The synthesis, structure, and electrochemical properties of Fe(C≡CC≡N)(dppe)Cp and related compounds

Article abstract of DOI:10.1139/V05-238

The cyanoacetylide complex Fe(C≡CC≡N)(dppe)Cp (3) is readily obtained from sequential reaction of Fe(C≡CSiMe₃)(dppe)Cp with methyllithium and phenyl cyanate. Complex 3 is a good metalloligand, and coordination to the metal fragments [RhCl(CO)₂], [Ru(PPh ₃)₂Cp]⁺, and [Ru(dppe)Cp*]⁺ affords the corresponding cyanoaceylide-bridged heterobimetallic complexes. In the case of the 36-electron complexes [Cp(dppe)Fe-C≡CC≡N-ML _n]ⁿ⁺, spectroscopic and structural data are consistent with a degree of charge transfer from the iron centre to the rhodium or ruthenium centre via the C₃N bridge, giving rise to a polarized ground state. Electrochemical and spectroelectrochemical methods reveal significant interactions between the metal centres in the oxidized (35 electron) derivatives, [Cp(dppe)Fe-C≡CC≡N-ML_n]⁽ⁿ⁺¹⁾⁺.

Full text of DOI:10.1139/V05-238

154
The synthesis, structure, and electrochemical
properties of Fe(CϵCCϵN)(dppe)Cp and related
compounds1
Mark E. Smith, Richard L. Cordiner, David Albesa-Jové, Dmitri S. Yufit,
FrantiÓek Hartl, Judith A.K. Howard, and Paul J. Low
Abstract: The cyanoacetylide complex Fe(CϵCCϵN)(dppe)Cp (3) is readily obtained from sequential reaction of
Fe(CϵCSiMe3)(dppe)Cp with methyllithium and phenyl cyanate. Complex 3 is a good metalloligand, and coordination
to the metal fragments [RhCl(CO)2], [Ru(PPh3)2Cp]+, and [Ru(dppe)Cp*]+ affords the corresponding cyanoaceylide-
bridged heterobimetallic complexes. In the case of the 36-electron complexes [Cp(dppe)Fe-CϵCCϵN-MLn]n+, spectro-
scopic and structural data are consistent with a degree of charge transfer from the iron centre to the rhodium or
ruthenium centre via the C3N bridge, giving rise to a polarized ground state. Electrochemical and spectroelectro-
chemical methods reveal significant interactions between the metal centres in the oxidized (35 electron) derivatives,
[Cp(dppe)Fe-CϵCCϵN-MLn](n+1)+
.
Key words: cyanide, cyanoacetylide, crystal structure.
Résumé : Le complexe cyanoacétylure Fe(CϵCCϵN)(dppe)Cp (3) est facilement préparé par la réaction séquentielle du
Fe(CϵCSiMe3)(dppe)Cp avec le méthyllithium et le cyanate de phényle. Le complexe 3 est un bon métalloligand et sa
coordination aux fragments métalliques [RhCl(CO)2], [Ru(PPh3)2Cp]+ et [Ru(dppe)Cp*]+ conduit à la formation des
complexes hétérobimétalliques à point cyanoacétylure. Dans le cas des complexes à 36 électrons [Cp(dppe)Fe-
CϵCCϵN-MLn]n+, les données spectroscopiques et structurales sont en accord avec un certain degré de transfert de
charge du centre ferrique à rhodium ou au centre ruthénium par le biais du pont C3N, ce qui conduit à un état fonda-
mental polarisé. Les méthodes électrochimiques et spectroélectrochimiques révèlent l’existence d’interactions importan-
tes entre les centres métalliques des dérivés oxydés (à 35 électrons) de formule générale [Cp(dppe)Fe-CϵCCϵN-
MLn](n+1)+
.
Mots clés : cyanure, cyanoacétylure, structure cristalline.
[Traduit par la Rédaction] Smith et al. 163
Introduction
Our interests in complexes featuring CϵC based ligands
stem from the rich electrochemical response of bimetallic
polyyndiyl systems (1), the coordinative flexibility of
(poly)yne ligands (2), and the chemical, physical, and spec-
troscopic properties of metal complexes featuring polycar-
bon ligands that depend heavily on the mixing of metal and
carbon orbitals (3). In seeking to extend these studies, and at
the same time to explore the relationships that exist between
acetylide, [CϵCR]–, and isoelectronic cyanide, [CϵN]–,
based unsaturated ligands, we were drawn to the cyano-
acetylide ligand, [CϵCCϵN]–.
The chemistry of organometallic complexes featuring un-
saturated ligands has been a source of interest for decades,
owing to the unusual physical properties and chemical reac-
tivity that arises from the mixing of metal d and ligand π
orbitals. Acetylides and acetylenes are among the most
structurally simple unsaturated ligands, offering cylindrical
symmetry about the CϵC moiety, and a polarizable π sys-
tem. However, these structurally simple ligands are remark-
ably electronically diverse, being capable of formally
donating one to six electrons to metal fragments and frame-
works through various combinations of the filled ligand σ
and π orbitals.
We have recently developed convenient syntheses of sev-
eral ruthenium complexes featuring the cyanoacetylide
ligand, such as [Ru(CϵCCϵN)(PPh3)2Cp] (4). In the process
Received 13 June 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 23 February 2006.
Dedicated to Arthur J. Carty, a respected mentor and valued friend, in recognition of his outstanding contributions to science.
M.E. Smith, R.L. Cordiner, D. Albesa-Jové, D.S. Yufit, J.A.K. Howard, and P.J. Low.2 Department of Chemistry, University of
Durham, South Rd, Durham DH1 3LE, UK.
