Temperature-Sensitive Localized Surface Plasmon Resonance of α-NiS Nanoparticles

The presented work shows a synthesis route to obtain nanoparticles of the hexagonal α-NiS phase and core-shell particles where the same material is grown onto previously prepared Au seeds. In the bulk, this nickel sulfide phase is known to exhibit a metal-insulator type phase transition (MIT) at 265 K which drastically alters its electrical conductivity. Since the produced nanoparticles show a localized surface plasmon resonance (LSPR) in the visible range of the electromagnetic spectrum, the development of their optical properties depending on the temperature is investigated. This is the first time an LSPR of colloidal nanoparticles is monitored regarding such a transition. The results of UV-vis absorbance measurements show that the LSPR of the particles can be strongly and reversibly tuned by varying the temperature. It can be switched off by cooling the nanoparticles and switched on again by reheating them above the transition temperature. Additional to the phase transition, the temperature-dependent magnetic susceptibility of α-NiS and Au-NiS nanoparticles suggests the presence of different amounts of uncompensated magnetic moments in these compounds that possibly affect the optical properties and may cause the observed quantitative differences in the LSPR response of these materials

Controlled ligand distortion and its consequences for structure, symmetry, conformation and spin-state preferences of iron(II) complexes

The ligand-field strength in metal complexes of polydentate ligands depends critically on how the ligand backbone places the donor atoms in three-dimensional space. Distortions from regular coordination geometries are often observed. In this work, we study the isolated effect of ligand-sphere distortion by means of two structurally related pentadentate ligands of identical donor set, in the solid state (X-ray diffraction, Fe-57-Mossbauer spectroscopy), in solution (NMR spectroscopy, UV/Vis spectroscopy, conductometry), and with quantum-chemical methods. Crystal structures of hexacoordinate iron(II) and nickel(II) complexes derived from the cyclic ligand L-1 (6-methyl-6-(pyridin-2-yl)-1,4-bis(pyridin-2-ylmethyl)-1,4-diazepane) and its open-chain congener L-2 (N-1,N-3,2-trimethyl-2-(pyridine-2-yl)-N-1,N-3-bis(pyridine-2-ylmethyl) propane-1,3-diamine) reveal distinctly different donor set distortions reflecting the differences in ligand topology. Distortion from regular octahedral geometry is minor for complexes of ligand L-2, but becomes significant in the complexes of the cyclic ligand L-1, where trans elongation of Fe-N bonds cannot be compensated by the rigid ligand backbone. This provokes trigonal twisting of the ligand field. This distortion causes the metal ion in complexes of L-1 to experience a significantly weaker ligand field than in the complexes of L-2, which are more regular. The reduced ligand-field strength in complexes of L-1 translates into a marked preference for the electronic high-spin state, the emergence of conformational isomers, and massively enhanced lability with respect to ligand exchange and oxidation of the central ion. Accordingly, oxoiron(IV) species derived from L-1 and L-2 differ in their spectroscopic properties and their chemical reactivity.DFG, EXC 314, Unifying Concepts in Catalysi

Realizing Shape and Size Control for the Synthesis of Coordination Polymer Nanoparticles templated by Diblock Copolymer Micelles

The combination of polymers with nanoparticles offers the possibility to obtain customizable composite materials with additional properties such as sensing or bistability provided by a switchable spin crossover (SCO) core. For all applications, a precise control over size and shape of the nanomaterial is highly important as it will significantly influence its final properties. By confined synthesis of iron(II) SCO coordination polymers within the P4VP cores of polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) micelles in THF we are able to control the size and also the shape of the resulting SCO nanocomposite particles by the composition of the PS-b-P4VP diblock copolymers (dBCPs) and the amount of complex employed. For the nanocomposite samples with the highest P4VP content, a morphological transition from spherical nanoparticle to worm-like structures was observed with increasing coordination polymer content, which can be explained with the impact of complex coordination on the self-assembly of the dBCP. Furthermore, the SCO nanocomposites showed transition temperatures of T1/2 = 217 K, up to 27 K wide hysteresis loops and a decrease of the residual high-spin fraction down to γHS = 14% in the worm-like structures, as determined by magnetic susceptibility measurements and Mössbauer spectroscopy. Thus, SCO properties close or even better (hysteresis) to those of the bulk material can be obtained and furthermore tuned through size and shape control realized by tailoring the block length ratio of the PS-b-P4VP dBCPs

Closing the Gap: Preparation and Characterization of the First Half-Open and Open Manganocene Complexes

The first preparations of half-open and open manganocenes were accomplished. Treatment of KPdl′ (Pdl′ = 2,4-(Me₃C)₂C₅H₅)) with [{(η⁵-Cp”)­Mn­(thf)­(μ-I)}₂] (Cp″ = 1,2,4-(Me₃C)₃C₅H₂) and MnI₂(thf)₂ resulted in the formation of [(η⁵-Cp″)­Mn­(Pdl′)] (<b>2</b>) and [(Pdl′)₂Mn] (<b>4</b>), respectively. Both compounds adopt a high-spin (<i>S</i> = 5/2) ground state. Maximum spin states are rather unusual for pentadienyl complexes, since these ligands generally stabilize transition metal complexes in their low-spin state. In addition, the electronic structure of <b>2</b> was compared to its closed analogue [(η⁵-Cp″)­Mn­(η⁵-Cp′)] (Cp′ = 1,3-(Me₃C)₂C₅H₃), which also adopts the high-spin configuration because of steric hindrance destabilizing the electronically more favorable low-spin state. Reaction of KPdl′ with [(C₅H₅)₂Mn] yields the trimetallic compound [{(η⁵-Pdl′)­Mn­(η⁵-C₅H₅)]}₂Mn] (<b>5</b>) concomitant with the formation of 2,4,7,9-tetra-<i>tert</i>-butyl-1,3,7,9-decatetraene (Pdl′₂), indicating reduction of two Mn atoms. Solid-state magnetic susceptibility studies and density functional theory computations suggest that the electronic structure in <b>5</b> is best described as two diamagnetic (<i>S</i> = 0) [(η⁵-Pdl′)­Mn­(η⁵-C₅H₅)]⁻ Mn­(I) anions, each being coordinated to a central Mn­(II) cation with a high-spin (<i>S</i> = 5/2) configuration

