From stable Sb- and Bi-centered radicals to a compound with a Ga=Sb double bond

Abstract

Neutral stibinyl and bismuthinyl radicals are typically short-lived, reactive species. Here we show the synthesis and solid-state structures of two stable stibinyl [L(Cl)Ga]₂Sb· 1 and bismuthinyl radicals [L(I)Ga]₂Bi· 4, which are stabilized by electropositive metal centers. Their description as predominantly metal-centered radicals is consistent with the results of NMR, EPR, SQUID, and DFT studies. The Lewis-acidic character of the Ga ligands allow for significant electron delocalization of the Sb- and Bi- unpaired radical onto the ligand. Single-electron reduction of [L(Cl)Ga]₂Sb· gave LGaSbGa(Cl)L 5, the first compound containing a Ga=Sb double bond. The π-bonding contribution is estimated to 9.56 kcal mol^-1 by NMR spectroscopy. The bonding situation and electronic structure is analyzed by quantum mechanical computations, revealing significant π backdonation from the Sb to the Ga atom. The formation of 5 illustrates the high-synthetic potential of 1 for the formation of new compounds with unusual electronic structures.

Fig. 1

Sb- and Bi-centered radicals. Persistent radicals (a, b), which have been identified in solution by electron paramagnetic resonance (EPR), and stable radicals of the heaviest group 15 elements (c–e), whose solid-state structures were determined by single-crystal X-ray diffraction

Fig. 2

Synthesis of 1–4. i stirring LGa and 0.5 eq. of Cp*SbCl₂ at 25 °C in benzene for 7 d with elimination of 0.5 Cp*₂. ii stirring LGa and 0.5 eq. of Cp*BiI₂ from −40 to 25 °C in toluene over 4 h with elimination of 0.5 Cp*₂. iii stirring LGa and an equimolar amount of Cp*SbCl₂ in benzene at 25 °C for 1 h. iv stirring 3 in benzene at 25 °C for 6 d

Fig. 3

Molecular structures of 1 and 4. Molecular structures of 1 (a) and 4 (b) in the solids of 1 and 4 · 2C₇H₈. H-atoms were omitted for clarity, displacement ellipsoids are drawn at the 50% probability level. Selected bond lengths and angles: 1, Ga–Sb 2.5899(4), 2.5909(3) Å, Ga–Sb–Ga 104.89(1)°; 4, Ga–Bi 2.6640(9), 2.6663(9) Å, Ga–Bi–Ga 106.68(3)°

Fig. 4

EPR spectroscopy. Continuous-wave EPR spectra of 1. as frozen solution obtained at X-band (9.634 GHz) and Q-band (34.202 GHz) in black with simulated spectra in red. High-field regions of each spectra are expanded as insets. g = [g₁, g₂, g₃] = [2.298 2.114 1.967], A(¹²¹Sb) = [A₁, A₂, A₃] = [−385, −496, 1138] MHz and 2 × A(⁶⁹Ga) = [132, 176, 134] MHz, lw (peak–peak) = 2.0 G. X-band conditions: temperature 15 K; modulation amplitude = 6 G; modulation frequency = 100 kHz; time constant = 20.48 ms; scan time = 167 s; single scan. Q-band conditions: temperature 10 K; modulation amplitude = 6 G; modulation frequency = 100 kHz; time constant = 14.65 ms; scan time = 60 s; number of scans = 24

Fig. 5

Geometry optimized structure of 1′ and 4′. The SOMO orbitals and spin densities exhibit the p-orbital character of the radical’s unpaired electron

Fig. 6

Single-electron oxidation and reduction reactions of 1. i sonicating 1 and 1 eq. of NO[BF₄] in benzene at 25 °C, followed (ii) by addition of a 2nd eq. of NO[BF₄] in benzene at 25 °C; iii reaction of 1 with 1 eq. of KC₈ in benzene at 25 °C over 1 h occurred with (iv) elimination of KCl

Fig. 7

Molecular and geometry optimized structures of 5 and 5′. (a) Molecular structure of 5. H-atoms were omitted for clarity, displacement ellipsoids are drawn at the 50% probability level. b Geometry optimized structure of 5′, with the HOMO plotted. Selected bond lengths and angles: 5, Ga–Sb 2.5528(2), 2.4629(2) Å, Ga–Sb–Ga 113.184(7)°; 5′, Ga–Sb 2.639, 2.535 Å, Ga–Sb–Ga 118.94°