Lakshmi Nilakantan's profile, University of Missouri, Columbia

Article

Commented on Facile Photoinduced Generation of Hydroxyl Radical on a Nitrocellulose Membrane Surface and its Application in the Degradation of Organic Pollutants

This is an interesting paper where hydroxyl anions generated by irradiation of nitrocellulose membrane (NCM), were used to photodegrade organic pollutants such as thiamphenicol (TAP) and bisphenol (BPA). The highlight of this study is that hydroxyl anions are produced exclusively by the membrane without major side products/competitors, by relatively inexpensive process. The ESR, HPLC and FTIR techniques thoroughly characterized the generated radicals and their practical applications on organic pollutants are well studied. A series of experiments were done to prove that hydroxyl radical generation was dominated by UVB region (280-320nm) and not UVA (320-400 nm). It was proposed that NCM contains nitrate esters that upon irradiation (280-400 nm) could produce nitrate and nitrite which in turn could generate hydroxyl radicals. The authors established that nitrate ions dominates the generation of hydroxyl radicals and the electrostatic interaction between NCM surface and the pollutants played an important role. A reasoning on the increase in hydroxyl generation rate obtained from the kinetic studies between first 60 min and the next 60 min would be useful for better understanding of the mechanism. Further experiments on testing the efficiency of NCM on the degradation of TAP and BPA at different pH as well as in the presence of common inorganic salts such as NaCl and sodium sulphate would be helpful. This are important in the practical point of view where the majority of effluents from industries be of either acidic or alkaline in nature and can eliminate the preliminary treatments before progressing for degradation of organic pollutants. Experimenting a continuous column mode operation using fixed bed NCM membranes and a UVB light source would be beneficial and can take this research into practical industrial level wastewater treatment.

Article

Commented on Dehydration of an Insoluble Urea Byproduct Enables the Condensation of DCC and Malonic Acid in Flow

The authors made a valiant attempt to convert the insoluble dicyclohexyl urea (DCU) into a soluble form, formed during the condensation of DCC and malonic acid in continuous flow conditions. The authors were successful in converting the insoluble DCU into soluble salt 5 by using POCl3. However, conversion of 5 to dicylohexylcarbodiimide was not effective in the presence of triethylamine. The proposed structure of 5 is debatable and raised several questions. 1H NMR and 31P NMR were used to support the structure of 5. When excess of equivalent of POCl3 was used, 31P NMR is expected to show the corresponding signal for the unreacted POCl3. However only one signal corresponding to the presumed structure of 5 was observed and the signal for excess POCl3 was absent. For the same chemistry, a patent (JPH10330344) proposed alternative salt structure for 5, where DCU and the salt exist in equilibrium. While 1HMR will remain the same for both proposed structures, 31P NMR of the present study is consistent with the proposed structure of patent with the observation of change in chemical shift, consistent with the equilibrium proposed. Further, assignment of 1664 cm-1 for C=N bond can well fit into the C=O functionality as C=N of DCC shows at 2100 cm-1 (https://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_disp.cgi?sdbsno=2412&spectrum_type=IR&fname=NIDA3700). It would be better to use 13C NMR and 17O NMR to conclusive establish the structure of 5 as it has C-Cl bond where as the patent proposed structure had C-O functionality. It also would be better to acknowledge the patent in this work. Scheme 2b and scheme 3 are the same and repetition could have been avoided. In scheme 2a, annotation of XY would be helpful.

Article

Commented on Oxidative Addition to Gold(I) by Photoredox Catalysis: Straightforward Access to Diverse (C ,N )-Cyclometalated Gold(III) Complexes

