Expedient Synthesis of a Metallostibine Os2(CO)8(μ-SbPh): An Unusual and Strong Two-Electron-Donor Ligand

Article abstract of DOI:10.1002/ejic.201700456

A metallostibine, Os₂(CO)₈(μ-SbPh) (2), was generated readily and in high yield and purity from the Os₃(CO)₁₁(μ-SbPh)(Cl)₂ (1) cluster through elimination of the terminal Os(CO)₃(Cl)₂ Lewis acid fragment with activated γ-alumina. Compound 2 is air sensitive in solution but relatively stable as a solid. The Sb atom in 2 adopts a trigonal pyramidal geometry in both the solid and solution states, with an active lone pair of electrons. Experimental and computational studies show that 2 is a strong two-electron donor with a donor strength comparable to that of alkylphosphines.

Full text of DOI:10.1002/ejic.201700456

A Journal of
Accepted Article
Title: Expedient Synthesis of a Metallostibine Os2(CO)8(mu-SbPh): An
Unusual and Strong Two-electron Donor Ligand
Authors: Weng Kee Leong, Yingzhou Li, and Rakesh Ganguly
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700456
Link to VoR: http://dx.doi.org/10.1002/ejic.201700456

10.1002/ejic.201700456
European Journal of Inorganic Chemistry
COMMUNICATION
Expedient Synthesis of a Metallostibine Os2(CO)8(-SbPh): An
Unusual and Strong Two-electron Donor Ligand
Ying-Zhou Li, Rakesh Ganguly, and Weng Kee Leong*
Abstract:
A
metallostibine Os2(CO)8(-SbPh), 2, is generated
metal fragment for the lone pair to be delocalised, the tendency
for metal-metal bond formation, and the steric bulk of the ligands
(L) coordinated to the metal atom.[5]
readily and in high yield and purity from the cluster Os3(CO)11(-
SbPh)(Cl)2, 1, through elimination of the terminal Lewis acid
fragment Os(CO)3(Cl)2 with activated -alumina. Compound 2 is air-
sensitive in solution but relatively stable as a solid. The Sb atom in 2
adopts a trigonal pyramidal geometry in both the solid and solution
states, with an active lone pair. Experimental and computational
studies show that 2 is a strong two-electron donor, with a donor
strength comparable to that of alkylphosphines.
While there is a fair amount of studies carried out on
electrophilic bi-metallopnictides of types B2 and B3,[1e, 6] less is
known about nucleophilic bi-metallopnictides of type B1. The bi-
metallophosphines Fe2Cp2(CO)2(-CO)(-PR) (Cp=η5-C5H5; R =
t
Cy, Ph, Mes*; Mes*=2,4,6-C6H2 Bu3) have been reported, and
their chemistry explored quite extensively.[7] In contrast, only
three metalloarsines of this type, viz., WCoCp(CO)5(-CO)[-
AsCH(SiMe3)2] and Ru2Cp2(CO)2(-CO)(-AsR) (Cp=η5-C5H5; R
=
Me, Ph), have been reported to date.[8] Several
Introduction
metallobismuthine complexes of this type are also known,
including number of iron-bismuth ring complexes
a
Phosphines, arsines and stibines are among the most
ubiquitous of ligands in organometallic chemistry. That their
stereoelectronic properties can be varied over a wide range
through the substituents have made them an important building
block in the toolkit of the organometallic chemist. Among them,
those in which one or two of the substituents are transition metal
fragments are of particular interest and several structural types
are known, obtained via a variety of synthetic routes (Figure 1).[1]
[Fe(CO)4BiR]2 (R = i-Bu, Me, Benzyl, Et) and one osmium-
bismuth complex Os2(CO)8(-BiPh). These were, however,
obtained in low to moderate yields. The former was obtained
from the reaction of the anionic complexes [Et4N]3{Bi[Fe(CO)4]4}
or [Et4N]2{Bi(Cl)[Fe(CO)4]3} with organic halides;[9] three other
miscellaneous reactions also afforded similar ring analogues.[10]
The osmium complex was obtained in low yield (5%),
presumably via thermal decomposition of Os3(CO)11(BiPh3).[11]
Interestingly, the synthetic methodologies reported for the
metallobismuthines could not be used to prepare the
corresponding metallostibine analogues. The reason for this
may lie with the much stronger donating ability of Sb compared
to Bi, making it difficult for the initially formed metallostibine
complex to be isolated from the complex reaction mixture
without coordination to a metal fragment.[12] The result is that
metallostibine complexes of type B1 is the least explored among
the group 15 elements. To our knowledge, only one such
metallostibine, viz., Fe2(CO)8{-Sb[CH(SiMe3)2]}, has been
structurally characterised.[13] The bulky CH(SiMe3)2 substituent
afforded steric protection of the lone pair on Sb, thus allowing for
its isolation. Herein we report the generation and isolation, in
high purity and yield, of a metallostibine of type B1 which is
sterically less encumbered and has a coordinating ability
comparable to that of alkylphosphines.
