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Acta Cryst. (2009). E65, i18-i19    [ doi:10.1107/S1600536809005601 ]

Synthesis, crystal structure and Raman spectrum of K2[(Pt2)(HPO4)4(H2O)2] containing (Pt2)6+ ions

K. Panagiotidis and R. Glaum

Abstract top

In the crystal structure of the acid platinum phosphate dipotassium di-[mu]-hydrogenphosphato-bis[aquaplatinum(III)](Pt-Pt), K2[Pt2(HPO4)4(H2O)2], the (Pt2)6+ dumbbells within the paddle-wheel complex show Pt-Pt distances of 2.4944 (5) and 2.4892 (5) Å. The pottassium ions are seven-fold coordinated by hydrogenphosphate groups. In the crystal, O-H...O hydrogen bonds help to establish the packing. The Raman spectrum was recorded.

Comment top

Information on phosphates of noble metals like platinum and palladium is scarcely found in literature. During our recent investigation of the ternary system Pd/P/O we obtained the diphosphate PdII2P2O7 (Panagiotidis et al., 2005) in addition to the already known Pd(PO3)2 (Palkina et al., 1978). In this context we were also interested in the crystal chemistry of platinum phosphates. Up to now, PtIVP2O7 (Wellmann & Liebau, 1981) is the only structurally characterized anhydrous phosphate of platinum. Since reactions starting from "PtO×3H2O" and P4O10 did not yield products suitable for closer investigation we tried an alternative synthetic approach by reacting K2PtCl4 with conc. H3PO4 (see Experimental). This led to the formation of orange crystals of K2[(Pt2)(HPO4)4(H2O)2]. The acid phosphate is isotypic to the sodium compound (Cotton et al., 1982a). In contrast to the structure of Na2[(Pt2)(HPO4)4(H2O)2] no disordered oxygen atoms are observed in the potassium compound. Distances d(Pt—Pt) for both structures are identical. The conventional residual as well as the standard deviations of the interatomic distances are slightly smaller for the refinement of K2[(Pt2)(HPO4)4(H2O)2]. The (Pt2)6+ binuclear complex was first observed in the crystal structure of K2[(Pt2(SO4)4(H2O)2] (Muraveiskaya et al., 1974; Muraveiskaya et al., 1976). By reaction of elemental platinum with concentrated sulfuric acid various platinum(III) sulfates were recently synthesized and structurally characterized (Pley & Wickleder, 2004a,b; Pley & Wickleder, 2005).

In K2[(Pt2)(HPO4)4(H2O)2] two crystallographically equivalent platinum atoms are connected to form (Pt2)6+ dinuclear complexes with surrounding (HPO4)2- groups (Fig. 1). The ligating oxygen atoms of the (HPO4)2- ions are arranged in a square-planar coordination around each platinum atom. Distances d(Pt—O) range from 1.978 Å to 2.030 Å. Angles 〈(O,Pt,O) are deviating only slightly from 90° and 180°, respectively. Due to their different crystal chemical environment, the oxygen atoms of the hydrogenphosphate anions show significantly different bond lenghts d(P—O). Oxygen atoms attached to platinum (coordination number of oxygen c.n.(O) = 2 (P, Pt)) show distances d(P—O) = 1.55 Å. Distances d(P—O) for those oxygen atoms which are coordinated to K+ ions (c.n.(O) = 2 (P, K)) range from 1.489 Å to 1.507 Å. Furthermore, for oxygen atoms which are attached to a hydrogen atom within the (HPO4)2- unit (c.n.(O) = 2, (P, H)), distances d(P—OH) around 1.565 Å are observed. The axial ligand positions of the Pt2 dumbbells are occupied by water molecules (Fig. 1 & 2). Distances d(Pt—O) = 2.135 Å observed for the water ligands are significantly longer than those within the [Pt2O8] entity. The hydrogen atoms of [HPO4] tetrahedra (H14, H24, H34, H42; numbering according to the oxygen atoms that cary the hydrogen) and the water molecules (H1A, H1B, H10A, H10B) are involved in hydrogen bonding with oxygen atoms of adjacent [(Pt2)(HPO4)(H2O)2]2- units. Interatomic distances d(OH···OP) range from 1.73 Å to 2.27 Å. They are in good accordance with those observed for strong hydrogen bonds (Steiner, 2002).

