The crystal structure of the title compound, [Zn{CO(NH2)2}6](NO3)2, has been determined at 110 and 250 K. The structure is stabilized by 12 individual hydrogen bonds, both intra- and intermolecular. Analysis of the thermal expansion tensor, based on unit cells determined over a temperature range of 180 K, shows uniaxial compression in the direction of the b axis during warming. The hydrogen bonds form layers perpendicular to this axis and these layers are connected by coordinative bonds parallel to the axis. As expected, the intermolecular hydrogen bonds expand during warming. Surprisingly, the coordinative bonds contract, accompanied by changes in the O—Zn—O angles. Overall, this behaviour can be described as an accordion-like effect.
Supporting information
CCDC references: 817037; 817038
Zinc nitrate hexahydrate was mixed with six equivalents of urea in water.
Evaporation at room temperature resulted in a highly viscous liquid, from
which crystals of (I) were obtained after several weeks.
As a starting model for the refinement, the coordinates of van de Giesen & Stam
(1972) were used, but it was decided to perform a unit-cell reduction with
PLATON (Spek, 2009). Further refinements were performed in the
conventional unit-cell setting.
For both compounds, data collection: COLLECT (Nonius, 1999); cell refinement: PEAKREF (Schreurs, 2008); data reduction: EVAL15 (Schreurs et al., 2010) and SADABS (Sheldrick, 2008a); program(s) used to solve structure: coordinates from literature (van de Giesen & Stam, 1972); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008b); molecular graphics: PLATON (Spek, 2009) and Jmol (Jmol, 2010); software used to prepare material for publication: manual editing of SHELXL97 cif file.
(Ia) Hexakis(urea-
κO)zinc(II) dinitrate
top
Crystal data top
[Zn(CH4N2O)6](NO3)2 | F(000) = 1136 |
Mr = 549.76 | Dx = 1.719 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 35466 reflections |
a = 17.0337 (6) Å | θ = 1.7–35.0° |
b = 18.0092 (5) Å | µ = 1.24 mm−1 |
c = 7.3550 (2) Å | T = 110 K |
β = 109.651 (2)° | Plate, colourless |
V = 2124.84 (11) Å3 | 0.36 × 0.20 × 0.12 mm |
Z = 4 | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 4678 independent reflections |
Radiation source: rotating anode | 4266 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.027 |
ϕ and ω scans | θmax = 35.0°, θmin = 1.7° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | h = −27→27 |
Tmin = 0.618, Tmax = 0.747 | k = −29→29 |
40566 measured reflections | l = −11→11 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.022 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.059 | All H-atom parameters refined |
S = 1.07 | w = 1/[σ2(Fo2) + (0.0319P)2 + 0.8657P] where P = (Fo2 + 2Fc2)/3 |
4678 reflections | (Δ/σ)max = 0.001 |
198 parameters | Δρmax = 0.46 e Å−3 |
0 restraints | Δρmin = −0.27 e Å−3 |
Crystal data top
[Zn(CH4N2O)6](NO3)2 | V = 2124.84 (11) Å3 |
Mr = 549.76 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 17.0337 (6) Å | µ = 1.24 mm−1 |
b = 18.0092 (5) Å | T = 110 K |
c = 7.3550 (2) Å | 0.36 × 0.20 × 0.12 mm |
β = 109.651 (2)° | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 4678 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | 4266 reflections with I > 2σ(I) |
Tmin = 0.618, Tmax = 0.747 | Rint = 0.027 |
40566 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.022 | 0 restraints |
wR(F2) = 0.059 | All H-atom parameters refined |
S = 1.07 | Δρmax = 0.46 e Å−3 |
4678 reflections | Δρmin = −0.27 e Å−3 |
198 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
Zn1 | 0.0000 | 0.178482 (6) | 0.2500 | 0.00913 (3) | |
O1 | −0.12486 (4) | 0.18499 (3) | 0.25260 (8) | 0.01319 (10) | |
O2 | 0.03229 (3) | 0.09309 (3) | 0.44918 (8) | 0.01242 (10) | |
O3 | 0.03092 (4) | 0.25533 (3) | 0.47623 (8) | 0.01304 (10) | |
N11 | −0.23117 (5) | 0.15549 (6) | 0.35631 (13) | 0.02612 (17) | |
H11A | −0.2539 (12) | 0.1333 (10) | 0.254 (3) | 0.048 (5)* | |
H11B | −0.2511 (10) | 0.1491 (9) | 0.447 (2) | 0.029 (4)* | |
N12 | −0.10996 (5) | 0.20428 (5) | 0.56689 (10) | 0.01953 (14) | |
