Conformational flexibility of tricine as a chelating agent in catena-poly[[(tricinato)copper(II)]-μ-chloro]

In the polymeric title compound, catena-poly­[[{N-[2-hydroxy-1,1-bis­(hydroxy­methyl)­ethyl]­glycinato-O,N,O′}copper(II)]-μ-chloro], [CuCl(C₆H₁₂NO₅)]_n, the Cu²⁺ ions are chelated by tridentate tricine anions and bridged by chloride ions. The metal ion is five-coordinated, with an approximately square-pyramidal geometry. The [CuCl(C₆H₁₂NO₅)] units form infinite zigzag chains running parallel to the c axis, with Cu—Cl distances of 2.2209 (14) and 2.7792 (19) Å. The tricine anion adopts a conformation intermediate between that found in the crystal structure of the neutral mol­ecule and those reported for the Ni²⁺ and Zn²⁺ complexes.

S1. Comment

Some of the best studied polynuclear copper compounds are planar dihydroxo-bridged copper(II) dimers in which the terminal groups are bidentate nitrogen-containing ligands. It was shown by Crawford et al. (1976) that the energy gap J between the singlet and triplet states of the Cu²⁺ dimer could be directly related to the bridging Cu—O—Cu angle; a difference of 1° in the bridging angle would result in a variation of about 74 cm^-1 in the singlet-triplet separation. Much effort has been expended in correlating the magnitude of the exchange interactions in halide-bridged copper(II) dimers or chains with the structural features, but a general magnetostructural correlation similar to the simple one detected for the di-µ-hydroxo-bridged copper dimers should not be expected (Rojo et al., 1987). The nature of the ligands trans to the bridging ligand is also important. Dimers having a chloride ligand in a trans position generally show greater antiferromagnetic interactions (Rojo et al., 1987). The linear chain compound [Cu(pyridine)₂Cl₂] has been studied extensively and it is generally accepted that this complex exhibits magnetic properties that can be explained by the one-dimensional Heisenberg model (Crawford & Hatfield, 1977). Another interesting chloride-bridged chain is found in cyclohexylammonium trichlorocopper(II), which exhibits metamagnetic behaviour and has chain bridging angles of 85.3 and 86°. In zero field, the ground state is antiferromagnetic, but in an applied field greater than 100 G (1 G = 10 ^-4 T) the ground state becomes ferromagnetic-like (Khan, 1993).

The title compound, (I), was synthesized as part of a study of the magnetic properties of Cu^II compounds, in particular dimers and polymeric chains where the transition metal ions are bridged through unsubstituted or substituted amino acids, providing a superexchange pathway via the carboxy groups. Tricine [N-tris(hydroxymethyl)methylglycine] is a good complexation agent for divalent alkaline-earth and transition metal cations (Good et al., 1966). Potentiometric studies have shown that stable complexation of several transition metal, rare-earth and actinide atoms occurs in solution. The stability constants of Th⁴⁺, Ce³⁺, La³⁺ and UO₂²⁺ have been determined in aqueous media and evidence was found for the presence of binary complexes of 1:1 and 1:2 proportions (El-Roudy et al., 1997). These studies suggest that tricine may act as a tridentate as well as a bidentate ligand. There is, however, little structural work on the chelating properties of this ligand. A survey of the April 2000 release of the Cambridge Structural Database (ref?) has shown that only two crystal structures, namely of Zn²⁺ and Ni²⁺ complexes, have been reported. In both, tricine indeed acts as a tridentate ligand (Menabue & Saladini, 1992). \sch

