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DOI: 10.1039/D2DT01496C (Paper) Dalton Trans., 2022, 51, 11776-11786

Trinuclear zinc calix[4]arenes: synthesis, structure, and ring opening polymerization studies†

Tian Xing a, Josef W. A. Frese b, Max Derbyshire b, Mollie A. Glenister b, Mark R. J. Elsegood b and Carl Redshaw *a
aPlastics Collaboratory, Department of Chemistry, University of Hull, Cottingham Road, Hull, HU6 7RX, UK. E-mail: c.redshaw@hull.ac.uk
bChemistry Department, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK

Received 13th May 2022 , Accepted 11th July 2022

First published on 11th July 2022

Abstract

The trinuclear zinc calix[4]arene complexes [Zn3(O2CCH3)2(L(O)2(OMe)2)2·xMeCN (x = 7.5, 1; x = 6, 1′), [Zn3(O2CCH3)2(L(O)2(OnPr)2)2·5MeCN (2·5MeCN), [Zn3(OEt)2(L(O)2(OMe)2)2]·4MeCN (3·4MeCN), [Zn3(OEt)2(L(Opentyl)2)2]·4.5MeCN (4·4.5MeCN) and [Zn3(OH)2(L(O)2(On-pentyl)2]·8MeCN (5·8MeCN) have been isolated from reaction of [(ZnEt)2(L(O)2(OR)2)2] (L(OH)2(OR)2 = 1,3-dialkoxy-4-tert-butylcalix[4]arene; R = methyl, n-propyl or pentyl) and the reagents acetic acid, ethanol, and presumed adventitious water, respectively. Attempts to make 5via a controlled hydrolysis led only to the isolation of polymorphs of (L(OH)2(Opentyl)2·MeCN. Reaction of [Zn(C6F5)2] with L(OH)2(Opentyl)2, in the presence of K2CO3, led to the isolation of the complex [Zn6(L(On-pentyl))2(OH)3(C6F5)3(NCMe)3]·3MeCN (6·3MeCN). The molecular structures of 1–6 reveal they all contain a near linear (163 to 179°) Zn3 motif. In 1–5, a central tetrahedral Zn centre is flanked by trigonal bipyramidal Zn centres, whilst in 6, for the linear Zn3 unit, a central distorted octahedral zinc centre is flanked by trigonal planar and a tetrahedral zinc centres. Screening for the ring opening polymerization (ROP) of ε-caprolactone at 90 °C revealed that they are active with moderate to good conversion affording low to medium molecular weight products with at least two series of ions. For comparative studies, the trinuclear aminebis(phenolate) complex [Zn3(Oi-Pr)2L/] (L/ = n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate) I was prepared. Kinetics revealed the rate order I > 4 > 6 ≈ 2 ≈ 1 > 3.

Introduction

The search for new, greener polymers as replacements for petroleum-based plastics continues at a pace, and progress has seen biodegradable synthetic polymers applied in areas such as the biomedical field.1 One avenue receiving much attention is the ring opening polymerization (ROP) of cyclic esters. Typically, this process requires the use of a catalyst/initiator and, when metal-based, these usually take the form of an alkoxide, or a species that can generate an alkoxide in situ.2 As part of the search for catalytic systems capable of such ROP, we and others have been investigating the use of metallocalix[n]arenes.3 The use of calixarenes is beneficial for a number of reasons, for example both upper and lower rims can readily be modified allowing for control over the sterics and electronics of the system as well as other properties such as solubility. Furthermore, employing calixarenes as ligands allows for the potential binding of multiple metal centres, which is attractive in terms of possible cooperative catalysis. The choice of metal is equally important, and earth-abundant metals are desirable. Given this, the metal zinc has been employed in a variety of ROP systems, usually in combination with Schiff-base type ligation, or ligands derived from diphenols.2g,4 Moreover, a limited number of zinc-containing calix[4]arene systems have been shown to be effective ROP catalysts, suggesting such systems retain sufficient nucleophilicity at the metal.5 Herein, we report the isolation of five trinuclear zinc complexes bearing dialkoxycalix[4]arene ligation (see Chart 1), including the serendipitous hydroxide-bridged product [Zn3(OH)2(L(O)2(On-pentyl)2]·8MeCN (5·8MeCN). The complex [Zn6(L(On-pentyl))2(OH)3(C6F5)3(NCMe)3]·3MeCN (6·3MeCN), which also contains a near linear Zn3 fragment, is also reported. Harrowfield et al. have reported a trimetallic zinc complex derived from p-tert-butyltetrathiacalix[4]arene.6 We also note that Ovsyannikov, Ferlay et al. have recently reported a trinuclear zinc complex bearing a sulfonylcalix[4]arene, however the zinc centres adopted a triangle-like arrangement.7a The same group have also reported the use of the sulfonylcalix[4]arene ligand set in combination with succinic acid to isolate a Zn6 complex in which cationic trinuclear Zn3 motifs are bridged by succinic carboxylate groups.7b A more linear arrangement was reported for a trimetallic zinc amino-bis(phenolate) complex I, which was also employed in the ROP of rac-lactide.8 We also note that trinuclear zinc complexes bound by carboxylate and pyridyl ligands have recently been exploited in the ROP of ε-CL and r-LA.9 Moreover, there is interest in ‘trapping’ zinc hydroxide/alkoxide/oxide species using organic ligands.10
image file: d2dt01496c-c1.tif
Chart 1 New pre-catalysts prepared herein (R = t-Bu) and I.6

The trinuclear zinc complexes herein have been screened for their ability to act as catalysts for the ROP of ε-caprolactone.

Results and discussion

Syntheses and solid-state structures

Acetate-bridged complexes. Reaction of 1,3-dimethylether-p-tert-butylcalix[4]arene(OH)2, L(OH)2(OMe)2, with [ZnEt2], and subsequent treatment with acetic acid led, following work-up, to the complex [Zn3(O2CCH3)2(L(O)2(OMe)2)2]·7.5MeCN (1·7.5MeCN). Single crystals suitable for X-ray diffraction were obtained from a saturated MeCN solution on standing (1–2 days) at ambient temperature. The molecular structure is shown in Fig. 1, with selected bond lengths and angles given in the caption. In the asymmetric unit, the whole molecule of 1·7.5MeCN is unique. The Zn(1) and Zn(3) centres are 5-coordinate and are approximately trigonal bipyramidal (tbp) as indicated by the structural index parameters of τ = 0.06 and 0.04, respectively.11 The central Zn(2) is 4-coordinate tetrahedral and is much less distorted than the central zinc centres found in 2 and 3 (see below). The Zn(1)–Zn(2)–Zn(3) angle is 174.539(11)°. One MeCN resides in each of the calixarene cavities, whilst there are 5.5 other MeCNs that lie exo to the molecule.
image file: d2dt01496c-f1.tif
Fig. 1 Molecular structure of [Zn3(O2CCH3)2(L(O)2(OMe)2)2]·7.5MeCN 1·7.5MeCN. H atoms and minor disorder components omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 1.9721(13), Zn(1)–O(2) 2.1879(13), Zn(1)–O(3) 1.8656(14), Zn(1)–O(4) 2.2657(14), Zn(2)–O(1) 1.9584(13), Zn(2)–O(6) 1.9545(16), Zn(2)–O(12) 1.9498(15); Zn(1)–O(1)–Zn(2) 110.54(6), Zn(2)–O(9)–Zn(3) 111.07(6), O(1)–Zn(2)–O(9) 122.43(6).

