Thermal Coupling Reactions of 1-Phenyl-3,4-dimethylphosphole within the Coordination Sphere of Palladium(II)

Article abstract of DOI:10.1021/ic951307f

The thermolyses of dihalobis(1-phenyl-3,4-dimethylphosphole)palladium(II) complexes [(DMPP)₂PdX₂, X = Cl, Br, I] were investigated in 1,1,2,2-tetrachloroethane solutions at 145°C and in the crystalline state at 140°C. For cis-(DMPP)<

Full text of DOI:10.1021/ic951307f

1486
Inorg. Chem. 1996, 35, 1486-1496
Thermal Coupling Reactions of 1-Phenyl-3,4-dimethylphosphole within the Coordination
Sphere of Palladium(II)
William L. Wilson,† Jean Fischer,‡ Roderick E. Wasylishen,§ Klaus Eichele,§
Vincent J. Catalano,† John H. Frederick,† and John H. Nelson*,†
Department of Chemistry, University of Nevada, Reno, Nevada 89557, Department of Chemistry,
Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3, and Laboratoire de Cristallochimie et de
Chimie Structurale (URA 424-CNRS), Universite´ Louis Pasteur, 67070 Strasbourg Cedex, France
ReceiVed October 12, 1995X
The thermolyses of dihalobis(1-phenyl-3,4-dimethylphosphole)palladium(II) complexes [(DMPP)2PdX2, X ) Cl,
Br, I] were investigated in 1,1,2,2-tetrachloroethane solutions at 145 °C and in the crystalline state at 140 °C.
For cis-(DMPP)2 PdCl2 and cis- or trans-(DMPP)2 PdBr2 four types of products were formed: (1) [4 + 2]
cycloaddition products, (2) [2 + 2] cycloaddition products, (3) compounds that result from 1,5-hydrogen migration
from a methyl group on one phosphole to the â-carbon of an adjacent phosphole (exo-methylene), and (4) products
that result from an intermolecular [4 + 2] coupling of two phospholes followed sequentially by phosphinidene
elimination and intramolecular [4 + 2] cycloaddition to another phosphole to give diphosphatetracyclotetradec-
atrienes (DPTCT). trans-(DMPP)2PdBr2 undergoes thermal isomerization to cis-(DMPP)2PdBr2 in the solid state,
and cis- and trans-(DMPP)2PdBr2 give the same products in both their solid- and solution-state thermolyses. In
contrast, trans-(DMPP)2 PdI2 neither isomerizes to the cis-isomer nor undergoes any of the phosphole coupling
reactions in either the solution or solid state. The crystal structures of trans-(DMPP)2PdX2 (X ) Br, I), {-
(DMPP)2[2 + 2]}PdBr2, {(DMPP)2(exo-methylene)}PdBr2, and (DPTCT)PdCl2 were determined. They crystallize
in the monoclinic P21/c, triclinic P1h, monoclinic P21/c, monoclinic P21/n, and orthorhombic P212121 space groups
in units cells of the following dimensions: a ) 10.158 (3) Å, b ) 14.876 (4) Å, c ) 16.829 (5) Å, â ) 104.25-
(2)°, Fcalc ) 1.732 g/cm3, Z ) 4; a ) 9.025(1) Å, b ) 11.023(1) Å, c ) 13.833 (1) Å, R ) 101.15(1)°, â )
98.82(1)°, γ ) 105.30(1)°, Fcalc ) 1.886 g/cm3, Z ) 2; a ) 13.090 (2) Å, b ) 17.637 (2) Å, c ) 21.834 (2) Å,
â ) 100.51 (1)°, Fcalc ) 1.738 g/cm3, Z ) 4, a ) 10.721 (1) Å, b ) 16.929 (1) Å, c ) 14.675(1) Å, â ) 97.86
(1)°, Fcalc ) 1.663 g/cm3, Z ) 4; and a ) 15.532 (3) Å, b ) 19.401 (4) Å, c ) 9.910 (2) Å, Fcalc ) 1.490 g/cm3,
Z ) 2, respectively. Least-squares refinements converged at final values of R(F) of 0.041, 0.0354, 0.0624, 0.0533,
and 0.035 for 2770, 2672, 2729, 2159, and 2525 independent observed reflections, respectively. Kinetic studies
suggest that the reaction mechanisms are the same in both the solid and solution states and that the reaction
mechanisms are substantially different from those previously reported for the thermolyses of the analogous cis-
(DMPP)2PtX2 complexes.
Introduction
Thermal1,2 and photochemical3,4 dimerization of 1-substituted-
3,4-dimethylphospholes occurs within the coordination spheres
of various transition metals. In each case where a [4 + 2]
cycloaddition product results, only the exo-diastereomer is
formed and these cycloaddition reactions are intramolecular. In
contrast, intermolecular cycloadditions of dienophiles to free
phospholes5 (reaction 1) or [4 + 2] cyclodimerizations of
produce only the endo-diastereomers.6-11 The latter are ther-
mally unstable12,13 and are converted to dihydrophosphindole
oxides and phosphindole sulfides, respectively, upon thermoly-
sis.
The differences in the products of thermolysis of
1
2
(DMPP)2NiX2 and (DMPP)2PtX2 are striking, particularly
since both reactions involve four coordinate complexes of metals
from the same family. Within a given family, second and third
row transition metals generally exhibit similar chemical reactions
which are usually distinct from those of their first row congeners.
The differences in the chemical behavior of the second and third
row transition elements are mostly due to differences in their
reaction rates which are usually 3-4 orders of magnitude faster
phosphole oxides and phosphole sulfides (reaction 2) normally
* Author to whom correspondence should be addressed.
† University of Nevada.
‡ Universite Louis Pasteur.
§ Dalhousie University.
X Abstract published in AdVance ACS Abstracts, February 15, 1996.
(1) Mercier, F.; Mathey, F.; Fischer, J.; Nelson, J. H. J. Am. Chem. Soc.
1984, 106, 425; Inorg. Chem. 1985, 24, 4141.
(2) Wilson, W. L.; Rahn, J. A.; Alcock, N. W.; Fischer, J.; Frederick, J.
H.; Nelson, J. H. Inorg. Chem. 1994, 33, 109.
(3) Santini, C. C.; Fischer, J.; Mathey, F.; Mitschler, A. J. Am. Chem.
Soc. 1980, 102, 5809.
(4) Ji, H.-L.; Nelson, J. H.; Fischer, J. Phosphorus, Sulfur Silicon Relat.
Elem. 1993, 77, 268.
(5) Mathey, F.; Mercier, F. Tetrahedron Lett. 1981, 22, 319.
0020-1669/96/1335-1486$12.00/0 © 1996 American Chemical Society

Thermal Coupling of 1-Phenyl-3,4-dimethylphosphole
Inorganic Chemistry, Vol. 35, No. 6, 1996 1487
of the ligand in the recrystallization process or trace quantities of excess
phosphole. Anal. Calcd for C24H26Cl2P2Pd: C, 52.08; H, 4.70; Cl,
12.81. Found: C, 51.89; H, 4.65; Cl, 12.68.
for the second row element. Marked differences in the reactions
of analogous platinum and palladium complexes are not
common.14 Therefore, the thermolyses of dihalobis(1-phenyl-
3,4-dimethylphosphole)palladium(II) complexes might be ex-
pected to mimic those of their platinum(II) counterparts.2 As
we show in this paper, there are differences in the intramolecular
reactions and the palladium complexes exhibit a novel inter-
molecular reaction as well.15
trans-Dibromobis(1-phenyl-3,4-dimethylphosphole)palladium-
(II), trans-(DMPP)2PdBr2. The preparation of this complex is similar
to that of cis-(DMPP)2PdCl2. To a vigorously stirred emulsion of 5.00
g (28.2 mmol) of PdCl2 dissolved in 150 mL of saturated aqueous
potassium bromide solution and 150 mL of dichloromethane was added,
under nitrogen via syringe, 10.5 mL (55.8 mmol) of 1-phenyl-3,4-
dimethylphosphole. Isolation, as above, afforded 15.03 g (83.9%) of
bright orange crystals of trans-dibromobis(1-phenyl-3,4-dimethylphos-
Experimental Section
phole)palladium(II), mp 234-236 °C. IR (Nujol cm-1) υPdP, 375, υPdBr
,
A. Reagents and Physical Measurements. All chemicals were
reagent grade and were used as received or synthesized as described
below. Solvents were dried by standard procedures and stored over
Linde type 4 Å molecular sieves. All syntheses were conducted under
a dry nitrogen atmosphere. The 1-phenyl-3, 4-dimethylphosphole was
prepared by the literature method.16 Elemental analyses were performed
by Galbraith Laboratories, Knoxville, TN. Infrared spectra were
recorded on a Perkin-Elmer 1800 FT-IR instrument as Nujol mulls on
polyethylene thin films. Melting points were obtained using a Mel-
Temp melting point apparatus and are uncorrected. All 31P MAS NMR
spectra were recorded on a Bruker MSL-200 spectrometer at 80.0 MHz.
Cross-polarization from protons to 31P under the Hartman-Hahn match
200 cm-1. Anal. Calcd for C24H26Br2P2Pd: C, 44.87; H, 4.05; Br,
24.88. Found: C, 44.58; H, 4.00; Br, 24.76.
cis-Dibromobis(1-phenyl-3,4-dimethylphosphole)palladium(II), cis-
(DMPP)2 PdBr2. A dilute solution of trans-dibromobis(1-phenyl-3,4-
dimethylphosphole)palladium(II) in an acetone-water mixture was
prepared by dissolving the complex in acetone and then slowly adding
water until the mixture became cloudy. Then acetone was added to
give a clear solution. The flask was covered with a tissue and set in
a place far from the trans complex. As the acetone slowly evaporated,
small bright yellow needles of cis-dibromobis(1-phenyl-3,4-dimeth-
ylphosphole)palladium(II) crystallized from the solution. These were
collected by gravity filtration and allowed to air-dry, mp 229-231 °C
(dec.). IR (Nujol cm-1) υPdP, 395, 376; υPdBr 203, 180. Anal. Calcd
for C24H26Br2P2Pd: C, 44.87; H, 4.05; Br, 24.88. Found: C, 44.69;
H, 4.02; Br, 24.72.
