Coordinating properties of mixed pyrrolyl/dicarbollide cobalt metallocene-type complexes

Article abstract of DOI:10.1039/b110918a

Mixed-sandwich Co(III) complexes incorporating an η⁵ pyrrolyl unit [η⁵-NC₄H₄]^- are very stable when the second η⁵ moiety is a dicarbollide [η⁵-C₂B₉H₁₁]^2-. The existence of a lone pair on N in the neutral closo-[3-Co(η⁵-NC₄H₄)-1,2-C₂B ₉H₁₁] (1) has allowed for the first time a study of the N-σ bonding capacity of a pyrrolyl ligand η⁵ bonded to a metal in an oxidation state higher than two. This capacity has been proven by reacting closo-[3-Co(η⁵-NC₄H₄)-1,2-C₂B ₉H₁₁] with Lewis acids (BH₃, BF₃ and Ag⁺) and a Co(III) macrocycle. In all cases the reaction went to completion proving the strength of the σ bond generated. The stability of the [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉ H₁₁] complexes vs. oxidation state Co(III)/Co(II) was studied by cyclic voltammetry. It was found that [3-Co(η⁵-NC₄H₄)1,2-C₂B₉ H₁₁]^- is not as stable as [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉ H₁₁]; however, when coordinated to cobalt(III), both [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉ H₁₁]^- and [3-Co(η⁵-NC₄H₄)-1,2-C₂B₉ H₁₁] are of comparable stability. The stability of the reduced form Co(II) in the mixed pyrrolyl/dicarbollide is enhanced when an electron withdrawing group is bonded to the cluster carbon atom in the dicarbollide.

Full text of DOI:10.1039/b110918a

View Article Online / Journal Homepage / Table of Contents for this issue
Coordinating properties of mixed pyrrolyl/dicarbollide cobalt
metallocene-type complexes
Jordi Llop,a Clara Viñas,a Francesc Teixidor*a and Lluís Victorib
a Institut de Ciència de Materials de Barcelona, CSIC, CampusUAB, E-08193 Bellaterra,
Spain. E-mail: teixidor@icmab.es
b Institut Químic de Sarrià, Via Augusta 380, E-08017 Barcelona, Spain
Received 29th November 2001, Accepted 25th January 2002
First published as an Advance Article on the web 20th March 2002
Mixed-sandwich Co() complexes incorporating an η5 pyrrolyl unit [η5-NC4H4]Ϫ are very stable when the second
η5 moiety is a dicarbollide [η5-C2B9H11]2Ϫ. The existence of a lone pair on N in the neutral closo-[3-Co(η5-NC4H4)-
1,2-C2B9H11] (1) has allowed for the first time a study of the N-σ bonding capacity of a pyrrolyl ligand η5 bonded to
a metal in an oxidation state higher than two. This capacity has been proven by reacting closo-[3-Co(η5-NC4H4)-
1,2-C2B9H11] with Lewis acids (BH3, BF3 and Agϩ) and a Co() macrocycle. In all cases the reaction went to
completion proving the strength of the σ bond generated. The stability of the [3-Co(η5-NC4H4)-1,2-C2B9H11]
complexes vs. oxidation state Co()/Co() was studied by cyclic voltammetry. It was found that [3-Co(η5-NC4H4)-
1,2-C2B9H11]Ϫ is not as stable as [3-Co(η5-NC4H4)-1,2-C2B9H11]; however, when coordinated to cobalt(), both
[3-Co(η5-NC4H4)-1,2-C2B9H11]Ϫ and [3-Co(η5-NC4H4)-1,2-C2B9H11] are of comparable stability. The stability
of the reduced form Co() in the mixed pyrrolyl/dicarbollide is enhanced when an electron withdrawing group
is bonded to the cluster carbon atom in the dicarbollide.
(Cc) atoms or at the α positions of the pyrrolyl unit. For
Introduction
comparing purposes, however, closo-[3-Co(η5-NC4H4)-1-C6H5-
The first cobalt sandwich complex incorporating one pyrrolyl
1,2-C2B9H10] (2) which contains a phenyl ring near the
and one dicarbollide unit was reported in 1996.1 Since this early
N-coordinating site, therefore hindering N coordination, was
work, a number of synthetic strategies have been developed to
examined in parallel.
