Liye Qu

Thierry Roisnel

Marie Cordier

Dan Yuan
email: yuandan@suda.edu.cn

Yingming Yao

Bei Zhao

Evgueni Kirillov
email: evgueni.kirillov@univ-rennes1.fr

Rare-Earth Metal Complexes Supported by Polydentate Phenoxy-Type Ligand Platforms: C-H Activation Reactivity and CO(2)/Epoxide Copolymerization Catalysis

published or not. The documents may come

INTRODUCTION

Phenolate-based ligand platforms constitute strong and convenient alternative to cyclopentadienyl-type ligands for the modern coordination chemistry of the early transition metals, largely thanks to their remarkable tunability allowing diverse sterico-electronic variations. 1 In particular, these hard, electronegative -donor ligands are attractive because they offer strong metal-oxygen bonds that are expected to stabilize complexes of highly oxophilic and electropositive metals (e.g. Group 3 metals). [START_REF] Edelmann | Synthesis and Structural Chemistry of Non-Cyclopentadienyl Organolanthanide Complexes[END_REF] Rare-earth metal alkyl complexes have shown great utility in many 100 % atom-economy catalytic reactions involving heteroatom-containing substrates, such as CH activation/functionalization of anilines, anisoles, amines and heterocycles, [START_REF] Arnold | CH Bond Activation by f-Block Complexes[END_REF]4 (co)polymerization of polar monomers (lactones, carbonates, epoxides) 5 and activation of CO 2 . [START_REF] Coates | Discrete Metal-Based Catalysts for the Copolymerization of CO 2 and Epoxides: Discovery, Reactivity, Optimization, and Mechanism[END_REF][START_REF] Cui | Alternating Copolymerization of Cyclohexene Oxide and Carbon Dioxide Catalyzed by Organo Rare Earth Metal Complexes[END_REF] As such, there is continued interest in designing better performing and more stable catalysts that also exhibit improved functional group tolerance. Herein we report the synthesis and characterization of group 3 metal complexes supported by bulky multidentate bis(imino)phenolate and bis(anilino)phenolate ligand systems (Scheme 1). The multidentate nature and high coordination abilities of these ligand platforms have initially been anticipated as beneficial for the preparation of dinuclear complexes or formation of dinuclear intermediates in catalytic processes. Special emphasis has been placed on (i) studies of the coordination chemistry of this ligand systems with scandium and yttrium, starting from tris(carbyl) precursors LnR 3 (THF) x , in order to achieve selective synthesis of the corresponding alkyl complexes; (ii) evaluation of potential of the titled alkyl complexes as precatalysts of CH activation of 2-phenylpyridine and hydroarylation of styrene; (iii) stoichiometric reactions between alkyl complexes and 2-phenylpyridine mimicking the CH activation step; (iv) evaluation of the efficacy of some complexes obtained during this study in copolymerization reactions of CO 2 with cyclohexene oxide (CHO).

RESULTS AND DISCUSSION

Synthesis of rare-earth metal complexes derived from proligands {N 2 O}H (1a-H) and {N 2 O}H 3 (1b-H 3 ). The proligands, bis(imino)phenol 1a-H and bis(anilino)phenol 1b-H 3,

were prepared in good yields (29-60 %) using the protocols reported in the literature. 8 To study the coordination ability of these ligand platforms with group 3 metals (Sc, Y), bond metathesis reactions between the corresponding proligands and metal tris(alkyls) MR 3 (THF) 2

were investigated.

The reaction between 1a-H and 1 equiv. of Sc(CH 2 SiMe 3 ) 3 (THF) 2 in C 6 D 6 (Scheme 2), monitored by 1 H NMR spectroscopy, proceeded readily in the 25 to 25 °C range, completely consuming both reagents. However, the final 1 H NMR spectrum (Figure S4) showed two series of resonances corresponding to two different products, mono-ligand bis(alkyl) 1a-Sc and bis(ligand) mono-alkyl 1a-Sc' in a respective 1:0.3 ratio. This observation is in line with that by Piers et al. describing the preferential formation of a related bis(phenoxy-imino) scandium monoalkyl complex {η 2 -1-[CH=N-Ar]-2-O-3-tBu-C 6 H 3 } 2 ScCH 2 SiMe 3 (I-Sc) along with small amounts of the monoligand bis(alkyl) congener in the reaction between the equimolar amounts of the corresponding proligand and Sc(CH 2 SiMe 3 ) 3 (THF) 2 . [START_REF] Emslie | Organometallic Complexes of Scandium and Yttrium Supported by a Bulky Salicylaldimine Ligand[END_REF] In our case, the increased steric bulk of {1a}  contributes to a better stabilization of the monoligand bis(alkyl) species. More comprehensive NMR data ( 13 C{ 1 H}, 1 H 1 H COSY) were collected for this sample that allowed understanding the solution structures of both products and making assignments of characteristic resonances. For example, for 1a-Sc, the key resonances in the room-temperature 1 H and 13 C{ 1 H} NMR spectra (Figures S4 andS7, respectively) include: (a) two broad signals from the two CH=N groups ( H 9.20, 8.16 ppm and  C 172.5, 158.6 ppm, respectively), (b) (c) in the 1 H NMR spectrum, two doublets from the diastereotopic ScCHH protons ( H 0.67 and 0.25 ppm) and a tiny singlet ( C 42.9 ppm) in the 13 C{ 1 H} NMR spectrum, (d) singlet resonance for the SiMe 3 groups ( H 0.00 ppm and  C 0.4 ppm, respectively). Upon increasing the temperature to 72 °C (Figure S5) both products appeared to be stable in solution, and no significant change in the pattern of signals was observed for 1a-Sc'. On the other hand, for 1a-Sc, substantial broadening was observed for the signals in the aromatic region, thus, suggesting the existence of some fluxional dynamic process probably related to the exchange between the coordinated and pending imino functions. [START_REF] Shannon | For the structurally close complexes {η 2 -1-[CH=N-Ar]-2-O-3-tBu-C 6 H 3 }Y(CH 2 SiMe 3 ) 2 (THF) n (n = 1 and 2 (I-Y)), a fluxional dynamic process was explained by a reversible decoordination of THF molecule and interconversion between isomers arising from the relative positioning of the two alkyl groups around the metal center, see ref. 9 . 11 Effective ionic radii for 6-coordinate centers: Sc 3+ , 0.745 Å; Y 3+[END_REF] A small amount (15 % yield) of pure complex 1a-Sc was isolated by recrystallization and characterized by X-ray crystallography (vide infra).

Similar NMR scale reaction between 1a-H and 1 equiv. of Y(CH 2 SiMe 3 ) 3 (THF) 2 in C 6 D 6 afforded 1a-Y quantitatively as judged by 1 H NMR spectroscopy. Thus, the scaled-up synthesis was repeated allowing isolation of pure 1a-Y in 67 % yield after recrystallization (Scheme 2). Complex 1a-Y was characterized by 1 H and [START_REF]C{ 1 H} NMR (125 MHz, C 7 D 8, 25 o C) (many aromatic signals were not observed due to fluxional dynamics and overlapping): δ 165[END_REF] C NMR spectroscopy and X-ray crystallography. For this compound, a complex fluxional behavior in solution was observed over a broad temperature range. For example, the room-temperature 1 H NMR spectrum of 1a-Y (Figure S8) exhibited a series of broadened signals, only a few of them could be unequivocally assigned: very broad signals at  H 9.007.50 ppm from the CH=N and C 6 H 2 groups, broad multiplet at  H 3.18 ppm from the CH(CH 3 ) 2 protons and singlets at  H 0.12 and 0.71 ppm from the Si(CH 3 ) 3 and CH 2 protons, respectively, of the YCH 2 Si(CH 3 ) 3 groups.

Upon lowering temperature to 40 °C 1a-Y (Figure S9), the broad signals in the aromatic regions split into two pairs of broad singlets, while the resonance from the CH 2 protons in the high-field split into a series of three broad signals. Though, more detailed information about the solution structure of 1a-Y could not be obtained, these observations are consistent with the existence of the same exchange processes as those depicted for 1a-Sc and I-Sc. [START_REF] Emslie | Organometallic Complexes of Scandium and Yttrium Supported by a Bulky Salicylaldimine Ligand[END_REF] The molecular structures of 1a-Sc (Figure S1) and 1a-Y (Figure 1) are comparable to those of the penta-coordinate {η Selected bond distances (Å) and angles (°): Y( 1)O (1), 2.1994(15); Y( 1 Both complexes featured fluxional behavior in a broad range of temperatures, apparently associated with a restricted rotation of bulky bis(isopropyl)phenyl groups. The 2.190(4); O( 1)Y( 1)N( 1), 83.12(15); O( 2)Y( 1)N( 2), 116.62 (18); N( 1)Y( 1)N( 2), 106.17 (17).

The by 0.140.24 Å, in line with the larger ionic radius of yttrium. 11 Also, the intermetallic MM distances in these dinuclear molecules (3.4832(12) and 3.9228( 16) Å, respectively) are not exceptional [START_REF] Duan | Synthesis and Characterization of Dinuclear Rare-Earth Complexes Supported by Amine-Bridged Bis(Phenolate) Ligands and Their Catalytic Activity for the Ring-Opening Polymerization of L -Lactide[END_REF] and are larger than the sum of the corresponding ionic radii (1.490 and 1.800 Å, respectively). 11

Stoichiometric and catalytic studies on the reactivity of bis(alkyl) complex 1a-Y

with 2-phenylpyridine. The CH bond addition of heterocycles (hydroarylation) to alkenes or imines, catalyzed by alkyl complexes of group 3 metals, is known to follow a multi-step mechanism [START_REF] Arnold | CH Bond Activation by f-Block Complexes[END_REF]4 involving the ortho-C(sp 2 )H activation reaction of an aromatic molecule affording aryl intermediate, followed by insertion of the C=C or C=N bond into the Maryl bond. Stoichiometric reactivity of group 3 metals alkyls with 2-phenylpyridine has also been the subject of several studies. For example, the formation of different CH bond activation products incorporating  2 -N,C-6-phenylpyridyl, 13  2 -N,C-2'-phenylpyridyl 13a,14,15 or the 2-phenypyridine-derived biheterocyclic 13a,b,c,14b ligands have been reported. In particular, Diaconescu et al. 13a observed in the corresponding yttrium and lutetium complexes the slow rearrangement process for the 2-phenylpyridine ligand from the  2 -N,C-6-phenylpyridyl (three-membered-ring metallocycle) to the more stable  2 -N,C-2'-phenylpyridyl (five-membered-ring metallocycle) coordination mode.

