Synthesis and solution properties of the boron-containing clusters HM4(CO)12BAu2(dppf) (M=Fe or Ru) and the competitive formation of 2(dppf) for M=Ru (dppf=1,1'-bis(diphenylphosphino)ferrocene). Molecular structure of HRu4(CO)12BAu2(dppf)*2CHCl3

Article abstract of DOI:10.1016/0022-328X(92)83455-Q

The reaction of (M=Fe or Ru) with ClAu(dppf)AuCl (dppf=1,1'-bis(diphenylphosphino)ferrocene) leads to HM4(CO)12BAu2(dppf) in which the environment of the boron atom depends upon the identity of M.For M=Ru, the boron atom is interstitial, with the hydride ligand wholly associated with the tetraruthenium framework.For M=Fe, an isomer equilibrium exists in solution between a metallaborane and a metallaboride cluster with the former, Fe4(CO)12BHAu2(dppf) (1) being the dominant species.The difference in structure is attributed to the increase in size of the tetrametal butterfly framework from Fe4 to Ru4 and follows the pattern previously noted for HM4(CO)12BAu2(PPh3)2.For M=Ru, the formation of HRu4(CO)12BAu2(dppf) (3) competes with that of the linked bicluster 2(dppf) (2).A crystallographic structural determination of 3 has revealed that the hexametal skeleton in 3 encapsulates the boron atom but is non-octahedral.The two Au atoms are related by a molecular C2 axis and each bridges one B-Ru_wingtip edge.The orientation of the ferrocenyl group with respect to the cluster core is controlled by the location of the gold atoms and the two C5 rings are locked in an approximately mutually eclipsed position.In solution, 1H NMR spectroscopy in the Cp region has been used to study the fluxional behaviour of 3; three processes are mutually dependent (i) a rocking motion of the two Au atoms with respect to the Ru4B core (ii) inversion at the phosphorus atoms (iii) cyclopentadienyl ring twisting.