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

Abstract

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)Å, 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)Å, 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)Å, 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(K_co/atm^–1)= 1.63 ± 0.05 and 2.11 ± 0.06, respectively.