Guilty as charged: Non-innocent behavior by a pincer ligand featuring a central cationic phosphenium donor

Article abstract of DOI:10.1039/c0cc05739h

The synthesis of a new pincer ligand containing a central cationic N-heterocyclic phosphenium donor is described. The electrophilic nature of this cationic ligand renders it non-innocent, and coordination of this ligand to a PtCl₂ fragment leads to chloride migration from Pt to the cationic phosphorus center.

Full text of DOI:10.1039/c0cc05739h

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COMMUNICATION
Guilty as charged: non-innocent behavior by a pincer ligand featuring a
central cationic phosphenium donorw
Gregory S. Day, Baofei Pan, Daniel L. Kellenberger, Bruce M. Foxman and
Christine M. Thomas*
Received 22nd December 2010, Accepted 28th January 2011
DOI: 10.1039/c0cc05739h
The synthesis of a new pincer ligand containing a central cationic
N-heterocyclic phosphenium donor is described. The electrophilic
nature of this cationic ligand renders it non-innocent, and
coordination of this ligand to a PtCl2 fragment leads to chloride
migration from Pt to the cationic phosphorus center.
open metal coordination site trans to the NHP moiety in
these meridional-coordinating ligands, and (3) the ability to
sterically modify the pendant donor functionalities to protect
the phosphenium unit from nucleophilic attack. Incorporation
of an NHP into a multidentate framework may allow
stabilization of catalytically relevant transition metal
complexes and more comprehensive comparisons with NHCs.
Herein we report the synthesis and characterization of the
first NHP-containing pincer ligand and its representative
coordination chemistry with PtII.
While N-heterocyclic carbene ligands (NHCs) have become
ubiquitous in the field of transition metal and organo-
catalysis,1–5 far less focus has been placed on the potential
applications of their isovalent group 15 analogues, N-hetero-
cyclic phosphenium cations (NHPs).6–8 In contrast to the
nucleophilic s-donor nature of NHCs, the weak s-donor
and strong p-acceptor properties of NHPs lead to electrophilic
character. Moreover, owing to their many possible resonance
structures and the potential to adopt either planar or pyramidal
geometries, Jones and coworkers have drawn a convincing
analogy between NHPs and NO+, highlighting the potential
non-innocent character of these ligands.9 Recent advance-
ments in the chemistry of NHPs include new preparative
methods10–13 and new reactivity patterns,9,10,14,15 but very
few catalytic applications of these ligands have been reported
to date.16,17 This is surprising in light of the anticipated
complementary reactivity of NHPs in comparison to
their electronically opposite NHC relatives. However, this
dearth of catalytic applications can likely be attributed to
the susceptibility of NHPs to nucleophilic attack.6–8
Synthesis of the targeted ligand was not as straightforward
as initially envisioned for several reasons including: (1) typical
glyoxal condensation routes proved problematic with
phosphine-substituted anilines,23 and (2) lithiation of aryl
bromides was complicated by the presence of acidic protons
on the ortho anilido nitrogen atoms which appeared to undergo
irreversible N-phosphination upon treatment with iPr2PCl
(see ESIw).24 Nonetheless, phosphenium ligand 4+ was synthe-
sized successfully via the route shown in Scheme 1. Synthesis
of the pincer ligand backbone was accomplished via alkylation
of o-fluoroaniline with 1,2-dibromoethane to generate diamine
1, followed by nucleophilic substitution of the aryl fluorides
with KPPh2 to generate the diphosphine 2 in modest yield
(Scheme 1). Interestingly, no side reaction of the amine
functionality with KPPh2 was detected. Treatment of 2 with
PCl3 in the presence of two equivalents of NEt3 leads to
formation of the phosphine chloride 3. Compound 3 has 31P
NMR shifts at ꢀ17.6 ppm and 147.9 ppm, characteristic of
triarylphosphine and chlorodiaminophosphine moieties, re-
spectively. Halide abstraction from 3 is accomplished using
either TlPF6 or NaB(ArF)4 (ArF = 3,5-CF3–C6H3) to obtain
A potential strategy for imparting stability in transition
metal phosphenium complexes is the incorporation of these
donors into chelating frameworks. Transition metal complexes
of multidentate ligands featuring NHPs are noticeably absent
from the literature, particularly in comparison to the growing
number of NHC-containing chelating ligands.18,19 In this
report we have chosen to incorporate an N-heterocyclic
phosphenium cation into the central position of a tridentate
pincer ligand, motivated by: (1) the stability imparted by the
rigid framework of pincer ligands,20–22 (2) the placement of an
Department of Chemistry, Brandeis University, 415 South Street,
Waltham, MA 02454, USA. E-mail: thomasc@brandeis.edu;
Fax: +1 781-736-2516; Tel: +1 781-736-2576
w Electronic supplementary information (ESI) available. CCDC
805666–805668. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc05739h
Scheme 1 Synthesis of pincer ligand 4+
.