obtain mixed-sandwich complexes of this type.2 Formally, the
three positive charged cobalt compensates the dinegative dicarb-
Results and discussion
ollide and the mononegative pyrrolyl. Therefore, the complex is
neutral, with an electron lone pair on the nitrogen atom of the
pyrrolyl unit. The result is a complex which is isoelectronic to
pyridine, imidazole or azaferrocene, among other nitrogen
bases. The question is whether the η5 coordination of the pyr-
rolyl unit to a highly electron-density demanding metal, Co(),
will not diminish the coordinating capacity of the pyrrolyl
nitrogen. The number of readily available η5 similar systems
is practically restricted to azaferrocene,3 or, less related, to
the semi-sandwich pyrrolyltricarbonyl manganese.4 Kuhn et al.5
have demonstrated that the coordination reaction of S(CH3)2
BH3 with permethylated 1,1Ј-diazaferrocene yields the corre-
sponding stable diadduct. Other examples involving N-σ
coordination to metal complexes can be found, although the
data available is restricted to low valent metals.6
Reaction with Lewis acids: formation of 1 BH3 (3), 1 BF3 (4)
and 2 BF3 (5)
It is well known that BH3 and BF3 form stable adducts with
THF, (CH3)2S and, in general, any molecule containing lone
pairs (i.e., Lewis bases). To prove that nitrogen σ-coordination
is feasible with closo-[3-Co(η5-NC4H4)-1,2-C2B9H11], this was
allowed to react with increasing amounts of THF BH3 (see
Scheme 1) ranging from 1 : 0.1 to 1 : 1 in the absence of co-
The similarity of closo-[3-Co(η5-NC4H4)-1,2-C2B9H11] to aza-
ferrocene provided a new system with a metal in ϩ3 oxidation
state able to evaluate the N-σ coordinating properties in sand-
wich η5 coordinated azaheterocycles. This pyrrolyl/dicarbollide
cobalt system offers interesting possibilities mostly derived
from its stability to air, easy preparation and purification and
high yield. Furthermore, use of dicarbollide has permitted the
synthesis of dicarbollide/pyrrolyl complexes where there are
no substituents on the α positions of the pyrrolyl group. The
absence of electron-donating groups on the pyrrolyl ligands
allows one to probe the influence of η5 bonding on the coordin-
ating character of this heterocycle.
Scheme 1 Reaction of formation of 1 BH3 (3).
ordinating solvent. For compound 1, sequential adduct form-
ation is illustrated in Fig. 1. The 11B{1H} NMR spectrum of
1 shows, in toluene, six resonances with a 1 : 1 : 2 : 2 : 2 : 1
pattern in the range ϩ7.3 to Ϫ22.2 ppm (Fig. 1a). Upon the
addition of THF BH3 seven new peaks appear (Figs 1b and
1c), which coexist with those for pure 1. When the stoichio-
metric amount of THF BH3 is added (Fig. 1d), all resonances
corresponding to “free” complex 1 dissappear and only the new
pattern 1 : 1 : 2 : 2 : 2 : 1 : 1 is observed in the range ϩ8.7 to
Ϫ20.8 ppm. This corresponds to pure compound 3. The reson-
ance at Ϫ20.8 ppm becomes a quartet in the 11B spectrum, and
is assigned to the BH3 boron atom.
From among the mixed dicarbollide/pyrrolyl complexes
described in the literature,1,2 we have chosen closo-[3-Co(η5-
NC4H4)-1,2-C2B9H11] (1) for study due to its geometrical sim-
plicity. It does not contain substituents on the cluster carbon
DOI: 10.1039/b110918a
J. Chem. Soc., Dalton Trans., 2002, 1559–1565
This journal is © The Royal Society of Chemistry 2002
1559

View Article Online
0.6 ppm that is a singlet in the 11B spectrum, is assigned to the
1
coordinated BF3. The H NMR spectrum of 1 BF3 displays
two resonances in the pyrrolyl region with chemical shift posi-
tions at 7.1 (2H) and 6.6 (2H) ppm slightly displaced to low
field with regard to 1. As expected, an excess of (C2H5)2O BF3
left unaltered the spectrum of 1 BF3 (1 : 1), indicating that
our former interpretation concerning the instability of the
reduced form of 1 was correct. This will later be seen with the
cyclic voltammogram studies reported.
We have observed by X-ray diffraction that the pyrrolyl
nitrogen atom is placed bisecting the C–C bond of the dicarb-
ollide (see Fig. 3). Considering that the presence of a bulky
Fig. 1 11B{1H} NMR spectra of the reaction of compound 1 with
THF BH3 at different 1 : THF BH3 ratios: (a) 1 : 0, (b) 1 : 0.25,
(c) 1 : 0.50, (d) 1 : 1.
Fig. 3 The nitrogen atom of the pyrrolyl unit in mixed cobalt
complexes incorporating one pyrrolyl and one dicarbollide unit is
placed bisecting the C–C bond of the dicarbollide.
Although the formation of the adduct was complete at the
ratio 1 : 1, excess of THF BH3 was added. Under these
reducing conditions the mixed-sandwich complex was not
group bonded to the dicarbollide carbon atom could difficult
the σ coordination to BF3, we decided to study the reaction of
closo-[3-Co(η5-NC4H4)-1-C6H5-1,2-C2B9H10] (2) with (C2H5)2-
stable. Most probably Co()
Co() reduction takes place,
leading to an unstable complex. To prove this hypothesis, the
same set of experiments were done with the non-reducing
(C2H5)2O BF3. In this case, CDCl3 was used as non-
coordinating solvent. 1 was mixed with increasing amounts of
(C2H5)2O BF3 covering the range 1 : 0.1 to 1 : 1, and the
results, illustrated in Fig. 2, are comparable to those found for
O
BF3 as it could provide a clear view of the strength of
such a bond. Experiments were conducted for 2 in equal way
as for 1, thus 2 was mixed with (C2H5)2O BF3 in different
ratios and their 11B{1H} NMR spectra recorded. The starting
1 : 1 : 4 : 1 : 2 pattern in the range ϩ8.3 to Ϫ15.5 ppm for 2
changed to 1 : 1 : 1 : 2 : 2 : 1 : 1 : 1 in the range ϩ14.4 to Ϫ14.2
ppm when the ratio 2 : (C2H5)2O BF3 was 1 : 1. The singlet at
0.6 ppm was assigned to the BF3 unit. As for the non-sterically
crowded 1 the reaction was fully shifted to the formation of the
adduct. The coexistence of the species in excess in solution and
the adduct, e.g. 1 and 1 BF3; 2 and 2 BF3 or 1 and 1 BH3
indicate the great stability of the adduct not reverting to the free
species in the NMR time scale.
