A New Unnatural Amino Acid Derived from the Modification of 4′-(p-tolyl)-2,2′:6′,2″-terpyridine and Its Mixed-Ligand Complexes with Ruthenium: Synthesis, Characterization, and Photophysical Properties

Round 1

Reviewer 1 Report

The authors describe the synthesis of various homoleptic as well as heteroleptic complexes of the type [RuL1L2](PF6)2 (L = terpyridine derivatives). The complexes were characterized by elemental analysis, NMR spectroscopic and HR-ESI-MS as well as photophysical studies. The experiments seem to be carried out without any problems.

Prior to publication the following points are requested to consider:

1. A very important concern is that coupling constants J are missing by all 1H NMR spectroscopic data. These have to be added in any case.

minor points:

line 302: "ABX proton spin system" change by "ABX spin splitting pattern"

line 376: "-terpyridin" change by "-terpyridine" 

Author Response

Reviewer#1

The authors describe the synthesis of various homoleptic as well as heteroleptic complexes of the type [RuL1L2](PF6)2 (L = terpyridine derivatives). The complexes were characterized by elemental analysis, NMR spectroscopic and HR-ESI-MS as well as photophysical studies. The experiments seem to be carried out without any problems.

Prior to publication the following points are requested to consider:

1. A very important concern is that coupling constants J are missing by all 1H NMR spectroscopic data. These have to be added in any case.

³J_HH spin-spin coupling constants have been calculated and added in the NMR data for every one of the synthesized compounds.

minor points:

line 302: "ABX proton spin system" change by "ABX spin splitting pattern"

The phrase "ABX proton spin system" has been corrected as "ABX spin splitting pattern"

line 376: "-terpyridin" change by "-terpyridine"

It has been corrected.

Reviewer 2 Report

I think the authors have prepared a lot of complexes but there are few studies to present. Half of the introduction is on unnatural amino acids and their aplications but the paper has nothing to do with this topic. More than half of the results section is devoted to description of common ligand synthectic procedures and 1H NMR of ligands. I think more characterization of the complexes (DRX for example) or other aplications of them are needed. The photophysical studies are incomplete.  

Author Response

Reviewer#2

I think the authors have prepared a lot of complexes but there are few studies to present. Half of the introduction is on unnatural amino acids and their aplications but the paper has nothing to do with this topic. More than half of the results section is devoted to description of common ligand synthectic procedures and 1H NMR of ligands. I think more characterization of the complexes (DRX for example) or other aplications of them are needed. The photophysical studies are incomplete. 

We do not agree. The ligand “phet” is a novel UAA, synthesized for the first time. As a chimeric compound it combines the chemical properties of both phenylalanine and terpyridine. Potentially, it can used in peptidomimetics either as a building block itself or coordinated with various metal ions. Thus, it is completely reasonable that a significant portion of the introduction is committed to UAAs and their appplications. The synthesis of the ligand is novel and reported herewith for the first time. Because we failed to obtain single crystal X-ray data, the solution characterization of the ligand and the complexes is necessary to have detailed NMR and high-resolution MS studies. Regarding the photophysical studies, we believe that the study is enough for such Ru(II)-terpyridine complexes given the expected results.

Reviewer 3 Report

The paper describes the synthesis and optical properties of new Ru(II)-terpyridine derivatives functionalied by alpha-aminoacid moieties. The reported compounds deserve a special interest due to their capabillity of peptide functionalization. Although crystal structures of new coordination compunds were not reported, a presented thorough characterization by means of NMR, ESI-MS and elemental analysis looks acceptable to prove their molecular structures and composition. Minor corrections are suggested for the improvement in the discussion part:

1. Please comment what is oxidized while Ru³⁺ is reduced to Ru²⁺ during the synthesis. 

2. In my opinion, it is quite surprising that a series of compounds with free NH2 and COOH groups emit much more stronger than a series containing substituted amino- and carboxylic groups, which have no mobile H atoms known to be the effective luminescence quenchers. I guess such unusual observation deserves some distinct discussion in the article. 

Author Response

Reviewer#3

The paper describes the synthesis and optical properties of new Ru(II)-terpyridine derivatives functionalied by alpha-aminoacid moieties. The reported compounds deserve a special interest due to their capabillity of peptide functionalization. Although crystal structures of new coordination compunds were not reported, a presented thorough characterization by means of NMR, ESI-MS and elemental analysis looks acceptable to prove their molecular structures and composition. Minor corrections are suggested for the improvement in the discussion part:

1. Please comment what is oxidized while Ru3+ is reduced to Ru2+ during the synthesis.

Usually, Ru(II)-polypyridine complexes are formed using RuCl3 xH2O, as the salt of the metal, and the polypyridine ligand in a boiling solution of ethylene glycol. Under these conditions Ru(III) reduces to Ru(II) and is stabilized by the polypyridine ligands which are pi-acceptors and tend to stabilize metals in low oxidation states. On the other hand, a part of the solvent oxidizes in several products such as glyoxal, glycolaldehyde, etc. The following comment was added in the manuscript text (line 267) «In boiling ethylene glycol the Ru(III) was reduced to Ru(II) while oxidation species, such as glyoxal, glycolaldehyde were formed». 