In order to assess the potential of bis(alkyl) complex 1a-Y in C(sp The five-membered-ring metallocyclic ring of complex 3a-Y exhibiting  2 -N,C-2'-phenylpyridyl coordination of the ligand parallels previous results obtained by different groups. 13a,14,15 It should be also mentioned that the existence of a possible isomerization process in solution similar to that described by Diaconsescu et al. 13a and consisting in our case in the change of the mode of coordination of the 2-phenyl-pyridyl ligand in 3a-Y from  2 -N,C-2'-phenylpyridyl to  2 -N,C-6-phenylpyridyl type could not be unequivocally established by our experimental techniques.

The molecular structure of 3a-Y (Figure 3) revealed the yttrium atom in a seven-coordinated environment provided with the oxygen atom and the nitrogen atom of the {N 2 O}  ligand, the two carbon and two nitrogen atoms of the two monoanionic phenyl-pyridine ligands and the oxygen atom of the coordinated THF molecule. In 3a-Y, the YO and YN distances (2.2225(16) and 2.519(2) Å) for the coordinated phenoxy-imino ligand are very close to those in the parent 1a-Y. The YC(aryl) and YN(aryl) bond lengths (2.502(2), 2.505(2) and 2.504(2), 2.583(2) Å, respectively) in the seven-coordinated 3a-Y are longer than those (2.4492.480 and 2.4532.506 Å, respectively) observed for the previously reported five-and six-coordinated yttrium complexes bearing the same  2 -N,C-2'-phenylpyridyl ligands. [START_REF]C{ 1 H} NMR (125 MHz, C 7 D 8, 25 o C) (many aromatic signals were not observed due to fluxional dynamics and overlapping): δ 165[END_REF]14,15 1)O (2), 85.46(6); O(1)Y( 1)C(47), 132.75 (7); O(1)Y( 1)N(3), 82.26 (7); N( 4)Y( 1)C(58), 66.40(8); N(3)Y( 1)C(47), 66.73 (7).

In order to evaluate the feasibility of the alkylation of styrene with 2-phenylpyridine in the presence of 1a-Y, stoichiometric reactions between complex 3a-Y and styrene were monitored by 1 H NMR spectroscopy. Unfortunately, no reaction was observed in the temperature range 2560 °C with 110 equiv. of styrene. Further increasing of the temperature to 100 °C resulted only in decomposition of 3a-Y, with no detectable insertion of styrene into the Y(pyridine-2'-ylbenzene) bond.

To gain a better insight into the mechanism of CH activation of 2-phenylpyridine with the bis(alkyl) complex 1a-Y, DFT computations were conducted (Scheme S2). The objectives of these non-exhaustive computations were to assess and compare the energy profiles for For benchmarking purposes, the catalytic performance of several bis(alkyl) complexes of scandium and yttrium CpM R 4b and {N 4 }M R 16 (Scheme 5) as reference catalyst precursors, was explored under our experimental conditions (70100 °C, C 6 D 6 ; Table S2). Among the most successful results, the combination (10 mol%) of CpSc R with B(C 6 F 5 ) 3 afforded selectively 2-phenyl-6-(2-phenylethyl)pyridine ( 5) with 53 % conversion after 96 h at 70 °C.

Also, the high selectivity in alkylation of 2-phenylpyridine with styrene was achieved with the binary system {N Under the same or modified conditions, poor catalytic results were achieved using complex 1a-Y as precatalyst (Table S4). For example, only the combination 1a-Y/[Ph 3 C] + [B(C 6 F 5 ) 4 ]  afforded small amounts (6 %) of 5 at 100 °C (Table S4, entries 4 and 5). Given the molecules of 1a-Y and 3a-Y both contain coordinated THF molecules, the absence of activity in catalysis, and particularly, reactivity towards styrene, could be attributed to a strong coordination of THF ligand, thus, impeding its displacement by such a poor donor as styrene. In order to address this issue, several catalytic experiments were conducted in the presence of substoichiometric or stoichiometric amounts of LiCl or AlMe 3 as possible scavengers of THF. However, only in one experiment using AlMe 3 (20 mol%) formation of only a small amount of 2-(2-(1-phenylethyl)-phenyl)-pyridine (6, 5 %) was observed (Table S4, entry 8). No visible formation of products were detected when other electrophilic substrates (e.g. N-R-1-phenyl-methanimine (R = Ph, iPr), PhSiH 3 , Et 3 SiH) were used in the place of styrene.

Studies on copolymerization of CO 2 with cyclohexene oxide. The mononuclear bis(alkyl) 1a-Y and dinuclear alkyl-free complexes [1b-M] 2 were then tested as initiators of copolymerization of CO 2 with cyclohexene oxide (Scheme 6). For benchmarking purposes, first the copolymerization reactions initiated by {BDI}Zn(N(SiMe 3 ) 2 ) [START_REF] Cheng | Single-Site β-Diiminate Zinc Catalysts for the Alternating Copolymerization of CO 2 and Epoxides: Catalyst Synthesis and Unprecedented Polymerization Activity[END_REF] was studied at 70 °C and 12 bar CO 2 pressure (Table 1, entry 1) affording 10 % conversion of monomer after 64 h.

The low activity compared with the result reported by Coates et al., [START_REF] Cheng | Single-Site β-Diiminate Zinc Catalysts for the Alternating Copolymerization of CO 2 and Epoxides: Catalyst Synthesis and Unprecedented Polymerization Activity[END_REF] which gave 44 % conversion at 50 °C after 2 h, is apparently due to the addition of solvent (toluene), as the copolymerization rate law features a first order dependence on monomer concentration. [START_REF] Moore | Mechanism of the Alternating Copolymerization of Epoxides and CO 2 Using β-Diiminate Zinc Catalysts: Evidence for a Bimetallic Epoxide Enchainment[END_REF] To our delight, using complex 1a-Y as catalyst under the same conditions gave 95 % conversion and 97 % selectivity for polymers (entry 2). The yields remained almost unchanged when the loading of 1a-Y decreased from 0.5 to 0.2 mol% (entries 3 and 5). Decreasing the concentration of 1a-Y to 0.1 mol % led to a significantly lower conversion of 57 % (entry 12), suggesting high sensitivity of the initiating system to the presence of impurities. Similar observation on the decrease of the polymer conversion upon reducing the catalyst loading to 0.1 mol% was reported for the series of -diimine bis(alkyl) yttrium complexes. [a] Entry Initiator (%) T ( o C) Time (h) [b] Conversion (%) [c] Polymer [c] Carbonate units [c] M n,GPC (M n,calc ) 10 3

g•mol 1

[e]

Ð 1.72 [a] Reaction conditions: solvent = toluene (1.0 mL); [CHO] 0 = 4.95 mol•L 1 ; 70 ºC; P CO2 = 12 bar; [CO 2 ]/[CHO] = 5.3; n.o. = not observed. [b] Reaction times were not necessarily optimized. [c] Conversion of product and selectivity were determined by 1 H NMR spectroscopy. [d] Determined by DSC.

[e] Determined by GPC; Ð = M w / M n . [f] Experiment conducted in the presence of cyclohexane-1,2-diol (2 equiv. vs 1a-Y).

The reaction temperature appeared to have a significant influence on the copolymerization activity. While the experiment conducted at room temperature resulted in no polymer formation (entry 4), at 50 °C, a conversion of 86 % was observed after 18 h (entry 6). The highest conversion of 92 %, using 500 equiv. of CHO (TON of 460), achieved after 5 In order to determine orders on CHO and catalyst, a series of kinetic studies was undertaken. Kinetic monitoring of copolymerization under regular conditions (Figure S25)

did not exhibit a significant induction period, regardless the catalyst concentration, suggesting rapid initiation with Y CH 2 SiMe 3 groups. The latter fact was corroborated by MALDI-ToF spectroscopy (Figure S31) demonstrating the presence of CH 2 SiMe 3 end-groups in the low molecular weight polycarbonate sample (Table 1, entry 9). The first-order dependence both on the CHO concentration (Figure 4) and on the concentration of catalyst (Figure S26) was determined from the corresponding linear plots, indicating a mononuclear pathway. [START_REF] Moore | Mechanism of the Alternating Copolymerization of Epoxides and CO 2 Using β-Diiminate Zinc Catalysts: Evidence for a Bimetallic Epoxide Enchainment[END_REF][START_REF] Thevenon | Indium Catalysts for Low-Pressure CO 2 /Epoxide Ring-Opening Copolymerization: Evidence for a Mononuclear Mechanism?[END_REF] ) compares well with those reported in the literature for the atactic analogues. [START_REF] Nakano | Spectral Assignment of Poly[Cyclohexene Oxide-Alt-Carbon Dioxide[END_REF] The thermal properties of several copolymer samples (entries 2 and 10)

were also examined by DSC analysis revealing the T g values of 88.7 (Figgure S30) and 87.3 o C, respectively. These values are typical for such low molecular weight polymers falling in the regular range of those (45.0118.9 o C) reported for polycarbonates. 22 No melting transitions were detected in these cases.

The fact that the experimentally determined by GPC average-number molecular weights, are systematically lower than the theoretical values, calculated from the initial monomer-to-initiator ratio and conversion values, are diagnostic of several possible phenomena operating under our conditions: initiation and growing of two polymer chains per one metal center, and occurrence of side transfer reactions, e.g. chain transfer to cyclohexane-1,2-diol generated from CHO in the presence of adventitious protic impurities (H 2 O). 23 The latter phenomenon can also be responsible for the broadening of the molecular weight distributions of polymers obtained in the experiments carried out with low amounts of 1a-Y (entry 12). In order to probe this hypothesis, copolymerization experiment with 2 equiv.

of cyclohexane-1,2-diol was carried out (entry 13). Thus, the lower CHO conversion (60 %)

was achieved as compared to that in the experiment without cyclohexane-1,2-diol (95% conversion, entry 5). The PDI value was found to be much broader (8.30 vs 1.99, respectively), which is diagnostic of poorly controlled transfer processes operating under these conditions and/or of multi-site behavior of the catalytic system.

