Synthesis of oligometallic compounds from tripodiron(II) species using cyano bridging ligands - Electronic coupling of two metal termini through metal dicyano and diisocyano entities

Article abstract of DOI:10.1002/1099-0682(200111)2001:11<2783::AID-EJIC2783>3.0.CO;2-R

Organometallic monocyanometalates [M-CN]^- (1^-) [M = Cr(CO)₅ 1a^-, Cr(η⁶-CF₃C₆H₅)(CO)₂ 1b^-, Mn(η⁵-CH₃C₅H₄)(CO)₂ 1c^-] produce tetrametallic (2^-) and trimetallic (3) aggregates upon reaction with tripod and iron(II) salts [tripod = CH₃C(CH₂PPh₂)₃]. The tetranuclear compound [tripodFe{NC-M}₃]^- (2^-) [M = Cr(CO)₅] or the trinuclear species [tripodFe(CO){NC-M}₂] (3) [M = Cr(CO)₅ 3a, Cr(η⁶-CF₃C₆H₅)(CO)₂ 3b, Mn(η⁵-CH₃C₅H₄)(CO)₂ 3c] can be selectively formed by following the appropriate reaction protocols. Organometallic dicyanometalates [NC-M′-CN]^- (4^-) [M′ = Mn(CO)₄ 4a^-, CpFe(CO) 4b^-, CpCo(CN) 4c^-] react in a similar manner to give pentanuclear species [tripodFe{NC-M′-CN}₃Fetripod]⁺ (5⁺). - In compounds 2^- and 3, two easily oxidizable organometallic groups are linked through a tripodFe(NC)₂ unit, while in 5⁺ two tripodiron(II) entities are linked through a less readily oxidizable M′(CN)₂ unit. For both types of compound, at least two distinct reversible oxidations are observed by cyclic voltammetry. The potentials at which these oxidations occur are characteristic of the terminal groups in each case. The potentials of the two consecutive oxidation steps are separated by between 150 and 350 mV. The oxidation products of compounds 2^-, 3, and 5⁺ can hence be classified as class III mixed-valence species with comproportionation constants K_C of between 10² and 10⁶. The extent of electronic coupling mediated by the M′(CN)₂ or M′(NC)₂ bridging entities is thus similar to that mediated by [RC(CN)₂]^- bridging ligands in otherwise analogous compounds, for which K_C values of around 10⁴ are observed. All the aforementioned compounds have been characterized by analytical and spectroscopic techniques, including cyclic voltammetry and UV/Vis spectroscopy. In addition, X-ray crystallographic data are reported for 2^-, 3a, 3b, 3c, and 5a⁺. Wiley-VCH Verlag GmbH, 2001.

Full text of DOI:10.1002/1099-0682(200111)2001:11<2783::AID-EJIC2783>3.0.CO;2-R

FULL PAPER
Synthesis of Oligometallic Compounds from tripodIron(II) Species Using
Cyano Bridging Ligands ؊ Electronic Coupling of Two Metal Termini
Through Metal Dicyano and Diisocyano Entities
Volker Jacob,[a] Gottfried Huttner,*[a] Elisabeth Kaifer,[a] Peter Kircher,[a] and
Peter Rutsch[a]
Dedicated to Professor Henri Brunner on the occasion of his 66th birthday
Keywords: Cyanides / Cyclic voltammetry / Tripod ligands / Iron
Organometallic monocyanometalates [M−CN]− (1−) [M =
Cr(CO)5
1a−, Cr(η6-CF3C6H5)(CO)2 1b−, Mn(η5-
CH3C5H4)(CO)2 1c−] produce tetrametallic (2−) and trimetal-
ible oxidations are observed by cyclic voltammetry. The po-
tentials at which these oxidations occur are characteristic of
the terminal groups in each case. The potentials of the two
lic (3) aggregates upon reaction with tripod and iron(II) salts consecutive oxidation steps are separated by between 150
[tripod = CH3C(CH2PPh2)3]. The tetranuclear compound
and 350 mV. The oxidation products of compounds 2−, 3, and
5+ can hence be classified as class III mixed-valence species
[tripodFe{NC−M}3]− (2−) [M = Cr(CO)5] or the trinuclear spe-
cies [tripodFe(CO){NC−M}2] (3) [M = Cr(CO)5 3a, Cr(η6- with comproportionation constants KC of between 102 and
CF3C6H5)(CO)2 3b, Mn(η5-CH3C5H4)(CO)2 3c] can be select-
ively formed by following the appropriate reaction protocols.
Organometallic dicyanometalates [NC−MЈ−CN]− (4−) [MЈ =
Mn(CO)4 4a−, CpFe(CO) 4b−, CpCo(CN) 4c−] react in a sim-
106. The extent of electronic coupling mediated by the
MЈ(CN)2 or MЈ(NC)2 bridging entities is thus similar to that
mediated by [RC(CN)2]− bridging ligands in otherwise ana-
logous compounds, for which KC values of around 104 are
ilar
manner
to
give
pentanuclear
species observed. All the aforementioned compounds have been
[tripodFe{NC−MЈ−CN}3Fetripod]+ (5+). − In compounds 2−
and 3, two easily oxidizable organometallic groups are linked
through a tripodFe(NC)2 unit, while in 5+ two tripodiron(II)
entities are linked through a less readily oxidizable MЈ(CN)2
unit. For both types of compound, at least two distinct revers-
characterized by analytical and spectroscopic techniques, in-
cluding cyclic voltammetry and UV/Vis spectroscopy. In ad-
dition, X-ray crystallographic data are reported for 2−, 3a,
3b, 3c, and 5a+.
Introduction
gands are quite effective in mediating electronic coupling
between the two terminal metal centers in an MϪ(µ-
CN)ϪMЈϪ(µ-CN)ϪM arrangement.
Dinitrile ligands are efficient building blocks for the syn-
thesis of oligonuclear coordination compounds.[1] Anionic
geminal dinitrile ligands [RC(CN)2Ϫ, N(CN)2Ϫ] have been
found to lead to efficient electronic coupling between the
As the efficiency of electronic coupling is easily measured
by cyclic voltammetry, and since the electrochemical be-
havior of dinuclear tripodiron compounds [tripod
ϭ
CH3C(CH2PPh2)3] in which the two iron centers are linked
through X(CN)2 entities [X ϭ RCϪ, NϪ] is well known,[2]
it was tempting to synthesize tripodiron derivatives having
the two iron(II) centers linked by MЈ(CN)2 bridging units
(Scheme 1, A). We report herein the synthesis of such com-
pounds (5؉) and the measurement of their degree of elec-
tronic coupling. Moreover, tripodiron(II) templates may
also serve to address the same type of problem in an
terminal
metal
centers
in
the
corresponding
MϪNCϪXϪCNϪM arrangements.[2] It would appear that
the nature of the linking unit X has little influence on the
efficiency of this coupling, since the separation between the
first and second oxidation potentials of the corresponding
compounds is relatively constant, irrespective of the nature
of X.[2] Even the replacement of RCϪ by NϪ as X does not
greatly affect this separation.[2] The distance between the
two metal centers in compounds of the type
MϪNCϪXϪCNϪM (X ϭ RCϪ, NϪ) is around 700 pm,
which is rather similar to the distance between two metal
centers linked by cis-dicyano complexes MЈ(CN)2.[3] Com-
pounds of this type have been extensively studied by Vah-
renkamp,[4] who showed that these inorganic bridging li-
inverted
version.
Compounds
of
the
type
[tripodFeL{NCϪM}2] contain the tripodFeIIL unit in a
central position (Scheme 1, B).
In these compounds, the terminal metal centers are
bridged by a CNϪFeϪNC ligand entity. Cyclic voltamme-
try of such compounds should reveal the ability of this type
of linker to electronically couple the two terminal metal
centers. The synthesis of such compounds (2؊ and 3;
Scheme 2) and the efficiency of electronic coupling through
the tripodFe(NC)2 linker are described herein.
[a]
Anorganisch-Chemisches Institut der Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: (internat.) ϩ 49-6221/545-707
E-mail: g.huttner@indi.aci.uni-heidelberg.de
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/1111Ϫ2783 $ 17.50ϩ.50/0
2783

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
Compound 1b؊ could be obtained by irradiation of the
parent (arene)tricarbonyl species[10] in methanol in the pres-
ence of excess potassium cyanide[11] as its intensely orange-
colored potassium salt. Its composition was unequivocally
confirmed by its analytical and spectroscopic data (see
Exp. Sect.).
