Some novel cobalt diphenylphosphine complexes: Synthesis, characterization, and behavior in the polymerization of 1,3‐butadiene

Article abstract of DOI:10.3390/molecules26134067

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.

Full text of DOI:10.3390/molecules26134067

molecules
Article
Some Novel Cobalt Diphenylphosphine Complexes: Synthesis,
Characterization, and Behavior in the Polymerization
of 1,3-Butadiene
Giovanni Ricci 1,
* ,
, Giuseppe Leone 1 , Giorgia Zanchin 1, Benedetta Palucci 1, Alessandra Forni 1,†
Anna Sommazzi 2 , Francesco Masi 3, Stefano Zacchini 4 , Massimo Guelfi 5 and Guido Pampaloni 5
1
CNR—Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” (SCITEC), Via Corti 12, I-20133 Milano, Italy;
giuseppe.leone@scitec.cnr.it (G.L.); giorgia.zanchin@scitec.cnr.it (G.Z.); benedetta.palucci@scitec.cnr.it (B.P.);
alessandra.forni@scitec.cnr.it (A.F.)
Versalis S.p.A.—Centro Ricerca Novara, Via Fauser 4, I-28100 Novara, Italy; anna.sommazzi@versalis.eni.com
2
3
Versalis S.p.A.—R&D Partner Catalysis, Piazza Boldrini 1, San Donato Milanese, I-20097 Milano, Italy;
francesco.masi@versalis.eni.com
Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4,
4
I-40136 Bologna, Italy; stefano.zacchini@unibo.it
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi 13, I-56124 Pisa, Italy;
5
massimo.guelfi@unipi.it (M.G.); guido.pampaloni@unipi.it (G.P.)
*
Correspondence: giovanni.ricci@scitec.cnr.it
†
Current address: CNR—Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” (SCITEC), Via Golgi 19,
I-20133 Milano, Italy.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: 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 polymeriza-
tion 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 signifi-
cant differences were observed depending on the MAO/Co ratio used, and a plausible interpretation
for such a different behavior is proposed.
Citation: Ricci, G.; Leone, G.;
Zanchin, G.; Palucci, B.; Forni, A.;
Sommazzi, A.; Masi, F.; Zacchini, S.;
Guelfi, M.; Pampaloni, G. Some
Novel Cobalt Diphenylphosphine
Complexes: Synthesis,
Characterization, and Behavior in the
Polymerization of 1,3-Butadiene.
Molecules 2021, 26, 4067. https://
doi.org/10.3390/molecules26134067
Keywords: cobalt phosphine complexes; catalysts; stereospecific polymerization; poly(1,3-butadiene);
X-ray structure
Academic Editor: Erika Bálint
Received: 8 June 2021
Accepted: 25 June 2021
Published: 2 July 2021
1. Introduction
Catalyst systems obtained by combining CoCl2(PRPh2)2 (R = methyl, ethyl, allyl,
normal-propyl, iso-propyl, tert-butyl, (S)-(+)neomenthyl, cyclohexyl and phenyl) with
MAO are well known to be able to give predominantly 1,2 polymers (~85%) from 1,3-
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iations.
butadiene [1–3]. The tacticity of the 1,2 polymers was found to depend on the type of
phosphine bonded to the cobalt atom, the syndiotacticity degree generally increasing when
increasing the steric hindrance of the phosphine ligand.
According to the diene polymerization mechanism previously proposed [
mation of 1,2 polymers was attributed to the presence of a phosphine ligand on the cobalt
atom [ ]; it is in fact worthwhile to note that naked CoCl2, 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
CoCl2/MAO, specific for cis-1,4 poly(1,3-butadiene), can be formed.
4], the for-
Copyright:
© 2021 by the authors.
5–7
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Molecules 2021, 26, 4067. https://doi.org/10.3390/molecules26134067
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 4067
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To check this hypothesis, we have now synthesized some novel cobalt diphenylphos-
phine complexes by reacting CoCl2 with (2-methoxyethyl)diphenylphosphine, (2-methoxy-
phenyl) diphenylphosphine, and 2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine
(Figure 1), that is ligands which contain within their structure heteroatoms such as O
and N and have some affinity for aluminum. We have examined their behavior in the
polymerization of 1,3-butadiene as the MAO/Co ratio varies, in comparison with that of
the systems CoCl2(PRPh2)2/MAO (R = Ph, nPr). Furthermore, we also examined in more
detail the influence of MAO/Co molar ratio on the polymerization of 1,3-butadiene with
CoCl2(PRPh2)2/MAO (R = Ph, nPr), even at very high MAO/Co ratios (up to 5000). The
results obtained are reported in the present paper, together with a plausible interpretation.
Figure 1. Chemical structures of CoCl2(Ph2PCH2CH2OCH3)2 (
and CoCl2(Ph2P(C5NH3(C5H11))2 (3).
1), CoCl2(Ph2P(C6H4OCH3)) 2 (2),
2. Results and Discussion
2.1. Synthesis and Characterization of Cobalt Complexes
The novel diphenylphosphine cobalt complexes, CoCl2(Ph2PCH2CH2OCH3)2 (
1),
CoCl2(Ph2P(C6H4OCH3))2 ( ), and CoCl2(Ph2P(C5NH3(C5H11))2 ( ) (Figure 1) were pre-
2
3
pared according to a general experimental procedure already reported in the literature [
8
].
CoCl2 was reacted with an excess of phosphine ligands using ethanol as a solvent.
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).
Figure 2. Molecular structure of 1 with key atoms labeled. Thermal ellipsoids are drawn at the 50%
probability level.

