Synthesis, spectroscopy, catalysis and crystal structure of [Rh(η4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (Ar=C6H5, 3-/4-MeOC6H4, and 4-BrC6H4)

• Published in: Inorganica Chimica Acta 2012-05, Vol.387, p.173-180
• Main Authors: Enamullah, Mohammed, Royhan Uddin, A.K.M., Hogarth, Graeme, Janiak, Christoph
• Format: Article
• Language: English
• Subjects: (η4-cod)Rh(I)-complexes; Catalysis; Chiral Schiff bases (R)-N-(Ar)ethyl-2-hydroxy-1-naphthaldimine; Crystal structure; Enantiopurity
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2012.01.013

The enantiopure hydroxy-1-naphthaldimine Schiff base ligand (HSB) reacts with dinuclear [Rh(η4-cod)(μ-O2CMe)]2 to give enantiopure [Rh(η4-cod)(SB)] complexes where the deprotonated Schiff base (SB) co-ordinates to the [Rh(η4-cod)]-fragment as a six-membered N,O-chelate ligand. [Display omitted] ► Condensation of hydroxy-naphthaldehyde with (R)-(Ar)ethylamine gives (R)-(Ar)ethylhydroxy-naphthaldimine (HSB). ► These Schiff bases react with [Rh(η4-cod)(O2CMe)]2 to afford [Rh(η4-cod)(SB)] (I–III). ► Deprotonated Schiff base coordinates to Rh(η4-cod)-fragment as six-membered N,O-chelate. ► Reduction of acetophenone with I and diphenylsilane into (±)-1-phenylethanol up to 93–97%. Condensation of 2-hydroxy-1-naphthaldehyde with (R)-(Ar)ethylamine yields the enantiopure Schiff bases, (R)-N-(Ar)ethyl-2-hydroxy-1-naphthaldimine (Ar=C6H5, 3-/4-MeOC6H4, 4-BrC6H4). These Schiff bases readily react with the dinuclear complex [Rh(η4-cod)(μ-O2CMe)]2 to afford the mononuclear complexes [Rh(η4-cod){(R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (Ar=C6H5 (I); 3-MeOC6H4 (II); 4-BrC6H4 (III)), respectively in C6H6/MeOH (5:1, v/v). The Schiff bases and complexes are characterized by IR, UV–Vis, 1H/13C NMR and mass spectrometry, polarimetry and HPLC. The polarimetric measurements show the enantiopurity of the Schiff bases as well as the complexes. The X-ray structure determination for III demonstrates that the deprotonated Schiff bases, (R)-N-(Ar)ethyl-2-oxo-1-naphthaldiminate, co-ordinate to the [Rh(η4-cod)]-fragment as a six-membered N,O-chelate ligand with distorted square planar geometry at the rhodium metal atom. Reaction of III with O2 leads to the formation of the oxidative aduct [Rh(η4-cod)(μ-O)]2 (IIIa). Compound I or [Rh(η4-cod){(S or R)-N-(phenyl)ethyl-salicylaldiminato}] were used for reduction of acetophenone with diphenylsilane into (±)-1-phenyl-ethanol, and conversions up to 93–97% have been achieved.