Bis(triphenylphosphine)palladium cycloheptadienynylium tetrafluoroborate: A palladium complex of tropyne

Article abstract of DOI:10.1021/om9600396

Palladium complexes of cyclohepta-3,5-dien-1-yne (4) and cyclohepta-1,5-dien-3-yne (5) have been prepared and converted to a palladium complex of tropyne (6) by treatment with triphenylcarbenium tetrafluoroborate.

Full text of DOI:10.1021/om9600396

Organometallics 1996, 15, 2465-2468
2465
Bis(tr ip h en ylp h osp h in e)p a lla d iu m
Cycloh ep ta d ien yn yliu m Tetr a flu or obor a te: A P a lla d iu m
Com p lex of Tr op yn e
J erzy Klosin,† Khalil A. Abboud, and W. M. J ones*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
Received J anuary 22, 1996X
Palladium complexes of cyclohepta-3,5-dien-1-yne (4) and cyclohepta-1,5-dien-3-yne (5)
have been prepared and converted to a palladium complex of tropyne (6) by treatment with
triphenylcarbenium tetrafluoroborate.
Sch em e 1
In tr od u ction
Transition metal complexes containing six-membered
arynes in their coordination sphere have generated
considerable interest in recent years due to their rich
chemistry and unique structural features.1 We have
been interested for some time in complexes containing
seven-membered ring arynes as ligands which led to the
preparation of a platinum tropyne complex 1,2 its
dibenzannelated analogue,3 a zirconium tropyne com-
plex 2,4 and a Pt-Mo bimetallic tropyne complex 3.5
ylphosphine)palladium in THF solution (Scheme 1). The
mixture of complexes was characterized by multinuclear
spectroscopy, HRMS, IR, elemental analysis, and, in the
1
case of 4, single-crystal X-ray analysis. In the H NMR
As a continuation of our work in this area we report
at this time the synthesis of a palladium tropyne
complex and its characteristics.
of the mixture of 4 and 5, the hydrogens attached to
the sp3 carbon of the seven-membered rings appear as
3
a doublet at δ 3.67 (H7, J H7-H6 ) 3.6 Hz) for 4 and a
triplet at δ 2.6 (H5′, J H5′-H4′/H6′ ) 6.3 Hz) for 5.
3
Resu lts a n d Discu ssion
Chemical shift assignments to the vinyl hydrogens of
each isomer are based on a 2D COSY experiments. The
31P{1H} NMR of the mixture of 4 and 5 exhibits two
Palladium cycloheptadienyne complexes 4 and 5 were
prepared (as a 7:1 mixture) from a mixture of 1-, 2-, and
3-bromocycloheptatrienes by dehydrobromination with
lithium diisopropylamide in the presence of tris(triphen-
high intensity doublets at δ 31.20 and 30.77 (2J P-P
)
4.6 Hz) for 4 and a singlet at δ 31.52 for 5 as expected
for complexes with C1 and Cs symmetry, respectively.
The IR spectrum of the mixture shows a peak of medium
intensity at 1769 cm-1 which is assigned to the coordi-
nated triple bond of 4. The corresponding peak in the
platinum analogue6 appears at 1712 cm-1. The signifi-
cantly lower value for the stretching frequency of the
coordinated triple bond in the platinum complex pre-
sumably results from a stronger bond between the
platinum and the alkyne ligand. A similar trend in
stretching frequencies is seen in palladium and plati-
num complexes of cyclohexyne (1780 vs 1721 cm-1) and
cycloheptyne (1848 vs 1770 cm-1).7 The triple bond
stretching frequency of 4 (1769 cm-1) is also consider-
† Present address: The Dow Chemical Co., Central Research &
Development, Catalysis Laboratory, Midland, MI 48674.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) (a) Bennett, M. A.; Schwemlein, H. P. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1320. (b) Buchwald, S. L.; Nielsen, R. B. Chem. Rev.
1988, 88, 1047. (c) Hartwig, J . F.; Bergman, R. G.; Andersen, R. A. J .
