Sebastian Dechert

A double-switching multistable Fe4 grid complex with stepwise spin-crossover and redox transitions.

One of the requirements for the development of novel materials for high-density information storage is switchability on the molecular level. Such molecular electronics can be realized by different intrinsic properties of the molecule. Spin-crossover (SCO) behavior, where molecular species (mostly iron(II) complexes) switch between high-spin (HS) and low-spin (LS) states, has long been discussed as a promising way of attaining bior even multistability. Whilst most SCO systems are mononuclear, in recent years significant progress has also been seen in the field of dinuclear (including pyrazolate bridged) and polynuclear SCO compounds. An interesting class of compounds in this context are grid complexes, which provide a matrix-like array of addressable sites whose size is even smaller than quantum dots. However, the number of grid-type Fe4 complexes has remained very limited, and only very few exhibit SCO bior multistability. 6,9] The first [2 2] Fe4 grid complexes of this type, which were based on pyrimidinederived compartmental ligand strands, were published by Lehn et al. in 2000 and showed continuous and incomplete spin transition between three magnetic levels. Some cyanide-bridged tetranuclear ferrous squares contain two lowspin and two SCO sites, where the latter show two-step spin conversion depending on the capping ligand. Recently Sato et al. reported an oxo-bridged Fe4 cluster that undergoes abrupt thermally induced and complete photoinduced spin transition, which surprisingly occurs on the same side of the ferrous square to give a cis-[LS-LS-HS-HS] geometric configuration at low temperatures. Alternatively, switching in molecular squares may be achieved electrochemically. A tetrairon [2 2] grid complex has recently been described that features four successive redox processes and forms a stable di-mixed-valence state. Such systems attract particular interest as potential “quantum cellular automata” (QCA) with four dots located at the vertices of a square. In these QCAs, binary information is encoded in the charge configuration of a molecular cell that is composed of differently charged redox centers, and not in the on/off state of a current switch. 11] Compartmental polypyridyl ligands are the most prominent class of ligand strands to induce the formation of gridtype complexes in a self-assembly process. We have recently developed a rigid pyrazole-bridged binucleating ligand L that provides two tridentate terpyridine-like binding pockets and forms very robust and compact grid complexes with various 3d transition metal ions, such as Co, Mn, and Cu. The anionic nature of this scaffold leads to a lower overall charge of the M4 square compared to, for example, the pyrimidine-based systems, which we expected to be beneficial for sequential oxidation up to M4 species. Herein we present a Fe4 [2 2] grid complex of L that shows an unprecedented multistability with respect to both spin transitions and redox transformations (Figure 1).

[NdI2(thf)5], the First Crystallographically Authenticated Neodymium(II) Complex

Two new starting materials for a new chemistry of low-valent lanthanide compounds are the first crystallographically characterized molecular complexes of the rare Nd2+ ion, [NdI2 (thf)5 ] (1), and [TmI2 (thf)(dme)2 ] (2; dme=1,2-dimethoxyethane). These have a pentagonal-bipyramidal structure in which the two iodine atoms are in axial positions.

Organometallic Compounds of the Lanthanides. 157 [1] The Molecular Structure of Tris(trimethylsilylmethyl)samarium, -erbium, -ytterbium, and -lutetium†

SmCl3 reacts with Me3SiCH2Li in THF yielding Sm(CH2SiMe3)3(THF)3 (1). The single crystal X-ray structural analyses of 1, Er(CH2SiMe3)3(THF)2 (2), Yb(CH2SiMe3)3(THF)2 (3), and Lu(CH2SiMe3)3(THF)2 (4) show the Sm atom in a fac-octahedral coordination and the heavier lanthanides Er, Yb, and Lu trigonal bipyramidally coordinated with the three alkyl ligands in equatorial and two THF molecules in axial positions. Metallorganische Verbindungen der Lanthanoide. 157 [1] Die Molekulstruktur von Tris(trimethylsilylmethyl)samarium, -erbium, -ytterbium und -lutetium SmCl3 reagiert mit Me3SiCH2Li in THF unter Bildung von Sm(CH2SiMe3)3(THF)3 (1). Die Einkristall rontgenstrukturanalysen von 1, Er(CH2SiMe3)3(THF)2 (2), Yb(CH2SiMe3)3(THF)2 (3) und Lu(CH2SiMe3)3(THF)2 (4) zeigen das Sm Atom fac-oktaedrisch und die schweren Lanthanoide Er, Yb und Lu trigonal bipyramidal koordiniert mit den drei Alkylliganden in aquatorialer und zwei THF Molekulen in axialer Position.