Alkyl compounds of main-group metals are known not only to promote the copolymerization of CO 2 with epoxides, 24 but also to contribute to the formation of highly active polynuclear systems derived from transition metals. 6d,e,g,h,19 The effect of the addition of such alkyl metal (Zn, Mg, Al) reagents to 1a-Y was also investigated in copolymerization (entries 1416) and, despite our expectations, appeared to be detrimental. The efficiency of the resulting binary systems was found to be much lower than that of the mono-component 1a-Y.

Dinuclear alkyl group-free complexes [1b-Sc] 2 and [1b-Y] 2 were also probed as promoters of copolymerization (entries 17 and 18, respectively). Under identical conditions (70 °C, 18 h), only small amounts of polymers were isolated (14 and 31 % conversion, respectively), which were identified by NMR spectroscopy as polyethers (Figure S32), 7a homopolymers of CHO. The different nature of initiating groups (alkyl vs phenolate group) may account for the reactivity and selectivity differences in these systems. 25 This observation also parallels the results reported by Chakraborty et al. for dinuclear titanium and zirconium complexes, which produced poly(cyclohexene oxide) polymers in the absence of a nucleophilic cocatalyst. [START_REF] Mandal | Synthesis and Structural Characterization of Titanium and Zirconium Complexes Containing Half-Salen Ligands as Catalysts for Polymerization Reactions[END_REF]

CONCLUSIONS

In summary, we investigated complexation of two types of ligand platforms, namely, bis(imino)phenoxy and bis(amido)phenoxy, with group 3 metals. During this study, a series of new complexes of scandium and yttrium were obtained by -bond metathesis (alkane elimination) and completely characterized. Among them, bis(alkyl) complex {N H, 8.65; N, 4.91. Found C, 75.84; H, 8.69; N, 4.73.

Reaction between 1b-H 3 and Y(CH

For Table of Contents Only

Synopsis

Mono and dinuclear group 3 metal complexes incorporating polydentate bis(imino)phenoxy {N 2 O}  and bis(amido)phenoxy {N 2 O} 3 ligands were synthesized. Bis(alkyl) complex of yttrium was used for copolymerization of cyclohexene oxide with CO 2 giving polycarbonates with high efficiency and selectivity, and also for the selective CH activation of 2-phenylpyridine affording the corresponding bis(aryl) product. The mechanism of the latter reaction was rationalized by DFT computations.

  Scheme 1. Proligands 1a-H and 1b-H 3 Used in This Study.

  Scheme 2. Formation of Complexes 1a-M (M = Sc, Y) and 1a-Sc'.

Figure 1 .

 1 Figure 1. Molecular structure of {N 2 O}Y(CH 2 SiMe 3 ) 2 (THF) 2 (1a-Y) (all hydrogen atoms and

10 room-temperature 1 H

 101 NMR spectra of [1b-Sc] 2 and [1b-Y] 2 (Figures S12 and S16, respectively) contained in each case a series of resonances consistent with the average C 2 -symmetry of molecules. For [1b-Sc] 2 , the four broadened characteristic signals from the two pairs of diastereotopic protons of the CH 2 N groups were observed at  H 5.29, 4.06, 3.85 and 2.97 ppm in the 1 H NMR spectrum and the corresponding two resonances at  C 62.8 and 55.8 ppm in the 13 C NMR spectrum. The same groups in [1b-Y] 2 afforded two well-resolved doublets  H 5.25 and 3.95 ppm ( 2 J HH = 14.6 Hz) and two resonances at  C 62.6 and 59.5 ppm at in the corresponding 1 H and 13 C{ 1 H} NMR spectra.

Figure 2 .

 2 Figure 2. Molecular structure of [1b-Y] 2 (all hydrogen atoms and iPr groups are omitted for

  molecular structures of [1b-Sc] 2 and [1b-Y] 2 are depicted in Figure S3 and Figure 2, respectively. In unit cell of [1b-Y] 2 contains three independent molecules featuring very similar geometrical parameters (bond lengths and angles) and overall organizations, and therefore the structural details of only one of them is discussed hereafter. Isostructural [1b-Sc] 2 and [1b-Y] 2 exhibit each the two five-coordinated metal atoms whose geometries are best described as distorted trigonal bipyramidal composed by two nitrogen and three oxygen atoms of the two {N 2 O} 3 ligands and one THF molecule. Thus, the MO and MN bonds (2.090(3)2.143(3) and 2.006(3)2.087(3) Å, respectively) with ligand in [1b-Sc] 2 are shorter than those in the yttrium analogue (2.278(4)2.337(4) and 2.190(4)2.212(5) Å, respectively)

Scheme 4 .

 4 Scheme 4. Formation of Complex 3a-Y From 1a-Y and 2-Phenylpyridine.

Figure 3 .

 3 Figure 3. Molecular structure of 3a-Y (all hydrogen atoms and iPr groups are omitted for

  Scheme 5. Hydroarylation of Styrene With 2-Phenylpyridine.

  7b

6 .

 6 Scheme 6. Copolymerization of Cyclohexene Oxide (CHO) and CO 2 Catalyzed by

Figure 4 .

 4 Figure 4. Plot of ln([CHO] 0 /[CHO] t ) as a function of time for the copolymerization of CO 2

2 2 .

 22 SiMe 3 ) 3 (THF) 2 . Formation of complex [1b-Y]Using a similar procedure, described for 1a-Y, complex [1b-Y] 2 was prepared from 1b-H 3 (0.053 g, 0.1 mmol) and Y(CH 2 Si(CH 3 ) 3 ) 3 (THF) 2 (0.050 g, 0.1 mmol). Yellow crystals of [1b-Y] 2 (0.024 g, 39 %) were obtained after the solution was kept for 7 days at -25 °C.1 H NMR (500 MHz, toluene-d 8, 25 o C): δ 7. 16H, Ar), 5.25 (d,[START_REF] Edelmann | Synthesis and Structural Chemistry of Non-Cyclopentadienyl Organolanthanide Complexes[END_REF] J HH = 14.6, 4H, CH 2 N),3.95 (d, 2 J HH = 14.6, 4H, CH 2 N), 3.803.00 (br m, 16H, -CH 2, THF + CH(CH 3 ) 2 ),1.34-1.11 (m, 74H, -CH 2 (THF) + CH(CH 3 ) 2 + C(CH 3 ) 3 ).[START_REF]C{ 1 H} NMR (125 MHz, C 7 D 8, 25 o C) (many aromatic signals were not observed due to fluxional dynamics and overlapping): δ 165[END_REF] C{ 1 H} NMR (125 MHz, calculation at the same level of theory, to be considered for the calculation of the total energy values at T = 298 K in the same way as in the approach used byCastro et al. 34 Typical procedure for hydroarylation of styrene with 2-phenyl-pyridine in the presence of group 3 metal complexes. In a typical experiment (TableS1, entry 4), in the glovebox, to a Teflon-valved NMR tube was placed 1a-Y (0.0093 g, 0.01 mmol), 2-phenylpyridine (14.20 µL, 0.10 mmol), styrene (11.50 µL, 0.10 mmol), and C 6 D 6 (0.5 mL) was added at 25 °C. The tube was sealed at room temperature and the mixture was heated at the required temperature for the desired time. Progress of the reaction was monitored by 1 H NMR spectroscopy.Typical Procedure for Copolymerization of CO 2 with CHO. In a typical experiment (Table1, entry 5), in the glovebox, a Schlenk tube was charged with CHO (1.0 mL, 9.9 mmol), 1a-Y (0.02 g, 0.05 mmol) and toluene (1.0 mL). The mixture was transferred to an autoclave equipped with a magnetic stirring bar under argon and, then, pressurized at CO 2(12 bar). The reaction mixture was stirred vigorously at the required temperature for the desired time. After cooling to room temperature, CO 2 was released and a small amount of the resulting mixture was analyzed by 1 H NMR spectroscopy to determine the conversion and selectivity. The reaction mixture was quenched by the addition of MeOH/HCl, then poured into a large amount of MeOH to precipitate the polymer, which was dried under vacuum at 40 °C and weighted.ASSOCIATED CONTENTSupporting Information. Crystallographic data; representative NMR and MALDI-TOF Calculations? Some Insights from Propylene Coordination and Insertion Reactions withGroup 3 and 4 Metallocenes. ACS Catal. 2015, 5, 416-425.

  two broad signals from the C 6 H 2 moiety ( H 9.02, 7.35 ppm and  C 131.6, 135.2 ppm, respectively), (c) one resonance for the ScCH 2 groups ( H 0.01 ppm and  C 40.4 ppm, respectively), (d) singlet resonance for the SiMe 3 groups ( H 0.08 ppm and  C 6 3.3 ppm, respectively). For 1a-Sc', similar key resonances include: (a) two sharp signals from the two pairs of the equivalent coordinated and non-coordinated CH=N groups ( H 7.93, 7.45 ppm and  C 172.5, 156.8 ppm, respectively), (b) two sharp signals from the non-equivalent protons of the two C 6 H 2 moieties ( H 8.90, 7.19 ppm and  C 132.3, 134.8 ppm, respectively),

-Sc and 1a-Y. In 1a-Y, the

		2 -1-[CH=N-Ar]-2-O-3-tBu-C 6 H 3 }Y(CH 2 SiMe 3 ) 2 (THF) and
	six-coordinate	{η 2 -1-[CH=N-Ar]-2-O-3-tBu-C 6 H 3 }Y(CH 2 SiMe 3 ) 2 (THF) 2	complexes,
	respectively, reported previously. 9 Thus, the respective distorted trigonal-bipyramidal and
	octahedral coordination environments around the metal centers are perfectly reproduced in the
	structures of 1aYC(carbyl) and YN(1) bond lengths (2.423(2),
	2.438(2) and 2.5159(18) Å, respectively) are in the range of those observed in the two yttrium
	complexes (2.3982.440 and 2.4662.661 Å, respectively), 9 while the YO(1) distance
	(2.1994(15) Å) is only slightly longer (2.1212.166 Å).

