Synthesis and X-ray molecular structure of (PNP)2[Mo IV(C6H4S2-1,2)3] completing the structural characterization of the series [Mo(C6H 4S2-l,2)3]n-(n = 0, 1, 2)

Article abstract of DOI:10.1002/ejic.200500086

The tris(benzene-1,2-dithiolato) complex (Li)₂[Mo ^IV(C₆H₄S₂-1,2)₃] {(Li)₂[Mo^IV(bdt)₃]} (bdt^2- = C ₆H₄S₂-1,2 dianion) was synthesized from [MoCl₄(CH₃CN)₂], (H₂bdt), and nBuLi in THF. Complex (Li)₂[Mo^IV(bdt)₃] undergoes aerial oxidation to give Li[Mo^V(bdt)₃]. Both complexes were crystallized from methanol as the salts (PNP)₂[Mo ^IV(bdt)₃] and (PNP)[Mo^V(bdt)₃] {PNP⁺ = bis(triphenylphosphoranylidene)ammonium cation), and their molecular structures were determined by X-ray diffraction. The molybdenum(IV) complex [Mo^IV-(bdt)₃]^2- assumes a distorted trigonal prismatic coordination geometry (average twist angle φ = 24.8°) while the coordination geometry of [Mo^V(bdt)₃] ^- is best described as distorted octahedral (average twist angle φ = 34.1°). Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200500086

FULL PAPER
Synthesis and X-ray Molecular Structure of (PNP)2[MoIV(C6H4S2-1,2)3]
Completing the Structural Characterization of the Series [Mo(C6H4S2-1,2)3]n–
(n = 0, 1, 2)
Christian Schulze Isfort,[a] Tania Pape,[a] and F. Ekkehardt Hahn*[a]
Keywords: Molybdenum / S ligands / X-ray diffraction / Coordination modes
The tris(benzene-1,2-dithiolato) complex (Li)2[MoIV(C6H4S2-
1,2)3] {(Li)2[MoIV(bdt)3]} (bdt2– = C6H4S2-1,2 dianion) was
synthesized from [MoCl4(CH3CN)2], (H2bdt), and nBuLi in
THF. Complex (Li)2[MoIV(bdt)3] undergoes aerial oxidation
to give Li[MoV(bdt)3]. Both complexes were crystallized from
methanol as the salts (PNP)2[MoIV(bdt)3] and (PNP)[MoV-
(bdt)3] {PNP+ = bis(triphenylphosphoranylidene)ammonium
cation}, and their molecular structures were determined by
X-ray diffraction. The molybdenum(IV) -
complex [MoIV
(bdt)3]2– assumes a distorted trigonal prismatic coordination
geometry (average twist angle φ = 24.8°) while the coordina-
tion geometry of [MoV(bdt)3]– is best described as distorted
octahedral (average twist angle φ = 34.1°).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
erally enhance the tendency for the formation of octahedral
complexes, and that the geometry of a particular complex
is not governed only by electronic factors. Ligand con-
straints,[10] matching of ligand and metal d orbital energies,
symmetry of ligand π orbitals, inter-donor interactions, and
the overall charge of the complex have been suggested to
be additional important factors.[11]
The situation is further complicated by the non-innocent
nature of the bdt2– ligand. Wieghardt has shown that an
intramolecular redox reaction can convert coordinated
bdt2– into a coordinated o-dithiobenzosemiquinonate(1–)
radical anion by the reduction of the metal center. Under
such conditions, the spectroscopic oxidation state of the me-
tal differs from the formal oxidation state. The geometrical
changes within the ligand that are associated with the intra-
molecular redox reaction have been outlined for square
planar bdt2– complexes.[12]
As part of our ongoing effort to shed light on the rela-
tionship between the formal oxidation state of the metal
and the molecular structure of [M(bdt)3]n– (M = Mo, W)
complexes, we present here the preparation and molecular
