Relationship between Structure and Zero-Field Splitting of Octahedral Nickel(II) Complexes with a Low-Symmetric Tetradentate Ligand

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper reports the synthesis, crystal structures, magnetic properties and calculations of two new octahedral nickel(II) complexes by using a low-symmetric tetradentate ligand. The reported nickel(II) complexes here are interesting and well-characterized. The structures were determined by single-crystal X-ray diffraction. The density functional theory (DFT) computations were also conducted for the obtained complexes to reveal their zero-field splitting parameters. Magnetic susceptibility was also measured for the complexes. Overall, I think this work will be of interest to the readership of Magnetochemistry.  I ask the authors to consider the following revisions:  

1. The static magnetic behaviors have been probed, however, the dynamic magnetic properties were not measured. I suggest the authors to probe the potential slow magnetic relaxation of the complexes under zero or a dc field.

2. Figure 6. The variation of the XmT from 300 to 100 K is large. Please check the data treatment. 

3. If there are other previously characterized complexes (if known in the literature) with the one ligand. Also, some additional sentences should be devoted to the previously studied coordination chemistry of this ligand with any metal.

4. Recently, several nickel(II) complexes with paramagnetic behaviors and other functions have been reported. Please insert related references to this study, such as

Inorg. Chem. 2023, 62, 16222−16227;

Cryst. Growth Des. 2023, 23, 5035−5042;

Inorg. Chem. 2023, 62, 9025–9034;

J Mol. Struct. 2024, 1305, 137823.

Comments on the Quality of English Language

no comment on the English

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

REFEREE’S REPORT ON THE PAPER: magnetochemistry-2917464

Title: Relationship between structure and zero-field splitting of octa- 2 hedral nickel(II) complexes with a low-symmetric tetradentate 3 ligand

Corresponding Author: Hiroshi Sakiyama et al.

The paper represents an interesting and solid piece of experimental and theoretical work, comprising synthesis of new octahedral nickel(II) complexes, single-crystal X-ray, magnetic susceptibility and magnetization studies as well as the density functional theory (DFT) computations. The purpose was to clarify the relationship between structure and zero-field splitting (ZFS) in this low-symmetric system. Interesting findings of this study are: (i) the axial zero-field splitting parameter, D was estimated to be positive for each complex and (ii) importance of considering g-anisotropy in magnetic analysis, even if g-anisotropy is small. This topic is important for several areas of current interests, e.g. EMR spectroscopy of transition ions and DFT/ab-initio modelling of ZFS parameters.

For these reasons the paper fully satisfies criteria for publications in Magnetochemistry. However, in order to enhance the scientific quality of the paper, clarification of some essential points and revisions are necessary as indicated below. In short, the paper should be acceptable for publication, provided that the original draft is adequately revised.

Below in Part A, I discuss general points to clarify and provide recommendations (R:), whereas in Part B, I discuss specific points, i.e. pertinent examples of the phrases, that need modifications or are not clear, are quoted (Q:), followed by my specific comments (C:). Written responses to my comments together with the revised MS would be appreciated. It would be helpful if the new changes in the revised MS are indicated, e.g. in blue color. This would speed up further reviewing of the draft.

Part A. General points to clarify:

A1. Usage of the crucial notions: ligand field (LF) [equivalently, crystal field (CF)]; spin Hamiltonian (SH) and zero-field splitting (ZFS); magnetic anisotropy (MA):

Major drawback of MS is that the crucial notions are used without providing pertinent references to sources where their proper definitions could be found. Importantly, a given crucial notion is often referred in literature to by one of three names that are not synonymous: CF (LF), SH (ZFS), or MA (SIA). Since two of three names must be incorrectly used, such cases constitute confusion of the type: CF=ZFS, ZFS=CF, or MA=ZFS as well as in some cases a compounded confusion: MA=CF/LF=ZFS. In view of the various types of serious confusion between crucial notions existing in literature, their meaning cannot be taken for granted. Hence, it is essential to adhere to the good practice of providing pertinent references.

R: Please cite some additional pertinent references indicated below.

(a) Distinction between the notions LF, ZFS and MA:

These crucial notions underlie this MS and have been invoked therein.

The notion ligand field is used 12 times, whereas zero-field splitting is used 9 times. Yet only one reference is cited for both notions, their ref. #1, i.e. Boča, R. Zero-field splitting in metal complexes. Coord. Chem. Rev. 2004, 248, 757–815. Additional references may be provided to some of the textbooks, see, e.g. [[1],[2],[3],[4],[5],[6]].

