Consecutive cyclometalation by platinum(II)

Article abstract of DOI:10.1021/om9600452

The mechanism of the selective formation of the new benzylic mono- and bicyclometalated platinum(II) complexes 2a,b and 3b has been studied. Monitoring the initial sp³ C-H activation of the ligand DIPPIDH (α²-(diisopropylphosphino)isodurene; 1) and (1,5-cyclooctadiene)dimethylplatinum(II) ((COD)PtMe₂) at room temperature by ³¹P{¹H} NMR reveals that, after displacement of 1,5-cyclooctadiene, the complex cis-(DIPPIDH)(DIPPID)-PtMe (2a) is formed selectively by oxidative addition of a benzylic C-H bond and subsequent reductive elimination of CH₄. This reaction is controlled by electronic factors. Heating of 2a results in isomerization to the thermodynamically more stable isomer trans-(DIPPIDH)-(DIPPID)PtMe (2b) as well as the formation of the double-C-H activated complex trans-(DIPPID)₂Pt (3b) with liberation of CH₄ in parallel pathways. Continuous heating results in the quantitative formation of the thermally stable 3b. Mechanistically, the second C-H activation proceeds analogously to the first one. The molecular structure of 3b, possessing two six-membered metallacycles, two methylene bridges, and two phosphines in mutually trans positions, was determined by complete single-crystal diffraction studies.

Full text of DOI:10.1021/om9600452

2562
Organometallics 1996, 15, 2562-2568
Con secu tive Cyclom eta la tion by P la tin u m (II)
Milko E. van der Boom,† Shyh-Yeon Liou,† Linda J . W. Shimon,‡
Yehoshoa Ben-David,† and David Milstein*,†
Departments of Organic Chemistry and Chemical Services, The Weizmann Institute of Science,
76100 Rehovot, Israel
Received J anuary 23, 1996X
The mechanism of the selective formation of the new benzylic mono- and bicyclometalated
platinum(II) complexes 2a ,b and 3b has been studied. Monitoring the initial sp3 C-H
activation of the ligand DIPPIDH (R2-(d iisop ropylp hosphino)isod urene; 1) and (1,5-
cyclooctadiene)dimethylplatinum(II) ((COD)PtMe2) at room temperature by 31P{1H} NMR
reveals that, after displacement of 1,5-cyclooctadiene, the complex cis-(DIPPIDH)(DIPPID)-
PtMe (2a ) is formed selectively by oxidative addition of a benzylic C-H bond and subsequent
reductive elimination of CH4. This reaction is controlled by electronic factors. Heating of
2a results in isomerization to the thermodynamically more stable isomer trans-(DIPPIDH)-
(DIPPID)PtMe (2b) as well as the formation of the double-C-H activated complex trans-
(DIPPID)2Pt (3b) with liberation of CH4 in parallel pathways. Continuous heating results
in the quantitative formation of the thermally stable 3b. Mechanistically, the second C-H
activation proceeds analogously to the first one. The molecular structure of 3b, possessing
two six-membered metallacycles, two methylene bridges, and two phosphines in mutually
trans positions, was determined by complete single-crystal diffraction studies.
In tr od u ction
Recently we demonstrated the selective activation and
functionalization of unstrained sp2-sp3 C-C bonds by
rhodium(I)1 and platinum(II),2 using bis(phosphino)-
xylene substrates under mild homogeneous conditions.
It was shown that the competing, kinetically favorable
benzylic C-H activation can be reversed or suppressed
by utilizing nonpolar or polar substrates. The sp3 C-H
activation precedes an interesting overall process, which
includes (i) the selective demethylation of an aromatic
system and (ii) transferring of the methylene group to
another substrate (eq 1)
ecule of 1 to coordinate to the metal, resulting in the
formation of a second six-membered chelating ring with
liberation of CH4. Previous studies on cyclometalation
provided much information on intra- and intermolecular
6a
C-H activation.5 It is known that σ-bonded Pt-CH3
groups and bulky substituents promote C-H
activation.6b,c In this paper we report on the mechanism
of two consecutive C-H activation processes involving
Pt(II), displaying interesting steric and electronic ef-
fects. The synthesis and the crystal structure of the
double-cyclometalated complex 3b, possessing two six-
membered chelates, two methylene bridges, and two
phosphines in mutually trans positions, are described.
Furthermore, the isolation (2a ) and formation (2a ,b) of
two monocyclometalated products are discussed.
R-CH3 + MLn -H-L8 R-CH2-MLn-1 +A-B8
R-MLn-1 + A-CH2-B (1)
To extend our findings and to obtain insight into the
nature of the bond activation processes involved, we
studied the formation of new methylene-bridged metal
complexes using the phosphine ligand DIPPIDH (R2-
(d iisop ropylp hosphino)isod urene; 1) and (1,5-cyclo-
octadiene)dimethylplatinum(II) ((COD)PtMe2).
The chelating diene in (COD)PtMe2 can be displaced
by phosphine ligands.3,4 Reaction with 1 followed by
cyclometalation of a benzylic C-H bond by the electron-
rich metal center is expected to result in the formation
of a six-membered ring and CH4, rendering the overall
process irreversible. Our system allows a second mol-
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions were carried out under
nitrogen in a Vacuum Atmospheres glovebox (DC-882) equipped
(5) (a) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789.
(b) Chipperfield, J . R. In Chemistry of the Platinum Group Metals;
Hartley, F. R., Ed.; Elsevier: Amsterdam, The Netherlands, 1991;
Chapter 7. (c) Hartley, F. R. In Comprehensive Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon
Press: Oxford, U.K., 1982; Vol. 6, Chapter 39, pp 592-606. (d) Ryabov,
A. D. Chem. Rev. 1990, 90, 403. (e) Evans, D. W.; Baker, G. R.;
Newkome, G. R. Coord. Chem. Rev. 1989, 93, 155. (f) Dehand, J .;
Pfeffer, M. Coord. Chem. Rev. 1976, 18, 344. (g) Omae, I. Coord. Chem.
