RE(III) 3-Furoate Complexes: Synthesis, Structure, and Corrosion Inhibiting Properties

In this study, two types of Rare Earth (RE) 3-furoate complexes were synthesized by metathesis reactions between RE chlorides or nitrates and preformed sodium 3-furoate. Two different structural motifs were identified as Type 1RE and Type 2RE. The Type 1RE monometallic complexes form 2D polymeric networks with the composition [RE(3fur)3(H2O)2]n (1RE = 1La, 1Ce, 1Pr, 1Nd, 1Gd, 1Dy, 1Ho, 1Y; 3furH = 3-furoic acid) while Type 2RE bimetallic complexes form 3D polymeric systems [NaRE(3fur)4]n (2RE = 2Ho, 2Y, 2Er, 2Yb, 2Lu). The stoichiometric mole ratio used (RE: Na(3fur) = 1:3 or 1:4) in the metathesis reaction determines whether 1RE or 2RE (RE = Ho or Y) is formed, but 2RE (RE = Er, Yb, Lu) were obtained regardless of the ratio. The corrosion inhibition behaviour of the compounds has been examined using immersion studies and electrochemical measurements on AS1020 mild steel surfaces by a 0.01 M NaCl medium. Immersion test results revealed that [Y(3fur)3(H2O)2]n has the highest corrosion inhibition capability with 90% resistance after 168 h of immersion. Potentiodynamic polarisation (PP) measurements also indicate the dominant behaviour of the 1Y compound, and the PP curves show that these rare earth carboxylate compounds act predominantly as anodic inhibitors.


