Coinage Metal Complexes of Tris(pyrazolyl)methanide-Based Redox-Active Metalloligands

A series
of coinage metal complexes containing the redox-active
metalloligands [RuCpX(κ3N-Tpmd)] {κ3N-Tpmd = κ3N-[C­(pz)3] with pz = pyrazolyl;
[RuCp­(Tpmd)] (2a) and [RuCp*­(Tpmd)] (2b)}
are presented. 2a and 2b are isolable, relatively
stable compounds, despite the fact that they feature a “naked”
carbanion at the bridgehead position of the κ3N-coordinated tris­(pyrazolyl)­methanide ligand scaffold.
As expected, both complexes act as κ1C ligands toward coinage metal fragments to yield dinuclear complexes
of the general formula [RuCpX(μ-Tpmd)­{MX}] (μ-Tpmd
= μ-κ1C:κ3N-[C­(pz)3]; M = Au, X = Cl, CpX = C5H5 (3a) or C5Me5 (3b); M = Au, X = CN, CpX = C5H5 (4a) or C5Me5 (4b); M = Cu, X = OC­(O)­Me, CpX = C5H5 (5a); M = Cu, X = Si­(SiMe3)3, CpX = C5H5 (6a) or C5Me5 (6b); M = Ag, X = SC­(S)­NEt2, CpX = C5H5 (7a), M = Au, X = CC–Ar, CpX = C5H5 {Ar = C6H5 (8a),
4-NH2-C6H4 (9a), 3,5-(CF3)2-C6H3 (10a)}). All complexes under study were fully characterized by common
spectroscopic techniques; the structural parameters of 2a, 3a, 5a, 6a, 7a, and 10a were determined by X-ray diffraction. Coordination
of the {MX} fragment leads to electronic effects on the metalloligand
unit, as reflected by the corresponding 1H and 13C NMR spectra. Density functional theory calculations were performed
in order to elucidate a conceivable interplay between the metal atoms.
The bonding characteristics within the {MX} fragment are only marginally
affected upon electronic excitation of the ruthenium-based metalloligand.
However, some effect of the influence of {MX} on the E01/2(RuII/RuIII) value
was detected with the aid of cyclic voltammetry measurements. A strong
Lewis-acidic metal fragment such as GaCl3 (11a) leads to an E01/2 value
of 0.37 V, while electron-richer coinage metal fragments facilitate
the oxidation of the ruthenium center significantly (E01/2 = 0.14–0.23 V). This dependence
suggests an interaction between both metals due to their close spatial
proximity.