Multinuclear Copper(I) Guanidinate Complexes

A series of multinuclear Copper­(I) guanidinate complexes
have been
synthesized in a succession of reactions between CuCl and the lithium
guanidinate systems Li­{L} (L = Me2NC­(iPrN)2 (1a), Me2NC­(CyN)2 (1b), Me2NC­(tBuN)2 (1c), and Me2NC­(DipN)2 (2d) (iPr = iso-propyl, Cy = cyclohexyl, tBu = tert-butyl, and Dip = 2,6-disopropylphenyl) made in situ, and structurally characterized.
The di-copper guanidinates systems with the general
formula [Cu2{L}2] (L = {Me2NC­(iPrN)2} (2a), {Me2NC­(CyN)2} (2b), and {Me2NC­(DipN)2} (2d) differed significantly from
related amidinate complexes because of a large torsion of the dimer
ring, which in turn is a result of transannular repulsion between
adjacent guanidinate substituents. Attempts to synthesis the tert-butyl derivative [Cu2{Me2NC­(tBuN)2}2] result in
the separate formation and isolation of the tri-copper
complexes [Cu3{Me2NC­(tBuN)2}2(μ-NMe2)] (3c) and [Cu3{Me2NC­(tBuN)2}2(μ-Cl)] (4c), both of which have been unambiguously characterized by single
crystal X-ray diffraction. Closer inspection of the solution state
behavior of the lithium salt 1c reveals a previously
unobserved equilibrium between 1c and its starting materials,
LiNMe2 and N,N′-di-tert-butyl-carbodiimide, for which activation enthalpy and entropy values
of ΔH⧧ = 48.2 ± 18 kJ
mol–1 and ΔS⧧ = 70.6 ± 6 J/K mol have been calculated using 1D-EXSY NMR spectroscopy
to establish temperature dependent rates of exchange between the species
in solution. The molecular structures of the lithium complexes 1c and 1d have also been determined and shown
to form tetrameric and dimeric complexes respectively held together
by Li–N and agostic Li···H–C interactions.
The thermal chemistry of the copper complexes have also been assessed
by thermogravimetric analysis.