Introduction
Mild steel is extensively used as an engineering material in diverse industries due to its relatively low cost, high machinability, and widespread availability [1]. However, these kinds of metals and alloys are vulnerable to corrosion degradation by chemical and/or electrochemical reactions [2,3]. A range of factors such as pressure, temperature, and aggressive acidic environmental conditions [3] can lead to premature failure in mild steel structures. Over 3% of the world's Gross Domestic Production equating to US $2.5 trillion is spent annually to replace and reduce the damage caused by corrosion [4,5]. A major part of this cost includes the maintenance and replacement of water recirculating pipelines, bridges, and water tanks associated with cooling systems [3,6]. Without a proper control method to mitigate corrosion, such assets would have a significantly limited lifetime [7]. The latest reports by NACE international's IMPACT researchers have revealed that the application of currently available corrosion control practices could save between US $375 and US $875 billion annually. Thus 15-35% of the global cost of corrosion could be saved [5]. To protect metals and alloys from corrosion, a range of approaches are used. Among them, one of the oldest and most cost-efficient protection methods is using corrosion inhibitors [8,9]. Corrosion inhibitors are widely incorporated in water treatment formulations, as pigments in paint coatings, pipeline streams, etc. [8,10]. Normally, these inhibitors make a protective barrier film on the metal surface to minimize corrosion attacks [11].
Among corrosion inhibitors, rare earth (RE) metal salts have been recognized as an effective and environmentally friendly nontoxic method to prevent corrosion [12][13][14][15][16][17]. The was explained based on the electron-donating ability of the well as the carboxylic oxygen [23]. Khaled tested three fura roate, ethyl 2-furoate, and amyl 2-furoate) on mild steel in 1 corrosive properties with respect to the changes in substitue pound. It was suggested that the protection of mild steel oc sharing electrons with the metal surface, forming a stable (chemisorption) [24].
The recent innovative approaches toward designing " have involved combining RE ions with an organic ligand to mixed inhibition functions [10,15,16]. RE-carboxylate comple to the highly oxophilic nature of REs towards carboxylate lig either dimeric or polymeric forms [25,26]. Over the years, se patterns have been observed with carboxylate ligands bindin So far, several trivalent rare-earth carboxylates have been for their corrosion inhibition properties on mild steel [7,10,15 performing RE carboxylate complexes reported in recent yea lanthanum 4-hydroxycinnamate [30] and yttrium 3-(4′-met The ligands feature an additional donor function, which ma to a steel surface. [10,15,16]. Our latest paper evaluated the i phene derivatives on mild steel by investigating the rare ear thiophenecarboxylate complexes [31], where the S atom p bridging to steel. In this paper, we discuss the synthesis of RE 3-furoate acid, Chart 1) and the corrosion inhibitory properties for A NaCl corrosive media, to examine whether the oxygen donor ficacious than sulfur. The synthesized compounds were also techniques including single crystal X-ray crystallography. M bition properties of the synthesized compounds were determ ments combined with potentiodynamic polarization measure Chart 1. Structure of 3-furoic acid (3FurH).
In all cases, single crystals of the compounds were obtained by slow evaporation of the mother liquor. The bulk samples of complexes [RE(3fur)3(H2O)2]n (1RE = 1La, 1Ce, 1Pr, 1Nd, 1Gd, 1Dy, 1Ho, 1Y) ( Figure 1) and [NaRE(3fur)4]n (2RE = 2Ho, 2Y, 2Er, 2Yb, 2Lu) ( Figure 2) gave X-ray powder diffraction patterns that are identical to each other and are in agreement with the patterns generated from the single-crystal data.   Satisfactory elemental analyses were obtained for all the compounds, with the exception of compound 2Yb, with a low % C value. This is a frequent issue recognized for rare earth and alkaline earth complexes within previous literature [32][33][34][35][36][37][38]. However, Xray crystallographic data together with additional % RE analyses and TGA data offer further evidence to confirm the final chemical compositions of the complexes.
The IR spectra of [RE(3fur)3(H2O)2]n (1RE = 1La, 1Ce, 1Pr, 1Nd, 1Gd, 1Dy, 1Ho, 1Y) ( Figure S1) and [NaRE(3fur)4]n (2RE = 2Ho, 2Y, 2Er, 2Yb, 2Lu) ( Figure S2) show strong similarities in major bands within each isostructural series. Thus, the significant IR bands for only one compound from each series (1Nd and 2Yb) are listed in Table 1. For the 1RE compounds broad O-H stretching bands in the range of 3200-3550 cm −1 are comparable and attributed to the presence of coordinated water molecules. IR spectra of all the RE complexes show strong asymmetric and symmetric carboxylate stretching bands at 1530-1350 cm −1 . Relatively small Δν(CO2 -) values are indicative of the presence of chelating and/or bridging carboxylate ligands [39]. A TGA study was conducted to establish the thermal behaviour of the complexes as well as the intermediates and products formed during the thermolysis. TGA curves of the 1RE bulk sample ( Figure S3) showed a steep initial weight loss (ca. 75-130 ℃), which corresponds to the loss of two molecules of water and the results are in agreement with the composition determined crystallographically as [RE(3fur)3(H2O)2]n. In line with the singlecrystal compositions of the 2RE compounds, their thermograms ( Figure S4) do not indicate any weight loss owing to the loss of coordinated or lattice solvent molecules. Further weight loss is observed for all the compounds at ca. 370-500 ℃. However, the TGA plot of 1La exhibits significantly different behaviour as the product formation starts and finishes at lower temperatures than for the rest of the compounds in the isostructural series. The products plausibly resulted from the decomposition of the anhydrous complexes to the RE2(CO3)3 in the 1RE series and Na3RE(CO3)3 in the 2RE series. The IR spectra of the Satisfactory elemental analyses were obtained for all the compounds, with the exception of compound 2Yb, with a low % C value. This is a frequent issue recognized for rare earth and alkaline earth complexes within previous literature [32][33][34][35][36][37][38]. However, X-ray crystallographic data together with additional % RE analyses and TGA data offer further evidence to confirm the final chemical compositions of the complexes.
The IR spectra of [RE(3fur) 3 (H 2 O) 2 ] n (1RE = 1La, 1Ce, 1Pr, 1Nd, 1Gd, 1Dy, 1Ho, 1Y) ( Figure S1) and [NaRE(3fur) 4 ] n (2RE = 2Ho, 2Y, 2Er, 2Yb, 2Lu) ( Figure S2) show strong similarities in major bands within each isostructural series. Thus, the significant IR bands for only one compound from each series (1Nd and 2Yb) are listed in Table 1. For the 1RE compounds broad O-H stretching bands in the range of 3200-3550 cm −1 are comparable and attributed to the presence of coordinated water molecules. IR spectra of all the RE complexes show strong asymmetric and symmetric carboxylate stretching bands at 1530-1350 cm −1 . Relatively small ∆ν(CO 2 − ) values are indicative of the presence of chelating and/or bridging carboxylate ligands [39]. A TGA study was conducted to establish the thermal behaviour of the complexes as well as the intermediates and products formed during the thermolysis. TGA curves of the 1RE bulk sample ( Figure S3) showed a steep initial weight loss (ca. 75-130°C), which corresponds to the loss of two molecules of water and the results are in agreement with the composition determined crystallographically as [RE(3fur) 3 (H 2 O) 2 ] n . In line with the single-crystal compositions of the 2RE compounds, their thermograms ( Figure S4) do not indicate any weight loss owing to the loss of coordinated or lattice solvent molecules. Further weight loss is observed for all the compounds at ca. 370-500°C. However, the TGA plot of 1La exhibits significantly different behaviour as the product formation starts and finishes at lower temperatures than for the rest of the compounds in the isostructural series. The products plausibly resulted from the decomposition of the anhydrous complexes to the RE 2 (CO 3 ) 3 in the 1RE series and Na 3 RE(CO 3 ) 3 in the 2RE series. The IR spectra of the residues of compounds 1Ce and 2Yb after thermolysis to 500°C support this interpretation [40,41] since they correspond well with the previously reported spectrum  Figure S5) [42]. However, for the residues of [RE(3fur) 3 (H 2 O) 2 ] n compounds, there is not sufficient weight loss for RE 2 (CO 3 ) 3 alone. Residual 3-furoate ligand could not be detected in the residue by IR spectra, so there must be an amorphous or IR silent coproduct such as RE(OH) 3 , RE 2 O 3 , or REC 2 . Metal carboxylates have a number of possible decomposition paths including the formation of carbonates and oxides [43,44].
Liquid chromatography-mass spectrometry (LCMS) studies have been used to investigate the composition of complexes 1Dy, 2Ho and 2Lu in solution. In the mass spectrum of monometallic complex 1Dy the molecular ion peak [Dy(3fur) 2 (H 2 O) 2 ] + can be detected at m/z 422.1 with the correct isotope patterns. Other than that, the related ion such as [Dy(3fur)(OH)EtOH] + , can be also observed. The bimetallic complexes 2Ho and 2Lu were also examined by LCMS. In the positive mode of complex 2Ho, [Ho 2 (3fur) 4 (HCO 3 )(H 2 O)] + and [Na 2 Ho(3fur) 3 (3furH)(HCO 3 )(H 2 O)] + were detected at m/z 853.5 and 735.2, respectively, with low intensity. Finally, compound 2Lu in LCMS led to a highly intense peak as [Na 2 (CO 3 )Lu(3fur) 4 (H 2 O) 2 ]in the negative mode at m/z 761. Interestingly, mass spectrometry results suggest that the bimetallic complexes remain considerably intact in solution. Furthermore, the observations of LCMS experiments have revealed that all the tested complexes retain some form of rare earth carboxylate fragments in solution.

