Some Novel Cobalt Diphenylphosphine Complexes: Synthesis, Characterization, and Behavior in the Polymerization of 1,3-Butadiene

Some novel cobalt diphenylphosphine complexes were synthesized by reacting cobalt(II) chloride with (2-methoxyethyl)diphenylphosphine, (2-methoxyphenyl)diphenylphosphine, and 2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine. Single crystals suitable for X-ray diffraction studies were obtained for the first two complexes, and their crystal structure was determined. The novel compounds were then used in association with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene, and their behavior was compared with that exhibited in the polymerization of the same monomer by the systems CoCl2(PnPrPh2)2/MAO and CoCl2(PPh3)2/MAO. Some significant differences were observed depending on the MAO/Co ratio used, and a plausible interpretation for such a different behavior is proposed.

According to the diene polymerization mechanism previously proposed [4], the formation of 1,2 polymers was attributed to the presence of a phosphine ligand on the cobalt atom [5][6][7]; it is in fact worthwhile to note that naked CoCl 2 , in combination with MAO, provides a poly(1,3-butadiene) with a very high cis content (~97%). The formation of a certain amount of cis-1,4 units (~15%) was attributed, in our opinion, to the fact that some phosphine during the polymerization process may be removed from the cobalt atom by the large excess of MAO; therefore, some active sites such as those originating from CoCl 2 /MAO, specific for cis-1,4 poly(1,3-butadiene), can be formed.
Crystalline products were obtained by cooling a toluene solution at low temperature and by a continuous extraction of the reaction products with boiling pentane. The structures of 1 and 2 have been determined by single-crystal X-ray diffraction (Figures 2 and 3).  In the case of complex 3, the presence of highly disordered cocrystallized solvent (diethyl ether) prevented obtaining an acceptable structure, though X-ray data confirmed the formation of the compound, as shown in Figure 1.
In both complexes, the Co(II) center displayed a distorted tetrahedral geometrical coordination, being bonded to two chlorides and two phosphine ligands. The Co-Cl and Co-P bond distances (Tables 1 and 2) were in the ranges reported for analogue CoCl 2 (PRPh 2 ) 2 complexes (R = ethyl [2], normal-propyl [2], iso-propyl [1], tert-butyl [3], CH 2 CH(OCH 3 ) 2 [9], CH 2 C(O)Ph [10], and Ph [11,12]), i.e., 2.21−2.24 and 2.36−2.43 Å, respectively. As for the P-C bonds, a distinction should be made according to the aliphatic/ aromatic nature of the bonded carbon atom. In fact, by considering average values within each structure, P-C aliphatic bonds (varying from 1.827 [2] to 1.888 Å [3]) were systematically longer than P-C aromatic ones (ranging from 1.812 [10] to 1.833 Å, the latter value observed in 1). In particular, as already pointed out in our previous analysis on CoCl 2 (PRPh 2 ) 2 complexes [3], the greater the steric hindrance of the aliphatic group, the larger the difference in P-C bond lengths. For compound 1, only a little difference was observed, the average P-C aliphatic bond distance, 1.838 Å, being only slightly longer than the average P-C aromatic one, 1.833 Å. In the case of 2, only P-C aromatic bonds were present, measuring on average 1.823 Å and virtually identical to those of the CoCl 2 (PPh 3 ) 2 structure [11,12]. In the solid state, several short intermolecular contacts were present, including CH 2 -O·HCHP (in 1), C phenyl ·H phenyl and C phenyl ·HCH 2 (in 2), and Co-Cl·H phenyl (in both structures) hydrogen bonds as well as normal van der Waals distances.