Molecules 2021, 26, 4067
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Figure 3. Molecular structure of
2
with key atoms labeled. Thermal ellipsoids are drawn at the 50%
probability level.
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
CoCl2(PRPh2)2 complexes (R = ethyl [2], normal-propyl [
2], iso-propyl [1], tert-butyl [3],
CH2CH(OCH3)2 [ ], CH2C(O)Ph [10], and Ph [11
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
9
,
12]), i.e., 2.21
−
2.24 and 2.36−2.43 Å
,
each structure, P–Caliphatic bonds (varying from 1.827 [
longer than P–Caromatic ones (ranging from 1.812 [10] to 1.833 Å, the latter value observed
in ). In particular, as already pointed out in our previous analysis on CoCl2(PRPh2)2 com-
plexes [ ], the greater the steric hindrance of the aliphatic group, the larger the difference
in P–C bond lengths. For compound , only a little difference was observed, the average
P–Caliphatic bond distance, 1.838 Å, being only slightly longer than the average P–Caromatic
one, 1.833 Å. In the case of , only P–Caromatic bonds were present, measuring on average
1.823 Å and virtually identical to those of the CoCl2(PPh3)2 structure [11 12]. In the solid
state, several short intermolecular contacts were present, including CH2-O HCHP (in ),
Cphenyl Hphenyl and Cphenyl HCH2 (in ), and Co-Cl Hphenyl (in both structures) hydrogen
2] to 1.888 Å [3]) were systematically
1
3
1
2
,
·
1
·
·
2
·
bonds as well as normal van der Waals distances.
Table 1. Selected bond lengths and angles for 1.
Bond Lengths (Å)
Co–Cl1
Co–P1
P1–C1
P1–C4
2.229(3)
2.361(3)
1.843(10)
1.821(12)
Co–Cl2
Co–P2
P2–C21
P2–C24
2.220(3)
2.356(3)
1.834(10)
1.848(12)
P1–C10
C1–C2
C2–O1
O1–C3
1.831(11)
1.514(15)
1.393(14)
1.408(14)
P2–C30
C21–C22
C22–O2
O2–C23
1.834(11)
1.526(15)
1.415(13)
1.431(14)
◦
Bond Angles ( )
Cl1–Co–Cl2
Co–P1–C1
C1–P1–C4
C4–P1–C10
114.72(12)
109.7(4)
105.2(5)
105.1(6)
P1–Co–P2
Co–P2–C21
C21–P2–C24
C24–P2–C30
107.54(11)
118.1(4)
104.7(5)
103.0(5)
C10–P1–Co
P1–C1–C2
C1–C2–O1
C2–O1–C3
122.1(4)
111.4(8)
106.0(10)
112.5(10)
C30–P2–Co
P2–C21–C22
C21–C22–O2
C22–O2–C23
112.5(4)
113.0(7)
106.0(8)
111.3(8)