Iron(I) and Iron(II) Amido-imidazolin-2-imine Complexes as Catalysts for H/D Exchange in Hydrosilanes

The unsymmetrical amino-imidazolin-2-imine ligand [HAmIm, 1,2-(DippNH)–C6H4–N=C(NiPrCMe)2] is employed in the synthesis of the iron(I) arene complex [(AmIm)Fe(η6-C6H6)] and the iron(II) neosilyl complex [(AmIm)Fe(CH2SiMe3)]. These compounds are highly efficient precatalysts in H/D exchange reactions with deuterium (D2) in hydrosilanes. The scope comprises primary to tertiary silanes at a catalyst loading of 1 mol % at ambient temperature. In-depth mechanistic studies including various control experiments and the syntheses of isolated iron-hydride and iron-silyl compounds are performed. These studies reveal that the activation of both Fe(I) and Fe(II) complexes generates Fe–H/D species as key catalytic intermediates. An alternative catalytic pathway involving an iron-silyl intermediate, although shown to be less feasible by DFT calculations, may also be operative

Iron(I) and Iron(II) Amido-imidazolin-2-imine Complexes as Catalysts for H/D Exchange in Hydrosilanes

The unsymmetrical amino-imidazolin-2-imine ligand [HAmIm, 1,2-(DippNH)–C6H4–N=C(NiPrCMe)2] is employed in the synthesis of the iron(I) arene complex [(AmIm)Fe(η6-C6H6)] and the iron(II) neosilyl complex [(AmIm)Fe(CH2SiMe3)]. These compounds are highly efficient precatalysts in H/D exchange reactions with deuterium (D2) in hydrosilanes. The scope comprises primary to tertiary silanes at a catalyst loading of 1 mol % at ambient temperature. In-depth mechanistic studies including various control experiments and the syntheses of isolated iron-hydride and iron-silyl compounds are performed. These studies reveal that the activation of both Fe(I) and Fe(II) complexes generates Fe–H/D species as key catalytic intermediates. An alternative catalytic pathway involving an iron-silyl intermediate, although shown to be less feasible by DFT calculations, may also be operative

Low-Coordinate Iron(II) Amido Half-Sandwich Complexes with Large Internal Magnetic Hyperfine Fields

The half-sandwich complex [Cp′Fe{N(dipp)(SiMe3)}] (Fe-dipp; Cp′ = 1,2,4-tri-tert-butylcyclopentadienyl and dipp = 2,6-diisopropylphenyl) and the mixed metallocene [Cp′Fe{(η5-C6H3iPr2)N(SiMe3)}] (Fe-chd) formed in the reaction between [{Cp′Fe(μ-I)}2] and [Li{N(dipp)(SiMe3)}]2 were characterized by NMR spectroscopy and X-ray diffraction analysis. Fe-dipp complements the series of low-coordinate, quasi-linear iron amido half-sandwich complexes [Cp′Fe{N(tBu)(SiMe3)}] (Fe-tBu) and [Cp′Fe{N(SiMe3)2}] (Fe-tms) reported earlier, and all three compounds were characterized in the solid state by zero-field 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements, confirming their S = 2 electronic ground state. Moreover, the Mössbauer absorption spectra reveal slow paramagnetic relaxation at low temperatures with large internal magnetic hyperfine fields of Bhf = 96.4 T (Fe-dipp, 20 K), Bhf = 101.3 T (Fe-tBu, 15 K), and Bhf = 96.9 T (Fe-tms, 20 K). The magnetic measurements further confirm that the presence of significant axial zero-field splitting and slow relaxation of magnetization is detected, which is revealed even in the absence of a static magnetic field in the case of Fe-tBu. Supplementary ab initio and density functional theory calculations were performed and support the experimental data

Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited

The pyrrolyl-based iron pincer compounds [(^<i>t</i>BuPNP)­FeCl] (<b>1</b>), [(^<i>t</i>BuPNP)­FeN₂] (<b>2</b>), and [(^<i>t</i>BuPNP)­Fe­(CO)₂] (<b>3</b>) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field ⁵⁷Fe Mössbauer and X-band electron paramagnetic resonance spectroscopy. While the iron­(II) starting material <b>1</b> adopts an intermediate-spin (<i>S</i> = 1) state, the iron­(I) reduction products <b>2</b> and <b>3</b> exhibit a low-spin (<i>S</i> = ¹/₂) ground state. Consistent with an intermediate-spin configuration for <b>1</b>, the zero-field ⁵⁷Fe Mössbauer spectrum shows a characteristically large quadrupole splitting (Δ<i>E</i>_Q ≈ 3.7 mm s^–1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|<i>D</i>| ≈ 37 cm^–1). The effective magnetic moments observed for the iron­(I) species <b>2</b> and <b>3</b> are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin–orbit coupling in the ground state. The experimental findings are complemented by density functional theory computations, which are in good agreement with the experimental data. Most notably, these calculations reveal a low-lying (<i>S</i> = 2) excited state for complex <b>1</b>; furthermore, the computed Mössbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings