Transient palladadiphosphanylcarbenes: Singlet carbenes with an "Inverse" electronic configuration (pπ2 instead of σ2) and unusual transannular metal-carbene interactions (πC→Pd donation and σPd→C bac

Article abstract of DOI:10.1021/ja066738j

Upon treatment with [PdCl(allyl)]₂, asymmetrically substituted α,α′-diphosphanyl diazo compounds eliminate dinitrogen to afford C-chlorodiphosphanylmethanide complexes in high yields. In the presence of a chloride-abstracting agent, such as sod

Full text of DOI:10.1021/ja066738j

Published on Web 01/03/2007
Transient Palladadiphosphanylcarbenes: Singlet Carbenes
with an “Inverse” Electronic Configuration (pπ instead of σ2)
2
and Unusual Transannular Metal-Carbene Interactions (πCfPd
Donation and σPdfC Back-donation)
Joan Vignolle,†,§ Heinz Gornitzka,† Laurent Maron,*,‡ Wolfgang W. Schoeller,#
Didier Bourissou,*,† and Guy Bertrand*,†,§
Contribution from the Laboratoire He´te´rochimie Fondamentale et Applique´e du CNRS (UMR
5069), UniVersite´ Paul Sabatier, 118, route de Narbonne, F-31062 Toulouse Cedex 9, France,
Laboratoire de Nanophysique, Magne´tisme et Optoe´lectronique (LNMO), INSA, UniVersite´ Paul
Sabatier, 135, aVenue de Rangueil, F-31077 Toulouse Cedex, France, Fakulta¨t fu¨r Chemie der
UniVersita¨t, Postfach 10 01 31, D-33615 Bielefeld, Germany, and UCR-CNRS Joint Research
Chemistry Laboratory (UMI 2957), Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403
Received September 25, 2006; E-mail: dbouriss@chimie.ups-tlse.fr
Abstract: Upon treatment with [PdCl(allyl)]2, asymmetrically substituted R,R′-diphosphanyl diazo compounds
eliminate dinitrogen to afford C-chlorodiphosphanylmethanide complexes in high yields. In the presence of
a chloride-abstracting agent, such as sodium tetraphenylborate, the C-chlorodiphosphanylmethanide
complexes react with pyridine and trimethylphosphine, readily affording the corresponding nitrogen and
phosphorus ylides. The postulated intermediate in this process, namely palladadiphosphanylcarbenes, could
not be spectroscopically characterized, but their transient formation was chemically supported further by a
Lewis base exchange reaction between pyridine and 4-dimethylaminopyridine. This hypothesis has also
been substantiated by computing the corresponding dissociation energy using two model systems featuring
methyl groups at the phosphorus. Of particular interest, density functional theory calculations reveal that
these palladadiphosphanylcarbenes have a singlet ground state with an “inverse” pπ2 electronic configuration
and a distorted geometry associated with unusual transannular metal-carbene interactions (πCfPd donation
and σPdfC back-donation).
Over the past two decades, transition metal complexes
featuring stable aminocarbenes,1 especially N-heterocyclic car-
benes (NHCs),2 as ligands have attracted considerable interest
and found numerous synthetic applications.3 Comparatively, the
coordination chemistry of the other well-known family of stable
carbenes,4 namely phosphinocarbenes (PCs), is much less
developed, and only a few PC complexes are known.5 Indeed,
despite the preceding discovery of stable PCs,6 it was not until
2002 that direct complexation of a stable PC was reported with
the isolation and structural characterization of complex A7
(Figure 1). In addition, the scope of stable PCs has been recently
extended to P-heterocyclic carbenes (PHCs), and the ensuing
complexes B have been described.8 Apart from direct complex-
ation of stable species, a few alternative routes give access to
† Laboratoire He´te´rochimie Fondamentale et Applique´e du CNRS,
Universite´ Paul Sabatier.
‡ Laboratoire de Nanophysique, Magne´tisme et Optoe´lectronique, Uni-
versite´ Paul Sabatier.
# Fakulta¨t fu¨r Chemie der Universita¨t.
§ Department of Chemistry, University of California, Riverside.
(1) For general reviews on stable carbenes, see: (a) Hahn, F. E. Angew. Chem.,
Int. Ed. 2006, 45, 1348. (b) Canac, Y.; Soleilhavoup, M.; Conejero, S.;
Bertrand, G. J. Organomet. Chem. 2004, 689, 3857. (c) Kirmse, W. Angew.
Chem., Int. Ed. 2004, 43, 1767. (d) Alder, R. W.; Blake, M. E.; Chaker,
M. E.; Harvey, J. N.; Paolini, F.; Schu¨tz, J. Angew. Chem., Int. Ed. 2004,
43, 5896. (e) Bourissou, D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39.
(4) Recently, cyclopropenylidenes, a third family of stable carbenes, have been
discovered: (a) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.;
Bertrand, G. Science 2006, 312, 722. (b) Lavallo, V.; Ishida, Y.; Donnadieu,
B.; Bertrand, G. Angew. Chem., Int. Ed. 2006, 45, 6652-6655.
(5) Apart from PC complexes, a few carbene complexes featuring σ4-
phosphorus substituents have been prepared from bis(iminophosphorano)-
and bis(thiophosphinoyl)methanediides: (a) Aparna, K.; Ferguson, M.;
Cavell, R. G. J. Am. Chem. Soc. 2000, 122, 726. (b) Cavell, R. G.; Babu,
R. P. K.; Aparna, K. J. Organomet. Chem. 2001, 617, 158. (c) Cantat, T.;
Mezailles, N.; Ricard, L.; Jean, Y.; Le, Floch, P. Angew. Chem., Int. Ed.
2003, 43, 6382. (d) Cantat, T.; Demange, M.; Mezailles, N.; Ricard, L.;
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Jaroschik, F.; Nief, F.; Ricard, L.; Mezailles, N.; Le Floch, P. Chem.
Commun. 2005, 5178. (f) Cantat, T.; Jaroschik, F.; Ricard, L.; Le, Floch,
P.; Nief, F.; Mezailles, N. Organometallics 2006, 25, 1329.
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1999, 32, 913.
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L.; Nolan, S. P. AdV. Organomet. Chem. 2001, 46, 181. (b) Enders, D.;
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9
978
J. AM. CHEM. SOC. 2007, 129, 978-985
10.1021/ja066738j CCC: $37.00 © 2007 American Chemical Society

Transient Palladadiphosphanylcarbenes
A R T I C L E S
2
Figure 2. σ2 and pπ electronic configurations of singlet carbenes and
structure of palladadiphosphanylcarbenes F and cyclic diborylcarbenes G.
Figure 1. Structure of (di)phosphino carbene complexes A and B (nbd )
2,5-norbornadiene, cod ) 1,4-cyclooctadiene).
