Symmetrical and unsymmetrical diphosphanes with diversified alkyl, aryl, and amino substituents

Article abstract of DOI:10.1039/c8dt03775b

We present the comprehensive study of diphosphanes with diversified substituents regarding their syntheses, structures, and properties. To this end, we have synthesized a series of novel unsymmetrical alkyl, aryl and amino-substituted diphosphanes of the general formula R₁R₂P-PR₃R₄ (where R₁, R₂, R₃, R₄ = tBu, Ph, Et₂N or iPr₂N) via a salt metathesis reaction of halophosphanes with metal phosphides in high yield. We vastly expanded this group of compounds by obtaining the first mono- A nd tri-amino-substituted systems. The structures of the isolated compounds were characterized by NMR spectroscopy and X-ray diffraction. The isolated unsymmetrical diphosphanes have no tendency to rearrange to the corresponding symmetrical species. Additionally, we proposed the general classification of diphosphanes based on the number of different groups attached to phosphorus atoms and their distribution within a molecule. To investigate the impact of substituents on the properties of P-centers and a molecule as a whole, we conducted a DFT study on the electronic and steric properties of the obtained systems. The experimental and theoretical results can be very useful for designing P-P systems with desired properties.

Full text of DOI:10.1039/c8dt03775b

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Symmetrical and unsymmetrical diphosphanes
with diversified alkyl, aryl, and amino substituents†
Cite this: Dalton Trans., 2018, 47,
16885
Natalia Szynkiewicz,
Łukasz Ponikiewski
and Rafał Grubba
*
We present the comprehensive study of diphosphanes with diversified substituents regarding their synth-
eses, structures, and properties. To this end, we have synthesized a series of novel unsymmetrical alkyl,
aryl and amino-substituted diphosphanes of the general formula R1R2P-PR3R4 (where R1, R2, R3, R4 = tBu,
Ph, Et2N or iPr2N) via a salt metathesis reaction of halophosphanes with metal phosphides in high yield.
We vastly expanded this group of compounds by obtaining the first mono- and tri-amino-substituted
systems. The structures of the isolated compounds were characterized by NMR spectroscopy and X-ray
diffraction. The isolated unsymmetrical diphosphanes have no tendency to rearrange to the corres-
ponding symmetrical species. Additionally, we proposed the general classification of diphosphanes based
on the number of different groups attached to phosphorus atoms and their distribution within a molecule.
To investigate the impact of substituents on the properties of P-centers and a molecule as a whole, we
conducted a DFT study on the electronic and steric properties of the obtained systems. The experimental
and theoretical results can be very useful for designing P–P systems with desired properties.
Received 18th September 2018,
Accepted 5th November 2018
DOI: 10.1039/c8dt03775b
rsc.li/dalton
sequently, to an asymmetric distribution of the electron
Introduction
density and polarization of the P–P bond.41 While simple, sym-
Diphosphanes are the simplest group of compounds con- metrical systems like Ph2P-PPh2 were found to be stable upon
taining the P–P bond.1,2 These compounds consisting of heating up to 200 °C,42 an asymmetric distribution of electron
two PR2 units are considered as organic derivatives of dipho- density over phosphorus atoms in unsymmetrical dipho-
3
sphane P2H4 in which one or more hydrogen atoms were sphanes may affect their stability besides the steric
replaced with e.g., a halogen,4,5 silyl,5–8 amine,5,9–17 alkyl,18–23 hindrance.43–45 The incorporation of sterically demanding and
alkanoyl,24 aryl,18–20,22,24 boryl25,26 or borazinyl27 group giving electron-withdrawing substituents on the P-atoms increases
diversified structures. Organo-substituted diphosphanes with the tendency of the P–P bond to dissociate homolytically, and
P–C, P–Si, P–Li or P–Cl bonds are widely applicable in organo- equilibrium between the radical and dimer form is observed
metallic chemistry as precursors of diphosphorous or polypho- in the solution.41,46,47 The type and size of substituents bound
sphorus ligands containing the P–P bond4,28–39 or precursors with phosphorus atoms determine their stereochemistry as
of bidentate PC⋯CP ligands.18,19 Recently, Gudat et al. have well.20,48–51 If two different groups are attached to one or both
developed a class of N-heterocyclic diphosphanes with P–N phosphorus atoms, symmetrical and unsymmetrical chiral
bonds and found that systems with a highly polarized P–P structures may be obtained. Furthermore, the presence of two
bond may serve as catalysts in the synthesis of dispho- chiral centers allows the formation of diastereomeric pairs. So
sphanes.10,11,40,41 The properties of diphosphanes strongly far, several approaches to the synthesis of diphosphanes have
depend on the nature of substituents at P-atoms. In the case of been developed and applied. The first organo-substituted
unsymmetrical species, differences in the electron-donating systems,52–54 including unsymmetrical species,55,56 were pre-
properties of substituents lead to the formation of more pared via a simple reaction of the secondary phosphine R2PH
nucleophilic and electrophilic sites of the P–P bond and, con- with chlorophosphane R2PCl (Scheme 1A). This method was
further improved by the addition of a tertiary amine capturing
hydrochloride (Scheme 1B)40,57 or replacing phosphine with its
silyl derivative (Scheme 1C).40 Symmetrical systems may be
Department of Inorganic Chemistry, Faculty of Chemistry, Gdańsk University of
Technology, G. Narutowicza St. 11/12, PL-80-233 Gdansk, Poland.