The notion anisotropy is invoked 3 times in the context of: ‘magnetic anisotropy’ and ‘the g-anisotropy and the χtip-anisotropy’, but no references were cited. Additional references may be provided to some of the key textbooks, see, e.g. [[7],[8],[9],[10],[11]].

For succinct review of these notions as well as elucidation of their origin and interrelationships, the authors may consult [[12]].

It is surprising that the book by Gatteschi et al. [11] is not cited. Regrettably, serious confusion between the notions CF, ZFS, and MA is evident in this book, as discussed in the reviews [12,[13],[14]].

On positive note, it is commendable that the authors use proper terminology, i.e. without explicitly confusing the notions: LF, MA & ZFS, by stating, quote: ‘the D parameter is related to magnetic anisotropy‘.

However, could the authors clarify how ‘the D parameter is related to magnetic anisotropy‘? This may require providing the definition of the notions ZFS and MA utilized in MS.

(b) Distinction between single molecule magnets [SMM] and single ion magnets [SIM]:

Actually, the octahedral nickel(II) complexes studied in this MS represent SIM and not SMM systems. Please consult Sec. 5.4 in [[15]]: The SMM and SCM are formed by polynuclear clusters assembled from mononuclear coordination complexes, whereas SIM are mononuclear complexes.

Part B. Specific comments:

Abstract:

Q: Judging from the resulting d-orbital related molecular orbitals, the octahedral coordination geometry for the complexes were both approximated as the D4 rotation group, contrary to the apparent low symmetric structures.

NOTE: Similar phrases also appear in Sec. 3.2 & 4.

C: D4 is not the ‘rotation group’ but the point symmetry group (PSG) of the Ni2+-polyhedron exhibiting tetragonal site symmetry.

C: It is not clear what do you mean by the underlined phrase.

R: Please discuss the structural symmetry aspects in terms of PSG. This is important since PSG determines the form of CF and ZFS Hamiltonians as well as the Zeeman Hamiltonian [12-15]

1. Introduction

Q1: The relationship between the coordination geometry around the nickel(II) ion and the sign of the axial zero-field splitting parameter, D [H = guβSuHu + D[Sz2 – S(S + 1)/3] (u = x, y, z)], has been studied [1], and the result indicates that D is negative when the axial ligand field is strong (z > x, y) and positive when the axial ligand field is weak (z < x, y).

C: In the context of LF theory, the meaning of the underlined symbols is not clear.

R: Please clarify and possibly relate to the microscopic SH theory of ZFSP, invoked in your ref #1, see, also [8,10, 12].

R: Please indicate in Figure 2 & 3 the reference frame (x, y, z), especially the tetragonal axis. This is important since meaningful comparison of various ZFSP (or CFP) sets requires knowledge of the reference frame in which they are expressed [12,8,10].

Q: On the other hand, it is not easy to predict the sign of D when the symmetry of the ligand field around the central metal is not clear.

C: Why this symmetry is not clear? Analysis of XRD data should yield not only the space group listed in Tab. 1 & 2 but also the PSG of the Ni2+-polyhedron?

C: It is not clear what do you mean by the underlined phrase.

R: Explain how such symmetry is related to PSG of the Ni2+-polyhedron.

3.2. Density Functional Theory (DFT) Computations for Complexes 1’ and 2

Q: Judging from the splitting pattern of the five d-orbital-related molecular orbitals, the coordination geometry can be well approximated as the D4 rotation group.

C: How this conclusion was derived?

R: Provide justification, and cite proper reference(s), that such splitting pattern does correspond to particular ‘coordination geometry’.

Q: Based on these DFT results, it is reasonable to assume that the symmetry around the nickel(II) ion in both complexes is axial in magnetic analysis.

C: The underlined phrase seems awkward.

3.3. Magnetic Properties of Complexes 1 and 2

Q: This behavior suggests very small [A=] intermolecular antiferromagnetic interaction or [B=] zero-field splitting of the ground state.

C: This phrase (repeated 2 times) implies equivalence of the two notions: [A] & [B]. To avoid such implied confusion, add ‘either’:

This behavior suggests either very small intermolecular antiferromagnetic interaction or zero-field splitting of the ground state.

C: Can the authors explain how to distinguish between the two options [A] & [B] and provide pertinent references?

