The first example of tetraosmium carbonyl clusters containing (μ3-NH) nitrene ligands: Syntheses and crystal structures

Article abstract of DOI:10.1021/om020767v

The reaction of [Os₄(μ-H)₄(CO)₁₂] with o-tert-butylhydroxylamine hydrochloride (^tBuONH₂·HCl) in the presence of trimethylamine-N-oxide (Me₃NO) in dichloromethane afforded [Os₄-(μ-H)₄(CO)₁₁{η¹-NH₂ O^tBu}] (1) in high yield. Upon the addition of tetrafluoroboric acid (HBF₄) into the solution of 1 in acetonitrile, the novel cationic cluster [Os₄(μ-H)₄(CO)₁₁(μ-NH₂)-(NCMe) ] [BF₄] (2) formed. Reaction of 1 with HBF₄ in the presence of diphenylacetylene gave two geometric isomers, [Os₄(μ-H)₂(CO)₁₁(μ-NH₂){μ- η³-Ph(CHC)Ph}] (3) and [Os₄(μ-H)₂(CO)₁₁-(μ-NH₂)(η ₁-Ph(CH=C)Ph)] (4). Treatment of 2 with excess Me₃NO in acetonitrile gave [Os₄-(μ-H)₄(CO)₁₀(μ-NH₂)(NCMe) ₂] [BF₄] (5). The carbonylation of 2 in the refluxing chloroform afforded the neutral clusters [Os₃(μ-H)₂(CO)₉(μ-NH₂)Cl] (6) and [Os₄(μ-H)₃(CO)₁₂(μ-NH₂)] (7). Protonation of 7 with trifluoromethylsulfonic acid, CF₃SO₃H, gave another cationic cluster, [Os₄(μ-H)₄(CO)₁₂(μ-NH₂)] [CF₃SO₃] (8), which is an analogue of 2. Refluxing of 7 in toluene under an argon atmosphere afforded [Os₄(μ-H)₂(CO)₁₂(μ₃-NH)] (9), which is the first example of (μ₃-NH) nitrene ligands containing tetraosmium carbonyl clusters. The solid-state structures of 2-9 were established by X-ray analysis. The ₁H NMR properties of these clusters were studied, and a correlation between the chemical shift and the structural geometry was established.

Full text of DOI:10.1021/om020767v

Organometallics 2003, 22, 1029-1037
1029
Th e F ir st Exa m p le of Tetr a osm iu m Ca r bon yl Clu ster s
Con ta in in g (µ3-NH) Nitr en e Liga n d s: Syn th eses a n d
Cr ysta l Str u ctu r es
Yat Li and Wing-Tak Wong*
Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, People’s Republic of China
Zhen-Yang Lin
Department of Chemistry, The University of Science and Technology, Clear Water Bay,
Hong Kong, People’s Republic of China
Received September 16, 2002
The reaction of [Os4(µ-H)4(CO)12] with o-tert-butylhydroxylamine hydrochloride (tBuONH2‚
HCl) in the presence of trimethylamine-N-oxide (Me3NO) in dichloromethane afforded [Os4-
(µ-H)4(CO)11{η1-NH2OtBu}] (1) in high yield. Upon the addition of tetrafluoroboric acid (HBF4)
into the solution of 1 in acetonitrile, the novel cationic cluster [Os4(µ-H)4(CO)11(µ-NH2)-
(NCMe)][BF4] (2) formed. Reaction of 1 with HBF4 in the presence of diphenylacetylene gave
two geometric isomers, [Os4(µ-H)2(CO)11(µ-NH2){µ-η3-Ph(CHC)Ph}] (3) and [Os4(µ-H)2(CO)11-
(µ-NH2)(η1-Ph(CHdC)Ph)] (4). Treatment of 2 with excess Me3NO in acetonitrile gave [Os4-
(µ-H)4(CO)10(µ-NH2)(NCMe)2][BF4] (5). The carbonylation of 2 in the refluxing chloroform
afforded the neutral clusters [Os3(µ-H)2(CO)9(µ-NH2)Cl] (6) and [Os4(µ-H)3(CO)12(µ-NH2)] (7).
Protonation of 7 with trifluoromethylsulfonic acid, CF3SO3H, gave another cationic cluster,
[Os4(µ-H)4(CO)12(µ-NH2)][CF3SO3] (8), which is an analogue of 2. Refluxing of 7 in toluene
under an argon atmosphere afforded [Os4(µ-H)2(CO)12(µ3-NH)] (9), which is the first example
of (µ3-NH) nitrene ligands containing tetraosmium carbonyl clusters. The solid-state
1
structures of 2-9 were established by X-ray analysis. The H NMR properties of these
clusters were studied, and a correlation between the chemical shift and the structural
geometry was established.
In tr od u ction
reactions, such as the Haber process6 and the oxidation
of ammonia.7 In some of these reactions, surface-
coordinated nitrogen atoms are the key intermediates.
The Haber process has been shown to involve the
dissociative chemisorption of molecular nitrogen to give
surface-adsorbed nitrogen atoms, which further react
with molecular hydrogen to form N-H bonds, followed
by the desorption of ammonia.8 The bonding relation-
ship between coordinated transition metal clusters and
the surface-absorbed NHx moieties is illustrated in
Scheme 1.9-12
The chemistry of late transition metal carbonyl
clusters possessingamino,1 amido,2 and nitrene ligands3-5
has been extensively investigated in recent years. Much
of the interest in those species stems from the compari-
son of their reactivity to that of nitrogen atoms bound
to metal surfaces. The formation and cleavage of N-H
bonds on metal surfaces is believed to be an integral
part of several important heterogeneously catalyzed
* Corresponding author. Fax: (+852)25472933. E-mail: wtwong@
hkucc.hku.hk.
