Is the 1JPSe coupling constant a reliable probe for the basicity of phosphines? A 31P NMR study

Article abstract of DOI:10.1080/10426507.2010.547892

The influence of different heteroaryl and functionalized aryl substituents on the electron-donating ability and basicity of the phosphorus atoms in heteroaryl phosphines and diphosphines has been determined by the use of the direct ¹J_PSe

Full text of DOI:10.1080/10426507.2010.547892

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Is the 1 J PSe Coupling Constant a Reliable
Probe for the Basicity of Phosphines? A
31P NMR Study
Udo Beckmann a , Diyana Süslüyan a & Peter C. Kunz a
a Institut für Anorganische Chemie und Strukturchemie, Lehrstuhl I:
Bioanorganische Chemie und Katalyse , Heinrich-Heine-Universität ,
Dusseldorf, Germany
Published online: 14 Oct 2011.
To cite this article: Udo Beckmann , Diyana Süslüyan & Peter C. Kunz (2011) Is the 1 J PSe Coupling
Constant a Reliable Probe for the Basicity of Phosphines? A 31P NMR Study, Phosphorus, Sulfur, and
Silicon and the Related Elements, 186:10, 2061-2070, DOI: 10.1080/10426507.2010.547892
To link to this article: http://dx.doi.org/10.1080/10426507.2010.547892
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Phosphorus, Sulfur, and Silicon, 186:2061–2070, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.547892
IS THE 1JPSe COUPLING CONSTANT A RELIABLE PROBE
FOR THE BASICITY OF PHOSPHINES? A 31P NMR STUDY
Udo Beckmann, Diyana Su¨slu¨yan, and Peter C. Kunz
Institut fu¨r Anorganische Chemie und Strukturchemie, Lehrstuhl I:
Bioanorganische Chemie und Katalyse, Heinrich-Heine-Universita¨t,
Dusseldorf, Germany
GRAPHICAL ABSTRACT
Abstract The influence of different heteroaryl and functionalized aryl substituents on the
electron-donating ability and basicity of the phosphorus atoms in heteroaryl phosphines and
1
diphosphines has been determined by the use of the direct JPSe coupling constants of the
corresponding selenides. The generality of the use of 31P–77Se spin–spin coupling constants as
probe for the basicity of phosphines is discussed as well as the scope and limits of this concept.
Keywords Phosphine selenide; heteroaryl phosphines; 31P–77Se spin–spin coupling constant
INTRODUCTION
Tertiary phosphines and diphosphines are widely used as ligands in transition metal
complexes for catalysis and biomedical applications. The properties and reactivity of the
Received 6 September 2010; accepted 22 November 2010.
D. Su¨slu¨yan thanks all the members of the institute for providing the organophosphorus compounds.
Address correspondence to Peter C. Kunz, Institut fu¨r Anorganische Chemie und Strukturchemie, Lehrstuhl
I: Bioanorganische Chemie und Katalyse, Heinrich-Heine-Universita¨t, Universita¨tsstr. 1, D-40225, Dusseldorf,
Germany. E-mail: peter.kunz@uni-duesseldorf.de
2061

