Facile photoinduced charge separation through a cyanoacetylide bridge in a heterobimetallic Fe(II)-Re(I) complex

Article abstract of DOI:10.1039/b811357b

Photoinduced Fe-to-bpy charge transfer in [{Cp(dppe)Fe}(μ- C≡CC≡N){Re(CO)₃(bpy)}]PF₆ has been observed by ps-TRIR spectroscopy, supported by UV-Vis/IR spectroelectrochemistry and DFT calculations. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811357b

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Facile photoinduced charge separation through a cyanoacetylide bridge
in a heterobimetallic Fe(II)–Re(I) complexw
Mark E. Smith,a Emma L. Flynn,a Mark A. Fox,a Alexandre Trottier,a
Eckart Wrede,a Dmitri S. Yufit,a Judith A. K. Howard,a Kate L. Ronayne,b
b
b
c
a
ˇ
Michael Towrie, Anthony W. Parker, Frantisek Hartl and Paul J. Low*
Received (in Cambridge, UK) 3rd July 2008, Accepted 28th August 2008
First published as an Advance Article on the web 7th October 2008
DOI: 10.1039/b811357b
Photoinduced Fe-to-bpy charge transfer in [{Cp(dppe)Fe}-
(l-CRCCRN){Re(CO)3(bpy)}]PF6 has been observed by
ps-TRIR spectroscopy, supported by UV-Vis/IR spectroelectro-
chemistry and DFT calculations.
relationship between {MLn}CRCCRN{M0Ln}+ and
‘‘all-carbon’’ buta-1,3-diyndiyl complexes {MLn}CRCCR
C{M0Ln} is also worthy of mention.6
With a view to exploring the capacity of the cyanoacetylide
fragment to mediate electronic effects between remote metal
centres we have prepared the heterometallic complex
[{Cp(dppe)Fe}(m-CRCCRN){Re(CO)3(bpy)}]PF6 ([3]PF6).
By correlating the shift in n(CO)7 and n(CRCCRN)5 bands
with changes in oxidation state, important evidence for the
contribution of the metal dp-system to the frontier orbitals of
these fascinating materials is obtained. Furthermore, the
rhenium fragment Re(CO)3(bpy) offers the potential for direct
monitoring of the excited state behaviour of [3]+, as photo-
excitation into charge transfer excited states associated with
the bpy ligand results in well-characterised shifts in the n(CO)
pattern. The photochemistry and excited state properties of
rhenium complexes of general form [ReX(CO)3(NN)] have
been studied in extensive detail, with the ligands X (halide,
pseudo halide, or other donor) and NN (chelating a-diimine
ligand) both playing a significant role in tuning the excited
state nature (MLCT and/or XLCT), energy and luminescent
properties of the complex.8
Polynuclear systems in which some bridging ligand permits
control over the spatial arrangement of the constituent metal
centres and electronic interactions between them are of immense
contemporary interest with a view to the construction of func-
tional molecular structures.1 In this regard, the ubiquitous
cyanide ligand, [CN]ꢀ, offers many useful structural and electro-
nic properties, including cylindrical symmetry and a strong
s-bonding framework complemented by p and p* orbitals of
appropriate symmetry to interact with metal d orbitals. Cyanide
also has a permanent dipole moment suited for promotion of
directional electron transfer. The electronic and physical structure
of the ligand can be monitored readily through the n(CRN) IR
signature.2 When employed as a bridging ligand in the assembly
of polymetallic systems, the structural and electronic properties of
the cyanide ligand give rise to compounds with a wealth of
interesting electronic, photochemical or magnetic properties.3 In
turn, this leads to a considerable interest in new cyanometallate
building blocks.4
Reaction of [Fe(CRCCRN)(dppe)Cp] (1) with [Re(NCMe)-
(CO)3(bpy)]PF6 ([2]PF6) gave the yellow heterobimetallic complex
[3]PF6 (Scheme 1). In the ground state, [3]PF6 offers a rich series of
Convenient synthetic routes to metal complexes featuring
the cyanoacetylide ligand, [CRCCRN]ꢀ, including bridging
examples, have been developed,5 and studies of the electronic
characteristics of metal compounds featuring this ‘‘extended’’
cyanide-like ligand are now possible. Given the numerous
interesting physical properties associated with cyanide
complexes,3 the introduction of a new cyanide-like ligand
offers a wealth of possibilities for study. The iso-electronic
n(CRC), n(CRN) and n(CO) bands between 1900–2250 cmꢀ1
,
which can readily be assigned by comparison with the IR spectra
of related mononuclear reference systems and DFT calculations
(see below). Thus, the broad band near 1930 cmꢀ1 is assigned to
the unresolved out-of-phase A0(2) and equatorial A00 n(CO)
vibrations, whilst the sharper n(CO) band near 2035 cmꢀ1 is
a Department of Chemistry, Durham University, South Rd, Durham,
UK DH1 3LE. E-mail: p.j.low@durham.ac.uk;
Fax: +44 (0)191 384 4737; Tel: +44 (0)191 334 2114
b Central Laser Facility, STFC Rutherford Appleton Laboratory,
Chilton, Didcot, Oxfordshire, UK OX11 0QX
c Van’t Hoff Institute for Molecular Sciences, University of
Amsterdam, Nieuwe Achtergracht 166, Amsterdam, 1018 WV,
The Netherlands
w Electronic supplementary information (ESI) available: Synthetic
procedures for [3]PF6 and [3]BF4, and representative CV plot, ob-
served and calculated vibrational frequencies, computational details,
including TD-DFT results and tables of bond lengths, orbital energies
and orbital composition for [3-H]+, and brief description of the
disorder model used in the refinement of the structure [3]BF4. CCDC
reference number 693768. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b811357b
Scheme 1
ꢁc
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Chem. Commun., 2008, 5845–5847 | 5845

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Fig.
