Chemical processes proceed much faster and more selectively in the presence of appropriate catalysts, and as such the field of catalysis is of key importance for the chemical industry, especially in light of sustainable chemistry. Enzymes, the natural catalysts, are generally orders of magnitude more selective than synthetic catalysts and a major difference is that they take advantage of well-defined cavities around the active site to steer the selectivity of a reaction via the second coordination sphere. Here we demonstrate that such a strategy also applies for a rhodium catalyst; when used in the hydroformylation of internal alkenes, the selectivity of the product formed is steered solely by changing the cavity surrounding the metal complex. Detailed studies reveal that the origin of the capsule-controlled selectivity is the capsule reorganization energy, that is, the high energy required to accommodate the hydride migration transition state, which leads to the minor product.

The synthesis and characterization of a series of nickel, palladium, and platinum complexes containing new achiral and chiral PCP pincer ligands based on 1,3-diaminobenzene, 5-trifluoromethyl-1,3-diaminobenzene, and 3,5-diamino-4-chloroisobutylbenzoate are reported. The new PCP ligands are prepared conveniently in high yield by treatment of the respective diaminobenzene with 2 equiv of a variety of achiral and chiral R2PCl compounds in the presence of base. PCP complexes of Ni(II), Ni(PCP)Cl, were synthesized by the reaction of NiCl2·6H2O with 1 equiv of a PCP ligand. In similar fashion, treatment of M(COD)X2 (M = Pd, Pt; X = Cl, Br) with 1 equiv of a PCP ligand yields the square-planar complexes M(PCP)X. Palladium PCP complexes featuring a coordinated TFA ligand (TFA = CF3COO-) are obtained by the reaction of Pd(TFA)2 with 1 equiv of a PCP ligand. Alternatively, palladium PCP complexes can also be generated via an oxidative addition route. Addition of 2 equiv of PCP ligands based on 3,5-diamino-4-chloroisobutylbenzoate to Pd2(dba)3 affords the respective Pd(PCP)Cl pincer complexes in high yields. X-ray structures of representative Ni, Pd, and Pt PCP complexes have been determined. Finally, the use of the palladium complexes as catalysts for the Suzuki−Miyaura coupling of some aryl bromides and phenyl boronic acid has been examined.

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The dynamics of iron tetracarbonyl olefin complexes has been investigated using two-dimensional infrared (2D-IR) spectroscopy. Cross peaks between all CO-stretching bands show that the CO-stretch modes are coupled, and from the cross-peak anisotropies we can confirm previous assignments of the absorption bands. From the pump-probe delay dependence of the diagonal peaks in the 2D-IR spectrum we obtain a correlation time of ∼3 ps for the spectral fluctuations of the CO-stretch modes. We observe a multi-exponential pump-probe delay dependence of the cross-peak intensities, with rate constants ranging from 0.1 ps(-1) to 0.6 ps(-1). To determine whether this delay dependence originates from fluxionality of the complex or from intramolecular vibrational relaxation (IVR), we modulate the free-energy barrier of fluxional rearrangement by varying the pi-backbonding capacities of the olefin ligand in two iron tetracarbonyl olefin complexes: Fe(CO)(4)(cinnamic acid) and Fe(CO)(4)(dimethyl fumarate). Since the pi-backbonding strongly influences the rate of fluxionality, comparing the dynamics in the two complexes allows us to determine to what extent the observed dynamics is caused by fluxionality. We conclude that on the time scale of our experiments (up to 100 ps) the cross-peak dynamics in the iron complexes is determined by intramolecular vibrational energy relaxation. Hence, in contrast to previously investigated irontricarbonyl and ironpentacarbonyl complexes, iron tetracarbonyl olefin complexes exhibit no fluxionality on the picosecond time scale.

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