Rhodium(III) complexes of a substituted 2,2 0 -bisoxazoline Nirmal K. Shee a , Samik Nag a , Michael G.B. Drew b , Dipankar Datta a,⇑ a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India b Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK article info Article history: Received 17 September 2012 Received in revised form 13 May 2013 Accepted 15 May 2013 Available online 25 May 2013 Keywords: Rh(III) Bisoxazoline Binding of alcoholic O pK a abstract Reaction of 4,4,4 0 ,4 0 -tetramethyl-2,2 0 -bisoxazoline (L) with RhCl 3 in methanol followed by addition of excess NaClO 4 yields cis-[RhL 2 Cl 2 ]ClO 4 (1). Complex 1 hydrolyses to yield the mixed ligand Rh(III) com- plex trans-[RhL(L 0 H)Cl 2 ]ClO 4 2H 2 O(2), where L 0 H is 2-amino-2-methyl-1-propanol (H: dissociable alco- holic proton). The X-ray crystal structures of 1 and 2 have been determined. The aminoalcohol L 0 H binds the metal in 2 in bidentate mode through the N and the O ends. The 1 H NMR of 2 in CD 3 CN shows that the alcoholic proton dissociates in solution. DFT calculations at the B3LYP/LanL2DZ level reveal that the dissociation of the alcoholic proton in 2 is highly exothermic and accompanied with elimination of a chloride ion from the metal coordination sphere giving rise to a square pyramidal five-coordinate Rh(III) species. The relevant pK a value has been determined in acetonitrile from solution conductance as 8.98 (±0.27). Both the complexes 1 and 2 display an irreversible Rh(III/I) couple in cyclic voltammetry in ace- tonitrile at a glassy carbon electrode around 0.35 V versus NHE. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Catalytic asymmetric homogeneous reactions have been recognized as the most promising methods for obtaining optically active compounds. The majority of ligands used so far in asymmetric hydrogenation catalysts with late transition metals have involved phosphines [1], and only a handful of reports involving rhodium(I) or palladium(II) complexes with nitrogen donors have been pub- lished [1–4]. In the last decade there has been a surge in interest in chiral nitrogen-donor ligands for use in asymmetric catalysis and bisoxazolines have been particularly successful [5–7]. We have re- cently reported few findings in the chemistry of 2,2 0 -bisoxazolines with various transition metals [8–14]. Herein we make a prelimin- ary report of our studies on the rhodium chemistry of a substituted 2,2 0 -bisoxazoline (L). The most common oxidation states of rhodium are I and III. A small number of monomeric complexes of Rh(II) are known [15]. Rhodium complexes of aromatic 1,4-diimines like 2,2 0 -bipyridine (bpy) and 1,10-phenanthroline are well studied [16,17]. Our L is a non-aromatic 1,4-diimine. O N N O H 2 N OH L L' L'H 2. Results and discussion Reaction of L with RhCl 3 in methanol under refluxing condition and subsequent addition of excess NaClO 4 afforded [RhL 2 Cl 2 ]ClO 4 (1) in 50% yield as a pale orange precipitate. Slow diffusion of diethyl ether into a dilute solution of 1 yielded pale yellow single crystals suitable for X-ray crystallography. It revealed that 1 con- tains discrete [RhL 2 Cl 2 ] + cations with crystallographic C 2 symmetry and disordered perchlorate anions. The cation in 1 is shown in Fig. 1 along with the atomic numbering scheme. The metal atom occupies a distorted octahedral coordination sphere with the main distortion arising from the small angle of 78.8(3)° subtended by the bidentate ligands, L. The two chloride atoms are mutually cis to each other, giving rise to a RhN 4 Cl 2 core. The two Rh–N bond dis- tances are 2.092(7) and 2.086(8) Å, the smaller one being trans to the Cl. The Rh(1)–Cl(2) distance is 2.322(3) Å. The Rh–Cl distances in cis-[Rh(bpy) 2 Cl 2 ] + cation are 2.341(2) and 2.328(2) Å [18]. The Cl–Rh–Cl angle in 1 is 94.51°. The same angle in cis-[Rh(bpy) 2 Cl 2 ] + is 90.86° [18]. Trans-[Rh(bpy) 2 Cl 2 ] + is known but it is a high energy species [19]. The filtrate after isolation of 1, furnished orange crystals on standing for a week. Single crystal X-ray crystallography revealed that the compound is [RhL(L 0 H)Cl 2 ]ClO 4 2H 2 O(2), where L 0 H is 2-amino-2-methyl-1-propanol (H is the dissociable alcoholic pro- ton). The structure of 2 (Fig. 2) contains discrete [RhL(L 0 H)Cl 2 ] + cat- ions and disordered perchlorate anions together with three solvent water molecules, two of which have 50% occupancy. The metal atom has a slightly distorted octahedral environment being bonded to two bidentate ligands that form the equatorial plane 0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.05.012 ⇑ Corresponding author. Tel.: +91 33 2473 5374; fax: +91 33 2473 2805. E-mail address: icdd@iacs.res.in (D. Datta). Inorganica Chimica Acta 405 (2013) 111–115 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica