Tricobalt carbonyl clusters containing Cn (n = 2, 4, 6) ligands

Article abstract of DOI:10.1021/om058056v

Treatment of Co₃(μ₃-CBr)(μ-dppm)(CO) ₇ with zinc dust in refluxing thf afforded Co₆(μ ₆-C₂)(μ-dppm)₂-(μ-CO)(CO)₁₂ (5), containing two Co₃ triangles which are linked by a short Co-Co bond and the C₂ ligand. The Pd(0)/Cu(I)-catalyzed reactions of Co₃(μ₃-CBr)(μ-dppm)(CO)₇ or Co ₃(μ₃-CBr)(CO)₉ with Co₃{μ ₃-C(C≡C)_nAu(PPh₃)}(μ-dpprn)(CO) ₇ (n = 1, 2, respectively) gave {Co₃(μ-dppm)(CO) ₇}₂(μ₃:μ₃-CC=CC) (6) and the unsymmetrical C₆ derivative {(OC)₉Co₃} {μ₃:μ₃-C(C≡C)₂C}{Co ₃(μ-dppm)(CO)₇} (4). The former reacted further with dppm to give {Co₃(μ-dppm)(CO)₇} ₂(μ₃:μ₃-CC≡CC)(μ-dppm) (7), in which the two Co₃ clusters are bridged by the third dppm ligand.

Full text of DOI:10.1021/om058056v

Organometallics 2006, 25, 4817-4821
4817
Tricobalt Carbonyl Clusters Containing Cn (n ) 2, 4, 6) Ligands
Michael I. Bruce,*,† Brian W. Skelton,‡ Allan H. White,‡ and Natasha N. Zaitseva†
Department of Chemistry, UniVersity of Adelaide, Adelaide, South Australia 5005, Australia, and
Chemistry M313, UniVersity of Western Australia, Crawley, Western Australia 6009, Australia
ReceiVed December 20, 2005
Treatment of Co3(µ3-CBr)(µ-dppm)(CO)7 with zinc dust in refluxing thf afforded Co6(µ6-C2)(µ-dppm)2-
(µ-CO)(CO)12 (5), containing two Co3 triangles which are linked by a short Co-Co bond and the C2
ligand. The Pd(0)/Cu(I)-catalyzed reactions of Co3(µ3-CBr)(µ-dppm)(CO)7 or Co3(µ3-CBr)(CO)9 with
Co3{µ3-C(CtC)nAu(PPh3)}(µ-dppm)(CO)7 (n ) 1, 2, respectively) gave {Co3(µ-dppm)(CO)7}2(µ3:µ3-
CCtCC) (6) and the unsymmetrical C6 derivative {(OC)9Co3}{µ3:µ3-C(CtC)2C}{Co3(µ-dppm)(CO)7}
(4). The former reacted further with dppm to give {Co3(µ-dppm)(CO)7}2(µ3:µ3-CCtCC)(µ-dppm) (7),
in which the two Co3 clusters are bridged by the third dppm ligand.
with mean Co-Co and Co-C separations of 2.457(1) and 1.95-
(1) Å, respectively. The X-ray structure of the analogous C4
cluster (2)13c shows that the C4 ligand linking the two Co3(CO)9
clusters has C-C separations of 1.37(1) and 1.24(2) Å, together
with mean Co-Co and Co-C distances of 2.47(1) and 1.92(1)
Å. In this complex, the carbon chain geometry is consistent with
Introduction
Interest in compounds containing unsaturated Cn chains end-
capped by transition-metal-ligand groups arises out of the
propensity for electronic interactions (electron or hole transport)
between the metal centers through the carbon chain.1-3 This is
particularly apparent when the metal-ligand groupings are redox
active, and much recent interest in systems containing Mn,4 Re,5
Fe,6,7 Ru,8 Ru2, 9 and Pt10 has been summarized in recent
reviews.11
Complexes containing C2 or C4 groups linking two metal
cluster moieties have been known for many years.12-14 The
molecular structures of both monoclinic15 and trigonal16 phases
of {(OC)9Co3}2(µ3:µ3-C2) (1) confirm the presence of a C-C
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C. Organometallics 1998, 17, 1928. (c) Coat, F.; Paul, F.; Lapinte, C.;
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Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949. (b) Bruce, M. I.; Ellis, B.
