Effect of Azide and Chloride Binding to Diamino ... - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Effect of Azide and Chloride Binding to Diamino-bis(phenolate) Chromium Complexes on CO2/Cyclohexene Oxide Copolymerization Kaijie Ni, Valentine Paniez-Grave, and Christopher M. Kozak* Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, A1B 3X7, Canada

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ABSTRACT: The affinity for nucleophile (azide, N3−, and chloride, Cl−) binding significantly influences the catalytic activity of chromium complexes for the copolymerization of cyclohexene oxide (CHO) and CO2. The binding of N3− and Cl− to two amino-bis(phenolate) Cr(III) chloride complexes [L1Cr(μ-Cl)]2 (1) and [L2CrCl(THF)] (2·THF), where L1 = methoxyethylamino-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) and L2 = dimethylaminoethylamino-N,N-bis(2methylene-4,6-di-tert-butylphenolate), were studied by both matrix assisted laser desorption/ionization time-of-flight and electrospray ionization mass spectrometry (MALDI-TOF-MS and ESI-MS, respectively). Upon reaction with [PPN][N3] ([PPN] = bis(triphenylphosphine)iminium), 1 exhibited greater ability to form six-coordinate bis-azide ions [L1Cr(N3)2]− and produced fewer five-coordinate species detected by MS than 2·THF. This corresponded to 2·THF having a significantly faster rate of polymerization with anionic nucleophile cocatalysts than 1. In the presence of 1 equiv of [PPN][N3], 1 showed a slow reaction rate with an induction period of 20 min, whereas 2·THF showed a much faster reaction rate with no induction period. Also, 1 in the presence of 1 equiv of [PPN][Cl] showed faster copolymerization than in the presence of 2 equiv of [PPN][Cl]. The binding of chloride to 1 from addition of [PPN][Cl] was studied by UV−vis spectroscopy, which showed an equilibrium constant of 1.6 × 103 M−1 favoring the formation of [L1Cr(Cl)2]−. The ring-opening process of CHO was observed in the mixture of 2·THF/CHO/[PPN][N3] via ESI-MS. Combined with MALDI-TOF-MS of the polymer product obtained within the first 60 min, it was shown that N3− preferentially initiates ring-opening of the epoxide in CHO. Upon consumption of N3− through generation of active polymer chains, the chloride originating from the metal complex can serve as initiator of epoxide ring-opening. This was confirmed by the occurrence of chloride-containing polymers later in the reaction. Taken together, these studies provide mechanistic insight into the copolymerization of CO2 and epoxide by diamino-bis(phenolate) Cr(III) chloride complexes.

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INTRODUCTION The use of carbon dioxide (CO2) as a carbon feedstock for production of materials is appealing, as CO2 is nontoxic, abundant, and renewable.1 However, the requirement of a large input of energy to overcome the thermodynamic stability of CO2 limits its utility. Reaction of CO2 with reactive epoxides to generate polycarbonates or cyclic carbonates (Scheme 1) has become a promising process that produces potentially valuable materials from CO2. A diverse collection of

homogeneous catalysts that perform this reaction have been reported.2−8 A particularly well-studied family of catalysts are those possessing salen (salicylimine/Schiff base) ligands in combination with Cr9−17 or Co.18−23 These complexes typically require an appropriate nucleophile as cocatalyst to achieve high activity and selectivity for either the cyclic or polymeric product. The most commonly used cocatalysts include chloride, bromide, or azide paired with bulky cations such as bis(triphenylphosphine)iminium (PPN) or tetrabutylammonium, or neutral bases such as dimethylaminopyridine (DMAP). Mechanistic understanding of this catalyzed reaction, of course, is important for the design of highly efficient catalyst systems.24−26 Mass spectrometry (MS) is useful in this regard because it affords insight into the molecular composition of transition metal complexes.27 Particularly, soft ionization methods such as ESI-MS28,29 and MALDI-TOF-MS27,30,31

Scheme 1. Reaction of Cyclohexene Oxide, CHO, and CO2 To Produce Poly(cyclohexene carbonate), PCHC, and Cyclohexene Carbonate, CHC

Received: May 8, 2018

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.8b00298 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics are well suited for mechanistic studies as they allow weakly bound ligands to remain coordinated to the metal within a complex ion. Chen and co-workers have employed electrospray tandem MS to study the binding of propylene oxide (PO) to a series of salen complexes of different metals and discussed these results in terms of catalytic activities toward the copolymerization of PO with CO2.32 ESI-MS has been used by Lu and co-workers to confirm intermediates in PO/CO2 copolymerization by a single-site cobalt(III) salen catalyst.33 MALDI-TOF-MS has also been used by Duchateau and coworkers to identify unexpected side reactions and chain transfer reactions for the zinc catalyzed copolymerization of cyclohexene oxide (CHO) and CO2 to produce poly(cyclohexene carbonate), PCHC.34,35 Darensbourg and co-workers have studied the role of cocatalyst such as DMAP in CHO/CO2 copolymerization by salen Cr(III) complexes.15 They proposed a catalytic cycle for activating DMAP, in which DMAP binds to the metal center of a salen chromium complex and interacts with CO2 to generate a carbamate zwitterion, followed by a reaction with epoxide to form another zwitterion, which dissociates to regenerate the salen Cr(III) complex. The formed carbamate zwitterion was identified by νCO2 bands at 2097 and 2017 cm−1. This proposed activating DMAP process elucidates the initiation time observed for the formation of polycarbonate. Regarding use of ionic initiators, that report mentioned in a footnote that mass spectrometry of low molecular PCHC (∼3000 Da) obtained using salenCrN3 and [PPN][Cl] showed both chloride and azide end groups. Lu and co-workers have used ESI-MS to study the binding of DMAP to salen Cr(III) and salan Cr(III) complexes.36 They clearly demonstrated that salen Cr complexes readily bind two DMAP molecules, whereas salan Cr complexes preferentially bind only one DMAP and the formation of a bis-DMAP adduct is only observed under a high DMAP:Cr ratio of 10:1. This difference in DMAP binding affinity was found to result in a significantly different catalytic activity for PO/CO2 copolymerization. That is, salen Cr/DMAP exhibited a long induction period of up to 2 h; however, a very short or no induction period was observed for salan Cr/DMAP. It was proposed that DMAP dissociation from the bis-DMAP adduct was difficult, thus hindering formation of the active species. This was also thought to be the cause of the long induction period exhibited by salen Cr/ DMAP. We have been investigating amino-bis(phenolate) Cr(III) chloride complexes for CO2/epoxide copolymerization37−41 and used MALDI-TOF-MS to study the binding of DMAP to amino-bis(phenolate) Cr(III) chloride complexes with different pendent donor groups (Figure 1).42 We found that the amino-bis(phenolate) Cr(III) complex containing a dimethylaminoethyl pendent group (2·THF) prefers to bind one DMAP, while the other derivatives (1·THF, 3·THF−5·THF) readily bind two DMAP molecules. Similar to the observations of Lu where slight differences in the electronic and steric properties of the two similar salen- and salan-containing catalyst systems impact the resulting activity for CO2/epoxide copolymerization, this difference in affinity for DMAP binding in amino-bis(phenolate) Cr complexes resulting from the nature of pendent donor group was found to influence their catalytic activity for this reaction. That is, the aminobis(phenolate) Cr(III) chloride complex with the dimethyl-

