The synthesis, structure and dynamic behaviour of disubstituted alkenylphosphine derivatives of [Rh6(CO)16]

Abstract

A series of [Rh₆(CO)₁₆] substituted derivatives containing Ph₂P(alkenyl) ligands has been synthesized starting from the [Rh₆(CO)_16−x(NCMe)_x] (x = 1, 2) clusters and Ph₂P((CH₂)_nCHCH₂) (n = 2, 3) phosphines. It was shown that the terminal alkenyl substituents in these phosphines easily undergo isomerization in the coordination sphere of the hexarhodium complexes to give the allyl –CH₂CHC(H)R (R = Me and Et) fragments coordinated through the double bond of the rearranged organic moieties. The solid-state structure of two clusters, [Rh₆(CO)₁₄(μ₂,κ³-Ph₂PCH₂CHC(H)CH₃)] (4) and [Rh₆(CO)₁₄(μ₂,κ³-Ph₂PCH₂CHC(H)CH₂CH₃)] (8), was established by X-ray crystallography. Solution structures of the products obtained were also characterized by IR and NMR (¹H, ³¹P, ¹H–¹H COSY and ¹H–¹H NOE) spectroscopy. It was shown that 4 and 8 exist in solution as mixtures of three isomers (A, B and C), which differ in the conformation of the coordinated allyl fragment. A similar (two species, A and B) equilibrium was found to occur in the solution of the [Rh₆(CO)₁₄(μ₂,κ³-Ph₂PCH₂CHCH₂)] (2) cluster. The dynamic behaviour of 2, 4, 8 and [Rh₆(CO)₁₄(μ₂,κ³-Ph₂PCHCH₂)] has been studied using VT ³¹P and ¹H–¹H NOESY NMR spectroscopy, rate constants and activation parameters of the (A↔B) isomerization processes were determined. It was shown that the most probable mechanism of this isomerization involves a dissociative {Rh₆(CO)₁₄(κ¹-Ph₂P(alkenyl))} intermediate and recoordination of the double bond to the same metal atom where the process started from. The conversion of the A and B species in 4 and 8 into the third isomer very likely occurs through the transfer of an allyl hydrogen atom onto the rhodium skeleton to give eventually cis conformation of the coordinated allyl fragment.