Pincer ligands with an all-phosphorus donor set: Subtle differences between rhodium and palladium

Article abstract of DOI:10.1039/c1dt10360a

The synthesis of a new, all-phosphorus pincer PP^NEt2P ligand L3^NEt2, which is derived from 2-indolylphosphine and features a central N₂P(NEt₂) core, is described. This 'PPP' species shows coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PP^HP ligands L1^H and L2 ^H, featuring a secondary phosphine core, show 'ambivalent' coordination, acting as persistent neutral triphosphine ligands with Rh, and as easily-formed monoanionic phosphido(bisphosphine) pincer ligands toward Pd. These subtle differences, which might be more general for group 9 and 10 metal complexes with this ligand set, are explained by comparative DFT calculations (BP86; def2-TZVP level of theory) for the Rh and Pd species involved, including those with the structurally related PN^HP ligands. The optimized structure for complex PdCl(L2) indicates minimal overlap of available Pd d-orbitals with the lone pair of the central, deprotonated phosphorus atom (formally a phosphido fragment), suggesting that it behaves predominantly like a bulky phosphine instead of a phosphido fragment. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10360a

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This article is published as part of the Dalton Transactions themed issue entitled:
Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
Published in issue 35, 2011 of Dalton Transactions
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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
ARTICLES:
Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
Gianluca Bossi, Elisabetta Putignano, Pierluigi Rigo and Walter Baratta
Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Pincer ligands with an all-phosphorus donor set: subtle differences between
rhodium and palladium†
Richard C. Bauer,a Yann Gloaguen,a Martin Lutz,b Joost N. H. Reek,a Bas de Bruina and
Jarl Ivar van der Vlugt*a
Received 2nd March 2011, Accepted 23rd May 2011
DOI: 10.1039/c1dt10360a
The synthesis of a new, all-phosphorus pincer PPNEt2P ligand L3NEt2, which is derived from
2-indolylphosphine and features a central N2P(NEt2) core, is described. This ‘PPP’ species shows
coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PPHP
ligands L1H and L2H, featuring a secondary phosphine core, show ‘ambivalent’ coordination, acting as
persistent neutral triphosphine ligands with Rh, and as easily-formed monoanionic
phosphido(bisphosphine) pincer ligands toward Pd. These subtle differences, which might be more
general for group 9 and 10 metal complexes with this ligand set, are explained by comparative DFT
calculations (BP86; def2-TZVP level of theory) for the Rh and Pd species involved, including those with
the structurally related PNHP ligands. The optimized structure for complex PdCl(L2) indicates minimal
overlap of available Pd d-orbitals with the lone pair of the central, deprotonated phosphorus atom
(formally a phosphido fragment), suggesting that it behaves predominantly like a bulky phosphine
instead of a phosphido fragment.
Introduction
Diphosphorus “PEP” pincer ligands have attracted substantial
interest and shown significant merits in studies of fundamental
C–H, C–C and C–N bond-activation processes, coordination
chemistry, metal-mediated reactivity and homogeneous catalysis
applications.1 Some of the characteristic ligand design features
include i) a rigid, tridentate binding pocket, ii) compatibility with
Fig. 1 Generic representations of monoanionic “PEP” pincer ligands; A:
a large number of transition metals, iii) a high degree of synthetic
tunability, and iv) modularity in the coordinating bridgehead
classic PC-P framework; B–D: various scaffolds with a central deproto-
nated heteroatom.
atom E. Besides the archetypical, monoanionic carbon-based
frameworks such as A,2,3 novel backbones incorporate amido (B),4
Interest in the design of novel pincer systems is spurred by recent
developments in the field of E–H bond activation processes. For
instance, the selective N–H bond cleavage of ammonia is one of the
silyl (C)5 or boryl (D)6 units that function as pivotal heterodonor
systems (Fig. 1). Furthermore, various neutral pincer ligands with
phosphorus side-groups have been intensively studied, including
most sought-after proponents.11 Successful metal-mediated N–H
noninnocent, cooperative systems based on pyridine as the central
activation of ammonia has been reported for a small number of
fragment.7 These systems undergo facile and reversible charge-
mononuclear late transition metal complexes, i.e. Ir(PC-P) (E),12
switching from a neutral to a monoanionic state, which can be
Ir(PSi-P) (F),13 and Ru(PN-P) (G),14 all featuring a pincer type
used to selectively activate substrates and catalyse novel reactions
ligand (Fig. 2). Additionally, we have preliminary data to suggest
efficiently,8 even with first row transition metals.9,10
that Ir-species (H), bearing a novel hybrid PN-N pincer ligand, is
also amenable to N–H bond activation.15
We wondered if the rich coordination chemistry displayed by
the versatile monoanionic frameworks with a central heteroatom
aSupramolecular & Homogeneous Catalysis, van ‘t Hoff Institute for
Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH,
donor B–D and the resulting reactivity with group 9 metals could
be extended to all-phosphorus based donor sets, of which there
are surprisingly few reports (Fig. 3).
