Synthesis, structure, electrochemistry, and Mössbauer effect studies of the ferraphosphadicarbollides [(C5R5)Fe(PC 2B8H10)] (R = H, Me)

Reaction of the phosphadicarbollide anion [7,8,9-PC2B 8H10]- (4) with [CpFe(C6H 6)]+ or [Cp*Fe(MeCN)3]+ gives the expected 12-vertex ferraphosphadicarbollides [1-C5R 5-1,2,3,4-FePC2B8H10] [R = H (5a), Me (5b); the metal atom is assigned number 1] in around 35% yield. Compounds 5a,b undergo selective polyhedral rearrangement at 150°C to give the corresponding isomers [1-C5R5-1,2,4,5-FePC 2B8H10] (6a,b). The structure of 5a has been determined by X-ray diffraction. An electrochemical investigation has revealed that all the compounds obtained display the FeII/FeIII oxidation process with features of chemical reversibility on the cyclic voltammetric timescale. However, only cation [6b]+ is reasonably stable on the long timescales of electrolysis. Ferraphospadicarbollides 5a and 6a are more difficult to oxidize than the related dicarbollide and tricarbollide derivatives [CpFe(C2B9H11)]- and [CpFe(C3B8H11)], respectively. Temperature-dependent Mössbauer effect studies show that the recoil-free fraction over the temperature interval 110-342 K for 5a is well accounted for by a linear regression, and a calculation of the root-mean-square-arnplitude-of- vibration (rmsav) of the iron atom, based on the Mössbauer effect (ME) data, is in reasonable agreement with the value calculated from the single-crystal X-ray study at 120 K. Similarly, the ME and X-ray rmsav data for 6a at 293 K are in excellent agreement with each other, and the value at 120 K calculated for 6a is in good agreement with that observed for 5a. The metal atom motion is nearly isotropic with respect to the major symmetry axis running through the Cp ring and the Fe atom. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.