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Cationic Terminal Gallylene Complexes by Halide Abstraction: Coordination Chemistry of a Valence Isoelectronic Analogue of CO and N-2

Lookup NU author(s): Emeritus Professor Bill Clegg, Dr Luca Russo, Lucinda Male

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Abstract

While N-2 and CO have played central roles in developing models of electronic structure, and their interactions with transition metals have been widely investigated, the valence isoelectronic diatomic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a ligand in a simple metal complex. As part of a program designed to address this deficiency, together with wider issues of the chemistry of cationic systems [LnM(ER)](+) (E = B, Al, Ga; R = aryl, amido, halide), we have targeted complexes of the type [LnM(GaX)](+). Halide abstraction is shown to be a viable method for the generation of mononuclear cationic complexes containing gallium donor ligands. The ability to isolate tractable two-coordinate products, however, is strongly dependent on the steric and electronic properties of the metal/ligand fragment. In the case of complexes containing ancillary pi-acceptor ligands such as CO, cationic complexes can only be isolated as base-trapped adducts, even with bulky aryl substituents at gallium. Base-free gallylene species such as [Cp*Fe(CO)(2)(GaMes)](+) can be identified only in the vapor phase by electrospray mass spectrometry experiments. With bis(phosphine) donor sets at the metal, the more favorable steric/electronic environment allows for the isolation of two-coordinate ligand systems, even with halide substituents at gallium. Thus, [Cp*Fe(dppe)(Gal)](+)[BAr4f](-) (9) can be synthesized and shown crystallographically to feature a terminally bound Gal ligand; 9 represents the first experimental realization of a complex containing a valence isoelectronic group 13/group 17 analogue of CO and N-2. DFT calculations reveal a relatively weakly bound Gal ligand, which is confirmed experimentally by the reaction of 9 with CO to give [Cp*Fe(dppe)-(CO)](+)[BAr4f](-). In the absence of such reagents, 9 is stable for weeks in fluorobenzene solution, presumably reflecting (i) effective steric shielding of the gallium center by the ancillary phosphine and Cp* ligands; (ii) a net cationic charge which retards the tendency toward dimerization found for putative charge neutral systems; and (iii) (albeit relatively minor) population of the LUMOs of the Gal molecule through pi overlap with the HOMO and HOMO-2 of the [Cp*Fe(dppe)](+) fragment.


Publication metadata

Author(s): Coombs ND, Vidovic D, Day JK, Thompson AL, Le Pevelen DD, Stasch A, Clegg W, Russo L, Male L, Hursthouse MB, Willock DJ, Aldridge S

Publication type: Article

Publication status: Published

Journal: Journal of the American Chemical Society

Year: 2008

Volume: 130

Issue: 47

Pages: 16111-16124

ISSN (print): 0002-7863

ISSN (electronic): 1520-5126

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/ja806655f

DOI: 10.1021/ja806655f


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