Embracing ligands. A synthetic strategy towards new nitrogen–thioether multidentate ligands and characterization of the cobalt(iii) complexes

Abstract

The synthesis of the hexadentate ligand 2,2,9,9-tetra(methyleneamine)-4,7-dithiadecane (EtN₄S₂amp) is reported. The ligand is of a type in which bifurcations of the chain occur at atoms other than donor atoms. The cobalt(III) complex [Co(EtN₄S₂amp)]³⁺ (1) was isolated and characterized. The synthetic methodology also results in a number of by-products, notably 2,9,9-tris(methyleneamine)-9-methylenehydroxy-4,7-dithiadecane (Et(HO)N₃S₂amp) and an eleven-membered pendant arm macrocyclic ligand 6,10-dimethyl-6,10-bis(methyleneamine)-1,4-dithia-8-azaacycloundec-7-ene (dmatue). The complexes [Co(Et(HO)N₃S₂amp)]³⁺ (2), in which the alcohol is coordinated to the metal ion, and [Co(dmatue)Cl]²⁺ (4) were isolated and characterized. Et(HO)N₃S₂amp also undergoes complexation with cobalt(III) to produce two isomers endo-[Co(Et(HO)N₃S₂amp)Cl]²⁺ (endo-3) and exo-[Co(Et(HO)N₃S₂amp)Cl]²⁺ (exo-3), both with an uncoordinated alcohol group. endo-3 has the alcohol positioned cis, and exo-3trans, to the sixth metal coordination site. Reaction of 1 with isobutyraldehyde, paraformaldehyde and base in dimethylformamide results in the encapsulated complex [Co(1,5,5,9,13,13-hexamethyl-18,21-dithia-3,7,11,15-tetraazabicyclo[7.7.6]docosa-3,14-diene)](ClO₄)₃·2H₂O ([Co(Me₆docosadieneN₄S₂)]³⁺ (5). All complexes have been characterized by single crystal X-ray study. The low-temperature (11 K) absorption spectrum of 1 has been measured in Nafion films with spin-allowed ¹A_1g → ¹T_1g and ¹A_1g → ¹T_2g and spin forbidden ¹A_1g → ³T_1g and ¹A_1g → ³T_2g bands observed. The octahedral ligand-field parameters were determined (10Dq = 22570 cm⁻¹, B = 551 cm⁻¹; C = 3500 cm⁻¹). For 5 10Dq and B were determined (20580 cm⁻¹; 516 cm⁻¹, respectively) and compared with those for similar expanded cavity complexes [Co(Me₈tricosatrieneN₆)]³⁺ and [Co(Me₅tricosatrieneN₆)]³⁺.