Nickel（Ⅱ）Coordination Preference Directed the Condensation Reaction of Schiff Base Ligands

YANG Tian－tian，ZHENG Bai－Feng，XIANG Jing （College of Chemistry and Environmental Engineering

Yangtze University，Hubei Jingzhou434023）

1 Introduction

Metal complexes bearing Schiff bases play important roles in the development of coordination chemistryfor over half a century，mainly due to their easy preparation，facile tunability and stabilization of high oxidation state of metal complexes.In particular，some manganese and ruthenium complexes bearing various Schiff base ligands are widely used as active catalyst for various organic transformations such as cyclopropanation［1］，alkene epoxidation［2］，aziridination［3］，sulfimidation［4］，and Diels－Alder reactions［5］.

As the coordinated ligands around metal ions in natural systems are unsymmetrical，the design，synthesis，and characterization of metal complexes containing unsymmetrical Schiff base ligands have attracted wide attentions［6－7］.The symmetrical tetradentate Schiff bases of 1，2－diamines with o－h(huán)ydroxyaldehydes／ketones are prepared and studied intensively.However，the unsymmetrical tetradentate Schiff bases derived from 1，2－diamines and different aldehydes／ketones are much less studied.It is worthwhile to mention that this class of asymmetrical Schiff bases are usually more difficult to obtain and isolate，as compared with the symmetric ones［8］.However，the metal complexes bearing the unsymmetrical Schiff bases ligands present significantly different reactivities［9］.Many 3d－metal ions such as Cu （Ⅱ）and Ni（Ⅱ）could be used as the template to synthesize the compounds containing the in－situ formed macrocyclic ligands，where the coordination preference of the metal ions controlls the ligand formation［10］.In order to further extend the template effect of 3d－metal ions towards the condensation of Schiff base ligands，a series of diamines and acetylpyridine（ACPY）are used as the starting material to investigate the reaction.Herein，the linear diamines including NH2NH2，ethylenediamine（EDA），diethylenetriamine（DETA），triethylenetetramine（TETA）and tetraethylenepentamine（TEPA）are selected as starting materials to react with the ACPY to investigate the dominated effect of Ni（Ⅱ）ion.Five mononuclear nickel（Ⅱ）compounds，［Ni（L1）3］（ClO4）2（1），［Ni（L2）2］ （ClO4）2（2）， ［Ni （L3）2］ （Cl）2·H2O （3），［Ni （L4）］ （ClO4）2（4）and［Ni （L5）］ （ClO4）2（5）are obtained and structurally characterized by X－ray crystallography（The structures of ligands L1－L5are shown in Figure 1）.In the case of NH2NH2，EDA and TEPA，the formed ligands are mono－azomethine ligands，L1，L2and L5with a—NH2group，whereas TETA is condensed with ACPY to give a bis－azomethine ligand L4.More interestingly，DETA is not condensed with ACPY to give corresponding Schiff base ligands in this condition.The above results indicate that Ni ion has a dominated effect on the formation of Schiff base ligands，which could be used as the potential synthetic strategy for synthesis of complexes containing the unsymmetrical Schiff base.

2 Experiment

2.1 Reagents and instrumentation

All the solvents are purified by conventional procedures and distilled prior to use.All other chemicals are of reagent grade and used without further purification.All manipulations are performed without precaution to exclude air or moisture unless otherwise stated.Infrared spectra from KBr pellets are collected on a Nicolet Avatar 360FTIR spectrometer in the range 4000－400cm－1.Electrospray ionization mass spectrometry（ESI－MS）is performed by using a PE－SCIEX API 365triple quadruple mass spectrometer.Elemental analyses of C，H and N are determined with a Perkin－Elmer 2400C Elemental Analyzer.Magnetic measurements are performed at room temperature by using a Sherwood magnetic balance（MarkⅡ）.

2.2 Synthesis of［Ni （L1）3］（ClO4）2 （1）

ACPY（2mmol，240mg），NH2NH2·H2O （2mmol，100mg），NiCl2·6H2O （0.75mmol，178mg）and NaClO4（1.5mmol，183mg）are added into methanol（30ml） .The mixtures are heated at reflux for 5hto generate the red precipitation，which was air－dried and collected.The product is dissolved in CH3CN （20ml）and slow evaporation of the CH3CN solution of 1gives the red block crystals suitable for X－ray crystallography.Yield：（330mg，75%）.Elemental analysis calcd（%）for C21H27Cl2N9NiO8：C，38.21；H，4.06；N，18.99；Found：C，38.04；H，4.10；N，19.01%.ESI／MS：m／z：231.6［M］2＋；562.1［M＋ClO4］＋.Selected IR（KBr）：ν（N—H）3403，3313cm－1；ν（C═ N）1601cm－1；ν（N—Ni）629cm－1；ν （Cl—O）1110cm－1.