2062
U. BECKMANN ET AL.
metal can be tuned by the steric demand and the electronic effects of the phosphine lig-
ands. Several concepts, e.g., the Tolman cone angle,1 exist to describe the steric demand
of phosphine and diphosphine ligands,2,3 whereas electronic effects are less studied. The
electron-donating ability and the electronic effect can be estimated using the ν(CO) stretch-
ing frequencies of the corresponding metal carbonyls1 or 1JPX spin–spin coupling constants,
X = 195Pt, 77Se, in the corresponding platinum complexes or selenides.4–7 Especially for
the determination of the electron-donating ability of multidentate heterophosphines, the
value of 1JPSe is the most available of these approaches. In such multitopic ligand systems,
the basicity of the phosphorus atom cannot be determined by titration as heteroatoms,
e.g., nitrogen atoms, are the stronger Brønsted bases. Furthermore, it is not necessarily the
phosphorus atom to act as the Lewis base and bind to, e.g., the platinum or nickel atoms,
whereas the reaction with selenium gives the phosphorus selenide selectively. The value
1
of the JPSe coupling constant for organophosphorus selenides depends on the nature of
substituents at the phosphorus atom. In general, the value increases with the substituents
becoming more electron-withdrawing. This value is also related to the basicity of the
phosphorus atom. Again, in the case of heterophosphine compounds, the Brønsted basicity
cannot be observed directly, as the heteroatoms, e.g., the N atoms of pyridinyl or imidazolyl
substituents, are by far more basic; thus, titration, e.g.,would lead to the pKB-value of the
corresponding pyridinium phosphine or imidazolium phosphine, respectively.
RESULTS AND DISCUSSION
We are interested in the chemistry of phosphine and diphosphine compounds as
ligands for water-soluble catalysts, nano-sized metal clusters, and metal-based pharmaceu-
ticals.8–10 We synthesized some functionalized phosphorus compounds containing func-
tionalized phenyl groups, imidazole, and thiazole substituents as well as alkylphosphonate
side chains.
In order to gain more information about the electron-donating abilities and P ba-
sicities of multifunctional phosphines and diphosphines, we have investigated the direct
phosphorus-31 selenium-77 spin–spin coupling constants (1JPSe) of a series of multifunc-
tional phosphorus compounds.
The selenium compounds were prepared by reaction of the corresponding phos-
phines with KSeCN or red selenium.5,11 Especially, when sterically more demanding 1-
methylimidazol-2-yl substituents were present in the phosphine, the reaction with KSeCN
was slow and gave poor yields as compared to selenation with elemental selenium.
1
For the correlation of the JPSe coupling constants of the phosphine selenides and
the Brønsted basicity (pKB) of the corresponding phosphines, we used literature-known
examples (Table 1). The phosphine PtBu3 was not included in the correlation, as the bulky
tert-butyl groups lead to a widening of the C-P-C angles and thus to a decrease of s-character
of the lone pair. The direct spin–spin coupling constant of phosphorus to a heteronucleus
is sensitive to the hybridization of the phosphorus atom: the coupling constant increases
with increasing s-character. As can be seen from Figure 1, the spin–spin coupling constant
1JPSe of the phosphine selenides and the Brønsted basicity (pKB) of the corresponding
organophosphorus compounds correlate well, although the two compounds PPh2Me and
P(4-MeOC6H4)3 have very similar pKB values (9.41 and 9.43), but their selenides display
a difference in 1JPSe of 17 Hz (with 725 and 708 Hz, respectively).
Based upon this correlation, we estimated the basicities of the phosphine and diphos-
phine compounds, as summarized in Tables 2 and 3. The organophosphorus compounds

IS 1JPSe A RELIABLE PROBE FOR THE BASICITY OF PHOSPHINES?
2063
Figure 1 Correlation of 1JPSe and pKB values of organophosphorus compounds. Linear regression gives 1JPSe
7.60 × pKB + 646 (R2 = 0.9492); PtBu3 was not included in the regression (data taken from refs. 11–17).
=
with heterocyclic substituents can be classified according to their substituents: compounds
2a–c and compound 4 bearing imidazol-4(5) and imidazol-4-yl substituents, respectively,
are the most basic ones (pKB = 9.2–10.0), whereas the corresponding imidazol-2-yl deriva-
tives 3a–c and 6 and the imidazol-5-yl derivative 5 are less basic (pKB = 11.8–12.9). A
pronounced change in the electronic properties and in the basicity of the phosphorus atom
is observed in 2-hydroxyphenyl diphenylphosphine 10 as compared to PPh3. The basic-
ity of phosphine 10 is more comparable to that of tris(alkyl) phosphines than to that of
arylphosphines. This can be explained by the orientation of the OH group, ideally suited
for the formation of a hydrogen bond toward the selenium atom and therefore favoring
1
P+–Se− over P Se.18,19 It has to be mentioned that JPSe can differ in various solvents,
Table 1 Organophosphorus compounds used for the correlation of 1JPSe and pKB (data taken from refs. 11–17)
Phosphine
1JPSe/Hz
pKB (exp.)
pKB (calc.)
PPh3
PPh2Me
PPhMe2
PMe3
732a
725a
710a
684a
691a
673b
689a
692a
722b
718b
747b
740b
711b
11.27
9.41
7.50
5.35
5.31
4.30
5.57
2.60
10.67
10.16
12.97
12.03
9.43
11.3
10.4
8.4
5.0
5.9
3.6
5.7
6.0
10.0
9.5
PEt3
PCy3
PnBu3
PtBu3
P(3-MeC6H4)3
P(4-MeC6H4)3
P(4-ClC6H4)3
P(4-FC6H4)3
P(4-MeOC6H4)3
13.3
12.4
8.6
aIn CH2Cl2. bIn CDCl3.