ReactionwithAg؉.Formationof[(3-Co(ꢀ5-NC4H4)-1,2-C2B9H11)2-
Ag][BF4] (6) and [(3-Co(ꢀ5-NC4H4)-1-C6H5-1,2-C2B9H10)2Ag]-
[BF4] (7)
The reaction of 1 and 2 with Ag[BF4] in ratio 3 : 1 led to the
formation, in CH2Cl2, of reasonably light- and air-stable com-
plexes (6 and 7, respectively), which were characterized by
NMR and MALDI-TOF mass spectroscopies. The proposed
structure for compound 6 is illustrated in Fig. 4. The species is a
1
Fig. 2 11B{1H} NMR and H{11B} NMR spectra of the reaction of
compound 1 with (C2H5)2O BF3 at different 1 : (C2H5)2O BF3
ratios: (a) 1 : 0, (b) 1 : 0.33, (c) 1 : 0.66, (d) 1 : 1.
THF BH3. Fig. 2a corresponds to compound 1. As before,
seven new signals are observed upon successive additions of
(C2H5)2O BF3 (Figs 2b and 2c). Fig. 2d corresponds to the
stoichiometric 1 : 1 relation or to 1 BF3 (4). The resonance at
Fig. 4 Proposed structure of compound 6.
1560
J. Chem. Soc., Dalton Trans., 2002, 1559–1565

View Article Online
Fig. 5 MALDI-TOF mass spectrum obtained for compound 6 (T = calculated, E = experimental).
cation compensated by the [BF4]Ϫ as evidenced by 11B{1H}
NMR. The 11B{1H} NMR spectrum of 6 displays six reson-
ances with a 2 : 2 : 1 : 8 : 4 : 2 pattern. The resonance at Ϫ0.5
ppm is a singlet in the 11B NMR spectrum, and is assigned to
[BF4]Ϫ. As a result of coordination, the pyrrolyl hydrogen
atoms are slightly shifted to low field relative to those found for
1 in the 1H NMR spectrum. The MALDI-TOF mass spectrum
of 6 is shown in Fig. 5. Typical patterns associated to boron
clusters are observed with the highest peaks, being at m/z = 623
and m/z = 365 which are assigned to [Ag(1)2]ϩ and [Ag(1)]ϩ,
respectively. A comparison of the experimental (E) and theor-
etical (T) distributions for the two peaks is included in Fig. 5.
The peak at m/z = 365 can be interpreted as a fragment of the
peak at m/z = 623.
Compound 7, which has lower symmetry than 6, displays
1
three resonances in the H{11B} NMR pyrrolyl region. This
diminished symmetry does not influence the 11B{1H} NMR
that is comparable to that for 6 with a 2 : 2 : 1 : 8 : 2 : 4 pattern,
the resonance at Ϫ0.5 ppm being assigned to [BF4]Ϫ.
As for 6, the MALDI-TOF also displayed peaks for [Ag(2)2]ϩ
and [Ag(2)]ϩ, respectively, further supporting the trimetallic
nature of compound 7.
Formation of 1 mcox (8) and 2 mcox (9)
Cobaloximes (cox) are readily available compounds7 much
easier to synthesize than porphyrines. Thus, they are suitable
to be studied in coordination reactions and their results are
expected to be comparable with those of phthalocyanines and
porphyrins.
Fig. 6 Proposed structure and reaction of formation for compound 8.
The reactions of 1 and 2 with methylaquocobaloxime (mcox)
in an anhydrous non-coordinating solvent (methylene chloride)
lead to the isolation of solids upon the addition of the same
volume of hexane. Fig. 6 illustrates the reaction of formation
and the proposed structure for complex 8. Crystallization was
used as a purification method, and red needles were obtained
from saturated methylene chloride–hexane (1 : 1) solutions for
both 8 and 9. The 1H NMR spectrum is consistent with a
modified methylcobaloxime whose water molecule has been
ing in the same way the four methyl groups of the macrocycle.
The 11B{1H} NMR spectrum pattern and frequency closely
resembles that for 1.
The easiness of complexation between 1 and methylaquo-
cobaloxime was monitored by 1H NMR spectroscopy. An
initial solution of methylaquocobaloxime in CDCl3 was
allowed to react with increasing amounts of 1. Fig. 7 shows the
1H NMR spectra at different ratios. As with (C2H5)2O BF3 or
THF BH3 the reaction was very quick. Therefore, total
coexistence of resonances attributed to independent mcox and
1
substituted. Therefore, the H NMR spectrum of 8 shows an
acidic hydrogen at 18.1 ppm and the axial CH3 at 2.2 ppm. In
addition, two resonances were found in the pyrrolyl region
slightly displaced to high field relative to those of 1. The fact
that only one resonance is found for the in-plane methyl groups
suggests that there is no preferential orientation of 1, interact-
1
mcox was observed at any ratio. A similar behavior is found
for 9, however the existence of the phenyl group breaks the
possible Cs symmetry of the complex and leads to the two
equal-intensity signals (6 : 6 pattern) corresponding to the
J. Chem. Soc., Dalton Trans., 2002, 1559–1565
1561

View Article Online
Fig. 8 Voltamperograms corresponding to compound 1 (curve a),
methylaquocobaloxime (curve b) and a mixture of the compounds in
1 : 1 ratio (curve c) in chloroform as a solvent.
