Synthesis, characterization, and cis-trans isomerization studies of cis-[PdCl2{Ph2P(CH2CH2O) 3CH2CH2PPh2-P,P′}] and trans-[PtCl2{Ph2P(CH2CH2O) 3CH2CH2PPh2-P,P′}] metallacrown ethers

Article abstract of DOI:10.1002/ejic.200800436

The metallacrown ether cis-[PdCl₂{Ph₂P(CH ₂CH₂O)₃CH₂-CH₂PPh ₂-P,P′}] (cis-1) has been kinetically trapped via precipitation during the reaction of PdCl₂ and Ph₂P(CH ₂CH₂O)₃-CH₂CH₂PPh ₂ in an acetonitrile/THF mixture. The kinetics of the cis-trans isomerization and oligomerization equilibria of cis-1 have been followed using quantitative ³¹P{¹H} NMR spectroscopy. The reaction follows reversible first-order kinetics, and an Eyring analysis was carried out to yield ΔH^?_f (81 ± 2 kJmol ^-1), ΔH^?_r (46 ± 2 kJmol^-1), ΔS^?_f (117 ± 4 Jmol^-1 K^-1), and ΔS^?_r (20 ± 6 Jmol^-1K^-1) for the isomerization reaction. The ΔH^?_f is consistent with a proposed associative transition state involving partial cleavage of a Pd-P bond and partial formation of Pd-O(ether) bond to an adjacent ether oxygen. For comparison, trans-[PtCl₂{Ph₂P(CH₂CH ₂O)₃CH₂-CH₂PPh₂-P, P′}] (trans-2) was isolated from a mixture of cis and trans isomers by column chromatography. An analysis of the isomerization process of trans-2 to cis-2 using quantitative ³¹P{¹H} NMR spectroscopy revealed that no isomerization occurs at 348 K in the absence of an acid catalyst and that the acid-catalyzed trans-cis isomerization is zero order in complex. Crystal structures of cis-1, cis-2, and trans-2 are also presented, and the conformations of the metallacrown ether rings in these structures have been related to the data presented above. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800436

FULL PAPER
DOI: 10.1002/ejic.200800436
Synthesis, Characterization, and cis–trans Isomerization Studies of
cis-[PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] and
trans-[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] Metallacrown Ethers
Samuel B. Owens Jr.,[a] Dale C. Smith Jr.,[b] Charles H. Lake,[c] and Gary M. Gray*[a]
Keywords: Isomerization / Platinum / Palladium / Crown ethers
The metallacrown ether cis-[PdCl2{Ph2P(CH2CH2O)3CH2-
oxygen. For comparison, trans-[PtCl2{Ph2P(CH2CH2O)3CH2-
CH2PPh2-P,PЈ}] (cis-1) has been kinetically trapped via pre- CH2PPh2-P,PЈ}] (trans-2) was isolated from a mixture of cis
cipitation during the reaction of PdCl2 and Ph2P(CH2CH2O)3-
CH2CH2PPh2 in an acetonitrile/THF mixture. The kinetics of
the cis–trans isomerization and oligomerization equilibria of
cis-1 have been followed using quantitative 31P{1H} NMR
spectroscopy. The reaction follows reversible first-order ki-
and trans isomers by column chromatography. An analysis of
the isomerization process of trans-2 to cis-2 using quantita-
tive 31P{1H} NMR spectroscopy revealed that no isomeriza-
tion occurs at 348 K in the absence of an acid catalyst and
that the acid-catalyzed trans–cis isomerization is zero order
in complex. Crystal structures of cis-1, cis-2, and trans-2 are
also presented, and the conformations of the metallacrown
ether rings in these structures have been related to the data
netics, and an Eyring analysis was carried out to yield ∆H‡
f
(81Ϯ2 kJmol–1), ∆H‡ (46Ϯ2 kJmol–1), ∆S‡ (117Ϯ4 Jmol–1
K–1), and ∆S‡ (20Ϯ6 Jmol–1 K–1) for the isomerization reac-
r
f
r
tion. The ∆H‡f is consistent with a proposed associative tran- presented above.
sition state involving partial cleavage of a Pd–P bond and
partial formation of Pd–O(ether) bond to an adjacent ether
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
which are needed for these studies, generally cannot be iso-
lated. For Pt complexes, the pure trans isomers can be iso-
lated, but they seem to be stable and do not isomerize.[1]
During our ongoing study of the coordination chemistry
of metallacrown ethers, we have prepared the pure cis-
PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ} (cis-1) and
trans-PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ} (trans-
2). The metallacrown ethers are excellent candidates for the
study of the kinetics and thermodynamics of cis–trans isom-
erizations because the pure isomers can be isolated, they
are stable in the solid state and they are quite soluble and
the isomerizations occurs at rates that allow the reactions
to be followed by 31P{1H} NMR spectroscopy. We have car-
ried out these studies using quantitative 31P{1H} NMR
spectroscopy, and the results are reported in this manu-
script. We have also determined the X-ray crystal structures
of both complexes and that of cis-2 to gain additional in-
sight into the factors that may be affecting the cis–trans
isomerizations.