1
condition and high-power decoupling were employed with typical H
90° pulses of 4.0-10.0 µs and contact times of 3-5 ms. Spectra were
referenced with respect to 85% H3PO4(aq) by setting the 31P resonance
of solid NH4H2PO4 to 0.81 ppm. Solution 1H, 1H{31P}, 13C{1H}, and
31P{1H} NMR spectra were recorded at 500, 500, 125, and 202.4 MHz,
respectively, on a Varian unity plus 500 MHz NMR spectrometer.
Proton and carbon chemical shifts are relative to internal Me4Si, while
phosphorus chemical shifts are relative to external 85% H3PO4(aq) with
positive values being downfield of the respective reference.
trans-Diiodobis(1-phenyl-3,4-dimethyphosphole)palladium(II),
trans-(DMPP)2PdI2. The preparation of this compound is similar to
that of cis-(DMPP)2PdCl2. To a vigorously stirred emulsion of 1.00 g
(5.64 mmol) PdCl2 dissovled in 30 mL of saturated aqueous sodium
iodide solution and 30 mL of dichloromethane was added, under
nitrogen via syringe, 2.1 mL (11.2 mmol) of 1-phenyl-3,4-dimeth-
ylphosphole. Isolation as for cis-(DMPP)2PdCl2 afforded 3.76 g
(91.5%) of deep red crystals of trans-diiodobis(1-phenyl-3,4-dimeth-
ylphosphole)palladium(II), mp 234-237 °C (dec.). 31P{1H} NMR
(CDCl3): δ 9.51. 1H NMR (CDCl3): δ 2.11(s, 12H, CH3), 7.06 (T,
2J(PH) + 4J(PH) ) 29.86 Hz, 4H, HR), 7.23-7.8 (m, 10H, Ph). 13C-
B. Syntheses. cis-Dichlorobis(1-phenyl-3,4-dimethylphosphole)-
palladium(II), cis-(DMPP)2PdCl2. To a vigorously stirred emulsion
of 5.00 g (28.2 mmol) of PdCl2 dissolved in 150 mL of 1 M aqueous
sodium chloride and 150 mL of dichloromethane was added, under
nitrogen via syringe, 10.5 mL (55.8 mmol) of 1-phenyl-3,4-dimeth-
ylphosphole. The aqueous phase became paler, and the organic phase
became yellow. The reaction mixture was stirred overnight, whereupon
the aqueous phase became pale pink and the organic phase deep yellow.
The organic phase was removed by separatory funnel, dried with
anhydrous magnesium sulfate, and gravity filtered into a 500 mL round-
bottom flask. The solvent was removed by rotary evaporation using a
water bath. The temperature of the water bath was gradually raised
from room temperature to boiling over a period of about 1 h. The
removal of the dichloromethane in this manner resulted in a foamy
glass. The flask was removed hot from the rotary evaporator, and just
enough benzene was added to the hot flask to dissolve the foamy glass.
The flask was allowed to cool undisturbed, whereupon 1-5 mm crystals
of the title compound slowly grew from the solution. These were
isolated by gravity filtration, rinsed with diethyl ether, and allowed to
air-dry (yield 12.85 g, 83.2%). Another crop of crystals could be
obtained by removing the solvent from the mother liquor, redissolving
in dichloromethane, and repeating the process (overall yield: 14.78 g,
95.7%). The compound isolated in this manner is identical to that
prepared previously17 but with no impurities derived from dimerization
5
{1H} NMR (CDCl3): δ 17.6 (T, 3J(PC) + J(PC) ) 12.7 Hz, CH3),
127.8 (T, 1J(PC) + 3J(PC) ) 48.0 Hz, Ci), 128.4 (T, 3J(PC) + 5J(PC)
) 10.7 Hz, Cm), 130.2 (T, 1J(PC) + 3J(PC) ) 53.7 Hz, CR), 130.5 (s,
Cp), 133.4 (T, 2J(PC) + 4J(PC) ) 13.7 Hz, Co), 152.5 (T, 2J(PC) +
4J(PC) ) 14.6 Hz, Câ). Anal. Calcd for C24H26I2P2Pd: C, 39.14; H,
3.53. Found: C, 39.26; H, 3.45.
Solution Preparation of (DPTCT)PdCl2. cis-Dichlorobis(1-phenyl-
3, 4-dimethylphosphole)palladium(II), 1.00 g, was dissolved in 125 mL
of 1,1,2,2-tetrachloroethane in a 250 mL round-bottom flask fitted with
a reflux condenser and an N2 outlet. The solution was heated at reflux
for 30 h. A 31P{1H} NMR spectrum of the reaction mixture showed
that the relative amounts of (DPTCT)PdCl2, the [2 + 2], and
exo-methylene products were 2.6:1:1.2. The reaction mixture was
filtered, and the solution was allowed to sit uncovered in air at ambient
temperature until all of the tetrachloroethane had evaporated, leaving
bright yellow crystals in a red tar. The tar was dissolved in
dichloroethane, leaving 0.20 g of (DPTCT)PdCl2
(6) Mathey, F.; Mankowski-Favelier, R. Org. Magn. Reson. 1972, 4, 171.
(7) Mathey, F., Lampin, J. P.; Thavard, D. Can. J. Chem. 1976, 54, 2402.
(8) Markl, G.; Potthast, R. Tetrahedron Lett. 1968, 1755.
(9) Mathey, F. Tetrahedron 1972, 28, 4171.
(10) Usher, D. A.; Westheimer, F. H. J. Am. Chem. Soc. 1964, 86, 4732.
(11) Quin, L. D.; Szewczyk, J. J. Chem. Soc., Chem. Commun. 1986, 844.
(12) Hughes, A. N.; Phisithkul, S.; Rukachaisirikul, T. J. Heterocycl. Chem.
1979, 16, 1417.
(13) Holah, D. G.; Hughes, A. N.; Kleemola, D. J. Heterocycl. Chem. 1977,
14, 705.
(14) Hartley, F. R. The Chemistry of Platinum and Palladium; Wiley: New
York, 1973.
(15) An abstract of some of this work has been published; Wilson, W. L.;
Nelson, J. H.; Rahn, J. A.; Alcock, N. W.; Fischer, J. Phosphorus,
Sulfur Silicon Relat. Elem. 1993, 77, 29.
(16) Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983.
(17) MacDougall, J. J.; Nelson, J. H.; Mathey, F.; Mayerle, J. J. Inorg.
Chem. 1980, 19, 709.
as bright yellow crystals which were recrystallized from methanol/water
to yield 0.18 g, mp 263-266 °C (dec.). 31P{1H} NMR (CDCl3): δ
34.84 (d, 2J(PP) ) 28.10 Hz, Pa), 97.60 (d, 2J(PP) ) 28.10 Hz, Pb). 1H
4
NMR (CDCl3): δ 1.27 (d, J(PbH) ) 1.20 Hz, 3H, CH3(17)), 1.35
4
4
(apparent t, J(PbH) ) J(H5CH3) ) 1.20 Hz, 3H, CH3(18)), 1.42 (d,
4J(PaH) ) 1.20 Hz, 3H, CH3(14)), 1.59 (s, 3H, CH3(16)), 2.07 (dd,

1488 Inorganic Chemistry, Vol. 35, No. 6, 1996
Wilson et al.
1
1
4J(PaH) ) 2.10 Hz, 4J(H1H) ) 1.20 Hz, 3H, CH3(13)), 2.28 (d, 4J(H2-
CH3) ) 1.50 Hz, 3H, CH3(15)), 2.33 (apparent dt, 3J(PbH) ) 1.50 Hz,
4J(H5H6) ) 3J(H4H5) ) 0.90 Hz, 1H, H5), 2.55 (dddd, 3J(PaH) ) 24.94
Hz, 3J(PbH) ) 21.94 Hz, 3J(H3H4) ) 8.41 Hz, 3J(H4H5) ) 0.9 Hz, 1H,
H4), 3.11 (ddd, 2J(PaH3) ) 13.22 Hz, 3J(H3H4) ) 8.41 Hz, 4J(PbH3) )
C12), 117.73 (d, J(PbC) ) 58.41 Hz, C4), 124.35 (d, J(PaC) ) 46.86
3 3
Hz, C10), 128.91 (d, J(PC) ) 11.79 Hz, Cm), 129.18 (d, J(PC) )
1
1
11.71 Hz, Cm), 130.06 (d, J(PC) ) 47.61 Hz, Ci), 131.12 (d, J(PC)
) 48.75 Hz, Ci), 132.11 (d, 4J(PC) ) 2.87 Hz, Cp), 132.55 (d, 4J(PC)
) 2.87 Hz, Cp), 132.57 (d, 2J(PC) ) 12.70 Hz, Co), 133.22 (d, 2J(PC)
) 12.62 Hz, Co), 146.96 (dd, 2J(PaC) ) 14.67 Hz, 3J(PbC) ) 7.03 Hz,
3
4
3.0 Hz, 1H, H3), 3.66 (dd, J(PbH6) ) 1.50 Hz, J(H5H6) ) 0.90 Hz,
2
4
1H, H6), 5.39 (q, 4J(H2CH3(15)) ) 1.50 Hz, 1H, H2), 6.19 (dq, 2J(PaH1)
C8), 157.64 (dd, J(PbC) ) 5.74 Hz, J(PaC) ) 0.68 Hz, C3), 163.81
4
2
4
) 27.05 Hz, J(H1CH3(13)) ) 1.20 Hz, 1H, H1), 7.2-8.1 (m, 10H,
(dd, J(PaC) ) 14.10 Hz, J(PbC) ) 2.50 Hz, C9). Anal. Calcd for
C24H26Br2P2Pd‚H2O: C, 43.65; H, 4.24; Br, 24.20. Found: C, 43.72;
H, 3.94; Br, 24.13.