X-ray Crystallography
There are three distinct carboxylate ligand binding modes, namely bridging µ-1κ ( residues of compounds 1Ce and 2Yb after thermolysis to 500 ℃ support this interpretation [40,41] since they correspond well with the previously reported spectrum of [Dy2(CO3)3].4H2O ( Figure S5) [42]. However, for the residues of [RE(3fur)3(H2O)2]n compounds, there is not sufficient weight loss for RE2(CO3)3 alone. Residual 3-furoate ligand could not be detected in the residue by IR spectra, so there must be an amorphous or IR silent coproduct such as RE(OH)3, RE2O3, or REC2. Metal carboxylates have a number of possible decomposition paths including the formation of carbonates and oxides [43,44]. Liquid chromatography-mass spectrometry (LCMS) studies have been used to investigate the composition of complexes 1Dy, 2Ho and 2Lu in solution. In the mass spectrum of monometallic complex 1Dy the molecular ion peak [Dy(3fur)2(H2O)2] + can be detected at m/z 422.1 with the correct isotope patterns. Other than that, the related ion such as [Dy(3fur)(OH)EtOH] + , can be also observed. The bimetallic complexes 2Ho and 2Lu were also examined by LCMS. In the positive mode of complex 2Ho, [Ho2(3fur)4(HCO3)(H2O)] + and [Na2Ho(3fur)3(3furH)(HCO3)(H2O)] + were detected at m/z 853.5 and 735.2, respectively, with low intensity. Finally, compound 2Lu in LCMS led to a highly intense peak as [Na2(CO3)Lu(3fur)4(H2O)2] -in the negative mode at m/z 761. Interestingly, mass spectrometry results suggest that the bimetallic complexes remain considerably intact in solution. Furthermore, the observations of LCMS experiments have revealed that all the tested complexes retain some form of rare earth carboxylate fragments in solution.

