Carbene complexes of phosphorus(V) fluorides by oxidative addition of 2,2-difluorobis(dialkylamines) to phosphorus(III) halides

Article abstract of DOI:10.1021/om2009827

Carbene-stabilized complexes of substituted and unsubstituted phosphorus(V) fluorides were obtained by oxidative addition of 2,2-difluorobis(dialkylamines) to phosphorus(III) halides. Octahedral and hydrolytically stable complexes were obtained in quantit

Full text of DOI:10.1021/om2009827

Communication
pubs.acs.org/Organometallics
Carbene Complexes of Phosphorus(V) Fluorides by Oxidative
Addition of 2,2-Difluorobis(dialkylamines) to Phosphorus(III) Halides
Tobias Bottcher,† Oleg Shyshkov,‡ Matthias Bremer,§ Bassem S. Bassil,†
̈
and Gerd-Volker Roschenthaler*,†
̈
†School of Engineering and Science, Jacobs University Bremen, Campus-Ring 1, 28759 Bremen, Germany
‡Dyneon GmbH, Industrieparkstraße 1, 84508 Burgkirchen, Germany
§Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt, Germany
S
* Supporting Information
ABSTRACT: Carbene-stabilized complexes of substituted and unsubstituted phosphorus(V) fluorides were obtained by
oxidative addition of 2,2-difluorobis(dialkylamines) to phosphorus(III) halides. Octahedral and hydrolytically stable complexes
were obtained in quantitative yields. All compounds were characterized in the solid state by single-crystal X-ray diffraction.
-heterocyclic diaminocarbenes (NHC) have attracted a
great deal of attention during the past two decades, not
N
only because of scientific curiosity but also for their excellent
applicability in numerous chemical, technical, and medicinal
processes.1 In the focus are primarily NHC complexes of late
transition metals, due to their catalytic activity, the most prom-
inent examples being the second-generation Grubbs catalysts.2,3 In
contrast, fewer compounds of main-group elements have been
reported, although many of them give insight into the kind of
chemistry that was only known for d-block elements.4 Moreover,
NHC ligands are used to stabilize main-group elements in high
oxidation states to form charged and uncharged stable complexes
as Lewis acid and base adducts.5,6 Our workgroup recently used
2,2-difluorobis(dialkylamines) as carbene precursors for oxidative
addition to Ge(II) and Sn(II) halides in order to obtain carbene-
stabilized complexes of Ge(IV) and Sn(IV).7
Figure 1. Cyclic (1) and acyclic (2) 2,2-difluorobis(dialkylamines).
Scheme 1. Synthesis of the Carbene Complexes of
Phosphorus(V) Fluorides
Here we applied the method mentioned above in order to
obtain carbene-coordinated complexes of phosphorus(V) fluo-
rides. The aim was to obtain complexes of carbene ligands which
are as small as possible: i.e., without bulky substituents at the N,N′
position (enhancing steric stabilization) and with no aromaticity
(causing electronic stabilization). The aim was also to obtain
acyclic carbene complexes. Therefore, compounds 1 and 2 were
used as sources of carbene ligands (Figure 1). The corresponding
free carbene is very reactive for 18 and not known in the case of 2.
The smallest known stable, acyclic, and isolated free carbene is
bis(diisopropylamino)carbene.9
The phosphorus in all complexes is hexacoordinated, and all
compounds were obtained as colorless solids, which are stable
and sparingly soluble in water. All reactions proceeded quan-
titatively in solution, which was confirmed through monitoring
by 31P NMR. For the preparation of compounds 4 and 5,
chlorophosphines were used as starting materials (see Scheme
1). In both cases 2-chloroimidazolidinium chloride precipitated
as a byproduct, which was easily removed by washing with
Compounds 1 and 2 are also prone to halide metathesis,
which allowed the use of chlorophosphines as well as fluoro-
phosphines as starting materials (see Scheme 1). All reactions
were carried out under Schlenk conditions in diethyl ether at
−40 °C for 12 h. The products are shown in Scheme 1.
Special Issue: Fluorine in Organometallic Chemistry
Received: October 18, 2011
Published: December 22, 2011
© 2011 American Chemical Society
1278
dx.doi.org/10.1021/om2009827 | Organometallics 2012, 31, 1278−1280

