# Homolytic Photocleavage of the B–N Bond in N-Heterocyclic Carbene Borane Scaffolds: Towards Efficient Type I Photoinitiators

Sofia Telitel, Nicolas Blanchard, Bernadette Graff, Jacques Lalevée,
Emmanuel Lacôte

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## To cite this version:

Sofia Telitel, Nicolas Blanchard, Bernadette Graff, Jacques Lalevée, Emmanuel Lacôte. Homolytic Photocleavage of the B–N Bond in N-Heterocyclic Carbene Borane Scaffolds: Towards Efficient Type I Photoinitiators. Synthesis: Journal of Synthetic Organic Chemistry, In press, 10.1055/s-0043-1773518. hal-04975615

HAL Id: hal-04975615

https://hal.science/hal-04975615v1

Submitted on 4 Mar 2025

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# Homolytic PhotoCleavage of the B–N bond in N-heterocyclic Carbene Borane Scaffold: Towards new Efficient Type I Photoinitiators

Sofia Telitel<sup>a,b</sup>

Nicolas Blanchard<sup>c</sup>

Bernadette Graff<sup>a,b</sup>

Jacques Lalevée<sup>*a,b</sup>

Emmanuel Lacôte<sup>*d</sup>

## Abstract

A new family of N-heterocyclic carbene boranes-based Type I photoinitiators relying on previously unknown homolytic B–N cleavages is introduced. Three original NHC-amino borane compounds were designed, synthesized, and their UV absorption properties assessed. They all exhibit high molar extinction coefficients and frontier orbitals on the N atom-bearing group. Their photolysis is rapid. ESR experiments clearly support the B–N homolytic bond cleavage to both NHC-BH2• and a delocalized radical mostly sulfur-centered. Outstanding polymerization profiles of a benchmark trifunctional acrylate monomer are noted both in laminate, where the final acrylate conversions reached 70% with filtered light (λ > 300 nm), and under air in the presence of isopropylthioxanthone (45% final acrylate conversions at wavelengths λ > 340 nm).

## Key words

radical reactions, photochemistry, polymerization, cleavage, N-heterocyclic carbene boranes.

In the field of radiation curing (e.g., varnishes, coatings, adhesives, inks), imaging (e.g., graphic arts, Laser Direct Imaging (LDI) technology, Computer-To-Plate (CTP) systems), 3D printing by stereolithography, and other related areas (e.g., dentistry, medicine, optics, microelectronics, micro- and nano-sized materials), a liquid monomer/oligomer matrix exposed to suitable light in the presence of a photoinitiator (PI) or a photoinitiating system (PIS) leads to the formation of a solid polymer material through a photopolymerization reaction.<sup>1–3</sup> One key factor in such light-induced free radical polymerization

(FRP) reactions is the PIs and PISs, which enable the starting formulation to absorb light and generate the initiating species.<sup>4–6</sup>

A Type I PI generates free radicals through a homolytic single bond cleavage process upon light irradiation. In contrast, a Type II PI functions differently, typically through hydrogen abstraction or electron/proton transfer between a PI and a hydrogen donor, known as a co-initiator. Today, the careful selection of PIs or PISs based on their chemical structures allows a good match between their absorption spectra and the emission spectra of light sources (UV lamps, lasers, Light Emitting Diodes (LEDs)). However, the (photo)chemical reactivity and practical efficiency (final monomer conversion (FC), rate of polymerization (Rp)) of PIs or PISs can vary significantly from one system to another without clear predictive insight. The search for high-performance PIs (Type I or Type II) or PISs therefore remains very active.

In the case of Type I photoinitiators (PIs) one needs monocomponent systems. This necessitates the design of easily cleavable compounds that generate radicals (R<sub>1</sub>•, R<sub>2</sub>•) upon light excitation (r1). At least one of these radicals must be highly reactive towards the addition to a monomer double bond (M) to facilitate a fast and efficient initiation of polymerization (r1). Over fifty years of research have led to an extensive exploration of various chromophores with photocleavable bonds.<sup>5,7–9</sup> Altering or modifying the chromophore can significantly impact the nature of the excited state processes, the (photo)chemical reactivity, and ultimately the polymerization initiating ability (FC and Rp).

$$PI + hv \rightarrow R_{1} \bullet + R_{2} \bullet \quad \text{and} \quad R_{1} \bullet (R_{2} \bullet) + M \rightarrow R_{1} \cdot M \bullet (R_{2} \cdot M \bullet) \quad (r1)$$

