catena-Poly[copper(II)-[mu]-L-tyrosyl-L-leucinato]

In the title compound, [Cu(C₁₅H₂₀N₂O₄)]_n, the copper(II) coordination is square planar. The anionic L-tyrosyl-L-leucinate ligand binds in an N,N',O-tridentate mode to one Cu^II cation on one side and in an O-monodentate mode to a second Cu^II cation on the other side, thus defining -Cu-O-C-O-Cu'- chains which run along the a axis. These chains are held together by a strong hydrogen bond involving the hydroxy H atom.

S1. Comment

Weak and long non-covalent interactions are important in the structure and function of biological macromolecules (Lippard & Berg, 1994). They contribute to the structure, optimize molecular reactivity, allow molecular recognition, and more. In electron-transfer proteins, these bonds are conveniently tuned to regulate the kinetics of the transfer process, and thus the function of the protein.

The best known and probably most important weak interactions between metal ions, radicals or redox centres are hydrogen bonds (Jeffrey & Saenger, 1991). Real chemical paths are usually a sequence of elemental weak interactions plus strong covalent bonds. They may be identified in protein structures (Perutz, 1993), assigned to a specific biological function (Calvo et al., 2000) and in some cases reproduced in model systems (Costa-Filho et al. 2001, 2004; Santana et al., 2005). Thus, characterization of these weak interactions in model systems is important, and metal compounds with amino acids and peptides are particularly relevant. In previous work (Costa-Filho et al., 2001, 2004; Santana et al., 2005), we characterized the properties of weak interactions between metal ions using electron paramagnetic resonance (EPR) and magnetic measurements. Recently, our work allowed comparison with results obtained in an electron-transfer protein (Calvo et al., 2000; Santana et al., 2005). Both structural and magnetic information about a compound are needed to progress in this direction. In line with the work performed by Costa-Filho et al. (2001, 2004), we are now involved in the study of magnetic interactions in the title compound, (I). So far, our EPR experiments have allowed the evaluation of the exchange interactions transmitted through a 13-step path (two coordination + ten covalent + one hydrogen bond) and which connects Cu^II ions 9.735 (1) Å apart (see below). Magnetic measurements at very low temperature displayed a magnetic phase transition of the compound, intimately connected to this path. Thus, a detailed structural determination is essential in order to be able to analyze these magnetic data, and this is the scope of the present report.

Fig. 1 presents a molecular view of complex (I). The L-tyrosyl–L-leucine (TyrLeu) ligand binds to atom Cu1 in a tridentate mode via the amino atom N1, the deprotonated peptide atom N2 and the carboxylate atom O3. The square-planar coordination of the metal centre is completed through the binding of the second carboxylate atom O4ⁱ [symmetry code: (i) x + 1/2, 1/2 − y, 1 − z] of a neighbouring ligand. This particular coordination leads to CuN₂O₂ polyhedra mounted onto a 2₁ screw axis parallel to the a axis, with the Cu^II cations bridged by carboxylato groups in a –Cu—O—C—O—Cu'- chain sequence. Similar one-dimensional structures have already been described in other Cu–dipeptide complexes [(II) (Nascimento et al., 2001), (III) (Tiliakos et al., 2002) and (IV) (Amirthalingam & Muralidharan, 1976)]. Table 1 presents selected bond distances and angles around the Cu^II cation in (I), which depart significantly from a regular pattern due to restraints imposed by chelation, a general trend (Cu—N_peptide < Cu—N_amine ≈ Cu—O_carbox) which is shared by other dipeptide complexes. There are, however, some distortions which are intrinsic of each structure and which depend on the particular interaction scheme. One of these is the departure from planarity of the CuN₂O₂ group. The copper coordination plane in (I) presents a slight tetrahedral distortion, with the mean plane through atom Cu1 leaving the donor atoms alternately above and below the mean plane by a mean of 0.10 (1) Å. This value lies somewhere in between the distortions presented in complexes (II) [0.05 (1) Å], (III) [0.12 (1) Å] and (IV) [0.22 (1) Å].

