Synthesis and redox activity of carbene-coordinated group 13 metal radicals †

Carbenes are known to stabilize main group element compounds with unusual electronic properties. Herein, we report the synthesis of carbene-stabilized group 13 metal radicals (cAAC)MX 2 (IPr) (M = Al, X = Br 3; M = Ga, X = Cl 4) and the corresponding cations [(cAAC)MX 2 (IPr)][B(C 6 F 5 ) 4 ] (M = Al, X = Br 5; M = Ga, X = Cl 6), which were characterized spectroscopically and by sc-XRD. Quantum chemical calculation gave insights into their electronic structures.

Radicals are important species in organic synthesis, 1 polymer 2 and plasma chemistry 3 and in bioorganic chemistry. 4 They typically tend to dimerize, hence kinetic stabilization by use of sterically demanding ligands or electronic stabilization by delocalization of the unpaired electron are major stabilizing strategies. In recent years, main group element radicals received increasing interest due to their unique physical and chemical properties, 5 and several bulky and/or p-conjugated ligands were established which allowed for the isolation of electronically and sterically stabilized reactive radicals.
In contrast to widely known boron-based radicals, 5f the number of heavier group 13 metal radicals is limited (Scheme 1) to dinuclear radical anions (I), 6,7 in which the unpaired electron is localized in the p M-M orbital, mononuclear radical anions (II), 8 cyclic Al biradical (III) 9 and boryl-substituted neutral radicals (V, M = Ga-Tl). 10 Recently, the N-heterocyclic carbene (NHC) coordinated radical cation IV was synthesized by one-electron oxidation reaction of the digallene. 11 Cyclic (alkyl)(amino)carbenes (cAAC) are also known to stabilize main group metal radicals 12 including group 13 metal radicals (VI, 13a,b VII), 13c but reactivity studies of such radicals are yet still missing.
In view of the splendid performance of NHCs in stabilizing low-valent main group compounds 14 and our general interest in main group metal radicals, we became interested in the synthesis and reactivity of group 13 metal radicals stabilized by one cAAC and one NHC, and we herein report on the syntheses of cAAC(IPr)MX 2 radicals and their oxidation reactions.
Single crystals of 5 and 6 were obtained by layering n-hexane on the top of fluorobenzene solutions at ambient temperature. Compounds 5 (Fig. S18, ESI †) and 6 ( Fig. 2) crystallize in the monoclinic space group P2 1 /n and P2 1 /c, respectively. The coordination geometry of cations 5 and 6 and radicals 3 and 4 is similar. The C-M-C bond angles (117.12(5)1 5, 117.59(5)1 6) are slightly wider than those of radicals 3 and 4, while the Al-C cAAC bond in cation 5 is elongated compared to that of 3, but close to those of adducts (cAAC)AlX 3 (X = Cl, 2.037(1) Å; X = I, 2.049(2) Å). 13b,20 Comparable findings were observed for cation 6, showing a longer Ga-C cAAC bond (2.037(1) Å) than radical 4 (1.932(3) Å), whereas comparable bond lengths were reported for (cAAC)GaCl 3 (2.039(2) Å), 16 (cAAC)GaHCl 2 (2.053(2) Å), 19 and (cAAC) 2 Ga 2 Cl 4 (2.078(2) Å), respectively. 21 The M-C IPr bonds (2.047(1) 5; 2.026(1) Å 6) are slightly shorter than those of neutral radicals 3 (2.078(1) Å) and 4 (2.059(3) Å), but still fall in the typical range of Al-C IPr and Ga-C IPr bond lengths. 11,15,[17][18][19][20] Electron paramagnetic resonance (EPR) spectroscopy was conducted on 3 and 4 to provide further insight into their electronic properties. The frozen solution (77 K) spectrum of 3 exhibits an isotropic S = 1/2 signal centred near g e = 2.0023 (Fig. 3). The spectrum of 3 is split by an isotropic 27 Al (100% nat. abund.) hyperfine constant (HFC) of a iso ( 27 Al) = 38 MHz, much smaller than the Al centred unpaired spin (A( 27 Al) = B1000 MHz), 22 but more consistent with Al-cAAC radical species VI (a iso ( 27 Al) = Scheme 2 Synthesis of radical species 3 and 4.  14.6-23.4 MHz) where the unpaired electron is localized to the cAAC ligand. 13a,b The room-temperature EPR of 3 (Fig. S22, ESI †) exhibits a similar broad isotropic spectrum to that of the 77 K spectrum, however, with some additional fine structure. Simulations of the room temperature spectrum estimate an Al a iso value of 35 MHz, in agreement with the low estimate from simulation of the 77 K spectrum (Fig. 3) and yielding a rough error estimate of AE5 MHz for the 77 K measurement. Additionally, a small 14 N hyperfine coupling of 25 MHz is estimated, consistent with the nitrogen hyperfine couplings observed in other cAAC centered radicals. 14a,23 The frozen solution EPR spectrum of 4 also exhibits an isotropic S = 1/2 signal centred near to g e , consistent with a light-atom centred radical and unlike the significant g-shifts observed in related two-coordinate Ga centred radicals. 10 The EPR spectrum of 4 is split into four dominant lines, where the two outermost lines are further split, arising from the two I = 3/2 isotopes of gallium ( 69 Ga (60.1%) and 71 Ga (39.9%)) which exhibit HFCs that scale by the nuclear gyromagnetic ratios g n ( 71 Ga)/g n ( 69 Ga) = 1.271. 