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 'ope n' (i.e. non-cluster) [BiFe4(CO)(16)](3-) (or [Bi{Fe(CO)(4)}(4)](3-)) ion f ollowing 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-), [Fe-2(CO)(8)](2-) and [Fe-4(CO)(13)](2-), wi th the distribution of species being cation dependent. The previously unkno wn [Bi2Fe5(CO)(17)](2-) ion was obtained as the [N(PPh3)(2)](+) salt, cryst allizing in the triclinic space group P1, with a 15.851(7), b 17.199(8), c 20.020(8) Angstrom, alpha 114.72(2), beta 95.51(3), gamma 113.01(2)degrees, Z 2 and V 4338(3) Angstrom (3). The structure was determined by X-ray diff raction methods to an R of 0.035 (R-w = 0.040) for 6778 independent observe d reflections. The anion consists of a central square-based pyramidal 'nido -Bi-2[Fe(CO)(3)](3)' cluster unit, with two external Fe( CO) 4 fragments at tached to the two trans four-coordinate bismuth atoms located in the base o f the pyramid. A thermogravimetric study of [NEt4](3)[BiFe4(CO)(16)] indica ted decomposition above 150 degreesC, with an apparent loss of three CO mol ecules per [BiFe4(CO)(16)](3-) ion below 160 degreesC. Decomposition was co mplete by 205 degreesC and also involved pyrolysis of the [NEt4](+) cations . Pyrolysis of solid [NEt4](3)[BiFe4(CO)(16)] at 160 degreesC for several h ours under N 2 implicated an intermediate species in the Bi/ Fe carbonyl an ion system, probably [Bi2Fe3(CO)(9)](2-), in addition to the [Fe-2(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 i n acetonitrile, in an attempt to selectively remove CO by chemical means, l ead 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 wi th (CH3)(3)NO to give [Bi2Fe4(CO)(13)](2-) as the major product. Conversion of [Bi2Fe5(CO)(17)](2-) to [Bi2Fe4(CO)(13)](2-) is likely initiated throug h nucleophilic attack at the axial carbon site of a pendant Fe(CO)(4) group , according to extended-Huckel 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 anio n can be produced by reaction of [Bi2Fe4(CO)(13)](2-) with Fe-2(CO)(9) in t etrahydrofuran, 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.