Frustrated Lewis pair-like splitting of aromatic C-H bonds and abstraction of halogen atoms by a cationic [(FPNP)Pt]+ species

Article abstract of DOI:10.1039/c2sc21385k

This manuscript explores new chemistry that can be related to the unobserved 14-electron [(^FPNP)Pt]⁺ transient ( ^FPNP = (4-F-2-(ⁱPr₂P)C₆H ₃)₂N). Its reactivity can be accessed via abstraction of triflate from (^FPNP)PtOTf (1) with K[B(C₆F ₅)₄] in various solvents serving as substrates. With benzene, toluene and fluorobenzene, net heterolytic splitting of an aromatic C-H bond across the N-Pt bond is observed, without any detectable intermediates, leading to the [(^FPN(H)P)Pt-Ar]⁺ products (Ar = Ph, 3a; Ar = C₆H₄Me, 3b (ortho), 3c (meta), 3d (para); Ar = o-C ₆H₄F, 3e). The latter can be alternatively prepared by protonation of the neutral (^FPNP)Pt-Ar compounds. Compounds 3a-3e do not release free arene under thermolysis at 80 °C, and compounds 3b/c/d do not interconvert under ambient temperature. With chlorobenzene and bromobenzene, the kinetic product is the κ¹-Cl or κ¹-Br adduct [(^FPNP)Pt-Cl-Ph]⁺ (4) or [(^FPNP)Pt-Br- Ph]⁺ (10). Compound 4 rearranges into a C-H splitting product [( ^FPN(H)P)Pt-C₆H₄Cl]⁺ (3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is observed upon the reaction of 1 with K[B(C₆F ₅)₄] in dichloromethane, producing [(^FPNP)PtCl] [B(C₆F₅)₄] (9a) which features an oxidized ^FPNP ligand framework. X-Ray diffractometry established structures of [(^FPN(H)P)Pt-C₆H₄F-o][B(C₆F ₅)₄] (3e, disordered rotamers), [(^FPN(H)P)Pt- C₆H₄Me][B(C₆F₅)₄] (disordered meta- and para-isomers 3c/d), and [(^FPNP)PtCl][HCB ₁₁Cl₁₁] (9b). DFT calculations at the PBE0 and M06-L levels on the free [(^FPNP)Pt]⁺ cation predict a relatively small (10-12 kcal mol^-1) separation between the singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the unexpected halogen atom abstraction reactions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2sc21385k

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Frustrated Lewis pair-like splitting of aromatic C–H
bonds and abstraction of halogen atoms by a cationic
[(FPNP)Pt]+ species†
Cite this: Chem. Sci., 2013, 4, 642
*
Jessica C. DeMott, Nattamai Bhuvanesh and Oleg V. Ozerov
This manuscript explores new chemistry that can be related to the unobserved 14-electron [(FPNP)Pt]+
transient (FPNP ¼ (4-F-2-(iPr2P)C6H3)2N). Its reactivity can be accessed via abstraction of triflate from
(FPNP)PtOTf (1) with K[B(C6F5)4] in various solvents serving as substrates. With benzene, toluene and
fluorobenzene, net heterolytic splitting of an aromatic C–H bond across the N–Pt bond is observed,
without any detectable intermediates, leading to the [(FPN(H)P)Pt–Ar]+ products (Ar ¼ Ph, 3a; Ar ¼
C6H4Me, 3b (ortho), 3c (meta), 3d (para); Ar ¼ o-C6H4F, 3e). The latter can be alternatively prepared by
protonation of the neutral (FPNP)Pt–Ar compounds. Compounds 3a–3e do not release free arene under
thermolysis at 80 ꢀC, and compounds 3b/c/d do not interconvert under ambient temperature. With
chlorobenzene and bromobenzene, the kinetic product is the k1-Cl or k1-Br adduct [(FPNP)Pt–Cl–Ph]+ (4)
or [(FPNP)Pt–Br–Ph]+ (10). Compound 4 rearranges into a C–H splitting product [(FPN(H)P)Pt–C6H4Cl]+
(3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is
observed upon the reaction of 1 with K[B(C6F5)4] in dichloromethane, producing [(FPNP)PtCl][B(C6F5)4]
(9a) which features an oxidized FPNP ligand framework. X-Ray diffractometry established structures of
[(FPN(H)P)Pt–C6H4F-o][B(C6F5)4] (3e, disordered rotamers), [(FPN(H)P)Pt–C6H4Me][B(C6F5)4] (disordered
meta- and para-isomers 3c/d), and [(FPNP)PtCl][HCB11Cl11] (9b). DFT calculations at the PBE0 and M06-L
levels on the free [(FPNP)Pt]+ cation predict a relatively small (10–12 kcal molꢁ1) separation between the
singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the
unexpected halogen atom abstraction reactions.
