Nirmal K. Shee

Angle isomerism, as exemplified in a five-coordinate, dimeric copper(II) Schiff base complex. Observation of Ostwald ripening

The 1:1 condensate of benzil and 2-hydrazinopyridine is the ligand (LH; H: a dissociable proton) here. Its reaction with CuCl2·2H2O in methanol at room temperature in equimolar proportion affords a mixture of two types of dark green (with metallic luster) single crystals—hexagonal (1a) and rectangular (1b). They are separated mechanically. The yield of 1a is higher. X-ray crystallography shows that 1a and 1b are penta-coordinate, dichloro-bridged dimers of the type Cu2L2Cl2 with very similar centrosymmetric structures. All the bonding parameters except for two mutually dependent bond angles in the N2OCl2 coordination sphere of Cu(II) are the same. Correspondingly, two different minima are located in DFT calculations on 1a and 1b. Energetically 1b is more stable than 1a in the gas phase by 3–4 kcal mol−1. Their X-band EPR spectra in the solid state at 77 K, which are axial, reveal that (dx2−y2)1 is the ground state in 1a (g∥ > g⊥) and (dz2)1 in 1b (g∥ < g⊥). In keeping with Ostwald ripening, the energetically less stable isomer 1a crystallizes first. As the crystallization time is allowed to be longer, more of 1b is formed. The transformation of 1a to 1b in methanol solution is found to follow the kinetics of a zero order reaction. The reverse transformation is not possible.

Tuning of the lowest excited states in mixed ruthenium(II) polypyridyl complexes having RuN6 cores by the conformation of the ancillary ligand. Emission from a 3ligand-to-ligand-charge-transfer state

Using the dihydrazone of 2,6-diacetylpyridine as L, two Ru(II) complexes of the type [Ru(bpy)2L](ClO4)2·0.5H2O (1) and [Ru(phen)2L](ClO4)2·1.5H2O (2) are synthesized. Their X-ray crystal structures have been determined. The ligand L, which is a potentially tridentate N donor, is found to bind Ru(II) in 1 and 2 in a bidentate mode – one of the two hydrazone arms remains free. The conformation of this unbound fragment is somewhat different in the two complexes. Complex 1 shows a symmetrical absorption band at 437 nm in the electronic spectrum and 2 a broad, unsymmetrical one around 400 nm. These are of charge transfer origin. Analyses of the frontier molecular orbitals pictorially by means of DFT calculations at the BP86/LanL2DZ level reveal that the HOMO in 1 is localized on L giving rise to an L(π) → bpy(π*) charge transfer, i.e. a ligand-to-ligand charge transfer (LLCT). The other complex has a metal based HOMO giving rise to an MLCT band, which is usual in Ru(II)-polypyridyl complexes. Upon excitation at 440 nm, complexes 1 and 2 display emissions at room temperature in acetonitrile with maxima at 608 and 598 nm respectively. The quantum yield of the emission is 0.076 in 1 with a lifetime of 157 ns and that in 2 is 0.067 with a lifetime of 87 ns. It is concluded that the emission in 1 is from a 3LLCT state and that in 2 from a 3MLCT state. Interestingly, when the conformation of L in 1 is theoretically changed to that observed crystallographically in 2, the HOMO becomes metal based. On the other hand, when the conformation of L in 2 is assumed to be that in 1, the HOMO in 2 becomes L based. Thus the conformation adopted by L in the two complexes determines the nature of their lowest excited state. Emission studies at 77 K in acetonitrile–toluene glass indicate that actually the 3LLCT state is in thermal equilibrium with the 3MLCT state.

A wine red copper(II) complex of a tetradentate nitrogen donor showing two-electron oxidation. Generation of a copper(II)-phosphine bond

Abstract Using the 1 : 2 condensate of benzil and 2-hydrazinopyridine as the ligand LH2 (H: dissociable NH proton), the red complex Cu(LH2)(ClO4)2 (1) was synthesized. The ligand also afforded the orange [Zn(LH2)(OH2)2](ClO4)2 (2). The X-ray crystal structures of the ligand, 1 and 2 have been determined. The metals in 1 and 2 have octahedral N4O2 environments. 1 is paramagnetic with μeff of one unpaired electron (1.63 μB and displays an axial EPR spectrum in the solid state with = 2.07, characteristic of a (dx2−y2)1 ground state (g|| > g⊥; A|| = 16 mT). In cyclic voltammetry, 1 displays a two-electron oxidation around 0.9 V versus NHE. The two-electron oxidized (coulometrically) solution of 1 (golden yellow) gives an EPR spectrum with = 2.17 and g|| < g⊥. The reaction of PPh3 with 1 yields the orange complex [Cu(LH2)(PPh3)](ClO4)2 (4). With the assumed chemical formula, the effective magnetization of 4 corresponds to one electron. Its EPR spectrum in the solid state is isotropic with g = 2.07. This g value yields a theoretical μeff of 1.80 μB at 298 K from Curie’s law, which matches very well with the experimental value of 1.89 μB at room temperature. Since single crystals of 4 could not be obtained, DFT calculations at the UBP86/6–311G(2d,p) level have been carried out and indicate that the cation in 4 is square pyramidal with the phosphine at the apex. The ease of the oxidation of the metal in 1 leads to the stabilization of the rare Cu(II)-P bond in 4.

A copper(II) complex with a Cu–S8 bond. Attenuated total reflectance, electron paramagnetic resonance, resonance Raman and atoms-in-molecule calculations

Green [Cu(1,10-phenanthroline)2OH2](ClO4)2 (1) reacts with yellow elemental sulfur at room temperature in methanol to yield turquoise blue [Cu(1,10-phenanthro-line)2(S8)](ClO4)2 (2). A comparative study of the EPR spectra of 1 and 2 in solid state and in methanol glass indicates that the S8 unit in 2 is bound to the metal. High level DFT calculations show that the cation in 2 is five coordinate, distorted square pyramidal with S8 occupying the apical position. The crucial Cu(II)-S bond is around 2.9Å. Such long Cu(II)-S bonds occur in oxidized plastocyanin where it is considered to be bonding. Presence of a weak Cu-S8 bond is revealed in the resonance Raman spectra of 2. Satisfactory matching of the calculated and experimental IR spectra vindicates the theoretically derived structure of the cation in 2.

Ligand substituents effect on 10Dq

Reaction of 5,6-dihydro-5,6-epoxy-1,10-phenanthroline (L) with Ni(ClO(4))(2)·6H(2)O in methanol in 3:1M proportion at room temperature yields [NiL(3)](ClO(4))(2)·2H(2)O. The X-ray crystal structure of the cation NiL(3)(2+) has been determined. Aminolysis of the three epoxide rings in NiL(3)(2+) by 4-substituted anilines in boiling water without any Lewis acid catalyst gives a family of Ni(II) complexes with octahedral NiL(6)(2+) core. In these complexes, crystal field splitting 10Dq varies from 11601 to 15798 cm(-1) in acetonitrile. The variation in 10Dq is found to be satisfactorily linear (r(2)=0.951) with the Hammett σ(R) parameter of the substituent on the anilino fragment. 10Dq increases with the increase in the electron donation ability of the substituent.

Isolation of a metal-to-ligand charge-transfer (MLCT) state of a tris 1,4-diimine complex of iron in the solid state: X-ray crystal structure and EPR

Using 5,6-dihydro-5,6-epoxy-1,10-phenanthroline as the ligand L, the X-ray crystal structure of [FeL3](ClO4)2·2H2O is determined. Its EPR and magnetic properties indicate entrapment of an MLCT state of the type Fe(III)–L−˙.