From a repeat synthesis, the structure of a different solvate [Zn3(O2CCH3)2(L(O)2(OMe)2)2]·6MeCN (1′·6MeCN) has also been determined, see Fig. S1, ESI.† In the asymmetric unit, a molecule of 1′·6MeCN lies on a 2-fold axis, so half is unique.

Use of 1,3-di-n-propylether-p-tert-butylcalix[4]arene(OH)2, L(OH)2(On-Pr)2, under similar conditions led, following work-up, to isolation of the complex [Zn3(O2CCH3)2(L(O)2(On-Pr)2)2]·5MeCN (2·5MeCN). The molecular structure is shown in Fig. 2, with selected bond lengths and angles given in the caption; an alternative view is given in Fig. S2, ESI.† This is half of the asymmetric unit, so there are two metal complexes, each with a MeCN in each calixarene cavity, plus six other MeCNs exo to the metal complexes, three per Zn3 complex. Data were non-merohedrally twinned via a 180° rotation about [0 0 1] direct & reciprocal. The twinning in conjunction with the very large structure result in the data quality being sub-optimal. However, the connectivity is clearly established as very similar to 1. Fine details about geometry are far less reliable and should not be emphasised. Zn(1) & Zn(3) have square-based pyramidal geometry, with calixarene oxygens in the square base and an acetate oxygen apical. The central Zn(2) is tetrahedral, binding to two acetate oxygens and two calixarene phenolate oxygens, one from each calixarene. The two acetate ligands and Zn(2) provide the bridge between the two calixarenes.


image file: d2dt01496c-f2.tif
Fig. 2 Molecular structure of [Zn3(O2CCH3)2(L(O)2(On-Pr)2)2]·5MeCN (2·5MeCN). H atoms, minor disorder components, and some MeCNs omitted for clarity. A second Zn3 complex in the asymmetric unit has similar geometry. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 1.978(7), Zn(1)–O(2) 2.215(9), Zn(1)–O(3) 1.862(7), Zn(1)–O(4) 2.259(8), Zn(1)–O(5) 1.951(9), Zn(2)–O(1) 1.942(7), Zn(2)–O(6) 1.959(8), Zn(2)–O(9) 1.956(7), Zn(2)–O(12) 1.968(9); Zn(1)–O(1)–Zn(2) 111.0(3), Zn(2)–O(9)–Zn(3) 113.6(4), O(1)–Zn(2)–O(9) 125.1(3).

These acetate-bridged complexes are soluble in common organic solvents such as MeCN, toluene, THF, acetone and dichloromethane. In toluene, the solubility of 1·7.5MeCN is about 0.28 g per mL, whilst that of 2·5MeCN is about 0.40 g per mL.

Ethoxide-bridged complexes. Similar treatment of L(OH)2(OMe)2 with [ZnEt2], followed by the addition of ethanol, led to the isolation of the complex [Zn3(OEt)2(L(O)2(OMe)2)2]·4MeCN (3·4MeCN) in good yield (ca. 71%). The molecular structure is shown in Fig. 3, with selected bond lengths and angles given in caption. As for 1, there are three Zn2+ ions present. Each pair is bridged by one ethoxide and one phenolate oxygen of a calix[4]arene. The Zn(1) centre is 5-coordinate and approximately tbp as indicated by a structural index parameter of τ = 0.22.11 Moreover, Zn(2) is also 5-coordinate and approximately tbp indicated by a structural index parameter of τ = 0.18.11 The central zinc Zn(3) is distorted tetrahedral with angles in the range 81.68(18) to 114.7(2)°. The Zn(1)–Zn(3)–Zn(2) angle is 175.92(3)°. One MeCN resides in each calixarene cavity, whilst the two others are exo to the complex.
image file: d2dt01496c-f3.tif
Fig. 3 Molecular structure of [Zn3(OEt)2(L(O)2(OMe)2)2]·4MeCN (3·4MeCN). H atoms and minor disorder components omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 2.200(4), Zn(1)–O(2) 2.005(4), Zn(1)–O(3) 2.241(4), Zn(1)–O(4) 1.870(5), Zn(1)–O(9) 1.970(5), Zn(3)–O(2) 1.979(4), Zn(3)–O(6) 1.985(4), Zn(3)–O(9) 1.929(5), Zn(3)–O(10) 1.928(5); Zn(1)–O(2)–Zn(3) 97.33(18), Zn(1)–O(9)–Zn(3) 100.2(2).

Reaction of the related 1,3-di-n-pentylether-p-tert-butylcalix[4]arene(OH)2, L(OH)2(On-pentyl)2, with [ZnEt2], followed by the addition of ethanol, led to the isolation of the complex [Zn3(OEt)2(L(Opentyl)2)2]·4.5MeCN (4·4.5MeCN). The molecular structure is shown in Fig. 4, with selected bond lengths and angles given in the caption. As for 1–3, whilst the central Zn(2) is 4-coordinate distorted tetrahedral, the other Zn centres in 4 are 5-coordinate tbp (τ = 0.296 at Zn(1) and 0.258 at Zn(3)).11 The Zn(1)–Zn(2)–Zn(3) angle is 178.92(2)°.


image file: d2dt01496c-f4.tif
Fig. 4 Molecular structure of [Zn3(OEt)2(L(O)2(Opentyl)2)2]·4.5MeCN (4·4.5MeCN). H atoms and minor disorder components omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 2.221(2), Zn(1)–O(2) 1.853(3), Zn(1)–O(3) 2.437(2), Zn(1)–O(4) 1.987(2), Zn(1)–O(9) 1.962(3), Zn(2)–O(4) 1.982(2), Zn(2)–O(8) 1.983(2), Zn(2)–O(9) 1.942(3), Zn(2)–O(10) 1.937(3); Zn(1)–O(4)–Zn(2) 97.14(10), Zn(1)–O(9)–Zn(2) 99.32(12), Zn(2)–O(10)–Zn(3) 99.08(11).

These ethoxide-bridged complexes are somewhat more soluble in toluene than 1 and 2; the solubility of 3·4MeCN is about 0.50 g per mL and that for 4·4.5MeCN about 1.20 g per mL.

