Hybrid Phosphine/Amino-Acid Ligands Built on Phenyl and Ferrocenyl Platforms: Application in the Suzuki Coupling of o-Dibromobenzene with Fluorophenylboronic Acid

We describe the synthesis and characterization of two classes of hybrid phosphino ligands functionalized with amino ester or amino acid groups. These compounds are built either on a rigid planar phenyl platform or on a functionalized - conformationally controlled - rotational ferrocene backbone. Modifications at the -PR₂ phosphino groups (R=aryl and alkyl, with various steric bulk, Ph, Mes, i-Pr, Cy) and at the amino acid/amino ester functions are reported, showing a valuable high modularity. The coordination chemistry of these compounds regarding palladium and gold was investigated, in particular with respect to the coordination mode of the phosphino groups and the preferred interaction with metals for the amino ester and amino acid functions. For all the hybrid ligands, based either on ferrocenyl or phenyl platforms, the (P,N)-chelating effect dominates in solution for coordination to Pd(II), while linear P-Au(I) complexes without interaction with the amino groups are assumed. The investigation of the catalytic activity of these new ligands in the demanding palladium-catalyzed Suzuki-Miyaura coupling of o-dibromoarenes with fluorophenylboronic acid underlined the importance of the amino ester dicyclohexylphosphinoferrocene for avoiding the deleterious homocoupling and arene oligomerization side-reactions that were otherwise observed with the other phosphine ligands.

Electronic structure and bridge geometric distortion in push–pull imine-bridged triads. A theoretical study

Intramolecular electron transfer (IET) in imine-bridged triads is studied by analyzing electric charge distribution and ferrocene and bridge distortion parameters.

Redox-Induced Hydrogen Bond Reorientation Mimicking Electronic Coupling in Mixed-Valent Diruthenium and Macrocyclic Tetraruthenium Complexes

We present the coordination-driven self-assembly of three tetranuclear metallacycles containing intracyclic NH₂, OH, or OMe functionalities through the combination of various isophthalic acid building blocks with a divinylphenylene diruthenium complex. All new complexes of this study were characterized by means of nuclear magnetic resonance spectroscopy, ultrahigh-resolution ESI mass spectrometry, cyclic and square wave voltammetry and, in two cases, X-ray diffraction. The hydroxy functionalized macrocycle 4-BOH and the corresponding half-cycle 2-OH stand out, as their intracyclic OH···O hydrogen bonds stabilize their mixed-valent one- (2-OH, 4-BOH) and three-electron-oxidized states (4-BOH). Despite sizable redox splittings between all one-electron waves, the mixed-valent monocations and trications do not exhibit any intervalence charge-transfer band, assignable to through-bond electronic coupling, but nevertheless display distinct IR band shifts of their charge-sensitive Ru(CO) tags. We ascribe these seemingly contradicting observations to a redox-induced shuffling of the OH···O hydrogen bond(s) to the remaining, more electron-rich, reduced redox site.

Direct solid-phase synthesis of molecular heterooligonuclear lanthanoid-complexes

Molecular lanthanoid complexes are highly valuable building blocks for a number of important technological applications, e.g. as contrast agents in magnetic resonance imaging (MRI) or as luminescent probes for bioassays. For the next generation of advanced applications based on molecular species, heterooligonuclear lanthanoid complexes with well-defined chemical and structural compositions are required. The great kinetic lability of trivalent lanthanoids so far prevents the realization of such molecular architectures with a universally applicable methodology. Here, we have developed functionalized molecular lanthanoid cryptates as monomeric building blocks which can be directly linked by standard solid-phase peptide synthesis to yield sequence-specific heterooligonuclear lanthanoid complexes. These molecular materials enable unique applications such as the generation of molecular codes with very convenient luminescence read-out.

Scalable Synthesis of Functionalized Ferrocenyl Azides and Amines Enabled by Flow Chemistry

A scalable access to functionalized ferrocenyl azides has been realized in flow. By halogen-lithium exchange of ferrocenyl halides and trapping with tosyl azide, a variety of functionalized ferrocenyl azides were obtained in high yields. To allow a scalable preparation of these potentially explosive compounds, a flow protocol was developed accelerating the reaction time to minutes and circumventing accumulation of potentially hazardous intermediates. The corresponding ferrocenyl amines were then prepared by a reliable reduction process.

Polysubstituted ferrocenes as tunable redox mediators

A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (1), 1,1’-bis(methoxycarbonyl)- (2), 1,1’,3-tris(methoxycarbonyl)- (3) and 1,1’,3,3’-tetrakis(methoxycarbonyl)ferrocene (4), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in 1–4 on the electrochemical potential E_1/2 is correlated with the sum of Hammett constants. The 1/1⁺–4/4⁺ redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV–vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV–vis absorptions of 1–4 and 1⁺–4⁺ correlate with the number of ester groups. Paramagnetic ¹H NMR redox titration experiments give access to the chemical shifts of 1⁺–4⁺ and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.

Application of the Curtius rearrangement to the synthesis of 1′-aminoferrocene-1-carboxylic acid derivatives

The shortest synthesis of N-protected 1′-aminoferrocene-1-carboxylic acid from readily available ferrocene-1,1′-dicarboxylic acid is reported.

Metal coordination of ferrocene–histidine conjugates

Synthesis and complete structural characterization of ferrocene–histidine dipeptides including detailed analysis of the ligand–metal complexation.

Diferrocenyl tosyl hydrazone with an ultrastrong NH⋯Fe hydrogen bond as double click switch

The intramolecular NH⋯Fe hydrogen bond in diferrocenyl hydrazone 2 raises the barrier for intramolecular electron transfer in its mixed-valent cation 2+ and is only disrupted by double oxidation to 22+.

Synthesis of novel ferrocenyl N/O-heterocycles, chiral P,N-ligand and α-dehydro-β-amino acid derived short peptides from Morita–Baylis–Hillman adducts of ferrocenealdehyde

The golden triangle of Fc, OH/NH, COO moieties created by classical/aza-MBH reaction of ferrocenealdehyde has been exploited for the first time for the synthesis of the title compounds.

Phosphaalkenylidene bridged ferrocenes

The lithiation of ferrocenylphosphane Fc-PH(2) (Fc = -C(5)H(4)FeC(5)H(5)) has been reinvestigated and both Fc-PHLi and Fc-PLi(2) have been identified by NMR-spectroscopy. The lithiated phosphanides have been converted to the corresponding mono and bis(silylated) species the latter of which gave synthetic access to an oligomer in which three ferrocene units are symmetrically connected by phosphaalkene units. The charge distribution within this oligomer and its isomers has been analyzed using DFT calculations which indicates that the iron atom of the central metallocene unit is slightly more positive than the terminal ones. These findings are supported experimentally by Mößbauer spectroscopy and cyclic voltammetry.