Highly Active Carbene Potassium Complexes for the Ring-Opening

Nov 16, 2017 - Synopsis. Two potassium complexes of aNHC and cAAC ligands have been synthesized, and their polymerization activities to cyclic esters ...
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Highly Active Carbene Potassium Complexes for the Ring-Opening Polymerization of ε‑Caprolactone Mrinal Bhunia,† Gonela Vijaykumar,† Debashis Adhikari,*,‡ and Swadhin K. Mandal*,† †

Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India Department of Chemical Sciences, Indian Institute of Science Education and Research Mohali, SAS Nagar 140306, India

‡

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S Supporting Information *

ABSTRACT: Herein we report the synthesis of two complexes of potassium employing strongly nucleophilic carbenes, such as cyclic “(alkyl)(amino)carbene (cAAC) and abnormal N-heterocyclic carbene (aNHC). Both complexes are dimeric in the solid state and the two potassium centers are bridged by trimethylsilylamide. In these complexes, the carbene- - -K interaction is predominantly electrostatic in character, which has been probed thoroughly by NBO and AIM analyses. Indeed, the delocalization energy of the cAAC lone pair calculated from the second-order perturbation theory was only 5.21 kcal mol−1, supporting a very weak interaction. The solution-state behavior of these molecules, as inferred from NOESY spectra, hints that the weak carbene- - -K interaction is retained in nonpolar solvents, and the bond is not dissociated at least on the NMR time scale. We took advantage of such a weak interaction to develop highly effective ring-opening polymerization catalysts for ε-caprolactone and rac-lactide. The efficacy of these catalysts is prominent from a very high substrate/metal-initiator ratio as well as very low dispersity index of the obtained polymer chains, reflecting significant control over polymerization.

T

tion.19 Moreover, there has been substantial interest in developing inexpensive, nontoxic alkali-metal catalysts, owing to the earth-abundant and benign nature of these metals. A thorough literature survey reveals that there have been only a few reports of s-block metals complexed by NHC and other related carbenes.20,21 Although there are some examples of potassium complexes of carbene ligands where a tethered anionic group provides further stabilization,22 examples of coordination by neutral carbenes to potassium are extremely scarce. A few such isolated examples involving potassium and nucleophilic neutral carbenes are presented in Chart 1. The first example reported by Alder involves a dimeric complex of potassium coordinated by six-membered diaminocarbenes where the potassium metals are bridged by trimethylsilyl amides (structure I, Chart 1).23 Hill and coworkers have isolated an identical potassium complex with a five-membered nNHC instead of a six-membered diaminocarbene (structure II, Chart 1).24 In the same report, they also observed the formation of a homoleptic carbene-K complex, where ipso-C of the 2,6-diisopropylphenyl substituent is interacting with the metal (represented by a dashed line in structure III). By a very similar effort involving cAAC ligands, recently Turner et al. isolated a homoleptic cAAC-K complex as a countercation of the Sr-amidate counteranion (structure IV,

he discovery of normal N-heterocyclic carbenes (nNHCs) by Arduengo and co-workers1,2 heralded a new era in organometallic chemistry. Bertrand modified this marvelous ligand class further, generating even more nucleophilic abnormal N-heterocyclic carbenes (aNHCs) and cyclic (alkyl)(amino)carbenes (cAACs).3−6 The stronger σ-donor and πacceptor abilities of aNHC7 and cAACs8 have made this class of ligands very unique in its stereoelectronic properties, and this family of ligands has been exploited to perform small-molecule activation,9,10 to stabilize both unusual main-group species11−13 and low-valent transition-metal complexes,14 and to carry out homogeneous catalysis.15−17 Despite these widespread research efforts involving a substantial variety of metals coordinated by these aNHC and cAAC carbenes, coordination complexes of s-block metals with these ligands remain very limited. Arguably, the lack of wellcharacterized alkali-metal complexes of the aNHC or cAACs stems from the very weak bond between the s-block metal and the carbene. The solution-state complexation behavior of sblock metals with carbenes is important, since carbenes are often prepared upon C-deprotonation of formamidinium and other related salts by bases with alkali-metal counterions. It has been documented earlier that when nonhindered diaminocarbenes were deprotonated by strong bases carrying alkali-metal counterions, the latter complexed with carbene.18 Additionally, coordination of carbenes to alkali metals is especially important regarding the rate and mechanism of the carbene dimeriza© 2017 American Chemical Society