Ph). CH3OH and H2O were detected in the 1H NMR spectrum in
approximately the correct ratios. 13C{1H} NMR (CDCl3): δ 14.55 (s,
CH3(18)), 16,66 (d, 3J(PbC) ) 3.02 Hz, CH3(16)), 17.49 (d, 3J(PaC) )
13.32 Hz, CH3(13)), 20.75 (s, CH3(15)), 22.09 (d, 3J(PaC) ) 6.66 Hz,
CH3(14)), 30.50 (d, 3J(PbC) ) 8.55 Hz, CH3(17)), 42.88 (d, 2J(PbC) )
Solid-State Preparation of (DPTCT)PdBr2, {(DMPP)2[2 + 2]}-
PdBr2, and [(DMPP)]2(exo-methylene)]PdBr2. cis-(DMPP)2PdBr2 or
trans-(DMPP)2PdBr2 (1.00 g) was sealed in a break-seal ampule under
nitrogen and heated in an oven at 140 °C for 30 h. The solid was
dissolved in CDCl3 and the solution was filtered. The relative amounts
of (DPTCT)PdBr2, the [2 + 2], and exo-methylene products were
determined to be 1:2:2 by 31P{1H} NMR spectroscopy. Slow and
careful fractional crystallization from CHCl3/ETOH afforded 64 mg
of (DPTCT)PdBr2 as the least soluble product, mp 308-312 °C (dec.).
31P{1H} NMR (CDCl3): δ 31.52 (d, 2J(PP) ) 23.42 Hz, Pa), 94.65 (d,
2J(PP) ) 23.42 Hz, Pb). 1H NMR (CDCl3): δ 1.27 (d, 4J(PbH) ) 1.00
2
1
18.86 Hz, C8), 47.07 (d, J(PbC) ) 20.49 Hz, C7), 49.01 (dd, J(PbC)
) 33.31 Hz, 3J(PaC) ) 21.37 Hz, C10), 50.38 (d, 2J(PaC) ) 11.19 Hz,
1
3
C3), 54.37 (dd, J(PaC) ) 37.31 Hz, J(PbC) ) 3.48 Hz, C4), 57.09
1
3
1
(dd, J(PbC) ) 33.27 Hz, J(PaC) ) 3.86 Hz, C9), 117.28 (d, J(PaC)
1
) 59.71 Hz, C1), 124.16 (s, C5), 127.97 (d, J(PC) ) 54.81 Hz, Ci),
3
3
128.15 (d, J(PC) ) 10.69 Hz, Cm), 129.28 (d, J(PC) ) 11.69 Hz,
3
Cm), 129.74 (d, J(PC) ) 48.27 Hz, Ci), 130.70 (s, Cp), 132.07 (d,
2J(PC) ) 8.05 Hz, Co), 132.72 (s, Cp), 134.62 (d, 2J(PC) ) 13.07 Hz,
Co), 135.58 (d, 2J(PbC) ) 23.13 Hz, C11 or C12), 141.56 (s, C6), 163.94
(d, 2J(PbC) ) 8.17 Hz, C12 or C11). Anal. Calcd for C30H34Cl2P2Pd‚0.5
CH3OH‚H2O: C, 54.87; H, 5.69; Cl, 10.62. Found: C, 54.67; H, 5.42;
Cl, 10.39.
4
4
Hz, 3H, CH3(17)), 1.31 (apparent, t J(PbH) ) J(H5CH3) ) 1.20 Hz,
3H, CH3(18)), 1.42 (d, 4J(PaH) ) 1.00 Hz, 3H, CH3(14)), 1.62 (s, 3H,
4
4
CH3(16)), 2.07 (dd, J(PaH) ) 2.25 Hz, J(H1CH3) ) 1.25 Hz, 3H,
CH3(13)), 2.28 (d, 4J(H2CH3) ) 1.50 Hz, 3H, CH3(15)), 2.30 (apparent
dt, 3J(PbH) ) 1.50 Hz, 4J(H5H6) ) 3J(H4H5) ) 0.90 Hz, 1H, H5), 2.57
(dddd, 3J(PaH) ) 24.49 Hz, 3J(PbH) ) 21.87 Hz, 3J(H3H4) ) 8.13 Hz,
3J(H4H5) ) 0.90 Hz, 1H, H4), 3.14 (ddd, 2J(PaH3) ) 13.00 Hz, 3J(H3H4)
Solid-State Preparation of (DPTCT)PdCl2, Thermolysis of Dichlo-
robis(1-phenyl-3,4-dimethylphosphole)palladium(II).
cis-
(DMPP)2PdCl2 (1.00 g) was sealed in a break-seal ampule under
nitrogen and heated in an oven at 140 °C for 30 h. The solid was
dissolved in 1,1,2,2-tetrachloroethane, and the solution was filtered.
The relative amounts of (DPTCT)PdCl2, the [2 + 2], and exo-methylene
products were determined by 31P{1H} NMR spectroscopy to be 2.4:1:
1.4. Isolation as above afforded 0.16 g of the title complex as bright
yellow crystals.
4
2
) 8.13 Hz, J(PbH3) ) 2.50 Hz, 1H, H3), 3.68 (dd, J(PbH6) ) 2.00
4
4
Hz, J(H5H6) ) 0.90 Hz, 1H, H6), 5.36 (q, J(H2CH3(15)) ) 1.50 Hz,
1H, H2), 6.27 (dq, 2J(PaH1) ) 26.99 Hz, 4J(H1CH3(13) ) 1.25 Hz, 1H,
H1), 7.2-8.1 (m, 10H, Ph). Limited solubility precluded obtention of
a 13C{1H} NMR spectrum. Anal. Calcd for C30H34Br2P2Pd: C, 49.87;
H, 4.70; Br, 22.12. Found: C, 49.62; H, 4.28; Br, 21.91. From the
next fraction was obtained 218 mg of {(DMPP)2[2 + 2]}PdBr2‚0.75
H2O,
Solution Preparation of [(DMPP)2(exo-methylene)]PdBr2.
mp 276-280 °C (dec.). 31P{1H} NMR (CDCl3): δ 108.76 (s). 1H
NMR (CDCl3): δ 1.53 (s, 6H, CH3(6, 11)), 2.01 (T, 4J(PH) + 6J(PH)
) 2.15 Hz, 6H, CH3(5,12)), 2.92 (AA′XX′, 3J(HH) ) 14.00 Hz, 2J(PH)
) 22.29 Hz, 3J(PH) ) 1.30 Hz, 2J(PP) ) 26.87 Hz, 2H, HC4C7), 6.32
(AA′XX′, 2J(PH) ) 28.00 Hz, 4J(PH) ) 1.00 Hz, 2J(PP) ) 26.87 Hz,
2H, HC1C10), 7.4-7.8 (m, 10H, Ph). H2O was detected in the 1H NMR
spectrum in approximately the correct ratio. 13C{1H} NMR (CDCl3):
δ 18.12 (T, 3J(PC) + 4J(PC) ) 14.08 Hz, C6,11), 22.74 (T, 3J(PC) +
trans-(DMPP)2PdBr2 (1.00 g) was dissolved in 125 mL of 1,1,2,2-
tetrachloroethane in a 250 mL round-bottom flask fitted with a reflux
condenser and an N2 outlet. The solution was brought to reflux and
heated at reflux for 18 h. A 31P{1H} NMR spectrum of the reaction
mixture showed that the relative amounts of (DPTCT)PdBr2, the [2 +
2] and exo-methylene products were 1:1:2. The reaction mixutre was
filtered, and the solution was allowed to sit at ambient temperature
until the tetrachloroethane had evaporated, leaving orange-brown
crystals in a brown tar. The tar was dissolved in 1,2-dichloroethane,
leaving 0.39 g of crystalline exo-methylene compound as orange-brown
crystals, mp 278-280 °C (dec.). 31P{1H} NMR (CDCl3): δ 108.47 (d,
3
5J(PC) ) 6.29 Hz, C5,12), 53.43 (T, 1J(PC) + J(PC) ) 46.39 Hz,
3
C4,7), 67.93 (T, 2J(PC) + J(PC) ) 9.05 Hz, C3,8), 122.79 (AXX′,
1J(PC) ) 51.41 Hz, 3J(PC) ) 0.25 Hz, 2J(PP) ) 26.87 Hz, C1,10), 128.70
5
(T, 3J(PC) + J(PC) ) 11.69 Hz, Cm), 131.91 (s, Cp), 132.81 (T,
2
2J(PP) ) 16.97 Hz, Pa), 94.62 (d, J(PP) ) 16.97 Hz, Pb). 1H NMR
4
1
2J(PC) + J(PC) ) 12.19 Hz, Co), 132.91 (apparent dd, J(PC) )
(CDCl3): δ 1.29 (d, 3J(H4CH3) ) 6.91 Hz, 3H, CH3(b)), 1.94 (apparent
dt, 4J(PbCH3) ) 2.4 Hz, 4J(H5CH3) ) 4J(H4H3) ) 1.2 Hz, 3H, CH3(a)),
2.32 (apparent t, 4J(PaCH3) ) 4J(H1CH3) ) 1.5 Hz, 3H, CH3(c)), 2.34
3
4
80.01 Hz, J(PC) ) 12.6 Hz, Ci), 160.18 (T, 2J(PC) + J((PC) )
9.55 Hz, C2,8). Anal. Calcd for C24H26Br2P2Pd‚0.75 H2O: C, 43.95;
H, 4.19; Br, 24.39. Found: C, 43.62; H, 3.89; Br, 24.58. Finally,
189 mg of [(DMPP)2(exo-methylene)]2PdBr2 was obtained as the most
soluble product.
3
3
4
(dqq, J(H3H4) ) 7.81 Hz, J(H4, CH3(b)) ) 6.91 Hz, J(H4, CH3(a))
) 1.2 Hz, 1H, H4) 2.87 (dddd, 3J(PaH3) ) 43.28 Hz, 2J(PbH3) ) 10.82
Hz, 3J(H2H3) ) 9.95 Hz, 3J(H3H4) ) 7.81 Hz, 1H, H3), 3.47 (dd,
2
C. Kinetic Studies. Solutions of cis-(DMPP)2PdCl2 (0.0036,
0.0072, and 0.0144M) in 1,1,2,2-tetrachloroethane solvent were heated
at gentle reflux in a two-neck round-bottom flask fitted with a rubber
septum, a reflux condenser, and an N2 inlet and outlet. Aliquots were
removed periodically, and their contents were determined by 31P{1H}
NMR spectroscopy at 121.65 MHz on a G.E. GN-300 NMR
spectrometer, with 60° pulses and duty cycles of 5 s which is
approximately 5 times the longest T1.
