Cheng Xu

Guangyu Li

Michel Etienne

Xuebing Leng

Yaofeng Chen

-C-C Agostic interactions and C-H bond activation in scandium cyclopropyl complexes

Introduction

The poor overlap of carbon based orbitals within a strained cyclopropyl ring raises their energy and make them spatially exposed, imparting them unique chemical properties. [1][2][3][4] One consequence in organometallic complexes [M](c-C3H5) is that so-called -C-C agostic distortions or interactions -formally three center-two electron bonds -are preferred to the much more common C-H agostic interactions whether they be with the -or -C-H bonds. 5,[START_REF] Harvey | Transition-Metal Complexes with (C-C)M Agostic Interactions[END_REF] This has been observed in unsaturated early transition [START_REF] Jaffart | An unprecedented α-C-C agostic interaction in a cyclopropyl tris(pyrazolyl)boratoniobium complex[END_REF][START_REF] Jaffart | C-H and C-C agostic interactions in cycloalkyl tris(pyrazolyl)boratoniobium complexes[END_REF][START_REF] Boulho | C-C Coupling Constants, JCC, Are Reliable Probes for α-C-C Agostic Structures[END_REF] and alkaline [START_REF] Dufrois | Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two-Coordinate Lithium Cations and CC Agostic Interactions 13 in Cyclopropyllithium Derivatives[END_REF][START_REF] Dufrois | α-CC agostic structures and aggregation diversity in cyclopropyllithium derivatives[END_REF] metal complexes especially. For rare-earth metals, the bimetallic complex [(C5Me5)2Y(-c-C3H5)2Li(thf)] exhibits C-C agostic interactions with both lithium and yttrium centers, and the interaction with lithium being more electrostatic in nature when that with yttrium being more covalent. [START_REF] Escudie | An Unsymmetrical bis C-C Agostic Heterobimetallic Lithium Yttrium Complex[END_REF] This represented a single example of a C-C agostic rare-earth metal complex since C-C agostic interactions between rareearth metals and remote cyclopropyl rings have only been computed so far. [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] In this paper, we present our efforts to synthesize monometallic scandium cyclopropyl complexes. Their characterization in the solid state and in solution indicate that C-C agostic interactions are present. DFT is used to better define the nature of the interactions.

An intramolecular C-H bond activation reaction degrades an ancillary ligand allowing the comparison between different types of alkyl ligands intramolecularly confirming that cyclopropyl groups are privilege structures to observe C-C agostic interactions.

Results and discussion

Synthetic and structural aspects

Scandium dichloride [LSc(μ-Cl)Cl]2 (L = [MeC(NDIPP)CHC(Me)(NDIPP)] -, DIPP = 2,6-( i Pr)2C6H3) and cyclopropyl lithium were prepared as reported. [START_REF] Hayes | Dialkylscandium Complexes Supported by β-Diketiminato Ligands: Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes[END_REF][START_REF] Denney | Preparation and some chemistry of tricyclopropylphosphine[END_REF] Reactions of [LSc(μ-Cl)Cl]2 with cyclopropyl lithium in a 1 : 2 or 1 : 4 molar ratio in toluene provided scandium cyclopropyl chloride complex [LSc(μ-Cl)(c-C3H5)]2 (1) and biscyclopropyl complex [LSc(c-C3H5)2] (2) in 38% and 80% yields, respectively (Scheme 1). Both complexes were characterized by NMR spectroscopy ( 1 H and [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C{ 1 H}), elemental analysis, and single-crystal X-ray crystallography. The 1 H NMR spectra in C6D6 indicated a time-averaged C2v symmetry of cyclopropyl group in 1 and 2 at room temperature. The CαH of cyclopropyl group in 1 resonates as a triplet of triplet at δ = -0.45 ppm ( 3 JH-H = 10.5, 8.1 Hz), the corresponding CαH in 2 appears shifted slightly downfield (δ = -0.06 ppm, 3 JH-H = 10.3, 8.2 Hz). The chemical shifts of ScCH3 in the scandium methyl chloride complex [LSc(μ-Cl)Me]2 and dimethyl complex [LScMe2(THF)] are 0.29 and -0.15 ppm, respectively. [START_REF] Hayes | Dialkylscandium Complexes Supported by β-Diketiminato Ligands: Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes[END_REF][START_REF] Knight | β-Diketiminato Scandium Chemistry: Synthesis, Characterization, and Thermal Behavior of Primary Amido Alkyl Derivatives[END_REF] All the CβH signals of the cyclopropyl groups in 1 and 2 appear as doublets, and their chemical shift values range from 0.38 to 0.61 ppm. The 13 C{ 1 H} NMR spectra confirm the C2v symmetry of cyclopropyl group in the complexes, with only one CβH signal at δ = 9.4 ppm for 1 and δ = 7.4 ppm for 2. Due to the quadrupolar relaxation, the Cα signals appear as broad singlets at δ = 33.2 and 30.8 ppm for 1 and 2, respectively; the assignment of these two signals was assisted with the HSQC spectra of the complexes.

Scheme 1 Synthesis of 1 and 2. Fig. 1 ORTEP drawings of 1 and 2 with thermal ellipsoids set at 30% probability level. Isopropyl groups of DIPP and hydrogen atoms have been omitted for clarity.

The single-crystal X-ray diffraction analysis showed that complex 1 exists as a dimer in the solid state, in which each pentacoordinated scandium ion is coordinated by two nitrogen atoms of L, two bridging chlorides and one carbon atom of cyclopropyl ligand (Fig. 1). Two Sc-Cl bond lengths are 2.558(1) and 2.540(1) Å. The Sc-C30 bond length is 2.195(2) Å, which is close to the Sc-C(methyl) bond length in [LScMe2(THF)] (2.210(9) and 2.245(9) Å). [START_REF] Hayes | Dialkylscandium Complexes Supported by β-Diketiminato Ligands: Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes[END_REF] The distances from the scandium ion to two Cβ atoms of the cyclopropyl ligand, 3.354 and 3.364 Å, are long, indicating no interaction between the scandium ion and the Cβ atoms. The two Sc-Cα-Cβ bond angles are close (132.3(2) o and 132.0(2) o ). Complex 2 is monomeric, with the scandium ion being coordinated by two nitrogen atoms of L and two cyclopropyl carbon atoms (Fig. 1) in a pseudotetrahedral environment. The two Sc-Cα bonds, Sc-C30 and Sc-C33, are 2.180(2) and 2.195(2) Å, respectively, similar to that in 1 (2.195(2) Å). There is an inconspicuous unsymmetrical coordination of one cyclopropyl ring in 2, which can be described by the difference between the ScC31 distance (ScC31: 2.904 Å) and other ScCβ distances (ScC32: 3.248 Å, ScC34: 3.389 Å and ScC35: 3.335 Å). Such distortion is also exhibited by the different Sc-Cα-Cβ angles, the Sc-C30-C31 (101. As observed with other dialkyl complexes of scandium, [START_REF] Hayes | Dialkylscandium Complexes Supported by β-Diketiminato Ligands: Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes[END_REF] complex 2 slowly eliminates cyclopropane at room temperature (δ = 0.14 ppm in the 1 H NMR spectrum) to give a complicated mixture in C6D6 at room temperature. Interestingly, in the presence of THF, complex 2 nearly quantitatively converts into a new complex 3 in 24 h at 65 o C with elimination of cyclopropane. A scaled-up reaction in hexane/THF provided 3 in 78% isolated yield as a yellow solid, in which a C-H bond of an isopropyl group in 2 has been activated to form a new dianionic ligand (L-H) as shown in Scheme 2. Complex 3 was characterized by NMR spectroscopy ( 1 H and 13 C{ 1 H}), elemental analysis and single-crystal X-ray crystallography. The latter revealed there are two crystallographically independent molecules in the unit cell of 3; these two molecules have very close structural parameters and one molecule was taken as the example to analyze the structural parameters (Fig. 2). In 3, the scandium ion is coordinated by two nitrogen atoms and one carbon atom of the newly formed ligand (L-H), one carbon atom of the cyclopropyl ligand and one oxygen atom of THF. The scandium atom is in a trigonal bipyramidal environment with O and N2 forming the apical sites [N2-Sc-O = 169.94 (11)°]. The distances from the scandium ion to the isopropyl carbon atom and cyclopropyl carbon atom are 2.273(4) and 2.184(4) Å, respectively. The cyclopropyl ring and its coordination to the scandium atom are significantly distorted. The Sc-C30-C31 angle (95.9(3) o ) is much smaller than the Sc-C30-C32 angle (132.7(3) o ). In addition, the ScC31 distance (2.793 Å) is 0.60 Å shorter than the ScC32 distance (3.391 Å). Within 3, C30-C31 (1.531(6) Å) is not statistically stretched as compared to C30-C32 (1.505(5) Å).

