Synthesis and characterization of an unsymmetrical 1,2-diphosphinoethanide complex

Article abstract of DOI:10.1021/om200069t

A palladium 1,2-ethylene-bisphosphine complex prepared by diphosphination of acrylonitrile and subsequent complexation undergoes facile and reversible C deprotonation at a backbone carbon atom. The 1,2-diphosphinoethanide complex that formed was character

Full text of DOI:10.1021/om200069t

NOTE
pubs.acs.org/Organometallics
Synthesis and Characterization of an Unsymmetrical 1,
2-Diphosphinoethanide Complex
Daniela F€orster,† Martin Nieger,‡ and Dietrich Gudat*,†
†Institut f€ur Anorganische Chemie, Universit€at Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany
‡Laboratory of Inorganic Chemistry, University of Helsinki, A. I. Virtasen aukio 1, Helsinki, Finland
S Supporting Information
b
ABSTRACT: A palladium 1,2-ethylene-bisphosphine complex
prepared by diphosphination of acrylonitrile and subsequent
complexation undergoes facile and reversible C deprotonation
at a backbone carbon atom. The 1,2-diphosphinoethanide com-
plex that formed was characterized by spectroscopic data and a
single-crystal X-ray diffraction study. The reaction may explain the
previously observed configurational lability of coordinated 1,
2-bisphosphine ligands with electron-withdrawing substituents.
nsymmetrically substituted N-heterocyclic diphosphanes
Scheme 1
U
such as 1a,b (Scheme 1) exhibit highly reactive PꢀP bonds
that add under very mild conditions to electron-poor double or
triple bonds to give the bidentate 1,2-bisphosphines 2 and 3,1
thus allowing the simultaneous introduction of two P-donor
moieties with different electronic properties at an organic
backbone.
Whereas the diphosphination of alkynes is Z stereospecific2ꢀ4
and yields a single stereoisomer, addition to alkenes produces
racemic mixtures of enantiomers with R and S configurations at
the newly formed stereogenic center at one of the backbone carbon
atoms.1,5 A particularly interesting case is provided by reactions of
diphosphane 4 with 1,2-disubstituted alkenes (Scheme 2), where a
pair of initially formed diastereomeric adducts with meso and dl
configurations at the two backbone carbon atoms gave a single
stereoisomer of complexes 5 and 6 after reaction with (cod)PdCl2
(cod = 1,5-cyclooctadiene).5 Although it had been suspected that
this epimerization may be catalyzed by either Lewis bases or Lewis
acids,5 its actual mechanism remained unknown. Here, we report on
a similar reaction sequence, involving N-heterocyclic diphosphane
addition to acrylonitrile and complexation of formed 1,2-bispho-
sphine to palladium, which allowed for the isolation of a reaction
intermediate that provides consistent proof for a base-catalyzed
epimerization mechanism.
of 2.221 ( 0.11 Å6) than those for 1b (2.334(1) Å1), 4 (2.312
(1) Å5), or symmetrical bis-diazaphospholenes (2.24ꢀ2.33 Å7),
which reflects the extra inductive stabilization of the diazapho-
spholenium cation fragment by the electron-releasing methyl
groups.8 The PꢀP bond lengthening had previously been identified
as a typical feature of CC-unsaturated N-heterocyclic diphosphanes
and is closely related to their high chemical reactivity.9
Reaction of diphosphane 8 with acrylonitrile at 50 °C pro-
duced the single addition product 9, which was detected by 31P
NMR (AX spin system, δ 111.1 (PN2), ꢀ16.0 (PPh2), 3JPP = 12.3
Hz) and directly trapped by addition of (cod)PdCl2 (Scheme 3).
Complex 10 was isolated as a yellow powder; its purity and
constitution were established by analytical and spectroscopic stud-
ies. The 31P{1H} NMR spectrum of 10 displays a characteristic AX-
type pattern whose chemical shifts allow easy assignment of the PN2
(δ 119.7, 3JPP = 20 Hz) and PPh2 (δ 55.6, 3JPP = 20 Hz) moieties.
’ RESULTS AND DISCUSSION
The N-heterocyclic diphosphane 8 was prepared by analogy to
the synthesis of 1a,b1 via metathesis of chlorodiazaphospholene
7 with lithium diphenylphosphide (Scheme 3).
