Audrey Beillard

Xavier Bantreil
email: xavier.bantreil@umontpellier.fr

Thomas-Xavier Métro
email: thomas-xavier.metro@umontpellier.fr

Jean Martinez

Frédéric Lamaty
email: frederic.lamaty@umontpellier.fr

Unraveling the synthesis of homoleptic [Ag(N,N-diaryl-NHC) 2 ]Y (Y = BF 4 , PF 6 ) complexes by ball-milling †

A user-friendly and general mechanochemical method was developed to access rarely described NHC (N-heterocyclic carbene) silver(I) complexes featuring N,N-diarylimidazol(idin)ene ligands and noncoordinating tetrafluoroborate or hexafluorophosphate counter anions. Comparison with syntheses in solution clearly demonstrated the superiority of the ball-milling conditions.

Introduction

Solvent-free mechanochemistry showed numerous advantages over traditional solution chemistry. 1 In a number of cases, molecules that are otherwise inaccessible by traditional synthesis in solution could be synthesized by using a ball-mill. 2 An impressive example of the superiority of ball-milling over solution chemistry was the reaction of unsymmetrical azobenzene derivatives with palladium(II) acetate that allowed to obtain a doubly cyclopalladated complex that could not be formed in solution. 3 Besides, working under solvent-free or under Liquid-Assisted Grinding (LAG) 4 conditions generally leads to reduced environmental impact and enhanced reaction rates. Simplified set-up of reactions and recovery of products are additional advantages provided by ball-mills. Many areas of chemistry have benefited from these aspects: inorganic chemistry, 5 organic synthesis, 1a catalysis 6 and organometallic chemistry. 7 Pursuing our quest to disclose advantages of ball-milling in synthetic chemistry, 8 we recently demonstrated that mechanochemistry could supersede solution chemistry in the synthesis of N,N-dialkylimidazolium halides and their corresponding NHC (N-heterocyclic carbene) silver(I) halide complexes. 9 We next turned our attention to the synthesis of the rarely described homoleptic [Ag(NHC) 2 ]Y complexes, featuring N,N-diarylimidazol(idin)ene ligands, a cationic silver center and a non-coordinating counter anion (Y = BF 4 , PF 6 ); we reasoned that solvent-free ball-milling could bring a new paradigm and become the user-friendly and general method for the production of these challenging [Ag(NHC) 2 ] + complexes. Indeed, only eight structures containing N,N-diarylimidazol(idin)ene ligands and BF 4 10 and PF 6 11 counter anions were reported over the years, highlighting the difficulty to obtain such complexes. Their relative scarcity could be explained by the rather laborious approaches described in literature to access [Ag(NHC) 2 ]Y complexes. For instance, direct metalation with silver(I) oxide required the use of toxic solvent at reflux (Scheme 1, pathway A). 12 Metalation of imidazolium halides followed with anion exchange required prolonged reaction times (up to 3 days, pathway B). 13 Direct metalation could also be performed using silver tetrafluoroborate or hexafluorophosphate, but these reactants are highly expensive compared to silver(I) oxide ( pathway C). 14 These silver salts could also be reacted with free NHC, which needs prior isolation and rigorous manipulation due to high moisture sensitivity ( pathway D). 15 The most convenient method so far was reported by the group of Cazin, 10c and involved the intermediate synthesis of a NHC silver(I) chloride, which was then reacted under microwave irradiation with an imidazolium tetrafluoroborate in the presence of an excess of sodium hydroxide ( pathway E). Nevertheless, this technique was limited to one example of homoleptic complex.

Results and discussion

When 1a•HBF 4 and 1a•HPF 6 were reacted with silver(I) oxide (0.55 equiv.) in a stainless steel bowl in a vibrating ball-mill (vbm) at 25 Hz for 2 h, 1 H NMR analysis of the crude mixture indicated 4% and 7% conversion, respectively (Table 1, entry 1). The presence of a non-coordinating counter anion, in comparison with more coordinating chloride anion, might slightly increase the pK a of the imidazolium, thus limiting its reactivity with silver oxide. 16 Thus, sodium hydroxide was added to enable the deprotonation of the imidazolium precursor. To our delight, the addition of 1.1 equivalent of sodium hydroxide provided full conversion regardless of the counter anion (Table 1, entry 3). Reducing the amount of silver to a stoichiometric amount was detrimental to the conversion (Table 1, entry 4). In fine, homoleptic complexes [Ag(1a) 2 ]BF 4 and

[Ag( 1a) 2 ]PF 6 could be readily obtained in 91% yield in solely 2 hours of ball-milling under solvent-free conditions. To compare, their synthesis in solution required inert atmosphere in refluxing acetonitrile with 1.5 equivalent of Ag 2 O (Y = BF 4 , Scheme 1, pathway A) 10b,17 and 2 days of reaction through pathway B (Y = PF 6 , Scheme 1). 11a Of note, utilisation of a pestle and mortar to mix either 1a•HBF 4 or 1a•HPF 6 with NaOH (1.1 eq.) and Ag 2 O (0.55 eq.) for 1 minute every 20 minutes during 2 hours led to very low conversion of the substrates (≤5% conversion), indicating that continuous agitation is required to reach complete conversion in a reasonable amount of time.

The optimized milling conditions were then applied to a wide array of imidazolium salts The generality of the conditions were confirmed by the successful metalation of N,N-dialkylimidazolium salts 1e•HY and 1f•HY. Corresponding homoleptic silver complexes could be isolated in excellent yield and in only 1-2 h of reaction. In comparison, previous literature described the synthesis of [Ag(1e) 2 ]PF 6 in 70% yield by reacting 1 equivalent of silver(I) oxide in refluxing acetonitrile for 17 h. 18 Finally, ball-milling conditions revealed once more efficient when starting with triazolium salts 1g•HY, furnishing the unprecedented complexes [Ag(1g) 2 ]BF 4 and [Ag(1g) 2 ]PF 6 in 74% and 73% yield respectively (Scheme 2). It is important to highlight that even though PF 6

-salts are less reactive than BF 4 -salts, the use of ball-mill allowed to obtain both families of complexes in similar yields.

To the best of our knowledge, among all the complexes synthesized, only four compounds were previously described in literature. 19 All the complexes were fully characterized by 1 H, 13 C and 19 F NMR. The cationic structure of [Ag(1d) 2 ]BF 4 and [Ag(1e) 2 ]BF 4 were confirmed by X-ray diffraction on single crystals obtained by slow diffusion of diethyl ether in a dichloromethane solution of each complex (Fig. 1). These complexes are stable provided they are protected from light, and can be stored for months in a freezer without observable degradation.

To clearly establish the superiority of ball-milling over solution chemistry for the synthesis of these complexes, we attempted the synthesis in solution of unprecedented complexes [Ag(1e) 2 ]BF 4 , featuring N,N-dialkyl ligands, as well as [Ag(1d) 2 ]BF 4 and [Ag(1d) 2 ]PF 6 , featuring highly sterically demanding N,N-diaryl ligands (Table 2). Imidazolium salts were submitted to two sets of conditions: conditions A, which were found to be efficient in literature for other substrates 18 and involving excess of silver(I) oxide in refluxing acetonitrile, and conditions B, which are the direct application of milling conditions but in acetonitrile as solvent and under magnetic stirring. 19 For the sake of comparison, conditions C, which are solvent-free ball-milling conditions (cf. Scheme 2), were included in Table 2. After 17 h reaction, 1 H NMR analysis of the crude mixtures indicated that full conversion of the imidazolium salts were not reached in solution. In comparison, the ball-mill permitted to fully convert the imidazoliums in less than 3 h, thus enabling the isolation of the complexes after filtration of the crude mixture on celite (Table 2, entries 3, 6 and9). The benefits of the ball-mill were unambiguously demonstrated with the use of N,N-diarylimidazolium salt 1d•HPF 6 . In solution, using either conditions A or B, less than 7% conversion of 1d•HPF 6 into [Ag(1d) 2 ]PF 6 were observed after 17 h of reaction while the complex could be isolated in 91% yield after 3 h of milling treatment. Of note, complete conversion of 1d•HPF 6 was not reached after 48 h of reaction both in conditions A and B. In these conditions, the corresponding complex [Ag(1d) 2 ]PF 6 was formed in very small quantities (7% and <5% under conditions A and B respectively)

