Mononuclear and Polynuclear Copper(I) Complexes with a New N,N‘,S-Donor Ligand and with Structural Analogies to the Copper Thionein Core

The N,N‘,S-donor ligand 4-methoxy-3,5-dimethyl-2-((3-(2-(methylthio)phenyl)-1H-pyrazol-1-yl)methyl)pyridine (L) was
prepared from 2-(chloromethyl)-4-methoxy-3,5-dimethylpyridine hydrochloride and 3-(2-(methylthio)phenyl)-1H-pyrazole.
The Cu(I) complexes [Cu2(L)2CH3CN][Cu(L)CH3CN](BF4)3 (1), [Cu(L)PPh3]BF4 (2), and [Cu6(L)2(C6F5S)6] (3) were
prepared and characterized by X-ray crystallography (PPh3 = triphenylphosphine, C6F5S- = pentafluorothiophenolate).
The unit cell of compound 1 consists of cocrystallized mononuclear and dinuclear entities in which all of the copper
atoms exhibit distorted tetrahedral coordination. Compound 2 is monomeric with L bound in the κ3-N,N‘,S mode
and a PPh3 molecule that completes the coordination environment. Compound 2 presents a fluxional behavior in
CDCl3 solution due to the boat inversion of the six-membered N,N‘ chelate ring (ΔH⧧ = +43.6(3) kJ mol-1, ΔS⧧
= −16(1) J mol-1 K-1). Crystallization of 3 in acetonitrile leads to a polynuclear structure that contains a CH3CN
molecule coordinated to one of the copper atoms:  [Cu6(L)2(C6F5S)6CH3CN] (3a). The core of 3a partially resembles
a {Cu4S6} adamantane-like moiety, the only difference being that the Cu−NCCH3 interaction leads to the opening
of the cluster by disrupting a Cu−Cu interaction. Part of this assembly is found in the yeast metallothionein copper(I)−cysteinate core whose crystal structure has recently been reported. Two additional [Cu(L)]+ peripheral moieties
interact with the cluster by means of bridging thiolates. ESI-mass spectrometry, conductivity measurements, and
1H/19F pulsed gradient spin echo (PGSE) NMR experiments suggest that 3a dissociates in acetonitrile solution:  3a
+ CH3CN → [Cu4(C6F5S)6]2- + 2[Cu(L)CH3CN]+. The stability of the cluster with respect to the hypothetical
mononuclear species, [Cu(L)(C6F5S)], is confirmed by DFT calculations (B3LYP), which illustrate the exergonic
character of the reaction:  6[Cu(L‘)(C6H5S)] → [Cu6(L‘)2(C6H5S)6] + 4L‘ (ΔG298 = −58.6 kJ mol-1, where L‘ and
C6H5S- are simplified models for L and C6F5S-, respectively). The energetics pertinent to the ionic dissociation of
the cluster in acetonitrile is computed using the polarizable continuum model (PCM) approach.