2J(H2H3) ) 9.95 Hz, J(PaH2) ) 5.11 Hz, 1H, H2), 5.38 (s, 1H, H6 or
2
4
H7) , 5.76 (s, 1H, H7 or H6), 6.33 (dq, J(PbH5) ) 27.95 Hz, J(H5-
CH3(a)) ) 1.2 Hz, 1H, H5), 6.76 (dq, 2J(PaH1) ) 28.55 Hz, 4J(H1, CH3-
(c)) ) 1.5 Hz, 1H, H1), 7.35-7.92 (m, 10H, Ph). H2O was detected
in the 1H NMR spectrum in approximately the correct ratio. 13C{1H}
3
NMR (CDCl3): δ 16.92 (d, J(PaC) ) 13.00 Hz, CH3(c)), 19.63 (d,
3J(PbC) ) 15.95 Hz, CH3(a)), 22.50 (dd, J(PbC) ) 5.78 Hz, J(PaC)
3
4
) 2.16 Hz, CH3(b)), 46.15 (d, 2J(PbC) ) 9.75 Hz, C2), 50.00 (dd,
1J(PaC) ) 37.30 Hz, J(PbC) ) 20.82 Hz, C7), 57.68 (dd, J(PbC) )
D. Numerical Simulation of Reaction Kinetics. The kinetics of
the reaction were simulated by numerically integrating the differential
2
1
2
3
34.01 Hz, J(PaC) ) 18.37 Hz, C1), 117.65 (d, J(PaC) ) 7.86 Hz,

Thermal Coupling of 1-Phenyl-3,4-dimethylphosphole
Inorganic Chemistry, Vol. 35, No. 6, 1996 1489
Table 1. Abridged Summary of Data Collection Parameters for (DPTCT)PdCl2 (1), trans-(DMPP)2PdBr2 (2), trans-(DMPP)2PdI2 (3),
[(DMPP)2(exo-methylene)]PdBr2 (4), and {(DMPP)2[2 + 2]}PdBr2 (5)a
1‚0.5CH3OH‚H2O
2
3
4‚H2O
5‚0.75H2O
formula
fw
C30H34Cl2P2Pd
667.58
C24H26Br2P2Pd
642.6
C24H26I2P2Pd
736.6
C24H26Br2P2Pd
642.6
C24H26Br2P2Pd
642.6
color and habit
space group
crystal size, mm
a, Å
b, Å
c, Å
yellow plates
P212121
0.26 0.32 0.38
15.532(3)
19.401(4)
9.910(2)
orange cubes
P21/c
0.18 0.18 0.35
10.158(3)
14.876(4)
16.829(5)
red-orange plates
P1h
yellow prisms
P21/n
0.2 0.2 0.4
10.72(1)
16.929(1)
14.675(1)
yellow plates
P21/c
0.02 0.4x0.1
13.090(2)
17.637(2)
21.834(2)
0.2 0.1 0.4
9.205(1)
11.023(1)
13.833(1)
101.15(1)
98.82(1)
105.30(1)
1296.9(2)
2
R, deg
â, deg
104.25(2)
97.86(1)
100.51(1)
γ, deg
V, Å3
2986.2
2
1.490
2464.7
4
1.732
2638.2(3)
4
1.663
4956.5(10)
4
1.738
Z
d(calc), g/cm3
linear abs coeff, cm-1
2θ range, deg
no. of obs reflcns
no. of variables
1.886
32.28
9.229
2-25
40.924
2-24
38.67
2-45
41.13
3.5-45
3.5-45
2729 (F > 5σ(F))
271
2525 (I > 3σ(I))
2770 (I > 3σ(I))
2672 (F > 4σ(F))
2159 (F > 4σ(F))
333
261
263
272
b
R,Rw
0.035, 0.057
0.041, 0.053
0.035, 0.045
0.053, 0.064
0.062, 0.075
a Data were collected at 298 K using an Enraf-Nonius CAD4-F diffractometer, Mo KR (λ ) 0.709 30 Å) for 1, a syntex P3 diffractometer, Mo
KR (λ ) 0.710 7 Å for 2; and a Siemens P4, Mo KR (λ ) 0.710 73 Å) for 3-5. b For 1 and 2 R(F) ) ∑( Fo - Fc /∑Fo ), Rw(F) ) [∑w( Fo
-
Fc 2/∑w Fo 1/2, w ) 1/σ2(F2) ) σ2 (counts + (pI)2; for 3-5 Rw(F) ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2
] .
2
2
2
rate laws (Vide infra). The integration was carried out using a fourth
were introduced in the structure calculations by their computed
coordinates (CH ) 0.95 Å) with isotropic temperature factors such as
B(H) ) 1.3Beqv(C) Å3 but were not refined. For (DPTCT)PdCl2 the
absolute structure was determined by comparing +x, +y, +z and -x,
-y, -z refinements (Flack absolute structure parameter X ) 0.37(7)).20
Solutions were obtained with full least-squares refinements with
weighting schemes given in Table 1. Final difference maps revealed
no significant maxima. The scattering factor coefficients and anomalous
dispersion coefficients come respectively from parts a and b of ref 21.
The trans-(DMPP)2PdI2, [(DMPP)2(exo-methylene)]PdBr2, and {-
(DMPP)2[2 + 2]}PdBr2 crystals were coated with epoxy, mounted on
glass fibers and placed on a Siemens P4 diffractometer. Two check
reflections monitored every 100 reflections, showed random (<2%)
variation during the data collection. Unit cell parameters were
determined by least-squares refinement of 24 reflections. The data were
corrected for Lorentz, polarization, and absorption effects (using an
empirical model derived from azimuthal data collections). Scattering
factors and corrections for anomalous dispersion were taken from a
standard source.22 Calculations were performed with the Siemens
SHELXTL Plus version 4.0 software package on a personal computer.
The structures were solved by direct methods. Anisotropic thermal
parameters were assigned to all non-hydrogen atoms. Hydrogen atoms
were refined at calculated positions with a riding model in which the
C-H vector was fixed at 0.96 Å. For {(DMPP)2[2 + 2]}PdBr2
anisotropic thermal parameters were assigned to palladium, phosphorus,
and bromine atoms. All other atoms were refined with isotropic thermal
parameters. Hydrogen atoms were added to appropriate atoms exclud-
ing the solvate and were refined at calculated positions with a riding
model as above. One phenyl ring (C37 to C42) was modeled as a
rigid group. There was disordered solvent equivalent to three-fourths
of a water molecule occupying two sites. Selected bond lengths and
angles are given in Tables 2 and 4-6.
order Gear integrator with a constant time step of 0.02 h for 2250 time
steps (about 45 h). This procedure conserved the material balance to
better than 1 part in 108. The routine was adjusted to interpolate
between integration points to obtain calculated mole fractions of each
of the reaction species for the same time points at which the
experimental data were collected. For a given trial set of rate constants,
the summed square deviation (variance) of the computed data relative
to the experimental data was accumulated, and then the rate constants
were iteratively adjusted until the variance had been minimized. All
experimental data were given equal weighing in this fitting procedure.
E. X-ray Data Collection and Processing. Bright yellow crystals
of (DPTCT)PdCl2 were obtained from CH3OH/H2O, orange crystals
of trans-(DMPP)2PdBr2 were obtained from benzene, dark red-orange
crystals of trans-(DMPP)2PdI2 were obtained from chloroform, and
yellow crystals of [(DMPP)2(exo-methylene)]PdBr2 and {(DMPP)2[2
+ 2]}PdBr2 were obtained from chloroform/ethanol. Crystal data and
details of the data collection are given in Table 1. In all cases, single
crystals were cut from clusters of crystals. Systematic searches in
reciprocal space with an Enraf-Nonius CAD4-F diffractometer showed
that the crystals of (DPTCT)PdCl2 belong to the orthorhombic system
and for trans-(DMPP)2PdBr2 data collected on a Syntex P3 diffracto-
meter showed that the crystals belonged to the monoclinic system, while
for trans-(DMPP)2PdI2, [(DMPP)2(exo-methylene)]PdBr2 and{(DMPP)2-
[2 + 2]}PdBr2 data collected on a Siemens P4 diffractometer showed
that the crystals belong to the triclinic, monoclinic, and monoclinic
systems respectively. Quantitative data for the first two compounds
were obtained at room temperature in the θ-2θ mode. The resulting
data sets were transferred to a VAX computer, and for all subsequent
calculations, the Molen package18 was used for (DPTCT)PdCl2 and
SHELXTL Plus19 was used for trans-(DMPP)2PdBr2. Three standard
reflections measured every 1 h during the entire data collection periods
showed no significant trends. The raw data were converted to intensities
and corrected for Lorentz, polarization, and absorption effects within
SHELXTL Plus for trans-(DMPP)2PdBr2 by the Gaussian method. For
(DPTCT)PdCl2, no absorption corrections were applied as the absorption
coefficient is small. The structures were solved by the heavy-atom
method. For (DPTCT)PdCl2 the CH3OH molecule is disordered over
two positions related by a 2-fold crystallographic axis superimposing
the C and O atoms; this disorder was treated by affecting to the C/O
position a mixed form factor of 50% C + 50% O. After refinement of
the heavy atoms, difference Fourier maps revealed maximas of residual
electronic density close to positions expected for hydrogen atoms. They
Results and Discussion
(DMPP)2PdX2 complexes are present in solution as equilib-
rium mixtures of the cis- and trans-isomers.17,23 These equilibria
are solvent and temperature dependent such that the generally
thermodynamically more stable cis isomers are stabilized by
(20) Flack, H. D. Acta Crystallogr. 1984, A41, 500.
(21) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch: Birmingham, England, 1974; Vol. IV, (a) Table
2.2b; (b) Table 2.3.1.
(18) Molen, An InteractiVe Structure Solution Procedure; Enraf-Nonius:
Delft, The Netherlands, 1990.
(19) Sheldrick, G. M. SHELXTL User Manual; Nicolet: Madison, WI,
1986.
