Bis[N-(2-hydroxy­ethyl)-β-alaninato]copper(II)

The Cu^II ion in the title complex, [Cu(C₅H₁₀NO₃)₂] or [Cu(He-ala)₂] [He-ala = N-(2-hydroxy­ethyl)-β-alaninate], resides at the inversion centre of a square bipyramid comprised of two facially arranged tridentate He-ala ligands. Each He-ala ligand binds to a Cu^II ion by forming one six-membered β-alaninate chelate ring in a twist conformation and one five-membered ethanol­amine ring in an envelope conformation, with Cu—N = 2.017 (2) Å, Cu—O_COO = 1.968 (1) Å and Cu—O_OH = 2.473 (2) Å. The [Cu(He-ala)₂] mol­ecules are involved in a network of O—H...O and N—H...O hydrogen bonds, forming layers parallel to the (10) plane. The layers are connected into a three-dimensional structure by van der Waals inter­actions, so that the mol­ecular centres form pseudo-face-centered close packing.

S1. Comment

The title compound, [Cu(He-ala)₂], was synthesized as part of our systematic study of the coordination ability of dipodal ligands derived from β-alanine (Skorik et al., 2005, 2004, 2003, 2002). The acid–base and complexation equilibria of N-(2-hydroxyethyl)-β-alanine with Cu^II, Ni^II and Co^II have been studied by means of pH–potentiometric titration in aqueous media. Evidence was found for the presence of the [M(He-ala)] and [M(He-ala)₂] complexes (Uhlig & Linke, 1964). In the case of Cu^II ions, the monoprotonated [CuH(He-ala)] complex can also be formed in strong acidic conditions. To the best of our knowledge, no complexes of He-ala have previously been structurally characterized. In order to determine the Cu^II coordination geometry and the chelating pattern, the present X-ray crystal structure determination has been carried out on the title complex, (I), and the results are presented here.

The molecular structure of the [Cu(He-ala)₂] complex, along with the atomic numbering scheme, is shown in Fig. 1, while Table 1 lists selected bond lengths and angles. The structure of (I) consists of isolated [Cu(He-ala)₂] units, with the Cu^II ion located at the inversion centre in a square-bipyramidal geometry (4 + 2). The basal plane of the bipyramid is occupied by two N atoms of secondary amino groups and by two carboxylate O atoms, and the apices are occupied by two hydroxyl O atoms of two symmetrically arranged He-ala ligands. The axial Cu—O bonds are typically longer than the other in-plane bonds. The trans O—Cu—O or N—Cu—N angles are 180°, as required by symmetry. The cis angles involving the basal atom O2 differ only slightly from 90°, while the largest angular distortions of the octahedron occur for the N1—Cu1—O1 angles (Table 1).

Each monodeprotonated He-ala ligand binds to a Cu^II centre as a tridentate NO₂ ligand in a fac fashion by the formation of two chelate rings (one β-alaninate and one ethanolamine). There are three possible fac conformations for He-ala in a square-bipyramidal coordination, as shown in the scheme.

The isolated complex turned out to be a fac₁ isomer (Fig. 1). The selective formation of this isomer can be rationalized by reasoning that the most thermodynamically preferable isomer consists of the stronger field ligands in the equatorial positions and the weaker ligands in the apices of an axially elongated octahedron. Taking into account the spectrochemical series RNH₂ > RCOO⁻ > ROH (Bersuker, 1996), the fac₁ conformation for He-ala appears to be the most thermodynamically stable. For the same reason, a fac₁ isomer is favourable for the glycine derivative of ethanolamine (Ananeva et al., 1975; Ammar et al., 2001).

The six-membered β-alaninate chelate ring adopts a twist conformation, with atoms C5 and N1 not involved in the distortion of the initial planar hexagon. The puckering parameters (Cremer & Pople, 1975) generated by PLATON (Spek, 2003) are Q = 0.7335 (19) Å, θ = 94.75 (15)°, ϕ = 29.83 (15)°. The sum of the internal angles [675.0 (4)°] has a positive deviation from the ideal value, 648° = 120 + (109.5 × 4) + 90, and this exerts a stress, resulting in the flattening. The five-membered ethanolamine chelate ring adopts an envelope conformation, with atom C2 tilted by 0.646 (3) Å away from the Cu1/O1/C1/N1 plane; the puckering parameters are Q = 0.475 (2) Å and θ = 305.4 (2)°. The dihedral angle formed by the r.m.s. planes of the two chelate rings is 70.69 (9)°.

