Group 1 and group 2 metal complexes supported by a bidentate bulky iminopyrrolyl ligand: synthesis, structural diversity, and ε -caprolactone polymerization study †

We report here a series of alkali and alkaline earth metal complexes, each with a bulky iminopyrrolyl ligand [2-(Ph 3 CN v CH)C 4 H 3 NH] ( 1-H ) moiety in their coordination sphere, synthesized using either alkane elimination or silylamine elimination methods or the salt metathesis route. The lithium salt of molecular composition [Li(2-(Ph 3 CN v CH)C 4 H 3 N)(THF) 2 ] ( 2 ) was prepared using the alkane elimination method, and the silylamine elimination method was used to synthesize the dimeric sodium and tetra-nuclear potassium salts of composition [(2-(Ph 3 CN v CH)C 4 H 3 N)Na(THF)] 2 ( 3 ) and [(2-(Ph 3 CN v CH)C 4 H 3 N)K(THF) 0.5 ] 4 ( 4 ) respectively. The magnesium complex of composition [(THF) 2 Mg(CH 2 Ph){2-(Ph 3 CN v CH)C 4 H 3 N}] ( 5 ) was synthesized through the alkane elimination method, in which [Mg(CH 2 Ph) 2 (OEt 2 ) 2 ] was treated with the bulky iminopyrrole ligand 1-H in 1 : 1 molar ratio, whereas the bis(iminopyrrolyl)magnesium complex [(THF) 2 Mg{2-(Ph 3 CN v CH)C 4 H 3 N} 2 ] ( 6 ) was isolated using the salt metathesis route. The heavier alkaline earth metal complexes of the general formula {(THF) n M(2-(Ph 3 CN v CH)C 4 H 3 N) 2 } [M = Ca ( 7 ), Sr ( 8 ), and n = 2; M = Ba ( 9 ), n = 3] were prepared in pure form using two synthetic methods: in the ﬁ rst method, the bulky iminopyrrole ligand 1-H was directly treated with the alkaline earth metal precursor [M{N(SiMe 3 ) 2 } 2 (THF) n ] (where M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent at ambient temperature. The complexes 7 – 9 were also obtained using the salt metathesis reaction, which involves the treatment of the potassium salt ( 4 ) with the corresponding metal diiodides MI 2 (M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent. The molecular structures of all the metal complexes ( 1-H , 2 – 9 ) in the solid state were established through single-crystal X-ray di ﬀ raction analysis. The complexes 5 – 9 were tested as catalysts for the ring-opening polymerization of ε -caprolactone. High activity was observed in the heavier alkaline earth metal complexes 7 – 9 , with a very narrow polydispersity index in comparison to that of magnesium complexes 5 and 6 .


Introduction
Biodegradable polyesters such as poly(ε-caprolactone) (PCL) and poly(lactic acid) (PLA) have attracted widespread interest from industrial and academic research groups, particularly polylactones, which are widely used in medicinal applications such as drug delivery systems, medical sutures, and as plastic modifiers in industries. 1-3 Although they can be obtained by traditional poly-condensation reactions, 4 these polymers are best prepared through the ring-opening polymerization (ROP) of cyclic esters. Macromolecular engineering of these polymers is gaining importance for the synthesis of telechelics, block, graft, and star-shaped polymers. 5 This, in turn, requires the synthesis of polyesters with predictable molecular weights, low polydispersity indices, and control over end groups. Various metal complexes such as aluminium, 6 titanium, 7 tin, 8 zinc, 9 magnesium, 10 and rare earth metals 11 have been used as initiators for the ROP of ε-caprolactone (ε-CL). In many cases, the molecular weight distribution of PCL became broader after the monomer was completely consumed, which suggested the occurrence of trans-esterification (Chart 1). 11b,12 These transesterifications occur very often during the polymerization of ε-CL by metal alkoxides, and the rate at which they occur are related to both the nature of the metal ion and the groups surrounding the ion. Subsequently, it was shown that sterically demanding groups attached to a metal ion can prevent PCL chains from coordinating to the ion, and therefore minimize trans-esterification reactions. 13 Thus, the development of very efficient metal initiators with strong control over the initiation, propagation, and termination steps is needed to synthesize polyesters with predictable molecular weights and narrow polydispersity indices. So far, the ROP process initiated by the heavier alkaline earth (Ae) metals calcium, strontium, and barium has been explored less when compared to other metal complexes, [6][7][8][9][10][11][12][13] and only a few well-defined neutral heteroleptic and homoleptic Ae ROP catalysts are mentioned in the literature. 