(22) International Tables for X-ray Crystallography; Reidel: Boston, MA,
1992; Vol. C.
(23) MacDougall, J. J.; Mathey, F.; Nelson, J. H. Inorg. Chem. 1980, 19,
1400.

1490 Inorganic Chemistry, Vol. 35, No. 6, 1996
Wilson et al.
17
Table 2. Selected Bond Lengths (Å) and Angles (deg) for trans-(DMPP)2PdBr2, trans-(DMPP)2PdI2 and cis-(DMPP)2PdCl2
Bond Lengths
trans-(DMPP)2PdBr2
Pd-Br1
Pd-Br2
Pd-P1
Pd-P2
P1-C1
P1-C4
P2-C13
P2-C16
trans-(DMPP)2PdI2
cis-(DMPP)2PdCl2
2.4334(8)
2.4349(8)
2.319(2)
2.313(2)
1.793(6)
1.782(6)
1.790(6)
1.789(6)
Pd1-I1
2.603(1)
2.609(1)
2.324(2)
2.312(2)
1.801(10)
1.804(7)
1.790(9)
1.788(9)
Pd-Cl1
Pd-Cl2
Pd-P1
2.350(3)
2.356(3)
2.243(3)
2.238(2)
1.799(9)
1.791(9)
1.793(1)
1.790
Pd1-I2
Pd1-P1
Pd1-P2
P1-C1
P1-C4
P2-C13
P2-C16
Pd-P2
P1-C11
P1-C14
P2-C21
P2-C24
Bond Angles
trans-(DMPP)2PdI2
trans-(DMPP)2PdBr2
cis-(DMPP)2PdCl2
Br1-Pd-Br2
Br1-Pd-P1
Br1-Pd-P2
Br2-Pd-P1
Br2-Pd-P2
P1-Pd-P2
177.57(3)
89.08(5)
90.20(4)
90.81(4)
90.05(4)
176.80(6)
91.9(3)
I1-Pd1-I2
167.8(1)
91.2(1)
89.9(1)
90.3(1)
89.7(1)
174.8(1)
91.0(4)
92.0(5)
Cl1-Pd-Cl2
C12-Pd-P1
Cl1-Pd-P1
Cl2-Pd-P2
Cl1-Pd-P2
P1-Pd-P2
91.3(1)
178.6(1)
88.90(9)
86.0(1)
172.3(1)
93.43(9)
92.7(5)
I1-Pd1-P1
I1-Pd1-P2
I2-Pd1-P1
I2-Pd1-P2
P1-Pd1-P2
C1-P1-C4
C13-P1-C16
C1-P1-C4
C13-P2-C16
C11-P1-C14
C21-P2-C24
91.5(3)
91.8(2)
Figure 1. ORTEP Drawing of trans-(DMPP)2PdBr2 showing the atom
numbering scheme (50% probability ellipsoids). Hydrogen atoms are
omitted.
Figure 2. Perspective drawing of trans-(DMPP)2PdI2 showing the atom
numbering scheme (50% probability ellipsoids). Hydrogen atoms have
an arbitrary radius of 0.1 Å.
high dipole moment solvents and the trans isomer population
increases with increasing temperature and increasing ligand
steric bulk. Thus, (DMPP)2PdCl2 is wholly cis; (DMPP)2PdBr2,
a 0.51:1.00 mixture of the trans- and cis-isomers; and (DMPP)2-
PdI2, wholly trans in CDCl3 at ambient temperature as shown
the bond angles around the metal are 360.14(5)° and 361.1(1)°
and the dihedral angles between the X1-Pd-P1 and X2-Pd-
P2 planes are small (1.2 and 13.2°), respectively. By way of
comparison, for cis-(DMPP)2PdCl2 with approximate C2 sym-
metry, the PdCl (2.350(3), 2.356(3) Å) and PdP (2.243(3), 2.238
(2) Å) distances (Table 2) show similar differences. This
complex exhibits a small degree of tetrahedral distortion as the
sum of the bond angles around palladium is 359.63(10)° and
the dihedral angle between the Cl-Pd-Cl and P-Pd-P planes
is 7.2°.
Dissolution of either cis- or trans-(DMPP)2PdBr2 in CDCl3
forms an equilibrium mixture of the two isomers as the 1H, 13C-
{1H}, and 31P{1H} NMR spectra of both isomers are identical
to those reported previously.17 The two isomers exhibit es-
sentially the same melting points (trans, 234-236 °C; cis, 229-
231 °C), suggesting that they isomerize in the solid state as
well. Solid-state isomerizations of L2PtX2 complexes are well-
documented.28-33
1
by H, 13C{1H}, and 31P{1H} NMR spectroscopy.17 Whereas,
the cis- and trans-isomers are both often easily isolated for the
robust (R3P)2PtX2 complexes,14,24,25 in only a few cases14,26,27
have both the cis- and trans-isomers been isolated for the more
labile (R3P)2PdX2 complexes. Thus, we have completely
characterized the cis- and trans-isomers of (DMPP)2PdBr2. The
cis-isomer was isolated from an acetone/water mixture as yellow
needles and the trans-isomer from benzene as orange cubes.
We were only able to obtain X-ray quality crystals for the trans-
isomer, and its structure is shown in Figure 1. We were also
able to obtain crystals of the analogous trans-(DMPP)2PdI2, and
its structure is shown in Figure 2. Both complexes crystallize
as discrete molecular entities with no unusual intermolecular
contacts. The molecules possess near, but not exact, C2h
symmetry. The PdBr (2.4334(8), 2.4349(8) Å), PdI (2.603(1),
2.609(1) Å), PdP (2.319(2), 2.313(2) Å), and (2.324(2), 2.312-
(2) Å) distances (Table 2) are not identical. The palladium
coordination geometries are essentially planar as the sums of
Cis- and trans-(DMPP)2PdBr2 may be distinguished by
infrared spectroscopy as follows. For the C2 symmetry cis-
isomer, two υPdP and two υPdBr vibrations are expected and
observed. For the C2h symmetry trans-isomer, only one υPdP
and one υPdBr vibration is expected and observed (see Experi-
mental Section). They may also be distinguished by CP/MAS
31P{1H} NMR spectroscopy.34-36
(24) Chatt, J.; Wilkins, R. G. J. Chem. Soc. 1952, 273, 4300.
(25) Chatt, J.; Wilkins, R. G. J. Chem. Soc. 1956, 525.
(26) Cheney, A. J.; Mann, B. E.; Shaw, B. L.; Slade, R. M. J. Chem. Soc.
(A) 1971, 3833.
(27) Allen, E. A.; Wilkinson, W. J. Inorg. Nucl. Chem. 1974, 36, 1663.
(28) Ellis, R.; Weil, T. A.; Orchin, M. J. Am. Chem. Soc. 1970, 92, 1078.

Thermal Coupling of 1-Phenyl-3,4-dimethylphosphole
Inorganic Chemistry, Vol. 35, No. 6, 1996 1491
Table 3. Solution and CP/MAS 31P NMR Data for (DMPP)2PdX2
Complexesa
31
X
δ(31P)(CDCl3)
δ
( P)
δ11
δ22
δ33
iso
Cl
Br
26.4 (cis)
24.2 (cis)
17.5 (trans)
9.5 (trans)
28
60
62
46
40
44
36
26
19
-18
-18
-12
-25
30,26b
19c
I
11
a Chemical shifts are reported with respect to 85% H3PO4. Errors
in the principal components are (5 ppm unless indicated otherwise.
b Two sites with tensor components equal within (10 ppm. c 2J(P,P)
) 540 ( 40 Hz.
CP/MAS 31P{1H} NMR Spectroscopy. The phosphorus
chemical shift tensors obtained from analysis of 31P MAS and
static NMR spectra of cis- and trans-complexes of DMPP with
palladium halides are summarized in Table 3. For the bromo
and iodo derivatives, relatively broad peaks (∆υ
300-600
Hz) were observed due to incomplete averaging of the dipolar
interaction involving 31P and the quadrupolar nuclei present in
these complexes (e.g., 79/81Br and 127I). From the data presented
in Table 3, it is clear that the difference in isotropic chemical
shifts between the cis- and trans-arrangements of phosphine
ligands results mainly from variations in the least and intermedi-
ate shielded principal components, δ11 and δ22, which are more
shielded in the latter by about 10 ppm (cf. cis- and trans-
(DMPP)2PdBr2). It has been found that differences in isotropic
chemical shifts for phosphorus nuclei in different polymorphs
or crystallographically nonequivalent positions may be 10 ppm
or more; even larger differences in the principal components
have been noted.37 Such large differences in the phosphorus
chemical shift with subtle structural variations make if difficult
to discuss the small changes in chemical shift with the
geometrical arrangement of phosphine ligands.
For cis-(DMPP)2PdCl2, two resonances at 28 and 27 ppm
were observed in the 31P CP/MAS NMR spectrum obtained at
9.4 T (see insert Figure 3). Note, that these values are different
from those reported earlier.34 On first sight, the observation of
two resonances is consistent with the crystal structure of this
compound, as the two phosphine ligands are crystallographically
nonequivalent.17 However, a detailed investigation revealed that
the peak-to-peak separation between these resonances is de-
pendent on the sample spinning frequency. The maximum peak
splitting observed at Bo ) 9.4 T was 165 Hz when the MAS
rate was 3.0 kHz. In the discussion that follows, it is necessary
to define crystallographic and magnetic equivalence. Also, it
is important to recognize how the solid-state 31P MAS NMR
Figure 3. Dependence of the peak-to-peak splitting in the 31P CP/
MAS NMR spectrum of cis-(DMPP)2PdCl2 on the spinning rate and
the strength of the external magnetic field. For the measurements at
9.4 T, a reduced spinning rate was used; i.e., the rates were divided by
2. The insert shows the isotropic region of the 31P CP/MAS NMR
spectrum obtained at 9.4 T and a spinning rate of 2.8 kHz. Note the
presence of 105Pd satellites.
spectra of mononuclear bisphosphine complexes will differ
depending on whether or not two phosphines are crystallo-
graphically or magnetically equivalent. Two phosphine ligands
are crystallographically equivalent if they are related by a
symmetry element of the space group. Thus, crystallographi-
cally equivalent phosphine ligands will have identical isotropic
chemical shifts. As well, the principal components of their
respective chemical shift tensors will be of identical magnitude.
