Yohan Champouret

K Cory

Cory Macleod

Ulrich Baisch

Brian O Patrick

Kevin M Smith

Rinaldo Poli

K Cory Macleod

Cyclopentadienyl Chromium β-Diketiminate Complexes: Initiators, Ligand Steric Effects, and Deactivation Processes in the Controlled Radical Polymerization of Vinyl Acetate

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

Introduction

The controlled polymerization of functionalized monomers like vinyl acetate continues to pose a formidable challenge to transition metal chemists. Early metal catalysts that effectively polymerize non-polar monomers by an insertion mechanism are incompatible with vinyl ester substrates. 1 More functional group tolerant late metal catalysts experience problems for copolymerization of vinyl acetate and ethylene due to low π-bonding affinity, chelate formation, and β-acetate elimination. [2][3][4] Vinyl acetate is also a difficult monomer for controlled radical polymerization methods due to the relatively high reactivity of the propagating radical species compared to more tractable substrates such as styrene or methyl acrylate. 5 Organometallic radical polymerization (OMRP) provides a new route to poly(vinylacetate). In the OMRP mechanism, unwanted bimolecular radical coupling reactions are effectively prevented by reversible formation and homolytic cleavage of metal-alkyl bonds. 6 Cobalt-mediated radical polymerization is the most well explored system for OMRP of vinyl acetate. The mechanistic details of these polymerizations are rather complex, with a range of termination and chain transfer steps playing a role dictated by the degree of solvent coordination and the spin state of the Co(II) and Co(III) intermediate species. 7 In principle, well-defined high spin Cr II and Cr III complexes could serve as a simpler system for preparing poly(vinylacetate) via OMRP. Chromium(II) species such as aqueous [Cr(H 2 O) 6 ] 2+ or CrCl 2 in coordinating aprotic solvents have long been known to effectively trap organic radicals at close to diffusion controlled rates to form Cr III -alkyl intermediates. 8 This fundamental reactivity mode is the key step in the chromium-mediated coupling of organic halides and aldehydes. 9,10 The homolytic bond dissociation energy (BDE) of a Cr III -alkyl bond is typically greater than that of a Co III -alkyl. Homolytic transfer of alkyl groups from Co III -R to Cr II is used in the Takai-Utimoto reaction where catalytic amounts of B 12 or cobalt phthalocyanine are used to activate alkyl halides and transfer the organic radical from Co to Cr prior to coupling with aldehydes. 11 While the trapping of organic radicals with Cr II to form octahedral Cr III organometallic species is usually considered to be rapid and irreversible, the reaction can be reversible for secondary or tertiary radicals due to the lowering of the Cr-R BDE through adverse steric interactions. 12,13 CpCr(nacnac) complexes are high spin, well characterized species that do not bind coordinating solvents but have been shown to trap CH 3 radicals. 14,15 CpCr(nacnac)(CH 3 ) complexes can be independently synthesized by the reaction of Grignard reagents with the corresponding Cr III triflate or halide compounds.

Modifying the size of the ortho substituents of the nacnac N-aryl groups should exert a dramatic influence on the Cr III -R BDE for CpCr(nacnac)R complexes, as indicated by preliminary DFT calculation. 16 This desired tunability of the M-R BDE is critical for both exploring structure-activity relationships for these reagents and for developing a class of well-defined organometallic complexes capable of mediating OMRP reactions for a range of activated olefin substrates. 6 In 2008, we communicated our preliminary studies towards OMRP of vinyl acetate using CpCr(nacnac) complexes and V-70. 16 In this paper, we further examine the structure-activity relationships of vinyl acetate polymerization using isolated CpCr(nacnac) compounds with V-70. Although the paramagnetic nature of the Cr II and Cr III complexes makes interpretation of their NMR spectra difficult, the UV-vis spectra of these intensely coloured species proved to be quite informative.

Employing elevated temperatures to accelerate initiation in the CpCr(nacnac)/V-70/vinyl acetate system unexpectedly led to a decrease in the observed rate of polymerization due to formation of an inactive Cr III acetate thermal decomposition product. As an alternative to thermolysis of CpCr(nacnac) with V-70, a well-defined Cr III neopentyl complex was developed as a single-component OMRP reagent. The choice of the neopentyl group was based on the high steric demand of this specific alkyl ligand, which has long been used to provoke unique organometallic reactivity, both of non-radical [17][18][19][20] and of radical type. and 5 16 were prepared according to literature procedures.

Characterizations. followed by the addition of THF (20 mL). The mixture was stirred overnight at room temperature in the absence of light, the solvent was evacuated in vacuo and the residue was extracted with 12 mL of a hexanes/dichloromethane mixture (3:1), filtered through Celite and rinsed with hexanes (3 × 3 mL). The green filtrate was concentrated slightly and cooled to -20˚C to yield 187 mg of black crystals over several days in three crops. Yield: 69%. UV/Vis (hexanes; λ max , nm (ε, M -1 cm -1 )): 411 (9160), 508 (446), 588 (574).

General procedures for the radical polymerization of vinyl acetate. (a)

OMRP procedure: Cr II + V-70. All polymerisations were conducted following the same experimental protocol. A typical experiment is described here as a representative example with complex 4 (Cr:V-70:VAc = 1:0.8:500). All operations were carried out under a protective argon atmosphere. Complex 4 (41 mg, 0.086 mmol, 1 equiv) and V-70 (18.7 mg 0.061 mmol, 0.8 equiv) were introduced in a Schlenk tube, followed by the addition of degassed vinyl acetate (4 mL, 43 mmol, 500 equiv). The Schlenk (b) OMRP procedure: Cr III -Np. The experimental protocol is similar to that described above for the OMRP procedure: Cr II + V-70. As an example, complex 8 (17.5 mg, 0.035 mmol, 1 equiv) and VAc (4 mL, 43 mmol, 1200 equiv) were introduced into a Schlenk flask and stirred at room temperature. At the desired time, a sample was removed from the Schlenk flask and analyzed as described above.

X-ray crystallography.