Rev. 1988, 83, 137. (h) Shaw, B. L. J . Organomet. Chem. 1980, 200,
311. (i) Bruce, M. I. Angew. Chem. 1977, 89, 75.
* To whom correspondence should be addressed.
† Department of Organic Chemistry.
‡ Department of Chemical Services.
X Abstract published in Advance ACS Abstracts, April 15, 1996.
(1) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699. (b) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman,
A.; Ben-David, Y.; Milstein, D. Nature 1994, 370, 42.
(2) Van der Boom, M. E.; Kraatz, H.-B.; Ben-David, Y.; Milstein, D.
To be submitted for publication.
(6) (a) Cheney, A. J .; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1972,
754. (b) Cheney, A. J .; Mann, B. E.; Shaw, B. L.; Slade, R. M. J . Chem.
Soc., Chem. Commun. 1970, 1176. (c) Cheney, A. J .; Mann, B. E.; Shaw,
B. L.; Slade, R. M. J . Chem. Soc. A 1971, 3833.
(3) Clark, H. C.; Manzer, L. E. J . Organomet. Chem. 1973, 59, 411.
(4) Fallis, S.; Anderson, G. K.; Rath, N. P. Organometallics 1991,
10, 3180.
S0276-7333(96)00045-3 CCC: $12.00 © 1996 American Chemical Society

Consecutive Cyclometalation by Pt(II)
Organometallics, Vol. 15, No. 10, 1996 2563
with a recirculation (MO-40) “Dri-Train” or under argon using
standard Schlenk techniques. Solvents were reagent grade
or better. Benzene was dried (Frutarom, Na/benzophenone),
distilled, and degassed before introduction into the glovebox,
where it was stored over activated 4 Å molecular sieves.
Deuterated solvents were purchased from Aldrich and were
degassed and stored over 4 Å activated molecular sieves in
the glovebox. (COD)PtMe2 and DIPPIDH (1) were prepared
by published procedures.3,7 Reaction flasks were washed with
water and acetone and oven-dried prior to use. GC analyses
were performed on a Varian 3300 gas chromatograph equipped
with a molecular sieve column. Field desorption (FD) mass
spectra were measured on a Varian MAT 711 double-focused
mass spectrometer at The Institute of Mass Spectrometry,
University of Amsterdam, Amsterdam, The Netherlands.
Elemental analyses were carried out at the Hebrew University,
J erusalem, Israel.
Sp ectr oscop ic An a lysis. The 1H, 31P, and 13C NMR
spectra were recorded at 400.19, 161.9, and 100.6 MHz,
respectively, on a Bruker AMX 400 NMR spectrometer. All
chemical shifts (δ) are reported in ppm and coupling constants
(J ) in Hz. The 1H and 13C NMR chemical shifts are relative
to tetramethylsilane; the resonance of the residual protons of
the solvent was used as an internal standard h1 (7.15 ppm
benzene) and all-d solvent peaks (128.00 ppm benzene),
respectively. 31P{1H} NMR chemical shifts are relative to 85%
H3PO4 in D2O at δ 0.0 (external reference), with shifts
downfield of the reference considered positive. Screw-cap 5
mm NMR tubes were used in NMR follow-up experiments.
Assignments in the 1H and 13C{1H} NMR were made using
1H{31P}, 1H-1H COSY, and 13C-DEPT-135 NMR (distortionless
enhancement by polarization transfer). All measurements
were carried out at 298 K unless otherwise specified.
complex 2a to 130 °C for 2 days. Quantitative analysis of the
gas phase of the latter reaction by GC showed formation of 1
equiv of CH4. Anal. Calcd for C32H52P2Pt: C, 55.40; H, 7.55.
Found: C, 55.70; H, 7.80. 1H NMR (C6D6): δ 6.55 (s, 2H, ArH),
6.40 (s, 2H. ArH), 2.91 (br s, 4H, ArCH2P), 2.54 (br s, 4H,
PCH(CH3)2), 2.25 (s, 6H, ArCH3), 2.18 (br s, 4H, ArCH2Pt),
3
3
2.05 (s, 6H, ArCH3), 1.21 (dd, 12H, J HH ) 6.9 Hz, J PH ) 6.8
Hz), 0.91 (br m, 12H, PCH(CH3)2). 31P{1H} NMR (C6D6): δ
1
73.26 (s, 2P, J PtP ) 3270.2 Hz).
F ollow -u p of th e F or m a tion of cis-(DIP P IDH)(DIP -
P ID)P tMe (2a ). A solution of DIPPIDH (1; 45 mg, 0.180
mmol) in 0.6 mL of C6D6 was added to (COD)PtMe2 (20 mg,
0.060 mmol) and loaded in a 5 mm screw-cap NMR tube, which
was equipped with a septum and tightly closed using Teflon
tape and Parafilm. After approximately 1 h at room temper-
ature the reaction mixture was monitored by 31P{1H} NMR,
showing the presence of free ligand 1 and intermediate A, δ
26.64 (s, 1J PtP ) 1765.1 Hz). The reaction mixture was heated
for 10 min at 80 °C, resulting in the formation of complex 2a .
Analysis of the gas phase by GC and comparison with an
authentic sample showed the formation of CH4, which was also
observed by 1H and 13C{1H} NMR. This experiment was
repeated under the same conditions at 58 °C and monitored
by 31P{1H} NMR measurements at room temperature. The
results are presented in Figure 4.