H12A | −0.0595 (10) | 0.2213 (9) | 0.584 (2) | 0.035 (4)* | |
H12B | −0.1281 (9) | 0.1953 (8) | 0.662 (2) | 0.024 (3)* | |
N21 | 0.10433 (5) | 0.01587 (4) | 0.68823 (11) | 0.01961 (14) | |
H21A | 0.0640 (10) | −0.0140 (9) | 0.660 (2) | 0.033 (4)* | |
H21B | 0.1493 (10) | 0.0043 (9) | 0.763 (2) | 0.029 (4)* | |
N22 | 0.16810 (4) | 0.12007 (4) | 0.62171 (10) | 0.01352 (11) | |
H22A | 0.1683 (8) | 0.1484 (8) | 0.532 (2) | 0.019 (3)* | |
H22B | 0.2138 (9) | 0.1034 (8) | 0.690 (2) | 0.022 (3)* | |
N31 | 0.02754 (5) | 0.37612 (4) | 0.55510 (12) | 0.01949 (14) | |
H31A | −0.0174 (10) | 0.3702 (9) | 0.572 (2) | 0.030 (4)* | |
H31B | 0.0428 (10) | 0.4195 (9) | 0.544 (2) | 0.034 (4)* | |
N32 | 0.11823 (5) | 0.33650 (4) | 0.40451 (12) | 0.01756 (13) | |
H32A | 0.1325 (10) | 0.3013 (9) | 0.347 (2) | 0.029 (4)* | |
H32B | 0.1263 (9) | 0.3827 (8) | 0.382 (2) | 0.026 (3)* | |
C1 | −0.15381 (5) | 0.18085 (4) | 0.39026 (11) | 0.01272 (12) | |
C2 | 0.09943 (5) | 0.07617 (4) | 0.58032 (10) | 0.01129 (11) | |
C3 | 0.05724 (5) | 0.32095 (4) | 0.47597 (10) | 0.01116 (11) | |
O1N | 0.09406 (4) | 0.50492 (4) | 0.39746 (10) | 0.02076 (12) | |
O2N | 0.21997 (4) | 0.48409 (4) | 0.58623 (13) | 0.03066 (17) | |
O3N | 0.16659 (4) | 0.59439 (3) | 0.57256 (9) | 0.01808 (11) | |
N1 | 0.16009 (4) | 0.52762 (4) | 0.51828 (10) | 0.01451 (12) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Zn1 | 0.01002 (5) | 0.00805 (5) | 0.00952 (5) | 0.000 | 0.00353 (4) | 0.000 |
O1 | 0.0122 (2) | 0.0178 (3) | 0.0113 (2) | −0.00135 (19) | 0.00615 (18) | −0.00085 (18) |
O2 | 0.0120 (2) | 0.0112 (2) | 0.0119 (2) | −0.00008 (17) | 0.00110 (18) | 0.00239 (17) |
O3 | 0.0166 (2) | 0.0103 (2) | 0.0139 (2) | −0.00319 (18) | 0.00730 (19) | −0.00175 (17) |
N11 | 0.0183 (3) | 0.0433 (5) | 0.0205 (3) | −0.0125 (3) | 0.0114 (3) | −0.0066 (3) |
N12 | 0.0212 (3) | 0.0273 (4) | 0.0127 (3) | −0.0071 (3) | 0.0091 (2) | −0.0030 (3) |
N21 | 0.0160 (3) | 0.0166 (3) | 0.0205 (3) | −0.0029 (2) | −0.0014 (3) | 0.0091 (2) |
N22 | 0.0110 (3) | 0.0151 (3) | 0.0132 (3) | −0.0012 (2) | 0.0025 (2) | 0.0027 (2) |
N31 | 0.0195 (3) | 0.0133 (3) | 0.0288 (4) | −0.0012 (2) | 0.0124 (3) | −0.0066 (3) |
N32 | 0.0201 (3) | 0.0129 (3) | 0.0243 (3) | −0.0040 (2) | 0.0134 (3) | −0.0027 (2) |
C1 | 0.0144 (3) | 0.0120 (3) | 0.0137 (3) | −0.0005 (2) | 0.0073 (2) | 0.0001 (2) |
C2 | 0.0124 (3) | 0.0110 (3) | 0.0104 (3) | 0.0005 (2) | 0.0036 (2) | 0.0001 (2) |
C3 | 0.0113 (3) | 0.0108 (3) | 0.0106 (3) | −0.0005 (2) | 0.0026 (2) | −0.0013 (2) |
O1N | 0.0157 (3) | 0.0167 (3) | 0.0230 (3) | −0.0035 (2) | −0.0027 (2) | −0.0006 (2) |
O2N | 0.0158 (3) | 0.0203 (3) | 0.0458 (4) | 0.0029 (2) | −0.0029 (3) | 0.0019 (3) |
O3N | 0.0195 (3) | 0.0143 (3) | 0.0227 (3) | −0.0048 (2) | 0.0101 (2) | −0.0055 (2) |
N1 | 0.0129 (3) | 0.0140 (3) | 0.0163 (3) | −0.0023 (2) | 0.0044 (2) | −0.0001 (2) |
Geometric parameters (Å, º) top
Zn1—O1 | 2.1366 (6) | N21—C2 | 1.3310 (10) |
Zn1—O2 | 2.0668 (5) | N21—H21A | 0.842 (16) |
Zn1—O3 | 2.0909 (6) | N21—H21B | 0.806 (16) |
Zn1—O2i | 2.0668 (5) | N22—C2 | 1.3589 (10) |
Zn1—O3i | 2.0909 (6) | N22—H22A | 0.833 (14) |
Zn1—O1i | 2.1366 (6) | N22—H22B | 0.828 (14) |
O1—C1 | 1.2688 (9) | N31—C3 | 1.3341 (10) |
O2—C2 | 1.2606 (9) | N31—H31A | 0.823 (16) |
O3—C3 | 1.2642 (9) | N31—H31B | 0.835 (16) |
N11—C1 | 1.3363 (11) | N32—C3 | 1.3420 (10) |
N11—H11A | 0.822 (19) | N32—H32A | 0.843 (16) |
N11—H11B | 0.851 (16) | N32—H32B | 0.868 (15) |
N12—C1 | 1.3308 (11) | O1N—N1 | 1.2448 (9) |
N12—H12A | 0.881 (16) | O2N—N1 | 1.2497 (10) |
N12—H12B | 0.871 (15) | O3N—N1 | 1.2601 (9) |
| | | |
O1—Zn1—O1i | 173.71 (3) | C2—N21—H21A | 119.5 (11) |
O2—Zn1—O2i | 83.84 (3) | C2—N21—H21B | 118.1 (11) |
O3—Zn1—O3i | 97.11 (3) | H21A—N21—H21B | 120.9 (16) |
O1—Zn1—O2 | 93.61 (2) | C2—N22—H22A | 114.3 (10) |
O1—Zn1—O3 | 86.66 (2) | C2—N22—H22B | 119.2 (10) |
O2—Zn1—O3 | 89.58 (2) | H22A—N22—H22B | 116.5 (13) |
O2—Zn1—O3i | 172.90 (2) | C3—N31—H31A | 118.6 (11) |
O2i—Zn1—O3i | 89.58 (2) | C3—N31—H31B | 118.6 (11) |
O2i—Zn1—O3 | 172.89 (2) | H31A—N31—H31B | 118.2 (15) |
O2—Zn1—O1i | 91.07 (2) | C3—N32—H32A | 115.5 (11) |
