Thermal Decomposition of Fluoroalkyl Pentacarbonylmanganese(I) Derivatives by α-Fluorine Elimination

Abstract

This contribution brings to light the first example of an α-F elimination process in manganese organometallic chemistry. As shown in a recent contribution (Chem. Eur. J., DOI: 10.1002/chem.201804007) the thermal decomposition of compounds [Mn(RF)(CO)5] (RF = CF3, 1; CHF2, 2; CH2CF3, 3) in C6D6 at 70-100 °C leads primarily to homolytic Mn-RF bond cleavage, followed by RF• trapping in the presence of (Me3Si)3SiH (TTMSS) to yield RF-H. However, the formation of other products was revealed by a detailed 19F NMR investigation. Compounds 1 and 2 also produced significant amounts of (Me3Si)3SiF and of F-poorer organic compounds (namely, CH2F2 and CH3F from 1; CH3F from 2). The extent of this additional process was rather important (67%) for 1 but minor (10%) for 2. In contrast, no traces of either (Me3Si)3SiF or CH3CHF2 were detected during the decomposition of 3. A DFT investigation shows that the process occurs via CO dissociation followed by α-F elimination from [Mn(RF)(CO)4] to yield [cis-MnF(CXF)(CO)4] (X = F for 1, H for 2). The latter can then react with TTMSS via a rate-determining transition state, which is located at a comparable energy to that of the homolytic bond breaking asymptote, to yield (Me3Si)3SiF and [Mn(CXHF)(CO)5] after CO addition. The greater extent of (Me3Si)3SiF formation produced by the H/F exchange in the order 1 > 2 agrees with the stronger Mn-RF bond in the same order. Analysis of the relative barriers for the two competitive processes at 100 °C confirms that the experimentally determined ΔH∂ for the thermal decompositions of 1, 2, and 3 are close estimates of the Mn-RF bond dissociation energies (BDEs).