Homoleptic carbene complexes: Part IX. Bis(imidazolin-2-ylidene-1-yl)borate complexes of palladium(II), platinum(II) and gold(I)

The in situ synthesis of the monoanionic chelating dicarbene ligand bis(imidazolin-2-ylidene-1-yl)borate (BIS^R, R=Me (2a), Et (3a), ⁱPr (4a)) from potassium bis(imidazol-1-yl)dihydridoborate (1) via dialkylation with alkyl iodide and deprotonation with LiⁿBu is described. Treatment of PdI₂ and PtCl₂ with THF solutions of BIS^R (1:2) leads to the first neutral homoleptic tetracarbene complexes of these metals 5–9, which are highly soluble in organic media. According to the X-ray structure analyses of 5, 6 and 9, the bischelates adopt centrosymmetric trans double-boat conformations exhibiting the usual stereochemical features. Neutral heteroleptic dicarbene–phosphine complexes of palladium and platinum of the general formula [M(BIS^R)(I)(PEt₃)] (14–19) result in good yields from the reaction of BIS^R with [M(μ-Cl)Cl(PEt₃)]₂ (2:1). From X-ray structure determinations of 15, 16 and 18 two findings should be emphasized: the heavy out-of-plane bending of the boat-shaped chelate ring (which is the cause of chirality of these complexes), and the existence of two rather different M to carbene carbon bond lengths (indicating the stronger trans influence of PEt₃ as compared to I⁻). With Au(Cl)(PPh₃), instead of forming small chelate rings, the bidentate ligands BIS^R switch to a μ₂-η¹:η¹-bridging ligand type of function giving rise to the compounds 10–12. An X-ray study of 11 reveals the structure of a 12-membered dimetallacycle in a twisted boat-like conformation with a trans-annular Au⋯Au separation of 3.3610(7) Å, i.e. only a weak interaction. In addition, two by-products — the three-coordinate iodobis(triphenylphosphine)gold(I) (13) and trans-diiodobis(triethylphosphine)palladium(II) (20) — have been structurally characterized.