In compound (I), the Cu²⁺ ion is five-coordinated with an approximately square-pyramidal geometry (Fig. 1). The basal atoms of the pyramid are a Cl^- ion, a carboxy O atom, the N atom and an O atom of one of the hydroxymethyl groups, which form a distorted square plane around the Cu atom [r.m.s. deviation 0.0682 (6) Å]. A symmetry-related Cl^- ion [Clⁱ; symmetry code: (i) x, 1 - y, z - 1/2] occupies the apical position of the pyramid. The Cu²⁺ ion lies 0.1015 (4) Å out of the least-squares plane of the pyramid base. The maximum deviation from the ideal value of 90° of the valency angles involving the transition metal atom is 7.70 (5)° for O1—Cu—Cl. The complex units form infinite zigzag chains parallel to the c axis, bridged through the Cl^- ions with asymmetric Cu—Cl distances of 2.2211 (15)* and 2.7792 (19) Å (Fig. 2). The Cu—Cl—Cu bridging angle is 122.08 (3)°. [Cuⁱ—Cl—Cu? Or Clⁱ—Cu—Cl? See query below.] * Please check value - given here as 2.2211 (15) Å, in tables as 2.2209 (14) Å.

Tricine is present in (I) in the anionic form, with a carboxylate group. Despite the fact that this group is ionized, there is considerable asymmetry between the carboxy C—O bond lengths, which may be explained by the chelating bond between O1 and Cu that lengthens the O1—C1 bond. Bond distances and angles within the anion compare well with those reported by Menabue & Saladini (1992) for the zinc and nickel complexes. In these compounds, the tricine ion is heavily folded (see below) to accommodate two symmetrical ions coordinating to the metal atom; in the present structure, only one tricine ion bonds to the Cu²⁺ ion and the conformation of the tricine ion is closer to that of the neutral molecule (Ramos Silva et al., 2000). It is important to note that, whereas in the nickel and zinc complexes the metal ion is bonded to one of the hydroxymethyl groups further away from the main skeleton (C1—C2—N—C3), in (I) the chelating hydroxymethyl group is the closest to the main skeleton (the chelating O4 is less than 0.6 Å away from the least-squares plane of C1—C2—N—C3). In the zinc and nickel complexes the metal ions are six-coordinated in a distorted octahedral environment and the O1-metal-O4 angles are 88.21 and 85.77° respectively, while for copper the angle is 164.92 (5)°, showing that here the ligand bridges the trans rather than the cis positions.

The main skeleton of the tricine molecule in (I) deviates from planarity, in contrast with the neutral molecule, but not by as much as in the nickel and zinc complexes, as shown by the C1—C2—N—C3 torsion angles [-134.13 (12), 90.57, -82.21 and 177.57 (7)°, for the copper, nickel and zinc complexes and pure tricine, respectively]. As shown by the C2—N—C3—C4, C2—N—C3—C5 and C2—N—C3—C6 torsion angles, the tris(hydroxymethyl)methyl group is rotated as a rigid group around the N—C3 bond by approximately 9°, much less than the rotation found in the nickel and zinc complexes or in pure tricine (where the rotation is 20, 25 and 22°, respectively). The N—C3—C5—O4, N—C3—C4—O3 and N—C3—C6—O5 torsion angles [-50.25 (15), 40.23 (17) and -59.75 (16)°, respectively] show that the three hydroxymethyl groups bonded to the N atom do not adopt the propeller-like conformation seen in the neutral molecule.

The polymers are linked together by an extended three-dimensional network of hydrogen bonds. Full saturation of every strong donor involved in hydrogen bonding is observed. The hydroxyl groups act as donors to three neighbouring tricine ions linking the polymeric chains, whereas a weak interaction between the NH group and the Cl^- ion occurs within the chain.

The magnetic susceptibility of (I) was measured using a SQUID magnetometer as a function of temperature on a powder sample which was free to rotate under the applied field. Unexpectedly, paramagnetic behaviour was found from room temperature down to the lowest attainable temperature of 2 K. A linear fit of the inverse susceptibility as a function of the temperature yielded an effective magnetic moment of 1.8 µ_B per Cu atom. The fact that the susceptibility obeys the paramagnetic Curie law down to such a low temperature shows that the magnetic exchange interaction between the Cu²⁺ ions is very small and that the Cl^- ions do not provide an effective superexchange pathway through the chain. This might be explained by the wide Cl—Cu—Cl bridging angle [Clⁱ—Cu—Cl? Or Cuⁱ—Cl—Cu? See query above.] and the fact that the magnetic planes of adjacent metal ions (planes with the four most strongly bound ligands, where the unpaired electron exists) do not share the bridging Cl atom.

