Pavel A Dub

Aurélien Béthegnies

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

ethylene catalyzed by Pt II or Pt 0 : transalkylation vs. hydroamination

Keywords: Ethylene hydroamination, Amine transalkylation, DFT calculations, Aqueous catalysis

Reactions of diethylamine and

Introduction

Amines are important compounds for the bulk and fine chemical industries. There is great current interest in optimizing the hydroamination reaction, an atom economical process that results in the N-H addition of ammonia or an amine to an unsaturated substrate to produce a higher amine. [START_REF] Brunet | Catalytic Heterofunctionalization[END_REF] Amongst all the hydroamination reactions, the most difficult one is the intermolecular version with non-activated olefins and solutions based on transition metal catalysis are actively pursued. [START_REF] Brunet | Catalytic Heterofunctionalization[END_REF][START_REF] Brunet | [END_REF][3] With regard to industrial applications, short chain aliphatic olefins and especially ethylene constitute the most attractive starting materials. In the last 6 years a number of papers have appeared on the hydroamination of ethylene and other -olefins catalysed in particular by Pt-based systems. These include the addition of substituted anilines to ethylene [4] and 1-hexene [5] catalyzed by PtBr2/n-Bu4PBr (I), including our recent extension to aqueous media (PtBr2/NaBr(aq), I'), [6] the addition of carboxamides to RHC=CH2 (R = H, Me) catalyzed by [PtCl2(H2C=CH2)]2/PPh3 (II) [7] and that of sulfonamides or anilines to more strained olefins catalyzed by (COD)Pt(OTf)2 (III). [8] For each type of catalyst the turnover number (TON) or product yield was shown to increase as the amine basicity decreases and no activity was found when the conjugate acid of the amine has a pKa value above a certain cut-off value. Although comparisons are complicated by the different conditions used for each of these three catalytic systems, I appears the most active one, allowing ethylene hydroamination with 4-MeOC6H4NH2 the most basic amine for which an activity has been reported for a Pt-based catalyst. [4] Attempts to probe more basic amines for system I were not performed, probably because of expectations on the basis of the observed pKa trend. Attempts to use more basic secondary amines were described only for system II, with negative results when using secondary amines. [7] The catalytic system III was shown to be inactive already when using aniline (pKa = 4.63). [8] In recent work, we have explored the details of the PtBr2/Br - catalyzed addition of aniline to ethylene at the experimental and computational levels, by isolating a number of low-energy intermediates and off-loop species and studying their solution equilibria, [9] and by carrying out a computational investigation of the catalytic cycle, [10][11] the main features of which are summarized in Scheme 1. The nucleophilic addition to the coordinated and activated ethylene substrate is more facile for the neutral compound trans-[PtBr2(NHEt2)(C2H4)] (2), and also for the bis(ethylene) complex trans-[PtBr2(C2H4)2] (not shown in Scheme 1),than for the anionic resting state [PtBr3(C2H4)] -(1), but the only zwitterionic intermediate capable to complete the catalytic cycle is [Br3Pt (2-) (CH2CH2N (+) H2Ph)] (4), obtainable directly from 1.

Scheme 1. Mechanism of the PtBr2/Br -catalyzed hydroamination of ethylene by aniline.

Since the zwitterionic intermediates invoked for this system, such as 3 and 4, are too high in energy to be experimentally observed, we have carried out model investigations using the more basic and nucleophilic Et2NH, obtaining direct evidence for the generation of the zwitterionic adduct [(Et2HN)Br2Pt (-) CH2CH2N (+) HEt2]. [12] An excess amount of NHEt2 was shown to promote an equilibrium deprotonation of this species, yielding the aminoalkyl derivative [(Et2N)Br2PtCH2CH2NEt2] -, which then decomposes by giving rise to the deposition of Pt 0 and to the generation of other uncharacterized species. This behaviour is similar to that of other complexes obtained by addition of basic amines to Pt-coordinated olefins. [13][14][15] This observation suggests that the catalyst deactivation process is triggered by the deprotonation of the intermediate zwitterionic species and this hypothesis has been confirmed by new catalytic tests: the addition of strong bases (KOH, NHEt2, NEt3, or DBU) to the catalytic medium completely suppresses the production of the PhNHEt product, while the Pt II catalyst is completely converted into metallic platinum. [16] In this report, we present two new principles. The first one, derived from a computational investigation on the catalytic cycle with NHEt2, is that more basic amines are indeed kinetically competent to complete the hydroamination catalytic cycle. Hence, the lack of catalysis is not due to an intrinsic lower catalytic efficiency but must be attributed to the above mentioned catalyst decomposition triggered by deprotonation, which is more favourable for stronger bases. The second one, derived from further catalytic experiments, deals with the discovery of a competitive amine transalkylation process, which is catalyzed by Pt 0 under the hydroamination conditions. We first attempted the ethylene hydroamination by HNEt2 under the same conditions previously used for the hydroamination of ethylene by aniline (PtBr2/nBu4PBr catalyst, I) [4] and also under our recent extension of this system to aqueous media (K2PtCl4/NaBr(H2O) catalyst, I'). [6] These experiments yielded triethylamine in only close to stoichiometric amounts, see Table 1 (runs 1 and 2), while the starting compound conversion was negligible according to NMR. Extensive deposition of metallic Pt, as also reported for the aniline addition experiments, [4][5][6] was observed at the end of the reaction. Surprisingly, ethylamine, the formal product of dehydroamination, was also observed as a coproduct in non negligible amounts. Table 1. Pt-catalyzed hydroamination of C2H4 by Et2NH. [a] run cat. Product (TON) Other products (TON)

Results and Discussion

1 I Et3N (~1) EtNH2 (3) 2 I' Et3N (~2) EtNH2 (5)
[a] Conditions: [Pt] = 0.3% relative to amine; [Br]/Pt = 150; 15 ml of H2O (only for system I'); 25 bar C2H4 at 298 K, 10 h, 150°C.

We shall now first address the catalytic cycle of ethylene hydroamination by NHEt2 through a computational investigation, before analyzing the reason for the formation of the EtNH2 coproduct.

(b) Computational investigation of the hydroamination mechanism with Et2NH.

The energetics of the ethylene hydroamination by Et2NH were analyzed by DFT, according to the established mechanism for the addition of aniline (Scheme 1). [11] In order to directly compare the results of the two system, the calculations were carried out at the same level of theory (see computational details). The addition of Et2NH to C2H4 is exoergic by ∆G°298 = -3.7 kcal/mol (cf. -6.0 for PhNH2) in the gas phase and ∆G°298 CPCM = -6.1 kcal/mol (cf. -8.0 for PhNH2) with inclusion of the solvent effect in aniline. The energetic details of the catalytic cycle are summarized in Figure 1. Beyond the generation of the zwitterion [(Et2HN)Br2Pt (-) -CH2CH2N (+) HEt2] (3'), which was also observed experimentally as already mentioned above, [12] ligand exchange occurs to yield the tribromido derivative [Pt (2-) Br3(CH2CH2N (+) HEt2)] -, 4'. Contrary to the PhNH2/C2H4 cycle, a local minimum could not be located for a 5-coordinate Pt IV hydride complex [PtHBr3(CH2CH2NEt2)] -(5') derived from 4' by intramolecular proton transfer. Every attempt to optimize such a complex led back to 4', consistent with the greater basicity of the N atom relative to the aniline system. However, a transition state was located for transfer of the ammonium proton to the carbon atom, directly transforming 4' into the -complex [PtBr3(η 2 -H-CH2CH2NEt2)] -, 6', from which the Et3N product may be released by exchange with C2H4 to start a new cycle. This proton transfer is however assisted by the Pt atom, as shown by the relative short Pt•••H contact in the transition state (Figure 1). The overall barrier between the resting state and the ratedetermining transition state [17] (34.4 kcal mol -1 ), is not significantly higher than that calculated for the corresponding PhNH2/C2H4 cycle (33.8 kcal mol -1 ). [11] Et2NH coordination to Pt II is more favorable than PhNH2 coordination, thus the resting state of the catalytic cycle in this case, according to the computational study, is the diamine complex trans-PtBr2(NHEt2)2, 7', rather than complex 1. Complex trans-PtBr2(PhNH2)2, 7, on the other hand, was calculated as 3.7 kcal mol -1 less stable than 1. [11] On the basis of this study, systems I and I' would be predicted as equally competent to catalyze the addition of Et2NH and PhNH2 to C2H4. Hence, the reason for their poorer performance for NHEt2 must be attributed to the catalyst decomposition, which is related to the zwitterion deprotonation by the excess base, as confirmed by our recent study. [16] Note that the protonation transfer step is endoergic on the ΔG CPCM scale for both systems according to the previously published studies, but less so for the NHEt2 system (+15.3 kcal/mol for 3'+NHEt2) [12] than for the PhNH2 system (+19.8 kcal/mol for 3+PhNH2). [11] The catalyst deactivation is then probably driven by the irreversibility of the subsequent steps leading to the metal reduction.

