Cavitand Scission by Transition-Metal Centres – Cleaved Cavitand Chirality and Its Consequences
Résumé
Two molecules of the phosphine cavitand 5-diphenylphosphanyl-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]arene (1) each underwent cleavage of an O–CH2O bond when treated with 1 equiv. of [Ni(η5-C5H5)(1,5-cyclooctadiene)]BF4 to form a bis(phosphino–phenolato) complex (2) without a cyclopentadienyl ligand. The partially cleaved (P,O) ligands in 2 are chiral, and a single-crystal X-ray structure determination of 2 has shown that the complex molecule has close to twofold rotational symmetry with both ligand units in the same absolute configuration. As expected, the primary coordination sphere has a cis-NiO2P2 geometry. In contrast, C–O bond breaking occurred only once per metal centre when 1 was treated with [RuBr2(p-cymene)]2. This led to a crystallographically characterised complex [RuBr(P,O)(p-cymene)] (4) along with a minor amount of a species (5) that is assumed to be a stereoisomer. Upon crystallisation of 4, two distinct types of crystals were isolated from the same solution; some of these were the racemic compound, while the others were the racemic conglomerate