Figure 1. Known structural types for metallophosphines, -arsines and -stibines.
Most of the studies to date have been on the mono-
metallopnictides (type A). Those of type A1 among them has
attacted more interest due to their potential use as
metalloligands for the efficient synthesis of bi- and multimetallic
complexes.[2] Early interests in them were concerned with the
conformational impact of a “transition metal gauche effect“,[3]
and the consequent enhanced solution-phase nucleophilicity of
the pnictogen atoms and increased tendency towards
dimerization or conversion to the electrophilic form A2.[4] The bi-
metallopnictides (type B) can similarly be either nucleophilic (B1)
or electrophilic (B2 or B3). Adoption of a pyramidal (A1 or B1) or
trigonal planar (A2, B2 or B3) geometry at the pnictogen atom
(E) depends on whether or not there are acceptor orbitals on the
Results and Discussion
We recently reported the synthesis of the osmium-antimony
carbonyl cluster Os3(CO)11(3-SbPh)(Cl)2, 1.[14] This cluster can
be viewed as an adduct of the metallostibine Os2(CO)8(-SbPh),
2, with the electron-deficient fragment Os(CO)3(Cl)2. We
reasoned that it may be possible to release 2 if it was substituted
by other two-electron donors. Cluster 1 failed to react with
excess PPh3 at room temperature but its reaction with PMe3
indeed afforded 2, albeit in low yield (< 10%), together with the
Division of Chemistry & Biological Chemistry,
Nanyang Technological University,
21 Nanyang Link, Singapore, 637371.
E-mail: chmlwk@ntu.edu.sg
mononuclear
complexes
Os(CO)3(Cl)2(PMe3),
3,
and
Supporting information for this article is given via a link at the end of the
document.
Os(CO)2(Cl)2(PMe3)2, 4 (Scheme 1). Complex 3 is known,[15] and
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10.1002/ejic.201700456
European Journal of Inorganic Chemistry
COMMUNICATION
4 was characterized spectroscopically and its identity proposed
on the basis of the similarity of its IR spectrum with those of
other phosphine analogues (Figure S16).[16]
W(CO)5, interconverted between a closed form with a W-W bond
(type B2) and an open form without a W-W bond (type B1); the
antimony was Lewis basic and acidic, respectively.[21]
Scheme 1. Preparation of 2. Reaction conditions: a. PMe3 (1.5 equivalents),
THF, R.T., 36 h; b. PMe3 (1.5 equivalents), alumina (100 wt%), THF, 0 oC, 6 h;
c. Alumina (2000 wt%), THF, R.T., 20 min.
Figure 2. ORTEP plot showing the molecular structure of 2. Thermal ellipsoids
are drawn at the 30% probability level. Organic hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (°): Os1‒Os2 =
2.935(2), Os2‒Sb5 = 2.7294(17), Os1‒Sb5 = 2.7472(17), Os1‒Sb5‒Os2 =
64.80(5), Os1‒Sb5‒C51 = 104.8(4), Os2‒Sb5‒C51 = 108.8(4).
