Coordination chemistry of [2 + 2] Schiff-base macrocycles derived from the dianilines [(2-NH₂C₆H₄)₂X] (X = CH₂CH₂, O): structural studies and ROP capability towards cyclic esters

Abstract

Reaction of the [2 + 2] Schiff-base macrocycles {[2-(OH)-5-(R)-C₆H₂-1,3-(CH)₂][CH₂CH₂(2-C₆H₄N)₂]}₂ (R = Me, L¹H₂; tBu, L²H₂) with FeBr₂ afforded the complexes [FeBr(L¹H₂)]₂[(FeBr₃)₂O]·2MeCN (1·2MeCN), [FeBr(L²H₂)][X] (X = 0.5(FeBr₃)₂O, 2·0.5MeCN, X = Br, 3·5.5MeCN), respectively. Reaction of L²H₂ with [KFe(OtBu)₃(THF)] (formed in situ from FeBr₂ and KOtBu), following work-up, led to the isolation of the complex [Fe(L²)(L²H)]·3MeCN (4·3MeCN), whilst with [CuBr₂] afforded [CuBr(L²H₂)][CuBr₂]·2MeCN (5·2MeCN). Attempts to form mixed Co/Ti species by reaction of [CoBrL²][CoBr₃(NCMe)] with TiCl₄ resulted in [L²H₄][CoBr₄]·2MeCN (6·2MeCN). Use of the related oxy-bridged Schiff-base macrocycles {[2-(OH)-5-(R)-C₆H₂-1,3-(CH)₂][O(2-C₆H₄N)₂]}₂ (R = Me, L³H₂; tBu, L⁴H₂) with CoBr₂ led to the isolation of the complexes [(CoBr)₂(L³)]·2C₃H₆O (7·2C₃H₆O), [Co(NCMe)₂(L⁴H₂)][CoBr₄]·5MeCN (8·5MeCN), [Co(NCMe)₆][CoBr₃(MeCN)]₂·2MeCN (9·2MeCN). For comparative structural/polymerisation studies, the complexes {CoBr(NCMe)L⁵}₂·2MeCN (10·2MeCN) and [Co(NCMe)₂L⁵]₂[CoBr₃(NCMe)]₂ (11), [FeBr(NCMe)L⁵]₂·2MeCN (12·2MeCN) where L⁵H = 2,6-(CHO)₂-4-tBu-C₆H₂OH, as well as the chelate-free salt [Fe(NCMe)₆][FeBr₃OFeBr₃] (13) have been isolated and structurally characterized. The ability of these complexes to act as catalysts for the ring opening polymerisation (ROP) of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) was investigated, as well as co-polymerisation of ε-CL with rac-lactide (r-LA) and vice versa.

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Introduction

Global issues over plastic pollution continue to drive the search for alternative, more environmentally-friendly materials.¹ As part of our search for new catalysts capable of affording biodegradable polymers via the ring opening polymerisation (ROP) of cyclic esters, we have initiated investigations into the coordination chemistry of Schiff-base macrocycles derived from the [2 + 2] condensation of the dianilines [(X)(2-C₆H₄NH₂)₂] (X = CH₂CH₂, O) with the diformylphenols 2,6-(CHO)₂-4-R-C₆H₂OH (R = Me, tBu) (Chart 1).² To date, we have reported how remote alkylaluminium centres bound to the macrocycle (X = CH₂CH₂) exhibited beneficial cooperative effects in the ROP of ε-caprolactone (ε-CL), whereas aluminoxane type (Al–O–Al) bonding proved to be detrimental.³ Manganese complexes bearing these macrocycles were far less active (conversions <15%).⁴ However, studies of mixed cobalt/zinc systems revealed interesting catalytic properties, with homo-dinuclear systems exhibiting inactivity while mixed-metal systems proved to be efficient for the ROP of ε-CL and δ-valerolactone (δ-VL).⁵ It is also noteworthy that the structural chemistry of macrocycles of this type remains underexplored; a search of the CSD revealed no hits,⁶ other than our reported aluminium, manganese and cobalt systems.^3–5 Given this, we have re-focused our efforts on such Schiff-base systems and have extended our studies to iron, cobalt and copper complexes bearing [2 + 2] macrocycles derived from the dianilines [(X)(2-C₆H₄NH₂)₂] (X = CH₂CH₂, O, Chart 2). Herein, we report the molecular structures of these complexes, and have screened a number of them for their capability in the ROP of ε-CL, δ-VL and rac-lactide (r-LA), and for the co-polymerisation of ε-CL with r-LA and vice versa. Poly(ε-caprolactone), PCL, and poly(lactide), PLA, are favoured polymers given their biodegradability properties, and their co-polymers are considered as potential environmentally-friendly commodity plastic.⁷

Chart 1 Synthesis of ligands L¹H₂–L⁵H prepared herein.

Chart 2 Structures of iron and cobalt complexes 1–13 prepared herein.

Results and discussion

–CH₂CH₂– bridged systems

Iron.
Use R = Me L¹H₂. Iron is a cheap, earth abundant metal and its complexes have shown potential in the ROP of cyclic esters.⁸ Given this, we have initiated studies on the iron chemistry of our [2 + 2] macrocyclic systems. The reaction of the macrocycle {[2-(OH)-5-Me-C₆H₂-1,3-(CH)₂][CH₂CH₂(2-C₆H₄N)₂]}₂ (L¹H₂) with two equivalents of FeBr₂ in refluxing toluene afforded, following work-up (MeCN), a brown crystalline solid in moderate yield. Single crystals were grown from a saturated solution of acetonitrile on standing at ambient temperature for 3 days. The molecular structure is shown in Fig. 1, with selected bond lengths and angles given in the caption. There are two macrocyclic iron complexes in the asymmetric unit, related by a pseudosymmetric translation of c/2 (see ESI†), plus an anion of [Br₃FeOFeBr₃] and two molecules of acetonitrile. Of the macrocyclic bound iron centres, both Fe1 and Fe2 adopt a distorted trigonal bipyramidal geometry (τ = 0.69),⁹ bound by a bromide and two nitrogen atoms and two oxygen atoms of the macrocycle, apical sites are occupied by O atoms, with the bromide and N atoms in the equatorial sites. The composition is thus [FeBr(L¹)]₂[Br₃FeOFeBr₃]·2(MeCN) (1·2MeCN). In terms of charge, the 2+ available from the anion which contains two Fe(III) centres, is balanced by the two cations (2× + 1), each of which contains an Fe(II) centre.

Fig. 1 Asymmetric unit for the structure of [FeBr(L¹)]₂[Br₃FeOFeBr₃]·2(MeCN) (1·2MeCN). Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (°): Fe(1)–O(1) 2.002(6), Fe(1)–O(2) 2.020(6), Fe(1)–N(1) 2.129(7), Fe(1)–N(3) 2.138(7), Fe(1)–Br(1) 2.5070(16), Fe(2)–O(3) 2.031(6), Fe(2)–N(5) 2.150(7), Fe(2)–N(7) 2.135(7), Fe(2)–Br(2) 2.5104(15); O(1)–Fe(1)–O(2) 177.0(2), N(1)–Fe(1)–N(3) 103.8(3), Br(1)–Fe(1)–N(1) 120.26(19), N(5)–Fe(2)–N(7) 102.9(3).