In 1a-Sc, the same bond lengths

	(2.2100(16), 2.2333(16), 2.0198(11) and 2.3502(13) Å, respectively) are shorter than those in
	the yttrium congener, which is in line with the decrease of the effective ionic radii of the
	Sc(3+) metal ion. 11

Scheme 3. Formation of Complexes [1b-M] 2 .

	unreacted tris(alkyl) precursor took place, as judged by 1 H NMR spectroscopic studies of the
	corresponding crude reaction mixtures.
	In attempts to synthesize dinuclear complexes, in which two metal centers are linked by
	a single ligand platform, regular alkane elimination reactions of 1b-H 3 with the corresponding
	rare-earth metal precursors M(CH 2 SiMe 3 ) 3 (THF) 2 (M = Sc, Y) in the corresponding 1:2 ratio
	were studied. However, only alkyl group-free dinuclear bis(ligand) complexes [1b-M] 2 were
	isolated (Scheme 3). For instance, monitoring by 1 H NMR spectroscopy of the reaction of
	1b-H 3 with 2 equiv. of M(CH 2 SiMe 3 ) 3 (THF) 2 confirmed the quantitative formation of these
	products, while 1 equiv. of the tris(alkyl) precursor remained intact in each case. The larger
	scale syntheses of complexes [1b-M] 2 were optimized by using equimolar amounts of the two
	reagents. Both compounds were found stable in benzene-d 6 or toluene-d 8 solutions at 60 °C
	for days.
			tBu
	N Y(1)N(1), 2.5159(18); Y(1)C(41), 2.423(2); Y(1)C(45), 2.438(2); Y(1)O(61), O N Ar Ar M(CH 2 SiMe 3 ) 3 (THF) 2 )O(51), 2.3779(18); -25 °C to R.T. N O N Ar Ar Hexane 1b-H 3 M M THF THF
		-Me 4 Si
	2.4637(17); O(51)Y(1)N(1), 160.20(6); C(45)Y(1)O(61), 86.17(7); C(41)Y(1)O(61),
			tBu
	168.96(8).	Ar = 2,6-iPr 2 C 6 H 3	M = Sc, [1b-Sc] 2 (45 %)
			Y, [1b-Y] 2 (39 %)
	In the attempts to synthesize dinuclear complexes, potentially incorporating two
	M-(CH 2 SiMe 3 ) x groups united by one multifunctional {N 2 O}  ligand scaffold, upon reacting
	1a-H with 2 equiv. of M(CH 2 SiMe 3 ) 3 (THF) 2 only the formation of 1a-M together with the

Table 1 . Copolymerization of CO 2 With CHO Catalyzed by Complex 1a-Y.

 1

and Sc(CH 2 SiMe 3 ) 3 (THF) 2 . Formation of complex 1a-Sc .

  [START_REF] Edelmann | Synthesis and Structural Chemistry of Non-Cyclopentadienyl Organolanthanide Complexes[END_REF] O}Y(CH 2 SiMe 3 ) 2 (THF) 2 (1a-Y) was used for studies on stoichiometric CH activation of 2-phenylpyridine selectively affording under mild conditions an original bis(aryl) product, complex {N 2 O}Y( 2 -N,C-2'-PhPy) 2 (THF) (3a-Y). The latter was found stable in solution under ambient conditions and completely reluctant in reactions with weak electrophiles, such Et 2 O) and Na/K alloy (toluene, pentane) under argon, degassed thoroughly, and stored under nitrogen prior to use. Deuterated solvents (benzene-d 6 , toluene-d 8 , THF-d 8, >99.5 % D, Deutero GmbH and Euroisotop) were vacuum transferred from Na/K alloy into storage tubes. The ligand precursors 1a-H, 8a 1b-H 3, 8b Me 3 SiCH 2 Li, 27 M(CH 2 SiMe 3 ) 3 (THF) 2 , M(CH 2 C 6 H 4 -o-NMe 2 ) 3 , 28 CpSc R , 4a CpY R 4b and {N 4 }M R 16 (M = Sc, Y) were prepared according to the published procedures. Styrene and cyclohexene oxide were distilled from CaH 2 and stored in the fridge at 25 o C. 2-Phenylpyridine, PhOMe, PhNMe 2 , PhSiH 3 , Et 3 SiH were dried with 4Å molecular sieves and stored under argon. Other starting materials were purchased from Acros, Strem and Aldrich, and used as received.Instruments and Measurements. NMR spectra of complexes were recorded on Bruker AM-400, and AM-500 spectrometers in Teflon-valved NMR tubes at 25 °C, unless otherwise indicated. 1 H,[START_REF]C{ 1 H} NMR (125 MHz, C 7 D 8, 25 o C) (many aromatic signals were not observed due to fluxional dynamics and overlapping): δ 165[END_REF] C chemical shifts are reported in ppm vs SiMe 4 (0.00), as determined by reference to the residual solvent peaks. The resonances of organometallic complexes were H and[START_REF]C{ 1 H} NMR (125 MHz, C 7 D 8, 25 o C) (many aromatic signals were not observed due to fluxional dynamics and overlapping): δ 165[END_REF] C NMR spectroscopy according to the published procedures.[START_REF] Nakano | Spectral Assignment of Poly[Cyclohexene Oxide-Alt-Carbon Dioxide[END_REF] MALDI-TOF spectra were acquired on a Bruker Ultraflex-III TOF/TOF mass spectrometer (Bruker Daltonics, Inc., SiMe 3 ) 3 (THF) 2 (0.023 g, 0.05 mmol). To this mixture, C 6 D 6 (ca. 0.5 mL) was vacuum-transferred in at 25 °C and the tube was shaken for 1 h at room temperature.

	H, 9.20; N, 3.01. Found: C, 67.30; H, 9.46; N, 2.92.
	Reaction between 1b-
	as styrene, imines and hydrosilanes. This reaction of formation of 3a-Y from 1a-Y and Billerica, MA) equipped with a Nd-YAG laser (355 nm). CH 3 COONa was added for
	2-phenylpyridine through a series of CH activation steps was studied by DFT calculations, facilitating ion formation, and trans-2-[3-(4-tert-butylphenyl)-2-methyl-
	and the inactivity of 3a-Y towards styrene was also rationalized. Attempts to use 1a-Y, in 2-propenylidene]malononitrile was used as matrix.
	combination with borane and borate activators for the hydroarylation of styrene and imines were also unsuccessful. Reaction between 1a-H In the glovebox, in a Teflon-valved NMR tube was placed 1a-H (0.026 g, 0.05 mmol),
	Gratifyingly, 1a-Y found application as initiator of copolymerization of CO 2 with cyclohexene epoxide under mild conditions (70 °C, 12 bar of CO 2 pressure, toluene). The corresponding polycarbonate polymers were obtained with nearly quantitative conversion over 5 h of polymerization and high selectivity (9799 % of carbonate units). Yet, this system constitutes a rare example of most efficient rare-earth metal alkyl complexes for copolymerization of CO 2 with epoxides (TON of 460) operating under mild conditions. These results again highlight the high potential of group 3 metal alkyl complexes in catalysis. Further studies of the development of new ligand platforms for CH activation chemistry and CO 2 transformations are ongoing in our laboratories. EXPERIMENTAL SECTION General Considerations. All manipulations were performed under a purified argon atmosphere using standard Schlenk techniques or in a glovebox. Solvents were distilled from constants are given in hertz. Elemental analyses (C, H, N) were performed using a Flash EA1112 CHNS Thermo Electron apparatus and are the average of two independent determinations. DSC measurements were performed on a SETARAM Instrumentation DSC 131 differential scanning calorimeter at a heating rate of 10 °/min, first and second runs were recorded after cooling to 30 °C. Size exclusion chromatography (SEC) of polycarbonate samples was performed in THF (1 mL•min 1 ) at 20 °C using a Polymer Laboratories PL50 apparatus equipped with PLgel 5 µm MIXED-C 300 × 7.5 mm columns, and combined RI and Dual angle LS (PL-LS 45/90°) detectors. The number average molecular weights (M n ) Sc(CH 2 be assigned unequivocally): 1 H NMR (500 MHz, C 6 D 6, 25 o C): δ 9.20 (br s, 1H, CH=N), 9.00 (br s, 1H, Ar), 8.16 (br s, 1H, CH=N), 7.35 (br s, 1H, Ar), 3.89 (br m, 4H, -CH 2 , THF), 3.29 (br m, 4H, CH(CH 3 ) 2 ), 1.34 (m, 4H, -CH 2 , THF), 1.391.26 (m, 24H, CH(CH 3 ) 2 ), 1.21 (s, 9H, C(CH 3 ) 3 ), 0.08 (s, 18H, Si(CH 3 ) 3 ), 0.01 (s, 4H, ScCH 2 ). 13 C{ 1 H} NMR (125 MHz, C 6 D 6, 25 o C): δ 172.4 (CH=N), 158.6 (CH=N), 150.9 (Ar), 148.9 (Ar), 140.9 (Ar), 137.4 (Ar), 135.2 (Ar), 131.6 (Ar), 127.1 (Ar), 124.1 (Ar), 122.9 (Ar), 122.3 (Ar), 68.6 (-CH 2 , THF), 40.4 (ScCH 2 ), 28.6 (CH), 28.2 (CH), 28.1 (CH 3 ), 25.8 (CH 3 ), 25.0 (-CH 2 , THF), 23.1 (CH 3 ), 22.2 (CH 3 ), 3.3 (Si(CH 3 ) 3 ). Compound 1a-Sc' (some resonances couldn't be assigned unequivocally): 1 H NMR Na/benzophenone (THF, assigned from 2D 1 H 1 H COSY, 1 H 13 C HSQC and HMBC NMR experiments. Coupling (500 MHz, C 6 D 6, 25 29

and polydispersities (Ð) of the polymers were calculated with reference to a universal calibration vs. polystyrene standards. The microstructure of polycarbonate was determined by

1 1

H NMR spectroscopy indicated quantitative consumption of both reagents and formation of 1a-Sc and 1a-Sc' in 1:0.3 ratio, respectively. Compound 1a-Sc (some resonances couldn't o C): δ 8.90

