The effect of solvent upon the structure of the linear trinuclear copper complex dichloro-tetrakis[m₃-bis(2-pyridyl)amido] tricopper(II)·ether

Received on Jul. 27, 2003. Supported by the National Natural Science Foundation of China(No. 20261004).

Abstract The linear trinuclear copper complex [Cu₃(m₃-dpa)₄Cl₂·ether], which involves solvent ether molecule in its unit cell, is apparently different from the previous reports. The differences are determined on both its crystal cell parameters and molecular structure. For the titled compound, the crystal system is monoclinic, space group P2(1)/c, Z = 4, and the crystallographic twofold symmetry has been deviated distinctly due to the effect of solvent molecule included. The bond lengths of Cu(1)-Cu(2) and Cu(2)-Cu(3) are not completely equal, Cu-Cl distances in both ends, with 2.4404(8)Å and 2.4740(8) Å respectively, are distinctly different.
Keywords solvent effect, complex structure, linear trinuclear copper complex

1. INTRODUCTION     
Metal string complexes are mostly used in the fundamental study of metal-metal interactions^[1,2] and their potential application as a molecular metal wire. In our past effort, a series of polynuclear metal string complexes have been successfully synthesized and characterized^[3-6]. The linear trinuclear copper(II) complex is the simplest metal string that was synthesized early by Brian Hathaway^[7] and Gloria J. Pyrka^[8], in which the Cu-Cu-Cu bond angle is very close to 180° and the central Cu atom occupies a two-fold special position. Here we report a trinuclear copper string complex that involves solvent molecule in its unit cell, which is apparently different from that of the previous reports due to the effect of the included ether molecule in cell units.

2. EXPERIMENTAL
2.1 Synthesis of the complex
Dpyam (Di-2-pyridylamine, 0.25 g, 1.46 mmol) and CuCl(0.18g, 1.8mmol) were placed in an Erlemeyer flask, and DMF(25 mL) was added as solvent. The mixture was heated for 10min at 120-130ºC, a solution of potassium t-butoxide (0.164 g, 1.46 mmol) in n-butanol (2 mL) was added dropwise. Heating lasted for 30 min, then kept the mixture overnight in air, reoxygenation to copper(II) species occurred, the oxidation of the metal centre resulted in the deprotonation of the dipyridylamine ligand at the amine nitrogen, the mechanism has been described in the literatures [7]. Evaporating and removing the solvent, then extracting the remaining solid with dichloromethane and re-crystallized from dichloromethane/ether, we obtained a black single crystal. MS(FAB) [m/z(%)] displays the mother peaks round about 941. IR(cm^-1): 1601(s), 1592(s), 1547(m), 1464(s), 1371(s), 1278(m), 1150(m), 758(s), 736(m).
2.2 Determination and analysis of the crystal structure
The unit-cell data and intensities were collected on Nonius KappaCCD diffractometer with graphite-monochromated Mo K_a radiation (l = 0.7107 Å). The structures were solved and refined by using the SHELXS-97 and SHELXTL-97 package, respectively. An empirical absorption correction based on three azimuthal scans was also applied. The structures were solved using direct methods and difference Fourier techniques and refined by least-squares analysis. The non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were included in an idealized geometry but not refined. The crystal data are summarized in Table 1. In which Hathaway's data about compound [Cu₃(m₃-dpa)₄Cl₂·2H₂O] have been cited^[7] and compared with our experimental data about compound [Cu₃(m₃-dpa)₄Cl₂·ether].

Table 1 Crystal data and structure refinement for two complexes

	Fig. 1 ORTEP plot with 50% probability ellipsoids of [Cu3(dpa)4Cl2]·ether, perpendicular to the Cl-Cu-Cu-Cu-Cl axis.
	Fig. 2 ORTEP plot with 50% probability ellipsoids of [Cu3(dpa)4Cl2]·ether, along the Cl-Cu-Cu-Cu-Cl axis.

3. RESULTS AND DISCUSSION
The structure of the complex [Cu₃(m₃-dpa)₄Cl₂ {dpa=di-2-pyridylamino, [(C₅H₄N)₂N]^-}] is shown in Fig. 1 (perpendicular to the Cl-Cu-Cu-Cu-Cl axis) and Fig. 2 (along the copper-copper axis), both included solvent ether molecule in unit cell. As other metal string complexes consisted of polypyridylamido ligand, the three Cu^II ions and the two chloride ions are collinear with all the angles of  M-M-M being ~180° and the linear metal chain is wrapped by four all-syn of deprotonated oligo-a-pyridylamido ligands. The mean central Cu-N distances are below 2.0 Å, while the mean terminal Cu-N distances are greater than 2.0 Å(see Table 2). These data are similar to the previous descriptions.
    By scrutinizing, we find that the Cu(1)-Cu(2) and Cu(2)-Cu(3) distances, with 2.4691(5) and 2.4774(5) Å respectively, are not completely equal, Dd=0.0083 Å, this value has gone beyond normal error range, furthermore the Cu-Cl lengths in both ends of metal string, with 2.4404(8) and 2.4740(8) Å respectively, have more evident differences. Selected geometric parameters have been listed in Table 2.

