As I understand, the problem is that the compound does not crystallize at all. Perhaps you should avoid the hydrolysis of complex during slow evaporation. Try to perform the synthesis and crystallization under Ar. Using the combination of 'good' and 'bad' solvents (e.g. DMSO and CCl4, benzene and ethanol, etc.) is also very powerful technique.

please set  up a column suitable to your complex system based on literature, to purify the system, if it is any impurities are there, we may not get crystal by using above said methods. similarly, You try to complex with co(II) at room temp with 2 to 3 days(may be low B.P). 

Not sure on the nature of your compounds, but you could try, is  to evaporate all the solvent to check if you obtain a powder or an oil. if you obtain an oil but you think it should be a powder, then add a low boiling point solvent (in which your compound is completely insoluble). Then wash and evaporate the solvent a few times. This way you might be able to "break the oil" and get a crystalline product next time you try

As I understand, the problem is that the compound does not crystallize at all. Perhaps you should avoid the hydrolysis of complex during slow evaporation. Try to perform the synthesis and crystallization under Ar. Using the combination of 'good' and 'bad' solvents (e.g. DMSO and CCl4, benzene and ethanol, etc.) is also very powerful technique.

I totally agree with Dr. Tsymbarenko.

Dear Prof., I thankful for your nice ideas.

It's always useful to have a range of crystallisation techniques to hand: http://www.chem.uzh.ch/linden/sample_prep.html

Dear Raj Kumar,

I agree with the suggestions of the previous authors, but I like to add from my experience

There are several basic reasons why your oily compounds may not crystallize.

1. The compound is oil and will not crystallize at all.

2. Your compounds are insoluble in the solvent that you are using, and the solvent-combinations 

3. Your oily compound consists of several compounds that prevent each other from crystallizing.

 I would first test this hypothesis. There are several possibilities. I have experienced the efficiency of paper- chromatography; ideal would be to use a solvent that dissolved your oil. Another method that I used is column chromatography, and another one which is electro-chromatography. It is similar to paper -chromatography, but the paper is soaked in a conductive solution and a voltage is applied between the solution and the top of the soaked paper.

4. If your complex is positively or negatively charged, try different counter-ions.

5. Avoid solvents that may form complexes with the ligands of the compounds.

6. Read up on the controlled crystallizations of evaporation, cooling, and anti-solvent precipitations (‘the three sisters of crystallization’). There are four publications (including the Introduction) on my ResearchGate site. You can down-load them free.  Ingo     

Dear Raj kumar I suggest you one more technique which I personally tried and got good results. Instead of using a flask or beaker for the reaction, use glass viol for the reaction. It is borosilicate glass viol with a lid at top. And temperature of the reaction should be slightly less than the b.p of the solvent, and 1/3rd of the viol must be empty. use concentrated solution of the reactants in it. and put it in oven for 3 days. due to close container,  pressure is produced which enhances the chances of crystallization. If u tell me about the reactants, I could suggest u the solvents.

Dear Rajkumar.

Waqas has a good idea to change the solvent and heating conditions for the synthesis.

I had problems during my thesis to synthesize dyes. Of course, changing counter-ions is a viable approach. I am hoping that you tried this.

I found that OH-containing solvents did interact with the reactants and inhibited both the synthesis and the crystallizations. MeOH is better than EtOH, which was not so good for synthesis.

I found that acetonitrile was a great solvent for synthesis. It is a great solvent for 'salts' . Its boiling temperature is around 80C.I found that there were fewer impurities formed since it did not seriously participate in the synthesis. Because it is a great solvent, The boiling temperature of acetonitrile for the synthesis is good. This solvent is unlikely to interfere with your synthesis.

Once the precipitation is finished, The next step would be to reduce the solvent by distillation. If crystals appear, you succeeded. Never remove all of the solvent, because any impurities will join your product. 

If your product is a salt, try ether. follow the suggestions of the anti-solvent  precipitation. If your product is neutral, evaporate to dryness and extract with benzene or a similar solvent.

if your product has a color, liquid chromatography may separate the impurities from your compound. It also gives you information about the number and quantity of the impurities. Catching the product will significantly improve your chance to crystallizing.

I am hoping that this may help your efforts and I am wishing you an easy time and great success.

Cheers, Into 

Dear Prof., Hopeful and grateful to all professor. My Complex contains Cobalt with alkylamine, n-hexylimidazole with uncoordinate 2 chlorine in their lattice. After a long time i hope it may possible for ionic liquid. Please send your comments.

I believe your complexes consist of mixture of different optical isomers or also possible that amines with long alkyl tails conformationally mobile, so there is a reasson why you have no suscess in crystallization. to be lucky in crystallization process you need to separate that isomers using apropriate techniques (such as chromagraphic colums may be with chiral stationary phase). Sometimes oil may be crystallized spontaneously during long ime (months or even years)

Good day everyone,

I aimed to synthesize schiff base complexe with CuCl2. 2H2O.

But I only have an aldhyde Ir spectrum from which its schiff base was made?!!!

Why this is happened?

The reaction was as follow :

I added cu metal unhydrous ethanolic sol. Dropwise to hot ethanolic sol. of schiff base then reflex for about 3 hrs.

Agree with Dr. Tsymbarenko