Nickel and Palladium Silyl Pincer Complexes: Unusual Structural Rearrangements that Involve Reversible SiC(sp³) and SiC(sp²) Bond Activation

Abstract

Transition-metal pincer complexes have been the subject of numerous reactivity studies that range from explorations of catalytic activity to investigations of challenging bond-activation reactions. [1] With the goal of extending the versatility of metal pincer chemistry, significant effort has been devoted to the development of new pincer-like tridentate ligands by variation of the steric and electronic properties of the donor fragments and/or the ligand backbone. [1,2] In this context, pincer ligands that feature a bis(o-phenylene) backbone have emerged as particularly attractive candidates for the synthesis of highly reactive early-and late-transition-metal complexes. [3][4][5][6] Such o-phenylene linkers provide a relatively rigid ligand architecture, and the lack of b-hydrogen atoms in the resulting complexes circumvents potential ligand degradation by b-hydride elimination. A variety of such phenylene-bridged pincer ligands have been reported, including examples where the central donor is an amido, [1a, 3] phosphido, [4] carbene, [5] or silyl group. [6,7] In this regard, we have recently introduced silyl pincers of the type [k 3 -(2-R' 2 PC 6 H 4 ) 2 SiMe] À ((R'ÀPSiP), R' = Ph, cyclohexyl (Cy)); [6a-c] we considered that the strongly electron-donating and translabilizing central Si donor may promote the formation of electron-rich and coordinatively unsaturated complexes that exhibit aggressive reactivity with s bonds. Indeed, we have demonstrated that [(Cy À PSiP)Ir] species undergo facile arene CÀH bond-activation chemistry. [6b] We have recently begun to investigate Group 10 metal (R'-PSiP) complexes and have reported the synthesis of [(R'À PSiP)Pt II ] species, including examples of square-planar complexes that feature alkyl, aryl, or silyl substitution trans to the pincer Si donor. [6c] In expanding this chemistry to Ni and Pd, we uncovered an unusual ligand rearrangement for [(Cy À PSiPM)(alkyl)] species (M = Ni, Pd) that involves remarkably facile SiÀC(sp 3 ) and SiÀC(sp 2 ) bond-cleavage processes; these results are reported herein. Notably, for M = Ni, these SiÀC bond-activation processes are reversible on the timescale of NMR spectroscopy in solution. Although SiÀC(sp 2 ) bond activation is well-documented, [8] examples of unstrained SiÀ C(sp 3 ) bond cleavage within the coordination sphere of a mononuclear metal complex are extremely rare, [9,10] and are unprecedented for Ni. [11] By comparison, this ligand rearrangement was not observed in our previously reported [(R'À PSiP)Pt II ] chemistry. [6c] In the pursuit of new Ni and Pd alkyl complexes, the [(CyÀ PSiP)MCl] species (M = Ni, 1; M = Pd, 2) were treated with alkyl lithium and Grignard reagents. In the case of 2, treatment with 1 equivalent of MeLi led to the formation of [(Cy À PSiP)PdMe] (3), which was isolated in 78 % yield (Scheme 1). [12] The NMR spectra of isolated 3 (in [D 6 ]benzene) are consistent with a C S -symmetric complex, as indicated by the presence of a single resonance in the 31 P NMR spectrum at d = 60.5 ppm. The ...

On the Stability of a POCsp3OP‐Type Pincer Ligand in Nickel(II) Complexes

“…A few recent examples include reports on complexes featuring PCP-, [4] PNP-, [5] and PSiP-type [6] pincer ligands, whereas Ni complexes featuring resorcinol-derived POCOP ligands have been shown to be prone to oxidative degradation. A few recent examples include reports on complexes featuring PCP-, [4] PNP-, [5] and PSiP-type [6] pincer ligands, whereas Ni complexes featuring resorcinol-derived POCOP ligands have been shown to be prone to oxidative degradation.…”

“…In this context, there have been sporadic reports describing pincer complexes in which one or more binding moiety undergoes unexpected reactions that introduce major modifications in the pincer framework. A few recent examples include reports on complexes featuring PCP-, [4] PNP-, [5] and PSiP-type [6] pincer ligands, whereas Ni complexes featuring resorcinol-derived POCOP ligands have been shown to be prone to oxidative degradation. [7] Indeed, the aliphatic analogues of POCOP ligands, POC sp3 OP (Scheme 1), are particularly vulnerable to complex side reactions leading to decomposition.…”

Sequential Barium‐Catalysed N−H/H−Si Dehydrogenative Cross‐Couplings: Cyclodisilazanes versus Linear Oligosilazanes

“…Although not common, the rupture of SiÀC bonds has already been documented under other circumstances. [35] The rhodium-catalysed coupling of 2-trimethylsilylphenylboronic acid with internal alkynes produces 2,3-disubstituted benzosilole derivatives. [33] More recently,s everal cases of metal-promoted SiÀCb ond ruptures have been disclosed for both C sp 2 and C sp 3 carbon atoms.…”

“…[34] Nickel andp alladium silyl pincerc omplexes can undergo structural rearrangements implicating reversible SiÀC sp 3 and SiÀC sp 2 bond activation. [35] The rhodium-catalysed coupling of 2-trimethylsilylphenylboronic acid with internal alkynes produces 2,3-disubstituted benzosilole derivatives. [36] 2 SiMeR (R = Ph, Me, Et);i tw as shown that the facility of bond activation increased with temperature and according to SiÀEt < SiÀMe < SiÀPh.…”

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

“…Over the last decade, pincer ligands with two phosphine donors and a central anionic Si donor (PSiP ligands) have attracted attention owing to the high trans influence of the Si atom and the rigidity of the pincer framework 7. Turculet and co‐workers have demonstrated that complexes supported by this ligand can undergo reversible SiC bond cleavage,7f whereas Takaya and Iwasawa utilized catalysts with PSiP ligands for the carboxylation of allenes under mild conditions 7d. Complexes supported by the PSiP ligand have not only given rise to interesting reactivity but have also led to the formation of structurally remarkable species.…”