As found for various other compounds containing the (Pt2)6+ dinuclear complex, the angle 〈(Pt,Pt,O) between the dumbbell and the axial oxygen atoms deviates only slightly from 180° (Cotton et al., 1982b; Pley & Wickleder et al., 2005). [HPO4] tetrahedra in [(Pt2)(HPO4)(H2O)2]2- show no significant angular distortion. Charge compensation of the anionic [PtIII2(HPO4)4(H2O)2]2- unit is achieved by two crystallographically independent K+ ions, which are surrounded by oxygen atoms of phosphate groups.

In addition to its structure refinement, we were able to record the Raman spectrum of the paddle-wheel complex [PtIII2(HPO4)4(H2O)2]2- (Fig. 3). An unequivocal assignment of the observed signals is yet impossible. Comparison of the Raman spectrum to those of the In24+ containing indium phosphates In3(PO4)2 and In2O(PO4) (Thauern & Glaum, 2004) suggests the Pt—Pt valence vibration to be at ν = 222 cm-1. In comparison to ν(Pt—Pt) in complexes [(PtIII2)L4L'2]n- (Stein et al., 1983) assignment of ν = 83 cm-1 to the Pt—Pt vibration appears to be unreasonable. This is the more so, since d (Pt—Pt) = 2.51 Å observed for K2[(Pt2)(HPO4)4(H2O)2] is close to the lower limit of 2.47 Å < d (Pt—Pt) < 2.695 Å found for a series of dinuclear platinum(III) complexes (Che et al., 1982, Stein et al., 1983, Muraveiskaya et al., 1974).

Related literature top

The structure of the title isotypic sodium compound, Na2[Pt2(HPO4)4(H2O)2], was determined by Cotton et al. (1982a). For platinum phosphates, see: Wellmann & Liebau (1981). For related compounds containing dinuclear platinum(III), see: Bancroft et al. (1984); Baranovskii & Schelokow (1978); Che et al. (1982); Cotton & Walton (1982b); Dikareva et al. (1982, 1987); Muraveiskaya et al. (1974, 1976); Pley & Wickleder (2004a,b, 2005); Stein et al. (1983); Woollins & Kelly (1985). For the ternary system Pd/P/O, see: Palkina et al. (1978); Panagiotidis et al. (2005); For hydrogen bonds, see: Steiner (2002). For the Raman spectra of In3(PO4)2 and In2O(PO4), see: Thauern & Glaum (2004). For the synthesis, see: Brauer (1978).

Experimental top

Aiming at the crystallization of "Pt2P2O7" a reaction starting from 150.0 mg K2PtIICl4, which were dissolved in water and mixed with 5.0 ml conc. H3PO4, was performed. After the obtained red solution was kept in a desiccator over P2O5 for two weeks, plate-like, orange crystals of K2[(Pt2)(HPO4)4(H2O)2] with edge-lengths up to 0.3 mm were deposited besides microcrystalline platinum (eq. 1). The synthesis of K2[PtIICl4] was performed according to the procedure given by Brauer (1978).