Received 29th August 2012
Accepted 23rd October 2012
DOI: 10.1039/c2sc21385k
www.rsc.org/chemicalscience
Pincer ligands15,16 offer a convenient framework for studies
of square-planar compounds in which only one coordination
Introduction
site is intended to be the “reactive” site.17 Pincer complexes are
perhaps best known for their thermal stability, but equally
important attributes include the prevention of undesired ligand
exchange and redistribution and a high degree of control over
the positioning of the ancillary donor atoms. The work we
present here was inspired and inuenced by two sets of ndings
in the literature. Peters and Harkins reported in 2002 that
reacting (NNN)PtOTf with a trialkylamine base in benzene led to
heterolytic C–H activation of the solvent (Scheme 1A).18 Similar
ndings were reported by the Liang group with (PNPPh)PtOTf.19
The mechanism that appears to be most consistent with the
results of both Liang et al. and Peters et al. involves displace-
ment of the triate ligand with a solvent molecule (via either a
dissociative or associative mechanism) to form a cationic Pt–
arene complex.20,21 This intermediate could either be sufficiently
acidic to be deprotonated by a relatively weak base or it could
undergo C–H oxidative addition, giving rise to an even more
acidic PtIV aryl/hydride species.20–22
Platinum chemistry, particularly some of the early work by
Shilov,1–4 has played a central role in the development of the
modern understanding of carbon–hydrogen bond activation
and the current techniques for performing such trans-
formations selectively and efficiently. Arranging for a hydro-
carbon to react with a three-coordinate d8 fragment is a
strategy that has been used in a number of successful
approaches to C–H activation and functionalization.5–8 Recog-
nized as important intermediates in C–H activation, hydro-
carbon s-complexes have been the subject of many studies:
direct spectroscopic observations in rare accessible cases,9–12
indirect investigations using the tools of kinetics,13 and
computational work.14
Department of Chemistry, Texas A&M University, College Station, TX 77842, USA.
E-mail: ozerov@chem.tamu.edu
† Electronic supplementary information (ESI) available: Details of experimental
and computational procedures and characterization, coordinate les for
calculated structures. CCDC 898629 (3c/d), 898630 (3e) and 898631 (9b). For ESI
and crystallographic data in CIF or other electronic format. See DOI:
10.1039/c2sc21385k
Additionally, the seminal 2009 report by Goldberg, Broo-
khart et al. demonstrated that a neutral pyridine/bis(phos-
phinite) PONOP ligand can be used to allow for the observation
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form of the analogous group 10 complex [(MePNP)Pd]+ functions
as a frustrated Lewis pair with the ability to heterolytically split
alkynyl C–H bonds,31 B–H bonds and B–B bonds (Scheme 2).32
We envisioned that the three-coordinate d8 fragment [(FPNP)Pt]+
(FPNP ligand drawn in Scheme 3) would be a promising design
for observing hydrocarbon complexes of cationic PtII and set out
to explore whether [(FPNP)Pt]+ could be generated with a weakly
coordinating counter-anion. Aromatic hydrocarbons and halo-
genated hydrocarbons seemed suitable as somewhat polar and
weakly coordinating solvents that could either give rise to C–H
complexes themselves, or permit an added hydrocarbon
substrate to bind. While we were easily able to remove an
anionic ligand from a four-coordinate Pt precursor, the results
we obtained were rather unexpected. Instead of hydrocarbon
complexes, we observed heterolytic splitting of aromatic C–H
bonds across the Pt–N bond of the [(FPNP)Pt]+ fragment. On the
other hand, certain halogenated hydrocarbons reacted with
[(FPNP)Pt]+ via the net transfer of a halogen atom – a very
unusual reaction for a PtII cation. Both of these pathways derive
from the non-innocence of the central amido ligand of FPNP: as
a Brønsted base in the rst case, and as an oxidizable ligand in
the other. Nitrogenous ligands have been ubiquitous in the
studies of Pd and Pt-mediated C–H activation, but a clear
demonstration of the potential for these new reactivity path-
ways has not been previously reported.
