Benjamin Oelkers

f-Element–Metal Bonding and the Use of the Bond Polarity To Build Molecular Intermetalloids

Metal-metal bonding in heterobimetallic complexes is of fundamental interest due to its implications to both bonding theory and new reactivities. In this Concept, structurally authenticated molecular compounds with direct bonds between rare-earth metals or actinoids and transition or main group metals are summarized. Special attention is given to the use of bond polarity as a tool for designing molecular intermetalloids incorporating rare-earth atoms and transition metals.

A new synthetic pathway to the second and third generation of superbasic bisphosphazene proton sponges: the run for the best chelating ligand for a proton.

We present the up to now strongest chelating neutral pincer ligand for the simplest electrophile of chemistry, the proton. Two novel bisphosphazene proton sponges, 1,8-bis(trispyrrolidinophosphazenyl)naphthalene (TPPN) and its higher homologue P2-TPPN, were obtained via a Staudinger reaction and investigated concerning their structural features and basic properties by experimental and computational means. They exhibit experimental pK(BH)(+) values in acetonitrile of 32.3 and 42.1, respectively, exceeding the existing basicitiy record for proton sponges by more than 10 orders of magnitude. We show that Schwesingers concept of homologization of phosphazene bases and Alders concept of proton chelation in a constrained geometry regime of basic centers can be combined in the design of highly basic nonionic superbases of pincer type.

Superbasic Alkyl‐Substituted Bisphosphazene Proton Sponges: Synthesis, Structural Features, Thermodynamic and Kinetic Basicity, Nucleophilicity and Coordination Chemistry

Herein we describe an easily accessible class of superbasic proton sponges based on the 1,8-bisphosphazenylnaphthalene (PN) proton pincer motif and P-alkyl substituents ranging from methyl (TMPN) to n-butyl (TBPN), isopropyl (TiPrPN) and cyclopentyl (TcyPPN). These neutral bases with a pK(BH)(+) value (MeCN) of ~30 were accessible via a Kirsanov condensation using commercially available 1,8-diaminonaphthalene, and in case of TMPN and TBPN, simple one-pot procedures starting from trisalkylphosphanes can be performed. Furthermore, the known pyrrolidinyl-substituted superbase TPPN previously synthesized via a Staudinger reaction could also be prepared by the Kirsanov strategy allowing its preparation in a larger scale. The four alkyl-substituted proton sponges were structurally characterized in their protonated form; molecular XRD structures were also obtained for unprotonated TiPrPN and TcyPPN. Moreover, we present a detailed description of spectroscopic features of chelating bisphosphazenes including TPPN and its hyperbasic homologue P2-TPPN on which we reported recently. The four alkyl-substituted superbases were investigated with respect to their basic features by computational means and by NMR titration experiments revealing unexpectedly high experimental pK(BH)(+) values in acetonitrile between 29.3 for TMPN and 30.9 for TBPN. Besides their thermodynamic basicity, we exemplarily studied the kinetic basicity of TMPN and TPPN by means of NMR-spectroscopic methods. Furthermore, the competing nucleophilic versus basic properties were examined by reacting the proton sponges with ethyl iodide. Insight into the coordination chemistry of chelating superbases was provided by reacting TMPN with trimethylaluminum and trimethylgallium to give cationic complexes of Group XIII metal alkyls that were structurally characterized.

Cu(I)/(II) based catalytic ionic liquids, their metallo-laminate solid state structures and catalytic activities in oxidative methanol carbonylation

Three types of copper-containing catalytic ionic liquids (CILs) have been synthesised and fully characterised by spectroscopy, elemental, thermal and, in most cases, XRD analyses. Whereas type I ILs comprise copper exclusively in the cation (e.g. [Cu(Im12)4][PF6], Im12 = 1-dodecylimidazole), type II ionic liquids contain Cu(I) in both the cation and the anion (e.g. [Cu(Im12)2][CuBr2]) and type III ILs incorporate copper solely in form of halocuprate(I)/(II) anions (e.g. [DMIM][CuBr2], DMIM = 1-dodecyl-3-methylimidazolium). A limitation concerning the preparation of a series of type II ionic liquids has been observed by the formation of the neutral compound [Cu(Im12)I]6 instead of the expected IL [Cu(Im12)2][CuI2]. The novel ILs reveal a metallo-laminate structure in the crystalline state. The salts have a liquidus range of up to 300 °C before decomposition takes place. Their catalytic properties in the synthesis of dimethyl carbonate from MeOH, CO and O2 have been studied.