The identity of 8 was established by spectroscopic data and a
single-crystal X-ray diffraction study (Figure 1). The molecular
structure of 8 is notable for showing a PꢀP bond even longer
(P1ꢀP2 = 2.347(1) Å vs a standard PꢀP distance in diphosphanes
Received: January 26, 2011
Published: April 13, 2011
r
2011 American Chemical Society
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Organometallics
NOTE
Scheme 2
Scheme 4a
a R1 = Dmp.
Scheme 3a
a R1 = 2,6-Me2C6H4 (Dmp).
Figure 2. Representation of one formula unit of 11 in the crystal (top) and
reduced plot without P substituents (bottom). H atoms and solvent
molecules are omitted for clarity, and thermal ellipsoids are drawn at the
50% probability level. Li1# and N26# denote atoms of adjacent units in the
coordination polymer generated by the symmetry operations ꢀ1þ x, y,zand
1 þ x, y, z, respectively. Selected distances (in Å): Li1#ꢀN26 = 1.971(11),
Pd1ꢀP1=2.242(1), Pd1ꢀP2=2.219(1), Pd1ꢀCl1 = 2.427(1), Pd1ꢀCl2 =
2.431(1), P1ꢀN2 = 1.697(5), P1ꢀN5 = 1.706(5), P1ꢀC24 = 1.716(5),
C24ꢀC25 = 1.514(8), C25ꢀP2 = 1.814(5), C24ꢀC26 = 1.407(7),
C26ꢀN26 = 1.147(6), Cl1ꢀLi1 = 2.322(10), Cl2ꢀLi1 = 2.334(10).
Figure 1. Molecular structure of 8 (H atoms omitted for clarity;
displacement ellipsoids at the 50% probability level). Selected bond
lengths (Å): P1ꢀN2 = 1.705(1), P1ꢀN5 = 1.706(1), P1ꢀP2 =
2.347(1).
In order to study the possible influence of a base on the
epimerization of the stereogenic center in the backbone of 10, we
performed the reaction of 7 with lithium diphenylphosphide,
acrylonitrile, and (cod)PdCl2 in a one-pot procedure in the presence
of triethylamine. Instead of the expected complex 10, we isolated in
this case a product which was later identified as dimetallic complex
11. The same product was also obtained from the reaction of pure
complex 10 with appropriate amounts of triethylamine and lithium
chloride (Scheme 4). The key step in this transformation is
R-deprotonation of a doubly activated methylene group to give a
nitrile-stabilized carbanion. Species of this type are useful inter-
mediates in organic synthesis.10 R-Phosphanyl anions are less
abundant but have also been prepared.11
Complex 11 was isolated after crystallization from acetonitrile.
A single-crystal X-ray diffraction study revealed the presence of
chloride-bridged dinuclear lithiumꢀpalladium complexes that
are connected via interaction of the nitrile lone pair with the
lithium ion of an adjacent formula unit to form a one-dimen-
sional coordination polymer (Figure 2). Each lithium ion is
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Organometallics
NOTE
further solvated by an additional acetonitrile molecule. The
C24ꢀC25 bond (1.514(8) Å) forming the backbone of the
bisphosphine ligand is a normal single bond, whereas the
C24ꢀC26 distance (1.407(7) Å) is markedly shortened, indicat-
ing some electron delocalization between the formal carbanion
center and the adjacent nitrile group. The C26ꢀN26 distance
(1.147(6) Å) and the N26ꢀLi1 contact of 1.971(11) Å are
typical for coordinated CN triple bonds.12 The chelate bite angle
(P1ꢀPd1ꢀP2 = 84.80(5)°) is smaller than those in 5 and 6,4
related complexes of ethylene-1,2-bisphosphines such as 2,5 or
similar specimens recently reported by Pringle et al.2 (PꢀPdꢀP =
88ꢀ90°). The PdP2C2 chelate ring displays a twist conforma-
tion, and the palladium coordination sphere shows a similar
deviation from planarity as had been observed for 5 and 6,5 with a
dihedral angle of 9° between the P1ꢀPdꢀP2 and Cl1ꢀPdꢀCl2
planes. As a consequence of the μ2-bridging chloride coordina-
tion, the PdꢀCl distances (Pd1ꢀCl1 = 2.427(1) Å, Pd1ꢀCl2 =
2.431(1) Å) are longer than those to the terminal chlorides in 5
and 6 (PdꢀP = 2.35ꢀ2.37 Å5).
(1H, 250.1 MHz; 13C, 62.8 MHz; 31P, 101.2 MHz) NMR spectrometers
at 303 K; chemical shifts are referenced to external TMS (1H, 13C) or
85% H3PO4 (Ξ = 40.480 747 MHz, 31P). Coupling constants are given
as absolute values. Elemental analyses were determined on a Perkin-
Elmer 24000CHN/O analyzer. Melting points were determined in
sealed capillaries.