along with an unidentified compound preventing isolation of pure [Ag(1d) 2 ]PF 6 . These results are clearly indicating that this ball-milling method enabled the synthesis of a complex for which solvent-based conditions were unsuccessful. As multistep sequences were previously described with success, 20 we envisioned using imidazolium chloride as starting material. To our delight, when 1a•HCl was reacted with NaBF 4 or KPF 6 in a vibrating ball-mill agitated during 30 min at 25 Hz, corresponding 1a•HBF 4 and 1a•HPF 6 salts were isolated in 95% and 94% yield, respectively (Scheme 3). A one-pot two-step procedure to obtain the NHC silver(I) complexes directly from 1a•HCl was then realized. Thus, homoleptic [Ag(1a) 2 ]BF 4 and [Ag( 1a) 2 ]PF 6 complexes were isolated in less than 3 h in 84% and 78% yield, respectively, without isolation of the intermediate (Scheme 3).

Conclusion

In summary, a general and user-friendly method that clearly outclasses solution conditions was developed thanks to ballmilling. Access to a wide panel of homoleptic cationic silver(I) complexes, featuring sterically hindered N,N-diaryl ligands, N,N-dialkyl ligands as well as triazolylidene ligands, was demonstrated. Of note, this strategy enabled the synthesis of a large number of unprecedented compounds, including complex [Ag(1d) 2 ]PF 6 that was inaccessible through classical solution conditions. In addition, a highly convenient and environmentally friendly one-pot two-step procedure allowed to yield homoleptic silver complexes directly from corresponding imidazolium chloride, without isolation of intermediates.

Experimental

General information

All reagents were purchased from Aldrich Chemical Co., Fluka and Alfa Aesar and used without further purification. The milling treatments were carried out either in a Retsch MM200 or MM400 vibrating ball-mill operated at 25 Hz or 30 Hz.

Milling load is defined as the sum of the mass of the reactants per free volume in the bowl. (2,4,6-trimethylphenyl)imidazolium tetrafluoroborate (126.6 mg, 0.323 mmol, 1.00 eq.), Ag 2 O (41.1 mg, 0.178 mmol, 0.55 eq.) and NaOH (14.2 mg, 0.355 mmol, 1.1 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours in the vibrating ball-mill operated at 25 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1, 3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver tetrafluoroborate (117.6 mg, 0.146 mmol, 91%) as a white solid.

From imidazolium chloride: 1,3-bis-(2,4,6-trimethylphenyl) imidazolium chloride (100.7 mg, 0.295 mmol, 1.00 eq.) and NaBF 4 (34.1 mg, 0.310 mmol, 1.05 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). The bowl was closed and subjected to grinding for 20 minutes in the vibrating ball-mill operated at 25 Hz. Afterwards, Ag 2 O (39.2 mg, 0.148 mmol, 0.50 eq.) and NaOH (13.0 mg, 0.325 mmol, 1.10 eq.) were added. Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours and 30 minutes. The black powder was recovered with CH 2 Cl 2 and filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1, 3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver tetrafluoroborate (100.0 mg, 0.124 mmol, 84%) as a white solid. 1 H NMR (300 MHz, DMSOd 6 ) δ 7.67 (d, J = 1.4 Hz, 4H), 6.99 (s, 8H), 2.41 (s, 12H), 1.67 (s, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 181. 4 (dd,J = 192.4,13.8 Hz),138.8,134.9,134.0,128.8,123.63,123.58,20.7,16. [1, 4,]imidazolium tetrafluoborate (131.8 mg, 0.314 mmol, 1.00 eq.), Ag 2 O (36.3 mg, 0.157 mmol, 0.50 eq.) and NaOH (13.8 mg, 0.345 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The Scheme 3 One-pot two-step synthesis of cationic silver complexes from 1a•HCl.

bowl was closed and subjected to grinding for 3 × 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis [(1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl)imidazol-2-ylidene]silver tetrafluoroborate (117.8 mg, 0.137 mmol, 86%) as a white solid. 1 H NMR (400 MHz, CDCl 3 ) δ 7.00 (s, 8H), 2.42 (s, 12H), 1.81 (s, 12H), 1.61 (s, 24H); 13 C NMR (101 MHz, CDCl 3 ) δ 178. 3 (dd, J = 190.6, 13.7 Hz), 138.7, 134.4, 133.2, 128.9, 125.94, 125.89, 20.7, 16.6, 8.5; 19 O (33.1 mg, 0.143 mmol, 0.50 eq.) and NaOH (12.6 mg, 0.315 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum. The yellow solid was washed with Et 2 O and dried under vacuum to afford bis[1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene]silver tetrafluoroborate (126.5 mg, 0.130 mmol, 91%) as a white solid. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.76 (s, 4H), 7.50 (t, J = 7.8 Hz, 4H), 7.18 (d, J = 7.8 Hz, 8H), 2.27-2.13 (m, 8H), 1.01 (d, J = 6.8 Hz, 24H), 0.75 (d, J = 6.9 Hz, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 181.4 (dd, J = 201.9, 14.4 Hz), 144. 7, 134.5, 130.3, 125.71, 125.65, 124.1, 28.1, 24.0, 23.7; 19 [1,3-Bis(2,6-diisopropylphenyl)-4,5-dimethyl]imidazolium tetrafluoroborate (138.2 mg, 0.274 mmol, 1.00 eq.), Ag 2 O (31.7 mg, 0.137 mmol, 0.50 eq.) and NaOH (12.1 mg, 0.301 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[(1,3-bis(2,6-diisopropylphenyl)-4,5-dimethyl)imidazol-2-ylidene]silver tetrafluoroborate (120.5 mg, 0.117 mmol, 86%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.53 (t, J = 7.8 Hz, 4H), 7.21 (d, J = 7.8 Hz, 8H), 2.13 (sept, J = 6.9 Hz, 8H), 1.69 (s, 12H), 1.02 (d, J = 6.9 Hz, 24H), 0.69 (d, J = 6.9 Hz, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 178.4 (dd, J = 204. 3, 14.7 Hz), 144.7, 132.7, 130.5, 127.2, 127.1, 124.4, 27.9, 25.0, 22.6, 9.2; 19 Bis[(1-benzyl-3-methyl)imidazol-2-ylidene]silver tetrafluoroborate [Ag(1e) 2 ]BF 4 . 1-Benzyl-3-methylimidazolium tetrafluoroborate (109.7 mg, 0.422 mmol, 1.00 eq.), Ag 2 O (53.7 mg, 0.232 mmol, 0.55 eq.) and NaOH (18.6 mg, 0.464 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[(1-benzyl-3-methyl)imidazol-2-ylidene] silver tetrafluoroborate (104.3 mg, 0.193 mmol, 92%) as a white powder. 1 H NMR (300 MHz, DMSO-d 6 ) δ 7.56 (s, 2H), 7.47 (s, 2H), 7.41-7.15 (m, 10H), 5.32 (s, 4H), 3.77 (s, 6H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 137. 5, 128.9, 128.1, 127.6, 123.4, 122.5, 54.1, 38.3 (the 13 1,3-Dibenzylimidazolium tetrafluoroborate (123.3 mg, 0.367 mmol, 1.00 eq.), Ag 2 O (42.5 mg, 0.183 mmol, 0.50 eq.) and NaOH (16.1 mg, 0.404 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis [1,3-dibenzylimidazol-2-ylidene]silver tetrafluoroborate (107.7 mg, 0.156 mmol, 85%) as a white powder. 1 H NMR (300 MHz, DMSO-d 6 ) δ 7.57 (s, 4H), 7.31-7.22 (m, 12H), 7.21-7.15 (m, 8H), 5.27 (s, 8H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 137. 3, 128.8, 128.1, 127.5, 122.9, 54.3 (the 13 Bis [1,3,4-triphenyl-1,2,4-triazol-2-ylidene]silver tetrafluoroborate [Ag(1g) 2 ]BF 4 . 1,3,4-Triphenyl-1,2,4-triazolium tetrafluoroborate (128.6 mg, 0.334 mmol, 1.00 eq.), Ag 2 O (38.7 mg, 0.167 mmol, 0.50 eq.) and NaOH (14.7 mg, 0.367 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 × 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis [1,3,4-triphenyl-1,2,4-triazol-2ylidene]silver tetrafluoroborate (97.7 mg, 0.124 mmol, 74%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.90 (d, J = 7.4 Hz, 4H), 7.60 (t, J = 7.4 Hz, 2H), 7.55-7.48 (m, 12H), 7.46-7.37 (m, 12H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 183.0 (d,J = 194.2 Hz),153.8,138.8,136.3,131.2,130.1,129.8,129.6 Bis[1, 3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver hexafluorophosphate [Ag(1a) 2 ]PF 6 . From imidazolium hexafluorophosphate: 1,3-bis- (2,4,6-trimethylphenyl)imidazolium hexafluorophosphate (131.8 mg, 0.293 mmol, 1.00 eq.), Ag 2 O (37.3 mg, 0.761 mmol, 0.55 eq.) and NaOH (12.9 mg, 0.322 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours in the vibrating ball-mill operated at 25 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1, 3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver hexafluorophosphate (115.3 mg, 0.144 mmol, 91%) as a white solid.