Organometallics
Communication
water. The stability of hexacoordinated phosphorus(V) fluo-
rides in water is known and was reported some time ago by our
workgroup, involving however a different acid−base adduct of
ammonia and phosphorus pentafluoride.10
A plausible mechanism for these reactions is proposed in
Scheme 2. In step A a fluoride is transferred from 2 to
Table 1. Selected Bond Lengths (pm) and Angles (deg) for
Compounds 3−6
3
4
5
6
C1−P1
P1−F5
190.3(2)
191.56(16)
192.03(16)
192.49(12)
161.04(8)
159.80(15)
P1−C6
183.14(18)
164.13(11)
110.40(14)
174.91(5)
175.31(5)
184.38(15)
163.51(11)
110.14(14)
175.89(5)
175.64(5)
av P1−Fcis
N1−C1−N2
F1−P1−F4
F2−P1−F3
160.64(17)
110.9(2)
161.03(9)
116.97(11)
179.47(6)
179.31(5)
Scheme 2. Proposed Mechanism for the Addition of 2 and
PF3
179.54(9)
179.79(9)
The most remarkable difference between all the compounds
are the bond lengths of the carbene ligand to the phosphorus
center. The shortest C1−P1 bond length was found in the
cyclic carbene−PF5 adduct 3 (190.3(2) pm) and the longest in
the acyclic carbene−PF5 adduct 6 (192.49(12) pm), having in
between the two trans-substituted derivatives 4 and 5 with
respective similar bond lengths of 191.56(16) and 192.03(16)
pm (Figure 2). The average N−C−N angle is 110.5(2)° for the
cyclic complexes and increases in the acyclic complex to
116.97(11)°. The distance of the phosphorus to the carbon
atom in the position trans to the NHC ligand is 179.82(7) pm
for the methyl group in 4 and 184.38(15) pm for the
ipso carbon of the phenyl substituent in 5. Furthermore, the
imidazolidine ring is planar and is in a staggered conformation
to the cis fluorine atoms. Moreover, the phenyl ring in 5 is in an
eclipsed conformation with the imidazolidine ring (Figure 2).
Derivatives of 3 and 5 were reported earlier by Arduengo and
co-workers by addition of free carbene to PF5 and PhPF4, yielding
compounds 7 and 8 (Figure 3).5,6 Complex 7 is isostructural
phosphorus(III) trifluoride to give the respective phosphor-
anide and 2-fluoro-amidinium ions. In step B the carbon in
the 2-position of the 2-fluoro-amidinium cation under
goes nucleophlic attack by the phosphoranide anion and a
carbon−phosphorus bond is formed. In step C the final redox-
rearrangement takes place, where the electron-poor and highly
Lewis-acidic phosphorus(III) abstracts the second fluoride from
the carbon in the 2-position and is oxidized to give
phosphorus(V), whereas carbon(IV) is reduced to yield the
carbenic carbon. Low temperature NMR measurements were
performed in order to confirm the existence of the
tetrafluorophosphoranide in solution.11 However, the reaction
was too fast and spontaneously resulted in formation of the
final product at −94 °C.
Crystals of all products were obtained by slow diffusion of
diethyl ether into a solution of the compounds in acetonitrile.
X-ray diffraction analysis confirmed the structure of the four
octahedral complexes 3−6 of phosphorus(V), as shown in
Figure 2. Selected bond lengths and angles are shown in Table
Figure 3. Earlier reported NHC−phosphorus(V) fluoride complexes.
with 3. Although 7 is coordinated by a very different NHC ligand
(aromatic ring with bulky substituents in N,N′ positions), the bond
lengths and angles differ only slightly between the two compounds
(the carbenic carbon−phosphorus bond length is 0.5 pm larger in
3, whereas P−Ftrans is 0.4 pm and P−Fcis is 1.1 pm larger).
1. All products were fully characterized by 1H, 13C, 19F, and 31
P
NMR spectroscopy and high-resolution mass spectrometry (see
the Supporting Information).
Figure 2. (top row) Solid-state molecular structures of 3−6 with thermal ellipsoids set at the 50% probability level (H atoms are omitted for clarity).
(bottom row) Views along the C1−P1 bond.
1279
dx.doi.org/10.1021/om2009827 | Organometallics 2012, 31, 1278−1280

