Insertion reactions of (benzyne)nickel(0) complexes with carbon monoxide: X-ray structure of a (phthalato)nickel(II) complex formed by oxidation of an η¹:η¹-phthaloyl intermediate

The reaction of (benzyne)nickel(0) complexes Ni(η²-C₆H₄)(dcpe) (1) and Ni((1,2-η)-4,5-F₂C₆H₂)(dcpe) (2) [dcpe = 1,2-bis(dicyclohexylphosphino)ethane, (C₆Hu)₂PCH₂CH₂P(C₆H ₁₁)₂] with carbon monoxide at low concentration gives Ni(CO)₂(dcpe) (3) and, as the main organic products, 9H-fluoren-9-one (4) and 2,3,6,7-tetrafluoro-9H-fluoren-9-one (5), 2,3,6,7-X₄C₁₂H₄-CO (X = H, F), respectively, resulting from monoinsertion into the nickel-benzyne bond. Higher concentrations of CO give rise to bis(acyl) intermediates, one of which, Ni(CO-4,5-F₂C₆H₂CO-2)(dcpe) (9), has been observed by ³¹P and ¹⁹F NMR spectroscopy. These intermediates reversibly form 3 and the corresponding benzocyclobutenediones OC-4,5-X₂C₆H₂CO [X = H (12), F (13)] by reductive elimination and react readily with oxygen to form the stable (phthalato)nickel(II) complexes Ni(OCOC₆H₄COO-2)(dcpe) (6) and Ni(OCO-4,5-F₂C₆H₂COO-2)(dcpe) (7). On treatment with iodine, the latter give the corresponding phthalic anhydrides C₆H₄C₂O₃ (10) and 4,5-F₂C₆H₂C₂O₃ (11), respectively. According to a single-crystal X-ray study, 7 is a planar nickel(II) complex containing a seven-membered chelate ring with monodentate carboxylato groups, the average Ni-O and Ni-P distances being 1.904(4) and 2.152(2) Å, respectively.