Another distinctive feature is the shape that the two five-membered coordination loops adopt upon chelation. In the present case, the description is simplified by the fact that, in both five-membered loops, four atoms lie very nearly in the same plane, with the fifth one departing significantly and thus giving each ring a well defined `envelope' appearance, viz. in the Cu1/O3/C11/C10/N2 and N2/C9/C8/N1/Cu1 groups, the first four atoms depart by a mean of 0.01 (1) and 0.02 (1) Å from planarity, respectively, while the fifth atom is 0.14 (1) or 0.38 (2) Å away, respectively. An alternative way to evaluate this is through the torsion angles calculated around the loops (Table 1). It can be seen that, in each cycle, one of these torsion angles is distinctly smaller than the rest and corresponds to the quasi-planar part of the cycle.

From the two possible H-donor groups present in the dipeptide (OH and NH₂), the first provides a strong intermolecular interaction [O1—H1A···O2(1 − x, y − 1/2, 1/2 − z)], while only one of the amino H atoms appears involved in a fairly weak contact N1—H1C···O3(x + 1/2, 1/2 − y, 1 − z) (Table 2).

In many related compounds where the Cu centre is complexed to dipeptides with aromatic groups in their side chains, some sort of Cu···π interaction has been observed [viz. glycyl-L–leucyl-L-tyrosine (Franks & Van der Helm, 1971), L-tyrosine (Van der Helm & Tatsch, 1972) and glycyl-L-tryptophan (Hursthouse et al., 1971)]. This does not seem to be the case in (I), where the benzene ring is at an angle of 103.5 (1)° to the CuN₂O₂ mean plane, and the nearest Cu···C_arom approach is 3.84 (1) Å. There is, however, a close approach of a methyl H atom to the centroid G of the benzene ring (Table 2).

As already stated, the elemental packing units in the structure are the chains running along the a axis. The syn–anti carboxylate bridges link symmetry related Cu^II ions to a nearest-neighbour distance along the chain of 4.981 (1) Å. Fig. 2 presents a simplified view, showing the chain `spine' in bold, as well as two intrachain non-bonding interactions providing the chain stability, namely the weak hydrogen bond involving the amino H atom (full broken lines) and the C—H···π interaction (double broken line).

These `S' shaped strips (Fig. 3) stack parallel to each other, the main link between neighbouring units being the strong hydrogen bond involving the hydroxy group. The bulky lateral wings act as effective chain spacers and, as a result, the second nearest-neighbour distance between cations [Cu1···Cu1(x + 1,y,z) 9.031 (1) Å] is also achieved along the chain and corresponds to one full unit-cell translation along a. The shortest chemical path joining cations from different chains goes through the hydroxy hydrogen bond and links Cu^II centres 9.735 (1) Å apart via a 19.12 (1) Å path made up of 12 covalent/coordination steps plus one hydrogen bond. We have detected a weak interchain magnetic interaction through this path, of a still unknown character, exhibiting J' (kB) ~0.05 K. This should be compared with the direct link along the chain, Cu1···Cu1(x + 1/2, 1/2 − y, 1 − z), of 4.981 (1) Å, through a four-step path of 6.39 (1) Å, along which a ferromagnetic interaction takes place with J (kB) ~2.5 K.

The way in which chains approach each other favours the appearance of a very short Cu···H contact [Cu1···H14B (1/2 − x, 1 − y, z + 1/2) 2.50 (1) Å, shown as double broken lines in Fig. 3]. A survey of the Cambridge Structural Database (November 2003 update; Allen, 2002) showed this to be a rather infrequent case: out of 4060 reported cases with a CuN_nO_4-n polyhedra (0< = n < = 4), only 30 presented shorter H···Cu distances, in the range 2.04–2.50 Å.

