Ligand substitution in HC(O)CCo3(CO)9 with 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd): Diphosphine ligand fluxionality, decarbonylation of the formyl moiety and competitive P-Ph bond cleavage reactivity

Article abstract of DOI:10.1016/j.jorganchem.2006.05.029

The reaction of the formyl-capped cluster HC(O)CCo₃(CO)₉ (1) with the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) in the presence of added Me₃NO leads to the production of the disubstituted cluster HC(O)CCo₃(CO)₇(bpcd) (2). Thermolysis of 2 in toluene at 60 °C gives the methylidyne-capped cluster HCCo₃(CO)₇(bpcd) (4) and the phosphido-bridged cluster Co₃(CO)₇[μ₂,η²,η¹-P(Ph)C{double bond, long}C(PPh₂)C(O)CH₂C(O)] (5). Cluster 4 has been independently prepared from HCCo₃(CO)₉ and bpcd and shown to serve as the precursor to 5. The new clusters 2, 4, and 5 have been isolated and characterized in solution by IR and NMR (¹H and ³¹P) spectroscopies and their solid-state structures have been established by X-ray diffraction analyses. Both clusters 2 and 4 contain 48e- and exhibit triangular Co₃ cores with a chelating and bridging bpcd ligand in the solid state, respectively. The structure of 5 provides unequivocal support for the loss of the methylidyne capping ligand and P-Ph bond cleavage attendant in the activation of 4 and confirms the presence of the face capping seven-electron μ₂,η²,η¹-P(Ph)C{double bond, long}C(PPh₂)C(O)CH₂C(O) ligand in the final product. The fluxionality displayed by the bpcd ligand in clusters 2 and 4 and the decarbonylation behavior of the formyl moiety in the former cluster are discussed relative to related alkylidyne-capped Co₃ derivatives.

Full text of DOI:10.1016/j.jorganchem.2006.05.029

Journal of Organometallic Chemistry 691 (2006) 3609–3616
www.elsevier.com/locate/jorganchem
Ligand substitution in HC(O)CCo3(CO)9 with
4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd):
Diphosphine ligand fluxionality, decarbonylation of the formyl
moiety and competitive P–Ph bond cleavage reactivity
a,
a
a
William H. Watson *, Satish G. Bodige , Krzysztof Ejsmont ,
b
b,
*
Jie Liu , Michael G. Richmond
a
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, United States
b
Department of Chemistry, University of North Texas, Denton, TX 76203, United States
Received 24 April 2006; received in revised form 8 May 2006; accepted 9 May 2006
Available online 27 May 2006
Abstract
The reaction of the formyl-capped cluster HC(O)CCo3(CO)9 (1) with the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopen-
ten-1,3-dione (bpcd) in the presence of added Me3NO leads to the production of the disubstituted cluster HC(O)CCo3(CO)7(bpcd) (2).
Thermolysis of 2 in toluene at 60 °C gives the methylidyne-capped cluster HCCo3(CO)7(bpcd) (4) and the phosphido-bridged cluster
Co3(CO)7[l2,g2,g1-P(Ph)C@C(PPh2)C(O)CH2C(O)] (5). Cluster 4 has been independently prepared from HCCo3(CO)9 and bpcd and
shown to serve as the precursor to 5. The new clusters 2, 4, and 5 have been isolated and characterized in solution by IR and NMR
(1H and 31P) spectroscopies and their solid-state structures have been established by X-ray diffraction analyses. Both clusters 2 and 4
contain 48e- and exhibit triangular Co3 cores with a chelating and bridging bpcd ligand in the solid state, respectively. The structure
of 5 provides unequivocal support for the loss of the methylidyne capping ligand and P–Ph bond cleavage attendant in the activation
of 4 and confirms the presence of the face capping seven-electron l2,g2,g1-P(Ph)C@C(PPh2)C(O)CH2C(O) ligand in the final product.
The fluxionality displayed by the bpcd ligand in clusters 2 and 4 and the decarbonylation behavior of the formyl moiety in the former
cluster are discussed relative to related alkylidyne-capped Co3 derivatives.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Tetrahedral clusters; Ligand substitution; P–Ph bond cleavage; Decarbonylation
1. Introduction
RCCo3(CO)7(P–P) [1–4]. This genre of trimetallic clusters
continues to command our attention due to the novel
reactivity that these systems exhibit as a result of the
ancillary diphosphine ligand. For example, the nondisso-
ciative diphosphine isomerization displayed by the bma
and bpcd ligands at many different cluster motifs allows
for facile interconversion between the chelating and bridg-
ing forms of the cluster; this isomerization behavior is
depicted in Eq. (1) for PhCCo3(CO)7(bma). The fluxional
process involving these and related diphosphine ligands
points to the under appreciated and generally overlooked
fact that tertiary phosphines are capable of participation
Coordination of the diphosphine ligands 2,3-bis(diph-
enylphosphino)maleic anhydride (bma) and 4,5-bis(di-
phenylphosphino)-4-cyclopenten-1,3-dione (bpcd) to the
tetrahedrane clusters RCCo3(CO)9 [R = Ph, Fc, Me, Cl]
affords the corresponding diphosphine-substituted clusters
*
Corresponding authors. Tel.: +1 817 257 7195 (W.H. Watson), +1 940
565 3548 (M.G. Richmond).