Figure 1. Chromium(III) complexes with different pendent donor groups.

aminoethyl donor group, combined with DMAP, exhibits higher catalytic activity than the other derivatives studied.42 Herein, we investigate the activity of 1, isolated as a basefree dimer, toward the copolymerization of CHO and CO2 to give PCHC. The binding of ionic nucleophiles in the cocatalysts [PPN][N 3 ] and [PPN][Cl] to amino-bis(phenolate) Cr(III) chloride complexes was also studied. In this report, MALDI-TOF-MS and ESI-MS were both used to study the binding of azide to amino-bis(phenolate) Cr(III) chloride complexes (Figure 1) possessing methoxyethyl, 1, and dimethylaminoethyl pendent donor groups, 2·THF. The difference in ionization method and sample introduction method (laser desorption of solid samples in the presence of a matrix vs applying a voltage to an aerosol of an analyte solution) will permit a comparison of the abundance of resultant ions while addressing the problem of artifacts particular to either method. The binding of chloride to 1 was studied by UV−vis spectroscopy, and the ring-opening step of epoxide was examined by ESI-MS and MALDI-TOFMS. These investigations, combined with kinetic studies using in situ infrared spectroscopy, provided mechanistic insight into the copolymerization of CHO and CO2 catalyzed by aminobis(phenolate) Cr(III) chloride complexes in the presence of ionic cocatalysts.

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RESULTS AND DISCUSSION Synthesis and Characterization of 1. The aminobis(phenolate) Cr(III) complex 1 was synthesized via the deprotonation of the proligand with sodium hydride in THF at −78 °C, followed by reaction with CrCl3(THF)3 in THF at −78 °C to give a green solid (Scheme 2). The paramagnetic complex was characterized by MALDI-TOF mass spectrometry, elemental analysis, UV−vis spectroscopy, and single crystal X-ray diffraction. We have previously reported the synthesis and characterization of 2·THF.41 The MALDI-TOF mass spectrum of 1 showed peaks at m/z 596 and 561, which correspond to the radical cation of [CrCl[L1]]+• and the [Cr[L1]]+ fragment resulting from a loss of a chloride ion (Figure S1). Two additional peaks were B


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Organometallics

the two bridging chloride ligands. One is trans to a phenolate oxygen and the other is trans to the amino donor. This geometry is identical to the previously reported structure of a related Cr(III) amino-bis(phenolate) complex with a pyridyl pendent donor.37 The bond angle of Cr(1)−Cl(1)−Cr(1) in 1 is 97.26(2)°, which is slightly larger than the analogue having a pyridyl group, which showed an angle of 95.66(3)°. The Cr− Cl distances are 2.4783(8) and 2.3922(8) Å, which are slightly shorter than the observed bond distances in the pyridyl derivative (2.4431(8) and 2.4050(8) Å). These bond distances and angles lie within the ranges observed in other sixcoordinate Cr(III) chloride-bridged species.43−45 Copolymerization of CHO and CO2 Catalyzed by 1. Complex 1 combined with [PPN][Cl] was used in the copolymerization of CHO and CO2 at 60 °C and 40 bar CO2 (Table 1, entries 1−6). The 1H NMR spectrum (Figure S2) of the resulting product showed PCHC was formed with a negligible amount of ether linkages and no cyclic carbonate was produced. The stereochemistry of the polycarbonate was atactic based on its 13C NMR spectrum (Figure S3). The initial reactions were investigated with different [PPN][Cl] loadings, running for 24 h (entries 1−4). The best results in terms of the conversion and molecular weight were obtained using 1 equiv of [PPN][Cl] (per Cr), where 87% conversion was obtained and the molecular weight of the resulting polymer reached 18.4 kg mol−1 with a narrow dispersity of ∼1.1 (entry 1). The use of 0.5 equiv of [PPN][Cl] showed a lower conversion (72%), but a higher molecular weight (entry 2). Complex 1 combined with 2 equiv of [PPN][Cl] showed a high conversion (87%); however, the molecular weight of the obtained polymer decreased and dispersity increased slightly (entry 3). Our previous report concerning complex 4 (also obtained as a chloride-bridge dimer in the solid state) also found that the polymer molecular weight decreased as the cocatalyst loading ratio increased.37 Rieger and co-workers proposed that the growing polymer chain more easily dissociates from the metal center with a high cocatalyst loading, hence leading to lower molecular weight polymers.46 Our previous report for 2·THF under similar conditions resulted in modestly higher conversions (91%) and yields (72%) but produced lower molecular weight polymers (Mn = 13 kg mol−1). However, as will be discussed below, the initial rates of polymerization for these two catalysts are significantly different. Binding of Azide to Amino-bis(phenolate) Cr(III) Chloride Complexes. We previously reported initial reaction rates in copolymerizations of CO2 with CHO by several amino-bis(phenolate) complexes via in situ FTIR spectroscopy.40,41,47 It was observed that the rate of CO2/CHO copolymerization by 2·THF was fastest with azide cocatalyst.41 In the present study, 1 was found to have the fastest initial rate when DMAP was used (Figure S4). MALDI-TOF-MS was, therefore, used to explore the binding of azide to 1 and 2·THF to complement our previous MALDI-TOF-MS investigation of DMAP binding to these and other chromium(III) aminobis(phenolates).42 MALDI-TOF-MS has been shown to be suitable for quantitative analysis of analytes by our colleagues at Memorial University. Linear responses of analyte peak height vs concentration were obtained by Kerton and coworkers, indicating relative peak intensities can be used for determining relative concentrations.48 Solutions of 1 or 2·THF with varying ratios of [PPN][N3] with the anthracene matrix were spotted on the MALDI plate in a nitrogen-filled glovebox, and mass spectra were collected in negative mode at high

