Dipotassium di-μ-hydro­xo-bis­[(nitrilo­triacetato-κ⁴N,O,O′,O′′)­chromium(III)] hexahydrate

The crystal structure of the title compound, K₂[Cr₂(C₆H₆NO₆)₂(OH)₂]·6H₂O or K₂[Cr₂(μ-OH)₂(nta)₂]·6H₂O (nta = nitrilo­tri­acetate), is composed of a dinuclear hydro­xo-bridged chromium(III) complex anion, potassium cation and water mol­ecules. Each Cr^III ion is in a distorted octahedral environment and coordinated by one nitro­gen and three carboxyl­ate O atoms of the nta³⁻ ligand plus two bridging hydro­xo O atoms. The dinuclear complex anion possesses 2/m crystallographic symmetry. The Cr—O(μ-OH) bond lengths are 1.937 (4) and 1.951 (4) Å. The Cr—N and Cr—O bond distances of the nta³⁻ ligand are 2.054 (4) and 1.968 (3)–1.983 (3) Å, while the HO—Cr—OH and Cr—OH—Cr angles are 80.46 (16) and 99.54 (16)°, respectively.

Key indicators

• Single-crystal X-ray study
• T = 296 K
• Mean [sigma](C-C) = 0.007 Å
• H-atom completeness 54%
• R factor = 0.056
• wR factor = 0.179
• Data-to-parameter ratio = 18.9

checkCIF/PLATON results

No syntax errors found



Alert level A
PLAT306_ALERT_2_A Isolated Oxygen Atom (H-atoms Missing ?) .......         O8

Author Response: Because the positions of hydrogen atoms for these hydrated water molecules could not be found in D-syntheses maps, we did not introduce the attached hydrogen atoms in the structural calculation.

PLAT306_ALERT_2_A Isolated Oxygen Atom (H-atoms Missing ?) .......         O9

Author Response: Because the positions of hydrogen atoms for these hydrated water molecules could not be found in D-syntheses maps, we did not introduce the attached hydrogen atoms in the structural calculation.

Alert level B
PLAT029_ALERT_3_B _diffrn_measured_fraction_theta_full Low .......       0.98
PLAT430_ALERT_2_B Short Inter D...A Contact  O4     ..  O9       =       2.84 Ang.
PLAT430_ALERT_2_B Short Inter D...A Contact  O8     ..  O9       =       2.72 Ang.
PLAT430_ALERT_2_B Short Inter D...A Contact  O8     ..  O9       =       2.72 Ang.

Alert level C
CHEMW03_ALERT_2_C The ratio of given/expected molecular weight as
            calculated from the _atom_site* data lies outside
            the range 0.99 <> 1.01
           From the CIF: _cell_formula_units_Z                    2
           From the CIF: _chemical_formula_weight           700.55
           TEST: Calculate formula weight from _atom_site_*
           atom     mass    num     sum
           Cr       52.00    2.00  103.99
           O        16.00   20.00  319.98
           N        14.01    2.00   28.01
           C        12.01   12.00  144.13
           H         1.01   14.00   14.11
           K        39.10    2.00   78.20
           Calculated formula weight              688.43
DIFMX01_ALERT_2_C  The maximum difference density is > 0.1*ZMAX*0.75
            _refine_diff_density_max given =      1.807
            Test value =      1.800
DIFMX02_ALERT_1_C  The minimum difference density is > 0.1*ZMAX*0.75
            The relevant atom site should be identified.
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT043_ALERT_1_C Check Reported Molecular Weight ................     700.55
PLAT044_ALERT_1_C Calculated and Reported Dx Differ ..............          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       2.00 Ratio
PLAT068_ALERT_1_C Reported F000 Differs from Calcd (or Missing)...          ?
PLAT094_ALERT_2_C Ratio of Maximum / Minimum Residual Density ....       2.54
PLAT152_ALERT_1_C Supplied and Calc Volume s.u. Inconsistent .....          ?
PLAT241_ALERT_2_C Check High   U(eq) as Compared to Neighbors ....         O4
PLAT341_ALERT_3_C Low Bond Precision on C-C bonds (x 1000) Ang ...          7
PLAT710_ALERT_4_C Delete 1-2-3 or 2-3-4 Linear Torsion Angle ... #          9
            O7  -CR  -N1  -C3    -60.90  0.20   1.555   1.555   1.555   1.555