structure of the yet unknown derivative (PNP)2[MoIV-
(bdt)3]2–. In addition, the previously described[6] complex
anion [MoV(bdt)3]– was prepared again, and the crystal
structure of the salt (PNP)[MoV(bdt)3] was elucidated and
compared to the molecular structure reported for (nBu4N)-
[MoV(bdt)3].[6a]
Introduction
The first trigonal prismatic (TP) tris(dithiolene) metal
complex,
tris(cis-1,2-diphenylethylene-1,2-dithiolato)rhe-
nium, was reported in 1965.[1] Ever since, the reasons for
the formation of complexes with trigonal prismatic coordi-
nation geometry over octahedral complexes has been a sub-
ject for discussion.[2] Early explanations based on molecular
orbital schemes indicated that metal centers with a formal
d0 electron configuration preferentially form trigonal pris-
matic tris(dithiolene) transition metal complexes. In the
series of tris(benzene-1,2-dithiolato) complexes, [WVI(bdt)3]
is trigonal prismatic (twist angle φ = 0°),[3] while the geome-
try of [WV(bdt)3]– was found to be closer to octahedral
(average twist angle φ = 33°) than to trigonal prismatic.[4]
Surprisingly, the [WIV(bdt)3]2– complex exhibits a geometry
close to perfect trigonal prismatic (average twist angle φ =
3.5°).[5] In the homologous series of molybdenum com-
plexes [MoV(bdt)3]– assumes a geometry analogous to the
WV complex, between octahedral and trigonal prismatic
(average twist angle φ = 33.5°),[6] while the complex [MoVI-
(bdt)3] is, as expected, trigonal prismatic (average twist an-
gle φ = 0°).[7] The molecular structure of [MoIV(bdt)3]2– has
not been reported. The pseudooctahedral geometry of the
d0 complex [ZrIV(bdt)3]2– is rather unexpected[8] when com-
pared to the trigonal prismatic[9] d0 complex [NbV(bdt)3]–.
It appears that the stepwise reduction of the formal d0 cen-
ter in [MVI(bdt)3] (M = Mo, W) complexes does not gen-
[a] Institut für Anorganische und Analytische Chemie, Westfälis-
che Wilhelms–Universität Münster,
Results and Discussion
Wilhelm Klemm Strasse 8, 48149 Münster, Germany
Fax: +49-251-833-3108
Transition metal tris(benzene-1,2-dithiolato) complexes
have been synthesized by the reaction of the hexahalides
E-mail: fehahn@uni-muenster.de
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DOI: 10.1002/ejic.200500086
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Schulze Isfort, T. Pape, F. E. Hahn
FULL PAPER
MX6 with an excess of benzene-1,2-dithiol (H2bdt) in CCl4, solution of PNPCl yielded green, air-stable crystals of
a reaction that liberates gaseous HCl.[2d] Later Sellmann et (PNP)[MoV(bdt)3] (Scheme 1).
al. showed that benzene-1,2-dithiol that is deprotonated
with nBuLi (Li2bdt) reacts with [MoCl4(THF)2] to give
(Li)2[MoIV(bdt)3], while the reaction of benzene-1,2-dithiol
with [MoCl4(THF)2] gives [MoVI(bdt)3].[6b] Complex
[MoVI(bdt)3] can be reduced to the complexes [MoV-
(bdt)3]– and [MoIV(bdt)3]2–with nBuLi.
We prepared (Li)2[MoIV(bdt)3] by the deprotonation of
H2bdt with nBuLi at –78 °C in THF followed by the the
dropwise addition of a DMF solution of [MoCl4(CH3CN)2]
at –78 °C (Scheme 1). Below –40 °C the reaction mixture
remained unchanged and colorless. Warming to above
–40 °C led to a color change in the reaction mixture to deep
Scheme 1. Preparation of (Li)2[MoIV(bdt)3] and (Li)[MoV(bdt)3].