Scheme 2
Scheme 1 a
a [Ru] ) Ru(t-BuNC)4; L ) Py, THT (tetrahydrothiophene).
configuration (Figure 2).15 Accordingly, “inverse” singlet car-
benes featuring a pπ ground state have neither been isolated
2
PC complexes, such as the heterofunctionalization of carbyne
complexes9 and the C-H activation of ligated PMe3.10 More
recently, Le Floch et al. took advantage of the propensity of
dithioacetals for desulfurization to prepare PHC zirconocene
complexes.11
Another significant breakthrough in this area has been
reported by Ruiz et al., with the generation of the transient
metalladiphosphanylcarbene D by iodide abstraction from C and
its in situ trapping with Lewis bases, leading to the correspond-
ing adducts E (Scheme 1).12,13
nor characterized, and only the putative cyclic diborylcarbenes
G have been predicted to adopt such an unusual configuration.16
This peculiar situation has been attributed to strong π-interac-
tions between the doubly occupied pπ orbital at carbon and the
vacant 2p orbitals at boron. Similarly, for palladadiphospha-
nylcarbenes F, DFT calculations reveal significant interactions
between the carbene lone pair and vacant orbitals at phosphorus,
but also unusual transannular PdC interactions, the nature of
which (πCfPd donation/σPdfC back-donation) is opposite to that
usually encountered in Fischer-type carbene complexes (σCfM
donation/πMfC back-donation).
Taking into account the extreme moisture sensitivity of free
PCs, we became interested in using their considerably more
robust diazo precursors and have reported the coordination of
an R-diazo phosphine to Rh(I) fragments with retention of the
diazo moiety.14 These results prompted us to investigate the
coordination properties of related diphospanyl diazo compounds,
and we describe here (i) the unexpected preparation of diphos-
phanylmethanide complexes related to C, (ii) a Lewis base
exchange reaction that supports the hypothesis of the transient
formation of palladadiphosphanylcarbenes F (Figure 2), and (iii)
the very unusual pπ2 electronic configuration of singlet carbenes
F predicted by density functional theory (DFT) calculations.
Results and Discussion
Synthesis and Characterization of Palladium Complexes
3. The introduction of bulky amino groups at phosphorus is
known to efficiently stabilize R-phosphino diazo compounds,17
but also to make the complexation of the phosphine to a metal
center rather difficult. With a view to studying the coordination
behavior of diphosphanyl diazo compounds, the unsymmetri-
cally substituted derivatives 2 were thus considered as promising
candidates due the combination of a bis(diisopropylamino)- (2a)
or bis(dicyclohexylamino)phosphino group (2b) with a less
sterically demanding and electronically active diisopropylphos-
phino group (Scheme 2). As detailed below, no significant
difference was found between the properties of 2a and 2b, but
the diisopropylamino- and dicyclohexylamino-substituted ver-
sions were developed in parallel for convenient NMR charac-
terization and X-ray diffraction analysis.
Hetero-functionalization of diazo compounds is most com-
monly achieved by reacting metalated diazo precursors with
electrophiles. Although this strategy may be limited, in some
instances, by concurrent formation of nitrile-imines resulting
from the reaction at the terminal nitrogen rather than at carbon,18
it proved efficient for the preparation of the desired diphosphino
diazo compounds. Treatment of the known precursors 117a with
n-BuLi or lithium diisopropylamide (LDA) in tetrahydrofuran
(THF) at -78 °C, followed by addition of chlorodiisopropy-
2
The pπ configuration is usually found among the excited
states of singlet carbenes, whose ground state adopts the σ2
(8) (a) Nyula´szi, L. Chem. ReV. 2001, 101, 1229. (b) Fekete, AÄ ; Nyula´szi, L.
J. Organomet. Chem. 2002, 278, 643-644. (c) Martin, D.; Baceiredo, A.;
Gornitzka, H.; Schoeller, W. W.; Bertrand, G. Angew. Chem., Int. Ed. 2005,
44, 1700. (d) Schoeller, W. W.; Schroeder, D.; Rozhenko, A. B. J.
Organomet. Chem. 2005, 690, 6079. (e) Jacobsen, H. J. Organomet. Chem.
2005, 690, 6068. (f) Jacobsen, H. Dalton Trans. 2006, 18, 2214. For a
related anionic four-membered P-heterocyclic carbene, see: (g) Niecke,
E.; Fuchs, A.; Nieger, M.; Schmidt, O.; Schoeller, W. W. Angew. Chem.,
Int. Ed. 1999, 38, 3031. (h) Fuchs, A.; Gudat, D.; Nieger, M.; Schmidt,
O.; Sebastian, M.; Nyula´szi, L.; Niecke, E. Chem. Eur. J. 2002, 8, 2188.
(9) (a) Fischer, E. O.; Reitmeier, R.; Ackermann, K. Angew. Chem., Int. Ed.
1983, 22, 411. (b) Kreissl, F. R.; Ostermeier, J.; Ogric, C. Chem. Ber. 1995,
128, 289. (c) Lehotkay, T.; Wurst, K.; Jaitner, P.; Kreissl, F. R. J.
Organomet. Chem. 1996, 523, 105.
(10) (a) Gibson, V. C.; Grebenik, P. D.; Green, M. L. H. J. Chem. Soc., Chem.
Commun. 1983, 1101. (b) Gibson, V. C.; Graimann, C. E.; Hare, P. M.;
Green, M. L. H. J. Chem. Soc., Dalton Trans. 1985, 2025. (c) Green, M.
L. H.; Hare, P. M.; Bandy, J. A. J. Organomet. Chem. 1987, 330, 61. (d)
Kee, T. P.; Gibson, V. C.; Clegg, W. J. Organomet. Chem. 1987, 325,
C14. (e) Anstice, H. M.; Fielding, H. H.; Gibson, V. C.; Housecroft, C. E.;
Kee, T. P. Organometallics 1991, 10, 2183.
(11) Cantat, T.; Me´zailles, N.; Maigrot, N.; Ricard, L.; Le Floch, P. Chem.
Commun. 2004, 1274.
(15) Jones, M. Jr.; Moss, R. A. In ReactiVe Intermediates Chemistry; Moss, R.
A., Platz, M. S., Jones, M., Jr., Eds.; Wiley: New York, 2004; pp 273-
328.
(12) Ruiz, J.; Mosquera, M. E. G.; Garc´ıa, G.; Patro´n, E.; Riera, V.; Garc´ıa-
Granda, S.; Van, der Maelen, F. Angew. Chem., Int. Ed. 2003, 42, 4767.
(13) Isocyanide and NHC adducts of manganadiphosphanylcarbenes have been
prepared by independent routes: (a) Ruiz, J.; Riera, V.; Vivanco, M.;
Lanfranchi, M.; Tiripicchio, A. Organometallics 1998, 17, 3835. (b) Ruiz,
J.; Mosquera, M. E. G.; Garc´ıa, G.; Marqu´ınez, F.; Riera, V. Angew. Chem.,
Int. Ed. 2005, 44, 102.
(16) (a) Budzelaar, P. H. M.; Schleyer, P. v. R.; Krogh-Jespersen, K. Angew.
Chem., Int. Ed. 1984, 23, 825. (b) Budzelaar, P. H. M.; Krogh-Jespersen,
K.; Clark, T.; Schleyer, P. v. R. J. Am. Chem. Soc. 1985, 107, 2773.
(17) (a) Baceiredo, A.; Bertrand, G.; Sicard, G. J. Am. Chem. Soc. 1985, 107,
4781. (b) Gillette, G. R.; Baceiredo, A.; Bertrand, G. Angew. Chem., Int.
Ed. 1990, 29, 1429.
(14) Despagnet-Ayoub, E.; Gornitzka, H.; Fawcett, J.; Dyer, P. W.; Bourissou,
D.; Bertrand, G. Organometallics 2003, 22, 1358.