1,3-Bis(20,60-dimethylphenyl)-4,5-dimethyl-2-diphenylphos-
phino[1.3.2]diazaphospholene (8). A solution of n-BuLi (2.5 M in
THF, 1.5 mL, 2.7 mmol) was added dropwise to a cooled (ꢀ78 °C)
solution of diphenylphosphine (0.45 mL, 2.7 mmol) in THF (10 mL).
After 15 min, the mixture was warmed to room temperature and stirred
for 1 h. This solution was then slowly added to a cooled (ꢀ78 °C)
solution of chloro-1,3-diazaphospholene 715 (0.95 g, 2.7 mmol). Stirring
was continued for 1 h after the addition was completed, and the solution
was evaporated under reduced pressure. The residue was extracted with
hexane (20 mL). The filtrate was filtered over Celite and concentrated to
5 mL. The product crystallized at 4 °C (yield 78%): mp 105 °C; 31P
NMR (C6D6) δ 147.4 (d, 3JPP = 288 Hz, N2P), ꢀ24.9 (d, 3JPP = 288 Hz,
PPh2). Anal. Calcd for C32H34N2P2 (508.57): C, 75.57; H, 6.74; N, 5.51.
Found: C, 74.62; H, 6.71; N, 5.52.
The molecular structure of 11 was further confirmed by 31P
solid-state NMR data. The 31P CP/MAS NMR spectrum consists
of two signals with chemical shifts of 108 and 55 ppm that are
connected by a mutual coupling of 28 Hz, which was derived from a
J-resolved 2D spectrum. Attempts to characterize 11 by 31P solution
NMR in THF produced spectra with several sets of signals, some of
which showed substantial exchange broadening. Although no
further signal assignment was attempted, these spectra indicate that
the complex undergoes some reaction in solution.
2-[1,3-Bis(20,60-dimethylphenyl)[1.3.2]diazaphospholenyl]-3-
(diphenylphosphino)propanenitrile (9) and Its Dichloropalla-
dium Complex (10). Acrylonitrile (0.14 mL, 2.1 mmol) was added to a
stirred solution of 8 (0.75 g, 2.1 mmol) in THF (15 mL), and the mixture
was stirred for 4 h at 50 °C. 31P NMR spectroscopy revealed the formation
of 9 (δ 111.1 (d, 3JPP = 12.3 Hz, N2P), ꢀ16.0 (d, 3JPP = 12.3 Hz, PPh2))
together with minor amounts of species resulting from hydrolysis of 8.4 A
solution of (cod)PdCl2 (0.29 g, 1.05 mmol) in CH2Cl2 (15 mL) was then
slowly added. The solution was stirred for a further 0.5 h after the addition
was completed and evaporated under reduced pressure. The residue was
extracted with diethyl ether (20 mL) and filtered. The solvent was
evaporated to leave an orange powder (yield 38%): mp 174 °C; 31P
NMR (C6D6) δ 120.6 (d, 3JPP = 22 Hz, N2P), 59.2 (d, 3JPP = 22 Hz, PPh2);
1H NMR (C6D6) δ 7.44ꢀ7.28 (m, 10 H, Hphenyl), 7.18ꢀ7.04 (m, 6 H,
Hphenyl), 3.24ꢀ3.09 (m, 1 H, CNꢀCH), 2.98 (s, 3 H, o-CH3), 2.82 (s, 3 H,
o-CH3), 2.45 (s, 3 H, o-CH3), 2.38(s, 3 H, o-CH3), 2.18ꢀ2.09 (m, 1 H,
CH2), 2.02ꢀ1.95 (m, 1 H, CH2), 1.69 (d, 6 H, 4JPH = 4.75 Hz, NꢀCH3);
In order to ascertain that complex 11 may be considered a
genuine intermediate in the epimerization of the stereogenic
center in 10, it is essential to prove also the feasibility of the
reverse reaction: i.e. reprotonation of 11 to give 10. To this end,
we reacted the THF solution of 11 with an excess of formic acid
and established by monitoring the reaction by 31P NMR that the
neutral complex 10 was indeed formed as the only detectable
product (Scheme 4).