Hydroxy-bridged complex. Interaction of L(OH)2(On-pentyl)2, [Zn(C6F5)2]·toluene12 and K2CO3 led, following work-up, to the complex [Zn3(OH)2(L(O)2(On-pentyl)2]·8MeCN (5·8MeCN) in low yield (<10%). The molecular structure is shown in Fig. 5, with selected bond lengths and angles given in the caption. The molecule lies on a 2-fold axis, and so half is unique. There are three Zn2+ ions, each pair bridged by an OH− and a calixarene phenolate O. Zn(1) is 5-coordinate and approximately tbp indicated by a structural index parameter of τ = 0.25.11 The central Zn(2) is 4-coordinate and is very distorted from tetrahedral. The Zn(1)–Zn(2)–Zn(1A) angle is very similar to that in 1·7.5MeCN, at 173.332(15)°. One unique MeCN hydrogen bonds to the OH− group, one MeCN resides in the calixarene cavity and the other two MeCNs lie exo to the molecule. We presume the formation of 5·8MeCN initially involves the formation of a bimetallic {[Zn(C6F5)]2L(O)(Opentyl)2} species which then undergoes a fortuitous hydrolysis reaction to form the hydroxide-bridged product 5·8MeCN. Attempts to prepare 5via the addition of H2O to intermediate alkylzinc L(O)2(Opentyl)2 species led only to the isolation of the parent calixarene ligand, namely L(OH)2(On-pentyl)2. The molecular structures of two polymorphs of L(OH)2(On-pentyl)2·MeCN are given in the ESI (Fig. S3, S4 and Table S1†). A new triclinic β polymorph crystallises in space group P[1 with combining macron] with Z′ = 2. We have previously reported the α monoclinic polymorph which crystallises in space group P21/n with Z′ = 1, which was obtained again during the present studies, and for which we now report a considerably more precise determination.13 In both polymorphs an acetonitrile molecule resides in the calixarene cavity.
image file: d2dt01496c-f5.tif
Fig. 5 Molecular structure of [Zn3(OH)2(L(O)2(On-pentyl)2]·8MeCN (5·8MeCN). H atoms, except hydroxy-H, and minor disorder components omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 2.2756(14), Zn(1)–O(2) 1.9889(14), Zn(1)–O(3) 2.2587(14), Zn(1)–O(4) 1.8755(15), Zn(2)–O(2) 1.9955(14), Zn(2)–O(5) 1.9134(16); Zn(1)–O(2)–Zn(2) 95.56(6), Zn(1)–O(5)–Zn(2) 99.79(7); H-bond geometry: O(5)–H(5)⋯N(1) 1.98(3) Å, 170(2)°.

Reaction of commercial [Zn(C6F5)2] with L(OH)2(On-pentyl)2 in the presence of K2CO3 led, following work-up (MeCN), to the colourless complex [Zn6(L(On-pentyl))2(OH)3(C6F5)3(NCMe)3]·3MeCN (6·3MeCN). This is the asymmetric unit. There are 12 positive charges, from the 6 Zn2+ ions, balanced by 2 calixarenes each with three negative charges, 3 C6F5− anionic ligands, and three OH− ions. The two calixarenes having different conformations (pinched and cone), see Fig. 6. The pinched cone houses a C6F5 ligand bound to Zn(6), while the conventional cone has an MeCN ligand bound to Zn(1) in the cavity. For the pinched cone, the opposite para C atoms in the rings, namely C(53) and C(75), are 7.130(5) Å apart, while the other pair, C(64) and C(86), are 10.715(5) Å apart. In the conventional cone, the two comparable distances are 7.956(6) Å for C(4) to C(26), and 8.015(5) Å for C(15) to C(37). The Zn(1) and Zn(5) centres are 6-coordinate octahedral, while Zn(6), in the pinched cone cavity, is 3-coordinate trigonal planar, but may be forming π-interactions with the nearby pair of calixarene aromatic rings bound to O(5) and O(7). The centres Zn(2), Zn(3), and Zn(4) are all 4-coordinate distorted tetrahedral. In the core, there are three Zn2O2 diamond motifs, with that involving Zn(5) and Zn(6) utilising two calixarene phenolate oxygens. Those involving Zn(1)/Zn(2) and Zn(1)/Zn(4) are linked, and involve one calixarene phenolate oxygen and the shared hydroxyl at O(9). There are also three Zn3O3 ring motifs, with that involving Zn(2)/Zn(4)/Zn(5) including two phenolate oxygens from different calixarenes and an OH−, while the two involving Zn(1)/Zn(3)/Zn(4) include two phenolate oxygens from the same conventional cone calixarene and an OH−, or two OH− ligands and one phenolate oxygen. Indeed, only the two calixarene oxygens bearing the pentyl groups are not involved in these ring motifs. In terms of a linear Zn3 fragment, the angles subtended at Zn(5), with Zn(2) and Zn(6) on either side, is 162.75(2)°.


image file: d2dt01496c-f6.tif
Fig. 6 Molecular structure of [Zn6(L(On-pentyl))2(OH)3(C6F5)3(NCMe)3]·3MeCN (6·3MeCN). Calixarene tBu groups, and H atoms except hydroxy-H have been omitted for clarity. The right-hand image shows the core of the molecule. Selected bond lengths (Å) and angles (°): Zn(1)–O(1) 2.207(3), Zn(1)–O(2) 1.996(2), Zn(1)–O(3) 2.033(3), Zn(1)–O(4) 2.014(3), Zn(1)–O(9) 2.184(3), Zn(1)–N(1) 2.180(3), Zn(2)–O(2) 1.986(2), Zn(2)–O(9) 2.045(3), Zn(2)–O(11) 1.915(3), Zn(2)–C(105) 2.007(4), Zn(3)–O(10) 1.947(3), Zn(3)–N(2) 2.087(4), Zn(4)–O(3) 1.948(2), Zn(4)–O(7) 1.898(2), Zn(4)–O(9) 2.021(3), Zn(4)–O(10) 1.929(3), Zn(5)–O(5) 2.335(2), Zn(5)–O(6) 2.069(3), Zn(5)–O(7) 2.082(2), Zn(5)–O(8) 2.031(2), Zn(5)–O(11) 1.921(3), Zn(5)–N(3) 2.167(3), Zn(6)–O(6) 1.914(2), Zn(6)–O(8) 1.900(3), Zn(6)–C(111) 1.950(4); Zn(1)–O(1)–Zn(2) 100.87(11), Zn(2)–O(11)–Zn(5) 128.10(14), Zn(5)–O(6)–Zn(6) 100.23(10). H-bond geometry: O(9)–H(9)⋯F(5) 2.28(2) Å, 158(4)°, O(9)–H(9)⋯F(6) 2.27(3) Å, 125(3)°, O(10)–H(19)⋯N(5) 2.08(2) Å, 109(4)°, O(11)–H(11)⋯N(4) 2.19(2) Å, 174(4)°.

There is intramolecular bifurcated H-bonding from the OH− ligand (O(9), which does not H-bond to an MeCN) to two F atoms of two different C6F5 ligands. Of the 3 molecules of MeCN of crystallization, two H-bond to OH− groups, whilst the other lies exo to the metal complex.

