Molybdenum 1,4-diazabuta-1,3-diene tricarbonyl solvento complexes revisited: From solvatochromism to attractive ligand-ligand interaction

Article abstract of DOI:10.1021/ic202517w

Various complexes of the types [Mo(DAD)(CO)₃L] (L = CO, MeCN, MeOH, THF, DMSO, DMF, Me₂CO, EtOAc, THT, Im^H, Im ¹, tBuNC, nBu₃P), (ER₄)[Mo(DAD)(CO) ₃X] (ER₄ = NEt₄⁺; X^- = Cl^-, Br^-, I^-, NCS^-, CN^- and ER₄⁺ = PPh₄⁺; X^- = N₃^-), and (ER₄)[{Mo(DAD)(CO)₃} ₂(μ-X)] (ER₄⁺ = NEt₄⁺; X^- = CN^-, OAc^- and ER₄⁺ = PPh₄⁺; X^- = N₃^-; DAD = N,N′-bis(2,6-dimethylphenyl)butane-2,3-diimine) were prepared by ligand exchange from cycloheptatriene molybdenum tricarbonyl. A total of 19 crystal structures were determined, including unprecedented structural characterization of molybdenum(0) coordination by dimethyl sulfoxide (DMSO), methanol, ethyl acetate (EtOAc), acetone, and N,N-dimethylformamide (DMF). Correlation of ¹³C NMR shifts with the complex geometry suggests a direct ligand-ligand interaction between DAD and O-bonded coligands with C=O and S=O double bonds, such as EtOAc, Me₂CO, DMF, and DMSO. Unexpectedly, the solvatochromic properties of these tricarbonyl complexes [Mo(DAD)(CO) ₃L] are unfavorable for the determination of Kamlet-Taft parameters of the corresponding solvent L. Contrastingly, the UV/vis absorption of [Mo(DAD)(CO)₄] is strongly correlated with the Kamlet-Taft parameter π*, which is shown for 22 solvents, including seven room temperature ionic liquids.

Full text of DOI:10.1021/ic202517w

Article
pubs.acs.org/IC
Molybdenum 1,4-Diazabuta-1,3-diene Tricarbonyl Solvento
Complexes Revisited: From Solvatochromism to Attractive
Ligand−Ligand Interaction
Benjamin Oelkers, Alexander Venker, and Jorg Sundermeyer*
̈
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
̈
S
* Supporting Information
ABSTRACT: Various complexes of the types [Mo(DAD)(CO)3L] (L = CO, MeCN, MeOH, THF, DMSO, DMF, Me2CO,
+
+
EtOAc, THT, ImH, Im1, tBuNC, nBu3P), (ER4)[Mo(DAD)(CO)3X] (ER4+= NEt4 ; X− = Cl−, Br−, I−, NCS−, CN− and ER4
=
PPh4 ; X− = N3 ), and (ER4)[{Mo(DAD)(CO)3}2(μ-X)] (ER4 = NEt4 ; X− = CN−, OAc− and ER4 = PPh4 ; X− = N3 ;
DAD = N,N′-bis(2,6-dimethylphenyl)butane-2,3-diimine) were prepared by ligand exchange from cycloheptatriene molybdenum
tricarbonyl. A total of 19 crystal structures were determined, including unprecedented structural characterization of molybdenum(0)
coordination by dimethyl sulfoxide (DMSO), methanol, ethyl acetate (EtOAc), acetone, and N,N-dimethylformamide (DMF).
Correlation of 13C NMR shifts with the complex geometry suggests a direct ligand−ligand interaction between DAD and O-bonded
coligands with CO and SO double bonds, such as EtOAc, Me2CO, DMF, and DMSO. Unexpectedly, the solvatochromic
properties of these tricarbonyl complexes [Mo(DAD)(CO)3L] are unfavorable for the determination of Kamlet−Taft parameters of
the corresponding solvent L. Contrastingly, the UV/vis absorption of [Mo(DAD)(CO)4] is strongly correlated with the Kamlet−Taft
parameter π*, which is shown for 22 solvents, including seven room temperature ionic liquids.