A single crystal of each compound was mounted on a glass fiber and centered on the optical path of a Bruker X8 APEX II diffractometer with graphite monochromated Mo-Kα radiation. The data were collected at a temperature of -100.0 + 0.1°C in a series of φ and ω scans in 0.50° oscillations. Data were collected and integrated using the Bruker SAINT software package [START_REF] Saint | Bruker Analytical X-ray System[END_REF] and were corrected for absorption effects using the multi-scan technique (SADABS), [START_REF] Sadabs | Bruker Nonius area detector scaling and absorption correction -V2.10[END_REF] and for Lorentz and polarization effects. All structures were solved by direct methods with SIR97. [START_REF] Altomare | [END_REF] and the disorder solvent atoms for 6) were refined anisotropically. All hydrogen atoms were placed in calculated positions but were not refined. All refinements were performed using the SHELXTL crystallographic software package of Bruker-AXS. [START_REF] Sheldrick | version 5.1[END_REF] The molecular drawings were generated by the use of ORTEP-3 [START_REF] Farrugia | [END_REF] and POV-Ray.

Crystal data and structure refinement parameters are collected in the Supporting Information.

Computational details. QM/MM calculations were carried out by use of the Gaussian03 suite of programs 38 with use of the B3LYPfunctional 39 within the DFT methodology for the QM part and of the UFF 40

Scheme 1

The new CpCr II (nacnac) complexes (2-4) were prepared following the literature procedure reported for 1, 14,16 which consists of the one-pot reaction of CrCl 2 (tmeda)

with one equivalent of NaCp, followed by one equivalent of the appropriate nacnacLi salt (Scheme 2). Compounds 1-5 are highly air sensitive. The configuration previously determined for 1. 14 As previously reported for compound 5, 14 solutions of the Cr II complexes 1-4 are green to incident light with a distinctive magenta color to transmitted light. They exhibit two very strong bands at 307-308 nm and 427-430 nm, comparable to those observed for three coordinate Cr II nacnac alkyl complexes. 42 Compounds 1-5 also have a less intense single band at 558-576 nm.

Scheme 2

Compounds 2 and 3 have also been characterized by single crystal X-ray diffraction. The geometry of the two Cr II compounds (Figure 1) can be described as a "two-legged piano stool", with the Cp ring centroid lying close to the plane defined by the Cr1, N1 and N2 or N1* atoms. Relevant bond distances and angles are collected in Table 1. The bonding parameters for compounds 2 and 3 are very similar to those found in previously reported structures for CpCr(nacnac) Cr II complexes. a Angle between the CNT-Cr vector and the CrN 2 plane

(b) VAc polymerizations under OMRP: V-70 initiator

Before presenting the new results, it is necessary to review those already described in a recent communication. 16 We now report subsequent studies with compounds 1 and 5, as well as with compounds 2-4, which revealed a more complex and interesting state of affairs.

Compounds 2-4 were synthesized with the idea of fine controlling the Cr III -PVAc BDE, thus allowing the development of a system capable of yielding a suitable

F o r R e v i e w . C o n f i d e n t i a l - A C S
polymerization rate and degree of control. However, a steric bulk increase on going from 1 to 2 and then to 3 and 4 did not produce any significant labilization of the Cr III -PVAc bond since the polymerization did not proceed in any of these cases beyond 12% conversion for experiments carried out at 90°C, see Figure 2. Indeed, the polymerization essentially stops in each case after an initial burst of monomer consumption, within the first 5-10 hours. On the other hand, new polymerization experiments with compound 5, run at higher temperatures, gave a lower apparent polymerization rate constant., only 34% conversion after 84 h at 90°C. The M n value increased linearly with conversion and the M w /M n decreased to reach a value of only 1.21 for the final sample (see Figure 3), although the M n was much greater than expected (54 800 vs. 14 600). A polymerization process that takes place rapidly under mild conditions (30°C) 16 cannot become slower at a higher temperature while maintaining the same mechanism. It is Well-defined Cr III -R complexes could serve as single-component OMRP reagents if they possessed a Cr-alkyl bond that was sufficiently weakened by steric interactions to readily undergo homolysis. 12,13 To date, the success of synthetic routes to CpCr(nacnac)X complexes has depended on the degree of steric bulk in the target molecule. For the 2,6-iPr 2 substituted system, CpCr(nacnac dipp,dipp )Cl was difficult to prepare by salt metathesis, and did not react cleanly with Grignard reagents. 14 In contrast, reducing the size of the ortho substituents in the nacnac ligands was found to greatly improve both the synthesis and salt metathesis reactivity. radical trap will thus be inefficient at room temperature. However, the observation of a small amount of uncontrolled polymerization leading to high M n is consistent with the slow release of CH 3 • radicals which then react rapidly in neat vinyl acetate. [START_REF]Solution phase measurements give the bimolecular rate constant for the reaction of methyl radical with vinyl acetate as 1.4 × 10 4 M -1 s -1 , while the same reaction with the more stabilized benzyl radical has k = 14 M -1 s -1 : (a) Zytowski[END_REF] For an efficient single component OMRP reagent, a Cr III alkyl complex is required that has a low Cr-R BDE and that generates a R• radical capable of reacting rapidly with vinyl acetate. The steric pressure exerted by the neopentyl ligand has often been used to encourage not only intramolecular deprotonation reactions, [17][18][19][20] but also metal-alkyl bond homolysis. [21][22][23] While several classes of even-electron monomeric chromium neopentyl complexes are known for Cr II , 31,45 Cr IV , 46,47 and Cr VI , [48][49][50] well-defined Cr III neopentyl complexes are relatively unexplored. [START_REF] Heintz | The metallacylobutane Cp*Cr(pyridine)(CH 2 ) 2 CMe 2 has been prepared via the route developed for the analogous CH 2 SiMe 3 reactions[END_REF] The synthesis of compound CpCr(nacnac 54 Unlike the geometries of 2 and 3, the Cr atom deviates significantly from the plane of the nacnac ligand, toward the Cl atom. The Cr in 6 lies 0.615(3) Å out of the imaginary plane defined by the N1-C2-C4-N2 atoms, which is smaller than the 0.72 Å out-of-plane distortion previously observed for the bulkier CpCr(nacnac dipp,dipp )Cl. 14 Relative to the Cr II structures, the Cr-CNT distance is slightly shorter, whereas the Cr-N distances are not significantly different. While 6 is a useful precursor to CpCr(nacnac xyl,xyl )Me, 15 attempts to install more sterically demanding alkyl ligands once again required the use of a better leaving group. The reaction of compound 6 with one equivalent of AgOTs provides compound 7 in high yields as an air stable crystalline solid (Figure 5). Preliminary reactions between 6 and AgOTf or AgOTs had indicated that the Cr III tosylate was less air sensitive and more crystalline than the Cr III triflate complex. The Cr-O bond length in 7, 1.9839(15) Å, is slightly shorter than the 2.030(1) observed for the CpCr(nacnac dipp,dipp )(OTf), 14 which may be attributable to the lower electron withdrawing power of the para-tolyl substituent compared to CF 3 , the decreased steric demand of the 2,6-Me 2 C 6 H 3 substituted nacnac, or both. 14,15 The structural characterization of 8 has been achieved by single crystal X-ray diffraction (Figure 6). The Cr-C( 27) bond is significantly elongated in 8 (2.136(3) Å) compared to the 2.076(2) Å observed previously for the corresponding Cr III methyl complex. 15,43 The Cr-N bond lengths in