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bond, measured at 1.37(2) and 1.426(9) Å, respectively. Each
carbon atom is also attached to a triangular Co3(CO)9 cluster
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Ren, T.; Zou, G.; Alvarez, J. C. Chem. Commun. 2000, 1197.
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B.; Arif, A. M.; Hampel, F.; Gladysz, J. A. In Coordination Chemistry at
the Turn of the Century; Ondrejovic, G., Sirota, A., Eds.; Slovak Technical
University Press: Bratislava, Slovakia, 1999; p 47. (d) Owen, G. R.; Stahl,
J.; Hampel, F.; Gladysz, J. A. Organometallics 2004, 23, 5889. (e) Owen,
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* To whom correspondence should be addressed. E-mail:
michael.bruce@adelaide.edu.au.
† University of Adelaide.
‡ University of Western Australia.
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10.1021/om058056v CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/24/2006

4818 Organometallics, Vol. 25, No. 20, 2006
Bruce et al.
“partial π-delocalization”, while the carbonyl groups shield the
CtC triple bond so that it does not react with Co2(CO)8, for
example. The next higher ethynologue, {Co3(CO)9}2(µ3:µ3-CCt
CCtCC) (3), has not yet been described, although the Co2-
(CO)6 derivative has been obtained and structurally characterized
by two groups.17,18 In connection with our studies on complexes
containing carbon chains end-capped by metal cluster moieties,
much of which has been centered on compounds containing
the Co3(µ-dppm)(CO)7 end group,19 we have attempted to obtain
complexes analogous to 1 and 2 containing these end groups.
This paper describes the results of our search and the structures
and properties of three complexes which we have characterized,
together with the asymmetric bis-cluster compound {(OC)9Co3}-
(µ3:µ3-CCtCCtCC){Co3(µ-dppm)(CO)7} (4).
Results and Discussion
Treatment of Co3(µ3-Br)(µ-dppm)(CO)7 with zinc dust in
refluxing thf afforded a Co6 cluster identified as Co6(µ6-C2)-
(µ-dppm)2(µ-CO)(CO)12 (5) by a single-crystal X-ray structure
determination, together with small amounts of Co3(µ3-CCO2H)-
(µ-dppm)(CO)7.20,21 In 5, the two Co3 clusters have been linked
by a further long Co-Co bond (2.6788(6) Å) at the expense of
loss of one of the CO ligands. This Co-Co bond is bridged by
a CO group, while the C2 fragment also links the two clusters
by interacting with all six metal atoms. The two dppm ligands
bridge Co-Co edges of the triangular faces and adopt a
conformation which places them in mutually orthogonal orienta-
tions; in one Co3 cluster the three basal Co-Co edges are also
bridged by CO ligands.
The C4 complex {Co3(µ-dppm)(CO)7}2(µ3:µ3-CCtCC) (6),
obtained in 63% yield by the Pd(0)/Cu(I)-catalyzed elimination
of AuBr(PPh3) from an equimolar mixture of Co3(µ3-CBr)(µ-
dppm)(CO)7 and Co3{µ3-CCtCAu(PPh3)}(µ-dppm)(CO)7,22
reacts further with dppm to give {Co3(µ-dppm)(CO)7}2(µ3:µ3-
CCtCC)(µ-dppm) (7) in 87% yield.23 The unsymmetrical com-
plex {(OC)9Co3}{µ3:µ3-C(CtC)2C}{Co3(µ-dppm)(CO)7} (4)
was formed similarly in 72% yield from Co3(µ3-CBr)(CO)9 and
Co3(µ3-CCtCCtCAu(PPh3)}(µ-dppm)(CO)7.24 All of the com-
plexes were identified by elemental microanalyses and mass
spectrometry.
The IR spectra of 3-7 each contained terminal ν(CO)
absorptions between 2063 and 1933 cm-1, with weak bands
between 1875 and 1785 cm-1 arising from the bridging CO
groups in 5. Comparison of the spectra of 2 (2107 w, 2090 s,
2065 vs, 2037 s, 1983 vw cm-1) and 6 is instructive, the stronger
absorptions in particular showing a pronounced shift to lower
wavenumbers, consistent with increased back-bonding from the
more electron-rich dppm-substituted clusters. For 3, ν(CO) bands
(14) (a) Worth, G. H.; Robinson, B. H.; Simpson, J. Organometallics
1992, 11, 3863. (b) Robinson, B. H.; Tham, W. S. J. Organomet. Chem.
