Pavel A Dub

Aurélien Béthegnies

Jean-Claude Daran

Rinaldo Poli
email: poli@lcc-toulouse.fr

Formation and Structure of a Platinum(II) Complex Containing Two trans Nonstabilized Phosphorus Ylide Ligands: Evidence for Reversible Ylide Dissociation

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

We have recently carried out synthetic and solution equilibrium studies involving a few tetra-n-butylphosphonium salts of anionic platinate(II) complexes, including 1 These and other neutral derivatives, namely cis-and trans-PtBr2(PhNH2)(C2H4) and cis-PtBr2(PhNH2)2 are low-energy intermediates of the catalytic intermolecular hydroamination of ethylene by aniline. [1][2][3][4][5][6][7] In an attempt to generate a diaryl derivative of type [Ar2PtBr2] 2-, which we wished to test for its potential as a hydroamination pre-catalyst, we have reacted a solution of [nBu4P]2[PtBr4] with a lithium aryl derivative, 2,2'-(C6H4Li)2. The reaction, however, led to deprotonation of the phosphonium cation and to the isolation of an unprecedented bis(ylide) derivative of platinum(II). The reaction leading to the formation of this product and its structural characterization are reported herein.

[nBu4P]2[PtBr4], [nBu4P]2[Pt2Br6], [nBu4P][PtBr3(PhNH2)] and [nBu4P][PtBr3(C2H4)].

Experimental section

General. The synthesis was carried out under argon. THF and Et2O were dried by conventional methods and distilled under argon prior to use. NMR investigations were carried out on Bruker DPX 300 spectrometer operating at 64.5 MHz ( 195 Pt) and 121.49 MHz ( 31 P{ 1 H}). The spectra were calibrated relative to external Na2PtCl6 ( 195 Pt) or 85% H3PO4 ( 31 P).

Generation and crystallization of complex trans-(R,S)-PtBr2[CH(nPr)-(PnBu3)]2.

To a cooled solution (-70°C) of 2,2'-(C6H4Br)2 (36 mg, 0.116 mmol) in Et2O (2 mL) was added dropwise BuLi (0.16 mL of a 1.6 M hexane solution, 2.2 equiv).

After stirring at -70°C for 5 min, this solution was transferred by cannula dropwise at -70°C into a separate Schenk flask where a suspension of [nBu4P]2[PtBr4] (100 mg, 0.097 mmol) in 3 ml of THF had been separately prepared. Subsequent warming to room temperature resulted in a clear yellow solution. After stirring for 1 h at r.t., a portion of the solution was withdrawn and used to measure a 195 Pt{ 1 H} NMR spectrum with overnight data collection (external lock with a C6D6 capillary). The spectrum showed only two broad resonances with equal intensity at δ -3471 and -3482. After one additional day of stirring, a second aliquot was measured in the same way, yielding an increased intensity for the resonance at δ -3471 and a decreased intensity for that at δ -3482. Yellow crystals were grown directly on the walls of this Schlenk flask by slow solvent evaporation under argon flow (ca. 2 hrs). One of these crystals was used for the X-ray structural analysis, revealing the meso (R,S) stereochemistry. NMR (CDCl3, 298K): 31 P{ 1 H}: 31.8 (s + Pt satellites, 2 JP-Pt = 60 Hz); 195 Pt{ 1 H}: -3465 ppm (t, 2 JPt-P = 60 Hz).

In a separate experiment run under the same conditions, the reaction was monitored by 31 P{ 1 H} NMR. After one hour, 0.2 mL of the reaction mixture diluted into 0.3 mL of CDCl3 revealed only one broad resonance at δ 31.0 with visible Pt satellites having JPPt ~ 60 Hz (see SI, Figure S1). The shape of this spectrum did not change over 3 weeks, except for the appearance of small resonances attributed to degradation products. The solution was worked up by addition of wet Et2O followed by filtration of the resulting white precipitate and evaporating to dryness. The 31 P{ 1 H} spectrum of the yellow crude product in acetone-d6 revealed two major resonances as closely spaced broad singlets at δ 31.64 and 31.47 (relative intensity ca. 70:30), in addition to other less intense resonances attributed to decomposition products (Figure S2). An analytically pure product could not be isolated.

X-ray diffractometric analysis.

A single crystal was mounted under inert perfluoropolyether at the tip of a glass fibre and cooled in the cryostream of a Bruker APEXII CCD diffractometer. The data were collected using the monochromatic MoK radiation (= 0.71073). The structure was solved by direct methods (SIR97 8 ) and refined by least-squares procedures on F 2 using SHELXL97. [START_REF] Sheldrick | Program for Crystal Structure refinement[END_REF] All H atoms attached to carbon were introduced at idealised positions and treated by the riding model. The drawing of the molecule was realised with the help of ORTEP32. [START_REF] Farrugia | [END_REF] Computational details. Computations were carried out using the hybrid Becke functional (B3) 11,12 for electron exchange and the correlation functional of Lee, Yang, and Parr (LYP), 13 as implemented in the GAUSSIAN 09 software package. [START_REF] Frisch | Revision A.02[END_REF] For platinum, the SDD basis set with the associated effective core potential was employed. [START_REF] Andrae | [END_REF] All other atoms were modelled at the 6-31G+(d) level of theory. Geometry optimizations were carried out on full models without system simplifications of the isolated molecules in the gas phase as well as in the tetrahydrofuran force field using the self-consistent reaction field (SCRF) with CPCM-UFF solvation model. In both cases, the identity of the optimized geometry as local energy minima was verified by frequency calculations under the harmonic approximation. NMR shielding tensors were computed with the the Gauge-Independent Atomic Orbital (GIAO) method. 16

Results and Discussion

The Platinum(II) chemistry with phosphorus ylides is well established, but the reaction reported here shows unusual features. The substitution of halide ligands by phosphorus ylides is relatively common for stabilized ylides, such as RCOCH=PPh3 (R = Ph, Me, EtO) in the case of palladium complexes. 18 When the ylide is not stabilized as in the present case, and even for stabilized ylides in the case of platinum complexes, the reaction typically proceed with transylidation or ortho-metallation and formation of a chelating anionic ylide complex and a phosphonium salt, for instance PtCl2(PMe3)2 and two equivalents of tBu2MeP=CH2 yield [Pt{(CH2)2PtBu2}(PMe3)2] + Cl -and tBu2Me2P + Cl -. 19 In the latter reaction, the second equivalent of ylide captures a proton from an α-C-H bond of the already coordinated ylide ligand. The reaction between PtCl2 and Ph3P=CHCOCH3 was initially incorrectly described as leading to the bis(ylide) adduct but was later shown to yield the ortho-metallation product [Pt(µ-Cl)(o-

C6H4PPh2CHCOCH3)]2. 20,21 For the reaction presented herein, on the other hand, both ylides substitute a bromide ligand. Platinum(II) complexes with neutral, non stabilized ylide ligands have previously been obtained by addition of the phosphine to a halomethyl precursor, for instance the addition of excess PPh3 to cis-Pt(COD)(CH2I)2 yielded cis-[PtI(PPh3)2(CH2PPh3)2] + I -, which was structurally characterized by X-ray diffraction. 22 Also, complex trans-[PtI(CH2PEt3)(PEt3)2] + I -was obtained by reacting Pt(PEt3)3 and CH2I2, presumably via the intermediate oxidative addition product, PtI(CH2I)(PEt3)2, which is further attacked by the released PEt3. 23 A possible reason for this unprecedented reactivity is related to the different coordination sphere and electronic environment around the Pt II center relative to the previous studies, which generally involved neutral complexes with phosphine ligands.

Once a first halide ligand is replaced to generate a cationic [Pt(ylide)X(PR3)2] + complex, there is a choice between substitution of the second halide ligand or deprotonation at the α-CH position of the already coordinated ylide ligand (see Scheme 2, L = PR3). For the cationic complex (L = neutral donor), the halide ligand is probably tightly bonded to Pt and the phosphorus atom of the already bonded ylide ligand has a high positive charge, exalting the acidity of the α-CH bond and favoring deprotonation (transylidation). On the other hand, the anionic complex obtained when L = X -(in the present case bromide) has reduced acidity at the ylide α-CH bond and greater lability for the coordinated X -ligands, especially the one located trans to the ylide. 2. The molecule has crystallographically imposed inversion symmetry, with the Pt atom sitting on the inversion center and half of the molecule in the asymmetric unit. This is an unprecedented structural arrangement for Pt II ylide complexes. [START_REF] Urriolabeitia | Transition Metal Complexes of Neutral Eta1-Carbon Ligands[END_REF] The only other structurally characterized κ 1 phosphorus ylide complexes of Pt II , according to the Cambridge Structural Database, are cationic complexes that contain only one ylide ligand (trans-[PtI(PEt3)2(CH2PEt3)] + , [START_REF] Kermode | [END_REF] and cis-

[PtX(PPh3)2(CH2PPh3)] + (X = Cl, I) 26,27 ) or two ylide ligands in a relative cis arrangement, cis-[PtI(PPh3)(CH2PPh3)2] + . 22 The complex described here, on the other hand, is neutral and features the two ylide ligands in a mutual trans arrangement. A related structure, which contains however a stabilized ylide ligand, is found for a palladium(II) complex, trans-PdCl2[CH(COPh)(PnBu)3]2. 28 Like for those previously investigated ylide complexes, the P-C bond to the Pt-bonded C atom is slightly shorter than the other P-C bonds. The Pt-C11 distance is rather long relative to those observed in the above mentioned compounds (all being shorter than 2.1 Å except for 2.12(2) Å in 27 . This is possibly related to the increased electron density at the metal center and the mutual trans arrangement of the strongly electron donating ylide ligands. a All computed parameters related by pseudo-symmetry are given as an averaged single entry. Symmetry code (for the X-ray experimental structure): (') -x+1,-y+1,-z+1.

cis-[PtI(PPh3)2(CH2PPh3)] + )
The most intriguing feature of this compound is the slow isomerization, interconverting the two diastereomers. Initially, the reaction yields a 1:1 mixture of the two isomers under kinetic control, as expected from statistical considerations since the trans arrangement of the two ylide ligands cannot lead to strong diastereoselection.

However, the slight energetic difference (see DFT section below) leads to a subsequent slow re-equilibriation to give a final solution richer in the more stable meso isomer, which is also preferentially crystallized. This isomerization could in principle be caused by the presence of excess base, which could invert the configuration at the Pt-bonded C atom via reversible proton transfer steps, involving a chelated anionic ligand intermediate and also a cis isomeric form, as shown in Scheme 3 (path A).

Scheme 3. Possible diastereoisomer interconversion mechanisms.