easily obtained by reductive coupling of two chlorophosphane
molecules in the presence of active metals like lithium,58
E-mail: rafal.grubba@pg.edu.pl
†Electronic supplementary information (ESI) available: Experimental, crystallo-
sodium,42,59,60 potassium42 or magnesium61 (Scheme 1D) or
graphic, spectroscopic and computational details. CCDC 1560634–1560642,
1576833–1576836 and 1585541. For the ESI and crystallographic data in CIF or
by the direct coupling reaction of phosphides R2PLi and the
respective chlorophosphanes R2PCl (Scheme 1E).18,62 Method
other electronic format, see DOI: 10.1039/c8dt03775b
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mostly related to the last step, removal of the BH3-protecting
group followed by the isolation of a product.
The main aim of our study on diphosphanes was to obtain
a range of new species with diversified nucleophilic properties
of P-atoms to apply them as basic components in frustrated
Lewis pairs (FLPs). Hence, we synthesized a series of novel
symmetrical and unsymmetrical alkyl-, aryl- and amino-substi-
tuted diphosphanes of the general formula R1R2P-PR3R4
(where R1, R2, R3, R4 = tBu, Ph, Et2N or iPr2N; Chart 1, com-
pounds 2, 6, and 9–29). Moreover, we found that there are no
reports on mono- and triamino-substituted diphosphanes. To
investigate the influence of substituents on the electronic and
steric properties of diphosphanes and to determine the impact
of selected groups, we conducted NMR, X-ray, and DFT studies
of the obtained systems. Hence, the previously synthesized
systems including the considered groups were also taken into
account (Chart 1, compounds 1,18,69,70 3,18 4/5,71–73 7,18,53,54 8,5
and 30 5,9). In general, we studied thirty compounds: alkyl–aryl-
substituted systems (1–7) and amino-substituted species with
fragments (iPr2N)2P (8–10), (Et2N)2P (11–13), (iPr2N)Ph (14–20),
and (iPr2N)tBu (21–27) as well as systems with four different
substituents 28/29 and a tetra-amino-substituted one 30. To
arrange the considered compounds 1–30, we proposed a simple
general classification of diphosphanes based on the number of
different groups attached to phosphorus atoms and their distri-
bution within a molecule, organized as follows (Chart 1).
Results and discussion
Synthesis and reactivity of diphosphanes
Searching for a simple and effective procedure to obtain
unsymmetrical P–P systems, firstly, we applied the approach
presented by Pringle et al. However, it turned out that the syn-
thesis of unsymmetrical amino-substituted species is not
Scheme 1 Methods of diphosphane synthesis.
E may also be applied for the synthesis of unsymmetrical achievable via this method.18 To overcome this issue, we
diphosphanes,18,19 however the obtained products are signifi- attempted to obtain these systems in a direct coupling reaction
cantly contaminated with symmetrical diphosphanes as in the between phosphide RR′PLi and chlorophosphane (iPr2N)2PCl.
case of method F where unsymmetrical systems are formed by It is worth mentioning that this method was previously used
mixing two different symmetrical species (Scheme 1F).20,51,63 for the preparation of symmetrical diphosphanes
Furthermore, symmetrical diphosphanes can be obtained in (Scheme 1E).18,62 Surprisingly, the 31P{1H} NMR spectra of the
dehydrocoupling reactions of secondary phosphines catalyzed reaction mixtures (8–10) revealed a complete conversion of
by transition metal complexes (Scheme 1G)64–66 or in P–P substrates into the products at a low temperature and, what is
reductive coupling reactions mediated by N-heterocyclic car- the most important, no rearrangement products were
benes (Scheme 1H).67,68
observed. Pure diphosphanes were isolated in 80–98% yields,
Pringle et al. proposed the synthesis of unsymmetrical and X-ray quality crystals were grown from toluene solutions.
