Exploiting non-innocent ligands to prepare masked palladium(0) complexes

Article abstract of DOI:10.1002/anie.201003946

Is it or isn't it: Reaction of [PdMe2(tmeda)] with pyridyl-N-di(tert-butyl) phosphinoimine spontaneously affords an unusual bimetallic palladium(I) ligand-based biradical complex, which behaves as a "masked" form of Pd0 in its reactions with neutral ligan

Full text of DOI:10.1002/anie.201003946

Communications
DOI: 10.1002/anie.201003946
Non-Innocent Ligands
Exploiting Non-Innocent Ligands to Prepare Masked Palladium(0)
Complexes**
Dan A. Smith, Andrei S. Batsanov, Karine Costuas, Ruth Edge, David C. Apperley,
David Collison, Jean-Franꢀois Halet, Judith A. K. Howard, and Philip W. Dyer*
The last decade has shown an upsurge of interest in “non-
innocent ligands” (NILs), a term originally coined by
Jørgensen to describe metal scaffolds that are subject to
redox changes at a site remote from the metal they support.[1]
Such systems are well-established in biochemistry, being at
the heart of many metalloproteins,[2] and are now found
throughout coordination chemistry due to their diverse
spectroscopic, magnetic, and electrochemical properties.[3,4]
This versatility has extended the application of NILs to
organometallic chemistry, where they have been used to
Scheme 1. Pyridyl-N-phosphinoimines 1.
enhance coordinated substrate reactivity.[5–8]
More recently, the term “NIL” has been expanded to
include non-redox-active systems, where the metalꢀs coordi-
nation sphere is perturbed through variation in ligand
coordination mode and/or structure, which occurs in
“response” to external stimuli.[9–12] These changes have been
demonstrated to affect not only the rates of co-ligand
dissociation, but also their intrinsic reactivity.[13,14]
An unusual class of “responsive” NIL are the pyridyl-N-
phosphinoimines 1, with the “open” bis(diisopropylamino)
derivative 1a existing in tautomeric equilibrium with a
“closed” anellated iminophosphorane form 1a’ (Scheme 1).
As a result, 1a has been shown to behave as either a k2-PN or
a k1-N ligand depending on the nature of the partner metal,
and to exhibit unusual redox behavior.[15,16] In contrast, the
diphenyl analogue 1b behaves only as a bidentate k2-PNPy
ligand and exhibits no tautomerism.[17]
Despite the importance of palladium catalysis in a variety
of organic transformations,[17] there have been very few
studies of Pd–NIL complexes.[13,19,20] This combination could
help surmount the contradictory electronic and steric needs of
the oxidative addition and reductive elimination steps key to
many Pd-centered catalytic cycles through changes in NIL
binding. To this end, we report here a study of the behavior of
compounds 1 in the coordination sphere of palladium and the
preparation of low-coordinate Pd0 complexes.
Since it is well-established that bulky ligands such as PtBu3
can promote the formation of unsaturated Pd0 species that
show enhanced reactivity in Pd-catalyzed cross-coupling
reactions, the PtBu2-substituted derivative 1c (Scheme 2)
[*] D. A. Smith, Dr. A. S. Batsanov, Dr. D. C. Apperley,
Prof. J. A. K. Howard, Dr. P. W. Dyer
Department of Chemistry, Durham University
South Road, Durham, DH1 3LE (UK)
Fax: (+44)1913784737
E-mail: p.w.dyer@durham.ac.uk
Homepage: http://www.dur.ac.uk/chemistry/staff/profile/
?id=1317
Dr. K. Costuas, Dr. J.-F. Halet
Laboratoire des Sciences Chimiques de Rennes, UMR 6226 CNRS,
Universitꢀ de Rennes, 1, Rennes (France)
Dr. R. Edge, Prof. D. Collison
School of Chemistry, University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
Scheme 2. Synthesis of complexes 2 and 6.