Scheme 2. Synthetic Route to Complex 1

observed in the higher mass region at m/z 1194 and 1157 corresponding to the dimeric species [CrCl[L1]]2+• and the fragment ion [Cr2Cl[L1]2]+ after loss of one chloride. Isotopic distribution patterns of the experimental ions were in good agreement with the calculated representations. We previously observed via MALDI-TOF-MS similar ions of a related chloride-bridged bimetallic amino-bis(phenolate) Cr(III) chloride complex.37 Single crystals of 1 suitable for X-ray diffraction were obtained by slow evaporation of a toluene solution under nitrogen. The solid-state molecular structure with selected bond distances and angles is shown in Figure 2. The structure of 1 was found to be a chloride-bridged dimer that exhibits distorted octahedral geometries in the two Cr centers. In each metal-ligand unit, four coordination sites are occupied by the tetradentate ligand with the two phenolate oxygen atoms in cis orientations. The other two coordination sites are occupied by

Figure 2. Molecular structure (ORTEP) and partial numbering scheme of 1. Ellipsoids are drawn at 50% probability. Hydrogen atoms omitted for clarity. Symmetry operations used to generate equiv atoms (*): −x, −y, −y + 1. Bond lengths (Å) Cr(1)−Cl(1)* 2.4783(8), Cr(1)−O(1) 1.9169(14), Cr(1)−Cl(1) 2.3922(8), Cr(1)−O(2) 1.8714(13), Cr(1)−N(1) 2.0758(13), Cr(1)−O(3) 2.0877(13). Bond angles (deg) O(1)−Cr(1)−N(1) 92.69(6), O(2)− Cr(1)−Cl(1)* 89.33(5), O(1)−Cr(1)−O(2) 94.16(6), N(1)− Cr(1)−O(2) 92.89(5), O(1)−Cr(1)−O(3) 91.97(6), O(1)− Cr(1)−Cl(1)* 174.13(4), O(3)−Cr(1)−Cl(1) 93.35(4), N(1)− Cr(1)−Cl(1) 171.97(4), O(2)−Cr(1)−O(3) 171.00(5), Cl(1)− Cr(1)−Cl(1)* 82.74(2), Cr(1)−Cl(1)−Cr(1)* 97.26(2). C


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Organometallics Table 1. Copolymerization of CO2 and CHO by 1 entrya

Cocat

[Cr]:[CHO]:[Cocat] (molar ratio)

time (h)

conv. (%)b

yield (%)c

TONd

TOFe (h−1)

Mnf (kg mol−1)

D̵ f (Mw/Mn)

1 2 3 4 5 6 7 8

[PPN][Cl] [PPN][Cl] [PPN][Cl] [PPN][Cl] [PPN][Cl] [PPN][Cl] [PPN][N3] DMAP

1:500:1 1:500:0.5 1:500:2 1:500:0 1:500:1 1:500:0.5 1:500:1 1:500:1

24 24 24 24 4 4 24 24

87 72 87 2 54 37 87 86

68 44 60 NDg 32 10 64 51

435 360 435 ND 270 185 435 430

18 15 18 ND 68 46 18 18

18.4 25.9 13.0 ND 9.5 6.2 13.0 20.6

1.12 1.18 1.46 ND 1.05 1.04 1.05 1.05

Copolymerization reactions were carried out in neat CHO (5 mL) at 60 °C and 40 bar CO2. bCalculated by 1H NMR. cIsolated yield: mass polymer/expected mass polymer × 100%. dTurnover number (TON): moles of repeating units produced per mole of Cr present. eTurnover frequency (TOF): moles of repeating units produced per mole of Cr per hour. fDetermined by triple detection GPC in THF, dn/dc = 0.0701 mL g−1. gND = not determined. a

compound will henceforth be referred to as 2 when not discussing the solid-state formula). In combination with [PPN][N3], salenCrCl has also been reported to form three ions: [salenCrCl2]−, [salenCr(N3)Cl]−, and [salenCr(N3)2]−, which were characterized by ESI-MS and X-ray crystallography.49 It is important to note when the mole ratio of [PPN][N3]:1 (per Cr) is 1:1, the bis-azide species, [L1Cr(N3)2]− (F6), at m/z 645 becomes the base peak (Figure 4C). However, in the case of 2/[PPN][N3], the bis-azide species of [L2Cr(N3)2]− (F6) ion at m/z 658 requires 2 equiv of [PPN][N3] to become the base peak (Figure 4D). Since the conditions of the preparation of these analyte mixtures and the instrumental conditions used to collect the mass spectra are identical, this observation suggests complex 1 in the presence of [PPN][N3] forms bis-azide species more easily than 2. Additionally, a striking difference in the relative abundance of the five-coordinate species [LCrCl]•− (F2) between 1/ [PPN][N3] and 2/[PPN][N3] was also observed (Figure S5). The relative abundance of the five-coordinate ion (F2) in the 1/[PPN][N3] mixture dramatically decreases with increasing proportion of [PPN][N3]. In contrast, the relative abundance of F2 in the 2/[PPN][N3] only slightly decreased and still showed more than 60% abundance even in the presence of 10 equiv of [PPN][N3]. This significant difference in the relative abundance of F2 ion by changing the pendent donor suggests that, in the presence of [PPN][N3], fivecoordinate species (F2) of complex 2 are more stable than for complex 1. This implies 2 should be more amenable than 1 for coordination by an incoming epoxide, thus leading to a faster initial reaction rate. This is indeed the case as shown by the following studies. ESI-MS of acetonitrile solutions of 1 and 2·THF with varying ratios of [PPN][N3] was used to measure the abundance of ions F1−F6 as a comparison to those observed by MALDI-TOF-MS. ESI-MS was chosen for its gentleness by which ions are formed, sensitivity, and dynamic range. It has also been extensively used for solution-phase organometallic chemistry to identify short-lived intermediates at low concentration in catalytic reactions.50 The negative mode ESI-MS of different ratios of 1/[PPN][N3] and 2/[PPN][N3] is illustrated in Figure S6, and a comparison of the relative abundance of the observed ions is shown in Figure S7. ESI-MS showed the presence of unmetalated ligand ion, [L1 + H]− (F1), is much higher in the case of 1/[PPN][N3] than 2/ [PPN][N3] (Figure S7). This may suggest L1 is more weakly coordinated to Cr than L2. When 0.5 equiv of [PPN][N3] was added to 1, the spectrum showed three new peaks at m/z

resolution (reflectron mode). The structures of observed ions are shown in Figure 3, and the mass spectra obtained using