1
13C{1H}NMR (C6D6) δ 138.4 (d, JPC = 62.3 Hz, i-Cphenyl), 138.3 (d,
1JPC = 62.1 Hz, i-Cphenyl), 136.0 (d, 2JPC = 50.5 Hz, i-CDmp), 135.9 (d, 2JPC
50.1 Hz, i-CDmp), 132.3 (d, 5JPC = 4.2 Hz, p-Cphenyl), 132.0 (d, 5JPC
11.9 Hz, o-Cphenyl), 131.8 (d, 5JPC = 4.9 Hz, p-Cphenyl), 131.7 (d, 5JPC
=
=
=
’ CONCLUSIONS
In summary, we have demonstrated the facile and fully
reversible interconversion between the neutral 1,2-bisphosphine
complex 10 and its C-deprotonation product, 11. The reaction
allows a mechanistic explanation of the previously observed5 back-
bone epimerization in the diphosphination of maleic esters. It
appears plausible that the epimerization may be catalyzed by other
bases (such as phosphines) and that anion formation is facilitated by
the accumulation of electron-withdrawing nitrile and phosphenium
substituents. In this respect, the results are not in contradiction with
the configurational stability of certain ethane-1,2-bisphosphines
such as Prophos, Chiraphos, Norphos,13 and others, which lack
any electron-withdrawing substituents, but suggest that other bis-
phosphines obtained by diphosphination of electron-poor alkenes
may also be liable to a similar isomerization. Even though this
behavior precludes using such species to control enantioselectivity
in a catalytic process, it offers opportunities to develop novel “non-
innocent” ligands for cooperative catalysis.14
11.5 Hz, o-Cphenyl), 130.6 (d, 3JPC = 3.1 Hz, o-CDmp), 128.6 (s, broad,
o-CDmp), 127.5 (d, 5JPC = 0.9 Hz, p-CDmp), 127.4 (s, broad, m-Cphenyl),
127.3 (s, m-CDmp), 127.2 (s, m-CDmp), 127.1 (s, broad, m-Cphenyl), 126.9
(d, 5JPC = 1.3 Hz, p-CDmp), 123.8 (d, 2JPC = 5.2 Hz, NꢀCCH3), 123.0
(d, 2JPC = 5.2 Hz, NꢀCCH3), 38.2 (dd, 2JPC = 5.6 Hz, 1JPC = 41.1 Hz,
CNꢀCH), 25.3 (dd, 2JPC = 21.2 Hz, 1JPC = 30.2 Hz, CH2), 21.7 (d, 4JPC
=
1.0 Hz, o-CH3), 21.1 (s, broad, o-CH3), 17.9 (d, 4JPC = 0.6 Hz, o-CH3),
17.8 (s, broad, o-CH3), 9.5 (d, 3JPC = 3.3 Hz, NꢀCH3), 9.4 (d, 3JPC
=
3.3 Hz, NꢀCH3); IR ν(CN) 2236 (w) cmꢀ1. Anal. Calcd for
h
C35H37Cl2N3P2Pd (738.96): C, 56.89; H, 5.05; N, 5.69. Found: C,
56.47; H, 5.26; N, 5.00.
Deprotonation of 10 To Give 11. Palladium complex 10 (0.2 g,
0.27 mmol), 2 equiv of triethylamine, and 1 equiv of LiCl were dissolved in
anhydrous THF (10 mL) and stirred for 3 h at 50 °C. The solution was
cooled to room temperature and evaporated under reduced pressure. The
residue was dissolved in acetonitrile (5 mL) and stored at 4 °C to yield red
crystals (yield 83%): dec pt 172 °C; 31P CP/MAS NMR δ 108 (N2P), 55
(Ph2P), 3JPP = 28 Hz. Anal. Calcd for C35H37Cl2N3P2PdLi (744.89): C,
56.43; H, 4.87; N, 5.64. Found: C, 56.48; H, 4.88; N, 6.18.
’ EXPERIMENTAL SECTION
All manipulations were carried out under an atmosphere of dry argon
using standard vacuum line techniques. Solvents were dried by standard
procedures. NMR spectra were recorded on Bruker Avance 400
(1H, 400.1 MHz; 13C, 100.5 MHz; 31P, 161.9 MHz) and Avance 250
Protonation of 11. Complex 11 (120 mg) and 1 drop of formic
acid were dissolved in d8-THF (1 mL) and stirred for 15 min.
Quantitative protonation to give 10 was confirmed by 31P NMR
spectroscopy. No attempt at isolation of the product was made.
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Organometallics
NOTE
31P NMR (THF-d8): δ 119.7 (d, 3JPP = 22 Hz, N2P), 55.6 (d, 3JPP = 22
Hz, PPh2).
