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

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, eta(2)-C2H4, eta(2)-C8H8, eta(2)-1,4-C8H6(SiMe3)(2), eta(2)-C4H6, eta(2)-PhC2Ph and eta(2)-MeC2Me]. Treatment of trans-[FeCl2(pdmp)(2)] with sodium-naphthalene generates the hydrido(naphthyl)iron(II) complex [FeH(eta(1)-C10H7)(pdmp)(2)] as a cis/trans-mixture in tautomeric equilibrium with a zerovalent iron naphthalene complex [Fe(eta(2)-C10H8)(pdmp)(2)]. Single crystal X-ray analysis shows that [Fe(eta(2)-C2H4)(pdmp)(2)] has a distorted octahedral structure in which the ethylene is twisted by ca. 15 degrees 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 Delta G double dagger at 324 K of 67 kJ mol(-1), as estimated from variable temperature H-1 and P-31 NMR spectra. In the cyclooctatetraene complexes [Fe(eta(2)-C8H8)(pdmp)(2)] and [Fe{(eta(2)-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 nu(CO) in [Fe(CO)(pdmp)(2)], the highly shielded H-1 and C-13 nuclei of coordinated ethylene in [Fe(eta(2)-C2H4)(pdmp)(2)], the Fe-C and C=C bond lengths in the Fe-C2H4 unit [2.052(8) and 1.418(10) Angstrom, respectively], and the facile reversible protonation of the coordinated ethylene by methanol or ethanol. Treatment of [Fe(eta(2)-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.