Fig. 3 1 H and [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR data for key protons and carbons in 3. Data color code: black, 1 H NMR data; red, [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR data.

The unusual distortion of the cyclopropyl ring prompted a detailed NMR investigation on the underlying agostic interaction in complex 3. First of all, a comprehensive assignment of all protons and carbons of the cyclopropyl ring in 3 was realized through 1 H and 13 C{ 1 H} NMR with the aid of HSQC, COSY and NOESY sequences, as shown in Fig. 3. The assignments indicated that, in solution, complex 3 adopts a comformation fully consistent with that in the solid state revealed by X-ray crystallography. The 1 H NMR spectrum showed five signals for the five protons of the cyclopropyl ring, [C30H: -0.48 (m, 1H), C31H: 0.13, -0.16 (m, 2H), C32H: 0.55, 0.19 (m, 2H)]. The 13 C{ 1 H} NMR spectrum showed two signals for the two Cβ atoms of the cyclopropyl ring, C31 at 5.5 ppm and C32 at 8.7 ppm when C (C30) was observed as a broad signal due to quadrupolar relaxtion at 29.0 ppm. The NOESY spectrum played a vital role in uncovering the spatial arrangement of above protons with the guidance of characteristic proton (C25H) of the L-H ligand. Apart from the detailed assignment of the protons in the cyclopropyl ring, the NOESY spectrum also depicted the greater intensity of coherent signal for the C25H'-C31H' than the C25H'-C32H' (Fig. 4), indicating that the cyclopropyl ring in 3 presents a distorted configuration with the C30-C31 bond approaching to the scandium ion in the solution phase. Subsequently, the 1 H-1 H EXSY and variable temperature 1 H NMR spectra (303 -343 K) were recorded, no proton exchange or rotation of the cyclopropyl ring in 3 was observed even at 343 K in C6D6, illustrating the relative stable geometry conformation of the cyclopropyl ring in 3.

Fig. 4 Expansion plot of the NOESY spectrum of 3 (400 MHz, C6D6).

The 1 JC-H values were then obtained from a gated [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR spectrum. The C31 and C32 atoms resonate as triplets with the 1 JC-H values of 158 and 155 Hz, respectively, which are similar to the 1 JC-H value of cyclopropane (161 Hz) and indicate the absence of agostic interaction between the scandium ion and the corresponding -C-H bonds of the cyclopropyl ring. Due to quadrupolar relaxation, 1 J could not be measured. The agostic interaction between the scandium ion and the C-C bond of the cyclopropyl ring was then investigated. A remarkable decrease of 1 JC-C value is the key evidence for the presence of such agostic interaction. [START_REF] Boulho | C-C Coupling Constants, JCC, Are Reliable Probes for α-C-C Agostic Structures[END_REF][START_REF] Escudie | An Unsymmetrical bis C-C Agostic Heterobimetallic Lithium Yttrium Complex[END_REF][START_REF] Valadez | Reaction of Cp * (Cl)M(Diene) (M = Ti, Hf) with Isonitriles[END_REF] Fortunately, owing to well resolved resonance of Cβ signals, good solubility of 3 (ca. 0.4 M in C6D6), and the utility of sensitivity boosted cryogenetic probe, inherently challenging 1D INADEQUATE spectra of high quality were successfully acquired and the 1 JC-C values of Cβ signals were obtained. The C32 signal displays two doublets with 1 JC-C values of 14.8 and 3.2 Hz; while the C31 signal appears as a doublet with 1 JC-C values of 14.8 Hz (Fig. 5a). Therefore, the 1 JC31-C32 value is 14.8 Hz and the 1 JC30-C32 value is 3.2 Hz. This small value is linked to the low electronegativity of the scandium. The observation of C31 signal as a single doublet instead of two can be rationalized in two ways: (a) the 1 JC31-C30 value is the same or very close to the 1 JC31-C32 value, so the peaks overlap; (b) the 1 JC31-C30 value is very small and indeed less than the line width of the experiment (< 2.0 Hz) hence not detectable. Therefore, we recorded the 13 C-13 C INADEQUATE spectrum of 3 optimized for J = 15 Hz (Fig. 5b), which showed that the intensity of the C31 doublet is not twice but equal to that of the C32 doublet. This result clearly indicated that the 1 JC31-C30 value is too small to be detected, and such small 1 JC31-C30 value revealed an agostic interaction between the scandium ion and the C30-C31 bond of the cyclopropyl ring. Unfortunately, this could not be confirmed in the absence of the C30 signal (however, see Computational Study below). There are three plausible pathways by which complex 2 eliminates cyclopropane to give complex 3 (Scheme 3):

(1) Path A, one of the cyclopropyl ligands abstracts a proton from the other cyclopropyl ligand first to form a scandium η 2 -cyclopropene intermediate, which is unstable and subsequently undergoes a σ-bond metathesis between the Sc-C bond and the C-H bond of the isopropyl group; (2) Path B, one of the cyclopropyl ligands abstracts a proton from the isopropyl group of L; (3) Path C, an -H abstraction to form a cycloropylidene intermediate, followed by a deprotonation of the isopropyl group of L. To investigate the reaction pathway, we decided to synthesize complex 2-d8 which contains two 2,2,3,3tetradeuteriocyclopropyl ligands and study its thermolysis. It can be reasonably envisioned that, if complex 3 is formed via Path A, the cyclopropenyl ligand in the η 2 -cyclopropene intermediate would abstract a proton from the isopropyl group to generate the complex 3-d3, which would display the cyclopropyl CβH signal in the 1 H NMR spectrum; on the contrary, if complex 3 is formed via Path B or C, the thermolysis of 2-d8 would give the complex 3-d4, in which no signal of the CβH of the cyclopropyl would be observed in the 1 H NMR spectrum.

Scheme 4 Synthesis of 2,2,3,3tetradeuteriocyclopropyl lithium and 1monodeuteriocyclopropyl lithium.

