Carbene Character in a Series of Neutral PCcarbeneP Cobalt(I) Complexes – Radical Carbenes Versus Nucleophilic Carbenes

Cobalt(I) complexes supported by a series of PCcarbeneP pincer ligands of varying donicity, differing in the aryl group linking the phosphine arms with the anchoring carbon donor, are described. Addition of the proligands to cobalt bromide results in the formation of a series of cobalt(II) tetrahedral complexes, Ln-1, which serve as excellent precur-sors to the corresponding PCalkylP and PCcarbeneP complexes. Square planar cobalt PCcarbeneP complexes, L2R-3-X (X = Cl, Br), are readily synthesized by addition of a bulky aryloxide radical to the corresponding PCalkylP complex, L1-2-Br or via addition of L2R to ClCo(PPh3)3 in the presence of trityl radical or by addition of NaHBEt3 and trityl radical to iso-lated L2R-1. For the L2NMe2 PCcarbeneP complexes, salt metathesis reactions with either CsOH·H2O, LiCH2TMS, or LiNH2 result in the corresponding hydroxo, alkyl, and amine complexes, L2NMe2-3-R (R = OH, CH2TMS, NH2). Reaction of L2NMe2-3-OH with benzoic acid affords the 2-O2CPh derivative The nature of the carbene bond in either ligand plat-form as well as the effects of the X-type capping ligand on the Co=C bond are explored computationally and show that triplet structures are relatively more stable in for the less electron donating ligand L1 while singlet Co(I) carbenes dominate for the more electron rich L2 derivatives. For L2NMe2 complexes, the effect of the trans ligand X was also probed. Pi donors imbue the carbene with singlet character while the strongly  donating alkyl derivative exhibits significant triplet character.

Uncovering a copper(II) Alkynyl Complex in C−C Bond Forming Reactions

<p>Copper(II) alkynyl species are proposed as key intermediates in numerous Cu−catalysed C−C coupling reactions. Supported by a β−diketiminate ligand, the three coordinate copper(II) alkynyl [Cu^II]−C≡CAr (Ar = 2,6−Cl₂C₆H₃) forms upon reaction of the alkyne H−C≡CAr with the copper(II) <i>tert</i>−butoxide complex [Cu^II]−O<i>^t</i>Bu. In solution, this [Cu^II]−C≡CAr species cleanly transforms the to the Glaser coupling product ArC≡C−C≡CAr and [Cu^I](solvent). Addition of nucleophiles R′C≡CLi (R′ = aryl, silyl) and Ph–Li to [Cu^II]−C≡CAr affords the corresponding C_sp−C_sp and C_sp−C_sp2coupled products RC≡C−C≡CAr and Ph–C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]−C≡CAr}⁻. Supported by DFT calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R−C≡CAr products. [Cu^II]−C<a>≡</a>CAr also captures the trityl radical Ph₃C• to give Ph₃C−C≡CAr. Radical capture represents the key C_sp−C_sp3 bond forming step in the copper catalysed C-H functionalization of benzylic substrates R−H with alkynes H−C≡CR′ (R′ = (hetero)aryl, silyl) that provide C_sp−C_sp3 coupled products R−C≡CR via radical relay with <i>^t</i>BuOO<i>^t</i>Bu as oxidant.</p>

Uncovering a copper(II) Alkynyl Complex in C−C Bond Forming Reactions

<p>Copper(II) alkynyl species are proposed as key intermediates in numerous Cu−catalysed C−C coupling reactions. Supported by a β−diketiminate ligand, the three coordinate copper(II) alkynyl [Cu^II]−C≡CAr (Ar = 2,6−Cl₂C₆H₃) forms upon reaction of the alkyne H−C≡CAr with the copper(II) <i>tert</i>−butoxide complex [Cu^II]−O<i>^t</i>Bu. In solution, this [Cu^II]−C≡CAr species cleanly transforms the to the Glaser coupling product ArC≡C−C≡CAr and [Cu^I](solvent). Addition of nucleophiles R′C≡CLi (R′ = aryl, silyl) and Ph–Li to [Cu^II]−C≡CAr affords the corresponding C_sp−C_sp and C_sp−C_sp2coupled products RC≡C−C≡CAr and Ph–C≡CAr with concomitant generation of [Cu^I](solvent) and {[Cu^I]−C≡CAr}⁻. Supported by DFT calculations, redox disproportionation forms [Cu^III](C≡CAr)(R) species that reductively eliminate R−C≡CAr products. [Cu^II]−C<a>≡</a>CAr also captures the trityl radical Ph₃C• to give Ph₃C−C≡CAr. Radical capture represents the key C_sp−C_sp3 bond forming step in the copper catalysed C-H functionalization of benzylic substrates R−H with alkynes H−C≡CR′ (R′ = (hetero)aryl, silyl) that provide C_sp−C_sp3 coupled products R−C≡CR via radical relay with <i>^t</i>BuOO<i>^t</i>Bu as oxidant.</p>

SET processes in Lewis acid–base reactions: the tritylation of N-heterocyclic carbenes

Carbenes as single electron donors: the tritylation of N-heterocyclic carbenes proceeds via an initial SET step, giving highly reactive carbene radical cations and the trityl radical.

Three-spin solid effect and the spin diffusion barrier in amorphous solids

Dynamic nuclear polarization (DNP) has evolved as the method of choice to enhance NMR signal intensities and to address a variety of otherwise inaccessible chemical, biological and physical questions. Despite its success, there is no detailed understanding of how the large electron polarization is transferred to the surrounding nuclei or where these nuclei are located relative to the polarizing agent. To address these questions we perform an analysis of the three-spin solid effect, and show that it is exquisitely sensitive to the electron-nuclear distances. We exploit this feature and determine that the size of the spin diffusion barrier surrounding the trityl radical in a glassy glycerol–water matrix is <6 Å, and that the protons involved in the initial transfer step are on the trityl molecule. 1H ENDOR experiments indicate that polarization is then transferred in a second step to glycerol molecules in intimate contact with the trityl.