F ollow -u p of th e F or m a tion of tr a n s-(DIP P IDH)(DIP -
P ID)P tMe (2b) a n d tr a n s-(DIP P ID)2P t (3b). A solution of
DIPPIDH (1; 32 mg, 0.128 mmol) in 0.6 mL of C6D6 was added
to (COD)PtMe2 (20 mg, 0.060 mmol) and loaded in a 5 mm
screw-cap NMR tube which was firmly sealed using Teflon
tape and Parafilm. The NMR tube was placed in an oil bath
at 100 °C and monitored by 31P{1H} NMR at room tempera-
ture. 31P{1H} NMR first showed formation of complex 2a and
intermediate A; the latter was only observed during the start
of the reaction. After prolonged heating, formation of com-
plexes 3b and 2b became visible, while complexes 2a and A
disappeared. The results are presented in Figure 2. Raising
the temperature to 130 °C resulted in disappearance of
complex 2b and an increase in the amount of 3b. This process
was completed within 2 h. This experiment was repeated by
starting with 2a (42 mg, 0.060 mmol) in 0.6 mL of C6D6 at
109 °C and monitored by 31P{1H} NMR at room temperature.
The results are presented in Figures 5 and 6.
X-r a y Cr yst a l St r u ct u r e Det er m in a t ion of tr a n s-
(DIP P ID)2P t (3b). A colorless transparent crystal (0.9 × 0.5
× 0.5 mm) was mounted on a glass fiber and flash-frozen under
a cold nitrogen stream (at 110 K) on a Rigaku AFC5R four-
circle diffractometer mounted on a rotating anode with Mo KR
radiation and a graphite monochromator. Accurate unit-cell
dimensions were obtained from a least-squares fit to setting
angles of 25 reflections in the range 2.1° e θ e 27.5°. The
SHELXS-928 and SHELXL-939 program packages installed on
a Silicon Graphics workstation were used for structure solution
and refinement. Structure 3b was solved using direct methods
(SHELXS-92) and refined by full-matrix least-squares tech-
niques based on F2 (SHELXL-93). The final cycle of the least-
squares refinement gave an agreement factor R of 0.0294
(based on F2) for all data. Hydrogens were calculated from
difference Fourier maps and refined in a riding mode with
individual temperature factors. An ORTEP view of the
molecular structure and the adopted numbering scheme is
shown in Figure 3. Table 1 gives details of the crystal
structure determination. Selected bond angles and distances
are listed in Table 3.
Syn t h esis of cis-(DIP P IDH)(DIP P ID)P t Me (2a ).
A
solution of DIPPIDH (1; 60 mg, 0.240 mmol) in 10 mL of
benzene was added dropwise to a stirred solution of (COD)-
PtMe2 (40 mg, 0.120 mmol) in 10 mL of benzene. After 3 days
of stirring at room temperature, the colorless solution was
submitted to high vacuum, yielding a colorless oil (80 mg, 94%).
Prolonged stirring for another 2 days did not result in the
formation of other products. Quantitative analysis of the gas
phase by GC showed formation of 1 equiv of CH4. FDMS
analysis showed m/ z 709 (calcd M•+ 709. Anal. Calcd for
C
33H56P2Pt: C, 55.84; H, 7.95. Found: C, 56.20; H, 8.25. 1H
2
NMR (C6D6): δ 6.7 (m, 4H, ArH), 3.26 (d, 2H, J PH ) 9.0 Hz,
3J PtH ) 19.0 Hz, ArCH2P), 2.84 (vt, 2H, J PH ) 10.4 Hz, J PtH
3
2
2
3
) 89.0 Hz, ArCH2Pt), 2.82 (d, 2H, J PH ) 9.2 Hz, J PPt ) 29.2
Hz, ArCH2P), 2.4-2.2 (m, 15H, ArCH3), 2.0 (m, 4H, PCH(CH3)2),
3
1.2-0.9 (m, 24 H, PCH(CH3)2), 0.90 (vt, 3H, J PH ) 6.4 Hz,
2J PtH ) 66.9 Hz, PtCH3). 31P{1H} NMR (C6D6): δ 65.97 (s, 1P,
1J PtP ) 2270.6 Hz), 31.21 (s, 1P, J PtP ) 1889.9 Hz). 13C{1H}-
1
3
2
NMR (C6D6): δ 150.2 (vt, J PC ) 7.1 Hz, J PtC ≈ 56 Hz, Cipso
-
CH2Pt), 138-124 (m, not resolved, CAr), 30-16 (m, not resolved
totally, ArCH3 and PCH(CH3)2), 30.63 (dd, 3J PC ) 7.6 Hz, 1J PC
2
2
) 248.9 Hz, J PtC ) 497.8 Hz, ArCH2PPt), 10.89 (vt, J PC,cis
)
2
2
6.5 Hz, ArCH2Pt), 5.02 (dd, J PC,cis ) 10.4 Hz, J PC,trans ) 90.3
1
Hz, J PtC ) 621.9 Hz, PtCH3).
Syn th esis of tr a n s-(DIP P ID)2P t (3b). A solution of
DIPPIDH (1; 30 mg, 0.120 mmol) in 10 mL of benzene was
added dropwise to a stirred solution of (COD)PtMe2 (20 mg,
0.060 mmol) in 10 mL of benzene. The solution was heated
to 110-120 °C using a pressure flask. After 3 days the
colorless solution was cooled to room temperature and was
concentrated in vacuo to approximately 1 mL, resulting in the
formation of colorless crystals of pure 3b, which were isolated
by filtration (25 mg, yield 59%). The 31P{1H} NMR of the
mother liquid showed only complex 3b. Quantitative analysis
of the gas phase by GC showed formation of 2 equiv of CH4.