(c) Catalysis of transalkylation processes.

Based on these experimental and theoretical premises, we wondered whether catalytic hydroamination with Et2NH could be induced through protection of the zwitterionic complex by the addition of acid. Indeed, replacing H2O/NaBr by HBraq gave marginally better results, with TON = 4 for Et3N, see run 3 (Table 2). However, the formation of EtNH2 (ca. 18 cycles) was also observed. In addition to Et3N, EtNH2, and residual Et2NH, the 1 H and 13 C{ 1 H} NMR monitoring also revealed the formation of a minor amount of EtOH (ca. 1 cycle). The next experiments were run with simplified catalytic systems. Run 4 was carried out in the presence of HBr (same conditions as run 3) without the Pt compound. The result demonstrates the absence of any acid-induced transformation. There are reports warning about the possible action of acids as hydroamination catalysts, [18][19] but they all concern less challenging processes such as intramolecular hydroaminations or the intermolecular addition across activated C=C bonds. Furthermore, the conditions employed here are not really acidic, since the amount of acid is a fraction of that of the amine (150 equiv. vs. 350 equiv), hence the reaction mixture contains a Et2NH2 + /Et2NH buffer. Run 5 was carried out in the absence of acid. These conditions are equivalent to those of run 2 (Table 1), except for the absence of NaBr. The results are also comparable to those of run 2. In fact, an even lower activity was recorded, in agreement with the known activating role of the bromide ions for this catalytic system. [4,6] Further simplification of the system by running the reaction in the absence of ethylene (run 6), unexpectedly gave a large conversion into Et3N and EtNH2. Therefore, no external ethylene is necessary to generate the observed products, proving that these are not resulting from a hydroamination or dehydroamination process. The observed transformation can be rationalized as a simple transalkylation process. Furthermore, comparison of runs 5 and 6 demonstrates that the presence of ethylene inhibits this process.

On the basis of this new evidence, we wondered whether ammonia may also be formed by further transalkylation from the EtNH2 primary product. The 14 N or 15 N analyses of the solution obtained from run 3 did not allow detection of any NH3. However, this result may be a consequence of the small conversion coupled with small sensitivity (for 15 N) or broad lines (for 14 N). An additional experiment was carried out starting from EtNH3 + Cl - rather than from a Et2NH/HBr mixture. K2PtCl4 was used as the catalyst, without the presence of ethylene, using the standard reaction time and temperature. The final mixture indeed revealed the formation of NH3 (NH4 + ) with a resonance at δ -357.6 in the 14 N NMR spectrum, see Figure 2. The spectrum also exhibited a strong resonance at δ -344.4 for the residual EtNH2/EtNH3 + , and a weak one at δ -329.1 for Et2NH/Et2NH2 + . The identity of all these resonances was verified by comparison with genuine samples. A resonance for Et3N/Et3NH + , which should appear around δ -325, is not visible, once again because of the small conversion for this transalkylation process. Transalkylation reactions for amine substrates have already been shown to occur under Pd 0 catalysis. [20][21][22][23][24][25] This intrigued us whether the same phenomenon may also occur in the presence of Pt 0 , because metallic platinum is generated in situ by decomposition of the Pt II system as shown above. Platinum-catalyzed amine translakylation has previously been reported only by Brunet et al., to the best of our knowledge, as a side process during the PtBr2catalyzed heterocyclization of aniline to quinolines in the presence of Bu3N, but the authors could not establish whether the process is catalyzed by Pt II or by the Pt 0 formed during the reaction. [26] Indeed, using Pt 0 in place of K2PtCl4 under otherwise identical conditions gave approximately the same conversion and product distribution of the transalkylation products (run 7, Table 2). The 13 C{ 1 H} NMR spectrum of the final reaction mixture is shown in Figure 3. Once again, the process is inhibited by ethylene (run 8). 2). 1: Et3N, 2: Et2NH, 3: EtNH2 (internal standard: 1,4-dioxane, 1.3 mmol).

The mechanism proposed by Murahashi for the Pd 0 -catalysed transalkylation process involves metal insertion into the α-CH bond with formation of a hydrido iminium intermediate. [21] This pathway is illustrated on the left hand side of Scheme 2 for the related Pt 0 catalyst. The iminium ligand in this intermediate is then attacked by a second amine molecule to generate a coordinated aminoalylammonium ligand, followed by exchange of the proton and the platinum centres and reversal of all steps. Following our recent computational investigation, [10][11]16] however, we can also imagine an alternative pathway involving the reverse steps of the hydroamination reaction, starting with insertion of Pt 0 into a β-CH bond and leading to a Pt 0 -olefin intermediate, followed by hydroamination by a second amine molecule. This pathway is illustrated on the right hand side of Scheme 2. Note, however, that the hydroamination process was shown to occur via Pt II /Pt IV complexes, whereas in this case the Pt 0 /Pt II couple would operate.

To test the possibility of the dehydroamination/hydroamination mechanism, we carried out two additional catalytic experiments, using only triethylamine as starting compound, under conditions identical to those of runs 7 and 8 of Table 2. Clearly, the α-CH bond pathway cannot lead to any reaction, whereas the β-CH bond pathway may result in the generation of Et2NH, with release of ethylene (dehydroamination). Under the conditions of run 8, namely in the presence of ethylene pressure, a minor but non negligible amount (ca. 1 cycle) of diethylamine was produced, whereas 5 cycles of the product were observed under the conditions of run 7 (no added ethylene). While these experiments do not exclude that the α-CH bond activation mechanism is the main pathway when starting from diethylamine, they confirm that the β-CH bond activation mechanism is also taking place under these conditions. The inhibiting effect of the ethylene pressure may be rationalized by a saturation effect of the olefin coordination to the Pt 0 surface, blocking or retarding access of the platinum atoms by the C-H bonds. This rationalization is compatible, however, with both mechanisms. Additional investigations would be necessary to confirm the presence of an α-CH bond pathway for this system, which are however outside of the scope of the present work.

Conclusions

The present study has revealed that the catalytic systems I and I', are not efficiently adding the N-H bond of Et2NH across the C=C bond of ethylene, although a computational investigation suggests that they should be as competent to catalyze this transformation as the addition of the aniline N-H bond. The reason is attributed to rapid catalyst degradation with reduction to metallic platinum, favoured by the deprotonation of the key zwitterionic intermediate (3'), which was shown to form readily by an independent recent study. [12] This deprotonation is favoured by the high basicity of Et2NH.

