Palladium-catalyzed phosphaketene decarbonylation: Diphosphaureylene intermediates in diphosphene formation

Article abstract of DOI:10.1021/om970761j

Pd(PPh₃)₄-catalyzed decarbonylation of the phosphaketene Mes* PCO (1, Mes* = 2,4,6-(t-Bu)₃C₆H₂) gives the diphosphene Mes* P=PMes* (2). Related reactions of 1 with zerovalent Pd and Pt phosphine compl

Full text of DOI:10.1021/om970761j

4768
Organometallics 1997, 16, 4768-4770
P a lla d iu m -Ca ta lyzed P h osp h a k eten e Deca r bon yla tion :
Dip h osp h a u r eylen e In ter m ed ia tes in Dip h osp h en e
F or m a tion
Marie-Anne David, Denyce K. Wicht, and David S. Glueck*
6128 Burke Laboratory, Department of Chemistry, Dartmouth College,
Hanover, New Hampshire 03755
Glenn P. A. Yap, Louise M. Liable-Sands, and Arnold L. Rheingold
Department of Chemistry, University of Delaware, Newark, Delaware 19716
Received August 27, 1997
Sch em e 1
Sch em e 2
Summary: Pd(PPh3)4-catalyzed decarbonylation of the
phosphaketene Mes*PCO (1, Mes* ) 2,4,6-(t-Bu)3C6H2)
gives the diphosphene Mes*PdPMes* (2). Related reac-
tions of 1 with zerovalent Pd and Pt phosphine complexes
afford diphosphaureylene complexes ML2[Mes*PC(O)-
PMes*] (L2 ) chelating diphosphine), whose structure
and properties depend markedly on the metal and
ancillary ligands; Pd(dppf)[Mes*PC(O)PMes*] (12, dppf
) 1,1′-bis(diphenylphosphino)ferrocene) also catalyzes
the title reaction.
Decarbonylation of the phosphaketene1 Mes*PCO (1,
Mes* ) 2,4,6-(t-Bu)3C6H2) by metal complexes usually
results in stoichiometric PdC bond cleavage and yields
products derived from the phosphinidene Mes*P.2 We
report here a new reaction of 1: Pd-catalyzed decar-
bonylation to give the diphosphene Mes*PdPMes* (2),
the formal result of phosphinidene coupling.3 Related
stoichiometric Pd and Pt chemistry yields diphosphau-
reylene [Mes*PC(O)PMes*] complexes, which appear to
be involved in P-P bond formation.
Pd(PPh3)4 catalyzes decarbonylation of 1 to yield 2
(∼5 turnovers/h, THF, room temperature). The reaction
is quantitative by 31P NMR, and diphosphene 2 was
isolated after recrystallization as orange crystals in 70%
yield (Scheme 1).4 Although the Pd catalyst could be
recovered, monitoring of the reaction by IR and 31P
NMR shows that Pd(PPh3)3(CO) (3)5 is formed and
decomposes on workup. Independently prepared 3 also
catalyzes the formation of 2, at a similar rate.6
the catalytic reaction. A variety of zerovalent metal
precursors react with 2 equiv of 1 to give the diphos-
phaureylene complexes ML2[Mes*PC(O)PMes*] (M )
Pd, L2 ) dcpe (4), dppe (5), dppp (6), η2-triphos (7), dmpe
(8); M ) Pt, L2 ) dppe (9), η2-triphos (10)) (Scheme 2).7
Complexes 8-10, like the previously reported Pt-
(dmpe)[Mes*PC(O)PMes*] (11),2d are orange to red, but
Pd compounds 4-7 are green. 31P NMR data also shows
large differences between 4-7 and 8-11 (AA′XX′ spin
systems, Table 1). The chemical shift (ppm) of the
diphosphaureylene P nuclei ranges from 14.0 to 51.8 for
8-11 but from 134.7 to 176.8 for 4-7. Moreover, the
green complexes show a larger cis J PP coupling within
the diphosphaureylene ligand (353-433 Hz) and a
smaller trans J PP coupling (89-126 Hz) than the red
analogs, for which J cis ranges from 191 to 158 Hz and
J trans from 217 to 196 Hz. In both sets of complexes,
these chemical shifts and cis J PP coupling constants
Related Pt and Pd chemistry with chelating diphos-
phine ligands provides insight into the mechanism of
(1) Appel, R.; Paulen, W. Angew. Chem., Int. Ed. Engl. 1983, 22,
785-786.