1. Introduction
Square planar transition metal complexes with phos-
phorus-donor ligands are important catalysts for a variety
of organic reactions.[1] These complexes can exhibit both
cis and trans geometrical isomers, and the isomer ratio can
dramatically affect the activities and selectivities of the ca-
talysis reactions. Thus, a large number of studies of these
equilibria including measurements of the ∆H and ∆S have
been carried out with the majority of these studies focusing
on square planar Pt and Pd complexes containing either
phosphane or phosphite ligands.[2–37]
Although a wealth of information has been obtained re-
garding the relative stabilities of the cis and trans isomers
of the complexes,[38,39] very little information has been ob-
tained regarding the kinetic parameters (∆H‡ , ∆H‡ , ∆S‡ ,
f
r
f
and ∆S‡ ) that would provide a better understanding of the
r
reaction mechanisms. For Pd complexes, the primary
reason for this is that the pure cis isomers of the complexes,
[a] Department of Chemistry, University of Alabama at
Birmingham,
2. Results and Discussion
901 14th Street South, Birmingham, Alabama 35294-1240, USA
Fax: +1-205-934-2543
2.1 Synthesis and Characterization
E-mail: gmgray@uab.edu
[b] Avanti Polar Lipids, Inc.,
We have previously demonstrated that the reaction of
Pd(PhCN)2Cl2 and Ph2P(CH2CH2O)3CH2CH2PPh2 (1) in
dichloromethane yields an equilibrium mixture of cis- and
trans-[PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}]n mono-
700 Industrial Park Drive, Alabaster, AL 35007, USA
[c] Department of Chemistry, Indiana University of Pennsylvania,
Indiana, PA 15705, USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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cis–trans Isomerizations in Pd Metallacrown Ether Complexes
mers (n = 1) and cyclic oligomers (n Ͼ 1).[38,40] In contrast,
when this reaction is run in a 2:1 acetonitrile/THF mixture,
the kinetic product cis-[PdCl2{Ph2P(CH2CH2O)3CH2CH2-
PPh2-P,PЈ}] (cis-1) precipitates from the reaction mixture.
We have also previously reported that the reaction of
Pt(1,5-cod)Cl2 and Ph2P(CH2CH2O)3CH2CH2PPh2 under
moderately high dilution conditions gives a high yield of
cis-[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] (cis-2)[41]
In contrast, the reaction of Ph2P(CH2CH2O)3CH2CH2PPh2
and Pt(PhCN)2Cl2 yields a mixture of cis- and trans-
[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] (cis-2 and
trans-2). The two isomers are easily separated via
chromatography on silica gel using a 5:1 dichloromethane/
ethyl acetate elutant with cis-2 remaining at the origin and
trans-2 eluting from the column. The trans geometry of
trans-2 is indicated by the relatively small one-bond Pt–P
coupling constant (2754 Hz).[25]
2.2 X-ray Crystal Structures of cis-1, cis-2 and trans-2
ORTEP diagrams showing the molecular structures of
cis-1, cis-2, and trans-2 are presented in Figures 1, 2, and
3, respectively. No anomalous intermolecular contacts were
observed in these molecular structures. Selected bond
lengths and angles for cis-1, cis-2, and trans-2 are collected
in Tables 1 and 2, respectively. The geometry associated
with the primary coordination sphere about the metal cen-
ter in each of the complexes is best described as a distorted
square-planar geometry. The metallacrown ether ring is cis-
chelated to the metal in cis-1 and cis-2 and trans-chelated
to the metal in trans-2. The P1–M–P2 bond angles are
98.7(1)° in both the cis structures, cis-1 and cis-2, suggesting
that this distortion from square-planar geometry most
likely results from the steric interactions between the adja-
cent diphenylphosphanyl groups. The average deviations
from the least-squares plane (defined by the two phos-
phorus atoms, the two chlorides and the metal) for cis-1,
cis-2 and trans-2 are 0.0502 Å, 0.01563 Å, and 0.0634 Å,
respectively. This indicates that neither cis or trans chelation
of the Ph2P(CH2CH2O)3CH2CH2PPh2 ligand causes a sig-
nificant out-of-plane distortion in the square-planar geome-
try.
Figure 1. ORTEP drawing of the molecular structure of cis-1. Ther-
mal ellipsoids are drawn at the 50% probability level, and hydrogen
atoms are omitted for clarity.
The coordination geometry does have a significant effect
on the metal–P and metal–Cl bond lengths. The metal–
chloride bond lengths in cis-1 and cis-2 are significantly
longer than are those in trans-2, while the metal–phosphane
bonds in cis-1 and cis-2 are significantly shorter than are
those in trans-2. These trends are consistent with the greater
σ donor ability of a phosphane ligand relative to that of a
chloride ligand.
Figure 2. ORTEP drawing of the molecular structure of cis-2. Ther-
mal ellipsoids are drawn at the 50% probability level, and hydrogen
atoms are omitted for clarity.
The conformations of the metallacrown ether rings in cis-
1, cis-2 and trans-2 are the most interesting aspects of the
structures. Values of selected torsion angles for cis-1, cis-2
and trans-2, given in Table 3, indicate that the three metall- pseudo-gauche. This is consistent with the tendency of such
acrown ethers have rather different ring conformations. One groups to orient in a cis conformation and has previously
similarity between the structures is that the torsion angles been observed in the X-ray crystal structures of related
around the ethylene groups (defined by O–C–C–O) are all metallacrown ether complexes.[42–46]
Eur. J. Inorg. Chem. 2008, 4710–4718
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. B. Owens Jr., D. C. Smith Jr., C. H. Lake, G. M. Gray
FULL PAPER
Table 2. Selected bond angles [°] for cis-1, cis-2, and trans-2.