diphosphanes based on the formation of a borane adduct of Moreover, this method was also applied to the synthesis of
the respective secondary phosphine in the first step, then alkyl–aryl-substituted systems. We obtained two new unsym-
lithiation of the intermediate complex, a coupling reaction metrical systems of this kind, 2 and 6 and repeated the syn-
with the appropriate chlorophosphane and finally the removal thesis of previously described 3.18 In the latter case, we
of the BH3 group by an amine (Scheme 1I).18,19 Given that the enhanced the yield of the reaction and isolated the product as
anionic phosphorus center is more crowded by the presence of colorless, X-ray quality crystals. Using the respective chloro-
BH3, the coupling of two bulky units and consequently, the phosphane and phosphide fragments as shown in Fig. 1, we
formation of the symmetrical system are less favored. obtained diphosphanes 2, 3, 6, 8–17, 21–24 and 28/29
Although this method enables us to obtain pure products, the (Scheme 2A). Although this method is a very efficient and
yield of this process is usually less than 50%. The low yield is effective way for the synthesis of diversified systems, it does
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Chart 1 General classification of the obtained diphosphanes into four types (I–IV) divided into seven groups (a–g) based on the number of
different substituents and their arrangement within a molecule. All considered compounds (1–30) were ordered into particular groups. The dipho-
sphanes belonging to d/d’, f/f’, and g/g’ groups were isolated as pairs of diastereomers. The structures of diastereomers were taken into account in
the classification as their spectroscopic and computational properties differ essentially. Structure drawings ignore the pyramidal geometry of the
P-atoms for simplicity. The previously obtained 1,18,69,70 3,18 4/5,71–73 7,18,53,54 8 5 and 30 5,9 are included in this classification.
have its limitations. At least one of the RR′P fragments build- Some experimental e.g., spectroscopic data were collected
ing the P–P bond must be incorporated as a phosphide deriva- when the respective chlorophosphanes were mixed in a
tive RR′PLi. As we could not lithiate amino-substituted phos- 1 : 1 molar ratio in THF solution with magnesium turnings,
phanes RR′PH to yield the respective RR′PLi, we were not able giving 20 and 27 as one of three products in the reaction
to obtain tri-amino-substituted species like 20 and 27 in this mixture (besides the corresponding symmetrical species, see
straightforward process. We have made a few attempts to the ESI Fig. S34 and S35†). By applying this approach, we syn-
obtain tri-amino-substituted species via methods A–E thesized and isolated symmetrical amino-substituted dipho-
(Scheme 1), however, in no case pure products were yielded. sphanes 18/19, 25/26 and 30 as analytically pure products
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contact with air leads to the formation of oxidation products,
the oxidation of 18/19 to (iPr2N)PhP(O)–O–P(O)(iPr2N)Ph
(ox18/19) may serve as an example of such reaction (see
Fig. S14† for an X-ray structure ox18/19).
Our attempts to synthesize unsymmetrical diphosphanes
revealed that under appropriate reaction conditions, it is poss-
ible to obtain these compounds in a good yield and high
purity without the usage of the intermediate boron adducts
(Scheme 1I). The crucial point is to isolate the diphosphanes
immediately after the reaction is completed. The 31P NMR ana-
lysis of reaction solutions indicates that generally the complete
conversion of substrates into products takes place after
30 minutes at a low temperature. It is worth noting that phos-
phides give coloured THF solutions which become colourless
as the equimolar quantity of chlorophosphine is added and
the respective diphosphane is formed. Hence, the colourless
or pale yellow reaction mixtures suggest the complete conver-
sion of substrates into products. We found that keeping the
reaction mixtures at room temperature for a prolonged time
promotes the metathesis reactions, thus resulting in the for-
mation of symmetrical species and other impurities. In con-
trast, in toluene and/or petroleum ether solutions of the iso-
lated products metathesis did not proceed, even if they were
stored over a few months at room temperature. Therefore, we
decided to study the factors that may promote rearrangement
in the group of nine selected systems (2, 3, 6, 8, 9, 14, 15/16,
17 and 24). We carried out a series of experiments to examine
the impact of solvent (THF), an excess of phosphide (≈10 mol%)
and chlorophosphane (≈30 mol%) used in the synthesis of
these compounds on their stability (Scheme 3).
Fig. 1 Fragments used for the synthesis of novel, unsymmetrical
diphosphanes.