K1b shows an irreversible oxidation wave in its cyclic vol-
tammogram at Epa ϭ Ϫ100 mV (CH3CN, vs. SCE). It is
thus easily oxidized and is similar in this respect to
Na1a.[12a] No data are available for K1c itself, but the par-
ent compound Na[CpMn(CO)2CN] is reversibly oxidized at
40 mV vs. SCE[4][12b] and the oxidation potential of 1c؊ can
be expected to be of the same order. All compounds 1؊ are
thus oxidized at much lower potentials than the tri-
podiron(II) entity in a coligand environment of nitrile
groups. The oxidation of [tripodFe(CH3CN)3]2ϩ occurs at
1400 mV (CH3CN, vs. SCE)[13] and oxidation of this entity
Scheme 1. Two types of inorganic groups linking two metal cen-
ters M
in
the
dinuclear
compounds
[tripodFe{µ-
minimum of
NCϪC(R)ϪCN}3Fetripod]ϩ requires
a
900 mV vs. SCE.[2c] This combination of facts makes com-
pounds 1؊ ideal ligands for testing the electronic coupling
properties of a tripodFe(NC)2 unit linking two redox-active
metal centers (Scheme 1, B).
The reaction of equimolar amounts of tripod and FeCl2
with three equivalents of Na1a in acetone gives a red solu-
tion, from which, after cation exchange with PPNCl and
crystallization, PPN2·0.75 CH2Cl2 is obtained in the form
of red needle-shaped crystals (Scheme 4).
Scheme 2. The tripodiron compounds described in this paper
Results and Discussion
It has been shown that the N-termini of nitrile groups in
organometallic cyano compounds are quite nucleophilic.[5]
The nucleophilicity of metal-bonded cyano groups has been
known since the early days of organic chemistry.[6] This ap-
pears to be a general property of anionic coordination com-
pounds containing cyano ligands and has been amply dem-
onstrated for species of type 1؊ by their reactions with
main-group-[7] or transition-metal-centered[8] electrophiles.
Compounds 1a؊ and 1c؊ (Scheme 3) are easily accessible as
their sodium or potassium salts by published procedures.[9]
Scheme 4. Synthesis of PPN2
The constitution of the tetranuclear anion 2؊ was un-
equivocally proven by the spectroscopic and analytical data
of PPN2 (Table 2). The signals of the axial and equatorial
carbonyl groups as well as of the nitrile groups are clearly
resolved in the 13C NMR spectrum of PPN2 (Table 3),
while its infrared spectrum shows weak bands attributable
to νCN absorptions at 2126 cmϪ1 and 2135 cmϪ1. The νCO
band pattern (Table 2) is consistent with the presence of
Cr(CO)5 groups, although the pattern is somewhat more
complicated than that generally observed for LCr(CO)5
compounds with undisturbed local C4v symmetry. This in-
dicates some deviation of these groups from this local sym-
metry in the tetranuclear anion. The structure of
PPN2·0.75 CH2Cl2, as determined by single-crystal X-ray
analysis (Figure 1; Tables 1 and 8), shows an overall close to
C3-symmetric arrangement for the anion 2Ϫ. The rotational
Scheme 3. Organometallic monocyanometalate ligands 1؊
2784
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
Table 1. Bond lengths and angles in the tetranuclear anion 2؊
positions of the phenyl groups, as well as those of the
Cr(CO)5 entities, are close to this idealized symmetry
(Table 1; torsion angles τ, ϕ, and χ).
FeϪP
FeϪN
CrϪCCN
CrϪCCO,ax
CrϪCCO,eq
225.3(2)Ϫ226.0(2)[a]
195.6(5)Ϫ197.7(5)
203.5(7)Ϫ207.2(7)
185.1(7)Ϫ186.8(6)
188.9(7)Ϫ192.7(8)
τ1
τ2
τ3
35.5(4)
32.6(4)
32.1(5)
[b]
[c] [d]
[e]
ϕa
ϕb
Ϫ36.4 to Ϫ68.5
Ϫ13.7 to Ϫ19.9
FeϪNϪC
CrϪCϪN
PϪFeϪP
NϪFeϪN
O···H[f]
170.2(4)Ϫ176.2(5)
177.6(5)Ϫ178.8(5)
84.7(2)Ϫ87.3(2)
87.4(1)Ϫ91.1(6)
262
χ1
χ2
χ3
Ϫ67/23
Ϫ156/112
Ϫ71/21
Ϫ163/108
Ϫ51/37
266
277
Ϫ147/120
[a] The values in parentheses are standard deviations in units of the
[b]
last decimal listed. Ϫ τ denotes the torsion angles FeϪPϪCϪC
[c]
within the chelate cage. Ϫ The suffixes a and b refer to the two
classes of aryl groups, which would be equivalent under exact C3
[d]
symmetry (Figure 1, A). Ϫ
ϕ designates a torsion angle
C
orthoϪCipsoϪPϪHz, where Hz is the end point of a vector vertical
to the plane spanned by the three phosphorus nuclei and fixed to
one phosphorus center, pointing in the direction of the observer
[e]
with respect to Figure 1, A. Ϫ χ denotes the torsional arrange-
ment of the Cr(CO)5 groups. It is defined as the torsion angle
HxϪFeϪCrϪOCO,eq, where Hx is the end point of a vector radiat-
ing from the iron center along the idealized C3 axis in a direction
[f]
away from the observer with respect to Figure 1, A. Ϫ
Shortest
distance between a carbonyl oxygen atom of any of the three penta-
carbonylchromium units and a hydrogen atom of a tripod phenyl
substituent.
of the compounds 3a, 3b (FeCr2), and 3c (FeMn2)
(Scheme 5) were unequivocally established from their spec-
troscopic data (Tables 2 and 3) and X-ray analyses (Tables
4 and 8; Figure 2).
Figure 1. Two views of the structure of the tetranuclear anion 2؊
Short O···H contacts are seen between some of the equat-
orial carbonyl groups and the protons of the phenyl rings.
Each pentacarbonylchromium group is involved in at least
one such contact with H···O distances in the range 260Ϫ300
pm (Table 1). Even though this is not indicative of hydrogen
bonding, it shows that the tripodiron entity and the
Cr(CO)5 groups are in steric interference with one another.
Scheme 5. Synthesis of compounds 3
The infrared spectra of compounds 3 each feature a band
Disturbance of the local C4v symmetry of the Cr(CO)5 at around 2030 cmϪ1 (Table 2), attributable to the iron-bon-
groups, as inferred from the IR-spectroscopic data, is thus ded carbonyl group. The νCN vibrations are observed as one
consistent with the observed structure.
band envelope at around 2100 cmϪ1. The νCO vibrations
When equimolar mixtures of tripod and FeCl2 in acetone of the Cr(CO)5 groups in 3a produce the expected pattern
are treated with 2 equiv. of either Na1a, K1b, or K1c, or- (Table 2). The CO groups of the Cr(η6-CF3C6H5)(CO)2 en-
ange-red solutions result. On bubbling CO through these tities in 3b give rise to two bands of equal intensity, as do
solutions, they undergo a color change to orange-brown.
those of the Mn(η5-CH3C5H4)(CO)2 entities in the FeMn2
The trinuclear compounds 3 were isolated as red-brown compound 3c (Table 2). The 31P NMR spectra of com-
crystalline materials in yields of around 80%. The identities pounds 3 each show a doublet of higher intensity and a
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2785

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
Table 2. Spectroscopic and analytical data for compounds PPN2 and 3
Ϫ1
˜
No.
Formula
( [g/mol])
Ccalcd., Hcalcd., Ncalcd., Pcalcd.
