Molybdenum 17- and 18-electron bis- and tris(Butadiene) complexes: Electronic structures, spectroscopic properties, and oxidative ligand substitution reactions

Article abstract of DOI:10.1021/ic400145f

New results on the electronic structures, spectroscopic properties, and reactivities of the molybdenum tris(butadiene) and tris(2,3-dimethylbutadiene) complexes [Mo(bd)₃] (1^bd) and [Mo(dmbd)₃] (1^dmbd), respectively, are reported. Importantly, the metal ligand bonding interaction can be weakened by oxidizing the metal center with ferrocenium salts. The addition of the bidentate phosphine ligand 1,2-bis(diphenylphosphino)ethane then leads to a new type of stable 17-electron complex, [Mo(dmbd)₂(dppe)](X) (2; X = BF₄^-, PF₆^-, BPh₄^-), where one of the butadiene ligands is exchanged by a chelating phosphine. Reduction of the cationic complexes 2 generates the corresponding 18-electron complex [Mo(dmbd)₂(dppe)] (3), thus establishing a new strategy for ligand substitution reactions in [Mo(bd)₃] complexes via one-electron oxidized intermediates. The new heteroleptic molybdenum complexes are characterized by X-ray structure analysis; vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT calculations are performed to explain the structural and specroscopic trends observed experimentally. For compound 1^bd, a normal coordinate analysis is presented, providing additional information on the bonding situation in this type of complex.

Full text of DOI:10.1021/ic400145f

Article
pubs.acs.org/IC
Molybdenum 17- and 18-Electron Bis- and Tris(Butadiene)
Complexes: Electronic Structures, Spectroscopic Properties, and
Oxidative Ligand Substitution Reactions
Gerald C. Stephan, Christian Nather, Gerhard Peters, and Felix Tuczek*
̈
Institut fur Anorganische Chemie, Christian Albrechts Universitat zu Kiel, Max-Eyth-Straße 2, D-24098 Kiel, Germany
̈
̈
S
* Supporting Information
ABSTRACT: New results on the electronic structures,
spectroscopic properties, and reactivities of the molybdenum
tris(butadiene) and tris(2,3-dimethylbutadiene) complexes
[Mo(bd)3] (1bd) and [Mo(dmbd)3] (1dmbd), respectively, are
reported. Importantly, the metal ligand bonding interaction
can be weakened by oxidizing the metal center with
ferrocenium salts. The addition of the bidentate phosphine
ligand 1,2-bis(diphenylphosphino)ethane then leads to a new
type of stable 17-electron complex, [Mo(dmbd)2(dppe)](X)
−
−
−
(2; X = BF4 , PF6 , BPh4 ), where one of the butadiene
ligands is exchanged by a chelating phosphine. Reduction of
the cationic complexes 2 generates the corresponding 18-
electron complex [Mo(dmbd)2(dppe)] (3), thus establishing a
new strategy for ligand substitution reactions in [Mo(bd)3] complexes via one-electron oxidized intermediates. The new
heteroleptic molybdenum complexes are characterized by X-ray structure analysis; vibrational, NMR, and EPR spectroscopy; and
electrochemistry. DFT calculations are performed to explain the structural and specroscopic trends observed experimentally. For
compound 1bd, a normal coordinate analysis is presented, providing additional information on the bonding situation in this type
of complex.
which are routinely used for other molybdenum complexes
have been established for the butadiene compounds to date.
The reason for these findings is the special bonding situation
in [Mo(bd)3]: due to backbonding from the molybdenum
center, an equalization of the C−C bond lengths occurs in the
butadiene ligands. This delocalized bonding mode causes a high
stability of the molybdenum butadiene complexes and renders
these systems inert toward exchange reactions with electron-
donating ligands like phosphines under thermal reaction
conditions. Kaupp et al. have analyzed this bonding situation
employing X-ray analysis and theoretical calculations,2 provid-
ing theoretical evidence for a significant transfer of charge from
the (formally zerovalent) molybdenum center to the (formally
neutral) butadiene ligands. This way, resonance structures with
a Mo(IV) center and two dinegative butadiene ligands were
found to provide the best description of the actual charge
distribution.
I. INTRODUCTION
Due to their unusual trigonal-prismatic structure, molybdenum
complexes with a tris(1,3-butadiene) coordination are interest-
ing compounds in the field of group 6 transition metal
chemistry.1,2 In comparison to other homoleptic molybdenum
compounds like [Mo(CO)6], [Mo(arene)2], or [Mo(PMe3)6],
the reactivity of these systems is less explored. [Mo(CO)6], e.g.,
undergoes various substitution reactions with nitriles, phos-
phines, arenes, and vinylketones.3−5 Depending on the reaction
conditions, up to four CO ligands can be substituted. Similar
reactivities can be observed for bis(arene) compounds leading
to different types of half-sandwich complexes.6 Besides ligand
exchange reactions of [Mo(CO)6] or [Mo(arene)2] complexes
under thermal or photochemical reaction conditions, also
exchange reactions under oxidizing conditions are known.7,8
For the highly reactive molybdenum complex [Mo(PMe3)]6, a
wide range of exchange reactions especially with heterocycles
has been described and analyzed in detail.9−11
The few reactions known for molybdenum and tungsten
tris(1,3-butadiene) complexes have been described by Wilke et
al. It was found that [Mo(bd)3] and [W(bd)3] undergo
substitution reactions of the butadiene (bd) ligands against
COT and CO accompanied by a trimerization of butadiene to
cyclododecatriene.12 Furthermore, the compounds were tested
as catalysts for polymerization and metathesis reactions.13
Nevertheless, none of the well-defined exchange reactions
In this paper, we describe how vibrational spectroscopy can
be used to obtain further insight into the electronic structure of
this class of compounds. Furthermore, we present our first
results on the reactivities of the homoleptic complexes
tris(butadiene)molybdenum [Mo(bd)3] (1bd) and tris(2,3-
dimethylbutadiene)molybdenum [Mo(dmbd)3] (1dmbd). Im-
Received: January 18, 2013
Published: April 29, 2013
© 2013 American Chemical Society
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Table 1. Selected Crystallographic Data and Results of the Structure Refinement for [Mo(dmbd)2(dppe)](BPh4) (2BPh4),
[Mo(dmbd)2(dppe)] (3), and [MoF2(dppe)2](BF4) (4)
compound
[Mo(dmbd)2(dppe)]BPh4 (2BPh4
)
[Mo(dmbd)2(dppe)] (3)
[MoF2(dppe)2](BF4) (4)
empirical formula
fw g mol−1
temp/K
cryst syst
space group
unit cell dimensions
a/Å
C62H64BMoP2
977.82
C38H44MoP2
658.61
C52H48BF6MoP4
1017.53
170(2)
170(2)
170(2)
triclinic
monoclinic
P21/n
monoclinic
Cc
P1
̅
11.856(1)
13.758(1)
16.281(2)
89.96(1)
85.09(11)
71.29(1)
2505.1(3)
2
9.711(1)
12.454(1)
26.280(2)
9.867(1)
22.528(2)
21.175(2)
b/Å
c/Å
α/deg
β/deg
91.70(1)
92.68(1)
γ/deg
vol/Å3
3177.0(3)
4
4701.5(5)
4
Z
D
calcd/Mg/m3
1.296
1.377
1.438
μ/mm−1
0.366
0.539
0.475
θ range/deg
index ranges
2.28−26.02
−14 ≤ h ≤14
−16 ≤ k ≤16
−20 ≤ l ≤20
24750
2.22−28.05
−12 ≤ h ≤12
−16 ≤ k ≤16
−34 ≤ l ≤34
39264
2.41−28.06
−12 ≤ h ≤13
−29 ≤ k ≤29
−27 ≤ l ≤27
23978
reflns collected
independent reflns
Rint
9632
7658
11122
0.0557
0.0399
