Investigating Chemical Bonding In Unusual Metal Compounds Using Experimental And Theoretical Techniques
AuthorSchmitt, Jennifer C.
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A series of luminescent Au-Au and Au-Cu complexes were investigated by computational methods. The complexes within the Au-Au series—AuI2(Ph2P(CH2)3PPh2)2I2 (n3), AuI2(Ph2P(CH2)5PPh2)2I2 (n5), and AuI2(Ph2P(CH2)6PPh2)2I2 (n6)—demonstrated a change in luminescence that could not be explained by changes in the complexes’ metric parameters. Complex n3 emits at 420 nm, n5 emits at 465 nm, and n6 emits at 499 nm. We modeled the emission through the spectroscopy-oriented configuration interaction (SORCI) methods using a truncated complex, [AuI(PPh2Me)2I]. The truncated model replicates the emission of the complexes and is computationally cheaper. The computational model of n3 emits at 425 nm, n5 emits at 486 nm, and n6 emits at 505 nm. We noted that only two metric parameters change significantly upon excitation from the ground to the excited state surface: the Au-I bond length and P-Au-P bond angle. The changes in these ground and excited state parameters influence the energy of the states and thus influence the luminescence. In order to separate out the effects of these two, we ran two potential energy surface (PES) scans to determine the effect. The first PES examined was the distortion of the P-Au-P bond angle from 105° to 180° in 2.5 increments with a constrained Au-I bond length of 3.000 Å. The second PES examined was the elongation of the Au-I bond length from 2.800 Å to 3.150 Å with the P-Au-P bond angle constrained to 160°. These PESs demonstrated that the Au-I bond length change caused the greater change in emission energies, with the P-Au-P bond angle having minor contributions. In the Au-Cu series, which contains the [AuCu(PPh2py)3]+ cation, two polymorphs with the formula [AuCu(PPh2py)3](BF4)2·2CH3OH and a pseudopolymorph with the formula [AuCu(PPh2py)3](BF4)2·3.5CH3OH were prepared. Each complex displays a different luminesce emission energy even though the cations are practically superimposable. We used time-dependent density functional theory (TD-DFT) and a quantum theory of atoms in molecules (QTAIM) analysis to investigate this system. We demonstrated that anion-π and anion-cation interactions have a dramatic influence on the emission energies of the Au-Cu series. The change in luminescence energies is due to the effects of the cation and anion placements. Finally, we modeled the [Ni-Fe]hydrogenase maturation helper protein HypB, which contains nickel in an S4 coordination motif, by modifying a nickel superoxide dismutase metallopeptide mimic SODM1 (SODM1=HCDLPCGVYDPA). The N-terminal histidine was changed to Ta, (2-mercapto-acetic acid), to create a terminal sulfur ligand. We established that Ni(II) is bound in an NS3 coordination motif. At physiological pH 7.4 the metallopeptide is in a distorted square planar coordination motif, while at pH 9.6 the metallopeptide is in a rigid square planar complex. We also observed a difference in the rate of oxidation of the metallopeptide solutions when we ran oxidative kinetic studies at the two different pHs. Upon further investigation, it was found that the metallopeptide mimic is protonated at pH 7.4, potentially protecting the thiolates from oxidation. The further suggests that these protonated nickel cysteines may be common in nature.