Phosphine, isocyanide, and alkyne reactivity at pentanuclear molybdenum/tungsten-iridium clusters

Article abstract of DOI:10.1039/c5dt00525f

The trigonal bipyramidal clusters M₂Ir₃(μ-CO)₃(CO)₆(η⁵-C₅H₅)₂(η⁵-C₅Me₄R) (M = Mo, R = Me 1a, R = H; M = W, R = Me, H) reacted with iso

Full text of DOI:10.1039/c5dt00525f

Dalton
Transactions
PAPER
Phosphine, isocyanide, and alkyne reactivity at
pentanuclear molybdenum/tungsten–iridium
clusters†
Cite this: Dalton Trans., 2015, 44,
7292
Peter V. Simpson, Michael D. Randles, Vivek Gupta, Junhong Fu, Graeme J. Moxey,
Torsten Schwich, Mahbod Morshedi, Marie P. Cifuentes and Mark G. Humphrey*
The trigonal bipyramidal clusters M2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-C5Me4R) (M = Mo, R = Me 1a, R = H;
M = W, R = Me, H) reacted with isocyanides to give ligand substitution products M2Ir3(μ-CO)3(CO)5(CNR’)-
t
(η5-C5H5)2(η5-C5Me4R) (M = Mo, R = Me, R’ = C6H3Me2-2,6 3a; M = Mo, R = Me, R’ = Bu 3b), in which
core geometry and metal atom locations are maintained, whereas reactions with PPh3 afforded M2Ir3-
(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me4R) (M = Mo, R = Me 4a, H 4c; M = W, R = Me 4b, H), with reten-
tion of core geometry but with effective site-exchange of the precursors’ apical Mo/W with an equatorial
Ir. Similar treatment of trigonal bipyramidal MIr4(μ-CO)3(CO)7(η5-C5H5)(η5-C5Me5) (M = Mo 2a, W 2b) with
PPh3 afforded the mono-substitution products MIr4(μ-CO)3(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) (M = Mo 5a;
M = W 5b), and further reaction of the molybdenum example 5a with excess PPh3 afforded the bis-substi-
tuted cluster MoIr4(μ3-CO)2(μ-CO)2(CO)4(PPh3)2(η5-C5H5)(η5-C5Me5) (6). Reaction of 1a with diphenylace-
tylene proceeded with alkyne coordination and CuC cleavage, affording Mo2Ir3(μ4–η2-PhC2Ph)-
(μ3-CPh)2(CO)4(η5-C5H5)2(η5-C5Me5) (7a) together with an isomer. Reactions of 2a and 2b with PhCuCR
afforded MIr4(μ3–η2-PhC2R)(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (M = Mo, R = Ph 8a; M = W, R = Ph 8b, H;
M = W, R = C6H4(C2Ph)-3 9a, C6H4(C2Ph)-4), while addition of 0.5 equivalents of the diynes 1,3-
C6H4(C2Ph)2 and 1,4-C6H4(C2Ph)2 to WIr4(μ-CO)3(CO)7(η5-C5H5)(η5-C5Me5) gave the linked clusters
[WIr4(CO)8(η5-C5H5)(η5-C5Me5)]2(μ6–η4-PhC2C6H4(C2Ph)-X) (X = 3, 4). The structures of 3a, 4a–4c, 5b, 6,
7a, 8a, 8b and 9a were determined by single-crystal X-ray diffraction studies, establishing the core iso-
merization of 4, the site selectivity for ligand substitution in 3–6, the alkyne CuC dismutation in 7, and
the site of alkyne coordination in 7–9. For clusters 3–6, ease of oxidation increases on increasing donor
strength of ligand, increasing extent of ligand substitution, replacing Mo by W, and decreasing core Ir
content, the Ir-rich clusters 5 and 6 being the most reversible. For clusters 7–9, ease of oxidation
diminishes on replacing Mo by W, increasing the Ir content, and proceeding from mono-yne to diyne,
although the latter two changes are small. In situ UV-vis-near-IR spectroelectrochemical studies of the
(electrochemically reversible) reduction process of 8b were undertaken, the spectra becoming increas-
ingly broad and featureless following reduction. The incorporation of isocyanides, phosphines, or alkyne
residues in these pentanuclear clusters all result in an increased ease of oxidation and decreased ease of
reduction, and thereby tune the electron richness of the clusters.
Received 5th February 2015,
Accepted 11th March 2015
DOI: 10.1039/c5dt00525f
www.rsc.org/dalton
Introduction
Research School of Chemistry, Australian National University, Canberra, ACT 2601,
Australia. E-mail: Mark.Humphrey@anu.edu.au; Fax: +61 2 6125 0750;
Tel: +61 2 6125 2927
Transition metal carbonyl clusters have attracted long-standing
interest.1 Due to ease of synthetic access to such species, con-
siderable effort has been expended defining the reactivity of
†Electronic supplementary information (ESI) available: Synthesis and spectro-
scopic characterization details for 3b, 4b, 4c, 4d, 5b, 8b, 8c, 9b, and 10b, X-ray
crystallographic data in CIF format for 3a, 4a–4c, 5b, 6, 7a, 8a, 8b, and 9a, low-nuclearity (Mn, n = 3, 4) homometallic clusters; medium-
crystal data for 3a, 4a–4c, 5b, 6, 7a, 8a, 8b, and 9a, ORTEP plots of 4b, 4c, 8a,
(n = 5, 6) and higher-nuclearity (n ≥ 7) examples are compara-
tively less-explored, although the increasing metal loading on
and 8b, and one of the two crystallographically distinct molecules of each of 6
and 7a, and UV-vis spectral data during reduction of 8b. CCDC 1018833 (3a),
an organic substrate that is possible with such clusters may
1018834 (4a), 1018835 (4b), 1018836 (4c), 1018837 (5b), 1018838 (6), 1018839
afford enhanced or heretofore unobserved forms of activation.2
(7a), 1018840 (8a), 1018841 (8b), and 1018842 (9a). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5dt00525f
In this context, heterometallic clusters are also less explored,
7292 | Dalton Trans., 2015, 44, 7292–7304
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
with the vast majority of examples studied thus far incorporat- W2Ir3(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me5) (4b), Mo2Ir3-
ing metal atoms from either the same transition series group (μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me4H) (4c), W2Ir3(μ-CO)4-
or from adjacent groups; mixed-metal clusters containing dis- (CO)4(PPh3)(η5-C5H5)2(η5-C5Me4H) (4d), WIr4(μ-CO)3(CO)6-
parate metals are still under-represented, despite the fact that (PPh3)(η5-C5H5)(η5-C5Me5) (5b), WIr4(μ3–η2-PhC2Ph)(μ3-CO)2-
the resultant (more) polar metal–metal bonds may provide (CO)6(η5-C5H5)(η5-C5Me5)
(8b),
WIr4(μ3–η2-HC2Ph)(CO)8-
additional avenues for substrate activation.3 The varying con- (η5-C5H5)(η5-C5Me5) (8c), WIr4(μ3–η2-PhCCC6H4(C2Ph)-4)(CO)8-
stituent metals of the cluster core in mixed-metal clusters (η5-C5H5)(η5-C5Me5) (9b) and [WIr4(CO)8(η5-C5H5)(η5-C5Me5)]2-
also afford the possibility of probing metallo-, bond-, and site- (μ6–η4-PhCCC6H4(CCPh)-4) (10b) and reactions of 2a with
selectivity for a variety of reagents. Reactivity studies of CNBut and CNC6H3Me2-2,6 and 2c and 2d with PPh3 are given
medium- and high-nuclearity mixed-metal clusters incorporat- in the ESI.†
ing disparate metals are therefore of considerable interest.
Instrumentation
Phosphines and alkynes are arguably the archetypal sub-
strates for exploring two-electron donor ligand and cluster- Infrared spectra were recorded on PerkinElmer System 2000
bound C-ligand chemistry, respectively, while isocyanides CNR and PerkinElmer Spectrum One FT-IR spectrometers using a
are carbonyl-like ligands with considerable flexibility in com- CaF2 solution cell and AR grade n-hexane or CH2Cl2 solvent;
position. We have previously reported extensive studies of the spectral features are reported in cm−1 1H NMR spectra were
.
phosphine, isocyanide, and alkyne chemistry of the tetrahedral recorded on a Varian Gemini-300 spectrometer at 300 MHz in
group 6-group 9 transition metal clusters MIr3(CO)11(η5-C5R5) CDCl3 (Cambridge Isotope Laboratories) and referenced to
and M2Ir2(CO)10(η5-C5R5)2 (M = Mo, W; R5 = H5, H4Me, HMe4, residual solvent (δ 7.26). 31P NMR spectra were recorded on a
Me5),4 defining the isomers’ structures, distribution and CO Varian Gemini-300 spectrometer at 121 MHz in CDCl3 and
fluxionality resultant upon mono- to tris-phosphine and iso- referenced to external 85% H3PO4. Unit resolution and high-
cyanide substitution, and affording an array of unique aryl- resolution ESI mass spectra were recorded on a Micromass-
diyne- and -triyne-linked oligoclusters. However, as mentioned Waters LC-ZMD single quadrupole liquid chromatograph-MS
above, the diversity of the chemistry is constrained by the low- instrument, and are reported in the form: m/z (assignment,
nuclearity of the cluster. We very recently reported the high- relative intensity). Microanalyses were carried out at the Micro-
yielding syntheses and structural characterization of the trigo- analysis Service Unit in the Research School of Chemistry,
nal bipyramidal clusters M2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-C5Me4R) ANU, or at the School of Human Sciences, Science Centre,
(R = Me, M = Mo 1a, W 1b; R = H, M = Mo 1c, W 1d) and London Metropolitan University, UK.
