Pentamethylcyclopentadienyl rhodium(III) trifluorovinyl phosphine complexes and attempted intramolecular dehydrofluorinative coupling of pentamethylcyclopentadienyl and trifluorovinyl phosphine ligands

Abstract

The trifluorovinyl phosphine complexes [Cp*RhCl₂{PR_3−x(CFCF₂)_x}] (1 x = 1, a R = Ph, b Prⁱ, c Et; 2 x = 2, R = Ph) have been prepared by treatment of [Cp*RhCl(μ-Cl)]₂ with the relevant phosphine. The salt [Cp*RhCl(CNBu^t){PPh₂(CFCF₂)}]BF₄, 3, was prepared by addition of Bu^tNC to 1a in the presence of NaBF₄. The salt [Cp*RhCl{κP,κS-(CF₂CF)PPh(C₆H₄SMe-2)}]BF₄ was prepared as a mixture of cis (5a) and trans (5b) isomers by treatment of [Cp*RhCl(μ-Cl)]₂ with the phosphine-thioether (CF₂CF)PPh(C₆H₄SMe-2), 4, in the presence of NaBF₄. The structures of 1a–c and 5a have been determined by single-crystal X-ray diffraction. Intramolecular dehydrofluorinative carbon–carbon coupling between pentamethylcyclopentadienyl and trifluorovinylphosphine ligands of 1a, 3 and 5 has been attempted. No reaction was observed on treatment of the neutral complex [Cp*RhCl₂{PPh₂(CFCF₂)}], 1a, with proton sponge, however, 5a underwent dehydrofluorinative coupling to yield [{η⁵,κP,κS-(C₅Me₄CH₂CFCF)PPh(C₆H₄SMe-2)}RhCl]BF₄, 6. Other reactions, in particular addition of HF across the vinyl bonds of 5, occurred leading to a mixture of products. The cation of 3 underwent similar reactions.

Introduction

In contrast to fluorinated aryl and alkyl phosphines [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], fluoroalkenylphosphines have received relatively little attention [11], [12], [13]. Following the development of a convenient synthetic route to [CF₂CF]⁻Li⁺ from the readily available HFC-134a (CF₃CH₂F), and subsequent syntheses of trifluorovinylphosphines R_3−xP(CFCF₂)_x [14], [15], [16], a number of complexes of trifluorovinylphosphines with molybdenum [15], platinum [15], [17] and gold [15] have been reported over the past decade.

The trifluorovinyl substituent is known to be susceptible to nucleophilic attack [16], [18], with substitution of the fluorine atom trans to the phosphorus atom, which allows the opportunity for functionalizing the phosphine. One attractive possibility is the intramolecular coupling of metal-bound cyclopentadienyl and trifluorovinylphosphine ligands. Rhodium complexes of chelating bi- and tri-functional cyclopentadienyl–phosphine ligands have been synthesized by intramolecular dehydrofluorinative carbon–carbon coupling [19], [20], [21], [22], [23], [24]. These ligands contain a three-carbon atom linkage between the cyclopentadienyl ring and the phosphorus atom. To date the complexes synthesized by this method have been restricted to those in which two of the carbon atoms in the linkage are part of a fluoroaromatic group: tetrafluorophenyl [19], [20], [21], [24], fluorophenyl [22] or trifluoropyridyl [23]. The coupling occurs on addition of proton sponge to the salts [Cp*RhCl(PL)]⁺ (PL = chelating fluoroarylphosphine) or [Cp*RhCl_L(P)]⁺, or by heating a benzene solution of [Cp*RhCl(μ-Cl)]₂ and PL. The reaction is postulated to proceed by generation of a nucleophilic exo methylene carbon atom by loss of a proton from the pentamethylcyclopentadienyl ring of the cation and subsequent attack at the ortho position of a fluoroarene [19], [25]. If this mechanism is correct then the trifluorovinyl group would be suitable as a substituent leading to a three-carbon atom linkage in which two of the carbon atoms are part of an alkene.

Here we report rhodium piano stool complexes comprising trifluorovinylphosphines and an investigation into the intramolecular coupling of η⁵-pentamethylcyclopentadienyl to phosphine and phosphine–thioether ligands bearing trifluorovinyl substituents.