Received: July 25, 2017 Published: November 16, 2017 14459

DOI: 10.1021/acs.inorgchem.7b01892 Inorg. Chem. 2017, 56, 14459−14466

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Inorganic Chemistry

The nature of the complexes has been probed by 1H and 13C NMR spectroscopy, CHN analysis, and single-crystal X-ray diffraction studies. To unambiguously understand the bond connectivity, colorless crystals were analyzed further via singlecrystal X-ray diffraction (Figure 1 and Table 1). Compound 1

Chart 1. Examples of Potassium Carbene Complexes

Chart 1).25 All of these complexes exhibit a common feature: a long carbene carbon to potassium bond with predominant ionic character. Arnold previously suggested that NHC-based complexes can initiate ring-opening polymerization of cyclic esters by the carbene, when the M−Ccarbene bond is sufficiently weak.21 We note that, despite some reports of carbene-based alkaline-earth metals25 to promote ring-opening polymerization (ROP), s-block elements are considerably underrepresented as catalysts. Herein we report two potassium complexes of highly nucleophilic cAAC and aNHC ligands. Upon assessing the weak coordination between a carbene and Group 1 metal center, we leveraged the lability of the bond and used these molecules as polymerization catalysts. As a part of our continued interest in developing aNHC- and cAAC-based catalysts,26−31 this report presents highly active carbene-based potassium complexes, which can catalyze ring-opening polymerization under very mild conditions.

Figure 1. Views of the molecular structures of 1 (top) and 2 (bottom). Ellipsoids are set at the 50% probability level; hydrogen atoms have been omitted for the sake of clarity. Selected bond distances (Å) and angles (deg) for 1: K1−K1, 3.7316(8); C1−K1, 2.9769(18); N1−C1, 1.301(2); K1−N2, 2.8806(15); C1−K1−K1, 160.16(4); N2−K1−C1, 111.98(5); N2−K1−C1, 149.59(5). Selected bond distances (Å) and angles (deg) for 2: K2−K1, 3.8075(6); K2−C40, 2.973(2); K2−N6, 2.7511(19); K2−N5, 2.864(2); K1−C1, 2.941(2); K1−N6, 2.8489(19); K1−N5, 2.7388(19); C40−K2−K1, 170.19(5); N6− K2−C40, 122.69(6); N5−K2−C40, 142.95(6); C1−K1−K2, 171.32(5); N6−K1−C1, 142.54(6); N5−K1−C1, 122.76(6).

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RESULTS AND DISCUSSION Synthesis and Characterization. The syntheses of 1 and 2 were accomplished by the treatment of free cAAC and aNHC ligands with commercially available potassium bis(trimethylsilyl)amide in an 1:1 stoichiometric ratio using toluene at room temperature (Scheme 1). Analytically pure colorless crystals of 1 and 2 were obtained in 49% and 38% isolated yields, respectively, from a concentrated toluene solution at −21 °C.32,33 Compounds 1 and 2 both readily dissolve in toluene, benzene, and THF as well as show some limited solubility in hexane and pentane.

crystallizes in the triclinic space group P1̅ with two molecules in the asymmetric unit, whereas compound 2 crystallizes in the orthorhombic space group P212121. The X-ray crystal structures of 1 and 2 show that the K ion is bonded to the cAAC and aNHC, respectively, by an elongated bond, where an N(SiMe3)2 bridge connects two potassium ions in a dimeric core. In the case of 1, the cAAC backbone is almost orthogonal to the diamond-shaped K1−N2−K1−N2 plane, most likely to avoid steric interaction between bulky aryl substituents and the silyl amide groups. In comparison, the aNHC backbone in 2 is only slightly tilted (∼19°), since the steric encumbrance is not as imposing as in the previous case. The average K− CcAAC bond distance in centrosymmetric 1 is 2.9769(18) Å and the same K−CaNHC bond is 2.973 Å (average distance) in 2. The K- - -C bond lengths (2.977 and 2.973 Å in 1 and 2, respectively) are slightly shorter than that observed by Hill in case of an nNHC-based complex (3.0291(17) Å), which is in agreement with the aNHC’s superior σ-donation capacity.24 In comparison to the C- - -K distance (3.002 Å) reported in Alder’s complex,23 our observed bond distances are also shorter, indicating a slightly stronger bond. The 13C{1H} NMR spectra of 1 and 2 reveal resonances at δ 307.5 and 197.2 ppm, assignable to the C-5 carbon bound to the potassium center, which is slightly shifted downfield for 1 and upfield for 2