(7) Abbreviations used: dba ) dibenzylideneacetone, triphos ) MeC-
(CH2PPh2)3, dppp ) Ph2PCH2CH2CH2PPh2, dppe ) Ph2PCH2CH2PPh2,
dmpe ) Me2PCH2CH2PMe2, dcpe ) Cy2PCH2CH2PCy2 (Cy ) cyclo-
C6H11), dppf ) Ph2PC5H4FeC5H4PPh2. Synthetic details and charac-
terization data for the new complexes are included in the Supporting
Information; an example follows. Syn th esis of 5. To a solution of Pd-
(dppe)2 (151 mg, 0.168 mmol) in THF (1 mL) was added Mes*PCO (102
mg, 0.335 mmol) dissolved in THF (1 mL). The mixture became deep
green immediately and was stirred at room temperature in the dark
for 1 h. The solvent was removed in vacuo. The green residue was
washed with petroleum ether (40 mL), filtered, dissolved in a minimum
of THF, layered with petroleum ether, and cooled to -25 °C to give 5
as a green powder (129 mg, 71% yield). Green needles can be obtained
by recrystallization from THF/petroleum ether or by slow evaporation
of a THF solution. 1H NMR (CD2Cl2): δ 7.48-7.42 (m, 8H), 7.34-7.30
(m, 8H), 7.16-7.11 (m, 8H), 2.19-2.13 (m, 4H), 1.47 (36H), 1.34 (18H).
(2) (a) Champion, D. H.; Cowley, A. H. Polyhedron 1985, 4, 1791-
1792. (b) Cowley, A. H.; Pellerin, B.; Atwood, J . L.; Bott, S. G. J . Am.
Chem. Soc. 1990, 112, 6734-6735. (c) David, M.-A.; Paisner, S. N.;
Glueck, D. S. Organometallics 1995, 14, 17-19. (d) David, M.-A.;
Glueck, D. S.; Yap, G. P. A.; Rheingold, A. L. Organometallics 1995,
14, 4040-4042.
(3) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J .
Am. Chem. Soc. 1981, 103, 4587-4589.
(4) P d -ca t a lyzed for m a t ion of 2. Addition of Pd(PPh3)4 (6 mg,
0.005 mmol) to an orange solution of Mes*PCO (33 mg, 0.11 mmol) in
1 mL of THF immediately gave a dark brown solution; the 31P NMR
spectrum of the mixture after
2 h showed that 2 was formed
quantitatively (11 turnovers). The solvent was removed, and the
residue was extracted with 5 mL of petroleum ether. Cooling the
resulting orange solution to -25 °C gave 21 mg of 2 (70%).
(5) (a) Kudo, K.; Hidai, M.; Uchida, Y. J . Organomet. Chem. 1971,
33, 393-398. (b) Morandini, F.; Consiglio, G.; Wenzinger, F. Helv.
Chim. Acta 1979, 62, 59-61.
(6) Since the title reaction is not synthetically useful, we did not
carry out detailed rate studies or attempt to maximize turnover
number. However, after catalysis by Pd(PPh3)4 was complete, if more
1 was added to the reaction mixture, it was also converted to 2 at a
similar rate.
1
13C{1H} NMR (CD2Cl2): δ 226.5 (t, J P-C ) 64 Hz, quat, CO), 157.2
(br, quat Ar), 149.8 (quat Ar), 134.8-134.0 (m, quat Ar), 133.8-133.6
(m, Ar), 132.9-132.4 (m, quat Ar), 130.7 (Ar), 128.9-128.8 (m, Ar),
122.9 (br, Ar), 39.4 (quat), 35.4 (quat), 34.1 (ortho Me), 31.6 (para Me),
26.3-25.8 (m, CH2). IR(KBr): 3053, 2953, 2903, 1590, 1521, 1478,
1434, 1391, 1358, 1309, 1278, 1211, 1100, 1025, 1000, 921, 876, 821,
743, 695, 651, 586, 524, 482 cm-1. Anal. Calcd. for C63H82OP4Pd: C,
69.69; H, 7.63. Found: C, 69.31; H, 7.64.
S0276-7333(97)00761-9 CCC: $14.00 © 1997 American Chemical Society

Communications
Ta ble 1. 31P NMR Da ta for P d a n d P t
Organometallics, Vol. 16, No. 22, 1997 4769
Dip h osp h a u r eylen e Com p lexesa
complex
δ(P1)
δ(P3)
2J 12
2J 13
2J 14
2J 34
4b
5c
61.8
41.5
3.5
134.7
142.8
172.3
176.8
51.8
48.5
50.9
14.0
75
80
131
126
50
17
39
10
-22.5
-33
123.6
126
89
90
211
202
196.3
216.5
363
353
428
433
170
175
190.8
158
6b
-40
7b,d
8b
5.2
-38
-18
-2.7
-5.5
-5.5
26.5
50.5
2.1
9e,f
10d,f,g
11b,f,h
F igu r e 1. ORTEP diagram of 9. A tert-butyl group (C2*,-
C3*) is rotationally disordered in two positions with a 50/
50 distribution. The disordered carbon atoms were refined
isotropically. Selected bond lengths (Å): Pt-P(1) 2.276(3);
Pt-P(2) 2.288(3); Pt-P(3) 2.377(4); Pt-P(4) 2.347(3); P(3)-
C(1) 1.805(12); P(4)-C(1) 1.794(12); O-C(1) 1.248(10).
