Phosphorus(V) complexes with acyclic monoaminocarbene ligands via oxidative addition

Article abstract of DOI:10.1021/ic400756z

(Difluoroorganyl)dimethylamines, RCF₂NMe₂ (R = H, Ph, tBu), can be used as carbene precursors for phosphorus trifluoride in an oxidative addition reaction. By this method, complexes of sterically nondemanding asymmetric and acyclic c

Full text of DOI:10.1021/ic400756z

Communication
pubs.acs.org/IC
Phosphorus(V) Complexes with Acyclic Monoaminocarbene Ligands
via Oxidative Addition†
Tobias Bottcher, Bassem S. Bassil, Lyuben Zhechkov, and Gerd-Volker Roschenthaler*
̈
̈
School of Engineering and Science, Jacobs University Bremen, Campus-Ring 1, 28759 Bremen, Germany
S
* Supporting Information
a
Scheme 1. 1 as a Carbene Source
ABSTRACT: (Difluoroorganyl)dimethylamines,
RCF2NMe2 (R = H, Ph, tBu), can be used as carbene
precursors for phosphorus trifluoride in an oxidative
addition reaction. By this method, complexes of sterically
nondemanding asymmetric and acyclic carbenes were
obtained that are otherwise not accessible.
a
Addition to PF3 yields the corresponding carbene complex of
phosphorus pentafluoride (2).
he isolation of “a stable crystalline carbene” by Arduengo
Tand co-workers is considered as a landmark discovery in
chemistry.1 Since then, N-heterocyclic carbenes (NHCs)
evolved from a curiosity to an established ligand in modern
coordination chemistry.2 Moreover, these NHCs took part in
the ongoing “renaissance” within main-group-element chem-
istry.3 Low oxidation states in reactive main-group compounds
were stabilized with the help of sterically demanding carbene
ligands, eventually allowing the synthesis of main-group
compounds in the formal oxidation state of zero on a
preparative scale.4 The NHCs used in these reactions were
readily available imidazolin-2-ylidenes, which are stable in their
free form and can be conveniently handled using standard
Schlenk techniques, e.g., C{N(Ar)CH}2, where Ar = 2,6-
diisopropylphenyl.5 Almost all of the recent examples of main-
group-element carbene complexes were synthesized by the
direct addition of such free uncoordinated carbene-ligand
types.6 As expected, this procedure is, however, limited to
carbene ligands that can be isolated in their free form. Shortly
after the initial report of free NHCs, Arduengo and others
followed the same synthetic method and successfully
synthesized and isolated different imidazoline-based NHC
ligands, ranging from free bulky groups at the N and N′
positions7 to the incorporation of a saturated ring system or an
acyclic diaminocarbene,8,9 even reaching an air-stable carbene.10
Despite these extreme examples, some carbenes are too reactive
to be isolated in their free form.
However, our group has recently introduced two different
methods for the synthesis of carbene complexes of main-group-
element compounds: carbene transfer and oxidative addition.11
For the latter method, difluorobis(dialkylamino)methane (1)
and its cyclic analogue were introduced as precursors that
allowed the synthesis of main-group-element complexes with
sterically nondemanding carbenes that are otherwise not
accessible via the free carbene route (Scheme 1). Although
free acyclic diaminocarbenes are known, the derivative with the
sterically least demanding groups reported so far is bis-
(diisopropylamino)carbene.9 The simplest carbene, methylene
(:CH2), however, is a very reactive compound and can only
exist as a short-lived intermediate.2e Previously, reactions of
Vilsmeier’s salt ([Me2NCHCl]Cl) with transition-metal com-
plexes in low oxidation states gave complexes with the
(dimethylamino)methylene ligand :C(H)(NMe2).12 The anal-
ogous oxidative addition reaction of this salt toward the
respective main-group-element compounds is, however, un-
known. Therefore, and in a continuation to our research over
new carbene precursors for main-group-element compounds,
we decided to test derivatives of 1, in which one dimethylamino
group is replaced by H (3-H), Ph (3-Ph), and tBu (3-tBu) as
carbene precursors (see Figure 1).
Figure 1. (Difluoroorganyl)dimethylamines (3-R) as carbene
precursors.
