Synthesis,Crystal Structure and Properties of Monomeric Copper（Ⅱ）Complexes of Pyrrole-Containing Tridentate Schiff-Base Ligands

LI Rong-Qing ZHAO Pu-Su ZHANG Yu ZHANG Zai-Chao LIU Zhong-Yu ZHAO Zhen

(Jiangsu Key Laboratory for the Chemistry of Low-Dimensional Materials,School of Chemistry and Chemical Engineering,Huaiyin Normal University,Huai′an,Jiangsu 223300,China)

Synthesis,Crystal Structure and Properties of Monomeric Copper（Ⅱ）Complexes of Pyrrole-Containing Tridentate Schiff-Base Ligands

LI Rong-Qing＊ZHAO Pu-Su ZHANG Yu ZHANG Zai-Chao LIU Zhong-Yu ZHAO Zhen

(Jiangsu Key Laboratory for the Chemistry of Low-Dimensional Materials,School of Chemistry andChemical Engineering,Huaiyin Normal University,Huai′an,Jiangsu223300,China)

Two copper（Ⅱ） complexes of the homologous tridentate Schiff-base ligands(L1)-and(L2)-,[CuL1I](1) and[CuL2I](2),have been synthesised and characterised by IR,ES-MS,UV-Vis,single crystal X-ray diffraction and thermogravimetric analysis((L1)-and (L2)-are the deprotonated forms of 2-(2-pyridyl)-N-[1-(1H-pyrrol-2-yl) ethylidene]ethanamine(HL1)andN-[1-(1H-pyrrol-2-yl)ethylidene](2-pyridyl)methanamine(HL2),respectively).X-ray crystal structure determination carried out on 1 reveals that it is a distorted square planar copper（Ⅱ）monomer. The crystal structure indicates that the intermolecular interaction is also a factor which can influence the structural properties of copper（Ⅱ）complexes in addition to ligand flexibility and steric constraints.CCDC:862124.

copper（Ⅱ）complex;Schiff-base;pyrrole;tridentate ligand

Over the years much attention has been paid to the complexes of a wide range of acyclic Schiff-base ligands,in particular the pyridine-containing systems. However,much less interest has been attracted to the complexes of pyrrole-analogues of such ligands,despite the potentially interesting similarities to porphyrins. Recently,our attention has been turned to the copper（Ⅱ）chemistry of NNN-donor tridentate Schiff base ligands, in particular the mixed pyrrole-imine-pyridine ligands. Ligands(L1)-,(L2)-,(L3)-and (L4)-,the deprotonated forms of 2-(2-pyridyl)-N-[1-(1H-pyrrol-2-yl)ethylidene] ethanamine (HL1),N-[1-(1H-pyrrol-2-yl)ethylidene](2-pyridyl)methanamine(HL2),2-(2-pyridyl)-N-(1H-pyrrol-2-ylmethylene)ethanamine (HL3)andN-(1H-pyrrol-2-ylmethylene)(2-pyridyl)methanamine(HL4)(Scheme 1), are of this type.These ligands are closely related to thepyridine-imine-pyridine ligands,which have been well studied[1-7].Recently,using ligands (L1)-to (L4)-and halide or pseudohalide coligands,we reported a series of monomeric,dimeric and one-dimensional chain polymeric copper（Ⅱ） complexes[8-13].These examples, together with those obtained with the pyridine-iminepyridine ligands mentioned above[1-7],show that the exact nature of the tridentate ligand used and the choice of the bridging halide or pseudohalide coligands influence the structural diversity of metal assembled complexes.These dimeric and polymeric complexes can exhibit interesting magnetic properties.

Scheme 1 Selection of tridentate Schiff-base ligands, including the ligands used in this research

Ligands (L1)-to (L4)-are very closely related to each other,with(L1)-and(L3)-being more flexible than (L2)-and (L4)-.In addition,attachment of a methyl substituent to the C atom of the imino C=N bond of(L3)-and(L4)-,which results in(L1)-and(L2)-,imposes some steric constraints on(L1)-and(L2)-.These differences in ligand flexibility and steric constraints are expected to influence the structural properties of the complexes of these ligands,e.g.the formation of a mono-,di-or polymeric complex.Crystal structures of our previously reported complexes of these ligands indicate that use of the ethylene-linked ligands, (L1)-, (L3)-and the brominated form of (L3)-,leads to the formation of halide-orpseudohalide-bridged dimeric oronedimensional chain polymeric copper（Ⅱ）complexes while the use of the methylene-linked ligands,(L2)-and(L4)-, tends to result in the formation of monomeric copper（Ⅱ）complexes.As a part of our ongoing study on the complexes of pyrrole-containing tridentate Schiff base ligands,here we wish to report the synthesis,structures and properties of two copper（Ⅱ）complexes of(L1)-and (L2)-in the form of[CuL1I](1)and[CuL2I](2).Crystal structure of 1 clearly shows that it is a monomeric iodo copper（Ⅱ）complex,rather than an iodo-bridged dimeric copper（Ⅱ）complex.This is different from the structures observed for our previously reported chloro and bromo copper（Ⅱ）complexes of(L1)-and (L3)-,which are all chloro-or bromo-bridged dimeric copper（Ⅱ）complexes.