In the course of chromatographic separation, we observed
that activated neutral -alumina efficiently catalyzed the
substitution reaction.[17] Thus, in the presence of 100 wt% of
neutral -alumina,[18] the reaction proceeded rapidly even at low
temperature (0 oC); the lower reaction temperature disfavored
the formation of 4 as well as the thermal decomposition of 1
(Figure S2).[19] It was subsequently found that 1 could afford 2
rapidly (20 min at R.T.) and in high yield (91%) simply by stirring
with a large excess (2000 wt%) of neutral -alumina; the
phosphine could be dispensed with (Figures S3-S5)! This
procedure worked in most of the common solvents, although the
reaction rate showed solvent-dependence, with THF being the
most favorable among those tested (Figure S6). The role of the
-alumina is not entirely clear but presumably, it could have
acted as a weakly nucleophilic surface to displace 2 from the
Os(CO)3Cl2 unit which would be bound as a surface species; in
the presence of PMe3, complex 3 is formed from it. This is
corroborated by the observation that acidic -alumina (pH = 6.0 ±
0.5) and basic -alumina (pH = 9.5 ± 0.5) showed lower and
higher conversion rates, respectively. Brönsted acidic silica, the
Lewis acid AlCl3, and other amphoteric metal oxides with weaker
nucleophilic surfaces such as TiO2 and CeO2, did not catalyze
the conversion under similar reaction conditions.
In contrast to these iron- and tungsten-metallostibines, which
were neither strongly nucleophilic nor electrophilic, the NMR
spectrum of 2 showed only one set of peaks, indicating the
presence of only one form in solution. A computational study
with DFT showed that a type B2 form of 2 lies ~64 kJ mol-1
higher in energy. These suggest that the same form persists in
solution; the pattern for the CO stretches in the calculated IR
spectrum for the B1 form was also in good agreement in the
experimental spectrum (Figure S7). The metallostibine 2 did not
react with PMe3, indicating that it was not electrophilic. The
adoption of the pyramidal B1 form presumably stems from the
saturated nature of the Os2(CO)8 moiety and the strength of the
Os-Os bond. We thus believe that, like in the case of the mono-
metallopnictides, this Os2(CO)8 moiety is important to the
electron donating ability of 2. There appears, however, to have
been no previous measurement of the “nucleophilicity
enhancement“ of bi-metallopnictides, partly because of the lack
of suitable model molecules. We attempted to evaluate the
donor ability of 2 in several ways.
The structure of 2 has been confirmed by a single crystal X-
ray diffraction study; the ORTEP plot depicting its molecular
structure is shown in Figure 2, together with selected bond
parameters. The geometry about the antimony atom is
pyramidal, with retention of the metal-metal bond between the
osmium atoms. This is clearly indicative that the antimony atom
The donor ability of group 15 ligands have been quantified
via Tolman’s electronic parameter (), via the shift in the CO
stretching frequency of Ni(CO)3(L) (where L = two-electron
donor ligand) relative to that for L = PtBu3.[22] Complexes such as
M(CO)5(L) (where M = Cr, Mo, W) and Rh(Cl)(CO)2(L) have also
possesses
a
lone pair of electrons. The only other
been used as proxies for the acutely toxic Ni complexes.[23]
more recent alternative measure of donor ability is the use of 13
A
C
crystallographically characterized metallostibine complex of type
B1, viz., Fe2(CO)8{-Sb[CH(SiMe3)2]}, existed as two forms in
solution - a closed form with an Fe-Fe bond of type B1, and an
open form without an Fe-Fe bond of type B2. Both forms were
isolated and characterized spectroscopically, though it was
proposed that the open form could gradually convert to the
closed form.[13b,20] It was also reported that the species
W2(CO)10(-SbtBu), a reaction intermediate which was not
isolated but characterized through its adducts with PPh3 and
chemical shifts for two-electron donor ligands such as
phosphines and N-heterocyclic carbenes (NHCs).[24] Good linear
correlations have been reported between the δCO in Ni(CO)3(L)
with (a) the CO(A1) (r = 0.962), and (b) the cis carbonyl chemical
shifts (δCOcis) in M(CO)5(L) complexes (M = Cr, r = 0.983; M = Mo,
r = 0.993).[25]
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10.1002/ejic.201700456
European Journal of Inorganic Chemistry
COMMUNICATION
As expected, the metallostibine 2 was found to react readily
with W(CO)5(NCCH3) to afford the adduct Os2(CO)8(3-
SbPh)W(CO)5, 5 (Scheme 2). The metallostibine 2 in the
resulting adduct cannot be displaced by PMe3 under similar
reaction conditions to that for the generation of 2 from 1, and
may be ascribed to the lower electrophilicity of the W centre in 5;
its structure has also been determined crystallographically
(Figure S1).
