Manabendra Ray

Magneto–structural studies of monohydroxo-bridged dicopper(II) complexes M[Cu2L2(OH)]·2H2O (M=Na+ (1) and K+ (2); H2L=2,6-bis[N-(phenyl)carbamoyl]pyridine). Effect of CuOHCu bridge angle on antiferromagnetic coupling

Abstract Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H2L), in its deprotonated form, two new monohydroxo-bridged dicopper(II) complexes M[Cu2L2(OH)]·2H2O (M=Na+ (1) and K+ (2)) have been prepared and characterised by a number of methods, including X-ray crystallography. Each copper(II) ion is terminally coordinated by one pyridyl and two amide nitrogen donors. The two copper(II) centres are bridged by a hydroxo group, with each copper(II) centre assuming a distorted square planar geometry. The observation of short CuNpy and long CuNamide bonds is caused by the steric requirement of the ligand. Interestingly, each cation Na+/K+ is coordinated to four different [Cu2L2(OH)]− units through the amide O-donors, in an uncommon distorted tetrahedral coordination environment. Temperature-dependent magnetic susceptibility measurements revealed that the compounds have S=0 ground state with singlet–triplet energy separation, 2J=−334 and −296 cm−1 for 1 and 2, respectively. The larger CuOHCu bridge angle in 1 (131.1(6)°) causes better antiferromagnetic exchange coupling than that in 2 (125.7(6)°).

Cobalt(III) complexes using in-plane tetradentate pyridinecarboxamide ligands and two monodentate axial ligands: Spectroelectrochemical correlation

Abstract A series of diamagnetic cobalt(III) complexes of bpb and bpc ligands [H 2 bpb = 1,2-bis(2-pyridinecarboxamido)benzene; H 2 bpc = 4,5-dichloro-1,2-bis(2-pyridinecarboxamido) benzene] with axial ligands (Cl − , N 3 − , SCN − , NO 2 − or MeCO 2 − ) has been synthesized. The trans geometry has been shown by 1 H NMR spectroscopy. The brown or green crystalline complexes display dominant ligand-to-metal charge-transfer transitions at ca 400 nm, while in the low-energy region ligand field transitions are observed. From an analysis of the d-d transition(s) ligand field parameters of the in-plane and axial ligands have been determined. In acetonitrile solution the complexes exhibit an irreversible Co III -Co II couple [ E pc −0.42 to −1.18 V vs saturated calomel electrode (S.C.E.)] and a quasi-reversible Co II -Co I couple ( E f −1.12 to − 1.27 V vs S.C.E.). When X = SCN − this couple is irreversible ( E pc = −1.40 V vs S.C.E.). These complexes display an additional quasi-reversible oxidative response (0.69-0.92 V vs S.C.E.) of primarily ligand oxidation origin. When X = NO 2 − and SCN − this couple is irreversible [ E pa = 0.88 V (NO 2 − ); E pa = 1.00 V (SCN − )]. A linear spectroelectrochemical correlation has been obtained between the ligand field strength of the axial ligands and the cathodic peak potential for the Co III -Co II couple.

Three Point Chiral Recognition and Resolution of Amino Alcohols Through Well-Defined Interaction Inside a Metallocavity

Recognition and separation of one enantiomer over its mirror image is important because of the different biochemical activity shown by chiral compounds. For example, a-limonene and linalool have a different smell depending on the chirality and one enantiomer of the antidepressant drug methylphenidate is 13 times more potent than its isomer. Ideally, recognition of an enantiomer requires recognition of three out of four different groups around an sp-hybridised carbon centre. Recognition of the groups in biological receptors often uses non-covalent interactions as observed in the case of adrenaline receptors. The involvement of hydrogen bonds and other weak interactions in the recognition process has the advantage of easy recovery of the guest, but the lability of the guest makes the isolation of the host– guest complex more difficult. Recognition and separation of enantiomers by using synthesised hosts has been approached from diverse angles, including a few that use a rigid metal–organic framework (MOF). Although separation by using a chiral adsorbent as the stationary phase has progressed substantially, the understanding of recognition at the molecular level is limited to molecular modelling due to the difficulty in structural characterisation of large organic or MOF hosts. For example, Kim and co-workers used structurally characterised porous channels of a chiral metal complex as a host to enhance the stereoselectivity of an organic reaction, but structural characterisation with the adduct was not possible. A low molecular weight rigid host would, in principle, facilitate structural characterisation, but it is challenging to accommodate three different recognition sites within a small host. Chin and co-workers, by using a chiral Co complex as the host, showed the chiral interaction between the host and chiral guest in a set of structurally characterised covalently bonded host–guest complexes, but chiral separation was not possible because the interactions were weak due to the open nature of the cavity. In a continuation of our earlier attempts at synthesising a rigid chiral host by using metal complexes, we now present the first report on the synthesis and structural characterisation of a medium-sized chiral metallocavity and its host– guest complexes in which two different chiral amino alcohols were recognised through well-defined three point recognition resulting in chiral separation. We chose amino alcohols as the target guest because of their structural similarity to adrenaline and noradrenaline, a hormone and neurotransmitter, respectively, (Scheme 1) and amino alcohols have two different hydrogen-bonding capable groups. Thus, binding with the host can be enhanced by electrostatic attraction between the cationic ammonium form of the amino alcohol and the use of an anionic host.

