Effect of Metals in Biomimetic Dimetal Complexes on Affinity and Gas-Phase Protection of Phosphate Esters

Although the biomimetic dimetal complex [LGa₂(OH)₂(H₂O)₂]³⁺ [L = 2,6-bis­((<i>N</i>,<i>N</i>′-bis­(2-picolyl)­amino)­methyl)-4-tertbutylphenolate] provides efficient protection against phosphate loss in phosphopeptides upon collision-induced dissociation tandem mass spectrometry (CID MS/MS), the underlying mechanism remains unknown. Here, we explored the mechanism in detail and investigated the selective binding to phosphate groups in solution. Dimetal complexes containing combinations of Ga³⁺, In³⁺, Fe³⁺, Co³⁺, Zn²⁺, Cu²⁺, and V²⁺ were reacted with HPO₄^2–, phosphoserine, and a phosphopeptide (FQ­pS­EEQ­QQT­EDE­LQ­DK, abbreviated “βcas”) and studied with isothermal titration calorimetry (ITC), CID MS/MS, and density functional theory (DFT). <i>K</i>_a for HPO₄^2– binding scaled with the metal charge and was 35-fold larger for [LGa₂(OH)₂(H₂O)₂]³⁺ (3.08 ± 0.31 × 10⁶ M^–1) than for [LZn₂(HCOO)₂]⁺. CID MS/MS of [LGa₂(βcas)]^<i>n</i>+ revealed protection against phosphate detachment (<3% of the total ion intensity). Phosphate detachment from βcas was 22–40% and increased to 42–71% when bound to dimetal complexes of lower charge than {LGa₂}⁵⁺. CID data suggests that facile metal–phosphate dissociation is associated with proton transfer from the intermediate oxazoline ring formed in the phosphopeptide to the metal–phosphate complex. The observed phosphate stabilization was attributed to a significant reduction in the gas-phase basicity (GB) of the phosphate group when bound to {LGa₂}⁵⁺/{LIn₂}⁵⁺ complex cores. Absence of proton transfer results in formation of an ion–zwitterion intermediate with a greater dissociation threshold. This hypothesis is supported by DFT calculations for [LGa₂(PO₄)]²⁺, [LGaZn­(PO₄)]⁺, [LZn₂(PO₄)], and 2,4-dimethyl-3-oxazoline showing that [LGa₂(PO₄)]²⁺ is the only compound with a substantial lower GB (321 kJ/mol less) than 2,4-dimethyl-3-oxazoline

Electrochemically Generated <i>cis</i>-Carboxylato-Coordinated Iron(IV) Oxo Acid–Base Congeners as Promiscuous Oxidants of Water Pollutants

The nonheme iron­(IV) oxo complex [Fe^IV(O)­(tpenaH)]²⁺ and its conjugate base [Fe^IV(O)­(tpena)]⁺ [tpena^– = <i>N</i>,<i>N</i>,<i>N</i>′-tris­(2-pyridylmethyl)­ethylenediamine-<i>N</i>′-acetate] have been prepared electrochemically in water by bulk electrolysis of solutions prepared from [Fe^III₂(μ-O)­(tpenaH)₂]­(ClO₄)₄ at potentials over 1.3 V (vs NHE) using inexpensive and commercially available carbon-based electrodes. Once generated, these iron­(IV) oxo complexes persist at room temperature for minutes to half an hour over a wide range of pH values. They are capable of rapidly decomposing aliphatic and aromatic alcohols, alkanes, formic acid, phenols, and the xanthene dye rhodamine B. The oxidation of formic acid to carbon dioxide demonstrates the capacity for total mineralization of organic compounds. A radical hydrogen-atom-abstraction mechanism is proposed with a reactivity profile for the series that is reminiscent of oxidations by the hydroxyl radical. Facile regeneration of [Fe^IV(O)­(tpenaH)]²⁺/ [Fe^IV(O)­(tpena)]⁺ and catalytic turnover in the oxidation of cyclohexanol under continuous electrolysis demonstrates the potential of the application of [Fe^III(tpena)]²⁺ as an electrocatalyst. The promiscuity of the electrochemically generated iron­(IV) oxo complexes, in terms of the broad range of substrates examined, represents an important step toward the goal of cost-effective electrocatalytic water purification

A Versatile Dinucleating Ligand Containing Sulfonamide Groups

Copper, iron, and gallium coordination chemistries of the new pentadentate bis-sulfonamide ligand 2,6-bis­(<i>N</i>-2-pyridylmethylsulfonamido)-4-methylphenol (psmpH₃) were investigated. PsmpH₃ is capable of varying degrees of deprotonation, and notably, complexes containing the fully trideprotonated ligand can be prepared in aqueous solutions using only divalent metal ions. Two of the copper­(II) complexes, [Cu₂(psmp)­(OH)] and [Cu₂(psmp)­(OAc)₂]⁻, demonstrate the anticipated 1:2 ligand/metal stoichiometry and show that the dimetallic binding site created for exogenous ligands possesses high inherent flexibility since additional one- and three-atom bridging ligands bridge the two copper­(II) ions in each complex, respectively. This gives rise to a difference of 0.4 Å in the Cu···Cu distances. Complexes with 2:3 and 2:1 ligand/metal stoichiometries for the divalent and trivalent metal ions, respectively, were observed in [Cu₃(psmp)₂­(H₂O)] and [M­(psmpH)­(psmpH₂)], where M = Ga^III, Fe^III. The deprotonated tridentate <i>N</i>-2-pyridylsulfonylmethylphenolato moieties chelate the metal ions in a meridional fashion, whereas in [Cu₃(psmp)₂­(H₂O)] the rare μ₂-<i>N</i>-sulfonamido bridging coordination mode is observed. In the bis-ligand mononuclear complexes, one picolyl arm of each ligand is protonated and uncoordinated. Magnetic susceptibility measurements on the doubly and triply bridged dicopper­(II) complexes indicate strong and medium strength antiferromagnetic coupling interactions, with <i>J</i> = 234 cm^–1 and 115 cm^–1 for [Cu₂(psmp)­(OH)] and [Cu₂(psmp)­(OAc)₂]⁻, respectively (in H_HDvV =...+<i>JS</i>₁<i>S</i>₂ convention). The trinuclear [Cu₃(psmp)₂­(H₂O)], in which the central copper ion is linked to two flanking copper atoms by two μ₂-<i>N</i>-sulfonamido bridges and two phenoxide bridges shows an overall magnetic behavior of antiferromagnetic coupling. This is corroborated computationally by broken-symmetry density functional theory, which for isotropic modeling of the coupling predicts an antiferromagnetic coupling strength of <i>J</i> = 70.5 cm^–1

Spin Crossover in Fe(II) Complexes with N₄S₂ Coordination

Reactions of Fe­(II) precursors with the tetradentate ligand <i>S,S</i>′-bis­(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE^– coligands afforded mononuclear complexes [Fe­(bpte)­(NCE)₂] (<b>1</b>, E = S; <b>2</b>, E = Se; <b>3</b>, E = BH₃) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS^– to NCSe^– to NCBH₃^–, the SCO shifts to higher temperatures. Complex <b>1</b> exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex <b>3</b> exhibits a complete SCO with <i>T</i>_1/2 = 243 K. While the γ-<b>2</b> polymorph also shows the complete SCO with <i>T</i>_1/2 = 192 K, the α-<b>2</b> polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with <i>T</i>_1/2 = 120 K and the second step proceeding incompletely in the 80–50 K range. The amount of residual HS fraction of α-<b>2</b> that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-<b>2</b> was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K