Fig. 7 1H{11B} NMR spectra of the reaction of compound 1 with
mcox at different 1 : mcox ratios: (a) 0 : 1, (b) 0.5 : 1, (c) 1 : 1.
Fig. 9 Schematic representation of the oxidation and reduction processes presented in Fig. 8.
methyl groups of the cobaloxime. This is the first indication
that an axial orientation can be obtained, although the number
of signals corresponding to methyl groups is too low to indicate
a fixed conformation at room temperature.
ing to NMR study. We can indicate that a 100% conversion
takes place when the stoichiometric amount of THF BH3 is
added as a consequence of the high stability of pure 1 BH3
adduct. Notwithstanding this, for ratios of 1 : THF BH3 <1
we noticed that the generated adduct 1 BH3 did decompose.
BH3 is a reducing agent and we interpreted that 1 BH3 was
not stable when the oxidation state of Co was . The question
was whether 1 itself was unstable under reducing conditions or
if the adduct was the unstable species.
Electrochemical characterization
The NMR studies have shown that a fast reaction takes place
between 1 or 2 with THF BH3, (C2H5)2O BF3, Agϩ or
mcox. Indeed, only the starting compound in excess and the
generated compounds, e.g. 1 and 1 BH3, were present accord-
For this purpose, we considered that complex 1 mcox (8)
1562
J. Chem. Soc., Dalton Trans., 2002, 1559–1565

View Article Online
Two peaks, E4 and E5, are observed about 230 mV to higher
values with respect to E3 and E6. The E4 and E5 peaks corre-
spond to the mcox cobalt redox process in 1 mcox. The shift
of the peaks to higher values indicates stabilization of the mcox
Co(), in 1 mcox caused by substitution of the hard water by
the softer nitrogen. Decomplexation of 1 mcox to 1 and mcox
takes place at high oxidation potentials. This is proven by the
unequal reduction (E5) and oxidation (E4) peaks of 1 mcox,
see Fig. 8c. The oxidation peak is more intense that the reduc-
tion one, indicating that a fraction of the oxidized form is not
reduced due to decomposition of the complex and formation of
pure 1 and mcox species.
Similar results were obtained studying 2 mcox; however
two points deserve special mention. Peaks E1 and E8 have the
same height; this was not true for 1, which means that 2 has
better reversibility than 1. Besides, a shift in E1/2 for 2 of about
100 mV to more positive values relative to 1 is found, indicating
that the ϩ2 formal oxidation state in 2 is more stable than in 1.
Both phenomena, peaks height and shift, are interpreted by the
presence of the phenyl ring which allows dissipation of the
excess of charge from the cobalt atom through the aromatic ring.
Fig. 10 Voltamperograms corresponding to compound 1 (curve a),
methylaquocobaloxime (curve b) and a mixture of the compounds in
1 : 1 ratio (curve c) in acetonitrile as a solvent.
and 2 mcox (9) would bring important information concern-
ing the stability of N-σ coordinated [3-Co(η5-NC4H4)-1,2-
C2B9H11] upon oxidation/reduction. This stability was tested in
both non-coordinating and coordinating media by using cyclic
voltammetry (CV).
(b) Stability in a coordinating medium. These CV studies were
carried out in CH3CN (Fig. 10). Immediate differences with
regard to the non-coordinating solvent are observed in the CV
for 1 where the peak associated with the oxidation process has
virtually disappeared. As expected, the CV for mcox, Fig. 10b,
is similar in appearance to the curve obtained in chloroform
(Fig. 8b). The greatest difference with the non-coordinating
solvent was found upon mixing equal ratios of 1 and mcox (Fig.
10c). The resulting voltammogram is the simple addition of
curves (a) and (b), indicating that no complex formation has
taken place. The σ-coordinating capacity of [3-Co(η5-NC4H4)-
1,2-C2B9H11] was not sufficient to compete with a large excess
of acetonitrile preventing the formation of the Co–pyrrolyl
(a) Stability in a non-coordinating medium. Complexes 8
and 9 contain two metallic centers suitable to undergo redox
processes. The Co() in the cobaloxime unit can be oxidized to
the  formal oxidation state, while the Co() in the mixed
cobaltacarborane unit can be reduced to its  formal oxidation
state. Thus, three pairs of oxidation states, /, /, and /,
are possible.
