The Role of Oleylamine Revisited: an Original Disproportionation

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The Role of Oleylamine Revisited: an Original Disproportionation Route to Monodispersed Cobalt and Nickel Nanocrystals Anthony Vivien, Maya Guillaumont, Lynda Meziane, Caroline Salzemann, Corinne Aubert, Stéphanie Halbert, Hélène Gérard, Marc Petit, and Christophe Petit Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04435 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Chemistry of Materials

The Role of Oleylamine Revisited: an Original Disproportionation Route to Monodispersed Cobalt and Nickel Nanocrystals Anthony Vivien,1,2,‡ Maya Guillaumont,3,‡ Lynda Meziane,2 Caroline Salzemann,2 Corinne Aubert,1 Stéphanie Halbert,3 Hélène Gérard,3,* Marc Petit,1,* Christophe Petit.2,* 1Sorbonne

Université, CNRS, Institut Parisien de Chimie Moléculaire, UMR 8232, 4 place Jussieu, 75005 Paris (France).2 Sorbonne Université, CNRS, MONARIS, UMR 8233, 4 place Jussieu, 75005 Paris (France).3 Sorbonne Université, CNRS, LCT, UMR 7616, 4 place Jussieu, 75005 Paris (France).

ABSTRACT: Herein, is highlighted an original disproportionation mechanism for formation of hcp cobalt nanoparticles starting from low-valent cobalt complexes. By coupling experimental and theoretical studies, we demonstrate that oleylamine promotes the disproportionation of the starting metal complexes and controls the growth of the nanocrystals, in contrast to the role of reducing reagent proposed elsewhere. We also decipher the sensitivity of the synthesis in regard to the nature of the phosphine and of the halogen ligands in the starting cobalt precursors. Understanding of the mechanism allows the development of a less expensive and less toxic synthesis of these nanoparticles by decreasing the amount of oleylamine in alkane solvent. Finally, an extension of this process is presented and allows reaching size and shape controlled nanospheres and nanorods nickel nanoparticles.

INTRODUCTION The preparation of well-defined nanostructures is becoming of utmost importance in the fields of sciences and technology for catalytic,1-4 magnetic,5-7 electronic8 or other applications.9-12 To reach the required standards in terms of selectivity (catalysis) or material efficiency (magnetism), a perfect control of the size, shape and crystallinity is now mandatory. In the case of metallic cobalt nanocrystals (NCs), several syntheses have been described giving access to controlled size and crystallinity. In particular, its hexagonal closed pack (hcp) structure displays one of the highest magnetic anisotropy for monometallic compounds and thus high potentiality for magnetic applications.13 However, several syntheses leading selectively to a specific crystalline structure with a good control of size and shape often involve a complex mixture of reducing reagent, surfactant, metal seeds and additives.14-21 The last few decades provided new strategies to simplify these syntheses using the so-called organometallic approach. Indeed, Bawendi,22 Murray23 and later Puntes24-25 have shown that Co2(CO)8 can decompose under thermal condition to form mainly epsilon-structure cobalt nanoparticles with the concomitant release of 8 equivalents of carbon monoxide.24-27 Using a hydrocarbyl complex such as Co(3-C8H13)(4-C8H12) under 3 bars of dihydrogen, Chaudret et al. were able to access to several shape of hcp-NPs depending on the nature of the additives such as rhodamine,28 polyphenyloxypolymer29 or a mixture of surfactants.30 However the tedious synthesis of the

starting complex makes this approach somehow difficult to reproduce whereas the need of dihydrogen pressure can be a limitation in term of safety.31 It was recently described by Sun et al. that oleylamine can be used together as a surfactant, reducing reagent and solvent.32 Yet, if this approach does not work using Co(II) salt precursors, it appeared efficient with a Co(I) metal precursor such as ClCo(PPh3)3. Indeed, we demonstrated that a mixture of ClCo(PPh3)3 and oleylamine at 190 °C for 1 hour or for 10 hours gives hcp-nanoparticles of sphere or rod shapes respectively.33 This approach using a simple mixture of an easily accessible and stable complex with oleylamine was a real breakthrough in the synthesis of cobalt NPs. However the exact role of oleyamine was not established and the reduction process at stake in the cobalt(0) formation was not clear. In this work, by coupling new experimental results and DFT calculations, we demonstrate that the synthesis of cobalt hcp-nanoparticles starting from ClCo(PPh3)3 as a Co(I) precursor can be explained through a disproportionation mechanism promoted by the presence of amine and does not require to invoke additionally reduction by oleylamine itself