Complex 3b is also formed by heating a benzene solution of
Resu lts a n d Discu ssion
a . F or m a tion a n d Id en tifica tion of Com p lexes
2a ,b a n d 3b. Displacement of COD from (COD)PtMe2
(8) Sheldrick, G. M. SHELXS-92 Program for Crystal Structure
Determination. University of Go¨ttingen, Go¨ttingen, Germany, 1992.
(9) Sheldrick, G. M. SHELXL-93 Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1993.
(7) Liou, S.-Y. Ph.D. Thesis, Weizmann Institute of Science, Rehovot,
Israel, 1995.

2564 Organometallics, Vol. 15, No. 10, 1996
van der Boom et al.
Ta ble 1. Cr ysta l Da ta for tr a n s-(DIP P ID)2P t (3b)
formula
fw
C32H52P2Pt
693.77
space group
cryst system
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
P1
triclinic
8.238(2)
9.677(2)
10.806(2)
66.07(3)
77.70(3)
89.39(3)
766.5 (3)
1.503
D
Z
calcd, g cm-3
1
47.0
0.9 × 0.5 × 0.5
µ(Mo KR), cm-1
crystal size, mm3
Ta ble 2. 31P {1H} NMR Da ta
P: δ, ppm
Pring: δ, ppm
(1J PPt, Hz)
∆(complex-ligand 1),
compd
(1J PPt, Hz)
ppm
1
A
2a
2ba
3b
4.60
26.64 (1765.1)
31.21(1889.9) 65.97 (2270.6) 26.61 (P), 61.37 (Pring
22.92 (2876.9) 71.85 (3319.5) 18.32 (P), 67.25 (Pring
F igu r e 1. Complexes 2a ,b and 3a ,b (3a is not observed;
the isopropyl substituents on P are omitted for clarity).
22.40 (P)
)
)
)
2
as a virtual triplet at δ 0.90 with J PtH ) 66.9 Hz and
73.26 (3270.2)
68.66 (Pring
3J PH ) 6.4 Hz, which collapses into a singlet upon 31P
a
2J PP ) 419.1 Hz.
decoupling.
in benzene at room temperature by the aromatic phos-
phine 1 results in quantitative formation of the sp3 C-H
activated cis product 2a (Figure 1) and 1 equiv of CH4.
Prolonged stirring, as well as changes in the metal/
ligand ratio, did not result in the formation of other
products. Compound 2a has been fully characterized
by various NMR techniques, FDMS, and elemental
Reacting (COD)PtMe2 with the ligand 1 in benzene
at higher temperatures (100-130 °C) in a pressure flask
results in quantitative formation of the double-C-H-
activated complex 3b and 2 equiv of CH4. Colorless
prismatic crystals are readily formed upon slow con-
centration of the reaction mixture. Complex 3b was
fully characterized by NMR, X-ray (vide infra), and
elemental analysis. Changing the metal/ligand ratio
does not influence the product formation, as observed
also in the formation of 2a . 31P{1H} NMR exhibits one
sharp singlet at δ 73.26 for both 31P nuclei, flanked by
1
analysis. CH4 was unambiguously detected by H and
13C{1H} NMR and quantified by GC. In the 31P{1H}
NMR (Table 2) two slightly broadened resonances of
equal intensity appear at δ 65.97 and 31.21 (w1/2 ) 10
and 40 Hz, respectively), flanked by 195Pt satellites.
These signals sharpen upon heating to 333 K. The
broadening is assumed to be a result of restricted bond
rotation caused by steric hindrance of the bulky phos-
phine ligands in cis positions and due to fluxionality of
the ring. The low-field chemical shift of δ 65.97 reflects
a deshielding effect of this phosphorus atom due to the
formation of the six-membered chelated ring.10,11,14 No
31P-31P coupling is observed, indicating that both
phosphorus atoms are coordinated mutually cis, which
is in agreement with the observed platinum-phospho-
rus coupling constants (1J PtP ) 2270.6 and 1889.9 Hz).
In addition, these coupling constants are characteristic
of phosphines trans to a strong σ donor such as an
alkyl.10-13 The structure of complex 2a is fully sup-
195Pt satellites, indicating that both phosphorus atoms
1
are magnetically equivalent. The observed J PPt
)
3270.2 Hz is typical of two mutually trans phosphorus
atoms.10 The low-field chemical shift of δ 73.26 reflects
a deshielding effect of both phosphorus atoms due to
1
the formation of two six-membered rings.10,11,14 The H
NMR shows slightly broadened resonances, which are
probably caused by fluxional behavior of these two rings.
The thermal stability of the double-cyclometalated
complex 3b is likely to be due to the presence of the
chelating rings, but it may be also kinetically inert due
to steric factors. The sterically hindered, but electroni-
cally favorable, cis isomer 3a was never observed.
However, monitoring the formation of 3b from 1 and
(COD)PtMe2 by 31P{1H} NMR at 100 °C (Figure 2)
shows the formation and disappearance of two species:
(i) the short-lived intermediate A, which is probably a
result of partial or full displacement of the diene by the
ligand(s) (Scheme 1), and (ii) the complex 2a . Mean-
while, complexes 2b and 3b are formed in parallel
pathways (vide infra). Raising the temperature to 130
°C at the end of the experiment for approximately 2 h
results in the disappearance of 2b.