It turned out that diphosphane 6 reacts with the corres-
ponding Ph2PCl, yielding symmetrical 7 and tBuPhCl. This
result was confirmed in the reaction with 150 mol% excess of
Ph2PCl in which 6 quantitatively converted into products after
an hour of mixing. Reactivity towards phosphide components
varies depending on the substituents bonded with RR′PLi. In
the case of bulky tBu2PLi, no products of the P–P bond clea-
vage were formed. In reactions of Ph2PLi with diphosphanes
Ph2P-PRR′ (8, 14) products of the P–P bond cleavage, sym-
metrical species (Ph2P)2 and (RR′P)2 were identified in the 31P
{1H} NMR spectra of reaction mixtures. In the case of a reac-
tion involving 8, about 31 mol% of diphosphane undergoes
Scheme 2 Synthesis of unsymmetrical diphosphanes via direct coup-
ling of RR’PLi and R’’R’’’PCl (A) and synthesis of symmetrical systems by
reductive coupling of chlorophosphane with magnesium (B).
(Scheme 2B). All isolated diphosphanes were obtained in high the P–P bond cleavage that leads to the formation of the
yields (65–98%), for most syntheses the yield of the reaction was respective symmetrical diphosphanes. Similar results were
higher than 80%. The purity of the products was confirmed by obtained in the reactions of tBuPhPLi with tBuPhP-PRR′
1
means of H, 13C, and 31P NMR spectroscopy (Fig. S15–S204 in (6, 15/16) that led to the formation of symmetrical diphosphanes
the ESI†) and elemental analysis. The compounds 3, 6, 8–12, (tBuPhP)2 and (RR′P)2. These observations suggest that phos-
14, 17, 21, 23, 24 and 25/26 were isolated as colourless crystals, phides can catalyze the formation of symmetrical species.
whereas 2, 11, 13, and 28/29 were obtained as colourless oils Therefore, during the synthesis of unsymmetrical dipho-
(2, 11) or yellowish oils (13, diastereomers 28 and 29).
sphanes, an excess of substrates should be avoided and pro-
Furthermore, diphosphanes 15/16, 18/19, and 22/23 were ducts should be isolated immediately after the reaction is
yielded as pairs of diastereomers forming white amorphic completed.
solids. The recrystallization of 22/23 and 28/29 gave a small
number of crystals of diastereomers 23 and 28. All isolated
Structures of diphosphanes
compounds were moisture and air sensitive, however they can A great majority of compounds were isolated in the crystalline
be handled using the standard Schlenk technique. Long-term form (3, 6, 8, 9, 10, 12, 14, 17, 21, 23, 24, 25, and 28) which
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allows us to discuss their structures in the solid state, includ-
ing previously reported 3 and 8 for which X-ray structures were
not determined before. Molecular structures representative of
six groups of diphosphanes (b–g): 12, 14, 17, 23, 25 and 28 are
presented in Chart 2 with their conformations in Newman pro-
jection. For X-ray structures of 3, 6, 8, 9, 10, 21, and 24, see
Fig. S1–S5, S9, and S11 (ESI†). The selected structural para-
meters of the obtained diphosphanes are collected in Table S1
(ESI†). It is worth noting that the crystal unit cell of compound
6 consists of two enantiomers whereas in the unit cell of 9,
there are three different eclipsed conformers found that resulted
from the rotation around the P–P bond (Fig. S4†). As expected,
the P-atoms in all analysed diphosphanes exhibit pyramidal
geometry. In diphosphanes consisting of iPr2N or Et2N groups,
the geometry of N-atoms is almost planar. According to NBO
analysis, this structural feature of amino-substituted com-
pounds 8, 9, 10, 12, 14, 23–25 and 28 may be explained by the
interaction of the molecular orbital associated with the lone
pair at the N-atom with the antibonding σ*(P–P) orbital.
The optimal conformations of diphosphanes vary depend-
ing on the substitution pattern and the bulkiness of the substi-
tuents bound with P-atoms. The idealized conformations of
diphosphanes with diversified substituents are shown in
Scheme 3 Reactions of selected diphosphanes with 10 mol% of
corresponding phosphide and 30 mol% of chlorophosphane in THF
solution. Reaction progress was monitored for four days.
Chart 2 Selected X-ray structures of diphoshanes representative of groups b–g together with their conformations.