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Table 2. Selected bond lengths and angles for 2.
Bond Lengths (Å)
2.2218(6)
Co–Cl1
Co–P1
P1–C1
P1–C7
P1–C13
Co–Cl2
Co–P2
2.2395(5)
2.4115(6)
1.820(2)
1.829(2)
1.817(2)
2.4064(6)
1.821(2)
1.821(2)
1.829(2)
P2–C19
P2–C25
P2–C31
◦
Bond Angles ( )
Cl1–Co–Cl2
Co–P1–C1
C1–P1–C7
C7–P1–C13
C13–P1–Co
124.01(2)
109.25(7)
104.87(9)
101.57(9)
114.16(7)
P1–Co–P2
Co–P2–C19
C19–P2–C25
C25–P2–C31
C31–P2–Co
111.02(2)
119.67(7)
104.18(9)
105.08(9)
110.80(7)
2.2. 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 CoCl2(PPh3)2/MAO and CoCl2(PnPrPh2)2/MAO are
reported for comparison.
1
Table 3. Polymerization of 1,3-butadiene with cobalt catalytic systems.
Time
(min)
N 2
(min
cis-1,4 3
(%)
Run
Co-Complex (µmol)
Al/Co
Yield (%)
1,2 3 (%)
[rr] 4 (%)
−1
)
1
2
3
4
5
6
7
8
9
1 (10)
1 (10)
1 (30)
2 (10)
2 (10)
2 (30)
3 (10)
3 (10)
1000
100
20
1000
100
20
1000
100
3000
1600
1000
500
300
100
5000
1000
500
100
50
30
30
60
60
60
60
10
10
30
4
4
2
2
2
88.5
61.8
84.9
92.9
78.2
50.4
90.1
81.7
87.3
58.8
88.9
90.7
93.5
94.8
30.8
80.6
72.4
100
4130
2884
661
2168
1825
87.3
19.7
18.9
79.4
19.4
21.7
94.7
36.3
48.2
33.4
28.7
26.0
21.8
21.3
54.1
26.0
26.4
24.0
23.7
23.4
12.7
80.3
81.1
19.6
80.6
78.3
5.3
63.7
51.8
66.6
71.3
74.0
78.2
78.7
45.9
74.0
73.6
76.0
76.3
76.6
51
50
66
65
392
12,614
11,438
13,580
41,160
62,230
63,490
65,450
66,360
3194
32,240
33,787
46,667
38,880
18,060
46
46
46
46
43
45
44
66
64
64
65
66
65
CoCl2(PnPrPh2)2 (3)
CoCl2(PnPrPh2)2 (5)
CoCl2(PnPrPh2)2 (5)
CoCl2(PnPrPh2)2 (10)
CoCl2(PnPrPh2)2 (10)
CoCl2(PnPrPh2)2 (10)
CoCl2(PPh3)2 (1)
CoCl2(PPh3)2 (5)
CoCl2(PPh3)2 (5)
CoCl2(PPh3)2 (5)
CoCl2(PPh3)2 (5)
CoCl2(PPh3)2 (10)
10
11
12
13
14
15
16
17
18
19
20
1
135
7
6
6
7
97.2
90.3
20
7
Polymerization conditions: toluene, total volume 16 mL; 1,3-butadiene, 2 mL; MAO; 22 ◦C. The polymer molecular weights are in
2
the range 100,000–200,000 g
·
mol−1, with a Mw/Mn in the range 2–3, N = moles of butadiene polymerized/moles of Co
×
minutes,
3
determined by FTIR and 1H NMR, 4 percentage of syndiotactic triads, determined by 13C NMR.
Catalysts based on the new cobalt complexes 1–3 were much less active than the
systems CoCl2(PnPrPh2)2/MAO and CoCl2(PPh3)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 CoCl2(PnPrPh2)2/MAO
and CoCl2(PPh3)2/MAO.
Concerning the selectivity, all five catalytic systems, the three new ones and the two
old ones, showed a very similar behavior operating at a MAO/Co molar ratio of 100 or