0.0370
reflns with [I > 2σ(I)]
params
GoF on F2
7840
6640
10612
600
372
578
1.054
1.033
1.029
R1 [I > 2σ(I)]
wR2 (all data)
δF (e/Å3)
0.0494
0.0268
0.0324
0.1381
0.0704
0.0817
2.126/−0.878
0.384/−0.559
0.457/−0.554
portantly, we find that the metal ligand bonding interaction can
be weakened by oxidizing the metal center with a ferrocenium
salt. The addition of the bidentate phosphine ligand 1,2-
bis(diphenylphosphino)ethane then leads to the formation of a
new type of stable cationic complex, [Mo(dmbd)2(dppe)](X)
study are characterized by X-ray structure analysis, spectrosco-
py, and electrochemical methods. Furthermore, the electronic
structure of complexes 2 and 3 is determined and compared to
the electronic structure of 1bd analyzed before.2
−
−
−
II. EXPERIMENTAL SECTION
(2; X = BF4 , PF6 , BPh4 ), where one of the butadiene ligands
is exchanged by the chelating phosphine. In contrast to other
mononuclear molybdenum 17-electron compounds which are
very sensitive, the molybdenum complexes with butadiene−
phosphine coordination exhibit a high stability and can be
stored under air as a solid or in solution over months without
any signs of decomposition. As a side product of the reaction,
the butadiene-free molybdenum complex [MoF2(dppe)2](BF4)
(4) was isolated. Reduction of the 17-electron complexes 2
leads to the corresponding neutral compound [Mo-
(dmbd)2(dppe)] (3), thus establishing a new strategy for
ligand substitution reactions in [Mo(bd)3] complexes via one-
electron oxidized intermediates. A comparable molybdenum
complex, [Mo(bd)2(PMe3)2], has already be synthesized by
Brookhart et al., employing an exchange reaction based on the
highly reactive complex [Mo(PMe3)6].14 Moreover, two related
monophosphine (PEt3, PMe2Ph) bis(1,3-butadiene) complexes
have been synthesized by Galindo et al. starting from
molybdenum(0) alkene or dinitrogen complexes.15 An
interesting aspect of the PEt3 containing complex [Mo-
(bd)2(PEt3)2] was the fact that it could be synthesized based
on a dimerization of ethylene.
General Methods. All reactions were performed under an inert
gas atmosphere using Schlenk techniques. The solvents were dried and
freshly distilled under inert gas. All other reagents were used without
further purification. [Mo(bd)3] (1bd) and [Mo(dmbd)3] (1dmbd) were
prepared using literature procedures.26 IR spectra were obtained from
KBr pellets using a Bruker IFS v66/S FT-IR spectrometer. Raman
spectra were obtained on a Bruker IFS 66/CS NIR FT-Raman
spectrometer. The setup involves a 350 mW Nd:YAG laser with an
excitation wavelength of 1064 nm. NMR spectra were recorded on a
Bruker Avance 400 pulse Fourier transform spectrometer operating at
a 1H frequency of 400.13 MHz (31P 161.98 MHz, 13C 106.62) using a
5 mm inverse triple-resonance probe head. References as substitutive
standards: H3PO4 85% pure, δ(31P) 0 ppm. Elemental analyses were
performed with a Euro Vector Euro EA 3000. The magnetic properties
were determined with a Bruker B-SU 20 Faraday balance containing a
Sartorius 4411 balance at 15 kGs.
The electrochemical measurements (cyclovoltammetry, difference-
pulse voltammetry) were performed with an EG&G PAR Model 273 A
potentiostat using a platinum-knob working electrode, a silver-rod
counterelectrode, and a platinum rod auxiliary electrode. The setup
was controlled by the EG&G PAR M270 software. For referencing,
ferrocene was added as an internal standard.
X-Ray Structure Analysis. Intensity data were collected using a
STOE imaging plate diffraction system (IPDS-1) with Mo Kα
radiation (λ = 0.71073 Å). All structures were solved with direct
methods using SHELXS-97, and refinement was performed against F2
−
The new complexes [Mo(dmbd)2(dppe)](X) (2; X = BF4 ,
−
−
PF6 , BPh4 ) and [Mo(dmbd)2(dppe)] (3) described in this
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using SHELXL-97. All non-hydrogen atoms were refined anisotropi-
cally. The hydrogen atoms were positioned with idealized geometries
(methyl H atoms allowed to rotate but not to tip) and were refined
using a riding model. The absolute structure of compound 4 was
determined and is in agreement with the selected setting (Flack x-
parameter: −0.024(18)). The crystal of compound 2BPh4 was slightly
nonmerohedrically twinned, but both individuals could not be
separated successfully. Selected crystal data and results of the structure
refinements are summarized in Table 1.
19.40/19.21 (−CH3). 31P{H} NMR (δ,thf-d8, 213 K, 213 K): 77.01 (2
P, dppe).
Isolation and Characterization of [MoF2(dppe)2](BF4) (4). If
fluoride-containing ferrocenium salts, especially FcBF4, were employed
in the oxidation of 1dmbd, a small amount of the molybdenum(III)
complex [MoF2(dppe)2](BF4) (4) could be isolated. Single crystals of
4 were characterized by X-ray structure analysis. This compound has
already been obtained as one of the reaction products of
[MoH4(dppe)2] with HBF4.24
CCDC-934953 (2BPh4), CCDC-934954 (3), and CCDC-934955
(4) contain the supplementary crystallographic data for this paper,
which can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.uk/data_request.cif.
Density Functional Calculations. DFT calculations were
performed for the complexes [Mo(bd)3] (1), [Mo(dmbd)2(dppe)]+
(2), and [Mo(dmbd)2(dppe)] (3) using Becke’s three parameter
hybrid functional with the correlation functional of Lee, Young, and
Parr (B3LYP).16 The LANL2DZ basis set was used for the
calculations. It applies Dunning/Huzinaga full double-ζ basis functions
on the first row and Los Alamos effective core potentials plus DZ
functions on all other atoms.17,18 All computational procedures were
used as implemented in the Gaussian 03 package.19 Wave functions
were plotted with GaussView03.20
III. RESULTS AND ANALYSIS
A. Vibrational Spectra and Assigments of [Mo(bd)3]
(1bd). Infrared and Raman spectra of molybdenum butadiene
complexes allow one to assess the degree of backbonding from
the metal to the ligands. In order to analyze these data, the
vibrational spectra of 1bd have been investigated in detail with
the help of DFT. The optimized structure of 1bd is shown in
Figure 1. The MIR regions of the measured and calculated IR
Normal Coordinate Analysis. Normal coordinate calculations
were performed using the QCPE computer program by Peterson and
McIntosh.21 It involves solutions of the secular equation GFL = ΛL by
the diagonalization procedure of Miyazawa. The calculations are based
on a general valence force field, and the force constants are refined
using the nonlinear optimization of the simplex algorithm according to
Nelder and Mead.22 The f-matrix that derives from the DFT
calculation has its force constants in Cartesian coordinates. These
Cartesian coordinates were transformed to internal coordinates using
the program REDONG.23
−
−
Preparation o−f [Mo(dmbd)(dppe)](X) (X= PF6 (2PF6), BF4
(2BF4), and BPh4 (2BPh4)). 2PF6: 10 mL of THF was added to a
mixture of 185 mg (0.54 mmol) of [Mo(dmbd)3] (1dmbd), 185 mg
(0.56 mmol) of FcPF6, and 216 mg (0.54 mmol) of dppe. The
suspension turned green after a few minutes and was stirred for 1 h.