O2i—Zn1—O1i | 93.61 (2) | C3—N32—H32B | 117.7 (10) |
O3i—Zn1—O1i | 86.66 (2) | H32A—N32—H32B | 122.5 (14) |
O3—Zn1—O1i | 89.18 (2) | O1—C1—N12 | 121.27 (7) |
O2i—Zn1—O1 | 91.07 (2) | O1—C1—N11 | 119.63 (8) |
O3i—Zn1—O1 | 89.18 (2) | N12—C1—N11 | 119.03 (7) |
C1—O1—Zn1 | 131.24 (5) | O2—C2—N21 | 120.82 (7) |
C2—O2—Zn1 | 132.76 (5) | O2—C2—N22 | 121.46 (7) |
C3—O3—Zn1 | 127.74 (5) | N21—C2—N22 | 117.67 (7) |
C1—N11—H11A | 117.9 (13) | O3—C3—N31 | 120.58 (7) |
C1—N11—H11B | 121.8 (11) | O3—C3—N32 | 121.16 (7) |
H11A—N11—H11B | 117.3 (16) | N31—C3—N32 | 118.19 (7) |
C1—N12—H12A | 116.7 (10) | O1N—N1—O2N | 119.97 (7) |
C1—N12—H12B | 119.5 (10) | O1N—N1—O3N | 120.10 (7) |
H12A—N12—H12B | 122.9 (14) | O2N—N1—O3N | 119.93 (7) |
| | | |
O2—Zn1—O1—C1 | 40.74 (7) | O3i—Zn1—O3—C3 | −26.68 (5) |
O2i—Zn1—O1—C1 | 124.64 (7) | O1i—Zn1—O3—C3 | 59.85 (6) |
O3i—Zn1—O1—C1 | −145.80 (7) | O1—Zn1—O3—C3 | −115.43 (7) |
O3—Zn1—O1—C1 | −48.63 (7) | Zn1—O1—C1—N12 | 31.21 (11) |
O2i—Zn1—O2—C2 | 122.48 (8) | Zn1—O1—C1—N11 | −151.71 (7) |
O3—Zn1—O2—C2 | −60.21 (7) | Zn1—O2—C2—N21 | −178.34 (6) |
O1i—Zn1—O2—C2 | 28.96 (7) | Zn1—O2—C2—N22 | 4.31 (11) |
O1—Zn1—O2—C2 | −146.83 (7) | Zn1—O3—C3—N31 | 136.97 (7) |
O2—Zn1—O3—C3 | 150.93 (6) | Zn1—O3—C3—N32 | −46.11 (10) |
Symmetry code: (i) −x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11A···O2Nii | 0.822 (19) | 2.415 (19) | 3.1377 (13) | 147.3 (17) |
N11—H11B···O3Niii | 0.851 (16) | 2.159 (16) | 2.9389 (10) | 152.1 (14) |
N12—H12A···O3 | 0.881 (16) | 2.048 (16) | 2.8504 (9) | 150.9 (14) |
N12—H12B···N22iv | 0.871 (15) | 2.357 (15) | 3.1705 (10) | 155.6 (13) |
N21—H21A···O2v | 0.842 (16) | 2.116 (17) | 2.9505 (10) | 171.0 (15) |
N21—H21B···O2Nvi | 0.806 (16) | 2.158 (16) | 2.9491 (11) | 167.1 (15) |
N22—H22A···O1i | 0.833 (14) | 2.081 (14) | 2.8504 (9) | 153.4 (13) |
N22—H22B···O3Nvi | 0.828 (14) | 2.200 (14) | 2.9901 (9) | 159.9 (13) |
N31—H31A···O3Nvii | 0.823 (16) | 2.487 (15) | 3.1638 (11) | 140.3 (14) |
N31—H31B···O1N | 0.835 (16) | 2.220 (16) | 2.9820 (11) | 151.7 (14) |
N32—H32A···O1i | 0.843 (16) | 2.208 (16) | 2.9802 (10) | 152.4 (14) |
N32—H32B···O1N | 0.868 (15) | 2.280 (15) | 3.0590 (10) | 149.4 (13) |
N32—H32B···O2N | 0.868 (15) | 2.553 (15) | 3.2089 (11) | 133.0 (12) |
Symmetry codes: (i) −x, y, −z+1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x−1/2, y−1/2, z; (iv) −x, y, −z+3/2; (v) −x, −y, −z+1; (vi) −x+1/2, y−1/2, −z+3/2; (vii) −x, −y+1, −z+1. |
(Ib) Hexakis(urea-
κO)zinc(II) dinitrate
top
Crystal data top
[Zn(CH4N2O)6](NO3)2 | F(000) = 1136 |
Mr = 549.76 | Dx = 1.692 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 33077 reflections |
a = 17.1082 (6) Å | θ = 1.7–35.0° |
b = 17.9456 (7) Å | µ = 1.23 mm−1 |
c = 7.46654 (16) Å | T = 250 K |
β = 109.701 (2)° | Plate, colourless |
V = 2158.18 (12) Å3 | 0.36 × 0.20 × 0.12 mm |
Z = 4 | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 4747 independent reflections |
Radiation source: rotating anode | 4013 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.030 |
ϕ and ω scans | θmax = 35.0°, θmin = 1.7° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | h = −27→27 |
Tmin = 0.668, Tmax = 0.747 | k = −28→28 |
41204 measured reflections | l = −12→11 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.029 | Hydrogen site location: difference Fourier map |
wR(F2) = 0.075 | All H-atom parameters refined |
S = 1.03 | w = 1/[σ2(Fo2) + (0.0386P)2 + 0.8626P] where P = (Fo2 + 2Fc2)/3 |
4747 reflections | (Δ/σ)max < 0.001 |
198 parameters | Δρmax = 0.31 e Å−3 |
0 restraints | Δρmin = −0.44 e Å−3 |
Crystal data top
[Zn(CH4N2O)6](NO3)2 | V = 2158.18 (12) Å3 |
Mr = 549.76 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 17.1082 (6) Å | µ = 1.23 mm−1 |
b = 17.9456 (7) Å | T = 250 K |
c = 7.46654 (16) Å | 0.36 × 0.20 × 0.12 mm |
β = 109.701 (2)° | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 4747 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 2008a) | 4013 reflections with I > 2σ(I) |
Tmin = 0.668, Tmax = 0.747 | Rint = 0.030 |
41204 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.029 | 0 restraints |