2.3 Synthesis of ［Ni（L2）2］（ClO4）2 （2）

Complex 2was obtained by similar methods to that of 1except that ethylenediamine（EDA，2mmol，120mg）is used instead of hydrazine.Yield：（294mg，70%） .Elemental analysis calcd （%）for C18H26Cl2N6NiO8：C，37.02；H，4.49；N，14.39.Found：C，37.24；H，4.52；N，14.50%.ESI／MS：m／z：192.1［M］2＋；483.1［M＋ClO4］＋.Selected IR（KBr）：ν（N—H）3338，3289cm－1；ν（C═ N）1593cm－1，ν （N—Ni）624cm－1.

2.3 Synthesis of［Ni（L3）2］Cl2·H2O （3）

Complex 3is obtained by similar methods of 1except that diethylenetriamine（DETA，2mmol，206mg）is used instead of hydrazine.Yield：（254mg，72%）.Elemental analysis calcd（%）for C8H26Cl2N6Ni·H2O：C，27.15；H，7.97；N，23.74%.Found：C，27.00；H，8.01；N，23.22.483.1.ESI／MS：m／z：132.1［M］2＋；363.1［M＋Cl］＋.Selected IR（KBr）：ν （O—H）3438，3349，3322cm－1；ν （N—H）3285，3263cm－1；ν （C—H）3185，3166cm－1.

2.4 Synthesis of［Ni （L5）］（ClO4）2 （4）

Complex 4is obtained by similar methods of 1except that triethylenetetraamine（TETA，2mmol，292mg）is used instead of hydrazine.Yield：（390mg，64%）.Elemental analysis calcd（%）for C20H28Cl2N6NiO8：C 39.37，H 4.63，N 13.78% .Found：C 39.45，H 4.52，N 13.55%；ESI／MS：m／z：205.1［M］2＋；509.1［M＋ClO4］＋.Selected IR（KBr）：ν（N—H）3142cm－1，ν（C═ N）1593cm－1；ν（N—Ni）629cm－1.

2.5 Synthesis of［Ni （L4）］（ClO4）2 （5）

Complex 5is obtained by similar methods of 1except that tetraethylenepentaamine（TEPA，2mmol，378mg）is used instead of hydrazine.Yield：（638mg，58%）.Elemental analysis calcd（%）for C15H28Cl2N6NiO8：C 32.76，H 5.13，N 15.28%）；Found：C 32.89，H 5.21，N 15.19%；ESI／MS：m／z：175.1［M］2＋；449.1 ［M＋ClO4］＋.Selected IR （KBr）：ν （N—H）3350，3305cm－1；ν （N—H）3199cm－1；ν（C═ N）1597cm－1；ν（N—Ni）629cm－1.

2.6 Structure determination

Measurements were collected on a Bruker CCD diffractometer using graphite－monochromated MoKa radiation（λ＝0.71073?）for all the compounds.Details of the intensity data collection and crystal data are given in Table 1.Absorption corrections are done by the multi－scan method.The structures are resolved by the heavy－atom Patterson method or direct methods and refined by full－matrix least－squares using SHELX－97and expanded by using Fourier techniques［11］.All non－h(huán)ydrogen atoms are refined anisotropically.Hydrogen atoms are generated by the program SHELXL－97.The positions of hydrogen atoms are calculated on the basis of riding mode with thermal parameters equal to 1.2times that of the associated C atoms，and participated in the calculation of final R indices.All calculations are performed by using the texsan crystallographic software.All the complexes except 3have two perchlorates as the counter－ions，which are seriously disorder and restrained to a standard geometry.CCDC references numbers（615485～615487and 284523）.

