Homogeneous Transition Metal Catalyzed Reactions - American

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Homogeneous Transition Metal Catalyzed Reactions Downloaded from pubs.acs.org by UNIV OF MELBOURNE on 01/03/16. For personal use only.

Homogeneous Bimetallic Hydroformylation Catalysis Two Metals Are Better Than One Scott A. Laneman and George G . Stanley

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Department of Chemistry, Louisiana State University, Baton Rouge, L A 70803

Homobimetallic rhodium complexes based on the electron-rich binucleating linear tetratertiary phosphine ligand (Et CH CH )(Ph)PCH P(Ph)(CHC PEt ) (eLTTP) are surprisingly active and selective hydroformylation catalysts. This behavior is remarkable because monometallic rhodium catalysts based on electron-rich chelating phosphine ligands are extremely poor hydroformylation catalysts. The proposed key rate-enhancing step in the bimetallic Rh (eLTTP) catalyst system is an intramolecularhydride transfer that facilitates the elimination of the aldehyde product. This proposal has been tested by preparing model binucleating tetraphosphine ligands of the gen eral type Et PCH CH P(Ph)Y(Ph)PCH CH PEt (Y is p-xylene or pro pylene) to space the two metal centers apart and limit the extent of bimetallic cooperativity. These spaced bimetallic complexes, as well as related monometallic model complexes, are very poor hydrofor mylation catalysts. This evidence clearly points to the most dramatic example of homobimetallic cooperativity ever seen for a major cat alytic process. 2

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T R A N S I T I O N M E T A L D I M E R A N D C L U S T E R S P E C I E S are of interest as ho

mogeneous catalysts because of the following advantages that multimetallic systems should have over mononuclear complexes: • the ability to form multicenter metal-to-ligand bonds to a sub strate, thus assisting in the activation of that species toward further reactions; 'Corresponding author

0065-2393/92/0230-0349$06.00/0 © 1992 American Chemical Society

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H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

• the capacity to support multielectron transfers (e.g., reduction of Ν 2 to Ν Η ) either as a transfer point or as an electron sink-reservoir;


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• the potential to use M - M bonds, particularly weak ones, as "disguised" sites of coordinative unsaturation allowing direct insertion of a substrate into the M - M bond, thus eliminating the need for prior ligand dissociation to open coordination sites; and • the use of mixed-metal systems, which offer the possibility for the selective and subsequent activation of two (or more) dif ferent substrate species. Despite these potential advantages, only a very few dimer or cluster catalysts are known to have activities or selectivities even remotely ap proaching those of well-known mononuclear systems ( i , 2). Considering the amount of work being done on dimer and cluster species, it might appear tempting to wonder if these types of complexes will ever display novel homogeneous catalytic behavior. Yet the enormous number of possible com binations of metal centers (different types and oxidation states), ligands, and framework geometries make it clear that the seemingly large body of work on multimetallic systems represents only the tip of the iceberg. A seminal report by Adams et al. (3) on the cluster-catalyzed amine metathesis describes a reaction that is uniquely homogeneously catalyzed by a multimetallic sys tem. It provides strong impetus for continuing work in the area of polymetallic catalysis. Two problems have traditionally hampered the study of polymetallic systems: the preparation of these complexes in high yields and their frag mentation under catalytically interesting conditions (e.g., medium to high pressures of C O or H ) . In the last decade, however, synthetic techniques for the rational preparation of polymetallic complexes have been developed by Stone (4, 5), Vahrenkamp (6), Richter (7), Osborn and co-workers (8, 9), Geoffroy and Gladfelter (10), and others. The unifying idea behind all of these synthetic methods is the use of suitably designed ligand systems that can act as a template for the assembly of polymetallic complexes. 2

Inhibition of fragmentation of dimer and cluster systems usually centers around the use of strong M - M bonds (e.g., osmium clusters, M - M multiple bonds) or bridging ligands such as bis(diphenylphosphino)methane (dppm). Strong M - M bonds, however, negate the advantage of using M - M bonds as reaction sites. Weak M - M bonds can be ideal reactive sites for interaction with substrates because the molecular orbitals (MO) associated with the M - M interaction are often the highest occupied ( H O M O ) and lowest un occupied M O ( L U M O ) . The breaking of a M - M bond can play an important role in the activation of a substrate species. Similarly, the re-formation of the M - M bond(s) at the