Copper(I) and Silver(I) Bis(trifluoromethanesulfonyl)imide and Their Interaction with an Arene, Diverse Olefins, and an NTf2−‐Based Ionic Liquid

The chemistry of coinage metal bis(triflyl)imides of technological interest, CuNTf(2) and AgNTf(2), their synthesis and complexes with excess of comparatively weakly coordinating NTf(2)(-) as well as with ether, olefins, and the arene mesitylene are described. The existence of the solvent-free pure phase [CuNTf(2)](∞) has not been evidenced so far. Contrary to the literature, in which the preparation of [CuNTf(2)](∞) is supposed to be carried out by reacting mesityl copper, [Cu(Mes)](5), and HNTf(2), we found that in fact this reaction leads reproducibly to the interesting copper diarene sandwich complex [Cu(η(3)-MesH)(2)][Cu(NTf(2))(2)] (1) (MesH = 1,3,5-trimethylbenzene). The unexpectedly stable molecular etherate [Cu(Et(2)O)(NTf(2))] (2) turned out to be the best precursor for CuNTf(2) having only an inert and easily exchangeable solvent ligand. The coordination mode of NTf(2)(-) in 1 and 2 as well as in the hitherto unknown crystalline phase of [AgNTf(2)](∞) (3) is described. The complex formation, which takes place when dissolving 2 or 3 in the room temperature ionic liquid (RTIL) [emim]NTf(2) ([emim](+) = 1-ethyl-3-methylimidazolium), has been studied. Furthermore, the reaction of 1-3 towards the diolefins 1,5-cyclooctadiene (COD), 2,5-norbornadiene (NBD) and isoprene (2-methylbuta-1,3-diene) and towards ethylene has been investigated. The products 4-13 of these conversions have been isolated and fully characterized by NMR- and IR spectroscopies, mass spectrometry, and elemental- and XRD analyses. The potential of [Cu(η(3)-MesH)(2)][Cu(NTf(2))(2)] (1), [Cu(Et(2)O)(NTf(2))] (2) and [AgNTf(2)](∞) (3) as well as of [emim][M(NTf(2))(2)] (M = Cu 4, Ag 5) as chemisorbers for ethylene was studied by NMR spectroscopy.

Pentaalkylmethylguanidinium methylcarbonates - versatile precursors for the preparation of halide-free and metal-free guanidinium-based ILs

Pentaalkylmethylguanidinium methylcarbonates 6 can easily be prepared from pentaalkylguanidines 5 and dimethyl carbonate (DMC) in a sustainable solvent-free synthesis. Most of the title compounds are room temperature ionic liquids (RTILs) which provide convenient access to halide-free guanidinium-based ILs (GILs) 7via acid–base reactions and subsequent decarboxylation, similar to industrially important imidazolium methylcarbonates 1.

Lanthanoid-transition-metal bonding in bismetallocenes.

Bismetallocenes [Cp2 LuReCp2 ] and [Cp*2 LaReCp2 ] (Cp=cyclopentadienyl; Cp*=pentamethylcyclopentadienyl) were prepared using different synthetic strategies. Salt metathesis-performed in aromatic hydrocarbons to avoid degradation pathways caused by THF-were identified as an attractive alternative to alkane elimination. Although alkane elimination is more attractive in the sense of its less elaborate workup, the rate of the reaction shows a strong dependence on the ionic radius of Ln(3+) (Ln=lanthanide) within a given ligand set. Steric hindrance can cause a dramatic decrease in the reaction rate of alkane elimination. In this case, salt metathesis should be considered the better alternative. Covalent bonding interactions between the Ln and transition-metal (TM) cations has been quantified on the basis of the delocalization index. Its magnitude lies within the range characteristic for bonds between transition metals. Secondary interactions were identified between carbon atoms of the Cp ligand of the transition metal and the Ln cation. Model calculations clearly indicated that the size of these interactions depends on the capability of the TM atom to act as an electron donor (i.e., a Lewis base). The consequences can even be derived from structural details. The observed clear dependency of the LuRu and interfragment LuC bonding on the THF coordination of the Lu atom points to a tunable Lewis acidity at the Ln site, which provides a method of significantly influencing the structure and the interfragment bonding.