Scheme 1 (A) C–H activation of benzene by pincer complexes of Pt by Peters
et al.18 and by Liang et al.19 (B) Synthesis of a Rh–methane complex by Goldberg,
Brookhart et al.23
of a unique square-planar d8 RhI–methane adduct (Scheme
1B).23 This nding and the above mentioned results of Peters
and Liang18,19 suggested the possibility that an anionic pincer
ligand may provide access to cationic hydrocarbon complexes of
PtII. h2-C,C–arene complexes of PtII have been observed and
even isolated, but primarily in systems where the ligands are
considerably less sterically encumbering than a pincer ligand
with bulky anking phosphine arms.20–22 Yamashita and Nozaki
very recently reported the structure of an h2-C,H–arene complex
involving a d8 (PBP)Rh fragment (PBP is a boryl/bis(phosphine)
pincer ligand)24 while Peters et al. reported a relatively long-
range interaction between a cationic PtII center supported by a
tripodal silyl/tris(phosphine) ligand in a C–H bond of toluene in
the solid state.25
Results and discussion
C–H activation of arenes
Desiring to test whether the (FPNP)Pt system performs similarly
to the NNN- and PNPPh-supported systems,18,19 we prepared
complex (FPNP)PtOTf analogously to the reported (MePNP)
PtOTf.30 We specically chose the FPNP ligand because of its
uorine substituents, which do not have much inuence33 on
the reactivity at the metal but provide a very convenient and
informative NMR spectroscopic probe.34 As expected, thermol-
ysis of 1 at 115 ꢀC with Et3N in C6D6 or protio-toluene produced
results (Scheme 3) analogous to the ndings of Peters et al.18
and Liang et al.19 The consumption of 1 was slow; 7 days were
required for completion of the reaction with toluene and in the
reaction with C6D6, only 83% conversion was seen aer 2 weeks.
The corresponding Pt–aryl complexes 2a–d were observed by
NMR spectroscopy, along with <10% of (FPNP)PtH.35 In the case
of toluene, a mixture of Pt–tolyl isomers was produced. An
o : m : p ratio of ca. 1 : 68 : 31 was determined by comparison
with independently synthesized authentic samples (vide infra).
Our group has frequently used diarylamido/bis(phosphine)
PNP pincer ligands.26–29 For example, we previously reported that
[(MePNP)Ni]+ acts as an efficient catalyst for the coupling of
acetonitrile and aldehydes.30 Additionally, wefoundthatamasked
Scheme 2 1,2-Addition reaction of alkynyl C–H and of B–B bonds with the
[(MePNP)Pd]+ fragment.31
Scheme 3 C–H activation of arenes with (FPNP)PtOTf and base.
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In order to access the [(FPNP)Pt]+ cation, we used K[B(C6F5)4]
(K-BARF) to abstract the triate from 1. Stirring a mixture of
(FPNP)PtOTf and K-BARF in C6H6 for 4 days at ambient
temperature resulted in complete consumption of 1 and full
conversion to 3a. The rate of the reaction was probably
depressed by the limited solubility and the low rate of dissolu-
tion of K-BARF in benzene. Heating accelerated the reaction,
and at 80 ꢀC, the analogous products 3a-D, 3b–d or 3e could be
synthesized from (FPNP)PtOTf and K-BARF in C6D6, toluene or
uorobenzene aer just 2–16 h.