14,15 This reflects the lack of information in the literature about the synthesis, stability, and reactivity of complexes of these highly electropositive metals. Recurring issues typical of Ca, Sr, and Ba complexes include their kinetic lability with Schlenk-type equilibria in solution. 16 However, significant efforts have been made of late in order to make them friendly and exploit the high reactivity of these complexes. Strategies aimed at suppressing solution distribution equilibria through the careful selection of ancillary ligands such as tris( pyrazolyl)borates, 17 β-diketiminates, 18 aminotrop-(on)iminates, 19,20 or bulky nucleophilic substituents have been invented, 21 while the range of synthetic precursors is growing steadily. 22,23 As a result, single-site Ae-based catalysts have shown an astounding ability for a variety of transformations involving σ-bond metathesis processes. 24 Recently, iminopyrrole ligands have been commonly employed in the synthesis of several transition metals and rare earth metal compounds. 25 The metal complexes including main group metals, transition metals as well as rare earth metals with 2-iminopyrrolyl in their coordination spheres act as efficient polymerization catalysts. 26 The Mashima group reported a series of alkaline earth metal complexes stabilized by the [2-(2,6-i Pr 2 C 6 H 3 NvCH)C 4 H 3 NH] ligand and those Aecomplexes act as efficient catalysts for the ROP of ε-CL. They also investigated the effects of ionic radii of metal ions on the rate of polymerization and concluded that it was rather good for heavier alkaline earth metal complexes. 27 However, we have noticed that PCLs obtained by heavier alkaline earth metal complexes supported by the [2-(2,6-i Pr 2 C 6 H 3 NvCH)C 4 H 3 NH] ligand have moderate polydispersity indices (PDI = 2.0 for Ba and 1.9 for Mg complexes). This could be due to intra or inter-molecular trans-esterification reactions or other side reactions, which are very common in the ROP of cyclic esters (Chart 1). In our ongoing research into heavier alkaline earth metal chemistry, we have previously reported a series of alkaline earth metal complexes with amidophosphine-chalcogenides/boranes [R 2 NHPh 2 P(E)] n (R = C(CH 3 ) 3 , CHPh 2 , CPh 3 , *CH(CH 3 )(Ph), -CH 2 -CH 2 -; E = O, S, Se, and BH 3 ; n = 1 or 2) in their coordination spheres. 28 In these complexes the anionic ligands presented novel molecular structural characteristics via coordination from the amido-nitrogen atom and coordination from the chalcogenide atom or borane group through the hydrogen atoms either in a η 1 or η 2 fashion. A few such complexes have presented excellent catalytic activity towards the ROP of ε-CL. 28d,f In this context, we introduce another sterically demanding ancillary ligand [2-(Ph 3 CNvCH)-C 4 H 3 NH] (1-H) into alkaline earth metal coordination chemistry. We envisage that the sterically demanding bulky substituent on the imine nitrogen atom will completely shield the metal ion, preventing trans-esterification or other side reactions to afford narrower polydispersity indices for PCL (Chart 2).
pyrrol-2-carboxyaldehyde with 1 equiv. of tritylamine in the presence of a catalytic amount of glacial acetic acid in methanol solvent (Scheme 1). The ligand 1-H was fully characterized using standard spectroscopic/analytical techniques and its solid-state structure was established using single-crystal X-ray diffraction analysis.
A strong absorption band observed at 1629 cm −1 in FT-IR spectra indicates a CvN bond in the ligand 1-H. This value is within the range reported in the literature. 29,30 The 1 H NMR spectrum of ligand 1-H showed a broad resonance signal at δ 9.56 ppm for the N-H proton of the pyrrole moiety. The singlet resonance signal at δ 7.71 ppm can be assigned to the imine NvC-H proton. In addition, the singlet at 6.96 ppm, doublet at 6.45 ppm, and multiplets centered at 6.29 ppm in the 1 H NMR spectrum clearly represent the resonance of pyrrole ring protons. In the 13 C{ 1 H} NMR spectrum, we observed a strong resonance signal at δ 150.2 ppm for the imine carbon atom -CvN, which is in good agreement for the compound [2-(2,6-i Pr 2 C 6 H 3 NvCH)-C 4 H 3 NH] (δ 153.2 ppm) and for the compound [2-(2-Ph 2 PC 6 H 4 NvCH)-C 4 H 3 NH] (δ 148.7 ppm) reported in the literature. 8 The resonance signal at δ 77.8 ppm corresponds to the tertiary carbon atom of the CPh 3 group.