The cyclic voltammetry response of 1 in chloroform is shown
in Fig. 8a. Two well-defined peaks assigned to the couple
[3-Co(η5-NC4H4)-1,2-C2B9H11]/[3-Co(η5-NC4H4)-1,2-C2B9H11]Ϫ
are observed. We observe that the oxidation peak (E1) is less
intense than the reduction peak (E8); that is in agreement with
the 1 BH3 results indicated above concerning the unstability
of the mixed cobalt complex under reducing conditions. The
CV for methylaquocobaloxime (mcox) in CHCl3 (Fig. 8b)
shows two well-defined peaks corresponding to the Co()/
Co() process of the central atom, with a half-wave potential
of 1.34 V. Fig. 8c displays the CV obtained when equimolar
quantities of 1 and mcox were mixed together. At this stage, the
three species corresponding to 1, mcox and 8 coexist. Therefore,
eight peaks are observed, four corresponding to oxidation pro-
cesses and four to reduction ones, or four corresponding to
starting materials and four to the 1 mcox. Fig. 9a summarizes
our interpretation of the processes associated with the cobalt
atom at the carborane unit and Fig. 9b the processes associated
with the cobalt atom in the cobaloxime unit. A main difference
is found between the CV and the NMR of 1 mcox. The NMR
has shown that there is only one species upon mixing equimolar
amounts of 1 and mcox. The CV shows that 1, mcox and
bond. This result was confirmed by means of 1H NMR and 11
B
NMR spectroscopy carried out in CD3CN. The resonances
corresponding to the pyrrolyl unit (1H NMR) and the boron
cage atoms (11B NMR) of the sandwich complex match these
for the reagents. A completely parallel behavior was found upon
mixing compound 2 and mcox in CH3CN.
Conclusions
Mixed-sandwich Co() complexes ([3-Co(η5-NC4H4)-1,2-
C2B9H11]) incorporating one unsubstituted pyrrolyl and one
dicarbollide units present good N-σ coordinating proper-
ties through the electron pair on the nitrogen atom of the
pyrrolyl unit. They are the first examples of N-σ coordin-
ation by a pyrrolyl η5-coordinated to a metal in an oxidation
state higher than 2. N-σ bond formation was observed when
closo-[3-Co(η5-NC4H4)-1,2-C2B9H11] was mixed with Lewis
acids ((C2H5)2O BH3, THF BF3 and Agϩ) and a Co()
macrocycle. In all cases the reaction went to completion, prov-
ing the strength of the σ-bond generated. The presence of
a bulky group (phenyl) on one of the cluster carbon atoms of
the dicarbollide unit did not inhibit the coordinating ability
of the mixed complex; however, it altered the electrochemical
properties of the two cobalt nuclei in the resulting dinuclear
complexes. This phenyl group also stabilized the Co() state in
the mixed complex, as a result of its electron-withdrawing
capacity. Therefore the Co()/Co() redox process for 2 is
more reversible than for 1 in non-coordinating solvent media.
1
mcox coexist under the same reaction conditions. Our
interpretation, consistent with the CV, is that by using this
technique we generate in situ 1 and mcox from 1 mcox.
The peaks at E1 and E8 in Fig. 8c correspond, respectively, to
oxidation and reduction of the cobalt atom in the mixed ligand
1. Similarly, E2 and E7 correspond to the same process in 1, this
time coordinated via the pyrrolyl to the cobaloxime. Comparing
the two equivalent redox processes on 1 and 1 mcox, an E1/2
shift of 300 mV to higher voltage values and an enlargement of
the oxidation peak E2 vs. E1 is observed, in such a way that the
height of E2 ∼ E7. These changes indicate a higher stability of
the Co() in 1 mcox than in free 1, essentially induced by σ-
coordination of the nitrogen atom. Peaks E3 and E6 in curve c
correspond, respectively, to oxidation and reduction processes
associated with the cobalt atom of the methylaquocobaloxime.
Experimental
Instrumentation
IR spectra were recorded with KBr pellets on a FTIR-8300
1
1
Shimadzu spectrophotometer. H and H{11B} NMR (300.13
J. Chem. Soc., Dalton Trans., 2002, 1559–1565
1563

View Article Online
MHz), 13C{1H} NMR (75.47 MHz) and 11B and 11B{1H} NMR
(96.29 MHz) spectra were recorded at room temperature with a
Bruker ARX 300 instrument equipped with the appropriate
solid which precipitated was collected. The mixture was filtered
off, washed with cold methylene chloride (2 mL) and vacuum-
dried. (0.030 g, 68%). (Found: C, 20.52; H, 4.15; N, 3.72.
C12B19H30Co2N2F4Ag requires: C, 20.31; H, 4.26; N, 3.95%);
ν/cmϪ1: (Cc–H) 3120, 3055, (B–H) 2617, 2590, 2567, 2536,
(B–F) 1089, 1060 (KBr pellet); δH 7.2 (4H, s, N–Cpyr–H), 6.7
decoupling accessories. Chemical shift values for H, H{11B}
and 13C{1H} NMR spectra were referenced to Si(CH3)4, and
those for 11B and 11B{1H} NMR spectra were referenced to
(C2H5)2O BF3. Chemical shifts are reported in ppm down-
field from Si(CH3)4 and coupling constants in Hz. Cyclic
voltammograms were recorded using an EG&G PAR 273 A
potentiostat/galvanostat. MS spectra were recorded using a
Bruker Biflex MALDI-TOF mass spectrometer.