Key structural parameters for the calixarenes 1–6 are presented in Table 1.

Table 1 Key structural data for 1–6
Compound 1·8MeCN 2·5MeCN 3·4MeCN 4·4.5MeCN 5·8MeCN 6·3MeCN
a Note this is the central zinc. b tbp = trigonal bipyramid; Reedijk et al. defined τ as τ = (p − r)/60 which is applicable to five-co-ordinate structures as an index of the degree of trigonality.11
Coordination number
Zn(1) 5 5 5 5 5 —
Zn(2) 4 4 5 4 4 4
Zn(3) 5 5 4a 5 5 —
Zn(4) — — — — — —
Zn(5) — — — — — 6
Zn(6) — — — — — 3
Geometry
Zn(1) tbp (τ = 0.06)b Tetragonal pyramid tbp (τ = 0.22)b tbp (τ = 0.296)b tbp (τ = 0.25)b —
Zn(2) Tetrahedral Tetrahedral tbp (τ = 0.18)b Tetrahedral Tetrahedral Tetrahedral
Zn(3) tbp (τ = 0.04)b Tetragonal pyramid Tetrahedral tbp (τ = 0.258)b tbp (τ = 0.25)b —
Zn(4) — — — — — —
Zn(5) — — — — — Octahedral
Zn(6) — — — — — Trigonal planar
Linearity of Zn3 motif 175° 174° 176° 179° 173° 163°

Ring opening polymerization studies

Based on our previous zinc calixarene ROP studies,4b we selected the conditions of 90 °C with a ratio of ε-CL to complex of 500[thin space (1/6-em)]:[thin space (1/6-em)]1 in the presence of one equivalent of benzyl alcohol over 24 h. For comparative studies, we also screened the known trinuclear complex I, which bears the aminebis(phenolate) ligand n-propylamine-N,N-bis(2-methylene-4,6-di-tert-butylphenolate).8 Data for the runs are presented in Table 2, where [Cat] represents the concentration of the complex employed, and it can be seen that low to medium molecular weight products are formed with good control. Interestingly, despite the narrow PDI values, MALDI-TOF spectra revealed at least two series of ions corresponding to polycaprolactone (PCL). For example, in Fig. S5 (for 2, entry 11, Table 2, ESI) and S6 (for 3, entry 14, Table 2, ESI),† the lower mass species is consistent with sodiated PCL with H/OH end groups. The OH end group likely forms an effective carboxylic acid end group with a carbonyl from the last repeat unit that can exchange the acidic proton with sodium, resulting in the higher mass sodiated series. Cyclic polymers are also evident, see ESI.† In the case of 6 (entry 19, Table 2), in the MALDI-TOF spectrum (Fig. S7, ESI†) there appear to be groups of three main series and so BnO–/–H end groups should correspond to the first series in each group i.e. m/z 4356, 4470, 3585, 4698, etc. Based on the average masses and inter-series mass differences, the middle series (m/z 4380, 4494, 4608, 4722, etc.) is likely to have HO–/–H end groups and the third series (m/z 4394, 4507, 4622, 4736, etc.) is likely CH3O–/–H end groups. For runs employing BnOH, end group analysis by 1H NMR spectroscopy (e.g. Fig. S8† for entry 17, Table 2) is consistent with the presence of a BnO end group (cf. Fig. S9† for entry 18, Table 2). From a kinetic study (Fig. 7) of 1–6 (not 5) and I, conducted in the presence of two equivalents of BnOH, it was observed that the polymerization rate exhibited first-order dependence on the ε-CL concentration (Fig. 7(a)), and the conversion of monomer achieved over 150 min was >70% for I and 4, whereas for the other systems, the conversion was <50% over 300 min.
image file: d2dt01496c-f7.tif
Fig. 7 (a) Relationship between conversion and time for the polymerization of ε-CL by using complexes 1–6 (not 5) and I; (b) plot of ln[CL]0/[CL]tvs. time for the polymerization of ε-CL by using complexes 1–6 (not 5) and I.
Table 2 Synthesis of polycaprolactone using complexes 1–6 and I
Entry Cat. [Monomer][thin space (1/6-em)]:[thin space (1/6-em)][Cat][thin space (1/6-em)]:[thin space (1/6-em)]BnOH T/°C t/h Conva (%) M n M n,Cal PDId
a Conversion was confirmed by 1H NMR spectroscopy. b Determined by GPC analysis calibrated with polystyrene standards and multiplied by correction factor of 0.56. c F.W.[M]/[BnOH] (conversion) + BnOH. d Polydispersity index (Mw/Mn) were determined by GPC. e Conducted in air.
1 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 79.7 11[thin space (1/6-em)]870 45[thin space (1/6-em)]590 1.18
2 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 50.7 43[thin space (1/6-em)]440 29[thin space (1/6-em)]040 1.44
3 1 100[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 96 6980 11[thin space (1/6-em)]060 1.37
4 1 250[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 99 14[thin space (1/6-em)]790 28[thin space (1/6-em)]350 1.29
5 1 1000[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 99 3860 113[thin space (1/6-em)]100 1.16
6e 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 5.7 910 3360 1.12
7 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 1 38.3 6790 21[thin space (1/6-em)]960 1.23
8 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 3.5 97 22[thin space (1/6-em)]430 55[thin space (1/6-em)]460 1.25
9 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 5 99 21[thin space (1/6-em)]030 56[thin space (1/6-em)]600 1.32
10 1 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 15 24 5.5 2160 3240 1.08
11 2 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 48.1 3260 27[thin space (1/6-em)]550 1.31
12 2 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 58.4 3520 33[thin space (1/6-em)]430 1.16
13 3 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 90 24 99.7 2895 57[thin space (1/6-em)]000 1.11
14 3 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 >99 2760 56[thin space (1/6-em)]600 1.09
15 4 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 >99 21[thin space (1/6-em)]275 56[thin space (1/6-em)]600 2.59
16 4 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 >99 34[thin space (1/6-em)]690 56[thin space (1/6-em)]600 2.30
17 5 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2 90 24 >99 29[thin space (1/6-em)]765 56[thin space (1/6-em)]600 2.25
18 5 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 56.9 19[thin space (1/6-em)]070 32[thin space (1/6-em)]580 2.10
19 6 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 90 48 97.5 5940 55[thin space (1/6-em)]750 1.25
20 I 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 20 24 29 4750 16[thin space (1/6-em)]650 1.24
21 I 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]0 90 24 99.5 17[thin space (1/6-em)]390 56[thin space (1/6-em)]890 1.98

