Activation of Dioxygen Using Group VIII Metal Complexes - Advances

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Activation of Dioxygen Using Group VIII Metal Complexes BRIAN R. JAMES Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5

The O -oxidation of inorganic and organic substrates catalyzed by Group VIII metal complexes is considered together with some analogies to enzyme systems; organometallic type and more bioinorganic type models are discussed. Net oxygen-atom transfer can occur, but rarely does this appear to involve transfer of coordinated dioxygen to a substrate coordinated at the same metal center. Other nonselective, free-radical pathways, including Haber-Weiss-catalyzed decomposition of trace hydroperoxides, usually are competitive and dominate with organic substrates, although formation of hydroperoxide or the HO radical via hydrogen abstraction by a metal-dioxygen moiety seems plausible. Nucleophilic substrates can release the oxidizing power of coordinated dioxygen as free peroxide or superoxide, and this (or genuine oxygen-atom transfer), if necessary coupled to a Wacker cycle, can lead to co-oxidation of substrates. Molecular H can be used to provide hydride as one substrate, and the use of O /H mixtures can result in oxidations that are formally very similar to P-450 systems. Olefin interaction with metalloporphyrins suggests possible oxidation via substrate-activation pathways. 2

2

2

2

S

2

ynthetic 0 -carriers have been known for some forty years and have maintained considerable interest because of the role of such centers in some naturally occurring oxygen-storage and -transport systems (1-8). Much of the earlier work centered around a range of formally divalent Co complexes with N- and/or O-donor ligands (3, 9), 2

0-8412-0514-0/80/33-191-253$06.00/0 © 1980 American Chemical Society

In Biomimetic Chemistry; Dolphin, David, et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

254

BIOMIMETIC CHEMISTRY

and indeed various forms of coordinated dioxygen moieties [monodentate, bent superoxide (1), and bridging superoxide or peroxide (2)] were first established by x-ray crystallography at Co centers.


i M

M—O

/ ° -

°\-° M

m

Vaska's report (10) in 1963 of reversible 0 -binding by transIrCl(CO)(PPh ) stimulated others to discover an extensive list of oxygen complexes of Group VIII metal systems with "organometalliclike" ligands (carbonyls, tertiary phosphines, etc.); these complexes, which were usually derived from d and d platinum metal and nickel systems, demonstrated the side-on geometry (3), the 0 moiety being likened either to coordinated ethylene (singlet 0 ), or more usually to peroxide when formation of the dioxygen complex is discussed under the general classification of oxidative-addition reactions (11, 12). Interest in dioxygen complexes of metalloporphyrins (I, 2, 3, 5, 6, 7, 8) is particularly intense because nature has evolved such systems not only for binding and reversibly carrying 0 (e.g., myoglobin, hemoglobin) but also for activating 0 via enzymic oxygenases, which incorporate one or two atoms of 0 to a substrate, or via oxidases that convert both atoms of 0 to water or hydrogen peroxide. The heme unit (an iron-porphyrin moiety) is particularly prevalent and, for example, is found in myoglobin and hemoglobin (13, 14, 15), the monooxygenase cytochrome P 450 (16-20), tryptophan dioxygenase (21), and in cytochrome c oxidase—the terminal enzyme in the respiratory redox chain that reduces 0 to water (21, 22). The enzymes catalase and peroxidase, both containing heme centers, utilize H 0 and are related to the dioxygen systems (21, 23). It is only within the last decade that protein-free metalloporphyrin-0 complexes have been recognized clearly, and of interest, this again stems, at least historically, from studies on a Co(II) system—the reaction of vitamin B with 0 at low temperature (24). Co(II) systems, of course, have the advantage of forming ESRdetectable superoxide species (Co —0 "). However, in this regard, Corwin and Bruck (25) over 20 years ago almost certainly (6) had oxygenated rather than oxidized bis(imidazole)ferrous porphyrin systems. Reaction of dioxygen with metalloporphyrins to give 1: 1 complexes with geometry 1 or 3 has been demonstrated with Cr(II), Mn(II), Fe(II), Co(II), and Ru(II) systems (7, 8, 26). The factors affecting the binding of dioxygen (usually "oxygen", for the sake of familiar2

3

2

8

10

2

2

2

2

2

2

2

2

2

1 2 r

an)

2

2


2


14.

