Hexaaqua­copper(II) dinitrate: absence of Jahn-Teller distortion

In the title compound, [Cu(H₂O)₆](NO₃)₂, the geometry around the Cu^II ion is approximately octa­hedral, formed by six O atoms from the coordinated water mol­ecules. The Cu-O distances are rather similar [2.014 (2)-2.084 (2) Å] and not related by symmetry. The Jahn-Teller effect is, at best, only weakly observed in this structure, in contrast to many other structures where the hexa­aqua­copper(II) ion has been characterized. An extensive mesh of hydrogen-bond inter­actions between the coordinated water mol­ecules and nitrate ions is a feature of the structure and may limit the degree to which the Jahn-Teller effect can be observed.

Key indicators

• Single-crystal X-ray study
• T = 93 K
• Mean [sigma](O-N) = 0.004 Å
• R factor = 0.034
• wR factor = 0.096
• Data-to-parameter ratio = 9.0

checkCIF/PLATON results

No syntax errors found



Alert level B
PLAT029_ALERT_3_B _diffrn_measured_fraction_theta_full Low .......       0.95

Alert level C
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT088_ALERT_3_C Poor Data / Parameter Ratio ....................       9.05
PLAT250_ALERT_2_C Large U3/U1 Ratio for Average U(i,j) Tensor ....       2.74
PLAT731_ALERT_1_C Bond    Calc     0.98(4), Rep   0.978(18) ......       2.22 su-Rat
              O6   -H6B     1.555   1.555
PLAT735_ALERT_1_C D-H     Calc     0.98(4), Rep   0.978(18) ......       2.22 su-Rat
              O6   -H6B     1.555   1.555
PLAT735_ALERT_1_C D-H     Calc     0.98(4), Rep   0.978(18) ......       2.22 su-Rat
              O6   -H6B     1.555   1.555
PLAT735_ALERT_1_C D-H     Calc     0.98(4), Rep   0.978(18) ......       2.22 su-Rat
              O6   -H6B     1.555   1.555

   0 ALERT level A = In general: serious problem
   1 ALERT level B = Potentially serious problem
   8 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   6 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 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
   0 ALERT type 4 Improvement, methodology, query or suggestion

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S1. Comment

During attempts to grow crystals of the copper complex of the ditopic ligand, 1-[4'-p-tolyl-(2,2':6',2''-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane, (I) (Padilla-Tosta et al., 2000), blue block-shaped crystals of [Cu(H₂O)₆](NO₃)₂, (II), formed instead from the reaction mixture. Attempts to grow similar crystals in the absence of the ditopic ligand proved unsuccessful, which leads us to speculate that the ditopic ligand may be influencing the crystallization process. Unfortunately, the vagaries of nucleation and crystal growth make it difficult to test this hypothesis. We report here the structure of hexaaquacopper complex as its dinitrate salt.

The asymmetric unit of (II) consists of a [Cu(H₂O)₆]²⁺ cation and two nitrate anions. The geometry around the Cu²⁺ can be best described as an octahedron, with bonds to six water molecules (Fig. 1 and Table 1). The Cu—O bond lengths are rather similar, falling in the range 2.014 (2)–2.084 (2) Å, and there is an extended hydrogen-bonding network that links the coordinated water molecules and the nitrate anions throughout the crystal lattice (Fig. 2 and Table 2). Bond lengths and angles in the nitrate anions [1.233 (4)–1.272 (4) Å and 118.5 (3)–121.1 (3)°, respectively] are unremarkable, there being only small deviations from the ideal geometry.