1 Experimental

1.1 Materials and instruments

All chemicals and solvents were of reagent grade and were used as received.Infrared spectra were obtained on a Nicolet Avatar 360 FTIR spectrometer as pressed KBr discs.MS spectra were collected on a ThermoFinnigan LCQ Advantage with an ESI probe. UV-Vis spectra were obtained on a GBC UV-Vis 916 spectrophotometer.Thermogravimetric measurements were carried out on a Mettler Toledo TGA/SDTA851e thermogravimetric analyser from room temperature to 1 000℃ in air with a heating rate of 10℃·min-1. Ligands HL1and HL2wereprepared usingour previously described procedure[9-10].

1.2 Synthesis of compounds 1 and 2

Compound 1:To a solution of ligand HL1(0.438 mmol)in methanol(5 mL)was added triethylamine (0.416 mmol)in methanol(2 mL).To this resulting solution was added a blue solution of copper（Ⅱ）tetrafluoroborate tetrahydrate(0.410 mmol)in methanol (2 mL),giving a deep green solution.Subsequent addition of potassium iodide(0.408 mmol)in methanol (2 mL)led to the formation of a precipitate.The resulting mixture was stirred overnight after which the solid was collected by filtration,washed with methanol and dried in vacuo(yield 0.110 g,67%based on copper（Ⅱ） tetrafluoroborate tetrahydrate used).Single crystals of complex 1 suitable for X-ray determination were obtained by vapour diffusion of diethyl ether into a dichloromethane solution.FTIR(KBr disc,cm-1):3123, 1568,1529,1437,1390,1342,1302,1031,751.ESMS(m/z):403.6[CuL1I+1]+,274.6[CuL1]+.

Compound 2:To a solution of ligand HL2(0.239 mmol)in methanol(5 mL)were added triethylamine (0.215 mmol)in methanol (2 mL)and copper（Ⅱ）tetrafluoroborate tetrahydrate(0.213 mmol)in methanol (2 mL).Subsequent addition of potassium iodide(0.213 mmol)in methanol(2 mL)led to the formation of a precipitate.The resulting mixture was stirred overnight after which the solid was collected by filtration,washed with methanol and dried in vacuo(yield 0.0511 g,62% based on copper(II)tetrafluoroborate tetrahydrate used).FTIR(KBr disc,cm-1):3422,1599,1529,1387,1345, 1 293,1 032,750.ES-MS(m/z):387.1[CuL2I-1]+,261 [CuL2]+.

1.3 X-ray crystallography

All X-ray data were collected on a Bruker Smart ApexⅡCCD diffractometer using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm).The data were reduced using the SAINT program and semiempiricalabsorption corrections (SADABS)were applied.The crystal structure was solved by direct methods and refined on F2by full-matrix least-squares methods with the SHELXTL-97 program[14].All nonhydrogen atoms were made anisotropic.Hydrogen atoms were placed at calculated positions and rode on the atoms to which they are attached (including their isotropic thermal parameters which were equal to 1.2 times the equivalent isotropic displacement parameter for the attached non-hydrogen atom).A summary of the key crystallographic information is given in Table 1.

CCDC:862124.

Table 1 Crystal data and structure refinement for complex 1

2 Results and discussion

2.1 Synthesis and characterisation

The homologous Ligands,HL1and HL2,are potentially two tridentate Schiff-base ligands containing both a pyridine and a pyrrole moiety.The pyrrole NH moiety in these ligands can be deprotonated in the presence of a base and this leads to tridentate coordination of(L1)-and(L1)-to the metal ion.

The copper（Ⅱ） complexes of the deprotonated tridentate ligands(L1)-and(L2)-,[CuL1I](1)and[CuL2I] (2),were obtained as precipitates by mixing HL1or HL2, triethylamine,copper（Ⅱ） tetrafluoroborate and potassium iodide in methanol.The X-ray crystal structure determination reveals that complex 1 is a monomer [CuL1I](Fig.1).Due to the likely similarities between 2 and 1 and our previously reported structurally characterised chloro copper（Ⅱ）monomer of the ligand (L2)-, [CuL2Cl](3)[10],the iodo complex 2 is also proposed to be a monomer[CuL2I].

