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

The in situ synthesis of the monoanionic chelating dicarbene ligand bis(imi dazolin-2-ylidene-1-yl)borate (BISR, R = Me (2a), Et (3a), Pr-i (4a)) from potassium bis(imidazol-1-yl)dihydridoborate (1) via dialkylation with alkyl iodide and deprotonation with (LiBu)-Bu-n is described. Treatment of PdI2 and PtCI2 with THF solutions of BISR (1:2) leads to the first neutral homol eptic 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 exhib iting the usual stereochemical features. Neutral heteroleptic dicarbene-pho sphine complexes of palladium and platinum of the general formula [M(BISR)( I)(PEt3)] (14-19) result in good yields from the reaction of BISR with [M(m u -CI)CI(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 complex es), and the existence of two rather different M to carbene carbon bond len gths (indicating the stronger trans influence of PEt3 as compared to I-). W ith Au(Cl)(PPh3), instead of forming small chelate rings, the bidentate lig ands BISR switch to mu (2)-eta (1):eta (1)-bridging ligand type of function giving rise to the compounds 10-12. An X-ray study of 11 reveals the struc ture of a 12-membered dimetallacycle in a twisted boat-like conformation wi th a trans-annular Au . . . Au separation of 3.3610(7) Angstrom, i.e. only a weak interaction. In addition, two by-products - the three-coordinate iod obis(triphenylphosphine)gold(I) (13) and trans-diiodobis(triethylphosphine) palladium(II) (20) - have been structurally characterized. (C) 2001 Elsevie r Science B.V. All rights reserved.