The similarity of the Cu—O bond lengths is relatively unusual in that Jahn–Teller distortion often leads to two of the copper-ligand bonds that lie along one axis being much longer than the remaining four copper–ligand bonds. A number of Jahn–Teller-distorted hexaaquacopper complexes have been characterized by X-ray crystallography, viz. X–3(C₂H₁₀N₂²⁺)–2(O₁₂P₄⁴⁻) (Averbuch-Pouchot & Durif, 1989), X–2(ClO₄⁻)–2(C₆H₁₀N₂O₂) (Benedetti et al., 1979, 1986), X–2(C₆H₄ClO₃S⁻) (Bernardinelli et al., 1991), X–2(C₇H₇O₃S⁻) (Couldwell et al., 1978), X–2(C₉H₉O₉S₃³⁻–1.3(H₂O) (Dalrymple et al., 2002), X–2(C₂H₁₀N₂²⁺)–O₁₈P₆⁶⁻ (Durif & Averbuch-Pouchot, 1989), X–C₆H₈CuO₁₀²⁻ (Filippova, 2000), X–2(C₁₂H₁₀O₄P⁻)–2(C₂H₅NO₂) (Glowiak & Podgorska, 1986), X–C₁₆H₁₆CuO₁₀²⁻ (Honghui et al., 1988), X–C₁₆H₁₆CuO₁₀²⁻ (Kennard & Smith, 1989), X–2(Cl₄⁻),2(H₂O) (Li et al., 2004), X–2 C l⁻–2(C₁₀H₈N₂O₂)–2(H₂O) (Ma et al., 2001), X–2(C₇H₅O₆S⁻)–2(H₂O) (Ma et al., 2003), X–2(NH₄⁺)–2(SO₄²⁻) (Maslen et al., 1988), X–2(C₂₄H₄₄H₁₆O₄Pt₄⁴⁺)–10(ClO₄⁻)–9(H₂O) (Navarro et al., 2000), X–(C₆H₈CuO₁₀²⁺) (Rodriguez-Martin or Rodriquez-Martin et al., 2002), X–2(C₈H₁₁N₄O⁺)–2(SO₄²⁻)–2(H₂O) (Shamuratov et al., 1993), X–(C₁₆H₁₆CuO₁₀²⁻) (Wang et al., 1988) and (X)_n–2n(C₅H₈O₄⁻)–4n(H₂O) (Zviedre et al., 1985), where X is [Cu(H₂O)₆]²⁺. In these cases, the axial Cu—O bond lengths fall in the range 2.202–2.423 Å, in comparison with the equatorial bond lengths (1.945–2.084 Å). The mean axial bond length is between 8.7 and 24% longer than the mean equatorial bond length in these structures (the mean value of these percentage differences is 18.6% over 20 structures). In our structure, the mean bond length along the longest axis (O2—Cu—O4) is only 1.6% longer than that along the remaining axes.

We are aware of only six crystallographic studies of copper(II) complexes where static Jahn–Teller distortions are not observed in complexes where all six donors are otherwise identical, viz. in X–(BrO₃)₂ (Blackburn et al., 1991), Cu(en)₃²⁺–SO₄²⁻ (Cullen & Lingafelter, 1970), 2 K⁺–Pb²⁺–Cu(NO₂)₆⁴⁻ (Cullen & Lingafelter, 1971), Cu{[(CH₃)₂N]₂P(O)OP(O)[N(CH₃)₂]₂}₃(Cl₄)₂ (Joesten et al., 1970), X–(SiF₆)²⁻–6(H₂O) (Ray et al., 1973) and 2 T l⁺–Pb²⁺–Cu(NO₂)₆²⁻ (Takagi et al., 1976), where X is [Cu(H₂O)₆]²⁺. The structure we report further stands out from these other six because, in this case, the Cu atom lies on a general position, with all Cu—O bonds lengths being independently refined. In the remaining six cases, the Cu atoms are located on the special positions in higher symmetry space groups (Pa3, P31c, Fm3, P3c1, R3 and Fm3, respectively).

Jahn–Teller distortion may not be observed in a crystallographic study if either there is disorder in the structure (so that a defined long axis is randomly distributed over the three orientations relation to the unit-cell axes), or there is sufficient thermal motion to allow the long and short bonds in a structure to exchange over time (sometimes referred to as the dynamic Jahn–Teller effect). In these cases, the averaging inherent in the X-ray experiment (over spatial location in the crystal in the first case or time in the second) might be expected to manifest itself in the crystallographic modelling process as larger than expected anisotropic displacement parameters for the donor atoms along the direction of the copper–ligand bond. This effect has been discussed (Cullen et al., 1970) and may be significant in a number of the literature cases (Blackburn et al., 1991; Cullen et al., 1971; Takagi et al., 1976). Table 3 presents the anisotropic displacement parameters of Cu and the water O atoms in the structure of (II). The largest principal axes of the ellipsoids are not directed along the Cu—O bonds (Fig. 1). Taken together, these data strongly suggest the lack of Jahn–Teller distortion (static or dynamic) in the structure of (II). Here, three marginally longer Cu—O bonds (Cu—O2, Cu—O3 and Cu—O4) are meridionally distributed around the Cu atom, as are the Cu—O shorter bonds. The variation in the Cu—O bond lengths of the structure, and the absence of any significant Jahn–Teller effect may be explained by the influence of the hydrogen-bonding network in the lattice of the complex (Fig. 3 and Table 2). All of the coordinated water molecules are involved in several hydrogen bonds, which means that, while the copper centre may not be in its lowest energy Jahn–Teller distorted state, this could be made up for by the large number of weak interactions that may each be marginally stronger in the less distorted structure.