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Scheme 2 Treatment of 3 with AgOTf.
solid state structure of 5 (Fig. 2) confirms that the P–Cl bond
remains intact and reveals little or no interaction between
the Ag center and the chloride-bound central phosphorus
consistent with the lack of Ag–P coupling in solution.y The
complex adopts a distorted trigonal planar geometry around
Ag and a clearly pyramidalized geometry about the central
phosphorus donor. In addition to a long interatomic Ag–P2
˚
distance (2.7230(13) A), the Ag–P2–Cl (153.09(7)1) angle is
Fig. 1 Displacement ellipsoid representation of 4+. PF6 counter-
ꢀ
significantly larger than that in reported transition metal
chlorophosphine complexes (based on a 2010 CSD search),
implying that there is negligible bonding between Ag and P2.
In an attempt to coordinate the chelating phosphenium
cation to a metal, [4][PF6] was treated with (COD)PtCl2
(COD = 1,5-cyclooctadiene, Scheme 3). The 31P NMR of
the resulting product revealed a singlet at 2.9 ppm for the aryl
phosphine sidearms and a singlet at 76.6 ppm for the central
phosphorus atom. While both resonances feature 195Pt satellites
˚
anion has been omitted for clarity. Relevant interatomic distances (A):
P1–P2, 2.2786(4); P1–P3, 2.858.
the phosphenium PP+P pincer ligand [4][PF6] or [4][B(ArF)4]
in 96.0% and 93.3% yields, respectively.
While the 31P NMR shift of the PPh2 moieties of 4+ appears
in the expected location (ꢀ11 ppm, doublet, J = 220 Hz), the
31P resonance for the central phosphenium phosphorus atom
is shifted remarkably upfield (B90 ppm, triplet, J = 220 Hz)
in comparison to both 3 and other phosphenium cations
reported in the literature.6,7 An intramolecular phosphenium–
phosphine Lewis acid/base interaction would account for this
upfield shift. The solid state structure of [4][PF6] shown in
Fig. 1 indeed reveals a well-defined interaction between one of
the triarylphosphine substituents and the central phosphenium
1
(1JPt–P = 2100 Hz (PR3), JPt–P = 4860 Hz (central P))
implying that both phosphorus-containing moieties are bound
to the Pt center, the central phosphorus resonance is too far
upfield to correspond to a metal-bound phosphenium unit.
˚
cation with a P1–P2 distance of 2.2786(4) A (cf. 2.858 A for the
˚
˚
unbound phosphine).z A similar P–P distance of 2.3065(9) A
was observed in the structure of the phosphenium–PMe3
adduct reported by Baker et al.13 While this asymmetry is
apparent in the solid state, it is in stark contrast to the room
temperature 31P NMR spectrum, which implies equivalence
of both phosphine arms on the NMR time scale at room
temperature. Since the room temperature 31P NMR signals are
an average of rapidly exchanging P+-bound and unbound
phosphines, the actual 1JP–P can be determined by the following
4
equation: Jobs
=
12(1JP–P
+
4JP–P). Assuming that JP–P is
1
negligible, JP–P is calculated to be 440 Hz, consistent with
Baker’s phosphenium–PMe3 adducts.13 At low temperature
the doublet at ꢀ11 ppm for the phosphine sidearms broadens
and eventually coalesces at ꢀ65 1C, while the triplet at 88 ppm
changes its shape as the temperature is lowered, presumably
morphing into a doublet (1JP–P = 440 Hz) as the central peak
of the triplet coalesces into the baseline (see ESIw).
Interestingly, treatment of phosphine chloride 3 with
AgOTf instead of TlPF6 leads to formation of the silver
coordination complex (PPClP)AgOTf (5) rather than halide
abstraction (Scheme 2). The 31P NMR resonances of the
phosphine sidearms shift downfield upon metal coordination
(ꢀ6.8 ppm), but the 31P shift corresponding to the central
phosphorus atom remains in the range for a halide-bound
phosphorus (134 ppm). The phosphines show distinct coupling
to 107/109Ag with 1JAg–P = 590 Hz, while no coupling between
the Ag center and the central phosphorus is observed. The
Fig. 2 Displacement ellipsoid diagram (50%) of 5. Relevant interatomic
˚
distances (A) and angles (1): Ag–P2, 2.7230(13); Ag–P1, 2.4457(13); Ag–P3,
2.4433(13); P2–Cl1, 2.1618(18); Ag–O1, 2.363(3); Cl1–P2–Ag, 153.09(7);
P1–Ag–P3, 139.39(4); P3–Ag–O1, 107.85(11); P1–Ag–O1, 109.34(11).