Use R = tBu L²H₂. Similar reaction of {[2-(OH)-5-(tBu)-C₆H₂-1,3-(CH)₂][CH₂CH₂(2-C₆H₄N)₂]}₂ (L²H₂) with 2.1 equivalents of FeBr₂ afforded, following work-up, red needles in good yield. Crystals suitable for X-ray diffraction were grown from a saturated solution of acetonitrile at ambient temperature. A view of the molecular structure is shown in Fig. 2, and bond lengths and angles are given in the caption. The crystal comprises FeBr(L²H₂) cations, (FeBr₃)₂O anions and MeCN solvent molecules. The anions lie about a centre of symmetry and the solvent molecule sites refine best with half-occupancy; the molecular formula is therefore [FeBr(L²H₂)], 0.5[(FeBr₃)₂O]·0.5(MeCN) (2·0.5MeCN), and like 1·2MeCN, is a mixed oxidation state Fe(II)/Fe(III) system. The iron atom in the cation is five-coordinate with a trigonal bipyramidal pattern (τ = 0.82);⁹ the apical sites are occupied by O atoms, with the bromide and N atoms in the equatorial sites. There is a pseudo two-fold symmetry axis along the Fe–Br bond. In the macrocyclic ligand, there are four N atoms, each involved in a double bond, viz. C11=N2 1.298(5) C52=N1 1.287(6) C26=N3 1.296 (6) C37=N4 1.308 (7), N1 and N3 are coordinated to the Fe atom, while N2 and N4 are bonded to two C atoms (one with a double bond) and a hydrogen atom. The hydrogen atoms were included in the structure factor calculations in a planar, trigonal fashion, and their U_iso values were refined freely and satisfactorily; it is believed that both these groups are charged C–N⁺H=C groups. Both these hydrogen atoms are involved in intramolecular hydrogen bonds.

Fig. 2 View of the [FeBr(L²)][(FeBr₃)₂O]_0.5·0.5(MeCN) (2·0.5MeCN) complex ion, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Symmetry equivalent atoms are generated by i = 1 − x, 1 − y, 1 − z. Selected bond lengths (Å) and angles (°): Fe(1)–O(1) 2.052(3), Fe(1)–O(2) 2.059(3), Fe(1)–N(1) 2.131(4), Fe(1)–N(3) 2.124(4), Fe(1)–Br(1) 2.4967(8); Br(1)–Fe(1)–O(1) 89.71(9), O(1)–Fe(1)–N(3) 94.83(14).

The (FeBr₃)₂O anion lies about a centre of symmetry. One of the bromide ligands is disordered over two sites, in an 82 : 18 occupancy ratio. The nearest neighbours of the bromide atoms are atoms of the disordered t-butyl group.

Similar treatment of {[2-(OH)-5-(tBu)-C₆H₂-1,3-(CH)₂][O(2-C₆H₄N)₂]}₂, but with limited FeBr₂ (1.1 equivalents), led to the isolation of the Fe(II) salt [FeBr(L²H₂)]Br·3MeCN (5.5MeCN). The molecular structure is provided in the ESI (Fig. S2†).

Use of [KFe(OtBu)₃(THF)]. Given alkoxide species play a central role in metal-catalysed ROP, we attempted to generate an iron alkoxide species. Reaction of L²H₂ with in situ generated [KFe(OtBu)₃(THF)] in refluxing toluene resulted, after work-up, in the isolation of a brown crystalline material. Crystals grown from a saturated solution of acetonitrile were found to be a bis-chelate structure [Fe(L²)(L²H)]·3MeCN (4·3MeCN) (see Fig. 3), in which a distorted octahedral iron(III) centre is bound to two of the macrocyclic ligands. The asymmetric unit contains one iron complex and 3 molecules of acetonitrile. The coordination at the iron is such that one macrocycle is bound only in chelate fashion via N,O-type ligation, whilst the second macrocycle utilizes four atoms to bind in 2× N,O-type fashion. We have observed similar binding modes recently for aluminium.^3c

Fig. 3 View of [Fe(L²)(L²H)]·3MeCN (4·3MeCN), indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. For clarity hydrogen atoms are not shown. Selected bond lengths (Å) and angles (°): Fe(1)–O(11) 1.936(4), Fe–O(41) 1.931(4), Fe–O(71) 1.905(4), Fe–N(18) 2.234(5), Fe–N(37) 2.189(5), Fe–N(78) 2.240(5); O(11)–Fe–O(41) 92.1(2), O(11)–Fe–N(37) 176.6(2), N(18)–Fe–N(78) 164.0(2).

Use of copper bromide. Copper is also an earth abundant metal, though it has had only limited success in the ROP of cyclic esters.¹⁰ For the successful systems, Schiff-base ligation appears beneficial, suggesting that interaction with the macrocyclic systems of the type herein could lead to a ROP active complex. Reaction of L²H₂ with two equivalents of CuBr₂ afforded, following work-up, the orange/brown Cu salt complex [CuBr(L²H₂)][CuBr₂]·2MeCN (5·2MeCN) in good yield. In the cation, the copper centre is distorted trigonal bipyramidal with oxygens at the apex and two nitrogens and a bromide in the equatorial plane (τ = 0.90).⁹ The anion Cu(I)Br₂⁻ resides close to a pocket formed by the macrocycle, see Fig. 4.

Fig. 4 Molecular structure of [CuBr(L²H₂)][CuBr₂]·2MeCN (5·2MeCN) with atoms drawn as 50% probability ellipsoids. Solvent molecules are not shown. Selected bond lengths (Å) and angles (°): Cu(1)–O(1) 1.955(3), Cu(1)–O(2) 1.948(3), Cu(1)–N(1) 2.106(3), Cu(1)–N(3) 2.074(3), Cu(1)–Br(1) 2.4790(6); O(1)–Cu(1)–O(2) 179.8(1), N(1)–Cu(1)–Br(1) 119.58(9), N(3)–Cu(1)–Br(1) 125.8(1).

Attempted mixed-metal systems

Having successfully isolated mixed cobalt/zinc systems, which were active for ring opening polymerization whereas the analogous homodinuclear species were not,⁵ we attempted to prepare mixed cobalt/titanium complexes. Our entry point was again the cobalt complex [CoBrL²H₂][CoBr₃(NCMe)], and reaction with [TiCl₄] resulted in [L²H₄][CoBr₄]·2MeCN (6·2MeCN) as the only crystalline product. The molecular structure of the salt 6 comprises a protonated macrocycle L²H₂ and the Co(II) containing anion [CoBr₄]²⁻; see ESI (Fig. S4†) for details.

–O– bridged systems

We have also initiated a programme to investigate the coordination chemistry of the oxy-bridged macrocycles {[2-(OH)-5-(R)-C₆H₂-1,3-(CH)₂][O(2-C₆H₄N)₂]}₂ (R = Me L³H₂, tBu L⁴H₂). Reaction of L³H₂ with two equivalents of CoBr₂ afforded, following work-up (acetone), the complex [(CoBr)₂L³]·2C₃H₆O (7·2C₃H₆O), containing two Co(II) centres, as black crystals in ca. 40% isolated yield. Single crystals can be grown from a saturated solution of acetone at ambient temperature, and the molecular structure is shown in Fig. 5; selected bond lengths and angles are given in the caption. Both cobalt centres adopt distorted trigonal bipyramidal with a bromide at each apex (τ = 0.59 and 0.53 for Co1 and Co₂, respectively).⁹

Fig. 5 Molecular structure of [(CoBr)₂(L³)]·2C₃H₆O (7·2C₃H₆O) with atoms drawn as 50% probability ellipsoids. Selected bond lengths (Å) and angles (°): Co(1)–O(1) 1.982(11), Co(1)–O(3) 2.216(11), Co(1)–N(1) 2.067(12), Co(1)–N(2) 2.091(13), Co(1)–Br(1) 2.421(3), Co(2)–O(1) 2.179(11), Co(2)–O(3) 1.977(11), Co(2)–N(3) 2.102(14), Co(2)–N(4) 2.079(13), Co(2)–Br(2) 2.421(3); O(1)–Co(1)–O(3) 70.0(4), N(1)–Co(1)–N(2) 112.3(5), O(1)–Co(1)–Br(1) 122.2(3), O(1)–Co(2)–N(3) 157.8(5), N(4)–Co(2)–Br(2) 111.2(4).