F. Hartl. Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands.
1This article is part of a Special Issue dedicated to Professor Arthur Carty.
2Corresponding author (e-mail: p.j.low@durham.ac.uk).
Can. J. Chem. 84: 154–163 (2006)
doi:10.1139/V05-238
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Smith et al.
155
Scheme 1.
BF4
TlBF4
LiCCH·TMEDA
-LiBF4
Fe I
PPh2
Fe OCMe2
PPh2
Fe C C H
PPh2
Ph2P
acetone
Ph2P
Ph2P
2a
Scheme 2.
i) MeLi
Me3SiCCH
THF / NEt3
Fe Cl
PPh2
Fe C C C N
PPh2
Fe C C SiMe3
PPh2
Ph2P
Ph2P
Ph2P
ii) PhOCN
2b
3
1
Fe C N
PPh2
Ph2P
4
of extending the synthetic methodology, we have had cause
to prepare the related iron complex [Fe(CϵCCϵN)(dppe)Cp]
(3). In this paper we describe the synthesis, molecular struc-
ture, and electrochemical response of 3, together with those
of several related complexes.
Further reaction of 2b with MeLi (1.6 mol/L in diethyl
ether) at low temperature gave an orange-yellow coloured
solution containing the intermediate acetylide anion
[Fe(CϵC–)(dppe)Cp], which was not isolated but treated in
situ with PhOCN to afford the cyanoacetylide complex
[Fe(CϵCCϵN)(dppe)Cp] (3) as a mustard-yellow coloured
solid in 88% yield after chromatography and crystallization
(Scheme 2). The cyanoacetylide complex was readily char-
acterized from solution spectroscopic data, which included
Results and discussion
1
sharp singlets in the H and 13C NMR spectra arising from
Acetylide complexes featuring the elementary structure
[Fe(CϵCR)(dppx)Cp] (dppx = bis(diphenylphosphino)meth-
ane, dppm; 1,2-bis(diphenylphosphino)ethane, dppe) are
well-known (5), yet in recent times the development of the
chemistry of this system has fallen behind that of the more
electron-rich and sterically encumbered [Fe(CϵCR)(dppe)Cp*]
the Cp ligand (δH 4.29, δC 80.42), the phosphine resonance
in the 31P NMR spectrum (δP 104.91), and three carbon reso-
nances arising from the cyanoacetylide ligand (δC 153.95,
JCP = 37 Hz (Cα); 106.13 (CN); 87.02 (Cβ)). The Cα reso-
nance, which was unambiguously assigned on the basis of
the coupling to the dppe phosphorus nuclei, falls at the
higher end of frequencies found in other acetylide com-
plexes, [Fe(CϵCR)(dppe)Cp*] (6c). The cyanoacetylide
ligand gave rise to two bands of moderate intensity at 2174
and 1991 cm–1, which can be approximated as isolated
ν(CN) and ν(CC) vibrations, but could also be a combina-
tion of vibrational modes.
The molecular structure of 3 (Fig. 1) was determined by
single crystal X-ray diffraction (Table 1), and selected bond
lengths and angles are summarized in Table 2. For purposes
of comparison, the structure of the related cyanide complex
[Fe(CϵN)(dppe)Cp] (4) (9) was also determined (Fig. 2),
while the complex [Fe(CϵCC6H4NO2)(dppe)Cp] (8) pro-
vides a convenient set of metrical parameters associated with
a metal acetylide featuring an electron-withdrawing group
(10). The structures of 3, 4, and 8 show the expected gross
similarities, with the iron centre adopting a pseudo-
octahedral environment supported by the Cp ring, the chelat-
ing dppe ligand, and the CnN (n = 1 (4), 3 (3)) or acetylide
fragments.
5
analogues (Cp* = η -C5Me5) (6). This is somewhat surpris-
ing given the quite straightforward synthesis of the key re-
agent [FeCl(dppe)Cp] (1a) (7).
In seeking to access an iron cyanoacetylide complex of
the general form [Fe(CϵCCϵN)(dppe)Cp], we required
access to a synthon for the nucleophilic acetylide anion
[Fe(CϵC–)(dppe)Cp]. Although Fe(CϵCH)(dppe)Cp (2a) is
a known compound (5b), the existing synthesis (reaction of
Fe(dppe)Cp]+ with [LiCϵCH·TMEDA]) (Scheme 1) was not
considered to be optimal given the many advances in syn-
thetic transition metal acetylide chemistry that have been
made since the original report of this compound.
We chose instead to examine the possibility of preparing
either 2a or [Fe(CϵCSiMe3)(dppe)Cp] (2b) by formation of
the analogous vinylidene, and deprotonation in situ (8).
Thus, reaction of [FeCl(dppe)Cp] with excess (4 equiv.)
HCϵCSiMe3 in THF–NEt3 (1:1 volumetric ratio) resulted in
a gradual colour change in the solution from dark purple to
orange. Workup gave the orange trimethylsily-protected
acetylide complex 2b in high (~80%) yield (Scheme 2).
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Can. J. Chem. Vol. 84, 2006
Table 1. Crystallographic data for 3, 4, [6]PF6, and [7]PF6.