Tetradeuteriocyclopropyl lithium was firstly synthesized, as shown in Scheme 4a. Ethyl 2,2,3,3tetradeuteriocyclopropanecarboxylate was prepared using THF-d8 as a starting material in three steps according to the procedures reported by Meijere and co-workers. [START_REF] Seebach | Stereoselective Preparation of Six Diastereomeric Quatercyclopropanes from Bicyclopropylidene and Some Derivatives[END_REF] Ethyl 2,2,3,3tetradeuteriocyclopropanecarboxylate was hydrolyzed in a H2O/MeOH solution of NaOH, and then treated with HCl aqueous solution to give 2,2,3,3tetradeuteriocyclopropanecarboxylic acid (66% yield). With tetradeuteriocyclopropanecarboxylic acid in hand, 2,2,3,3-tetradeuteriocyclopropyl bromide was synthesized using a Cristol-Firth reaction. 19 After the completion of the reaction, excess bromine was removed by treatment with sodium thiosulfate, and benzene was added. The addition of benzene helps the distillation of 2,2,3,3-tetradeuteriocyclopropyl bromide in a small scale. The distilled tetradeuteriocyclopropyl bromide/benzene mixture was reacted with a suspension of finely divided lithium in diethylether to provide the desired 2,2,3,3-tetradeuteriocyclopropyl lithium as a white solid; the total yield of the above two steps is 13%. Following the procedure for complex 2, the salt metathesis of [LSc(μ-Cl)Cl]2 with 2,2,3,3tetradeuteriocyclopropyl lithium in a 1:4 molar ratio in toluene provided the deuterated complex 2-d8 as a yellow solid in a 77% isolated yield. The 1 H NMR spectrum of 2-d8 in C6D6 resembles that of 2, except the absence of resonances at 0.61 and 0.41 ppm, indicating the deuteration on Cβ atoms. In the 2 H NMR spectrum of 2-d8 in C6H6, two resonances were observed at 0.52 and 0.31 ppm for the CβD of the tetradeuteriocyclopropyl ligands. The thermolysis of 2-d8 in THF-d8/C6D6 at 65 o C was investigated, which clearly indicated the formation of 3-d4 (Scheme 5a). The H NMR spectral monitoring showed the appearance of the CαH signal at -0.61 ppm and the absence of the CβH signal (Fig. S20 in the ESI). In the 2 H NMR spectrum of the product in C6H6, the resonances were observed in the region of -0.3 ~ 0.5 ppm (Fig. S21 in the ESI). Therefore, the complex 3 is not formed via the Path A, which is different from the reported cyclopropane elimination mechanism for zirconium [START_REF] Hu | β-H Abstraction/1,3-CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers[END_REF] and niobium 21-23 cyclopropyl complexes. For example, in biscyclopropyl complex [Cp2Zr(c-C3H5)2 proton from the other cyclopropyl ligand to release cyclopropane and generate the zirconium η 2cyclopropenyl species [Cp2Zr(η 2 -c-C3H4)]. [START_REF] Hu | β-H Abstraction/1,3-CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers[END_REF] To discriminate between Path B and Path C, we need to synthesize complex 2-d2 which contains two 1monodeuteriocyclopropyl ligands and study its thermolysis. If complex 3 is formed via Path B, the thermolysis of 2-d2 would give the complex 3-d1, in which no signal of the CαH of the cyclopropyl would be observed in the 1 H NMR spectrum, and the 2 H NMR spectrum of the complex would displays a CαD signal; if complex 3 is formed via Path C, the thermolysis of 2-d2 would give the complex 3-d0, in which no signal of the CαD of the cyclopropyl would be observed in the 2 H NMR spectrum, and the 1 H NMR spectrum of the complex would shows a CαH Scheme 5 Thermolysis of 2-d8 into 3-d4, and 2-d2 into 3-d1.

signal. Methyl cyclopropyl ketone-α, α, α, α'-d4 with deuterium isotopic content to 90% was prepared by protons exchange of methyl cyclopropyl ketone with 40 wt% NaOD in D2O, [START_REF] Wolf | Properties of small-ring free radicals. 2. Thermal decomposition of alicyclic percarboxylates[END_REF] and this ketone undergoes a haloform reaction [START_REF] Vogel | A Textbook of Practical Organic Chemistry[END_REF] with NaOBr to provide 1monodeuteriocyclopropanecarboxylic acid (90 atom % D). From 1-monodeuteriocyclopropanecarboxylic acid, 1-monodeuteriocyclopropyl lithium (90 atom % D) was synthesized in a total yield of 31% using the similar approach as that of 2,2,3,3-tetradeuteriocyclopropyl lithium (Scheme 4b), and was then subjected to the salt metathesis with 0.25 equiv. [LSc(μ-Cl)Cl]2. The reaction produced complex 2-d2 as a yellow solid in 84% yield. The 1 H NMR spectrum of 2-d2 in C6D6 shows ~90% deuteration on the α-C, and no 3 JH-H spin-spin splitting was obseved for the CβH (0.61 and 0.41 ppm). In the 2 H NMR spectrum of 2-d2 in C6H6, one resonance was observed at -0.10 ppm for the CαD of the 1monodeuteriocyclopropyl ligands. The 1 H and 2 H NMR spectral monitoring on the thermolysis of 2-d2 in THF-d8/C6D6 or THF/C6H6 showed the transformation of 2-d2 into 3-d1 (Scheme 5b). In the 1 H spectrum (Fig. S26 in the ESI), four CβH signals were clearly observed at 0.41, 0.10, -0.04 and -0.33 ppm, while the CαH signal is very weak in line with a deuterium isotopic content of 90%. The 2 H NMR showed two signals, one for the CαD (δ = -0.74 ppm) in 3-d1 and the other for the CHD (δ = 0.06 ppm) in eliminated monodeuteriocyclopropane (Fig. S27 in the ESI). Therefore, the complex 3 is formed via a direct proton abstraction of the isopropyl group in L by cyclopropyl group (Path B).

Computational study

We have used a computational approach to better define the C-C agostic interactions in complex 3 and possibly in complex 2. We first used the experimental data for complex 3 to benchmark our study and then considered the distorted solid state molecular structure of complex 2 whose solution structure is time averaged. Optimizations were carried out in the framework of DFT with the PBE0 functional coupled to empirical dispersion corrections. Details of basis sets can be Fig. 6 Computed structures and coupling constants J for complexes 3 and 2 and comparison with experimental values when available. Only the core coordination sphere and the cyclopropyl groups are drawn for clarity. Experimental values (distance in Å, coupling constants in Hz) are boldface. Key data for agostic interactions are highlighted in red. Element color code: violet, Sc; blue, N; red, O; grey, C; off white, H. found in the experimental section and in the Electronic Supporting Information. The results for complex 3 are summarized in Fig. 6 where it is seen that a remarkable agreement with the structural and spectroscopic data is reached, the Sc-C-C angles as well as the dihedral angle C25-Sc-C30-C31, ie the conformation of the cyclopropyl ring, are correctly modelled. Considering the C-C bond lengths within the cyclopropyl ring, a small but clear elongation of C30-C31 bond relative to C30-C32 bond is seen which was not statistically meaningful experimentally. As to the coupling constants, not only the trend but also the absolute values are faithfully computed (IGLOOII basis set) including JC-H as references and JC-C. This approach has been validated in similar studies. [START_REF] Boulho | C-C Coupling Constants, JCC, Are Reliable Probes for α-C-C Agostic Structures[END_REF][START_REF] Dufrois | Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two-Coordinate Lithium Cations and CC Agostic Interactions 13 in Cyclopropyllithium Derivatives[END_REF][START_REF] Dufrois | α-CC agostic structures and aggregation diversity in cyclopropyllithium derivatives[END_REF][START_REF] Escudie | An Unsymmetrical bis C-C Agostic Heterobimetallic Lithium Yttrium Complex[END_REF] For the latter, the computed values of 6 and 15 Hz are in very good agreement with the experimental values of 3 and 15 Hz found for the coupling constants involving C30-C32 and C31-C32, respectively. Therefore, the computed value of -3 Hz is consistent with our assignment of JC30-C31 < 2 Hz. The computational results together with the experimental data are consistent with the presence of an -C-C agostic interaction in complex 3. 5,[START_REF] Harvey | Transition-Metal Complexes with (C-C)M Agostic Interactions[END_REF][START_REF] Boulho | C-C Coupling Constants, JCC, Are Reliable Probes for α-C-C Agostic Structures[END_REF] The results of similar calculations for complex 2 are summarized in Fig. 6 as well. The geometrical parameters are again very well reproduced. The C30 based cyclopropyl group is clearly distorted as compared to the one based on C33 which remains regular. The C30-C31 bond (1.53 Å) is indeed longer than the other CC bond (C30-C32 (1.50 Å)). The associated JC-C, unavailable experimentally, are -5 and 6 Hz, respectively. The elongated C30-C31 bond associated with the reduced JC-C are consistent with the presence of an Q-C-C agostic interaction in complex 2.