S2. Experimental

To a solution containing cupric acetate monohydrate (0.25 mmol) (Merck, Darmstadt, Germany) and L-tyrosyl–L-leucine (0.25 mmol) (Sigma, St Louis, Missouri, USA) in water (20 ml), we added ethanol (20 ml) and 0.1 N NaOH solution (5 ml). After one week, a number of well shaped, though small and extremely poorly diffracting, needles appeared by evaporation of the solution at room temperature.

S3. Refinement

H atoms attached to C and N atoms were included in calculated positions, with idealized distances to their hosts (C—H = 0.98, C—H₂ = 0.97, C—H₃ = 0.96, C—H_arom = 0.93 and N—H₂ = 0.92 Å), and allowed to ride; in the case of terminal CH₃, they were allowed to rotate as well. The H atom attached to an O atom was found in the difference density map and refined with a restrained O—H distance of 0.82 (1) Å. In all cases, H atoms were ascribed an isotropic displacement factor U_iso(H) = xU_eq(parent), with x = 1.2 for non-methyl H atoms and x = 1.5 for methyl ones.

Computing details

Data collection: P3/P4-PC (Siemens, 1991); cell refinement: P3/P4-PC; data reduction: XDISK in SHELXTL/PC (Sheldrick, 1994); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL/PC; software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of (I). The independent part of the molecule is shown in bold and with atom-numbering. Displacement ellipsoids are drawn at the 50% level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) x + 1/2, 1/2 − y, 1 − z.] Please check added symmetry code.
	Fig. 2. A lateral view of a chain, with the main path connecting Cu atoms shown in bold. Full broken lines denote the hydrogen bond involving the amino H atom and double broken lines indicate the C—H···π interactions. [Symmetry code: (i) x + 1/2, 1/2 − y, 1 − z.] Please check added symmetry code.
	Fig. 3. A packing view of (I), down the a axis. For clarity, chains are shown in alternate line weights. Full broken lines denote the hydrogen bond involving the hydroxy H atom, and double broken lines indicate the short H···Cu contact (see text). [Symmetry code: (i) x + 1/2, 1/2 − y, 1 − z.] Please check added symmetry code.

catena-Poly[copper(II)-µ-L-tyrosyl-L-leucinato]

Crystal data

[Cu(C15H20N2O4)]	F(000) = 740
Mr = 355.87	Dx = 1.553 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 25 reflections
a = 9.0307 (9) Å	θ = 7.5–12.5°
b = 10.4375 (12) Å	µ = 1.45 mm^−1
c = 16.1471 (18) Å	T = 295 K
V = 1522.0 (3) Å^3	Needle, blue
Z = 4	0.55 × 0.14 × 0.12 mm

