Cationic Terminal Gallylene Complexes by Halide Abstraction: Coordination Chemistry of a Valence Isoelectronic Analogue of CO and N₂

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 [L(n)M(ER)](+) (E = B, Al, Ga; R = aryl, amido, halide), we have targeted complexes of the type [L(n)M(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)(GaI)](+)[BAr(f)(4)](-) (9) can be synthesized and shown crystallographically to feature a terminally bound GaI 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 GaI ligand, which is confirmed experimentally by the reaction of 9 with CO to give [Cp*Fe(dppe)(CO)](+)[BAr(f)(4)](-). 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 GaI molecule through pi overlap with the HOMO and HOMO-2 of the [Cp*Fe(dppe)](+) fragment.

Structure and bonding analysis of dimethylgallyl complexes of cobalt, rhodium and iridium [Me(PMe3)2(Me3GaCl)M(GaMe2)] (M = Co, Rh, Ir) and [Me(PMe3)2ClIr(GaMe2)]: A theoretical study

“…Recently, a structurally characterized dimethylgallyl iridium complex [(Me)Ir(Me3GaCl)(GaMe 2 )(PCy 3 ) 2 ] has been reported by Merino and coworkers [24]. Fischer [27,29]. A number of theoretical studies of the terminal metal gallyl complexes have been reported [5,30e35].…”

The role of the bis-trimethylsilylamido ligand, [N{SiMe3}2]−, in main group chemistry. Part 2: Structural chemistry of the metallic p-block elements

“…5) were formed [35]. These disubstituted species, isolated from the reaction of GaCl 2 (N{SiMe 3 } 2 )·THF and Na[Fe(Á-C 5 R 5 )(CO) 2 ], were formed irrespective of the reaction stoichiometry and conditions.…”

“…These include (i) the impressively large [Ga 84 Br 2 (N{SiMe 3 } 2 ) 20 ] 4− ( [35]) [37], and it's oxidized analogue, [Ga 84 Br 2 (N{SiMe 3 } 2 ) 20 ] 3− ( [36]) [38], each consisting of 64 × "naked" Ga atoms, with a further 20 that are bonded to -N{SiMe 3 } 2 ligands; (ii) the dianionic cluster [Ga 22 (N{SiMe 3 } 2 ) 10 ] 2− ( [37]) [39], which contains a central Ga atom surrounded by 11 × Ga atoms, with 10 × Ga-N{SiMe 3 } 2 groups around the periphery; (iii) the bromo/amido anions [Ga 22 Br( -Br) 10 (N{SiMe 3 } 2 ) 10 ] 3− ( [38]) and [Ga 22 Br 2 ( -Br) 10 [40], both of which contain an icosahedral Ga 12 unit without a central Ga atom, which is surrounded by a further 10 × Ga atoms and (iv) the neutral cluster, Ga 23 (N{SiMe 3 } 2 ) 11 (40) [41], which also contains a central Ga 12 core but is surrounded by 11 × Ga-N{SiMe 3 } 2 groups.…”

Structure and bonding in haloarylgallyl complexes of iron, ruthenium and osmium [(η5-C5H5)(CO)2M{Ga(X)(Ph)}]: A theoretical study

“…Recently, a structurally characterized dimethylgallyl iridium complex [(Me)Ir(Cl.GaMe 3 )(GaMe 2 )(PCy 3 ) 2 ] has been reported by Merino and coworkers [42]. Fischer [44,45,47]. Pandey and coworkers have investigated in detail the bonding situation in dihalogallyl complexes of Ni, Pd, Pt, Fe, Ru and Os [48,49].…”

“…Fe(GaI 2 )] [dppe ¼ 1,2-bis(diphenylphosphino)ethane][44,45]. In addition, Braunschweig et al recently isolated dibromogallyl and diiodogallyl complexes of platinum, trans-X(PCy 3 ) 2 Pt(GaX 2 ) (X ¼ Br, I), using an oxidative addition methodology[46].…”