New organoarsino-bridged cobalt carbonyl complexes derived from reactions of [Co2(CO)8] or [Co2(n-alkyne)(CO)6] with cych-(AsPh)6

Article abstract of DOI:10.1039/a706576k

Reaction of hexaphenylcyclohexaarsane (PhAs)6 with [Co₂(CO)8] in toluene at ambient temperature resulted in As-As bond cleavage and the new complexes [Co4(u4-AsPh)2(CO)IO] 1 and [Co4(uj-AsPh)(u4-T|2:r|2:r|'-As4Ph4)(CO)10] 2. In contrast, reaction of c>r/o-(PhAs)6 with the alkyene-bridged complexes [Co2(u-R'C=CR2)(CO)6] in toluene or in benzene at 65 °C afforded the new complexes [Co2(u-R'GCR2){u-c>'c/0-(PhAs)6}(CO)4] (R1 = R2 = H 3; or Ph 4 and 5 (isomers); R1 = Ph, R2 = H 6; R1 = Me, R2 = H 7) in which the As6 ring remains intact. All seven products have been characterised spectroscopically and, in addition, compounds 1-4 have been structurally characterised by X-ray diffraction analysis. The molecular structure of 1 comprises a rectangle of cobalt atoms capped above and below the plane by two quadruply bridging AsPh ligands, giving a pseudooctahedral Co4As2 core. The molecular structure of 2 consists of two discrete Co2(CO)5 units linked by an As4Ph4 chain and an AsPh unit. In complexes 3-7 the six-membered As6 ring remains intact and acts as a bidentate bridging ligand replacing two equatorial carbonyl groups, one on each Co atom.

Full text of DOI:10.1039/a706576k

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New organoarsino-bridged cobalt carbonyl complexes derived from
reactions of [Co2(CO)8] or [Co2(ì-alkyne)(CO)6] with cyclo-(AsPh)6
Rohini M. De Silva, Martin J. Mays,* John E. Davies, Paul R. Raithby, Moira A. Rennie and
Gregory P. Shield
Department of Chemistry, Lensfield Road, Cambridge, UK CB2 1EW
Reaction of hexaphenylcyclohexaarsane (PhAs)6 with [Co2(CO)8] in toluene at ambient temperature resulted in
As᎐As bond cleavage and the new complexes [Co4(µ4-AsPh)2(CO)10] 1 and [Co4(µ3-AsPh)(µ4-η2 :η2 :η1-As4Ph4)-
1
2
᎐
(CO) ] 2. In contrast, reaction of cyclo-(PhAs) with the alkyene-bridged complexes [Co (µ-R C᎐CR )(CO) ]
᎐
10
6
2
6
1
2
᎐
in toluene or in benzene at 65 ЊC afforded the new complexes [Co (µ-R C᎐CR ){µ-cyclo-(PhAs) }(CO) ]
᎐
2
6
4
(R1 = R2 = H 3; or Ph 4 and 5 (isomers); R1 = Ph, R2 = H 6; R1 = Me, R2 = H 7) in which the As6 ring remains
intact. All seven products have been characterised spectroscopically and, in addition, compounds 1–4 have been
structurally characterised by X-ray diffraction analysis. The molecular structure of 1 comprises a rectangle of
cobalt atoms capped above and below the plane by two quadruply bridging AsPh ligands, giving a pseudo-
octahedral Co4As2 core. The molecular structure of 2 consists of two discrete Co2(CO)5 units linked by an
As4Ph4 chain and an AsPh unit. In complexes 3–7 the six-membered As6 ring remains intact and acts as a
bidentate bridging ligand replacing two equatorial carbonyl groups, one on each Co atom.
The reactions of transition-metal carbonyl complexes with
cyclopolyarsines have attracted considerable attention due to
the interesting bonding modes and structural features of the
derived products. These include complexes with co-ordinated
rings containing five to ten RAs units,1–6 complexes with chains
of two to eight RAs units7–10 and complexes with naked As
atoms obtained as a result of loss of the organic group R. These
can contain naked Asn units ranging from one,11 two12 and
three As atom chains13 to rings of five As atoms.14 Another type
of process involves the transfer of R groups between As atoms
and this is exemplified by the reaction of hexaphenylcyclo-
hexaarsane with metal carbonyl complexes, which leads to
pathways involved in the formation of the products reported
in earlier studies. Accordingly we now report (i) the reaction
at ambient temperature of cyclo-(PhAs)6 with [Co2(CO)8],
which leads to the formation of two new tetracobalt complexes,
one of which contains µ3-AsPh, and µ4-As4Ph4 fragments,
and (ii) the reactions at 65 ЊC between cyclo-(PhAs)6 and the
1
2
᎐
alkyne-bridged dicobalt complexes [Co (µ-R C᎐CR )(CO) ]
᎐
2
6
(R1 = R2 = H or Ph; R1 = Ph, R2 = H; or R1 = Me, R2 = H),
which afford new complexes containing an intact cyclo-(PhAs)6
ring.
Results and Discussion
16
16
products containing AsPh,11,15 AsPh2 and As(Ph)AsPh2
fragments.