Fig.1 Perspective view of the monomeric complex 1 with thermal ellipsoids drawn at the 30%probability level

In the infrared spectra of complexes 1 and 2 the imine bands occur at 1 568 and 1 599 cm-1,respectively.These values are very close to those found for the other halido or pseudohalido copper（Ⅱ） complexes of ligands(L1)-and(L2)-,[Cu2(L1)2(μ-Cl)2](4)(1 574 cm-1)[9], [CuL2Cl](3)(1 601 cm-1)and[CuL2(NCS)](5)(1 605 cm-1)[11],and also close to those observed for the monomeric and dimeric copper（Ⅱ）complexes of ligands (L3)-and(L4)-,[Cu2(L3)2(μ-X)2](X=Cl-,6;X=Br-,7;X=,8;X=NCS-,9)[8],[CuL4X](X=Cl-,10;X=Br-,11) and[Cu2(L4)2(μ1,3-NCS)2](12)[12].

The electrospray mass spectra of the complexes [CuL1I](1)and[CuL2I](2)show similar fragmentation patterns.The fragments [CuL1]+and [CuL2]+are associated with the loss of their respective coligand, iodide ion.Complexes 1 and 2 also have the fragments [CuL1I+1]+and [CuL2I-1]+,respectively,the latter provides support that 2 is a monomeric copper（Ⅱ）complex.

The UV-Vis spectra of complexes 1 and 2 in N,N-dimethylformide solution exhibit a broad absorption band,centred at 628 nm for 1 and at 611 nm for 2 due to the copper（Ⅱ）d-d transition.In addition to this visible d-d transition,an intense band,at 369 nm for 1 and 379 nm for 2,is found,due to either charge transfer or π-π* transition.

2.2 Crystal structure description

The structure of complex 1 is shown in Fig.1 and the selected bond lengths and angles are listed in Table 2.It consists of isolated neutral monomeric[CuL1I] molecules.The copper（Ⅱ）ion in the structure is bound to three nitrogen atoms(comprised of one deprotonated pyrrole nitrogen donor,one pyridine nitrogen donor and one imine nitrogen donor)of the deprotonated tridentate ligand (L1)-and to one iodide ion,giving a distorted square planar N3I coordination sphere.In contrast to the formation of an iodo copper（Ⅱ）monomer,1,with ligand (L1)-,crystal structures of the chloro or bromo copper（Ⅱ）complexes of ligands(L1)-and (L3)-show that they are all doubly chloro-or bromo-bridged square pyramidal copper（Ⅱ）dimers,[Cu2(L1)2(μ-Cl)2](4)[9],[Cu2(L3)2(μ-Cl)2] (6)and[Cu2(L3)2(μ-Br)2](7)[8].So the coordination mode and geometry about the copper（Ⅱ）ion in the monomeric complex 1 are different from those observed for the dimeric complexes 4,6 and 7,but are similar to those found for the monomeric copper（Ⅱ）complexes of ligands (L2)-and(L4)-,[CuL2Cl](3)and[CuL4Cl](10)[12].However, the low completeness of the dataset for complex 6 prevents detailed comparison ofthe geometrical parameters between structures 1 and 6.

Of the three Cu-N bond lengths in 1,the shortest one(N1-Cu1)occurs between the copper atom and the deprotonated negatively charged pyrrole nitrogen atom. This phenomenon was also seen in 3,4,7 and 10.The lengths of other two bonds(N2-Cu1 and N3-Cu1)are the same within experimental error and are slightly longer than that of N1-Cu1 bond.As expected,in 1 the N2-Cu1-N3 angle is much larger than the N1-Cu1-N2 angle since the former forms in the six-membered pyridine-imine chelate while the latter forms in the fivemembered pyrrole-imine chelate.The two cis N-Cu-I angles in 1 are similar to each other but both are bigger than a right angle.The same phenomenon about the four cis basal angles was also observed in 3,4,7 and 10.These bond lengths and angles in 1 are similar to the corresponding ones in 3,4,7 and 10 and also compare well with the values reported for related copper（Ⅱ）complexes[2,15-18].

Within the ligand (L1)-in complex 1 the pyridine and pyrrole rings are not coplanar and the twist angle between the mean planes of these two rings is 61.1(2)°, which is much larger than it in complexes 4 and 7 (44.3(2)°for 4 and 37.3(1)°for 7).This twist is facilitated by the flexibility afforded by the ethylene group.