Molecules 2021, 26, 4067
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lower, providing poly(1,3-butadiene)s with a predominantly 1,2 structure. A completely
different behavior was instead observed at MAO/Co ratio of 1000: CoCl2(PPh3)2/MAO
and CoCl2(PnPrPh2)2/MAO still gave predominantly 1,2 poly(1,3-butadiene)s, whereas the
catalytic systems 1/MAO, 2/MAO, and 3/MAO gave highly cis-1,4 poly(1,3-butadiene)s.
Taking into consideration that the CoCl2/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 [1–3,5,13], is that shown in
Figure 4A, with only one phosphine ligand on the cobalt atom, the incoming monomer cis-
η4 coordinated, and the growing chain bonded to the cobalt atom through a η3−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.
Figure 4. Possible structure of the active sites with the phosphine ligand on the cobalt atom (
A) and
on MAO (B).
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 CoCl2(PPh3)2/MAO and CoCl2(PnPrPh2)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(1,3-
butadienes). 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 CoCl2
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 CoCl2(PPh3)2/MAO
and CoCl2(PnPrPh2)2/MAO at higher MAO/Al molar ratio (up to 5000) give poly(1,3-
butadiene)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.

Molecules 2021, 26, 4067
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3. Materials and Methods
3.1. General Procedure and Materials
2-(1,1-Dimethylpropyl)-6-(diphenylphosphino)pyridine (Aldrich, Merck KGaA (Darm-
stadt, Germany 98% pure (HPLC)), (2-methoxyphenyl)diphenylphosphine (Aldrich (Merck
KGaA (Darmstadt, Germany) 98% pure (HPLC)), anhydrous cobalt dichloride (Aldrich,
Merck KGaA (Darmstadt, Germany) 99,9% pure), and methylaluminoxane (MAO) (Aldrich,
Merck KGaA (Darmstadt, Germany), 10 wt% solution in toluene) were used as received.
(2-Methoxyethyl)diphenylphosphine [14], CoCl2(PnPrPh2)2 [2], and CoCl2(PPh3)2 [8] were
prepared according to that reported in the literature. Ethanol (Aldrich, Merck KGaA
(Darmstadt, Germany)) was degassed under vacuum, then by bubbling dry dinitrogen
and kept over molecular sieves; pentane (Aldrich, Merck KGaA (Darmstadt, Germany),
99% pure) was refluxed over Na/K alloy for 8 h, then distilled and stored over molecular
sieves under dry dinitrogen; toluene (Aldrich, Merck KGaA (Darmstadt, Germany), 99,8%
pure) was refluxed over Na for 8 h, then distilled and stored over molecular sieve under
dry dinitrogen. 1,3-Butadiene (Aldrich, Merck KGaA (Darmstadt, Germany), >99,5% pure)
was evaporated from the container before each run, dried by passing through a column
packed with molecular sieves, and condensed into the reactor, which had been precooled to
−
20 ◦C. The two novel phosphine cobalt complexes were synthesized as indicated below,
following a general procedure already reported in the literature [8].
3.2. Synthesis of Cobalt Phosphine Complexes
3.2.1. [(2-Methoxyethyl)diphenylphosphine] Cobalt Dichloride (1), Toluene as Medium
(2-Methoxyethyl)diphenylphosphine (0.605 g, 2.48
toluene (20 mL), and CoCl2 (0.161 g, 1.24
10−3 mol) was successively added. A blue
suspension was gradually formed; after 24 h it was filtered, then washed with toluene
10 mL). The blue solid isolate was then dried in vacuum
×
10−3 mol) was dissolved in
×
(
2 × 5 mL) and heptane (2 ×
at room temperature. The blue supernatant solution was removed, concentrated, and
cooled at
crystallographic studies. Yield: 0.552 g (72.0% based on CoCl2).
Anal. Calcd. for C30H34Cl2CoO2P2: C, 58.27; H, 5.54; Cl, 11,47; Co, 9.53; Found: C,
58.00; H, 5.19; Cl, 11,84; Co, 9.12.
◦
−
18 C, causing the precipitation of a crystalline product suitable for X-ray
Spectroscopic data: IR (KBr)
ν
(cm−1) 3051 w, 2885 w, 1584 w, 1572 w, 1485 m, 1433 s,
1386 m, 1100 vs, 953 s, 741 vs, 690 vs (see Supplementary Materials, Figures S1 and S1A).
3.2.2. [(2-Methoxyphenyl)diphenylphosphine] Cobalt Dichloride (2),
Tetrahydrofurane/Ethanol as Medium
(2-Methoxyphenyl)diphenylphosphine (1.00 g, 3.42
tetrahydrofurane (30 mL) and successively added to a solution of CoCl2 (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
×
10−3 mol) was dissolved 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 CoCl2).
Anal. Calcd. for C38H34Cl2CoO2P2: C, 63.88; H, 4.80; Cl, 9.92; Co, 8.25; Found: C, 64.0;
H, 4.7; Cl, 9.8; Co, 8.4.
Spectroscopic data: IR (KBr)
757 s, 746 s, 693 s, 505 s, 492s (see Supplementary Materials, Figure S2).
ν
(cm−1). 1475mw, 1464 mw, 1435 m, 1101 m, 906 m,

