Platinum Metals Rev., 1978, 22, (4), 126
Iridium Compounds in Homogeneous Hydrogenation
Iridium is generally considered to be a less effective catalyst than the other platinum metals but when highly co-ordinatively unsaturated species of it are prepared they appear to be exceptionally active hydrogenation catalysts, even for reduction of sterically hindered olefins. A mechanism for this reaction has been postulated based on low temperature magnetic resonance evidence. As the reaction proceeds, catalytically inactive hydrogen bridged cluster compounds are formed but work is in progress to counter this deactivation by physically separating the catalytic centres on a polymer support.
The generally accepted order of catalyst activities for the elements of the transition series and their compounds is 2nd row ≽ 1st row ≫ 3rd row. A characteristic example is that of the complexes MCl(PPh3)3. Where M=Rh, this complex is a versatile homogeneous catalyst for hydrogenation (1), decarbonylation (2), and cyclisation (3). The cobalt and iridium analogues are entirely inactive for these catalytic reactions. In the case of homogeneous hydrogenation, the cobalt complex is inactive because it does not react at all with hydrogen (4) while the iridium analogue reacts with hydrogen to give [IrH2Cl(PPh3)3], which, unlike the rhodium analogue, fails to dissociate PPh3 to allow the substrate access to the active site (5).
We believed that once access of the substrate to the active site was assured, compounds of third row elements might prove to be highly active catalysts. What was needed was a ligand which would occupy a coordination site on a metal, and give readily isolable, stable complexes for use as catalyst precursors. However, this ligand would have to be irreversibly removed from the coordination sphere under the conditions of the catalytic reaction. Osborn briefly described (6) the catalyst precursors [Ir(cod)L2]+ (i); where cod=1,5-cyclo-octadiene; L=tertiary phosphine. He found that under H2 and in a co-ordinating solvent, the complexes [IrH2(solvent)2L2]+ (ii), (solvent=Me2CO, EtOH or thf) were formed. These were moderately active hydrogenation catalysts but less active than the rhodium analogues which were studied in much more detail (7).
Cod might therefore serve as such a ligand, since it was irreversibly removed by hydrogenation to cyclooctane in the catalytic reaction. However, in a co-ordinating solvent, the cod was merely replaced by a different co-ordinating ligand, the solvent itself.
We decided to look at the behaviour of complexes such as (i) in polar, but non-co-ordinating, solvents, such as CH2Cl2. The results were strikingly different from those found in co-ordinating solvents, showing the profound effect of solvent on a process as complicated as catalysis. No solvate such as (ii) was formed, instead, when (i) reacts with hydrogen in CH2Cl2, highly co-ordinatively unsaturated species were formed. In the presence of olefin, the resulting solutions constitute hydrogenation catalysts of unprecedented activity. They are even capable of rapidly reducing tetrasubstituted olefins such as Me2C=CMe2. The Table shows some of the results obtained with different catalysts (8). The corresponding rhodium catalysts [Rh(cod)L2]+ can be seen to be far less active, in sharp contrast to their behaviour in co-ordinating solvents.
|Rate(b)||Extent of hydrogenation(c)||Rate(b)||Extent of hydrogenation(c)|
|[Ir(cod) (dpe)]PF6 (d)||1300||100||640||72|
|[Ir(cod) (PMe2Ph)3]PF6 (e)||1200||100||370||65|
This increase in activity, gained by using highly unsaturated iridium species as catalysts, is accompanied by a loss of catalyst stability. When the olefin has been consumed, or, in the case of olefins which are poor ligands, nearly consumed, the catalyst deactivates to give a series of remarkable hydrogen-bridged cluster complexes which are catalytically inactive. For example:
(L=PPh3 or PMePh2)
This complex (iii) has the shortest iridium-iridium distance known (8). The bridge bonding probably resembles that in diborane so that both iridium atoms can achieve completed valence shells (9). The complexes [Ir(cod)LL′]PF6 (10) (L=tertiary phosphine, L′=pyridine) react with hydrogen to give a yellow crystalline substance, which preliminary spectral and crystallographic evidence suggests is an unusual trimeric iridium complex, [(IrLL′H2)3(μ3-H)](PF6)2, in which the unique hydrogen atom bridges three iridium atoms.