In the crystal structure of (I), the [Cu(He-ala)₂] molecules are involved in an extended two-dimensional system of hydrogen bonds, forming layers parallel to the (101) plane (Fig. 2a). Six intralayer molecules are hydrogen-bonded to the reference molecule; two of them form two N—H···O contacts each, while the other four form only O—H···O contacts (Table 2). The whole molecular packing may be represented as a superposition of these layers. Its topology was characterized with coordination sequences (O'Keeffe, 1995) calculated using the TOPOS4.0 professional program suite for crystallochemical analysis (Blatov et al., 2000). The molecular centres of gravity form the coordination sequence 12, 42, 92. In other words, the first, second and third coordination sphere of any molecule contains 12, 42 and 92 molecules, respectively. This sequence corresponds topologically to three-layered face-centred cubic (fcc) packing (O'Keeffe, 1995) which is slightly distorted geometrically. The simplified three-layered packing motif in the structure is shown in Fig. 2(b), where the centres of the molecules are represented as balls. Three molecules of the upper and lower layers and six molecules of the middle layer are shown. Thus, the hydrogen-bonded layers are joined by van der Waals interactions to the distorted fcc packing that is typical for molecular compounds (Kitaigorodskii, 1973; Peresypkina & Blatov, 2000; Braun & Huttner, 2005).

S2. Experimental

N-(2-Hydroxyethyl)-β-alanine was prepared using a modification of the literature procedure of Salov et al. (1985). A mixture containing acrylic acid (4.1 ml, 0.060 mol) and ethanolamine (10.8 ml, 0.18 mol) in water (56 ml) was heated under reflux for 8 h. The solvent and the excess ethanolamine were then evaporated on a water bath under vacuum. The resulting solid product was recrystallized from methanol [yield 2.28 g, 36%, m.p. 420 K (literature value 419–420 K; Salov et al., 1985)]. Analysis, found: C 44.92, H 8.62, N 10.43%; calculated for C₅H₁₁NO₃: C 45.10, H 8.33, N 10.52%. Spectroscopic analysis: ¹H NMR (400 MHz, D₂O, δ, p.p.m.): 3.84 (t, J = 5.20 Hz, 2H), 3.26 (t, J = 6.69 Hz, 2H), 3.20 (t, J = 5.20 Hz, 2H), 2.58 (t, J = 6.69 Hz, 2H). The title complex was prepared as follows. A mixture containing N-(2-hydroxyethyl)-β-alanine (4.7 g, 0.035 mol), (CuOH)₂CO₃ (9.0 g, 0.041 mol) and water (20 ml) was stirred at room temperature for 48 h. After filtration, the resulting solution was maintained at room temperature until evaporation resulted in the formation of blue–violet crystals of (I) suitable for X-ray diffraction analysis. Analysis, found: C 36.53, H 6.38, N 8.55, Cu 19.14%; calculated for C₁₀H₂₀N₂O₆Cu: C 36.64, H 6.15, N 8.55, Cu 19.38%.

S3. Refinement

Atom H2 of the OH group was found in a difference electron-density map and refined with the O—H distance constrained to 0.82 (2) Å and with U_iso(H) = 1.2U_eq(O). All other H atoms were located geometrically and refined using a riding model, with C—H = 0.97 and N—H = 0.91 Å, and with U_iso(H) = 1.2U_eq(C,N). [Please check added text] Intermolecular interactions and features of crystal packing were investigated according to Peresypkina & Blatov (2000) using the TOPOS4.0 professional program suite for crystallochemical analysis (Blatov et al., 2000).

Computing details

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2; data reduction: APEX2; program(s) used to solve structure: SHELXTL (Bruker, 2005); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: local programs.