RESULTS AND DISCUSSION ROLE OF THE OLEYLAMINE Oleylamine is a polyfunctional compound with a high boiling point (350°C) featuring a long chain (C18) primary amine and including an alkene function at the C9-C10 junction. It has been

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shown to act at high temperature as an electron-donor.34 Since a commercially available 70% purity oleylamine was used in our previous study,33 we first ruled out the role of an impurity by running the same reaction using a higher purity of oleylamine (98%). No difference is observed in terms of structure, shape, crystallinity and kinetic of the reaction, demonstrating the robustness of this procedure (Figure 1A). Then, in order to better understand the role of the various oleylamine functions in our reaction procedure, we ran several experiments using amines, alkenes and alkanes analogs. In the following study all the experiments have been run using a 98 % purity oleylamine (98 % assay and 98% primary amine). It should be noticed that the impurities for the 70% purity oleylamine are only other primary amines (70% assay, 98% primary amine). To evaluate the role of the double bond, octadecylamine was used instead of oleylamine. Applying exactly the same procedure as for the oleylamine, we observed the formation of spherical nanoparticles quite similar in size (about 8.5 nm compared to 8.9 nm for oleylamine) with higher polydispersity (15% compared to 7.8% of with oleylamine) (Figure 1). The alkene function is thus not mandatory for the nanoparticles formation whereas the increase in polydispersity highlights the role of the amine in nanocrystals growth, possibly due to a lower stability of the capping with octadecylamine. In addition, the lower inter-particles distance in the case of octadecylamine confirms that the bent of oleylamine chain (presence of a double bond) disfavors the interdigitation and limits the aggregation.35

Figure 1: TEM images and size distribution of spherical cobalt NCs obtained at 190°C using (A) oleylamine with 98% purity and (B) octadecylamine.

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the electron beam. This underlines the importance of a long chain to protect the nanocrystals.36

Figure 2: TEM images (A) and zoom (C) of the cobalt NCs obtained with octylamine at 130 °C (B) electronic diffraction of the cobalt NCs (D) size distribution.

Two experiments were next reported to evaluate the role of the amine function in the NPs formation. Reactants without amine functions are used, either with 1-octadecene (C18 carbon chain with a double bond in a terminal position), or tetradecane (C14H30). In the classical conditions, in both cases only magnetic aggregated material of Co(0) was obtained, stuck on the stirring bar after precipitation with ethanol. As a conclusion, the amine function is not responsible for the reduction of the Co(I) complex but appears essential for controlling size, shape and stability of NPs in solution. In fact, whatever the solvent used (amine, alkane or others), similar typical colors were obtained at the end of the reaction (Figure 3): a black precipitate, corresponding to the nanomaterial or to the aggregate of Co(0) and a blue solution, which is classically associated and can be visually compared with Co(II) solutions. The slightly different blue color observed in presence of oleylamine can be associated to ligand exchange within the Co(II) complex (Figure 3A and 3C compare to Figure 3B and 3D).

The effect of the chain length was next examined by replacing the octadecylamine (C18) by the octylamine (C8), using once again the same procedure (for C8 the temperature of the reaction was 130 °C instead of 190 °C due to the lower boiling point of this amine). Spherical cobalt nanoparticles were obtained with an even smaller size of 4.9 nm (probably due to the lower temperature, which limits the diffusion of the monomers) and a polydispersity of 11.7 % (Figure 2-A-D). A typical electronic diffraction pattern obtained from a collection of nanocrystals confirms the hcp structure (Figure 2-B). However, due to the shorter chain of the surfactant (C8), these NCs are less stable than the previous ones. In particular melting evolution can appear during the time of exposition to


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Chemistry of Materials obtained as a reaction product.41 Description of these two steps from a computational point of view was next undertaken.