Alert level G
FORMU01_ALERT_2_G  There is a discrepancy between the atom counts in the
            _chemical_formula_sum and the formula from the _atom_site* data.
            Atom count from _chemical_formula_sum:C12 H26 Cr2 K2 N2 O20
            Atom count from the _atom_site data:  C12 H14 Cr2 K2 N2 O20
CELLZ01_ALERT_1_G Difference between formula and atom_site contents detected.
CELLZ01_ALERT_1_G WARNING: H atoms missing from atom site list. Is this intentional?
           From the CIF: _cell_formula_units_Z    2
           From the CIF: _chemical_formula_sum  C12 H26 Cr2 K2 N2 O20
           TEST: Compare cell contents of formula and atom_site data

           atom    Z*formula  cif sites diff
           C         24.00     24.00    0.00
           H         52.00     28.00   24.00
           Cr         4.00      4.00    0.00
           K          4.00      4.00    0.00
           N          4.00      4.00    0.00
           O         40.00     40.00    0.00

   2 ALERT level A = In general: serious problem
   4 ALERT level B = Potentially serious problem
  14 ALERT level C = Check and explain
   3 ALERT level G = General alerts; check

  10 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
  10 ALERT type 2 Indicator that the structure model may be wrong or deficient
   2 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion

---

S1. Comment

The nitrilotriacetate ion (nta³⁻) may coordinate to a metal ion as a tridentate or tetradentate ligand via the one N and three carboxylate O atoms. The crystal structure of di(µ-hydroxo)bis[(nitrilotriacetato)chromium(III)] with the Cs⁺ cation was reported previously (Visser et al., 1999). This is another example of a [Cr₂(µ-OH)₂(nta)₂]²⁻ complex anion with a different cation, K⁺. The sharp-line electronic transitions and their splittings in chromium(III) complexes are very sensitive to the exact bond angles around the metal. Thus, it may be possible to extract the structural information from the electronic spectroscopy without a full X-ray structure determination (Hoggard, 1986; Choi, 1994); the relationship between environmental changes and geometric distortion may be found in the chromium(III) complexes. In order to examine the influence of cations and water contents of the crystal on the conformations of the Cr^III complexes, the title complex, (I), was prepared and its crystal structure was determined.

The structure analysis showed that the crystal of (I) consists of a [Cr₂(µ-OH)₂(nta)₂]²⁻ anion, two K⁺ cations, and six hydrated waters. Each Cr^III ion displays a distorted octahedral environment, formed by two bridging hydroxo O7 atoms and one N and three O atoms of the nta ligand. It is also found that with decreasing ionic radius of the cations the water content increases from four for Cs⁺ to six for K⁺ (Golic & Bulc, 1988).

S2. Experimental

The starting material, K₃[Cr(nta)₂]·2H₂O, was synthesized according to the literature (Visser et al., 1999). This complex was suspended in water and a blue–black precipitate was deposited after several days. Recrystallization of the precipitate from water afforded brown crystals of (I) suitable for X-ray analysis.

S3. Refinement

H atoms of nta³⁻ and bridging OH⁻ were placed geometrically, with O–H distances of 0.91 Å and C–H distances of 0.97 Å, and these atoms were refined using a riding model, U_iso(H) = U_eq(O or C). Other H atoms of hydrated water molecules were not included. In the final difference synthesis map, four rather large peaks (more than 0.65 e Å⁻³) were found at (a) (0.0331, 0.0000, 0.4198), (b) (0.1122, 0.3315, 0.6691), (c) (0.3252, 0.1676, 0.4900), and (d) (0.0425, 0.3444, 0.4254). Peaks a and b were close to O7 and O4 at distances of 0.666 and 1.142 Å, respectively, while peaks c and d were located around the K atom at distances of 1.283 and 1.075 Å, respectively. Thus, these peaks are not related to the H atoms of hydrated water molecules.

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Rigaku, 1985); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN (Molecular Structure Cooperation and Rigaku, 2000); program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP (Johnson, 1970); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Perspective view of the anionic part of (I). Displacement ellipsoids are drawn at 50% probability level. [Symmetry codes: (i) x, −y, z; (ii) −x, −y, 1 − z; (iii) −x, y, 1 − z.]