The reaction of cyclo-(PhAs)6 with [Co2(CO)8] in a 1:4 molar
ratio in toluene or in benzene at ambient temperature afforded,
after 2 h, the red crystalline solid [Co4(µ4-AsPh)2(CO)10] 1 and
the brown crystalline solid [Co4(µ3-AsPh)(µ4-η2 :η2 :η1-As4Ph4)-
(CO)10] 2 as the only isolable products. Compounds 1 and 2
have been characterised spectroscopically (Table 1) and their
Most of the above reactions were carried out using elevated
temperatures and it seemed of interest to investigate reactions
of this type at ambient temperature, or at temperatures
lower than those used previously, in the hope of isolating
new products which might provide an insight into the reaction
Table 1 Spectroscopic and microanalytical data for the new complexes 1–7
Microanalysis (%)c
IR (ν co/cmϪ1
)
1H NMR (δ)a
13C NMR (δ)b
FAB mass spectrum
820 (Mϩ)
C
H
1 2033vs, 2023s, 2007s,
1864w
2 2058s, 2037vs, 2023vs,
2004m, 1995m, 1822w
7.6–7.4 (m, 10 H, Ph)
133–128 (m, Ph), 209 (br, CO)
32.13
(32.33)
37.67
1.42
(1.22)
2.25
8.2–6.8 (m, 25 H, Ph)
134–128 (m, Ph), 239 (s, CO),
235 (s, CO), 203 (br, CO), 202
(br, CO), 200 (br, CO), 198
(br, CO)
137–128 (m, Ph), 203 (br, CO),
202 (br, CO)
142–125 (m, Ph), 205 (br, CO),
201 (br, CO)
138–125 (m, Ph), 206 (br, CO),
203 (br, CO)
1249 (Mϩ Ϫ CO)
(37.65)
(1.97)
3 2025s, 1996vs, 1969s
4 2021s, 1996vs, 1969s
5 2022s, 1995vs, 1967s
6 2023s, 1997vs, 1970s
7 2020s, 1995vs, 1964s
7.9–6.6 (m, 32 H, Ph ϩ C2H2)
8.1–6.6 (m, 40 H, Ph)
1168 (Mϩ Ϫ nCO,
n = 1–4)
42.87
(43.18)
48.94
(49.1)
49.1
(49.1)
46.19
(46.33)
43.35
(43.69)
2.68
(2.76)
3.11
(3.0)
3.07
(3.0)
2.79
(2.92)
2.89
1320 (Mϩ Ϫ nCO,
n = 1–4)
7.5–7.0 (m, 40 H, Ph)
1320 (Mϩ Ϫ nCO,
n = 1–4)
7.9–6.4 (m, 36 H, Ph), 5.0 (s, H)
142–126 (m, Ph), 112 (s, CPh),
206 (br, CO), 201 (br, CO)
1226 (Mϩ Ϫ nCO,
n = 1–4)
7.9–6.6 (m, 30 H, Ph), 5.0
[q, 4JHH 0.7, 1 H HCCMe];
2.5 (d, 3 H, HCMe)
137–128 (m, Ph), 117 (s, CCH3), 1154 (Mϩ Ϫ nCO,
24 (s, CCH3), 207 (br, CO), 202
(br, CO)
n = 1–4)
(2.89)
a Chemical shifts in ppm relative to SiMe4 (0.0), coupling constants in Hz, in CDCl3 at 293 K. b Chemical shifts in ppm relative to SiMe4 (0.0), in
CDCl3 at 293 K. c Calculated values in parentheses.
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446
439

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Fig. 1 Molecular structure of [Co4(µ4-AsPh)2(CO)10] 1 including the atom numbering scheme. Hydrogen atoms have been omitted for clarity
range from 2.3551(9) Å to 2.3636(12) Å, with the mean value of
2.358 Å being comparable to the mean value of 2.39 Å for the
corresponding µ4-PhAsCo4 distances in the Co16 cluster
[{Co8(µ6-As)(µ4-As)(µ4-AsPh)2(CO)16}2]11 and within the range
(2.24–2.54 Å) of Co᎐E distances found in analogous Co4E2
clusters.18,19,21 The Co᎐As᎐Co bond angles are also comparable
with Co᎐E᎐Co angles found in related Co4E2 clusters. The
As ؒ ؒ ؒ As distance of 2.7670(11) Å is only 0.3 Å longer than a
typical As᎐As single bond, a remarkable feature also found in
other complexes of the type [Co4(CO)10E2]. Thus, for example,
the P ؒ ؒ ؒ P separation in the phosphorus analogue of 1 is
2.537(6) Å, only 0.3 Å greater than a P᎐P single bond.19
It has been suggested that an attractive direct E ؒ ؒ ؒ E inter-
action accounts for these short distances but the need to
optimise the M᎐M and M᎐E distances is also acknowledged
to be an important factor.19,20,22 The cluster [Fe3(µ3-AsPh)2-
(CO)9]23 contains a similar short As ؒ ؒ ؒ As separation of
2.789(2) Å.
The spectroscopic properties of complex 1 are in accord with
the solid-state structure being maintained in solution. The IR
spectrum in CH2Cl2 exhibits three carbonyl bands at 2033vs,
2023s and 2007s together with a band at 1864w cmϪ1 in the
bridging carbonyl region. The 13C NMR spectrum at 20 ЊC
shows, in addition to the multiplet for the two phenyl groups, a
single broad resonance at δ 209 due to the two bridging and
eight terminal carbonyl groups. This is in good agreement with
the observed value of δ 208 for the weighted average chemical
shifts of the terminal and bridging carbonyl in studies of the
temperature-dependent carbonyl spectra of related tetracobalt
clusters.24
The molecular structure of complex 2 has also been deter-
mined by crystal structure analysis. Suitable crystals were
grown by diffusion of hexane into a dichloromethane solution
of 2. The molecular structure together with the atom number-
ing scheme is shown in Fig. 2, selected bond distances and
angles in Table 3. The structure consists of two discrete metal–
metal bonded Co2(CO)5 units in each of which one of the CO
ligands occupies a bridging position. The two units are linked
by an AsPh4 chain and by an AsPh group, both fragments being
derived from opening and breaking of the cyclo-(PhAs)6 ring.
