Triorganylphosphoranides: Realization of an Unusual Structural Motif Utilizing Electron Withdrawing Pentafluoroethyl Groups

Phosphoranides comprising three phosphorus carbon bonds are scarcely represented in the literature. We now utilized tris(pentafluoroethyl)phosphane, P(C₂ F₅ )₃ , and cyanobis(pentafluoroethyl)phosphane, P(C₂ F₅ )₂ (CN), featuring electron withdrawing pentafluoroethyl groups, to synthesize such compounds. Metal fluorides MF (M=Ag, Cs) add to P(C₂ F₅ )₃ yielding respective M[P(C₂ F₅ )₃ F] salts. Those M[P(C₂ F₅ )₃ F] subsequently suffer a loss of C₂ F₄ , furnishing M[P(C₂ F₅ )₂ F₂ ]. The cesium salt decomposes instantaneously when warmed to rt, whereas the silver salt decomposes slowly over several days at rt. Treatment of Ag[P(C₂ F₅ )₃ F] with 2,2'-bipyridine facilitated the isolation and structural characterization of [Ag(bipy){P(C₂ F₅ )₃ F}]. With P(C₂ F₅ )₂ (CN), AgF reacts under substitution of the cyano group yielding P(C₂ F₅ )₂ F, rather than phosphoranide formation. However, [K(18-crown-6)]F adds to P(C₂ F₅ )₂ (CN) furnishing [K(18-crown-6)][P(C₂ F₅ )₂ (CN)F]. Its structural characterization was successful, despite its tendency to undergo an exchange of substituents, yielding [K(18-crown-6)][P(C₂ F₅ )₂ F₂ ] and presumably [K(18-crown-6)][P(C₂ F₅ )₂ (CN)₂ ]. The latter forms an equilibrium with [K(18-crown-6)]CN and P(C₂ F₅ )₂ (CN) which lies well on side of the phosphane and cyanide salt.

Free Difluorobis(pentafluoroethyl)phosphoranide Ion and Its Ligand Properties

In this contribution, we report on the "free" [P(C₂F₅)₂F₂]^- ion and its ligand properties in transition metal complex chemistry. For this purpose, Ag[P(C₂F₅)₂F₂] was treated with [{(Et₂N)₃P═N}₃P═NHC(CH₃)₃]Cl ([EtP₄H]Cl) to yield [EtP₄H][P(C₂F₅)₂F₂], featuring a weakly coordinating phosphazenium cation. Due to the weak interaction between the cation and anion, the [P(C₂F₅)₂F₂]^- ion meets the so-called pseudo-gas-phase conditions. To determine the Tolman electronic parameter, [EtP₄H][Ni(CO)₃{P(C₂F₅)₂F₂}] was prepared from [EtP₄H][P(C₂F₅)₂F₂] and [Ni(CO)₄], facilitating the classification of the P(C₂F₅)₂F₂ moiety as a moderately π-acidic ligand. By treatment of [EtP₄H][P(C₂F₅)₂F₂] with [AuCl(tht)], the neutral tetrahydrothiophene ligand was substituted by the phosphoranide ion, yielding [EtP₄H][AuCl{P(C₂F₅)₂F₂}]. When Ag[P(C₂F₅)₂F₂] was treated with [AuCl(tht)], on the other hand, the chloride was substituted. Transmetalation reactions of this type proved to be an efficient transfer method of the P(C₂F₅)₂F₂ moiety, as further demonstrated by the reactions of Ag[P(C₂F₅)₂F₂] with [FeCl(CO)₂Cp], [FeCl(CO)₂Cp*], and [PdCl₂(NCMe)₂]. Surprisingly, P(C₂F₅)₂F demonstrated fluorinating abilities toward [FeCl(CO)₂Cp], [FeCl(CO)₂Cp*], [AuCl(tht)], and [PdCl₂(NCMe)₂]. Apparently, fluorido transition metal complexes were generated in situ under the formation of P(C₂F₅)₂Cl. The fluorido iron and palladium complexes transfer their fluoride ions onto P(C₂F₅)₂F, yielding the respective phosphoranido complexes, featuring the P(C₂F₅)₂F₂ moiety.

Difluorobis(pentafluoroethyl)phosphoranide: A Promising Building Block for Phosphoranidometal Complexes

Despite the fact that different metal tetrafluorophosphoranides, M[PF₄] (M = Cs, Ag, K), decompose readily, we successfully enhanced the stability of such species by the replacement of two fluorine atoms with electron withdrawing pentafluoroethyl groups. Thus, AgF adds to P(C₂F₅)₂F, yielding Ag[P(C₂F₅)₂F₂], which served as a starting material for the synthesis of mono-, bis-, and tris[difluorobis(pentafluoroethyl)phosphoranido]silver complexes. Addition of 2,2'-bipyridine allowed for the isolation of stable [Ag(bipy){P(C₂F₅)₂F₂}], whereas the reaction with the chlorides [NMe₄]Cl and CoCl₂ afforded the bis- and trisphosphoranidoargentates [NMe₄][Ag{P(C₂F₅)₂F₂}₂(OEt₂)] and [Co(NCMe)₆][Ag{P(C₂F₅)₂F₂}₃]·2MeCN, respectively. Altogether, the difluorobis(pentafluoroethyl)phosphoranido moiety serves as a novel, small, noncyclic phosphoranido ligand. It provided access to the first homoleptic phosphoranidometal complex, [Co(NCMe)₆][Ag{P(C₂F₅)₂F₂}₃]·2MeCN, which itself features the unusual structural motif of an [AgX₃]^2- ion.