Take sal: en as 2:1

Make reaction of both reagents in a ratio of 1: 1 in acetic acid or in an alcohol (MeOH, EtOH?), when you use 1 eq of sale, 0.5 eq of en and 0.5 eq of en.2HCl (= 1 eq of en.HCl) . See  https://www.researchgate.net/publication/256078555_Direct_N1-monosubstitution_of_piperazine for an inspiration. TLC controll, MeOH as eluent, detection by ninhydrine agent+heat.

Conference Paper Direct N1-monosubstitution of piperazine

All reactions of ethylenediamine with aldehydes or ketones will result in the formation of mixtures of mono and bis-structures (Major), or sometimes the bis one is only product. This is also with acyl halides.

To prepare your target compound:

1- Reflux 1:1 mole of ethylene diamine and ethyl acetate in THF to obtain mainly mono acetyl ED.  You can also prepare the mono-N-acetyl by mixing 1:1 moles of Ethylene diamine and acetic anhydride (as diluted solutions in THF (at room temp., or preferred in ice-bath with stirring)

2- Now you are in need to remove the minor (bis) from your product by chromatography.

3- Use 1:1 of your mono acetylated product with your aldehyde.

Good luck and happy new year.

In the following references you might find some hints for the synthesis of an imidazolidine from an aldehyde with en (extracted from ChemBrain IXL (http://neuronix.ch/chembrain)):

Houben-Weyl XI/2, 86;

M. Schultz, K, Jaross, Chem. Ber. 34, 15504 (1901);

A. W. Titherly, G. E. K. Branch, J. Chem. Soc. 103, 330 (1913);

J. van Alphen, Rec. Trav. Chim. 54, 93 (1935); 55, 412, 669, 835 (1936);

J. T. L. B. Rameau, Rec. Trav. Chim. 57, 194 (1938);

M. Senkus, J. Am. Chem. Soc. 68, 1611 (1946).

With en and sale you will get an azo compound. Depending on whether you want a cyclic struture or a chain, methods are different. you can follow the method suggested by Abdel Nasse Ahmad for open compound.

Follow the recommendations of Pavel Pezdera, and treat the en with one equivalent of HCl in MeOH to obtain the monosalt. Besides prepare a low concentration solution of sale in MeOH. Finally drop the sale solution to the en-HCl salt solution to obtain the desired product. Best reactions. Hiram  

Thanks for your response.