Role of Spin State and Ligand Charge in Coordination Patterns in Complexes of 2,6-Diacetylpyridinebis(semioxamazide) with 3d-Block Metal Ions: A Density Functional Theory Study

Abstract

We report here a systematic computational study on the effect of the spin state and ligand charge on coordination preferences for a number of 3d-block metal complexes with the 2,6-diacetylpyridinebis(semioxamazide) ligand and its mono- and dianionic analogues. Our calculations show excellent agreement for the geometries compared with the available X-ray structures and clarify some intriguing experimental observations. The absence of a nickel complex in seven-coordination is confirmed here, which is easily explained by inspection of the molecular orbitals that involve the central metal ion. Moreover, we find here that changes in the spin state lead to completely different coordination modes, in contrast to the usual situation that different spin states mainly result in changes in the metal-ligand bond lengths. Both effects result from different occupations of a combination of π- and σ-antibonding and nonbonding orbitals.

Can the Nonheme Iron Oxygenase Thymine Hydroxylase Catalyze Three Successive Reaction Cycles on One Substrate?

“…A small OH rebound barrier 5 TS2 TH of Δ G = 3.2 kcal mol −1 above 5 IM1 TH then leads to the hydroxylated products 5hmU with large exothermicity. This mechanism matches previous calculations on nonheme iron and αKG‐dependent dioxygenases that generally give a rate‐determining hydrogen atom abstraction as well as experimental work that determined a large kinetic isotope effect for analogous systems [66–89]. A triplet spin mechanism was also considered but found to be considerably higher in free energy as compared to the quintet spin mechanism.…”

“…The 5 R TH structure has a short Fe−O distance of 1.616 Å, which forms a hydrogen bond to a nearby water molecule at a distance of 2.04 Å. The structural features of the reactant complex match previous calculations on the iron(IV)‐oxo species of various nonheme iron dioxygenases and biomimetic complexes well that found similar Fe─O bond length [66–89]. The calculated free energy landscape on the quintet spin state surface is shown in Figure 4.…”

Can the Nonheme Iron Oxygenase Thymine Hydroxylase Catalyze Three Successive Reaction Cycles on One Substrate?

“…A small OH rebound barrier 5 TS2 TH of Δ G = 3.2 kcal mol −1 above 5 IM1 TH then leads to the hydroxylated products 5hmU with large exothermicity. This mechanism matches previous calculations on nonheme iron and αKG‐dependent dioxygenases that generally give a rate‐determining hydrogen atom abstraction as well as experimental work that determined a large kinetic isotope effect for analogous systems [66–89]. A triplet spin mechanism was also considered but found to be considerably higher in free energy as compared to the quintet spin mechanism.…”

“…The 5 R TH structure has a short Fe−O distance of 1.616 Å, which forms a hydrogen bond to a nearby water molecule at a distance of 2.04 Å. The structural features of the reactant complex match previous calculations on the iron(IV)‐oxo species of various nonheme iron dioxygenases and biomimetic complexes well that found similar Fe─O bond length [66–89]. The calculated free energy landscape on the quintet spin state surface is shown in Figure 4.…”

Sulfur-ligated iron( iv )-imido and iron( iv )-oxo complexes, which one is more reactive?

“…Typically, iron( iv )-oxo complexes have Fe–O bond lengths ranging from 1.60 to 1.67 Å. 106–121 In contrast, the Fe–N bonds in iron-tosylimido structures are longer as indeed observed here. 44,45,68–70 Interestingly, the equatorial and axial ligand distances from the metal are virtually the same for 3 2b / 3 2c and 5 2b / 5 2c .…”

Density functional approximations for consistent spin and oxidation states of oxoiron complexes

“…The best performance for the correct description of spin ground state is obtained with OPBE, S12g and SSB‐D. These three DFAs are especially designed to deal with this delicate and demanding task, and they showed success in many similar works from the past . Spin ground states determined with OPBE, S12g and SSB‐D are in accordance with the experiment for all examined iron‐oxo complexes, except for the [Fe II (OH)(Hdidpa)(CH 3 CN)] 2+ (1) complex.…”

Can the Nonheme Iron Oxygenase Thymine Hydroxylase Catalyze Three Successive Reaction Cycles on One Substrate?