blue, which is typical for solutions containing the
[MoIV(bdt)3]2– anion.[6b] The solution was warmed to room
The 1H NMR spectrum of complex (Li)2[MoIV(bdt)3] ex-
temperature, stirred at this temperature overnight, and then hibits two multiplets at δ = 7.37 ppm and δ = 6.57 ppm for
heated under reflux for 1 h. Standard workup procedures the benzene protons, and the coupling pattern corresponds
gave (Li)2[MoIV(bdt)3] in 76% yield. Blue, air-sensitive crys- to a symmetrical AAЈXXЈ spin system (Figure 1). Two reso-
tals of (PNP)2[MoIV(bdt)3]·6MeOH suitable for X-ray nances for the benzene carbon atoms were observed in the
analysis were obtained by addition of a methanolic solution 13C NMR spectrum at δ = 119.8 ppm and δ = 126.5 ppm
of PNPCl to a methanolic solution of (Li)2[MoIV(bdt)3]. in addition to the signal for the ipso-carbon atom at δ =
Aerial oxidation of (Li)2[MoIV(bdt)3] in methanol gave a 155.1 ppm. The difference in the line widths of these signals
green solution of (Li)[MoV(bdt)3]. Addition of a methanolic appears to be an artifact of the data acquisition method.
1
Figure 1. 13C NMR (top) and H NMR spectra (bottom) of (Li)2[MoIV(bdt)3] (* = solvent).
Figure 2. Left: molecular structure of the [MoIV(bdt)3]2– anion with the crystallographic numbering scheme. Hydrogen atoms have been
omitted. The anion resides on a twofold axis which bisects the C9–C9* bond (symmetry code *: 1 – x, y, 1/2 – z). Selected bond lengths
[Å] and angles [°]: Mo–S1 2.3952(14), Mo–S2 2.3981(12), Mo–S3 2.3940(12), S1–C1 1.765(3), S2–C2 1.761(3), S3–C7 1.750(4); S1–Mo–
S2 81.06(4), S1–Mo–S3 115.68(4), S1–Mo–S1* 152.33(5), S1–Mo–S2* 84.93(5), S1–Mo–S3* 85.96(4), S2–Mo–S3 83.22(5), S2–Mo–S1*
84.93(5), S2–Mo–S2* 118.70(5), S2–Mo–S3* 153.06(3), S3–Mo–S3* 81.31(6). Right: drawing of the MoS6 core showing the twist angle
φ between the S3 planes.
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Synthesis and X-ray Molecular Structure of (PNP)2[MoIV(C6H4S2-1,2)3]
FULL PAPER
1
The H NMR resonances correspond well to the values re-
In both the PNP+ and the nBu4N+ salts, the structure
ported for (NEt4)2[W(bdt)3].[5] The molecular structure of of the anion [Mo(bdt)3]– is an unsymmetrical polyhedron,
(PNP)2[Mo(bdt)3]·6MeOH was determined by X-ray dif- exhibiting a geometry between trigonal prismatic and octa-
fraction (Figure 2).
hedral (average twist angle φ = 34.1° for the PNP+ salt and
The overall solid state molecular structure of the com- φ = 33.5° for the nBu4N+ salt). However, the [MoV(bdt)3]–
plex anion [MoIV(bdt)3]2– differs from that of the heavier anions in the two salts show a considerable difference re-
homologue [WIV(bdt)3]2–. The molybdenum(iv) ion in garding the orientation of the S3 planes that define the twist
[Mo(bdt)3]2– is coordinated by six sulfur atoms from three angle φ (Figure 2). While the anion in the nBu4N+ salt exhi-
bdt2– ligands in a strongly distorted trigonal prismatic fash- bits three rather similar angles (30.4, 34.4, 35.6°), which in-
ion. The average twist angle φ[13] (see Figure 2, right) is dicates an almost coplanar arrangement of the S3 planes,
24.8° for [Mo(bdt)3]2– compared to φ = 3.5° for [W- three significantly different values were observed for
(bdt)3]2–.[5] The three different twist angles observed for (PNP)[Mo(bdt)3] (18.4, 36.3, 47.8°). The S1/S3/S5 plane in
[Mo(bdt)3]2– result from the noncoplanar arrangement of this complex is tilted by 7.2° with respect to the S2/S4/S6
the two S3 planes. The bending of the S–C–C–S plane away plane. Crystal packing effects might be responsible for this
from the S–Mo–S–plane is small in [MoIV(bdt)3]2– (about observation, which again illustrates that electronic effects
0.8°) while large bending angles were observed for- are not solely responsible for the molecular structure of
[MoVI(bdt)3] (13.1, 21.1, 30.0°)[7] and [WVI(bdt)3] (13.5, [M(bdt)3]n– complexes.