S2. Experimental

A solution of Cu(NO₃)₂·3H₂O (50 mg) in ethanol (5 ml) was added to a cooled filtered solution of ligand L, or (I) (0.15 g) in ethanol (5 ml). The reaction mixture was heated at reflux for 1 h, and upon cooling to room temperature afforded a blue–green insoluble precipitate (0.11 g). The precipitate was suspended in ethanol–water (1:1, 5 ml), then the mixture was filtered after it was heated to reflux for 1 h. The solution was allowed to cool to room temperature overnight. The solution was kept in the refrigerator for about two months during which time blue crystals of (II) suitable for X-ray analysis were produced. No crystals of (I) were produced in this way.

S3. Refinement

The H atoms were located in a difference Fourier map. The O—H distances were constrained to 1.0 Å, with U_iso(H) = 1.2U_eq(O). The highest peak in the final difference map is located 0.99 Å from Cu and the deepest hole 0.89 Å from the same atom.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SAINT-Plus (Bruker, 1999); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL and Mercury (Version 1.4; Bruno et al., 2002); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A perspective view of diagram of the title Cu^II complex, (II), showing the atom-labelling scheme with 50% probability displacement ellipsoids. H atoms are drawn as small spheres of arbitrary radii.
	Fig. 2. A packing diagram, showing hydrogen-bonding interactions (dashed lines) within the lattice of the complex. Key: Cu, N, O, and H atoms are shown with light blue (large), purple, red, and light blue (small) circles, respectively.
	Fig. 3. A perspective view of diagram of the Cu^II complex, (II), showing the hydrogen-bonding interactions (dashed lines) involving the dication. [Symmetry codes: (i) −x, −y, −z; (ii) x − 1, y − 1, z; (iii) −x + 1, −y + 1, −z + 1; (iv) x, y − 1, z; (v) x − 1, y, z.]

Hexaaquacopper(II) dinitrate

Crystal data

[Cu(H2O)6](NO3)2	Z = 2
Mr = 295.67	F(000) = 302
Triclinic, P1	Dx = 2.126 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 5.7404 (8) Å	Cell parameters from 2722 reflections
b = 7.6452 (10) Å	θ = 2.9–26.4°
c = 11.4655 (15) Å	µ = 2.43 mm^−1
α = 106.428 (2)°	T = 93 K
β = 98.399 (2)°	Block, blue
γ = 101.504 (2)°	0.55 × 0.34 × 0.12 mm
V = 461.84 (11) Å^3

Data collection

Bruker SMART CCD diffractometer	1556 independent reflections
Radiation source: fine-focus sealed tube	1494 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
Detector resolution: 8.192 pixels mm^-1	θmax = 25.1°, θmin = 3.7°
ϕ and ω scans	h = −6→6
Absorption correction: multi-scan (SADABS; Bruker, 1999)	k = −8→8
Tmin = 0.341, Tmax = 0.744	l = −13→13
2917 measured reflections

