Synthesis of a Ga-Stabilized As-Centered Radical and a Gallastibene by Tailoring Group 15 Element–Carbon Bond Strengths

Abstract

A convenient synthetic route to Ga-stabilized pnictogen-centered radicals and gallapnictenes by manipulation of pnictogen-carbon bond strengths is presented. Two equivalents of LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) react with Cp Ar AsCl2 (Cp Ar = C5(4-t-BuC6H4)5) with formation of the arsenic-centered radical [L(Cl)Ga]2As• 1. In contrast, the analogous reaction with TerSbCl2 (Ter = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) yields the gallastibene LGa=SbTer 2 containing a Ga-Sb double bond, whereas reactions of DipSbCl2 with one and two equivalents of LGa give the monoinsertion and bisinsertion products L(Cl)GaSb(ClDip 3 and [L(Cl]Ga]2SbDip 4, respectively. 1 -4 were structurally characterized by single crystal X-ray diffraction. The description of 1 as arsenic-centered radical is consistent with results of EPR and DFT studies. The π-bonding in LGa=SbTer 2 is estimated to 10.68 kcal mol -1 by variable-temperature (VT) NMR spectroscopy, and DFT studies reveal a significant π-bonding interaction between Sb and Ga. ASSOCIATED CONTENTThe Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, 1 H, 13 C NMR, IR and EPR spectra, crystallographic details, bond lengths and angles of 1 -4 and results from quantum chemical calculations (pdf).

Ligand Effects on the Electronic Structure of Heteroleptic Antimony‐Centered Radicals

“…The Ga−Sb bond lengths ( 2‐B 2.5959(7) Å, 2‐C 2.635(3) Å, 2‐N 2.6258(3) Å) compare well to those of VI (2.5899(4) Å, 2.5909(3) Å), [L(Cl)Ga] 2 Sb 2 (2.58178(19) Å), {[L(Cl)Ga] 2 (μ,η 1:1 ‐Sb 4 )} (2.6008(13) Å, 2.6044(14) Å), and the sum of the computed covalent radii (∑ r cov (Ga−Sb)=2.64 Å) . The Sb−B bond length in 2‐B (2.245(5) Å) is comparable to that of [DipNC( t ‐Bu)NN(Ph)B]SbPh 2 (2.257(5) Å) and the sum of the covalent radii (∑ r cov (Sb−B)=2.25 Å), while the Sb−C bond length in 2‐C (2.1716(16) Å) is virtually identical with those of LGa=Sb‐ C (2.180(3) Å) and C ‐Sb=Sb‐ C (2.169(4) Å) . The Sb−N distance in 2‐N (2.0502(18) Å) is close to the average value of Sb−N bond lengths observed in aminostibanes (2.036 Å) containing a SbN(Ar)(SiR 2 X) group (Ar=2,4,6‐ t ‐Bu 3 C 6 H 2 , 2,6‐Mes 2 C 6 H 3 , 2,4,6‐CHPh 2 ‐4‐ i ‐PrC 6 H 2 , Dip; R=Me, i ‐Pr; X=Me, i ‐Pr, Cl, N 3 , OSO 2 CF 3 , N(H)Dip).…”

“…VI (104.89(1)°), but fairly agree with the Ga‐As‐Ga bond angle in [L(Cl)Ga] 2 As . (109.43(6)°) . The Ga−Sb bond lengths ( 2‐B 2.5959(7) Å, 2‐C 2.635(3) Å, 2‐N 2.6258(3) Å) compare well to those of VI (2.5899(4) Å, 2.5909(3) Å), [L(Cl)Ga] 2 Sb 2 (2.58178(19) Å), {[L(Cl)Ga] 2 (μ,η 1:1 ‐Sb 4 )} (2.6008(13) Å, 2.6044(14) Å), and the sum of the computed covalent radii (∑ r cov (Ga−Sb)=2.64 Å) .…”

“…In 2‐C , an interaction of the SOMO with the π‐system of the central aryl ring of the terphenyl ligand is not observed. Furthermore, it was shown that the π‐acidic nature of the [L(Cl)Ga] ligand, as confirmed by EPR spectroscopy, efficiently stabilizes main group element radical centers …”

“…The hyperfine tensor of the Sb centers, including signs, has been determined as previously published . For each, a minimal isotropic hyperfine coupling value, a iso , is observed as expected for minimal s‐orbital spin density .…”

“…For each, a minimal isotropic hyperfine coupling value, a iso , is observed as expected for minimal s‐orbital spin density . The resultant anisotropic Sb hyperfine tensor, T = A − a iso , observed for 2‐R are all nearly axial, [‐ t , ‐ t , 2 t ], characteristic of the unpaired electron centered in a p orbital, with an estimated p‐orbital density from +0.87 ( 2‐B ) to +0.98 ( 2‐N ) . The anisotropic tensor of 2‐B exhibits the largest degree of rhombicity, indicating small perturbations to the Sb spin density induced by the π‐accepting character of the boron substituent, which facilitates delocalization of spin density onto the ligand.…”