E-mail addresses: w.watson@tcu.edu (W.H. Watson), cobalt@unt.edu
(M.G. Richmond).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.05.029

3610
W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
in nondissociative terminal-bridge-terminal phosphine
ligand scrambling about a polynuclear metal cluster [5].
Such a dynamic phosphine exchange allows for the net
permutation of a given phosphine moiety between metal
centers, avoiding the generation of a high-energy, unsatu-
rated cluster through a dissociative release of the phos-
phine moiety, followed its recoordination to the cluster
polyhedron. The migratory behavior that has been pro-
posed for these phosphine isomerizations parallels that of
the CO ligand, whose fluxionality at numerous metal clus-
ters is a well-established phenomenon [6]
(l2-PPh2) [1,2], while the Me- and Cl-capped derivatives
display gross decomposition under comparable thermolysis
conditions [5]. The last manifold for cluster/ligand activa-
tion involves the clusters HC(O)CCo3(CO)7(P–P) and
HCCo3(CO)7(P–P), whose reaction chemistry serves as
the basis of this report. The course of reactivity for the
H- and HC(O)-capped clusters RCCo3(CO)7(bpcd) is illus-
trated in lower right-hand portion of Scheme 1. Here the
formal loss of the capping carbyne moiety and one of the
phenyl groups from the diphosphine ligand furnishes the
phosphido-bridged cluster Co3(CO)7[l2,g2,g1-P(Ph)C@
C(PPh2)C(O)CH2C(O)]. These data suggest that subtle ste-
ric and electronic effects strongly influence the final product
outcome in the thermolysis reactions of these RCCo3-
(CO)7(P–P) clusters in a manner that is not fully under-
stood at this time. Definitive proof for the enhanced
diphosphine ligand reactivity in RCCo3(CO)7(bma) and
RCCo3(CO)7(bpcd) derives from the synthesis of the re-
lated clusters containing the archetypal diphosphine ligand
(Z)-Ph2PCH@CHPPh2. The structurally similar clusters
RCCo3(CO)7[(Z)-Ph2PCH@CHPPh2] [7] have been shown
to be stable under conditions comparable to those used to
promote the transformations illustrated in Scheme 1.
Herein we present our data on the ligand substitution in
the tricobalt clusters HC(O)CCo3(CO)9 and HCCo3(CO)9
with the diphosphine ligand bpcd, which initially give the
cluster compounds RCCo3(CO)7(bpcd) and ultimately fur-
nish the phosphido-bridged cluster Co3(CO)7[l2,g2,g1-
P(Ph)C@C(PPh2)C(O)CH2C(O)] (5) upon thermolysis.
The solution spectroscopic data and the X-ray diffraction
structures of clusters 2, 4, and 5 confirm the identities of
the substitution products and allow for the delineation of
a working reaction sequence leading to cluster 5.
Ph
Ph
C
Ph2
P
C
O
O
Co
Co
PPh2
O
Co
Co
P
O
Ph2
O
Co
Co
P
Ph2
O
chelating
bridging
ð1Þ
Apart from the dynamics associated with the diphosphine
isomerization, the tetrahedrane clusters RCCo3(CO)7(bma)
and RCCo3(CO)7(bpcd) have also been found to undergo
facile cluster/diphosphine activation over the temperature
range 30–60 °C, with the exact mode of reaction being
influenced by the capping R group. Scheme 1 summarizes
the known reactivity pathways for the tricobalt cluster fam-
ily, where three different modes of reaction have been
found. When the capping RC group is Ph or ferrocenyl
(Fc), the RCCo3(CO)7(P–P) clusters decompose with the
formation of the benzylidene- and ferrocenylidene-capped
clusters Co3(CO)6[l2,g2,g1-C(R)C@C(PPh2)C(O)XC(O)]-
R
C
Co
PPh2
x = O (bma), CH2 (bpcd)
O
P
Ph2
X
O
R = Ph, Fc
R = H, HC(O)
R = Me, Cl
O
O
R
C
Ph
P
X
X
decomposition
O
Ph2
P
O
Co
Co
Co
Co
PPh2
PPh2
Co
Co
Scheme 1.

W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
3611
2. Experimental
under vacuum and the crude product was purified by
low-temperature (ꢀ78 °C) column chromatography over
silica gel using CH2Cl2/petroleum ether (1:1) as the eluent.