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.096	H-atom parameters constrained
S = 0.91	w = 1/[σ^2(Fo^2) + (0.0546P)^2 + 2.7318P] where P = (Fo^2 + 2Fc^2)/3
1556 reflections	(Δ/σ)max < 0.001
172 parameters	Δρmax = 0.60 e Å^−3
18 restraints	Δρmin = −1.14 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
Cu	0.17237 (7)	0.14519 (6)	0.23243 (4)	0.00973 (18)
O1	0.2520 (4)	−0.0653 (4)	0.1031 (2)	0.0095 (5)
H1A	0.230 (7)	−0.058 (5)	0.018 (2)	0.011*
H1B	0.180 (7)	−0.189 (4)	0.106 (3)	0.011*
O2	0.5425 (4)	0.2400 (4)	0.3146 (2)	0.0094 (5)
H2A	0.607 (7)	0.372 (3)	0.323 (3)	0.011*
H2B	0.581 (7)	0.216 (5)	0.394 (2)	0.011*
O3	0.0978 (4)	0.3582 (4)	0.3683 (2)	0.0102 (5)
H3A	0.185 (7)	0.479 (4)	0.365 (3)	0.012*
H3B	0.138 (7)	0.343 (5)	0.449 (2)	0.012*
O4	−0.1905 (4)	0.0364 (3)	0.1453 (2)	0.0088 (5)
H4A	−0.291 (6)	0.107 (4)	0.190 (3)	0.011*
H4B	−0.234 (7)	−0.097 (3)	0.137 (4)	0.011*
O5	0.2220 (4)	0.3095 (4)	0.1238 (2)	0.0105 (5)
H5A	0.388 (4)	0.340 (5)	0.111 (4)	0.013*
H5B	0.144 (6)	0.412 (4)	0.134 (4)	0.013*
O6	0.1346 (4)	−0.0050 (4)	0.3531 (2)	0.0096 (5)
H6A	−0.037 (4)	−0.048 (5)	0.352 (4)	0.012*
H6B	0.221 (6)	−0.102 (5)	0.356 (4)	0.012*
N1	0.7929 (5)	0.5522 (4)	0.1275 (3)	0.0093 (6)
O11	0.6585 (4)	0.6598 (4)	0.1232 (2)	0.0119 (5)
O12	0.7080 (4)	0.3869 (4)	0.1234 (2)	0.0124 (5)
O13	1.0202 (4)	0.6086 (4)	0.1371 (2)	0.0132 (6)
N2	0.5813 (5)	0.7634 (4)	0.3903 (3)	0.0102 (6)
O21	0.6533 (5)	0.9126 (4)	0.3697 (2)	0.0124 (5)
O22	0.3683 (4)	0.7234 (4)	0.4107 (2)	0.0145 (6)
O23	0.7148 (4)	0.6530 (4)	0.3925 (2)	0.0125 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu	0.0058 (3)	0.0106 (3)	0.0126 (3)	0.00079 (17)	0.00374 (17)	0.00344 (18)
O1	0.0078 (12)	0.0084 (13)	0.0119 (12)	0.0012 (9)	0.0045 (10)	0.0020 (10)
O2	0.0050 (11)	0.0102 (13)	0.0130 (12)	0.0003 (9)	0.0024 (9)	0.0047 (10)
O3	0.0091 (12)	0.0099 (13)	0.0123 (12)	0.0027 (10)	0.0053 (10)	0.0032 (10)
O4	0.0052 (11)	0.0079 (13)	0.0134 (12)	0.0013 (9)	0.0035 (9)	0.0031 (10)
O5	0.0067 (12)	0.0120 (13)	0.0160 (13)	0.0021 (10)	0.0064 (10)	0.0076 (10)
O6	0.0058 (12)	0.0114 (13)	0.0137 (12)	0.0014 (9)	0.0046 (10)	0.0063 (10)
N1	0.0081 (14)	0.0111 (17)	0.0077 (14)	0.0011 (12)	0.0033 (11)	0.0015 (11)
O11	0.0095 (12)	0.0124 (14)	0.0152 (13)	0.0043 (10)	0.0052 (10)	0.0043 (10)
O12	0.0095 (12)	0.0079 (14)	0.0216 (14)	0.0007 (10)	0.0067 (10)	0.0068 (10)
O13	0.0051 (12)	0.0120 (14)	0.0223 (14)	−0.0007 (10)	0.0054 (10)	0.0058 (11)
N2	0.0080 (14)	0.0121 (16)	0.0088 (14)	0.0016 (12)	0.0032 (11)	0.0009 (12)
O21	0.0119 (12)	0.0108 (14)	0.0180 (13)	0.0023 (10)	0.0081 (10)	0.0078 (10)
O22	0.0065 (12)	0.0137 (15)	0.0227 (14)	−0.0003 (10)	0.0065 (10)	0.0049 (11)
O23	0.0106 (12)	0.0135 (14)	0.0143 (13)	0.0054 (10)	0.0043 (10)	0.0037 (10)

Geometric parameters (Å, º)

