NacNac-zinc-pyridonate mediated ε-caprolactone ROP

Herein we report the synthesis, isolation and polymerisation activity of two new zinc compounds based on a 2,6-diisopropylphenyl (Dipp) β-diiminate (NacNac) ligand framework with zinc also ligated by an amidate (2-pyridonate or 6-methyl-2-pyridonate) unit. The compounds crystallised as either monomeric (6-Me-2-pyridonate derivative) or dimeric (2-pyridonate) species, although both were found to be monomeric in solution via1H DOSY NMR spectroscopy, which was supported by DFT calculations. These observations suggest that both complexes initiate ring-opening polymerisation (ROP) through a single-site monometallic mechanism. High molecular weight poly ε-caprolactone (PCL) was achieved via exogenous initiator-free ROP conditions with both catalysts. An increase in the 2-pyridonate initiator steric bulk (6-Me- vs. 6-H-) resulted in an improved catalytic activity, facilitating complete monomer conversion within 1 h at 60 °C. Pyridonate end-groups were observed by MALDI-ToF mass spectrometry, contrasting with previous observations for DippNacNac-Zn acetate complexes (where no acetate end groups are observed), instead this more closely resembles the reactivity of DippNacNac-Zn alkoxide complexes in ROP (where RO end groups are observed). Additional major signals in the MALDI-ToF spectra were consistent with cyclic PCL species, which are attributed to back-biting ring-closing termination steps occuring in a process facilitated by the pyridonate unit being an effective leaving group. To the best of our knowledge, these complexes represent the first examples of pyridonate, and indeed amidate, initated ROP.

a EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK *E-mail: j.garden@ed.ac.ukA marked downfield shift in proton resonance was noted c.f. the free pyridone in DMSO-d6. 3 Note this compound is poorly soluble, hence NMR spectra were recorded at higher temperatures where the solubility is somewhat improved.

Figure S11. MALDI-ToF MS analysis of crude PCL made in the presence of 2 after 2 h in toluene at RT (table 2, entry 7). MALDI-
ToF preparation method b) was used.

DOSY parameters and instrumentation
The Diffusion-Ordered Spectroscopy (DOSY) NMR experiments were performed at 298 K on a Bruker Ascend 2 channel instrument operating at a frequency of 500 MHz for proton resonance under TopSpin (version 3.2, Bruker Biospin, Karlsruhe) and equipped with a z-gradient DCH/5mm tuneable "CryoProbe" TM probe and a GRASP II gradient spectroscopy accessory providing a maximum gradient output of 53.5 G/cm (5.35G/cmA).Diffusion ordered NMR data was acquired using the Bruker pulse program dstebpgp3s with a spectral width of 10330 Hz (centred on 6.175 ppm) and 32768 data points.
A relaxation delay of 2 s was employed along with a diffusion time (large delta) of 100 ms and a longitudinal eddy current delay (LED) of 5 ms.Bipolar gradients pulses (little delta/2) of 1.5 ms and homospoil gradient pulses of 0.  S3].Adamantane was run a total of four times and the average observed log(D) was used.Furthermore, adamantane was used as an internal standard for all other standards used.From the diffusion coefficients of the external standards, linear calibration graphs were obtained by plotting log(D) vs log(MW) (Figure S20).Following DOSY analysis of the product, the diffusion coefficient obtained for the signals corresponding to the product allowed an estimate of the MW of the species present in solution through use of the equation y = mx + c.
Table S3.Diffusion coefficients of standards in toluene-d8 solution compared to their molecular weights.11.Computational details for the structures of 1 and 2 All of the calculations were performed using the Gaussian09 program 6 at B3PW91 level of theory 7,8 using LANL2DZ basis set for Zn atom and 6-311G(d,p) basis set for all other atoms.All geometry optimizations were full, with no restrictions.Stationary points located in the potential energy surface were characterized as minima (no imaginary frequencies) by vibrational analysis.Solvent effects of dichloromethane were introduced using the self-consistent field approach, by means of the integral equation formalism polarizable continuum model (IEFPCM). 9

Figure S12 .
Figure S12.MALDI-ToF MS analysis of crude PCL made in the presence of 2 after 2 h in THF at RT. MALDI-ToF preparation method b) was used.

Figure S13 .
Figure S13.MALDI-ToF MS analysis of purified PCL (cold MeOH/H + ) made in the presence of 2 after 4 h in toluene at RT. MALDI-ToF preparation method b) was used

Table S4 .
Bond lengths and angles calculated for monomeric 1 and 2