cis-1
cis-2
trans-2
About metal
P1–M–P2
Cl1–M–Cl2
Cl1–M–P1
Cl1–M–P2
Cl2–M–P2
Cl2–M–P1
98.68(6)
98.76(6)
173.01(4)
178.99(5)
87.50(5)
89.88(5)
91.13(5)
91.50(5)
88.91(6)
177.75(6)
83.53(6)
170.71(6)
88.85(6)
87.79(8)
167.53(8)
88.41(7)
170.38(7)
86.61(7)
About P
M–P1–C1
M–P1–C9
117.86(19)
113.10(18)
108.23(18)
102.5(2)
107.6(3)
106.9(3)
111.8(2)
117.8(2)
115.1(2)
103.5(3)
101.1(3)
105.7(3)
114.7(2)
115.9(2)
112.9(2)
104.8(3)
106.7(3)
100.3(3)
111.25(18)
116.34(16)
115.07(16)
102.0(2)
107.8(2)
103.2(2)
111.83(17)
114.91(16)
118.01(17)
105.6(2)
100.8(2)
104.0(2)
M–P1–C15
C1–P1–C9
C1–P1–C15
C9–P1–C15
M–P2–C8
M–P2–C21
M–P2–C27
C8–P2–C21
C8–P2–C27
C21–P2–C27
110.0(2)
124.16(18)
107.51(19)
106.5(3)
106.7(3)
100.5(3)
Figure 3. ORTEP drawing of the molecular structure of trans-2.
Thermal ellipsoids are drawn at the 50% probability level, and hy-
drogen atoms are omitted for clarity.
Around ring
Table 1. Selected bond lengths [Å] for cis-1, cis-2, and trans-2.
P1–C1–C2
C1–C2–O1
C2–O1–C3
O1–C3–C4
C3–C4–O2
C4–O2–C5
O2–C5–C6
C5–C6–O3
C6–O3–C7
O3–C7–C8
C7–C8–P2
115.7(4)
110.5(5)
115.4(5)
113.7(5)
111.0(6)
113.7(6)
111.2(6)
108.8(6)
115.3(5)
108.5(5)
114.1(4)
121.2(5)
112.5(7)
110.3(7)
111.6(9)
116.1(11)
121.4(13)
120.0(15)
115.3(9)
115.4(8)
106.3(6)
114.9(5)
118.8(4)
111.3(5)
117.1(4)
113.7(5)
109.2(5)
111.8(4)
109.8(5)
114.4(5)
114.7(4)
110.1(4)
117.6(4)
cis-1
cis-2
trans-2
About metal
M–Cl2
M–Cl1
M–P1
M–P2
2.3425(15)
2.3595(15)
2.2568(16)
2.2763(15)
2.3526(19)
2.339(2)
2.2476(19)
2.2447(19)
2.2838(13)
2.3098(13)
2.3118(14)
2.3143(14)
About P
P1–C1
P1–C15
P1–C9
P2–C8
P2–C21
P2–C27
1.821(5)
1.815(5)
1.826(5)
1.827(6)
1.834(5)
1.821(6)
1.838(7)
1.821(7)
1.816(7)
1.803(7)
1.805(7)
1.826(7)
1.824(5)
1.817(5)
1.817(5)
1.834(5)
1.819(5)
1.828(5)
Table 3. Selected torsion angles [°] for cis-1, cis-2, and trans-2.
cis-1 cis-2 trans-2
159.2(4) –32.4(7) 30.5(5)
Around ring
M–P1–C1–C2
P1–C1–C2–O1
C1–C2–O1–C3
C2–O1–C3–C4
O1–C3–C4–O2
C3–C4–O2–C5
C4–O2–C5–C6
O2–C5–C6–O3
C5–C6–O3–C7
C6–O3–C7–C8
O3–C7–C8–P2
C7–C8–P2–M
C8–P2–M–P1
P2–M–P1–C1
C1–C2
C2–O1
O1–C3
C3–C4
C4–O2
O2–C5
C5–C6
C6–O3
O3–C7
C7–C8
1.518(7)
1.418(7)
1.418(7)
1.495(9)
1.411(7)
1.401(7)
1.485(10)
1.404(7)
1.416(7)
1.526(8)
1.494(10)
1.369(9)
1.453(7)
1.441(6)
1.398(7)
1.500(8)
1.423(7)
1.407(6)
1.494(8)
1.405(6)
1.439(6)
1.498(7)
168.7(4)
102.0(6)
–93.0(7)
72.9(7)
175.0(6)
164.6(6)
–73.8(8)
–171.2(6)
172.8(5)
65.3(6)
–72.8(8)
–177.4(7)
–174.2(9)
59.2(15)
86.1(17)
175.3(11)
47(2)
60.2(15)
–176.2(8)
174.0(6)
–35.8(7)
–84.4(3)
141.5(3)
–176.8(4)
86.5(6)
93.6(6)
1.445(11)
1.403(13)
1.411(13)
1.276(15)
1.473(16)
1.391(12)
1.407(9)
–75.9(7)
179.4(5)
177.3(5)
74.4(6)
–87.3(6)
–85.4(6)
–174.4(3)
–34.0(5)
11.8(4)
1.500(10)
40.0(5)
–123.7(2)
39.1(2)
The oxygen–oxygen distances for cis-1, cis-2 and trans-2
and for the related metallacrown ethers, cis-[Mo(CO)4-
{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}], cis-4, and trans-
[Mo(CO)4Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}], trans-4,
–11.1(4)
(Table 4) allow the cavity sizes in the various metallacrown plexes with cis-chelated phosphanes (cis-1, cis-2, cis-4) have
ether rings to be compared. The distances between adjacent shorter O3···O9 distances than do the complexes with trans-
oxygens (O3···O6 and O6···O9) in all of the complexes chelated phosphane (trans-2, trans-4). The differences in the
range from 2.842 to 3.052 Å and are on average 2.942 Å. O3···O9 distances reflect the larger chelate bite angles of the
These distances are consistent with pseudo-gauche configu- trans-chelated bis(phosphane)-polyether ligands. The fact
rations about the O–C–C–O groups. The distances between that this distance is larger in trans-4 than in trans-2 is most
nonadjacent oxygens strongly depend on the coordination likely a result of the steric interactions between axial CO
geometry at the metal center.[42–46] As expected, the com- ligands with the metallacrown ring in trans-4.
