Dear Gourab Mukherjee many thanks for posting this very interesting technical question on RG. I assume that solubility problems are the main reason why the synthesis of iridium porphyrin complexes usually starts with organometallic iridium complexes and not with the chlorides. Iridium trichloride exists in two forms. The anhydrous IrCl3 is insoluble in water and common organic solvents. Then there is the trihydrate, IrCl3(H2O)3. This compound is well soluble only in water, but not in organic solvents. Thus both trichlorides are more or less unsuitable for the synthesis of iridium porphyrin complexes which is normally done in high-boiling organic solvents. Basically the same is true for ruthenium trichloride. In contrast, compounds like Ru3(CO)12 and [Ir(COD)Cl]2 are well soluble in a variety of organic solvents. Particularly the iridium complex [Ir(COD)Cl]2 is every easily accessible in high yields from IrCl3(H2O)3 and COD (= 1,5-cyclooctadiene). You can regard [Ir(COD)Cl]2 as a soluble form or iridium(I) chloride which is otherwise not accessible. Ru3(CO)12 is more difficult to make and should perhaps better be obtained from commercial sources, although it is very expensive. Both complexes can be used to synthesize porphyrin complexes of the mono- and trivalent metals. For a typical literature reference please have a look at the following article:

Syntheses and reactions of iridium complexes of octaethylporphyrin

Article Syntheses and reactions of iridium complexes of octaethylporphyrin

Good luck with your research and best wishes, Frank Edelmann

Dear Prof. Frank T. Edelmann, thank you so much for your insights. It was really helpful.

Hi, Iridium(III)Cl3.trihydrate is an 18e octahedral complex and it is less reactive. As Prof. Edelmann had mentioned, you need to look for a labile complex which is soluble in organic solvents and that should be electronically and coordinatively unsaturated. In many reactions anhydrous complex is less reactive and you need drastic conditions as they will be having in some case, di, tri, tetra or polymeric structures. Ligands such as acetonitrile, norbornadiene, cyclooctadiene, benzonitrile etc., form mixed ligand labile complexes and in case of dimeric compounds they cleave symmetrically at halo-bridges, to have three coordinated species and automatically fourth coordination site can be occupied by the ligand chosen. Another advantage with [IrCODCl]2 (dimer) is it can cleave chloro-bridges or dissociate COD ligands depending on the reaction conditions and stiochiometry and nature of ligands and the reactions can be carried out in nonaqueous medium. So it is always wise to use labile complexes with metals in their low oxidation states and low coordination number. Many such labile complexes can be made easily starting from cheaper metal salts.

Good wishes

Balakrishna

Thank you Prof. Balakrishna Maravanji for your answer. In the same context, I have another query. Even while using the [Ir(COD)Cl]2 for complexation of porphyrins, the resulting Ir-porphyrin complex contains a CO bound to the metal. One justification is that it might come from the oxidation of the alkene to the corresponding aldehyde by low valent Ir(I) via a dioxetane intermediate, and that the aldehyde liberates CO, which goes on to form the Ir-carbonyl porphyrin adduct.

Is this justification correct? or is there any other source of the CO? Prof. Frank T. Edelmann, would you also like to comment, please.

Dear Gourab Mukherjee the formation of carbonyl complexes like (OEP)IrIIICl(CO) from [Ir(COD)Cl]2 and octaethylporphyrin is indeed rather dubious. As you already mentioned, it is stated in one of the original publications from 1978 that "Although the detailed reaction pathway is not known, the carbonyl group is probably derived from the coordinated cyclooctadiene. It is suggested that low-valent metal complexes catalyze the oxidation of olefins to aldehydes via intermediate formation of dioxetane resulted in the liberation of carbon monoxide from aldehyde." Article Syntheses and reactions of iridium complexes of octaethylporphyrin

To me this sounds rather speculative if not nonsensical. Why should a low-valent iridium complex act as an oxidation catalyst?? The compound dioxetane is known to occur in two isomers (https://de.wikipedia.org/wiki/Dioxetane). Was this intermediate detected by the authors? I don't think so. It's all speculation.

