ω scans	θmax = 28°, θmin = 2.9°
Absorption correction: multi-scan SADABS (Bruker, 2004)	h = −9→9
Tmin = 0.149, Tmax = 0.613	k = −9→7
6602 measured reflections	l = −16→17
1774 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.029	w = 1/[σ^2(Fo^2) + (0.0272P)^2 + 0.602P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.065	(Δ/σ)max < 0.001
S = 1.06	Δρmax = 1.07 e Å^−3
1774 reflections	Δρmin = −0.95 e Å^−3
54 parameters

Special details

Experimental. The intensity data were collected on a Siemens SMART 1 K CCD diffractometer using an exposure time of 30 s/frame. A total of 1315 frames were collected with a frame width of 0.3° covering up to θ = 28.00° with 100% completeness accomplished. The first 50 frames were recollected at the end of each data collection to check for decay; none was found.

We screened the CSD for the crystal class distribution [Belsky, V. K., Zorkaya, O. N. & Zorky, P. M. (1995). Acta Cryst. A51, 473–481],which contains information about the molecular point group symmetry. The search was done using the CONQUEST software package (Bruno et al., 2002).

DFT-calculations were done using Turbomole 5.5 (Alrichs et al., 1989)

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 (Ų)

	x	y	z	Uiso*/Ueq
Pt1	1	0	1	0.03096 (11)
I1	0.86489 (6)	0.08880 (7)	1.16451 (3)	0.04635 (14)
I2	0.72171 (6)	0.17370 (7)	0.87853 (4)	0.05010 (15)
S1	1.1315 (2)	0.2911 (2)	1.02060 (14)	0.0422 (4)
C1	1.1860 (11)	0.3703 (11)	0.9039 (6)	0.058 (2)
H1A	1.2408	0.486	0.9152	0.087*
H1B	1.0775	0.3783	0.8518	0.087*
H1C	1.2687	0.2891	0.8823	0.087*
C2	1.3581 (10)	0.2808 (12)	1.0972 (7)	0.061 (2)
H2A	1.425	0.1898	1.0708	0.092*
H2B	1.3531	0.2534	1.1669	0.092*
H2C	1.4166	0.3939	1.0948	0.092*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pt1	0.02545 (16)	0.0390 (2)	0.02898 (17)	0.00422 (14)	0.00694 (12)	0.00138 (15)
I1	0.0446 (2)	0.0603 (4)	0.0380 (2)	0.0054 (2)	0.01724 (19)	−0.0030 (2)
I2	0.0371 (2)	0.0593 (3)	0.0506 (3)	0.0123 (2)	0.0016 (2)	0.0076 (2)
S1	0.0395 (8)	0.0416 (11)	0.0481 (10)	−0.0003 (7)	0.0152 (7)	−0.0022 (8)
C1	0.060 (5)	0.064 (6)	0.055 (5)	−0.006 (4)	0.023 (4)	0.010 (4)
C2	0.049 (4)	0.068 (6)	0.063 (5)	−0.014 (4)	0.003 (4)	−0.007 (4)

Geometric parameters (Å, º)

Pt1—S1^i	2.3904 (18)	S1—C2	1.805 (7)
Pt1—S1	2.3904 (18)	C1—H1A	0.96
Pt1—I1	2.6918 (7)	C1—H1B	0.96
Pt1—I1^i	2.6918 (7)	C1—H1C	0.96
Pt1—I2^i	2.7070 (8)	C2—H2A	0.96
Pt1—I2	2.7070 (8)	C2—H2B	0.96
S1—C1	1.793 (8)	C2—H2C	0.96
S1^i—Pt1—S1	180.00 (9)	C1—S1—C2	98.2 (4)
S1^i—Pt1—I1	95.74 (4)	C1—S1—Pt1	111.6 (3)
S1—Pt1—I1	84.26 (4)	C2—S1—Pt1	110.4 (3)
S1^i—Pt1—I1^i	84.26 (4)	S1—C1—H1A	109.5
S1—Pt1—I1^i	95.74 (4)	S1—C1—H1B	109.5
I1—Pt1—I1^i	180	H1A—C1—H1B	109.5
S1^i—Pt1—I2^i	83.23 (5)	S1—C1—H1C	109.5
S1—Pt1—I2^i	96.77 (5)	H1A—C1—H1C	109.5
I1—Pt1—I2^i	90.62 (2)	H1B—C1—H1C	109.5
I1^i—Pt1—I2^i	89.38 (2)	S1—C2—H2A	109.5
S1^i—Pt1—I2	96.77 (5)	S1—C2—H2B	109.5
S1—Pt1—I2	83.23 (5)	H2A—C2—H2B	109.5
I1—Pt1—I2	89.38 (2)	S1—C2—H2C	109.5
I1^i—Pt1—I2	90.62 (2)	H2A—C2—H2C	109.5
I2^i—Pt1—I2	180	H2B—C2—H2C	109.5

Symmetry code: (i) −x+2, −y, −z+2.