2
The UV-Vis spectroelectrochemical conversion of [3]n+
(n = 1, 2) in an OTTLE cell (CH2Cl2/0.1 M NBu4PF6). The arrows
show the trends on the anodic cycle.
Fig.
1 The IR spectra of [{Cp(dppe)Fe}(CRCCRN){Re(CO)3-
(bpy)}]n+ (n = 1, 2) collected spectroelectrochemically in an OTTLE cell
(CH2Cl2/0.1 M NBu4PF6). The arrows the trends on the anodic cycle.
assigned to the in-phase A0(1) vibration. The bands at 1970 and
2190 cmꢀ1 are attributed to the coupled n(CRC) and n(CꢂN)
modes of the cyanoacetylide ligand, respectively (cf.
n(CRC/CRN) 1991/2174 cmꢀ1).
1
Compound [3]PF6 undergoes reversible one-electron oxida-
tion (E1/2 = +0.23 V vs. ferrocene/ferrocenium; Ic/Ia = 1)
and irreversible one-electron reduction (E1/2 = ꢀ1.67 V;
Ic/Ia o 0.37 at n r 100 mV sꢀ1) on a cyclic voltammetric
scan at a platinum microelectrode in tetrahydrofuran (thf) at
293 K, which on the basis of IR spectroelectrochemical
experiments can be assigned to a largely Fe centred oxidation
and a p*(bpy) localized reduction (see below). The reduction
becomes more chemically reversible below 270 K.
Fig. 3 Difference ps-TRIR spectra of [Cp(dppe)Fe}(CRCCRN)-
{Re(CO)3(bpy)}]PF6 in CH2Cl2 measured after 490 nm excitation. The
legend shows the delay times (ps) for each spectral acquisition.
The oxidation of [3]PF6 results in a small (5–10 cmꢀ1
)
increase in the n(CO) wavenumbers, pointing to only a minor
decrease in electron density at the rhenium centre (Fig. 1).
The n(CRC) and n(CRN) bands also show relatively
small shifts (ca. 30 cmꢀ1) to higher wavenumbers. The limited
positive shifts in the ligand stretching frequencies clearly
indicate that the electron density removed on oxidation origi-
nates largely from the iron centre, with a minor contribution
from the cyanoacetylide ligand and Re based orbitals. At
293 K the bpy-localised reduction of [3]PF6 in CH2Cl2 results
in cleavage of the axial Re–NCCC bond, affording
1 (n(CRC/CRN) at 1994/2175 cmꢀ1) and two unidentified
[ReL(CO)3(bpy)] species (A0(1) n(CO) at 2024 and 2007 cmꢀ1).
The UV-Vis absorption spectrum of [3]PF6 is characterized
by broad, overlapping features centred near 27 000 cmꢀ1
(e = 7700 Mꢀ1cmꢀ1) and 22 200 cmꢀ1 (e = 1600 Mꢀ1cmꢀ1).
The oxidation of [3]PF6 gives rise to a new absorption band
from [3]2+ at 15 000 cmꢀ1 (e = 2000 Mꢀ1cmꢀ1), assigned to a
Re - Fe charge transfer band (Fig. 2). With a view to probing
the low energy excitation processes in more detail, ps-TRIR
spectroscopy was employed.9 The ps-TRIR spectra of [3]PF6
measured after excitation at 490 nm are shown in Fig. 3. In the
transient, the n(CO) bands shift to smaller wavenumbers by
ca. 20 cmꢀ1, consistent with one-electron reduction of the bpy
ligand. In contrast, the n(CRC) and n(CRN) bands in the
ground and excited states are almost coincident. Analysis of
the ps-TRIR spectra yields a transient decay lifetime of
B30 ps, consistent with the lifetime of 30 ꢃ 4 ps determined
from transient absorption (TA) spectroscopy (lex = 460 nm).
The initial TA spectrum shows a prominent band at ca.