Cfound, Hfound, Nfound, Pfound
MS (FAB)
m/z [fragment]
IR (CH2Cl2): ν [cm
νCN
]
Yield
(%)
νCO
PPN2
3a
C95H69Cr3FeN4O15P5
(1873.3)
C54H39Cr2FeN2O11P3
(1144.5)
C62H49Cr2F6FeN2O5P3
(1268.8)
C60H53FeMn2N2O5P3
(1140.7)
60.91, 3.71, 2.99, 8.27
60.97, 3.93, 2.99, 8.10
56.66, 3.43, 2.45, 8.12
56.63, 3.91, 2.43, 8.00
59.08, 3.85, 2.19, 7.25
58.80, 3.97, 2.25, 7.21
63.17, 4.68, 2.46
Ϫ1336 [tripodFe(1a)3]Ϫ
2135 w
2126 vw
2123 w
2062 m
1938 vs
2061 m
2030 m
2024 w
1920 vs
1887 vs
1937 vs
1923 vs
1907 vs
1844 vs
1917 vs
1851 vs
52
80
84
83
Ϫ
3b
ϩ 960 [tripodFe1b]ϩ
2087 m
2089 m
3c
Ϫ
2022 w
61.73, 4.79, 3.32
coupling between these two types of phosphorus nuclei is
2JPP ഠ 70 Hz in each case (Table 3). The Cs symmetry of
the coordination polyhedron around the iron center is cor-
roborated by the 1H NMR spectroscopic data of com-
pounds 3; two types of methylene groups are observed for
the tripodal framework in the 1H{31P} NMR spectra of
compounds 3 (Table 3). While the signal with a relative in-
tensity of two would be expected to be a singlet, the other
one with the observed relative intensity of four might be
expected to show a more complicated pattern due to the
diastereotopicity of the relevant methylene protons. How-
ever, the differentiation between these two types of protons
is evidently too small to be observable with the resolution
of our experiment and only an apparent singlet signal is
observed for these protons (Table 3). The 13C NMR spectra
of the trinuclear complexes 3 show signals due to the iron-
bonded carbonyl group, the carbonyl groups bonded to the
chromium (FeCr2 compounds 3a and 3b) or manganese
centers (FeMn2 compound 3c), and the bridging cyanide
groups (Table 3).
The presence of the (η6-CF3C6H5) moieties in 3b is evid-
1
ent from their NMR signature. The H NMR signals ob-
served for this group in the spectrum of the potassium salt
K1b are also observed in that of 3b, with only very slight
shift differences (Table 3). The 13C NMR spectrum of 3b
shows signals due to the CF3C6H5 ligands in the same spec-
tral region as observed for K1b. In the 13C NMR spectrum
of 3b, the CF3 groups give rise to a quadruplet, which is
partly obscured by the group of signals due to the tripod
aryl carbon atoms. The arene carbon atoms of the
CF3C6H5 ligands, giving one signal each for the ipso, ortho,
meta, and para carbon atoms in the spectrum of K1b, ap-
pear in part as pairs of signals in the spectrum of 3b
(Table 3). The reason for this side differentiation is not
clear. It appears improbable that the two Cr(η6-
CF3C6H5)(CO)2 units would be fixed in 3b such as to have
different time-averaged chemical environments. A loss of
the Cs symmetry of the environment due to the chirality of
the {tripodFe(CO)ϪNCϪ(η6-CF3C6H5)Cr(CO)2} residue
would appear to offer a more probable explanation.
Figure 2. Two views of the structure of complex 3b
triplet of lower intensity (Table 3). This is consistent with
the static idealized octahedral coordination geometry
The presence of the π-bonded CH3C5H4 group in the
1
around the iron centers, with two phosphorus nuclei lying FeMn2 compound 3c is evident from its characteristic H
trans to the nitrile ligands and the third being trans to the NMR signals (Table 3). The methyl substituent gives rise to
iron-bonded carbonyl group (Scheme 5, Figure 2). The a signal at δ ϭ 2.0, while the C5H4 fragment gives a partly
2786
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
Table 3. NMR-spectroscopic data for compounds PPN2 and 3
PPN2
3a
3b
3c
1H{31P} NMR
tripod-CH3
tripod-CH2
1.37 s, 3 H
2.19 s, 6 H
1.64 s, 3 H
2.35 s, 4 H
2.75 s, 2 H
1.61 s, 3 H
2.29 s, 4 H
2.68 s, 2 H
4.7Ϫ5.2 m, 10 H
1.61 s, 3 H
2.30 s, 4 H
2.67 s, 2 H
1.98 s, 3 H
ligand[a]
Ϫ
4.3Ϫ4.6 m, 8 H
7.0Ϫ8.1 m, 30 H
CHarom
7.1Ϫ7.7 m, 30 H
6.9Ϫ7.9 m, 30 H
7.0Ϫ7.9 m, 30 H
13C{1H} NMR
tripod-CH3
tripod-CH2
38.0 q
33.9 m
37.7 q
30.9 m
37.8 q
30.6 m
38.0 m
31.0 m
32.8 m
36.5 br
33.3 m
36.4 m
33.5 m
36.7 br
tripod-Cq
36.5 s
tripod-Carom
tripod-Cq,arom
ligand-C[b]
132Ϫ135 m
137Ϫ138 m
Ϫ
128Ϫ134.5 m
135.5Ϫ137.5 m
Ϫ
128Ϫ134 m
136Ϫ138 m
126.0 q (CF3)
(1JCF ϭ 270 Hz)
90.0 s (p)
128Ϫ135 m
136Ϫ138 m
14.7 (CH3)
ligand-Carom
Ϫ
Ϫ
82 br
88.4 q (i)
(2JCF ϭ 35 Hz)
86.3 s/85.9 s (m)
85.2 m/84.1 m (o)[c]
195.4 s
239.7 s
239.8 s
CN
ligand-CO
172.1 s
222.2 s
218.0 s
Ϫ
182.8 s
220.6 s
217.3 s
191 br
235 br
Fe-CO
213.5 dt
214.5 dt
216 m
(2JCPcis ϭ 20 Hz)
(2JCPtrans ϭ 50 Hz)
(2JCPcis ϭ 20 Hz)
(2JCPtrans ϭ 50 Hz)
31P{1H} NMR
tripod-PPh2
37.5 s
36.5 d, 2 P
36.1 d, 2 P
36.0 d, 2 P
10.7 t, 1 P
9.0 t, 1 P
8.2 t, 1 P
(2JPPcis ϭ 68 Hz)
(2JPPcis ϭ 69 Hz)
(2JPPcis ϭ 68 Hz)
[a]
1
The H NMR resonances of the ligands are found at δ ϭ 4.5Ϫ5.1 (K1b, [D6]acetone) and δ ϭ 4.4 (C5H4), 1.9 (CH3) (K1c, [D6]ace-
tone)[9b,12b]. Ϫ The 13C NMR resonances of the ligands are found at δ ϭ 221.5, 218.5 (CO) and 153.8 (CN) (Na1a, CD3CN)[12]; δ ϭ
241.3 (s, CO), 168.0 (s, CN), 126.4 (q, CF3; JFC ϭ 270 Hz), 89.6 (s, Car), 86.5 (q, Cipso; JFC ϭ 35 Hz), 86.3 (s, Car), 84.3 (s, Car) (K1b,
[D6]acetone). Ϫ The signals are split due to JCF coupling.
[b]
1
2
[c]
3
resolved multiplet. In the spectrum of K1c, the expected similar for these four groups in the three compounds 3. This
A2B2 multiplet is unresolved,[12b] as is often observed for means that three of the carbonyl groups of the pentacar-
compounds of this type. The form of this signal seen for 3c, bonylmetal entities in 3a are formally replaced by η6-arene
like that observed for 3b, is suggestive of side differentiation units (FeCr2 compound 3b) or η5-cyclopentadienyl units
induced by the prochirality of the compound. The struc- (FeMn2 compound 3c) without a significant change in the
tures of the trinuclear complexes 3 have been elucidated by position of the two remaining carbonyl groups. These car-
X-ray crystallography (Tables 4 and 8). The basic molecular bonyl groups, like those in 2؊, show short contacts to some
arrangement is the same in all three compounds, hence only of the phenyl protons of the tripod entity (Table 4). Geo-
the representative structure of 3b is shown in Figure 2. The metrically, these contacts are again rather similar for all
coordination around the iron center is idealized octahedral three compounds. Thus, the rotational positions of the aryl
with the tripod ligand occupying one face of the octahed- groups (Figure 2) of the tripodiron entity are also rather
ron. The organometallic ligands are consequently in a mu- similar for all three species (Table 4). Overall, in fact, the
tually cis arrangement with the coordination octahedron crystals of 3a, 3b (FeCr2), and 3c (FeMn2) are isotypic
being completed by the iron-bonded carbonyl ligand (Fig- (Table 8). The FeϪP bond lengths are in the normal range,
ure 2).
