Manfred Zabel

Synthesis of Cyclometallated Platinum Complexes with Substituted Thienylpyridines and Detailed Characterization of Their Luminescence Properties

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (B = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of (3)pi pi* character, which experience only weak spin-orbit couplings to higher lying singlet states.

Acylguanidines as Bioisosteres of Guanidines : NG-Acylated Imidazolylpropylguanidines, a New Class of Histamine H2 Receptor Agonists

N1-Aryl(heteroaryl)alkyl-N2-[3-(1H-imidazol-4-yl)propyl]guanidines are potent histamine H2-receptor (H2R) agonists, but their applicability is compromised by the lack of oral bioavailability and CNS penetration. To improve pharmacokinetics, we introduced carbonyl instead of methylene adjacent to the guanidine moiety, decreasing the basicity of the novel H2R agonists by 4-5 orders of magnitude. Some acylguanidines with one phenyl ring were even more potent than their diaryl analogues. As demonstrated by HPLC-MS, the acylguanidines (bioisosteres of the alkylguanidines) were absorbed from the gut of mice and detected in brain. In GTPase assays using recombinant receptors, acylguanidines were more potent at the guinea pig than at the human H2R. At the hH1R and hH3R, the compounds were weak to moderate antagonists or partial agonists. Moreover, potent partial hH4R agonists were identified. Receptor subtype selectivity depends on the imidazolylpropylguanidine moiety (privileged structure), opening an avenue to distinct pharmacological tools including potent H4R agonists.

A New Strategy for the Stereoselective Synthesis of 1,2,3-Trisubstituted Cyclopropanes

The stereoselective synthesis of highly functionalized 1,2,3-trisubstituted cyclopropanes 1 and 2, starting from readily available furans 3 or N-protected pyrrole 4, is described. Furthermore, exceptionally high diastereocontrol in agreement with the Felkin-Anh model was observed for the addition of nucleophiles to the title compounds.

[(Cy)Ru(LL*)Cl] and related half-sandwich compounds—two diastereomers in the same single crystal

Abstract The ruthenium compound [(Cy)Ru(LL*)Cl] ( 1 ) crystallizes as a 1:1 mixture of the two diastereomers ( R Ru , S C )- 1 and ( S Ru , S C )- 1 in the same single crystal, whereas the corresponding osmium compound [(Cy)Os(LL*)Cl] ( 2 ), almost identical to 1 in the molecular parameters, crystallizes as the pure diastereomer ( R Os , S C )- 2 . 1:1 diastereomer co-crystallization is traced back to a molecular recognition motif, the pair formation of fragments (Ar)MXY, in which X and Y are electronegative substituents. Thus, compounds of the type [(Ar)MXYZ], in particular (Cy)Ru derivatives, tend to form inversion pairs of enantiomers and also of diastereomers. Fifteen cases of 1:1 diastereomer co-crystallization in half-sandwich complexes are presented. The discussion is extended to [(Cy)RuLCl 2 ] and related complexes. It is shown that there is a delicate balance between 1:1 diastereomer co-crystallization and crystallization as a pure diastereomer.

Optically active transition-metal complexes ☆: Part 124. Chiral 1-phospha[1]ferrocenophanes and 1,12-diphospha[1.1]ferrocenophanes — synthesis, characterization and ring-opening polymerization

Abstract The phosphorus(III)-bridged [1]ferrocenophanes 1,1′-ferrocenediylphenylphosphine ( 1 ), (−)-1,1′-ferrocenediylmenthylphosphine ( 2 ) and (−)-bornyl-1,1′-ferrocenediylphosphine ( 3 ) have been synthesized via the reaction of 1,1′-dilithioferrocene (TMEDA adduct) and Cl 2 PR (R=Ph, Men, Bor). Compounds 1 and 2 have been used as ligands in the preparation of the complexes Cp*Mn(CO) 2 [Fe(η 5 -C 5 H 4 ) 2 PPh] ( 4 ) and (−)- trans -PdCl 2 [Fe(η 5 -C 5 H 4 ) 2 PMen] 2 ( 5 ). The new compounds 2 – 5 were characterized by multinuclear NMR, by MS, and 2 , 4 and 5 by single-crystal X-ray diffraction. Remarkably, the cyclic dimer anti - exo,exo -1,12-dimenthyl-1,12-diphospha[1.1]ferrocenophane ( 6 ) could be isolated and structurally characterized. The thermal ring-opening polymerization of 1 , 2 and 3 yielded the poly(ferrocenediylphosphines) 7 , 8 and 9 . Compounds 2 and 8 were used as chiral ligands in the Rh-catalyzed diastereoselective hydrogenation of folic acid.

