Pseudo-merohedrally twinned praseodymium hexa­cyano­ferrate(III) tetrahydrate

Crystals of the title compound, di­aqua­hexa-[mu]-cyano-ferrate(III)­praseo­dym­ium(III) dihydrate, Pr[Fe(CN)₆]·4H₂O or [PrFe(CN)₆(H₂O)₂]·2H₂O, are twinned with three components. The Pr atom is coordinated by eight atoms, viz. six N and two symmetry-related water O atoms. The Pr polyhedron (Pr has site symmetry m2m, Wyckoff position 4c) is linked to an FeC₆ octahedron (Fe located on a site with imposed 2/m symmetry, Wyckoff position 4b) through N atoms, forming an infinite array. The second (symmetry independent) water mol­ecule lies on a mirror plane, is not included in coordination and is weakly hydrogen bonded to N atoms.

S1. Comment

The crystal structures of Pr[Fe(CN)₆]·5H₂O and Nd[Fe(CN)₆]·5H₂O solved in hexagonal cells (P6₃/m) have been reported (Wang et al., 1999) to contain a water molecule with C_2v symmetry located on a threefold axis. Such an arrangement is rather unusual and implies a disorder of H atoms. A very similar hexagonal structure has been found for a related compound, Pr[Co(CN)₆]·5H₂O, by Yukawa et al. (1996). For Nd[Co(CN)₆]·5H₂O, however, the same authors reported the non-centrosymmetric orthorhombic space group C222₁, even though the structures were expected to be similar.

In contrast with pentahydrates, the structures of tetrahydrates have usually been described in the orthorhombic space group Cmcm, where no such possible conflict between molecular and crystal symmetry is possible. Such structures are Er[Fe(CN)₆]·4H₂O (Mullica et al., 1989; Marsh, 1989; Gramlich et al., 1990), Gd[Fe(CN)₆]·4H₂O (Mullica & Sappenfield, 1991), Nd[Co(CN)₆]·4H₂O (Mullica et al., 1996), Sm[Fe(CN)₆]·4H₂O and Sm[Co(CN)₆]·4H₂O (Mullica & Sappenfield, 1989), and Bi[Fe(CN)₆]·4H₂O and Bi[Co(CN)₆]·4H₂O (Petter & Gramlich, 1990). The only exception was the structure of La[Fe(CN)₆]·4H₂O (Mullica et al., 1980), which was reported, like La[Fe(CN)₆]·5H₂O (Bailey et al., 1973), to be hexagonal, space group P6₃/m.

The diffraction data of the title compound were first processed as orthorhombic C-centred, with R_sym = 0.076 before an absorption correction, which is quite acceptable for a crystal containing a heavy atom. However, when evaluating the data, the mean value of Σ(E²-1) was observed to be rather low, 0.539 [expected value 0.968 for centrosymmetric and 0.736 for non-centrosymmetric space groups (Wilson, 1985)]. This is usually an indicator of twinned crystals (Herbst-Irmer & Sheldrick, 1998). In attempting to solve the structure in the orthorhombic system, the best result that could be achieved was in space group Cmc2₁ (No.36), with R = 0.151 on observed data and large residuals on the difference Fourier synthesis (14 and −17 e Å⁻³).

As the data indicated a pseudohexagonal symmetry with R_sym = 0.164, an attempt was made to solve the structure in space group P6₃ (No. 173). The best result achieved gave R = 0.213, and again large residuals were found on the difference Fourier synthesis (6 and −9 e Å⁻³). Therefore, it was decided to use the hexagonal cell to solve the structure in the monoclinic system, space group P2₁ (No. 4). Even though the structure could be `seen', the refinement was unsuccessful until the introduction of a twinning law (TWIN 0 0 1 0 1 0 − 1 0 − 1 3) for a twinned crystal with three components. The refinement was than satisfactory, with R = 0.024 and acceptable residuals on the difference Fourier synthesis (0.7 and −1.5 e Å⁻³). At this stage, the analysis of the resulting structure by PLATON (Spek, 2001) clearly indicated the centrosymmetric space group Cmcm (No. 63). To complete the refinement in the Cmcm space group the twinning law had to be reformulated as TWIN −0.5 0.5 0 − 1.5 − 0.5 0 0 0 1 3. The volume fractions of the twin refined to 0.580 (2), 0.210 (1) and 0.210 (2).

The crystal structure of Pr[Fe(CN)]₆·4H₂O consists of [Fe(CN)₆] octahedra, the N atoms of which also coordinate Pr atoms (Fig. 1). The polyhedron around the Pr atoms is completed by two water molecules (O2). The N atoms formally provide links between these two polyhedra, forming an infinite array (Fig. 2). The second water molecule (O1), as well as bonding to O2, also bonds weakly to three N atoms through hydrogen bonds, with atom H12 forming a symmetrical bifurcated bridge (Fig. 3).

The discrepancy between the expected and observed chemical composition can be best explained by the fact that at least a part of the batch of crystals dehydrated by losing a water molecule, and thus a more stable tetrahydrate was formed. Such a process has also been reported previously for metastable Nd[Co(CN)₆]·5H₂O and Pr[Co(CN)₆]·5H₂O (Yukawa et al., 1996).