Polymerization of 1,3-Butadiene
The results concerning the polymerization of 1,3-butadiene with the catalysts obtained by combining the three novel diphenylphosphine complexes 1-3 with MAO are shown in Table 3; the results obtained with CoCl 2 (PPh 3 ) 2 /MAO and CoCl 2 (P n PrPh 2 ) 2 /MAO are reported for comparison. Catalysts based on the new cobalt complexes 1-3 were much less active than the systems CoCl 2 (P n PrPh 2 ) 2 /MAO and CoCl 2 (PPh 3 ) 2 /MAO, and their activity seemed to decrease with decreasing the MAO/Co molar ratio. The polybutadienes had a molecular weight in the range 100,000-200,000 g·mol −1 and a molecular weight distribution around 2-3, values quite similar to those of the polymers obtained with CoCl 2 (P n PrPh 2 ) 2 /MAO and CoCl 2 (PPh 3 ) 2 /MAO.
Taking into consideration that the CoCl 2 /MAO system produces from 1,3-butadiene a polymer with a cis content of about 97%, we may formulate the following working hypothesis to justify the different behavior exhibited by the above catalysts by varying the MAO/Co molar ratio.
In the polymerization of 1,3-butadiene with cobalt phosphine complex-based catalysts, the structure of the active site, as reported in our previous papers [1][2][3]5,13], is that shown in Figure 4A, with only one phosphine ligand on the cobalt atom, the incoming monomer cisη 4 coordinated, and the growing chain bonded to the cobalt atom through a η 3 −allyl bond. Most likely, a sort of equilibrium between cobalt and aluminum (MAO) was established (Figure 4), so that the phosphine ligand, initially on the cobalt atom ( Figure 4A), may migrate onto the aluminum atom ( Figure 4B), causing a drastic change in the selectivity of the catalytic center from specific 1,2 to specific cis-1,4. Notably, this equilibrium was more displaced towards cobalt or aluminum according to (i) the MAO/Co molar ratio value and (ii) the affinity level of the ligand for cobalt or aluminum, strongly affected by the presence of heteroatoms on the phosphine ligand. At low MAO/Co molar ratios (≤100), the equilibrium mentioned above clearly moved towards the cobalt atom ( Figure 4A), with the phosphine mainly on the cobalt atom, and almost exclusively 1,2 units formed; some migration of the phosphine however may take place, with formation of a small amount of cis-1,4 units.
When increasing the MAO/Co molar ratio to 1000, the situation did not seem to change for the CoCl 2 (PPh 3 ) 2 /MAO and CoCl 2 (PnPrPh 2 ) 2 /MAO systems; that is, the phosphine ligand remained coordinated to the cobalt atom, while the same was not valid for the 1/MAO, 2/MAO, and 3/MAO systems, which provide highly cis-1,4 poly(1,3butadienes). In this case, it is likely that the presence of nitrogen or oxygen donor atoms within the phosphine ligand structure, associated with the high concentration of MAO, caused the phosphine to migrate onto the aluminum atoms ( Figure 4B), with generation of a cis-1,4 specific catalytic center, quite similar to the one obtained by reacting naked CoCl 2 with MAO.
The possibility that the phosphine ligand, under certain polymerization conditions, may migrate from cobalt to aluminum, causing a drastic change in the selectivity of the catalytic center, is supported by the fact that the catalytic systems CoCl 2 (PPh 3 ) 2 /MAO and CoCl 2 (P n PrPh 2 ) 2 /MAO at higher MAO/Al molar ratio (up to 5000) give poly(1,3butadiene)s with a mixed cis-1,4/1,2 structure. Evidently, in this case such a high MAO/Co molar ratio caused a migration of part of the ligand onto the aluminum atom.
Anal  Figures S1 and S1A). (2), Tetrahydrofurane/Ethanol as Medium (2-Methoxyphenyl)diphenylphosphine (1.00 g, 3.42 × 10 −3 mol) was dissolved in tetrahydrofurane (30 mL) and successively added to a solution of CoCl 2 (0.204 g, 1.57 × 10 − 3 mol) in ethanol (30 mL). A blue suspension was gradually formed; after 20 h it was filtered, washed with ethanol (2 × 10 mL) and pentane (2 × 10 mL), and then dried in vacuum at room temperature. The isolated blue solid was transferred on the filter of a Soxhlet for solids and extracted continuously with boiling diethylether. The extraction was practically complete in two days; at the end, a microcrystalline blue powder was formed on the bottom of the extraction Schlenk-tube. The blue supernatant solution was removed, concentrated, and cooled at − 30 • C, causing the precipitation of a crystalline product. Further crops of crystals were obtained by repeating this workup operation several times. Yield: 0.897 g (79.6% based on CoCl 2 ).

X-ray Crystallographic Studies
A summary of the experimental details concerning the single crystal X-ray diffraction studies on complexes 1 and 2 is reported in Table 4. The crystals used for data collection were entirely covered with perfluorinated oil to reduce crystal decay. Data were recorded on Bruker Photon 100 (1) or APEX II (2) area detector diffractometers (Bruker AXS Inc., Madison, WI, USA) using Mo-Kα radiation. Data were corrected for Lorentz polarization and absorption effects by SADABS [15]. The structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 [16]. Hydrogen atoms were fixed at calculated positions and refined by a riding model.