The black-green solid that was obtained was then recrystal-
lized from CH2Cl2 and heptane. Yield of 2: 0.12 g (68%).
IR (CH2Cl2): m(CO) 2071 (vs), 2022 (vs), 1753 (w, symm
dione), 1718 (m, antisymm dione), 1650 (b, vw, formyl)
2.1. General methods
The starting clusters HC(O)CCo3(CO)9 and HCCo3-
(CO)9 were synthesized from Co2(CO)8 (Strem Chemical
Co.) according to the procedure of Seyferth [8], while the
diphosphine ligand bpcd was prepared from 4,5-dichloro-
4-cyclopenten-1,3-dione and Ph2PSiMe3 [9,10]. All reaction
and NMR solvents were distilled under argon from a suit-
able drying agent and stored in Schlenk storage vessels [11].
The combustion analyses were performed by Atlantic
Microlab, Norcross, GA.
cmꢀ1 1H NMR (CD2Cl2; 298 K): d 3.28 (s, 2H, CH2),
.
7.25–7.42 (m, 20H, aryl), 10.43 (s, CHO). 31P NMR
(THF; 298 K): d 43 (broad); (THF; 187 K): d 64.19 (chelat-
ing, 45%), 37.39 (bridging, 55%). Anal. Calcd. (found) for
C38H23Co3O10P2: C, 52.59 (52.62); H, 2.60 (2.88).
All reported infrared data were recorded on a Nicolet 20
SXB FT-IR spectrometer in 0.1 mm amalgamated NaCl
cells, using PC control and OMNIC software, while the 1H
(300 MHz) and 31P NMR (121 MHz) spectra were recorded
on a Varian 300-VXR spectrometer. The 31P NMR data
were acquired in the proton-decoupled mode and are
reported relative to external H3PO4, taken to have d = 0.
2.3. Reaction between HCCo3(CO)9 (3) and bpcd in the
presence of Me3NO
To 0.22 g (0.50 mmol) of HCCo3(CO)9 and 0.23 g
(0.50 mmol) of bpcd in 30 mL of CH2Cl2 was added
75 mg (1.0 mmol) of Me3NO. The solution immediately
changed from purple to black and stirring was continued
for an additional 0.5 h, after which the solvent was
removed under vacuum. Pure 4 was isolated by low-tem-
perature (ꢀ78 °C) column chromatography using CH2Cl2
as the eluent. Yield: 0.10 g (23%). Due the relative instabil-
ity of 4, the product was spectroscopically characterized in
solution. IR (CH2Cl2): m(CO) 2063 (vs), 2008 (vs), 1748 (w,
2.2. Reaction between HC(O)CCo3(CO)9 (1) and bpcd in
the presence of Me3NO
To 0.094 g (0.20 mmol) of HC(O)CCo3(CO)9 and
0.093 g (0.20 mmol) of bpcd in 20 mL of CH2Cl2 at 0 °C
was added 34 mg (0.45 mmol) of Me3NO in one portion
under argon flush. The reaction solution was stirred with
warming to room temperature (<1 h) at which time TLC
analysis revealed the presence of new spot (Rf = 0.40 in
CH2Cl2) belonging to cluster 2. The solvent was removed
1
symm dione), 1717 (m, antisymm dione) cmꢀ1. H NMR
(CD2Cl2; 298 K): d 3.29 (s, 2H, CH2), 7.35–7.49 (m, 20H,
aryl), 10.99 (s, l3-CH). 31P NMR (THF; 298 K): d 51
(broad); (THF; 187 K): d 65.57 (chelating, 58%), 41.20
(bridging, 42%).
