Alkane Elimination Reactions between Transition Metal Hydrides and Rare-Earth Alkyls
In the present work various transition metal hydrides were examined regarding their potential to form unsupported bonds towards rare-earth metals. Selected rare-earth alkyls were reacted with the metal hydrides in question. Motivated by previous results of the Kempe group in which the alkane elimination reaction of [Cp*RuH2]2 with [Cp2Y(CH2SiMe3)(thf)] yielded [H(Cp*Ru)2H2YCp2], it was assumed that the reaction of the formal dihydride [Cp*RuH2]2 with two equivalents of a rare-earth bis(alkyl) should lead to products with unsupported metal-metal bonds. [Cp*RuH2]2 was reacted with two equivalents of [Ln(CH2SiMe3)2(OC6H3tBu2-2,6)(thf)2] (Ln = Y, Lu). However the heteromultimetallic polyhydride complexes [(Cp*Ru)3(μ-H)4Ln(OC6H3tBu2-2,6)(μ- H)2RuCp*] (Ln = Y, Lu) were obtained. Regardless the stoichiometry (0.5-2.0 equiv [Cp*RuH2]2) the cluster compounds were obtained selectively. Solid state structures of both compounds could be established by XRD analyses. The isostructural compounds feature four Ln-Ru intramolecular distances of which one is significantly shorter than the three others. Six bridging hydride ligands are located between the metal centers. As the reaction of [Cp*RuH2]2 towards rare-earth alkyls forms polyhydride complexes further studies focused on transition metal monohydrides. Inspired by the use of Rp and Fp fragments (Fp = [CpFe(CO)2], Rp = [CpRu(CO)2]) in metal-metal bonding the reactivity of [HW(CO)3Cp] towards yttrium alkyls was examined. The relatively high acidity of the metal-hydrogen bond should allow alkane elimination reactions. Indeed, reactions of [HW(CO)3Cp] with yttrium alkyls proceeded rapidly with evolution of tetramethylsilane. However, isocarbonyl bridged products were isolated. Several bonding modes in RE-TM carbonyl complexes are possible, namely unsupported metal-metal bonds, isocarbonyl linkages and solvate separated ion-pairs. Which bonding mode is preferred mainly depends on the nucleophilicity of the transition metal, the carbonyls’ oxygen atoms and the used solvent. Reactions in several solvents were performed, always yielding isocarbonyl bridged compounds of type [{CpW(CO)2(μ-CO)}3Y(thf)5] which was obtained by the reaction of [Y(CH2SiMe3)3(thf)2] with three equivalents of [HW(CO)3Cp] in THF. The nucleophilicity of the carbonyls’ oxygen atoms appears to be higher than that of the tungsten atom. Thus, isocarbonyl linkages are preferred. To avoid the observed isocarbonyl linkages the carbonyl ligands in [HRu(CO)2Cp] were replaced by a chelating phosphine ligand. Thus, identifying [HRu(dmpe)Cp] as a possible candidate for the formation of unsupported rare-earth metal–transition metal bonds by alkane elimination. Reaction of [HRu(dmpe)Cp] with the rare-earth monoalkyls [Cp2Ln(CH2SiMe3)(thf)] (Ln = Y, Lu) led to the formation of C-H bond activated products by deprotonation of the Cp ligands on Ru. Heterometallic hydride complexes of the type [Cp2Ln(μ-H)(μ-eta1:eta5-C5H4)Ru(dmpe)] (Ln = Y, Lu) were isolated. The reaction of bis(alkyl) complexes [Ln(CH2SiMe3)2(OC6H3tBu2-2,6)(thf)2] (Ln = Y, Lu) with [HRu(dmpe)Cp] gave the products [(OC6H3tBu2-2,6)Ln(μ-H)(μ-eta1:eta5-C5H4){kappa3C,- P,P’-CH2(Me)P(CH2)2PMe2}Ru]2 (Ln = Y, Lu) by double C-H bond activation. In addition to deprotonation at the Cp ligand a methyl-group of the phosphine was deprotonated. As these results show, however, substitution of the carbonyls in [HRu(CO)2Cp] by an electron-rich phosphine changed the electronic properties of the resulting hydride complex. The hydride became unreactive towards rare-earth alkyls. Instead, the protons of the Cp ligand showed higher acidity. Thus, C-H bond activated products were obtained. To overcome this problem the carbonyls in [HRu(CO)2Cp] were replaced by electron-poor phosphines to mimic the electronic properties of the parent compound. Fluorinated diphosphines serve as bidentate CO analogues. Primarily the readily available bidentate (perfluoroalkyl)phosphine dfmpf was chosen. The hydride complexes [HRu(dfmpf)Cp] and [HCo(dfmpf)(CO)2] were prepared. Preliminary reactivity studies showed [HCo(dfmpf)(CO)2] to be more reactive towards rare-earth alkyls than [HRu(dfmpf)Cp]. This is consistent with DFT calculations which predicted a weakend metal-hydrogen bond in [HCo(dfmpf)(CO)2].
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