The addition of 10 mL of n-hexane to the mixture causes a complete
precipitation of the product. The green solid was filtered from the
yellow-orange solution, washed with 2 × 5 mL n-hexane, and dried
under vacuum conditions. Yield: 350 mg (0.44 mmol, 87%). Anal.
Calcd for C38H44P3F6Mo: C, 56.81%; H, 5.48%. Found: C, 57.10%; H,
4.49%.
2
BF4: The compound has been prepared in analogy to compound
2PF6 (based on 0.5 mmol of [Mo(dmbd)3]). Yield: 270 mg (0.36
mmol, 72%). Anal. Calcd for C38H44P2F4BMo: C, 61.23%; H, 5.90%.
Found: C, 60.93%; H, 5.91%.
Figure 1. Optimized structure of [Mo(bd)3] (1bd).
2
BPh4: The compound has been prepared in analogy to compound
2PF6 (based on 0.5 mmol of [Mo(dmbd)3]). Yield: 200 mg (0.21
mmol, 42%). Anal. Calcd for C62H64P2BMo: C, 76.15%; H, 6.55%.
Found: C, 76.41%; H, 6.53%.
spectra are shown in Figure 2; corresponding Raman spectra
are given in Figure 3. In general, the calculated spectrum is
shifted to somewhat higher energy compared to the experi-
ment; the average deviation in vibrational energies is about 2 to
3%. Nevertheless, the relative energies and intensities of the
calculated and measured IR and Raman spectra match quite
well and enable an unequivocal assignment of the vibrational
modes.
Preparation of [Mo(dmbd)(dppe)] (3). A suspension of 400 mg
(0.5 mmol) of [Mo(dmbd)2(dppe)](PF6) (2PF6) in 15 mL of THF
was added to sodium amalgam (150 mg Na in 20 g Hg). The mixture
was stirred overnight. The brown solution was decanteted, filtred, and
reduced under vacuum conditions to 3 mL. After the addition of 8 mL
of methanol, the solution was reduced again, and another 8 mL of
methanol was added. The orange precipitate was filtered and dried
under vacuum conditions. Yield: 180 mg crude product (0.27 mmol,
55%). Recrystallization by layering a benzene solution with n-hexane
leads to orange crystals. Anal. Calcd for C38H44P2Mo: C, 69.30%; H,
6.68%. Found: C, 69.51%; H, 6.80%.
The tris(butadiene) complex 1bd exhibits 3N − 6 = 87
normal modes. Selected eigenvectors are shown in Figure S1,
Supporting Information. In the C3h point group of the complex,
the modes transform according to the A′, A″, E′, or E″
irreducible representations. The complex has 18 C−H
stretching modes (3 × A′, 3 × A″, 3 × E′, and 3 × E″, 87−
70 in Table 2). The most intense bands in the Raman spectrum
at 3054 cm−1 and 2976 cm−1 can be assigned to the
combination of the antisymmetric CH2 stretch with the
symmetric CHCH stretching mode (85, A′, calcd. 3240
cm−1) and the antisymmetric CH2 stretching mode (70, A′,
1H NMR (δ, thf-d8, 213 K): 7.90−7.06 (m, 20H, Ph), 2.59 (broad
m, 2H, CH2-dppe), 2.60 (broad m, 2H, CH2-dppe), 1.73 (s, 6H,
−CH3), 1.64 (s, 6H, −CH3), 0.58 (s, 2H, syn-CH2, H2), 0.40 (d, J(P−
H) = 21.24 Hz, 2H, syn-CH2, H4), −1.26 (d, J(H−H) = 6.28 Hz, 2 H,
anti-CH2, H1), −2.64 (dd, J(H−H) = 6.28 Hz, J(P−H) = 10.85 Hz,
2H, anti-CH2, H3). 13C{1H} NMR(δ, thf-d8, 213 K): 135−126 (Ph),
91.99 (Cc), 83.64 (Cb), 51.25 (Cd), 36.64 (Ca), 28.86 (dppe-CH2),
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Table 2. Selected Frequencies of [Mo(bd)3] (1bd)
calculated experimental
frequency/
frequency/
symmetry
in C3h
mode
no.
cm−1
cm−1
ν(CC)central + δ(CH2) + 69
δ(CHCH) +
1531
1530
1488 (RA)
1488 (IR)
1456 (RA)
A′
E′
ν(CC)sym,terminal
ν(CC)central + δ(CH2) + 68/67
δ(CHCH) +
ν(CC)sym,terminal
ν(CC)asym,terminal
+
66/65
64
1514
1509
1506
1503
1422
1422
1257
1253
1212
1198
1088
1086
1080
1078
990
E″
A″
E′
δ(CH2) + ρr(CHCH)
ν(CC)asym,terminal
δ(CH2) + ρr(CHCH)
+
ν(CC)central + δ(CH2) + 63/62
δ(CHCH)
1439 (RA),
1435 (IR)
ν(CC)central + δ(CH2) + 61
δ(CHCH)
A′
A″
E″
A′
E′
δ(CH2) + ρr(CHCH) + 60
δ(CC)asym,terminal
1374 (IR)
1367 (RA)
1219 (RA)
1213 (IR)
1178 (IR)
1171 (RA)
1067 (RA)
δ(CH2) + ρr(CHCH) + 59/58
ν(CC)asym,terminal
Figure 2. Measured (black) and calculated IR spectra (red) of
[Mo(bd)3] (1bd).
ρr(CHCH) + δ(CH2) + 57
ν(CC)sym,terminal
ρr(CHCH) + δ(CH2) + 56/55
ν(CC)sym,terminal
ρr(CHCH) + δ(CH2) + 54
δ(CCC)asym
A″
E″
A′
E′
ρr(CHCH) + δ(CH2) + 53/52
δ(CCC)asym
δ(CHCH) + δ(CH2) +
δ(CCC)sym
51
δ(CHCH) + δ(CH2) +
δ(CCC)sym
50/49
ρr(CHCH) + δ(CH2) + 48/47
ν(CC)asym,terminal
E″
A″
E″
A″
E′
ρr(CHCH) + δ(CH2) + 46
ν(CC)asym,terminal
1059 (IR)
984 (RA)
ρt(CHCH)sym
ρw(CH2)sym
+
45/44
ρt(CHCH)sym
ρw(CH2)sym
+
43
980
ρw(CH2) + δ(CHCH) + 42/41
972
947 (IR)
943 (RA)
ν(CC)sym,terminal
ν(CC)sym,central
+
ρw(CH2) + δ(CHCH) + 40
964
A′
Figure 3. Measured (black) and calculated Raman spectra (red) of
[Mo(bd)3] (1bd).