wR(F2) = 0.075 | All H-atom parameters refined |
S = 1.03 | Δρmax = 0.31 e Å−3 |
4747 reflections | Δρmin = −0.44 e Å−3 |
198 parameters | |
Special details top
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 (Å2) top | x | y | z | Uiso*/Ueq | |
Zn1 | 0.0000 | 0.178335 (9) | 0.2500 | 0.02057 (5) | |
O1 | −0.12432 (5) | 0.18478 (5) | 0.25497 (11) | 0.02926 (16) | |
O2 | 0.03312 (5) | 0.09291 (4) | 0.44724 (11) | 0.02645 (14) | |
O3 | 0.03126 (5) | 0.25578 (4) | 0.47169 (12) | 0.02915 (16) | |
N11 | −0.23087 (9) | 0.15887 (11) | 0.3556 (2) | 0.0599 (4) | |
H11A | −0.2527 (17) | 0.1382 (15) | 0.257 (4) | 0.083 (8)* | |
H11B | −0.2520 (14) | 0.1518 (12) | 0.441 (3) | 0.061 (6)* | |
N12 | −0.10885 (8) | 0.20246 (8) | 0.56398 (16) | 0.0437 (3) | |
H12A | −0.0579 (14) | 0.2191 (12) | 0.580 (3) | 0.063 (6)* | |
H12B | −0.1270 (11) | 0.1934 (10) | 0.656 (3) | 0.043 (5)* | |
N21 | 0.10473 (8) | 0.01632 (7) | 0.68312 (18) | 0.0427 (3) | |
H21A | 0.0651 (12) | −0.0134 (11) | 0.657 (3) | 0.046 (5)* | |
H21B | 0.1506 (12) | 0.0044 (11) | 0.761 (3) | 0.048 (5)* | |
N22 | 0.16754 (6) | 0.12051 (6) | 0.61859 (15) | 0.03044 (19) | |
H22A | 0.1677 (10) | 0.1479 (10) | 0.532 (2) | 0.035 (4)* | |
H22B | 0.2129 (11) | 0.1045 (9) | 0.688 (2) | 0.036 (4)* | |
N31 | 0.02890 (9) | 0.37577 (7) | 0.5549 (2) | 0.0449 (3) | |
H31A | −0.0142 (13) | 0.3686 (11) | 0.570 (3) | 0.049 (5)* | |
H31B | 0.0432 (13) | 0.4168 (12) | 0.547 (3) | 0.055 (5)* | |
N32 | 0.11980 (8) | 0.33707 (7) | 0.40947 (19) | 0.0391 (2) | |
H32A | 0.1328 (11) | 0.3030 (11) | 0.346 (3) | 0.044 (5)* | |
H32B | 0.1271 (12) | 0.3822 (11) | 0.390 (3) | 0.048 (5)* | |
C1 | −0.15310 (7) | 0.18119 (6) | 0.38989 (16) | 0.02787 (19) | |
C2 | 0.09971 (6) | 0.07648 (6) | 0.57690 (14) | 0.02376 (17) | |
C3 | 0.05820 (6) | 0.32118 (6) | 0.47556 (15) | 0.02454 (17) | |
O1N | 0.09580 (6) | 0.50736 (6) | 0.40249 (16) | 0.0474 (2) | |
O2N | 0.22007 (7) | 0.48642 (7) | 0.5856 (2) | 0.0655 (4) | |
O3N | 0.16831 (6) | 0.59699 (5) | 0.57163 (15) | 0.0413 (2) | |
N1 | 0.16133 (6) | 0.53019 (6) | 0.51959 (15) | 0.03199 (19) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Zn1 | 0.02267 (8) | 0.01753 (7) | 0.02228 (7) | 0.000 | 0.00856 (5) | 0.000 |
O1 | 0.0262 (3) | 0.0392 (4) | 0.0261 (3) | −0.0030 (3) | 0.0136 (3) | −0.0021 (3) |
O2 | 0.0263 (3) | 0.0227 (3) | 0.0266 (3) | −0.0018 (3) | 0.0040 (3) | 0.0046 (3) |
O3 | 0.0372 (4) | 0.0228 (3) | 0.0322 (4) | −0.0074 (3) | 0.0179 (3) | −0.0056 (3) |
N11 | 0.0405 (7) | 0.1006 (13) | 0.0488 (7) | −0.0278 (8) | 0.0282 (6) | −0.0148 (8) |
N12 | 0.0468 (6) | 0.0615 (8) | 0.0294 (5) | −0.0129 (6) | 0.0217 (5) | −0.0048 (5) |
N21 | 0.0351 (5) | 0.0358 (6) | 0.0445 (6) | −0.0065 (4) | −0.0033 (5) | 0.0195 (5) |
N22 | 0.0248 (4) | 0.0340 (5) | 0.0298 (4) | −0.0028 (4) | 0.0056 (3) | 0.0069 (4) |
N31 | 0.0468 (7) | 0.0281 (5) | 0.0680 (8) | −0.0038 (5) | 0.0302 (6) | −0.0160 (5) |
N32 | 0.0442 (6) | 0.0273 (5) | 0.0559 (7) | −0.0104 (4) | 0.0300 (6) | −0.0073 (5) |
C1 | 0.0309 (5) | 0.0265 (5) | 0.0315 (5) | −0.0014 (4) | 0.0176 (4) | 0.0008 (4) |
C2 | 0.0263 (4) | 0.0220 (4) | 0.0224 (4) | 0.0007 (3) | 0.0075 (3) | 0.0010 (3) |
C3 | 0.0249 (4) | 0.0219 (4) | 0.0257 (4) | −0.0010 (3) | 0.0071 (3) | −0.0029 (3) |
O1N | 0.0365 (5) | 0.0354 (5) | 0.0546 (6) | −0.0067 (4) | −0.0053 (4) | −0.0014 (4) |
O2N | 0.0363 (6) | 0.0463 (6) | 0.0927 (10) | 0.0054 (5) | −0.0061 (6) | 0.0003 (6) |
O3N | 0.0439 (5) | 0.0333 (5) | 0.0520 (5) | −0.0107 (4) | 0.0230 (4) | −0.0121 (4) |
N1 | 0.0286 (4) | 0.0305 (5) | 0.0364 (5) | −0.0051 (4) | 0.0104 (4) | −0.0012 (4) |
Geometric parameters (Å, º) top
Zn1—O1 | 2.1428 (8) | N21—C2 | 1.3251 (14) |
Zn1—O2 | 2.0682 (7) | N21—H21A | 0.83 (2) |
Zn1—O3 | 2.0882 (8) | N21—H21B | 0.83 (2) |
Zn1—O2i | 2.0682 (7) | N22—C2 | 1.3507 (14) |
Zn1—O3i | 2.0882 (8) | N22—H22A | 0.815 (18) |
Zn1—O1i | 2.1428 (8) | N22—H22B | 0.826 (17) |
O1—C1 | 1.2633 (12) | N31—C3 | 1.3275 (15) |
O2—C2 | 1.2563 (12) | N31—H31A | 0.79 (2) |
O3—C3 | 1.2578 (12) | N31—H31B | 0.79 (2) |
N11—C1 | 1.3291 (17) | N32—C3 | 1.3364 (15) |
N11—H11A | 0.80 (3) | N32—H32A | 0.849 (19) |