Examination of JC-H values indicates that for the -C-H

pointing towards the scandium atom, a value of 136 Hz is computed. This value is somewhat reduced with respect to the other JC-H which are in the narrow, normal range of 147-151 Hz. In complex 2, an -C-C agostic interaction is accompanied by a -C-H agostic interaction. In order to probe the nature of these distortions, we carried out a Natural Bond Orbital (NBO) [START_REF] Reed | Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint[END_REF] study of the electronic structure of complex 2. The NBO analysis shows two significant interactions at the second order perturbation theory involving (i) the -orbital of a -C-H bond on C31 with the empty antibonding Sc-C33 with strong d character of 17 kJ/mol, and (ii) the -C30-C31 with d-orbitals of the scandium atom of 33 kJ/mol. ‡ These interactions are also nicely pictured in NLMOs which reveal participation of scandium d-orbitals in the C-H (ca 1.0 %) and C-C (ca 1,3 %) based NLMO, respectively (Fig. 7).

Although the absolute values might be questioned, interactions of similar magnitude have been observed in zirconium [START_REF] Hu | β-H Abstraction/1,3-CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers[END_REF] and yttrium 12 cyclopropyl complexes. The involvement of both C-C and C-H bonds in the interaction with the rare-earth metal has been observed previously in the bimetallic complex [(C5Me5)2 -c-C3H5)2Li(thf)] albeit with different weights. [START_REF] Escudie | An Unsymmetrical bis C-C Agostic Heterobimetallic Lithium Yttrium Complex[END_REF] It reinforces the view that there is a continuum between pure -C-H and Q-C-C agostic structures with both types of interactions complementing each other depending on the metals, 27 the type of ligands, electron count, coordination number and geometry. The significant involvement of the C-C orbital is a distinctive property of the cyclopropyl group.

Conclusions

Discrete electron deficient scandium cyclopropyl complexes have been synthesized and fully characterized for the first time. They show distortions of their coordination sphere in the solid state. In one case, solution NMR studies allow the direct measurement of the reduced JC-C coupling constant. Together with DFT modelling, -C-C agostic interactions possibly accompanied by -C-H agostic interactions have been established. They reinforce the view that a continuum of stabilizing secondary interactions exists in the strongly electron deficient species. For the thermolysis of scandium biscyclopropyl complex, the cyclopropyl ligand prefers to abstract a proton from the isopropyl group of -diketiminato ligand rather than from the other cyclopropyl ligand in the complex.

Experimental

General methods

All operations were carried out under an atmosphere of argon using Schlenk techniques or in nitrogen or argon filled gloveboxes. Toluene, THF, hexane, THF-d8 and C6D6 were dried over Na/K alloy, transferred under vacuum, and stored in the gloveboxes. Scandium dichloride [LSc(μ-Cl)Cl]2 (L = [MeC(NDIPP)CHC(Me)(NDIPP)] -, DIPP = 2,6-( i Pr)2C6H3), [START_REF] Hayes | Dialkylscandium Complexes Supported by β-Diketiminato Ligands: Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes[END_REF] cyclopropyl lithium, [START_REF] Denney | Preparation and some chemistry of tricyclopropylphosphine[END_REF] ethyl 2,2,3,3tetradeuteriocyclopropanecarboxylate [START_REF] Seebach | Stereoselective Preparation of Six Diastereomeric Quatercyclopropanes from Bicyclopropylidene and Some Derivatives[END_REF] and 1monodeuteriocyclopropanecarboxylic acid [START_REF] Vogel | A Textbook of Practical Organic Chemistry[END_REF] were prepared as reported. 1 H, 2 H and [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR spectra were recorded on a Bruker AV III 400 spectrometer, a Varian MR400 spectrometer or an Agilent MR400 spectrometer. Variable-temperature NMR spectra, 1 H-1 H EXSY and NOESY spectra were acquired on a Bruker NEO 600 spectrometer. INADEQUATE experiments were performed on a Bruker NEO 500 spectrometer equipped with a BBO cryoprobe. All chemical shifts were reported in δ units with references to the residual solvent resonances of the deuterated solvents for proton and carbon chemical shifts, and 2 H NMR was referred to CDCl3 in chloroform, C6D6 in benzene and THF-d8 in THF, respectively. Elemental analysis was performed by the Analytical Laboratory of Shanghai Institute of Organic Chemistry. Synthesis 1: A solution of [LSc(μ-Cl)Cl]2 (100 mg, 0.094 mmol) in 8 mL toluene was added by a slurry of 28.4 mg cyclopropyl lithium • lithium bromide (cyclopropyl lithium: 28.5% by weight, 0.17 mmol) in 3 mL toluene at -35 o C under stirring, and then the reaction mixture was gradually warmed to room temperature. After stirring at room temperature for 4 h, the precipitate was removed by centrifugation. The volatiles of the solution were removed under vacuum to give a yellow solid. The solid was washed with hexane (2 mL × 3) and toluene (2 mL), and dried under vacuum to give 1 as a pale yellow solid (35 mg, 38% yield). Due to the poor solubility of 1 in C6D6, its 1 H and [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C{ 1 H} NMR spectra were recorded in THF-d8. 1 H NMR (400 MHz, THF-d8, 25 o C): δ (ppm) 7.20 (m, 6H, ArH), 5.39 (s, 1H, MeC(N)CH), 3.39 (sept, 4H, ArCHMe2), 1.82 (s, 6H, MeC(N)), 1. 30 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), 1. 19 (d, 3 JH-H = 6.6 Hz, 12H, ArCHMe2), 0.48 (d, 3 JH-H = 8.1 Hz, 2H, CβH2), 0.38 (d, 3 JH-H = 10.5 Hz, 2H, CβH'2), -0.45 (tt, 3 JH-H = 10.5, 8.1 Hz, 1H, CαH). [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF]

2:

A solution of [LSc(μ-Cl)Cl]2 (500 mg, 0.47 mmol) in 10 mL toluene was added by a slurry of 284 mg cyclopropyl lithium • lithium bromide (cyclopropyl lithium: 28.5% by weight, 1.69 mmol) in 5 mL toluene at room temperature under stirring. After stirring at room temperature for 1.5 h, the precipitate was removed by centrifugation. The volatiles of the solution were removed under vacuum to give a yellow solid. This solid was extracted with 2 mL hexane, the extract was stored at -35 o C to give 2 as yellow needle crystals (365 mg, 80% yield). [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR (100 MHz, C6D6, 25 o C): δ (ppm) 30.8 (br, CαH, 1 JC-H was not observed due to the quadrupolar relaxation), 7.4 ( 1 JC-H = 159 Hz, CβH2). Anal. Calcd (%) for C35H51N2Sc: C, 77.17; H, 9.44; N, 5.14. Found: C, 76.75; H, 9.01; N, 5.09.