Table 1
X-ray crystallographic data and processing parameters for the tricobalt clusters 2, 4, and 5
Compound number
CCDC entry number
Space group
2
4
5
277904
277905
277906
ꢀ
Monoclinic, P21/c
8.854(2)
19.468(5)
20.867(5)
Triclinic, P1
11.5588(9)
12.493(1)
12.797(1)
98.268(1)
100.914(1)
101.228(1)
1748.1(2)
C37H23Co3O9P2
850.28
Monoclinic, Cc
10.395(1)
33.233(4)
9.204(1)
˚
a (A)
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
92.895(4)
108.501(2)
3
˚
V (A )
3593(1)
C38H23Co3O10P2
878.29
3014.3(6)
C30H17Co3O9P2
760.17
Molecular formula
Formula weight
Formula units per cell (Z)
Dcalcd (Mg/m3)
4
2
4
1.624
1.615
1.675
˚
k (Mo Ka) (A)
0.71073
1.517
0.71073
1.554
0.71073
1.791
Absorption coefficient (mmꢀ1
)
Maximum/minimum transmission
Total reflections
0.6249/0.8675
15,341
0.2849/0.4418
11,012
0.554/0.889
9580
Independent reflections
5159
7730
4924
Data/restraints/parameters
5159/0/478
0.0354
0.0741
7730/0/460
0.0300
0.0810
4924/2/397
0.0486
0.1175
R
Rw
GOF on F2
Weights
0.938
1.038
1.033
[0.04F2 + (rF)2]ꢀ1
0.498 and ꢀ0.348
[0.04F2 + (rF)2]ꢀ1
0.472 and ꢀ0.351
[0.04F2 + (rF)2]ꢀ1
0.747 and ꢀ0.870
3
˚
Largest difference in peak and hole (e/A )

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W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
2.4. Synthesis of Co3(CO)7[l2,g2,g1-
P(Ph)C@C(PPh2)C(O)CH2C(O)] (5) from the
thermolysis of cluster 2
Table 2
˚
Selected bond distances (A) and angles (°) for the tricobalt clusters 2, 4, and
5a
Cluster 2
Bond distances
To 0.10 g (0.10 mmol) of HC(O)CCo3(CO)7(bpcd) in a
Schlenk tube was added 20 mL of toluene, after which
the vessel was heated at 60 °C for 3.0 h. Upon cooling
the reaction was analyzed by TLC analysis, which revealed
the complete consumption of cluster 2 and the presence of
cluster 5 and decomposed material that remained at the
origin of TLC plate. 5 was subsequently isolated by column
chromatography at room temperature using CH2Cl2 as the
eluent. The resulting green solid was recrystallized from a
CH2Cl2 solution of 5 that had been layered with hexane.
Yield: 0.030 g (39%). IR (CH2Cl2): m(CO) 2072 (vs), 2028
(vs), 1712 (m, symm dione), 1686 (m, antisymm dione)
Co(1)–Co(2)
Co(2)–Co(3)
Co(1)–P(2)
Co(1)–C(37)
Co(2)–C(31)
Co(2)–C(37)
Co(3)–C(34)
Co(3)–C(37)
C(1)–C(2)
2.4944(7)
2.4922(8)
2.203(1)
1.884(3)
1.800(4)
1.921(3)
1.805(4)
1.932(3)
1.502(4)
1.336(4)
1.510(4)
Co(1)–Co(3)
Co(1)–P(1)
Co(1)–C(30)
Co(2)–C(33)
Co(2)–C(32)
Co(3)–C(36)
Co(3)–C(35)
C(37)–C(38)
C(1)–C(5)
2.4933(7)
2.220(1)
1.795(4)
1.793(4)
1.833(4)
1.786(4)
1.806(4)
1.444(4)
1.514(4)
1.510(4)
3.108(2)
C(2)–C(3)
C(4)–C(5)
C(3)–C(4)
P(1)ꢁ ꢁ ꢁP(2)
Bond angles
C(37)–Co(1)–P(2)
P(2)–Co(1)–P(1)
P(1)–Co(1)–Co(3)
P(1)–Co(1)–Co(2)
C(3)–P(2)–Co(1)
C(2)–C(3)–P(2)
114.3(1)
C(37)–Co(1)–P(1)
P(2)–Co(1)–Co(3)
P(2)–Co(1)–Co(2)
C(2)–P(1)–Co(1)
C(3)–C(2)–P(1)
106.1(1)
89.28(4)
110.47(3)
155.34(3)
103.7(1)
117.8(2)
156.97(3)
97.17(3)
104.1(1)
118.6(2)
1
cmꢀ1. H NMR (CDCl3; 298 K): d 3.38 (AB quartet, 2H,
CH2, JH–H = 20 Hz), 7.18–8.20 (m, 15H, aryl). 31P NMR
(THF; 187 K): d 171.58 (s, phosphido), 23.25 [s, Ph2P-
(dione)]. Anal. Calcd. (found) for C30H17Co3O9P2 Æ 1/5hex-
ane: C, 48.18 (48.29); H, 2.57 (2.45).
C(38)–C(38)–Co(1) 140.7(2)
C(38)–C(37)–Co(3) 122.2(2)
C(38)–C(37)–Co(2) 129.1(2)
Cluster 4
Bond distances
Co(1)–Co(2)
Co(2)–Co(3)
Co(2)–P(2)
Co(1)–C(31)
Co(2)–C(36)
Co(2)–C(37)
Co(3)–C(34)
Co(3)–C(37)
C(1)–C(2)
2.5. X-ray structural determinations
2.4740(4)
2.4837(4)
2.1889(5)
1.814(2)
1.773(2)
1.873(2)
1.791(2)
1.918(2)
1.500(3)
1.518(3)
3.493(1)
Co(1)–Co(3)
Co(1)–P(1)
Co(1)–C(30)
Co(1)–C(37)
Co(2)–C(35)
Co(3)–C(32)
Co(3)–C(33)
C(1)–C(5)
2.4937(4)
2.2052(5)
1.770(2)
1.890(2)
1.797(2)
1.789(2)
1.827(2)
1.520(2)
1.505(3)
1.357(3)
Tables 1 and 2 contain the X-ray data and processing
parameters and selected bond distances and angles, respec-
tively, for clusters 2, 4, and 5. The reported X-ray data
were collected on a Bruker SMARTe 1000 CCD-based
diffractometer at 213 K. The frames were integrated with
the available SAINT software package using a narrow-frame
algorithm [12], and the structures were solved and refined
using the SHELXTL program package [13]. The molecular
structures were checked by using PLATON [14], and solved
by direct methods with all nonhydrogen atoms refined
anisotropically. All carbon-bound hydrogen atoms were
assigned calculated positions and allowed to ride on the
attached heavy atom, unless otherwise noted.
