Phosphaalkene vs. phosphinidene: The nature of the P-C bond in carbonyl-decorated carbene → PPh adducts

Article abstract of DOI:10.1039/c3cc45134h

Treatment of dichlorophenylphosphine with two equivalents of carbonyl-decorated carbenes results in a two-electron reduction of the phosphorus centre concomitant with carbene oxidation to afford novel phosphaalkenes as confirmed via crystallographic, spectroscopic, and DFT analyses.

Full text of DOI:10.1039/c3cc45134h

Showcasing research from T. W. Hudnall’s Research Laboratory,
Department of Chemistry and Biochemistry, Texas State
University, San Marcos, Texas USA
As featured in:
Phosphaalkene vs. phosphinidene: the nature of the P–C bond
in carbonyl-decorated carbene→PPh adducts
Carbonyl-decorated carbenes afford phosphaalkenes: Carbonyl
moieties can act as pulleys in carbene→PPh adducts by
pulling electron density away from the phosphorus centre. This
enhances π-bond character, and results in a shortening of the
P–C interaction.
See Todd W. Hudnall,
Chem. Commun., 2014, 50, 162.
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Phosphaalkene vs. phosphinidene: the nature
of the P–C bond in carbonyl-decorated
carbene - PPh adducts†
Cite this: Chem. Commun., 2014,
50, 162
Received 8th July 2013,
Accepted 30th July 2013
Roberta R. Rodrigues, Christopher L. Dorsey, Chelsee A. Arceneaux and
Todd W. Hudnall*
DOI: 10.1039/c3cc45134h
www.rsc.org/chemcomm
Treatment of dichlorophenylphosphine with two equivalents of respectively.16 Moreover, the p-accepting properties of carbenes
carbonyl-decorated carbenes results in a two-electron reduction can be quantified by evaluating the 31P NMR of these carbene -
of the phosphorus centre concomitant with carbene oxidation to PPh adducts. Indeed, for strongly p-acidic carbenes, the phos-
afford novel phosphaalkenes as confirmed via crystallographic, phaalkene form dominates in compounds such as II as revealed
spectroscopic, and DFT analyses.
through a short P–C bond and low-field 31P resonance (Table 1).16
We and others have recently been exploring the ability to modu-
The preparation of isolable phosphaalkenes (R2CQPR0) bearing a late carbene reactivity via decoration with carbonyl-containing func-
variety of substituents continues to be a contemporary research area tional groups.17,18 We recently disseminated our results which
due to the growing interest of utilizing PQC bonds in applications demonstrate that these electrophilic carbenes readily activate white
ranging from materials science1–3 to ligands in catalysis.4,5 There phosphorus (P4) to afford novel meta stable P4 and P8 allotropes that
are currently several synthetic routes for preparing PQC bonds6 feature PQC bonds.19 Given that carbenes are well-suited to stabiliz-
including: Becker condensation/1,3-silatropic rearrangement;7 ing P(I) species,16,20,21 we sought to investigate the utility of carbonyl-
1,2-elimination;8 and phospha-Peterson9–13 methods as well decorated carbenes (CDCs) as phosphaalkene precursors.
as others.14 However, these methods require large bulky phos-
phorus substituents, limiting their utility.
We initially attempted Bertrand’s two-step synthesis16,20 by treat-
ing carbenes 1–3 with dichlorophenylphosphine (PhPCl2, 1 eq.)
Alternatively, Arduengo showed that carbene - phosphinidene followed by reduction with magnesium (2 eq.) to afford the desired
(PR) adducts such as I can be regarded as two canonical forms A phosphaalkenes. However, treatment of a diamidocarbene (DAC) 1
and B (Fig. 1).15 Crystallographic and 31P NMR spectroscopic data or 2 with PhPCl2 in C6D6 was sluggish and did not afford the
(Table 1) however, determined that phosphinidene form A domi- phosphonylated salt required for the Mg reduction.‡ Addition of
nated. Bertrand recently described that the nature of the P–C bond trimethylsilyl triflate (TMSOTf) greatly increased the reaction rates;
in these adducts lies on a continuum between the forms A and B, however, 1H NMR (CDCl3) of the product revealed two carbene-
containing compounds in a 1 : 1 molar ratio. We found that the
phosphorus-containing product could be extracted from the crude
reaction mixture using hexanes, suggesting the formation of a neutral
phosphorus compound. Multinuclear NMR of the second carbene-
containing species revealed the formation of a triflate salt (19F: d =
ꢀ79 ppm, CDCl3) that did not exhibit a signal in the 31P spectrum.
