Synthesis and Characterization of Silver(I) Pyrazolyl­methyl­pyridine Complexes and Their Implementation as Metallic Silver Thin Film Precursors

A series of light- and air-stable
silver­(I) pyrazolyl­methyl­pyridine complexes [Ag­(LR)]n(BF4)n (L = pyrazolylmethylpyridine; R = H, 1; R = Me, 2; R = i-Pr, 3) and [Ag­(LR)­(NO3)]2 (L = pyrazolylmethylpyridine;
R = H, 4; R = Me, 5; R = i-Pr, 6) has been synthesized and structurally and spectroscopically characterized.
In all of the molecular structures, the pyrazolylmethylpyridine ligands
bridge two metal centers, thus giving rise to dinuclear (2, 4, 5, and 6) or polynuclear
structures (1 and 3). The role played by
the counteranions is also of relevance, because dimeric structures
are invariably obtained with NO3– (4, 5, and 6), whereas the less-coordinating
BF4– counteranion affords polymeric structures
(1 and 3). Also, through atoms-in-molecules
(AIM) analysis of the electron density, an argentophilic Ag···Ag
interaction is found in complexes 2 and 4. Thermogravimetric analysis (TGA) shows that the thermolytic properties
of the present complexes can be significantly modified by altering
the ligand structure and counteranion. These complexes were further
investigated as thin silver film precursors by spin-coating solutions,
followed by annealing at 310 °C on 52100 steel substrates. The
resulting polycrystalline cubic-phase Ag films of ∼55 nm thickness
exhibit low levels of extraneous element contamination by X-ray photoelectron
spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron
microscopy (SEM) indicate that film growth proceeds primarily via
an island growth (Volmer–Weber) mechanism. Complex 4 was also evaluated as a lubricant additive in ball-on-disk tribological
tests. The results of the friction evaluation and wear measurements
indicate a significant reduction in wear (∼ 88%) at optimized Ag complex concentrations with little change in
friction. The enhanced wear performance is attributed to facile shearing
of Ag metal in the contact region, resulting from thermolysis of the
silver complexes, and is confirmed by energy-dispersive X-ray analysis
of the resulting wear scars.