Synthesis, structure, and reactivity of mixed-metal clusters bearing η1-acetylide groups, (η5-C5H5)MFe2(μ3-E )2(CO)6(η1-CCPh) (M = Mo or W and E = Se or Te)

Article abstract of DOI:10.1021/om020064d

Reaction of Fe₃(CO)₉(μ₃-E)₂ (E = Se, Te) with (η⁵-C₅H₅)M(CO)₃(C≡CPh) (M = Mo, W), in the presence of trimethylamine-N-oxide (TMNO), in acetonitrile solvent at 60 °C yi

Full text of DOI:10.1021/om020064d

Organometallics 2002, 21, 2215-2218
2215
Syn th esis, Str u ctu r e, a n d Rea ctivity of Mixed -Meta l
Clu ster s Bea r in g η1-Acetylid e Gr ou p s,
(η5-C5H5)MF e2(µ3-E)2(CO)6(η1-CCP h ) (M ) Mo or W a n d E
) Se or Te)
Pradeep Mathur,* Anjan K. Bhunia, Arvind Kumar, Saurav Chatterjee, and
Shaikh M. Mobin
Chemistry Department, Indian Institute of Technology, Powai, Bombay 400 076, India
Received J anuary 29, 2002
Reaction of Fe3(CO)9(µ3-E)2 (E ) Se, Te) with (η5-C5H5)M(CO)3(CtCPh) (M ) Mo, W), in
the presence of trimethylamine-N-oxide (TMNO), in acetonitrile solvent at 60 °C yields the
novel mixed-metal clusters (η5-C5H5)MFe2(µ3-E)2(CO)6(η1-CCPh) (1, E ) Se, M ) Mo; 2, E )
Se, M ) W; 3, E ) Te, M ) Mo; 4, E ) Te, M ) W) bearing η1-acetylide groups. The reaction
of 1 or 3 with dicobalt octacarbonyl at room temperature gives the mixed-metal clusters
(η5-C5H5)MoFe2Co2(µ3-E)2(CO)9(µ-CCPh) (5, 6). Structures of 1 and 6 have been established
crystallographically.
In tr od u ction
work atoms. Under aerobic conditions some clusters
bearing both the oxo and the acetylide units are
obtainable.18-20 Formation of chalcogen-bridged and
acetylide-bearing mixed-metal clusters from the ther-
molytic reactions of [Fe3(CO)9(µ3-E)2] (E ) chalcogen
atom) and [M(η5-C5H5)(CO)3(CtCPh)] (M ) Mo or W)
may be thought to occur by a mechanism involving an
initial decarbonylation at the iron center followed by
attachment of the electron-deficient iron atom at the
triple bond of the coordinated acetylide group of the
mononuclear acetylide complex, or by loss of one or more
carbonyls from the metal acetylide compound and
complexation by lone pairs of the triply bridging chal-
cogen atoms of the tri-iron cluster. Formation of new
metal-metal bonds, acetylide coupling, and rearrange-
ments to give the most stable cluster framework would
follow to yield the final products. The nature of the
acetylide coupling on the newly formed mixed-metal
cluster would be a consequence of the relative orienta-
tions of the adding metal acetylide units as well as the
nature of the chalcogens. Importantly, since decarbo-
nylation is a necessary step in the overall mechanism,
the nature of the reaction conditions is also expected to
play an important role.
Metal acetylide complexes evince interest from several
perspectives: as useful precursors for preparation of
organometallic polymers,1-6 as components in NLO
materials,7,8 and as new liquid crystals.9,10 Formation
of the cluster-bound polycarbon units occurs as a result
of coupling of acetylide units from two different mono-
nuclear acetylide molecules11-15 or coupling of two
acetylide units with CO to form a pentanone ligand.16,17
Acetylide coupling is found to be sensitive to reaction
conditions used as well as the nature of cluster frame-
(1) Khan, M. S.; Kakker, A. K.; Ingham, S. L.; Raithby, P. R.; Lewis,
J .; Spencer, B.; Wittmann, F.; Friend, R. H. J . Organomet. Chem. 1994,
247, 472.
(2) Faulkner, C. W.; Ingham, S. L.; Khan, M. S.; Lewis, J .; Long, N.
J .; Raithby, P. R. J . Organomet. Chem. 1994, 139, 482.