In the case of L⁴H₂, reaction with CoBr₂ afforded, following work-up (MeCN), two sets of crystals (∼90 : 10). Both sets were subjected to single crystal X-ray diffraction, and the major product, as shown in Fig. 6, was found to be the Co(II) containing salt [Co(NCMe)₂(L⁴H₂)][CoBr₄]·5MeCN (8·5MeCN). In the cation, the cobalt centre is distorted octahedral, and is bound by two phenoxide oxygens and the two nitrogens N(1) and N(3) of the macrocycle plus two bound acetonitrile ligands. In the solid-state, there are C–H⋯Br interactions present. The minor product was found to be Co(II) containing salt [Co(NCMe)₆][CoBr₃NCMe]₂·2MeCN (9·2MeCN), details of this structure can be found in the ESI (Fig. S4†).

Fig. 6 Molecular structure of [Co(NCMe)₂(L⁴)][CoBr₄]·5MeCN (8·5MeCN) with atoms drawn as 50% probability ellipsoids. Selected bond lengths (Å) and angles (°): Co(1)–O(1) 2.016(3), Co(1)–O(3) 2.056(3), Co(1)–N(1) 2.156(3), Co(1)–N(3) 2.156(3), Co(1)–N(5) 2.121(3), Co(1)–N(6) 2.114(3); O(1)–Co(1)–O(3) 91.98(11), N(1)–Co(1)–N(3) 169.15(12), O(1)–Co(1)–N(6) 174.71(13), O(3)–Co(1)–N(5) 174.77(13)°.

‘Dialdehyde’ systems

To probe the role played by the presence of the macrocycle during catalysis (see ROP section), we have also prepared cobalt complexes bearing chelate ligands derived from the 4-tert-butyl-2,6-diformylphenol 2,6-(CHO)₂-4-tBu-C₆H₂OH (L⁵H). Reaction of L⁵H with CoBr₂ in the presence of excess Et₃N afforded, after work-up (MeCN), the Co(II) complex {[CoBr(NCMe)]L⁵}₂·2MeCN (10·2MeCN) as orange/brown prisms in good yield. The molecular structure is shown in Fig. 7, with selected bond lengths and angles given in the caption. Each cobalt centre is distorted octahedral, and are bound by oxygens from two L ligands and a bromide and acetonitrile ligand. The ligand arrangement is such that one bromide and an MeCN ligand reside above the Co₂O₆ plane and one bromide and an MeCN ligand are below this plane.

Fig. 7 Molecular structure of {[CoBr(NCMe)]L⁵}₂·2MeCN (10·2MeCN) with atoms drawn as 50% probability ellipsoids (unbound solvent not shown). Selected bond lengths (Å) and angles (°): Co(1)–O(1) 2.052(2), Co(1)–O(2) 2.079(2), Co(1)–Br(1) 2.5751(6), Co(1)–N(1) 2.163(3); O(1)–Co(1)–O(2) 87.17(8), Co(1)–O(2)–Co(1_i) 100.59(9), Br(1)–Co(1)–N(1) 169.51(8).

If the same reaction is conducted in the absence of Et₃N, then, following work-up (MeCN), green prisms are isolated in good yield. The molecular structure (see Fig. 8 for the cation) revealed the structure of the complex to be Co(II) containing salt [Co(NCMe)₂L⁵]₂[CoBr₃(NCMe)]₂ (11). The dimer is centrosymmetric – the second half of the dimer is generated by the symmetry operation −x, 1 − y, 1 − z. Each cobalt centre in the cation is distorted octahedral, but unlike 10, there is no bound bromide.

Fig. 8 Molecular structure of [Co(NCMe)₂L⁵]₂[CoBr₃(NCMe)]₂ (11) with atoms drawn as 50% probability ellipsoids. Symmetry operation used to generate equivalent atoms: i = −x, 1 − y, 1 − z. Selected bond lengths (Å) and angles (°): Co(1)–O(1) 2.046(2), Co(1)–O(2) 2.050(2), Co(1)–N(1) 2.108(3), Co(1)–N(2) 2.143(3); O(1)–Co(1)–O(2) 88.56(9), Co(1)–O(2)–Co(1i) 99.59(9), N(1)–Co(1)–N(2) 172.07(11).

If a similar reaction is conducted with FeBr₂ in the presence of Et₃N, then, following work-up (MeCN), black prisms are isolated in good yield. The molecular structure (Fig. S5†) revealed the structure of the Fe(II) complex to be [FeBr(NCMe)L⁵]₂·2MeCN (12·2MeCN). Each iron centre in the cation is distorted octahedral, bound by oxygens from two L ligands, a bromide and an acetonitrile ligand.

Finally, for comparative catalytic studies, the salt [Fe(NCMe)₆] [FeBr₃OFeBr₃] (13) was prepared from L¹H₂ and FeBr₃.

Ring opening polymerisation (ROP) of ε-CL and δ-VL

The Fe, Cu and Co complexes 1, 2, 4, 5, 9–13 have been screened for their ability to ring open polymerise ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL); runs were conducted in the presence of benzyl alcohol (BnOH). For complexes 1, 2 and 4, a variety of conditions were used in the attempted ROP of ε-CL including differing ratios of [ε-CL] : [Cat] : [BnOH] and run times, and it was found that the Fe compounds were far more active than the other metal complexes screened herein. ROP systems based on Fe,⁸ Cu¹⁰ and Co^5,11 have been reported with mixed success.

The polymerisation data for the ε-CL runs is given in Table 1. Highest conversion was achieved using 2 in run 5 using the ratio 500 : 1 : 1 at 130 °C over 24 h, albeit with less control than observed in run 6 using 250 : 1 : 1. Systems employing 1 and 4 were also less controlled, whilst the non-macrocyclic cobalt systems 9–11 proved to be inactive under the conditions employed herein. Interestingly, the non-macrocyclic iron complexes 12 and 13 proved to be poorly active with low conversions for ε-CL. However, for δ-VL, a 32% conversion (affording small molecular weight oligomers) was observed for 13, whereas 12 was inactive. The MALDI-TOF spectra can be interpreted using the formula C₆H₆–CH₂O[O(CH₂)₅CO]_nOH, for example see run 5 (Fig. S7, ESI†). The ¹H NMR spectrum of the PCL indicated the presence of benzyloxy and hydroxyl end groups (Fig. S8, ESI†). There was evidence of significant transesterication, whilst all observed M_n values were significantly lower than the calculated values.

