Pyramidal Stability of Chiral-at-Metal Half-Sandwich 16-Electron Fragments [CpRu(P−P′)]

Hapticity of asymmetric rhodium-allyl compounds in the light of real-space bonding indicators

Rhodium boryl complexes are valuable catalysts for hydro- or diboration reactions of alkenes, but can also react with ketones (R2C=O) and imines (R2C=NR′) giving rise to insertion products having formally Rh–R2C–O/NR′–B linkages. The resulting molecular structures, however, may show complex metal–ligand and ligand–ligand interaction patterns with often unclear metal–ligand connectivities (hapticities, ηn). In order to assign the correct hapticity in a set of asymmetric rhodium-allyl compounds with molecular structures indicating η1−5bonding, a comprehensive DFT study was conducted. The study comprises determination of a variety of real-space bonding indicators derived from computed electron and pair densities according to the AIM, ELI-D, NCI, and DORI topological and surface approaches, which uncover the metal–ligand connectivties and suggest an asymmetric ligand–metal donation/metal–ligand back-donation framework according to the Dewar–Chatt–Duncanson model.

Stereoretentive Ligand Exchange Reactions of N-Fused Porphyrin Ruthenium(II) Complexes

The ligand exchange reactions of the ruthenium(II) complex of N-fused tetraphenylporphyrin, Ru(NFp)(CO)₂Cl (2), with various anions were investigated. The chloride ligand of the isomers 2a-c was stereoretentively exchanged with bromide (Br^-), iodide (I^-), and acetate (AcO^-) anions in toluene at 100 °C, structures of which were confirmed by ¹H NMR as well as single crystal X-ray diffraction analysis. The silver (AgOAc, AgOTf) and boron (NaBPh₄) reagents also afforded the corresponding stereoretentive products. On the other hand, the reaction with NaBH₄ afforded the hydride complex Ru(NFp)(CO)₂H (7) with low stereospecificity, showing a higher reactivity of 2c than other isomers. The ligand dissociation mechanism was proposed with the help of theoretical calculations on the plausible five-coordinated intermediates.

Kinetics of phosphine substitution in CpRu(PPh3)2X (X = Cl, Br, I, N3, and NCO)

The kinetics of phosphine substitution in CpRu(PPh₃)₂X (X = Cl, Br, I, N₃, and NCO) is consistent with a dissociative mechanism. The complexes react with chloroform to yield CpRu(PPh₃)₂Cl by a mechanism that involves phosphine loss.

Chirality and diastereoselection of Δ/Λ-configured tetrahedral zinc complexes through enantiopure Schiff base complexes: combined vibrational circular dichroism, density functional theory, 1H NMR, and X-ray structural studies

The metal-centered Δ/Λ-chirality of four-coordinated, nonplanar Zn(A(^)B)(2) complexes is correlated to the chirality of the bidentate enantiopure (R)-A(^)B or (S)-A(^)B Schiff base building blocks [A(^)B = (R)- or (S)-N-(1-(4-X-phenyl)ethyl)salicylaldiminato-κ(2)N,O with X = OCH(3), Cl, Br]. In the solid-state the (R) ligand chirality induces a Λ-M configuration and the (S) ligand chirality quantitatively gives the Δ-M configuration upon crystallization as deduced from X-ray single crystal studies. The diastereoselections of the pseudotetrahedral zinc-Schiff base complexes in CDCl(3) solution were investigated by (1)H NMR and by vibrational circular dichroism (VCD) spectroscopy. The appearance of two signals for the Schiff-base -CH═N- imine proton in (1)H NMR indicates an equilibrium of both Δ- and Λ-diastereomers with a diastereomeric ratio of roughly 20:80% for all three ligands. VCD proved to be very sensitive to the metal-centered Δ/Λ-chirality because of a characteristic band representing coupled vibrations of the two ligand's C═N stretch modes. The absolute configuration was assigned on the basis of agreement in sign with theoretical VCD spectra from Density Functional Theory calculations.

The impact of ion pair association upon the 1H NMR spectra of cationic complexes akin to Grubbs catalysts

A reconsideration of the (1)H NMR spectra of cationic transition metal complexes exhibiting the features of cyclophanes on the basis of Mislow's classification of topic groups arrives at the conclusion that the spectral changes triggered by chiral counterions reflect an intracationic diastereotopication of enantiotopic protons rather than the existence of pairs of long-lived diastereomeric ion pairs.

Ligand dissociation: planar or pyramidal intermediates?

In organometallic chemistry, ligand dissociation is a key intermediate step in many useful processes. The dissociation of halide from an 18-electron half-sandwich complex (that is, with a single cyclopentadienyl ligand) of the type [CpRu(P-P')Hal] leaves an unsaturated 16-electron intermediate [CpRu(P-P')](+), which is then ready for subsequent addition reactions. Does the intermediate maintain its structure with a vacant site in place of the dissociated ligand, or does it rearrange, either concurrent with its formation or subsequently? In other words, are the 16-electron species planar or pyramidal? The outcome is relevant for chiral-at-metal compounds [CpML(1)L(2)L(3)] because an intermediate [CpML(1)L(2)] retains its chirality with respect to the metal atom as long as it is pyramidal, whereas it loses its chiral information if it becomes planar. In this Account, we address experimental results and theoretical calculations that help illuminate the energetics of structural rearrangements after halide dissociation. The rate-determining step in the halide exchange and racemization reactions of (R(Ru))- and (S(Ru))-[CpRu(P-P')Cl] is the cleavage of the Ru-Cl bond to give pyramidal intermediates (R(Ru))- and (S(Ru))-[CpRu(P-P')](+), which have kept the original metal configurations. These unsaturated intermediates can react with added ligands, such as Br(-) or I(-) (k(2) paths). The substitution products form with retention of the metal configuration. However, the pyramidal intermediates (R(Ru))- and (S(Ru))-[CpRu(P-P')](+) can also invert to their mirror images (k(3) paths). For (R(Ru))- and (S(Ru))-[CpRu(P-P')](+), the barrier of the pyramidal inversion (k(3)) is much higher than that of the halide addition (k(2)). The competition ratio k(3)/k(2) determines how much racemization occurs in a ligand exchange reaction. The competition ratio k(3)/k(2) can be determined from the ratio of the (R(Ru))- and (S(Ru))-products, which is constant throughout the course of the reaction. For compounds like [CpRu(P-P')Hal], k(3)/k(2) is much smaller than 1, resulting in an energy profile that resembles a basilica. These results, established with chiral-at-metal compounds, are supported by calculations that show that 16-electron half-sandwich intermediates with sigma-donating ligands, such as [CpM(NH(3))(2)], adopt planar structures, whereas strongly pi-bonding ligands, as in [CpM(CO)(2)], lead to pyramidal intermediates. The computed activation energies for the pyramidal inversion are on the order of 10 kcal/mol, with the planar species being transition states. The ligand dissociation behavior of 18-electron transition metal complexes is compared with nucleophile dissociation in main group compounds with octet configurations; here we include new computational results. Without exception, unsaturated main group intermediates, such as the carbenium ions formed in S(N)1 reactions, are planar. Our results and analysis help put transition metal chemistry on a firmer mechanistic foundation, and chiral-at-metal compounds are invaluable to this end.