[AsCCAs]-Coordinated Rhenium Hydrides and Their Reactivities Toward Unsaturated Hydrocarbons, Heterocumulenes, and CO2

An [AsCCAs] ligand featuring a central alkyne and two flanking arsenic donors was employed for the synthesis of a trihydrido rhenium complex, while the corresponding phosphorus ligand was shown to be less suited. The reactivity of the former trihydride [AsCCAs]ReH3 (3) was examined in detail, which revealed that two alternative reaction channels may be entered in dependence of the substrate. Upon reaction of 3 with PhC≡CPh, ethylene, and CS2, monohydrides of the general formula [AsCCAs]Re(L)H with L = η2-PhC≡CPh (4), η2-H2C═CH2 (5), and η2-CS2 (6) were formed along with H2. In contrast, insertion products of the type [AsCCAs]Re(X)H2 (7-9) were obtained upon treatment of 3 with CyN═C═NCy, PhN═C═O, and Ph2C═C═O, while CO2 failed to react with 3 under identical reaction conditions. Given that several productive reactions between CO2 and hydrido rhenium carbonyls have been reported in the literature, 3 was further derivatized by introducing CO and tBuNC coligands, respectively. This led to the isolation of trans-[AsCCAs]ReH(CO)2 (trans-10) and trans-[AsCCAs]ReH(CNtBu)2 (trans-11), which were shown to thermally isomerize to the corresponding cis-configured products, cis-10 and cis-11. Interestingly, only the cis-complexes were found to react with CO2, which was rationalized by evaluating the relative nucleophilicities of the hydrides in cis-10, trans-10, cis-11, and trans-11 via Fukui analysis. The formates cis-[AsCCAs]Re(OCHO)(CO)2 (12) and cis-[AsCCAs]Re(OCHO)(CNtBu)2 (13) were isolated and shown to contain κ1-O-coordinated formate moieties. Treatment of 12 with [LutH]Cl/B(C6F5)3 (or with Ph3SiCl) led to the liberation of [LutH][OCHO···B(C6F5)3] (or triphenylsilyl formate) with concomitant formation of the expected chloro complex cis-[AsCCAs]ReCl(CO)2 (14). In a closed synthetic cycle, hydride 12 was regenerated from the latter chloride using NaBEt3H as a hydride source.