Neutral triphosphorus ligands that induce a characteristic mer-
coordination are relatively rare16 compared to systems such as
‘triphos’ (I) that prefer a fac arrangement,17 whilst there are
Amsterdam, The Netherlands. E-mail: j.i.vandervlugt@uva.nl; Fax: +31-20-
5255604; Tel: +31-20-5256459
bCrystal and Structural Chemistry, University of Utrecht, Padualaan 8,
Utrecht, 3584 CH, The Netherlands
† Electronic supplementary information (ESI) available. CCDC reference
numbers 815799. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10360a
8822 | Dalton Trans., 2011, 40, 8822–8829
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The Royal Society of Chemistry 2011
©

Scheme 1 Generic synthesis route to PPHP ligands LH.20
Fig. 2 Recent pincer complexes capable of NH3 activation.
This scaffold has previously been utilized for the construction
of tripodal tetradentate frameworks.27 Deprotonation of 1 with
nBuLi followed by treatment with 0.5 equiv Cl2PNEt2 led to the
new tridentate PPP pincer ligand L3NEt2 (Scheme 2), which was
isolated as a pure white solid in good yield after purification by
chromatography. The 31P NMR spectrum exhibited a triplet at d
96.1 ppm (PNEt2) and a doublet at -29.4 ppm, (PPh2), both with
a coupling constant 3JPP of 168.5 Hz.
Fig. 3 Overview of known PPP pincer ligand systems.
also intermediate cases (J).18 Togni developed various chiral PPP
ligands, termed Pigiphos (K), which were successfully applied in
the Ni-catalyzed hydroamination of electron-poor olefins.19 Only
very recently scaffold LH was described, and it was shown to act
as a monoanionic bridging ligand to CuI.20 Subsequently, LH was
found to coordinate as a monoanionic pincer ligand to Pd.21
To date, group 9 metal complexes of diphenylphosphine-
derived PPHP ligands L1H and L2H have not been reported,
whereas the coordination chemistry and reactivity with analogous
diphenylamine-derived PNHP ligands have been very successfully
investigated and exploited.4,22 Gru¨tzmacher has shown that the
acidity of secondary amines is significantly increased upon coor-
dination to group 9 metals.23 The difference in acidity of HPPh2
(pKa: exp. 21.7; calc. 22.9) vs. HNPh2 (pKa: exp. 25.0; calc. 26.4),24
which may be assumed to persist after ortho-functionalization of
the phenyl rings, suggests that extension of the amine-based nonin-
nocent reactivity to secondary phosphine derived skeletons might
be possible. This could lead to similar rich coordination chemistry
and reversible deprotonation behaviour for PPHP-type ligands
such as L1H and L2H, especially with group 9 transition metals.
For instance, E–H bond splitting might be promoted via similar
cooperative ligand–metal activation mechanisms as observed for
certain neutral, tridentate PNP systems,8–10 and monoanionic PNP
ligand frameworks.25 To facilitate spectroscopic characterization,
we focused on Rh complexes with these all-phosphorus tridentate
donor ligands, which also enabled a direct comparison with its
group 10 congener Pd.21
Scheme 2 Synthesis of novel triphosphorus ligand L3NEt2. Reaction
t
conditions: i) 1 equiv nBuLi, CO2; ii) 1 equiv BuLi, ClPR2; iii) 1 equiv
nBuLi, 0.5 equiv P(NEt2)Cl2.
Unfortunately, selective P–N bond cleavage was not possible
when indolyl-based phosphine L3NEt2 was subjected to standard
hydride reduction methodologies used for the conversion of di-
ethylaminophosphines or chlorophosphines to the corresponding
secondary phosphines, and only the initial indolylphosphine 1
was recovered. Selective removal of the diethylamino group via
alcoholysis or protonation was also not successful.