This is an interesting paper where the authors have performed oxidative addition of diazomium salts to Au(I) complex to yield gold (III) complexes. Another significant aspect about this study is the use of visible light to carry out the oxidative addition in the presence of a photocatalyst. These types of cationic gold (III) complexes are expected to act as lewis acid catalysts in organic transformations. All the complexes were thoroughly characterized by XRD and NMR spectroscopy. The proposed photocatalytic oxidative addition radical mechanism (Scheme 3) is interesting and judicious for these complexes. To illustrate involvement of radical mechanism, a control reaction using TEMPO was done (Supporting information 3.2). A few lines about this observation would be worthy in the mechanistic proposal. In Scheme 1c, notation of “C-(Aryl)-X addition to Au(I) complex” is not convincing as the chloride is already coordinated to Au(I). Instead, it would be clear/easy on readers to mention “C-(Aryl) addition”. Scheme 1c, 2 and chemdraw figure (Table 1) refer to the same reaction, hence repetitions can be avoided. Expansion for Table 1 acronyms would be ideal for novice readers and structural elucidation of complexes for improved understanding. Citation of few literature's regarding the synthesis of (C,C) cyclometalated gold(III) species would be ideal and might be interesting on the readers. In these references, biphenyl was used as a reactant in contrast to aryldiazomium salts. 1. Usón, R.; Vicente, J.; Cirac, J. A.; Chicote, M. T. J. Organomet.Chem. 1980, 198, 105−112. 2. Dyadchenko, V. P.; Krasik, P. E.; Grandberg, K. I.; Kuz’mina, L.G.; Dvortsova, N. V.; Porai-Koshits, M. A.; Perevalova, E. G.Metalloorg. Khim. 1990, 3, 1260−1265 3. David, B.; Monkowius, U.; Rust, J.; Lehmann, C. W.; Hyzak, L.; Mohr, F. Dalton Trans. 2014, 43, 11059−11066. 4. Nilakantan, L.; Sharp, P.; McMillin, D.R. Organometallics 2016, 35, 2339-2347

Article

Commented on B12 H12 2- -Based Metal (Cu2+ , Ni2+ , Zn2+ ) Complexes as Hypergolic Fuels with Superior Hypergolicity

Trials of (B12H12)2- based metal (Cu2+, Ni2+, Zn2+) complexes as hypergolic fuels are good targets towards achieving stable hypergolic propellants with short ignition delay time. Characterization of complexes by IR, elemental analysis and X-ray were suffice to establish their structure. However it would be better to augment the molecular characterization by 1H NMR, 13C NMR and 11B NMR techniques which are common and essential nowadays. Decaborane is one of the important member in the borane family for its effectiveness as hypergolic propellants. Robin Rogers group has done a significant amount of work in that direction (ACS Omega 2018, 3, 8491-8496, Inorg. Chem. 2014, 53, 4770−4776). Referencing these in the introduction would give a holistic picture on the application of borane family as hypergolic propellants. Choice of earth abundant metals Cu2+, Ni2+, Zn2+ is a good idea. An explanation on why only these three metals and not metals such as Al, would be beneficial. The hypergolic tests recorded by high-speed camera established that the reported complexes had indeed shorter ignition delay time when compared with the conventional ones. Analysis and quantification of remnants after the hypergolic test will be helpful as the reported complexes have metals in it, which will remain with the residue. In this aspect, these complexes are in contrast with the organic hypergolic propellants such as hydrazine where most of its goes into vapor form, upon ignition.

Article

Commented on Insertion of Molecular Oxygen into the Metal–Methyl Bonds of Platinum(II) and Palladium(II) 1,3-Bis(2-pyridylimino)isoindolate Complexes

Conversion of methane into methanol using an inexpensive oxidant “oxygen” under mild reaction conditions is indeed a holy grail in chemistry. Dr. Goldberg group’s continuous effort in that direction using pyridine based pincer complexes of Pt and Pd is a welcoming step. The authors rightly recognized Dr. Britovsek’s contribution in this arena. Choice of 1,3-bis(2-pyridylimino)isoindole (BPI) ligand for this purpose was smart as their stability towards oxygen was a proven fact. Characterization of Pt and Pd complexes of BPI as well as their O2 reaction products were thorough. Well-defined kinetic studies and identification of reaction chemistry products duly supported the radical reaction pathway. It would be interesting to explore whether the reported metal complexes are photolytically active. This in turn will open up the possibility of alkane activation by the same complexes upon irradiation to form Pt-Me bond (Chem. Rev. 2005, 105, 2471-2526). A combination of alkane activation combined with controlled oxidation will be an ideal pathway to achieve the intended target of converting methane to methanol. Use of oxygen is environmental friendly. However, batch reactions and pressure reactors are not effective towards commercialization, considering the huge volume requirement and inexpensive nature of methanol. In this regard, exploration of viable alternatives using H2O2 and continuous process using the same molecular catalysts embedded in supported systems such as alumina or zeolite would be a good idea. Use of H2O2 is industrially practiced as well as greener, as water is the coproduct.

