Novel Cobalt Dichloride Complexes with Hindered Diphenylphosphine Ligands: Synthesis, Characterization, and Behavior in the Polymerization of Butadiene

Two novel cobalt diphenylphosphine complexes were synthesized by reacting cobalt(II) chloride with tert-butyl(diphenyl)phosphine (PtBuPh2) and (S)-(+)neomenthyldiphenylphosphine [(S)-NMDPP]. The crystal structure of the former was determined by single-crystal X-ray diffraction studies. The two complexes were then used in combination with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene: crystalline highly syndiotactic 1,2 poly(1,3-butadiene)s were obtained, with a 1,2 content and a syndiotactic index (percentage of syndiotactic triads [rr]) up to 95% and 85%, respectively. The results obtained further support and confirm what was already observed in the polymerization of 1,3-butadiene with CoCl2(PRPh2)2−MAO (R = methyl, ethyl, normal-propyl, iso-propyl, and cyclohexyl): the nature of the phosphine ligand strongly affects the polymerization stereoselectivity, the polymer syndiotacticity increasing with increasing phosphine ligand steric hindrance.


Synthesis and Characterization of Cobalt Complexes
The two novel cobalt complexes were prepared according to a general experimental procedure already reported in the literature [23]. CoCl2 was dissolved in ethanol and reacted with an excess of phosphine. The solutions so obtained were kept under stirring at room temperature for one day, then the solvent was removed under vacuum. The residues were washed with cold pentane, then dried again under vacuum. Crystalline products were obtained by continuous extraction of the residues with boiling pentane. Single crystals suitable for X-ray diffraction studies were obtained for complex 1, allowing its molecular structure to be determined ( Figure 2). Selected bond lengths and angles are reported in Table 1. The molecular structure of 1 can be conveniently discussed together with those of the previously reported related CoCl2(PRPh2)2 complexes with R = ethyl (Et), normal-propyl ( n Pr), and iso-propyl ( i Pr) [8,9]. In fact, in these compounds, the phosphorus atoms bear the same aromatic groups (two phenyl rings) but different aliphatic substituents, going from small (Et, n Pr) to larger ( i Pr, t Bu) groups. As expected, a clear differentiation between bonds connecting phosphorus and

Synthesis and Characterization of Cobalt Complexes
The two novel cobalt complexes were prepared according to a general experimental procedure already reported in the literature [23]. CoCl 2 was dissolved in ethanol and reacted with an excess of phosphine. The solutions so obtained were kept under stirring at room temperature for one day, then the solvent was removed under vacuum. The residues were washed with cold pentane, then dried again under vacuum. Crystalline products were obtained by continuous extraction of the residues with boiling pentane. Single crystals suitable for X-ray diffraction studies were obtained for complex 1, allowing its molecular structure to be determined ( Figure 2). Selected bond lengths and angles are reported in Table 1.

