Coordination chemistry of bidentate phosphine ligands with hydrogen-bonding arms: Picket-fence rhodium complexes Ilknur Babahan a,b , James T. Engle c , Nishant Kumar b , Christopher J. Ziegler c,⇑ , Li Jia a,⇑ a The University of Akron, Good year Polymer Science, Akron, OH 44325, USA b Adnan Menderes University, Faculty of Science and Art, Department of Chemistry, 09010 Aydin, Turkey c The University of Akron, Department of Chemistry, Akron, OH 44325, USA article info Article history: Received 25 September 2013 Accepted 18 November 2013 Available online 4 December 2013 Keywords: Bidentate phosphine Secondary interaction Rhodium complex abstract A bidentate phosphine ligand with two amide arms, designed to form hydrogen bonds with electron- donating moieties, was synthesized and isolated in high diastereomeric excess (95% de). The hydrogen-bonding abilities of the ligand and its diastereomer were demonstrated with two rhodium complexes containing these ligands. The structures of the rhodium compounds are reminiscent of the well-studied picket-fence porphyrin systems. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Secondary ligand–ligand and ligand-substrate interactions play important roles in the catalytic transformations promoted by tran- sition metal complexes [1] and as enzymatic models [2]. Among the various types of interactions, hydrogen bonds are useful, or even critical, in many cases of catalysis involving substrates con- taining oxygen and nitrogen atoms [3]. Given our interest in het- erocycle carbonylation [4], we explored along this line in a limited scope. Here, we document the synthesis of a bidentate phosphine ligand with a capacity for hydrogen bonding and its rhodium coordination chemistry. The resultant Rh complexes resemble picket-fence porphyrins [5], which have been used as models for enzymatic active sites and for selective substrate binding. 2. Experimental 2.1. General remarks All reactions and manipulations were performed under an inert atmosphere. All solvents were anhydrous and purchased from Sig- ma–Aldrich. NMR spectra were recorded on a Varian 300 MHz spectrometer. Bis(phenylphosphino)ethane, PhP(H)(CH 2 ) 2 P(H)Ph, was prepared according to a literature procedure [6]. 2.2. Synthesis of 1-C s A 50-mL flask was placed in an oil bath at 100 °C. Ph(H)CH 2 CH 2- P(H)Ph (1.24 g, 5.04 mmol), AIBN (50 mg), and CH 2 = CHCONH i Pr (1.26 g, 11.2 mmol) were added to this flask. The mixture was stir- red at 100 °C for 1 h under a nitrogen atmosphere and then under vacuum for 24 h to remove the slightly excess amount of CH 2 =- CHCONH i Pr. The resultant oil was dissolved in boiling toluene (20 mL) and filtered while the solution was hot. The filtrate was kept at room temperature (RT) to allow precipitation of a white crystalline solid, which was washed with acetonitrile (2  5 mL) and isolated after filtration (yield, 1.08 g, 45% in 95% diastereo- meric excess). 1 H NMR (CDCl 3 ): d 1.10 (‘‘triplet’’ arising from two partially overlapping doublets, J = 7.7 Hz, 12 H, –CH 3 ), 1.63–1.75 (m, 4H, –PCH 2 CH 2 CO–), 1.96 (m, 4H, –PCH 2 CH 2 P–), 2.07 (m, 4H, –CH 2 CO– ), 4.02 (heptet, J = 8.1 Hz, 2H, –CH–), 5.24 (b, 2H, –NH–), 7.34 (m, 6 H, CH aryl ), and 7.42 (m, 4H, CH aryl ). 13 C{ 1 H} NMR (CDCl 3 ): d 171.3, 137.1, 132.4, 119.2, 128.5, 41.4, 32.9, 23.5, 23.1, 22.8. 31 P{ 1 H} NMR (CDCl 3 ): d 19.9. 2.3. Collection of 1-C s /1-C 2 mixture The toluene filtrate from the above experiment was kept at 30 °C. A white crystalline solid again formed, and the isolated so- lid contained a mixture of 1-C s and 1-C 2 in about a 1:3 ratio (yield, 0.30 g, 12%). 1 H NMR (CDCl 3 ): d 1.10 (‘‘triplet’’ arising from two partially overlapping doublets, J = 7.7 Hz), 1.62–1.77 (m, –PCH 2 CH 2 CO–), 1.96–1.99 (m, –PCH 2 CH 2 P–), 2.05–2.12 (m, –CH 2 CO–), 3.97–4.05 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.11.018 ⇑ Corresponding authors. Tel.: +1 330 972 7511; fax: +1 330 972 5290 (L. Jia). Tel.: +1 330 972 2531; fax: +1 330 972 6085 (C.J. Ziegler). E-mail addresses: ziegler@uakron.edu (C.J. Ziegler), ljia@uakron.edu (L. Jia). Polyhedron 69 (2014) 156–159 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly