Tetraacylstannanes as Long‐Wavelength Visible‐Light Photoinitiators with Intriguing Low Toxicity

Abstract The first tetraacylstannanes Sn[(CO)R]4 (R=2,4,6‐trimethylphenyl (1 a) and 2,6‐dimethylphenyl (1 b)), a class of highly efficient Sn‐based photoinitiators, were synthesized. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. The UV/Vis absorption spectra of 1 a, b reveal a significant redshift of the longest wavelength absorption compared to the corresponding germanium compounds. In contrast to the known toxicity of organotin derivatives, the AMES test and cytotoxicity studies reveal intriguing low toxicity. The excellent performance of 1 as photoinitiators is demonstrated by photobleaching (UV/Vis) and NMR/CIDNP investigations, as well as photo‐DSC studies.

From the Bronze Age (3500 BC) onward, tin has been ah ighly appreciated metal and raw materialf or furtherp rocessing. Its acquisition marks an important part in the evolution of mankind. Pioneering works in organometallic tin chemistry by Frankland, [1] Cadet, [2] or Zeise [3] sparked the interest into organic tin derivatives. Advances in analytical techniques, including Sn NMR, X-ray diffraction, or appropriate computational methods aided organotin chemistry to be at hriving field of research. Besides being used as fungicides, insecticides or stabilizers in polyvinyl chloride (PVC), organostannanes are widely used in chemical synthesis. However, due to the acute toxicity of organotin compounds, their use in synthetic chemistry has been connoted quite negatively and gradually diminished re-lated research activities. Hitherto, the interest in acylstannane chemistry remained modest because of al ack of suitable synthetic methods towards such. For al ong time, acylstannanes were virtually unknowna nd believed to be extremely labile. For the sake of completeness, these derivatives were mentioned as as ide note when reporting on acylsilanes and acylgermanes. [4] Recent years have witnessedi ntense scientifice fforts in the chemistryo fk eto-derivatives of main group IV organometalloids (mainly,G eb ased) and their applicationa sv isible light (VL) photoinitiators. [5][6][7][8] The search for new photoinitiators (PIs) for radicalp olymerization exhibiting enhanced reactivity remains of great interest. [9] To date, phosphorousbased initiators (mono-a nd bisacylphosphane oxides) are well established, although UV light is required for sufficient photocuring and the parentc ompounds, as well as their photoproducts are in most cases highly toxic. [10][11][12] Thus, germanium-centered PI systems have emerged over the past few years as promising alternatives due to their low toxicity andt he bathochromic shift of their longest-wavelength absorption bands. [6,[13][14][15] However,asignificant drawback of the latter is the low abundance of germanium in the earth crust, resulting in high prices of germanium-based PIs. Therefore, this system cannotf ully meet the requirements forp hotoinitiators in highthroughput polymers ynthesis.
To overcome the above-mentioned restrictions, we investigated the implementation of other central atoms. The replacement of germanium by silicon leads to the formationo ft etraacylsilanes. But these derivatives appeared to be ineffective for free-radical photopolymerization. [16,17] Herein,w ep ioneer the use of tin as the central atom in metallo-acyl-based photoinitiators, tremendously extending the scope of previously established systemsi nt erms of long-wavelength absorption in combination with cost-effective synthesis and excellent biocompatibility.
Recently,o ur group published as eries of papers on previously unknown acylgermanes. [6,[24][25][26] Exploiting the multiple silyl abstraction enabled the targeted synthesis of these highly efficient PIsi nh igh yields. Moreover,t he strength of this reaction sequence lies in its high functional-group tolerance. [24,25] Following the resultso btained for tetraacylgermanes, we discovered that the reaction of potassium stannide 2K [27] with 4.1 molar equivalent of acid fluorides FÀ(CO)R (R = aryl) leads to the formation of tetraacylstannanes 1.M echanistically speaking, 2K undergoes as alt metathesis reaction with the respectivea cidf luoride, in which KF and tris(trimethylsilyl)acylstannane 3 are formed.
