Redox-induced reversible P-P bond formation to generate an organometallic σ4λ4-1,2-biphosphane dication

Article abstract of DOI:10.1002/anie.201208682

Twice the fun: Dimeric Fe^II diphosphonium bis(alkynyl) complexes are formed by oxidation of the corresponding Fe^II alkynyl phosphine complexes. The structure of these organometallic diphosphonium salts and their monomeric precursors

Full text of DOI:10.1002/anie.201208682

Angewandte
Chemie
DOI: 10.1002/anie.201208682
Alkynyl Diphosphane Complexes
À
Redox-Induced Reversible P P Bond Formation to Generate an
Organometallic s4l4-1,2-Biphosphane Dication**
Ayham Tohmꢀ, Guillaume Grelaud, Gilles Argouarch, Thierry Roisnel, Stꢀphanie Labouille,
Duncan Carmichael,* and Frꢀdꢀric Paul*
Many well-established methods exist for fine-tuning the
properties of phosphorus compounds, and these allow the
creation of tailored stereoelectronic environments that exert
exact and predefined control over systems for catalysis[1] or
molecular electronics.[2] In principle, the capacity to change
the properties of a phosphorus center actively during a process
creates further important potential, such as on–off switching
in catalysis,[3] or profound gating of the properties of
electronic materials.[4] Studies of how phosphorus centers
can be efficiently modulated using ferrocenes,[5] cobalto-
cenes,[6] TTF derivatives,[7] or other redox-active groups have
appeared; however, these redox-active groups are often
spatially close to the phosphorus center, so the electronic
outcome of the redox change can be conflated with significant
steric effects. This is obviously undesirable if geometrical
properties need to be maintained, so the possibility of
modulating the properties of a phosphorus atom by a molec-
ular wire[8] is attractive.[9] Herein we report a study that
concerns metallophosphanes 1a,b (Scheme 1) having a [Fe-
(dppe)(h5-C5Me5)] redox center linked to a s3,l3-phosphorus
atom by an alkyne that functions as a short molecular wire.[10]
Within such a system, oxidation of the redox-active organo-
iron(II) center is expected to trigger significant changes at
phosphorus because of the very efficient electronic commu-
nication that occurs across alkyne bridges.[11]
Scheme 1. Compounds 1–4[PF6]2 showing their dominant valence-bond
isomers as calculated by B3PW91 DFT calculations.
The prototype molecular wire-based metallophosphanes
1a,b were synthesized and characterized.[12] The crystal
structure analysis of 1a (Figure 1a) reveals a nearly linear
Fe-C-C-P arrangement (Fe-C37-C38 178.2(2)8, C37-C38-P3
ꢀ
167.7(2)8), a short C C distance (1.892(2) ꢀ), and a Ph-P-Ph
angle of 98.31(8)8, all of which are well reproduced by DFT
calculations at the B3PW91 level (see the Supporting
[*] A. Tohmꢀ, Dr. G. Grelaud, Dr. G. Argouarch, Dr. T. Roisnel,
Dr. F. Paul
Institut des Sciences Chimiques de Rennes, CNRS (UMR 6226),
Universitꢀ de Rennes 1
Campus de Beaulieu, 35042 Rennes Cedex (France)
E-mail: frederic.paul@univ-rennes1.fr
Dr. S. Labouille, Dr. D. Carmichael
Laboratoire Hꢀtꢀroꢀlꢀments et Coordination, CNRS (UMR 7653),
Ecole Polytechnique
91128 Palaiseau Cedex (France)
E-mail: duncan.carmichael@polytechnique.edu
[**] G.G. thanks the Rꢀgion Bretagne for a scholarship. The ANR Blanc
program is acknowledged for financial support (ANR 2010 BLAN
719). The Polytechnique laboratory is part of the EC COST funded
network “PhoSciNet” CM0802.
Figure 1. Crystal structures for the neutÀral complex 1a (a) and oxidized
dication 4a[PF6]2 (b), for which one PF6 counterion is omitted.
Ellipsoids are set at 50% probability; hydrogen atoms are omitted for
clarity.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208682.
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Information). Comparisons of the calculated structure for 1a
with those for the model organometallic complex 2 and the
simple alkynylphosphane 3[13] confirm that the complex is
uncontroversially characterized by the simple valence-bond
formulation given for 1a,b. It therefore shows the character-
istics of a classical phosphane and has the metal center in the
FeII state.