The activity trend in this case revealed that I was the most active and then 4 > 6 ≈ 2 ≈ 1 > 3. Complex I is thought to be more active given the more open (i.e. less sterically congested) environment around the three 4-coordinate tetrahedral zinc centres, which contrasts with the tbp geometries exhibited by two of the Zn centres found in the Zn3 calixarene complexes. Given that the NMR data suggests the solid-state structures are retained in solution, we have examined the structural data. In addition to that presented in Table 1, for 1–5, the Zn–O bond lengths to the alkylated Os are longer than to the non-alkylated Os, which generates a small distortion in the cone conformation. Looking at all the cross-cone distances C(4)–C(26) and C(15)–C(37) (and their equivalents on the other calixarenes), there is one set of short distances [7.3–7.9 Å] and one longer set [7.9–8.6 Å]. In the case of the most active complex 4, it possesses both the smallest and largest of these distances [7.311 and 8.594 Å] on the lower numbered cone. It is thought that this distortion allows the Zn centre to be more accessible, which combined with the higher solubility of 4 leads to the observed higher activity. Complex 6 also possesses asymmetric calixarene cones, and this, together with the presence of the labile MeCN ligands, is thought to be the reason for its high observed activity. The activities of the two acetate complexes 1 and 2 are similar, despite the differing R/groups at the lower rim, but both are slightly more active than the ethoxide-bridged complex 3.

Conclusions

In conclusion, we have isolated and structurally characterized a number of rare examples of trinuclear zinc-bearing 1,3-dialkoxycalix[4]arene ligands. The formation of products such as 5 from fortuitous syntheses suggests that the adoption of such trinuclear structures is thermodynamically driven. The products are capable of the ROP of ε-caprolactone affording low to medium molecular weight products. The products contain multiple series of ions with a variety of end groups and can contain either cyclic or linear species, or both. The congested geometries at the outer zinc centers in the calixarene complexes, compared with those in the complex I bearing the aminebis(phenolate), results in a slower ROP process.