Table 2 Selected bond distances (Å) and angles (°)

Cu(1)-Cu(2)	2.4691(5)	Cu(2)-Cu(3)	2.4774(5)
Cu(1)-Cl(1)	2.4404(8)	Cu(3)-Cl(2)	2.4740(8)
Cu(1)-N(1)	2.065(3)	Cu(2)-N(2)	1.962(2)
Cu(3)-N(3)	2.076(3)	O(1)-Cu(3)	6.594
Cu(1)-Cu(2)-Cu(3)	178.94(2)	N(1)-Cu(1)-Cu(2)	81.99(7)
N(2)-Cu(2)-Cu(1)	90.29(7)	N(3)-Cu(3)-Cu(2)	81.49(7)
Cl(1)-Cu(1)-Cu(2)	177.56(3)	Cl(2)-Cu(3)-Cu(2)	178.95(3)

    It is clear that the complex molecule structure has deviated from crystallographic twofold symmetry described in Hathaway and Pyrka's paper. We deduce that the solvent molecule involved in the unit cell resulted in the change of the complex molecular geometry. As shown in Fig.1, solvent ether molecule is lying on Cu(3) side of the Cl(1)-Cu(1)-Cu(2)-Cu(3)-Cl(2) chain and Cu(3)-O(1A) distance is 6.58 Å. It is noteworthy that intermolecular hydrogen bonds would impact on the geometry of the complex. First, the second ether molecule would elongate bond length of Cu(2)-Cu(3) lying on this side through hydrogen bond C(19)-H(19A)-O(1A) with distance D-A 3.326(4) Å [here, the O(1A) is an atom from adjacent symmetry-related unit]. Second, two terminal Cl atoms are taken as acceptor and form intermolecular hydrogen bonds C(2)-H(2)-Cl(1) and C(39)-H(39A)-Cl(2), respectively [also, the C(2), H(2), C(39) and H(39A) are atoms from adjacent symmetry-related unit], the unequals on hydrogen bonds action would result in the difference of bond lengths Cu(1)-Cl(1) and Cu(3)-Cl(2) on two ends, the multi-hydrogen bonds would consequently affect the structure symmetry of the complex Cu₃(m₃-dpa)₄Cl₂. Related data are listed in Table 3. Similar cases can be found in compounds [Co₃(m₃-dpa)₄Cl₂·CH₂Cl₂], [Co₃(m₃-dpa)₄Cl₂·2CH₂Cl₂] and [Co₃(m₃-dpa)₄Cl₂·2CH₂Cl₂·H₂O] ^[9,10], the first one is symmetric geometry and the latter two are unsymmetric structures.
    The difference of involved solvent molecule in unit cell also resulted in the difference of the crystal cell parameters. Related data can be examined in Table 1, in which, the data of complex [Cu₃(m₃-dpa)₄Cl₂·2H₂O] are Hathaway's results⁷. It is obvious that many parameters such as crystal system, space group, unit cell dimensions and so on are different, by this token, it is not suitable to distinguish crystal structure just by cell parameters.

Table 3. Related hydrogen-bonding geometry(Å, °)

D- H··· A	D- H	H··· A	D---A	D- H··· A
C(2)- H(2)··· Cl(1)	0.95	2.80	3.681(3)	154
C(39)- H(39A)··· Cl(2)	0.95	2.76	3.676(3)	162
C(19)- H(19)··· O(1)	0.95	2.52	3.326(4)	143

4. CONCLUSION
The solvent species and the amount involved in crystal unit cell would result in the difference of the crystal cell parameters. Also, the complex structure and geometry maybe changed in varying degree because of the interaction between complex and solvent molecules through the hydrogen bonds and Van der Waals forces.

REFERENCES
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[3] Sheu J T, Liu C C, Peng S M et al. Chem. Commun., 1996, 315.
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[5] Chang H C, Lin T W, Peng S M. European Journal of Inorganic Chemistry, 1999, 1243.
[6] Peng S M, Wang C C, Jang Y L et al. J. Magnet. & Magnet. Mater., 2000, 209, 80.
[7] Wu L P, Field P, Hathaway B. J. Chem. Soc. Dalton Trans., 1990, 3835.
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[10] Cotton F A, Daniel G T et al. Chem. Commun. 1997, 421.

   

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