(Figure 2; NPA charges computed for IVa2+: QFe À0.14, QP
+ 1.45; for Ia: QFe À0.13, QP + 0.96).[21]
Redox reactivity of 1a,b was investigated. [Fe(dppe)(h5-
ꢀ
C5Me5)(C C)] endgroups normally undergo chemically
reversible oxidations at half-wave potentials between 0 to
À0.3 V (in CH2Cl2/SCE) to give the corresponding FeIII
complexes,[10] and the cyclovoltammetric analysis of 1a,b
under these conditions pointed to the rapid and clean
formation of a new species upon oxidation at about À0.1 V
(see the Supporting Information).[14] The corresponding bulk
chemical oxidation of 1a,b using [FcH][PF6] instantaneously
gave purple solutions, from which diamagnetic purple dinu-
clear dications 4a,b[PF6]2 could be isolated in yields of about
80% (Scheme 2).
Figure 2. B3PW91 calculated Kohn–Sham delocalized orbitals for
IVa2+. LUMO (left) and HOMO (right).
Cyclic voltammetry experiments performed on isolated
4a,b[PF6]2 are consistent with those performed on 1a,b.
However, even when the scan is halted before reaching the
expected reduction potential (close to À1 V), the reductive
cycle for 4a[PF6]2 gives a persistent peak. This peak is
attributed to the reduction of 1a+ and implies that a rapid
equilibration of 4a2+ and 1a+ occurs in solution. This assign-
ment of a monomer–dimer equilibrium is corroborated by
UV/Vis spectra; these show the development of characteristic
LMCT transitions that are classically associated with mono-
III
5
+
ꢀ
nuclear [Fe (dppe)(h -C5Me5)(C C)] species when CH2Cl2
solutions of 4a2+ are diluted to 10À5 m at 208C.[22] Further-
more, simple exchange experiments showed that redistribu-
tion occurs when the two complexes 4a[PF6]2 and 4b[PF6]2 are
mixed at room temperature, whereupon a near-statistical
equilibrium mixture of the symmetrical starting materials and
a new compound assigned as the dissymmetrically substituted
dimer 4c[PF6]2 is established over two hours (Scheme 3).[23]
Scheme 2. Interconversion reactions of 1 and 4[PF6]2 complexes.
Reagents and conditions: i) [FcH][PF6], (2 equiv), CH2Cl2, 208C, 1 h;
ii) [CoCp2] (3 equiv), CH2Cl2, 208C, 1 h.
The dication 4a[PF6]2 features singlet 31P NMR peaks at
À
95 ppm (dppe) and À43 ppm (PPh2), a PF6 multiplet at
À144 ppm, and exhibits an apparent diamagnetic proton
NMR spectrum showing only pentamethylcyclopentadienyl,
aromatic, and methylene components; it also gives an IR
stretch at about 1850 cmÀ1 [15]
Its X-ray diffraction structure is
.
given in Figure 1b. The two halves of the dimer are crystallo-
graphically identical and almost perfectly staggered about the
[16]
À
P P bond. Dicationic hexacoordinated biphosphanes are
normally expected to show stronger but slightly longer bonds
than the corresponding biphosphanes[16a] (compare P2Me4
2.212(1) ꢀ[17] and [P2Me6]2+ 2.198(2) ꢀ); in 4a[PF6]2, the
À
very long P P distance (2.264(4) ꢀ) is significantly greater
than in P2Ph4 (2.217(1 ꢀ)[18] and lies close to the value for
P2Mes4 (2.260(1) ꢀ).[19] The anticipated lengthening[16d] of the
Scheme 3. Exchange between 4a[PF6]2, 4b[PF6]2, and 4c[PF6]2. Equilibri-
À
P C bonds upon passing from P2Ph4 to 4a[PF6]2 is not found,
and a small shortening which is on the limit of statistical
um established after 2 h at room temperature in dichloromethane.
À
significance is observed instead (P C(Ar) mean: 1a 1.845,
4a[PF6]2 1.825, P2Ph4 1.851 ꢀ). Within the alkynyl compo-
nent, the shortening of the bond to phosphorus upon passing
from 1a to 4a[PF6]2 is pronounced (0.047 ꢀ) but the
Calculations to support the formulation of the putative
cation 1a+ were made upon a simplified model Ia+ that has
H2PCH2CH2PH2 and Cp ligands. The calculations give little
spin density at phosphorus for Ia+ (computed Mulliken spin
densities: Fe + 1.27, C37 À0.14, C38 + 0.16, P À0.01), and
they also show that the positive charge increases significantly
at iron (by 0.43) but not at phosphorus (À0.01) upon moving
from Ia to Ia+. The calculations therefore imply that the initial
oxidation for 1a is largely metal-centered, and this agrees well
with the experimental first electrochemical oxidation poten-
ꢀ
associated C C bond lengthening is small, so the linker
largely retains the structural characteristics of an alkyne.