with the bonds to the phosphorus atom trans to the car-
All three organometallic ligands have at least two mutu- bonyl groups being longer than the other two FeϪP bonds
ally cis-carbonyl groups that are also cis to the cyano (Table 4). The FeϪN bond lengths are in the range
bridging group. These carbonyl groups show roughly the 195.7(3)Ϫ198.5(4) pm (Table 4) and are distinctly longer
same orientation with respect to the tripodiron(II) entity in than the FeϪCCO bonds [179.5(5)Ϫ180.5(2) pm] in each
all three cases (Table 4). The torsion angles χ defined by the case. The FeϪNϪC entities deviate from linearity, as is gen-
rotational position of the carbonyl groups around an erally observed for tripodiron(II) nitrile compounds.[1,2,13]
FeϪM axis [M ϭ Cr (3a and 3b), Mn (3c)] with respect to The deviation is smaller in 3a than in 3b or 3c (Table 4),
the idealized trigonal axis of the tripodiron entity are very reflecting the lower symmetry of the organometallic termini
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2787

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
Table 4. Bond lengths and angles in the trinuclear complexes 3
3a
3b
3c
FeϪPcisCO
FeϪPtransCO
FeϪN
228.9(1)/226.2(1)[a]
230.2(1)
196.3(3)/195.8(3)
180.1(4)
204.2(3)/203.7(4)
115.2(4)/115.3(4)
185.8(3)/185.7(4) (ax)
188.8(4)Ϫ192.2(4) (eq)
Ϫ
169.9(3)/169.1(3)
179.0(3)/176.4(3)
86.83(4)Ϫ90.77(4)
84.2(1)
227.0(1)/225.8(1)
230.3(1)
198.2(2)/195.9(2)
180.5(2)
199.9(2)/199.0(2)
116.1(3)
Ϫ
183.7(3)Ϫ184.7(3)
166.8/167.2
164.2(2)/157.1(2)
177.8(2)/175.2(2)
87.00(3)Ϫ91.28(3)
84.1(1)
226.6(1)/224.8(1)
229.9(1)
198.5(4)/195.7(4)
179.5(5)
191.5(5)/190.0(5)
115.5(6)/116.4(6)
Ϫ
173.4(4)Ϫ176.6(4)
177.3/177.3
165.1(4)/158.5(4)
177.9(4)/177.5(4)
86.74(4)Ϫ90.88(4)
84.5(1)
FeϪC
MϪCCN
[b]
CϪN
MϪCCO
MϪMP[c]
FeϪNϪC
MϪCϪN
PϪFeϪP
NϪFeϪN
O···H[d]
271/288
Ϫ
26.8(3)
262/284
265/292
26.7(2)
276/297
Ϫ
26.4(3)
F···H[e]
[f]
τ1
τ2
τ3
ϕ1
16.2(3)
29.7(3)
Ϫ61.6
19.0(2)
33.4(2)
Ϫ62.4
16.8(4)
30.9(3)
Ϫ58.7
[g]
ϕ2
ϕ3
ϕ4
ϕ5
ϕ6
Ϫ37.9
ϩ22.6
ϩ36.6
ϩ21.9
Ϫ5.3
Ϫ156
Ϫ27.9
ϩ24.1
ϩ39.0
ϩ15.4
Ϫ9.4
Ϫ146
Ϫ27.9
ϩ25.6
ϩ38.7
ϩ18.4
Ϫ6.6
Ϫ154
[h]
χa
χb
χc
χd
Ϫ69
Ϫ72
ϩ18
Ϫ65
Ϫ80
ϩ10
Ϫ63
Ϫ80
ϩ16
[a]
[b]
[c]
The values in parentheses are standard deviations in units of the last decimal listed. Ϫ M ϭ Cr (3a, 3b), Mn (3c). Ϫ MP is the
center of the π-coordinated cyclic ligand in the organometallic units of complexes 3b and 3c. Ϫ [d] Shortest distances between a carbonyl
oxygen atom of each of the two carbonylmetal moieties and a hydrogen atom of a tripod phenyl substituent. Ϫ
between a fluorine atom and a hydrogen atom of a tripod phenyl substituent. Ϫ τ denotes the torsion angles FeϪPϪCϪC within the
chelate cages. Ϫ [g] ϕ designates the torsion angles CorthoϪCipsoϪPϪHz, where Hz is the endpoint of a vector vertical to the plane spanned
by the three phosphorus nuclei and fixed to one phosphorus center, pointing in the direction of the observer with respect to Figure 2, A.
Ϫ [h] χ ϭ HxϪFeϪMϪOCO denotes the torsion angle HxϪFeϪMϪOCO,eq, where Hx is the endpoint of a vector radiating from the iron
center along the idealized C3 axis in a direction away from the observer with respect to Figure 2, A.
[e]
Shortest distances
[f]
in the latter compounds. The geometries of the organomet-
allic termini themselves are unremarkable (Table 4).
When an equimolar solution of tripod and FeCl2 in acet-
one is treated with an equimolar amount of K4a, the ini-
While compounds 2؊ (FeCr3) and 3 (FeCr2, FeMn2) con- tially faintly yellow solution immediately turns red.
tain organometallic groups linked by
a
tripod- Whereas we had hoped to obtain a tetrametal square with
ironϪdiisocyano group (Scheme 1, B), the reaction of tri- two tripodFeL entities and two Mn(CO)4 constituents at its
podiron(II) entities with organometallic dinitriles should corners by applying this stoichiometry, compound 5a؉
lead to compounds containing two tripodiron(II) entities (Scheme 7) was obtained as the only characterizable prod-
linked by a metalϪdinitrile species (Scheme 1, A). Quite a uct in very low yield (see Exp. Sect.).
number of anionic organometallic dinitriles are known and
some of them have been shown to be suitable building
blocks for the construction of oligometallic cage com-
pounds.[3b,3c,14] The dicyanometalates 4؊ (Scheme 6) are
obtained by standard procedures.[15]
Scheme 7. The constitution of the pentanuclear cation 5a؉
Ϫ
5a؉ was identified by X-ray crystallography of its FeCl4
salt, which was serendipitously found to crystallize. 5a؉ is
a pentanuclear Fe2Mn3 compound[16] composed of two tri-
Scheme 6. Dicyanometalate ligands 4؊
podiron(II) entities, which are bridged by the cis-dicyano-
2788
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
Figure 3. Structure determination of 5aFeCl4: A: view of the pentanuclear cation 5a؉; B: standardized view of the two tripodiron(II) cen-
ters
manganese parts of three (tetracarbonyl)dicyanomanganese (tetracarbonyl)dicyanomanganate units, a strain-free square
bridging units (Figure 3).
arrangement with linear FeϪNϪC groups would result,
5a؉ has approximate C2 symmetry (C2 axis along the bi- with the vacant coordination site at each tripodiron entity
sector of Fe1ϪMn3ϪFe2 in Figure 3, A). This symmetry being perpendicular to the plane of this square. These coor-
is not imposed by crystal symmetry, but is evident from dination sites have to be brought into an arrangement that
inspection of Figure 3, A, as well as from the values of the allows them to be spanned by the additional Mn(CN)2 unit.
geometric parameters, pairs of which should be equal under This induces some strain in the scaffolding, which results in
this symmetry (Table 5).
a bending of the FeϪNϪC angles. As is well known, these
angles are rather tolerant to this type of deformation (Fig-
ure 3; Table 5).
Table 5. Bond lengths and angles in the pentanuclear cation 5a؉
In view of the fact that pentanuclear compounds such as
the Fe2Mn3 complex 5a؉ are formed even under stoichi-
ometric conditions, which should favor the formation of
square tetranuclear aggregates, the stoichiometry was
FeϪP
FeϪN
226.1(2)Ϫ228.4(2)[a] Fe···Mn
507Ϫ511
[b]
195.1(7)Ϫ197.5(7)
198(1)Ϫ201(1)
186(1)Ϫ188(1)
τ1
τ2
40.4(6)/43.0(6)
27.7(7)/31.7(7)
26.3(6)/28.2(6)
ϩ1.5/Ϫ2.0
Ϫ25.8/Ϫ30.0
Ϫ61.6/Ϫ53.7
Ϫ43.2/Ϫ42.8
ϩ9.6/Ϫ34.5
Ϫ16.3/Ϫ28.0
MnϪCCN
MnϪCCO,cis
MnϪCCO,trans 182(1)Ϫ185(1)
FeϪNϪC
MnϪCϪN
PϪFeϪP
NϪFeϪN
CϪN
τ3[c]
Ϫ
changed to tripod/FeII/MЈ(CN)2 ϭ 2:2:3. According to
ϕI
168.6(6)Ϫ173.2(7)
172.2(7)Ϫ177.3(7)
85.8(1)Ϫ91.4(1)
83.8(3)Ϫ85.5(3)
114(1)Ϫ116(1)
ϕII
this protocol, the pentanuclear compounds 5bBF4 (Fe5) and
5cBF4 (Fe2Co3) were prepared (Scheme 8).