Synthesis and Characterization of Cobalt Complexes
The two novel cobalt complexes were prepared according to a general experimental procedure already reported in the literature [23]. CoCl2 was dissolved in ethanol and reacted with an excess of phosphine. The solutions so obtained were kept under stirring at room temperature for one day, then the solvent was removed under vacuum. The residues were washed with cold pentane, then dried again under vacuum. Crystalline products were obtained by continuous extraction of the residues with boiling pentane. Single crystals suitable for X-ray diffraction studies were obtained for complex 1, allowing its molecular structure to be determined ( Figure 2). Selected bond lengths and angles are reported in Table 1. The molecular structure of 1 can be conveniently discussed together with those of the previously reported related CoCl2(PRPh2)2 complexes with R = ethyl (Et), normal-propyl ( n Pr), and iso-propyl ( i Pr) [8,9]. In fact, in these compounds, the phosphorus atoms bear the same aromatic groups (two phenyl rings) but different aliphatic substituents, going from small (Et, n Pr) to larger ( i Pr, t Bu) groups. As expected, a clear differentiation between bonds connecting phosphorus and The molecular structure of 1 can be conveniently discussed together with those of the previously reported related CoCl 2 (PRPh 2 ) 2 complexes with R = ethyl (Et), normal-propyl ( n Pr), and iso-propyl ( i Pr) [8,9]. In fact, in these compounds, the phosphorus atoms bear the same aromatic groups (two Molecules 2019, 24, 2308 3 of 8 phenyl rings) but different aliphatic substituents, going from small (Et, n Pr) to larger ( i Pr, t Bu) groups. As expected, a clear differentiation between bonds connecting phosphorus and aliphatic (P−C aliph ) or aromatic carbons (P−C ar ) was found, the former being on average longer than the latter. Moreover, they both depended on the bulkiness of the aliphatic substituents, increasing from the complexes with R = Et and n Pr to 1. Greater variation was clearly observed for the P−C aliph distances, which measured on average 1.829(2), 1.827(2), 1.850(2), and 1.888(1) Å for R = Et, n Pr, i Pr, and t Bu, respectively. On the other hand, the average P-C ar bond lengths showed a modest increase from 1.814(3) (both Et and n Pr) and 1.816(2) ( i Pr) to 1.826(1) Å for t Bu. The greater steric hindrance of the t Bu group also implies larger Co−P and Co−Cl distances in 1 than those observed in the other three complexes. In particular, the average Co-P bond lengths, measuring 2.370(1), 2.380(1), and 2.363(1) Å in the complexes with R = Et, n Pr, and i Pr, respectively, became as large as 2.433(1) Å in 1. Table 1. Selected bond lengths (Å) and angles ( • ) for CoCl 2 (P t BuPh 2 ) 2 (1).

Bond Lengths
Bond Angles 1.8195 (13) Note that the different dimensions of the aliphatic substituents could explain the different conformations observed in complexes with R = t Bu and i Pr with respect to those of R = n Pr and Et derivatives. While in the former the aliphatic groups were in trans along the P−Co−P−R sequence, in the latter they were in gauche along the same sequence. This involves a different arrangement of the phenyl rings, which were both gauche in the complexes with R = t Bu and i Pr, granting a more efficient accommodation of the entire phosphinic group.
The observed difference in the conformations of CoCl 2 (PRPh 2 ) 2 complexes may in turn account for the significantly larger P-Co-P angle for the derivatives with bulkier aliphatic groups. This angle in fact decreased from 113.69(1) • (R = t Bu) to 110.64(3) • ( i Pr), 104.85(4) • ( n Pr), and 102.89(3) • (Et). Widening the P−Co−P angle allows for a partial reciprocal distancing of the phenyl rings of different phosphines, which in R = t Bu and i Pr derivatives happened to be in pairs in a face-to-face repulsive arrangement, only partially relieved by a partial rotation around the P−C ar bonds. The other bond angles were instead almost unchanged in the series of examined structures.

