Zwitterionic dithiocarboxylates derived from N-heterocyclic carbenes: coordination to gold surfaces

The zwitterionic dithiocarboxylates 1(+)-CS2--4(+)-CS2- were prepared by reacting the corresponding N-heterocyclic carbenes 1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (1), 1,3-diisopropylimidazol-2-ylidene (2), 1,3-dibenzylimidazol-2-ylidene (3) and 1,3-diethylbenzimidazol-2-ylidene (4) with CS2. In the latter two cases, the corresponding N-heterocylic carbene was generated in situ. Compounds 2(+)-CS2--4(+)-CS2- were structurally characterised by single-crystal X-ray diffraction studies. The chemisorption of these zwitterionic dithiocarboxylates on solid gold substrates was investigated in situ and in real time by optical second harmonic generation (SHG). The resulting thin films were exemplarily characterised by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the case of 1(+)-CS2- and 2(+)-CS2-, revealing the formation of almost contamination-free self-assembled monolayers, which exhibit a remarkable degree of orientational order.