GHADWAL RESEARCH GROUP
Molecular Inorganic Chemistry and Catalysis
advancing low-valent main-group chemistry for catalysis and materials
2017
60. Expanding the Scope of Cu(I) Catalyzed “Click Chemistry” with Abnormal-NHCs: Threefold Click to Tris-triazoles
Ho, N. Kim T., Reichmann, S. O., Rottschäfer,D., Herbst-Irmer, R., Ghadwal,* R. S.
Catalysts 2017, 7, 262. doi:10.3390/catal7090262 . Open Access
(Invited Article for the Special Issue "Tailor-Made NHC Ligands")
Three-fold Click...Cationic copper(I) complexes [Cu(aIPrPh)(IPr)]I (3) and [Cu (aIPrPh)2]I (4) featuring an abnormal N-heterocyclic carbene (aNHC) (aIPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene) and/ or an NHC (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) ligand(s) are reported. Treatment of Cu(aIPrPh)I (2) with IPr affords complex 3. Reaction of (IPrPh)I (1) (IPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazolium) with CuI in the presence of K{N(SiMe3)2} leads to the formation of 4. Complexes 3 and 4 represent rare examples of mixed aNHC-NHC and bis-aNHC metal complexes, respectively. Complexes 2, 3, and 4 are excellent catalysts for the threefold cycloaddition of a tris-azide with various alkynes. This catalytic protocol offers a high yield access to tris-triazoles in a shorter reaction time and considerably reduces the experimental workup compared to the classical synthetic method.
59. Nickel-Catalysed Direct C2-Arylation of N-Heterocyclic Carbenes
Ho, N. Kim T., Neumann, B., Stammler, H.G., Menezes da Silva, V. H., Watanabe, D., Braga, A. A. C., Ghadwal,* R. S.
Dalton Trans. 2017, 46, 12027–12031.
Nickel outperforms palladium...A highly efficient nickel catalysed method for the direct C2- arylation of N-heterocyclic carbenes (NHCs) is reported. This protocol enables a facile access to C2-arylated imidazolium salts (NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical studies suggest the viability of a Ni(I)/Ni(II) catalytic pathway in which the dinuclear Ni(I) species [(NHC)NiX]2 plays a key role.
58. N-Heterocyclic Vinylidene-Stabilized Phosphorus Biradicaloid
Rottschäfer, D.; Neumann, B.; Stammler, H.-G. Stammler, Ghadwal,* R. S.
Chem. Eur. J. 2017, 23, 9044–9047.
e-Greening the blue... Two electrons reduction of [(IPr)CP)2Cl]Cl (2) (IPr = C[N(2,6-iPr2-C6H3)CH]2) affords the N-heterocyclic vinylidene stabilized singlet biradicaloid compound [(IPr)CP)]2 (3). Compound 3 is diamagnetic and exhibits temperature-independent sharp NMR resonances.
57. Normal-to-Abnormal Rearrangement of an N-Heterocyclic Carbene with a Silylene Transition Metal Complex
Ghadwal,* R. S.; Rottschäfer, D.; Andrada, D. M.; Frenking,* G.; Schürmann C. J.; Stammler H.-G.
Dalton Trans. 2017, 46, 7791–7799.
Serendipity and No-serendipity...The synthesis and characterization of the N-heterocyclic carbene (NHC) stabilized dichlorosilylene Group 6 metal complexes {(IPr)SiCl2}W(CO)5 (3-W), {(IPr)SiCl2}2Cr(CO)4 (4-Cr), and {(IPr)SiCl2}2W(CO)4 (4-W) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) are reported. Treatment of 3-W with CsOH in the presence of IPr leads to the formation of an abnormal-NHC (aNHC) metal complex (aIPrH)W(CO)5 (6-W) (aIPrH = 1,3-bis(2,6-diisopropylphenyl)imidazol-4-ylidene), unveiling an unprecedented normal-to- abnormal transformation route of an NHC. DFT calculations support the proposed mechanism that involves CsOH mediated deprotonation of the IPr-backbone of 3-W to yield a ditopic carbanionic-NHC (dcNHC) complex 5a-W. Subsequent 1,4-migration of the W(CO)5 moiety and hydrolysis of the unmasked SiCl2 rationalize the formation of 6-W. The desired H2O molecule is generated in the initial step on deprotonation of IPr with CsOH. In contrast to the literature precedents, the calculations indicate that the abnormal complex 6-W is 13.5 kcal/mol thermodynamically higher in energy than the normal counterpart (IPr)W(CO)5 (8-W). Interestingly, as the aNHC-compounds reported so far are more stable than their normal counterparts, this finding showcases an opposite trend. Moreover, reaction pathways to the synthesized and related complexes have been investigated by DFT calculations.
56. Facile Routes to Abnormal-NHC-Cobalt(II) Complexes
Ghadwal,* R. S.; Lamm, J.-H.; Rottschäfer, D.; Schürmann, C. J.; Demeshko, S.
Dalton Trans. 2017, 46, 7664–7667.
Main Group aNHC-transfer agent... Deprotonation of [IPrPh]I (1) with Co{N(SiMe3)2}2 readily affords the abnormal N-heterocyclic carbene (aNHC) complex (aIPrPh)2CoI2 (2) (aIPrPh = 1,3-bis(2,6-iPr2C6H3)-2-phenyl-imidazol-4-ylidene). Treatment of 1 with NaHBEt3 yields (aIPrPh)BEt3 (3) that serves as an aNHC-transfer agent and yields (aIPrPh)Co{N(SiMe3)2}2 (4) on reaction with Co{N(SiMe3)2}2.
