Phosphasalen vs. Salen Ligands: What Does the Phosphorus Change?

Bis(salicylamidine) Ligands (FAlen): A Variant of Salen with "à la Carte" Denticity

Ethylene- and phenylene-bridged bis(salicylamidine) ligands have been readily prepared from ethylene or phenylenediamine and iminium chloride derivatives generated in situ from N,N-dialkylsalicylamides. The former, in its diprotonated form (FAlenH₂), reacts with AlMe₃ to afford a zwitterionic dimethyldiphenoxyaluminate complex with the FAlen ligand monoprotonated and in a bidentate κ²O,O' fashion. A phenylene-bridged proligand behaves differently, yielding a neutral methylaluminum complex bearing a κ³O,N,O'-coordinated FAlen ligand. From these complexes, methyl anion abstraction with B(C₆F₅)₃ or a reaction with Schrock's alcohol leads to the corresponding aluminum cationic or alkoxy complexes in which a κ⁴O,N,N',O'-coordination mode of the FAlen ligand is observed. X-ray diffraction studies of the proligands and of the complexes show that the amidine functions feature a trans configuration when the N-amidine atom is not coordinated to the metal and conversely a cis configuration when it is. Density functional theory calculations show that trans-cis isomerization of the amidine functions occurs upon coordination with the metal ion with very low energy barriers. They also confirm the intuition that the denticity of the FAlen ligands in the complexes is directly related to the electron richness of the metal ion. At last, FAlen Al complexes are used as initiators for the controlled ring-opening polymerization of rac-lactide to afford poly(lactic acid) with slight isotactic bias.

Phosphonium Ylides vs Iminophosphoranes: The Role of the Coordinating Ylidic Atom in cis-[Phosphine-Ylide Rh(CO)2] Complexes

The coordinating properties of two families of ylides, namely, phosphonium ylides and iminophosphoranes, differently substituted at the ylidic center (CH₂^- vs NiPr^-), have been investigated in structurally related cationic phosphine-ylide Rh(CO)₂ complexes obtained from readily available phosphine-phosphonium salt precursors derived from an ortho-phenylene bridge. However, while the Rh(CO)₂ complex bearing the P⁺-CH₂^- donor moiety proved to be stable, the P═NiPr donor end appeared to induce lability to one of the CO groups. All of the Rh^I carbonyl complexes in both ylide series were fully characterized, including through X-ray diffraction analysis. Based on the experimental and calculated infrared (IR) CO stretching frequencies in Rh(CO)₂ complexes, we evidenced that the phosphonium ylide ligand is a stronger donor than the iminophosphorane ligand. However, we also found that the difference in the intrinsic electronic properties can be largely compensated by the introduction of an iPr substituent on the N atom of the iminophosphorane, hence pointing to the noninnocent role of the peripheral substituent and opening novel possibilities to tune the properties of metal complexes containing ylide ligands.

Comparison of Imine- and Phosphinimine-Supported Indium Complexes: Tuning the Reactivity for the Sequential and Simultaneous Copolymerization of Lactide and ε-Caprolactone

Imine- and phosphinimine-supported indium complexes were used as catalysts in the polymerization of racemic lactide and ε-caprolactone as well as their copolymerization by the sequential and simultaneous addition of monomers. Tuning the electronics and sterics of the indium centers by either (i) changing the nature of the nitrogen donors and (ii) coordinating a hemilabile side group had a significant effect on the reactivity of the complexes, their stability, and their control in the synthesis of block copolymers. Specifically, the imine-supported complex (5) showed the highest activity in the homo- and copolymerization of the cyclic esters, in contrast to the phosphinimine-supported complex (7), which was significantly slower and less stable. The presence of morpholine and thiomorpholine hemilabile side groups either reduced the activity or prevented the formation of alkoxide complexes.

Indium phosphasalen catalysts showing high isoselectivity and activity in racemic lactide and lactone ring opening polymerizations

Study of a series of phosphasalen indium alkoxide complexes reveals that the substitution pattern at the phosphorus atoms can deliver outstanding isoselectivity with high rates.

Phosphasalen group IV metal complexes: synthesis, characterization and ring opening polymerization of lactide

A series of discrete mononuclear neutral and cationic phosphasalen Ti and Zr complexes has been isolated and characterized. For the first time, a cationic group IV metal complex was found active in the ROP of LA.

One-Electron Reduction of 2-Mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian)

The electrochemical characteristics of 2-mono(2,6-diisopropylphenylimino)acenaphthene-1-one (dpp-mian) have been investigated. One-electron reduction of dpp-mian involves the iminoketone fragment, which is revealed by the EPR spectrum obtained after the electrolysis of the dpp-mian solution in tetrahydrofuran (THF). The reduction of dpp-mian with one equivalent of metallic potassium leads to a similar EPR spectrum. The sodium complex [(dpp-mian)Na(dme)]₂ (1) produces an EPR signal with hyperfine coupling on the nitrogen atom of the iminoketone fragment of the dpp-mian ligand. Dpp-mian can also be reduced in a one-electron process by SnCl₂ ×(dioxane). In this case, complex (dpp-mian)₂ SnCl₂ (2) is formed, with the tin atom displaying an oxidation state of +4. Tin(II) chloride dihydrate, SnCl₂ ×2(H₂ O), also reduces dpp-mian, but the two ligands bound to tin in the product form a new carbon-carbon bond between the ketone moieties of the dpp-mian monoanions to form complex (bis-dpp-mian)HSnCl₃ (3). Metallic tin reduces dpp-mian to form the (bis-dpp-mian)₂ Sn (4) species. Compounds 1-4 were characterized by X-ray diffraction.

Zinc-catalyzed regioselective C–P coupling of p-quinol ethers with secondary phosphine oxides to afford 2-phosphinylphenols

A highly regioselective C–P coupling reaction of p-quinol ethers with secondary phosphine oxides is developed as a new synthesis method for 2-phosphinylphenols by using Zn(OTf)₂ as the catalyst.

Ruthenium complexes featuring cooperative phosphine–pyridine–iminophosphorane (PNN) ligands: synthesis, reactivity and catalytic activity

Ruthenium complexes with an iminophosphorane based (PNN) ligand; the NP substituent influences the coordination and the reactivity of the formed complexes.

Two isomers of a bis(diphenylphosphino)phosphinine, and the synthesis and reactivity of Ru arene/Cp* phosphinophosphinine complexes

Two isomers of a bis(diphenylphosphino)phosphinine have been synthesised, and the Ru coordination chemistry of a 2-phosphinophosphinine extended to include reactions of H₂O across a PC double bond.