Platinum(II) and palladium(II) dibenzo[a,e]cyclooctatetraene (DBCOT) oxo and halide complexes: Comparison to 1,5-COD analogues

Article abstract of DOI:10.1021/om050713w

The dialkene complexes L₂MCl₂ (M = Pt, Pd, L ₂ = dibenzo[a,e]cyclooctatetraene (DBCOT)) were prepared by treatment of MCl₄^2- salts with L₂. Comparison of the solid-state structures with those of the analogous 1,5-cyclooctadiene (COD) complexes shows strong congruence of the structures. Competition experiments between DBCOT and COD show that COD bonds more strongly to the Pt(II) center. In contrast, Ir(I) bonds slightly better to DBCOT. Reaction of PtCl ₂(DBCOT) with NaI gives PtI₂(DBCOT), which reacts with PhMgBr to give PtPh₂(DBCOT). ¹⁹⁵Pt NMR data indicate a less electron-rich Pt center in the DBCOT complexes as compared to the analogous COD complexes. Treatment of PtCl₂(DBCOT) with [(Ph ₃PAu)₃(μ³-O)](BF₄) yields trimeric [Pt(μ³-O)(DBCOT)(AuPPh₃)]₃(BF ₄)₃, which contains a six-membered Pt₃O ₃ ring in a twist-boat configuration. Au-Au and Au-Pt interactions may help stabilize the solid-state structure, but NMR and mass spectral data indicate a dynamic structure in solution and the likely presence of other oligomers.

Full text of DOI:10.1021/om050713w

678
Organometallics 2006, 25, 678-683
Platinum(II) and Palladium(II) Dibenzo[a,e]cyclooctatetraene
(DBCOT) Oxo and Halide Complexes: Comparison to 1,5-COD
Analogues
Anupam Singh and Paul R. Sharp*
125 Chemistry, UniVersity of Missouri, Columbia, Missouri 65211
ReceiVed August 17, 2005
The dialkene complexes L2MCl2 (M ) Pt, Pd, L2 ) dibenzo[a,e]cyclooctatetraene (DBCOT)) were
prepared by treatment of MCl42- salts with L2. Comparison of the solid-state structures with those of the
analogous 1,5-cyclooctadiene (COD) complexes shows strong congruence of the structures. Competition
experiments between DBCOT and COD show that COD bonds more strongly to the Pt(II) center. In
contrast, Ir(I) bonds slightly better to DBCOT. Reaction of PtCl2(DBCOT) with NaI gives PtI2(DBCOT),
which reacts with PhMgBr to give PtPh2(DBCOT). 195Pt NMR data indicate a less electron-rich Pt center
in the DBCOT complexes as compared to the analogous COD complexes. Treatment of PtCl2(DBCOT)
with [(Ph3PAu)3(µ3-O)](BF4) yields trimeric [Pt(µ3-O)(DBCOT)(AuPPh3)]3(BF4)3, which contains a six-
membered Pt3O3 ring in a twist-boat configuration. Au-Au and Au-Pt interactions may help stabilize
the solid-state structure, but NMR and mass spectral data indicate a dynamic structure in solution and
the likely presence of other oligomers.
previously prepared Pt oxo complexes with phosphine ligands,8-10
none of these new complexes show the alkene oxidation reac-
tivity of 1. Attempts to prepare Pt oxo complexes with NBD, a
cyclic diene that has yielded many COD analogues, were not
successful nor were attempts to prepare NBD or COD Pd oxo
complexes. Free NBD and COD were observed in the Pd
reaction mixtures, suggesting that a more strongly bonding
analogue of COD is needed.
In searching for a strongly bonding chelating diene ligand,
we came across dibenzo[a,e]cyclooctatetraene (DBCOT).
DBCOT is believed to bond more strongly to metal centers
than COD.11 Strong bonding and its resistance to hydrogena-
tion has led to the use of DBCOT as a poison for homogene-
ous catalysts.12 A recent report of an improved DBCOT
synthesis13 and a facile preparation of substituted DBCOTs14
makes this dialkene ligand particularly attractive for investi-
gation. Here we report the synthesis, characterization, and
structures of Pt(II) and Pd(II) complexes with the DBCOT ligand
and compare the stability and reactivity of PtCl2(DBCOT) with
PtCl2(COD).
Introduction
The chelating diene ligand 1,5-cyclooctadiene (COD) has
been prominent in transition metal chemistry.1 This is espe-
cially true for the late transition metals where COD has provided
a variety of stable but labile complexes many of which are
starting materials for further chemistry.2 Our group has used
COD as a supporting ligand for Pt, Rh, and Ir oxo chemistry.3
The COD Pt oxo chemistry has proved to be particularly
interesting, producing the oxo complex [Pt2(µ-O)Cl(COD)2]2
(1)4 that oxidizes alkenes.5 Remarkably, the COD ligand is not
itself oxidized in this system but seems to provide unique
properties.
Results
In our efforts to understand and expand this chemistry we
have attempted to substitute COD with other ligands and to
prepare a Pd analogue of 1. Pd and Pt oxo complexes6,7 with
diimine ligands have been successfully prepared, but, like our
Yellow PtCl2(DBCOT) (2) and orange PdCl2(DBCOT) (3)15
are readily prepared in good yields from K2PtCl4 and Na2PdCl4
by procedures similar to those reported for the COD ana-
logues.16,17 Both complexes are somewhat more soluble than
* To whom correspondence should be addressed. E-mail: SharpP@
missouri.edu.
(1) ComprehensiVe Organometallic Chemistry II; Pergamon: Oxford,
(8) Li, J. J.; Sharp, P. R. Inorg. Chem. 1994, 33, 183-184.
(9) Li, J. J.; Li, W.; Sharp, P. R. Inorg. Chem. 1996, 35, 604-613.