S2. Experimental

Copper(II) chloride dihydrate (10 mmol) was added to tricine (10 mmol; Aldrich, 99%) in a solution of ethanol (50 ml). After a few days blue crystals formed.

S3. Refinement

All H atoms could be located in a difference Fourier map at an intermediate stage of the refinement. The coordinates of the H atoms bonded to an O or N atom were refined freely with an isotropic displacement parameter constrained to that of the parent atom. H atoms bonded to C atoms were refined as riding on their parent atoms, using the SHELXL97 (Sheldrick, 1997) defaults. The absolute structure was determined by refinement of the Flack parameter (Flack, 1983) using 1326 Friedel pairs.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97; molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. ORTEPII (Johnson, 1976) plot of (I). Displacement ellipsoids are drawn at the 50% level and H atoms are shown as small spheres of arbitrary radii [symmetry code: (i) x, 1 - y, z - 1/2].
	Fig. 2. Projection of the structure of (I) along the b axis showing one of the chloride-bridged chains.

catena-poly[[{N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycinato-O,N,O'} copper(II)]-µ-chloro]

Crystal data

[CuCl(C6H12NO5)]	F(000) = 564
Mr = 277.16	Dx = 2.019 Mg m^−3
Monoclinic, Cc	Mo Kα radiation, λ = 0.71073 Å
a = 7.8836 (12) Å	Cell parameters from 25 reflections
b = 20.134 (3) Å	θ = 9.9–16.2°
c = 6.089 (4) Å	µ = 2.68 mm^−1
β = 109.32 (3)°	T = 293 K
V = 912.0 (6) Å^3	Prism, intense blue
Z = 4	0.27 × 0.17 × 0.14 mm

Data collection

Enraf-Nonius CAD4 diffractometer	2603 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.016
Graphite monochromator	θmax = 30.0°, θmin = 3.8°
profile data from ω/2θ scans	h = −11→11
Absorption correction: ψ-scan (North et al., 1968)	k = −28→28
Tmin = 0.652, Tmax = 0.687	l = −8→8
2843 measured reflections	3 standard reflections every 180 min
2662 independent reflections	intensity decay: 7%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.016	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.042	w = 1/[σ^2(Fo^2) + (0.0266P)^2 + 0.3297P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max = 0.001
2662 reflections	Δρmax = 0.22 e Å^−3
139 parameters	Δρmin = −0.36 e Å^−3
2 restraints	Absolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.004 (6)