DFT CALCULATION

Figure 3: Pictures of a vial after precipitation of the Co(0) with ethanol in the reaction with (A) oleylamine with (B) tetradecane. Picture of vial of the solubilization of Cl2Co(PPh3)2 in ethanol in presence of (C) oleylamine or not (D).

All these results suggest that the formation of the cobalt NPs does not go through an external reduction by oleylamine but via a “self-reduction” of Co(I), namely its disproportionation into a mixture of a Co(II) by-product and a Co(0) complex (Scheme 1, first step). Such a process would lead to a maximum possible yield of 50% in Co NPs with respect to the Co precursor, in contrast to a reduction pathway which can lead up to 100%. We evaluated the yield of the reported procedure at most to 40%,37 which is thus consistent with the disproportionation mechanism proposed. The lower value is attributed to the difficulty for recovering all the NP material. In addition, the very low polydispersity is in accordance with a single mechanism pathway. Indeed, two different mechanisms, with their own nucleation kinetics, should lead to a larger size distribution. Disproportionation has already been reported for the synthesis of copper and gold nanoparticles.38,39,40 For the copper,38 a two-step process was invoked which is transposed for Co in Scheme 1. First, disproportionation of the XCo(PR3)3 complex yields Co(II) and Co(0) complexes in solution (Scheme 1). Second, the Co(0) species evolves toward nucleation, NPs growth and precipitation (Scheme 1). The first step, namely the disproportionation of XCo(PR3)3, was suggested in the literature in 1969 by Rossi41 (X = Cl, Br, I and R = Ph) whereas the reverse reaction, the comproportionation, was reported by Klein in 1975 (same X, R = Me).42 It thus seems that the disproportionation is highly dependent to the phosphine, the solvent and to a lesser extent to the halide.

Scheme 1: Proposed disproportionation process, by analogy with reference 38. In addition the stability of Co(0) complex intermediate depends on the reaction conditions. It is indeed similar to the species involved in the comproportionation reaction described by Klein,42 and can be compared to the “monomer” proposed in the classical LaMer model. In contrast, the “naked” cobalt species described by Rossi is closer to the Co(0) precipitate

In a first part, we examine the disproportionation step. Since no computational method is unanimously recognized to yield a quantitative evaluation of electronic, steric and dispersive effects of phosphine ligands,43-45 we are limited to a semiquantitative study. We thus focus on the understanding of the global electronic effects at stake. In this connection, simple ligand models are used: PH3 as a phosphine model for the various PR3 ligands and NH3 as an amine model for the NH2R oleylamine. In a first step, only phosphine ligands and chlorine anions are considered to be bounded to Co centers (Eq. 1). As no steric effects can be taken into account in such a simple approach, tetracoordination of Co centers is assumed, in line with the experimentally described structures for Co(I) and Co(II) phosphine complexes.46,47 All ground states exhibit (sometimes strongly) distorted tetrahedral geometry. The CoCl(PH3)3 precursor is in a triplet ground state (16.7 kcal.mol-1 lower in energy than the singlet). The CoCl2(PH3)2 distorted tetrahedral quadruplet is 11.4 kcal mol-1 more stable than the square planar doublet. Finally, Co(PH3)4 is in a doublet state, lying 26.7 kcal.mol-1 below the quadruplet. Considering those structures, the disproportionation reaction (Eq. 1) is energetically disfavored (∆Edis(1) = 22.5 kcal.mol-1). In addition, entropic effects are expected to be minimal (as the number of molecules is similar in the right and left hands of the equation). 2 CoCl(PH3)3  CoCl2(PH3)2 + Co(PH3)4

(Eq. 1)