(18) (a) Re´au, R.; Veneziani, G.; Bertrand, G. J. Am. Chem. Soc. 1992, 114,
6059. (b) Bertrand, G.; Wentrup, C. Angew. Chem., Int. Ed. 1994, 33, 527.
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 979

A R T I C L E S
Vignolle et al.
bonded to the two phosphorus atoms. The CP bond distances
(1.73 Å) are rather short and in the range typical of those
observed in phosphorus ylides, indicating noticeable interaction
between the negative carbon atom and the σ4 phosphorus centers
(negative hyperconjugation). Finally, the η3-allyl moiety is
symmetrically coordinated to the palladium (Pd-C4 and Pd-
C2 bond lengths of 2.22 Å/C2-C3 and C3-C4 bond lengths
of 1.38 Å), despite the electronic and steric dissymmetry of the
PCP ligand.
From a mechanistic viewpoint, the formation of complexes
3 most probably results from the initial cleavage of the chloride
bridges of [Pd(µ-Cl)(allyl)]2 induced by the diisopropyl phos-
phino group of 2, leading to complexes 4 (Scheme 3). A possible
route to 3 would then involve nitrogen elimination to give the
phosphinocarbenes 5a,b,20 followed by ring closure and chlorine
migration. Alternatively, the proximity of the second phosphino
group to the metal center in 4 may favor the formation of the
cyclic ion pair complexes 6, and then after elimination of N2,
the chloride counteranion could add to the carbene center.
Aiming at discriminating experimentally between these two
pathways, the reaction between 2 and [Pd(µ-Cl)(allyl)]2 was
monitored by 31P NMR. Unfortunately, no intermediate could
be detected even at low temperature (-80 °C), suggesting that
the first step is rate determining. However, when the reaction
was performed in THF instead of DCM, and in the presence of
sodium tetraphenylborate, we were pleased to detect by 31P
NMR a new compound, which was rapidly converted into the
final complex 3. The 31P NMR data for this intermediate (Table
3) are very comparable to those of 3, suggesting a similar four-
membered metallacyclic structure. In addition, a strong infrared
Figure 3. Thermal ellipsoid diagram (50% probability) of 3b. For clarity,
the cyclohexyl and isopropyl groups are simplified and only one position
of the disordered meso carbon atoms of the allyl group is shown. Selected
bond lengths (Å) and angles (°): P1-C1 1.727(3), P2-C1 1.733(3), C1-
Cl1 1.778(3), P1-Pd1 2.3160(7), P2-Pd1 2.2886(7), Pd1-C2 2.216(3),
Pd1-C3 2.164(4), Pd1-C4 2.218(3), C2-C3 1.366(5), C3-C4 1.384(5),
P1-C1-P2 103.80(14), P1-C1-Cl1 130.18(16), P2-C1-Cl1 125.91(15),
P1-Pd1-P2 72.49(2).
lphosphine, afforded 2a (viscous red oil, 84% yield) and 2b
(orange powder, 83% yield).
Derivatives 2 readily and cleanly reacted with [Pd(µ-Cl)-
(allyl)]2 in dichloromethane (DCM) at room temperature. The
decomposition of the diazo moiety was apparent from the visible
evolution of N2, while the high solubility in pentane of the
resulting complexes 3 suggested a neutral structure. The
coordination of the P(i-Pr)2 group to palladium was indicated
by the deshielding of the corresponding 31P NMR signal (from
δ ) -2.5 to +40 ppm for 3a). Although the signal of the
P(NR2)2 group was only slightly shielded (2a, +66 ppm; 3a
+61 ppm), the presence of doublets of doublets for the three
allylic carbon atoms suggested that both phosphorus centers
were coordinated to the palladium. The η3 coordination of the
allyl fragment was retained, as deduced from the 13C chemical
shifts of the CH2 (63 and 54 ppm) and CH moieties (116 ppm).
Despite the elimination of N2, the signal for the central carbon
atom was only marginally affected by coordination (2a, 33 ppm,
JPC ) 71 and 36 Hz; 3a, 39 ppm, JPC ) 47 and 39 Hz). In
order to unambiguously establish the structure of complexes 3,
single crystals of 3b were grown from a saturated pentane
solution at -30 °C, and an X-ray diffraction study was
performed (Figure 3, Tables 1 and 2). Both phosphorus atoms
are indeed coordinated to the palladium center, and the chlorine
atom was found to have migrated from the palladium to the
central carbon. Overall, complexes 3 adopt a zwitterionic four-
membered metallacyclic structure, with formal negative and
positive charges located on carbon and palladium, respectively.
This situation is reminiscent of that observed in diphosphanyl-
methanide complexes (R2PCR′PR2)MLn that are generally
prepared by deprotonation of ligated methylenediphosphines.19
The P1C1P2Pd four-membered ring of 3b is perfectly planar,
with a very acute P1PdP2 angle (72.5°) and a rather wide
P1C1P2 angle (103.8°). The central carbon atom is in a planar
environment (sum of bond angles ) 360°) and symmetrically
absorption for a CNN vibration was observed at 2048 cm-1
,
indicating the presence of the diazo moiety. All these data are
consistent with the second pathway involving intermediates 6.
Nucleophilic Displacement from the Palladium Complexes
3. Ruiz et al. have demonstrated that C-halogeno diphospha-
nylmethanide complexes react with Lewis bases such as pyridine
and tetrahydrothiophene in the presence of silver salts to give
the corresponding substitution products.12 Complexes 3 were
also found to react cleanly with pyridine in the presence of silver
trifluoromethanesulfonate or sodium tetraphenylborate, quan-
titatively affording the corresponding ion pair complexes 7a,b
(Scheme 4). Complex 7b was subjected to a single-crystal X-ray
diffraction analysis (Figure 4, Tables 1 and 2). Accordingly, it
was found that replacement of the chlorine atom for a pyridine
moiety at carbon has only a limited geometric impact. In
particular, the P1C1P2 angle decreases by less than 2° (from
103.8° to 102.0°) and the P1PdP2 angle increases only slightly
(from 72.5° to 73.9°). The fairly small twist of the pyridine
ring relative to the PdPCP plane (32°) and the rather short C1N3
bond length (1.42 Å) indicate noticeable CN double bond
character. A more sterically hindered Lewis base such as
trimethylphosphine also reacts with complexes 3 in the presence
of sodium tetraphenylborate, affording complex 8b, which has
been fully characterized, including an X-ray analysis. The bond
lengths and angles measured for 8b are very similar to those of
3b and 7b, but the four-membered P1C1P2Pd ring slightly
deviates from planarity (angle between the PPdP and PCP
(19) For selected examples, see: (a) Ruiz, J.; Arauz, R.; Riera, V.; Vivanco,
M.; Garc´ıa-Granda, S.; Mene´ndez-Vela´zquez, A. Organometallics 1994,
13, 4162. (b) Falvello, L. R.; Fornie´s, J.; Navarro, R.; Rueda, A.;
Urriolabeitia, E. P. Organometallics 1996, 15, 309. (c) Ruiz, J.; Riera, V.;
Vivanco, M.; Garc´ıa-Granda, S.; D´ıaz, M. R. Organometallics 1998, 17,
4562.
(20) For related (phosphino)(phosphonio)carbenes, see: (a) Soleilhavoup, M.;
Baceiredo, A.; Treutler, O.; Ahlrichs, R.; Nieger, M.; Bertrand, G. J. Am.
Chem. Soc. 1992, 114, 10959. (b) Dyer, P.; Baceiredo, A.; Bertrand, G.