Table 1 Synthesis of polycaprolactone using catalysts 1, 2, 4, 9–13

Run	Cat.	[Monomer] : [cat] : BnOH	T (°C)	t/h	Conv.^a (%)	M n × 10^3 ^b	M nCalcd × 10^4 ^c	PDI^d
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.
1	1	500 : 1 : 1	130	24	63.2	3.70	3.61	1.95
2	1	250 : 1 : 1	130	24	88.7	2.95	2.54	3.28
3	1	500 : 1 : 1	130	12	79.4	0.97	4.53	1.11
4	1	500 : 1 : 1	130	12	66.3	0.70	3.79	1.17
5	2	500 : 1 : 1	130	24	99.3	5.92	5.66	1.56
6	2	250 : 1 : 1	130	24	55.0	2.60	1.58	1.06
7	4	500 : 1 : 1	130	24	68.9	8.67	3.94	1.75
8	4	250 : 1 : 1	130	24	87.6	7.52	2.50	1.58
9	9	500 : 1 : 1	130	24	—	—	—	—
10	10	500 : 1 : 1	130	24	—	—	—	—
11	11	500 : 1 : 1	130	24	—	—	—	—
12	12	500 : 1 : 1	130	24	8.4	—	0.45	—
13	13	500 : 1 : 1	130	24	15.3	—	0.87	—

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In the case of δ-VL (Table 2), complexes 1 and 2 exhibited better conversions versus4 and 12, whilst 5, 9, 10 and 11 proved to be inactive. In the MALDI-TOF mass spectra of the PVL, a number of families of peaks were observed separated by 100 mass units (Fig. S22, ESI†) can be assigned by the formula C₆H₆–CH₂O[O(CH₂)₄CO]_nOH; the highest were typically about 6000 (run 4, Fig. S21†). The ¹H NMR spectra of the PVL (e.g. Fig. S20†) revealed the presence of benzyloxy and OH end groups. The observed molecular weights were lower than the calculated values, suggesting the presence of a chain transfer agent (H₂O or BnOH).

Table 2 ROP of δ-VL using complexes 1, 2, 4, 5, 9–13

Run	Cat.	[VL] : [Cat] : BnOH	T (°C)	t/h	Conv.^a (%)	M n × 10^3 ^b	M nCalcd × 10^4 ^c	PDI^d
a Conversion was confirmed by ^1H NMR spectroscopy. b Determined by GPC analysis calibrated with polystyrene standards and multiplied by correction factor of 0.58. c F.W. ([M]/[BnOH])(conversion) + BnOH. d Polydispersity index (Mw/Mn) were determined by GPC.
1	1	500 : 1 : 1	130	24	98.6	2.52	4.94	1.14
2	1	250 : 1 : 1	130	24	94.3	3.10	2.36	1.28
3	1	500 : 1 : 1	130	12	39.7	—	2.00	—
4	2	500 : 1 : 1	130	24	96.1	6.00	4.92	1.38
5	2	250 : 1 : 1	130	24	93.2	4.53	2.41	1.23
6	2	500 : 1 : 1	130	12	83.8	—	4.20	—
7	4	500 : 1 : 1	130	24	34.5	2.59	1.84	1.25
8	4	250 : 1 : 1	130	24	23.5	2.51	0.60	1.27
9	5	500 : 1 : 1	130	24	—	—	—	—
10	9	500 : 1 : 1	130	24	—	—	—	—
11	10	500 : 1 : 1	130	24	—	—	—	—
12	11	500 : 1 : 1	130	24	—	—	—	—
13	12	500 : 1 : 1	130	24	—	—	—	—
14	13	500 : 1 : 1	130	24	32.0	0.76	2.18	1.26

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From a kinetic study (Fig. S11 and S18†), it was observed that the polymerisation rate exhibited near first order dependence on the ε-CL or δ-VL concentration at 130 °C, and complex 2 displayed the best rate in both δ-VL [K_obs = 7.29 × 10⁻³ h⁻¹] and ε-CL polymerisations [K_obs = 4.70 × 10⁻³ h⁻¹].

Synthesis of block co-polymers

The co-polymerisation of ε-CL with r-LA and of δ-VL with r-LA was also examined (Table 3). Noteworthy, the co-polymerisation of ε-CL with r-LA is much easier than δ-VL with r-LA when using the Fe complexes. In the presence of complex 2, the M_n of the block co-poly(ε-CL + r-LA) reached around 12 000. Complexes 1 and 4 were less active in these co-polymerisations. As observed by ¹H NMR spectroscopy (Fig. S14 and S24†), the co-polymers were also capped by benzyloxy and hydroxyl end groups. The composition of the copolymer was further illustrated by ¹³C NMR spectroscopy (Fig. S17 and S25†). From the MALDI-ToF mass spectrum (positive mode) of poly(ε-CL + r-LA) a gap of 114 corresponding to the molecular weight of ε-CL was evident, whilst running the spectra in negative method revealed a gap of 144 corresponding to the molecular weight of r-LA (Fig. S12 and 13†). The MALDI-ToF mass spectrum for co-poly (δ-VL + r-LA) was also recorded, see Fig. S26.† The 2D J-resolved ¹H NMR spectrum for the copolymer was recorded and the peaks were assigned to the corresponding tetrads (see Fig. S17 and S22, ESI†) according to literature reports,¹² which revealed an atactic LA–LA–LA chain.

Table 3 Synthesis of block copolymers from cyclic ester monomers using the Fe catalysts 1, 2 and 4

Run	Composition	Catalyst	t (h)	Incorporated amount^a	M n × 10^3 ^b	PDI
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.
1	Poly (ε-CL + r-LA)	1	24 + 24 :	ε-CL : r-LA = 47 : 53	3.75	2.22
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 500 : 500 : 1 : 1
2	Poly (δ-VL + r-LA)	1	24 + 24	δ-VL : r-LA = 14 : 86	5.51	1.73
	[δ-VL] : [r-LA] : [cat] : [BnOH] = 500 : 500 : 1 : 1
3	Poly (r-LA + ε-CL)	2	24 + 24	r-LA : ε-CL = 49 : 51	10.55	1.51
	[r-LA] : [ε-CL] : [cat] : [BnOH] = 500 : 500 : 1 : 1
4	Poly (r-LA + ε-CL)	2	24 + 24	r-LA : ε-CL = 93 : 7	6.20	1.82
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 250 : 250 : 1 : 1
5	Poly (ε-CL + r-LA)	2	24 + 24	ε-CL : r-LA = 60 : 40	9.31	1.64
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 500 : 500 : 1 : 1
6	Poly (ε-CL + r-LA)	2	24 + 24	ε-CL : r-LA = 63 : 37	12.63	3.02
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 250 : 250 : 1 : 1
7	Poly (δ-VL + r-LA)	2	24 + 24	δ-VL : r-LA = 55 : 45	3.40	1.26
	[δ-VL] : [r-LA] : [cat] : [BnOH] = 500 : 500 : 1 : 1
8	Poly (δ-VL + r-LA)	2	24 + 24	δ-VL : r-LA = 88 : 12	2.57	2.56
	[δ-VL] : [r-LA] : [cat] : [BnOH] = 250 : 250 : 1 : 1
9	Poly (ε-CL + r-LA)	4	24 + 24	ε-CL : r-LA = 100 : 0	4.61	1.23
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 500 : 500 : 1 : 1
10	Poly (ε-CL + r-LA)	4	24 + 24	ε-CL : r-LA = 90 : 10	3.61	1.18
	[ε-CL] : [r-LA] : [cat] : [BnOH] = 250 : 250 : 1 : 1

---

Conclusion

In conclusion, we have successfully synthesised iron, cobalt and copper complexes bearing [2 + 2] Schiff-base macrocycles derived from dianilines containing CH₂CH₂ bridges. Cobalt complexes were prepared from a related dianiline containing an oxy bridge. A number on non-macrocyclic Fe and Co complexes were also prepared for comparative catalytic studies. The iron complexes outperformed the other metal systems herein for the ring opening polymerisation of both ε-caprolactone and δ-valerolactone. Best results were obtained using the iron salt 2, however the non-macrocyclic system 12 also performed well. Complex 2 was also capable of the copolymerisation of ε-CL (or δ-VL) with rac-lactide, affording copolymers appreciable amounts of each monomer incorporated.