Parallel studies have also shown that the produced metallic platinum is responsible for a transmetallation reaction, equilibrating Et2NH with Et3N, EtNH2, and even NH3. This appears to be the first report of Pt 0 -catalyzed amine transalkylation, a process that has previously been well established for Pd 0 . This process is inhibited by the olefin, presumably because of a surface saturation effect, blocking its access to the substrate C-H bonds. Among two different mechanistic pathways involving respectively the activation of the amine α-CH and β-CH bonds, the latter was shown to be present by a control experiment, although the former could also play a role as previously suggested for the Pd 0 -catalyzed process. [21]

Experimental Section

General. All solvents were of HPLC grade and were used as received. HNEt2 (Fluka) was distilled and kept under argon in the dark. PtBr2 (Alfa Aesar), K2PtCl4 (Alfa Aesar) and nBu4PBr (Acros Organics) were used as received. Ethylene (purity ≥ 99.5%) was purchased from Air Liquide. Instrumentation. NMR investigations were carried out on Bruker AV400 spectrometer at 298 K operating at 400.1 MHz ( 1 H), 100.6 MHz ( 13 C), 28.9 MHz ( 14 N). The spectra were calibrated with the residual solvent resonance relative to TMS ( 1 H, 13 C) and MeNO2 ( 14 N). D2O was used for lock. Typical procedures for the attempted hydroamination of ethylene by Et2NH. (a) With PtBr2/ n Bu4PBr. The autoclave was charged with PtBr2 (46 mg, 0.13 mmol) and n Bu4PBr (440 mg, 1.3 mmol, 10 equiv), closed, and submitted to argon/vacuum cycles. Et2NH (4.7 mL, 45.5 mmol, 350 equiv) was syringed into the autoclave. Finally, the ethylene pressure was adjusted to 25 bar (ca. 100 mmol, 333 equiv) at RT. The temperature was then raised to 150°C. After 10 h, the autoclave was allowed to cool to room temperature and then slowly vented. An external standard (EtOH or 1,4dioxane, 1.3 mmol) was added to the mixture. The solution was then analyzed by NMR spectroscopy using traces of CDCl3 for instrument lock. (b) with K2PtCl4/HBr. The autoclave was charged with K2PtCl4 (54 mg, 0.13 mmol), closed, and submitted to argon/vacuum cycles. Water (or D2O), then Et2NH (4.7 mL, 45.5 mmol, 350 equiv), then aqueous HBr (gently) if necessary, were syringed into the autoclave and the ethylene pressure was adjusted to 25 bar (ca. 100 mmol, 333 equiv) at RT. The remainder of the experiment was carried out as above, except that D2O was used for the NMR lock. The amount of D2O was the minimum needed to homogenize the system. Typical procedures for the Et2NH transalkylation. The autoclave was charged with K2PtCl4 or Pt black (0.13 mmol), closed, and submitted to argon/vacuum cycles. Et2NH or Et3N (45.5 mmol, 350 equiv) was then added and the ethylene pressure (if needed) was adjusted to 25 bar (ca. 100 mmol, 333 equiv) at RT. The remainder of the experiment was carried out as above. A minor amount of D2O was added for the NMR lock. Computational Details. The DFT calculations were carried out using the same methodology described in detail in our recent studies of the C2H4/PhNH2 reaction, [10][11] namely using the B3LYP functional and the standard 6-31+G* basis set for all atoms except Pt, for which the LANL2TZ(f) basis was used. [27] The calculations included a frequency analysis for all minima and transition states and solvation effects by singlepoint C-PCM [28][29] corrections on the gas-phase optimized geometries, from which the thermodynamic ΔG CPCM values were derived. All new optimized geometries are available in the SI.

Supporting Information (see footnote on the first page of this article): List of optimized Cartesian coordinates for all new geometries (2 pages).

Entry for the

Figure 1 .

 1 Figure 1. G CPCM (aniline) profile in kcal mol -1 for the C2H4 hydroamination by Et2NH on the PtBr2/Br -catalytic system at 298.15 K.

Figure 2 .

 2 Figure 2. 14 N NMR spectrum of the solution obtained from EtNH3 + Cl -in D2O, after 10 h at 150°C in the presence of K2PtCl4 (0.3 %).

Figure 3 .

 3 Figure 3. 13 C{ 1 H} NMR spectrum of the final reaction mixture for run 7 (Table2). 1: Et3N, 2: Et2NH, 3: EtNH2 (internal standard: 1,4-dioxane, 1.3 mmol).

Scheme 2 .

 2 Scheme 2. Two alternative pathways for the Pt 0 -catalyzed transalkylation reaction.

Table 2 .

 2 Additional reactions between C2H4 and Et2NH.[a]

	run	cat.	C2H4 (bars) Et2NH (% conv) Et3N (TON) EtNH2 (TON)
	3 [b] K2PtCl4	25	7	~4	~18
	4 [c]	-	25	0	0	0
	5	K2PtCl4	25	~ 0	~1	0
	6 [d] K2PtCl4	-	52	92	23
	7	Pt 0	-	45	76	18
	8	Pt 0	25	~ 0	~1	0

[a] The experimental conditions are identical to those in Table

1

, except for the nature and amount of catalyst, olefin and additive as indicated. [b] With 150 eq of HBraq. (48%) relative to Pt; a minor amounts of aromatic products (< 1%, resonances in the δ 110-150 range) and EtOH (ca. 1%, resonances at 17.7 and 57.2) were visible by 13 C{ 1 H} NMR. [c] Same conditions as for run 3, without catalyst. [d] Minor amounts (< 2 %) of other products were also detected by 13 C{ 1 H} NMR in the aliphatic region.

Table of Contents Hydroamination vs. transalkylation Pavel A. Dub and Rinaldo Poli*

  …….... Page No. -Page No.

