Stibinidene

Stibinidenes are a class of organoantimony compounds in which the antimony center exhibits a formal oxidation state of +1. The parent stibinidenes have the formula R–Sb, with the antimony center possessing two lone pairs of electrons and a vacant 5p orbital (Figure 1). Reflecting their unusual low coordination number (i.e., 1) at antimony, stibinidines cannot be isolated. Instead, their oligomers or their adducts are often robust.

Synthesis

Attempted synthesis of stibinidenes, like carbenes, gives cyclic oligomeric forms. 6-, 5-, 4-, and 3-membered rings have been characterized. They are orange solids. These species exist in equilibrium:

5 (ArSb)6 ⇌ 6 (ArSb)5 4(ArSb)5 ⇌ 5 (ArSb)4 Distibinidenes, in principle, can be produced by reduction of the corresponding dichlorides. The following idealized equations apply:.

RSbCl2 + Mg → RSb + MgCl2 2,4,6-Tris[bis(trimethylsilyl)methyl]phenyl, 2,6-bis-[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl, and various m-terphenyl ligands, exist as dimers with the formula RSb=SbR.

2 RSb → RSb=SbR RSb=SbR + RSb → (RSb)3 When R is bulky, the product "RSb" is obtained as ring with Sb-Sb bonds. Larger substituents give smaller rings, otherwise 5- and 6-membered rings form. In some cases, a dimer with an Sb=Sb bond is isolated.

Base-stabilized stibinidene Monomeric stibinidenes were first obtained by Dostál reported a Sb(I) center stabilized by an N,C,N-pincer ligand. The ligand employed was L = 2,6-bis[N-(2',6'-dimethylphenyl)ketimino]phenyl. The synthesis of this complex was achieved by reducing LSb(III)Cl2 with two equivalents of t K[B(iBu)3H], resulting in the formation of isolable crystals of the stable monomeric stibinidene [C6H3-2,6-(C(Me)=N-2',6'-Me2C6H3)2]Sb via dihydrogen elimination (Scheme 2). In this system, coordination from the nitrogen centers provides thermodynamic stabilization to the Sb(I) center by delocalizing electron density, while the bulky N,C,N ligand introduces significant steric hindrance, which kinetically stabilizes the monomeric stibinidene by preventing dimerization or further reactions. Subsequently, other N,C,N-coordinating ligands were developed to produce stibinidenes, such as ArSb (where Ar = C6H3-2,6-(CH=NtBu)2 & Ar = C6H3-2,6-(CH=NDipp)2) which gained prominence in studies on stibinidene reactivity.

Carbene stabilized stibinidene Diamidocarbene (DAC) stabilize monomeric stibinidenes. The synthesis involved the reaction of phenylantimony dichloride, stabilized by a DAC, with magnesium powder in THF (Scheme 3). This process yielded stable, isolable, fluorescent red crystals of the carbene-stabilized stibinidene, (DAC)Sb-Ph. Despite the exocyclic Sb(I) center being exposed, the compound exists as a monomer, with its stability attributed to the strong backbonding between the DAC and the antimony center. The steric bulk of the mesityl group in the carbene further contributes to the compound's kinetic stability. Density functional theory (DFT) calculations revealed that the stability of the compound arises from partial double bond character between the carbene carbon and the Sb(I) center. This is attributed to backbonding from the antimony center into the vacant p orbital of the carbene. Chloro-substituted stibinidenes have been trapped using a cyclic alkyl(amino)carbene (CAAC) ligand. The synthesis involved reduction of CAAC-coordinated SbCl3 with KC8. Subsequently, the phosphine stabilized stibinidene (o-PPh2)C6H4(Ar*)Ge(Cl)Sb (E, where Ar* = 2,6-Trip2C6H3), was reported.

Reactivity Theoretically, singlet stibinidenes are ambiphilic due to the presence of both empty and filled 5p orbitals, which respectively confer Lewis acidic and Lewis basic character. However, N,C,N-pincer-coordinated stibinidenes exhibit diminished Lewis acidity because of nN → p*Sb donor-acceptor interactions. Despite this reduction in Lewis acidity, Dostál's stibinidene remains widely utilized in reactivity studies. In contrast, carbene-stabilized stibinidenes show significantly reduced reactivity as strong electron donation from the carbene ligand diminishes the Lewis acidic nature, while strong back-donation from the Sb center to the carbene weakens their Lewis basicity. Due to their ambiphilic nature, Dostál's stibinidenes are capable of activating small molecules, like disulfides, through oxidative addition. This reactivity arises from their ability to donate electron density to the LUMO of small molecules while simultaneously accepting electron density into the vacant 5p orbital. Dostál's N,C,N-coordinated stibinidene ArSb (where Ar = C6H3-2,6-(CH=NtBu)2) has been reported to act as a catalyst in the hydroboration of disulfides (Scheme 5). This reactivity exploits the ability of the stibinidene to reversibly interconvert between Sb(I) and Sb(III) oxidation states under the reaction conditions. The catalytic cycle involves the oxidative addition of disulfides to the Sb(I) center, followed by reductive elimination to regenerate the active species, enabling efficient hydroboration. As of 2024, this is the only reported example of catalysis involving stibinidene, demonstrating its potential in organometallic catalysis. Notably, triplet stibinidenes exhibit a distinct mode of reactivity. Acting as diradicals, they can react with small molecules such as alkynes and butadienes, forming antimony-substituted heterocycles, including three-membered and five-membered rings respectively (Scheme 4).

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