Compounds 3a–f are the products resulting from formal
additions of aromatic C–H bonds across the Pt–N bond in
[(FPNP)Pt]+. In our previous work,31 we pointed out that the
analogous [(MePNP)Pd]+ fragment can be viewed as a frustrated
Lewis pair.36 The lone pair on N and the empty orbital on the
metal are orthogonal to each other, while the steric bulk
prevents the Namido from bridging, resulting in a “frustrated”
arrangement. We did not observe aromatic C–H splitting with
[(MePNP)Pd]+. A single product 3a/3a-D was observed in each of
the reactions with C6H6 or C6D6. The assigned structures of 3a
and 3a-D are consistent with the NMR spectra, including the NH
1
proton resonance at 7.26 ppm in the H NMR spectrum of 3a
1
and the overall time-averaged Cs symmetry in the H, 13C{1H},
31P{1H} and 19F NMR spectra. We have previously commented
on the sensitivity of the 19F NMR chemical shi exhibited by the
FPNP ligand to the environment at the metal, particularly with
respect to the amido–metal or amine–metal nature of the N
donor.34 In 3a, the 19F NMR resonance was detected at ꢁ111.7
ppm, which is consistent with a protonated amine form of the
PNP ligand.
Scheme 4 1,2-Addition of arenes.
Similar reactions run in uorobenzene or toluene resulted in
mixtures of products. Each of these products displayed general
NMR spectroscopic features that are similar to those of 3a,
including downeld NH signals, Cs symmetry at RT, and 19F
NMR chemical shis around ꢁ111.3 to ꢁ111.7 ppm that can be
assigned to the FPNP ligand. For uorobenzene, only the ortho-
uorophenyl isomer was detected, but two spectroscopically
distinct rotamers were seen (F either syn or anti with respect to
NH). Deprotonation of 3e with KOtBu led to a single neutral Pt–
aryl isomer (2e).
For toluene, three structural isomers were formed (8% ortho
and 92% meta + para37). The benzyl isomer 3g was not detec-
ted.38 This set of conditions yielded a higher, albeit still small,
proportion of the ortho-isomer 3b than the 1/Et3N reaction in
Scheme 3. However, the regio-discriminating step may not be
identical for these two reactions. Deprotonation (Scheme 4) of
this product mixture resulted in a corresponding mixture of
neutral ortho-, meta- and para-tolyl–Pt isomers. Each of these
isomers 2b–d was independently synthesized (Scheme 5), and
protonation of 2b–d with [H(OEt2)2][B(C6F5)4] yielded pure
samples of each corresponding (FPN(H)P)Pt–tolyl isomer 3b–d.
Analogous protonation and deprotonation reactions were also
carried out for other Pt–aryls and for the Pt–benzyl complex 2g
(Schemes 4 and 5).
We were able to obtain an X-ray quality single crystal of 3e.
The solid-state structure supported the proposed assignment of
uorine in the ortho-position (Fig. 1). Moreover, the F atom of
Scheme 5 Synthesis and protonation of (FPNP)Pt–R compounds.