S2. Experimental

Potassium hexacyanoferrate(III), K₃[Fe(CN)₆], was purchased from Waco pure Chemical Ltd., and praseodymium nitrate hexahydrate, Pr(NO₃)₃·6H₂O, from Shin-etu Chemical Co. The complex praseodymium hexacyanoferrate(III) pentahydrate, Pr[Fe(CN)₆]·5H₂O, was prepared by adding equimolar quantities of an aqueous solution of K₃[Fe(CN)₆] (0.25 M) to a solution of Pr(NO₃)₃·6H₂O (0.25 M). Single crystals of Pr[Fe(CN)]₆·4H₂O were obtained from the mixture by keeping it at room temperature for several days. Although the synthesis was conducted to provide the pentahydrate, the analysed crystal turned to be the tetrahydrate.

S3. Refinement

H atoms were refined isotropically with a common atomic displacement factor of 0.059 (9) and with O—H distances restrained to 0.82 (2) Å. Please check added text.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT and SADABS (Sheldrick, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXTL (Bruker, 2001); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the structure of Pr[Fe(CN)]6·4H2O, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x, y, 1/2 − z; (ii) −x, y, z; (iii) x, y, 1/2 − z; (iv) −x, 1 − y, 1 − z; (v) x, 1 − y, 1 − z; (vi) 1/2 − x, 1/2 − y, 1 − z.]
	Fig. 2. The structure of Pr[Fe(CN)]6·4H2O viewed along the a axis. Large spheres represent Pr atoms coordinated by N atoms and two molecules of water (O2). FeC6 octahedra are pictured as dark.
	Fig. 3. The hydrogen-bonding pattern in Pr[Fe(CN)]6·4H2O (dashed lines). Symmetry codes and further details are given in Table 2.

Diaquahexa-µ-cyano-ferrate(III)praseodymium(III) dihydrate

Crystal data

[PrFe(CN)6(H2O)2]·H2O	F(000) = 812
Mr = 424.94	Dx = 2.113 Mg m^−3
Orthorhombic, Cmcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2c 2	Cell parameters from 8192 reflections
a = 7.4920 (1) Å	θ = 1.5–33.1°
b = 12.9293 (1) Å	µ = 4.71 mm^−1
c = 13.7904 (1) Å	T = 173 K
V = 1335.83 (2) Å^3	Irregular plate, dark red
Z = 4	0.05 × 0.04 × 0.02 mm

Data collection

Siemens SMART 1K CCD area-detector diffractometer	1408 independent reflections
Radiation source: fine-focus sealed tube	1388 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.062
ω scans	θmax = 33.1°, θmin = 1.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	h = −11→11
Tmin = 0.816, Tmax = 0.929	k = −19→19
12392 measured reflections	l = −20→20

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.017	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.039	Only H-atom coordinates refined
S = 1.02	w = 1/[σ^2(Fo^2) + (0.0233P)^2] where P = (Fo^2 + 2Fc^2)/3
1408 reflections	(Δ/σ)max = 0.001
61 parameters	Δρmax = 0.52 e Å^−3
3 restraints	Δρmin = −1.30 e Å^−3

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Pr1	0.0000	0.177159 (15)	0.2500	0.00780 (5)
Fe1	0.0000	0.5000	0.5000	0.00721 (9)
O1	0.5000	0.34575 (19)	0.39831 (16)	0.0226 (4)
O2	0.2628 (4)	0.2885 (2)	0.2500	0.0243 (6)
N1	0.2034 (3)	0.07313 (15)	0.36094 (14)	0.0188 (4)
N2	0.0000	0.2822 (2)	0.4084 (2)	0.0168 (6)
C1	0.1840 (3)	0.45482 (16)	0.58811 (14)	0.0119 (4)
C2	0.0000	0.3637 (2)	0.4434 (2)	0.0123 (5)
H11	0.5000	0.317 (4)	0.452 (2)	0.059 (9)*
H12	0.5000	0.4093 (16)	0.407 (4)	0.059 (9)*
H2	0.321 (5)	0.299 (3)	0.2992 (19)	0.059 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pr1	0.00740 (10)	0.00826 (10)	0.00773 (7)	0.000	0.000	0.000
Fe1	0.0069 (2)	0.0074 (3)	0.00737 (18)	0.000	0.000	0.00050 (17)
O1	0.0283 (16)	0.0215 (13)	0.0180 (8)	0.000	0.000	−0.0012 (7)
O2	0.0178 (12)	0.0359 (14)	0.0193 (11)	−0.0161 (10)	0.000	0.000
N1	0.0182 (10)	0.0218 (8)	0.0163 (8)	0.0049 (7)	−0.0022 (7)	0.0024 (7)
N2	0.0184 (15)	0.0155 (13)	0.0165 (13)	0.000	0.000	−0.0029 (10)
C1	0.0132 (9)	0.0111 (8)	0.0115 (8)	−0.0005 (7)	0.0014 (7)	0.0012 (7)
C2	0.0118 (13)	0.0129 (12)	0.0121 (12)	0.000	0.000	0.0008 (11)

