Is the R₃Si Moiety in Metal–Silyl Complexes a Z ligand? An Answer from the Interaction Energy

Abstract

The computation of metal-silyl interaction energies indicates the existence of situations in which the silyl group behaves as a Z-type ligand according to the Green method of covalent-bond classification. There is a scale of relative intrinsic silylicity Π, defined as the ratio of the intrinsic silyl-to-triflate interaction energy of a silyltriflate as a reference compound relative to the silyl-to-metal interaction of given complex, that can reveal in a straightforward manner the propensity of SiR groups to behave chemically as metal-bound "silylium" ions, namely, [SiR ] . Emblematic cases, either taken from the Cambridge Structural Database (CSD) or constructed for the purpose of this study, were also investigated from the viewpoints of extended transition-state natural orbitals for chemical valence (ETS-NOCV) and quantum theory of atoms in molecules (QTAIM) analyses. It is shown in the case of POBMUP-which is the iridium 1,3-bis[(di-tert-butylphosphino)oxy]benzene (POCOP) complex isolated by Brookhart et al.-how slight variations of molecular charge and structure can drastically affect the relative intrinsic silylicity of the SiEt group that is weakly bonded to the hydrido-iridium motif.

Entrapment of THF‐Stabilized Iridacyclic IrIII Silylenes from Double H−Si Bond Activation and H2 Elimination

“…Similar conclusions can be drawn for [ 2 d ] + and [ 4 ] + (silylicity related to [Si n BuH 2 ] + Π ≈1.3). Such high intrinsic silylicities Π were observed essentially for cationic metal–silane adducts showing catalytic activity in hydrosilylation reactions.…”

“…[18] Interestingly,t he productso ft he reactiono f[ 1a,b] [BArF 24 ]w ith HSiEt 3 , [ 2a,b] [BArF 24 ]a re neithers o-called s complexes [3f, 19] nor the products of the oxidative addition of the SiÀHb ond to the Ir center [3f, 19a, b, 20] or any of its arrested states, [21] but [(IrH)!SiEt 3 ] + donor-acceptor complexes (Scheme 1) resulting from concerted transfero fh ydride from the silane to the electrophilic Ir III centera nd captureo ft he labile silylium moiety. [17][18]22] The present investigation of the reactiono f[ 1b] [BArF 24 ]w ith phenylsilane and n-butylsilane revealedt hat not only such [(IrH)!SiRH 2 ] + adducts are formed in solutionb ut also that THF assists actively their transformation into highly unstable cationic Ir III silylenes, of whicht wo iridacyclice xamples were successfully trapped by reactive crystallization. The silylene complex arising from the reaction of [1b] [BArF 24 ]w ith H 3 SiPh was found to promote the catalytic hydrodefluorination(HDF) of fluoroalkanes.…”

“…Figure shows the electron‐density redistribution (red arrows) in the H 2 ‐release process according to the ETS‐NOCV analysis, which entails evolution from a donor–acceptor Ir→Si interaction (wbi≈0.44, silylicity Π related to the [SiPhH 2 ] + moiety ≈1.3) in [ 2 c ] + to a reversed dative Ir←Si or a coordinative Ir−Si bond bearing some Ir→Si π character in [ 3 ] + . Similar conclusions can be drawn for [ 2 d ] + and [ 4 ] + (silylicity related to [Si n BuH 2 ] + Π ≈1.3).…”

“…By virtue of the microreversibility principle one can state from deformation density D1 1 (Figure 8) that the departure of H 2 entails:1 )electron-density donation to the Ir center and the IrÀSi s bond from the s IrÀH Ir and SiÀH Si bonds, 2) minor donation fromt he Si-boundT HF oxygen atom, and 3) crucial transfer of density to H Ir ,w hich rebalances the charge density between this hydrogen atom and H Si in the process of H 2 formation. Figure 9s hows the electron-density redistribution (red arrows) in the H 2 -release processa ccording to the ETS-NOCV analysis, which entails evolution from ad onor-acceptor Ir!Si interaction [17,18] (wbi % 0.44, silylicity P [22] [22] were observed essentiallyf or cationic metal-silane adducts showing catalytic activity in hydrosilylation reactions.…”

“…Complexes [ 1 a , b ][BArF 24 ] are efficient precatalysts for the alcoholysis of silanes and the hydrosilylation of carbonyls and nitriles . Interestingly, the products of the reaction of [ 1 a , b ][BArF 24 ] with HSiEt 3 , [ 2 a , b ][BArF 24 ] are neither so‐called σ complexes nor the products of the oxidative addition of the Si−H bond to the Ir center or any of its arrested states, but [(IrH)→SiEt 3 ] + donor–acceptor complexes (Scheme ) resulting from concerted transfer of hydride from the silane to the electrophilic Ir III center and capture of the labile silylium moiety . The present investigation of the reaction of [ 1 b ][BArF 24 ] with phenylsilane and n ‐butylsilane revealed that not only such [(IrH)→SiRH 2 ] + adducts are formed in solution but also that THF assists actively their transformation into highly unstable cationic Ir III silylenes, of which two iridacyclic examples were successfully trapped by reactive crystallization.…”