+
−
+
+
+
−
done concerning phen and bipy complexes, especially covering
strong donor/acceptor ligands,1h,11 (weakly) donating solvents,9f,12
and (pseudo)halide anions,13 less is known for DAD-based
complexes. Only neutral coligands with large ligand-field splitting
(P-based ligands,1d,f,2d,5b,6a,7c,d,14 isonitriles,15 cyclooctene,16 and
acetonitrile1f,14a,b,15b) have been applied in this case (tables listing
all known tricarbonyl complex preparations and crystal structures
are given in the Supporting Information, SI).
In contrast to a plethora of X-ray diffraction (XRD) studies on
tetracarbonyl complexes [Mo(N−N)(CO)4], surprisingly little
structural information is available in the literature on the cor-
responding tricarbonyl complexes [Mo(N−N)(CO)3L], being
limited to phosphanes,17 pyridine,18 isonitriles,17b,19 tetrahydrofur-
an (THF),17b and sulfur dioxide.20 No complexes of the type
[Mo(N−N)(CO)3X]− have been structurally characterized; this
also holds for molybdenum 1,4-diazabuta-1,3-diene tricarbonyl
complexes with any coligand. Concerning (weak) donor solvents
INTRODUCTION
■
Carbonyl complexes of zerovalent group VI metals, especially
of molybdenum, bearing unsaturated diaza ligands are one of
the most studied compound classes since the early days of
organometallic chemistry. Mainly, the tetracarbonyl complexes
[Mo(N−N)(CO)4] have attracted a lot of interest concerning
all four major types of planar 1,4-diaza ligands (N−N), i.e., 1,4-
diazabutadienes (DAD),1 bipyridines (bipy),1h,2 phenanthrolines
(phen),2m,3 and iminopyridines (pyridine-2-carbaldimines,
PyCa).1h,4 Among the most interesting features of these complexes
are characteristic metal−ligand charge-transfer (MLCT) bands that
show strong solvatochromic behavior1c,4b,5 and have thus been used
for the empirical determination of the solvent polarity.5a,6 Further
studies include photoexcitation,7 multimetallic complexes,8 electro-
chemistry,9 and polymerization of methyl methacrylate.10
Formal substitution of one carbonyl ligand by a neutral (L)
or anionic coligand (X−) leads to tricarbonyl complexes
[Mo(N−N)(CO)3(L/X)]0/− that are found to be fac-configured
for all but the most π-accepting coligands, which is due to the
pronounced trans effect of CO. While considerable work has been
Received: November 22, 2011
Published: March 28, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
for 60 min. Afterward, the product was precipitated by the addition of
hexane (25 mL), isolated by filtration, and dried in vacuo. Yield: 412 mg
like amides, carboxylates, ketones, alcohols, dimethyl sulfoxide
(DMSO), or tetrahydrothiophene (THT), little to no structural
information is available for molybdenum(0) complexes of any kind.
Because the tricarbonyl complexes [Mo(N−N)(CO)3(L/X)]0/−
are known to easily exchange the monodentate coligand (L/X−) in
equilibrium reactions,12a,13a,14a,b we decided to investigate whether
the corresponding, up to now unknown, DAD complexes are acces-
sible. If so, the complex fragment [Mo(DAD)(CO)3] could serve as
a spectroscopic tool for solvent-polarity measurements with
behavior presumably different from that of the corresponding
tetracarbonyl complex [Mo(DAD)(CO)4] (1).21 The latter was
also tested for comparison, including the first reported use of such
chromophores for the solvatochromic description of ionic liquids
(ILs). NMR spectra and solid-state structures could be obtained
for most compounds and provide a reliable basis on which the
observed UV/vis spectra will be discussed.
1
(63%), blue powder. H NMR (300 MHz, CD2Cl2): δ 1.25 (tt, 12H,
3
3JHH = 7.2 Hz, JNH = 1.7 Hz, CH3CH2N), 1.96, 2.06, 2.45 (3 × s, 3 ×
3
6H, CH3), 3.21 (q, 8H, JHH = 7.3 Hz, CH3CH2N), 6.98−7.10 (m, 6H,
CaromH). 13C NMR (75 MHz, CD2Cl2): δ 7.9 (CH3CH2N), 18.5, 19.3,
1
20.6 (CH3), 53.1 (t, JCN = 3 Hz, CH3CH2N), 124.8, 128.1, 128.47,
128.53, 131.0, 151.7 (Carom), 165.7 (CN). IR (neat): ν 2946 (w), 1895
(m), 1799 (s), 1774 (vs), 1535 (w), 1482 (w), 1441 (w), 1386 (m), 1313
(m), 1258 (w), 1215 (m), 1170 (w), 1093 (w), 975 (m), 846 (m), 785
(m), 702 (m), 629 (m), 617 (w), 564 (w), 547 (w), 470 (w), 434 (w),
417 (m) cm−1.