Am. Chem. Soc. 1991, 111, 3404. (d) Cockcroft, K. J .; Gibson, C. V.;
Howard, J . A. K.; Poole, A. D.; Siemeling, U.; Wilson. C., J . Chem.
Soc., Chem. Commun. 1992, 1668. (e) Houseknecht, K. L.; Stockman,
K.. E.; Sabat, M.; Finn, M. G.; Grimes, R. N., J . Am. Chem. Soc. 1995,
117, 1163. (f) Bennett, M. A.; Wenger, E. Organometallics 1995, 14,
1267. (g) Bennett, M. A.; Hockless, D. C. R.; Wenger, E. Organome-
tallics 1995, 14, 2091. (h) Mashima, K.; Tanaka, Y.; Nakamura, A.
Organometallics 1995, 14, 5642.
(2) Lu, Z.; Abboud, K. A.; J ones, W. M. J . Am. Chem. Soc. 1992,
114, 10991.
(3) Lu, Z.; J ones, W. M. Organometallics 1994, 13, 1539.
(4) Klosin, J .; Abboud, K. A.; J ones, W. M. Organometallics 1995,
14, 2892.
(5) Klosin, J .; Abboud, K.; A.; J ones, W. M. Organometallics 1996,
15, 596.
(6) Lu, Z.; Abboud, K. A.; J ones, W. M. Organometallics 1993, 12,
1471.
(7) Bennett, M. A.; Yoshida, T. J . Am. Chem. Soc. 1978, 100, 1750.
S0276-7333(96)00039-8 CCC: $12.00 © 1996 American Chemical Society

2466 Organometallics, Vol. 15, No. 10, 1996
Klosin et al.
Ta ble 2. Cr ysta llogr a p h ic Da ta for Com p lex 4
A. Crystal Data (298 K)
a, Å
b, Å
c, Å
13.543(3)
17.244(2)
16.124(2)
107.82(1)
3585(1)
â, deg
V, Å3
d
calc, g cm-3 (298 K)
1.336
empirical formula
fw
C43H36P2Pd
721.06
cryst system
space group
Z
monoclinic
P21/c
4
F(000), electrons
1480
0.76 × 0.23 × 0.23
cryst size, mm3
B. Data Collection (298 K)
radiation, λ (Å)
mode
scan range
Mo KR, 0.710 73
ω-scan
symmetrically over 1.2°
about KR1,2 max
offset 1.0 and -1.0 in ω
from KR1,2 max
3-6
bkgd
F igu r e 1. Structure and labeling scheme for 4 with 40%
probability of thermal ellipsoids.
scan rate, deg min-1
2θ range, deg
range of hkl
3-50
0 e h e 0
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 4
-0 e k e 0
-0 e l e 0
6827
6299
0.64
tot reflcns measd
unique reflcns
Bond Lengths (Å)
Pd-P1
Pd-C1
P1-C21
P1-C31
P1-C41
C1-C2
C3-C4
C5-C6
C7-C1
2.305(1)
2.031(4)
1.834(4)
1.838(4)
1.826(4)
1.258(6)
1.364(8)
1.360(11)
1.458(7)
Pd-P2
Pd-C2
P2-C51
P2-C61
P2-C71
C2-C3
C4-C5
C6-C7
2.313(1)
2.039(4)
1.832(3)
1.827(4)
1.833(4)
1.438(8)
1.398(9)
1.407(10)
abs coeff/µ(Mo KR), cm-1
min, max transm
0.852, 0.902
C. Structure Refinement
S, goodness-of-fit
reflcns used, I > 2σ(I)
no. of variables
R, wR, %a
1.477 08
5010
415
4.11, 4.68
0.0000
0.0000
-0.3
R
int, %
max shift/esd
Bond Angles (deg)
min peak in diff Fourier map, e Å-3
max peak in diff Fourier map, e Å-3
P1-Pd-P2
P2-Pd-C1
C1-Pd-C2
C(1)-C7-C6
C2-C1-C7
C5-C4-C3
C7-C6-C5
109.53(4)
104.0(1)
36.0(2)
112.5(5)
134.7(5)
128.1(6)
130.0(6)
P1-Pd-C1
P2-Pd-C2
C2-Pd-P1
C1-C2-C3
C4-C3-C2
C6-C5-C4
146.3(1)
139.7(1)
110.3(1)
134.4(4)
117.4(5)
132.8(6)
0.4
a
Relevant expressions are as follows, where in the footnote Fo
and Fc represent, respectively, the observed and calculated
structure-factor amplitudes. Function minimized was w(|Fo| -
|Fc|)2, where w ) (σ(F))-2. R ) ∑(||Fo - |Fc||)/∑|Fo|. wR ) [∑w(|Fo|
- |Fc|)2/∑|Fo| ]1/2. S ) [∑w(|Fo| - |Fc|)2/(m - n)]1/2
.
