Long range charge transfer in trimetallic mixed-valence iron complexes mediated by redox non-innocent cyanoacetylide ligands

Article abstract of DOI:10.1039/c4dt00614c

The reaction of Fe(CCCN)(dppe)Cp (1) with one-half equivalent of [trans-Fe(NCMe)₂(dppx)₂][BF₄]₂ (dppx = dppe ([2][BF₄]₂) or dppm ([3][BF₄] ₂)) affords trimetallic [trans-Fe{NCCCFe(dppe)Cp}₂(dppx) ₂][BF₄]₂ (dppx = dppe [4][BF₄] ₂; dppx = dppm [5][BF₄]₂). Both [4][BF ₄]₂ and [5][BF₄]₂ undergo three, one-electron oxidation processes, arising from sequential oxidation of the two terminal Fe(CCR)(dppe)Cp moieties and finally the central Fe(NCR) ₂(dppx)₂ fragment. The redox products [4]ⁿ⁺ and [5]ⁿ⁺ (n = 3, 4) have been characterised by UV-vis-NIR and IR spectroelectrochemistry. The shifts in the characteristic ν(CCCN) bands upon oxidation demonstrate not only the localised electronic structure of the trications, but also the redox non-innocence of the cyanoacetylide ligands. The trimetallic [formally Fe(ii/ii/iii) mixed-valence] complexes [4]³⁺ and [5]³⁺ feature two distinct IVCT transitions, one associated with charge transfer from the central 18-electron {Fe(NCR)₂(dppx) ₂}²⁺ to terminal {Fe(CCR)(dppe)Cp}⁺ moiety, and a lower energy transition involving charge transfer between the terminal Fe fragments which are separated by the redox active 9-atom, 10-bond -CC-CN{Fe(dppx)₂}NC-CC- bridge. The tetracationic complexes [4] ⁴⁺ and [5]⁴⁺ generated by a further stepwise oxidation exhibit a single {Fe(NCR)₂(dppx)₂}²⁺→ {Fe(CCR)(dppe)Cp}⁺ IVCT transition. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00614c

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Long range charge transfer in trimetallic mixed-
valence iron complexes mediated by redox non-
innocent cyanoacetylide ligands†
Cite this: Dalton Trans., 2014, 43,
6291
Received 28th February 2014,
Accepted 4th March 2014
Josef B. G. Gluyas,a Andrew J. Boden,a Samantha G. Eaves,a Herrick Yua and
Paul J. Low*a,b
DOI: 10.1039/c4dt00614c
www.rsc.org/dalton
The reaction of Fe(CuCCuN)(dppe)Cp (1) with one-half equivalent based framework materials and bimetallic complexes, the
of [trans-Fe(NuCMe)2(dppx)2][BF4]2 (dppx = dppe ([2][BF4]2) or properties of discrete trimetallic compounds of the form
dppm ([3][BF4]2)) affords trimetallic [trans-Fe{NuCCuCFe(dppe)- M–CuN–M′–NuC–M have also attracted interest. For example,
Cp}2(dppx)2][BF4]2 (dppx = dppe [4][BF4]2; dppx = dppm [5][BF4]2). recent investigations have shown that electronic interactions
Both [4][BF4]2 and [5][BF4]2 undergo three, one-electron oxidation between the peripheral metal centres exist in [(py)5Ru]–CuN–
processes, arising from sequential oxidation of the two terminal [Ru(py′)4]–NuC–[Ru(py)5] (py
= pyridine, py′ = pyridine,
Fe(CuCR)(dppe)Cp moieties and finally the central Fe(NuCR)2- 4-methoxypyridine),1 and magnetic interactions are present
(dppx)2 fragment. The redox products [4]n+ and [5]n+ (n = 3, 4) have between the external iron centres in compounds such as
been characterised by UV-vis-NIR and IR spectroelectrochemistry. Cp(dppe)Fe–CuN–[Ru]–NuC–Fe(dppe)Cp (dppe
= 1,2-bis-
The shifts in the characteristic ν(CuCCuN) bands upon oxidation (diphenyl-phosphino)ethane, [Ru]
=
Ru(2,2′-bipyridine)2).2
demonstrate not only the localised electronic structure of the tri- Although similar studies of complexes based on isoelectronic
cations, but also the redox non-innocence of the cyanoacetylide dicarbon bridging ligands, –CuC–, and its valence isomers are
ligands. The trimetallic [formally Fe(II/II/III) mixed-valence] com- rather more rare,3–5 longer polyyndiyl ligands –(CuC)n– have
plexes [4]3+ and [5]3+ feature two distinct IVCT transitions, one been used in the construction of some truly impressive linear
associated with charge transfer from the central 18-electron polymetallic complexes,6–8 with computational and spectro-
{Fe(NuCR)2(dppx)2}2+ to terminal {Fe(CuCR)(dppe)Cp}+ moiety, scopic methods being used to demonstrate the presence of an
and a lower energy transition involving charge transfer between extended ⋯π–d–π⋯ system along the molecular backbone.