(NEt4)[Mo(DAD)(CO)3Br] (15), (NEt4)[Mo(DAD)(CO)3I]
(16), (NEt4)[Mo(DAD)(CO)3(NCS)] (17), (PPh4)[Mo(DAD)-
(CO)3(N3)] (20), (PPh4)[{Mo(DAD)(CO)3}2(μ-N3)] (21), and
(NEt4)[{Mo(DAD)(CO)3}2(μ-OAc)] (22) were obtained using the
same protocol; see the SI.
[Mo(DAD)(CO)3(tBuNC)] (12; Method D). 2 (203 mg, 0.40 mmol,
1.00 equiv) was suspended in toluene (15 mL). tert-Butylisonitrile
(35 mg, 0.42 mmol, 1.05 equiv) was added to the dark-blue mixture.
After stirring for 30 min, the precipitated product was isolated by
filtration and dried in vacuo. Yield: 162 mg (74%), microcrystalline,
dark-blue powder. 1H NMR (300 MHz, CD2Cl2): δ 1.41 (s, 9H,
(CH3)3CNC), 1.99 (s, 6H, CH3), 2.18 (s, 12H, CH3), 7.07−7.19 (m,
6H, CaromH). 13C NMR (75 MHz, CD2Cl2): δ 18.7, 18.9 (CH3), 30.7
DAD ligands are easily tunable because of their facile pre-
paration from various α-diketones and primary amines. In order
to get a consistent data set, only one member of this ligand
family was used throughout our study, namely, N,N′-bis(2,6-
dimethylphenyl)butane-2,3-diimine, which will in the following
be abbreviated to DAD.
1
((CH3)3CNC), 56.8 (t, JCN = 4 Hz, (CH3)3CNC), 125.4, 128.3, 128.8,
EXPERIMENTAL SECTION
Synthesis of Compounds. Preparation and characterization data
are given for one example of each synthetic protocol applied. Full
experimental details for all compounds are given in the SI.
150.9 (Carom), 164.8 (CN). IR (neat): ν 2988 (w), 2112 (m), 1912 (s),
1838 (s), 1813 (vs), 1555 (w), 1458 (m), 1370 (w), 1325 (m), 1218 (m),
1194 (w), 1162 (w), 1093 (w), 1035 (m), 849 (w), 769 (s), 701 (w), 630
(w), 585 (w), 521 (m), 492 (w), 446 (m), 421 (m) cm−1.
■
[Mo(DAD)(CO)3(ImH)] (10; ImH = imidazole), [Mo(DAD)(CO)3-
(Im1)] (11; Im1 = N-methylimidazole), and [Mo(DAD)(CO)3(nBu3P)]
(13) were obtained according to the same protocol; see the SI.
[Mo(DAD)(CO)3(DMSO)] (5; Method A). A mixture of [Mo(C7H8)-
(CO)3] (288 mg, 1.06 mmol, 1.03 equiv) and DAD (302 mg, 1.03
mmol, 1.00 equiv) was dissolved in toluene (15 mL). DMSO (0.1 mL,
1.3 mmol, 1.3 equiv) was added, resulting in a dark-blue reaction
mixture. After stirring overnight, the precipitated product was isolated
by filtration and dried in vacuo. Yield: 506 mg (89%), microcrystalline,
(NEt4)[Mo(DAD)(CO)3(CN)] (18; Method E). A mixture of 2
(114 mg, 0.22 mmol, 1.00 equiv) and NEt4CN (46 mg, 0.29 mmol,
1.32 equiv) was dissolved in methanol (15 mL). After stirring
overnight, the product was precipitated by the addition of diethyl ether
(30 mL), isolated by filtration, and dried in vacuo. Yield: 50 mg (36%),
microcrystalline, violet powder. 1H NMR (300 MHz, CD2Cl2): δ 1.25
(tt, 12H, 3JHH = 7.3 Hz, 3JNH = 1.8 Hz, CH3CH2N), 1.91 (s, 6H, CH3),
1
violet-blue powder. H NMR (300 MHz, CD2Cl2): δ 1.99 (s, 6H,
CH3), 2.18 (s, 12H, CH3), 2.65 (s, 6H, (CH3)2SO), 7.06−7.17 (m,
6H, CaromH). 13C NMR (75 MHz, CD2Cl2): δ 18.5, 18.9, 19.5 (CH3),
39.3 ((CH3)2SO), 125.6, 128.5, 128.6, 128.9, 150.0 (Carom), 169.9
(CN). IR (neat): ν 2920 (w), 1890 (s), 1781 (s), 1754 (vs), 1602
(w), 1517 (w), 1465 (m), 1421 (w), 1375 (w), 1308 (m), 1258 (w),
1215 (m), 1095 (w), 1021 (w), 990 (m), 956 (w), 940 (m), 846 (w),
769 (m), 699 (w), 640 (w), 626 (w), 507 (w), 425 (w) cm−1.