These results suggested that a redox process occurred in which
a sacrificial equivalent of carbene was oxidized concomitant with
phosphorus reduction. Optimization found that treating PhPCl2
with two equivalents of carbenes 1–3 in the presence of TMSOTf
(1.0 eq.) afforded the desired phosphaalkenes 1ꢁPPh–3ꢁPPh§ along
with their 2-chloropyrimidinum (iminium) triflate salts¶ in modest
yields (Scheme 1).
1ꢁPPh–3ꢁPPh were fully characterized by multinuclear NMR
spectroscopy, as well as single crystal X-ray diffraction for 1ꢁPPh
and 2ꢁPPh (Fig. 2). Most notably, the phosphorus atoms in 1ꢁPPh
and 2ꢁPPh resonate down-field (+85.4 and +78.6 ppm, respectively)
Fig. 1 Two extreme representations and representative examples of carbene -
PPh adducts. Mes = 2,4,6-(CH3)3–C6H2; Dipp = 2,6-(iPr)2-C6H3.
Department of Chemistry and Biochemistry, Texas State University, San Marcos,
TX, 78666, USA. E-mail: hudnall@txstate.edu
† Electronic supplementary information (ESI) available: General experimental
information, synthetic procedures, computational data, and additional figures.
CCDC 946638–946640. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc45134h
162 | Chem. Commun., 2014, 50, 162--164
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Table 1 Pertinent metrical and spectroscopic data associated with phosphaalkenes derived from singlet carbenes
PQC (Å)
(X-ray)
C–P–C o (1)
PQC (Å)
(calculated)
C–P–C o (1)
Torsion
angle (X-ray)
Torsion
angle (calculated)
31P NMR
(ppm)
13C NMRb
(ppm)
Compound
(X-ray)
(calculated)
1ꢁPPh
2ꢁPPh
3ꢁPPh
Id
1.726(3)
1.691(4)
—
1.763(6)
1.7336(15)
—
108.22(13)
106.65(17)
—
99.9(3)
104.94(6)
—
1.738
1.721
1.755
1.768
1.737
1.685
110.59
108.10
110.45
105.88
108.07
105.99
9.83
0.77
—
26.5
4.25
—
10.78
0.93
19.11
14.22
0.43
+83.0a
+78.6a
+37.7a
ꢀ23.0a
+68.9a
—
172.0 (75.1)c
169.9 (66.6)
180.1 (78.8)
170.0 (49.7)
208.1 (66.0)
—
IIe
III
2.31
a
b
c
d
NMR data obtained in C6D6. P–C coupling constants J (in parenthesis) are given in Hz. NMR data obtained in CDCl3. Experimental data
e
obtained from ref. 15. Experimental data obtained from ref. 16.
(1.65–1.68 Å; 105–1111).6,22,23 Additionally, the CPh–P1–C2–N2 (see
Fig. 2 for numbering) torsion angles for 1ꢁPPh and 2ꢁPPh were 9.83
and 0.771, respectively. These values compare well with II (4.251),16
indicating increased planarity at the phosphorus centre when
compared to I (torsion angle = 26.51).15
Remarkably, the short P–C distance of 1.691(4) Å observed in
2ꢁPPh is the shortest P–C bond reported to date for a carbene - PPh
adduct, and attests to the high p-acidity of DACs. To further
interrogate the nature of the P–C bond, compounds 1ꢁPPh–3ꢁPPh,
I, II, and a model complex III (Me2CQPPh) were investigated
computationally using density functional theory.
Scheme 1 Synthesis of phosphaalkenes 1ꢁPPh–3ꢁPPh. Yields: 1ꢁPPh
(47.4%), 2ꢁPPh (31.9%), 3ꢁPPh (55.4%).
from 3ꢁPPh (+39.7 ppm) due to the greater p-acidity of DACs 1 and 2
when compared 3.18 The 31P signals of 1ꢁPPh–3ꢁPPh were observed
at high-field in comparison to the chemical shift of non-polarized
phosphaalkenes (d = +230–420 ppm),6,22 but are considerably down-
field with respect to I (31P for = ꢀ23.0 ppm)15 indicating a more
electron-deficient phosphorus centre. Moreover, the 31P chemical
shift of compounds 1ꢁPPh and 2ꢁPPh is approximately 10 ppm
downfield relative to II, indicating that DACs are stronger p acceptors
than cyclic alkyl amino carbenes (CAAC).
Geometry optimizations† of all compounds were performed,
and the computed structures compared to the respective X-ray
structures (when available, see Table 1). Though there are some
slight variations in the overall structure, the geometry about the
Ccarbene and P centres appears to be well modelled in all cases.