X-ray Crystallography
There are three distinct carboxylate ligand binding modes, namely bridging μ-   Table S4. Lanthanoid contraction [45] is observed across the period from La to Y with a decrease in the average RE-O bond distance (Table S2)  the adjacent metal centres. The 1D polymeric chain propagates into a 2D network by synanti bridging bidentate ligands (O5, 5#2) between Nd1 and Nd1#2 metal atoms. Experimentally found H bonds of the compound [Nd(3fur)3(H2O)2]n are shown in Figure 4a and H bond lengths are listed in Table S4. Lanthanoid contraction [45] is observed across the period from La to Y with a decrease in the average RE-O bond distance ( Single crystals of X-ray crystallography quality were obtained for all five Ho, Y, Er, Yb and Lu analogues. The isostructural series of complexes forms a 3D polymeric network and crystallizes in the tetragonal P-4n2 space group. All RE 3+ are eight coordinated with two chelating ligands and four bridging ligands in distorted square antiprismatic geometry, Na1 is six-coordinate with six bridging ligands, and the donor atoms have an octahedral arrangement. Specific refinement parameters for individual structures are given in the Supplementary Material. A representative structure [NaYb(3fur) 4 ] n of the Type 2RE can be found in Figure 5. The Yb1 and Yb2 atoms are bridged by the O1-Na1-O7 moiety present in the asymmetric unit of the crystal structure (Figure 5a). The Yb1 metal centre is bonded by chelating carboxylate oxygen atoms O1,2 and O1#1,2#1 of two

Immersion Tests
Immersion studies were conducted over seven days in 0.01 M NaCl control solution and in the presence of inhibition systems to access the longer-term performance of these inhibitors for mild steel. The testing was conducted for the compounds at 800 ppm as the solubility limit for [Y(3fur)3(H2O)2]n in 0.01 M sodium chloride solution is close to 800 ppm. However, [Ce(3fur)3(H2O)2]n and [Pr(3fur)3(H2O)2]n compounds had maximum solubilities as high as 3000 ppm. Table 2 presents a summary of the weight loss measurements after 168 h of immersion in specific inhibitor solutions. The corrosion rate (R) of each inhibitor compound was evaluated and then percentage corrosion inhibition (η) was calculated using Eq 1.  (C11-Yb2-C11#4). The metal-metal distance between Yb1 . . . .Na1 and Yb2 . . . Na1 are 3.3826(18) and 3.3720 (18), respectively, with Yb1-Na1-Yb2 at an angle of 175.26 (7) • . The effect of lanthanoid contraction [45] is reflected across the series with the reduction of average RE-O bond distances in 1D chains (Table S5) (4) Å and these dimers propagate to form 1D channels. However, the successive Yb2 chains are perpendicular to each other, thus constructing two types of chains. As mentioned above, symmetric bridging between adjacent Yb1 (III) atoms leads to the formation of dimeric Yb1 channels that crosslink the neighbouring Yb2 channels running along the perpendicular direction, into a two-dimensional grid. Finally, sodium metal atoms connect all dimers into a 3D coordination polymer, namely [NaYb(3fur) 4 ] n . (Figure 5c).

