Coordination Chemistry with Cu(I)

A big thank you goes to my graduate supervisors Emily Tsui and Sibo Lin and the rest of the Agapie group for making my research experience productive and enjoyable. Treatment of the Cu(I) species with O2 or deprotonation of the ligand followed by metalation with Cu(II) generates polynuclear copper cores supported by bridging alkoxides. Mono-, di-, tetra- and hexanuclear copper complexes were synthesized by varying the protonation state and backbone geometry of the ligand.

Structural differences between complexes of meta and para ligand variants have been characterized by single-crystal X-ray diffraction and 31P-NMR spectroscopy. One of the first synthetic complexes to demonstrate oxygen activation was a copper(I) complex supported by a tetradentate, tripodal amine-based ligand.11,12 In the presence of oxygen, the formation of a μ-1,2-peroxo dicopper ( II ) complex was observed, a reaction that is reversible if oxygen is removed under reduced pressure and heating. Furthermore, if transition of the reduced oxygen species to water is desired, the dicopper core must be disassembled, resulting in a large rearrangement and thus a large energy barrier to catalysis.

Changing the substitution around the central aryl ring gives a different steric environment to the copper center and allows us to explore the relationship between the ligand configuration and the reactivity of the complexes. In particular, the distance between the two coordinated coppers is expected to affect the geometry of each oxygen intermediate as well as potentially the resulting complexes.

Figure 1.2. Binding modes of O 2 at dicopper sites. Figure from Hatcher and Karlin

Synthesis of dipyridyl carbinol ligands

Coordination Chemistry with Cu(I)

The modular synthetic approach easily adapts to possible modifications of the terphenyl or biphenyl backbone or pyridines. This indicates that both copper centers are temporarily located on the same side of the terphenyl backbone and therefore cooperative binding of small molecule substrates is feasible. Reactions of m-H2L and HL with the same Cu(I) source also led to bright yellow complexes believed to be structurally analogous, although we could not obtain solid structures.

Based on the comparison of 1H-NMR spectra, we conclude that Cu(I) coordinates with m-H2L and HL in the same way as with p-H2L.

Figure 1.4. Solid state structure of [p-H 2 LCu(I) 2 (CH 3 CN) 4 ](2OTf - ) shown as 50%

Coordination Chemistry with Cu(II)

Summary of metallation reactions with ligand p-H 2 L
Summary of metallation reactions with ligand HL
O 2 Cores Ligand
O 3 Cores Ligand

The crystal structure of the resulting complex, [p-LCu(II)2(CH3CN)2]2(4OTf-), reveals a D2-symmetric dimer with two alkocose-bridged dichloro(II) cores (Figure 1.7). The structure reveals that the copper-oxygen core is slightly puckered—the other oxygen is 162° out of the Cu-O-Cu plane. Similar reactions were performed with the 3-armed version of the ligands presented here, but no peroxotitanyl assay.

Preliminary support for this hypothesis is provided by the observation that the reaction of the dicopper(I) complex with excess H2O2 immediately gives the copper(II) alkoxide dimer, as evidenced by an identical 1H-NMR spectrum. Although disordered, the solid structure of the product reveals the formation of a similar Cu(II) alkoxide core, albeit with an unexpected geometry (Figure 1.8). Instead of the previously observed dimeric Cu2O2 diamond-shaped core, this structure shows a trimer with two Cu3O3 cores that appear to be covered by triflates.

Perhaps not surprisingly, the reaction of Cu(I) complex HL with oxygen or hydrogen peroxide also gives the same complex as upon deprotonation of the ligand and metalation with Cu(OTf)2. This is probably due to the fact that the copper binding sites of the paraligand are ideally positioned to form an “X”-shaped dimer, whereas a trimer is formed with the meta ligand due to its reduced bite angle.

Figure 1.6. Solid state structure of [HLCu(II)(CH 3 CN) 2 ](2OTf - ) shown as 50% thermal ellipsoids

Supporting Information

Chemical shifts are reported relative to the internal solvent: 1.94 ppm (CD3CN) for 1H-NMR data. The reaction was allowed to run overnight and then quenched with 11.28 mL (11.28 mmol, 2 equiv) of 1M HCl. The reaction was allowed to run overnight and then quenched with 3.7 mL of 1M HCl.

The reaction was allowed to run overnight and subsequently quenched with 3.2 mL of 1 M HCl. An immediate color change to bright yellow was accompanied by complete solubilization of the ligand. The solution was filtered through a plug of glass wool and crystals were grown from acetonitrile with liquid diffusion of diethyl ether.

Within 15 min, a color change to dark blue was accompanied by complete solubilization of the ligand. The resulting tan-colored deprotonated ligand was redissolved in 10 mL acetonitrile and stirred vigorously while Cu(CF 3 SO 3 ) 2 (48.1 mg, 0.133 mmol, 1 equiv) in 5 mL acetonitrile was added. The same product can be obtained by reacting the [p-H2LCu(I)2](2OTf) complex with excess O2 or H2O2 in acetonitrile (confirmed by 1H-NMR and crystal structure).

Purification was achieved by filtration through a plug of glass wool and repeated precipitation of the complex with diethyl ether. The immediate color change to purple and then dark blue was accompanied by complete solubilization of the ligand. An immediate color change to lavender and then dark blue was accompanied by complete solubilization of the ligand.

The resulting pale blue deprotonated ligand was redissolved in 15 mL of acetonitrile and stirred vigorously while Cu(CF3SO3)2 (107 mg, .296 mmol, 1 equiv.) in 15 mL of acetonitrile was added. Crystals of both complexes were grown from acetonitrile and diethyl ether liquid diffusion; LCu(II)]2(2OTf) is green and [LCu(II)]3(3OTf) is green/blue. The same product can be obtained by reaction of the [HLCu(I)](OTf) complex with excess O2 or H2O2 in acetonitrile (confirmed by 1H-NMR).

Figure A2. Proton assignments for complexes made with m-H 2 L.

Curie Law Plot for [LCu(II)] 2 (2OTf)

Curie Law Plot for [p-LCu(II) 2 ] 2 (4OTf)

Temp (K)

Supporting Information

All air and moisture sensitive compounds were manipulated using standard Schlenk techniques or in a glovebox under a nitrogen atmosphere. Solvents for air and moisture sensitive reactions were dried using the method of Grubbs.1 Benzene-d6 was purchased from Cambridge Isotopes and vacuum transferred from sodium benzophenone ketyl. Proton chemical shifts are reported with respect to internal solvent (7.16 ppm for C6D6) while 31P shifts are reported with respect to 85% aq.

Synthesis of p-P2ReCl(CO)2: Ligand p-P2 (synthesized according to the literature procedure3) (0.01 g, 0.021 mmol) was dissolved in 2 mL of xylene in a small Schlenk flask. All scans were made for 0.5 mM complex in THF with 0.1 M TBAP electrolyte using a glassy carbon electrode and Ag/Ag+ reference electrode using dry solvents under N2 atmosphere unless otherwise noted.