Importantly, the computed P–C distances follow the trend
observed in the X-ray structures, and decrease on going from I
(1.768 Å) to 2ꢁPPh (1.721 Å), with the model phosphaalkene III
having shortest calculated PQC distance of 1.685 Å. The calcu-
lated data echoed the experimental data and revealed that com-
pounds III, 1ꢁPPh, and 2ꢁPPh lie more toward the phosphaalkene
form B on the continuum of structures, whereas compounds II
and 3ꢁPPh lie in the middle between A and B, and compound I lies
at the phosphinidene form A as previously described.15 The
nature of the bonding in 1ꢁPPh–3ꢁPPh was further examined by
subjecting the X-ray structures for all compounds to a natural
bonding orbital (NBO) analysis.†824 The NBO analysis showed
that for all carbene - PPh adducts, with the exception of I, the
P–C bond was comprised of both s and p bonds. The %C and %P
localization of the p bond orbital was also determined from the
NBO analyses (see Fig. 3 and Table S2).†
The NBO data concluded that model compound III has the most
developed p bond, with equal localization on the C and P centres
(50.19 and 49.81%, respectively), followed by 2ꢁPPh and 1ꢁPPh (%C =
47.11% and 46.30%, respectively). Moreover, it was observed that
both 3ꢁPPh and II feature p bonds that are more polarized toward
the phosphorus centre (%C = 39.95% and 39.71%, respectively). The
NBO analyses also showed that the Wiberg bond indices (WBI) for
the phosphorus atom in each compound ranged from 2.912–2.822,
which indicated that the phosphorus centres form three bonds (two
s and one p) in all of the complexes with the exception of I for which
no p bond could be located in the output file. Instead, the phosphorus
centre in I formed one s bond to the Ccarbene atom, one s bond to the
To further substantiate the presence of a PQC bond in 1ꢁPPh–
3ꢁPPh, we note a barrier of rotation about the P–C bond as indicated
1
1
by the H NMR of each phosphaalkene. Indeed, the H NMR of
1ꢁPPh–3ꢁPPh exhibited inequivalent mesityl substituents in solution,
consistent with considerable multiple bond character. In contrast,
the 1H NMR of I exhibited equivalent mesityl groups, thus illustrat-
ing a poorly developed multiple bond between the carbene and
phosphorus centres.15
The X-ray structures of phosphaalkenes 1ꢁPPh and 2ꢁPPh also
corroborated the existence of multiple bond character between the
Ccarbene and the P centres. Indeed the short P–C distances (1.726(3)
and 1.691(4) Å for 1ꢁPPh and 2ꢁPPh, respectively) coupled with the
C–P–C angles (108.22(33) and 106.65(17)1) are comparable with
what has been observed for nonconjugated phosphaalkenes
Fig. 2 ORTEP diagrams of 1ꢁPPh (left) and 2ꢁPPh (right). Ellipsoids are
drawn at 50% probability and H-atoms have been omitted for clarity.
Pertinent metrical parameters are provided in the text.
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12876–12877; C.-W. Tsang, B. Baharloo, D. Riendl, M. Yam and D. P. Gates,
Angew. Chem., Int. Ed., 2004, 43, 5682–5685; C.-W. Tsang, B. Baharloo,
D. Riendl, M. Yam and D. P. Gates, Angew. Chem., 2004, 116, 5800–5803.
2 R. C. Smith and J. D. Protasiewicz, J. Am. Chem. Soc., 2004, 126, 2268–2269;
R. C. Smith and J. D. Protasiewicz, Eur. J. Inorg. Chem., 2004, 998–1006.
3 S. Kawasaki, J. Ujita, K. Toyota and M. Yoshifuji, Chem. Lett., 2005, 724–725.
4 P. L. Floch, Coord. Chem. Rev., 2006, 250, 627–681; M. Doux, A. Moores,
´
N. Mezailles, L. Ricard, Y. Jean and P. L. Floch, J. Organomet. Chem.,
2005, 690, 2407–2415.
5 F. Ozawa and M. Yoshifuji, Dalton Trans., 2006, 4987–4995; L. Weber,
Angew. Chem., Int. Ed., 2002, 41, 563–572; L. Weber, Angew. Chem., 2002,
114, 583–592; J. Dugal-Tessier, S. C. Serin, E. B. Castillo-Contreras, E. D.
