Phosphenium Versus Pro-Phosphide Character of P-tert-butyl-dicyclopropeniophosphine: Zwitterionic Palladate Complexes of a Dicationic Phosphido Ligand

Abstract

With the view to enhancing the unique coordinating ability of the known phenyl-tetrakis(diisopropylamino)dicyclopropeniophosphine (Ph-DCP), replacement of the phenyl substituent by a tert-butyl substituent was envisaged. Both α-dicationic R-DCP phosphines, with R = Ph and Bu, were prepared in 54%-55% yield by substitution of RPCl with two equivalents of bis(diisopropylamino)-dicyclopropenylidene (BAC) and metathesis with NaBF. This method is implicitly consistent with the representation of R-DCPs as BAC-phosphenium adducts. The R-DCP salts were found to coordinate hard and soft Lewis acids such as a promoted oxygen atom (in the singlet spin state) in the corresponding R-DCP oxides, and electron-rich transition-metal centers in η-R-DCP complexes with AuCl, PtCl, or PdCl, respectively. Coordination of Ph-DCP with PdCl, which is a more electron-deficient Pd(II) center, leads to pentachlorinated dinuclear complexes [(Ph-DCP)PdCl]Cl, where the dicoordinate Cl bridge screens the repelling pairs of positive charges from each other. The same behavior is inferred for the Bu-DCP ligand, from which addition of an excess of (MeCN)PdCl was found to trigger a heterolytic cleavage of the DCP-Bu bond, releasing Bu and a dicationic phosphide, DCP: the latter is evidenced as a ligand in a tetranuclear complex ion [(μ-DCP)PdCl], which, upon HCl treatment, dissociates to a doubly zwitterionic dipalladate complex. All the complexes were isolated in 82%-97% yield, and five of them were characterized by X-ray crystallography.

Assessing the Ligand Properties of NHC‐Stabilised Phosphorus(I) Cations

“…Simultaneously, Weigand reported the preparation of coinage metal complexes of a much more bulky, asymmetrically substituted variant . Remarkably it has recently been shown that a dicationic phosphine may act as a cyclopropenylidene‐stabilised P I pro‐ligand in the presence of excess PdCl 2 by heterolytic cleavage of a tert ‐butyl group . Considering the intensifying interest in this field and in the field of cationic ligands in general, we now report the isolation of various cationic metal–carbonyl complexes of the NHC‐stabilised phosphorus(I) ligand [L] + ([L] + =[bis(1,3,4,5‐tetramethylimidazol‐2‐ylidene)phosphanide] + ).…”

“…[77] Remarkably it has recentlyb een shown that ad icationic phosphine may act as ac yclopropenylidene-stabilised P I pro-ligand [78] in the presenceo fe xcess PdCl 2 by heterolytic cleavage of a tert-butyl group. [79] Considering the intensifying interesti nt his field and in the field of cationic ligands in general, we now report the isolation of various cationic metal-carbonyl complexes of the NHC-stabilised phos-…”

α-Cationic Phosphines: from Curiosities to Powerful Ancillary Ligands

“…1b, 14 An alternative route for the synthesis of ligands 15, which utilizes the reaction of chlorophosphines with bis(diisopropylamino)dicyclopropenylidene 18, was subsequently described. 15 Although both 16 and 17 are catalytically competent in alkyne hydroarylation reactions, the Scheme 2 Effect of the cationic phosphine in a) hydroformylation of 1-octene, b) Sonogashira coupling with acid chlorides, and c) cycloisomerization of allendienes. Reagents and conditions: (i) 4, 5, or6 (0.2 mol%), [Rh(CO) 2 acac] (0.1 mol%), CO/H 2 (1:1), 10 bar, 100 °C; (ii) 7 or 8 (0.5 mol%), CuI (1 mol%), Et 3 N, THF, RT; (iii) [LAuCl]…”

Bis(N-cyclopropenio)-imidazol-2-ylidene: An N-Heterocyclic Carbene Bearing Two N-Cationic Substituents

“…Analogy between mono- ( A + – B + ) and dicationic phosphines ( A 2+ – B 2 + ) , (left) vs mono- ( C + ) and dicationic NHCs 1 2+ (right).…”