C(2)–C(3)
C(4)–C(5)
C(3)–C(4)
P(1)ꢁ ꢁ ꢁP(2)
Bond angles
C(37)–Co(1)–P(1)
P(1)–Co(1)–Co(3)
P(2)–Co(2)–Co(1)
C(4)–P(1)–Co(1)
C(5)–C(4)–P(1)
109.50(6)
P(1)–Co(1)–Co(2)
C(37)–Co(2)–P(2)
P(2)–Co(2)–Co(3)
C(5)–P(2)–Co(2)
C(4)–C(5)–P(2)
102.72(2)
158.20(2)
103.19(2)
119.01(6)
126.4(1)
99.78(6)
149.59(2)
112.88(6)
124.6(1)
Cluster 5
3. Results and discussion
Bond distances
Co(1)–Co(2)
Co(2)–Co(3)
Co(2)–P(2)
Co(1)–C(24)
Co(1)–C(26)
Co(2)–C(28)
Co(3)–C(30)
Co(3)–C(5)
C(1)–C(2)
2.562(1)
2.5441(9)
2.146(1)
1.776(8)
1.797(7)
1.814(6)
1.786(5)
2.069(4)
1.477(7)
1.517(9)
Co(1)–Co(3)
Co(1)–P(1)
Co(2)–P(2)
Co(1)–C(25)
Co(2)–C(27)
Co(3)–C(29)
Co(3)–C(1)
C(1)–C(2)
2.576(1)
2.146(1)
2.199(2)
1.785(7)
1.793(6)
1.777(6)
2.055(5)
1.435(7)
1.522(7)
1.478(7)
3.1. Synthesis, bpcd fluxional behavior, and molecular
structure of HC(O)CCo3(CO)7(bpcd) (2) and
HCCo3(CO)7(bpcd) (4)
Treatment of an equimolar mixture of HC(O)C-
Co3(CO)9 and bpcd in CH2Cl2 with a slight excess of
Me3NO at room temperature led to an immediate reaction
and formation of the disubstituted cluster HC(O)CCo3-
(CO)7(bpcd). Attempts to purify cluster 2 at room
temperature by column chromatography led to extensive
decomposition of 2, necessitating the use of low-tempera-
ture (ꢀ78 °C) chromatography over silica gel using
CH2Cl2/petroleum ether as the eluent. Isolated samples
of HC(O)CCo3(CO)7(bpcd) are thermally and air sensitive
in both the solid state and in solution; 2 has been observed
to undergo slow decarbonylation to furnish the hydrogen-
capped cluster HCCo3(CO)7(bpcd) (4) on prolonged stor-
C(2)–C(3)
C(4)–C(5)
C(3)–C(4)
Bond angles
P(1)–Co(2)–P(2)
C(5)–P(1)–C(6)
C(6)–P(1)–Co(2)
C(6)–P(1)–Co(1)
C(27)–Co(2)–P(2)
C(24)–Co(1)–P(1)
C(26)–Co(1)–P(1)
C(28)–Co(2)–P(1)
89.86(5)
109.4(2)
126.4(2)
128.5(2)
108.9(2)
103.6(2)
139.0(2)
152.2(2)
C(1)–Co(3)–C(5)
C(5)–P(1)–Co(2)
C(5)–P(1)–Co(1)
Co(2)–P(1)–Co(2)
C(28)–Co(2)–P(2)
C(25)–Co(1)–P(1)
C(27)–Co(2)–P(1)
40.7(2)
101.3(2)
111.7(2)
73.29(4)
105.9(2)
99.7(2)
97.3(2)
a
Numbers in parentheses are estimated SDs in the least significant digits.

W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
3613
age at room temperature under argon (vide infra). The
transformation of 2 to 4 is inhibited under CO (1 atm).