Conrad, G. R. Dake and D. P. Gates, Chem.–Eur. J., 2012, 18, 6349–6359;
J. Dugal-Tessier, G. R. Dake and D. P. Gates, Org. Lett., 2010, 12, 4667–4669.
6 M. Yam, J. H. Chong, C.-W. Tsang, B. O. Patrick, A. E. Lam and D. P.
Gates, Inorg. Chem., 2006, 45, 5225–5234.
Fig. 3 NBOs of 1ꢁPPh–3ꢁPPh and I–III depicting p bond (PLP for I).
7 M. Brym and C. Jones, Dalton Trans., 2003, 3665–3667; A. Mack,
E. Pierron, T. Allspach, U. Bergstraesser and M. Regitz, Synthesis, 1998,
1305–1313; A. Gruenhagen, U. Pieper, T. Kottke and H. W. Roesky,
Z. Anorg. Allg. Chem., 1994, 620, 716–722.
8 T. C. Klebach, R. Lourens and F. Bickelhaupt, J. Am. Chem. Soc., 1978,
100, 4886–4888; S. M. Cornet, K. B. Dillon, A. E. Goeta, J. A. K. Howard,
M. D. Roden and A. L. Thompson, J. Organomet. Chem., 2005, 690,
3630–3637; K. Toyota, S. Kawasaki and M. Yoshifuji, Tetrahedron Lett.,
2002, 43, 7953–7959; A. Decken, C. J. Carmalt, J. A. C. Clyburne and
A. H. Cowley, Inorg. Chem., 1997, 36, 3741–3744.
CPh atom and featured two lone-pairs in nonbonding orbitals of
primarily s- (67.46% s-, 32.51% p-, and 0.04% d-) and p- (0.67%
s-, 99.11% p-, and 0.22% d-) character, which indicated 100% P
localization.
The WBI of the phosphorus centre in I (2.629) is misleading
as the ‘‘three bonds’’ that the phosphorus forms are derived
from the Ccarbene–P s bond, the P–CPh s bond and a donor–
acceptor interaction involving the p-type lone-pair on the
phosphorus atom with a Ccarbene–N p* orbital.
9 O. Daugulis, M. Brookhart and P. S. White, Organometallics, 2002,
21, 5935–5943.
10 G. Becker, W. Uhl and H. J. Wessely, Z. Anorg. Allg. Chem., 1981, 479, 41–56.
11 A. Jouaiti, M. Geoffroy and G. Bernardinelli, Tetrahedron Lett., 1992,
33, 5071–5074; A. Jouaiti, M. Geoffroy and G. Bernardinelli, Tetra-
hedron Lett., 1993, 34, 3413–3416; A. Jouaiti, M. Geoffroy and
G. Bernardinelli, Chem. Commun., 1996, 437–438.
12 A. S. Ionkin and W. J. Marshall, Heteroat. Chem., 2002, 13, 662–666.
13 A. Termaten, d. S. M. van and F. Bickelhaupt, Eur. J. Org. Chem.,
2003, 2049–2055.
14 J. I. Bates, B. O. Patrick and D. P. Gates, New J. Chem., 2010, 34,
1660–1666; J. I. Bates, J. Dugal-Tessier and D. P. Gates, Dalton
Trans., 2010, 39, 3151–3159.
15 A. J. Arduengo, J. C. Calabrese, A. H. Cowley, H. V. R. Dias, J. R. Goerlich,
W. J. Marshall and B. Riegel, Inorg. Chem., 1997, 36, 2151–2158.
16 O. Back, M. Henry-Ellinger, C. D. Martin, D. Martin and G. Bertrand,
Angew. Chem., Int. Ed., 2013, 52, 2939–2943.
17 T. W. Hudnall and C. W. Bielawski, J. Am. Chem. Soc., 2009, 131,
16039–16041; J. P. Moerdyk and C. W. Bielawski, J. Am. Chem. Soc.,
2012, 134, 6116–6119; J. P. Moerdyk and C. W. Bielawski, Nat. Chem.,
2012, 4, 275–280; T. W. Hudnall, J. P. Moerdyk and C. W. Bielawski,
Chem. Commun., 2010, 46, 4288–4290; A. Makhloufi, W. Frank and
C. Ganter, Organometallics, 2012, 31, 2001–2008; M. Braun, W. Frank
and C. Ganter, Organometallics, 2012, 31, 1927–1934; K. M. Wiggins,
J. P. Moerdyk and C. W. Bielawski, Chem. Sci., 2012, 3, 2986–2992;
M. Braun, W. Frank, G. J. Reiss and C. Ganter, Organometallics, 2010, 29,
We have shown that CDCs may be used to prepare novel
phosphaalkenes in a one-pot redox reaction which affords the
desired phosphaalkenes in moderate yields. The nature of the P–C
bond was fully interrogated using a combination of 31P NMR, X-ray
crystallographic, and DFT methods. These results demonstrated
that DAC - PPh adducts lie toward the canonical phosphaalkene
end of the continuum between forms A and B, whereas the
monoaminoamido carbene (MAAC) and CAAC-derived adducts,
that exhibited weaker p bonds, lie in the middle, and compound I
lies toward the established phosphinidene structure.15 Our current
efforts are focused on exploring the reactivity of compounds
1ꢁPPh–3ꢁPPh to compare to other known phosphaalkenes.