[Mo(DAD)(CO)3(DMF)] (6) and [Mo(DAD)(CO)3(THT)] (9)
were obtained using the same protocol; see the SI.
3
2.24 (br s, 12H, CH3), 3.20 (q, 8H, JHH = 7.3 Hz, CH3CH2N), 6.99−
7.12 (m, 6H, CaromH). 13C NMR (75 MHz, CD2Cl2): δ 7.9 (CH3CH2N),
1
18.4 (CH3), 19.1 (br, CH3), 53.1 (t, JCN = 3 Hz, CH3CH2N), 124.5,
128.3, 129.5, 152.3 (Carom), 161.0 (CN). IR (neat): ν 2981 (w), 2078
(w), 1899 (s), 1797 (vs), 1589 (w), 1531 (w), 1480 (m), 1438 (m), 1387
(s), 1319 (s), 1258 (w), 1216 (m), 1170 (m), 1157 (w), 1092 (w), 976
(m), 848 (m), 792 (s), 703 (m), 624 (m), 591 (w), 561 (w), 547 (w),
516 (w), 496 (m), 438 (m), 424 (m) cm−1.
(NEt4)[{Mo(DAD)(CO)3}2(μ-CN)] (19) was obtained using the
same protocol; see the SI.
UV/Vis Measurements. Instrumentation and experimental results
[Mo(DAD)(CO)3(MeCN)] (2; Method B). To a mixture of [Mo-
(C7H8)(CO)3] (825 mg, 3.03 mmol, 1.00 equiv) and DAD (885 mg,
3.03 mmol, 1.00 equiv) was added acetonitrile (30 mL). The dark-blue
reaction mixture was stirred overnight. Afterward, the precipitated product
was isolated by filtration and dried in vacuo. Yield: 1276 mg (82%),
1
microcrystalline, dark-blue powder. H NMR (300 MHz, CD2Cl2): δ
are given in the SI.
2.00, 2.09 (2 × s, 2 × 6H, CH3), 2.17 (s, 3H, CH3CN), 2.21 (s, 6H,
CH3), 7.07−7.19 (m, 6H, CaromH). 13C NMR (75 MHz, CD2Cl2): δ 3.9
(CH3CN), 18.2, 19.0, 19.4 (CH3), 125.6, 128.3, 128.5, 128.7, 129.1, 150.7
Single-Crystal XRD Studies. Crystallographic details are available
in the SI, including ORTEP-style figures, crystal parameters, and data
collection and refinement results for all compounds studied by XRD.
All ORTEP-style figures are shown for 50% probability; carbon-
bound hydrogen atoms as well as solvent molecules and cations are
omitted for clarity.
1
(Carom), 167.8 (CN). H NMR (300 MHz, CD3CN): δ 1.96 (s, 3H,
CH3CN), 2.02, 2.14, 2.23 (3 × s, 3 × 6H, CH3), 7.06−7.20 (m, 6H,
CaromH). 13C NMR (75 MHz, CD3CN): δ 1.8 (CH3CN), 18.3, 19.3, 19.8
(CH3), 126.3, 128.5, 129.4, 129.5, 151.0 (Carom), 171.5 (CN). IR
(neat): ν 2978 (w), 2920 (w), 1903 (s), 1826 (sh), 1812 (s), 1776 (vs),
1555 (w), 1462 (m), 1433 (m), 1409 (m), 1376 (m), 1307 (s), 1257 (w),
1216 (m), 1164 (w), 1092 (w), 1032 (w), 971 (m), 847 (m), 774 (m),
700 (m), 635 (m), 558 (w), 529 (w), 510 (m), 477 (w), 434 (m) cm−1.