MIr4(μ-CO)3(CO)7(η5-C5H5)(η5-C5Me4R) (R = Me, M = Mo 2a,
Cyclic voltammetry
W 2b; R = H, M = Mo 2c, W 2d).5 We report herein the results
of studies exploring the isocyanide and phosphine substitution Measurements were recorded at room temperature using an
chemistry of these pentanuclear clusters, as well as their reac- EA161 potentiostat and e-corder from eDaq Pty Ltd, with plati-
tivity towards arylalkynes, with the products from these num disk working, platinum wire auxiliary, and Ag/AgCl reference
studies featuring cluster core isomerization, site selectivity for electrodes, such that the ferrocene/ferrocenium redox couple was
ligand substitution, and alkyne CuC dismutation.
located at 0.56 V (ipc/ipa = 1; ΔEp = 0.09 V). Scan rates were typi-
cally 100 mV s−1. Solutions contained 0.1 M (NnBu4)PF6 and ca.
10−3 M complex in dried, distilled dichloromethane, and were
deoxygenated and maintained under a nitrogen atmosphere.
Experimental
General conditions and reagents
Spectroelectrochemical studies
Reactions were performed under an atmosphere of nitrogen Electronic spectra of 8b were recorded using a Cary 5 spectro-
using standard Schlenk techniques, although no special pre- photometer. Solution spectra of the oxidized species were
cautions were taken to exclude air in the work-ups. Reactions obtained at 298 K by electrogeneration in a custom-built opti-
were monitored regularly by IR spectroscopy to ensure con- cally transparent thin-layer electrochemical (OTTLE) cell with
sumption of the starting cluster. Solvents used in reactions potentials of ca. 200 mV beyond E1/2 to ensure complete elec-
were AR grade and distilled under nitrogen using standard trolysis. The solution was made using 0.3 M (NnBu4)PF6 in
methods: CH2Cl2 over CaH2, toluene and THF over sodium dried, distilled dichloromethane under a nitrogen atmosphere.
benzophenone ketyl. All other solvents and other reagents
Syntheses
were obtained commercially and were used as received. Petrol
refers to a fraction of boiling range 60–80 °C. Cluster products
Synthesis of Mo2Ir3(μ-CO)3(CO)5(CNC6H3Me2-2,6)(η5-C5H5)2-
were purified by preparative thin-layer chromatography (TLC) (η5-C5Me5) (3a). CNC6H3Me2-2,6 (3.0 mg, 23.3 μmol) was
on 20 × 20 cm glass plates coated with Merck GF254 silica gel added to a brown solution of Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-
(0.5 mm). Analytical TLC was conducted on aluminium sheets C5Me5) (1a) (10.0 mg, 7.8 μmol) in THF (10 mL) and the resul-
coated with 0.25 mm Merck GF254 silica gel. Literature pro- tant mixture was heated at reflux for 20 h. The solution was
cedures were used to synthesize 1a–1d and 2a–2d.5 The syn- taken to dryness in vacuo, and the crude residue dissolved in
theses of Mo2Ir3(μ-CO)3(CO)5(CNtBu)(η5-C5H5)2(η5-C5Me5) (3b), the minimum amount of CH2Cl2 and applied to a silica pre-
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 7292–7304 | 7293

Paper
Dalton Transactions
parative TLC plate. Elution with CH2Cl2–petrol (4 : 1) afforded C5Me5). 31P NMR:
δ
16.5 (PPh3). MS (ESI): calc.,
four bands. The contents of the first (Rf = 0.7, yellow), second C42H35Ir4MoO9P, 1579 ([M]+); found, 1579 ([M]+, 100). Anal: calc.
(Rf = 0.4, brown) and third (Rf = 0.2, brown) bands were in for C42H35Ir4MoO9P: C 31.94, H 2.23%: found C 32.18, H 2.00%.
trace amounts and were not isolated. The contents of the
Synthesis of MoIr4(μ3-CO)2(μ-CO)2(CO)4(PPh3)2(η5-C5H5)(η5-
fourth band (Rf = 0.1, brown) were extracted into CH2Cl2, C5Me5) (6). PPh3 (2.2 mg, 8.4 mmol) was added to a solution
which was then reduced in volume to afford a brown solid of MoIr4(μ-CO)3(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) (5a, 4.4 mg,
identified as 3a (5.9 mg, 55%). IR (CH2Cl2): ν(NC) 2111 w, 2.8 mmol) in THF (5 mL), and the resultant solution was
1
ν(CO) 2003 s, 1963 w br, 1947 w sh, 1869 w, 1822 w cm−1. H heated at reflux for 6 h. The solution was taken to dryness
NMR: δ 7.10–7.16 (m, 3H, C6H3), 5.15 (s, 5H, C5H5), 5.00 (s, in vacuo, and the crude residue dissolved in the minimum
5H, C5H5), 2.41 (s, 6H, Me), 1.92 (s, 15H, C5Me5). MS (ESI): amount of CH2Cl2 and applied to a silica preparative TLC
calc., C37H34Ir3Mo2NO8, 1389 ([M]+); found, 1389 ([M]+, 7), plate. Elution with CH2Cl2–petrol (4 : 1) afforded three bands.
1361 ([M − CO]+, 5), 1333 ([M − 2CO]+, 14), 1305 ([M − 3CO]+, Bands appearing in trace amounts were not isolated. The con-
35), 1277 ([M − 4CO]+, 100), 1249 ([M − 5CO]+, 16). Anal: calc. tents of the second band (Rf = 0.6, red) were extracted with
for C37H34Ir3Mo2NO8: C 31.99, H 2.47, N 1.01%: found CH2Cl2 and the extracts reduced in volume to afford a red
C 31.61, H 2.77, N 1.00%.
solid (0.7 mg, 16%), identified as 5a from IR spectral compar-
Synthesis of Mo2Ir3(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me5) ison to an authentic sample. The contents of the third band
(4a). PPh3 (13.5 mg, 51.5 μmol) was added to a brown solution (Rf = 0.2) were extracted into CH2Cl2, which was then reduced
of Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-C5Me5) (1a) (14.0 mg, in volume to afford a brown solid, identified crystallographi-
10.9 μmol) in THF (10 mL) and the resultant mixture was cally as 6 (3.7 mg, 73%). IR (CH2Cl2): ν(CO) 1999 m,
heated at reflux for 20 h. The solution was taken to dryness 1974 m, 1943 m, 1870 w, 1786 w, 1734 w, 1671 w cm−1 1H
.
in vacuo, and the crude residue dissolved in the minimum NMR: δ 7.23–7.45 (m, 30H, PPh3), 4.49 (s, 5H, C5H5), 1.94 (s,
amount of CH2Cl2 and applied to a silica preparative TLC 15H, C5Me5). 31P NMR: δ 33.3, −3.7 (PPh3). MS (ESI): calc.,
plate. Elution with CH2Cl2–petrol (4 : 1) afforded two bands. C59H50Ir4MoO8P2, 1818.0553 ([M]+); found, 1818.0544 ([M]+).
The contents of the first (Rf = 0.4, brown) band were in trace Anal: calc. for C59H50Ir4MoO8P2: C 39.07, H 2.78%: found
amounts and were not isolated. The contents of the second C 38.97, H 2.79%.
band (Rf = 0.2, red-brown) were extracted into CH2Cl2, which
Synthesis of Mo2Ir3(μ4–η2-PhC2Ph)(μ3-CPh)2(CO)4(η5-C5H5)2-
was then reduced in volume to afford a red-brown solid identi- (η5-C5Me5) (7a). Diphenylacetylene (2.3 mg, 12.9 μmol) was
fied as 4a (14.2 mg, 86%). IR (CH2Cl2): ν(CO) 2006 s, 1956 s br, added to a brown solution of Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2-
1
1868 w, 1771 m br, 1712 m br cm−1. H NMR: δ 7.58–7.43 (m, (η5-C5Me5) (1a) (16.7 mg, 13.0 μmol) in toluene (10 mL) and
15H, Ph), 4.80 (s, 10H, Cp), 1.84 (s, 15H, C5Me5). 31P NMR the resultant mixture was heated at reflux for 1 h, monitoring
(CDCl3): δ 19.1 (s, PPh3). MS (ESI): calc., C46H40Ir3Mo2O8P, by IR spectroscopy. The solution was taken to dryness in vacuo,
1520 ([M]+); found, 1559 ([M + K]+, 9), 1543 ([M + Na]+, 17), and the crude residue was dissolved in the minimum amount
1520 ([M]+, 9), 1492 ([M − CO]+, 10), 1464 ([M − 2CO]+, 37), of CH2Cl2 and applied to a silica preparative TLC plate.