Dipotassium di-µ-hydroxo-bis[(nitrilotriacetato-κ⁴N,O,O',O'')chromium(III)] hexahydrate

Crystal data

K2[Cr2(C6H6NO6)2(OH)2]·6H2O	F(000) = 716
Mr = 700.55	Dx = 1.863 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C 2y	Cell parameters from 25 reflections
a = 7.311 (2) Å	θ = 14.7–15.0°
b = 13.321 (2) Å	µ = 1.30 mm^−1
c = 12.862 (2) Å	T = 296 K
β = 94.45 (2)°	Block, brown
V = 1248.8 (5) Å^3	0.30 × 0.22 × 0.20 mm
Z = 2

Data collection

Rigaku AFC-5R diffractometer	1277 reflections with I > 2σ(I)
Radiation source: rotating anode	Rint = 0.025
Graphite monochromator	θmax = 30.0°, θmin = 3.1°
ω–2θ scans	h = 0→9
Absorption correction: integration (Coppens et al., 1965)	k = 0→17
Tmin = 0.755, Tmax = 0.790	l = −17→17
2000 measured reflections	3 standard reflections every 150 reflections
1869 independent reflections	intensity decay: 2.1%

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)] = 0.056	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.179	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.1161P)^2 + 0.9272P] where P = (Fo^2 + 2Fc^2)/3
1869 reflections	(Δ/σ)max = 0.001
99 parameters	Δρmax = 1.81 e Å^−3
0 restraints	Δρmin = −0.71 e Å^−3

Special details

Experimental. none

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
Cr	0.14512 (10)	0.0000	0.58737 (5)	0.0176 (2)
K	0.0000	0.32166 (9)	0.5000	0.0340 (3)
O1	0.4150 (4)	0.0000	0.6034 (3)	0.0220 (7)
O2	0.6559 (5)	0.0000	0.7198 (3)	0.0335 (9)
O3	0.1490 (4)	0.1477 (2)	0.60712 (19)	0.0270 (5)
O4	0.0745 (5)	0.2649 (2)	0.7190 (3)	0.0468 (8)
O7	0.1218 (5)	0.0000	0.4363 (3)	0.0309 (9)
O8	0.3125 (9)	0.5000	0.9839 (4)	0.0605 (15)
O9	0.1534 (9)	0.3207 (4)	0.9305 (4)	0.0926 (19)
N1	0.1642 (5)	0.0000	0.7475 (3)	0.0189 (7)
C1	0.3636 (7)	0.0000	0.7860 (4)	0.0325 (12)
C2	0.4909 (7)	0.0000	0.6985 (4)	0.0220 (9)
C3	0.0719 (5)	0.0945 (3)	0.7772 (3)	0.0243 (7)
C4	0.1018 (5)	0.1772 (3)	0.6966 (3)	0.0271 (7)
H1	0.3888	−0.0588	0.8292	0.032*
H3A	0.1218	0.1160	0.8457	0.024*
H3B	−0.0584	0.0825	0.7804	0.024*
H7	0.2097	0.0000	0.3899	0.031*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr	0.0133 (3)	0.0250 (4)	0.0145 (3)	0.000	0.0004 (2)	0.000
K	0.0441 (7)	0.0242 (6)	0.0347 (6)	0.000	0.0101 (5)	0.000
O1	0.0122 (14)	0.0345 (19)	0.0192 (15)	0.000	0.0009 (12)	0.000
O2	0.0121 (16)	0.057 (3)	0.032 (2)	0.000	0.0010 (14)	0.000
O3	0.0333 (14)	0.0255 (12)	0.0220 (11)	0.0008 (11)	0.0020 (10)	0.0017 (10)
O4	0.070 (2)	0.0262 (14)	0.0443 (18)	0.0056 (15)	0.0035 (17)	−0.0016 (13)
O7	0.0150 (16)	0.062 (3)	0.0161 (15)	0.000	0.0030 (12)	0.000
O8	0.067 (4)	0.069 (4)	0.044 (3)	0.000	−0.004 (3)	0.000
O9	0.142 (5)	0.064 (3)	0.067 (3)	0.002 (3)	−0.022 (3)	−0.013 (2)
N1	0.0160 (17)	0.0262 (19)	0.0147 (16)	0.000	0.0020 (13)	0.000
C1	0.016 (2)	0.064 (4)	0.018 (2)	0.000	0.0003 (16)	0.000
C2	0.018 (2)	0.031 (2)	0.018 (2)	0.000	0.0015 (16)	0.000
C3	0.0246 (16)	0.0269 (16)	0.0220 (15)	0.0009 (13)	0.0059 (12)	−0.0060 (12)
C4	0.0285 (18)	0.0263 (17)	0.0261 (16)	−0.0005 (14)	−0.0008 (13)	−0.0006 (13)