Scheme 1. Syntheses of Complexes 1 and 2

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DOI: 10.1021/acs.inorgchem.7b01892 Inorg. Chem. 2017, 56, 14459−14466

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Inorganic Chemistry Table 1. Crystallographic Data for Complexes 1 and 2 empirical formula fw cryst syst a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (K) space group Z λ (Å) ρcalc (g cm−3) μ (mm−1) total no. of rflns no. of obsd rflns R1 wR2

1

2

C52H98K2N4Si4 969.90 triclinic 9.1952(5) 11.4794(7) 14.3463(9) 83.927(5) 80.950(5) 82.166(5) 1476.01(16) 293(2) P1̅ 1 0.71073 1.091 0.276 11666 6691 0.0439 0.1004

C90H124K2N6Si4 1480.51 orthorhombic 15.2987(3) 29.5861(7) 19.6789(4) 90.00 90.00 90.00 8907.3(3) 100.00(10) P212121 4 0.71073 1.104 0.205 26411 17380 0.0415 0.0930

Figure 2. Computationally optimized structures of the slightly truncated complexes of 1 (top) and 2 (bottom) at the B3LYP/631G* level of theory. Natural charges from the NBO analysis are shown along respective atoms. Color code: gray, carbon; blue, nitrogen; magenta, potassium.

from the corresponding chemical shifts of free cAAC (δ 304.2 ppm) and aNHC (δ 201.9 ppm). Such a small 13C (∼3.3 ppm for cAAC and 4.7 ppm for aNHC) NMR chemical shift in comparison to the free carbene carbon resonances is attributable to the weak interaction between the metal and carbene carbon and is also consistent with previously reported potassium complexes.21 Since both complexes display a mild shift in the carbene carbon resonance in the solution state in comparison to their chemical shifts observed in free carbene, it can be assumed that, despite the weak nature of the K−Ccarbene interaction, it is retained in solution. To substantiate this assertion further, we conducted a 2D-correlation spectroscopic experiment (NOESY) for 2 in C6D6. A clear, strong NOE signal was observed between the silylmethyl resonances (0.21 ppm) of the bridging amide and the methyl protons of the oisopropyl substituents (1.44 ppm) in the aNHC backbone (Figure S5 in the Supporting Information). This finding along with the change in chemical shift of 13C NMR data on coordination of the carbene strongly indicates that the carbenes remain bound to the potassium amide in solution and do not dissociate, at least on the NMR time scale. Computational Analysis of Bonding. To explore the nature of the K−Ccarbene bond, high-level quantum chemistry calculations were undertaken using the B3LYP DFT functional. Computationally optimized structures of slightly truncated models were validated as true minima by following up with frequency calculations. The CcAAC- - -K distance from the computationally optimized structure 1 is 2.97 Å, which is in very good agreement with the crystallographically obtained value and portrays a predominantly ionic bond. The primarily electrostatic interaction is intuitive, given that a hard metal is interacting with a soft and polarizable carbene. In the case of 2, structural optimization further reproduces the crystallographically derived metrical parameters as well. Natural bond orbital (NBO) analyses34 support that most of the positive charge is located on the Group 1 metal center and only minimally dissipated onto the ancillary ligands. Both the optimized structures and the NBO charges at the major atoms are presented in Figure 2. Furthermore, a detailed analysis of the

molecular orbitals for 2 can locate the carbene lone pair as the orbital HOMO-6, which is engaged in an ionic interaction with potassium (Figure 3). The HOMO for 2 is the π-bond of the

Figure 3. Frontier molecular orbitals of 2. HOMO-6 represents the pz lone pair of the carbene, which is donated to the Lewis acidic potassium. The HOMO is the π-orbital involving trimethylsilylamide. The LUMO is centered to the aromatic substituents of the abnormal carbene backbone.