1436 ([M − 3CO]+, 35), 1408 ([M − 4CO]+, 47), 1380 ([M − Elution with CH2Cl2–petrol (1 : 1) afforded four bands. The
5CO]+, 100), 1352 ([M
−
6CO]+, 46). Anal: calc. for contents of the first band (Rf = 0.7, orange) were extracted with
C46H40Ir3Mo2O10P: C 36.34, H 2.65%: found C 36.45, H 2.35%. CH2Cl2 and reduced in volume to afford an orange solid,
Synthesis of MoIr4(μ-CO)3(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) identified as 7a (2.1 mg, 11%). IR (CH2Cl2): ν(CO) 1992 vs,
1
(5a). PPh3 (1.9 mg, 7.36 μmol) was added to a brown solution 1939 s cm−1. H NMR: δ 7.38–6.38 (m, 20H, Ph), 4.69 (s, 5H,
of MoIr4(μ-CO)3(CO)7(η5-C5H5)(η5-C5Me5) (2a) (9.9 mg, Cp), 4.40 (s, 5H, Cp), 1.42 (s, 15H, C5Me5). MS (ESI): calc.,
7.36 μmol) in CH2Cl2 (5 mL) and the resultant mixture was C52H45Ir3Mo2O4, 1502 ([M]+); found, 1541 ([M + K]+, 45), 1525
stirred for 20 h. The solution was taken to dryness in vacuo, ([M + Na]+, 100), 1502 ([M]+, 51), 1474 ([M − CO]+, 39). Anal:
and the crude residue dissolved in the minimum amount of calc. for C52H45Ir3Mo2O4: C 41.57, H 3.02%: found C 41.45, H
CH2Cl2 and applied to a silica preparative TLC plate. Elution 2.74%. The contents of the second band (Rf = 0.5, purple) were
with CH2Cl2–petrol (4 : 1) afforded four bands. The contents of extracted into CH2Cl2, which was then reduced in volume to
the first (Rf = 0.7, yellow) and fourth (Rf = 0.1, pink) bands afford a purple solid identified as 7b (2.4 mg, 12%). IR
were in trace amounts and were not isolated. The contents of (CH2Cl2): ν(CO) 1993 vs, 1936 s, 1754 w, 1715 m cm−1 1H
.
the second band (Rf = 0.6, brown) were extracted into CH2Cl2 NMR: δ 7.03–6.35 (m, 20H, Ph), 5.18 (s, 10H, C5H5), 1.92 (s,
and the extract then reduced in volume, to afford an orange 15H, C5Me5). MS (ESI): calc., C52H45Ir3Mo2O4, 1502 ([M]+);
solid identified as MoIr3(μ-CO)3(CO)6(PPh3)2(η5-C5H5) (2.7 mg, found, 1541 ([M + K]+, 34), 1525 ([M + Na]+, 100), 1502 ([M]+,
48%) by MS and IR spectral comparison to an authentic 32), 1474 ([M − CO]+, 21). Anal: calc. for C52H45Ir3Mo2O4: C
sample.6 The contents of the third band (Rf = 0.4, red-brown) 41.57, H 3.02%: found C 41.78, H 2.74%. The contents of the
were extracted into CH2Cl2 and the extract was then reduced in third band (Rf = 0.4, green) were extracted into CH2Cl2, which
volume, to afford a red-brown solid identified as 5a (5.6 mg, was then reduced in volume to afford a green solid, identified
48%). IR (CH2Cl2): ν(CO) 2056 m, 2024 w sh, 2006 sh, 1995 s, as Mo2Ir2(μ4–η2-PhC2Ph)(CO)8(η5-C5H5)2 (0.4 mg, 3%) by IR
1963 w sh, 1879 w br, 1811 w, 1767 w, 1740 m cm−1. 1H NMR: δ spectral comparison to an authentic sample.7 The contents of
7.36–7.38 (m, 15H, PPh3), 5.01 (s, 5H, C5H5), 1.85 (s, 15H, the fourth band (Rf = 0.3, dark brown) were extracted into
7294 | Dalton Trans., 2015, 44, 7292–7304
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Paper
CH2Cl2, which was then reduced in volume to afford a brown Elution with acetone–petrol (2 : 3) afforded two bands; the con-
solid, identified as unreacted Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2- tents of the first band (Rf = 0.5, green) occurred in trace
(η5-C5Me5) (1a) (4.6 mg, 28%) by IR spectral comparison to an amounts and were not isolated. The contents of the second
authentic sample.5
and major band (Rf = 0.4, green) were extracted into CH2Cl2,
Synthesis of MoIr4(μ3–η2-PhC2Ph)(μ3-CO)2(CO)6(η5-C5H5)(η5- which was then reduced in volume to afford a green solid,
C5Me5) (8a). Diphenylacetylene (7.0 mg, 39.3 μmol) was added identified as 10a (1.6 mg, 7%). IR (CH2Cl2): ν(CO) 2035 w sh,
to a red-brown solution of MoIr4(μ-CO)3(CO)7(η5-C5H5)(η5- 2015 s, 2002 s, 1976 m, 1944 w, 1682 w, 1662 w br cm−1. MS
C5Me5) (2a) (4.5 mg, 3.3 μmol) in CH2Cl2 (5 mL) and the resul- (ESI): calculated, C68H54Ir8O16W2, 3037.9466 ([M]+); found,
tant mixture was heated at reflux for 3 h, the extent of reaction 3037.9460 ([M]+). The low yield precluded NMR spectroscopic
being monitored by IR spectroscopy. The solution was taken to and microanalytical analysis.
dryness in vacuo, and the crude residue dissolved in the
minimum amount of CH2Cl2 and applied to a silica prepara-
X-ray crystallographic studies
tive TLC plate. Elution with CH2Cl2–petrol (1 : 1) afforded one
General. The crystal and refinement data for compounds
band (Rf = 0.30, brown); extraction into CH2Cl2 and reduction 3a, 4a–4c, 5b, 6, 7a, 8a, 8b and 9a are summarized in
in volume of the extract afforded a brown solid identified as 8a Table S1.† Crystals suitable for the X-ray structural analyses
(4.0 mg, 82%). IR (CH2Cl2): ν(CO) 2034 vw, 2015 s, 1999 s, 1977 were grown by liquid diffusion of methanol into a dichloro-
w, 1943 w, 1682 m, 1671 w cm−1 1H NMR: δ 7.06–6.78 (m, methane solution (3a, 4b, 4c, 8b, 9a), liquid diffusion of
.
10H, Ph), 5.45 (s, 5H, Cp), 1.99 (s, 15H, C5Me5). MS (ESI): calc., ethanol into a dichloromethane solution (4a, 7a, 8a), liquid
C37H30Ir4MoO8, 1468 ([M]+); found, 1507 ([M + K]+, 54), 1491 diffusion of methanol into a chloroform solution (5b) or liquid
([M + Na]+, 100), 1468 ([M]+, 69). Anal: calc. for C37H30Ir4MoO8: diffusion of hexane into a dichloromethane solution (6) at
C 30.28, H 2.06%: found C 30.11, H 1.90%.
277 K. Suitable crystals were mounted on fine glass capillaries,
Synthesis of WIr4{μ3–η2-PhC2C6H4(CuCPh)–3}(μ3-CO)2- and intensity data were collected on a Nonius KAPPA CCD
(CO)6(η5-C5H5)(η5-C5Me5) (9a). 1,3-Bis(phenylethynyl)benzene diffractometer at 200 K using graphite-monochromated Mo-Kα
(2.0 mg, 7.2 μmol) was added to a solution of WIr4(μ-CO)3- radiation (λ = 0.71073 Å). Nt (total) reflections were measured
(CO)7(η5-C5H5)(η5-C5Me5) (2b) (10.0 mg, 7.0 μmol) in CH2Cl2, by using psi and omega scans and were reduced to No unique
and the resultant solution heated at reflux for 17 h, the extent reflections, with Fo > 2σ(Fo) being considered to be observed.
of reaction being monitored by IR spectroscopy. The solution Data were initially processed and corrected for absorption by
was taken to dryness in vacuo, and the crude residue dissolved using the programs DENZO8 and SORTAV.9 The structures
in the minimum amount of CH2Cl2 and applied to a silica pre- were solved using direct methods and observed reflections
parative TLC plate. Elution with CH2Cl2–petrol (3 : 1) afforded were used in least-squares refinement on F2, with anisotropic
at least six bands; bands appearing in trace amounts were not thermal parameters refined for non-hydrogen atoms. Hydro-
isolated. The contents of the fifth band (Rf = 0.2, green) were gen atoms were constrained in calculated positions and
extracted with CH2Cl2 and the extracts reduced in volume to refined with a riding model. Structure solutions and refine-
afford a green solid, identified as 9a (5.2 mg, 44%). IR ments were performed by using the CRYSTALS10 software
(n-hexane): ν(CO) 2040 w, 2025 vs, 2006 s, 1983 m, 1952 w, package or the programs SHELXS-97 and SHELXL-9711
1
1692 w, 1673 m cm−1. H NMR: δ 7.51–6.77 (m, 14H, C6H4 + through the graphical interface Olex2,12 which was also used
Ph), 5.60 (s, 5H, C5H5), 1.89 (s, 15H, C5Me5). MS (ESI): calcu- to generate the figures. In all structures, the largest peaks in
lated, C45H35Ir4O8W, 1659.0359 ([M + H]+); found, 1659.0354 the final difference electron map are located near the metal
([M + H]+). Slow decomposition of 9a precluded elemental atoms. Crystallographic data for the structural analyses have
analysis.
been deposited with the Cambridge Crystallographic Data
Synthesis of [WIr4(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5)]2(μ6–η4- Centre, CCDC numbers 1018833–1018842.
PhC2C6H4(C2Ph)-3)
(10a). 1,3-Bis(phenylethynyl)benzene
Variata. 4a. Disordered lattice solvent could not be success-
(1.0 mg, 3.6 μmol) was added to a solution of WIr4(μ-CO)3- fully modelled, so was removed from the refinement using
(CO)7(η5-C5H5)(η5-C5Me5) (2b) (10.0 mg, 7.0 μmol) in CH2Cl2, PLATON SQUEEZE.13 4b. Anisotropic displacement parameter
and the resultant solution was heated at reflux for 19 h, the restraints were applied to phenyl rings C21–C26, C27–C32 and
extent of reaction being monitored by IR spectroscopy. The C33–C38 of the triphenylphosphine moiety. Disordered lattice
solution was taken to dryness in vacuo, and the crude residue solvent could not be successfully modelled and was therefore
dissolved in the minimum amount of CH2Cl2 and applied to a removed from the refinement using PLATON SQUEEZE.13 6.
