Lewis Acid Coordination Redirects S-Nitrosothiol Reduction

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology and are responsible for a myriad of physiological responses. While copper enzymes promote NO release from RSNOs by serving as Lewis acids capable of 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(C6F5)3 coordinates to the RSNO oxygen atom in adducts RSNO-B(C6F5)3, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials (-1.0 to -1.1 vs. NHE), B(C6F5)3 coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction results in formation of the Lewis acid stabilized hyponitrite dianion trans-[LA–O–N=N–O–LA]2– (LA = B(C6F5)3) that releases N2O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C6F5)3]•/- radical-anion susceptible to N-N coupling prior to loss of RSSR

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 O₂ to give cysteine sulfinic acid. The synthesis of a new structural and functional model of the cysteine-bound CDO active site, [Fe^II(N3PyS)­(CH₃CN)]­BF₄ (<b>1</b>) is reported. This complex was prepared with a new facially chelating 4N/1S­(thiolate) pentadentate ligand. The reaction of <b>1</b> with O₂ resulted in oxygenation of the thiolate donor to afford the doubly oxygenated sulfinate product [Fe^II(N3PySO₂)­(NCS)] (<b>2</b>), which was crystallographically characterized. The thiolate donor provided by the new N3PyS ligand has a dramatic influence on the redox potential and O₂ reactivity of this Fe^II model complex

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

One-electron reduction of [Fe­(NO)-(N3PyS)]­BF₄ (<b>1</b>) leads to the production of the metastable nonheme {FeNO}⁸ complex, [Fe­(NO)­(N3PyS)] (<b>3</b>). Complex <b>3</b> is a rare example of a high-spin (<i>S</i> = 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 <b>3</b> involves addition of Piloty’s acid, an HNO donor, to an Fe^II precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases

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)(CH(3)CN)](BF(4))(2) (1) and [Fe(II)(N3Py(amide)SR)](BF(4))(2) (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

CCDC 976173: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 976174: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 976172: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Unsaturated Vicinal Frustrated Lewis Pair Formation by Electrocyclic Ring Closure and Their Reaction with Nitric Oxide

The Lewis acidic β-styryl-B­(C₆F₅)₂ reagent <b>6a</b> undergoes a clean 1,1-carboboration reaction with 1-(PMes₂)-2-cyclohexenyl acetylene <b>9</b> at 60 °C to give the vicinal P/B-substituted conjugated triene product <b>10a</b>. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system <b>11</b>. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product <b>12</b>. Both <b>11</b> and <b>12</b> are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP <b>11</b> reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product <b>13</b>. Compound <b>12</b> cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion <b>14</b>. Both the FLPs <b>11</b> and <b>12</b> cooperatively add P/B to the nitrogen atom of nitric oxide (NO) within minutes at room temperature to give the persistent P/B FLPNO^• radicals <b>19</b> and <b>21</b>, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs <b>11</b> and <b>12</b> are thermally robust. At elevated temperatures (<b>11</b>: 75 °C, <b>12</b>: 100 °C) they undergo a coupling reaction with dimethyl acetylenedicarboxylate with carbon–carbon bond activation at a P-mesityl substituent

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 (<b>3</b>), the first structural and electronic model of NO-bound cysteine dioxygenase. The nearly isostructural all-N-donor analogue [Fe­(NO)­(N4Py)]­(BF₄)₂ (<b>4</b>) was also prepared, and comparisons of <b>3</b> and <b>4</b> 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 <b>3</b>, but not from <b>4</b>