2
ably lower than that of the corresponding palladium
complex of cycloheptyne (1848 cm-1), which, presum-
ably, reflects both greater ring strain in cyclohepta-
dienyne relative to cycloheptyne (MMX calculations
predict 3,5-cycloheptadien-1-yne to be about 5 kcal/mol
more strained than cycloheptyne) and conjugation with
the double bonds.
the shape of a shallow boat as shown by a small dihedral
angle [35.0(6)°] between planes C1sC7sC6 and C2s
C3sC4sC5. Corresponding dihedral angles in cyclo-
heptatriene derivatives are much larger [for example,
93.1° for 2,5-dimethyl-3,4-diphenyl-1,3,5-cycloheptatriene9
and 98.3° for 3-anilino-2-(2,4,6-cycloheptatrien-1-yl)-2-
propenal].10 This difference in dihedral angle values is
presumably caused by significant sp character at the
alkyne carbons of 3 which increases bond angles
C7sC1sC2 [134.7(5)°] and C1sC2sC3 [134.4(4)°].
This distortion, in turn, leads to a flattening of the
seven-membered ring.
For the X-ray crystal structure analysis, complex 4
was crystallized from a toluene/hexane mixture at -16
°C. It crystallizes in a monoclinic P21/c space group.
The structure of 4 is isomorphic with that of the
corresponding platinum complex.6 The thermal el-
lipsoid drawing of 4 is displayed in Figure 1, while
selected bond lengths and angles are listed in Table 1.
Crystal structure data are provided in Table 2. The
coordination geometry around the palladium atom in 4
is square planar. The alkyne bond distance (C1sC2)
in 4 is 1.258(6) Å and is equal, within experimental
error, to alkyne distances in platinum complexes of
cycloheptyne8 [1.283(5) Å], 3,5-cycloheptadien-1-yne6
[1.31(2) Å], and cyclohexyne8 [1.297(8) Å]. Consistent
with two double bonds, the bond lengths of C3sC4 and
C5sC6 are 1.364(8) and 1.360(11) Å, respectively. The
bond angle of 112.5(5)° for C1sC7sC6 is consistent with
sp3 hybridization for C7. The seven-membered ring has
The tropyne complex 6 was prepared by treating a
CD2Cl2 mixture of 4 and 5 with triphenylcarbenium
tetrafluoroborate at -78 °C (Scheme 1). Similar to the
platinum analogue,2 reaction occurred rapidly to give a
deep red solution. After 1 h at -78 °C, multinuclear
NMR spectra revealed clean formation of the palladium
tropyne complex 6 and triphenylmethane. With the
exception of no coupling to Pd which has no magneti-
1
cally active isotope, the H NMR of 5 is almost identical
to that of the platinum tropyne complex 1. As in the
(9) Stegemann, J .; Lindner, H. J . Acta Crystallogr., Sect. B 1979,
35, 2161.
(10) Reichardt, C.; Yun, K.-Y.; Massa,W.; Birkhahn, M.; Schmidt,
R. E. Liebigs Ann. Chem. 1985, 1969.
(8) Robertson, G. B.; Whimp, P. O. J . Am. Chem. Soc. 1975, 97, 1051.