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Fig. 2. Anti conformations are preferable for most systems in
the crystalline form (3, 6, 8, 10, 12, 14, 21, 23 and 28). In the
case of diphosphanes with bulky substituents, the most privi-
leged conformation is determined by the size of these groups,
rather than by the antiperiplanar alignment of lone pairs.74
Steric hindrance that is a consequence of intermolecular
interactions between both RR′P fragments may force them to
adapt eclipsed conformations weakening the overlapping of σ-P
orbitals and elongating the P–P bond. Indeed, diphosphanes
9, 17, 24 and 25/26 in which both P-atoms are substituted with
bulky tBu or/and iPr2N exhibit anticlinal conformations. The
P–P bond distances of the obtained compounds vary from
2.225(1) to 2.314(3) Å for 6 and 25, respectively. The longest
P–P bonds are observed in sterically crowded diphosphanes
that adopt eclipsed conformations (9, 25). DFT calculations of
enthalpy ΔHdiss and free energy ΔGdiss of homolytic P–P bond
dissociation (Table 1) revealed that these highly congested
systems (1, 9, 24/25 or 30) have expectantly the least stable P–P
bonds. Nevertheless, we cannot predict the stability of the P–P
bond only on the basis of its length. It may be noted for the
moderately congested systems that no strong correlation
between the P–P bond lengthening and the decrease of ΔHdiss
and ΔGdiss (compare e.g. 1, 3 and 28, 17 and 21) is visible.
Fig. 2 Possible, idealized conformations of diphosphanes belonging to
groups a–g with distinction in two different diastereomeric forms.
Table 1 Selected experimental (a) and calculated (b) properties of diphosphanes 1–30: 31P{1H} NMR data, P–P bond length, ΔHdiss – enthalpy of the P–P
bond dissociation, ΔGdiss – the free energy of the P–P bond dissociation, and EF1–F2 – energy of dispersion interaction between two PR2 units
1JP–P
[Hz]
δP1
δP2
ΔHdiss
ΔGdiss
ΔEF1–F2
a
a
a
b
b
b
No.
Diphosphane
[ppm]
[ppm]
P–Pa [Å]
P–Pb [Å]
[kJ mol−1
]
[kJ mol−1
]
[kJ mol−1
]
1
2
3
4
5
6
7
8
tBu2P-PtBu2
tBu2P-PtBuPh
tBu2P-PPh2
rac-tBuPhP-PtBuPh
meso-tBuPhP-PtBuPh
tBuPhP-PPh2
Ph2P-PPh2
(iPr2N)2P-PPh2
(iPr2N)2P-PtBu2
(iPr2N)2P-PtBuPh
(Et2N)2P-PPh2
(Et2N)2P-PtBu2
(Et2N)2P-PtBuPh
(iPr2N)PhP-PPh2
p-meso-(iPr2N)PhP-PtBuPh
p-rac-(iPr2N)PhP-PtBuPh
(iPr2N)PhP-PtBu2
meso-(iPr2N)PhP-P(iPr2N)Ph
rac-(iPr2N)PhP-P(iPr2N)Ph
(iPr2N)PhP-P(iPr2N)2
(iPr2N)tBuP-PPh2
p-meso-(iPr2N)tBuP-PtBuPh
p-rac-(iPr2N)tBuP-PtBuPh
(iPr2N)tBuP-PtBu2
meso-(iPr2N)tBuP-P(iPr2N)tBu
rac-(iPr2N)tBuP-P(iPr2N)tBu
(iPr2N)tBuP-P(iPr2N)2
p-meso-(Et2N)(iPr2N)P-PtBuPh
p-rac-(Et2N)(iPr2N)P-PtBuPh
(iPr2N)2P-P(iPr2N)2
—
39.6
30.8
33.0
1.9
−4.4
9.7
39.6
1.4
−25.9
1.9
2.234(1)70
—
2.237(1)
—
2.235
2.244
2.264
2.257
2.252
2.252
2.252
2.262
2.296
2.271
2.265
2.275
2.250
2.261
2.257
2.250
2.256
2.246
2.241
2.259
2.265
2.252
2.282
2.252
2.294
2.252
2.299
2.264
2.258
2.300
166.6
201.3
200.1
223.2
226.2
210.3
200.3
185.5
145.8
183.2
186.8
174.8
202.0
193.2
206.1
204.5
178.7
181.5
170.8
162.8
202.2
202.3
186.8
167.2
154.4
157.4
155.8
205.1
205.6
123.8
96.0
131.0
137.9
153.2
156.9
152.2
143.2
122.5
66.6
107.8
123.7
103.5
130.7
133.1
139.4
139.2
107.9
115.9
102.8
88.6
−34.5
−33.6
−27.3
−27.8
−26.4
−23.6
−30.1
−38.0
−42.0
−40.7
−21.3
−26.6
−25.5
−27.5
−30.0
−26.2
−37.4
−33.7
−29.1
−43.2
−32.8
−37.6
−37.7
−39.0
−41.5
−46.7
−48.8
−32.6
−29.7
−54.3
370.3
254.3
—
—
158.5
—
119.3
358.2
155.3
135.0
193.7
145.3
144.8
138.0
145.3
303.2
—
−4.4
−30.8
−14.9
−38.0
62.6
−9.5
−38.3
11.8
−15.7
−36.0
−7.5
−11.1
36.8
23.6
21.5
16.1
−30.7
14.9
7.7
65.6
2.229(1)73
2.225(1)
2.2519(6)53
2.2444(6)
2.295(2)
2.252(1)
—
−14.9
71.8
88.2
72.2
108.4