(d) OMRP of VAc with complex 8

Polymerizations of VAc in the presence of complex 8 (VAc/8 = 1200:1) at room temperature gave a linearly growing M n up to a monomer conversion of 14% after 400 h (M n,SEC = 16200 vs. the expected value of 15100, M w /M n = 1.46), see Figure 7.

These results contrasts with the uncontrolled polymerization observed with CpCr(nacnac xyl,xyl )Me under the same conditions, as discussed above. There is no visible induction period and the good agreement between the observed and calculated M n demonstrates the increased initiator efficiency compared with the OMRP reactions

F o r R e v i e w . C o n f i d e n t i a l - A C S
with the V-70 initiator. However, the progressive decrease of the polymerization rate constant (decreasing slope in Figure 7a) suggests that partial deactivation of the growing chains occurs, which also causes the broad molecular weight distribution. UV/Vis spectrum further evolved to yield a shift of the major band toward higher frequency (412 nm), while the 556 nm band decreased in intensity, see Figure 8. This spectral evolution parallels the observed decrease in polymerization rate (Figure 7).

After 400 h of polymerization process, the sample was heated at 70°C for 3.5 h to produce a colour change from purple to green (incident light) and orange (transmitted light) with the higher energy absorption band increasing in intensity and the 556 nm band disappearing and being replaced by a less intense band at 575 nm. The results suggested that, even though the rate of polymerization had slowed down significantly (see Figure 7), there was still some CpCr III (nacnac xyl,xyl )(PVAc) compound present after 400 h of polymerization, which underwent further reactivity once heated. It is striking to compare the behaviour of the polymerization initiated by compound 8 on one side (14% in 400 h at room temperature, Figure 7) with that initiated by the 1/V-70 mixture (5% in 50 h at 90°C, Figure 2), both leading in principle to the same OMRP equilibrium. This clearly proves that the CpCr III (nacnac xyl,xyl )(PVAc) dormant chain can be reversibly reactivated under mild conditions to sustain the OMRP of vinyl acetate, but it also suffers irreversible thermal deactivation, even at room temperature at a slow rate, and much faster at more elevated temperatures.

It is interesting to compare the relatively slow polymer growth in the presence of the CpCr(nacnac xyl,xyl ) system and the previously reported 16 faster polymer growth in the presence of the CpCr(nacnac dipp,dipp ) system (70% conversion in 46 h at 30°C). This difference confirms the previously proposed steric effect on the homolytic bond dissociation energy, which is further investigated at the theoretical level in the next section.

(e) DFT study of the OMRP trapping processes

The previous communication reported geometry optimizations at the full QM level using density functional theory (DFT) for CpCr II (nacnac Ar,Ar ) and CpCr III (nacnac Ar,Ar )(CHMeOOCMe) (a model of the OMRP dormant chain), leading to the calculation of the Cr III -C BDE values of 28.4 and 19.7 kcal/mol for Ar = Ph and Xyl, respectively. 16 This shows a tremendous steric effect of the nacnac aryl We have also optimized the transition state leading from the CpCr II (nacnac Ar,Ar ) complex plus free radical to the OMRP dormant species. The coordination geometry is quite close to that of the Cr II complex with a rather long Cr II -C distance, i.e. the transition state is early for the radical trapping process, consistent with the very low calculated energy barrier. The barrier is lowest, as expected, for the less encumbered Ph derivative, whereas the most encumbered Dipp system yields a lower barrier than the Xyl system with intermediate steric demand. Although the barrier for trapping the radical is very low for all three systems, the unexpectedly lower barrier for the Dipp derivative may be attributable to the structure of the Cr II complex. The calculated ground state structure of CpCr(nacnac dipp,dipp ) faithfully reproduces the slight bowing of the N-aryl substituents out of the plane defined by the nacnac ligand that was observed in the X-ray structure of 1. 14 As suggested by a helpful reviewer, this sterically-induced initial deformation of the Cr II Dipp complex may permit a slightly lower energy approach of the alkyl radical compared to CpCr(nacnac xyl,xyl ) which does not display this distortion.

(f) Isolation of the deactivated complex

As discussed above, UV-vis analysis of the Cr III neopentyl compound in vinyl acetate suggested that while 8 was consumed within minutes at room temp, a subsequent thermal decomposition reaction occurred and that this is accelerated by heating. To determine the ultimate fate of the organochromium complex, the These results suggested a more complicated mechanism for the formation of CpCr(nacnac)OAc. The proposed mechanism for the formation of 9 is that of β-acetate elimination arising from a 2,1-insertion (head-to-head) of the monomer, as shown in Scheme 3. Head-to-head insertion is known to be particularly problematic for the radical polymerization of vinyl acetate, due to the relatively poor regioselectivity of radical addition. 1,5 Note that the 2,1-insertion step may lead to the generation of a dormant species with a presumably stronger Cr III -C bond, which could also contribute for a gradual slow down of the polymerization process.

F

  by three freeze-vacuum-thaw cycles and then immersed in an oil bath preheated at 50°C. At the desired time, the Schlenk flask was rapidly cooled to room temperature by immersion into iced water before sample withdrawal. The monomer conversion was determined gravimetrically after removal of the unconverted monomer under reduced pressure and the resulting residue was used for SEC characterization.

