reaction of multi-spin isoindoline nitroxides followed by EPR spectroscopy †

Institute of Organic Chemistry (IOC), K Fritz-Haber-Weg 6, D-76131 Karlsruhe, Germ Institute of Functional Interfaces (IFG), K Hermann-von-Helmholtz-Platz 1, D-76344 E Institute of Physical Chemistry (IPC), Karls Haber-Weg 2, D-76131 Karlsruhe, Germany ETH Zurich, Laboratory of Physical Chem Zurich, Switzerland Institute of Toxicology and Genetics (ITG), Hermann-von-Helmholtz-Platz 1, D-76344 E † Electronic supplementary information ( synthetic procedures, details on E 10.1039/c6ra06510d ‡ Current address: International Iberian Mestre José Veiga, 4715-330, Braga, Portu Cite this: RSC Adv., 2016, 6, 55715


Introduction
The synthesis of rigid organic building blocks containing multiple functional groups is of high interest to create molecular tectons for the synthesis of crystalline or amorphous covalently linked materials as well as for the assembly of non-covalently bound supramolecular architectures. 1 The functionalization of these molecular tectons with nitroxide moieties lead to attractive model systems for EPR distance measurements 2 and would allow the assembly of the tectons via halogen bonding 3 or nitroxide exchange reaction. 4 The nitroxide exchange reaction, 5 which belongs to the class of dynamic covalent chemistry, has been incorporated in dynamic polymers or macromolecules 6 and as a tool to trigger the self-assembly of micro-crystals. 7 In addition, the nitroxide exchange reaction has been utilized to functionalize polymers, self-assembled monolayers 8 or surfaces. 9 Moreover, the incorporation of prouorescent nitroxides allows following the kinetics of the nitroxide exchange reaction. 4 However, in all the reported cases, only the product formation could be followed, not the exchange process as such. Herein we report the synthesis of a rigid tetrahedral organic tecton containing multiple nitroxide moieties. Aerwards we performed a nitroxide exchange reaction and demonstrate that progress of the reaction can be followed by EPR spectroscopy.