(1) Lin, Y. W.; Gau, H. M.; Wen, Y. S.; Lu, K. L. Organometallics
1992, 11, 1445. Lu, K. L.; Chen, C. J .; Lin, Y. W.; Gau, H. M.; Hong,
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10.1021/om020767v CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/30/2003

1030 Organometallics, Vol. 22, No. 5, 2003
Li et al.
Sch em e 1
95 mass spectrometer, using m-nitrobenzyl alcohol as matrix
solvent. Microanalyses were performed by Butterworth Labo-
ratories, UK.
The examples of transition metal carbonyl clusters
containing nitrene ligands are rare compared to those
clusters containing imido groups (µ3-NR or µ4-NR).
Several efficient routes to the isolation of (µ3-NH and
µ4-NH) nitrene clusters have been reported. These
include protonation of nitrido complexes,12,13 thermoly-
sis, and pyrolysis of nitrosyl complexes.4 However, these
synthetic methodologies are limited to the iron and
ruthenium systems only. To our knowledge, there is no
structurally established example of an osmium cluster
containing (µ3-NH and µ4-NH) nitrene ligands. Herein,
we report the preparation and characterization of a
series of new novel osmium metal clusters containing
amino and (µ-NH2) amido ligands and the first example
of (µ3-NH) nitrene ligands containing osmium clusters.
The new synthetic methodology of nitrene metal clusters
from amino metal clusters through amido metal clusters
is demonstrated.
P r ep a r a tion of [Os4(µ-H)4(CO)11{η1-NH2OtBu }] (1). The
complex [Os4(µ-H)4(CO)12] (110 mg, 0.1 mmol) was dissolved
in dichloromethane (30 mL) and mixed with 1 equiv of o-tert-
butylhydroxylamine hydrochloride (tBuONH2‚HCl) at 0 °C.
One equivalent of trimethylamine-N-oxide was added drop-
wise. Upon stirring for 1 h, the solution turned dark yellow.
The solvent was removed under reduced pressure. Chroma-
tography of the residue on preparative TLC plates eluting with
n-hexane/dichloromethane (1:1, v/v) afforded the complex [Os4-
(µ-H)4(CO)11{η1-NH2OtBu}] (1) (40 mg, 34%). Anal. Calcd for
C
15H15NO12Os4: C, 15.50; H, 1.30; N, 1.20. Found: C, 15.4;
H, 1.3, N, 1.2. IR (n-hexane): ν(CO) 2093(w), 2060(s), 2033-
(s), 2010(m), 2003(m), 1983(w). 1H NMR (CD2Cl2, 298 K): δ
-18.65 (s, br, 4H, hydride), 1.25 (s, 9H, methyl), 6.08 (s, 2H,
NH2); at 233K, 1a -13.64 (s, 1H, hydride), -15.36 (s, 1H,
hydride), -18.28 (s, 1H, hydride), -18.41 (s, 1H, hydride); 1b
-14.28 (s, 1H, hydride), -14.93 (s, 1H, hydride), -18.18 (s,
1H, hydride), -20.86 (s, 1H, hydride). FAB-MS (m/z): 1162.
Rea ction of 1 w ith Tetr a flu or obor ic Acid (HBF 4‚Et2O).
Three drops of tetrafluoroboric acid were added into a solution
of 1 (23 mg, 0.02 mmol) in acetonitrile (30 mL) under an argon
atmosphere. The yellow solution was stirred for 1 h, and the
solvent then reduced in vacuo. The residue was separated by
preparative TLC on silica, with an eluent of n-hexane/
dichloromethane (1:1, v/v). The yellow band was isolated as
[Os4(µ-H)4(CO)11(µ-NH2)(NCMe)][BF4] (2) (18 mg, 80%). Anal.
Calcd for C13H9BF4N2O11Os4: C, 12.83; H, 0.75; N, 2.30.
Found: C, 12.6; H, 0.7, N, 2.2. IR (CH2Cl2): ν(CN) 2132(m);
ν(CO) 2105(s), 2078(s), 2062(s), 2041(w), 2032(w), 2008(w),
1993(w). 1H NMR (CD2Cl2): δ -21.25 (s, 1H, hydride), -16.78
(s, 1H, hydride), -16.14 (s, 1H, hydride), -15.55 (s, 1H,
hydride), 1.10 (s, br, 1H, NH2), 1.84 (s, br, 1H, NH2), 2.72 (s,
3H, methyl). FAB-MS (m/z): 1130.
Rea ction of 1 w ith HBF 4 in th e P r esen ce of Dip h en y-
la cetylen e. A solution of 1 (23 mg, 0.02 mmol) in dichlo-
romethane (30 mL) was mixed with an excess amount of
diphenylacetylene under an argon atmosphere. Several drops
of tetrafluoroboric acid were added, and the resultant solution
was stirred for 2 h. The solvent was concentrated under
reduced pressure and subjected to TLC separation using
n-hexane/dichloromethane (1:1, v/v) as eluent. Two consecutive
yellow bands afforded [Os4(µ-H)2(CO)11(µ-NH2){µ-η3-Ph(CHC)-
Ph}] (3) (4 mg) and [Os4(µ-H)2(CO)11(µ-NH2)(η1-Ph(CHdC)Ph)]
(4) (3 mg) in 15 and 12% yields, respectively. 3 Anal. Calcd
for C25H15NO11Os4: C 23.71; H 1.19; N 1.11. Found: C, 23.6;
Exp er im en ta l Section
All reactions and manipulations were carried out under
argon using standard Schlenk techniques, except for the
chromatographic separations. Solvents were purified by stan-
dard procedures and distilled prior to use.14 Reactions were
monitored by analytical thin-layer chromatography (Merck
Kieselgel 60 F254), and the products were separated by thin-
layer chromatography on plates coated with silica (Merck
Kieselgel 60 F254). All chemicals, unless otherwise stated, were
purchased commercially and used as received. [Os4(µ-H)4-
(CO)12]15 was prepared by the literature methods.