[Mo(DAD)(CO)3(MeOH)]·3MeOH (3·3MeOH), [Mo(DAD)-
(CO)3(Me2CO)] (7), and [Mo(DAD)(CO)3(EtOAc)] (8) were
obtained according to the same protocol; see the SI.
RESULTS AND DISCUSSION
■
Syntheses and Reactivities. Molybdenum(0) diazadiene
tricarbonyl complexes with phosphanes as coligands have been
prepared by thermal displacement of one carbonyl ligand from
the corresponding tetracarbonyl complexes.14b However, this
method seems unsuitable for thermally less robust products and
ligands that are considerably weaker than carbon monoxide.
Therefore, substitution of labile ligands at ambient temperature
was chosen as a synthetic concept (Scheme 1).12a,13a,14b
(NEt4)[Mo(DAD)(CO)3Cl] (14; Method C). A mixture of [Mo(C7H8)-
(CO)3] (278 mg, 1.02 mmol, 1.00 equiv), DAD (298 mg, 1.02 mmol,
1.00 equiv), and NEt4Cl (202 mg, 1.21 mmol, 1.19 equiv) was dissolved
in dichloromethane (15 mL). The dark-blue reaction mixture was stirred
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Inorganic Chemistry
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Scheme 1. Synthetic Procedures Using Methods A−E
Most complexes were prepared by the reaction of
molybdenum cycloheptatriene tricarbonyl, [Mo(C7H8)(CO)3],
with 1 equiv of DAD and the desired coligand L or X−, similar
to the corresponding phen and bipy complexes.12a,13a While
weakly donating and/or hard ligands were applied in large
excess, i.e., as solvents of the reaction (method B), 1 equiv of
the stronger donating ligands DMSO, N,N-dimethylformamide
(DMF), and THT in a toluene solution was sufficient for com-
plete conversion (method A). Anionic coligands X− were intro-
duced as tetraethylammonium salts (tetraphenylphosphonium in
the case of azide) using a similar procedure (method C). Dichloro-
methane was found to be a reasonable compromise between the
solubility of the (ionic) starting materials and products, stability of
the products, and coordination behavior of the solvent. It was also
used for NMR measurements (vide infra).
subsequent ligand exchange on further tricarbonyl complex
molecules.
Polarity Measurements. A range of 22 solvents, including
7 room temperature ILs (RTILs), was used to determine the
solvatochromic behavior of 1 and the complex fragment [Mo-
(DAD)(CO)3]. Complex 1 shows strong negative solvatochromism
concerning its MLCT band, similar to a variety of molybdenum(0)
diaza ligand tetracarbonyl compounds known in the literature.5 2
was used as the second probe molecule, showing significantly
smaller differences in its MLCT band. As expected, donor solvents
(L) show the same visible spectra for any labile complex fragment
source applied (2, [Mo(DAD)(CO)3L], [Mo(C7H8)(CO)3] +
DAD). The triene and free DAD do not interfere because their
absorption maxima lie at significantly higher energies compared to
the complexes under investigation. The results suggest that ligand-
exchange reactions lead to the quantitative in situ formation of
[Mo(DAD)(CO)3L] due to fast equilibration. This conclusion is
consistent with NMR spectra recorded in acetonitrile showing
complete conversion toward the acetonitrile complex 2 upon dis-
solution of compounds with weak or hard donor ligands, e.g.,
acetone, EtOAc, and methanol.
For the ligands with the highest affinity toward molybde-
num(0), overreaction was sometimes observed, yielding
products not containing any DAD ligands. IR spectra indicated
the formation of [Mo(CO)3L3] in these cases.22 To overcome
this problem, 2 was used as a precursor, giving good yields of
the desired complexes (methods D and E).