However, when redissolving the isolated pure (R,S) crystals in CDCl3, a slow isomerization process still occurs. The 195 Pt{ 1 H} NMR spectrum recorded after 1 month on the solution prepared from the crystallized meso isomer is shown in Figure 2. This process cannot take place by pathway A, since no external base is present under these conditions. An alternative and simpler possibility consists of a dissociation/reassociation process (path B in Scheme 3). A dissociative exchange of non stabilized ylides, which are normally considered as strongly bonding ligands, is apparently unprecedented. A precedent for the interconversion of diastereoisomers was only reported, to the best of our knowledge, for complexes containing two stabilized chiral ylides, for instance for the similar palladium system trans-PdCl2[CH(COAr)(SMe2)]2, where equilibration took place over a few hours, 29 and for a palladium compound containing the chelating phosphine-ylide Ph2PCH2CH2PPh2=CHC(O)R and a second asymmetric center in the chelating o-C6H4CH(Me)NMe2 ligand. 30 In the latter case, the diastereomeric ratio was DFT calculations have addressed the thermodynamic preference and have given indications about the isomerization mechanism. The calculations were carried out on the full model of the structurally characterized (R,S) isomer (no ligand simplification) and also on its (R,R) diastereoisomer. The molecular geometries were fully optimized both in the gas phase and in THF solution by the CPCM method (selected optimized values are collected in Table 2. Although no symmetry restrictions were used in the calculations, the energy minima were obtained for geometries with approximately Ci and C2 point group symmetry (shown in Figure 3) for the first coordination sphere in the (R,S) and (R,R) molecules, respectively. The ideal C2 axis for the (R,R) isomer runs through the Br-Pt-Br vector. This ideal symmetry is significantly broken only by the conformation adopted by the outermost nBu and nPr groups. Each pair of parameters related to each other by the pseudo-symmetry element in Table 2 display nearly equivalent values. The optimized parameters are very similar for the two diastereoisomers and are very close to those determined experimentally by X-ray diffraction for the meso isomer and all trends are reproduced, notably the slightly shorter P-C11 distance relative to the P-C distances involving the nBu groups.

Optimization in solvent yields substantially equivalent geometries to the gas phase. The calculations indicate greater stability for the crystallographically characterized meso diastereoisomer. The stabilization relative to the dl isomer on the ΔG CPCM (ΔEZPVE CPCM ) scale is 4.8 (4.2) kcal/mol in the gas phase and 4.3 (3.7) kcal/mol in solution. The reason for this energy difference is probably caused by less favorable van de Waals contacts between the nPr and the PnBu3 groups of the two different ylide ligands on the opposite sides of the molecule. These bulky groups prefer to occupy the space above and below the coordination plane (the CH bond of the Pt-bonded atom lies close to the coordination plane) and thus the meso form is better able to accommodate these groups further away from each other, as may be appreciated by inspecting a space filling model of the two optimized geometries, although the difference is not very large.

The calculated energy difference would lead to a meso/dl ratio of ca. 0.03% at equilibrium at room temperature, namely an unobservable relative amount, whereas a substantial amount of dl isomer is generated as shown in Figure 2. However, DFT calculations are believed to yield relative energies for related systems within an accuracy of a few kcal/mol (in our experience ±2 kcal/mol when the two systems are very closely related as in the present case). Therefore, the computational result is not necessarily in disagreement with the experiment.

Following the geometry optimizations, the DFT approach was also used to estimate the 195 Pt chemical shifts for the two optimized geometries, yielding values of -28.8 ppm (R,S) and -37.6 ppm (R,R). Relative to the calculated chemical shift value for the Na2PtCl6 reference, these numbers become -205.2 ppm for the (R,S) isomer and -214.0 for the (R,R) isomer. These values are quite far from those experimentally determined for the two compounds, probably because of the very different chemical nature of the complexes (neutral square planar Pt II for the bis(ylide) complexes, dianionic octahedral Pt IV for the reference) and limitations in the DFT accuracy for the calculation of the Pt shielding tensors. However, the chemical shift difference between the two isomers (ca. 10 ppm) matches quite well in both magnitude and sign with the experimentally observed one.

The calculations on the separate ylide and three-coordinate complex PtBr2[CH(nPr)(PnBu3)] (intermediates of the dissociative isomerization mechanism, path B in Scheme 3) yield an energy only 11.8 kcal/mol higher than the more stable meso isomer (7.5 kcal/mol higher than the dl isomer), see Figure 3. The energy of the transition state is not much higher, as could have been predicted a priori from the dissociative nature of the process. In fact, for the ylide dissociation from the more stable meso isomer this is found at even lower energy than the dissociation asymptote on the CPCM(THF) energy scale. In other words, the dissociative 4-coordinate intermediate has an essentially zero barrier for reassociation of the ylide and this perfectly justifies the initial formation a 50:50 diastereomeric mixture under kinetic control. Therefore, the results of the DFT calculations are consistent with the occurrence of this pathway.

Concerning pathways C and D, attempts to optimize the 5-coordinate Pt IV hydride intermediate of path C led to loss of the Br -ligand and optimization of a square planar geometry for [PtHBr{CH(nPr)(PnBu3)}{C(nPr)(PnBu3)}] -in close contact with the outer sphere Br -ion. Hence, the preference of Pt II for a square planar geometry induces loss of the weakest ligand in the coordination sphere. The energy of this intermediate species is +32.5 kcal/mol relative to the starting meso compound (on the ΔG scale in THF). The hydrido Pt IV intermediate of path D, on the other hand, leads to smooth convergence to the expected pseudo-octahedral 6-coordinate geometry but is again found at very high energy relative to the meso compound (+34.8 kcal/mol on the ΔG scale in THF). Figures of these two high-energy minima are shown in Figure S3. In conclusion, the DFT study points strongly toward a dissociative mechanism for the diastereoisomer interconversion through the free non stabilized ylide.

Conclusions

We report here an unprecedented trans square planar bis(ylide) derivative of Pt II with a non stabilized ylide, generated by deprotonation of tetra-n-butylphosphonium.

Given the central chirality of the Pt-bound C atom, meso and dl diastereoisomers are generated by the synthesis as observed by 31 P and 195 Pt NMR, only the meso isomer being crystallized and structurally characterized. The observed slow interconversion of the two isomers in solution is suggested by a DFT study to take place via a dissociative mechanism with release of the non stabilized ylide. To the best of our knowledge, this is the first reported evidence for reversible dissociation of a non stabilized ylide, which is considered to be a strongly binding ligand, from a transition metal complex. The relative trans arrangement of two such ligands in the Pt coordination sphere is probably

Table of Content Graphic

Synopsis

Deprotonation of tetra-n-butylphosphonium in association with the tetrabromoplatinate(II) dianion yields a unique example of a trans-bis(ylide) complex of Pt II with non stabilized phosphorus ylide ligands.

  reaction between [nBu4P]2[PtBr4] and 2,2'-(C6H4Li)2 in a 1:1 ratio was intended to yield a 2,2'-biphenyldiyl platinum(II) derivative. However, the reaction produced instead a mixture of two compounds, of which the major one was shown to correspond to the bis(ylide) derivative trans-(R,S)-PtBr2[CH(nPr)(PnBu3)]2, see Scheme 1 as confirmed by an X-ray structural analysis (vide infra). The reaction initially gave the two compounds in a 1:1 ratio, as shown by the relative intensity of their 195 Pt resonances at δ -3471 and -3482. Subsequently, the more upfield shifted resonance decreased very slowly (over several days) in favour of the other resonance. However, the 31 P{ 1 H} NMR of the reaction mixture (after diluted into CDCl3) revealed only one broad resonance with Pt satellites at δ 31.0, see FigureS1, showing that the chemical environment around the P nucleus in the two compounds is very similar. The two products could be distinguished in the 31 P spectrum only when the crude isolated product was redissolved in acetone-d6 (FigureS2).A few attempts were made to generate the bis(ylide) compound by using nBuLi as a base. Although evidence for the formation of the same two products was obtained, this reaction led to multiple products according to the NMR analysis. The procedure using 2,2'-(C6H4Li)2, on the other hand, is selective leading to the formation of the two bis(ylide) isomers only, according to NMR. The outcome of this reaction suggests that the combination of the strong basicity of the lithium reagent and the relative acidity of the α-CH2 protons in the nBu4P + cations results in deprotonation to generate a phosphorus ylide intermediate, nBu3P=CH(nPr), which then undergoes ligand substitution at[PtBr4]

Scheme 2 .

 2 Scheme 2. Competing reaction pathways for the attack of a second ylide ligand to [PtX(ylide)(L)2] + .

Figure 1 .

 1 Figure 1. ORTEP view of the structure. Ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity.

  shown to change reversibly with temperature indicating rapid equilibrium. The occurrence of a dissociation for a non stabilized ylide ligand appears remarkable and may be facilitated in the present case by the strong mutual trans effect of two such ligands in relative trans position. Two other possibilities for the isomerization mechanism may be imagined, involving C-H activation processes. In the first one, an α-H elimination from the ylide ligand could yield a 5-coordinate hydrido carbene intermediate, which could reverse to the opposite configuration at the C atom after stereochemical rearrangement of the Pt coordination sphere (Scheme 3, path C). The second one would entail a γ-C-H oxidative addition to generate the chelating [(CHnPr)2P(nBu)2] -ligand on a Pt IV hydrido intermediate, followed by the reductive elimination to yield the same Pt II cis isomer as in path A, and then the reverse process to generate the opposite configuration at the C atom (Scheme 3, path D).

Figure 2 .

 2 Figure 2. 195 Pt{ 1 H} NMR spectrum of a CDCl3 solution of redissolved crystals of trans-(R,S)-PtBr2[CH(nPr)(PnBu3)]2, after standing for 1 month at r.t.

Figure 3 .

 3 Figure 3. Views of the optimized geometries and relative energies computed at DFT/B3LYP/SDD(Pt)/6-31+G*(C,H,P,Br)/CPCM(THF) level of theory for the meso (left) and dl (right) diastereomers of trans-PtBr2[CH(nPr)(PnBu3)]2, and for the dissociative PtBr2[CH(nPr)(PnBu3)] + nPrCH=PnBu3 (center).

Table 1 .

 1 Crystal data and structure refinement parameters.

	Crystal data and

Table 2 .