This is in full accordance with the comproportionation observed by Klein for PMe3 ligands in ether as solvent. Such an equation is not appropriate when disproportionation takes place in aminated solvents (oleylamine and octadecylamine) as ligand exchange can take place either in reactants or in products. To evaluate this possibility, the energy associated with the substitution of phosphine ligands by amine ones are examined, using our simple model. Changing PH3 to NH3 does not change the spin ground state of the Co complexes (see Supporting Information) and results associated with this ground state as a function of the number of substituted PH3 are gathered in Table 1. The electronic changes associated with the Hard (NH3) vs Soft (PH3) nature of the ligand influence the Co complexes stability, depending on the Co oxidation state. Following the expected Hard and Soft Acids and Bases (HSAB) principles, the “hard” Co(II) center favours coordination of the “hard” NH3 ligand so that complete exchange of the PH3 by NH3 ligand to yield CoCl2(NH3)2 is energetically favorable.48 On the opposite, exchange of PH3 by NH3 are systematically endothermic for Co(0), in line with the preferential coordination of the “soft” PH3 ligand to the “soft” Co(0). The Co(PH3)4 complex is thus the lower in energy. Co(I) exhibits an intermediate behavior, as the two first substitutions are quasi-athermic. As a consequence, CoCl(NH3)(PH3)2 is used in the following as it allows writing an isodesmic (no ligand exchange) disproportionation process (see Supporting Information for other possible disproportionation equations).



CoClx(PH3)y(NH3)z + NH3  CoClx(PH3)y-1(NH3)z+1 + PH3 First substitution

Second substitution

Third substitution

Fourth substitution

z

0

1

2

3

y

4-x

3-x

2-x

1-x

Co(0): x=0

9,7

10,9

4,3

4,1

Co(I) : x=1

0.3

1.0

7.7

Non existing

Co(II) x=2

-9.9

-9.8

Non existing

Non existing

At this stage, we conclude that using amine solvents shifts the disproportionation equilibrium to the right, whatever the metal at stake, whereas replacing chlorine by iodine should shift it to the left. We next examined the effect of this shift on the formation of NPs. This requires examining the decomposition of the Co(0) complex (Scheme 1, right side). As evidenced in Equation 3, this decomposition can be viewed thermodynamically as ligand decoordination (step (a)) followed by formation of the “naked” Co clusters (step (b)). (𝑎)

Table 1: SCF energies in kcal mol-1 for the substitution reaction given above the Table. In amine ligand, Eq. 2 is retained to model the disproportionation reaction. 2 CoCl(NH3)(PH3)2  CoCl2(NH3)2 + Co(PH3)4

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Eq. 2

In these frameworks, disproportionation is quasi-athermic (ΔEdis(2) = 2.3 kcal.mol-1). Disproportionation energy is thus lowered due the stabilization of the Co(II) obtained by an amine ligand. In these conditions, the disproportionation equilibrium is shifted to the right compared to Eq. 1, which models a system in the absence of amine. We also evaluate the generality of this HSAB driven effect by looking at the same reaction for the Ni analogue of the Co precursor. In this case, in the absence of amine, disproportionation is found to be favored (exothermic by -1.1 kcal mol-1, see Table 2, ΔEdis(1)). A similar influence of the Ni oxidation state is found when looking at the substitution of PH3 by NH3 (see Supporting Information), as it is highly exothermic (over -10 kcal mol-1) for Ni(II), highly endothermic (over +10 kcal mol-1) for Ni(0)) and quasiathermic for Ni(I). As a consequence, working with aminated solvent also shifts the disproportionation equilibrium to the right as exothermicity increases to ΔEdis(2) = -17.6 kcal mol-1. The observed effect of L ligand hardness on the equilibrium suggests that a shift of the disproportionation equilibrium could also be obtained through halogen exchange. Computationally, changing hard Cl to softer X ligands (Cl  Br  I) increases the disproportionation energy (Table 2). The softer the X ligand, the less favored the formation of the CoX2L2 species, compared to the unaltered CoXL3 precursor. CoCl(PH3)3

NiCl(PH3)3

CoBr(PH3)3

CoI(PH3)3

ΔEdis(1)

22.8

-1.1

25.0

27.1

ΔEdis(2)

2.2

-17.6

4.8

7.4

Table 2: SCF disproportionation energies in kcal.mol-1 for the Eq. 1 and Eq. 2 as a function of the precursor.