Figure 3. Anions and their masses observed in the negative mode MALDI-TOF mass spectra of 2 with added [PPN][N3]. The corresponding ions of 1 are represented by analogous structures to fragments F1−F6 with masses of 511.4025, 596.2963, 603.3366, 631.2651, 638.3055, and 645.3459, respectively.

varying amounts of [PPN][N3] are shown in Figure 4. The mass spectra of 1 and 2·THF with no added azide (Figure 4A) show the existence of the five-coordinate anion (F2) and the six-coordinate dichloride “ate” species (F4). The spectrum of 1 in the presence of 0.5 equiv of [PPN][N3] (Figure 4B) exhibits three new species at m/z 603, 638, and 645, which correspond to [L1CrN3]− (F3), [L1CrClN3]− (F5), and [L1Cr(N3)2]− (F6), respectively. Similarly, the mass spectra of 2·THF combined with varying amounts of [PPN][N3] also show three ions at m/z 616, 651, and 658 representing [L1CrN3]•− (F3), [L2CrClN3]− (F5), and [L2Cr(N3)2]− (F6), respectively. These observations demonstrate replacement of chloride in amino-bis(phenolate) Cr(III) chloride complexes by azide occurs in both 1 and 2·THF (where the THF ligand in 2·THF is easily displaced in solution and gas phase experiments; the D


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Figure 4. Negative mode MALDI-TOF-MS of different molar ratios of 1/[PPN][N3] and 2/[PPN][N3]. Ratios are given in moles of Cr to moles of azide (A) 1:0, (B) 1:0.5, (C) 1:1, (D) 1:2, (E) 1:4, and (F) 1:10. Intensity of five-coordinate anion F2 ion for 1 (m/z 596.3, left) is observed to decrease with increased azide loading, whereas F2 for 2 (m/z 609.3, right) remains abundant even at high azide loading (spectra E and F).

510.4, 638.3, and 645.4 (Figure S6B), which correspond to [L1H]− (F1), [L1CrClN3]− (F5), and [L1Cr(N3)2]− (F6), respectively. When the ratio of 1 to [PPN][N3] is 1 to 1 (Figure S6C), the bis-azide species of [L1Cr(N3)2]− (F6) at m/z 645.4 becomes the most abundant. In contrast, when 0.5 equiv of [PPN][N3] was added to 2 (Figure S6B), the bisazide species (F6) at m/z 658.4 was negligible and only becomes the most abundant ion in the presence of 2 equiv of [PPN][N3] (Figure S6D). These observations also suggest complex 1 more easily binds two azide ligands than 2, which is consistent with the observation by MALDI-TOF-MS as discussed above. UV−vis Titration of 1 with [PPN][Cl] and CHO. Dichloromethane solutions of 1 showed a color change from green to pink upon addition of [PPN][Cl]; therefore, the binding of chloride from added [PPN][Cl] to 1 was studied by UV−visible spectroscopy. Titration of a solution of 1 in

dichloromethane with [PPN][Cl] in various ratios shows the peak at 616 nm corresponding to 1 decreased commensurate with the growth of a new peak at 537 nm (see Figure S8). The new peak at 537 nm did not further increase in intensity after addition of 2 equiv of [PPN][Cl] per Cr. These observations suggest that the dimer of 1 dissociates to form a six-coordinate anion in the presence of [PPN][Cl] (Scheme 3), which was also observed by negative mode MALDI-TOF-MS (Figure S9). The equilibrium constant of this reaction was calculated to be 1.6 × 103 M−1. Performing a similar procedure for titration of 1 with [PPN][N3] was troubled by the presence of multiple mixed-anion species (as reported above for MS studies) giving overlapping absorbances and no apparent isosbestic point. The reported equilibrium constants of reactions between [PPN][N3] and porphyrin aluminum complexes were 1.6 × 102 M−1, 9.3 M−1, and 6.6 M−1 depending on the substituents on the porphyrin ligand E


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Organometallics

m/z 651.3, and [L2Cr(N3)2]− at m/z 658.4 collectively as the major ions (Figure S11). The respective ions were also observed for mixtures of 1/[PPN][N3] by MALDI-TOF-MS (Figure S12). However, three new species at m/z 742.4, 749.4, and 756.5 with relatively low abundance were also observed for 2/[PPN][N3], corresponding to the alkoxide-containing anions [L2CrCl(C6H10O)Cl]−, [L2CrCl(C6H10O)N3]−, and [L2CrN3(C6H10O)N3]−, respectively (Figure 5). These three

Scheme 3. Equilibrium Reactions of 1 with [PPN][Cl] (A) and CHO (B)

