A specific formation of an iridium(III) hydrido complex bearing 8-(diphenylphosphino)quinoline

Highlights

• Iridium(III) hydrido complex with 8-(diphenylphosphino)quinoline was characterized.

• A possible mechanism for the hydrido complex formation was suggested.

• Crystal structures of [Cp*IrCl(Ph₂Pqn, diphos or phen)]PF₆were analyzed.

Abstract

A reaction of [Cp*IrCl(Ph₂Pqn)]PF₆ {Cp* = η⁵-pentamethylcyclopentadienyl; Ph₂Pqn = 8-(diphenylphosphino)quinoline} and Ag(CF₃SO₃) in methanol afforded orange crystals of the corresponding hydrido complex, [Cp*IrH(Ph₂Pqn)]PF₆, which was identified by ¹H, ³¹P{¹H} NMR and IR spectroscopy as well as X-ray structural analysis. The reactions in deuterated solvents indicated that formation of the hydrido complex proceeded via β-hydrogen elimination of the coordinated methanol molecule. It was also revealed that the hydrido formation was specific for the complex bearing Ph₂Pqn ancillary ligand; the analogous complex with 1,2-bis(diphenylphosphino)benzene (diphos) or 1,10-phenanthroline (phen) did not give the corresponding hydrido complex by a similar reaction with Ag⁺ in methanol. In order to elucidate the reason for the different reactivity among these complexes, the crystal structures of the precursor chlorido complexes, [Cp*IrCl(Ph₂Pqn)]PF₆, [Cp*IrCl(diphos)]PF₆ and [Cp*IrCl(phen)]PF₆, as well as an acetonitrile complex of [Cp*Ir(Ph₂Pqn)(CH₃CN)](PF₆)₂, were also determined by X-ray analysis. The resulting structural information suggested that a specific formation of the hydrido complex with Ph₂Pqn could be originated from the facile formation of the corresponding methanol complex and the hemilabile nature of ancillary Ph₂Pqn ligand, which induced the reactivity of the coordinated methanol toward β-hydrogen elimination.

Introduction

Pentamethylcyclopentadienyl-iridium(III) or -rhodium(III), Cp*M^III (M = Ir or Rh), are versatile and useful metal fragments for many aspects in coordination and organometallic chemistry, i.e., activation of unreactive bonds or small molecules [1], [2], [3], [4], catalytic or stoichiometric chemical transformations [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], construction of molecular box or cage compounds [15], [16], [17], and so on [18], [19], [20]. Octahedral Ir^III and Rh^III complexes are in general very inert for ligand-substitution, but their Cp* complexes become somewhat hemilabile due to the strong trans effect of the Cp* ligand. Also, because the Cp*M^III fragment induces the products to be kinetically and/or thermodynamically stable, the reactive intermediates can often be isolated or detected spectroscopically. Furthermore, these properties lead to prepare a series of Cp*M^III complexes with analogous ligands easily and systematically, and allow to compare their structures and properties in terms of the ancillary ligand effects [20], [21], [22]. We have been investigated a number of Cp*Ir^III complexes in this context. For example, photolysis product of the azido complexes of [Cp*Ir^III(N₃)(L–L)] were largely dependent on the ancillary ligand, L–L [23], [24], [25]. In the dinuclear complexes of [{Cp*M(L–L)}₂(μ-MeCN₄)]ⁿ⁺ (MeCN₄⁻ = 5-methyltetrazolate) it was found that the bridging mode of MeCN₄⁻ was dependent on the L–L ligands [25], [26]. When we tried to prepare several derivatives of Cp*Ir complexes having 1,8-naphthyridine (napy: Scheme 1), we have obtained an interesting hydrido-bridged dinuclear product from a reaction mixture of [Cp*IrCl₂(napy)] and an equimolar of AgPF₆ in a mixture of methanol and dichloromethane. The yield of the complex, [(Cp*IrCl)₂(μ-napy)(μ-H)]PF₆ was small, but an isolable amount of the red crystals were deposited, together with a main product of orange [Cp*IrCl(napy)]PF₆ and another minor by-product of pale yellow [Cp*Ir(napy)₂](PF₆)₂ [27]. Such a formation of Cp*Ir^III hydrido complexes in alcohols was rarely observed [1](a), [28] [28] and limited to some specific ancillary ligand systems. In this study we will report another example of formation of a Cp*Ir^III hydrido complex when the abstraction of coordinated Cl⁻ is taken place by Ag⁺ ion from [Cp*IrCl(Ph₂Pqn)]PF₆ {1; Ph₂Pqn = 8-(diphenylphosphino)quinoline} in methanol. The molecular structure and spectroscopic properties of the product as well as a plausible formation mechanism are described.