Small molecule activation by nickel complexes: Selenium, white Phosphorus, Nitrogen oxides, and dioxygen

Low-valent transition metal complexes are powerful and effective reagents capable of activating small molecules. The purpose of this study is to extend the examination of low-valent nickel complexes to a range of small molecules. In this thesis, results on O2, Se, P4, and NOx - (x = 2, 3) have been included. Elemental selenium activation by a nickel(I) complex yielded [(PhTttBu)Ni]2(mu-&eegr; 2:&eegr;2-Se2) 1, which was also accessible by reaction of a nickel(II) precursor with Na2Se2. This complex exhibits a planar core structure containing two nickel(II) ions antiferromagnetic coupled. Another dinuclear complex [(PhTttBu)Ni]2(mu-Se) 2 was prepared by reaction of the same nickel(II) precursor with Na2 Se. The core structure of 2 adopts a linear arrangement with Ni-Se-Ni bond angle of 180o. Both complexes 1 and 2 possess diamagnetic ground states (S = 0) and are thermally stable in the solid state. Chapter 3 describes a series of high-spin monomeric nickel selenate complexes. Transition metals in sulfur-rich coordination environments featuring metal-selenium bonds are found at the active sites of several metalloenzymes ([NiFeSe] hydrogenase, Mo-formate reductase, and W-formate reductase). Experimental and computational studies suggest the existence of metal-selenium bond not only responsible for O2 tolerance, but also involved in small molecule activation in some cases. To study the unique features of a metal-selenium bond, a series of [PhTttBu]Ni(SeAr) complexes were prepared, featuring a single Ni-SeAr bond with different substituents group on phenyl rings. Three [PhTttBu]Ni(SeR) complexes (R = Ph 3, p-C6H4Cl 4, and C6F5 5) were prepared by reaction of [PhTttBu ]Ni(NO3) with the corresponding selenol and Et3N. Chapter 4 describes white phosphorus (P4) activation by [PhTt tBuNiI](PPh3). Reaction in THF yielded diamagnetic [PhTttBu]Ni(&eegr;3-cyclo-P 3) 8, which exhibited diamagnetic shifted 1H NMR features and distinct 31P NMR feature at delta = -136. Alternatively, when the reaction was performed in diethyl ether or hexane using solid P 4, a different diamagnetic species 9, was detected. Further study showed that complex 9 converted to 8 in THF upon heating to 70°C. Comparison with similar cyclo-P3 complexes suggests that 9 may be a dinickel complex with intact P4 unit bridged between two nickel centers. Studies in Chapter 5 showed that [PhTttBuNi](NO3) was reduced affording [PhTttBuNi](NO) upon heating in toluene for several hours. Similarly, the nitrite ion, in [PhTttBuNi](NO 2), was reduced yielding [PhTttBu]Ni(NO)upon heating. Those two reactions mimic the activities of the natural enzymes, nitrate reductase and nitrite reductase, which reduce nitrate and nitrite to NO, respectively. However, the current reactions proceeded in an aprotic medium. Chapter 6 continued the exploration of the electronic structures of nickel-dioxygen complex [Ni(tmcyclen)NiO2](OTf) 10, which can be prepared either by reaction of nickel(I) with dry O2, or by reaction of a nickel(II) precursor with excess H2O2 and Et3N. Previous characterization included a v(O-O) stretch of 1002 cm-1, MS m/z = 318.0 [M +], and Uv-vis features of (lambdamax (nm) (epsilon (M-1 cm-1)) in acetonitrile): 347 (172), 398 (142 ), 639 (56), 918 (40), which indicate the nature of nickel(II)-superoxo core structure. A newly obtained crystal structure of 10 showed an O-O bond length of 1.386 (4) A. The Ni L-edge XAS spectra showed 10 had an average L3 absorption centroid at 853.4 eV, insistent with the values of previously reported nickel(II) oxidation states. These results provided compelling evidence consistent with the assertion that the electronic structure of 10 is best described as nickel(II)-superoxo.