Rolf W. Saalfrank

Ligand-to-metal ratio controlled assembly of tetra- and hexanuclear clusters towards single-molecule magnets.

A simple template-mediated route, starting from triethalolamine 1, sodium hydride or caesium carbonate, and iron(III) chloride led to the six- and eight-membered iron coronates [Na c [Fe6[N(CH2CH2O)3]6]]+ (2) and [Cs c (Fe8[N(CH2CH2O)3]8]]+ (3). In the reaction of N-methyldiethanolamine 4 (H2L1) or N-(2,5-dimethylbenzyl)iminodiethanol 6 (H2L2) with calcium hydride followed by addition of a solution of iron(III) chloride, the neutral unoccupied coronands [Fe6Cl6(L1)6] (5) and [Fe6Cl6(L2)6] (7) were formed. Subsequent exchange of the chloride ions of 7 by bromide or thiocyanate ions afforded the ferric wheels [Fe6Br6(L2)6] (8) or [Fe6(NCS)6(L2)6] (9), respectively. Titration experiments of solutions of dianion (L1)2- with iron(III) chloride in THF revealed interesting mechanistic details about the self-assembling process leading to 5. At an iron/ligand ratio of 1:1.5 star-shaped tetranuclear [Fe[Fe(L1)2]3] (11) was isolated. However, at an iron/ligand ratio of 1:2, complex 11 was transformed into the ferric wheel 5. It was shown, that the interconversion of 5 and 11 is reversible. Based on the mechanistic studies, a procedure was developed which works for both the synthesis of homonuclear 11 and the star-shaped heteronuclear clusters [Cr[Fe(L1)2]3] (12) and [Al[Fe(L1)2]3] (13). The structures of all new compounds were determined unequivocally by single-crystal X-ray analyses.

Crown Ethers, Double‐Decker, and Sandwich Complexes: Cation‐Mediated Formation of Metallatopomer Coronates

Metallacoronates or metallacrown ether sandwich complexes? As with organocrown ethers, the size of the encapsulated cation (Na+, Ca2+, or K+) determines which product is formed—a crown ether, a dimeric crown ether, or a sandwich complex (see below). H2L = ketipinate.

Self-assembly of tetrahedral and trigonal antiprismatic clusters [Fe4(L4)4] and [Fe6(L5)6] on the basis of trigonal tris-bidentate chelators.

In a one-pot reaction, the tetranuclear iron chelate complex [Fe4(L4)4] 6 was generated from benzene-1,3,5-tricarboxylic acid trichloride (4), bis-tert-butyl malonate (5a), methyllithium, and iron(II) dichloride under aerobic conditions. Alternatively, hexanuclear iron chelate complex [Fe(L5)6] 7 was formed starting from bis-para-tolyl malonate (5b) by employing identical reaction conditions to those applied for the synthesis of 6. The clusters 6 and 7 are present as racemic mixtures of homoconfigurational (delta,delta,delta,delta)/(lambda,lambda,lambda,lambda)-fac or (delta,delta,delta,delta,delta,delta)/(lambda,lambda,lambda,lambda,lambda,lambda)-fac stereoisomers. The structures of 6 and 7 were unequivocally resolved by single-crystal X-ray analyses. The all-iron(III) character of 6 and 7 was determined by Mössbauer spectroscopy.

The {FeIII[FeIII(L1)2]3} star-type single-molecule magnet

Star-shaped complex {FeIII[FeIII(L1)2]3} (3) was synthesized starting from N-methyldiethanolamine H2L1 (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mossbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3·4CHCl3 contains the enantiomers (Δ)-[(S,S)(R,R)(R,R)] and (Λ)-[(R,R)(S,S)(S,S)], whereas in case of 3·3CH2Cl2 four independent molecules, forming pairs of the enantiomers [Λ-(R,R)(R,R)(R,R)]-3 and [Δ-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

A meso-Helical Coordination Polymer from Achiral Dinuclear [Cu2(H3CCN)2(μ-pydz)3][PF6]2 and 1,3-Bis(diphenylphosphanyl)propane—Synthesis and Crystal Structure of {[Cu(μ-pydz)2][PF6]} (pydz=pyridazine)

Reaction of achiral [Cu2(H3CCN)2(mu-pydz)3][PF6]2 (1) (pydz = pyridazine) with bidendate 1,3-bis(diphenylphosphanyl)propane (2) in acetonitrile at room temperature in a 1:1 ratio yielded the mononuclear copper(I) complex [Cu[CH2(CH2PPh2)2]2][PF6] (3) together with new one-dimensional coordination polymer 1 to infinity[[Cu(mu-pydz)2][PF6]] (4). Air-sensitive single crystals of 4, suitable for X-ray structure determination, were grown from a mixture of dichloromethane/ hexane [crystal system: monoclinic; space group: C2/c: a = 21.910(3), b = 12.130(2), c = 25.704(3) A,beta = 110.08(10) degrees, V = 6416.65(16) A3]. The one-dimensional coordination polymer 1 to infinity[[Cu(mu-pydz)2][PF6]] (4) exhibits as outstanding feature the rare structure of a meso-helix.