Immersion Tests
Immersion studies were conducted over seven days in 0.01 M NaCl control solution and in the presence of inhibition systems to access the longer-term performance of these inhibitors for mild steel. The testing was conducted for the compounds at 800 ppm as the solubility limit for [Y(3fur) 3 Table 2 presents a summary of the weight loss measurements after 168 h of immersion in specific inhibitor solutions. The corrosion rate (R) of each inhibitor compound was evaluated and then percentage corrosion inhibition (η) was calculated using Equation (1). The corrosion rates of the samples placed in the inhibitor solutions were found to be significantly lower than that of control coupons. As shown in Table 2, [Y(3fur) 3 (H 2 O) 2 ] n compound has the highest inhibitor efficiency of 90%. Furthermore, the visual inspection of the surface immersed in [Y(3fur) 3 (H 2 O) 2 ] n containing solutions also suggests a minimum corrosive attack on the steel surface with relatively less surface roughness ( Figure S6). As a group, Type 1RE [RE(3fur) 3 (H 2 O) 2 ] n complexes showed better inhibition performance than the Type 2RE bimetallic [NaRE(3fur) 4 ] n species. This was evident from the better level of protection achieved by Type 1RE monometallic Y III and Ho III inhibited solutions compared to the Type 2RE Na-Y and Na-Ho bimetallic compounds. This may be a consequence of the presence of coordinated sodium in the compounds in the latter instance. The influence of sodium on inhibition efficiency can be seen by comparing the immersion test results of sodium 3-furoate (Na(3fur)) with the rare earth carboxylate complexes in Table 2. It has also been shown in previous literature that the corrosion rates of sodium carboxylates on steel are generally higher than that of rare earth analogues [46]. However, the 2Yb complex is a positive outlier since it gave the second-best performance with an η of 88%.
When considering lighter rare earths (La-Nd) the efficiency decreased from La-Nd. The level of protection by the heavy rare earths correlates with the decreasing atomic radius of the rare earth metals since the inhibitive effect increased from Gd-Y among the [RE(3fur) 3 (H 2 O) 2 ] n class. A similar trend was observed to some extent with [NaRE(3fur) 4 ] n complexes with improved protection from Ho to Yb, but Lu is a negative outlier. The low solubility of most of the complexes has hampered attempts at concentration studies. The performance was better than of the analogous rare earth 3-thiophenecarboxylates [31], suggesting that an oxygen substituent may be more beneficial than sulfur, but the 3fur complexes were tested at somewhat higher concentration owing to greater solubility.

Potentiodynamic Polarisation
The results of the potentiodynamic polarisation (PP) experiments are shown in Table 3.
[Y(3fur) 3 (H 2 O) 2 ] n 1Y showed the lowest i corr , at 0.56 µA/cm 2 , and 1Y > 2Yb > 1La in effectiveness. After 24 h immersion, with the compounds present in the solution there is a shift in corrosion potential (E corr ) values towards more positive direction compared to the control solution, indicative of anodic inhibition dominating the reduction in i corr . Representative PP scans in Figure 6 show the marked reduction in the anodic branch with the addition of inhibitors, and some reduction in the cathodic kinetics. According to the previous literature [46], this is a typical behaviour of these types of compounds. However, the efficiency of the [Y(3fur) 3 (H 2 O) 2 ] n was somewhat inferior to that of yttrium 3-(4 -methylbenzoyl)propanoate [6], which though is limited by low solubility. For this reason, yttrium 3-furoate may potentially be more useful. in corrosion potential (Ecorr) values towards more positive direction compared to the control solution, indicative of anodic inhibition dominating the reduction in icorr. Representative PP scans in Figure 6 show the marked reduction in the anodic branch with the addition of inhibitors, and some reduction in the cathodic kinetics. According to the previous literature [46], this is a typical behaviour of these types of compounds. However, the efficiency of the [Y(3fur)3(H2O)2]n was somewhat inferior to that of yttrium 3-(4′methylbenzoyl)propanoate [6], which though is limited by low solubility. For this reason, yttrium 3-furoate may potentially be more useful.

General Consideration
All reagents and solvents that were used are of standard commercial grade and used without further purification. IR spectra were collected using a Nicolet™ iS™ 5 FTIR Spectrometer in the range of 4000-500 cm −1 . Elemental analyses were performed by the Elemental Analysis Service Team, Science Centre, London Metropolitan University, England. Metal analysis was conducted by complexometric titration with 0.01 M EDTA using Xy-

General Consideration
All reagents and solvents that were used are of standard commercial grade and used without further purification. IR spectra were collected using a Nicolet™ iS™ 5 FTIR Spectrometer in the range of 4000-500 cm −1 . Elemental analyses were performed by the Elemental Analysis Service Team, Science Centre, London Metropolitan University, England. Metal analysis was conducted by complexometric titration with 0.01 M EDTA using Xylenol Orange indicator and hexamethylenetetramine buffer. Thermogravimetric analysis (TGA) was conducted on a TA instrument SDT 650 using standard 90 µL alumina metal pans under an N 2 atmosphere (50 mL min −1 ) from room temperature up to 750 • C (with a ramp of 10 • C min −1 ). Melting points were determined in glass capillaries and are reported uncalibrated. Powder XRD measurements were obtained at room temperature using a Bruker D2 PHASER diffractometer in the range of 2-60 • with a 0.2 • divergence slit and at 0.02 • increments. X-ray powder simulations were generated using the Mercury program provided by Cambridge Crystallographic Data Centre [47], from the obtained single-crystal X-ray diffraction data. LCMS was collected on an Agilent 6100 Series Single Quad LC/MS coupled with an Agilent 1200 Series HPLC.