The bulky iminopyrrolyl ligand 1-H readily crystallizes in CH 2 Cl 2 at room temperature and therefore, the solid-state structure was established using single-crystal X-ray diffraction analysis. The molecular structure of ligand 1-H is shown in Fig. 1 and details of the structural parameters are given in Table TS1 in the ESI. † Ligand 1-H crystallizes in the monoclinic space group P2 1 /c with two independent molecules in the asymmetric unit. The bond distance of 1.357(6) Å observed for C1-N1 is in good agreement with the value reported for the compound [2-(2,6-i Pr 2 C 6 H 3 NvCH)-C 4 H 3 NH] 31
The sodium complex 3 crystallizes in the monoclinic space group P2 1 /n, with two molecules in the unit cell. The details of the structural parameters are given in TS1 in the ESI. † The solid-state structure of complex 3 confirms its dimeric structure owing to the larger size of the sodium ion than the lithium ion when compared to the monomeric lithium complex 2. The solid-state structure and selected bond lengths and bond angles are shown in Fig. 3. In the dimeric sodium complex 3, each sodium ion is surrounded by one anionic ligand moiety in a bidentate (κ 2 ) fashion and one THF molecule. Each sodium ion further has π-interactions with pyrrole ring carbons in η 3 mode in the dimeric sodium complex 3. Therefore, each ligand in the dimeric sodium complex 3 bonds to the sodium ions in (σ + π) mode. The geometry of each sodium ion is best described as distorted tetrahedral, formed due to the coordination between two nitrogen atoms of the iminopyrrolyl ligand, one oxygen atom of the THF molecule, and η 3 -coordination from the pyrrolyl ring of the dimer fragment. The bite angles of 95.46(5)°for N1-Na1-N2, 75.44(5)°for N1 i -Na1-N2, and 85.56(6)°for N1-Na1-N1 i were observed for each of the iminopyrrolyls chelated to the sodium atoms. The Na-N bond distances of 2.3586 (17) and 2.4641(16) Å were in the range similar to the Na-N distances observed in the compound [µ 2 :κ 2 -2-(2,6-Me 2 C 6 H 3 NvCH)C 4 H 3 NNa(OEt 2 )] 2 [2.405(3) and 2.4285(3) Å]. 31 The distance between the sodium ion and the pyrrole ring atoms (C1, N1, and C4 or C1 i , N1 i , and C4 i ) were found to be 2.790(2), 2.6998 (17), and 2.8670(19) Å respectively. These distances are somewhat longer when compared to the Na-pyrrolyl centroid distances of 2.447(3) Å and 2.494(3) Å found in the polymeric sodium compounds of the type [{Na(μ 2 :κ 2 -N,N′-iminopyrrolyl)} 2n (OEt 2 ) 2x ] (n ≥ 1; x = 0 or 1), (aryl = C 6 H 5 or 2,6-Me 2 C 6 H 3 ), 31 indicating that moderate π-interactions exist between the sodium ions and pyrrolyl rings in the dimeric sodium complex 3. The bond distances of 1.349(2), 1.436(3), and 1.289(2) Å for C1-N1, C4-C5, and C5-N2 respectively were almost unchanged compared to that of the free ligand [C1-N1: 1.357(6), C4-C5: 1.433(6), and C5-N2: 1.260(6) Å] upon coordination to the sodium ion. Therefore, each bidentate iminopyrrolyl ligand forms a fivemembered metallacycle Na1-N1-C4-C5-N2 or Na1 i -N1 i -C4 i -C5 i -N2 i with the sodium ion, where the sodium ions are slightly deviated from the planarity. Each sodium ion in the dimeric complex 3 is further stabilized by the coordination from one THF molecule. The Na-O bond distance of 2.3315(17) Å fits well with reports in the literature. A short contact Na⋯H between the sodium ion and one of the phenyl protons (Na1⋯H13 2.707 Å) is observed, which can be characterized as a remote or secondary M⋯H interaction. 35 However, in solution all phenyl protons appear equivalent, as observed in the 1 H NMR study, presumably due to the dynamic behavior of the complex.