From imidazolium chloride: 1,3-bis-(2,4,6-trimethylphenyl) imidazolium chloride (89.3 mg, 0.262 mmol, 1.00 eq.) and KPF 6 (50.8 mg, 0.275 mmol, 1.05 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). The bowl was closed and subjected to grinding for 20 minutes in the vibrating ball-mill operated at 25 Hz. Afterwards, Ag 2 O (30.3 mg, 0.131 mmol, 0.50 eq.) and NaOH (11.5 mg, 0.288 mmol, 1.10 eq.) were added. Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours and 30 minutes. The black powder was recovered with CH 2 Cl 2 and filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1, 3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene]silver hexafluorophosphate (83 mg, 0.0963 mmol, 78%) as a white solid. 1 H NMR (300 MHz, DMSO-d 6 ) δ 7.67 (d, J = 1.2 Hz, 1H), 7.00 (s, 2H), 2.41 (s, 3H), 1.68 (s, 6H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 181. 4 (dd, J = 192.4, 13.8 Hz), 138.7, 134.9, 134.0, 128.8, 123.62, 123.55, 20.7, 16.6; 31 P NMR (162 MHz, DMSO-d 6 ) δ -144.2 (sept, J = 711.4 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.1 (d,J = 711.4 Hz).

Bis [(1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl)imidazol-2ylidene]silver hexafluorophosphate [Ag(1b) 2 ]PF 6 . [1, 4,imidazolium hexafluorophosphate (136.4 mg, 0.285 mmol, 1.00 eq.), Ag 2 O (33.0 mg, 0.143 mmol, 0.50 eq.) and NaOH (12.5 mg, 0.314 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 × 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis [(1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl)imidazol-2-ylidene]silver hexafluorophosphate (119.0 mg, 0.130 mmol, 91%) as a beige solid. 1 H NMR (400 MHz, DMSO-d 6 ) δ 6.99 (s, 8H), 2.41 (s, 12H), 1.80 (s, 12H), 1.61 (s, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 178. 3 (dd, J = 190.6, 14.1 Hz), 138.7, 134.4, 133.2, 128.9, 126.0, 20.8, 16.6, 8.6; 31 O (30.3 mg, 0.131 mmol, 0.50 eq.) and NaOH (11.5 mg, 0.288 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum. The white solid was dissolved in a minimum of CH 2 Cl 2 , washed three times with water and concentrated under vacuum. The solid was washed with water and Et 2 O and dry under vacuum to afford bis[1, 3bis(2,6-diisopropylphenyl)imidazol-2-ylidene] silver hexafluorophosphate (96.9 mg, 94.1 µmol, 72%) as a white solid. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.77 (d, J = 0.9 Hz, 4H), 7.51 (t, J = 7.8 Hz, 4H), 7.18 (d,J = 7.8 Hz,8H), 2.20 (sept, J = 6.9 Hz, 8H), 1.01 (d, J = 6.9 Hz, 24H), 0.75 (d, J = 6.9 Hz, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 181. 4 (dd,J = 201.6,14.5 Hz),144.6,134.5,130.3,125.7,125.6,124.0,28.1,24.0,23. Bis[1,3-bis(2,6-diisopropylphenyl)-4,5-dimethyl]silver hexafluorophosphate [Ag(1d) 2 ]PF 6 . [1,3-Bis(2,6-diisopropylphenyl)-4,5-dimethyl]imidazolium hexafluorophosphate (141.7 mg, 0.252 mmol, 1.00 eq.) Ag 2 O (29.2 mg, 0.126 mmol, 0.50 eq.) and NaOH (11.1 mg, 0.277 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1,3-bis(2,6diisopropylphenyl)-4,5-dimethyl]silver hexafluorophosphate (124.1 mg, 0.114 mmol, 91%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.53 (t, J = 7.8 Hz, 4H), 7.20 (d, J = 7.8 Hz, 8H), 2.18-2.05 (m, 8H), 1.69 (s, 12H), 1.01 (d, J = 6.9 Hz, 24H), 0.69 (d, J = 6.9 Hz, 24H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 145. 2, 133.1, 130.9, 127.6, 124.8, 28.4, 25.4, 23.1, 9.7; 31 Bis[(1-benzyl-3-methyl)imidazol-2-ylidene]silver hexafluorophosphate [Ag(1e) 2 ]PF 6 . 1-Benzyl-3-methylimidazolium hexafluorophosphate (126.9 mg, 0.321 mmol, 1.00 eq.), Ag 2 O (40.9 mg, 0.177 mmol, 0.55 eq.) and NaOH (14.1 mg, 0.353 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[(1-benzyl-3-methyl) imidazol-2-ylidene]silver hexafluorophosphate (90 mg, 0.151 mmol, 94%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.55 (d, J = 1.7 Hz, 2H), 7.46 (d, J = 1.7 Hz, 2H), 7.34-7.24 (m, 10H), 5.32 (s, 4H), 3.77 (s, 6H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 137. 4, 128.8, 128.0, 127.5, 123.3, 122.4, 54.0, 38.2 (the 13 Bis[1,3-dibenzylimidazol-2-ylidene]silver hexafluorophosphate [Ag(1f) 2 ]PF 6 . 1,3-Dibenzylimidazolium hexafluorophosphate (129.5 mg, 0.328 mmol, 1.00 eq.), Ag 2 O (38.0 mg, 0.164 mmol, 0.50 eq.) and NaOH (14.4 mg, 0.361 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 2 hours in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis[1,3-dibenzylimidazol-2-ylidene]silver hexafluorophosphate (104.7 mg, 0.140 mmol, 85%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.56 (s, 4H), 7.31-7.23 (m, 12H), 7.21-7.15 (m, 8H), 5.27 (s, 8H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 137. 2, 128.7, 128.0, 127.4, 122.8, 54.2 (the 13 Bis [1,3,4-triphenyl-1,2,4-triazol-2-ylidene]silver hexafluorophosphate [Ag(1g) 2 ]PF 6 . 1,3,4-Triphenyl-1,2,4-triazolium hexafluorophosphate (133.7 mg, 0.302 mmol, 1.00 eq.), Ag 2 O (35.0 mg, 0.151 mmol, 0.50 eq.) and NaOH (13.3 mg, 0.332 mmol, 1.10 eq.) were introduced in a 10 mL stainless steel grinding bowl with one stainless steel ball (10 mm diameter). Total mass of the reagents was calculated so that milling load equals 19.2 mg mL -1 . The bowl was closed and subjected to grinding for 3 × 1 hour in the vibrating ball-mill operated at 30 Hz. The black powder was recovered with CH 2 Cl 2 and the suspension was filtrated over celite. The filtrate was concentrated under vacuum to afford bis [1,3,4-triphenyl-1,2,4-triazol-2-ylidene]silver hexafluorophosphate (93.8 mg, 0.111 mmol, 73%) as a white powder. 1 H NMR (400 MHz, DMSO-d 6 ) δ 7.90 (d,J = 7.6 Hz,2H),7.59 (t,J = 7.6 Hz,1H),6H),6H); 13 C NMR (101 MHz, DMSO-d 6 ) δ 183.1 (d, J = 200.0 Hz), 153.8, 138.8, 136.3, 131.2, 130.1, 129.8, 129.5, 129.1, 128.8, 126.8, 124.1, 122.6; 31