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3.2.3. [2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine] Cobalt Dichloride (3)
2-(1,1-Dimethylpropyl)-6-(diphenylphosphino)pyridine (0.997 g, 2.99
10−3 mol)
was dissolved in ethanol (30 mL) and successively added to a solution of CoCl2 (0.180 g,
1.38
10−3 mol) in ethanol (20 mL). A deep blue solution was gradually formed; after
48 h the solvent was removed under vacuum, obtaining a blue oil product, which was
washed several times at low temperature with pentane (4 20 mL) and then dried under
×
×
×
vacuum at room temperature. A blue microcrystalline powder was at the end obtained:
yield 0.685 g (62.3% based on CoCl2). Single crystals of the complex were obtained by
dissolving the complex in diethylether and by recrystallizing at low temperature.
Anal. Calcd. for C44H48Cl2CoN2P2: Co, 7.40; Cl, 8.90; P, 7.78; C, 66.34; H, 6.07; N, 3.52.
Found: Co, 7.6; Cl, 9.0; P, 7.7; C, 66.2; H, 6.2; N, 3.6.
Spectroscopic data: IR (KBr)
ν
(cm−1) 1599 s, 1435 ms, 1260 m, 1095 m, 808 m, 742 s,
692 s, 504 m (see Supplementary Materials, Figure S3).
3.3. 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.
Table 4. Crystal data, data collection, and refinement details for 1 and 2.
1
2
Crystal data
Chemical formula
Mr
Crystal system
Space group
Temperature [K]
a [Å]
C30H34Cl2CoO2P2
618.34
C38H34Cl2CoO2P2
714.42
triclinic
triclinic
P−1 (No. 2)
P−1 (No. 2)
100(2)
120(2)
9.922(4)
9.3758(7)
9.927(4)
12.3136(9)
15.7602(12)
99.7760(10)
104.3110(10)
100.9440(10)
1685.5(2)
b [Å]
16.872(6)
101.510(9)
90.902(9)
110.168(7)
1522.1(10)
2
c [Å]
◦
α [ ]
◦
β [ ]
◦
γ [ ]
V [Å3]
Z
2
−1
0.869
0.796
µ(MoKα) [mm
]
Crystal size [mm]
Crystal color, habit
0.19 × 0.14 × 0.11
0.15 × 0.12 × 0.10
blue, plate
light blue, prism
Data collection
Tmin, Tmax
No. of measured reflns
No. of independent refls
No. of observed refls [I >
2σ(I)]
0.509, 0.746
10,336
0.708, 0.746
33,407
5272
10,240
3640
7397
Rint
0.0827
0.1307
0.595
0.0467
0.0551
0.715
Rσ
−1
(sin θ/λ)max [Å
]
Refinement
R[F2 > 2σ(F2)]
0.1221
0.3066
1.145
5272
336
0.0463
0.1138
1.007
10,240
408
wR(F2)
S
No. of reflections
No. of parameters
No. of restraints
42
0
−3
2.130, −0.926
1.080, −0.295
∆ρmax, ∆ρmin (e Å
)

