UC Davis

This dissertation describes the synthesis and characterization of a series of copper(I) thiolates and Tin-containing derivatives featuring as attractive London dispersion effects and rare structures with unusual chemical behavior.

Chapter 2 describes the synthesis of the first two-coordinate, dimeric copper thiolato complexes {CuSAr*}2 and comparison of copper thiolates with their lithium congeners. Reactions of the large terphenyl thiols HSAr* (Ar* = very bulky terphenyl ligands) with stoichiometric amounts of mesitylcopper(I) afforded the corresponding two-coordinate, dimeric copper thiolato species {CuSAr*}2. These complexes feature centrosymmetric dimeric structures with planar Cu2S2 core in which the two Cu atoms are bridged by sulfurs from two terphenyl thiolato ligands, which bear a resemblance to the CuA site in nitrous oxide reductase in which two cysteines also bridge two copper atoms. Single X-ray crystallography shows that the Cu derivatives feature much longer Cu- -Cu distances than the Li--Li distances in their dimeric Li congeners. Given the very similar effective ionic radii of the Li+ (0.59 Å) and Cu+ (0.60 Å) ions, this unusual phenomenon is probably a consequence of stronger dispersion interactions which are caused by a higher degree of instantaneous induced dipoles between copper atoms.

Chapter 3 is an extension of the studies in Chapter 2. Further investigation of the reactions described in Chapter 2 revealed that the adjustment of the amount of mesitylcopper(I) afforded a new type of heteroleptic aryl Cu(I) thiolato compounds with formula {Cu2(SAr*)Mes}2 (Ar* = very bulky terphenyl ligands). The molecular structures of those compounds show they all display tetrametallic Cu4 cores in which two thiolato or two mesityl ligands bridge the metals. It is noteworthy that the use of relatively less bulky thiolato ligands yielded products that feature the expected conventional alternating thiolato and mesityl bridging patterns. Nevertheless, switching to bulkier ligands afforded a previously unknown structural arrangement in which the two thiolato ligands are adjacent to each other. The new complex with cis arrangement of the ligands is sterically counterintuitive which is likely due to London dispersion (LD) energy effects. Dispersion corrected DFT calculations have been carried out to show that 3 has the highest LD effect stabilization arising from the increased numbers of C-H···H-C interactions of the isopropyl ligand substituents.

Chapter 4 depicts the London dispersion effects in an equilibrium of distannene and a tristannane with identical substituents. {Sn(C6H2-2,4,6-Cyp3)2}3, a new cyclotrisyannane, was produced by reacting lithium salt {LiC6H2-2,4,6-Cyp3·Et2O}2 with SnCl2 in Et2O at -78oC. Variable temperature (VT) 119Sn NMR spectroscopy shows {Sn(C6H2-2,4,6-Cyp3)2}3 can be converted to the distannene {Sn(C6H2-2,4,6-Cyp3)2}2 upon heating in hydrocarbon solution. However, the presence of the corresponding monomer :Sn(C6H2-2,4,6-Cyp3)2 at elevated temperature was not detected neither by VT 119Sn NMR spectroscopy nor by 1H-DOSY NMR spectroscopy. The suppression of the formation of the monomeric stannylene is a reflection of enhanced London dispersion energy between the two Sn(C6H2-2,4,6-Cy3)2 units in the {Sn(C6H2-2,4,6-Cyp3)2}3 and {Sn(C6H2-2,4,6- Cyp3)2}2, as evidenced by multiple computational studies. Van’t Hoff analysis revealed a cyctristannane {Sn(C6H2-2,4,6-Cyp3)2}3 to distannene {Sn(C6H2-2,4,6-Cyp3)2}2 conversion ΔHconv. energy value of 33.36 kcal mol−1 and a ΔSconv. value of 0.102 kcal mol−1 K-1, which gives ΔGconv.300 K = 2.86 kcal mol−1.

Chapter 5 described novel reactivities of stannylene :Sn(C6H3-2,6-(C6H3-2,6-iPr2)2)2 with phenylacetyene and diphenylacetylene. Stannylenes have shown high reactivities towards unsaturated small molecules (CO, alkene, alkyne, azide, etc...) due to their relatively modest HOMO-LUMO gap. However, only a few reactions of stannylene (:SnR2) with alkynes have been investigated so far. This chapter depicts the reactions of diaryl stannylene Sn(C6H3-2,6-(C6H3-2,6- iPr2)2)2 with terminal or non-terminal alkynes in benzene at elevated temperature, from which aryl alkyl stannylene products were isolated and characterized by X-ray crystallography and spectroscopy.