ϕIII
ϕIV
ϕV
ϕVI
[a] The values in parentheses are standard deviations in units of the
[b]
last decimal listed. Ϫ
The τ values refer to the torsion angles
FeϪPϪCϪC within the chelate cages; they are arranged in pairs
[c]
to reflect the approximate C2 symmetry of the cation 5a؉. Ϫ
ϕ
describes the torsion of the phenyl rings of the tripodiron entities.
The torsion relates to the rotation around the corresponding
PϪCipso axis with the idealized trigonal axis of the tripodiron frag-
ment as the reference axis. The first value ϕI describes the rotation
of the phenyl group at P1, which points towards the observer in
Figure 3, A, with the companion phenyl group at P6 again pointing
towards the observer. Starting from this phenyl group at P1, the
six pairs of values ϕIϪϕVI are listed in a counterclockwise sequence,
when looking at Fe1 from the location of Fe2 (Figure 3, A and B).
Scheme 8. Synthesis of 5bBF4 and 5cBF4
Thus, even the torsion angles ϕ describing the orientation
Upon work-up of the respective reaction mixtures, micro-
of the phenyl groups at the two tripodiron entities, as well crystalline red powders were obtained, which were shown
as the twist angles τ describing the skew of the chelate cage, by NMR spectroscopy to contain different diastereomers.
are in good pairwise agreement (Figure 3, B; Table 5). The With 5b؉ (Fe5), the reaction product showed four signals
1
FeϪNϪC angles deviate from linearity in a systematic way. in the cyclopentadienyl region of the H NMR spectrum.
If, as initially expected, only two sites at each tripodiron By repeated recrystallization, a product was obtained that
entity were to be coordinated by the cyano groups of two showed one of these Cp peaks with 95% of the total intens-
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2789

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
ity. The cyclopentadienyl resonance of the crude product is diastereotopic and give rise to a doublet-of-doublets signal
1
clearly composed of two sets of signals, one being a sharp in the H{31P} NMR spectrum (see Exp. Sect.). Likewise,
singlet at δ ϭ 4.1 and the other one being composed of the phenyl groups of the PPh2 donors of the tripod ligands
three signals of equal intensity (see Exp. Sect.).
are diastereotopic. The 13C NMR signals fall into two well-
The intensity ratio of these two sets of signals is initially separated groups, consisting of a multiplet due to the ortho,
2:1, whereas it becomes 19:1 following the recrystallization meta, and para carbon atoms and a well-resolved signal due
process. These observations may be explained as follows: to the ipso carbon atoms (see Exp. Sect.). The pentanuclear
There are two possible basic arrangements for the mutual structure of 5؉ was unequivocally established from these
orientation of the Cp groups in a compound of type 5b؉ findings, as were the structures of 5a؉ (Fe2Mn3), 5b؉ (Fe5),
(Fe5). One of them conforms to the constitutional C3 sym- and 5c؉ (Fe2Co3) by the aforementioned combination of
metry (Scheme 9, A), while in the other this symmetry is analytical methods.
lost (Scheme 9, B). There are six different possibilities for
The FABϩ mass spectra show peaks due to the molecular
creating this unsymmetrical type of arrangement ion 5؉ in each case, accompanied by peaks originating from
(Scheme 9, B), whereas there are only two possibilities for [5 Ϫ tripod]ϩ (Table 6). The infrared spectrum of 5a؉
generating a C3-symmetric arrangement (Scheme 9, A). The (Table 6) shows the presence of the organometallic
statistical probability of formation of the C3-symmetric spe- (CO)4Mn(CN)2 building blocks through the characteristic
cies is hence 0.25. The C3-symmetric species will give rise patterns of its νCO and νCN bands, which are similar to
to just one single Cp resonance, while for the unsymmet- those seen for K4a. Although the νCO absorptions are seen
rical species three Cp signals of equal intensity can be ex- at wavelengths similar to those found for K4a, they show a
1
pected. This is indeed what is observed in the H NMR somewhat more complicated structure than in the case of
spectrum of crude 5b؉. The ratio of the symmetrical to the this starting material (Table 6). This observation may be
unsymmetrical diastereomers is initially 2:1, which is quite similarly explained as previously proposed for the pair of
different from the ratio expected on statistical grounds. This compounds Na1a and PPN2 (FeCr3). 5bBF4 shows νCN
suggests that the C3-symmetric product is formed under bands at 2140 cmϪ1 and 2106 cmϪ1, positions not dissimilar
considerable diastereoselective control. Crystallization en- from those of the bands reported for K4b.[17] The νCO band
riches the sample in the symmetrical product (Scheme 9, A), of 5bBF4 is observed at 1961 cmϪ1, and is thus shifted to
which is present to an extent of 95% after three recrystal- higher energies relative to the νCO absorption of K4b. The
lizations.
(cyclopentadienyl)tricyanocobalt(III) units of 5cBF4
(Fe2Co3) give rise to a band at 2168 cmϪ1, which is broader
and more intense than a second band at 2126 cmϪ1. This
latter band is attributed to the νCN vibrations of the termin-
ally bonded cyano ligands, while the former is interpreted
as being the envelope of the two νCN vibrations of the
bridging cyano groups.
Electrochemistry and UV/Vis Spectroscopy
All compounds PPN2, 3, and 5BF4 contain the tri-
podiron(II) entity coordinated to at least two nitrile func-
tions. A band at λ ഠ 20000 cmϪ1 appears to be character-
istic of iron(II) compounds of this type (Table 7). Irrespect-
ive of whether the nitrile functions originate from bridging
cyano groups or from organic nitriles, this is a constant
signature for all [tripodFeII(NCR)3]nϩ species and obviously
for all [tripodFeII(NCR)2L]nϩ species as well. The energy of
this transition is fairly constant for all compounds of this
type.[2,13] A band at shorter wavelengths at λ ഠ 25000 cmϪ1
is somewhat more variable with respect to its energy and
extinction coefficient (Table 7), but also appears to be char-
acteristic of compounds containing a tripodFeII(NCR)3 or
a tripodFeII(NCR)2L (3a) moiety.[2,13] Based on the intensit-
ies of the relevant bands, the underlying electronic trans-
Scheme 9. Diastereomers of 5b؉ and 5c؉
Diastereoselectivity is also observed in the formation of itions must involve some charge-transfer character. At the
5c؉ (Fe2Co3). In this case, the C3-symmetric isomer is isol- short wavelength side, a band is present at λ ഠ 30000 cmϪ1
,
ated as the sole product in 40% yield after just one recrys- although it is not always observable as it may be obscured
tallization step. 5cBF4 (Scheme 9, A) shows only one sharp by the steep rise of shorter wavelength bands. A band in
1
Cp signal at δ ϭ 4.7 in its H{31P} NMR spectrum. The this position has also been observed for other tripod-
protons of the methylene groups of the tripod ligands are FeII(NCR)2L compounds.[2,13]
2790
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
Table 6. Spectroscopic and analytical data of compounds 5aFeCl4, 5bBF4, and 5cBF4
Ϫ1 [a]
˜
No.
Formula
( [g/mol])
MS
m/z
Ccalcd., Hcalcd., Ncalcd.
Cfound, Hfound, Nfound
IR (CsI): ν [cm
νCN
]
[Frag.]
νCO
5aFeCl4
5bBF4
5cBF4
C100H78Cl4Fe3Mn3N6O12P6
(2215.8)
C106H93BF4Fe5N6O3P6
(2050.8)
C106H93BCo3F4Fe2N9P6
(2054.1)
2018
1680
1963
1339
1966
1342
[M]ϩ
54.21, 3.55, 3.79
53.30, 4.45, 3.59
62.07, 4.57, 4.10
61.72, 4.65, 4.25
61.97, 4.56, 6.14
60.55, 4.76, 6.07
2155 w
2091 m
2005 vs, 1987 vs
1961 vs
[M Ϫ 12 CO]ϩ
[M]ϩ
2140 s
2106 m
2168 s
2126 m
[M Ϫ tripod]ϩ
[M]ϩ
Ϫ
Ϫ
[M Ϫ tripod]ϩ
[a]
IR absorptions of the potassium salts of the organometallic ligands [cmϪ1]: 2145 w, 2103 vw, 2030 vs, 1952 vs[15a] (K4a); 2094, 2088,
1949[17] (K4b); 2130 s, 2124 vs, 2120 vs[15c] (K4c).