Compound code
3
4
[6]PF6
[7]PF6
Empirical formula
C35H31Cl2FeNP2
C33H31Cl2FeNP2
C75.50H65ClF6FeN P5Ru
C75.50H65ClF6 FeNP5Ru
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
654.30
120(2)
Triclinic
P-1
9.718(2)
12.415(3)
13.013(3)
87.444(4)
88.471(4)
76.206(4)
1 523.1(5)
2
630.28
120(2)
1 447.50
30(2)
Monoclinic
P21/c
14.6918(6)
23.6085(9)
19.5215(7)
90.00
106.9440(10)
90.00
1 447.50
120(2)
Monoclinic
P21/c
14.6968(4)
23.7690(7)
19.6918(6)
90.00
106.61(1)
90.00
Monoclinic
P21/n
12.8588(4)
8.8080(3)
25.7235(9)
90.00
90.862(2)
90.00
2 913.12(17)
4
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
6 477.1(4)
4
1.437
6 591.9(3)
4
1.459
Dc (Mg/m3)
1.427
0.802
1.437
0.835
µ (mm–1)
0.649
0.673
Crystal size (mm)
θ Range (°)
Reflections collected
Independent reflections
R(int)
0.25 × 0.24 × 0.16
2.81–30.51
18 007
9 055
0.0151
0.26 × 0.24 × 0.22
1.58–30.52
23 428
8 857
0.0427
0.40 × 0.32 × 0.16
1.45–30.45
60 279
19 302
0.1633
0.20 × 0.18 × 0.18
1.68–28.00
59 855
15 905
0.0446
wR(F2) (I > 2σ(I))
R(F) (I > 2σ(I))
Refined parameters
GOF
0.0829
0.0317
458
1.068
0.1229
0.0549
352
1.154
0.0856
0.0755
882
1.227
0.1719
0.0768
807
1.192
∆ρmin,max (e Å–3)
0.832 and –0.466
0.841 and –0.578
1.376 and –1.327
1.014 and –1.338
Fig. 1. The molecular structure of Fe(CϵCCϵN)(dppe)Cp (3),
showing the atom labelling scheme. In this and all subsequent
figures hydrogen atoms are omitted for clarity. Selected bond
lengths (Å): Fe—C(1), 1.8526(13); C(1)—C(2), 1.2278(19);
C(2)—C(3), 1.3672(19); C(3)—N(1), 1.1547(19); Fe—P(1),
2.1726(5); Fe—P(2), 2.1736(5); Fe—Cp(centroid), 1.711(1). Se-
lected bond angles (°): Fe-C(1)-C(2), 176.03(11); C(1)-C(2)-C(3),
176.22(15); C(2)-C(3)-N(1), 178.97(17); P(1)-Fe-P(2), 86.27(2).
Table 2. Spectroelectrochemically generated IR data
for compounds 3, 3+, [6]+, [6]2+, [7]+, and [7]2+.
Compound
ν(CϵCCϵN) (cm–1)
3
2174, 1991
2201 (No other band observed)
2191, 1976
2136, 1932
2189, 1984
3+
[6]+
[6]2+
[7]+
[7]2+
2121, 1946
are the the Fe—P (1, 2) bond lengths (3: 2.1736(5),
2.1726(5) Å; 4: 2.1798(7), 2.1807(7) Å; 8: 2.158(2),
2.157(2) Å). Interestingly, the intraring C—C distances in
the cyclopentadienyl ring (3: avg. 1.421 Å; 4: avg. 1.423 Å;
8: avg. 1.376 Å) also display a degree of variation between
the CnN and acetylide complexes. Given the significance of
back-bonding effects in M—Cp and M—P interactions, the
structural trends in M—P and M—Cp bond lengths are con-
sistent with either the decreased σ donation from the CnN
ligands relative to the acetylide ligand in 8, and (or) a
greater degree of iron-to-CnN back-bonding. The Fe—Cα
distance in all three complexes (3, 4, and 8) are similar (3:
1.853(1) Å; 4: 1.893(2) Å; 8: 1.856(8) Å), while the
acetylide CϵC distance in 3 (1.228(2) Å) is indistinguish-
able from that in 8 (1.216(10) Å). Both observations argue
against any statistically significant structural variation owing
to a significant increase in back-bonding in 3 relative to 8.
Numerous donor ligands (L) are known to react with the
dimeric complex [RhCl(CO)2]2 resulting in cleavage of the
chloride bridge, affording monomeric species cis-
The Fe—Cp distances in 3 (2.086(1)–2.105(1) Å, avg.
2.095 Å) and 4 (2.093(2)–2.107(2) Å, avg. 2.101 Å) are lon-
ger than those in 8 (2.060(9)–2.088(10) Å, avg. 2.078 Å), as
© 2006 NRC Canada

Smith et al.
157
Fig. 2. The molecular structure of Fe(CϵN)(dppe)Cp (4),
showing the atom labelling scheme. Selected bond lengths (Å):
Fe—C(1), 1.893(2); C(1)—N(1), 1.167(3); Fe—P(1), 2.1807(7);
Fe—P(2), 2.1798(7); Fe—Cp(centroid), 1.717(1). Selected bond
angles (°): Fe-C(1)-N(1), 178.6(2); P(1)-Fe-P(2), 84.84(2).
Scheme 3.
CO
CO
OC
OC
Cl
Rh
Rh
Cl
+
CO
Fe C C C N Rh CO
Ph2P
Cl
PPh2
5
Fe C C C N
PPh2
Ph2P
3
the cyanoacetylide-bridged complex, [{Cp(dppe)Fe}(µ-
CϵCCϵN){Ru(PPh3)2Cp]PF6 ([6]PF6).
Reaction of 3 with RuCl(PPh3)2Cp in MeOH containing
NH4PF6 afforded a red solution from which [6]PF6 was iso-
lated as a bright yellow crystalline solid after extraction
(CH2Cl2) and precipitation (hexane) (Scheme 4). Coordina-
tion of the Ru(PPh3)2Cp fragment to the nitrogen centre in 3
was evident from comparison of the spectroscopic data with
the properties of model complexes such as 3 and
[Ru(NCPh)(PPh3)2Cp]PF6 (13, 14). IR spectroscopy re-
vealed a shift in the characteristic cyanoacetylide bands from
2174 and 1991 cm–1 in 3 to 2192 and 1977 cm–1 in [6]PF6.
