Coordination and organometallic complexes of iron containing o-phenylenebis(dimethylphosphine) (pdmp). Crystal and molecular structure of [Fe(η2-C2H4)(pdmp)2

Abstract

Routes to hydrido- and organo-iron complexes of the bidentate ligand o-phenylenebis(dimethylphosphine), o-C6H4(PMe2)2 (pdmp), have been developed, starting from trans-[FeCl2(pdmp)2]. Reduction of trans-[FeCl2(pdmp)2] with LiAlH4 gives the dihydride, cis-[FeH2(pdmp)2], there being no evidence for an intermediate monohydride [FeHCl(pdmp)2]. Treatment of trans-[FeCl2(pdmp)2] with an excess of methyllithium gives [FeMe2(pdmp)2], isolated as a cis/trans-mixture, the methyl groups of which are cleaved by an excess of HX to give [FeX2(pdmp)2] (X = Cl, Br), predominantly in the form of the cis-isomers. Other new complexes include [FeCl(Me)(pdmp)2] (cis/trans-mixture) and [Fe(L)(pdmp)2] [L = CO, η2-C2H4, η2-C8H8, η2-1,4-C8H6(SiMe3)2, η2-C4H6, η2-PhC2Ph and η2-MeC2Me]. Treatment of trans-[FeCl2(pdmp)2] with sodium–naphthalene generates the hydrido(naphthyl)iron(II) complex [FeH(η1-C10H7)(pdmp)2] as a cis/trans-mixture in tautomeric equilibrium with a zerovalent iron naphthalene complex [Fe(η2-C10H8)(pdmp)2]. Single crystal X-ray analysis shows that [Fe(η2-C2H4)(pdmp)2] has a distorted octahedral structure in which the ethylene is twisted by ca. 15° relative to the FeP4 skeleton, probably because of steric effects. In solution the coordinated ethylene rotates about the coordination axis with a free energy of activation ΔG‡ at 324 K of 67 kJ mol−1, as estimated from variable temperature 1H and 31P NMR spectra. In the cyclooctatetraene complexes [Fe(η2-C8H8)(pdmp)2] and [Fe{(η2-1,4-C8H6)(SiMe3)2}(pdmp)2] there is also a lower energy process in which the Fe(pdmp)2 unit migrates between the double bonds of the eight-membered ring. The strong electron-donating ability of the Fe(pdmp)2 fragment is indicated by the low value of ν(CO) in [Fe(CO)(pdmp)2], the highly shielded 1H and 13C nuclei of coordinated ethylene in [Fe(η2-C2H4)(pdmp)2], the Fe–C and CC bond lengths in the Fe–C2H4 unit [2.052(8) and 1.418(10) Å, respectively], and the facile reversible protonation of the coordinated ethylene by methanol or ethanol. Treatment of [Fe(η2-C2H4)(pdmp)2] with HBF4·OEt2 gives initially a complex cis-[Fe(BF4)(Et)(pdmp)2], in which the ethyl group may be agostic, and finally cis-[Fe(BF4)2(pdmp)2]. The coordinated BF4 groups of the latter compound are displaced by acetonitrile to give cis-[Fe(NCMe)2(pdmp)2](BF4)2. The BPh4 salts of the cations [FeBr(NCMe)(pdmp)2]+ and [Fe(NCMe)2(pdmp)2]2+ have also been isolated as mixtures of cis/trans-isomers from cis-[FeX2(pdmp)2] (X = Cl, Br) and acetonitrile.