Coordination chemistry to Rh and comparison with Pd
Reaction of L1H with [Rh(m-Cl)(COE)2]2 (COE = cyclooctene)
in THF or toluene resulted in the formation of a light orange
solid (Scheme 3, top). Strikingly, the 31P NMR spectrum of the
corresponding Rh-complex showed a doublet-of-triplets at 48.9
ppm for the central P atom (free L1H: dP = -49.2 ppm20) and
H
a doublet-of-doublets at 65.5 ppm for the diisopropylphosphine
iPr
groups (free L1H: dP = 0.0 ppm), which results in substantial Dd
shifts of up to 98.1 ppm upon complexation. The 1JRh-P coupling
3
constants of 136 Hz (PiPr-Rh) and 167 Hz (PH-Rh) and a JPP
3
coupling constant of 32 Hz (free L1H: JPP = 131 Hz) reveal the
Results and discussion
coordination of phosphines to Rh to yield complex 2. This implies
that deprotonation of the central -PH fragment of the pincer
ligand, to generate a phosphido-unit, does not occur. Genuine
terminal rhodium-phosphido species are very rare; to the best of
our knowledge only rhodium complexes with bridging phosphides
are well-characterized.28
Ligand synthesis
Tridentate PPHP ligands L1H and L2H were prepared as pre-
viously described in the literature (Scheme 1).20 Furthermore,
we developed a new all-phosphorus pincer ligand based on the
recently described ‘indolylphosphine’ building block,26 which is
easily prepared from 3-methylindole using a convenient two-step
protocol involving in situ CO2 protection of the indole nitrogen.
Notably, upon closer inspection of the 1H NMR spectrum,
1
a doublet was observable at d 6.30 ppm, with a JPH coupling
1
of 342 Hz (free ligand: d 6.02 ppm, JPH = 219 Hz), which is
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Dalton Trans., 2011, 40, 8822–8829 | 8823
©

(doublet-of-doublet-of-triplets) at d -16.91 ppm in the 1H NMR
spectrum, with JPH coupling constants of 6.7 (-PiPr) and 10.1 Hz
(-PH). We propose that oxidative addition of HCl has occurred,
leading to octahedral RhIII-hydride species 2HCl with the hydride
acting as axial ligand, i.e. cis to all phosphorus groups, although
this was not investigated further.
The tridentate all-phosphorus ligand L3NEt2, which bears a
stronger P–N bond at the bridging atom, also acts as a neutral
ligand when complexed with Rh. Treatment of L3NEt2 with [Rh(m-
Cl)(COE)2]2 or [Rh(m-Cl)(CO)2]2 in THF resulted in the facile
and clean formation of complex 3 as an orange solid (Scheme
3, bottom). Both Rh reagents form the same pincer complex,
as evidenced from the spectral data, e.g. signals at 142.4 ppm
1
2
1
(dt, JRhP = 223.9 Hz; JPP = 42.6 Hz) and 17.1 ppm (dd, JRhP
=
144.6 Hz; 2JPP = 42.6 Hz) in the 31P NMR spectrum. In this case,
the chemical shift differences were more moderate and typical for
archetypical phosphine coordination with Dd’s of up to 46.3 ppm.
Single crystals suitable for X-ray diffraction were obtained from
toluene and the resulting molecular structure is depicted in Fig.
4. The four-coordinate Rh–Cl pincer complex, which shows a
distorted square-planar geometry around the Rh-center, features a
central trisamidophosphine group (connectivity around atom P1).
The distortion is evident from the ∠P2–Rh–P3 of 154.301(17)◦,
which is induced by the tetrahedral geometry around the sp3-
hybridized PNEt2-atom that pushes both indole-fragments below
the PNEt2-Rh–Cl plane (angle N2–P1–N3 is 111.30(7)◦). There is a
pseudo-helical out-of-plane twist of the indole-heterocycles due
to steric constraints, similar to what was observed in tetradentate
analogues.27a The intramolecular Rh–P1 distance is slightly shorter
than for Rh–P2 and Rh–P3, which could be an electronic effect of
PN3 vs. PC3 substitution.
Scheme 3 Synthesis of neutral Rh complexes 2 (top, with L1H) and 3
(bottom, with L3NEt2).
strong evidence for the existence of a species with the overall
formulation Rh(L1H)Cl, species 2.29 To confirm this, we also
measured the IR spectrum of species 2, which showed a band
at n 2272 cm-1 for the P–H moiety (free HP(PiPr)2 ligand: 2282
cm-1). Unfortunately, we were unable to recrystallize compound 2
as it proved very unstable, with significant decomposition after
24 h and complete decomposition within 7 days at 4 ◦C; it
was therefore only characterized spectroscopically. Complexation
of the diphenylphosphine analogue L2H to Rh or Ir led to
multiple species, as determined by 31P NMR spectroscopy, with
no indication for P–H activation.
Ligand L1H was subjected to several Rh sources‡ under
varying conditions, in order to achieve insertion into the P–H
bond but none of these reactions led to a well-defined product.
