A Push–Pull Pd^II Complex with a Ternary Pd–P–C⁺ Accepting End and a Key N‐Heterocyclic Carbene–Imid­azoliophosphine Ligand

Abstract

Neutral and cationic PdCl2 complexes based on the neutral frame of a cis‐chelating N‐heterocyclic carbene (NHC)–imidazolophosphine ligand involving an o‐phenylene bridge have been devised for comparison of their relative push–pull character. Both target complexes, which are globally isosteric in the Pd coordination sphere, have been prepared in 58–68 % yield by direct coordination of the corresponding cis‐chelating NHC‐imidazol(i)ophosphine “free ligand”, or by extrusion of a Ph2P+ phosphenium moiety from (imidazoliophosphine)PdII complex precursors. The cationic complex has also been obtained in 64 % yield by selective N‐methylation of the neutral counterpart. The charge transfer from the NHC donating end (LD) to the imidazol(i)ophosphine accepting end (LA) has been investigated by electronic spectroscopy and by calculations of the first excited states at the time‐dependent (TD)‐DFT level. Although all the excitations involving the LD and LA ends of the neutral complex converge to the Pd center, the excited states |S7> (305 nm) and |S8> (303 nm) of the cationic complex exhibit a net LD[M]LA charge‐tranfer (CT) character, which is clearly visualized by particle–hole density analysis. The push–pull complex involving a Pd–P–C+ π‐accepting end is regarded as a “ternary” counterpart of “binary” phosphine–borane complexes involving two‐center Pd–B σ‐accepting ends.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

“…By contrast, the neutral complex 28 is mainly characterized by the presence of excitations from [PdCl 2 ] MOs to the LUMO orbital localized on the Pd atom, probably due to too little difference in donating character between the neutral NHC and imidazolophosphine extremities. The existence of similar contributions in the first excited states of cationic complex 29 suggests, however, that if the electron‐poor imidazoliophosphine constitutes an actual option for the accepting end, more electron‐rich fragments have to be considered on the donating side …”

“… Preparation of the dipolar NHC‐imidazoliophosphine PdCl 2 complex 29 through three alternate routes: from the chelating imidazoliophosphine 26 , from the trans ‐dicationic PdCl 2 complex 27 , or from the cis ‐neutral PdCl 2 complex 28 …”

“…Based on these target characteristics, the cationic PdCl 2 complex 29 bearing a cis-chelating NHC-imidazoliophosphine ligand was synthesized in 59-67 %y ield following three different routes,n amely,b yd irect coordination of the chelating imidazoliophosphine 26,b ys elective cleavage of the N 2 C + ÀP bond of the trans-bis(imidazoliophosphine)PdCl 2 complex 27, or by N-methylation of the neutral cis-PdCl 2 complex 28 (Scheme 8). [58] UV/Vis spectroscopicm easurements and time-dependentD FT calculationso ft he first excited states led to the conclusion that cationic PdCl 2 complex 29 exhibits as ignificant L D [M]L A CT character,a se videnced by the presence of excitations from NHC molecular orbitals (MOs) to imidazoliophosphine-centered MOs. By contrast, the neutralc omplex 28 is mainlyc haracterizedb yt he presence of excitations from [PdCl 2 ]M Os to the LUMO orbitall ocalized on the Pd atom, probablyd ue to too little differencei nd onating character between the neutral NHC and imidazolophosphine extremities.…”

“…The existence of similar contributions in the first excited states of cationic complex 29 suggests, however,t hat if the electronpoor imidazoliophosphine constitutes an actual option for the accepting end, more electron-richf ragments have to be considered on the donating side. [58]…”

“…Scheme8.Preparation of the dipolar NHC-imidazoliophosphine PdCl 2 complex 29 through threealternater outes:from the chelating imidazoliophosphine 26, from the trans-dicationic PdCl 2 complex 27,orfromt he cis-neutral PdCl 2 complex 28. [58] Scheme9.Ligand effect on the Pt-catalyzed hydroarylation of propargyl aryl ether to chromene. [17] were finally prepared in 55-75 %y ield after selective chlorenium abstraction at the electrophilicC -2 positioni nduced by bulky tertiary phosphines such PPh 3 or PCy 3 throughaS N 2(Cl)type mechanism.…”