Search Results (6)

In our recent work, we revisited C–H and C–C bond activation in rhodium (I) complexes of pincer ligands PCP, PCN, PCO, POCOP, and SCS. Our findings indicated that an η³-C_sp2C_sp3H agostic intermediate acts as a common precursor to both C–C and C–H bond activation in these systems. We explore the electronic structure and bonding nature of these precleavage complexes using electron density and molecular orbital analyses. Using NBO, IBO, and ESI-3D methods, the bonding in the η³-CCH agostic moiety is depicted by two three-center agostic bonds: Rh–C_sp2–C_sp3 and Rh–C_sp3–H, with all three atoms datively bound to Rh(I). IBO analysis specifically highlights the involvement of three orbitals (CC→Rh and CH→Rh σ donation, plus Rh→CCH π backdonation) in both C–C and C–H bond cleavages. NCIPLOT and QTAIM analyses highlight anagostic (Rh–H) or β-agostic (Rh–C_sp2–H) interactions and the absence of Rh–C_sp³ interactions. QTAIM molecular graphs suggest bond path instability under dynamic conditions due to the nearness of line and ring critical points. Several low-frequency and low-force vibrational modes interconvert various bonding patterns, reinforcing the dynamic η³-CCH agostic nature. The kinetic preference for C–H bond breaking is attributed to the smaller reduced mass of C–H vibrations compared to C–C vibrations. Full article

Homogeneous catalysis involving a transition metal agostic interaction (TM^…H^…C) is an attractive strategy for C–H bond activation, in which the transition metal agostic intermediates serve as the critical component. To investigate the roles of manganese agostic intermediates in the unusual migration of the Mn(CO)₃ fragment in the (exo-phenyl)(η³-cyclohexenyl)manganese tricarbonyl [(Ph)(η³-C₆H₈)Mn(CO)₃] (complex 1) under the protonation of tetrafluoroboric acid–diethyl ether (HBF₄.Et₂O), a comprehensive density functional theory (DFT) theoretical study was performed. The computational results showed that formation of the [(cyclohex-3-enyl)-η⁶-benzene]manganese tricarbonyl complex [(C₆H₉)(η⁶-Ph)Mn(CO)₃⁺][BF₄] (complex 2) was achieved via a series of mono-agostic and di-agostic intermediates. The overall rate-limiting step for this unusual migration of the Mn(CO)₃ fragment is the formation of the di-agostic (η²-phenyl)manganese complex 8 (4 → 5 → 8) with a Gibbs barrier of 15.4 kcal mol⁻¹. The agostic intermediates with TM^…H^…C agostic interactions were well-characterized by geometry parameters, Atoms-In-Molecules (AIM) analyses, and the Natural Adaptive Orbitals (NAdOs). The located pathways in the current study successfully explained the experimental observations, and the findings on the TM^…H^…C agostic interaction provided a new aspect of the catalytic reaction with the manganese complex. Full article

Multiple fluxional processes of 6-monomethylcyclohexenylmanganese tricarbonyl [(6-MeC₆H₈)Mn(CO)₃, complex 1] and 5-monomethylcyclohexenylmanganese tricarbonyl [(5-MeC₆H₈)Mn(CO)₃, complex 2] have been explored using density functional theory (DFT) computations. The contributions of four agostomers—1, 2, 3, and 4—to the (MeC₆H₈)Mn(CO)₃ exchange processes were revealed. The computational results demonstrated that the 1, 2-agostic isomerization only occurred via the η⁴-diene hydride transition state (TS-1-2, 14.0 kcal/mol), which is consistent with the experimentally proposed high-energy exchange process (16.0 kcal/mol). Excellent agreement is observed (R² = 0.9862) when comparing the computed and experimentally observed variable temperature ¹H NMR chemical shifts. With these results, important insights into the role of agostic interaction in the homogeneous catalysis process could be made, especially with regard to transition metal catalyzed C-H activation. Full article

