Unlocking New Chemistry with Persistent Iridium Complexes: A Game Changer!

You know, in the wild world of organometallic chemistry, where we play with metals and organic molecules, iridium(III) complexes have always been pretty important. For a long time, the stars of the show were these guys called “cyclic” Cp*Ir(III) complexes. They’re like the reliable workhorses, great for all sorts of reactions, from making things more symmetrical (asymmetric hydrogenation) to building complex molecules (C-H functionalization). Their story really kicked off back in the late 60s and early 80s, thanks to some pioneering work by Maitlis and his crew.

But there’s another type, the “acyclic” Cp*Ir(III) complexes. Think of them as the less-explored cousins. Even though folks started looking into them around the same time, they’ve been a bit trickier to handle. They’re less rigid, a bit more… fragile, I guess you could say. This made them harder to make, harder to study, and definitely harder to use in the kind of reactions we chemists dream about, like cross-coupling – that’s where you stitch two different molecular pieces together.

The Breakthrough: Making Acyclic Iridium Stable

So, for a long time, the acyclic iridium complexes were kind of sitting on the sidelines. We really needed a way to make them more robust, more accessible, maybe even from simple starting materials. And guess what? We think we’ve found a pretty neat trick!

Our journey started with a bit of a surprise. We were actually trying to make one of those cyclic iridium complexes from benzaldehyde, a common chemical building block. We mixed benzaldehyde with a standard iridium source ([Cp*IrCl2]2) and a few other bits and bobs. We expected a ring-shaped molecule involving the iridium. But nope! Instead, we got this acyclic phenyl Cp*Ir(III)(CO)Cl complex. It turns out that during the reaction, carbon monoxide (CO) was generated *in situ* – right there in the reaction flask – and it decided to latch onto the iridium. This CO ligand acted like a little molecular shield, stabilizing the acyclic structure beautifully.

This was a game changer! We optimized the conditions, and suddenly, we had a general and efficient way to synthesize a whole bunch of these persistent aryl Cp*Ir(III)(CO)Cl complexes directly from various aryl aldehydes. We could use aldehydes with all sorts of decorations on them – electron-donating groups, electron-withdrawing groups, even sensitive functional groups like chlorine, bromine, iodine, esters, amides, and nitro groups. The reaction was super selective and worked for multi-substituted aldehydes and even naphthaldehydes. The only things that didn’t play along were cinnamaldehyde and hexanal, which makes sense given the mechanism.

These new complexes are surprisingly stable in air and moisture. You can purify them easily, characterize them fully, and really get to know them. We even got beautiful X-ray crystal structures of several of them, confirming their acyclic nature with the Cp* ligand, a chloride, the crucial CO, and the aryl group all bound to the iridium center. The Ir-CO bond is particularly strong, which likely contributes to their impressive stability. This whole process happens via a concerted metallation–deprotonation (CMD) mechanism, which is different from the usual way metals handle decarbonylation. Pretty cool, right?

Opening Up Possibilities: Reactivity with Everything!

Okay, so we had these stable acyclic iridium complexes. Now, what can they *do*? The next big step in many reactions, especially cross-coupling, is called transmetallation. This is where an organic piece hops from one metal to another. It’s a crucial step, but catching and studying the intermediate species after this hop is usually really tough. Because our new acyclic iridium complexes are so stable, they gave us a unique chance to peek behind the curtain.

And boy, did they react! These stable complexes turned out to be fantastic platforms for transmetallation. We tested them with a wild variety of partners, called nucleophiles, and they were incredibly versatile. We successfully reacted them with *eight* different classes of nucleophiles!

Think about it:

• Aryl and alkenyl boronic acids: These are common partners in cross-coupling. We got aryl-Ir-aryl and aryl-Ir-alkenyl complexes.
• Aryl and alkyl magnesium reagents (Grignards): Classic carbon nucleophiles.
• Aryl and alkyl lithium reagents: Even stronger carbon nucleophiles.
• Arylzinc reagents: Another type of organometallic partner.
• Arylsilanes and aryltin reagents: More ways to transfer aryl groups.
• Terminal alkynes: We could even attach alkynes, forming aryl-Ir-alkynyl complexes, often with a little help from a copper catalyst.
• Carboxylic acids: Yes, even non-carbon partners like carboxylic acids worked, giving us aryl-Ir-acyloxy complexes.

We could isolate and characterize these resulting “diorganoiridium” species – the ones where the iridium is now holding onto two organic pieces (the original aryl group and the piece from the nucleophile). And here’s the amazing part: these diorganoiridium complexes are also remarkably stable in air and moisture! This is a big deal because these intermediates are usually fleeting and hard to study. We got more beautiful X-ray structures, confirming the structures and showing how the bond lengths change depending on what’s attached to the iridium. The Ir-alkynyl bond was the shortest, while the Ir-alkyl bond was the longest, which tells us something about how tightly they’re held.

