An investigation of steric effects along a cyclopentadienyl substituent: a study by 1H, 13C and 29Si NMR spectroscopy of bis(trimethylsilyl)cyclopentadienyl iron complexes

Abstract

The syntheses of a range of complexes, [(η 5-C 5H 3(SiMe 3) 2)Fe(CO)(L)I], (L  tBuNC, 2,6-Me 2C 6H 3NC (xylNC), P(OMe) 3, PMe 2Ph, P(O- o-tol) 3 PPh 3, P( m-tol) 3, P( p-tol) 3, and P(CH 2Ph) 3) are reported. Three separate cyclopentadienyl ring proton resonances were observed in the NMR spectra of the iron complexes, and their assignments determined from NOE experiments (L  P(OMe) 3, P(O- o-tol) 3). The 13C NMR spectra were recorded and assignments made from a knowledge of the 1H spectra by use of CH correlated spectroscopy. 29Si and 31P NMR data were also obtained. Three-dimensional correlations were observed between separations of pairs of NMR resonances ( 29Si, 13C and 1H) and (i) steric effects as measured by the Tolman cone angle, θ, and (ii) electronic effects as measured by ν CO, the stretching frequency of the carbonyl group. The degree of correlation varied with the distance of the NMR-active nuclei from the iron atom (ring C and Si(CH 3) 3 Si, C and H atoms), and the result is taken as an indication of the best region, in space, in which the Tolman cone angle concept is most appropriate. The best correlation occurred between the chemical shift difference of the two 29Si resonances and θ and ν(CO) ( R 2 = 0.96, mse = 0.00640). Conformational data obtained from the NOE spectra suggest that the Group 15 donor ligand resides close to a SiMe 3 group and near the two adjacent ring protons. The steric demand of the two bulky SiMe 3 groups hinders rotation of the P(OMe) 3 and P(O- o-tol) 3 ligands around the ring, resulting in a windscreen wiper motion of the ligand between the two SiMe 3 groups.