Article

Commented on Fluorescent antitumor titanium(iv) salen complexes for cell imaging

The author’s choice of titanium salen complexes over the existing platinum complexes for tumor cell imaging is an innovative one considering the challenges involved in the solubility and stability of titanium complexes in biological environments. Two new water soluble Ti(IV) salen complexes were reported and confocal fluorescence microscopy was used to study their bio-distribution. Characterization of all complexes were thorough and well done. In scheme 1, R= CN to represent complex 1b was fine. However denotation of R=C4H4 for complex 2b is not convincing as both R denotes one phenyl ring. Though X-ray structure clears this ambiguity, it would be better to give the entire structure of 2b for more clarity. Instead of using the general terminology “spectra” in the text for representing NMR and mass spectroscopy, it would be better to mention as NMR spectra, Mass spectra for better following up of the paper. As the complexes are already labelled as a,b,c, a different notation on confocal fluorescence figures would have been better for ease of understanding. A few sentences about why these complexes have low IC50 values would be beneficial the novice readers. Though the molecule forms dimer in the aqueous medium due to the labile axial phenyl ligand, intact of titanium metal center with salen moiety in the dimer confirmed that the reported molecules are indeed potential candidates for cancer treatment. The Ti(IV) complexes are generally fluorescent inactive due to d0 configuration of Ti(IV) complexes. Nonetheless, these Ti(IV) salen complexes showed red and green–yellow emission. A separate detailed spectroscopic investigation on their photo chemical properties supported by DFT analysis would be a photo chemist’s dream.

Article

Commented on Metal-Free and Alkali-Metal-Catalyzed Synthesis of Isoureas from Alcohols and Carbodiimides

The authors added another feather to the cap of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) which is widely used as an organic base for variety of organic transformations, by utilizing it as a catalyst for the synthesis of isoureas from carbodiimides and alcohols. It is noteworthy to mention that TBD is a product of DCC itself (US 9,108,968). Increasing the bulkiness of alcohol from methanol to isopropyl alcohol decreased the yield suggesting that the steric plays an important role. It would be better if the authors would have done the reaction with bulkier analogs of DIC such as dicyclohexyl carbodimide (DCC) and diisopropylphenyl carbodiimide (DIPC) and compared the results with DIC to reconfirm the steric effects. DCC and DIPC are commercially available and are relatively easy to use when compared with DIC. Longer reaction times gave better yields of isoureas. Using excess of alcohols instead of longer reaction times could also have same impact and the alcohols can be easily removed by evaporation. Characterization of reaction intermediates and use of DFT for predicting the reaction pathway were well done. Kinetic studies such as deuterium isotope effects using MeOH/MeOD and determination of rates from pseudo first order reactions such as using excess of alcohol would further help to find the rate determining step as well as support the DFT predicted reaction pathway. In the experimental point of view, information on safe handling of DIC in the experimental section would be helpful as these compounds are notorious as eye irritants (C&EN 1990, November, Page 2).

Article

Commented on Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

Ease of ligand synthesis coupled with utilization of non-precious metals such as Co and Ni for catalysis is a good acumen towards commercialization. In this regard, Dr. Balaraman group’s effort towards the use of Co based phosphine free pincer ligand for acceptorless dehydrogenative coupling of aminoalcohols with alcohols is a welcoming step. Reported SNS-cobalt(II) pincer complexes were well characterized and a wide range of pyrrole, pyridine and pyrazine derivatives were made by using the same catalysts. The authors proposed a mechanism in the supporting information based on the previous literature. However, a brief discussion in the main manuscript on the mechanism would also be helpful for the readers. Interestingly, the reaction and the proposed mechanism raises intriguing questions. (i) Authors mentioned "reflux" for the reaction temperature in the text. However, in supporting information, the reaction temperature was 180 C which was 40 C more than the boiling point of xylene and a pressure vessel was used for the reaction. Mentioning of these details in the scheme 2 would be helpful. It would be better for the readers to have the details of pressure vessel used, as obtaining the temperature of 180 C with toluene and THF in common type of ace pressure vessels, which can withstand only maximum of 3 Kg pressure, would be challenging. (ii) Xylene gave best yield of 77% followed by n-octane, toluene and THF, which did not follow the polarity order of the solvent. An insight on this would be helpful (iii) Use of Argon atmosphere for the catalytic reactions is intriguing as the catalysts are stable in air. It would be interesting to compare whether the reaction happens in atmospheric conditions also. (iv) Use of mercury test to verify that the reaction followed a homogeneous catalytic pathway, was a good idea. However, for such confirmation in general, the rate of reaction needs to be compared with and without mercury. A detailed study on the kinetics, characterization of intermediates and DFT analysis on these reactions to establish the most probable mechanism would be very interesting.

