Efforts to generate organomanganese reagents under ball-milling conditions have led to the serendipitous discovery that manganese metal can mediate the reductive dimerization of arylidene malonates. The newly uncovered process has been optimized and its mechanism explored using CV measurements, radical trapping experiments, EPR spectroscopy, and solution control reactions. This unique reactivity can also be translated to solution whereupon pre-milling of the manganese is required.
This paper describes the design considerations for a dual mode X-band continuous wave (CW) Electron Paramagnetic Resonance (EPR) cavity, for simultaneous EPR measurement and microwave heating of the same sample. An elliptical cavity geometry is chosen to split the degeneracy of the TM110 mode, allowing for a well resolved EPR signal with the TM110,a and TM110,b modes resonating at around 10 GHz and 9.5 GHz, respectively, the latter of which is used for EPR measurements. This geometry has the benefit that the TM010 mode used for microwave heating resonates at 6.1 GHz, below the cut off frequency of the X-band waveguide used for the EPR channel, providing effective isolation between the heating and EPR channels. The use of a pair of 9 µm thick copper clad laminates as the flat cavity walls allows for sufficient penetration of the modulation field (Bmod) into the cavity, as well as maintaining a high cavity Q factor (> 5700) for sensitive EPR measurements. Locating the heating port at an angle of 135° to the EPR port provides additional space for easier coupling adjustment and for larger sample access to be accommodated. The associated decrease of EPR signal strength is fully compensated for by using a 7.2 GHz low pass filter on the heating port. EPR spectra using 1.6 mm and 4.0 mm sample tubes are shown at room temperature (298 K) and 318 K for a standard Cu(acac)2 solution, demonstrating the effectiveness of this dual-mode EPR cavity for microwave heating during EPR detection.
Efforts to generate organomanganese reagents under ball-milling conditions have led to the serendipitous discovery that manganese metal can mediate the reductive dimerization of arylidene malonates. The newly uncovered process has been optimized and its mechanism explored using CV measurements, radical trapping experiments, EPR spectroscopy, and solution control reactions. This unique reactivity can also be translated to solution whereupon pre-milling of the manganese is required.
Ground state changes of (R,R’)‐N,N’‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexane‐diamino Co(II), following coordination of various pyridyl substrate has been examined by CW EPR, pulsed relaxation measurements and DFT. The solution‐based Co(II) complex possesses a low spin (LS) state |yz,2A2⟩ (with g‐values of 1.96, 1.895, 3.14). Upon coordination of the pyridyl substrate, the resulting bound adduct reveals a distribution of LS ‘base‐on’ species, possessing a |z2,2A1⟩ electronic ground state (with g‐values of 2.008, 2.2145, 2.46) and a high spin (HS) species (with geff=4.6). DFT indicated that the energy gap between the LS and HS state is dramatically lowered (ΔE<25 kJ mol−1) following substrate coordination. DFT suggests the main geometrical difference between the LS and HS systems is the severe puckering of the N2O2 ligand backbone. The results revealed a tentative dependency on the pKa−H of the substrates for the spin distribution where, in most cases, the higher pKa−H substrate values favoured the HS species.
A custom-built dual-mode EPR resonator was used to study the radical chemistry of AIBN thermal decomposition. This resonator enables both simultaneous in situ heating using microwaves and EPR measurements to be performed. The thermal decomposition of AIBN was compared following conventional heating methods and microwave-induced (or dielectric) heating methods. Under both heating conditions, the radicals formed and detected by EPR include the 2-cyano-2-propyl (CP●) and 2-cyano-2-propoxyl (CPO●) radicals. Under aerobic conditions, the observed relative distribution of these radicals as observed by EPR is similar following slow heating by conventional or dielectric methods. In both conditions, the kinetically favoured CPO● radicals and their adducts dominate the EPR spectra up to temperatures of approximately 80–90 °C. Under anaerobic conditions, the distribution can be altered as less CPO● is available. However, the observed results are notably different when rapid heating (primarily applied using a MW-induced T-jump) is applied. As the higher reaction temperatures are achieved on a faster timescale, none of the ST●-CPO adducts are actually visible in the EPR spectra. The more rapid and facile heating capabilities created by microwaves may therefore lead to the non-detection of radical intermediates compared to experiments performed using conventional heating methods.
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