Why Does Cyanide Pretend to be a Weak Field Ligand in [Cr(CN)5]3−?

Lord and Baik*

Generally speaking, it is difficult to study Cr chemistry because many Cr complexes are paramagnetic. Computational modeling is also not easy due to spin contamination issues. This paper by Lord and Baik provides a rationale for the observation of (NEt4)3[Cr(CN)5] in a high-spin (S = 2) ground state, suggesting that one must consider both ligand−ligand electrostatic interactions and classical ligand−field arguments in order to have a full picture of spin-state energetic ordering in a transition metal complex. What they found here was that the Coulombic repulsion between the anionic cyanide ligands dominates the overall energetics of [Cr(CN)5]3−., making longer ligand−ligand separation more favorable and eventually leading to the high-spin complex.

Nickel-Catalyzed Decarbonylative Addition of Phthalimides to Alkynes

Kajita, Matsubara,* and Kurahashi*

This is an interesting reaction to make isoquinolones. Their design was quite logical and it did work at the first try, although the yield of the desired product was only 18%. The authors managed to improve the yield by the introduction of electron withdrawing Ar groups. The mechanism was proposed to proceed by a nucleophilic attack of Ni(0) to the amide moiety (an oxidative addition of C-N to Ni) followed by decarbonylation, insertion of the alkyne to the phenyl C-Ni bond, and finally reductive elimination of the product to regenerate the Ni(0) catalyst. The fate of CO was not mentioned, but I guess the bonded CO after the decarbonylation step was kicked out by the excess PMe3.

Unusually Weak Metal-Hydrogen Bonds in HV(CO)4(P-P) and Their Effectiveness as H atom Donors

Choi, Pulling, Smith, and Norton*

The Norton group has been interested in developing new catalysts for Sn free radical cyclization reactions. One hypothesis from their previous observations is that decreasing the strength of the M-H bond may result in an increased rate for the H-atom transfer from M to carbon. Inspired by Landis' work, they have now studied a series of seven-coordinate complexes HV(CO)4(P-P) where P-P = Ph2P(CH2)nPPh2. It was confirmed that these complexes possess unuaual weak M-H bonds (BDE = 54.9~57.9 kcal/mol), and undergo faster H-atom transfer reaction to styrene than CpCr(CO)3H (BDE(Cr-H) = 62.2 kcal/mol) does. However, the V-H complexes show little catalytic reactivities for the radical cyclization reactions.

Cobalt Dinitrosoalkane Complexes in the C-H Functionalization of Olefins

Schomaker, Boyd, Stewart, Toste,* and Bergman*

This is an interesting idea that broadens the scope of a 20 plus-year-old chemistry. The cobalt dinitrosoalkane complexes were applied in the C-H functionalization of alkenes. Reaction of a series of strained alkenes with Me4CpCo(CO)2 in the presence of NO generates intermediate cobalt dinitrosoalkane complexes that can be deprotonated by LHMDS and added to various Michael acceptors in the presence of a Lewis acid, Sc(OTf)3. Then the resultant functionalized "alkene" can be replaced by the origin alkene via retrocycloaddition reactions and the starting cobalt dinitrosoalkane complex can be regenerated.

Mild, Rhodium-Catalyzed Intramolecular Hydroamination of Unactivated Terminal and Internal Alkenes with Primary and Secondary Amines

Liu and Hartwig*

The complexes that can catalyze hydroamination of olefins are usually air and/or moisture sensitive. This also means not many functional groups can be tolerated, especially free hydroxyl groups. The Hartwig group has screened a series of ligands and Rh(I) catalyst precursors and found that the combination of [Rh(COD)2]BF4 and the biarylphosphine ligand L2 formed the most active catalyst. The intramolecular hydroamination reactions were conducted at 70 C with 2.5 mol% of catalyst to give excellent to good yields for a variety of amines. Aryl chloride, nitrile, ester, and free hydroxyl groups do not poison the catalyst.

I am particularly interested in the different reactivities between t-BuXantphos and L1. It seems that the selectivity depends largely on the electronic property of the phosphine.

A Rhodium-Catalyzed C-H Activation/Cycloisomerization Tandem

Aıssa and Furstner*

Hey, I am back from a conference in Japan. It was a fun trip. We had a pre-conference symposium in Kyoto followed by a 3-day meeting in Tateshina. This was the most enjoyable conference I have attended. Relaxed and friendly atmosphere and small size (~70 people or so) made the interaction between attendees barrierless.

The metal of the choice for the first review after a long break is Rh. In this communication, the authors first employed a pyridine group as a directing group to induce a Csp2-H bond oxidative addition to Rh. The resulting intermediate (B) later undergo hydrometalation, cyclopropane C-C bond cleavage for ring expansion, and reductive elimination to give the product and to regenerate the catalyst. (PPh3)3RhCl was found to be more reactive than the phosphine-free complexes, [Rh(CO)2Cl]2 and [Rh(coe)2Cl]2. The addition of AgSbF6 could help to achieve cleaner conversions.