  HY-1g•HY (Y = BF 4 , PF 6 ), including N,N-diaryl and N,N-dialkyl imidazolium as well as triazolium salts (Scheme 2). 1b•HBF 4 and 1b•HPF 6 , although featuring high steric hindrance caused by the presence of the methyl groups on the imidazolium, were efficiently converted into complexes [Ag(1b) 2 ]BF 4 and [Ag(1b) 2 ]PF 6 in 87% and 91% yield, respectively. Similarly, highly encumbered N,N-(2,6-diisopropylphenyl)imidazolium derivatives 1c•HY and 1d•HY were transformed to [Ag(1c) 2 ]Y in 73-74% yield and to [Ag(1d) 2 ]Y in 86-91% yield after 3 h milling at 30 Hz (Scheme 2).

Scheme 2

 2 Scheme 2 Mechanosynthesis of various homoleptic cationic silver complexes. Reaction conditions: reactions performed under air, in a vibrating ball-mill (vbm) at 30 Hz, with 1.1 eq. of NaOH. Total mass of reactants accounted for 182 mg. a Reaction performed at 25 Hz.

Fig. 1

 1 Fig. 1 ORTEPs ( probability at 50% level) of silver complexes [Ag(1d) 2 ] BF 4 and [Ag(1e) 2 ]BF 4 . H atoms are omitted for clarity.

  7; 19 F NMR (376 MHz, DMSO-d 6 ) δ -148.3; HRMS calcd for C 42 H 48 N 4 Ag [M -BF 4 -] + : 715.2930; found: 715.2932. Bis[(1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl)imidazol-2ylidene]silver tetrafluoroborate [Ag(1b) 2 ]BF 4 .

  7; 31 P NMR (162 MHz, DMSO-d 6 ) δ -144.2 (hept, J = 711.3 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.2 (d, J = 711.3 Hz).

  C of carbene was not observed due to the low concentration of the sample); 31 P NMR (162 MHz DMSO-d 6 ) δ -144.2 (sept, J = 711.3 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.2 (d, J = 711.4 Hz); HRMS calcd for C 22 H 24 N 4 Ag [M -PF 6 -] + : 451.1052; found: 451.1052.

  C of carbene was not observed due to the low concentration of the sample); 31 P NMR (162 MHz, DMSO-d 6 ) δ -144.2 (sept, J = 711.2 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.1 (d, J = 711.2 Hz); HRMS calcd for C 34 H 32 N 4 Ag [M -PF 6 -] + : 603.1678; found: 603.1680.

Table 1 Optimization of the mechanosynthesis of cationic silver complexes a

 1

				NMR conv. (%)
	Entry	NaOH (equiv.)	Ag 2 O (equiv.)	[Ag(1a) 2 ]BF 4	[Ag(1a) 2 ]PF 6
	1	0	0.55	4	7
	2 3	0.5 1.1	0.55 0.55	56 100 (91) b	46 100 (91) b
	4	1.1	0.27	91	82

Table 2

 2 Comparative synthesis of [Ag(1d) 2 ]BF 4 , [Ag(1d) 2 ]PF 6 and [Ag(1e) 2 ]BF 4

	a
	Entry	Substrate	Conditions	NMR conv. (%)
	1 2 3	1e•HBF 4	A B C	75 67 100 (92) b
	4 5 6	1d•HBF 4	A B C	32 86 100 (86) b
	7 8 9	1d•HPF 6	A B C	7 <5 100 (91) b

  Ag(1a) 2 ]BF 4 . From imidazolium tetrafluoroborate: 1,3-bis-

	1 H NMR spectra were recorded on
	a Bruker Avance DPX 200 MHZ, 300 MHz or 400 MHz spectro-
	meter and are reported in ppm using deuterated solvent for
	calibration (CDCl 3 at 7.26 ppm or DMSO-d 6 at 2.50 ppm). Data
	are reported as s = singlet, d = doublet, t = triplet, q = quadru-
	plet, qt = quintuplet, sept = septuplet, m = multiplet; coupling
	constant in Hz; integration.

13

C NMR spectra were recorded on Bruker Avance AM 50 MHz, 75 MHz or 101 MHz spectrometers and are reported in ppm using deuterated solvent for calibration (CDCl 3 at 77.2 ppm or DMSO-d 6 at 39.5 ppm). HRMS analysis were performed on a Q-Tof

(Waters, ESI, 2001)

spectrometer.

Synthesis of [Ag(NHC) 2 ]BF 4

Bis[1,

3-bis(2,4,6-trimethylphenyl)

imidazol-2-ylidene]silver tetrafluoroborate [

  F NMR (376 MHz, CDCl 3 ) δ -143.6; HRMS calcd for C 46 H 56 N 4 Ag [M -BF 4

	-] + : 771.3536; found: 771.3565.
	Bis[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]silver
	tetrafluoroborate [Ag(1c) 2 ]BF 4 . 1,3-Bis-(2,6-diisopropylphenyl)
	imidazolium tetrafluoroborate (136.2 mg, 0.286 mmol,
	1.00 eq.), Ag 2

  F NMR (376 MHz, DMSO-d 6 ) δ -148.3; HRMS calcd for C 58 H 80 N 4 Ag [M -BF 4

	-] + : 939.5434; found:
	939.5429.

  C of carbene was not observed due to the low concentration of the sample); 19 F NMR (376 MHz, DMSO-d 6 ) δ -148.3; HRMS calcd for C 22 H 24 N 4 Ag [M -BF 4

	-] + : 451.1052; found: 451.1050.
	Bis[1,3-dibenzylimidazol-2-ylidene]silver tetrafluoroborate
	[Ag(1f ) 2 ]BF 4 .

  C of carbene was not observed due to the low concentration of the sample); 19 F NMR (376 MHz, DMSO-d 6 ) δ -148.3; HRMS calcd for C 34 H 32 N 4 Ag [M -BF 4 -]

+ : 603.1678; found: 603.1681.

,

  This journal is © The Royal Society of Chemistry 2016 Dalton Trans., 2016, 45, 17859-17866 | 17863 Published on 24 October 2016. Downloaded by University of Montpellier on 17/01/2017 16:26:00.129.1, 128.8, 126.8, 124.1, 122.6; 19 F NMR (376 MHz, DMSOd 6 ) δ -148.3; HRMS calcd for C 40 H 30 N 6 Ag [M -BF 4 -] + : 701.1578; found: 701.1584.

	Synthesis of [Ag(NHC) 2 ]PF 6

  P NMR (162 MHz, DMSO-d 6 ) δ -144.2 (hept, J = 711.4 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.2 (d, J = 711.4 Hz) HRMS calcd for C 46 H 56 N 4 Ag [M -PF 6

			-] + : 771.3536;
	found: 771.3545.
	Bis[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]silver
	hexafluorophosphate [Ag(1c) 2 ]PF 6 . 1,3-Bis-(2,6-diisopropyl-
	phenyl)imidazolium	hexafluorophosphate	(140.1	mg,
	0.262 mmol, 1.00 eq.) Ag 2

  P NMR (162 MHz, DMSO-d 6 ) δ -133.2 to -157.4 (m). 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.1 (d, J = 711.2 Hz); HRMS calcd for C 58 H 80 N 4 Ag [M -PF 6 -]

+ : 939.5434; found: 939.5433.