Selected bond angles (deg): P(1)-Pt-P(2) 85.02(11); P(1)-
Pt-P(4) 172.24(10); P(2)-Pt-P(4) 102.52(12); P(1)-Pt-
P(3) 100.79(12); P(2)-Pt-P(3) 171.24(11); P(4)-Pt-P(3)
71.95(10); C(1)-P(3)-Pt 92.8(4); C(1)-P(4)-Pt 94.1(4);
O-C(1)-P(4) 128.9(11); O-C(1)-P(3) 130.2(11); P(4)-
C(1)-P(3) 100.9(5).
32.0
a
Chemical shifts in ppm (85% H3PO4 external reference);
coupling constants in Hertz. In CD2Cl2. c In CD2Cl2/C6D6. Un-
coordinated triphos arm resonance: for 7, δ -25; for 10; δ -26.5.
e In THF-d8. f Pt-P couplings: for 9, J 15 ) 2477, J 35 ) 1145; for
10, J 15 ) 2459, J 35 ) 1085; for 11, J 15 ) 2435, J 35 ) 1103. In
C6D6. David, M.-A.; Glueck, D. S.; Yap, G. P. A.; Rheingold, A.
b
d
g
h
L. Organometallics 1995, 14, 4040-4042.
increase with the cone angle and the bite angle of the
ancillary diphosphine ligand.
These spectroscopic observations suggest a structural
difference between the two sets of complexes, which was
confirmed by X-ray crystallography.8 The PtPC(O)P
ring in Pt complex 9 (Figure 1) is essentially planar (fold
angle between PMP and PCP planes ) 5.4°), but in Pd
analog 6 (Figure 2) this ring is puckered with fold angles
of 29.3° and 31.0° in the two independent molecules.
This puckering (Figure 3) suggests a contribution from
a π-allyl resonance structure as in analogous oxatri-
methylenemethane complexes.9
Diphenylphosphino-ligated palladium complexes 5-7
decompose in THF solution to afford diphosphene 2 (at
room temperature for 6 and 7 and 65 °C for 5). This
suggests a possible mechanism for the catalysis (Scheme
F igu r e 2. ORTEP diagram of 6, showing one of the two
independent molecules in the asymmetric unit. Data for
one of these molecules, selected bond lengths (Å): Pd-P(1)
2.382(5); Pd-P(2) 2.348(5); Pd-P(3) 2.398(5); Pd-P(4)
2.356(6); P(3)-C(4) 1.95(2); P(4)-C(4) 1.79(2); O(4)-C(4)
1.11(2). Selected bond angles (deg): P(1)-Pd-P(2) 90.3-
(2); P(1)-Pd-P(4) 160.2(2); P(2)-Pd-P(4) 97.2(2); P(1)-
Pd-P(3) 97.7(2); P(2)-Pd-P(3) 166.8(2); P(4)-Pd-P(3)
72.0(2); C(4)-P(3)-Pd 89.0(6); C(4)-P(4)-Pd 94.5(7); O(4)-
C(4)-P(4) 136(2); O(4)-C(4)-P(3) 127(2); P(4)-C(4)-P(3)
96.5(11).
(8) Crystal data for 6: C64H84OP4Pd, fw ) 1099.59, monoclinic, P21,
a ) 10.539(2) Å, b ) 20.538(3) Å, c ) 27.777(5) Å, â ) 91.607(4)°, V )
6010(2) Å3, Z ) 4, T ) 218(2) K, Dcalc ) 1.215 g/cm3, R(F) ) 12.98%,
R(wF2) ) 26.66% for 10 148 independent observed reflections (3° e 2θ
e 58°). Despite several recrystallizations, the best crystals that could
be obtained were extremely weak diffractors and the reflections were
diffuse. Because systematic absences in the data contained weak
violations (ca. 1.5σ) to the 21 screw axis and showed the absence of a
glide plane, the space group options P2, P2/m, Pm, P21, and P21/m
were allowed. From these options, those without mirror plane sym-
metry were preferred and those with mirror plane symmetry were
initially rejected, because the potential mirror-plane is misaligned with
the crystallographic ac plane. Of the remaining options, solution and
refinement was successful in P21 only. There are two crystallographi-
cally independent but chemically equivalent molecules in the asym-
metric unit. The refined value of the Flack parameter suggested the
possibility of a racemic twin, and a 60/40 model minimized R. Disorder
was observed in the methyl carbons of the Mes* tert-butyl groups, but
attempts to model it were incomplete; they were fixed as rigid
tetrahedra and refined isotropically. The carbonyl carbon atom in each
of the two molecules, C(4) and C(4′), was also refined isotropically.
