Synthesis of 1-(2-aminopyridine)-4-phenyl-1-azabuta-1,3-diene and 1-(3-aminopyridine)-4-phenyl-1-azabuta-1,3-diene as heterodienes for iron carbonyl complexes

Although a large number of p- (1,3-diene) iron tricarbonyl complexes have been reported and their reactivity investigated [1], less is known of the corresponding heterodiene compounds. In such compounds, which may be regarded as derived from the basic butadiene unit by the replacement of one or more of the carbon atoms by the oxygen or nitrogen, the possibility arises that the lone pair of electrons of the heteroatom is involved with the metal-ligand bond [2,3,4,5].In this study we report the synthesis of two new Schiff bases derived from 2-aminopyridine, 3-aminopyridine and cinnamaldehyde.These compounds are potentially good ligands for iron carbonyl complexes.

A solution of 2-aminopyridine 1 (0.94g, 10.00mmol) in dry benzene (10mL) was added with stirring to a solution of cinnamaldehyde 2 (2.64g, 20.00mmol) in benzene (10mL). The reaction mixture was stirred at c.a. 40 °C for 4 hours. Most of solvent was removed by splash-guard adaptor attached to vessel. The residue of solvent was removed under reduced pressure. After returning to room temperature and addition of 100 mL of n-heptane, the mixture was allowed to stand overnight in a refrigerator. The upon layer was extracted and after removal of solvent, the remaining oil was allowed to stand for 2 days under hood at room temperature. The yellow crystals were separated by filtration and washed with two 3 mL portions of cold ether, yielding (1.42g, 68%) of 3. The procedure for preparing 3' was similar to 3.Compound 3' was a brown solid and its yield was 98%.Its physiochemical properties were almost the same with 3. The IR spectrum showed the characteristic absorption of Schiff base C=N at 1580 cm^-1 and the C=C at 1551 cm^-1.. The ¹H-NMR spectrum showed a multiplet for aromatic protons at 7.20-7.70 p.p.m. H^e had a doublet at 9.01 with a J= 8.80. A doublet appeared at 8.46 for H2. H3 and H4 had a multiplet at 6.75. The ¹³C-NMR spectrum showed C=N group at 164.20.The mass spectrum showed the (M+1) peak at 209 and the M⁺ at 208.Their ability for coordination to metal ions are under study.

Melting Point: 68°C.

TLC (dichloromethane-acetone 20/1): R_f= 0.56

IR (KBr, ν, cm^-1): 1580 (s, C=N); 1551 (s, C=C).

¹H-NMR (250 MHz, CDCl₃): δ= 9.01 (1H, d, J=8.8, H^e); 8.46 (1H, d, H2); 7.2-7.7(8H, m, aromatic); 6.75 (2H, m, H3 and H4).

¹³C-NMR (62.9 MHz, CDCl₃): δ= 164.2 (C2); 160.7 (C^a); 146.7 (C^e); 148.8 (C3); 146.5 (C4); 138.2 (C^c); 121.9 (C^d); 120.3 (C^b); 127.7-129.9 and 135.5 (Ph carbon atoms).

UV (EtOH; λ_max nm; ε (dm³.mol^-1.cm^-1): 319 (18000); 226 (13000).

MS (m/z, %): 209 (M⁺ +1, 5.9); 208 (M⁺, 38.3); 207 (M⁺-1, 50.5); 131 (100); 130 (77); 79 (87.5); 78 (54.2); 52 (49.4); 51 (52.6).

Elemental Analysis: Calculated for C₁₄H₁₂N₂: C, 80.77%; H, 5.76%; N, 13.46%. Found: C, 80.75%; H, 5.89%; N, 13.49%.