Scheme 3 Treatment of 4+ with (COD)PtCl2.
Chem. Commun., 2011, 47, 3634–3636 3635
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Notes and references
z Crystal data for [4][PF6]: C38H32F6N2P4, M = 754.57, monoclinic,
˚
˚
˚
a = 20.0011(5) A, b = 24.3395(6) A, c = 16.1163(4) A, a = 901,
3
˚
b = 115.9030(10)1, g = 901, V = 7057.5(3) A , T = 120 K, space
group C12/c1, Z = 8, Rint = 0.040. The final R1 values were 0.0449
(I 4 2s(I)). The final wR(F2) values were 0.1038 (I 4 2s(I)).
y Crystal data for 5: C47H48Ag1Cl1F3N2O5P3S1, M = 1046.21, orthor-
˚
˚
˚
hombic, a = 11.9962(5) A, b = 15.9208(7) A, c = 25.0428(10) A,
3
˚
a = 901, b = 901, g = 901, V = 4782.9(3) A , T = 120 K, space group
P212121, Z = 4, Rint = 0.062. The final R1 values were 0.0420
(I 4 2s(I)). The final wR(F2) values were 0.0968 (I 4 2s(I)).
z Crystal data for 6: C38H32Cl2N2P3PtꢁF6P, M = 1020.56, monocli-
˚
˚
˚
nic, a = 12.5297(7) A, b = 13.4232(7) A, c = 22.7095(13) A, a = 901,
3
˚
b = 96.303(2)1, g = 901, V = 3796.4(4) A , T = 120 K, space group
P121/n1, Z = 4, Rint = 0.028. The final R1 values were 0.0182
(I 4 2s(I)). The final wR(F2) values were 0.0435 (I 4 2s(I)).
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ꢀ
Fig. 3 Displacement ellipsoid diagram (50%) of 6. PF6 counter-
˚
anion has been omitted for clarity. Relevant interatomic distances (A)
and angles (1): Pt–P1, 2.2980(5); Pt–P2, 2.1553(5); Pt–P3, 2.3018(5);
Pt–Cl1, 2.3339(5); P2–Cl2, 2.0588(6); Pt–P2–Cl2, 110.06.
The X-ray structure of the product (Fig. 3), indeed, revealed
that one of the Pt-bound Clꢀ ligands had migrated to the
central phosphenium unit, resulting in the cationic Pt complex
[(PPClP)PtCl][PF6] 6.z Chloride attack on the electrophilic
phosphenium unit is not particularly surprising, and similar
halide and alkyl migration events have been previously
reported.13,25–27 In contrast to Ag complex 5, the central
chlorophosphine unit in 6 is bound tightly to Pt, with a P2–Pt
˚
distance (2.1553(5) A) even shorter than that observed for the
˚
˚
Pt-bound phosphines (Pt–P1: 2.2980(5) A, Pt–P3: 2.3018(5) A).
Notably, this Pt–P2 distance is also remarkably similar to that
in the Pt phosphenium complex reported by Baker and co-
15
˚
workers (2.116(3) A). This short interaction is likely due to
extensive p-backbonding from Pt into the s* orbital of the
strongly acidic diamidochlorophosphine unit. Indeed, both the
1
Pt–P distance and the JPt–P coupling constant associated with
the central phosphorus donor are in the range reported for
Pt–P(OMe)3 complexes.28 Notably, no reaction was observed
between 6 and TlPF6, implying that chloride ion abstraction is
not feasible once coordinated to a metal center.
In conclusion, a new pincer ligand containing a central
N-heterocyclic phosphenium cation has been synthesized.
Preliminary investigations into the coordination chemistry of
this ligand with the PtCl2 fragment suggest that the electro-
philic nature of the central phosphenium donor facilitates
halide migration from Pt to P. Further modification of the
ligand framework with more sterically encumbering substituents
may prevent this intramolecular halide transfer. Additional
investigations into the coordination chemistry of 4+ with a
variety of other transition metals are currently underway and
will be reported in subsequent publications.
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This work was funded by startup funds from Brandeis
University.
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3636 Chem. Commun., 2011, 47, 3634–3636
This journal is The Royal Society of Chemistry 2011