(10) Li, J. J.; Li, W.; James, A. J.; Holbert, T.; Sharp, T. P.; Sharp, P.
R. Inorg. Chem. 1999, 38, 1563-1572.
(11) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 621-627.
(12) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855-859.
(13) Chaffins, S.; Brettreich, M.; Wudl, F. Synthesis 2002, 1191-1194.
(14) La¨ng, F.; Breher, F.; Stein, D.; Gru¨tzmacher, H. Organometallics
2005, 24, 2997-3007.
(15) Avram, M.; Dinu, D.; Mateescu, G.; Nenitzescu, C. D. Chem. Ber.
1960, 93, 1789-1794.
1995.
(2) See the subject indexes of Inorg. Synth. volumes 25 and 28 under
1,5-cyclooctadiene.
(3) Sharp, P. R. J. Chem. Soc., Dalton Trans. 2000, 2647-2657.
(4) Shan, H.; James, A. J.; Sharp, P. R. Inorg. Chem. 1998, 37, 5727-
5732.
(5) Szuromi, E.; Hui, S.; Sharp, P. R. J. Am. Chem. Soc. 2003, 124,
10522-10523.
(6) Singh, A.; Sharp, P. R. Dalton Trans. 2005, 2080-2081.
(7) Anandhi, U.; McGee, C.; Sharp, P. R. Unpublished results.
10.1021/om050713w CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/20/2005

Pt(II) and Pd(II) (DBCOT) Oxo and Halide Complexes
Organometallics, Vol. 25, No. 3, 2006 679
their COD analogues, dissolving readily in halogenated solvents
but less readily in aromatic solvents.
spectrum of 4, like 2, shows a quintet (JPtH ) 59.0 Hz) at
-2439.7 ppm but to higher frequency than that of 2.
The diphenyl complex PtPh2(DBCOT) (5) is obtained from
1
the reaction of 4 with phenylmagnesium bromide.19 The H
1
The H NMR spectrum of 2 shows the resonances of the
NMR spectrum of complex 5 shows aromatic resonances at
7.63-7.04 and 6.90-6.77 ppm with the olefinic protons at 6.14
ppm with Pt satellites (JPt-H ) 33.0 Hz). The 195Pt NMR
resonance for 5 is at -3260.2 ppm. This is at lower frequency
than those for 2 and 4, but the smaller Pt-H coupling is not
resolved and the signal is a singlet.
aromatic protons of the DBCOT ligand as a singlet at 7.17 ppm,
indicating accidental coincidence of the two aromatic proton
signals. The olefinic protons appear as a sharp singlet at 6.56
ppm with Pt satellites (J
) 59.3 Hz). The 195Pt NMR
Pt-H
spectrum of 2 shows a quintet at -2974 ppm (JPt-H ) 59.3
Hz), confirming the coupling between the olefinic protons and
the Pt center. The 1H NMR spectrum of 3 shows the resonances
for the aromatic protons as a symmetric pattern at 7.28-7.13
ppm, similar to that seen in the free ligand. The olefinic protons
are found as a singlet at 6.95 ppm.
Yellow 6 is isolated from the oxo-chloro exchange reaction3
between gold oxo complex [(AuPPh3)3(µ3-O)](BF4) and PtCl2-
(DBCOT) in an equimolar ratio in THF (eq 6). Further addition
of PtCl2(DBCOT), which in the case of COD gave 1,4 does not
give any new products. The solid-state structure of 6 was
determined by an X-ray crystal structure determination (see
below and Figure 3). In contrast to the analogous COD reaction,
which yields the dimer [Pt(µ3-O)(COD)(AuPPh3)]22+,4 6 is the
trimer [Pt(µ3-O)(DBCOT)(AuPPh3)]33+. However, spectroscopic
data suggest the presence of other species such as a dimer in
solutions of 6 probably in equilibrium with the isolated trimer
(eq 6).
To investigate the stability of DBCOT complex 2 relative to
the COD analogue, an equimolar mixture of 2 and COD in
methylene chloride in a flame-sealed NMR tube was moni-
1
tored by H NMR spectroscopy. There was no change in the
mixture after 14 h at room temperature. Heating the mixture
at 60-90 °C for 2 days gave complete conversion to PtCl2-
(COD) and free DBCOT with no detectable concentrations of
2 or COD. Addition of 20 equiv of DBCOT to a fresh mixture
dramatically slowed the reaction such that after 10 days at 90
°C the reaction was 50% complete and still slowly progressing.
A mixture of PtCl2(COD) and 20 equiv of DBCOT showed no
change after 7 days at 90 °C. In none of these reactions were
there any visual or spectroscopic signs of decomposition.
Assuming a detection limit of 1%, this places a lower limit of
ca. 105 on the equilibrium constant for the reaction shown in
eq 3.
The 31P NMR spectrum of 6 at room temperature shows a
singlet at 27.5 ppm. At -70 °C the peak slightly broadens but
does not resolve into the 1:2 pattern expected from the solid-
1
state structure. In contrast the H NMR spectrum of 6 is com-
plex and highly temperature and solvent dependent. At ambient
probe temperature the spectrum in CDCl3 shows two sets of
multiplets at 7.4 and 7.1 ppm and two broad overlapping sing-
lets in a ca. 3:1 ratio at 6.75 and 6.68 ppm. Coupling to Pt
characteristic of the olefinic protons is not discernible. In
CD2Cl2 three sets of multiplets at 7.45, 7.1, and 6.8 ppm are
observed along with two singlets at 6.70 and 6.28 ppm in a 2:1
ratio. The singlet at 6.28 ppm is broad. Again, coupling to Pt is
not visible. Cooling the CD2Cl2 sample results in a broadening
and collapse of the signals such that at -70 °C there is an
irregular broad peak from 6.1 to 7.6 ppm under a somewhat
sharper multiplet at 7.4 ppm. We attribute this behavior partly
to exchange of the different Ph3PAu fragments, but equilibrium
Despite the weaker bonding of the DBCOT ligand, 2 still
undergoes chemistry similar to PtCl2(COD).18 Dichloride 2 is
readily converted to the diiodide PtI2(DBCOT) (4) in 98% yield
by addition of NaI to a suspension of 2 in acetone. The 1H NMR
spectrum of 4 is similar to that for 2 (including the accidental
coincidence of the two aromatic signals) except that the signals
are at higher frequency than those for 2. The 195Pt NMR
(16) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem.
