Infrared Spectra of CX2═CoX2 and CX3−CoX Complexes from Reactions of Laser-Ablated Cobalt Atoms with Halomethanes

Transition-metal difluorocarbene complexes

Although transition metal carbenes have found widespread applications and difluorocarbene has served as a versatile intermediate, it is still quite challenging to make use of transition-metal difluorocarbenes in synthetic chemistry due to their unpredictable reactivities. In this Highlight, we review recent developments in the transition-metal-catalyzed or -mediated transfer of difluorocarbene and the reactivies and conversions of transition-metal difluorocarbene complexes. We start with the MCF₂ bonding, then provide the progress in the transfer of difluorocarbene, and finally briefly discuss the conversions of MCF₂ into other metal complexes. The understanding of the interesting reactivities of MCF₂ may help design the catalytic transfer of difluorocarbene for various reactions.

Explaining the singlet complexes detected for the reaction Zr(3F) + CH3CH3 through a non-spin flip scheme

Energy profiles for the lowest lying triplet and singlet electronic pathways that link the reactants Zr + CH₃CH₃ with the products observed under matrix-isolation conditions were obtained from DFT and CASSCF-MRMP2 calculations. The insertion of the metal into the C-H bond of the organic molecule to yield the oxidative addition product is not favorable for any of the investigated channels. However, the inserted structure H-Zr-CH₂CH₃ can be obtained from two sequential reactions involving the radical species ZrH and CH₂CH₃. According to this scheme, a first reaction produces the radical fragments from the ground state of the reactants. Then, the radicals can recombine themselves in a second reaction to form the inserted species H-Zr-CH₂CH₃. As the triplet and singlet radical asymptotes ZrH + CH₂CH₃ that vary only in spin of the non-metallic fragment are degenerate, the rebounding of the radicals can occur through both multiplicity channels. It is shown that the low spin channel leads to the most stable structures of the dihydride ZrH₂-(CH₂)₂ and the vinyl metal trihydride complexes ZrH₃-CH=CH₂ experimentally determined for this reaction under matrix-isolation conditions. The description attained for this interaction does not invoke interactions between the triplet and singlet electronic states emerging from the reactants, as proposed by other authors.

Using a non-spin flip model to rationalize the irregular patterns observed in the activation of the C-H and Si-H bonds of small molecules by CpMCO (M = Co, Rh) complexes

The activation of the C-H and Si-H bonds of CH(CH₃)₃ and SiH(CH₃)₃ molecules by organometallic compounds CpMCO (M = Co, Rh) has been investigated through DFT and CASSCF-MRMP2 calculations. In particular, we have analyzed the pathways joining the lowest-lying triplet and singlet states of the reactants with the products arising from the insertion of the metal atom into the C-H or Si-H bonds of the organic molecules. Channels connecting the reactants with the inserted structure Cp(CO)H-M-C(CH₃)₃ through the oxidative addition of the C-H bond of the organic molecule to the metal fragment were found only for the reaction CpRhCO + CH(CH₃)₃. However, inserted structures could also be obtained for the interactions of SiH(CH₃)₃ with CpCoCO and CpRhCO by two sequential reactions involving the formation and rebounding of the radical fragments Cp(CO)H-M + Si(CH₃)₃. According to this two-step reaction scheme, the complex CpCoCO is unable to activate the C-H bond of the CH(CH₃)₃ molecule due to the high energy at which the radical fragments Cp(CO)H-M + C(CH₃)₃ are located. The picture attained for these interactions is consistent with the available experimental data for this kind of reaction and allows rationalization of the differences in the reactivity patterns determined for them without using spin-flip models, as has been proposed in previous studies.

Computational insights into CH3MX (M = Cu, Ag and Au; X = H, F, Cl, Br and I)

The thermodynamically stability of CH₃MX with respect to CH₃X + M is CH₃CuX > CH₃AuX > CH₃AgX. Some stable CH₃MX have not been identified experimentally because their vibrational fingerprints (ν_C−M and v_M−X) are too low to be detected.

Spin-flip reactions of Zr + C2H6 researched by relativistic density functional theory

Density functional theory (DFT) with relativistic corrections of zero-order regular approximation (ZORA) has been applied to explore the reaction mechanisms of ethane dehydrogenation by Zr atom with triplet and singlet spin-states. Among the complicated minimum energy reaction path, the available states involves three transition states (TS), and four stationary states (1) to (4) and one intersystem crossing with spin-flip (marked by -->): (3) Zr + C 2 H 6 → (3) Zr-CH 3 -CH 3 ((3)1) → (3)TS 1/2 → (3) ZrH-CH 2 -CH 3 ((3)2) → (3) TS 2/3 --> (1) ZrH2-CH2 = CH2 ((1) 3) → (1) TS 3/4 → (1) ZrH 3 -CH = CH 2 ((1)4). The minimum energy crossing point is determined with the help of the DFT fractional-occupation-number (FON) approach. The spin inversion leads the reaction pathway transferring from the triplet potential energy surface (PES) to the singlet's accompanying with the activation of the second C-H bond. The overall reaction is calculated to be exothermic by about 231 kJ mol(-1). Frequency and NBO analysis are also applied to confirm with the experimental observed data.

Redox noninnocence of carbene ligands: carbene radicals in (catalytic) C-C bond formation

In this Forum contribution, we highlight the radical-type reactivities of one-electron-reduced Fischer-type carbenes. Carbene complexes of group 6 transition metals (Cr, Mo, and W) can be relatively easily reduced by an external reducing agent, leading to one-electron reduction of the carbene ligand moiety. This leads to the formation of "carbene-radical" ligands, showing typical radical-type reactivities. Fischer-type carbene ligands are thus clearly redox-active and can behave as so-called "redox noninnocent ligands". The "redox noninnocence" of Fischer-type carbene ligands is most clearly illustrated at group 9 transition metals in the oxidation state II+ (Co(II), Rh(II), and Ir(II)). In such carbene complexes, the metal effectively reduces the carbene ligand by one electron in an intramolecular redox process. As a result, the thus formed "carbene radicals" undergo a variety of radical-type C-C and C-H bond formations. The redox noninnocence of Fischer-type carbene ligands is not just a chemical curiosity but, in fact, plays an essential role in catalytic cyclopropanation reactions by cobalt(II) porphyrins. This has led to the successful development of new chiral cobalt(II) porphyrins as highly effective catalysts for asymmetric cyclopropanation with unprecedented reactivity and stereocontrol. The redox noninnocence of the carbene intermediates results in the formation of carbene-radical ligands with nucleophilic character, which explains their effectiveness in the cyclopropanation of electron-deficient olefins and their reduced tendency to mediate carbene dimerization. To the best of our knowledge, this represents the first example in which the redox noninnocence of a reacting ligand plays a key role in a catalytic organometallic reaction. This Forum contribution ends with an outlook on further potential applications of one-electron-activated Fischer-type carbenes in new catalytic reactions.