Infrared spectra were recorded on a Bio-Rad FTS-135 IR
spectrometer, using 0.5 mm calcium fluoride solution cells, and
1H NMR spectra were recorded on a Bruker DPX300 spec-
trometer using CD2Cl2 and referenced to SiMe4 (δ 0). Variable-
temperature 1H NMR spectra were obtained on a Bruker
DPX500 spectrometer. Positive ionization fast atom bombard-
ment (FAB) mass spectra were recorded on a Finnigan MAT
(13) Fjare, D. E.; Gladfelter, W. L. Inorg. Chem. 1981, 20, 3533.
Fjare, D. E.; Gladfelter, W. L. J . Am. Chem. Soc. 1981, 1573.
(14) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 4th ed.; Butterworth Heinemann: Bath, 1996.
(15) Kaesz, H. D.; Knox, S. A. R.; Koepke, J . W.; Saillant, R. B. J .
Chem. Soc., Chem. Commun. 1971, 477. J ohnson, B. F. G.; Lewis, J .;
Raithby, P. R.; Scheldrick, G. M.; Wong, K. J . Chem. Soc., Dalton
Trans. 1978, 673. Zuccaro, C.; Pampaloni, G.; Calderazzo, F. Inorg.
Synth. 1989, 26, 293.

Tetraosmium Carbonyl Clusters
Organometallics, Vol. 22, No. 5, 2003 1031
H, 1.2, N, 1.1. IR (CH2Cl2): ν(CO) 2095(w), 2064(s), 2047(s),
2032(m), 1997(w). 1H NMR (CD2Cl2): δ -16.79 (s, 1H, hy-
dride), -13.35 (s, 1H, hydride), -0.03 (s, br, 1H, NH2), 0.32
(s, br, 1H, NH2), 2.60 (s, 1H, methyl), 6.73 (dd, J HH ) 7.8, 2H,
phenyl), 7.24 (m, 6H, phenyl), 7.43 (m, 2H, phenyl). FAB-MS
(m/z): 1266. 4 Anal. Calcd for C25H15NO11Os4: C, 23.71; H,
1.19; N 1.11. Found: C, 23.7; H, 1.2, N, 1.2. IR (CH2Cl2): ν(CO)
2103(w), 2074(s), 2055(s), 2029(s), 2008(w). 1H NMR (CD2-
Cl2): δ -19.10 (s, 1H, hydride), -15.90 (s, 1H, hydride), 4.00
(s, br, 1H, NH2), 4.83 (s, br, 1H, NH2), 2.12 (s, 1H, methyl),
6.81 (d, J HH ) 6.9, 2H, phenyl), 7.02 (m, 6H, phenyl), 7.31 (t,
J HH ) 7.9, 2H, phenyl). FAB-MS (m/z): 1087. [M - Ph(CHd
C)Ph]+.
P r ep a r a tion of [Os4(µ-H)4(CO)10(µ-NH2)(NCMe)2][BF 4]
(5). Solid 2 (23 mg, 0.02 mmol) was dissolved in acetonitrile
(30 mL) to give a yellow solution. One equivalent of trimethy-
lamine-N-oxide was added dropwise into the yellow solution.
Upon stirring for 3 h, the yellow solution darkened. The solvent
was removed under reduced pressure. The residue was redis-
solved in dichloromethane and separated by TLC eluting with
n-hexane/dichloromethane (1:3, v/v) affording [Os4(µ-H)4(CO)10-
using a Bruker SMART CCD 1000 diffractometer with graphite-
monochromated Mo KR radiation using ω scan type. Details
of the intensity data collection and crystal data are given in
Table 1. The data were corrected for Lorentz and polarization
effects. The structures were solved by direct methods (SIR92
or SHELXS86)16 and expanded using Fourier techniques
(DIRDIF94),17 refined by full-matrix least-squares analysis.
Refinements for all non-hydrogen atoms anisotropically have
been attempted for all the structures. However, some thermal
parameters of these atoms were found to be nonpositive
definite, and their thermal parameters were refined in an
isotropical manner. This is a common problem encountered
for strongly absorbing crystals, as the absorption correction
is imperfect. Hydrogen atoms on the organic moieties were
introduced in their idealized position, while hydride atoms
were located either by difference Fourier syntheses using low
angle data or from potential energy calculations.18 They were
included in the structure factors but not refined. The absolute
structures of 8 and 9 were also established from the refined
Flack parameters. All calculations were performed using the
teXsan19 crystallographic software package of the Molecular
Structure Corporation.
(µ-NH2)(NCMe)2][BF4] (5) (15 mg, 66%). Anal. Calcd for C14H12
-
BF4N3O10Os4: C, 13.67; H, 0.98; N, 3.42. Found: C, 13.8; H,
1.0, N, 3.5. IR (CH2Cl2): ν(CN) 2107(m); ν(CO) 2072(s), 2059-
(s), 2022(m), 2001(w), 1989(w). 1H NMR (CD2Cl2): δ -21.98
(s, 2H, hydride), -15.53 (s, 2H, hydride), 0.64 (s, 2H, NH2),
2.69 (s, 6H, methyl). FAB-MS (m/z): 1143.