+
ꢀ
Simple [R3P C C] ligands are considered to be strong s-
donors and weak p-acceptors,[20] and the relative charge
densities obtained here from B3PW91 DFT studies point
clearly to dominant valence-bond structure for 4a2+ that has
FeII endgroups and a dicationic biphosphane functionality
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1
710 Hz, PF6). H NMR (300 MHz, CD2Cl2): d = 7.74–6.90 (m, 60H,
HAr), 2.38 (m, 8H, CH2), 1.25 ppm (s, 30H, C5(CH3)5). 4b2+: Yield
tial for 1a,b, which lies within the region that is classically
associated with the FeII to FeIII oxidation step for [Fe(dppe)-
ꢀ À
5
[24]
94%. IR (KBr): n˜ = 1849 cmÀ1 (vs, C C P). 31P NMR (121 MHz,
ꢀ
(h -C5Me5)(C C)] endgroups.
CDCl3): d = 95.2 (s, 4P, dppe), À41.4 (s, 2P, P-P), À144.4 ppm (sept,
2P, JPF = 710 Hz, PF6). 1H NMR (300 MHz, CD2Cl2): d = 7.45–6.86
(m, 56H, HAr), 2.46 (s, 12H, CH3), 2.38 (m, 8H, CH2), 1.24 ppm (s,
30H, C5(CH3)5).
Reduction of 4a[PF6]2: A solution of 4a[PF6]2 (20 mg, 0.01 mmol)
and triphenylphosphane used as internal standard (5 mg, 0.02 mmol)
in CH2Cl2 was added under argon to cobaltocene (6 mg, 0.03 mmol)
and stirred for 1 h. The reaction was monitored by NMR spectros-
copy.
Reaction between 4a[PF6]2 and 4b[PF6]2: A solution of 4a[PF6]2
(20 mg, 0.01 mmol) in dry CD2Cl2 (0.5 mL) was added under argon to
a solution of 4b[PF6]2 (20 mg, 0.01 mmol) in dry CD2Cl2 (0.5 mL), and
the mixture was left to react for 2 h and checked by NMR
spectroscopy.
Finally, a series of experiments conducted at room
temperature in CH2Cl2 confirmed that treatment of 4a[PF6]2
with cobaltocene leads cleanly and near-quantitatively to the
regeneration of the monomeric complex 1a within an hour
(Scheme 2).
In conclusion, we report a chemically robust redox system
involving a prototypical molecular-wire-based phosphane and
demonstrate that initial oxidation of the neutral precursors
1a,b occurs within an electrochemically clement window at
the iron center; the products are the complexes 4a,b[PF6]2,
which are obtained as isolable dimers.[25] These compounds
À
show a weak P P bond and a capacity for redistribution in
solution. This redox-induced dimerization established for
1a,b provides support for the involvement of diphosphane
intermediates that is often proposed to occur in the complex
solution chemistry that follows the oxidation of more classical
metallocene-containing phosphanes.[26] Compound 1a can be
regenerated from 4a[PF6]2 by reduction, so that a powerful
change in the properties of the phosphorus atom can be
effected reversibly through redox chemistry. This process is
unprecedented in that none of the few known oxidatively
induced dimerization reactions of metal acetylide complexes
have yet been found to be reversible.[27] Work is in progress to
investigate further aspects of the chemistry of 4a,b[PF6]2.
Received: October 29, 2012
Revised: December 21, 2012
Published online: March 14, 2013
Keywords: iron · dimerization · molecular electronics ·
.
phosphorus · radical reactions
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Experimental Section
All of the reactions and workup procedures were carried out under
argon using standard Schlenk techniques with freshly distilled
solvents.
Synthesis of 1a,b: [Fe(Cp*)(dppe)Cl][28] (625 mg, 1 mmol), HC
ꢀ
C PAr2 (Ar= Ph, 4-Tol; 1.2 equiv),[29] and KPF6 (184 mg, 1 mmol)
were dissolved in THF (15 mL) and MeOH (15 mL) and stirred
overnight. After removal of the solvents, the residue was extracted
with CH2Cl2, concentrated, and precipitated by addition of n-pentane.