The end arsenic atoms of the As4 chain, As(3) and As(4), act as
three-electron donors; As(3) bridges the Co(1)᎐Co(3) edge in
one of the dimetal units, while As(4) bridges Co(3) from this
unit and Co(2) from the other dimetal unit. Additional co-
ordination of one of the interior arsenic atoms in the chain,
As(5), to Co(4) completes the linking of the two Co2(CO)5 units
Table 2 Selected bond distances (Å) and angles (Њ) for complex 1
As(1)᎐Co(1)
As(1)᎐Co(2)
As(1) ؒ ؒ ؒ As(1A)
Co(1)᎐As(1A)
Co(1)᎐Co(2)
Co(2)᎐As(1A)
C(1)᎐Co(2A)
2.3551(9) As(1)᎐Co(1A)
2.360(2) As(1)᎐Co(2A)
2.7670(11) Co(1)᎐C(1)
2.3572(9) Co(1)᎐Co(2A)
2.8078(13) Co(2)᎐C(1A)
2.3636(12) Co(2)᎐Co(1A)
1.949(4)
2.3572(9)
2.3635(12)
1.959(5)
2.592(2)
1.949(4)
2.592(2)
Co(1)᎐As(1)᎐Co(1A) 108.09(3) Co(1)᎐As(1)᎐Co(2)
Co(1A)᎐As(1)᎐Co(2) 66.67(2) Co(1)᎐As(1)᎐Co(2A)
73.09(2)
66.64(4)
Co(1A)᎐As(1)᎐Co(2A) 72.99(3) Co(2)᎐As(1)᎐Co(2A) 108.28(5)
Co(1)᎐As(1)᎐As(1A) 54.08(3) Co(2)᎐As(1)᎐As(1A) 54.20(2)
Co(1A)᎐As(1)᎐As(1A) 54.01(2) Co(2A)᎐As(1)᎐As(1A) 54.08(5)
As(1)᎐Co(1)᎐As(1A)
As(1)᎐Co(1)᎐Co(2)
Co(2A)᎐Co(1)᎐Co(2)
As(1)᎐Co(2)᎐Co(1A)
As(1)᎐Co(2)᎐Co(1)
Co(1A)᎐Co(2)᎐Co(1)
71.91(3) As(1)᎐Co(1)᎐Co(2A)
53.53(4) As(1A)᎐Co(1)᎐Co(2)
90.21(6) As(1)᎐Co(2)᎐As(1A)
56.61(5) As(1A)᎐Co(2)᎐Co(1A) 56.52(4)
53.37(3) As(1A)᎐Co(2)᎐Co(1)
56.72(4)
53.61(2)
71.72(5)
53.40(3)
89.79(6)
Symmetry relation: A Ϫx ϩ 1, Ϫy, Ϫz ϩ 2.
structures confirmed by X-ray crystallographic analysis. The
synthesis of cluster 1 has been reported previously17 but no
details of the synthesis were given nor was it structurally
characterised.
The molecular structure of compound 1 is shown in Fig. 1
together with the atomic numbering scheme. Selected bond
lengths and angles are given in Table 2. The four Co atoms are
coplanar and form a rectangle, with a crystallographic centre of
symmetry as its centre, which is capped above and below the
plane by quadruply bridging AsPh ligands, with the apical As
atoms being situated 1.383(1) Å from the plane. The central
pseudo-octahedral Co4As2 core conforms closely to a D2h sym-
metry and compound 1 is a member of the [Co4(CO)10E2]
family where E = S,18 Te,19 PPh19,20 or GeMe.21 Each Co atom is
bonded to the two As atoms and also to one bridging and two
terminal CO ligands. The bridging CO groups span opposite
sides of the cobalt rectangle and the bridged Co᎐Co bonds are
0.21 Å shorter than the unbridged Co᎐Co bonds [2.592(2) Å
versus 2.8078(13) Å]. The unbridged Co᎐Co distances of
2.8078(13) Å can be compared to the corresponding distances
of 2.88(2) Å in the tellurium analogue of 119 and it is clear that
Co᎐Co distances in the [Co4(CO)10E2] family increase with
increasing covalent radius of the µ4-bridged hetero atom
(covalent radii: As, 1.22; Te, 1.37 Å). The Co᎐As distances in 1
440
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446

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Fig. 2 Molecular structure of [Co4(µ3-AsPh)(µ4-η2 :η2 :η1-As4Ph4)(CO)10] 2. Details as in Fig. 1
characterised clusters such as [{Co8(µ6-As)(µ4-As)(µ4-AsPh)-
(CO)16}2],11 [(OC)9Co3(µ4-As)Co4(CO)11]25 and [{Co3(µ4-As)-
(CO)8}3].26 The Co(2)᎐As(4) and Co(3)᎐As(4) bond distances
of 2.429(3) and 2.431(3) Å, respectively, are slightly longer than
Co᎐As bonds in many other related complexes but comparable
to the average Co᎐As distance of 2.439 Å recorded for
[CoAs3(CO)3].27 The cobalt–carbonyl carbon (terminal) bond
lengths are almost equal (average = 1.819 Å) and comparable
with previously reported values.27 The average As᎐As distance
[As(3)᎐As(5)᎐As(2)᎐As(4)] in the As4 chain of 2.45 Å is in
accord with the average As᎐As distance of 2.456 Å for cyclo-
(PhAs)6,28 but the As(5)᎐As(2)᎐As(4) angle of 83.63(8)Њ is
smaller than any angle in the free arsane, the distortion may be
related to the general strain of the whole molecule caused by
the close non-bonded approaches of the carbonyl and Ph
groups. The Co᎐As᎐Co angles are in the range 64–66Њ for the
arsenic atom bridging the two metal–metal bonded Co atoms
from the same bimetal unit and in the range 116–131Њ for the
arsenic atom which bridges the two non-bonded Co atoms from
different units. These angle ranges are typical for correspond-
ingly bonded As atoms in the related structures mentioned
above. Complex 2 resembles [{Co8(µ6-As)(µ4-As)(µ4-AsPh)2-
(CO)16}2]11 in having an AsCo3 unit of the type As(1)᎐Co(1)᎐
C(2)᎐Co(3) and an As2Co2 of the type As(5)᎐Co(2)᎐Co(4)᎐
As(4).