bond lengths and bond angles are shown in Fig. 4. The asymmetric unit of potassium complex 4 contains two iminopyrrolyls, two potassium ions, and one coordinated THF molecule. It must be noted that the coordination spheres of both the potassium ions are different. The ion K1 is chelated by two iminopyrrolyl ligands in a bidentate fashion, and one THF molecule, whereas the second ion, K2, is chelated by two nitrogens from a iminopyrrolyl ligand in a κ 2 fashion and π interaction (η 5 -mode) from one pyrrole ring of the adjacent iminopyrrolyl ligand. Therefore, in the grown structure, two potassium ions (K2 and K2 i ) are observed as sandwiched between two pyrrolyl ring π-electron densities in a η 5 -fashion and further chelated by imine-nitrogen atoms of the iminopyrrolyls. The other two potassium atoms (K1 and K1 i ) are surrounded by iminopyrrolyl moieties in a bidentate fashion and one THF molecule. Further, K1 has weak interactions with the aromatic ring hydrogen atoms (K1⋯H24 and K1⋯H48). Therefore, the geometry of K1 is best described as distorted trigonalbipyramidal, while that of K2 as distorted tetrahedral. The K2-N1 pyrrolyl and K2-N3 pyrrolyl bond distances, 3.155(3) and 3.041(3) Å respectively, fit well with the K-N distances observed in [K(THF) 2 {Ph 2 P(Se)N(CMe 3 )}] n (3.047(3) Å). 28e The K1-N1 pyrrolyl and K1-N3 pyrrolyl bond distances of 2.971(3) and 2.667(3) Å respectively, which are in good agreement with K-N distances, were observed in the complexes [{Ph 2 P(Se)NCHPh 2 }K(THF) 2 ] 2 [(2.725(3) Å] and [{Ph 2 P(BH 3 )NCHPh 2 }K(THF) 2 ] 2 [(2.691(2) Å] previously reported by our group. 28 The K2-N2 imine and K1-N4 imine bond distances of 2.946(3) and 3.013(3) Å respectively were also observed. An average distance of 2.913 Å for the potassium-pyrrolyl ring centroid was observed in the potassium complex 4, which indicates that highly electropositive and larger potassium atoms have considerable interactions with pyrrolyl π-electron density. In addition, K1 has very weak interactions with the aromatic ring hydrogen atoms (K1⋯H24 3.047 Å and K1⋯H48 2.891 Å), reducing their coordination unsaturation. The observed K1-O1 bond distance is 2.668(3) Å, which is in a range similar to that reported in the literature. To the best of our knowledge this is the only example observed of a µ 2 -(η 1 -η 5 )binding mode between a pyrrole ring and potassium atoms when considering different binding modes such as μ 2 -(η 1 -η n )-reported many times in the literature. 36
The two magnesium complexes 5 and 6 were fully characterized using spectroscopic and analytical techniques. The molecular structures of complexes 5 and 6 were established by single-crystal X-ray diffraction analysis. In the 1 H NMR spectra of 5 and 6 recorded in C 6 D 6 , the resonance of the imine proton was observed as a singlet at δ 7.91 ppm (5) and 7.66 ppm (6). The resonances of the two benzyl protons of the -CH 2 Ph group were obtained as singlets at δ 1.73 ppm Scheme 2 Synthesis of heteroleptic (5) and homoleptic (6) magnesium complexes. for complex 5. In the 13 C{ 1 H} NMR spectra, resonance at δ 146.3 ppm (for 5) and 145.8 ppm (for 6) can be assigned to the imine carbon (HCvN) present in the ligand moiety. However, these values are significantly up-field shifted compared to the free ligand 1-H (150.3 ppm). In addition, a resonance signal at δ 39.3 ppm was observed for the benzylic carbon atom in complex 5.