  P NMR (162 MHz, DMSO-d 6 ) δ -144.2 (sept, J = 711.4 Hz); 19 F NMR (376 MHz, DMSO-d 6 ) δ -70.2 (d, J = 711.4 Hz); HRMS calcd for C 40 H 30 N 6 Ag [M -PF 6 -] + : 701.1583; found: 701.1584.

a Reactions performed under air, in a vibrating ball-mill (vbm) at 25 Hz, total mass of reactants accounted for 182 mg. b Isolated yield is given in brackets. Scheme 1 Synthetic pathways to homoleptic [Ag(NHC) 2 ]Y (Y = BF 4 /PF 6 ) complexes.

a Conditions A: Ag 2 O (1 equiv.), CH 3 CN, reflux, 17 h; conditions B: Ag 2 O (0.5-0.55 equiv.), NaOH (1.1 equiv.), CH 3 CN, rt, 17 h; conditions C: Ag 2 O (0.5-0.55 equiv.), NaOH (1.1 equiv.), vbm, 1-3 h. b Isolated yield is given in brackets.

This journal is © The Royal Society of Chemistry 2016

This journal is © The Royal Society of Chemistry 2016 Published on 24 October 2016. Downloaded by University of Montpellier on 17/01/2017 16:26:00. View Article Online

Acknowledgements

Université de Montpellier and CNRS are acknowledged for funding. Pascal Retailleau (Institut de Chimie des Substances Naturelles) is gratefully acknowledged for the XRD analysis of [Ag(1d)]BF 4 .


C Gestraud

Benoit Glorieux

Jonathan Massera

Laëtitia Petit
email: laeticia.petit@tut.fi

Alexandre Fargues

Marc Dussauze

Thierry Cardinal

Leena Hupa

Effect of partial crystallization on the structural and Er3+ luminescence properties of phosphate-based glasses

Keywords: Er doped glasses, heat treatment, nucleation and growth, micro-Raman & Infrared spectroscopies, Er luminescence properties 1. Introduction

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Glasses and glass-ceramics doped with rare-earth (RE) ions have been intensively studied for their use in telecommunications systems, such as up-converter, fibers, optical amplifiers, solid state lasers and 3D displays [1]. Among the RE ions, Er 3+ doped materials have attracted great interest because of their potential applications as optical fiber amplifiers due to their ability to increase the signal at the wavelength of 1530nm, which corresponds to the intra-4f transition of Er 3+ from 4 I 13/2 to 4 I 15/2 [2]. Oxide glasses have been found to be stable hosts for obtaining efficient luminescence from RE ions [3].

In the development of optical devices containing rare-earth (RE) ions, the optical properties are mainly determined by the local environment around the RE [4]. Locally ordered nuclei of ErPO 4 were found to form in germano-silicate glasses and exhibit luminescence properties typical of RE-doped crystals [5]. Those nanometer-sized particles in glass are of great interest as they offer promising perspectives in terms of resolution and luminescence efficiency [6]. Additionally, the photoluminescence intensity of phase separated Er 3+ doped sodium borosilicate glass was found to be higher than that for the as-prepared glasses [7]. In this glass system, the phase separation led to a borate phase and a silicate phase, the Er 3+ ions being segregated into the borate-rich phase. The work on the processing of glass-ceramics with nanoparticles small enough to avoid strong light scattering and with rare-earth ions incorporated into a crystalline phase, has recently opened a new route to increase the efficiency of glass as active hosts [8]. Those new glass-ceramic materials combine some features of glasses (easier and cheaper processing than for single crystals) and some advantages of rare-earth doped single crystals (higher absorption/emission cross-section and longer lifetime of luminescent levels) [9].

The crystallization behavior of phosphate-based glasses with composition 50P 2 O 5 -(40x)CaO-xSrO-10Na 2 O with x=0, 20 and 40 (in mol%) was reported recently [10]. Two phases crystallized in the glasses: Ca(PO 3 ) 2 and NaCa(PO 3 ) 3 in the glass with x=0, Sr(PO 3 ) 2 and NaSr(PO 3 ) 3 in the glass with x=40 and a solid solution of (Ca,Sr)(PO 3 ) 2 and Na(Ca,Sr)(PO 3 ) 3 in the glass with x=20. All crystals were found to preferentially nucleate and grow from the surface.

In order to increase the tendency of the glass to crystallize, nucleation agent can be added in the glass network. Previous studies have revealed that B 2 O 3 , TiO 2 or ZnO can act as good nucleation agent in many glasses [11][12]. The role of those elements as nucleation agent in phosphate glasses has not been fully studied. Additionally, to the best of our knowledge, there is only little information on understanding the impact of the nucleation and growth of phosphate glasses on Er 3+ spectroscopic properties.

Therefore, in this study, we focused on the understanding the influence of these elements introduced in low concentration (<5mol%) on the structure and crystallization behavior of the glasses. We also checked the impact of the glass crystallization on the luminescence properties of erbium. Our main goal of this study will be to process a phosphate glass which upon heat treatment exhibits a volume crystallization associated with the precipitation of evenly distributed Er 3+ doped nanocrystals. In this paper, different Er 3+ doped phosphate glasses within similar glass system (P 2 O 5 -CaO-SrO-Na 2 O) were heat treated at different temperatures in order to study the spectroscopic properties of the (partially to fully) crystallized glasses. The changes of the structure and luminescence properties were investigated.

Experimental

2.1

Glass preparation

Glasses with the composition 100-x(0.5P [10]. The polished glass pieces were placed on a platinum foil to prevent contamination from the sample holder. The heat treatments were performed in air.

Thermal and physical properties

The glass transition temperature (T g ) and crystallization temperature (T p ) were measured by differential thermal analysis (STA449 F1 JUPITER from Netzsch) at the heating rate of 10 o C/min. The T g was taken at the inflection point of the endotherm, as obtained by taking the first derivative of the DSC curve. T p was defined as the maximum of the exothermic peak. The temperatures were determined with an accuracy of ±3 o C.

The density of the bulk glass materials was measured after Archimedes' method in water.

The accuracy was better than ±0.02g/cm 3 .

Optical properties

The UV-visible-IR absorption spectra of the glass samples were recorded using a Perkin Elmer Lambda 900 UV-vis-NIR spectrometer with an integrating sphere setup over the 250-2500 nm spectral region. The measurements were performed at room temperature and were corrected for the Fresnel losses and the glass thickness. The absorption cross-sections ( abs ) were calculated from the experimentally measured absorption coefficients and from Er 3+ ions concentration in the glass, using Equation (1).

(

) 1
where log(I 0 /I) is the absorbance, N the rare-earth ion concentration (ions.cm -3 ) calculated from the batch composition and the glass density and L the thickness of the sample (cm).

The absorption spectra over the 2500-4000 cm -1 range were performed on a FTIR Bruker Alpha-T spectrometer with a spectral resolution of 2 cm -1 . The measurements were performed at room temperature and were corrected for the Fresnel losses.

The emission spectra in the 1400-1700 nm range were measured with an Edinburgh

Instruments monochromator (M300) and a liquid nitrogen cooled germanium detector (ADC 403L) at room temperature using an excitation source emitting at 980 nm.

Structural properties

The crystalline phases were identified using an X-ray diffraction analyzer (Philips X'pert)

with Cu Kα radiation (= 1.5418 Å). The scans were performed from 2θ = 0 to 60° with a step size of 0.02°.

Raman spectra were recorded with a Xplora Horiba micro-Raman spectrometer using a 785 nm excitation line with 10 mW of incident power. Raman spectra in the range of 200-2000 cm-1 were recorded with a resolution of 2.5 cm-1.

