Synthesis and structure of ansa-cyclopentadienyl pyrrolyl titanium complexes: [(η-C5H4)CH2(2-C4H3N)]Ti(NMe2)2 and [1,3-{CH2(2-C4H3N)}2(η-C5H3)]Ti(NMe2)

Reaction of ansa-cyclopentadienyl pyrrolyl ligand (C₅H₅)CH₂(2-C₄H₃NH) (2) with Ti(NMe₂)₄ affords bis(dimethylamido)titanium complex [(η⁵-C₅H₄)CH₂(2-C₄H₃N)]Ti(NMe₂)₂ (3) via amine elimination. A cyclopentadiene ligand with two pendant pyrrolyl arms, a mixture of 1,3- and 1,4-{CH₂(2-C₄H₃NH)}₂C₅H₄ (4), undergoes an analogous reaction with Ti(NMe₂)₄ to give [1,3-{CH₂(2-C₄H₃N)}₂(η⁵-C₅H₃)]Ti(NMe₂) (5). Molecular structures of 3 and 5 have been determined by single crystal X-ray diffraction studies.     

Are cyclopentadienyl complexes more stable than their pyrrolyl analogues?

Metal-η⁵-cyclopentadienyl (M-Cp) and metal-η⁵-pyrrolyl (M-pyr) bond dissociation enthalpies in group 4 complexes were determined from DFT/B3LYP calculations with a VTZP basis set. Thermochemical cycles involving calculated enthalpies of ligand exchange reactions and experimental values of ligand electron affinities and M-Cl bond dissociation enthalpies were applied to [M(η⁵-X)Cl₃] piano stool complexes (M = Ti, Zr, Hf; X = pyr, Cp), allowing a comparative study of those metal-ligand bond strengths. The results indicate that both ligands establish weaker bonds with Ti than with the heavier elements, Zr and Hf. Very similar bond dissociation enthalpies were obtained for pyrrolyl and cyclopentadienyl (within 1 kcal mol⁻¹), suggesting that the well known difference in reactivity between those families of complexes should derive from kinetic rather than thermodynamic causes.     

Insertion reactions of phenyl isocyanate into hafnium nitrogen bonds: synthesis and reactivity of hafnium complexes bearing substituted pyrrolyl ligands

A bis(diethylamido)hafnium compound [C₄H₃N(CH₂NMe₂)-2]₂Hf(NEt₂)₂ (1) has been prepared in 79% yield by reacting Hf(NEt₂)₄ with 2 equiv. of [C₄H₃NH(CH₂NMe₂)-2] in heptane via deamination. Reacting compound 1 with 2 equiv. of phenyl isocyanate at room temperature in diethyl ether results in the PhNCO being inserted seletively into hafnium-NEt₂ bonds to generate [C₄H₃N(CH₂NMe₂)-2]₂Hf[PhNC(NEt₂)O]₂ (2) in 56% yield. Similarly, while reacting 1 with 2 equiv. of phenyl isocyanate for a week in toluene produces a mixture of 2 and [C₄H₃N(CH₂NMe₂)-2]Hf[PhNC(NEt₂)O]₃ (3). For comparison, reacting Hf(NEt₂)₄ with 4 equiv. of PhNCO in a toluene solution at room temperature results in the PhNCO inserted into Hf-N bonds, and forms a tetrakis-ureato hafnium compound Hf[PhNC(NEt₂)O]₄ (4) in 88% yield. A theoretical calculation found that the unpaired electrons of the ureato fragments of 2 are resonance delocalized between the C-O, C-NPh, and C-NEt₂ bonds, which are all partially doubly bonded.     