Cu(II)-mediated synthesis of a new fluorescent pyrido[1,2-a]quinoxalin-11-ium derivative.

Quinoxalines are heterocyclic compounds with potential application as drugs or fluorophores. However, few quinoxalinylium salts have been reported in the literature. This manuscript describes the synthesis and structural characterization of a previously unknown quinoxalinylium derivative, 2-aminopyrido[1,2-a]quinoxalin-11-ylium ([1](+)), as perchlorate and thiocyanate salts from the Cu(II)-mediated reaction of a Schiff base. The reaction is most efficient with Cu(II). The formation of a small quantity was observed spectroscopically in the presence of either a strong oxidizer or Mn (II). No product formation was observed with Zn(II), Cd(II), Fe(II), or Ni(II). [1](+) emits at 580 nm, with a quantum yield estimated as 0.23, upon excitation at 470 nm.

Synthesis and characterization of a trigonal monopyramidal nickel(II) complex

An unusual trigonal monopyramidal nickel(II) complex of a new tripodal tris-(N-tert-butylcarbamoylmethyl)aminato (3–) ligand is reported; this ligand sterically protects the NiII centre preventing further ligand coordination.

Manganese(III) complexes of 1,2-bis(2-pyridinecarboxamido)benzene: synthesis, spectra, and electrochemistry

The synthesis and solution properties of the high-spin (µeff.= 4.78–4.86 at 298 K) manganese(III) complexes [Mn(bpb)X][X = Cl–, N3–, or SCN–; H2bpb = 1,2-bis(2-pyridinecarboxamido)benzene], are described. The brown crystalline complexes display ligand-to-metal charge-transfer transitions at ca. 430 nm, while in the near-infrared region crystal-field transitions are observed. In N,N-dimethylformamide solution the complexes exhibit a quasi-reversible MnIII–MnII couple [E298°–0.03 to + 0.03 V vs. saturated calomel electrode (s.c.e.)]. The complexes [Mn(bpb)Cl] and [Mn(bpb)(N3)] display an additional quasi-reversible MnIV–MnIII couple [E298°+0.87 (Cl–); + 0.49 V (N3–)vs. s.c.e.].

Retention of Cs–Cl bond induces coordination polymer formation over trinuclear chiral assembly of copper(II) complexes of L-leucine derived ligand

The use of caesium is a less trodden path in terms of the synthesis of coordination polymers. The chemical similarity and mild toxic nature of caesium salts does not provide much impetus to work with these compounds compared to their potassium analogues. However, are the potassium and caesium salts really close in terms of coordination polymer formation? The result presented in this manuscript shows that while neither potassium halides or potassium nitrate inhibit the formation of a trinuclear chiral assembly of a copper(II) complex from a mononuclear Cu(II) complex, caesium chloride preferentially forms a coordination polymer while keeping the Cs–Cl bond intact. Under the same conditions, caesium nitrate does not inhibit the assembly formation. Structural characterization of {[CsCl{Cu(HLL-leu)2}(H2O)]}n (2) showed it to be a two-dimensional coordination polymer in the ab plane, where penta-coordinated Cu(HLL-leu)2 complex units (HLL-leu = reduced Schiff base of L-leucine and the salicylaldehyde condensation product) were interconnected through the formation of Cu–Cl–Cs bonds. Multitudes of intermolecular H-bonds were observed as well. The use of KCl, KBr, KI or CsNO3 in lieu of CsCl to the [Cu(HLL-leu)2(CH3CN)] (1) facilitated the formation of the trinuclear assembly, [M{Cu(HLL-leu)2}3]X, where M = K+ or Cs+ and X = Cl, Br, I or NO3 depending on the salt used. The Cu(HLL-leu)2 units in these assemblies are hexacoordinated Cu(II) complexes.

Control of the stereochemistry of four-co-ordinate copper(II) complexes by pyridinecarboxamide ligands: crystal structure, spectral and redox properties

Three copper(II) complexes [CuL1]·H2O 1, [CuL22]2 and [CuL32]3[H2L1=N,N′-o-phenylenebis-(pyridine-2-carboxamide); HL2=N-phenylpyridine-2-carboxamide; HL3=N-2,4,5-trichlorophenyl-pyridine-2-carboxamide] have been prepared and their stereochemical properties investigated. Complex 2 has been characterized by X-ray crystallography: space group P21/n, a=10.853(4), b=19.015(6), c=10.293(3)A, β= 105.22(3)°, Z= 4, R= 0.030, R′= 0.034 for 2938 observed reflections. Compared to the known square-pyramidal structure of 1 it is revealed that the deprotonated bidentate ligand L2 exerts a measurable degree of geometric control over the co-ordination sphere of 2(distorted towards tetrahedral). Similar effects are also observed from absorption and EPR spectral results. The CuII–CuI redox potentials in dimethylformamide solution have been determined by cyclic voltammetry. Upon replacement of the tetradentate L1 ligand by two uninegative bidentate L2 or L3 ligands of similar donor set, a marked positive shift (ca. 0.6–0.7 V) was observed, implying the predominance of structural effects.