(𝑏)

Co(PR3)n Co + n PR3 Cocluster + n PR3 (Eq.3) The step b has been extensively studied from both experimental49,50 and theoretical51-53 points of view. The energy associated with formation of Co cluster from isolated Co, ∆E(3b), can be evaluated from the literature between -83.6 and -101.6 kcal mol-1, depending whether cluster or bulk values are used. Decoordination of the four phosphine ligands, ∆E(3a), is theoretically evaluated to 78.5 kcal mol-1 for PH3 model ligands, 85.9 kcal mol-1 for PMe3 and 61.3 kcal mol-1 for PPh3. Considering the controversies and approximation associated with a quantitative evaluation of the phosphine binding energies46-47 as well as our simple model (phosphine or oleylamine – Cocluster interactions, entropic effects associated to ligand release and precipitation, …), we believe it is not possible to get better insight into the energy for the nucleation/ growth step (Scheme 1). We thus just conclude that the energy for this step, computed as ∆E(3) = ∆E(3a)+ ∆E(3b), is much more negative for PPh3 (between -29.9 and 47.9 kcal mol-1 depending on the value used for ∆E(3b)) than for PMe3 (between +2.3 and -15.7 kcal mol-1). This latest evaluation is totally consistent with the fact that Co(PMe3)4 complex can be experimentally isolated and is stable at room temperature for months, yielding no Co(0) precipitate in these conditions. To strengthen this result, Co(PMe3)4 is used as precursor in our standard reaction conditions (heating at 190 °C). Neither change of color nor formation of nanoparticles is observed. This confirms that no nucleation / growth step can occur on its own using PMe3 as a phosphine ligand. It also allows to understand the formation of Co(0) reported above in non-coordinating solvents (tetradecane and octadecene). In this case, the disproportionation equilibrium is strongly displaced toward the left (mediamutation, ΔEdis(1) around 25 kcal mol-1 ) but, when PPh3 ligands are used, the nucleation/growth step is sufficiently exothermic (more than -30 kcal mol-1) to overcome the endothermicity of disproportionation. Nevertheless, in these conditions, reduction and growth are not properly separated in time, which justifies the absence of size control. In order to confirm the DFT calculations, several analogs of the ClCo(PPh3)3 complex were examined by replacing (i) the triphenylphosphine by trimethylphosphine to yield the ClCo(PMe3)3 complex and (ii) by the chlorine atom by iodine to yield ICo(PPh3)3. Using ClCo(PMe3)3 as precursor in our classical conditions leads to important change as no control of the size and the shape can be observed after 1 hour or 3 hours. Only polydispersed nanoparticles were obtained (Figure 4A). Formation of Co(0) nanomaterial suggests that, in line with theoretical data, replacing PPh3 by PMe3 does not prevent disproportionation but is detrimental to nucleation/growth. Since a quantitative comparison of PMe3 and PPh3 cannot be done computationally, it is examined experimentally. A 31P NMR analysis study is conducted (see Supporting Information


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for details) to evaluate phosphine / oleylamine exchange in CoCl(PR3)3 and CoCl2(PR3). Substitution of PPh3 in the presence of an excess of oleylamine is fast, even at room temperature, for both Co(I) and Co(II) complexes. At the opposite, no exchange of PMe3 ligands in the presence of an excess of oleylamine at room temperature is observed neither for Co(I) nor for Co(II). Thus, using a more labile ligand (PPh3 rather the PMe3) on the cobalt precursor allows at the same time a faster ligand exchange leading to a faster disproportionation, but also a better control of the nucleation/growth step. Using ICo(PPh3)3 complex in our classical conditions did not give the formation of well-defined spherical NCs after 1 hour as usually obtained with chloride. Only small nanoparticles were observed (Figure 4B). Putting these results in the perspective of the DFT calculations (disproportionation is energetically less favored by about 6 kcal mol-1) suggests that replacing Cl by I shifts the disproportionation equilibrium to the left, decreasing the concentration of Co(0) complex in solution (see Scheme 1). Thus, the kinetics of nucleation decreases, yielding to smaller NPs for standard reaction conditions (1 hour) . This was confirmed by running the reaction for 3 hours instead of 1 hour. In these conditions, we observed larger nanoparticles (10 nm) with a very low polydispersity (5.5 %, Figure 4C and 4D).