The main family of (commercial) cleavable scaffolds relies on α or β-scission of excited carbonyl compounds, leading to C-

<footnote><sup>a</sup> Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France.</footnote>

<footnote><sup>b</sup> Université de Strasbourg, France.</footnote>

<footnote><sup>c</sup> Université de Haute-Alsace, Université de Strasbourg, CNRS, LIMA, UMR 7042, F-68000 Mulhouse, France.</footnote>

<footnote><sup>d</sup> Universite Claude Bernard Lyon 1, CNRS, CNES, ArianeGroup, LHCEP, Bât. Raulin, 2 rue Victor Grignard, F-69622 Villeurbanne, France.</footnote>

<footnote>jacques.lalevee@uha.fr; emmanuel.lacote@cnrs.fr</footnote>
and/or P-centered radicals.⁴⁻⁶ Many combinations of σ-bonded p block elements leading to free radicals capable of efficient initiation have been introduced, e. g. sulfonyl ketones (C–S cleavage),¹⁰ thiobenzoates (C–S),¹¹,¹² peroxides (O–O),¹³ keto-oxime esters, hydroxamic and thiohydroxamic esters (N–O),¹⁴⁻¹⁷ alkoxyamines (C–O),¹⁸ acylgermanes and bis germyl ketones (C–Ge),¹⁹,²⁰ disilanes and polysilanes (Si–Si),²¹,²² tellurides (C–Te),²³,²⁴ etc.

Chemists have also focused on various organo metallic compounds,²⁵,²⁶ as in titanocenes (C–Ti bond cleavage),²⁷ or organocobalt complexes (C–Co).²⁸ Some organometallic complexes can also generate radicals by ligand decomposition under irradiation.²⁹ This huge effort allows to improve molar extinction coefficients, target specific wavelengths, introduce designer functional end groups, and adjust the PIs to the materials requirements (solubility, hydrophilicity, etc.).

This is why there is a continued interest in exploring the reactivity of new cleavable bonds, especially those leading to new initiating radicals. Our interest lies in borane chemistry. From the radical cleavage angle it has been shown that organoboranes lead to C-radicals in the presence of oxygen. This happens via B–C bond breaking on the peroxyboryl species.³⁰⁻³⁴ Additionally, silicon-boron compounds can undergo photochemical B–Si bond cleavage, although in that case that was meant to generate Si-radicals.³⁵,³⁶ Lewis-base complexes boryl radicals are especially attractive because the variation of the Lewis base can help tune the spin density at boron, and therefore the reactivity.³⁷⁻⁴¹ In particular, when the latter is a N-heterocyclic carbene (NHC), the resulting NHC-boryl radicals become extremely nucleophilic and add very fast to acrylate double bonds, which we exploited to initiate photopolymerizations.⁴² They are also able to repair the formation of peroxyl radicals.⁴³,⁴⁴ Thus far we have mostly relied on a H-atom abstraction from an N-heterocyclic carbene-borane (NHC-BH₃) to generate the initiating species, i.e. we used the NHC-boranes as co-initiators.⁴³⁻⁴⁹ We nonetheless introduced a family of NHC-borosulfides,⁵⁰ where the B–S bond was weakened by complexation by the NHC. Its homolytic photo-cleavage produces NHC-boryl, NHC-thioboryl, and thyl radicals, resulting in Type I initiation.⁵¹ We were interested in extending this approach to other B–X bonds, especially those which cannot be cleaved in the absence of a Lewis base.

To the best of our knowledge no homolytic cleavage of a B–N bond has been reported, let alone its application in FRP photoinitiation. In this paper, we introduce three NHC-aminoboranes (BNa-c, Figure 1) that can function as Type I PIs under UV light. Their excited state behavior will be investigated through steady-state photolysis, molecular orbital calculations, and electron spin resonance spin trapping (ESR-ST) experiments.