We have also calculated the HOMO energies for the free
ligands using DFT with the B3LYP density functional. The
HOMO corresponds to the lone pair and thus the energy of the
HOMO may be expected to reflect -donating ability. In
comparison with the free phosphines and stibines, that for 2 lies
between those of PMe3 and PnBu3; the aryl phosphine PPh3
does not appear to follow the trend, probably due to significant
delocalization of the phosphorus lone pair into the aromatic
ring.[27] The energies of the HOMO for some related
metallopnictides, viz., M2(CO)n(-EPh) (where E = P, Sb; M = W,
n = 10; M = Os or Fe, n = 8) were also computed (Figure S17
(b)), although all except 2 presented in this work (when M = Os,
E = Sb, n = 8) have only been observed as their coordination
complexes.[28] A comparison shows that the Os(CO)4 metallo-
fragment is more electron-rich than W(CO)5 and Fe(CO)4.
The steric bulk of 2 has also been evaluated using its
percent buried volume (%VBur) in 5;[29] it is less bulky (20.5%)
than PPh3 (23.2%), Fe2(CO)8{-Sb[CH(SiMe3)2]} (25.4%) and
PtBu3 (27.2%) (Figure S18).
Scheme 2. Preparation of 5.
The IR absorption at 2064 cm-1 for 5 has been identified as
the A1 mode for the W(CO)5 fragment, by comparison with the
calculated spectrum.[26] This is tabulated together with the same
vibrational mode for a series of phosphine and stibine analogues
W(CO)5(L) [5n, where L = P(OMe)3 (a); PPh3 (b); SbPh3 (b’);
PMe3 (c); SbMe3 (c’); PnBu3 (d); PtBu3 (e); 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (IMes) (f) (Table 1). The
lower vibrational frequency for 5 suggests that the metallostibine
2 is a strong donor ligand, with donor strength more like the
alkylphosphines; the calculated values are also supportive of
this (Figure S13). Similarly, the 13C chemical shift for the
carbonyl ligand that is cis to the metallostibine ligand in 5 is to
lower field than most of the complexes 5n, lying in between that
of 5d (L = PnBu3) and 5e (L = PtBu3) (Table 1 and Figure S14).
Conclusions
We have shown that the metallostibine 2 can be isolated in
high purity and yield. It is a very strong donor ligand, with a
donor ability that is greater than phosphines such as PPh3 and
PMe3, and yet is sterically less bulky than PPh3. Together with
the fact that it is stable in the solid state, the metallostibine 2
should be useful in the preparation of multimetallic clusters. The
findings also alter the prevailing view that stibines are weaker
electron donating ligands than phosphines. Metallostibines such
as 2 represent a new class of two-electron donor ligands
[M2(L)n(-SbR)], with electron donating ability that should be
tunable via the substituent R as well as the ligands L and the
metal M. Both sets of investigations are currently underway.
Table 1. CO(A1) and δCOcis for 5 and 5n, and calculated HOMO energies for
the corresponding free ligands.