(3) Abe, A.; Kinura, N.; Tabata, S. Macromolecules 1991, 24, 6238.
(4) Fyfe, H. B.; Mlekuz, M.; Zergarian, D.; Taylor, N. J .; Marder, T.
B. Chem. Commun. 1991, 188.
(5) Gia, G.; Puddephatt, R.; Vittal, J . J .; Payne, N. C. Organome-
tallics 1993, 12, 263.
(6) Gia, G.; Payne, N. C.; Vittal, J . J .; Puddephatt, R. J . Organo-
metallics 1993, 12, 4771.
(7) Marder, S. R.; Sohn, J . E.; Stucky, G. D. ACS Symp. Ser. 1991,
605, 455, and references therein.
(8) Prasad, P. N.; Williams, D. J . Nonlinear Optical Effects in
Molecules and Polymers; Wiley: New York, 1991.
(9) Kaharu, T.; Matsabara, H.; Takahashi, S. J . Mater. Sci. 1991,
32, 732.
(10) Takahashi, S.; Takai, Y.; Morimoto, H.; Sonogashira, K. Chem.
Commun. 1984, 3.
(11) Blenkiron, P.; Enright, G. D.; Carty, A. J . Chem. Commun.
1997, 483.
Our previous attempts at preparation of mixed Fe/
Mo or Fe/W clusters bearing uncoupled, η1-acetylide
moieties have been unsuccessful; under anaerobic con-
ditions, either dimerization of the mononuclear acetylide
complexes takes place or clusters bearing coupled
acetylide units are formed.21 Under aerobic and mild
(12) Chi, Y.; Carty, A. J .; Blenkiron, P.; Delgado, E.; Enright, G.
D.; Wang, W.; Peng, S.-M.; Lee, G.-H. Organometallics 1996, 15, 5269.
(13) Carty, A. J .; Hogarth, G.; Enright, G. D.; Frapper, G. Chem.
Commun. 1997, 1883.
(14) Wu, C.-H.; Chi, Y.; Peng, S.-M.; Lee, G.-H. Organometallics
1991, 10, 1676.
(18) Mathur, P.; Ahmed, M. O.; Dash, A. K.; Kaldis, J . H. Organo-
metallics 2000, 19, 941.
(19) Mathur, P.; Mukhopadhyay, S.; Ahmed, M. O.; Lahiri, G. K.;
Chakraborty, S.; Walwalakar, M. G. Organometallics 2000, 19, 5787.
(20) (a) Park, J . T.; Chi, Y.; Shapely, J . R.; Churchill, M. R.; Ziller,
J . W. Organometallics 1994, 13, 813. (b) Blenkiron, P.; Carty, A. J .;
Peng, S.-M.; Lee, G.-H.; Su, C.-J .; Shiu, C.-W.; Chi, Y. Organometallics
1997, 16, 519.
(21) Mathur, P.; Mukhopadhyay, S.; Ahmed, M. O.; Lahiri, G. K.;
Chakraborty, S.; Puranik, V. G.; Bhadbhade, M. M.; Umbarkar, S. B.
J . Organomet. Chem. 2001, 629, 160.
(15) Huang, G.-K.; Chi, Y.; Peng, S.-M.; Lee, G.-H. J . Organomet.
Chem. 1990, C7, 389.
(16) Mathur, P.; Ahmed, M. O.; Dash, A. K.; Walawalkar, M. G.;
Puranik, V. G. J . Chem. Soc., Dalton Trans. 2000, 2916.
(17) (a) Mathur, P.; Ahmed, M. O.; Kaldis, J . H.; McGlinchey, M. J .
J . Chem. Soc., Dalton Trans. 2002, 619. (b) Mathur, P.; Ahmed, M.
O.; Dash, A. K.; Walawalkar, M. G. J . Chem. Soc., Dalton Trans. 1999,
1795.
10.1021/om020064d CCC: $22.00 © 2002 American Chemical Society
Publication on Web 04/11/2002

2216 Organometallics, Vol. 21, No. 11, 2002
Mathur et al.
Sch em e 1
thermolytic reaction conditions, oxo-bridged mixed-
metal complexes are obtained in which the acetylide
units remain uncloupled, but are π-bonded, and hence
the acetylide triple bond is unutilizable for further
reactions.18-20 We report here on the first example of a
mixed-metal cluster bearing a η1-bound acetylide group
in which the CtC triple bond remains intact and, hence,
susceptible to attack by electrophiles.
