Kartik Chandra Mondal

Coexistence of Distinct Single‐Ion and Exchange‐Based Mechanisms for Blocking of Magnetization in a CoII2DyIII2 Single‐Molecule Magnet

Two ways to relax: A defect-dicubane Co2Dy2 single-molecule magnet (SMM) displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a 3d–4f-based SMM. Analysis of the relaxation reveals two distinct blocking regimes, one of which is intraionic, localized on the DyIII ions, while the other is exchange-based.

A Stable Singlet Biradicaloid Siladicarbene: (L:)2Si

Silicon, the congener of carbon, frequently shows different chemistry than that of its sister element. It has been realized that silicon prefers a positive charge if bonded to a more electronegative atom. Compounds with silicon in lower oxidation states are very important, since they can activate small organic molecules which cannot be activated by transition metals. In 2008 Robinson et al. reported on the adduct of two N-heterocyclic carbene (NHC) molecules with disilicon A (Scheme 1). This unusual compound was prepared

Defect-Dicubane Ni2Ln2 (Ln = Dy, Tb) Single Molecule Magnets

Two pairs of Ni(2)Dy(2) and Ni(2)Tb(2) complexes, [Ni(2)Ln(2)(L)(4)(NO(3))(2)(DMF)(2)] {Ln = Dy (1), Tb (2)} and [Ni(2)Ln(2)(L)(4)(NO(3))(2)(MeOH)(2)]·3MeOH {Ln = Dy (3), Tb (4)} (H(2)L is the Schiff base resulting from the condensation of o-vanillin and 2-aminophenol) possessing a defect-dicubane core topology were synthesized and characterized. All four complexes are ferromagnetically coupled, and the two Dy-analogues are found to be Single Molecule Magnets (SMMs) with energy barriers in the range 18-28 K. Compound 1 displays step-like hysteresis loops, confirming the SMM behavior. Although 1 and 3 show very similar structural topologies, the dynamic properties of 1 and 3 are different with blocking temperatures (3.2 and 4.2 K at a frequency of 1500 Hz) differing by 1 K. This appears to result from a change in orientation of the nitrate ligands on the Dy(III) ions, induced by changes in ligands on Ni(II).

Acyclic Germylones: Congeners of Allenes with a Central Germanium Atom

The cyclic alkyl(amino) carbene (cAAC:)-stabilized acyclic germylones (Me2-cAAC:)2Ge (1) and (Cy2-cAAC:)2Ge (2) were prepared utilizing a one-pot synthesis of GeCl2(dioxane), cAAC:, and KC8 in a 1:2:2.1 molar ratio. Dark green crystals of compounds 1 and 2 were produced in 75 and 70% yields, respectively. The reported methods for the preparation of the corresponding silicon compounds turned out to be not applicable in the case of germanium. The single-crystal X-ray structures of 1 and 2 feature the C-Ge-C bent backbone, which possesses a three-center two-electron π-bond system. Compounds 1 and 2 are the first acyclic germylones containing each one germanium atom and two cAAC: molecules. EPR measurements on compounds 1 and 2 confirmed the singlet spin ground state. DFT calculations on 1/2 revealed that the singlet ground state is more stable by ~16 to 18 kcal mol(-1) than that of the triplet state. First and second proton affinity values were theoretically calculated to be of 265.8 (1)/267.1 (2) and 180.4 (1)/183.8 (2) kcal mol(-1), respectively. Further calculations, which were performed at different levels suggest a singlet diradicaloid character of 1 and 2. The TD-DFT calculations exhibit an absorption band at ~655 nm in n-hexane solution that originates from the diradicaloid character of germylones 1 and 2.

Three New Cu−Azido Polymers and Their Systematic Interconversion: Role of the Amount of the Blocking Amine on the Structural Diversity and Magnetic Behavior

Three new coordination polymers [Cu5(N3)10(en)2]n (1), [Cu6(N3)12(en)4]n (2), and [Cu4(N3)8(en)4]n (3) have been synthesized in a controlled manner by treatment of a 1:2 mixture of Cu(NO3)2 and NaN3 with varying amount of ethylenediamine (en). Single-crystal structure analyses clearly indicated that the puckered Cu4 biscubane unit in 1 gradually opens to a slightly more open Cu4 macrocyclic unit in 2 when more en approaches to the Cu4 core. Upon addition of further en, an open Cu4 linear secondary building unit was obtained in complex 3. Complex 1 contains four different kinds of bridging modes of the azide anion and is a complicated 3D polymer. Similarly, complexes 2 and 3 are 3D and 2D polymers, respectively, containing three different kinds of bridging azides. Complex 3 contains two very rare cis end-to-end (EE) and single-end-on (EO) azido modes. Structural transformation from 1 to 3 was monitored and explained qualitatively. Variable-temperature magnetic studies in the temperature range of 300-2 K reveal the existence of dominant ferromagnetic behavior in all the three cases with a metamagnetic-type behavior in complex 1 with the critical field of transition at 0.8 T. The purity of all the complexes were established by elemental analyses, as well as by the powder XRD patterns that matched well with the expected patterns from the single-crystal structure analysis.

