Palladacycles of novel bisoxazoline chelating ligands based on the dimeric cyclobutadiene linked cobalt sandwich compound [(η5-Cp)Co(η4-C4Ph3)]2

Abstract

The reaction of 1,4-diphenylbutadiyne along with diphenylacetylene, when carried out with η⁵-[MeOC(O)Cp]Co(PPh₃)₂ resulted in the cyclobutadiene linked dimeric cobalt sandwich compound {[η⁵-MeOC(O)Cp]Co(η⁴-C₄Ph₃)}₂ (1) along with the known monomeric compound η⁵-[MeOC(O)Cp]Co(η⁴-C₄Ph₄). Compound 1, on treatment with KOH gave the dicarboxylic acid {η⁵-[HOC(O)Cp]Co(η⁴-C₄Ph₃)}₂ (2) which on reaction with oxalyl chloride followed by (S)-2-amino-3-methyl-1-butanol, triethylamine and mesylchloride was converted to the bis(cobalt oxazoline) based chiral ligand {[η⁵-(4-iPr-2-Ox)Cp]Co(η⁴-C₄Ph₃)}₂ (3) (Ox = Oxazolinyl). Compound 3 on reaction with palladium acetate gave an NCN bound bis(cobalt-oxazolinyl) palladacycle {[η⁵-(4-iPr-2-Ox)Cp]Co(η⁴-C₄Ph₃)}₂PdOAc (4) having both the oxazolinyl groups bound to the palladium in a chelate mode and one of the Cp rings forming a five membered [C,N] palladacycle. Reaction of 4 with aq. NaCl resulted in the chloro analogue of 4, {[η⁵-(4-iPr-2-Ox)Cp]Co(η⁴-C₄Ph₃)}₂PdCl (5). A nonchiral bidentate bis cobalt oxazoline ligand {[η⁵-(Ox)Cp]Co(η⁴-C₄Ph₃)}₂ (6) was also prepared by replacing (S)-2-amino-3-methyl-1-butanol with 2-amino-1-ethanol in the procedure used for the preparation of 3. A reaction of 1,4-diphenylbutadiyne, diphenylacetylene and (η⁵-Cp)Co(PPh₃)₂ resulted in the dimer [(η⁵-Cp)Co(η⁴-C₄Ph₃)]₂ (7) and small amounts of a trimer [(η⁵-Cp)Co(η⁴-C₄Ph₃)]₂[(η⁵-Cp)Co(η⁴-C₄Ph₂)] (8) having no substituents on the Cp rings. All the compounds except 2 were structurally characterized. Structural studies on palladacycles 4 and 5 showed interesting anagostic C–H⋯Pd interactions between the ortho hydrogen of one of the Cp rings and the palladium centre. Preliminary studies carried out on the palladacycles showed promising catalytic activity in the asymmetric rearrangement of trichloroacetimidates.