A Palladium Complex of Tropyne
Organometallics, Vol. 15, No. 10, 1996 2467
were obtained on the Finnigan Mat 95Q. Elemental analyses
were performed in the microanalysis lab in the Chemistry
Department at the University of Florida. Melting points were
measured in open capillaries and were not corrected. The
following compounds were prepared as described in the
literature without any modification: Pd(PPh3)3;11 bromocyclo-
heptatrienes.12
platinum analogue, all of the remote ring hydrogens are
shifted significantly upfield from the tropylium ion (δ
9.55) with the hydrogens on C5 and C4/C6 appearing
as a triplet at δ 8.52 (3J H5-H4/H6 ) 9.6 Hz) and a
multiplet at δ 8.32, respectively, and the C3/C7 hydro-
gens appearing as a doublet at δ 7.83 (3J H3/H7-H4/H6
)
6.3 Hz). The chemical shift assignment for the C5
hydrogen is based on its area (one-half that of the other
two), and further chemical shift assignments are based
on decoupling experiments. The corresponding reso-
nances for the remote ring hydrogens in the platinum
complex appear at δ 8.64, 8.34, and 7.66, respectively.2
From these proton chemical shifts, it would appear that
bis(triphenylphosphine)palladium is about as effective
as an electron donor as is its platinum counterpart.
Resonances in the 13C{1H} NMR for the palladium
complex are also very similar to those of the platinum
analogue. Probably the most characteristic resonance
is that of the carbons attached to the palladium atom
which appear at δ 173.34 ppm as a doublet of doublets
due to coupling with two nonequivalent phosphorus
P r ep a r a t ion of (1,2-η2-cycloh ep t a -3,5-d ien -1-yn e)b is-
(tr ip h en ylp h osp h in e)p a lla d iu m (4) a n d (1,2-η2-cycloh ep -
ta -1,5-d ien -3-yn e)bis(tr ip h en ylp h osp h in e)p a lla d iu m (5).
Tris(triphenylphosphine)palladium (0.5 g, 0.56 mmol) and
LDA (100 mg, 0.934 mmol) were dissolved in a minimum
amount of THF (ca. 16 mL). To this solution was added very
slowly (1 h) 160 mg (0.934 mmol) of bromocycloheptatriene
(mixture of three isomers) in 3 mL of THF at room tempera-
ture. After addition was complete the mixture was stirred for
1 h and then the solvent was removed in vacuum. The residue
was dissolved in 14 mL of toluene. To this solution 36 mL of
hexane was added, and the solution was filtered through a
cannula. The filtrate was set aside in the freezer (-16 °C) for
2 days to give 210 mg (52% yield) of pure 4 and 5, mp 114-
115 °C (dec). IR (KBr): 3053, 3001, 1769 (coordinated CtC),
1478, 1433, 1307, 1181, 1093, 1026, 744, 696 cm-1
.
1H NMR
nuclei (2J C1/C2-Ptrans ) 85.3 Hz, 2J C1/C2-P ) 4.2 Hz). The
data for 4 (C6D6): δ 7.6-7.8 and 6.95-7.1 (m, PPh3), 5.96 (m,
1H, H3), 5.8 (m, 2H, H4 and H5), 5.23 (m, 1H, H6) 3.7 (d, 2H,
3J H7-H6 ) 3.6 Hz, H7). 1H NMR data for 5 (C6D6): δ 7.6-7.8
cis
31P{1H} NMR and the 19F NMR exhibit singlets at δ
24.32 and -151.84, respectively.
3
and 6.95-7.1 (m, PPh3), 6.74 (d, 2H, J H3′/H7′-H4′/H6′ ) 8.1 Hz,
In contrast to its platinum analogue which is stable
to at least 70 °C, the palladium tropyne complex is very
thermally unstable. For example, in CD2Cl2 it decom-
poses at -35 °C with a half-life of about 3 h and
decomposes slowly, even at temperatures as low as -50
°C.