	C	2.513747000 0.449389000	2.426003000	H	4.479459000 1.746570000 -1.607079000
	H	3.294263000 0.058542000 3.088709000	Br	-0.768344000 2.527908000 0.139985000
	H	3.146009000 -0.697867000 0.702766000	Pt	-0.620385000 0.041713000 -0.113132000
	H	2.318076000 1.490354000 2.713172000	Br	-0.354392000 -2.440329000 -0.371164000
	H	1.601550000 -0.131995000 2.588342000	H	2.966313000 -2.707741000 0.828742000
	C	2.990007000 -0.071219000 -2.098985000	C	2.594798000 -0.394482000 -0.924018000
	H	3.355632000 0.230925000 -3.087541000	Br	-3.038931000 -0.220545000 0.419948000
	H	3.802312000 -0.604244000 -1.593790000
	H	2.158509000 -0.767937000 -2.231290000	trans-[PtF2(C2H4)(PhNH2)] 5
	H	1.783057000 1.716163000 -1.893261000	SCF DONE: -685.150356
	H	3.409850000 1.854795000 -1.204839000	78	-1.003000000 0.000556000 -0.094129000
				9	-0.957709000 1.975517000 -0.142570000
	[PtBr3(C2H4NHEt2)] -4		7	Reactions of diethylamine and ethylene 0.722778000 0.013187000 -1.330241000
	Pt II salts and Pt 0 promote different	catalyzed by Pt II or Pt 0 : transalkylation -0.944481000 -1.973790000 -0.152465000 hydroamination process, whereas Pt 0 9
	vs. hydroamination -3.059819000 -0.007446000 0.474548000 promotes transalkylation, a reaction 6 6 -2.267246000 -0.011668000 1.628413000 which is inhibited by the olefin. -2.104365000 3.182480000 2.643258000 reactions for the amine/ethylene combination. Pt II is a catalyst for the SCF DONE: -8126.038077 H 1 -2.082288000 0.911774000 2.172500000
	H	-0.985991000 1.798682000 2.580794000	1	Keywords: Platinum / ethylene -2.076889000 -0.940319000 2.161645000
	C	-1.696418000 2.394545000 1.998264000	1	hydroamination / amine transalkylation -3.494540000 -0.932360000 0.102205000
	H C	-3.329584000 1.008874000 2.319265000 -2.844775000 1.527831000 1.485591000	1 6	-3.498969000 1.981448000 0.006284000 -0.600006000 0.919392000 0.112257000 / DFT calculations / aqueous catalysis
	H	-3.608691000 2.126870000 0.977219000	1	0.649515000 -0.810354000 -1.930303000
	N	-2.378493000 0.478445000 0.517194000	1	0.647749000 0.849823000 -1.911776000
	H	-3.256952000 0.941364000 -1.364721000	6	2.564827000 -1.213044000 -0.248144000
	H	-1.926917000 2.010390000 -0.848656000	6	3.766929000 -1.213218000 0.463163000
	H	-1.337036000 0.244029000 0.690546000	6	4.375866000 -0.007365000 0.824261000
	C	-1.181638000 0.151700000 -1.649473000	6	3.772263000 1.205310000 0.477540000
	H	-0.879370000 0.722426000 -2.537086000	6	2.570265000 1.218740000 -0.233928000
	H	-1.582681000 -0.812310000 -1.988609000	1	2.071748000 -2.146346000 -0.507333000
	H	-2.578626000 -1.541854000 0.016566000	1	4.223840000 -2.159999000 0.737754000
	C	-3.109046000 -0.828566000 0.652034000	1	5.311320000 -0.012698000 1.376722000
	H	-2.949035000 -1.154960000 1.684873000	1	4.233327000 2.146788000 0.763201000
	C	-4.598830000 -0.760950000 0.328873000	1	2.082241000 2.157507000 -0.482909000
	H	-5.136338000 -0.042029000 0.959191000
	H	-5.036263000 -1.750058000 0.508132000	[PtBr3(CH3CH2NEt2)] -5
	H	-4.784598000 -0.507012000 -0.719644000
	Br	-0.083000000 -2.573941000 -0.273127000	SCF DONE: -8126.031875
	Pt	0.458235000 -0.128152000 -0.397091000
	Br	0.962242000 2.315335000 -0.719374000	H	4.284230000 -2.872651000 1.770679000
	H	-1.127794000 2.855983000 1.185313000	H	3.307991000 -1.545087000 2.452857000
	C	-2.252005000 0.970351000 -0.923565000	C	3.683570000 -1.986447000 1.522727000
	Br	2.424404000 -0.403268000 1.275244000	H	5.360966000 -0.641196000 1.407734000
				C	4.536353000 -0.967271000 0.759497000
	[PtBr3(C2H4NHEt2)] -TS		H	5.007241000 -1.457718000 -0.116705000
				N	3.780004000 0.226367000 0.375375000
	SCF DONE: -8126.017479		H	3.445995000 0.215841000 -1.737897000
				H	2.596878000 -1.019592000 -0.813617000
	H	4.431289000 -2.828971000 1.824518000	H	1.062668000 0.578673000 0.166355000
	H	3.042498000 -1.847482000 2.365568000	C	1.644825000 0.884163000 -0.753612000
	C	3.632913000 -2.150041000 1.494035000	H	1.076810000 0.763014000 -1.680932000
	H	4.867582000 -0.387816000 1.532235000	H	1.813687000 1.955089000 -0.607798000
	C	4.236714000 -0.919237000 0.807997000	H	3.894989000 2.290237000 0.142706000
	H	4.911166000 -1.242616000 -0.010943000	C	4.576059000 1.452814000 0.334936000
	N	3.220563000 0.021285000 0.331336000	H	4.982267000 1.617399000 1.342829000
	H	3.265835000 -0.234385000 -1.794269000	C	5.735800000 1.505185000 -0.682885000
	H	2.401956000 -1.468194000 -0.865599000	H	6.478463000 0.720909000 -0.490298000
	H	0.918916000 0.174516000 0.254494000	H	6.247944000 2.474498000 -0.616247000
	C	1.269178000 0.306892000 -1.223532000	H	5.383063000 1.387391000 -1.714151000
	H	0.849623000 -0.122226000 -2.137791000	Br	-1.345096000 2.478063000 0.183747000
	H	1.374329000 1.385166000 -1.347657000	Pt	-0.827127000 0.065005000 -0.083691000
	H	2.736519000 2.046939000 0.218158000	Br	-0.061464000 -2.278182000 -0.445760000
	C	3.623238000 1.429400000 0.395370000	H	2.814669000 -2.316700000 0.943876000
	H	3.924821000 1.629959000 1.431878000	H C	-3.841836000 0.600657000 1.369572000 2.909262000 0.027625000 -0.787382000
	C	4.755759000 1.871992000 -0.553414000	H Br	-3.365738000 -0.110802000 2.915873000 -3.113933000 -0.608494000 0.293189000
	H	5.679058000 1.305058000 -0.377355000	C	-3.002209000 0.159243000 1.917418000
	H	4.979443000 2.935297000 -0.395518000	C	-2.464044000 -1.094874000 1.239493000
	H	-4.542268000 -0.729073000 -2.269058000	H	-1.682458000 -1.557138000 1.847515000
	H	-3.716283000 -2.181533000 -1.687123000	H	-3.265116000 -1.835589000 1.119337000
	H	-4.684750000 -1.193886000 -0.571183000	Pt	0.066123000 0.027448000 -0.029007000
	C	-3.994261000 -1.153677000 -1.420202000	Br	-0.761224000 2.381085000 -0.102415000
	H	-3.039300000 0.714621000 -0.818520000	Br	0.892481000 -2.328617000 0.037704000
	C	-2.762673000 -0.290150000 -1.139577000	N	1.975072000 0.915211000 0.024775000
	H	-2.163997000 -0.176913000 -2.047276000	H	1.729554000 1.845155000 0.380508000
	N	-1.838172000 -0.862951000 -0.109228000	C	2.554710000 1.175992000 -1.338821000
	H	-1.576739000 -1.798488000 -0.437661000	H	3.918729000 0.895746000 0.837516000
	H	-2.227166000 0.922679000 2.021065000	C	2.977765000 0.344084000 0.977302000

Acknowledgments

We are grateful to the CNRS (Centre National de la Recherche Scientifique), the ANR (Agence Nationale de la Recherche, Grant No. NT09_442499), the IUF (Institut Universitaire de France) for financial support, as well as the CINES (Centre Informatique National de l'Enseignement Supérieur) and the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP) for granting free computational time. PD thanks the MENESR (Ministère de l'Éducation Nationale, de l'Enseignement Supérieur et de la Recherche, France) for a Ph.D. fellowship. ____________


Zhigang Xue

Jean-Claude Daran

Yohan Champouret

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

Ligand adducts of bis(acetylacetonato)iron(II): a 1 H NMR study

Keywords: . Iron, acetylacetonate, coordination chemistry, paramagnetic NMR

We report here a thorough 1 H NMR study of Fe(acac)2 solutions in a wide variety of non coordinating and coordinating solvents, as well as the interaction of this complex with Et3N, pyridine, PMe2Ph, and R2PCH2CH2PR2 [R = Ph (dppe), Et (depe)] in C6D6. The study reveals that Fe(acac)2 is readily transformed into Fe(acac)3 in solution under aerobic conditions and that the commercial compound is usually contaminated by significant amounts of Fe(acac)3. The 1 H NMR resonances of Fe(acac)2 are rather solvent-dependent and quite different than those reported in the literature. The compound is unstable in CDCl3 and stable in CD2Cl2, C6D6, CD3CN, acetone-d 6 , DMSO-d 6 , THF-d 8 , and CD3OD. The addition of the above mentioned ligands (L) reveals only one paramagnetically shifted band for each type of acac and L proton, the position of which varies with the L/Fe ratio, consistent with rapid ligand exchange equilibria on the NMR timescale. A fit of the NMR data at high L/Fe ratio allows the calculation of the expected resonances for all protons in the Fe(acac)2L2 molecules. The system with the bidentate depe ligand shows evidence for a slow ligand exchange at low depe/Fe ratios, proposed to involve a species with the cis-chelated mononuclear Fe(acac)2(depe) structure, whereas the fast exchange at higher ratio is proposed to involved a trans-Fe(acac)2(κ 1 -depe)2 complex. Complex Fe(acac)2(dppe) cannot be investigated in solution because of low solubility in non coordinating solvent and because of the poor dppe competition for binding in coordinating solvents. The compound was crystallized and its X-ray structure reveals a 1-dimensional polymeric structure with dppe-bridged Fe centers having the trans-octahedral Fe(acac)2(κ 1 -dppe)2 coordination environment.

