N‑Benzoylbenzamidinate Complexes of Magnesium: Catalysts for the Ring-Opening Polymerization of ε‑Caprolactone and CO 2 /Epoxide Coupling B. Raghavendra, K. Bakthavachalam, Buthanapalli Ramakrishna, and N. Dastagiri Reddy* Department of Chemistry, Pondicherry University, Pondicherry 605 014, India * S Supporting Information ABSTRACT: A series of amidinate-based N,O-chelated magnesium complexes [( L 1 ) 2 (THF) 2 Mg] ( 1 ), [( L 2 ) 2 (THF) 2 Mg] ( 2 ), [( L 3 ) 2 (THF) 2 Mg] ( 3 ), and [(L 4 ) 2 Mg] (4) were prepared by treating N-benzoyl-N′- arylbenzamidines (L 1-4 H) with 0.5 equiv of di-n-butylmagne- sium in THF. Analogous CH 3 CN-coordinated complexes [(L 1 ) 2 (CH 3 CN) 2 Mg] (5) and [(L 3 ) 2 (CH 3 CN) 2 Mg] (6) were prepared in a similar way using CH 3 CN as solvent. All of the compounds were characterized by 1 H/ 13 C NMR spectroscopy, and the molecular structures of 1, 2, and 4-6 were further confirmed by single-crystal X-ray diffraction studies. Complexes 1, 2, 5, and 6 displayed good catalytic activity toward the ring-opening polymerization (ROP) of ε-caprolactone. In addition, 1, 5, and 6 were also found to be excellent catalysts for making cyclic carbonates from CO 2 and epoxides in the presence of a cocatalyst, n-Bu 4 NBr. ■ INTRODUCTION Employing the coupling reaction of CO 2 and epoxide to generate cyclic/poly carbonates is not only 100% atom economical but also reduces the burden on nonrenewable resources used in the industry for making these highly important materials. 1 For example, CO 2 , which is a renewable feedstock, contributes 43 kg to every 100 kg of the propylene carbonate produced by this method. Recently reported analyses on the production of poly/cyclic propylene carbonate from CO 2 and propylene oxide show that the process is sustainable, as it fixes the carbon on high-value chemicals, and may also be economically viable, depending upon the selling price of the product as well as the efficiency of the catalyst. 2 Owing to their high boiling points and polarity, cyclic carbonates have been used as polar aprotic solvents and as electrolytes in lithium ion batteries. 3 In addition to being intermediates 4 in the manufacture of fine chemicals, they also find applications in the cosmetics and plastics industries. 5 There are numerous main-group- and transition-metal-based catalysts reported for the synthesis of cyclic carbonates from CO 2 and epoxides. 6 Among these, Al and Mg have been attractive because not only are they nontoxic and earth-abundant but also they have afforded highly efficient catalysts with excellent turnover frequencies (TOFs). 6a,b A few of these catalysts have also shown high activity in the production of polycaprolactone (PCL) via the ring-opening polymerization of ε-caprolactone. 7 PCL is a widely explored synthetic biodegradable and biocompatible polymer having a broad spectrum of biomedical and pharmaceutical applications. 8 Recently, we reported a few Al complexes of N-benzoyl-N′-arylbenzamidinates, which are highly active in the ROP of ε-caprolactone. 9 These complexes were found to be more active than the structurally analogous ketiminate Al complexes (Figure 1). 10 However, these Al complexes are not suitable for the synthesis of cyclic/poly carbonates, as they polymerize the epoxides to polyethers instantly. The enhanced activity of N-benzoyl-N′-arylbenzami- dinate complexes can be attributed to the replacement of C by N in the ligand backbone, which makes the metal center more acidic. Encouraged by these results, we decided to synthesize magnesium complexes of these ligands and explore their catalytic efficiency toward CO 2 /epoxide coupling and the ROP of ε-caprolactone. Herein, we describe in detail the synthesis and characterization of N-benzoyl-N′-arylbenzamidinate mag- nesium complexes and their catalytic activity. ■ RESULTS AND DISCUSSION Synthesis and Structural Characterization of Proli- gands. N-Benzoyl-N′-phenylbenzamidine (L 1 H) was synthe- Received: August 11, 2017 Figure 1. Isostructural features of N-benzoylbenzamidinate and ketiminate ligands. Article pubs.acs.org/Organometallics © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.7b00617 Organometallics XXXX, XXX, XXX-XXX Cite This: Organometallics XXXX, XXX, XXX-XXX