Q: Equation 1 must be used for simulations in magnetic data analysis for magnetically anisotropic compounds.

Q: … for the principal magnetic susceptibilities χz and χx, previously derived Equations 9– 14 [20] are useful for S = 1 system.

C: Since no refs given for Eqs (1-8), one may presume that these Eqs represent new derivations – is it really the case of new theory? Only for Eqs (9-14) a reference is given.

[20] Sakiyama, H.; Yamamoto, Y.; Hoshikawa, R.; Mitsuhashi, R. Magnetochemistry 2023, 9, 14.

But if the same set of Eqs have already been published, there is no need to reproduce these Eqs, only provide source refs.

Q: The positive D value suggests that the axial ligand field is weaker than the equatorial ligand field [1], which is concordant with the DFT result.

Q: Similar to 1, the D value was found to be positive, suggesting the weak axial ligand field [1], which was concordant with the DFT result.

Note: see also Q1 in Introduction.

C: In the context of LF theory, the meaning of the underlined terms is not clear and must be provided.

C: How the LF strength (weaker, weak) is quantified?

Q: [A=] Although it is difficult to obtain precise g-values from magnetic data, considering g-anisotropy improves the fitting of magnetization in the high field region. The resulting anisotropic g-factor relationship (gz < gx) agreed to the positive D value [1]. [B=] This study shows the importance of considering g-anisotropy in the analysis, even if g-anisotropy is small.

C: Can the authors explain convincingly how the phrase [A] proves the phrase [B]?

4. Conclusions

Q: Crystal structures were determined for 1’ and 2’, and the DFT computations based on the crystal structures indicated that the octahedral coordination geometry around the nickel(II) ions in 1’ and 2’ are both approximated as D4 rotation group, and the weak axial ligand field (z < x, y) was suggested.

C: DFT computations cannot determine the PSG. Rather, the results of DFT computations, e.g. the ZFSPs and g-tensor components may only reflect a specific site symmetry, here tetragonal site symmetry given by D4 PSG.

[[1]]  R.B. Burns, Mineralogical Applications of Crystal Field Theory, Cambridge University Press, Cambridge, 1993.

[[2]]  A.B.P. Lever, E.I. Solmon, Ligand Field Theory and the Properties of Transition Metal Complexes, Inorganic Electronic Structure and Spectroscopy, Eds.  E.I. Solmon, A.B.P. Lever, Wiley, New York, 1999.

[[3]]  R.C. Powell, Physics of Solid-State Laser Materials, Springer, New York, 1998.

[[4]] B.N. Figgis, M.A. Hitchman, Ligand Field Theory and its Applications, Wiley-VCH, New York, 2000.

[[5]] B. Henderson, R.H. Bartram, Crystal-Field Engineering of Solid-State Laser Materials, Cambridge Univ. Press, Cambridge, 2000.

[[6]] J. Mulak, Z. Gajek, The Effective Crystal Field Potential, Elsevier, Amsterdam, 2000.

[[7]] K.W.H. Stevens, Magnetic Ions in Crystals, Princeton Univ. Press, Princeton, 1997.

[[8]] R. Boča, Theoretical Foundations of Molecular Magnetism. Elsevier, Amsterdam, 1999.

[[9]] K.H.J. Buschow, F.R. de Boer, Physics of Magnetism and Magnetic Materials, Kluwer Academic, New York, 2003.

[[10]] R. Boča, Magnetic parameters and magnetic functions in mononuclear complexes beyond the spin-Hamiltonian formalism, Struct. Bond. 117 (2006) 1.

[[11]] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford Univ Press, Oxford, 2006.

[[12]] C. Rudowicz, M. Karbowiak, “Disentangling intricate web of interrelated notions at the interface between the physical crystal field Hamiltonians and the effective spin Hamiltonians”, Coordination Chemistry Reviews, 287, 28-63 (2015)

[[13]] C. Rudowicz, M. Karbowiak, Physica B 451 (2014) 134.

[[14]] C. Rudowicz, M. Karbowiak, Physica B 456 (2015) 330.