N11—H11B | 0.84 (2) | N32—H32B | 0.84 (2) |
N12—C1 | 1.3210 (17) | O1N—N1 | 1.2362 (14) |
N12—H12A | 0.89 (2) | O2N—N1 | 1.2393 (15) |
N12—H12B | 0.860 (19) | O3N—N1 | 1.2535 (13) |
| | | |
O1—Zn1—O1i | 173.81 (5) | C2—N21—H21A | 119.8 (13) |
O2—Zn1—O2i | 84.32 (4) | C2—N21—H21B | 118.9 (13) |
O3—Zn1—O3i | 96.55 (5) | H21A—N21—H21B | 120.1 (19) |
O1—Zn1—O2 | 93.61 (3) | C2—N22—H22A | 113.7 (12) |
O1—Zn1—O3 | 86.61 (3) | C2—N22—H22B | 119.5 (12) |
O2—Zn1—O3 | 89.62 (3) | H22A—N22—H22B | 117.1 (16) |
O2—Zn1—O3i | 173.33 (3) | C3—N31—H31A | 116.9 (14) |
O2i—Zn1—O3 | 173.33 (3) | C3—N31—H31B | 118.8 (14) |
O2i—Zn1—O3i | 89.62 (3) | H31A—N31—H31B | 120 (2) |
O2i—Zn1—O1i | 93.61 (3) | C3—N32—H32A | 116.2 (13) |
O2—Zn1—O1i | 90.98 (3) | C3—N32—H32B | 116.7 (13) |
O3i—Zn1—O1i | 86.61 (3) | H32A—N32—H32B | 121.1 (18) |
O3—Zn1—O1i | 89.28 (3) | O1—C1—N12 | 121.46 (11) |
O2i—Zn1—O1 | 90.98 (3) | O1—C1—N11 | 119.72 (12) |
O3i—Zn1—O1 | 89.28 (3) | N12—C1—N11 | 118.75 (12) |
C1—O1—Zn1 | 131.84 (8) | O2—C2—N21 | 120.76 (10) |
C2—O2—Zn1 | 133.02 (7) | O2—C2—N22 | 121.72 (9) |
C3—O3—Zn1 | 129.29 (7) | N21—C2—N22 | 117.47 (10) |
C1—N11—H11A | 116.3 (19) | O3—C3—N31 | 120.54 (11) |
C1—N11—H11B | 123.8 (15) | O3—C3—N32 | 121.28 (10) |
H11A—N11—H11B | 117 (2) | N31—C3—N32 | 118.09 (11) |
C1—N12—H12A | 116.2 (14) | O1N—N1—O2N | 119.46 (12) |
C1—N12—H12B | 119.4 (13) | O1N—N1—O3N | 120.34 (11) |
H12A—N12—H12B | 123.6 (19) | O2N—N1—O3N | 120.20 (11) |
| | | |
O2i—Zn1—O1—C1 | 125.43 (10) | O3i—Zn1—O3—C3 | −27.64 (8) |
O2—Zn1—O1—C1 | 41.05 (10) | O1i—Zn1—O3—C3 | 58.86 (10) |
O3i—Zn1—O1—C1 | −144.96 (10) | O1—Zn1—O3—C3 | −116.52 (10) |
O3—Zn1—O1—C1 | −48.35 (10) | Zn1—O1—C1—N12 | 28.53 (17) |
O2i—Zn1—O2—C2 | 123.52 (11) | Zn1—O1—C1—N11 | −154.65 (13) |
O3—Zn1—O2—C2 | −59.28 (10) | Zn1—O2—C2—N21 | −178.59 (9) |
O1i—Zn1—O2—C2 | 29.99 (10) | Zn1—O2—C2—N22 | 4.09 (16) |
O1—Zn1—O2—C2 | −145.85 (10) | Zn1—O3—C3—N31 | 137.16 (11) |
O2—Zn1—O3—C3 | 149.84 (9) | Zn1—O3—C3—N32 | −46.38 (16) |
Symmetry code: (i) −x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11A···O2Nii | 0.80 (3) | 2.54 (3) | 3.229 (2) | 145 (2) |
N11—H11B···O3Niii | 0.84 (2) | 2.16 (2) | 2.9471 (16) | 155 (2) |
N12—H12A···O3 | 0.89 (2) | 2.06 (2) | 2.8722 (14) | 150.9 (19) |
N12—H12B···N22iv | 0.860 (19) | 2.41 (2) | 3.2239 (16) | 157.9 (16) |
N21—H21A···O2v | 0.83 (2) | 2.14 (2) | 2.9671 (14) | 171.9 (17) |
N21—H21B···O2Nvi | 0.83 (2) | 2.14 (2) | 2.9605 (17) | 166.4 (18) |
N22—H22A···O1i | 0.815 (18) | 2.120 (18) | 2.8751 (13) | 153.9 (16) |
N22—H22B···O3Nvi | 0.826 (17) | 2.215 (18) | 3.0058 (14) | 160.1 (15) |
N31—H31A···O3Nvii | 0.79 (2) | 2.56 (2) | 3.2196 (18) | 141.2 (18) |
N31—H31B···O1N | 0.79 (2) | 2.29 (2) | 3.0101 (18) | 152.0 (19) |
N32—H32A···O1i | 0.849 (19) | 2.239 (19) | 3.0078 (15) | 150.5 (17) |
N32—H32B···O1N | 0.84 (2) | 2.32 (2) | 3.0815 (16) | 151.5 (17) |
N32—H32B···O2N | 0.84 (2) | 2.57 (2) | 3.2147 (17) | 134.8 (16) |
Symmetry codes: (i) −x, y, −z+1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x−1/2, y−1/2, z; (iv) −x, y, −z+3/2; (v) −x, −y, −z+1; (vi) −x+1/2, y−1/2, −z+3/2; (vii) −x, −y+1, −z+1. |
Experimental details
| (Ia) | (Ib) |
Crystal data |
Chemical formula | [Zn(CH4N2O)6](NO3)2 | [Zn(CH4N2O)6](NO3)2 |
Mr | 549.76 | 549.76 |
Crystal system, space group | Monoclinic, C2/c | Monoclinic, C2/c |
Temperature (K) | 110 | 250 |
a, b, c (Å) | 17.0337 (6), 18.0092 (5), 7.3550 (2) | 17.1082 (6), 17.9456 (7), 7.46654 (16) |
β (°) | 109.651 (2) | 109.701 (2) |
V (Å3) | 2124.84 (11) | 2158.18 (12) |
Z | 4 | 4 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 1.24 | 1.23 |
Crystal size (mm) | 0.36 × 0.20 × 0.12 | 0.36 × 0.20 × 0.12 |
|
Data collection |
Diffractometer | Nonius KappaCCD area-detector diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 2008a) | Multi-scan (SADABS; Sheldrick, 2008a) |
Tmin, Tmax | 0.618, 0.747 | 0.668, 0.747 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 40566, 4678, 4266 | 41204, 4747, 4013 |
Rint | 0.027 | 0.030 |