Polymerization of 1,3-Butadiene
The results obtained in the polymerization of 1,3-butadiene with the two new cobalt complexes based catalysts are shown in Table 2; the results obtained with CoCl 2 (PRPh 2 ) 2 −MAO (R = iso-propyl, cyclohexyl) [8,9], which were found to be the systems providing 1,2 poly(1,3-butadiene) with the highest syndiotactic content, are added for comparison.
With a MAO/Co molar ratio in the range 10−100, the CoCl 2 (P t BuPh 2 ) 2 −MAO and CoCl 2 [PPh 2 (NMDPP)] 2 −MAO systems both gave polybutadienes with a predominantly 1,2 structure (up to 94.5%) and a syndiotactic content up to 85.5%. The 13 C-NMR spectrum of the polybutadiene obtained with the system CoCl 2 (P t BuPh 2 ) 2 −MAO at MAO/Co = 25, 0 • C, and in heptane as solvent (Table 2, run 4) is shown in Figure 3; the 13 C NMR spectra of the other polybutadienes of Table 2 ( Figures S1-S6), together with the FTIR spectra of the same polymers ( Figures S7-S13) are reported in the Supplementary Materials. The 1,2 content and the syndiotacticity degree seemed to be only slightly affected by the MAO/Co molar ratio (in the range examined) (cfr. runs 1−2), whereas they seemed to be more influenced by the nature of the solvent (cfr. runs 2−3 and 5−6) and the polymerization temperature in particular (cfr. runs 3-4 and 6-7). This behavior was already observed for the analogous systems CoCl 2 (PRPh 2 ) 2 −MAO (R = methyl, ethyl, normal-propyl, iso-propyl, and cyclohexyl), and a plausible interpretation for this behavior has already been given [8,9]. The syndiotactic degrees of the polybutadienes obtained with the two systems CoCl 2 (P t BuPh 2 ) 2 −MAO and CoCl 2 [PPh 2 (NMDPP)] 2 −MAO were higher than those exhibited by CoCl 2 (P i PrPh 2 ) 2 −MAO and CoCl 2 (PCyPh 2 ) 2 −MAO (cfr. runs 1,8 and 5,9), which, as mentioned above, in the series CoCl 2 (PRPh 2 ) 2 −MAO (R = methyl, ethyl, normal-propyl, iso-propyl, and cyclohexyl) were those giving the highest syndiotactic content. Therefore, the results obtained with the two novel systems described in the present paper confirm once more that the steric hindrance of the ligand exerts a strong influence on the mutual orientation of the allylic unit of the growing chain and of the new incoming monomer that is responsible for the polymerization stereoselectivity.
Molecules 2019, 24, 2308 4 of 8 iso-propyl, and cyclohexyl), and a plausible interpretation for this behavior has already been given [8,9]. The syndiotactic degrees of the polybutadienes obtained with the two systems CoCl2(P t BuPh2)2−MAO and CoCl2[PPh2(NMDPP)]2−MAO were higher than those exhibited by CoCl2(P i PrPh2)2−MAO and CoCl2(PCyPh2)2−MAO (cfr. runs 1,8 and 5,9), which, as mentioned above, in the series CoCl2(PRPh2)2−MAO (R = methyl, ethyl, normal-propyl, iso-propyl, and cyclohexyl) were those giving the highest syndiotactic content. Therefore, the results obtained with the two novel systems described in the present paper confirm once more that the steric hindrance of the ligand exerts a strong influence on the mutual orientation of the allylic unit of the growing chain and of the new incoming monomer that is responsible for the polymerization stereoselectivity.
All the obtained polybutadienes were semicrystalline (the X-ray powder spectra are reported in the Supplementary Materials, Figures S14−S18), with a melting point in the range from 141 to 161 °C, and the crystallinity degree essentially depending on the polymer syndiotacticity.

General Procedure and Materials
Tert-butyl(diphenyl)phosphine (P t BuPh 2 ) (Aldrich, St. Louis dry dinitrogen and kept over molecular sieves; pentane (Aldrich, ≥99.5%) was refluxed over Na/K alloy for ca. 8 h, then distilled and stored over molecular sieves under dry dinitrogen; toluene (Aldrich, ≥99.5% pure) was refluxed over Na for ca. 8h, then distilled and stored over molecular sieves under dry dinitrogen. 1,3-Butadiene (Aldrich, ≥99.5%) was evaporated from the container prior to each run, dried by passing through a column packed with molecular sieves and condensed into the reactor which had been precooled to −20 • C. All the phosphine cobalt complexes were synthesized as indicated below, following a general procedure already reported in the literature [23].

Synthesis of Cobalt Phosphine Complexes
Cobalt dichloride (CoCl 2 ) anhydrous (1.24 g, 9.6 mmol) and 40 mL of ethanol were placed in a 250 mL flask: the blue solution obtained was kept under stirring, at 22 • C, for about 2 hours. Subsequently, tert-butyldiphenylphosphine (P t BuPh 2 ) (5.1 g, 21 mmol) dissolved in ethanol (30 mL) was added: everything was kept under stirring, at room temperature, for 24 hours and subsequently the solvent was removed almost completely under vacuum. Then, pentane (40 mL) was added and the obtained suspension was kept under stirring at room temperature for 2 hours: at the end, the light blue/blue suspension obtained was filtered, the residue obtained was washed further with cold pentane (2 × 10 mL) and then dried under vacuum at room temperature. The blue solid obtained was then continuously extracted with boiling pentane; crystals of CoCl 2 (P t BuPh 2 ) 2 were formed directly on the bottom of the Schlenk tube during the extraction and further crops of crystals were obtained by cooling the supernatant pentane solution at −30 • C.
The crystallographic data obtained are shown in Table 3. The FTIR spectrum of 1 is shown in Figure S19 of the Supplementary Materials.

