Synthesis and coordinative properties of a hybrid bis(diphenylphosphino)methane-terpyridine

Article abstract of DOI:10.1039/b002586k

A newly-synthesized 4'-[bis(diphenylphosphino)methane]2,2':6',2''- terpyridine-based (dppm-terpy) ligand forms complexes by selective complexation of the terpy or diphos segment, respectively, with Fe(II) or Pd(II) salts; in the latter case, an X-ray stru

Full text of DOI:10.1039/b002586k

Synthesis and coordinative properties of a hybrid
bis(diphenylphosphino)methane–terpyridine
Guillaume Pickaert,a Michèle Cesario,b Laurent Doucea and Raymond Ziessel*a
a Laboratoire de Chimie, d’Electronique et Photonique Moléculaires, Ecole Européenne de Chimie, Polymères et
Matériaux, Université Louis Pasteur, 25 rue Becquerel, F-67087 Strasbourg Cedex 02, France.
E-mail: ziessel@chimie.u-strasbg.fr
b Institut de Chimie des Substances Naturelles, CNRS, F-91128 Gif-sur-Yvette, France
Received (in Cambridge, UK) 31st March 2000, Accepted 15th May 2000
A newly-synthesized 4A-[bis(diphenylphosphino)methane]–
2,2A+6A,2B-terpyridine-based (dppm–terpy) ligand forms
complexes by selective complexation of the terpy or diphos
segment, respectively, with Fe(II) or Pd(II) salts; in the latter
case, an X-ray structural analysis reveals the formation of a
neutral square-planar complex assembled from the deproto-
nated ligands.
corresponding phosphine oxide L(O)2, obtained under phase
transfer conditions with NaIO4 as oxidant, shows a 31P NMR
signal at d 29.5, with the CH proton appearing as a triplet at d
= 5.00, JHP 14.4 Hz. A strong IR absorption band is found at
1210 cm21 (nP=O).
The ambivalent reactivity of dppm–terpy is demonstrated by
virtue of its interaction with Fe and Pd precursors. Thus,
.
reaction of L with Fe(ClO4)2 6H2O gives a deep-violet complex
The 2,2A+6A,2B-terpyridine (terpy) molecule is widely used in
transition metal chemistry as a meridionally coordinating and
tridentate chelating ligand. Multiple applications have been
found in the areas of analysis and the design of molecular
electronic devices.1 Phosphine-functionalized terpy entities,
whose structure is assigned as C1.† Both the intense MLCT
absorption band centred at 562 nm (e = 20450 dm3 mol21
cm21) and the slightly shifted singlet observed (d 3.2 in CDCl3)
in the 31P NMR spectrum are indicative of complexation at the
terpy segment. Note that the related Fe(II) complex formed from
L(O)2 gives a singlet at d 30.0 (CD3CN). In contrast, ligand L
reacts with [Pd(acac)2] (acac = acetylacetonate) to form a
sparingly soluble, deep-yellow compound (lmax = 407 nm, e =
43450 dm3 mol21 cm21) that lacks the signal characteristic of
such as terpyPPh2 and terpyPO3H2,3 readily complex many
2
different transition metals and bind strongly to certain semi-
conductors. We have shown recently that terpyCH2P(O)Ph2 can
be deprotonated under mild conditions to produce a useful
synthon for the preparation of carotenoid-based photoactive
molecular-scale wires.4 In light of the rich coordination
chemistry of bis(diphenylphosphino)methane (dppm), a species
well known in organometallic chemistry as a monodentate,
bidentate or bridging ligand,5 it was anticipated that the hitherto
unreported terpy analogue (dppm–terpy) might be a valuable
building block for the construction of multimetallic networks.
Interest in such polynuclear assemblies is stimulated by the
study of intramolecular electron or energy transfer processes6 as
well as by the coupling of luminophoric metal centers to
photoactive catalytic sites.
the CH fragment in the proton NMR spectrum but displays a 31
P
NMR signal at d 229.9 (singlet in CD2Cl2). The presence of
uncomplexed terpy fragments within this latter complex,
labelled as C2 in Scheme 2, was confirmed by an X-ray
diffraction study (vide infra). The dppm–terpy ligands are
deprotonated during the process so that the overall product is a
neutral palladium(II) complex. It is likely that the acac anion
operates as a buffer to deprotonate the ligand.7
We present here, the synthesis and coordination behaviour of
a hybrid diphos/terpy ligand where the complexation behaviour
is governed by the choice of the metal precursor. Ligand L is
prepared in a stepwise fashion from the deprotonated mono-
phosphine-terpy ligand 1, followed by nucleophilic substitution
with PPh2Cl as illustrated in Scheme 1. The dppm–terpy ligand
L is characterized by a 31P NMR signal at d 23.7 (singlet),
compared to intermediate 1 which gives a singlet at d 28.5. The
.
Scheme 2 Reagents and conditions: i, Fe(ClO4)2 6H2O, methanol–
dichloromethane; ii, [Pd(acac)2], THF; iii, [Ru(terpy)(dmso)Cl2], methanol,
AgBF4, 80 °C; all reactions were carried out using argon degassed
solutions.
Scheme 1 Reagents and conditions: i, BunLi, Pri2NH, THF, 278 °C; ii,
Ph2PCl, THF.
DOI: 10.1039/b002586k
Chem. Commun., 2000, 1125–1126
This journal is © The Royal Society of Chemistry 2000
1125

