Base free N-alkylation of anilines with ArCH2OH and transfer hydrogenation of aldehydes/ketones catalyzed by the complexes of η5-Cp*Ir(iii) with chalcogenated Schiff bases of anthracene-9-carbaldehyde

Abstract

The condensation of anthracene-9-carbaldehyde with 2-(phenylthio/seleno)ethylamine results in Schiff bases [PhS(CH₂)₂CN-9-C₁₄H₉](L1) and [PhSe(CH₂)₂CN-9-C₁₄H₉] (L2). On their reaction with [(η⁵-Cp*)IrCl(μ-Cl)]₂ and CH₃COONa at 50 °C followed by treatment with NH₄PF₆, iridacycles, [(η⁵-Cp*)Ir(L-H)][PF₆] (1: L = L1; 2: L = L2), result. The same reaction in the absence of CH₃COONa gives complexes [(η⁵-Cp*)Ir(L)Cl][PF₆] (3–4) in which L = L1(3)/L2(4) ligates in a bidentate mode. The ligands and complexes were authenticated with HR-MS and NMR spectra [¹H, ¹³C{¹H} and ⁷⁷Se{¹H} (in the case of L2 and its complexes only)]. Single crystal structures of L2 and half sandwich complexes 1–4 were established with X-ray crystallography. Three coordination sites of Ir in each complex are covered with η⁵-Cp* and on the remaining three, donor atoms present are: N, S/Se and C⁻/Cl⁻, resulting in a piano-stool structure. The moisture and air insensitive 1–4 act as efficient catalysts under mild conditions for base free N-alkylation of amines with benzyl alcohols and transfer hydrogenation (TH) of aldehydes/ketones. The optimum loading of 1–4 as a catalyst is 0.1–0.5 mol% for both the activations. The best reaction temperature is 80 °C for transfer hydrogenation and 100 °C for N-alkylation. The mercury poisoning test supports a homogeneous pathway for both the reactions catalyzed by 1–4. The two catalytic processes are most efficient with 3 followed by 4 > 1 > 2. The mechanism proposed on the basis of HR-MS of the reaction mixtures of the two catalytic processes taken after 1–2 h involves the formation of an alkoxy and hydrido species. The real catalytic species proposed in the case of iridacycles results due to the loss of the Cp* ring.