Synergistic Effect of Serendipity and Rational Design in Supramolecular Chemistry

Many of the higher nuclearity Werner-type clusters are fortuitous discoveries. This clearly suggests that the principles which control cluster formation are still poorly understood from the point of view of rational design. However, the predictable nature of coordination chemistry has been used successfully for the specific generation of the metalla-topomers of the well-known organic-based coronates and cryptates. Furthermore, it has been shown that the metal coordination geometry and the orientation of the interacting sites in a given ligand provide the instruction, or blueprint, for the rational design of high symmetry coordination clusters. Based on these principles, and the interplay with serendipity, directed syntheses for polynuclear metalla-coronates, sandwich complexes, manifold metalla-cryptates, and metalla-cylinders have been developed. Only recently, the combination of detailed symmetry considerations with the basic protocols of coordination chemistry have made the design of rational strategies for the construction of a variety of nanoscale systems with procured shape and size feasible. Highlights among these species are cube-like clusters and multicompartmental cylindrical nanostructures.

SELF-ASSEMBLY OF A THREE-DIMENSIONAL GA6(L2)6 METAL-LIGAND CYLINDER

A near trigonal antiprism with metal–metal distances in the nanometer regime is formed by the six metal ions in the crystalline, homochiral [Ga6(L2)6] (see structure). This metal–ligand “cylinder” is based on a threefold symmetric, β-diketone ligand, and represents a new geometry for metal–ligand clusters.

Enantiomerisation of tetrahedral homochiral [M4L6] clusters: Synchronised four Bailar twists and six atropenantiomerisation processes monitored by temperature-dependent dynamic 1H NMR spectroscopy

Temperature-dependent 1H NMR studies prove homochiral, racemic [([symbol: see text])/([symbol: see text])]-((NH4)4[symbol: see text] [Mg4(L1)6]) (1) to be kinetically stable on the NMR timescale. Due to steric reasons, rotation around the central C-C single bond in (L1)2- is blocked, which prevents 1 from enantiomerisation. Most interestingly, however, the 1H NMR spectrum of racemic 2a reveals dynamic temperature dependence. This phenomenon can be explained by simultaneous Bailar twists at the four octahedrally coordinated magnesium centres, synchronised with the sterically unhindered atropenantiomerisation processes around the C-C single bonds of the six ligands (L2)2-, leading to the unprecedented enantiomerisation ([symbol: see text])-2a [symbol: see text] ([symbol: see text])-2a. The profound nondissociative rearrangement occurs without the formation of diastereoisomers. Supplementary support for the interpretation of the temperature-dependent dynamic 1H NMR spectra of 2a is presented by additional studies of [([symbol: see text])/([symbol: see text])]-((EtNH3)4 [symbol: see text] [Mg4(L2)6]) (2b). In 2a and 2b, the ether methylene protons exhibit identical temperature dependence. However, with addition, the methylene protons of the ethyl ammonium groups of 2b display similar temperature dependence as the ligand ether methylene protons.

Self‐Assembly of {2}‐Metallacryptands and {2}‐Metallacryptates

Reaction of H2L (1) with potassium or strontium hydride, or lanthanum(III) chloride, followed by iron(III) chloride, yielded the {2}-ironcryptates 2a–c. The mono-, di-, and trivalent guest cations are endohedrally encapsulated. In contrast, the dinuclear trispyridinium ironcryptand 2d was generated from the reaction of H2L (1) with only iron(III) chloride. The potassium metallacryptate 2a′ was formed from the triple-helicate 2d by addition of potassium carbonate. The new compounds 2b, 2c, and 2d were unequivocally characterised by X-ray diffraction analyses.

Metal-Directed Formation of Tetra-, Hexa-, Octa-, and Nonanuclear Complexes of Magnesium, Calcium, Manganese, Copper, and Cadmium

Efficacious metal control of self-assembly of dialkylketipinate dianions leads to completely different supramolecular assemblies. The structures of grid 1, double-decker 2, triple-decker 3, and metalla-spherand 4 were characterized by X-ray crystallographic analyses or by NMR spectroscopy.