The consecutive nucleophilic attack of the fluoride ion on 4, originating from the formation of 2K,i ns itu generates Me 3 SiF 5 and tBuO À ,w hichf urtherr eacts with 3 to form the stannenolate 6.Accordingly, 4 and 5 could be detected by NMR spectroscopyi nt he reaction mixture (d 29 Si = 30.3 and 6.92 ppm, respectively). [28] Subsequently,t he appliede xcess of acid fluoride immediately reacts with 6 to the respective bisacylstannane 7 releasing again KF and 4.A cylation proceeds until all trimethylsilyl groups are abstracted, and the final product 1 is formed (Scheme 2). It appears that di-ortho substitution at the phenyl ring is necessary for the successful preparationoftetraacylstannanes. To date, no derivatives with ad ifferent substitution pattern werei solated. We assumet hat the di-ortho substitution prevents the nucleophilic attack of any reactive anionic inter-mediate formed (F À or tBuO À ). Ac haracteristicf eature of tetraacylstannanes 1 is the 13 CNMR resonance for the carbonyl group,w hich appearsb etween 243 and 244 ppm. As imilar tendency was found for tetraacylgermanes and -silanes. [6,16] The analytical data for 1a and b (see the Supporting Information) are consistent with the proposed structures. As ar epresentative example, the structure of 1a is depicted in Figure1.T he structural features of these tin derivatives correlate with the resultso btained for acylgermanes.D i-ortho substitutioni nduces as ignificant torsion between the carbonyl group and the aromatic ring plane.T he SnÀCb ondl ength is similart ot he values found for other SnÀCsingle bonds. [29] As wasm entioned above,p hotopolymerization of biocompatible materials requires non-toxic PIs and non-toxic irradiation sources (visible light). In the course of our studies, we found that the absorption maximum of non-toxic tetraacylgermaness how valuesb etween 363 and4 22 nm. [24] However, tetraacylgermanes lack sufficientc uring efficiency upon irradiation with light sourceso perating above 450 nm. Nevertheless, this type IP Is ystem represents the most bathochromic shifted absorption bands to date. This border is shattered by the introductiono ft etraacylstannanes 1.F igure 2i llustrates the lon-Scheme1.Synthesis of tetraacylstannanes 1a and b.
Scheme2.Proposed mechanism for the synthesis of tetraacylstannanes.  gest-wavelength absorption bands, which were computationally assigned to the HOMO-LUMO transition and show considerable charge-transfer character.U pon excitation, electron density is displaced from the n(C=O)/s(SnÀC) bonding HOMO to the p*(C=O/Aryl) antibonding LUMO (Figure 3), which results in the population of an orbital with antibonding character between the SnÀCb ond.
In comparison to the respectiveg ermanium analogues 2a and b and to bisacylphosphine oxide 3 (Scheme 3), the extinction coefficient of 1a and b are significantly increased.T he bathochromic shifted l max ,a sw ell as the increased extinction coefficient, induce am ore pronounced tailing into the visiblelight region > 450 nm, enabling a-cleavage upon irradiation with light sourceso perating > 450 nm. Hence, 1 represents a radicals ource with most promising absorption properties for visible-light applications (Table 1).
To assess the efficiency of 1a and b as radicalp hotoinitiators, we have performed photobleaching experiments. [6,7] Steady-state photolysis (SSP) upon irradiation with an LED operated at 470 nm reveals remarkably fast bleaching of 1a and b compared to germanium- [5][6][7]15] and phosphorus-based [10,30] photoinitiators 2 and 3 (Figure 4). Fast photobleaching of the initiator is an indication for efficient radicalf ormation and is moreoverc rucial fora chieving high curing depthsa nd colorless polymers. [31] Compound 1b exhibits the fastestp hotobleaching, whereas bisacylphosphane oxide 3 shows almost no bleaching, being unsuitable for curing applications in this wavelength region. The superior photobleaching performance of tetraacylstannanes 1 over the reference initiators 2 and 3 upon irradiation with high wavelength visible light (470 nm) is consistentw ith the bathochromically shifted absorption spectra ( Figure 2).