For structures 2 , 3

 23 and 6 the Cp ring was found disordered among different orientations (four for 2 and two for 3 and 6). Two of the four orientations in 2 are symmetry-related to the other two since the molecule sits on a twofold axis with half molecule in the asymmetric unit. Refinement of the population of each of these near equivalent values of 0.25. For compounds 3 and 6, the two Cp orientations had equal population. In addition, one disordered half-molecule of hexane is present in the asymmetric unit of 6. This disorder was modeled in two orientations, with restraints employed to maintain similar geometries. Compound 8 crystallizes with two independent molecules in the asymmetric unit. Compound 9 crystallizes as a two-component split crystal with the two components related by a 51° rotation about the (0 0 1) real axis. Data were integrated for both twin components, including both overlapping and non-overlapping reflections. The structure was solved using non-overlapped data from the major twin component. Subsequent refinements of 9 were carried out using HKLF 5 format data set containing complete from component 2 and all overlapped reflections from component 1. The batch scale refinement showed a roughly 53:47 ratio between the major and minor twin components. All non-hydrogen atoms (except those of the Cp ring for compound 2

Figure 1 .

 1 Figure 1. Views of compounds 2 (a) and 3 (b), with thermal ellipsoids drawn at the 50% probability level. All H atoms are omitted and only one Cp orientation is shown for clarity.

  Compound 1 was shown to trap radicals produced by V-70 in the presence of VAc, since a polymerization test with -70/1 = 500:0.8:1 at 50°C for 4 h, and then at 90°C gave only an 11% monomer conversion after 8 h (M n,SEC = 11500 vs. the expected value of 4730, M w /M n = 1.81), which no longer increased upon warming at 90°C for 66 h. Conversely, compound 5 gave a much higher monomer conversion, since a polymerization with VAc/V-70/5 = 500:0.8:1 gave a linearly growing M n up to a conversion of 70% (M n,SEC = 67300 vs.the expected value of 30100, M w /M n = 1.80) in 46 h under much milder conditions (T = 30°C). Although the controlling ability was poor (low initiator efficiency, high polydispersity), the sustained polymerization and the M n growth with conversion demonstrated the reversibility of radical trapping. The hypothesis that both complexes Ar,Ar )Cr III -PVAc, with a much weaker Cr III -PVAc bond when Ar = dipp (compound 5) relative to Ar = xyl (compound 1) seemed fully consistent with DFT calculations of the bond strengths. The larger system with Ar = dipp was not calculated, but a comparison of the bond dissociation energy for the two related systems with Ar = xyl and Ph (19.7 and 28.4 kcal mol -1 at the B3LYP/6-31G* level, respectively) indicated a strong effect of the steric congestion created by the aryl substituents on the ability of the organometallic dormant chain to release free radicals in solution.

Figure 2 .

 2 Figure 2. Conversion as a function of time for the VAc polymerization initiated by V-70 in the presence of compounds 1-5. Conditions: VAc/V-70/Cr II = 500:0.8:1. T = 50°C for 4 h, then 90°C.

  think of a deactivation process (vide infra).

Figure 3 .

 3 Figure 3. Number average molecular weight and polydispersity index for the PVAc obtained in the presence of compound 5. Conditions are as in Figure 2.

Figure 4 . 23 Cr( 1 )

 4231 Figure 4. Thermal ellipsoid diagram (50%) of compound 6. All H atoms are omitted and only one Cp orientation is shown for clarity. Selected bond lengths (Å):

Figure 5 .

 5 Figure 5. Thermal ellipsoid diagram (50%) of compound 7. All H atoms are omitted for clarity. Selected bond lengths (Å): Cr(1)-N(1), 2.0038(16); Cr(1)-N(2), 2.0019(17); Cr(1)-CNT, 1.896; Cr(1)-O(1), 1.9839(15). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.17(7); N(1)-Cr(1)-O(1), 91.45(7); N(2)-Cr(1)-O(1), 92.48(7);

8 1 . 25 compound 1

 1251 are also slightly longer, and the Cr-C(27)-C(28) angle is 135.1(2)º, indicative of the strain imposed by the t Bu substituent of the neopentyl ligand. Solutions of 8 are purple, exhibiting strong bands in the UV-visible spectrum at 404 nm and 567 nm.The strong absorbance around 550 nm seems characteristic of CpCr(nacnac)(alkyl) complexes: the increase in absorbance at 530 nm due to formation of CpCr(nacnac)Me was used to monitor the kinetics of iodomethane activation with CpCr(nacnac) complexes 1-3.15 Although 8 was stable in solution to allow for recrystallization from hexanes, decomposition took place upon dilution (necessary for UV/Vis characterization) to yield CpCr(nacnac xyl,xyl ), The decrease in stability of 8 at high dilution is consistent with the facile homolytic Cr-C bond cleavage, paralleling the reactivity of other neopentyl complexes,[21][22][23] with formation of and an extremely reactive neopentyl radical, and indicated the potential for complex 8 to be used as a single component reagent to initiate and control the radical polymerization of vinyl acetate.

Figure 6 .

 6 Figure 6. Thermal ellipsoid diagram (50%) of compound 8. Compound 8 has two independent molecules in the crystal lattice, only one is shown and all H atoms are omitted for clarity. Selected bond lengths (Å): Cr(1)-N(1), 2.046(2); Cr(1)-N(2), 2.051(2); Cr(1)-CNT, 1.949; Cr(1)-C(27), 2.136(3). Selected bond angles (deg): N(1)-Cr(1)-N(2), 89.71(9); N(1)-Cr(1)-C(27), 93.50(10); N(2)-Cr(1)-C(27), 93.63(10); CNT-Cr(1)-N(1), 123.03; CNT-Cr(1)-N(2), 122.29; CNT-Cr(1)-C(27), 125.39; Cr(1)-C(27)-C(28), 135.1(2).

Figure 7 .

 7 Figure 7. (a) ln([VAc] 0 /[VAc]) as a function of time for the VAc polymerization initiated by compound 8. Conditions: VAc/8 = 1200:1. T = room temp. (b) Variation of the M n,SEC (diamonds) and M w /M n (triangles) as a function of conversion; the solid line represents M n,th .

Figure 8 .

 8 Figure 8. Evolution of the UV/Vis properties during the VAc polymerization controlled by compound 8 (conditions as shown in Figure 7). (a) Initial spectrum of compound 8; (b) after 10 min of polymerization; (c) after 400 h of polymerization; (d) after warming to 70°C (spectrum of the decomposition product).

Figure 9 .

 9 Figure 9. Optimized geometries and QM/MM relative energies (in kcal/mol) of geometry-optimized CpCr II (nacnac Ar,Ar ) and CpCr III (nacnac Ar,Ar )(CHMeOOCMe) for

  of 35.8 mg of 8 in 4 mL of vinyl acetate was evacuated and the unknown Cr species was extracted from the PVAc with diethyl ether. After filtration, concentration, and storage at -35 °C for two weeks, 7.9 mg of X-ray quality crystals of CpCr(nacnac xyl,xyl )OC(O)Me (9) (Figure10) were obtained, corresponding to a 23% isolated yield based on 8.

