Homogeneous Hydrogenation and Isomerisation of Olefins catalysed by Pentamethylcyclopentadienyl-rhodium and -iridium complexes

Abstract

<p>The catalytic activity of a series of pentamethylcyclopentadienyl-rhodium and -iridium complexes was explored to determine which catalytic species were significant. The halide complexes [M(C₅Me₅)x₂]₂ ((=Cl, Br, I) were inactive and required the presence of base as co-catalyst for the homogeneous hydrogenation of olefins.</p> <p>As a part of this investigation, a number of new mono-ρ-hydrido [(M(C₅Me₅))₂HX₃], di-ρ-hydrido [Ir(C₅Me₅)HX]₂, and related complexes were isolated and successfully synthesised and characterised.</p> <p>The hydrido complexes [(M(C₅Me₅))₂HX₃] and [lr(C₅Me₅)HX]₂ catalysed the hydrogenation of olefins in the absence of base at 1 atm and ambient temperature. However, they did show some co-catalysis by base and the effect of base on catalytic activity of [M(C₅Me₅)Cl₂]₂, [(M(C₅Me₅))₂HCl₃] and [Ir(C₅Me₅)HCl]₂ was carefully examined. The iridium complexes showed some deactivation on addition of excess triethylamine due to the formation of the caralytically inactive cationic complex [(Ir(C₅Me₅))₂H₃]. No such deactivation and formation of cationic complex was observed for the rhodium system.</p> <p>In general iridium complexes were better catalysts than rhodium ones. The hydrogenation appeared to be favoured by polar and weakly coordinating solvents, particularly isopropanol.</p> <p>A premliminary study of the mechanism of the reactions catalysed by these complexes indicated that the mono-μ-hyrdrio complexes reacted mainly as dimers and dichlorides and di-μ-hydrido complex [Ir(C₅Me₅)HCl]₂ reacted predominantly as monomer; mechanisms are presented to account for these observations.</p> <p>The hydrido complexes [(M(C₅Me₅))₂(D)HCl₃] and [Ir(C₅Me₅)HCl]₂ were also active catalysts for the isomerisation of olefins. During the isomerisation of 4-methylcyclohexene by [(Ir(C₅Me₅))₂DCl₃], appearance of a high field hydride resonance was observed and these reactions are explained in terms of a hydride addition-elimination mechanism.</p> <p>ecies "cre stgnific.ant. The hal ide c~~lexes pl(CSMeS)xzlz ( =. Cl, B "<br />,I) ~e.re ina~tiv,e'and ,required the presence, of base as co~cata1.yst forth'<br />hC!':JQeneous 'hydroQen~ -.' .,<br />'I ,~a' part of thls investi~ation.a numb!J-r 0t, ne\~ oono-~-hydrido_<br />[(M(~SI~e5) i ZHX 3]. d;"-;,-hydr,ido [I~(C5Me5)HX]Z" and'Tflated complexes \~ere"<br />isola~e:1 and s,uccessfull y syntheslsed ~ni1 .c~aracteris:d. . .'<br />.Thehydrido comple~es ['M!Cs~es~)zHX3] and [l~(c5r~e's)HX}L c<1talysed<br />the hydrogenation of olefins in t~e absence of base at 1 atm and a~bient_<br />"temperature. However. they did show some co-catalysis by bas~ and ~he<br />effect Of, !and:[lr(C5~leS)HC.1]2wascarefully, exar.1ined. The iridium compl,,1.'!?s sho','led<br />'. some deactivation. on addit~on of ~xC:~ss trtethYl,ar.line due to t~o~!"'.ation<br />of thecatalytlC-:lly InactIve catlon'\ccomple~ [(!r(CS~1e5)12Hp'\""'iu-- .<br />such deactivation,and formation of cationic complex was observed for the<br />I'hod i urn .sys tem. "<br />In general iridium compleJones. The hydrogenation appea'red to be favoured 'by polar 'and weakly co-<br />I ,ordinating sOflvents. partic:ul.arl y isopropanol: ' I.<br />l,.- A prel.,jminary stud/of the ;,echaniS!:1of the reactlons catalysed<br />l' ,<br />'} by these ccmplexes indicated that the mono-~-hydrido complexes reacted mainly as ,dimers and dichlorides ~nd di-u-,hydridocomplex [Ir(CSf.leS)HC1]Z<br />. \ ' 'reacted predominantly 4S monomer; mec~anisms are. pres~nted to dCco~nt for<br />these observations, !. (0)<br />.'- The hydrido complexes [(H(CSMeS)i Z<br />HC1<br />3<br />] and [lr(.CSMeS)HC1]Z 4'e<br />.also active catafysts for the isomerisation of olefins, .During the isom- . . ~<br />eri sa ti on of 4-methylcyc Iohexene by ['(l r( CSfi,eS)) ZOeI<br />3<br />], a ppeari\nce of a<br />hi gh fi el d hydride resonance was observed and thes,e re&c t ions are ~xp I ai ned<br />in" terms of a hydride addition~eliminationinechanism.</p>