Resu lts a n d Discu ssion
Syn th esis. The preparation of 1-9 is summarized
in Scheme 2. The parent amino cluster 1 was simply
prepared by the reaction of [Os4(µ-H)4(CO)12] with o-tert-
butylhydroxylamine hydrochloride (tBuONH2‚HCl). An
amino transfer to the cluster can be achieved upon
elimination of the tBuO group as tBuOH in acidic
medium. This would result in the further coordination
of NH2 ligand to the metal core. The novel cationic
amido cluster 2 was isolated by using this strategy. The
amido-supported butterfly geometry of 2 is thought to
be favorable for the formation of a nitrene cluster after
removing the labile acetonitrile ligand. The carbonyla-
tion of 2 in the refluxing chloroform afforded the neutral
clusters 6 and 7. The formation of trinuclear cluster 7
is attributed to the decomposition of 2. Cluster 7, the
deprotonated analogue of 2, was protonated with trif-
luoromethylsulfonic acid (CF3SO3H) to give the cationic
8. Clusters 2 and 8 are isoelectronic, and the protonation
and deprotonation processes are found to be reversible.
The first example of a (µ3-NH) nitrene osmium cluster
9 was isolated by the thermolysis of 7 in toluene. The
transformation of an amino ligand to a nitrene ligand
through an amido ligand was established. In addition,
the reaction of 2 with excess Me3NO in acetonitrile gave
another cationic cluster, 5.
Ca r bon yla tion of 2. A solution of 2 (23 mg, 0.02 mmol) in
chloroform (30 mL) was heated to refluxing temperature with
carbon monoxide bubbled through the solution for 10 h. With
the aid of IR monitoring, the reaction was halted until all of 2
was consumed. The solvent was then removed in vacuo, and
the residue was chromatographed on TLC using n-hexane/
dichloromethane (1:1, v/v) as eluent. A minor yellow band of
[Os3(µ-H)2(CO)9(µ-NH2)Cl] (6) (2 mg, 10%) and a pale yellow
band of [Os4(µ-H)3(CO)12(µ-NH2)] (7) (12 mg, 55%) were
isolated. 6: Anal. Calcd for C10H4ClNO9Os3: C, 13.52; H, 0.45;
N, 1.58. Found: C, 13.4; H, 0.44, N, 1.59. IR (CH2Cl2): ν(CO)
2124(w), 2116(w), 2091(m), 2063(s), 2049(m), 2005(m). 1H
NMR (CD2Cl2): δ -15.37 (t, J HH ) 2.0, 2H, hydride), 5.30 (s,
br, 2H, NH2). FAB-MS (m/z): 876. 7: Anal. Calcd for C12H5-
NO12Os4: C, 12.91; H, 0.45; N, 1.25. Found: C, 12.8; H, 0.46,
N, 1.25. IR (CH2Cl2): ν(CO) 2107(w), 2078(s), 2060(s), 2014-
(s), 1989(m). 1H NMR (CD2Cl2): δ -16.38 (s, 3H, hydride),
-0.57 (s, br, 2H, NH2). FAB-MS (m/z): 1116.
R ea ct ion of
7 w it h Tr iflu or om et h ylsu lfon ic Acid
(CF 3SO3H). To a yellow dichloromethane solution of 7 (22 mg,
0.02 mmol) were added several drops of trifluoromethylsulfonic
acid (CF3SO3H). The yellow solution was stirred at ambient
temperature for 3 h, and then the solvent was reduced in
vacuo. Some colorless precipitate appeared and was character-
ized as [Os4(µ-H)4(CO)12(µ-NH2)][CF3SO3] (8) (8 mg, 36%). Anal.
Calcd for
Found: C, 12.4; H, 0.46, N, 1.09. IR (CH2Cl2): ν(CO) 2126(s),
2105(s), 2084(vs), 2049(s), 2018(w). 1H NMR (CD2Cl2):
C13H6F3NO15Os4S: C, 12.33; H, 0.48; N, 1.11.
The main stimulus for the study of 1 toward diphe-
nylacetylene stems from the success of the isolation of
a series of µ4-nitrene clusters in the reaction of [Ru3-
(CO)10(µ3-NOMe)] with diphenylacetylene.20 The diphe-
nylacetylene is suggested to be important structural
stabilizers in the formation of a square planar metal
core that contains a µ4-NH ligand. However, only two
δ
-17.20 (s, 4H, hydride), 2.63 (s, br, 2H, NH2). FAB-MS (m/z):
1117.
Th er m olysis of [Os4(µ-H)3(CO)12(µ-NH2)]. A solution of
7 (22 mg, 0.02 mmol) in toluene (30 mL) was refluxed under
an argon atmosphere for 24 h. After cooling, the solvent was
removed in vacuo and the residue was separated by prepara-
tive TLC on silica, with an eluent of n-hexane/dichloromethane
(1:1, v/v). The pale yellow band was isolated and identified as
[Os4(µ-H)2(CO)12(µ3-NH)] (9) (5 mg, 24%). Anal. Calcd for C12H3-
NO12Os4: C, 12.94; H, 0.27; N, 1.26. Found: C, 12.9; H, 0.26;
N, 1.27. IR (CH2Cl2): ν(CO) 2078(s), 2030(s), 2006(m). 1H NMR
(CD2Cl2): δ -16.82 (d, J HH ) 0.8, 2H, hydride), 9.35 (s, br,
1H, NH). FAB-MS (m/z): 1115.
(16) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi A.; Polidori, G. SIR92, J . Appl. Crystallogr.
1994, 27, 435.
(17) SHELXS 86, Program for Crystal Structure Solution: Sheld-
rick, G. M. Acta Crystallgr., Sect. A 1990, 46, 467.
(18) Orpen, A. G. J . Chem. Soc., Dalton Trans. 1980, 2509.