2. In my opinion, it is quite surprising that a series of compounds with free NH2 and COOH groups emit much more stronger than a series containing substituted amino- and carboxylic groups, which have no mobile H atoms known to be the effective luminescence quenchers. I guess such unusual observation deserves some distinct discussion in the article.

The following phrase was added in line 365 at the manuscript: “Similar results have been reported previously by Zhang et.al [10.1021/ic100810z] where Ru(II) complexes with 4,4’ substituted 2,2’-bipyridine were applied as sensitive luminescence probes for detection of cysteine (Cys) and homocysteine (Hcy). The reaction of the non-luminescent probe with Cys and Hcy accompanied by notable luminescence increase.”

Reviewer 4 Report

Dear Editor

The manuscript describes the synthesis of two new terpyridine ligand derivatives which where utilized to synthesize a number of ruthenium complexes which were characterized by spectroscopic means while their photophysical properties were investigated. The manuscript is well written and presented while the results merit publication. Therefore, I am glad to suggest publication after minor revision.

11.       Given that the ligands utilized are neutral within the complexes, the oxidation state of the ruthenium in complexes 4-11 is (II) although the Authors began their reactions with starting materials of Ru(III). A comment of this will be helpful for the non-familiar to ruthenium chemistry readers.

22.       In page 5, lines 207-209 the Authors might have mistakenly written “In a 207 typical experiment 50 mg of the corresponding parent complex (1) - (3) and (5) was transferred”. Compound 1 is one of the ligands, so those numbers must change in the revised text.

33.       Why did the Authors utilized different excitation wavelength for the excitation of the complexes 8-11 and compound 2 in solution and in the solid-state?

44.       In page 11, lines 368-371 the Authors state “All spectra showed vibronic structure with vibronic spacing about 1750 and 800 cm-1 due to”. Are these numbers (1750 and 800 cm-1) correct? Please check.

Author Response

Adjudicatory Reviewer

Dear Editor

The manuscript describes the synthesis of two new terpyridine ligand derivatives which where utilized to synthesize a number of ruthenium complexes which were characterized by spectroscopic means while their photophysical properties were investigated. The manuscript is well written and presented while the results merit publication. Therefore, I am glad to suggest publication after minor revision.

11. Given that the ligands utilized are neutral within the complexes, the oxidation state of the ruthenium in complexes 4-11 is (II) although the Authors began their reactions with starting materials of Ru(III). A comment of this will be helpful for the non-familiar to ruthenium chemistry readers.

Usually, Ru(II)-polypyridine complexes are formed using RuCl3 xH2O, as the salt of the metal, and the polypyridine ligand in a boiling solution of ethylene glycol. Under these conditions Ru(III) reduces to Ru(II) and is stabilized by the polypyridine ligands which are pi-acceptors and tend to stabilize metals in low oxidation states. On the other hand, a part of the solvent oxidizes in several products such as glyoxal, glycolaldehyde, etc. The following comment was added in the manuscript text (line 267) «In boiling ethylene glycol the Ru(III) was reduced to Ru(II) while oxidation species, such as glyoxal, glycolaldehyde were formed». 

22. In page 5, lines 207-209 the Authors might have mistakenly written “In a 207 typical experiment 50 mg of the corresponding parent complex (1) - (3) and (5) was transferred”. Compound 1 is one of the ligands, so those numbers must change in the revised text.

Indeed, there is a mistake at the numbering of complexes joining in the reaction which has been corrected.

33. Why did the Authors utilized different excitation wavelength for the excitation of the complexes 8-11 and compound 2 in solution and in the solid-state?

The excitation wavelengths result from the absorbance spectra which were recorded at solution and solid state, where λ_max (MLCT) were observed at 488 and 466 respectively. This phenomenon is commonly due to the presence of solvents [10.1039/c6ce00890a].

44. In page 11, lines 368-371 the Authors state “All spectra showed vibronic structure with vibronic spacing about 1750 and 800 cm-1 due to”. Are these numbers (1750 and 800 cm-1) correct? Please check.

The space, in cm^-1 , between the successive λ maxima of the spectra of the complexes, was calculated as 1750 and 800 cm^-1. This is normal for pyridine-carboxyl- ligands [10.1021/ic500267b]