Introduction

Nuclear magnetic resonance spectroscopy is one of the most useful characterization tools for molecular chemistry, mostly applied for the analysis of the proton distribution in molecular compounds in solution. [START_REF] Wrackmeyer | Guide to Multinuclear Magnetic Resonance[END_REF][START_REF] Horst | Basic One-and Two-Dimensional NMR Spectroscopy[END_REF] It is also a powerful tool for the study of dynamic phenomena occurring in solution when the rate of chemical exchange between non equivalent sites falls in the same range as the chemical shift difference. [START_REF] Sandström | Dynamic NMR Spectroscopy[END_REF] Most routine analyses are limited to diamagnetic compounds, because the strong dipolar coupling between the magnetic moments associated to the observed nuclear spin and the electronic spin in paramagnetic systems causes very rapid relaxation of the nuclear states and consequent broadening of the resonance signal, sometimes so severe that the peak disappears below the noise level. However, under suitable conditions (rapid relaxation of the electronic spin system, generally restricted to spin systems different from the doublet state, S = ½) the relaxation of the nuclear state is sufficiently slow to yield visible signals, albeit characterized by a large paramagnetic shift. Thus, informative spectra can be obtained when opening the data collection window to a much wider frequency range relative to diamagnetic compounds. [START_REF] Mar | NMR of paramagnetic molecules. Principles and Applications[END_REF] For instance, [START_REF] Wrackmeyer | Guide to Multinuclear Magnetic Resonance[END_REF] H NMR resonances, usually restricted within the 0-15 ppm range for diamagnetic compounds, may be spread over several hundreds of ppm for paramagnetic compounds. The 1 H NMR investigation of paramagnetic molecules in solution is now a very powerful tool, [START_REF] Bertini | [END_REF] especially for the investigation of the coordination sphere of paramagnetic metals and metal clusters in proteins. 6 NMR is also a preferred technique for the investigation of ligand exchange processes for paramagnetic complexes. 7,8 Yet, the technique is seldom employed for the investigation of the structure and speciation of simple paramagnetic coordination compounds. [START_REF] Crans | Applications of paramagnetic NMR spectroscopy for monitoring transition metal complex stoichiometry and speciation[END_REF] Certain simple complexes that have attracted considerable attention many decades ago in terms of their basic coordination chemistry have never been investigated by [START_REF] Wrackmeyer | Guide to Multinuclear Magnetic Resonance[END_REF] H NMR, even though they exhibit sufficiently resolved resonances, or the investigation has not been carried out in sufficient detail. This is the case for Fe(acac)2.

Compound Fe(acac)2 has been known for a long time, the typical synthetic procedure being the reaction of aqueous FeCl2 with acetylacetone in the presence of a base. [START_REF] Emmert | [END_REF] It is commercially available and inexpensive. The study of its structure in the solid state and in solutions of coordinating solvents as well as non coordinating ones has attracted considerable attention in the 1960's and 70's. The compound crystallizes in the form of a tetramer with two O-bridged dinuclear units that are further held together by a rather long Fe-C bond between the inner Fe atom of one unit with one acetylacetonate central C atom of the adjacent one, as shown in Chart 1 (I). 11 In the presence of neutral donor molecules, which may also be weakly binding solvent molecules, mononuclear bis-ligand adducts are formed. The bis-aqua derivative with a trans geometry (II) has been crystallographically chartacterized, 12 and other derivatives with the same geometry have been reported for related bis(βdiketonato) compounds Fe(RCOCHCOR)2 with R = CF3 (L = THF, 13 o-diaminobenzene, 14 and the cyano function the metalloligand CpFe(dppe)CN 15 ), Ph (L = Et2O) 16 and tBu (L = 1,4-dioxane) 16 .

Derivatives with a cis geometry (III) have also been crystallographically characterized in the presence of chelating ligands, such as Fe(CF3COCHCF3)2(L2) with L2 = Me2NCH2CH2NMe2 17 and 2-(2-pyridyl)-4,6-bis(trifluoromethyl)pyrimidine. 18 Chart 1. Structures observed for Fe(acac)2 and its ligand adducts.

From the above data, it is clear that ligand binding to Fe(acac)2 is not very strong, particularly with weak donors. When this compound is dissolved in a coordinating solvent, it is then possible to imagine that solvent-dependent equilibria between different species may be established. The solution behavior of Fe(acac)2 has been studied by a variety of methods. Cryoscopic and ebullioscopic measurements indicate that the compound is monomeric in dilute benzene solutions and a tetrahedral structure was deduced from powder X-ray photographs. 19 The titration of a dilute benzene solution with pyridine, monitored by UV-visible spectroscopy, was shown to lead successively to three different species, formulated as [Fe(acac)2]2py, [Fe(acac)2py]2 and Fe(acac)2py2. 20 Possible structures for the dinuclear species are IV and V (Chart 1), identical to those of cobalt and nickel analogues. 21 There is only one report of the 1 H NMR properties of Fe(acac)2, in the absence of donor molecules (C6D6 solution), according to which the compound exhibits two resonances at δ 0.8 for the CH and 17.5 for the CH3 protons. 22 In this contribution, we report a more detailed 1 H NMR study of Fe(acac)2 in the presence of a variety of donor ligands including phosphines, and the X-ray structure of the adduct with the bidentate diphosphine ligand bis(diphenylphosphino)ethane (dppe). The results highlight the power of 1 H NMR for assessing the behavior of Fe(acac)2 in solution and the relative binding strength of different ligands.

The study has also unexpectedly revealed that commercially available Fe(acac)2 is generally contaminated by a significant amount of Fe(acac)3.

Experimental Section

General. All solvents used in the reactions were distilled under argon. The NMR solvents were purchased from Euriso-top as single vials packaged under argon and used as received. The NMR spectra were recorded on Bruker ARX250 or DPX300 instruments. Chemical shifts are expressed in ppm downfield from Me4Si. All spectra were recorded at a temperature of 298 K, which was maintained in the NMR probe by an automatic regulation device and was regularly checked by the 4% methanol and 80% glycol 1 H NMR methods; the observed maximum temperature fluctuation in the probe was ±2 K.

The Fe(acac)2 material used for this study was either purchased from Aldrich or synthesized from FeCl2 according to the published procedure, [START_REF] Emmert | [END_REF] working under the most rigorous exclusion of air using Schlenk line techniques. The white anhydrous FeCl2 precursor was prepared immediately prior to the synthetic procedure by refluxing anhydrous FeCl3 and metallic Fe in dry THF. 23 All the 1 H NMR investigations of the ligand addition equilibria were carried out using the commercial Fe(acac)2, since the Fe(acac)3 impurity present in this sample (see Results and Discussion) was found to be completely inert toward the addition of any of the ligands used. The Fe(acac)2 concentration was calculated taking into account the amount of Fe(acac)3 impurity (estimated from the integration of the CH3 resonances of the two compounds).

Synthesis of Fe(acac)2(dppe).

Fe(acac)2 (200 mg, 0.787 mmol) and dppe (376 mg, 0.944 mmol) were dissolved in 20 mL of CH2Cl2.

Stirring at room temperature produced a golden yellow precipitate within a few minutes. After 2 h, the solid was collected by filtration and dried (260 mg, 50.6 %). The solid was dissolved in 10 mL of THF and the resulting solution layered with 50 mL of pentane. Black crystals (150 mg) were obtained by diffusion overnight.

X-ray crystallography.