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Eur. J. Inorg. Chem. 2008, 4710–4718

cis–trans Isomerizations in Pd Metallacrown Ether Complexes
Table 4. Oxygen–oxygen distances in cis-1, cis-2, trans-2, cis-4, and which the Cl– ligands are opposite to the phosphanes and
trans-4.
the positive ∆S suggests that the cis isomer is more ordered
due to crowding. Such crowding is observed in the X-ray
crystal structures of cis-2, discussed above, and of cis-4,[42]
where the Ph2P(CH2CH2O)3CH2CH2PPh2 ligand is packed
only in one quadrant of the complex. In contrast, in the X-
ray crystal structures of trans-2, discussed above, and of
trans-4,[47] the Ph2P(CH2CH2O)3CH2CH2PPh2 ligand is
spread out to cover a hemisphere of the complex. Crowding
in cis-metallacrown ethers has also been used to explain
the unexpected conformation equilibria in solutions of cis-
Mo(CO)4{Ph2P(CH2CH2O)5CH2CH2PPh2} and of cis-PtCl2-
{Ph2P(CH2CH2O)5CH2CH2PPh2} metallacrown ethers.[47]
O1···O3 [Å]
O1···O2 [Å]
O2···O3 [Å]
cis-1
5.067
4.766
5.309
4.480
5.527
3.003
2.863
3.001
2.842
3.052
2.911
2.865
2.997
2.883
3.031
cis-2
trans-2
cis-4
trans-4
The relative orientations of the metallacrown ether ring,
as defined by the plane containing the ether oxygens, and
the metal complex, as defined by the least-squares plane of
the metal and the two phosphorus atoms, are also of inter-
est. For the complexes with cis-chelated phosphanes (cis-1,
cis-2, cis-4), the dihedral angles between these planes are
95.5°, 94.6°, and 95.1°, respectively. The similar angles sug-
gest that there is little interaction between the metallacrown
ether ring and the metal center in the cis-metallacrown
ethers. In contrast, the dihedral angles between the two pla-
nes in the trans-metallacrown ethers, trans-2 (3.6°) and
trans-4 (68.0°), are very different. This difference is most
likely due to the fact that the metallacrown ether ring in a
complex with trans-chelated phosphanes wraps around the
metal centers, and thus their orientations are strongly influ-
enced by the nature of the metal center.
Figure 4. Plot of –RlnK vs. 1/T for the cis–trans isomerization of
cis-1.
2.3 Thermodynamics of the cis–trans Isomerization of cis-1
2.4 Kinetics of the cis–trans Isomerization of cis-1
The cis–trans equilibrium constants were calculated at
four temperatures from the equilibrium concentrations of
monomeric cis-1 and of both monomeric and oligomeric
trans-1 and are given in Table 5. The cis–trans equilibrium
constants increase with increasing temperature.
The kinetic parameters for the forward and reverse reac-
tions of the cis–trans isomerization of cis-1, have been de-
termined using an Eyring analysis. The conversion of cis-1
to trans-1 at four temperatures (331, 341, 348, and 356 K)
was followed with 31P{1H} NMR spectroscopy. As shown
in Figure 5, although the concentration of the solution was
low, a small amount of the oligomeric trans-1 was still
formed. The concentration of cis-1 and trans-1 were deter-
mined from the integrations of the resonances at δ =
27 ppm (monomeric cis-1), 16 ppm (monomeric trans-1)
and 15 ppm (oligomeric trans-2) and the initial concentra-
tion of the cis-1. The reversible first-order rate constants
for the forward reactions at each of the temperatures were
extracted from plots of –ln(1-[trans-1]/[trans-1]e) vs. time. A
plot of this type for the 331 K data is shown in Figure 6.
The reversible first-order rate constants for the reverse reac-
Table 5. Equilibrium constants for the cis–trans equilibrium in 1 at
various temperatures.
Temp. [K]
Integration Integration [cis-1]
[trans-2]
()
K
cis-1
trans-1
()
326
341
348
356
392
422
1.00
1.00
1.00
1.00
1.00
1.00
0.323
0.560
0.713
0.983
2.93
0.0677
0.0574
0.0523
0.0452
0.0228
0.0121
0.0219
0.0322
0.0373
0.0444
0.0668
0.0775
0.323
0.560
0.713
0.983
2.93
6.42
6.42
A plot of –RlnK vs. 1/T is linear over the temperature tions were calculated from the reversible first-order rate
range, as shown in Figure 4, and gives both the enthalpy constants for the forward reactions and the equilibrium
(∆H) and entropy (∆S) changes for the cis–trans equilib- constants.
rium of 1 (∆H = +35.8Ϯ0.2 kJ/mol; ∆S = +100.2Ϯ0.8 J/
The kinetic parameters for the activation process were
molK). These values are similar to those previously re- determined from plots of the type shown in Figure 7. The
ported by Verstuyft and Nelson for cis–trans equilibria of ∆H and ∆S for the equilibrium determined from these
Pd{p-XC6H4)2PMe}2Cl2 (X = Cl, H, Me) complexes in the kinetic parameters (∆H = ∆H‡ – ∆H‡ = +36Ϯ3 kJ/mol;
f
r
same solvent (∆H = +14.6, 26.8 and 33.9 kJ/mol; ∆S = ∆S = ∆S‡f – ∆S‡r = +98Ϯ7 J/molK) are in excellent agree-
+44.4, 79.1 and 103.8 J/molK, respectively).[27] As was pre- ment with the ∆H and ∆S obtained from the data reported
viously suggested by Verstuyft and Nelson, the positive ∆H in Table 5 (∆H = +35.8Ϯ0.2 kJ/mol; ∆S = +100.2Ϯ0.8
indicates that stronger bonding occurs in the cis isomer in J/molK).