ν(CC)sym,terminal
ν(CC)sym,central
+
ρw(CH2)asym
+
39
958
949
948
916
912
888
A′
E′
A″
E″
A′
E′
ρw(CHCH)
calcd. 2976 cm−1), respectively. The IR spectrum in the region
>2800 cm−1 is dominated by two peaks at 3047 cm−1 and 2978
cm−1. The vibration at 3047 cm−1 corresponds to a
combination of an antisymmetric CH2 vibration with an
antisymmetric CHCH vibration (84, A″, calcd. 3234 cm−1),
whereas the band at 2978 cm−1 is due to a degenerate
symmetric vibration of the terminal CH2 groups (72/71, E′,
calcd. 3132 cm−1).
ρw(CH2)asym
ρw(CHCH)
+
38/37
36
910 (IR)
ρw(CH2)asym
ρt(CHCH)
+
ρw(CH2)asym
ρt(CHCH)
+
35/34
33
882 (RA)
ρw(CHCH)sym
ρw(CH2)sym
+
+
ρw(CHCH)sym
ρw(CH2)sym
32/31
872 (IR, RA)
At lower frequencies than the C−H stretching vibrations, the
modes with C−C stretching and C−H deformation contribu-
tions are located. The vibration at highest energy in this region
is observed at 1488 cm−1 in the Raman spectrum (69, calcd.
1531 cm−1; Figure 3). It can be assigned to the A′ vibration
(69) that is dominated by the stretching motion of the central
C−C bond. Additionally, deformations of the CH2 and CH
groups contribute to this vibration. The corresponding
vibration mode in E′ symmetry is associated with a band at
1488 cm−1 in the IR spectrum (68/67, calcd. 1530 cm−1). The
stretching vibrations of the terminal C−C bonds mixed with
C−H deformations in E″ symmetry cause a band at 1456 cm−1
ρt(CH2)sym
ρt(CH2) +
30
29
765
760
760 (IR)
732 (RA)
A″
A′
ρw(CHCH)sym
ρt(CH2) + δ(CCC)
28/27
25/26
751
743
E″
E′
ρt(CH2) + ρw(CHCH)
+ δ(CCC)
732 (IR, RA)
δ(CCC)
δ(CCC)
24
650
630
650 (IR)
626 (RA)
E″
E″
23/22
in the Raman spectrum (66/65, calcd. 1514 cm−1). The
corresponding A″ mode 64 which is calculated at 1509 cm−1
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could neither be identified in the IR nor in the Raman
spectrum. The E′ vibrations 63 and 62 are dominated by the
stretching vibration of the central C−C bond and the
deformation of the terminal CH2 group. They give rise to a
band in the Raman spectrum at 1439 cm−1 and in the IR
spectrum at 1435 cm−1, respectively (calcd. 1506 cm−1). The
calculations show that the corresponding A′ vibration (61,
calcd. 1503 cm−1) has no intensity in the IR and only weak
intensity in the Raman spectrum. Therefore, it could not be
identified in the experimental spectra.
terminal C−C force constants of the bd ligands in the complex
[Mo(bd)3] (1bd) were calculated. To this end, the set of
Cartesian force constants derived from the DFT vibrational
analysis was converted into a set of internal force constants by
the program REDONG.23 A force field was employed which
describes each coordinated bd ligand in terms of a central and a
terminal C−C force constant, a Mo−Cint metal−carbon force
constant, and six C−H force constants (Scheme 1). Around
Scheme 1. Force Field Employed for the Normal Coordinate
a
The bands in the IR spectrum at 1374 cm−1 (calcd. 1422
cm−1) and 1367 cm−1 (calcd. 1422 cm−1) can be assigned the
A″ vibration 60 and the E″ vibrations 59 and 58, respectively.
They represent combinations of deformations of the terminal
CH2 and both central CH groups. The vibrations following to
lower energy are combinations of these deformations as well.
Vibration 57 in A′ symmetry gives rise to an intense band at
1219 cm−1 (calcd. 1257 cm−1). The vibrations in E′ symmetry
56/55 cause a band in the IR spectrum at 1213 cm−1 (calcd.
1253 cm−1). At lower wavenumbers, the combinations of C−C
deformations of the butadiene backbone can be found.
Specifically, the bands at 1178 cm−1 (54, A″, calcd. 1212
cm−1) in the IR and at 1171 cm−1 (53/52, E″, calcd. 1198
cm−1) in the Raman are due to combinations of δ(CCC) and
CH deformations. While these vibrations are antisymmetric
with respect to the mirror plane of the complex, the symmetric
modes cause an intense band in the Raman spectrum at 1067
cm−1 (51, A′, calcd. 1088 cm−1). The calculations show that the
corresponding E′ (50/49, calcd. 1086 cm−1) has low intensity
and can therefore not be identified in the experimental spectra.
The CH out-of-plane vibrations can be found in the region
below 1000 cm−1. The most intense peaks are at 943 cm−1 in
the Raman and at 872 cm−1 in the IR spectrum. The band at
943 cm−1 can be assigned to the A′ mode dominated by a
wagging vibration of the terminal CH2 group (40, calcd. 963
cm−1). The band at 872 cm−1 has E′ symmetry and is caused by
out of plane CH vibrations (32/31, calcd. 888 cm−1). It can
also be identified in the Raman spectrum, giving rise to a weak
peak at 873 cm−1. The calculation shows that the bands in the
lower region are due to further CH out-of-plane vibrations and
CC torsion modes. Vibrations with metal ligand stretching
character, finally, can be found below 400 cm−1.
Analysis of [Mo(bd)3] (1bd)
a
For clarity, H atoms are omitted on two of the three bd ligands.
each central and each terminal C atom of a coordinated bd
ligand, six (respectively three) bending force constants are
defined. Each terminal C−C bond is associated with a torsion.
In order to describe the relative motions of the three bd ligands
around the Mo(0) center, six Cout−Mo−Cout bends and six
torsions around the Mo−Cout bonds are introduced. The
resulting force field exhibits 18 redundancies.
On the basis of the Cartesian force constants calculated by
DFT by the described procedure are given internal force
constants f of 4.79 mdyn/Å for the central and 3.97 mdyn/Å
for the terminal C−C bonds. Uncoordinated (trans-)
butadiene, in contrast, exhibits an fcentral of 5.20 mdyn/Å and
an fterminal of 8.87 mdyn/Å.25 Upon coordination to the Mo(0)
center, both C−C force constants of free butadiene thus are
lowered, but fterminal about 7 times more (−55%) than fcentral
(−8%). The large decrease of fterminal and the weak decrease of
fcentral are due to the fact that metal → ligand backbonding
involves the ψ3 orbital of butadiene, which is antibonding with
respect to the terminal C−C bonds and bonding with respect
to the central C−C bond (see below).