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Bimetallic Hydroformylation Catalysis

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end of a catalytic cycle can assist in the elimination of products and the temporary stabilization of an unsaturated catalyst. The second approach, using bridging ligands such as dppm, often fails either because the ligands do not coordinate strongly enough to stop fragmentation or because they impose metal coordination geometries that are not as reactive. To fully exploit both template and antifragmentation concepts, we designed and synthesized a new hexatertiary phosphine ligand ( E t P C H C H ) 2

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P C H P ( C H C H P E t ) (abbreviated eHTP). This polyphosphine ligand has Homogeneous Transition Metal Catalyzed Reactions Downloaded from pubs.acs.org by UNIV OF MELBOURNE on 01/03/16. For personal use only.

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a number of features favoring it as a binucleating ligand system: the ability to both bridge and bichelate two transition metal centers; alkylphosphine moieties (rather than arylphosphines), which improve solubility and metal coordinating strength; and a straightforward, high-yield synthetic procedure (Ii). As might be expected, e H T P is a powerful binucleating ligand system. Every time we have treated it with two equivalents of a simple mononuclear metal halide or carbonyl we have obtained a bimetallic system, typically in high yields (12-17). Although e H T P was designed to form closed-mode binuclear complexes of the general type l a , we have found that the openmode complexes of the general types l b , lc, and Id are produced.

The preceding work has clearly demonstrated that e H T P is a powerful and quite rugged binucleating ligand system. It confirms our choice of combining bridging and chelating functionalities into a single ligand system. One of our primary concerns with eHTP, however, was that the tridentate, bichelating nature of the ligand ties up too many coordination sites on a metal

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center. As a result, for a closed-mode ligand conformation such as l a , access to the metal centers by a substrate molecule is essentially limited to the "top-side" of the e H T P - d i m e r system. O u r work on Group VIII square planar complexes of e H T P also clearly showed that steric factors would prevent these M (eHTP) complexes from readily accessing closed-mode ge2


ometries.

A Binucleating Tetraphosphine Ligand System One approach to reducing some of the unfavorable steric factors in e H T P is to conceptually backtrack and remove two of the chelate arms from e H T P to prepare a binucleating tetratertiary phosphine ligand of the general type ( R P C H C H ) ( R ) P C H P ( R ) ( C H C H P R ) . This ligand would still have the 2

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bridging-chelating framework of eHTP, yet would provide a considerably more open environment about the metal centers for reactions to occur. A tetratertiary phosphine of this type is chiral at the two internal phosphorus atoms. This conformation results in both the racemic meso (H,S) diastereomers shown as rac-eLTTP

(R,R; S,S) and

and m^so-eLTTP. We refer

to this general class of ligands as L T T P (for linear tetratertiary phosphine). The chirality of this system can be a desirable feature for promoting potential enantioselective reactions, but will reduce our overall synthetic yields and lead to more difficult separations of the tetraphosphine itself.

P

E

t

Et P

2

PEt

2

Ph

Et P

Ph

2

Ph

2

rac-eLTTP

meso-eLTTP

We developed a straightforward synthetic route to L T T P that is quite amenable to a wide variety of structural modifications. Our synthetic procedure

for

preparing the

LTTP

ligand ( R P C H C H ) ( P h ) P C H P ( P h ) 2

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( C H C H P R ) (R is E t or Ph) is shown in Scheme I. It involves the building 2

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of the central bis(phosphino)methane unit from the reaction of KP(H)Ph with C H C 1 . The Ph(H)PCH P(H)Ph species thus produced is isolated and then 2

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treated with 2 equiv of R P ( C H = C H ) under free radical-catalyzed condi2

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tions (18) to produce L T T P (19). We decided to use ethylene-linked terminal phosphines in L T T P because they simplify the synthetic procedure and give higher yields of the final tetraphosphine (8&-92% isolated yields based on P h ( H ) P C H P ( H ) P h , 2

39-43% isolated yields based on starting P h P H ) . The presence of phenyl 2

groups on the central P - C H - P bridge is a designed feature of this ligand 2

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DMF

CH,Cl

t

2PhPH"

2PhPH + 2K0H 2

(H)PhPCH PPh(H) 2

PEt,

Et P

PEt,

Ph

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LANEMAN & STANLEY

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AIBN \ Homogeneous Transition Metal Catalyzed Reactions Downloaded from pubs.acs.org by UNIV OF MELBOURNE on 01/03/16. For personal use only.