was investigated.[21] Compound 1c was prepared by modifi-
cation of a known procedure and, as for 1b, was found to exist
purely in the “open” form.[17,22] Access to the target Pd0
complex was attempted through reaction of 1c with equi-
molar quantities of [PdMe2(tmeda)] (tmeda = N,N,N’,N’-tet-
ramethylethylenediamine) using a variation on known meth-
ods.[23,24] The reaction proceeded to completion slowly over a
[**] Durham University, the EPSRC, and Sasol Technology UK Ltd. are
acknowledged for financial support of this work and Johnson
Matthey for the loan of palladium salts. We thank Prof. R. Rꢀau
(Rennes) for invaluable discussions. The EPSRC is thanked for
access to mass spectrometry, EPR, and solid-state NMR facilities.
K.C. and J.-F.H. thank the IDRIS and CINES for computing facilities.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003946.
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period of 7 d at room temperature, affording 2 as an intensely
colored red/brown solid in 73% yield (Scheme 2). Recrystal-
lization from deuterobenzene afforded crystals of 2·4C6D6. X-
ray diffraction revealed a centrosymmetric dimer structure in
À
which a short Pd Pd bond is supported by two P,N,N ligands
Scheme 3. Two possible redox states for ligand 1c.
each coordinated in a k-NN,k1-P fashion (Figure 1).[22,25–27]
Figure 2. X-Band second derivative EPR (9.4 GHz, power=0.02 mW,
293 K) spectrum of 2 (powder).
Figure 1. X-ray structure of 2·4C6D6 (50% probability thermal ellip-
soids, H atoms and solvate C6D6 molecules are omitted).[22,25] Primed
atoms are generated by the inversion centre. Bond lengths (ꢁ;
calculated values in brackets[22]): Pd–Pd’ 2.4565(3) [2.511], Pd–P
2.8577(5) [2.922], Pd–P’ 2.2533(5) [2.300], Pd–N(1) 2.1376(15) [2.162],
Pd–N(2) 2.0614(16) [2.100], P–N(2) 1.6795(16), N(2)–C(6) 1.348(2)
[1.357], N(1)–C(5) 1.383(2) [1.395], C(5)–C(6) 1.433(2) [1.438]; angles
[8]: Pd’-Pd-P’ 74.57(1) [74.7], Pd’-Pd-N(2) 84.94(4) [84.8], N(1)-Pd-P’
121.21(5) [121.4], N(1)-Pd-N(2) 79.26(6) [79.0], N(2)-P-Pd’ 101.38(6)
[101.3], Pd-N(2)-P 99.11(8) [99.1].
presents a 1:2:3:2:1 pattern consistent with an isolated ligand-
centered unpaired electron coupling with two 14N centers
(splitting 8.80 G).[22] Any phosphorus contribution to the EPR
pattern is assumed to be lost due to the large linewidths
encountered. In conjunction with the experimentally deter-
mined gaxial values (g(xy) = 2.0062; g(z) = 1.9894), these data are
consistent with an unpaired electron being localized on each
of the P,N,N ligands. As a result, the distance between the two
radical centers of 2 is significant (ca. 7 ꢁ), which accounts for
the absence of a half-field EPR signal, even at S-band
frequency, where the relative intensity of this signal versus the
main signal would be expected to be greatest.[22]
In agreement with the proposed diradical character, an
effective magnetic moment (meff) of 2.75 mB was obtained from
a benzene solution of 2 using Evansꢀ method. This value is
intermediate between that expected for a broken symmetry
(BS) biradical singlet (2.45 mB) and a high-spin (HS) triplet
state (2.82 mB).[22,36] Despite repeated attempts, reliable values
of meff for 2 in the solid state were unforthcoming due to very
low signal intensity. However, a broad, contact-shifted
À
The Pd Pd’ distance of 2 is at the lowest end of the range
typically found for s-bonds between square planar palladium
centers (2.42–2.84 ꢁ),[28] with few examples below 2.48 ꢁ
having been reported.[29–31]
The Pd atoms and two PN bridges of 2 are coplanar within
experimental error, with the pyridine and phenyl rings being
inclined to this plane by 38 and 848, respectively; both C6 and
N2 are planar (ꢀang = 360.00 and 359.758). The geometry of
the Pd’-P-N2-Pd bridge is dictated by an acute Pd-N2-P angle
of 99.11(8)8, similar to that found in [Cp2Zr{k2-PN-N-
(tBu)PEt2}IrH(Cp*)].[32] Although 2 displays a long Pd P
resonance centered at d = À338.2 (n = = 3880 Hz) ppm was
À
1
2
contact of 2.8577(5) ꢁ reminiscent of semi-bridging phos-
phine coordination,[33,34] here this palladium–phosphorus
interaction is a consequence of geometric constraints imposed
by the four-membered metallacycle, rather than a formal
bonding interaction.