1
1
(4H, s, Cpyr–Cpyr–H), 4.9 (4H, s, Cc–H); δH{B} 7.2 (4H, s, N–Cpyr
–
H), 6.7 (4H, s, Cpyr–Cpyr–H), 4.9 (4H, s, Cc–H), 3.1 (6H, br s,
B–H), 1.9 (4H, br s, B–H), 1.7 (8H, br s, B–H); δC{H} 111.1 (s,
N–Cpyr), 94.0 (s, Cpyr–Cpyr), 55.7 (s, Cc); δB 7.7 [2B, d, 1J(B,H) =
1
151], 4.4 [2B, d, J(B,H) = 145], Ϫ0.5 (s, 1B), Ϫ4.2 [8B, d,
1
1J(B,H) = 149], Ϫ15.3 [4B, d, J(B,H) = 157], Ϫ21.6 [2B, d,
1J(B,H) = 176]. MS-MALDI-TOF: m/z = 622 [Ag(1)2]ϩ and 365
[Ag(1)]ϩ.
Materials
closo-[3-Co(η5-NC4H4)-1,2-C2B9H9]1 and methylaquocobal-
oxime7 were prepared according to literature methods.
(C2H5)2O BF3, THF BH3 and tetrabutylammonium hexa-
fluorophosphate (Fluka), and Ag[BF4] and tetrabutyl-
ammonium chloride (Aldrich) were used as received without
further purification. Diethyl ether and toluene were freshly
distilled from sodium benzophenone. Other solvents were of
reagent grade purity and were used without further purification
and stored over molecular sieves. Experiments were carried out
under dry, oxygen-free dinitrogen atmosphere, using standard
Schlenk techniques, with some subsequent manipulation in the
open laboratory.
Synthesis of 7. The procedure was the same as for 6 but using
compound 2 (0.102 g, 0.31 mmol) and Ag[BF4] (0.030 g,
0.15 mmol) as starting materials (0.035 g, 63%) (Found: C,
33.71; H, 4.33; N, 3.23. C24B19H38Co2N2F4Ag requires C, 33.45;
H, 4.44; N, 3.25%); ν/cmϪ1: (Cc–H) 3118, 3045, (B–H) 2593,
2565, 2555, (B–F) 1076, 1051 (KBr pellets); δH 7.7–7.3 (10H, m,
Car–H), 6.9 (2H, s, N–Cpyr–H), 6.5 (4H, s, Cpyr–Cpyr–H), 6.1
(2H, s, N–Cpyr–H), 5.9 (2H, s, Cc–H); δH{B} = 7.7–7.3 (10H, m,
Car–H), 6.9 (2H, s, N–Cpyr–H), 6.5 (4H, s, Cpyr–Cpyr–H), 6.1
(2H, s, N–Cpyr–H), 5.9 (2H, s, Cc–H), 3.9 (2H, br s, B–H), 3.3
(6H, br s, B–H), 1.8 (4H, br s, B–H), 1.7 (6H, br s, B–H); δC{H}
=
143.2, 136.7, 128.2, 125.6 (s, C6H5), 115.1, 112.8 (s, N–Cpyr),
93.7, 92.1 (s, Cpyr–Cpyr), 54.7 (s, Cc); δB{H} = 8.3 [2B, d, 1J(B,H) =
Electrochemical measurements
1
149], 5.6 [2B, d, J(B,H) = 150], Ϫ0.5 (s, 1B), Ϫ4.0 (br s, 8B),
Electrochemical measurements were performed in a standard
double-compartment three-electrode cell. Ag/AgCl/[N(C4H9)4]-
Cl (0.1 M in CH3CN) was used as a reference electrode. A
4-mm2 platinum plate and a platinum wire were used as work-
ing and counter electrode, respectively. All measurements were
performed both in acetonitrile and in chloroform with 0.1 M
tetrabutylammonium hexafluorophosphate as supporting elec-
trolyte. Cyclic voltammograms were recorded with a scan rate
1
1
Ϫ11.3 [2B, d, J(B,H) = 159], Ϫ15.9 [4B, d, J(B,H) = 158];
MS-MALDI-TOF: m/z = 774 [Ag(2)2]ϩ and 441 [Ag(2)]ϩ.
Synthesis of 8. To a solution of methylaquocobaloxime
(0.065 g, 0.201 mmol) in 5 mL of methylene chloride, com-
pound 1 (0.104 g, 0.402 mmol) was added. After 5 min of
stirring at room temperature, the orange solution was vacuum-
dried. The red residue was washed twice with 5 mL of ice-cold
methylene chloride–hexane (1 : 1). The solid was dried under
vacuum overnight. Recrystallization from CH2Cl2–C6H14 (1 : 1)
gave red needles. (100 mg, 88%) (Found: C, 32.23; H, 5.54; N,
12.32. C15B9O4H32Co2N5 requires: C, 32.08; H, 5.74; N, 12.47);
ν/cmϪ1: (Car–H) 3035, 3006, (C–H) 2910, (B–H) 2597, 2576,
of 80 mV sϪ1
.