Molecules 2021, 26, 4067
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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–K
α
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 F2 [16]. Hydrogen atoms were fixed at calculated positions and refined by a
riding model.
3.4. 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.
3.5. Polymer Characterization
The polymer microstructure was determined through FT-IR and NMR (1H and
13C) analyses (see Supplementary Materials, Figures S4–S15) as reported in the litera-
ture [17–23].13C NMR and 1H NMR measurements were performed with a Bruker AM
400 ◦instrument (Bruker Italia Srl, Milano, Italy). The spectra were obtained in C2D2Cl4 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 Sup-
plementary 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. CuK
was adopted, and each spectrum was measured with steps of 0.05 2
α
radiation with power use of 40 KV
×
40 mA
◦
θ
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 (Mw) and the molecular weight distribution (Mw/Mn) 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 Mw/Mn poly(styrene) standards with
.
Mws ranging from 162 to 5.6
dissolved in 5 mL of o-DCB with 0.05% of BHT as antioxidant.
×
106 g mol−1. For SEC analysis, about 12 mg of polymer was
4. 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 pro-
cess 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-

Molecules 2021, 26, 4067
9 of 10
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].
Supplementary Materials: Tables of atomic coordinates, anisotropic thermal parameters, bond
lengths and angles of
Road, Cambridge CB2 1 EZ, UK, on quoting the deposition number CCDC 1905516 (compound
and CCDC 2074828 (compound ), the names of the authors and the journal citation (fax: +44-1223-
336-033; e-mail: deposit@ccdc.cam.ac.uk; web site: http://www.ccdc.cam.ac.uk). FT-IR spectra of
the cobalt complexes
; FT-IR spectra, NMR (1H and 13C) spectra, and XRD spectra of some of the
1 and 2 may be obtained free of charge from The Director CCDC, 12 Union
1
)
2
1-3
polybutadienes of Table 3. Figure S1. FT-IR spectrum of (2-methoxyethyl)diphenylphosphine; Figure
S1A. FT-IR spectrum of [(2-methoxyethyl)diphenylphosphine]2 cobalt dichloride (1), Figure S2. FT-IR
spectrum of [(2-methoxyphenyl)diphenylphosphine]2 cobalt dichloride (2); Figure S3. FT-IR spectrum
of [2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine]2 cobalt dichloride (3); Figure S4. FT-IR
spectrum of the polybutadiene of Table 3, run 1; Figure S5. FT-IR spectrum of the polybutadiene of
Table 3, run 2; Figure S6. FT-IR spectrum of the polybutadiene of Table 3, run 3; Figure S7. FT-IR
spectrum of the polybutadiene of Table 3, run 4; Figure S8. FT-IR spectrum of the polybutadiene of
Table 3, run 5; Figure S9. FT-IR spectrum of the polybutadiene of Table 3, run 6; Figure S10. FT-IR
spectrum of the polybutadiene of Table 3, run 7; Figure S11. FT-IR spectrum of the polybutadiene of
Table 3, run 8; Figure S12. NMR spectra (1H down, 13C up) of the polybutadiene of Table 3, run 2;
Figure S13. NMR spectra (1H down, 13C up) of the polybutadiene of Table 3, run 3; Figure S14. NMR
spectra (1H down, 13C up) of the polybutadiene of Table 3, run 5; Figure S15. NMR spectra (1H down,
13C up) of the polybutadiene of Table 3, run 6; Figure S16. XRD spectrum of the polybutadiene of
Table 3, run 5; Figure S17. XRD spectrum of the polybutadiene of Table 3, run 6.
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.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: Authors wish to thank Fulvia Greco and Daniele Piovani for their support in
NMR and GPC analyses of the polymers.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the cobalt compounds and of the polymers are not available from
the authors.
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