F igu r e 1. ORTEP diagram of 1 with 30% probability
ellipsoids. Selected bond distances (Å) and bond angles
(deg): C(1)-C(2) ) 1.221(12), Mo(1)-Fe(1) ) 2.9085(15),
Mo(1)-Fe(2) ) 2.8228(15), Mo(1)-Se(2) ) 2.5277(12), Mo-
(1)-Se(1) ) 2.5557(11), Fe(1)-Se(2) ) 2.3625(17), Fe(2)-
Se(1) ) 2.3866(17), C(1)-C(2)-C(3) ) 178.2(10), Fe(1)-
Mo(1)-Fe(2) ) 74.65(4), Mo(1)-Se(1)-Fe(1) ) 72.58(5),
Mo(1)-Se(2)-Fe(2) ) 70.03(4).
Resu lts a n d Discu ssion
Clusters 1-4 were obtained in yields of 23-43% by
reacting Fe3(CO)9(µ3-E)2 (E ) Se, Te) with (η5-C5H5)M-
(CO)3(CtCPh) (M ) Mo, W) in the presence of tri-
methylamine-N-oxide (TMNO) in acetonitrile solvent
at 60 °C in an evacuated sealed tube (Scheme 1). There
was some decomposition, but no observation in the
reaction mixture of the known dimers [{(η5-C5H5)M-
(CO)2}2{µ-1,2-PhC-CCtCPh}]22 and [{(η5-C5H5)M-
(CO)2}2{µ-1,2-PhC-C(CO)CtCPh}].16 Apart from 1-4,
the only other compounds to be observed and isolated
from the reaction were trace amounts of the starting
materials. The new compounds are air stable in solid
but decompose in solution over a period of 2-3 days.
Infrared spectra of 1-4 show a similar four-band
mechanistic details are not known, certain observations
lead us to suggest possible steps likely to be involved
in the formation of 1-4. In the presence of TMNO, Fe3-
(CO)9(µ3-E)2 is known to convert to Fe2(CO)6(µ-E2) in
about 30% yield. We investigated the direct reaction of
Fe2(CO)6(µ-E2) with (η5-C5H5)M(CO)3(CtCPh) (room
temperature stirring, in the presence of and without
TMNO), but did not observe formation of compounds
1-4. Also, the fact that we do not observe any formation
of the known dimers [{(η5-C5H5)Mo(CO)2}2{µ-1,2-PhC-
CCtCPh}]22 and [{(η5-C5H5)Mo(CO)2}2{µ-1,2-PhC-
C(CO)CtCPh}]16 leads us to believe that, in the pres-
ence of TMNO/NCMe, (η5-C5H5)M(CO)3(CtCPh) forms
(η5-C5H5)M(CO)(NCMe)2(CtCPh) (A) as a possible in-
termediate (formation of (η5-C5H5)M(CO)2(NCMe)(Ct
CPh) would have yielded the dimers). Intermediate A
can now react in one of two ways: Pathway 1 shows its
reaction with the in-situ formed Fe2(CO)6(µ-E2) (E ) Se,
Te), which involves loss of the two acetonitrile ligands
and addition of the coordinatively unsaturated (C5H5)M-
(CO)(CtCPh) unit across the E-E bond of Fe2(CO)6(µ-
E2) to form intermediate B in much the same way as
has been observed in the formation of (η5-C5H5)CoFe2-
(CO)6(µ3-Se)2 from the reaction of (η5-C5H5)Co(CO)2 with
Fe2(CO)6(µ-Se2).24 Intermediate B rearranges to 1-4 by
a process involving cleavage of the Fe-Fe bond and
formation of new Fe-M bonds. An alternate option
available for A is its reaction with the starting iron
cluster Fe3(CO)9(µ3-E)2. Pathway 2 shows that upon loss
of the two acetonitrile ligands, the unsaturated (C5H5)M-
(CO)(CtCPh) unit can add to the triply bridging Se
atoms to form intermediate C, as for instance in the
formation of Fe4(CO)11(µ4-Se)2 and Fe3Ru(CO)11(µ4-Se)2
from reactions of Fe3(CO)9(µ3-Se)2 with Fe(CO)5 and Ru-
(CO)4(C2H4), respectively.25 Formation of new Fe-M
bonds and simultaneous loss of one Fe(CO)3 unit would
result in 1-4.