Preparations
Synthesis of 1 BH3 (3). THF BH3 (0.1 mL g, 0.100 mmol)
was added to 1 (0.026 g, 0.100 mmol) in dry toluene (1 ml) at
room temperature, resulting a bright orange solution. δB 8.7
2553, 2528, (C–H) 1564, (C᎐N) 1236 (KBr pellets); δ 18.1
᎐
H
1
1
[1B, d, J(B,H) = 138], 7.4 [1B, d, J(B,H) = 149], Ϫ2.0 (2B),
Ϫ2.7 (2B), Ϫ14.4 [2B, d, 1J(B,H) = 153], Ϫ17.9 [1B, d, 1J(B,H) =
149], Ϫ20.8 [1B, q, 1J(B,H) = 142].
(2H, s, OH), 6.6 (2H, s, N–Cpyr–H), 6.4 (2H, s, Cpyr–Cpyr–H), 4.4
(2H, s, Cc–H), 2.2 (12H, s, –CH3), 0.8 (3H, s, Co–CH3); δH{B}
=
18.1 (2H, s, OH), 6.6 (2H, s, N–Cpyr–H), 6.4 (2H, s, Cpyr–Cpyr
–
H), 4.4 (2H, s, Cc–H), 3.4 (1H, br s, B–H), 3.1 (3H, br s, B–H),
2.2 (12H, s, –CH3), 1.8 (5H, br s, B–H), 0.8 (3H, s, Co–CH3);
δC{H} = 150.0 (s, CH3–Co), 109.5 (s, N–Cpyr), 92.7 (s, Cpyr–Cpyr),
54.3 (s, Cc), 12.1 (s, C–CH3); δB 9.9 (1B), 7.8 (1B), Ϫ2.2 (2B),
Ϫ3.7 (2B), Ϫ13.8 [2B, d, 1J(B,H) = 162], Ϫ20.5 [1B, d, 1J(B,H) =
169].
Synthesis of 1 BF3 (4). (C2H5)2O BF3 (0.016 g, 0.100
mmol) was added to 1 (0.026 g, 0.100 mmol) in CD3Cl (1 ml) at
room temperature, producing a bright orange solution. δH 7.1
(2H, s, N–Cpyr–H), 6.6 (2H, s, Cpyr–Cpyr–H), 4.9 (2H, s, Cc–H);
δC{H} 104.8 (s, N–Cpyr), 92.5 (s, Cpyr–Cpyr), 59.1 (s, Cc); δB 11.8
[1B, d, J(B,H) = 129], 10.7 [1B, d, J(B,H) = 126], 0.6 (s, 1B),
Ϫ0.5 (2B), Ϫ1.2 (2B), Ϫ12.5 [2B, d, 1J(B,H) = 163], Ϫ18.8
[1B, d, 1J(B,H) = 174].
1
1
Synthesis of 9. The procedure was as for 8 but with com-
pound 2 (0.134, 0.402 mmol) as starting material (92 mg, 70%)
(found: C, 39.50; H, 5.84; N, 10.62. C21B9O4H36Co2N5 requires:
C, 39.55; H, 5.69; N, 10.98); ν/cmϪ1 (Car–H) 3097, (Cc–H) 3016,
Synthesis of 2 BF3 (5). (C2H5)2O BF3 (0.016 g, 0.100
mmol) was added to 2 (0.033 g, 0.100 mmol) in CD3Cl (1 ml)
producing a bright orange solution. δH 7.6–7.3 (5H, m, –C6H5),
7.2 (1H, s, N–Cpyr–H), 6.7 (1H, s, N–Cpyr–H), 6.0 (2H, s,
Cpyr–Cpyr–H), 4.9 (1H, s, Cc–H); δC{H} = 135.1, 129.3, 125.2,
125.0 (s, –C6H5), 113.9 (s, N–Cpyr), 101.1 (s, N–Cpyr), 94.1
(s, Cpyr–Cpyr), 93.2 (s, Cpyr–Cpyr), 62.0 (s, Cc–H); δB 14.4 [1B, d,
1J(B,H) = 138], 12.1 [1B, d, 1J(B,H) = 166], 0.6 (s, 1B), 0.1 (2B),
Ϫ1.7 (2B), Ϫ6.1 [1B, d, 1J(B,H) = 165], Ϫ11.6 [1B, d, 1J(B,H) =
166], Ϫ14.2 [1B, d, 1J(B,H) = 171].
(C–H) 2896, (B–H) 2599, 2559, 2530, 2516, (C–H) 1554, (C᎐N)
᎐
1234 (KBr pellets); δH 18.3 (2H, s, OH), 7.6–7.3 (5H, m, Car–H),
6.8 (1H, N–Cpyr–H), 6.6 (1H, N–Cpyr–H), 5.7 (1H, s, Cpyr
–
Cpyr–H), 5.5 (1H, s, Cpyr–Cpyr–H), 4.3 (1H, s, Cc–H), 2.2 (6H, s,
–CH3), 2.1 (6H, s, –CH3), 0.9 (3H, s, Co–CH3); δH{B} 18.3 (2H, s,
OH), 7.6–7.3 (5H, m, Car–H), 6.8 (1H, N–Cpyr–H), 6.6 (1H,
N–Cpyr–H), 5.7 (1H, s, Cpyr–Cpyr–H), 5.5 (1H, s, Cpyr–Cpyr–H),
4.3 (1H, s, Cc–H), 3.6 (2H, br s, B–H), 3.4 (1H, br s, B–H), 2.9
(1H, br s, B–H), 2.2 (6H, s, –CH3), 2.1 (6H, s, –CH3), 1.9 (3H, br
s, B–H), 1.8 (2H, br s, B–H), 0.9 (3H, s, Co–CH3); δC{H} 151.0 (s,
CH3–Co), 149.5, 144.5, 129.1, 128.4 (s, Car), 105.8 (s, N–Cpyr),
117.8 (s, N–Cpyr), 95.2 (s, Cpyr–Cpyr), 94.9 (s, Cpyr–Cpyr), 57.6 (s,
Cc–H), 12.2 (s, C–CH3), 12.0 (s, C–CH3); δB 11.5 [1B, d, 1J(B,H)
Synthesis of 6. Compound 1 (0.054 g, 0.206 mmol) was dis-
solved in CH2Cl2 (5 ml) and Ag[BF4] (0.020 g, 0.103 mmol)
added. The solution was stirred for 30 min and the dark orange
1564
J. Chem. Soc., Dalton Trans., 2002, 1559–1565

View Article Online
= 153], 8.9 [1B, d, 1J(B,H) = 159], Ϫ2.6 (4B), Ϫ8.0 [1B, d,
395, C77; J. Zakrzewski and C. Giannotti, J. Photochem. Photobiol.