(sin θ/λ)max (Å−1) | 0.807 | 0.807 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.022, 0.059, 1.07 | 0.029, 0.075, 1.03 |
No. of reflections | 4678 | 4747 |
No. of parameters | 198 | 198 |
H-atom treatment | All H-atom parameters refined | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 0.46, −0.27 | 0.31, −0.44 |
Selected geometric parameters (Å, º) for (Ia) topZn1—O1 | 2.1366 (6) | O1—C1 | 1.2688 (9) |
Zn1—O2 | 2.0668 (5) | O2—C2 | 1.2606 (9) |
Zn1—O3 | 2.0909 (6) | O3—C3 | 1.2642 (9) |
| | | |
O1—Zn1—O1i | 173.71 (3) | O2—Zn1—O3 | 89.58 (2) |
O2—Zn1—O2i | 83.84 (3) | O2—Zn1—O3i | 172.90 (2) |
O3—Zn1—O3i | 97.11 (3) | C1—O1—Zn1 | 131.24 (5) |
O1—Zn1—O2 | 93.61 (2) | C2—O2—Zn1 | 132.76 (5) |
O1—Zn1—O3 | 86.66 (2) | C3—O3—Zn1 | 127.74 (5) |
| | | |
Zn1—O1—C1—N12 | 31.21 (11) | Zn1—O2—C2—N22 | 4.31 (11) |
Zn1—O1—C1—N11 | −151.71 (7) | Zn1—O3—C3—N31 | 136.97 (7) |
Zn1—O2—C2—N21 | −178.34 (6) | Zn1—O3—C3—N32 | −46.11 (10) |
Symmetry code: (i) −x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) for (Ia) top
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11A···O2Nii | 0.822 (19) | 2.415 (19) | 3.1377 (13) | 147.3 (17) |
N11—H11B···O3Niii | 0.851 (16) | 2.159 (16) | 2.9389 (10) | 152.1 (14) |
N12—H12A···O3 | 0.881 (16) | 2.048 (16) | 2.8504 (9) | 150.9 (14) |
N12—H12B···N22iv | 0.871 (15) | 2.357 (15) | 3.1705 (10) | 155.6 (13) |
N21—H21A···O2v | 0.842 (16) | 2.116 (17) | 2.9505 (10) | 171.0 (15) |
N21—H21B···O2Nvi | 0.806 (16) | 2.158 (16) | 2.9491 (11) | 167.1 (15) |
N22—H22A···O1i | 0.833 (14) | 2.081 (14) | 2.8504 (9) | 153.4 (13) |
N22—H22B···O3Nvi | 0.828 (14) | 2.200 (14) | 2.9901 (9) | 159.9 (13) |
N31—H31A···O3Nvii | 0.823 (16) | 2.487 (15) | 3.1638 (11) | 140.3 (14) |
N31—H31B···O1N | 0.835 (16) | 2.220 (16) | 2.9820 (11) | 151.7 (14) |
N32—H32A···O1i | 0.843 (16) | 2.208 (16) | 2.9802 (10) | 152.4 (14) |
N32—H32B···O1N | 0.868 (15) | 2.280 (15) | 3.0590 (10) | 149.4 (13) |
N32—H32B···O2N | 0.868 (15) | 2.553 (15) | 3.2089 (11) | 133.0 (12) |
Symmetry codes: (i) −x, y, −z+1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x−1/2, y−1/2, z; (iv) −x, y, −z+3/2; (v) −x, −y, −z+1; (vi) −x+1/2, y−1/2, −z+3/2; (vii) −x, −y+1, −z+1. |
Selected geometric parameters (Å, º) for (Ib) topZn1—O1 | 2.1428 (8) | O1—C1 | 1.2633 (12) |
Zn1—O2 | 2.0682 (7) | O2—C2 | 1.2563 (12) |
Zn1—O3 | 2.0882 (8) | O3—C3 | 1.2578 (12) |
| | | |
O1—Zn1—O1i | 173.81 (5) | O2—Zn1—O3 | 89.62 (3) |
O2—Zn1—O2i | 84.32 (4) | O2—Zn1—O3i | 173.33 (3) |
O3—Zn1—O3i | 96.55 (5) | C1—O1—Zn1 | 131.84 (8) |
O1—Zn1—O2 | 93.61 (3) | C2—O2—Zn1 | 133.02 (7) |
O1—Zn1—O3 | 86.61 (3) | C3—O3—Zn1 | 129.29 (7) |
| | | |
Zn1—O1—C1—N12 | 28.53 (17) | Zn1—O2—C2—N22 | 4.09 (16) |
Zn1—O1—C1—N11 | −154.65 (13) | Zn1—O3—C3—N31 | 137.16 (11) |
Zn1—O2—C2—N21 | −178.59 (9) | Zn1—O3—C3—N32 | −46.38 (16) |
Symmetry code: (i) −x, y, −z+1/2. |
Hydrogen-bond geometry (Å, º) for (Ib) top
D—H···A | D—H | H···A | D···A | D—H···A |
N11—H11A···O2Nii | 0.80 (3) | 2.54 (3) | 3.229 (2) | 145 (2) |
N11—H11B···O3Niii | 0.84 (2) | 2.16 (2) | 2.9471 (16) | 155 (2) |
N12—H12A···O3 | 0.89 (2) | 2.06 (2) | 2.8722 (14) | 150.9 (19) |
N12—H12B···N22iv | 0.860 (19) | 2.41 (2) | 3.2239 (16) | 157.9 (16) |
N21—H21A···O2v | 0.83 (2) | 2.14 (2) | 2.9671 (14) | 171.9 (17) |
N21—H21B···O2Nvi | 0.83 (2) | 2.14 (2) | 2.9605 (17) | 166.4 (18) |
N22—H22A···O1i | 0.815 (18) | 2.120 (18) | 2.8751 (13) | 153.9 (16) |
N22—H22B···O3Nvi | 0.826 (17) | 2.215 (18) | 3.0058 (14) | 160.1 (15) |
N31—H31A···O3Nvii | 0.79 (2) | 2.56 (2) | 3.2196 (18) | 141.2 (18) |
N31—H31B···O1N | 0.79 (2) | 2.29 (2) | 3.0101 (18) | 152.0 (19) |
N32—H32A···O1i | 0.849 (19) | 2.239 (19) | 3.0078 (15) | 150.5 (17) |
N32—H32B···O1N | 0.84 (2) | 2.32 (2) | 3.0815 (16) | 151.5 (17) |
N32—H32B···O2N | 0.84 (2) | 2.57 (2) | 3.2147 (17) | 134.8 (16) |
Symmetry codes: (i) −x, y, −z+1/2; (ii) x−1/2, −y+1/2, z−1/2; (iii) x−1/2, y−1/2, z; (iv) −x, y, −z+3/2; (v) −x, −y, −z+1; (vi) −x+1/2, y−1/2, −z+3/2; (vii) −x, −y+1, −z+1. |
Tensor components (10 -6 K-1) of the unit strain tensor of thermal expansion
for warming from 110 to 250 K. Tensors are in a Cartesian coordinate system.