Deprotonation of the PPHP ligand with nBuLi at -78 ◦C, evidenced
by the characteristic bright red/pink colour of the THF solution,
and subsequent treatment with [Rh(m-Cl)(COE)2]2 did not result
in the formation of the desired product either, but rather led to
decomposition with no detectable 31P NMR signals. Reaction of
in situ prepared Rh complex 2 with strong bases such as NaOtBu,
nBuLi or KHMDS only resulted in decomposition of the ligand,
as indicated by 31P NMR spectroscopy, whereas LiNH2, LiNHPh,
and triethylamine did not noticeably react with the Rh species.
These results are remarkable, given the higher intrinsic acidity of
R2PH vs. R2NH in their free form,24 and the reported effect on
the –NH acidity upon amine coordination to Rh.23 This must
mean that the acidity of the P–H bond of unactivated secondary
phosphines is not substantially altered upon coordination to Rh.
Abstraction of the chloride ligand from complex 2 using silver
salts such as AgSbF6 in either THF or CH2Cl2 resulted in
decomposition of the complex. Chloride abstraction from the
initial Rh reagent, prior to addition of pincer ligand L1H,30 also
resulted in unwanted side-reactions. Treatment of Rh complex 2
with 2.0 M HCl in diethylether at 0 ◦C afforded a hydride signal
Fig. 4 ORTEP plot (50% probability displacement ellipsoids) of complex
3, [RhCl(L3NEt2)]; left–top view; right–side view. Hydrogen atoms and
disordered toluene solvent molecules have been omitted for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): Rh–P1 2.1516(4); Rh–P2 2.2677(4);
Rh–P3 2.2514(4); Rh–Cl 2.3948(4); P1–N1 1.6774(14); P1–N2 1.7271(14);
P1–N3 1.7298(14); P1–Rh–P2 83.620(16); P1–Rh–P3 87.092(16), P2–Rh–P3
154.301(17), P1–Rh–Cl 176.474(17), Rh–P1–N1 126.06(5); N1–P1–N2
99.56(7), N1–P1–N3 100.78(7), N2–P1–N3 111.30(7).
As encountered with complex 2, attempts to obtain a cationic
species from 3 via Ag+-induced halide abstraction led to ill-defined
products, presumably involving undesired redox-chemistry.
Treating Rh complex 3 with 2.0 M HCl in ether at 0 ◦C did
not appear to result in protonation of the diethylamino-group and
subsequent P–N rupture, prohibiting this ligand from acting as an
in situ generated monoanionic analogue of LH, also when already
‡ Other precursors employed to induce P–H cleavage were [Rh(cod)2]BF4,
[Rh(m-OH)(coe)2]2, Rh(acac)(CO)2, [Rh(CO)2Cl]2, [Rh(cod)(MeCN)2]-
BF4, [Rh(OAc)2]2 and RhCl3·3H2O.
8824 | Dalton Trans., 2011, 40, 8822–8829
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©

Table 1 Selected structural data for the DFT optimized geometries
(BP86, def2-TZVP) of Rh and Pd complexes with L2H and L2
coordinated to Rh. This further illustrates the propensity of both
types of tridentate PPP-compounds to act as neutral triphosphine
ligands to Rh.
Rh(L2H)
Rh(L2)
Pd(L2H)
Pd(L2)
These results markedly contrast the facile way by which Pd
coordinates to the tridentate monoanionic PP-P ligand. The
palladium complex PdCl(L2) 4 can be prepared simply by heating
PPHP ligand L2H with PdCl2(COD) and Hu¨nig’s base in THF at 50
◦C (Scheme 4).21 The 1H NMR signal for a secondary phosphine is
absent (no 1JP-H coupling detected), and no P–H stretch is observed
in the IR spectrum. The molecular structure for complex 4 has
been reported before.21
M-PH (A)
2.2952
2.3155
2.3242
174.03
82.03
84.21
113.77
162.81
2.3399
2.2931
2.3036
174.35
81.82
83.75
108.98
159.25
2.2267
2.3300
2.3346
178.67
84.66
85.31
116.27
168.46
2.2882
2.2985
2.3144
172.11
83.62
85.84
107.82
162.45
˚
M-PPh (A)
˚
PH-M-L (◦)
PH-M-PPh (◦)
C-PH-C (◦) ◦
PPh-M-PPh ( )
complex 4 when optimized at the BP86, def2-TZVP level agree
well with the X-ray structural data available, with the Pd-P-
intramolecular distance being slightly shorter than the Pd-PR bond
-
R
21,32
˚
˚
lengths (Pd-P : ~2.25 A; Pd-P ~2.30 A).
