Iron Nanoparticles Catalyzing the Asymmetric Transfer Hydrogenation of Ketones

Investigation into the mechanism of transfer hydrogenation using <i>trans-</i>[Fe­(NCMe)­CO­(PPh₂C₆H₄CHNCHR)₂]­[BF₄]₂, where R = H (<b>1</b>) or R = Ph (<b>2</b>) (from <i>R,R</i>-dpen), has led to strong evidence that the active species in catalysis are iron(0) nanoparticles (Fe NPs) functionalized with achiral (with <b>1</b>) and chiral (with <b>2</b>) PNNP-type tetradentate ligands. Support for this proposition is given in terms of <i>in operando</i> techniques such as a kinetic investigation of the induction period during catalysis as well as poisoning experiments using substoichiometric amounts of various poisoning agents. Further support for the presence of Fe(0) NPs includes STEM microscopy imaging with EDX analysis, XPS analysis, and SQUID magnetometry analysis of catalytic solutions. Further evidence of Fe NPs acting as the active catalyst is given in terms of a polymer-supported substrate experiment whereby the NPs are too large to permeate the pores of a functionalized polymer. Final support is given in terms of a combined poisoning/STEM/EDX experiment whereby the poisoning agent is shown to be bound to the Fe NPs. This paper provides evidence of a rare example of asymmetric catalysis with nonprecious metal, zerovalent nanoparticles

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π‑Radical and Its N–N σ‑Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at <i>T</i>_c↓ = 128(2) K and <i>T</i>_c↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the <i>P</i>1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (<i>S</i> = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (<i>S</i> = 0) and half retain their <i>S</i> = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π‑Radical and Its N–N σ‑Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at <i>T</i>_c↓ = 128(2) K and <i>T</i>_c↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the <i>P</i>1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (<i>S</i> = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (<i>S</i> = 0) and half retain their <i>S</i> = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π‑Radical and Its N–N σ‑Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at <i>T</i>_c↓ = 128(2) K and <i>T</i>_c↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the <i>P</i>1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (<i>S</i> = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (<i>S</i> = 0) and half retain their <i>S</i> = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition

Bimetallic Iron(3+) Spin-Crossover Complexes Containing a 2,2′-Bithienyl Bridging bis-QsalH Ligand

We describe the synthesis of a new 3,3′-diethynyl-2,2′-bithienyl bridging bis-QsalH ligand (5), and the preparation of four bimetallic iron(3+) complexes containing 5 with Cl− (6), SCN− (7), PF6− (8), and ClO4− (9) counteranions. We show with variable temperature magnetic susceptibility, Mössbauer, and electron paramagnetic resonance (EPR) spectroscopy that each complex undergoes a spin-crossover in the solid state. In all four complexes, we observe very gradual and incomplete S = 5/2, 5/2 to S = 1/2, 1/2 spin-crossover processes, with three of the four complexes exhibiting nearly identical magnetic properties. We investigated the electronic properties of the complexes by cyclic and differential pulse voltammetry, and attempted electropolymerization reactions with acetonitrile solutions of the complexes, which were not successful. Each complex features a single iron(3+) reduction wave at approximately −0.7 V (versus ferrocene), and the oxidation of the 2,2′-bithienyl substituent occurs at +1.1 V. These materials represent a new structural paradigm for the study of rare bimetallic iron(3+) spin-crossover complexes

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π‑Radical and Its N–N σ‑Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at <i>T</i>_c↓ = 128(2) K and <i>T</i>_c↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the <i>P</i>1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (<i>S</i> = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (<i>S</i> = 0) and half retain their <i>S</i> = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition

Bimetallic Iron(3+) Spin-Crossover Complexes Containing a 2,2′-Bithienyl Bridging bis-QsalH Ligand