silica preparative TLC plate. Elution with CH2Cl2–petrol (3 : 1) The asymmetric unit contains two cluster molecules and three
afforded at least six bands; bands appearing in trace amounts molecules of dichloromethane. The Flack14 parameter refined
were not isolated. The contents of the fifth band (Rf = 0.2, to 0.491(8) indicating a nearly 50 : 50 inversion twin. A check
green) were extracted with CH2Cl2 and the extracts reduced in in PLATON ADDSYM15 confirmed that no additional
volume to afford a green solid, identified as 9a (4.0 mg, symmetry elements needed to be added to the space group,
2.4 μmol, 34%) by IR spectroscopic comparison with an auth- and thus Pca21 was the most suitable space group. Bond dis-
entic sample. The baseline was collected, extracted with tance restraints were applied to the two pentamethyl-
CH2Cl2 and re-applied to a silica preparative TLC plate. cyclopentadienyl
ligands
and
the
three
lattice
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Dalton Trans., 2015, 44, 7292–7304 | 7295

Paper
Dalton Transactions
dichloromethane molecules. Anisotropic displacement para-
meter restraints were applied to atoms C222, O222, C551,
O551, C881 and O881 (carbonyl ligands) and the cyclopenta-
dienyl and triphenylphosphine ligands. 7a. The asymmetric
unit contains one and a half cluster molecules; the second
cluster molecule is generated through a mirror plane that
passes through Mo6, Ir8 and Mo9. Disordered lattice solvent
could not be successfully modelled and was therefore removed
from the refinement using PLATON SQUEEZE.13 8a. The Cp
ligand (C1–C5) exhibited positional disorder. This was success-
fully modelled using a two-level model with 0.51(0.01) : 0.49
occupancy levels, in conjunction with anisotropic displace-
ment parameter restraints. 6b. The asymmetric unit contains
one cluster molecule, and two half molecules of dichloro-
methane. The Cp ligand (C1–C5) exhibited positional disorder,
successfully modelled using a two-level model with 0.59
(0.01) : 0.41 occupancy levels, in conjunction with anisotropic
displacement parameter restraints. Anisotropic displacement
parameter restraints were also applied to the lattice dichloro-
methane molecules. 9a. Anisotropic displacement parameter
restraints were applied to atoms C16–C37 (alkyne ligand),
atoms C1–C5, C6–C15 (Cp ligands), and the C and O atoms of
all carbonyl ligands. Disordered lattice solvent could not be
successfully modelled and was therefore removed from the
refinement using PLATON SQUEEZE.13
Scheme 1 Syntheses of 3–6.
in CH2Cl2 are analogous, with five ν(CO) bands spanning the
range 2006–1708 cm−1; the close visual similarity of the IR
spectra is suggestive of isostructural species. The 1H NMR
spectra of 4a and 4c each possess a single resonance for the
two cyclopentadienyl ligands, consistent with a symmetrical
structure for the clusters, whereas those of 4b and 4d each
contain two cyclopentadienyl resonances. The 31P NMR
spectra reveal singlet resonances for each cluster that move
downfield on proceeding from co-ligand C5Me5 to C5Me4H
and on replacing tungsten with molybdenum. The ESI mass
spectra each show a molecular ion with the correct character-
istic isotope pattern (see Fig. S1† for a representative ESI MS
molecular ion and its sodium adduct and their calculated
counterparts (those of 4d)).
Addition of one equivalent of PPh3 to 2a or 2b afforded
the phosphine-substituted products MIr4(μ-CO)3(CO)6(PPh3)-
(η5-C5H5)(η5-C5Me5) (M = Mo 5a, M = W 5b), isolated following
preparative TLC in yields of 48% and 40%, respectively. Clus-
ters 5a and 5b have been characterized by IR, 1H and 31P NMR
spectroscopies, ESI mass spectrometry and, in the case of 5b, a
single-crystal X-ray diffraction study. The solution IR spectra of
5a and 5b contain nine ν(CO) bands, six of which correspond
to terminally bound carbonyls (2056–1879 cm−1) and three to
bridging carbonyl modes (1821–1727 cm−1). As is the case with
4a–4d, the similarity in the spectra of 5a and 5b is suggestive
of isostructural clusters. The 1H NMR spectra of 5a and 5b
contain signals due to the Cp and Cp* ligands and the phenyl
groups of the triphenylphosphine unit, as expected, while 5a
and 5b show signals in the 31P NMR spectrum at δ 16.5 and
13.0, respectively, due to the triphenylphosphine ligand. The
ESI mass spectra of 5a and 5b confirm the cluster composition
with peaks showing the characteristic isotope pattern at 1579
([M]+) and 1670 ([M + Na]+) mass units, respectively. In contrast
Results and discussion
Syntheses and spectroscopic characterization of 3–6
The reaction of Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-C5Me5) (1a)
with three equivalents of 2,6-dimethylphenyl isocyanide or
t-butyl isocyanide in refluxing THF or CH2Cl2 led to the substi-
tution of one carbonyl ligand from one of the less-sterically-
hindered iridium atoms and the subsequent formation of 3a
and 3b, isolated as brown solids after preparative TLC in yields
of 55 and 24%, respectively (Scheme 1). 3a and 3b exhibit
ν(CN) bands in the solution IR spectra at 2111 and 2146 cm−1
,
respectively, while the ESI mass spectra of 3a and 3b reveal
molecular ion peaks with the correct characteristic isotope
pattern at 1389 and 1347 mass units, respectively. The mole-
cular structure of 3a was determined by a single-crystal X-ray
diffraction study (see below), with that of 3b presumed similar
due to the spectral similarities. In contrast to these obser-
vations, MoIr4(μ-CO)3(CO)7(η5-C5H5)(η5-C5Me5) (2a) reacted
with three equivalents of t-butyl isocyanide or five equivalents
of 2,6-dimethylphenyl isocyanide with loss of the Ir(η5-C5Me5)
vertices to give MoIr3(μ-CO)3(CNR)(CO)7(η5-C5H5).
Reaction of 1a with an excess of PPh3 in refluxing THF for
20 h led to the formation of 4a, which was isolated by prepara-
tive TLC as a red/brown solid in 86% yield (Scheme 1). Similar
reactions of 1b–1d with PPh3 in either refluxing CH2Cl2 or
refluxing THF afforded 4b–4d in good yields. The new clusters
1
were characterized by IR, H and 31P NMR spectroscopies, ESI
MS, satisfactory microanalyses and, in the case of 4a–4c,
single-crystal X-ray diffraction studies. The IR spectra of 4a–4d
7296 | Dalton Trans., 2015, 44, 7292–7304
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Dalton Transactions
Paper
to these observations, reactions of PPh3 with the tetramethyl-
cyclopentadienyl-ligated clusters 2c and 2d under the same
conditions did not afford any identifiable cluster species.
Amongst the products obtained following the addition of
one equivalent of PPh3 to a solution of 2a was the previously
reported tetranuclear cluster MoIr3(μ-CO)3(CO)6(PPh3)2(η5-
C5H5).6 To explore the possibility that in situ attack of 5a by a
second equivalent of PPh3 led to the displacement of the Ir(η5-
C5Me5) group, and subsequent formation of the bis-substi-
tuted tetranuclear product, a sample of 5a was treated with
one equivalent of PPh3 in CH2Cl2 at room temperature, but no
change was observed after 24 h. Similarly, addition of excess
PPh3 had no noticeable effect. When 5a was heated in reflux-
ing THF for 6 h with three equivalents of PPh3, however, a new
species began to form, which was isolated as a brown solid in
73% yield, and identified as the bis-substituted pentanuclear
cluster
MoIr4(μ-CO)3(CO)5(PPh3)2(η5-C5H5)(η5-C5Me5)
(6)
Fig. 1 ORTEP plot and atom numbering scheme for Mo2Ir3(μ-CO)3-
(CO)5(CNC6H3Me2-2,6)(η5-C5H5)2(η5-C5Me5) (3a). Displacement ellip-
soids are shown at the 40% probability level. Hydrogen atoms and lattice
dichloromethane molecule have been omitted for clarity. Selected bond
lengths: Mo1–Ir2 2.7670(5), Mo1–Ir3 2.8147(5), Mo1–Ir4 2.8349(5),
Mo1–Mo5 3.0536(7), Ir2–Ir3 2.7209(3), Ir2–Ir4 2.7369(3), Ir2–Mo5
2.8490(6), Ir3–Ir4 2.6859(3), Ir3–Mo5 2.7871(5), Mo1–C111 1.976(7),
Mo1–C141 1.966(6), Ir2–C21 1.967(6), Ir2–C221 1.881(6), Ir2–C241
1.994(6), Ir3–C331 1.859(6), Ir3–C332 1.870(6), Ir3–C351 2.456(7), Ir4–
C141 2.375(6), Ir4–C241 2.049(6), Mo5–C351 1.986(7), Mo5–C551
1.991(7), N1–C21 1.168(8) Å.
(Scheme 1), ruling out the possibility of 6 being an intermedi-
ate in the formation of MoIr3(μ-CO)3(CO)6(PPh3)2(η5-C5H5).
The solution IR spectrum of 6 contains seven ν(CO) bands,
four of which correspond to terminally bound carbonyls
(1999–1870 cm−1) and three to bridging/face-capping carbonyl
modes (1786–1671 cm−1). The 1H NMR spectrum of 6 is as
expected, while the 31P NMR spectrum shows two signals at δ
33.3 and −3.7 (note that the 31P NMR chemical shifts are not a
definitive indicator of ligation site in Mo/W–Ir clusters, spec-
tral assignment of which necessitates further studies: see, for
example, ref. 4e). The ESI mass spectrum of 6 reveals a peak
with the correct characteristic isotope pattern at 1818 ([M]+).
The structure of 6 was conclusively established from a single-
crystal X-ray diffraction study.
X-ray structural studies of 3a, 4a–4c, 5b, and 6
Single-crystal X-ray diffraction studies confirmed the molecular
compositions of 3a, 4a–4c, 5b, and 6. ORTEP plots with the
molecular structure and atomic labeling schemes for 3a, 4a,
5b, and one of the two crystallographically distinct molecules
of 6 are shown in Fig. 1–4, respectively. Fig. S2–S4† contain
ORTEP plots from the structural studies of 4b and 4c, and the
remaining crystallographically independent molecule of 6,
respectively; 4b and 4c are isostructural with 4a. Table S1 in
the ESI† contains crystallographic data acquisition and refine-
ment parameters, Table 1 lists selected bond distances for 4a–
4c, with selected bond distances for 3a, 5b, and the two mole-
cules of 6 being given in the respective figure captions.