obtained for 2 by solid-state MAS 31P NMR spectroscopy,
consistent with its paramagnetic character.
The bonding situation in 2 was explored computationally
using a combined extended Hꢂckel theory (EHT)/density
functional theory (DFT) study.[22,37] Complex 2 may be
regarded as comprising two 15-electron T-shaped [PdL3]+
fragments bound through a single metal–metal bond, each
of which possesses five occupied d-type frontier orbitals (FO)
and a vacant orbital that has significant ligand character
(Figure 3).[38] Interaction of the FOs of the two PdL3 frag-
ments leads to a set of ten metallic molecular orbitals (MO)
and two ligand MOs. One metallic MO is strongly antibond-
ing and destabilized in such a way that it becomes empty after
À
The N2 C6 bond of 2 (1.348(2) ꢁ) is longer than in the
related PdII complex [PdCl2(k2-PN-1b)] (1.278(2) ꢁ) in which
1b acts as a neutral PN-chelate, and is accompanied by a
[17]
À
shortening of the C5 C6 bond (D = 0.08 ꢁ). These data
reflect ligand 1c having undergone a one-electron reduction,
behaving as an open-shell monoanion 1cCÀ rather than as a
neutral P,N ligand 1c (Scheme 3).[35] Thus 2 is best described
I
I
À
as possessing a Pd Pd core bearing a reduced organic
scaffold.
À
interaction (LUMO+1). Consequently, Pd Pd bonding
The formulation of 2 as [{PdI(k2-NN,k1-P-1cCÀ)}2] is
supported by its solid-state EPR spectrum (Figure 2), which
occurs through a filled s-type M M bonding (HOMO 1)
and a vacant s*-type antibonding combination, implying that
À
À
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Scheme 4. Reactions of complex 2.
coordination is apparent from 1H and 13C NMR spectroscopic
analysis since no characteristic high frequency coordination
chemical shifts for the NCH resonances of the pyridyl moiety
of 1c are observed (cf. 5).[17] Although crystals of 4 suitable for
X-ray analysis were not forthcoming, a DFT study revealed
that the most stable structure is indeed a 14-electron near-
linear P-Pd-P complex.[22,37] Here, k1-P rather than k2-P,N
Figure 3. Qualitative EHT MO interaction diagram for 2 with C2v
symmetry.
two electrons are located on the ligand (HOMO). This is coordination of 1c is presumed to be favored due to the “soft”
consistent with two 15-electron [PdL3]+ fragments bound to character of the Pd0 center, something observed previously
each other through a single metal–metal bond giving rise to for a related P,N-scaffold.[39]
the expected 16-valence electron count.