A single crystal was mounted under inert perfluoropolyether on the tip of a cryoloop and cooled in the cryostream of an Oxford-Diffraction XCALIBUR SAPPHIRE-I CCD diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structure was solved by direct methods (SIR97) 24 and refined by least-squares procedures on F 2 using SHELXL-97. 25 All H atoms attached to carbon were introduced in idealized positions and treated as riding on their parent atoms in the calculations. The drawing of the molecule was realized with the help of ORTEP3. [START_REF] Burnett | Report ORNL-6895[END_REF][START_REF] Farrugia | [END_REF] Crystal data and refinement parameters are shown in Table 1. Our own synthetic efforts following the original procedure 10 confirmed the facile contamination of Fe(acac)2 by Fe(acac)3, yielding a sample very much like that obtained from commercial sources. A major obstacle is the use of FeCl2 that may be contaminated by Fe III impurities. The use of a modified synthetic procedure 19 that makes use of sodium dithionite to reduce the Fe 3+ impurities to Fe 2+ indeed yielded a drastic reduction of the Fe(acac)3 content, but a minor band at δ ca. 21 due to this impurity persisted. The cleanest Fe(acac)2 sample, in our hands, was obtained by the standard procedure 10 when carried out under the most rigorous exclusion of air and from an anhydrous FeCl2 sample generated in situ from anhydrous FeCl3 and metallic Fe in boiling dry THF, a procedure also used by Wilkinson for an improved synthesis of ferrocene. 23 We have not found suitable conditions for efficient removal of the Fe(acac)3 impurity from Fe(acac)2 by recrystallization. The material obtained using this procedure shows the spectrum illustrated in In addition to a warning about the purity of commercially available Fe(acac)2, the above observations lead to two conclusions. The first one is that, under any circumstance, all Me and CH protons are equivalent, which disagrees with the known solid state structure 11 but agrees with the solution molecular weight study that indicates a mononuclear formulation in dilute solution. 19 Clearly, if aggregation occurs in solution, it must be weak and leading to rapid dynamic averaging of all chemically non equivalent positions. The second conclusion is that our observed 1 H NMR spectrum for Fe(acac)2 in C6D6 is dramatically different from that reported in the literature (see Introduction), whereas those of Fe(acac)3 are in relatively good agreement (the spectra of the two compounds were reported in the same contribution). 22 The behavior of Fe(acac)2 in CDCl3 deserved additional discussion. As seen in Figure 1, the resonances of Fe(acac)2 are not present, whereas those of Fe(acac)3, identical to those of the genuine sample (Figure 2) remain. On the other hand, a new relatively strong resonance is observed at ca. δ ca.

1.9. This resonance is obtained reproducibly, but its line width appears quite concentration dependent (broader at lower concentrations). Hence, a chemical reaction completely consuming Fe(acac)2 has occurred. One possibility that comes to mind is oxidation to FeCl(acac)2, a known compound 28 (note that chloroform is a well known Cl atom donor), according to equation 2. Dichloromethane is less prone to Cl atom transfer because of its stronger C-Cl bonds 29 and indeed Fe(acac)2 is stable in this solvent.

The 1 H NMR spectrum for compound FeCl(acac)2 has apparently not been reported, therefore we have prepared it by one of the reported methods, 30 which consists of the comproportionation of Fe(acac)3 and anhydrous FeCl3 in a 2:1 ratio. However, the 1 H NMR spectrum of this compound in CDCl3 shows a strong and broad resonance at δ 16.4, rather than at δ ca. 1.9, and a smaller broad resonance at δ -38.9.

These resonance are rather close to those of Fe(acac)3 and it is thus possible that FeCl(acac)2 is present, together with the Fe(acac)3 impurity, in the spectrum shown in Figure 1. However, this compound does not account for the observed band at δ 1.9. We have also considered the possibility that FeCl(acac)2, once formed, further reacts with Fe(acac)2 to yield a mixed-valence adduct (equation 3). However, an independent test shows that the two compounds do not react with each other in dichloromethane.

Therefore, the nature of this product remains for the moment undetermined, but it is most definitely resulting from a reaction between Fe(acac)2 and chloroform.

Fe(acac)2 + CHCl3 FeCl(acac)2 + CHCl2 • (2)
FeCl(acac)2 + Fe(acac)2 Fe2Cl(acac)4

(b) Addition of neutral ligands to Fe(acac)2

The coordination chemistry of Fe(acac)2 with respect to a few neutral ligands (Et3N, pyridine, and the phosphine ligands PhPMe2, Ph2PCH2CH2PPh2,and Et2PCH2CH2PEt2) was investigated by means of Pyridine also binds the compound reversibly with rapid ligand exchange processes. As shown in Figure 6, the pyridine resonances are very much shifted from their normal position, all in the downfield direction (positive spin density on all aromatic protons). The assignment is based on the relative intensity (2:2:1 for o,m and p), the broader and more paramagnetically shifted resonance being attributed to the ortho protons on the reasonable ground that its greater proximity to the paramagnetic center allows the transmission of a higher spin density. The same hypothesis was previously made for the related Co(acac)2(py)x adducts, 32 and agrees with the assignments made on the basis of the comparison with picoline derivatives. 33 The resonances of the acac protons experience a more dramatic change relative to the previously discussed addition of NEt3. Unfortunately, the methyne resonance is no longer discernible beyond a py/Fe ratio of 5 because of its large band width, of a small relative intensity and of its overlap with the stronger pyridine bands. The more clearly visible acac CH3 resonance exhibits (like the methyne resonance) a significant upfield shift, up to a py/Fe ratio of 3 and then essentially no more change. This is an indicator of the stronger binding ability of pyridine relative to NEt3, a result that parallels that of the related study with Co(acac)2. 32 The observation of relatively weak binding of Fe(acac)2 by a relatively strong field ligand such as PMe2Ph encouraged us explore the coordination behavior of a phosphine ligand with a greater electron donating power and a bidentate chelating coordination mode, namely Et2PCH2CH2PEt2 (depe), hoping to obtain a less labile adduct. Contrary to the complexes containing monodentate ligands, which presumably preferentially generate symmetrical trans adducts, a mononuclear adduct with a short bite chelating ligand such as depe must necessarily adopt a cis configuration. In spite of the stronger anticipated binding properties, however, this ligand also leads to dynamic properties similar to those of the above examined monodentate ligands, as shown in Figure 8. Note in particular the continuous shift of the acac resonances up to depe/Fe ratios greater than 10. The paramagnetic shifts experienced by the phosphine α-H nuclei are quite similar to that experienced by the Me protons in the PMe2Ph adduct (cf. with Figure 7) whereas the β-H nuclei are much less shifted. As pointed out above for the PMe2Ph adduct, the pattern of chemical shifts for the Et2PCH2CH2PEt2 ligand in the Fe(acac)2 adduct is also very similar to that reported for the tetrahedral complex Fe(Se-2,6-C6H3iPr2)2(Et2PCH2CH2PEt2). 34 A specific point of interest is the observation of a single resonance for the methylene protons of the Et substituent, whereas two are observed for complex Fe(Se-2,6-C6H3iPr2)2(Et2PCH2CH2PEt2) as expected because these methylene protons become inequivalent (diastereotopic) upon coordination. This is in perfect agreement with the observed fast exchange between free and coordinated Et2PCH2CH2PEt2 in the Fe(acac)2 complex, whereas a rigid structure is obtained for the tetrahedral bis(selenolate) compound. A curious phenomenon is observed for these spectra at low depe/Fe ratios in the region of the depe Me protons (near 0 ppm), see Figure 9. If the complex were fully dynamic, only one resonance should be observed for each type of proton. However, the Me region of the depe ligand is characterized by several resonances, the most upfield shifted of which changes position, moving downfield, and increases in intensity as the depe/Fe ratio is increased, whereas the chemical shift and relative intensity of the other ones appear to be less sensitive to the depe/Fe ratio. For greater than 1 equiv of depe per Fe, only one resonance remains discernible. We tentatively interpret this phenomenon as indicating a slower chemical exchange between free phosphine (responsible for the resonances at δ 3-1) and a ciscoordinated chelating phosphine. This slower exchange would be a consequence of the chelate effect on the rate of chemical exchange. On the other hand, when the depe/Fe ratio increases, additional phosphine may coordinate and drive the system to the more favorable trans coordination, leading to fast exchange. This state of affairs is summarized in Scheme 1 (the 5-coordinate intermediate may also adopt a dinuclear acac-bridged structure, as shown in Chart 1). Note that, contrary to the methyl protons, the methylene protons of the depe ethyl groups and backbone in the far downfield region show evidence for only one resonance for each different set. This may result from the much larger broadening of these resonances, not allowing the detection of any fine structure for the chemically inequivalent protons in the cis-chelated structure. The above proposition cannot be proven in the absence of isolated samples of the 1:1 and 1:2 adducts.