1968, 16, P45. (c) Matheson, T. W.; Robinson, B. H.; Tham, W. S. J. Chem.
Soc. A 1971, 1457.
(15) Brice, M. D.; Penfold, B. R. Inorg. Chem. 1972, 11, 1381.
(16) Geiser, U.; Kini, A. M. Acta Crystallogr. 1993, C49, 1322.
(17) Seyferth, D.; Spohn, R. J.; Churchill, M. R.; Gold, K.; Scholer, F.
J. Organomet. Chem. 1970, 23, 237.
(18) (a) Robinson, B. H.; Spencer, J. L. J. Organomet. Chem. 1971, 30,
267. (b) Dellaca, R. J.; Penfold, B. R. Inorg. Chem. 1971, 10, 1269.
(19) (a) Bruce, M. I.; Smith, M. E.; Zaitseva, N. N.; Skelton, B. W.;
White, A. H. J. Organomet. Chem. 2003, 670, 170. (b) Bruce, M. I.; Skelton,
B. W.; White, A. H.; Zaitseva, N. N. J. Organomet. Chem. 2003, 683, 398.
(c) Antonova, A. B.; Bruce, M. I.; Ellis, B. G.; Gaudio, M.; Humphrey, P.
A.; Jevric, M.; Melino, G.; Nicholson, B. K.; Perkins, G. J.; Skelton, B.
W.; Stapleton, B.; White, A. H.; Zaitseva, N. N. Chem. Commun. 2004,
960.
(22) A mixture of Co3(µ3-CBr)(µ-dppm)(CO)9 (34 mg, 0.04 mmol), Co3-
{µ3-CCtCAu(PPh3)}(µ-dppm)(CO)9 (50 mg, 0.04 mmol), Pd(PPh3)4 (6 mg,
0.005 mmol), and CuI (1 mg, 0.005 mmol) in thf (4 mL) was stirred at
room temperature for 2 h. After removal of solvent under reduced pressure,
the residue was taken up in CH2Cl2 and separated by preparative tlc (acetone/
hexane 3/7). One major brown band (Rf 0.28) was collected to give dark
brown crystals of {(OC)7(µ-dppm)Co3}2(µ3:µ3-CCtCC) (6; 40 mg, 63%).
Anal. Found: C, 52.30; H, 2.66. Calcd (C68H44Co6O14P4; Mr ) 1562): C,
52.27; H, 2.84. IR (Nujol, cm-1): ν(CtC) 2113 w. IR (CH2Cl2): ν(CO)
2063 m, 2049 m, 2009 vs (br), 1975 (sh), 1961 (sh). 1H NMR: δ 3.59,
4.08 (2 × m, 2 × 2H, dppm), 7.06-7.45 (m, 40H, Ph). 13C NMR: δ 40.8
(s (br), CH2), 96.1 (Câ), 128.0-132.6 (m, Ph), 204.1, 217.45 (2 × s (br),
CO). 31P NMR: δ 31.1 (dppm). ES MS (positive ion, MeOH, m/z): 1562,
M+; 1534-1478, [M - nCO]+ (n ) 1-3). ES MS (negative ion, MeOH
+ NaOMe, m/z): 1561, [M - H]-.
(23) A solution containing 6 (70 mg, 0.045 mmol) and dppm (17.2 mg,
0.045 mmol) in thf (7 mL) was treated with Me3NO (6.8 mg, 0.09 mmol),
and the mixture was stirred at room temperature for 1 h. After removal of
solvent under reduced pressure, a dichloromethane extract of the residue
was purified by preparative tlc (acetone/hexane 3/7). The major purple band
(Rf 0.31) contained {(OC)7(µ-dppm)Co3}2(µ3:µ3-CCtCC)(µ-dppm) (7; 73.6
mg, 87%), which formed dark red crystals (CH2Cl2/MeOH). Anal. Found:
C, 57.64; H, 3.40. Calcd (C91H66Co6O12P6; Mr ) 1891): C, 57.74; H, 3.49.