Table 2 Selected bond lengths(nm)and angles(°)for complex 1

In addition an intermolecular π-π interaction is observed between the two parallel pyridine rings (dihedral angle of 0°)of two neighbouring molecules of complex 1(Fig.2).The perpendicular distance between the mean planes of these two parallel pyridine rings is 0.344 nm and the distance between centroids of these two rings is 0.357 nm.

Fig.2 A view of the crystal packing of complex 1 down the b axis

Ligands (L1)-to (L4)-are clearly very closely related to each other,however,complexation of them with Cu（Ⅱ）ions in the presence of a coligand,Cl-,Br-or I-,gives two different structural types,monomer and dimer.Examination of these ligands shows that(L1)-and (L3)-,with ethylene links between the imine nitrogen and the 2-pyridyl ring of ligands,have similar flexibility but both ligands are more flexible than (L2)-and (L4)-which feature a methylene link.Correspondingly, copper（Ⅱ） dimer[Cu2(L1)2(μ-Cl)2](4)forms with(L1)-, and dimers[Cu2(L3)2(μ-Cl)2](6)and[Cu2(L3)2(μ-Br)2](7) form with(L3)-,while copper（Ⅱ）monomers[CuL2Cl](3) and[CuL4Cl](10)were obtained with(L2)-and(L4)-, respectively.It is probably the reduced flexibility of (L2)-and (L4)-that results in the formation of such monomers.However,the iodo copper（Ⅱ）complex of the more flexible ligand (L1)-,1,is also a copper（Ⅱ）monomer [CuL1I],rather than an iodo-bridged dimer [Cu2(L1)2(μ-I)2].This is different from the chloro-or bromo-bridged dimeric structures of 4,6 and 7.Comparison of the structural parameters and the packing of complex 1 with those of complexes 4 and 7 suggests that it is probably the strong twisting of ligand (L1)-(twist angle of 61.1(2)°between the mean planes of the pyrrole and pyridine rings)and the packing effects(π-π stacking)in the unit cell(Fig.2),as discussed above, that result in the formation of such a copper（Ⅱ）monomer 1.

2.3 Thermal stability

Thermogravimetric analyses (TGA)were carried out on complexes 1 and 2 from room temperature to 1 000℃in air with a heating rate of 10℃·min-1.The TGA curves are shown in Fig.3.The TGA curve for complex 1 shows that it decomposed in four steps.The first and second steps in the temperature range of 25～320℃accompanied with weight loss of 13.62%(calcd.13.70%)which may be attributed to the decomposition of the linker between the pyrrole and pyridine rings and the loss of CCH3and CH2CH2groups.The next two steps,which occurred within the temperature range 320～680℃,may involve the loss of an iodide ion,gain of an oxygen atom and decomposition of the pyrrole and pyridine rings with observed weight loss of 56.18% (calcd.56.10%,the corresponding residue is presumably CuN3O).The final residue was calculated to be CuO with found weight loss of 9.05% (calcd.10.44%).The TGA curve for complex 2 shows a similar decomposition pattern to complex 1 and the final residue is also CuO.Comparison of the TGA curves of both complexes clearly shows that complex 2 is less stable than complex 1.

Fig.3 TGA curves of complexes 1 and 2

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含吡咯三齿席夫碱配体的单核铜（Ⅱ）配合物的合成、晶体结构和性质

李荣清＊赵朴素 张 宇 张载超 刘仲雨 赵 振

(淮阴师范学院化学化工学院，江苏省低维材料化学重点建设实验室，淮安 223300)

合成和表征了两个类似的三齿席夫碱配体(L1)-和(L2)-的铜（Ⅱ）配合物[CuL1I](1)和[CuL2I](2)(HL1为2-(2-pyridyl)-N-[1-(1H-pyrrol-2-yl)ethylidene]ethanamine；HL2为N-[1-(1H-pyrrol-2-yl)ethylidene](2-pyridyl)methanamine)，并用红外光谱、电喷雾质谱、紫外-可见光谱、单晶X-射线衍射和热重分析等手段对配合物进行了表征。对配合物1的X-射线晶体结构测定表明它是一个畸变的平面正方形单核铜（Ⅱ）配合物。晶体结构还表明，除了配体的柔性和空间限制外，分子间的相互作用也是影响铜（Ⅱ）配合物结构的因素。

铜（Ⅱ）配合物；席夫碱；吡咯；三齿配体

O614.121

A

1001-4861(2012)05-1025-06

2011-07-21。收修改稿日期：2011-12-30。

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