Cracking the Code: Getting the Product Out (Reductive Elimination)

So, we could make these stable diorganoiridium complexes. The final step in a cross-coupling reaction is usually “reductive elimination.” This is where the two organic pieces bound to the metal ditch the metal and hook up with each other, forming the new bond (like a C-C or C-O bond), and the metal goes back to a lower oxidation state, ready for the next cycle.

But here was the puzzle: our super stable diorganoiridium complexes didn’t want to do this step easily. They were kinetically stable, meaning they just sat there. This suggested that the usual pathway – transmetallation followed by direct reductive elimination – wasn’t working for these iridium guys.

We hypothesized that maybe they needed a little push, an *oxidation* step, to get them to react. We thought, “What if we oxidize the iridium center to a higher oxidation state? Would that make the reductive elimination easier?”

To test this, we turned to electrochemistry – basically, using electricity to control the oxidation state of the metal. We did cyclic voltammetry (CV) experiments on the aryl-Ir-aryl complexes. These experiments showed us that the iridium could be oxidized, first from Ir(III) to Ir(IV), and then potentially to Ir(V). The ease of oxidation depended on the electronic nature of the groups attached to the aryl ring – electron-donating groups made it easier to oxidize, which makes sense because they push electron density onto the metal.

Armed with this knowledge, we tried using electricity to trigger the reductive elimination. We applied a voltage to the solution containing our diorganoiridium complexes. And it worked! We successfully formed the desired C-C coupled products. By carefully controlling the voltage, we found that oxidizing the iridium to the Ir(IV) state seemed to be enough to kickstart the reductive elimination. Pushing it too far (towards Ir(V)) actually lowered the yield, probably due to unwanted side reactions.

We also confirmed this using chemical oxidants. Only oxidants with a sufficiently high oxidation potential could trigger the reaction, mirroring what we saw with the electrochemistry. This really cemented our hypothesis: for these stable acyclic iridium complexes, reductive elimination happens from a high-valent Ir(IV) intermediate.

We then tested this electrochemical reductive elimination on all the different types of diorganoiridium complexes we made (aryl-Ir-alkenyl, aryl-Ir-alkyl, aryl-Ir-alkynyl, and aryl-Ir-acyloxy). They all worked! The yields varied a bit, and interestingly, this variation correlated nicely with their oxidation potentials measured by CV. Complexes that were harder to oxidize (higher potential) gave lower yields under constant electrical conditions. This highlights how important it is to understand the redox behavior of these intermediates.

Putting it Together: Enabling Cross-Coupling Reactions

With all this understanding of how to make these stable acyclic complexes, how they undergo transmetallation with a wide range of partners, and how to trigger the crucial reductive elimination step via oxidation, we were ready to try and make some actual cross-coupling reactions happen.

Could we combine the transmetallation and reductive elimination steps into one process? We tried a tandem reaction with our aryl Cp*Ir(III)(CO)Cl complexes and aryl boronic acids. In the presence of a base and an oxidant (AgOAc), we successfully achieved formal Suzuki-Miyaura cross-coupling, producing C-C coupled products in good yields. This showed that our stable intermediates could indeed be channeled into a catalytic cycle.

Even though the energy barrier for reductive elimination from the aryl-Ir-acyloxy complexes was calculated to be a bit higher by DFT calculations, we were still able to develop a catalytic version of the decarbonylative cross-coupling between aryl aldehydes and carboxylic acids. By using a silver oxidant and higher temperatures, we could couple aryl aldehydes directly with carboxylic acids under iridium catalysis to form C-O coupled products. This is pretty significant, as it takes simple starting materials and builds a new bond using our iridium system.

Wrapping Up: A New Platform for Iridium Chemistry

So, what’s the big takeaway from all this? Well, we’ve basically figured out how to make those elusive acyclic aryl Cp*Ir(III)(CO)Cl complexes stable and accessible directly from simple aryl aldehydes. The *in situ* generated CO is the key player in keeping them together.

We’ve shown that these stable complexes are incredibly versatile, reacting with a huge variety of nucleophiles to form stable diorganoiridium intermediates – intermediates that were previously super hard to get your hands on.

And we’ve gained crucial insights into the final step, reductive elimination, showing that for these complexes, it likely proceeds through a high-valent Ir(IV) intermediate, and we can trigger it electrochemically or chemically.

Most importantly, these fundamental studies weren’t just academic exercises. They directly led to the development of new iridium-catalyzed cross-coupling reactions, specifically decarbonylative C-C and C-O couplings of aryl aldehydes.

This work really establishes a robust new platform for exploring the chemistry of acyclic Cp*Ir(III) complexes. It opens up exciting possibilities for designing new iridium-catalyzed reactions in the future. I’m really excited to see where this takes us!

Source: Springer