Table 3 Selected bond distances (Å) and angles (Њ) for complex 2
As(1)᎐Co(2)
As(1)᎐Co(4)
As(2)᎐As(4)
As(3)᎐As(5)
As(4)᎐Co(2)
As(5)᎐Co(4)
Co(2)᎐Co(4)
Co(3)᎐C(3)
Co(4)᎐C(8)
2.355(3)
2.376(3)
2.473(2)
2.450(2)
2.429(3)
2.388(3)
2.516(3)
1.92(2)
As(1)᎐Co(1)
As(2)᎐As(5)
As(3)᎐Co(1)
As(3)᎐Co(3)
As(4)᎐Co(3)
Co(1)᎐Co(3)
Co(1)᎐C(3)
2.395(3)
2.436(2)
2.278(3)
2.307(3)
2.431(3)
2.522(4)
1.97(2)
Co(2)᎐C(8)
Co᎐C (terminal)
1.91(2)
1.80 (av.)
1.97(2)
Co(2)᎐As(1)᎐Co(4)
Co(2)᎐As(1)᎐Co(1) 122.94(11)
Co(1)᎐As(3)᎐Co(3) 66.71(9)
Co(3)᎐As(3)᎐As(5) 115.74(10)
Co(2)᎐As(4)᎐Co(3) 116.54(10)
As(2)᎐As(5)᎐As(3)
C(3)᎐Co(1)᎐Co(3)
As(3)᎐Co(1)᎐Co(3)
As(1)᎐Co(1)᎐Co(3)
C(8)᎐Co(2)᎐As(4)
C(8)᎐Co(2)᎐Co(4)
64.21(9)
Co(4)᎐As(1)᎐Co(1) 131.62(10)
As(5)᎐As(2)᎐As(4) 83.63(8)
Co(1)᎐As(3)᎐As(5) 117.86(10)
Co(3)᎐As(4)᎐As(2) 99.75(9)
Co(2)᎐As(4)᎐As(2) 114.57(9)
Co(4)᎐As(5)᎐As(2) 117.66(9)
Co(4)᎐As(5)᎐As(3) 110.38(9)
87.50(8)
48.7(5)
57.20(9)
98.95(11)
149.7(5)
50.7(5)
As(3)᎐Co(1)᎐As(1)
C(8)᎐Co(2)᎐As(1)
As(1)᎐Co(2)᎐As(4)
As(1)᎐Co(2)᎐Co(4)
C(3)᎐Co(3)᎐As(3)
As(3)᎐Co(3)᎐As(4)
88.49(10)
89.8(5)
92.05(9)
58.30(8)
82.0(6)
As(4)᎐Co(2)᎐Co(4) 100.28(10)
C(3)᎐Co(3)᎐As(4)
C(3)᎐Co(3)᎐Co(1)
As(3)᎐Co(3)᎐Co(1)
C(8)᎐Co(4)᎐As(5)
C(8)᎐Co(4)᎐Co(2)
As(5)᎐Co(4)᎐Co(2)
152.6(6)
50.5(6)
56.09(8)
145.4(5)
48.5(5)
87.44(10)
As(4)᎐Co(3)᎐Co(1) 102.93(11)
C(8)᎐Co(4)᎐As(1)
As(1)᎐Co(4)᎐As(5)
As(1)᎐Co(4)᎐Co(2)
78.1(5)
90.29(10)
57.49(9)
The spectroscopic properties of complex 2 are in accord with
the solid-state structure being maintained in solution. Five
ν(CO) bands are visible in the IR spectrum including an
absorption at 1822 cmϪ1 corresponding to the presence of
bridging CO groups in the molecule. The 13C NMR spectrum
displays two singlets at δ 239 and 235 for the two bridging
carbonyls and four broad signals at δ 203, 202, 200 and 198 for
the eight terminal carbonyl groups.