 2 Selected

		meso isomer		dl isomer
		X-Ray	gas	THF	gas	THF
	Pt-Br1	2.4560(11) 2.532 2.531 2.527 2.532
	Pt-C11	2.141(11) 2.174 2.182 2.173 2.182
	P1-C11	1.794(10) 1.804 1.812 1.804 1.812
	P1-C21	1.805(12)	1.838	1.838	1.833	1.842
	P1-C31	1.813(12)	1.857	1.852	1.861	1.857
	P1-C41	1.799(12)	1.848	1.847	1.843	1.842
	Br1-Pt-Br2 180.0	179.6 179.5 178.4 177.5
	C11-Pt-C11' 180.0	179.6 179.5 174.9 174.5
	Br1-Pt-C11 91.9(3)	92.5	92.4	91.8	91.8
	Br1-Pt-C11' 88.1(3)	87.4	87.6	88.3	88.3
	Pt-C11-P1	109.7(5)	110.7 112.5 110.9 113.1
	Pt-C11-C12 119.0(8)	117.4 117.2 116.2 116.0
	C11-P1-C21	112.8(6)	110.8	110.6	114.0	113.9
	C11-P1-C31	111.1(5)	112.4	112.2	113.6	113.1
	C11-P1-C41	113.2(5)	114.2	114.3	110.6	110.7

structural parameters (distances in Å, angles in °) for the two diastereoisomers of trans-PtBr2[CH(nPr)(PnBu3)]2. a

Acknowledgements. We are grateful to the ANR (Agence Nationale de la Recherche, Grant No. NT09_442499), to the CNRS (Centre National de la Recherche Scientifique) and to the IUF (Institut Universitaire de France) for support of this work. This work was granted access to the HPC resources of CINES under the allocation 2010-086343 made by GENCI (Grand Equipement National de Calcul Intensif) and to the resources of the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP). PD thanks the MENESR (Ministère de l'Éducation Nationale, de l'Enseignement Supérieur et de la Recherche, France) for a Ph.D. fellowship.

at the origin of this unexpected behavior.


Yohan Champouret

Stefano Gulli

Jean-Claude Daran

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Preparation of Diamine-β-diketiminato Copper(II) Complexes and Their Application in the Reverse Atom-Transfer Radical Polymerization of Styrene

Keywords: diketiminato ligands, controlled radical polymerization, polystyrene, ARGET ATRP

come   L'archive ouverte pluridisciplinaire

Introduction

Since its discovery in 1995, [1][2] metal-catalyzed radical polymerization has grown to become a powerful method to access high added-value functionalized polymers for a multitude of specialty applications. [3][4][5][START_REF] Matyjaszewski | Macromolecular Engineering: Precise Synthesis, Materials Properties, Applications[END_REF][START_REF] Ouchi | [END_REF][8] The technique, also named Atom Transfer Radical Polymerization (ATRP) rests on controlling the radical concentration at very low levels, thereby reducing the impact of irreversible bimolecular terminations, through the establishment of a shuttle between the active radical chain and a dormant chain which is capped by a halogen atom. The equilibrium between these two species is insured by a metal complex that can reversibly abstract the halogen atom from the dormant chain and therefore generate a new metal complex with one additional ligand (the halide) with an increase of the formal oxidation state by one unit. This mechanism is illustrated in Scheme 1. Two common approaches to start the polymerization are from either the left hand side of the equilibrium (the M x /Ln complex plus a suitable alkyl halide initiator, so called "ATRP conditions") or from the right hand side (the oxidized Ln/M x+1 -X complex plus a suitable radical generating initiator, so-called "reverse ATRP conditions"). The most versatile and convenient metal system for this process has proven to be Cu I /Cu II because of low cost, efficiency, facility of catalyst assembly in situ from simple copper halide salts and suitable ligands, and availability of a large variety of neutral multidentate ligands that allow the system to be tuned for application to a variety of different monomers and operating conditions, even though limitations exist. The combination of CuX (X = halide; mostly Cl and Br have been used) with neutral supporting ligands is the simplest method to afford a wide variety of catalysts but can only lead to systems that contain a halogen atom in the coordination sphere or to cationic complexes when the halide ion dissociates. [9] Little attention has so far devoted to the development of ATRP Cu I catalysts containing the anionic functionality embedded in the multidentate ligand itself, mostly limited to tris(pyrazolyl)borate derivatives. [10][11][12] This leads to halide-free neutral catalysts that are of interest because they may avoid ligand redistribution processes that are observed for the CuX/Ln systems under certain conditions. [13] We have recently explored the use of a tridentate "XL2" type ligand, the amino-β-ketiminato ligand obtained by deprotonation of R2NCH2CH2NHC(Me)CHC(Me)O (R = Me, Et) and have reported the synthesis of corresponding CuCl(XL2) complexes and their preliminary application to the reverse ATRP of styrene. [14] In this contribution, we wish to extend the study to a tetradentate diamino-β-diketiminato ligand. We report the synthesis of unprecedented Cu II complexes containing the ligand [Et2NCH2CH2NC(Me)CHC(Me)NCH2CH2NEt2] -(L -), our attempts to generate a corresponding Cu I complex, and the performance of the Cu II complex CuClL, 1, as a controlling agent for the polymerization of styrene.

Results and Discussion

(a) Synthesis and characterization

The pro-ligand HL was prepared by a previously reported twostep procedure as shown in Scheme 2. [15] The reaction of HL with MeLi in ether affords the lithium salt LiL that can react in situ with anhydrous CuCl2 to give purple LCuCl (1) in moderate yields (Scheme 2), isolated as a hygroscopic microcrystalline purple powder, which changes color to brown upon absorption of water. However, well formed crystals of 1 are stable in air without any color change even after a few months. Scheme 2. Procedure for the synthesis of HL and LCuCl (1).

Complex 1 is soluble in most organic solvents yielding deep purple solutions. It has been characterized by mass spectrometry, by IR and EPR spectroscopies, by elemental analysis and by single crystal X-ray diffraction. The molecular ion is not revealed by the mass spectrometric study, the highest peak corresponding to the loss of one chloride ligand, [M -Cl] + at m/z = 358. The IR spectrum (see figure 1S in the SI) does not change upon exposure to moist air, except for the appearance of bands corresponding to coordinated water. Hence, the color change can be attributed to a change of the coordination environment of the Cu II ion but the chemical nature of the CuLCl moiety remains intact. The C=N stretching vibrations, which are observed as the most intense bands at 1619 and 1561 cm -1 for the free ligand, become less intense and are red-shifted to 1557 and 1523 cm -1 in the Cu II complex.

Single crystals of 1 suitable for the X-ray analysis could be grown from pentane solutions by slow evaporation or by hot filtration followed by cooling to -20°C. The molecular structure of 1 is depicted in Figure 1. The complex is mononuclear with a four coordinated copper(II) centre bound by one chlorine atom and three N atoms of L (two N atoms of the -diketiminato moiety and one of the two amino N atoms) while one amine arm remains pendant. The geometry around the copper centre can best be described as a distorted square plane with trans N1-Cu-Cl1 and N2-Cu-N3 angles significantly smaller than 180° [161.87(13)° and 154.94(10)°, respectively]. The Cl1-Cu-N3 plane makes a dihedral angle of 25.48° with the N1-Cu-N2 plane. This structural type is quite typical for a 4-coordinated Cu II complex, [16][17][18][19][20][21][22][23] fully analogous to that of the complex with a tridentate amine-βketiminato ligand, [Et2NCH2CH2NC(Me)CHC(Me)O]CuCl, which also displays similar Cu-Cl (2.2458(4) Å) and Cu-N distances (1.9404(12) Å to the ketiminato N atom, 2.0563(12) Å to the amino N atom). [14] In the latter contribution, it was specifically shown that this 4-coordinate geometry is stabilized by bulky groups against the possible dimerization with formation of loose Cu•••Cl axial interactions, which were on the other hand seen for the related [Me2NCH2CH2NC(Me)CHC(Me)O]CuX complexes (X = Cl, [14] O2CPh [START_REF] Zhang | Nankai Daxue Xuebao[END_REF] ) as well as for other related complexes. [25][26][27] For the present structure, the bulky ethyl groups and the pendant diethylaminoethyl arm are presumably preventing the formation of such loose Cl-bridged dimer. The EPR spectra of 1 in toluene solution at room temperature and frozen at 115K are shown in Figure 2. In the isotropic room temperature spectrum, only the coupling to the 63 Cu and 65 Cu nuclei (I = 3/2) is observed. The frozen spectrum shows an essentially tetragonal tensor (with a slight orthorhombic distortion) with gx,y >> gz > 2.0 and A║ Cu > A┴ Cu (the latter is not discernible), which is characteristic of a ground state with the unpaired electron in the dx2-y2 orbital. This is common to other Cu complexes with similar coordination geometry, notably Cu II complexes with isoindoline based ligands that also display a 4-coordinate CuClN3 environment. [28] The fine structure that can be discerned on the perpendicular component (see inset of Figure 2b) is due to superhyperfine coupling to the 14 N nuclei. The spectrum has been simulated to a reasonable level (see SI). However, the presence of three chemically distinct N atoms is not unambiguously demonstrated by the simulation. The best fitting is obtained when using three g values, two A values for Cu (x, z) and only one A value for two undistinguishable N atoms (details in the Experimental section). Coupling to the third N atom (probably the less strongly coordinated amino arom) was not visible.

The synthesis of the copper(I) complex CuL was attempted by a strategy completely analogous to that described in Scheme 2, using CuCl instead of CuCl2. The reaction, however, resulted in the formation of a copious amount of Cu 0 . The same result was obtained upon using [Cu(CH3CN)4]BF4 as substrate in toluene. This result could be caused by disproportionation of the initially formed Cu I product, or by overreduction because of the interaction of the formed CuL with the Li + salt that is still present in solution. An alternative strategy consisted of reacting HL with CuCl in the presence of NEt3 to trap the HCl by-product. Indeed, when using THF as solvent, the precipitation of white Et3NH + Cl -occurred as expected, but a disproportionation reaction was still observed, although more slowly (over several hours) under these conditions. This indicates that overreduction accelerates the formation of Cu 0 , but the CuL product is also prone to slow disproportionation. Upon rapid work-up of the solution after filtration of Et3NH + Cl -, we were not able to isolate the putative CuL compound. Only a few crystals of the mixed-valence Cu I /Cu II compound 2 (from the CuCl/HL/NEt3 reaction) or the Cu II complex 3 (from the LiL/[Cu(CH3CN)4]BF4 reaction) were obtained from the various crystallization attempts. Disproportionation of β-diketiminato Cu I complexes has also been previously observed by other authors and attributed to an insufficient steric bulk of the substituent on the ligand N donor atoms. [29] In the present case, it is likely that the coordination of the two NH2NH2NEt2 arms is not sufficiently strong to stabilize the Cu I center in a 4-coordinate environment, particularly in view of the fact that the planarity of the βdiketiminato ligand, which extends to the first C atom on each aminoethyl arm, does not allow the structure to adopt the preferred tetrahedral coordination geometry.

