[PdCl2(dppfO2-O,O')]: A simple palladium(II) complex with a rare tetrahedral structure

Article abstract of DOI:10.1039/a904016a

Single-crystal X-ray diffraction analysis of PdCl₂(dppfO₂) [dppfO₂ = 1,1'-bis(oxodiphenylphosphoranyl)ferrocene] shows a rare tetrahedral geometry at a d⁸ metal centre; physical and spectroscopic characterisations indicate a paramagnetic species.

Full text of DOI:10.1039/a904016a

[PdCl2(dppfO2-O,OA)]: a simple palladium(ii) complex with a rare tetrahedral
structure
Jeremy S. L. Yeo, Jagadese J. Vittal and T. S. Andy Hor*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543.
E-mail: chmandyh@nus.edu.sg
Received (in Cambridge, UK) 19th May 1999, Accepted 1st June 1999
Single-crystal X-ray diffraction analysis of PdCl2(dppfO2)
neutral complex with a chelating dppfO2 and two terminal
chlorides (Fig. 1). The near-ideal tetrahedral geometry at the
Pd(ii) centre is exemplified by the large intra- and inter-ligand
angles [O1–Pd1–O2, Cl1–Pd1–Cl2 and O–Pd–Cl angles of
104.3(3), 120.4(2) and 107.8° (av.), respectively]. The dihedral
angle between the {PdO2} and {PdCl2} planes (84.2°) also
reflects a tetrahedral rather than a ‘distorted square-planar’
description. O-donation leads to lengthening of the P–O bonds
(av. 1.512 Å) compared to those in dppfO2 (1.495 Å3). This is
also consistent with a stronger P–C (C5 plane) bond (av. P–C
1.772 Å) compared to those in [PdCl2(dppf)] (av. P–C
1.804 Å2d). There is little geometric distortion of the ferrocenyl
C5, moieties, the C5 rings being planar with tilts of 3.6(7)° and
torsional twist of < 2° (close to mirror symmetry). C1–P1 and
C6–P2 deviate slightly from the mean plane by +0.12 and
+0.008°, respectively.
The analogous NiCl2(dppfO2) complex can be obtained only
in poor yields from NiCl2·6H2O and dppfO2 in propan-2-ol at
room temperature or under reflux. Attempts to synthesise the
platinum analogue have been unsuccessful. This is not surpris-
ing in view of the even stronger preference of Pt(ii) for square
planar coordination. The tetrahedral geometry of 1 is probably
imposed by the large bite angle as a consequence of the
sterically demanding chelating ring in dppfO2. Its isolation
demonstrates, for the first time, that tetrahedral Pd(ii) is
sustainable even in a mononuclear structure in which the metal
centre is not constrained by neighbouring metal moieties.
[PdCl2(dppf)] is probably the most well known dppf complex
and finds extensive uses in catalysis (e.g. Grignard synthe-
sis,2b–d hydrodehalogenation,2f etc.). Although this catalyst has
demonstrated superior activity in many situations,2d,e in certain
cases, its effectiveness is poor2b,c and reasons for this are
[dppfO2
=
1,1A-bis(oxodiphenylphosphoranyl)ferrocene]
shows a rare tetrahedral geometry at a d8 metal centre;
physical and spectroscopic characterisations indicate a
paramagnetic species.
The coordination and catalytic chemistry of 1,1A-bis(diphenyl-
phosphino)ferrocene (dppf) has been reviewed.1 There are
ample examples in the recent literature that demonstrate the
significance of this diphosphine ligand.2 Like other phosphines,
it can be readily oxidised to its mono- and di-oxide forms.3 The
structure of the latter, 1,1A-bis(oxodiphenylphosphoranyl)ferro-
cene (dppfO2) is of interest to a number of groups, notably those