Figure 10 .

 10 Figure 10. Thermal ellipsoid diagram (50%) of 9. All H atoms are omitted for clarity. Selected bond lengths (Å): Cr(1)-N(1), 2.010(5); Cr(1)-N(2), 2.004(6); Cr(1)-CNT, 1.914; Cr(1)-O(1), 1.952(5); C(27)-O(1), 1.278(9); C(27)-O(2), 1.230(9). Selected bond angles (deg): N(1)-Cr(1)-N(2), 90.7(2); N(1)-Cr(1)-O(1), 92.7(2); N(2)-Cr(1)-O(1), 88.6(2); CNT-Cr(1)-N(1), 124.94; CNT-Cr(1)-N(2), 123.34; CNT-Cr(1)-O(1), 125.92; Cr(1)-O(1)-C(27), 133.6(5).

  ) 2 CH] complexes (Ar = 2,6-diisopropylphenyl, 2,6-xylyl, 2,4,6-mesityl or 2,6-diethylphenyl) and a mixed-aryl nacnac complex CpCr[ArNC(Me)CHC(Me)NAr'] (Ar = 2,6-diisopropylphenyl and Ar'= 2,6-xylyl) have been studied as moderators for the polymerization of vinyl acetate initiated by V-70. The isolated, structurally-characterized Cr III neopentyl complex CpCr[(XylNCMe) 2 CH](CH 2 CMe 3 ) has been used to polymerize vinyl acetate at room temperature, while elevated temperatures lead to deactivation via formation of the Cr III acetate.

  used as received. Compounds Mg(CH 2 CMe 3 ) 2 •x(1,4-dioxane) 30 CrCl 2 (tmeda), 31 116

	Experimental Section
	F o r Materials. All reactions, unless otherwise stated, were carried out under dry, oxygen-free argon or nitrogen, using standard Schlenk and glove box techniques. F o r
	21-23 i o . e e R v i w C n f d e n t i a l Solvents were dried by using the method of Grubbs 24 or they were distilled under argon R from appropriate drying agents and degassed by three freeze-vacuum-thaw cycles prior e v i e w . C o n f i d e to use. CHC(Me)N(2,6-Me 2 C 6 H 3 ), 26 n t i a (2,6-Et 2 C 6 H 3 )NHC(Me)CHC(Me)N(2,6-Et 2 C 6 H 3 ), 27 l
	-(2,4,6-Me 3 C 6 H 2 )NHC(Me)CHC(Me)N(2,4,6-Me 3 C 6 H 2 ) 27 and the mixed N-aryl -
	A C S A β-diketiminato ligand, (2,6-iPr 2 C 6 H 3 )NHC(Me)CHC(Me)N(2,6-Me 2 C 6 H 3 ) 28 were prepared according to the literature procedure. NaCp was prepared according to the literature procedure, 29 or was purchased from Aldrich as a 2.0 M solution in THF and C S
	6

[START_REF] Armarego | Purification of Laboratory Chemicals[END_REF]

Celite (Aldrich) was dried overnight at 110 ºC before being evacuated and then stored under argon or nitrogen. Vinyl acetate (VAc, 99 %, Alfa Aesar or 99+%, Aldrich) was passed through a neutral alumina column to remove the stabilizer, dried over calcium hydride, distilled at 90 °C, degassed by three freeze-vacuum-thaw cycles and stored under argon or nitrogen at -20 °C. 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70, 96%, Wako) was used as received. n-BuLi (1.6 M in hexanes), p-toluenesulfonic acid monohydrate, acetylacetonate, 2,6-dimethylaniline, 2,6-diisopropylaniline, 2,6-diethylaniline, 2,4,6-trimethylaniline, CrCl 3 (anhydrous), 1,4-dioxane, silver p-toluenesulfonate and silver acetate were purchased from Aldrich and used as received. The symmetric β-diketiminato ligands, (2,6-Me 2 C 6 H 3 )NHC(Me)-

Synthesis of CpCr(nacnac dep,dep ) (3): Following a procedure similar to that

  1 H NMR spectra were recorded on a Bruker ARX 250 or a

	previously reported for compound 1, 16 compound (2,4,6-Me 3 C 6 H 2 )NHC(Me)-contents stirred for 20 min at room temperature. To this solution, the lithium salt
	CHC(Me)N(2,4,6-Me 3 C 6 H 2 ) (744 mg, 2.23 mmol) was dissolved in THF (12 mL). prepared above was added dropwise via a cannula and the mixture was stirred at room
	Bruker DPX 300 spectrometer. A Varian Cary 100 Bio UV-visible spectrophotometer F o r n-BuLi (1.60 mL, 2.56 mmol, 1.15 equiv) was added dropwise and the resulting yellow solution was stirred for 30 min at room temperature. In a separate Schlenk temperature overnight. The solvent was evaporated in vacuo and the residue was F r o extracted with pentane, followed by filtration through Celite. The solvent was again
	was used to conduct measurements using a specially constructed cell for air-sensitive samples: a Kontes Hi-Vac Valve with PTFE plug was attached by a professional glassblower to a Hellma 10 mm path length quartz absorption cell with a R e v flask, CrCl 2 (tmeda) (531 mg, 2.22 mmol, 1 equiv) was suspended in THF (35 mL) followed by the addition of NaCp (1.25 mL, 2.50 mmol, 1.13 equiv). The resulting mixture was stirred for 30 min at room temperature. To this solution, the lithium salt removed in vacuo and the complex was dissolved in the minimum amount of pentane, R filtered and cooled to -80°C overnight to yield 462 mg of black crystals. Yield: 35%. e v µ eff (Evans, C 6 D 6 ) 4.6(1) µ B . Anal. Calcd. for C 30 H 38 CrN 2 : C, 75.28; H, 8.00; N, 5.85.
	i e prepared above was added dropwise and the mixture was stirred at room temperature i e Found: C, 74.97; H, 8.30; N, 5.96. UV/Vis (hexanes; λ max , nm (ε, M -1 cm -1 )): 308
	w . overnight. The solvent was evaporated in vacuo and the residue was extracted with . w (12200), 428 (7910), 576 (411).
	C o n f i d e hexanes, followed by filtration through Celite. The solvent was again removed in C vacuo and the complex was dissolved in hexanes (20 mL), filtered and cooled to -35°C for several days. 730 mg of black crystals were isolated in two crops. Yield: o n f i 73%. µ eff (Evans, C 6 D 6 ) 4.8(1) µ B . Anal. Calcd. for C 28 H 34 CrN 2 : C, 74.64; H, 7.61; N, 6.22. Found: C, 74.83; H, 7.96; N, 6.42. UV/Vis (hexanes; λ max , nm (ε, d e
	n t i a l n t i a previously reported for compounds 1, 16 compound (2,6-Et 2 C 6 H 3 )NHC(Me)-M -1 cm -1 )): 308 (11700), 427 (7210), 573 (376). l
	-CHC(Me)N(2,6-Et 2 C 6 H 3 ) (1.00 g, 2.76 mmol) was dissolved in THF (10 mL) and -
	A cooled to -40 °C in an acetonitrile/liquid nitrogen bath. n-BuLi (1.75 mL, 2.80 mmol, A
	C S 1.01 equiv) was added dropwise and the resulting yellow solution was stirred for one hour at -40°C. In a separate Schlenk flask, CrCl 2 (tmeda) (660 mg, 2.76 mmol, 1 equiv) C S
	and NaCp (243 mg, 2.76 mmol, 1 equiv) were suspended in THF (15 mL) and the
	8