Figures

Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. The axial bonds to Cu are shown by dashed lines.

Fig. 2. The crystal structure of (I). (a) The hexagonal layer parallel to the (101) plane. H atoms have been omitted for clarity. (b) A fragment of the spatial arrangement of the centres of gravity of molecules of (I), illustrating the three-layered fcc packing motif. Upper, medium and lower levels are shown as dark-, medium- and light-grey balls, respectively. Solid and dashed lines show the intra- (hydrogen bonding) and interlayer (van der Waals) distances between the molecular centres.

Bis[N-(2-hydroxyethyl)-β-alaninato]copper(II)

Crystal data

[Cu(C5H10NO3)2]	F(000) = 342
Mr = 327.82	Dx = 1.636 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1481 reflections
a = 9.5257 (6) Å	θ = 2.6–28.1°
b = 5.6597 (3) Å	µ = 1.67 mm^−1
c = 12.4288 (7) Å	T = 295 K
β = 96.779 (3)°	Block, blue-violet
V = 665.38 (7) Å^3	0.35 × 0.33 × 0.23 mm
Z = 2

Data collection

Bruker X8 APEX CCD area-detector diffractometer	1553 independent reflections
Radiation source: fine-focus sealed tube	1204 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.019
Detector resolution: 25 pixels mm^-1	θmax = 28.2°, θmin = 2.6°
ω and ϕ scans	h = −12→7
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	k = −7→6
Tmin = 0.477, Tmax = 0.682	l = −16→16
3238 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)] = 0.027	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.069	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0293P)^2 + 0.3245P] where P = (Fo^2 + 2Fc^2)/3
1553 reflections	(Δ/σ)max < 0.001
91 parameters	Δρmax = 0.37 e Å^−3
1 restraint	Δρmin = −0.26 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
Cu1	0.0000	0.0000	0.5000	0.02268 (11)
C1	0.3231 (2)	0.1144 (5)	0.4834 (2)	0.0435 (6)
H1A	0.3532	0.0410	0.4194	0.052*
H1B	0.4063	0.1765	0.5268	0.052*
C2	0.2224 (2)	0.3136 (4)	0.44997 (19)	0.0348 (5)
H2A	0.2083	0.4073	0.5131	0.042*
H2B	0.2636	0.4152	0.3992	0.042*
C3	0.0926 (2)	0.1347 (4)	0.28765 (17)	0.0330 (5)
H3A	0.1740	0.0314	0.2887	0.040*
H3B	0.1064	0.2667	0.2402	0.040*
C4	−0.0391 (2)	0.0004 (4)	0.24275 (17)	0.0334 (5)
H4A	−0.1218	0.0922	0.2548	0.040*
H4B	−0.0394	−0.0185	0.1652	0.040*
C5	−0.0488 (2)	−0.2399 (4)	0.29391 (17)	0.0297 (5)
N1	0.08360 (17)	0.2246 (3)	0.39879 (13)	0.0247 (4)
H1	0.0250	0.3520	0.3914	0.030*
O1	0.2578 (2)	−0.0569 (4)	0.54400 (16)	0.0502 (5)
H2	0.305 (3)	−0.074 (6)	0.5983 (18)	0.060*
O2	−0.00510 (16)	−0.2610 (3)	0.39498 (11)	0.0300 (3)
O3	−0.0940 (2)	−0.4119 (3)	0.23863 (15)	0.0540 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.03225 (19)	0.01987 (18)	0.01574 (18)	0.00165 (16)	0.00207 (11)	−0.00013 (16)
C1	0.0319 (12)	0.0633 (17)	0.0340 (14)	0.0060 (12)	−0.0016 (10)	−0.0032 (14)
C2	0.0352 (12)	0.0367 (12)	0.0323 (13)	−0.0073 (10)	0.0037 (9)	−0.0001 (11)
C3	0.0415 (12)	0.0385 (13)	0.0202 (11)	0.0022 (10)	0.0082 (9)	0.0036 (10)
C4	0.0442 (12)	0.0378 (12)	0.0167 (10)	0.0062 (11)	−0.0030 (8)	0.0027 (11)
C5	0.0340 (11)	0.0294 (11)	0.0250 (11)	0.0088 (9)	−0.0003 (8)	−0.0026 (10)
N1	0.0288 (9)	0.0234 (8)	0.0218 (9)	0.0050 (7)	0.0029 (6)	0.0030 (7)
O1	0.0499 (11)	0.0511 (11)	0.0440 (12)	0.0056 (8)	−0.0179 (8)	0.0083 (9)
O2	0.0481 (9)	0.0231 (7)	0.0178 (8)	0.0034 (6)	0.0001 (6)	−0.0004 (6)
O3	0.0840 (14)	0.0357 (9)	0.0359 (10)	0.0033 (9)	−0.0202 (9)	−0.0092 (9)