<figure><img src="image_3.png" /><figcaption>Figure 1 Structures of the BN compounds Investigated</figcaption></figure>

i) *Light absorption properties of the new proposed B–N compounds.* The three BN compounds exhibit strong light absorption properties in the UV range (Figure 2). They feature high molar extinction coefficients at their maximum absorption wavelengths: $ε_{324nm} = 3.5 \times 10^4 \text{ M}^{-1}\text{cm}^{-1}$ for BNa, $ε_{288nm} = 1.5 \times 10^4 \text{ M}^{-1}\text{cm}^{-1}$ for BNb, and $ε_{322nm} = 3.2 \times 10^4 \text{ M}^{-1}\text{cm}^{-1}$ for BNc. In contrast, the parent carbene borane (DiMe-Imd-BH₃) does not absorb above 250 nm, highlighting the significant role of the N-heterocycle in enhancing the light absorption properties of these new NHC-borane derivatives. This is reminiscent of what was observed with the similar NHC-borosulfides,⁵¹ where the substituent on the sulfur atom had a crucial influence on the molecular orbitals responsible for the observable electronic transitions.

<figure><img src="image_4.png" /><figcaption>Figure 2 UV absorption spectra of BNa, BNb, BNc and NHC-BH₃ in acetonitrile.</figcaption></figure>

Molecular orbital calculations were performed to better understand the electronic transitions between frontier orbitals that are associated with the absorption peaks observed (Figure 3). Both the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) are localized on the N-heterocycles. Consequently, their architecture has a significant impact on the UV-vis spectrum of BNa-c, consistent with our experimental findings (Figure 2).
<figure><img src="image_5.png" /><figcaption>**Figure 3** Frontier orbitals (Highest Occupied Molecular Orbitals (HOMOs) and Lowest Unoccupied Molecular Orbitals (LUMOs) for **BNa-c**; calculated at</figcaption></figure>

UB3LYP/6-31G* level).⁵²

ii) *Photochemical reactivity.* The steady-state photolysis of **BNa-c** in acetonitrile is depicted in Figure 4. Since all compounds absorb light up to 370 nm, there is a good overlap with the emission spectrum of the Xe-Hg lamp, which has emission peaks at 313, 334, and 365 nm (see Figure S1). Remarkably, UV irradiation leads to very rapid photolysis, with a significant decrease in the absorption peaks of all BNs. For **BNa**, complete photolysis was observed in less than 60 s, indicating high photoreactivity and photosensitivity.
<figure><img src="image_6.png" /><figcaption>Figure 4 Photolysis of the BN compounds in acetonitrile upon a Xe-Hg lamp exposure; A) BNa; B) BNb; C) BNC.</figcaption></figure>

The control experiments in the absence of light showed that the absorption spectra of **BNa-c** remain unchanged, and therefore that the process is light-dependent. Isobestic points were observed in all cases, indicating that the photolysis proceeds without significant side reaction. **BNb** and **BNc** underwent a similarly rapid photolysis (Figure 4; see Scheme S1 and Figure S2 in the supporting information for the other BNs).

Spin-trap electron spin resonance (ESR-ST) experiments were conducted on **BNb** so as to confirm that homolysis was indeed taking place at the B-N bond. This was achieved by irradiating **BNb** in the ESR cavity in the presence of a spin trap (PBN). The hyperfine coupling constants (hfcs) for a first nitroxide radical were: aN = 15.2 G, aH = 2.1 G, and aB = 4.1 G (Figure 5 and Scheme 1). They correspond to the **DiMe-Imd-BH2•** adduct with PBN.<sup>43,46</sup> A second nitroxide radical was detected, with hyperfine coupling constants of aN = 13.8 G and aH = 2.1 G. These parameters are like those of a thiyl radical/PBN adduct.<sup>46</sup>

The overall observations confirm that the B-N bond was homolytically cleaved. The NHC-boryl radical **DiMe-Imd-BH2•** adds directly to PBN, while the formally N-centered radical **Nb•** is really centered on the sulfur atom (**Sb•**, Scheme 1). Both **DiMe-Imd-BH2•** and **Sb•** are stabilized. The former by spin delocalization onto the NHC, and the latter by aromaticity. This is the key that explains why homolysis of an otherwise way too strong B-N bond could take place. In support of this the calculated B-N BDE's in Me2N-BH2 and Ph(Me)N-BH2 were 134 kcal.mol⁻¹ and 122 kcal.mol⁻¹, respectively, while the same

bonds in their corresponding DiMe-Imd adducts Me2N-BH2-NHC and Ph(Me)N-BH2-NHC were both 63 kcal.mol⁻¹. This shows that NHC complexation has a huge influence over this BN bond strength.