Dalton Transactions Paper
In the solid state, complexes 5 and 6 crystallize in the triclinic space group P1, with four molecules of 5 and one molecule of 6 in the unit cell. The details of the structural parameters are given in TS1 in the ESI. † The solid-state structures of complexes 5 and 6 confirmed the attachment of one (for 5) and two (for 6) iminopyrrolyl ligands to the magnesium ion through the κ 2 -NN mode. Fig. 5 and 6 show the molecular structures of complexes 5 and 6 respectively. The central magnesium ion in complex 5 is chelated via two nitrogen atoms of the iminopyrrolyl moiety, one benzyl carbon of the -CH 2 Ph group, and two oxygen atoms from two THF molecules. Thus, the geometry of the magnesium ion in this complex can be best described as distorted trigonal bipyramidal, with two oxygen atoms in the apical position, and two nitrogen atoms and one carbon atom in the basal plane. In contrast, the central magnesium atom in complex 6 is coordinated by two iminopyrrolyl moieties and two THF molecules to adopt a distorted octahedral geometry around the magnesium ion. Both complexes 5 and 6 display two sets of Mg-N distances: one short and one long. The short bond distances Mg-N pyr , 2.070 (2) (for 5) and 2.0813 (14) 37 Recently, our group also synthesized magnesium complexes of the type [Mg{C 2 H 4 (NPh 2 P(Se)) 2 }-(THF) 3 ] in which we observed Mg-N distances of 2.066(3) and 2.083(3) Å, which are in good agreement with the observed values of 2.070(2) and 2.0813(14) Å for the complexes 5 and 6 respectively. 28e The slightly elongated Mg1-N2 distance of 2.200(2) Å in complex 5 represents the coordination bond between the imine nitrogen and the magnesium ion. The Mg-N bond distances also agree well with the Mg-N imine bond distances [2.194(16) Å for [{C 4 H 3 N(2-CH 2 NMe 2 )}Mg{N(SiMe 3 ) 2 }] 2 ; 2.225(10) Å for [{C 4 H 3 N(2-CH 2 NEt 2 )}Mg{N(SiMe 3 ) 2 }] 2 reported by Ting-Yu Lee et al. 38 In contrast, the Mg-N imine bond length of 2.5422(14) Å in complex 6 is longer than the Mg-N imine distance of 2.200(2) Å observed in complex 5 and literature reports. 28e,37-39 The elongated Mg-N imine bond length in complex 6 can be explained by the steric congestion-created moiety around the magnesium ion by the presence of two bulky triphenyl groups attached to the imine nitrogen atoms of the bulky iminopyrrolyl. In complex 5, the coordinating nitrogen atoms N1 and N2 formed a bite angle of N1-Mg1-N2 80.98(8)°with the magnesium ion. However, the bite angle of N1-Mg1-N2 75.72(5)°is slightly reduced in complex 6. In complex 5, the Mg1-C25 bond distance of 2.185(3) Å is in good agreement with the Mg-C bond distances of 2.1697(17) Å in [(tmeda)Mg(CH 2 Ph) 2 ] and 2.1325(18) Å in [η 2 -HC{C(CH 3 )-NAr′} 2 Mg(CH 2 Ph)(thf )] (Ar′ = 2,6-diisopropylphenyl) observed and reported by P. J. Bailey et al. 40 In complexes 5 and 6, the Mg-O bond distances of 2.204 and 2.225 Å (for 5) and 2.1517(12) Å (for 6) fit well with the values we previously reported. 28e, 38,39 For complex 5, a five-membered magnesium metallacycle N1-C4-C5-N2-Mg1 and for complex 6, two five-  membered metallacycles, N1-C4-C5--N2-Mg1 and N1 i -C4 i -C5 i -N2 i -Mg1, are observed due to the κ 2 -NN coordination of the iminopyrrolyl ligand 1.
