Transformations in Transition-Metal Carbonyls Containing Arsenic: Exploring the Chemistry of [Et4N]2[HAs{Fe(CO)4}3] in the Search for Single-Source Precursors for Advanced Metal Pnictide Materials

The chemistry of [Et4N](2)[HAs{Fe(CO)(4)}(3)] ([Et4N](2)[I]) has been explored with the goal of preparing heterometallic compounds for use as single-source precursors to metal pnictide phases. Reaction of AgPF6 with [Et4N](2)[I] in THF produces [Et4N] [HAs {Fe-2(CO)(6)(mu-C) (mu-H)}{Fe(CO)(4)}] in 12% yield, but the yield is improved to 71% if the reaction is carried out with added [Et4N][HFe(CO)(4)]. The structure possesses rare As-H and {Fe-2(CO)(6)(mu-CO)(mu-H)} fragments. The addition of [CPh3][BF4] to [Et4N](2)[I] in THF gives [Et4N][Fe-3(CO)(9)(mu-CO){mu(3)-AsFe(CO)(4)}] in 45% yield. This compound is also obtained as a minor product from oxidation of [Et4N](2)[I] with 1 equiv of AgPF6, AsCl3 SbCl3, or BiCl3 in THF. The cluster consists of an Fe3As tetrahedron with an isolated Fe(CO)4 unit bound to the arsenic in a spiked tetrahedral arrangement. Refluxing [Et4N](2)[I] with 1 equiv of AgPF6 in THF produces [Et4N][(mu-H)(2)Fe-3(CO)(9)){mu 3AsFe(CO)(4)}]. This cluster is similar to [Et4N] [Fe-3(CO)(9)(u-CO){mu(3)-AsFe(CO)(4)}] with the bridging CO being replaced by two bridging hydride ligands. Treatment of [Et4N](2)[I] with 1 equiv of Mn(CO)(5)Br in THF gives the spirocyclic compound [Et4N][{FeMn(CO)(8)}(mu(4)-As){Fe-2(CO)(6)(mu-CO)(mu-H)}] in 52% yield. Refluxing this cluster in the presence of triethylamine and an additional 1.8 equiv of Mn(CO)(5)Br gives [Et4N][Fe-3(CO)(9)(mu-CO){mu(3)-AsMn(CO)(4)Br-cis}] in low yield. Alternatively, treating [Et4N][{FeMn(CO)(8)}(mu(4)-As){Fe-2(CO)(6)(mu-CO)(mu-H)}] with triflic acid or an oxidant ([Ox] = Cu+, Ag+, Cu2+, CPh3+) in dichloromethane produces the two neutral compounds (mu-H)(2)Fe-3(CO)(9){mu(3)-AsMn(CO)(6)} and {FeMn(CO)(8)}(mu(4)-As){Fe-2(CO)(8)} in trace amounts and high yield, respectively. The former could be removed by treatment with base in nonpolar solvents. Neutral {FeMn(CO)(8)}(mu(4)-As){Fe-2(CO)(8)} can then be isolated by filtration and evaporation in 74% yield. An improved synthesis of the known [Fe-3(CO)(9)(mu-CO){mu(3)-PFe(CO)(4)}](-) as its tetraethylammonium salt is also included. All compounds were characterized by spectroscopy, by ESI-mass spectrometry, and by single-crystal X-ray diffraction.