3
H3′/H7′), 5.15 (m, 2H, H4′/H6′), 2.65 (t, 2H, J H5′-H4′/H6′ ) 6.3
Hz, H5′). 13C{1H} NMR (C6D6): δ 137.65 (m, PPh3-ipso),
134.36, (dd, J C-P ) 15.1, J C-P ) 4.5 Hz, PPh3), 129.11 (s, PPh3),
128.26 (d, J C-P ) 9.4 Hz, PPh3), 127.26 (d, J C-P ) 9 Hz), 126.4,
123.78 (t, J C-P ) 9.5 Hz), 120.34 (m), 114.89 (m), 114.1 (d, J C-P
) 4 Hz), 113.23 (d, J C-P ) 4.8 Hz), 30.31, 29.72 (t, J C-P ) 9.4
Hz). 31P{1H} NMR (C6D6): δ 31.52 (s, complex 5), 31.20 (d,
2J P-P ) 4.6 Hz), 30.77 (d, 2J P-P ) 4.6 Hz). HRMS (FAB): calcd
for (M + 1)+, m/ e 721.1405; found, m/ e 721.1528. Anal. Calcd
for C43H36P2Pd: C, 71.62; H, 5.03. Found; C, 71.77; H, 5.07.
Attempts to effect productive reactions with 6 failed.
Treatment of 6 with KBEt3H or LiAl(O-t-Bu)3H at -78
°C followed by slow warming to room temperature
showed no detectable amount of 4 or 5; only a complex
mixture was formed from which no single product could
be characterized. Both of these reagents reduce 1 to
its cycloheptadienyne precursors.2 Similarly, treatment
of 6 with HBr at -78 °C followed by warming gave no
σ complex comparable to the platinum analogue. And,
finally, on the chance that the reason for decomposition
might be low-temperature dissociation of the tropyne
moiety from the metal center, a CD2Cl2 solution of 6
was warmed in the presence of cyclopentadiene which
would be expected to trap the electorphilic tropyne
dienophile. Again, only decomposition was observed.
P r epar ation of (1,2-η2-tr opyn e)bis(tr iph en ylph osph in e)-
p a lla d iu m (6). An NMR tube equipped with a joint and a
stopcock (mini Schlenk tube) was charged with 70 mg (0.097
mmol) of 4 and 5. This complex was dissolved in 0.2 mL of
CD2Cl2, and subsequently the solution was frozen in liquid N2.
To this apparatus 33 mg (0.098 mmol) of triphenylcarbenium
tetrafluoroborate dissolved in 0.4 mL of CD2Cl2 was added via
a syringe, and this solution was frozen. While keeping the
mixture under a nitrogen atmosphere, it was allowed to warm
up to -78 °C. After the mixture had become homogeneous it
was frozen in liquid N2, the apparatus was evacuated and the
NMR tube was sealed. This NMR tube was then warmed to
-78 °C in a dry ice bath before NMR measurements which
were performed at -90 °C. 1H NMR (CD2Cl2): δ 8.52 (t, 1H,
3J H5-H4/H6 ) 9.6 Hz, H5), 8.32 (m, 1H, H4/H6), 7.83 (d,
3J H3/H7-H4/H6 ) 6.3 Hz, H3/H7), 7.0-7.5 (m, 30H, PPh3). 13C-
Exp er im en ta l Section
Gen er a l Meth od s. All experiments involving organome-
tallic compounds were carried out under an atmosphere of
purified N2 using Schlenk, vacuum line, and drybox tech-
niques. Solvents were distilled under nitrogen prior to use,
toluene and THF from sodium benzophenone ketyl, hexane
from sodium benzophenone ketyl/tetraglyme mixture, and
methylene chloride from CaH2. Potassium tert-butoxide,
hydrogen bromide (30 wt % solution in acetic acid), potassium
triethylborohydride (1.0 M solution in THF), lithium tri-tert-
butoxyaluminohydride (1.0 M solution in THF), and triphen-
ylcarbenium tetrafluoroborate were purchased from Aldrich
Chemical Co. and were used as received. NMR spectra were
measured on a Varian XL-300 (FT 300 MHz, 1H; 75 MHz, 13C;
282 MHz, 19F; 121 MHz, 31P). 1H NMR and 13C{1H} NMR
spectra were referenced to the residual solvent peaks and are
reported in ppm relative to tetramethylsilane. 19F NMR
spectra were referenced to external CFCl3. 31P{1H} NMR
spectra were referenced to external 85% H3PO4 in D2O.
