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.