Data collection

Siemens R3m diffractometer	1066 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.055
Graphite monochromator	θmax = 25.0°, θmin = 2.3°
ω/2θ scans	h = 0→10
Absorption correction: ψ scan (SHELXTL/PC; Sheldrick,1994)	k = 0→12
Tmin = 0.785, Tmax = 0.845	l = 0→19
1662 measured reflections	2 standard reflections every 98 reflections
1557 independent reflections	intensity decay: 2%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: geom+difmap
R[F^2 > 2σ(F^2)] = 0.048	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.111	w = 1/[σ^2(Fo^2) + (0.0517P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max = 0.05
1557 reflections	Δρmax = 0.32 e Å^−3
202 parameters	Δρmin = −0.36 e Å^−3
0 restraints	Absolute structure: Flack (1983), no Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.07 (5)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cu1	0.40970 (10)	0.35067 (8)	0.49942 (8)	0.0315 (3)
O1	0.6050 (10)	0.4465 (6)	0.0854 (4)	0.059 (2)
H1A	0.636 (12)	0.370 (4)	0.090 (7)	0.089*
O2	0.3197 (7)	0.7021 (5)	0.4223 (4)	0.0414 (16)
O3	0.2077 (6)	0.2797 (4)	0.5017 (5)	0.0381 (13)
O4	−0.0165 (7)	0.3041 (5)	0.4511 (4)	0.0374 (15)
N1	0.5822 (7)	0.4670 (5)	0.4986 (6)	0.0482 (17)
H1B	0.6222	0.4698	0.5497	0.058*
H1C	0.6512	0.4362	0.4637	0.058*
N2	0.3198 (8)	0.4842 (6)	0.4387 (4)	0.0318 (17)
C1	0.6188 (10)	0.4933 (9)	0.1640 (6)	0.040 (2)
C2	0.5624 (11)	0.6135 (8)	0.1815 (6)	0.046 (3)
H2A	0.5175	0.6607	0.1396	0.055*
C3	0.5716 (11)	0.6637 (8)	0.2594 (5)	0.041 (2)
H3A	0.5317	0.7444	0.2695	0.049*
C4	0.6381 (10)	0.5984 (8)	0.3237 (6)	0.040 (2)
C5	0.7009 (11)	0.4791 (9)	0.3044 (6)	0.048 (3)
H5A	0.7497	0.4332	0.3456	0.058*
C6	0.6916 (11)	0.4282 (8)	0.2252 (6)	0.047 (2)
H6A	0.7352	0.3495	0.2137	0.056*
C7	0.6477 (9)	0.6557 (9)	0.4098 (5)	0.041 (2)
H7A	0.6298	0.7472	0.4059	0.049*
H7B	0.7477	0.6441	0.4304	0.049*
C8	0.5399 (10)	0.5990 (8)	0.4726 (5)	0.034 (2)
H8A	0.5431	0.6534	0.5220	0.041*
C9	0.3778 (10)	0.5976 (8)	0.4415 (5)	0.033 (2)
C10	0.1662 (9)	0.4623 (7)	0.4151 (5)	0.032 (2)
H10A	0.1047	0.5353	0.4315	0.038*
