Organometallic chemistry of phosphine complexes of iron and ruthenium

Abstract

This thesis describes two projects involving the organometallic chemistry of iron and ruthenium complexes with DMPE ligands [DMPE = 1,2-bis(dimethylphosphino)ethane]. The first study involves an investigation into the kinetics and mechanisms of OH bond activation reactions of [Fe(DMPE)2]. The second project involves an investigation into the synthesis of RuH2(DMPE)2, the formation and properties of trans-[RuH(n2-H2)(DMPE)2]*, and the reactions of RuH2(DMPE)2 with alkyl and aryl thiols.

In Part I of this work, the kinetics of the cis/trans isomerization of FeH(C6H5)(DMPE)2 and FeD(C6D5)(DMPE)2 were measured by 31P NMR spectroscopy in pentane and THF. The isomerization reactions follow first-order reversible kinetics. FeH(C6H5)(DMPE)2 and FeD(C6D5)(DMPE)2 also undergo exchange with added arenes in a concerted fashion at the iron centre. The rate of exchange is comparable to the rate of isomerization. From the equilibrium constant for the exchange reaction, it was found that FeH(C6H5)(DMPE)2 is thermodynamically more stable than FeD(C6D5)(DMPE)2 by approximately 3 kJ mol'1 in pentane.

FeH(C6H5)(DMPE)2 and FeD(C6D5)(DMPE)2 react with diethyl disulfide to give Fe(SEt)2(DMPE)2. The reaction proceeds via loss of benzene or benzene-d6 followed by addition of [Fe(DMPE)2] to the 8-8 bond of EtSSEt. By following the kinetics of the reactions of EtSSEt with FeH(C6H5)(DMPE)2 and FeD(C6D5)(DMPE)2 in THF separately, the rates of reductive elimination of benzene OH and GD bonds at 283 K were found to be 3.9 x 10‘ s-1 and 6.5 x 104 s-1 respectively. The inverse deuterium isotope effect (k”/kD = 0.6) can be rationalized by the presence of a n-benzene intermediate in the elimination reaction. In solution, the phenyl ring in cis'FeH(C6H5)(DMPE)2 assumes a fixed orientation and is constantly flipping at 240 K.

During this work, it was discovered that [Fe(DMPE)2] is capable of catalyzing the hydrogenation of alkenes to alkanes under photochemical conditions. The hydrogenation reaction competes with a significantly slower dehydrogenation reaction. A quantitative analysis of the efficiency of [Fe(DMPE)2] as a hydrogenation catalyst was carried out. The hydrogenation of cyclopentene is faster than that of tenninal alkenes. A reaction cycle is proposed for the hydrogenation-dehydrogenation reactions mediated by Fe(DMPE)2 complexes. Treatment of an irradiated sample of FeH(cyclopenteny1)(DMPE)2 with dibromomethane afforded FeBr2(DMPE)2 and trans-[Fe(cyclopentenyl)Br(DMPE)2]Br.2H20 whose crystal structures are presented.

In Part II, a synthesis of Rqu(DMPE)2 from trans-RuC12(DMPE)2 by reduction with sodium/Z-propanol is presented. Protonation of RuH2(DMPE)2 with weak organic acids such as methanol, ethanol and thiols affords the molecular hydrogen complex trans-[RuH(T]2-H2)(DMPE)2]+ which has a nZ-bound H2 ligand and a 6-bound hydride ligand. T1 measurements and 1JHD coupling in nZ-HD ligand confirm the 'non-classical' structure. Between 220 and 300 K, the molecular hydmgen complex continuously undergoes intermolecular exchange with the protonating solvent and all the rutheniumbound hydrides undergo intramolecular exchange. In methanol, a previously unreported five-coordinate ruthenium(II) complex, trans-[RuH(DMPE)2]+, exists in equilibn'um with the molecular hydrogen complex. Reactions of the ruthenium dihydride with alkyl- and arylthiols afford trans-monothiolate hydrides. Aromatic thiols react more rapidly than alkanethiols. The reaction is believed to proceed via protonation of the dihydride (by the acidic thiol group) to give the molecular hydrogen complex, followed by substitution of the 'r12-H2 ligand with the conjugate base of the thiol. The dithiolate complex trans-[Ru(SPh)2(DMPE)2] has been isolated and its X-ray crystal structure is presented. In dithiols, dithiaruthenocycles are not formed, which is in contrast with the formation of the iron analogues. Although protonation of RuH2(DMPE)2 with alcohols is facile, substitution of trans-[RuH(T]2-H2)(DMPE)2]* by alkoxide ions does not take place in the presence of thiolate ions.