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

The synthesis of ferrocenylphosphine-borane adducts 1,2-fc(E)(PPh·BH) (E = SnRR', 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(SnMeCl)(PPh·BH), 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(SnMe)(PPh·BH) and 1,2-fc(PPh)[1,2-fc(PPh·BH)], in which the Lewis acidity of the tin atom is weaker than in 1,2-fc(SnMeCl)(PPh·BH), the B-HSn distances are much longer but still within the sum of the van der Waals radii of Sn and H (∑ = 3.27 Å). The chloromercury-substituted ferrocenylphosphine-borane 1,2-fc(HgCl)(PPh·BH) shows a similarly short B-HHg contact of 2.615(5) Å (∑ = 3.15 Å). Inspection of the extended structure of 1,2-fc(HgCl)(PPh·BH) reveals that the Lewis acidic mercury atom is also involved in intermolecular HgCl interactions with a neighboring molecule. An analysis of P and B 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(SnMe)(PPh·BH) and 1,2-fc(SnMeCl)(PPh·BH) 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 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.