(19) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; Gelder, R. de.; Israel, R.; Smits, J . M. M. The DIRDIF-
94 program system; Technical Report of the Crystllography Laboratory;
University of Nijmegen: The Netherlands, 1994.
X-r a y Cr ysta llogr a p h y. Single crystals of 2-9 were grown
from their appropriate solvent systems under favorable condi-
tions. Intensity data were collected at ambient temperature
(20) Ho, E. N. M.; Wong, W. T. J . Chem. Soc., Dalton Trans. 1998,
4215.

1032 Organometallics, Vol. 22, No. 5, 2003
Li et al.
amido clusters, 3 and 4, were isolated in the reaction of
1 and diphenylacetylene and no µ4-NH nitrene osmium
cluster can be detected under a similar reaction condi-
tion employed for the ruthenium system.
Solid -Sta te Str u ctu r es of 2-9. The solid-state
structures of 2-9 were established by single-crystal
X-ray diffraction analyses, and the selected bond pa-
rameters are illustrated in Table 2. The novel cationic
cluster [Os4(µ-H)4(CO)11(µ-NH2)(NCMe)][BF4], 2, is the
sole product isolated in the reaction of 1 toward tet-
rafluoroboric acid (HBF4) in acetonitrile at ambient
temperature, where the tert-butylhydroxy ligand (tBuO)
is eliminated. This amido metal cluster is significant,
in part because it is one of the very rare cationic
carbonyl cluster complexes, and more importantly be-
cause it is believed to be a good candidate for synthesiz-
ing nitrene metal clusters. It is anticipated that the
acetonitrile ligand can be removed under mild condi-
tions, which might trigger the amido nitrogen to further
coordinate to the other two metal centers.
The molecular structure of 2 is illustrated in Figure
1. Four osmium atoms of 2 adopt the butterfly geometry,
coordinated with 11 terminal carbonyl ligands. The two
triosmium triangles share the common Os(1)-Os(3)
hinge edge, and the dihedral angle between the Os(1)-
Os(2)-Os(3) and the Os(1)-Os(3)-Os(4) planes is 95.51°.
This is similar to the corresponding value of 97.7°
observed in [Os4(µ-H)4(CO)12(OH)]+.21 The hinge Os(1)-
Os(3) bond distance [2.839(2) Å] is the shortest, while
the four hydride-bridged “hinge” to “wingtip” Os-Os
bond distances are within the range 2.983(2)-3.041(2)
Å. The butterfly geometry is supported by the amido
ligand through two wingtip atoms Os(2) and Os(4),
having Os-N bond distances of 2.17(2) and 2.08(3) Å,
respectively. The terminal acetonitrile ligand coordi-
nated to the Os(4) atom with a distance of 2.06(2) Å,
showing only a slight deviation from linearity, with Os-
(4)-N(1)-C(12) and N(1)-C(12)-C(13) angles of 173°
and 174°, respectively.
The success of the isolation of a series of µ4-nitrene
clusters in the reaction of [Ru3(CO)10(µ3-NOMe)] with
diphenylacetylene or phenylacetylene suggests that the
alkynes are important structural stabilizers. Several
drops of tetrafluoroboric acid were added to a mixture
of 1 and diphenylacetylene in dichloromethane, leading
to the formation of two new clusters, [Os4(µ-H)2(CO)11-
(µ-NH2){µ-η3-Ph(CHC)Ph}], 3, and [Os4(µ-H)2(CO)11(µ-
NH2){η1-Ph(CHdC)Ph}], 4, in moderate yields. These
are geometric isomers; the diphenylacetylene ligand acts
as a three-electron and one-electron donor in 3 and 4,
respectively.
The solid-state structures of 3 and 4 are depicted in
Figures 2 and 3, respectively. X-ray analysis of 3
illustrates that the amido-supported butterfly geometry
of 2 is retained. The dihedral angle between the two
wings of the triosmium planes Os(1)-Os(2)-Os(3) and
Os(2)-Os(3)-Os(4) is 86.97°, which is slightly smaller
than the value observed in 2. The diphenylacetylene
bridges Os(3) and Os(4) with a µ-η3 coordination mode
have an Os(3)-Os(4) bond distance of 2.856(1) Å, which
is comparable to the value observed in [HOs3(CO)10{µ-
η3-Ph(CHC)Ph}].22 Cluster 4 has a distorted tetrahedron
(21) J ohnson, B. F. G.; Lewis, J .; Raithby, P. R.; Zuccaro, C. J . Chem.
Soc., Dalton Trans. 1980, 716.

Tetraosmium Carbonyl Clusters
Organometallics, Vol. 22, No. 5, 2003 1033
Sch em e 2
metal framework with a characteristic 60 cluster va-
lence electron count. The amido ligand asymmetrically
bridges Os(3) and Os(4) with Os-N distances of 2.12-
(3) and 2.03(2) Å, respectively. The dihedral angle
between planes Os(3)-N(1)-Os(4) and Os(1)-Os(3)-
Os(4) is 172.35°. The diphenylacetylene is terminally
bonded to Os(4), with a Os(4)-C(18) bond of 2.12(4) Å.
A similar bonding mode was observed in [Ir4(CO)11{η1-
Ph(CHdC)Ph}][NEt4].23
Through the treatment of an acetonitrile solution of
2 with an excess of trimethylamine-N-oxide, the orange
cluster [Os4(µ-H)4(CO)10(µ-NH2)(NCMe)2][BF4], 5, was
isolated. Cluster 5 is another example of a cationic metal
cluster, which is structurally similar to the unbridged
butterfly cluster [Os4(µ-H)4(CO)10(NCMe)2]+.24 It seems
that the acetonitrile group is a good stabilizer for the
cationic metal cluster because it is a good σ-donor and
a poor π-acceptor. The metal-carbon bonds can be
strengthened, as more back-π-electrons can be shared
by the remaining carbonyl ligands.