3 K2[PtIICl4] + 4 H3PO4 + 2 H2O → K2[(PtIII2)(HPO4)4(H2O)2] + Pts + 8H+ + 12 C l- + 4 K+ (eq. 1)

Computing details top

Data collection: COLLECT (Hooft, 2004); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. Pa ddle-wheel complex [(PtIII2)(HPO4)4(H2O)2] with anisotropic displacement parameters given at the 50% level (Progr. ATOMS v.6.2).
[Figure 2] Fig. 2. Projection of the crystal strucure of K2[(Pt2)(HPO4)4(H2O)2] along [110]. [PO4] tetrahedra (yellow), Pt26+ (red), K+ (violet), H+ (blue), O2- (white) (Progr. ATOMS v.6.2).
[Figure 3] Fig. 3. Raman spectrum of K2[(Pt2)(HPO4)4(H2O)2].
dipotassium di-µ-hydrogenphosphato-bis[aquaplatinum(III)](Pt—Pt) top
Crystal data top
K2[Pt2(HPO4)4(H2O)2]F000 = 812
Mr = 888.32The lattice parameters of K2[Pt2(HPO4)4(H2O)2] were determined from single crystal diffraction data.
Triclinic, P1Dx = 3.800 Mg m−3
Hall symbol: -P 1Mo Kα radiation
λ = 0.71073 Å
a = 7.8852 (2) ÅCell parameters from 5589 reflections
b = 7.9657 (2) Åθ = 1.5–32.5º
c = 13.7739 (4) ŵ = 19.05 mm−1
α = 82.358 (1)ºT = 123 K
β = 81.509 (1)ºCell measurement pressure: 101.3 kPa
γ = 65.528 (1)ºPlate, orange
V = 776.32 (4) Å30.24 × 0.14 × 0.08 mm
Z = 2
Data collection top
Nonius Kappa CCD
diffractometer		18915 measured reflections
Radiation source: MoKα5589 independent reflections
Monochromator: graphite4077 reflections with I > 2σ(I)
Detector resolution: 9 pixels mm-1Rint = 0.064
T = 123 Kθmax = 32.5º
P = 101.3 kPaθmin = 1.5º
CCD rotation images scansh = −11→11
Absorption correction: multi-scan
(Blessing, 1995)		k = −12→12
Tmin = 0.060, Tmax = 0.224l = −19→20
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.033H atoms treated by a mixture of
independent and constrained refinement
wR(F2) = 0.086  w = 1/[σ2(Fo2) + (0.0423P)2]
where P = (Fo2 + 2Fc2)/3
S = 0.98(Δ/σ)max = 0.001
5589 reflectionsΔρmax = 3.46 e Å−3
259 parametersΔρmin = −2.87 e Å−3
10 restraintsExtinction correction: none
Primary atom site location: structure-invariant direct methods
Crystal data top
K2[Pt2(HPO4)4(H2O)2]γ = 65.528 (1)º
Mr = 888.32V = 776.32 (4) Å3
Triclinic, P1Z = 2
a = 7.8852 (2) ÅMo Kα
b = 7.9657 (2) ŵ = 19.05 mm−1
c = 13.7739 (4) ÅT = 123 K
α = 82.358 (1)º0.24 × 0.14 × 0.08 mm
β = 81.509 (1)º
Data collection top
Nonius Kappa CCD
diffractometer		5589 independent reflections
Absorption correction: multi-scan
(Blessing, 1995)		4077 reflections with I > 2σ(I)
Tmin = 0.060, Tmax = 0.224Rint = 0.064
18915 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03310 restraints
wR(F2) = 0.086H atoms treated by a mixture of
independent and constrained refinement
S = 0.98Δρmax = 3.46 e Å−3
5589 reflectionsΔρmin = −2.87 e Å−3
259 parameters
Special details top

