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Markus Albrecht

Publications and source records attributed to Markus Albrecht.

At least 19 recordsLinked to original sources
PubMed2006

Dicatechol cis-dioxomolybdenum(VI): a building block for a lithium cation templated monomer-dimer equilibrium.

Dinuclear helicate-type complexes form from 3-carbonyl catechol derivatives and MoO2 units. The two monomeric subunits are non-covalently bound through templating Li+ ions. The monomeric and dimeric complexes as well as a hydrolysis product have been investigated in the solid state by X-ray crystallography, in solution by NMR experiments, and in the gas phase by ESI mass spectrometry. Dimerization constants can easily be extracted from NMR experiments. A particular property of these complexes is the mutual homochiral recognition between the two halves of the dimers which only occurs between homochiral monomers.

Journal Article↗
PubMed2006

Self-assembly and host-guest chemistry of big metallosupramolecular M4L4 tetrahedra.

Metallosupramolecular tetrahedra M8[L4Ti4] are easily obtained by self-assembly from the triangular ligands L-H6 and titanoyl bis(acetylacetonate) in the presence of alkali metal carbonates as base. All the complexes can be well characterized by 1H NMR in combination with ESI FT-ICR MS. Force field calculations reveal that the tetrahedra show Ti-Ti separations of 17 angstroms ([L1(4)Ti4]8-) and 23.5 angstroms ([L2(4)Ti4]8-), respectively, leading to huge internal cavities. The cavity is readily shielded in the case of L1 but possesses big pores with the bigger ligand L2. [L1(4)Ti4]8- was used to investigate the host-guest chemistry of these container molecules and it was found that cationic organic guest species like anilinium can be introduced in the interior of the complex. Inclusion is nicely followed by NMR spectroscopy. Upon addition of one equivalent of guest the symmetry of the tetrahedron is lost but is regained after addition of significantly more than four equivalents.

Journal Article↗
PubMed2005

2-[(8-Hydroxyquinolinyl)methylene]hydrazinecarboxamide: expanding the coordination sphere of 8-hydroxyquinoline for coordination of rare-earth metal(III) ions.

The semicarbazone of 8-hydroxyquinoline-2-carbaldehyde can be easily synthesized and is an effective tetradentate ligand for the coordination of rare-earth(III) ions. Investigations with yttrium(III) and lanthanum(III) in solution and in the solid state show, that the small yttrium ion can form 2 : 2 (1 : 1 stoichiometry) and 2 : 1 ligand to metal complexes (X-ray structures: [LY(NO3)(DMF)2]2Cl2 x 2DMF and [LL'Y] x 3MeOH x Et2O). With the larger lanthanum(III) ion only a well defined 1 : 1 complex (X-ray structure: [LLa(NO3)(MeOH)2]2(NO3)2) can be observed but probably 2 : 1 complexes are also formed. The X-ray structure analyses of [(L-H)MCl3] x MeOH (M = Er, Ho) and Na[(micro-NO3){LEu(NO3)2}2] x 2DMF show different coordination modes of the ligand. It can coordinate in its deprotonated but also in the protonated form.

Journal Article↗
PubMed2005

5,5'-Diamino-2,2'-bipyridine: a versatile building block for the synthesis of bipyridine/catechol ligands that form homo- and heteronuclear helicates.

Herein we present an improved synthesis of 5,5'-diamino-2,2'-bipyridine (1) starting from the pyrrole-protected aminopyridine 4. By standard reactions 1 can easily be transformed into the imine- or amide-bridged dicatechol-bipyridine ligands L1-H4 and L2-H4. Whereas ligand L1 readily forms homodinuclear helicates [(L1)3Ti2]4-, the attempted formation of mono-, tri-, or even oligonuclear coordination compounds from this ligand did not work. However, the amide-connected ligand L2 affords mononuclear ([(L2-H4)PdCl2], [(L2-H4)3Zn]2+), dinuclear ([(L2)3Ti2]4-), and heterotrinuclear coordination compounds ([(L2)3Ti2Zn]2-).

Journal Article↗
PubMed2005

Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes.

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.

Journal Article↗
PubMed2005

Metallacyclopeptides: artificial analogues of naturally occurring peptides.

Naturally occurring metalloproteins contain metal ions either to introduce a special reactivity or to stabilize a peptide structure. Since the early 1990s, chemists have been trying to use metal coordination for the fixation of short artificial peptides in well-defined cyclic structures. In this tutorial review a survey of the general approaches towards metallacyclopeptides as small cyclic peptide derivatives or as a part of a bigger alpha-helix (or beta-sheet) structure is given. In three case studies it is shown how naturally occurring compounds can be mimicked by metal coordination to non-natural peptide derivatives.

Metalloproteins↗
PubMed2004

Catechol imine ligands: from helicates to supramolecular tetrahedra.