HC(O)CCo3(CO)7(bpcd) was characterized in solution
by IR and NMR spectroscopies. The IR spectrum for 2
exhibits two strong signature terminal m(CO) bands at
2071 and 2022 cmꢀ1 typical of diphosphine-substituted
clusters RCCo3(CO)7(P–P) [7,15]. The two lower frequency
carbonyl stretches at 1753 and 1718 cmꢀ1 belong to the
vibrationally coupled C@O groups of the dione ring [16].
phine ligand at the one unique axial site and one of the two
equatorial sites [1,3,17]. In the cases of such chelating iso-
mers, two distinct 31P resonances are observed in the
absence of diphosphine equilibration between the equato-
rial and axial sites. This rocking motion of the diphosphine
ligand serves to equilibrate the two phosphine moieties
between the equatorial and axial sites, giving rise to a
weighted-average 31P resonance [18]
O
O
1
H
H
The H NMR spectrum of 2 in CD2Cl2 shows resonances
C
C
C
C
O
Ph2
P
at d 3.28 (s), 7.25–7.42 (m), and 10.43 (s) for the methylene,
aromatic, and formyl hydrogens, respectively. Cluster 2
reveals the presence of a single, broad 31P resonance at
room temperature in THF at ca. d 49, whose location is
strongly suggestive of a bridging bpcd ligand. Cooling this
sample to 187 K led to the production of two sharp 31P res-
onances at d 64.19 and 37.39 attributed to the bpcd chelat-
ing (45%) and bridging (55%) isomers of 2. Samples of
cluster 2 that were cycled several times over the tempera-
ture range of 187–298 K afforded 31P NMR spectral
changes consistent with a reversible equilibrium between
the isomeric bpcd clusters, as depicted below in Eq. (2)
and previously demonstrated by us for PhCCo3(CO)7(bma)
[1]. The Keq value at 187 K is 1.22 in favor of the bridging
isomer. The observation of a single 31P resonance for the
chelating isomer of 2 indicates that the two phosphorus
atoms are bound at the two equatorial sites, as expected
for a cluster having idealized Cs symmetry. An alternative
coordination mode for the bpcd ligand at a single cobalt
center in cluster 2 would involve the binding of the diphos-
Co
Co
PPh2
O
O
Co
Co
P
Ph2
Co
Co
P
Ph2
O
chelating 2
bridging 2
ð2Þ
The molecular structure for 2 was established by X-ray
crystallography. The ORTEP diagram of 2 is depicted in
the left-hand portion of Fig. 1. Cluster 2 contains 48-
valence electrons and displays a chelating bpcd ligand that
is attached to the Co(1) center. The three cobalt atoms in 2
form an equilateral triangle that is capped by the l3-
CCH(O) moiety. The three Co–Co bonds exhibit a mean
˚
distance of 2.4933 A, consistent with those distances in
the nonacarbonyl clusters RCCo3(CO)9 [19] and numerous
phosphine-substituted clusters RCCo3(CO)9ꢀnPn and
˚
RCCo3(CO)7(P–P) [20]. The Co(1)–P(1) [2.203(1) A] and
Fig. 1. Thermal ellipsoid plots of HC(O)CCo3(CO)7(bpcd) (2; left) and HCCo3(CO)7(bpcd) (4; right) showing the thermal ellipsoids at the 50% probability
level.

3614
W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
˚
Co(1)–P(2) [2.220(1) A] bond distances and the P(1)–
Co(1)–P(2) bond angle of 89.28(4)° are unexceptional with
respect to other bpcd-substituted compounds.
Cluster 4 was characterized in solution by the usual
solution methods and X-ray diffraction analysis. The IR
spectrum of 4 mirrors that of cluster 2 in the terminal car-
bonyl region with two strong m(CO) bands at 2063 and
2008 cmꢀ1, whose ca. 11 cmꢀ1 shift to lower frequency is
consistent with the presence of a hydrogen versus an elec-
tron-withdrawing formyl capping group. The two dione
carbonyl stretches in 4 are unaffected by the nature of the
capping carbyne moiety and appear at 1748 (w) and 1717
The chelation of the bpcd ligand in cluster 2 is of
interest because the vast majority of structurally charac-
terized diphosphine-substituted clusters RCCo3(CO)7-
(P–P) contain a bridging P–P ligand. Other than 2, the
only other clusters containing a chelating diphosphine
ligand that have been structurally characterized are
PhCCo3(CO)7(P–P) (where P–P = bma, bpcd) and
PhCCo3(CO)5(dmpe)2, all of which have been prepared
by our groups [1,3,21]. Moreover, cluster 2 is unique
because the seven ancillary carbonyl groups are all termi-
nal in nature, with the bpcd ligand bound to the Co(1)
center at both equatorial sites. The diphosphine ligand
in benzylidyne-capped clusters PhCCo3(CO)7(bpcd) and
PhCCo3(CO)7(bma) is coordinated at axial and equatorial
sites and is accompanied by three l2-CO groups (see Eq.
1) [22], each of which spans a Co–Co bond in solution at
low temperatures and the solid state of recrystallized
samples.