Experimental

General

All manipulations were carried out under an atmosphere of nitrogen using standard Schlenk and cannula techniques or in a conventional nitrogen-filled glove-box. Toluene was refluxed over sodium, whilst acetonitrile was refluxed over calcium hydride. IR spectra (nujol mulls, KBr windows) were recorded on a Nicolet Avatar 360 FT-IR spectrometer; ¹H and ¹³C NMR spectra were recorded at room temperature on a Varian VXR 400 S spectrometer at 400 MHz or a Gemini 300 NMR spectrometer or a Bruker Advance DPX-300 spectrometer. The ¹H NMR spectra were calibrated against the residual protio impurity of the deuterated solvent. Elemental analyses were performed by the elemental analysis service at the London Metropolitan University, the Chemistry Department at the University of Hull or Nanjing University. The precursors 2,6-(CHO)₂-4-R-C₆H₂OH (R = Me, tBu) and 2,2′-ethylenedianiline (or 2,2′-oxydianiline) and the Schiff-base pro-ligands were prepared by the literature.^3a,c,13–15 For the iron and cobalt complexes, all manipulations were carried out under an atmosphere of dry nitrogen using conventional Schlenk and cannula techniques or in a conventional nitrogen-filled glove box. All solvents were distilled and degassed prior to use (Table 2).

Synthesis of [FeBr(L¹H₂)]₂[(FeBr₃)₂O]·2(MeCN) (1·2MeCN)

L¹H₂ (0.52 g, 0.77 mmol) and FeBr₂ (0.34 g, 1.58 mmol) were combined in a Schlenk and toluene (20 mL) was added. After refluxing for 12 h, the volatiles were removed in-vacuo, and the residue was extracted into MeCN (20 mL). Prolonged standing at room temperature afforded orange/brown prisms. Yield: 0.72 g, 78%. Elemental analysis calculated for C₉₆H₈₂Br₈Fe₄N₁₀O₅: required: C 49.32% H 3.60% N 5.00% Found: 49.88% H 3.58% N 5.09%. IR (KBr, cm⁻¹): 3378 (s), 2923 (s), 2726 (w), 2671 (w), 2360 (w), 1633 (m), 1585 (m), 1538 (m), 1462 (s), 1377 (s), 1300 (w), 1280 (w), 1238 (m), 1102 (m), 977 (w), 873 (w), 801 (m), 754 (m), 722 (m), 688 (w), 622 (w), 574 (w), 534 (w), 502 (m), 486 (w), 455 (w). M.S. (MALDI-ToF): 817 (M⁺). Magnetic moment: 6.35 B.M.¹⁶

Synthesis of [FeBr(L²H₂)]₂[(FeBr₃)₂O]·0.5MeCN (2·0.5MeCN)

To the pro-ligand L²H₂ (0.50 g, 0.65 mmol) in toluene was added 2.1 equivalents of FeBr₂ (0.30 g, 1.39 mmol), and the system was refluxed for 12 h. On cooling, the volatiles were removed under vacuum, and the residue was extracted into acetonitrile (30 ml) to afford 2·MeCN as red needles (0.48 g, 56%). Elemental analysis calculated for C₁₀₆H₁₀₇Br₈Fe₄N₉O₅: C 51.97, H 4.40, N 5.15%; found: C 51.21, H 4.43, N 5.55%. IR (KBr, cm⁻¹): 3168 (m), 2925 (s), 2854 (s), 2726 (w), 1704 (m), 1620 (m), 1587 (m), 1542 (s), 1462 (s), 1377 (s), 1317 (w), 1260 (w), 1233 (m), 1214 (m), 1180 (m), 1130 (w), 933 (w), 890 (w), 839 (w), 799 (m), 744 (m), 722 (m), 526 (m), 449 (m). M.S. (MALDI-ToF): 901 (M − anion). Magnetic moment: 5.86 B.M.¹⁶

Synthesis of [FeBr(L²H₂)]Br·3MeCN (5.5MeCN)

As for 2, but using L²H₂ (0.52 g, 0.68 mmol) and FeBr₂ (0.16 g, 0.74 mmol) affording 3·5.5MeCN as brown prisms. Yield: 0.52 g, 65%. IR (KBr, cm⁻¹): 3168 (m), 2925 (s), 2854 (s), 2726 (w), 1636 (s), 1619 (s), 1599 (m), 1587 (m), 1537 (s), 1463 (s), 1377 (s), 1333 (m), 1283 (w), 1241 (m), 1183 (m), 1101 (m), 1063 (s), 1007 (m), 976 (m), 880 (s), 794 (m), 753 (s), 722 (m), 689 (m), 622 (w), 595 (w), 575 (w), 535 (w), 513 (m), 494 (m), 476 (w). M.S. (MALDI-ToF): 901 (M − 5.5MeCN − anion).

Synthesis of [Fe(L²)(L²H)]·3MeCN (4·3MeCN)

To FeBr₂ (1.00 g, 4.64 mmol) was added KOtBu (1.04 g, 9.27 mmol) in THF (30 ml) at 0 °C and the system was stirred for 5 h. Following removal of the volatiles, L²H₂ (3.57 g, 4.64 mmol) and toluene (20 ml) was added and the system was refluxed for 12 h. On cooling, the volatiles were removed and the residue was extracted into cold acetonitrile (30 ml). 4·3MeCN formed. Yield 1.62 g, 41%. Elemental analysis calculated for C₁₀₄H₁₀₁FeN₈O₄ (sample dried in-vacuo for 2 h): C 78.92, H 6.43, N 7.08%. Found C 79.09, H, 6.49, N On prolonged standing at ambient dark brown block. Yield: 1.84 g, 46.6%. IR (KBr, cm⁻¹): 3377 (w), 2958 (s), 2924 (s), 2854 (s), 2726 (w), 1630 (m), 1587 (m), 1461 (s), 1415 (w), 1377 (s), 1260 (s), 1202 (w), 1093 (s), 1019 (s), 863 (m), 800 (s), 755 (w), 740 (w), 723 (m), 705 (w), 662 (w), 566 (w), 530 (w), 504 (w), 465 (w). MS (Maldi): 1584 (M + H)⁺. Magnetic moment: 5.07 B.M.¹⁶

Synthesis of [CuBr(L²H₂)][CuBr₂]·2MeCN (5·2MeCN)