Compound 1 (as well as its imidazoline-based analogue) has
been described as a very potent fluorinating agent, caused by
negative hyperconjugation of the lone pairs of the two nitrogen
atoms into the σ* orbitals of the C−F bonds.13 In contrast to 1,
the −CF2− function of compounds 3-R is only activated by one
dimethylamino group, and eventually the carbene carbon atom
is stabilized by only one adjacent amine group through the
“push−pull” effect. Compounds 3-R have been reported
previously, and an improved synthesis for 3-Ph and 3-tBu is
given in the Supporting Information [SI; chlorination of
Me2N−C(O)R with oxalyl chloride and subsequent fluorina-
tion with triethylamine trihydrofluoride].14
Herein we report the successful use of the asymmetric
(difluoroorganyl)dimethylamines 3-R as carbene precursors.
Received: March 27, 2013
Published: April 26, 2013
© 2013 American Chemical Society
5651
dx.doi.org/10.1021/ic400756z | Inorg. Chem. 2013, 52, 5651−5653

Inorganic Chemistry
Communication
The addition of 3-R to PF3 in diethyl ether at −196 °C and
slowly allowing the reaction mixture to reach room temperature
gave the corresponding carbene complexes of phosphorus
pentafluoride 4-R as colorless solids (Scheme 2).
Table 1. Selected Bond Lengths (pm) and Angles (deg) for
Compounds 4-R and, for Comparison, 2, the Acyclic
Diaminocarbene Complex of PF5 (Average Values for the
Two Independent Molecules of 4-H)
4-H
4-Ph
4-tBu
2
Scheme 2. Synthesis of Complexes with Asymmetric
Carbene Ligands and Phosphorus Pentafluoride
C1−P1
C1−N1
C1−R
P1−F1
av P1−Fcis
C1−P1−F1
N1−C1−P1
186.27(12)
127.50(15)
93(2)
191.32(15)
129.49(19)
149.70(19)
159.72(10)
161.60(7)
178.36(6)
126.02(11)
193.71(15)
130.46(19)
155.7(2)
192.49(12)
133.67(16)
159.27(9)
161.20(10)
176.26(5)
131.00(9)
160.39(11)
161.01(8)
179.01(7)
120.08(12)
161.04(8)
161.03(9)
179.74(5)
121.52(9)
Compared to the quasi-quantitative reactions following
Scheme 1, the oxidative addition products 4-R were obtained
in considerably lower yields: 55% for 4-H, 14% for 4-Ph, and
6% for 4-tBu. Whereas 4-H and 4-Ph were fully characterized
by multinuclear NMR spectroscopy and high-resolution mass
spectrometry, the low yield of 4-tBu allowed partial character-
ization only. Contrary to the previously reported compound 2,
which was found to be stable toward and insoluble in water, 3-
R showed decomposition (monitored by 31P NMR) and had to
be washed with cold ethanol instead. All three products were
each recrystallized by slow diffusion of diethyl ether into a
saturated acetonitrile solution of the respective material, leading
to crystals suitable for single-crystal X-ray diffraction.
Compound 4-H crystallized in the monoclinic space group
P21/c and both 4-Ph and 4-tBu in the orthorhombic space
group Pnma.
The crystal structures showed that, in all compounds, the
phosphorus is in the expected octahedral environment (Figure
2). The C−P bond length varied significantly between the three
title compounds; the shortest was found in 4-H [186.27(12)
pm], followed by 4-Ph [191.32(15) pm] and 4-tBu
[193.71(15) pm]. In comparison, the corresponding bond
length of the acyclic diaminocarbene adduct 2 is 192.49(12)
pm, more than 6 pm longer compared to the shortest one in 4-
H. The variation in the C1−P1 bond length as well as the N1−
C1−P1 angle (see Table 1) might be attributed to steric rather
than electronic effects. Diaminocarbenes are exceptionally
stable carbenes. On the one hand, the lone pairs of the two
adjacent nitrogen atoms donate electron density into the empty
p orbital of the carbene via the formation of a three center/4π
mesomeric system. On the other hand, the two electronegative
nitrogen atoms pull σ electron density from the carbene carbon
atom, stabilizing therefore further the singlet ground state by
lowering the energy of the carbene’s σ orbital.15 In the case of
4-R, the carbene carbon atom is stabilized by only one amino
function, leading to a significant shortening in the N1−C1
bond length compared to the average C−N bond length in 2
[133.67(16) pm]. The related bond length in 4-H was found to
be 127.50(15) pm, that in 4-Ph 129.49(19) pm, and that in 4-
tBu 130.46(19) pm. For compound 4-H, the bond length lies
in the typical region of imines and can already be attributed to a
double bond.16 Finally, the cis and trans P−F bond lengths
differ only slightly between the different compounds (including
2; see Table 1).
