Pt(PEt3)(κ2-P,C-PEt2CHMe)(C 6Cl5): An unexpected intermediate in the synthesis of cis-Pt(PEt3)2(Cl)(C6Cl5)

Article abstract of DOI:10.1021/om1010879

Addition of 1 to 6 equiv of LiC₆Cl₅ to cis-Pt(PEt₃)₂Cl₂ gives the unexpected phosphine ligand C-H bond activation product Pt(PEt₃)(κ²-P,C- PEt₂CHMe)(C₆Cl₅) (1). Aqueous HCl addition to 1 gives the previously reported product of the reaction, cis-Pt(PEt ₃)₂(Cl)(C₆Cl₅) (2).

Full text of DOI:10.1021/om1010879

NOTE
pubs.acs.org/Organometallics
Pt(PEt3)(K2-P,C-PEt2CHMe)(C6Cl5): An Unexpected Intermediate in the
Synthesis of cis-Pt(PEt3)2(Cl)(C6Cl5)
Robert Robinson, Jr., Joseph M. Clarkson, Morgan A. Moody, and Paul R. Sharp*
125 Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211, United States
S Supporting Information
b
ABSTRACT: Addition of 1 to 6 equiv of LiC6Cl5 to cis-Pt(PEt3)2Cl2
gives the unexpected phosphine ligand C-H bond activation product
Pt(PEt3)(κ2-P,C-PEt2CHMe)(C6Cl5) (1). Aqueous HCl addition
to 1 gives the previously reported product of the reaction, cis-
Pt(PEt3)2(Cl)(C6Cl5) (2).
’ INTRODUCTION
of 1, and P1 is assigned to the metalated phosphine ligand. The
doublet at δ 19.9 is assigned to the “nonmetalated” phosphine,
P2. 195Pt-31P couplings are also consistent with this assign-
ment.7,8 P1 is trans to the strongly donating anionic C6Cl5 ligand
and has the smallest coupling while P2, with larger coupling, is
trans to the less strongly donating phosphorus-bonded CHMe
group. In the 195Pt NMR spectrum of 1 in CH2Cl2, the expected
A common method for the synthesis of Pt(II) alkyl and aryl
complexes involves addition of an organolithium or organomag-
nesium reagent to a Pt(II) halide followed by an aqueous, often
acidic workup to remove excess reagent and coproduct salts.1,2 It
is usually assumed that the Pt alkyl or aryl complex is formed
directly in the reaction and is present before and after the
workup. Herein, we report an example where this is not the
case, and the final Pt(II) aryl complex is obtained only after acidic
treatment of an initially formed phosphine ligand metalation
product.3
doublet of doublets appears at δ -3252 (JP1Pt = 1588 Hz, JP2Pt
=
3088 Hz). The 1H NMR spectrum of 1 is less readily interpreted
and contains multiple overlapping peaks in the PEt3 region that
could only be assigned with the assistance of COSY and HMQC
experiments (see Experimental Section). On the other hand, the
13C NMR spectrum of 1 is well resolved, and the three-
membered-ring carbon atom (C7) is apparent as the lowest
frequency signal (δ 4.3) consisting of a doublet of doublets with
satellites (JPC7 = 59 and 13 Hz, JPtC7 = 197 Hz). The ipso carbon
atom (C1) of the C6Cl5 ligand is the highest frequency signal (δ
167.4) and is also a doublet of doublets with satellites (JPC1 = 83
and 6 Hz; JPtC1 = 1150 Hz).
’ RESULTS AND DISCUSSION
The reported synthesis of Pt(II) aryl complex cis-Pt(PEt3)2-
(Cl)(C6Cl5) (2) employs the reaction of cis-Pt(PEt3)2Cl2 with
excess LiC6Cl5 or C6Cl5MgCl with an HCl(aq) workup.4 (The
steric bulk of the C6Cl5 ligand was assumed to prevent a second
arylation.) In repeating this synthesis, we left out the acidic
workup and were surprised to isolate not 2, but rather a new
complex, Pt(PEt3)(κ2-P,C-PEt2CHMe)(C6Cl5) (1). Thus, 1
was obtained by reaction of cis-Pt(PEt3)2Cl2 with 1 to 6 equiv
of LiC6Cl5 in THF followed by a neutral aqueous workup
(Scheme 1).