In order to clarify the nature of the observed solvatochromic
effects, regression analyses were performed against the
simplified linear solvation energy relationship of Kamlet et al.23
While the halide anions were found to act exclusively as
monodentate ligands toward the complex fragment [Mo-
(DAD)(CO)3], different reactivities were observed for the
multiatomic pseudohalides CN−, SCN−, and N3− as well as for
OAc−. These anions are capable of bridging two Mo centers;
pure samples of such binuclear complexes were obtained for
ν(probe) = ν + aα + bβ + sπ*
o
The solvent parameters α, β, and π* in this model correspond
to the solvent’s ability to donate hydrogen bonds (α, acidity) and
to accept hydrogen bonds (β, basicity) and to its overall dipolarity
and polarizability (π*). The solvent-independent, solute-character-
istic coefficients a, b, and s describe the degree to which the
corresponding solvent parameter affects the chromophore’s
absorption band ν(probe). Of special interest is the strength of
correlation with each parameter that can be quantified as the ratio
of the corresponding coefficient and its error.
As can be seen from Table 1, the tetracarbonyl complex 1
shows significant correlation with π* because of its large value
and small relative error of s. While the dependence on α and
β is comparably small, the coefficient of determination seems
reasonable, leading to the conclusion that the observed MLCT
band can be approximated as a function of π* alone, as shown
in Figure 1.
CN−, N3 , and OAc− using the same synthetic protocols as
−
those mentioned above except for the necessarily different
stoichiometry. In the case of OAc−, only the binuclear complex
−
could be obtained, while for CN− and N3 , both mono- and
dinuclear complexes could be prepared selectively.
All complexes were obtained as blue-to-violet, sometimes almost
black, microcrystals or powders that give intensely blue-colored
solutions. While complexes bearing strong donor/acceptor
coligands such as nBu3P or tBuNC are moderately stable toward
oxygen both in solution and in the solid state, those compounds
with weakly bound coligands such as EtOAc or iodide readily
decompose upon contact with air. Slow degradation is also observed
in a dichloromethane solution. The tetracarbonyl complex 1 was
found to be the dominating decomposition product, presumably
formed by CO release from an oxidized tricarbonyl complex and
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Table 1. Linear Regression Results
All spectra reveal Cs symmetry for the complex molecules, as
expected for the fac isomer. Thus, three methyl group signals of
equal intensity are usually found in the 1H NMR spectra. While
the carbonyl ligands are mostly not observed because of
solubility limitations, the quaternary carbon signals Cimine and
Cipso are found to be strongly dependent on the coligand L or
X−. These signals correspond to the two DAD carbon atoms
directly bound to the ligating nitrogen atoms and are thus
strongly influenced by the metal environment. When any two
complexes are compared, it becomes evident that, as the imine
resonance is shifted downfield, the aromatic ipso signal is
invariably shifted upfield. The relative shift difference Δδ =
δ(Cimine) − δ(Cipso) of these two signals can therefore be used
as a quantitative measure for the ligand’s electronic properties.
When the shift differences Δδ of the halide and nonbridged
pseudohalide complexes are sorted by increasing values, the
same order as that in the spectrochemical series25 is observed
for all of these anionic ligands but cyanide (cf. Table 2).
[Mo(DAD)(CO)4] (1; n = 22; R2 = 0.813)
ν0
a
b
s
coefficient
error
16918
222
187
237
0.8
656
286
2.3
1856
293
6.3
|coefficient/error|
76.3
[Mo(DAD)(CO)3(MeCN)] (2; n = 18; R2 = 0.567)
ν0
a
b
s
coefficient
error
15514
281
153
205
0.7
−799
259
3.1
619
329
1.9
|coefficient/error|
55.3
A similar, roughly linear correlation has already been observed
for a related molybdenum(0) iminopyridine tetracarbonyl complex
and Reichardt’s ET scale.6a Because of the fact that this parameter
is, in turn, strongly correlated with π* for all solvents with low
acidity α,24 this result meets the expectations. Figure 1 also shows
Table 2. Selected Spectroscopic and Solid-State Parameters
a
b
Δδ/
N−Mo−L /
Mo−L/
ν1 (CO) /
compound coligand
ppm
deg
pm
cm−1
18
19
16
13
15
19
12
14
10
11
21
20
9
CN−
μ-CN−
I−
8.69
8.90
c
c
1899
1909
1905
1915
1895
1909
1912
1895
1895
1896
1895
1888
1897
1884
1903
1887
90.3
c
218.8(3)
c
9.99
nBu3P
Br−
10.17
12.76
13.24
13.89
14.05
14.22
14.30
15.28
15.60
15.98
16.42
17.19
17.77
106.7
91.8
88.4
91.1
90.9
88.1
89.2
c
257.5(2)
273.0(2)
220.8(3)
214.8(5)
259.6(2)
227.0(4)
228.7(2)
c
μ-CN−
tBuNC
Cl−
ImH
Im1
Figure 1. Maximum absorption energy ν(1) as a function of π*.