Surface carbon deposits deactivate Ni and Co catalysts in reactions involving hydrocarbons and CO_x. Electronic properties, adsorption energies of H, C, and CH_x species, and the energetics of the hydrogenation of surface C atom to methane are studied for (100) and (111) surfaces of monometallic Ni and Co, and bimetallic NiCo. The bimetallic catalyst exhibits a Co→Ni electron donation and a concomitant increase in the magnetization of Co atoms. The CH_x species resulting from sequential hydrogenation are more stable on Co than on Ni atoms of the NiCo surfaces due to more favorable (C-H)–Co agostic interactions. These interactions and differences between Co and Ni sites are more significant for (111) than for (100) bimetallic surfaces. On (111) surfaces, CH is the most stable species, and the first hydrogenation of C atom exhibits the highest barrier, followed by the CH₃ hydrogenation steps. In contrast, on (100) surfaces, surface C atom is the most stable species and CH₂ or *CH₃ hydrogenations exhibit the highest barriers. The Gibbs free energy profiles suggest that C removal on (111) surfaces is thermodynamically favorable and exhibits a lower barrier than on the (100) surfaces. Thus, the (100) surfaces, especially Ni(100), are more prone to C poisoning. The NiCo(100) surfaces exhibit weaker binding of C and CH_x species than Ni(100) and Co(100), which improves C poisoning resistance and lowers hydrogenation barriers. These results show that the electronic effects of alloying Ni and Co strongly depend on the local site composition and geometry. Full article

Trinuclear copper(II) complex [Cu^II₃(NIT₂PhO)₂Cl₄] was synthesized with p-cresol-substituted bis(α-nitronyl nitroxide) biradical: 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (NIT₂PhOH). The crystal structure of this heterospin complex was determined using single-crystal X-ray diffraction analysis and exhibits four unusual seven-membered metallocycles formed from the coordination of oxygen atoms of the N-O groups and of bridging phenoxo (µ-PhO⁻) moieties with copper(II) ions. The crystal structure analysis reveals an incipient agostic interaction between a square planar copper center and a hydrogen-carbon bond from one methyl group carried on the coordinated nitronyl-nitroxide radical. The intramolecular Cu∙∙∙H-C interaction involves a six-membered metallocycle and may stabilize the copper center in square planar coordination mode. From the magnetic susceptibility measurements, the complex, which totals seven S = 1/2 spin carriers, has almost a ground state spin S = 1/2 at room temperature ascribed to strong antiferromagnetic interaction between the nitronyl nitroxide moieties and the copper(II) centers and in between the copper(II) centers through the bridging phenoxo oxygen atom. Full article

This article discusses the properties of as many as 30 carbene–ZnX${}_2$ (X = H, Me, Et) complexes featuring a zinc bond C⋯Zn. The group of carbenes is represented by imidazol-2-ylidene and its nine derivatives (labeled as IR), in which both hydrogen atoms of N-H bonds have been substituted by R groups with various spatial hindrances, from the smallest Me, ${}^i$Pr, ${}^t$Bu through Ph, Tol, and Xyl to the bulkiest Mes, Dipp, and Ad. The main goal is to study the relationship between type and size of R and X and both the strength of C⋯Zn and the torsional angle of the ZnX${}_2$ plane with respect to the plane of the imidazol-2-ylidene ring. Despite the considerable diversity of R and X, the range of $d_{\mathrm{C}\cdots \mathrm{Zn}}$ is quite narrow: 2.12–2.20 Å. On the contrary, $D_0$ is characterized by a fairly wide range of 18.5–27.4 kcal/mol. For the smallest carbenes, the ZnX${}_2$ molecule is either in the plane of the carbene or is only slightly twisted with respect to it. The twist angle becomes larger and more varied with the bulkier R. However, the value of this angle is not easy to predict because it results not only from the presence of steric effects but also from the possible presence of various interatomic interactions, such as dihydrogen bonds, tetrel bonds, agostic bonds, and hydrogen bonds. It has been shown that at least some of these interactions may have a non-negligible influence on the structure of the IR–ZnX2 complex. This fact should be taken into account in addition to the commonly discussed R⋯X steric repulsion. Full article