Electronic Structure and Slow Magnetic Relaxation of Low-Coordinate Cyclic Alkyl(amino) Carbene Stabilized Iron(I) Complexes

Cyclic alkyl(amino) carbene stabilized two- and three-coordinate Fe(I) complexes, (cAAC)2FeCl (2) and [(cAAC)2Fe][B(C6F5)4] (3), respectively, were prepared and thoroughly studied by a bouquet of analytical techniques as well as theoretical calculations. Magnetic susceptibility and Mössbauer spectroscopy reveal the +1 oxidation state and S = 3/2 spin ground state of iron in both compounds. 2 and 3 show slow magnetic relaxation typical for single molecule magnets under an applied direct current magnetic field. The high-frequency EPR measurements confirm the S = 3/2 ground state with a large, positive zero-field splitting (∼20.4 cm(-1)) and reveal easy plane anisotropy for compound 2. CASSCF/CASPT2/RASSI-SO ab initio calculations using the MOLCAS program package support the experimental results.

Spin-Canting and Metamagnetic Behavior in a New Species from the Hydrothermal Co(II)-trans-3-Pyridylacrylate System

Hydrothermal reaction of CoCl(2) with trans-3-pyridylacrylic acid (3-pycaH) and Et(3)N results in the formation of a new three-dimensional network formulated as [{Co(4)(mu-H(2)O)(2)(3-pyca)(8)}(0.94){Co(5)(mu(3)-OH)(2)(3-pyca)(8)}(0.06)]. Magnetic measurements reveal that spin-canting and metamagnetic behavior coexist in this compound with a critical temperature at 9.5 K.

Cyclic Alkyl(amino) Carbene Stabilized Complexes with Low Coordinate Metals of Enduring Nature

N-Heterocyclic carbenes (NHCs) are known to stabilize some metal atoms in different oxidation states mostly by their strong σ-donation. After the successful syntheses of cyclic alkyl(amino) carbenes (cAACs), they have been proven to be much more effective in stabilizing electron rich species. In cAAC, one of the σ-withdrawing and π-donating nitrogen atoms of NHC is replaced by a σ-donating quaternary carbon atom leading to a lower lying LUMO. This makes the acceptance of π-back-donation from the element bound to the carbene carbon atom of cAAC energetically more advantageous. Further evidence suggests that the carbene carbon of cAAC can use the lone pair of electrons present on the adjacent nitrogen in a more controlled way depending on the accumulation of electron density on the bound metal. It has been found that cAAC can be utilized as excellent ligand for the stabilization of a complex with three coordinate metal center [(cAAC)2M(I)-Cl; M = Fe, Co, Cr]. Complex (cAAC)2M(II)Cl2 [M = Fe, Co, Cr] was prepared by reacting anhydrous M(II)Cl2 with two equiv of cAAC followed by treatment with one equiv of KC8 (reducing agent) to obtain (cAAC)2M(I)-Cl. The corresponding cation (cAAC)2M(+) was isolated when (cAAC)2M(I)-Cl was reacted with sodium-tetraarylborate (lithium) in toluene or fluorobenzene. The CV of cation (cAAC)2M(+) [M = Co, Fe] suggests that it can reversibly undergo one electron reduction. The cations of Co and Fe were reduced with Na(Hg) or KC8, respectively. (cAAC)2Co(I)Cl can be directly reduced to (cAAC)2Co(0) when reacted with one equiv of KC8. Analogous (cAAC·)2Zn(II) and (cAAC)2Mn complexes are prepared by reduction of (cAAC)MCl2 [M = Zn, Mn] with two equiv of KC8 in the presence of one equiv of cAAC. The square planar (cAAC)2NiCl2 complex was directly reduced by two equiv of LiN(iPr2) (KC8) to (cAAC)2Ni(0). The (cAAC)2Pd(0) and (cAAC)2Pt(0) complexes are prepared by substituting all four triphenylphosphines of (Ph3P)4M(0) [M = Pd, Pt] by two cAACs. Cation (cAAC)2M(+) [M = Cu, Au] was reduced with sodium/potassium to obtain the neutral analogue [(cAAC)2Cu, (cAAC)2Au]. Two coordinate Zn/Mn/Cu/Au are stabilized by two neutral carbene ligands possessing radical electrons on the carbene carbon atoms, while analogous complexes of Co/Fe/Ni/Pd/Pt contain metals in the zero oxidation state. The ground electronic structure of (cAAC)2M was thoroughly studied by theoretical calculations. In this Account, we summarize our developments in stabilizing metal complexes with low coordinate metal atoms in two, one, and most significantly in their zero oxidation states by utilizing cAACs as ligands.

C4 Cumulene and the Corresponding Air‐Stable Radical Cation and Dication

A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(·+) , and dication 1(2+) have been synthesized. The redox property of 1(·+) was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1(·+) is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis.

Easy Access to Silicon(0) and Silicon(II) Compounds

Two different synthetic methodologies of silicon dihalide bridged biradicals of the general formula (L(n)•)2SiX2 (n = 1, 2) have been developed. First, the metathesis reaction between NHC:SiX2 and L(n): (L(n): = cyclic akyl(amino) carbene in a 1:3 molar ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C-C bond formation. The formation of the coupled NHCs (L(m) = cyclic alkyl(amino) carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction between NHC:SiBr2 and L(n): when compared with that of NHC:SiCl2. Second, the reaction of L(1):SiCl4 (8) (L(1): =:C(CH2)(CMe2)2N-2,6-iPr2C6H3) with a non-nucleophilic base LiN(iPr)2 in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical (L(1)•)2SiCl2 (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 °C. The dibromide (6, 7) analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution. Decomposition of the products has been observed in the UV-vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated (L(n):)2Si(0) (n = 1 (9), 2 (10)) under KC8 reduction. Compounds 2 and 4 were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.