However, supporting evidence comes from the result of a reaction with the structurally related bis(diphenylphoshino)ethane ligand, dppe. An attempt to carry out a 1 H NMR monitoring of this reaction, by analogy with those described above, was hampered by the negligible solubility of the reaction product in C6D6 and in CD2Cl2. An analogous study could be carried out in d 8 -THF, but this did not reveal any interaction between dppe and the Fe complex (the signals of dppe were observed at their regular diamagnetic position), probably because the THF solvent has a stronger binding capacity than dppe. The Fe(acac)2(dppe) adduct could be crystallized from a THF solution by diffusion of pentane, affording suitable single crystals for an X-ray analysis. Note that this compound had previously been reported and proposed to adopt a mononuclear cis-chelated structure. 35 On the other hand, our crystallographic study reveals a 1-dimensional polymer, see Figure 10. The coordination geometry of each Fe center is trans-octahedral, as typically found in bis-adducts of bis(β-diketonato)iron(II) compounds. [12][13][14][15][16] This structure is the first one for this family of bis-adducts with a P-atom donor ligand. The Fe atom and the center of the C-C bond in the PCH2CH2P moiety sit on inversion centers. Consequently, all trans X-Fe-X angles are 180°. The symmetry-equivalent Fe-P distances of 2.6496(4) Å are rather long. The only other crystallographically characterized Fe II complex with a P2O4 coordination sphere appears to be Fe[P(CH2CH2OEt)2(CH2Ph)]2(O3SCF3)2, with Fe-P distances of 2.583(2) Å. 36 The FeX2P4 coordination geometry (X = halogen atom) is more represented, with 35 hits in the Cambridge crystallographic database. These structures are nicely divided into two sets as a function of the spin state, with 27 low spin (diamagnetic) compounds clustering around an average Fe-P distance of 2.26 Å with a standard deviation of 0.04 Å, while 8 high spin (S = 2) compounds yield an average Fe-P distance of 2.62(5) Å. Most interesting are the two spin-crossover complexes FeX2(Ph2PCH=CHPPh2)2 (X = Cl, Br), for which the structure was recorded both above and below the transition temperature, yielding Fe-P distances of 2.312(8) and 2.289(9) Å for the low spin structure at 130 K, 2.592(2) and 2.576(2) Å for the high spin one at 295 K (X = Cl); 37 2.326(4) and 2.294(4) Å for the low spin structure at 149 K, 2.586(3) and 2.575(3) Å for the high spin one at 193 K (X = Br). 38 Thus, the Fe-P distance in an octahedral Fe II complex may be taken as a direct gauge of the spin state and the long distance observed for Fe(acac)2(dppe) is in good agreement with its paramagnetism. It is interesting to note that complex FeCl2(depe)2 was shown to have short Fe-P distances consistent with diamagnetism in the solid state, although a small population of the high-spin state occurs in solution. 39 Compound Fe(acac)2(depe) investigated in the present contribution, on the other hand, is clearly paramagnetic.

Discussion

The NMR data collected during the present study clearly indicate that, for all tested neutral ligands except perhaps for depe at low ligand/Fe ratio, there is a very rapid exchange averaging each type of proton in the acac ligand (between all Fe(acac)2Lx complexes) and in the neutral ligand (between all Fe(acac)2Lx complexes and residual free L). Given our knowledge of the total metal and ligand concentrations and a model for the number and stoichiometry of the complexes present in solution, the data are amenable to a partial quantitative analysis. The major problem is the ratio between the number of parameters to be optimized (equilibrium constants and chemical shifts for each species) and the number of available data (observed chemical shifts). The observed chemical shifts δH,obs for each different type of proton are averages of the chemical shifts of the same proton in each species, δH,i, weighted by the population xi of the species (equation 4, where the sum is extended over all species containing the observed proton). For a mixture of all species I-V shown in Chart 1 plus the free ligand (5 species for each type of proton and 4 independent equilibria), 9 independent parameters are needed.

We may perhaps exclude species III from consideration, given that the trans isomer II is shown to be preferred, except for the chelating ligands at small L/Fe ratios (see Results). This reduces the number of independent parameters to 7 for each type of proton. Even though the equilibrium constants are common for all different types of protons within the same ligand system, a global solution cannot be obtained with the small amount of available data points.

δH,obs = Σi(xiδH,i)

Restricting ourselves to the data points obtained with [L] ≥ 5 (or ≥ 2 for the more strongly binding pyridine), this analysis gives excellent fits and values of δH,L in excellent agreement with the known spectra of the free ligands. The δH,compl values for the Fe(acac)2L2 complexes resulting from these analyses are reported in This type of analysis, leading to the extrapolation of paramagnetic shifts for rapidly exchanging ligands on paramagnetic complexes, does not appear to be frequently carried out, the majority of NMR studies for paramagnetic substances being limited to stable complexes with firmly bonded ligands. [START_REF] Crans | Applications of paramagnetic NMR spectroscopy for monitoring transition metal complex stoichiometry and speciation[END_REF][40][41][42][43][44][45][46][47][48][49] For this reason, the values reported in Table 2 cannot be compared with literature precedents, because high-spin Fe(II) complexes that contain the same ligands have either not been investigated by [START_REF] Wrackmeyer | Guide to Multinuclear Magnetic Resonance[END_REF] ligand. The effect of L bonding on the spin density transmitted to the acac ligands is also interesting.

The effect is greater, as may be expected, on the CH resonance than for the CH3 resonance, given that the former is directly linked to the pseudo-aromatic 6-membered cycle where π transmission of the spin density is possible. L binding greatly decreases the positive spin density on the CH proton whereas it slightly increases the negative spin density on the CH3 protons. The effect is less important for NEt3, intermediate for pyridine, and greater for the phosphine ligands. This effect does not seem to correlate with the ligand binding strength, since pyridine appears to be the more strongly binding of the four ligands as discussed above. However, the effect may correlate with the transmission of electron density from L to Fe.

Another interesting observation, revealed by comparing the data in Table 2 with the plots of each δH,obs in the whole L/Fe range (Figures 3, 5 greater paramagnetic shift than those extrapolated for pure Fe(acac)2L2. However, this is essentially never the case. For instance, the PMe resonance in Fe(acac)2(PMe2Ph)2 is calculated as δ 22.7, whereas this resonance is shifted up to δ 55.8 for a PMe2Ph/Fe ratio of 0.2 (Figure 7). It can be concluded that a complex with a L/Fe ratio of 1 (like V in Chart 1) or 0.5 (like IV in Chart 1), or both, is characterized by greater paramagnetic shifts for each δH than II. This appears reasonable, since the spin density in II is transmitted from the metal to two L molecule and the overall effect on each should be smaller. As mentioned above, the number of data points for each resonance was too small for a more detailed analysis according to equation 4. However, this NMR titration method has the potential to provide fuller speciation information, if it is so desired, by carefully collecting a greater number of data points. We decided not to pursue such detailed study, but have outlined here the procedure allowing this to be done.

Conclusion

We have applied for the first time the powerful 1 H NMR spectroscopic analysis to the investigation of the coordination chemistry of a well known compound, Fe(acac)2. The study has not only corrected the erroneous assignment of the Fe(acac)2 [START_REF] Wrackmeyer | Guide to Multinuclear Magnetic Resonance[END_REF] H NMR resonances in an older report and highlighted the possible contamination of commercial Fe(acac)2 by Fe(acac)3, but also unveiled previously unappreciated features of the coordination chemistry of this compound with a variety of N-based and Pbased ligands. The presence of rapid ligand exchange phenomena for paramagnetic substances is particularly amenable to investigation by the NMR technique and a more general application of this approach merits consideration for the analysis of paramagnetic coordination compounds.

Results (a) Solution properties of Fe(acac)2 in various solvents 1 H

 1 NMR spectra of a commercially available sample of Fe(acac)2 in a variety of different solvents are shown in Figure 1. With the exception of CDCl3 (to be discussed further down), three broad resonances are observable, a first one (strongly solvent dependent) in the δ 50 -25 range, a second one (solvent independent) at δ 21, and a third one (slightly solvent dependent) at δ -25 --31. The possibility that these resonances result from the non equivalence of protons in a structure as observed in the solid state (I in Chart 1) is discarded by the observed behavior in the presence of donor molecules, known to form symmetric mononuclear adducts. In this context, the solvent independent resonance at δ 21 is puzzling. Note that the latter resonance becomes the major one observed in CDCl3, accompanied by another strong and broad resonance at δ 2.0 which is absent in the other solvents, and by a much weaker and broad resonance at ca. δ -29, whereas the resonances in the δ 50 -25 (weak) and -25 --31 (strong) ranges seen in the other solvents are absent in CDCl3.

