Governing the oxidative addition of iodine to gold(i) complexes by ligand tuning

Abstract

While several gold(I) complexes of the type (L)AuX (X = Cl, Br) are known to undergo oxidative addition of elemental chlorine and bromine (X₂), respectively, to give the corresponding gold(III) complexes (L)AuX₃, the addition of iodine to (iodo)gold(I) compounds is strongly ligand-dependent, suggesting a crucial threshold in the oxidation potentials. A systematic investigation of this particular oxidative addition of iodine using a large series of tertiary phosphines as ligands L has shown that both electronic and steric effects influence the course of the reaction. The reactions were followed by ³¹P NMR spectroscopy and the products crystallized from dichloromethane–pentane solutions. Complexes with small triakylphosphines (PMe₃, PEt₃) are readily oxidized, while those with more bulky ligands (PⁱPr₃, P^tBu₃) are not. With L taken from the triarylphosphine series [PPh₃, P(2-Tol)₃, P(3-Tol₃), P(4-Tol)₃] no oxidation takes place at all, but mixed alkyl/aryl-phosphines [PMe_nPh_3−n] induce oxidation for n = 3 and 2, but not for n = 1 and 0. However, in cases where no oxidation of the gold atoms is observed, the synthons may crystallize as adducts with molecular iodine of the polyiodide type instead, which have an iodine rich stoichiometry. This fact explains inconsistent reports in the literature. The metal atoms in (L)AuI coordination compounds with L representing a tri(heteroaryl)phosphine [P(2-C₄H₃E)₃, E = O, S], a phosphite [P(OR)₃] or a trialkenylphosphine [PVi₃] are all not subject to oxidative addition of iodine. The dinuclear complex of the ditertiary phosphine Ph₂PCH₂PPh₂, (dppm)(AuI)₂, gives an iodine adduct (without oxidation of the metal atoms), but with 1,2-Ph₂P(C₆H₄)PPh₂ (dppbe) an ionic complex [(dppbe)AuI₂]⁺I₃⁻ with a chelated gold(III) centre is obtained. The gold(I) bromide complexes with tertiary phosphines are readily oxidized by bromine to give the corresponding gold(III) tribromide complexes, as demonstrated for (BzMePhP)AuBr and (Ph₃P)AuBr. With (dppm)(AuBr)₂ the primary product with mixed oxidation states was also isolated: (dppm)AuBr(AuBr₃). The crystal structures of the following representative examples and reference compounds have been determined: (Me₃P)AuI₃, (Me₂PhP)AuI₃, (ⁱPr₃P)AuI·1.5I₂, (Ph₃P)AuI·I₂, {[(2-Tol)₃P]AuI}₂·I₂, [(2-Tol)₃P]AuI, (dppm)(AuX)₂ (with X = Br, I), (dppm)AuBr(AuBr₃) and [(dppbe)AuI₂]⁺I₃⁻. The structures are discussed focusing on the steric effects. It appears that e.g. the reluctance of (Ph₃P)AuI to add I₂ is an electronic effect, while that of (ⁱPr₃P)AuI has its origin in the steric influence of the ligand.