Geometry. All e.s.d.'s 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 and 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
xyzUiso*/Ueq
K10.1242 (4)−0.3657 (4)0.6326 (2)0.0790 (9)
K20.3845 (3)−0.1884 (3)0.90856 (15)0.0472 (5)
O10.6393 (7)0.2464 (8)0.6286 (4)0.0289 (11)
H1A0.566 (8)0.299 (10)0.676 (4)0.043*
H1B0.734 (6)0.175 (9)0.656 (5)0.043*
O120.6784 (7)−0.1304 (6)0.6372 (3)0.0238 (10)
O140.4660 (7)−0.2761 (7)0.7170 (4)0.0305 (11)
H140.486 (13)−0.387 (4)0.717 (6)0.046*
O110.8028 (7)−0.4742 (7)0.6678 (3)0.0256 (10)
O130.5849 (7)−0.3216 (6)0.5370 (3)0.0245 (10)
O220.7832 (6)0.0412 (7)0.4628 (4)0.0247 (10)
O240.9754 (7)−0.3003 (7)0.4569 (3)0.0276 (11)
H241.045 (10)−0.372 (10)0.415 (5)0.041*
O210.9903 (7)−0.0893 (7)0.3117 (4)0.0298 (11)
O230.6936 (6)−0.1446 (7)0.3608 (3)0.0267 (10)
O100.2526 (7)0.5993 (7)0.1437 (3)0.0249 (10)
H10A0.207 (11)0.719 (3)0.141 (5)0.037*
H10B0.226 (11)0.582 (9)0.2058 (19)0.037*
O32−0.1067 (6)0.5907 (6)0.1622 (3)0.0212 (9)
O33−0.3103 (6)0.5226 (6)0.0567 (3)0.0199 (9)
O34−0.4078 (7)0.8408 (7)0.1168 (4)0.0286 (11)
H34−0.520 (4)0.896 (11)0.107 (6)0.043*
O31−0.4038 (6)0.5674 (7)0.2362 (3)0.0231 (10)
O44−0.0110 (6)0.8018 (7)0.0001 (3)0.0236 (10)
O41−0.2456 (6)1.0558 (6)−0.0996 (3)0.0233 (10)
O43−0.2059 (6)0.7307 (6)−0.1068 (3)0.0210 (9)
O420.0530 (7)0.8330 (7)−0.1816 (4)0.0310 (12)
H420.050 (13)0.930 (7)−0.162 (6)0.047*
P10.6381 (2)−0.3051 (2)0.63814 (12)0.0220 (3)
P20.8592 (2)−0.1187 (2)0.39488 (12)0.0215 (3)
P3−0.3082 (2)0.6280 (2)0.14389 (12)0.0180 (3)
P4−0.1056 (2)0.8585 (2)−0.09741 (12)0.0187 (3)
Pt10.54549 (3)0.09217 (3)0.549475 (17)0.01966 (7)
Pt20.09528 (3)0.53533 (3)0.052197 (16)0.01543 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
K10.101 (2)0.087 (2)0.0691 (17)−0.0526 (18)−0.0488 (16)0.0208 (15)
K20.0525 (12)0.0601 (13)0.0406 (10)−0.0346 (11)0.0048 (9)−0.0133 (9)
O10.030 (3)0.030 (3)0.029 (3)−0.013 (2)0.000 (2)−0.010 (2)
O120.030 (2)0.020 (2)0.021 (2)−0.011 (2)−0.0040 (19)0.0035 (18)
O140.032 (3)0.028 (3)0.028 (3)−0.014 (2)0.007 (2)0.001 (2)
O110.028 (2)0.021 (2)0.019 (2)−0.003 (2)−0.0023 (19)0.0031 (18)
O130.029 (2)0.019 (2)0.021 (2)−0.006 (2)−0.0018 (19)0.0019 (18)
O220.019 (2)0.023 (2)0.030 (3)−0.007 (2)0.0021 (19)−0.005 (2)
O240.029 (3)0.026 (3)0.019 (2)−0.005 (2)0.0028 (19)0.0021 (19)
O210.022 (2)0.031 (3)0.029 (3)−0.008 (2)0.0055 (19)0.007 (2)
O230.021 (2)0.028 (3)0.025 (2)−0.005 (2)0.0031 (19)−0.006 (2)