Since it was apparent that displacement of the bridging k1-
This situation is fully supported by DFT calculations, with P motif of 2 by neutral donor ligands led to oxidation of 1cCÀ
optimized bond distances and angles computed for 2 compar- and metal reduction, other reactions leading to dimer
ing well with the experimentally determined values. The cleavage were explored. A common transformation of
HOMO and LUMO are close in energy and have a significant LnPd–PdLn dimers is binuclear oxidative addition of organo-
contribution localized on the ligand across the NCCN frag- halides RX to afford equimolar amounts of LnPd-X and LnPd-
ment (Figure 3).[22] Geometry optimization of both the HS R.[27,40,41] Hence, 2 was treated with neat chlorobenzene at
triplet and the BS biradical singlet magnetic states of 2 reveals 508C for 2 h, which gave [PdCl(Ph)(k2-PN-1c)] (5) in 95%
that they are close in energy (0.05 eV in favor of the BS state, yield as
a
single regioisomer [d(31P) = + 97.3 ppm]
although this value is probably somewhat overestimated by (Scheme 4). Complex 5 is structurally analogous to the
this functional), in agreement with the experimentally methyl derivative [PdCl(Me)(k2-PN-1b)] reported previ-
determined magnetic moment. The computed spin density is ously.[17] The conversion of complex 2 into 5 in which both
mostly localized on the nitrogen atoms and on one carbon the organo- and halo-fragments are incorporated at the same
atom over two of the ligands (0.13, 0.17, 0.18, 0.08, 0.33 Pd centre, is suggestive of a combined redox/k1-P dissociation
electrons on N1/N1’, N2/N2’, C2/C2’, C4/C4’, C6/C6’, respectively, process generating a low-coordinate Pd0 intermediate, fol-
for the triplet state).
lowed by two-electron oxidative addition, and not a binuclear
The intriguing electronic structure and unusual synthesis process. Hence, complex 2 may be regarded as a “masked”
of 2 prompted us to explore its chemistry in greater detail, in form of Pd0 in terms of its reactivity.
particular whether disruption of the bridging k1-P interaction
In contrast to the behavior of 1c, reaction of P-diphenyl-
À
(potentially important in supporting the Pd Pd bond) would substituted 1b with [PdMe2(tmeda)] under conditions iden-
result in a change in structure. This was indeed the case with tical to those used for the preparation of 2 rapidly afforded
the addition of two equivalents of PPh3 to a [D6]benzene the dimethyl complex [PdMe2(k2-PN-1b)] (6) [d(31P) = +
solution of 2 leading to the quantitative formation of [Pd0- 64.3 ppm] as the only product; 6 was isolated in 61% yield
(PPh3)(1c)] (3) according to 31P NMR spectroscopy [AX spin and fully characterized in solution and the solid state
system: d(31P)
=
+ 26.7 (d), + 106.6 ppm (d) {2JPP
=
(Scheme 2).[22] In contrast to the reactivity associated with
140 Hz}].[22] Further characterization and isolation of complex 1c, attempts to isolate the corresponding Pd0 species were
3 was precluded by its facile decomposition affording “Pd unsuccessful. Although reductive elimination of ethane from
black” within 1 h either in the solid state or in solution.
6 was apparent [1H NMR: d = 0.78 ppm (s)], over a period of
In an attempt to counter this problem and to further probe 3 h the precipitation of “Pd black” occurred, with no tractable
the reversibility of the 1c/1cCÀ interconversion, a toluene phosphorus-containing products being isolable.
solution of 2 was treated with two equivalents of 1c. This gave
Despite repeated attempts, no reaction was found to take
cleanly the stable, diamagnetic Pd0 complex [Pd(k1-P-1c)2] place between the bis(diisopropylamino)-substituted deriva-
(4) [d(31P) = + 97.5 ppm; cf. d = 81.2 ppm for 1c], which was tive 1a/1a’ and [PdMe2(tmeda)]. Contrastingly, 1a/1a’ reacts
isolated in 84% yield (Scheme 4). The absence of pyridyl smoothly with an isoelectronic RhI complex [{RhCl(CO)2}2],
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Angewandte
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to afford the P,N-chelated derivative [RhCl(CO)(k2-PN-1a)]
in high yield.[15] This lack of reactivity has tentatively been
ascribed to the significantly weaker Lewis basicity of the
diamino phosphine moiety, which cannot displace the chelat-
ing tmeda.