However, the orientations of the two chemical shift tensors will
not, in general, be coincident. In order to be magnetically
equivalent, the nuclei must be crystallographically equivalent
and the orientations of the principal axes of both tensors must
coincide. This requirement is generally only fulfilled if both
phosphine ligands are related by a center of inversion or a
translation. Only for a trans-arrangement of ligands is a center
of inversion possible at the metal center. For an isolated pair
of magnetically equivalent nuclei, theory predicts the MAS line
shapes to be independent of the sample spinning frequency.38
For cis-complexes, the indirect spin-spin coupling between the
two 31P nuclei is in most cases smaller than 50 Hz and hence
smaller than the typical line widths in 31P CP/MAS NMR
spectra.39 However, the direct dipolar coupling for typical P-P
distances is still sizable (200-400 Hz). Depending on the
relative orientations of the two CS tensors, the direct dipolar
interaction may become homogeneous. This in turn may result
(29) Kukushkin, Y. N.; Budanova, V. F.; Sedove, G. N.; Pogareva, V. G.;
Fadeev, Y. V.; Soboleva, M. S. Zh. Neorg. Khim, 1976, 21, 1265;
Russ. J. Inorg. Chem. (Engl. Transl.) 1976, 21, 689.
(30) Sedova, G. N.; Demchenko, L. N. Zh. Neorg. Khim. 1981, 26, 435;
Russ. J. Inorg. Chem. (Engl. Transl.) 1981, 26, 234.
(31) Kukushkin, Y. N.; Budanova, V. F.; Sedova, G. N.; Pogareva, V. G.
Zh. Neorg. Khim. 1977, 22, 1305; Russ. J. Inorg. Chem. (Engl. Transl.)
1977, 22, 710.
(32) Kukushkin, Y. N.; Sedova, G. N.; Antonov, P. G.; Mitronina, L. N.
Zh. Neorg. Khim. 1977, 22, 2785; Russ. J. Inorg. Chem. (Engl. Transl.)
1977, 22, 1512.
(33) Andronov, E. A.; Kukushkin, Y. N.; Lukicheva, T. M.; Kunovalou,
L. V.; Bakhireva, S. I.; Postnikova, E. S. Zh. Neorg. Khim. 1976, 21,
2443; Russ. J. Inorg. Chem. (Engl. Transl.) 1976, 21, 1343.
(34) Nelson, J. H.; Rahn, J. A.; Bearden, W. H. Inorg. Chem. 1987, 26,
2192.
(35) Rahn, J. A.; O’Donnell, D. J.; Palmer, A. R.; Nelson, J. H. Inorg.
Chem. 1989, 28, 3631.
(36) Rahn, J. A.; Baltusis, L.; Nelson, J. H. Inorg. Chem. 1990, 29, 750.
(37) (a) Naito, A.; Sastry, D. L.; McDowell, C. A. Chem. Phys. Lett. 1985,
115, 19. (b) Power, W. P.; Lumsden, M. D.; Wasylishen, R. E. J. Am.
Chem. Soc. 1991, 113, 8257. (c) Jarrett, P. S.; Sadler, P. J. Inorg.
Chem. 1991, 30, 2098. (d) Lindner, E.; Fawzi, R.; Mayer, H. A.;
Eichele, K.; Hiller, W. Organometallics 1992, 11, 1033. e) Szalontai,
G.; Bakos, J.; Aime, S.; Gobetto, R. J. Organomet. Chem. 1993, 463,
223.
(38) (a) Wu, G.; Wasylishen, R. E. Inorg. Chem. 1994, 33, 2774. (b) Wu,
G.; Wasylishen, R. E. J. Chem. Phys. 1993, 99, 6391. (c) Wu, G.;
Wasylishen, R. E. J. Chem. Phys. 1993, 98, 6138. (d) Eichele, K.;
Wu. G.; Wasylishen, R. E. J. Magn. Reson., Ser. A 1993, 101, 156.
(39) Pregosin, P. S.; Kunz, R. W. In NMR Basic Principles and Progress;
Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1976;
Vol. 16.

1492 Inorganic Chemistry, Vol. 35, No. 6, 1996
Wilson et al.
in the observation of spinning-rate dependent splittings. As
previously shown,38a in order to determine whether or not two
nuclei are crystallographically equivalent, it is advantageous to
study the spinning-rate dependence of the splittings at different
external applied magnetic fields. If the two nuclei are crystal-
lographically equivalent, the spinning-rate dependence of the
splittings is independent of the reduced sample spinning
frequency, ωRred. Here, we define ωRred ) ωR (4.7/Bo), where
ωR is the sample spinning frequency measured at Bo (in tesla).
To determine whether or not the two phosphorus nuclei of cis-
(DMPP)2PdCl2 are crystallographically equivalent, the spinning-
rate dependence of this splitting was studied at 9.4 T. Using a
reduced spinning rate, the spinning-rate dependencies at both
fields are superimposable and imply that the two phosphorus
nuclei are crystallographically equivalent within experimental
error. Note, however, that this criterium for crystallographic
equivalence only means that they are equivalent within experi-
mental error, as only the establishment of nonequivalence is
unambigous.
The two samples of cis-(DMPP)2PdCl2 examined by CP/MAS
31P NMR spectroscopy are polymorphic. That examined
earlier34 was obtained from CH3OH solution as yellow mono-
clinic crystals,17 space group Cc, with a unit cell of the following
dimensions: a ) 10.427(4) Å, b ) 15.070(6) Å, c ) 15.910-
(6) Å, â ) 92.88(2)°, Z ) 4. Those investigated in this work
were obtained from benzene as yellow monoclinic crystals,
space group P21/c, with a unit cell of the following dimen-
sions: a ) 11.425(2) Å, b ) 8.687(1) Å, c ) 26.212(4) Å, â
) 100.64(1)°, Z ) 4. Structure refinement of these latter
crystals did not proceed below R ) 0.107 due to severe disorder
of both phosphole ligands.
Figure 4. Isotropic region of the 2D homonuclear J-resolved 31P CP/
MAS NMR spectrum of trans-(DMPP)2PdBr2, obtained at 4.7 T and a
spinning rate of 1.8 kHz. The time increment in F1 was synchronized
with the sample spinning frequency. The J(PP) coupling constant is
540 ( 40 Hz.
2
Scheme 1
The 31P CP/MAS NMR spectrum of cis-(DMPP)2PdCl2 also
revealed some interesting features as small satellites (see Figure
3). We attribute these satellites to dipolar and indirect spin-
5
spin coupling of 31P to 105Pd (I ) /2, 22.3%). Due to the
complex nature of the uncoupled central peak (i.e., the spinning-
frequency dependent splitting), no attempt has been made to
analyze this effect. However, this appears to be the first report
of spin-spin coupling involving 105Pd.
For trans-(DMPP)2PdBr2, only one broad peak was discern-
able in the 31P CP/MAS NMR spectrum but with some
interesting features in the spinning side bands; i.e., their shape
differs from the isotropic peak and indicates strong coupling
effects. The crystal structure shows two nonequivalent phos-
phole ligands. A two-dimensional J-resolved spectrum40 re-
vealed J(PP)
540 ( 40 Hz, comparable to other values of
This is in contrast to the thermolyses of cis-(DMPP)2PtX2,
where only the platinum analogues of A-C were formed and
A and B were ultimately converted to C.2
2J(PP) trans-coupling constants in palladium phosphine com-
plexes39, hence establishing the trans-geometry of this com-
pound. The 2D J-resolved spectrum is shown in Figure 4.
X-ray crystal structures were obtained for an example of three
of these compound types. Figures 5 and 6 show the structures
of the [2 + 2] cycloaddition product and the exo-methylene
product, respectively, formed from the thermolysis of trans-
(DMPP)2PdBr2, and Figure 7 shows the structure of [DPTCT]-
PdCl2 formed from the thermolysis of cis-(DMPP)2PdCl2.
Tables 4-6 list relevant bond distances and bond angles. Each
of the compounds crystallizes as discrete molecular entities with
no unusual intermolecular contacts. The coordination geometry
about palladium in each compound is essentially planar as the
sums of the bond angles around palladium are very near 360°.
The metrical parameters for the analogous palladium and
platinum compounds are very similar, yet their chemical
reactivities are quite different. The Pd-P bond distances in
the [2 + 2] product are slightly different (2.238(5), 2.224(8)
Å); they are equal in the exo-methylene product (2.219(3),
2.213(4) Å) and in both cases are considerably shorter than those
in trans-(DMPP)2PdBr2 (2.319(2), 2.313(2) Å).
Thermolyses Reactions. Both cis-(DMPP)2PdCl2 and cis-
or trans-(DMPP)2PdBr2 undergo thermal coupling reactions of
their coordinated phospholes, yielding four products, while
thermolysis of trans-(DMPP)2PdI2 gives none of these products.
These products are a [2 + 2] cycloaddition product, A; a [4 +
2] cycloaddition product, B, a third compound, C, in which a
carbon-carbon bond is formed between carbons 1 and 1′ and
a hydrogen is transferred to carbon 2 from the methyl group on
carbon 2′, which is referred to as the exo-methylene product;
and a fourth compound, D, that is possibly formed by the
following sequence of reactions: intermolecular endo-[4 + 2]
cyclodimerization followed by phosphinidene elimination and
then intramolecular exo-[4 + 2] cycloaddition to another
phosphole to give a diphosphatetracyclotetradecatriene referred
to as DPTCT (see Scheme 1).
(40) Wu, G.; Wasylishen, R. E. Organometallics 1992, 11, 3242.