IR (Nujol): ν(CtC) 2133 vw. IR (CH2Cl2): ν(CO) 2012 vs, 1993 s (sh),
1982 m, 1962 m, 1933 (sh) cm-1. 1H NMR: δ 3.37, 3.54, 3.81 (3 × m, 3
× 2H, dppm), 7.13-7.37 (m, 60H, Ph). 13C NMR: δ 30.65 (s (br), CH2),
95.75 (Câ), 128.1-132.7 (m, Ph), 202.6, 213.1 (2 × s (br), CO). 31P
NMR: δ 31.2 (dppm). ES MS (positive ion, MeOH, m/z): 1891, M+; 1863,
1835, [M - nCO]+ (n ) 1, 2).
(20) Zinc dust (excess) was added to a solution of Co3(µ3-CBr)(µ-dppm)-
(CO)9 (50 mg, 0.06 mmol) in thf (6 mL), and the mixture was heated at the
reflux point for 4 h. The filtered solution was evaporated, and a CH2Cl2
extract of the residue was separated by preparative tlc (acetone/hexane 3/7).
The first black band (Rf 0.46) contained Co3(µ3-CH)(µ-dppm)(CO)9 (5.3
mg, 11%), and the second green band (Rf 0,31) afforded Co3(µ3-CCO2H)-
(µ-dppm)(CO)9 (2.9 mg, 6%), both identified by comparison with authentic
samples. A slow-moving brown-purple band (Rf 0.21) yielded Co6(µ6-C2)-
(µ-dppm)2(µ-CO)(CO)12 (5; 16.3 mg, 37%), obtained as dark crystals (CH2-
Cl2/MeOH). Anal. Found: C, 51.70; H, 2.92. Calcd (C65H44Co6O13P4; Mr
) 1510): C, 51.65; H, 2.91. IR (CH2Cl2, cm-1): ν(CO) 2045 s, 2015 vs,
1991 m, 1875 vw, 1848 vw, 1819 vw, 1785 vw. 1H NMR: δ 2.84, 4.39 (2
× m, 2 × 2H, dppm), 7.37-7.87 (m, 40H, Ph). 31P NMR: δ 32.25, 44.8
(2 × s (br)). ES MS (positive ion, MeOH + NaOMe, m/z): 1533, [M +
Na]+. ES MS (negative ion, MeOH + NaOMe, m/z): 1509, [M - H]-;
1491, [M - H - CO]-.
(21) Bruce, M. I.; Kramarczuk, K. A.; Perkins, G. J.; Skelton, B. W.;
White, A. H.; Zaitseva, N. N. J. Cluster Sci. 2004, 15, 119.

Co3 Carbonyl Clusters Containing Cn Ligands
Organometallics, Vol. 25, No. 20, 2006 4819
are at 2099 m, 2058 vs, 2039 m, 2016 vs, 1971 (sh), and 1954
(sh) cm-1 and a weak band at 2115 cm-1 is assigned to ν(CC).
For 4, ν(CC) is at 2115 cm-1 and the ν(CO) bands are found
between 2099 and 1954 cm-1 (cf. 2089 cm-1 and between 2064
and 1974 cm-1, respectively, for {Co3(µ-dppm)(CO)7}2{µ-
C(CtC)2C}19a).
1
In the H and 31P NMR spectra, resonances at δH ca. 2.85
and 4.35 (CH2), δH 7.0-7.9 (Ph), and δP 31-32 are assigned
to the dppm ligands. In the case of 5, two 31P resonances are
found at δP 32.25 and 44.8, which can be assigned to the dppm
ligands on the non-CO-bridged and CO-bridged Co3 clusters,
respectively, by comparison with related complexes. For 6 and
7, 13C NMR spectra could be obtained and showed the dppm
CH2 (δC ca. 40) and Ph (δC ca. 128-132.5), together with the
inner carbons of the C4 chains at δC 96.13 (6) or 95.75 (7). The
Co-CO ligands are found as broad singlets at δC ca. 203 and
215. Three 13C resonances from the C6 chain in 4 are found at
δ 95.8, 112.5, and 123.0, with the CO ligands of the two clusters
at δ 199.2 (Co3(CO)9) and 201.5, 209.4 (Co3(dppm)(CO)7). As
is often the case, the carbons attached to the Co3 clusters were
not found, probably as a result of broadening by the cobalt
quadrupole. The electrospray mass spectra (ES MS) were mea-
sured in solutions containing NaOMe to aid ionization and con-
tained cations such as [M + Na]+ and [M - nCO]+ or anions
such as [M - H - nCO]-, consistent with their compositions.