Article

Commented on Function-Integrated Ru Catalyst for Photochemical CO2 Reduction

The author’s work on CO2 capturing and converting into HCOOH by ruthenium complexes is a welcoming research considering the impact of greenhouse emission on the global warming and the resulting climate change. Activating the CO2 is the major energy consuming step in the CO2 capture which often offset the benefit of converting it. In majority of cases, electrochemical conversion of CO2 is the favored option. Here the authors judiciously used the photolytic property of terpyridine moiety to activate the molecule with the energy from a light source and then activated the CO2 molecule by the traditional Ru metal chemistry. The mechanistic investigation of CO2 activation showed that increasing the water quantity lowered the catalytic performance of Ru catalyst. This can be attributed to the stronger coordination ability of water in comparison with DMA, which inhibits the replacement of solvent by CO2 in their catalytic cycle (Scheme 1) leading to the reduction in catalytic efficiency. It would be a better idea to establish the trans influence of phosphine ligand by comparing the Ru-C(O) bond length with complexes not having it in the literature. It would be easy on the readers to have the S7 figure (UV-Vis spectra of the complex 1 with BIH and subsequently with CO2) in the text instead of Figure 4 (mass spectra) for easy follow up to understand the mechanism. A few words about nonsensitized and sensitized photocatalysts followed by literature references would be good on novice readers in this field. It is worthy to note that these kind of dual active (photo+catalytic) systems can be extended to many catalytic applications such as C-H activation, water splitting, and acceptorless dehydrogenation.

Article

Commented on Improved Synthesis of [CpRRhCl2]2 Complexes

Report on the improved synthesis of [CpRRhCl2]2 complexes is an useful one for producing them in a commercial scale as this methodology offers good yield in shorter reaction time. The authors asserted that changing the solvent from methanol/ethanol to isopropyl alcohol reduced the reaction time significantly and gave better or equivalent yields. Interestingly, table 2 data shows improvement only in [Cp*3iprRhCl2]2 in yield and others remained more or less the same. The reduction in reaction time can be attributed to the increase in reaction temperature as the boiling point of isopropyl alcohol is higher than ethanol at which the reaction takes place. It would be better to explain/find the reasoning of variation in yields 0-35% for [Cp*3iprRhCl2]2 and 21-47 % for [Cp*tBuRhCl2]2when ethanol was used as a solvent. For example, experiments such as controlling the water content in ethanol would be helpful to identify the cause. Ambient atmospheric reaction conditions are always beneficial as the reaction described in this paper can be carried out in bench top. However, experiments comparing yields between ambient and inert atmospheric/degassed conditions would be helpful to make sure that the presence of oxygen does not have an impact.

Article

Commented on Rhenium and Manganese Complexes Bearing Amino-Bis(phosphinite) Ligands: Synthesis, Characterization, and Catalytic Activity in Hydrogenation of Ketones

The use of rhenium complex of amino bisphosphenite ligand for the heterolytic cleavage of dihydrogen by metal ligand cooperative mechanism is interesting. Characterization of new rhenium complexes was through and wide range of ketones were trialed for the catalysis. NMR analysis of intermediates, in establishing the mechanism of reduction of ketones was well done and supported the proposed mechanism. It is note-worthy to mention the contribution of Abu-Omar’s group in PNN pincer rhenium chemistry in the introduction (Organometallics 2017, 36(9), 1688, Organometallics 2014, 33(7), 1672, Angew. Chem. Int. Ed. 2014, 53(32), 8320). The lower yields of ortho and para methoxy substituted alcohols (76% and 55%) were intriguing. It would be interesting to find out whether there was any demethoxylation took place, as a base was used in the reaction. 2 bar pressure was sufficient for the quantitative reaction of metal complex with hydrogen in the mechanistic study in NMR tube. However 50 bar pressure was used for the catalytic reactions in autoclave. Author’s insight on this large difference in H2 pressure would be helpful. In addition, it would be interesting to explore whether there is occurrence of ring flipping in the reported metal complexes by Variable-temperature Dynamic NMR spectroscopy (DNMR) as pincer metal complexes are known to exhibit that property (J. Organomet. Chem. 2017, 843, 62). Expanding the acronym of KHMDS as potassium bis(trimethylsilyl)amide would be easy on the readers for easy follow up.