Et,P'

2Et P(CH=CHj) 2

raoeLTTP

meso-eLTTP

Scheme I. Synthetic procedure for preparing the LTTP ligand. to allow more facile crystallizations of transition metal complexes. Although the all-phenyl-substituted L T T P ligand was prepared, our primary interest is in the ethyl-substituted L T T P (eLTTP) ligand. Its electron-rich alkylated terminal phosphines will coordinate strongly to transition metal centers and be far more effective at inhibiting ligand dissociation and dimer fragmentation processes.

Bimetallic Complexes Based on eLTTP The meso and racemic diastereomers of e L T T P are powerful binucleating ligands that can both bridge and chelate two metal centers. They form complexes that will have different overall orientations of the phosphines about the two metal centers for idealized M - M bonded dimer systems. The racemic diastereomer of e L T T P has an anti orientation of the chelate rings; the meso diastereomer has the chelate rings directed syn to one another.

rae-M (eLTTP) 2

meso-M (eLTTP) 2

We were able to separate the two e L T T P diastereomers by reacting them with metal complexes to produce bimetallic M ( e L T T P ) systems. Then we took advantage of the differences between the rac-M (eLTTP) and mesoM ( e L T T P ) coordination geometries to effect separations by column chromatography or by fractional crystallizations. This approach worked particularly well for the N i C l ( e L T T P ) complexes (20). From them we can isolate pure rac- or meso-eLTY? by treatment of the appropriate diastereomerically 2

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pure nickel complex with excess K C N in refluxing H 0 - h e p t a n e . The cyanide displaces the nickel atoms to form water-soluble N i C N ~ and free eLTTP, which quantitatively extracts into the organic phase. We believe that the e L T T P ligands can also be separated directly by H P L C techniques, but have not yet been able to study this feature in detail. 2

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The reaction of R h ^ - C l ) ( C O ) with e L T T P produces the bimetallic Rh(I) system R h C l ( C O ) ( e L T T P ) in about 40-50% yield (19). The first diastereomer of this complex to crystallize out of the tetrahydrofuran (THF) or toluene solution is the racemic system (see structure). Not too surprisingly, the rae-Rh Cl (CO) (eLTTP) structure is closely related to the racN i C l ( e L T T P ) system (20) with a R h - P l - P l - R h ' torsional angle of 123° and a Rh~Rh separation of 5.813(2) Â. We now almost exclusively use a higher yield synthetic route to bimetallic rhodium-eLTTP complexes. It involves the use of [Rh(norb) ](BF ) (norb is norbornadiene) as a starting material to give 80-90% isolated yields of [Rh (norb) (eLTTP)](BF ) . 2

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racemic-RhaCljCCO^eLTTP)

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mes^Co ^-CO) (CO) (eLTTP) 2

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We characterized bimetallic systems based on e L T T P that formally possess M - M bonds. For example, the Co(0) dimer C o ^ - C O ) ( C O ) ( e L T T P ) was prepared by the reaction of the Co(0) dimer system Co (|xCO) (CO) (norb) with e L T T P (21). Single-crystal X-ray analysis on the orange crystals that initially form from the slow evaporation of a T H F solution confirms the presence of meso-eLTTF in a cradle geometry that is bridging and chelating a C o - C o dimer. The m £ s o - C o ^ - C O ) ( C O ) ( e L T T P ) (see structure) molecule lies on a crystallographic mirror plane that passes through the bridging carbonyl ligands and the central methylene group of the e L T T P ligand. The C o - C o bond length of 2.513(4) A is typical for a C o - C o single bond (22-24). The meso-eLTTF ligand symmetrically bridges and chelates both cobalt centers with the two five-membered chelate rings eclipsed and oriented syn to one another. Because of the crystallographic mirror plane, all four phosphorus atoms lie in the same plane. 2