The heavier alkaline earth metal complexes of composition [M(2-(Ph 3 CNvCH)C 4 H 3 N) 2 (THF) n ] [M = Ca (7), Sr (8), and n = 2; M = Ba (9), n = 3] were synthesized using two methods. In the first method, the bulky iminopyrrolyl ligand 1-H was directly charged with the corresponding alkaline earth metal bis(trimethylsilyl)amides [M{N(SiMe 3 ) 2 } 2 (THF) n ] (where M = Ca, Sr, and Ba) in 2 : 1 molar ratio in THF solvent at ambient temperature. The same alkaline earth metal complexes 7-9 were also obtained using the salt metathesis reaction involving the treatment of potassium salt 4 with the corresponding alkaline earth metal diiodides MI 2 (M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent (Scheme 3). 33 In the 1 H NMR spectra, each of the complexes 7-9 shows a sharp singlet resonance at δ 7.95 (for 7), 8.04 (for 8), and 7.89 (for 9) ppm, indicating the presence of the imine -C-H proton in the metal complexes, which is slightly downfield shifted compared to the free ligand (7.67 ppm). The coordinated THF molecules can be easily recognized by the 1 H NMR spectra as two multiplets centered at 3.61 and 1.76 ppm (for 7), 3.38 and 1.17 ppm (for 8), and 3.56 and 1.40 ppm (for 9). One set of resonance signals was observed for the aromatic ring protons in each metal complex, indicating dynamic behavior in the solution state. The solid-state structures of complexes 7-9 were established through single-crystal X-ray diffraction analysis. The centro-symmetric calcium complex 7 crystallizes in the monoclinic space group P2 1 /n, with two molecules in the unit cell. In contrast, both the strontium and barium complexes 8 and 9 crystallize in the non-centro-symmetric triclinic space group P1, with two molecules each in their respective unit cells. The details of the structural parameters are given in TS1 in the ESI. † The molecular structures of complexes 7-9 are shown in Fig. 7a-c respectively. The selected bond lengths and bond angles of complexes 7-9 are given in Table 1. The calcium complex 7 is iso-structural to the corresponding magnesium complex 6, in which the central calcium ion is surrounded by two anionic iminopyrrolyl ligands and two THF molecules trans to each other. Each ligand moiety coordinates to the metal center through the N pyrrolyl and N imine atoms, and forms two five-membered metallacycles, N1-C4-C5-N2-Ca1 and N1i-C4i-C5i-N2i-Ca1, with a bite angle of 71.76(13)°. In complex 8, the strontium ion was ligated by two chelating bulky iminopyrrolyls and two THF molecules. However, the two THF molecules are cis to each other. In both complexes 7 and 8, the geometry around the central metal ion can be best described as distorted octahedral. In complex 7, the Ca-N pyr bond distance of 2.423(4) Å and Ca-N imin bond distance of 2.567 (  and Imp Me = 2-(2,6-C 6 H 3 Me 2 -CNvCH)C 4 H 3 N). 27 The Ca1 In the barium complex 9, the Ba-N pyr bond distances [Ba1-N1 2.731(5) Å; Ba1-N3 2.762(5) Å] are the longest among all the complexes 7-9 due to the largest ionic radius of Ba 2+ among three metal ions. However, these values fit well to the Ba-N bond distances reported for the complexes of composition [Ba((Dipp) 2 DAD)(μ-I)(THF) 2 ] 2 [2.720(4) and 2.706(4) Å]. 21 The Ba-N imine bond distances of Ba1-N2 2.946(5) Å and Ba1-N4 2.933(5) Å are slightly longer than the Ba-N distances reported for the complexes of composition [(Imp Dipp ) 2 Ba(THF) 2 ] [Ba-N1 2.821(5) Å and (Ba-N2 2.823(4) Å] and [Ba((Dipp) 2 DAD)-(μ-I)(THF) 2 ] 2 (2.720(4) and 2.706(4) Å] (where Imp Dipp = 2-(2,6-C 6 H 3 i Pr 2 -CNvCH)-C 4 H 3 N). 27,42 The Ba-N bond distances are also comparable to the barium complexes we reported previously. 28 Each ligand moiety is ligated to the barium ion through the N pyr and N imine atoms to form two five-membered metallacycles N1-C1-C5-N2-Ba1 with a bite angle of 63.78 (14)

ROP studies on ε-CL