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	0.82160 (2)	0.421684 (8)	0.31659 (2)	0.01892 (5)
Cl	0.93053 (6)	0.458416 (19)	0.68042 (6)	0.02664 (8)
N	0.70685 (15)	0.38437 (6)	−0.00166 (19)	0.0149 (2)
H0	0.698 (3)	0.4196 (10)	−0.094 (4)	0.018*
O1	1.03736 (14)	0.37702 (6)	0.3058 (2)	0.0234 (2)
O2	1.12564 (16)	0.29568 (6)	0.1245 (2)	0.0308 (3)
O4	0.56516 (15)	0.44589 (6)	0.2879 (2)	0.0215 (2)
H4	0.527 (4)	0.4316 (13)	0.364 (5)	0.032*
O5	0.38935 (16)	0.40180 (7)	−0.4197 (2)	0.0248 (2)
H5	0.287 (4)	0.4042 (14)	−0.492 (5)	0.037*
O3	0.67260 (17)	0.30740 (7)	0.3435 (2)	0.0288 (2)
H3	0.660 (4)	0.2813 (15)	0.417 (5)	0.043*
C1	1.01010 (18)	0.33469 (7)	0.1401 (3)	0.0181 (2)
C2	0.82562 (19)	0.33251 (7)	−0.0425 (2)	0.0178 (2)
H2A	0.8367	0.3388	−0.1951	0.021*
H2B	0.7723	0.2892	−0.0401	0.021*
C3	0.52138 (17)	0.36354 (7)	−0.0191 (2)	0.0164 (2)
C5	0.4425 (2)	0.42350 (7)	0.0673 (3)	0.0205 (3)
H5A	0.4208	0.4592	−0.0458	0.025*
H5B	0.3285	0.4114	0.0848	0.025*
C6	0.4057 (2)	0.34627 (8)	−0.2688 (3)	0.0219 (3)
H6A	0.4597	0.3094	−0.3241	0.026*
H6B	0.2872	0.3326	−0.2710	0.026*
C4	0.52857 (19)	0.30187 (8)	0.1302 (3)	0.0226 (3)
H4A	0.4161	0.2973	0.1611	0.027*
H4B	0.5451	0.2626	0.0472	0.027*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu	0.01654 (7)	0.02293 (8)	0.01593 (7)	0.00295 (7)	0.00354 (5)	−0.00490 (7)
Cl	0.03617 (19)	0.02661 (16)	0.01582 (14)	−0.00146 (14)	0.00681 (13)	−0.00366 (13)
N	0.0137 (5)	0.0150 (5)	0.0158 (5)	−0.0008 (4)	0.0046 (4)	0.0007 (4)
O1	0.0161 (4)	0.0277 (6)	0.0222 (5)	0.0029 (4)	0.0006 (4)	−0.0087 (4)
O2	0.0197 (5)	0.0303 (6)	0.0408 (7)	0.0064 (4)	0.0077 (5)	−0.0110 (5)
O4	0.0206 (5)	0.0266 (5)	0.0188 (5)	0.0020 (4)	0.0085 (4)	−0.0020 (4)
O5	0.0188 (5)	0.0347 (6)	0.0180 (5)	−0.0020 (4)	0.0022 (4)	0.0052 (4)
O3	0.0301 (6)	0.0298 (6)	0.0215 (5)	−0.0035 (5)	0.0017 (4)	0.0113 (4)
C1	0.0154 (5)	0.0179 (6)	0.0215 (6)	−0.0005 (5)	0.0066 (5)	−0.0022 (5)
C2	0.0168 (6)	0.0192 (6)	0.0174 (6)	0.0002 (5)	0.0057 (5)	−0.0045 (5)
C3	0.0124 (5)	0.0186 (6)	0.0171 (5)	0.0001 (4)	0.0036 (4)	0.0022 (4)
C5	0.0169 (6)	0.0241 (7)	0.0197 (6)	0.0043 (5)	0.0051 (5)	0.0015 (5)
C6	0.0189 (6)	0.0232 (6)	0.0196 (6)	−0.0032 (5)	0.0009 (5)	0.0000 (5)
C4	0.0186 (6)	0.0228 (6)	0.0248 (7)	−0.0028 (5)	0.0049 (5)	0.0061 (5)

Geometric parameters (Å, º)