quartz-to-glass graded seal. Size exclusion chromatography (SEC) of poly (vinyl acetate) was carried out in filtered THF (flow rate: 1 ml/min) at 35°C on a 300x7,5 mm PL gel 5micrometer mixed-D column (polymer laboratories), equipped with multiangle light scattering (minidawn Tristar, Wyatt Technology Corporation) and refractive index (RI2000, Sopares) detectors, with a waters column pack (300x7,5 mm, ultrastyragel 104,103,100 Å), equipped with multiangle light scattering (miniDawn Tristar,Wyatt Technology Corp.) and refractive index (waters 410) detectors or at 30°C on a Polymer Labs PL-GPC 50 plus (two PLgel mixC columns in series) with a PL-AS RT autosampler and PL-RI detector. The isolated polymer samples were dissolved in THF and the polymer solutions were filtered (pore size = 0.45 µm) before chromatographic analysis. The columns were calibrated against linear polystyrene standards (Polymer Laboratories). Elemental analyses were performed by Guelph Chemical Laboratories, Guelph, ON, Canada. Solution magnetic susceptibilites were determined by the Evans method. 32 Synthesis of CpCr(nacnac mes,mes ) (2): Following a procedure similar to that

Synthesis of CpCr(nacnac xyl,dipp ) (4). Using

	a procedure identical to that
	described above for compound 2, compound CpCr(nacnac xyl,dipp ) (4, 223 mg) was
	obtained as black crystals from (2,6-iPr 2 C 6 H 3 )NHC(Me)CHC(Me)N(2,6-Me 2 C 6 H 3 )
	(316 mg, 0.872 mmol). Yield: 54%. µ eff (Evans, C 6 D 6 ) 4.4(1) µ B . Anal. Calcd for
	C 30 H 38 CrN 2 : C, 75.28; H, 8.00; N, 5.85. Found: C, 75.00; H, 8.38; N, 5.62.

Synthesis of CpCr(nacnac xyl,xyl )Cl (6). Compound

	overnight. The solvent was evaporated in vacuo and the residue was extracted with 40
	mL of a hexanes/dichloromethane mixture (3:1), filtered through Celite and rinsed with
	hexanes (3 × 5 mL). The green to incident and orange to transmitted light filtrate was F r o concentrated and cooled to -20˚C to yield 1.66 g of dark green crystals over several
	days in three crops. Yield: 78%. Anal. Calcd. for C 26 H 30 CrN 2 Cl: C, 68.19; H, 6.60; R N, 6.12. Found: C, 67.88; H, 6.50; N, 5.73. UV/Vis (hexanes; λ max , nm (ε, M -1 cm -1 )): e v 418 (7220), 581 (504).
	i e
	w .
	C
	o n f i
	d e
	(2,6-Me 2 C 6 H 3 )NHC(Me)-CHC(Me)N(2,6-Me 2 C 6 H 3 ) (1.43 g, 4.67 mmol) was added to a Schlenk flask and n t i a dissolved in THF (30 mL) and cooled to 0˚C in an ice-water bath. n-BuLi (3.20 mL, l
	5.12 mmol, 1.10 equiv) was added dropwise and the resulting yellow solution was -
	allowed to warm to room temperature while stirring for 1 hour. The lithium salt was A
	then cannulated into a suspension of CrCl 3 (744 mg, 4.70 mmol, 1.00 equiv) in THF (20 mL) and stirred at room temperature overnight. NaCp (2.60 mL, 5.20 mmol, 1.11 C S
	equiv) was added to the solution which was again stirred at room temperature
	10

Synthesis of CpCr(nacnac xyl,xyl )OTs (7). Compound 6 (

	F o r
	R
	e v
	i e	1.28 g, 2.79 mmol)
	and AgOTs (781 mg, 2.80 mmol, 1.00 equiv) were placed in a Schlenk flask followed w .
	C
	o n f i
	d e
	n t i a
		l
		-
		A
		C S

by the addition of THF (60 mL). The mixture was stirred overnight at room temperature, filtered through Celite and the solvent was evaporated in vacuo. The residue was extracted with 32 mL of a hexanes/dichloromethane mixture (4:1), filtered through Celite and rinsed with hexanes (2 × 5 mL). The green to incident and orange to transmitted light filtrate was cooled to -20˚C to yield 1.40 g of black crystals over several days in four crops. Yield: 84%. Anal. Calcd. for C 33 H 37 CrO 3 N 2 S: C, 66.76; H, 6.28; N, 4.72. Found: C, 66.50; H, 6.20; N, 4.34. UV/Vis (diethyl ether; λ max , nm (ε, M -1 cm -1 )): 411 (8140), 571 (548).

Synthesis of CpCr(nacnac xyl,xyl )CH 2 CMe 3 (8).

Compound 7 (600 mg, 1.01 mmol) was added to a Schlenk flask followed by the addition of diethyl ether (30 mL).

