Published: March 03, 2011 r2011 American Chemical Society 2720 dx.doi.org/10.1021/ic1025262 | Inorg. Chem. 2011, 50, 2720–2722 COMMUNICATION pubs.acs.org/IC Salen Complexes of Zirconium and Hafnium: Synthesis, Structural Characterization, Controlled Hydrolysis, and Solvent-Free Ring-Opening Polymerization of Cyclic Esters and Lactides Tanmoy Kumar Saha, Venkatachalam Ramkumar, and Debashis Chakraborty* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India b S Supporting Information ABSTRACT: Dinuclear salen compounds of zirconium and hafnium are efficient initiators for the solvent-free ring- opening polymerization of cyclic ester monomers and lactides. There is a correlation between the theoretical and experimental number-average molecular weights (M n 's) in these polymerizations. Polymerization of β-butyrolactone gives poly(3-hydroxybutyrate) with a good M n and molec- ular weight distribution. T he nonrenewable and rapidly depleting petrochemical re- sources have manifested a mandatory impact on expediting research toward the search for new polymeric materials that are derived from renewable feedstock. 1 These polymers are synthe- sized from annually renewable natural resources such as corn and sugar beet. 2 These polymers have found vital applications in biomedical and pharmaceutical sectors such as drug delivery, fabrication of implants, and scaffolds for tissue engineering. 3 In addition, the film and fiber industry have also benefited by employing such polymers in suitable applications. 4 Such an impact is the result of their permeability, biocompatibility, and biodegradability. 5 The most common and rapidly used strategy in the synthesis of such polymers is the catalytic ring-opening polymerization of suitable cyclic ester monomers and lactides that are mediated by metal catalysts. 6 Among several options available for ring-opening polymerization, coordination-inser- tion polymerization has been extensively popular because of its capability of producing high polymers with narrow molecular weight distributions (MWDs). 6 The polymerization initiates rapidly, and there are minimal risks of unwanted transesterifica- tion reactions. The synthesis of new metal catalysts containing rigid and polydentate ancillary ligands with suitable pendant substituents as initiating groups has been the common metho- dology of choice for catalyst construction. 6 It is understood that the nuclearity of the metal catalyst is controlled by this ancillary ligand and that the catalyst nuclearity plays a vital role in the coordination-insertion growth mechanism. This often has an influence on the proximity of observed number-average molec- ular weights (M n 's) to that of theoretical M n 's. We were inter- ested in group 4 metals for the catalyst development toward the synthesis of green polymers. 7 Our ongoing studies are aimed at understanding the role of the pendant ligand in controlling M n 's and MWDs. With zirconium(IV) and hafnium(IV), a few initiators have already been reported. 8 The direct reactions of the (R, R)-(-)- N, N 0 -bis(3,5-di- t -butylsalicylidene)-1,2-cyclohexanedi- amine (salen) ligand and their derivatives are known with group 4 metal halides, resulting in the synthesis of mononuclear metal complexes. 9 However, the synthesis of dinuclear complexes with such ligands and zirconium(IV) and hafnium(IV) alcoholates still remains unexplored. We surmised that the dinuclear struc- ture of the metal precursors must have an impact on controlling the nuclearity of the new complexes. 7a,10 Herein, we report the synthesis and structural characterization of new zirconium and hafnium compounds containing the salen ligand backbone. These compounds are potent initiators for the solvent-free ring- opening polymerization of rac-lactide (rac-LA), L-lactide (L-LA), ε-caprolactone (CL), δ-valerolactone (VL), and β-butyrolactone (BL). The theoretical M n 's match very well with the observed M n 's in these polymerizations. With rac-LA, the resulting poly- mer is atactic. Using our derivatives, the polymerization of BL results in the formation of poly(3-hydroxybutyrate). We have done controlled hydrolysis studies on these compounds and successfully characterized the hydrolysis product from the haf- nium compound. This product is also a viable initiator for the polymerization of the above-mentioned monomers. The salen ligand was reacted with Zr(O i Pr) 4 (HO i Pr) or Hf(O t Bu) 4 in a 1:2 stoichiometric ratio, leading to the formation of 1 and 2 (Scheme 1; see also the Supporting Information, SI). These compounds were purified by crystallization from to- luene and isolated as yellow crystalline solids in high yields and purity. A stoichiometric reaction between salen and Zr(O i Pr) 4 - (HO i Pr) or Hf(O t Bu) 4 revealed the formation of the required product 1 or 2 along with the presence of an unreacted ligand. The compounds were completely characterized using various spectroscopic techniques, and their purity was assured by the correct elemental analysis results. The 1 H and 13 C NMR clearly reveal signals corresponding to the terminal and bridging alkoxy groups. Electrospray ionization mass spectrometry (ESI-MS) spectra reveal that these compounds have a dinuclear structure (see the SI). A single-crystal X-ray structure of 1 (Figure 1; see also the SI) proves that the complex is dinuclear in the solid state and each Zr center adopts a distorted octahedral geometry. The two N centers from the ligand are coordinated to the Zr centers. The two metal centers are bridged by two -O i Pr moieties, resulting in the formation of a dinuclear core that resembles the one from the starting material Zr(O i Pr) 4 (HO i Pr). 7a,9b All bond lengths and angles are in agreement with literature precedents. 8g,h Received: December 18, 2010