Inorg. Chem. 1996, 35, 46.
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980 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007

Transient Palladadiphosphanylcarbenes
A R T I C L E S
Table 1. Selected Experimental (and Theoreticala) Geometric Data for Compounds 3b, 7b, and 8b (VII′ and IX′)
compound
R/R
′
L
P1C
P2C
P1Pd
P2Pd
CPd
P1CP2
P1PdP2
∑CR
3b
7b
c-Hex2N/i-Pr
c-Hex2N/i-Pr
Me/Me
Me2N/Me
c-Hex2N/i-Pr
Me/Me
Me/Me
Me2N/Me
Me2N/Me
1.727
1.769
1.80
1.79
1.764
1.71
1.82
1.69
1.82
1.733
1.776
1.80
1.80
1.771
1.71
1.82
1.71
1.81
2.316
2.297
2.34
2.33
2.314
2.38
2.37
2.37
2.36
2.289
2.286
2.34
2.34
2.285
2.38
2.37
2.35
2.36
2.86
103.80
102.0
102.1
101.1
99.01
129.7
110.1
135.1
108.3
72.49
73.89
73.7
72.8
71.55
80.8
77.8
83.2
77.2
359.9
360.0
360.0
360.0
360.0
Py
Py
Py
PMe3
VII
VII′
8b
3.00
3.01
1IX
3IX
1IX′
3IX′
2.25
2.88
2.27
2.89
a At the B3PW91 level.
Table 2. Crystallographic Data for Complexes 3b, 7b, and 8b
3b
7b
8b
formula
formula weight
C34H63ClN2P2Pd
703.65
C65H92BCl4N3P2Pd
1236.37
C61H92BN2P3Pd
1063.49
cryst system, space group
a, Å
b, Å
c, Å
triclinic, P1h
10.364(1)
12.560(2)
14.176(2)
91.410(2)
99.142(2)
101.852(2)
1779.9(4)
2
1.313
0.711
15032
7235
orthorhombic, Pna21
36.164(4)
12.575(1)
monoclinic, P21/n
17.119(4)
16.313(4)
13.970(2)
21.541(5)
R, deg
â, deg
γ, deg
V, Å3
Z
110.272(4)
6352.6(12)
4
1.293
0.552
28987
5643(2)
4
1.252
0.453
32210
d
calcd, Mg/m3
absorp coeff, mm-1
no. total reflections
no. unique reflections
R1 (I > 2σ(I))
8692
11621
0.0333
0.0891
0.898 and -0.418
0.0516
0.1062
0.631 and -0.713
0.0427
0.1075
0.655 and -0.679
wR2 (all data)
(∆/r)max (e Å-3
)
Table 3. Selected 31P NMR and IR Data for Compounds 2a, 3a,
and 6a
The structure of the resulting exchange adduct 10b was
unambiguously deduced by the similarity of its spectroscopic
data with those of 7b. This reaction is most likely driven by
the enhanced donating properties of 4-dimethylaminopyridine
(DMAP), resulting in a stronger CN bond. From a mechanistic
viewpoint, nucleophilic attack of DMAP at the carbon atom of
7b seems rather unlikely, and the reaction most probably
proceeds via a dissociative pathway involving carbene 9b.
In order to further support the transient formation of metal-
ladiphosphanylcarbenes 9 in this exchange reaction, the dis-
sociation of the pyridine adducts was investigated computa-
tionally (eq 1). Pyridine and the model compounds VII and
δ
(ppm)
1
(i-Pr2N)2P
P(i-Pr)2
J
PP (Hz)
ν
CN2 (cm-
)
2a
3a
6a
66.4
61.2
50
-2.5
40.1
36
128
22
13
2012
2048
planes, 170.9°). This distortion most probably results from steric
crowding between the PMe3 group and the neighboring sterically
demanding i-Pr and c-Hex substituents. Finally, noticeable
negative hyperconjugation between the central carbon atom and
the three phosphorus centers is suggested by the shortness of
the associated CP bonds (1.74-1.77 Å).
As pointed out by Ruiz et al.,12 complexes such as 7 and 8
might be envisaged as Lewis base adducts of the corresponding
metalladiphosphanylcarbenes 9. All our efforts to spectroscopi-
cally characterize these carbenes upon chloride abstraction
failed. Aiming at demonstrating their possible formation by
dissociation of adducts 7 and 8, we turned our attention to Lewis
base exchange reactions. Indeed, the pyridine adduct 7b was
found to react spontaneously and cleanly with dimethylami-
nopyridine (1 equiv) in DCM at room temperature (Scheme 5).
IX, featuring methyl groups at phosphorus, were optimized
(Table 1). The dissociation energy for the Lewis base adduct
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 981

A R T I C L E S
Vignolle et al.
Figure 4. Thermal ellipsoid diagrams (50% probability) of 7b (left) and 8b (right). For clarity, the cyclohexyl and isopropyl groups are simplified. Only
one position is shown for the disordered isopropyl groups at phosphorus and meso carbon atoms of the allyl group. The tetraphenylborate anion has been
omitted. Selected bond lengths (Å) and angles (°): 7b, P1-C1 1.769(5), P2-C1 1.776(6), C1-N3 1.424(7), P1-Pd1 2.2969(15), P2-Pd1 2.2862(15),
P1-C1-P2 102.0(3), P1-C1-N3 129.8(4), P2-C1-N3 128.2(4), P1-Pd1-P2 73.89(5); 8b, P1-C1 1.764(3), P2-C1 1.771(3), C1-P3 1.741(3), P1-
Pd1 2.3142(9), P2-Pd1 2.2845(10), P1-C1-P2 99.01(16), P1-C1-P3 133.41(18), P2-C1-P3 127.58(18), P1-Pd1-P2 71.55(3).
Scheme 3
Scheme 4
Figure 5. Optimized structures for singlet and triplet metalladiphosphi-
nocarbenes IX′ (hydrogen atoms have been omitted for clarity).
significant electronic stabilization, the precise nature of which
is not obvious, since both phosphorus lone pairs are engaged
in the coordination of the metal and are therefore not prone to
interact with the carbene center.
Metalladiphosphanylcarbenes 9 (IX). Careful attention was
paid to the electronic structure of the model metalladiphospha-
nylcarbene IX′ (Figure 5), and an unprecedented stabilization
Scheme 5
mode was thereby evidenced for the singlet state. The corre-
sponding triplet state was also optimized as a reference structure,
only weak interaction between the carbene center and its
surroundings being anticipated in this case. The first unexpected
geometric feature of singlet carbene 1IX′ is the noticeable
deviation from planarity of the four-membered PCPPd ring
(angles of 154.5° between the PPdP and PCP planes). This
situation is in marked contrast with the perfectly planar
conformations found for the corresponding triplet state 3IX′ as
was thereby estimated to be ∆G ) 29.7 kcal‚mol-1. A slightly
well as the pyridine adduct VII′. Moreover, the PCP angle of
smaller dissociation energy (28.6 kcal‚mol-1) was found when
metalladiphosphanylcarbenes is noticeably wider in the singlet
amino groups were introduced at one of the phosphorus centers
state (135.1°) than in the triplet state (108.3°), in contrast to
(model compounds VII′ and IX′). These values are suggestive
the situation found for most carbenes.22 Importantly, a consider-
of spontaneous exchange reactions at room temperature. For
3
able shortening of the PdC distance on going from IX′ (2.89
comparison, a dissociation energy of 63.3 kcal‚mol-1 was
1
Å) to IX′ (2.27 Å) is observed. This observation raises the
predicted for the extrusion of the parent singlet carbene CH2
question of the possible involvement of a transannular interaction
from the corresponding pyridinium ylide.21 This suggests that
between the carbene center and the metal (sum of van der Waals
metalladiphosphanylcarbenes 9 should benefit from some
radii, 3.33 Å). This question also arises from the rather large
(21) Combined with laser flash photolysis techniques, the formation of pyri-
dinium ylides has proven to be a powerful tool to study transient carbenes:
Jackson, J. E.; Platz, M. In AdVances in Carbene Chemistry; Brinker, U.
