Structural and electronic variations in cobalt-alkyne clusters

Article abstract of DOI:10.1039/b008445j

The complexes [Co₂(μ-η²-HC₂C₆H ₄X-4)(CO)₄(dppm)] (X = H, NMe₂, NO₂, CN or C=C{Ru(PPh₃)₂Cp}) and [Co₂(μ-η²-RC₂C=C

Full text of DOI:10.1039/b008445j

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Structural and electronic variations in cobalt–alkyne clusters†
Tom J. Snaith,a Paul J. Low,*a Roger Rousseau,b Horst Puschmanna and Judith A. K. Howarda
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
b Steacie Institute for Molecular Sciences, 100 Sussex Drive, Ottawa, Ontario, K1A 0R6,
Canada
Received 19th October 2000, Accepted 23rd November 2000
First published as an Advance Article on the web 15th January 2001
2
᎐
The complexes [Co (µ-η -HC C H X-4)(CO) (dppm)] (X = H, NMe , NO , CN or C᎐C{Ru(PPh ) Cp}) and
᎐
2
2
6
4
4
2
2
3 2
2
᎐
[Co (µ-η -RC C᎐C{Ru(PPh ) Cp})(CO) (dppm)] (R = H or SiMe ) have been prepared and characterised
᎐
2
2
3
2
4
3
crystallograpically. Electrochemical and spectroscopic evidence has been used to help formulate an empirical
MO scheme and thereby explain the nature of the electronic interactions that occur between the pendant group
and the Co2C2 cluster core.
Introduction
Results
In recent times the search for molecular systems that are able
to convey electronic effects between two or more remote sites
efficiently has become an area of increasing endeavor, primarily
as a result of the interest in the construction of molecular
scale electronic devices.1–5 Electronic interactions between the
sites may be manifest in the spectroscopic or electrochemical
response of the assembly, and in appropriate cases the mechan-
ism of interaction may be identified, or at least alluded to, by
analysis of these parameters.
Synthesis
The series of complexes [Co2(µ-η2-HC2C6H4X-4)(CO)4(dppm)]
were prepared by reaction of the appropriately substituted
phenyl acetylene HC2C6H4X-4 with [Co2(CO)6(dppm)] (X = H,
56%, 1; NMe2, 61%, 2; NO2, 59%, 3; CN, 89%, 4).18 The NMR
data for these complexes were unremarkable (Table 1). The
cluster bound proton Co2C2H was found as a triplet (JHP = 7–8
Hz) near δ 5.8, the precise position of which showed some
dependence on the electronic nature of the X group (1, 5.78; 2,
5.72; 3, 5.84; 4, 5.83). The positive and negative ion electrospray
mass spectra (ES MS) collected in methanolic solutions
containing a small amount of NaOMe were characterised by
highest molecular weight ions corresponding to [M ϩ Na]ϩ and
[M Ϫ H]Ϫ respectively.19
Interactions between identical mono-nuclear or cluster-based
redox active probe groups bridged by ynyl and polyynyl spacers
have been studied extensively.6–12 The (C᎐C) π* orbitals are
᎐
᎐
n
generally too high in energy to interact significantly with filled
metal-based orbitals in the systems examined to date, and
classical back-bonding mechanisms are unlikely to play any
significant role in the electronic structure of these complexes.
Instead, it has been concluded that the strong electronic com-
munication in these systems is a result of efficient mixing
between filled metal fragment and polyyne-based orbitals. As
a result of these strong orbital interactions the properties of the
assembly may differ significantly from those of the individual
components.
The metal acetylide substituted analogue [Co2(µ-η2-Me3Si-
᎐
C C H C᎐C{Ru(PPh ) Cp})(CO) (dppm)]
5 was prepared
᎐
2
6
4
3
2
4
᎐
᎐
from the reaction of [Ru(C᎐CC H C᎐CSiMe )(PPh ) Cp] with
᎐
᎐
6
4
3
3 2
[Co2(CO)6(dppm)] in benzene, and isolated as a tan coloured
crystalline solid in good yield. The NMR spectra were charac-
terised by the resonances expected for the SiMe3 and Cp ligands
and the 13C resonances of the cluster core and alkynyl carbon
centres, which have been assigned on the basis of the magnitude
of the JCP coupling constants (Table 1). The positive and
negative ion mass spectra of 5 are characterised by [M ϩ Na]ϩ
and [M Ϫ H]Ϫ ions, respectively.
Several recent studies have identified unusual spectroscopic
and electrochemical features associated with complexes which
feature two dissimilar redox active fragments linked by (poly)-
ynyl bridges and interpreted in terms of varying degrees of
Complexes in which the cobalt–carbon carbonyl cluster
was attached directly to a pendant metal acetylide moiety were
readily prepared from the cluster substituted terminal alkynes
electronic “communication” between the fragments.13–16 In
2
᎐
᎐
the case of [Co (µ-η -Me SiC C᎐CC᎐C{Ru(PPh ) Cp})(CO) -
᎐
᎐
2
3
2
3
2
4
2
᎐
᎐
(dppm)] and [Co (µ-η -Me SiC᎐CC C᎐C{Ru(PPh ) Cp})-
᎐
᎐
2
2
3
2
3 2
᎐
[Co (µ-η -RC C᎐CH)(CO) (dppm)] (R = SiMe 7 or H, 8).
᎐
2
2
4
3
(CO)4(dppm)] electronic interactions between the mononuclear
and cluster centres were found to result from strong filled
orbital–filled orbital interactions along the length of the
Treatment of 7 or 8 with one equivalent of [RuCl(PPh3)2Cp] in
the presence of NH4PF6 in methanol afforded the vinylidene
complex [Ru{=C᎐C(H)[C Co (R)(CO) (dppm)]}(PPh ) Cp]PF
6
which was not isolated but rather deprotonated in situ by
addition of a methanolic solution of sodium methoxide to
afford the strikingly green complexes [Co (µ-η -RC C᎐C{Ru-
᎐
2
2
4
3
2
᎐
Ru(C᎐C) C Co fragment, which gave rise to a delocalised
᎐
n
2
2
᎐
HOMO with significant Ru, C᎐C and cluster character, and the
᎐
dipolar nature of the ground state.17 Now, in order to explore
further the role played by the bridge in these systems, we have
examined the structural, electrochemical and spectroscopic
2
᎐
᎐
2
2
(PPh3)2Cp})(CO)4(µ-dppm)] (R = SiMe3 9 or H, 10) (Table 1).