Geometric parameters (Å, º)

Cu1—N1^i	2.0172 (17)	C2—N1	1.486 (3)
Cu1—N1	2.0172 (17)	C3—H3A	0.9700
Cu1—O1^i	2.4733 (18)	C3—H3B	0.9700
Cu1—O1	2.4733 (18)	C3—C4	1.516 (3)
Cu1—O2	1.9682 (14)	C3—N1	1.484 (2)
Cu1—O2^i	1.9682 (14)	C4—H4A	0.9700
C1—H1A	0.9700	C4—H4B	0.9700
C1—H1B	0.9700	C4—C5	1.509 (3)
C1—C2	1.507 (3)	C5—O2	1.282 (2)
C1—O1	1.416 (3)	C5—O3	1.239 (3)
C2—H2A	0.9700	N1—H1	0.9100
C2—H2B	0.9700	O1—H2	0.772 (17)
N1^i—Cu1—N1	180.0	N1—C2—H2B	109.3
N1^i—Cu1—O1^i	76.42 (7)	H3A—C3—H3B	107.9
N1—Cu1—O1^i	103.58 (7)	C4—C3—H3A	109.2
N1^i—Cu1—O1	103.58 (7)	C4—C3—H3B	109.2
N1—Cu1—O1	76.42 (7)	N1—C3—H3A	109.2
O1^i—Cu1—O1	180.00 (9)	N1—C3—H3B	109.2
O2—Cu1—N1^i	87.70 (7)	N1—C3—C4	112.25 (17)
O2^i—Cu1—N1^i	92.30 (7)	C3—C4—H4A	109.0
O2—Cu1—N1	92.30 (7)	C3—C4—H4B	109.0
O2^i—Cu1—N1	87.70 (7)	H4A—C4—H4B	107.8
O2—Cu1—O1^i	90.28 (6)	C5—C4—C3	112.71 (18)
O2^i—Cu1—O1^i	89.72 (6)	C5—C4—H4A	109.0
O2—Cu1—O1	89.72 (6)	C5—C4—H4B	109.0
O2^i—Cu1—O1	90.28 (6)	O2—C5—C4	117.91 (19)
O2—Cu1—O2^i	180.0	O3—C5—C4	120.61 (19)
H1A—C1—H1B	108.1	O3—C5—O2	121.4 (2)
C2—C1—H1A	109.5	Cu1—N1—H1	106.2
C2—C1—H1B	109.5	C2—N1—Cu1	110.09 (13)
O1—C1—H1A	109.5	C2—N1—H1	106.2
O1—C1—H1B	109.5	C3—N1—Cu1	115.76 (13)
O1—C1—C2	110.73 (19)	C3—N1—C2	111.62 (17)
C1—C2—H2A	109.3	C3—N1—H1	106.2
C1—C2—H2B	109.3	Cu1—O1—H2	132 (2)
H2A—C2—H2B	107.9	C1—O1—Cu1	106.29 (13)
N1—C2—C1	111.72 (19)	C1—O1—H2	108 (2)
N1—C2—H2A	109.3	C5—O2—Cu1	124.04 (14)

Symmetry code: (i) −x, −y, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2^ii	0.91	2.21	3.030 (2)	150
O1—H2···O3^iii	0.77 (2)	1.89 (2)	2.659 (2)	173 (3)

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