H. Ed.; JAI Press: Greenwich, CT, 1994; Vol. 1, p 89.
(22) (a) Hoffmann, R.; Zeiss, G. D.; Van Dine, G. W. J. Am. Chem. Soc. 1968,
90, 1485. (b) Baird, N. C.; Taylor, K. F. J. Am. Chem. Soc. 1978, 100,
1333.
9
982 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007

Transient Palladadiphosphanylcarbenes
A R T I C L E S
Figure 7. Schematic representations of the MC interactions involved in
Fischer-type carbene complexes (a) and in metalladiphosphinocarbenes (b).
noticeable contribution of Pd-centered d-orbitals in the HOMO
and LUMO of 1IX′ strongly suggested the presence of transan-
nular interactions between the carbene and the metal. This
situation was confirmed by Natural Bonding Analysis (NBO).24
At the second-order donor-acceptor interaction level, the
carbene lone pair was indeed predicted to be delocalized over
empty orbitals centered at both phosphorus (40 kcal‚mol-1) and
palladium (20 kcal‚mol-1). In addition to this π donation
from the carbene to the palladium center, some σ back-donation
from the metal to the carbene was also apparent (7 kcal‚mol-1).
This bonding situation is completely opposite to that normally
encountered in Fischer-type carbene complexes25 (σCfM dona-
tion/πMfC back-donation) (Figure 7). Although the πCfM/σMfC
interactions involved in metalladiphosphanylcarbenes are not
comparable in strength to those present in normal Fischer-type
carbene complexes, they are apparently strong enough to enforce
2
the butterfly geometry and to favor to some extent the pπ
configuration. The resulting stabilization of such carbenes should
also be related to the relative propensity of the corresponding
pyridine adducts for dissociation.
1
Figure 6. Plots of the frontier orbitals for IX′.
Conclusion
singlet/triplet gap predicted for metalladiphosphanylcarbenes,
1IX′ being about 14.5 kcal‚mol-1 more stable than 3IX′. Indeed,
this value is comparable in magnitude to that predicted for
diphosphinocarbenes,23 although both phosphorus lone pairs in
IX′ are engaged in coordination and therefore cannot efficiently
stabilize the singlet state.
A new route to C-chlorodiphosphanylmethanide complexes
has been evidenced by reacting the diphosphino diazo com-
pounds 2 with [Pd(µ-Cl)(allyl)]2. The resulting complexes 3
were found to react readily with pyridine and trimethylphosphine
in the presence of a chloride-abstracting agent to give the
corresponding adducts 7 and 8. The spontaneous Lewis base
exchange reaction observed between pyridine and DMAP
substantiates the possible dissociation of such adducts and
therefore the transient formation of the corresponding metalla-
diphosphanylcarbenes 9. Although these carbenes could not be
spectroscopically characterized, DFT calculations supported their
possible formation, since relatively low dissociation energies
are predicted for model systems. According to DFT calculations
and NBO analyses, singlet metalladiphosphanylcarbenes adopt
a distorted geometry associated with an “inverse” pπ2 electronic
configuration. This peculiar situation has been attributed to
stabilizing interactions between the carbene lone pair and vacant
orbitals at phosphorus and to transannular PdC interactions, the
nature of which (πCfPd donation and σPdfC back-donation) is
opposite to that usually encountered in Fischer-type carbene
complexes.
More insight into the electronic structure of singlet metalla-
diphosphanylcarbene 1IX′ was gained from its frontier orbitals.
Indeed, the carbene lone pair (HOMO) was found to be of p
symmetry with an almost perpendicular orientation relative to
the carbene coordination plane, while the carbene vacant orbital
(LUMO) is located approximately along the CPd axis (Figure
6). This pπ2 electronic configuration is in marked contrast with
the σ2 electronic configuration normally observed for singlet
carbenes (Figure 2). As mentioned in the introduction, only the
putative cyclic diborylcarbenes G have been predicted to adopt
2
such a pπ configuration due to strong π-interaction between
the doubly occupied pπ orbital at carbon and the vacant 2p
orbitals at boron.16 At first glance, a comparable situation may
1
be found in IX′. Indeed, it is clearly apparent from the short
CP bond lengths (1.69-1.71 Å) that the doubly occupied pπ
orbital at carbon interacts with the σ4 phosphorus centers
(negative hyperconjugation). Although these interactions prob-
ably play an important role in the “inverse” electronic config-
uration of metalladiphosphanylcarbenes, they cannot explain the
geometric distortion predicted for 1IX′. At this stage, the
From these results, the characterization of singlet carbenes
adopting a pπ2 electronic configuration appears to be a realistic
goal. In this regard, metalladiphosphanylcarbenes should be
considered as promising candidates, since variation of the metal
fragment and phosphorus substituents may allow further stabi-
lization of such species.
(23) (a) Hoffmann, M. R.; Kuhler, K. J. J. Chem. Phys. 1991, 94, 8029. (b)
Dixon, D. A.; Dobbs, K. B.; Arduengo, A. J., III; Bertrand, G. J. Am. Chem.
Soc. 1991, 113, 8782. (c) Nyulaszi, L.; Szieberth, D.; Reffy, J.; Veszpremi,
T. J. Mol. Struct. (THEOCHEM) 1998, 453, 91. (d) Schoeller, W. W. Eur.
J. Inorg. Chem. 2000, 369.
(24) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(25) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons:: New York, 1994. (b) Vyboishchikov, S. F.; Frenking,
G. Chem. Eur. J. 1998, 4, 1428.
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 983

A R T I C L E S
Vignolle et al.