Table 1 Crystal data and structure refinement details for compounds 2－5

3 Results and discussion

3.1 Synthesis and characterization of complexes 1－5

In order to investigate the template effect of Ni（Ⅱ）ion towards the condensation reactions of Schiff base ligands，all the reactions are conducted in one－pot.Compounds 1－5are obtained with high yields by treatment of ACPY，hydrated NiCl2，and corresponding diamines in the presence of NaClO4.Compound 3has been previously reported by us as its special 1－D helical［Cl…H2OH－bonding chain in the solid state［12］.These complexes are isolated as paramagnetic，air－stable，block crystals.The IR spectra of these compounds show the sharpν（N—H）stretching bands in the range of 3200～3400cm－1.Except that of 3，theν（C═ N）stretching bands locate in the range of 1593～1601cm－1andν（Cl—O）stretching bands of anion ClO－4occur around 1090cm－1.The IR spectrum of 3does not show theν（Cl—O）stretching bands of anion ClO－4like the other compounds，but it shows a strong stretching band in the 3200～3600cm－1region that origins from a 1－D helical Cl－…H2O chain.This result further confirms the stability of such anionic chain as discussed previously［13］.Compounds 1－5have a room－temperature magnetic moment in the range 2.80～2.95μB（Gouy method，solid sample），which are consistent with their formulations as the high spin d8NiII complexes in a six－coordination octahedral symmetry.Their geometric structures could be further confirmed by their crystal structural characterizations（see the below section）.The ESI／MS in MeOH （＋mode）of these complexes show two predominant peaks at m／z231.6，562.1 in 1，m／z192.1and 483.1in 2，m／z205.1and 509.1in 4，m／z175.1and 449.1in 5，which are assigned to parent cation［M］2＋and［M＋ClO4］＋，respectively.The ESI／MS of 3in MeOH（＋mode）shows two predominant peaks at m／z132.1，363.1，which are assigned to［M］2＋and［M＋Cl］＋，respectively.

Table 2 Selected bond lengths（?）and angles（°）for compounds 2－5

3.2 Metal dominated formation of ligand L1－L5

Herein，the template effect of Ni（Ⅱ）ion towards the condensation reactions of ACPY with these diamines are emphasized.Usually，condensation of aldehyde or ketone with amines is facile.Thus，the symmetric bis （azomethine）ligands should be easily prepared，but such class of asymmetrical Schiff bases are always more difficult to be obtained，especially laborious to isolate；however，the condensation of ACPY with diamines is controlled by the coordination preference of Ni（Ⅱ）ion that readily forms a steady six－coordinated octahedral geometry.The L1in 1bonds the Ni（Ⅱ）ion via the a pyridine and an imine N atoms，which actually act as a bidentate ligand.The L2in 2bonds the Ni（Ⅱ）via a pyridine，an imine and an amine N atoms and acts as a tridentate ligand.Both the L4in 4（two pyridine，two imine and two secondary amine N atoms）and L5in 5 （apyridine，an imine，three secondary amine and a primary amine N atoms）act as hexadentate ligands.More importantly，the diamine ligand L3does not react with ACPY in this condition.The dentate number of these ligands L1－L5is the submultiple of six.Moreover，in order to further investigate the dominated effect of the Ni（Ⅱ）ion，different mole ratios of starting ACPY and diamines are tried.All the reactions are monitored by the ESI／MS investigations and these experiments shows the same results；namely，these reactions always give exclusively the same products.These data further indicate the significant template effect of Ni（Ⅱ）ion.It is well known that a range of coordination geometries are observed for Ni（Ⅱ）complexes with coordination numbers from 4to 6 being common；octahedral and square plannar geometries are most usual.For the present cases，the metal centres are all coordinated in the distorted octahedral geometries，possibly due to the flexibility of linear polyamines.The other 3dmetal ions such as Zn （Ⅱ）and Cu （Ⅱ）possessing similar coordination preference are used instead of Ni（Ⅱ）；similar results are obtained.Usually，it is difficult to obtain the unsymmetric Schiff base of diamines due to their hard control and laborious purification.Herein，the condensed products are controlled by the coordination preference of metal ions.Thus，this method may be used as a potential synthetic strategy for the unsymmetric Schiff base ligands.

3.3 Structural description of complexes 1－5

Complexes 1－5are determined by X－ray crystallography.However，compound 1shows serious disorder and thus the bond parameters around the Ni（Ⅱ）are not available.Their cationic structures are shown in Figure 1 and Figure 2.Selected bond parameters of these compounds are summarized in Table 2.All the complexes are mononuclear and the Ni（Ⅱ）ions are six－coordinated in distorted octahedral geometries，which are completed by six nitrogen atoms of L1－L5.The bond lengths of Ni－Npyare in the range of 2.008 （3）～2.121 （3）?，which are equivalent to those of Ni－N（imine）in the narrow range of 2.012 （3）～2.088 （8）?.These bond lengths are slightly longer than those of Ni－N（amine）in the range of 2.081 （1）～2.158 （1）?. These parameters are comparable with the previous reported related nickel （Ⅱ）complexes［8，13－14］.The bite angles of N－Ni－N of these complexes in the range of 77.1 （2）～81.6 （2）°are significantly smaller than 90°，in line with the distorted octahedrons in complexes 1－5.

Fig.1 Structures of L1－L5

Fig.2 The cationic structures of compounds 2（a），3（b），4（c）and 5（d）.Thermal ellipsoids are drawn at 30%probability and H atoms are omitted for clarity

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