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This was interpreted to mean that the larger substituent size
perturbs the equilibrium constant far away from unity. In
compounds that have a hydrogen atom at both ortho positions
(3a, 3e and 3d), we did not observe resolved rotamers, but the
aromatic signals were broad, which is consistent with a rota-
tion rate about the Pt–C bond that is on the order of the NMR
timescale. Restricted rotation about a metal–phenyl bond is
common in complexes where an aryl ligand is cis to two
phosphine ligands of this or greater size.41,42 However, in
pincer complexes containing PiPr2 groups and an aryl
substituent of comparable size to a phenyl ring, this is usually
observed only when the aryl group is also cis to the central
pincer atom.43
Fig. 1 ORTEP drawings (50% probability ellipsoids) of 3e (left) and 3c/d (right)
showing selected atom labeling. Hydrogen atoms (except for N–H) and the BARF
counter-ion are omitted for clarity. Only one of the two disordered orientations is
shown for the fluorophenyl (3e) and tolyl (3d shown) groups. Selected bond
ꢀ
˚
In contrast, the neutral (FPNP)Pt–aryl complexes 2a–f do not
display any evidence of separately observable rotamers,
showing only one sharp set of signals for each compound at
ambient temperature. It is likely that the puckering of the PNP
ligand upon quaternization of the N atom exacerbates the steric
interactions between the isopropyl groups and the Pt–aryl bond,
leading to a lower rate of rotation about Pt–Caryl in the
protonated complexes.
distances (A) and angles ( ) for 3e: N1–Pt1 2.159(2), P1–Pt1 2.2828(8), P2–Pt1
2.2780(8), Pt1–C44 2.012(3); P1–Pt1–P2 160.96(3), N1–Pt1–C44 170.46(11), C44–
Pt1–P1 95.22(8), C44–Pt1–P2 99.05(8), N1–Pt1–P1 83.34(7), N1–Pt1–P2 84.59(7),
C29–N1–Pt1 108.82(18), C31–N1–Pt1 116.10(18), C31–N1–C29 119.5(2).
ꢀ
˚
Selected bond distances (A) and angles ( ) for 3d: N1–Pt1 2.183(7), P1–Pt1
2.288(2), P2–Pt1 2.270(2), Pt1–C25 2.072(6); P1–Pt1–P2 161.35(8), N1–Pt1–C25
166.8(3), C25–Pt1–P1 93.9(3), C25–Pt1–P2 100.9(3), N1–Pt1–P1 83.13(17), N1–
Pt1–P2 84.81(17), C1–N1–Pt1 107.4(5), C7–N1–Pt1 115.1(5), C1–N1–C7 119.9(6).
the FC6H4 ligand was found to be disordered over the syn-/anti-
positions (vs. NH), consistent with the observation of these
rotamers in solution. An X-ray quality single crystal was also
obtained from the product mixture of the reaction of 1 with K-
BARF in toluene (Fig. 1). Remarkably, in this case as well, we
saw solid-state disorder between the two major isomers
observed in solution: 3c and 3d. The structure was successfully
modeled with a para (3d) to meta (3c) isomer ratio of 53 : 47.
Both the structure of 3e and that of 3c/d showed a distorted
square-planar environment about Pt and a tetrahedral one
about N.
The reaction with C6H5Cl (Scheme 4) ultimately produced a
single ortho isomer39 (and a single rotamer) of 3f. However, an
intermediate was observed in the initial stages of the reaction
that we tentatively assigned as 4. Although we do not have rm
structural evidence, we believe it is more likely to be the k1-Cl
chlorobenzene complex than a p-complex (i.e., an h2-C,C–arene
complex), because similar intermediates were not observed in
reactions with benzene, toluene or uorobenzene. In addition,
we observed what we believe to be a similar k1-Br adduct in the
reaction with C6D5Br, which will be discussed in another
section. Halocarbon (CH2Cl2, PhBr and PhI, but not PhCl)
adducts of cationic PtII fragments have been studied by Kubas
et al.40
Probing inter- and intramolecular exchange
We designed several experiments to probe whether the C–H
addition across N–Pt is reversible. The rst set of experiments
involved thermolysis of isolated arene addition products 3a-D
and 3b–e in a different arene (Scheme 6). We did not observe
arene exchange for any of the combinations, indicating that the
complete reversal of arene addition (including liberation of the
free arene) is not kinetically accessible. However, heating 3a-D
in neat acetonitrile for 24 h at 80 ꢀC did result in the formation
of the cationic acetonitrile adduct [(FPNP)Pt(NCMe)][BARF] (6)
and loss of C6D6 from 3a-D. It is unclear whether the apparent
elimination of C6D6 is simply faster in MeCN as solvent rather
than arenes, or if it involves a more direct interaction of MeCN
with 3a-D that makes benzene elimination kinetically
accessible.