C. Synthesis and Reactivities of the Cationic
Molybdenum Complexes 2. The 17-electron complex
[Mo(dmbd)(dppe)](BPh4) (2BPh) was synthesized by one-
electron oxidation of [Mo(dmbd)3] (1dmbd) in the presence of
1,2-bis(diphenylphosphino)ethane (dppe) in thf. Ferrocinium
salts (FcX, X = BF4 , PF6 , BPh4 ) were used as oxidizing
reagents. All compounds could be isolated in good yields as air
stable green powders. The complexes are well soluble in
CH2Cl2, CHCl3, and nitriles, whereas they are insoluble in
nonpolar solvents like toluene and hexane and in alcohols. If
fluoride-containing ferrocenium salts, especially FcBF4, are
employed, sometimes small amounts of impurities appear that
The vibrational properties of [Mo(dmbd)3] (1dmbd) were
evaluated in a similar way. The optimized structure of 1dmbd as
well as the measured and calculated spectra are given in the
Supporting Information, Figures S2−S4. However, due to the
strong coupling of the vibrations of the diene unit with the
methyl groups at the central carbon atoms, the assignment of
the individual modes is less clear-cut.
The identification of the bands containing C−C stretching
modes is useful for the interpretation of the bonding situation
in the coordinated butadiene units. The highest-energetic C−C
vibration of free trans-1,3-butadiene is the ν4 mode at 1644
cm−1 containing CC stretching motions of the two double
bonds.25 The importance of backbonding is indicated by the
fact that the corresponding modes appear at much lower
frequencies in the complex (Table 2). The vibrational analysis
further shows that none of the C−C bonds can be described as
a pure double or single bond but that these vibrations are in fact
strongly mixed.
−
−
−
B. Normal Coordinate Analysis of [Mo(bd)3] (1bd). In
order to obtain further insight into the coordination of the
butadiene ligands to molybdenum centers, the central and
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can be separated by crystallization. Under these conditions, a
few crystals of the complex [MoF2(dppe)2](BF4) (4) were
isolated and characterized by X-ray structure analysis. This
compound is already known as one of the reaction products of
Scheme 2. Reaction Scheme for the Oxidative Substitution
of Tris(2,3-dimethylbutadiene)molybdenum, 1dmbd
[MoH4(dppe)2] with HBF4.24 The addition of FcBF4 to 2BF4
however, did not lead to the formation of 4.
,
By reduction of 2 with sodium amalgam in thf overnight, the
molybdenum complex [Mo(dmbd)2(dppe)] (3) was synthe-
sized. The orange compound is unstable under aerobic
conditions in solution; i.e., it decomposes under precipitation
of a green compound which could be characterized by X-ray
crystallography and identified as the 17-electron complex
[Mo(dmbd)2(dppe)]2(Mo2O7) (data not shown).
Importantly, 3 could not be obtained by ligand exchange of
[Mo(dmbd)3] (1dmbd) under thermal conditions, even by
refluxing [Mo(dmbd)3] with dppe in acetonitrile or toluene.
The fact that 1dmbd is inert toward substitution and the
occurrence of the butadiene-free complex [MoF2(dppe)2](BF4)
(4) in the reaction product suggest the following reaction
mechanism (Scheme 2): The unreactive complex [Mo-
(dmbd)3] is oxidized first to a [Mo(dmbd)3]+ cation. In this
oxidation state, one of the weaker bound diene ligands can be
substituted by a bidentate dppe, forming the stable 17e−
complex [Mo(dmbd)2(dppe)]+ (2). As a side reaction, also
the substitution of one dmbd against fluoride can occur. This
fluoride-containing intermediate can further be oxidized and
fluorine-substituted, leading to the complex [MoF2(dppe)2]+
(4).
The molybdenum complex [Mo(bd)3] (1bd) exhibits a
different reactivity as compared to its methylated counterpart.
Use of FcBF4 or FcPF6 as oxidants leads to an undefined
mixture of products with small fractions of a green compound.
This observation suggests that the nonmethylated complex has
a much higher reactivity toward fluoride ions, leading to
different fluorinated compounds as main products. Using the
halide-free oxidizing reagent FcBPh4, on the other hand, was
found to lead to a green oily reaction product which can be
isolated as a foamy solid after drying under vacuum conditions.
No further characterization of this compound could be
achieved. Exchange of the anion might possibly lead to a
crystalline product.
The differences in reactivity between 1bd and 1dmbd indicate
that steric shielding imposed by the methyl groups must have a
large influence. Especially the 17-electron trisdiene species
seems to be much more reactive toward fluoride substitution
for the bd as compared to dmbd intermediate.
position to C3 and C4; P2 is located in trans position to C13
and C14.
The C−C bonds of the butadiene ligands show an
equalization of the bond lengths due to extensive backbonding
into the π* orbitals of the diene units. In contrast to the parent
compound [Mo(dmbd)3] (1dmbd), where the central C−C
bond (1.396(3) Å) is shorter than the terminal bonds
(1.439(3) Å), the C−C bonds in 2BPh4 have similar values
ranging from 1.418(5) Å to 1.433(5) Å.26,27 Due to this strong
backbonding effect, the terminal CH2 groups of the 2,3-
dimethylbutadiene ligands are not planar anymore, indicating a
change from sp2 to sp3 character (see below). The distances
from the metal to the terminal C atoms have comparable values
for both dmbd ligands, ranging from 2.272(4) Å to 2.289(4) Å.
The distances from the metal to the central C atoms C2
(2.344(3) Å) and C12 (2.344(4) Å) are slightly shorter as
compared to C3 (2.372(4) Å) and C13 (2.367(4) Å), which
approximately are in trans position to the phosphorus atom P2.
Crystals of a corresponding 17-electron complex were isolated
by the diffusion of diethylether into an acetonitrile solution of
D. X-Ray Structure Analysis of 2BPh4, 3, and 4.
Crystalline substances of 2 were isolated by diffusion of
diethylether into an acetonitrile solution. The compounds 2PF6
and 2BF4 gave no crystals suitable for X-ray diffraction. 2BPh4
crystallizes in the triclinic space group P1 with two complexes
̅
and two tetraphenylborate counterions per unit cell (cf Table
1). The complex is C2-symmetric with the 2-fold axis bisecting
the angle P1−Mo1−P2. The structure is shown in Figure 4.
Both cis-2,3-dimethylbutadiene ligands are bound in an η4
mode to the molybdenum center with a basically planar
butadiene skeleton. The ligands are not coordinated in an
eclipsed fashion but are twisted by an angle of 52.9° (the twist
angle is defined as the angle between the projections on the P−
Mo−P plane of the vectors from the centers of the butadiene
ligands to the Mo center). The dppe ligand exhibits a bite angle
of 77.68(1)°. The P1 atom of the dppe ligand is bound in trans
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Figure 5. Crystal structure of [Mo(dmbd)2(dppe)] (3; 50% ellipsoids,
hydrogen atoms omitted). Selected bond lengths (Å) and angles
(deg): Mo(1)−C(14) 2.271(2), Mo(1)−C(12) 2.283(9), Mo(1)−
C(2) 2.285(4), Mo(1)−C(1) 2.287(9), Mo(1)−C(4) 2.289(3),
Mo(1)−C(11) 2.291(6), Mo(1)−C(13) 2.307(3), Mo(1)−C(3)
2.323(5), Mo(1)−P(1) 2.466(3), Mo(1)−P(2) 2.476(6), C(1)−
C(2) 1.440(5), C(2)−C(3) 1.420(3), C(3)−C(4) 1.428(3), C(11)−
C(12) 1.432(5), C(12)−C(13) 1.420(3), C(13)−C(14) 1.435(3),
C(3)−C(2)−C(1) 114.63(16), C(2)−C(3)−C(4) 116.92(15), P(1)−
Mo(1)−P(2) 81.17(1), C(2)−Mo(1)−C(1) 36.70(6).
