Metal–Indolizine Zwitterion Complexes as a New Class of Organometallic Material: a Spectroscopic and Theoretical Investigation

Abstract

Indolizine zwitterion coordinated metal species have been commonly proposed as intermediates in the mechanisms of metal-catalyzed cycloisomerization of propargylic pyridines for indolizines. Yet, it is only recently that the first metal–indolizine complexes have been isolated by our group. Considering from the perspective of molecular materials, the π-interaction between the dπ(M) and the π-system of the indolizine skeleton in the electronic ground or excited states may allow charge delocalization and offer functionalities for optoelectronic applications. We herein report the synthesis and spectroscopic and theoretical investigations on two classes of Ru–indolizine zwitterion complexes. The synthetic strategy employed, i.e., cycloisomerization of propargylic pyridines, represents a general preparation method for stable metal–indolizine complexes. Indolizine zwitterions in this work have been found to exhibit strong trans effect. Spectroscopic studies on these complexes reveals the tunability of the π*(indolizine) level and its impact on the luminophores nearby. Overall, indolizine zwitterion represents a new class of organometallic ligand with high potential in the design of functional molecular electronic/photonic elements.

A Ruthenium(II) Complex Supported by Trithiacyclononane and Aromatic Diimine Ligand as Luminescent Switch-On Probe for Biomolecule Detection and Protein Staining

“…Excitation of 1 ′ (31 μM) at 450 nm in acetonitrile resulted in an emission with λ max at 692 nm, a quantum yield of 2.334 × 10 −3 , and a lifetime of 0.272 μs. This emission was assigned as the triplet d π (Ru II ) → π*(N ∧ N) charge-transfer in nature based on comparison with analogous complexes reported previously 7 37 38 39 40 41 42 43 44 45 46 47 . The replacement of the bpy ligand with dcmb has been previously reported to red-shift the emission maximum of ruthenium(II) complexes 48 .…”

Ruthenium‐Induced Alkyne Cycloisomerization: Construction of Metalated Heterocycles, Revelation of Unconventional Reaction Pathways, and Exploration of Functional Applications

“…The scope of this reaction was later extended to pyrimidine-and quinoline-substituted propargylic alcohols, and another Ru II precursor [Ru( [14]-aneS4)Cl 2 ]( 2a-2c in Scheme 2b; [ 14]aneS4 = 1,4,8,11-tetrathiacyclotetradecane). [5] The X-ray crystal structures of cations 1a, 1b,a nd 2c ( Figure 1) clearly reveal that the propargylic substratesc ycloisomerizei nto indolizines and metalate at the C-2 positions. The RuÀCd istances (2.048(3)-2.073(2) )a re indicative of RuÀCs ingle bond character.A lthough the C-1 atoms are sp 3 hybridized, the nine non-hydrogen atoms in each indolizine skeleton are essentially coplanar.…”

Ruthenium‐Induced Cyclization of Heteroatom‐Functionalized Alkynes: Progress, Challenges and Perspectives

“…Around the same time, Wong and co‐workers initiated a project to synthesize metalated heterocyclic complexes through metal‐induced cycloisomerization of heteroatom‐functionalized alkynes, and isolated some Ru–heterocyclic complexes arising from the “non‐vinylidene‐involving” pathways. In 2013 and 2014, they isolated Ru–indolizine complexes 7 from the reactions between pyridine‐substituted propargylic alcohols HC≡CC(OH)(2‐py)R ( S7 ) and Ru II precursors [Ru([9]aneS3)(bpy)(OH 2 )] 2+ and [Ru([14]aneS4)Cl 2 ] (Scheme ; [9]aneS3=1,4,7‐trithiacyclononane, [14]aneS4=1,4,8,11‐tetrathiacyclotetradecane) . For comparative studies, non‐pyridine‐substituted propargylic alcohol S8 was employed to react with the same Ru precursors, and the vinylidene‐derived products Ru–acyl complex 8 a and Ru–carbene complexes 8 b were obtained.…”