Thermal Coupling of 1-Phenyl-3,4-dimethylphosphole
Inorganic Chemistry, Vol. 35, No. 6, 1996 1493
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
2
{(DMPP)2[2 + 2]}PdBr2 and {(DMBtP)2[2 + 2]}PtBr2
Bond Lengths
{(DMPP)2[2 + 2]}PdBr2
{(DMBtP)2[2 + 2]}PtBr2
Pd1-Br1
Pd1-Br2
Pd1-P1
Pd1-P2
C1-C2
C2-C3
C3-C4
C3-C8
C4-C7
C7-C8
C8-C9
C9-C10
2.476(2)
2.480(3)
2.238(5)
2.224(8)
1.365(27)
1.507(29)
1.573(23)
1.617(25)
1.521(24)
1.560(22)
1.495(25)
1.299(24)
Pt-Br1
2.488(1)
2.491(1)
2.237(8)
2.244(3)
1.32(2)
1.53(2)
1.55(1)
1.59(2)
1.53(1)
1.54(1)
1.52(1)
1.31(1)
Pt-Br2
Pt-P1
Pt-P2
C1-C2
C2-C3
C3-C4
C3-C8
C4-C7
C7-C8
C8-C9
C9-C10
Figure 5. Perspective drawing of {(DMPP)2[2 + 2]}PdBr2 showing
the atom numbering scheme (50% probability ellipsoids). Hydrogen
atoms have an arbitrary radius of 0.1 Å.
Bond Angles
{(DMPP)2[2 + 2]}PdBr2
{(DMBtP)2[2 + 2]}PtBr2
Br1-Pd1-Br2
Br2-Pd1-P2
Br1-Pd1-P2
P1-Pd1-P2
C1-P1-C4
C7-P2-C10
P1-C1-C2
C1-C2-C3
C2-C3-C4
C2-C3-C8
C4-C3-C8
P1-C4-C3
P1-C4-C7
C3-C4-C7
P2-C7-C4
P2-C7-C8
C4-C7-C8
C3-C8-C7
C3-C8-C9
C8-C9-C10
C7-C8-C9
P2-C10-C9
95.3(1)
90.2(2)
88.6(1)
84.8(2)
93.8(9)
Br1-Pt-Br2
Br1-Pt-P1
Br2-Pt-P2
P1-Pt-P2
86.76(5)
92.78(8)
93.94(8)
86.1(1)
92.7(5)
91.9(5)
113.8(8)
117(1)
108.2(9)
116.9(9)
89.4(8)
108.3(7)
113.8(7)
90.3(8)
115.3(7)
109.2(7)
91.8(7)
C1-P1-C4
C7-P2-C10
P1-C1-C2
C1-C2-C3
C2-C3-C4
C2-C3-C8
C4-C3-C8
P1-C4-C3
P1-C4-C7
C3-C4-C7
P2-C7-C4
P2-C7-C8
C4-C7-C8
C3-C8-C7
C3-C8-C9
C8-C9-C10
C7-C8-C9
P2-C10-C9
91.9(8)
113.6(16)
115.8(16)
109.2(15)
116.1(14)
90.2(12)
107.4(13)
113.7(12)
89.2(12)
114.01(1)
107.0(1)
94.4(13)
86.3(12)
116.7(16)
116.4(16)
108.6(14)
115.9(14)
88.4(8)
117.5(9)
117.4(9)
107.8(9)
113.5(8)
Figure 6. Perspective drawing of [(DMPP)2(exo-methylene)]PdBr2
showing the atom numbering scheme (50% probability ellipsoids).
Hydrogen atoms have an arbitrary radius of 0.1 Å.
2.356(3) Å). The C2-C3 (1.33(1) Å), C14-C15 (1.33(1) Å),
and C19-C20 (1.32(1) Å) distances are all in the range expected
for double bonds. The final reaction step in the formation of
(DPTCT)PdCl2 is diastereospecific, forming only a racemic
mixture of the exo-diastereomer, as expected for an intramo-
lecular [4 + 2] cycloaddition between a metal-coordinated diene
and dienophile.41
The NMR spectroscopic data for (DPTCT)PdCl2 are consis-
tent with the structure. The 31P chemical shifts41 suggest the
presence of a coordinated phospholene phosphorus atom in a
six-membered chelate ring (δ 34.84) and a 7-phosphanorbornene
phosphorus atom that is also part of a six-membered chelate
ring (δ 97.60). The magnitude of 2J(PP) ) 28.01 Hz is typical
for cis-square planar palladium(II) diphosphine complexes. The
1H NMR spectrum contains six methyl resonances, three vinyl
proton resonances, and four methine resonances, two of which
(H3 and H4) exhibit large magnitude J(PH) couplings typical
of methine hydrogens anti to phosphorus in rigid ring systems.41
The 13C{1H} NMR spectrum contains six methyl resonances,
Figure 7. ORTEP drawing of (DPTCT)PdCl2 showing the atom
numbering scheme (50% probability ellipsoids). Hydrogen atoms are
omitted.
3
one of which, CH3(16) displays a J(PC) of 3.02 Hz for the
same reason. There are also four aliphatic methine, two olefinic
methine, and four quarternary olefinic resonances. The proton
and carbon assignments were confirmed by 31P decoupling,
appropriate two-dimensional, and APT spectra.
For (DPTCT)PdCl2 there is some tetrahedral distortion as the
dihedral angle formed between the P-Pd-P and Cl-Pd-Cl
planes is 6.1(4)°. The Pd-P distances (2.208(2), 2.218(2) Å)
are shorter than those in cis-(DMPP)2PdCl2 (2.243(3), 2.238-
(2) Å) as a result of the formation of a six-membered chelate
ring. The Pd-Cl distances (2.381(1), 2.369(2) Å) are slightly
lengthened relative to those in cis-(DMPP)2PdCl2 (2.350(3),
(41) Nelson, J. H. In Phosphorus-31 NMR Spectral Properties in Compound
Characterization and Structural Analysis, Quin, L. D., Verkade, J.
G., Eds.; VCH: Deerfield, Beach, FL, 1994; pp. 203-214.

1494 Inorganic Chemistry, Vol. 35, No. 6, 1996
Wilson et al.
Table 5. Selected bond lengths (Å) and angles (deg) for [(DMPP)2-
(exo-methylene)]PdBr2 and [(DMBtP)2(exo-methylene)]PtBr2
Bond Lengths
[(DMPP)2(exo-methylene)]PdBr2 [(DMBtP)2(exo-methylene)]PtBr2
Pd-Br1
Pd-Br2
Pd-P1
Pd-P2
P1-C1
P1-C4
P2-C7
P2-C10
C1-C2
C1-C7
C2-C3
C2-C5
C3-C4
C7-C8
C8-C9
C8-C12
C9-C10
2.489(2)
2.487(2)
2.219(3)
2.213(4)
1.860(12)
1.779(12)
1.85(12)
1.801(16)
1.557(8)
1.555(18)
1.506(19)
1.527(18)
1.330(20)
1.523(22)
1.489(24)
1.336(22)
1.342(25)
Pt-Br1
Pt-Br2
Pt-P1
2.484(1)
2.491(2)
2.214(3)
2.219(3)
1.86(1)
1.77(1)
1.84(1)
1.79(1)
1.48(2)
1.55(2)
1.49(2)
1.40(2)
1.36(2)
1.46(2)
1.43(2)
1.42(2)
1.29(2)
Pt-P2
P1-C1
P1-C4
P2-C7
P2-C10
C1-C2
Cl-C7
C2-C3
C2-C5
C3-C4
C7-C8
C8-C9
C8-C12
C9-C10
Figure 8. First-order plot for the disappearance of cis-(DMPP)2PdCl2
in Cl2CHCHCl2 at 145°: (, 0.0036 M; O, 0.0072 M; 2, 0.0144 M.
The average apparent value of the rate constant is (4.25 ( 0.3) 10-5
s-1
.
Bond Angles
[(DMPP)2(exo-methylene)]PdBr2 [(DMBtP)2(exo-methylene)]PtBr2
Br1-Pd-Br2
Br1-Pd-P2
Br2-Pd-P1
P1-Pd-P2
C1-P1-C4
C7-P2-C10
P1-C1-C2
P1-C1-C7
C2-C1-C7
C1-C2-C3
C1-C2-C5
C3-C2-C5
C2-C3-C4
P1-C4-C3
P2-C7-C1
P2-C7-C8
C1-C7-C8
C7-C8-C9
C7-C8-C12
C9-C8-C12
C8-C9-C10
P2-C10-C9
97.2(1)
90.7(1)
87.2(1)
85.3(1)
92.9(6)
Br1-Pt-Br2
Br1-Pt -P2
Br2-Pt-P1
P1-Pt-P2
88.90(6)
91.42(8)
92.58(8)
86.5(1)
93.5(7)
92.0(6)
107.7(9)
113.5(6)
114(1)
110(1)
123(2)
114(1)
118(1)
C1-P1-C4
C7-P2-C10
P1-C1-C2
P1-C1-C7
C2-C1-C7
C1-C2-C3
C1-C2-C5
C3-C2-C5
C2-C3-C4
P1-C4-C3
P2-C7-C1
P2-C7-C8
C1-C7-C8
C7-C8-C9
C7-C8-C12
C9-C8-C12
C8-C9-C10
C2-C10-C9
94.0(6)
104.4(8)
109.8(8)
118.7(11)
108.8(11)
112.6(10)
111.7(11)
117.1(11)
112.1(10)
111.6(8)
103.4(9)
113.7(11)
112.3(13)
127.7(16)
120.0(18)
115.2(16)
111.4(11)
Figure 9. Percent composition as a function of time for the thermolysis
of cis-(DMPP)2PdCl2 in Cl2CHCHCl2 at 145 °C: (, starting material;
b, [4 + 2]; O, exo-methylene; 2, [2 + 2]; 0, (DPTCT)PdCl2. The
lines were calculated from the following rate constants: k1 ) (1.5 (
0.3) 10-5 s-1; k2 ) (2.9 ( 0.3) 10-5 s-1; k3 ) (3.4 ( 0.3) 10-5
110(1)
113.8(7)
105.5(8)
114.8(9)
113(1)
119(2)
120(1)
s-1; k4 ) (2.5 ( 0.3) 10-5 s-1; k5 ) (1.0 ( 0.3) 10-4 M-1 s-1
.
