[Pd₈(CH₃COO)₈(NO)₈]: solution from X-ray powder diffraction data

The water-insoluble title compound, octakis([mu]-acetato-[kappa]²O:O)­octakis([mu]-nitro­so-[kappa]²N:O)­octapalladium(II), [Pd₈(CH₃COO)₈(NO)₈], was precipitated as a yellow powder from a solution of palladium nitrate in nitric acid by adding acetic acid. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques. Patterson and Fourier syntheses were used for atom locations, and the Rietveld technique was used for the final structure refinement. The crystal structure is of a molecular type. The skeleton of the [Pd₈(CH₃COO)₈(NO)₈] mol­ecule is con­structed as a tetragonal prism with Pd atoms at the vertices. The eight NO^- groups are in bridging positions along the horizontal edges of the prism. The N and O atoms of each nitro­so group coordinate two different Pd atoms. The vertical edges present Pd...Pd contacts with a short distance of 2.865 (1) Å. These Pd atoms are bridged by a pair of acetate groups in a cis orientation with respect to each other. The complex has crystallographically imposed 4/m symmetry; all C atoms of the acetate groups are on mirror planes. The unique Pd atom lies in a general position and has square-planar coordination, consisting of three O and one N atom.

S1. Comment

Nitroso–acetate and nitrito–acetate complexes of palladium are, frequently, intermediate compounds in the synthesis of palladium acetate, which is an important industrial catalyst. They can be obtained at various ratios of acetate and nitroso groups and present an example of stable cluster complexes with various palladium nucleation numbers. Therefore, these compounds are rather interesting models for studying polynuclear complexes. A search of the Cambridge Structural Database (Allen, 2002) for palladium acetate compounds with NO^- or NO₂^- ligands revealed structures of [Pd₃(CH₃COO)₅NO₂] (Chiesa et al., 1990), [Pd₆(CH₃COO)₈(NO)₂] (Chiesa et al., 1990) and [Pd₄(CH₃COO)₆(NO)₂]·CH₂Cl₂ (Podberezskaya et al., 1981), as well as one nitroso–acetate compound of platinum, [Pt₄(CH₃COO)₆(NO)₂]·2CH₃COOH (Meester & Skapski, 1973). In the present paper, the crystal structure of the novel octanuclear complex compound [Pd₈(CH₃COO)₈(NO)₈], (I), as determined from powder diffraction data (Fig. 1), is communicated.

The [Pd₈(CH₃COO)₈(NO)₈] structure is of molecular type with relatively large molecules consisting of 48 non-H atoms (Fig. 2). The skeleton of the molecule is constructed as a tetragonal prism with Pd in the vertices. However, the eight Pd atoms are not directly bonded in a single cluster unit. Rather, there are four Pd₂ fragments in vertical edges of the prism. The 2.8648 (10) Å Pd···Pd contacts in the fragments can be considered as metal–metal bonds. These Pd₂ groups are connected via NO^- ligands, which form the eight horizontal edges of the prism. The Pd···Pd bonds are enforced by two acetate groups positioned across the bond in a bridging mode and cis oriented to each other.

The nitroso groups demonstrate the bidentate bonding mode in the compound, coordinating one Pd atom by the O atom and another by the N atom. The exact positions of the NO^- ligands are not coincident with the lines connecting the Pd atoms. Rather, they lie a little higher or lower than this line, allowing the Pd coordination geometry to remain very close to ideal square planar (Table 1). The very short N—O distance, 1.06 (3) Å, is consistent with the triple bond expected in a negatively charged NO^- anion. The packing arrangement in the crystal conforms to an I-centered cell with the centers of the Pd₈(CH₃COO)₈(NO)₈] molecules situated on the lattice points and the molecules rotated approximately 60° around the z axis with respect to the Pd₈ cages (Fig. 3). This packing results in the formation of columns from stacked complexes with 3.9856 (10) Å intermolecular Pd···Pdⁱⁱⁱ distances along the z direction (symmetry operation iii is x,y,-z).

The known nitroso-acetate compounds of palladium and platinum can be separated into two groups in terms of the number of metal atoms in the cluster. The compounds Pd₂(CH₃COO)₂(NO)₂, [Pd₄(CH₃COO)₆(NO)₂]·CH₂Cl₂ and [Pt₄(CH₃COO)₆(NO)₂]·2CH₃COOH have two atoms in clusters with the Pd—Pd bond shorter than 2.92 Å. [Pd₃(CH₃COO)₅NO₂] and [Pd₆(CH₃COO)₈(NO)₂] have triangular clusters with the Pd···Pd contact in the range 3.03–3.20 Å. In this respect, [Pd₈(CH₃COO)₈(NO)₈] can be related to first group. Another point of classification is the coordination of the NO^- groups. Bidentate bonding of NO^- is a specific feature of the studied structure. The title compound is the first example of such coordination in the family of nitroso–acetate compounds of palladium and platinum.

S2. Experimental

The synthesis of (I) was carried out by adding acetic acid to a solution of palladium nitrate in nitric acid. The solution was heated at 378 K. The precipitated yellow product was filtered off, washed with water and dried in air.