X-ray Crystallography
Single crystals were mounted on loops using viscous hydrocarbon oil. Data were collected on the MX1 beamline at the Australian Synchrotron. The data integration was completed using Blue-ice [48] and XDS [49] software programs. Structures were solved by SHELXT and refined by full-umatrix least-squares methods against F2 using SHELX2018 [50], utilizing the Olex2 [51] graphical user interface. All hydrogen atoms were placed in calculated positions using the riding model. The graphical representations were generated using bitmap images GUI of Olex2 [51]. Crystal data and refinement details are given in Table S1.

Corrosion Testing
To evaluate the general corrosion and inhibition behaviour of synthesized compounds, corrosion immersion experiments were conducted according to the standard method ASTM G31-72 [52]. Mild steel alloy AS 1020 coupons used in the weight loss tests were cut to approximately 20 × 20 × 1.5 mm and abraded progressively with sanding sheets of 80, 120, 240, 360, 800, 1200 and 2000 grits. The specimens were rinsed with distilled water followed by ethanol and dried under flowing N 2 gas. The coupons were used immediately after polishing and washing to do a series of immersion tests, up to 168 h (7 days). The sample and the control coupons were suspended in beakers containing 0.01 M NaCl solutions with and without 800 ppm (1.25-1.75 mM) of the inhibitor compounds. In each setup, replicates were done by fully immersing the coupons at mid-depth with the use of Teflon strings. Upon the completion of the test, firstly, the corrosion product that clung to the substrate was removed by mild sonication in clean distilled water followed by using the finest sanding papers with minimum force to avoid the removal of sound material. Experiments were done in duplicate. Lastly, the coupons were washed with ethanol and dried with N 2 gas.
The compounds containing the REs Y, Yb and La were used to conduct Potentiodynamic Polarisation (PP) experiments, as they showed the best performance in weight loss experiments. The tests were performed using a Bio-Logic VMP3 multi-channel potentiostat with solutions containing the same concentrations as those used in the weight loss experiments. A three-electrode cell with a titanium mesh counter electrode, a Ag/AgCl reference electrode and an AS1020 mild steel rod as the working electrode surface was used. The working electrode consisted of a 10mm diameter steel rod steel rod in epoxy, polished to a 1200 grit finish. A test solution volume of 100mL was used, with the solution open to air. Open Circuit Voltage (OCV) was monitored for 24 h following which the PP scan was conducted over a scan range of 150 mV below to 250 mV above OCV at 0.167 mV/s. A Tafel extrapolation was used to determine the corrosion current densities (i corr ) and potentials (E corr ) from the PP curves using EC Lab software V11.27.

Conclusions
A series of novel rare earth 3-furoate complexes was synthesized and characterized. The 13 structures identified in this study could be separated into two groups based on their molecular composition as below. The ability to obtain two different structural motifs from HoCl 3 and YCl 3 at 1:3 and 1:4 (rare earth: ligand) mole ratios indicates that the stoichiometry used in the metathesis reaction affects the final complex formation. The lanthanoid contraction was evident with a reduction of the average RE-O bond distance across each series.
The immersion studies showed that at 800 ppm inhibitor concentrations for mild steel in 0.01 M NaCl solutions, [Y(3fur) 3 (H 2 O) 2 ] n (2Y) was the best performing inhibitor with [NaYb(3fur) 4 ] n (2Yb) and [La(3fur) 3 (H 2 O) 2 ] n (1La) exhibiting comparable inhibition performance. Potentiodynamic polarisation confirmed the superior performance of [Y(3fur) 3 (H 2 O) 2 ] n (1Y). The compounds act predominantly as anodic inhibitors. A comparison of Y complex data suggests that the introduction of a furoate moiety into a carboxylate is more effective than the thiophene group but not as effective as a 3-(4 -methylbenzoyl) group [6,31]. Application of the rare earth 3-furoates as inhibitors would involve dissolution of the solid complexes in the water of cooling towers or radiators, etc., to give a very dilute solution. In protective coating, a suspension of the complexes in paint would be used, but the effectiveness in this role has still to be investigated.