Table 7. Cyclovoltammetric and UV/Vis-spectroscopic data of compounds PPN2, 3, and 5BF4; reference data for dinuclear compounds
with dicyanamido- (A؉) and dicyanmethanido-bridged (B؉) dinuclear tripodiron(II) complexes (Scheme 11)[2c]
CV
UV
λ [cmϪ1
]
Bridging unit
[a]
[b]
No.
E11/2
[mV]
E21/2
[mV]
∆E1/2
[mV]
KC
δ∆G298
[kJ/mol]
(ε [Ϫ1cmϪ1])
2؊
425[c]
780
355
1·106 [d]
34
30200
(11000)
Ϫ
30000
(6000)
Ϫ
31900
(5000)
Ϫ[e]
ഠ 27000[e]
20100
(820)
Ϫ
19800
(1000)
Ϫ
19900
(1000)
19900
(1800)
19100
(2100)
19600
(1900)
ϪCNϪFeIIϪNCϪ
2
3a
780[f]
825[f]
900
993
120
168
1·102
6·102
12
16
Ϫ
ϪCNϪFeIIϪNCϪ
ϪCNϪFeIIϪNCϪ
24700
(1600)
Ϫ
3b
88[f]
252
164
213
6·102
4·103
16
21
ϪCNϪFeIIϪNCϪ
ϪNCϪFeIIϪCNϪ
5b؉
652[g]
865[g]
25400
(1100)
26000
(5000)
24200
(900)
24300
(1100)
5c؉
A؉
B؉
939[g]
910[g]
1120[g]
1084[g]
1150[g]
1400[g]
145
240
280
3·102
1·104
5·104
14
23
27
ϪNCϪCoIIIϪCNϪ
ϪNCϪNϪϪCNϪ
ϪNCϪC(R)ϪϪCNϪ
Ϫ[e]
Ϫ[e]
[a]
Equilibrium constants KC for the comproportionation reaction (Scheme 10) calculated from KC ϭ exp{∆E1/2/25.69}; ∆E1/2 in mV. Ϫ
[b] δ∆G298 calculated from the differences ∆E1/2 between the oxidation potentials of the first and second oxidation steps. Ϫ [c] The negative
charge of 2؊ leads to a value for the first oxidation potential about 400 mV lower than that observed for 3a. Ϫ [d] The extent of electronic
coupling is greatest for the negatively charged complex 2؊. Ϫ Band is obscured by the steep rise of shorter wavelength bands. Ϫ
[e]
[f]
The Cr(CO)5CN groups of 2؊ and 3a are less easily oxidized than the Cr(CF3C6H5)(CO)2CN group of 3b: Epa{K[Cr(η6-
CF3C6H5)(CO)2CN], K1b} ϭ Ϫ100 mV (vs. SCE, CH3CN), E1/2{Na[Cr(CO)5CN], Na1a} ϭ ϩ690 mV (vs. SCE; reversible, CH3CN).[12]
Ϫ
[g] The oxidation potentials of the dinuclear compounds increase with the estimated electronegativity of the central atom (right column)
in the bridging moieties.
Cyclic voltammetry shows a minimum of two reversible neutral compound compared with the oxidation of an an-
one-electron oxidation steps for compounds PPN2, 3, and ion. The appearance of two well-separated one-electron ox-
5BF4. Compounds PPN2 (FeCr3) and 3 (FeCr2) contain a idation steps for 3a and 3b implies electronic conjugation
tripodFeIIL entity at the bridging position between two or- between the two organometallic termini. Purely electrostatic
ganometallic cyano compounds. [tripodFeII(NCMe)3]2ϩ [13] coupling cannot account for the difference in ∆E1/2 of
is only oxidized at 1400 mV (CH3CN; reversible vs. SCE), around 165 mV observed between these two cases[18]
while the organometallic nitriles bonded to this bridging (Table 7) for two centers about 600 pm apart as they are
group in 3 are more easily oxidized [E1/2{Na[(CO)5CrCN]; in 3.
Na1a} ϭ ϩ690 mV (CH3CN; reversible vs. SCE);[12]
The comproportionation constant (Scheme 10) for the re-
Epa{K[(η6-CF3C6H5)Cr(CO)2CN]; K1b} Ϫ100 mV action is 6 ϫ 102 for both FeCr2 compounds 3a and 3b
ϭ
(CH3CN; vs. SCE; see Exp. Sect.)]. The oxidation waves (Table 7). It is thus in a range characteristic of class III
observed for 3a and 3b (Table 7) are thus in the character- mixed-valence compounds[19] and may be taken as proof of
istic range for the one-electron oxidations of the terminal
groups. Compared to the corresponding alkali metal salts,
the oxidation potentials of the iron-coordinated organomet-
allic nitriles are shifted to higher potentials by around 200
mV (Table 7), as would be expected for the oxidation of a Scheme 10. Comproportionation equilibrium
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2791

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
a conjugative interaction between the termini through the amples of 2؊, 3, and 5؉, the mixed-valence species are def-
tripodFe(NC)2 linking groups.
initely of type III.[19]
Compound PPN2 (FeCr3) undergoes three reversible
A comparison can now be made between the conjugation
one-electron oxidation steps, which is consistent with its capacities of dicyanometalate bridges and main-group-
composition. The three Cr(CO)5CN entities are oxidized in based anionic dicyano bridges. Several compounds con-
a stepwise manner 2Ϫ Ǟ 2, 2 Ǟ 2ϩ, and 2ϩ Ǟ 22ϩ taining two tripodiron(II) entities coupled by dicyanamido
(Table 7). Oxidation of the first Cr(CO)5CN entity occurs or dicyanomethanido ligands are known[2] (Scheme 11; A؉,
at E11/2 ϭ 425 mV and is thus about 400 mV easier to oxid- B؉). Their electrochemical properties are very similar to
ize than with the corresponding neutral FeCr2 species 3a. those observed for 5b؉ and 5c؉ (Table 7). The relevant
The second and third one-electron oxidation steps are comproportionation constants resulting from conjugation
found at E21/2 ϭ 780 mV and E13/2 ϭ 900 mV and are thus through the organic linkers are KC ഠ 104 and are thus not
in a range well below the potential needed for the oxidation significantly different from the values observed with either
of tripodFeII(NCR)3 entities.[13] The occurrence of three organometallic dicyanometalate or tripodFeII(CN)2 linking
one-electron oxidation steps in the observed range leaves no groups. It appears that the HOMO of such compounds
doubt that the individual oxidations occur at the organo- does not have a large orbital coefficient at the central atom
metallic termini. Here again, the separation between the ox- of the bridging group, be it a main-group element or a
idation potentials can only be explained by assuming elec- transition metal. In each case studied, this central group is
tronic conjugation between the organometallic termini far more difficult to oxidize than the termini.
through the tripodFe(NC)3 linker. The comproportionation
constant deduced from the first two oxidation steps of 2؊
(Kc ϭ 106) is the largest for the series of compounds ana-
lyzed (Table 7). The comproportionation constant deduced
Conclusion
With monocyanometalates [MϪCN]Ϫ (1؊), the tri-
from the second and third oxidation potentials is still Kc ϭ
102 (Table 7). Both constants are characteristic of conjug-
podiron(II) template selectively forms tetranuclear species
[tripodFe{NCϪM}3]Ϫ
(2؊)
or
trinuclear
entities
ated systems, and it appears that increasing the negative
charge of the molecule tends to increase conjugation.
Compounds 5bBF4 (Fe5) and 5cBF4 (Fe2Co3) contain
two tripodiron(II) termini coupled through organometallic
dicyano linkers MЈ(CN)2. Two reversible one-electron ox-
idation steps are observed for these compounds (Table 7).