(d, 4

J = 2.6, 2H, Ar),

7.93 (s, 2H, CH=N), 7.45 (s, 2H, CH=N), 7.307.19 (m, 10H, Ar), 7.19 (d, 4

J = 2.6, 2H, Ar),

6.93 (d, J = 7.5, 2H, Ar), 6.82 (d,

H 3 and Sc(CH 2 SiMe 3 ) 3 (THF) 2 . Formation of complex

  THF), 62.8 (CH 2 N), 55.8 (CH 2 N), 31.1 (CHCH 3 ), 29.3 (CHCH 3 ), 28.1 (CH 3 ), 26.8 (CH 3 ), 23.8 (-CH 2, THF), 22.6 (CH 3 ). Anal. Calcd for C 72 H 98 N 4 O 2 Sc 2 : C, 75.76;

	[1b-Sc] 2. Using a similar procedure, described for 1a-Y, complex [1b-Sc] 2 was prepared from
	1b-H 3 (0.0528 g, 0.1 mmol) and Sc(CH 2 SiMe 3 ) 3 (THF) 2 (0.0450 g, 0.1 mmol). Colorless
	crystals of [1b-Sc] 2 (0.029 g, 45 %) were obtained after the solution was kept for 3 days at
	25 °C. 1 H NMR (500 MHz, C 6 D 6, 25 o C): δ 7.39 (br m, 6H, Ar), 7.326.74 (br m, 8H, Ar),
	6.74 (br m, 2H, Ar), 5.60 (br m, 2H, CH 2 N), 5.29 (br m, 2H, CH 2 N), 4.41 (br m, 2H,
	CH(CH 3 ) 2 ), 4.06 (br m, 2H, CH 2 N), 4.00 (br m, 4H, -CH 2, THF), 3.85 (br m, 2H, CH 2 N),
	3.53 (br m, 6H, -CH 2, THF + CH(CH 3 ) 2 ), 2.97 (br m, 4H, CH(CH 3 ) 2 ), 1.77 (br m, 6H,
	C(CH 3 ) 3 ), 1.600.70 (br m, 62H, CH(CH 3 ) 2 + -CH 2 (THF) + C(CH 3 ) 3 ), 0.43 (br m, 6H,
	C(CH 3 ) 3 ) . 13 C{ 1 H} NMR (125 MHz, C 6 D 6, 25 o C) (due to a strong fluxional dynamics some
	of the aromatic signals could not be observed): δ 153.7 (Ar), 152.2 (Ar), 145.3 (Ar), 123.6
	(Ar), 71.6 (-CH 2,

ACKNOWLEDGEMENTS

The authors are grateful to the PHC Cai Yuanpei program between Campus France and Chinese Research Council. EK thanks ENSCR and the CTI group of ISCR for computational facilities, and acknowledges support from ANR-17-CE06-0006-01 "CO22CHEM". We are grateful to Dr Vincent Dorcet (Univ Rennes) for resolving the structure of 1a-Y and to Lihua Hu (Soochow University) for MALDI-ToF analyses.

Crystal Structure Determination of Complexes 1a-M, 2a-M , [1b-M] 2 and 3a-Y (M

= Sc and Y). Diffraction data were collected at 150(2) K using a Bruker APEX CCD diffractometer with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Ǻ). The crystal structures were solved by direct methods, remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F2 (programs SIR97 and SHELXL-97). 29 Many hydrogen atoms could be located from the Fourier difference analysis. Other hydrogen atoms were placed at calculated positions and forced to ride on the attached atom. The hydrogen atom positions were calculated but not refined. All non-hydrogen atoms were refined with anisotropic displacement parameters. Crystal data and details of data collection and structure refinement for the different compounds are given in Table S1. Crystal data, details of data collection and structure refinement for all compounds (CCDC 2016816-2016822, respectively) can be obtained from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Studies. The calculations were performed using the Gaussian 09 [START_REF] Frisch | Revision D.01[END_REF] program employing B3PW91 31 functional, and using a standard double- polarized basis set, namely the LANL2DZ set, augmented with a single polarization f function on yttrium (0.835) and a single polarization d function on silicon (0.296). The solvent effects, in our case for toluene, were taken into account during all the calculations by means of the SMD model. [START_REF] Marenich | Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions[END_REF] All stationary points were fully characterized via analytical frequency calculations as either true minima (all positive eigenvalues) or transition states (one imaginary eigenvalue). The IRC procedure was used to confirm the nature of each transition state connecting two minima. [START_REF] Gonzales | An Improved Algorithm for Reaction Path Following[END_REF] Zero-point vibrational energy corrections (ZPVE) were estimated by a frequency


R Bahi

C Nouveau

N E Beliardouh
email: beliardouh_23@yahoo.fr

C E Ramoul

S Meddah

O Ghelloudj

Surface performances of Ti-6Al-4V substrates coated PVD multilayered films in biological environments

Keywords:

Multilayered and nanostructured coatings of Ti based alloys (oxides and nitrides) are elaborated and tested for increasing protective properties such as corrosion and wear resistances. A pin-on-disc tribometer was used to evaluate the wear resistance in Hank's solution against bovine bone. Corrosion behavior in Hank's solution was determined by potentiodynamic and electrochemical impedance spectroscopy techniques. Besides, the specimen surfaces were characterized by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) microanalyses. The results showed that optimal tribological properties were obtained in the case of coatings having TiN as top layer. The main wear mechanism was abrasive third body wear. In vitro corrosion tests at 37 °C showed that the better corrosion resistance was obtained when TiN was the top layer. However all of them exhibited good tribological properties, good corrosion resistance and then may be promising options for biomedical applications.

Introduction

The challenge in biomedical implants is to improve mechanical properties and corrosion resistance as well as biocompatibility at low cost. Ti-6Al-4V alloy is one of the most common materials used as orthopedic implants into the human body. It has good mechanical properties and favorable biocompatibility in addition to an excellent resistance against wear and corrosion in aggressive environments [START_REF] Mendonça | Advancing dental implant surface technology -from micron-to nanotopography[END_REF][START_REF] Yao | Anodization: a promising nano-modification technique of titanium implants for orthopedic applications[END_REF]. However, it was indicated that Aluminum and Vanadium had a negative biological responses because of their toxicity effects caused by the dissolution of Al and V ions in the human body [START_REF] Cui | A graded nano-TiN coating on biomedical Ti alloy: low friction coefficient, good bonding and biocompatibility[END_REF][START_REF] Kaur | Review on titanium and titanium based alloys as biomaterials for orthopaedic applications[END_REF]. On another hand, poor tribological behavior of Ti-6Al-4V alloy leads to a fine wear debris causing allergy and inflammation. Another disadvantage is that caused by the high difference between elastic moduli of the antagonists in contact i.e. implant and bone; E (Ti-6Al-4V) is around 114 GPa [START_REF] Ching | Effect of surface coating on reducing friction and wear of orthopedic implants[END_REF], while E (bone) is nearly 30 GPa [START_REF] Wang | Nano-and macro-scale characterisation of the mechanical properties of bovine bone[END_REF]. Consequently, inconveniences mentioned above lead to a decrease in the service life of the prosthesis [START_REF] Tamilselvi | In vitro and in vivo studies of alkali-and heat-treated Ti-6Al-7Nb and Ti-5Al-2Nb-1Ta alloys for orthopedic implants[END_REF][START_REF] Guo | The effect of hydrofluoric acid treatment of TiO2 grit blasted titanium implants on adherent osteoblast gene expression in vitro and in vivo[END_REF].

To enhance mechanical properties, biocompatibility, osseointegration, wear and corrosion resistance, protecting the Ti-6Al-4V substrates by PVD layers was proven to be a promising solution. TiN and TiO 2 are among coatings used as protective because of their non-toxicity, good stability, ease of mass production and high controllability [START_REF] Hernández-Montes | Titanium dioxide coatings on magnesium alloys for biomaterials: a review[END_REF][START_REF] Shukla | Structural, mechanical and corrosion resistance properties of Ti/TiN bilayers deposited by magnetron sputtering on AISI 316L[END_REF]. Since the last two decades, many researchers have shown that multilayer coatings, especially metal/ceramic sequences, improve the mechanical properties of the coating and their resistance to higher corrosion compared to monolayer coatings [START_REF] Chenglong | Corrosion resistance and hemocompatibility of multilayered Ti/TiN-coated surgical AISI 316L stainless steel[END_REF][START_REF] Herranen | Corrosion behaviour of Ti/ TiN multilayer coated tool steel[END_REF][START_REF] Nordin | Influence of lamellae thickness on the corrosion behaviour of multilayered PVD TiN/CrN coatings[END_REF][START_REF] Tian | Corrosion behavior of TiN and TiN/Ti composite films on Ti6Al4V alloy in Hank's solution[END_REF][START_REF] Yanfeng | Effect of multilayered structure on properties of Ti/TiN coating[END_REF]. Several interlayers should be deposited on the substrate and below external coating surface. It's found that ductile titanium in the intermediate layers in coatings can reduce internal stresses and improve toughness of hard coatings [START_REF] Jiang | Influence of titanium interlayer thickness distribution on mechanical properties of Ti/TiN multilayer coatings[END_REF]. The surface of titanium alloy is naturally covered by a passive layer of titanium dioxide (TiO 2 ), when in contact with biological environments. Thus, passive layer create an interface between the surrounding tissue and the implant promoting osseointegration [START_REF] Bogdanov | The particles precipitation and osseointegration of a TiO2 thin-film coating by ion beam deposition -an in vivo study[END_REF][START_REF] Longo | Improving osteoblast response in vitro by a nanostructured thin film with titanium carbide and titanium oxides clustered around graphitic carbon[END_REF][START_REF] Heinrichs | Physical vapour deposition and bioactivity of crystalline titanium dioxide thin films[END_REF].

In the present work, new design of multilayer coatings was elaborated by DC magnetron sputtering and evaluated for their suitability as protective coatings for biomedical implants. The tribological behavior (against bone) and corrosion behavior in Hank's solution of the (Ti/ TiN) 9 and (Ti/TiN) 9 /TiO 2 systems have been analyzed and compared.