A single crystal of 1, suitable for X-ray diffraction analysis, was
obtained by slow vapor diffusion of EtOH into a CH2Cl2 solution
of 1 at room temperature. Complex 1 (Figure 1) shows a planar
geometry at the Pt(II) metal center; however, the typical square
shape is distorted by the presence of the Pt,P,C ring.
The 31P NMR spectrum for 1 in CDCl3 (see Figure 1 for atom
numbering) exhibits two doublets at δ -22.4 (JPtP1 = 1545 Hz,
JP1P2 = 29 Hz, P1) and 19.9 (JPtP2 = 3007 Hz, P2), a pattern not
unlike that reported for 2.5 However, due to the large negative
shift of P1, the chemical shift difference between the doublets
(Δδ = 42.3) is very much larger than expected for 2. The shift for
P1 is consistent with P1 being part of the three-membered ring6
Received: November 17, 2010
Published: February 23, 2011
r
2011 American Chemical Society
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|

Organometallics
NOTE
Scheme 1
solvent and adventitious water interacting with the SiO2 surface.)
A single crystal of 2 for X-ray diffraction was obtained from slow
evaporation of CH2Cl2 solution of 2 at room temperature (SI).15
The above NMR and structural data match well with that
reported for related Pt and Pd complexes containing M,P,C
three-membered rings.9-19 Most closely related are the com-
plexes Pt(PEt3)(κ2-P,C-PEt2CHMe)(R) (R = a carboranyl, e.g.,
PhC2B10H10-), which are formed in exactly the same way as 1,
that is, from the reaction of PtL2Cl2 (L = a phosphine) and a
carboranyl lithium reagent.9-13 (The first report of these
complexes9 incorrectly assigns the metalation to the β-carbon
of the phosphine ligands.)
The formation of 1 and its carboranyl analogues is rather
curious, and we conducted experiments to learn more about the
formation of 1. The reaction of Pt(PEt3)2Cl2 and LiC6Cl5 was
repeated with incremental additions of LiC6Cl5 and 31P NMR
monitoring. Other than trace amounts of 2, 1 and C6Cl5H (1H
NMR: δ 7.54) are the only detected products. With less than
stoichiometric amounts of the Li reagent the products are the
same but with remaining Pt(PEt3)2Cl2. Complex 2 is not an
intermediate in the formation of 1, as there is no reaction between
isolated 2 and LiC6Cl5 under similar conditions (Scheme 1). It
seems that either phosphine ligand metalation precedes Pt center
arylation or a species other than 2 is formed in the arylation
reaction and undergoes rapid phosphine metalation.
Two possible pathways are shown in Scheme 2. A key feature
for pathway (a) is PEt3 dissociation, which would probably need
to precede the bulky aryl anion attack. Chloride loss from anionic
3 with coordination of a triethylphosphine C-H bond would
activate the C-H bond to deprotonation20 in 4. Deprotonation
by the second aryl anion gives 5, and PEt3 displacement of
chloride, with a geometric rearrangement, would then yield 1.
For this pathway to be successful, free PEt3 must not capture 3 or
4, or 2 would be the product. While this may be the case with low
PEt3 concentrations, capture would be much more likely with
added PEt3. In addition, added PEt3 should slow the reaction rate
by suppressing PEt3 dissociation in the first step. We added 1
equiv of free PEt3 to the reaction and observed no change in the
product distribution or in the qualitative reaction rate, suggesting
that pathway (a) is not operative.
Pathway (b) occurs by aryl anion attachment on a coordinated
PEt3 to give 7, either by deprotonation (R-CH deprotonation
of alkyl phosphines has been reported21-24) to give 7 directly or
by attack at the P atom to form metallaphosphorane25 6 followed
by RH elimination. We favor direct deprotonation given the
steric crowding around the P center in 6. Chloride displacement
by the ylidic carbon center in 7 then gives 8, which would be
sterically much more open than PtCl2(PEt3)2, allowing aryl
anion/chloride exchange to give 1. Added PEt3 should not affect
this pathway.