(1)-N(2)-C(11) and Os(3)-N(3)-C(13) angles are 174°
and 176°, respectively. The molecular structure of 5
possesses a Cs symmetry, where the mirror plane bisects
the atoms O(8), C(8), Os(4), N(1), Os(2), C(4), O(4).
A chloroform solution of 2 was heated to reflux with
carbon monoxide bubbling for 10 h, and the new clusters
[Os3(µ-H)2(CO)9(µ-NH2)Cl], 6, and [Os4(µ-H)3(CO)12(µ-
NH2)], 7, were isolated. Cluster 6 is believed to be a
fragment of 2, and the chloride atom probably originates
from the chloroform solution. The molecular structure
of 6 is depicted in Figure 5, which consists of an isosceles
triangle of osmium metal atoms. The three Os-Os bond
distances are within the range 2.826(1)-3.0551(9) Å,
where the nonbridged Os(1)-Os(2) edge is the shortest
one. The dihedral angle of the plane Os(2)-Os(3)-N(1)
with reference to the triosmium plane is 101.41°. The
characteristic 48 cluster valence electron count is ob-
served.
The amido-supported butterfly geometry is also ob-
served in 7 (Figure 6), which has a dihedral angle of
94.49° between the planes Os(1)-Os(2)-Os(4) and Os-
(2)-Os(3)-Os(4). Each osmium metal center is coordi-
nated by three terminal carbonyl ligands. The charac-
teristic 62 cluster valence electron count was observed
for 7. The labile acetonitrile group of 2 is substituted
by a carbonyl ligand, together with metal hydride
elimination. It is noteworthy that the deprotonation
occurs preferentially at the metal hydride sites instead
The molecular architecture of 5 is very similar to that
of 2, where one more carbonyl ligand is replaced by the
acetonitrile ligand. Two labile acetonitrile ligands are
terminally bonded to two wingtip osmium atoms, which
show only a slight deviation from linearity, and the Os-
(22) Clauss, A. D.; Tachikawa, M.; Shapely, J . R.; Pierpont, C. G.
Inorg. Chem. 1981, 20, 1528.
(23) Della Pergola, R.; Garlaschelli, L.; Martinengo, S.; Manassero,
M.; Sansoni, M. J . Organomet. Chem. 2000, 593, 63.
(24) J ohnson, B. F. G.; Lewis, J .; Nelson, W. J . H.; Puga, J .; Raithby,
P. R.; Whitmire, K. H. J . Chem. Soc., Dalton Trans. 1983, 1339.
1
of at the hydrogen atoms of the amido ligand. The H
NMR spectrum of 2 reveals that the significantly

1034 Organometallics, Vol. 22, No. 5, 2003
Ta ble 2. Selected Bon d Dista n ces [Å] a n d An gles [d eg] for 2-9
Li et al.
2
3
4
5
6
7
8
9
Os(1)-Os(2)
Os(1)-Os(3)
Os(1)-Os(4)
Os(2)-Os(3)
Os(2)-Os(4)
Os(3)-Os(4)
Os(4)-Os(5)
Os(4)-Os(6)
Os(5)-Os(6)
Os(4)-N(1)
Os(3)-N(1)
Os(4)-N(2)
Os(2)-N(2)
Os(3)-N(3)
Os(1)-N(2)
Os(2)-N(1)
Os(1)-N(1)
Os(4)-N(2)
Os(5)-N(2)
Os(3)-C(12)
Os(4)-C(12)
Os(4)-C(18)
Os(1)-Cl(1)
3.025(2)
2.839(2)
3.041(2)
3.015(2)
2.827(1)
3.028(1)
2.770(2)
2.908(2)
2.752(2)
2.964(2)
2.972(2)
2.791(2)
3.0727(7)
2.826(1)
3.0551(9)
2.8367(9)
3.024(1)
3.028(1)
2.762(1)
2.851(2)
3.0001(7)
3.0622(7)
2.8178(7)
3.0005(7)
2.977(1)
3.0263(9)
2.812(1)
3.0490(9)
2.899(1)
2.888(1)
2.856(1)
2.8178(9)
2.834(1)
3.036(1)
3.020(1)
2.958(1)
2.983(2)
2.764(1)
2.856(2)
2.969(1)
2.06(2)
2.09(1)
2.03(2)
2.12(3)
2.11(2)
2.12(1)
2.113(10)
2.11(1)
2.14(1)
2.15(1)
2.15(2)
2.08(3)
2.17(2)
2.06(1)
2.08(1)
2.05(1)
2.16(2)
2.15(3)
2.05(1)
2.13(1)
2.132(9)
2.19(2)
2.18(2)
2.12(4)
84.5(9)
2.472(4)
Os(1)-N(2)-C(11)
Os(3)-N(3)-C(13)
Os(4)-N(1)-C(12)
Os(3)-C(12)-Os(4)
Os(3)-N(1)-Os(4)
Os(1)-N(1)-Os(4)
Os(1)-N(1)-Os(3)
Os(2)-N(1)-Os(3)
Os(2)-N(1)-Os(2*)
Os(1)-N(1)-Os(2)
Os(5)-N(1)-Os(5*)
Os(4)-N(1)-Os(5)
174(1)
176(1)
109.9(7)
81.8(6)
114(1)
115.0(6)
114.1(4)
111.5(6)
83.1(4)
130(1)
81.9(7)
130(1)
82.1(7)
shielded amido proton signals are located at δ ) 1.10
and 1.84. The electron-rich amido ligand lowers the
polarity of the NH bond to a certain extent, so that the
elimination of the NH proton becomes very difficult.