Samples of 2 were observed to decompose over the
course of weeks at room temperature, as initially assessed
by TLC analysis. Gently heating solutions of 2 at temper-
atures <50 °C led to the complete destruction of 2 and the
formation of a new cluster whose IR spectrum supported
the formation of a new bpcd-substituted cluster. Since the
decarbonylation of the acyl clusters RC(O)CCo3(CO)9
(where R = various aromatic groups) has been shown to
furnish the corresponding RCCo3(CO)9 clusters [23], it
seemed reasonable that decarbonylation of the formyl moi-
ety could yield the methylidyne-capped cluster HCCo3-
(CO)7(bpcd) (4). Our premise was subsequently confirmed
by the independent synthesis of 4 from the reaction of bpcd
with HCCo3(CO)9 (3). Treatment of cluster 3 with bpcd in
the presence of Me3NO afforded cluster 4 as a temperature-
and oxygen-sensitive black solid in low yields after
chromatographic purification at ꢀ78 °C. The direct ther-
molysis of 3 and bpcd also produced small amounts of 4
and the phosphido-bridged cluster Co3(CO)7[l2,g2,g1-
P(Ph)C@C(PPh2)C(O)CH2C(O)] (5), along with extensive
decomposition. Scheme 2 outlines the relationship between
clusters 2, 4, and 5.
(m) cmꢀ1 1H NMR resonances recorded at d 3.29 (s),
.
7.35–7.49 (m), and 10.99 (s) for the methylene, aromatic,
and methylidyne hydrogens are in full agreement with the
structure of 4. A single, broad 31P NMR resonance at d
51 was recorded at room temperature in THF. This reso-
nance sharpened slightly as the temperature was lowered
and split into two new resonances at d 65.57 and 41.20 at
187 K that are assigned to the chelating (58%) and bridging
(42%) isomers analogous to the isomeric behavior found
with cluster 2.
The thermal ellipsoid plot of cluster 4 shown in Fig. 1
(right-hand side) confirms the coordination of the bpcd
ligand across the Co(1)–Co(2) vector. The Co–Co bonds
in 4 are highly asymmetric in length, ranging from
˚
˚
2.4720(4) A [Co(1)–Co(2)] to 2.4937(4) A [Co(1)–Co(3)]
˚
and exhibiting a mean distance of 2.4831 A. The internu-
clear P(1)ꢁ ꢁ ꢁP(2) distance of 3.493(1) A is only slightly
˚
longer than the corresponding internuclear distance found
in the chelating bpcd ligand in cluster 2 and indicates that
the bridging coordination mode observed here exerts no
adverse perturbation on the cluster core. The observation
of a bridging bpcd ligand in the solid-state structure of 4
is of interest as the benzylidyne-capped cluster PhCCo3-
(CO)7(bpcd) displays a chelating diphosphine ligand [3].
The adoption of a bridging bpcd ligand in cluster 4 may
be attributed, in part, to the smaller methylidyne capping
`
ligand (HC) vis-a-vis PhCCo3(CO)7(bpcd); however, we
also note that the crystallization thermodynamics,
electronic effects from the capping ligand, and the partici-
pation of the ancillary carbonyls in the redistribution of
electron density about the molecular polyhedron may also
influence the outcome of the recrystallization. Clearly addi-
tional examples of RCCo3(CO)7(bpcd) clusters bearing dif-
ferent RC capping groups are required before any gross
O
H
O
Ph
C
C
H
C
P
O
heat/-CO
Co
PPh2
Co
O
Co
Co
Co
PPh2
PPh2
Co
Co
O
P
Ph2
P
O
Ph2
O
cluster 4
cluster 2
cluster 5
Scheme 2.

W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
3615
generalizations can be made concerning the preferred solid-
state structure in this genre of cluster. The seven CO groups
in 4 are all linear, and the alkene bond in the bpcd that is
defined by the C(4) and C(5) atoms exhibits a distance of
˚
1.357(3) A.