1
pattern in the terminal carbonyl region, and H NMR
spectra confirm the presence of (η5-C5H5) and phenyl
groups.
The X-ray crystal analysis of 1 (Figure 1) shows a
distorted square pyramidal core for Fe2MoSe2 in which
the Mo atom sits at the apical site, as in the previously
reported cluster Fe2MoSe2(CO)10.23 The Mo atom of 1
differs in having a single carbonyl group, a (η5-C5H5)
ligand, and a η1-CtCPh group. The acetylide C-C bond
distance in 1 is longer (1.221(12) Å) than the acetylide
triple bond distances seen in (η5-C5H5)Mo(CO)3(CtCPh)
(1.196(9) Å), [{(η5-C5H5)Mo(CO)2}2{µ-1,2-PhC-CCt
CPh}]22 (1.208(5) Å), and [{(η5-C5H5)Mo(CO)2}2{µ-1,2-
PhC-C(CO)CtCPh}]16 (1.197(5) Å) but still within the
acetylenic triple-bond range. Formal replacement of
three carbonyls on the Mo atom of Fe2MoSe2(CO)10 by
one (η5-C5H5) ligand and a η1-CtCPh group in 1 lead
to a marginal increase in the Mo-Fe bond lengths (Mo-
Fe in Fe2MoSe2(CO)10, 2.823(4) and 2.794(4) Å; in 1,
2.9085(15) and 2.8228(15) Å). All other bond distances
are similar in the two compounds.
Formation of 1-4 involves a formal conversion of the
Fe3E2 unit to an Fe2E2 unit and addition of a (η5-
C5H5)M(CO)(CtCPh) group to it. Although the exact
(22) Ustynyuk, N. A.; Vinogradova, V. N.; Korneva, V. N.; Kravtsov,
D. N.; Andrianov, V. G.; Struchkov, Y. T. J . Organomet. Chem. 1984,
277, 285.
(23) Mathur, P.; Sekar, P.; Satyanarayana, C. V. V.; Mahon, M. F.
J . Chem. Soc., Dalton Trans. 1996, 2173.
(24) Mathur, P.; Sekar, P.; Satyanarayana, C. V. V.; Mahon, M. F.
Organometallics 1995, 14, 2115.
(25) Mathur, P.; Hossain, M. M.; Rashid, R. S. J . Organomet. Chem.
1993, 460, 83.

Mixed-Metal Clusters Bearing η1-Acetylide Groups
Organometallics, Vol. 21, No. 11, 2002 2217
Ch a r t 1. P a th w a y 1
F igu r e 2. ORTEP diagram of 6 with 30% probability
ellipsoids. Selected bond distances (Å) and bond angles
(deg): C(12)-C(13) ) 1.368(9), Mo(1)-Fe(1) ) 2.6918(10),
Mo(1)-Fe(2) ) 2.7328(11), Co(1)-Co(2) ) 2.5387(13), Co-
(1)-Te(2) ) 2.5267(10), Mo(1)-Te(1) ) 2.6614(7), C(11)-
C(12)-C(13) ) 126.1(6), Fe(1)-Mo(1)-Fe(2) ) 58.05(3),
Co(2)-Co(1)-Mo(1) ) 110.76(4), Mo(1)-Te(1)-Fe(2) )
64.42(3), Fe(1)-Co(1)-Te(2) ) 85.89(4).
Ch a r t 2. P a th w a y 2
Ch a r t 3. P a th w a y 3
Sch em e 2
electron precise, if it is assumed that the Co-Mo bond
is a donor acceptor bond.
Con clu sion
The new mixed-metal clusters 1-4 were prepared in
reasonable yields and were shown to contain a η1-
acetylide group, which should be attractive for cluster
growth reaction as well as coupling with acetylide
moieties in other complexes. Preliminary investigations
of the reactivity of 1 and 3 have led to isolation of the
mixed Fe/Mo/Co clusters 5 and 6. We are presently
exploring the utility of 1-4 for addition of various units
to the η1-acetylide group as well as to the triply bridging
chalcogens in the cluster core.