“…A small OH rebound barrier 5 TS2 TH of Δ G = 3.2 kcal mol −1 above 5 IM1 TH then leads to the hydroxylated products 5hmU with large exothermicity. This mechanism matches previous calculations on nonheme iron and αKG‐dependent dioxygenases that generally give a rate‐determining hydrogen atom abstraction as well as experimental work that determined a large kinetic isotope effect for analogous systems [66–89]. A triplet spin mechanism was also considered but found to be considerably higher in free energy as compared to the quintet spin mechanism.…”

“…The 5 R TH structure has a short Fe−O distance of 1.616 Å, which forms a hydrogen bond to a nearby water molecule at a distance of 2.04 Å. The structural features of the reactant complex match previous calculations on the iron(IV)‐oxo species of various nonheme iron dioxygenases and biomimetic complexes well that found similar Fe─O bond length [66–89]. The calculated free energy landscape on the quintet spin state surface is shown in Figure 4.…”

Sulfur-ligated iron( iv )-imido and iron( iv )-oxo complexes, which one is more reactive?

“…Typically, iron( iv )-oxo complexes have Fe–O bond lengths ranging from 1.60 to 1.67 Å. 106–121 In contrast, the Fe–N bonds in iron-tosylimido structures are longer as indeed observed here. 44,45,68–70 Interestingly, the equatorial and axial ligand distances from the metal are virtually the same for 3 2b / 3 2c and 5 2b / 5 2c .…”

Density functional approximations for consistent spin and oxidation states of oxoiron complexes

“…The best performance for the correct description of spin ground state is obtained with OPBE, S12g and SSB‐D. These three DFAs are especially designed to deal with this delicate and demanding task, and they showed success in many similar works from the past . Spin ground states determined with OPBE, S12g and SSB‐D are in accordance with the experiment for all examined iron‐oxo complexes, except for the [Fe II (OH)(Hdidpa)(CH 3 CN)] 2+ (1) complex.…”

Can the Nonheme Iron Oxygenase Thymine Hydroxylase Catalyze Three Successive Reaction Cycles on One Substrate?

“…A small OH rebound barrier 5 TS2 TH of Δ G = 3.2 kcal mol −1 above 5 IM1 TH then leads to the hydroxylated products 5hmU with large exothermicity. This mechanism matches previous calculations on nonheme iron and αKG‐dependent dioxygenases that generally give a rate‐determining hydrogen atom abstraction as well as experimental work that determined a large kinetic isotope effect for analogous systems [66–89]. A triplet spin mechanism was also considered but found to be considerably higher in free energy as compared to the quintet spin mechanism.…”

“…The 5 R TH structure has a short Fe−O distance of 1.616 Å, which forms a hydrogen bond to a nearby water molecule at a distance of 2.04 Å. The structural features of the reactant complex match previous calculations on the iron(IV)‐oxo species of various nonheme iron dioxygenases and biomimetic complexes well that found similar Fe─O bond length [66–89]. The calculated free energy landscape on the quintet spin state surface is shown in Figure 4.…”

Sulfur-ligated iron( iv )-imido and iron( iv )-oxo complexes, which one is more reactive?

“…Typically, iron( iv )-oxo complexes have Fe–O bond lengths ranging from 1.60 to 1.67 Å. 106–121 In contrast, the Fe–N bonds in iron-tosylimido structures are longer as indeed observed here. 44,45,68–70 Interestingly, the equatorial and axial ligand distances from the metal are virtually the same for 3 2b / 3 2c and 5 2b / 5 2c .…”

Density functional approximations for consistent spin and oxidation states of oxoiron complexes

“…The best performance for the correct description of spin ground state is obtained with OPBE, S12g and SSB‐D. These three DFAs are especially designed to deal with this delicate and demanding task, and they showed success in many similar works from the past . Spin ground states determined with OPBE, S12g and SSB‐D are in accordance with the experiment for all examined iron‐oxo complexes, except for the [Fe II (OH)(Hdidpa)(CH 3 CN)] 2+ (1) complex.…”