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Both the rac- and meso-M (LTTP) binuclear systems have significantly less steric hindrance than the corresponding M ( H T P ) complexes. They represent new geometric arrangements for phosphine ligands about two metal centers. We believe that the more open ligand environment and 2

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rotational flexibility in these M (LTTP) dimers will promote a greater number of interesting reactions. 2

Bimetallic Hydroformylation Chemistry Hydroformylation (also called the oxo reaction) is the chemical process of converting alkenes into aldehydes by using H and C O , typically with soluble rhodium- or cobalt-based transition metal catalysts. It is the largest homogeneous catalytic process in the world, with more than 9 billion pounds of industrially important aldehydes and alcohols produced each year (25). H y droformylation is also a reaction that is potentially well suited to bimetallic systems (26, 27).


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Heck and Breslow (28), for example, proposed in 1963 that there could be an intermolecular hydride transfer in the Co (CO) -catalyzed hydroformylation cycle (Scheme II). Instead of adding H to a Co(acyl)(CO) species and then eliminating the product aldehyde, they suggested that another H C o ( C O ) species does an intermolecular hydride transfer to the acyl spe2

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Scheme 11. Heck-Breslow hydroformylation mechanism.

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H O M O G E N E O U S TRANSITION

METAL CATALYZED

REACTIONS

cies. This transfer eliminates aldehyde and forms a C o - C o bonded carbonyl system, which then goes on to react with H to form 2 equiv of H C o ( C O ) . Initially, kinetic and high-pressure IR data supported this mechanism for cobalt-catalyzed hydroformylation (29-35), but more recently careful highpressure IR studies (36, 37) provided strong support for the dominance of the monometallic pathway shown in Scheme II. Mechanistic data for hydroformylation by rhodium-based catalysts such as HRh(CO)(PPh ) is considerably less firm about details such as direct H addition or intermolecular hydride transfer from another rhodium hydride species (38). Sanger's group (39-43) demonstrated rate enhancements for hydroformylation with bimetallic rhodium catalysts. However, the linearto-branched aldehyde selectivities are quite poor for these systems. 2


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Several reports (44-47) have described the effects of heterobimetallic homogeneous catalysts on the rate and selectivity of hydroformylation reactions. Kovacs et al. (48), for example, proposed that the rate-limiting step in the mechanistic study of a mixed H C o ( C O ) - H M n ( C O ) catalyst system is the bimolecular reaction of an unsaturated cobalt acyl with the manganese hydride to give a binuclear reductive elimination of the product aldehyde. For these reasons, we examined Rh (eLTTP)-type bimetallic complexes for hydroformylation catalysis. O u r reaction studies on Rh (CO) (norb) ( e L T T P ) (norb is norbornadiene) show that it is a remarkable hydroformylation catalyst. The rates reported for our bimetallic complexes have all been divided by 2 to convert from a per-mole basis to a per-rhodium-atom basis. This conversion allows straightforward comparisons to monometallic catalyst systems. Table I lists the rates and selectivities of our bimetallic system and several other conventional monometallic hydroformylation catalysts for 1-hexene. The bimetallic Rh (eLTTP)-based catalyst is quite active and shows remarkable product selectivity with a 25-30:1 linear-to-branched aldehyde ratio. This result is particularly impressive considering that we are not adding any excess phosphine ligand to our catalyst. Virtually every other commercial 4

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Table I. Hydroformylation Catalysts for 1-Hexene Catalyst

Turnover (h-')

Linear-to-Branched (% isomer)

0.5:1 1:1 10:1 958:1

390 10 4900 875

>25:1 (8%) 3:1 (85%) 4:1 (10%) 14:1 (4%)

a

Rh (norb) (eLTTP) Rh(norb)(depmpe) 2

P:Rh 2

2+

+ d

HRh(CO)(PPh )3 i

Homogeneous Transition Metal Catalyzed Reactions - American

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