Davison N, Quirk JA, Wills C, Dixon C, Waddell PG, Dawson JA, Lu E. Elucidating Solution-State Coordination Modes of Multidentate Neutral Amine Ligands with Group-1 Metal Cations: Variable-Temperature NMR Studies.
Inorg Chem 2022;
61:15204-15212. [PMID:
36109881 PMCID:
PMC9516690 DOI:
10.1021/acs.inorgchem.2c02457]
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Abstract
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Multidentate neutral amine ligands play vital roles in
coordination
chemistry and catalysis. In particular, these ligands are used to
tune the reactivity of Group-1 metal reagents, such as organolithium
reagents. Most, if not all, of these Group-1 metal reagent-mediated
reactions occur in solution. However, the solution-state coordination
behaviors of these ligands with Group-1 metal cations are poorly understood,
compared to the plethora of solid-state structural studies based on
single-crystal X-ray diffraction (SCXRD) studies. In this work, we
comprehensively mapped out the coordination modes with Group-1 metal
cations for three multidentate neutral amine ligands: tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane
(Me3TACN), tetradentate tris[2-(dimethylamino)ethyl]amine
(Me6Tren), and hexadentate N,N′,N″-tris-(2-N-diethylaminoethyl)-1,4,7-triaza-cyclononane
(DETAN). The macrocycles in the Me3TACN and DETAN are identified
as the rigid structural directing motif, with the sidearms of DETAN
providing flexible “on-demand” coordination sites. In
comparison, the Me6Tren ligand features more robust coordination,
with the sidearms less likely to undergo the decoordinating–coordinating
equilibrium. This work will provide a guidance for coordination chemists
in applying these three ligands, in particular, the new DETAN ligand
to design metal complexes which suit their purposes.
Combining variable-temperature nuclear
magnetic resonance
(VT NMR) and DFT calculations, this work elucidates the solution-state
coordination modes of three multidentate neutral amine ligands with
Group-1 metal cations. Our studies prove that two ligand-design building
blocks, that is, N-macrocycle (TACN) and N-sidearms, act as structure-dictating
and hemilabile coordinating sites, respectively. The concept could
be utilized in designing new catalytic systems, which anchor the metal
center on the macrocycle, while the sidearms serve as “on-demand”
protective coordination sites.
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