system.51 The larger value shown by this amino-bis(phenolate) complex suggests a much higher affinity for the external nucleophile. The reaction of 1 with different amounts of CHO in dichloromethane was also monitored by UV−vis spectroscopy (see Figure S10). The peak corresponding to 1 at 616 nm decreased with the addition of CHO and a new peak at 481 nm appeared, which suggests dissociation of the dimer into the likely six-coordinate CHO adduct, 1·CHO (Scheme 3). The equilibrium constant of this reaction was found to be 8.5 × 102 M−1; therefore, the formation of the six-coordinate dichloride anion is moderately favored over epoxide adduct formation. During catalysis conditions, however, the 500-fold excess of CHO at the start of the reaction would permit formation of a significant quantity of this the CHO-adduct. X-ray molecular structures of the THF and oxetane adducts of similar sixcoordinate Cr epoxide complexes, such as salenCr(N3)·THF and salenCrCl·oxetane, have been reported,16,49 and we also previously reported structural characterization of THF adducts of monometallic amino-bis(phenolate) Cr(III) complexes,40,41,52 and a propylene oxide adduct of a Co(II) amino-bis(phenolate) compound.53 Epoxide Ring-Opening Steps Examined by ESI-MS and MALDI-TOF-MS. Polymer end group analysis by MALDI-TOF-MS or ESI-MS is a common method to identify the initiating species, particularly if the bond formed is stable to hydrolysis.3,17,18,54,55 Alternatively, Darensbourg and coworkers monitored the initial ring-opening step by analyzing the mixture of CHO/salenCrCl/n-Bu4NN3 with solution infrared spectroscopy to show azide-initiated ring-opening of CHO.49 To investigate the initial ring-opening species of our catalyst system, a mixture of 2/[PPN][N3]/CHO in a ratio of 1:1:20 was studied by negative ion mode ESI-MS. No polycyclohexene oxide formation was observed. The ESI mass spectrum observed for the mixture of 2·THF and [PPN][N3] showed [L2CrCl2]− at m/z 644.3, [L2CrClN3]− at

Figure 5. Expanded region of the negative mode ESI mass spectrum of the mixture of 2/[PPN][N3]/CHO in a ratio of 1:1:20.

observed ions indicate both the chloride from the Cr(III) complex and the azide from [PPN][N3] can ring-open CHO, which is consistent with the MALDI-TOF mass spectrum of the resulting polymer obtained using 2·THF and [PPN][N3], which showed azide occurs as end groups of the higher molecular weight polymer, whereas chloride end groups (and hydroxyl groups arising from chain transfer to adventitious water) are found in the lower mass products (Figure S13). In the case of the ion [L2CrCl(C6H10O)N3]− at m/z 749.4, either chloride or azide may be the nucleophile responsible for epoxide ring-opening, but given the observed stability of the Cr−Cl bond in 2 and the azide end group found in the higher molecular weight polymer, it is likely that azide is the initiator and the chloride from the metal complex serves as a secondary initiator once the concentration of external initiator is decreases or is depleted. We have previously found, however, that, in the absence of an external nucleophile, aminobis(phenolate)CrCl complexes are ineffective at copolymerization of CO2 and CHO; therefore, chloride displacement by the external nucleophile may be required for liberation of the initial Cr-bound chloride.37,40 Previously, Lu and co-workers observed the propagating polymer species having DMAP as the end group at various intervals in the copolymerization of PO and CO2 catalyzed by salenCrNO3/DMAP or salanCrNO3/DMAP using ESI-MS. This demonstrated DMAP initiation in this reaction rather than the nitrate group of the Cr complex.36 To further assess which nucleophile (added DMAP, azide, or the chromiumbound chloride ion) participated in the primary initiation of F


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Organometallics

Figure 6. MALDI-TOF mass spectra of aliquots obtained at the specified times from the copolymerization of CHO and CO2 by 2·THF and DMAP (DHBA matrix). Li+ ions are adventitious from residual cationizing agents in the mass spectrometer. Conditions: [Cr]:[CHO][Cocat] = 1:500:1, 60 °C, and 40 bar CO2.

∼5100 Da. This is consistent with our previous work that showed azide gives a faster rate of copolymerization than DMAP for 2·THF.41 Comparing the polycarbonate propagation for 1/DMAP under the same conditions revealed that, here too, DMAP exists as a chain end with the expected hydroxyl terminus (Figure S15). No chloride end groupcontaining polymer is observed, but it is evident by the lack of polymer molecular weight increase after removal of the first aliquot (giving an average mass of ∼1900 Da after 20 min) that polymerization by 1 proceeds more slowly (or is more sensitive to air and moisture, causing slowed polymerization) than 2· THF. Furthermore, the observed propagating species indicates the external nucleophile is most likely the primary initiator of epoxide ring-opening; i.e., when [PPN][N3] is used with 1 and 2·THF, the azide initiates polymerization rather than the chloride. Mechanistic Interpretation. The mechanism of epoxide/ CO2 copolymerization catalyzed by salen complexes and

epoxide ring-opening, we conducted short-run reactions and analyzed the end groups of the polymer products. To investigate the propagating oligomers that are present during initial stages of the copolymerization of CHO and CO2 catalyzed by 2·THF in combination with DMAP and [PPN][N3], respectively, aliquots were taken at various intervals and analyzed by MALDI-TOF-MS. The resulting mass spectra of DMAP initiated polymerization are shown in Figure 6. The mass spectrum of the polymer obtained after 20 min clearly showed one polymer chain with DMAP as one of the end groups. Polymer chain propagation is observed over time with a shift of the envelope of peaks to higher m/z. Similarly, in the case of [PPN][N3] as a cocatalyst, the mass spectrum showed one polymer chain with azide and hydroxyl end groups (Figure S14). The azide-initiated polymerization shows masses of ∼6000 Da after 20 min. Masses of ∼3800 Da were obtained after 20 min for DMAP-initiated polymerization, which required 60 min to obtain polymer masses of G


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Organometallics

when 1 was paired with 0.5 equiv of [PPN][Cl], no induction period was observed. Use of 1.0 equiv of [PPN][Cl] showed a brief induction period of ∼5 min, but the overall rates of reaction for these two conditions were nearly identical. In the presence of 2 equiv of [PPN][Cl], the reaction exhibited a much slower rate following an induction period of approximately 10 min (Figure 8). Taking also into account the UV−

anions coupled with PPN or tetraalkylammonium cations has been well studied.3,15 Darensbourg and co-workers have proposed the six-coordinate metal anion derived from addition of ionic cocatalyst to salen complex is the active species, which is responsible for epoxide binding and concomitant ringopening of epoxide by the displaced nucleophile. Due to the fast formation of this six-coordinate metal anion, the most striking feature of using ionic cocatalysts in the salencontaining catalyst systems is the lack of an induction period. In situ infrared spectroscopy (ATR-FTIR) was used to investigate the copolymerization rates of CO2/CHO catalyzed by 1 and 2·THF in the presence of [PPN][N3]. The resulting time profile of the absorption at ∼1750 cm−1 corresponding to the formation of polycarbonate is shown in Figure 7, which