97.84(10)
by the As4 chain. The µ3-arsinidene group, As(1)Ph, caps Co(1),
Co(2) and Co(4), acting as a four-electron donor. The Co᎐Co
distances within the dimetal units of 2.516(3) and 2.522(4) Å
and the observed bond lengths of 2.278(3) and 2.307(3) Å
for As(3)᎐Co(1) and As(3)᎐Co(3) respectively are similar to
the lengths of corresponding bonds in related structurally
1
2
᎐
The reaction of cyclo-(PhAs) with [Co (µ-R C᎐CR )(CO) ]
᎐
6
2
6
in a 1:2 molar ratio in toluene or in benzene does not proceed
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446
441

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᎐
Fig. 3 Molecular structure of [Co2(µ-HC᎐CH){µ-cyclo-(PhAs)6}(CO)4] 3. Details as in Fig. 1
᎐
Table 4 Selected bond distances (Å) and angles (Њ) for complex 3
Table 5 Selected distances (Å) and angles (Њ) for complex 4
Co(1)᎐C(3)
Co(1)᎐Co(2)
Co(2)᎐C(3)
Co(2)᎐As(5)
As(1)᎐As(6)
As(3)᎐As(4)
As(5)᎐As(6)
C᎐O (mean)
1.927(14)
2.496(2)
1.927(14)
2.371(2)
2.459(2)
2.462(2)
2.452(2)
1.140
Co(1)᎐C(4)
Co(1)᎐As(1)
Co(2)᎐C(4)
As(1)᎐As(2)
As(2)᎐As(3)
As(4)᎐As(5)
Co᎐C (mean)
As᎐C (mean)
1.971(13)
2.362(2)
1.947(13)
2.466(2)
2.455(2)
2.476(2)
1.780
Co(1)᎐C(3)
Co(1)᎐Co(2)
Co(2)᎐C(3)
Co(2)᎐As(5)
As(1)᎐As(6)
As(3)᎐As(4)
As(5)᎐As(6)
C᎐O (mean)
1.930(7)
2.486(2)
1.953(7)
2.373(2)
2.4504(13)
2.4422(13)
2.468(2)
1.144
Co(1)᎐C(4)
Co(1)᎐As(1)
Co(2)᎐C(4)
As(1)᎐As(2)
As(2)᎐As(3)
As(4)᎐As(5)
Co᎐C (mean)
As᎐C (mean)
1.995(7)
2.395(2)
1.963(7)
2.4739(13)
2.451(2)
2.611(13)
1.784
1.957
1.962
C(3)᎐Co(1)᎐C(4)
C(3)᎐Co(1)᎐As(1)
C(3)᎐Co(1)᎐Co(2)
C(3)᎐Co(2)᎐C(4)
C(4)᎐Co(2)᎐As(5)
C(4)᎐Co(2)᎐Co(1)
Co(1)᎐As(1)᎐As(6)
As(3)᎐As(2)᎐As(1)
As(3)᎐As(4)᎐As(5)
As(6)᎐As(5)᎐As(4)
Co(2)᎐As(5)᎐As(4)
39.8(5)
143.3(4)
49.6(4)
40.1(5)
103.6(4)
50.8(4)
116.30(7)
95.16(6)
98.61(6)
90.52(6)
125.72(8)
C(4)᎐Co(1)᎐As(1)
C(4)᎐Co(1)᎐Co(2)
As(1)᎐Co(1)᎐Co(2)
C(3)᎐Co(2)᎐As(5)
C(3)᎐Co(2)᎐Co(1)
As(5)᎐Co(2)᎐Co(1)
Co(1)᎐As(1)᎐As(2)
As(6)᎐As(1)᎐As(2)
As(2)᎐As(3)᎐As(4)
Co(2)᎐As(5)᎐As(6)
As(5)᎐As(6)᎐As(1)
106.0(4)
50.0(4)
101.48(8)
140.4(4)
49.6(4)
98.36(7)
124.71(8)
96.65(6)
91.13(7)
117.97(7)
84.8(6)
C(3)᎐Co(1)᎐As(1)
C(3)᎐Co(1)᎐Co(2)
As(1)᎐Co(1)᎐Co(2) 100.28(5)
C(3)᎐Co(2)᎐C(4)
C(4)᎐Co(2)᎐As(5)
C(4)᎐Co(2)᎐Co(1)
Co(1)᎐As(1)᎐As(6) 116.92(5)
Co(1)᎐As(1)᎐As(2) 129.88(4)
147.0(2)
50.6(2)
C(3)᎐Co(1)᎐C(4)
C(4)᎐Co(1)᎐As(1)
C(4)᎐Co(1)᎐Co(2)
C(3)᎐Co(2)᎐As(5)
C(3)᎐Co(2)᎐Co(1)
As(5)᎐Co(2)᎐Co(1) 100.32(5)
As(6)᎐As(1)᎐As(2)
As(4)᎐As(3)᎐As(2)
40.3(3)
111.6(2)
50.5(2)
143.9(2)
49.8(2)
40.4(3)
107.4(2)
51.7(2)
88.51(4)
93.67(5)
As(3)᎐As(2)᎐As(1)
Co(3)᎐As(4)᎐As(5)
As(4)᎐As(5)᎐As(6)
94.22(4)
97.24(4)
93.64(5)
Co(2)᎐As(5)᎐As(6) 115.16(5)
Co(2)᎐As(5)᎐As(4) 128.61(5)
As(5)᎐As(6)᎐As(1)
85.74(5)
at ambient temperature but at 65 ЊC afforded the complexes
and close to the value of that in free cyclo-(PhAs)6 (91.03Њ).28
However the As(5)᎐As(6)᎐As(1) angle of 84.8(6)Њ in 3 and the
corresponding angle of 85.74(5)Њ in 4 are smaller than any of
the As᎐As᎐As angles in the free arsane. This allows the two
arsenic donor atoms in the 1,5 positions in the arsenic rings to
approach each other more closely, providing a better match to
the Co᎐Co distances. The Co᎐Co distances of 2.496(2) and
2.486(2) Å in 3 and 4, respectively, are identical within experi-
mental error with the observed values for other bis-equatorially
and bis-pseudo-equatorially substituted alkyne-bridged dico-
balt complexes.29,30 The mean cobalt to carbonyl carbon
distance of 1.78 Å in both compounds is comparable to the
corresponding distance of 1.77 Å in [Co2(µ-PhC2Ph)(dppm)-
(CO)4] (dppm = Ph2PCH2PPh2),29 1.75 Å in [Co2(µ-PhC2Ph)-
(dpam)2(CO)2] (dpam = Ph2AsCH2AsPh2)29 and 1.78 Å in
[Co2(µ-HC2H)(PPh3)2(CO)4].30 The cobalt to arsenic average
bond lengths of 2.36 and 2.38 Å, respectively, for complexes 3
and 4 agree closely with Co᎐As distances in related species.29
1
2
1
2
᎐
[Co (µ-R C᎐CR ){µ-cyclo-(PhAs) }(CO) ] (R = R = H 3; or
᎐
2
6
4
Ph 4 and 5 (isomers); R1 = Ph, R2 = H 6; R1 = Me, R2 = H 7).
