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((PrN)-Pr-i)(2) (1a), Me2NC(CyN)(2) (1b), Me2NC((BuN)-Bu-t)(2) (1c), and Me2NC(DipN)(2) (2d) = (Pr-i = iso-propyl, Cy = cyclohexyl, Bu-t = tert-butyl, and Dip = 2,6-disopropylphenyl) made in situ, and structurally characterized. The di-copper guanidinates systems with the general formula [Cu-2{L}(2)] (L = {Me2NC-((PrN)-Pr-i)(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 [Cu-2{Me2NC((BuN)-Bu-t)(2)}(2)] result in the separate formation and isolation of the tri-copper complexes [Cu-3{Me2NC(BuN)(2)}(2)(mu-NMe2)] (3c) and [Cu-3{Me2NC((BuN)-Bu-t)(2)}(2)(mu-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,Delta H-double dagger = 48.2 +/- 18 kJ mol(-1) and Delta S-double dagger = 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 Id have also been determined and shown to form tetrameric and dimeric complexes respectively held together by Li-N and agostic Li center dot center dot center dot H-C interactions. The thermal chemistry of the copper complexes have also been assessed by thermogravimetric analysis.