Synthesis and Crystal Structure of Salen Ligands Derived from Unsymmetrical Vicinal Diamine

A. Gayathri1, T. Vidhyasagar1 and K. Rajeswari2

1Department of Chemistry,Annamalai University, Annamalai Nagar-608002, INDIA.
2PG & Research Department of Chemistry, Government College for Women (A), Kumbakonam – 612 001, INDIA.
email: rraajjii2006@gmail.com

(Received on: November 8, Accepted: November 17, 2017)


Salen type Schiff bases possessing an unsymmetrical vicinal diamine backbone are promising nominees in view of their diversified applications. Synthesis of such Schiff bases viz., 2'-((1E,1'E)-((1-phenylpropane-1,2-diyl)bis(azanylylidene)) bis(methanylylidene))diphenol and 2,2'-((1E,1'E)-((1-(p-tolyl)propane-1,2-diyl) bis (azanylylidene))bis(methanylylidene))diphenol were achieved. Their single crystals were developed using ethanol as solvent by slow evaporation technique and subjected for XRD-studies. Interestingly, both the crystals crystallize with similar pattern i.e., in triclinic crystal system with space group P-1 and both found to have E-configuration at each imine bond. XRD data reveals the molecular structure of both the compounds exhibit similar intra & inter- molecular hydrogen bonding interactions.

Keywords:Crystal structure, Unsymmetrical vicinal diamines, Salen ligand, Schiff base, CCDC: 1448832 & 1448833.


Salen ligands are privileged entrants both in synthetic and medicinal fields. Though Schiff bases are known for several decades1-5, chemists are still interested in design and synthesis of new Schiff bases6,7, as these are capable of coordinating with various metals, which find applications in fields of synthesis8, medicine9 and material study.10 Numerous studies were conducted in order to understand the properties of salen and its derivatives.11-14 Owing to the importance of salen type ligands, the understanding of their structure becomes significant. Hence, the present report adds one more feather on the crown to the world of single crystals of salen ligands.


Synthesis and Crystallization

2,2'-((1E,1'E)-((1-phenylpropane-1,2-diyl)bis(azanylylidene))bis(methanylylidene)) diphenol:Synthesis of salen ligand 1, (C23H21ClN2O2), was achieved from the condensation 2-hydroxybenzaldehyde (0.02 mol) and 1-phenylpropane-1,2-diamine (0.01 mol) in ethanol (25 ml, 99%). The mixture was refluxed for 3h and the resulting solid product was collected by filtration. The solid was purified by recrystallization using ethanol which afforded yellow needles of pure salen ligand. Single crystal of the compound was developed using ethanol as solvent by slow evaporation technique.

2,2'-((1E,1'E)-((1-(p-tolyl)propane-1,2-diyl)bis(azanylylidene))bis(methanylylidene)) diphenol:The above procedure was repeated for the synthesis of salen ligand 2, (C24H24N202) with 2-hydroxybenzaldehyde(0.02 mol) and 1-(p-tolyl)propane-1,2-diamine (0.01 mol) as reagents. The single crystal was developed from ethanol by slow evaporation technique.

XRD study

For Salens 1 & 2 the crysatal data were collected on a Bruker Proteum 2CCD diffractometer with X-ray generator operating at 45 kV and 10 mA using CuKα radiation of wavelength 0.71073 Å. The cell refinement and data thus obtained were processed using APEX2 and SAINT15; Data reduction by SAINT and XPREP.16 The structure was solved by direct method and refined by full-matrix least squares on F2 using SHELXS and SHELXL programs.17 The geometrical parameters were calculated by using PLATON.18 Crystal Data, collection procedure and refinement results were summarized in Table 1. Supplementary crystallographic data can be obtained from CCDC using numbers:1448833 (salen1) & 1448832 (salen 2).


Salen ligands 1 and 2 were synthesized by the condensation of salicylaldehyde with corresponding 1,2-diamines in ethanolic medium. Both 1 and 2 were subjected for single crystal growth using ethanol as solvent by slow evaporation technique. The single crystals of 1 as well as 2 were subjected for XRD study [Salen1: CCDC: 1448833 & Salen2: CCDC: 1448832]. The Crystal Data for salens 1 and 2 were presented in Table 1.The ORTEP of salens 1 and 2 were presented in Fig.1 & 2, respectively. The packing diagrams for both were given in Fig.3 & 4. In salen 1, the bonding parameters of C=N [C7-N1(1.272 Å), C17-N2 (1.268 Å)] & C-O [C1-O1(1.352 Å), C23-O2(1.355 Å)] signify an exclusive formation of enol-imine form over the other tautomeric forms(keto-amine) of Schiff base. It is observed that, the C-N & C-O bond lenths are consistent with normal C=N & C-O bond lengths, respectively. Similarly, from the crystal data of salen 2, C=N [C7-N1(1.26Å), C18-N2 (1.268Å)] & C-O [C1-O1(1.34 Å), C24-O2(1.346 Å)] the existence of its enol-imine form are established. Inter- and intra- molecular interactions observed from the crystal data of Salen 1 & 2 are presented in Table 2 & 3, respectively. Both Salens exhibit N….H-O type hydrogen bonding.