Axial functionalization of sterically hindered titanium phthalocyanines.

Several axially functionalized, weakly aggregating titanium phthalocyanines (Pc) have been synthesized and characterized. Soluble titanium dichlorido tetrakis-(1,1,4,4-tetramethyl-6,7-tetralino)-porphyrazine [Pc(#)TiCl(2)] (5) has been prepared by reductive cyclotetramerization of the respective dinitrile precursor in the presence of TiCl(4). 5 and the analogous oxido compound [Pc(#)TiO] (1) are versatile starting materials for the formation of other axially functionalized titanium phthalocyanines such as organoimido (6, 7), alkoxido and aryloxido (8, 9), peroxido (10), sulfido (12), disulfido (11), selenido (14) or diselenido (13) species. Furthermore the deprotonated ligand salts [Pc(#)M(2)] (M = Li (2), Na (3), K (4) are described. The reactivity of the titanium compounds was studied in atom group transfer reactions and ethene polymerization. The crystal structures of 5 and the free ligand Pc(#)H(2) are reported. 5 crystallizes from dichloromethane in the cubic space group Im3. The two chlorido ligands exhibit a cis arrangement. The free ligand Pc(#)H(2) crystallizes in the trigonal space group R3.

Soluble Molybdenum(V) Imido Phthalocyanines and Pyrazinoporphyrazines: Crystal Structure, UV–vis and Electron Paramagnetic Resonance Spectroscopic Studies

Soluble alkyl and aryl imido phthalocyanines [Pc(#)Mo(NR)Cl] (R = tBu, Mes) with molybdenum(V) as central metal were prepared and studied by UV-vis and electron paramagnetic resonance (EPR) spectroscopy. As structural analogue to the weakly aggregating, soluble alkyl substituted Pc(#) ligand, a new, more electron deficient octaazaphthalocyanine, the pyrazinoporphyrazine ligand Ppz(#), was designed. The respective alkyl and aryl imido complexes [Ppz(#)Mo(NR)Cl] are the first examples of molybdenum pyrazinoporphyrazines. UV-vis and EPR spectra revealed unexpected differences between the alkyl and the aryl imido complexes, indicating different electronic structures depending on the nature of the axial ligand. The octahedral coordination of the molybdenum atoms by the axial NR and Cl ligands and the equatorial macrocycles could be verified by EPR spectroscopy. This result was also confirmed by the crystal structure of [Pc(#)Mo(NMes)Cl], which crystallizes from CH2Cl2 in the cubic space group Im3.

Constrained-geometry bisphosphazides derived from 1,8-diazidonaphthalene: synthesis, spectroscopic characteristics, structural features, and theoretical investigations.

Investigations on the Staudinger reaction between 1,8-diazidonaphthalene and phosphorous(III) building blocks, a key step in the synthesis of superbasic bisphosphazene proton sponges, yielded a set of bisphosphazides with a constrained geometry 1,8-disubstituted naphthalene backbone. This compound class has attracted our interest not only due to their surprisingly high stability, but in particular because of their theoretically predicted basicity in the range of their bisphosphazene analogues that can be referred to the constrained geometry interaction of two highly basic nitrogen atoms. Eleven new bisphosphazides bearing simple P-amino groups as well as P-guanidino substituents, azaphosphatrane moieties, P2 building blocks, or chiral P-amino substituents derived from L-proline are presented. They were studied concerning their spectroscopic properties and partly also their chromophoric and structural features. In the case of the pyrrolidino-substituted TPPN(2N2) (TPPN = 1,8-bis(trispyrrolidinophosphazenyl)naphthalene), the stepwise nitrogen elimination is investigated theoretically and experimentally, which led to the isolation and structural characterization of TPPN(1N2) bearing a phosphazide and a phosphazene functionality in one molecule. Attempts to protonate the obtained bisphosphazides and to prove the computationally predicted pKBH(+) values through NMR titration reactions resulted in their decay, which again was rationalized by theoretical calculations. Altogether we present the so far most extensive spectroscopic, structural and theoretical investigation of constrained geometry bisphosphazides and their Brønsted and Lewis basic properties.