The presence of the distinct Cp ligands associated with iron
[RhCl(L)(CO)2] or cis-[RhCl(L2)(CO)] (11). The reaction of
3 with [RhCl(CO)2]2 proceeded smoothly in MeOH to afford
cis-[RhCl{NϵCCϵCFe(dppe)Cp}(CO)2] (5) as a pale orange
solid after purification of the reaction mixture by chroma-
tography (Scheme 3).
1
and ruthenium were clearly observed in the H NMR spectra
([6]PF6: δH(Fe) 4.35; δH(Ru) 4.21; 3: δH(Fe) 4.29; δH
[Ru(NCPh)(PPh3)2Cp]PF6 4.55), while the dppe and PPh3
ligands in [6]PF6 gave rise to two singlet resonances in the
31P NMR spectrum at 103.91 (cf. 3: δP 104.91) and
42.20 ppm (cf. [Ru(NCPh)(PPh3)2Cp]PF6: δP 42.89 ppm),
respectively. The complex cation [6]+ was observed in the
ES(+)-MS at m/z 1260.
The complex [{Cp(PPh3)2Ru}(µ-CϵCCϵN){Fe(dppe)Cp}]PF6
([7]PF6), a coordination (C/N bond) isomer of [6]PF6, was
prepared on a previous occasion from the reaction of
[Ru(CϵCCϵN)(PPh3)2Cp] with 1a (4) (Scheme 5). The
spectroscopic properties of [6]PF6 and [7]PF6 are distinct,
which rules out the possibility of ligand isomerism processes
occurring during the syntheses. However, as complexes
[6]PF6 and [7]PF6 are the first C/N bond isomers featuring
the cyanoacetylide ligand, a concerted effort was made to
crystallize both complexes, resulting in samples suitable for
single crystal X-ray diffraction (Table 1).
The cis geometry about the rhodium centre was unequivo-
cally established from the IR spectrum, which revealed
ν(CO) bands at 2085 and 2015 cm–1 in addition to the char-
acteristic bands of the CϵCCϵN fragment at 2199 and
1978 cm–1, and the 13C NMR spectrum that contained a pair
of doublets at 182.28 and 179.25 ppm arising from the two
carbonyl ligands that are coupled to the Rh nucleus (Fig. 3).
Similar data are associated with complexes derived from
aminophosphines and point to the considerable donor char-
acter of the iron cyanoacetylide “metalloligand” (11). The
13C NMR spectrum also revealed the remarkably high-
frequency Cα resonance, which was observed as a triplet
(JCP = 36 Hz) at 177.23 ppm. Whilst the IR data clearly ar-
gue against a cumulenic description of the cyanocarbon
ligand, the high-frequency 13C shift of Cα supports the con-
cept of a considerably polarized structure, with the electron-
rich Fe(dppe)Cp fragment donating considerable electron
density to the RhCl(CO)2 fragment via the CϵCCϵN bridge,
which also carries a dipole moment in the same orientation.
We have recently described the synthesis of the 36-
electron ruthenium and mixed iron–ruthenium diyndiyl
complexes [{Cp′(L2)Ru}(µ-CϵCCϵC){Ru(L2)Cp′}] and
[{Cp*(dppe)Fe}(µ-CϵCCϵC){Ru(L2)Cp′}] (L = PPh3, Cp′ =
Cp; L2 = dppe, Cp′ = Cp*), together with the properties of
the electrochemically accessible 35- and 34-electron oxida-
tion products (1b, 1c, 12). The demonstration of consider-
able interaction between the heterometallic centres in 5 and
predominant role of the ruthenium centre in determining the
electronic properties of the mixed-metal diyndiyl system
prompted us to consider the use of 3 in the preparation of
The structures of the cations [6]+ (Fig. 4) and [7]+ (Fig. 5)
illustrate a number of unusual features. The compounds are
isostructural and in both of them the molecules of the metal
complex are disordered over two positions (Fig. 4, Fig. 5).
The disorder is rather unusual and the two possible orienta-
tions differ by interchange of the positions of the ethane
bridge and FeCp moiety. Inevitably, three out of four atoms
of the central C3N link in both compounds are also disor-
dered, while the atom coordinated to the Ru centre (N1
([6]+); C1 ([7]+)) has large anisotropic displacement parame-
ters. At the same time, the P(1)Ph2 and P(2)Ph2 fragments,
which are coordinated to the iron centre, are identical in
both conformations. The disorder is static, with the structure
of [6]PF6 remaining disordered in the same fashion at tem-
peratures between 120 and 30 K. The structures of [6]PF6
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Can. J. Chem. Vol. 84, 2006
Fig. 3. The 175–185 ppm region of the 13C NMR spectrum of 5 showing the distinct CO and Cα resonances.
186
185
184
183
182
181
180
179
178
177
176
175
ppm
Scheme 4.
PF6
RuCl(PPh3)2Cp
NH4PF6
Fe C C C N Ru
PPh2
Fe C C C N
Ph2P
PPh3
PPh3
Ph2P
PPh2
3
[6]PF6
Scheme 5.
PF6
FeCl(dppe)Cp
NH4PF6
Ru C C C N Fe
Ru C C C N
Ph3P
Ph3P
Ph3P
PPh2
Ph3P
Ph2P
[7]PF6
and [7]PF6 also contain disordered PF6 anions and mole-
cules of the crystallization solvent, CH2Cl2.