Soc. 1976, 98, 6521-6528.
(17) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346-349.
(18) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-
428.
(19) For a new preparation of PtPh2(COD) directly from PtCl2(COD)
see: Shekhar, S.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 13016-
13027.

680 Organometallics, Vol. 25, No. 3, 2006
Table 1. Crystallographic and Data Collection Parameters for 1-4a
Singh and Sharp
1
2
3
4
formula
fw
cryst syst
space group
a, Å
C16H12Cl2Pt‚2CH2Cl2
640.1
triclinic
C16H12Cl2Pd‚2CH2Cl2
520.84
triclinic
C16H12I2Pt‚0.5CH2Cl2
695.61
orthorhombic
Pbcn
13.1530(12)
16.0334(14)
32.836(3)
90
[C102H81Au3O3P3Pt3][BF4]3‚10CHCl3
4077.86
monoclinic
P21/c
14.5038(9)
36.472(2)
26.1887(16)
90
97.8320(10)
90
13724.0(15)
4
P1h
P1h
8.7025(15)
11.418(2)
11.949(2)
64.326(3)
88.254(3)
78.493(3)
1046.4(3)
2
8.6556(5)
11.4257(6)
11.9665(7)
63.9610(10)
87.5790(10)
78.2270(10)
1039.44(10)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
90
90
6924.7(11)
16
2.67
11.815
0.0464, 0.0969
d
calc, g/cm3
2.03
7.47
0.0376, 0.0984
1.76
1.66
0.0326, 0.0791
1.974
6.92
0.0827, 0.1808
µ, mm-1
R1,b wR2c
a λ ) 0.71070 Å (Mo), T ) -100 °C. b R1 ) (∑||Fo| - |Fc||)/∑|Fo|. c wR2 ) [(∑w(Fo - Fc )2)/∑w(Fc )2]1/2
.
2
2
2
Table 2. Selected Distances and Angles for 2-4
between a dimer and trimer form of 6 (eq 7) probably also
contributes, as suggested by the ESI mass spectrum of an
acetonitrile solution of 6.
PtCl2(DBCOT) PdCl2(DBCOT) PtI2(DBCOT) PtI2(DBCOT)
(2)
(3)
(4)
(4)a
M-X1
2.2999(16)
2.3110(16)
2.181(6)
2.162(6)
2.161(6)
2.173(6)
2.169(10)
1.388(9)
1.486(9)
1.405(8)
1.487(8)
1.390(8)
1.497(8)
1.383(9)
1.491(9)
2.2914(8)
2.3000(7)
2.206(3)
2.202(3)
2.207(3)
2.207(3)
2.206(2)
1.385(4)
1.486(4)
1.397(4)
1.485(4)
1.380(4)
1.490(4)
1.395(4)
1.487(4)
2.6121(7)
2.6007(8)
2.204(9)
2.197(10)
2.209(10)
2.200(9)
2.6119(9)
2.5968(9)
2.193(10)
2.211(10)
2.208(10)
2.204(10)
M-X2
M-C1
M-C2
M-C9
M-C10
M-C (av)
C1-C2
C2-C3
C3-C8
C8-C9
C9-C10
C10-C11
C11-C16
C1-C16
2.203(6)
1.391(14)
1.474(13)
1.387(15)
1.472(14)
1.433(15)
1.497(14)
1.385(14)
1.490(14)
1.382(15)
1.499(14)
1.398(15)
1.478(16)
1.440(14)
1.480(13)
1.390(14)
1.493(14)
The first three most intense peaks in the mass spectrum (ESI/
APC-direct infusion) of 6 are assigned to [Ph3PAuNCMe]+,
[(Ph3P)2Au]+, and [(Ph3PAu)2Cl]+. The next most intense peak
at m/z 1289 is attributed to the dimer fragment [Pt2(O)2-
(DBCOT)2(AuPPh3)]+ produced by loss of Ph3PAu+ from the
dimer [Pt(µ3-O)(DBCOT)(AuPPh3)]22+. The isotope pattern
match for all of these peaks is excellent. The next most intense
peak at m/z 874 could be due to the trimer [Pt(µ3-O)(DBCOT)-
X1-M-X2
89.55(6)
88.1
90.10(3)
87.2
91.43(3)
86.7
90.18(3)
86.6
b
||-M-||
C2-C1-M
C1-C2-M
C10-C9-M
C9-C10-M
C16-C1-M
C3-C2-M
C11-C10-M
C8-C9-M
C2-C1-C16
C1-C2-C3
C10-C9-C8
70.6(3)
72.1(4)
71.8(3)
70.8(3)
107.5(4)
108.5(4)
108.2(4)
108.4(4)
123.7(6)
124.5(6)
123.1(5)
71.52(17)
71.85(17)
71.80(17)
71.78(17)
107.5(2)
107.21(18)
107.81(19)
107.15(19)
124.4(3)
124.8(3)
123.8(3)
124.1(3)
71.3(5)
71.9(6)
70.7(6)
71.4(5)
108.8(7)
107.7(7)
108.4(6)
107.5(7)
125.2(9)
124.2(8)
123.3(9)
123.7(9)
72.5(6)
71.0(6)
70.8(6)
71.1(5)
107.7(7)
109.4(7)
108.6(6)
108.8(6)
123.7(9)
123.2(9)
123.7(9)
122.7(9)
(AuPPh3)]3 or the dimer [Pt(µ3-O)(DBCOT)(AuPPh3)]2
.