1
1J(B,H) = 177], Ϫ13.4 [1B, d, J(B,H) = 145], Ϫ15.1 [1B, d,
A: Chem., 1991, 57, 479; J. Zakrzewski and C. Giannotti, J. Chem.
Soc., Chem. Commun., 1993, 662; J. Zakrzewski and C. Giannotti,
J. Chem. Soc., Dalton Trans., 1993, 1629; J. Zakrzewski and C.
Giannotti, J. Chem. Soc., Chem. Commun., 1993, 1109; M. Cesario,
C. Giannotti, J. Guilhem and J. Zakrzewski, Acta Crystallogr., Sect.
C, 1992, 48, 798.
1J(B,H) = 153].
References
1 M. Lamrani, S. Gómez, C. Viñas, F. Teixidor, R. Sillanpää and
R. Kivekäs, New. J. Chem., 1996, 20, 909.
4 N. I. Pyshnograeva, V. N. Setkina, V. G. Andrianov, Y. T. Struchkov
and D. N. Kursanov, J. Organomet. Chem., 1977, 128, 381; N. I.
Pyshnograeva, V. N. Setkina, V. G. Andrianov, Y. T. Struchkov and
D. N. Kursanov, J. Organomet. Chem., 1978, 157, 431; N. I.
Pyshnograeva, V. N. Setkina, V. G. Andrianov, Y. T. Struchkov and
D. N. Kursanov, J. Organomet. Chem., 1980, 186, 331; N. I.
Pyshnograeva, V. N. Setkina, V. G. Andrianov, Y. T. Struchkov and
D. N. Kursanov, J. Organomet. Chem., 1981, 206, 169.
2 F. Teixidor, S. Gómez, M. Lamrani, C. Viñas, R. Sillanpää and
R. Kivekäs, Organometallics, 1997, 16, 1278; S. Gómez, C. Viñas,
M. Lamrani, R. Kivekäs and R. Sillanpää, Inorg. Chem., 1997, 36,
3565; J. Bertran, C. Viñas, S. Gómez, L. Lamrani, F. Teixidor,
R. Sillanpää and R. Kivekäs, Collect. Czech. Chem. Commun., 1997,
62, 1263; J. Llop, C. Viñas, F. Teixidor, L. Victori, R. Kivekäs and
R. Sillanpää, Organometallics, 2001, 19, 4024.
3 J. Zakrzewski and C. Giannotti, Coord. Chem. Rev., 1995, 140, 169;
C. Janiak and N. Kuhn, Adv. Nitrogen Heterocycl., 1996, 2, 179;
S. Wolowiec and L. Latos-Grazynski, New J. Chem., 1996, 20(9), 939;
M. Cesario, C. Giannotti, J. Guilhem, J. Silver and J. Zakrzewski,
J. Chem. Soc., Dalton Trans., 1997, 47; M. Cesario, C. Giannoti,
J. Guilhem and J. Zakrzewski, Acta Crystallogr. Sect. C, 1992, 48,
798; J. Zakrzewski and C. Giannotti, J. Organomet. Chem., 1990, 385,
C23; J. Zakrzewski and C. Giannotti, J. Organomet. Chem., 1990,
5 N. Kuhn, E. V. Horn, R. Boese and N. Augart, Angew. Chem., Int. Ed.
Engl., 1989, 28, 342.
6 N. I. Pyshnograeva, V. N. Setkina and D. N. Kursanov, Izv. Akad.
Nauk. SSSR Ser. Khim., 1984, 12, 2778; S. P. Best, R. J. H. Clark, A. J.
Deeming, R. C. S. McQuenn, N. I. Powell, C. Acuña, A. J. Arce and
Y. De Sanctis, J. Chem. Soc., Dalton Trans., 1991, 1111; J. Silver, J.
Zakrzewsky, A. Tosik and M. Bukowska-Strzyzewska, J. Organomet.
Chem., 1997, 540(1–2), 169.
7 G. N. Schrauzer, Inorg. Synth, 1968, 11, 61–70.
J. Chem. Soc., Dalton Trans., 2002, 1559–1565
1565