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. 2018 Nov 6;10(5):1360-1367.
doi: 10.1039/c8sc03719a. eCollection 2019 Feb 7.

Reversible homolytic activation of water via metal-ligand cooperativity in a T-shaped Ni(ii) complex

Mu-Chieh Chang  1 , Kate A Jesse  1 , Alexander S Filatov  1 , John S Anderson  1
Affiliations

Reversible homolytic activation of water via metal-ligand cooperativity in a T-shaped Ni(ii) complex

Mu-Chieh Chang et al. Chem Sci. .

Abstract

A T-shaped Ni(ii) complex [Tol,PhDHPy]Ni has been prepared and characterized. EPR spectra and DFT calculations of this complex suggest that the electronic structure is best described as a high-spin Ni(ii) center antiferromagnetically coupled with a ligand-based radical. This complex reacts with water at room temperature to generate the dimeric complex [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] which has been thoroughly characterized by SXRD, NMR, IR and deuterium-labeling experiments. Addition of simple ligands such as phosphines or pyridine displaces water and demonstrates the reversibility of water activation in this system. The water activation step has been examined by kinetic studies and DFT calculations which suggest an unusual homolytic reaction via a bimetallic mechanism. The ΔH ‡, ΔS ‡ and KIE (k H/k D) of the reaction are 5.5 kcal mol-1, -23.8 cal mol-1 K-1, and 2.4(1), respectively. In addition to the reversibility of water addition, this system is capable of activating water towards net O-atom transfer to substrates such as aromatic C-H bonds and phosphines. This reactivity is facilitated by the ability of the dihydrazonopyrrole ligand to accept H-atoms and illustrates the utility of metal ligand cooperation in activating O-H bonds with high bond dissociation energies.

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Figures

Scheme 1
Scheme 1. Synthesis of complexes 2–5. RO˙ = 2,4,6-tri-tert-butylphenoxy radical. a Previous work.
Fig. 1
Fig. 1. Crystal structures of 2-Py, 3-Py, 4, and 5. Ellipsoids are shown at 50% and hydrogen atoms, solvent molecules, and counterions have been omitted for clarity (with the exception of the N–H in 2-Py which was located in the difference map). C atoms shown in gray, N in blue, and Ni in green. The labeling scheme for all complexes is as shown for 2-Py.
Scheme 2
Scheme 2. Top: Synthesis of [Tol,PhDHPy]Ni(μ-OH)Ni[Tol,PhDHPyH] (6) and Bottom: Constituent oxidized (6[ox]) and reduced (6[red]) halves of the dimer.
Fig. 2
Fig. 2. Crystal structure of 6. Ellipsoids are shown at 50% except for aryl rings which are shown in wireframe. Hydrogen atoms bound to C have been omitted for clarity, and only one of the two molecules in the asymmetric unit is shown. The dashed line indicates a hydrogen bonding interaction. C atoms shown in gray, N in blue, and Ni in green. See Table 2 for bond distances and angles.
Scheme 3
Scheme 3. Proposed mechanism of water activation by 5 and calculated reaction free energies (kcal mol–1) of different activation pathways. Note that Ar is the same for all complexes as shown for 3-H2O.
Fig. 3
Fig. 3. UV-vis kinetic trace of the reaction of 5 with excess H2O at room temperature. Each line represents a one-minute increment. The inset shows a plot of the dependence of rate on temperature from which the activation parameters have been obtained.
Scheme 4
Scheme 4. Reactivity of 6.
Fig. 4
Fig. 4. Crystal structure of 8. Note that the dimeric structure is shown although the asymmetric unit only contains one Ni center. Ellipsoids are shown at 50% except for aryl rings which are shown in wireframe. Hydrogen atoms bound to C have been omitted for clarity. Dashed lines indicate a hydrogen bonding interaction. C atoms shown in gray, N in blue, and Ni in green. Selected bond distances (Å) and angles (°) (* indicates a bond to the symmetric equivalent): Ni–N2 = 1.908(1), Ni–N6 = 1.813(1), Ni–N9 = 1.929(1), Ni–Ni* = 2.624(5), Ni–N2* = 1.929(1), N2–N3 = 1.451(1), N3–C4 = 1.302(2), C25–C26 = 1.453(2), C8–N9 = 1.317(2), N9–N10 = 1.413(1), N10–H···N3* = 2.02(2), N3–N10* = 2.815(2), N2–Ni–N10 = 171.45(5), N2–Ni–N6 = 91.41(5), N10–Ni–N6 = 82.13(5).
Scheme 5
Scheme 5. Proposed mechanisms for the reactivity of 6. L = phosphines or pyridine as shown in Scheme 4.
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