A series of alkaline-earth metal complexes supported by a bulky iminopyrrolyl ligand were studied as initiators for the living ROP of ε-CL. The typical ring-opening polymerization process is depicted in Scheme 4. We mostly concentrated on living ROP and not on immortal ROP of ε-CL in order to understand the influence of steric bulk on rate of polymerization and the influence of the nature of the metal centre. In the living ROP of cyclic esters, the metal complex acts an initiator, that is, each metal center produces only one polymer chain. On the other hand, an immortal ROP is performed upon addition of a large excess of a protic agent (typically an alcohol) acting as an exogenous initiator and a chain transfer agent, and the complex acts as a catalyst: if the transfer between growing and dormant macromolecules is fast and reversible, the number of polymer chains generated per metal center is equal to the [transfer agent] 0 -to-[metal] 0 . The selected data obtained with alkaline-earth metal complexes (5-9) as initiators for living ROP of ε-CL are shown in Table 2. The catalytic efficiency of newly synthesized heteroleptic and homoleptic magnesium complexes 5 and 6 to promote ROP of ε-CL was first evaluated (Table 2, entries 1-6). Indeed, although some previously reported studies on similar magnesium complexes having less-bulky iminopyrrolyls in their coordination sphere gave poor results under similar polymerization conditions, 27 our preliminary investigations on magnesium complexes 5 and 6 showed that they are active in the ROP of ε-CL at 25°C in toluene with conversion over 90% within 15 minutes ( Table 2, entries 1-6). The molar mass distribution PDI values obtained from GPC analysis are narrow (PDI < 1.8, for entries 1-6) and controlled molecular weight distribution was observed. We noticed that  (17) 84.29(16) O1-M-N2 89.32 (15) 78.967 (7) 75.024 (5) the heteroleptic Mg complex (5) is more active than the homoleptic Mg complex (6). The difference in reactivity could be understood by the initiation steps in both the cases. In the case of complex 5 polymerization follows a nucleophilic route and is initiated by the transfer of an alkyl ligand to the monomer, with cleavage of the acyl-oxygen bond and formation of a metal alkoxide-propagating species. 43 A similar mechanism was also suggested by the A. M. Rodrıguez group for the magnesium complex of composition [Mg(CH 2 SiMe 3 )-(κ 2 -η 5 -bpzcp)] (where bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl cyclopentadienyl) as an initiator for the living ROP of ε-CL. 44 The results obtained therein (PDI < 1.5 with controlled molecular weights) are comparable to our observations (see Table 2) suggesting that the ligand steric bulk and the nature of the metal centre play a crucial role in the ROP of ε-CL. The calcium complex 7 also showed comparable activity towards the ROP of ε-CL with magnesium analogues (5 and 6) with narrow PDI values and controlled molecular weight distributions (Table 2, entries 7-11). Indeed, the sluggish reactivity of the calcium complexes is very similar to that observed in some previously reported studies using other calcium complexes for ROP of ε-CL, 45,46 we have noted living polymerization characteristics at room temperature without using any initiating agent like alcohol (entry 9, PDI = 1.5 and M w = 50 483) indicating that the triphenylmethyl group on the ligand backbone strongly influences the activity of calcium complexes towards the ROP of ε-CL. We anticipated that strontium (8) and barium (9) complexes could be more active than those of magnesium and calcium complexes having bulky iminopyrrolyls due to the larger ionic radii of Sr 2+ and Ba 2+ ions. 47,48 Both strontium and barium analogues showed higher reactivity towards the conversion of ε-caprolactone to poly-caprolactone and up to 600 ε-CL units were successfully converted in high yields (90 to 98%) within 5-10 minutes at 25°C. The control over the ROP process was rather good, affording PCLs, with controlled molar mass values, as well as very narrow dispersity data (PDI < 1.4, entries 12-21). Therefore, the overall catalytic efficiency of ringopening polymerization by heavier alkaline-earth metal complexes (Sr 2+ and Ba 2+ ) supported by sterically hindered iminopyrrolyl ligands was much better and afforded polycaprolactone with controlled molecular weights and narrow PDI values. From the 1 H NMR spectrum of low-molecular weight PCL by 9 (run 17), we found resonance signals assignable to a terminal iminopyrrolyl group (Fig. S19 †), indicating that in the case of amido complexes of alkaline-earth metal complexes (6-9) the initial step of the polymerization was a nucleophilic attack of the pyrrolyl nitrogen atom towards the carbonyl carbon of the monomer followed by acyl-oxygen cleavage.