Molecular Structures. The molecular structures of 5-7 have
been determined by single-crystal X-ray diffraction studies: 5
modeled as its 3CH2Cl2 solvate, 6 as 4PhMe and 2CH2Cl2‚2C6H6
solvates, as well as the unsolvated form, results from the latter
being presented in Figure 2 and the main text. In addition, the
structure of 2 has been redetermined from a specimen obtained
during the course of another study.25
In 5 (Figure 1), the two Co3 clusters have been linked by a
further long Co-Co bond (2.6788(6) Å) at the expense of loss
of one of the CO ligands. This Co-Co bond is bridged by a
CO group, while the C2 fragment also links the two clusters by
interacting with all six metal atoms. The two dppm ligands
bridge Co-Co edges of the triangular faces and adopt a confor-
mation which places them in mutually orthogonal orientations
(interplanar dihedral angle 89.79(4)°); in one Co3 cluster the
three basal Co-Co edges are also bridged by CO ligands. The
Co-Co separations in the two Co3 fragments of 5 (labeled A
and B here) fall into four types and range between 2.4069 and
2.5412(6) Å. The shortest, Co(4)-Co(5) in cluster B, is bridged
by both CO and one dppm ligand, while the two other CO-
bridged edges are longer, at 2.4682 and 2.4806(6) Å. In the
Figure 1. Plot of a molecule of Co6(µ6-C2)(µ-dppm)2(µ-CO)(CO)12
(5), showing the atom-numbering scheme. Selected distances (Å)
and angles (deg): Co(1)-Co(2) ) 2.4991(7), Co(1)-Co(3) )
2.5079(6), Co(2)-Co(3) ) 2.5412(6), Co(3)-Co(6) ) 2.6788(6),
Co(4)-Co(5) ) 2.4069(7), Co(4)-Co(6) ) 2.4682(6), Co(5)-Co-
(6) ) 2.4806(6), Co(1)-P(1) ) 2.1929(9), Co(2)-P(2) ) 2.2243-
(8), Co(4)-P(4) ) 2.2348(9), Co(5)-P(5) ) 2.2416(8), Co(1,2,3)-
C(1) ) 1.927(3), 1.873(2), 2.001(3), Co(4,5,6)-C(2) ) 1.946(3),
1.938(2), 2.024(3), C(1)-C(2) ) 1.367(3); Co(1,2,3)-C(1)-C(2)
) 136.3(2), 141.4(2), 102.2(2), Co(4,5,6)-Co(2)-Co(1) ) 138.1-
(2), 143.4(2), 115.7(2).
other Co3 cluster A, the dppm-bridged edge is 2.4991(7) Å,
some 0.093 Å longer than Co(4)-Co(5). Surprisingly, the two
nonbridged Co(1,2)-Co(3) bonds differ by 0.04 Å, reflective
of a more general asymmetry about the Co(3,6)C(1,2) plane.
Somewhat longer Co-P bonds (2.2348, 2.2416(8) Å) are found
for cluster B as compared to those for for cluster A (2.1929,
2.2243(8) Å). This is a result of these Co atoms being also
involved in bonding to the bridging CO ligands and hence not
back-bonding as efficiently into the phosphorus ligand. Both
sets of values may be compared with those found in the
prototypical complex Co3(µ3-CH)(µ-dppm)(CO)7, which has
Co-Co separations of 2.462(2) Å (dppm-bridged) and 2.452,
2.474(2) Å and Co-P distances of 2.162(2) Å.21
The most interesting feature of the structure of 5 is the C2
ligand, which interacts with all six metal atoms (Co-C )
1.873-2.024(3) Å). The somewhat unsymmetrical bonding
appears to result from formation of shorter bonds to the cobalt
atoms also supporting the dppm ligand, separations to atoms
Co(3,6) being considerably longer at 2.001 and 2.024(3) Å as
well as from the Co(3)-Co(6) interaction and its CO bridge.
