M. Y. Chavan

Structural, spectroscopic, and electrochemical characterization of tetrakis-µ-(2-pyrrolidinonato)-dirhodium(II) and tetrakis-µ-(δ-valerolactamato)-dirhodium(II)

The structure, chemical properties, and electrochemistry of two tetralactamato bridged dirhodium(II) complexes is reported. These complexes were formed by a ligand-exchange reaction involving 2-pyrrolidinone (Hpyro) and δ-valerolactam (Hvall)(2-piperidinone) with the acetate groups of [Rh2(O2CCH3)4]. Compound [Rh2(pyro)4(Hpyro)2]·2CH2Cl2(1a) crystallizes in space group P(triclinic) with cell constants a= 9.186(1), b= 9.569(1), c= 10.458(1)A, α= 107.28(1), β= 99.15(1), γ= 95.07(1)°, and Z= 1. The structure refinement converged to R= 0.032 and R′= 0.038. Compound [Rh2(vall)4(Hvall)2]·2Hvall (2a) crystallizes in the monoclinic space group C2/c with cell constants a= 19.990(3), b= 10.567(1), c= 21.784(7)A, β= 99.47(2)°, and Z= 4. The sample decayed rapidly, which limited the amount of available data, but the refined model converged at R= 0.035 indicating good accuracy. The prominent feature common to compounds (1a) and (2a) is the ‘cis’ arrangement of the bridging ions, where two cis nitrogens and two cis oxygens are bound to each Rh ion. The Rh–Rh bond lengths are 2.445(1) and 2.392(1)A, respectively. The oxidation potentials for [Rh2(pyro)4](1) and [Rh2(vall)4](2) in CH3CN are +0.15 and +0.04 V vs. s.c.e. Carbon monoxide binding to (1) and (2) is rapid and reversible in binding solvents such as CH3CN. However, in non-bonding solvents CO adduct formation is fast but CO dissociation is very slow. Formation constants for CO binding to (1) and (2) in CH3CN were log(Kco/atm–1)= 1.63 ± 0.05 and 2.11 ± 0.06, respectively.

Nickel(II), cobalt(II), and iron(II) lacunar complexes having very small cavities: syntheses, properties, and reactions with dioxygen

The syntheses and properties of novel lacunar macrobicyclic complexes of a ligand containing a very small cavity around one of the metal-ion axial co-ordination sites are described. The cobalt(II) and iron(II) complexes do not reversibly bind dioxygen under any explored conditions, unlike all other congenors previously reported. This behaviour is presumed to be a consequence of the restrictive volume accessible for dioxygen co-ordination within the cavity of the structure. Despite the lack of dioxygen binding, the iron(II) complex autoxidizes extremely fast, with a second-order rate constant of (7.5 ± 0.5)× 10–5 Torr–1 s–1 at –31 °C; this behaviour constitutes evidence for an outer-sphere electron-transfer oxidation mechanism.