Lewis Acid Coordination Redirects S-Nitrosothiol Signaling Output.

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C6 F5 )3 coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C6 F5 )3 coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA]2- [LA=B(C6 F5 )3 ], which releases N2 O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C6 F5 )3 ] radical anion, which is susceptible to N-N coupling prior to loss of RSSR.

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O.

One-electron reduction of [Fe(NO)-(N3PyS)]BF4 (1) leads to the production of the metastable nonheme {FeNO}8 complex, [Fe(NO)(N3PyS)] (3). Complex 3 is a rare example of a high-spin (S = 1) {FeNO}8 and is the first example, to our knowledge, of a mononuclear nonheme {FeNO}8 species that generates N2O. A second, novel route to 3 involves addition of Piloty's acid, an HNO donor, to an FeII precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases.

A Frustrated and Confused Lewis Pair.

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates iPr2 ArNCO (iPr2 Ar=2,6-iPr2 C6 H3 ) and Ph2tBu ArNCO (Ph2tBu Ar=2,6-Ph2 -4-tBuC6 H2 ) with Piers' borane (HB(C6 F5 )2 ). While hydroboration of smaller isocyanates such as iPr2 ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier Ph2tBu ArNCO allows isolation of the substrate-free aminoborane with a short, covalent N-B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a "normal" FLP along the reaction pathway, supported by high-level DFT studies and variable-temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid-base interaction.

An Ethylene-Bridged Phosphane/Borane Frustrated Lewis Pair Featuring the -B(Fxyl)2 Lewis Acid Component.

Hydroboration of dimesitylvinylphosphane with bis[3,5-bis(trifluoromethyl)phenyl]borane [HB(Fxyl)2 ] gave the intramolecular ethylene-bridged P/B frustrated Lewis pair (FLP) Mes2 PCH2 CH2 B(Fxyl)2 . The new compound underwent a variety of typical FLP reactions such as P/B-addition to the carbonyl group of p-chloro-benzaldehyde. Cooperative N,N-addition to nitric oxide gave the respective persistent P/B FLPNO(.) radical, which readily reacted with 1,4-cyclohexadiene by H-atom abstraction to yield the corresponding P/B FLPNOH product. The B(Fxyl)2 -containing FLP reacted as a template for the HB(C6 F5 )2 reduction of carbon monoxide to the formyl stage to give the respective FLP(η(2) -formylborane) product. Most products were characterized by single-crystal X-ray crystal structure analysis.

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}(7) Complex with Dioxygen.

The nonheme iron complex, [Fe(NO)(N3PyS)]BF4, is a rare example of an {FeNO}(7) species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm(-1) in CD3CN for [Fe(NO)(N3PyS)]BF4 at room temperature. This {FeNO}(7) complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe(III)(NO2)(N3PyS)](+), which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the Fe(III)-NO2 product. The O2 reactivity of [Fe(NO)(N3PyS)]BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe(III)-superoxo and Fe(III)-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe(III)-nitrite complex is limited, and decay of [Fe(III)(NO2)(N3PyS)](+) leads to {FeNO}(7) species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}(7) complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.

Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes.

The dimesitylpropargylphosphanes mes2 P-CH2 -C≡C-R 6 a (R=H), 6 b (R=CH3 ), 6 c (R=SiMe3 ) and the allene mes2 P-C(CH3 )=C=CH2 (8) were reacted with Piers' borane, HB(C6 F5 )2 . Compound 6 a gave mes2 PCH2 CH=CH(B(C6 F5 )2 ] (9 a). In contrast, addition of HB(C6 F5 )2 to 6 b and 6 c gave mixtures of 9 b (R=CH3 ) and 9 c (R=SiMe3 ) with the regioisomers mes2 P-CH2 -C[B(C6 F5 )2 ]=CRH 2 b (R=CH3 ) and 2 c (R=SiMe3 ), respectively. Compounds 2 b,c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11. The systems 2 b,c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b,c. Compound 13 c transferred the H(+) /H(-) pair to a small series of enamines. Compound 13 c was also a metal-free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C6 F5 )3 to give the zwitterionic phosphonium/borate 17. The -PPh2 -substituted mes2 P-propargyl system 6 d underwent a typical 1,2-P/B-addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20. Several products were characterized by X-ray diffraction.

