Room-Temperature Intermolecular Hydroamination of Vinylarenes Catalyzed by Alkali-Metal Ferrate Complexes

Alkene hydroamination of multiple bonds represents a valuable and atom-economical approach to accessing amines, using simple and widely available starting materials. This reaction requires a metal catalyst, and despite the success of noble transition metals, s-block, or f-block elements, iron organometallic complexes have found limited applications. Partnering iron with an alkali metal and switching on bimetallic cooperativity, we report the synthesis and characterization of a series of highly reactive alkali-metal alkyl ferrate complexes, which can deprotonate amines and activate them toward the catalytic hydroamination of vinylarenes. An alkali-metal effect has been observed, with the sodium analogue being the best for an efficient hydroamination of different styrene derivatives and amines. Stoichiometric studies on the reaction of the sodium tris(alkyl) ferrate complex with 3 mol equiv of piperidine evidenced the ability of the three alkyl groups on Fe to undergo amine metalation, furnishing a novel tris(amido) sodium ferrate which is postulated as a key intermediate in these catalytic transformations. The enhanced reactivity of these alkali-metal ferrates contrasts sharply with that of the Fe(II) bis(alkyl) precursor which is completely inert toward alkene hydroamination.

Anilido-Oxazoline-Ligated Iron Alkoxide Complexes for Living Ring-Opening Polymerization of Cyclic Esters with Controllability

Anilido-oxazoline-ligated iron complexes, including bis(anilido-oxazolinate) iron(II), mononuclear iron(II) alkyl and aryloxide, as well as the dinuclear analogues, were synthesized, and their catalytic performance on ring-opening polymerization (ROP) has been studied. Transmetalation of FeCl2(THF)1.5 with in situ-generated anilido-oxazolinate lithium afforded the bis(anilido-oxazolinate) iron complexes 1 and 2. Half-sandwich anilido-oxazolinate iron trimethylsilylalkyl complexes 3 and 4 could be synthesized in good yields via taking pyridine as an L-type ligand. Treatment of 3 with benzyl alcohol and 4-phenoxyphenol, respectively, generated the dimeric alkoxide or aryloxide complexes 5 and 6, whereas the reaction with 2,4,6-trimethylphenol and 2,6-di-tert-butyl-4-methylphenol yielded the mononuclear aryloxide complexes 7 and 8, respectively. The iron alkoxide and aryloxide complexes were active single component catalysts for the ROP of ε-caprolactone (CL). Remarkably, the dinuclear complex 5 exhibited excellent controllability, livingness, and high initiation efficiency for ROP of CL. ROP of CL derivatives by 5 produced the corresponding polycyclic esters with good controllability, and the well-defined block copolymers could be generated by sequentially feeding different monomers. The chain initiation and propagation processes were investigated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and kinetics analysis. In addition, a computational study was conducted to rationalize the mechanism and synergistic effect of the alkoxide-bridged bimetallic iron centers.

Earth-Abundant 3d Transition Metal Catalysts for Hydroalkoxylation and Hydroamination of Unactivated Alkenes

This review summarizes the most noteworthy achievements in the field of C–O and C–N bond formation by hydroalkoxylation and hydroamination reactions on unactivated alkenes (including 1,2- and 1,3-dienes) promoted by earth-abundant 3d transition metal catalysts based on manganese, iron, cobalt, nickel, copper and zinc. The relevant literature from 2012 until early 2021 has been covered.

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T_1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, ⁵⁷Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)₂Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T_1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

A series of Fe(II) complexes with formazanate ligands are reported, and ligand substituent effects on structure and spin-crossover properties are examined. These ligand modifications allow isolation of compounds with tetrahedral geometries in both low- and high-spin ground states as well as an intermediate-spin square-planar complex. Steric properties, π-stacking interactions, and additional donor substituents lead to a wide range of spin-crossover temperatures (T_1/2) in this class of compounds.

Unexpected intramolecular N-arylcyano-β-diketiminate cyclization in new aminoquinoline derivative complexes of aluminium for CO2 fixation into cyclic carbonates

exo-Dig intramolecular cyclization of β-diketiminates allowed to obtain 4-amino-3-iminoquinolines ligands through a novel pathway. Al(III) complexes bearing these ligands where active towards the synthesis of cyclic carbonates from epoxides and CO₂.