Although complex 2b was not isolated, the structure
of 2b is unambiguous. Two different phosphorus signals
of equal intensity appear as sharp doublets at δ 71.85
and 22.92 in the 31P{1H} NMR and are accompanied
by satellites due to coupling with 195Pt. The large
phosphorus-phosphorus coupling (2J PP ) 419.1 Hz)
indicates that both nuclei are mutually trans, which is
in agreement with the observed 195Pt-31P coupling
1
ported by H and 13C{1H} NMR. For instance, the Pt-
CH3 group appears in the 13C{1H} NMR spectrum as a
double doublet at 5.02 ppm, flanked by 195Pt satellites
2
(1J PtC ) 621.9 Hz) with trans J PC ) 90.3 Hz and cis
2J PC ) 10.4 Hz. In 1H NMR this alkyl group appears
(10) (a) Pregosin, P. S. In Phosphorus-31 NMR Spectroscopy in
Stereochemical Analysis; Verkade, J . G., Quin, L. D., Eds.; VCH:
Deerfield Beach, FL, 1987. (b) Pregosin, P. S.; Kunz, R. W. 31P and
13C NMR of Transition Metal Complexes; Springer-Verlag: Berlin,
1979.
(11) Allen, F. H.; Pidcock, A. J . Chem. Soc. A 1968, 2700.
(12) (a) Garrou, P. E. Chem. Rev. 1981, 81, 229. (b) Linder, E.; Fawzi,
R.; Hermann, A. M.; Eichele, K.; Hiller, W. Organometallics 1992, 11,
1033.
(13) (a) Perera, S. D.; Shaw, B. L.; Thornton-Pett, M. J . Chem. Soc.,
Dalton Trans. 1993, 3653. (b) Hii, K. K.; Perera, S. D.; Shaw, B. L.;
Thornton-Pett, M. J . Chem. Soc., Dalton Trans. 1994, 103.
(14) Perera, S. D.; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1995,
641 and references therein.

Consecutive Cyclometalation by Pt(II)
Organometallics, Vol. 15, No. 10, 1996 2565
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 3b
Bond Lengths
Pt(1)-C(11)
Pt(1)-C(10)
Pt(1)-P(3)
Pt(1)-P(2)
P(2)-C(20)
P(2)-C(26)
P(2)-C(23)
2.146(11)
2.149(12)
2.262(3)
2.273(3)
1.82(2)
P(3)-C(36)
P(3)-C(33)
P(3)-C(30)
C(10)-C(312)
C(11)-C(212)
C(36)-C(37)
C(26)-C(27)
1.838(12)
1.853(12)
1.86(2)
1.52(2)
1.47(2)
1.843(12)
1.866(12)
1.54(2)
1.47(2)
Bond Angles
C(11)-Pt(1)-C(10) 178.4(6) P(3)-Pt(1)-P(2)
179.9(2)
92.5(3)
87.4(3)
112.2(4)
C(11)-Pt(1)-P(3)
C(11)-Pt(1)-P(2)
C(26)-P(2)-Pt(1)
91.8(3) C(10)-Pt(1)-P(2)
88.3(3) C(10)-Pt(1)-P(3)
110.9(4) C(36)-P(3)-Pt(1)
C(212)-C(11)-Pt(1) 113.7(7) C(312)-C(10)-Pt(1) 112.4(8)
C(212)-C(27)-C(26) 119.1(10) C(312)-C(37)-C(36) 116.5(11)
interesting to note that the sp2-sp3 carbon-carbon bond
lengths between C(10)-C(312) and C(11)-C(212) (1.52
and 1.47 Å, respectively) are significantly different. The
Pt(1)‚‚‚C(312) and the Pt(1)‚‚‚C(212) distances are rela-
tively short (3.066 and 3.047 Å, respectively), although
there is no additional spectroscopic evidence for any
metal-carbon interaction and the aromaticity is not
distorted in this complex. The C(312)-C(10)-Pt(1) and
C(212)-C(11)-Pt(1) angles (112.4 and 113.7°, respec-
tively) are indicative of an approximate tetrahedral
geometry around C(10) and C(11). The X-ray crystal
structure shows that both of the six-membered rings
Pt(1)-C(10)-C(312)-C(37)-C(36)-P(3) and Pt(1)-
C(11)-C(212)-C(27)-C(26)-P(2) have a distorted-boat
configuration. The planarity of the coordination geom-
etry around platinum has been determined by least-
squares plane analysis through the atoms Pt(1), P(2),
P(3), C(10), C(11), C(26), and C(36), which showed
deviations from the plane of 0.011, 0.010, 0.032, -0.004,
-0.030, 0.016, and -0.017 Å, respectively (mean devia-
tion from plane 0.0171 Å). The aromatic backbones of
both cyclometalated ligands are mutually trans.
F igu r e 2. 31P{1H} NMR Follow-up of the reaction of 1 (2.1
equiv) with (COD)PtMe2 (1 equiv) in C6D6 at 100 °C.
F igu r e 3. ORTEP view of complex 3b, showing that the
platinum atom has inserted into two C-H bonds, with
retention of the metal oxidation state.
constants (1J PtP ) 3319.5 and 2876.9 Hz).10,11 The low-
field chemical shift of δ 71.85 reflects the deshielding
effect of this phosphorus atom due to the six-membered
chelated ring,10,11,14 as also observed with 2a and 3b.
The chemical shift of δ 22.92 is similar to the one
assigned to the η1-bound phosphine 1 in 2a and A (Table
2). The σ-bonded Pt-CH3 group appears in 1H NMR
as a virtual triplet at δ 0.24 (3J PH ) 5.3 Hz) with
platinum satellites (2J PtH ) 53.2 Hz), which collapsed
into a singlet upon 31P decoupling.13
b. X-r a y Cr ysta l Str u ctu r e of (DIP P ID)2P t (3b).