Table 7. 31P{1H} NMR Data at 121.65 MHz for the Products of
Thermolysis of (DMPP)2PdX2 Complexes in CDCl3 at 300 K (δ,
ppm; J Hz)
117(1)
112(1)
δ31P
2J(PP)
Table 6. Selected Bond Lengths (Å) and Angles (deg) for
(DPTCT)PdCl2
X ) Cl
[4 + 2]
133.67, 58.58
107.42
12.29
Bond Lengths
[2 + 2]
Pd-Cl1
Pd-P1
P1-C1
P2-C19
C2-C3
C19-C20
2.369(2)
2.208(7)
1.844(6)
1.770(8)
1.33(1)
Pd-Cl2
2.381(2)
2.218(2)
1.839(7)
1.824(6)
1.33(1)
[exo-methylene]
[DPTCT]
107.26, 93.26
97.60, 34.84
10.22
28.10
Pd-P2
P1-C4
X ) Br
P2-C22
C14-C15
[4 + 2]
134.16, 58.10
108.76
11.50
[2 + 2]
1.32(1)
[exo-methylene]
[DPTCT]
108.47, 94.62
94.65, 31.52
16.97
23.42
Bond Angles
Cl1-Pd-Cl2
Cl1-Pd-P1
Cl2-Pd-P1
C19-P2-C22
P2-C22-C16
95.48(7)
175.34(7)
86.49(6)
92.3(3)
Cl1-Pd-P2
Cl2-Pd-P2
P2-Pd-P1
C1-P1-C4
C1-C16-C22
92.09(7)
171.36(6)
86.27(6)
82.4(3)
in Scheme 1 were not isolated but were only characterized by
31P{1H} NMR spectroscopy of the reaction mixtures. Assign-
ments (Table 7) were made by comparison with the data for
the analogous platinum complexes.2
117.8(4)
109.5(5)
Kinetics. Detailed kinetics of the solid-state thermolyses
were not obtained. The same products in the same final ratios
were formed in both the solid- and solution-state thermolyses,
suggesting that the mechanisms are the same.
Detailed kinetic runs were performed on reactions of cis-
(DMPP)2PdCl2 in 1,1,2,2-tetrachloroethane at 145 °C. The
results are shown in Figures 8 and 9. The overall loss of starting
material is clearly first order (Figure 8), implying a unimolecular
mechanism. A proposed mechanism is shown in Scheme 2.
Though the [2 + 2], [4 + 2], and exo-methylene products could
The structure of [(DMPP)2(exo-methylene)]PdBr2 was also
confirmed by multinuclear NMR spectroscopy (see Experimen-
tal Section). Of particular note is the fact that this molecule,
like its platinum analogues,2 is formd as a racemic mixture of
a single diastereomer by stereospecific hydrogen transfer of H4.
This means that the C-C coupling to form a biradical
intermediate, as previously discussed,2 occurs in a syn-fashion.
This results in H3 and H4 being syn in the products. Except for
(DPTCT)PdCl2, the other palladium chloride products illustrated

Thermal Coupling of 1-Phenyl-3,4-dimethylphosphole
Inorganic Chemistry, Vol. 35, No. 6, 1996 1495
Scheme 2
Scheme 3
eqs 3-8 were integrated numerically to find the temporal
behavior of the concentration of each species. The calculated
percent composition of the reaction mixture was based on the
relative amounts of A, C, D, E, G, and unidentified products,
with the amounts of G and unidentified products being assumed
to be equal to each other. The rate constants were then adjusted
to give the fit to the experimental data.
The validity of this mechanism is shown in the concentration
Vs time graph of the various components (Figure 9). Since the
rate of formation of the [4 + 2] cycloaddition product is
approximately one-fourth its rate of conversion to the [2 + 2]
and exo-methylene products, its concentration peaks early in
the reaction and ultimately decreases to zero. Note also that
the initial rate of formation of the exo-methylene product is only
1.2 times that of the [2 + 2] product and this is the approximate
final ratio of these two products.
also be formed by independent electrocyclic and ene reactions,
we favor the mechanism shown in Scheme 2 for its simplicity.
The starting material (A) may react by forming a bond
between carbons 1 and 1′ of two adjacent phospholes to generate
a diallyl 1,4-biradical transition state (B). This transition state
could collapse by additional carbon-carbon bond formation to
the observed [4 + 2] species (C). As can be seen from Figure
9, the [4 + 2] product is subsequently converted to the [2 + 2]
(D) and exo-methylene (E) products. We assume, in contrast
to what was found2 for the thermolyses of cis-(DMPP)2PtCl2,
that both the [2 + 2] and exo-methylene products are formed
irreversibly. Thus, the unimolecular reaction is ultimately driven
to the [2 + 2] and exo-methylene products.
In addition to this unimolecular reaction pathway there is also
a parallel bimolecular reaction pathway for the formation of
(DPTCT)PdCl2, a proposed mechanism for which is shown in
Scheme 3.
These two pathways give rise to the following rate laws, eqs
3-8.
An attempt was made to model the kinetics of the thermolysis
of cis-(DMPP)2PdCl2 by the same rate laws that were found2
for the thermolysis of cis-(DMPP)2PtCl2 but without success.
Also, in contrast to what was observed for the thermolysis of
the platinum complex where the solid-state (k1 ) (6.1 ( 0.3)
10-5 s-1) reaction was slightly slower than the reaction in
solution (k1 ) (8.33 ( 0.3)
10-5 s-1), for the palladium
complex the reaction in the solid state is much faster than that
in solution. This is probably related to the fact that for the
reactions of the palladium complexes there are two parallel
reaction pathways, whereas for the reactions of the platinum
complexes there is only one pathway. The complex concentra-
tion is maximized in the solid state such that the reaction in the
solid state might be expected to be faster than that in solution.
The thermolysis of trans-(DMPP)2PdBr2 is faster in solution
than in the solid state. This is probably because isomerization
to the cis-isomer must first occur before the intramolecular
phosphole coupling reactions may proceed. The two isomers
are in rapid dynamic equilibrium in solution, but the solid-state
isomerization is slow.
-d[A]/dt ) (k1 + k4)[A] + k5[A][F]
d[C]/dt ) k1[A] -(k2 + k3)[C]
d[D]/dt ) k2[C]
(3)
(4)
(5)
(6)
(7)
(8)
d[E]/dt ) k3[C]
The exact reasons why the palladium reactions contain two
pathways and the platinum reactions do not are unknown. If
one compares the crystal packing diagrams for cis-
d[F]/dt ) k4[A] - k5[A][F]
d[G]/dt ) d[X]/dt ) k5[A][F]
17
(DMPP)2PdCl2 and cis-(DMPP)2PtCl2,42 we see that in fact
two complex molecules are somewhat closer in space for the
palladium complex, where the distance between the centroids
of two phosphole rings on adjacent complexes is 7.08 Å, than
According to this mechanism, the loss of starting material
should be first order in starting material and an approximate
value of (k1 + k4) ) (4.25 ( 0.3)
determined. Approximate values of k2 and k3 were determined
10-5 s-1 is directly
(42) Holt, M. S.; Nelson, J. H.; Alcock, N. W. Inorg. Chem. 1986, 25,
from the initial rates of formation of D and E, respectively. Then
2288.

1496 Inorganic Chemistry, Vol. 35, No. 6, 1996
Wilson et al.
for the platinum complex where the corresponding distance is
8.26 Å. trans-(DMPP)2PdBr2 undergoes thermal isomerization
to cis-(DMPP)2PdBr2 in the solid state, most likely by dissocia-
tion and reassociation of a phosphole. This observation is
consistent with the bimolecular mechanism for the formation
of (DPTCT)PdX2 shown in Scheme 3. If the rate determining
step in this process is the dissociation of phosphole, then the
disappearance of (DMPP)2PdX2 would be first order and the
unimolecular and ultimately bimolecular reactions would be
parallel processes. The fate of (DMPP)PdX2 and the phosphin-
idene, PhP, are unknown. There are, however, unidentified
phosphorus containing products formed together with some
elemental palladium. No elemental platinum was observed in
the thermolyses of the platinum complexes.
thermodynamically stable the trans isomer. There are minimal
differences in the reaction rates and product distributions
between the chloride and bromide complexes in solution. In
the solid state there are notable differences in the rates but not
the final product distributions. No reaction occurs for the iodide
complex, which has the trans geometry in both states. The
palladium complexes slowly dissociate phosphole to some
extent, and this leads to bimolecular products. The platinum
complexes do not dissociate phosphole, and for them no
bimolecular products are observed. The reaction rates are not
influenced by added halide in the form of [Me4N]+X- (X )
Cl, Br), but phosphole inhibition studies could not be undertaken
because the presence of excess phosphole in solution leads to
rapid ligand exchange through formation of pentacoordinate
(DMPP)3MX2 complexes.44
Conclusions
The thermolyses of (DMPP)2PdX2 complexes proceed Via
Acknowledgment. We are grateful to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for financial support, to the National Science
Foundation (Grant CHE-9214294) for funds to purchase the 500
MHz NMR spectrometer, and to Professor Derek J. Hodgson
and Dr. Urszula Rychlewska of the University of Wyoming for
assistance with the crystal packing diagrams.
parallel bimolecular (k5 ) (1.0 ( 0.3)
10-4 M-1 s-1) and
branched unimolecular (k1 ) (1.5 ( 0.3) 10-5 s-1) processes.
The branches in the unimolecular process may evolve from a
common 1,4-butanediyl biradical43 transition state. In both the
solution and solid states the reaction proceeds only from the
cis-(DMPP)2PdX2 complexes. For (DMPP)2PdBr2 this results
from trans h cis isomerization equilibria extant in both the
solution and solid states. The trans-geometry precludes in-
tramolecular carbon-carbon bond formation. The nature of the
halide affects the reaction only in the determination of the
geometry of the complexes. The larger the halide, the more
Supporting Information Available: For the crystal structures,
tables listing crystal and refinement data, bond distances and angles,
H atom coordinates, and thermal parameters (23 pages). Ordering
information is given on any current masthead page.
IC951307F
(43) For recent discussions of biradical intermediates, see: Peterson, T.
H.; Carpenter, B. K. J. Am. Chem. Soc. 1992, 114, 1496. Ariel, S.;
Evans, S. V.; Garcia-Garisbay, M.; Harkness, B. R.; Omkaram, N.;
Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1988, 110, 5591.
(44) MacDougall, J. J.; Nelson, J. H.; Mathey, F. Inorg. Chem. 1982, 21,
2145.