Figure 4. Crystal structure of [Mo(dmbd)2(dppe)](BPh4) (2BPh4
;
50% ellipsoids, hydrogen atoms omitted). Selected bond lengths (Å)
and angles (deg): Mo(1)−C(1) 2.272(4), Mo(1)−C(14) 2.282(3),
Mo(1)−C(11) 2.288(4), Mo(1)−C(4) 2.289(4), Mo(1)−C(12)
2.344(4), Mo(1)−C(2) 2.344(3), Mo(1)−C(13) 2.367(4), Mo(1)−
C(3) 2.371(4), Mo(1)−P(2) 2.5395(10), Mo(1)−P(1) 2.5556(10),
C(1)−C(2) 1.423(5), C(2)−C(3) 1.423(5), C(3)−C(4) 1.433(5),
C(11)−C(12) 1.425(6), C(12)−C(13) 1.418(5), C(13)−C(14)
1.429(5), C(1)−C(2)−C(3) 117.4(3), C(2)−C(3)−C(4) 117.9(4),
P(2)−Mo(1)−P(1) 77.68(3), C(1)−Mo(1)−C(4) 74.12(14).
Å). A similar bonding situation has also been determined for
the complex [Mo(bd)2(PMe3)2].14
Single crystals of the byproduct [MoF2(dppe)2](BF4) (4)
could be isolated by diffusion of diethylether into an
acetonitrile solution of the reaction product of [Mo(dmbd)-
(dppe)](BF4). The complex crystallizes in the monoclinic space
−
the decomposition product of [Mo(dmbd)2(dppe)] (3). By X-
ray crystallography, these crystals were identified as [Mo-
(dmbd)2(dppe)]2(Mo2O7)·2NCCH3 (data not shown).
Single crystals of the corresponding 18-electron complex
[Mo(dppe)(dmbd)2] (3) suitable for X-ray diffraction could be
isolated by layering a benzene solution with n-hexane. The
compound crystallizes in the monoclinic space group P21/n
with four molecules per unit cell. The structure of the complex
is given in Figure 5; crystallographic data are collected in Table
1.
group Cc with four complexes and four BF4 counterions per
unit cell (Table 1). The Mo center is octahedrally coordinated
with the two diphos ligands in the equatorial plane and two F−
ligands in axial positions (Figure 6).
The Mo−P distances have values from 2.5260(6) Å to
2.5549(6) Å. The metal flouride distances are 1.8605(16) Å
and 1.8732(17) Å. The bite angles of the dppe ligand are
79.29(2) Å; the F−Mo−F angle is 178.33(8) Å.
E. Spectroscopic and Magnetic Properties of Com-
pounds 2 and 3. The 17-electron complex 2PF6 gives rise to a
heavily split signal in the room temperature EPR spectrum at g
= 2.0146 (Figure 7). The complicated signal structure is caused
by hyperfine coupling of the unpaired spin with the two
phosphines and the four different CH2 protons; the I = 5/2
isotopes 95Mo and 97Mo cause an additional splitting. Magnetic
measurements of 2PF6 indicate a paramagnetic behavior with a
magnetic moment of 1.73 μB per formula unit (Figure S5,
Supporting Information). Both data sets thus are consistent
with the presence of one unpaired electron.
With two η4 bonded 2,3-dimethylbutadienes and one
bidentate dppe ligand, the complex has a similar C2-
symmetrical structure to its 17-electron counterpart. In
comparison to the cationic complex, the twist of the diene
ligands has increased in the neutral complex to 81.44°. Another
structural difference to the cationic complex is the larger bite
angle of the dppe ligand (81.17(1)°). Interestingly the
oxidation state of the metal center has no significant influence
on the C−C bond lengths of the diene ligands. As in the neutral
trisbutadiene complex 1 and in the cationic complex 2PPh4, an
equalization of the C−C bond lengths is observed for 3. In
contrast to 1dmbd, where the central C−C bond lengths are
slightly shorter than the terminal ones (vide supra), the C−C
bond lengths in 2 have comparable values ranging from
1.420(3) Å to 1.440(5) Å. The molybdenum−carbon distances
for the terminal C atoms have similar values ranging from
2.271(2) Å (Mo−C(14)) to 2.291(6) Å (Mo−C(11)). Like in
compound 2, the distances from molybdenum to the central
carbon atoms approximately in trans position to the P atoms,
C3 (2.323(5) Å) and C13 (2.307(3) Å), are slightly longer as
compared to Mo−C2 (2.285(4) Å) and Mo−C12 (2.283(9)
The corresponding neutral complex 3 is, of course,
1
diamagnetic but exhibits a temperature-dependent H NMR
spectrum revealing significant structural dynamics in solution.
The denominations of the C and H atoms of 3 are given in
Figure 8. Figure 9 shows the signals of the endo-CH2 protons
H1 and H3 in thf-d8 solution (endo hydrogens are placed away
from the molybdenum, cf. Figure 8). Due to the C2 symmetry
of the complex, these protons give rise to two resonances at
−1.20 ppm and −2.46 ppm; at room temperature, these
resonances are broad. Cooling the solution to below 240 K
leads to better resolved signals located at −1.26 ppm (broad d,
H1, J(H−H) = 6.2 Hz) and −2.65 ppm (broad dd, H3, J(H−H)
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Figure 8. Assignment of C and H atoms of 3 for the analysis of 1H and
13C NMR spectra.
Figure 6. Crystal structure of [MoF2(dppe)2](BF4) (4; 50% ellipsoids,
hydrogen atoms omitted). Selected bond lengths (Å) and angles
(deg): Mo(1)−F(2) 1.8605(16), Mo(1)−F(1) 1.8732(17), Mo(1)−
P(1) 2.5518(6), Mo(1)−P(2) 2.5418(6), F(2)−Mo(1)−F(1)
178.33(8), P(2)−Mo(1)−P(1) 79.29(2), P(3)−Mo(1)−P(4)
79.29(2).
Figure 9. Temperature-dependent 1H NMR spectrum of 3 (endo-CH2
protons).
static. This is also reflected by the 31P NMR spectra; i.e., upon
cooling from 300 to 213 K, the line width of the resonance at
76.9 ppm decreases from 50 to 20 Hz.
Figure 7. EPR spectrum of [Mo(dmbd)2(dppe)](PF6) (2PF6).