500 nm and a weak broad absorption rising between
650–800 nm, which can safely be assigned to IL transitions
of the bpy radical anion.10 These observations indicate a net
increase in the electron density at the rhenium-bpy centre
and are not consistent with the 460–490 nm excitation into a
Re(d)-to-bpy(p*) MLCT transition. Instead, the lowest energy
excited state can be attributed charge-separated (CS) charac-
ter, involving fast electron transfer from the donor CpFe(d)
moiety to the acceptor bpy(p*)Re site, mediated by the
conjugated cyanoacetylide ligand.
Electronic structure calculations were also undertaken to
support these observations using the simplified model
[{Cp(dHpe)Fe}(CRCCRN){Re(CO)3bpy}]+ [3-H]+ (dHpe
= 1,2-diphosphinoethane). The geometry of [3-H]+ is in good
agreement with the structure of the cation in [3]BF4 (Fig. 4),z
and vibrational frequency calculations based on the optimized
geometry of [3-H]+ are in good agreement with the IR
spectrum of [3]PF6, which gives confidence in the accuracy
of the models (calculated values: [3-H]+ n(CRN) 2204,
n(CRC) 1980, n(CO) 2016, 1941, 1934 cmꢀ1).
The HOMO in [3-H]+ has the same general composition as
d6/d6 bimetallic butadiyndiyl bridged complexes,6 and is
derived from the out-of-phase mixing of the Fe(d), CRC
and CRN (p) and Re(d) orbitals (Fig. 5). The HOMO is
largely (40%) Fe in character, admixed with appreciable
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ps-TRIR spectroscopy. Given the appreciable Fe character
in these high lying orbitals, the photoproduct can be described
+
ꢀ
as
a
{CpFeꢄ }(CRCCRN)Re(bpyꢄ
)
CS state. This
description is consistent with the spectroscopic properties of
the complex upon one-electron electrochemical oxidation and
reduction. The cyanoacetylide ligand offers a p-orbital struc-
ture similar to that of both cyanide and buta-1,3-diyndiyl, and
is therefore appropriate for use as a p-donating bridging
ligand. With the development of convenient synthetic routes
to complexes featuring this fragment, there is considerable
scope for the development of the ‘‘cyanide-like’’ cyanoacety-
lide ligand as a bridging ligand in inorganic systems. The
fascinating potential of cyanoacetylide ligand to act as a
mediating unit for magnetic effects is high on a long list of
future studies.
We gratefully acknowledge the EPSRC, LSF and Royal
Society for financial support of this work.
Fig.
4
A plot (50% ellipsoids) of the cation [{Cp(dppe)Fe}-
(CRCCRN){Re(CO)3(bpy)}]+ from [3]BF4. Hydrogen atoms are
omitted for clarity. Selected bond lengths (A) and angles (1): Fe(1)–P(1)
2.1803(8); Fe(1)–P(2) 2.2033(7); Fe(1)–C(1) 1.828(3); C(1)–C(2) 1.235(4);
C(2)–C(3) 1.348(4); C(3)–N(1) 1.161(3); N(1)–Re(1) 2.120(2); Re(1)–N(2)
2.180(2); Re(1)–N(3) 2.172(2); P(1)–Fe(1)–P(2) 87.59(3); Fe(1)–C(1)–C(2)
Notes and references
z Crystal data for [3]BF4: C47H37FeN3O3P2Re BF4, M = 1082.60,
monoclinic, space group P21/c(no.14), a = 10.9091(2), b = 32.1612(6),
c = 12.9704(2) A, b = 106.00(1)1, U = 4374.38(22) A3, F(000) = 2144,
Z = 4, Dc = 1.644 mg mꢀ3, m = 3.232 mmꢀ1 (Mo-Ka, l = 0.71073 A),
T = 120.0(1) K. 58630 reflections were collected yielding 12 770 unique
data (Rmerg = 0.0249). Final wR2(F2) = 0.0.723 for all data (507 refined
parameters), conventional R1(F) = 0.0303 for 11686 reflections with
I Z 2s, GOF = 1.033. Crystallographic data for the structure have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC number 693768.w
177.9(2);
C(1)–C(2)–C(3)
170.1(3);
C(2)–C(3)–N(1)
179.6(3);
C(3)–N(1)–Re(1) 169.2(2).
contributions from the CRCCRN (27%) and Re (15%)
fragments. The HOMO-1, which is approximately orthogonal
to the HOMO, is similarly comprised. The bpy p* system
makes up the LUMO, and is well separated from the other
frontier orbitals.
TD-DFT calculations support the interpretation of the photo-
chemical results. In [3-H]+, a band with significant (HOMO-1)
- LUMO (Fe-to-bpy p*) character and reasonable intensity
(f = 0.0492) is calculated at 461 nm (21 700 cmꢀ1). A predomi-
nantly Re-to-bpy MLCT is calculated at shorter wavelengths
409 nm (24 400 cmꢀ1, f = 0.0016). At shorter wavelengths, a
series of transitions associated with the Fe(dHpe)Cp fragment
can be identified.
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Fig. 5 The (a) HOMO (b) HOMO-1 (c) LUMO of [3-H]+
.
ꢁc
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Chem. Commun., 2008, 5845–5847 | 5847