Multilayer coatings, used in this study, alternating successively hard phase/ductile phase are a biomimetic design inspired from nature (nacre-inspired materials and coatings [START_REF] Lin | Growth and structure in abalone shell[END_REF][START_REF] Park | Biomaterials: Principles and Applications[END_REF]). Hank's solution is the most widely used simulated human body fluid, in corrosion tests as an aggressive medium and in wear tests as a bio-lubricant. Wang et al. [START_REF] Wang | Tribological behavior of Ti-6Al-4V against cortical bone in different biolubricants[END_REF] reported that bio-lubricants should be considered to be an important factor on the tribological behavior of bone and titanium alloys. The extreme conditions of tests, used in this study, are far from being the case in vivo, however accidents may result and they could be an effective tool when choosing the appropriate material for the specific implant.

Materials and methods

Coatings deposition

Coatings were elaborated by reactive DC magnetron sputtering (KENOSISTEC KV40 system), using a high-purity Ti target (99.95%). Ti-6Al-4V substrates were intended for tribological and corrosion tests (Ø = 20 mm × 5 mm, R a = 165 ± 5 nm) and Silicon (100) wafer substrates (10 mm × 10 mm × 0.38 mm, R a = 1 nm) were used for nondestructive analysis. Coating specimens deposited on Si substrates were cut longitudinally with a diamond tip before SEM observation of the cross section. The Ti-6Al-4V and Si (100) substrates were ex-situ ultrasonically cleaned for 10 min in acetone and deionized water successively. Fig. 1 shows a scheme of the multilayer coatings elaborated in the present work. The design consists of a pure Ti metal under layer (~100 nm) for all the samples. Afterwards, alternating (Ti/TiN) layers were performed by controlling the reactive nitrogen flow (purity: 99.999%). Specimens "type C1" are composed of substrate + nine repeating Ti/TiN layers (~180 nm thick); which means a total of eighteen (18) layers. Specimens "type C2" and "type C3" were elaborated firstly as well as C1. Then, the nitrogen flow was stopped and oxygen (purity: 99.995%) was introduced into the chamber. The process was conducted with a continuous gas flow (mixing Ar + O 2 ) ensured by an MKS massflow controller during 720 s. The pressure was kept constant at the level of 0.5 Pa. The focus is to form a TiO 2 film as a top layer (~200 nm). Two O 2 partial pressures are employed i.e. O 2 /Ar ratio = 0.12 and O 2 / Ar ratio = 0.26 during the elaboration of C2 and C3 specimens respectively. The deposition parameters are summarized in Table 1.

Characterizations

Coating adhesion

Coating adhesion was evaluated by scratch test. The lower critical load L C1 (cohesive failure) and the critical load L C2 (adhesive failure) were measured with a CSM Instruments scratch tester equipped with a diamond stylus (Rockwell C, 200 μm radius tip) under progressive loading conditions of 1-50 N, with a 5 mm scratch length. All specimens are tested in the same conditions (Table 2).

Corrosion

Electrochemical investigations were carried out with Autolab potentiostat/galvanostat in 100 ml of the Hanks' solution at 37 °C (pH = 7, and composition in (g/l): 8.0 NaCl, 0.4 KCl, 0.2 MgSO 4 •7H 2 O, 0.16 CaCl 2 •H 2 O, 0.04 Na 2 HPO 4 , 0.06 KH 2 PO 4 , 0.35 NaHCO 3 , and 1.0 Glucose). The temperature of the electrolyte was controlled by thermostat with water recirculation at 37 ± 1 °C. Global measurements were performed using a classical three-electrode cell; Pt and Ag/AgCl (3M KCl solution) are employed as counter and reference electrodes, respectively. The electrochemical impedance spectroscopy (EIS), was performed at open circuit potential (OCP) after a time period of 1 h. The frequency range was from 10 5 kHz to 10 -2 Hz at a rate of 5 points/ decade in E ocp vs Ag/AgCl and the amplitude was set at 10 mV. The potentiodynamic polarization test was raised from -1 V to +1 V, at a scan rate of 1 mV/s. Before the test, specimens were immersed in the electrolyte for 2 h to attain stable open circuit potential (OCP). The polarization resistance (R p ) was estimated from Tafel curves using Stern-Geary equation as follows:

= × + × R β β a βc I 2.3039(β ) 1 p a c corr (1)
where β a , β c are the slopes of anodic and cathodic Tafel plots.

Tribology

Bovine cortical bone was designed to be the upper friction counterpart against rotating Ti-6Al-4V, C1, C2, and C3 disc specimens to further simulate their (bone) in vivo tribological behavior. As this is not possible to have spherical ball from organic matter of animal origin (cartilage, bone, etc.), the pin-on-disc configuration provided by ©CSM Instruments (Switzerland) tribometer is perfectly adapted for this case and is described in details in a previous work [START_REF] Ramoul | Surface performances of PVD ZrN coatings in biological environments[END_REF]. The bone pins were prepared (cylinder Ø = 6 mm × 20 mm, Ra = 0.73 μm) from a bovine femur. Wear tests were conducted in accordance to the ASTM G99-95a to determine the coefficient of friction (COF). The geometry of the pinon-disc tests is shown in Fig. 2. During the tests, discs were completely immerged in Hank's solution. The flowing parameters were kept constant: applied load (N = 3 N), sliding speed (v = 5 cm/s) and track radius (r = 4 mm). The sliding distance of 100 m was long enough to produce a breakdown of the coating. The specific wear rate was obtained by the classical formula as follows:

= W V NL (mm /Nm) s 3 (2)
where W s is the specific wear rate, V is the wear volume [mm 3 ], N is the normal load [N] and L is the sliding distance [m]. The W s estimation was automatically obtained by short software [START_REF] Ramoul | Surface performances of PVD ZrN coatings in biological environments[END_REF] using 3D profilometry image (Optical profilometer VEECO, Wyko NT-1100). The surface morphologies of discs and pins were examined by SEM/EDS (JEOL JSM-5900-LV microscope).

Surface roughness and structure

The roughness of coatings on Ti-6Al-4V substrates was determined using a 3D profilometer while roughness of the coatings on Si (100) substrates was obtained by using an atomic force microscope (AFM, Explorer, VEECO Topometrix). The scanning area of each image was set at 5 μm × 5 μm with a scanning rate of 1.0 Hz to obtain a detailed surface image. The structure of the coatings was analyzed by X-ray Diffraction (Bruker diffractometer; radiation source (λ) Co (Kα) = 0.1789 Å). The diffraction scanning angle ranges from 20 to 80°at a scanning rate of 1°/min.

Results and discussion

Microstructural characterization

Prior to any microstructural analysis, the microstructural phases must be identified. XRD analyses were carried out by using the conventional symmetrical Bragg-Brentano configuration on C1, C2 and C3 samples to determine the structure of the top layer on the specimen. 113) at 2ϴ = 72.94°) according to the standard ICDD cards N°01-074-1214, 00-044-1294 and 01-071-1169 respectively and compared with available literature [START_REF] Shukla | Structural, mechanical and corrosion resistance properties of Ti/TiN bilayers deposited by magnetron sputtering on AISI 316L[END_REF][START_REF] Jiang | Influence of titanium interlayer thickness distribution on mechanical properties of Ti/TiN multilayer coatings[END_REF]. Peaks at 2ϴ = 38.63°and 73.413°are related to the substrate i.e. Si (200) and Si (400) respectively (ICDD card N°00-027-1402). Our findings provide a unique anatase phase for the two oxygen flow. It was suggested that the selective crystal phase of TiO 2 thin films (rutile, anatase) is based on the oxygen concentration during reactive magnetron sputtering [START_REF] Rafieian | Controlled formation of anatase and rutile TiO2 thin films by reactive magnetron sputtering[END_REF].

Remark: the strongest picks related to Si substrate (2θ > 80°) were voluntary removed from XRD patterns.

Fig. 4 shows the cross sections and surface morphology of the multilayer films. One can observe that the coatings fully cover the Ti-6Al-4V substrates. Fig. 4(a-c) depicts the cross-section SEM images of C1, C2 and C3, respectively. It is shown the typical columnar structure of PVD coatings i.e. vertically aligned nanocolumns; the largest horizontal section, are between 70 and 120 nm. The sequence (horizontal) of the alternating layers appears clearly; the sharp boundaries are seen at the different interfaces; it is then possible to count directly the alternated layers of TiN and to estimate approximately the size of the nanocrystallites. The SEM images in Fig. 4(d-f) show that TiN (Fig. 4d) presents a different surface morphology in comparison to the TiO 2 one (Fig. 4(e-f)). Moreover, "protuberances" and micropores are observed at both TiN and TiO 2 surfaces. A fine microstructure constituted by small grains with narrow size distribution was shown in the case of TiN films. Surfaces of TiO 2 films show almost spherical agglomerates of columns. They also present a compact and homogeneous surface morphology but some irregularities such as pores are clearly seen. The composition of the surface layers was analyzed by EDS microanalyses. The total thickness of the coatings and the final chemical compositions are listed in Table 2.

Fig. 5 shows the three dimensional AFM images of TiN (C1) and TiO 2 (C2 and C3) coatings surface. It is obvious that all the films have a compact structure with several "keen peaks" i.e. column tops. The roughness of these thin films was characterized according to R a (Roughness average) and R q (R q = Root Mean Square). As presented in Fig. 5, the deposited films have very low surface roughness. There are no significant differences in roughness between films except C1 that has a slightly superior R q in comparison to C2 and C3. It is also shown in Fig. 5 that the size of the columnar surface related to the TiO 2 upper layer (C2 or C3) are higher when compared to TiN upper layer (C1). Besides, C2 and C3 present similar surface topography (R a , R q ). So, in the case of the TiO 2 film, the oxygen content seems to have no influence upon "nanofeatures of surface". More details are presented in Table 3. The AFM quantitative results are within the range obtained by similar works [START_REF] Wiatrowski | Comparison of the physicochemical properties of TiO2 thin films obtained by magnetron sputtering with continuous and pulsed gas flow[END_REF].

Electrochemical analysis

One of the basic criteria to choose the material to be implanted into human (animal) body is the corrosion resistance in Hank's solution. Body fluids are aggressive and corrosive media for biomaterials. In terms of corrosion, a large number of ions (Na + , Cl -, Ca 2+ , SO 4 2-

…)

in addition to organic acids in Hank's solution would reduce the stability of the passive film, resulting in the most negative potential owing to more exposed metal surfaces, and finally damage and deterioration of the implant.