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In summary, it has been established that pyridyl-N-
phosphino imine 1c acts as an unusual type of non-innocent
ligand in combination with Pd0, behaving as both a “classical”
redox-active as well as a “structurally responsive” NIL. Both
theoretical and experimental investigations of complex 2
confirm its ligand-based biradical character over a range of
temperatures in solution and the solid state. Significantly,
perturbation of the palladiumꢀs coordination sphere in 2 by
other donor ligands induces both a reversible electronic
(ligand oxidation) and structural (k2-NN,k1-P to k2-PN to k1-
P) reorganization, generating a palladium species that acts as
a “masked” source of Pd0 complex. Once again, this study
highlights how slight modifications of the phosphorus sub-
stituents can substantially affect the chemical behavior of
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currently under investigation.
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Experimental Section
All reactions were carried out under an atmosphere of dry nitrogen
using standard Schlenk/glovebox techniques.
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2: [PdMe2(tmeda)] (173 mg, 0.69 mmol) and 1c (224 mg,
0.69 mmol) were dissolved in C6H6 (10 mL) and allowed to react for
7 d. The crystals that formed were isolated, washed with benzene (3 ꢃ
2 mL), and dried in vacuum (24 h), giving 2·4C6D6 as a dark red/
brown material (217 mg, 73%). Elemental analysis, found: C 63.70, H
8.37, N 4.79%; calcd: C 63.94, H 8.55, N 4.66%.
3: To a C6D6 (0.8 mL) solution of 2 (10 mg, 0.01 mmol) was added
PPh3 (6 mg, 0.02 mmol). Following heating at 508C for 0.5 h a brown
solution containing 3 was obtained. 31P{1H} NMR (161.91 MHz,
2
C6D6): d = 26.7 (1P, d, JPP = 140), 106.6 ppm (1P, d, 2JPP = 140).
[20] J. Zhou, H. Sun, K. Harms, J. Sundermeyer, Z. Anorg. Allg.
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[25] CCDC 723960 (6) and 723961 (2) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
4: A toluene (5 mL) solution of 2 (80 mg, 0.09 mmol) was added
to 1c (60 mg, 0.18 mmol) in toluene (5 mL) and stirred for 4 d.
Removal of volatile components in vacuum gave 4 as a red solid
(117 mg, 84%). Elemental analysis, found: C 63.09, H 7.11, N 7.43%;
calcd: C 63.28, H 7.17, N 7.38%.
5: A chlorobenzene solution of 2 (18 mg, 0.02 mmol) was heated
at 508C for 2 h. MS (FAB+) m/z: 509 [MÀCl]+, MS (EI+) m/z: 469
[MÀPh]+.
6: [PdMe2(tmeda)] (150 mg, 0.595 mmol) and 1b (217 mg,
0.595 mmol) were each dissolved in toluene and the solutions
combined, resulting in the precipitation of an orange solid (2 d).
This was isolated by filtration, washed with cold (08C) toluene, and
dried in vacuum, leaving 6 as an orange solid (305 mg, 61%).
Elemental analysis, found: C 61.92, H 5.06, N 5.49%; calcd: C 62.10,
H 5.01, N 5.57%.
[26] Data from The Cambridge Structural Database, version 5.30,
Feb. 2009: F. H. Allen, Acta Crystallogr. Sect. B 2002, 58, 380 –
Received: June 29, 2010
388.
Published online: August 16, 2010
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Keywords: coordination chemistry · non-innocent ligands ·
.
P,N ligands · palladium · redox reactions
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Communications
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[40] C. M. Fafard, D. Adhikari, B. M. Foxman, D. J. Mindiola, O. V.
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[41] R. H. Crabtree, The Organometallic Chemistry of the Transition
Metals, Wiley, New York, 1988, p. 122 – 123.
[36] D. F. Evans, D. A. Jakubovic, J. Chem. Soc. Dalton Trans. 1988,
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