Preparation of cobalt sandwich diphosphine ligand [(η5-C5H4iPr)Co(η4-C4(PPh2)2Ph2)] and its chelated palladium complex: Application of diphosphine ligand in the preparation of mono-substituted ferrocenylarenes

The reaction of (η5-C5H4iPr)Co(PPh3)2 with PhCCPPh2 furnished two isomeric cyclobutadiene-substituted Cp′CoCb diphosphines, [(η5-C5H4iPr)Co(η4-1,2-(PPh2)2C4Ph2)] (5-cis) and [(η5-C5H4iPr)Co(η4-1,3-(PPh2)2C4Ph2)] (5-trans). Further reaction of 5-cis with one molar equivalent of Pd(COD)Cl2 gave palladium complex [(η5-C5H4iPr)Co(η4-1,2-(PPh2)2C4Ph2)-PdCl2] (6) in good yield. Both of the molecular structures of 5-cis and 6 were determined by single-crystal X-ray diffraction methods. Unexpectedly, the palladium complex 6 was found to be more efficient than the combination of the commonly used Buchwald’s ligand, biphenyl-2-yl-di-tert-butyl-phosphane, with Pd(OAc)2 as the catalytic precursor in the Suzuki–Miyaura reaction between ferroceneboronic acid and 4-bromoaldehyde. The X-ray structural analysis and DFT study of several palladium complexes containing sandwich-type diphosphine chelating ligands revealed that the variations in bite angles are much larger than those in bite distances. The more energetically favorable conformation in the Pd(II) complexes is the one with bite angle close to 90°.