The combination of spectroscopic and structural data as-
sociated with the bimetallic complexes provides clear evi-
dence for a degree of interaction between the metal centres
through the polarized CϵCCϵN bridging ligand. In an effort
to further quantify these interactions, combined electrochem-
ical and spectroelectrochemical (UV–vis–NIR, IR) investiga-
tions were carried out. Potentials reported here are
referenced against internal ferrocene or decamethyl
ferrocene standards as detailed in the Experimental section.
Complex 3 undergoes a single, reversible oxidation event
at +0.53 V, which compares to that of the cyanide complex 4
at +0.55 V (16). Under the same conditions, the oxidation
potential of [Ru(CϵCCϵN)(PPh3)2Cp] falls at +0.92 V (4).
The bimetallic complexes [6]PF6 and [7]PF6 each undergo
two oxidation processes, the reversibility of which improves
at subambient temperatures. The oxidation potentials of both
heterobimetallic complexes, [6]PF6 (+0.66, +1.34 V) and
[7]PF6 (+0.62, +1.22 V), are similar and do not correlate
with a simple model based upon metal-centred oxidation
events, but rather indicate a significant donor–acceptor inter-
action between the iron and ruthenium centres in agreement
with the structural data described above.
Although variations in the relative disposition of the metal
fragments about a diyndiyl (CϵCCϵC) ligand are common
(15), demonstrating the low energy of rotation of the metal
fragment about the M—Cα bond, the observation of both
cisoid and transoid forms within the same crystal lattice in
the case of [6]PF6 and [7]PF6 is most unusual. It is also un-
usual that the same disorder results from the rotation of the
Fe(dppe)Cp fragment about both Fe—C ([6]PF6) and Fe—N
([7]PF6) bonds.
In spite of the disorder, the distinct metal–C/N bond
lengths in [6]PF6 and [7]PF6 reflect the different metal frag-
ment coordinated at the C and N termini of the C3N bridge,
e.g., Ru—N(1) = 2.054(4) Å in [6]PF6 and Ru—C(1) =
1.971(5) Å in [7]PF6; Fe—C(1) = 1.845(8) Å (transoid) and
1.86(1) (cisoid) Å in [6]PF6, Fe—N(1) = 1.904(6) Å
(transoid) and 1.92(1) Å (cisoid) in [7]PF6. The C(2)ϵN(1)
bond length in each conformation of [6]+ (1.183(9),
1.18(1) Å) is longer than the C(3)ϵN(4) bond length in the
corresponding conformation of [7]+ (1.16(2), 1.161(9) Å),
but the other parameters are generally similar. Curiously, the
Fe—P bond lengths are significantly longer in the cisoid
conformation of [6]+ (2.138(2), 2.140(2) Å) than the transoid
form (2.117(2), 2.116(2) Å). Back-bonding interactions will
influence these Fe—P bond lengths, and we speculate that
these Fe—P bond lengths indicate the different conforma-
tions observed in the solid state might arise from better
transmission of electronic effects among the metal centres in
the transoid geometry.
To access spectroscopic data from the mono- and di-
oxidized forms of 3, [6]PF6, and [7]PF6 we turned to
spectroelectrochemical methods. Although the oxidation
events associated with each complex were reversible on the
CV timescale at room temperature, bulk electrolysis reac-
tions in the spectroelectrochemical cells were carried out at
ca. –30 °C to restrict complications arising because of de-
composition of the electrogenerated oxidation products. Oxi-
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Smith et al.
159
Fig. 4. Plots of the cation [6]+ illustrating (a) the atom labelling
scheme and (b) the disordered nature of the iron end-cap. Selected
bond lengths (transoid/cisoid) (Å): Fe—C(1), 1.849(7)/1.864(10);
C(1)—C(2), 1.237(10)/1.217(14); C(2)—C(3), 1.374(10)/1.370(13);
C(3)—N(1), 1.184(8)/1.183(10); Ru(1)—N(1), 2.056(4); Fe—
P(1), 2.1265(17)/2.151(2); Fe—P(2), 2.1248(14)/2.1493(19);
Ru—P(3), 2.3423(10); Ru—P(4), 2.3367(11). Selected bond an-
gles (°): Fe-C(1)-C(2), 163.7(5)/179.2(9); C(2)-C(3)-N(1),
175.7(8)/170.8(11); C(3)-N(1)-Ru, 163.7(5)/159.9(6); P(1)-Fe-
P(2), 88.95(6)/87.68(7); P(3)-Ru-P(4), 104.21(4).
Fig. 5. An illustration of the disordered cation [7]+ showing
(a) the atom labelling scheme and (b) the disordered nature of
the iron end-cap. Selected bond lengths (transoid/cisoid) (Å):
Ru—C(1), 1.971(5); C(1)—C(2), 1.226(9)/1.318(16); C(2)—C(3),
1.374(10)/1.36(2); C(3)—N(1), 1.161(9)/1.16(3); Fe—N(1),
1.904(6)/1.916(13); Ru—P(3), 2.3093(13); Ru—P(4), 2.3090(13);
Fe—P(1), 2.1310(18)/2.117(3); Fe—P(2), 2.147(2)/2.136(3). Se-
lected bond angles (°): Ru-C(1)-C(2), 172.1(6)/157.1(8); C(1)-
C(2)-C(3), 173.0(8)/168.6(16); C(2)-C(3)-N(1), 178.4(8)/176.7(17);
C(3)-N(1)-Fe, 172.8(6)/175.9(13).