cis
Complex
Ligand (L)

CO(A1)
δCO
EHOMO (ev)[c]
Experimental Section
(cm-1)[a]
(ppm)[b]
General Methods. All reactions were carried out under an argon
atmosphere with standard Schlenk techniques. Reagent grade solvents
were dried by the standard procedures and were freshly distilled prior to
use. Compound 1 was prepared through an adapted method from
literature.[14] The tungsten complexes 5a-f were prepared according to
literature method.[30] All activated neutral, basic, and weakly acidic -
Al2O3 (Brockmann activity I, particle size: 150 mesh, pore size: 58 Å)
were purchased from Sigma-Aldrich and used as received without further
activation. TLC separations were carried out on 20 × 20 cm2 plates
coated with silica gel 60 F254 under ambient conditions while the neutral
-Al2O3 column purifications carried out in glovebox with dry solvents as
eluents. NMR spectra were recorded on a 400 MHz NMR spectrometer
at room temperature unless otherwise specified. All 1H and 13C chemical
shifts were referenced to the residual proton resonance, and the 13C
resonance, of the deuterated solvent used; 31P chemical shifts were
referenced to external 85% aq. H3PO4. High-resolution mass spectra
5
2
2064
2079
2072
2072
198.3
195.3
196.3
197.4
-6.07
-6.87
-6.24
-6.00
5a
5b’
5b
P(OMe)3
SbPh3
PPh3
5c’
5c
5d
5e
5f
SbMe3
PMe3
PnBu3
PtBu3
IMes
-
-
-6.19
-6.19
-5.72
-5.67
-
2069
2067
2064
2059
197.1
197.5
199.4
-
were recorded in ESI mode on
a Waters UPLC-Q-TOF mass
spectrometer. Elemental analyses were carried out in-house.
[a] A1 mode of the W(CO)5 fragment, recorded in dichloromethane solutions.
[b] 13C chemical shifts of the cis carbonyl ligands in the W(CO)5 fragment in
CDCl3. [c] Calculated HOMO energies of the optimized structures of the free
ligands L.
Reaction of 1 with PMe3 in the absence of Al2O3: Cluster 1 (20 mg, 17
mol) was dissolved in dry THF (8 ml) and PMe3 (2.0 mg, 25 mol) was
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10.1002/ejic.201700456
European Journal of Inorganic Chemistry
COMMUNICATION
dropwise added. The reaction mixture was stirred at room temperature
for 36 h to give a yellow solution; IR monitoring showed 1 was completely
consumed. The solvent was then removed and the residue separated by
TLC on silica with DCM/hexane (2:1, v/v) as the eluent to give two
separable bands. The first, colorless, band was identified as
Os(Cl)2(CO)2(PMe3)2, 4 (Rf = 0.35; yield = 4.0 mg, 49%). IR (CH2Cl2):
ν(CO) 2033s, 1960s cm-1. 1H{31P} NMR (CDCl3) δ 1.78(s, 18H, P(CH3)3).
31P{1H} NMR (CDCl3): δ -33.34. ESI-MS+ (m/z): 488 [M+H3O]+, 434 [M-
Cl]+. The second, colorless, band was identified as the known
Os(Cl)2(CO)3(PMe3), 3 (Rf = 0.25; yield = 2.0 mg, 27%). IR (CH2Cl2):
ν(CO) 2127m, 2054s, 2012m cm-1. 1H NMR (d-THF) δ 1.80 (s, 9H,
P(CH3)3). When the residue was separated on neutral -Al2O3 column
with DCM/Hex (1:10, v/v) as eluent, the first fraction, yellow solution,
afforded small amount of 2 (not quite pure as determined by IR spectrum;
yield < 1.0 mg, < 10%).
Supporting Information (see footnote on the first page of this article):
Crystallographic data of 2 and 5, IR, NMR and MS spectra of all the new
compounds, and details of the DFT calculations.
Acknowledgements
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No. M4011158).
We also acknowledge Dr. Pei-Zhou Li for assistance with the X-
ray diffraction data collection on cluster 5.
Keywords: Metallostibine • Stibinidene • Strong two-electron
donor • Alkylphosphine • Osmium carbonyl
Reaction of 1 with PMe3 in the presence of Al2O3 (100 wt%): Cluster 1
(20 mg, 17 mol) was dissolved in dry THF (8 ml) at 0 oC. To the
yellowish solution PMe3 solution in toluene (0.1 M) (0.2 ml, 20 mol) and
neutral -Al2O3 (20 mg) were sequentially added. The reaction mixture
was stirred at 0 oC for 6 h to give a light yellow solution; IR monitoring
showed 1 was completely consumed. The solvent was then removed and
the residue subjected to a neutral -Al2O3 column with dry DCM/Hex
(1:10, v/v) as eluent to give 2 in the first fraction (Yield = 10 mg, 72%).
