First π-linker featuring mercapto and isocyano anchoring groups within the same molecule: synthesis, heterobimetallic complexation and self-assembly on Au(111)

Abstract

Mercapto (–SH) and isocyano (–NC) terminated conducting π-linkers are often employed in the ever-growing quest for organoelectronic materials. While such systems typically involve symmetric dimercapto or diisocyano anchoring of the organic bridge, this article introduces the chemistry of a linear azulenic π-linker equipped with one mercapto and one isocyano terminus. The 2-isocyano-6-mercaptoazulene platform was efficiently accessed from 2-amino-6-bromo-1,3-diethoxycarbonylazulene in four steps. The 2-NC end of this 2,6-azulenic motif was anchrored to the [Cr(CO)₅] fragment prior to formation of its 6-SH terminus. Metalation of the 6-SH end of [(OC)₅Cr(η¹-2-isocyano-1,3-diethoxycarbonyl-6-mercaptoazulene)] (7) with Ph₃PAuCl, under basic conditions, afforded X-ray structurally characterized heterobimetallic Cr⁰/Au^I ensemble [(OC)₅Cr(μ-η¹:η¹-2-isocyano-1,3-diethoxycarbonyl-6-azulenylthiolate)AuPPh₃] (8). Analysis of the ¹³C NMR chemical shifts for the [(NC)Cr(CO)₅] core in a series of the related complexes [(OC)₅Cr(2-isocyano-6-X-1,3-diethoxy-carbonylazulene)] (X = –NC, Br, H, SH, SCH₂CH₂CO₂CH₂CH₃, SAuPPh₃) unveiled remarkably consistent inverse-linear correlations δ(¹³CO_trans) vs. δ(¹³CN) and δ(¹³CO_cis) vs. δ(¹³CN) that appear to hold well beyond the above 2-isocyanoazulenic series to include complexes [(OC)₅Cr(CNR)] containing strongly electron-withdrawing substituents R, such as CF₃, CFClCF₂Cl, C₂F₃, and C₆F₅. In addition to functioning as a sensitive ¹³C NMR handle, the essentially C_4v-symmetric [(–NC)Cr(CO)₅] moiety proved to be an informative, remote, ν_NC/ν_CO infrared reporter in probing chemisorption of 7 on the Au(111) surface.