(a)
(a)
(b)
(b)
dation of [Fe(CϵCCϵN)(dppe)Cp] (3) resulted in the
collapse of the characteristic ν(CϵCCϵN) bands of the un-
saturated ligand and observation of a band at 2201 cm–1 of
significantly lower intensity (Table 2). The pseudo ν(CϵC)
band in this oxidized species, [3]+, was not apparent, per-
haps because of its low intensity. In contrast, two bands
were observed following oxidation of the bimetallic com-
plexes [6]PF6 and [7]PF6, which were shifted to lower en-
ergy by ca. 40–70 cm–1 in comparison with the spectra of
the precursors, and therefore supporting the notion of an oxi-
dation event that involves an orbital with an appreciable de-
gree of ligand as well as metal character. In each case the
original spectral profile was recovered after back-reduction
of the oxidized samples.
The electronic (UV–vis–NIR) spectrum of 3 changed little
upon oxidation, and recovery of the original spectrum after
back-reduction demonstrated the chemical reversibility of
the electrochemical oxidation. Crucially, the NIR region of
the spectrum remained transparent before and after oxidation
of 3. More interestingly, the one-electron oxidation products
derived from [6]PF6 and [7]PF6 exhibited intense absorp-
tions in the NIR region, each of which could be decon-
voluted into three Gaussian-shaped absorption bands (Fig. 6,
Table 3). Of these, the particularly low energy band of low
intensity (vmax(cm–1)/εmax = 5260 cm–1/140 (mol/L)–1 cm–1
([6]2+); 5980 cm–1/190 (mol/L)–1 cm–1 ([7]2+)) was approxi-
mated as a forbidden ligand–field transition associated with
the oxidized iron centre (6h, 6k, 17), which indicates that the
© 2006 NRC Canada

160
Can. J. Chem. Vol. 84, 2006
Table 3. Selected experimental and calculated band-shape parameters associated with the NIR bands
exhibited by the 35-electron species, [6]2+ and [7]2+.
v (cm–1), ε
∆v1/2
∆v1/2
Hab
(class II)a
Band
((mol/L)–1 cm–1)
(cm–1, obs)
(cm–1, Hush)
[6]2+ Band 1
[6]2+ Band 2
[7]2+ Band 1
[7]2+ Band 2
9 175, 1 990
11 185, 840
9 310, 4 780
11 040, 1 665
2650
4550
2445
3815
4600
5080
4640
5050
590
555
890
710
aCalculated using eq. [1] and assuming dMM′ = 7.62 Å, the average Fe—Ru separation obtained for both crystallo-
graphically determing cisoid and transoid conformations of [6]PF6 and [7]PF6.
approximated by the crystallographically determined metal–
metal separation, a coupling parameter (Hab) may be calcu-
lated for each band using the expression give in eq. [1] (Ta-
ble 3) (19).
Fig. 6. The deconvoluted NIR band envelopes associated with
(a) [6]2+ and (b) [7]2+.
3000
2500
(a)
2000
1500
1000
500
0
0.0205 vmaxε∆v
1/ 2
[1]
Hab =
′
dMM
Typically, values of Hab for heterometallic cyanide
bridged d5/d6 bimetallic complexes are in the range 1000–
1700 cm–1 when the NIR band shape data is treated in an en-
tirely similar fashion. We note, however, that the use of the
metal–metal distance is only a crude approximation of the
electron-transfer distance, and given the involvement of the
ligand orbitals in the SOMO, the real electron-transfer dis-
tance is likely to be considerably shorter. The values of Hab
reported in Table 3 represent a lower limit for the value of
this parameter.
3000
5000
7000
9000
11000
13000
15000
ν (cm–1)
7000
6000
5000
4000
3000
2000
1000
(b)
Whilst the availability of these 35-electron species as iso-
lated samples would have greatly facilitated this study by al-
lowing ready measurement of the solvent dependence of the
NIR bands and permitted access to Mössbauer and magnetic
data, it is unfortunate that at the present time we have been
unable to chemically isolate samples of [6]2+ or [7]2+ as salts
with common counterions. However, efforts in this area are
ongoing, and with the possibility of exchanging the Cp for
Cp* and related substituted cyclopentadienyl ligands, to-
gether with the large variety of phosphine ligands that may
be introduced at either metal centre, we are hopeful that fur-
ther information regarding the electronic structure of these
unusual cyanoacetylide-bridged heterobimetallic complexes
will become available in due course.
0
3000
5000
7000
9000
11000
13000
15000
ν (cm–1)
first oxidation event has some iron character in both C/N
bond isomers.
Demandis et al. (18) have recently reviewed the properties
of strongly coupled, ligand-bridged bimetallic mixed valence
complexes, and described the various contributions of
metal–ligand orbital mixing, spin-orbit coupling, and low
symmetry, which lead to the three transitions that are in
principle allowed between d6 and d5 metal ions to gather ap-
preciable intensity. While the 35-electron, heterobimetallic
dications [6]2+ and [7]2+ are not true mixed-valence species,
similar considerations apply to the electronic spectra of
these complexes. Since spin-orbit effects are relatively minor
in lighter first and second row transition elements, the obser-
vation of multiple transitions in the NIR region of both spe-
cies provides further evidence that the electronic structure of
these heterometallic systems features a considerable degree
of metal–ligand mixing. The band energy and shape are re-
markably similar for both [6]2+ and [7]2+, and consequently
similar transitions, and hence underlying electronic struc-
tures, are involved in both isomers.
Conclusion
The readily prepared substrate FeCl(dppe)Cp is a conve-
nient reagent for the preparation of both acetylide and
cyanoacetylide iron complexes. The iron–cyanoacetylide
complex, [Fe(CϵCCϵN)(dppe)Cp], is a good metalloligand,
readily coordinating to RhCl(CO)2 or Ru(PPh3)2Cp frag-
ments to afford heterometallic complexes featuring the µ-
1
1
η (C),η (N)-CϵCCϵN ligand. Spectroscopic data are consis-
tent with a significant degree of polarization in the structures
of the 36-electron species, and relatively large values of the
coupling constant Hab in the electrochemically generated
35-electron (d5/d6) dication, [{Cp(dppe)Fe}(µ-CϵCCϵN)-
{Ru(PPh3)2Cp}]2+, and its C/N bond isomer, [{Cp(PPh3)2-
Ru}(µ-CϵCCϵN){Fe(dppe)Cp}]2+.