Organometallics
Communication
(10) Storzer, W.; Schomburg, D.; Roschenthaler, G.-V. Chem. Ber.
̈
In compound 8, on the other hand, the phenyl substituent is
twisted by 90° and is therefore in the staggered conformation
to the imidazolidine ring, whereas the phenyl ring in 5 is in the
eclipsed conformation. In analogy with 3 and 7, the bond
lengths and angles are only slightly affected by the different
NHC ligand used in 8 (the carbenic C−P bond length is
1.0 pm shorter in 5, whereas the average P−Fcis bond length is
2.3 pm longer, and the C−P bond to the ipso carbon of the phenyl
substituent is equal within the standard deviation to that of 8).
In summary, we have used 2,2-difluorobis(dialkylamines) as
precursors for carbene-coordinated complexes of substituted
and unsubstituted phosphorus(V) fluorides and were able to
obtain four different compounds, the crystal structure of each
being determined by single-crystal XRD.
1983, 116, 367−374.
(11) Christe, K. O.; Dixon, D. A.; Mercier, H. P. A.; Sanders, J. C. P.;
Schrobilgen, G. J.; Wilson, W. W. J. Am. Chem. Soc. 1994, 2850−2858.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving experimental details
for synthesis of compounds 3−6 and X-ray crystallographic
and NMR spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: g.roeschenthaler@jacobs-university.de.
ACKNOWLEDGMENTS
■
G.-V.R. and T.B. acknowledge the financial support of the
Deutsche Forschungsgemeinschaft (Grant Ro 362/47/1-0).
REFERENCES
■
(1) For synthesis, structural study, and application of carbenes and
carbene complexes, see: (a) Hermann, W. A.; Kocher, C. Angew. Chem.
̈
1997, 109, 2256−2282. (b) Hahn, F. E.; Jahnke, M. C. Angew. Chem.,
Int. Ed. 2008, 47, 3122−3172. (c) Diez-Gonzales, S.; Marion, N.;
Nolan, S. P. Chem. Rev. 2009, 109, 3612−3676. (d) Hindi, K. H.;
Panzner, M. J.; Tessier, C. A.; Cannon, C. L.; Youngs, W. J. Chem. Rev.
2009, 109, 3859−3884. (e) Arduengo, A. J. III Acc. Chem. Res. 1999,
32, 913−921. (f) Kuhn, N.; Al-Sheikh, A. Coord. Chem. Rev. 2005, 249,
829−857.
(2) Glorius, F., N-Heterocyclic Carbenes in Transition Metal Catalysis;
Springer: Berlin, 2007.
(3) Grubbs, R. H. Angew. Chem. 2006, 118, 3845−3850.
(4) Selected examples of group elements in low oxidation states:
(a) Wang, Y. Z.; Robinson, G. H. Chem. Commun. 2009, 5201−5213.
(b) Wang, Y. Z.; Xie, Y. M.; Wei, P. R.; King, R. B.; Schaefer, H. F.;
Schleyer, P. V.; Robinson, G. H. Science 2008, 321, 1069−1071.
(c) Wang, Y. Z.; Xie, Y. M.; Wei, P. R.; King, R. B.; Schaefer, H. F.;
Schleyer, P. V.; Robinson, G. H. J. Am. Chem. Soc. 2008, 130, 14970−
14971. (d) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G.
J. Am. Chem. Soc. 2007, 129, 14180−14181. (e) Dyker, C. A.; Bertrand,
G. Nat. Chem. 2009, 1, 265−266. (f) Alcarazo, M.; Lehmann, C. W.;
Anoop, A.; Thiel, W.; Furstner, A. Nat. Chem. 2009, 295−391. (g) Dyker,
̈
C. A., Bertrand, G. Science, 321, 1050−1051. (h) Weigand, J. J.; Feldmann,
K.-O.; Henne, F. D. J. Am. Chem. Soc. 2010, 132, 16321−16323.
(5) Arduengo, A. J. III; Krafczyk, R.; Marshall, W. J.; Schmutzler, R.
J. Am. Chem. Soc. 1997, 119, 3381−3382.
(6) Arduengo, A. J. III; Davidson, F.; Krafczyk, R.; Marshall, W. J.;
Schmutzler, R. Monatsh. Chem. 2000, 131, 251−265.
(7) Bottcher, T.; Bassil, B. S.; Roschenthaler, G.-V. Submitted for
̈
̈
publication in Inorg. Chem.
(8) Denk, M. K.; Thadani, A.; Hatano, K.; Lough, A. Angew. Chem.,
Int. Ed. 1997, 36, 2607−2609.
(9) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew.
Chem., Int. Ed. 1996, 35, 1121−1123.
1280
dx.doi.org/10.1021/om2009827 | Organometallics 2012, 31, 1278−1280