The structures of 3 and 4 have been determined by single-
crystal X-ray analysis. Suitable crystals were grown by diffusion
of hexane into dichloromethane solutions of 3 and 4. The
molecular structures are illustrated in Figs. 3 and 4 and selected
bond lengths and angles are given in Tables 4 and 5. In both 3
and 4 the (PhAs)6 moiety acts as a bidentate ligand through the
two arsenic atoms in 1,5 positions in the ring, replacing two
equatorial carbonyls, one on each cobalt atom, and bridging
the Co᎐Co vector. The six-membered ring of arsenic atoms
remains intact and the geometry of this ring is close to that of
the free arsane. The solid-state structures of 3 and 4 are similar,
both in the orientations adopted by the aromatic rings attached
to the unco-ordinated arsenic atoms and in the orientation of
the alkyne bridge. The average As᎐As᎐As angle of 92.82Њ in the
arsenic ring in 3 is almost equal to the average angle 92.16Њ in 4
442
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446

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᎐
Fig. 4 Molecular structure of [Co2(µ-PhC᎐CPh){µ-cyclo-(PhAs)6}(CO)4] 4. Details as in Fig. 1
᎐
The spectroscopic properties of complexes 3 and 4 are in
accord with the solid-state structures being maintained in
solution. Three strong ν(CO) bands are visible in the terminal
carbonyl region of the IR spectrum similar to that observed for
other equatorially disubstituted alkyne-bridged dicobalt deriv-
atives.29 The 1H NMR spectrum of 3 shows a complex multiplet
at δ 7.94–6.60 which can be assigned as being due to the thirty
aromatic protons in the six phenyl rings. There are no separate
identifiable peaks corresponding to the two acetylenic protons
and the resonances for these two protons are presumably
masked by the aromatic protons. For the compound
[Co2(PPh3)2{µ-HC2C(Me)Ph(OH)}(CO)4]31 the corresponding
1H signal is at δ 6.62.
The spectroscopic properties of complexes 5–7 are closely
similar to those of 3 and 4, suggesting that they have analogous
structures. The question then arises as to how the structures of
4 and 5 differ from each other. Unfortunately it did not prove
possible to obtain crystals of 5 suitable for a single-crystal
X-ray diffraction study. However, the IR spectrum of 5 is iden-
tical to that of complex 4, suggesting that the bridging of the
ligand across the metal centres and the carbonyl arrangement
around the metal atoms is similar in both cases. The mass spec-
troscopic data and elemental analyses confirm that the two
complexes have the same empirical formulae and it is clear from
these data that they are isomers. A possible isomeric structure
for 5 is shown in Scheme 1(b). It differs from 4 in the orienta-
tion adopted by the alkyne bridge relative to the co-ordinated
(PhAs)6 ring. In 4 the X-ray diffraction study shows that the
alkyne bridge leans towards that part of the (PhAs)6 ring which
contains the three unco-ordinated arsenic atoms. It is suggested
that in 5 the alkyne bridge leans towards that part of the
(PhAs)6 which contains the one unco-ordinated arsenic atom.
An alternative possibility, which we cannot exclude, is that 5 is
related to 4 by inversion of the phenyl group and the lone pair
at one or more of the unco-ordinated As atoms. One such pos-
sible inversion is shown in Scheme 1(c). This type of inversion
isomerism has been observed previously for cyclo-(PhP)5 co-
ordinated to a triosmium framework in the complex [Os3-
{cyclo-(PhP)5}(CO)10].32,33
Attempts via thermolysis to convert 1 and 2 into any of the
complexes obtained previously in the reaction of cyclo-(PhAs)6
with [Co2(CO)8] in a sealed tube at higher temperatures and
pressures, such as [{Co8(µ6-As)(µ4-As)(µ4-AsPh)2(CO)16}2],11
were not successful, and we were unable to demonstrate that 1
or 2 is present as an intermediate in this earlier reaction.
Conclusion
The dependence of the products on the reaction temperature
indicates that there must be more than one reaction pathway
involved in the reaction between cyclo-(PhAs)6 and [Co2(CO)8].
It is clear that As᎐As bond cleavage in cyclo-(PhAs)6 is such a
facile process that it is promoted by [Co2(CO)8] even at ambient
temperature. It is then at first sight surprising that the reaction
1
2
᎐
between cyclo-(PhAs) and [Co (µ-R C᎐CR )(CO) ] does not
᎐
6
2
6
also lead to products derived from As᎐As bond cleavage, since
34
the cleavage of P᎐P and P᎐S bonds in the ligands PhPPPh2
and Ph2PSPh35 in substituted dinuclear complexes of the type
[Co (µ-R C᎐CR )(µ-Ph PXR )(CO) ] (XRn = PPh2 or SR)
1
2
᎐
᎐
2
2
n
4
occurs under relatively mild conditions (and As᎐As bonds are
in general weaker than P᎐P bonds). Thus the complex [Co2-
᎐
(µ-PhC᎐CPh)(µ-Ph PPPh )(CO) ] is readily converted into
᎐
2
2
4
[Co2{µ-PPh2C(Ph)C(Ph)CO}(µ-PPh2)(CO)4] on thermolysis at
55 ЊC in toluene.34 Complexes 3–7 are, however, stable to
thermolysis at temperatures up to 100 ЊC and thermolysis of 3
and 4 at 120 ЊC resulted in complete decomposition. This dif-
ference in thermolytic stability may be due to the fact that e.g. in
᎐
[Co (µ-PhC᎐CPh)(µ-Ph PPPh )(CO) ] there is considerably
᎐
2
2
2
4
more strain in the four-membered Co2P2 dimetallic ring than in
the corresponding five-membered Co2As3 ring present in
1
2
᎐
[Co (µ-R C᎐CR ){µ-cyclo-(PhAs) }(CO) ].