C11	0.1152 (10)	0.3401 (9)	0.4601 (5)	0.033 (2)
C12	0.1479 (11)	0.4352 (8)	0.3223 (5)	0.038 (2)
H12A	0.2144	0.3660	0.3075	0.045*
H12B	0.0476	0.4051	0.3130	0.045*
C13	0.1764 (12)	0.5458 (10)	0.2646 (6)	0.054 (3)
H13A	0.2716	0.5846	0.2794	0.064*
C14	0.1862 (16)	0.4958 (14)	0.1739 (6)	0.099 (5)
H14A	0.2797	0.4538	0.1656	0.148*
H14B	0.1778	0.5666	0.1362	0.148*
H14C	0.1073	0.4362	0.1638	0.148*
C15	0.0574 (13)	0.6476 (11)	0.2709 (8)	0.083 (4)
H15A	0.0615	0.6870	0.3245	0.125*
H15B	−0.0381	0.6090	0.2631	0.125*
H15C	0.0733	0.7114	0.2290	0.125*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0352 (4)	0.0263 (4)	0.0330 (4)	−0.0013 (5)	−0.0003 (8)	0.0057 (7)
O1	0.100 (6)	0.036 (3)	0.042 (4)	0.008 (4)	0.013 (5)	−0.004 (3)
O2	0.055 (4)	0.024 (3)	0.045 (4)	0.006 (3)	0.001 (3)	0.001 (3)
O3	0.040 (3)	0.031 (3)	0.044 (3)	−0.009 (2)	0.000 (5)	0.013 (5)
O4	0.034 (3)	0.031 (3)	0.047 (4)	−0.002 (3)	−0.002 (3)	0.016 (3)
N1	0.043 (4)	0.034 (3)	0.067 (4)	−0.002 (3)	−0.019 (7)	0.012 (5)
N2	0.035 (4)	0.027 (4)	0.033 (4)	−0.003 (3)	0.004 (4)	0.008 (3)
C1	0.039 (6)	0.031 (5)	0.049 (6)	−0.007 (5)	0.006 (5)	−0.009 (4)
C2	0.062 (7)	0.036 (5)	0.040 (5)	0.003 (5)	−0.001 (5)	0.011 (4)
C3	0.058 (6)	0.021 (4)	0.044 (5)	0.001 (5)	0.011 (5)	0.007 (4)
C4	0.036 (5)	0.030 (4)	0.055 (6)	−0.007 (4)	0.014 (5)	−0.002 (5)
C5	0.046 (6)	0.043 (6)	0.054 (6)	0.006 (5)	−0.003 (5)	0.002 (5)
C6	0.055 (6)	0.027 (5)	0.058 (6)	0.013 (5)	0.001 (6)	−0.005 (5)
C7	0.039 (5)	0.035 (5)	0.049 (5)	−0.014 (5)	−0.008 (4)	−0.008 (5)
C8	0.047 (5)	0.020 (4)	0.035 (5)	−0.004 (4)	−0.003 (4)	0.003 (3)
C9	0.044 (6)	0.029 (4)	0.026 (4)	−0.006 (4)	0.002 (4)	0.005 (4)
C10	0.035 (5)	0.022 (4)	0.037 (5)	0.002 (4)	0.008 (4)	0.011 (4)
C11	0.039 (6)	0.037 (5)	0.023 (4)	0.003 (5)	0.002 (4)	0.001 (4)
C12	0.051 (5)	0.037 (5)	0.025 (4)	−0.003 (5)	0.009 (4)	0.008 (4)
C13	0.047 (6)	0.058 (7)	0.056 (6)	−0.011 (6)	−0.014 (5)	0.020 (6)
C14	0.139 (13)	0.128 (11)	0.028 (6)	−0.045 (12)	−0.014 (8)	0.025 (8)
C15	0.077 (9)	0.057 (7)	0.116 (10)	−0.005 (8)	−0.028 (8)	0.045 (8)