Experimental

General

All manipulations were carried out under an atmosphere of dry nitrogen using Schlenk and cannula techniques or in a conventional nitrogen-filled glove box. Ethanol was dried over molecular sieves (3 Å). Toluene was dried over sodium, acetonitrile and triethylamine were dried over calcium hydride, and dmso and acetone were dried over molecular sieves. All solvents were distilled and degassed prior to use. ε-Caprolactone was stored over activated 4 Å molecular sieves and then distilled prior to use. Benzyl alcohol was distilled, and then a stock solution prepared containing 0.01 mmol per mL. Compound I was made by the method of Kozak.8 [Zn(C6F5)2]·toluene was made by the method of Bochmann.12 All other chemicals were purchased from Sigma-Aldrich or TCI UK and used as received. IR spectra (Nujol mulls, KBr windows) were recorded on a Nicolet Avatar 360 FT IR spectrometer. Matrix Assisted Laser Desorption/Ionization Time of Flight (MALDI-TOF) mass spectrometry was performed in a Bruker autoflex III smart beam in linear mode, and the spectra were acquired by averaging at least 100 laser shots. 2,5-Dihydroxybenzoic acid was used as the matrix and THF as solvent. Sodium chloride was dissolved in methanol and used as the ionizing agent. Samples were prepared by mixing 20 μL of matrix solution in THF (2 mg mL−1) with 20 μL of matrix solution (10 mg mL−1) and 1 μL of a solution of ionizing agent (1 mg mL−1). Then 1 mL of these mixtures was deposited on a target plate and allowed to dry in air at ambient temperature. Elemental analyses were performed by the elemental analysis service at the London Metropolitan University, the Department of Chemistry & Biochemistry, University of Hull, and at Nanjing University of Information Science & Technology.
Synthesis of [Zn3(O2CCH3)2(L(O)2(OMe)2)2]·7.5MeCN (1·7.5MeCN). ZnEt2 (2.96 mL, 1.0 M, 2.96 mmol) was added to L4(OH)2(OMe)2 (1.00 g, 1.48 mmol) in toluene (30 mL) and the system was refluxed for 12 h. On cooling, acetic acid (0.17 mL, 3.0 mmol) was added and the system was stirred for 12 h. The volatiles were then removed in vacuo, and the residue was extracted into hot MeCN (30 mL). On standing at ambient temperature (1–2 days), colourless prisms of 1·7.5MeCN formed (0.93 g, 63%). C98H125Zn3NO12 (−6.5MeCN) requires C 69.03, H 7.39, N 0.82%. Found C 68.80, H 7.82, N 0.87%. IR: 3456w, 3342bm, 3179bw, 1681m, 1596w, 1297s, 1261s, 1208s, 1168m, 1100m, 1017s, 987m, 943w, 920w, 871m, 802s, 722s, 634w. 1H NMR (CDCl3) δ: 7.08 (AB quarter, J = 7.8 Hz, J′ = 2.4 Hz, 4H, arylH), 7.00 (overlapping d, J = 2.8 Hz, 2H, arylH), 6.92 (AB quartet, J = 12.2 Hz, J′ = 2.4 Hz, 4H, arylH), 6.82 (d, J = 2.4 Hz, 2H, arylH), 6.72 (d, J = 2.8 Hz, 2H, arylH), 5.15 (d, J = 12.4 Hz, 2H, endo-CH2), 4.47 (d, J = 12.4 Hz, 2H, endo-CH2), 4.38 (d, J = 12.0 Hz, 2H, endo-CH2), 4.23 (s, 6H, OCH3), 3.86 (s, 6H, OCH3), 3.33 (overlapping d, J = 12.4 Hz, 4H, endo- + exo-CH2), 3.24 (d, J = 12.0 Hz, 2H, exo-CH2), 3.17 (d, J = 12.0 Hz, 2H, exo-CH2), 2.39 (s, 6H, OAc), 2.16 (d, J = 12.8 Hz, 2H, exo-CH2), 1.27 (s, 18H, C(CH3)3), 1.19 (s, 18H, C(CH3)3), 1.17 (s, 18H, C(CH3)3), 1.12 (s, 18H, C(CH3)3). The other solvate, namely 1′·6MeCN, was prepared as above.
Synthesis of [Zn3(O2CCH3)2(L(O)2(On-Pr)2)2]·5MeCN (2·5MeCN). As for 1, but using ZnEt2 (2.73 mL, 1.0 M, 2.73 mmol), L4(OH)2(On-Pr)2 (1.00 g, 1.36 mmol) and acetic acid (0.17 mL, 3.0 mmol) affording 2 as a white crystalline solid (1.10 g, 75%). C104H138Zn3O12 (sample dried in vacuo for 3 h, −5MeCN) requires C 70.32, H 7.83%. Found C 70.30, H 8.40%. IR: 2247w, 1573s, 1364s, 1323w, 1306s, 1261s, 1244w, 1196s, 1125m, 1090m, 1062w, 1025m, 989w, 953w, 947w, 936w, 917w, 907w, 889w, 868m, 836w, 820m, 800s, 750w, 722m, 670w, 636w, 619w, 523w. 1H NMR (CDCl3) δ: 8.13 (m, 2H, arylH), 7.15 (s, 1H arylH), 7.02 (m, 1H arylH), 6.92 (m, (s, 6H arylH), 6.85 (d, J = 2.8 Hz, 1H, arylH), 5.25 (d, J = 12.4 Hz, 2H, endo-CH2), 4.56 (m, 2H, OCH2), 4.36 (d, J = 11.6 Hz, 2H, endo-CH2), 4.30 (d, J = 12.8 Hz, 4H, endo-CH2), 4.18 (m, 2H, OCH2), 4.04 (m, 2H, OCH2), 3.82 (m, 2H, OCH2), 3.31 (d, J = 12.8 Hz, 2H, exo-CH2), 3.26 (d, J = 12.0 Hz, 2H, exo-CH2), 3.17 (d, J = 12.0 Hz, 2H, exo-CH2), 3.14 (d, J = 12.0 Hz, 2H, exo-CH2), 2.24 (s, 6H OAc), 2.06 (m, 6H, OCH2CH2CH3), 1.66 (m, 2H, OCH2CH2CH3), 1.29 (s, 6H, MeCN), 1.24 (s, 18H, C(CH3)3), 1.19 (s, 18H, C(CH3)3), 1.16 (s, 18H, C(CH3)3), 1.12 (s, 18H, C(CH3)3), 1.06 (s, 6H, MeCN), 1.03 (s, 3H, MeCN), 0.99 (t, J = 7.2 Hz, 6H, CH3), 0.89 (t, J = 7.2 Hz, 6H, CH3). Mass spec. (MALDI): 1717 ((M − 5MeCN − OAc).
Synthesis of [Zn3(OEt)2(L(O)2(OMe)2)2]·4MeCN (3·4MeCN). As for 1, but using L4(OH)2(OMe)2 (1.00 g, 1.48 mmol) [ZnEt2] (2.96 ml, 1.0 M, 2.96 mmol) and EtOH (0.17 mL, 3.0 mmol) affording colourless prisms (0.94 g, 71%). C100H131Zn3N2O10 (sample dried in vacuo for 2 h, −2MeCN) requires C 69.94, H 7.69, N 1.63%. Found C 68.52, H 7.59, N 1.59%. IR: 3457w, 3319bm, 3180w, 2247w, 1681m, 1664m, 1586m, 1482s, 1364s, 1299s, 1260s, 1207s, 1168m, 1124m, 1101m, 1092m, 1005s, 946w, 870m, 785s, 764w, 755w, 722w, 671w, 637w. 1H NMR (C6D6) δ: 7.26 (bm, 4H, arylH), 7.20 (m, 2H, arylH), 7.07 (bm, 1H, arylH), 7.03 (bm, 3H, arylH), 6.93 (bm, 4H, arylH), 4.77 (bq, 4H, J obscured, OCH2CH3), 4.49 (d, 2H, J = 14.4 Hz, endo-CH2), 4.41 (d, 2H, J = 12.4 Hz, endo-CH2), 3.86 (s, 6H, OMe), 3.77 (d, 2H, J = 12.4 Hz, endo-CH2), 3.41 (s, 6H, OMe), 3.37 (overlapping d, 6H, endo + exo-CH2), 3.21 (d, 2H, J = 12.0 Hz, exo-CH2), 3.08 (d, 2H, J = 12.8 Hz, exo-CH2), 1.84 (t, 6H, J = 6.8 Hz, OCH2CH3), 1.47 (s, 18H, C(CH3)3), 1.24 (s, 18H, C(CH3)3), 0.93 (s, 18H, C(CH3)3), 0.84 (s, 18H, C(CH3)3), −0.32 (s, 6H MeCN). Mass spec. (MALDI): 1525 (M − 4MeCN − Zn − OEt), 1413 (M − 4MeCN − 2Zn − 2OEt).
Synthesis of [Zn3(OEt)2(L(Opentyl)2)2]·4.5MeCN (4·4.5MeCN). As for 1, but using L4(OH)2(On-pentyl)2 (1.00 g, 1.27 mmol), [ZnEt2] (2.54 ml, 1.0 M, 2.54 mmol) and EtOH (0.15 mL, 2.5 mmol) affording colourless prisms. Yield: 1.03 g, 79%. C117H165.5N2.5Zn3O10 (sample dried in vacuo for 2 h, −2MeCN) requires C 71.58, H 8.50, N 1.78%. Found C 71.80, H 8.94, N 1.73%. IR: 2288w, 2248w, 1746w, 1584m, 1189s, 1172m, 1125s, 1110s, 1093s, 1045s, 1025m, 980s, 947w, 937w, 917w, 900w, 891w, 871s, 825m, 801s, 763w, 723m, 670w, 635w, 591w, 538m, 502w, 458w, 429w. Mass spec. (MALDI): 1133 (M − 3MeCN − L(Opentyl)2), 1092 (M − 3MeCN − L(Opentyl)2). 1H NMR (C6D6) δ: 7.25 (bm, 4H arylH), 7.21 (s, 1H, arylH), 7.16 (bm, 4H, arylH), 7.10 (bm, 6H, arylH), 6.97 (s, 1H, arylH), (4.89m, 4.69m 4.49 d, 4.20 bm, 4.08 bm; 16H, endo-CH2 + 4× OCH2), (3.81 overlapping d, 3.70m, 3.50bm, 3.33bm; 12H, exo-CH2 + 2× OCH2), 1.46 (s, 18H, C(CH3)3), 1.39 (m, 4H, CH2-pentyl), 1.32 (m, 2H, CH2-pentyl), 1.29 (s, 18H, C(CH3)3), 1.27–1.07 (overlapping m, 10H, CH2-pentyl), 1.02, (s, 36H, C(CH3)3), 0.93 (overlapping m, 6H, CH3), 0.83 (m, 6H, CH3).
Synthesis of [Zn3(OH)2(L(O)2(On-pentyl)2]·8MeCN (5·8MeCN). As for 1, but using L(OH)2(On-pentyl)2 (1.00 g, 1.27 mmol) [Zn(C6F5)2]·toluene (1.01 g, 2.54 mmol) and K2CO3 (0.06 g, 0.43 mmol) affording colourless prisms. Yield: 0.16 g, 8.8%. C118H165Zn3N5O10 (sampled dried for 2 h in vacuo, −3MeCN) requires C 70.52, H 8.28, N 3.45%. Found C 69.82, H 8.66, N 2.70%. IR: 3387bs, 3176w, 1598w, 1303s, 1260s, 1241m, 1195s, 1170m, 1097s, 1076m, 999s, 980m, 946w, 919w, 893m, 801s, 722s, 635w.
Synthesis of [Zn6(L(On-pentyl))2(OH)3(C6F5)3(NCMe)3]·3MeCN (6·3MeCN). As for 1, but using L(OH)2(On-pentyl)2 (1.00 g, 1.27 mmol) [Zn(C6F5)2] (1.01 g, 2.54 mmol) and K2CO3 (0.06 g, 0.43 mmol) affording pale yellow crystals (0.81 g, 73%). C122H138F15Zn6N3O11 (sample dried in vacuo for 2 h, −3MeCN) requires C 58.62, H 5.57, N 1.68%. Found C 57.23, H 5.89, N 1.46%. IR: 3352bm, 1659w, 1598w, 1533w, 1503m, 1363s, 1301m, 1261s, 1204s, 1123s, 1097s, 1048s, 1023s, 962w, 951m, 919w, 871m, 801s, 722m, 635w, 527w. 1H NMR (C6D6) δ: 8.96 (s, 1H, OH), 8.72 (s, 2H, OH), 7.29 (s, 1H, arylH), 7.16 (m, 2H, arylH), 7.13 (m, 2H, arylH), 7.08 (overlapping m, 4H, arylH), 7.04 (m, 2H, arylH), 6.99 (m, 1H, arylH), 6.90 (s, 4H, arylH), 4.64 (d, 2H, J = 13.2 Hz, endo-CH2), 4.47 (overlapping d, 5H, endo-CH2), 3.72 (m, 4H, OCH2 of pentyl), 3.37 (overlapping d, 6H, exo-CH2), 3.29 (d, 2H, J = 12.4 Hz, exo-CH2), 1.89 (m, 4H, CH2 of pentyl), 1.54 (m, 4H, CH2 of pentyl), 1.40 (s, 18H, C(CH3)3), 1.39 (s, 9H, C(CH3)3), 1.30 (overlapping m, 4H, CH2 of pentyl) 1.19 (s, 9H, C(CH3)3), 1.11 (s, 9H, C(CH3)3), 0.94 (t, 3H, J = 7.2 Hz, CH3 of pentyl), 0.84 (t, 3H, J = 7.2 Hz, CH3 of pentyl), 0.83 (s, 18H, C(CH3)3), 0.61 (s, 9H, C(CH3)3), −0.20 (bs, 9H, MeCN). 19F NMR (C6D6) δ: −113.65, −138.99, −153.99, −155.12, −157.78, −160.32, −162.18, −162.20.