111.1
99.5
45.5
28.3
27.6
38.7
23.6
21.5
63.3
68.5
57.7
70.3
69.5
88.8
82.0
92.1
79.5
79.3
83.5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
2.2603(6)
—
2.229(1)
—
—
2.2445(5)
—
—
—
101.7
185.8
348.8
283.4
489.5
—
—
2.2432(9)
—
2.265(2)
2.2508(6)
2.314(3)
—
138.4
130.9
109.5
96.2
80.4
76.8
88.8
82.0
88.7
−13.7
−14.9
83.5
—
327.0
143.1
140.4
—
—
74.4
2.228(1)
—
133.2
134.2
36.4
2.2988(8)9
Unambiguous attribution of signals in the 31P{1H} NMR spectrum of 28/29 to isomers p-meso and p-rac is not possible. As signals in the 31P{1H}
NMR spectra of 17, 24 and 25/26 recorded at room temperature are broad, low-temperature NMR experiments were applied to determine the spec-
tral data of these species.
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Calculations indicated that the most stable systems are those the most sterically crowded systems (1, 2, 9, 17, 22/23, 24, 25/
involving the PtBuPh unit as the one site of the P–P bond: 4–6, 26 and 30) with a predominance of iPr2N and tBu groups have
13, 15, 16 or 28–29. The structural and electronic features of the greatest values of EF1–F2. As the size of substituents
the PtBuPh moiety constitute an optimal compromise between decreases, EF1–F2 decreases (less negative) and diphosphanes
the steric hindrance and electron-donating properties of sub- adopt classical gauche or anti-lone-pairs conformations (see
stituents necessary for obtaining highly stable species.
ESI, Chart S1†). We noticed that EF1–F2 decreases with the
To gain a better insight into the structural features of number of interacting H-atoms in neighbouring fragments in
diphosphanes 1–30, we performed conformational analysis the order (iPr2N)2 > (iPr2N)tBu > tBu2 > (iPr2N)Ph > tBuPh >
using DFT methods, including the obtained systems for which Ph2. For 4 and 7, an eclipsed conformation is energetically
X-ray structures were not determined and known species 1, 3, favored due to the π–π interactions of coplanar Ph groups of
4/5, 7, 8 and 30. The optimal conformations calculated for neighboring P-atoms.
1–30 are presented in Chart S1.† The calculated molecular
structures are in good agreement with those determined
experimentally also for 1, 2, 9, 17, 23, 24, 25/26, 27 and 30
where an antiperiplanar arrangement of lone pairs or adapting
any staggered conformation is not attainable due to the pres-
Experimental part
Materials and methods
ence of a few sterically demanding groups, e.g. tBu and iPr2N. All manipulations were carried out under a dry argon atmo-
It is worth mentioning that diastereomers 22 and 23 have sphere by using flame-dried Schlenk-type glassware on a
different optimal conformations (anticlinal and anti, respect- vacuum line or in a glove-box. Solvents were dried by standard
ively). Hence, isomer 22 has lower energy than isomer 23 procedures over Na(K)/K/Na/benzophenone and distilled under
which exhibits
a
non-typical conformation for sterically argon. 1D (31P, 13C, and 1H) and 2D NMR spectra in C6D6 solu-
crowded systems. Moreover, we observed a correlation between tion were recorded on a Bruker AV400 MHz spectrometer
1
the spatial orientation of the lone pairs at P-atoms and the (external standard TMS for H and 13C; 85% H3PO4 for 31P) at
magnitude of 1JP–P coupling constants (Table 1). ambient temperature. Low-temperature 31P, 31P{1H} and 1H
Unsymmetrical diphosphanes that adopt eclipsed confor- NMR experiments were performed for toluene-d8 solutions of
mations (2, 9, 17, 22, 24, 27) which result in the proximity of 24 and 25/26 with data collected at 298 K, 273 K, 248 K and
1
lone pairs at P-atoms display the largest magnitudes of JP–P
.
