Rh Pincer Research Articles

Catalysis of Kumada–Tamao–Corriu coupling by a (POCOP)Rh pincer complex

A pincer-based (P(O)C(O)P)Rh catalyst is demonstrated to be an active and well-defined catalyst for the coupling of select aryl and alkyl Grignards with aryl iodides. The proposed intermediacy of oxidative addition of aryl halides to (P(O)C(O)P)Rh(I) is supported by the isolation of the oxidative addition product.

Synthesis and characterization of a free phenylene bis(N-heterocyclic carbene) and its di-Rh complex: Catalytic activity of the di-Rh and CCC–NHC Rh pincer complexes in intermolecular hydrosilylation of alkynes

1,3-Bis(3-butylimidazolium-1-yl)benzene diiodide ( 1) was reacted with Li(2,2,6,6-tetramethylpiperidine) yielding the free bis-carbene, 1,3-bis(3-butylimidazol-2-ylidene-1-yl)benzene ( 3), which has been spectroscopically characterized. Combining the free bis-carbene with [Rh(COD)Cl] 2 yielded the corresponding di-Rh bis(N-heterocyclic carbene) complex ( 4) that was structurally characterized. The di-Rh bis-carbene complex was found to exhibit complex solution 13C and 1H NMR spectra that have been assigned as a mixture of diastereomers. The crystal structure of the di-Rh bis-carbene compound 4 was composed of a pair of enantiomeric atropisomers. The diastereomeric atropisomers were assigned as the source of the spectral complexities. The di-Rh di-carbene complex 4 and the CCC–NHC Rh pincer complex 2 were applied as catalysts in hydrosilylation reactions of terminal and internal alkynes. Both catalysts are highly active, regioselective, stereoselective, and chemoselective: terminal alkynes give predominantly the β-( Z) isomer and internal alkynes afford the β-( E) isomer in chloroform or benzene. One of the strongest attributes of the catalyst systems is that the results were achieved without exclusion of air and without purification of commercially available reagents.

Toward a general method for CCC N-heterocyclic carbene pincer synthesis: Metallation and transmetallation strategies for concurrent activation of three C–H bonds

A general solution to the preparation of pincer complexes that require the formation of two N-heterocyclic carbenes (NHC’s) and the activation of an aryl C–H bond is reported. The reaction of a phenylene-bridged bis(imidazolium) salt with Zr(NMe 2) 4 generated the requisite CCC–NHC Zr pincer complex, which has been characterized by X-ray crystallography. Subsequent manipulation of the Zr coordination sphere by reaction with MeI demonstrated the robustness of the ligand geometry at Zr and led to the isolation and structural characterization of a CCC–NHC Zr triiodide pincer complex. A variation of the methodology has been applied to a saturated NHC analog to produce the corresponding CCC–NHC Zr pincer complex. Importantly, it has been found that CCC–NHC Zr pincer complexes can be generated in situ and transmetallated with an appropriate Rh source to generate CCC–NHC Rh pincer structures. These two methodologies, metallation of CCC–NHC precursors with transition metal amido complexes combined with transmetallation, hold great promise for opening general synthetic pathways to a wide variety of transition metal CCC–NHC pincer complexes.

DFT study of the fixation of CO by SPS-based pincer Rh(I) and Ir(I) complexes: Regioselectivity and reactivity

Summary The regioselectivity of the addition of CO on SPS-type pincer-based metal complexes is studied by means of DFT calculations on the unsubstituted model complexes [M(SPS)(PH 3 )] (M=Rh and Ir). The reaction leads to the decoordination of a sulphur centre and to the formation of the four-coordinate square-planar complex [M(SP)(PH 3 )(CO)], but the five-coordinate syn -SS complex [M(SPS)(PH 3 )(CO)] is found to be very close in energy (Δ E −1 ). MQ/MM calculations on the real complex [M(SPS)(PPh 3 )(CO)] with phenyl substituents however favor this five-coordinated structure, in good agreement with the X-ray structure reported for the Rh compound. The influence of the nature of the metal centre on the thermicity of the CO addition is related through a thermodynamic cycle to the electronic properties of the metal complex and to the energetics of the M–CO bond. It is shown that the larger exothermicity predicted for the Ir complex is due to the energetics of the metal–carbon bond under formation (Δ E bond (Ir–CO)>Δ E bond (Rh–CO)).