Crystal data for 9: C63H82OP4Pt, fw ) 1174.26, monoclinic, P21/c, a )
10.736(3) Å, b ) 19.13(2) Å, c ) 29.16(2) Å, â ) 99.44(3)°, V ) 5908(8)
Å3, Z ) 4, T ) 298(2) K, Dcalc ) 1.320 g/cm3, R(F) ) 4.07%, R(wF2) )
4.50% for 3605 independent observed reflections (4° e 2θ e 42°). The
carbon atoms of one of the tert-butyl groups on the supermesityl ligand
were equally disordered over two positions, so its carbon atoms were
refined isotropically. The structure of 5 was also determined, but only
serves to establish the connectivity: C63H82OP4Pd, triclinic, P1h, a )
10.394(4) Å, b ) 13.493(6) Å, c ) 25.17(2) Å, R ) 75.75(5)°, â ) 88.41-
(5)°, γ ) 78.80(3)°, V ) 3355(4) Å3, Z ) 2, T ) 293(2)K, Dcalc ) 1.074
g/cm3, R(F) ) 14.42%, R(wF2) ) 34.70%.
3), in which diphosphene 2 is formed from related
diphosphaureylene intermediates by CO extrusion from
the diphosphaureylene ring10 and PdP bond formation,
in a formal reductive elimination from Pd(II).11 Con-
sistent with this description of the reactivity, the more
electron-rich Pt and dialkylphosphino-ligated Pd com-
plexes 4 and 8-11 do not form 2. The increase in
reactivity with diphosphine bite angle from 5 to 6 and
7 may be rationalized by a destabilization of the
(10) For related reactions, see: (a) Becher, H. J .; Langer, E. Angew.
Chem., Int. Ed. Engl. 1973, 12, 842-843. (b) Appel, R.; Paulen, W.
Chem. Ber. 1983, 116, 2371-2373. (c) Appel, R.; Paulen, W. Chem.
Ber. 1983, 116, 109-113. (d) King, R. B.; Wu, F.-J .; Holt, E. M. J .
Organomet. Chem. 1990, 383, 295-305.
(11) Reductive elimination of diphenylphosphido groups from Pd-
(II) to afford Ph2PPPh2 has been proposed. Tunney, S. E.; Stille, J . K.
J . Org. Chem. 1987, 52, 748-753.
(9) Kemmitt, R. D. W.; Moore, M. R. Transition Met. Chem. 1993,
18, 348-352. Since the Pd-C distance in 6 (3.068(10) and 3.100(10)
Å) is nonbonding (and similar to that in 9 (3.055(6) Å)), the contribution
from such an allyl structure is likely to be minor.

4770 Organometallics, Vol. 16, No. 22, 1997
Communications
ketene 1; under these conditions, 1 is catalytically
converted to 2 and, once 1 is consumed, 12 decomposes.
Complex 12 also decomposed slowly in solution at -60
°C but could be isolated as a pure green solid by
recrystallization in the presence of excess 1 and stored
in the solid state at room temperature for days. Isolated
12 catalyzes formation of 2 from 1, but not as quickly
as Pd(PPh3)4 (∼1 turnover/h).6
In conclusion, we have observed the novel Pd-
catalyzed decarbonylative coupling of phosphaketene 1.
Related chemistry with chelating diphosphines suggests
that the catalysis proceeds via diphosphaureylene in-
termediates, whose structure and reactivity depend both
on the metal and on the ancillary ligands. Further
mechanistic studies on these and related compounds
with M-P bonds will be required to provide support for
this hypothesis and to compare metal-mediated reac-
tions which lead to formation of bonds to phosphorus15
to the well-studied processes that make bonds to
carbon.16
F igu r e 3. ORTEP diagram of 6, showing one of the two
independent molecules. Phenyl and Mes* groups have been
removed for clarity, to emphasize the pucker of the PdPCP
ring.