silyl amide bridge, whereas the LUMO is the in-phase πinteraction of the carbene C−N bond along with some aromatic π-interaction in the substituted rings (Figure 3). A very close pattern of bonding is also shown by 1, where HOMO-4 clearly depicts the carbene based lone pair (Figure S14 in the Supporting Information). Precisely, the metal− ligand interaction in both complexes can be portrayed as dative σ-donation from the carbene to the metal center from a predominantly pz orbital. The second-order perturbation analysis of the donor−acceptor linkages from NBOs clearly suggests that the cAAC and aNHC ligands act as essentially pure σ-donors. The principal character of the C−M interaction is carbon-based, where the pz lone pair of electrons is interacting with the potassium 4s orbital. The delocalization 14461

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Inorganic Chemistry

2. When 700 equiv of ε-CL was introduced for polymerization, 78% and 82% conversions were realized in 5 and 10 min, respectively (entries 4 and 5, Table 2). Given the weak coordination of potassium and the carbenes discussed in this report, it is a pertinent question as to whether any free carbene might be dissociating in solution to initiate polymerization. A previous report on an nNHC-Zn complex also revealed that free carbene generated during the reaction was responsible for displaying polymerization activity, although the outcome in tacticity remained distinctly different.38 To ensure this, we conducted polymerization of ε-CL in the presence of free carbenes. No polymerization was observed during the use of free cAAC as an organocatalyst (entry 3, Table 2). This observation is also in full accord with Turner’s report where cAAC-based alkaline-earth metals were used as polymerization catalysts, but the free cAAC was totally inactive.25 The lack of polymerization activity in the presence of only cAAC clearly indicates that the combination of carbene and potassium amide plays a vital role in initiating the ring-opening polymerization of ε-CL. In contrast to the complete inactivity of the free cAAC, free aNHC is active to some extent as an organocatalyst under identical conditions, offering a 27% conversion after 1 h when 1000 equiv of ε-CL was subjected to polymerization (entry 6, Table 2). Furthermore, the relative catalytic activities of 1 and 2 were tested by studying the kinetics of polymerization reactions. The progress of polymerization reactions was monitored by 1H NMR spectroscopy at different time intervals after quenching the polymerization by addition of hexane. The APC analysis of the PCL exhibits a narrow molecular weight distribution with dispersity (Đ) values close to 1.04 (Table 2).39 Maintaining a low Đ for a high substrate to catalyst ratio is suggestive of the polymerization being “living” in nature. The controlled nature of the polymerization reaction is also clearly demonstrated in the plots of molecular weight (Mn(APC)) vs percent conversion, which shows the linear increase of molecular weight with the progress of the reaction for both 1 and 2 (Figure 4) Achieving such low Đ values strongly suggests that the initiation step associated with the polymerization is clean and much faster than the chain propagation step. We attribute such a clean and fast initiation to the weak bond between the carbene and potassium, whose labile nature facilitates the ring opening of the ε-CL. It is noteworthy that a high conversion to PCL can be achieved by the metal catalysts in absence of any additional initiator, unlike the case for many polymerization catalysts with a metal catalyst/BnOH combination.40 However, we were curious to test whether an external initiator could improve the polymerization yield further. Addition of BnOH to the polymerization mixture showed that 1 is not stable in BnOH. However, 2 remains intact in BnOH and the polymerization efficiency

energy in 1, calculated by NBO analysis, was obtained as mere 5.21 kcal mol−1, which is in strong agreement with a very weak bond. Such a weak bonding for Group 1 metal complexes with carbene is fully consistent with Hill’s report, where the carbene−M bond becomes progressively weaker on descending along the Group.24 Additionally, we undertook an “atoms in molecules” (AIM) analysis35 to obtain the quantitative structural and bonding feature present in these complexes. In 1, at a bond critical point (3, −1) the electron density ρ(r) value is small (0.014 e Å−3) and the Laplacian of the electron density ∇2ρ(rb) is a small positive number (0.048 e Å−5). These values unambiguously prove that the C- - -K interaction is closed-shell in nature and there is minimal covalent component present. A similar observation from AIM analysis is noted for the atomic interaction in 2, and the conclusions for its bonding pattern remain essentially the same. Polymerization of ε-Caprolactone (CL). The weak ionic interaction between the carbene lone pair and the potassium ion motivated us to test these molecules as polymerization initiators. At first, complexes 1 and 2 were tested as initiators (Scheme 2) for the ring-opening polymerization (ROP) of εScheme 2. ROP of ε-CL Catalyzed by Binuclear K Complexes 1 and 2