3+
2+
However, the peak pattern does not match either species or a
combination of the two, making a positive assignment out of
reach.
Structures. A summary of crystal data and data collection
and processing is given in Table 1. Selected distances and angles
are given in Tables 2 and 3. Drawings of the solid-state
structures of 2 and 6 are given in Figures 1 and 2. Those for 3
and 4 are provided in the Supporting Information.
C9-C10-C11 123.8(6)
b
a Molecule 2 of 2. || represents C1-C2 and C9-C10 centroids.
the angle of the coordinated olefinic unit to the Pd coordination
plane. This is again at nearly 90° for 3 (89.8 and 89.2) but canted
in PdCl2(COD) (83.6 and 84.9, 83.2 and 83.3).
Crystals of PtI2(DBCOT) (4) are not isomorphous with those
of 2 and 3 and contain two independent molecules in the
asymmetric unit. However, 4 is isostructural with 2 and 3 but
with the expectedly longer metal-iodine distances for 4 (Table
2). All other features match those of 2 and 3.
The core structure of the cationic portion of oxo complex 6
is shown in Figure 2 (a complete drawing is available in the
Supporting Information) and is a trimer of [(DBCOT)Pt-O-
AuPPh3]+ units linked through Pt-O bonds. The result is a six-
membered Pt-O ring in a distorted twist-boat conformation with
Pt1 and O2 in the unique “up” positions and Au2 occupying a
The structures of 2 and PtCl2(COD)20 are highly congruent,
as shown by a comparison of the averaged metrical parameters
in Table 4. The average Pt-C, Pt-Cl, and coordinated CdC
double bond lengths are identical within experimental error, as
are the angles around the Pt atoms. The one feature that is
different is the angle of the coordinated olefinic unit to the Pt
coordination plane. The double bonds are essentially perpen-
dicular to the plane (89.9° and 89.2°) in 2 but somewhat canted
in PtCl2(COD) (83.4° and 83.3°).
Crystals of PdCl2(DBCOT) (3) are isomorphous with those
of 2. The compounds are isostructural but with the expectedly
longer metal-carbon distances for 3 (Table 2). As with 2 a
comparison with the COD analogue21-23 reveals no differences
in the essential features of the structure with the exception of
(22) T. Kumar, R.; Maverick, A. W.; Fronczek, F. R.; Doyle, J. R.;
Baenziger, N. C.; Howells, M. A. M. Acta Crystallogr., C 1993, C49, 1766-
1767.
(23) Benchekroun, L.; Herpin, P.; Julia, M.; Saussine, L. J. Organomet.
Chem. 1977, 128, 275-290.
(20) Syed, A.; Stevens, E. D.; Cruz, S. G. Inorg. Chem. 1984, 23, 3673-
3674.
(21) Rettig, M. F.; Wing, R. M.; Wiger, G. R. J. Am. Chem. Soc. 1981,
103, 2980-2986.

Pt(II) and Pd(II) (DBCOT) Oxo and Halide Complexes
Organometallics, Vol. 25, No. 3, 2006 681
Figure 1. Plot of the solid-state structure of PtCl2(DBCOT) (50%
probability ellipsoids, hydrogen atoms omitted).
Table 3. Selected Distances and Angles for 6
Au1-Pt1
Au2-Pt2
Au3-Pt3
Au1-Au3
Au1-P1
Au2-P2
Au3-P3
Au1-O1
Au2-O2
3.0057(12)
3.1150(12)
3.0556(12)
3.1087(12)
2.213(6)
2.214(6)
2.217(6)
2.075(15)
2.076(14)
Au3-O3
Pt1-O1
Pt1-O3
Pt2-O1
Pt2-O2
Pt3-O2
Pt3-O3
Pt-O (av)
Pt-C (av)
2.084(14)
2.022(15)
1.975(13)
1.981(15)
2.028(14)
1.976(14)
1.998(14)
2.00(2)
Figure 2. Drawing of the core structure of the cationic portion of
oxo complex 6 (50% probability ellipsoids, hydrogen and carbon
atoms omitted).
Table 4. Comparison of the Average Metrical Parameters
for PtCl2(DBCOT) (2) and PtCl2(COD)
2.18(2)
PtCl2(DBCOT) (2)
PtCl2(COD)
O1-Au1-P1
O2-Au2-P2
O3-Au3-P3
O1-Au1-Au3
P1-Au1-Au3
Pt1-Au1-Au3
O1-Au1-Pt1
P1-Au1-Pt1
O2-Au2-Pt2
P2-Au2-Pt2
O3-Au3-Pt3
P3-Au3-Pt3
O3-Au3-Au1
P3-Au3-Au1
Pt3-Au3-Au1
O1-Pt1-Au1
O3-Pt1-Au1
171.8(4)
173.4(4)
160.3(4)
79.6(4)
103.18(17)
73.47(3)
42.1(4)
130.91(17)
40.1(4)
137.28(15)
40.5(4)
125.26(17)
75.7(4)
123.44(17)
107.88(3)
43.5(4)
O3-Pt1-O1
O1-Pt2-O2
O2-Pt3-O3
O2-Pt2-Au2
O1-Pt2-Au2
O2-Pt3-Au3
O3-Pt3-Au3
Pt2-O1-Pt1
Pt3-O2-Pt2
Pt1-O3-Pt3
Pt2-O1-Au1
Pt1-O1-Au1
Pt3-O2-Au2
Pt2-O2-Au2
Pt1-O3-Au3
Pt3-O3-Au3
90.4(6)
95.0(6)
89.6(5)
41.2(4)
92.0(4)
83.4(4)
42.6(4)
122.6(8)
134.5(7)
112.1(6)
118.1(7)
94.4(6)
116.6(7)
98.7(6)
128.6(7)
96.9(6)
Pt-C1
Pt-C
2.305(8)
2.169(10)
1.389(9)
89.55(6)
88.1(2)
2.312(4)
2.170(11)
1.381(8)
89.78(5)
87.3(2)
CdC
Cl-Pt-Cl
a
||-Pt-||
C-CdC
123.8(6)
125.0(6)
a
|| represents C1-C2 and C9-C10 centroids.