The metal cores of 3a, 4a–4c, 5b, and 6 possess trigonal
bipyramidal geometries with the group 6 metal atom(s) and
one of the iridium atoms ligated by η5-cyclopentadienyl
ligands with varying degrees of alkylation. Whereas 3a, 5b, and
6 possess the same core metal disposition as their starting
clusters, the cores of 4a–4c result from a formal isomerization
Fig. 2 ORTEP plot and atom numbering scheme for Mo2Ir3(μ-CO)4-
(CO)4(PPh3)(η5-C5H5)2(η5-C5Me5) (4a). Displacement ellipsoids are
shown at the 40% probability level. Hydrogen atoms have been omitted
for clarity.
corresponding to interchange of an apical Mo/W and equator- four (4a–4c), or two (6) bridging carbonyls, and, in the case of
ial Ir. The structural studies reveal that the incoming ligand is 6, two face-capping carbonyl ligands. All possess 72 cluster
coordinated at equatorial (3a), apical (4a–4c, 5b), and equator- valence electrons, and so are EAN-precise for M5 clusters with
ial and apical (6) iridium atoms. The clusters contain five (3a), nine M–M bonds. While the core bond distances fall within
four (4a–4c, 6) or six (5b) terminal carbonyls, three (3a, 5b), the range of literature precedents, the M1–Ir5 (4a–4c) and
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Paper
Dalton Transactions
Table 1 Selected bond lengths (Å) for 4a–4c
4a M = Mo
4b M = W
4c M = Mo
M1–M2
M1–Ir3
M1–Ir4
M1–Ir5
M2–Ir3
M2–Ir4
M2–Ir5
Ir3–Ir4
Ir3–Ir5
M1–C111
M1–C141
M2–C241
M2–C251
Ir3–C331
Ir3–C332
Ir3–C351
Ir4–C141
Ir4–C241
Ir5–C551
Ir5–C251
Ir5–C351
Ir5–P1
2.9622(7)
2.8420(5)
2.7926(5)
2.8406(6)
2.8015(5)
2.8194(6)
2.8573(5)
2.7652(3)
2.7706(3)
1.993(6)
2.053(6)
1.997(7)
1.992(7)
1.914(7)
1.865(7)
2.125(6)
2.096(6)
2.261(6)
1.845(7)
2.396(6)
2.011(6)
2.3134(16)
2.9399(13)
2.8330(8)
2.7813(11)
2.8305(8)
2.7968(10)
2.8154(8)
2.8606(10)
2.7683(8)
2.7731(10)
2.010(10)
2.019(10)
1.992(9)
2.002(10)
1.903(10)
1.875(12)
2.123(10)
2.065(9)
2.9553(9)
2.8246(7)
2.8235(9)
2.8713(7)
2.8180(8)
2.7644(7)
2.8425(7)
2.7656(6)
2.7148(6)
1.969(6)
2.064(6)
1.994(5)
1.985(6)
1.913(6)
1.865(6)
2.171(6)
2.046(5)
2.301(6)
1.859(6)
2.481(6)
1.990(6)
2.3155(14)
Fig. 3 ORTEP plot and atom numbering scheme for WIr4(μ-CO)3-
(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) (5b). Displacement ellipsoids are shown
at the 40% probability level. Hydrogen atoms and lattice chloroform
molecule have been omitted for clarity. Selected bond lengths: W1–Ir2
2.8474(6), W1–Ir3 2.7710(6), W1–Ir4 2.7284(5), W1–Ir5 2.8677(5), Ir2–Ir3
2.7130(5), Ir2–Ir4 2.7621(5), Ir2–Ir5 2.7828(5), Ir3–Ir4 2.7118(5), Ir3–Ir5
2.7937(5), W1–C111 1.984(13), W1–C141 2.085(11), Ir2–C221 1.891(11),
Ir2–C222 1.883(14), Ir2–C251 2.057(10), Ir3–C331 1.933(14), Ir3–C332
1.857(12), Ir3–C351 2.090(11), Ir4–C141 2.079(11), Ir5–C251 2.146(10),
Ir5–C351 2.064(13), Ir5–C551 1.863(13), Ir5–P1 2.337(3) Å.
2.319(10)
1.862(11)
2.411(10)
2.000(10)
2.316(3)
Syntheses and spectroscopic characterization of 7–10
Reaction of one equivalent of diphenylacetylene with 1a pro-
ceeded in refluxing toluene over one hour, affording two main
products that were isolated following preparative TLC in yields
of 11% (7a) and 12% (7b) (Scheme 2). An X-ray structural
study of a single crystal of 7a identified the compound
as
Mo2Ir3(μ4–η2-PhC2Ph)(μ3-CPh)2(CO)4(η5-C5H5)2(η5-C5Me5)
1
(7a) (Fig. 5); its ESI mass spectrum and H NMR spectroscopy
data are consistent with this formulation. From a comparison
Fig. 4 ORTEP plot and atom numbering scheme for one of the two
crystallographically distinct molecules of MoIr4(μ3-CO)2(μ-CO)2-
(CO)4(PPh3)2(η5-C5H5)(η5-C5Me5) (6). Displacement ellipsoids are shown
at the 40% probability level. Hydrogen atoms and lattice dichloro-
methane molecules have been omitted for clarity. Selected bond
lengths: Mo1–Ir2 2.9194(13), Mo1–Ir3 2.7914(13), Mo1–Ir4 2.8359(13),
Mo1–Ir5 2.7743(13), Ir2–Ir3 2.7023(7), Ir2–Ir4 2.8145(8), Ir2–Ir5
2.7737(7), Ir3–Ir4 2.6830(8), Ir3–Ir5 2.7379(8), Mo1–C124 2.656(15),
Mo1–C134 1.978(15), Mo1–C151 2.102(19), Ir2–C124 2.157(13), Ir2–C221
1.881(15), Ir2–C251 2.164(12), Ir2–P1 2.369(4), Ir3–C134 2.522(15),
Ir3–C331 1.86(2), Ir3–C332 1.857(16), Ir4–C124 1.972(15), Ir4–C134
2.430(15), Ir5–C151 2.197(16), Ir5–C251 2.065(14), Ir5–C551 1.835(16),
Ir5–P2 2.335(4) Å.
Ir1–W5 (5b) bonds are the longest M–Ir vectors in these struc-
tures, consistent with their trans dispositions to the incoming
PPh3 ligand.
Scheme 2 Syntheses of 7–10.
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Dalton Transactions
Paper
and 9b in fair to good yields and 10a and 10b in poor yields
(Scheme 2), the products being isolated following preparative
TLC as brown (8a) or green solids and characterized by IR and
1H NMR spectroscopy and HR-ESI mass spectrometry, and, in
the case of 8a, 8b, and 9a, single-crystal X-ray diffraction
studies. Five ν(CO) bands (2041–1943 cm−1) corresponding to
terminally bound carbonyl ligands are observed in the solution
IR spectra; the IR spectra also display two weak bands corres-
ponding to face-capping carbonyl modes. The ESI mass
spectra contain molecular ions (8a–8c, 10a, 10b) or protonated
molecular ions (9a, 9b) with the correct isotope pattern.
X-ray structural studies of 7a, 8a, 8b, and 9a
Single-crystal X-ray diffraction studies revealed the molecular
compositions of 7a, 8a, 8b, and 9a. ORTEP plots with the
molecular structure and atomic labeling schemes for one of
the two crystallographically distinct molecules of 7a and the
molecule of 9a are shown in Fig. 5 and 6, respectively. Fig. S5–
S7 in the ESI† contain ORTEP plots from the structural studies
of the remaining crystallographically independent molecule of
7a, and the molecules of 8a and 8b, respectively; the connec-
tivity of the cluster cores of 8a and 8b mimic that of 9a.
Table S1 in the ESI† contains crystallographic data acquisition
and refinement parameters, and Table 2 lists selected bond
distances for 8a, 8b, and 9a, while selected bond distances for
the two molecules of 7a are given in the relevant figure
caption.
Fig. 5 ORTEP plot and atom numbering scheme for one of the two
crystallographically distinct molecules of Mo2Ir3(μ4–η2-PhC2Ph)(μ3-
CPh)2(CO)4(η5-C5H5)2(η5-C5Me5) (7a). Displacement ellipsoids are shown
at the 40% probability level. Hydrogen atoms have been omitted for
clarity. Selected bond lengths: Mo1–Ir2 2.8968(4), Mo1–Ir3 2.8429(5),
Mo1–Ir4 2.6969(4), Mo1–Mo5 2.6016(6), Ir2–Ir4 2.7636(3), Ir2–Mo5
2.8014(4), Ir3–Ir4 2.7664(3), Ir3–Mo5 2.8377(4), Mo1–C35 2.065(5),
Mo1–C42 2.047(5), Ir2–C221 1.865(6), Ir2–C222 1.887(6), Ir2–C27
2.352(5), Ir2–C28 2.291(5), Ir2–C42 2.164(4), Ir3–C331 1.855(6), Ir3–
C332 1.896(5), Ir3–C27 2.319(5), Ir3–C28 2.284(5), Ir3–C35 2.148(5),
Ir4–C27 2.048(5), Mo5–C28 2.182(5), Mo5–C35 2.041(5), Mo5–C42
2.064(5), C27–C28 1.472(6) Å.
