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43048-AC3
Activation of Nitric Oxide at Low-Coordinate Fe, Co, Ni, and Cu Centers
Nitric oxide (NO) is an undesirable product of high-temperature hydrocarbon combustion, reacting swiftly with oxygen in the air to form nitrogen dioxide (NO2), a brown gas which is a principle component of smog, irritating to mucous membranes.� With the introduction of automobile catalytic converters, NOx emissions have dropped by 70 � 90 %.� These three way catalysts consist of oxide supported late transition metal atoms which both bind NO as well as assist its disproportionation to surface-bound N and O atoms which recombine to give the thermodynamically favored N2 and O2.� Nonetheless, the traditional three-way catalysts based on Pt and Rh are not effective in the O2-rich exhaust of diesel and lean-burn engines, motivating a new zeolite-based catalysts incorporating less expensive first-row metals.
During the second year of this two-year PRF grant, we have explored the ligand variations in three-coordinate nickel nitrosyl complexes to understand their effect on the coordinated nitrosyl ligand.� Addition of the thallium b-diketiminate Tl[H2NNF] to NiI(NO)(THF)2 in the presence of 2,4-lutidine results in the structurally characterized tetrahedral [H2NNF]Ni(NO)(2,4-lutidine) which binds lutidine rather weakly in solution.� The use of BF3�OEt2 cleanly removes the substituted pyridine to afford the three-coordinate [H2NNF]Ni(NO).� The electron-withdrawing nature of the fluorinated b-diketiminate ligand results in a significantly increased nNO stretching frequency of 1827 cm-1.�
We have also begun to explore the use of monodentate supporting ligands to achieve low-coordinate metal nitrosyls.� The addition of a free N-heterocyclic carbene (NHC) to NiI(NO)(THF)2 results in the formation of the three-coordinate [NHC]Ni(I)(NO) which exhibits a nNO stretch at 1766 cm-1. �This is amenable to further functionalization via reactions with a variety of nucleophiles X- to deliver [NHC]Ni(X)(NO) species (SBut, with nNO = 1750 cm-1 or Cp with nNO = 1810 cm-1).� Moreover, [NHC]Ni(I)NO undergoes reduction with sodium amalgam to give the dinuclear species {[NHC]Ni}2(m-I)(m-NO) (vNO = 1683 cm-1) with a very short Ni-Ni distance of 2.297 Ǻ)
Towards the goal of generating isolable, unprecedented two-coordinate transition metal-nitrosyls, we performed an anion exchange of the iodide in [NHC]NiI(NO) with AgOTf to provide [NHC]Ni(OTf)NO.� This possesses a somewhat T-shaped geometry (C-Ni-N = 121.59˚, C-Ni-O = 101.06˚, N-Ni-O = 137.27˚) that could be reproduced by high-level DFT calculations on the full molecule (C-Ni-N = 123.2˚, C-Ni-O = 101.7˚, N-Ni-O = 135.0˚). �Its nNO stretching frequency of 1826 cm-1 is significantly higher than that of the iodide, reflecting the poorer donating ability of the triflate.� Reaction with NaBArF4 (ArF = 3,5-(CF3)2C6H3 in either ether or CH2Cl2 results in identical NMR spectra in CDCl3.� Moreover, solution IR spectra of this putative cationic species in either ether or CH2Cl2 reveal one nNO of 1846 cm-1, suggesting that neither solvent coordinates to the metal, or they coordinate in a very similar manner.� Further spectroscopic and isolation attempts continue in an effort to better understand this curious, extremely low-coordinate species.
To identify the reactivity of NO and NO2 in the absence of a redox-active metal center, we utilized zinc tris(pyrazolyl)borate complexes.� While the zinc-thiolates iPr2TpZn-SBut are stable towards free NO, they react instantaneously with NO2 (or aerobic NO) to give tBuSNO and iPr2TpZn(NO3).� The observation of the zinc-nitrate indicates that NO2 (N2O4 ≡ [NO]+[NO3]-) is the active nitrosating agent.� Moreover, transnitrosation with S-nitrosothiols RSNO readily takes place at iPr2TpZn-SR' complexes in CDCl3 to provide equilibrium mixtures of R'SNO and iPr2TpZn-SR. A second-order rate constant of 2.0(0) M-1 s-1 was obtained at 60 �C for the degenerate exchange between PhCH2SNO and iPr2TpZn-SCH2Ph.� Further work with copper is in progress, especially given the success of the copper-exchanged zeolite Cu-ZSM-5 to serve as a photocatalyst for NO decomposition as well as the synthetic accessibility of TpCu(NO) complexes.
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