Synthesis of aluminum alkoxide and bis-alkoxide compounds containing bidentate pyrrolyl ligands

A series of aluminum alkoxide and bis-alkoxides compounds were synthesized and characterized. Reacting 1 with 1 and 2 equiv. of t-butanol in methylene chloride generates [C₄H₃N(CH₂NMe₂)-2]₂Al(O-t-Bu) (2) and [C₄H₃N(CH₂NMe₂)-2-H-C₄H₃N(CH₂NMe₂)-2]Al(O-t-Bu)₂ (3) in 47% and 54% yield, respectively. The ¹H NMR spectrum of 2 exhibits two singlets for NMe₂ and CH₂N at δ 2.52 and 3.84, respectively, representing the symmetrical manner of molecular structure 2 in a solution. Compound 3 is not thermal stable in solution which decompose into substituted pyrrole ligand C₄H₄N(CH₂NMe₂)-2 and unknown aluminum alkoxides. Reacting 1 with 2 equiv. of triphenylsilanol in methylene chloride generates a tetra-coordinated aluminum “ate” compound [C₄H₃N(CH₂NMe₂)-2-H- C₄H₃N(CH₂NMe₂)-2]Al(OSiPh₃)₂ (4) in 49% yield. The ¹H NMR spectra of 4 at room temperature show a broad signal at δ 1.57 for NMe₂ fragments and the signals for CH₂N were not observed. VT ¹H NMR spectra of 4 show the NMe₂ fragments became two singlets (δ 1.27 and 2.12) and the CH₂N exhibited two doublets (δ 2.44 and 3.56) at 240 K. The fluxional energy barrier (ΔG^‡) is estimated at ca. 50 kJ/mol. The molecular structures of compounds 3 and 4 are determined by single-crystal X-ray diffractometer.     

Carbenhomologe Verbindungen von Germanium,Zinn und Blei mit 2‐substituierten N‐Pyrrolyl‐Liganden

Carbene Homologues of Germanium, Tin, and Lead with 2‐substituted N ‐Pyrrolyl Ligands A series of germylenes, stannylenes, and plumbylenes could be prepared by reacting the appropriate bis(trimethylsilyl)amino‐substituted carbene homologue E[N(SiMe₃)₂]₂ (E = Ge, Sn, and Pb) with an α‐carbonyl substituted pyrrole derivative under elimination of bis(trimethylsilyl)amine. The isolated compounds have been analysed spectroscopically, and the resulting NMR and IR data were contrasted with parameters obtained from quantumchemical calculations. The good agreement between experimental and theoretical results gives us the opportunity to discuss the vibrations in more detail, particularly those in which the group 14 element is involved. X‐ray crystal structure analyses obtained for five examples show the title compounds essentially to be monomers with primary E–N bonds and, in addition to that, coordinative E ← O contacts.     

Synthesis and structural characterization of unsymmetrical osmocenes containing the pentamethylcyclopentadienyl ligand

Treatment of dibromo(pentamethylcyclopentadienyl)osmium(III) dimer with alkali metal salts of a variety of cyclopentadienyl derivatives provides a simple approach to the synthesis of unsymmetrical osmocenes containing the pentamethylcyclopentadienyl ligand. Furthermore, the reaction of dibromo(pentamethylcyclopentadienyl)osmium(III) with alkali metal salts of pyrrole and 3,5-di-tert-butylpyrazole afforded the corresponding pentamethylcyclopentadienylosmium complexes containing η⁵-pyrrolyl or η⁵-3,5-di-tert-butylpyrazolato ligands. This overall synthetic approach afforded pentamethylosmocene (64%), (η⁵-pentamethylcyclopentadienyl)(η⁵-indenyl)osmium (36%), (η⁵-pentamethylcyclopentadienyl)(η⁵-fluorenyl)osmium (30%), (η⁵-pyrrolyl)(η⁵-pentamethylyclopentadienyl)osmium (30%), and (η⁵-3,5-di-tert-butylpyrazolato)(η⁵-pentamethylcyclopentadienyl)osmium (38%). The new complexes were characterized by spectroscopic and analytical techniques, and by single crystal X-ray structural determinations. In the solid state, all of the new complexes exist as eclipsed metallocenes.     