of the IR, 1H NMR, and ESI mass spectral data, the second
product (7b) is proposed to be an isomer of 7a. The solution
IR spectrum of 7a in CH2Cl2 contains only two ν(CO) bands,
both corresponding to terminally-bound carbonyls (1992 and
1939 cm−1). In contrast, that of 7b contains four ν(CO) bands,
two corresponding to terminally-bound carbonyls (1993 and
1936 cm−1) and two corresponding to face-capping carbonyl
modes (1754 and 1715 cm−1). The 1H NMR spectrum of 7a
contains two resonances for the two cyclopentadienyl groups
at δ 4.69 and 4.40, while that of 7b contains a single resonance
for the two cyclopentadienyl groups at δ 5.18; for both clusters,
The metal core of 7a adopts a distorted square-based pyra-
midal geometry in which one of the equatorial Ir–Ir bonds
from the trigonal bipyramidal precursor has been cleaved
(Ir2⋯Ir3 = 3.698 Å). As with 3a, 5b and 6, the metal cores of
8a, 8b, and 9a possess trigonal bipyramidal geometries with
the same core metal disposition as their starting clusters. In
the structures of 7a, 8a, 8b, and 9a, the group 6 metal atom(s)
and one of the iridium atoms are ligated by η5-cyclopenta-
the
phenyl,
cyclopentadienyl,
and
pentamethyl-
cyclopentadienyl protons are found in the ratio 20 : 10 : 15
(4Ph : 2Cp : 1Cp*). The ESI mass spectra of both clusters
contain intense molecular ion peaks at 1502 ([M]+), in addition
to sodium and potassium adducts of the molecular ion at
1525 and 1541 mass units, respectively. While the data are sug-
gestive of isomers, attempts to interconvert the species, both
photochemical (short and long wavelength lamps) and
thermal (refluxing toluene, 16 h), were unsuccessful, unreacted
7a and 7b being recovered and confirmed by IR. This result,
coupled with the ambiguity of the existing spectral data, and
our inability thus far to obtain suitable single crystals for an
X-ray diffraction study, renders further speculation of the iden-
tity of 7b unwarranted.
Fig. 6 ORTEP plot and atom numbering scheme for WIr4{μ3–η2-
PhC2C6H4(CuCPh)-3}(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (9a). Displace-
ment ellipsoids are shown at the 40% probability level. Hydrogen atoms
Reactions of more than one equivalent of arylalkynes with
2a or 2b proceeded under milder conditions (refluxing CH2Cl2)
than the aforementioned reaction with 1a, affording 8a–8c, 9a, have been omitted for clarity.
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Dalton Transactions
3 × 5(Cp/Cp*) + 4 × 1(Ph) + 4 × 2(CO) = 82 CVE). The electron
deficiency of 7a may be ameliorated by an exceptionally short
Mo–Mo bond of 2.6008(7) Å cf. typical Mo–Mo single bond dis-
tances of 2.95–3.10 Å.4f–h,5
Table 2 Selected bond lengths (Å) for 8a, 8b and 9a
8a: M = Mo
8b: M = W
9a: M = W
M1–Ir2
M1–Ir3
M1–Ir4
M1–Ir5
Ir2–Ir3
Ir2–Ir4
Ir2–Ir5
Ir3–Ir4
2.9333(9)
2.8949(9)
2.6904(10)
2.6443(9)
2.6026(5)
2.8075(5)
2.6957(5)
2.8479(5)
2.7119(5)
2.030(11)
2.051(12)
1.924(11)
1.905(12)
2.532(10)
2.148(10)
1.938(12)
1.892(12)
2.517(12)
2.131(9)
2.9485(5)
2.8741(5)
2.6836(5)
2.6299(5)
2.6066(4)
2.8211(5)
2.6869(5)
2.8658(5)
2.7059(5)
2.015(10)
2.034(9)
1.921(10)
1.895(10)
2.606(9)
2.104(9)
1.935(9)
1.895(10)
2.530(10)
2.087(9)
2.158(8)
2.126(8)
1.877(10)
1.847(10)
2.208(8)
2.220(8)
1.426(12)
2.9161(13)
2.9134(12)
2.6728(13)
2.6276(12)
2.5952(13)
2.8171(14)
2.6907(13)
2.8456(13)
2.6959(13)
2.05(2)
2.06(3)
1.87(2)
1.93(3)
2.52(3)
2.22(3)
1.87(2)
1.83(2)
2.63(3)
2.08(3)
2.12(2)
2.17(2)
1.94(3)
Electrochemical and spectroelectrochemical studies
The electrochemical behavior of the new clusters was investi-
gated using cyclic voltammetry; a representative scan (that of
8a) is shown in Fig. 7, and details from all clusters are col-
lected in Table 3. Under the experimental conditions, all MIr4
clusters possess a reversible or semi-reversible first oxidation
process, and an irreversible second oxidation process at ca. 1 V
(accompanied in the case of 9a and 9b by a third, irreversible,
oxidation process). The first oxidation processes of the phos-
phine-containing MIr4 clusters are at lower potentials than
those of the alkyne clusters, reflecting the strong electron-
donating character of the phosphine ligands; introduction of a
second phosphine, in proceeding from 5a to 6, results in a
further 0.25 V increase in ease of oxidation. The MIr4 clusters
also exhibit reversible or semi-reversible reductions at ca. −1 V,
with the reduction process of the alkyne-containing clusters
8a–8c being two-electron in nature (see Fig. 7 for the cyclic vol-
tammogram of 8a). The M2Ir3 clusters exhibit two semi-revers-
ible oxidations, together with one irreversible reduction
process at large negative potentials. Across clusters 3–6, the
ease of oxidation increases on increasing the donor strength of
the isocyanide (proceeding from CNC6H3Me2-2,6 to CNBut)
and replacing Mo by W, while increasing the core Ir content in
proceeding to the Ir-rich clusters 5 and 6 affords the clusters
with the most reversible first oxidation processes. For clusters
8 and 9, ease of oxidation diminishes on introduction of a
pendant alkyne, in proceeding from 8 to 9, although the poten-
tial differences are small.
Ir3–Ir5
M1–C124
M1–C134
Ir2–C221
Ir2–C222
Ir2–C124
Ir2–C23
Ir3–C331
Ir3–C332
Ir3–C134
Ir3–C22
Ir4–C124
Ir4–C134
Ir5–C551
Ir5–C552
Ir5–C22
Ir5–C23
C22–C23
2.116(10)
2.138(10)
1.860(13)
1.877(11)
2.206(9)
2.182(10)
1.343(14)
1.83(2)
2.21(3)
2.24(4)
1.46(5)
dienyl ligands with varying degrees of alkylation. The clusters
contain four (7a) or six (8a, 8b, 9a) terminal carbonyls, and, for
the latter, two face-capping carbonyl ligands. The structural
studies reveal that alkyne ligands are coordinated in a classic
μ3–η2-“parallel” fashion across an Ir3 face in 8a, 8b, and 9a,
the face that is least sterically hindered, and in a μ4–η2-fashion
in 7a oriented perpendicular to the non-bonding Ir2-Ir3 vector;
for the latter, phenylmethylidyne residues resulting from CuC
cleavage cap the Mo2Ir faces. Assuming four electrons contri-
buted to each cluster from the alkyne ligands and three from
each alkylidyne moiety, 8a, 8b, and 9a possess 72 cluster
valence electrons, EAN-precise for M5 clusters with nine M–M
bonds. Alternatively, considering the alkyne carbon atoms in
8a, 8b, and 9a as core atoms, the cores of 8a, 8b, and 9a can
be described from the condensed polyhedra perspective as
square-based pyramids (74 CVE) fused via a triangular face
(−48 CVE) to a trigonal bipyramid (72 CVE) with two main
group atoms within the cores (−2 × 10 CVE) = 78 CVE, consist-
ent with the electron count (1 × 6(M) + 4 × 9(Ir) + 2 × 4(core C)
+ 2 × 5(Cp/Cp*) + 8 × 2 (CO) + 2 × 1(Ph)). In contrast, 7a pos-
sesses 72 CVE, which is two electrons deficient for an M5
cluster with 8 M–M bonds. Alternatively, one can view 7a as a
bicapped (PhC35, PhC42) pentagonal bipyramid (apices Ir2,
Ir3, and equatorial atoms Mo1, Ir4, C27, C28, Mo5); the con-
densed polyhedra-predicted count for a pentagonal bipyramid
(100 CVE) with two main group atoms (−2 × 10 CVE) fused via
triangular faces (−2 × 48 CVE) to two tetrahedra (2 × 60 CVE)
with one main group atom each (−2 × 10 CVE) is 84 CVE, and
thus the cluster is two electrons deficient from the condensed
polyhedra perspective (2 × 6(Mo) + 3 × 9(Ir) + 4 × 4(core C) +
The reduction process of 8b was probed at room tempera-
ture using UV-vis-NIR spectroelectrochemistry. The progressive
electronic absorption of 8b → 8b− in an optically transparent
Fig. 7 Cyclic voltammogram of MoIr4(μ3–η2-PhC2Ph)(μ-CO)2(CO)6(η5-
C5H5)(η5-C5Me5) (8a) at room temperature in CH2Cl2, using the ferro-
cene/ferrocenium redox couple at 0.56 V as an internal standard.
