Metal-Diazo Radicals of α-Carbonyl Diazomethanes

Metal-diazo radicals of α-carbonyl diazomethanes are new members of the radical family and are precursors to metal-carbene radicals. Herein, using electron paramagnetic resonance spectroscopy with spin-trapping, we detect diazo radicals of α-carbonyl diazomethanes, induced by [RhICl(cod)]2, [CoII(por)] and PdCl2, at room temperature. The unique quintet signal of the Rh-diazo radical was observed in measurements of α-carbonyl diazomethane adducts of [RhICl(cod)]2 in the presence of 5,5-dimethyl-pyrroline-1-N-oxide (DMPO). DFT calculations indicated that 97.2% of spin density is localized on the diazo moiety. Co- and Pd-diazo radicals are EPR silent but were captured by DMPO to form spin adducts of DMPO-N∙ (triplet-of-sextets signal). The spin-trapping also provides a powerful tool for detection of metal-carbene radicals, as evidenced by the DMPO-trapped carbene radicals (DMPO-C∙, sextet signal) and 2-methyl-2-nitrosopropane-carbene adducts (MNP-C∙, doublet-of-triplets signal). The transformation of α-carbonyl diazomethanes to metal-carbene radicals was confirmed to be a two-step process via metal-diazo radicals.

. Control experiments suggest that in the absence of metal catalysts, α-carbonyl diazomethane and DMPO, no EPR signals could be detected, which confirms that the radicals are generated during the interaction of the metal-catalysts and the diazocompounds. Figure 2A. Experimental (green line) and simulated (red line) EPR spectra obtained from the [RhCl(cod)] 2 -PDA system (detected at 1 min).

Analysis:
The EPR spectra shown in Figure 2a displays more splitting besides the quintet signal, partly due to the radical motion in solvent, and more importantly as a consequence of the superposition by other paramagnetic species in low concentration. These spectra are further simulated (red line) as a combination of the quintet hyperfine pattern (blue line), the sextet of the DMPO-C· radical (green line) and the triplet DMPOX signal (Figure.

Analysis:
We calculated the N 2 release process from Co-diazo radical to Co-carbene radical and obtained a transition state of Co-diazo radical, which holds higher energy level (+61.02 KJ/mol). DFT calculations assigned 63.1% of the spin density to the diazo moiety (26.0% on the terminal nitrogen atom, 4.3% on the central nitrogen atom and 32.8% on the carbon atom, reported in the manuscript). The results clearly proved that spin transfer process occurred from Co center to the diazo moiety.
Supplementary Figure 8a. Calculation on the N 2 release process from Co-diazo adduct to Co-carbene radical via the transition state of Co-diazo radical.
Supplementary Figure 8b. Calculated molecular structure for the transition state of Co-diazo radical.

Analysis:
To exclude the possibility that the detected signals may stem from minor paramagnetic impurities, spin counting was performed (double integration of the detected signals referenced against a reference compound). DPPH (as the reference compound; 1.5*10 -2 mol/L) and Rh-diazo radical detected in the [RhCl(cod)] 2 -PDA system (the catalyst concentration is about 1.2*10 -2 mol/L). The details of the detection parameters: For DPPH (supplementary Fig. 14 Fig. 16): Center Field 3510.00 G; Static Field 3410.00 G; Microwave Frequency 9.863 GHz; Microwave power 20.08 mW; Receiver Gain 1.00*10 5 ; Modulation Frequency 100.00 KHz; Modulation Amplitude 1.00 G. The Bruker WinEPR (software) was used to double integration of the signal (see below). Although the signal-to-noise of the quintet signal observed with Rh-diazo radical is weak than that of DPPH reference sample, the absolute signal intensity of Rh-diazo radical is lower than that of DPPH, since DPPH is a stable standard -NN▪ radical while Rh-diazo radical is an active radical intermediate that cannot be presented in high amount. Besides, we also traced the gradual disappearance of the Rh-diazo radical and DMPO-N· signals along with an increase in the DMPO-C· signal. Taking the two points into account, we are convinced that the detected signals cannot be stem from minor paramagnetic impurities. Figure 14. EPR spectrum of DPPH.

Analysis of the different EPR signals detected with Rh-diazo radical and Co-diazo radical in the presence of DMPO:
The 'quintet' EPR signals of Rh-diazo radicals come from the thermodynamical stabilities of Rh-diazomethane complexes and the stabilization effect of DMPO. For example, by the DFT-calculated results, N-centered Rh-diazo radical is a predominant structure and can be formed from diazocompounds and [Rh I Cl(cod)] 2 , but to the Co-diazo radical the predominant structure is metal-centered radical. Obviously, it is impossible to detect the quintet EPR spectra of Co-diazo radical.

Supplementary Figure 20b.Potential barrier between Rh-diazo radical and its TS
In the presence of DMPO the linkage between Rh and carbon atom is relaxed by the interaction between Rh and negative oxygen ion of DMPO (c.f. E. Jellema, et al., J. Am. Chem. Soc., 2007, 129, 11631-11641) and the linkage between the carbon atom of diazomethane and diazo group is reinforced, by which the nitrogen release is retarded. Obviously, Rh-diazo radical is stabilized by DMPO (DFT calculations indicate that the binding energy of DMPO-Rh-diazo radical complex (-139.82 KJ mol -1 ) is 35.62 KJ mol -1 lower than that of Rh-diazo radical (-104.2 KJ mol -1 )). The possible structure of DMPO-involved Rh-diazo radical is suggested as below: The steric effect of involved DMPO molecule prevents diazo radical being trapped by other DMPO molecules. So, Rh-diazo radical can be detected in the presence of DMPO but not captured by DMPO.

Supplementary
In the case of Co-diazo complexes, the spin density can transform from the metal atom into the -N=N-C-group to form a N-centered [M]-diazo radical, which is then captured by DMPO. The process is dynamically controlled and competes with the N 2 -losing. Figure 21.Spin density transformation from the metal atom into -N=N-Cgroup.