There is an earlier paper from 1968 in which this issue is mentioned, but not further discussed:

Article Studies on some new metalloporphyrins: Rh(III) and Ir(III) d...

To me the formation of the carbonyl ligand in this reaction remains rather mysterious. 😳

P.S. For a good overview on iridium porphyrin complexes please also have a look at the introduction of the following PhD thesis:

Iridium porphyrin synthesis, reactivity and catalysis, and the nanogold-catalyzed synthesis of lactams

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=6153&context=etd

The thesis is freely accessible as public full text on the internet (see attached pdf file). It also has a number of potentially useful references to original research papers in this field.

Dear Gourab,

What solvent you used. Is it ethanol or methanol or something else?

Thank you very much Prof. Frank T. Edelmann

Yes, the oxidation of aldehydes by low valent iridium sounds confusing indeed. But no other discussion has been found on this topic. It has remained a mystery as such. Thank you again for the thesis reference.

The solvent used is ortho-xylene. Prof. Balakrishna Maravanji

Dear Gourab, many thanks for your kind response. Perhaps we must accept that the origin of the carbonyl ligand in (OEP)IrIIICl(CO) has not yet been clarified. It is particularly unclear when [Ir(COD)Cl]2 is used as precursor. Perhaps we should just acknowledge this as a fact, but in my opinion it remains rather mysterious. Other researchers don't seem to care about this. For example, please have a look at the attached Supplementary Information. For the synthesis of (OEP)IrIIICl(CO) the authors cite here references S1 and S2 (see page S17). When you look up these references, you will find that the authors of those papers cite the old 1978 work. Thus apparently nobody has ever revealed the origin of the CO ligand in (OEP)IrIIICl(CO).

In most of these papers, it is quite possible that small amounts of ethanol found their way into the reaction. I don't see any indication of how solvents were purified or how glassware was cleaned or dried. At the scale of solvent and with the small molecular amount if Ir, the oxidation of an alcohol and subsequent decarbonylation of the aldehyde can easily result in the formation of CO. We have seen some of our iridium complexes decarbonylate benzaldehyde . Unless there is scrupulous attention paid to solvent purity and reaction conditions, the formation of CO with Ir, Rh, Ru etc compounds can easily creep into a reaction.

Dear Gourab Mukherjee I think that Joseph S Merola raised a very good point. The standard procedures for preparing compounds like (OEP)IrCl(CO) are rather old, and there is often no mention of rigorous purification and drying of the solvents. I fully agree with Joseph in that small amounts of ethanol and its oxidation product acetaldehyde could be the source of the carbonyl ligand in compounds like (OEP)IrCl(CO). It has even been reported in the literature that organoiridium complexes can catalyze the decarbonation of acetaldehyde;

The iridium-catalyzed decarbonylation of aldehydes under mild conditions

Article The iridium-catalyzed decarbonylation of aldehydes under mil...

When we then consider the high molar mass of "(OEP)IrCl" it's easy to understand that even minor amounts of CO can lead to formation of (OEP)IrCl(CO) in high yield.

The reason I had asked about the solvent was to know whether the solvent used, is capable of generating in situ CO during the reaction. O-xylene is unlikely to produce CO under the conditions you have used. Of course, at a very high reaction temperature, under oxidizing conditions, o-xylene undergo oxidation to form phthalic anhydride which can be the source of CO. Now please go back to your lab note book and analyze everything. Whether the glass-wares were rinsed in a base bath and, if so, whether they were washed thoroughly later and dried in oven for a minimum of 8 h around 150 deg C. How solvents were dried etc., will definitely give you the answer and also performing the reaction again and analyzing/monitoring the reaction mixture using mass should give more insight into this mystery.

Good Luck

Balakrishna

Thank you very much Prof. Frank T. Edelmann Prof. Balakrishna Maravanji and Prof. Joseph S Merola for your insights. We can therefore assume that the source of the mysterious CO could be the solvent employed. Thorough washing of the glassware and proper drying has to be conducted to check if we still see the CO ligated to the metal ion or not.