Single-crystal X-ray diffraction of complex 3b unam-
biguously confirms its structure and also indirectly
supports the structure of precursors 2a ,b. An ORTEP
view of the molecular structure and the adopted num-
bering scheme is shown in Figure 3. Table 1 gives
details of the crystal structure determination. Selected
bond angles and distances are listed in Table 3. The
structure of 3b shows a monomeric neutral complex in
which the Pt(II) center has an almost perfect square-
planar coordination geometry. The phosphines as well
as the methylene groups are in a trans arrangement
with almost perfect linear angles of P(3)-Pt(1)-P(2)
(179.9°) and C(11)-Pt(1)-C(10) (178.4°). The Pt(1)-
P(2) (2.273 Å), Pt(1)-P(3) (2.262 Å), Pt(1)-C(10) (2.149
Å), and Pt(1)-C(11) (2.146 Å) distances are in the range
normally found for such bonds.15 The ligand arrange-
ment is the one expected on steric grounds; however,
the strongest trans directors, the methylene groups, are
situated trans, which is unfavored electronically. It is
c. Mech a n ism of th e F or m a tion of Com p lexes
2a ,b a n d 3b. 31P{1H} NMR follow-up of the reaction
of (COD)PtMe2 with 1 in benzene at room temperature
shows the formation of intermediate A by coordination
of the ligand 1 to the metal center (Scheme 1). The
postulated structure of A is analogous to those of
previously reported platinum-olefin complexes.16
31P{1H} NMR shows the presence of free 1 and 2a and
a broad singlet at δ 26.64 ppm (w1/2 ) 37 Hz), flanked
with its platinum satellites. This chemical shift is in
the normal range found for similar phosphine ligands
coordinated to platinum10 and is similar to the ones
assigned to the η1-bound phosphines in complexes 2a ,b.
In addition the platinum-phosphorus coupling (1J PtP
1765.1 Hz) is characteristic of phosphines trans to a
)
strong σ donor such as a CH3 group (as in complex
1
2a ).10-13 The H NMR is not conclusive but shows the
presence of a new signal at 4.79 ppm which is partially
overlapped with a signal of the starting material (COD)-
PtMe2. However, the structure of the postulated A
might also consist of two ligands 1 coordinated mutually
cis to the metal center, although this is not expected to
be favorable for steric reasons. Raising the temperature
(15) International Tables for Crystallography; Wilson, Ed.; A. T. C.,
Kluwer Academic: Dordrecht, The Netherlands, 1992; Vol. C, Math-
ematical, Physical and Chemical Tables.
(16) Chaloner, P. A.; Broadwood-Strong, G. T. L. J . Organomet.
Chem. 1989, 362, C21.

2566 Organometallics, Vol. 15, No. 10, 1996
van der Boom et al.
Sch em e 1. P r op osed Mech a n ism for th e Selective
F or m a tion of 2a (X ) COD, DIP P IDH (1))
F igu r e 4. 31P{1H} NMR follow-up of a reaction of 1 (3
equiv) with (COD)PtMe2 (1 equiv) at 58 °C in C6D6.
example, Whitesides and co-workers reported that bis-
(neopentyl)platinum(II) complexes undergo facile cyclo-
metalation.22 The reaction proceeds by initial dissocia-
tion of a phosphine ligand to generate a 14-electron
intermediate, followed by oxidative addition of a C-H
bond and finally by reductive elimination of neopentane.
Dissociation of the monocoordinated COD or ligand 1
from A results in the formation of the unsaturated 14-
electron complex B, which undergoes oxidative addition
of the C-H bond to give the five-coordinated alkyl
hydride C. Rapid C-H reductive elimination gives 1
equiv of CH4, rendering the overall process irreversible.
Complexes such as C are proposed as intermediates in
the protonolysis of platinum(II)-carbon σ-bonds, which
is a subject of current interest.23-26 A six-coordinated
alkyl(hydrido)platinum(IV) complex was isolated re-
cently by Tesauro.24 However, five-coordinate alkyl-
(hydrido)platinum(IV) species such as C are expected
to be very unstable toward C-H reductive elimination,
as reported very recently by Bercaw25 and Puddephatt.26
The resulting 14-electron complex D forms the more
stable 16-electron complex 2a by coordination of the
additional ligand 1. We believe that the selective
formation of the sterically hindered cis isomer 2a is due
to strong electronic effects in the proposed Pt(IV)
to 80 °C for 10 min results in the disappearance of A,
the quantitative formation of complex 2a , and the
production of 1 equiv of CH4. Thus, association of 1 and
opening of the chelated olefinic ring occurs before the
rate-determining step. Monitoring the disappearance
of 1 at 58 °C by 31P{1H} NMR shows formation of 2a as
the only product (Figure 4).
Bulky phosphines promote C-H activation by stabi-
lizing a conformation in which the C-H bond is in
proximity or in direct contact with the metal.6b,c In
many reported cases the C-H bond approaches the
metal at a short M‚‚‚H distance, which may be viewed
as a bent three-center-two-electron (3c-2e) bond in-
cluding C, H, and the metal.17 Although this process
(1 f 2a ) was also monitored by 1H NMR at room
temperature, no evidence was obtained for the presence
of so-called “agostic” interactions. A plausible oxidative-
addition-reductive-elimination pathway is presented in
Scheme 1. Cope and Siekmann18 reported for the first
time cyclometalation involving aryl C-H bond activa-
tion by platinum(II). Oxidative addition and reductive
elimination of C-H bonds by d8 L4 species generally
requires an unsaturated, three-coordinated, 14-electron
complex (d8 L3), as exemplified in many stoichiometric
reactions19 and supported by theoretical studies,20 al-
though direct C-H activations by square-planar 16-
electron complexes (which are likely to have a relative
large kinetic barrier) have also been reported.21 For
intermediate C. The σ-bonded benzyl ligand ArCH2PtIV
,
which is formed upon C-H activation, labilizes the Pt-
CH3 bond trans to it. Formation of CH4 by C-H
reductive elimination involving this alkyl group is
therefore preferred over the CH3 group trans to P. Since
CH4 formation is irreversible, only intermediate D is
likely to be formed, which is captured by 1 to form
complex 2a at a rate higher than its isomerization. An
alkyl(hydrido)platinum(IV) intermediate having one
methyl in the apical position will lead to the formation
of compound 2b. This selective transformation repre-
sents an unusual case of an organometallic reaction
which is entirely driven by electronic factors. Only the
sterically hindered, but electronically preferred, com-
pound is obtained. Since intermediate A is observed
prior to the C-H activation step, the rate-determining
(17) Crabtree, R. H. Chem. Rev. 1985, 85, 245 and references
therein.