1
As shown previously, the J(C−H) coupling constants of
= 6.2 Hz, J(P−H) = 10.4 Hz; line width ∼5 Hz). The exo-CH2
protons H2 and H4, on the other hand, give two resonances at
0.40 ppm (broad d, H4, J(P−H) = 21.2 Hz) and 0.58 ppm
(broad s, H2), and the CH3 groups (H5, H6) lead to two signals
at 1.60 ppm and 1.73 ppm, respectively (all values at room
temperature). The dppe ligand, finally, causes two broad
multiplets for the CH2 protons (H7, H8, 2.66 ppm, 3.49 ppm)
and multiplets ranging from 7.05 ppm to 7.93 ppm for the
phenyl protons.
butadiene and dimethylbutadiene complexes can be correlated
with the hybridization at the corresponding C atoms, providing
important information with respect to the degree of back-
bonding in these compounds.28 For the J(H1−Ca) coupling
1
constant of compound 3 (cf Figure 8), a value of 149 Hz is
obtained. Importantly, this value is very similar to those found
for the homoleptic complexes [Mo(bd)3] and [Mo(dmbd)3]
(cf Table 3; representative sections of NMR spectra are shown
in Figure S6, Suppurting Information) as well as [W(bd)3] and
[W(dmbd)3].29 This indicates that the amount of backbonding
from the metal center to the butadiene ligands in 3 is very
similar to that of the neutral Mo and W tris(diene)
counterparts. Using Newton’s empirical relation, a coupling
constant of 150 Hz can be correlated with 29.54% s character
The temperature dependence of the 1H NMR spectrum of 3
is remarkable in view of the fact that the corresponding
trimethylphosphine butadiene complex [Mo(bd)2(PMe3)2]
does not show comparable behavior.14 It can be assumed that
the line broadening at 300 K is caused by a conformational
change of the complex with the two diene ligands exchanging
their position, which is accompanied by a twist of the dppe
ligand. At lower temperatures, the structure becomes more
and, correspondingly, a hybridization of sp2.385 30 indicative of a
,
significant charge transfer from the Mo center to the double
bonds of the butadiene ligands.
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1
which is also compatible with the C−C bond lengths and the
Table 3. J(C−H) Coupling Constants for the Terminal C
1
observed J(C−H) coupling constants.
Atoms of Butadiene and Dimethyl-Butadiene Ligands
Coordinated to Mo and W Complexes (at Room
Temperature; n.r. = Not Resolved; Shifts in Parentheses; for
3 Parameters at 213 K Are Given)
F. Electrochemical Properties. The cyclic voltammogram
of compound 2PF6 (Figure 11) shows a quasi-reversible one-
1
1
compound
endo J(C−H)
exo J(C−H)
ref.
[Mo(bd)3] (1bd)
150 Hz
(0.48 ppm)
154 Hz
(1.6 ppm)
this work
[W(bd)3]
151 Hz
(0.35 ppm)
151 Hz
(1.42 ppm)
Benn et
al.29
[Mo(dmbd)3] (1dmbd
[W(dmbd)3]
)
152 Hz
n.r. (1.46 ppm) this work
(−0.27 ppm)
147 Hz
(−0.33 ppm)
147 Hz
Benn et
al.29
(1.13 ppm)
[Mo(dmbd)2(dppe)]
(3)
149 Hz
(−1.26 ppm)
n.r. (0.40 ppm) this work
n.r. (−2.65 ppm) n.r. (0.58 ppm)
As shown for the parent compound [Mo(bd)3] (1bd; vide
supra), important information on the bonding situation of the
butadiene ligands coordinated to molybdenum centers is also
provided by vibrational spectroscopy. The Raman spectra of
compounds 2 and 3 (Figure 10) exhibit almost the same signals
Figure 11. Cyclic voltammogram of 2PF6
.
electron redox wave at −0.57 V vs SCE which can be assigned
to the Mo(I) ↔ Mo(0) redox process. Furthermore, four
additional irreversible redox processes could be observed at
1.00, 1.21, and 1.49 V vs SCE, which may be caused by the
oxidation to higher oxidation states of the molybdenum center.
The difference pulse voltammogram of 2PF6 (Figure S7,
Supporting Information) shows two more signals at −1.22 V
and −1.64 V which appear after running several cycles from −2
V to +2 V. After applying a constant voltage of 2 V for 5 min,
the signals disappear. Therefore, they can be assigned to
decomposition products on the electrode surface.
G. Electronic Structures of [Mo(bd)3] (1bd), [Mo-
(dmbd)2(dppe)]+ (2), and [Mo(dmbd)2(dppe)] (3). Further
insight into the electronic structures of the heteroleptic Mo(I)
and Mo(0) complexes 2 and 3 is provided by DFT, allowing a
comparison with the electronic structures of the parent
homoleptic molybdenum tris(butadiene) compounds. The
optimized structure of 1bd is shown in Figure 1 (vide supra).
The electronic structure of this complex has been treated
before.2 Table 4 collects structural parameters given in ref 2 in
comparison to those calculated in the present study and those
determined experimentally; no major differences are observed.
Figure 10. Raman spectrum of [Mo(dmbd)(dppe)](BPh4) (2BPh4
top) and [Mo(dmbd)(dppe)] (3, bottom).
,
Table 4. Selected Bond Lengths of the Optimized Structure
of [Mo(bd)3] (1bd)
as 1bd and 1dmbd in the region from 1500 cm−1 to 1000 cm−1,
which is characteristic for the C−H-deformation and C−C-
stretching vibrations; the IR spectra of compounds 2BPh4 and 3
are given in the Supporting Information, Figure S6. A detailed
vibrational assignment on the basis of DFT calculations, as
performed for 1bd, is difficult for these compounds due to the
strong mixing of the C−C stretching vibrations with the CH3
deformations. Nevertheless, the Raman spectra clearly indicate
that the bonding of the diene ligands does not change upon
one-electron oxidation of the metal center in 3, leading to 2.
This is compatible with the C−C bond lengths determined
from the X-ray structures. Furthermore, vibrational spectros-
copy reveals that the bonding of the diene ligands is very similar
in the homoleptic complexes 1 and the heteroleptic complex 3,
B3LYP/LANL2DZ B3LYP2 BP862
exptl.2
Mo−Cterminal
Mo−Ccentral
Cterminal−Ccentral
Ccentral−Ccentral
2.308
2.397
1.442
1.414
2.302
2.367
1.427
1.401
2.294
2.360
1.434
1.408
2.284/2.273
2.325/2.330
1.414
1.403/1.388
Optimized structures of 2 and 3 are given in the Supporting
Information, Figure S8. In agreement with the crystal
structures, they show a twisting of the butadiene ligands. In
the cationic complex 2, a twist angle (vide supra, C) of 56.55° is
calculated (crystal structure 52.9°); the neutral molybdenum
complex 3 shows a corresponding angle of 83.00° (crystal
structure 81.44°). The trends of the dppe bite angle are
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correctly reproduced as well; i.e., the calculated angle for 2 is
77.1° (crystal structure: 77.68(1)°), and the calculated value for
3 is 80.6° (crystal structure: 81.17(1)°). Selected bond lengths
and angles of the optimized structures are collected in Table 5.
represent the bonding interactions of the phosphine lone pairs
with dyz (143) and dxz (141), respectively. MO 142 is a dppe
orbital that is nonbonding with respect to the metal. All three
MO’s (141, 142, 143) have strong phenyl-π contributions. The
LUMO (149) and LUMO+1 (150), on the other hand, are
dominated by phenyl-π orbitals. The LUMO (149) additionally
Table 5. Selected Structural and Calculated Parameters of 2
and 3
2
2
has a nonbonding dx −y contribution, whereas the LUMO+1
(150) contains a nonbonding dxz contribution.
[Mo(dmbd)2(dppe)]+ [Mo(dmbd)2(dppe)]
The corresponding cationic complex 2 shows a comparable
MO scheme (Figure S9, Supporting Information). The SOMO
148 is metal−ligand nonbonding like the HOMO in complex 3.
The MOs 147 and 146 mediate the backbonding interactions
with the ψ3 butadiene orbitals. SOMO−3 (145) is equivalent to
the bonding phosphine orbital 141 of 2; the bonding
combinations of the ψ2 orbitals are located in the MOs 144
and 136. The energetic sequence of the orbitals in 2 thus is
slightly different as compared to 3, which is due to the change
in the coordination sphere, i.e., the different arrangement of the
butadiene ligands.