Improved energies
were obtained with single point calculations employing COSMO
(e = 7.58; tetrahydrofuran) dielectric solvent corrections, using
both BP86/def2-TZVP and b3lyp/def2-TZVP level of theory for
each of the PPHP and PNHP complexes (Table 2). The computed
data clearly confirm that deprotonation is much less favorable for
the PPHP based species. For both PPHP and PNHP, deprotonation
of the RhCl complexes is disfavored relative to deprotonation
of the corresponding PdCl complexes. To balance the effect of
comparing complexes with different overall charges (neutral vs.
monoanionic), the corresponding rhodium-carbonyl complexes
were included in the analysis. However, even after elimination of
charge differences and introducing the p-accepting carbonyl ligand
(which should stabilise the amido-metal interaction through push-
pull charge delocalisation33), deprotonation of the Pd complexes
is still favored (Table 2). This is most likely a local charge effect
resulting from the higher metal oxidation state of Pd (RhI vs. PdII).
The calculations agree with our experimental observations that a
monoanionic framework is easily accessed for PdII, but difficult to
achieve for RhI.
From inspection of the geometries and orbitals of the depro-
tonated PN-P and PP-P complexes it is clear that the nitrogen
atom of the PN-P complexes readily adopts an sp2 hybridisation,
thus allowing a clear p-interaction between the nitrogen atom p-
orbital and the metal orbitals (Fig. 6). Although these are weakly
attractive interactions, they do contribute to some stabilisation
of the amido complexes. Similar stabilising interactions cannot
be achieved for the corresponding PP-P phosphido complexes,
because the P-atom does not rehybridise from sp3 to sp2, and thus
the lone pair on the central phosphorus atom points away from
the metal (Fig. 5 and 6), giving the P-atom considerable phosphine
character. Taken together with the higher electronegativity of
nitrogen, this explains why the R2PH ligand complexes are weaker
acids than the corresponding R2NH ligand complexes.
Scheme 4 Synthesis of Pd complex 4 featuring deprotonated monoan-
ionic ligand L1.
DFT calculations on Rh and Pd species with L2H and L2
To shed more light on the fact that the Rh complexes of ligands
L1H and L2H are difficult to deprotonate, while the corresponding
phosphido Pd complexes are easily obtained, we calculated the
optimized geometries of the PPHP phosphine and PPP phosphido
complexes [M(L2H)(L)]n and [M(L2)(L)]n-1 (M = Rh, Pd; L = Cl,
CO; n = +1, 0) with DFT (BP86, def2-TZVP). For comparison, we
also optimized the analogous PNHP amine and PNP amido com-
plexes, as both Rh(PNP) and Pd(PNP) complexes are known.31
A comparative analysis of the relative energies for deprotonation
of the ligand backbone by triethylamine was performed using a
full geometry optimization (BP86, def2-TZVP) of all the species
involved (Scheme 5).
This observation opens up the interesting possibility to use
these phosphido-palladium complexes as sterically encumbered
metallo-ligands for other metals.
Scheme 5 Computed (DFT) deprotonation of the PPHP and PNHP
ligands in the complexes [MCl(PEHP)]n and [Rh(PEHP)(CO)]+ (M = Rh,
Pd; E = P, N) with NEt3 to give the corresponding PE-P complexes.
Conclusions
In summary, the preparation and coordination chemistry of
the neutral, tridentate all-phosphorus ligand L1H and L3NEt2 to
group 9 metals is described, including a molecular structure for
RhCl(L3NEt2). In contrast to the known reactivity of structurally
Quite similar geometries were obtained for the four PPHP
based complexes Rh(L2H)(CO)-cation, Rh(L2)(CO), Pd(L2H)Cl-
cation and Pd(L2)Cl (Table 1). The geometric data obtained for
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Dalton Trans., 2011, 40, 8822–8829 | 8825
©

Table 2 Computed proton transfer energies (kcal mol-1)a
BP86
BP86 + cosmob
b3-lyp + cosmob
[(PPHP)ML]m + [(PNP)ML]n → [(PNHP)ML]n + [(PPP)MCl]m
M = Rh, L = Cl
+18.0
+12.9
+12.4
+16.3
+11.0
+10.2
+15.8
+11.1
+9.5
M = Rh, L = CO
M = Pd, L = Cl
[(PNHP)ML]n + NEt3 → [(PNP)ML]n-1 + [HNEt3]+
M = Rh, L = Cl
+74.4
+6.7
+1.4
+6.3
-13.8
-18.2
+10.3
-10.3
-16.2
M = Rh, L = CO
M = Pd, L = Cl
[(PPHP)ML]n + NEt3 → [(PPP)ML]n-1 + [HNEt3]+
M = Rh, L = Cl
+92.4
+19.6
+13.9
+22.6
-2.8
-8.0
+26.2
+0.8
-6.7
M = Rh, L = CO
M = Pd, L = Cl
a def2-TZVP basis set. b e = 7.58 (tetrahydrofuran)
Fig. 6 Simplified orbital interaction scheme explaining the stabilisation
of the amido complexes (A) relative to the phosphido complexes (B).