caprolactone (ε-CL). Table 2 summarizes the ROP results of εCL catalyzed by 1 and 2 and further details are given in Tables S5 and S6 in the Supporting Information. All polymerization screenings of ε-CL were carried out under a dry nitrogen atmosphere in toluene at room temperature, and the percentage of conversion of ε-CL to polycaprolactone (PCL) was monitored through 1H NMR spectroscopy by taking out a certain amount of sample from the reaction mixture at different time intervals. The synthesized polymers were analyzed by an advanced polymer chromatography (APC) instrument. Ring-opening polymerization reactions conducted using catalyst 1 (1 mM) with 700 equiv of ε-CL at room temperature in toluene resulted in 79% and 83% conversions in 5 and 10 min, respectively (entries 1 and 2, Table 2). Notably, the activity of our metal initiator is much better in comparison to some well-known polymerization catalysts for ε-CL.36,37 Ringopening polymerization reactions of ε-CL also proceed well with the abnormal carbene based binuclear potassium complex Table 2. ROP of ε-CL at Room Temperaturea entry

cat.

[CL]0/[Cat.]

time

conversnb (%)

Mn,expc

Mn,calcb

Đc

1 2 3 4 5 6

1 1 free cAAC 2 2 free aNHC

700 700 700 700 700 1000

5 min 10 min 3h 5 min 10 min 1h

79 83 0 78 82 27

59243 63997 0 58227 62760 28273

63781 66514 0 64001 65651 30818

1.07 1.04 0 1.09 1.07 1.09

In toluene at 25 °C. [Cat.] = 1 mM. bConversions were determined by 1H NMR spectroscopy. Mn,calc = molecular weight of chain end + 114.14 g mol−1 × ([CL]0/[Cat.]) × conversion. cObtained from APC analysis and calibrated by poly(methyl methacrylate). a

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Surprisingly, catalyst 1 is inactive in polymerizing rac-LA, whose exact reason is not yet clear to us. In contrast, 2 polymerizes rac-LA in toluene at room temperature showing 96% conversion within 2.5 h (entry 2, Table 3). The time dependence to the extent of conversion revealed 35%, 48% and 50% conversions were realized in 50 min, 2 h 15 min, and 2 h 50 min, respectively (entries 3−5, Table 3) with 300 equiv of rac-LA in toluene. The catalytic activity of 2 is comparable to some of the well-known indium,47 scandium48 and Al-salen catalysts.49 Unlike cAAC, free aNHC itself can polymerize racLA to a small percentage (35% within 2.5 h when 100 equiv of rac-LA was used) corroborating to our previous observation.40 Like the ε-CL polymerization by 1 and 2, rac-LA polymerization is also very controlled in nature, as evidenced from the plot of molecular weight (Mn(APC)) vs percent of conversion, which exhibits a linear increase (Figure 5). To shed light on the mechanism of the polymerization reaction of ε-CL, we conducted end-group analysis of the oligomer, initiated by 2. When 10 equiv of ε-CL monomer was treated with 2, and the reaction mixture was analyzed by 1H NMR spectroscopy, the aromatic protons from aNHC underwent a shift, possibly indicating that the carbene acts as a nucleophile to facilitate the ring opening of the ε-CL. The labeled aromatic peaks (with asterisks) are shifted by 0.15−0.2 ppm in the oligomeric product, in comparison to the corresponding peaks present in 2 (Figure 6). In complete agreement with this 1H NMR spectroscopic observation, the 13C NMR spectrum also displays a shift for the carbene carbon from 201.9 to 172.5 ppm (Figure S7), which may be attributed to the carbene carbon attached to a ring-opened acyl group. Furthermore, the integration of the backbone methylene groups of the PCL clearly reveals that 10 equiv of monomer has been inserted in the oligomeric product. These experiments indicate that the ring opening is initiated by the carbene rather than the trimethylsilyl amide. On the basis of these preliminary observations, it is proposed that the carbonyl group of the ε-CL is activated by Lewis acidic potassium and the carbene promotes ring opening by the acyl bond cleavage. The resulting alkoxide likely acts as the chain-propagating end. The most plausible mechanism is depicted in Scheme 4. A similar pathway was proposed earlier by Turner for rac-lactide polymerization using an alkaline-earth-metal carbene complex.25