Figure 8. Time profiles of the absorbance at 1750 cm −1 corresponding to the PCHC obtained by using 1 with 0.5 (dashed green line), 1.0 (solid pink line), and 2.0 (dashed-dotted blue line) equiv of [PPN][Cl].

vis titration results where the concentration of the sixcoordinate anionic species became maximized under 2 equiv of [PPN][Cl], this suggests the six-coordinate anionic species is relatively abundant in CHO solutions. Indeed, the equilibrium constants measured show “ate” complex formation is only slightly favored over epoxide adduct formation (in dichloromethane), but even in neat CHO it is sufficient to cause the observed induction period. Dissociation of one chloride to produce a vacant site for CHO coordination may be slow, thus causing an induction period and an overall slower reaction rate due to the lower concentration of active sites. These observations differ from those found for salen Cr(III) complexes with anionic cocatalysts where the fastest catalytic CO2/epoxide copolymerization activity was obtained with no induction period by using 2 equiv of anionic cocatalyst.15 The different complex geometry found in 1 compared to salen Cr(III) complexes may be significant, as it directs any incoming nucleophile to occupy a site cis to the ancillary chloride ligand, thus forming the cis-six-coordinate anionic species, whereas salen Cr(III) complexes exhibit trans orientation of the added nucleophile. Indeed, a salan Cr(III) complex exhibiting cis-β stereochemical orientation, thereby creating cis-oriented binding sites for ancillary nucleophiles or epoxide, has shown a decrease in reaction rate when an additional equivalent of [PPN][N3] was added.56 On the basis of the above spectroscopic analyses, the observed alkoxide intermediates and end groups of the produced polycarbonates, a mechanism can be proposed for CO2/epoxide copolymerization by amino-bis(phenolate) Cr(III) complexes and ionic cocatalysts. An equilibrium is established in the binary system of amino-bis(phenolate) Cr(III) chloride complexes and [PPN][N3], which includes [LCrCl2]−, [LCrCl(N3)]−, [LCr(N3)2]−, and LCrCl. The fivecoordinate species LCrCl plus a “free azide” is most likely the real active species, which provides the main pathway for the initiation reaction (Scheme 4, pathway A). That is, epoxide

−1

Figure 7. Time profiles of the absorbance at 1750 cm corresponding to PCHC obtained using complex 1 (solid blue line) and 2·THF (dashed pink line) in the presence of 1 equiv of [PPN][N3].

confirms the initial rate of polymerization by 2·THF is significantly faster than that by 1. 1/[PPN][N3] exhibited a slow growth of the polycarbonate band at 1750 cm−1 following a 20 min induction period, whereas 2/[PPN][N3] showed a significantly faster rate of polycarbonate formation and no prolonged induction period. This notable difference in catalytic activities, combined with the mass spectrometry results described above, provide strong evidence that the formation of the bis-azide species, which is more facile in 1 than 2·THF, hinders copolymerization catalysis by 1 and explains why DMAP results in faster polymerization with 1 (bis-DMAP adduct formation was less abundant under the same conditions compared to bis-azide complexes) but azide is the most active cocatalyst for 2·THF.41 The observed induction period is therefore cocatalyst dependent rather than as a result of compound 1 being dimeric in the solid state. As shown by UV−vis (see above), the dimer is easily disrupted in the presence of epoxide so that, even in dichloromethane solution, 2 equiv of CHO is sufficient to suppress dimerization; therefore, the monomer−dimer equilibrium is not likely causing the observed induction period. Also, we have previously reported the polymerization rate is first-order in catalyst concentration for amino-bis(phenolate) chromium chloride complexes.47 To study the effect of the anionic cocacatalyst on the induction period observed for 1, the effect of [PPN][Cl] loading (i.e., chloride rather than azide-initiated epoxide ringopening) on the copolymerization rate was monitored via in situ FTIR spectroscopy. The time profiles demonstrated that, H


Article

Organometallics Scheme 4. Proposed Initiation Pathways for the Copolymerization of CHO and CO2 by 2 and [PPN][N3]

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coordinates to the five-coordinate species, followed by the free azide ring-opening the epoxide, which leads to an azidefunctionalized alkoxide. This proposed initiation is consistent with the observed propagating species possessing azide termini at various intervals by MALDI-TOF-MS (Figure S13). If [PPN][Cl] is employed, a similar pathway is followed. DMAP was found to give the fastest polymerization rates with 1, but azide was the fastest initiator for 2·THF. This, we propose, is due to larger steric and increased electronic donating ability of the dimethylaminoethyl group in 2·THF, which destabilizes “ate” complex formation (represented by the [LCrCl(N3)]− ion within the box in Scheme 4) compared to the methoxyethyl group of 1. Our previous work with pyridylfunctionalized amine-bis(phenolate) Cr(III) complexes showed DMAP coordination is possible in these compounds but still may act as the initiator of epoxide ring-opening, along with the initial chromium-bound chloride.42,47 On the other hand, six-coordinate anions, such as the bis-azide, dichloride, or mixed ligand anions, hinder the reaction in which epoxide competes with the coordinated chloride and/or azide to bind to the chromium center. Only if the epoxide successfully replaces the coordinated chloride or azide can ring-opening of the epoxide occur (Scheme 4, pathway B). This process provides a minor initiation pathway and results in the presence of chloride as the end group, as observed in the low m/z region of the MALDI-TOF mass spectrum (Figure S13), and in our experience, this only occurs later in the copolymerization, i.e., once a significant amount of the azide has been consumed as polymer chain ends. For the catalyst system of 1/[PPN][N3], due to the smaller steric effect of the pendent donor group of 1, the equilibrium reaction favors the formation of sixcoordinate species, which are believed to show lower catalytic activity than five-coordinate species. Similar enhanced catalytic activity of salen complexes has been observed by increasing the electron density at the metal center through introducing more electron donating substituents.12 The rate of polymerization was found previously to be first-order in catalyst concentration for amino-bis(phenolate) chromium chloride complexes;47 therefore, bimetallic intermolecular pathways are not used by these catalysts.8