Conventional solvents are shown in black and ILs in blue. Data are
given in tabular form in the SI. Notation: [BMIM]+, 1-butyl-3-methyl-
imidazolium; [BMMIM]+, 1-butyl-2,3-dimethylimidazolium; [BMpyr]+, 1-
butyl-1-methylpyrrolidinium; [HMIM]+, 1-hexyl-3-methylimidazolium.
−
μ-N3
−
N3
c
c
THT
92.0
87.7
84.7
80.6
83.4
82.6
99.1
96.9
95.4
96.5
80.5
88.7
82.3
80.0
76.7
261.7(1)
220.8(3)
223.3(3)
224.2(3)
221.8(4)
224.9(2)
201.8(2)
204.4(2)
204.0(4)
203.6(4)
226.6(2)
231.9(3)
226.8(6)
226.6(5)
228.5(2)
17
2
NCS−
MeCN
μ-OAc−
22
that the used ILs all have π* values around 1.0 and thus similar
absorption bands for 1.
6
1
DMF
CO
19.12
19.74
1893
1993
2006
While the behavior of 1 is consistent with the prediction, the
complex fragment [Mo(DAD)(CO)3] exhibits complicated UV/
vis behavior. As stated above, all experimental evidence leads to
the conclusion that, in donor solvents L, [Mo(DAD)(CO)3L] is
quantitatively formed. On the other hand, the linear regression
shows no strong correlation with any of the Kamlet−Taft para-
meters (cf. Table 1). This failure of the tested complex fragment as
a solvatochromic probe intrigued us and led us to investigate the
solution and solid-state properties of the tricarbonyl complexes in
some detail.
NMR Spectroscopy. The only (affordable) NMR solvent
with sufficient solubilization for a broad range of neutral as well as
anionic tricarbonyl complexes was found to be dichloromethane.
While donor solvents lead to ligand-exchange reactions and are
only suitable for indirect complex characterization, nonpolar
solvents do not dissolve most of the complexes, especially in the
case of salt-like substances. However, the stability of the tricarbonyl
complexes in dichloromethane is limited so that spectra had to be
recorded within a couple of hours. Some complexes with weakly
bound ligands (acetone, EtOAc, THF, and MeOH) could only be
characterized indirectly via NMR spectroscopy using acetonitrile.
In these cases, the free coligand and in situ formed acetonitrile
complex 2 are found in the correct ratio.
5
4
3
7
8
DMSO
THF
19.93
1890
1902
1895
1893
1896
c
c
c
c
MeOH
Me2CO
EtOAc
a
Average angle between the imine nitrogen, molybdenum, and
coligand. High-energy CO stretching band. No value available.
b
c
These ligands do not participate in significant π-back-bond-
ing; in fact, the heavier halides are even known to be π donors.
The observed trend can thus be readily explained: Coordina-
tion of a weaker (stronger π-donating) ligand leads to an accu-
mulation of the electron density at the metal, which, in turn,
causes stronger back-bonding to the DAD ligand. The latter can
be observed as an upfield shift of the imine carbon resonance,
leading to a small Δδ.
Table 2 also shows that this concept does not hold for ligands
with π-accepting properties like cyanide, tri-n-butylphosphine,
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DMF, or DMSO. While the former two exhibit unexpectedly small
Δδ values, the latter two lead to a DAD ligand with electronic
characteristics similar to those in the tetracarbonyl complex. This
effect is indicative of some influence of a different nature than pure
ligand-field splitting.
while the DAD moiety undergoes minor changes in the bond
lengths and angles. The bite angle N−Mo−N remains almost
constant at 71−72°, while the bond length Mo−N varys between
216 and 224 pm. Most interesting is the angle N−Mo−L between
DAD and the coligand, which varies greatly from 76.7° to 110.2°
and will be discussed in detail (vide infra). Because of the different
ligating atoms, the distances Mo−L also differ strongly. These
mostly unprecedented bond lengths are tabulated in Table 2,
hopefully providing a useful source of information for comparison
with other zerovalent molybdenum compounds in the future.