(18) Cope, A. C.; Siekmann, R. W. J . Am. Chem. Soc. 1965, 87, 3272.
(19) (a) J ones, W. D.; Kosar, W. P. J . Am. Chem. Soc. 1986, 108,
5640. (b) Neve, F.; Ghedini, M.; Tiripicchio, A.; Ugozzoli, F. Inorg.
Chem. 1989, 28. 3084. (c) Farnetti, E.; Nardin, G.; Graziani, M. J . Chem
Soc., Chem. Commun. 1989, 1264. (d) J ones, W. D.; Feher, F. J . Acc.
Chem. Res. 1989, 22, 91 and references therein. (e) Ghosh, C. K.;
Hogano, J . K.; Krentz, K.; Graham, W. A. G. J . Am. Chem. Soc. 1989,
111, 5480. (f) Price, R. T.; Andersen, R. A.; Muetterties, E. L. J .
Organomet. Chem. 1989, 376, 407. (g) Hoffmann, P.; Heiss, H.; Neiteler,
P.; Muller, G.; Lachmann, J . Angew. Chem., Int. Ed. Engl. 1990, 29,
880. (h) Maguire, J . A.; Boese, W. T.; Goldman, M. E.; Goldman, A. S.
Coord. Chem. Rev. 1990, 97, 179. (i) McCamley, A.; Perutz, R.; Stahl,
S.; Werner, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1690. (j)
Freedman, D. A.; Mann, K. R. Inorg. Chem. 1991, 30, 836. (k) Cavell,
K. J .; J in, H. J . Chem. Soc., Dalton Trans. 1995, 4081.
(21) Gregory, T.; Harper, P.; Desrosiers, P. J .; Flood, T. C. Organo-
metallics 1990, 9, 2523 and references therein.
(22) (a) Foley, P.; Whitesides, G. M. J . Am. Chem. Soc. 1979, 101,
2732. (b) Foley, P.; Whitesides, G. M. J . Am. Chem. Soc. 1980, 102,
6713.
(23) Canty, A. J .; van Koten, G. Acc. Chem. Res. 1995, 28, 406 and
references therein.
(24) De Felice, V.; De Renzi, A.; Panunzi, A.; Tesauro, D. J .
Organomet. Chem. 1995, 488, C13.
(25) Stahl, S.; Labinger, J . A.; Bercaw, J . E. J . Am. Chem. Soc. 1995,
117, 9371.
(26) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J . Organometallics
1995, 14, 4966.
(20) Saillard, J .-Y.; Hoffmann, R. J . Am. Chem. Soc. 1984, 106, 2006.

Consecutive Cyclometalation by Pt(II)
Organometallics, Vol. 15, No. 10, 1996 2567
Sch em e 3. P r op osed Mech a n ism for th e Selective
F or m a tion Of Com p lex 3b
F igu r e 5. 31P{1H} NMR follow-up of thermolysis of 2a at
109 °C in C6D6.
Sch em e 2. P r op osed Mech a n ism for th e
Isom er iza tion of Com p lexes 2a T 2b.
forming the 14-electron complex F , which readily un-
dergoes C-H oxidative addition of the second η1-
coordinated ligand 1 to form the Pt(IV) species G.
Reductive elimination of CH4 gives H, which is followed
by association of the phosphine to give the thermally
stable complex 3b. It is likely that, in analogy with the
formation of 2b from 2a , first the cis isomer 3a is
formed, which isomerizes to the thermally stable trans
3b. However, 3a was not detected upon monitoring of
the second cyclometalation by 31P{1H} NMR. Only the
electronically unfavorable, but sterically preferred, com-
pound is obtained. Shaw and co-workers reported the
synthesis of two analogous cis and trans Pt(II) com-
plexes bearing two chelating five-membered rings.27
31P{1H} NMR follow-up of this process shows that the
isomerization process forming 2b is kinetically prefer-
able over formation of the double-cyclometalated 3b
(Figures 2 and 5). Dissociation of the η1-coordinated
phosphine from 2a is preferable over opening of the six-
membered chelate, although the latter is expected to
open readily in order to reduce the steric hindrance. Our
observations show that the isomerization process 2a f
2b is faster than the irreversible cyclometalation reac-
tion 2a f 3b, which suggests that formation of an
unsaturated 14-electron Pt(II) complex (D or F ) by
disassociation of the bulky ligand 1 (2a f 2b) or opening
of the chelate (2a f 3b) is rate determining. This
process is first order in 2a with kobs
step probably involves a later step such as the formation
of a 14-electron complex (A f B) by dissociation of the
olefin or the C-H cleavage itself (B f C).
d . Mech a n ism of th e F or m a tion of Com p lex 2b.
31P{1H} NMR follow-up of the second C-H activation
process shows that the cis complex 2a , which is formed
selectively at room temperature, undergoes two parallel
reactions upon heating: isomerization to the trans
complex 2b and C-H activation to form the double-
cyclometalated trans 3b (Figures 1, 2 and 5). Thus,
complex 2b is thermodynamically more stable than its
cis isomer 2a .