To test the reactivity of the η1-acetylide group of 1-4,
we investigated the reaction of compounds 1 and 3 with
Co2(CO)8 and observe the formation of (η5-C5H5)MoFe2-
Co2(µ3-E)2(CO)9(µ-CCPh) (E ) Se, 5; E ) Te, 6) in yields
of 66% and 63%, respectively (Scheme 2). IR and NMR
spectroscopy indicate 5 and 6 to be isostructural. The
structure of 5, determined crystallographically and
shown in Figure 2, shows the expected addition of the
Co2 unit across the CtC triple bond of 3. Decarbony-
lation at the metal centers leads to formation of Fe-
Fe, two Fe-Co, and one Co-Mo bond, as well as one
new E-Co bond (Pathway 3). All the metal centers are
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Reactions and manipulations
were carried out using standard Schlenk line techniques under

2218 Organometallics, Vol. 21, No. 11, 2002
Mathur et al.
an atmosphere of prepurified argon. Solvents were purified,
dried, and distilled under an argon or nitrogen atmosphere
prior to use. Reactions were monitored by TLC as well as by
FT-IR spectroscopy. Infrared spectra were recorded on a
Nicolet Impact 400 FT spectrophotometer, as hexane solutions
of the sample in a 0.1 mm path length NaCl cell, and NMR
(1H) were recorded on a Varian VXR-300S spectrometer in
CDCl3. Elemental analyses were performed using a Carlo Erba
automatic analyzer. The compounds [Fe3(CO)9(µ3-E)2]26,27 and
[M(η5-C5H5)(CO)3(CtCPh)]28 (M ) Mo, W) were prepared by
established procedures.
composed materials, and the solvent was removed in vacuo.
The residue was subjected to chromatographic workup on silica
gel TLC plates using hexane-dichloromethane (50:50 v/v) to
give the pure products (5, 6).
5. Yield ) 40 mg, 66%. Anal. Found for C22H10O9Se2Fe2-
MoCo2: C, 29.60; H, 1.10. Calcd: C, 29.30; H, 1.11. IR (hexane)
ν cm-1 (terminal CO): 2090s, 2058s, 2034s, 2016w, 2005w. 1H
NMR (CDCl3): δ 5.32 (s, 5H, C5H5), 7.34-7.70 (m, 5H, C6H5).
Mp (°C): 148-150 (dec).
6. Yield ) 42 mg, 63%. Anal. Found for C22H10O9Te2Fe2-
MoCo2: C, 26.30; H, 0.97. Calcd: C, 26.42; H, 1.00. IR (hexane)
ν cm-1 (terminal CO): 2085s, 2053s, 2032s, 2016w, 2000w. 1H
NMR (CDCl3): δ 5.30 (s, 5H, C5H5), 7.34-7.70 (m, 5H, C6H5).
Mp (°C): 150-152 (dec).
Rea ction of [F e3(CO)9(µ3-E)2] w ith TMNO to F or m [F e2-
(CO)6(µ-E2)]. In a typical reaction, 250 mg of [Fe3(CO)9(µ3-
E)2] was dissolved in acetonitrile (30 mL), and to this was
added dropwise an acetonitrile solution of TMNO (10 mg in
10 mL) over a period of 30 min. The mixture was stirred for a
further 15 min. The solvent was removed in vacuo, and the
residue was subjected to column chromatography using silica
gel. Elution with hexane yielded an orange band of [Fe2(CO)6-
(µ-E2)] (yield: E ) Se, 28%; E ) Te, 33%).
X-r a y Cr ysta l Str u ctu r e Deter m in a tion s. Single crystals
of 1 and 6 were grown from dichloromethane-hexane solvent
mixtures at 0 °C. Data were collected on a Nonius MACH3
four-circle diffractometer (graphite-monochromatized Mo KR
radiation) for the cell determination and intensity data col-
lection. The unit cell parameters were derived and refined by
using randomly selected reflections in the 2θ range 2-50°. The
structure was solved by direct methods using the SHELXS97
program and refined by using SHELXL97 software.29,30 The
non-hydrogen atoms were refined with anisotropic thermal
parameters. All of the hydrogen atoms were geometrically fixed
and refined using a riding model. Absorption correction was
employed using ψ scans.31 The size of the crystals were 0.25
× 0.20 × 0.10 mm and 0.4 × 0.25 × 0.15 mm for 1 and 6,
respectively. Final R values: I > 2σ(I), R1 ) 0.603, wR2 )
0.1477; all data, R1 ) 0.0760, wR2 ) 0.1643 (1); I > 2σ(I), R1
) 0.0422, wR2 ) 0.1088; all data, R1 ) 0.0474, wR2 ) 0.1137
(6).