Potentiodynamic polarization curves

Fig. 6 presents the polarization curves of uncoated (Ti-6Al-4V substrate) and coated (C1, C2 and C3) Ti-alloy samples in Hank's solution at 37 °C. The multilayer coatings improve the corrosion resistance of substrate. Lower corrosion current density (i corr ) and higher corrosion potential (E corr ) indicate that the sample has relatively excellent corrosion resistance. From Fig. 6, it was obvious that the C1 surface sample exhibited the highest corrosion resistance which can be observed by a shift in the noble direction of the whole polarization curves towards the region of lower current density and higher potential. However, when passive films are on their steady state (+0.2 V/(Ag/AgCl)), the passive current density tendency of C2 sample increases slightly relatively to Ti-6Al-4V sample. The electrochemical corrosion parameters, including the corrosion current density (i corr ), corrosion potential (E corr ) and R p are displayed in Table 4. Quantitative parameters from Table 4 indicate that i corr of C1 is almost 0.05 μA/cm 2 and E corr is practically equal to zero V (Ag/AgCl). The current density of C1 specimen was lower by two (02) orders in magnitude at least, when compared to the others. The uncoated Ti-6Al-4V substrate exhibited the highest corrosion potential (E corr = -0.38 V/(Ag/AgCl)) and a slightly lower i corr than C2. Based on quantitative R p values the obtained results show that: C1 > C3 > Ti-6Al-4 V > C2 (Table 4).

From the potentiodynamic polarization curves, it can be concluded that C1 coating has the best chemical stability in the corrosive environment, suggesting that nitride coating surface improved enough the corrosion resistance of Ti-6Al-4V substrate. This can be explained by the compact nitride layer acting as a barrier to corrosion [START_REF] Wu | Wear and corrosion resistance of anti-bacterial Ti-Cu-N coatings on titanium implants[END_REF]. The protective nature of the multilayer coating Ti/TiN surface (higher number of bilayers) in Hank's solution could be attributed to their lower porosity, among others surface characteristics as suggested by Shukla et al. [START_REF] Shukla | Structural, mechanical and corrosion resistance properties of Ti/TiN bilayers deposited by magnetron sputtering on AISI 316L[END_REF]. This is confirmed by the obtained results for C1 when compared to TiO 2 and Ti-6Al-4V surfaces. The finding corrosion parameters are in the same order of magnitude and in good agreement with literature [START_REF] Shukla | Structural, mechanical and corrosion resistance properties of Ti/TiN bilayers deposited by magnetron sputtering on AISI 316L[END_REF], [START_REF] Chellappa | Improved corrosion resistant and mechanical behavior of distinct composite coatings (silica/titania/zirconia) on Ti-6Al-4V deposited by EPD[END_REF][START_REF] Wang | Comparison of tribological and electrochemical properties of TiN, CrN, TiAlN and a-C:H coatings in simulated body fluid[END_REF][START_REF] Masdek | Effect of protein concentration on corrosion of Ti-6Al-4V and 316L SS alloys[END_REF][START_REF] Runa | Tribocorrosion response of the Ti6Al4V alloys commonly used in femoral stems[END_REF]. 3.2.2. Electrochemical impedance spectroscopy Fig. 7 displays the electrochemical impedance spectroscopy results obtained for Ti-6Al-4V substrate and the coatings tested in Hank's solution at 37 °C. The Nyquist plot for each tested specimen appears to be arc of a semicircle (Fig. 7a). It can be easily confirmed that: (i) all the coatings (C1, C2 and C3) show larger radius of curvature than Ti-6Al-4V substrate; (ii) two loops are clearly shown in the low and in the medium frequency range of the Nyquist plots (insert image in Fig. 7a). The larger diameter of the curvature sample suggested that it offers higher resistance to charge transfer process. EIS data in Bode plots (Fig. 7(b-c)) confirm the highly electrochemical performances of C1 and the lowest performance of Ti-6Al-4V substrate. Two time constants are observed for all of the specimens. Then two capacitive behaviors occurred for the entire tested specimens. Such behaviors indicate improved passive film properties in such conditions, already observed in potentiodynamic technique (Fig. 6).

Fig. 7d indicates the selected electrical equivalent circuit model for all the specimens. This model was proposed in the literature for Ti-6Al-4V [START_REF] Lavos-Valereto | Electrochemical impedance spectroscopy characterization of passive film formed on implant Ti-6AI-7Nb alloy in Hank's solution[END_REF], Ti-6Al-4V coated Ti/TiN multilayer coatings [START_REF] Shukla | Structural, mechanical and corrosion resistance properties of Ti/TiN bilayers deposited by magnetron sputtering on AISI 316L[END_REF] and Ti-6Al-4V coated TiO 2 [START_REF] Chellappa | Improved corrosion resistant and mechanical behavior of distinct composite coatings (silica/titania/zirconia) on Ti-6Al-4V deposited by EPD[END_REF].

Ti-6Al-4V substrate specimen

Reviewed literatures showed some controversy about the selected equivalent circuit (EC) modeling the electrochemical behavior of Ti-6Al-4V alloy in Hank's solution at 37 °C. However, all of them [START_REF] Chellappa | Improved corrosion resistant and mechanical behavior of distinct composite coatings (silica/titania/zirconia) on Ti-6Al-4V deposited by EPD[END_REF][START_REF] Wang | Comparison of tribological and electrochemical properties of TiN, CrN, TiAlN and a-C:H coatings in simulated body fluid[END_REF][START_REF] Masdek | Effect of protein concentration on corrosion of Ti-6Al-4V and 316L SS alloys[END_REF][START_REF] Runa | Tribocorrosion response of the Ti6Al4V alloys commonly used in femoral stems[END_REF][START_REF] Lavos-Valereto | Electrochemical impedance spectroscopy characterization of passive film formed on implant Ti-6AI-7Nb alloy in Hank's solution[END_REF][START_REF] Movassagh-Alanagh | Improving the wear and corrosion resistance of Ti-6Al-4V alloy by deposition of TiSiN nanocomposite coating with pulsed-DC PACVD[END_REF][START_REF] Alves | In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti-6Al-4V in simulated body fluid at 25 °C and 37 °C[END_REF][START_REF] Milošev | Passive film on orthopaedic TiAlV alloy formed in physiological solution investigated by X-ray photoelectron spectroscopy[END_REF][START_REF] Pohrelyuk | Corrosion resistance of Ti-6Al-4V alloy with nitride coatings in Ringer's solution[END_REF], agree that there was evidence of the formation of a twolayer oxide film (essentially TiO 2 [START_REF] Pohrelyuk | Corrosion resistance of Ti-6Al-4V alloy with nitride coatings in Ringer's solution[END_REF]) composed of dense inner layer and porous outer layer. In this case, the EC model consists with Hank's solution resistance R s . R1, CPE1 are resistance and capacitance of porous layer (outer) with the electrolyte inside pore and R2, CPE2; resistance and capacitance of the barrier layer (inner). Detailed fitting data are listed in Table 5. The resistance of the outer layer (R1 ~6 kΩ) is lower than that of the inner layer by a factor of eleven (R2 = 66 kΩ), supporting the hypothesis that the corrosion resistance is determined by the dense inner layer.

Ti-6Al-4V coated specimens

In the case of C1, C2 and C3 coatings, R1 is the charge transfer resistance produced by coating layer and R2 is the charge transfer resistance that occurs on surface of the substrate. CPE1 and CPE2 are constant phase elements used to replace the double layer capacitance of the barrier layer. In other words, R1 represents the resistance of the coating layer resulted from the coating defects (pores) and R2 is the charge transfer resistance of the coating/substrate interface. The CPE1 and CPE2 are the capacitances at the solution/coating and coating/ substrate interfaces, respectively. As can be clearly seen in Fig. 7a, data adjusted well to the proposed equivalent circuit (curve in red). The chisquared (χ 2 ) average values were of the order of 10 -3 and the error percentages corresponding to each component of the equivalent circuit was < 8%. A perfect concordance, between the experimental curve and the fitted curve was shown in Fig. 7a.

The EIS results (Nyquist and Bode) show that: C1 > C3 > Ti-6Al-4V > C2. From Table 5, the inner layer of C2 specimen (R2 = 24.65 kΩ) has a slightly higher value than the resistance related to the porous layer (R1 = 23.2 kΩ) and represents the lowest resistance when compared to the other specimens. At the interface coating C2/passive film formed in Hank's solution, the inner layer doesn't play its role as a "barrier layer". The reason was maybe due to the low stability of this passive film. Indeed, Daviðsdóttir et al. [START_REF] Daviðsdóttir | Investigation of DC magnetron-sputtered TiO2 coatings: effect of coating thickness, structure, and morphology on photocatalytic activity[END_REF] reported, in the case of magnetron-sputtered titanium dioxide (TiO 2 ), that the transport of charge between the coating and substrate depends on the activation depth of the anatase coating, and any interface oxide layer present on the substrate surface.

Tribological behavior

Quantitative results (wear volume/weight loss) of bone specimens are not so easy to obtain by simple sliding tests. In any cases they are not reproducible. The principal reason was the structural constitution of bone. As defined in literature, bone is built up of mineralized collagen fibrils [START_REF] Weiner | THE MATERIAL BONE: structure-mechanical function relations[END_REF]. The term of "structure" refers to its constitution based on the circular osteons as demonstrated in previous works [START_REF] Rafieian | Controlled formation of anatase and rutile TiO2 thin films by reactive magnetron sputtering[END_REF].

