Syntheses, crystal structures and properties of [PPh4]2[SbFe3H(CO)12] and [PPh4]2[AsFe3H(CO)12]

Abstract

Reaction of Sb₂O₃ or As₂O₃ with methanolic solutions of Fe(CO)₅–OH^– have resulted in the formation of the a nionic species [SbFe₃H(CO)₁₂]^2– or [AsFe₃H(CO)₁₂]^2–, which maybe precipitated as their PPh₄⁺ salts. The anions have also been obtained as the NEt₄⁺, NMe₄⁺ or N (PPh₃)₂⁺ salts. The PPh₄⁺ salts of the anions are not isomorphous, with [PPh₄]₂[SbFe₃H(CO)₁₂] crystallizing in the monoclinic space group P2₁/c, with a= 11.165(5), b= 49.223(9), c= 10.903(5)Å, β= 108.13(2)° and Z= 4, while [PPh₄]₂-[AsFe₃H(CO)₁₂] is triclinic, space group P, with a= 12.088(5), b= 13.257(5), c= 19.389(8)Å, α= 80.43(3), β= 83.55(3), γ= 70.83(3)° and Z= 2. Each anion consists of a central antimony or arsenic atom surrounded by three Fe(CO)₄ groups in a trigonal-pyramidal arrangement, together with a hydrogen atom, although this was not crystallographically located and is inferred on the basis of charge considerations and, more importantly, ¹H NMR data. The Sb–Fe and As–Fe bond distances average 2.596(1) and 2.459(2)Å, respectively. The ¹H NMR spectra exhibit slightly broad singlets at δ–1.39 for [SbFe₃H(CO)₁₂]^2– and δ 1.34 for [AsFe₃H(CO)₁₂]^2–, both with W_½, of ca. 6 Hz, which are indicative of hydrogen attached to antimony and arsenic. Attempts to synthesize the corresponding bismuth species were unsuccessful, which may be attributed to the poor Lewis basicity of the lone pair on bismuth and to the lack of stability displayed by a Bi–H bond. The reaction in this case generally results in the formation of [BiFe₄(CO)₁₆]^3– and bismuth metal, likely involving the disproportionation of an anionic Bi^III–Fe_m(CO)_n intermediate such as [BiFe₃(CO)₁₂]^3–, the deprotonated bismuth analogue of [SbFe₃H(CO)₁₂]^2– and [AsFe₃H(CO)₁₂]^2–.