3:

Complex 2 (500 mg, 0.92 mmol) in a mixed solvent of hexane (10 mL) and THF (200 mg) was heated at 65 o C for 24 h. After cooled to room temperature, the volatiles of the reaction solution were removed under vacuum. The residue was washed with hexane (1 mL × 3), and dried under vacuum to give 3 as a yellow solid (360 mg, 78% yield). .6 (br, Sc-CH2CH), 39.9, 29.3, 28.9, 28.8 (ArCHMe2 and Sc-CH2CH), 29.0 (br, CαH), 26.7~23.9 (ArCHMe2 and Sc-CH2CHCMe), 8.7(Cβ2H2), 5.5 (Cβ1H2). [START_REF] Tobisch | Organolanthanide-Mediated Ring-Opening Ziegler Polymerization (ROZP) of Methylenecycloalkanes: A Theoretical Mechanistic Investigation of Alternative Mechanisms for Chain Initiation of the Samarocene-Promoted ROZP of 2-Phenyl-1-methylenecyclopropane[END_REF] C NMR (100 MHz, C6D6, 25 o C): δ (ppm) 29.0 (br, CαH, 1 JC-H was not observed due to quadrupole relaxation), 8.7 ( 1 JC-H = 155 Hz, Cβ2H2), 5.5 ( 1 JC-H = 158 Hz, Cβ1H2). Anal. Calcd (%) for C36H54N2OSc: C, 75.10; H, 9.45; N, 4.87. Found. C, 75.08; H, 9.38; N, 5.03.

2,2,3,3-Tetradeuteriocyclopropanecarboxylic acid:

Ethyl 2,2,3,3-tetradeuteriocyclopropanecarboxylate (2.60 g, 22 mmol) in 10 mL MeOH was added to a NaOH aqueous solution (3.52 g NaOH (88 mmol) in 10 mL H2O) at room temperature under stirring. After stirring under reflux for 4 h, the reaction mixture was cooled to room temperature and MeOH was removed under vacuum. The reaction mixture was cooled to 0 o C, and was carefully brought to pH ≈ 6 by adding a 1.0 M HCl solution. The mixture was extracted with ethyl acetate 50 mL × 3, the combined organic extracts were washed with 50 mL brine, dried over anhydrous sodium sulfate and distilled under reduced pressure to give 2,2,3,3-tetradeuteriocyclopropanecarboxylic acid (b. 2,2,3,3-Tetradeuteriocyclopropyl lithium: To a vigorously stirring susppension of HgO (1.56 g, 7.20 mmol) in 1,1,2,2-tetrachloroethane (10 mL) was added dropwise a mixture of 2,2,3,3tetradeuteriocyclopropanecarboxylic acid (1.30 g, 14.4 mmol) and bromine (2.31 g, 14.4 mmol) in 1,1,2,2tetrachloroethane (10 mL) over a period of 0.5 h. The reaction mixture was stirred at 35 ~ 40 °C until the evolution of carbon dioxide ceased. Stirring in the sealed flask was maintained at room temperature for another 12 h. After the precipitate was removed by centrifugation, the mixture was washed with sat. aq. Na2S2O3 (10 mL), and the residual aqueous phase was extracted with 5 mL benzene. The organic extracts in tetrachloroethane and benzene were combined, and washed with 10 mL brine, dried over anhydrous sodium sulfate and distilled under ambient pressure (b.p. 65 ~ 80 o C) to provide 2,2,3,3-tetradeuteriocyclopropyl bromide/benzene mixture as a colorless liquid. This mixture was added dropwise to a suspension of finely divided lithium (200 mg, 28.8 mmol) in 10 mL Et2O at room temperature. After stirring for 12 h, the precipitate was removed by centrifugation. The volatiles of solution were removed under vacuum, the residue was washed with toluene (3 mL × 3) and hexane (1 mL), and dried under vacuum to provide 2,2,3,3-tetradeuteriocyclopropyllithium • lithium bromide as a white solid 320 mg (2,2,3,3tetradeuteriocyclopropyllithium: 31% by weight). The total yield of the above two steps is 13%. 1 1-Monodeuteriocyclopropyl lithium: Following the procedure described for 2,2,3,3tetradeuteriocyclopropyl lithium, from 1monodeuteriocyclopropanecarboxylic acid (4.50 g, 51.7 mmol) , 1.91 g 1-monodeuteriocyclopropyl lithium • lithium bromide was obtained as a white solid (1monodeuteriocyclopropyllithium: 30% by weight) in 31% yield. 1 H NMR (400 MHz, THF-d8, 25 o C): δ (ppm) 0.36 (s, 2H, CβH2), -0.16 (s, 2H, CβH2'). 2 H NMR (60 MHz, THF, 25 o C): δ (ppm) -2.52 (s, CαD).

2-d2: Following the procedure described for complex 2, reaction of [LSc(μ-Cl)Cl]2 (500 mg, 0.470 mmol) with 290 mg 1-monodeuteriocyclopropyl lithium • lithium bromide (1-monodeuteriocyclopropyllithium: 30% by weight, 1.78 mmol) gives 2-d2 as a yellow solid (410 mg, 84% yield).

X-ray Crystallography

Single crystals of 1 were grown from a toluene solution; single crystals of 2 and 3 were grown from hexane solutions. The suitable single crystals were mounted under a nitrogen atmosphere on a glass fiber, and data collection was performed at 170(2) K on a Bruker D8 Venture diffractometer with graphite-monochromated Ga Kα radiation (λ = 1.34139 Å). The SMART program package was used to determine the unit cell parameters. The absorption correction was applied using SADABS program. [START_REF] Sheldrick | SADABS: An Empirical Absorption Correction Program for Area Detector Data[END_REF] The structures were solved by direct methods and refined on F 2 by full-matrix leastsquares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculation. Calculations were carried out using the SHELXL-2015 and Olex2 program. 29 Crystallographic data and refinement parameters are listed in Table S1 of ESI.

Computational details

Calculations were performed at the DFT level with the software Gaussian09, revision D.01. 30 Geometry optimizations were carried out without symmetry constraints in the gas phase using the PBE0 functional [START_REF] Adamo | Toward reliable density functional methods without adjustable parameters: The PBE0 model[END_REF] and including dispersion corrections (GD3-BJ). [START_REF] Grimme | Effect of the damping function in dispersion corrected density functional theory[END_REF] The scandium atom was described using the Stuttgart/Dresden ECP (SDD) pseudo-potential and its associated basis set [START_REF] Dolg | Energy-adjusted ab initio pseudopotentials for the first row transition elements[END_REF] to which was added an f polarization function. [START_REF] Ehlers | A set of f-polarization functions for pseudo-potential basis sets of the transition metals Sc-Cu, Y-Ag and La-Au[END_REF] All other atoms were described with the def2-SVP basis set except those directly bonded to the scandium atom (N, O) and the carbon atoms of the cyclopropyl groups for which the def2-TZVP basis set was used. [START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] The nature of the stationary points was ascertained by a vibrational analysis within the harmonic approximation (1 atm and 298 K). Minima were identified by a full set of real frequencies. Computation of the NMR coupling constants was realized with the IGLOII basis set for carbon and hydrogen atoms. [START_REF] Schindler | Theory of magnetic susceptibilities and NMR chemical shifts in terms of localized quantities. II. Application to some simple molecules[END_REF] NBO calculations were carried out with NBO6 as implemented in Gaussian09. 37 Drawings were produced with the software Chemcraft. 38

Conflicts of interest

There are no conflicts to declare

  Scheme 2 Thermolysis of 2 into 3.

Fig. 5

 5 Fig. 5 13 C-13 C INADEQUATE spectra of 3, optimized for J = 3 Hz (a) and J = 15 Hz (b), respectively.

Fig. 7

 7 Fig. 7 NLMOs describing (a) the α-C-C interaction, (b) the α-C-H interaction in complex 2.

  C{ 1 H} NMR (100 MHz, THF-d8, 25 o C): δ (ppm) 169.1 (MeC(N)CH), 147.5, 144.4 (i-ArC and o-ArC), 127.2, 125.3 (p-ArC and m-ArC), 99.1(MeC(N)CH), 33.2 (br, CαH), 29.5 (ArCHMe2), 26.0, 25.7 (MeC(N) and ArCHMe2), 9.4 (CβH2). Anal. Calcd (%) for C32H46ClN2Sc: C, 71.29; H, 8.60; N, 5.20. Found: C, 70.53; H, 8.12; N, 5.03.