The calculated B-N bond dissociation energies for **BNa-c** were respectively 74, 83, and 70 kcal.mol⁻¹. These values are slightly higher than those of the B-S bonds in the NHC-boron sulfides, which typically range from 50 to 60 kcal.mol⁻¹,<sup>51</sup> but still lower than for the B-H bonds in the NHC-BH3 family.<sup>53,54</sup> The B-N bond is weak enough to be homolytically cleaved.

<figure><img src="image_7.png" /><figcaption>Figure 5 ESR-spin trapping spectrum upon irradiation of a **BNb/PBN** solution. Experimental (a) and simulated (b) spectra. [PBN] = 0.01 M. Xe-Hg lamp. In tert-butylbenzene. See text for the hfcs.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Scheme 1 Elementary steps undergone by **BNb** in the ESR-ST cavity.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 6** Photopolymerization profiles of acrylate functions of TMPTA in laminate conditions for different photoinitiating systems: (A) upon a Xe-Hg lamp irradiation (λ > 300 nm) (1) TMPTA without initiating system; (2) BNa (2% w/w); (3) BNb (2% w/w); (4) BNc (1% w/w); (B) upon a Xe-Hg lamp irradiation (λ > 340 nm) (1) TMPTA without initiating system; (2) BNa (2% w/w). The irradiation starts for t = 5 s.</figcaption></figure>

iii) Reactivity of BNa-c as Type I photoinitiators for free radical polymerization (FRP). Given that the BNs under irradiation are able to generate the DiMe-Imd-BH₂• radical, which is an extremely efficient initiating species for the polymerization of (meth)acrylates,⁴³,⁴⁵ we decided to engage the former as Type I photoinitiators.⁵¹ The FRP of TMPTA in the presence of BNa-c under irradiation of a Xe-Hg lamp (all λ < 300 nm were filtered off) was achieved by direct exposure of a monomer film sandwiched between two polypropylene pieces (laminate conditions) to the light beam. This led to excellent polymerization profiles with high final acrylate function conversions of 60-70% - i. e. close to the maximum achievable for this rapidly cross-linking monomer - (Figure 6A, curves 2, 3, and 4), compared to less than 15% conversion in the absence of BNs (Figure 6A, curve 1). No polymerization occurred in the absence of light, indicating that the polymerization is a photochemical process.

The photoinitiating abilities of BNa, BNb, and BNc are quite similar, but given their different absorption profiles, BNa and BNc may be more effective than BNb at longer wavelengths. For example, BNa is still able to lead to reasonably high acrylate conversion with a light filtered off below λ = 340 nm (45%, Figure 6B, curve 2). On the other hand, the polymerizations in vessels open to air led to no observable conversions above 340 nm, likely due to strong oxygen inhibitions.

Photosensitization is an attractive way to enable photochemical reactions at a higher wavelength than those normally adapted to the chromophores used and is therefore a strategy well adapted for photopolymerizations.⁵ We focused on Isopropylthioxanthone (ITX) because it has a notable absorption in the 380-420 nm range, making it an effective photosensitizer (PS) when used in conjunction with photoinitiators (PIs).⁵⁵ ITX sensitization reaction can involve either energy transfer (r2a & r2b) or electron transfer (both oxidative or reductive, see r3a & r3b) processes, which opens a wide range of options. Given that the calculated B-N bond dissociation energy is higher (>70 kcal.mol⁻¹) than the excited state energy level of ITX (~62 kcal.mol⁻¹),⁵⁶ the dissociation after energy transfer reaction (r2) is energetically unfavorable. Therefore, the sensitization will likely favor the electron transfer mechanisms (r3a, r3b).

$$ \text{ITX (hv)} \rightarrow *\text{ITX} \qquad (\text{r2a}) $$

$$ *\text{ITX} + \text{PI} \rightarrow \text{ITX} + *\text{PI} \rightarrow \text{initiating radicals} \qquad (\text{r2b}) $$

$$ *\text{ITX} + \text{Ox} \rightarrow \text{ITX*} + \text{Ox*} \rightarrow \text{initiating radicals} \qquad (\text{r3a}) $$

$$ *\text{ITX} + \text{Red} \rightarrow \text{ITX*} + \text{Red*} \rightarrow \text{initiating radicals} \qquad (\text{r3b}) $$

<figure><img src="image_10.png" /><figcaption>**Figure 7** Photopolymerization profiles of TMPTA (acrylate functions conversion vs. irradiation time) under air using different photoinitiating systems upon a Xe-Hg lamp irradiation (filtered for irradiation with λ > 340 nm): (1) ITX (1% w/w); (2) BNa + ITX (2%/1% w/w); (3) BNb + ITX (2%/1% w/w). The irradiation starts for t = 5 s.</figcaption></figure>