For 9, the H and 13C NMR spectra contained the anticipated
1
properties of a series of compounds [Co2(µ-η2-HC2C6H4X-4)-
resonances of the SiMe3 and Cp ligands, while a triplet reson-
ance at δC 36.88 (JCP = 20 Hz) was readily assigned to the
methylene carbon of the dppm ligand. The quaternary carbons
of the alkynyl bridge and Co2C2 cluster core were not observed.
The ES MS of 9 obtained from solutions containing a small
amount of NaOMe as an in situ aid to chemical ionisation
afforded the ions [M ϩ Na]ϩ, [M]ϩ and a fragment ion derived
᎐
(CO) (dppm)] (X = H, NMe , NO , CN or C᎐C[Ru(PPh ) Cp])
᎐
4
2
2
3 2
and compared them with the simple alkynyl complexes [Co2-
2
᎐
(µ-η -RC C᎐C{Ru(PPh ) Cp})(CO) (dppm)] (R = H or SiMe ).
᎐
2
3
2
4
3
† Dedicated to the memory of Ron Snaith, a much loved product of
this Department and father to Tom.
292
J. Chem. Soc., Dalton Trans., 2001, 292–299
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b008445j

Table 1 Spectroscopic data for complexes 1–5, 9, 10
Analysis (%)b
δ( J/Hz)
Yield
(%)
Complex
Colour
Red
ν(CO)a/cmϪ1
C
H
N
ES MSc m/z
1H NMRe
13C NMRe
1
56
2027vs, 1999s, 1975s, 1956w
62.56
(62.03)
4.28
(3.94)
ES(ϩ): 739, [M ϩ Na]ϩ;
711, [M ϩ Na Ϫ CO]ϩ;
688, [M Ϫ CO]ϩ
3.10 (dt, 1H, JHP = 13, JHH = 10,
CHP2); 3.61 (dt, 1H, JHP = 13,
JHH = 10, CHP2); 5.78 (t, 1H,
JHP = 7, Co2C2H); 7.20–7.67 (m,
25H, Ph)
41.23 (t, JCP = 21, CH2P2); 96.30 [t,
JCP = 16, C(3/4)]; 126.09–142.87 (m, Ph);
203.00, 207.04 (2 × br, CO)
ES(Ϫ): 715, [M Ϫ H]Ϫ;
687, [M Ϫ H Ϫ CO]Ϫ
2
3
4
5
9
Purple
Dark red
Purple
Tan
61
59
89
60
2023vs, 1995s, 1971vs, 1952w
2030s, 2003vs, 1970s, 1963w
61.55
(61.88)
4.39
(4.38)
1.67
(1.84)
ES(ϩ): 782, [M ϩ Na]ϩ;
759, [M]ϩ; 731, [M Ϫ CO]ϩ
ES(Ϫ): 758, [M Ϫ H]Ϫ;
730, [M Ϫ H Ϫ CO]Ϫ
2.98 (m, 6H, 2 × Me); 3.05 (dt,
1H, JHP = 13, JHH = 10, CHP2);
3.55 (dt, 1H, JHP = 13, JHH = 10,
CHP2); 5.72 (t, 1H, JHP = 8,
Co2C2H);6.70–7.58(m,24H,Ph)
72.64 [s, C(3/4)]; 97.98 [t, JCP = 16, C(3/4)];
112.20 [s, C(31)]; 128.17 – 149.22 (m, Ph);
203.34, 207.63 (2 × br, 2 × CO)
57.80
(58.36)
3.57
(3.57)
1.78
(1.84)
ES(ϩ): 784, [M ϩ Na]ϩ
ES(Ϫ): 760, [M Ϫ H]Ϫ;
732–704, [M Ϫ H Ϫ
nCO]Ϫ (n = 1 or 2)
3.15 (dt, 1H, JHP = 12, JHH
10, PCHP2); 3.68 (dt, 1H, JHP
=
=
41.75 (t, JCP = 21, CH2P2); 74.21 [t, JCP = 8,
C(3/4)]; 91.78 [t, JCP =16, C(3/4)]; 123.73,
[s, C(31)]; 128.30–152.37 (m, Ph); 202.18,
206.33 (2 × br, 2 × CO)
12, JHH = 10, PCHP2); 5.84
(t, 1H, JHP = 8, Co2C2H), 7.23–
8.15 (m, 24H, Ph)
2030s, 2006sh, 2002vs,
1978s, 1961w
61.30
(61.54)
3.62
(3.64)
1.84
(1.89)
ES(ϩ): 795, [M ϩ Naϩ
NCMe]ϩ, 764, [M ϩ Na]ϩ
3.14 (dt, 1H, JHP = 12, JHH = 10,
CHP2); 3.67 (dt, 1H, JHP = 12,
JHH = 10, CHP2); 5.83 (t, 1H,
JHP = 7, Co2C2H); 7.21–1.69 (m,
24H, Ph)
41.60 (t, JCP = 21, CH2P2); 73.93 [t, JCP = 8,
C(3/4)]; 92.43 [t, JCP = 17, C(3/4)]; 108.47
[s, CN)]; 119.78 [s, C(31)]; 128.22–152.37
(m, 24H. Ph); 202.29, 206.76 (2 × br,
2 × CO)
(2075w, br), 2019s, 1993vs,
1968s, 1948w
66.19
(66.36)
4.71
(4.70)
ES(ϩ): 1525, [M ϩ Na]ϩ
ES(Ϫ): 1501, [M Ϫ H]Ϫ
0.35 (s, 9H, SiMe3); 3.22 (dt,
JHP = 13, JHH = 11, CHP2); 3.46
(dt, JHP = 13, JHH = 11, CHP2);
4.33 (s, 5H, Cp); 6.93–7.52 (m,
54H, Ph)
0.89 (s, SiMe3); 35.61 (t, JCP = 21, CH2P2);
85.27 (s, Cp); 88.04 [t, JCP = 10, C(4)];
107.23 [t, JCP = 8, C(2)]; 115.20 (s, C(3);
116.68 [t, JCP = 23, C(1)]; 127.49–139.62
(m, Ph); 203.36, 207.78 (2 × br, CO)
Dark green 51
(2028w), 2004s, 1984vs,
1957s, 1941w
64.75
(64.85)
4.57
(4.66)
ES(ϩ): 1449, [M ϩ Na]ϩ;
1426, [M]ϩ; 1398, [M Ϫ CO]ϩ
ES(Ϫ): 1425, [M Ϫ H]Ϫ
0.29 (s, 9H, SiMe3); 3.36 (dt,
1H, JHP = 12, JHH = 10, CHP2);
3.65 (dt, 1H, JHP = 12, JHH = 10,
CHP2); 4.23 (s, 5H, Cp); 6.78–
7.43 (m, 50H, Ph)
1.46 (s, SiMe3); 36.88 (t, JCP = 20, CH2P2);
86.24 (s, Cp); 112.29 [s, C(1/2)]; 127.19–
139.18 (m, Ph); 206.51, 207.17 (2 × br, CO)
d
10
Dark green 58
2030s, 2004w, 1981vs, 1954s
ES(ϩ): 1377, [M ϩ Na]ϩ;
1349, [M ϩ Na Ϫ CO]ϩ
ES(Ϫ): 1353, [M Ϫ H]Ϫ
3.20 (br, 1H, CHP2), 3.45 (br,
1H, CHP2); 4.28 (s, 5H, Cp);
5.63 (s, 1H, Co2C2H); 7.11–7.51
(m, 50H, Ph)
39.95 (t, JCP = 18, CH2P2); 86.00 (s, Cp);
112.85 [s, C(1/2)]; 127.46–139.05 (m, Ph);
204.81, 208.42 (2 × br, CO)
a
In cyclohexane, ν(C᎐C) given in parentheses. b Required values given in parentheses. c In MeOH containing NaOMe. d Despite repeated attempts, an accurate microanalysis could not be obtained. e In CDCl3 and labelled
᎐
᎐
in accord with the corresponding ORTEP figure.

