Are Metal Complexes “Organic,” “Inorganic,” “Organometallic,” or “Metal-Organic” Materials? A case Study for the Use of Trinuclear Coinage Metal Complexes as “Metal-Organic Coatings” for Corrosion Suppression on Aluminum Substrates

Figures & data

Figure 1 Molecular structure of {[3,5-(CF₃)₂Pz]Cu}₃ (a.k.a., “Cu trimer”).

Figure 2 FTIR spectra of the Cu trimer powder (a) and Cu trimer films before (b) and after (c) corrosion test.

Figure 3 Top view of SEM images of uncoated aluminum alloy 3003 (a) vs that coated by the Cu trimer (b); EDX for the same coated substrate (c) used in B; side view of SEM images of two other aluminum alloy 3003s coated by the Cu trimer (d and e), showing both the substrate and coating simultaneously; and optical images of the luminescent Cu trimer film on the coated substrate before (f) and after (g) the corrosion test.

Figure 4 TGA for Cu trimer powder and film before and after corrosion test.

Figure 5 (a) Contact angle for uncoated aluminum surface; (b) contact angle for Cu trimer film on aluminum surface.

Figure 6 (a) Potentiodynamic polarization for uncoated (bare aluminum) and aluminum coated by a single coating of the Cu trimer and Ag trimer films; (b) potentiodynamic polarization for uncoated (bare aluminum) and aluminum coated by a double coating of the Cu trimer film.

Table 1 Potentiodynamic polarization plot data for uncoated aluminum and aluminum coated by Cu trimer

Data Samples	Ecorr (mV)	icorr (µA/cm^2)	Ba (mV/dec)	Bc (mV/dec)	Rp (Ω/cm^2)	Corrosion Rate (mpy)	PE (%)
Uncoated Al	–777	2.50 x 10^–2	229	191	1.80 x 10^3	1.00 x 10^–2	—
Cu trimer film on Al	–151 ± 1	9.64 x 10^–5 ± 1.26 x 10^–6	514 ± 18	596 ± 4	1.34 x 10^6 ± 4.53 x 10^4	4.50 x 10^–3 ± 5.85 x 10^–5	99.61 ± 0.01

Figure 7 OCP submersion studies in 3.5% NaCl.

Figure 8 Tape test for adhesion measurements for films of the Cu trimer (left) vs Ag trimer (right) on aluminum substrates.

Table 2 Binding energy of the Al atom and distance between the Al atom and centroid of the M₃ unit in a half-sandwich adduct of the Al atom with various unsubstituted and substituted cyclotrimers, according to M06/CEP-31G(d) simulations

Trimer	Binding Energy	Al-Centroid distance (Å)
[Au (μ-Pz)]3	–9.99	2.06
[Ag (μ-Pz)]3	–12.87	2.13
[Cu (μ-Pz)]3	–11.89	2.03
[Ag (μ-Pz(Me)2)]3	–12.95	2.11
[Ag (μ-Pz(CF3)2)]3	–15.02 (–19.73)^a	2.11 (2.12)^a
[Cu (μ-Pz(CF3)2)]3	–15.89 (–19.96)^a	2.10 (2.01)^a

^aValues in parentheses refer to B97D/CEP-31G(d) results.

Figure 9 Top: Geometries of the Al atom binding to the Cu (left) and Ag (right) trimers. Bottom: MEP surfaces of the trimers alone. Results are according to M06/CEP-31G(d) simulations. The Q_zz quadrupole tensor values are labeled for the trimers alone, representing the highest magnitude (z axis is normal to the cylotrimer plane).

Figure 10 Left panel: Molecular structures (top), electrostatic potential showing positive and negative regions in space (ESP/middle), and molecular electrostatic potential (MEP/bottom) of unsubstituted coinage metal pyrazolate cyclotrimers. Right panel: Positive charge attraction (PCA) curves for the three trimers. Results are according to M06/CEP-31G(d) simulations using Gaussian 09 for ESP/MEP and using GAMESS for PCA. The Q_zz quadrupole tensor values for the three trimers are –17, –15, and –20 Debye.Å with M = Cu, Ag, and Au, respectively, when unsubstituted, whereas the magnitude becomes positive upon CF₃ substitution (see Figure 9).