Then the eluent was changed to DCM/Hex (1:3, v/v) to give 3 (Yield =
6.0 mg, 82%) and further 4 (trace amount, unweighed).
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Reaction of 1 with THF in the presence of Al2O3 (2000 wt%): Cluster 1
(80 mg, 70 mol) was dissolved in dry THF (15 ml) followed by addition
of neutral -Al2O3 (1.60 g) in one portion. The suspension was stirred at
room temperature for 20 min to give a light yellow solution. The
suspension was then filtered and Al2O3 further washed with cold THF (<
0
oC) under inert atomosphere. The solvent of the combined filtrate was
removed in vacuo to give 2 in a pure enough form (Yield = 51 mg, 91%).
IR (CH2Cl2): ν(CO) 2118m, 2076s, 2035s, 2021sh, 2006w, 1990w cm-1.
1H NMR (CD2Cl2, -5 oC) δ 7.39 (d, 2H, Ph-H), 7.15 (t, 2H, Ph-H), 7.08 (t,
1H, Ph-H). 1H NMR (C6D6) δ 7.35 (dd, 2H, Ph-H), 6.96 (m, 2H, Ph-H),
6.88 (m, 1H, Ph-H). 13C{1H} NMR (CD2Cl2, -5 oC): δ(CO) 179.33, 178.80,
174.09, 171.99; δ(Ph-C) 138.40, 128.20, 127.90, 127.38. Anal. Calcd for
C14H5O8Os2Sb: C 20.93, H 0.63. Found: C 20.74, H 1.19. ESI-MS+ (m/z):
821 [M+H2O]+, 737 [M-3CO+H2O]+. Note: This reaction and the
subsequent work-up can also be easily performed under ambient
conditions using normal THF as solvent, but the purity will be slightly
lower due to the moderately air-sensitive nature of 2!
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Reaction of 2 with W(CO)5(CH3CN): Cluster 2 (15 mg, 19 mol) was
dissolved in dry THF (8 ml), and W(CO)5(CH3CN) (10 mg, 27 mol) was
added. The reaction mixture was stirred at room temperature for 12 h to
give a yellow solution. The solvent was then pumped off and the residue
separated by TLC with DCM/hexane (1:2, v/v) as the eluent to give one
main yellow band affording Os2W(CO)13(3-SbPh), 5 (Rf = 0.50; yield =
18 mg, 86% ). IR (CH2Cl2): ν(CO) 2130s, 2092s, 2064s, 2049s, 2022w,
2008w, 1935s cm-1. 1H NMR (C6D6) δ 7.26-7.28(m, 2H, Ph-H), 6.88-6.92
(m, 2H, Ph-H), 6.81-6.85 (m, 1H, Ph-H). 13C{1H} NMR (CDCl3): δ (CO)
200.91 (1C), 198.25 (4C), 175.02 (2C), 174.86 (2C), 170.94 (2C), 170.10
(2C);
δ (Ph-C) 135.47, 129.27, 129.16, 128.90. Anal. Calcd for
C19H5O13Os2SbW: C 20.24, H 0.45. Found: C 19.80, H 0.99. ESI-MS+
(m/z): 1128 [M+H]+.
CCDC-1534534 (for 5) and -1534535 (for 2) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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10.1002/ejic.201700456
European Journal of Inorganic Chemistry
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Activated neutral -Al2O3 (Brockmann activity I, particle size: 150 mesh,
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10.1002/ejic.201700456
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Strong two-electron donor:
The stable metallostibine
Os2(CO)8(-SbPh) can be
obtained in high yield and
purity via a one-step reaction.
Experimental and
computational data show that
its donor strength is
comparable to that of
Metallostibine
Ying-Zhou Li, Rakesh,
Ganguly, and Weng Kee
Leong*
Page No. – Page No.
Expedient Synthesis of a
Metallostibine Os2(CO)8(-
SbPh): An Unusual and
Strong Two-electron Donor
Ligand
alkylphosphines.
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