α12 = α23 = 0, due to symmetry. Orthogonalization matrix: x//a, z//c*,
z//yx (Dunitz, 1995). topT (K) | α11 | α22 | α33 | α13 |
110–130 | 25.15 | -21.76 | 79.08 | -12.08 |
130–150 | 33.84 | -19.49 | 86.59 | -13.35 |
150–170 | 33.11 | -22.16 | 91.61 | -14.87 |
170–190 | 27.78 | -28.89 | 100.08 | -19.81 |
190–210 | 32.89 | -28.73 | 110.79 | -16.38 |
210–230 | 27.96 | -24.56 | 128.82 | -20.99 |
230–250 | 32.03 | -32.78 | 129.35 | -20.36 |
Eigenvalues of the unit strain tensor of the thermal expansion (10 -6 K-1)
and corresponding angles with the unit-cell axes (°) for warming from 110 to
250 K. Orthogonalization matrix: x//a, z//c*, z//yx (Dunitz, 1995). topT (K) | Principal axis | Eigenvalue | Angle with a | Angle with b | Angle with c |
110–130 | α1 | 82 (2) | 102.1 (13) | 90 | 7.6 (13) |
130–150 | α1 | 90 (2) | 103.4 (13) | 90 | 6.2 (13) |
150–170 | α1 | 95.2 (19) | 103.5 (12) | 90 | 6.2 (12) |
170–190 | α1 | 105.1 (18) | 104.4 (9) | 90 | 5.3 (9) |
190–210 | α1 | 114 (2) | 101.4 (6) | 90 | 8.3 (6) |
230–250 | α1 | 133 (2) | 101.3 (7) | 90 | 8.3 (7) |
| | | | | |
110–130 | α2 | 23 (2) | 12.1 (13) | 90 | 97.6 (13) |
130–150 | α2 | 31 (2) | 13.4 (13) | 90 | 96.2 (13) |
150–170 | α2 | 30 (2) | 13.5 (12) | 90 | 96.2 (12) |
170–190 | α2 | 23 (2) | 14.4 (9) | 90 | 95.3 (9) |
190–210 | α2 | 30 (2) | 11.4 (6) | 90 | 98.3 (6) |
210–230 | α2 | 24 (2) | 11.3 (4) | 90 | 98.4 (4) |
230–250 | α2 | 28 (2) | 11.3 (7) | 90 | 98.3 (7) |
| | | | | |
110–130 | α3 | -22 (2) | 90 | 0 | 90 |
130–150 | α3 | -19 (3) | 90 | 0 | 90 |
150–170 | α3 | -22.2 (19) | 90 | 0 | 90 |
170–190 | α3 | -29 (2) | 90 | 0 | 90 |
190–210 | α3 | -28.7 (18) | 90 | 0 | 90 |
210–230 | α3 | -25 (2) | 90 | 0 | 90 |
230–250 | α3 | -33 (2) | 90 | 0 | 90 |
In complex chemistry, urea is a well studied model compound for the coordination of biologically relevant ligands to transition metals via the C═O and/or NH2 groups. According to Mak & Zhou (1992), urea usually acts in metal complexes as a monodentate O-bonded ligand, although sometimes the bidentate N,O-coordination mode is found. Additionally, the Cambridge Structural Database (CSD, August 2010 update; Allen, 2002) contains ten urea complexes showing µ-O bridging coordination. In total, the CSD contains 143 urea–transition metal complexes, in 35 of which the metal cation is surrounded by urea as the only ligand.
The structure of hexakis(urea-κO)zinc(II) dinitrate, (I), was determined at room temperature by van de Giesen & Stam (1972). The compound crystallizes in space group C2/c, with the ZnII atom on a twofold axis. Zhongyuan et al. (1986) described the crystal structure of hexakis(urea-κO)zinc sulfate with cocrystallized solvent water. Prior & Kift (2009) reported the structure of diaqua-tetrakis(urea-κO)zinc dinitrate, measured at 150 K. We redetermined the structure of (I) at 110 K, (Ia), and 250 K, (Ib), in order to obtain more accurate geometries and to determine the thermal expansion behaviour. Unit-cell determinations were performed during cooling from 290 to 110 K and warming from 110 to 250 K, in 20 K intervals.
The overall shape of the cation in (I) is approximately spherical, with a nearly isotropic tensor of inertia. The ZnII atom is surrounded by six urea ligands coordinated by their O atoms (Fig. 1). This is in contrast with urea cadmium nitrate (Catesby, 1961), where the central Cd atom is surrounded by four O-coordinated urea ligands. In (I), the ZnO6 polyhedron has an exact C2 symmetry and an approximate Oh environment, with r.m.s. deviations of 0.1787 and 0.1307 Å2, respectively, as calculated using the MOLSYM routine (Pilati & Forni, 1998). The Zn—O1 bond is oriented in the direction of the a axis and the Zn—O2 and Zn—O3 bonds are perpendicular to it. The Zn—O1 bond of 2.1366 (6) Å at 110 K is significantly longer than the Zn—O2 and Zn—O3 bonds of 2.0668 (5) and 2.0909 (6) Å, respectively (Tables 1 and 3). The most likely explanation is that atom O1 is an acceptor of two hydrogen bonds, while atoms O2 and O3 accept only one hydrogen bond each. The Zn—O3 bond is slightly longer than Zn—O2, which can be explained by a slightly stronger hydrogen bond with an H···O distance of 2.049 (17) Å, versus 2.116 (17) Å at 110 K (Tables 2 and 4). The Zn—O distances are similar to those found in the diaqua complex (Prior & Kift, 2009). There, each urea ligand accepts a single hydrogen bond, resulting in Zn—Ourea bonds of 2.0893 (15) and 2.0753 (14) Å.
The Zn—O3 bond fails the Hirshfeld rigid-bond test (Hirshfeld, 1976) by 8.50σ at 110 K and 5.50σ at 250 K. The absolute values for the differences are 0.0017 and 0.0022 Å2 at 110 and 250 K, respectively, which is only slightly larger than the value of 0.0010 Å2 suggested by Hirshfeld for a rigid bond. We therefore still consider the anisotropic displacement parameters (ADPs) as reliably determined.
The urea ligands are essentially planar, with a maximum deviation of 0.0150 Å from the least-squares planes through their non-H atoms. The planarity at the N atoms has been assessed by evaluating their angle sums. All but one of the N atoms are planar, with angle sums between 355 (2) and 359 (2)° at 110 K. Atom N22 has an angle sum of 350.0 (19)°, indicating a slight pyramidalization. This is probably due to a close intermolecular contact with atom H12B(-x, y, -z + 3/2), with an N···H distance of 2.358 (15) Å at 110 K. In the publication of van de Giesen & Stam (1972), this interaction was described as a hydrogen bond. In our opinion, the sp2-hybridized N atom of urea is not capable of accepting hydrogen bonds, but we still consider this interaction responsible for the slight pyrimidalization.
During warming from 110 to 250 K, the O3—Zn—O3i angle [symmetry code: (i) -x, y, -z + 1/2] decreases from 97.11 (3) to 96.55 (5)°. This equates to a movement of atom O3 towards the b axis. At the same time, the O2—Zn—O2i angle increases from 83.84 (3) to 84.32 (4)°. The O2—Zn—O3 angle stays constant within experimental error [89.58 (2) and 89.62 (3)°].
The crystal packing is stablized by a hydrogen-bond network, consisting of a total of 12 independent hydrogen bonds: three intramolecular N—H···O bonds within the cation (involving atoms H12A, H22A and H32A), and nine intermolecular N—H···O bonds (Tables 2 and 4). The nitrate anion accepts eight of the nine intermolecular hydrogen bonds. Atom H32B is involved in a bifurcated hydrogen bond, with atoms O2N and O1N of the nitrate anion as acceptors. All other H atoms are involved in single hydrogen bonds. Atom H12B has a short intermolecular contact with atom N22, but we do not consider this as a hydrogen bond (see above). The intermolecular hydrogen bonding results in the formation of layers in the crystallographic ac plane (Fig. 2). These layers are interconnected by coordinative bonds from the urea O atoms to the ZnII atoms, and by the intramolecular hydrogen bonds.
