Research Interests

Main Group Chemistry

First stable singlet carbene (N-heterocyclic carbene; NHC) was isolated in 1991 and following years it was utilized as an efficient ligand in different areas of chemistry. The electronic property and the binding ability of carbene depend upon ring size, substituents and the presence of hetero-atoms adjacent to the carbene carbon atom. The singlet-triplet energy gap of a carbene changes with the variation of any of these above mentioned factors which have crucial effects on the sigma-donation and pi-backacceptance properties of a carbene. Till now, a plethora of carbenes have been reported. Each one can be utilized to serve different purposes of the chemists. For example NHCs are known as strong sigma-donor and poor pi-acceptor ligands. They can be employed as favorable ligands for stabilization of zero valence metal and metalloids. However, NHCs are not suitable for stabilizations of unstable radicals and electron rich metal atoms due lack of sufficient pi-backaccepting ability. One nitrogen containing carbene (cyclic (alkyl)(amino) carbene; cAAC) was reported in 2005. This cAAC was experimentally shown to be efficient ligand for activation and stabilization of white phosphorus in numerous allotropic forms. Afterwards, cAACs were employed as strong sigma-donating and pi-accepting ligands for the stabilization of borylene (:BH) and its radical cation. Moreover, cAAC was able to activate strong chemical bonds like H-H, Si-H and N-H. The cAAC coordinated Au(I)-Cl complex and its cation were shown to mediate unusual catalytic organic transformations.

cAAC ligands are able to stabilize several radicals, radical cations, diradicals and radicaloids of metalloids and metals having low coordination numbers and low oxidation states.1-2 cAACs can keep great control over its sigma-donation and pi-backacceptance properties in a fascinating manner. The balance between these two properties such that sometimes it is hard to distinguish the nature of bonds between a carbene carbon atom and an element (E). Several experimental and theoretical efforts are necessary to unambiguously state the true bonding picture. cAAC forms different types of bonds such as dative bond, electron sharing sigma bond and partial double bonds. The latter two types of bonds are favored specially when the nature of covalent character and low valency of an element are reached. cAAC prefers to form above mentioned three types of bonds depending upon the accumulation of electron density on silicon atom1 while mostly dative bonds are formed between cAAC and metal/germanium/tin.2 cAAC has been successfully employed as ligand where NHC is unable to stabilize several species. The replacement of NHC by cAAC can lead to drastic change in electronic structures and bonding scenarios.

Our major research objective will be to employ cAAC as a ligand to develop the chemistry of main group elements beyond the reach of NHC ligand. The key controlling factor will be its significance pi-backaccepting property as shown in Figure 1.

Figure 1. Stabilization of suspected inter-stellar Si3 cluster in the laboratory.3

References:

1. Kartik Chandra Mondal, Sudipta Roy, Herbert W. Roesky. Silicon Based Radical, Radical ion and Diradical. Chem. Soc. Rev. 2016, 45, 1080-1111.

2. Sudipta Roy, Kartik Chandra Mondal, Herbert W. Roesky. Cyclic Alkyl(amino) Carbene Stabilized Low Coordinate Metal Complexes of Enduring Nature. Acc. Chem. Res., 2016, 49, 357–369.

3. Kartik Chandra Mondal,* Sudipta Roy, Birger Dittrich,* Diego M. Andrada, Gernot Frenking,* Herbert W. Roesky*. A Triatomic Silicon(0) Cluster Stabilized by a Cyclic Alkyl(amino) Carbene. Angew. Chem. Int. Ed. 2016, 55, 3158-3161.

Metal-radical Based Single Molecule Magnets:

The molecules which act as magnet below their blocking temperature (Tb) are called single molecule magnets (SMMs). The reorientations of magnetization of SMMs slow down below this temperature (Tb) and below this temperature SMMs behave as classical magnets. The SMM property was first observed in famous Mn12-Ac coordination cluster which is the most studied class of molecular magnet studied so far. The action of magnetic anisotropy (D) with well defined ground state spin (S) of Mn12-Ac led to the high thermal energy barrier (Ueff) of relaxation of magnetization (presence of in-phase (χ´) and out-of-phase magnetic susceptibility (χ´´)) below 10 K. The origin of magnetic anisotropy is Jahn-Teller of MnIII (d4) ions. Mn12-Ac displays step like hysteresis loop (memory effect) below 3.5 K which is required to store or process data. Such molecule also exhibits several other properties such as quantum tunneling of magnetization and quantum phase interference which are the key properties needed for materials to function as quantum bits (qubits). Coordination compounds of mono nuclear DyIII ion most frequently display slow relaxation of magnetization due to large unquenched angular orbital momentum. TbIII analogue also displays the same property and often at higher temperature since it has perpendicular magnetic anisotropy (oblate spheroid) when compared with that of DyIII ion (proplate spheroid). It has been recently concluded that coordination compounds of anisotropic metal ions (such as Dy/Tb) are advantageous when they are coupled with radicals. Radicals can interact with the metal ions strongly leading to a so-called exchange bias which is capable of significantly enhancing the hysteresis. The origins of this effect can be understood by regarding the radical ligand as providing an additional, “internal” magnetic field-akin to the application of an external dc field that lifts the degeneracy of mJ pairs and significantly reduces the rate of QTM. [K(18-crown-6)(thf)2][TbIII2{N(SiMe3)2}4(thf)2(μ,η22-N2)] exhibit hysteresis loop at 14 K which is till now the highest temperature when compared with those of SMMs reported till date. The corresponding temperature of Dy-analogue was recorded to be 8 K. The sticking point of these two SMMs is that TbIII or DyIII ions are magnetically coupled very strongly by trianionic N23- radical anion, hindering zero-field fast relaxation pathways, forcing thermally activated relaxation behavior over a much broader temperature range.

Our research goal is to synthesize low nuclearity lanthanide-radical based SMMs taking the advantage of strong magnetic interactions between lanthanide ions and radicals (Figure 2). Additionally, the blocking temperature of 3d-4f based SMM4 will be increase by introducing radical containing bridges.

Figure 2. Lanthanide ions chelated by radical ligands in cubic (hypothetical, left, φ = 0) and square antiprism (right, φ ≠ 0; ۞) coordination geometry. The red arrow indicates radical center and blue arrow shows anisotropic easy axis of lanthanide ion. Hysteresis loop will confirm the SMM behavior.

Reference:

4. Kartik Chandra Mondal, Alexander Sundt, Yanhua Lan, George E. Kostakis, Oliver Waldmann, Liviu Ungur, Liviu F. Chibotaru, Christopher E. Anson, Annie K. Powell. Defect-Coexistence of Distinct Single-Ion and Exchange-Based Mechanisms for Blocking of Magnetization in a CoII2DyIII2 Single-Molecule Magnet. Angew. Chem. Int. Ed. 2012, 51, 7550 –7554.