Au atoms closer to one of the two Pt atoms in the oxygen-
centered Pt2Au triangles.
The trimeric structure of 6 with Pt-Au and Au-Au interac-
tions is in contrast to the analogous COD product, which is
the dimer [Pt(µ3-O)(COD)(AuPPh3)]2(BF4)2.4 There are no
Au-Au or Pt-Au interactions in the dimer, and the Pt2O2Au2
core is nearly planar. As a result of the smaller ring size for the
COD complex (4 vs 6), the internal angles are much smaller.
For example, the O-Pt-O angle is 79.5(3)° in the dimer as
compared to the 112-135° range observed for 6. Despite these
differences, the average Pt-O distances are the same in both
complexes, at 2.012(7) Å for the COD complex and 2.00(2) Å
for 6. On the other hand, the Au-O distances are somewhat
shorter in the COD complex (2.026(6) Å) than in 6 (2.078(5)
Å). The longer distances in 6 may result from the distortions
caused by the Au-Au and Au-Pt interactions.
79.7(4)
flagpole position on O2. Au1 and Au3 occupy adjacent “axial”
positions, allowing them to participate in aurophilic24 inter-
actions. There are also short Pt-Au distances (Pt1-Au1,
Pt3-Au3, Pt2-Au2), indicating related closed shell Pt-Au
interactions.25-29 Despite the presence of two different types
of Pt and O centers, the Pt-O distances (1.975(13)-2.028(14)
Å) are identical within experimental error, as are the Au-O
distances (2.075(15)-2.084(14) Å). However, the O-Pt-O
angles do differ, with the unique O1-Pt2-O2 angle signifi-
cantly greater (95.6(6)°) than the corresponding angles of Pt1
(90.4(6)°) and Pt3 (89.6(5)°). The angles around the oxo groups
vary considerably. Among the Pt-O-Pt angles, that for unique
O2 is largest at 134.5(7)° followed by O1 (122.6(8)°) and O3
(112.1(6)°). The Pt-O-Au angles occur in sets of one large
and one small for each oxo ligand (e.g., 118.1(7)° and 94.4(6)°
for O1). This is due to the Au-Pt interactions, which draw the
Discussion
The weaker bonding of the DBCOT ligand in PtCl2(DBCOT)
in comparison to PtCl2(COD) is in sharp contrast to the reported
observations on related Ir(I) complexes.11 [Ir(µ-Cl)(DBCOT)]2
was readily obtained from the room-temperature reaction of
[Ir(µ-Cl)(COD)]2 with 1 equiv of DBCOT, implying that the
DBCOT complex is more stable than the COD complex.
However, [Ir(µ-Cl)(DBCOT)]2 is much less soluble than
[Ir(µ-Cl)(COD)]2 and precipitates from the reaction mixture.
Precipitation could shift an unfavorable equilibrium such that
[Ir(µ-Cl)(DBCOT)]2 could be isolated even if it is less stable
than the COD complex. To investigate this possibility, we
examined the reaction of [Ir(µ-Cl)(COD)]2 with DBCOT in
(24) Schmidbaur, H. Gold Bull. 2000, 33, 3-10.
(25) Pyykko¨, P. Chem. ReV. 1997, 97, 597-636.
(26) Jeffery, J. C.; Jelliss, P. A.; Stone, F. G. A. Inorg. Chem. 1993, 32,
3943-3947.
(27) Hadj-Bagheri, N.; Puddephatt, R. J. Inorg. Chim. Acta 1993, 213,
29.
(28) Tanase, T.; Toda, H.; Yamamoto, Y. Inorg. Chem. 1997, 36, 1571.
(29) Yip, H. K.; Lin, H. M.; Wang, Y.; Che, C. M. J. Chem. Soc., Dalton
Trans. 1993, 2939-2944.

682 Organometallics, Vol. 25, No. 3, 2006
Singh and Sharp
various ratios and at two concentrations. The reactions were
monitored by 1H NMR spectroscopy. In the more concentrated
samples precipitation occurred 3-24 h after mixing. There was
no change in the mixture up to precipitate formation. In the
more dilute sample no precipitation was observed up to 1 week
and the 1H NMR spectrum did not change over this time period,
suggesting that equilibrium of the complexes with each other
in solution is achieved quickly. Free COD and [Ir(µ-Cl)-
(DBCOT)]2 were observed along with remaining DBCOT and
[Ir(µ-Cl)(COD)]2. Another COD species with peaks very close
to those for [Ir(µ-Cl)(COD)]2 was also observed. This is assigned
to the mixed DBCOT/COD dimer Ir2(µ-Cl)2(COD)(DBCOT).
This same complex is formed from a mixture of [Ir(µ-Cl)-
(COD)]2 and [Ir(µ-Cl)(DBCOT)]2. By integration of the peaks
for complexed and free ligands an equilibrium constant of 8 (
1 was obtained for the equilibrium shown in eq 8, where
(DBCOT)Ir(I) and (COD)Ir(I) represent the amount of each
complexed ligand in both the pure dimers and the mixed dimer.