Figure 5: Cobalt NCs obtained by dismution of CoCl(PPh3)3 precursor in a mixture of (A) tetradecane:oleylamine (9:1), (B) the eletronic diffraction and (C) the corresponding size distribution.

Figure 6: Cobalt NCs obtained by dismution of CoCl(PPh3)3 presursor in a mixture of (A) DMF:oleylamine (9:1) and (B) the corresponding size distribution. These results confirm that a slight excess of oleylamine (oleylamine/triphenylphosphine: 4/1) is sufficient to promote and control the growth of the nanoparticles, and that the polarity of the solvent mixture does not play a significant role.

Figure 4: TEM images of the cobalt NCs obtained with (A) ClCo(PMe3)3 after 1 hour , with (B) ICo(PPh3)3 after 1 hour and with (C and D) ICo(PPh3)3 after 3 hours. (E) Size distribution.

Finally, it appears from the experimental and computational studies that the main roles of oleylamine are (i) to increase the rate of disproportionation and (ii) to control the growth of the nanoparticles. We can thus propose to go from a fully oleylamine solvent reaction to a mixture of a less expensive solvent with just an excess of oleylamine as additive. Experiments using either a 9/1 tetradecane/oleylamine mixture, and DMF/oleylamine one, yielded spherical nanoparticles with a size of 10.6 nm and 8.4 nm respectively and a polydispersity of 8.4 % and 12.7% respectively (Figure 5 and 6).

EXTENSION TO NICKEL NCs We demonstrated, a new pathway to reach size and shape controlled hcp cobalt nanospheres starting from XCo(PR3)3. It involves a disproportionation process which was also recently proposed in an electrochemical study.54 In this article Yakhvarov et al. demonstrated that nickel but also iron nanoparticles can be synthesized using the disporportionation mechanism. In order to study the versatility of our synthesis, the analogous ClNi(PPh3)3 complex was tested for which a favored disproportionation process is proposed computationally (see Table 2). By heating the nickel complex in oleylamine at 190 °C for 1 hour according to the standard procedure, we observed the formation of spherical nanoparticles of nickel with a 9.7 nm diameter and 10 % of polydispersity (Figure 7A and 7C). However, the typical electronic diffraction pattern evidences fcc-nickel nanoparticles (Figure 7B).



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sustainable synthesis of metallic nanocrystals with controlled shape and size.

ASSOCIATED CONTENT Supporting Information Figure 7: TEM images (A) of the nickel NCs obtained with oleylamine at 190 °C. (C) dispersity histogram. (D) electronic diffraction of the nickel NCs. Typical experimental evidences allow to suppose once more a disproportionation mechanism. Indeed, starting from a bright red Ni(I) complex, a yellow solution is obtained at the end of the reaction in addition to nanoparticles. The yellow color is typical of the Ni(II) complex analog (see Supporting Information). More interestingly, by heating the solution for longer time (here 60 hours), nickel nanorods (Figure 8A) were obtained, with a fcc structure (Figure 8B), 11.6 nm length and 3.6 nm large with a low polydispersity of 13% (Figure 8C).

Figure 8: TEM images (A) of the nickel nanorods obtained with oleylamine after 60 hours of heating at 190°C. (B) electron diffraction typical of the fcc structures (C) distribution of length and diameter of the nanorods. Hence we demonstrated that formation of cobalt and nickel NPs can be explained without invoking a reducing role for oleylamine. Coupled theoretical and experimental studies show that oleylamine play a key role in both the kinetics and the thermodynamics of disproportionation. Several ligand exchanges are proposed and oleylamine appears necessary in the growth and the stability of the NPs. We also demonstrated that changing the ligand and the halogen of the starting complexes can be detrimental in the control of the size and on the shape of the nanocrystals. Finally, the standard reaction conditions were optimized by significantly decreasing the amount of oleylamine required and we showed that this process could be generalized to other metallic precursors prone to disproportionation. In particular, the synthesis of nickel nanocrystals following the same protocol is reported with low size polydispersity and controlled shape. Studies are undergoing in our laboratory to understand the mechanism of formation of nickel nanorods and to evaluate the potential benefits of mixing Ni and Co precursors to control nucleation and growth. Finally, this opens new opportunities in the