A plausible mechanism of this isomerization process
is presented in Scheme 2. Upon mild heating of the
kinetic isomer 2a , the η1-coordinated ligand 1 is likely
to dissociate due to (i) the strong trans influence of the
σ-bonded Pt-CH3 group and (ii) the steric hindrance of
the mutually cis phosphine ligands. We propose that
the trigonal species D is formed again and isomerizes
to E. Coordination of DIPPIDH (1) gives the observed
trans square-planar complex 2b.
) 2.3 × 10-3 s-1 at
109 °C (Figure 6).
It is tempting to assume that this isomerization
process is reversible (2a T 2b) and that 2a is the only
precursor of 3b. Chatt and Wilkins28 showed that the
cis isomers of (R3P)2PtCl2 complexes are in equilibrium
with their trans isomers. However, we cannot exclude
the possibility that 3b is formed by two parallel path-
ways (3b r 2a T 2b f 3b). The direct formation of 3b
from 2a and not from 2b is preferable if we consider
the loss of rotational entropy of the phosphine ligand
1, which occurs on cyclometalation. In 2a rotation is
restricted due to steric interaction, whereas in 2b it is
not. Thus, metalation of 2a will cause a relatively small
loss of rotational entropy compared to cyclometalation
The driving force of this isomerization process is
undoubtedly the decrease of steric hindrance between
the mutually cis phosphines in 2a . Complex 2a , having
two high trans-effect alkyls mutually cis, is favored
electronically. Compatible with the reduced steric
hindrance in complex 2b, the 31P{1H} NMR spectrum
of 2b exhibits sharp resonances, as compared with the
broad ones observed with complex 2a , indicating no
restriction of bond rotation. Thus, in this isomerization
process steric factors prevail over electronic ones in
controlling the relative stability of the products.
e. Mech a n ism of th e F or m a tion of Com p lex 3b.
Complex 3b can be formed from 2a (Scheme 3) by
following almost the same argumentation as in the
formation of complexes 2a ,b (Schemes 1 and 2). Upon
moderate heating, the six-membered ring opens up,
(27) Cheney, A. J .; Shaw, B. L. J . Chem. Soc., Dalton Trans. 1972,
754.
(28) Chatt, J .; Wilkins, R. G. J . Chem. Soc. 1952, 273, 4300.

2568 Organometallics, Vol. 15, No. 10, 1996
van der Boom et al.
F igu r e 6. 31P{1H} NMR follow-up of thermolysis of 2a at
109 °C in C6D6.
of 2b. The chelated ring of 2a is also expected to open
more readily, reducing the larger steric hindrance.
Therefore, the barrier to form 3b directly from 2b is
higher, if it occurs at all, which agrees with our
observations.
F igu r e 7. Relative free energy barriers for the formation
of complexes 2a ,b and 3b.
at the transition state, the second C-H activation is
directed by steric factors. The monometalated cis
complex 2a reacts in two competing pathways to give
the trans complex 2b and the double-cyclometalated
trans complex 3b. The cis-trans isomerization of the
monometalated complexes 2a f 2b is kinetically prefer-
able over the second cyclometalation 2a f 3b. Eventu-
ally the trans complex 2b is also selectively converted
to 3b. Complex 3b is directly generated from 2a and
might be directly generated form 2b, although the
isomerization process (2a f 2b) is likely to be reversible.
Our observations lead to the following estimations for
the relative free energy barriers for the formation of
2a ,b and 3b: ∆Gq < ∆Gq << ∆Gq (Figure 7).
Con clu sion s
Six-membered chelated benzylic platinum(II) com-
plexes are easily accessible by sp3 C-H activation using
(phosphino)arenes such as DIPPIDH (1) and (COD)-
PtMe2. Activation of two benzylic C-H bonds with
formation of two new M-C(sp3) σ-bonds results in
selective loss of 2 equiv of methane, with retention of
the metal oxidation state (eq 2).
2R-CH3 + MII(CH3)2 f (R-CH2)2MII + 2CH4 (2)
1
2
3
At relatively low temperatures, the distribution of the
products is governed by their relative rates of formation
(the order of rates of formation being 2a > 2b > 3b),
because under these conditions the rates of the reverse
reactions are negligible. The overall process shows that
the σ-bonded methylene group (ArCH2Pt) has a major
electronic effect on the course of the reaction but at the
end steric demands prevail over the electronic ones.
Our results indicate that the stepwise metal insertion
into both C-H bonds proceeds by a similar mechanism.
We believe that in both cases a coordinatively unsatu-
rated 14-electron complex is involved, possessing three
relatively strong σ-donors (two alkyl groups and one
phosphine ligand), which undergoes oxidative addition
to form alkylhydridoplatinum(IV) intermediates. This
process is followed by C-H reductive elimination to give
the ring-closed platinum(II) products and CH4. The
initial C-H activation takes places at room temperature
due to the relatively easy dissociation of the coordinated
olefin; the second C-H activation follows first-order
kinetics and requires more vigorous conditions, since
opening up of the six-membered platinacycle is involved.
The second cyclometalation is expected to be more
favorable for entropic reasons, although the first cyclo-
metalation reaction involves formation of two relatively
strong phosphorus-platinum(II) bonds. Both C-H
activation processes proceed in a regioselective manner;
whereas the first process is directed by electronic factors
Ack n ow led gm en t. We thank Roel H. Fokkens
(University of Amsterdam) for performing the FDMS
experiments. This work was supported by the U.S.-
Israel Binational Science Foundation, J erusalem, Israel.
D.M. is the holder of the Israel Matz professorial chair
of organic chemistry.
Su p p or tin g In for m a tion Ava ila ble: X-ray structure
analysis of 3b, including tables of crystal data and structure
refinement of 3b, atomic coordinates, bond lengths and bond
angles, and anisotropic displacement parameters (6 pages).
Ordering information is given on any current masthead page.
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