As for 1, but using L²H₂ (0.52 g, 0.68 mmol) and CuBr₂ (0.30 g, 1.34 mmol), affording 5 as brown prisms. Yield 0.69 g, 84%. Elemental analysis calculated for C₅₂H₅₂Br₃Cu₂N₄O₂: C 55.18, H 4.63, N 4.95%. Found C 54.59, H, 4.48, N 4.93%. IR (KBr, cm⁻¹): 3171 (m), 2922 (s), 2853 (s), 2727 (s), 2671 (w), 2350 (w), 1633 (s), 1618 (s), 1594 (s), 1538 (s), 1463 (s), 1377 (s), 1336 (w), 1285 (m), 1260 (w), 1249 (m), 1239 (w), 1183 (m), 1156 (w), 1103 (m), 1062 (m), 1022 (w), 977 (w), 950 (w), 938 (w), 876 (s), 846 (m), 830 (m), 794 (m), 754 (s), 738 (w), 722 (s), 688 (m), 623 (m), 594 (w), 575 (m), 558 (m), 550 (m). M.S. (ESI): 908 (M − anion), 827 (M − anion − Br). Magnetic moment: 1.19 B.M.¹⁷

Synthesis of [L²H₄][CoBr₄]·2MeCN (6·2MeCN)

To [CoBrL²][CoBr₃(NCMe)] (1.00 g, 0.80 mmol) in toluene (30 mL) was added [TiCl₄] (0.80 mL, 1.0 M, 0.80 mmol) and the system was refluxed for 12 h. On cooling, the volatiles were removed and the residue was extracted into acetonitrile (30 ml). On prolonged standing at 0 °C small red blocks of 6·2MeCN formed. Yield: 0.38 g, 83%. Elemental analysis calculated for C₅₂H₅₄Br₄CoN₄O₂: C 54.52, H 4.75, N 4.89%. Found C 53.34, H, 5.19, N 4.95%. IR (KBr, cm⁻¹): 3381 (m), 2925 (s), 2924 (s), 2854 (s), 2726 (w), 2359 (w), 1636 (w), 1618 (s), 1594 (m), 1574 (w), 1538 (s), 1462 (s), 1377 (s), 1333 (s), 1285 (m), 1260 (m), 1183 (s), 1102 (m), 1063 (m), 1021 (m), 873 (m), 799 (s), 756 (m), 722 (m), 688 (w), 668 (w), 624 (w), 576 (w), 558 (w), 522 (w). M.S. (ESI): 847 (M − CoBr₃). 829 (M − Br₄). 766.97 (M − CoBr₄). Magnetic moment: 5.05 B.M.¹⁸

Synthesis of [(CoBr)₂(L³)]·2C₃H₆O (7·2C₃H₆O)

To the pro-ligand L³H₂ (1.00 g, 1.52 mmol) in toluene was added 2.1 equivalents of CoBr₂ (0.70 g, 3.20 mmol), and the system was refluxed for 12 h. On cooling, the volatile were removed under vacuum, and the residue was extracted into acetone (30 ml) to afford 7·2C₃H₆O as black crystals (0.98 g, 62%). Single crystals can also be grown from a saturated acetone solution of 7. Elemental analysis calculated for C₄₂H₃₀Br₂Co₂N₄O₄: C 54.10, H 3.24, N 6.01%. Found C 53.88, H 3.19, N 6.25%. IR (KBr, cm⁻¹): 2924 (s), 2853 (s), 2727 (w), 1746 (w), 1693 (w), 1620 (m), 1587 (m), 1537 (m), 1463 (s), 1377 (s), 1261 (m), 1235 (m), 1215 (m), 1151 (w), 1106 (w), 1072 (m), 1030 (w), 891 (w), 867 (w), 842 (w), 800 (m), 757 (w), 722 (m), 534 (w), 478 (w), 449 (w). M.S. (ESI): 851 (M − 2C₃H₆O − Br), 714 (M − 2C₃H₆O − CoBr). Magnetic moment: 6.10 B.M.¹⁸

Synthesis of [Co(NCMe)₂(L⁴H₂)]·[CoBr₄] 5MeCN (8·5MeCN)

To the pro-ligand L⁴H₂ (1.00 g, 1.36 mmol) in toluene was added 2.1 equivalents of CoBr₂ (0.62 g, 2.83 mmol), and the system was refluxed for 12 h. On cooling, the volatile were removed under vacuum, and the residue was extracted into acetonitrile (30 ml) to afford 8·3MeCN as red crystals, yield 0.86 g, 44%, and a smaller amount of green crystals of 9, yield ca. 5%. Elemental analysis calculated for C₆₂H₆₅Br₄Co₂N₁₁O₄: C 49.55, H 4.00, N 6.67% Found for 8 C, 49.35, H 3.96, N 6.61%. IR (KBr, cm⁻¹): 3172 (m), 2956 (s), 2923 (s), 2853 (s), 2726 (w), 1619 (m), 1595 (m), 1536 (m), 1461 (s), 1377 (s), 1260 (s), 1155 (w), 1096 (s), 1060 (s), 1022 (s), 892 (w), 874 (w), 800 (s), 722 (s). M.S. (ESI): 933 (M − 7MeCN − 3Br), 817 (M − 7MeCN − 3Br − 2Co). Magnetic moment: 6.94 B.M.¹⁸

IR (KBr, cm⁻¹) for Complex 9: 2922 (s), 2854 (s), 2727 (w), 2308 (m), 2282 (m), 2247 (m), 1658 (s), 1635 (s), 1580 (w), 1531 (s), 1464 (s), 1416 (s), 1377 (s), 1367 (s), 1354 (s), 1327 (w), 1287 (m), 1255 (m), 1244 (w), 1222 (m), 1149 (w), 1130 (w), 1098 (w), 1040 (m), 1014 (s), 930 (w), 917 (w), 846 (s), 801 (m), 772 (m), 757 (m), 727 (s), 627 (w), 550 (s), 531 (m), 430 (m).

Synthesis of {CoBr(NCMe)L⁵}₂·2MeCN (10·2MeCN)

To L⁵H (1.00 g, 4.85 mmol) and [CoBr₂] (1.05 g, 4.85 mmol) was added toluene (20 mL) and Et₃N (0.3 ml, 0.22 mmol) and the system was refluxed for 12 h. Following removal of volatiles in-vacuo, the residue was extracted into MeCN (20 mL), and on standing at ambient temperature large orange/brown prisms of 10·3MeCN formed. Isolated yield: 1.41 g, 68%. Elemental analysis calculated for C₃₂H₃₈Br₂Co₂N₄O₆ (sample dried in-vacuo for 2 h): C 43.66, H 4.19, N 3.64. Found C 43.81, H 4.23, N 3.67%. IR (KBr, cm⁻¹): 2957 (s), 2924 (s), 2360 (s), 2342 (m), 2312 (m), 2284 (s), 1696 (m), 1641 (s), 1566 (m), 1535 (s), 1464 (s), 1402 (w), 1377 (s), 1367 (s), 1352 (w), 1259 (s), 1225 (m), 1093 (s), 1038 (s), 939 (m), 911 (m), 867 (w), 845 (m), 800 (s), 768 (s), 729 (s), 668 (m), 620 (m), 556 (s), 532 (m), 435 (m). M.S. (ESI): 733 (M − MeCN), 585 (M − 3MeCN − Co), 573 (M − MeCN − 2Br). Magnetic moment: 6.35 B.M.¹⁸

Synthesis of [Co(NCMe)₂L⁵]₂[CoBr₃(NCMe)]₂ (11)

