• Goutam Kumar Lahiri

Articles written in Journal of Chemical Sciences

• Metal ion-mediated selective activations of C-H and C-Cl bonds. Direct aromatic thiolation reactions via C-S bond cleavage of dithioacids

The reactions of potassium salt of dithiocarbonate, R′OCS2K,4 (R′=Me, Et,nPr,nBu,iPr,iBu, -CH2Ph) with the low-spinctc-RuII(L)2Cl21,ctc-OsII(L)2Br22 andmer-[CoII(L)3](ClO4)2·H2O3 [L=2-(arylazo)pyridine, NC5H4-N=N-C6H4(R), R=H,o-Me/Cl,m-Me/Cl,p-Me/Cl;ctc: cis-trans-cis with respect to halides, pyridine and azo nitrogens respectively) in boiling dimethylformamide solvent resulted in low-spin diamagnetic RuII(L′)2,5, OsII(L′)26 and [CoIII(L′)2]ClO47 respectively (L′=o-S-C6H3(R)N=NC5H4N). In the complexes5, 6 and7 ortho carbon-hydrogen bond of the pendant phenyl ring of the ligands (L′) has been selectively and directly thiolated via the carbon—sulphur bond cleavage of4. The newly formed tridenate thiolated ligands (L′) are bound to the metal ion in a meridional fashion. In the case of cobalt complex (7), during the activation process the bivalent cobalt ion in the starting complex3 has been oxidised to the trivalent CoIII state. The reactions are highly sensitive to the nature and the location of the substituents present in the active phenyl ring. The presence of electron donating Me group at the ortho and para positions of the pendant phenyl ring with respect to the activation points can only facilitate the thiolation process. The complexes1c, 2c and3c) having chloride group at the ortho position of the active phenyl ring underwent the thiolation reaction selectively via the carbon—chloride bond activation process. The rate of carbon—chloride activation process has been found to be much faster compared to the C-H bond activation. The reactions are sensitive to the nature of the solvent used, taking place only in those having high boiling and polar solvents. The rate of the reactions is also dependent on the nature of the R′ group present in4, following the order: Me∼ Et&gt;nPr&gt;nBu&gt;iPr&gt;iBu≫-CH2Ph. The molecular geometry of the complexes in solution has been established by1H and13C NMR spectroscopy. The thiolated complexes (5, 6, 7) exhibit metal to ligand charge-transfer transitions in the visible region and intraligandπ-π* andn-π* transitions in the UV region. In acetonitrile solution the complexes display reversible MIII⇄MII reductions at 0.43 V for Ru (5a), 0.36 V for Os (6a) and −0.13 V for Co (7a) vs saturated calomel electrode (SCE).

• Bridging function mediated intermetallic coupling in diruthenium-bis(bipyridine) complexes

The interactions of potentially dinucleating bridging functionalities (I–VI) with the ruthenium-bis(bypyridine) precursor [RuII(bpy)2(EtOH)2]2+have been explored. The bridging functionsI,II andVI directly result in the expected dinuclear complexes of the type [(bpy)2RuIILnRuII(bpy)2]z+ (1,2,7 and 8) (n = 0,z =4 andn = -2,z = 2). The bridging ligandIII undergoes N-N or N-C bond cleavage reaction on coordination to the RuII(bpy)2 core which eventually yields a mononuclear complex of the type [(bpy)2RuII(L)]+,3, where L =-OC6H3(R)C(R′)=N-H. However, the electrogenerated mononuclear ruthenium(III) congener, 3+in acetonitrile dimerises to [(bpy)2RuIII {-OC6H3(R)C(R′)=N-N=(R′)C(R)C6H3O-}RuIII(bpy)2]4+ (4). In the presence of a slight amount of water content in the acetonitrile solvent the dimeric species (4) reduces back to the starting ruthenium(II) monomer (3). The preformed bridging ligandIV undergoes multiple transformations on coordination to the Ru(bpy)2 core, such as hydrolysis of the imine groups ofIV followed by intermolecular head-to-tail oxidative coupling of the resultant amino phenol moieties, which in turn results in a new class of dimeric complex of the type [(bpy)2RuII-OC6H4-N=C6H3(=NH)O-RuII(bpy)2]2+ (5). In5, the bridging ligand comprises of twoN,O chelating binding sites each formally in the semiquinone level and there is ap-benzoquinonediimine bridge between the metal centres. In complex6, the preformed bridging ligand, 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2-dihydro-1,2,4,5-tetrazine, H2L (V) undergoes oxidative dehydrogenation to aromatic tetrazine based bridging unit, 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, L. The detailed spectroelectrochemical aspects of the complexes have been studied in order to understand the role of the bridging units towards the intermetallic electronic coupling in the dinuclear complexes.