7300 | Dalton Trans., 2015, 44, 7292–7304
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Paper
Table 3 Cyclic voltammetry data of 3a–b, 4a–d, 5a–b, 6, 7a, 8a–c, and 9a–ba
First oxidation
Second oxidation
Reduction
E1/2 (Ec) [ipc/ipa]
E1/2 [ipc/ipa
]
E1/2 (Ea) [ipc/ipa
]
Mo2Ir3(CNC6H3Me2-2,6)(μ-CO)3(CO)5(η5-C5H5)2(η5-C5Me5) (3a)
Mo2Ir3(CNtBu)(μ-CO)3(CO)5(η5-C5H5)2(η5-C5Me5) (3b)
Mo2Ir3(μ-CO)3(CO)5(PPh3)(η5-C5H5)2(η5-C5Me5) (4a)
0.42 [0.5]
0.34 [0.6]
0.44 [0.7]
0.40 [0.9]
0.47[0.7]
0.43 [0.9]
0.59 [1]
0.56 [0.9]
0.34 [1]
0.51 [0.9]
0.72 [1]
0.7 [0.5]
0.62 [0.6]
0.73 [0.4]
0.65 [0.6]
0.73 [0.5]
0.71 [0.6]
(0.90)c
(−1.36)c
(−1.43)c
(−1.47)c
W2Ir3(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me5) (4b)
(−1.52)c
Mo2Ir3(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me4H) (4c)
W2Ir3(μ-CO)4(CO)4(PPh3)(η5-C5H5)2(η5-C5Me4H) (4d)
MoIr4(μ-CO)3(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) (5a)
(−1.40)c
(−1.44)c
−1.14 [0.7]
−1.15 [1]
−1.04c
WIr4(μ-CO)3(CO)6(PPh3)(η5-C5H5)(η5-C5Me5) (5b)
(0.86)c
MoIr4(μ3-CO)2(μ-CO)2(CO)4(PPh3)2(η5-C5H5)(η5-C5Me5) (6)
Mo2Ir3(μ4–η2-PhC2Ph)(μ3-CPh)2(CO)4(η5-C5H5)2(η5-C5Me5) (7a)
MoIr4(μ3–η2-PhC2Ph)(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (8a)
WIr4(μ3–η2-PhC2Ph)(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (8b)
WIr4(μ3–η2-HC2Ph)(CO)8(η5-C5H5)(η5-C5Me5) (8c)d
(0.67)c
0.91 [0.5]
(−1.53)c
(1.11)c
−0.99b [1]
−1.03b [1]
−1.15b [0.9]
−0.97 [1]
−0.95 [1]
0.74 [0.7]
0.74 [0.7]
0.76 [0.9]
0.77 [0.9]
(1.08)c
(1.03)c
WIr4{μ3–η2-PhC2C6H4(CuCPh)-3}(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (9a)
WIr4{μ3–η2-PhC2C6H4(CuCPh)-4}(μ3-CO)2(CO)6(η5-C5H5)(η5-C5Me5) (9b)
1.08c, 1.24c
1.05c, 1.23c
a CH2Cl2, Ag/AgCl reference electrode, E1/2 for FcH/FcH+ process at 0.56 V. b 2e process. c Irreversible process, no measurable return peak. d Ref. 5.
thin-layer electrochemical (OTTLE) cell is displayed in Fig. S8.† C5Me5) (2a) reacted with isocyanides in CH2Cl2 with rapid clea-
The reduction results in the disappearance of the band cen- vage of the Ir(η5-C5Me5) vertex, evinced by the change in color
tered at 15 010 cm−1, increases in intensity in other regions, from dark brown to pale yellow (in ca. 30 s), and the formation
with an isosbestic point at 16 230 cm−1, and overall an increas- of MoIr3(μ-CO)3(CNC6H3Me2-2,6)(CO)7(η5-C5H5) or MoIr3(μ-
ingly featureless spectrum following reduction.
CO)3(CNtBu)(CO)7(η5-C5H5); this cleavage of the Ir(η5-C5Me5)
vertex occurred regardless of the stoichiometry of the isocya-
nide employed.
11a, 11b, 12a, and 12b reacted with one equivalent of tri-
phenylphosphine to afford mono- to di- or mono- to tri-substi-
tuted clusters, respectively.4a,b,6,7 In the present work, in
contrast, 1a–1d reacted with excess PPh3 to afford mono-sub-
stituted products only and, while 2a reacted with the same
substrate to afford mono- and di-substitution products, the
initial substitution is accompanied by a competitive loss of the
Ir(η5-C5Me4R) vertex and the formation of MoIr3(μ-CO)3
(CO)6(PPh3)2(η5-C5H5). Formation of the substitution products
4a–4d involves a core rearrangement not possible in the tetra-
nuclear system: both group 6 metal atoms now lie in the trigo-
nal bipyramidal clusters’ equatorial planes, presumably to
accommodate the bulky PPh3 ligand which is ligated to an
apical iridium atom. While the 1H NMR spectra of the tung-
sten-containing clusters 4b and 4d are consistent with the
solid-state structure (two η5-C5H5 resonances), those of the
molybdenum-containing clusters 4a and 4c have only one
η5-C5H5 resonance in their 1H NMR spectra, suggestive of
enhanced ligand fluxionality of the 4d metal-containing
clusters.
Discussion
Phosphines are the archetypical two-electron ligands, isocya-
nides CNR are carbonyl-like ligands with peripheral R group
flexibility, and alkynes have been the cluster ligand of choice
to probe C–C cleavage and formation processes relevant to
Fischer–Tropsch and other catalytic transformations; as
such, the phosphine, isocyanide, and alkyne chemistry of
carbonyl clusters is of fundamental importance. With this in
mind, the isocyanide, phosphine, and alkyne chemistry of
the trigonal bipyramidal clusters 1a–1d, 2a–2d described
above can be contrasted with that of the related lower-nuclear-
ity tetranuclear clusters M2Ir2(CO)10(η5-C5H5)2 (M = Mo 11a, W
11b) and MIr3(CO)11(η5-C5H5) (M = Mo 12a, W 12b) to shed
light on the impact of incorporation of the Ir(η5-C5Me4R)
vertex.
In the present studies, Mo2Ir3(μ-CO)3(CO)6(η5-C5H5)2(η5-
C5Me4R) (1a, 1b) reacted with excess isocyanide to give only
mono-substitution products Mo2Ir3(μ-CO)3(CO)5(CNR′)(η5-
C5H5)2(η5-C5Me4R) (R = Me, R′ = C6H3Me2-2,6 3a; R = Me, R′ =
tBu 3b), in which core geometry, ligand disposition, and metal
atom locations are maintained. In contrast, 11a reacted with
two equivalents of CNC6H3Me2-2,6 to afford both mono- and
di-substitution products, and reacted with either one or two
equivalents of CNBut to give only the di-substituted Mo2Ir2-
(μ-CO)2(CNBut)2(CO)6(η5-C5H5)2.16 12a reacted with one or two
equivalents of CNBut to afford mixtures of mono- and di- or
mono- to tri-substitution products, respectively.17 In the
present studies, in contrast, MoIr4(μ-CO)3(CO)7(η5-C5H5)(η5-
WIr3(CO)11(η5-C5H4R) (R = H 12b, Me) reacted with the
internal alkyne PhCuCPh to afford a mixture of the tetrahe-
dral bis(alkyne) adduct WIr3(μ3–η2-PhC2Ph)2(CO)7(η5-C5H4R),
with alkyne addition at the WIr2 and Ir3 faces, and the butter-
fly cluster WIr3(μ3-CPh){μ–η4-C(Ph)C(Ph)C(Ph)C(Ph)}(μ-CPh)-
(CO)5(η5-C5H4R) via alkyne C–C coupling and W–Ir and CuC
cleavage, together with the (iridacyclopentadienyl)iridium
complex Ir2{μ–η4-C(Ph)C(Ph)C(Ph)C(Ph)}(CO)5 18 In contrast, in
the present work 2a and 2b reacted with alkynes by μ3–η2-
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Dalton Transactions
coordination at the non-cyclopentadienyliridium-containing solubility of Ir4(CO)12, which retards direct phosphine and iso-
Ir3 face only. cyanide substitution, also impedes its reactivity towards
11a and 11b reacted in a relatively clean fashion with both alkynes; this is restricted to the activated alkyne MeO2CCu
internal and terminal alkynes via insertion into the M–M bond CCO2Me (necessitating irradiation) and phosphinoalkynes that
to give butterfly clusters M2Ir2(μ4–η2-RC2R′)(μ-CO)4(CO)4(η5- react via initial P-coordination.25 In contrast, 11a, 11b, 12a,
C5H5)2.4g–i,7 In contrast, 1a reacted with the internal alkyne and 12b react with a broad range of alkynes between room
PhCuCPh to give several products, one corresponding to equa- temperature and refluxing toluene (milder conditions for 11a,
torial Ir–Ir cleavage, μ4–η2-coordination of one equivalent of 11b, and for M = Mo) to afford a diverse range of products
alkyne, CuC cleavage of a second equivalent of the alkyne, incorporating one to three alkyne residues.4c,g–k,7,17,26 Thus,
and coordination of μ3-phenylmethylidyne ligands to each of the incorporation of the heterometal(s) result(s) in signifi-
the two Mo2Ir faces, a second product presumably an isomer cantly enhanced reactivity at tetranuclear core nuclearity.
of the first, and a third product corresponding to a small
The contrast is also stark with pentanuclear clusters. The
few extant pentairidium clusters27 possess square pyrami-
amount of Mo2Ir2(μ4–η2-PhC2Ph)(μ-CO)4(CO)4(η5-C5H5)2.