  1 H NMR (400 MHz, C6D6, 25 o C): δ (ppm) 7.15 (m, 6H, ArH), 4.91 (s, 1H, MeC(N)CH), 3.43 (sept, 4H, ArCHMe2), 1.56 (s, 6H, MeC(N)), 1.44 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), 1.19 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), 0.61 (d, 3 JH-H = 10.7 Hz, 2 H, CβH2), 0.41 (d, 3 JH-H = 8.2 Hz, 2H, CβH'2), -0.06 (tt, 3 JH-H = 10.3, 8.2 Hz, 1H, CαH). 13 C{ 1 H} NMR (100 MHz, C6D6 , 25 o C): δ (ppm) 168.0 (MeC(N)CH), 143.6, 143.0 (i-ArC and o-ArC), 127.3, 124.9 (p-ArC and m-ArC), 94.9 (MeC(N)CH), 30.8 (br, CαH), 29.5 (ArCHMe2), 26.1, 25.1, 24.6 (ArCHMe2 and MeC(N)), 7.4 (CβH2).

1H

  NMR (400 MHz, C6D6, 25 o C): δ (ppm) 7.42, 7.31 and 7.03 (m, 6H, ArH), 5.20 (s, 1H, MeC(N)CH), 3.58, 3.43 and 3.11 (m, 3H, ArCHMe2, overlapped with THF-H signals), 3.25 (m, 1H, Sc-CH2CH), 1.87 and 1.68 (s, 6H, MeC(N)), 1.62 (d, 3H, Sc-CH2CHCMe) 1.48, 1.39, 1.30, 1.10 and 1.00 (d, 18H, ArCHMe2, overlapped with THF-H signals), 0.59 and 0.37 (m, 2H, Sc-CH2CHAr), 0.55 and 0.19 (m, 2H, Cβ2H2), 0.13 and -0.16 (m, 2H, Cβ1H2), -0.48 (m, 1H, CαH). 13 C{ 1 H} NMR (100 MHz, C6D6, 25 o C): δ (ppm) 165.9, 165.8 (MeC(N)CH), 147.6, 145.8, 145.5, 144.4, 144.3, 143.0 (i-ArC and o-ArC), 126.7, 126.3, 124.9, 124.7, 124.2, 122.4 (p-ArC and m-ArC), 98.3 (MeC(N)CH), 71.0 (THF-C), 59

  1 H NMR (400 MHz, C6D6, 25 o C): δ (ppm) δ 7.16 (m, 6H, ArH), 4.93 (s, 1H, MeC(N)CH), 3.45 (sept, 3 JH-H = 6.8 Hz, 4H, ArCHMe2), 1.58 (s, 6H, MeC(N)), 1.45 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), 1.20 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), 0.61 (s, 2H, CβH2), 0.41 (s, 2H, CβH'2). 2 H NMR (60 MHz, C6H6, 25 o C): δ (ppm) -0.10 (s, CαD).

  p. 76 o C, 15 mmHg) as a colorless liquid (1.31 g, 66% yield). 1 H NMR (400 MHz, CDCl3, 25 o C): δ (ppm) 11.96 (br, 1H, COOH), 1.58 (s, 1H, CαH). 2 H NMR (60 MHz, CHCl3, 25 o C): δ (ppm) 1.03, 0.90 (s, CβD2).

  MeC(N)), 1.45 (d,3 JH-H = 6.8 Hz, 12H, ArCHMe2), 1.20 (d, 3 JH-H = 6.8 Hz, 12H, ArCHMe2), -0.08 (s, 1H, CαH). 2 H NMR (60 MHz, C6H6, 25 o C): δ (ppm) 0.52, 0.33 (s, CβD2).

	mg, 77% yield). 1 H NMR (400 MHz, C6D6, 25 o C): δ (ppm)
	δ 7.16 (m, 6H, ArH), 4.92 (s, 1H, MeC(N)CH), 3.45 (sept,
	4H, ArCHMe2), 1.57 (s, 6H,
	H NMR (400
	MHz, THF-d8, 25 o C): δ (ppm) -2.54 (s, 1H, CαH). 2 H NMR
	(60 MHz, THF, 25 o C): δ (ppm) 0.35, -0.17 (s, CβD2).
	2-d8: Following the procedure described for complex 2,
	reaction of [LSc(μ-Cl)Cl]2 (200 mg, 0.19 mmol) with 115
	mg 2,2,3,3-tetradeuteriocyclopropyllithium • lithium
	bromide (2,2,3,3-tetradeuteriocyclopropyllithium: 31%
	by weight, 0.68 mmol) gives 2-d8 as a yellow solid (160

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 21732007, 21890721, 21821002), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), and the Program of Shanghai Academic Research Leader.

Notes and references ‡ As a reference, we used the NBO delocalizations of -C-C orbitals in the *-C-C within a cylopropyl ring which describes the bonding in any cyclopropane derivative. These interactions are in the range 17


Céline Hubert

Cyril Chomette

Pierre-Etienne Rouet

Antony Désert

Mona Treguer- Delapierre

Jérôme Majimel

Stéphane Mornet

Adeline Perro

Etienne Duguet

Serge Ravaine
email: ravaine@crpp-bordeaux.cnrs.fr

Regioselective functionalization of dimpled silica particles

Keywords: surface modification, regioselectivity, patchy particles, silica, polystyrene

Multipod-like particles made of a silica core surrounded by polystyrene nodules have been used as precursors to synthesize silica particles with 3 or 12 dimples through the selective growth of the dielectric core. Using the polymer nodules as temporary protecting masks, we regioselectively grafted amino groups onto the unprotected silica surface, as revealed by the adsorption of gold markers. After dissolution of the PS nodules, some polymer chains remained grafted onto the silica surface, forming organic bumps at the bottom of the dimples. These residues were also selectively functionalized, leading to silica particles with both entropic and enthalpic patches.

Introduction

The self-assembly of size-monodisperse isotropic colloidal particles to form colloidal crystals is one of the existing examples that justifies the analogy between atoms and hard spherical particles, that may be called colloidal atoms. Going one step further, one can consider the molecular world as a source of inspiration for designing colloidal analogues of simple molecules. Such colloidal molecules (CMs) are defined as robust discrete clusters of colloidal atoms. Many different strategies have been developed during the last decade to produce such "colloidal molecules" [START_REF] Van Blaaderen | Chemistry: Colloidal molecules and beyond[END_REF][START_REF] Yi | Monodisperse micrometer-scale spherical assemblies of polymer particles[END_REF][START_REF] Zerrouki | Preparation of doublet, triangular, and tetrahedral colloidal clusters by controlled emulsification[END_REF][START_REF] Stöber | The aerodynamic diameter of aggregates of uniform spheres[END_REF][START_REF] Duguet | Design and elaboration of colloidal molecules: an overview[END_REF]. One approach is to start with preformed spherical particles and to control their aggregation into polyhedral clusters. Examples of this approach include colloidal aggregation on micro-patterned substrates [START_REF] Masuda | Self-assembly and micropatterning of sphericalparticle assemblies[END_REF] or in evaporating emulsions droplets [START_REF] Manoharan | Dense packing and symmetry in small clusters of microspheres[END_REF].

Another strategy consists in nucleating and growing satellite particles on a preformed seed.