In Co3(µ3-CH)(µ-dppm)(CO)7, the Co-C distances are between
1.836 and 1.898(9) Å, with no particular relationship to whether
the Co is bonded to dppm.21 The C(1)-C(2) bond length is
1.367(3) Å, the same as that in the monoclinic phase (1.37(2)
Å)15 but shorter than that found in the trigonal phase of 1 (1.426-
(9) Å).16
Complexes 6 and 7 (Figures 2 and 3) are derived from 2 by
replacement of two CO ligands by two and three dppm ligands,
respectively. In the former, the Co-Co edges bridged by dppm
are somewhat longer, at 2.4826 and 2.4755(8) Å, than the other
two in each cluster (2.4501-2.4672(6) Å) while, in comparison,
the basal Co3 triangle in 2 is considerably expanded (Co-Co-
(av) ) 2.522(10) Å). The Co-P distances are in the range
2.178-2.187(1) Å, there being no essential difference from those
found in 5 above. The third dppm ligand in 7 bridges the
nonbonded Co(3)-Co(6) vector with Co-P distances of 2.216
and 2.242(1) Å. In this molecule, one of the non-dppm-bridged
Co-Co bonds in each cluster is considerably longer (2.4948-
(24) A solution containing Co3(µ3-CBr)(CO)9 (41.6 mg, 0.08 mmol), Co3-
{µ3-CCtCCtCAu(PPh3)}(µ-dppm)(CO)9 (100 mg, 0.08 mmol), Pd(PPh3)4
(9 mg, 0.008 mmol), and CuI (1 mg, 0.006 mmol) in thf (7 mL) was left
at room temperature for 1 h. After removal of solvent, the residue was
taken up in CH2Cl2 and purified by preparative tlc (acetone/hexane 3/7).
The major brown band (Rf 0.57) afforded {(OC)9Co3}(µ3:µ3-C(CtC)2C)-
{Co3(µ-dppm)(CO)7} (8; 70.8 mg, 72%) as small red platelike crystals.
Anal. Found: C, 44.88; H, 1.68. Calcd (C47H22Co6O16P6; Mr ) 1258): C,
44.83; H, 1.75. IR (CH2Cl2, cm-1): ν(CtC) 2115 vw. IR (CH2Cl2): ν-
1
(CO) 2099 m, 2058 vs, 2039 m, 2016 s, 1971 (sh), 1954 m. H NMR: δ
3.54, 4.37 (2 × m, 2 × 1H, dppm), 7.30-7.40 (m, 20H, Ph). 13C NMR:
δ 44.65 (t, J(CP) ) 19 Hz, dppm), 95.8, 112.5, 123.0 (carbon chain),
128.55-135.3 (m, Ph), 199.2 (s (br), Co3(CO)9), 201.5, 209.4 (2 × s (br),
Co3(CO)7(dppm)). 31P NMR: δ 35.2 (s (br), dppm). ES MS (positive ion,
MeOH + NaOMe, m/z): 1281, [M + Na]+. ES MS (negative ion, MeOH
+ NaOMe, m/z): 1289, [M + OMe]-; 1230, [M + OMe - CO]-. A red
band (Rf 0.50) contained {Co3(µ-dppm)(CO)7}2{µ3:µ3-C(CtC)4C} (5.2 mg,
8%), identified by comparison with an authentic sample.19a
(25) A serendipitous preparation of 2 enabled a structural determination
to be made that was considerably more precise than that described over 30
years ago.13,15 Structural parameters from the present determination are used
in the discussion above.

4820 Organometallics, Vol. 25, No. 20, 2006
Bruce et al.
Table 1. Redox Chemistry of Cobalt Clustersa
complex
E1°/V
E2°/V
∆E°/mV
KC
1b
2b
5
-0.77
-0.68
-1.18
-1.45
-1.33
-0.41
-0.39
+0.22
-1.21
+0.06
360
210
1400
240
1390
1.21 × 106
3.54 × 103
6c
7
1.14 × 104
a Conditions: 10-3 M complex, 0.1 M [NBu4]PF6 in CH2Cl2, referenced
to FeCp2/[FeCp2]+ ) +0.46 V, three-electrode cell equipped with Pt-disk
working electrode, Pt-gauze auxiliary electrode, and Pt-wire pseudo-
reference electrode, scan rate 0.2 V s-1 b References 26 and 27. c E3° )
.
