BARF & Advances in Macrocyclic Coordination in the S Group

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[note: Figures are in the source article and not highlighted in this summary]

This is a summary of work reported in the Journal of Inorganic Chemistry concerning macrocyclic coordination chemistry in the periodic S group. In the literature concerning these molecules we see that large cyclic chelate-type ligands, termed macrocyclic ligands, are often useful in transition metal chemistry due to their multidentate properties. Their multiple bonding allowing for higher achievable stabilities through the macrocyclic effect. The macrocyclic effect follows the entropy driven principle set forth by the chelate effect, however it is enhanced by the locked cyclic structure of the ligand itself. The area of macrocyclic research Champion1 and associates reports on the use of thioether ligands bonding with s group elements, primarily sodium. A type of bonding typically reserved for crown ethers and a handful of thio-oxa and selena-oxa macrocyclic complexes1.

Since s block complexes containing thioether only coordination bonds are completely unknown1, the group’s goal was to determine possibility of this coordination. Their method focused on a thioether macrocycle in reaction with a sodium cation bonded with the weak [BArF]-1 ([BArF]-1 here, termed “BARF”, corresponds to: [B[3,5-(CF3)2C6H3]4]) anion in tetrahydrofuran (THF) solution. The BARF anion being so weak  provides less competition in response to the macrocycle reacting with the sodium atom.

The group reports the first group 1 thioether only complex, seen as an unexpected cation geometry in [Na([24]aneS8)]+. The BARF anion provides charge balance here. Due to the instability of the metal-ligand bonds, unless supporting ligands were used, isolation of the unexpected cation would be improbable. These group 1 hard cation-soft ligand bonds being one of the reasons behind the lack of previous discoveries concerning these types of complexes. Isolation and crystallography was acquired for a partial sandwich complex of Na-S tridentate coordination and two THF ligands.1 In order to explore the electronic and orbital properties of the complex computational chemistry was used.

At minimal energy state, the complex has S4 Symmetry, and in experimental or solid state occupies a dodecahedral geometry.1 Bond lengths and angles were matched with crystallography data. Further computations were ran to understand the charge distribution across the macrocyclic ligand, which showed that Na+ attracts the lone pairs found on the S atoms. These are then stabilized via resonance across the neighboring C-C and C-H bonds.1 The density functional theory (DFT) calculations showed the S atoms to have a positive charges and the C atoms to have negative charges.1 H atoms exhibit a slight positive charge which is within the norm. Orbital plots were also developed using the optimized, minimum energy, structures HOMO and LUMO calculations, and were visualized in Figure 3. HOMO orbitals show electron transfer into the 3s and 3p of the Na+.

It is proposed that using the above data, further group 1 macrocyclic coordination complexes can be achieved. High bonding complexes would prove most capable at displacing any possible ligand competition, such as that found in THF. In essence higher denticity in soft donors leads to higher stability of group 1 s block acceptor complexes.

 

Summary: 2/13/2016

Reference

  1. Champion, M., Dyke, J., Levason, W., et al; Sodium Thioether Macrocyclic Chemistry: Remarkable Homoleptic Octathia coordination to Na+, J. Inorganic Chemistry, 2015, 54, 2497-2499.

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