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Table 2 Selected bond lengths (Å) and angles (Њ) for complexes 1–5, 9, 11
1
2
3
4
5
9
11a
Ru–C(1)
Ru–P(3)
Ru–P(4)
2.003(2)
2.2849(7)
2.3021(7)
1.218(4)
1.432(3)b
1.465(3)
1.359(3)
2.5116(5)
1.948(2)
1.997(3)
1.981(2)
1.971(2)
1.411(3)
1.383(3)
1.408(3)
1.400(3)
1.387(3)
1.410(4)
2.012(2)
2.2853(6)
2.2935(6)
1.223(3)
1.410(3)
1.994(5)
2.297(2)
2.301(2)
1.202(8)
1.432(7)
C(1)–C(2)
C(2)–C(3)
C(3)–C(34)
C(3)–C(4)
Co(1)–Co(2)
Co(1)–C(3)
Co(1)–C(4)
Co(2)–C(3)
Co(2)–C(4)
C(31)–C(32)
C(32)–C(33)
C(33)–C(34)
C(34)–C(35)
C(35)–C(36)
C(36)–C(31)
1.468(2)
1.348(2)
1.465(3)
1.350(3)
1.458(3)
1.356(3)
1.463(2)
1.351(3)
1.368(3)
2.4947(4)
1.972(2)
1.981(2)
1.990(2)
1.986(2)
2.4875(3)
1.9727(15)
1.9508(16)
1.9470(15)
1.9660(16)
1.391(3)
1.390(2)
1.403(2)
1.401(2)
1.395(2)
2.4657(4)
1.9857(19)
1.9577(19)
1.9434(19)
1.9674(19)
1.410(3)
1.391(3)
1.402(3)
1.404(3)
1.384(3)
2.4950(4)
1.9591(14)
1.9609(14)
1.9591(14)
1.9609(14)
1.394(3)
1.387(3)
1.412(3)
1.406(3)
1.392(3)
2.4707(3)c
1.9516(14)
1.9662(15)
1.9516(14)c
1.9662(15)c
1.392(3)
1.41(1)
1.392(3)
1.405(3)
1.402(3)
1.386(3)
1.370(7)
1.363(8)
1.37(1)
1.373(8)
1.400(8)
1.388(3)
1.409(3)
1.387(3)
1.397(3)
Ru–C(1)–C(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
171.7(2)
174.6(3)b
138.2(2)b
166.61(18)
177.6(2)
144.7(2)
175.9(4)
175.0(9)
142.60(15)d
143.41(18)d
141.81(17)d
144.82(17)d
a From ref. 14. b Read C(31) for C(3). c Read Co(1)#1 for Co(2). d Read C(34) for C(2).
from the loss of a carbonyl ligand [M Ϫ CO]ϩ. The negative
ion spectrum of the same solution contained the [M Ϫ H]Ϫ ion
at m/z 1425. Metallation of 8 by the Ru(PPh3)2Cp fragment
occurred only at the terminal acetylenic position as indicated by
NMR data, with Co2C2H and Cp resonances being detected
along with those of the dppm ligand. Again, the quaternary
carbons could not be identified. The positive and negative ion
ES MS of 10 contained [M ϩ Na Ϫ nCO]ϩ (n = 0 or 1) and
[M Ϫ H]Ϫ ions, respectively.
Molecular structures
Single-crystal X-ray diffraction studies were performed on
complexes 1–5 and 9. Important bond lengths and angles are
summarised in Table 2. These structural studies serve the
obvious purpose of confirming the structures and highlight
the role of steric influences on the overall molecular shape.
As the primary interest in these complexes lies in the
exploration of electronic interactions between the various
fragments, it is tempting to search the metrical parameters
Fig. 1 Molecular structure of complex 1 showing the atom labelling
scheme. Phenyl hydrogen atoms are omitted.
carefully for evidence of structural distortion, particularly
᎐
in the M–C, C᎐C and C–C Co bond lengths. However,
᎐
2
2
the structural perturbations associated with these fragments
in different electronic environments are subtle and such
differences as there are often fall within the range of the
experimental errors.20 Suffice to say any conclusions based
solely on very small differences in structure must be made with
caution.