S3. Refinement

The sample was prepared by top-loading the standard quartz sample holder. Corundum was used as the external standard. X-ray powder diffraction data are deposited in JCPDS-ICDD PDF2 data base. Cell parameters were obtained from d-spaces by indexing and refining using programs described by Visser (1969) and Kirik et al. (1979). The space group was determined from the analysis of systematic absences. The intensities of 85 reflections were estimated from the powder pattern by means of the full-profile fitting procedure (Le Bail et al., 1988) and used in the Patterson synthesis. The Pd atoms were located directly from the Patterson map. The positions of light atoms O, N and C were obtained from a difference Fourier synthesis. H atoms were not located, but they were included in the refined structure models and rigidly connected to their C atoms having in view the special positions of C atoms. The final refinement was carried out by the Rietveld (1969) method.

Computing details

Data collection: DRON-4 data collection software; cell refinement: POWDER (Kirik et al., 1979); data reduction: ?; program(s) used to solve structure: Modified DBWM (Wiles & Young, 1981); program(s) used to refine structure: Modified DBWM; molecular graphics: XP (Siemens, 1989).

Figures

	Fig. 1. The observed (dots), calculated (superimposed solid) and difference profiles after the Rietveld refinement. The reflection positions are marked by ticks.
	Fig. 2. The molecular complex [Pd8(NO)8(CH3COO)8].
	Fig. 3. The arrangement of complexes in the crystal structure.

octakis(µ-acetato-κ²O:O)octakis(µ-nitroso-κ²N:O)octapalladium(II)

Crystal data

[Pd8(C2H3O2)8(NO)8]	Cell parameters are obtained from the Rietveld refinement
Mr = 1563.79	Dx = 2.461 Mg m^−3
Tetragonal, I4/m	Cu Kα radiation
Hall symbol: -I 4	T = 293 K
a = 17.5504 (3) Å	Particle morphology: thin powder
c = 6.8504 (2) Å	yellow
V = 2110.04 (8) Å^3	circular flate plate, 20.0 × 20.0 mm
Z = 2	Specimen preparation: Prepared at 293 K and 101 kPa, cooled at 0 K min^−1
F(000) = 1472.0

Data collection

DRON-4 powder diffractometer	Specimen mounting: packed powder pellet
Radiation source: conventional sealed tube	Data collection mode: reflection
Graphite monochromator	2θmin = 5.0°, 2θmax = 90.0°, 2θstep = 0.02°

Refinement

Refinement on F^2	Profile function: Pearson VII
Least-squares matrix: full	45 parameters
Rp = 0.106	0 restraints
Rwp = 0.146	0 constraints
Rexp = 0.105	H-atom parameters constrained
RBragg = 0.053	Weighting scheme based on measured s.u.'s
R(F^2) = 0.040	(Δ/σ)max = 0.05
Excluded region(s): none	Preferred orientation correction: March-Dollase correction

Special details

Refinement. Shift/su_max coeficient related with positional coordinates. R_prof-backgr = 0.106

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pd	0.1956 (2)	0.0365 (2)	0.2909 (1)	0.0091*
C11	0.319 (1)	−0.043 (2)	0.5	0.0267*
C12	0.386 (1)	−0.087 (2)	0.5	0.0290*
O1	0.289 (1)	−0.025 (2)	0.3341 (5)	0.0295*
H12A	0.40186	−0.09606	0.63209	0.0800*	0.50
H12B	0.42544	−0.06016	0.43144	0.0800*	0.50
H12C	0.37642	−0.1348	0.43647	0.0800*	0.50
C21	0.281 (2)	0.164 (1)	0.5	0.0361*
C22	0.324 (1)	0.232 (1)	0.5	0.0372*
O2	0.257 (1)	0.131 (1)	0.3398 (6)	0.0305*
H22A	0.33486	0.24669	0.36791	0.0800*	0.50
H22B	0.29594	0.27187	0.56351	0.0800*	0.50
H22C	0.37091	0.22336	0.56858	0.0800*	0.50
O3	0.104 (1)	0.101 (1)	0.2519 (4)	0.0283*
N	0.135 (1)	−0.054 (1)	0.2611 (5)	0.0310*

Geometric parameters (Å, º)

Pd—O1	1.99 (2)	O3^i—N	1.06 (3)
Pd—O2	2.01 (2)	Pd—Pd^ii	2.8648 (10)
Pd—O3	1.98 (2)	C12—H12A	0.96
Pd—N	1.92 (2)	C12—H12B	0.96
O1—C11	1.29 (2)	C12—H12C	0.96
O2—C21	1.31 (2)	C22—H22A	0.96
C11—C12	1.41 (3)	C22—H22B	0.96
C21—C22	1.41 (3)	C22—H22C	0.96
O2—Pd—O1	88.9 (6)	C11—C12—H12A	109.4
O2—Pd—O3	89.2 (6)	C11—C12—H12B	109.4
O3—Pd—N	90.5 (6)	C11—C12—H12C	109.4
O1—Pd—N	91.3 (7)	C21—C22—H22A	109.4
O1—C11—O1^ii	123.2 (2)	C21—C22—H22B	109.4
O2—C21—O2^ii	113.8 (3)	C21—C22—H22C	109.4

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