Mixed 3d-3d’-Metal Complexes: A Dicobalt(III)Iron(III) Coordination Cluster Based on Pyridine-2-Amidoxime

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript describes the synthesis and characterization of a Fe-Co heterometallic complex using the tridentate ligand pyridine-2-amidoxime (NH2paoH). The synthesis of the CoIII2FeIII(NH2pao)6(NO3)2 complex is clearly presented. The employment of spectroscopic techniques, including IR, Raman, UV/VIS, and Mössbauer spectroscopy, provides valuable insights into the electronic properties, coordination modes, and oxidation states of the metal ions within the complex. Additionally, the magnetic studies offer a comprehensive understanding of the magnetic behavior of the isolated FeIII center. Overall, this work contributes to the study of heterometallic complexes. I recommend the publication of this work.

Please explain Alert B in the Checkcif doucument.

Author Response

We thank the reviewer for her/his time to study the ms and her/his warm comments.

Concerning her/his comment about Alert B, this alert concerns only 3 reflections among more than 12000, and it is most likely the consequence of a quite poor crystal quality and/or the disorder of the nitrate, perchlorates and water (both being most likely related).

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript describes synthesis, structure, spectroscopic characterization and magnetic properties of a new 3d-3d’ metal complex [Co^III₂Fe^III(NH₂pao)₆](ClO₄)₂(NO₃) (1). This is the first heterometallic trinuclear complex with (NH₂pao)ˉ ligands that includes two trivalent 3d metals. The compound can be prepared using three different synthetic strategies starting from Co^II/Fe^III or Co^II/Fe^II-containing reagents and possesses properties of isolated high-spin Fe^III. The paper includes discussion on promising features of chelating pyridine-2-amidoxime ligand, NH₂paoH, and its anionic forms NH₂paoˉ and NHpao²ˉ for the synthesis of homo- and heterometallic complexes, as well as a review of the known compounds in the field. The manuscript itself is well-written and organized, and the work is, in my opinion, of high quality and suitable for Inorganics. I recommend this paper for the publication after minor modification.

1) Single crystal structural study and magnetic measurements were performed on the freshly isolated 1·5MeOH·1.5H₂O sample whereas further characterization by elemental analysis, IR, Raman, UV/VIS, Mössbauer methods was done for the dried 1·H₂O sample. Were the X-ray powder diffraction patterns obtained for the fresh product (to confirm its purity) and for the dried one? It would be useful to compare them with each other and with the single crystal data.

2) Check chemical formula of the crystal in CIF and text, the amount of oxygen is incorrect.

3) Check grammar in lines 99, 190, 533, 590.

Author Response

We thank the reviewer for her/his time to study the ms, her/his warm comments and the valuable revision points/comments/suggestions provided. Concerning the points she/he has raised, our answers are listed below, after the relevant comment (in italics).

Single crystal structural study and magnetic measurements were performed on the freshly isolated 1·5MeOH·1.5H₂O sample whereas further characterization by elemental analysis, IR, Raman, UV/VIS, Mössbauer methods was done for the dried 1·H₂O sample. Were the X-ray powder diffraction patterns obtained for the fresh product (to confirm its purity) and for the dried one? It would be useful to compare them with each other and with the single crystal data.

The comment is correct. Our Department and University lack powder XRD facilities. So, the purity of the samples used for IR, Raman, UV/VIS and ⁵⁷Fe-Mössbauer methods is ensured by pulverizing and drying only single crystals of the product (which represent the pure compound). Then the sample is subjected to microanalyses (C, H, N) [in the present case the compound, in the form of a powder, was satisfactorily analyzed as 1^.H₂O] and used for the measurements. The magnetic measurements were performed on freshly isolated crystals (taken directly from the mother liquor) in order to detect intermolecular exchange interactions (if any) between the paramagnetic species (the trinuclear cations in the present case) mediated through the diamagnetic counterions and the lattice solvent molecules. This has been the research policy in our laboratories in the last 10 years or so (with more than 150 publications). All this information has now been added in the revised version of the ms to make things clear and remove any ambiguity.

Check chemical formula of the crystal in CIF and text, the amount of oxygen is incorrect.

The comment of the reviewer for the mistake in the chemical formula of the crystal in CIF and text is absolutely correct. We have corrected the amount of oxygen in the crystallographic Table 1 and CIF, i.e. from O₄₈ to O₄₇. Thus, we have deposited in the CCDC the CIF with the correct formula and we are providing the new CheckCif document.

