A Readily Recyclable Homogeneous Catalyst

Morris Bullock

Thermal, Photo- and Radiation-Induced Reactions in Condensed Media Group

Transition metal hydrides are ubiquitous in homogeneous catalysis, performing the crucial role of delivering hydrogen to a wide variety of organic substrates.  A goal of our research is to seek an improved understanding of the factors that influence the rupture of M‑H bonds.  Formal cleavage of M‑H bonds can occur with delivery of a proton (H+), a hydrogen atom (H•) or a hydride (H–) from the metal to an unsaturated organic compound.  Our research seeks to better understand the factors governing each of these pathways of M‑H bond rupture through kinetic and mechanistic studies.


We then try to use this information to guide the design of new catalysts that utilize these reactions.  We are designing new catalysts that function by an ionic hydrogenation mechanism. These catalysts use inexpensive metals like molybdenum, and they function by a mechanism entirely different from those established for traditional reactions that use precious metals.  Ionic hydrogenation occur by proton transfer from a cationic metal dihydride, followed by hydride transfer from a neutral metal hydride, as shown below:


A series of Mo and W complexes of formula [Cp(CO)2(PR3)M(O=CEt2)]+ was shown to serve as catalyst precursors for the ionic hydrogenation of ketones.  The stability of these Mo and W catalysts is limited by dissociation of a phosphine, which is protonated to give HPR3+ under the reaction conditions.  Suppression of phosphine dissociation was sought by using a two-carbon bridge to chelate the phosphine to the cyclopentadienyl ligand.  Improved performance was found using catalysts obtained by reaction of HMo(CO)2[h5:h1-C5H4(CH2)2PR2] (R = Ph, Cy, tBu) with Ph3C+BAr'4 –.  Advantages found with these new complexes include low catalyst

loadings (< 0.4 mole %), higher thermal stability, substantially longer lifetimes (hundreds of turnovers), and hydrogenation of liquid ketones under solvent-free conditions.

            Recent efforts have focused on N-heterocyclic carbene ligands rather than phosphines.  The tungsten complex CpW(CO)2(IMes)H (IMes = the carbene ligand 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) was prepared by substitution of the phosphine ligand in CpW(CO)2(PMe3)H by IMes.  Hydride abstraction from CpW(CO)2(IMes)H  produces CpW(CO)2(IMes)+.  An x-ray crystal structure carried out

at the National Synchrotron Light Source indicated a weak bonding interaction between the tungsten and one of the C=C bonds of a mesityl ring.   This cationic complex is a catalyst precursor for hydrogenation and  hydrosilylation of ketones.  An unusual property is observed in the hydrosilylation of aliphatic substrates – the catalyst precipitates from solution at the end of the reaction, so that the liquid product can be separated from the catalyst by pouring off the liquid.  (link to photos)  The recovered catalyst can then be readily recycled and used again.  These hydrosilylations are carried out in neat ketone/HSiEt3, so that no solvent is used in either the reaction or the catalyst recycling.

Last Update on Friday April 23, 2004