Infrared spectra were measured in KBr pellets on a Perkin-
Elmer 1600 FTIR spectrometer. Mass spectra (positive FAB)
{1H} NMR (CD2Cl2): δ 173.34 (dd, 2J C4-Ptrans ) 85.3 Hz, 2J C4-P
cis
) 4.2 Hz, C1/C2), 149.67 (s, br, C4/C6), 143.05 (s, C5), 139.47
(t, 3J C3-P ) 10.2 Hz, C3/C7), 133.35 (t, J C-P ) 6.8, PPh3), 131.91
(m, PPh3-ipso), 130.29 (s, PPh3), 128.33 (t, J C-P ) 4.6, PPh3).
31P{1H} NMR (CD2Cl2): δ 24.32 (s). 19F NMR (CD2Cl2): δ
-151.84 (s). The spectra for 6 are almost identical to those of
the platinum analog (1); therefore peak assignments in the
13C NMR for 6 were based on simple comparison with 1.
Cr ysta llogr a p h ic An a lysis of Com p lex 4. Data were
collected at room temperature on a Siemens R3m/V diffrac-
tometer equipped with a graphite monochromator utilizing Mo
KR radiation (λ ) 0.710 73 Å). A total of 32 reflections with
20.0° e 2θ e 22.0° were used to refine the cell parameters of
each crystal. Four reflections were measured every 96 reflec-
tions to monitor instrument and crystal stability for each data
(11) Giannoccaro, P.; Sacco, A.; Vasapollo, G. Inorg. Chim. Acta
1979, 37, L455.
(12) Fohilsh, B.; Haug, E. Chem. Ber. 1971, 104, 2324.

2468 Organometallics, Vol. 15, No. 10, 1996
Klosin et al.
set (maximum correction on I was 2%). Absorption corrections
were applied on the basis of measured crystal faces using
SHELXTL plus.13 The structure was refined in SHELXTL
plus using full-matrix least squares. The structure of 3 was
solved by the heavy-atom method in SHELXTL plus from
which the location of the Pd atoms were obtained. The rest
of the non-hydrogen atoms were obtained from subsequent
difference Fourier maps. All non-H atoms were refined with
anisotropic thermal parameters, while positions of all of the
H atoms were calculated in ideal positions and their isotropic
thermal parameters were fixed. The linear absorption coef-
ficients were calculated from values from ref 14. Scattering
factors for non-hydrogen atoms were taken from Cromer and
Mann15 with anomalous-dispersion corrections from Cromer
and Liberman,16 while those of hydrogen atoms were from
Stewart, Davidson, and Simpson.17
Ack n ow led gm en t. Support of this research by the
National Science Foundation and the Chevron Research
and Technology Co. is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: A thermal ellipsoid
drawing showing full numbering schemes and tables of atomic
positions and U values, anisotropic thermal parameters for
non-hydrogen atoms, and comprehensive bond lengths and
angles (10 pages). Ordering information is given on any
current masthead page.
OM9600396
(14) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham, England, 1974; Vol. IV, p 55. (Present distributor: D.
Reidel, Dordrecht, The Netherlands.)
(15) Cromer, D. T.; Mann, J . B. Acta Crystallogr. 1968, A24, 321.
(16) Cromer, D. T.; Liberman, D. J . Chem. Phys. 1970, 53, 1891.
(17) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J . Chem. Phys.
1965, 42, 3175.
(13) Sheldrick, G. M. SHELXTL plus, version 4.21/V, Siemens XRD,
Madison, WI, 1990.