223 K. Literature methods were used to synthesize tBu2PLi,
For these species, JP–P values are in the range of 303.2–489.5 tBuPhPLi, Ph2PLi,76,77 tBu2PCl,77 iPr2NPCl,78 (iPr2N)2PCl79 and
Hz. Unlike bulky species, less crowded unsymmetrical dipho- (Et2N)2PCl.79 Methods described for tBuPhPCl,80 (iPr2N)
sphanes (3, 6, 8, 10–16, 20, 21, 23, 28/29) exhibit smaller mag- PhPCl81 and (iPr2N)BuPCl82 were modified at the stage of
1
1
nitudes of JP–P within the range of values 101.4 Hz and 283.4 purification of a crude product. tBuPhPCl and (iPr2N)tBuPCl
Hz. It is caused by the antiperiplanar spatial orientation of were purified by distillation under reduced pressure, collecting
lone pairs at P-atoms in the most energetically favoured con- pure chlorophosphanes at 62–57 °C (2 mmHg) and 48–52 °C
formers. Computational data confirmed the elongation of the (0.1 mmHg), respectively. (iPr2N)PhPCl was dried under
P–P bond in 1, 2, 9, 17, 23, 24, 25/26, 27 and 30 compared to vacuum (0.01 mmHg) at ambient temperature for 1 h giving
other systems. Additionally, conformational analysis of the cal- greenish crystals. (Et2N)(iPr2N)PCl was synthesized via the
culated structures revealed an interesting feature: the most method described in the ESI† (see Part A). PhPCl2, Ph2PCl,
stable conformer of bulky diphosphanes was the one with the and iPr2NH Et2NH were purchased from Aldrich. Commercial
shortest P–P bond. In the systems with smaller steric hin- reagents were distilled prior to use. Reaction progress was
drance that may adopt a staggered conformation with anti-lone monitored by 31P{1H} NMR spectroscopy of the reaction
pair alignment, it is not the case (see ESI, Fig. S205–S234 and mixtures.
Table S6†). In general, as the proximity of diphosphane lone
Diffraction data of compounds 3, 6, 8, 9, 10, 12, 14, 17,
pairs increases, the P–P bond shortens. As the spatial orien- ox18/19, 21, 23, 24, 25, and 28 were collected on a diffract-
tation of lone pairs changes from antiperiplanar to eclipsed, ometer equipped with a STOE image plate detector system
the energy of conformers increases for less sterically crowded IPDS2 T using MoKα radiation (λ = 0.71073 Å) for 3, 6, 8, 9, 10,
systems and decreases for bulky ones. It is because of the 12, 14, 17, ox18/19, 21, 24, 25, and 28 and CuKα Kα radiation
London dispersion forces which play a crucial role in the stabi- (λ = 1.54178 Å) for 23 with graphite monochromatization (λ =
lization of bulky systems.45,75 Therefore, to understand the 0.71073 Å). Good quality single-crystal specimens were selected
stability of different isomers, we need to take into account not for the X-ray diffraction experiments at 120 K for 3, 6, 8, 9 10,
only the spatial orientation but also through-space interactions 12, 14, ox18/19, 21, and 25, at 130 K for 17, 24, and 28 and at
of substituents. By calculating the energy of the interaction 150 K for 23. The structures were solved by direct methods and
EF1–F2 between two RR′P fragments contained in Table 1, we refined against F2 using the Shelxs-97 and Shelxl-9783 pro-
confirmed that attractive dispersion interactions have a defin- grams run under WinGX.84 Non-hydrogen atoms were refined
ing structural role and account for the increased stability of with anisotropic displacement parameters; hydrogen atoms
bulky systems. Since these interactions have additive character were usually refined using the isotropic model with Uiso(H)
and increase with the number of interacting hydrogen atoms, values fixed to 1.5 times Ueq of C atoms for –CH3 or 1.2 times
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Ueq for –CH, –CH2 groups and aromatic H. The crystallo- recorded after 4 days of mixing. A detailed description of the
graphic details for 3, 6, 8, 9, 10, 12, 14, 17, ox18/19, 21, 23, 24, reactions including the NMR data and composition of the
25, and 28 can be found in the ESI.†
final reaction mixtures is presented in the ESI† (Part A).