Sch em e 3
Ack n ow led gm en t. We thank Dartmouth College
and the Petroleum Research Fund, administered by the
American Chemical Society, for partial support and
J ohnson-Matthey/Alfa/Aesar for loans of Pd and Pt
salts.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and characterization data for complexes 4 and
6-10 and tables of the crystal data and structure refinement,
atomic coordinates, bond lengths and angles, anisotropic
displacement coefficients, and H-atom coordinates for 6 and
9 (29 pages). Ordering information is given on any current
masthead page.
Sch em e 4
OM970761J
(13) P d (d p p f)[Mes*P C(O)P Mes*] (12). Mes*PCO (48 mg, 0.16
mmol) was added to a red-purple solution of Pd(dba)2 (30 mg, 0.052
mmol) and dppf (36 mg, 0.065 mmol) in CH2Cl2 (5 mL). After the
solution turned green (a few minutes), the solvent was removed under
vacuum and the resulting green residue was washed with petroleum
ether (in which it is sparingly soluble), then dissolved in acetonitrile.
Addition of petroleum ether gave an immiscible mixture; green flakes
of the product, which formed at the interface at -20 °C, were collected,
washed with petroleum ether, and dried in vacuo to give 20 mg (31%
yield) of 12. An analytical sample (green crystals of a methylene
chloride solvate, as confirmed by 1H NMR) was obtained by further
recrystallization from CH2Cl2/petroleum ether in the presence of
Mes*PCO. 1H NMR (C6D6): δ 7.76 (br, 7H), 7.42 (4H), 7.0-6.86 (m,
13H), 4.15 (4H), 3.76 (4H), 1.69 (36H), 1.35 (18H). 31P{1H} NMR
(C6D6): δ 212.7 (t, J ) 15 Hz), 15.6 (t, J ) 15 Hz). IR(KBr): 2959,
1595, 1478, 1436, 1390, 1361, 1094, 740, 693, 489 cm-1. Anal. Calcd
for C71H86FeOP4Pd‚CH2Cl2: C, 65.18; H, 6.70. Found: C, 64.94; H,
6.48. FAB-MS (3-NBA): m/z 1305, 1241 (MH)+, 937 (M - Mes*PCO)+,
660 (M - Mes*PCOPMes*). HRMS calcd for C71H87FeOP4Pd (MH)+
1241.4092, found 1241.4075.
(14) Sillett, G.; Ricard, L.; Patois, C.; Mathey, F. J . Am. Chem. Soc.
1992, 114, 9453-9457. It is also possible that the spectrum is a special
case of the AA′XX′ system where some of the couplings are equivalent
(see Schlaf, M.; Lough, A. J .; Morris, R. H. Organometallics 1997, 16,
1253-1259) or that P-P bond formation has occurred and 12 is a
pseudotetrahedral Pd(0) complex of a three-membered ring ligand.
(15) For related studies, see: Wicht, D. K.; Kourkine, I. V.; Lew, B.
M.; Nthenge, J . M.; Glueck, D. S. J . Am. Chem. Soc. 1997, 119, 5039-
5040.
distorted square planar ground state by increasing
steric interactions between the PPh2 and PMes* groups.
We have not yet carried out kinetic studies or ob-
tained direct evidence for intramolecular P-P bond
formation, but this mechanistic hypothesis suggested
related studies of the dppf ligand, whose large bite angle
is known to promote reductive elimination in Pd(0)-
catalyzed coupling reactions.12 Bright green Pd(dppf)-
[Mes*PC(O)PMes*] (12), rapidly formed on addition of
2 equiv of 1 to a mixture of Pd(dba)2 and dppf (Scheme
4),13 displays an apparent A2X2 31P NMR spectrum
(C6D6: δ 212.7; 15.6 (J AX ) 15 Hz)), in contrast to the
AA′XX′ spectra seen for the rest of this series. This
observation is consistent with the pseudotetrahedral C2v
structure illustrated in Scheme 4, as in Mathey’s
analogous metalladiphospholene complexes.14 In solu-
tion, complex 12 decomposes to 2 and unidentified Pd
complexes in hours at room temperature but it is stable
when generated in the presence of excess phospha-
(16) (a) Gillie, A.; Stille, J . K. J . Am. Chem. Soc. 1980, 102, 4933-
4941. (b) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J . K. Bull.
Chem. Soc. J pn. 1981, 54, 1857-1867. (c) Low, J . J .; Goddard, W. A.,
III J . Am. Chem. Soc. 1986, 108, 6115-6128.
(12) (a) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi,
T.; Hirotsu, K. J . Am. Chem. Soc. 1984, 106, 158-163. (b) Brown, J .
M.; Guiry, P. J . Inorg. Chim. Acta 1994, 220, 249-259.