Synthesis of Quinoline-Based Pt-Sb Complexes with L- or Z-Type Interaction: Ligand-Controlled Redox via Anion Transfer

Intramolecular Interactions between the Pnictogen Groups in a Rigid Ferrocene Phosphinostibine and the Corresponding Phosphine Chalcogenides, Stiboranes, and Their Complexes

Differences in the chemical properties of phosphorus and antimony enable the synthesis of heteroditopic derivatives whose properties can be modified by altering the pnictogen substituents. In this work, 1-(diphenylstibino)-2-(dicyclohexylphosphino)ferrocene, [Fe(η5-1-Ph2Sb-2-Cy2PC5H3)(η5-C5H5)] (1), and the corresponding phosphine chalcogenides [Fe(η5-1-Ph2Sb-2-Cy2P(E)C5H3)(η5-C5H5)] (E = O, S, Se) and catecholatostiboranes [Fe(η5-1-Ph2(Cl4C6O2)Sb-2-Cy2P(E)C5H3)(η5-C5H5)] (E = void, O, S, Se) were examined, with a focus on the intramolecular donor-acceptor interactions between the antimony and the phosphorus substituents. Experimental data and theoretical analysis consistently indicated that these interactions can be described as pnictogen bonding between the Lewis acidic antimony and the lone pair at the phosphorus substituent (either at the phosphorus or at the chalcogen atom) and that they are significantly stronger in the stiboranes due to the increased Lewis acidity of the Sb atom. Noncovalent interactions were also observed in the chlorogold(I) complexes obtained from 1 and catecholatostiborane [Fe(η5-1-Ph2(Cl4C6O2)Sb-2-Cy2PC5H3)(η5-C5H5)] as P-donors. As shown by experiments in Au-mediated cyclization of N-propargylbenzamide, the noncoordinated antimony group influenced the catalytic properties of the Au(I) complexes. Notably, an intramolecular Cl → Sb pnictogen bond affected the molecular geometry of the Pd(II) complex [PdCl2(1-κ2P,Sb)], which in turn suggested that the structural influence exerted by ligands of this type needs to be assessed with care.

Mechanistic Studies of Alkyl Chloride Acetoxylation by Pt-Sb Complexes

The bis-acetate complexes (SbQ3)Pt(OAc)2 (1) and (SbQ2Ph)Pt(OAc)2 (2) (Q = 8-quinolinyl) were used to study C-Cl acetoxylation of 1,2-dichloroethane (DCE) to generate 2-chloroethyl acetate and the complexes (SbQ3)PtCl2 (1b) and (SbQ2Ph)PtCl2 (2b), respectively. The first acetoxylation step produced the intermediates (SbQ3)Pt(Cl)(OAc) (1a) and (SbQ2Ph)Pt(Cl)(OAc) (2a). The reaction was studied using pseudo first order kinetics (excess DCE) in order to compare the rates of reaction of 1 and 2, which revealed that k obs = 2.44(6) × 10-4 s-1 for 1 and 0.51(2) × 10-4 s-1 for 2. The intermediate 1a was synthesized independently, and the solid-state structure was determined using single crystal X-ray diffraction. A non-Sb containing control complex, (tbpy)Pt(OAc)2 (3) (tbpy = 4,4'-di-tert-butyl-2,2'bipyridine), was studied for the acetoxylation of DCE to form (tbpy)Pt(Cl)(OAc) with k obs = 0.46(1) × 10-4 s-1. Density Functional Theory (DFT) calculations were used to examine possible Pt-mediated mechanisms for the reactions of 1, 2, or 3 with DCE. The lowest energy calculated substitution mechanism occurs with nucleophilic attack by the Pt center on the C-Cl bond followed acetate reaction with the Pt-C bond. However, close in energy and potentially also a viable mechanism is a direct substitution mechanism where the coordinated acetate anion directly reacts with the C-Cl bond of DCE. In addition, the rate of acetoxylation for complex 1 in heated dichloromethane-d 2 and chloroform-d was determined (0.43(1) × 10-4 s-1 for dichloromethane-d 2 and 0.37(1) × 10-4 s-1 for chloroform-d) and compared to the rate of acetoxylation of DCE.

Antimony centre in three different roles: does donor strength or acceptor ability determine the bonding pattern?

A set of antimony(III) compounds containing a ligand (Ar) with a pendant guanidine function (where Ar = 2-[(Me2N)2CN]C6H4) was prepared and characterized. This includes triorgano-Ar3Sb, diorgano-Ar2SbCl and monoorgano-ArSbCl2 compounds and they were characterized by 1H and 13C NMR spectroscopy and by single-crystal X-ray diffraction analysis (sc-XRD). The coordination capability of Ar3Sb and Ar2SbCl was examined in the reactions with either cis-[PdCl2(CH3CN)2] or PtCl2 and complexes cis-[(κ2-Sb,N-Ar3Sb)MCl2] (M = Pd 1, Pt 2) and [(κ3-N,Sb,N-Ar2SbCl)MCl2] (M = Pd 3, Pt 4) were isolated, while their structures were determined by sc-XRD. Notably, the ligands Ar3Sb and Ar2SbCl exhibit different coordination modes - bidentate and tridentate, respectively - and the antimony exhibits three distinct bonding modes in complexes 1-4, which were also subjected to theoretical studies.

Pnictogen Bonding at the Core of a Carbene-Stiborane-Gold Complex: Impact on Structure and Reactivity

Our interest in the design of ambiphilic ligands and their coordination to gold has led us to synthesize an indazol-3-ylidene gold chloride complex functionalized at the 4-position of the indazole backbone by a stibine functionality. The antimony center of this new complex cleanly reacts with o-chloranil to afford the corresponding stiborane derivative. Structural analysis indicates that the stiborane coordination environment is best described as a distorted square pyramid whose open face is oriented toward the gold center, allowing for the formation of a long donor-acceptor, or pnictogen, Au → Sb bonding interaction. The presence of this interaction, which has been probed computationally, is also manifested in the enhanced catalytic activity of this complex in the cyclization of N-propargyl-4-fluorobenzamide.

Rh → Sb Interactions Supported by Tris(8-quinolyl)antimony Ligands

The ligands tris(8-quinolyl)stibine and tris(6-methyl-8-quinolyl)stibine have been synthesized and complexed to rhodium using (MeCN)3RhCl3. The resulting complexes feature an unusual [RhSb]VI core as a result of the formal insertion of the antimony center into one of the Rh-Cl bonds. Computational analysis using density functional theory (DFT) methods reveals that the resulting Rh-Sb σ bond is polarized toward the Rh atom, suggesting a description of this linkage as a Rh → Sb Z-type interaction.