The Attenuated Total Reflectance Infrared spectra were recorded on a Thermo Scientific Nicolet iS50 FT-IR spectrometer equipped with a DLaTGS detector (KBr window) using the diamond iS50 ATR accessory. Each spectrum was obtained in the 4000-400 cm-1spectral range, at a resolution of 4 cm-1, by co-adding 200 scans.

Results and discussion

The goal of this work is to investigate the effect of nucleation and growth on the structural and Er 3+ luminescence properties of phosphate-based glasses prepared with different dopants: 2.5 mol% of B 2 O 3 and ZnO were added in glasses (the glasses were respectively labeled B-Er and Zn-Er glasses) while 2.5 and 5mol% of TiO 2 were used (the glasses were labeled Ti2.5-Er and Ti5-Er glasses).

Effect of the glass composition on the thermal, structural and luminescence properties

The density of the glasses and some of their thermal and optical properties are listed in Table 1. The addition of B 2 O 3 , TiO 2 , and ZnO leads to a slight increase in the glass density. This could be attributed to the smaller ionic radius of B, Ti and Zn as compared to the ionic radius of the other elements in the glasses, which has an influence on the compaction of the network. The addition of ZnO reduces T g and T p whereas the addition of B 2 O 3 and TiO 2 increases T g and T p .

The increase in T g could indicate that the addition of B 2 O 3 and TiO 2 improves the strength of the network whereas the reduction in T g for the glass containing ZnO indicates that ZnO acts as a network modifier, in agreement with [13]. Table 1 shows also ∆T, the temperature difference between T g and T p . Typically, a largeT value is an indication of a small probability for overlapping between the nucleation and growth curves. While the addition of ZnO increases T, the addition of B 2 O 3 does not have any significant impact on T. However, the addition of the TiO 2 leads to an increase of T g and a decrease in T. This suggests that titanium cations have a higher impact on the bond strength of the glass network than the other elements.

The absorption spectra of the glasses are shown in figure 1a. The UV absorption edge is not clearly affected by the addition of B 2 O 3 or ZnO. However, it shifts to longer wavelength with the addition of TiO 2 due to the presence of an additional broad band centered at ~500 nm which can be assigned to the absorption band of Ti 3+ ions as suggested in [14]. In the 250-1600 nm range, several bands are observed. These bands are characteristics of the Er 3+ ion 4f-4f transitions from the ground state to various excited levels. As seen in Figure 1b, the shape of the band centered at 1530 nm remained almost unchanged with the changes in the glass composition.

One can notice that the addition of the various dopants leads to a similar increase in the absorption coefficient at 1530 nm as compared to the reference Er glass (x=0). The absorption cross-sections at 980nm and 1530 nm were measured from the absorption coefficient using equation 1 (Table 1). Within the accuracy of the measurement, the addition of the dopant in the phosphate glass has no noticeable impact on the absorption cross-sections at 1530nm. A small decrease of the absorption cross-section at 980nm was observed with the addition of TiO 2 . Based on those small changes in the absorption cross-sections, the sites of the Er 3+ ions do not seem to be strongly influenced by the changes in the glass composition.

The emission spectra after excitation at 980 nm are shown in Figure 2a. They were collected on powder between quartz plates. The spectra exhibit a broad band centered at 1530 nm which arises from the 4 I 13/2  4 I 15/2 transition typical of the Er 3+ emission in oxide glasses. The shape of the emission band is almost unaffected by the changes in glass composition confirming the small impact of the dopants on the sites of the Er 3+ ions (Figure 2b). As seen in Figure 2a, the addition of dopants leads to a reduction in the intensity of the emission. The larger the TiO 2 concentration, the larger the decrease in the emission intensity. This is in agreement with the small reduction in the absorption cross-section at 980nm with the addition of TiO 2 seen in Table 1. However, while the B-Er and Zn-Er glasses have similar absorption cross-sections at 980 and 1532nm than the Reference Er glass (Table 1), they exhibit a lower emission at 1532nm after 980nm excitation than the Reference Er glass. As explained in [15], the decrease of the emission at 1530nm with the addition of B 2 O 3 could be due to an increase of the multiphonon decay rate by coupling of larger number of borate groups. In order to understand the emission properties of the Zn-Er glass, the IR absorption spectra the glasses were measured as the intensity of the emission at ~1530nm is related to the impurities such as OH hydroxyl groups, known as serious quenchers of Er 3+ ions luminescence 16. Figure 2c shows the IR absorption spectra of the glasses. The spectra exhibit a broad absorption band between 2800 and 3500 cm -1 which is usually attributed to OH groups in several oxide glasses [16]. No direct correlation between the OH content and the intensity of the emission at 1530nm could be established for the Zn-Er glass as this glass has the lowest amount of OH as compared to the other glasses. Therefore, it is possible to think that the addition of the ZnO in the phosphate glass changes the structure of the glass which then cannot incorporate the same amount of Er 3+ ions than the reference Er glass without forming ions pairs. Aggregation and clustering of RE ions are known to increase the interaction between RE ions which results in luminescence quenching [17]. A study on the Er 3+ solubility in the investigated glasses is in progress. We plan to provide more information about the impact on the glass composition on Er 3+ coordination and clustering as performed in [18][19].

The structure of the investigated glasses was, then, analyzed using Raman and IR spectroscopies. The Raman spectra of the glasses are presented in Figure 3a. All spectra are normalized at the band with maximum intensity peaking at ~1175 cm -1 . The spectra exhibit well defined bands at ~700, 1175 and 1200-1300 cm -1 . Various bands appear between 800 and 1110 cm -1 . The bands at around 1250 and 1175 cm -1 is ascribed to the asymmetric and symmetric stretching of non-bridging (PO 2 ) of Q 2 groups, respectively [20][21][22]. The band at 700 cm -1 is related to the symmetric stretching of bridging  s (O-P-O) of Q 2 groups [23]. The same author refers the band at 1020 cm -1 as relevant of the stretching (P-O) of terminal groups (Q 1 ). From the spectra of the glasses, the investigated glasses are expected to have a metaphosphate structure, as suggested by [24] and many other publications. There is no Q 3 which are usually revealed at Raman Shift higher than 1300 cm -1 and a few amount of terminal group Q 1 .

The addition of B 2 O 3 , TiO 2 and ZnO leads to a broadening of the main band at 1175 cm -1 which is assigned to the symmetric stretching of non-bridging (PO 2 ) of Q 2 . This broadening is most likely related to a disordering of the phosphate network induced by the addition of B 2 O 3 , TiO 2 and ZnO. A shift to lower wavenumber of this band and also of the band at 700 cm -1 which is assigned to symmetric stretching of bridging  s (O-P-O) of Q 2 is observed and could be related to a modification of the iono-covalent state of the P-O bound [25] or to a mass effect of the cations surrounding the phosphate network [26]. The intensity of the bands between 900 and 1110 cm -1 increases with the addition of the dopants which is an indication of an increase of the Q 1 units.

The increase in intensity of those bands is more visible in the spectrum of the Ti-Er glasses probably due to the formation of P-O-Ti and to the increase of the Q 1 units coordinated to titanium in the glasses [25]. In metaphosphate glass, the titanium cation is in an octahedral environment TiO 6 [27]. Titanium may even create a TiO 6 sub-network through Ti-O-Ti-O-Ti bonding. This chain structure, mentioned by Cardinal et al [28], induces vibrations which can be seen by the increase in intensity of the 900-1100 cm -1 band in the Raman spectra. Since the progressive addition of TiO 2 leads to an increase in T g , it is possible that the formation of P-O-Ti linkages distorts the glass network and forms three-dimensional network of P-O-Ti linkages as suggested in [14]. In the Raman spectrum of the B-Er glass, the bands in the 600-650 cm -1 and 1050-1150 cm -1 range can also be related to ring-type metaborate units and diborate group, respectively. In agreement with [29], the bands in the 1050-1150cm -1 range can be also related to the symmetric stretching vibration (A 1 ) of the [PO 4 ] 3-tetrahedron surrounded by B 3+ ions (PO 4 -BO 4 groups). According [30], the B cations are thought to enter in the phosphate network in BO 4 tetrahedra, which share oxygen with the phosphate network. The negative charge surrounding the tetragon is then compensated by a positive charge, certainly carried by Na + . In that case, the sodium ions around the boron units have less impact on the phosphate network leading the shift of the Q 2 bands the in Raman spectrum. As the addition of B 2 O 3 was found to also increase the T g of the glass, the increase in connectivity cannot be due to the increase of the P-O-P bonds as the number of such bonds decreases. The increase in T g is therefore likely to be due to the formation of P-O-B links which replace the P-O-P bonds as seen in [31]. The formation of the P-O-B links is also suggested by the increase in intensity of the shoulder at 630 cm -1 which is related to the vibrations of metaborate units bonded to P-O as in [32].

