Mg metal reduction of the divalent precursor PdCl(2)(CNAr(Dipp2))(2) (Dipp = 2,6-diisopropylphenyl) provides the isolable, two-coordinate Pd(0) bis-isocyanide, Pd(CNAr(Dipp2))(2), which is the first stable monomeric isocyanide complex of zerovalent palladium. Variable temperature (1)H NMR and FTIR studies on Pd(CNAr(Dipp2))(2) in the presence of added CNAr(Dipp2) revealed that free and coordinated isocyanide undergo rapid exchange, but the components do not form a stable tris-isocyanide complex. Bis-isocyanide Pd(CNAr(Dipp2))(2) is active for oxidative addition reactions and readily reacts with benzyl chloride and mesityl bromide to form Pd(Cl)(Bz)(CNAr(Dipp2))(2) and Pd(Br)(Mes)(CNAr(Dipp2))(2), respectively. Room-temperature Suzuki-Miyaura cross-coupling reactions are mediated by Pd(CNAr(Dipp2))(2). Coordinatively and electronically unsaturated substrates also react with Pd(CNAr(Dipp2))(2). Addition of thallium(I) triflate (TlOTf) to Pd(CNAr(Dipp2))(2) results in the salt [TlPd(CNAr(Dipp2))(2)]OTf, while addition of O(2) results in the peroxo complex (eta(2)-O)Pd(CNAr(Dipp2))(2). Most remarkably, 2 equiv of nitrosobenzene react with Pd(CNAr(Dipp2))(2) to form the square planar complex (kappa(1)-N-PhNO)(2) Pd(CNAr(Dipp2))(2), the geometry of which strongly suggests the formation of a divalent Pd center. With the aid of density functional theory calculations, this valence change is rationalized in terms of a formal reduction of the bond order in each NO unit to 1.5.
Blocking the pass: Low-valent Ni centers readily bind Tl(I) ions in a synthetically reversible fashion. The Tl units, in turn, serve as coordination site protection agents for Ni with respect to incoming Lewis basic ligands. This synthetic sequence allows for the isolation of a reactive zero-valent Ni tris-isocyanide complex.
Detailed herein are synthetic, spectroscopic and reactivity studies for two isolable four-coordinate iridium(I) monohydride complexes of the simple formulation HIrL(3). Such complexes have been postulated as reactive species in several transformations, but definite evidence for their existence has remained elusive. To stabilize these complexes, the methyleneadamantyl substituted phosphine ligand P(CH(2)(1)Ad)(i-Pr)(2) (abbreviated L(mAd)) was employed because of the resistance of the adamantane cage toward cyclometalation reactions. Treatment of the dihydride-chloride complex, H(2)IrCl(L(mAd))(2) with PhMgBr under N(2) afforded the square planar complex HIr(N(2))(L(mAd))(2). Contrastingly, treatment of H(2)IrCl(L(mAd))(2) with Li[HBEt(3)] under N(2) generates the trihydride complex H(3)Ir(L(mAd))(2), which possesses an agostic interaction between the L(mAd) ligand and the Ir center. Dissolution of HIr(N(2))(L(mAd))(2) in Et(2)O or C(6)D(12) rapidly establishes an equilibrium mixture with the cyclometalated complex H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)). Despite the equilibrium between HIr(N(2))(L(mAd))(2) and H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)), addition of 2 equiv of H(2) or 1 equiv of H(2)O to the mixture cleanly generates the pentahydride complex H(5)Ir(L(mAd))(2) or the dihydride hydroxide complex H(2)Ir(OH)(L(mAd))(2), respectively. Sequential addition (n)BuLi and 12-crown-4 (12-c-4) to a HIr(N(2))(L(mAd))(2)/H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)), mixture provides the salt [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))], which contains another four-coordinate Ir(I) monohydride. (31)P{(1)H} NMR studies provide evidence that four-coordinate HIr(N(2))(L(mAd))(2) is deprotonated en route to [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))]. [Li(12-c-4)(2)][HIr(kappa(2)-P,C-L(mAd))(L(mAd))] deprotonates both H(2)N(2,6-(i-Pr)(2)C(6)H(3)) and HOC(6)F(5) under an N(2) atmosphere to regenerate HIr(N(2))(L(mAd))(2)/H(2)Ir(kappa(2)-P,C-L(mAd))(L(mAd)) equilibrium mixtures.
Abgeschirmt: Niedervalente Nickelzentren binden leicht und synthetisch reversibel Thallium(I)‐Ionen, die wiederum die Koordinationsstellen des Nickelzentrums vor angreifenden Lewis‐basischen Liganden schützen. Diese Synthesesequenz ermöglicht die Isolierung eines reaktiven, nullwertigen Nickel‐ Tris(isocyanid)‐Komplexes.
Binding of thallium(I) by low-valent nickel establishes the use of main-group ions as coordination site protection agents for transition-metal centers. As described by J. S. Figueroa and co-workers in their Communication on page 3473 ff., Tl(I) coordination provides a route to the preparation of a coordinatively unsaturated nickel trisisocyanide complex. Thallium functions as a protecting group towards Lewis bases and is readily removed upon addition of halide ions.
Die Bindung von Thallium(I) durch niedervalentes Nickel ist ein Beispiel für die Verwendung von Hauptgruppenionen zum Schutz von Koordinationsstellen von Übergangsmetallzentren. Wie J. S. Figueroa et al. in der Zuschrift auf schildern, eröffnet die Koordination von Thallium(I) einen Zugang zu einem koordinativ ungesättigten Nickel‐Tris(isocyanid)‐Komplex. Thallium wirkt als Schutzgruppe gegen Lewis‐Basen und kann durch Zugabe von Halogenidionen leicht entfernt werden.
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