The nature of compounds 2 and 3 was revealed by single crystal X-ray diffraction analyses. The two structures contain the same cation and will be therefore discussed together. Both complexes contain two independent cations and two independent anions in the asymmetric unit, each pair of like ions having essentially the same geometrical parameters. For the structure of 2 both cation-anion pairs are located in a general position whereas one pair is in a general position and the second one sits on a crystallographic twofold axis for the structure of 3. A view of one of these independent pairs is provided in Figure 3 for compound 2 and in Figure 4 for compound 3. In the case of complex 2, the typical coordination geometry observed for each fragment unambiguously identifies the CuCl2 fragment as an anion of Cu I because of the linear geometry [Cl-Cu-Cl angles of 179.08(8)° around Cu(3) and 179.49(8)° around Cu(4)]. Consequently, the CuL fragment must consist of a cation of Cu II , which is identical to the [CuL] + fragment in complex 3. The [CuCl2] -anion is precedented in over 100 other salts, of which we found fourteen containing a Cu II cation in the Cambridge Structural Database. These cations are mostly 5-coordinate complexes, [30][31][32][33][34][35][36][37][38][39][40][41] but there are also two examples of 4-coordinate complexes like the cation of compound 2 in the structures of the chloro-(2,6-bis((1,3-di-tert-butylimidazolin-2-imino)methyl)pyridine)copper(II) and [Cu(Ph3AsO)4] 2+ salts. [42- 43] The relevant geometric parameters of the cations in the two structures are reported and compared in Table 1. For both complexes, L acts as a tetradentate ligand through the four nitrogen atoms to yield a slightly distorted (towards a butterfly) square coordination geometry, very close to that adopted by 1 (Figure 1) and by many other 4-coordinate Cu II complexes as already cited above. The range of the trans N-Cu-N angles is between 161 and 163° for compound 2 and between 156 and 165° for compound 3 The (β-diketonato)Cu plane (e.g. N11-Cu1-N14 in Figure 3) and the other CuN2 plane involving the two amino groups (e.g. N12-Cu-N13 in Figure 3) form dihedral angles of 22.7° and 19.5° in complex 2 and 20.5° and 29.7° in complex 3. (6) 85.89( 6) N(n2)-Cu(n)-N(n3) 83.60 (18) 84.25 (18) 82.49( 6) N(n2)-Cu(n)-N(n4) 161.65 (18) 162.14 (18) 164.99( 6) N(n3)-Cu(n)-N(n4) 105.01 (16) 104.00 (16) 104.24 (6) 103.85 (8) Once the formulation of 2 as a mixed-valence Cu I -Cu II salt became evident from the X-ray structural analysis, the compound was prepared in bulk quantities by a logical approach, which consists of the reaction of equivalent amounts of complex 1 and CuCl in THF at room temperature (Scheme 3). In this reaction, CuCl acts as a Lewis acid by extracting the chlorido ligand from compound 1, which allows the pendant NEt2 group to coordinate and yield the 4-coordinate [CuL] + cation. The success of this synthetic strategy might also hint to the possible mechanism of its formation in the initial attempt aimed at synthesizing CuL. The mass spectrum of compound 2 in the positive mode reveals the m/z peak of the CuL + ion, identical to the spectrum of compound 1, while the spectrum in negative mode shows the m/z peak of the CuCl2 -ion.

(b) Controlled radical polymerization

The new copper complex 1 was tested in the controlled radical polymerization of styrene by the atom transfer strategy. Since the target copper(I) complex could not be isolated and the synthetic studies revealed its high propensity to disproportionation, the polymerization was carried out using 1 under ARGET (Activators ReGenerated by Electron Transfer) ATRP conditions. [44][45][46] The reducing agent chosen to generate the copper(I) in situ as well as to regenerate it during the polymerization process was glucose, while (1-bromoethyl)benzene (BEB) was selected as the initiator. The principle of the ARGET ATRP involves the in situ generation of the Cu I catalyst from the corresponding oxidatively stable Cu II halide by the action of a reducing agent. Subsequently, the Cu I complex activates the initiator by the normal ATRP equilibrium. [8] The use of an excess of reducing agent serves to insure that the Cu II species accumulating when the radicals irreversibly terminate is continuously reduced to regenerate the Cu I activator species. The process is summarized in Scheme 4 for the action of the 1/BEB combination. A well controlled polymerization should reveal a first order consumption of monomer (constant radical concentration insured by the persistent radical effect), [47] a linear growth of the molecular weight with the conversion in close agreement with the value calculated from the molar monomer/initiator ratio, and a narrow dispersity (Đ = Mw/Mn). Scheme 4. Mechanism of action of compound 1 in ARGET ATRP.

The results of ARGET ATRP of styrene using 1 are presented in Figure 5. The first-order kinetics plot (Figure 5, top) shows better linearity when the polymerization is carried out at 80°C. As expected, the polymerization is faster at 110°C initially, but subsequently slows down at higher conversions, eventually becoming even slower than the polymerization conducted at 80°C. The polymer number average molecular weight increases linearly with conversion at both temperatures, as noticeable in Figure 5 (bottom) and the polymer dispersities are reasonably low (smaller than 1.40 at > 60% conversion). However, the observed Mn are greater than the theoretical values (for instance, at 80°C and 63% conversion Mn,exp = 96640 g/mol vs. Mn,theo = 32740 g/mol, f = 0.34; at 110°C and 67% conversion Mn,exp = 80960 g/mol vs. Mn,theo = 35085 g/mol, f = 0.43). This observation shows that the polymerization is negatively affected by a high tendency to bimolecular terminations, reducing the number of active chains to a greater extent as the polymerization progresses. This must result from an insufficient rate of radical trapping (kd in Scheme 4). Note that the catalyst disproportionation process would only lead to a decrease of the catalyst concentration and this may potentially affect the polymerization rate but not the initiator efficiency factor (f). The greater Mn (low initiator efficiency) must necessarily be related to a decrease of active chains through bimolecular terminations. It seems that this phenomenon is more of less equally important at the two temperatures. In fact, f is greater at the higher temperature, suggesting that trapping becomes more efficient (greater kd) as the temperature increases. The trend observed for the polymerization rate, on the other hand, can be rationalized by a faster catalyst loss through the disproportionation process at the higher temperature. The amount of catalyst and the degree of disproportionation were apparently too small to visually confirm the formation of metallic copper. We have recently reported a slow trapping rate also for the reverse ATRP of styrene with related Cu II complexes containing the tridentate β-ketoiminato ligands R2NCH2CH2NC(Me)-CHC(Me)O (R = Me, Et), which adopt a distorted square planar geometry similar to that of compound 1, even when run in the presence of additional ligands such as pyridine or bipy. [14] In that case, we argued that the addition of a second amino arm could improve the catalyst performance by providing a smaller reorganizational energy upon halogen atom transfer from CuXL to CuL. However, as shown by the structural analysis of compound 1 in this contribution, this compound is not able to accept the coordination of the additional amino arm and therefore still requires a significant reorganization during the atom transfer process, thereby maintaining a high activation energy. Furthermore, the geometry of the tetracoordinated L ligand on the Cu I complex does not appear suitable for the stabilization of the preferred tetrahedral geometry of Cu I , whereas it fits quite well with the distorted square planar geometry preferred by [Cu II L] + . This factor is presumably responsible for the insufficient stabilization of the CuL complex, leading to disproportionation.

Conclusions

Cu II complexes containing the tetradentate mononegative diamino-β-diketiminato ligand Et2NCH2CH2NC(Me)CHC(Me)-NCH2CH2NEt2 (L -), namely CuClL and [CuL] + , have been synthesized and fully characterized. An analogous Cu I complex CuL could be generated in solution but could not be isolated because of its susceptibility to disproportionation. Contrary to our hope, this CuL/CuClL system is not suitable for an efficient control of atom transfer radical polymerization and we have identified two reasons for this shortcoming. The first one is the tendency of the Cu I species to disproportionate. The second one is a slow trapping rate for the CuXL (X = Cl, Br) species as shown by the results of the styrene polymerization run under ARGET ATRP conditions. Both these reasons are probably related to the unsuitable coordination geometry of the L ligand. Improvement of this "halogen-free" CuN4 coordination environment for application to ATRP requires further ligand design, in order to create a coordination environment better suited to Cu I and work in this directly is currently underway.

Experimental Section

General. All solvents were freshly distilled under argon prior to use. NMR spectra were recorded with Bruker DPX 300 and AV 300 instruments. Chemical shifts are expressed in ppm downfield from Me4Si. EPR spectra were measured with a Bruker Elexsys E 500 spectrometer (X-band) equipped with both a frequency meter and a gauss meter. The elemental analyses were performed on a Perkin Elmer 2400 instrument by the analytical services of the LCC Toulouse. Size exclusion chromatography (SEC) of polystyrene was carried out in filtered THF (flow rate: 1 ml/min) at 35°C on a 300x7.5 mm PL gel 5micrometer mixed-D column (polymer laboratories), equipped with multiangle light scattering (minidawn Tristar, Wyatt Technology Corporation) and refractive index (RI2000, Sopares) detectors. The isolated polymer samples were dissolved in THF and the polymer solutions were filtered (pore size = 0.45 μm) before chromatographic analysis. The columns were calibrated against linear polystyrene standards (Polymer Laboratories). Acetylacetone, N,Ndiethylethylenediamine, 1-bromoethyl(benzene), glucose, MeLi (1.6 M solution in Et2O), CuCl and CuCl2 were purchased from Sigma-Aldrich and used as received. [Et3O][BF4] was prepared as previously described. [48] Styrene (Sigma-Aldrich) was dehydrated by passing through a neutral alumina column and then stirred in the presence of CaH2 for 1 day, followed by distillation under argon. Synthesis of CuLCl, 1. To a cooled solution (-78°C) of HL (697 mg, 2.35 mmol) in dried Et2O (15 ml), MeLi (1.46 mL, 1 equiv) was added dropwise.