This is probably the reason for the deprotonation process
in the synthesis of 7 occurring preferentially at metal
hydride sites instead of at the amido ligand, which
might disfavor the formation of µ3- or µ4-nitrene clusters.
To investigate the properties of the bridging amido
ligand, molecular orbital calculations at the B3LYP level
of density functional theory based on the experimental
geometry of 7 were performed to examine its orbital
bonding characteristics in the HOMO-LUMO region.
F igu r e 1. Molecular structure of [Os4(µ-H)4(CO)11(µ-NH2)-
(NCMe)][BF4] (2) with the atom-numbering scheme.
F igu r e 2. Molecular structure of [Os4(µ-H)2(CO)11(µ-NH2)-
{µ-η3-Ph(CHC)Ph}] (3) with the atom-numbering scheme.

Tetraosmium Carbonyl Clusters
Organometallics, Vol. 22, No. 5, 2003 1035
F igu r e 5. Molecular structure of [Os3(µ-H)2(CO)9(µ-NH2)-
Cl] (6) with the atom-numbering scheme.
F igu r e 3. Molecular structure of [Os4(µ-H)2(CO)11(µ-NH2)-
(η1-Ph(CHdC)Ph)] (4) with the atom-numbering scheme.
F igu r e 6. Molecular structure of [Os4(µ-H)3(CO)12(µ-NH2)]
(7) with the atom-numbering scheme.
bonds range from 0.18 to 0.32, while the indices for the
N-H bonds are about 0.85. Clearly, the preference for
the protonation or deprotonation site depends heavily
on the relative bond strength.
Protonation of 7 with a few drops of trifluoromethyl-
sulfonic acid in dichloromethane gave some colorless
crystalline materials, which were identified as [Os4(µ-
H)4(CO)12(µ-NH2)][CF3SO3], 8. As expected, the proto-
nation occurs at the metal hydride sites, which is
consistent with molecular orbital calculations. They are
the analogue of 2 and 5 (Figure 8). Each osmium metal
is bonded with three carbonyl ligands, and four hydrides
are bridged on the “hinge to wing” Os-Os bond. The
dihedral angle between the wings of Os(1)-Os(2)-Os-
(3) and Os(2)-Os(3)-Os(4) is 83.67°. It is a highly
symmetric structure in comparison to 2 and 5 and
possesses C2 symmetry. The C2 axis passes through the
amido nitrogen to the center point of the hinge osmium-
osmium edge. This cationic cluster experiences orbital
contraction due to the electron deficiency, which makes
the orbital overlapping less efficient. Therefore, the
F igu r e 4. Molecular structure of [Os4(µ-H)4(CO)10(µ-NH2)-
(NCMe)2][BF4] (5) with the atom-numbering scheme.
It was observed that the highest occupied molecular
orbitals (HOMOs) correspond to the σ bonds of the
unbridged Os-Os bonds, followed by the “t2g” set d
orbitals of the four metal centers. The lowest unoccupied
molecular orbitals (LUMOs) are π* of the carbonyl
ligands, together with the Os-Os/Os-CO σ* antibond-
ing orbitals. The spatial plots25 of the two highest
occupied and one lowest unoccupied orbital are shown
in Figure 7. The natural bond order (NBO) analysis,26
based on the molecular orbital calculations, indeed
shows that the Os-H bonds are much weaker than the
NH bonds. The Wiberg bond indices27 for the Os-H
(25) Schaftenaar, G. Molden v3.5, CAOS/CAMM Center Nijmegen:
Toernooiveld, Nijmegen, Netherlands, 1999.
(26) Glendening, E. D.; Reed, A. E.; Carpenter, J . E.; Weinhold, F.
NBO version 3.1.
(27) Wiberg, K. B. Tetrahedron 1968, 24, 1083.

1036 Organometallics, Vol. 22, No. 5, 2003
Li et al.
F igu r e 9. Molecular structure of [Os4(µ-H)2(CO)12(µ3-NH)]-
(9) with the atom-numbering scheme.
osmium nitrene cluster. The nitrene ligand acts as a
four-electron donor, and an amido proton and a metal
hydride are eliminated during the thermolysis. The
characteristic 62 cluster valence electron count of but-
1
terfly geometry is retained. The H NMR spectrum of 9
shows that the nitrene proton signal is located at δ )
9.35, which is highly deshielded in comparison to amido
protons. This implies that the nitrene ligand is relatively
electron deficient, which makes the NH bond polar,
suggesting that the deprotonation process may occur
more preferentially on the nitrene ligand as compared
to 7. This might be a good precursor for the formation
of the nitrido or µ4-nitrene clusters.
F igu r e 7. Spatial plots of the two highest occupied and
the lowest unoccupied molecular orbitals for [Os4(µ-H)3-
(CO)12(µ-NH2)] (7).