favored over deprotonation of the PPHP ligand complexes, which
is partly due to the higher electronegativity of nitrogen and partly
due to a stabilising p-interaction between the amido fragment and
the metal. In all cases deprotonation of the palladium complexes
is favored over deprotonation of the rhodium complexes. This is
most likely due to a better stabilisation of the negative charge
of the ligand by the divalent palladium centre. Furthermore, the
HOMO of the resulting complex PdCl(L2) displayed a high degree
of P lone pair character, implying that the pivotal phosphorus
atom of the PPHP scaffold LH, which is formally a phosphido
unit when coordinated to Pd as a monoanionic ligand, is actually
more like a metallophosphine fragment that should be amenable
to form (hetero)bimetallic complexes. We foresee very interesting
applications for catalysis with these very bulky, electron-rich
monophosphines. Furthermore, in light of the increased interest in
adaptive catalyst systems, it is worthwhile to investigate if complex
PdCl(L2), 4, or (other group 10 metal) analogues thereof are
susceptible to reversible protonation of the pivotal ‘phosphido’
unit, as has previously been described for Pd-complexes with a
structurally related PNHP ligand.25a
Fig. 5 DFT optimized geometry of complexes (A) [Rh(CO)(L2H)]-cation
(left) and [Rh(CO)(L2)] (right) and (B) [PdCl(L2H)]-cation (left) and
[PdCl(L2)] (right) (DFT; BP86 functional; def2-TVZP). Color code:
orange – P; red – O; green – Cl, dark gray – metal.
related PNHP, PNHN, PSiHP, and PCHP ligand backbones, L1H
displays neither oxidative addition nor proton-transfer activation
of the central P–H bond under various reaction conditions with
several RhI precursors, and only neutral tridentate PPHP ligation
is seen. This is mirrored by the reactivity of the electronically and
sterically related, novel tridentate ligand L3NEt2. To understand
the apparent dichotomy of the reactivity of the PPHP scaffold to
group 9 (Rh) and previously reported group 10 (Pd) precursors,
we performed DFT calculations on both cationic RhI and PdII
complexes with L2H and the related neutral complexes with the
deprotonated monoanionic analogue L2. Related PNHP analogs
were included in the analysis as well. The computations reveal
that in all cases deprotonation of the PNHP ligand complexes is
Experimental section
General
All reactions were carried out under an atmosphere of nitrogen
using standard Schlenk techniques. Reagents were purchased
8826 | Dalton Trans., 2011, 40, 8822–8829
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The Royal Society of Chemistry 2011
©

1
2
1
from commercial suppliers and used without further purification.
The following compounds were prepared following literature
procedures: L1H and L2H,20 2-diphenylphosphino-3-methylindole
1,26 and [{Rh(m-OH)(COE)2}2].34 THF, pentane, hexane and
Et2O were distilled from sodium benzophenone ketyl. CH2Cl2
was distilled from CaH2, toluene and NEt3 from sodium under
nitrogen. NMR spectra (1H, 31P and 13C) were measured on a
Varian INOVA 500 MHz or a Varian MERCURY 300 MHz. IR
spectra (in C6D6) were measured on a FT-IR Vertex-70 (Bruker).
(ddd, JRhP = 97 Hz; JPP = 11.0 Hz;); 64.5 (ddt, JRhP = 120 Hz;
2JPP = 11.0 Hz). 31P NMR (121 MHz, CD2Cl2): d 64.5 (br dd, 1JPP
393 Hz).
=
Complex RhCl(PPNEt2P) (3). To a stirred THF solution (4 mL)
of L3NEt2 (211 mg, 0.29 mmol) was added solid [Rh(m–Cl)(COE)2]2
(103.5 mg, 0.14 mmol). After 1 h the volatiles were removed
under vacuum; washing with hexanes removed COE to yield a
pure orange powder (216 mg, 86%). Complex 3 was recrystallized
from toluene at -4 ◦C to afford orange crystals suitable for X-ray
analysis. 1H NMR (300 MHz, C6D6): d 8.17–8.04 (m, 6H); 7.87–
7.78 (m, 4H); 7.32 (d, J = 7.4 Hz, 2H); 7.23–7.03 (m, 8H); 7.02–6.89
(m, 8H); 2.41–2.18 (m, 4H); 1.93 (s, 6H); 0.69 (t, J = 7.5 Hz, 6H).
