Synthesis and Structure of PNP-Supported Iridium Silyl and Silylene Complexes: Catalytic Hydrosilation of Alkenes

Abstract

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)) iridium complexes (PNP)IrH(2) and (PNP)Ir(COE) (11) afforded iridium silyl hydride complexes (PNP)Ir(H)(SiRR'R'') (3-8). Addition of 2 equiv of PhSiH(3) or (3,5-Me(2)C(6)H(3))SiH(3) to (PNP)IrH(2) or 11 yielded disilyl complexes (PNP)Ir(SiH(2)R)(2) (R = Ph (9), 3,5-Me(2)C(6)H(3) (10)). Hydride abstraction from (PNP)Ir(H)(SiH(2)R) (R = Trip (5), Dmp (6)) by [Ph(3)C][B(C(6)F(5))(4)] afforded iridium silylene complexes [(PNP)(H)Ir=SiR(H)][B(C(6)F(5))(4)] (R = Trip (12), Dmp (13)) exhibiting downfield (29)Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield (1)H NMR resonances for the Si-H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)Ir=SiPh(2)][A] (A = (-)B(C(6)F(5))(4) (14), (-)CB(11)H(6)Br(6) (16)) were isolated via hydride abstraction from (PNP)Ir(H)(SiHPh(2)). The X-ray structure of 16 confirmed sp(2) hybridization at silicon and revealed a short Ir-Si bond of 2.210(2) A. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)Ir=SiMes(H)][B(C(6)F(5))(4)] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H(3)SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))IrH(SiH(2)Mes)][B(C(6)F(5))(4)] (19), featuring a beta-agostic interaction demonstrated by a J(SiH) of 102 Hz for the N-SiH hydrogen. Similarly, addition of H(2)SiPh(2) to 16 afforded the structurally characterized Ir(V) disilyl complex [(PNP)(SiPh(2))Ir(SiPh(2)H)(H)(2)][CB(11)H(6)Br(6)] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[415] Theuse of WCA precursors to effect the formation of cationic transition-metal silylenes through abstraction of halides,p seudohalides,o ra lkyl substituents permitted the [405] 2011 Inorganic/organometalliccatalysts and initiators involving weakly coordinating anions for isobutene polymerisation [406] 2013 Cationic Polymerization of IsobutyleneatR oom Temperature [407] 2014 DevelopmentofCatalysts and Ligands for Enantioselective Gold Catalysis [408] Angewandte Chemie Reviews discovery of an alternative mechanism of hydrosilylation (Scheme 20). [420,421] Ther elated neutral Lewis acid B(C 6 F 5 ) 3 remains the most widely utilized main-group catalyst for the hydrosilylation of aromatic aldehyde,k etone,a nd ester substrates.T he reaction mechanism has been shown, through aseries of detailed studies,t op roceed through the activation of the SiÀHb ond by the B(C 6 F 5 ) 3 Lewis acid not through the activation of the hydrosilylation substrate. Thea lkene inserts into the Si À Hb ond to yield exclusively the products of anti-Markovnikov addition.…”

“…Thea lkene inserts into the Si À Hb ond to yield exclusively the products of anti-Markovnikov addition. [420,421] Ther elated neutral Lewis acid B(C 6 F 5 ) 3 remains the most widely utilized main-group catalyst for the hydrosilylation of aromatic aldehyde,k etone,a nd ester substrates.T he reaction mechanism has been shown, through aseries of detailed studies,t op roceed through the activation of the SiÀHb ond by the B(C 6 F 5 ) 3 Lewis acid not through the activation of the hydrosilylation substrate. [422][423][424] More recently,t he ferrocenyl-substituted silylium ion [FcSi( t Bu)-(Me)] + supported by the [B-(C 6 F 5 ) 4 ] À ion has even been shown to effect the reduction of ketones without competing deoxygenation pathways.…”

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

“…The structures of [ 3 ][BArF 24 ] and [ 4 ][BArF 24 ] were obtained from crystals grown in a narrow tube by diffusion of a heptane solution of silane into a solution of [ 1 b ] + in CH 2 Cl 2 /THF through a buffer layer of dry benzene, that is, reactive crystallization, which proved to be the only way to trap crystals of the key Ir silylene complexes. The structures (Figure a and b) reveal that Si−Ir interatomic distances remain somewhat in the range of those reported for exogenous‐donor‐devoid Ir III silylene complexes (Ir−Si ≈2.25 Å), of the triflate‐bonded Ir silylene of Klei, Tilley, and Bergman (Ir−Si ≈2.30 Å), and other known neutral donor‐bound metal silylene complexes (M−Si(donor)RR ≈2.26–2.32 Å). The Ir−Si distances are consistently shorter by about 0.2 Å in [ 3 ] + and [ 4 ] + than in the structures of [ 2 a , b ] + …”

“…A pertinent reference case reported by Calimano and Tilley is the cationic Ir III silylene complex devoid of any donor–acceptor interaction with the Si center shown in Figure , for which the ETS‐NOCV analysis also carried out under the assumption of an interaction between a neutral :SiPh 2 silylene and a cationic Ir‐centered residue indicates that Ir‐to‐Si π‐backdonation represents only 14 % of the total interfragment orbital interaction energy, which is less than for the fictitious THF‐devoid [ 3 ] + and most probably a consequence of the weaker donating properties of the ligand set in the PNP complex (Figure ).…”

“…Main deformationdensity isosurface plots (0.005 ebohr À3 )r esulting from the ETS-NOCV analysisoft he interaction of aneutral prepared silylene fragment SiPh 2 with the cationic Ir III group of an Ir silylene complex reported by Calimano and Tilley. [6] The doublered arrows uggests which bond is cleaved in the fragmentationmode. D1 1 corresponds to the establishment of the SiÀIr s bond.…”