Electrooxidation of pyrrole-terminated self-assembled lipoic acid derivatives

New pyrrole derivatives, pyrrolyl lipoic acid (Py-LA 3) and dipyrrolyl lipoic acid (Py₂-LA 2) have been used for surface attachment and immobilisation on gold surfaces, by self-assembly. The electrooxidation of the surface-confined pyrroles was analysed by cyclic voltammetry and the modified electrodes morphological and thickness changes addressed by scanning probe microscopy and ellipsometry. The data support the formation of oligomers as a result of the pendant-pyrrolyl units ease oxidation but provide no evidence of an effective subsequent polymerisation.     

Lewis Base Assisted Magnesium Complexes Incorporating Pyrrolyl and Ketiminate Ligands: Synthesis,Structural Diversity and Characterization

A series of four, five and six‐coordinated magnesium derivatives integrating with substituted pyrrole and ketimine ligands are conveniently synthesized. Reaction of two equiv of 2‐dimethylaminomethyl pyrrole with Mg[N(SiMe₃)₂]₂ in THF affords the monomeric magnesium complex Mg[C₄H₃N(2‐CH₂NMe₂)]₂ (THF)₂ ( 1 ) in high yield along with elimination of two equiv of HN(SiMe₃)₂. Similarly, the reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe₃)₂]₂ in THF renders the magnesium derivative, Mg[C₄H₃N(2‐CH₂NH^tBu)]₂(THF)2₂( 2 ) in good yield. Interestingly, reaction between two equiv of 2‐t‐butylaminomethyl pyrrole and Mg[N(SiMe₃)₂]₂ in toluene, instead of THF, generates Mg[C₄H₃N(2‐CH2NH^tBu)]₂ ( 3 ), also in high yield. Furthermore, the assembly of two equiv of ketimine ligand, HOCMeCHCMeNAr (Ar = C₆H₃‐2,6‐ⁱPr₂) and Mg[N(SiMe₃)₂]₂, yields five‐coordinated magnesium derivatives, Mg(OCMeCHCMeNAr)₂(THF) ( 4 ) and Mg(OCMeCHCMeNAr)₂(OEt₂) ( 5 ), using THF and diethyl ether, respectively. All the aforementioned derivatives are characterized by ¹H and ¹³C NMR spectroscopy as well as 1 , 3 , 4 and 5 are subjected to X‐ray diffraction analysis in solid state.     

The fluorescence of 5-cyano-2-(1-pyrrolyl)-pyridine (CPP) in different solvents and in solid argon: An experimental and theoretical study

The fluorescence spectrum of 5-cyano-2-(1-pyrrolyl)-pyridine (CPP) was measured in several solvents as well as in an argon matrix. Based on comparison with other compounds and on ab initio calculations it is proposed that the fluorescence in the argon matrix and in non-polar solvents is due to two electronic excited states: one is of locally excited nature, the other a charge transfer (CT) state. In polar solvents the spectrum is dominated by the CT emission. The photo-physical behavior of CPP is discussed by comparison with that of other molecules exhibiting dual fluorescence and in view of a recent model developed for the benzene analog.     

α-Acrylaldehyde 3-Pyrrolyl BODIPY as a Specific Optical Sensor for Cysteine and Homocysteine

A new fluorophore, α-acrylaldehyde 3-pyrrolyl BODIPY was synthesized by treating 3-pyrrolyl BODIPY with a mixture of 3-(dimethylamino) acrolein and POCl₃ under Vilsmeier–Haack reaction conditions. The X-ray structure revealed that the fluorophore was almost planar, and the appended pyrrole was in the same plane with a small deviation from the mean plane. We investigated the potential use of α-acrylaldehyde 3-pyrrolyl BODIPY for sensing thiol containing amino acids such as cysteine/homocysteine (Cys/Hcy). Our studies showed that the α-acrylaldehyde- 3-pyrrolyl BODIPY was found to be useful for exclusive sensing of Cys/Hcy and to exhibit different optical signaling responses to Cys and Hcy at physiological pH in aq. CH₃CN (1 : 1 v/v, PBS) medium. The enhancement in optical properties for Cys and quenching in same properties for Hcy was attributed to different binding modes of Cys/Hcy with α-acrylaldehyde 3-pyrrolyl BODIPY.