Cu—O5	2.014 (2)	O4—H4A	0.970 (18)
Cu—O1	2.034 (2)	O4—H4B	0.974 (18)
Cu—O6	2.041 (2)	O5—H5A	0.974 (18)
Cu—O4	2.064 (2)	O5—H5B	0.963 (18)
Cu—O3	2.074 (2)	O6—H6A	0.973 (18)
Cu—O2	2.084 (2)	O6—H6B	0.978 (18)
O1—H1A	0.981 (18)	N1—O11	1.241 (4)
O1—H1B	0.966 (18)	N1—O12	1.245 (4)
O2—H2A	0.976 (18)	N1—O13	1.268 (4)
O2—H2B	0.981 (18)	N2—O21	1.233 (4)
O3—H3A	0.977 (18)	N2—O23	1.252 (4)
O3—H3B	0.966 (18)	N2—O22	1.272 (4)
O5—Cu—O1	89.49 (10)	H2A—O2—H2B	112 (2)
O5—Cu—O6	175.94 (10)	Cu—O3—H3A	108 (2)
O1—Cu—O6	93.58 (10)	Cu—O3—H3B	110 (2)
O5—Cu—O4	91.38 (10)	H3A—O3—H3B	113 (2)
O1—Cu—O4	88.81 (10)	Cu—O4—H4A	111 (2)
O6—Cu—O4	91.34 (10)	Cu—O4—H4B	108 (2)
O5—Cu—O3	91.72 (10)	H4A—O4—H4B	113 (2)
O1—Cu—O3	178.31 (10)	Cu—O5—H5A	114 (2)
O6—Cu—O3	85.17 (10)	Cu—O5—H5B	118 (2)
O4—Cu—O3	92.35 (10)	H5A—O5—H5B	114 (3)
O5—Cu—O2	89.50 (10)	Cu—O6—H6A	110 (2)
O1—Cu—O2	87.93 (10)	Cu—O6—H6B	122 (2)
O6—Cu—O2	87.96 (10)	H6A—O6—H6B	112 (2)
O4—Cu—O2	176.61 (9)	O11—N1—O12	120.8 (3)
O3—Cu—O2	90.89 (10)	O11—N1—O13	120.7 (3)
Cu—O1—H1A	116 (2)	O12—N1—O13	118.5 (3)
Cu—O1—H1B	113 (2)	O21—N2—O23	121.1 (3)
H1A—O1—H1B	113 (2)	O21—N2—O22	118.9 (3)
Cu—O2—H2A	112 (2)	O23—N2—O22	120.0 (3)
Cu—O2—H2B	113 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O4^i	0.98 (2)	1.91 (2)	2.894 (3)	179 (3)
O1—H1B···O13^ii	0.97 (2)	1.79 (2)	2.741 (4)	168 (4)
O2—H2B···O22^iii	0.98 (2)	2.12 (2)	3.038 (4)	156 (3)
O2—H2A···O23	0.98 (2)	2.00 (2)	2.940 (4)	162 (3)
O2—H2B···O21^iv	0.98 (2)	2.38 (3)	2.912 (4)	113 (3)
O3—H3A···O22	0.98 (2)	1.83 (2)	2.779 (4)	162 (3)
O3—H3B···O23^iii	0.97 (2)	1.88 (2)	2.827 (4)	167 (3)
O4—H4A···O2^v	0.97 (2)	1.99 (2)	2.942 (4)	167 (4)
O4—H4A···O1^v	0.97 (2)	2.60 (4)	3.070 (3)	110 (3)
O4—H4B···O11^ii	0.97 (2)	1.79 (2)	2.763 (4)	175 (3)
O5—H5A···O12	0.97 (2)	1.78 (2)	2.735 (3)	166 (4)
O5—H5A···N1	0.97 (2)	2.50 (3)	3.417 (4)	156 (3)
O5—H5A···O11	0.97 (2)	2.58 (3)	3.285 (3)	130 (3)
O5—H5B···O13^v	0.96 (2)	1.78 (2)	2.740 (4)	172 (4)
O5—H5B···N1^v	0.96 (2)	2.47 (2)	3.365 (4)	155 (3)
O5—H5B···O12^v	0.96 (2)	2.45 (3)	3.123 (3)	126 (3)
O6—H6B···O21^iv	0.98 (2)	2.44 (3)	3.154 (3)	130 (3)
O6—H6B···O22^iv	0.98 (2)	1.91 (2)	2.860 (4)	162 (4)
O6—H6B···N2^iv	0.98 (2)	2.51 (2)	3.436 (4)	157 (3)

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