the terminal Fe fragments which are separated by the redox active
Simple synthetic routes to complexes containing cyano-
9-atom, 10-bond –CuC–CuN{Fe(dppx)2}NuC–CuC– bridge. acetylide, –CuCCuN,9–11 and related cyanobutadiynyl,
The tetracationic complexes [4]4+ and [5]4+ generated by a further –CuCCuCCuN,12 ligands have been established offering
stepwise oxidation exhibit
{Fe(CuCR)(dppe)Cp}+ IVCT transition.
a
single {Fe(NuCR)2(dppx)2}2+
→
entry to systems with obvious structural and electronic
relationships with both cyanide and polyynyl ligands. Cyano-
acetylide ligands are not only of interest as novel conduits for
the propagation of electronic and magnetic effects between
remote metal centres,9–11 but also for the potential to realise a
wide range of polymetallic complexes through synthetic strat-
egies based on coordination driven self-assembly.
Cyanide-bridged bi- and polymetallic complexes have attracted
attention for decades, a time frame extended considerably if
one considers the early fascination with the colours of, and
indirectly the Fe(II/III) IVCT processes in, Prussian Blue and its
analogues. Whilst most attention has focussed on cyanide-
Complex Fe(CuCCuN)(dppe)Cp (1) can be prepared by
reaction of Fe(CuCLi)(dppe)Cp with phenyl cyanate,10 or more
5
conveniently by treatment of [Fe(vCvCH2)(dppe)Cp]PF6
aDepartment of Chemistry, Durham University, South Road, Durham DH1 3LE,
United Kingdom
bSchool of Chemistry and Biochemistry, University of Western Australia, Crawley,
with 1-cyano-4-dimethyl-amino pyridinium tetrafluoroborate
([CAP]BF4).12,13† The reaction of 1 with one half-equivalent of
[Fe(NuCMe)2(dppe)2][BF4]2 ([2][BF4]2)14 or [Fe(NuCMe)2-
(dppm)2][BF4]2 ([3][BF4]2)† in acetonitrile gave trimetallic
[trans-Fe{NuCCuCFe(dppe)Cp}2(dppe)2][BF4]2 ([4][BF4]2, khaki
green, 95%) or [trans-Fe{NuCCuCFe(dppm)Cp}2(dppm)2]-
[BF4]2 ([5][BF4]2, brick red, 62%) as analytically pure precipi-
tates (Scheme 1). A small red-shift in the lowest energy absorp-
tion band of [4][BF4]2 relative to [5][BF4]2 is apparently
Perth 6009, Australia. E-mail: paul.low@uwa.edu.au
†Electronic supplementary information (ESI) available: Details of the prepa-
ration of [1][BF4]2, [3][BF4]2, [4][BF4]2 and [5][BF4]2; estimation of molecular
length and Vis spectra of [4]2+ and [5]2+; UV-Vis-NIR and IR spectroelectrochem-
ical details and spectral plots; Gaussian deconvolutions of the NIR spectra of
[4]3+ and [5]3+; voltammetric details and CV plots; crystal structure analysis of
[3][BF4]2. CCDC 973687. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4dt00614c
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Table 1 IR and NIR spectroscopic data obtained spectroelectrochemi-
cally in CH2Cl2/0.1
(5200 cm−1 (1000 M−1 cm−1)); [5]4+ (4400 cm−1 (800 M−1 cm−1))
M
NBu4PF6. Weak dπ–dπ transitions: [4]4+
Infrared (IR)
Near infrared (NIR)a
ν(CuC)/
ν(CuN)/
νmax(π–π*)/cm−1
νmax(MLCT)/m−1
cm−1
cm−1
(ε/M−1 cm−1
)
(ε/M−1 cm−1
)
[4]2+
[4]3+
1958s
1875s
1957s
2175vs
2092s
2154(sh)
2177s
—
—
5400 (7000)
[5500]b
—
9350 (3000)
[9300]b
—
[4]4+
2138m
9040 (12 000)
—
—
[9400]b
[5]2+
[5]3+
1963s
1902m
1963vs
2177vs
2111m
2155(sh)
2180s
—
5600 (5000)
—
—
10 300 (5000)
—
—
[5]4+
2151m
10 300 (14 200)
a From the apparent NIR band centres. b Data obtained in MeCN/0.1 M
NBu4PF6.
