Technetium Hydrides Revisited: Syntheses, Structures, and Reactions of [TcH3(PPh3)4] and [TcH(CO)3(PPh3)2]

Optimized synthetic approaches to [TcH3(PPh3)(4)] (1) and mer-trans-[TcH(CO)(3)(PPh3)(2)] (5) as key compounds of the technetium hydrido chemistry are reported. They give access to pure and stable samples of the complexes. The solid-state structures of the two title compounds have been determined. Reactions of [TcH3(PPh3)(4)] with monodentate basic phosphines such as PMe2Ph or PMe3 lead to mixtures of mixed-phosphine complexes of the compositions [TcH3(PR3)(n)(PPh3)(4-n)] (n = 1-3), from which the mixed-phosphine trihydride complex [TcH3(PPh3)(2)(PMe3)(2)] (2) could be isolated. Spectroscopic data and DFT calculations suggest a fluxional structure between a capped trigonal prism and a pentagonal bipyramid. Reactions of the monohydride mer-trans-[TcH(CO)(3)(PPh3)(2)] (5) with HX (X = Cl, Br, I) give [TcX(CO)(3)(PPh3)(2)] (6) complexes in good yields, while mer-trans-[Tc{eta(1)-O(CR)O}(CO)(3)(PPh3)(2)] (7) or cis-trans[Tc{eta(2)-OO(CR)}(CO)(2)(PPh3)(2)] (8) complexes are formed with carboxylic acids depending on the substituent R and the conditions applied. Chelate formation of the formato ligand can also be obtained by thermal decarbonylation of the isolated mertrans-[Tc(eta(1)-O(CH)O)(CO)(3)(PPh3)(2)] (7a) complex The latter reaction is reversible, and the tricarbonyl compound is reformed when [Tc(eta(2) -OO(CH))(CO)(2)(PPh3)(2)] (8a) is exposed to CO gas. Reactions of [TcH(CO) (PPh3)(2)] (5) with phenyl seleninic acid (PhSeOOH) in methanol give the tetranuclear cluster [{Tc(CO)(3)}(3)(mu(3)-OH)(mu(2)-O(SePh)O)(3) {Tc(CO)(3)}] (10) and a small amount of the bridged dinuclear oxalato complex [Tc-2(CO)(6)(ox)(OPPh3)(2)] (11). The latter compound is the result of a complex reaction, which involves a metal-induced oxidation of methanol to formate and a subsequent C-C coupling of two formato ligands. The plausibility of the proposed mechanism is supported by the fact that the dimeric oxalato complex is much more efficiently formed when the formato complex [Tc{eta(1)-O(CH)O}(CO)(3)(PPh3)(2)] (7a) is directly reacted with PhSeOOH. Exclusively the monomeric oxalato complex [Tc(eta(2)-oxH)(CO)(2)(PPh3)(2)] (12) is formed during a reaction of mer-trans-[TcH(CO)(3)(PPh3)(2)] (5) with oxalic acid. The remaining proton of the Hof ligand of [Tc(eta(2)-oxH)(CO)(2)(PPh3)(2)] (12) can be removed by NEt3, and the ion pair (HNEt3)[Tc(eta(2)-ox)(CO)(2)(PPh3)(2)] (13) is formed.

Automated summary

Optimized synthetic approaches to key technetium hydrido compounds, [TcH3(PPh3)(4)] (1) and mer-trans-[TcH(CO)(3)(PPh3)(2)] (5), have been reported, providing pure and stable complexes. The solid-state structures of these compounds have been determined, and experiments coupled with DFT calculations suggest a fluxional structure for some mixed-phosphine complexes.