Rotamerism
The observation of two rotamers by NMR spectroscopy
suggests slow (on the NMR timescale) rotation about the Pt–C
bond in 3e. On the timescale of experimental handling,
however, these two rotamers are in apparent equilibrium; the
ratio changes reversibly with temperature, and recrystallization
does not change the ratio of rotamers. One would expect even
slower rotation about the Pt–C bond in the analogous
compounds 3b and 3f, which have larger ortho substituents,
but in both of these cases, only a single rotamer was observed. Scheme 6 Reactions probing intramolecular isomerization.
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The second set of experiments was based on the notion that
the C–H addition process and its reverse should proceed via the
intermediacy of a [(FPNP)Pt(arene)]+ adduct (7). Re-forming this
arene adduct from a C–H addition product such as 3a could
hypothetically reverse C–H cleavage without liberating any free
arene. In the case of 3b–d, this would lead to an intramolecular
scrambling of isomers. However, solutions of pure 3b, 3c or 3d
did not produce any other isomers in NMR-detectable concen-
trations aer 2 weeks at ambient temperature (Scheme 6). The
same was true for the benzyl isomer 3g (not observed in the
reactions of 1 with K-BARF in toluene). There was also no
evidence of isomerization when 3c or 3d was exposed to (FPNP)
Pt(m-C6H4Me), Me2S–BF3 or B(C6F5)3.
Halogen atom abstraction
Although complexes 3 were stable when dissolved in dichloro-
methane for extended periods of time at ambient temperature,
dichloromethane could not be used as a solvent for their
synthesis. Treatment of 1 with K-BARF in dichloromethane led
to the formation of a blue solution that clearly contained a
paramagnetic product, which we assigned as 9a. The addition of
decamethylferrocene to this solution resulted in the formation
of the diamagnetic compound 5, which was observed in a 50%
yield vs. an integration standard aer ltration.
An alternative synthesis of the 9 cation was achieved via
oxidation of a pure sample of 5 with Ag[HCB11Cl11]. This reac-
tion allowed for the isolation of 9b as an analytically pure solid
in 44% (unoptimized) yield aer work-up. An X-ray diffraction
study on a single crystal of 9b revealed its solid-state structure
(Fig. 2). In this structure, Pt maintains an aꢁpproximately square-
On the other hand, treatment of 3b–d in CD2Cl2 with an
HOTf–PhF solution did result in some isomerization and the
formation of [(FPN(H)P)PtOTf]+ (8).44 We propose that triic acid
can reversibly protonate 3b–d and 3g to give unobserved [(FPN(H)
P)Pt(toluene)]2+ (A, Scheme 6), causing ortho/meta/para scram-
bling, and that the arene in A can be irreversibly but relatively
slowly be replaced by triate. Notably, [H(OEt2)2][BARF] or triic
acid in ether failed to induce the isomerization. Evidently,
protonated Et2O is not a strong enough Brønsted acid. Related
observations were made by Tilset et al., who reported that
protonation of a cationic [(diimine)Pt(Ph)(NCMe)]+ complex in
acetonitrile was orders of magnitude faster with HOTf than with
HBF4/Et2O.22 [(FPN(H)P)PtOTf]+ appears to be unstable in
dichloromethane, producing [(FPN(H)P)PtCl]+ (9) over time.