[Co2(ꢀ-ꢁ2-HC2C6H4X-4)(CO)4(ꢀ-dppm)] (X ؍ H, 1; NMe2, 2;
NO2, 3 or CN, 4). The molecular structures of the phenyl
substituted Co2C2 clusters 1–4 are illustrated in Figs. 1–4. The
dppm ligand occupies an exo position relative to the pendant
phenyl ring, in response to steric demands. Across the series,
and within experimental error, the C(3)–C(4) bond length is
invariant (Table 2). The Co(1)–Co(2) separations range from
2.4657(4) (X = p-NMe2, 2) to 2.4950(4) Å (X = p-NO2, 3).
Remaining cluster parameters including Co(1,2)–C(3,4) are
comparable across the series and show no clear variation with
the electronic nature of the phenyl substituent. Most of the
differences in C–C bond lengths within the C(31)–(36) phenyl
ring are within the 3σ level of confidence, although C(32,35)–
C(33,36) appear to be relatively short (Table 2).
Fig. 2 Molecular structure of complex 2. Details as in Fig. 1.
Perhaps the most significant aspect of the structural
determination is the orientation adopted by the phenyl ring
Fig. 3 Molecular structure of complex 3. Details as in Fig. 1.
294
J. Chem. Soc., Dalton Trans., 2001, 292–299

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Fig. 4 Molecular structure of complex 4. Details as in Fig. 1.
Fig. 6 Molecular structure of complex 9. Details as in Fig. 1.
2
᎐
[Co (ꢀ-ꢁ -Me SiC C᎐C{Ru(PPh ) Cp})(CO) (ꢀ-dppm)]
9.
᎐
2
3
2
3
2
4
An ORTEP projection of complex 9 is shown in Fig. 6 and
selected bond lengths and angles are given in Table 2. The
dppm ligand spans the Co(1)–Co(2) vector occupying co-
ordination sites approximately trans to the C(1) and C(2)
atoms of the bridge. This presumably arises from the steric con-
straints imposed by the bulky triphenylphosphine ligands
of the ruthenium fragment which force the dppm ligand to
occupy a site closer to the bulky trimethylsilyl group capping
the cluster than might otherwise be expected. Further to relieve
steric interactions between the Cp, PPh3 and cluster bound
SiMe3 ligands, the Ru–C(1)–C(2) bond angle is distorted
from linearity [166.6(2)Њ]. The P(3)–Ru–P(4) bond angle
[100.78(2)Њ] is somewhat smaller than in the case of 11. Other
bonding parameters of the mononuclear ruthenium frag-
ment are comparable with those of 11 with the average
Ru–Cp (2.240 Å), Ru–C(1) [2.012(2) Å] and Ru–P bond
lengths being equivalent within experimental error. The C(2)–
Fig. 5 Molecular structure of complex 5. Details as in Fig. 1.
relative to the plane normal to the Co–Co vector. In the case
of complexes 1 and 2 the C(4)–C(3)–C(34)–C(33) torsion angles
are 46.2 and 29.5Њ, respectively. For 3 and 4 which feature the
electron-withdrawing NO2 and CN groups the phenyl ring lies
in the crystallographic mirror plane and the C(4)–C(3)–C(34)–
C(33) torsion angle is therefore 180Њ in both cases. There are no
obvious close intermolecular separations within the unit cells
of 1–4 and given the correlation with the electronic properties
of the substituent it appears unwise to attribute categorically
this variation in orientation of the phenyl ring simply to crystal
packing forces. We defer further comment on this point to the
Discussion (see below).
C(3) bond length [1.410(3) Å] is the same as the analogous
2
᎐
᎐
bond in [Co (µ-η -(Me SiC᎐CC C᎐C{Ru(PPh ) Cp})(CO) -
᎐
᎐
2
3
2
3
2
4
(dppm)] [1.407(3) Å].17 Similarly, there are no significant
deviations in the bond lengths of 9 when compared with those
of the analogous compound [Co (µ-η -HC C᎐C{Ru(PPh ) -
2
᎐
᎐
2
2
3 2
Cp})(CO)4(µ-dppm)] 10,25 although the relatively low quality
of the data obtained in the latter case precludes detailed com-
parisons. The elongation of the Co(1)–C(3), Co(2)–C(4) bonds
and contraction of Co(1)–C(4), Co(2)–C(3) bonds in 5 relative
to 9 is consistent with the small twist in the direction of the
Co(1)–Co(2) vector relative to that of C(3)–C(4) (89.6, 9;
91.9Њ, 5).
2
᎐
[Co (ꢀ-ꢁ -Me SiC C H C᎐C{Ru(PPh ) Cp})(CO) (ꢀ-dppm)]
᎐
2
3
2
6
4
3
2
4
5. The structure of compound 5 is shown in Fig. 5. It clearly
shows a mononuclear half-sandwich cyclopentadienyl bis(tri-
phenylphosphine) ruthenium fragment attached to the Co2C2
tetrahedron via a phenylacetylide bridge. The conformation
adopted is that which minimises the steric interactions between
the dppm ligand and trimethylsilyl group. The cluster core
parameters are consistent with those of 1 and 2, although
twisting of the C(3)–C(4) vector relative to the Co–Co bond
makes direct comparison of the metrical parameters difficult.
Theoretical analysis of dicobalt alkyne complexes suggests that
the orientation of the alkyne bond relative to the Co–Co vector
is sensitive to electronic effects,21 and pronounced rotation of
the alkyne vector has been observed following oxidation of
[Co2(µ-η2-MeC2Me)(CO)2(dppm)2].22
Electrochemistry
The phenyl-substituted cluster 1 displayed two redox waves, the
reversibility of which improved at sub-ambient temperatures.
At Ϫ30 ЊC a fully chemically reversible reduction and an
oxidation process which was only partially chemically reversible
were observed (Table 3). Complex 2 displayed well behaved
electrochemical behaviour at room temperature and the cyclic
voltammogram displayed two closely spaced oxidation waves
and a single reversible reduction process. In contrast, the NO2
analogue 3 displays a single oxidation wave and two reduction
Ϫ
The Ru–P(3,4) and C(1)–C(2) bond lengths found in complex
waves. The reduction of 3 to 3 was found to be reversible
᝽
᎐
5 are in better agreement with the analogous bonds in [Ru(C᎐C-
when examined in isolation and on the basis of the trends in
EЊ found throughout this series of compounds is assigned to the
Co2C2 core. The second totally irreversible reduction at Ϫ1.49
V is assigned to the C6H4NO2-4 moiety. The CV of 5 appears
grossly similar to that of 2 and involves a single reversible one-
electron reduction and two one-electron oxidation processes.
