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.

A General Pathway for the Synthesis of Gallastibenes containing Ga=Sb Double Bonds

“…To gain a deeper mechanistic understanding, we systematically investigated reactions of LGa with SbX 3 (X=F, Cl, Br, I) in different molar ratios and different reaction conditions. Reactions of three equivalents of LGa with SbX 3 at ambient temperature yielded LGaSbGa(X)L (X=F 1 , Cl 2 (= C ), Br 3 , I 4 ), which contain Ga=Sb double bonds (Scheme ).…”

“…The strength of the Ga=Sb double bond in 1 and 3 was determined by variable‐temperature (VT) 1 H NMR spectroscopy. As observed in compound 2 , the γ ‐CH signal of 3 splits into two peaks at −80 °C (Figure S12 in Supporting Information), with a maximum peak separation of 46.5 Hz, giving Δ G =9.35 kcal mol −1 for the rotation about the Ga=Sb bond . The resonance peaks corresponding to the i Pr groups are split into three signals with a relative intensity of 1:1:2.…”

“…These compounds crystallize with one molecule in the asymmetric unit in the monoclinic space groups P 2 1 / n ( 1 , 4 ) and P 2 1 / c ( 3 ) . The crystal structure of 2 was previously reported …”

“…Both Ga atoms in the C 3 N 2 Ga rings of 1 – 4 are slightly out of plane. Ga2−Sb1 bond lengths (Table ) in 1 (2.4601(3) Å), 3 (2.4502(3) Å), and 4 (2.4496(8) Å) are comparable to that observed in 2 (Ga2−Sb1 2.4629(2) Å) . Due to the π ‐bonding contribution, Ga2−Sb1 bonds are substantially shorter than the Ga1−Sb1 bonds of the tetrahedrally‐coordinated Ga atom (2.5596(3) Å 1 ; 2.5528(2) Å 2 ; 2.5662(3) Å 3 ; 2.5478(6) Å 4 ).…”

“…To the best of our knowledge, [{Li(thf) 3 } 2 Ga 2 {As(Si‐ i Pr 3 )} 4 ] A , LGaAsCp* {L=HC[C(Me)N(2,6‐ i Pr 2 C 6 H 3 )] 2 , Cp* = C 5 Me 5 } B , resulting from the reaction of Cp*AsCl 2 with two equivalents of LGa, and LGa=SbGa(Cl)L C , which was formed by single electron reduction of the stibinyl radical L(Cl)Ga) 2 Sb . with KC 8 (Scheme ), represent the only structurally characterized compounds containing Ga=E (E=As, Sb) double bonds, to date.…”