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This grant focused on the design and synthesis of a new kind of ligand derived from the famous dye molecule indigo. Somewhat surprisingly, despite the interesting electronic structure of indigo (very low energy absorption) and a structure with obvious potential as a bridging ligand, the literature on indigo metal complexes is nearly non-existent. This was probably because of the very poor solubility of the indigo molecule. We found that reactions with a wide range of aniline derivatives gave the corresponding diimines which we dubbed "Nindigo" by analogy to the colloquial "NacNac" term used for the ubiquitous b-diketiminates. In fact the Nindigo structure possesses two "NacNac" binding sites which are fused together. This structure has been reported a few times in the literature but no reliable synthesis has ever been reported nor has any The Nindigos show intense visible absorption and are actually similarly colored to the parent indigo molecule. However the presence of imine substituents renders the Nindigos much more soluble than indigo, in some cases by a factor of 100,000 moreso in a huge range of solvents.

The coordination chemistry of the Nindigo ligand has been explored. Our first forays focused on Pd(II) substrates. We were able to prepare binuclear Pd complexes with the Nindigo ligand adopting the anticipated bis-bidentate binding mode – most of the time; when the aniline substituents are relatively large, reactions with Pd(hfac)2 led to ligand isomerization and isolation of mononuclear Nindigo-Pd species in which the Nindigo core has switched from a trans to a cis alkene. The physicochemical properties of the complexes turn out to be extremely interesting. The pi-pi* absorption of the ligand persists in the metal complexes but is shifted into the near infrared region; aborsption intensities remain extremely high (~10E4 M-1 cm-1). Even more interesting is the discovery of several reversible oxidation/reduction processes (monitored  by cyclic voltammetry) which are attributable to the coordinated Nindigo ligand. As such, the Nindigo structure represents a brand new type of bridging ligand which (a) can be readily tuned  (in terms of steric and electronic properties) by simple choice of the aniline reagent, (b) holds two metal centres in very close proximity, which bodes well for strong metal-metal interactions mediated by the bridging ligand, and (c) exhibits a range of functionalities such as near infrared absorption and redox activity.

Overall this project has worked out more successfully than I had imagined it could in a relatively short period of time; our recent discoveries have opened up a huge range of opportunities and ‘new directions’ which I am now enthusiastically pursuing. This project would not have been possible without funding support from PRF and I am immensely grateful for the opportunity provided by this grant.