Photolytic and Reductive Activations of 2‐Arsaethynolate in a Uranium‐Triamidoamine Complex: Decarbonylative Arsenic Group‐Transfer Reactions and Trapping of a Highly Bent and Reduced Form

Little is known about the chemistry of the 2-arsaethynolate anion, but to date it has exclusively undergone fragmentation reactions when reduced. Here, we report the synthesis of [U(TrenTIPS)(OCAs)] (2, TrenTIPS = N(CH2CH2NSiPri 3)3), which is the first isolable actinide-2-arsaethynolate linkage. UVphotolysis of 2 results in decarbonylation, but the putative [U(TrenTIPS)(As)] product was not isolated and instead only [{U(TrenTIPS)}2(μ-η2:η2-As2H2)] (3) was detected. In contrast, reduction of 2 with [U(TrenTIPS)] gave the mixed-valence arsenido [{U(TrenTIPS)}2(μ-As)] (4) in very low yield. Complex 4 is unstable which precluded full characterisation, but these photolytic and reductive reactions testify to the tendency of 2-arsaethynolate to fragment with COrelease and As-transfer. However, addition of 2 to an electride mixture of potassium-graphite and 2,2,2-cryptand gives [{U(TrenTIPS)}2{μ-η2(OAs):η2(CAs)- OCAs}][K(2,2,2-cryptand)] (5). The coordination mode of the trapped 2-arsaethynolate in 5 is unique, and derives from a new highly-reduced and -bent form of this ligand with the most acute O-C-As angle in any complex to date (O-C-As ∠ ~128°). The trapping rather than fragmentation of this highly reduced O-C-As unit is unprecedented, and quantum chemical calculations reveal that reduction confers donor-acceptor character to the O-C-As unit.