We have recently reported the high-yield synthesis of binary polystyrene (PS)/silica multipods with a controlled morphology by a seeded-growth emulsion polymerization of styrene [START_REF] Perro | A chemical synthetic route towards "colloidal molecules[END_REF][START_REF] Désert | High-yield preparation of polystyrene/silica clusters of controlled morphology[END_REF][START_REF] Désert | Synthesis and site-specific functionalization of tetravalent, hexavalent, and dodecavalent silica particle[END_REF][START_REF] Désert | Multipod-like silica/polystyrene clusters[END_REF].

In this paper, we show that these multipod-like particles can be used as endotemplates to produce silica particles with 3 or 12 dimples. Using the PS pods as temporary masks [START_REF] Perro | Towards large amounts of Janus nanoparticles through a protection-deprotection route[END_REF], we regioselectively functionalized the unprotected silica surface. We also report that we are able to selectively functionalize the organic residues located at the bottom of the dimples that remain after the dissolution of the PS nodules. From a technological point of view, particles with heterogeneously patterned surfaces can offer a route to the production of target materials with specific (ordered as well as disordered) structures, thus being amenable in many technological applications.

Materials and methods

Materials

Tetraethoxysilane (TEOS, ≥ 99 %, Fluka), N- [3-(trimethoxysilyl)propyl]ethylenediamine (EDPS, 97%, Aldrich), n-propyltriethoxysilane (PTES, 97%, Aldrich), paraformaldehyde (reagent grade, crystalline, Sigma-Aldrich), tin(IV) chloride (98%, Aldrich), ethylenediamine (99.5%, Fluka), hydrochloric acid (37%, Sigma-Aldrich) and ammonia (28-30% in water, SDS) were used as received. Deionized water with a resistivity of 18.2 MOhm.cm at 25°C was obtained with a MilliQ system (Millipore). Tetrahydrofuran (THF), dimethylformamide (DMF), chloroform and absolute ethanol were purchased from Sigma-Aldrich.

Methods

Synthesis of dimpled silica particles.

Multipods made of a central silica core and three, or twelve PS satellite nodules were prepared by seeded-growth emulsion polymerization of styrene, according to an already published procedure [START_REF] Perro | A chemical synthetic route towards "colloidal molecules[END_REF][START_REF] Désert | High-yield preparation of polystyrene/silica clusters of controlled morphology[END_REF]. The regrowth of the silica cores was performed in the following conditions: a certain volume of TEOS, V TEOS , was diluted ten times in absolute ethanol. The resulting solution was introduced continuously in a mixture made of 1 mL of an aqueous suspension of multipods, 45.5 mL of absolute ethanol and 3.2 mL of ammonia thanks to a single-syringe pump (0.5 mL.h -1 ). The mixture was stirred at room temperature during 12 h. The dissolution of the PS nodules was performed in 100 mL of THF at room temperature under stirring for 1 h. After washing by three cycles of centrifugation/redispersion in THF, the multivalent silica particles were finally redispersed in absolute ethanol.

Functionalization of the surface area between the dimples.

To graft amino groups onto the unprotected silica surface, a given volume of EDPS corresponding to a nominal surface density of 20 functions.nm -2 was added directly into the particle suspension and the mixture was let to age at 40°C for 2 hours. The particle dispersion was purified by 3 cycles of centrifugation/redispersion in absolute ethanol. 5-nm gold nanoparticles, synthesized as previously described [START_REF] Duff | A new hydrosol of gold clusters[END_REF], were added into 20 mL of the functionalized multipod-like particles dispersion. The mixture was stirred at room temperature overnight, and then the particle assemblies were collected by centrifugation and washed three times with absolute ethanol. The dissolution of the PS nodules was performed in 100 mL of THF at room temperature under stirring for 1 hour. After washing by three cycles of centrifugation/redispersion in THF, the multivalent silica particles were finally redispersed in absolute ethanol.

Functionalization of the PS residues at the bottom of the dimples.

To graft propyl groups onto the unprotected silica surface, a given volume of PTES corresponding to a nominal surface density of 20 functions.nm -2 was added dropwise into the particle suspension and the mixture was let to age at room temperature for 3 h. The solvents were removed by using a rotavapor. An excess of THF was added to the solid residue to dissolve the PS nodules.

After washing by three cycles of centrifugation/redispersion in THF, the silica particles were redispersed in chloroform. Finally, the particles were washed by three cycles of centrifugation/redispersion in 40 mL of chloroform. The chloromethylation of the PS chains was done by adding 5 mL of a 3M butyl chloromethylether solution in chloroform, prepared according to Warshawsky et al. [START_REF] Warshawsky | Halomethyl octyl ethers: Convenient halomethylation reagents[END_REF], and 0.3 mL of SnCl 4 into 20 mL to the silica particles and the mixture was aged overnight at 45°C. After washing by three cycles of centrifugation/redispersion in HCl solution (4 wt.% in water) and three cycles of centrifugation/redispersion in a water/EtOH (50/50 wt %), the silica particles were redispersed in 20 mL of DMF. Then, 1 mL of ethylenediamine was added and the mixture was aged overnight at 90°C. After three cycles of centrifugation/redispersion in water, the pH was adjusted to 4 with HCl. Finally, 5-nm gold nanoparticles were added into the suspension.

The mixture was stirred at room temperature overnight, and then the particle assemblies were collected by centrifugation and washed three times with absolute ethanol.

Characterization techniques. Electron microscopy experiments and statistical

calculations. TEM images were obtained with a Hitachi H600 microscope operating at 75 kV and with a Hitachi H7650 operating at 120 kV. Typically, the collected samples were diluted 100 times in ethanol and one drop was deposited on a copper grid coated with a carbon membrane. Statistical analyses of the cluster morphology were performed on a minimum of 100 clusters per batch.

Results

Synthesis of the tripod-and dodecapod-like particles

We have already described the seeded-growth emulsion polymerization of styrene in the presence of silica [START_REF] Perro | A chemical synthetic route towards "colloidal molecules[END_REF][START_REF] Désert | High-yield preparation of polystyrene/silica clusters of controlled morphology[END_REF][START_REF] Taveau | New insights into the nucleation and growth of PS nodules on silica nanoparticles by 3D cryo-electron tomography[END_REF][START_REF] Reculusa | Hybrid dissymmetrical colloidal particles[END_REF][START_REF] Perro | Planar submicronic silica-polystyrene particles obtained by substrate-directed shaping[END_REF][START_REF] Chaduc | Towards a one-step method for preparing silica/polymer heterodimers and dimpled polymer particles[END_REF][START_REF] Bourgeat-Lami | Nucleation of polystyrene latex particles in the presence of gammamethacryloxypropyltrimethoxysilane: functionalized silica particles[END_REF]. When the surface of the silica seeds is treated by a compatibilizer, e.g. methacryloxymethyltriethoxysilane (MMS), some organophilic loci promoting the surface capture of the growing macromolecules are created. Therefore the nucleation/growth of the PS nodules occurs preferentially onto the surface of the seeds. As previously shown, using the optimal nominal surface density of MMS onto the silica seeds, i.e. 0.5 fct.nm -2 , we observed the preferential surface nucleation and growth of a limited number of PS nodules per seed. We also reported that the size of the silica seeds, D silica , influences the final number of PS nodules per seed, , as it conditions the maximal number of satellites which can co-exist on the same seed for steric reasons. For instance, when the fraction of sodium dodecyl sulfate (SDS) was equal to 5 wt.%, tetrapods and hexapods were obtained with morphology yield over 80 % from 55-nm and 85-nm silica seeds, respectively (Figures 1a and1c and Table 1, Entries #1.2 and #1.3) [START_REF] Désert | High-yield preparation of polystyrene/silica clusters of controlled morphology[END_REF].