To investigate the intermolecular interactions further, temperature-dependent unit-cell determinations were performed by cooling the crystal from 290 to 110 K and then warming from 110 to 250 K, in 20 K intervals. To minimize diffractometer errors in the cell determinations, the PHI/PHI-CHI routine was used (Duisenberg et al., 2000) and the position of the detector was kept fixed. The cell axes change linearly with temperature (Fig. 3). The magnitude of the thermal expansion and contraction was assessed by calculation of the expansion tensors using the STRAINANAL routine in PLATON (Spek, 2009), which uses the algorithm of Ohashi & Burnham (1973). The thermal expansion tensor is a symmetric second-rank tensor usually expressed in a Cartesian coordinate system (Lovett, 1999). Due to the monoclinic symmetry, two off-diagonal components of the tensor are equal to 0 (Table 5) and one of the eigenvectors (α3) is parallel to the b axis/twofold rotation axis (Table 6). The largest eigenvalues are found for the α1 direction, which is nearly collinear with the c axis. The eigenvalues for the α2 and α3 directions have approximately the same magnitude. Interestingly, the eigenvalues for the α3 direction are negative, as a consequence of a uniaxial compression along the b axis during warming (Fig. 4).
Analysis of the thermal expansion tensor can give insight into the strengths of intermolecular interactions (Salud et al., 1998; Küppers, 2003). The largest expansion is expected in the direction of the weakest intermolecular interactions. In (I), the intermolecular hydrogen bonds manifest as two-dimensional layers in the crystallographic ac plane. Eigenvalues α1 and α2 of the expansion tensor are indeed located in this plane. As expected, the lengths of the intermolecular hydrogen bonds increase during warming. Eigenvalue α3 is perpendicular to this plane and mainly reflects the interlinkage of the planes via coordinative bonds to the Zn. This eigenvalue is negative, corresponding to a contraction during warming. Overall, while the layer of hydrogen bonds expands, the distance between the layers shortens, leading to an accordion-like movement. Parallel to the layers, the Zn···Zn(1/2 - x, 1/2 - y, -z) distance increases from 10.6659 (4) Å at 110 K to 10.7358 (4) Å at 250 K. Perpendicular to the layers, the Zn···Zn (x, 1 - y, z + 1/2) distance is shortened from 12.1505 (5) Å at 110 K to 12.1336 (5) Å at 250 K. The combination of the negative eigenvalue of the thermal expansion tensor with the two positive values results in an overall expansion of the unit-cell volume during warming.
Negative thermal expansion is not uncommon in crystals of inorganic compounds. A famous example is the family of cyanide-bridged nanoporous frameworks (Phillips et al., 2008), where transverse vibrations of the cyanide bridges shorten the metal···metal distances. Other framework materials such as ZrW2O8, ZrV2O7 and Sc2(WO4)3 also show strong negative thermal expansion (Evans, 1999), and a framework-based model has also been used to explain the negative thermal expansion observed in the cuprites Cu2O and Ag2O (Artioli et al., 2006). In organic compounds, negative thermal expansion is seldom observed. The rigid aromatic molecule pentacene has a very anisotropic molecular shape with an anisotropic tensor of inertia. This can be related to the anisotropy of the libration tensor and the uniaxial negative thermal expansion (Haas et al., 2007). In the monohydrate of the dipeptide tryptophylglycine, the uniaxial negative thermal expansion could be explained by the increased ordering of the solvent water molecule (Birkedal et al., 2002).
Rigid-body analyses were performed on the anisotropic displacement parameters of the cations of (Ia) and (Ib) using the program THMA11 (Schomaker & Trueblood, 1998). In total, 12 rigid-body parameters were refined against 78 independent observations. The weighted R values of the resulting TLS model are 0.198 at 110 K and 0.173 at 250 K (R = {[Σ(wΔU)2]/[Σ(wUobs)2]}1/2, with w = 〈σ〉/σ). Such high R values indicate significant non-ridigity of the complex. This non-rigidity can also be detected by a comparison of the equivalent isotropic displacement parameters [Ueq = 1/3Σi,j(Uija*ia*jaiaj)]. The Ueq values of the N atoms are much larger than those of the other atoms. PEANUT plots (Hummel et al., 1990) of the difference between the observed Uij and the Uij from the TLS model indicate movement in the out-of-plane directions for the urea ligands (Fig. 5). The largest differences are observed for urea molecule 1 (atoms O1, C1, N11 and N12) and the smallest on urea molecule 3 (atoms O3, C3, N31 and N32).
The non-rigidity of the cation in (I) can be treated with a segmented rigid-body model, allowing rotations about the O—C bonds. Here, three additional parameters are refined together with the 12 rigid-body parameters. The weighted R values for the TLS models improve significantly to 0.138 and 0.126 for (Ia) at 110 K and (Ib) at 250 K, respectively. It remains unclear whether this improvement is due to a better model or is just a consequence of more degrees of freedom. Measurements over more temperatures, together with a normal coordinate analysis (Bürgi & Capelli, 2000), will be necessary for a final judgement on this question.
To analyse further the non-rigidity of the molecule, we looked at the difference between the ADPs of (Ia) at 110 K and (Ib) at 250 K. In the first step, the atomic coordinates of (Ia) were fitted to those of (Ib) using a quaternion fit (Mackay, 1984). The ADPs of (Ia) were then transformed accordingly and the difference was visualized using PEANUT (Fig. 6). The plot shows the differences in mean-square displacements (U250 K - U110 K) as a consequence of the temperature increase. It is clearly visible that the non-rigidity of the urea ligands mostly originates from libration around the O—C bond. The largest eigenvectors of the difference ADPs are as good as perpendicular to the urea ligand planes. These directions are different from the THMA result (Ucalc - Uobs), which is shown in Fig. 5.
The C, N and O atoms in (I) have rather large anisotropicities, as calculated by the ratio between the highest and lowest eigenvalues (λ3/λ1) of the ADPs. They are in the ranges 1.23–4.80 at 110 K and 1.27–5.48 at 250 K. These ratios are larger than in the diaqua compound (Prior & Kift, 2009), which has quite isotropic C, N and O atoms with a maximum (λ3/λ1) of 2.85 at 150 K. Restraints on the displacement parameters of two atoms had been used in the refinement of this structure. A redetermination of the structure of the diaqua compound in our laboratory at 150 K gave essentially the same result as that obtained by Prior & Kift (2009), but refinement without restraints on the displacement parameters led to a (λ3/λ1) range between 1.40 and 4.07 (Lutz, 2011).