22.6, 32.1°).[3] On the basis of Fenske–Hall MO calculations
The “innocent” and “non-innocent” nature of ligands
it has been suggested that the bending of the ligand can be like bdt2– has recently been discussed intensively.[12] For
viewed as a second-order Jahn–Teller distortion, which is O,OЈ-coordinated catecholates or o-benzosemiquinonate-
most favored for d0 metal centers in a trigonal prismatic (1–)-π-radicals a number of typical structural and spectro-
coordination environment.[14] While the electronic factors scopic features have been described, which allow the detec-
provide the principal driving force, the bulk of the dithiolate tion and identification of such ligands in coordination com-
ligand also plays a role in determining the molecular struc- pounds.[15] The picture was not so clear for the correspond-
tures. This becomes obvious when the three different bend- ing o-dithio derivatives like bdt2–. Recently square-planar
ing angles for [MVI(bdt)3] (M = Mo, W) complexes are con- complexes of the type [M(bdt)2]n– (M = AuIII, NiII)[12] have
sidered.
been described, and structural parameters associated with
For a comparison of the bond parameters in [MoIV- dithiolato(2–) and o-dithiobenzosemiquinonato(1–) coordi-
(bdt)]2– and [MoV(bdt)]–, which have been obtained under nation have been worked out. This allows, in principle, the
similar conditions (153–173 K) and with identical counter- assignment of the oxidation level of a benzene-1,2-dithiol-
ions, we determined the molecular structure of ato ligand in a complex. For complexes of the type [M-
(PNP)[MoV(bdt)3] (Figure 3). The molecular structure of (bdt)2]n– this assignment becomes complicated if a benzene-
(nBu4N)[MoV(bdt)3] has been determined previously at 1,2-dithiolate and an o-dithiobenzosemiquinonate ligand
room temperature.[6a]
are coordinated at the same time, as the structural differ-
ences between the two ligand types are small. Statistical dis-
order problems may hamper the identification of the dif-
ferent oxidation levels of the ligand and this becomes even
more complicated for complexes of the type [M(bdt)3]n–
with three bdt2– ligands, each of which can undergo intra-
molecular oxidation.
We have studied the structural parameters in the series
[Mo(bdt)3]n– (n = 0, 1, 2, Figure 4) and found some trends
in the Mo–S, S–C and C–C bond lengths. On the basis of
the geometric parameters of the o-dithiolato ligands in
[MoVI(bdt)3] and the paramagnetism observed for the com-
plex, Bennet et al.[7] proposed, as early as 1976, the presence
of a dithioketonic ligand species in [MoVI(bdt)3]. On the
basis of Wieghardt’s investigations,[12] we believe that the
ligand in [MoVI(bdt)3] is best described as an o-dithioben-
zosemiquinonate(1–) radical anion.
Figure 3. Molecular structure of [MoV(bdt)3]– with the crystallo-
graphic numbering scheme. Hydrogen atoms have been omitted.
Selected bonds lengths [Å] and angles [°]: Mo–S1 2.3880(8), Mo–
S2 2.4020(8), Mo–S3 2.3702(7), Mo–S4 2.3778(8), Mo–S5
2.3999(8), Mo–S6 2.3785(7); S1–Mo–S2 81.81(3), S1–Mo–S3
95.42(3), S1–Mo–S4 160.05(3), S1–Mo–S5 82.89(3), S1–Mo–S6
102.93(3), S2–Mo–S3 107.72(3), S2–Mo–S4 79.91(3), S2–Mo–S5
162.41(3), S2–Mo–S6 92.28(2), S3–Mo–S4 82.64(3), S3–Mo–S5
82.31(3), S3–Mo–S6 154.62(3), S4–Mo–S5 116.31(3), S4–Mo–S6
85.78(3), S5–Mo–S6 82.73(2).
