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

Routes to hydrido- and organo-iron complexes of the bidentate ligand o-phen ylenebis(dimethylphosphine), o-C6H4(PMe2)(2) (pdmp), have been developed, s tarting from trans-[FeCl2(pdmp)(2)]. Reduction of trans-[FeCl2(pdmp)(2)] wi th LiAlH4 gives the dihydride, cis-[FeH2(pdmp)(2)], there being no evidence for an intermediate monohydride [FeHCl(pdmp)(2)]. Treatment of trans-[FeCl 2(pdmp)(2)] with an excess of methyllithium gives [FeMe2(pdmp)(2)], isolate d as a cis/trans-mixture, the methyl groups of which are cleaved by an exce ss of HX to give [FeX2(pdmp)(2)] (X=Cl, Br), predominantly in the form of t he 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-C 8H6(SiMe3)(2), eta(2)-C4H6, eta(2)-PhC2Ph and eta(2)-MeC2Me]. Treatment of trans-[FeCl2(pdmp)(2)] with sodium-naphthalene generates the hydrido(naphth yl)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)-C2H 4)(pdmp)(2)] has a distorted octahedral structure in which the ethylene is twisted by ca. 15 degrees relative to the FeP4 skeleton, probably because o f steric effects. In solution the coordinated ethylene rotates about the co ordination axis with a free energy of activation Delta G double dagger at 3 24 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(pd mp)(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(e ta(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 pr otonation of the coordinated ethylene by methanol or ethanol. Treatment of [Fe(eta(2)-C2H4)(pdmp)(2)] with HBF4. OEt2 gives initially a complex cis-[F e(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 compoun d 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)(pd mp)(2)](2+) have also been isolated as mixtures of cis/trans-isomers from c is-[FeX2(pdmp)(2)] (X=Cl, Br) and acetonitrile.