Dear Gourab Mukherjee yes, it would be really interesting to run an experiment under conditions where no CO can be formed. As you mentioned, it would be very important to use carefully dried glassware and purified starting materials and solvents. In any case you should start with [Ir(COD)Cl]2. Then I think that the choice of solvent would be crucial. A solvent like ortho-xylene could be oxidized e.g. to phthalic anhydride. My suggestion of a solvent which can never be oxidized would be hexamethyldisiloxane, Me3SiOSiMe3. My suspicion, however, is that the reaction will not work under conditions where (OEP)IrCl(CO) cannot be formed. Chances are that the porphyrin ligand will either not react at all or that you will get an insoluble polymer. BNevertheless, I think it would be worth a try.

I agree with everything previously stated by other researchers. I would like to add some considerations about porphyrin metalation. On the one hand, metal salts soluble in the solvent in which porphyrin is soluble are usually used, but it must be borne in mind that if the solvent is very polar, such as H2O, DMF, DMSO, it can coordinate to metalloporphyrin. On the other hand, oxidation states lower than that desired in metalloporphyrin are used, for example, for Fe (III) P, Fe (II) salt is used, the reason may have to do with the lability of the metal ions in the metalation reaction, ie the kinetics is more favorable.

Thus, in the example given to us, (Ir (COD) Cl) 2 the oxidation state of Ir is lower than in the compound IrCl3.xH2O.

It is also possible to use salts or forms of the metal ion insoluble in the solvent used, it is used in excess and then it is removed by filtration. For example, to obtain M (III) P an oxide in oxidation state (II) can be used.

Thank you very much Dr. Hermás R Jiménez This is an important observation. But how does the oxidation of the metal ion happen upon coordination with the porphyrin? How Ir(I) gets oxidized to Ir(III), or Fe(II) to Fe(III) upon complexation?

Dear Gourab Mukherjee "How Ir(I) gets oxidized to Ir(III), or Fe(II) to Fe(III) upon complexation?" Formally, elemental metals can be regarded as reducing agents, and also Fe(II) often acts as reducing agent. Thus these reactions can been seen as redox reactions in which the neutral porphyrin is reduced to the porphyrin dianion. One of the driving forces is certainly the highly stable 4-coordinate geometry around the metal inside the planar porphyrin ring system.

One possibility for oxidation is as indicated by F T Edelmann. It could also occur in part from trace amounts of O2 in the solvent, unless it is 100% degassed.

Size Ir and Ru minimizes stearic hendrence makes feasible.

[Ir(COD)Cl]2 is having iridium(I), whereas IrCl3.nH2O (preferably n = 3) has iridium in +3 state. Porphyrins usually behave as dianionic as diagonally opposite nitrogens are in >NH form and the other two are >N: (neutral 2e donors). Prior to reacting porphyrins, they are converted into dianionic by treating with an appropriate base or performing the reaction under elevated temperature, unless M--X bonds highly polarised and halide can come out easily; that is not the case in many instances.

Iridium(I) salts on treatment with porphyrins under appropriate reaction conditions, undergo oxidative addition reaction to form IrIII complexes which is rather easy starting from IrI and not using IrIII reagents. If you use directly IrIII salts, you need a reducing agent to reduce IrIII to IrI prior to its reaction with porphyrins and that can make reaction complicated and you may or may not get what you want or it can give a mixture of compounds.

Further, COD (1,5-cyclooctadiene) is a labile ligand that can be readily substituted with most of the nitrogen and phosphorus donor ligands, whether they are mono- or bidentate or even tri- or tetradenate (triphos, tetraphos, triethylenetetramine etc.). If you see some literature or standard text books, you will notice people using: [Rh(CO)2Cl]2, MCl2(NCPh)2, MCl2(COD), (M = Pd or Pt), Pd(PPh3)4 etc., for the same reason.

All the best

Balakrishna

In case of metal acetates, complex formation can happen without formal change in the oxidation state of the metal as acetate ion extracts H from >NH of porpyhrins and leaves as acetic acid (two equivalents). with other salts, reaction follows the sequence what Edelmann and others have opined.

The role of leaving groups is very important in substitution and redox reactions. Often they provide driving force required for the reaction and push the equilibrium towards right so that product formation is smooth.

Balakrishna