Stirring at -78°C for 2 h and then at r.t. overnight afforded a yellow solution of the lithium salt LiL. The solution was evaporated to dryness under vacuum and the residue was dissolved in toluene (15 mL). In a separated Schlenk flask, CuCl2 (316 mg, 1 equiv) was suspended in toluene (2 ml) and the lithium salt solution was added dropwise through a cannula to afford a purple solution. After 2 h stirring at room temperature, the solvent was removed under vacuum. The residual solid was recrystallized by dissolution in hot pentane, filtration through Celite and standing overnight at -80 °C to afford 1 as purple crystals. Yield: 370 mg (40%). Anal. % Calcd for C17H35CuClN4: C, 51.76; H, 8.94; N, 14.20. Found: C, 51.7; H, 8.9; N, 14.0. MS (FAB, MNBA): m/z 358 [M-Cl] + . EPR (CH2Cl2, X band): room temperature: g = 2.100 (aCu = 75 G); 115K: gx = 2.07 (Ax Cu = 10 G), gy = 2.04 (Ay 2N = 18 G), gz = 2.006 (Ax Cu = 179 G) (the reported g and A values fort he 115K spectrum derive from the simulation, see SI). Attempted syntheses of complex CuCl. (a) From CuCl, HL and NEt3. To a stirred solution of CuCl (0.5 g, 5.05 mmol) in 5 mL of dry THF, a solution of HL (1.5 g, 5.05 mmol in 5 mL of dry THF) was added and the resulting solution was stirred for 30 minutes. The solution became dark yellow. Then, 0.7 mL (5.05 mmol) of dry Et3N was added to the solution and stirring was continued for another hour, during which time the color turned to red. The white Et3NH + Cl -precipitate that had formed was removed by filtration. The solvent was then partially removed under reduced pressure to about 5 mL and slow diffusion of dry ether at -20°C afforded a few brownish crystals of [CuL][CuCl2], 2. (b) From [Cu(CH3CN)4]BF4 and LiL. To a cooled solution (-78°C) of HL (410 mg, 1.38 mmol) in dried Et2O (6 mL), MeLi (0.86 mL, 1 equiv) was added dropwise. Stirring at -78°C for 2 h and then at r.t. overnight afforded a yellow solution of the lithium salt LiL. The solution was evaporated to dryness under vacuum and the residue was dissolved in toluene (6 mL). In a separated Schlenk flask, [Cu(CH3CN)4]BF4 (434 mg, 1 equiv) was suspended in toluene (2 mL) and the lithium salt solution was added dropwise through a cannula to afford a dark-green solution that turned night blue after 15 min stirring. After 3 h stirring at room temperature, the solution was filtered and placed overnight at -20°C to afford a few red crystals of [ Procedure for ARGET-ATRP. Complex 1 (17.3 mg, 0.044 mmol), glucose (15.9 mg, 2 equiv), BEB (6 L, 1 equiv), styrene (2.5 mL, 500 equiv) and anisole (2.5 mL) were introduced in a Schlenk flask under a protective argon atmosphere. The flask was degassed by three freeze-pumpthaw cycles and then immersed in an oil bath preheated at the desired temperature. At the desired time, the Schlenk flask was rapidly immersed in an ice bath before sample withdrawal. The monomer conversion was determined gravimetrically after removal of the solvent and unconverted monomer under vacuum. X-ray Crystallography. A single crystal of each compound was mounted under inert perfluoropolyether on the tip of a glass fiber and cooled in the cryostream of either an Oxford-Diffraction XCALIBUR CCD diffractometer (for 1 and 3) or an Oxford-Diffraction GEMINI CCD diffractometer (for 2). The data were collected by using monochromatic Mo-Kα radiation (λ = 0.71073 Å) at 180(2) K. The structures were solved by direct methods (SIR97) [49] and refined by least-squares procedures on F2 using SHELXL-97. [50] All H atoms attached to carbon atoms were introduced in calculated idealized positions and treated as riding models. The drawing of the molecules was realized with the help of ORTEP32. [51][52] The crystal data and structure refinement parameters for the three compounds are listed in Table 2. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 852833 -852835. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Scheme 1 .

 1 Scheme 1. Equilibrium controlling the radical polymerization by the atom transfer mechanism.

Figure 1 .

 1 Figure 1. ORTEP view of compound 1. Thermal ellipsoids are drawn at the 50% probability level. Main geometrical parameters: Cu1-Cl1 2.2641(10), Cu1-N3 2.067(3), Cu1-N1 1.914(3), Cu1-N2 1.938 (3); Cl1-Cu1-N1 154.94(10) Å; Cl1-Cu1-N3 93.20(9), Cl1-Cu1-N2 97.46(10), N3-Cu1-N2 161.87(13), N3-Cu1-N1 83.88(12), N1-Cu1-N2 92.40(14)°.

Figure 2 .

 2 Figure 2. X-band EPR spectra of compound 1 in toluene. (a) Room temperature. (b) Frozen solution at 115K.

Figure 3 .

 3 Figure 3. ORTEP view of one of the two independent cation-anion pairs in the structure of complex 2. Thermal ellipsoids are drawn at 50% probability level.

Figure 4 .

 4 Figure 4. ORTEP view of one of the two independent cation-anion pairs in the structure of complex 3. Only one orientation of the disordered BF4 -ion is shown. Thermal ellipsoids are drawn at 50% probability level.

Scheme 3 .

 3 Scheme 3. Synthesis of [CuL][CuCl2] (2).

Figure 5 .

 5 Figure 5. First-order kinetic plot (top) and Mn (filled symbols) and dispersity index (Đ; empty symbols) versus conversion (bottom) for the radical polymerization of styrene in the presence of CuClL. T = 110°C (triangles) or 80°C (squares). [Styrene]:[BEB]:[Glucose]:[Cu] = 500:1:2:1. The straight line in the second plot indicates the theoretical molecular weight with Mn,theo = ([M]0/[BEB]0)*conversion.

Table 1 .

 1 Selected metric parameters (distances in Å, angles in °) for the [CuL]+ cations in the structures of compounds 2 and 3.

		Complex 2	Complex 3
		n = 1	n = 2	n = 1	n = 2
	Cu(n)-N(n1)	1.927(4)	1.930(4)	1.9252(15) 1.9053(14)
	Cu(n)-N(n2)	1.934(5)	1.923(4)	1.9260(14)
	Cu(n)-N(n3)	2.104(4)	2.109(4)	2.1140(15)
	Cu(n)-N(n4)	2.124(4)	2.130(4)	2.0936(14) 2.1040(14)
	N(n1)-Cu(n)-N(n2) 92.85(18) 92.58(18) 93.04(6)	93.80(8)
	N(n1)-Cu(n)-N(n3) 161.28(17) 162.88(17) 161.37(6) 156.11(6)
	N(n1)-Cu(n)-N(n4) 83.89(17) 83.92(17) 84.57

Table 2 .

 2 Crystal data and refinement parameters for all compounds.Supporting Information (see footnote on the first page of this article): IR spectrum of compound 1 before and after exposure to moist air.

	Compound	CuClL (1)	[CuL][ClCl2] (2) [CuL][BF4] (3)
	Empirical formula C17H35ClCuN4 C34H70Cl4Cu4N8 C17H35CuN4
	Mr	394.48	986.94	445.84
	Crystal system	Orthorhombic Monoclinic	Monoclinic
	Space group	Pbca	P21/c	C 2/c
	a [Å]	17.1959(13)	18.7208(5)	29.3723(15)
	b [Å]	8.0359(6)	17.3653(4)	13.6327(4)
	c [Å]	29.479(2)	13.0819(3)	16.8855(7)
	β[°]	90	90.978(2)	111.723(5)
	V [Å 3 ]	4073.5(5)	4252.21(18)	6281.2(5)
	Z	8	4	12
	Dcalcd. [Mgm -3 ]	1.286	1.542	1.414
	Abs. coeff. [mm -1 ] 1.209	2.260	1.087
	F(000)	1688	2056	2820
	Crystal size [mm] 0.50x0.21x0.02 0.62x0.50x0.40 0.58x0.41x0.24
	θ range [°]	3.01 to 26.37	2.94 to 26.37	2.88 to 28.41
	Refl collctd	22479	53358	23865
	Unique refl [R(int)] 4152 (0.0876) 8685 (0.0479)	6416 (0.0262)
	Completeness [%] 99.8	99.9	99.9
	Abs corr	Semi-empirical Semi-empirical Semi-empirical
	Max./min. transm 1.0000/0.7144 1.0000/0.5296	1.0000/0.6865
	Refinm method	F 2	F 2	F 2
	Data/restr/param	4152/10/252	8685/0/463	6416/76/412
	Goodnss-of-fit (F 2 ) 1.107	1.349	1.055
	R, wR2[I>2σ(I)]	0.0593, 0.1037 0.0562, 0.1402	0.0276/0.0734
	R, wR2 (all data)	0.0888, 0.1115 0.0685, 0.1451	0.0337/0.0734
	Resid dens [eÅ -3 ] 0.455, -0.437 1.041, -1.094	0.582,-0.422

Acknowledgments

We are grateful to the ANR (project OMRP, grant ANR 2010 BLANC 7101), to the Centre National de la Recherche Scientifique (CNRS) and to the Institut Universitaire de France (IUF) for support of this work. ____________

Entry for the Table of Contents


Jana Pisk

Biserka Prugovecki

Dubravka Matkovic-Calogovic

Rinaldo Poli

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr.[

Visnja Vrdoljak
email: visnja@chem.pmf.hr.[

Biserka Prugovečki

Dubravka Matković-Čalogović

Višnja Vrdoljak

Charged Dioxomolybdenum(VI) Complexes with Pyridoxal Thiosemicarbazone Ligands as Molybdenum(V) Precursors in Oxygen Atom Transfer Process and Epoxidation (pre)Catalysts

Keywords: Molybdenum, Thiosemicarbazones, Pyridoxal, Epoxidation, Solvent-free catalysis, OAT

published or not. The documents may come

Introduction

Oxygen atom transfer (OAT) processes, as well as oxidation processes, are important in Nature and in the chemical industry [1]. Molybdenum enzymes catalyze a number of such reactions. These oxotransferases contain oxo-(or dioxo)-molybdenum units additionally coordinated by sulfur atoms [2].

We are currently interested in dioxomolybdenum(VI) complexes with ligands that mimic the coordination environment of the metal ion in enzymes. Among them, tridentate ligands containing ONS donor atoms are attracting our interest.

Transition metal complexes with thiosemicarbazone ligands are of particular importance because of their interesting biological properties, e.g. antitumoral, antifungal or antiviral properties [3]. Ligands derived from pyridoxal have attracted considerable attention due to different coordination modes to the central metal atom [4][5][6][7][8]. In these complexes, the pyridoxal thiosemicarbazone ligand can be in neutral (H2L), singly-deprotonated (HL) -, or doubly-deprotonated (L) 2-form [4]. The protonation state of these ligands in metal complexes plays an important role since it offers fine-tuning of properties such as electrochemical, photophysical or catalytic activity [9]. Although numerous transition metal complexes (Cu, Fe, Ni, Mn, Co) [8a,10] with pyridoxal thiosemicarbazone derivatives are known, related molybdenum complexes are scarce [11]. Mo VI complexes are also known to be very active as epoxidation catalysts, generally in solution of an organic solvent, especially for neutral complexes [12].

