Synthesis, characterisation and some reactions of alkynyl derivatives [Ni(η5-C5H5)(PPh3)CCX]

Abstract

Oxalic acid catalyses the hydrolysis of the Ni(II) acetylide [Ni(η⁵-C₅H₅)(PPh₃)CCCH(OEt)₂] 1, to the alkynylaldehyde [Ni(η⁵-C₅H₅)(PPh₃)CCCHO] 2, in high yield. Condensation reactions of 2 with phenylhydrazine and dinitrophenylhydrazine in the presence of acetic acid, and with malononitrile and 3-phenyl-5-isoxazolone (C₉H₇NO₂) in the presence of triethylamine yield [Ni(η⁵-C₅H₅)(PPh₃)CCX] derivatives where X = CHNNHC₆H₅3, CHNNHC₆H₃(NO₂)₂-2,4 4, CHC(CN)₂5, and CHC₉H₅NO₂6. The reactivity of [Ni(η⁵-C₅H₅)(PPh₃)CCX] complexes towards [Co₂(CO)₈] is a function of X. Thus 1 and 2, where X = CH(OEt)₂ or CHO, react readily to give the bridging alkyne derivatives [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)CCCH(OEt)₂}{Co₂(CO)₆}] 7, and [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)CCCHO}{Co₂(CO)₆}] 8, but 5, where X is the strongly electron-withdrawing CHC(CN)₂ group, does not react even after 24 h at room temperature. Furthermore, coordination of the alkyne to a Co₂(CO)₆ fragment appears to inhibit the normal reactions of the group X in 7 and 8. Thus the acetal grouping in 7 does not undergo oxalic acid-catalysed hydrolysis to an aldehyde in 8, and the aldehyde function in 8 does not undergo a Knoevenagel condensation with CH₂(CN)₂. The IR spectra of 1, and 3–6 show a single ν(CC) band the frequency of which decreases along the series X = CH(OEt)₂ > CHNNHC₆H₅ > CHNNHC₆H₃(NO₂)₂-2,4 > CHC(CN)₂ ≈ CHC₉H₅NO₂; that of 2 is anomalous in that it can show two ν(CC) bands. The UV-visible spectra of 1–6 show a strong charge transfer absorption band which increases in wavelength 1 < 3 < 2 < 4 < 5 < 6. These spectroscopic data and the ¹³C chemical shifts suggest that the (η⁵-C₅H₅)(Ph₃P)Ni moiety is a donor and, when X is an acceptor, charge separated cumulenic mesomers such as Ni⁺CCX⁻ contribute to the description of the bonding in 1–6. This is not reflected in the molecular dimensions of 1, 2 and 5 as determined by X-ray diffraction. However, the crystal structure of [{μ-η¹:η¹-Ni(η⁵-C₅H₅)(PPh₃)CCCHO}{Co₂(CO)₆}], 8, shows that the C₂Co₂ cluster core is severely distorted because of the strong donor (Ni) and acceptor (CHO) substituents on the acetylenic carbon atoms.