Studies of the first oxidation process in isolation revealed it to
be fully reversible.
᎐
C6H4NO2-4)(PPh3)2Cp] 1123 (Table 2) than [Ru(C᎐CC H )-
᎐
᎐
6
5
(PPh3)2Cp].24 However, the C(32)–C(33) [1.383(3) Å] and
C(35)–C(36) [1.387(3) Å] separations are shorter than C(31)–
C(32,36) [1.411(3); 1.410(4) Å] and C(34)–C(33,35) [1.408(3);
1.400(3) Å]. Bond length alternation is not apparent in 11. The
plane of the phenyl ring in 5 bisects the P(1)–Ru–P(2) bond
angle and lies in the pseudo mirror plane of the ruthenium
fragment. The C(4)–C(3)–C(34)–C(33) torsion angle is of
comparable magnitude, but opposite sign (Ϫ48.4Њ), to that
found for 1.
2
᎐
Compounds [Co (µ-η -Me SiC C᎐CSiMe )(CO) (dppm)] 6,
᎐
2
3
2
3
4
7 and 8 form a structurally similar series containing a single
Co2C2 redox-active site bearing a pendant acetylide ligand.
J. Chem. Soc., Dalton Trans., 2001, 292–299
295

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Table 3 Electrochemical data for complexes 1–3 and 5–10a
Discussion
EЊOx1/V (ipa :ipc)
Complex [∆Ep]/V
EЊOx2/V (ipa :ipc)
[∆Ep]/V
EЊRed1/V (ipa :ipc)
[∆Ep]/V
Computational work has shown that the visible spectra of
[Co2(µ-RC2RЈ)(CO)4(dppm)] complexes feature three distinct
absorption bands corresponding to excitations from the Co2C2
centered HOMO to unoccupied Co2C2 based unoccupied
orbitals.17 The position of the visible absorption bands of
complexes such as 1–10 therefore provides an indication of
the degree of orbital reorganisation within the cluster core
and hence a measure of the interactions between the various
substituents and the cluster core.
1
2
0.79 (0.74) [0.42]
0.46 (1.0)b [0.21]
0.85 (0.64) [0.22]
0.47 (1.0)b [0.21]
0.90 (1.0) [0.15]
0.97 (1.0) [0.23]
0.95 (0.59) [0.20]
0.21 (1.0)b [0.21]
0.17 (1.0)b [0.31]
Ϫ1.73 (1.0) [0.45]
Ϫ1.80 (1.0) [0.24]
Ϫ1.07 (1.0) [0.20]d
Ϫ1.76 (1.0) [0.24]
Ϫ1.53 (1.0) [0.14]
Ϫ1.65 (1.0) [0.24]
Ϫ1.60 (0.95) [0.21]
Ϫ1.98 (1.0) [0.26]
Ϫ2.08 (0.70) [0.36]
0.66c [0.27]
0.75c [0.21]
3
5
6
7
8
9
0.59c [0.32]
0.40c [0.29]
The absorption spectra of complexes 1–3 exhibited a pro-
nounced red shift in the energy of the visible band and
increased absorption coefficient in comparison with those
of the ethynyl substituted clusters 6–8. For 2, the electronic
spectral data, together with the electrochemical measurements,
indicate that the HOMO for the NMe2 substituted cluster
lies at higher energy than for the simple phenyl cluster 1. The
UV-Vis spectrum of 3, which features the electron-withdrawing
NO2 group, has a curiously similar λmax value to that of 2.
Given the low potential of the first reduction potential of 3
and comparable oxidation potentials of 1 and 3, the red shift
observed in the spectrum of 3 relative to 1 is attributed to
transitions from the HOMO to the relatively low-lying LUMO.
The solvatochromic behaviour of complexes 3 and 4 is consist-
ent with a large change in polarity of the excited state relative
the ground state. The other complexes reported in this
work feature a more pronounced ground state dipole [Xδϩ (or
Ruδϩ)→Co2C2δϪ] and negligible solvatochromic behaviour,
which is consistent with a more limited change in dipole
moment upon excitation. We therefore suggest that the LUMOs
of the complexes examined in this study are predominantly
centred on the Co2C2 cluster core. Cluster centred LUMOs were
also found in our earlier study on related (poly)ynyl bridged
systems.17
10
a All data collected from thf solutions containing 0.1 M [nBu4N]PF6
as a supporting electrolyte at Ϫ30 ЊC. Scan rate 100 mV sϪ1, platinum
disk working electrode, platinum wire counter and pseudo reference
electrodes. Internal ferrocene–ferrocenium reference couple at 0.56 V.
EЊOx1 = first oxidation potential; EЊOx2 = second oxidation potential;
EЊRed1 = first reduction potential; EЊRed2 = second reduction potential;
ipa :ipc = ratio of anodic to cathodic peak current; ∆Ep = peak-to-peak
separation. b Measured from the isolated wave. c Unresolved. d Also
EЊRed2 Ϫ1.49 (unresolved) [0.47].
In each case the CV response was characterised by a single
oxidation and a single reduction process. Compounds 6 and 7,
which feature the cluster bound SiMe3 group, exhibited
chemically reversible redox chemistry, while the less sterically
hindered 8 displayed less chemically reversible redox behaviour,
even at low temperature.
For each of the RuII-substituted complexes 9 and 10 a single
reduction, which was reversible only in the case of the
SiMe3 cluster capped complex 9, was observed. The oxidation
sweeps contained two waves, the first of which were shown to
be reversible by switching the direction of the voltammetric
sweep prior to the onset of the second oxidation. In the
case of 9 the second oxidation is complicated by an ECE
(electrochemical step–chemical step–electrochemical step)
The frontier orbital composition of the tetrahedral
core common to all the compounds described in this work has
extensively been studied by Hoffmann and others.26 The
HOMO was found to present a p-type orbital fragment parallel
to the Co–Co vector. A qualitative diagram showing this
HOMO interacting with the highest occupied orbital of a
phenyl group is illustrated in Fig. 7. The Figure shows that the
Cpπ orbital interactions which occur along the C(3)–C(34) bond
(as defined in Figs. 1–6) contain both occupied bonding and
occupied anti-bonding components. Thus, to a first approxi-
mation, C(3)–C(34) should be essentially a single (σ) bond with
no net π-bonding contribution. However, the net π-orbital
contribution on C(34), and thus the degree to which these
π-bonding and π-antibonding components cancel each other
out, is sensitive to the electronic nature of the para substituent
via the introduction of mixing effects amongst the benzene-
type orbitals.27 In the most qualitative of terms, electron-
withdrawing groups will reduce the π-electron density at C(34),
whereas electron-donating groups will increase the π density
at the same atom. The corollary to this is that electron-
withdrawing groups should increase the net π-bonding inter-
action along the C(3)–C(34) bond, whereas the presence of
electron-donating groups will lead to the opposite behaviour.