Scheme 1 Synthesis of the trimetallic iron compounds [4][BF4]2 and
[5][BF4]2.
spatial separation of the half-sandwich moieties, and under-
lines the significance of through-bond contributions to the
propagation of electronic information along the molecular
backbone. The E1/2(3) redox couple associated with the central
{Fe(NCR)2(dppx)2}2+ fragment, which yields [4]5+ and [5]5+, is
also sensitive to the ion-pairing interactions with the electro-
lyte, and moves beyond the accessible solvent window in
NBu4BArF4 electrolyte solutions.
sufficient to lead to the difference in colour of these
complexes.†
The electrochemical response of [4][BF4]2 and [5][BF4]2 in
CH2Cl2/0.1 M NBu4PF6 was broadly similar, each exhibiting
two, reversible one-electron oxidations and a third quasi-
reversible process close to the limit of the accessible solvent
window.† The E1/2 values determined for the dppm-containing
derivative [5][BF4]2 (E1/2(1) −0.05; E1/2(2) 0.07; E1/2(3) 0.83 V) are
slightly less positive than those of [4][BF4]2 (E1/2(1) 0.02; E1/2(2)
0.15; E1/2(3) 0.92 V), which reflects the difference in electron
donating properties of the dppm and dppe ligands at the
central iron moiety. Comparison of the redox potentials with
those of 1 (0.07 V vs. FeCp2/[FeCp2]+)10 and [2][BF4]2 (+1.05 V
vs. FeCp2/[FeCp2]+)15 permitted assignment of the first two
redox processes in the trimetallic complexes to sequential oxi-
dation of the peripheral Fe(CuCR)(dppe)Cp fragments, gener-
ating tricationic ([4]3+, [5]3+) and tetracationic ([4]4+, [5]4+)
species, which would be classically regarded as Fe(II/II/III) and
Fe(III/II/III) mixed valence complexes, respectively. The 120 mV
difference in the redox potentials associated with the periph-
eral Fe(CuCR)(dppe)Cp fragments (|E(1)1/2 − E1/2(2)|) suggests
that there is some through-bond and/or through space inter-
action between these moieties, which are estimated to be sep-
arated by ca. 15 Å.† The third quasi-reversible process is
associated with the central iron moiety to give formally homo-
valent Fe(III/III/III) compounds [4]5+ and [5]5+.