CoCl 2 [PPh 2 (NMDPP)] 2 (2)
Cobalt dichloride (CoCl 2 ) anhydrous (0.182 g, 1.4 mmol) and 20 mL of ethanol were placed in a 100 mL flask: the blue solution obtained was kept under stirring at room temperature for 1 hour. Subsequently, (S)-(+)neomenthyl-diphenylphosphine [(S)-NMDPP] (1.0 g, 3.08 mmol) dissolved in ethanol (30 mL) was added: the solution obtained was brought to 60 • C and kept under stirring at this temperature for 24 hours. The solvent was removed under vacuum, then pentane (30 mL) was added and the suspension was kept under stirring at room temperature for 2 hours. At the end, the light blue/blue suspension obtained was filtered and the residue obtained was further washed with pentane (2 × 10 mL) and then dried under vacuum at room temperature. Yield, 0.950 g of a light blue powder, 87% conversion with respect to the loaded cobalt dichloride.
The FTIR spectrum of 2 is shown in Figure S20 of the Supplementary Materials.

X-ray Crystallographic Studies
A summary of the experimental details concerning the X-ray diffraction study of 1 is reported in Table 3. The crystals used for data collection were entirely covered with perfluorinated oil to reduce crystal decay. X-ray data were collected on a Bruker Smart Apex CCD area detector (Bruker AXS Inc., Madison, WI, USA) equipped with fine-focus sealed tube operating at 50 kV and 30 mA, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data reduction was made using SAINT programs [24]; absorption corrections based on multiscan were obtained by SADABS [24]. The structures were solved by SHELXS-97 [25] and refined on F2 by full-matrix least-squares using SHELXL-14 [26]. The program ORTEP-III [27] was used for molecular graphics.

Polymerization
All operations were carried out under an atmosphere of dry dinitrogen. A standard procedure is reported. 1,3-Butadiene was condensed into a 25-mL dried glass reactor kept at −20 • C, then solvent was added and the obtained solution was brought to the desired polymerization temperature. MAO and the cobalt compound were then added as toluene solutions in the order given. The polymerization was terminated with methanol containing a small amount of hydrochloric acid, the polymer was coagulated and repeatedly washed with methanol, and then dried in vacuum at room temperature.  [28,29]. Differential scanning calorimetry (DSC) scans were carried out on a Perkin Elmer Pyris 8000 (Waltham, MA, USA). Typically, ca. 10 mg of polymer was analyzed in each run, while the scan speed was 20 • C/min under a dinitrogen atmosphere. Wide-angle X-ray diffraction (XRD) experiments were performed at 25 • C under nitrogen flux, using a Siemens D-500 diffractometer equipped with Soller slits (2 • ) placed before samples, 0.3 • aperture and divergence windows, and a VORTEX detector with extreme energy resolution specific for thinner films. Cu Kα radiation with 40 kV × 40 mA power used was adopted, and each spectrum was carried out with steps of 0.05 • 2θ, and 6 s measure 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 . 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
We synthesized and characterized two novel cobalt diphenylphosphine complexes which, in combination with MAO, gave from 1,3-butadiene highly syndiotactic 1,2 polymers. The results obtained were as expected on the basis of the mechanistic hypotheses previously formulated [12,13], confirming their validity.
Supplementary Materials: Tables of atomic coordinates, anisotropic thermal parameters, bond lengths and angles of 1 may be obtained free of charge from The Director CCDC, 12 Union Road, Cambridge CB2 1 EZ, UK, on quoting the deposition numbers CCDC 1908992 the names of the authors and the journal citation (http://www.ccdc.cam.ac.uk). The following are available online: Figures S1−S6: 13 C NMR spectra (olefinic region) of the obtained poly(1,3-butadiene)s, Figures S7−S13: FTIR spectra of the obtained poly(1,3-butadiene)s, Figures S14−S18: X-ray powder spectra of some selected poly(1,3-butadiene)s, Figure S19: FTIR spectrum of 1, Figure S20: FTIR spectrum of 2.
Funding: This research received no external funding.