Thus, for complexes featuring electron-withdrawing groups
one might expect a preferential orientation of the phenyl
group in a configuration which leads to better π-orbital over-
lap with the cluster, whereas for electron-donating groups
one should expect relatively free rotation around the C(3)–
C(34) bond with the lowest energy conformation being dictated
largely by steric considerations. We note that similar sugges-
tions have been offered previously to explain the restricted
rotation of the phenyl group observed in the alkyne complex
[Co2(µ-η2-HC2Ph)(CO)6].28
process as evidenced by the ratio of peak currents ipa2 ≈ 2ipa1
.
The electrochemical data suggest that the ruthenium() centres
in 9 and 10 result in systems that are more easily oxidised and less
easily reduced than 2, which contains the C6H4NMe2-4 group.
Electronic spectra
The visible absorption spectrum of complex 1 exhibited a dis-
tinct visible d–d type band near 543 nm. For 2 the presence
of the NMe2 group resulted in a red shift in the lowest
energy absorption band (565 nm). In the case of 3 the lowest
energy absorption at 552 nm (CH2Cl2) displayed significant
solvatochromic behaviour [λmax = 543 (cyclohexane), 563 nm
(DMF)]. In addition an intense new absorption was observed
at 424 nm and is attributed to π–π* transitions associated with
the C6H4NO2-4 group. A similar effect of smaller magnitude
was found for the cyano substituted complex 4 [λmax(cyclo-
hexane) = 540, (DMF) = 550 nm]. Exchange of the organic
substituents on the aromatic ring for the organometallic
᎐
fragment Cp(PPh ) RuC᎐C (5) gave a spectrum similar in
᎐
3
2
profile to that of the C6H4NMe2-4 substituted cluster, with a
smaller absorption coefficient for the visible transitions.
The UV-Vis spectra of complexes 6–8 were dominated by the
characteristic UV bands below 350 nm associated with π–π*
transitions within the phenyl groups of the bisphosphine
ligand. The visible region contained broad low intensity absorp-
tion bands with λmax between 450 and 500 nm.
᎐
Complexes 9 and 10 with the Cp(PPh ) RuC᎐C moiety and
᎐
3
2
SiMe3 or H groups adorning the carbon vertices of the C2Co2
core respectively show very similar absorption profiles. The
relatively intense absorptions in the visible region (λmax = 600,
9; 599 nm, 10) are substantially red shifted in comparison
with those found in the spectra of 6, 7 and 8. The spectra of
6–10 showed virtually no solvatochromic behaviour.
In keeping with this qualitative MO description, the solid
state structures of complexes 1 and 2 illustrate a re-orientation
296
J. Chem. Soc., Dalton Trans., 2001, 292–299

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᎐
Fig. 8 Interaction diagram for the Cp(PPh3)2RuC᎐C and aryl π
᎐
systems [adapted from ref. 30].
᎐
phenyl acetylide ring of Ru(C᎐CPh)(PPh ) Cp is disordered
᎐
3
2
over two sites,24 each conjugated with one of the two ruthenium
Fig. 7 Pictorial representation of the π-interactions between the Co2-
(alkyne)(CO)6 and aryl fragments.
dπ components.
A qualitative examination of Figs. 7 and 8 suggests that
frontier orbital overlap between the Cp(PPh ) RuC᎐CC H and
᎐
᎐
3
2
6
4
Co2C2 moieties will occur optimally when the C6H4 plane
bisects both the P(3)–Ru–P(4) bond angle and the Co–Co
bond. Therefore the geometry of this complex in the solid
of the plane of the phenyl ring from that in which orbital
overlap would occur and indicates that a limited degree of
π character is associated with the C(3)–C(34) bond. Likewise,
the introduction of an electron-withdrawing substituent in the
para position (3, 4) gives complexes in which the phenyl ring
is found lying in the crystallographic mirror plane, bisecting
the Co–Co bond and co-planar with the C(3)–C(4) vector.
Furthermore we note the contraction of the C(34)–C(3) bond
in 3 [1.458(3) Å] relative to the other members of the series, and
that the C(31)–N separation [1.460(2) Å] is significantly shorter
᎐
state (Fig. 5) suggests that while the RuC᎐CPh π system is
᎐
conjugated, conjugation does not extend to the cluster. This
proposal is supported by the relevant bond lengths, with 5
featuring a shorter C(2)–C(31) bond [1.432(3) Å] than the
᎐
simple phenylacetylide complex [Ru(C᎐CC H )(PPh ) Cp]
᎐
6
5
3 2
[1.456(4) Å]24 and a C(34)–C(3) bond length identical to that
found in 1 (Table 2). The lack of an extended conjugation
π system is likely the reason that the spectral properties of 6
are not as prominent as those of 9 and 10 (see below).
than that found in nitrobenzene [1.492 Å].29
᎐
Complex 5 may be considered to be a RuC᎐CPh fragment
᎐
The electronic structures of the complexes [Co2(µ-η2-RC2-
fused at the para position to a Co2C2 tetrahedral core. While
᎐
᎐
(C᎐C) {Ru(PPh ) Cp})(CO) (dppm)] (n = 1, R = C᎐CSiMe ;
᎐
᎐
n
3
2
4
3
᎐
it has been established that Ru–C and C᎐C bond lengths are
᎐
n = 2, R = SiMe3) have been described recently.17 In both cases
relatively insensitive probes of the electronic character of
the HOMO was shown to include significant contributions
᎐
[Ru(C᎐CR)(PR ) Cp] complexes, the Ru–P bond lengths are
᎐
᎐
᎐
from the Ru(PR ) Cp, (C᎐C) and the Co C cluster core, and
3
2
3 2 n 2 2
more susceptible to minor changes in the electron density of the
metal centre.23,24 The structural information presented above
is consistent with a decrease in the electron density at the Ru
as a result is extensively delocalised across the entire molecule.