Preparation of N,N¢-bis(2-(diphenylphosphino)-3-(methyl)-
indolyl)diethylaminophosphine (L3NEt2). To
a THF solution
(10 mL, -78 ◦C) of indolylphosphine 1 (1.0 g, 3.17 mmol) was
added 2.5 M nBuLi (1.27 mL, 3.17 mmol) dropwise. The solution
was removed from the cold bath for 25 min, then cooled to -78
◦C and treated dropwise with Et2NPCl2 (231 mL, 1.59 mmol).
The solution was slowly allowed to warm up to r.t. and stirred
for 16 h, whereafter volatiles were removed in vacuo and the
crude material chromatographed through neutral alumina using
a gradient elution from 15% to 25% CH2Cl2 in hexanes to obtain
L3NEt2 as a white solid (858 mg, 74%). 1H NMR (400 MHz,
CDCl3): d 7.43 (d, J = 7.0 Hz, 2H); 7.27–7.20 (m, 10H); 7.15 (t,
J = 7.6 Hz, 2H); 7.11–7.04 (m, 8H); 7.04–6.93 (m, 6H); 3.49–3.31
(m, 4H); 1.67 (s, 6H); 0.86 (t, J = 7.3 Hz, 6H). 13C APT (100 MHz,
CDCl3): d 10.7 (s, CH3); 14.5 (s, CH3); 45.4 (d, J = 23 Hz, CH2);
113.1 (s, CHar); 119.0 (s, CHar); 119.8 (s, CHar); 123.5 (s, CHar);
124.9 (br, CAr); 127.4 (s, CHar); 127.8 (s, CHar); 128.0 (dd, Ja =
37 Hz, Jb = 6 Hz, CHo-phenyl); 132.1 (br, CAr); 132.5 (dd, Ja = 19 Hz,
Jb = 3 Hz, CHar); 135.0 (br, CAr); 136.0 (br, CAr); 140.0 (br, CAr).
31P NMR (121 MHz, C6D6): d 142.4 (dt, 1JRhP = 223.9 Hz; 2JPP
=
42.6 Hz); 17.1 (dd, 1JRhP = 144.6 Hz; 2JPP = 42.6 Hz). HR-MS (FAB)
calcd. for [M + H] C46H45N3ClP3Rh, 869.1492; found 869.1498.
Upon addition of a 2.0 M HCl solution, these signals shift to d
134.4 (PPh2) and 22.1 ppm (PNEt2), indicating that P–N rupture
by protonation did not occur.
DFT geometry optimizations and spectral parameter calcu-
lations. Geometry optimizations were carried out with the
Turbomole program35a,b coupled to the PQS Baker optimizer.36
All geometries were fully optimized as minima at the BP8637
level using the polarized triple-x def2-TZVP basis38 (small-core
pseudopotential39 on Rh or Ir). Solvent corrections from single
point cosmo calculations (e = 7.58; thf) were applied for all species
as well.40 For comparison, single point calculations (using the
optimized geometries obtained at the BP86/def2-TZVP level)
using the b3-lyp41 and the same def2-TZVP basis set and cosmo
corrections were performed as well. Thus obtained (SCF) energies
(kcal mol-1) are reported in Table 2. Optimized geometries of all
species are supplied as separate files in .pdb and .xyz format in the
supporting information.†
3
31P{H} NMR (162 MHz, CDCl3): d 95.6 (t, JPP = 163.0 Hz);
3
-30.2 (d, JPP = 163.0 Hz). 31P{H} NMR (162 MHz, CDCl3): d
95.6 (t, 3JPP = 163.0 Hz); -30.2 (d, 3JPP = 163.0 Hz).
Complex RhCl(PPHP) (2). To a stirred solution of L1H
(82.8 mg, 198.0 mmol) in THF (10 mL) was added [{Rh(m–
Cl)(COE)2}2] (71 mg, 99 mmol) at r.t. and the reaction mixture
was stirred for 10 min to generate an orange-colored solution.
Evaporation of the solvent in vacuo yielded 1 with persistent
COE29 in near-quantitative yield as an orange powder. Complex
1 decomposes significantly within 24 h under inert atmosphere.