IR spectroelectrochemistry (CH2Cl2/0.1 M NBu4PF6)† was
used to refine the proposed redox assignments and highlight
the important role of the cyanoacetylide ligand in the initial
redox processes. The two band ν(CuCCuN) pattern observed
initially for [4]2+ and [5]2+ splits into a superposition of
ν(CuCCuN) features associated with {Cp(dppe)Fe}(Cu
CCuN){M′Ln} and {Cp(dppe)Fe}+(CuCCuN){M′Ln} complexes
on one-electron oxidation,9–11 clearly supporting the notion of
a ‘localised’ [{Cp(dppe)FeCuCCuN}+–{Fe(dppx)2}2+–{NuCCu
CFe(dppe)Cp}] electronic structure in [4]3+ and [5]3+ (Table 1).†
Although the oxidation processes are clearly not completely
metal in character, with an appreciable degree of ligand contri-
bution evidenced by the ca. 80 cm−1 shift in ν(CuCCuN) fre-
quencies upon oxidation, the general pattern is consistent
with the formal Fe(II/II/III) mixed valence description, which is
a useful if not particularly accurate way of describing the redox
product.17 The ligand non-innocence also highlights the care
that must be exercised in correlations between geometric sep-
aration of the metal centres and the real electron transfer dis-
tance. Further oxidation to the tetracations causes a loss in
intensity of the IR features, and only a single band at 2138
([4]4+) or 2151 ([5]4+) cm−1 is observed.
In CH2Cl2/0.1 M NBu4BArF (BArF = B{C6H3(CF3)2-3,5}4)16
4
4
small increases in E1/2(1) and E1/2(2) were observed in the case
of both [4][BF4]2 and [5][BF4]2, although the difference between
the first and second redox potentials is only modestly larger in
On one-electron oxidation to the trications [4]3+ and [5]3+,
two new bands were observed in the NIR spectra ([4]3+ 5400
and 9350 cm−1; [5]3+ 5600 and 10 300 cm−1), which were better
resolved in the case of [5]3+ (Fig. 1).† On further oxidation to
[4]4+ and [5]4+ the lower energy band collapsed revealing an
underlying pseudo-Fe(III) dd band (probably better described
the electrolyte solution containing the poorly ion-pairing
−
[BArF
]
anion (|E1/2(1) − E1/2(2)| = 180 mV) than in NBu4PF6
4
(120 mV). This suggests that Coulombic and ion-pairing
effects only play a small role in stabilising the charge in the tri-
cationic species: this result is perhaps unsurprising given the
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The effectiveness of the 9-atom, 10-bond –CuC–CuN–Fe–
NuC–CuC– bridge to promote interaction between remote
metal centres, which is no doubt related to the redox-activity
of the ligand fragment, is comparable with cyanide-based
M–CuN–M′–NuC–M type structures,1 and highlights the
great potential of the –CuCCuN moiety in the assembly of
electro-optically active polymetallic structures.
Acknowledgements
We thank Dr Dmitry Yufit for determining the crystal structure
of [3][BF4]2, and the EPSRC and ARC (FT120100073) for
funding.
Notes and references
1 G. E. Pieslinger, P. Albores, L. D. Slep, B. J. Coe,
C. J. Timpson and L. M. Baraldo, Inorg. Chem., 2013, 52,
2906–2917.
Fig. 1 The NIR spectra of [5]n+ (n = 2, 3, 4) obtained spectroelectro-
chemically in CH2Cl2/0.1 M NBu4PF6.
2 X. Ma, S.-M. Hu, C.-H. Tan, Y.-F. Zhang, X.-D. Zhang,
T.-L. Sheng and X.-T. Wu, Inorg. Chem., 2013, 52, 11343–
11350.
3 J. A. Ramsden, W. Weng, A. M. Arif and J. A. Gladysz, J. Am.
Chem. Soc., 1992, 114, 5890–5891.
as an FeCuC dπ–dπ transition given the contribution of the
ligand fragment to the redox processes),18 and the higher
energy absorption bands grew appreciably in intensity, but
with negligible shift in energy.