Check grammar in lines 99, 190, 533, 590.

We have corrected the grammar mistakes in the indicated lines.

Reviewer 3 Report

Comments and Suggestions for Authors

This article by Skiadas et al. describes the synthesis and structure of heterometallic complexes with Co(+3)-Fe(+3)-Co(+3) arrays using pyridine-2-amidoxime (NH2PAO). Based on HSAB, the authors attempted to synthesize a heterometallic complex of the 3d-3d' type and successfully prepared complex 1 in a moderate yield. The X-ray structural analysis showed that the oxygen atoms surrounded the hard Fe(3+) ion and the pyridyl-imine moiety chelates the relatively soft Co(3+) ion. Complex 1 has been well characterized by NMR, IR, UV/VIS, and Mössbauer spectra. From this point of view, the reviewer considered this article suitable for publication in Inorganics. However, the points listed below need to be commented on.

Although the presence of neighboring Co(+3) ions is inferred by the relatively large ΔE_Q value observed in the Mössbauer spectrum, the distance between Fe and Co (3.488(2) Å) indicates no direct interaction between these atoms. Actually, the VT magnetic susceptibility measurements revealed the non-interacting high-spin Fe(+3) nature. However, this is different from the related Ni(+2)-Fe(+3)-Ni(+2) complex in Ref. 39, which displayed anti-ferromagnetic behavior. The authors should comment on this point.

The XRD analysis of 1 shows that a large residual peak above 2 eÅ^-3 remains (2.51eÅ^-3) and  an alert at the B level has been reported. Both of which require further comment.

Author Response

We thank the reviewer for her/his time to study the ms, her/his warm comments and the valuable revision points/comments/suggestions provided. Concerning the points that she/he has raised, our answers are listed below, after the relevant comment (in italics).

Although the presence of neighboring Co(+3) ions is inferred by the relatively large ΔE_Q value observed in the Mössbauer spectrum, the distance between Fe and Co (3.488(2) Å) indicates no direct interaction between these atoms. Actually, the VT magnetic susceptibility measurements revealed the non-interacting high-spin Fe(+3) nature. However, this is different from the related Ni(+2)-Fe(+3)-Ni(+2) complex in Ref. 39, which displayed anti-ferromagnetic behavior. The authors should comment on this point.

The Ni^II-Fe^III-Ni^II complex in ref. [39] is antiferromagnetically coupled because the “terminal” Ni^II atoms are paramagnetic (each having S = 1); thus, there are interactions between the central high-spin Fe^III atom (S = 5/2) and the “terminal” Ni^II atoms. In the case of our complex the “terminal” sites are low-spin (and hence diamagnetic) Co^III atoms, therefore precluding any magnetic exchange interactions within the molecule and resulting in an isolated high-spin Fe^III atom. We have added this information in the end of the magnetochemical paragraph in the revised version of the ms.

The XRD analysis of 1 shows that a large residual peak above 2 eÅ^-3 remains (2.51eÅ^-3) and  an alert at the B level has been reported. Both of which require further comment.

The residual electronic density of 2.51 e/ų is located at 0.71 Šfrom atom N20 of the lattice (i.e., counterion) nitrate and at equidistance of O23 and O25 from the same nitrate, too close to be a missing atom. It probably results from the disorder of the nitrate ion and water molecules, for which the elaboration of a more complex model would be too artificial. We have added these sentences in the crystallographic paragraph 3.3 of the revised ms, thus providing an explanation for this correct comment by the reviewer.

Reviewer 4 Report

Comments and Suggestions for Authors

This paper describes the synthesis, characterizations, the crystal structure, and the magnetic property of the trinuclear Co^IIIFe^IIICo^III complex with pyridine-2-amidoxime. This is a carefully done study, and the results are well discussed. In addition, the findings are interesting. Thus, this paper is worth publishing in Inorganics with minor revisions. An additional comment is shown below.

1) page 5−6: Co(II), Co(III), Fe(II) and Fe(III) should be replaced by Co^II, Co^III, Fe^II and Fe^III, respectively.

Author Response

We thank the reviewer for her/his time to study the ms, her/his warm comments and the valuable suggestion provided. Concerning her/his single suggestion, we have followed the advice in the revised version of the ms.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The new version answers the comments appropriately and is considered suitable for publication in Inorganics.