Further studies on the interconversion of large ring and cyclometallated complexes of rhodium, with the diphosphines But2P(CH2)5PBut2and But2PCH2CHCHCH2PBut2

Abstract

Improved routes to the fluxional cyclometallated hydride [[graphic omitted]Bu^t ₂)] are reported. Treatment of this hydride with carbon monoxide and sodium tetraphenylborate in methanol gives the fluxional salt [[graphic omitted]Bu^t ₂)][Bph₄], together with a small amount of the 16-atom ring complex, trans-[Rh₂Cl₂(CO)₂{Bu^t ₂P(CH₂)₅PBu^t ₂)₂], more readily prepared by treating [Rh₂Cl₂(CO)₄] with the diphosphine. This 16-atom ring complex exists as two rotamers in solution at 25 °C. Although [[graphic omitted]Bu^t ₂)] gives [[graphic omitted]Bu^t ₂)] with sodium propan-2-oxide and carbon monoxide on the other hand the corresponding complex [[graphic omitted]Bu^t ₂)] on similar treatment gives a complex mixture (probably containing rotamers of large-ring chelates). Treatment of [[graphic omitted]Bu^t ₂)] with MeNC and sodium propan-2-oxide also gives mixtures, probably containing large-ring chelates. Treatment of [[graphic omitted]Bu^t ₂)] with MeNC or Bu^tNC in the presence of sodium tetraphenylborate or ammonium hexafluorophosphate gives 16-atom ring chelate salts [Rh₂(CNR)₄{Bu^t ₂P(CH₂)₅PBu^t ₂}₂][anion]₂ which can also be made by treating [Rh(CNR)₄][anion] with the diphosphine. Rotation of trans-MeNC–Rh–CNMe moieties around P–Rh–P bonds has been studied by variable-temperature n.m.r. spectroscopy. The complex [Rh₂(CNBu^t)₄(Bu^t ₂PCH₂CHCHCHCH₂PBu^t ₂)₂][PF₆]₂ has also been prepared. Treatment of the cyclooctene complex [Rh₂Cl₂(C₈H₁₄)₄] with Bu^t ₂P(CH₂)₅PBu^t ₂ and MeNC or Bu^tNC (1 mol per rhodium atom) gives 16-atom ring complexes of type [Rh₂Cl₂(CNR)₂{Bu^t ₂P(CH₂)₅PBu^t ₂}₂]. The complex [Rh₂Cl₂(CNMe)₂{Bu^t ₂P(CH₂)₅PBu^t ₂}₂] gives only one rotamer in solution in which the two rhodiums are chemically non-equivalent. The diphosphine Bu^t ₂PCH₂CHCHCH₂PBu^t ₂ reacts similarly with [Rh₂Cl₂(C₈H₁₄)₄] and MeNC but also gives a small amount of another (unidentified) product. Infrared and ¹H, ³¹P, and ¹⁰³Rh n.m.r. data are given.