Given the assignment of the first two oxidation processes to
the peripheral Fe(CuCR)(dppe)Cp moieties, these NIR bands
in the (formally) Fe(II/II/III) and Fe(III/II/III) mixed-valence com-
plexes are attributed to two separate IVCT processes, the lower
energy band corresponding to charge transfer between the
distant Fe(CuCR)(dppe)Cp moieties (out-to-out) which is
unique to the Fe(II/II/III) form, and the one at higher energy to
charge transfer from the central iron(II) Fe(NuCR)2(dppx)2
moiety to the peripheral {Fe(CuCR)(dppe)Cp}+ fragment(s). In
keeping with this assignment, the energy of this latter ‘in-to-
out’ IVCT process shifts to the red on replacement of the dppe
ligands on the central iron atom ([4]3+) with dppm ([5]3+)
Additional spectroelectrochemical investigation of [4][BF4]2 in
acetonitrile showed solvatochromic behaviour of these NIR
bands, again consistent with the proposed IVCT character.†
The extraction of coupling information from the unique
‘out-to-out’ charge transfer processes in [4]3+ and [5]3+ is com-
plicated by the redox non-innocent character of the iron cyano-
acetylide metalloligands. However, taking the metal–metal
distances as proxy for the electron transfer distance, and using
the Hush relationships derived for the two-state case19 with
spectral parameters derived from Gaussian deconvolution,†
coupling parameters Hab can be estimated for [4]3+ (out-to-out
4 W. Weng, T. Bartik, M. Brady, B. Bartik, J. A. Ramsden,
A. M. Arif and J. A. Gladysz, J. Am. Chem. Soc., 1995, 117,
11922–11931.
5 M. I. Bruce, K. Costuas, B. G. Ellis, J.-F. Halet, P. J. Low,
B. Moubaraki, K. S. Murray, N. Ouddaï, G. J. Perkins,
B. W. Skelton and A. H. White, Organometallics, 2007, 26,
3735–3745.
6 M. I. Bruce, B. Le Guennic, N. Scoleri, N. N. Zaitseva and
J.-F. Halet, Organometallics, 2012, 31, 4701–4706.
7 F. Lissel, T. Fox, O. Blacque, W. Polit, R. F. Winter,
K. Venkatesan and H. Berke, J. Am. Chem. Soc., 2013, 135,
4051–4060.
8 Q. Zheng, F. Hampel and J. A. Gladysz, Organometallics,
2004, 23, 5896–5899.
9 R. L. Cordiner, M. E. Smith, A. S. Batsanov, D. Albesa-Jové,
F. Hartl, J. A. K. Howard and P. J. Low, Inorg. Chim. Acta,
2006, 359, 946–961.
10 M. E. Smith, R. L. Cordiner, D. Albesa-Jové, D. S. Yufit,
F. Hartl, J. A. K. Howard and P. J. Low, Can. J. Chem., 2006,
84, 154–163.
11 R. L. Cordiner, D. Corcoran, D. S. Yufit, A. E. Goeta,
J. A. K. Howard and P. J. Low, Dalton Trans., 2003, 3541–
3549.
610 cm−1, in-to-out 1845 cm−1) and [5]3+ (out-to-out 665 cm−1
,
12 S. Bock, S. G. Eaves, M. Parthey, M. Kaupp, B. Le Guennic,
J.-F. Halet, D. S. Yufit, J. A. K. Howard and P. J. Low, Dalton
Trans., 2013, 42, 4240–4243.
13 N. J. Brown, P. K. Eckert, M. A. Fox, D. S. Yufit,
J. A. K. Howard and P. J. Low, Dalton Trans., 2008, 433–
436.
in-to-out 1469 cm−1). Despite the over-estimation of the
electron transfer distance, these values compare well with
analogous data from the trimetallic, cyanide-bridged
complexes [(py)5Ru]–CuN–[Ru(py′)4]–NuC–[Ru(py)5] (Hab
700–800 cm−1, 1100 cm−1).1
=
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14 R. B. Bedford, E. Carter, P. M. Cogswell, N. J. Gower, 16 F. Barriere and W. E. Geiger, J. Am. Chem. Soc., 2006, 128,
M. F. Haddow, J. N. Harvey, D. M. Murphy, E. C. Neeve and 3980–3989.
J. Nunn, Angew. Chem., Int. Ed., 2012, 52, 1285– 17 W. Kaim, Eur. J. Inorg. Chem., 2012, 2012, 343–348.
1288.
18 F. Paul, L. Toupet, J.-Y. Thépot, K. Costuas, J.-F. Halet and
C. Lapinte, Organometallics, 2005, 24, 5464–5478.
19 C. Creutz, Prog. Inorg. Chem., 1983, 30, 1–74.
15 D. L. DuBois and A. Miedaner, Inorg. Chem., 1986, 25,
4642–4650.
6294 | Dalton Trans., 2014, 43, 6291–6294
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