+0.85 V.
separations of 1.412 and 1.258(4) Å, and angles at the central
carbons of 178.9(3)°. In 7, the range of Co-C distances (1.920-
1.960(4) Å) does not show any dependence on the nature of
the dppm attachment. Again the C-C distances along the chain
(1.396, 1.236, 1.387(7) Å) show evidence of electron delocal-
ization, with angles at C(2) and C(3) of 168.3 and 172.0(5)°,
respectively.
In these three complexes, it is instructive to compare the
torsion angles between the two dppm-bridged Co-Co vectors
in each complex, as indicated by Co(3)-C(1)-C(2 or 4)-Co-
(6) values of -6.2(2), -50.4(2), and 41.7(3)° for 5-7,
respectively. While the simplest explanation for this varying
twisting would appear to be steric interference between the dppm
Ph substituents, also involving the equatorial CO groups to a
degree, an electronic effect cannot be ruled out in the case of
5, in which the basal Co-Co bonds are also CO-bridged. In
the CCo3 clusters, the presence of bridging CO groups has been
interpreted in terms of a more electron-rich cluster, as found,
for example, in the cases of Co3(µ-CCl)(CO)9 and its PCy3-
substituted analogues.25
Figure 2. Plot of a molecule of {Co3(µ-dppm)(CO)7}2(µ3:µ3-CCt
CC) (6), showing the atom-numbering scheme. Selected distances
(Å) and angles (deg): Co(1)-Co(2) ) 2.4826(6), Co(1)-Co(3) )
2.4501(7), Co(2)-Co(3) ) 2.4590(7), Co(4)-Co(5) ) 2.4755(8),
Co(4)-Co(6) ) 2.4672(6), Co(5)-Co(6) ) 2.4697(8), Co(1)-P(1)
) 2.183(1), Co(2)-P(2) ) 2.178(1), Co(4)-P(4) ) 2.180(1), Co-
(5)-P(5) ) 2.187(1), Co(1,2,3)-C(1) ) 1.895(3), 1.902(3), 1.958-
(3), Co(4,5,6)-C(4) ) 1.921(3), 1.896(4), 1.952(3), C(1)-C(2) )
1.381(5), C(2)-C(3) ) 1.232(4), C(3)-C(4) ) 1.370(4); Co-
(1,2,3)-C(1)-C(2) ) 136.4(2), 135.0(2), 123.6(3), C(1)-C(2)-
C(3) ) 171.8(4), C(2)-C(3)-C(4) ) 169.9(4), C(3)-C(4)-
Co(4,5,6) ) 129.4(3), 141.6(3), 123.8(3).
Electrochemistry. Earlier extensive electrochemical studies
of 1 and 2 have shown that each undergoes two distinct one-
electron-reduction processes, a shift of the first to more negative
potentials occurring as the carbon chain is lengthened from C2
to C4.26,27 The reduction products are generally unstable at room
temperature, and electron-transfer-catalyzed substitution of CO
groups by ligands such as tertiary phosphines does not occur
(in contrast to the situation with mono-cluster species).26,28 At
low temperatures, chemically reversible processes are found.
Spectroscopic studies have been interpreted as showing conver-
sion of the initial reduced species to a CO-bridged isomer which
can be oxidized to the neutral 1.26 In their CVs (Table 1), the
two 1e waves are separated by ca. 360 mV for 1 and by ca.
200 mV for 2, with comproportionation constants, KC, of 1.21
× 106 and 3.54 × 103, respectively. Both results suggest that
there is effective electronic communication between the two
sites of reduction, the CCo3 clusters, dependent upon the
separation between the clusters (separation between CCo3
centers ca. 4.6 Å in 1 and ca. 7.1 Å in 2). For 2, a reversible
oxidation is found at ca. +0.85 V.