Di- and tricatechol imines are easily accessible by condensation of appropriate amines with 2,3-dihydroxybenzaldehyde. Dicatechol imines can be used for the alkali metal template-directed self-assembly of dinuclear triple-stranded helicates or meso-helicates with high diastereoselectivity. Tricatechol imines lead in self-assembly processes to metallosupramolecular 4 [ratio] 4 tetrahedra with a huge internal cavity, which is able to encapsulate guest species. Hereby the special features of the imine unit can be used to control the outcome of the self-assembly process.

Journal Article↗
PubMed2004

Preparation of tripeptide-bridged dicatechol ligands and their macrocyclic molybdenum(VI) complexes: fixation of the RGD sequence and the WKY sequence of urotensin ii in a cyclic conformation.

Dicatechol ligands were prepared with caprylic acid (6-H(4)) or the naturally occurring RGD (23-H(4)) or WKY sequences (32-H(4)) as spacers. 6-H(4) was prepared by solution-phase amide coupling chemistry, while 16, the precursor of 23-H(4), was obtained by solution-phase and solid-phase preparation. In the latter case, a polystyrene resin with a hydrazine benzoate linker was used as the solid support. The last coupling step was performed simultaneously with cleavage of the peptide from the resin. The protecting groups of 16 were all removed in one step to yield the free ligand 23-H(4). The WKY-bridged derivative 32-H(4) was obtained by a similar solid-phase synthesis followed by deprotection. The reaction of all three ligands with dioxomolybdenum(VI) bis(acetylacetonate) afforded 19-membered metallamacrocycles in which the short peptides are conformationally fixed in a turn-type structure. Hereby, the side-chain functionalities of the peptides do not interfere in the metal complexation.

Amino Acid Sequence↗
PubMed2004

Long-range stereocontrol in the self-assembly of two-nanometer-dimensioned triple-stranded dinuclear helicates.

A series of bisimine-bridged dicatechol ligands 2-H(4)-5-H(4) were synthesized and were used to prepare triple-stranded dinuclear helicate-type complexes with a length of up to more than 2 nm. X-ray structural analyses of Na(4)[(2)(3)V(2)], Na(4)[(3)(3)Ti(2)], Na(4)[(4)(3)Ti(2)], and Na(4)[(5)(3)Ti(2)], as well as temperature-dependent NMR investigations of Na(4)[(4)(3)Ti(2)] and Na(4)[(5)(3)Ti(2)] show that, in the case of the rigid linear ligands 2 and 3, and of the ligand 5, which possesses C(2h) symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4, on the other hand, with idealized C(2v) symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso-helicate. This is attributed to the ability of ligand 4 to adopt a less-restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na(4)[(4)(3)Ti(2)] invert (LambdaDelta-->DeltaLambda) simultaneously, while in the case of Na(4)[(5)(3)Ti(2)] a stepwise racemization proceeds.

Catechols↗
PubMed2004

Approaching supramolecular functionality.

Functional molecules require a high degree of complexity which is difficult to achieve by covalent synthesis. This article discusses supramolecular approaches to the creation of larger architectures through noncovalent bonds, self-assembly, and template strategies. It highlights selected examples for the structural and conformational control of function and attempts to identify difficulties and challenges which may arise in future.

Journal Article↗
PubMed2003

Dicatechol-diimines: easily accessible ligands for the self-assembly of dinuclear triple-stranded helicates.

Dicatechol ligands 3b-g-H4 are simply prepared by imine formation of 2,3-dihydroxybenzaldehyde 2 with a series of different diamines 1b-g . An X-ray structural analysis was obtained for the butyl-bridged compound 3e-H4, showing an intramolecular proton transfer and the formation of a chinoidic "keto-amine" structure. The dicatechol derivatives 3b-g-H4 form dinuclear triple-stranded helicates M4[(3)3Ti2] with titanium(IV) ions in the presence of alkali-metal carbonate. For the phenyl- and the trans-1,4-cyclohexyl-bridged complexes, K4[(3b)3Ti2] and Na4[(3f)3Ti2], X-ray structures were obtained.

Journal Article↗
PubMed2002

"Size-selectivity" in the template-directed assembly of dinuclear triple-stranded helicates.

The self-assembly of supramolecular structures depends on a subtle interplay of a series of different control mechanisms. The geometric as well as electronic complementarity of the molecular building blocks is crucial for the specific formation of defined supramolecular species. In addition, secondary effects, like templating, also have an important function. The templating ability of different cations in the formation of triple-stranded helicate-type complexes from alkyl-bridged di(8-hydroxyquinoline) ligands is investigated by introduction of alkyl chains of different length as ligand spacers. Hereby a "size-selectivity" between the cations and the dinuclear helicate-type complexes [(ligand)(3)M(2)] is observed. Large cations support the formation of big dinuclear complexes, whereas small cations are able to template the formation of small complexes.

Aluminum↗