Figure 4. Plot of percentage conversion vs Mn and Đ for polymerization of ε-caprolactone by 1 (top) and 2 (bottom) with [CL]0:[Cat.] = 700 in 5.0 mL of toluene at 25 °C. Triangles (▲) and squares (■) represent Mn and Đ values, respectively.

actually deteriorates in the presence of BnOH (see Table S7 in the Supporting Information). Polymerization of rac-Lactide (LA). Arguably, the most commonly studied cyclic ester is lactide, which can be generated from a renewable source such as corn starch. This also suggests that polylactide could be a viable alternative to conventional petrochemical polymers.41,42 Recently, Group 1 based metal complexes have emerged as potent polymerization catalysts43−45 for rac-LA, replacing the widely used toxic Sn(II)based initiators.46 To test the performance of complexes 1 and 2 as initiators for the ROP of rac-LA, we conducted polymerization studies in toluene (Scheme 3). Table 3

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CONCLUSIONS In summary, complexes 1 and 2 are two relatively rare entries in carbene-based Group 1 complexes. In these complexes, the K- - -Ccarbene distances are long and represent mostly electrostatic interactions between cationic potassium and very nucleophilic carbenes. An array of calculations substantiates the predominant ionic interaction, suggesting that the bond could be very labile. This feature has been utilized to develop highly efficient ring-opening polymerization catalysts at room temperature within a very short span of time. Strikingly, cAAC fails to polymerize any cyclic ester, while its potassium complex 1 is remarkably active to perform ROP of ε-CL. Moreover, these catalysts also showcase significant control over the polymerization to maintain very low Đ of the synthesized polymeric chains. We believe that the Group 1 metal mediated activation of the cyclic ester and successive ring opening by the carbene generates an alkoxide, which manages further chain propagation. It is assumed that this polymerization study comprising a cheap and benign Group 1 metal initiator will help in the further design of very active catalysts.

Scheme 3. ROP of rac-LA Catalyzed by Binuclear K Complexes 1 and 2

summarizes the polymerization results of rac-LA catalyzed by 1 and 2, and further details on polymerization results initiated by 2 are given in Tables S8 and S9 in the Supporting Information. Encouragingly, the APC analysis of the PLA also exhibits a narrow molecular weight distribution, with Đ values ranging from 1.29 to 1.44 in toluene (Table S8) and further improved values, 1.04 to 1.09, in THF (Table S9). 14463

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Inorganic Chemistry Table 3. ROP of rac-LA at Room Temperaturea entry

Cat.

[LA]0/[Cat.]

solvent

time

conversnb (%)

Mn, expc

Mn,calcb

Đc

1 2 3 4 5 6 7 8

1 2 2 2 2 2 2 free aNHC

100 100 300 300 300 500 500 100

toluene toluene toluene toluene toluene THF THF toluene

1 day 2.5 h 50 min 2 h 15 min 2 h 35 min 50 min 2 h 35 min 2.5 h

0 96 35 48 50 15 20 35

0 10725 15307 20797 21531 6322 8436 4170

0 13836 20451 29851 31130 6629 9183 5044

0 1.29 1.33 1.43 1.44 1.04 1.08 1.21

In toluene at 25 °C. [2] = 1 mM. bConversions were determined by 1H NMR spectroscopy. Mn,calc = molecular weight of chain end + 144.13 g mol−1 × ([LA]0/[Cat.]) × conversion. cObtained from APC analysis and calibrated by poly(methyl methacrylate).

a

Scheme 4. Plausible Mechanism for ROP of ε-Caprolactone Using 2a

Figure 5. Plot of percentage conversion vs Mn and Đ values for polymerization of rac-LA by complex 2 with [LA]0:[Cat] = 300 in 2.0 mL of toluene at 25 °C. Squares (■) and solid circles (●) represent Mn and Đ values, respectively.