Figure 1. Drawing of the solid-state structure of Pt(PEt3)(κ2-P,C-
PEt2CHMe)(C6Cl5) (1) (50% probability ellipsoids). Hydrogen atoms
are omitted. The methylene groups of the PEt3 ligand are disordered
over two positions. Only the major (70%) positions are shown. Selected
bond distances (Å) and angles (deg) for 1: Pt-P1, 2.226(2); Pt-P2,
2.281(2); Pt-C1, 2.057(5); Pt-C7, 2.158(6); P1-C7, 1.757(6); C7-
C8, 1.514(7); P1-Pt-P2, 116.28(5); P2-Pt-C1, 96.5(1); C1-Pt-
C7, 100.1(2); C7-Pt-P1, 61.7(1); Pt-C7-P1, 42.8(1); C7-P1-Pt,
64.4(2); Pt-C7-C8, 118.5(3); P1-C7-C8, 122.9(3).
The Et2PCHMe ligand can be formulated in the extremes as a
metalated phosphine ligand where the C atom bonded to the Pt is
essentially an anionic alkyl group (A) or as a metal-substituted
phosphorus ylide ligand with π-donation to the Pt center from the
P-C double bond (B). The P1-C7 distance of 1.757(6) Å is
shortened relative to the other P-C distances (average = 1.83),
consistent with double-bond character and the ylide formulation
B. Also in agreement with the ylide formulation is the Pt-C7
distance (2.158(6) Å), which is larger than that expected for an
alkyl-typeinteraction (compare tothePt-C1 distanceof2.057(5)
Å). The long distance also agrees with the 31P NMR data, which
indicate a relatively low trans influence for C7 (see above).
The stability and reactivity of 1 were briefly examined. Heating
solutions of 1 in THF at 90 °C overnight under inert or
atmospheric conditions gave no visible or spectroscopic indica-
tion of decomposition. An identical result was observed when 1
was heated in wet THF. As shown in Scheme 1, excess HCl,
gaseous or aqueous, gives 2, consistent with the unrecognized
conversion of 1 into 2 by the acidic workup in the reported
preparation of 2.4 Complex 2 was also acquired from a thick-layer
SiO2 chromatographic plate by eluting with hexanes/CH2Cl2
(90:10) in air. (The source of the HCl is presumably the CH2Cl2
In conclusion, we find that in the reported synthesis of aryl
complex cis-Pt(PEt3)2(Cl)(C6Cl5) (2) the primary product is
phosphine metalation complex Pt(PEt3)(κ2-P,C-PEt2CHMe)-
(C6Cl5) (1), and 2 is only formed after an acidic workup. The
analogous formation of 1 and previously reported Pt(PEt3)(κ2-P,
C-PEt2CHMe)(R) (R = a carboranyl) complexes probably occurs
by direct deprotonation of the coordinated PEt3 ligand.
’ EXPERIMENTAL SECTION
General Procedures. All procedures were performed under a
dinitrogen atmosphere in a Vacuum Atmospheres Corporation drybox
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Organometallics
NOTE
Scheme 2
unless otherwise stated. All solvents were dried and degassed by standard
techniques and stored under dinitrogen over 4 Å molecular sieves or
sodium metal. n-Butyllithium and hydrochloric acid were obtained from
commercial sources (Aldrich or Acros) and used as received. Commer-
cial hexachlorobenzene was recrystallized from benzene. cis-PtCl2-
under the signals around δ 1.1 and adds 1H to the integration in this
region. One line of the expected complex multiplet may be seen at δ
1.09. There is a possibility that the PEt2CHCH3 and PEt2CHMe peak
assignments are reversed. The 1H-13C HMQC spectrum did not show
1
a clear cross-peak for either H NMR signal, and our assignment was
26
(PEt3)2 and lithiopentachlorobenzene27 were prepared by literature
based on integration values (see SI). A previously reported Ph2PCHMe
Pt complex14 showed the CH peak at δ 1.63 and the CH3 peak at δ 0.90,
the reverse of our assignment order in δ. 13C{1H} NMR (CDCl3, 125
MHz): 167.4 (dd with satellites, JPtC = 1150, JPC = 83 and 6, C1), 137.2
(d with satellites, JPtC = 28, JPC = 42, C6Cl5), 132.3 (s), 129.1 (dd with
satellites, JPtC = 73, JPC = 83 and 6, C6Cl5), 124.9 (s, C6Cl5), 20.0 (dd
with satellites, JPC = 28 and 4, JPtC = 45, P(CH2CH3)3), 13.5 (pseudo t,
apparent JPC = 7, PEt2CHCH3), 13.3 (dd, JPC = 25, PEt(CH2CH3)-
CHMe)), 11.6 (d with satellites, JPC = 25, JPtC ≈ 7, PEt(CH2CH3)-
0CHMe)), 10.0 (d with satellites, JPC = 4, JPtC = 28, PEt(CH2CH3)-
methods. Hydrogen chloride gas was prepared from NaCl and sulfuric
acid. NMR spectra were recorded on Bruker AMX-250, -300, or -500
spectrometers at ambient probe temperature unless otherwise stated.
