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

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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.