[[15]] C. Rudowicz, P. Cecot, M. Krasowski, “Selection rules in EMR spectroscopy and related techniques: fundamentals and applications to modern case systems", Physica B, 608, 412863-21pp (2021)

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Sakiyama and colleagues' manuscript delineates the synthesis of two octahedral Ni(II) complexes featuring weak axial field ligands, resulting in positive axial zero-field splitting parameters (D). The authors corroborate the weak ligand field's predominance along the z-axis over the x and y axes through an analysis of d-orbital splitting patterns via Density Functional Theory (DFT). The manuscript exhibits commendable clarity, and I endorse its acceptance pending a few considerations:

1.       The introduction is brief and lacks sufficient references to relevant literature on octahedral Ni(II) complexes. The authors are encouraged to expand this section by incorporating additional pertinent articles to provide a more comprehensive overview of the field.

2.       Could the authors elucidate the significant discrepancy in the gz and gx values (gx-gz) between complex 1 (0.42) and complex 2 (0.08).

3.       The abundance of equations in the results and discussion section warrants consideration. It may be prudent for the authors to relocate these to the supporting information or the experimental section.

4.       Additionally, it would be beneficial for the authors to compute zero-field splitting parameters (D and E) and gx, gy, gz values through computational methods, such as CASSCF/NEVPT2/CASPT2. A thorough analysis via ab initio calculations will enhance comprehension of the disparities between g and D values.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

See pdf file

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The quality of English is much worse in sections 2.1 - 2.3. There are also inaccurate expressions and errors in verb tenses in the text as a whole.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript is well improved and meets the reviewer's comments and requests. Therefore, the manuscript is now acceptable for publication.

Author Response

Thank you for reviewing the manuscript. We appreciate it.

Reviewer 2 Report

Comments and Suggestions for Authors

Accept.

Author Response

Thank you for reviewing the manuscript. We appreciate it.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have successfully addressed the comments in the present version of the manuscript. After the inclusion of the changes in the present version, the manuscript in my opinion is suitable for publication.

Author Response

Thank you for reviewing the manuscript. We appreciate it.

Reviewer 4 Report

Comments and Suggestions for Authors

Unfortunately, the authors did not provide a complete set of information so that the compounds described in the article can be considered fully characterized.

The method of synthesis of the first-derived ligand (Schiff base) is not given, hence other scientists will not be able to reproduce the synthesis of this interesting ligand.

The removal of the magnetic properties of one of the compounds from the manuscript does not solve the problem of incomplete characterization of the obtained complexes.

The authors must convince the reader that the synthesis described by them allows a reliable reproduction of these new complexes and that their magnetically investigated powders are monophasic and correspond to the single crystal for which the structure was solved. This can only be confirmed by providing diffractograms of polycrystalline samples of the obtained complexes.

In addition, since the compounds contain solvated molecules and coordinated water, only TG-DTA studies can show how easily these molecules can be weathered out of the polycrystalline samples. This information is important to determine the molecular weight of the powder and also the diamagnetic contribution when processing magnetic measurement data.

Since the compounds have not been fully characterized, the paper cannot be published.

Author Response

Thank you for reviewing the manuscript. We appreciate it. The manuscript has been revised to take into account the comments.

Unfortunately, the authors did not provide a complete set of information so that the compounds described in the article can be considered fully characterized.
=> The compound (complex 1 in the revised manuscript) was characterized by elemental analysis and IR. In the elemental analysis of the bulk sample, the observed values were in best agreement with the calculated values assuming Ni:omp-:BPh4-:dmso:H2O in a 1:1:1:3:1 ratio. Since the water molecules are derived from the starting material and are bound to nickel(II) ions in the single crystal sample (complex 2 in the revised manuscript) obtained in a similar manner, it is reasonable to analogize that the water molecules are also bound to nickel(II) ions in the compound (complex 1).

The method of synthesis of the first-derived ligand (Schiff base) is not given, hence other scientists will not be able to reproduce the synthesis of this interesting ligand.
=> A description of the synthesis of the onp ligand has been added. (The ligand is not a Schiff base one.) This onp ligand is synthesized by the Mannich reaction, which is generally said to be not very reproducible. Professor David E. Fenton, who synthesized lots of ligands by the Mannich reaction, often said that the Mannich reaction is maniac. So, in the previous revision, I included information on synthesis in the introduction.

The removal of the magnetic properties of one of the compounds from the manuscript does not solve the problem of incomplete characterization of the obtained complexes.
=> Complex 1 was characterized by elemental analysis and IR. 

