Abstract
A series of P-NHC-type hybrid ligands containing both PR2 and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L(Cy), L(tBu) and L(Ph) (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L(Ph)·n(HBr) (n = 1, 2) with Ag2O gave the pseudo-cubane [Ag4Br4(L(Ph))2], isostructural to [Ag4Br4(L(R))2] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of ·HBF4 (R = Ph, tBu) led to the dinuclear complexes [Ag2(L(R))2](BF4)2 which, in the solid state, feature heteroleptic Ag centres and a 'head-to-tail' (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic 'head-to-head' (HH) isomers is facilitated by ligand fluxionality. 'Head-to-tail' [Cu2Br2(L(R))2] (R = Cy, tBu) dinuclear complexes were obtained from L(R)·HBr and [Cu5(Mes)5], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L(R)l ed to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag4Br4(L(tBu))2] to AuI by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L(tBu) transfer and to the dinuclear complex [Au2Cl2L(tBu)] with one L(tBu) ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.
Highlights
Phosphine and N-heterocyclic carbene (NHC) donors are often compared because they readily coordinate to metal centres and display bonding analogies and tuneable stereo-electronic properties.[1]
Extending our previous work on P-based NHC hybrid ligands,12a,c,e we report an efficient and modular access to the RP–NHC-type (RP = PCy2, PtBu2 or PPh2) ligands and their coinage metal complexes
We have already reported the synthesis of tetranuclear, laddertype phosphine-functionalised NHC (P–NHC)-type CuI complexes by transmetallation from [Ag4Br4(LR)2] or by reaction of the phosphonium–imidazolium LR·2HBr salts with mesitylcopper(I) [Cu5(Mes)5],12e which has been used before to form CuI NHC complexes from imidazolium salts.[38]
Summary
Phosphine and NHC donors are often compared because they readily coordinate to metal centres and display bonding analogies and tuneable stereo-electronic properties.[1]. We have already reported the synthesis of tetranuclear, laddertype P–NHC-type CuI complexes by transmetallation from [Ag4Br4(LR)2] or by reaction of the phosphonium–imidazolium LR·2HBr salts with mesitylcopper(I) [Cu5(Mes)5],12e which has been used before to form CuI NHC complexes from imidazolium salts.[38] The coordination chemistry of the LR ligands with CuI was further investigated by using the monoprotic proligands LR·HBr. Reaction of LR·HBr (R = Ph, tBu, Cy) with [Cu5(Mes)5] resulted in the formation of the corresponding [Cu2Br2(LR)2] complexes in good yields (Scheme 4).
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