The IR spectra of the glasses are presented in Figure 3b. The spectra are normalized to the band with maximum intensity peaking at ~880 cm -1 . The spectra exhibit bands between 650 and 800 cm -1 and at ~880, 1080 and 1240 cm -1 . Various bands appear between 950 and 1010 cm -1 . The impact of the oxygen-(Er, Zn, B, Ti) vibrations on the bands in the low wavenumber range [33] is insignificant as compared to that of the phosphate vibrations and will not be discussed in this paper. The bands at around 1240 cm -1 is ascribed to the asymmetric stretching mode  as (PO 2 -) of Q 2 groups [23]. The shoulder at 1160 cm -1 was assigned by Meyer et al. [24] as symmetric modes

(PO 2 -
) of the Q 2 groups. The band at 1080 cm -1 is related to asymmetric stretching mode of Q 1 group,  as (PO 3 2-

) [34]. The band at 880 cm -1 corresponds to the asymmetric vibrational modes  as (POP) of P-O-P bridges of Q 2 units in chains [23]. The various bands between 930 and 1010 cm -1 are often related to rings-type formation in the glass network [35]. The broad bands between 650 and 800 cm -1 may include symmetric vibrational modes  s (POP) of P-O-P bridges of Q 2 units [23]. No bands are revealed at higher wavenumbers than 1300 cm -1 , where the (P=O) of Q 3 groups typically locate. This confirms that there are no Q 3 groups in the experimental glasses in this work, in agreement with the analysis of the Raman spectra.

The addition of B 2 O 3 , TiO 2 and ZnO leads to a decrease in intensity and a shift to lower wavenumber of the IR band at 1240cm -1 and also to an increase in intensity of the bands in the at the expense of the other Q n units. Zn is then suspected to act as a modifier, leading to a depolymerization of the phosphate network and to a less cross-linked network. This is in agreement with the low density and T g of the Zn-Er glass as compared to the other glasses. It is possible to think that it is this change in the glass structure induced by the incorporation of Zn which reduces the emission at 1530nm of the glass.

Effect of the partial crystallization on the structural and luminescence properties of the glasses

All glasses were heat treated first at their respective (T g +20°C) for 6 and 17h followed by a hold at their respective (T p -40°C) for 10, 30 and 60min. The duration of the heat treatments was chosen based on the study of the crystallization mechanism of the glasses with the composition 50P 2 O 5 -(40-x)CaO-xSrO-10Na 2 O (in mol%) with x=0, 20 and 40 [10]. It was found that all these glasses when heat treated above (T p -40 o C) for 30 minutes or more were fully crystallized. Two phases were found to crystallize in the glass with x=20: (Ca,Sr)(PO 3 ) 2 and Na(Ca,Sr)(PO 3 ) 3 .

No visible sign of crystallization could be seen in any investigated glasses after 10 min at their respective (T p -40°C). The reference Er-glass became opaque after at least 30min at (T p -40°C), which is a clear sign of crystallization. The B-Er, Zn-Er and Ti2.5-Er glasses became translucent after 60min at their respective (T p -40°C), whereas the Ti5-Er glass had still a glassy appearance.

The crystalline phases were identified using an X-ray diffraction analyzer (Philips X'pert)

with Cu Kα radiation (= 1.5418 Å). The XRD patterns of the glasses prior to and after the heat From the analysis of the XRD patterns and of the SEM pictures of the heat treated glasses, it seems like the reference Er glass is the most sensitive glass to crystallization, while the Ti5-Er glass is the least sensitive composition of the experimental glasses. This difference in the crystallization tendency of the glasses can be related to the different structure of the glasses:

i) the addition of Zn ions, which act as modifiers, most likely reduces the amount of nucleation sites and decrease the crystallization rate.

ii) the increase in the glass connectivity induced by the formation of P-O-Ti/B links seems to delay crystallization in the B-and Ti-containing glasses. Therefore, an increase in the TiO 2 content increases the glass connectivity reducing further the glass crystallization tendency. This study is in agreement with [36][37]: TiO 2 was found to increase the glass stability and to promote surface crystallization in phosphate glasses when introduced in low quantities (below 4mol%).

Based on the XRD patterns of the B-Er glass, we suspect the crystals to precipitate already at the surface after 30min at (T p -40 o C). The absence of crystals in the SEM micrograph of the B-Er glass heat treated 30min at (T p -40 o C) might be due to inhomogeneity of the crystals precipitation.

The intensities of the emission at 1530 nm of the glasses prior to and after the heat treatments are shown in Figure 6. Bulk samples were used to collect the emission from the crystalscontaining surface when using an excitation at 980nm. No increase in the emission intensity was observed after the different heat treatments. We observed a significant decrease in the intensity of the emission from the reference Er glass after 30min at (T p -40 o C). When the duration of the heat treatment increased to 60min, the intensity of the emission continued to slightly decrease. A slight decrease in the emission intensity was also observed after the heat treatment of the Zn-Er glass, while no noticeable change in the emission intensity was detected for the B-Er and Ti-Er glasses. Surprisingly, all the glasses heat treated for 60min at (T p -40 o C) exhibited similar emission intensity within the accuracy of the measurement. As seen in Figure 7, the shape of the emission band of Er 3+ in the reference Er glass changed after the heat treatment indicating that changes in the sites of the Er 3+ ions in the glass occur during the formation and growth of the crystals. However, no significant changes in the emission band shape could be noticed after heat treatment of the other glasses indicating that the crystals precipitation in B-Er, Ti-Er and Zn-Er glasses has no impact on the sites of the Er 3+ ions. Additionally, no sharp peaks can be seen in any of the emission spectra indicating that the crystals precipitating in all the investigated glasses are Er 3+ ions free. The Er 3+ ions are, therefore, suspected to remain in the glass matrix.

Conclusion

The effect of the composition on the crystallization behavior of Er containing glasses in the system P 2 O 5 -CaO-SrO-Na 2 O was investigated. Using Raman and IR spectroscopies, we suspect i) B 2 O 3 to lead to a cross-linking between the metaphosphate chains through the formation of P-O-B links.

ii) TiO 2 to form P-O-Ti linkages distorting the glass network and forming threedimensional networks of P-O-Ti linkages iii) ZnO to act as a modifier leading to the depolymerization of the phosphate network and to a less cross-linked network.

However, while B 2 O 3 , TiO 2 and ZnO have a noticeable impact on the glass structure, they do not have a significant impact of the absorption and emission properties of Er 3+ .

The glasses were heat treated at their respective (Tg+20°C) for 6 to 17 h and at their respective (Tp-40°C) from 10 to 60 min. The glass, used as a reference, and the Ti-containing glasses are, respectively, is the most and least sensitive glass to crystallization. Two different crystals, the composition of which depends on the glass composition, are suspected to precipitate in the glasses. NaCa(PO 3 ) 3 is thought to be one of the crystals to precipitate in all glasses. A reduction in the Er 3+ emission intensity was observed after heat treatment and from the absence of sharp peak in the Er 3+ emission band, we suspect the Er 3+ ions to remain in the glassy matrix in the partly crystallized samples.