The authors must convince the reader that the synthesis described by them allows a reliable reproduction of these new complexes and that their magnetically investigated powders are monophasic and correspond to the single crystal for which the structure was solved. This can only be confirmed by providing diffractograms of polycrystalline samples of the obtained complexes.
=> For the ligand, a description has been added. The reader who wishes to synthesize this ligand should be able to do so with certainty. 
Only a small amount of single crystal sample was obtained, which was insufficient for various measurements. The elemental compositions obtained by elemental analysis differ from those obtained by single crystal structure analysis by one dmso. Although the structure of the magnetic measurement sample may not be identical to the structure of the single crystal sample, the structures are considered to be similar due to the steric requirement of the ligand.

In addition, since the compounds contain solvated molecules and coordinated water, only TG-DTA studies can show how easily these molecules can be weathered out of the polycrystalline samples. This information is important to determine the molecular weight of the powder and also the diamagnetic contribution when processing magnetic measurement data.
=> In the composition estimated from elemental analysis, the water molecules have a weight ratio of only 1.97%, which would be difficult to observe with TG-DTA.

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

The authors did not present diffractograms for polycrystalline samples. Consequently, the phase purity of the complex obtained by them was not proved, and, accordingly, the compound was not fully characterized. In this connection, it is not clear what the real molecular mass and diamagnetic contribution of the sample subjected to magnetic measurements is. 

Usually physicists automatically take the formula from the SC XRD analysis, even though the solvent is removed during storage or vacuuming during magnetic measurements. The shape of the magnetic curves cannot and does not affect this, but not in the case of nickel complexes! Furthermore, depending on the Mw and Correction(dia) used, the parameters of the theoretical curves will be different, since the values of all the points of temp. Especially since both cation and anion contain aromatic fragments, for which we need to choose the correct diamagnetic correction values for atoms and chemical bonds. https://doi.org/10.1021/ed085p532) Judging by the value of M(H) at H=7 T at 2 K, the authors clearly used an overestimated molecular mass.

Unfortunately, in the absence of PXRD data, I cannot recommend the manuscript for publication.

Author Response

=> Thanks for the comments.
((The authors did not present diffractograms for polycrystalline samples. Consequently, the phase purity of the complex obtained by them was not proved, and, accordingly, the compound was not fully characterized. In this connection, it is not clear what the real molecular mass and diamagnetic contribution of the sample subjected to magnetic measurements is.)) 
=> I disagree with this comment.
=> Powder diffraction analysis is a qualitative analysis, and broken small crystals or amorphous samples may not show significant diffraction. For this reason, powder X-ray diffraction is not suitable for the purpose of determining the purity of magnetic samples. We have stated from the beginning that the composition of the sample was determined by elemental analysis.
=> If the magnetic centers were significantly interacting in three dimensions, the crystal phase might be important, but since the complexes in this study are zero-dimensional discrete compounds, the local structure around the magnetic centers is more important. In the revised manuscript, we have added powder electronic spectra to discuss symmetry and ligand field.
=> Furthermore, the compound in this study is a complex with a dmso ligand. dmso complexes are widely known to exhibit phase transitions due to the molecular motion of the dmso ligand. Therefore, dmso complexes are not suitable for powder X-ray diffraction studies because the crystalline phase of the same crystal often changes depending on the temperature, or the crystal breaks down due to the phase transition, and diffraction data cannot be obtained.

((Usually physicists automatically take the formula from the SC XRD analysis, even though the solvent is removed during storage or vacuuming during magnetic measurements. The shape of the magnetic curves cannot and does not affect this, but not in the case of nickel complexes! Furthermore, depending on the Mw and Correction(dia) used, the parameters of the theoretical curves will be different, since the values of all the points of temp. Especially since both cation and anion contain aromatic fragments, for which we need to choose the correct diamagnetic correction values for atoms and chemical bonds. https://doi.org/10.1021/ed085p532) Judging by the value of M(H) at H=7 T at 2 K, the authors clearly used an overestimated molecular mass.))
=> What is indicated in the first sentence of this comment is not favorable. To prevent this we determine the composition of a compound by elemental analysis. We use dried samples whose compositions have been determined by elemental analysis for magnetic measurements, and the data are appropriately corrected for diamagnetism using Pascal's constants. The reference given by the reviewer is already cited in the manuscript as [15]. 
=> I disagree with the last sentence of the reviewer's comment. The proper reason is not stated by the reviewer. The values of magnetization are appropriate. Could the reviewer have miscalculated or misunderstood?