Eur. J. Inorg. Chem. 2008, 4710–4718
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S. B. Owens Jr., D. C. Smith Jr., C. H. Lake, G. M. Gray
FULL PAPER
Figure 5. 31P{1H} NMR spectra of the isomerization of cis-1 to trans-1 at 331 K.
2.5 Mechanism of cis–trans and Monomer–Oligomer
Equilibria of cis-1
The most interesting observation of our previous studies
of the simultaneous cis–trans and monomer–oligomer equi-
libria of cis-PdCl2{Ph2P(CH2CH2O)nCH2CH2PPh2-P,PЈ}
(n = 3 (cis-1), 4, 5) metallacrown ethers was that the dimer-
ization and oligomerization equilibrium constants of the
metallacrown ethers were much smaller than were those for
cis-PdCl2{Ph2P(CH2)12PPh2-P,PЈ} even though the chelate
rings sizes in the three metallacrown ethers bracketed those
in the bis(phosphane) complex.[39,40] The dimerization and
oligomerization reactions for any complex require that a
phosphane dissociate from a palladium and then attack an-
other palladium to form the next higher oligomer (oligo-
merization). This attack could form either cis or trans cen-
ters in the oligomer. If the dissociated phosphane instead
recombines with the original palladium, this can either re-
generate the monomeric cis complex (recombination) or
form a monomeric trans complex (isomerization).
Figure 6. Reversible first order kinetics plot of the conversion of
cis-1 to trans-1 at 331 K.
Although the dimerization and oligomerization reactions
must involve cleavage of a Pd–P bond, other mechanisms
are possible for the isomerization reaction. However, during
the studies of the cis–trans isomerization of cis-1 that are
reported in this paper, we observed that cis-1 simultaneously
converts to a mixture of trans-1 and cyclic oligomers of
trans-1 at higher concentrations and that this conversion
still obeys reversible first order kinetics. This behavior sug-
gests that the rate-determining step for both the cis–trans
and monomer–oligomer equilibria is the same and thus
must involve cleavage of a Pd–P bond.
The large differences in the equilibrium constants for the
dimerization and oligomerization reactions for the cis-
PdCl2{Ph2P(CH2CH2O)nCH2CH2PPh2-P,PЈ} (n = 3 (cis-1),
4, 5) and cis-PdCl2{Ph2P(CH2)12PPh2-P,PЈ} complexes can-
not be due to differences in the palladium-phosphane bond
strengths because the phosphanes are nearly identical in the
four complexes. Instead, the differences in the equilibrium
constants must be due to differences in the relative rates of
the recombination, isomerization and oligomerization reac-
Figure 7. Results from an Eyring analysis of the cis–trans isomer-
ization of cis-1.
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cis–trans Isomerizations in Pd Metallacrown Ether Complexes
tions. These reactions would be expected to proceed by an 2.6 Kinetics of the trans–cis Isomerization of trans-2
associative mechanism because the complexes are 16 val-
ence electron square-planar complexes.[48] In the metalla-
As mentioned in the Introduction, trans-Pt(phosphane)2-
crown ether complexes, the transition state might be ex- Cl2 complexes, unlike cis-Pd(phosphane)2Cl2 complexes,
pected to involve partial dissociation of the palladium- can be isolated and purified without isomerization occur-
phosphane bond and partial formation of a palladium–oxy- ring. To gain insight into the mechanisms of the isomeriza-
gen bond to an adjacent ether oxygen. The hemilabile coor- tion reactions of the two types of complexes, we have
dination of one of the ether oxygens to the palladium would studied the kinetics of the trans–cis isomerization of 2 using
keep the free phosphane relatively close to the palladium, 31P{1H} NMR spectroscopy. No isomerization occurred in
favoring recombination and isomerization over oligomer- pure 1,2-[D2]tetrachloroethane at 348 K, however, upon ad-
ization. In contrast, in the bis(phosphane) complex, this dition of a drop of a 12  solution of DCl in D2O to a
hemilabile oxygen coordination could not occur, and the solution of trans-2 in 1,2-[D2]tetrachloroethane, isomeriza-
recombination reactions would be less favored.
tion of the complex is observed. The relative amounts of
If the transition state for both reactions involves partial each isomer were measured as a function of the integrals of
dissociation of the palladium-phosphane bond and partial trans peaks located at δ = 13 ppm and the cis peaks located
formation of a palladium–oxygen bond, the ∆H‡ value at 7 ppm. The time-resolved 31P{1H} NMR of trans-
f
should be significantly less than the bond dissociation en- [PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] in 1,2-[D2]-
ergy for a palladium-phosphane bond. There is little data tetrachloroethane at 348 K are shown in Figure 8. Analysis
available in the literature for the strengths of Pd–P of this data showed that the acid-catalyzed isomerization of
bonds[49–53] and none for Pd–O(ether) bonds; however, one 2 followed zero-order kinetics and exhibited an excellent fit
study has reported
a “ligand bonding power” of to a plot of [trans-2] vs. time. (Figure 9).