of Hor,3b Pilloni,3c Postel3d and Hashmi.3e Phosphine oxides in
general have attracted attention in view of their weak coordinat-
ing ability4 and their function as co-catalysts,5 crystallisation
aids,6 etc. The chemistry of Ph3PO, the most familiar phosphine
oxide, is well established.4 The coordination ability of some
diphosphine oxides7 is also known, although less so for
dppfO2.3 Our interest in PdII–dppf chemistry stems from the
rich catalytic activities shown by many dppf complexes of
Pd(ii).2b–h Some of these complexes are susceptible to oxidation
in solution and the decomposition product is generally assumed
to be chiefly free dppfO2.3a There is hitherto no report on the
coordination chemistry of dppfO2 with Pd(ii) although such
complexes with other diphosphine oxides are known.7 Herein,
we report the isolation of a simple Pd(ii)–dppfO2 complex with
a rare mononuclear tetrahedral Pd(ii) centre which, to our
knowledge, has not been reported in the literature. It is well
known that d8 platinum metals overwhelmingly favour square
planar structures and only Ni(ii) forms a significant number of
tetrahedral complexes.
Addition of dppfO2 to [PdCl2(MeCN)2] in CH2Cl2 gives
[PdCl2(dppfO2-O,OA)] 1 in 81% yield.† This complex was
originally obtained as a by-product in the reaction of
[Pd2(dppf)2(m-S)2] with COS. Its isolation contrasts with an
earlier unsuccessful attempt to crystallise a palladium-contain-
ing triphenylphosphine oxide complex.8 The IR spectrum of 1
(CH2Cl2) exhibits two strong n(P–O) peaks at 1260 and 1269
cm21. The 31P NMR spectrum in CD2Cl29 shows a broad peak
at d 34.0 and a minor resonance at d 47.2, tentatively assigned
to [PdCl2(m-dppfO2)]2. Upon standing for a week, this solution
decomposes readily to give predominantly free dppfO2 (d 28.2).
The electronic spectrum shows two bands at 20 264 [3T1(F) ?
3A2(F)] and 26 900 cm21 [3T1(F) ? T1(P)], similar to that of a
3
typical tetrahedral nickel(ii) complex.10 The magnetic mo-
ment11 (2.48 mB) is lower than that of a typical Ni(ii) tetrahedral
complex (ca. 3.2 mB) but still consistent with paramagnetic
Pd(ii) with two unpaired electrons. The cyclic voltammogram
shows a reversible one-electron couple with E1/2 = 0.410 V (vs.
[FeCp2]+–[FeCp2]). Similar behaviour has been reported for
PdCl2(dppf) (0.430 V)2a and dppfO2 (0.573 V).3a The remote
possibility of an internal electron transfer that generates Pd(i)
and Fe(iii) is thereby eliminated. Similar observations were
reported for some Cu(ii)–dppfO2 complexes3f and point to an
oxygen-donating dppfO2 ligand coordinated to a paramagnetic
Pd(ii) centre. The rarity of the postulated structure merited a
crystallographic analysis.‡ This confirmed a mononuclear
Fig. 1 Molecular structure of [PdCl2(dppfO2-O,OA)]. Selected bond lengths
(Å) and angles (°): Pd(1)–O(1) 1.974(8), Pd(1)–O(2) 1.968(7), Pd(1)–Cl(1)
2.222(4), Pd(1)–Cl(2) 2.204(3), P(1)–O(1) 1.495(9), P(2)–O(2) 1.529(6),
P(1)–C(1) 1.780(9), P(1)–C(11) 1.781(10), P(1)–C(17) 1.781(9), P(2)–C(6)
1.764(11), P(2)–C(23) 1.785(13), P(2)–C(29) 1.791(10), O(1)–Pd(1)–O(2)
104.3(3), Cl(1)–Pd(1)–Cl(2) 120.4(2), O(1)–Pd(1)–Cl(1) 100.2(3), O(1)–
Pd(1)–Cl(2) 112.7(3), O(2)–Pd(1)–Cl(1) 110.8(3), O(2)–Pd(1)–Cl(2)
107.3(2), P(1)–O(1)–Pd(1) 158.3(6), P92)–O(2)–Pd(1) 139.9(5).
Chem. Commun., 1999, 1477–1478
1477

