Reducing GaH and GaC Bonds in Close Proximity to Oxidizing Peroxo Groups: Conflicting Properties in Single Molecules

Abstract

Treatment of [Li(H(2)Ga{CH(SiMe(3))(2)}(2))]2 OEt(2) (12 OEt(2)) with two equivalents of tert-butyl hydrogen peroxide, H-O-O-CMe(3), afforded the organogallium peroxide [({(Me(3)Si)(2)HC}(2)Ga(OH)(OOCMe(3))Li)(2)] (3), which possesses oxidizing peroxo groups in close proximity to reducing Ga-C bonds. The lithium atoms of the dimeric formula units are coordinated by both oxygen atoms of the peroxides and by two hydroxo groups. The cleavage of the Ga--C bond was not observed, even when an excess of H-O-O-CMe(3) was applied. Instead, the adduct [{(Me(3)Si)(2)HC}(2)Ga(OH)(OOCMe(3))(2)Li(2)(HOOCMe(3))] (4) was isolated, which has an intact H-O-O-CMe(3) molecule terminally attached to lithium. A similar structural motif was found for the compound [(LiOOCMe(3))(2)(HOOCMe(3))(2)] (5). The trihydrido gallanate [Li(H(3)Ga-{CH(SiMe(3))(2)})]OEt(2) (2) yielded the unique peroxide [({(Me(3)Si)(2)HC}--Ga(H)(OOCMe(3))(2)Li)(2)] (6) under similar conditions that possesses Ga-C and even more reactive Ga-H bonds beside peroxo groups. It decomposed at room temperature by the insertion of oxygen atoms into the Ga-H bonds and the formation of [({(Me(3)Si)(2)HC}-Ga(OH)(OCMe(3))(OOCMe(3))Li)(2)] (7), which was isolated in a low yield. Further decomposition gave the complete degradation of all peroxo groups with the formation of a relatively complicated Li(4)Ga(4)O(8) cage (8).

New Organogallium Peroxides

“…The molecular symmetry approaches the point group C 2v . The O-O bond lengths (146 pm on average) are in the normal range of peroxo compounds such as hydrogen peroxide [14] or the organoelement alkylperoxides described earlier [3,6,9,10]. Longer distances were expectedly observed for dianionic peroxo ligands which bridge two gallium atoms [7,8].…”

“…The longest one is to the peroxo oxygen atom O21. The distorted coordination sphere with one relatively long Li-O distance resembles that of other lithium or mixed lithium-gallium peroxides [9]. The O-O bond lengths (147.8 pm on average) correspond to standard values.…”

“…Treatment of lithium gallanates Li[GaR x H 4−x ] with tert-butyl hydrogenperoxide represents a facile method for the generation of organogallium peroxides by hydrogen release [9]. However, in most cases these peroxides are thermally relatively unstable and decompose slowly at room temperature in solution or in the solid state.…”

“…One of the most intriguing structures had three dianionic peroxo ligands bridged by three Ga-CH(SiMe 3 ) 2 groups [7]. In another case we were able to isolate a compound which had an intact Ga-H bond despite two peroxo ligands attached to the same gallium atom [9]. The latter compound is expectedly very reactive and unstable with respect to the insertion of an oxygen atom into the Ga-H bond.…”

“…Our strategy comprised the application of bis(trimethylsilyl)methyl compounds because their E-C bonds proved to be rather insensitive towards an attack of oxygen in previous experiments. Three methods for the introduction of peroxo ligands were applied: (i) Insertion of dioxygen molecules into In-C bonds [6], (ii) reactions of corresponding element hydrides with the dihydrogen peroxide-DABCO adduct by hydrogen release [7,8], and (iii) reactions of the same starting compounds with tert-butyl hydrogen peroxide [9,10]. One of the most intriguing structures had three dianionic peroxo ligands bridged by three Ga-CH(SiMe 3 ) 2 groups [7].…”