All in all, our observations imply that the C–H addition prod-
ucts 3a–f are separated from the arene adducts 7 by a barrier that
is insurmountable under 100 ꢀC. This means that (in the absence
of very strong acid, which can access a different pathway throughA)
the regioselectivity of the C–H addition is achieved under kinetic
control. That is interesting, considering that the ortho-hal-
ophenyl isomers (3e–f) are almost certainly thermodynamically
preferred as well,45 but usually are not kinetically preferred.46
To probe the relative barriers to the activation of C–H bonds
in various substrates, we set up a few competition experiments
in which 1 was treated with K-BARF at 80 ꢀC (3–24 h) in the
presence of two different arenes. In all of these reactions, full
conversion to the corresponding products 3a–e and 3a-D was
observed. C6H6 was activated three times as fast as C6D6, which
may indicate the importance of the C–H/C–D breaking step.
However, this apparent kinetic isotope effect of approximately 3
may be a composite value,13,47 and we do not have enough
information for a more conclusive analysis. The ortho-C–H bond
in uorobenzene was decidedly more reactive than the C–H or
C–D bonds of C6H6 or C6D6. Based on prior studies, we would
expect the activation of the ortho-C–H bond in uorobenzene to
be thermodynamically more favorable,45 but it is apparently also
kinetically more facile than activation of a C–H bond in
benzene. The fact that C6H6 was approximately three times
more competitive than C6D6 against C6H5F is consistent with
the 3 : 1 ratio for the C6H6–C6D6 competition experiment. In the
C6H6–C6H5CH3 competition reaction, the observed ratio of
products was close to the ratio between the number of C–H
bonds in C6H6 and the number of meta + para C–H bonds in
toluene.
planar environment, and the [HCB11Cl11
] anion is well sepa-
rated from the cation. Mindiola et al. previously studied the
reversible oxidation of (MePNP)NiCl and concluded that the
oxidation takes place primarily on the ligand.48 We made
similar observations in collaboration with the Nocera group
concerning the oxidation of (MePNP)M(CO)3 (M ¼ Mn, Re).49
Oxidation of 5 is expected to be a ligand-centered event as well,
and so 9a/9b should not be viewed as PtIII compounds, but
rather as PtII compounds with an oxidized PNP ligand.
The reaction of 1 with K-BARF in C6D5Br led to the complete
disappearance of 1 in 12 h at ambient temperature and the
F
formation of
a compound tentatively identied as 10.
Compound 10 is analogous to 4, which is observed in the
reaction with C6H5Cl, and showed a single 19F NMR resonance
at ꢁ122.9 ppm (along with three normal BARF resonances),
which is nearly identical to the spectrum of 4. In the 1H and 13
C
{1H} NMR spectra, 10 exhibited C2v-symmetry, which is consis-
tent with a k1-Br adduct. Treatment of 10 with acetonitrile
resulted in the clean formation of 6 (19F NMR resonance at
ꢁ125.1 ppm). Aer two weeks in a C6D5Br solution, the intensity
Fig. 2 ORTEP drawing (50% probability ellipsoids) of 9b showing selected atom
˚
labeling. Hydrogen atoms are omitted for clarity. Selected bond distances (A) and
angles (ꢀ): N1–Pt1 1.987(6), P1–Pt1 2.285(3), P2–Pt1 2.304(3), Pt1–Cl1 2.296(2);
P1–Pt1–P2 166.72(7), N1–Pt1–Cl1 171.92(16), Cl1–Pt1–P1 93.99(7), Cl1–Pt1–P2
97.67(7), N1–Pt1–P1 85.20(16), N1–Pt1–P2 84.13(16), C1–N1–Pt1 119.9(4), C7–
N1–Pt1 118.6(4), C1–N1–C7 121.2(6).
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of the NMR resonances assigned to 10 were diminished (with no
new 19F NMR signals emerging), and broad resonances remi-
niscent of 11 were evident in the 1H NMR spectrum. Addition of
decamethylferrocene to this aged solution resulted in the
appearance of NMR resonances indicative of 12 (20%). We
propose that Br atom abstraction from the bound C6D5Br takes
place over time, producing 11. We have not yet attempted to
establish the fate of the organic fragments aer the abstraction
of the Br atom from bromobenzene or of the Cl atom from
dichloromethane.
