Download Ruthenium Porphyrins David Dolphin Heme the iron complex of protoporphyrin,

Ruthenium Porphyrins
David Dolphin
5
Heme (
1 ) ,
the iron complex of protoporphyrin,
when bound to a number of proteins such as hemoglobin,
myoglobin and cytochrome P-450, reversibly binds molecular oxygen when the iron is in the ferrous oxidation
state. When, however, the heme is not bound to the
protein it undergoes irreversible oxidation to the
ferric stage which no longer binds oxygen. A number
of approaches, by ColIman, Traylor and others, which
involve modifications of the porphyrin or low temperature, have been used to stabilise the iron-porphyrin
oxygen adducts. More recently another approach, by
Farrell, Dolphin and James, using simple ruthenium
porphyrins, has provided systems which reversibly bind
molecular oxygen to give oxygen complexes which are
stable at room temperature.
Complexes ( 2 ) (where L=L1=CH3CN) were prepared
from both octaethyl- (OEP) and meso_-tetrapheny!porphyrins by photolyses of the corresponding complexes
( 2 ; where L=CO and L1 = CH3CH2OH) in acetonitrile.
The course of a photolysis is shown in Fig. 1 and the
isosbestic points indicate the smoothness of the reaction. Photolysis in other solvents including pyrrole,
dimethylformamide (DMF), dimethylacetamide (DMA) and a
variety of nitrogenous bases gives rise to similar
bis-solvated adducts.
When simple ferrous-iron porphyrins are reacted
with molecular oxygen an initial 1:1 complex ( 3 ) is
formed.
By itself 3 does not undergo irreversible
oxidation since oxygen is a two electron oxidising
agent and ferrous iron a one-electron reductant. However, if a second iron porphyrin coordinates to the
oxygen to give 4 (this can be prevented by sterically
encumbering the porphyrin, which is the case when heme
is bound to a protein) an irreversible reaction occurs
to give the bridging peroxide ( 5 ) .
When RU(II)(OEP)(CH3CN)2 in toluene reacts with
molecular oxygen a slow irreversible oxidation occurs
to give a ruthenium(III) complex. However, solutions
of RU(II)(OEP)(CH3CN)2 in DMF, DMA, or pyrrole reversibly absorb 1.0 mole of O2 per ruthenium at room temperature. Figure 2 shows the spectral changes when
the reaction is carried out in pyrrole. The rate of
oxygenation depends upon the axially coordinated
solvent, at 1 atm pressure of O2 the reactions are
pseudo-first-order with the reaction being almost
instantaneous in DMA but having t½=30 min in DMF.
The reverse deoxygenation is much slower and requires
pumping for 24 h. Addition of CO to the oxygenated
complex generates Ru(II)(OEP)(CO)L with no change in
the gaseous volume. All of the above chemistry is
consistent with that shown in Scheme 1.
The stability of the oxygen adducts is probably
due to the slow off-rate for loss of oxygen, which is
to be expected from substitution inert ruthenium(II)
complexes, and the correspondingly slow formation of
the dimeric peroxide species analogous to 5 .
Apo myoglobin has been reconstituted with both
Ru(II) and Ru(III) mesoporphyrin (D. Paulson, A.W.
Addison, D. Dolphin and B.R. James, unpublished
results). Ru(II)-mesoporphyrin and reduction of the
Ru(III)-Containing complex gives ruthenomyoglobin
which reacts with carbon monoxide, Fig. 3, to give the
CO-adduct similar to that of native myoglobin. At the
present time addition of oxygen to ruthenomyoglobin
appears to bring about an irreversible oxidation of
the metal.
In a manner analogous to complexes of type
2 , ruthenomyoglobin also reversibly binds molecular
nitrogen.
Oxidation of ruthenium(II)-containing porphyrins
follows two paths.
Brown et al. have shown that oxi-
dation of 2 (where L=L1=pyridine) results in oxidation
of the metal to give the ruthenium(III) species. When
CO is axially bonded its strong -rr-accepting properties
raise the potential for oxidation of the metal to.a
value higher than that of the porphyrin ring which is
instead oxidised to the porphyrin p-cation radical.
In those cases where the axial ligands of 2 are ones
other than CO (including CH3CN, DNIF, DMA) oxidation
initially occurs at the metal.
Bibliography
J.P. Collman, Acc Chem. Res., 10, 265 (1976).
T.G. Traylor, in "Bioorganic Chemistry, Vol. IV,
Electron transfer and energy conversion; cofactors,
probed," ed. E.E. van Tamelen, Academic Press, New
York, N.Y., 1978
N.Farrell, D.H. Dol
phin, and B.R. James,
Soc., 100, 324 (1978).
J. Am. Chem.
G.M. Brown, F.R. Hopf, J.A. Ferguson, T.J. Meyer, and
D.G. Whitten, J. Am. Chem. Soc., 95, 5939 (1973).
Figure 3 .
Spectral changes during the conversion of
Ru(III)-myoglobin, curve 1, to carbonmonoxyruthenomyoglobin by sodium dithi onite and CO, curve 2 .
Fig 2
Fig 3