Calculated equilibrium constants for dilute solution (no pre-
cipitate) and the more concentrated solutions agree.
oxo complex gave instead of the dimer observed with COD,
trimer 6. Solution data including mass spectral results suggest
that the DBCOT dimer does exist in solution in equilibrium
with the trimer and perhaps other oligomers. This may also be
true of the COD complex, though no evidence for this has been
detected. Solubility would then determine which oligomer is
isolated. This is reminiscent of Pt(II) hydroxo complexes of
formula [Pt(µ-OH)L2]nn+, which have been isolated as dimers
(n ) 2), trimers (n ) 3), and tetramers (n ) 4).35-40 As the
Ph3PAu+ fragment is isolobal with the proton,41 oxo complexes
of empirical formula [L2Pt(µ-OAuL)]+ are analogues of the
hydroxo complexes.
The greater electrophilicity of the Pt center in the
Pt(DBCOT)2+ unit may be responsible for the isolation of the
trimer. The COD dimer does not have metal-metal interactions.
In contrast, DBCOT oxo complex 6 has both Pt-Au and
Au-Au interactions. These interactions may contribute to the
stability of the trimer structure in comparison to the dimer
structure. While Au-Pt interactions are still possible in the more
rigid dimer structure with its four-membered Pt2O2 ring,3
Au-Au interactions would be difficult to attain with the strong
steric interactions that would be present with Ph3PAu+ units
on the same side of the Pt2O2 ring.
(COD)Ir(I) + DBCOT h (DBCOT)IR(I) + COD (8)
Thus, DBCOT appears to bond slightly better to Ir(I) than
COD. This difference between Ir(I) and Pt(II) may arise from
differences in the relative importance of the donor and acceptor
components of the bonding to the olefinic unit. DBCOT is a
weaker σ-donor but better π-acceptor than COD11 and the donor
component is likely more important for Pt(II) than for Ir(I).
Consistent with weaker σ-donor but better π-acceptor character,
IR studies have shown that the DBCOT ligand is substantially
more electron-withdrawing than COD.11
A more significant difference between the DBCOT and COD
chemistry is the failure of trimer 6 to react further with the gold
oxo complex [(Ph3PAu)3(µ-O)]+. The COD dimer yields the
all-Pt oxo complex 1 when treated with additional gold oxo
complex. Oxo complex 1 is unique in its reactions with alkenes,
and we anticipated analogous chemistry with DBCOT. The
failure of the DBCOT system to yield an analogue of 1 is
unexplained, as is the failure of the NBD ligand system.
Weaker bonding of DBCOT to Pt(II) is supported by the
195Pt NMR shifts. The 195Pt NMR signal for PtCl2(DBCOT)
(-2975) is at higher frequency by 362 ppm from that for PtCl2-
(COD) (-3337 ppm). A correlation of 195Pt NMR shifts with
the strength of alkene bonding has previously been noted for a
series of Zeise’s salt analogues, with the more stable complexes
showing the lowest frequency shifts.30,31 Similarly, a comparison
Conclusions
Simple DBCOT analogues of Pt and Pd COD complexes are
readily prepared using procedures similar to those for the COD
complexes. The DBCOT ligand is more electron-withdrawing
than the COD ligand and, in contrast to Ir(I), bonds more weakly
to Pt(II) than COD.
of the Pt-C coupling constants of PtCl2(DBCOT) (JPt-C
)
136.3 Hz) and PtCl2(COD) (JPt-C ) 152.5 Hz) indicates a
weaker Pt-C interaction.
Experimental Section
General Procedures. DBCOT (dibenzo[a,e]cyclooctatetraene),13
[(LAu)3(µ3-O)]BF4 (L ) PPh3),42 and [Ir(µ-Cl)(COD)]2 were
Comparison of the structures of PtCl2(DBCOT) and PtCl2-
(COD) does not reveal any significant differences in the
structures that would indicate weaker bonding for the DBCOT
ligand. The lower stability of PtCl2(NBD) as compared to PtCl2-
(COD) has been attributed to the smaller bite angle of NBD
(70.3°),32 but the essentially identical structures of PtCl2-
(DBCOT) and PtCl2(COD) with bite angles of 88.1° and 87.3°
eliminates this possibility, indicating that electronic factors are
involved. The lack of structural indicators for weaker bonding
is consistent with the structures of σ-donor/π-acceptor com-
plexes, which show that complex geometry is a poor indicator
of ligand bond strength.33,34
43
prepared by literature procedures. Phenylmagnesium bromide (3
M in diethyl ether) was purchased from Aldrich. Experiments were
performed under a dinitrogen atmosphere in a Vacuum Atmospheres
Corporation drybox or on a Schlenk line with dried and degassed
solvents stored under dinitrogen over 4 Å molecular sieves or
sodium metal. NMR spectra were recorded on a Bruker AMX-
(35) Macquet, J.-P.; Cros, S.; Beauchamp, A. L. J. Inorg. Biochem. 1985,
25, 197.
(36) Rochon, F. D.; Morneau, A.; Melanson, R. Inorg. Chem. 1988, 27,
10.
(37) Faggiani, R.; Lippert, B.; Lock, C. J. L.; Rosenberg, B. Inorg. Chem.
1978, 17, 1941-1945.
Despite the weaker bonding, our brief investigation of PtCl2-
(DBCOT) has revealed chemistry generally analogous to that
of PtCl2(COD). However, the oxo chemistry that interested us
most is significantly different. The 1:1 reaction with the gold
(38) Faggiani, R.; Lippert, B.; Lock, C. J. L.; Rosenberg, B. Inorg. Chem.