Abstraction of a halogen atom is not a reaction associated
with low-spin, closed-shell transition-metal complexes. Metal-
loradicals with odd numbers of electrons such as (porphyrin)Rh
or (porphyrin)Ir species are known to abstract halogen atoms
from halocarbons.50 In contrast, Bullock’s [(PCP)Pt]+ or
Peters’[(SiP3)Pt]+ simply coordinated dichloromethane and did
not show chlorine atom abstracting reactivity.25,51 Since PtII is
reliably low-spin in its complexes, the halogen-atom abstraction
observed with [(FPNP)Pt]+ appeared likely related to the non-
innocence of the FPNP ligand (Scheme 7). To probe this further,
we initiated a theoretical study.
Fig. 3 Depictions of the frontier orbitals of the singlet (left) and triplet (right)
states of [(PNP)Pt]+ calculated with the PBE0 functional.
by a relatively small margin (PBE0: by 12.6 kcal molꢁ1 in enthalpy
and by 11.0 kcal molꢁ1 in free energy; M06-L: by 10.7 and 10.2
kcal molꢁ1, respectively). The triplet can be best described as
arising from the promotion of an electron from the lone pair of N
to the empty orbital at Pt (see Fig. 3). We do not interpret the
singlet/triplet difference as evidence that the triplet [F(PNP)Pt]+ is
ever present in a kinetically competent concentration in solu-
tion; indeed, we do not know whether free [F(PNP)Pt]+ in any spin
state is an intermediate at all. However, this modest difference
indirectly suggests that [F(PNP)Pt]+ and its singlet four-coordi-
nate adducts may have reaction pathways accessible to them that
lead to products of apparent triplet reactivity.
DFT studies
The contrast between the typical low-spin reactivity for Pt in C–H
activation and the apparent metalloradical-type reactivity in
halogen atom abstraction led us to consider the nature of the
ground state of [(FPNP)Pt]+. Although we have not observed this
species directly, we thought that a comparison of the singlet and
triplet energies might assist in understanding the divergent
reaction pathways that can formally be dened as reactions of
[(FPNP)Pt]+ with a substrate. We turned to DFT calculations using
either the PBE052 or the M06-L53 functionals with Gaussian 09,54
which predicted that the singlet state is more stable than the
triplet. However, the triplet was calculated to be higher in energy
Conclusions
In summary, we have described the reactivity taking place upon
removal of one of the ligands from a square-planar complex of
PtII. The putative [F(PNP)Pt]+ fragment reacted with aromatic C–
H bonds by net heterolytic C–H addition across the N–Pt bond
when only C–H and strong C–Hal bonds were present in the
substrate. When weaker C–Hal bonds were present, such as in
bromobenzene or dichloromethane, the [F(PNP)Pt]+ fragment
exhibited a tendency to abstract a halogen atom, a reaction quite
unexpected of a PtII complex. The key to understanding the
divergent reactivity of [F(PNP)Pt]+ is the non-innocence of the
supporting FPNP ligand, which can function as an intra-
molecular component of a frustrated Lewis pair, or as a source of
a single electron. This work was originally conceived with the
thought of using [F(PNP)Pt]+ as a scaffold for forming hydro-
carbon complexes. It is now clear that a different pincer ligand is
necessary to achieve this goal, namely one with much lower
Bronsted basicity and with lesser tendency to give up an electron.
Acknowledgements
Scheme 7 Halogen atom abstraction and redox chemistry of [(FPNP)PtX]–[(FPNP)
PtX]+ complexes.
We are grateful for the support of this work by the US National
Science Foundation (grants CHE-0809522 and CHE-0944634),
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the Dreyfus Foundation (Camille Dreyfus Teacher-Scholar 24 M. Hasegawa, Y. Segawa, M. Yamashita and K. Nozaki,
Award to O. V. O.), and the Welch Foundation (grant A-1717 to Angew. Chem., Int. Ed., 2012, 51, 6956.
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cationic (PNP)Pt complexes, Dr Dan Smith for contributions to 2010, 132, 13975.
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