With this study, we clearly show that the transformation of an Er 3+ doped glass into glassceramic via nucleation and growth process does not necessarily lead to an increase in absorption and emission cross-section of rare-earth as seen in many papers. Therefore, it is crucial to continue to investigate the impact of the crystal precipitation on the rare-earth spectroscopic properties in different glass systems to further understand how to control the crystalline environment around the rare-earth ion for the crystals to actually act as effective rare-earth sensitizers.

  930-1010 cm -1 range indicating an increase of ring structure amount into the glasses. The bands in the 930-1010 cm -1 range in the IR spectrum of the B-Er glass can be also related to the creation of P-O-B-O-P as discussed in the previous paragraph. The decrease in intensity of the IR band at 1085 cm -1 seen with the incorporation of B 2 O 3 and TiO 2 might be related to the decrease of non-bridging oxygen probably due to the formation of P-O-B and P-O-Ti bonds. However, the addition of ZnO increases the intensity of this band at 1085cm -1 indicating an increase in Q 1 units

  treatments are shown in Figures 4. The XRD pattern of the heat treated reference-Er glass (Figure4a) clearly exhibits sharp peaks after the heat treatments. One can notice that the intensity of the peak increases dramatically when the duration of the heat treatment at (T p -40 o C) is increased from 10 to 30min. However, no significant impact of the increase in the duration of the heat treatment at (T g +20 o C) is seen on the intensity of the peaks. The peaks are mainly related to NaCa(PO 3 ) 3 based on [ICDD 00-023-0669] and Sr(PO 3 ) 2 based on [ICDD 00-012-0360]. As explained in the previous paragraph, similar crystals were found to precipitate in the glass with the composition 50P 2 O 5 -20CaO-20SrO-10Na 2 O[10]. While some peaks could be seen in the XRD patterns of the B-Er and Zn-Er glasses (Figure4b and c, respectively), no peaks could be seen in the XRD pattern of the Ti-Er glasses after 30min at (T p -40 o C) (Figures5d and e). The number and intensity of the peaks seen in the XRD pattern of the B-Er and Zn-Er glasses after 30min at (T p -40 o C) are too low to identify unambiguously the crystalline phases form in the glasses. This clearly shows that the addition of the B, Zn and Ti dramatically reduces the crystallization rate. As seen in Figures5b, c and d, an increase in the duration of the heat treatment at (T p -40 o C) from 30 to 60 min leads to an increase in intensity of the peaks in the XRD patterns of the B-Er and Zn-Er glasses. Peaks appear in the XRD pattern of the Ti2.5-Er glass whereas no peaks can be seen in the XRD pattern of the Ti5-Er glass (Figure4e). At least two phases are suspected to crystallize in the partially crystallized glasses. NaCa(PO 3 ) 3 is suspected to precipitate in all partially crystallized compositions. Sr(PO 3 ) 2 is thought to be the second phase precipitating in the Zn-Er and Ti2.5-Er glasses and Ca 2 P 2 O 7 in the B-Er glass based on [00-009-0345].

Figure 5

 5 Figure5present the SEM images of the investigated glasses after heat treatment, polished from the side, with the external surface at the top of the image. Clear signs of surface crystallization can be seen. Similar surface crystallization was observed in the glasses within the same glass system[10]. In all samples, crystals form a layer growing from the surface. The thickness of the crystallized layer increases with the duration of the heat treatment at (T p -40 o C) as seen in[10]. It is interesting that the Zn-Er glass behaves in a similar manner as the reference Er glass: after 10 minutes at (T p -40 o C), large crystals with a needle like shape could be evidenced. The only difference between the reference Er and Zn-Er glasses is that the crystals grew faster in the reference Er glass. The crystals precipitating in the B-Er and Ti-Er glasses grew dendritically and slower than the crystals in the reference Er glass. A large number of small crystals are seen in the B-Er and Ti-Er glasses after 60min at (T p -40 o C) (Figure5).

Figure 1 :

 1 Figure 1: Absorption spectra (a) and absorption band centered at 1530nm (b) of the investigated glasses

Figure 2 :

 2 Figure 2: Emission (a), normalized emission (b) and IR absorption (c) spectra of the investigated glasses

Figure 3 :

 3 Figure 3: Raman (a) and IR (b) spectra of the investigated glasses.

Figure 4 :Figure 5 :Figure 6 :Figure 7 :

 4567 Figure 4: X-Ray Diffraction patterns of the glasses prior to and after heat treatment (a: Reference Er, b: B-Er, c: Zn-Er, d: Ti2.5-Er and e:Ti5-Er glasses) (* NaCa(PO 3 ) 3 , +Ca 2 P 2 O 7 and -Sr(PO 3 ) 2 ) Figure 5: SEM images of the cross-section of the glasses after heat treatment (note: the dark round spots are bubbles in the glass) Figure 6: Intensity of the emission at 1.5m intensity of the glasses as a function of the heat treatment Figure 7: Normalized Emission spectra of the glasses prior to and after heat treatment

  Ca(PO 3 ) 2 and Sr(PO 3 ) 2 were elaborated by mixing CaCO 3 or SrCO 3 with (NH 4 ) 2 HPO 4 (Merck, ACS Reagent). These precursors were then used in combination with NaPO 3 , Er 2 O 3 and H 3 BO 3 or TiO 2 or ZnO to prepare the final glass. The raw materials were

	measurements performed on all as-prepared glasses did not show diffraction peaks, characteristic
	of the amorphous state of the as-prepared glasses.
	The glass compositions were checked by using a Scanning Electron Microscope (Leo 1530
	Gemini, Zeiss) coupled with an Energy Dispersive X-Ray Analyser (SEM/EDXA) (Vantage by
	Thermo Electron Corporation).
	The investigated glasses were polished and then heat treated first at T g +20°C for 6 and 17 h
	to form nuclei and then at T p -40°C for 10 to 60 min to grow the nuclei into crystals. The duration
	of the heat treatments was chosen based on the study of the crystallization mechanism of the
	glasses with the composition 50P 2 O 5 -(40-x)CaO-xSrO-10Na 2 O (in mol%) with x=0, 20 and 40
	(Sigma-Aldrich,	≥99.9%),	CaCO 3	(Sigma-Aldrich,	≥99.0%),	Er 2 O 3	(MV
	LABORATORIES,INC., 99.999%), H 3 BO ground and mixed to prepare a 25g batch, then placed in a silica crucible and melted at 1100°C
	for 45min. After quenching, the glasses were annealed for 4h at 40 o C below their respective T g to
	remove the internal stress induced by the quenching. Finally, the glasses were cut and polished,
	or ground depending on the experimental characterization targeted. All the glasses have a slight
	pink coloration except the Ti-containing glasses which have a dark pink coloration. X-ray

2 O 5 -0.2SrO-0.2CaO-0.1Na 2 O)-0.0025Er 2 O 3 -xB 2 O 3 /TiO 2 /ZnO (x=0, 2.5 or 5) (mol %) were studied. B 2 O 3 and ZnO were added within 2.5 mol% (the glasses were labeled B-Er and Zn-Er glasses) while 2.5 and 5mol% of TiO 2 were used (the glasses were labeled Ti2.5-Er and Ti5-Er glasses). The glass with x=0 is labeled as

Reference-Er glass. The glasses were prepared using NaPO 3 (Alfa Aesar, ACS Reagent), SrCO 3 3 (Sigma-Aldrich, ≥99.5%) or TiO 2 (Sigma-Aldrich,99-100.5%) or ZnO (Sigma-Aldrich,≥99.0%) and using standard melting method in silica crucible. First,

Table Caption Table 1 :

 Caption1 Density, thermal and optical properties of Er doped glasses

Acknowledgments

The Magnus Ehrnrooth foundation and the Academy of Finland (grant for J. Massera and for L. Petit "Competitive Funding to Strengthen University Research Profiles; funded by Academy of Finland, decision number 310359) are gratefully acknowledged.

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