This mixing was shown to destabilise the predominantly
Co2C2-centred orbital with respect to that of the parent cluster,
which accounts for the red-shifted optical spectra when
compared with those of simple [Co2(µ-RC2RЈ)(CO)4(dppm)]
species. Partial charge transfer from the Ru-based electron
donor to the cluster-based electron acceptor in the ground state
results in an accumulation of negative charge at the Co2C2
core and a depletion of charge at the mononuclear terminus,
while the charge on the acetylenic bridge remains essentially
invariant. The structural and spectroscopic similarity of these
complexes to 9 and 10 suggests that an analogous mixing of
in 5 relative to that of the simple phenyl acetylide complex
᎐
[Ru(C᎐CPh)(PPh ) Cp]. This observation, together with the red
᎐
3
2
shift in the lowest energy absorption band associated with the
cobalt fragment and the electrochemical data, clearly indicates
that there is a degree of interaction between the mononuclear
ruthenium fragment and the cluster core.
᎐
The orbital structure of the Cp(PPh ) RuC᎐CPh fragment
᎐
3
2
has been established, and in the orientation found in the solid
state structure of complex 5 the ruthenium dπ component
perpendicular to the plane of the ring is able to interact with the
HOMO of the phenyl group (Fig. 8).30 As with the interaction
between the phenyl ring and the C2Co2 cluster, the Cpπ orbitals
along the C(2)–C(31) bond in 5 have both filled bonding and
antibonding contributions and thus, to a first approximation,
a net π-bond order of zero. However, as before substitution on
the benzene ring and the resulting electronic effects can modify
the degree to which these components cancel each other and
lead to a preferred conformation that allows for a small net
bonding π interaction. It is interesting that in the solid state the
᎐
RuC᎐C and Co C fragment orbitals is likely to contribute to
᎐
2
2
the HOMO. The similar spectroscopic and electrochemical
properties of 9 and 10 suggest that there is little contribution
from silicon based orbitals to the frontier orbitals of these
molecules.
Conclusion
The spectroscopic, electrochemical and structural features of
a series of redox-active complexes featuring a dicobalt alkyne
J. Chem. Soc., Dalton Trans., 2001, 292–299
297

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Table 4 Crystallographic details for complexes 1–5, 9
1
2
3
4
5
9
Formula
M
Crystal system
Space group
a/Å
b/Å
c/Å
C37H28Co2O4P2 C39H33Co2NO4P2 C37H27Co2NO6P2 C38H27Co2NO4P4 C83H70Co2O4P4RuSiؒCH2Cl2 C77H66Co2O4P4RuSi
716.39
Monoclinic
C2/c
39.6115(10)
12.2498(3)
14.6345(4)
92.074(2)
7096.9(3)
100(2)
759.46
Monoclinic
P21/c
14.301(10)
15.716(10)
15.727(10)
98.792(2)
3493.2(4)
100(2)
761.40
Orthorhombic
Pbcm
14.4825(11)
13.6742(10)
16.8860(14)
90
741.41
Monoclinic
P21/m
9.3295(2)
16.6546(4)
11.1253(3)
92.212(1)
1727.35(7)
100(2)
1587.22
Monoclinic
P21/n
19.4088(9)
19.2738(9)
20.0688(10)
97.390(1)
7445.0(6)
100(2)
1426.20
Orthorhombic
P212121
13.7464(5)
19.9035(6)
24.8998(8)
90
β/Њ
U/Å3
T/K
3344.0(4)
100(2)
6812.6(4)
100(2)
Z
8
4
4
2
6
4
µ/mmϪ1
1.061
1.083
1.136
1.093
1.298
0.861
Reflections measured, 38965, 9374
35790, 8011
(0.0585)
0.0820
39675, 4608
(0.1189)
0.0794
21205, 4731
(0.0293)
0.0676
58747, 19733
(0.0374)
0.1133
86568, 18038
(0.0452)
0.0635
unique (Rint)
)
(0.0269)
0.0793
wR(F2) (all data)
cluster core and a phenyl-based spacing unit have been rational-
ised using a qualitative MO argument. The phenyl bridge
permits a degree of π conjugation only when strongly electron-
withdrawing groups are placed para to the redox-active
cluster probe group. In the absence of a π-conjugation pathway,
electronic interactions with the cluster probe group must be
more electrostatic in origin. In combination with the particular
organometallic fragments examined in this work, the inherent
stability of the phenyl ring π system does not favor the for-
mation of compounds in which orbitals extend over the entire
molecule. This is in direct contrast to the situation found for
pure ynyl bridges.
residue purified by preparative TLC (30% acetone in hexane) to
give a single band, which was crystallised (CH2Cl2–MeOH) to
afford large crystals of the desired product.
2
᎐
[Co (ꢀ-ꢁ -Me SiC C H C᎐C{Ru(PPh ) Cp})(CO) (ꢀ-dppm)]
᎐
2
3
2
6
4
3
2
4
5. A solution of [Co2(CO)6(dppm)] (100 mg, 0.15 mmol) and
᎐
᎐
[Ru(C᎐CC H C᎐CSiMe )(PPh )Cp] (133 mg, 0.15 mmol) in
᎐
᎐
6
4
3
3
benzene (12 ml) was heated at reflux. The bright yellow-orange
solution darkened slowly to give a brown solution and after
1.5 h TLC analysis indicated only one brown product. The
solvent was removed and the residue purified by preparative
TLC (30% acetone in hexane). The sole brown band yielded
complex 5 as tan coloured crystals (CH2Cl2–MeOH) suitable
for X-ray study.
Experimental
2
General conditions
᎐
[Co (ꢀ-ꢁ -Me SiC C᎐C{Ru(PPh ) Cp})(CO) (ꢀ-dppm)] 9. A
᎐
2
3
2
3
2
4
solution of [RuCl(PPh3)2Cp] (100 mg, 0.14 mmol) and complex
2 (101 mg, 0.14 mmol) in MeOH (15 ml) was treated with
NH4PF6 (22 mg, 0.14 mmol) and heated at reflux for 1 h. Note:
thf (0.5 ml) may be added to aid solvation of the starting
materials if poorly solvated after some 15 minutes of reflux.