Journal

Commented on JOURNAL OF ORGANOMETALLIC CHEMISTRY

I had a pleasant experience with my publication in Journal of Organometallic Chemistry (JOMC) (2017, 844, 30-34). I was a student and was the corresponding author in that paper. The reviewers were very quick and got back to us in exactly two weeks from the date of submission. Both reviewers were positive on the chemistry front with useful comments. It was exciting and surprising with such quick reviewing process. Another significant aspect was, when we had difficulties in submitting the CIF files for the final publication I had contacted the customer support center for help. They tried their best however in vain; they suggested e-mailing the editor. The editor’s response surprised me; he got back in just twenty minutes to our request, and helped in successful submission of CIF files to complete the publication process. Based on the above personal experience, I am looking forward publishing my future papers in JOMC.

Article

Commented on Low-symmetry phthalocyanine cobalt bis(dicarbollide) conjugate for hydrogen reduction

This authors connected a zinc phthalocyanine with cobalt dicarbollide with an ethereal linkage (compound 4) and studied its hydrogen evolution performance (HER). Characterization of the targeted compound 4 was through without ambiguity, confirming its structure. The authors established that compound 4 gave better HER performance than its cobalt dicarbollide absent precursor 3. However, the reasoning for improved performance was not convincing with the statement of “extending the redox richness of the MPcs”. Cobalt dicarbollide is separated from the PC by non conjugated ethereal linkage, thereby eliminating its electronic influence. This also consistent with the similar uv-vis absorption bands of 4 and 3 except the addition of band corresponding to cobalt dicarbollide in 4. Therefore, the exact reason for better HER performance of 4 over 3 is not known. Also it will be intriguing to analyze the HER performance of cobalt dicarbollide used here and compare with 4 to check the contribution of cobalt dicarbollide. Following are few other minor observations in the paper. The authors can use the terminology “11B decoupled 1H NMR” instead of “11B NMR decoupling”. Mentioning of sharpening of cage C-H protons signals in the 11B decoupled 1H NMR in comparison with 11B coupled 1H NMR for compound 4 will be helpful (Figure 1). Providing 11B NMR information in the experimental section in addition to the other characterization details will also helpful.

Article

Commented on Syntheses, Crystal Structures, and Photoluminescence of a Series of Iridium(III) Complexes Containing the Pentafluorosulfanyl Group

The authors nicely described the synthesis of a series of Iridium complexes having SF5 groups and studied their photo-physical properties, metal deduction capabilities, and supported their findings by DFT analysis. Their choice of ligands to get from red to blue emissions is commendable. Suggestions with the terminologies used: high reaction yields (high yields), peaking from (lamda max/highest intensity signals), dimmers (dimers) suppress the accumulation of molecules (suppress the agglomeration/aggregation). Scheme 1 and Scheme 2 mentions a,b c etc to represent the reaction conditions and it would be better to mention what they are, as a note below the schemes for better following up of the paper. In the ORTEP diagrams, it would be better to represent the atoms by its symbols such as Ir, F etc for easy follow up of the structure. Since The DFT pictures do not differ much within the group, representation of one from each group with bigger picture would give better clarity, with the rest given in supporting information. In the experimental part, details such as solvents used for column chromatography separation, vacuum sublimation temperature would be ideal for reproducibility by other researchers.

Article

Commented on Palladium catalyzed/counter ion tuned selective methylation of o-carboranes

This paper nicely described the selective methylation of carborane cage via palladium catalyzed alkylation reactions using MeI in the presence of AgCOOCH3 or AgOTf. Choice of 9-benzamide-o-carborane was a good one to activate the specific cage borons (B4 and B12) as it is a strong electron withdrawing group facilitating the difference in electron deficiency between the cage boron atom. It is known that the boron cage is delocalized analogous to benzene. Wide range of substituted benzamides were used to support and broaden the conclusions. While direct methylation of cage borons are desirable, it would be beneficial to inform the readers that selective functionalization of cage borons were also reported starting with decaborane, such as 9-iodo carboranes, 8-iodocarboranes (Inorg. Chem. 2012, 51, 4, 2629-2637). The authors reported that the yields were ~50% in majority of examples. It is intriguing what happened to the rest: are there any formation of nido carboranes as side products, as carboranes are susceptible to form their nido analogs in the presence of a base. Table 2 and 3 can be presented better by just giving R= amide group with yields for easy following up of the paper instead of giving the complete structure. The selectivity between AgOTf or AgCOOCH3 is intriguing. However, the author’s explanation of difference in the electron withdrawing ability between acetate and triflates was not convincing as the triflate should have given them a mixture of both B4 and B12 substitutions rather than selective towards B12. Also it would be better if they could have done experiments starting with B12 substituted materials to place Me group on the B4 position parallel to scheme 2 using AgCOOCH3.