Tungsten hexacarbonyl and molybdenum hexacarbonyl were
purchased from Strem Chemical Co., phenylacetylene and
cyclopentadiene were purchased from Aldrich Chemical Co.,
and these were used without further purification.
Gen er a l P r oced u r e for th e P r ep a r a tion of [(η5-C5H5)-
MF e2(CO)6(µ3-E)2(η1-CtCP h ] (M ) Mo, W a n d E ) Se, Te)
(1-4). In a typical procedure 0.044 mmol of [CpM(CO)3(CCPh)]
[M ) Mo, W] and 0.138 mmol of Fe3E2(CO)9 [E ) Se and Te]
were dissolved in 20 mL of acetonitrile. To this dark purple
colored solution was added dropwise 5 mg of freshly sublimed
trimethylamine oxide (TMNO) in acetonitrile over a period of
20 min. The reaction mixture was allowed to stir at room
temperature for 10 min. At this point vacuum was applied and
the reaction mixture was heated to 60 °C, yielding a purple
residue. This was dissolved in CH2Cl2 and filtered through
Celite. The clear purple filtrate was concentrated and subjected
to column chromatography. Elution with hexane-acetone (97:3
v/v) gave the pure products (1-4).
1. Yield ) 10 mg, 39%. Anal. Found for C20H10O7Se2Fe2Mo:
C, 33.25; H, 1.45. Calcd: C, 32.97; H, 1.37. IR (hexane) ν cm-1
(terminal CO): 2072m, 2044s, 2010m, 1987m. 1H NMR
(CDCl3): δ 5.48 (s, 5H, C5H5), 7.09-7.18 (m, 5H, C6H5). Mp
(°C): 118-120 (dec).
2. Yield ) 10 mg, 28%. Anal. Found for C20H10O7Se2Fe2W:
C, 29.62; H, 1.30. Calcd: C, 29.41; H, 1.22. IR (hexane) ν cm-1
(terminal CO): 2070m, 2041s, 2008m, 1983m. 1H NMR
(CDCl3): δ 5.56 (s, 5H, C5H5), 7.09-7.17 (m, 5H, C6H5). Mp
(°C): 125-128 (dec).
3. Yield ) 15.6 mg, 43%. Anal. Found for C20H10O7Te2Fe2-
Mo: C, 28.78; H, 1.15. Calcd: C, 29.08; H, 1.21. IR (hexane) ν
1
cm-1 (terminal CO): 2059m, 2034s, 1999m, 1977m. H NMR
(CDCl3): δ 5.36 (s, 5H, C5H5), 7.17-7.27 (m, 5H, C6H5). Mp
(°C): 140-142 (dec).
4. Yield ) 9 mg, 23%. Anal. Found for C20H10O7Te2Fe2W:
C, 26.33; H, 1.00. Calcd: C, 26.28; H, 1.09. IR (hexane) ν cm-1
(terminal CO): 2055m, 2029s, 1995m, 1970m. 1H NMR
(CDCl3): δ 5.41 (s, 5H, C5H5), 7.10-7.25 (m, 5H, C6H5). Mp
(°C): 147-150 (dec).
Gen er a l P r oced u r e for th e P r ep a r a tion of 5 a n d 6. In
a typical procedure, to a dichloromethane solution of 0.067
mmol of 1 or 3 was added 3 equiv of Co2(CO)8. The reaction
mixture was allowed to stir at room temperature for 12 h. The
reaction mixture was filtered through Celite to remove de-
Ack n ow led gm en t. A.K.B. and A.K. are grateful to
the Council of Scientific and Industrial Research, Gov-
ernment of India, for fellowships.
Su p p or tin g In for m a tion Ava ila ble: Details of the struc-
ture determination for 1 and 6, including tables listing atomic
coordinates, thermal parameters, and bond distances and
angles and figures showing structures. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
OM020064D
(29) Sheldrick, G. M. SHELXS 93, Program for crystal structure
solution and refinement; University of Go¨ttingen, 1993.
(30) Sheldrick, G. M. SHELX 97, Program for crystal structure
solution and refinement; University of Go¨ttingen, 1997.
(31) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
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