DYNAMIC BEHAVIOR OF SOME BORYL-SUBSTITUTED FERROCENES

The H-1, C-13 and B-11 NMR solution spectra of selected mono- and 1,1' -diborylated ferrocene derivatives, Fe(C5H5)(C(5)H(4)BRR') (3a, b, e, f, g, j) and Fe(C(5)H(4)BRR'), (4a-j), have been measured [BRR' = BBr2 (a), B(NMe(2))(2) (b), B(NPr2i)Br (c), B(Me)Br (d), B(NMe(2))Cl (e), B(NMe(2))Bu(t) (f), B(Bu(t))Cl (g), B(NPr2i)Cl (h), B[N(Me)CH2.](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 charact erized: (i) All compounds containing dialkylamino-substituted boryl gr oups show hindered rotation about the B-N bond; the activation barrier , Delta G(#), 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 a bsence of the electron-donating dialkylamino groups, a hindered rotati on about the B-C(1) bond between the boryl group and the cyclopentadie nyl ring: can be deduced from the temperature-dependent H-1 and C-13 N MR spectra of 4d [Delta G(#) (213) = 41.2 +/- 1 kJ mol(-1)] and the 9- BBN compounds 3j [Delta G(#): (213 K) = 40.5 +/- 1 kJ mol(-1)] and 4j [Delta G(#) (218 K) = 42.5 +/- 1 kJ mol(-1)] (iii) Finally a low-barri er rotation of nl the cyclopentadienyl rings about the axis defined by the iron atom and the two ring centres becomes evident from H-1/H-1 N OE difference experiments. This indicates that the favoured low-temper ature conformation in compounds such as 4b and 4i is the centrosymmetr ic trans arrangement of the ring substituents, although the limiting l ow-temperature spectrum could not be reached.