CONCLUSIONS A combination of spectroscopic techniques (UV−vis, FT-IR, and mass spectrometry) have allowed us to unravel the initiation process for amino-bis(phenolate) Cr(III) complexes in CO2/epoxide copolymerization. The negative mode MALDI-TOF and ESI mass spectra of amino-bis(phenolate) Cr(III) chloride complexes in the presence of various amounts of azide demonstrated the binding behavior of azide to the chromium center is influenced by the nature of the pendent donor group. Complex 1 with a methoxyethyl pendent donor shows a higher abundance of bis-azide-containing ions using both ionization methods and generates a much lower abundance of five-coordinate ions than 2, which possesses the dimethylaminoethyl pendent donor. This different behavior in the binding of azide correlates with a striking difference in the initial rates of 1 and 2 for the copolymerization of CHO and CO2. In the presence of 1 equiv of [PPN][N3], 1 exhibits a slow reaction rate with an induction period, whereas 2 shows a much faster reaction rate and no induction period is observed. The formation of sixcoordinate species hindering the initiation process is also confirmed by UV−vis and MS studies of the binding of chloride to 1 using [PPN][Cl]. This differs from the proposed mechanisms for Cr(III) and Co(III) salen complexes where the formation of six-coordinate compounds/anions is not believed to hinder catalysis, but allows for a labilizing trans effect to generate a vacant site for epoxide binding. In aminobis(phenolates), the forced cis-geometry of the monodentate ligands prevents this labilizing effect in their six-coordinate compounds; thus the electronic and steric influences of the pendent donor are important factors. Using MS methods to evaluate the stability of the five-coordinate compounds in the presence of varying amounts of added nucleophiles can, therefore, be extremely useful for the screening of new catalysts for CO2/epoxide copolymerization. The alkoxide species observed by MS and end groups of the produced polycarbonate indicate an external nucleophile (e.g., N3−) initiates the reaction wherein the five-coordinate species plus a free nucleophile is an active catalyst combination. Initiation by the chloride originating from the Cr(III) complex may also occur, but only later in the polymerization process once the I


Article

Organometallics

anthracene (6.0 mg, 3.3 × 10−5 mol) was prepared in 1.2 mL of dichloromethane. Appropriate amounts (2.9, 5.8, 11.6, 23.2, and 58.1 mg) of [PPN][N3] were added into the solution and allowed to stir for 5 min, giving analytes with different ratios of complex 1 (per Cr) to [PPN][N3] (1:0.5, 1:1, 1:2, 1:4, 1:10). Sample preparations of complex 2·THF and [PPN][N3] followed the same procedure. Laser intensity of 4526 au and reflectron (high resolution) mode were used in the experiments. Observation of Propagating Species. Aliquots taken at different time intervals from the copolymerization reactions catalyzed by 2· THF in combination with DMAP or [PPN][N3] were dissolved in THF at approximately 10 mg mL−1. DHBA was dissolved in THF at approximately 16 mg mL−1. The polymer and matrix solutions were combined in a volume ratio of 1:3. Laser intensity of 5406 au and mid linear mode were applied in the experiments. ESI-MS Methods. ESI-MS was performed using an Agilent 6230 TOF LC/MS. The ESI capillary voltage was maintained at 3500 V. All the mass spectra were recorded using MassHunter workstation software (version B.06.00). Binding of Azide to 1 and 2·THF Monitored via ESI-MS. Solutions of complex 1 (3.0 mg, 5.0 × 10−6 mol (per Cr)) were made in 3.0 mL of acetonitrile. A stock solution of 58.4 mg of [PPN][N3] in 2 mL of acetonitrile was prepared. Appropriate volumes (50, 100, 200, 400, 1000 μL) of [PPN][N3] solution were added into the complex 1 solutions and stirred for 5 min, giving the molar ratios of complex 1 (per Cr) to [PPN][N3]: 1:0.5, 1:1, 1:2, 1:4, 1:10. The resulting solutions were diluted with acetonitrile to a concentration of 0.75 mg mL−1. The sample preparations of complex 2·THF and [PPN][N3] followed the same procedure. Observation of Cr-alkoxide Species. The solutions of complex 2· THF (3.0 mg, 4.5 × 10−6 mol) and [PPN][N3] (2.6 mg, 4.5 × 10−6 mol) were made in 4 mL of acetonitrile. Appropriate amounts (8.8, 13.2, 26.4, 52.8 μL) of CHO were added into the solutions and stirred for 5 min, giving analytes with different ratios among 2, [PPN][N3], and CHO (1:1:20, 1:1:30, 1:1:60, 1:1:120). UV−vis Method. UV−visible spectroscopy was conducted on a dual-beam Thermo Scientific Evolution 300 UV−vis spectrophotometer equipped with a xenon lamp. Complex 1 (30 mg, 5.0 × 10−2 mmol (per Cr)) was dissolved in 50 mL of dichloromethane. Because 1 is air sensitive, an air-free cuvette attached to an ampule was used. The appropriate amount of [PPN][Cl] was successively added into solutions of 1 for analysis. Synthesis of 1.

added azide is consumed by formation of azide-terminated polymer chains.