The relationships originally derived by Hush (19) provide
a vehicle for comparison of the electron-transfer processes
occurring through the cyanoacetylide ligand in the com-
plexes reported here with those observed in a large number
of cyanide-bridged examples (20). Using the deconvoluted
band shapes, and assuming that electron-transfer distance is
© 2006 NRC Canada

Smith et al.
161
CH2 dppe), 79.52 (s, Cp), 103.85 (s, Cβ), 127.87 (dt, JCP
=
58, 4 Hz, m-Ph), 129.13 (d, JCP = 42 Hz, p-Ph), 133.14 (dt,
JCP = 248, 5 Hz, o-Ph), 138.00–142.64 (m, i-Ph), 151.39 (t,
JCP = 39 Hz, Cα). 31P{H} NMR (CDCl3) δ: 107.71 (s, dppe).
ES(+)-MS (m/z): 616.9 [M + H]+. Anal. calcd. for
C36H38FeP2Si (%): C 70.13, H 6.21; found: C 69.94, H 6.32.
Experimental
All reactions were carried out using standard Schlenk
techniques under dry high-purity nitrogen. Solvents were
dried using an Innovative Technologies solvent purification
system, and degassed prior to use. Preparative TLC was car-
ried out on 20 cm × 20 cm glass plates coated with silica gel
(Merck G254, 0.5 mm thick). Reagents were purchased and
used as received. Compounds 1a (7), 4 (9), and [7]PF6 (4)
were prepared according to the literature methods. Crystals
of 4 and [7]PF6 suitable for X-ray diffraction were obtained
from CH2Cl2–hexane and CH2Cl2–MeOH, respectively. IR
spectra were recorded on a Nicolet Avatar FT IR
spectrophotometer using solution cells fitted with CaF2 win-
dows. NMR spectra were obtained from solutions in CDCl3
using Varian VXR-400 (1H, 399.97 MHz; 13C, 100.57 MHz;
31P, 161.1 MHz) or Bruker DRX-400 (1H, 400.13 MHz; 13C,
100.61 MHz; 31P, 162.05 MHz) spectrometers. Electrochem-
ical experiments were conducted in CH2Cl2 solution contain-
ing 0.1 mol/L NBu4PF6 using a standard three-electrode cell
(all Pt electrodes) and an EcoChemie Autolab PGSTAT-30.
Potentials were corrected to SCE using an internal
ferrocene/ferrocinium (Fc/Fc+ = 0.46 V) or decamethylferrocene/
[Fe(CϵCCϵN)(dppe)Cp] (3)
A Schlenk flask was charged with [Fe(CϵCSiMe3)-
(dppe)Cp] (500 mg, 0.81 mmol) in THF (20 mL). The
orange/yellow solution was cooled to –78 °C and MeLi
(0.6 mL, 0.96 mmol of a 1.6 mol/L solution in Et2O) was
added at such a rate as to prevent the temperature exceeding
–50 °C. After stirring for 1 h the solution was warmed to
–20 °C before being cooled again. To the orange/yellow so-
lution was added PhOCN (0.5 mL, 0.9 mmol) and the solu-
tion allowed to come slowly to room temperature before the
solvent was removed. The dark red/brown residue was dis-
solved in CH2Cl2 and purified by column chromatography
on silica, the product eluting with acetone–hexane (30:70).
Concentration of the dark red fraction resulted in the forma-
tion of a mustard yellow precipitate, which was collected,
washed with hexane, and air dried to afford 3, which was
recrystallized from CH2Cl2–Et2O (yield: 400 mg, 88%). IR
decamethylferrocinium couple as standard (Fc*/Fc*+
0.084 V).
=
1
(CH2Cl2, cm–1): 2174, 1991 ν(CϵCCϵN). H NMR (CDCl3)
δ: 2.32 (m, 2H, dppe), 2.60 (m, 2H, dppe), 4.29 (s, 5H, Cp),
7.74–6.85 (m, 20H, Ph). 13C{H} NMR (CDCl3) δ: 27.88–
28.35 (m, CH2 dppe), 80.42 (s, Cp), 87.02 (s, Cβ), 106.13 (s,
IR spectroelectrochemical studies
IR spectroelectrochemical experiments at low tempera-
tures were performed with a cryostated, optically transparent
thin-layer electrochemical (OTTLE) cell equipped with CaF2
windows and a Pt minigrid working electrode (32 wires/cm)
(21). The electrolyses at room temperature were conducted
with another homemade demountable OTTLE cell (22). The
CH2Cl2 employed solutions were typically 3 × 10–1 mol/L in
the supporting electrolyte (NBu4PF6) and 10–3 mol/L in the
analyte. The working electrode potential of the spectro-
electrochemical cell was controlled with a PA4 potentiostat
(EKOM, Polná, Czech Republic). The IR spectra were re-
corded with Bio-Rad FTS-7 and Bruker Vertex 70 FT IR
spectrometers (16 scans, 1 to 2 cm–1 spectral resolution).