We have recently demonstrated that these epoxidation reactions show very good results under solventfree conditions [13].

Scheme 1. Pyridoxal based ligands.

In this contribution, we present the syntheses and characterization of dioxomolybdenum(VI) complexes with different pyridoxal thiosemicarbazone charged ligands (Scheme 1), as well as their activity as OAT agents and as epoxidation catalysts under solvent-free conditions. The influence of protonation on the spectroscopic and structural properties of the metal complexes was studied by UV-Vis and NMR spectroscopies, as well as by a single crystal X-ray diffraction.

Experimental Section

Measurements.

Thermogravimetric (TG) analyses were performed using a Mettler TG 50 thermobalance with aluminum crucibles. All experiments were recorded with a heating rate of 5 °C min -1 in a dynamic atmosphere with a flow rate of 200 cm 3 min -1 . Differential scanning calorimetry (DSC) measurements were carried out with a Mettler-Toledo DSC823e calorimeter and analyzed by the Mettler STARe 9.01. software.

Infrared spectra were recorded in KBr pellets with a Perkin-Elmer 502 spectrophotometer in the 4500-450 cm -1 region. Electronic absorption spectra were recorded at 25 °C on a Cary 100 UV/Vis spectrophotometer. Magnetic measurements were made by the standard Gouy method. NMR spectra were obtained using a Bruker Avance DRX500 spectrometer. The spectra were recorded in DMSO-d 6 with TMS as internal standard. 1 H and 13 C resonances were assigned by the combination of 1D ( 1 H and APT sequences) and 2D homo-and heteronuclear experiments (COSY, HSQC and HMBC sequences).

Elemental analyses (C, H, N, S) were provided by the Analytical Services Laboratory of the Ruđer Bošković Institute, Zagreb. Chlorine was determined by the gravimetric method as silver chloride [START_REF] Vogel | Textbook of Quantitative Inorganic Analysis[END_REF].

Starting materials

The starting complex [MoO2(acac)2] was prepared as described in the literature [START_REF] Chen | [END_REF]. Acetonitrile was dried over P2O5 and then distilled. Methanol was dried using magnesium turnings and iodine, and then distilled. Pyridoxal hydrochloride, thiosemicarbazides (H2NNHC(S)NHR, R = C6H5, CH3 or H) and triphenylphosphine (PPh3) were commercially available. Ligands H2L 1-3 •HCl were prepared according to procedures described in the literature [16].

Synthesis of [MoO2(HL 1 )(CH3OH)]Cl•1.5CH3OH (1a)

A mixture of [MoO2(acac)2] (0.16 g, 0.5 mmol) and pyridoxal 4-phenylthiosemicarbazone hydrochloride H2L 1 •HCl (0.14 g, 0.4 mmol) in dry acetonitrile (10 mL) and dry methanol (10 mL) was refluxed for 5 hours. The solution was left at room temperature for one day and the obtained orange crystalline precipitate was filtered, rinsed with acetonitrile and dried in a desiccator down to constant weight. Yield: 0.14 g (55%). Anal. Calc. for C17.5H25ClMoN4O6.5S (Mr = 558.868): C,37.61;H,4.50;Cl,6.34;N,10.03;S,5.74%. Found: C,37.73;H,3.95;Cl,6.55;N,10.56;S,6.05%. TG: CH3OH,13.9% (Calcd. 14.3%);MoO3,26.5% (Calcd. 26.5%). Selected IR data (cm -1 ): 1602, 1553 (C=N), 1367 (C-O), 939 (MoO2)asym, 904 (MoO2)sym, 639 (C-S).

Synthesis of [{MoO2(HL)}Cl]n complexes

The appropriate pyridoxal thiosemicarbazone ligand H2L 1-3. HCl (0.4 mmol) was dissolved in dry acetonitrile (20 mL),[MoO2(acac)2] (0.16 g, 0.5 mmol) was added and the resulting solution was refluxed for the specified time. The precipitate that formed was filtered, washed with acetonitrile and dried in a desiccator down to constant weight. R = Ph: Complex [{MoO2(HL 1 )}Cl]n (1): Dark red product. Reaction time: 5 hours. Yield: 0.22 g (94%). Anal. Calc. for C15H15ClMoN4O4S (Mr = 478.76): C,37.63;H,3.16;Cl,7.41;N,11.70;S,6.70%. Found: C,37.41;H,3.11;Cl,7.14;N,11.80;S,6.32%. TG: MoO3,29.8% (Calcd. 30.1%).

Selected IR data (cm -1 ): 1604, 1557 (C=N), 1362 (C-O), 954 (MoO2)asym, 935 (MoO2)sym, 641 (C-S).

UV-VIS(methanol): /nm (/dm 3 mol -1 cm -1 ): 354 (14888) and 434 (8368).

R = Me:

Complex [{MoO2(HL 2 )}Cl]n (2): Brown product. Reaction time: 12 hours. Yield: 0.087 g (43%). Anal. Calc. for C10H13ClMoN4O4S (Mr = 416.69): C, 28.82;H,3.14;Cl,8.51;N,13.45;S,7.70%. Found: C,28.66;H,3.25;Cl,8.21;N,13.50;S,7.65%. TG: MoO3,34.3 (Calcd. 34.5%).

Selected IR data (cm -1 ): 1591, 1540 (C=N), 1358 (C-O), 930 (Mo=O), 801 (Mo=OMo), 621 (C-S).

UV-VIS(methanol): /nm (/dm 3 mol -1 cm -1 ): 325 (17992) and 405 (2336). Complex [{MoO2(HL 3 )}Cl]n (3): Dark brown product. Reaction time: 12 hours. Yield: 0.08 g (46 %). Anal. Calc. for C9H11ClMoN4O4S (Mr = 402.66): C,26.85;H,2.75;Cl,8.80;N,13.91;S,7.96%. Found: C,26.53;H,2.92;8.54;N,13.65;S,7.18%. TG: MoO3,35.5% (Calcd. 35.8%). Selected IR data (cm -1 ): 1617, 1559 (C=N), 1357 (C-O), 930 (Mo=O), 815 (Mo=OMo), 621 (C-S). UV-VIS(methanol):

R = H:

/nm (/dm 3 mol -1 cm -1 ): 342 (177500) and 427 (1927).

X-Ray Crystallography.

The single-crystal X-ray diffraction data of 1a was collected by -scans on an Oxford Diffraction Xcalibur 3 CCD diffractometer with graphite-monochromated Mo-K radiation ( = 0.71073 Å). The data reduction was performed using the CrysAlis software package [START_REF]Xcalibur CCD system, CRYSALIS Software system[END_REF]. Solution, refinement and analysis of the structure were done using the programs integrated in the WinGX system [START_REF] Farrugia | [END_REF]. The structure was solved using SHELXS [19] by the Patterson method. The refinement procedure was performed by the full-matrix least-squares method based on F 2 against all reflections using SHELXL-97 [START_REF] Sheldrick | SHELXL-97, Program for the Refinement of Crystal Structures[END_REF]. One methanol molecule was found to be disordered over two sites and over a center of symmetry. The geometry of the two disordered methanol molecules were restrained to be as that of the methanol that is not disordered. The occupancy of the two sites refined to 0.263(8) and 0.237(8), while they had a common temperature factor. The non-hydrogen atoms were refined anisotropically, except for those of the disordered methanol molecule which were refined isotropically. All hydrogen atoms, except those on the disordered methanol molecule, were located in the difference Fourier maps.

Because of poor geometry for some of them they were placed in calculated positions and refined using the riding model. Only the hydrogen atoms of one free and the bound methanol hydroxyl group were fixed. Geometrical calculations were done using PLATON [21]. The structure drawings were prepared using PLATON and MERCURY [START_REF] Macrae | [END_REF] programs. The crystallographic data are summarized in Table 1, whereas the selected bond distances and angles are listed in Table 2.

R = Fo -Fc/Fo, b wR = (Fo 2 -Fc 2 ) 2 /w(Fo 2 ) 2  1/2

Reaction of complexes 1-3 with PPh3

All reactions were performed under a dry argon atmosphere. PPh3 (0.02 g, 0.08 mmol) and -picoline (8 L) were dissolved in dry acetonitrile (15 mL) and then 1, 2 or 3 (0.08 mmol) was added. The suspension was heated for 10 h. The dark red-brown microcrystalline product was collected by filtration, washed with acetonitrile and dried. : Complex [Mo2O3(HL 1 )2]Cl2 (4): Yield: 0.020 g (52%) (obtained from 1). Anal. Calcd. for C30H30Cl2Mo2N8O9S2 (Mr = 941.52): C, 38.27;H,3.21;Cl,7.53;N,11.90;S,6.81. Found: C,38.11;H,3.42;Cl,7.21;N,11.85;S,6.55. TG: 30.8% MoO3 (Calcd. 30.6%). Selected IR data (cm -1 ): 1601, 1552

R = Ph

(C=N), 1363 (C-O), 948 (Mo=O), 788 (Mo-O-Mo), 625 (C-S). R = Me: Complex [Mo2O3(HL 2 )2]Cl2 (

General procedure for the epoxidation of cyclooctene by aqueous TBHP

A mixture of cyclooctene (2.76 mL, 20 mmol), acetophenone (0.1 mL, internal reference) and Mo (pre)catalyst (0.01 mmol Mo) was stirred and heated up to 80 °C before adding aqueous TBHP (70 wt %, 5.48 mL, 40 mmol). The reaction was followed for 6 h. Aliquots (0.1 mL) of the organic phase were taken at required times from the reaction media, mixed with 2 mL of Et2O and a small quantity of MnO2 was added. The mixture was then filtered through silica and analyzed by 1 H NMR in CDCl3.

Computational Details

All geometries were optimized without any symmetry constraint with the Gaussian 03 program suite [START_REF] Frisch | Gaussian 03, Revision D.01[END_REF]. The input geometries were adapted from the X-ray structures of [MoO2(L 1 )(MeOH)]•CH3OH (1a*) [11] and [MoO2(HL 1 )(MeOH)]Cl•1.5CH3OH (1a). The calculations used the standard B3LYP three-parameter functional [24] with the 6-31G** basis set [25] for C, H, N and O atoms and the CEP-31G* basis set [26] for molybdenum. All optimized geometries were confirmed to be local minima by frequency analyses (Table S1 in the Supporting Information). The calculated IR spectra were generated from the DFT-generated frequencies and intensities by applying Lorentzian functions and adjusting the linewidth to the best fit with experimental spectra.