(2)
(3)
bond lengths/Å, bond
angles/deg
exptl.
calcd.
exptl.
calcd.
Mo(1)−C(1)
Mo(1)−C(2)
Mo(1)−P(1)
P(2)−Mo(1)−P(1)
C(1)−C(2)
2.272(4)
2.344(3)
2.5556(10)
77.68(3)
1.423(5)
1.423(5)
2.31
2.41
2.68
77.09
1.44
1.44
2.287(9)
2.285(4)
2.466(3)
81.17(1)
1.440(5)
1.423(5)
2.31
2.34
2.59
80.58
1.45
1.44
C(2)−C(3)
In contrast to compounds 2 and 3, all molybdenum d
orbitals contribute to the metal−ligand π-bonding the
homoleptic tris(butadiene) complex [Mo(bd)3] (1bd) (Figure
13).
The MO scheme of the neutral molybdenum complex 3 is
shown in Figure 12. The HOMO 148 is the dz orbital with
2
almost metal−ligand nonbonding character. The HOMO−1
147 and HOMO−2 146 are the bonding combinations of the
dx −y and the dxz metal orbitals with the ψ3 orbitals of the
butadiene ligands that form the LUMO in the free ligands (cf.
Figure 12). MOs 147 and 146 thus represent the metal−
butadiene backbonding interaction in 3.
2
2
Due to the fact that the ψ3 orbitals are antibonding with
respect to the terminal C−C bond and bonding with respect
the central C−C bond, an equalization of the butadiene C−C
bonds occurs in the complex. As a consequence of the low
symmetry of the coordination sphere, the d orbitals of the
metal center are strongly deformed. HOMO−3 and HOMO−4
are the bonding combinations of the butadiene orbital ψ2 with
the metal orbitals dxy (145) and dyz (144). ψ2 is bonding with
respect to the terminal C−C bonds but antibonding with
respect to the central C−C bond and represents the HOMO of
free butadiene (cf Figure 12). Molecular orbitals 143 and 141
Figure 13. Highest occupied MOs of [Mo(bd)3] (1bd; cf. ref 2).
Figure 12. MO scheme of complex [Mo(dmbd)2(dppe)] (3).
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2
Specifically, the HOMO (2a′) is the combination of dz and
C2 symmetric structure. Importantly, the oxidation state has no
influence on the characteristic equalization of the butadiene C−
C bond lengths. These results were confirmed by vibrational
spectroscopy and density functional calculations and could be
explained by the electronic structure. Both in 2 and in 3, the
ψ3; HOMO−1 and HOMO−2 (2e′) are bonding combinations
2
3
2
2
of dx −y and dxy with and ψ . This is similar to 2 and 3 with the
exception that these heteroleptic complexes have a metal−
ligand nonbonding orbital as the HOMO (vide supra). As a
result, one-electron oxidation of the complex (HOMO doubly
vs singly occupied) does not significantly influence the bonding
situation of the butadiene ligands.
2
highest occupied molecular orbital is a metal dz orbital with
almost metal−ligand nonbonding character. Therefore, the
oxidation state, particularly, the single vs double occupancy of
the HOMO, has no influence on the backbonding interaction
from the metal into the unoccupied orbitals of the diene
ligands. However, two other metal orbitals in 2 and 3 are
involved in a metal → backbonding interaction with the two
butadiene ligands, causing a similar elongation of their C−C
bonds as encountered in the homoleptic [Mo(bd)3] and
[Mo(dmbd)3] complexes, where three doubly occupied metal
orbitals are involved in a backbonding interaction with three
butadiene ligands.
IV. SUMMARY AND CONCLUSION
In the present paper, the first oxidative ligand substitution
reaction performed on molybdenum tris(2,3-dimethylbuta-
diene) complexes has been reported. On the basis of this
reaction, the cationic complexes [Mo(dmbd)2(dppe)](X) (X =
−
−
−
BF4 , PF6 , BPh4 ) (2) have been isolated and characterized.
Compounds 2 represent a new type of 17-electron
molybdenum complex being coordinated by two butadiene
ligands and one bidentate phosphine. By reduction of 2, the
corresponding neutral molybenum complex [Mo-
(dmbd)2(dppe)] (3) could be synthesized. Importantly, this
heteroleptic complex was not directly accessible by a ligand
exchange reaction of [Mo(dmbd)3]. The bonding situation of
the coordinated diene ligands in the heteroleptic systems as
well as in the parent molybdenum trisbutadiene complexes has
been investigated by a combination of spectroscopy and DFT.
For the complex [Mo(bd)3] (1bd), a normal coordinate analysis
has been performed. Following a procedure previously
employed by us for the analysis of dinitrogen and −NNHx
complexes,31 force constants for the central and the terminal
butadiene C−C bonds of 4.97 and 3.79 mdyn/Å, respectively,
have been determined, reflecting the fact that the backbonding
interaction predominantly involves the terminal C−C bonds of
these ligands. Similar results are obtained for 1dmbd and new
heteroleptic 17- and 18-electron complexes 2 and 3, all of
which approximately exhibit the same degree of backbonding.
Note that in the previous discussion, no reference to the
actual charge distribution in the molybdenum complexes 1−3
has been made. In the homoleptic complex [Mo(bd)3] (1bd),
the Mo center has been assigned a charge of +IV by Kaupp et
2
al. (vide supra). The two electrons in the metal-based dz -type
HOMO of 3 would correspondingly give rise to a d2
configuration. Ionization from this orbital would then lead to
a d1 configuration, i.e., a Mo(+V) charge state, in 2. As found
computationally in previous d2 vs d1 comparisons,36 this entails
only little structural consequence, in full agreement with the
results of this paper.
ASSOCIATED CONTENT
* Supporting Information
■
S
Normal modes of [Mo(bd)3]; vibrational spectra of [Mo-
(dmbd)3]; magnetic data of 2; NMR spectra and structures of
1bd, 1dmbd, and 3; and MO scheme of 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
This also becomes evident from a consideration of J(C−H)
AUTHOR INFORMATION
Corresponding Author
*Tel.: ++49 (0)431 880 1410. Fax: ++49 (0)431 880 1520. E-
mail: ftuczek@ac.uni-kiel.de.
coupling constants at the terminal carbon atoms of the
coordinated bd/dmbd ligands of 1bd, 1dmbd, and 3, all of
which have values around 150 Hz, corresponding to hybrid-
izations around sp2.4.
■
Seventeen-electron molybdenum complexes are rare. Their
preparation by oxidation of appropriate 18-electron molybde-
num complexes is, however, well established and has been
applied to bis(arene) and hexaphosphine compounds.8,32−34 In
contrast to these compounds, which form stable 17-electron
complexes, the one-electron oxidized counterparts of [Mo-
(bd)3] and [Mo(dmbd)3] easily undergo substitution reactions.
The parent complexes, on the other hand, are quite unreactive.
The reason for the different reactivities can be inferred from the
electronic struture of [Mo(bd)3] in which all metal d orbitals
contribute to the metal diene bonding.2,35 Oxidation of the
metal center thus weakens the metal−butadiene bond and
renders (at least) one ligand substitutable.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
F.T. thanks CAU Kiel and the State of Schleswig-Holstein for
support of this research.
■
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