2
2
(d, JCP2 ) 6 Hz, CH3i-Pr), 20.8 (d, JCP2 ) 6 Hz, CH3i-Pr), 24.5 (d,
Experimental Section
3JCP1 ) 3 Hz, CH3Ni-Pr), 24.6 (d, JCP1 ) 2 Hz, CH3Ni-Pr), 24.9 (d,
3
All manipulations were performed under an inert atmosphere of
argon using standard Schlenk techniques. Dry, oxygen-free solvents
were employed. 1H, 13C, and 31P NMR spectra were recorded on Bruker
3JCP1 ) 3 Hz, CH3Ni-Pr), 25.3 (d, JCP1 ) 3 Hz, CH3Ni-Pr), 27.6
3
(dd,1JCP2 ) 18 Hz, JCP1 ) 5 Hz, CHi-Pr), 27.8 (dd,1JCP2 ) 18 Hz,
3
3JCP1 ) 4 Hz, CHi-Pr), 39.2 (dd, JCP1 ) 39 Hz, JCP2 ) 47 Hz, CCl),
AC200, WM250, AMX400, and AMX500 spectrometers. H and 13C
1
2
2
47.4 (d, JCP1 ) 10 Hz, CHN), 47.7 (d, JCP1 ) 11 Hz, CHN), 53.8
(dd, JCP1 ) 42 Hz, JCP2 ) 5 Hz, CH2(allyl)), 63.2 (dd, JCP1 ) 12 Hz, JCP2
) 30 Hz, CH2(allyl)), 116.4 (dd, JCP1 ) 9 Hz, JCP2 ) 5 Hz, CH(allyl)). 3b
was obtained as yellow crystals by recrystallization from toluene at
-30 °C (367 mg, 87%), mp 148 °C. 31P{1H} NMR (C6D6): δ 62.2 (d,
chemical shifts are reported in ppm relative to Me4Si as external
standard. 31P NMR downfield chemical shifts are expressed with a
positive sign, in ppm, relative to external 85% H3PO4. Infrared spectra
were recorded on a Perkin-Elmer 1725X FT-IR spectrometer. Mass
spectra were obtained on a Ribermag R10 10E instrument. The bis-
(diisopropylamino)- and bis(dicyclohexylamino)phosphinodiazomethanes
1a,b were prepared according to the literature procedure.17a
2JPP ) 17 Hz, PNc-Hex2), 36.5 (d, JPP ) 17 Hz, Pi-Pr2). MS (FAB):
2
705 [M + H]+.
Synthesis of Palladium Complexes 7a,b. At -80 °C, pyridine (0.52
mmol, 0.042 mL) was added to a DCM solution (3 mL) of 3a (0.52
mmol, 283 mg) or 3b (0.52 mmol, 364 mg). The solution was warmed
to -30 °C and transferred by canula to a suspension of NaBPh4 (0.57
mmol, 195 mg) in DCM (5 mL). The resulting suspension was warmed
to room temperature and stirred for 30 min. NaCl was filtered off, and
the solvent was evaporated. Recrystallization from CH2Cl2/Et2O af-
forded 7b as yellow crystals (482 mg, 87%), mp 167-169 °C. 31P-
{1H} NMR (CH2Cl2): δ 30.9 (d, JPP ) 4.4 Hz, Pi-Pr2), 66.0 (br, PNc-
Hex2). 7a was also isolated and characterized (400 mg, 85%). 31P{1H}
NMR (CDCl3): δ 34.1 (d, JPP ) 4.9 Hz, Pi-Pr2), 68.7 (d, JPP ) 4.9
Synthesis of Diazo Compound 2a. n-BuLi (8.3 mmol) was added
at -80 °C to a THF solution (20 mL) of bis(diisopropylamino)-
phosphinodiazomethane (8.3 mmol, 2.26 g). After 30 min, chlorodi-
isopropylphosphine (8.3 mmol, 1.32 mL) was added at -80 °C, and
the solution was stirred for 15 min. After the solution warmed to room
temperature, the solvent was removed under vacuum. The crude residue
was extracted with pentane (20 mL) and quickly filtered through neutral
activated alumina. After evaporation of the solvent, 2a was obtained
as a red oil (2.7 g, 84%). 31P{1H} NMR (C6D6): δ 66.4 (d, 2JPP ) 128
2
1
Hz, PNi-Pr2), -2.5 (d, JPP ) 128 Hz, Pi-Pr2). H NMR (C6D6): δ
1.27 (ddd, 6 H, 3JHH ) 7.2 Hz, 3JHP2 ) 16.4 Hz, 5JHP1 ) 1.1 Hz, CH3i-
1
3
3
5
Hz, PNi-Pr2). H NMR (CDCl3): δ 1.00 (br, 3 H, CH3i-Pr), 1.14 (br,
Pr), 1.28 (ddd, 6 H, JHH ) 7.2 Hz, JHP2 ) 16.4 Hz, JHP1 ) 1.3 Hz,
3
3 H, CH3i-Pr), 1.21 (br, 3 H, CH3i-Pr), 1.24 (br, 3 H, CH3i-Pr), 1.24-
1.32 (br, 12 H, CH3N), 1.42 (br, 12 H, CH3N), 2.41 (br, 1 H, CHi-Pr),
2.50 (br, 1 H, CHi-Pr), 2.76 (br, 2 H, CH2(allyl)), 3.86 (br, 2 H, CHN),
3.95 (br, 2 H, CHN), 4.23 (br, 1 H, CH2(allyl)), 4.32 (br, 1 H, CH2(allyl)),
5.16 (m, 1 H, CH(allyl)), 6.85 (br, 3 H, CHPy), 6.95 (br, 4 H, CHBPh),
7.10 (br, 8 H, CHBPh), 7.52 (br, 8 H, CHBPh), 7.82 (br, 2 H, CHPy).
13C{1H} NMR (CDCl3): δ 20.8 (s, CH3i-Pr), 21.0 (s, CH3i-Pr), 22.5
(d, JCP2 ) 8.7 Hz, CH3i-Pr), 22.9 (d, JCP2 ) 8.1 Hz, CH3i-Pr), 24.3 (d,
JCP1 ) 4.7 Hz, CH3Ni-Pr), 24.4 (d, JCP1 ) 4.0 Hz, CH3Ni-Pr), 26.0 (s,
CH3Ni-Pr), 26.2 (s, CH3Ni-Pr), 28.9 (dd, JCP1 ) 5.7 Hz, JCP2 ) 30.4
Hz, CHi-Pr), 29.2 (dd, JCP1 ) 3.9 Hz, JCP2 ) 30.5 Hz, CHi-Pr), 49.6
(d, JCP1 ) 11.0 Hz, CHNi-Pr), 50.6 (d, JCP1 ) 11.5 Hz, CHNi-Pr),
CH3i-Pr), 1.31 (d, 12 H, JHH ) 6.4 Hz, CH3Ni-Pr), 1.43 (d, 12 H,
3JHH ) 6.4 Hz, CH3Ni-Pr), 1.90 (sept, 2 H, JHH ) 7.2 Hz, CHi-Pr),
3
3.49 (sept d, 4 H, JHH ) 6.4 Hz, JHP1 ) 1.6 Hz, CHN). 13C{1H}
3
3
NMR (C6D6): δ 19.5 (d, 2JCP2 ) 8 Hz, CH3i-Pr), 20.4 (dd, 2JCP2 ) 20
4
3
Hz, JCP1 ) 3 Hz, CH3i-Pr), 24.7 (d, JCP1 ) 7 Hz, CH3Ni-Pr), 25.3
(dd,1JCP2 ) 3 Hz, 3JCP1 ) 8 Hz, CHi-Pr), 25.6 (dd,1JCP2 ) 18 Hz, 3JCP1
) 2 Hz, CHi-Pr), 33.0 (dd,1JCP1 ) 36 Hz,1JCP2 ) 71 Hz, CN2), 48.0
(d, JCP1 ) 13 Hz, CHNi-Pr). IR (diethyl ether): νCN2 2012 cm-1
.
2
Synthesis of Diazo Compound 2b. A freshly prepared LDA solution
from diisopropylamine (1.71 mmol, 0.240 mL) and BuLi (1.71 mmol,
1.12 mL) in THF (5 mL) was added at -80 °C to a solution of bis-
(dicyclohexylamino)phosphino diazomethane (1.71 mmol, 738 mg) in
THF (10 mL). After 30 min, chlorodiisopropylphosphine (1.71 mmol,
0.272 mL) was added at -80 °C and stirred for a further 15 min. The
solution was warmed to rt and the volatiles were removed under
vacuum. The crude residue was extracted with pentane and quickly
filtered through celite and neutral activated alumina. After evaporation
of the solvent, compound 2b was obtained as an orange powder.