Mg(CH 2 CMe 3 ) 2 •1.05(1,4-dioxane) (143 mg, 0.554 mmol, 0.549 equiv) in diethyl ether (5 mL) was added dropwise to the Schlenk. The mixture was stirred for 1.5 h at room temperature, the solvent was evacuated in vacuo and the residue was extracted with 11 hexanes (30 mL), filtered through Celite and rinsed with hexanes (3 × 5 mL). The solvent was again evacuated in vacuo and the residue was extracted with hexanes (15 mL), filtered and cooled to -35˚C to yield 335 mg of black crystals over several days in four crops. Yield: 67%.

Anal

. Calcd. for C 31 H 41 CrN 2 : C, 75.42; H, 8.37; N, 5.67. Found: C, 75.27; H, 8.69; N, 5.66. UV/Vis (hexanes; λ max , nm (ε, M -1 cm -1 )): 404 (5170), 567 (1050). Synthesis of CpCr(nacnac xyl,xyl )OC(O)Me (9). Compound 7 (259 mg, 0.566 mmol) and AgOAc (95.2 mg, 0,570 mmol, 1.01 equiv) were place in a Schlenk flask

  31G** set for the H atoms, and the SDD set, which includes a pseudopotential, augmented by an f polarization function with the optimized41 1.941 coefficient for the Cr atom. The cutoff between the QM and MM parts was placed at the level of the Ar-N bonds, with the N=C(Me)-CH-C(Me)=N diketiminato moiety being treated quantomechanically together with the Cr atom, the Cp ring, and the CH(OAc)CH 3 ligand, while the entire aryl substituents were handled at the MM level. The input

	chosen for the QM calculations comprised the 6-31G* set for the C, N and O atoms, Results and Discussion
	F o r (a) Syntheses and characterization of CpCr II (nacnac) the 6-F The CpCr II (nacnac) compounds used in this study are shown in Scheme 1 (xyl = o r 2,6-dimethylphenyl; mes = 2,4,6-trimethylphenyl; dep = 2,6-diethylphenyl; dipp =
	R e v 2,6-diisopropylphenyl). Of these, only compounds 1 16 and 5 14, 16 have previously been R v e described in the literature.
	i e w . i Cr e N N w 1 . CpCr(nacnac xyl,xyl )	Cr 2 CpCr(nacnac mes,mes ) N N	Cr 3 CpCr(nacnac dep,dep ) N N
	C C
	o n f i d e Cr o N N n f e d CpCr(nacnac xyl,dipp ) Cr N CpCr(nacnac dipp,dipp ) N i 4 5
		n t i a n t i a
			l l
			--
			A A
			C S C S
		14

for the MM part. The basis set geometries were obtained or adapted from the crystallographically characterized compounds. Spin contamination was negligible, all calculations converging with <S 2 > close to the expected values (6 for the quintet state of Cr II -6.022 for all three compounds; 3.75 for the quartet state of Cr III -3.824 for the Dipp derivative). The values reported are the electronic energies without ZPVE correction.

Table 1 .

 1 Selected

		2	3
	Distances
	CNT-Cr	2.00(1)	2.013(12)
	Cr-N	2.007(2)	2.0276(12)
	Cr-N		2.0260(12)
	Angles
	CNT-Cr-N	134.0(3)	134.8(3)
	CNT-Cr-N		134.5(3)
	CNT-(CrN 2 ) a	177.7(3)	173.8(3)
	N-Cr-N	91.2(1)	90.37(5)

bond distances (Å) and angles (°) for compounds 2 and 3.

  The lack of steric pressure on the small methyl ligand and the relative instability of the CH 3 • radical should make the Cr III -CH 3 BDE high and the homolytic dissociation unfavorable. Initiation to form CH 3 • and the CrII

	Synthesis of the corresponding CpCr(nacnac dipp,dipp )Me had necessitated the use of the
	corresponding Cr III triflate complex. 14
	Isolated CpCr(nacnac xyl,xyl )Me does not serve as an effective single-component F r o OMRP reagent. When 16.1 mg of this Cr III methyl complex was dissolved in 4 mL of
	neat vinyl acetate, only a 9% mass conversion was observed after 48 hours at room R temperature. The resulting polymer had both a high M n (83900 compared to 9140 e v expected M n ) and a high PDI of 3.4.
	i e
	w .
	C
	o n f i
	d e
	n t i a
	l
	-
	A
	C S
	21

15,43

For example, CpCr(nacnac xyl,xyl )Me was readily prepared from CpCr(nacnac xyl,xyl )Cl with MeMgI.

15

  legged piano stool" and is ubiquitous of half-sandwich Cr III complexes. The Cr-Cl bond length of 2.3082(5) Å is similar to that found in CpCr(nacnac dipp,dipp )Cl,14 as well as other cylcopentadienyl Cr III complexes with terminal Cl groups, like

	F r o CpCr(acac)Cl and Cp*Cr(acac)Cl (Cr-Cl of 2.299(1) and 2.307(1) Å, respectively). 52
	It is also similar to the Cr-Cl distance in 6-coordinate Cr III nacnac complexes with R terminal Cl ligands (2.346(1) Å for Cr(nacnac Ph,Ph )Cl 2 (THF) 2 53 and 2.2947(12) Å for e v Cr(nacnac dipp,dipp )(O 2 CMe)Cl(THF) 54 ), while 5-coordinate Cr III nacnac complexes
	i e display shorter Cr-Cl bond lengths (2.233(2) Å and 2.2294(14) for the terminal
	. w chloride ligands in [Cr(nacnac dipp,dipp )Cl(µ-Cl)] 2 and Cr(nacnac dipp,dipp )[(OCPh) 2 CH]Cl,
	respectively).	C
		o n f i
		d e
		n t i a
		l
		-
		A
		C S

xyl,xyl

)Cl

(6)

was achieved by reacting CrCl 3 with one equivalent of Li(nacnac xyl,xyl ), followed by one equivalent of NaCp to yield an air stable crystalline solid. The geometry of 6 (Figure

4

) can be described as a 22 "three-

  CMe 3 . The use of the halide-free dialkyl Mg reagent was required to avoid unwanted substitution of the tosylate ligand in 7 with a halide prior to alkylation to form 8. Complex 8 was found to be highly soluble in non-polar solvents, similar to the previously reported CpCr(nacnac xyl,xyl )Me.