162 kJmol–1 for the Pd–PEt3 bonds in cis-[Pd(PEt3)2Cl2]
The fact that the isomerization of trans-2 does not occur
complex based on microcalorimetry data,[53] and several at an observable rate at 348 K in 1,2-[D2]tetrachloroethane
computational studies performed on these types of sys- in the absence of an acid catalyst is consistent with our
tems[49,52] have given similar calculated bond strengths. The earlier observations that the reactions of PtCl2(cod) and
fact that the ∆H‡ for cis-1 (81Ϯ2 kJmol–1) is significantly Ph2P(CH2CH2O)nCH2CH2PPh2 (n = 3, 4, 5) yield only the
f
less than the “ligand bonding power” of 162 kJmol–1 is monomeric cis metallacrown ethers. Based on the above
consistent with the proposed transition state.
studies with cis-1, the kinetic product of the reactions
The fact that the ∆H‡ for cis-1 is less that the “ligand would be expected to be the cis-PtCl2{Ph2P(CH2CH2O)n-
f
bonding power” cannot be explained by solvation effects. If CH2CH2PPh2-P,PЈ} (n = 3, 4, 5) metallacrown ethers, and
solvation is important, the decrease in solvation in the polar these complexes should neither isomerize nor oligomerize
1,2-[D2]tetrachloroethane upon isomerization should result at ambient reaction temperature.
in ∆H‡ for cis-1 being larger than the “ligand bonding
Although the isomerization of trans-2 does not occur in
f
power” because cis-1 is more polar than is trans-1.
1,2-[D2]tetrachloroethane at 348 K in the absence of acid,
Figure 8. 31P{1H} NMR spectra of the isomerization of trans-2 to cis-2.
Eur. J. Inorg. Chem. 2008, 4710–4718
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4715

S. B. Owens Jr., D. C. Smith Jr., C. H. Lake, G. M. Gray
FULL PAPER
cis-Mo(CO)4(phosphane)2 complexes and the 31P{1H} NMR inte-
grations of the 31P{1H} NMR resonances were observed to be pro-
portional to the molarities of the complexes in the solutions.
[40]
The starting materials: Ph2P(CH2CH2O)3CH2CH2PPh2
Pt(PhCN)2Cl2
and
[54,55]
and the cis-[PtCl2{Ph2P(CH2CH2O)3CH2CH2-
PPh2-P,PЈ}] (cis-2) metallacrown ether[41] were prepared using
literature procedures.
cis-[PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] (cis-1): A mix-
ture of 0.434 g (2.45 mmol) of palladium(II) chloride and 1.30 g
(2.45 mmol) of Ph2P(CH2CH2O)3CH2CH2PPh2 was dissolved in
25 mL of a 2:1 acetonitrile/tetrahydrofuran mixture at ambient
temperature. After stirring overnight, a pale yellow precipitate had
formed. The solid was collected, washed with acetonitrile
(3ϫ50 mL) and dried in vacuo (0.02 Torr at 298 K) for 48 h yield-
ing 1.21 g (69.8%) of cis-1. The filtrate was evaporated to dryness
(21 Torr at 315 K), and the orange oily residue was purified by
column chromatography on silica gel with ethyl acetate as the elut-
ant (Rf = 0.35) to yield 0.450 g (26.0%) of an equilibrium mixture
Figure 9. Plot of the concentration of [trans-2] vs. time for the
trans–cis isomerization of trans-2 showing that the isomerization is
zero order in trans-2.
it does occur in [D]chloroform at the same temperature. The
most likely explanation is that the chloroform contains a
small amount of acid and not that the mechanism is dif-
ferent in the two solvents.
1
of 1. H NMR of cis-1 ([D]chloroform, 300 MHz): δ = 2.18–2.92
(m, 4 H, P-CH2) 3.42–4.23 (m, 12 H, O-CH2), 7.79–7.09 (m, 20 H,
C6H5-P). 31P{1H} NMR of cis-1 ([D]chloroform, 121.5 MHz): δ =
24.37 (s) ppm. C32H36Cl2O3P2Pd (706.05): calcd. C 54.32, H 5.08,
Cl 10.02; found C 54.18, H 5.19, Cl 9.94.
trans-[PtCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,PЈ}] (trans-2):
A
3. Conclusions
solution of 0.723 g (1.36 mmol) of Ph2P(CH2CH2O)3CH2CH2PPh2
in 125 mL of dichloromethane was slowly added to a solution of
0.644 g (1.36 mmol) of Pt(PhCN)2Cl2 in 400 mL of dichlorometh-
ane, and the resulting solution was stirred at ambient temperature
for 20 h. Then, the solution was reduced in volume to 10 mL, and
the concentrate was poured into 500 mL of hexanes. The pale yel-
low precipitate was collected by filtration, washed with hexanes
(3ϫ30 mL) and dried in vacuo (0.02 Torr at 298 K) for 48 h to
yield 0.868 g (80.1%) of a mixture of cis-2 and trans-2. Chromatog-
raphy on silica with 5:1 dichloromethane/ethyl acetate mixture (Rf
= 0.45) yielded trans-2. Recrystallization from a hexanes and
dichloromethane solution yielded 0.431 grams (39.8%) of analyti-
cally pure trans-2. C32H36Cl2O3P2Pt (795.11): calcd. C 48.25, H
4.55, Cl 8.90; found C 48.18, H 4.60, Cl 8.99. 1H NMR ([D]chloro-
form, 300 MHz): δ = 2.18–2.92 (m, 4 H, PCH2), 3.42–4.53 (m, 12
H, OCH2), 7.79–7.09 (m, 20 H, C6H5P) ppm. 31P{1H} NMR ([D]
chloroform, 121.5 MHz): δ = 11.03, (s&d, |1J(Pt-P)| = 2574 Hz)
ppm.