In the series of [Mo(bdt)3]n– complexes, the C–S bond
lengths increase as the metal oxidation state changes from
MoVI to MoIV (Figure 4). MoVI is capable of oxidizing the
o-dithiolato ligand to a higher degree than MoIV, leading to
a thioketonic ligand which is detectable by the decrease in
the C–S bond lengths. This postulate is corroborated by an
evaluation of the C–C bond lengths within the aromatic
ring. In summary, the trends in the structural parameters
Eur. J. Inorg. Chem. 2005, 2607–2611
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Schulze Isfort, T. Pape, F. E. Hahn
FULL PAPER
= 2.4 Hz, 6 H, Ar–H). 13C NMR (50 MHz, [D7]DMF): δ = 155.1
(ipso-C), 126.5 (Ar–C), 119.8 (Ar–C).
(PNP)2[MoIV(bdt)3]·6MeOH: Slow diffusion of a methanolic solu-
tion of PNPCl (PNPCl = bis(triphenylphosphoranylidene)ammo-
nium chloride) into a solution of (Li)2[MoIV(bdt)3] in methanol led
to the precipitation of deep blue, air-sensitive single crystals of
(PNP)2[MoIV(bdt)3]·6MeOH which were suitable for an X-ray dif-
fraction study. These crystals were characterized by X-ray diffrac-
tion only.
(PNP)[MoV(bdt)3]: Complex (Li)2[MoIV(bdt)3] dissolved in meth-
anol was oxidized by exposure to air. A green solution of Li-
[MoV(bdt)3] resulted. Green crystals of (PNP)[MoV(bdt)3] were ob-
tained by the slow addition of a methanolic solution of PNPCl.
X-ray Crystallographic Study of (PNP)2[MoIV(bdt)3]·6MeOH: A
suitable crystal (0.20×0.10×0.07 mm3) was mounted on a Bruker
AXS APEX diffractometer equipped with a rotating molybdenum
anode (λ = 0.71073 Å), a cooling device, and a graphite monochro-
mator. (PNP)2[Mo(bdt)3]·6MeOH, C96H96N2MoO6P4S6, Mr
=
1785.93 gmol-1, monoclinic, C2/c, a = 22.901(14), b = 16.245(8), c
= 25.800(13) Å, β = 113.385(14)°, V = 8810(8) Å3, Z = 4, ρcalcd.
=
Figure 4. Average bond lengths [Å] for complexe anions of the type
1.346 gcm–3, μ = 0.419 mm–1. 34636 structure factors (–27 Յ h Յ
27, –19 Յ k Յ 19, –30 Յ l Յ 30, 2θ-range 3.2–50.0°) were collected
[Mo(bdt)3]n– (n = 2, 1, 0) including literature data for [MoVI
-
(bdt)3].[7]
at –100 °C. An empirical absorption correction applied (Tmin
=
0.921, Tmax = 0.971) before merging the data gave 7754 unique
intensities (Rint = 0.058). Structure solution was performed with
SHELXS,[16] and the subsequent refinement of 522 parameters
against F2 with 7754 unique intensities [6289 observed intensities I
Ͼ 2σ(I)] was conducted with SHELXL.[17] The anisotropic thermal
parameters for all non-hydrogen atoms were refined. Hydrogen
atoms were added to the structure model at calculated positions.
Final residuals: Rall = 0.0600, Robs = 0.0460, Rw,obs = 0.1182, GOF
= 1.106. The molybdenum atom resides on a special position, na-
mely, a twofold axis which bisects one of the benzene-1,2-dithiolato
indicate that the bdt2– ligand is at least partially oxidized
upon coordination to transition metals in high oxidation
states (MoVI).