Chemical shifts are given as δ, and coupling constants (J) in Hz. 1H and
13C NMR shifts are relative to internal TMS (0 ppm) as referenced
to solvent signals. 31P NMR shifts are relative to external 85% H3PO4
(0 ppm), and 195Pt NMR shifts are relative to external K2PtCl4/D2O
(-1624 ppm). NMR peak assignments were assisted by COSY, DEPT,
and HMQC experiments (SI).
Pt(PEt3)(K2-P,C-PEt2CHMe)(C6Cl5) (1). A suspension of C6Cl6
(50.3 mg, 0.177 mmol) in 0.5 mL of THF was cooled to -30 °C in a
refrigerator. The C6Cl6 mixture was removed from the refrigerator, and
nBuLi (0.11 mL, 0.176 mmol, 1.6 M in hexanes) was added dropwise
into the stirred C6Cl6 mixture over a 5 min period.27 The resulting dark
brown solution was stirred for another 20 min and then replaced in the
refrigerator for ca. 20 min. The cold solution was then slowly added
dropwise to a stirred suspension of cis-Pt(PEt3)2Cl2 (40.8 mg, 0.0812
mmol) in 0.5 mL of THF, previously cooled to -30 °C in the
refrigerator. The dark brown mixture was stirred for 20 min and allowed
to warm to room temperature. The mixture was filtered through
diatomaceous earth, and the volatiles were removed under reduced
pressure to give a dark brown oil. The oil was dissolved in CHCl3 and
washed 3ꢀ with H2O. The volatiles were removed under reduced
pressure from the organic fraction, and the resulting dark brown solid
was washed with 0.2 mL of EtOH, collected by filtration, and dried under
reduced pressure to give crude brown solid 1. Yield: 33 mg (60%). The
brown material is nearly pure by NMR and may be recrystallized from
CH2Cl2/EtOH to give a yellow solid. Yellow single crystals for X-ray
analysis were grown by slow vapor diffusion of EtOH into a CH2Cl2
solution at room temperature.
CHMe)), 9.4 (s with satellites, JPtC = 25), 8.5 (s with satellites, JPtC =
20), 4.3 (dd with satellites, JPtC = 197, JPC = 59 and 13, PEt2CHMe
(C7)). The C7 signal was clearly established as a CH in the DEPT
experiments. 31P{1H} NMR (CDCl3, 101 MHz): 19.7 (d with satellites,
JPtP = 3004, JPP = 29, P2); -22.4 (d with satellites, JPtP = 1542, JPP = 29,
P1). Essentially identical spectra are obtained in CH2Cl2, hexanes, THF,
and C6D6. 195Pt NMR (CH2Cl2, 64 MHz): -3252 (q, JPPt = 3088 and
1588). 195Pt NMR (hexanes, 64 MHz): -3260 (q, JPPt = 3063 and
1525).