Cu—O1	1.9444 (11)	O3—C4	1.419 (2)
Cu—N	1.9943 (16)	O3—H3	0.72 (3)
Cu—O4	2.0302 (12)	C1—C2	1.511 (2)
Cu—Cl	2.2209 (14)	C2—H2A	0.9700
Cu—Cl^i	2.7792 (19)	C2—H2B	0.9700
N—C2	1.4774 (17)	C3—C4	1.529 (2)
N—C3	1.4908 (16)	C3—C5	1.529 (2)
N—H0	0.89 (2)	C3—C6	1.532 (2)
O1—C1	1.2830 (18)	C5—H5A	0.9700
O2—C1	1.2301 (18)	C5—H5B	0.9700
O4—C5	1.443 (2)	C6—H6A	0.9700
O4—H4	0.69 (3)	C6—H6B	0.9700
O5—C6	1.425 (2)	C4—H4A	0.9700
O5—H5	0.78 (3)	C4—H4B	0.9700
O1—Cu—N	85.04 (6)	H2A—C2—H2B	108.0
O1—Cu—O4	164.92 (5)	N—C3—C4	110.10 (11)
N—Cu—O4	83.58 (6)	N—C3—C5	104.74 (11)
O1—Cu—Cl	97.70 (5)	C4—C3—C5	112.08 (12)
N—Cu—Cl	174.81 (4)	N—C3—C6	112.48 (12)
O4—Cu—Cl	92.94 (5)	C4—C3—C6	107.38 (12)
C2—N—C3	116.77 (11)	C5—C3—C6	110.14 (12)
C2—N—Cu	108.70 (9)	O4—C5—C3	110.07 (11)
C3—N—Cu	107.11 (9)	O4—C5—H5A	109.6
C2—N—H0	112.3 (14)	C3—C5—H5A	109.6
C3—N—H0	107.9 (15)	O4—C5—H5B	109.6
Cu—N—H0	103.1 (14)	C3—C5—H5B	109.6
C1—O1—Cu	114.39 (10)	H5A—C5—H5B	108.2
C5—O4—Cu	110.28 (9)	O5—C6—C3	111.03 (12)
C5—O4—H4	101 (2)	O5—C6—H6A	109.4
Cu—O4—H4	119 (2)	C3—C6—H6A	109.4
C6—O5—H5	106 (2)	O5—C6—H6B	109.4
C4—O3—H3	106 (2)	C3—C6—H6B	109.4
O2—C1—O1	123.32 (14)	H6A—C6—H6B	108.0
O2—C1—C2	118.79 (13)	O3—C4—C3	109.98 (12)
O1—C1—C2	117.89 (12)	O3—C4—H4A	109.7
N—C2—C1	111.41 (11)	C3—C4—H4A	109.7
N—C2—H2A	109.3	O3—C4—H4B	109.7
C1—C2—H2A	109.3	C3—C4—H4B	109.7
N—C2—H2B	109.3	H4A—C4—H4B	108.2
C1—C2—H2B	109.3
O1—Cu—N—C2	14.14 (9)	O1—C1—C2—N	2.99 (18)
O4—Cu—N—C2	−155.95 (9)	C2—N—C3—C4	50.83 (16)
Cl—Cu—N—C2	−107.9 (4)	Cu—N—C3—C4	−71.24 (13)
O1—Cu—N—C3	141.13 (9)	C2—N—C3—C5	171.51 (11)
O4—Cu—N—C3	−28.96 (9)	Cu—N—C3—C5	49.43 (11)
Cl—Cu—N—C3	19.1 (5)	C2—N—C3—C6	−68.88 (15)
N—Cu—O1—C1	−13.48 (11)	Cu—N—C3—C6	169.05 (9)
O4—Cu—O1—C1	27.6 (3)	Cu—O4—C5—C3	26.88 (14)
Cl—Cu—O1—C1	162.08 (10)	N—C3—C5—O4	−50.25 (15)
O1—Cu—O4—C5	−40.2 (2)	C4—C3—C5—O4	69.10 (15)
N—Cu—O4—C5	1.02 (9)	C6—C3—C5—O4	−171.42 (12)
Cl—Cu—O4—C5	−175.12 (9)	N—C3—C6—O5	−59.75 (16)
Cu—O1—C1—O2	−170.08 (13)	C4—C3—C6—O5	178.97 (12)
Cu—O1—C1—C2	9.20 (17)	C5—C3—C6—O5	56.68 (16)
C3—N—C2—C1	−134.13 (12)	N—C3—C4—O3	40.23 (17)
Cu—N—C2—C1	−12.89 (13)	C5—C3—C4—O3	−75.92 (16)
O2—C1—C2—N	−177.70 (14)	C6—C3—C4—O3	163.00 (13)

Symmetry code: (i) x, −y+1, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N—H0···Cl^ii	0.89 (2)	2.74 (2)	3.3671 (16)	127.9 (18)
O5—H5···O1^iii	0.78 (3)	2.02 (3)	2.7703 (19)	160 (3)
O3—H3···O2^iv	0.72 (3)	2.07 (3)	2.7908 (19)	178 (3)
O4—H4···O5^v	0.69 (3)	2.05 (3)	2.741 (2)	171 (3)

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