Results and Discussion

New mononuclear [MoO2(HL 1 )(CH3OH)]Cl•1.5CH3OH (1a), and polynuclear molybdenum(VI) complexes [{MoO2(HL 1-3 )}Cl]n (1-3) were prepared by the reaction of [MoO2(acac)2] with different pyridoxal thiosemicarbazone hydrochlorides (Scheme 2). Dry solvents (methanol and acetonitrile) were used to prevent ligand hydrolysis.

Scheme 2. Synthesis of Mo VI complexes (1a and 1-3) and Mo V complexes (4-6).

Complexes 1a and 1 were identified by the presence of (MoO2) bands at 939 and 904 cm -1 for 1a, and 954 and 935 cm -1 for 1 [27]. A band around 1071 cm -1 for both complexes was assigned to the (C-O)

of the coordinated methanol molecule in 1a or the hydroxymethyl group in 1 (Scheme 2) [28]. On the other hand, the sixth coordination site in [{MoO2(HL 2,3 )}Cl]n (2 and 3) is occupied by the oxygen atom from the Mo=O unit from the neighbouring complex molecule, thus forming a coordination polymer. This is suggested by the intense IR bands observed at 801 cm -1 (2) and 815 cm -1 (3), as well as by a single strong absorption band at 930 cm -1 assigned to the Mo=O•••Mo bridging and terminal Mo=O groups, respectively [29]. The broad absorption bands in the 3000-2600 cm -1 range, appearing in the spectra of all complexes, confirm the presence of the pyridinium moiety. Two vibration bands in the 1610-1540 cm -1 range were assigned to the stretching vibrations of the C=N groups of the thiosemicarbazones [30], whereas those at about 625 cm -1 were attributed to the coordinated C-Sthiol group [31].

Polynuclear complexes 1-3 are not solvated. Upon heating the [{MoO2(HL S1) [32]. This was also confirmed by comparison of the IR spectrum of the non solvated product with those observed for 1a and 1 (Figure S2 in the Supporting Information). Also, transformation of 1a into a μ-oxo bridged dimer is not possible in the solid-state, since the Mo=Ot group is not directed toward the vacant coordination site of the neighbouring [MoO2(HL)] + cation. This is additionally supported by absence of a strong band at ~800 cm -1 , which is characteristic for a molybdenumoxygen interaction.

On the other hand, transformation of the mononuclear complex 1a into the coordination polymer 1 can be achieved by dissolution of 1a in acetonitrile. Similarly, the reverse transformation can be obtained after exposure of the samples of 1 to methanol. Depending on the solvent nature, the methanol molecule in [MoO2(HL 1 )(CH3OH)] + cation can be easily replaced by the hydroxymethyl group of a neighbouring molecule and vice versa as already observed for the related neutral complexes [33].

NMR Spectroscopy

The NMR spectra of compounds 1-3 were recorded in DMSO-d 6 because of their very low solubility in non-coordinating solvents (the numbering scheme is given in Scheme 3 and all data are collected in Table S2, in Supporting Information). The solubilisation of the coordination polymers 1-3 is probably related to the coordination of DMSO to the Mo VI center, breaking the infinite chain structure and yielding mononuclear [MoO2(HL 1-3 )(DMSO)] + complexes. The spectra of 1a and 1 are very similar, the only difference being the presence of the MeOH molecule signal for the solution of 1a.

Scheme 3. NMR numbering scheme of H2L 1-3 •HCl. The 1 H and 13 C chemical shift values of [{MoO2(HL 1-3 )}Cl]n (1-3) may be compared to those previously reported for the related complexes containing the doubly deprotonated ligands [MoO2(L 1-3 )]n (Table S2 in SI, Figure S3). The spectral variations can be correlated with the protonation of the pyridoxal ligand.

The signal at ca. 12 ppm (assigned to H1) found in the spectra of compounds 1-3 is absent in the spectra of [MoO2(L 1-3 )]n. This confirms protonation of the pyridine ring N atom and the presence of the ligand as (HL) -. Such protonation induces a shift of all pyridine ring signals, particularly those of H6 (up to +0.13 ppm) and C6 atoms (up to -6.15 ppm). Additionally, the broad signal assigned to the hydroxymethyl H10 proton is also fairly shifted downfield by approximately 0.25 ppm. There is also a significant downfield shift of the H14 signal (up to 0.41 ppm) in complexes 1-3 with respect to the corresponding deprotonated complexes, which is probably the result of hydrogen bonding between the thiosemicarbazone NH group and chloride [34]. This suggests that the chloride counterion maintains this interaction in solution, as also found in the solid state (see next section).

X-Ray Crystallography

In the molecular structure of 1a the coordination sphere around the Mo atom is a distorted octahedron with a tridentate ligand (HL 1 ) -bound to the cis-{MoO2} 2+ core. The ligand is coordinated to molybdenum as ONS-donor through the phenolic-oxygen, the imine-nitrogen and the thiol-sulphur atoms. The sixth coordination site is occupied by the methanol molecule. The C1-S bond of 1.737(4) Å indicates the thiol form. Two chelate rings are formed by complexation of the ligand to Mo, a five-(Mo, S, C1, N2, N3) and a six-membered (Mo, N3, C2, C3, C7, O1) chelate ring that form a dihedral angle of 8.70( 13)°. Therefore, the ligand is relatively planar with π-electron delocalization throughout the ligand.

The substituted pyridine and phenyl rings form an angle of only 3.9(2)°. An ORTEP drawing of the complex cation and the chloride anion is shown in Figure 1. Relevant bond lengths and angles are given in Table 2. The crystal structure of 1a consists of [MoO2(HL 1 )(CH3OH)] + cations, chloride anions and molecules of methanol of which one is in a general position while the other one is disordered over a center of symmetry. Both protonated nitrogen atoms (N1 and N4) and the hydroxyl group from the pyridoxal moiety (O2) are involved in hydrogen bonding with the chloride ion, thus connecting three neighbouring complex cations and forming double-chains along the c-axis. The chains are linked through hydrogen bonding with one of the free methanol molecules along the a-axis. The solvent methanol molecule that is not disordered is a hydrogen bond acceptor from the bound methanol and a donor to the pyridoxal hydroxyl group. Hydrogen bonds are given in Table 3. Interactions between chains along the b-axis involve the disordered methanol molecule. The packing diagram is shown in Figure 2 (and Figure S4 in the Supporting Information).

Table 3. Geometry of the hydrogen bonds (Å, ) for compound 1a. In contrast to 1a where N4 is a hydrogen bond donor, the same atom in the non protonated complex, [MoO2(L1)(CH3OH)]•CH3OH (1a*), is a hydrogen bond acceptor [11]. Significant changes in the geometry of 1a in comparison to 1a* include angles of the Mo coordination polyhedron and the C5-N4-C6 angle in the pyridoxal ring, which increases upon protonation from 119.6(2)° to 124.6(3)°, Figure 3.

D-H

… A D-H (Å) H … A (Å) D … A (Å) D-H … A() N1-
2-x,1-y,1-z; b 2-x,1-y,2-z, c 1-x,1-y,1-z
The largest difference in the coordination polyhedron concerns the S-Mo-O5 angle, most probably caused by involvement in different hydrogen bonding pattern and also different packing in the crystal.

DFT calculations

DFT geometry optimizations were carried out to assign the IR absorption bands in 1a and 1a*, in order to assess the effect of ligand protonation. Before examining these results, we present a brief description of the optimized geometries. Relevant results are collected and compared to the experimental crystallographic data in Table 4. The agreement between experimental and calculated geometries for the core is generally very good and small differences may be at least in part attributed to crystal packing, which is not accounted for in the single molecule calculation. All calculated distances are overestimated, very little for the terminal oxido ligands (maximum deviation 0.01 Å), to an intermediate degree for Mo-O(1), Mo-S and Mo-N (0.03 Å to 0.06 Å), and to a greater extent for the coordinated methanol O atom (by 0.14 Å on average). The elongation/shortening of bond lengths experimentally observed between neutral and charged complexes (1a* and 1a) is reproduced by the DFT calculations, with similar differences. Only the most relevant data is listed in Table 4.

The experimental IR spectra of 1a and 1a* are compared with the calculated ones for A and A* in Figure 4. Although shifted by around 60 cm -1 on average, the patterns are similar. Three vibrations at 981 (asym), 1006 (sym), 1020 (sym) cm -1 for A can be attributed to MoO2 (the symmetric vibration is coupled with other vibrations and contributes significantly to two normal modes) and two vibrations at 968 (asym) and 1001 (sym) cm -1 for A*. The calculations are also useful for the assignment of other vibrational modes, such as the C-S vibrations (685 cm -1 for A and 679 cm -1 for A*) the N-N vibrations (1190 cm -1 for A and 1152 cm -1 for A*), the CH=N vibrations (coupled with other vibrations in bands at 1617, 1656 and 1658 cm -1 for A and at 1560, 1624 and 1653 cm -1 for A*) and finally the pyridoxal N-H vibration in case of A (3569 cm -1 ). The shift relatively to the experimental values is observed in all cases, as typically observed for calculations carried out at this level of theory.

UV-Vis Spectroscopy

The electronic spectra of the thiosemicarbazone complexes 1-3 recorded in methanol exhibited S(pπ)-Mo(dπ) LMCT in the regions 405 -434 nm as well as N(pπ)-Mo(dπ) and O(pπ)-Mo(dπ) LMCT bands at 325-354 nm [35]. These spectral characteristics are influenced by the nature of the ligand and by its degree of protonation. Titrations of 1-3 with HCl or Et3N were performed by adding aliquots of HCl or Et3N solution (2 μL) into cuvettes in methanol with UV-Vis monitoring and compared with those of the corresponding H2L 1-3 •HCl ligands. A titration experiment was also carried out by adding HCl to 1a*.

Spectral changes associated to system 1 and the ligand H2L 1 •HCl are presented in Figure 5.

The addition of HCl to the solution obtained from 1 resulted in a shift of the 434 and 354 nm bands to higher wavelengths (461 and 363 nm, respectively) with a change of molar absorptivity. These changes are presumably due to the protonation of the ligand and formation of the [MoO2(H2L 1 )(CH3OH)] 2+ cation. Therefore, the corresponding bands of 1a*, containing the ligand in the most deprotonated form, are at lower wavelengths than for 1, exhibiting two bands at 416 and 349 nm.

The addition of Et3N to the methanolic solution of 1 resulted in the bands at 390 and 312 nm. The spectrum of the obtained solution is different from the spectrum of the [MoO2(L 1 )(CH3OH)] complex.

However, it is similar to that of the deprotonated ligand (L 1 ) 2-obtained upon addition of Et3N to the solution of H2L 1 •HCl. Apparently, addition of Et3N to the solution destabilizes the association between the ligand and the molybdenum atom, probably yielding the free doubly deprotonated ligand and the molybdate ion, MoO4 2-. The same result was obtained also using KOH in methanol instead of Et3N.