To L⁵H (1.00 g, 4.85 mmol) and [CoBr₂] (1.05 g, 4.85 mmol) was added toluene (20 mL) and the system was refluxed for 12 h. Following removal of volatiles in-vacuo, the residue was extracted into MeCN (20 mL), and on standing at ambient temperature large green prisms of 11 formed. Isolated yield: 1.53 g, 46%. Elemental analysis calculated for C₃₆H₄₄Co₄N₆O₆ (sample dried in-vacuo for 2 h): C 47.43, H 4.73, N 6.91%. Found C 47.82, H 4.79, N 6.98%. IR (KBr, cm⁻¹): 2926 (s), 2854 (s), 2360 (m), 2342 (w), 2312 (s), 2284 (m), 1641 (s), 1621 (s), 1566 (m), 1535 (s), 1464 (s), 1402 (w), 1377 (s), 1367 (s), 1352 (w), 1259 (s), 1225 (m), 1093 (s), 1038 (s), 1019 (s), 845 (s), 800 (s), 768 (m), 758 (m), 729 (s), 668 (m), 620 (m), 556 (s), 532 (s), 435 (s). M.S. (ESI): 412 (M − 4MeCN − 2Co). Magnetic moment: 6.69 B.M.¹⁸

Synthesis of [FeBr(NCMe)L⁵]₂·2MeCN (12·2MeCN)

To L⁵H (1.00 g, 4.85 mmol) and [FeBr₂] (1.06 g, 4.85 mmol) was added toluene (20 mL) and Et₃N (0.7 ml, 0.485 mmol) and the system was refluxed for 12 h. Following removal of volatiles in-vacuo, the residue was extracted into MeCN (20 mL), and on standing at ambient temperature large orange/brown prisms of 10·3MeCN formed. Isolated yield: 0.837 g, 40%. Elemental analysis calculated for C₂₈H₃₂Br₂Fe₂N₂O₆ (sample dried in-vacuo for 2 h): C 44.02, H 4.22, N 3.67. Found C 44.61, H 4.52, N 3.72%. IR (KBr, cm⁻¹): 3352 (w), 2954 (s), 2923 (s), 2854 (s), 2727 (w), 2360 (w), 2341 (w), 1643 (m), 1620 (m), 1528 (m), 1462 (s), 1377 (s), 1259 (m), 1160 (m), 1037 (s), 1016 (m), 842 (w), 800 (s), 769 (w), 755 (w), 722 (m), 668 (w), 618 (m), 539 (m). M.S. (ESI): 519 (M − 4MeCN − 2Br). Magnetic moment: 6.70 B.M.¹⁸

Synthesis of [Fe(NCMe)₆][FeOBr₃]₂ (13)

L¹H₂ (0.52 g, 0.77 mmol) and [FeBr₃] (0.47 g, 1.58 mmol) were combined in a Schlenk and toluene (20 mL) was added. After refluxing for 12 h, the volatiles were removed in-vacuo, and the residue was extracted into MeCN (20 mL). Prolonged standing at room temperature afforded brown prisms; isolated yield 0.39 g 81%. Elemental analysis calculated for C₁₂H₁₈Br₆Fe₃N₆O (sample dried in-vacuo for 2 h): C 15.85, H 2.00, N 9.24%. Found C, 15.61 H, 2.12 N 9.31%. IR (KBr, cm⁻¹): 2925 (s), 2854 (s), 2361 (s), 2339 (s), 1868 (s), 1844 (s), 1830 (m), 1792 (m), 1772 (w), 1750 (m), 1734 (m), 1717 (m), 1700 (m), 1684 (m), 1669 (m), 1653 (m), 1646 (m), 1635 (m), 1616 (s), 1576 (s), 1559 (s), 1540 (s), 1521 (s), 1506 (s), 1497 (m), 1489 (m), 1457 (s), 1419 (m), 1377 (s), 1260 (s), 1089 (s), 800 (s), 668 (s).

ROP procedure

ε-Caprolactone and δ-valerolactone. Typical polymerisation procedure in the presence of one equivalent of benzyl alcohol (Table 1, run 1) is as follows. A toluene solution of 1 (0.010 mmol, in 1.0 mL toluene) and BnOH (0.010 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 (2.5 mmol) or δ-valerolactone 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 polymerisation 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 cold methanol (200 mL). The resultant polymer was then collected on filter paper and was dried in vacuo.

Co-polymerisations

A toluene solution of catalyst (0.010 mmol, in 1.0 mL toluene) was added into a Schlenk tube in the glove-box at room temperature. The solution was stirred for 2 min, and then 1 equivalent of BnOH (from 1 mmol BnOH in 100 ml toluene) and monomer were added, after 24 h the other monomer was added to the solution. The reaction mixture was then placed into an oil bath pre-heated to the 130 °C, and the solution was stirred for another 24 h. The polymerisation 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 cold methanol (200 mL). The resultant polymer was then collected on filter paper and was dried in vacuo.

Kinetic studies

The polymerisations were carried out at 130 °C in toluene (1 mL) using 0.010 mmol of complex. The molar ratio of monomer to initiator was fixed at 500 : 1, 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 by ¹H NMR spectroscopy.

Mass spectrometry

Polymer samples were run at the University of Hull using MALDI-TOF MS analysis. Samples were dissolved in THF, and the matrix, 2-(4-hydroxyphenylazo) benzoic acid (HPABA) with added NaOAc was employed, which was dissolved in THF to give a saturated solution. 50 μL of the sample solution was then mixed with 50 μL matrix solution, and 1 μL of the mixed solution applied to the sample target. The sample was allowed to dry in air before analysis. The co-poly (δ-VL + r-LA) (run 7) sample was analysed by MALDI in positive-linear and reflectron modes, with DCTB matrix and NaOAc additive. The sample was fully soluble in THF at 10 mg mL⁻¹.

X-ray crystallography

Single-crystal X-ray diffraction data for the structures (except for 4 and 11) were collected at the UK National Crystallography service on a range of Rigaku Oxford Diffraction ultra-high intensity instruments employing modern areas detectors. Samples were held at 100 K for data collection. For 4, data were from a Rigaku Oxford Diffraction Xcalibur-3 CCD diffractometer at 140 K. For 11, data were from a Stoe IPSD2 image plate diffractometer utilising monochromated Mo radiation (λ = 0.71073 Å). In all cases standard procedures were employed for integration and processing of data. Crystallographic data for all samples are collated in Table 4.

