Ligand K-edge X-ray absorption spectroscopy of transition metal diphosphorus complexes and ligand-centered reactivity of triaminoborane-bridged diphosphine complexes

Kyounghoon Lee

doi:10.17077/etd.005950

P K-edge X-ray absorption spectroscopy (XAS) was used to investigate and compare the electronic structure of Rh(I) PNP pincer complexes with different linker groups attached to phosphorus (O vs. CH2). Changing the linker groups from O to less electronegative CH2 caused a uniform increase in occupied molecular orbital energies in the d-manifold that corresponded to pronounced differences in the P K-edge XAS spectra. These electronic structure variations account for known differences in the reactivity of related Rh(I) PNP complexes towards small molecules such as CO, H2, and CH2Cl2. Notably, my investigation confirmed that Rh(I) pincer complexes with less-electronegative CH2 showed a higher degree of metal π-backbonding with substrates due to better orbital energy-matching between ligand π-acceptor orbitals and occupied metal π orbitals. Octahedral TiCl4 complexes with the diphosphine ligands dppm, dppe, and dppp were studied using P K-edge XAS. Our group previously showed that changing the length of the diphosphine bridging linker yields a stepwise change in Pd-P covalency in Pd(II) diphosphines complexes. My goal was to evaluate if this change was observed in metal complexes with different coordination geometries, coordination number, oxidation state, and metal type (early vs. late transition metal). Despite these differences, the Ti(IV) diphosphine complexes showed a remarkably similar covalency trend when compared to Pd(II) complexes with the same ligands. Furthermore, I demonstrated that individual σ- and π-contributions to Ti-P bonds can be directly measured using P K-edge XAS. The effect of phosphorus substituent variations (cyclohexyl vs. phenyl vs. trifluoromethyl) on covalent Ti-P σ and π bonding was also studied and showed the expected changes in response to their predicted influence on ligand field strength. P K-edge XAS was also used to probe electronic structure variations in nontrigonal phosphorus compounds and related Ru complexes known to exhibit unconventional acceptor reactivity at a single phosphorus site. XAS data showed that the energy of phosphorus acceptor orbitals systematically decreased when the local geometry around phosphorus was distorted from C3v → Cs → C2v. The structure-induced electronic differences accounted for the co-localized donor and acceptor reactivity (i.e. biphilic reactivity) in this unique class of phosphorus compounds. While studying metal-phosphorus bond covalency, I designed and prepared a new class of diphosphine ligand called TBDPhos that contain a triaminoborane backbone derived from 1,8,10,9-triazaboradecalin (TBD). Despite the very low Lewis acidity of tricoordinate boron in TBDPhos, I discovered that (PhTBDPhos)NiCl2 can undergo ligand-centered reactions with water, alcohols, or hydrated fluoride salts to yield trans H-O or H-F addition across the bridgehead N-B bonds. The reactions do not proceed using anhydrous alkoxide or fluoride reagents, which suggested that protonation of the bridgehead nitrogen is crucial to the ligand-centered reactivity. Further investigations with TBDPhos complexes revealed evidence that latent borenium ion formation was important to the observed ligand-centered reactivity. Treating (PhTBDPhos)NiCl2 with the anhydrous strong Bronsted acid HBF4·Et2O resulted in F- abstraction from BF4- to form BF3, a much stronger Lewis acid than TBD. This suggested a tri-coordinate borocation called a borenium ion was formed upon protonation of the bridgehead TBD nitrogen. In addition, HCl was added to TBDPhos when (PhTBDPhos)NiCl2 was treated with HNTf2 or HOTf. Protonation of nitrogen formed a reactive borenium ion that captured chloride originally bound to Ni. Reactions of related Ni TBDPhos complexes with HNTf2 or HOTf revealed further spectroscopic evidence of the borenium ion being formed, but the associated complexes could not be isolated. However, reactions with the hexacoordinate Mo(0) complex (PhTBDPhos)Mo(CO)4 allowed the borenium ion to be isolated and structurally characterized using single-crystal X-ray diffraction. Furthermore, treating (PhTBDPhos)Mo(CO)4 with 2 equiv. of HBF4·Et2O yielded the first example of a four-coordinate boron cation called a boronium ion on the TBD backbone. Fundamental ligand-centered reactivity studies with TBDPhos slowly gave way to investigations into their potential use for chemical sensing and imaging. The photoluminescent Cu(I) TBDPhos complex (PhTBDPhos)CuCl was prepared and exhibited green photoemission with a high quantum yield and long phosphorescent lifetime. Ligand-centered reactions with MeOH change the color of the photoemission from green to blue while maintaining the long photoluminescent lifetime. Another application of TBDPhos complexes with platinum was investigated for potential 18F PET imaging studies due to selective and robust fluoride binding to TBDPhos. To accommodate aqueous studies, a water-soluble derivative of TBDPhos was prepared with methoxy groups attached to phosphorus (MeOTBDPhos). Platinum metallodrug candidates with F-bound MeOTBDPhos were prepared and their water-solubility and stability were established. Preliminary results of cell viability and clonogenic assays revealed that Pt MeOTBDPhos complexes are as effective as FDA-approved cisplatin at killing some cancer cells. Finally, covalent metal-phosphorus bonding in Ni and Pd TBDPhos complexes was studied using P K-edge XAS to determine the influence of the TBD backbone and ligand-centered reactions on electronic structure at the metal. Combining the results with previously reported P K-edge XAS data of Ni and Pd diphosphine complexes, I discovered a remarkably linear correlation between M-P bond covalency and bond distances. These results demonstrated that small changes in M-P bond distance is a reliable indicator of changes in M-P covalency. Furthermore, extrapolation of the regression data were used to estimate an “ionic radius” of P. Using the sum of ionic radii for Ni2+, Pd2+, and P, the bond distances were integrated into a single plot, which demonstrated that relative changes in bond covalency could be compared across both metals.

Ligand K-edge X-ray absorption spectroscopy of transition metal diphosphorus complexes and ligand-centered reactivity of triaminoborane-bridged diphosphine complexes

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