The oxidation potentials are well below that observed for
[tripodFe(NCMe)3]2ϩ, as would be expected for a reduction
[tripodFe(CO){NCϪM}2] (3) under the appropriate stoichi-
ometric control. Dicyanometalates [MЈ(CN)2]Ϫ (4؊) form
pentanuclear compounds [tripodFe{NCϪMЈϪCN}3-Fe-
tripod]ϩ (5؉). The organometallic termini M in compounds
2؊ and 3 [e.g. Cr(CO)5] are more easily oxidizable than the
tripodiron(II) entity. Cyclic voltammetry reveals stepwise
one-electron oxidation of these termini with potential dif-
ferences characteristic of conjugative coupling between
them. In compounds 5؉, with MЈ being CpFe(CO) (5b؉) or
CpCo(CN) (5c؉), the tripodiron(II) entities are more easily
oxidized than the organometallic bridging units. Cyclic vol-
tammetry shows two well-separated, reversible, one-electron
oxidation steps, which is consistent with strong conjugative
coupling of the iron centers through the organometallic
MЈ(CN)2 linkers. It has been observed that the coupling
of the charge of
a
compound containing two
[tripodFe(NCR)3]2ϩ entities by three units in 5؉. The sep-
aration between the oxidation potentials is again rather
large and the comproportionation constants indicate strong
conjugation between the tripodiron(II) termini through the
organometallic dicyanometalate bridges. Thus, in all ex-
Ϫ
efficiency of MЈ(CN)2 is similar to that of [RC(CN)2]Ϫ in
structurally analogous compounds.
Experimental Section
General Remarks: All manipulations were carried out under argon
by means of standard Schlenk techniques. All solvents were dried
by standard methods[20] and distilled under argon. Silica gel and
kieselguhr used for filtrations and chromatography were degassed
at 1 mbar for 24 h and saturated with argon. The compounds
Na1a,[9] K1c,[9] tripod,[21] K4a,[15a] K4b,[15b] K4c,[15c] and
Fe(BF4)2·6H2O[22] were prepared according to or by adaptation of
literature procedures. All other chemicals were obtained from com-
mercial suppliers and were used without further purification. Ϫ
NMR: Bruker Avance DPX200 spectrometer operating at
200.13 MHz (1H), 50.323 MHz (13C), 81.014 MHz (31P); T ϭ
303 K; chemical shifts (δ) in ppm with respect to CD2Cl2 (1H: δ ϭ
Scheme 11. Dinuclear tripodiron(II) complexes A؉ and B؉ (Table 7) 5.32; 13C: δ ϭ 53.5), [D6]acetone (1H: δ ϭ 2.05; 13C: δ ϭ 20.8,
2792 Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
206.1), or [D6]DMSO (1H: δ ϭ 2.50; 13C: δ ϭ 39.4) as internal the form of brown microcrystalline powders by addition of petro-
standards; 31P chemical shifts (δ) in ppm with respect to 85%
leum ether (boiling range 40Ϫ60 °C) to the concentrated brown-
H3PO4 (31P: δ ϭ 0) as an external standard; CD2Cl2, [D6]acetone, red filtrates. Crystals of 3a, 3b, and 3c suitable for X-ray analyses
and [D6]DMSO used for NMR-spectroscopic measurements were
were grown by layering dilute solutions in acetone with petroleum
degassed by three successive freeze-pump-thaw cycles and dried ether (boiling range 40Ϫ60 °C) at room temperature. For analytical
˚
with 4 A molecular sieves. Ϫ IR: Biorad Excalibur FTS 3000 spec-
trophotometer (samples in CsI discs or as solutions between CaF2
windows). Ϫ UV/Vis spectra: PerkinϪElmer Lambda 9 UV/Vis
spectrophotometer; ca. 10Ϫ3  solutions in CH2Cl2. Ϫ MS: Finni-
gan MAT 8320 operated in FAB mode (xenon; matrix: 4-nitro-
benzyl alcohol). Ϫ Cyclic voltammetry: EG&G Princeton Applied
Research model 273 potentiostat; potentials in mV versus SCE at
and spectroscopic data of compounds 3a, 3b, and 3c, see Tables 2
and 3.
[(tripodFe)2{NC؊MЈ؊CN}3]BF4 (5BF4); MЈ ؍ Fe(η5-C5H5)(CO)
(5bBF4), Co(η5-C5H5)(CN) (5cBF4): To a solution of FeIIaq(BF4)2
(1.0 mmol, 338 mg) and tripod (1.0 mmol, 625 mg) in dichlorome-
thane/ethanol (1:1, 40 mL), the potassium salt of the appropriate
ligand, K4b (5bBF4) or K4c (5cBF4) (1.5 mmol), was added as a
solid in a single portion. A color change from pale-yellow to red
was observed. After stirring for 1 h at room temperature, the reac-
tion mixture was concentrated in vacuo. The red residue was fil-
tered through 1 cm of kieselguhr with dichloromethane. Evapora-
tion of the solvent from the red filtrate and subsequent washing of
the residue with diethyl ether left a red microcrystalline solid. The
C3-symmetric isomers (Scheme 9, A) of the pentanuclear complexes
were purified by recrystallization from dichloromethane/ethanol.
Yields: 5bBF4: 42% (crude product), ca. 10% (symmetric isomer of
5bBF4, three recrystallizations); 5cBF4: 67% (crude product), 40%
(symmetric isomer of 5cBF4, one recrystallization).
a glassy carbon electrode at 25 °C; sample solutions: ca. 10Ϫ3
in 0.1  Bu4NPF6 in CH3CN or CH2Cl2. Ϫ Elemental analyses:
Microanalytical Laboratory of the Organisch-Chemisches Institut,
Universität Heidelberg.

Potassium
[Dicarbonyl(cyano)(η6-trifluoromethylbenzene)chro-
mate(0)] (K1b): Tricarbonyl(η6-trifluoromethylbenzene)chromium
(1.72 g, 6.1 mmol) and potassium cyanide (1.14 g, 17.5 mmol) were
dissolved in methanol (200 mL) in an irradiation vessel. The vessel
was cooled to 0 °C and the solution was irradiated for 3 h with UV
light from a high-pressure mercury lamp (Hanau TQ 150). After
allowing the dark-brown reaction mixture to warm to room tem-
perature, it was filtered and the filtrate was concentrated to dryness.
The residue was washed with diethyl ether (100 mL) and filtered
through 5 cm of kieselguhr with tetrahydrofuran (100 mL). Evap-
oration of the solvent from the orange-red filtrate and subsequent
crystallization of the residue from acetone/petroleum ether (boiling
range 40Ϫ60 °C) yielded K1b as a microcrystalline solid with an
intense orange color (900 mg, 46%). Ϫ C10H5CrF3KNO2 (319.2):
calcd. C 37.62, H 1.58, N 4.39; found C 36.93, H 1.89, N 4.48. Ϫ
5bBF4: 1H{31P} NMR (CD2Cl2): δ ϭ 1.53 (s, CH3), 2.00Ϫ2.35 (m,
CH2), 4.10 (s, C5H5), 6.9Ϫ7.8 (m, Har). Ϫ 31P NMR (CD2Cl2): δ ϭ
37.4 (s). Ϫ The unsymmetrical isomer of 5bBF4 was not isolated in
pure form. Its Cp resonances were seen as singlets at δ ϭ 4.14,
4.04, and 3.86 in the 1H{31P} NMR spectrum of the crude product.
5cBF4: 1H{31P} NMR (CD2Cl2): δ ϭ 1.43 (s, CH3), 2.0Ϫ2.4 (m,
CH2), 4.7 (s, C5H5), 7.0Ϫ7.8 (m, Har). Ϫ 31P NMR (CD2Cl2): δ ϭ
39.1 (s). Ϫ 13C NMR ([D6]DMSO): δ ϭ 138.6 (dd, PCar), 136.9
(dd, PCar), 134.5Ϫ128.0 (m, Car), 133.7 (s, CoϪCNϪFe), 117.9 (s,
CoϪCN), 88.9 (s, C5H5), 37.6 (m, CH3), 36.4 (m, Cq), 32.8 (m,
CH3). Ϫ For further analytical and spectroscopic data of com-
pounds 5bBF4 and 5cBF4, see Tables 6 and 7.
1H NMR ([D6]acetone): δ ϭ 4.5Ϫ5.1 (m). Ϫ 13C NMR ([D6]ace-
1
tone): δ ϭ 241.3 (s, CO), 168.0 (s, CN), 126.4 (q, CF3, JFC
ϭ
270 Hz), 89.6 (s, Car), 86.5 (q, CarCF3, 2JFC ϭ 35 Hz), 86.3 (s, Car),
˜
84.3 (s, Car). Ϫ IR (CH3OH): ν ϭ 2041 w, br (CN), 1910 s, 1856 s
(CO). Ϫ MS (FABϪ): m/z ϭ 280.0 [M]Ϫ. Ϫ CV (CH3CN, SCE):
Epa ϭ Ϫ100 mV (irrev. ox.).