Experimental procedures, NMR analysis and DFT informations. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION Computational details All computations are carried out using the Gaussian 09 package (D01 version). 55 The Density functional theory (DFT) is used at the B3PW9156-57 level in its unrestricted formalism. The complexes showing the largest spin contamination are (i) Co(PH3)x(NH3)y complexes (y≠0), obtained upon substitution of PH3 by NH3, which is found to be unfavored ; (ii) both CoCl2L2 Cobalt (II) complexes (L=PH3 and NH3) in doublet state, which is not the ground spin state (as confirmed experimentally49-50) ; (iii) CoCl(PH3)3 in the singlet state, which is not the ground spin state (as confirmed experimentally49-50). None of these structures are retained to make the energetic evaluations in the RESULTS AND DISCUSSION part, which allows us to be confident with the reported ΔE. Some complementary calculations in the restricted open-shell formalism are added in Supporting Information to confirm the Co(0) results, for which no experimental data are available. The quasi-relativistic effective pseudo-potential (RECP) from the Stuttgart group58,59 are used for Co, Ni, Br and I and Cl (when halogen effect is studied) in conjunction to the associated basis set augmented by an f or d polarization function. The 6-31++G(d,p)60,61 basis set is used for all the other atoms (H, C, N, O, P) and Cl when substitutions effects are studied. Calculations including tri(phenyl)phosphine are performed using a 6-31G(d,p) basis set. For the sake of coherence, all values compared to tri(phenyl)phosphine complexes were recomputed at this computational level. No major differences are found. Full geometry optimization is carried out in the gas phase and vibrational analysis within the harmonic approximation is used to confirm the nature of the minima. Since most of our conclusions rely on energy trends (see for instance Cl, Br and I), we focused on differences in electronic energies ∆E. G(298) can be found in Supporting Information but are not adapted to conclude as experimental temperature is close to 463 K. Complexes synthesis All reactions were carried out in dry glassware under an argon atmosphere or using standard Schlenk techniques under an argon atmosphere. All solvents degassed by argon bubbling. All chemicals (cobalt and nickel salt, phosphine and oleylamine 98% and 70% were bought from Sigma-Aldrich and were used as arrived after degassing. General procedure for the synthesis of X(PPh3)3Co(I) type complexes: On CoX2.nH2O (X = Cl, n = 6; X = Br, I, n = 0) (40 mmoles) and triphenylphosphine (122 mmoles) were added 600 mL of degassed ethanol. The resulting heterogeneous solution was stirred vigorously at 60-70 °C for about 30 minutes to form in situ the complex X2(PPh3)2 Co(II). The mixture was then cooled down to 30 °C and sodium borohydride (34 mmoles) were added in 10 portions every 10 minutes. After 2 hours the brown precipitate


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was filtrated under argon and washed sequentially with ethanol, water, ethanol and finally hexane. The brown solid was dried under vacuum to give the desired X(PPh3)3 Co(I) complex. Procedure for the synthesis of ClNi(PPh3)3 complexe: On NiCl2.6H2O (1.2 g 5.1 mmol, 1 equiv.) and triphenylphosphine (5.35 g, 20.4 mmol, 4 equiv.) were added 50 mL of EtOH. Then a solution of lithium borohydride (133 mg, 6.12 mmol, 1.2 equiv.) in 10 mL of EtOH was added in 30 minutes. The reaction was stirred at room temperature for 2 hours the solution was removed via cannula and the precipitate was washed sequentially with ethanol (5 mL) 3 times. The orange solid was dried under vacuum to give 4 g (91 %) of ClNi(PPh3)3 complex. This catalyst has already been fully characterized in the literature General procedure for the synthesis for cobalt and nickel nanocrystals In a glove-box under nitrogen atmosphere 10 ml of degassed oleylamine (OA) were mixed with XM(PPh3)3 (X = Cl, Br, I; M = Co, Ni) or ClCo(PMe3)3 or Co(PMe3)4 (0.25 mmoles) in a 25 ml vial caped with a septa pierced by a pipette (to provide overpressure). The solution was stirred and heated using a 50 ml « drysyn » filled with sand. The mixture was heated by controlling the increase of the temperature from 20 °C to 190 °C in 1 hour. Then the solution was cooled down to room temperature and the nanoparticles were washed by adding 20 ml of ethanol and then centrifuged during 5 minutes at 2500 rpm. The blue or yellow supernatant was removed and the resulting black solid was then dispersed in toluene. For synthesis, in OA/tetradecane a mixture of 1 ml of OA and 9 ml of tetradecane was used instead of pure OA.