1977, 16, 1192-1196.
(39) Wimmer, S.; Castan, P.; Wimmer, F. L.; Johnson, N. P. J. Chem.
Soc., Dalton Trans. 1989, 403-412.
(40) Gill, D. S.; Rosenberg, B. J. Am. Chem. Soc. 1982, 104, 4598-
4604.
(30) O¨ hrstro¨m, L. Comments Inorg. Chem. 1996, 18, 305-323.
(31) Erickson, L. E.; Hayes, P.; Hooper, J. J.; Morris, K. F.; Newbrough,
S. A.; Van Os, M.; Slangan, P. Inorg. Chem. 1997, 36, 284-290.
(32) Butikofer, J. L.; Kalberer, E. W.; Schuster, W. C.; Roddick, D. M.
Acta Crystallogr., C 2004, C60, m353-m354.
(33) Huang, J. K.; Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy,
K. G. J. Am. Chem. Soc. 1998, 120, 7806-7815.
(34) Massera, C.; Frenking, G. Organometallics 2003, 22, 2758-2765.
(41) Evans, D. G.; Mingos, D. M. P. J. Organomet. Chem. 1982, 232,
171-191.
(42) Nesmeyanov, A. N.; Perevalova, E. G.; Struchkov, Y. T.; Antipin,
M. Y.; Grandberg, K. I.; Dyadchenko, V. P. J. Organomet. Chem. 1980,
201, 343-349.
(43) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18-20.

Pt(II) and Pd(II) (DBCOT) Oxo and Halide Complexes
Organometallics, Vol. 25, No. 3, 2006 683
Table 5. Calculated Equilibrium Constants for Eq 8
250, -300, or -500 spectrometer at ambient probe temperatures.
Shifts are given in ppm with positive values to higher frequency
reactant ratio
of TMS (1H and 13C), external H3PO4 (31P), or external K2PtCl4(aq)
1:0.5
9.7
1:1
7.6
1:1.5
7.9
1:3
6.8
1
(
195Pt, -1630 ppm). 13C and H NMR spectra were recorded in
K
av ) 8(1)
proton-decoupled mode. Desert Analytics performed the mi-
croanalyses (inert atmosphere). The presence of solvent of crystal-
lization in the analyzed sample was confirmed by NMR spectros-
copy. The Chemistry Mass Spectrometry Facility at The Ohio State
University collected the mass spectral data for the organic products.
Dichloro(dibenzo[a,e]cyclooctatetraene)platinum(II), PtCl2-
(DBCOT) (2). K2PtCl4 (50.0 mg, 0.120 mmol) was dissolved in
3.00 mL of water and then filtered. Glacial acetic acid (4.00 mL)
and dibenzo[a,e]cyclooctatetraene (24.5 mg, 0.120 mmol) were
added, and the mixture was brought to reflux with stirring. Over 3
h the deep red solution slowly became yellow and a light yellow
solid was deposited. The yellow solid was collected by filtration,
washed with water, ethanol, and diethyl ether, and dried at 90 °C
for 15 min. Yield: 46.0 mg (82%). Crystals for X-ray analysis were
obtained by recrystallization from CD2Cl2/ether at -30 °C. Anal.
Calcd (found) for C16H12PtCl2‚1/2CH2Cl2: C, 38.59 (38.97); H. 2.53
(81%). Anal. Calcd (found) for C28H22Pt‚0.5CH2Cl2: C, 57.38
(57.02); H. 3.85 (3.90). 1H NMR (300 MHz, CD2Cl2): 7.63-7.04
(m, 30H, aromatic CH) 6.90-6.77 (m, 8H, aromatic CH), 6.14 (s
with satellites, JPt-H ) 33.0 Hz, 4H, olefinic CH). 195Pt NMR (64.5
MHz, CD2Cl2): -3260 (s).
[Pt(µ3-O)(DBCOT)(AuPPh3)]3(BF4)3 (6). A solution of PtCl2-
(DBCOT) (94.0 mg, 0.200 mmol) in THF (50 mL) was vigorously
stirred with solid [(Ph3PAu)3(µ3-O)]BF4 (296 mg. 0.200 mmol).
The mixture became homogeneous and orange within 40 min.
Stirring was continued for 2 h, and then the volume of the solution
was reduced in vacuo to ca. 25 mL. Ether (50 mL) was added
and the mixture cooled at -30 °C for 24 h. The resulting pale
yellow-orange microcrystalline solid was isolated by filtration
and dried in vacuo. Yield: 158 mg (82%). Crystals for X-ray
analysis were obtained by recrystallization from CDCl3/CD2Cl2.
Anal. Calcd (found) for C102H81Au3B3F12O3P3Pt3‚1/2CHCl3: C,
41.77 (41.67); H. 2.76 (2.87). 1H NMR (250 MHz, CD2Cl2): 7.50-
7.35 (m, 45H, Ph), 7.17-7.06 (m, 24H, CH), 6.83-6.80 (m, 12H,
CH). 31P{1H} NMR (101.2 MHz, CD2Cl2): 27.4 (s). 195Pt
NMR (64.5 MHz, CD2Cl2): -2420 (s). MS (ESI/APC-direct
infusion, MeCN, m/z): 500.06 (100%, [Ph3PAuNCMe]+), 721.1
(65%, [(Ph3P)2Au]+), 953.09 (20%, [(Ph3PAu)2Cl]+), 1289.54
1
(2.33). H NMR (250 MHz, CD2Cl2): 7.17 (s, 8H, aromatic CH),
6.56 (s with satellites, JPt-H ) 59.3 Hz, 4H, olefinic CH). 195Pt
NMR (64.5 MHz, CD2Cl2): -2975 (quintet, JPt-H ) 59.3 Hz).
