under construction

General Research Statement:

Research interests in my group focus on the following fields: (i) organometallic chemistry, (ii) coordination chemistry with rare earth metals and to some degree with copper and main group elements, and (iii) applications of these compounds in catalysis and the design of novel materials. Special emphasis is given to the usage of sterically crowded aryl substituents, x-ray crystallography as analytic tool, and the application of quantum chemical methods (ab initio, DFT) for the interpretation of spectroscopic data and the analysis of week bonding forces (e.g. agostic and metal-p-arene interactions).

Biphenyl and m-Terphenyl Based Ligand Systems:

Ligands with the m-terphenyl scaffold (systematic name: 1,1':3',1''-terphenyl-2'-yl) have been used quite successfully in the last couple of years for the stabilization of unusual, in particular main group element compounds [1, 2]. Some advantages of these ligands and related sterically crowded biphenyl systems [3] may be summarized as follows: (i) easy large-scale synthesis, (ii) variation of the substituents R on the peripheric arene rings allows to control parameters like steric and electronic properties, solubility, and crystallization behaviour, (iii) additional p-arene interactions of bonded metal atoms to the flanking aryl groups often account for the isolation of s-donorfree compounds and therefore allow avoidance of coordinating solvents. We use biphenyl and m-terphenyl based or modified ligands for the synthesis of lanthanide, copper or main group metal organyls, chalcogenolates, triazenides, and silyls with unusual low coordination numbers and reactivity. Details on some running and finished projects are given below.

[1]   B. Twamley, S. T. Haubrich, P. P. Power, Adv. Organomet. Chem., 1999, 44, 1.
[2]   J. A. C. Clyburne, N. McMullen, Coord. Chem. Rev., 2000, 210, 73.
[3]   N. Hartmann, M. Niemeyer, Synth. Commun., 2001, 31, 3839. (DOI)

Sterically Crowded Triazenides as Novel Ancillary Ligands:

The design and development of alternative ligand systems capable of stabilizing monomeric metal complexes while provoking novel reactivity remains one of the most intensely studied areas of organometallic chemistry. Exploration of this field is driven by the potential use of these compounds in catalysis and organic synthesis. Examples of monoanionic chelating N-donor ligands which have received much recent attention include the b-diketiminate (I) and the amidinate (II) ligand systems. Much less attention has been given to the closely related triazenides (III). This may be attributed to the lack of suitable ligands which are sterically crowded enough to prevent undesirable ligand redistribution reactions and allow better control of the electronic and steric properties at the metal.

We recently succeeded in the preparation of sterically crowded diaryl triazenes. Steric, electronic, and solubility properties are easily controlled by the variation of the substituents on the pending aryls groups. The obtained diaryl triazenes were used to stabilize novel base-free pentafluorophenyl compounds of europium and ytterbium [14] and to prepare the first well-characterized metal aryls of the heavier alkaline earth metals calcium, strontium and barium [15]. In the solid-state and in solution kinetic stabilization through steric and electronic saturation of the metal centers is achieved by additional p-arene interactions to the pendent aryl substituents. As a result decomposition pathways involving ortho-fluoride elimination are effectively blocked.

Higher aggregated alkali-metal compounds are usually obtained with increasing radius of the metal. Alkali-metal salts derived from the sterically crowded triazenido ligand Tph2N3H [Tph = C6H3- 2,6-(C6H2-2,4,6-iPr3)2] do not obey this principle [16]. Interestingly, these compounds show inverse aggregation behavior in the solid state: the potassium and cesium salts crystallize as discrete monomers in which the cations interact with flanking arene rings of the diaryltriazenido ligands, whereas the lithium derivative is dimeric with a more conventional heteroatom-bridged structure.