Figure 1 . 1 H

 11 Figure 1. 1 H NMR spectra of commercial Fe(acac)2 in a variety of solvents. The starred resonances belong to the solvent.

Figure 2 . 1 H

 21 Figure 2. 1 H NMR spectra of Fe(acac)3 is a variety of solvents. The starred resonances belong to the solvent and the other sharp resonances in the δ 0-5 range are attributed to minor diamagnetic impurities.

Figure 3 .Figure 3 . 1 H

 331 Figure 3. 1 H NMR spectrum of 'clean' Fe(acac)2 in C6D6. Note the small resonance at δ ca. 20, due to a very small residual amount of Fe(acac)3.

the 1 H

 1 NMR technique. We start our analysis with the study of the NEt3 addition to Fe(acac)2 in C6D6, shown in Figure4. Binding of NEt3 to the Fe center is indicated by the paramagnetic shift experienced by the triethylamine methylene and methyl resonances at low NEt3/Fe ratios. The shift is of opposite sign for the methylene (positive spin density) and methyl (negative spin density) resonances. Even for the maximum ratio of 30:1 used in the study, the resonance of the NEt3 protons has not yet reached the typical value of the free ligand in a diamagnetic environment (e.g. δ 3.09 instead of 2.40 for the methylene resonance)31 while the broadening caused by the exchange with the paramagnetic center is still sufficient to hide the typical H-H coupling, see Figure5. The observation of a single resonance for each type of acac proton and also for each type of NEt3 proton signals the presence of chemical exchange between Fe(acac)2 and all its possible Fe(acac)2Lx adducts, on one side, and between the Fe(acac)2Lx adducts and free L, on the other side, at the fast exchange limit on the NMR timescale. The stronger paramagnetic shift for the CH2 protons than for the CH3 protons in NEt3 is the result of closer proximity of the former to the paramagnetic center when the ligand is coordinated, with transmission of greater spin density. Concerning the resonances of the acac ligand, both continue to move up to the maximum NEt3/Fe ratio used in the experiment. Both shift initially upfield with the acac CH3 resonance being less affected than the CH resonance. The latter moves slightly back downfield for NEt3/Fe ratios greater than 3, probably because of a solvent effect on the Fe(acac)2(NEt3)2 resonance and not because of changes in the equilibrium between the different Fe(acac)2(NEt3)x species.

Figure 4 . 1 H

 41 Figure 4. 1 H NMR chemical shifts observed for a solution of Fe(acac)2 in C6D6 in the presence of variable amounts of NEt3. [Fe(acac)2] = 8•10 -2 M.

Figure 5 .

 5 Figure 5. Excerpts in three different regions of the 1 H NMR spectra measured for a solution of Fe(acac)2 in C6D6 in the presence of variable amounts of NEt3. [Fe(acac)2] = 8•10 -2 M. The NEt3/Fe ratios on going from bottom to top are: 0, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 5, 10, 20, 30. The starred resonance belongs to the solvent.

Figure 6 . 1 H

 61 Figure 6. 1 H NMR chemical shifts observed for a solution of Fe(acac)2 in C6D6 in the presence of variable amounts of pyridine. [Fe(acac)2] = 8•10 -2 M.

Figure 7 . 1 H

 71 Figure 7. 1 H NMR chemical shifts observed for a solution of Fe(acac)2 in C6D6 in the presence of variable amounts of PMe2Ph. [Fe(acac)2] = 8•10 -2 M.

Figure 8 . 1 H

 81 Figure 8. 1 H NMR chemical shifts observed for a solution of Fe(acac)2 in C6D6 in the presence of variable amounts of Et2PCH2CH2PEt2. [Fe(acac)2] = 8•10 -2 M.

Figure 9 . 8 . 1 .

 981 Figure 9. Representative regions of the 1 H NMR spectra of Fe(acac)2 + depe in C6D6. [Fe(acac)2] = 8•10 -2 M and depe/Fe = 0.2, 0.4, 0.6 and 0.8.

Figure 10 .

 10 Figure 10. An ORTEP view of a fragment of the 1-dimensional infinite chain observed for compound Fe(acac)2(dppe). The thermal ellipsoids are shown at the 50% probability level and the H atoms as well as the phenyl rings have been removed for clarity. Relevant distances (Å) and angles (°): Fe-P, 2.6496(4); Fe-O1, 2.0439(10); Fe-O2, 2.0617(10); P-Fe-O1, 90.72(3); P-Fe-O2, 92.19(3); P-Fe-O1 i , 89.28(3); P-Fe-O2 i , 87.81(3); O1-Fe-O2, 86.49(4); O1-Fe-O2 i , 93.51(4). Symmetry transformations used to generate equivalent atoms: (i) -x+1,-y+1,-z+1; (ii) -x,-y+1,-z+1; (iii) 1+x, y, z.

A

  relatively simple analysis is possible limited to the L resonances in the presence of a large ligand excess. Under these conditions, the bis-adduct should be by far the dominant species amongst all Fe complexes, hence to a first approximation [Fe(acac)2L2] ~ total Fe concentration. This reduces equation 4 to equation 5, where there are no equilibrium constants and only two adjustable chemical shift parameters for the [Fe(acac)2L2] complex and the free ligand L. The molar fractions xcompl and xL are related to the ligand distribution and are known from the L/Fe ratio. δH,obs (ligand protons) = xcomplδH,compl + xLδH,L

  H NMR or their investigation does not consider the possible effect of reversible ligand dissociation/exchange on the NMR chemical shift. Interesting observations are the positive spin density transmitted to all aromatic protons of the coordinated pyridine, for which direct delocalization via ligand-to-metal and metal-to-ligand π interactions is possible, whereas alternating negative and positive spin density is found for the o-, mand paromatic protons of the PMe2Ph ligand, for which the spin density is presumably transmitted mainly through a σ polarization scheme. The positive spin density on the aliphatic α-H atoms is greater and nearly equivalent in all P-atom donors (δ 22.7 in PMe2Ph, 25.4 for the backbone and 17.4 for the Et methylene protons in depe) than in NEt3 (δ 10.1). The aliphatic β-H atoms in the Et substituents display a much smaller and negative shift, this time more important for the NEt3

  , 6 and 7), concerns the need to invoke the presence of intermediate complexes Fe(acac)2Lx with x < 2. If the species present under all circumstances were just Fe(acac)2 and Fe(acac)2L2, then the δH,obs values for the ligand resonances could never experience a

Table 1 .

 1 Crystal data and structure refinement for Fe(acac)2(dppe).

	Empirical formula	C36 H38 Fe O4 P2

Table 2 .

 2 Plots showing the data fits and the extrapolated δH,L values are available in the Supporting Information. The table also reports the values of the acetylacetonate ligands in the same Fe(acac)2L2 complexes, from extrapolations of the observed δH,obs to infinite L/Fe ratios, as well as the reference resonances of ligand-free Fe(acac)2.

Table 2 .

 2 Calculated chemical shifts (δ) for complexes Fe(acac)2L2 in C6D6 solution at 298±2 K.

	Proton type	L

Acknowledgement. We are grateful to the Centre National de la Recherche Scientifique (CNRS), the Institut Universitaire de France (IUF), the European Commission Marie Curie IIF program (project METMED-CRP, grant PIIF-GA-2009-235249) and the Agence National de la Recherche (project OMRP, grant ANR 2010 BLANC 7101) for support of this work.

Supporting Information Available. Fitting of the chemical shift data for ligand resonances in the Fe(acac)2 + L systems (L = Et3N, pyridine, PMe2Ph, depe) at high L/Fe ratios (2 pages).

Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 833545. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).