24 The hyperfine pattern is predominantly isotropic, with a iso ( 69 Ga) = 154 MHz and simulation of the EPR spectrum with a fairly large 17 Gauss linewidth (peak-to-peak) satisfactorily reproduces the experimental spectrum. For both 3 and 4, the relatively small Al/Ga HFCs and lack of resolvable/significant anisotropic hyperfine contributions in the frozen solution EPR spectra indicate that the unpaired electron is not Al/Ga p orbital centred but rather localized on the cAAC ligand. The small isotropic Al/Ga hyperfine coupling observed rather arises from spin polarization through the C-Al/ Ga bonds, where the radical is C centred. Additionally, no significant 14 N hyperfine coupling is resolved, and inclusion of up to 25 MHz 14 N coupling by simulation remains unresolved in the broad EPR lines of the experimental spectrum (Fig. S21, ESI †). The lack of a large 14 N HFC further supports a C centred radical species in both 3 and 4.
The bonding and electronic structure of radicals 3 and 4 and the cationic congeners 5 and 6 was evaluated by quantum chemical calculations. 25 Calculated bond lengths within the C-MX 2 -C skeleton (Table S2, ESI †) are in good agreement with the experimental values. The SOMO of the parent radicals correlates well to the LUMO of cations (Fig. S24, ESI †), in line with reversible reduction presented in CV. The DFT calculations at the TPSSh/def2-TZVP level of theory show that most of the spin density resides on the C atom of the coordinating cAAC ligand in both 3 (0.73) and 4 (0.76), with significant spin density present on the adjacent N atoms (0.18) ( Fig. 4a and b). Only a small amount of the total spin density is located at the Al/Ga centres (0.02), which is in line with the small isotropic HFCs used to simulate the experimental spectra but in contrast with the homoleptic carbene-coordinated Al radical VI, 14a,14b in which the spin density is distributed unsymmetrically on the two carbenes, but still more than 17% spin density is located on the carbene with the minor contribution. It is noted that although the total spin density at the nitrogen is much larger than at the Al or Ga atoms, the Al and Ga hyperfine interaction dominate the EPR spectra due, in part, to their larger nuclear spins (more transitions) and their larger isotropic hyperfine coupling parameters (a 0 [ 14 N] = 1811 MHz, a 0 [ 27 Al] = 3911 MHz, a 0 [ 69 Ga] = 12210 MHz). 24 The DFT calculated spin densities and larger a 0 values for Ga and Al in comparison to N supports the exclusion of a large N HFC (425 MHz) in the simulations of the experimental spectra (Fig. S21, ESI †). The magnetic orbitals for 3 and 4 are displayed in Fig. 4c and d, respectively, and show that electron density is localized to the cAAC moiety and is well represented by the C cAAC -N p*-orbitals.
Upon oxidation, one electron is removed from the C cAAC -N p*-orbital, resulting in increased WBIs (1.23 3, 1.23 4, 1.62 5, 1.63 6) and natural charges (-0.92 3, -0.87 4, -0.32 5, -0.28 6). In addition, the M-C cAAC bond is elongated in 5 and 6 despite the Fig. 3 EPR spectra of 3 (left) and 4 (right) at 77 K in toluene. Simulation parameters for 3: g = 2.0023, a iso (Al) = 38 MHz, line-width (peak-to-peak) = 27 Gauss. Simulation parameters for 4: g = 2.0025, a iso ( 71 Ga) = 196 MHz, a iso ( 69 Ga) = 154 MHz, line-width (peak-to-peak) = 17 Gauss. increasing charge, whereas the M-X and M-C IPr bonds become slightly shorter (WBI, Tables S3-S6, ESI †) as was observed in the solid state structures. Second order perturbation analysis of 3 and 4 showed a p-type interaction between a populated C cAAC -centred p-type orbital and the M-X s* orbitals (3: 9.8 kcal mol À1 ; 4: 9.7 kcal mol À1 ) exclusively for the alpha spin orbitals, which is missing in the corresponding cations 5 and 6 and therefore explains the elongated C cAAC -M and shortened M-X bonds in 5 and 6. The change in charge upon oxidation also has only a negligible influence on the C cAAC -M bond length as the natural charge of the metal is almost unaffected (Tables S3-S6, ESI †) and the C cAAC -M bonds of 5 (2.0549(13) Å) and 6 (2.0367(12) Å) are almost identical with those of cAAC-AlBr 3 (2.055(4) Å) 26 and cAAC-GaCl 3 (2.039(2) Å), 16 respectively. The WBI of the C cAAC -Al bond is much lower compared to the C cAAC -Ga bond, which might explain the lower thermal stability of 5.
In conclusion, one electron oxidation of carbene-centred neutral radicals 3 and 4 gave cationic compounds 5 and 6, which are rare examples of carbene-coordinated group 13 cations.
Financial support by the University of Duisburg-Essen (S. S.) and the Max Planck Society (G. E. C.) is acknowledged. We thank Mrs J. Krüger for supporting quantum chemical calculations.

Conflicts of interest
There are no conflicts to declare.