The corresponding behaviour of compounds 2 and 3 is identical. In the case of 2, addition of HCl caused a shift of the 405 and 325 nm bands to 442 and 360 nm, respectively. Similarly, the spectrum of 3 has absorption maxima at 427 and 342 nm, whereas acidification with HCl shifted the bands to 438 and 361 nm, respectively. The IR spectra of 4-6 exhibit bands around 950 cm -1 and 780 cm -1 , assigned to terminal Mo=O and Mo-O-Mo bridging groups, respectively (Figure S5 Supporting Information) [36]. All compounds were obtained pure with a 40% yield when the reaction proceeded for 10 hours. The diamagnetism, which is characteristic for oxo bridged Mo(V) dimers [37], was confirmed by magnetochemical measurements.

Our results are consistent with findings reported by Young et al. [38]. As it has been previously shown, formation of {Mo V 2O3} 4+ species from {Mo VI O2} 2+ seems to be the result of OAT from [Mo VI O2(HL 1-

3 )]Cl (1-3) to PPh3 [38,39]. Weight losses corresponding to the complex decomposition were observed by TGA (under a pure oxygen atmosphere) in the 246-446 °C (4), 239-431 °C (5) and 223-448 °C (6) ranges, leading to MoO3.

The reactivity of complexes 4-6 towards oxygen donors is revealed by dissolution in DMSO, which leads to an immediate colour change from dark red to pale orange, as a consequence of the reoxidation to 1-3 [40]. For this reason, NMR spectra of these compounds could not be recorded. Unfortunately, all other attempts to characterize 4-6 were hampered by their poor solubility even in very polar solvents.

Catalytic Epoxidation Reactions

We have recently reported the catalytic activity of the neutral [MoO2(L 1 )(MeOH)] (1a*) and [MoO2(L 1-3 )]n (1*-3*) complexes for the epoxidation of cyclooctene by aqueous TBHP (Scheme 4 and Scheme S1 in the Supporting Information) under solvent-free conditions and with a low Mo:olefin ratio (1:2000) [13]. We have now carried out a comparative study of the same catalytic reactions under the same conditions for the cationic complexes in 1-3 and 1a in order to evaluate and clarify the effect of ligand protonation.

Like the corresponding neutral complexes, the protonated analogues were sparingly soluble in cyclooctene and insoluble in water at room temperature, but completely dissolved in the organic phase after addition of aqueous TBHP at 80 °C. The reactions were followed by 1 H NMR during 6 hours. The relevant results are summarized in Table 5 and the kinetic profiles are shown in Figure 6. The ligands alone were not active and the reference [MoO2(acac)2] (pre)catalyst gives a conversion of 54% with a 82% epoxide selectivity under these conditions [13]. Complex 1a 1a* [13] 1 1* [13] 2 2* [13] 3 3* [13] [MoO2(acac)2] [13] Conversion (%) ) may be compared with those of the corresponding neutral systems (1040 for 1*, 1420 for 2* and 1380 for 3*, respectively [13]). Hence, the ligand protonation has a strongly positive influence for L 1 , a moderately negative one in the case of L 2 and a weakly positive one for L 3 . However, ligand protonation has a lower effect on the activity than methanol coordination. A visual comparison of the conversion and selectivity within the four complexes (mononuclear or polynuclear, neutral or cationic) containing ligand L 1 is shown in Figure 7. All these results might be linked to the nature of the ligands, to the protonation state and to the energy required to remove methanol in case of mononuclear compounds. To our knowledge, under solvent-free conditions, neutral compounds with similar ligands and based on vanadium instead of molybdenum as the catalytic metalperformed less efficiently as greater metal loadings were required [41]. Several literature references concern the epoxidation of olefins using molybdenum containing complexes (1% Mo vs. subsrate being the usual amount) with similar tridentate ligands as catalyst. Some of these catalysts exhibit very high TOF and are highly selective, but most of them were only used in solution of organic solvents (often dichloroethane) and/or with TBHP in decane as the oxidant [42]. The results presented herein are relevant because the process does not use solvent end employs TBHP in water as oxidant with a low catalysts loading. ligand. The results for the neutral compounds are taken from ref [13].

Conclusion

Mononuclear and polynuclear charged molybdenum(VI) complexes with singly-deprotonated pyridoxal thiosemicarbazonato ligands have been synthesised and characterized. They have been used as precursors for the synthesis of corresponding dinuclear oxo-bridged molybdenum(V) complexes by reaction with PPh3. The catalytic activity has been tested for cyclooctene epoxidation by aqueous TBHP under solvent-free conditions. The emphasis was placed on very low Mo loading (0.05% vs. substrate).

These compounds are quite active although generally not as much as the corresponding neutral analogues. Nevertheless, the results obtained with the charged polymeric materials provide a good basis for further optimization aiming at efficient catalyst recovery and recycling.

  1 H and13 C chemical shifts are compared with those of the free ligand H2L 1-3 •HCl. In the 1 H spectra of compounds 1-3, the azomethine CH=N proton signal H11 is shifted downfield up to 0.23 ppm as a result of the complexation. The absence of signals due to OH and =N-NH (H8 and H12, respectively) suggests coordination through the oxygen atom and thioenolization of the -N-(C=S)-group into -N=(C-S)-.Also, the H14 protons exhibit an upfield shift (up to 0.89 ppm) as a consequence of the coordination through the sulphur atom from the deprotonated thiol group. The 13 C chemical shifts exhibit larger changes, up to 7.35 ppm as observed for C11 and 11.49 ppm for C13. These effects are downfield for the former and upfield for the latter site and confirm the coordination of imine-nitrogen and thiol-sulphur atoms to the {MoO2} 2+ core. The effects observed for C3 are all downfield and appreciably smaller (up to 1.57 ppm).

Figure 1 .

 1 Figure 1. ORTEP plot of the structure of 1a with displacement ellipsoids of non-hydrogen atoms drawn at the 30% probability level. Molecules of crystal methanol are omitted. Hydrogen bond is shown by a dashed line.

Figure 2 .

 2 Figure 2. Packing of 1a in the unit cell. Hydrogen bonds are shown by dotted lines. All four possible molecule sites of the disordered methanol molecule are shown (without their hydrogen atoms).

Figure 3 .

 3 Figure 3. Superimposed molecules of 1a (light grey) and 1a* (dark grey).

Figure 4 .

 4 Figure 4. Comparison of experimental (1a and 1a*) and theoretical (A and A*) IR vibration pattern in the range 700-1200 cm -1 .

Figure 5 .

 5 Figure 5. (a) UV-Vis spectra of thiosemicarbazone complexes [MoO2(HL 1 )(CH3OH)]Cl•1.5CH3OH (1a) and [MoO2(L 1 )(CH3OH)]•CH3OH (1a*) in methanol at 25 °C. (b) UV-Vis spectrum of H2L 1 •HCl in methanol at 25 °C.

Scheme 4 .Figure 6 .

 46 Scheme 4. General equation of Mo-catalyzed epoxidation of cyclooctene.

Figure 7 .

 7 Figure 7. Conversion and selectivity comparison in epoxidation of cyclooctene by aqueous TBHP catalyzed by charged/neutral and/or monomeric/polymeric Mo VI complexes synthesized from the H2L 1

Table 1 .

 1 Crystallographic data for compound 1a.

	chemical formula	C17.5H25ClMoN4O6.5S
	Mr	558.87
	crystal colour, habit	orange-brown, plate
	crystal size (mm 3 )	0.593, 0.208, 0.042
	crystal system	Triclinic
	space group	P -1
	unit cell parameters
	a (Å)	7.2147(5)
	b (Å)	12.3246(10)
	c (Å)	13.8100(11)
	, ,  ()	101.115(7), 100.204(6), 103.460(6)
	V (Å3)	1139.67(16)
	Z	2
	Dcalc (g cm -3 )	1.623
	temperature (K)	295
	 (mm -1 )	0.827
	F(000)	566
	number of unique data	4807
	number of data [Fo  4(Fo)] 3755
	number of parameters	290
	R1 a , [Fo  4(Fo)]	0.0486
	wR2 b	0.1278
	Min. and max.

electron density (e Å -3 ) -0.86, 0.49 a

Table 2

 2

		Bond lengths (Å)		Bond angles ()
	Mo1-S	2.3936(10)	S-Mo-O1	149.40(10)
	Mo1-O1 1.943(3)	S-Mo-O3	98.59(10)
	Mo1-O3 1.684(4)	S-Mo-O4	92.97(10)
	Mo1-O4 1.696(4)	S-Mo-O5	79.33(8)
	Mo1-O5 2.332(4)	S-Mo-N3	76.11(7)
	Mo1-N3 2.312(4)	O1-Mo-O3 100.22(14)
	S-C1	1.737(4)	O1-Mo-O4 105.04(13)
	N1-C1	1.350(5)	O1-Mo-O5 77.22(12)
	N1-C10 1.407(6)	O1-Mo-N3 80.84(11)

. Selected bond lengths (Å) and angles () for compound 1a.

Table 4 .

 4 Comparison of relevant bond lengths (Å) between experimental data for 1a and 1a* and related

	calculated structures [MoO2(HL 1 )(MeOH)] + (A) and [MoO2(L 1 )(MeOH)] (A*). For A, only the cation
	had been taken into account for the calculation. Atom labels refer to the crystallographic structure.
	Bonds	1a	A	1a*	A*
	Mo-O(1) 1.943 1.987 1.918 1.975
	Mo-O(3) 1.684 1.691 1.689 1.693
	Mo-O(4) 1.696 1.697 1.706 1.706
	Mo-O(5) 2.332 2.455 2.336 2.484
	Mo-N(3) 2.312 2.373 2.280 2.339
	Mo-S(1)	2.394 2.448 2.415 2.449
	N(3)-N(2) 1.378 1.345 1.364 1.370

Table 5 .

 5 Comparison of epoxidation data for dioxomolybdenum(VI) (pre)catalysts.

Acknowledgments

All authors acknowledge the CNRS, the University Paul Sabatier (Institut Universitaire Paul Sabatier) and the Ministry of Science and Technology of the Republic of Croatia (Grant Nos. 119-1191342-1082 and 119-1193079-1084) for all research facilities. JP thanks the Ministry of Science and Technology of the Republic of Croatia, the National Foundation of Science and High Education of Croatia (03.01/O-3511-2010) and the French Embassy in Croatia for a research fellowship.

Supporting Information

Appendix A. Supplementary data -CCDC 829512 contains the supplementary crystallographic data for 1a. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.