13C{1H} NMR (300 MHz, CD2Cl2): 139.2 (s, aromatic), 129.0 (s,
aromatic), 126.8 (s, aromatic), 98.0 (s with satellites, JPt-C ) 136.3
Hz, olefinic C).
Dichloro(dibenzo[a,e]cyclooctatetraene)palladium(II), PdCl2-
(DBCOT) (3). Na2PdCl4 (50.0 mg, 0.170 mmol) was dissolved in
4.00 mL of dry methanol and then filtered. The dark red filtrate
was vigorously stirred with solid dichlorodibenzo[a,e]cyclo-
octatetraene (34.6 mg, 0.170 mmol). The color of the mixture
change from red to yellow within 15 min. Stirring was continued
for 4 h. Half of the solvent was removed in vacuo, and ether (8
mL) was added. The resulting microcrystalline yellow solid was
isolated by filtration, washed with methanol and diethyl ether, and
dried in vacuo. Yield: 52.3 mg (79%). Crystals for X-ray analysis
were obtained by recrystallization from CD2Cl2/ether at -30 °C.
Anal. Calcd (found) for C16H12PdCl2‚MeOH: C, 49.39 (49.41); H.
(10%, [C50H39AuO2PPt2]2+), 874.26 (5%, [C102H81Au3O3P3Pt3]3+
or [C68H54Au2O2P2Pt2]2+).
-
Reaction of [Ir(µ-Cl)(COD)]2 with DBCOT. Four 5 mm NMR
tubes were prepared with [Ir(µ-Cl)(COD)]2 (10.0 mg, 0.014 mmol)
dissolved in 0.7 mL of CDCl3. Solid DBCOT was added to each
tube in a ratio ([Ir(µ-Cl)(COD)]2:DBDOT) of 1:0.5, 1:1, 1:1.5, or
1:3. Each mixture changed from yellow to yellow-orange within
10 min. 1H NMR spectra were recorded repeatedly over a 3 h period
and did not change from the first spectrum. For the 1:3 reaction a
small amount of precipitate appeared after 1 h. Only after a day
did the others deposit a solid. The integration ratios from the NMR
spectra taken 20 min after mixing were used to calculate equilibrium
constants for eq 8 given in Table 5. A second set of two samples
was prepared with less [Ir(µ-Cl)(COD)]2: A solution of [Ir(µ-Cl)-
(COD)]2 (2.0 mg, 0.0029 mmol) in CD2Cl2 (0.4 mL) was added to
a solution of DBCOT (1.2 mg, 0.0059 mmol) in CD2Cl2 (0.3 mL).
The mixture changed from orange to red within 5 min and stayed
homogeneous for a least a week. The 1H NMR did not change over
this time period and gave the same equilibrium constant as the more
concentrated samples.
1
3.87 (3.48). H NMR (250 MHz, CD2Cl2): 7.28-7.13 (m, 8H,
aromatic CH), 6.95 (s, 4H, olefinic CH).
Diiodo(dibenzo[a,e]cyclooctatetraene)platinum(II),
PtI2-
(DBCOT) (4). Solid NaI (30.0 mg, 0.200 mmol) was added to a
suspension of PtCl2(DBCOT) (47.0 mg, 0.100 mmol) in acetone
(4 mL). The solution immediately turned orange. The mixture was
further stirred for 10 min and then concentrated in vacuo to ca. 2
mL. The resulting orange solid was collected by filtration and
washed with water and diethyl ether. Yield: 64.0 mg (98%).
Crystals for the X-ray analysis were obtained by recrystallization
from CD2Cl2/ether at -30 °C. Anal. Calcd (found) for C16H12PtI2‚
1H NMR (300 MHz, CD2Cl2) for Ir2(µ-Cl)2(COD)(DBCOT):
7.05 (m, 4H, DBCOT), 6.92 (m, 4H, DBCOT), 5.33 (s, 4H, olefinic
DBCOT), 4.29 (s, 4H, olefinic COD), 2.14 (m, 4H, COD), 1.50
(m, 4H, COD).
1
1/2CH2Cl2: C, 28.44 (28.72); H. 1.86 (1.90). H NMR (250 MHz,
CD2Cl2): 7.16 (s, 8H, aromatic CH), 6.87 (s with satellites, JPt-H
) 59.0 Hz, 4H, olefinic CH). 195Pt NMR (64.3 MHz, CD2Cl2):
-2440 (quintet, JPt-H ) 59.0 Hz).
Diphenyl(dibenzo[a,e]cyclooctatetraene)platinum(II), PtPh2-
(DBCOT) (5). In a drybox a diethyl ether solution of phenylmag-
nesium bromide (3 M, 0.26 mmol) was added dropwise to a stirred
solution of PtI2(DBCOT) (73.00 mg, 0.112 mmol) in 6.00 mL of
diethyl ether, and the reaction mixture was allowed to stir at room
temperature for 12 h. The reaction mixture was brought outside
the drybox and hydrolyzed with an ice-cold saturated aqueous
solution of ammonium chloride. The ether layer was separated, and
the water layer was extracted with two 20 mL portions of diethyl
ether. The combined ether extracts were dried over anhydrous
MgSO4 containing a small amount of activated charcoal. The
solution was filtered through diatomaceous earth, and the volatiles
were removed in vacuo. The yellow solid residue was crystallized
from a mixture of dichloromethane and ether. Yield: 50.0 mg
Acknowledgment. Support from the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, U.S. Department of Energy (DE-
FG02-88ER13880), is gratefully acknowledged. Grants from the
National Science Foundation (CHE-9531247, -922183526,
-8908304) provided a portion of the funds for the purchase of
the NMR equipment.
Supporting Information Available: Details of the crystal
structure determinations and crystallographic information files
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
OM050713W