In order to get clusters with a small number of PS satellites, e.g. tripods, as the main population, we explored a route by rendering the nucleation process less favorable to decrease the number of initial nuclei per seed . Assuming that the higher the SDS fraction in the surfactant mixture, the smaller and the more numerous the PS particles, we performed an experiment with 55-nm silica seeds by decreasing the fraction of SDS from 5 to 1 wt.% (Table 1, Entry #1.1). The statistical analysis of the TEM images showed that the distribution morphology was dramatically narrowed with essentially tripods (70 %) (Figure 1b).

Another challenge was to produce already-observed clusters but for a different seed size. For instance, we attempted the synthesis of dodecapods from 85-nm silica seeds, while these multipods were previously obtained with 255-nm seeds when no SDS was used [START_REF] Perro | A chemical synthetic route towards "colloidal molecules[END_REF]. We first performed an experiment by increasing the fraction of SDS from 5 to 10 wt.% but a complex mixture with mostly octopods (38 %) and "multi-silica" clusters (26 %) was obtained. We thus developed another strategy to increase the value by lowering the monomer concentration, i.e. increasing the surfactant-to-monomer ratio. An experiment was performed by decreasing the styrene concentration from 100 to 45 g.L -1 and dodecapods were indeed readily obtained with a yield of 74 % (Table 1, Entry #1.4, and Figure 1d).

Dimpled particles

We have already shown [START_REF] Désert | Synthesis and site-specific functionalization of tetravalent, hexavalent, and dodecavalent silica particle[END_REF] that (i) the slow addition of a dilute solution of tetraethoxysilane in ethanol into a hydroalcoholic suspension of multipods, in presence of ammonia, led to the growth of the silica core conforming to the shape of the polystyrene nodules, (ii) the subsequent dissolution of the PS nodules gave rise to silica particles with a fixed number of dimples. We studied the influence of the quantity of silica precursor by running a series of experiments during which 100 L, 200 L or 300 L of TEOS were added to a suspension of dodecapods. As shown in Figure 2, the size of the dimpled silica particles increases with V TEOS . The dependence of the size of the silica particles on V TEOS is reported in Erreur ! Source du renvoi introuvable.. Table 2. Average diameter values of the silica particles with 12 dimples after the injection of various volumes of TEOS and dissolution of the PS nodules using THF.

Regioselective functionalization

In order to regioselectively functionalize the surface of the silica particles, we first used the PS nodules as temporary protecting masks (Figure 3, route A). We added EDPS at the end of the TEOS condensation to graft amino groups onto the unprotected silica surface. We then mixed the functionalized multipods with an aqueous suspension of THPC-stabilized 5-nm gold nanoparticles [START_REF] Duff | A new hydrosol of gold clusters[END_REF], which are known to adsorb specifically on amine groups. After the dissolution of the PS nodules, Figure 4a-b shows that the gold nanoparticles packed densely on the amine-modified silica surface, whereas no gold nanoparticles was visible on the nonmodified surface of the concave faces of the silica particles, indicating highly selective functionalization.

We also took benefit of the fact that the PS chains which are covalently bonded to the silica surface thanks to the copolymerization of the methacrylate groups of the MMS grafts with styrene remain at the bottom of the dimples after the dissolution of the PS nodules [START_REF] Hubert | Synthesis of multivalent silica nanoparticles combining both enthalpic and entropic patchiness[END_REF].

Indeed, the dissolution of the PS nodules by THF molecules consisted in the disentanglement of the chains and especially from those which remained covalently bonded to the silica surface. Therefore, the structure of the dimpled silica particles was more complex than initially expected: to their complex shapes shall be added their biphasic nature. The silica dimples can be considered as entropic patches as well as potential enthalpic ones due to the discrepancy in term of chemical reactivity exhibited by the silica from one side and the residual PS chains on the other side. We selectively modified the residual PS chains located at the bottom of the dimples through a chloromethylation reaction of their phenyl groups (see inset of Figure 4c) and the subsequent grafting of ethylenediamine molecules onto the chloromethylated macromolecules (Figure 3, route B). Here again, this regioselective functionalization was revealed by the specific adsorption of gold nanoparticles within the dimples (Figure 4c-d).

Silica core regrowth

Adsorption of gold PS dissolution

PS residues derivatization

Adsorption of gold NPs

Conclusion

In summary, we synthesized tripods and dodecapods by modifying the SDS fraction in the surfactant mixture and of the styrene concentration during the emulsion polymerization in the presence of silica seeds. Silica particles with 3 or 12 dimples were synthesized through the growth of the silica core of the corresponding binary PS/silica multipods and the subsequent dissolution of the PS nodules. The size of these new dimpled silica particles can be easily controlled by adjusting the volume of the injected TEOS solution. We also demonstrated the successful functionalization of both the non-protected surface of the grown silica core and of the organic residues remaining at the bottom of each dimple, thereby providing the possibility of getting particles with patches combining both enthalpic and entropic characters. These particles are promising building blocks for the fabrication of new lattices by self-assembly or as "lock" particles able to be combined with a precise number of "key" particles in each dimple in order to obtain new supracolloids [START_REF] Sacanna | Lock and key colloids[END_REF][START_REF] Sacanna | Engineering shape: the novel geometries of colloidal self-assembly[END_REF][START_REF] Wang | Threedimensional lock and key colloids[END_REF].

Acknowledgements

Figure 1 .

 1 Figure 1. Typical TEM images of multipods obtained by styrene emulsion polymerization

Figure 2 .

 2 Figure 2. TEM images of silica particles with 12 dimples obtained after the addition of: V TEOS = 100L, 200L and 300L (from left to right). Scale bars: 100 nm. On the bottom row, the white dotted circle represents the initial 85-nm silica seed and the yellow, orange and red dotted circles illustrate the sizes of the particles after the addition of the different amounts of TEOS.

Figure 3 .

 3 Figure 3. Schematic representation of the regioselective functionalization of dimpled silica particles.

Figure 4 .

 4 Figure 4. Top row: TEM images of the gold-decorated dimpled silica particles after the dissolution of the polystyrene nodules. Bottom row: TEM images of silica particles with c) 3, d) 12 gold-decorated bumps at the bottom of the dimples. The inset shows an Electron energy loss spectroscopy (EELS) mapping after chloromethylation of the residual PS chains (Silicon in red, Carbon in blue and Chlorine in green).

  TEM experiments were performed at the Plateforme de Caract risation des at riaux and ordeaux ma in enter of the ni ersity of ordeaux This work was supported by the onseil ional d uitaine, the Agence Nationale de la Recherche (grant ANR-07-BLAN-0271), the LabEx AMADEus (ANR-10-LABX-42) and the IdEx Bordeaux (ANR-10-IDEX-03-02), i.e. the n estissements d enir pro ramme of the French o ernment mana ed by the Agence Nationale de la Recherche.

Table 1 .

 1 Influence of the SDS fraction in the surfactant mixture and of the styrene concentration on the morphology and geometrical parameters of the PS/silica clusters ([Na 2 S 2 O 8 ]

	Entry		#1.1	#1.2	#1.3	#1.4
		D silica (nm)	55	55	85	85
		N silica (10 15 L -1 )	19	18	7.3	7.3
	Experimental conditions	S silica (m 2 .L -1 ) [styrene] 0 (g.L -1 )	174 100	171 100	166 100	166 45
		wt.% of SDS	1	5	5	5
		Conversion (%)	90	89	92	89
		% mono/bipods	17	2	-	-
		% tripods	70	11	-	-
	Final batch composition in clusters a	% tetrapods	2	84	3	-
		% pentapods % hexapods	--	--	13 83	}4

0 = 0.5 g.L -1 ; [surfactant] = 3 g.L -1 ). Bold numerals concern the main cluster population.

a as extracted from the statistical analysis of TEM images