Crystallographic data for the structures of 3, 6, 8, 9, 10, 12,
General procedure for investigation of reactivity of dipho-
14, 17, ox18/18, 21, 23, 24, 25, and 28 reported in this paper sphanes towards R″R′′′PCl chlorophosphanes. A diphosphane
have been deposited with the Cambridge Crystallographic RR′P-PR″R′′′ (0.170 mmol) and the respective R″R′′′PCl chloro-
Data Centre as supplementary publication No. 1560634– phosphane (30 mol%, 0.051 mmol) were dissolved in 2 cm3 of
1560642, 1576833–1576836, and 1585541.†
THF and mixed at room temperature. Reaction progress was
monitored by 31P{1H} NMR spectroscopy performed after
1 hour, 24 hours and 4 days. The approximate composition of
Synthesis procedures
General procedure for preparation of 2, 3, 6, 8, 9, 10, 11, 12, the final reaction mixture was determined by means of 31P
13, 14, 15/16, 17, 21, 22/23, 24, and 28/29. To a solution of NMR spectra recorded after 4 days of mixing. A detailed descrip-
phosphide RR′PLi in 40 cm3 of THF cooled to −50 °C, a chloro- tion of the reactions including the NMR data and composition
phosphane R″R′′′PCl was added dropwise. The reaction of the final reaction mixtures is presented in the ESI† (Part A).
mixture was successively stirred at −50 °C for 30 minutes and
then allowed to warm up to room temperature for another
30 minutes. Then the solvent was evaporated and the residue
was dried under vacuum (0.01 mmHg) for 30 minutes at 50 °C
Conclusions
We obtained and fully characterized a set of novel symmetrical
to remove all volatiles. The crude product was dissolved in
15 cm3 of petroleum ether and filtered. Removal of the solvent
and unsymmetrical diphosphanes with diversified substitu-
ents on the phosphorus atoms. We have found that the syn-
under vacuum resulted in the pure product as oil or solids. In
thesis of such systems in a good yield and high purity is
the latter case, X-ray quality crystals were grown from pet-
readily achievable by the reaction of phosphides with chloro-
roleum ether or toluene solutions. A detailed description of
phosphanes. Our experiments showed that for obtaining
the syntheses including NMR data, elemental analyses, and
unsymmetrical systems, using a boron protecting group is not
crystallization conditions is presented in the ESI† (Part A).
necessary. The metathesis side-reactions during the syntheses
of unsymmetrical diphosphanes can be eliminated by keeping
General procedure for preparation of 18/19 and 25/26.
Magnesium turnings in 40 cm3 of Et2O previously activated by
iodine and a solution of chlorophosphane in 5 cm3 of Et2O
the reaction mixture at low temperature, using strictly stoichio-
metric amounts of reagents and isolation of the product
were mixed at room temperature and vigorously stirred over-
directly after the reaction is completed. The obtained sym-
night. The solvent was removed in a vacuum, and the residue
was extracted with 15 cm3 of toluene/petroleum ether and fil-
metrical and unsymmetrical diphosphanes with diversified
substituents may be applied as P-donor ligands for transition
tered. The filtrate was evaporated to dryness giving the
metal complexes. Furthermore, the unsymmetrical species
product. A detailed description of the syntheses including
with a polarized P–P bond can be used as reagents in organo-
NMR data, elemental analyses, and crystallization conditions
is presented in the ESI† (Part A).
General procedure for preparation of 20 and 27. Magnesium
phosphorus chemistry or in the activation of small molecules.
The studies of application of these highly nucleophilic systems
turnings in 30 cm3 of Et2O previously activated by iodine solu-
in FLPs as basic components are in progress and will be pub-
tions of chlorophosphanes A and B in 5 cm3 of Et2O were
lished in due course.
added simultaneously at room temperature and vigorously
stirred overnight. The 31P{1H} NMR spectra revealed that
diphosphanes 20/27 were formed as one of three products
Conflicts of interest
with
the
corresponding
symmetrical
diphosphanes.
There are no conflicts to declare.
Diphosphanes 20/27 were obtained only in the reaction
mixture, and pure compounds were not isolated. The approxi-
mate composition of the final reaction mixtures was estimated
by 31P NMR spectroscopy. A detailed description of the synth-
eses including NMR data and the composition of the reaction
mixtures is presented in the ESI† (Part A).
General procedure for investigation of reactivity of dipho-
sphanes towards RR′PLi phosphides. A diphosphane RR′P-PR″
R′′′ (0.170 mmol) and the respective RR′PLi phosphide (10%
mol, 0.017 mmol) were dissolved in 2 cm3 of THF and mixed
at room temperature. Reaction progress was monitored by 31P
{1H} NMR spectroscopy performed after 1 hour, 24 hours and
4 days. The approximate composition of the final reaction
mixture was determined by means of 31P NMR spectra
Acknowledgements
R. G., N. S., and Ł. P. thank the National Science Centre NCN,
Poland (Grant 2016/21/B/ST5/03088) for financial support. The
authors thank the TASK Computational Center for the access
to computational resources.
Notes and references
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