Linear and non-linear optical properties of areneFeCp complexes

Abstract Ionic [areneFeCp]+ fragments were used as electron acceptors in combination with dimethylamino or pyridoneiminyl donors to form push–pull substituted chromophores. While a tolane derivative showed high second-order non-linear optical activity (measured by hyper-Rayleigh scattering), an analogous pyridoneimine derivative did not. With the help of an X-ray crystal structure analysis this discrepancy was traced back to a cyanine-like structure of the imine. Semiempirical INDO/S calculations support this conclusion.

Stepwise Expansion of a Cp* Ring at Pentelidene Complexes and Stereoselective Formation of Triphosphines†

Stepwise opening and final removal of the η1-bound Cp* substituents in the bridged pentelidene complexes [Cp*E{W(CO)5}2] (E=P, As) occurs by the reaction with primary phosphines. Not only novel diphospha- and arsaphosphanorbornenes are obtained, but also diastereomerically pure complexed triphosphines. All reaction steps were monitored by NMR spectroscopy, and compounds structurally characterized.

Optically active transition metal complexes ☆: Part 125. Preparation and epimerization of chiral-at-metal pentamethylcyclopentadienyl-rhodium(III) and iridium(III) half-sandwich complexes

Abstract The optically active bidentate Schiff base ligands (+)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine NN*H and (+)-N-[(S)-1-phenylethyl]salicylaldimine ON*H react with [Cp*MCl2]2 (M=Rh, Ir) to give the half-sandwich complexes Cp*Rh(NN*)Cl (1), Cp*Ir(NN*)Cl (2), Cp*Rh(ON*)Cl (3) and Cp*Ir(ON*)Cl (4). In all four cases two diastereomers (RM,SC) and (SM,SC) arise, differing only in the metal configuration. However, in each of the systems 1–4, only the (RM,SC) diastereomer crystallized. X-ray structure analyses established the absolute configuration in the crystals of all four complexes. In solution the compounds epimerize via a change of the metal configuration and approach equilibria (RM,SC) (SM,SC) in which for 1 and 2 the (SM,SC) diastereomers dominate. Time dependent integration of 1H NMR signals of the complexes 1 and 2 revealed that the epimerization is a clean first-order reaction. The half-lives in CD2Cl2 solution at −50°C were 19.4 min for 1 and 30.6 min for 2. From the temperature dependence ΔH≠ was determined as 62.4 kJ mol−1 for 1 and 44.3 kJ mol−1 for 2 and ΔS≠ as 148 J K−1 mol−1 for 1 and 65 J K−1 mol−1 for 2. For the compounds 3 and 4, the kinetic data for the epimerization with respect to the metal center were derived from 1H NMR coalescence experiments: half lives 35 ms for 3 at 21±3° (CH2Cl2 solution) and 58 ms for 4 at −1.8±3°C (CHCl3 solution). These epimerization studies show that the metal configuration is labile in complexes 1 and 2 and extremely labile in complexes 3 and 4.

Efficient Aerobic Wacker Oxidation of Styrenes Using Palladium Bis(isonitrile) Catalysts

The palladium-catalyzed aerobic oxidation of alkenes and especially styrenes (Wacker oxidation) by using chiral pseudo C(2)-symmetrical bis(isonitrile) ligands in the absence of further cocatalysts gives rise to methyl ketones in a highly chemoselective manner. The palladium bis(isonitrile) catalyst was characterized by NMR spectroscopy and X-ray structure analysis, revealing a dissymmetric coordination of palladium by the two isonitrile moieties.

Synthesis, structure and catalytic activity of new chiral nitrogen-containing ligands

The synthesis of different nitrogen-containing ligands was developed during this work. The presence of the chiral (1R, 2R)-trans-cyclohexanediamine is the common structural element of all synthesised ligands. The synthetic methods selected allow to obtain the ligands in very good yields and in one to three steps. The catalytic activity of the ligands was investigated in the following asymmetric reactions: cyclopropanation of styrene with Cu(I), epoxidation of styrene with Mn(III), Mukaiyama Aldol reaction with Lewis acid and addition of diethylzinc to aldehydes. In the latter reaction secondary alcohols were obtained in quantitative yields and with enantioselectivities up to 74 ee.