By keeping the catalyst species apart from one another, deactivation can be prevented or, at least, slowed down. Since the catalyst precursors and the catalysts themselves appear to be entirely air-stable, one effective method is to inject the catalyst solution into the reaction vessel drop by drop. Choosing ligands L so large that they prevent dimerisation also works, but at the cost of almost complete loss of activity. A more elegant solution would be to use a polymer support, incorporating relatively widely spaced phosphine groups, to anchor the catalyst precursor. Work along these lines is in progress.
While co-ordinatively unsaturated iridium species can be much more reactive than their rhodium analogues, co-ordinatively saturated complexes of iridium are often more stable than the rhodium or cobalt analogues. We uncovered a striking example of this effect in our catalytic systems. When cod itself is used as a substrate for the catalyst system, co-ordinatively saturated species are rapidly formed and the catalysis slows down abruptly. The unexpected product of this reaction is cis-trans -[IrH2(cod)L2]+ (iv) which can be isolated as a crystalline solid (11).
At −80°C another isomer of (iv), namely cis -[IrH2(cod)L2]+ (v) is formed by the direct addition of hydrogen to (i) (12). On heating this solution to room temperature in the presence of cod, hydrogenation proceeds rapidly and then slows abruptly with the formation of (iv).
These complexes are the first examples of dihydrido olefin complexes. Such complexes had been assumed (1, 13) to be intermediates in the hydrogenation of olefins but never directly observed. They may be formed either, as above, by hydrogen addition to an olefin complex or by olefin addition to a hydride complex (11):
The direct addition of H2 to an olefin complex had been considered unlikely (1). The olefin, as an electron-withdrawing ligand, was considered to deactivate the metal towards the "oxidative" addition of hydrogen, so called because the metal was assumed to be partially oxidised in the product, implying the presence of hydridic hydrogens.
We now know that [Ir(cod)2]+, containing only olefinic ligands, reacts readily with H2 at −80°C to give a complex type (v) (L2=cod) (11). In contrast [Ir(cod)(pyridine)2]+, in which a cod ligand of [Ir(cod)2]+ has been replaced by two strongly donor ligands, fails to add H2 at any temperature. This may mean that the addition of hydrogen does not greatly affect, or even slightly lowers the partial positive charge on the metal, and that the addition of H2 is not oxidative, at least in the case of this iridium system. Certainly, the Pauling electronegativity of all the platinum metals (2.2–2.3) is very close to that of hydrogen (2.2), so that one would not expect a large charge separation in any platinum metal-hydrogen bond.
In the catalytic solutions themselves, the intermediates may well be of the type [IrH6−2x(mono-olefin)xL2]+ (x=0, 1, 2 or 3). They are probably formed by both of the routes mentioned above.
It is significant that the crystalline cis-trans -complex (iv) decomposes thermally at about 80°C to give largely (i) and hydrogen while (v) decomposes at around −20°C to give largely cyclooctene. Similarly, in a hydrogenation of cod by (i) (see Scheme below), the initially formed intermediate, (v), readily hydrogenates the co-ordinated cod. Free cod subsequently co-ordinates to give the cis-trans -isomer (iv) and the hydrogenation rate falls 40-fold. It would seem that coplanar M(C=C)H arrangement is required in order for the olefin to insert into the metal-hydrogen bond, since (v) possesses this arrangement, while (iv) does not.
The unusual intermediate (vi) can only be isolated where L is a bulky ligand such as PCy3, which is known (14) to stabilise unusual hydride complexes. Where L is a smaller phosphine (vi) rapidly dimerises to give (iii). From NMR and hydrogen evolution studies, the value of x seems to be six, in which case (vi) is a unique example of heptavalent iridium. Treatment of (vi) with NEt3 gives the known (15) complexes IrH5L2.
Iridium therefore seems to be an element whose catalytic potential has yet to be fully realised. Given the right conditions, in particular, co-ordinative unsaturation, iridium can provide catalysts as active as those of any other element.
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