3.2. Activation of clusters 2 and 4 and molecular structure of
the phosphido-bridged cluster Co3(CO)7[l2,g2,g1-
P(Ph)C@C(PPh2)C(O)CH2C(O)] (5)
Heating samples of cluster 2 led to the loss of the for-
myl carbonyl group and formation of methylidyne cluster
4 and the concomitant formation of an additional prod-
uct that was later identified as Co3(CO)7[l2,g2,g1-
P(Ph)C@C(PPh2)C(O)CH2C(O)] (5). That 4 serves as
the direct precursor for 5 was independently verified from
thermolysis reactions using 4 as a starting material. Two
strong terminal m(CO) bands at 2072 and 2028 cmꢀ1 were
recorded for 5, along with two moderately intense dione
carbonyl groups at 1712 and 1686 cmꢀ1. The observed
low energy shift of ca. 35 cmꢀ1 in the latter two carbonyl
stretching bands supports the coordination of the dione
ring to the cluster frame through the carbon–carbon p
bond [24]. Unequivocal proof for the activation of the
bpcd ligand was seen in the 31P NMR spectrum of 5,
where two singlets at d 171.58 and 23.25 that are readily
ascribed to a bridging phosphido group and phosphine
moiety, respectively [25]. Examination of the same sam-
ple by 1H NMR spectroscopy shed no information on
the nature of the organic derived by-product(s) derived
from the activation of the methylidyne-capping group
and P–Ph bond due to extensive broadening of the 1H
resonances by trace amounts of paramagnetic Co(II)
species. The solution spectroscopic data for isolated 5
closely match those data reported by us for the related
tricobalt cluster Co3(CO)7[l2,g2,g1(Ph)C@C(PPh2)C(O)-
OC(O)], which has been isolated as one of four products
from the thermolysis of Co4(CO)9(mesitylene) and bma
[26].
Fig. 2. Thermal ellipsoid plot of Co3(CO)7[l2,g2,g1-P(Ph)C@C(PPh2)-
C(O)CH2C(O)] (5) showing the thermal ellipsoids at the 50% probability
level.
ing bond distances and angles are unexceptional and
require no comment.
4. Conclusions
The reaction between the formyl-capped cluster
HC(O)CCo3(CO)9 and the rigid diphosphine ligand bpcd
yields the substituted cluster HC(O)CCo3(CO)7(bpcd),
which is shown to undergo facile decarbonylation upon
heating to give the methylidyne-capped cluster HCCo3-
(CO)7(bpcd), followed by the loss of the capping HC moiety
and one phenyl ligand from the bpcd ligand to furnish the
phosphido-bridged cluster Co3(CO)7[l2,g2,g1-P(Ph)C@C-
(PPh2)C(O)CH2C(O)]. The fluxional behavior of the ancil-
lary bpcd ligand in the former two clusters has been verified
by VT 31P NMR spectroscopy, and the molecular struc-
tures of all three cluster products have been established
by X-ray crystallography. The stability and course of
cluster/diphosphine ligand activation in different RCCo3-
(CO)7(P–P) clusters will be investigated as a function of
the RC capping moiety.
The X-ray structure of 5 is shown in Fig. 2. Cluster 5
possesses 48-electrons and the structure consists of a trian-
gular array of cobalt atoms, where the mean Co–Co bond
˚
distance of 2.561 A is in good agreement with those
distances found in other tricobalt clusters and Co3-
(CO)7[l2,g2,g1-P(Ph)C@C(PPh2)C(O)OC(O)]. The pres-
ence of the seven-electron face-capping l2,g2,g1-P(Ph)C@
C(PPh2)C(O)CH2C(O) moiety confirms the loss of one of
the phenyl groups belonging to the bpcd ligand and the
methylidyne capping CH group. The fate of these moieties
has not been ascertained but the formation of the transient
carbene species ‘‘PhCH’’ from the phenyl and methylidyne
cap would not be unreasonable, which could subsequently
dimerize to give stilbene [27]. Coordination of the C(1)–
5. Supporting information available
˚
C(5) bond to Co(3) leads to a ca. 0.07 A elongation relative
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Center, CCDC No. 277904 for 2; 277905 for 4; and 277906
to the free p bond found in clusters 2 and 4, a fact that reaf-
firms the Duncanson–Dewar–Chatt bonding model
between an alkene and a transition metal [28]. The remain-

3616
W.H. Watson et al. / Journal of Organometallic Chemistry 691 (2006) 3609–3616
(c) S.G. Bott, J.C. Wang, M.G. Richmond, J. Chem. Crystallogr. 28
(1998) 401.
[16] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared
and Raman Spectroscopy, third ed., Academic Press, New York,
1990.
for 5. Copies of these data may be obtained free of charge
from the Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ UK [fax: +44 1223 336 033; email: deposit@ccdc.
ac.uk or http://www:ccdc.cam.ac.uk.
[17] For examples of this phenomenon in related mixed-metal clusters, see:
(a) S.G. Bott, K. Yang, S.-H. Huang, M.G. Richmond, J. Chem.
Crystallogr. 34 (2004) 883;
Acknowledgments
(b) S.G. Bott, K. Yang, M.G. Richmond, J. Organomet. Chem. 690
(2005) 3067.
[18] (a) H. Shen, S.G. Bott, M.G. Richmond, Inorg. Chim. Acta 241
(1996) 71;
Financial support from the Robert A. Welch Founda-
tion (Grant Nos. P-0074 to W.H.W. and B-1093 to
M.G.R) is appreciated.
(b) S.G. Bott, H. Shen, M.G. Richmond, J. Organomet. Chem. 689
(2004) 3426;
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