Procedure for ROP

Typical polymerization procedure in the presence of one equivalent of benzyl alcohol (Table 2, entry 1) is as follows. A toluene solution of 1 (0.01 mmol, in 1.0 mL toluene) and, if required, BnOH (0.02 mmol) were added into a Schlenk tube in the glove-box at room temperature. The solution was stirred for 2 min, and then ε-caprolactone (typically 5 mmol unless stated otherwise) along with 1.5 mL toluene was added to the solution. The reaction mixture was then placed into an oil bath pre-heated to the required temperature, and the solution was stirred for the prescribed time. The polymerization mixture was then quenched by addition of an excess of glacial acetic acid (0.2 mL) into the solution, and the resultant solution was then poured into methanol (200 mL). The resultant polymer was then collected on filter paper and was dried in vacuo.

Kinetic studies

The polymerizations were carried out at 130 °C in toluene (2 mL) using 0.010 mmol of complex. The molar ratio of monomer to initiator to co-catalyst was fixed at 500[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]2, and at appropriate time intervals, 0.5 μL aliquots were removed (under N2) and were quenched with wet CDCl3. The percent conversion of monomer to polymer was determined using 1H NMR spectroscopy.

X-ray crystallography

The diffraction data were collected at low temperature on either sealed tube or rotating anode systems, or in the case of 5·8MeCN using synchrotron radiation at Daresbury Laboratory Station 9.8.14,15 All data sets were corrected for absorption, Lp effects and, where significant, for extinction effects. CCD or hybrid pixel array detectors were employed. Structures were solved by direct methods or via a charge flipping algorithm and all non-H atoms were refined anisotropically.16,17 H atoms were constrained except those on hetero atoms if the data quality allowed. Further details are provided in Table 3, in the deposited cif files, and the ESI.† Non-routine aspects are described below for each structure. Where disorder was modelled, this was supported by using geometric and anisotropic displacement parameter restraints.
Table 3 Crystal structure data for 1·7.5MeCN, 1′·6MeCN, 2·5MeCN, 3·4MeCN, 4·4.5MeCN, 5·8MeCN, 6·3MeCN, and L(OH)2(Opentyl)2
Compound 1·7.5MeCN 1′·6MeCN 2·5MeCN 3·4MeCN
Formula C111H144.5Zn3N7.50O12 C108H140Zn3N6O12 C114H153Zn3N5O12 C104H137Zn3N4O10
Formula weight 1971.94 1910.36 1981.51 1799.28
Crystal system Monoclinic Monoclinic Triclinic Monoclinic
Space group P21/c I2/a P[1 with combining macron] P21/c
Unit cell dimensions
a (Å) 18.65096(9) 22.336(2) 18.5416(2) 12.3700(15)
b (Å) 23.45772(11) 19.6856(18) 23.5115(3) 22.934(3)
c (Å) 25.30949(11) 23.872(4) 25.1000(3) 18.132(2)
α (°) 90 90 91.7320(11) 90
β (°) 105.0808(11) 90.4320(12) 90.6899(10) 106.849(2)
γ (°) 90 90 93.3356(10) 90
V (Å3) 10691.77(10) 10[thin space (1/6-em)]496(2) 10917.5(2) 4923.1(10)
Z 4 4 4 2
Temperature (K) 100(2) 150(2) 100(2) 150(2)
Wavelength (Å) 1.54178 0.71073 1.54178 0.71073
Calculated density (g cm−3) 1.225 1.209 1.206 1.214
Absorption coefficient (mm−1) 1.27 0.74 1.24 0.78
Crystal size (mm3) 0.10 × 0.09 × 0.04 0.35 × 0.15 × 0.15 0.16 × 0.10 × 0.06 0.46 × 0.26 × 0.05
θ (max.) (°) 70.5 28.3 68.5 25.0
Reflections measured 220[thin space (1/6-em)]405 53[thin space (1/6-em)]490 311[thin space (1/6-em)]775 39[thin space (1/6-em)]466
Unique reflections 20[thin space (1/6-em)]337 13[thin space (1/6-em)]090 91[thin space (1/6-em)]977 17[thin space (1/6-em)]245
Reflections with I > 2σ(I) 18[thin space (1/6-em)]519 9277 70[thin space (1/6-em)]748 13[thin space (1/6-em)]412
R int 0.042 0.053 0.196 0.052
Number of parameters 1164 654 2542 1247
R 1 [F2 > 2σ(F2)] 0.041 0.053 0.172 0.049
wR2 (all data) 0.118 0.146 0.492 0.118
GOOF, S 1.05 1.02 1.08 1.00
Largest difference peak and hole (e Å−3) 0.63 and −0.65 1.15 and −0.59 2.85 and −1.40 0.55 and −0.29
Compound 4·4.5MeCN 5·8MeCN 6·3MeCN β-L(OH)2(Opentyl)2
Formula C121H171.5Zn3N4.5O10 C124H174Zn3N8O10 C128H147F15Zn6N6O11 C56H79NO4
Formula weight 2045.23 2132.81 2622.73 830.20
Crystal system Triclinic Monoclinic Triclinic Triclinic
Space group P[1 with combining macron] C2/c P[1 with combining macron] P[1 with combining macron]
Unit cell dimensions
a (Å) 13.71849(16) 15.8101(6) 14.8407(3) 11.5697(2)
b (Å) 17.62314(17) 22.6338(9) 18.8517(3) 17.7560(6)
c (Å) 25.3027(3) 34.2001(14) 25.1416(4) 24.6362(3)
α (°) 70.0705(10) 90 70.6500(15) 94.3452(19)
β (°) 84.4086(10) 103.2650(5) 79.6723(14) 90.0311(13)
γ (°) 81.2304(9) 90 67.7528(16) 99.536(2)
V (Å3) 5676.87(12) 11911.7(8) 6130.9(12) 4976.3(2)
Z 2 4 2 4
Temperature (K) 100(2) 150(2) 100(2) 100(2)
Wavelength (Å) 1.54178 0.6942 0.71073 1.54178
Calculated density (g cm−3) 1.196 1.189 1.421 1.108
Absorption coefficient (mm−1) 1.19 0.62 1.24 0.52
Crystal size (mm3) 0.20 × 0.15 × 0.10 0.20 × 0.10 × 0.08 0.06 × 0.05 × 0.04 0.16 × 0.12 × 0.08
θ (max.) (°) 70.7 27.6 26.4 68.6
Reflections measured 105[thin space (1/6-em)]494 59[thin space (1/6-em)]374 126[thin space (1/6-em)]539 58[thin space (1/6-em)]481
Unique reflections 21[thin space (1/6-em)]180 14[thin space (1/6-em)]789 25[thin space (1/6-em)]065 26[thin space (1/6-em)]581
Reflections with I > 2σ(I) 18[thin space (1/6-em)]602 10[thin space (1/6-em)]953 16[thin space (1/6-em)]952 18[thin space (1/6-em)]806
R int 0.036 0.055 0.080 0.167
Number of parameters 1455 729 1536 1134
R 1 [F2 > 2σ(F2)] 0.075 0.048 0.052 0.127
wR2 (all data) 0.226 0.134 0.127 0.344
GOOF, S 1.06 1.03 1.03 1.04
Largest difference peak and hole (e Å−3) 0.82 and −0.68 0.51 and −0.49 1.13 and −0.68 1.25 and −0.42