A new family of 1D exchange biased heterometal single-molecule magnets: observation of pronounced quantum tunneling steps in the hysteresis loops of quasi-linear {Mn2Ni3} clusters.

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.

Metallocenes of the alkaline earth metals and their carbene complexes

Abstract Novel alkaline earth metallocenes with various cyclopentadienyl ligands and their carbene adducts with 1,3-di-iso-propyl-4,5-dimethylimidazol-2-ylidene (iPr-carbene) are described. The syntheses of the magnesocenes (C5Me4H)2Mg (1), (C5Me4tBu)2Mg (2), and (C5H3-1-(SiMe3)-3-tBu)2Mg (3) were carried out by reaction of the corresponding cyclopentadiene with dibutylmagnesium. The calcocenes (C5Me4H)2(NH3)2Ca (6), (C5Me4H)2(NH3)(THF)Ca (7) and the barocene (C5Me4tBu)2(THF)2Ba (13) were prepaired by reaction of calcium and barium, respectively, with the corresponding cyclopentadienes. Refluxing 7 in THF–toluene yielded the ammonia-free complex (C5Me4H)2(THF)Ca (8). Metathesis reaction of (C5Me4iPr)Na with CaI2 gave the calcocene (C5Me4iPr)2(THF)Ca (9). (C5Me4tBu)2(py)2Ba (14) results from dissolving 13 in pyridine. 1,3-Di-iso-propyl-4,5-dimethylimidazol-2-ylidene reacted with 1, 3, 7, 8, 13, and (C5Me5)2(THF)2Sr in toluene or THF to give the air- and moisture-sensitive adducts (C5Me4H)2(iPr-carbene)Mg (4), (C5H3-1-(SiMe3)-3-tBu)2(iPr-carbene)Mg (5), (C5Me4H)2(iPr-carbene)Ca (10), (C5Me4iPr)2(iPr-carbene)Ca (11), (C5Me5)2(iPr-carbene)Sr (12), and (C5Me4tBu)2(iPr-carbene)Ba (15). The novel compounds have been characterized by elemental analysis, mass spectrometry and NMR spectroscopy as well as the X-ray structures of the metallocenes (1, 2, 6, 9, 13) and the carbene adducts (4, 10, 11, 12, 15).

Reversible Solvatomagnetic Effect in Novel Tetranuclear Cubane-Type Ni4 Complexes and Magnetostructural Correlations for the [Ni4(μ3-O)4] Core

A new family of tetranuclear nickel cube complexes [Ni(4)L(4)(solv)(4)] (1, solv = MeOH; 2, solv = H(2)O; H(2)L = pyrazole-based tridentate {ONO} ligand) has been studied in detail, in particular by X-ray diffraction and superconducting quantum interference device (SQUID) magnetometry. Different solvates 1·H(2)O, 2·4C(3)H(6)O, 2·CH(2)Cl(2), and 2·H(2)O were obtained in crystalline form. Only small structural variations were found for the Ni-O-Ni angles of the [Ni(4)O(4)] cores of those compounds, but these slight variations have dramatic consequences for the magnetic properties. [Ni(4)L(4)(MeOH)(4)]·H(2)O (1·H(2)O) and [Ni(4)L(4)(H(2)O)(4)]·H(2)O (2·H(2)O) can be reversibly interconverted in the solid state by exposure to the respective solvent, MeOH or H(2)O, and this goes along with a switching of the spin ground state from magnetic (S(T) = 4) to diamagnetic (S(T) = 0). Likewise the (irreversible) loss of lattice solvent in [Ni(4)L(4)(H(2)O)(4)]·4C(3)H(6)O (2·4C(3)H(6)O) to give 2·2C(3)H(6)O changes the ground state from S(T) = 4 to S(T) = 0. In view of these dramatic solvatomagnetic effects for the present [Ni(4)L(4)(solv)(4)] complexes, which occur upon extrusion of lattice solvent or facile exchange of coordinated solvent molecules while keeping the robust [Ni(4)O(4)] core intact, a note of care is issued: whenever magnetic data are obtained for powdered material or for crystals that easily loose lattice solvent molecules, the magnetic properties may not necessarily reflect the situation observed in the corresponding single crystal diffraction study. Finally, a thorough analysis of the present series of complexes as well as other {Ni(4)(μ(3)-OR)(4)} cubes reported in the literature confirms that a correlation between the (Ni-O-Ni)(av) bond angle and J in [Ni(4)O(4)] cubane complexes does indeed exist.