Geometric parameters (Å, º)

Pr1—O2^i	2.439 (2)	Fe1—C1	1.928 (2)
Pr1—O2	2.439 (2)	Fe1—C1^iv	1.928 (2)
Pr1—N1^i	2.5438 (18)	Fe1—C1^v	1.928 (2)
Pr1—N1^ii	2.5438 (18)	Fe1—C1^ii	1.928 (2)
Pr1—N1^iii	2.5438 (18)	O1—H11	0.834 (19)
Pr1—N1	2.5438 (18)	O1—H12	0.830 (19)
Pr1—N2^iii	2.572 (3)	O2—H2	0.817 (18)
Pr1—N2	2.572 (3)	N1—C1^vi	1.156 (3)
Fe1—C2^iv	1.928 (3)	N2—C2	1.159 (4)
Fe1—C2	1.928 (3)	C1—N1^vi	1.156 (3)
O2^i—Pr1—O2	107.67 (15)	N1^iii—Pr1—N2	142.18 (5)
O2^i—Pr1—N1^i	80.13 (8)	N1—Pr1—N2	76.61 (7)
O2—Pr1—N1^i	142.71 (4)	N2^iii—Pr1—N2	116.25 (14)
O2^i—Pr1—N1^ii	80.13 (8)	C2^iv—Fe1—C2	180.0
O2—Pr1—N1^ii	142.71 (4)	C2^iv—Fe1—C1	91.25 (9)
N1^i—Pr1—N1^ii	73.95 (8)	C2—Fe1—C1	88.75 (9)
O2^i—Pr1—N1^iii	142.71 (4)	C2^iv—Fe1—C1^iv	88.75 (9)
O2—Pr1—N1^iii	80.13 (8)	C2—Fe1—C1^iv	91.25 (9)
N1^i—Pr1—N1^iii	73.59 (9)	C1—Fe1—C1^iv	180.0
N1^ii—Pr1—N1^iii	116.16 (10)	C2^iv—Fe1—C1^v	88.75 (9)
O2^i—Pr1—N1	142.71 (4)	C2—Fe1—C1^v	91.25 (9)
O2—Pr1—N1	80.13 (8)	C1—Fe1—C1^v	88.73 (13)
N1^i—Pr1—N1	116.16 (10)	C1^iv—Fe1—C1^v	91.27 (12)
N1^ii—Pr1—N1	73.58 (10)	C2^iv—Fe1—C1^ii	91.25 (9)
N1^iii—Pr1—N1	73.95 (8)	C2—Fe1—C1^ii	88.74 (9)
O2^i—Pr1—N2^iii	71.84 (5)	C1—Fe1—C1^ii	91.27 (13)
O2—Pr1—N2^iii	71.84 (5)	C1^iv—Fe1—C1^ii	88.73 (13)
N1^i—Pr1—N2^iii	76.61 (7)	C1^v—Fe1—C1^ii	180.0
N1^ii—Pr1—N2^iii	142.18 (5)	H11—O1—H12	108 (5)
N1^iii—Pr1—N2^iii	76.61 (7)	Pr1—O2—H2	122 (3)
N1—Pr1—N2^iii	142.18 (5)	C1^vi—N1—Pr1	165.47 (19)
O2^i—Pr1—N2	71.84 (5)	C2—N2—Pr1	146.5 (3)
O2—Pr1—N2	71.84 (5)	N1^vi—C1—Fe1	178.35 (19)
N1^i—Pr1—N2	142.18 (5)	N2—C2—Fe1	179.3 (3)
N1^ii—Pr1—N2	76.61 (7)
O2^i—Pr1—N1—C1^vi	−91.4 (7)	O2^i—Pr1—N2—C2	−58.17 (7)
O2—Pr1—N1—C1^vi	14.8 (7)	O2—Pr1—N2—C2	58.17 (7)
N1^i—Pr1—N1—C1^vi	159.3 (7)	N1^i—Pr1—N2—C2	−102.38 (12)
N1^ii—Pr1—N1—C1^vi	−138.4 (6)	N1^ii—Pr1—N2—C2	−142.00 (5)
N1^iii—Pr1—N1—C1^vi	97.3 (7)	N1^iii—Pr1—N2—C2	102.38 (12)
N2^iii—Pr1—N1—C1^vi	57.1 (7)	N1—Pr1—N2—C2	142.00 (5)
N2—Pr1—N1—C1^vi	−58.7 (7)	N2^iii—Pr1—N2—C2	0.000 (1)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H11···N2^vii	0.83 (3)	2.31 (4)	3.137 (4)	173 (5)
O1—H12···N1^viii	0.83 (2)	2.69 (2)	3.351 (3)	138 (2)
O2—H2···O1	0.82 (3)	2.01 (3)	2.809 (2)	166 (3)

Symmetry codes: (vii) x+1/2, −y+1/2, −z+1; (viii) x+1/2, y+1/2, z.