The respective chemical shifts in the 31P and 19F NMR
spectra for the three complexes are very similar, giving therefore
no conclusive statements in relation to the donor strength of
the carbene ligand. All shift values and coupling constants are
given in the SI.
The mechanism of the oxidative addition reaction of the
symmetric precursor (1) and PF3 was proposed earlier and
involves a fluoroamidinium cation, which undergoes nucleo-
philic attack by a phosphoranide.11b The situation for the
asymmetric precursors 3-R is supposed to follow the same
scheme. However, the fluoroiminium cation formed during this
process is a stronger electrophile, which may cause unwanted
side reactions. For 3-Ph and 3-tBu, steric hindrance is likely to
explain the low yields. All products are sensitive (unlike their
diaminocarbene counterparts) and tend to show decomposition
if kept in the reaction mixture. The pure compounds are stable
under a nitrogen atmosphere at room temperature for at least
several weeks.
Figure 2. Crystal structures of compounds 4-H, 4-Ph, and 4-tBu (thermal ellipsoids set at the 50% probability level; hydrogen atoms fixed in size).
Only one of the two independent molecules in the asymmetric unit of compound 4-H is shown.
5652
dx.doi.org/10.1021/ic400756z | Inorg. Chem. 2013, 52, 5651−5653

Inorganic Chemistry
Communication
F.; Krafczyk, R.; Marshall, W. J.; Tamm, M. Organometallics 1998, 17,
3375−3382. (h) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.;
Mahler, W.; Marshall, W. J. Z. Anorg. Allg. Chem. 1999, 625, 1813−
1817. (i) Kirmse, W. Eur. J. Org. Chem. 2005, 2005, 237−260.
(j) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank, L.;
Malacria, M.; Lacote, E. Angew. Chem., Int. Ed. 2011, 50, 10294−
10317. (k) Levason, W.; Reid, G.; Zhang, W. Coord. Chem. Rev. 2011,
255, 1319−1341.
In summary, we have successfully extended the family of
carbene precursors for main-group-element compounds to now
include the asymmetric (difluoroorganyl)dialkylamines with
only one amino group attached to the −CF2 function. We have
also presented an improved synthesis of the starting materials.
ASSOCIATED CONTENT
* Supporting Information
■
S
(7) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530−5534.
Text, figures, tables, and CIF files giving experimental details
for the synthesis of compounds 3-R and 4-R and X-ray
crystallographic and NMR spectroscopic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(8) Arduengo, A. J., III; Goerlich, J. R.; Marshall, W. J. J. Am. Chem.
Soc. 1995, 117, 11027−11028.
(9) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew.
Chem., Int. Ed. 1996, 35, 1121−1123.
(10) Arduengo, A. J., III; Davidson, F.; Dias, H. V. R.; Goerlich, J. R.;
Khasnis, D.; Marshall, W. J.; Prakasha, T. K. J. Am. Chem. Soc. 1997,
119, 12742−12749.
AUTHOR INFORMATION
Corresponding Author
*E-mail: g.roeschenthaler@jacobs-university.de.
Notes
■
(11) (a) Bottcher, T.; Bassil, B. S.; Zhechkov, L.; Heine, T.;
̈
Roschenthaler, G.-V. Chem. Sci. 2013, 4, 77−83. (b) Bottcher, T.;
̈
̈
Shyshkov, O.; Bremer, M.; Bassil, B. S.; Roschenthaler, G.-V.
̈
The authors declare no competing financial interest.
Organometallics 2012, 31, 1278−1280. (c) Bottcher, T.; Bassil, B. S.;
̈
Roschenthaler, G.-V. Inorg. Chem. 2012, 51, 763−765.
̈
ACKNOWLEDGMENTS
We gratefully acknowledge financial support of the Deutsche
Forschungsgemeinschaft (Grant RO 362/52-1).
(12) (a) Cetinkaya, B.; Lappert, M. F.; Turner, K. J. Chem. Soc.,
Chem. Commun. 1972, 851−852. (b) Cetinkaya, B.; Lappert, M. F.;
McLaughlin, G. M.; Turner, K. J. Chem. Soc., Dalton Trans. 1974,
1591−1599. (c) Hartshorn, A. J.; Lappert, M. F.; Turner, K. J. Chem.
Soc., Dalton Trans. 1978, 348−356. (d) Lappert, M. F. J. Organomet.
Chem. 1975, 100, 139−159.
(13) Hayashi, H.; Sonoda, H.; Fukumura, K.; Nagata, T. Chem.