Overall, the progression from tetrahedral to trigonal bipyra- dal27a,b or trigonal bipyramidal27c–e cores, with all except trigo-
midal group 6-group 9 mixed-metal cluster has resulted in nal bipyramidal [Ir5(μ-H)(μ-CO)2(CO)10]− being derived from a
species with considerable differences in reactivity: the P-ligated lower nuclearity precursor. [Ir5(μ-H)(μ-CO)2(CO)10]2−
additional Ir(η5-C5Me4R) vertex in the trigonal bipyramidal has limited stability and requires careful handling, necessitat-
clusters is prone to cleavage, being removed to varying extents ing the use of large cations, and is converted into tetrahedral
in reactions with isocyanides, phosphines or alkynes. Isocya- [Ir4(μ-H)(μ-CO)3(CO)8]− in the presence of CO and into
nides and phosphines react rapidly with the tetrahedral clus- [Ir6(μ-CO)3(CO)12]2− when reacted with mild oxidants such as
ters to afford mixtures of products with varying levels of AuCl(PPh3).27c In contrast, the trigonal bipyramidal clusters
substitution, while the trigonal bipyramidal clusters favor 1a–1d, 2a and 2b are stable and easy to handle. Indeed, the
mono-substitution, the sole example of a di-substituted cluster present studies have shown that increasing group 6 metal
being formed under comparatively forcing conditions. The content enhances core stability towards donor ligands isocya-
C–C bond formation between two alkyne units that forms η3-C nides and phosphines; ligated-iridium vertex removal
(Ph)C(Ph)C(Ph) and η4-C(Ph)C(Ph)C(Ph)C(Ph) ligands on the diminishes on proceeding from pentairidium to molybdenum/
tetrahedral clusters has not thus far been observed with the tri- tungsten–tetrairidium and then dimolybdenum/ditungsten–
gonal bipyramidal clusters and, for the latter, only one low- triiridium clusters.
yielding example of CuC cleavage has been conclusively esta-
The only phosphine-ligated pentairidium clusters Ir5(μ-P,C-
blished (7a). Examples of μ4–η2-PhC2Ph ligands have been PPh2C6H4-2)(μ-CO)4(CO)7(PPh3) and Ir5(μ-CO)3(Ph)(CO)9(PPh3)
identified with both the tetrahedral and trigonal bipyramidal both bear apical iridium-coordinated phosphine ligands,27d
clusters; in the former case [Mo2Ir2(μ4–η2-PhC2Ph)- similar to 4a–4d and 5a, 5b in the present study. Reaction of
(μ-CO)4(CO)4(η5-C5H5)2], the C2 vector is parallel to an intact Ir5(μ-CO)3(Ph)(CO)9(PPh3) with PPh3 necessitated refluxing
Ir–Ir linkage, forming a pseudo-octahedral Mo2Ir2C2 core, benzene and afforded Ir5(μ-P,C-PPh2C6H4-2)(μ-CO)4(CO)7(PPh3)
while in the latter case (7a), the C2 vector is perpendicular to a following orthometallation of a phosphine phenyl ring and
cleaved Ir–Ir linkage, the Mo2Ir3C2 forming a distorted penta- loss of benzene.27d Both phosphorus atoms in the latter
gonal bipyramid.
cluster are found at apical iridium sites, in contrast to 6 for
A fundamental interest in progression from homometallic which apical, equatorial ligation is seen due to pentamethyl-
to heterometallic clusters is the anticipation that introduction cyclopentadienyl ligation of one of the apical iridium atoms.
of the heterometal(s) should enhance or modify reactivity. The
There is no extant pentairidium isocyanide or alkyne chem-
present system permits examination of this possibility via a istry.28 C-ligands have been introduced into pentairidium clus-
contrast of homometallic iridium clusters with isostructural ters via reaction of the cyclooctadiene (COD)-ligated {Ir(μ-Cl)-
mixed group 6-iridium clusters. Phosphine substitution at (η4-1,2,5,6-C8H12)}2 with a tetrairidium precursor at 80 °C,27e
Ir4(CO)12 under thermal conditions necessitates temperatures the resultant species bearing one or two apical iridium-
of ca. 110 °C and affords tri- or tetra-substitution products.19 coordinated COD ligands. Thermolysis at 98 °C resulted in
In contrast, as mentioned above, phosphine substitution at Ir–Ir bond cleavage and a change in core nuclearity. In con-
11a, 11b and 12a, 12b proceeds stoichiometrically at room trast, alkyne addition with the mixed group 6-iridium clusters
temperature with one or two equivalents (one to three for the proceeded with no change in core nuclearity and, in the case
latter) to afford mono- and di-substitution products (mono- of the molybdenum/tungsten–tetrairidium clusters, reaction
to tri-substitution for the latter) in excellent yields.4a,b,6,7 under very mild conditions to afford alkyne adducts.
Examples of isocyanide substitution at Ir4(CO)12 are
Electrochemical studies of 1a–1d, 2a and 2b revealed both
sparse,20,21 and require forcing conditions; reactions with oxidation and reduction is accessible (with respect to the
CNBut proceed in refluxing chlorobenzene to afford mono- resting state), the oxidation processes being the more revers-
and di-substituted products in low yields.22,23 In contrast, 11a, ible, and increasingly so on decreasing Ir content of the clus-
11b, 12a, and 12b react at room temperature to afford mono- ters, replacing W by Mo, and increasing alkylation of the
or di-substitution products in excellent yields.16,24 The lack of cyclopentadienyl ligands.5 There is no report of the electroche-
7302 | Dalton Trans., 2015, 44, 7292–7304
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Dalton Transactions
Paper
mical behavior of [Ir5(μ-H)(μ-CO)2(CO)10]2−, but its aforemen- T. Schwich, A. L. Criddle, S. Petrie, J. G. MacLellan,
tioned oxidative sensitivity27c is consistent with this trend. S. R. Batten, R. Stranger, M. P. Cifuentes, M. G. Humphrey,
Introduction of isocyanide, phosphine or alkyne residue in Inorg. Chem., 2013, 52, 11256.
proceeding to the current ligand-substituted series results in
an increased ease of oxidation and increased difficulty in
reduction (although many of the latter processes are irrevers-
ible, so comment must be cautious). Incorporation of these
Abbreviations
Cp
(η5-C5H5)
ligands therefore serves to tune the electron richness at the
cluster. Finally, the reductive spectroelectrochemical study of
8b complements the oxidative spectroelectrochemical studies
of 1a and 2a in our previous report,5 the former cluster being
the molybdenum-containing homologue of the tungsten-con-
taining precursor to 8b. Oxidation of MoIr4(μ-CO)3(CO)7(η5-
C5H5)(η5-C5Me5) resulted in a UV-vis-NIR spectrum similarly
featureless to that observed following reduction of 8b; thus,
spectral features become increasingly indistinct with both oxi-
dation and reduction in this system.
Cp* (η5-C5Me5)
Cp# (η5-C5Me4H)
TLC Thin-layer chromatography
Acknowledgements
We thank the Australian Research Council (ARC) for financial
support. M.D.R. was the recipient of an Australian Postgradu-
ate Award, J.F. is the recipient of a China Scholarship Council
ANU Postgraduate Scholarship, M.P.C. thanks the ARC for an
Australian Research Fellowship, and M.G.H. held an ARC
Australian Professorial Fellowship.
Conclusions
The studies described herein have demonstrated facile
functionalization of pentanuclear molybdenum/tungsten–
iridium clusters and thereby access to a range of systemati-
cally-derivatized species not currently accessible in the homo-
metallic pentairidium system. The ligand substitution studies
have shown that cluster core stability and ease of derivatization
is diminished on proceeding from tetranuclear to pentanuc-
lear clusters, but is enhanced on increasing the group 6 metal
content, and have thereby demonstrated the benefit in pro-
ceeding from homometallic to heterometallic environment.
Ligand substitution generally proceeds at sites that can be
rationalized on steric grounds (non-cyclopentadienyl-ligated
iridium vertices, and initial substitution at an apical site if tri-
phenylphosphine and an equatorial site if isocyanide). As
judged by the potential of the first oxidation processes
accessed from the cluster resting states, the electron richness
of the present series of ligand-substituted clusters is tuned in
a rational fashion.
The current studies have also highlighted significant differ-
ences in alkyne reactivity between tetranuclear group 6-iridium
clusters and the pentanuclear analogues, but also illustrate the
desirable metallo-, bond-, and face-selectivity for reactions at
mixed-metal clusters; in addition to the aforementioned
iridium-specific ligand substitution, clusters M2Ir2(μ4–η2-
RC2R′)(μ-CO)4(CO)4(η5-C5H5)2 form in high yield by bond-
specific insertion of an alkyne into the M–M linkage of the pre-
cursor (with M–Ir and Ir–Ir, one of three distinct core bond
types), while clusters MIr4(μ3–η2-PhC2R)(μ3-CO)2(CO)6(η5-C5H5)-
(η5-C5Me5) (8, 9) form in good yield by face-specific addition at
Ir3 (with MIr2, one of two distinct core face types).
Notes and references
1 (a) The Chemistry of Metal Cluster Complexes, ed.
D. F. Shriver, H. D. Kaesz and R. D. Adams, VCH,
New York, 1990; (b) Physics and Chemistry of Metal Cluster
Complexes, ed. L. L. de Jongh, Kluwer, Amsterdam, 1994;
(c) P. Braunstein, L. A. Oro and P. R. Raithby, Metal Clusters
in Chemistry, Wiley-VCH, Weinheim, Germany, 1999.
2 (a) R. D. Adams and F. A. Cotton, Catalysis by Di- and Poly-
nuclear Metal Cluster Complexes, Wiley-VCH, New York,
1998. Higher-nuclearity clusters continue to attract interest.
See, for example: (b) E. G. Mednikov, M. C. Jewell and
L. F. Dahl, J. Am. Chem. Soc., 2007, 129, 11619;
(c) E. G. Mednikov and L. F. Dahl, J. Am. Chem. Soc., 2008,
130, 14813; (d) C. Femoni, M. C. Iapalucci, G. Longoni,
J. Wolowska, S. Zacchini, P. Zanello, S. Fedi, M. Ricco,
D. Ponticoli and M. Mazzani, J. Am. Chem. Soc., 2010, 132,
2919; (e) D. S. Dolzhnikov, M. C. Iapalucci, G. Longoni,
C. Tiozzo, S. Zacchini and C. Femoni, Inorg. Chem., 2012,
51, 11214.
3 (a) M. C. Comstock and J. R. Shapley, Coord. Chem. Rev.,
1995, 143, 501; (b) S. M. Waterman, N. T. Lucas and
M. G. Humphrey, Adv. Organomet. Chem., 2000, 46, 47.
4 See, for example: (a) J. Lee, M. G. Humphrey,
D. C. R. Hockless, B. W. Skelton and A. H. White, Organo-
metallics, 1993, 12, 3468; (b) S. M. Waterman,
M. G. Humphrey, V.-A. Tolhurst, D. C. R. Hockless,
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Mixed-Metal Cluster Chemistry. Part 34. Part 33:
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