Thioether-ligated iron(II) and iron(III)-hydroperoxo/alkylperoxo complexes with an H-bond donor in the second coordination sphere.

The non-heme iron complexes, [Fe(II)(N3PySR)(CH3CN)](BF4)2 () and [Fe(II)(N3Py(amide)SR)](BF4)2 (), afford rare examples of metastable Fe(iii)-OOH and Fe(iii)-OOtBu complexes containing equatorial thioether ligands and a single H-bond donor in the second coordination sphere. These peroxo complexes were characterized by a range of spectroscopic methods and density functional theory studies. The influence of a thioether ligand and of one H-bond donor on the stability and spectroscopic properties of these complexes was investigated.

Preparation of non-heme {FeNO}7 models of cysteine dioxygenase: sulfur versus nitrogen ligation and photorelease of nitric oxide.

We present the synthesis and spectroscopic characterization of [Fe(NO)(N3PyS)]BF4 (3), the first structural and electronic model of NO-bound cysteine dioxygenase. The nearly isostructural all-N-donor analogue [Fe(NO)(N4Py)](BF4)2 (4) was also prepared, and comparisons of 3 and 4 provide insight regarding the influence of S vs N ligation in {FeNO}(7) species. One key difference occurs upon photoirradiation, which causes the fully reversible release of NO from 3, but not from 4.

Sulfur oxygenation in biomimetic non-heme iron-thiolate complexes.

The S-oxygenation of cysteine with dioxygen to give cysteine sulfinic acid occurs at the non-heme iron active site of cysteine dioxygenase. Similar S-oxygenation events occur in other non-heme iron enzymes, including nitrile hydratase and isopenicillin N synthase, and these enzymes have inspired the development of a class of [N(x)S(y)]-Fe model complexes. Certain members of this class have provided some intriguing examples of S-oxygenation, and this article summarizes these results, focusing on the non-heme iron(II/III)-thiolate model complexes that are known to react with O(2) or other O-atom transfer oxidants to yield sulfur oxygenates. Key aspects of the synthesis, structure, and reactivity of these systems are presented, along with any mechanistic information available on the oxygenation reactions. A number of iron(III)-thiolate complexes react with O(2) to give S-oxygenates, and the degree to which the thiolate sulfur donors are oxidized varies among the different complexes, depending upon the nature of the ligand, metal geometry, and spin state. The first examples of iron(II)-thiolate complexes that react with O(2) to give selective S-oxygenation are just emerging. Mechanistic information on these transformations is limited, with isotope labeling studies providing much of the current mechanistic data. The many questions that remain unanswered for both models and enzymes provide strong motivation for future work in this area.

Addition of dioxygen to an N4S(thiolate) iron(II) cysteine dioxygenase model gives a structurally characterized sulfinato-iron(II) complex.

The non-heme iron enzyme cysteine dioxygenase (CDO) catalyzes the S-oxygenation of cysteine by O(2) to give cysteine sulfinic acid. The synthesis of a new structural and functional model of the cysteine-bound CDO active site, [Fe(II)(N3PyS)(CH(3)CN)]BF(4) (1) is reported. This complex was prepared with a new facially chelating 4N/1S(thiolate) pentadentate ligand. The reaction of 1 with O(2) resulted in oxygenation of the thiolate donor to afford the doubly oxygenated sulfinate product [Fe(II)(N3PySO(2))(NCS)] (2), which was crystallographically characterized. The thiolate donor provided by the new N3PyS ligand has a dramatic influence on the redox potential and O(2) reactivity of this Fe(II) model complex.