[14]   S.-O. Hauber, M. Niemeyer, Inorg. Chem., 2005, 44, 8644. (DOI)
[15]   S.-O. Hauber, F. Lissner, G. B. Deacon, M. Niemeyer, Angew. Chem. Int. Ed., 2005, 44, 5871. (DOI)
[16]   H.-S. Lee, M. Niemeyer, Inorg. Chem., 2006, 45, 6126. (DOI)

m-Terphenyl-substituted Chalcogenolates:

We have prepared a series of base-free, hydrocarbon-soluble complexes Ln(SAr*)2 (Ln = Sm, deep purple; Eu, orange; Yb, purple; Ar* = 2,6-Trip2C6H3; Trip = 2,4,6-iPr3C6H2) by protolysis reaction of the Grignard-analogue compound RLnI(thf)x (R = 2-CF3C6H4) with HSAr* or metathesis of SmI2 with KSAr* [10]. A remarkable feature in their solid-state structures is the apparent low coordination number of two for the lanthanide atoms accompanied by the shortest Ln(II)–S bonds known so far. However, in each compound additional Ln•••h6-p-arene interactions are observed to two arene rings of the flanking Trip substituents. These interactions are responsible for the remarkably low tendency of the thiophenolates to coordinate s-donor solvents such as tetrahydrofuran or even dimethoxyethane.

According to variable-temperature NMR experiments for the unsolvated, diamagnetic ytterbium complex the Yb•••h6-p-arene interactions persist in solution. Experimental data and quantumchemical calculations on suitable model systems show that these interactions are strong enough (50–60 KJ·mol–1) to explain the easy loss of coordinated donor solvent molecules.

We recently extented this work to the preparation of some unsolvated, heteroleptic Ln(III) thio- and selenophenolates [11-13]. These compounds are promising candidates for the synthesis of novel luminescent materials.

[10]   M. Niemeyer, Eur. J. Inorg. Chem., 2001, 1969. (DOI)
[11]   A. Cofone, M. Niemeyer, Z. anorg. allg. Chem., 2006, 632, 1930. (DOI)
[12]   S.-O. Hauber, M. Niemeyer, Chem. Commun., 2006, 275; (DOI)
[13]   D. Bubrin, M. Niemeyer, manuscript in preparation.

Low-aggregate Copper(I) and Thallium(I) Aryls:

Our research in this field has afforded several m-terphenyl-substituted low-coordinate thallium and copper organyls. The unique thallium(I) aryl [4] depicted below crystallizes with monomeric units and was at that time the first compound with a singly-coordinate metal atom in the solid state. We have also prepared the first examples of s-donorfree copper(I) organyls with an assoziation degree smaller than 4 [5] . The observed tri- and dimeric molecules aggregate via unusual metal-h2-p-arene interactions.

More details on the preparation and reactivity [6-8] of these compounds are found here.

[4]   M. Niemeyer, P.P. Power, Angew. Chem Int. Ed., 1998, 37, 1277. (DOI)
[5]   M. Niemeyer, Organometallics, 1998, 17, 4649. (DOI)
[6]   J. Klett, K. W. Klinkhammer, M. Niemeyer, Chem. Eur. J., 1999, 2531.
[7]   M. Niemeyer, Z. anorg. allg. Chem., 2003, 629, 1535. (DOI)
[8]   M. Niemeyer, Z. anorg. allg. Chem., 2004, 630, 252. (DOI)

Low-coordinate s-Bonded Lanthanide(II) Aryls

The direct synthesis between organyl halides and lanthanide metals is a long known but rarely used method for the preparation of s-bonded rare earth metal organyls. We have used this convenient approach to synthesize the first examples of structurally characterized lanthanide(II) aryls [9]. Like the well-known Grignard compounds the primarily obtained THF-solvated aryl lanthanide iodides ArLnI show a Schlenk-like equilibrium with the diaryls LnAr2 and the diiodides LnI2. All species could be detected using 171Yb NMR spectroscopy as a probe. Depending on the solvents or substituents used, either the heteroleptic compounds ArLnI(thf)x or the diaryls LnAr2(thf)x may be crystallized from solution.
171Yb-NMR spectrum of Yb(Dpp)I(thf)3 (Dpp = m-Terphenyl-2'-yl) molecular structure of Yb(Dpp)2(thf)2

In the solid-state structures of the aryllanthanide iodides or the diaryls secondary Ln•••C(aryl) interactions to ortho-carbon atoms of the flanking phenyl groups are observed. These interactions have been examined by quantum-chemical calculations on suitable model systems and may be viewed as the first step of a C–H-activation. This interpretation is supported by the isolation of a unique orthometallation product.

More details are found here.

[9]   G. Heckmann, M. Niemeyer, J. Am. Chem. Soc., 2000, 122, 4227. (DOI)

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