Effect of steric crowding : Synthesis, spectral characterization and redox studies of isomers of ruthenium(II) N(1)-ethyl-2-(arylazo)- imidazoles

Abstract

Arylazoimidazoles are N,N-chelators (N(imines), N and N(azo), N’). Ruthenium(II) complexes of <i style="mso-bidi-font-style: normal">o</i>-substituted arylazoimidazoles [N( l )-ethyl-2-(7-methylphenylazo)imidazole (L¹), N( l)-ethyl-2-(7,8-dimethylphenylazo)imidazole (L³)] (RuL₂Cl₂) give four different

isomers while non-orthosubstituted ligands [N(1)-ethyl-2-(S -methylphenylazo)imid azole(L²), N( l )-ethyl-2-(8,9-

<span style="font-size:14.0pt;font-family:" times="" new="" roman";mso-fareast-font-family:="" "times="" roman";color:#302d2e;mso-ansi-language:en-us;mso-fareast-language:="" en-us;mso-bidi-language:ar-sa"="">dimethylphenylazo)irnidazole (L⁴)] give only two isomers. Considering the coordinating pairs in the order of Cl, N, N', the isomers are characterised as <i style="mso-bidi-font-style:normal">trans-cis-cis </i>(tcc), <i style="mso-bidi-font-style:normal">cis-cis-trans</i> (cct), <i style="mso-bidi-font-style:normal">cis-trans-cis</i> (ctc) and <i style="mso-bidi-font-style:normal">cis-cis-cis</i> (ccc) for [Ru(L¹)₂Cl₂]/[Ru(L³)₂Cl₂. The two isomers for [Ru(L²)₂Cl₂)/[Ru(L⁴)₂Cl₂] are tcc and ctc type. They exhibit t₂(Ru)→π*(L) transition (MLCT) in the visible region and the energy of transition follows the order ccc > ctc > cct > tcc. The ¹H NMR spectra give one quartet and one triplet corresponding to N(1)CH₂-and (N(l)-CH₂)-CH₃respectively in tcc-RuL₂CI₂. The N(l)-CH₂ part in other isomer gives complex spectra of AB type multiplets and the geminal coupling constant follows the order cct (<i style="mso-bidi-font-style:normal">J</i> = 14.0 Hz) < ctc (<i style="mso-bidi-font-style:normal">J</i> = 20.0 Hz) < ccc (<i style="mso-bidi-font-style:normal">J</i> = 28.0 Hz). The Ar-Me (7- or 8-Me) or Ar-Me2 (7,8 or 8,9Me₂) exhibit one or two equally intense signal(s) for tcc, cct and ctc isomers and support Cl-symmetry in the complexes. The ccc-RuL₂Cl₂ shows two equally intense signals for (<b style="mso-bidi-font-weight:normal">1d</b>)/(<b style="mso-bidi-font-weight:normal">3d</b>) while three signals for (<b style="mso-bidi-font-weight:normal">2d</b>)/(<b style="mso-bidi-font-weight: normal">4d</b>). This may be due to inequivalence of Me group in ccc isomers (<i style="mso-bidi-font-style:normal">C</i>₁ -symmetry). Redox studies show ruthenium(III)/ruthenium(II) couple at 0.4 V (vs SCE) for tcc isomers and 0.5 - 0.6 V (vs SCE) for other isomers (cct/ctc/ccc).</span>