We describe the synthesis of a new 3,3′-diethynyl-2,2′-bithienyl bridging bis-QsalH ligand (5), and the preparation of four bimetallic iron(3+) complexes containing 5 with Cl− (6), SCN− (7), PF6− (8), and ClO4− (9) counteranions. We show with variable temperature magnetic susceptibility, Mössbauer, and electron paramagnetic resonance (EPR) spectroscopy that each complex undergoes a spin-crossover in the solid state. In all four complexes, we observe very gradual and incomplete S = 5/2, 5/2 to S = 1/2, 1/2 spin-crossover processes, with three of the four complexes exhibiting nearly identical magnetic properties. We investigated the electronic properties of the complexes by cyclic and differential pulse voltammetry, and attempted electropolymerization reactions with acetonitrile solutions of the complexes, which were not successful. Each complex features a single iron(3+) reduction wave at approximately −0.7 V (versus ferrocene), and the oxidation of the 2,2′-bithienyl substituent occurs at +1.1 V. These materials represent a new structural paradigm for the study of rare bimetallic iron(3+) spin-crossover complexes

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π‑Radical and Its N–N σ‑Bonded Dimer

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at <i>T</i>_c↓ = 128(2) K and <i>T</i>_c↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the <i>P</i>1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (<i>S</i> = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (<i>S</i> = 0) and half retain their <i>S</i> = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition

A Bimodal Oxobenzene-bridged Bisdithiazolyl Radical Conductor

The preparation and structural characterization of the methyl-substituted oxobenzene-bridged bisdithiazolyl radical <b>3b</b> is described. Crystals of <b>3b</b> belong to the monoclinic space group <i>C</i>2/<i>c</i> and contain two distinct radical environments, <b>A</b> and <b>B</b>. There are eight <b>A</b> radicals in the unit cell, which occupy general positions and form alternating twisted π-stacks running parallel to the <i>c</i>-axis. The four <b>B</b> radicals also adopt an alternating π-stack pattern, but each molecule lies on a crystallographic 2-fold rotation axis, and the overlay of neighboring radicals is centrosymmetric. Stacks of <b>A</b> radicals are linked by close intermolecular S···O′ and S···N′ contacts into ribbon-like arrays that weave along the <i>y</i>-direction, and the <b>B</b> radical stacks are located in columnar cavities generated by the out-of-register alignment of the ribbons of <b>A</b> radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, a result in accord with DFT estimated exchange energies for intrastack radical–radical interactions. Four-probe conductivity measurements indicate a conductivity σ­(300 K) = 9.0 × 10^–4 S cm^–1, with a thermal activation energy <i>E</i>_act = 0.13 eV

Structure and Property Correlations in Heavy Atom Radical Conductors

The synthesis and solid-state characterization of the resonance-stabilized heterocyclic thia/selenazyl radicals 1a−4a is described. While all the radicals crystallize in undimerized slipped π-stacked arrays, the four crystal structures do not constitute an isomorphous set; crystals of 1a and 3a belong to the orthorhombic space group P212121, while those of 2a and 4a belong to the monoclinic space group P21/n. The origin of the structural dichotomy can be traced back to the packing of the radicals in the P21/n structure, which maximizes intermolecular Se−Se′ contacts. There are marked differences in the transport properties of the two groups. Variable temperature conductivity measurements reveal high, but activated, conductivity for the monoclinic pair (2a/4a), with σ(298 K) > 10−3 S cm−1. The application of physical pressure increases the conductivity of both compounds, with σ(298 K) at 5 GPa reaching 0.5 S cm−1 for 2a and 2 S cm−1 for 4a. Variable-temperature magnetic susceptibility measurements indicate strong antiferromagnetic (AFM) coupling for the monoclinic pair 2a and 4a, the behavior of which has been modeled in terms of a molecular-field modified 1D Heisenberg chain of AFM coupled S = 1/2 centers. Extended Hückel theory band structure calculations and density functional theory first principles methods have been used to develop a qualitative understanding of the conductive and magnetic properties of radicals of the type 1−4 as a function of the degree and direction of slippage of the radical π-stacks