Commun. 2002, 1618−1619.
(14) (a) Fawcett, F. S.; Tullock, C. W.; Coffman, D. D. J. Am. Chem.
Soc. 1962, 84, 4275−4285. (b) Hasek, W. R.; Smith, W. C.;
Engelhardt, V. A. J. Am. Chem. Soc. 1960, 82, 543−551. (c) Ebenbeck,
W., Hilgers, P., Marhold, A., Barten, J. A., Kolomeitsev, A.,
■
REFERENCES
■
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361−363.
(2) (a) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913−921.
(b) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122−
3172. (c) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248,
2247−2273. (d) Kuhn, N.; Alsheikh, A. Coord. Chem. Rev. 2005, 249,
829−857. (e) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed.
̈
Roschenthaler, G.-V. Bayer Chemicals AG, Leverkusen, Germany;
̈
1997, 36, 2162−2187. (f) Droge, T.; Glorius, F. Angew. Chem., Int. Ed.
2010, 49, 6940−6952. (g) Chauvin, R.; Canac, Y. Transition Metal
Complexes of Neutral η1-Carbon Ligands; Springer: New York, 2010;
Vol. 30. (h) Liu, B.; Zhang, Y.; Xu, D.; Chen, W. Chem. Commun.
2011, 47, 2883−2885. (i) Nolan, S. P. N-Heterocyclic Carbenes in
Synthesis; Wiley-VCH: Weinheim, Germany, 2006. (j) Glorius, F. N-
Heterocyclic Carbenes in Catalysis; Springer: Berlin, 2007.
Eur. Pat. Appl. 2004, EP 1437342.
(15) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem.
Rev. 2000, 100, 39−92.
(16) Burg, A. B. Inorg. Chem. 1989, 28, 1295−1300.
(3) Chivers, T.; Konu, J. Comments Inorg. Chem. 2009, 30, 131−176.
(4) (a) Masuda, J. D.; Schoeller, W. W.; Donnadieu, B.; Bertrand, G.
J. Am. Chem. Soc. 2007, 129, 14180−14181. (b) Dyker, C. A.;
Bertrand, G. Science 2008, 321, 1050−1051. (c) Wang, Y.; Xie, Y.;
Wei, P.; King, R. B.; Schaefer, H. F., 3rd; Schleyer, P. v. R.; Robinson,
G. H. Science 2008, 321, 1069−1071. (d) Wang, Y.; Xie, Y.; Wei, P.;
King, R. B.; Schaefer, H. F., 3rd; Schleyer, P. v. R.; Robinson, G. H. J.
Am. Chem. Soc. 2008, 130, 14970−14971. (e) Back, O.; Kuchenbeiser,
G.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2009, 48,
5530−5533. (f) Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.;
Frenking, G. Angew. Chem., Int. Ed. 2009, 48, 9701−9704. (g) Wang,
Y.; Robinson, G. H. Chem. Commun. 2009, 5201−5213. (h) Wolf, R.;
Uhl, W. Angew. Chem., Int. Ed. 2009, 48, 6774−6776. (i) Abraham, M.
Y.; Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F., 3rd; Schleyer, P. R.;
Robinson, G. H. Chem.Eur. J. 2010, 16, 432−435. (j) Wang, Y.;
Robinson, G. H. Dalton Trans. 2012, 41, 337−345.
(5) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55,
14523−14534.
(6) (a) Arduengo, A. J., III; Dias, H. V. R.; Calabrese, J. C.; Davidson,
F. J. Am. Chem. Soc. 1992, 114, 9724−9725. (b) Arduengo, A. J., III;
Dias, H. V. R.; Calabrese, J. C.; Davidson, F. Inorg. Chem. 1993, 32,
1541−1542. (c) Arduengo, A. J., III; Dias, H. V. R.; Davidson, F.;
Harlow, R. L. J. Organomet. Chem. 1993, 462, 13−18. (d) Herrmann,
W. A.; Runte, O.; Artus, G. J. Organomet. Chem. 1995, 501, C1−C4.
(e) Kuhn, N.; Kratz, T.; Blaser, D.; Boese, R. Chem. Ber. 1995, 128,
̈
245−250. (f) Kuhn, N.; Bohnen, H.; Fahl, J.; Blaser, D.; Boese, R.
̈
Chem. Ber. 1996, 129, 1579−1586. (g) Arduengo, A. J., III; Davidson,
5653
dx.doi.org/10.1021/ic400756z | Inorg. Chem. 2013, 52, 5651−5653