Labile Hydrido Complexes of Iridium(III):  Synthesis, Dynamic Behavior in Solution, and Reactivity toward Alkenes

Abstract

The trisacetonitrile complexes [IrClH(PiPr3)(NCCH3)3]BF4 (1) and [IrH2(PiPr3)(NCCH3)3]BF4 (2) have been prepared in one-pot reactions with high yields by reaction of the iridium(I) dimers [Ir(μ-Cl)(coe)2]2 and [Ir(μ-OMe)(cod)2]2 with the phosphonium salt [HPiPr3]BF4. The rates of exchange between free acetonitrile and the labile acetonitrile ligands of complexes 1 and 2 have been measured by NMR spectroscopy. This kinetic study has shown that both complexes readily dissociate one acetonitrile ligand trans to hydride, giving rise to fluxional five-coordinate intermediates. Substitution products 3−7 have been obtained by treatment of complexes 1 and 2 with CO and PMe3. The structures determined for 3−7 can be rationalized on the basis of the steric requirements of the ligands, indicating that the products are formed by thermodynamic control. Ethene inserts reversibly into the Ir−H bond of 1 to give the compound [IrCl(Et)(PiPr3)(NCCH3)3]BF4 (8), which has been used for the preparation of the stable ethyliridium(III) complexes [IrCl(Et)(PiPr3)(Py)2(NCCH3)]BF4 (9) and [Ir(η2-O2CCH3)Cl(Et)(PiPr3)(NCCH3)3] (10), respectively. The molecular structure of 10 has been determined by X-ray crystallography. The reaction of 2 with ethene, at low temperature, results in the sequential formation of the ethene complex [IrH2(η2-C2H4)(PiPr3)(NCCH3)2]BF4 (11) and the diethyl derivative [Ir(Et)2(PiPr3)(NCCH3)3]BF4 (14). At room temperature in solution, 14 undergoes reductive elimination of ethane to form the iridium(I) species [Ir(PiPr3)(NCCH3)3]BF4 (15) and [Ir(PiPr3)(η2-C2H4)(NCCH3)2]BF4 (16). These cations readily react with H2 to regenerate 2, closing a cycle for ethene hydrogenation in which several participating species have been identified. The reaction of 2 with propene in solution also allows the characterization of products of propene coordination (17) and insertion (18). In this case, the species obtained after elimination of propane are products of allylic C−H activation: [IrH(η3-C3H5)(PiPr3)(NCCH3)2]BF4 (19) and [IrH(η3-C3H5)(η2-C3H6)(PiPr3)(NCCH3)]BF4 (20). The structure of complex 19 has been determined by X-ray diffraction, and the kinetics of dissociation of its two labile acetonitrile ligands have been studied by NMR spectroscopy. Complex 19 undergoes electrophilic activation of H2 to give propene and reform the starting complex 2.

Unusual 1‐Alkyne Dimerization/Hydrogenation Sequences Catalyzed by [Ir(H)2(NCCH3)3(P‐i‐Pr3)]BF4: Evidence for Homogeneous‐Like Mechanism in Imidazolium Salts

“…Thus, the reaction with t-BuC CH selectively gave compound (5) in quantitative yield. The structure of 5 determined by X-ray diffraction is shown in Figure 1.…”

“…[5] This compound has also been found to facilitate the spectroscopic observation and characterization of labile species likely involved in the catalytic cycles, as illustrated by the intermediates of ethene hydrogenation depicted in Scheme 1. A noticeable feature of this cycle is the formation of the bis-ethyl compound 3, which results from an unusual double insertion of ethene into the two Ir-H bonds of 1.…”

“…t-BuCCH (Lancaster) was used as received without further purification. 1-Butyl-3-methylimidazolium tetrafluoroborate (BMIM ¥ BF 4 ), [10] and the complex [Ir(H) 2 (NCCH 3 ) 3 (P-i-Pr 3 )]BF 4 (1), [5] were prepared as previously reported. BMIM ¥ BF 4 was dried by azeotropic distillation with benzene several times, followed by prolonged evacuation.…”

Evidence for a Dinuclear Mechanism in Alkyne Hydrogenations Catalyzed by Pyrazolate-Bridged Diiridium Complexes

“…The Ir À N(5) bond length 2.051(4) is shorter than those found for other labile acetonitrile ligands trans to terminal hydrides (2.13 ± 2.15 ), [5,9] but similar is to that found in complex 1 (2.04(2) ) in which the dissociation of acetonitrile with low activation energies is still possible. [4] In agreement with this observation, the acetonitrile ligand of 2 is readily substituted by [D 3 ]acetonitrile in benzene at room temperature.…”

“…The latter chelates the metal with a bite angle of 65.74(18)8, which is in the range found for related chelating ligands such as h 3 -allyl or acetate. [5] As a result of this small bite angle, the angles around C(6), the central carbon of the chelate, strongly deviate from the sp 2 ideal values, as also found in other complexes containing related ortho-metallated ligands. [6] The hydride ligand was located in the difference Fourier maps, bridging the iridium atoms with remarkably different distances of 1.52(4) and 1.99(4) .…”