Coefficient of friction (COF)

Fig. 8 displays tribological results of tested samples against bone pins under Hank's solution lubrication condition at 37 °C. Fig. 8a shows the COF as a function of the sliding distance. At the beginning of the test, the initial COF's values are the lowest but then, they increase abruptly to a second stage. The friction coefficient of Ti-6Al-4V substrate was unstable and weaves around 0.6 during the total sliding distance of 100 m. It is a sign of a severe wear. C1 specimen showed the lowest values; 0.2 < COF < 0.3 before 50 m and becomes 0.3 < COF < 0.4 until the end of the test. Its average value was 0.38 (Fig. 8b). C2 and C3 reached the steady state at the beginning of the test (after a few meters of sliding) when compared to C1. Fig. 8b displays the average value of COF at the steady-state and specific wear rates estimated based on the volume loss. It reveals that the best performance was shown by C1 specimen as expected while the worst was for the Ti-6Al-4V substrate. In the first stage, friction was controlled by the difference in hardness and surface roughness. Bone pin was the first damaged antagonist pair because it has the lowest mechanical properties. After more and more sliding cycles, an increase of temperature occurs and the surface asperities of bone are worn. Consequently the wear debris would form an adhesive oxide film on the interface between the antagonist pair. Hence, the contact area increases leading to a higher friction force. Besides, hard and abrasive debris are generated from the entire tribological pair in contact (debris mixture between bone and disc) leading to the second stage characterized by a severe abrasive wear. This result (Fig. 8b) supports the COF evolution observed in Fig. 8a. Nevertheless, the results of Fig. 8b should be taken with care due to the lack of data and because they are substantially lower than others obtained in the literature [START_REF] Ramoul | Surface performances of PVD ZrN coatings in biological environments[END_REF]. Besides, it is clear that Hank's solution doesn't play any role of lubrication in this contact pair.

Wear mechanisms analysis

The SEM images (Fig. 9) of wear tracks of two representative specimens i.e. C1 and C2 show the wear mechanisms involved during the tribological tests. From Fig. 9a-b, grooves, scratches and furrows are clearly shown, running parallel to the sliding direction (white arrows) indicating that samples have undergo a severe abrasive wear. That is the reason of a relatively high COF observed in Fig. 8. Abrasion zones as well as areas outside the wear track were covered by significant quantities of wear debris (circled areas). Black arrows indicate traces of coating delamination at the edges of the wear track; that means disappearance of the coating after sliding tests.

The coating delamination was usually caused by the residual stresses in the substrate. However after wear tests, the detachment of the coating on either side of the wear track can occur [START_REF] Esguerra-Arce | An approximation of tribological behavior of Ti1-xAlxN coatings against animal bone in ringer's solution[END_REF]. The film delamination in this case is probably due to: (i) the deterioration (abrasion) of the coating during the sliding test, layer by layer, becoming very thin in the area of the wear track, after more and more cycles. As a consequence, the delamination of the "residual thin coating" occurred at the border of the wear track maybe when the fatigue strength limit of the sublayers located under the wear track was reached i.e. at the interface limit between sub-surface areas undergoing load/unload cycles and free sub-surface areas; (ii) the bonding strength (adhesion between substrate/coating) provided by the used PVD method was not sufficient enough to avoid such delamination in these specific areas.

Further, Fig. 9(a′-b′) shows on one hand, deeply imbedded debris, pores and voids (traces of removal particles) inside grooves. On another hand, EDS analyses carried out in the wear tracks reveal the presence of bone elements (inside groove (part a′) and in dark zone (part b′)), coating elements (inside groove (part a′) and in gray zone (part b′)) and oxygen. It suggests a transfer from bone pin to coating surface. Fig. 10 shows SEM images of wear scar of one bone pin taken as an example (counterpart against C1 specimen). From Fig. 10a, one can note a higher damage and deterioration of the pin in comparison to the disc. The magnified area of the worn surface shows large cracks (Fig. 10b), scratches and wear debris (Fig. 10c). EDS analyses of the wear debris (particle inside the area labeled 3 in Fig. 10c) indicates the presence of oxygen, organic elements (Na, Ca, Cl, P…) and Ti (Fig. 10d). These results are signs of a third body abrasive wear mechanism.

Finally, sliding wear of Ti-6Al-4V coated specimens according to C1 or C2 design, in Hank's solution at 37 °C undergo an oxidative wear in the initial stage followed by a third body abrasive wear. The damage process seems to be as follows: at the beginning of the test, debris products come from the bone pin. After that, the pin surface in contact against the coating specimen becomes harder and rougher as a consequence of friction and increase in temperature. Thus, an "adhesive film" could be created between the antagonist surfaces in contact containing hard and abrasive particles (third body). The "adhesive film" consists of a mixture between molecules (Hank's solution), fragments of biological tissues, collagen (bone) and Ti-nitrides, Ti-oxides particles (tested specimen) sticking together. As a result, when specimen rub against bone, abrasion of the tribological pair in contact occurs.

Conclusion

Multilayered coatings were deposited on Ti-6Al-4V samples by DC reactive magnetron sputtering. Two types of coatings were studied: some films have TiN as upper layer (C1) while TiO 2 with 2 different oxygen content was the top layer of the others (C2 and C3). Obtained microstructures reveal the crystallinity of TiN single phase for C1 and anatase titanium dioxide for C2 and C3 coatings. All the specimens tested in Hank's solution at 37 °C, exhibit similar electrochemical

Fig. 1 .

 1 Fig. 1. Scheme of the studied multilayered coatings on Ti-6Al-4V substrates.

Fig. 3

 3 shows the XRD patterns that highlight the crystallinity of the deposited films on Si (100) substrates. It shows, for C1 C2 and C3 specimens, typical diffraction peaks of TiN phase ((111) at 2ϴ = 41.313°and (200) at 2ϴ = 47.16°), Ti (200) at 2ϴ = 44.96°and TiO 2 anatase phase ((112) at 2ϴ = 54.65°, (211) at 2ϴ = 56.45°, (220) at 2ϴ = 62.68°, (221) at 2ϴ = 65.95°, and (

Fig. 2 .

 2 Fig. 2. Schematic of the pin-on-disc test.

Fig. 3 .

 3 Fig. 3. XRD patterns of C1, C2 and C3 multilayer films. Symbols •, ▲, ■ indicated the 2 ϴ standard positions (S = substrate).

Fig. 4 .Fig. 5 .

 45 Fig. 4. SEM cross sections of the deposited films: (a) C1, (b) C2 and (c) C3. SEM surface morphologies of (d), C1, (e) C2 and (f) C3.

Fig. 6 .

 6 Fig. 6. Polarization resistance of the tested specimens in Hank's solution at 37 °C.

Fig. 7 .

 7 Fig. 7. Electrochemical test results (EIS) in Hank's solution at 37 °C: (a) Nyquist plots, (b) Bode plots, (c) phase angle Bode plots and (d) equivalent circuit model (WE = working electrode and RE = reference electrode). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Fig. 8 .

 8 Fig. 8. Tribological test results: (a) COF evolution and (b) estimated specific wear rates and COF values at the steady state.

Fig. 9 .

 9 Fig. 9. SEM images of the wear tracks after tribological test in Hank's solution at 37 °C of: (a) C1, (b) C2. Images (a′-b′) are the magnification of the corresponding areas in (a) and (b).

Fig. 10 .

 10 Fig. 10. SEM images of bone pin after tribological test against specimen C1: (a) overview of the wear scar, (b) and (c) magnification of areas labeled 1 and 2, and (d) EDS Spectrum of particles analyzed in area labeled 3.

Table 1

 1 Deposition parameters.

	Gas mixture	Coatings
		Ti	TiN	TiO 2
	Ar flow (sccm)	95	68.8	90
	N 2 flow (sccm)	-	33.3	-
	O 2 flow (sccm)	-	-	(C2 = 11), (C2 = 24)
	Target-substrate distance	120 mm
	Substrate bias voltage	-350 V
	Residual pressure (Pa)	2.10 -5
	Working pressure (Pa)	0.5

Table 2

 2 Thickness, adhesion and chemical composition of the layers.

			L C1 (N)	L C2 (N)	N	O	Ti
	C1	1.6	9.02 ± 1.5 30.16 ± 0.6 4.2	6.3	89.5
	C2	2.6	9.27 ± 0.7 32.24 ± 0.5 0	13.5	86.5
	C3	2.0	9.5 ± 0.5	32.5 ± 0.5	0	27.9	72.2

Samples Thickness (μm) Adhesion test

Contents (at.%)

Table 3

 3 AFM Roughness average values.

	Sample	Si substrate		T-6Al-4V substrate
		R a	R q	R a	R q
		(nm ± 2)	(nm ± 2)	(nm ± 3)	(nm ± 3)
	C1	5	8	65	95
	C2	7	6	70	97
	C3	5	6	58	75

Table 4

 4 Potentiodynamic polarization parameters of different coatings in Hank's solution at 37 °C. Data are the average values of three tests.

	Sample	β a (V/dec)	β c (V/dec)	E corr (V)	i corr	R p (Ω)
					(A/cm 2 ) • 10 -6
	Ti-6Al-4V	0.309	0.185	-0.345	0.117	490 841.17
	C1	0.169	0.252	-0.071	0.049	896 077.23
	C2	0.222	0.320	-0.156	0.152	364 074.30
	C3	0.243	0.317	-0.142	0.105	568 623.21

Table 5

 5 Electrochemical impedance parameters of coatings and uncoated Ti-6Al-4V in Hank's solution at 37 °C. Data are the average values of three tests.

	Sample	R s	CPE1	α 1	R 1	CPE2	α 2	R 2
		(Ω)	(μF/cm 2 ) • E -6		(kΩ)	(μF/cm 2 ) • E -6		(kΩ)
	Ti-6Al-4V	45.82	3.5	0.85	5.5	92.9	0.79	66.32
	C1	99.87	1.1	0.98	75.1	43.8	0.52	107.32
	C2	45.93	4.3	0.84	23.2	22.2	0.72	24.65
	C3	88.67	1.6	0.94	10.4	13.6	0.51	94.590

Acknowledgements

The authors are grateful to colleagues from LaBoMaP (Arts & Métiers -Institute of Technology, Cluny, France) and LIS (Annaba, Algeria) laboratories for providing facilities to carry out this work. The authors want also to thank Dr. Aouadi Khalil for his help during the coating depositions.

corrosion properties. The PVD layers could effectively enhance the corrosion resistance. The TiN upper layer presents the best tribological performances when against bovine cortical bone. The mean mechanism of wear involved during sliding tests was a severe third body abrasion wear. The TiO 2 upper layer doesn't play any role in the reduction of friction as well as any inhibition of corrosion, regardless biocompatibility performances, in the conditions adopted in this study.

CRediT authorship contribution statement R. Bahi: Formal analysis. C. Nouveau: Formal analysis. N.E. Beliardouh: Supervision, Writing -original draft, Formal analysis. C.E. Ramoul: Formal analysis. S. Meddah: Investigation. O. Ghelloudj: Investigation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.