᎐
2
6
4
Experimental
Unless otherwise stated, all reactions were performed under an
atmosphere of dry oxygen-free nitrogen, using solvents which
had been freshly distilled from the appropriate drying agent.
Infrared spectra were recorded in dichloromethane solution 0.5
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446
443

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As2
Ph
Ph
As
Ph
Ph
As5
As3
(OC)2
Co
Ph
(CO)2
Co
O
C
Co3
As4
(CO)2
[Co2(CO)8]
OC
CO
O
C
Co
Co
(CO)2
(OC)2Co1
(OC)2
(OC)2Co4
As1
Co2(OC)2
As
Ph
(a)
Ph
2
1
cyclo-(PhAs)6
R1
Ph
CO
CO
C
(b)
C
CO
OC
OC
CO
C
Co
Co
C
Co
Co
Ph
R2
As3
OC
Ph
As4
As2
CO
As5
As1
As3
Ph
As4
[Co2(µ-R1C≡CR2)(CO)6]
As5
As1
Ph
Ph
As6
Ph
Ph
As6
Ph
Ph
Ph
As2
Ph
Ph
Ph
3 R1 = R2 = H
4 R1 = R2 = Ph
6 R1 = Ph, R2 = H
7 R1 = Me, R2 = H
5
Ph
CO
C
CO
CO
Co
Co
C
Ph
CO
(c)
Ph
Ph
As5
As1
Ph
As4
As3
As6
As2
Ph
Ph
Ph
5
1
2
᎐
Scheme 1 Products derived from the reaction of cyclo-(PhAs)6 with (a) [Co2(CO)8] and (b) [Co2(µ-R C᎐CR )(CO)6. (c) An alternative structure for
᎐
complex 5, an isomer of 4
1
2
᎐
mm NaCl cells, using a Perkin-Elmer 1710 Fourier-transform
spectrometer, fast atom bombardment (FAB) mass spectra on a
Kratos MS890 instrument using 3-nitrobenzyl alcohol as a
matrix and 1H and 13C NMR spectra on either a Bruker
WM250 or AM-400 spectrometer. Microanalyses were per-
formed by the microanalytical department, University of
Cambridge. Preparative TLC was carried out on 1 mm silica
plates prepared at the University of Cambridge. Products are
given in order of decreasing Rf values. All reagents were
obtained from commercial suppliers and used without further
purification. The compounds [Co2(µ-R1CCR2)(CO)6] (R1 =
R2 = H or Ph; R1 = Ph, R2 = H; R1 = Me, R2 = H)36–38 and cyclo-
(PhAs)6 39 were prepared by published procedures.
Reaction of cyclo-(PhAs) with [Co (ì-R C᎐CR )(CO) ]
᎐
6 2 6
᎐
(a) [Co (ì-HC᎐CH){ì-cyclo-(PhAs) }(CO) ] 3. To a solution
᎐
2
6
4
᎐
of [Co (µ-HC᎐CH)(CO) ] (0.312 g, 1 mmol) in toluene or in
᎐
2
6
benzene (50 cm3) was added cyclo-(PhAs)6 (0.456 g, 0.5 mmol)
and the resulting solution heated at 65 ЊC for 20 h. The solution
was then cooled to room temperature and filtered. After
removal of solvent at reduced pressure, the residue was redis-
solved in CH2Cl2, silica added and the mixture pumped dry.
The solid was added to the top of a silica chromatography
column. Elution with hexane–CH2Cl2 (4:1) gave red crystalline
[Co2(µ-HC᎐CH){µ-cyclo-(PhAs)6}(CO)4] 3 (0.335 g, 57%).
᎐
᎐
᎐
(b) [Co (ì-PhC᎐CPh){ì-cyclo-(PhAs) }(CO) ] 4 and 5. To
᎐
2
6
6
᎐
a solution of [Co (µ-PhC᎐CPh)(CO) ] (0.464 g, 1 mmol) in
᎐
2
6
Reaction of cyclo-(PhAs)6 with [Co2(CO)8]
toluene or in benzene (50 cm3) was added cyclo-(PhAs)6 (0.912
g, 1.0 mmol) and the resulting solution heated at 65 ЊC for 24 h.
The solvent was then removed under reduced pressure, the resi-
due dissolved in the minimum volume of CH2Cl2 and applied to
To a solution of [Co2(CO)8] (0.171 g, 0.5 mmol) in toluene (25
cm3) was added cyclo-(PhAs)6 (0.114 g, 0.125 mmol) and the
resulting solution stirred at ambient temperature for 2 h then
filtered. After removal of solvent at reduced pressure, the resi-
due was dissolved in the minimum volume of CH2Cl2 and sep-
arated by preparative TLC. Elution using hexane–CH2Cl2 (4:1)
gave red crystalline [Co4(µ4-AsPh)2(CO)10] 1 (0.021 g, 20%), a
green solid (decomposed, uncharacterised) and brown crystal-
the base of TLC plates. Elution with hexane–CH2Cl2 (3:1) gave
green crystalline [Co (µ-PhC᎐CPh){µ-cyclo-(PhAs) }(CO) ] 4
᎐
᎐
2
6
4
᎐
(0.297 g, 45%) followed by red solid [Co (µ-PhC᎐CPh){µ-cyclo-
᎐
2
(PhAs)6}(CO)4] 5 (0.23 g, 35%).
line [Co4(µ3-AsPh)(µ4-η2 :η2 :η1-As4Ph4)(CO)10]
2
(0.038 g,
(c) [Co (ì-PhC᎐CH){ì-cyclo-(PhAs) }(CO) ] 6. To a solution
of [Co (µ-PhC᎐CH)(CO) ] (0.388 g, 1 mmol) in benzene (50
᎐
2 6
᎐
᎐
2
6
6
᎐
23%).