1.34 V vs. SCE (DEp = 80 mV) and two irreversible terpy-
based reductions at Epc(1) = 21.28 V and Epa(2) = 21.49 V.
It is surmised that the unusual irreversibility of the ligand
reduction is due to the strong electron density provided by the
¯
anionic Ph2PCPPh2 fragment appended to the terpy units. As
expected, complex C1 exhibits three well-defined and reversible
redox processes; namely, a single oxidation at 1.26 V (DEp
=
70 mV) and two ligand-centered reductions at 21.20 V (DEp =
74 mV) and 21.39 V vs. SCE (DEp = 66 mV). The easier Fe(II
)
oxidation vs. Ru(II) in C3 is in keeping with related un-
substituted terpy complexes.11
Preliminary steady-state emission studies show that complex
C2, in the solid state (0.5% dispersed in MgSO4), exhibits an
intense but structureless emission band at 590 nm when excited
at 400 nm. This emission is not observed in deoxygenated
acetonitrile solution. Additional photophysical measurements
will be carried out in order to explore the photoreactivity of
C3.
In summary, we describe a simple strategy for the synthesis
of hybrid ligands carrying hard and soft complexation centres.
The diphos or the terpy part of the ligand can be complexed with
good selectivity, using either Pd(II) or Fe(II), respectively.
Further complexation of the free terpy centers with redox-active
Ru(II) fragments facilitates preparation of linear heterotrinu-
clear complexes in a controlled manner. On-going experiments
will study the chemistry of these novel multitopic systems.
Fig. 1 ORTEP drawing of complex C2 (Displacement ellipsoids are shown
at the 50% probability level); Hydrogen atoms have been omitted for clarity.
Selected bond lengths (Å) and angles (°): Pd–P(1) 2.317(2), Pd–P(2)
2.315(2), P(1)–C(terpy) 1.756(4), P(1)–C(phenyl) 1.826(5), P(1)–C(phe-
nyl) 1.822(5), P(2)–C(terpy) 1.764(4), P(2)–C(phenyl) 1.818(4), P(2)–
C(phenyl) 1.823(5), P(2)*–Pd–P(2) 180.0, P(2)–Pd–P(1)* 109.09(5), P(2)–
Pd–P(1) 70.91(5), P(1)–C(terpy)–P(2) 99.5(2).
The single-crystal X-ray analysis‡ (Fig. 1) indicates that the
palladium atom is in a square-planar environment with the four
P atoms coordinated to the metal centre and with two
uncoordinated terpyridines. The crystals consist of discrete
neutral centrosymmetric molecules, the Pd atom being located
on an inversion centre. As expected, all six N-atoms are in a
transoidal arrangment that minimizes electronic interactions.8
An angle of 16.0(0)° between the planes defined by the two
pyridine rings containing N(5A) and N(15A) and a dihedral
angle of 16.8(0)° between the planes of the external rings
containing N(15A) and N(9A) illustrate twisting about the
interannular C–C bonds, as well as the slight distortion within
the terpy subunit. Owing to the anionic nature of the coordinated
diphos–terpy ligands the C–P bond distances (ca. 1.76 Å) are
shorter than in related Pd(II)-phosphine complexes.9 The bite
angle P(1)–Pd–P(2) of 70.91(5)° and the twist of the phenyl