General consideration
All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen and moisture in flamedried Schlenk-type glassware either on a dual manifold Schlenk line, interfaced to a high vacuum (10 −4 Torr) line, or in an argon-filled M. Braun glove box. THF was pre-dried over Na wire and distilled under nitrogen from sodium and benzophenone ketyl prior to use. Hydrocarbon solvents (toluene and n-pentane) were distilled under nitrogen from LiAlH 4 and stored in the glove box. 1 H NMR (400 MHz) and 13  and LiCH 2 SiMe 3 were prepared according to procedures prescribed in the literature. 30,49,50 The NMR solvent C 6 D 6 and CDCl 3 were purchased from Sigma Aldrich and dried under either Na/K alloy (for C 6 D 6 ) or a molecular sieve prior to use.
Other heavier alkaline earth bis(iminopyrrolyl) complexes 8 and 9 were prepared in a manner similar to complex 7 using two routes.  H 6.65,N 4.80. Found C 69.48,H 6.16,N 4.53.

Typical polymerization experiment
In a glove box under an argon atmosphere, the catalyst was dissolved in appropriate amount (1.0 mL) of dry toluene. ε-CL in 1.0 mL of toluene was then added along with vigorous stirring. The reaction mixture was stirred at room temperature for 5-10 minutes, after which the reaction mixture was quenched by addition of a small amount of (1.0 mL) methanol, to which a slight excess of acidified methanol was then added. The polymer was precipitated in excess methanol and it was filtered and dried under vacuum. The final polymer was then analyzed using NMR and GPC.
Single crystals of compound 1-H suitable for X-ray measurements were obtained from CH 2 Cl 2 at room temperature. For compounds 1-9 (except 5 and 7) a crystal of suitable dimensions was mounted on a CryoLoop (Hampton Research Corp.) with a layer of light mineral oil and placed in a nitrogen stream at 150(2) K and all measurements were made on an Agilent Supernova X-calibur Eos CCD detector with graphitemonochromatic Cu-Kα (1.54184 Å) radiation. For compounds 5 and 7, data were collected at 113(2) K and measurements were made on a Rigaku RAXIS RAPID imaging plate area detector or a Rigaku Mercury CCD area detector with graphite-monochromated Mo-Kα (0.71075 Å) radiation. Crystal data and structure refinement parameters are summarized in Table TS1 in the ESI. † The structures were solved by direct methods (SIR92) 51 and refined on F 2 by full-matrix least-squares methods; using SHELXL-97. 52 Non-hydrogen atoms were anisotropically refined. H atoms were included in the refinement in calculated positions riding on their carrier atoms. The function minimized was [∑w(F o 2 − F c 2 ) 2 ] (w = 1/[σ 2 (F o 2 ) + (aP) 2 + bP]), where P = (Max(F o 2 ,0) + 2F c 2 )/3 with σ 2 (F o 2 ) from counting statistics.

Conclusion
We have successfully prepared alkali metal complexes of the bulky iminopyrrolyl ligand and their solid-state structures reveal the flexibility and multidentate behavior of the iminopyrrolyl ligand 1. In the case of the sodium complex, we observed a dimeric structure whereas, due to lower charge density of the potassium ion, a tetranuclear structure was found in the solid state. The heteroleptic and homoleptic magnesium complexes 5 and 6 respectively were successfully synthesized and characterized using X-ray crystallography. In the solid state, complex 5 is five-fold coordinated and shows a trigonal bipyramidal geometry around the magnesium ion, whereas the magnesium ion in complex 6 adopts a octahedral arrangement due to the six-fold coordination from ligand 1 and THF molecules. The heavier alkaline earth metal complexes 7-9 were synthesized either by silylamine elimination or salt metathesis routes using [M{N(SiMe 3 ) 2 } 2 (THF) n ] or MI 2 (M = Ca, Sr, and Ba) as starting materials. In the solid state, the effect of the ion radii of the central metal ions as well as the steric bulk of the ligand backbone determined the metal coordination sphere. The calcium complex 7 is centro-symmetric and adopts an octahedral geometry, whereas the strontium and barium complexes (8 and 9), due to their larger size, are non-centro-symmetric and adopt distorted octahedral and distorted pentagonal-bipyramidal geometries respectively. In addition, the M-N pyr and M-N imine bonds are relatively longer than those of the other amido-metal complexes of barium and strontium. The bis(iminopyrrolyl)complexes of strontium (8) and barium (9) were highly active for the ROP of ε-CL, affording high molecular weight PCLs compared to the polymers produced by the calcium and magnesium complexes.