Modeling Reactivity of Nitrite and Nitrous Acid at a Phenolate Bridged Dizinc(II) Site: Insights into NO Signaling at Zinc

Abstract

Although nitrite‐to‐NO transformation at various transition metals including Fe and Cu are relatively well explored, examples of such a reaction at the redox‐inactive zinc(II) site are limited. The present report aims to gain insights into the reactivity of nitrite anion, nitrous acid (HONO), and organonitrite (RONO) at a dizinc(II) site. A phenolate‐bridged dizinc(II)‐aqua complex [LHZnII(OH2)]2(ClO4)2 (1H‐Aq, where LH = tridentate N,N,O‐donor monoanionic ligand) is illustrated to react with tBuONO to provide a metastable arene‐nitrosonium charge‐transfer complex 2H. UV‐vis, FTIR, multinuclear NMR, and elemental analyses suggests the presence of a 2:1 arene‐nitrosonium moiety. Furthermore, the reactivity of a structurally characterized zinc(II)‐nitrite complex [LHZnII(ONO)]2 (1H‐ONO) with a proton‐source demonstrates HONO reactivity at the dizinc(II) site. Reactivity of both RONO (R = alkyl/H) at the phenolate‐bridged dizinc(II) site provides NO+ charge‐transfer complex 2H. Subsequently, the reactions of 2H with exogenous reductants (such as ferrocene, thiol, phenol, and catechol) have been illustrated to generate NO. In addition, NO yielding reactivity of [LHZnII(ONO)]2 (1H‐ONO) in the presence of above‐mentioned reductants have been compared with the reactions of complex 2H. Thus, this report sheds light on the transformations of NO2–/RONO (R = alkyl/H) toNO/NO+ at the redox‐inactive zinc(II) coordination motif.

Reduction of nitrite to nitric oxide and generation of reactive chalcogen species by mononuclear Fe(ii) and Zn(ii) complexes of thiolate and selenolate

“…S24-S33 †). The IR spectrum of 3 Zn showed ν NO 2 values of 1374, 1253 and 1168 cm −1 (assigned by comparison with the IR spectrum of 5 Zn ) and were comparable with the ν NO 2 values of 1424, 1271 and 1103 cm −1 reported for a binuclear Zn(II)-ONO compound 50 and with ν NO 2 values of 1392 and 1271 cm −1 reported for a mononuclear Zn-κ 2 -ONO compound. 68…”

“…1d) are 1.070(7) and 1.326(6) Å, respectively. The Zn–O nitrite distance in 3 Zn is very similar to the Zn–O nitrite distance of 2.013(3)–2.140(3) Å (a binuclear Zn( ii )–bis(ONO) complex) 50 and 1.974(4) Å (a mononuclear Zn( ii )–ONO complex) 100 featuring terminal nitrite units, while some other reported Zn( ii )–nitrite complexes possess a κ 2 -O,O′ binding mode of nitrite and display Zn–O distances of 1.981(2) Å, 2.430(2) Å; 86 2.18(3), 2.41(2) Å; 68 2.221(3) and 2.240(5) Å. 101 The Zn–Br distance in 6 Zn (2.4594(4) Å) is very similar to those observed (2.4652(9) and 2.4324(8) Å) in the distorted square pyramidal compound 4 Zn .…”

“…1d) are 1.070(7) and 1.326(6) Å, respectively. The Zn-O nitrite distance in 3 Zn is very similar to the Zn-O nitrite distance of 2.013(3)-2.140(3) Å (a binuclear Zn(II)-bis(ONO) complex) 50 and 1.974(4) Å (a mononuclear Zn(II)-ONO complex) 100 featuring terminal nitrite units, while some other reported Zn(II)nitrite complexes possess a κ 2 -O,O′ binding mode of nitrite and display Zn-O distances of 1.981(2) Å, 2.430(2) Å; 86 In addition to the single crystal X-ray structure determination, all the compounds were further characterized by elemental analysis, 1 H and 13 C NMR spectroscopy (Zn(II) compounds; Fig. S1-S17 †), mass spectrometry (5 Zn and 5 Fe , Fig.…”

“…Examples include the reaction of iPr2 TpZn(ONO) with RSH (R = p-MeC 6 H 4 ) to produce iPr2 TpZn (SR) and NO (via the formation of RSNO), 48 the reaction of [(Bn 3 Tren)Zn(ONO)] 1+ with t BuBnSH to generate NO (via the formation of t BuBnSNO), 49 and the reaction of the binuclear Zn(II)-nitrite complex, [L H Zn(ONO)] 2 (L H is a tridentate N,N,Odonor monoanionic ligand) with t BuBnSH to generate NO. 50 A reasonable number of iron, cobalt and copper based bioinspired model complexes for the reduction of NO 2…”

“…Examples include the reaction of iPr2 TpZn(ONO) with RSH (R = p -MeC 6 H 4 ) to produce iPr2 TpZn(SR) and NO ( via the formation of RSNO), 48 the reaction of [(Bn 3 Tren)Zn(ONO)] 1+ with t BuBnSH to generate NO ( via the formation of t BuBnSNO), 49 and the reaction of the binuclear Zn( ii )-nitrite complex, [L H Zn(ONO)] 2 (L H is a tridentate N,N,O-donor monoanionic ligand) with t BuBnSH to generate NO. 50 A reasonable number of iron, cobalt and copper based bioinspired model complexes for the reduction of NO 2 − have been reported in the literature. 38,51–60 Chemical reduction of nitrite at first-row transition metal centres involving exogenous reducing agents such as thiols, 61,62 H 2 S, 63 phenols, 64–66 ene-diols, 67,68 VCl 3 69 and phosphines 59 is also available in the literature.…”