Table 1. The Crystal Data for salens1 and 2

Salen 1 2
CCDC 1448833 1448832
Empirical formula C23H22N202 C24H24N202
Formula weight 358.42 372.45
T (K) 296(2) 296(2)
Wavelength (Å) 0.71073 Ao 0.71073 Ao
Crystal system, space group Triclinic P-1 TriclinicP-1
a (Å) 9.6534(2 9.5158(2)
b (Å) 10.1308(3) 11.0960(3)
c (Å) 10.6936(3) 11.1132(2)
α(o) 85.985(3) 97.9510(10)
β (o) 79.586(3) 94.7630(10)
γ (o) 66.714(2) 112.081(2)
Volume (Å3) 944.79 1065.17
Z, Calculated density D (Mg/m3) 2,1.260 2,1.161
F(000) 380 396
Absorption coefficient μ (mmA-1) 0.081 0.074
h range for data collection (o) 2.189 to 25.000 2.015 to 24.998
hkl range -11<=h<=11/-12<=k<=12/ 11<=h<=11/-13<=k<=13
-12<=1<=12 -13<=1<=13
Reflections collected 17130 19711
Unique Rint 3323(0.0320) 3743(0.0270)
Crystal size 0.300×0.200× 0.200 0.350×0.300×0.250
Data / restraints / parameters 3323/0/245 3743/0/254
Goodness-of-fit on F2 1.015 1.102
Absolute structure parameter R(F) (I > 2σ(I)) 0.1(7) 0.1
Extinction coefficient wR(F2)(all data) 0.015(3) 0.010(3)
Largest diff. peak and hole
max/min. Δσ(e/Å3)
0.137/-0.212 0.264/-0.180

Table 2. Hydrogen bonds [Å and o] of Salen 1

D-H...A d(D-H) d(H...A) d(D...A) <(DHA)
C(22)-H(22)...O(2)#1 0.93 2.64 3.556(3) 169.5
O(1)-H(1A)...N(1) 0.82 1.88 2.600(2) 146.2
O(2)-H(2A)...N(2) 0.82 1.87 2.596(2) 146.5

Symmetry transformations used to generate equivalent atoms / #1 -x+1,-y+1,-z+1

Table 3. Hydrogen bonds [Å and o] of Salen 2

D-H...A d(D-H) d(H...A)d(D...A) <(DHA)  
O(1)-H(1)...N(1) 0.82 1.85 2.584(3) 148.3
O(2)-H(2A)...N(2) 0.82 1.83 2.563(3) 147.6

Symmetry transformations used to generate equivalent atoms

The selected bond length (Ao), bond angle (o) and Torsion angles (o) of salens 1and 2 are presented in Table 4 & 5, respectively. From the torsional angles of salens 1 & 2, it is observed that both the salens, exist in E-configuration at each imine bonds.

Table 4: Selected bond length (Ao), bond angle (o) and Torsion angles (o) of salen 1

Bond length (Ao) (Ao) Bond angle (o) Torsion angle (o)
C1-O1 1.352 O2-C23-C18 121.2 C2-C1-C6-C7 -178.79
C23-O2 1.355 O1-C1-C2 118.5 C9-C8-C15-C16 70.3
C15-C16 1.521 O1-C1-C6 121.3 N1-C8-C15-C16 -167.09
C8-C9 1.512 O2-C23-C22 118.6 N2-C15-C8-C9 -167.88
C8-N1 1.465 N2-C15-C16 110.6 N2-C17-C18-C23 10.2
C15-N2 1.468 N2-C15-C8 109.3 N2-C17-C18-C19 -170.24
C17-N2 1.268 N1-C8-C9 110.5 C16-C15-C8-C9 70.3
C15-C8 1.530 C16-C15-C8 110.24 C15-C8-N1-C7 136.2
C9-C14 1.387 C8-C9-C14 121.54 C17-N2-C18-C23 10.2
C12-C13 1.366 N2-C17-C18 118.8 C8-C9-C14-C13 179.7
C18-C23 1.392 N1-C7-C6 122.5 C8-C9-C10-C11 179.5
C7-N1 1.272 C12-C11-C10 120.4 C16-C15-N2-C17 -73.4

Table 5: Selected bond length (Ao), bond angle (o) and Torsion angles (o) of salen 2

Bond length (Ao) Bond angle (o) Torsion angle (o)
C8-C10 1.527 C10- C8-N1 110.0 C8-N1-C7-C6 178.7
C8-N1 1.458 C10-N2-C18 109.2 C10-N2-C18-C19 176.8
C10-N2 1.459 C8-N1-C7 118.1 C10-C11-C16-C15 -178.9
N2-C18 1.26 N1-C7-C6 122.7 C9-C8-N1-C7 -127.7
N1-C7 1.26 C6-C1-O1 120.8 C17-C14-C15-C16 -179.1
O1-C1 1.34 C23-C24-O2 118.6 C3-C2-C1-O1 -179.6
C24-O2 1.346 C10-C11-C12 121.2 N1-C7-C6-C5 179.6
C17-C14 1.514 C13-C14-C17 120.4 N2-C18-C19-C20 -179.6
C9-C8 1.517 N1-C8-C9 109.0 C17-N2-C18-C23 179.5
C10-C11 1.500 C10-C11-C16 120.8 C8-C9-C14-C13 179.5


Salen ligands 1 and 2 were synthesized by the condensation of salicylaldehyde with corresponding 1,2-diamines in ethanolic medium. Single crystals of both were achieved by slow evaporation method. Surprisingly, both the crystals crystallize with similar pattern, in triclinic crystal system with space group P-1, exhibiting N….H-O type hydrogen bonding interaction. Both salens found to have E-configuration at each imine bonds. The bonding parameters of C=N & C-O signify an exclusive formation of enol-imine form over the other tautomeric forms of Schiff base.


Authors thank SAIF, IIT-Madras for the support in single crystal data collection and gratefully acknowledge UGC for financial support under MRP scheme.


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