Photolytic, Pyrolytic and Trimethylamine N-Oxide Induced CO Removal from [BiFe4(CO)16]3–: Synthesis, X-Ray Crystal Structure and Properties of [N(PPh3)2]2[Bi2Fe5(CO)17]

The formation of anionic-cluster species from compounds containing the ‘open’ (i.e. non-cluster) [BiFe4(CO)16]3– (or [Bi{Fe(CO)4}4]3–) ion following loss of CO as a result of photolytic, pyrolytic and chemical means has been investigated. Photolysis of salts of the [BiFe4(CO)16]3– ion in acetonitrile, where the associated cations are [N(PPh3)2]+ or [NEt4]+, produce a range of species including [Bi2Fe5(CO)17]2–, [Bi2Fe4(CO)9]2–, [Bi2Fe3(CO)9]2–, [Fe2(CO)8]2– and [Fe4(CO)13]2–, with the distribution of species being cation dependent. The previously unknown [Bi2Fe5(CO)17]2– ion was obtained as the [N(PPh3)2]+ salt, crystallizing in the triclinic space group P 1, with a 15.851(7), b 17.199(8), c 20.020(8) Å, α 114.72(2), β 95.51(3), γ 113.01(2)o, Z 2 and V 4338(3) Å3. The structure was determined by X-ray diffraction methods to an R of 0.035 (Rw = 0.040) for 6778 independent observed reflections. The anion consists of a central square-based pyramidal ‘nido-Bi2[Fe(CO)3]3’ cluster unit, with two external Fe(CO)4 fragments attached to the two trans four-coordinate bismuth atoms located in the base of the pyramid. A thermogravimetric study of [NEt4]3[BiFe4(CO)16] indicated decomposition above 150˚C, with an apparent loss of three CO molecules per [BiFe4(CO)16]3– ion below 160˚C. Decomposition was complete by 205˚C and also involved pyrolysis of the [NEt4]+ cations. Pyrolysis of solid [NEt4]3[BiFe4(CO)16] at 160˚C for several hours under N2 implicated an intermediate species in the Bi/Fe carbonyl anion system, probably [Bi2Fe3(CO)9]2–, in addition to the [Fe2(CO)8]2– ion, although the final product obtained was found to be [Bi2Fe4(CO)13]2–. Reaction of [NEt4]3 [BiFe4(CO)16] with trimethylamine N-oxide in acetonitrile, in an attempt to selectively remove CO by chemical means, lead to the slow formation of [Bi2Fe4(CO)13]2– and a little [Bi2Fe5(CO)17]2–, although the [Bi2Fe5(CO)17]2– was observed to react slowly with (CH3)3NO to give [Bi2Fe4(CO)13]2– as the major product. Conversion of [Bi2Fe5(CO)17]2– to [Bi2Fe4(CO)13]2– is likely initiated through nucleophilic attack at the axial carbon site of a pendant Fe(CO)4 group, according to extended-HÜckel molecular orbital calculations. Calculations also show that there is no instability introduced in having a second pendant Fe(CO)4 attached to the central cluster unit, and the anion can be produced by reaction of [Bi2Fe4(CO)13]2– with Fe2(CO)9 in tetrahydrofuran, through the addition of an Fe(CO)4 fragment to the three-coordinate bismuth atom in the anion. The other product of the reaction is volatile Fe(CO)5, which is easily separated from the desired product.