The reactions of some σ-alkynylnickel complexes with 7,7,8,8-tetracyanoquinodimethane

Abstract

The σ-alkynyl complexes Ni(η 5-C 5H 5)(PPh 3)–C C–R ( 1), Ni(η 5-C 5H 5)(PPh 3)–C C–X–C CH ( 2) and Ni(η 5-C 5H 5)(PPh 3)–C C–X–C C–Ni(η 5-C 5H 5)(PPh 3) ( 3), reactwith 7,7,8,8-tetracyanoquinodimethane, TCNQ, at 30 °C by insertion of the alkyne C C into a C C(CN) 2 bond to give Ni(η 5-C 5H 5)(PPh 3)–C{ C 6H 4 C(CN) 2}–C{ C(CN) 2}–R ( 4), from 1, Ni(η 5-C 5H 5)(PPh 3)–C{ C 6H 4 C(CN) 2}–C{ C(CN) 2}–X–C CH ( 5), from 2, and Ni(η 5-C 5H 5)(PPh 3)–C{ C 6H 4 C(CN) 2}–C{ C(CN) 2}–X–C C–Ni(η 5-C 5H 5)(PPh 3) ( 6),and Ni(η 5-C 5H 5)(PPh 3)–C{ C 6H 4 C(CN) 2}– C{ C(CN) 2}–X–C{ C(CN) 2}–C{ C 6H 4 C(CN) 2}–Ni(η 5-C 5H 5)(PPh 3) ( 7),from 3 {R = (a) C 6H 5, (b) 4-PhC 6H 4, (c) 4-Me 2NC 6H 4, (d) 1-C 10H 7 (1-naphthyl), (e) 2-C 10H 7 (2-naphthyl), (f) 9-C 14H 9 (9-phenanthryl), (g) 9-C 14H 9 (9-anthryl), (h) 3-C 16H 9 (3-pyrenyl), (i) 1-C 20H 11 (1-perylenyl), (j) 2-C 4H 3S (2-thienyl), (k) C 10H 9Fe (ferrocenyl = Fc) and (l) H; X = (a) nothing, (b) 1,4-C 6H 4, (c) 1,3-C 6H 4 and (d) 4,4′-C 6H 4–C 6H 4}. The reaction is regiospecificand the other possible insertion product, R–C{ C 6H 4 C(CN) 2}–C{ C(CN) 2}–Ni(η 5-C 5H 5)(PPh 3) etc., is not formed. Under the same conditions, there is no evidencefor the reaction of TCNQ with the –C CH of 2, PhC CH, 1,4-C 6H 4(C CH) 2 or FcC CH, or for the reaction of more than one C C(CN) 2 of TCNQ with a Ni-alkynyl moiety. Complexes 4– 7 are all air-stable, purple solids which have been characterised by elemental analysis and spectroscopy (IR, UV–Vis, 1H NMR and 13C NMR),and by X-ray diffraction for 4a, 4b and 4l. The UV–Vis spectra of 4– 7 are very similar. This implies that all contain the same active chromophore which, it is suggested, is Ni–C(5) C 6H 4 C(CN) 2 and not R–C(4) C(CN) 2. This isconsistent with the molecular structures of 4a, 4b and 4l which show that the first of these potentially chromophoric fragments is planar or close to it with an in-built potential for delocalisation, whilst in the second the aryl group R is almost orthogonal to the C C(CN) 2 plane. The molecular structures of 4a, 4b and 4l also reveal a short Ni⋯C(4) separation, indicative of a Ni → C(4) donor–acceptor interaction. The electrochemistry of 4a shows aquasi reversible oxidation at ca. 1 V and complicated reduction processes. It is typical of most 4, but 4l is different in that it shows the same quasi reversible oxidation at ca. 1 V but two reversible reductions at −0.26 and −0.47 V (vs. [Fe(η 5-C 5Me 5) 2] +/0 0.0 V).