During this time the orange-red solution turned dark green and
a similarly coloured precipitate was observed. The solution was
allowed to cool and treated with NaOMe solution. Filtration
gave the crude dark green product 9. Recrystallisation from
CH2Cl2–EtOH (slow evaporation) gave crystals suitable for
X-ray study.
All reactions were carried out under dry high-purity nitrogen
using standard Schlenk techniques. Solvents were dried and
distilled with the exception of AnalR methanol, prior to use.
Preparative TLC was performed on 20 × 20 cm glass plates
coated with silica gel (Merck GF254, 0.5 mm thick). Literature
31
᎐
methods were used to prepare the alkynes HC᎐CC H X,
᎐
6
4
2
[Co2(CO)6(dppm)],32
[Co (µ-η -RC C᎐CRЈ)(CO) (µ-dppm)]
᎐
2 2 4
25
᎐
6, 7, 833 and [Ru(C᎐CC H C᎐CSiMe )(PPh ) Cp]. Other
᎐
᎐
᎐
᎐
6
4
3
3 2
reagents were purchased and used as received.
Instrumental measurements
2
Infrared spectra were recorded using calcium fluoride cells
of 0.5 mm path length on a Perkin-Elmer 1600 Series FT-
IR spectrometer, UV-Vis spectra from CH2Cl2 solutions in a
1 cm path length quartz cuvette on a Varian Cary 5 UV-Vis-
NIR spectrometer and NMR spectra with a Varian VXR-400s
at (1H) 399.97 MHz and (13C) 100.57 MHz in CDCl3 and
referenced against the solvent resonances. ES MS spectra
were obtained using a Fisons instrument with a cone voltage of
30 V. Solutions of compounds in CH2Cl2 in MeOH were intro-
duced via an induction loop with sodium methoxide solution
added in situ as an aid to ionisation. Cyclic voltammetry
experiments were recorded using an EG&G Versastat II instru-
ment in thf containing 0.1 M [NBu4]PF6. Solutions were purged
with nitrogen and measured with a platinum working electrode
with platinum wire reference and counter electrodes at low
temperature such that the ferrocene–ferrocenium redox couple
was located at 0.56 V. Microanalyses were performed in house.
᎐
[Co {ꢀ-ꢁ -HC C᎐C{Ru(PPh ) Cp})(CO) (ꢀ-dppm)] 10.
A
᎐
2
2
3
2
4
solution of [RuCl(PPh3)2Cp] (100 mg, 0.14 mmol) and complex
3 (93 mg, 0.14 mmol) in MeOH (15 ml) was treated with
NH4PF6 (22 mg, 0.14 mmol). The mixture was refluxed for
3 h, by which time it had darkened, the orange [RuCl(PPh3)2Cp]
had been consumed and a dark green precipitate was evident.
Treating the cooled solution with NaOMe gave further green
precipitate which was filtered off affording crude 10 which
was recrystallised from CH2Cl2–EtOH (slow evaporation).
Crystallography
Data were collected on a Bruker SMART CCD diffractometer,
using graphite-monochromated Mo-Kα radiation (λ = 0.71073
Å). The structures were solved by direct methods and refined
by full-matrix least squares against F2 on all data, using the
SHELXTL suite of programs.34 All phenyl group hydrogen
atoms were placed in calculated positions and refined using a
riding model. Crystallographic data are collected in Table 4.
The structure of complex 1 contained disordered solvent
of crystallisation (CH2Cl2/methanol). This residual electron
density was treated successfully with the utility “Squeeze”
available in the crystallographic suite PLATON.35 Prior to
treatment, the volume of the solvent accessible void as reported
Preparations
[Co2(ꢀ-ꢁ2-HC2C6H4X-4)(CO)4(ꢀ-dppm)] (X ؍ H, 1; NMe2, 2;
NO2, 3 or CN, 4). A solution of [Co2(CO)4(dppm)] (200 mg,
᎐
0.30 mmol) and HC᎐CC H X (0.30 mmol) in benzene (15 ml)
᎐
6
4
was heated at reflux for 1 h. The solvent was removed and the
298 J. Chem. Soc., Dalton Trans., 2001, 292–299

View Article Online
by the Squeeze routine was 816.8 Å3 and the residual electron
count per cell was 235. The refinement with all the electron
density in the solvent void intact (i.e. left as Q-peaks) gave a R1
value of 0.0699 and Rw2 value of 0.2176.
CCDC reference number 186/2280.
See http://www.rsc.org/suppdata/dt/b0/b008445j/ for crystal-
lographic files in .cif format.
14 F. Paul, W. E. Meyer, L. Toupet, H. J. Jiao, J. A. Gladysz and
C. Lapinte, J. Am. Chem. Soc., 2000, 122, 9405.
15 Y. B. Zhu, O. Clot, M. O. Wolf and G. P. A. Yap, J. Am. Chem. Soc.,
1998, 120, 1812.
16 M. C. B. Colbert, J. Lewis, N. J. Long, P. R. Raithby, A. J. P. White
and D. J. Williams, J. Chem. Soc., Dalton Trans., 1997, 99.
17 P. J. Low, R. Rousseau, P. Lam, K. A. Udachin, G. D. Enright,
J. S. Tse, D. D. M. Wayner and A. J. Carty, Organometallics, 1999,
18, 3885.
18 R. S. Dickson and P. J. Fraser, Adv. Organomet. Chem., 1974, 12,
323.
19 W. Henderson, B. K. Nicholson and L. J. McCaffrey, Polyhedron,
1998, 17, 4291.
20 J. Manna, K. D. John and M. D. Hopkins, Adv. Organomet. Chem.,
1995, 38, 79.
21 D. L. Thorn and R. Hoffmann, Inorg. Chem., 1978, 17, 126.
22 R. P. Aggarwal, N. G. Connelly, M. C. Crespo, B. J. Dunne,
P. M. Hopkins and A. G. Orpen, J. Chem. Soc., Dalton Trans., 1992,
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23 I. R. Whittal, M. G. Humphrey, D. C. R. Hockless, B. W. Skelton
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24 J. M. Wisner, T. J. Basrtczak and J. A. Ibers, Inorg. Chim. Acta,
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Acknowledgements
This work was supported by the Department of Chemistry,
Durham University. We gratefully acknowledge a generous gift
of the substituted acetylenes used in this study by Professor
T. B. Marder and Dr R. L. Thomas.
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