cis-Pt(PEt3)2(Cl)(C6Cl5) (2) from cis-(PEt3)Pt(κ2-P,C-PEt2
CHMe)(C6Cl5) (1). (a). Complex 1 (15.7 mg, 0.0231 mmol) dissolved
in 0.2 mL of CDCl3 was placed into a screw-capped NMR tube fitted
with a rubber septum. HCl(g) (2.0 mL, 0.065 mol) was injected by
syringe into the NMR tube, and the tube was shaken. The volatiles were
removed under reduced pressure to give 2 as a brown solid. Yield: 15.1
mg (91.3%). HCl(aq) could be used in place of HCl(g). (b) Complex 1
was eluted with hexanes/CH2Cl2 (90:10) on a thick chromatographic
plate and then extracted with C6H6. The volatiles were removed under
reduced pressure to give 2 as a brown solid. Light yellow single crystals
for X-ray analysis were grown by slow vapor evaporation of a CH2Cl2
solution at room temperature. Details of the structure are available in the
Supporting Information. The 195Pt and 31P NMR data match the
literature report.5
MS (ESI/APCI) m/z (rel intensity %): [M þ H]þ 679 (50), 681
(45), 680 (39), 683 (22), 682 (17), 678 (27), 677 (28), 684 (11), 685
(12), 686 (4), 687 (3); [M - C6Cl5]þ 429 (40), 430 (39), 431 (29), 433
(10), 432 (7), 434 (2), 427 (22). 1H NMR (CDCl3, 500 MHz): 2.10-
1.87 (overlapping m’s, 5H, PEt(CH2Me)CHMe and PEt2CHCH3),
1.68 (br m, 6H, P(CH2Me)3), 1.28 (overlapping dq, JHH = 7, JPH = 9,
1H NMR (CDCl3, 300 MHz): 2.03-1.95 (m, JHH = 7.5, 6H,
P(CH2Me)3 trans to Cl), 1.79-1.62 (m, JHH = 7.8, 6H, P(CH2Me)3
cis to Cl), 1.24-1.12 (m, JHH = 7.5, 18H, P(CH2CH3)3 trans to Cl),
1.14-1.04 (m, JHH = 8.1, 9H, P(CH2CH3)3 cis to Cl). 13C NMR
(CDCl3, 125 MHz): 137.3 (s, C6Cl5), 130.4 (s, C6Cl5), 128.3 (s, C6Cl5),
128.0 (s, C6Cl5), 18.2 (d, JPC = 37.7, P(CH2Me)3 cis to Cl), 14.6 (d,
JPC = 31.4, P(CH2Me)3 trans to Cl), 8.4 (d, JPC = 21.4, P(CH2CH3)3).
31P{1H} NMR (CDCl3, 101 MHz): 7.8 (d with satellites, JPtP = 2034,
2H, PEt(CH2Me)0CHMe), 1.23 (dt overlapping with 1.28 signal, JHH
=
8, JPH = 18, 3H, PEt(CH2CH3)CHMe)), 1.06 (dt, 3H, JHH = 7, JPH = 19,
PEt(CH2CH3)0CHMe)), 0.99 (overlapping dt, 9H, P(CH2CH3)3). The
COSY spectrum indicates that the PEt2CHMe resonance is buried
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Organometallics
NOTE
JPP = 19, PEt3 cis to Cl); -0.18 (d with satellites, JPtP = 3937, JPP = 19,
PEt3 trans to Cl). 31P{1H} NMR (CH2Cl2, 101 MHz): 7.6 (d with
satellites, JPtP = 2053, JPP = 19, PEt3 cis to Cl), -0.12 (d with satellites,
JPtP = 3903, JPP = 19, PEt3 trans to Cl). 31P{1H} NMR (C6H6, 101
MHz): 7.8 (d with satellites, JPtP = 2037, JPP = 19, PEt3 cis to Cl), -0.80
(d with satellites, JPtP = 3855, JPP = 19, PEt3 trans to Cl). 195Pt NMR
(CDCl3, 64 MHz): -2,889 (pseudo q, JPPt = 3989 and 2075). 195Pt
NMR (CH2Cl2, 64 MHz): -2,889 (pseudo q, JPPt = 3928 and 2037).
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic files in
b
CIF format for 1 and 2 and selected NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: SharpP@missouri.edu.
’ ACKNOWLEDGMENT
Support from the U.S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-88ER13880), is gratefully acknowl-
edged. A grant from the National Science Foundation (9221835)
provided a portion of the funds for the purchase of the NMR
equipment. We thank Nandita M. Weliange for 195Pt NMR
assistance and a reviewer for suggesting Scheme 2b.
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