a

molecular weight distributions (dispersity (Đ)) of polymers were determined by Waters ACQUITY advanced polymer chromatography (APC). The instrument contains a 1500 series HPLC pump, an ACQUITY refractive index (RI) detector, and an ACQUITY APCTM XT 2002.5 μm (4.6 × 7.5 mm) column. The samples were analyzed in THF at 45 °C at a flow rate of 0.25 mL/min. Poly(methyl methacrylate) (PMMA) standards were used to calibrate the instrument. Synthesis of 1, Dimeric cAAC Carbene Complex of K. In a glovebox, an oven-dried 50 mL Schlenk flask was charged with KN(SiMe3)2 (199 mg, 1 mmol) and cAAC (286 mg, 1 mmol), and then dry toluene (30 mL) was added via cannula at 25 °C under an argon atmosphere. The reaction mixture was stirred overnight, and the solution was filtered through a Celite pad and the reaction mixture concentrated to ca. 10 mL. Storage of this reaction mixture at −21 °C for 1 day afforded colorless block-shaped crystals (120 mg, 0.247 mmol, 49%). 1 H NMR (C6D6, 500 MHz, 298 K): δ 7.21 (dd, 1H, JH−H = 8 Hz), 7.13−7.14 (m, 2H), 3.11 (sept, 2H, CHCH3, JH−H = 7 Hz), 1.51 (s, 2H), 1.43 (s, 6H), 1.22 (dd, 12H, JH−H = 3 Hz), 1.06 (s, 6 H), 0.09− 0.15 ppm (s, 18H). 13C NMR (C6D6, 125 MHz, 298 K): δ 307.5 (C), 145.8, 137.5, 128.4, 128.3, 124.1, 82.5, 57.8, 50.4, 29.2, 29.1, 28.1, 26.8, 22.0, 7.3 ppm. Anal. Calcd for C52H98K2N4Si4: C, 64.39; H, 10.18; N, 5.76. Found: C, 64.42; H, 10.22; N, 5.80. Synthesis of 2, Dimeric aNHC Carbene Complex of K. In a glovebox, an oven-dried 50 mL Schlenk flask was charged with KN(SiMe3)2 (99.5 mg, 0.5 mmol) and aNHC (271 mg, 0.5 mmol) and then dry toluene (20 mL) was added via cannula at 25 °C under an argon atmosphere. The reaction mixture was stirred overnight, and the solution was filtered through a Celite pad and the reaction mixture concentrated to ca. 6 mL. Storage of this reaction mixture at −21 °C for 5 days afforded colorless block-shaped crystals (140 mg, 0.189 mmol, 38%). 1 H NMR (C6D6, 500 MHz, 298 K): δ 7.80 (dd, 2H, JH−H = 7 Hz), 7.22 (t, 1H, JH−H = 8 Hz), 7.16 (s, 1H), 7.14 (t, 1H, JH−H = 4 Hz), 7.12 (s, 1H), 7.11 (s, 1H), 7.10−7.11 (m, 1H), 6.98- 6.99 (m, 2H), 6.96 (t,

Figure 6. 1H NMR spectrum of the oligomer upon mixing 2 and 10 equiv of ε-CL in C6D6. The labeled peaks (with asterisks) show distinguished 1H NMR signals that have been used for peak integration. N′′ denotes N(SiMe3)2.

■

N′′ denotes N(SiMe3)2.

EXPERIMENTAL SECTION

Methods. The ligands aNHC and cAAC were prepared following the literature method.6,7 The HRMS data were obtained using a Finnigan MAT 8230 instrument. Elemental analyses were carried out using a PerkinElmer 2400 CHN analyzer, and samples were prepared by keeping them under reduced pressure (10−2 mbar) overnight. Crystallographic data for structural analysis of 1 and 2 have been deposited at the Cambridge Crystallographic Data Center (CCDC), as file numbers 1563423 and 1563422. These data can be obtained free of charge from the Cambridge Crystallographic Data Center. NMR spectra were recorded on a JEOL ECS 400 MHz spectrometer and on a Bruker Avance III 500 MHz spectrometer. Molecular weights and 14464