Filtration and drying in vacuo gave the corresponding vinylidene as
an orange solid. The vinylidene complex (0.9 mmol) was dissolved in
THF (20 mL), and DBU (0.2 mL, 1.3 mmol) was added dropwise.
After 2 h of stirring, the solvent was removed and the residue was
taken up with toluene and purified on pacified silica gel. After
removal of the toluene, the red-orange solid was washed with n-
pentane and dried in vacuo.
À
1a: Yield 70%. X-ray quality crystals were grown by slow
diffusion of methanol into
a dichloromethane solution of the
complex. The complex 1a was identified by comparison with
published data.[12] 1b: Yield 95%. IR (KBr): n˜ = 1964 cmÀ1 (s, C
[7] N. Avarvari, K. Kirakci, R. Llusar, V. Polo, I. Sorribes, C. Vicent,
Inorg. Chem. 2010, 49, 1894 – 1904.
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C). 31P NMR (121 MHz, C6D6): d = 100.0 (s, 2P, dppe), À20.1 ppm (s,
1P, P(p-Tol)2). 1H NMR (300 MHz, CDCl3): d = 7.96 (t, 4H, JHH
=
[8] A molecular wire is a one-dimensional molecule allowing
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[9] See also, for example: a) G. Grelaud, A. Tohme, G. Argouarch,
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[11] K. Costuas, F. Paul, L. Toupet, J. F. Halet, C. Lapinte, Organo-
metallics 2004, 23, 2053 – 2068.
8 Hz, HAr), 7.63 (t, 4H, JHH = 8 Hz, HAr), 7.21–6.93 (m, 20H, HAr),
2.56 (m, 2H, CH2), 2.08 (s, 6H, CH3), 1.78 (m, 2H, CH2), 1.47 ppm (s,
15H, C5(CH3)5).
Synthesis of 4a,b[PF6]2: The complex 1a,b (0.25 mmol) and
[FcH][PF6] (0.23 mmol) were dissolved in CH2Cl2 (20 mL) and stirred
for 1 h. After concentration of the solution to ca. 5 mL, the product
was precipitated by addition of n-pentane, filtrated, and dried
in vacuo to afford a purple solid.
4a2+: X-ray quality crystals were grown by slow diffusion of n-
pentane into a dichloromethane solution of the complex. Yield 89%.
IR (KBr): n˜ = 1852 cmÀ1 (vs, C C P). 31P NMR (121 MHz, CD2Cl2):
ꢀ À
d = 95.0 (s, 4P, dppe), À42.8 (s, 2P, P-P), À144.4 ppm (sept, JPF
=
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[12] Compound 1a has been reported previously: L. Dahlenburg, A.
Weiss, M. Bock, A. Zahl, J. Organomet. Chem. 1997, 541, 465 –
471.
[13] For recent surveys of alkynylphosphanes, see: a) J. R. Beren-
guer, E. Lalinde, M. T. Moreno, P. Montano, Eur. J. Inorg. Chem.
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[21] a) Computational models used for 1a, 1a+, and 42+ have been
denoted Ia, Ia+, and IVa2+, respectively (see the Supporting
Information for details); b) for a discussion of charge distribu-
tions in more classical biphosphane dications, see Ref. [16a].
[22] F. Paul, L. Toupet, J.-Y. Thꢁpot, K. Costuas, J.-F. Halet, C.
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ꢀ
[14] Note that no oxidation could be detected for Ph2PC CH
between 0 V and 1.2 V (vs. SCE) under identical conditions.
[15] This lies close to the bands found for allenylidene ligands in
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[23] The solvated energy the formation of the dimer is positive at the
B3PW91 level (+26.2 kcalmolÀ1) but becomes negative when
better-adapted calculations incorporating dispersion effects are
used (À9.3 kcalmolÀ1 at the M06 level; À10.2 kcalmolÀ1 at the
wB97XD level in the absence of entropy effects). The overall
dimerization can therefore be assumed to be slightly exergonic.
[24] Further circumstantial evidence for the intermediacy of a clas-
sical FeIII species, such as 1a+, comes from the oxidation reaction
of 1a in THF; here, a complexed vinylidene, a class of side-
products that are often associated with the preparation of
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[25] For a related oxidation of a much less conventional s2,l3-
5
+
ꢀ
phosphacumulene to
a
1,2-dialkynyl-s3,l3-substituted-1,2-
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