Dynamic behaviour of some boryl-substituted ferrocenes

Abstract

The 1H, 13C and 11B NMR solution spectra of selected mono- and 1,1′-diborylated ferrocene derivatives, Fe(C 5H 5)(C 5H 4BRR′) ( 3a, b, e, f, g, j) and Fe(C 5H 4BRR′) 2 ( 4a-j), have been measured [BRR′ = BBr 2 ( a), B(NMe 2) 2 ( b), B(NPr 2 1)Br ( c), B(Me)Br ( d), B(NMe 2)Cl ( e), B(NMe 2)Bu t ( f), B(Bu t)Cl ( g), B(NPr 2 i)Cl ( h), B[N(Me)CH 2 −] 2 ( i) and 9-bora-bicyclo [3.3.1]nonyl ( j)]. Depending on the nature of the BRR′ groups, three different types of dynamic processes can be characterized: (i) All compounds containing dial-kylamino-substituted boryl groups show hindered rotation about the BN bond; the activation barrier, ΔG #, is higher in the mono(dimethylamino) compounds [61.1 ± 1 ( 3f) and 62.7 ± 1 ( 4f) kJ mol −1] than in the bis(dimethylamino)boryl compounds ( ca 38.5 ±_1 kJ mol −1 in 3b and 4b). (ii) In the absence of the electron-donating dialkylamino groups, a hindered rotation about the BC(1) bond between the boryl group and the cyclopentadienyl ring can be deduced from the temperature-dependent 1H and 13C NMR spectra of 4d [ ΔG # (213) = 41.2 ± 1 kJ mol −1] and the 9-BBN compounds 3j [ ΔG # (213 K) = 40.5 ± 1 kJ mol −1] and 4j [ ΔG # (218 K) = 42.5 ± 1 kJ mol −1] (iii) Finally a low-barrier rotation of the cyclopentadienyl rings about the axis defined by the iron atom and the two ring centres becomes evident from 1H/ 1H NOE difference experiments. This indicates that the favoured low-temperature conformation in compounds such as 4b and 4i is the centrosymmetric trans arrangement of the ring substituents, although the limiting low-temperature spectrum could not be reached.