2064
U. BECKMANN ET AL.
Table 2 31P NMR spectroscopic data and calculated P basicities for organophosphorus compounds and their
selenides
Phosphane
PPh3
δ/ppm PIII
δ/ppm PV
1JPSe/Hz
pKb(calcd.)
1
2a
−7.12
−22.5
33.71
18.3
732d
720a
11.3
9.7
NH
Ph2P
N
iPr
2b
2c
3a
−55.6
−80.2
−30.6
0.31
−18.5
18.2
716a
720a
9.2
9.7
NH
PhP
P
N
iPr
2
NH
N
iPr
3
736d
741a
11.8
12.5
N
N
Ph2P
3b
3c
−44.2
−61.6
3.25
738d
741a
12.1
12.5
N
N
PhP
2
−12.5
741d
741a
12.5
N
P
3
N
4
5
−29.5
−34.3
18.2
15.5
722a
744a
10.0
12.9
N
Ph2P
Ph2P
N
N
N
6
−22.6
15.9
744c
12.9
H
N
Ph2P
Ph2P
N
7a
7b
−3.9
−2.5
32.5
31.8
749a
746a
13.6
13.2
N
PhP
2
N

IS 1JPSe A RELIABLE PROBE FOR THE BASICITY OF PHOSPHINES?
2065
Table 2 31P NMR spectroscopic data and calculated P basicities for organophosphorus compounds and their
selenides (Continued)
Phosphane
δ/ppm PIII
δ/ppm PV
1JPSe/Hz
pKb(calcd.)
7c
8a
−2.6a
−1.9d
30.5a
27.8d
750a
750d
13.7
P
3
N
−3.8a
−5.8e
36.5a
35.4e
738a
707e
12.1
8.0
SO3Na
Ph2P
PhP
P
8b
8c
−5.0
−5.4
35.1
35.3
694e
707e
6.3
8.0
SO3Na
2
SO3Na
3
9
−6.1
34.5
32.7
665e
683a
2.5
4.9
Ph2P
PO3Na2
10
−16.3
HO
Ph2P
aIn methanol. bIn ethanol. cIn tetrahydrofuran (THF). dIn CH2Cl2. eIn H2O.
The values of pKB are calculated by regression data of Figure 1.
1
Table 3 Direct spin–spin coupling constants JPSe of homologous series of phosphines PPhnR3−n [with
1JPSe(PPh3) = 732 Hzc]
1JPSe/Hz
R
PPh2R
PPhR2
PR3
Me
Cy
tBu
o-Tol
p-Tol
2-MeOC6H4
2-furyl
2-thienyl
2-imNMe
4(5)-im2-iPr
2-py
725c
725b
717b
732c
726b
721b
754c
743c
736a
720a
720b
710c
701b
708b
719c
–
718b
774c
752c
738a
716a
746a
684b
673b
692c
705b
715b
717b
793c
757c
741a
720a
750a
aIn CH3OH. bIn CDCl3. cIn CH2Cl2.
Data taken from this work (see Table 2) and refs. 4, 5, 11, 14, 16, 20, and 23–26.

2066
U. BECKMANN ET AL.
Figure 2 Direct spin–spin coupling constants 1JPSe of homologous series of phosphines PPhnR3−n
.
especially if a direct interaction of the solvent with the selenium atom of the P Se group
occurs.14,20–22 This has to be taken into account especially when comparing basicities of
different phosphines determined in different solvents. To minimize misinterpretation of the
basicities due to this effect, 1JPSe of homologous series of phosphines has been determined
in the same solvent. The results showed that the trend of the corresponding basicities is
significant.
The selenides of all compounds with three (hetero)aromatic substituents (1–8, with
exception of 10) show values for 1JPSe in the range of 716–750 Hz. This correlates to basici-
ties of 9–14 in pKB. A clear separation of electronic and steric effects can only be taken into
account for a series of homologous compounds, e.g., in 4-substituted triphenylphosphine
derivatives or in the series PPh3−nMen (n = 0–3) (Figure 1). Even in the homologous series
1
1, 2a–c and 1, 3a–c, no trend in JPSe can be observed. Electronic and steric effects may
contribute in opposite directions. Thus, if steric parameters are altered, the 1JPSe spin–spin