JAMES

255

Dioxygen Activation

ity) to transition-metal centers, whether the organometallic type systems or the more biological type porphyrin and related macrocyclic systems, are being elucidated slowly. These studies include kinetic, thermodynamic, structural, and theoretical studies, and especially a consideration of the role of the metal, ligands, and solvent. The area has been reviewed extensively in recent years (1-8), and other articles in this volume (15, 27, 28) are concerned with this topic. Studies on protein-free systems are proving extremely informative as exemplified by the elegant work of Traylor s group (27) and Collman's group (29), which is leading to a better understanding of the cooperativity phenomenon in 0 -binding by hemoglobin (15). The chemical behavior of 0 after coordination, that is the activation of 0 , is a diverse and complex subject because of the many pathways available for 0 reactions. The search for more selective oxidations, especially using 0 , is a prime goal that has immense potential on an industrial scale (4, 30), and is thus an important research area. The ultimate aim would be to mimic the (Deactivation ability of enzyme systems, for example the ability of cytochrome P 450 to catalyze Reaction 1, where R is a hydrocarbon molecule. 2

2

2

2

2

RH + 0

2

+ N A D H + H + ^ R O H + H Q + NAD+ 2

(1)

Excellent reviews on 0 activation by Sheldon and Kochi (31), and Lyons (4), have appeared recently, and I have used these extensively. This chapter, reflecting the authors interests, will concentrate on Group VIII metal systems, bringing in comparisons with enzymatic systems. 2

Oxygenation via Coordination Catalysis The autoxidation of organic substrates catalyzed by transitionmetal salts has been used widely in the petrochemical industry for many years (32), but the oxidations are frequently nonselective since they operate by free-radical pathways that are sometimes initiated by the transition-metal ion. An example is the chain reaction of Reactions 2, 3, 4, and 5 (4). Propagation is maintained via Reactions 3 and 4, and any interaction between the metal ion and 0 would be incidental. 2

M + + RCHO w

RCO • + M

( w _ 1 )

+ H+

RCO • + 0 - » R C 0 0 2

RC00 2

2

+ R C H O -> R C 0 0 H + R C O 2

R C O Q H + R C H O -> 2 R C O O H 2


(2) (3) (4) (5)

256


The discovery of the d and d metal 7r-bonded dioxygen complexes (formally d - and d -peroxide systems), and the accompanying chemistry exemplified in Scheme 1, showed the possibility of attaining net oxygen-atom transfer to both inorganic and organic substrates. 8

6

10

8

Scheme 1. Chemistry typical of d , d Pt metal systems: M = Rh(I), Ir(I), Ni , Pd , Pt , with ancillary ligands L, typically a tertiary phosphine type ligand, CO, etc. (4, 33, 34) 8


(0)

(0)

10

L M0 —C—C-

(14)

M(oxide) + C 0 4- R •

(15)

n

2

2

We had invoked formation of hydroperoxide via the mechanism outlined in Scheme 2 for oxidation of a Rh(I) cyclooctene complex in dichloroethane, the important step being hydrogen abstraction from the substrate by the coordinated dioxygen, although it was emphasized that neither the olefin nor hydroperoxide was necessarily coordinated to the Rh (54). The whole sequence shown in Scheme 2 was based on 0 -uptake data ( 0 : Rh = 1.0) and changes in IR spectra. A catalyzed autoxidation of the solvent occurred in dimethylacetamide (DMA) solution; a Rh(II)-0 species was detected by E S R and the same type of mechanism was invoked (55). There was no induction period, little inhibition by free radical inhibitors, and, interestingly, P P h was not oxidized; these data tend to rule out the presence of free hydroperoxide in solution and are more consistent with catalysis via coordination, at least of the dioxygen. Olefins are unlikely to be strong enough nucleophiles to displace the dioxygen as peroxide (36). The catalytic activity of the system diminished through the formation of an inactive Rh(II) species, that was later isolated as R h C l ( D M A ) (56). 2

2

2

_

3

2

6


2

2 _

260


Scheme 2.

The 0 oxidation of a Rh(I) cyclooctene complex (54, 55) 2

JUi


v

O,

HQ

2

+ cyclooctene \

V

Cl

,„0,

Activation of Dioxygen Using Group VIII Metal Complexes - Advances

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