They also showed that this series of reactions can be triggered by a pendent aldehyde group, since the OA reaction of a formyl C-H bond to Rh has been well established. Interestingly, although in most of the literature, the resulting acyl-Rh-hydride intermediate tends to undergo decarbonylation reaction, their results showed that the hydrometalation to the adjacent alkylidenecycloprpane is faster and/or more favorable.

This is a long wait....sorry!

I have been away because of the big move (over 9800 miles) to land my new position. Now everything seems to go well and Organimetallic Current is expected to be back in service very soon. Thanks for your patience. Have a nice weekend.

Snapshot of a Chelation-Assisted C-H/Alkyne Coupling: A Ruthenium Complex Caught in the Act of C-C Bond Formation

A review by blatnoi

Benhamou, Ce´sar, Lugan, and Lavigne*

Hello Readers,

I really enjoy the type of paper where the authors managed to isolate an intermediate in a catalytic process by changing a few things around in their system in order to stabilize these elusive intermediates. In the case of this Organometallics paper, the catalytic reaction is hydroformylation of an alkyne, where an aldehyde C-H bond is presumably broken via oxidative addition to a late transition metal and an alkyne inserts into the M-H bond followed by reductive CC bond formation.

The authors set out to model hydroformylation via a Ru complex. In the paper, they partly set out to prove that Rauchfuss, who made a prediction in 1979 that an oxidative addition mechanism was not operative in the first step since RuCl3 did not react with aldehydes, was wrong. And of course, they did prove him wrong since as the authors mention, oxidative addition reactivity depends very much on the oxidation state of the metal, as is now widely known.

Starting with a Ru(0) complex modified by three triphenylphosphine ligands and two carbonyls is the key. One of the triphenylphosphines was changed to have an aldehyde in the ortho position of the phenyl ring. This promptly led to fast CH activation and the generation of a Ru(II) hydride species. This hydride subsequently reacted with diphenylacetylene. Well... only after heating because the alkyne required a vacant coordination site. Which brought up an interesting point since it was predicted that a CO will leave instead of PPh3 to create that site, which is what happens when you look at the crystal structure of this product.

The product unfortunately underwent many transformations including insertion into the M-H bond and CC coupling to give back Ru(0). It would have been nice to see one of those intermediates as well, but based on my own experience with one of these transformations, it's a very difficult proposition indeed. The reaction stops there because the product is stuck to the metal via the modified phosphine ligand. As an interesting aside, a crystal structure of the product shows that the CO is bound in a side on fashion.

Thanks for reading!

Addition of Ammonia, Water, and Dihydrogen Across a Single Pd-Pd Bond

A review by blatnoi

Fafard, Adhikari, Foxman, Mindiola and Ozerov*

Hello readers,

The present communication has a very interesting reaction with a Pd supported PNP complex. PNP based and associated complexes have a very rich chemistry (a review to a Science arcticle of a PNN supported Ru complex can be found just below). The basic theme involves a meridionally coordinating trichelator. In this case, the medium atom is an anionic nitrogen trans to an alkyl ligand on the starting Pd(II) complex.

The authors found that although the complex is thermally stable, upon exposure to light or UV radiation, homolytic cleavage occurs to produce organic radical coupling products and a dinuclear Pd complex with a Pd-Pd bond. The authors surmised that these complexes can act as important precursors for activation of small molecules. Indeed, treating the dimer with hydrogen gas resulted in Pd-H monomer after formal insertion of the small molecule into the Pd-Pd bond. Water resulted in a Pd-H and a Pd-OH complex. Perhaps most intriguing, ammonia, a molecule that possesses a free electron pair on the nitrogen and thus usually simply binds to the metal without doing much of anything, was split into Pd-NH2 and Pd-H. A recent report that was highlighted in CEN news noted that organic carbenes can perform this reaction easily and made light of the inability of organometallic chemists to effect this transformation (except for just a few cases). In light of this result, it seems that the celebration in the 'pure organic' camp is a bit premature.

The mechanism of the reaction is not yet clear and may involve attack of one of the reagents on the metal center of a monomer formed by dissociation, or a sigma metathesis pathway involving the two metal centers. Since the bond length in the dimer is 2.58 Angstroms, I'm leaning towards the first pathway or an intermediate path where the small molecule bond is significantly weakened and elongated by one of the metal centers.

Going back to the middle of the article the authors also believed that some amount of the monomer was present in solution since a weak EPR signal from the dimers was detected. The NMR looked just fine, and so to test if monomers were present in solution, two dimer complexes with different ligand back bones were heated together. At the end, the Pd atoms swapped partners resulting in a 1:2:1 statistical distribution, proving that there was an equilibrium between the radical monomer and the dimer in solution.

Overall, I thought this was a neat communication with a number of important results that was an easy and enjoyable read. It also lends itself well to a 15 minute ACS oral presentation.

Neoflags

You may notice that I installed a Neoflags at the bottom. It shows that to date we have visitors from 23 countries.