CN), 127.99 (dt, JCP = 26, 4 Hz, m-Ph), 129.58 (d, JCP
=
53 Hz, p-Ph), 132.23 (dt, JCP = 167, 4 Hz, o-Ph), 135.55–
140.21 (m, i-Ph), 153.95 (t, JCP = 37 Hz, Cα). 31P{H} NMR
(CDCl3) δ: 104.91 (s, dppe). ES(+)-MS (m/z): 570.1 [M +
H]+. Anal. calcd. for C34H29FeNP2 (%): C 71.72, H 5.13, N
2.46; found: C 71.67, H 5.07, N 1.72.
[{Cp(dppe)Fe}(␮-CϵCCϵN){cis-RhCl(CO)2}] (5)
A Schlenk flask was charged with [Fe(CϵCCϵN)(dppe)Cp]
(73 mg, 0.129 mmol), [RhCl(CO)2]2 (25 mg, 0.065 mmol),
NH4PF6 (35 mg, 0.215 mmol), and MeOH (25 mL), and the
resulting solution stirred for 48 h. The solvent was then re-
moved and the residue extracted with CH2Cl2 and filtered
through a small pad of silica. Addition of hexane to the
CH2Cl2 solution and concentration resulted in an orange pre-
cipitate, which was collected, washed with pentane, and air
dried (yield: 50 mg, 51%). IR (CH2Cl2, cm–1): 2199, 1991
UV–vis–NIR spectroelectrochemical studies
UV–vis–NIR were carried out using a Varian Cary 5
spectrophotometer in an OTTLE cell similar to that de-
scribed previously (23) from solutions in CH2Cl2 containing
10–1 mol/L NBu4BF4 as supporting electrolyte. Typically
analyte concentrations were 10–1 mmol/L.
1
ν(CϵCCϵN); 2085, 2015 ν(CO). H NMR (CDCl3) δ: 2.37
(m, 2H, dppe), 2.60 (m, 2H, dppe), 4.36 (s, 5H, Cp), 7.68–
7.17 (m, 20H, Ph). 13C{H} NMR (CDCl3) δ: 28.24–28.59
(m, CH2 dppe), 81.58 (s, Cp), 85.32 (s, Cβ), 106.27 (s, CN),
128.30–139.41 (m, Ph), 177.23 (t, JCP = 36 Hz, Cα), 179.25
(d, JRhC = 70 Hz, CO cis to NC), 182.28 (d, JRhC = 70 Hz,
CO trans to NC). 31P{H} NMR (CDCl3) δ: 103.45 (s, dppe).
Anal. calcd. for C36H29ClFeNO2P2Rh (%): C 56.61, H 3.83,
N 1.83; found: C 56.22, H 3.96, N 1.37.
[Fe(CϵCSiMe3)(dppe)Cp] (2b)
A Schlenk flask was charged with [FeCl(dppe)Cp] (2.0 g,
3.60 mmol), NaBPh4 (1.48 g, 4.32 mmol), THF (75 mL),
and NEt3 (75 mL), and the resulting dark solution treated
with Me3SiCϵCH (1.77 g, 2.54 mL, 18.0 mmol). After stir-
ring for 16 h the solution colour had turned deep orange.
The solvent was removed and the residue extracted with
Et2O and pentane added. The combined solvent was re-
moved resulting in precipitation of a dark orange solid
[{Fe(dppe)Cp}(CϵCCϵN){Ru(PPh3)2Cp}]PF6 ([6]PF6)
A Schlenk flask was charged with [Fe(CϵCCϵN)-
(dppe)Cp] (75 mg, 0.132 mmol), [RuCl(PPh3)2Cp] (96 mg,
0.132 mmol), and NH4PF6 (86 mg, 0.528 mmol). The mix-
ture was suspended in MeOH (15 mL) and refluxed for 1 h
after which time a dark red solution had formed. This was
(1.75 g, 79%). IR (CH2Cl2, cm–1): 1984 ν(CϵC). H NMR
(CDCl3) δ: 0.06 (s, 9H, SiMe3), 2.12 (m, 2H, dppe), 2.81
(m, 2H, dppe), 4.32 (s, 5H, Cp), 7.04–8.11 (m, 20H, Ph).
13C{H} NMR (CDCl3) δ: 1.00 (s, SiMe3), 27.96–28.32 (m,
1
© 2006 NRC Canada

162
Can. J. Chem. Vol. 84, 2006
allowed to cool and the solvent removed. The red/brown res-
idue was extracted with CH2Cl2 and filtered into hexane.
The resulting yellow precipitate was collected, washed with
hexane, and air dried to afford [6]PF6 (yield: 105 mg, 57%).
Crystals suitable for X-ray diffraction were obtained from
CH2Cl2–hexane. IR (CH2Cl2, cm–1): 2192, 1977 ν(CϵCCϵN).
1H NMR (CDCl3) δ: 1.62 (br, 8H, dppe), 4.21, 4.35 (2 × s,
10H, 2 × Cp), 6.97–7.64 (m, 40H, Ph). 31P{H} NMR
University of Durham Chemistry EPSRC Doctoral Training
Account.
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Crystal data and experimental details are listed in Ta-
ble 1.3
Acknowledgements
We thank Taasje Mahabiersing (University of Amsterdam)
for assistance with the electrochemical and spectroelectro-
chemical work. Financial assistance from the Engineering
and Physical Sciences Research Council (EPSRC),
OneNorthEast, and the EU ESSD HPMT-CT-2001-0311 Ma-
rie Curie Training Site (Amsterdam, The Netherlands) is
gratefully acknowledged. RLC held a scholarship from the
3 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 4087. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 274245–274248 contain the crystallo-
graphic data for this manuscript. These data can be obtained, free of charge, via http://www.ccdc.cam.ac.uk/conts/retrieving.html (Or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or de-
posit@ccdc.cam.ac.uk).
© 2006 NRC Canada

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