O100.028 (2)0.032 (3)0.020 (2)−0.018 (2)−0.0068 (19)0.0045 (19)
O320.018 (2)0.023 (2)0.020 (2)−0.0061 (19)−0.0014 (17)−0.0005 (18)
O330.018 (2)0.025 (2)0.016 (2)−0.0101 (19)0.0043 (16)−0.0019 (17)
O340.020 (2)0.025 (3)0.038 (3)−0.006 (2)−0.003 (2)−0.001 (2)
O310.024 (2)0.026 (3)0.019 (2)−0.013 (2)0.0055 (18)−0.0032 (18)
O440.021 (2)0.023 (2)0.028 (2)−0.011 (2)−0.0080 (18)0.0054 (19)
O410.021 (2)0.017 (2)0.031 (3)−0.0073 (19)−0.0051 (19)0.0016 (19)
O430.022 (2)0.020 (2)0.019 (2)−0.0077 (19)−0.0011 (17)0.0025 (17)
O420.025 (2)0.025 (3)0.034 (3)−0.007 (2)0.008 (2)0.004 (2)
P10.0237 (8)0.0200 (8)0.0183 (8)−0.0061 (7)−0.0002 (6)0.0009 (6)
P20.0194 (8)0.0218 (9)0.0189 (8)−0.0066 (7)0.0020 (6)0.0024 (6)
P30.0150 (7)0.0204 (8)0.0172 (7)−0.0071 (6)0.0016 (6)−0.0005 (6)
P40.0167 (7)0.0176 (8)0.0190 (7)−0.0057 (6)−0.0011 (6)0.0033 (6)
Pt10.02042 (12)0.01997 (13)0.01736 (12)−0.00806 (10)0.00056 (9)−0.00042 (9)
Pt20.01457 (11)0.01729 (12)0.01430 (11)−0.00707 (9)−0.00109 (8)0.00101 (8)
Geometric parameters (Å, °) top
K1—O24i2.739 (6)O24—P21.570 (5)
K1—O31ii2.860 (5)O21—P21.489 (5)
K1—O11i2.957 (6)O23—P21.549 (5)
K1—O42iii3.047 (6)O23—Pt1iv2.014 (5)
K1—O22iv3.053 (5)O10—Pt22.132 (5)
K1—O32ii3.156 (5)O32—P31.544 (5)
K1—O12i3.222 (6)O32—Pt21.978 (4)
K2—O142.735 (6)O33—P31.561 (5)
K2—O34v2.822 (6)O33—Pt2vii2.030 (4)
K2—O41v2.858 (5)O34—P31.564 (5)
K2—O33ii2.920 (5)O31—P31.507 (4)
K2—O42iii2.990 (6)O44—P41.555 (5)
K2—O43vi3.000 (5)O44—Pt22.005 (5)
K2—O44iii3.215 (5)O41—P41.500 (5)
O1—Pt12.143 (5)O43—P41.553 (5)
O12—P11.549 (5)O43—Pt2vii2.014 (4)
O12—Pt11.992 (4)O42—P41.541 (5)
O14—P11.564 (5)Pt1—O13iv2.010 (4)
O11—P11.493 (5)Pt1—O23iv2.014 (5)
O13—P11.549 (5)Pt1—Pt1iv2.4944 (5)
O13—Pt1iv2.010 (4)Pt2—O43vii2.014 (4)
O22—P21.541 (5)Pt2—O33vii2.030 (4)
O22—Pt11.985 (4)Pt2—Pt2vii2.4892 (5)
O11—P1—O12110.2 (3)O22—Pt1—O23iv179.02 (19)
O11—P1—O13110.9 (3)O12—Pt1—O23iv90.37 (19)
O12—P1—O13111.5 (3)O13iv—Pt1—O23iv89.97 (19)
O11—P1—O14110.2 (3)O22—Pt1—O185.5 (2)
O12—P1—O14104.8 (3)O12—Pt1—O188.0 (2)
O13—P1—O14109.0 (3)O13iv—Pt1—O190.0 (2)
O21—P2—O22111.1 (3)O23iv—Pt1—O193.6 (2)
O21—P2—O23113.2 (3)O22—Pt1—Pt1iv90.98 (14)
O22—P2—O23109.6 (3)O12—Pt1—Pt1iv91.86 (14)
O21—P2—O24106.9 (3)O13iv—Pt1—Pt1iv90.05 (14)
O22—P2—O24107.7 (3)O23iv—Pt1—Pt1iv89.91 (15)
O23—P2—O24108.1 (3)O1—Pt1—Pt1iv176.49 (14)
O31—P3—O32108.9 (3)O32—Pt2—O4490.05 (19)
O31—P3—O33109.3 (3)O32—Pt2—O43vii89.88 (18)