For 1·7.5MeCN: tBu groups C(40), C(55), and C(66) within the calix[4]arene ligands were modelled with their methyl groups two-fold disordered, with major site occupancies of 50.0(9)%, 56.5(9)%, and 56.4(5)%, respectively. 5.5 MeCNs were accounted for with the Platon Squeeze18 procedure which recovered 235 electrons in each of two equal voids.

For 1·6MeCN: atoms C(41) > C(43) were modelled as split over 2 sets of positions in a tBu group, with major site occupancy 72.5(16)%. In a MeCN group N(2) was modelled as split over 2 positions with major component 78.4(15)%. There is a possible, <10%, OH− alternative to the acetate anion at O(5), but this could not be successfully modelled.

For 2·5MeCN: the tBu groups at C(7) and C(111) were modelled with Me groups split over two sets of positions with major occupancy 83(2) and 67(3)%, respectively. The central C atom in one of the n-Pr groups, C(153), was similarly modelled with major component occupancy 61(3)%. The diffraction data were non-merohedrally twinned via a 180° rotation about [0 0 1] in both direct and reciprocal space with component ratio of 0.5095[thin space (1/6-em)]:[thin space (1/6-em)]0.4905(18). This problem, along with it being a very large structure resulted in the data quality being fairly poor, but the connectivity is clearly established. Several crystals were analysed, all with the same difficulties.

For 3·4MeCN: the tBu groups at C(18), C(53), and C(75) were modelled with 2-fold disorder of the methyl groups. Major site occupancies in each case were 76.9(11), 64(2), and 68(2)%, respectively. For the MeCN including N(2), the methyl group was common to both of two disorder components while the other atoms were split with major component occupancy 66(4)%. The MeCN at N(3) was modelled with all atoms split over two sets of positions with major occupancy 68(3)%. The absolute structure was reliably determined with x = −0.001(6).

For 4, the tBu groups at C(7), C(72), and C(94) were modelled with 2-fold disorder of the methyl groups. The major component occupancy was 76.7(6), 62.1(10), and 76.0(11)%, respectively. Pentyl chain atoms C(48) and C(49), C(51) > C(54), and C(105) > C(108) were modelled as split over two sites with major occupancies 53.9(10), 62.8(6), and 70.3(6)%, respectively. There is a distance of around 3 Å between C(49) and C(54′) in a symmetry-related molecule, hence the disorder of the pentyl chains containing those atoms.

For 5·8MeCN: atoms C(46) > C(49) were modelled as split over two sets of positions with major occupancy 81.9(4)%. Atoms C(52) & C(53) were modelled as split over two positions with major site occupancy 59.9(7)%.

The diffraction data for the new triclinic polymorph β-L(OH)2(Opentyl)2·MeCN were non-merohedrally twinned via a 180° rotation about [1 0 0] in direct space or [1 −0.25 0] in reciprocal space with component ratio of 0.5179[thin space (1/6-em)]:[thin space (1/6-em)]0.4821(19).

CCDC 2133113–2133121† contain the supplementary crystallographic data for this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Chinese Scholarship Council (CSC) is thanked for the award of a PhD scholarship to Tian Xing. We thank the National Mass Spectrometry Service at Swansea, Wyatt Analytical Ltd, and the EPSRC Crystallographic Service Centre at Southampton for data. Data for 5·8MeCN were collected at Daresbury Laboratory (Station 9.8) for which we thank the STFC for beam time. CR thanks the EPSRC for financial support in the form of a PRIF grant (EP/S025537/1).

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Footnote

† Electronic supplementary information (ESI) available: molecular structure of 1′·6MeCN, alternative view of 2·5MeCN, molecular structure of the triclinic polymorph of L(OH)2(On-pentyl)2·MeCN, packing structure of L(OH)2(On-pentyl)2·MeCN, crystal structure data for α-L(OH)2(Opentyl)2, MALTI-TOF spectrum of PCL from 2 (entry 11, Table 2), MALTI-TOF spectrum of PCL from 3 (entry 14, Table 2), MALTI-TOF spectrum of PCL from 6 (entry 19, Table 2), 1H NMR spectrum of PCL using 5/BnOH (entry 17, Table 2), 1H NMR spectrum (CDCl3, 400 MHz, 298 K) of the PCL synthesized using 5 in the absence of BnOH (entry 18, Table 2). CCDC 2133113–2133121. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt01496c
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