Table 4 Crystallographic data

Compound	1·2MeCN	2·0.5MeCN	3·5.5MeCN	4·3MeCN
Formula	C96H86Br8Fe4N10O5	C52H52BrFeN4O2, C2H3N, 0.5(Br6Fe2O)	C52H52BrFeN4O2, Br, 5.5(C2H3N)	C104H101FeN8O4, (3MeCN)
Formula weight	2322.42	1224.841185.89	1185.89	1705.9
Crystal system	Monoclinic	Triclinic	Monoclinic	Monoclinic
Space group	P21/n	P1̄	P21/n	P21/c
Unit cell dimensions
a (Å)	15.6606(4)	13.6454(5)	14.2077(6)	12.6268(5)
b (Å)	16.0326(4)	13.9070(7)	27.6241(9)	30.3249(10)
c (Å)	37.4062(11)	15.6432(8)	15.1579(8)	24.6577(10)
α (°)	90	67.263(5)	90	90
β (°)	99.896(3)	88.642(4)	112.484(6)	100.817(3)
γ (°)	90	81.638(4)	90	90
V (Å^3)	9252.2(4)	2707.0(2)	5496.9(5)	9273.8(6)
Z	4	1	4	4
Temperature (K)	100(2)	100(2)	100(2)	140(1)
Wavelength (Å)	1.54184	0.71075	0.71073	0.71073
Calculated density (g cm^−3)	1.667	1.503	1.185	1.222
Absorption coefficient (mm^−1)	9.439	3.531	1.766	0.223
T min, Tmax	0.755, 1.000	0.592, 1.000	0.555, 1.000	0.749, 1.185
Crystal size (mm^3)	0.08 × 0.05 × 0.02	0.30 × 0.015 × 0.01	0.26 × 0.045 × 0.01	0.74 × 0.18 × 0.16
θ(max) (°)	68.2	27.5	27.5	20.0
Reflections measured	68 070	12 337	47 911	51 064
Unique reflections	16 805	12 337	12 466	8583
R int	0.107	0.067	0.056	0.143
Number of parameters	1127	604	556	1129
R 1 [F^2 > 2σ(F^2)]	0.070	0.061	0.088	0.057
wR2 (all data)	0.178	0.164	0.168	0.123
GOOF, S	1.011	1.049	1.050	0.913
Largest difference peak and hole (e Å^−3)	1.42 and −0.86	1.53 and −0.80	0.90 and −1.61	0.30 and −0.25

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Compound	5·2MeCN	6·2MeCN	7·2C3H6O	8·3MeCN
Formula	C56H58Br3Cu2N6O2	C52H54Br4CoN4O2·2(C2H3N)	C42H30Br2Co2N4O4·2(C3H6O)	C62H65Br4Co2N11O4
Formula weight	1213.89	1145.56	1048.56	1465.8
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	P21/c	Ia
Unit cell dimensions
a (Å)	10.0808(2)	12.2629(5)	11.4774(8)	15.77810(10)
b (Å)	15.0015(4)	13.2489(6)	14.5787(10)	25.0018(2)
c (Å)	18.6604(5)	17.0384(7)	26.021(3)	16.28380(10)
α (°)	71.442(2)	96.850(3)	90	90
β (°)	82.738(2)	91.115(3)	100.235(8)	91.911(10)
γ (°)	76.600(2)	99.672(4)	90	90
V (Å^3)	2598.2(1)	2707.2(2)	4284.6(6)	6420.08(8)
Z	2	2	4	4
Temperature (K)	100(2)	100(2)	100(2)	100(2)
Wavelength (Å)	1.54184	0.71075	1.54178	0.71073
Calculated density (g cm^−3)	1.552	1.405	1.535	1.516
Absorption coefficient (mm^−1)	4.572	3.309	8.652	3.060
T min, Tmax	0.149, 1.000	0.652, 1.000	0.404, 1.000	0.698, 1.000
Crystal size (mm^3)	0.70 × 0.60 × 0.15	0.10 × 0.04 × 0.02	0.14 × 0.03 × 0.01	0.25 × 0.18 × 0.05
θ(max) (°)	68.3	21.7	68.2	28.7
Reflections measured	44 092	10 619	6241	147 825
Unique reflections	9337	10 619	3471	16 480
R int	0.083	0.179	0.054	0.047
Number of parameters	622	564	526	762
R 1 [F^2 > 2σ(F^2)]	0.066	0.114	0.128	0.032
wR2 (all data)	0.184	0.297	0.371	0.064
GOOF, S	1.062	1.098	1.226	1.023
Largest difference peak and hole (e Å^−3)	3.45 and −1.24	1.02 and −1.54	2.52 and −2.02	0.52 and −0.46

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Compound	9	10·2MeCN	11·2MeCN	12·2MeCN	13
Formula	C20H30Br6Co3N10	C28H32Br2Co2N2O6·2(C2H3N)	C36H44Br6Co4N6O6	C32H38Br2Fe2N4O6	C12H18Br6Fe3N6O
Formula weight	1066.79	852.34	1371.95	846.18	909.33
Crystal system	Monoclinic	Triclinic	Triclinic	Triclinic	Trigonal
Space group	P21/n	P1̄	P1̄	P1̄	R3̄:H
Unit cell dimensions
a (Å)	8.34720(10)	8.4718(5)	10.416(2)	8.51990(10)	12.39520(10)
b (Å)	18.8457(3)	9.2109(3)	11.156(4)	9.20980(10)	12.39520(10)
c (Å)	12.5811(2)	12.2495(5)	11.738(4)	12.1881(2)	30.5277(3)
α (°)	90	101.186(3)	78.04(3)	101.2110(10)	90
β (°)	108.941(2)	102.715(4)	70.59(2)	102.4660(10)	90
γ (°)	90	97.208(4)	87.78(2)	97.4090(10)	120
V (Å^3)	1871.95(5)	900.47(7)	1257.8(7)	901.29(2)	4061.92(8)
Z	2	1	1	1	6
Temperature (K)	100(2)	100(2)	150(2)	100(2)	100(2)
Wavelength (Å)	0.71075	1.54184	0.71073	1.54184	1.54184
Calculated density (g cm^−3)	1.893	1.572	1.811	1.559	2.230
Absorption coefficient (mm^−1)	7.743	10.197	6.105	9.433	23.225
T min, Tmax	0.154, 0.733	0.423, 1.000	0.725, 0.820	0.646, 1.000	0.154, 0.733
Crystal size (mm^3)	0.216 × 0.125 × 0.10	0.32 × 0.16 × 0.10	0.31 × 0.13 × 0.09	0.30 × 0.20 × 0.10	0.16 × 0.14 × 0.03
θ(max) (°)	27.49	68.2	29.2	70.3	67.8
Reflections measured	48 302	15 481	11 602	32 046	24 700
Unique reflections	4301	3240	6632	3387	1644
R int	0.0235	0.066	0.044	0.0342	0.0363
Number of parameters	183	217	268	217	88
R 1 [F^2 > 2σ(F^2)]	0.017	0.042	0.035	0.0299	0.0188
wR2 (all data)	0.0037	0.119	0.072	0.0822	0.0474
GOOF, S	1.170	1.072	0.823	1.091	1.111
Largest difference peak and hole (e Å^−3)	0.79 and −0.35	0.85 and −0.83	0.84 and −0.63	0.413 and −0.69	0.38 and −0.51

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Crystal structures were solved using direct methods (in SHELXS^19a) or dual space methods implemented within SHELXT.^19b Completion of structures was achieved by performing least squares refinement against all unique F² values using SHELXL-2018.²⁰ All non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed using a riding model. Where the location of hydrogen atoms was obvious from difference Fourier maps, C–H and O–H bond lengths were refined subject to chemically sensible restraints. Minor disorder was treated using standard methods.

The crystal of 5 examined was the best of those available. The crystal was split and intensity data from the major component were used for refinement. The final refinement has good quality of fit (R₁ = 6.66%) but it is not perfect because of the imperfect nature of the crystals available.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the Chinese Scholarship Council for a PhD studentship (to KW). CR thanks the EPSRC (grant EP/S025537/1) for financial support. We thank the EPSRC National Crystallographic Service at Southampton for data collection and the Physical Sciences Data Service Centre for access to the CCDC. We also thank the National Mass Spectrometry Facility (NMSF), Swansea for data.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1849422, 2063728 and 2054028–2054038. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1dt00711d

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