We have investigated the radical reactionp athways upon irradiation of 1a and b in presence of monomers by using chemically induced dynamic nuclear polarization (CIDNP) NMR spectroscopy.T his methoda llows following the a-cleavage of the photoinitiators 1a and b and provides information about reaction products formed through radical pairs (SnC/BC; Scheme4). Radical-pair-based phenomena lead to enhanced absorptive or emissive NMR signals of reactionp roducts, causedb yanon-Boltzmann population of magnetic energy levels. [5,23] Figure 5c ompares the 1 HNMR andC IDNP spectra of 1a recorded in presenceo fb utyl acrylate BA (see the Supporting Information for the corresponding spectra of 1b). Polarized signals of the hydrogen atoms of the parent compound (at d = 2.25 ppm (1), d = 6.67 ppm (2), and d = 1.99 ppm (3)) can    Figure 5). Analogous signals have been observed in the CIDNP spectra of several photoinitiatings ystems containing ab enzoyl moiety. [32] The formation of BH is attributedt oadisproportionationr eaction (b-hydrogen transfer) between ab enzoyl-typer adicala nd radicals, which are able to donate hydrogen atoms (growing polymer chain). [32,33] Figure 5a dditionally shows as trongly polarized emissive multiplet at d = 4.49 ppm and an absorptive multiplet at d = 3.37 ppm. These signals are tentatively assigned to the methylene protons of species SnBAB,w hich is formed upon addition of benzoyl radical BC to ac hain radical initiatedb ySnC ( Figure 5). The analogous photoproduct has been observed for tetra-acylgermanes. [6] We assume that rotation around the CÀCb ond derived from BA (marked in pink in the structure of SnBAB,F igure 5) is sterically hindered by the adjacent bulky mesitoyl group, makingt he signal at d = 4.49 ppm appear as ad oublet of doublets, instead of at riplet. The prepared compounds were also investigated by means of photo-DSC measurements to get fast and accurate information on their initiation efficiency.W ith as ingle photo-DSCmeasurement, variouss ignificant parameters are accessible. From the height of the exothermic peak, the rate of polymerization R P (mol L À1 s À1 )c an be calculated. The overall heat evolved (DH P )g ives information on the final double conversion (DBC). Furthermore, the time to reach the maximum heat flow (t max ) can be derived from the photo-DSC plots. Herein,t he photocuring of the crosslinking monomer 1,6-hexandiol diacrylate (HDDA) with 1a was investigated (1a: t max % 4.5 s, R P, max > 0.31 mol L À1 s À1 ,D BC % 59 %). The previously reported tetraacylgermane 2a was used as reference compound, showing comparable reactivity to 1a under the chosen experimental condi-tions (2a: t max % 5.1s, R P, max > 0.25 mol L À1 s À1 ,D BC % 57 %). Photo-DSC and conversion plots are depicted in Figure 6. A summary of the DSC parameters can be found in the Supporting Information.
In summary,w ecould synthesize and fully characterize the first examples of tetraacylstannanes, representing ac lass of highly efficient VL PIs. Unprecedentedly,t he found photochemicalp roperties are beneficial over those knownf or Ge-or P-based PIs. Additionally,t he low toxicity gives rise to any application, in which biocompatibility and costs are an issue. SSP experiments show that the tetraacylstannanes 1a and b are promising initiators for high-wavelength visible-light curing. CIDNP experimentsc onfirm a-cleavage of 1a and b and efficient addition of tin-centered radicals and benzoyl radicals to monomer double bonds. High activity as photoinitiator was furtherverifiedb yphoto-DSC experiments.