Recristallisation from Et2O at -30 °C led to compound 2b as orange
crystals (778 mg, 83%): mp 54 °C; 31P{1H} NMR (C6D6): δ 67.6 (d,
2JPP ) 132 Hz, PNc-Hex2), -3.8 (d, 2JPP ) 132 Hz, Pi-Pr2); IR (THF)
60.3 (dd, JCP1 ) 42.0 Hz, JCP2 ) 5.3 Hz, CH2(allyl)), 65.5 (dd, JCP1
13.1 Hz, JCP2 ) 30.7 Hz, CH2(allyl)), 119.2 (dd, JCP1 ) 10.1 Hz, JCP2
)
)
6.2 Hz, CH(allyl)), 122.1 (s, CHBPh), 126.0 (q, JCB ) 2.7 Hz, CHBPh),
127.8 (s, CHPy), 131.7 (s, CHPy), 133.3 (s, CHPy), 136.8 (s, CHBPh),
164.6 (q, JCB ) 49.3 Hz, CBPh); Cylide was not observed.
Synthesis of Palladium Complex 8b. To a THF solution (5 mL)
of 3b (0.37 mmol, 258 mg) was added trimethylphosphine (0.37 mmol,
0.370 mL) at r oom temperature. This solution was transferred by canula
to a suspension of NaBPh4 (0.40 mmol, 137 mg) in 1.5 mL of THF
previously cooled to -80 °C. The suspension was warmed to room
temperature and stirred for 1 h. The solvent was evaporated, and the
solid was washed with Et2O (2 × 5 mL) and extracted with DCM (5
mL). Crystallization from a THF solution and slow diffusion of Et2O
at room temperature afforded 8b as pale yellow crystals (322 mg, 82%),
mp 214-216 °C. 31P{1H} NMR (CDCl3): δ 2.5 (dd, JP3P1 ) 16 Hz,
JP3P2 ) 7 Hz, PMe3), 26.9 (dd, JP2P1 ) 82 Hz, JP2P3 ) 7 Hz, Pi-Pr2),
63.8 (dd, JP1P2 ) 82 Hz, JP1P3 ) 16 Hz, PNc-Hex2). 1H NMR (CDCl3):
δ 1.17 (dd, 3 H, 3JHH ) 7.2 Hz, JHP2 ) 19.2 Hz, CH3i-Pr), 1.22 (m, 12
νCN2 2015 cm-1
.
Synthesis of Palladium Complexes 3a,b. A CH2Cl2 solution (4 mL)
of [PdCl(allyl)]2 dimer (0.3 mmol, 110 mg) was added at -80 °C to a
solution of diazo derivative 2a (0.7 mmol) or 2b (0.7 mmol) in 4 mL
of CH2Cl2. The solution was warmed to room temperature and stirred
for 1 h. After evaporation of the solvent, the crude residue was extracted
with 4 × 10 mL of pentane, yielding 3a as a yellow oily residue (277
2
mg, 85%). 31P{1H} NMR (CD2Cl2): δ 61.2 (d, JPP ) 22 Hz, PNi-
Pr2), 40.1 (d, 2JPP ) 22 Hz, Pi-Pr2). 1H NMR (CD2Cl2): δ 1.10 (dd, 3
3
3
3
3
H, JHH ) 7.2 Hz, JHP2 ) 18.0 Hz, CH3i-Pr), 1.16 (dd, 3 H, JHH
)
H, CH2c-Hex), 1.28 (dd, 3 H, JHH ) 6.8 Hz, JHP2 ) 18.8 Hz, CH3i-
7.2 Hz, 3JHP2 ) 18.0 Hz, CH3i-Pr), 1.18 (dd, 3 H, 3JHH ) 7.2 Hz, 3JHP2
) 15.2 Hz, CH3i-Pr), 1.27 (d, 6 H, 3JHH ) 6.8 Hz, CH3Ni-Pr), 1.28 (d,
Pr), 1.29 (dd, 3 H, 3JHH ) 7.4 Hz, JHP2 ) 17.6 Hz, CH3i-Pr), 1.31 (dd,
3
2
3 H, JHH ) 7.8 Hz, JHP2 ) 16.9 Hz, CH3i-Pr), 1.63 (d, 9 H, JHP3
)
3
3
6 H, JHH ) 6.8 Hz, CH3Ni-Pr), 1.31 (d, 6 H, JHH ) 6.8 Hz, CH3Ni-
12.4 Hz, PMe3), 1.68-2.04 (m, 28 H, CH2c-Hex), 2.15 (sept d d, 1 H,
Pr), 1.34 (d, 6 H, 3JHH ) 6.8 Hz, CH3Ni-Pr), 2.18 (sept d, 1 H, 3JHH
)
3JHH ) 7.2 Hz, JHP1 ) 3.2 Hz, JHP2 ) 10.4 Hz, CHi-Pr), 2.28 (sept d
4
3
3
7.2 Hz, JHP1 ) 1.2 Hz, CHi-Pr), 2.24 (sept d, 1 H, JHH ) 7.2 Hz,
d, 1 H, JHH ) 7.2 Hz, JHP1 ) 3.2 Hz, JHP2 ) 10.4 Hz, CHi-Pr), 2.70
4JHP1 ) 1.2 Hz, CHi-Pr), 2.42 (ddd, 1 H, 3JHH ) 13.2 Hz, JHP1 ) 12.4
(m, 2H, CH2(allyl)), 3.51 (m, 4 H, CHNc-Hex), 4.04 (m, 1 H, CH2(allyl)),
4.14 (tlike, JHH ) 8.4 Hz, JHP1 ) 8.4 Hz, CH2(allyl)), 5.12 (m, 1 H, CH(allyl)),
6.95 (t, 4 H, JHH ) 7.2 Hz, CHBPh), 7.10 (tlike, 8 H, JHH ) 7.2 Hz,
CHBPh), 7.38 (br, 8 H, CHBPh). 13C{1H} NMR (CDCl3): δ 16.9 (d,1JCP
3
Hz, JHP2 ) 1.2 Hz, CH2(allyl)), 2.60 (ddd, 1 H, JHH ) 13.2 Hz, JHP1
)
5.2 Hz, JHP2 ) 8.0 Hz, CH2(allyl)), 3.82 (d, 2 H, 3JHH ) 7.2 Hz, CH2(allyl)),
3
3
4.11 (sept d, 2 H, JHH ) 6.8 Hz, JHP1 ) 18.4 Hz, CHN), 4.15 (sept
d, 2 H, 3JHH ) 6.8 Hz, 3JHP1 ) 18.8 Hz, CHN), 4.91 (m, 1 H, CH(allyl)).
