DFT calculations of NMR JC–H coupling constants: An additional tool to characterize the α-agostic interaction in high oxidation state M-alkylidene complexes (M=Re, Mo and Ta)

Insights into the Fluxional Processes of Monomethylcyclohexenyl Manganese Tricarbonyl

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.

Diazomethane umpolung atop anthracene: an electrophilic methylene transfer reagent

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H₂CN₂A (1, A = C₁₄H₁₀ or anthracene).

Formal addition of diazomethane's terminal nitrogen atom to the 9,10-positions of anthracene yields H₂CN₂A (1, A = C₁₄H₁₀ or anthracene). The synthesis of this hydrazone is reported from Carpino's hydrazine H₂N₂A through treatment with paraformaldehyde. Compound 1 has been found to be an easy-to-handle solid that does not exhibit dangerous heat or shock sensitivity. Effective umpolung of the diazomethane unit imbues 1 with electrophilicity at the methylene carbon center. Its reactivity with nucleophiles such as H₂CPPh₃ and N-heterocyclic carbenes is exploited for C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond formation with elimination of dinitrogen and anthracene. Similarly, 1 is demonstrated to deliver methylene to a nucleophilic singlet d² transition metal center, W(ODipp)₄ (2), to generate the robust methylidene complex [2 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH₂]. This behavior is contrasted with that of the Wittig reagent H₂CPPh₃, a more traditional and Brønsted basic methylene source that upon exposure to 2 contrastingly forms the methylidyne salt [MePPh₃][2 Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH].

DFT calculations of d0M(NR)(CHtBu)(X)(Y) (M = Mo, W; R = CPh3, 2,6-iPr–C6H3; X and Y = CH2tBu, OtBu, OSi(OtBu)3) olefin metathesis catalysts: structural, spectroscopic and electronic properties

DFT(B3PW91) calculations have been carried out to rationalise the structural, electronic and spectroscopic properties of Mo and W imido M(NR1)(CHR2)(X)(Y) olefin metathesis catalysts by using either simplified or actual ligands of the experimental complexes. The calculated structures, energetics (preference for the syn isomer and alkylidene rotational barrier for the syn/anti interconversion), and spectroscopic properties (NMR J(C-H) coupling constants) are in good agreement with available experimental data. Additionally, the alkylidene nu(C-H) stretching frequencies, not available experimentally, have been calculated. These quasi-tetrahedral complexes have a linear imido group and a C-H alkylidene agostic interaction, which stabilizes the syn isomer. Whether looking at M(NR1)(CHR2)(X)(Y), M = Mo, W, or the isolobal Re complexes, Re(CR1)(CHR2)(X)(Y), a linear correlation is obtained between both the alkylidene nu(C-H) stretching frequencies and J(C-H) coupling constants with the calculated alkylidene C-H bond lengths. These correlations show that the strength of the alpha-C-H agostic interaction increases from alkylidyne Re to imido group 6 complexes and from Mo to W. The NBO and AIM Bader analyses show firstly that the imido and alkylidyne groups are both triply bonded to the metal, but that the triply bonded imido ligand is a weaker electron donor than the alkylidyne, hence the stronger alpha-C-H agostic interaction for group 6 imido complexes. Secondly, one of the pi bonds of the triply bonded ligand is weakened at the transition state of the alkylidene rotation: while no lone pair is formed, the metal-ligand triple bond is polarized. This is more favourable for an imido than for an alkylidyne ligand, hence the lower alkylidene rotational barrier for the former complexes. Conversely, the aryl imido is even less of an electron donor than the alkyl imido group, which in turn strengthens the alpha-C-H agostic interaction and lowers the alkylidene rotational barrier even more.