The cis–trans isomerization equilibrium of the palladi-
um(II) metallacrown ether cis-1 follows reversible first-or-
der kinetics, and both the thermodynamic and kinetic pa-
rameters for this reaction have been determined. These pa-
rameters, when analyzed in light of results from studies of
monomer–oligomer equilibria and the X-ray crystal struc-
tures of cis-1, cis-2, and trans-2, suggest that the transition
state involves cleavage of a Pd–P bond and formation of
Pd–O(ether) bond to an adjacent ether oxygen. In contrast,
the trans–cis isomerization of platinum(II) metallacrown
ether trans-2 to cis-2 does not occur in the absence of an
acid catalyst, and the acid catalyzed trans–cis isomerization
is zero order in complex. These studies clearly demonstrate
that palladium(II) and platinum(II) complexes with iden-
tical ligands and coordination geometries can exhibit very
different reaction mechanisms for the same reactions under
identical reaction conditions. If this is a general trend, it
could help to explain why similar palladium(II) and plati-
num(II) complexes can exhibit very different catalytic ac-
tivities and selectivities for the same reactions.
4.2 Collection, Solution and Refinement of X-ray Diffraction Data
All crystals in this study were grown by slow evaporation of their
dichloromethane/hexanes solutions. Suitable X-ray quality single
crystals of each compound were individually sealed into thin-
walled glass capillaries under aerobic conditions. Each crystal was
then mounted and aligned upon an Enraf–Nonius CAD4 single
crystal diffractometer with κ-geometry (graphite monochromated
Mo-Kα radiation, λ = 0.71073 Å). Details of the data collections
are collected in Table 6. All data were corrected for Lorentz and
polarization effects.
4. Experimental Section
4.1 General Procedures
Multinuclear 31P{1H}, 13C{1H}, and 1H NMR spectra were re-
corded on a Bruker ARX300 FT-NMR variable temperature spec-
trometer. All downfield chemical shifts are reported as positive. The
1H NMR and 13C{1H} chemical shifts were referenced either to
tetramethylsiliane or [D]chloroform (δ =77.23 ppm). The 31P{1H}
chemical shifts were referenced to external 85% phosphoric acid
The lattice parameters and the systematic absences of cis-1 and
trans-2 (h0l for h + l = 2n + 1 and 0k0 for k = 2n + 1) uniquely
defined the space group of both these crystals as P21/n (No. 14).
The lattice parameters and systematic absences of cis-2 (hkl for h
+ k = 2n +1 and h0l for l = 2n + 1) were consistent with either the
(capillary) in [D]chloroform. Atlantic Microlabs of Norcross, GA centrosymmetric space group C2/c (No. 15) or the noncentrosym-
performed all elemental analyses. Quantitative 31P{1H} NMR spec-
tra were obtained using an inverse gate pulse sequence with an 30°
pulse and a delay of 20 s. This method was tested on mixtures of
metric space group Cc (No. 9). Intensity statistics favored the cen-
trosymmetric choice of C2/c. The successful solution and refine-
ment of the crystal structure later verified this.
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cis–trans Isomerizations in Pd Metallacrown Ether Complexes
Table 6. Crystal and structure refinement data for cis-1, cis-2, and trans-2.
cis-1
cis-2
trans-2
Empirical formula
CCDC deposit no.
Formula weight
Temperature
C32H36Cl2O3P2Pd
661698
707.85
293(2) K
0.71073 Å
C32H36Cl2O3P2Pt
661700
796.54
293(2) K
0.71073 Å
C32H36Cl2O3P2Pt
661699
796.54
293(2) K
0.71073 Å
Wavelength
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/n
C2/c
P21/n
a = 11.780(2) Å
b = 16.159(3) Å
c = 16.911(3) Å
α = 90°
a = 28.277(6) Å
b = 11.603(2) Å
c = 21.706(4) Å
α = 90°
a = 16.496(3) Å
b = 10.080(2) Å
c = 19.789(4) Å
α = 90°
β = 101.23(3)°
γ = 90°
β = 109.08(3)°
γ = 90°
β = 109.79(3)°
γ = 90°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
3157.3(11) Å3
4
6730(2) Å3
3096.4(11) Å3
4
8
1.489 Mg/m3
0.890 [mm]–1
1448
1.572 Mg/m3
1.709 Mg/m3
4.840 mm–1
1576
4.453 mm–1
3152
0.2ϫ0.2ϫ0.15 mm3
2.17 to 22.48°
0 Յ h Յ 12, 0 Յ k Յ 17,
–18 Յ l Յ 17
4348
0.25ϫ0.2ϫ0.1 mm3
1.52 to 22.47°
–20 Յ h Յ 30, –8 Յ k Յ 12,
–23 Յ l Յ 22
5626
0.25ϫ0.2ϫ0.2 mm3
2.19 to 22.50°
0 Յ h Յ 17, –10 Յ k Յ 10,
–21 Յ l Յ 20
8111
Reflections collected
Independent reflections
Completeness to θ = 22.48°
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
4116 [R(int) = 0.0410]
99.7%
4389 [R(int) = 0.0533]
100.0%
4034 [R(int) = 0.0306]
99.8%
none
full-matrix least-squares on F2
4389/0/362
4116/0/362
0.995
R1 = 0.0337, wR2 = 0.0759
R1 = 0.0834, wR2 = 0.0901
4034/0/362
1.071
R1 = 0.0231, wR2 = 0.0565
R1 = 0.0327, wR2 = 0.0591
0.982
R1 = 0.0321, wR2 = 0.0783
R1 = 0.0555, wR2 = 0.0839
All crystallographic calculations were performed with the Siemens
SHELXTL-PC program package.[56] Heavy atoms were located
using a Patterson method and the remaining atoms were located
from difference Fourier analysis. All hydrogen atoms were placed
in calculated positions with the appropriate molecular geometry
and a δ(C–H) = 0.96 Å. The isotropic thermal parameter associated
with each hydrogen atom was fixed equal to the Ueq of the carbon
atom to which it was bound. The positional and anisotropic ther-
mal parameters for all non-hydrogen atoms were refined. The tool
SQUEEZE, from PLATON,[57,58] was used to calculate the number
of electrons of disordered solvent for the one void in trans-2. The
total number of electrons was determined to be 42.1, which equates
to approximately one molecule of dichloromethane.