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Inorganic Chemistry 3H, JH−H = 8 Hz), 6.62- 6.63 (m, 1H), 6.61- 6.62 (m, 2H), 3.13 (sept, 2H, JH−H = 7 Hz), 2.88 (sept, 2H, JH−H = 7 Hz), 1.44 (d, 6H, JH−H = 7 Hz), 0.94 (d, 6H, JH−H = 7 Hz), 0.84 (d, 6H, JH−H = 7 Hz), 0.78 (d, 6H, JH−H = 7 Hz), 0.21 ppm (s, 18H). 13C NMR (C6D6, 125 MHz, 298 K): δ 197.2 (C), 145.7, 145.0, 144.8, 141.9, 139.4, 135.3, 133.6, 130.6, 129.4, 129.3, 128.9, 128.6, 128.3, 126.5, 126.2, 125.1, 124.4, 29.2, 28.8, 25.7, 23.9, 23.5, 22.6, 7.4 ppm. Anal. Calcd for C90H124K2N6Si4: C, 73.01; H, 8.44; N, 5.68. Found: C, 73.03; H, 8.45; N, 5.70. General Procedure for the Ring-Opening Polymerization (ROP) of ε-Caprolactone. In a 15 mL screw-cap vial, a toluene (2.0 mL) solution of catalyst (0.005 mmol) and ε-caprolactone (399.5 mg, 3.5 mmol, 700 equiv) along with 3.0 mL of toluene were mixed inside the N2-filled glovebox. The reaction mixture was subsequently stirred at room temperature for an appropriate time. Aliquots were taken from the reaction mixture under a nitrogen atmosphere and quenched with wet hexane (1−1.5 mL), and the solvent was evaporated under reduced pressure. The crude product was analyzed by 1H NMR spectroscopy and advanced polymer chromatography (APC). The conversion of ε-CL to PCL was determined by integration of the methylene proton peaks at δ 2.1−2.5 ppm in the 1H NMR spectra. The PCL number-averaged molecular weight, Mn, and dispersity (Mw/ Mn; Đ) were determined through APC analysis in THF. General Procedure for the Ring-Opening Polymerization (ROP) of rac-Lactide. In a 15 mL screw-cap vial, a toluene (1.0 mL) solution of catalyst 2 (1.48 mg, 0.002 mmol) and rac-lactide (86.5 mg, 0.6 mmol, 300 equiv) along with 1.0 mL of toluene were mixed inside the N2-filled glovebox. The reaction mixture was subsequently stirred at room temperature for an appropriate time. Aliquots were taken from the reaction mixture under a nitrogen atmosphere and quenched with wet hexane (1−1.5 mL), and the solvent was evaporated under reduced pressure. The crude product was analyzed by 1H NMR spectroscopy and APC. The conversion of LA to PLA was determined by integration of the methylene proton peaks at δ 5.00−5.30 ppm in 1 H NMR spectra. The PLA number-averaged molecular weight, Mn, and dispersity (Mw/Mn; Đ) were determined through APC analysis in THF.

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ACKNOWLEDGMENTS

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REFERENCES

We thank the SERB of India (Grant No. SR/S1/IC-25/2012) for financial support. M.B. and G.V. thank the UGC, New Delhi, India, for their research fellowships. M.B. thanks Dr. Sayam Sen Gupta and Dr. Tamal K. Sen for constructive suggestions during this work. M.B. thanks Mr. Rajan Kumar for APC. S.K.M. thanks Dr. Raja Shunmugam for his useful suggestion regarding APC measurements. D.A. sincerely thanks the computational facility at Indiana University, Bloomington, IN, USA.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01892. Experimental procedure, spectroscopic data, scanned spectra, Cartesian coordinates of calculated structures, and X-ray crystallographic data for 1 and 2 (PDF) Cartesian coordinates of calculated structures (XYZ) Accession Codes

CCDC 1563422−1563423 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for D.A.: [email protected]. *E-mail for S.K.M.: [email protected]. ORCID

Debashis Adhikari: 0000-0001-8399-2962 Swadhin K. Mandal: 0000-0003-3471-7053 Notes

The authors declare no competing financial interest. 14465

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Article

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