X-ray analyses show that there are two crystallo-
graphically independent, but essentially identical, mol-
ecules in each asymmetric unit. The molecular structure
of only one of these will be described and is shown in
Figure 9. The molecule consists of a butterfly arrange-
ment of four osmium atoms with a triply bridging
nitrene ligand spanning one of the open triangular faces,
Os(1)-Os(2)-Os(2*). This kind of coordination moiety
is also observed in [Os4(µ-H)2(CO)12(µ3-PC6H11)],28 [Os4-
(CO)12(µ-SCH2CMe2CH2Cl)(µ-H)],29 and [Os4(CO)12(µ-
H)2(µ3-S)].30 The molecule contains a plane of symmetry
that is imposed by the crystal lattice and passes through
the atoms N(1), H(2), O(1), C(1), Os(1), Os(3), C(7), and
O(7). The dihedral angle between the two wings of the
butterfly, Os(1)-Os(2)-Os(3) and Os(1)-Os(2*)-Os(3),
is 97.90°, which is similar to the values observed in
other amido butterfly clusters. The dihedral angles of
the planes Os(1)-Os(2)-Os(3) and Os(1)-N(1)-Os(2*)
with reference to the plane Os(1)-N(1)-Os(2) are 95.56°
and 133.14°, respectively. The observed dihedral angle
is probably a consequence of the preferred bite of the
coordinatively flexible capping nitrene ligand, rather
than an electronic requirement of the bonding within
F igu r e 8. Molecular structure of [Os4(µ-H)4(CO)12(µ-
NH2)][CF3SO3] (8) with the atom-numbering scheme.
metal-metal bond strength decreases and the average
bond distances of 8 are relatively lengthened. A similar
observation is also found in 2 and 5. This probably
suggests that 8 is more likely to form clusters with a
64 valence electron count under nucleophilic attack, by
breaking the relatively weak metal-metal bond.
The novel cluster [Os4(µ-H)2(CO)12(µ3-NH)], 9, was
isolated in the thermolysis of 7 in toluene. To the best
of our knowledge, this is the first example of a µ3-NH
(28) Colbran, S. B.; J ohnson, B. F. G.; Lahoz, F. J .; Lewis, J .;
Raithby, P. R. J . Chem. Soc., Dalton Trans. 1988, 1199.
(29) Adams, R. D.; Belinski, J . A.; Pompeo, M. P. Organometallics
1992, 11, 3129.
(30) Adams, R. D.; Wang, S. Inorg. Chem. 1986, 25, 2534.

Tetraosmium Carbonyl Clusters
Organometallics, Vol. 22, No. 5, 2003 1037
Sch em e 3
Sch em e 4
which might cause a positive shift in the 1H NMR signal
of the NH group. A similar highly deshielded NH signal
(δ 9.50) was observed in [Fe3(CO)9(µ3-CO)(µ3-NH)].31
1
The H NMR signal of the NH group can serve as a
reference showing the electronic environment of amino,
amido, and nitrene ligands in the cluster, to determine
the polarity of the NH bond. The results might provide
some information on possible reactivities of these clus-
ters.
the cluster core. The trend of Os-N bond distances in
9 for the hinge Os(1)-N(1) [2.05(1) Å] is marginally
shorter than wingtip Os(2)-N(1) [2.16(2) Å]. The Os-
(2)-N(1)-Os(2*) and Os(1)-N(1)-Os(2) angles are 130-
(1)° and 81.9(7)°, respectively, which are slightly larger
than those values observed in other similar systems.28-30
1H NMR P r op er ties of Tetr a osm iu m Ca r bon yl
Clu ster s Con ta in in g Am in o, Am id o, a n d Nitr en e
Con clu sion
Osmium metal clusters containing (µ-NH2) amido and
(µ3-NH or µ4-NH) nitrene are relatively rare. They are
believed to be important cluster analogues for the
reactivity study of surface-bound nitrogen atoms toward
molecular hydrogen. In light of this, the syntheses,
structural characterization, and various reactivities of
a series of amino, amido, and nitrene tetraosmium metal
clusters have been examined and discussed in this
paper.
The first example of a tetraosmium cluster containing
a (µ3-NH) nitrene ligand was reported. The transforma-
tion of an amino cluster into a nitrene cluster through
an amido cluster has also been demonstrated in this
work and seems to be a good synthetic route to the
1
Liga n d s. The range of chemical shifts of the H NMR
signals of amino, amido, and nitrene protons in 1-9 is
illustrated in Scheme 3. Clearly, there is a negative shift
of NH signal as the amino clusters are transformed to
amido clusters. This is probably because the electron-
rich osmium metal center tends to donate electrons to
the electronegative nitrogen atom. Therefore, the elec-
tron density around the amido nitrogen increases as it
1
1
formation of nitrene clusters. The H NMR properties
bonds to more metal centers; the H NMR signal of NH
of these clusters were studied, and a correlation between
the chemical shift and the structural geometry has been
established.
becomes relatively upfield. The amido clusters are
believed to have similar chemical shifts for NH signals.
However, highly shielded amido proton signals of the
supported butterfly clusters are observed in the 1H NMR
spectrum in comparison to other amido clusters. There
is no obvious explanation for this observation, but it
suggests that the chemical shift of the NH protons, at
least to a certain extent, depends on their coordination
geometry. In contrast to the supported butterfly amido
clusters, the NH signal of the triply bridged nitrene
cluster is highly deshielded. This suggests that the
nitrene nitrogen might involve sp hybridization, in
which the px and py orbitals of nitrogen are not involved
in hybridization, but overlap directly with the d orbitals
of the osmium metal; see Scheme 4. Under this circum-
stance, the Ha is significantly influenced by the aniso-
tropic-induced magnetic field of the Os3-N triple bond,
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Hong Kong Research Grants
Council and the University of Hong Kong. Y. Li ac-
knowledges the receipt of a postgraduate studentship
(1999-2002) and the Li Po Chun Scholarship (2001-
2002) administered by the University of Hong Kong and
the Sir Edward Youde Memorial Fellowship (2001-
2002) awarded by the Sir Edward Youde Memorial
Trustees.
Su p p or tin g In for m a tion Ava ila ble: Details of data
collection parameters, bond lengths, bond angles, fractional
atomic coordinates, and anisotropic thermal parameters
for 2-9 are available free of charge via the Internet at
http://pubs.acs.org.
(31) Fjare, D. E.; Gladfelter, W. L. J . Am. Chem. Soc. 1981, 103,
1573.
OM020767V