13C{1H} NMR (CD2Cl2): δ 19.9 (s, CH3i-Pr), 20.0 (s, CH3i-Pr), 20.7
) 57.3 Hz, PMe3), 20.0 (d, JCP2 ) 2.6 Hz, CH3i-Pr), 20.1 (d, JCP2
)
2.7 Hz, CH3i-Pr), 22.6 (d, JCP2 ) 6.0 Hz, CH3i-Pr), 22.7 (d, JCP2 ) 6.1
9
984 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007

Transient Palladadiphosphanylcarbenes
A R T I C L E S
Hz, CH3i-Pr), 26.1 (s, CH2c-Hex), 26.2 (s, CH2c-Hex), 27.9 (s, CH2c-
Hex), 28.0 (br, CH2c-Hex), 29.0 (dd, JCP1 ) 5.0 Hz, JCP2 ) 18.6, CHi-
crystal on a Bruker-AXS CCD 1000 diffractometer (λ ) 0.71073 Å).
Semiempirical absorption corrections were employed.26 The structures
were solved by direct methods (SHELXS-97)27 and refined using the
least-squares method on F2.28
Pr), 29.1 (dd, JCP1 ) 5.0 Hz, JCP2 ) 18.6, CHi-Pr), 33.7 (ddd, JCP1
)
33.2 Hz, JCP2 ) 23.4 Hz, JCP3 ) 58.3, Cylide), 35.5 (d, JCP1 ) 3.3 Hz,
CH2c-Hex), 35.6 (d, JCP1 ) 3.5 Hz, CH2c-Hex), 35.8 (d, JCP1 ) 2.7
Computational Details. Palladium and phosphorus were treated with
a Stuttgart-Dresden pseudopotential in combination with their adapted
basis set.29 In all cases, the basis set has been augmented by a set of
polarization function (f for Pd and d for P).30 Carbon, nitrogen, and
hydrogen atoms have been described with a 6-31G(d,p) double-ú basis
set.31 Calculations were carried out at the DFT level of theory using
the hybrid functional B3PW91.32 Geometry optimizations were carried
out without any symmetry restrictions; the nature of the extrema
(minimum) was verified with analytical frequency calculations. All
computations described herein were performed with the Gaussian 9833
suite of programs. The electronic density was analyzed using the Natural
Bonding Analysis (NBO) technique.24
Hz, CH2c-Hex), 36.0 (d, JCP1 ) 2.3 Hz, CH2c-Hex), 57.8 (dd, JCP1
)
45.6 Hz, JCP2 ) 7.0 Hz, CH2(allyl)), 59.1 (d, JCP1 ) 10.2 Hz, CHNc-
Hex), 59.4 (d, JCP1 ) 10.8 Hz, CHNc-Hex), 65.8 (dd, JCP1 ) 15.5 Hz,
JCP2 ) 33.0 Hz, CH2(allyl)), 119.2 (dd, JCP1 ) 11.6 Hz, JCP2 ) 6.7 Hz,
CH(allyl)), 122.2 (s, CHBPh), 126.1 (q, JCB ) 2.7 Hz, CHBPh), 136.3 (s,
CHBPh), 164.4 (q, JCB ) 49.3 Hz, CBPh). MS (FAB): 743 [M]+.
Synthesis of Palladium Complex 10b. To a DCM solution (1 mL)
of 7b (0.085 mmol, 60 mg) was added DMAP (0.085 mmol, 11 mg)
in DCM (1 mL) at -80 °C. The solution was warmed to room
temperature and stirred for 1 h. After evaporation of the solvent, the
solid was washed with Et2O, affording 10b as a yellow powder (89
mg, 94%), mp 188-190 °C. 31P{1H} NMR (CDCl3): δ 31.4 (d, JPP
)
1
Acknowledgment. We are grateful to the Centre National
de la Recherche Scientifique, the University Paul Sabatier
(France), and the National Institutes of Health (R01 GM 68825)
for financial support of this work. CalMip (CNRS, Toulouse,
France) is acknowledged for calculation facilities.
8 Hz, Pi-Pr2), 60.6 (br, PNc-Hex2). H NMR (CDCl3): δ 1.01 (dd, 3
H, 3JHH ) 7.2 Hz, JHP2 ) 16.2 Hz, CH3i-Pr), 1.14 (dd, 6 H, 3JHH ) 7.1
3
Hz, JHP2 ) 17.4 Hz, CH3i-Pr), 1.18 (dd, 3 H, JHH ) 6.9 Hz, JHP2
)
3
18.0 Hz, CH3i-Pr), 2.19 (sept d, 1 H, JHH ) 7.0 Hz, JHP2 ) 7.1 Hz,
3
CHi-Pr), 2.29 (sept d, 1 H, JHH ) 7.1 Hz, JHP2 ) 7.1 Hz, CHi-Pr),
2.55 (s, 6 H, NMe2), 2.62 (m, 2 H, CH2(allyl)), 3.46 (m, 4 H, CHNc-
Hex), 4.08 (m, 2 H, CH2(allyl)), 5.05 (m, 1H, CH(allyl)), 6.02 (d, 2 H, JHH
) 7.8 Hz, CHDMAP), 6.90 (t, 4 H, JHH ) 7.2 Hz, CHBPh), 7.05 (tlike, 8
Supporting Information Available: Computational details (Z-
matrices) and complete ref 33 (PDF); X-ray crystallographic
data for complexes 3b, 7b, and 8b (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
H, JHH ) 7.5 Hz, CHBPh), 7.46 (br, 8 H, CHBPh), 7.69 (d, 2 H, JHH
)
7.8 Hz, CHDMAP). 13C{1H} NMR (C6D6): δ 20.2 (s, CH3i-Pr), 20.3 (s,
CH3i-Pr), 20.8 (d, JCP2 ) 5.7 Hz, CH3i-Pr), 21.2 (d, JCP2 ) 5.5 Hz,
CH3i-Pr), 25.8 (s, CH2Nc-Hex), 25.9 (s, CH2Nc-Hex), 27.4 (s, CH2Nc-
Hex), 27.5 (s, CH2Nc-Hex), 27.6 (s, CH2Nc-Hex), 27.7 (s, CH2Nc-
Hex), 28.3 (dd, JCP1 ) 5.3 Hz, JCP2 ) 17.9 Hz, CHi-Pr), 28.4 (dd, JCP1
) 6.2 Hz, JCP2 ) 17.9 Hz, CHi-Pr), 35.6 (d, JCP1 ) 2.9 Hz, CH2c-
Hex), 35.7 (d, JCP1 ) 2.9 Hz, CH2c-Hex), 35.9 (s, CH2c-Hex), 36.0 (s,
CH2c-Hex), 39.7 (s, NMe2), 56.3 (dd, JCP1 ) 43.9 Hz, JCP2 ) 5.1 Hz,
CH2(allyl)), 58.0 (d, JCP1 ) 9.8 Hz, CHNc-Hex), 58.5 (d, JCP1 ) 9.8 Hz,
CHNc-Hex), 64.3 (dd, JCP1 ) 13.7 Hz, JCP2 ) 31.1 Hz, CH2(allyl)), 65.3
(dd, JCP1 ) 36.2 Hz, JCP2 ) 42.3 Hz, Cylide), 107.4 (s, CHDMAP), 117.6
(dd, JCP1 ) 10.2 Hz, JCP2 ) 5.9 Hz, CH(allyl)), 121.6 (s, CHBPh), 125.5
(s, CHBPh), 136.3 (s, CHBPh), 141.5 (s, CHDMAP), 164.2 (q, JCB ) 49.3
Hz, CBPh).
JA066738J
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X-ray Crystallographic Studies of Complexes 3b, 7b, and 8b.
Crystal data for all structures are presented in Table 2. Data for all
structures were collected at 193(2) K using an oil-coated shock-cooled
9
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