Crystals of compound 5aFeCl4 suitable for an X-ray crystallo-
graphic structure determination were obtained in very low yield
(15 mg) from the reaction of FeCl2 (135 mg, 1.07 mmol), tripod
(680 mg, 1.09 mmol), and potassium tetracarbonyl(dicyano)man-
ganate(I) (280 mg, 1.08 mmol) in acetone. After concentrating the
reaction solution to dryness and filtration of the residue through
1 cm of kieselguhr with dichloromethane, the red filtrate was con-
centrated to dryness. Red parallelepiped-shaped crystals of 5aFeCl4
were grown by vapor diffusion of diethyl ether into a solution of
the crude product in tetrahydrofuran/ethanol at room temperature.
PPN[tripodFe{NC؊Cr(CO)5}3] (PPN2): To a solution of FeCl2
(0.29 mmol, 34 mg) and tripod (0.29 mmol, 184 mg) in acetone, a
solution of Na1a (216 mg, 0.90 mmol) in acetone was added drop-
wise by means of a syringe. An immediate color change from pale-
yellow to red was observed. After the addition of bis(triphenylpho-
sphanyl)iminium chloride (PPNCl; 176 mg, 0.31 mmol), the reac-
tion mixture was concentrated to dryness in vacuo. The red residue
was redissolved in dichloromethane (20 mL) and the resulting red
solution was stirred for 2 h at room temperature to complete the
cation exchange. Filtration through 3 cm of kieselguhr gave a red
solution, which was concentrated to a volume of 5 mL and layered
with ethanol (20 mL). PPN2·0.75 CH2Cl2 was obtained as red
needle-shaped crystals (280 mg, 52%). For analytical and spectro-
scopic data of PPN2, see Tables 2 and 3.
1
Ϫ H{31P} NMR (CD2Cl2): δ ϭ 1.5 (br. s, CH3), 2.3 (br. m, CH2),
7.0Ϫ7.8 (m, Harom); all signals are broad because of the paramag-
netism of the FeCl4 anion. Ϫ 31P NMR (CD2Cl2): δ ϭ 32.7 (s).
Ϫ
Ϫ For further analytical and spectroscopic data, see Tables 5, 6,
and 7.
[tripodFe(CO){NC؊M}2] (3);
M
؍
Cr(CO)5 (3a), Cr(η6-
Salts of 5a؉ with counteranions other than tetrachloroferrate(III)
could not be obtained in analytically pure form, neither by the
procedure described above for 5bBF4 and 5cBF4 nor under strictly
anhydrous conditions. This is mainly due to problems associated
with the purification of these compounds, which could not be ef-
fected by means of crystallization and chromatography.
CF3C6H5)(CO)2 (3b), Mn(η5-CH3C5H4)(CO)2 (3c): To a solution
of FeCl2 (0.65 mmol, 34 mg) and tripod (0.65 mmol, 184 mg) in
acetone, a solution of the alkali metal salt Na1a (3a), K1b (3b), or
K1c (3c) (1.30 mmol) in acetone was added dropwise by means of
a syringe. An immediate color change from pale-yellow to orange-
red was observed. After bubbling carbon monoxide through the
reaction mixture for 5 min, the turbid brown solution was concen- X-ray Crystallographic Studies: Suitable crystals were taken directly
trated to dryness in vacuo leaving a brown-red solid residue. This from the mother liquor, immersed in perfluorinated polyether oil,
was redissolved in dichloromethane and the resulting solution was and fixed to a glass capillary. Data were collected with a
filtered through 3 cm of kieselguhr. The products were isolated in
NoniusϪKappa CCD diffractometer (low-temperature unit, graph-
Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2793

V. Jacob, G. Huttner, E. Kaifer, P. Kircher, P. Rutsch
FULL PAPER
Table 8. Crystallographic data for compounds PPN2, 3, and 5aFeCl4
Compound
PPN2
3a
3b
3c
5aFeCl4
Solvate
0.75 CH2Cl2
Ϫ
Ϫ
Ϫ
0.4 THF ·0.5 Et2O
Empirical formula
(without solvate)
Molecular mass [g/mol]
Crystal size [mm]
Crystal system
Space group
C
95H69Cr3FeN4O15P5 C54H39Cr2FeN2O11P3 C62H49Cr2F6FeN2O5P3 C60H53FeMn2N2O5P3 C100H78Cl4Fe3Mn3N6O12P6
1873.3
1144.5
1268.8
1140.7
2215.8
0.30ϫ0.10ϫ0.05
monoclinic
P21
0.10ϫ0.15ϫ0.15
monoclinic
P21/n
0.15ϫ0.25ϫ0.12
monoclinic
P21/n
0.15ϫ0.20ϫ0.20
monoclinic
P21/n
0.25ϫ0.10ϫ0.03
monoclinic
P21/n
Lattice constants:
a [pm]
1438.3(3)
1449.1(3)
2198.7(4)
90
1342.9(3)
2255.0(5)
1717.8(3)
90
1308.5(3)
2252.5(5)
1863.7(4)
90
1307.3(3)
2211.1(4)
1802.4(4)
90
2076.5(4)
2459.7(5)
2250.3(5)
90
b [pm]
c [pm]
α [°]
β [°]
γ [°]
95.31(3)
90
92.01(3)
90
91.12(3)
90
90.42(3)
90
114.64(3)
90
V [106·pm3]
4563.00
2
1.410
5198.70
4
1.462
5492.00
4
1.535
5209.80
4
1.454
10447.00
4
1.455
Z
dx [g·cmϪ3
T [K]
]
200
200
200
200
200
Scan range
Method
Scan speed
No. of measured reflections
No. of unique reflections
No. of observed reflections
Observation criterion
No. of params. refined
Resid. el. density [10Ϫ6 e·pmϪ3
R1/Rw [%] (refinement on F2)
3.4° Յ 2θ Յ 52.0°
ω-Scan, ∆ω ϭ2.0°
18 s/frame
70626
17826
11825
I Ն 2σ
987
0.91
3.0° Յ 2θ Յ 52.1°
ω-Scan, ∆ω ϭ1.0°
10 s/frame
71226
10221
5734
I Ն 2σ
662
0.53
2.8° Յ 2θ Յ 52.1°
ω-Scan, ∆ω ϭ1.0°
10 s/frame
89959
10815
8611
I Ն 2σ
734
0.40
2.9° Յ 2θ Յ 52.0°
ω-Scan, ∆ω ϭ1.0°
10 s/frame
84087
10254
7205
I Ն 2σ
525
1.52
6.2/16.7
3.5° Յ 2θ Յ 50.0°
ω-Scan, ∆ω ϭ1.0°
30 s/frame
162573
18392
9339
I Ն 2σ
1155
1.41
7.9/22.9
]
6.0/13.1
5.2/8.7
3.5/8.5
B. Antelmann, A. Driess, B. Schiemenz, Z. Naturforsch., Teil
B 2000, 55, 638.
ite-monochromated Mo-Kα radiation). The data were processed by
the standard Nonius software.[23] All calculations were performed
using the SHELX software package. Structures were solved by dir-
ect methods using the program SHELXS-97[24] and refined using
the program SHELXL-97.[24] Graphical handling of the structural
data during solution and refinement was performed with
XPMA.[25] Atomic coordinates and anisotropic thermal parameters
of the non-hydrogen atoms were refined by full-matrix least-squares
calculations. Data relating to the structure determinations are com-
piled in Table 8. Figures 1, 2, and 3 were generated using WinRay-
32 and WinRay-GL.[26] Crystallographic data (excluding structure
factors) for the structures reported in this paper have been depos-
ited with the Cambridge Crystallographic Data Centre as supple-
mentary publication nos. CCDC-156056 to -156060. Copies of the
data can be obtained free of charge on application to the CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. [Fax: (internat.) ϩ 44-
1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
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Eur. J. Inorg. Chem. 2001, 2783Ϫ2795

Oligometallic Compounds from tripodIron(II) Species
FULL PAPER
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Eur. J. Inorg. Chem. 2001, 2783Ϫ2795
2795