Homoleptische Amide von Zink, Cadmium und Quecksilber

ZnCl2, CdCl2 und HgCl2 reagieren mit den Lithiumsalzen (1 a–5 a) der sterisch anspruchsvollen sekundaren Amine HN(SiMe3)Ph (1), HN(SiMe3)C6H3Me2-2,6 (2), HN(SiMe3)C6H3iPr2-2,6 (3), HN(SiMe3)C6H3tBu2-2,5 (4) und HN(SiMe2NMe2)C6H3iPr2-2,6 (5) unter Bildung der entsprechenden homoleptischen Metallamide Zn[N(SiMe2R′)R]2 (1 b–5 b), Cd[N(SiMe2R′)R]2 (1 c, 5 c) und Hg[N(SiMe2R′)R]2 (1 d–5 d). Mit Ausnahme des dimeren {Zn[N(SiMe3)Ph]2}2 (1 b) sind alle Verbindungen monomer. Die Charakterisierung der Verbindungen erfolgte durch Elementaranalysen, Molekulargewichtsbestimmungen, NMR- und Massenspektren. Die daruberhinaus durch Einkristall-Rontgenstrukturanalysen charakterisierten Zink- (1 b–5 b) und Quecksilberverbindungen (1 d–3 d und 5 d) zeigen mit Ausnahme von 1 b und 5 b eine lineare N–M–N Anordnung. Homoleptic Amides of Zinc, Cadmium, and Mercury ZnCl2, CdCl2 and HgCl2 react with the lithium salts (1 a–5 a) of the sterically demanding secundary amines HN(SiMe3)Ph (1), HN(SiMe3)C6H3Me2-2,6 (2), HN(SiMe3)C6H3iPr2-2,6 (3), HN(SiMe3)C6H3tBu2-2,5 (4), and HN(SiMe2NMe2)C6H3iPr2-2,6 (5) yielding the corresponding homoleptic metal amides Zn[N(SiMe2R′)R]2 (1 b–5 b), Cd[N(SiMe2R′)R]2 (1 c, 5 c), and Hg[N(SiMe2R′)R]2 (1 d–5 d), respectively. Except the dimeric {Zn[N(SiMe3)Ph]2}2 (1 b), all complexes are monomeric. The compounds were characterized by elemental analyses, molecular weight determinations, NMR and mass spectra. Furthermore, the zinc amides (1 b–5 b) and the mercury amides 1 d–3 d and 5 d were characterized by single crystal X-ray structure analysis. Except 1 b and 5 b, they show a linear N–M–N arrangement.

Genuine Redox Isomerism in a Rare‐Earth‐Metal Complex

Redox isomerism is observed for a lanthanide complex for the first time. Upon lowering the temperature, an electron of [{(dpp-bian)Yb(μ-Cl)(dme)}(2)] (1) is transferred from the metal to the ligand (see picture), giving rise to marked shortening of Yb-N bonds and a hysteretic jump in the magnetic moment. The crystal packing is of a crucial importance, as two other crystal modifications of 1 do not undergo this effect.

Targeted construction of azido-bridged Ni4 complexes with decisive effect of µ-1,3-azide torsion on the spin ground state

Highly preorganized pyrazolate-based dinickel(II) systems are shown to constitute suitable building blocks for the targeted assembly of azido-bridged Ni4 complexes with rectangular arrangement of the metal ions. A set of such complexes has been prepared and structurally characterized. mu-1,1-Azide binding within the bimetallic sub-units is controlled by the chosen topology of the pyrazolate ligand scaffold and gives rise to the anticipated ferromagnetic intradimer coupling. Overall magnetic properties of the Ni4 array, however, are mainly determined by the Ni-NNN-Ni torsion of the interdimer mu-1,3-azido linkages. According to the crystallographic results, these torsion angles vary over a wide range, and partial disorder of the mu-1,3-azide bridge in one of the compounds indicates high structural flexibility even in the solid state. Two of the compounds represent rare examples of molecular complexes with a Ni-NNN-Ni torsion angle of almost exactly 90 degrees . The resulting magnetic ground state (neglecting zero-field splitting) is either S = 0 or S = 4 depending on the Ni-NNN-Ni torsion, and in one case a drastic change is observed upon extrusion of lattice solvent.

Hydrotris(indazolyl)borate, Tp4Bo, a surprisingly effective Tp ligand for supramolecular assembly

The assembly of TlTp 4Bo and M(Tp 4Bo )2 (M e.g. Co, Ni) in the crystal is dictated by aromatic p-p and C-H˘p interactions. This leads to the formation of chiral C2-symmetrical pairs (for Tl) or solvent-filled, about 30%-porous, non-covalent framework structures(for Co and Ni). The potassium salt KTp 4Bo features a kN,h-C-C,h 2 -C-N cation-p coordination mode — curious for a Tp-type ligand. q2000 Elsevier Science S.A. All rights reserved.