(8) Burck, S.; G€otz, K.; Kaupp, M.; Nieger, M.; Weber, J.; Schmedt
auf der G€unne, J.; Gudat, D. J. Am. Chem. Soc. 2009, 131, 10763.
(9) Gudat, D. Acc. Chem. Res. 2010, 43, 1307.
(10) Arseniyadis, S.; Kyler, K. S.; Watt, D. S. Org. Reactions
1984, 31.Schmidt, A. Comprehensive Organic Functional Group Trans-
formations II; Elsevier: Amsterdam, 2005; Vol. 2, p 1059.
(11) Izod, K. Coord. Chem. Rev. 2002, 227, 153.
(12) Standard CN distance in coordinated nitriles 1.146(14) Å:
Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.;
Taylor, R. Dalton Trans. 1989, S1.
(13) Prophos: Fryzuk, M. D.; Bosnich, B. J. Am. Chem. Soc. 1978,
100, 5491. Chiraphos: Bergens, S. H.; Whelan, J.; Bosnich, B. Inorg.
Synth. 1997, 31, 131. Norphos: Marchand, A. P.; Romanski, J.; Kumar,
T. P. e-EROS Encycl. Reagents Org. Synth. 2001.
Crystallography. Crystal structure determinations of 8 and 11
were performed on a Nonius KappaCCD diffractometer at T = 123(2) K
using Mo KR radiation (λ = 0.710 73 Å). Direct methods (SHELXS-
97)16 were used for structure solution, refinement was carried out using
SHELXL-9716 (full-matrix least-squares on F2), and hydrogen atoms
were refined using a riding model. A semiempirical absorption correc-
tion was applied for 11. Data for 8: orange crystals, C32H34N4P2,
Mr = 508.55, crystal size 0.35 ꢁ 0.30 ꢁ 0.25 mm, triclinic, space group
P1 (No. 2), a = 8.051(2) Å, b = 12.213(3) Å, c = 14.472(1) Å,
R = 84.28(2)°, β = 80.99(2)°, γ = 78.72(2)°, V = 1374.8(5) Å3, Z =
2, F = 1.229 Mg mꢀ3, μ(Mo KR) = 0.182 mmꢀ1, T = 123(2) K, F(000) =
540, 2θmax = 55°, 30 636 reflections, 6280 of which were independent
(Rint = 0.031), 331 parameters, R1 = 0.037 (for I > 2σ(I)), wR2 = 0.093
(14) Van der Vlugt, J. I.; Reek, J. N. H. Angew. Chem., Int. Ed. 2009,
48, 8832.
(15) Burck, S.; Gudat, D.; N€attinen, K.; Nieger, M.; Niemeyer, M.;
Schmid, D. Eur. J. Inorg. Chem. 2007, 5112.
(16) Sheldrick, G. M. Acta Crystallogr. 2008, A46, 112.
(all data), S = 1.03, largest difference peak/hole 0.340/ꢀ0.236 e Åꢀ3
.
Data for 11: red crystals, C37H39Cl2LiN4P2Pd 2CH3CN, Mr = 868.01,
3
crystal size 0.40 ꢁ 0.20 ꢁ 0.10 mm, triclinic, space group P1 (No. 2),
a = 10.471(1) Å, b = 14.436(1) Å, c = 15.933(2) Å, R = 107.57(1)°,
β = 102.66(1)°, γ = 109.74(1)°, V = 2018.0(3) Å3, Z = 2, F = 1.429 Mg
m
ꢀ3, μ(Mo KR) = 0.709 mmꢀ1, T = 123(2) K, F(000) = 892, 2θmax
=
55°, 22 821 reflections, 9125 of which were independent (Rint = 0.077),
487 parameters, R1 = 0.070 (for I > 2σ(I)), wR2 = 0.191 (all data), S =
1.02, largest difference peak/hole 1.616/ꢀ0.926 e Åꢀ3
.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving X-ray structur-
b
al information on 8 (CCDC-807699) and 11 (CCDC-807700).
This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ49 711 68564241. E-mail: gudat@iac.uni-stuttgart.de.
’ ACKNOWLEDGMENT
We thank S. Plebst for recording the IR spectra. We thank the
Academy of Finland for a Research Fellowship and Deutscher
Akademischer Austauschdienst (DAAD) for funding. The
Deutsche Forschungsgemeinschaft (DFG, grant Gu 415/12-2)
and COST (action CM0802) are acknowledged for financial
support.
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