N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Palladium(ii) complexes with highly basic imidazolin-2-imines and their reactivity toward small bio-molecules

Pd(ii) complexes with chelating imidazolin-2-imine ligands were synthesized, and the pK_a values and reactivity of these complexes were investigated.

16-Electron pentadienyl- and cyclopentadienyl-ruthenium half-sandwich complexes with bis(imidazol-2-imine) ligands and their use in catalytic transfer hydrogenation

Ligand exchange from “protonated open ruthenocene” afforded 16-electron ruthenium half-sandwich complexes as efficient transfer hydrogenation catalysts.

The mixed-valent copper thiol-ate complex hexa-kis-{μ3-2-[(1,3-dimethyl-imidazol-idene)amino]-benzene-thiol-ato}dicopper(II)tetra-copper(I) bis-(hexa-fluoridophosphate) acetonitrile disolvate dichloro-methane disolvate

The mol-ecular structure of the title compound, [Cu4(I)Cu2(II)(C11H14N3S)6](PF6)2·2CH3CN·2CH2Cl2, shows a mixed-valent copper(I/II) thiol-ate complex with a distorted tetra-hedral coordination of the Cu(I) and Cu(II) cations by one guanidine N atom and three S atoms each. Characteristic features of the Cu6S6 skeleton are a total of six chemically identical μ3-thiol-ate bridges and almost planar Cu2S2 units with a maximum deviation of 0.110 (1) Å from the best plane. Each Cu2S2 unit then shares common Cu-S edges with a neighbouring unit; the enclosed dihedral angle is 60.14 (2)°. The geometric centre of the Cu6S6 cation lies on a crystallographic inversion centre. Cu-S bond lengths range from 2.294 (1) to 2.457 (1) Å, Cu-N bond lengths from 2.005 (3) to 2.018 (3) Å and the non-bonding Cu⋯Cu distances from 2.5743 (7) to 2.5892 (6) Å. C-H⋯F hydrogen-bond inter-actions occur between the PF6(-) anion and the complex mol-ecule and between the PF6(-) anion and the acetonitrile solvent mol-ecule.

Bis(2-{[bis-(dimethyl-amino)-methyl-idene]amino-κN}benzene-sulfonato-κN)copper(II)

The mol-ecular structure of the title compound, [Cu(C11H16N3O3S)2], shows the Cu(II) atom with a distorted square-planar coordination geometry from the N2O2 donor set of the two chelating 2-{[bis-(dimethyl-amino)-methyl-idene]amino}-benzene-sulfonate ligands. The Cu(II) atom lies 0.065 (1) Å above the N2O2 plane and the Cu-O [2 × 1.945 (2) Å] and Cu-N bond lengths [1.968 (3) and 1.962 (3) Å] lie in expected ranges. The two aromatic ring planes make a dihedral angle of 85.48 (1)°.

N-Trityl-2-(tritylsulfan-yl)aniline

The title compound, C₄₄H₃₅NS, is a derivative of amino­thio­phenol and possesses a protected S-triphenyl­methyl thio­ether and an N-triphenyl­methyl­amine functional group. The trityl groups show an anti orientation, with C—C—N—C and C—C—S—C torsion angles of −151.0 (3) and −105.3 (2)°, respectively. There is an intra­molecular N—H⋯S hydrogen bond.

2-[2-(Benzyl-sulfan-yl)phen-yl]-1,1,3,3-tetra-methyl-guanidine

The mol-ecular structure of the title compound, C(18)H(23)N(3)S, shows it to be a derivative of an amino-thio-phenol possessing a tetra-methyl-guanidine group with a localized C=N double bond of 1.304 (2) Å and a protected thiol functional group as an S-benzyl thio-ether. The two aromatic ring planes make a dihedral angle of 67.69 (6)°.

N-[Bis(dimethyl-amino)-methyl-idene]-2-[(triphenyl-meth-yl)sulfan-yl]ethanaminium hexa-fluoro-phosphate

The mol-ecular structure of the title compound, C(26)H(32)N(3)S(+)·PF(6) (-), shows a protonated guanidyl group bridged by an ethyl-ene linker with a tritylsulfanyl unit. The guanidinium (gua) unit displays charge delocalization over the three N-C(gua) bonds. The N-C-C-S group shows a folded nonplanar conformation with a torsion angle of 158.4 (1)°. In the crystal, the cation and anion are linked by an N-H⋯F inter-action.