From the variation of the twist angle φ in the series
[Mo(bdt)3]n– (n = 0, 1, 2) it appears that the formal d elec-
tron configuration is not the only governing principle for
the formation of certain coordination polyhedra. These re-
sults indicate once more that the stabilization of a trigonal
prismatic coordination geometry does not only depend on ligands. The asymmetric unit contains one half [MoIV(bdt)3]2–
the electronic situation at the central metal atom, but in-
anion, one PNP+ cation, and three molecules of MeOH.
stead must be considered a complicated process involving
various factors, including the counterions that are present.
X-ray Crystallographic Study of (PNP)[2]: A suitable crystal
(0.48×0.08×0.02 mm3) was mounted on a Bruker AXS APEX dif-
fractometer equipped with a rotating molybdenum anode (λ =
0.71073 Å), a cooling device, and a graphite monochromator.
(PNP)[Mo(bdt)3], C54H42MoNP2S6, Mr = 1055.13 gmol–1, triclinic,
Experimental Section
¯
P1, a = 11.036(1), b = 13.882(2), c = 16.041(2) Å, α = 87.969(3),
General Remarks: Unless stated otherwise, all manipulations were
carried out under dry argon by means of standard Schlenk tech-
niques. THF was freshly distilled from Na/benzophenone prior to
use. Methanol was dried over magnesium and freshly distilled. The
DMF purchased contained molecular sieves and was used as re-
ceived. [MoCl4(CH3CN)2] was purchased from Aldrich.
β = 75.861(3), γ = 87.512(3)°, V = 2380.0(5) Å3, Z = 2, ρcalcd.
=
1.472 gcm–3, μ = 0.645 mm–1. 19324 structure factors (–13 Յ h Յ
13, –16 Յ k Յ 16, –19 Յ l Յ 19, 2θ-range 3.0–50.0°) were collected
at –120 °C. An empirical absorption correction applied (Tmin
=
0.747, Tmax = 0.987) before merging the data gave 8382 unique
intensities (Rint = 0.035). Structure solution was performed with
SHELXS,[16] and subsequent refinement of 577 parameters against
F2 with 8382 unique intensities [6757 observed intensities I Ͼ 2σ(I)]
was conducted with SHELXL.[17] The anisotropic thermal parame-
ters for all non-hydrogen atoms were refined. Hydrogen atoms were
added to the structure model at calculated positions. Final residu-
als: Rall = 0.0476, Robs = 0.0335, Rw,obs = 0.0757, GOF = 1.007.
The asymmetric unit contains one molecule of (PNP)[Mo(bdt)3].
(Li)2[MoIV(bdt)3]:
A sample of benzene-1,2-dithiol (423 mg,
3.0 mmol) was dissolved in THF (20 mL). The solution was cooled
to –78 °C and nBuLi (6.0 mmol, 2.4 mL of a 2.5 m solution in hex-
ane) was added dropwise. [MoCl4(CH3CN)2] (320 mg, 1 mmol) in
DMF (5 mL) was then added, and the reaction mixture was al-
lowed to warm slowly to ambient temperature. While warming, the
color of the solution turned deep blue. The reaction mixture was
stirred overnight and then heated under reflux for 1 h. Thereafter
all solvents were removed and the crude reaction product was
washed with diethyl ether and dried in vacuo. Yield: 403 mg
(0.76 mmol, 76%). C18H12Li2MoS6 (530.48): calcd. C 40.76, H
CCDC-260986 (for (PNP)2[MoIV(bdt)3]·6MeOH) and CCDC-
260987 (for (PNP)[MoV(bdt)3]) contain the supplementary crystal-
lographic 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.
1
2.28; found C 41.43, H 2.40. H NMR (200 MHz, [D7]DMF): δ =
3
4
3
4
7.37 (dd, J = 8.8, J = 2.4 Hz, 6 H, Ar–H), 6.57 (dd, J = 8.8, J
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Synthesis and X-ray Molecular Structure of (PNP)2[MoIV(C6H4S2-1,2)3]
FULL PAPER
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Acknowledgments
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (IGK, 673).
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Received: January 26, 2005
Published Online: May 24, 2005
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