Polymerization
A standard procedure is reported. 1,3-Butadiene was condensed into a 25 mL dried glass reactor kept at −20 • C, then toluene was added, and the solution obtained was brought to the desired polymerization temperature. MAO and the cobalt compound were then added, as toluene solutions, in that order. The polymerization was terminated with methanol containing a small amount of hydrochloric acid, and the polymer was coagulated and repeatedly washed with methanol, then dried in vacuum at room temperature to constant weight.

Polymer Characterization
The polymer microstructure was determined through FT-IR and NMR ( 1 H and 13 C) analyses (see Supplementary Materials, Figures S4-S15) as reported in the literature [17][18][19][20][21][22][23]. 13 C NMR and 1 H NMR measurements were performed with a Bruker AM 400 instrument (Bruker Italia Srl, Milano, Italy). The spectra were obtained in C 2 D 2 Cl 4 at 103 • C (hexamethyldisiloxane, HMDS, as internal standard). The concentration of polymer solutions was about 10 wt.%. Wide-angle X-ray diffraction (XRD) experiments (see Supplementary Materials, Figures S16 and S17) were performed at 25 • C under nitrogen flux, using a Siemens D-500 diffractometer equipped with Soller slits (2 • ) placed before sample, 0.3 • aperture and divergence windows, and a VORTEX detector with extreme energy resolution specific for thinner films. CuKα radiation with power use of 40 KV × 40 mA was adopted, and each spectrum was measured with steps of 0.05 • 2θ and 6s measurement time. FTIR spectra were acquired using a Perkin-Elmer (Waltham, MA, USA) Spectrum Two in attenuated total reflectance (ATR) mode in the spectral range of 4000-500 cm −1 . The molecular weight average (M w ) and the molecular weight distribution (M w /M n ) were obtained by a high-temperature Waters GPCV2000 (Milford, MA, USA) size-exclusion chromatography (SEC) system equipped with a refractometer detector. The experimental conditions consisted of three PL Gel Olexis columns, ortho-dichlorobenzene (o-DCB) as the mobile phase, a 0.8 mL/min flow rate, and a 145 • C temperature. The calibration of the SEC system was constructed using eighteen narrow M w /M n poly(styrene) standards with M w s ranging from 162 to 5.6 × 10 6 g mol −1 . For SEC analysis, about 12 mg of polymer was dissolved in 5 mL of o-DCB with 0.05% of BHT as antioxidant.

Conclusions
Three novel cobalt diphenyl phosphine complexes were synthesized, and the crystal structure of two of them was determined by single crystal X-ray diffraction. The behavior of these complexes in combination with MAO in the polymerization of 1,3-butadiene was examined, and it was found to be strongly affected by the MAO/Co ratio, giving predominantly 1,2 polymers at low MAO/Co molar ratios (up to 100) and essentially cis-1,4 polymers at higher Al/Co molar ratios (1000). The different behavior by varying the MAO/Co ratios was attributed to the presence of donor heteroatoms within the ligand structure, making easier the displacement of the phosphine ligand from the cobalt atom, resulting in a drastic change in selectivity of the catalytic center, from 1,2 specific to cis-1,4 specific.
The possibility of modifying the catalytic selectivity during the polymerization process simply by varying the MAO/Co ratio could be interesting since it could permit the preparation of poly(1,3-butadiene)s consisting of polymeric blocks with different stereoreg-ularity, having elastomeric or thermoplastic features depending on the block microstructure. Examples of this type, in which the catalytic selectivity can be adjusted by varying the aluminum-alkyl ratio, have already been reported in the literature, for example in the case of iron [24] and neodymium-based catalysts [25,26].
Author Contributions: G.L., G.Z., and B.P. planned and carried out the polymerization experiments and part of the organometallic syntheses. A.S., F.M., M.G., and G.P. performed the synthesis of the ligands and of the organometallic complexes and their characterization, and contributed to preparing the manuscript. A.F. and S.Z. determined the X-ray molecular structures of the complexes and contributed to writing the paper. G.R. was the principal investigator, conceived and designed the experiments, interpreted the results, and wrote the paper. All authors have read and agreed to the published version of the manuscript.