1H NMR (300 MHz, C6D6): d 7.90–7.82 (m, 2H), 7.47–7.40 (m,
2H), 7.26–7.03 (m, 4H), 6.30 (d, 1JPH = 342 Hz, PH), 3.13 (br sept,
2H), 2.37 (br sept, 2H), 1.24 (app q, J = 7.1 Hz, 12H), 0.90 (app
q, J = 7.1 Hz, 12H). 31P{H} NMR (121 MHz, C6D6): d 65.5 (dd,
X-ray
crystal
structure
determination
1008.31, orange blocks,
of
3.
C46H44ClN3P3Rh·1.5(C7H8), Fw
=
¯
0.21 ¥ 0.27 ¥ 0.33 mm, triclinic, P1(no. 2), a = 11.4830(6), b =
˚
12.4619(4)◦, c = 18.6500(6) A, a = 90.887(2), b = 99.603(2), g =
3
-3
˚
109.560(2) , V = 2472.24(17) A , Z = 2, Dc = 1.355 g cm , m = 0.537
mm-1. 47552 Reflections were measured on a Nonius KappaCCD
diffractometer with rotating anode (graphite monochromator,
-1
˚
˚
l = 0.71073 A) up to a resolution of (sin q/l)max = 0.65 A at a
temperature of 150(2) K. The crystal appeared to be cracked into
two fragments. Therefore two orientation matrices were used for
the intensity integration with Eval15.42 TWINABS43 was used for
absorption correction and scaling based on multiple measured
reflections (0.69–0.71 correction range). 11346 Reflections were
unique (Rint = 0.02), of which 10282 were observed [I > 2s(I)].
The structure was solved with Direct Methods using the program
SHELXS-97.44 The structure was refined with SHELXL-9744
against F2 of all reflections taking the presence of two fragments
into account.45 Non hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were introduced
in calculated positions and refined with a riding model. The
toluene solvent molecules were disordered on inversion centers.
681 Parameters were refined with 420 restraints concerning the
disordered toluene molecules. R1 [I > 2s(I)] = 0.0255; wR2 [all
refl.] = 0.0647; S = 1.04. Residual electron density between -0.43
1JRhP = 136.4 Hz; 3JPP = 32.0 Hz); 48.9 (dd, 1JRhP = 167.1 Hz; 3JPP
32.0 Hz). 31P NMR (121 MHz, C6D6): d 65.5 (dd); 48.9 (ddt, 1JPH
=
=
341.7 Hz). IR (THF): n = 2271 cm-1. The analogous complex with
L2H was prepared using the same procedure.30
Complex Rh(H)Cl2(L1H) (2HCl). To a stirred THF solution
(2 mL, 0 ◦C) of L1H (24.5 mg, 58.5 mmol) and [{Rh(m-Cl)(COE)2}2]
(21 mg, 29.2 mmol) was added HCl in ether (35 mL, 2.0 M,
0.070 mmol). After 16 h at room temperature, volatiles were
removed under vacuum to afford the product, which is stable in
solution with no evidence of decomposition for 3 days. 1H NMR
(300 MHz, CD2Cl2): d 8.31–8.20 (br s, 2H); 7.91–7.78 (br s, 2H);
1
7.75–7.55 (br m, 4H); 6.96 (d, JPH = 393 Hz, 1H); 3.25 (br sept,
2H); 2.82 (br sept, 2H); 1.40–1.18 (m, 18H); 1.09 (d, J = 6.8 Hz,
1
2
6H); -16.91 (ddt, JRhH = 10.1 Hz; JPH = 10.1 Hz (PH);2JPH
=
6.7 Hz (PiPr2), 1H). 31P{H} NMR (121 MHz, CD2Cl2): d 71.1
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8822–8829 | 8827
©

3
˚
and 0.56 e/A . Geometry calculations and checking for higher
symmetry was performed with the PLATON program.46
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a highlight of this work, see: T. Braun, Angew. Chem., Int. Ed., 2005,
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Acknowledgements
This work has been financially supported by the NWO-ACTS
consortium (Advanced Chemical Technologies for Sustainability)
via research grant ASPECT 053.62.029 and by the University of
Amsterdam. J.I.v.d.V. also acknowledges the Netherlands Council
for Scientific Research – Chemical Sciences (NWO-CW) for a
VENI Innovative Research grant 2007–2010 (NWO 700.56.043)
and the EU-FP7 COST network CM0802 ‘PhoSciNet’ for ad-
ditional support. We thank Dr Jeroen Wassenaar for synthetic
suggestions regarding the preparation of indolylphosphines.
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8828 | Dalton Trans., 2011, 40, 8822–8829
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The Royal Society of Chemistry 2011
©

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