1995, ch. 1, p. 3; S.-W. A. Fong and T. S. A. Hor, J. Cluster Sci., 1998,
9, 351.
presently not well understood. It is commonly accepted that
PdCl2(dppf) loses its catalytic efficiency after losing its
phosphine ligand (in the form of dppfO2) via PdCl2(dppfO2).
Isolation of 1 and its reasonable stability suggested that the
current perception is not necessarily correct. The loss of activity
rather may be linked to the geometric changes for Pd(ii) upon
oxidation of its dppf ligand. For example, we have recently
demonstrated that C–C coupling and hydride transfer2g,h occur
very efficiently on a planar Pd(ii) core. Oxidative addition of
tetrahedral Pd(0) gives a Pd(ii) intermediate which must be
geometrically rigid12 for effective reductive elimination to
occur. Loss of such rigidity could adversely affect the catalytic
efficiency. Oxidation of the phosphine ligand also leads to a
change in bite angle which would have an effect on catalytic
activities.13
The authors acknowledge the National University of Singa-
pore (NUS) for support (Grant RP 960664/A). We thank G. K.
Tan for assistance in X-ray analysis, Y. P. Leong for assistance
in the preparation of this manuscript and the technical staff of
the department for professional support. Discussions with Y. K.
Yan and Z. H. Loh are much appreciated. We thank the
reviewers for some constructive feedback.
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9 Brüker AMX 500 spectrometer at 202.46 MHz; chemical shifts are
externally referenced to 85% H3PO4.
Notes and references
† Synthesis of [PdCl2(dppfO2-O,OA)]: addition of dppfO2 (0.074 g, 0.126
mmol) to a CH2Cl2 solution (40 cm3) of [PdCl2(MeCN)2] (0.032 g, 0.125
mmol) resulted in a brown suspension that was stirred for 12 h. Filtration
and concentration gave an orange solution. Layering of hexane on this
solution gave red crystals of 1, which were collected and washed with
hexane and Et2O (yield 81%). Found: C, 53.42; H, 4.02; Cl, 10.17. Calc. for
C34H28Cl2O2P2FePd: C, 53.47; H, 3.70; Cl, 9.28%.
‡ Crystallographic data: C34H28Cl2FeO2P2Pd 1: M = 763.65, crystal
dimensions: 0.33 3 0.18 3 0.13 mm, monoclinic, space group Cc (no. 9),
a = 10.0444(2), b = 19.4070(3), c = 16.3157(1) Å, b = 91.340(2), V =
10 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements,
Pergamon Press, New York, 1984, ch. 27, p. 1345.
3179.58(8) Å3, Z
= 4, m(Mo-Ka) =
1.321 mm21. 7985 reflections
11 Magnetic measurements were obtained on a Johnson Matthey MKII
magnetic susceptibility balance. Observed magnetic susceptibility
values were corrected for diamagnetism using Pascal’s constants (B. N.
Figgis and J. Lewis, The Magnetochemistry of Complex Compounds, in
Modern Coordination Chemistry, ed. J. Lewis and R. G. Wilkins,
Interscience, New York, 1960, ch. 6, p. 400) except for the diamagnetic
contribution of dppfO2 which is determined experimentally as 2240 3
measured, 4010 unique (Rint = 0.0297), final R1 and wR2 values 0.0679 and
0.2104 for 3799 independent reflections [I ! 2s(I)] and 344 parameters.
The data collection was performed at 295 K on a Bruker SMART CCD area-
detector by the w-scan method, within the limits 2.1 @ q @ 25.0°. The data
were corrected for absorption using an empirical method (SADABS14a) and
the structure was solved by direct methods and refined by full-matrix least
squares (SHELXTL14b). One of the phenyl rings (C11–C16) attached to
P(1) was found to be disordered. Two disorder models (occupancies 0.7/0.3)
were included in the least-squares refinements. Individual isotropic thermal
parameters were refined for the major disorder component and common
isotropic thermal parameter was refined for the other ring. Both the rings
were treated as regular hexagons. Riding models were used to place all the
hydrogen atoms in their idealized positions. CCDC 182/1307. See
http://www.rsc.org.suppdata/cc/1999/1477/ for crystallographic data in .cif
format.
1026 cm3 mol21
.
12 Geometric rigidity enables the cis ligands to be in close proximity and
their labilisation under the influence of the trans ligands.
13 B. C. Hamann and J. F. Hartwig, J. Am. Chem. Soc., 1998, 120,
3694.
14 (a) SMART & SAINT Software Reference Manuals, version 4.0,
Siemens Energy & Automation, Inc., Analytical Instrumentation,
Madison, WI, 1996; (b) G. M. Sheldrick, SADABS, a software for
empirical absorption correction, University of Göttingen, 1993.
1 K.-S. Gan and T. S. A. Hor, in Ferrocenes—Homogeneous Catalysis,
Organic Synthesis, Materials Science, ed. A. Togni and T. Hayashi,
Communication 9/04016A
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Chem. Commun., 1999, 1477–1478