rings around the P atom versus the plane defined by the square
containing the palladium (71–75°) are in good agreement with
values expected for a regular square-planar coordination
polyhedron.
Notes and references
† Synthetic details will be reported elsewhere. All new compounds gave
satisfactory elemental analyses and were authenticated by 1H and 13C NMR,
FTIR and MS. All 31P NMR chemical shift are referenced using H3PO4
(85% in water) as internal standard. Selected data: for C1; FAB m/z (m-
NBA) 1386.0 [M 2 ClO4]+. Found: C, 64.49, H, 4.13, N, 5.41.
C
80H62N6O8P4Cl2Fe requires C, 64.66; H, 4.21; N, 5.66%. For C2; FAB+
(m-NBA): m/z 1335.0 [M+H]+. Found: C, 71.85, H, 4.47, N, 6.17.
C
80H60N6P4Pd requires C, 71.94; H, 4.53; N, 6.29%. For C3; MALDI-TOF
m/z 2439.4 [M 2 PF6]+, 2294.9 [M 2 2PF6]. Found: C, 51.57, H, 3.48, N,
7.53. C110H82N12P8PdRu2F24.2C2H3N requires C, 51.35; H, 3.33; N,
7.35%.
‡ Crystal data for C2: C80H60N6P4Pd, M = 1335.62, monoclinic, space
group P21/n, yellow crystals, a = 11.477(4), b = 25.327(9), c = 11.595(5)
Å, b = 98.54(4)°, V = 3333(2) Å3, Z = 2, T = 293 K, Dc = 1.331 g cm23
,
m = 0.425 mm21, F(000) = 1376. The final conventional R1 factor is
0.0705 for 4185 data and 398 parameters, and 0.11 for all data, wR2
=
0.1423 (all data), goodness of fit = 1.068; largest peak and hole in the final
difference map were within +0.59 and 20.45 e Å3. CCDC 182/1638. See
http//www.rsc.org/suppdata/cc/b0/b002586k for crystallographic files in
.cif format.
This mononuclear Pd(II) complex is of interest as a potential
metallo-synthon for the construction of more elaborate molec-
ular architectures in which the electronic interaction between
both sites could, in principle, be tuned by the oxidation state of
the central metal. In order to demonstrate this principle, we
chose to complex the free terpy moieties with [Ru(terpy)(dm-
so)Cl2]10 under mild conditions. The heterotrinuclear complex
C3 has a characteristic MLCT absoption band at 482 nm (e =
43000 dm3 mol21 cm21), and exhibits a singlet in the 31P NMR
spectra at d 210.6 (CD3CN). The MALDI-TOF mass spectra is
in keeping with the proposed structure.†
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1988, 61, 1593.
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The redox behaviour of these complexes was studied by
cyclic voltammetry in MeCN (for C1 and C3) or dichloro-
methane (for C2) containing NBun PF6 (0.1 mol dm23) as
4
supporting electrolyte. The Pd(II) complex C2 exhibits two
irreversible oxidation waves at Epa(1) = 0.69 and Epa(2) = 1.04
V vs. SCE using ferrocene as internal reference (Fc/Fc+ = 0.41
V). No peaks are seen upon reductive scans, at least above 22.0
V vs. SCE. The oxidative processes can be ascribed to the
successive oxidation of the anionic terpy–diphos ligands.
Within the mixed Ru/Pd complex C3, ligand-based oxidation
steps are found at Epa(1) = 0.87 and Epa(2) = 1.06 V vs SCE.
This complex also exhibits a single metal-based oxidation at
1126
Chem. Commun., 2000, 1125–1126