IS 1JPSe A RELIABLE PROBE FOR THE BASICITY OF PHOSPHINES?
2067
Table 4 31P NMR spectroscopic data and calculated basicities for diphos-type compounds and their selenides
δ/ppm
Compound
R2P(CH2)2PR2
1JPSe
Hz
/
pKB
R
PIII,a
PIII,b
PV
(calcd.)
11a
12
Ph
−12.3
−12.9
37.1b
36.9c
3.8b
731b
741c
740b
11.2b
12.1c
12.4
−50
−63
−72
−54.7
N
N
13
14
n.o.
4.6c
731c
749b
11.2
13.6
H
N
N
−71
−4.6b
N
N
15
16
−20
−15
−20
−15
17.0b
21.2b
780b
787b
17.6
18.6
N
S
N
S
PhRP(CH2)2PRPh
17
−33
n.o.
21.0c
751c
13.8
R2P(CH2)3PR2
11b
Ph
−17.2
−59
−18.2
33.7b
33.4c
3.0c
728b,c
727c
10.8
10.6
18
n.o.
aDiphosphane. bMonoselenide. cDiselenide. n.o.: not observed.
coupling constant is not solely sensitive to the electron-donating ability or basicity of the
phosphorus atom.
1
Figure 2 shows the dependency of the coupling constant JPSe on the degree of
substitution within a homologous series of phosphine selenides. A clear trend is observed
for substituents that are not able to form hydrogen bonds or that form only weak hydrogen
bonds on heteroatoms.
Hydrogen bonding in the selenide of compound 10, observed in the solid state of the
corresponding oxide,27 is possible and may cause the trend to be far from linear. Despite
those irregularities, the general trend observed here is that the introduction of electron-
withdrawing substituents causes the values of 1JPSe to increase and that of electron-donating
substituents to decrease.

2068
U. BECKMANN ET AL.
The selenides of diphos-type ligands were prepared analogously to those mentioned
before. An excess of selenium reagent was used to yield the diselenide compounds. How-
ever, using diphos compounds 12 and 14–16, the monoselenides were obtained as the major
products. The 1JPSe coupling constants were determined either from the mono or diselenide
(Table 4).
1
The selenides of the imidazolyl compounds 12–18 show JPSe coupling constants
comparable to those of the selenides of dppe (11a) and dppp (11b). However, 14 and 17 are
slightly less basic as would be expected from the electron-withdrawing properties of the
heteroaryl substituents. The selenides of the thiazol-2-yl and benzothiazol-2-yl derivatives
15 and 16 show remarkably high values of 1JPSe. In the case of Se = P(2-furyl)3 (1JPSe
=
787 Hz), an interaction between O and Se was discussed as an explanation for its unusual
behavior.4 Increasing the spacer length between the phosphorus atoms from (CH2)2 to
(CH2)3 as in the selenides of dppe (11a) and dppp (11b) or in the selenides of compounds
1
12 and 18 leads to a decrease in JPSe of 10 3 Hz. This should not be due to different
electronic properties of the corresponding phosphorus atoms but to different electronic
repulsion of the P Se groups. However, the 1JPSe coupling constant is used as a degree for
the electron-donating ability and basicity of phosphines independent of steric interference.
1
For diphos-type compounds, this seems not to be the case, as JPSe is dependent on the
degree of selenation.
CONCLUSION
1
We conclude that the direct spin–spin coupling constant JPSe correlates to the
Brønsted basicity (pKB) of an organophosphine only if intramolecular or intermolecular
hydrogen bonding interaction is not dominant. If this is true, the direct spin–spin coupling
constants 1JPSe of homologous series of phosphines PRnRꢁ3−n correlate in a linear manner
to the degree of substitution.
EXPERIMENTAL
The phosphines were prepared according to the published procedures; the preparation
of 17 and 18 proceeded analogously and will be described in detail in further publications.
All experiments were carried out under an atmosphere of dry nitrogen. Solvents were puri-
fied and dried using standard techniques. 31P NMR spectra were recorded on a Brucker DRX
200 and DRX 500 spectrometer. For measurement of δP of organophosphorus compounds
and their corresponding selenides, aqueous NH4PF6 was used as an external standard (a
1
sealed capillary was immersed in solution and centered). The measurement of JPSe was
carried out at 20 ◦C ( 1 ◦C). The accuracy of measurement of the coupling constant was
0.5 Hz. The temperature and concentration dependence of 1JPSe for the selenides was not
1
studied. The sign of the JPSe coupling constants has not been determined, but is known
to be negative for phosphorus(V) selenides.16 The EI mass spectra were recorded with a
double-focussing mass spectrometer, model 311 A Varian MAT, ionization energy 70 eV.
The fast atom bombardment (FAB) mass spectra were recorded with a mass spectrometer
Finnigan, model MAT 8200, in a NBA matrix. Infrared spectra were recorded with a Bruker
IFS 66 FT-IR spectrophotometer.