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EXPERIMENTAL SECTION

General Materials. All manipulations were performed under an atmosphere of dry, oxygen-free nitrogen by means of standard Schlenk techniques or using an MBraun Labmaster DP glovebox. CrCl3(THF)3 was prepared by the standard method.57 The ligands of H2[L1] and H2[L2] were prepared by a modified literature procedure by using water instead of methanol as the reaction medium,58,59 and dried over sodium sulfate in tetrahydrofuran under nitrogen. Complex 2·THF was prepared as previously reported.41 Anhydrous tetrahydrofuran was distilled from sodium/benzophenone ketyl under nitrogen. Anhydrous dichloromethane was obtained by purification using an MBraun Manual Solvent Purification System. Acetonitrile was purchased from Fisher Scientific and dried over 4 Å molecular sieves. Cyclohexene oxide was purchased from Aldrich and freshly distilled from CaH2 under nitrogen. [PPN][Cl] was purchased from Alfa Aesar and used without further purification. [PPN][N3] was prepared following the reported procedure.60 General Instrumentation. Molecular weight determination of copolymer was performed on an Agilent Infinity HPLC instrument connected to a Wyatt Technologies triple detector system (light scattering, viscometry, and refractive index) equipped with two Phenogel 103 Å 300 × 4.60 mm columns with THF as eluent. Copolymer samples were prepared in THF at a concentration of 4 mg/mL and filtered through 0.2 μm syringe filters. The sample solution was then eluted at a flow rate of 0.30 mL·min−1. The values of dn/dc were calculated online (columns detached) assuming 100% mass recovery using the Astra 6 software package (Wyatt Technologies) giving dn/dc of poly(cyclohexene carbonate) = 0.0701 mL·g−1. 1 H and 13C NMR spectra were recorded in CDCl3 at 300 and 75.0 MHz, respectively, on a Bruker Avance III spectrometer with BBFO probe. Elemental analysis was performed at Guelph Chemical Laboratories, Guelph, ON, Canada. In situ FTIR monitoring was performed using a 100 mL Parr Instruments 4560 stainless steel mini reactor vessel with motorized mechanic stirrer and a heating mantle. The vessel was modified with a bottom-mounted Mettler Toledo SiComp Sentinel ATR sensor, which was connected to a ReactIR 15 base unit through a silver-halide Fiber-to-Sentinel conduit. Profiles of the absorbance height at 1750 cm−1 were measured every 60 s. Similar methods for in situ reaction via FTIR have been reported elsewhere.11,61 A calibration curve to demonstrate a linear response of absorbance to concentrations was obtained and reported elsewhere.41,62 The peak height of the polycarbonate signal at 1750 cm−1 was found to increase linearly with concentration in the neat cyclohexene oxide. The use of peak height to relate to polycarbonate concentration has been practiced by others.11,61 MALDI-TOF-MS Methods. MALDI-TOF-MS was performed using an Applied Biosystems 4800 MALDI TOF/TOF mass spectrometer equipped with a reflectron, delayed ion extraction and high-performance nitrogen laser (200 Hz firing rate operating at 355 nm). Chromium complex samples were prepared in the glovebox and sealed under nitrogen in a Ziploc bag for transport to the instrument. Anthracene was used as the matrix for the metal complexes. Anthracene and complex were each dissolved in toluene at concentrations of 10 mg·mL−1, and the solutions were combined in a ratio of 1:1 for the analyzed sample. 2,5-Dihydroxybenzoic acid (DHBA) was used as the matrix for the copolymers. 2,5Dihydroxybenzoic acid (DHBA) was dissolved in THF at approximately 16 mg mL−1, and polymer was dissolved in THF at approximately 10 mg mL−1. The matrix and polymer solutions were combined in a ratio of 3:1 as the analyzed sample. 1 μL aliquots of these samples were spotted on the MALDI plate and left to dry. Images of mass spectra were prepared using mMass software (www. mmass.org). Binding of Azide to 1 and 2·THF Monitored via MALDI-TOF-MS. A solution of complex 1 (6.0 mg, 1.0 × 10−5 mol (per Cr)) and

H2[L1] (2.00 g, 3.90 mmol) and sodium hydride (0.38 g, 15.60 mmol) in a Schlenk tube were cooled to −78 °C. THF (∼50 mL) was transferred to the mixture, which was then warmed to room temperature and further stirred for 2 h. The solution was separated from the excess NaH via filter cannula into a suspension of CrCl3(THF)3 (2.50 g, 6.67 mmol) in THF (∼50 mL) at −78 °C to give a pink/purple mixture. The mixture was warmed to room temperature and stirred for 18 h to give a dark purple solution (indicative of a THF adduct). The solvent was removed under vacuum. The solid residue was extracted into toluene and filtered through a pad of Celite. The complex is moderately soluble in toluene, so the extraction was repeated twice to dissolve the remaining product on the pad of Celite. The toluene was removed under vacuum, and the residue was washed with pentane and evaporated to dryness to yield 1.14 g (49%) of green powder (indicative of the J


Article

Organometallics THF-free compound). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of 1 in toluene at room temperature in a glovebox under nitrogen. Calculated (%) for C66H102Cl2Cr2N2O6: C, 66.37; H, 8.64; N, 2.35. Found (%): C, 66.13; H, 8.42; N, 2.62. MS (MALDI-TOF) m/z (%, ion): 596.3 (100, CrCl[L1]+•), 561.3 (50, Cr[L1]+), 1157.5 (9, [Cr[L2]]2Cl+) and 1194.5 (3, [CrCl[L1]]2+•). Copolymerization Conditions. Complex 1 (59 mg, 4.9 × 10−2 mmol) and the appropriate amount of ionic cocatalyst were first dissolved in 3 mL of dichloromethane and allowed to stir for 10 min before removal of solvent under vacuum. Cyclohexene oxide (4.9 g, 50 mmol) was added to the residue and stirred for 10 min. The reaction mixture was added via syringe to a 100 mL stainless steel Parr autoclave at 25 °C, which was predried by heating to 80 °C under vacuum overnight. The autoclave was heated to 60 °C, then charged with 40 bar CO2 and stirred. After the desired time, the autoclave was cooled in an ice bath and vented in the fume hood. An aliquot for NMR was taken immediately after opening for the determination of conversion. The copolymer was extracted into dichloromethane and precipitated using cold methanol. The product was then dried at 80 °C in a vacuum oven overnight. Crystallographic Experiment. Crystallography was performed by Dr. Louise N. Dawe of the C-CART X-ray Diffraction Laboratory, Memorial University of Newfoundland. Single crystals of C80H118Cl2Cr2N2O6 (1) were obtained by slow evaporation of a toluene solution. A suitable crystal was mounted in Paratone N on a MiTeGen MicroMount on a Rigaku Saturn70 diffractometer. The crystal was kept at 163 K during data collection. Using CrystalStructure,63 the structure was solved with the SIR200464 structure solution program using Dual Space and refined with the ShelXL65 refinement package using least-squares minimization.

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Memorial University of Newfoundland for a School of Graduate Studies Fellowship (K.N.), and Institut Universitaire de Technologie de Bethune for a research internship at Memorial University (V.P.-G.).

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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.organomet.8b00298. X-ray data (CCDC 1830447) and spectroscopic characterization data (PDF) Accession Codes

CCDC 1830447 contains 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1-709-864-8082. ORCID

Christopher M. Kozak: 0000-0001-8205-4130 Author Contributions

The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the Canada Foundation for Innovation (CFI), Newfoundland and Labrador Research Development Corporation (RDC), the Natural Sciences and Engineering Research Council (NSERC) of Canada (C.M.K.), K


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