Results and discussion
The synthesis of the rigid multi-spin molecule is based on tetrakis(4-azidophenyl)methane (1) as a core, which is functionalized with nitroxide moieties via fourfold copper-catalysed azide alkyne click chemistry 10 between the azide functions of the core 1 and the alkyne moiety of an isoindoline nitroxide 2. Scheme 1 shows the synthesis of tetraphenylmethane nitroxide (TPM-NO) 3. The TPM-NO 3 can be further functionalized using dynamic and reversible nitroxide exchange reaction. Nitroxide exchange reactions are based on thermal C-O bond homolysis of alkoxyamines, which leads to transient carbon-centred radicals and persistent nitroxide radicals. Usually these carbon centred radicals are quickly trapped by the nitroxide radicals and reform the alkoxyamines. If homolysis of an alkoxyamine is performed in presence of an additional nitroxide radical, also this additional nitroxide can trap the carbon centred nitroxide, and a mixture of the two possible nitroxides will be formed. The ratio will depend on the relative thermodynamic stabilities of the different alkoxyamines (see Fig. 1 for a schematic representation of the bond homolysis and of the nitroxide exchange reaction). 11 If the EPR (electron paramagnetic resonance) spectra of the two nitroxide moieties (depicted as red and green in Fig. 1) differ, i.e. in the hyperne coupling constant as in the present case, the exchange process can be followed via EPR measurements. In order to evaluate the possibility to follow the exchange process between the isoindoline nitroxide of TPM-NO 3 and 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO, 5), we recorded the EPR spectra of the individual compounds and mixtures thereof in toluene at room temperature. The EPR spectra show the typical tumbling induced line prole. The TEMPO nitroxide 5 instead has a hyperne coupling constant of about 15.5 Gauss. Due to the difference in the hyperne coupling constants of the isoindoline and TEMPO nitroxides, the exchange reaction can be easily followed by means of EPR.
We performed the nitroxide exchange reaction of the multispin system TPM-NO 3 with the TEMPO-alkoxyamine 4 shown in Scheme 2. Firstly, we followed the exchange reaction between TPM-NO 3 and TEMPO-alkoxyamine 4 using an equimolar ratio (with respect to the nitroxide moieties). Compounds 3 and 4 were dissolved in toluene and mixed at room temperature. The resulting solution was divided in several aliquots, inserted in closed ampules and degassed via bubbling with argon. Aerwards the different aliquots were heated under argon at 80 C for either 15 min, 1 h, 2 h, 14 h, 48 h or 96 h. Aer cooling to room temperature the continuous wave EPR spectra were recorded. Fig. 3 (top) shows the CW EPR spectra for the different points in time.
Scheme 1 Synthesis of TPM-NO 3 using fourfold copper-catalysed azide alkyne click chemistry. Fig. 1 (a) Homolysis of an alkoxyamine into a nitroxide radical (red) and a carbon centred radical (black); (b) thermodynamic product formation for homolysis of an alkoxyamine in presence of an additional nitroxide radical (green). In the EPR spectra of Fig. 3 (top), one can clearly see the progress of the exchange reaction over time, with a decrease in relative intensity of the species with lower hyperne coupling constant (TPM-NO 3) and an increase of TEMPO 5, showing a larger hyperne coupling constant. From the tting of the spectra (for details see the ESI †), we could obtain the different relative percentages of the two species and could plot the exchange percentage as function of time (see Fig. 3, bottom). The plot reveals that the exchange process is a fast initial reaction (within the rst two hours) followed by a deceleration. The deceleration is most likely due to the decreasing concentration of TEMPO-alkoxyamine 4 in combination with the competition for radical trapping between an increasing concentration of liberated TEMPO 5 and decreasing concentration of TPM-NO 3.
Even aer 96 h, the exchange reaction does not seem to have reached the thermodynamic limit, where both species adapt their nal equilibrium concentrations. In order to nd conditions under which the equilibrium is reached faster and in order to optimize the yield of TPM-NO-alkoxyamine 6, we investigated the inuence of both the reaction temperature and the equivalents of TEMPO-alkoxyamine 4 on the exchange process. Fig. 4 shows the comparison of the CW EPR spectra of TPM-NO 3 and TEMPO-alkoxyamine 4 in mixtures of 1 : 1 (a), 1 : 2 (b), and 1 : 5 (c) molar ratios (related to nitroxide moieties), aer heating at 100 C for 1 h and 24 h.
The EPR spectra show that the exchange reaction is almost quantitative aer 24 h at 100 C in the presence of 5 equivalents of TEMPO-alkoxyamine 4 with 96% yield (and 77% yield aer 1 h), whereas the 1/1 (20% yield aer 1 h and 65% aer 24 h) and 1/2 mixtures (39% yield aer 1 h and 80% yield aer 24 h) still contain large amounts of isoindoline nitroxide moieties of TPM-NO 3. To verify if the exchange reaction reaches equilibrium at 100 C aer 24 h, we determined the expected thermodynamic limit of exchange for the investigated molar ratios. For this, we rst estimated the Gibbs free energy DG by performing quantum chemical calculations. Employing density functional theory methods as implemented in the TURBOMOLE program package, 12 we obtain values between À4 kJ mol À1 and +5 kJ mol À1 . Considering the error in the calculated DG, we evaluated the limit of exchange for several values in the range of AE15 kJ mol À1 (for details of the calculations see Section 5 in the ESI †). The experimental results for T ¼ 100 C aer 24 h and the calculated results for selected values of DG are summarized in Table 1. The calculated results for slightly negative values of DG t well to the experimentally observed data. The calculations suggest that the exchange process is approaching its thermodynamic limit of exchange aer heating at 100 C for 24 h and that a nearly quantitative exchange can be reached if the TEMPO-alkoxyamine 4 is present in excess.

Experimental
The EPR spectra in this work were recorded on a Bruker ESP300E spectrometer. The compounds were dissolved in toluene and deoxygenated by bubbling argon for several minutes. Aer treatment of the samples as written in the text the spectra were taken at 298 K.
The instrument settings were as follows: microwave power 2.00 mW, modulation amplitude 0.0452 mT, modulation frequency 100 kHz, scan time 180 s. Further details as well as the synthesis of the compounds are described in the ESI. †

Conclusions
Here we describe the successful synthesis of novel rigid isoindoline based multispin nitroxides via fourfold click reaction. These nitroxides were further converted into alkoxyamines via exchange reaction with a TEMPO-alkoxyamine and characterized using EPR spectroscopy. Our results demonstrate that EPR spectroscopy is a versatile tool to follow the exchange process and to determine several factors inuencing the kinetics of the exchange process and optimize the experimental conditions to have the maximum yield. The presented approach could be used to study the nitroxide exchange process of various systems and the presented molecular components can be used as initiators for nitroxide mediated polymerization (NMP) 13 or as   tectons in the construction of supramolecular and covalently linked organic networks. Further investigations in this regards are currently ongoing.