Shanti G. Patra

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.

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.

The effect of an ancillary ligand proton on the photophysical properties of some RuIIN6 cores: a proton valve

Reaction of benzil mono-(2-pyridyl)hydrazone (LH; H is the dissociable proton) with cis-[Ru(N–N)2Cl2]·2H2O (N–N ≡ 2,2′-bipyridine and 1,10-phenanthroline) in methanol yields complexes of the type [Ru(N–N)2(LH)](ClO4)2 (1a and 2a). Their pKa values in CH3CN are determined from solution conductivity as 4.35 and 3.83. Deprotonation of the two complexes by the addition of triethylamine in methanol gives complexes of the type [Ru(N–N)2L]ClO4 (1b and 2b). The X-ray crystal structures of 1a and 2b have been determined to confirm the presence/absence of the LH proton. In the electronic spectra of the deprotonated complexes 1b and 2b in CH3CN, a prominent metal centered transition is observed at 340 nm, which is generally not seen clearly in complexes containing the RuIIN6 core. A fast electron transfer process Ru(III) + e− = Ru(II) is observed in cyclic voltammetry with an E1/2 of 0.8 V vs. NHE in 1b and 2b with a heterogeneous rate constant (ks,h) of 1.39 × 10−2 and 1.29 × 10−2 cm s−1, respectively. Weak emissions at 615 nm with a quantum yield of 7 × 10−4–1 × 10−4 are observed in the fluorescence spectra of 1a and 2a when excited at 430 nm in deaerated CH3CN. But 1b and 2b do not fluoresce. Such different photophysical behaviour is explained in terms of the fact that, while a 3MLCT is the lowest excited state in 1a and 2a (where the LH proton is present), the reactive 3MC is the lowest excited state in 1b and 2b (where the LH loses the proton). It is pointed out that the proton acts as a valve in the conduit for the radiative pathways. Various experimental results obtained here are supported by detailed density functional theory (DFT) calculations at the BP86/LanL2DZ level.