With this in mind we wanted to address two issues. (i) achieve photopolymerizations at 340 nm with BNb, which absorbs only at lower wavelengths; (ii) make the initiation process more efficient, and therefore able to overcome oxygen inhibition in open vessels. The photopolymerization experiments were therefore conducted under air, where no polymerization occurs above 340 nm with the BNs (Figure 7). Remarkably, good conversions were observed in the presence of ITX, with both BNa and BNb (curves 2&3). Most importantly conversion is not due to ITX alone, as the background polymerization led to a lower conversion (e.g. 42% for BNa/ITX vs. 28% with ITX alone, compare curves 1&2). Thus, our two goals were met, as BNs/ITX-based formulations improved both the acrylate conversion and
the rate of polymerization (e. g. Rp/[M₀]*100 = 11 s⁻¹ with **BNb/ITX** vs. only 1.9 with **ITX** alone).

In conclusion we have shown for the first time that the B–N bond
could be homolytically cleaved. This was achieved by installing
on both atoms groups that could give some thermodynamic
stabilization to the resulting B and N radicals. This allowed us to
introduce a new family of FRP photoinitiators based on the B–N
bond cleavage. They exhibit strong, red-shifted UV absorptions
compared to the starting NHC-borane. They also undergo fast
photolysis (less than 60 seconds for complete cleavage), leading
to facile formation of NHC-boryl radicals, a process that could
also be sensitized. This led to excellent polymerization profiles of
a trifunctional acrylate monomer in laminate and under air. That
makes them leads for designing high-performance type I Pls.
Ongoing work aims to further extend their spectral sensitivity
toward the visible range, as well as designing water-soluble
compounds for emulsion photopolymerizations.

The experimental section has no title; please leave this line here.

*Reagents.* The BN compounds (**BNa-c**) were obtained using our established NHC-borane amination methodology.⁵⁰ Trimethylolpropane triacrylate (TMPTA) was used as the benchmark acrylate monomer for the radical polymerizations and was sourced from Arkema-Sartomer (Verneuil-en-Halatte, France). The phenyl-N-tert-butyl-nitrone (PBN, see structure in Scheme 1) spin trap was obtained from Sigma-Aldrich.

*Steady State Photolysis.* The NHC-boranes **BNa-c** were irradiated in solution with a Xe-Hg lamp, and their UV-Vis absorption spectra were recorded at different irradiation times using a JASCO V-730 UV/Vis spectrophotometer.

**ESR Spin Trapping (ESR-ST) Experiment.** The ESR-ST experiments were carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at room temperature using a UV (Xe-Hg) lamp (LC5 from Hamamatsu) under N₂ and trapped by phenyl-N-tert-butylnitrone (PBN). The ESR spectra simulations were carried out using the WINSIM software.

*Computational Procedure.* Molecular orbital calculations were performed using the Gaussian 09 package.⁵² Electronic absorption spectra for **BNa-c** were calculated using time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G* level, based on geometries optimized at the UB3LYP/6-31G* level. The molecular orbitals involved in these transitions were extracted. The geometries of the optimized structures were checked for frequency. The B–N bond dissociation energies were determined as the energy difference between the relaxed compound and the optimized radicals resulting from the cleavage process.

**Procedures**

*Photopolymerization Experiments.* The precise compositions of the photosensitive formulations are given in the relevant captions. The formulations (25 µm thick) were deposited on a BaF₂ pellet either in laminate (i.e. they were sandwiched between two polypropylene films to avoid re-oxygenation during the photopolymerization) or directly exposed to air. The irradiation was carried out with a LC5 Hamamatsu Xe-Hg UV lamp, whose intensity at the sample position has been measured ~100 mW/cm² for λ > 300 nm with a powermeter. For λ > 340 nm it was ~70 mW/cm². The conversion was monitored *in situ* by following the TMPTA double bond signal (~1630 cm⁻¹) using a real-time JASCO FTIR 4100 spectrometer.

**Funding Information**

We thank CNRS and UHA for funding, as well as the Agence Nationale de la Recherche (ANR) for the grants IMPACT (ANR-11-BS08-0016) and NHCX (ANR-11-BS07-0008).

**Acknowledgment**

Dr. Anne-Laure Vallet is acknowledged for preparing some batches of **Ba-c**.

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