3)

Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer−Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels, Chem. Rev. 2007, 107, 169.

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Kim, S. W.; Son, S. U.; Lee, S. S.; Hyeon, T.; Chung, Y. K. Colloidal cobalt nanoparticles: a highly active and reusable Pauson–Khand catalyst, Chem. Commun. 2001, 21, 2212.

5)

Frey, N. A.; Sun, S. Magnetic Nanoparticle for Information Storage Applications, in: Altavilla, C.; Ciliberto, E. (Eds.), Inorganic

Nanoparticles:

Synthesis,

Applications,

and

Perspectives, CRC Press, 2010. 6)

Papaeftymiou, G. C. Nanoparticle magnetism, Nano Today 2009, 4, 438.

7)

Lu, A. H.; Salabas, E. L.; Schuth, F. Magnetic nanoparticles: synthesis,

protection,

functionalization,

and

application,

Angew. Chem. Int. Ed. 2007, 46, 1222. 8)

Karmakar, S.; Kumar, S.; Rinaldi, R.; Maruccio, G. Nanoelectronics and spintronics with nanoparticles, J. Phys.: Conf. Ser. 2011, 292, 012002.

9)

Krishnan, K. M.; Pakhomov, A. B. Bao ; Y.; Blomqvist, P.; Chun, Y.; Gonzales, M.; Griffin, K.; Ji, X.; Roberts, B. K. Nanomagnetism and spin electronics: materials, microstructure and novel properties J. Mater. Sci. 2006, 41, 793.

10) Sun, C.; Lee, J. S.; Zhang, M. Nanomagnetism and spin electronics: materials, microstructure and novel properties, Adv.

AUTHOR INFORMATION

Drug Deliv. Rev. 2008, 60, 1252.

Corresponding Author

11) Sonvico, F.; Mornet, S.; Vasseur, S.; Dubernet, C.; Jaillard, D.;

[email protected] [email protected] [email protected]

Degrouard, J.; Hoebeke, J.; Duguet, E.; Colombo, P.; Couvreur, P. Folate-Conjugated Iron Oxide Nanoparticles for Solid Tumor Targeting as Potential Specific Magnetic Hyperthermia

Author Contributions

Mediators: Synthesis, Physicochemical Characterization, and in

‡These authors contributed equally.

Vitro Experiments, Bioconjugate Chem. 2005, 16, 1181. 12) Podmiljsak, B.; McGuiness, P. J.; Mattern, N.; Ehrenberg, H.;

ACKNOWLEDGMENT

Kobe. S. Magnetocaloric Properties in the Gd5(Si2Ge2)1−zFez(

This work was supported by CNRS, MRES, Sorbonne University and by the LabEx MiChem, part of French state funds managed by the ANR within the “Investissements d'Avenir” program under reference ANR-11-IDEX-0004-02 which we gratefully acknowledge. The authors also thank Mario Alberto Avila Gutiérrez for its help with the electronic diffraction.

Z=0−0.75) System and the Magnetic Properties of the Newly Formed Gd5(Si,Ge)3 Phase, IEEE Trans. Magn. 2009, 45, 4364. 13) Gambardella, P.; Rusponi, S.; Veronese, M.; Dhesi, D. D.; Grazioli, C.; Dallmeyer, A.; Cabria, I.; Zeller, R.; Dederichs, P. H.; Kern, K.; Carbone, C.; Brune, H. Giant Magnetic Anisotropy of Single Cobalt Atoms and Nanoparticles, Science 2003, 300, 1130.

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The Role of Oleylamine Revisited: an Original Disproportionation

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