Chelating versus bridging behaviour and NMR fluxionality of dppf in the nido clusters [M3Se2(CO)7(dppf)] [M = Fe or Ru, dppf = Fe(η5-C5H4PPh2)2]. Crystal structure of the chelated ruthenium derivative†

Abstract

The reactions of [M₃(CO)₁₂] (M = Fe or Ru) with the diphosphine diselenide dppfSe₂ [dppfSe₂ = Fe(η⁵-C₅H₄P(Ph₂)Se)₂], in the presence of Me₃NO afforded the disubstituted square pyramidal selenido clusters [Fe₃Se₂(CO)₇(µ-dppf)] 1 and [Ru₃Se₂(CO)₇(dppf)] 2. In the previously reported structure of 1 the dppf ligand bridges the two non-bonded iron atoms, whereas in 2 the same ligand chelates to one ruthenium atom of the basal plane of the square pyramidal cluster. Compound 2 represents the first reported example of chelating dppf to a carbonyl cluster. Both compounds exhibit fluxional behaviour in solution, consisting in the rocking motion of the bidentate bridging ligand below the square basal plane of the iron clusters in 1 and in the exchange of the axial and equatorial positions between the two chelating P atoms in 2. The dynamic behaviour of both compounds was studied by variable temperature 1-D and 2-D COSY and EXSY ¹H NMR. The solid state structures correspond to the static structures in solution below 258 K for 1 and 213 K for 2. The cluster [Ru₃Se₂(CO)₇(µ-dppe)] 3 [dppe = (CH₂PPh₂)₂] has also been synthesized from dppeSe₂, and its structure determined for comparison.