Figure 3. Plot of a molecule of{Co3(µ-dppm)(CO)7}2(µ3:µ3-CCt
CC)(µ-dppm) (7), showing the atom-numbering scheme. Selected
distances (Å) and angles (deg): Co(1)-Co(2) ) 2.4983(10), Co-
(1)-Co(3) ) 2.4654(9), Co(2)-Co(3) ) 2.4948(9), Co(4)-Co(5)
) 2.4827(9), Co(4)-Co(6) ) 2.5097(9), Co(5)-Co(6) ) 2.4723-
(9), Co(1)-P(1) ) 2.199(1), Co(2)-P(2) ) 2.186(1), Co(3)-P(7)
) 2.216(2), Co(4)-P(4) ) 2.217(1), Co(5)-P(5) ) 2.201(2), Co-
(6)-P(8) ) 2.242(3), Co(1,2,3)-C(1) ) 1.923(5), 1.920(4), 1.960-
(4), Co(4,5,6)-C(4) ) 1.937(5), 1.930(4), 1.946(5), C(1)-C(2) )
1.396(7), C(2)-C(3) ) 1.236(7), C(3)-C(4) ) 1.387(7); Co-
(1,2,3)-C(1)-C(2) ) 141.7(4), 129.2(4), 124.0(3), C(1)-C(2)-
C(3) ) 168.3(5), C(2)-C(3)-C(4) ) 172.0(5), C(3)-C(4)-
Co(4,5,6) ) 126.1(3), 141.9(4), 127.6(3).
The CVs of 5-7 are also given in Table 1. For 5 and 7, only
one reduction wave is found, assigned to the cluster core. An
oxidation wave for 5 at +0.22 V, assigned to [Co(CO)4]-,
together with some minor waves may indicate that these
processes are not fully reversible at room temperature. Two 1e
processes are found for 6, which may be compared with the
(9), 2.5097(9) Å) than the other two (2.4654, 2.4723(9) Å); the
dppm-bridged Co-Co bonds are 2.4983 and 2.4827(9) Å.
The Co-C bonds in 6 range between 1.895 and 1.921(3) Å
for Co(1,2,4,5), i.e., those attached to dppm, while the remaining
Co-C bonds are longer (1.958, 1.952(3) Å). The four-carbon
chain has interatomic distances of 1.381(5), 1.232(4), and 1.370-
(4) Å, suggesting some delocalization of the π-electron density
along the chain. Angles at the two central carbons are 171.8(4)
and 169.9(4)°, indicating a larger than normal distortion away
from linearity. In 2, the geometry is somewhat different, with
slightly longer C-Co bonds (average 1.964(10) Å), C-C
(26) (a) Worth, G. H.; Robinson, B. H.; Simpson, J. Organometallics
1992, 11, 3863. (b) Brooksby, P. A.; Duffy, N. W.; McQuillan, A. J.;
Robinson, B. H.; Simpson, J. J. Organomet. Chem. 1999, 582, 183.
(27) (a) Osella, D.; Gambino, O.; Nervi, C.; Ravera, M.; Bertolino, D.
Inorg. Chim. Acta 1993, 206, 155. (b) Osella, D.; Milone, L.; Nervi, C.;
Ravera, M. J. Organomet. Chem. 1995, 488, 1.
(28) Dawson, P. A.; Robinson, B. H.; Simpson, J. J. Chem. Soc., Dalton
Trans. 1979, 1762.

Co3 Carbonyl Clusters Containing Cn Ligands
Organometallics, Vol. 25, No. 20, 2006 4821
two processes observed for the non-phosphine-substituted
complex 2 at -0.62 and -0.82 V (separation 210 mV). It is
more difficult to reduce 6 than it is to reduce 2 (E ) -1.21,
-1.45 V, ∆E ) 240 mV), and no oxidation wave is found.
Addition of the bis-tertiary phosphine ligand to the Co3 cluster
is expected to increase the electron density within the CCo3
core, in comparison with 2, as also shown by the ν(CO) values,
resulting in a system which is more difficult to reduce. The
magnitude of the ∆E values (210 mV for 2 vs 240 mV for 6,
with KC in the order 1 > 6 > 2) suggests that electronic
communication between the cluster cores through the carbon
chain is marginally more pronounced in 6 than in 2. For 7, on
the other hand, which is easier to reduce than 6 (E ) -1.33
V), only one process is found, although a second, oxidation,
event is found at +0.06 V.
Acknowledgment. We thank Professor Brian Nicholson
(University of Waikato, Hamilton, New Zealand) for providing
the mass spectra and the ARC for support of this work.
Supporting Information Available: Full details of the crystal
structure determinations of 2 and 5-7 (CCDC 278100-278103,
281788) as CIF files. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM058056V