Geometric parameters (Å, º)

Cu1—N2	1.887 (7)	C5—C6	1.388 (13)
Cu1—O4^i	1.922 (5)	C5—H5A	0.9300
Cu1—O3	1.969 (5)	C6—H6A	0.9300
Cu1—N1	1.975 (6)	C7—C8	1.525 (11)
O1—C1	1.365 (11)	C7—H7A	0.9700
O1—H1A	0.85 (4)	C7—H7B	0.9700
O2—C9	1.251 (9)	C8—C9	1.547 (12)
O3—C11	1.244 (10)	C8—H8A	0.9800
O4—C11	1.256 (10)	C10—C12	1.533 (11)
O4—Cu1^ii	1.922 (5)	C10—C11	1.538 (11)
N1—C8	1.491 (10)	C10—H10A	0.9800
N1—H1B	0.9000	C12—C13	1.505 (11)
N1—H1C	0.9000	C12—H12A	0.9700
N2—C9	1.295 (10)	C12—H12B	0.9700
N2—C10	1.456 (10)	C13—C15	1.515 (14)
C1—C6	1.368 (12)	C13—C14	1.558 (15)
C1—C2	1.383 (12)	C13—H13A	0.9800
C2—C3	1.366 (12)	C14—H14A	0.9600
C2—H2A	0.9300	C14—H14B	0.9600
C3—C4	1.379 (12)	C14—H14C	0.9600
C3—H3A	0.9300	C15—H15A	0.9600
C4—C5	1.403 (12)	C15—H15B	0.9600
C4—C7	1.517 (12)	C15—H15C	0.9600
N2—Cu1—O4^i	170.4 (3)	N1—C8—C7	112.5 (7)
N2—Cu1—O3	83.6 (3)	N1—C8—C9	109.0 (7)
O4^i—Cu1—O3	89.8 (2)	C7—C8—C9	113.1 (7)
N2—Cu1—N1	83.2 (3)	N1—C8—H8A	107.3
O4^i—Cu1—N1	104.2 (3)	C7—C8—H8A	107.3
O3—Cu1—N1	164.2 (2)	C9—C8—H8A	107.3
C1—O1—H1A	103 (8)	O2—C9—N2	128.2 (8)
C11—O3—Cu1	114.9 (5)	O2—C9—C8	118.0 (7)
C11—O4—Cu1^ii	122.1 (6)	N2—C9—C8	113.8 (7)
C8—N1—Cu1	111.6 (5)	N2—C10—C12	112.9 (7)
C8—N1—H1B	109.3	N2—C10—C11	107.0 (7)
Cu1—N1—H1B	109.3	C12—C10—C11	106.0 (6)
C8—N1—H1C	109.3	N2—C10—H10A	110.3
Cu1—N1—H1C	109.3	C12—C10—H10A	110.3
H1B—N1—H1C	108.0	C11—C10—H10A	110.3
C9—N2—C10	122.6 (7)	O3—C11—O4	123.2 (8)
C9—N2—Cu1	118.8 (6)	O3—C11—C10	118.3 (7)
C10—N2—Cu1	115.5 (5)	O4—C11—C10	118.5 (8)
O1—C1—C6	122.5 (8)	C13—C12—C10	116.4 (7)
O1—C1—C2	118.7 (9)	C13—C12—H12A	108.2
C6—C1—C2	118.7 (9)	C10—C12—H12A	108.2
C3—C2—C1	120.9 (9)	C13—C12—H12B	108.2
C3—C2—H2A	119.5	C10—C12—H12B	108.2
C1—C2—H2A	119.5	H12A—C12—H12B	107.3
C2—C3—C4	122.0 (8)	C12—C13—C15	112.0 (9)
C2—C3—H3A	119.0	C12—C13—C14	109.6 (9)
C4—C3—H3A	119.0	C15—C13—C14	109.8 (10)
C3—C4—C5	116.6 (9)	C12—C13—H13A	108.5
C3—C4—C7	121.3 (8)	C15—C13—H13A	108.5
C5—C4—C7	122.0 (9)	C14—C13—H13A	108.5
C6—C5—C4	121.3 (9)	C13—C14—H14A	109.5
C6—C5—H5A	119.4	C13—C14—H14B	109.5
C4—C5—H5A	119.4	H14A—C14—H14B	109.5
C1—C6—C5	120.4 (9)	C13—C14—H14C	109.5
C1—C6—H6A	119.8	H14A—C14—H14C	109.5
C5—C6—H6A	119.8	H14B—C14—H14C	109.5
C4—C7—C8	114.8 (7)	C13—C15—H15A	109.5
C4—C7—H7A	108.6	C13—C15—H15B	109.5
C8—C7—H7A	108.6	H15A—C15—H15B	109.5
C4—C7—H7B	108.6	C13—C15—H15C	109.5
C8—C7—H7B	108.6	H15A—C15—H15C	109.5
H7A—C7—H7B	107.5	H15B—C15—H15C	109.5
Cu1—N1—C8—C9	−8.6 (8)	Cu1—O3—C11—C10	−1.9 (10)
N1—C8—C9—N2	−5.0 (10)	O3—C10—C11—N2	4.2 (10)
C8—C9—N2—Cu1	17.9 (9)	C10—C11—N2—Cu1	−172.0 (7)
C9—N2—Cu1—N1	−18.7 (7)	C11—N2—Cu1—O3	−2.7 (3)
N2—Cu1—N1—C8	14.0 (6)	N2—Cu1—O3—C10	4.5 (3)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···O2^iii	0.85 (4)	1.81 (4)	2.642 (8)	168 (12)
N1—H1C···O3^i	0.90	2.38	2.813 (7)	110
C15—H15B···G^iv	0.96	3.08	4.02	167

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