IS 1JPSe A RELIABLE PROBE FOR THE BASICITY OF PHOSPHINES?
2069
Bis(2-isopropylimidazol-4(5)-yl)phenylphosphine (2b)
To a solution of 1-diethoxymethyl-2-isopropylimidazole (10.6 g, 50.0 mmol) in
300 mL of diethyl ether, 33 mL (53 mmol) of 1.6 M n-butyl lithium in hexane was added at
−78 ◦C. The solution was stirred at −78 ◦C for 30 min and additionally for 30 min at room
◦
temperature. After cooling to −78 C, PhPCl2 (3.39 mL, 25 mmol) in 10 mL of diethyl
ether was added. The white suspension was stirred overnight at room temperature, 75 mL
of conc. NH4OH was added, and the organic layer was separated, washed with water, and
dried over MgSO4. After removal of the solvent, the oily residue was redissolved in 100
mL of acetone:water (10:1) and heated to reflux for 12 h. The resulting white precipitate
1
was collected and recrystallized from methanol/acetone. Yield: 3.51 g (43%). H NMR
(200 MHz, CDCl3): δ = 1.39 (d, 3JHH = 7 Hz, 12H), 3.12 (sept, 3JHH = 7 Hz, 2H), 7.24
1
1
(m, 5H), 7.43 (d, JPH = 0.8 Hz). 31P{ H} NMR (81 MHz, CDCl3): δ = −63 (s). 31P{ H}
NMR (81 MHz, methanol): δ = −55.6 (s). EI MS (70 eV, 240 ◦C): m/z (%) = 326 (100)
[M]+, 249 (18) [M-Ph]+, 217 (61) [M-imiPr]+. C18H23N4P (326.38): Calcd. C, 66.2; H, 7.1;
N, 17.2. Found C, 65.6; H, 7.1; N, 17.1%.
1-Methylimidazol-4-yl-diphenylphosphine (4)
A solution of 4-iodo-1-methylimidazole (1.00 g) in THF was treated with EtMgBr
◦
(0.21 g). This solution was stirred at ambient temperature for 2 h, cooled to 0 C, and
Ph2PCl (0.96 g) was added. The suspension was stirred overnight, the solvent was removed
in vacuo, and the residue was extracted with NH3-saturated dichloromethane. The organic
phase was filtered and the solvent was removed in vacuo. The residue was recrystallized
from ethanol. Yield 1.01 g (78%). 1H NMR (200 MHz, CDCl3): δ = 4.00 (s, 3H, NCH3),
1
6.70 (s, 1H, Him), 7.40–7.53 (m, 11H, Ph, Him). 31P{ H} NMR (81 MHz, CDCl3): δ =
◦
−29.5. EI-MS (Pt, 210 C): m/z = 266 [M]+, 188 [M-Ph]+, 183 [M-imNMe]+. IR (KBr):
ν = 2963, 1581, 1478, 1433, 1261, 1095, 800 cm−1. C16H15N2P · 4/3 H2O (290.30): Calcd.
C, 66.5; H, 6.7; N 9.7. Found C, 66.9; H, 6.7; N, 10.0%.
1-Methylimidazol-5-yl-diphenylphosphine (5)
A solution of 5-bromo-1-methylimidazole (1.00 g) in THF was treated with EtMgBr
◦
(0.21 g). This solution was stirred at ambient temperature for 2 h, cooled to 0 C, and
Ph2PCl (0.96 g) was added. The suspension was stirred overnight, the solvent was removed
in vacuo, and the residue was extracted with NH3-saturated dichloromethane. The organic
phase was filtered and the solvent was removed in vacuo. Yield: 0.50 g (38%). 1H NMR (200
MHz, CDCl3): δ = 3.61 (s, 3H, N-CH3), 6.75 (s, 1H, Him), 7.12–7.45 (m, 10H, Ph), 7.68
1
(s, 1H, Him). 31P{ H} NMR (81 MHz, CDCl3): δ = −34.3. Electrospray ionization (ESI)
(CH3OH) m/z (rel. Int.) = 283 (15) [MO+H]+, 267 (100) [M+H]+, 219 (20) [OPPh2OH]+,
183 (50) [M-ImNMe]+. C16H15N2P · CH2Cl2 · 2 H2O (290.30): Calcd. C, 52.7; H, 5.5; N,
7.2. Found C, 52.3; H, 5.3; N, 6.7%.
Synthesis of Selenides
The synthesis was carried out using known procedures.

2070
U. BECKMANN ET AL.
• Route A: A solution of the phosphine (0.1 mmol) in 1 mL of the corresponding solvent
(Tables 1–4) was stirred for 16 h with selenium at room temperature and the excess of
selenium was removed by filtration.
• Route B: A solution of phosphine (0.1 mmol) in 1 mL of the corresponding solvent
(Tables 1–4) was stirred for 6 h with an excess KSeCN at room temperature and the
precipitate was removed by filtration. To the resulting solution, the external standard was
added and the 31P NMR spectrum was recorded.
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