Geometric parameters (Å, º)

Cr—O7	1.937 (4)	N1—C1	1.502 (6)
Cr—O7^i	1.951 (4)	C1—C2	1.514 (7)
Cr—O1	1.968 (3)	C1—H1	0.9700
Cr—O3	1.983 (3)	C3—C4	1.541 (5)
Cr—N1	2.054 (4)	C3—H3A	0.9700
O1—C2	1.304 (6)	C3—H3B	0.9700
O2—C2	1.216 (6)	K—O1^ii	2.815 (2)
O3—C4	1.289 (5)	K—O3	2.868 (3)
O4—C4	1.223 (5)	K—O4	2.926 (3)
O7—H7	0.9100	K—O3^iii	3.033 (3)
N1—C3	1.492 (4)
Cr···Cr^i	2.9687 (15)
O7—Cr—O7^i	80.46 (16)	N1—C1—C2	113.1 (4)
O7—Cr—O1	96.56 (15)	O2—C2—O1	123.6 (5)
O7^i—Cr—O1	177.02 (14)	O2—C2—C1	119.3 (4)
O7—Cr—O3	97.34 (7)	O1—C2—C1	117.1 (4)
O7^i—Cr—O3	91.38 (8)	N1—C3—C4	109.9 (3)
O1—Cr—O3	89.00 (8)	O4—C4—O3	124.2 (4)
O3—Cr—O3^iv	165.32 (14)	O4—C4—C3	119.4 (4)
O7—Cr—N1	178.84 (16)	O3—C4—C3	116.4 (3)
O7^i—Cr—N1	98.38 (15)	Cr—O7—H7	130.2
O1—Cr—N1	84.60 (15)	N1—C1—H1	109.0
O3—Cr—N1	82.67 (7)	C2—C1—H1	109.0
C2—O1—Cr	116.6 (3)	H1—C1—H1^iv	107.8
C4—O3—Cr	114.5 (2)	N1—C3—H3A	109.7
Cr—O7—Cr^i	99.54 (16)	C4—C3—H3A	109.7
C3^iv—N1—C3	115.1 (4)	N1—C3—H3B	109.7
C3—N1—C1	111.2 (2)	C4—C3—H3B	109.7
C3—N1—Cr	105.1 (2)	H3A—C3—H3B	108.2
C1—N1—Cr	108.6 (3)
O3—Cr—O1—C2	82.73 (7)	O3—Cr—N1—C3	29.4 (2)
O7—Cr—O3—C4	157.6 (3)	O3—Cr—N1—C1	−89.68 (8)
O7^i—Cr—O3—C4	77.0 (3)	C3—N1—C1—C2	−115.1 (2)
O1—Cr—O3—C4	−106.0 (3)	C3^iv—N1—C3—C4	−148.0 (3)
O3^iv—Cr—O3—C4	−23.8 (8)	C1—N1—C3—C4	84.3 (4)
N1—Cr—O3—C4	−21.3 (3)	Cr—N1—C3—C4	−33.0 (3)
O3—Cr—O7—Cr^i	−90.17 (8)	Cr—O3—C4—O4	−170.9 (3)
O3—Cr—N1—C3^iv	151.2 (2)	Cr—O3—C4—C3	6.7 (4)
O7—Cr—N1—C3	−60.9 (2)	N1—C3—C4—O4	−163.2 (4)
O1—Cr—N1—C3	119.1 (2)	N1—C3—C4—O3	19.1 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O7—H7···O2^v	0.91	1.78	2.680 (5)	169

Symmetry code: (v) −x+1, −y, −z+1.