We are grateful to the Welch Foundation for a Department
Research Grant (AI-0045), NSF for MRI awards (CHE-0821254,
CHE-0946998), the Research Corporation for Science Advance-
ments (20092, TWH), and to Texas State University for their
generous support. CAA was supported through the NSF REU
program (CHE-1156579).
´
4418–4420; V. Cesar, N. Lugan and G. Lavigne, Eur. J. Inorg. Chem., 2010,
´
361–365; L. Benhamou, N. Vujkovic, V. Cesar, H. Gornitzka, N. l. Lugan
Notes and references
and G. Lavigne, Organometallics, 2010, 29, 2616–2630; M. G. Hobbs,
T. D. Forster, J. Borau-Garcia, C. J. Knapp, H. M. Tuononen and R. Roesler,
New J. Chem., 2010, 34, 1295–1308.
‡ Treatment with (PhP)5 according to ref. 15 was also unsuccessful.
§ During the course of our work, Bertrand and co-workers independently
prepared 3ꢁPPh using a different methodology (ref. 16). In our report, 3ꢁPPh
was prepared in higher yield using our TMSOTf one-pot redox route.
¶ All triflate salts were characterized by 1H and 19F NMR, and the trilfate salt
[1ꢁCl][OTf ] was further characterized by X-ray diffraction. During the syn-
thesis of 2ꢁPPh, approximately 29% of the crude product mixture was the
dimer of carbene 2, resulting in a lower yield of the desired phosphaalkene.
8 Due to the lack of crystallographic data for compounds 3ꢁPPh and III,
the optimized geometries from the DFT analyses were used as input
geometries for NBO analyses.
18 G. A. Blake, J. P. Moerdyk and C. W. Bielawski, Organometallics,
2012, 31, 3373–3378; R. M. Mushinski, B. M. Squires, K. A. Sincerbox
and T. W. Hudnall, Organometallics, 2012, 31, 4862–4870.
19 C. L. Dorsey, B. M. Squires and T. W. Hudnall, Angew. Chem., Int. Ed., 2013,
52, 4462–4465; C. L. Dorsey, B. M. Squires and T. W. Hudnall, Angew.
Chem., 2013, 125, 4558–4561; C. D. Martin, C. M. Weinstein, C. E. Moore,
A. L. Rheingold and G. Bertrand, Chem. Commun., 2013, 49, 4486–4488.
20 O. Back, M. A. Celik, G. Frenking, M. Melaimi, B. Donnadieu and
G. Bertrand, J. Am. Chem. Soc., 2010, 132, 10262–10263.
21 O. Back, G. Kuchenbeiser, B. Donnadieu and G. Bertrand, Angew.
Chem., Int. Ed., 2009, 48, 5530–5533.
1 P. W. Siu, S. C. Serin, I. Krummenacher, T. W. Hey and D. P. Gates, Angew.
Chem., Int. Ed., 2013, 52, 6967–6970; J. I. Bates, J. Dugal-Tessier and 22 J. Dugal-Tessier, G. R. Dake and D. P. Gates, Angew. Chem., Int. Ed.,
D. P. Gates, Dalton Trans., 2010, 39, 3151–3159; C.-W. Tsang, M. Yam 2008, 47, 8064–8067.
and D. P. Gates, J. Am. Chem. Soc., 2003, 125, 1480–1481; K. J. T. Noonan 23 R. Appel, Multiple Bonds and Low Coordination in Phosphorus Chemistry,
and D. P. Gates, Angew. Chem., Int. Ed., 2006, 45, 7271–7274; K. J. T. Noonan Thieme, Stuttgart, 1990.
and D. P. Gates, Angew. Chem., 2006, 118, 7429–7432; K. J. T. Noonan, 24 E. D. Glendening, A. E. Reed, J. E. Carpenter and F. Weinhold,
B. H. Gillon, V. Cappello and D. P. Gates, J. Am. Chem. Soc., 2008, 130,
NBO Version 3.1, 2010.
164 | Chem. Commun., 2014, 50, 162--164
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