• Tuning intermetallic electronic coupling in polyruthenium systems via molecular architecture

A large number of polynuclear ruthenium complexes encompassing selective combinations of spacer (bridging ligand,BL) and ancillary (AL) functionalities have been designed. The extent of intermetallic electronic communication in mixed-valent states and the efficacy of the ligand frameworks towards the tuning of coupling processes have been scrutinised via structural, spectroelectrochemical, EPR, magnetic and theoretical investigations. Moreover, the sensitive oxidation state features in the complexes of non-innocent quinonoid bridging moieties have also been addressed.

• A water soluble heteropolyoxotungstate as a selective, efficient and environment friendly oxidation catalyst

A series of water soluble Keggin type heteropolyoxotungstates have been tested as oxidation catalysts in aqueous-biphasic media with dilute H2O2 (30%) as the oxygen atom donor, without using any phase transfer agent. The Zn substituted polyoxoanion {(NH4)7Zn0.5[𝛼-ZnO4W11O30ZnO5(OH2)]$.n$H2O} has been found to be the most efficient catalyst, which oxidizes a wide range of organic functionalities with good turnovers and high selectivities. The functionalities that undergo oxidations are: organic sulfides, pyridines, anilines, benzyl alcohols and benzyl halides. The oxidations of sulfides to sulfoxides and/or sulfones have been studied in detail, and a simple kinetic model consisting of two consecutive reactions, is shown to give good fit with the experimental data. In the catalytic system described here product isolation is easy, and the aqueous catalyst solution can be re-used several times with little loss in its efficiency.

• Electronic structures of ruthenium complexes encircling non-innocent ligand assembly

Electronic structural forms of selected mononuclear and dinuclear ruthenium complexes encompassing redox non-innocent terminal as well as bridging ligands have been addressed. The sensitive valence and spin situations of the complexes have been established in the native and accessible redox states via detailed analysis of their crystal structures, electrochemistry, UV/VIS/NIR spectroelectrochemistry, EPR signatures at the paramagnetic states and DFT calculations. Mononuclear complexes exhibit significant variations in valence and spin distribution processes based on the simple modification of the non-innocent ligand frameworks as well as electronic nature of the co-ligands, 𝜎-donating or 𝜋-accepting. Dinuclear complexes with modified pyrazine, 𝑝-quinone and azo-derived redox-active bridging ligands show complex features including redoxinduced electron-transfer (RIET), remote metal to metal spin-interaction in a three-spin metal-bridge-metal arrangement as well as electron-transfer driven chemical transformation (EC).

• Manganese-salen catalyzed oxidative benzylic chlorination

Metalloporphyrins are well-known to serve as the model for mimicking reactivities exhibited by cytochrome P450 hydroxylase. Recent developments on selective C–H halogenation using Mn-porphyrins provided the way for understanding the reactivity as well as mechanism of different halogenase enzymes. In this report, we demonstrated a method for benzylic C–H chlorination using easily prepared Mn(salen) complex as the catalyst, which shows a complementary reactivity of Mn-porphyrins. Here, NaOCl has been used as a chlorinating source as well as the oxidant. Efforts towards understanding the mechanism suggested the formation of the high-valent Mn(V)=O species which is believed to be the key intermediate to conduct this transformation.

• # Journal of Chemical Sciences

Volume 135, 2023
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019