444
J. Chem. Soc., Dalton Trans., 1998, Pages 439–446

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Table 6 Crystal data* for complexes 1–4
1
2
3
4
Molecular formula
M
C22H10As2Co4O10
819.86
C40H25As5Co4O10
1275.92
C42H32As6Co2O4
1168.06
C54H40As6Co2O4
1320.24
Crystal system
Space group
Monoclinic
P21/n
Triclinic
Pc
Monoclinic
P21/n
Triclinic
Pc
a/Å
b/Å
c/Å
α/Њ
9.713(3)
12.649(12)
10.649(3)
13.418(3)
16.443(3)
12.751(3)
110.22(3)
107.31(3)
89.04(3)
2508.7(9)
2
13.231(3)
24.380(5)
13.278(3)
14.277(6)
14.720(4)
13.607(5)
105.41(3)
111.12(3)
89.04(3)
2561.7(16)
2
β/Њ
96.74(2)
100.76(3)
γ/Њ
U/Å3
1299.3(13)
2
2.096
0.20 × 0.20 × 0.10
Red block
792
4207.8(16)
4
1.844
0.24 × 0.22 × 0.18
Red block
2272
Z
Dc/Mg mϪ3
Crystal size/mm
Crystal habit
F(000)
1.689
1.712
0.20 × 0.20 × 0.10
Brown block
1236
0.30 × 0.25 × 0.15
Green block
1296
µ/mmϪ1
5.090
4.619
5.511
4.537
Maximum, minimum
relative transmission
Data collection range/Њ
Index ranges
0.761, 1.000
0.808, 0.999
0.343, 0.260
1.000, 0.810
5.02 < 2θ < 49.98
Ϫ10 р h р 12,
0 р k р 16,
Ϫ13 р l р 13
4360
5.16 < 2θ < 45.0
0 р h р 14,
Ϫ17 р k р 17,
Ϫ13 р l р 13
7039
7.08 < 2θ < 47.0
Ϫ14 р h р 14,
0 р k р 27,
Ϫ3 р l р 14
6238
5.02 < 2θ < 55.0
0 р h р 18,
Ϫ19 р k р 19,
Ϫ17 р l р 16
12 220
Reflections measured
Independent reflection (Rint
Parameters
wR2 (all data)
x, y
)
2986 (0.0467)
172
0.190
0.0255, 0.0129
0.0325
6559 (0.066)
532
0.2658
0.1568
0.0653
6198 (0.038)
487
0.1662
0.0756, 7.838
0.0649
11 748 (0.037)
595
0.1790
0.100, 0.871
0.0656
R1[I > 2σ(I )]
Observed reflections [I > 2σ(I )]
Goodness of fit on F 2 (all data)
Maximum shift/σ
2098
1.025
Ϫ0.001
0.666, Ϫ0.785
Ϫ2
4028
1.024
0.001
1.586, Ϫ1.044
12
3829
1.051
0.004
0.610, Ϫ0.755
0
7533
1.009
0.001
1.197, Ϫ1.040
0
Peak, hole in final difference map/e ÅϪ3
Decay (%)
¹
4
* Data in common: graphite-monochromated Mo-Kα radiation, λ = 0.710 73 Å; T = 293(2) K; R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|, wR2 = [Σw(Fo2 Ϫ Fc2)2/ΣwFo ]²,
¹
w = 1/[σ2(Fo)2 ϩ (xP)2 ϩ yP], P = (Fo2 ϩ 2Fc2)/3, where x and y are constants adjusted by the program; goodness of fit = [Σw(Fo2 Ϫ Fc2)2/(n Ϫ p)]²
where n is the number of reflections and p the number of parameters.
cm3) was added cyclo-(PhAs)6 (0.912 g, 1.0 mmol) and the
resulting solution heated at 65 ЊC for 24 h. The solution was
then cooled to room temperature and filtered. After removal of
the solvent at reduced pressure, the residue was redissolved in
CH2Cl2, silica added and the mixture pumped dry. The solid
was then added to the top of a silica chromatography column
and elution using hexane–CH2Cl2 (4:1) gave traces of starting
material followed by uncharacterised unstable products.
non-hydrogen atoms by full-matrix least squares on F 2
(SHELXL 9344). Hydrogen atoms were placed in geometrically
idealised positions and refined using a riding model. In the final
cycles of refinement a weighting scheme was introduced which
produced a flat analysis of variance. Crystal data are given in
Table 6.
CCDC reference number 186/796.
Further elution with CH2Cl2 gave red crystalline [Co2-
Acknowledgements
We gratefully acknowledge the financial support of the
Cambridge Commonwealth Trust and the United Kingdom
Committee of Vice Chancellors and Principals (to R. M. D. S.).
᎐
(µ-PhC᎐CH){µ-cyclo-(PhAs) }(CO) ] 6 (0.650 g, 52%).
᎐
6
4
᎐
(d ) [Co (ì-MeC᎐CH){ì-cyclo-(PhAs) }(CO) ] 7. To a solu-
᎐
2
6
4
᎐
tion of [Co (µ-MeC᎐CH)(CO) ] (0.326 g, 1 mmol) in toluene
᎐
2
6
(50 cm3) was added cyclo-(PhAs)6 (0.456 g, 0.5 mmol) and the
resulting solution heated at 65 ЊC for 24 h. The procedure as in
References
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᎐
2
6
4
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