O32—P3—O33111.8 (3)O44—Pt2—O43vii178.49 (18)
O31—P3—O34111.6 (3)O32—Pt2—O33vii177.57 (17)
O32—P3—O34106.3 (3)O44—Pt2—O33vii90.65 (19)
O33—P3—O34108.9 (3)O43vii—Pt2—O33vii89.36 (18)
O41—P4—O42110.7 (3)O32—Pt2—O1087.17 (19)
O41—P4—O43109.3 (3)O44—Pt2—O1089.05 (19)
O42—P4—O43110.0 (3)O43vii—Pt2—O1089.45 (19)
O41—P4—O44110.6 (3)O33vii—Pt2—O1090.51 (18)
O42—P4—O44106.4 (3)O32—Pt2—Pt2vii91.55 (13)
O43—P4—O44109.7 (2)O44—Pt2—Pt2vii90.52 (13)
O22—Pt1—O1289.21 (19)O43vii—Pt2—Pt2vii90.99 (13)
O22—Pt1—O13iv90.42 (19)O33vii—Pt2—Pt2vii90.77 (13)
O12—Pt1—O13iv178.06 (19)O10—Pt2—Pt2vii178.65 (14)
Symmetry codes: (i) x−1, y, z; (ii) −x, −y, −z+1; (iii) x, y−1, z+1; (iv) −x+1, −y, −z+1; (v) −x, −y+1, −z+1; (vi) x+1, y−1, z+1; (vii) −x, −y+1, −z.
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O1—H1A···O31v0.83 (6)1.73 (6)2.559 (7)173 (8)
O1—H1B···O21viii0.84 (6)2.09 (4)2.860 (7)153 (8)
O14—H14···O31ii0.83 (2)1.82 (5)2.548 (7)146 (9)
O24—H24···O11ix0.84 (7)1.73 (7)2.562 (7)177 (9)
O34—H34···O41x0.83 (6)1.74 (3)2.546 (6)162 (9)
O10—H10A···O41xi0.86 (2)1.94 (4)2.713 (7)148 (7)
O10—H10B···O11iv0.86 (2)1.86 (2)2.694 (6)164 (6)
O42—H42···O21xii0.84 (7)2.27 (9)2.475 (7)94 (6)
Symmetry codes: (v) −x, −y+1, −z+1; (viii) −x+2, −y, −z+1; (ii) −x, −y, −z+1; (ix) −x+2, −y−1, −z+1; (x) −x−1, −y+2, −z; (xi) −x, −y+2, −z; (iv) −x+1, −y, −z+1; (xii) −x+1, −y+1, −z.
Table 1
Hydrogen-bond geometry (Å, °) top
D—H···AD—HH···AD···AD—H···A
O1—H1A···O31i0.83 (6)1.73 (6)2.559 (7)173 (8)
O1—H1B···O21ii0.84 (6)2.09 (4)2.860 (7)153 (8)
O14—H14···O31iii0.83 (2)1.82 (5)2.548 (7)146 (9)
O24—H24···O11iv0.84 (7)1.73 (7)2.562 (7)177 (9)
O34—H34···O41v0.83 (6)1.74 (3)2.546 (6)162 (9)
O10—H10A···O41vi0.86 (2)1.94 (4)2.713 (7)148 (7)
O10—H10B···O11vii0.86 (2)1.86 (2)2.694 (6)164 (6)
O42—H42···O21viii0.84 (7)2.27 (9)2.475 (7)94 (6)
Symmetry codes: (i) −x, −y+1, −z+1; (ii) −x+2, −y, −z+1; (iii) −x, −y, −z+1; (iv) −x+2, −y−1, −z+1; (v) −x−1, −y+2, −z; (vi) −x, −y+2, −z; (vii) −x+1, −y, −z+1; (viii) −x+1, −y+1, −z.
Acknowledgements top

We thank G. Schnakenburg (University of Bonn) for the single-crystal data collection and D. Ernsthäuser (University of Bonn) for the measurement of the Raman spectrum. This work was financially supported by Deutsche Forschungsgemeinschaft (DFG). A noble metal donation by UMICORE AG (Hanau-Wolfgang, Germany) is gratefully acknowledged.

references
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