1 at δ = 15 ppm (Itrans oligo) and the initial concentration of cis-1
([cis-1]i) using Equations (1) and (2).
(1)
(2)
The cis–trans equilibrium constants were calculated from the equi-
librium concentrations using Equation (3) and are given in Table 5.
The cis–trans equilibrium constants increase with increasing tem-
perature.
CCDC-661698 (for cis-1), -661700 (for cis-2), and -661699 (for
trans-2) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.).
(3)
Rate Constants: Rate constants for the cis–trans isomerizations of
cis-1 were also measured at four temperatures. At each temperature,
a 5-mm screw-capped NMR tube containing 0.042 g (0.058 mmol)
of cis-1 dissolved in 0.60 mL of 1,2-[D2]tetrachloroethane was in-
serted into the NMR probe, which had been preheated at either
58, 68, 75, or 83 °C for 30 min. The sample was then thermally
equilibrated in the probe for an additional 15 min after which
quantitative 31P{1H} NMR spectra were taken at 5 min intervals.
Concentrations of the cis- and trans-metallacrown ethers were cal-
culated from the relative integrations of the 31P{1H} NMR reso-
4.3 31P NMR Studies of the Isomerization Reactions of the Metalla-
crown Ethers
Equilibrium Constants: Equilibrium constants for the cis–trans
isomerizations of cis-1 were measured at four temperatures. At each
temperature, a 5 mm, screw-capped NMR tube containing 0.042 g
(0.058 mmol) of cis-1 dissolved in 0.60 mL of 1,2-[D2]tetrachloro-
ethane was allowed to stand until no change in the ratio of cis-1
to trans-1 was observed (at least 18 h). The equilibrium concentra-
tions of cis-1 and trans-1 were calculated from the integrals of the
31P{1H} NMR resonances of monomeric cis-1 at δ = 27 ppm (Icis), nances of the metallacrown ethers and the initial concentration of
monomeric trans-1 at δ = 16 ppm (Itrans mono) and oligomeric trans- the metallacrown ether as discussed above. Plots of –ln(1 – [trans-
Eur. J. Inorg. Chem. 2008, 4710–4718
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4717

S. B. Owens Jr., D. C. Smith Jr., C. H. Lake, G. M. Gray
FULL PAPER
1]/[trans-1]e) vs. time were linear and gave the reversible 1st-order
rate constants for the forward reactions, kf. The reversible 1st-order
rate constants for the reverse reactions, kr, were calculated from K
= kf/kr using the previously determined equilibrium constants.
[20] D. A. Redfield, J. H. Nelson, J. Am. Chem. Soc. 1974, 96, 6219.
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For trans-2, a drop of a 12  solution of DCl in D2O was added
toasolutionof0.042 g(0.053 mmol)oftrans-2in0.60 mLof1,2-[D2]-
tetrachloroethane in a 5 mm, screw-capped NMR tube under an
inert atmosphere. The tube was then inserted into the NMR probe,
which had been preheated at 75 °C for 30 min. The sample was
then thermally equilibrated in the probe for an additional 15 min
after which quantitative 31P{1H} NMR spectra were taken at
20 min intervals. Concentrations of the cis- and trans-metallacrown
ethers were calculated from the relative integrations of the 31P{1H}
NMR resonances of the metallacrown ethers. A plot of [trans-2] vs.
time was linear and gave the zero-order rate constant.
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Eyring Analysis: Plots of ln(kf/T) vs. (1/T) using the rate constants
obtained at the four temperatures for cis-1 gave the ∆H‡ (slope)
and ∆S‡ (intercept) for the forward reaction while plots of ln(kr/T)
vs. (1/T) using the rate constants obtained at the four temperatures
for cis-1 gave the ∆H‡ (slope) and ∆S‡ (intercept) for the reverse
reaction. Both plots were linear with r values greater than 0.995.
Errors for the kinetic parameters were determined from linear least-
squares analyses of the data.
Supporting Information (see also the footnote on the first page of
this article): Thermodynamic and kinetic data for isomerizations.
[38] D. C. Smith, C. H. Lake, G. M. Gray, Dalton Trans. 2003,
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Acknowledgments
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677.
The authors thank the Petroleum Research Fund, ACS
PRF#35349-AC3, for partial financial support of this project.
D. C. S. Jr. and S. B. O. Jr. also thank the Graduate School at the
University of Alabama at Birmingham for support through a grad-
uate fellowship.
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Received: April 28, 2008
Published Online: September 10, 2008
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