	CNT-Cr(1)-N(1), 125.74; CNT-Cr(1)-N(2), 123.82; CNT-Cr(1)-O(1), 123.32;
	Cr(1)-O(1)-S(1), 135.77(9).
	The F o r reaction Mg(CH 2 CMe 3 ) 2 •1.05(1,4-dioxane) gave CpCr(nacnac xyl,xyl )Np (8), where Np = of compound 7 with one-half equivalent of
	R neopentyl, CH 2
	e v
	i e
	w .
	C
	o n f i
	d e
	n t i a
	l
	-
	A
	C S
	24

Table 2 .

 2 Selected bond distances (Å) and angles (°) for the B3LYP//UFF optimized geometries. Angle between the CNT-Cr vector and the CrN 2 planeThe energetic results in terms of DBE(Cr III -C) for the Ph and Xyl systems are quite close to those previously obtained at the full QM level, although the steric labilization exerted by the four Me groups is predicted as less severe by the QM/MM calculations relative to the full QM level. The present calculation uses a polarized SDD basis set for the Cr atom, which is considered as more balanced than the LANL2DZ basis set without polarization functions previously used in the full QM calculations.16 The presence of a steric effect is confirmed on going from the Xyl to the Dipp substituents, where the four iPr groups induce a further Cr III -C bond labilization by 2.0 kcal/mol relative to four Me groups. This labilization is accompanied by a significant lengthening of the Cr III -C bond, in the order Ph (2.091 Å)

	Complex	CpCr II (nacnac Ar,Ar )		TS		CpCr III (nacnac Ar,Ar )(R) a
	Ar	Ph	Xyl	Dipp	Ph	Xyl	Dipp	Ph	Xyl	Dipp
	Distances
	CNT-Cr	2.081	2.081 2.070 2.072	2.080	2.079 2.070 2.081 2.096
	Cr-N	2.014	2.009 2.028 2.021	2.024	2.051 2.018 2.033 2.062
		2.014	2.009 2.030 2.020	2.021	2.040 2.024 2.034 2.088
	Cr-C	-	-	-	4.290	3.440	3.396 2.091 2.115 2.139
	Angles
	CNT-Cr-N	133.3	133.6 132.3 134.2	131.7	131.9 121.6 123.5 123.8
		133.2	133.6 133.9 133.3	131.0	132.1 124.4 124.5 124.5
	CNT-Cr-C				108.7	106.0	101.4 117.6 114.9 114.1
	CNT-(CrN 2 ) b	175.8	179.5 178.5 178.6	161.4	167.5 141.8 143.3 143.7
	N-Cr-N	93.3	92.7	93.8	92.4	91.6	93.4	92.3	90.1	91.7
	N-Cr-C	-	-	-	77.5	92.2	92.3	97.5	100.2 102.0
					75.1	91.4	89.1	97.2	97.8	94.8

a R = CHMeOOCMe.

b

  35alternative is acetate group transfer from the β-C atom of the radical chain to the Cr center, as shown in Scheme 4. Unfortunately, we were unable to observe vinyl chain end resonances in the13 C NMR spectrum of the polymer isolated from thermolysis of 8 in vinyl acetate to provide additional support for the mechanism proposed in Scheme 4. Homolytic bond rupture followed by atom (or group) transfer has been observed for R-Co(porphyrin) complexes where the coordination geometry has no cis vacant site available to accommodate the migrating atom or group.59 It is quite possible that, following a 2,1-insertion, the resulting CH 2 -terminated radical undergoes competitive β-acetate transfer as shown in Scheme 4 or Cr III -C bond formation as shown in Scheme 3, thus yielding a mixture of acetate complex and a less active Cp(nacnac xyl,xyl )Cr-CH 2 CH(OAc)-PVAc dormant species. Only upon warming the latter can be reactivated and eventually be completely transformed into the final acetate product.The present study has revealed a more complex situation than previously appreciated for the OMRP of vinyl acetate mediated by half-sandwich Cr II complexes of type CpCr(nacnac). The new data reported here confirm the sterical labilization of the Cr III -PVAc bond, with the more encumbering nacnac dipp,dipp ligand resulting in a faster apparent rate constant for polymer growth than the nacnac xyl,xyl ligand. However, the relatively stronger bond of Cp(nacnac xyl,xyl )Cr-PVAc is still sufficiently labile to sustain the OMRP of vinyl acetate even at room temperature. On the other hand, an irreversible deactivation process comes into play, slowly at room temperature and faster upon warming, to yield a new material now firmly identified as the acetate complex, CpCr(nacnac xyl,xyl )OAc. and 9, and complete crystallographic data for complexes 2, 3, 6, 7, 8, and 9. This material is available free of charge via the Internet at http://pubs.acs.org.

	energies of all calculated complexes, UV-visible spectra of complexes 1, 2, 3, 6, 7, 8,
	o r F o r F o r F o r F o r F o r F o r
	R R R R R R R
	e v e v e v e v e v e v e v
	i e i e i e i e i e i e i e
	w . w . w . w . w . w . w .
	C C C C C C C
	o n f i o n f i o n f i o n f i o n f i o n f i o n f i
	d e d e d e d e d e d e d e
	n t i a n t i a n t i a n t i a n t i a n t i a n t i a
	l l l l l l l
	-------
	A A A A A A A
	Scheme 4	C S C S C S C S C S C S C S
	Conclusions
	36 37

40 Graphical Abstract for Table of Contents Symmetric CpCr[(ArNCMe

ACS Paragon Plus EnvironmentSubmitted to Organometallics

Acknowledgement

RP thanks the Agence Nationale de la Recherche (contract ANR No. NT05-2_42140) and the Institut Universitaire de France for financial support, and the Centre Interuniversitaire de Calcul de Toulouse (CICT, project CALMIP) for granting free CPU time. KMS thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support, and thanks Jeffrey A. Therrien and Julia L.

Conway for obtaining single crystals of 3 and 7, respectively, suitable for X-ray diffraction.

Elimination of β-acetate groups has been well documented as a decomposition mode in attempts to copolymerize ethylene and vinyl acetate with Ni and Pd catalysts. 2,3 We point out, however, that the β-acetate elimination process from the newly formed dormant species is not likely to follow a classical β-elimination mechanism, because this requires an open coordination site on Cr capable of accepting the two additional electrons furnished by the incoming acetate group. While the metal is electronically unsaturated (15 electrons) and metal-based orbitals are indeed available, the latter are however half-occupied because of the ubiquitous spin quartet configuration of half-sandwich Cr III . Expansion to a 17-electron configuration would require an energetically costly (for Cr III ) electron pairing process. [56][57][58] A likely Supporting Information Available: A listing of the Cartesian coordinates and final