Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C-Cl bond activation.

Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd0 complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (pSpR, meso-isomer) and 1b (pSpS-isomer) differ in the orientation of the ferrocene moieties relative to the central Ph2PC5H3-Hg-C5H3PPh2 bridging entity. The structurally distinct trigonal LPd0(dba) complexes 2a (meso) and 2b (pSpS) are obtained upon treatment with Pd(dba)2. A competition reaction reveals that 1b reacts faster than 1a with Pd(dba)2. Unexpectedly, catalytic interconversion of 1a (meso) into 1b (rac) is observed at room temperature in the presence of only catalytic amounts of Pd(dba)2. Both Pd0 complexes, 2a and 2b, readily undergo oxidative addition into the C-Cl bond of CH2Cl2 at moderate temperatures with formation of the square-planar trans-chelate complexes LPdIICl(CH2Cl) (3a, 3b). Kinetic studies reveal a significantly higher reaction rate for the meso-isomer 2a in comparison to (pSpS)-2b.

Diferrocenylmercury diphosphine diastereomers with unique geometries: trans-chelation at Pd(ii) with short Hg(ii)Pd(ii) contacts.

Diphosphine chelate ligands are essential in many catalytic processes with both the electronic structure and bite angle having a dramatic influence on the coordination behavior and catalytic performance. The synthesis of a new class of diferrocenylmercury-supported diphosphine chelate ligands was accomplished by the reaction of (ortho-diphenylphosphino)ferrocenyl sulfinate (2) with t-BuLi, followed by treatment with mercury(ii) chloride. Two diastereomers, 4a (pSpR-, meso-isomer) and 4b (pSpS-isomer), differ in the orientation of the ferrocene moiety relative to the central Ph2PC5H3-Hg-C5H3PPh2 bridging entity. They were isolated independently and fully characterized in solution and in the solid state by single crystal X-ray diffraction analysis. Key characteristics of these ligands are their exceptionally wide and flexible bite angles and the unique stereochemical environment that is achieved upon coordination to transition metals. Complexation to Pd(ii)Cl2 gives rise to unusual square-planar trans-chelate complexes 5a (meso) and 5b (pSpS). In competition reactions, 4a and 4b show similar reactivity toward Pd(ii)Cl2. The molecular structures of 5a and 5b exhibit short PdHg contacts, possibly indicating secondary metallophilic interactions as further evidenced by bond-critical points between Pd and Hg that were identified by AIM analyses.

Diferrocenylmercury-Bridged Diphosphine: A Chiral, Ambiphilic, and Redox-Active Bidentate Ligand.

A diphosphine chelate ligand with a wide and flexible bite angle, a unique stereochemical environment, and redox-active and ambiphilic character is reported. Initially generated as its HgCl2 complex by reaction of 1,2-fc(PPh2 )(SnMe3 ) (fc=ferrocenediyl) with HgCl2 in acetone, treatment with [n-Bu4 N]CN readily liberates the free chiral bidentate ligand. An intermolecular ClHg-Cl→Hgfc2 (2.9929(13) Å) interaction that is unprecedented in ambiphilic ligand chemistry is seen in the solid structure of Hg(fcPPh2 )2 ⋅HgCl2 where the bridging mercury atom acts as a σ-acceptor. Furthermore, a bis-[Rh(COD)Cl] complex is introduced, which displays relatively short Rh⋅⋅⋅Hg contacts of 3.4765(5) and 3.4013(1) Å. Wiberg indices of 0.12 are determined for these Rh⋅⋅⋅Hg interactions and an AIM analysis reveals bond paths with an electron density ρ(r) of 1.2×10-2 and 1.4×10-2  e/a03 at the bond critical points.

Planar-chiral ferrocenylphosphine-borane complexes featuring agostic-type B-HE (E = Hg, Sn) interactions.

The synthesis of ferrocenylphosphine-borane adducts 1,2-fc(E)(PPh2·BH3) (E = SnR2R', HgX; 1,2-fc = 1,2-ferrocenediyl) that are substituted with organotin or organomercury Lewis acid moieties in ortho-position is presented. Several compounds that feature two ferrocenylphosphine-borane moieties bridged by Sn or Hg are also introduced. The products are fully characterized by multinuclear NMR spectroscopy, high-resolution MALDI-TOF mass spectrometry and elemental analysis. The attachment of the Lewis acid substituent to the same Cp ring of the ferrocene results in planar-chirality and the close proximity between the boron hydride group and the Lewis acid is expected to allow for agostic-type B-HE (E = Sn, Hg) interactions. Structural investigations by X-ray diffraction reveal a short B-HSn contact of 2.755(4) Å for 1,2-fc(SnMe2Cl)(PPh2·BH3), which is only the second reported example of such a short agostic-type contact involving a coordinatively saturated tin(iv) center. In contrast, for the tetraorganotin derivatives 1,2-fc(SnMe3)(PPh2·BH3) and [1,2-fc(PPh2)](µ-SnMe2)[1,2-fc(PPh2·BH3)], in which the Lewis acidity of the tin atom is weaker than in 1,2-fc(SnMe2Cl)(PPh2·BH3), the B-HSn distances are much longer but still within the sum of the van der Waals radii of Sn and H (∑vdW = 3.27 Å). The chloromercury-substituted ferrocenylphosphine-borane 1,2-fc(HgCl)(PPh2·BH3) shows a similarly short B-HHg contact of 2.615(5) Å (∑vdW = 3.15 Å). Inspection of the extended structure of 1,2-fc(HgCl)(PPh2·BH3) reveals that the Lewis acidic mercury atom is also involved in intermolecular HgCl interactions with a neighboring molecule. An analysis of 31P and 11B NMR data reveals a correlation between the chemical shifts and the Lewis acidity of the adjacent organotin/mercury substituent. Structure optimization of 1,2-fc(SnMe3)(PPh2·BH3) and 1,2-fc(SnMe2Cl)(PPh2·BH3) by density functional theory (DFT) indicates B-HSn contacts respectively of 3.129 and 2.631 Å that are close to the experimental values. Natural bond orbitals (NBO) and atom in molecules (AIM) analyses reveals a B-H→Sn donor-acceptor interaction energy of 5.46 kcal mol-1 and a B-HSn bond path for the chlorodimethyltin-substituted derivative with a modest electron density ρ(r) of 0.0082 a.u. and a positive Laplacian at the bond critical point.

How big is a Cp? Novel cycloheptatrienyl zirconium complexes with tri-, tetra- and pentasubstituted cyclopentadienyl ligands.

The new bulky cyclopentadienyl anions 1,2,4-tri(cyclopentyl)cyclopentadienide and 2,3-diisopropyl-1,4-dimethyl-5-trimethylsilyl-cyclopentadienide were prepared. These and the already known 1,2,4-tri(cyclohexyl)-, 1,2,4-tri(isopropyl)-, 2,3-diisopropyl-1,4-dimethyl-, 1,3,4-triisopropyl-2,5-dimethyl-, pentaphenyl-, and p-butylphenyl-tetraphenyl-cyclopentadienide as well as tert-butylindenide were coordinated to the cycloheptatrienylzirconium fragment [(CHT)ZrCl(tmeda)]. The nine zirconium complexes of the [(CHT)Zr(Cp)] type were characterized by elemental analysis and NMR spectroscopy. For five of the sandwich complexes X-ray crystal structure determination could be carried out; structures of the four others were obtained by DFT calculations. The data serve as a basis for cone angle measurements of cyclopentadienyl ligands to evaluate the steric demand of these ligands.