The bond length Mo−P in 13 [257.5(2) pm] is consistent
with those found for other molybdenum tricarbonyl complexes
([Mo(bipy)(CO)3(PPh3)], Mo−P = 260.3(1) pm;17d [Mo-
(phen)(CO)3(PPh3)], Mo−P = 259.7(1) pm;17c [Mo(phen)-
(CO)3(η1-dppm)] [dppm = 1,1-bis(diphenylphosphino)-
methane], Mo−P = 257.2(3) pm;17a [Mo(PyCaOH)-
(CO)3(PPh3)] (PyCaOH = 4-hydroxyphenyl-pyridine-2-
carbaldimine), Mo−P = 257.0(1) pm;17b [Mo(PyCaOH)-
(CO)3(PPh3)]·Et2O, Mo−P = 253.3(1) pm17b). However, all
of the known structures show angles N−Mo−P in the range of
85.5(2)−92.3(6)°, which deviates substantially from the
averaged value found for 13 (106.7°), most likely because of
steric reasons (vide infra). Similarly, the bond lengths Mo−L
[4, L = THF, Mo−O = 231.9(3) pm; 10, L = ImH, Mo−N =
227.0(4) pm; 11, L = Im1, Mo−N = 228.7(2) pm; 12, L =
tBuNC, Mo−C = 214.8(5) pm] are found to compare well with
reported values [[Mo(PyCaOH)(CO)3(THF)], Mo−O =
228.9(3) pm;17b [Mo(bipy)(CO)3(py)], Mo−N = 231.1(9)
pm;18a [Mo(bipy)(CO)3(η1-dipyam)] [dipyam = N,N-bis(2-
pyridyl)amine], Mo−N = 235.2(6) pm;18b [Mo(PyCaOH)-
(CO)3(tBuNC)], Mo−C = 216.6(3) pm;17b [Mo(PyCaOH)-
(CO)3(CyNC)], Mo−C = 215.1(2) pm17b], while the angles
N−Mo−L seem to strongly depend on the respective
coordination sphere of the metal.
IR Spectroscopy. All tricarbonyl complexes show two strong
CO stretching vibrations attributed to the symmetry species A1 and
E1, as expected for a fac-[M(CO)3L2L′] complex of local pseudo-
C3v symmetry.12a,26 As this approximation gets less accurate, e.g.,
when going from the light halide Cl− to the heavy halide I−, the
band of lower energy (E1) progressively splits, leading to three well-
separated bands for 16. A comparison of the high-energy band ν1
(A1) with the results from NMR measurements shows similar
characteristics: While the observed values of the nonbridging
(pseudo)halides except cyanide are consistent with the spectro-
chemical series, this concept fails for π-accepting ligands.
In general, the differences in the CO bands between different
complexes are relatively small and presumably affected by packing
effects in the solid state, as suggested by the two different modi-
fications of the tetracarbonyl complex 1 that show different CO
bands in the solid state. These findings render IR spectroscopy less
helpful in understanding the bonding situation in this class of
compounds. However, the clearly separated band of 1, which is
the main contamination arising from decomposition (vide supra),
proved to be rather useful in the purity determination of the
desired tricarbonyl products.
Crystal Structures. Most complexes could be characterized
by single-crystal XRD. In addition to being essential for the
current investigation, many of the obtained structures represent
the first structurally characterized examples of the coordination
of various weakly bound ligands to molybdenum(0).
All complexes exhibit fac-oriented carbonyl ligands in a distorted
octahedral metal environment, as was already concluded from IR
and NMR measurements. In order to compress the amount of
data to discuss, a statistical analysis was performed on the data
sets,27 yielding the extreme values shown in Table 3.
While the different Mo−C distances can be explained by
varying back-bonding situations with different coligands, the
variation in the bond angles N−Mo−C reflects the distortion
caused by different angles N−Mo−L stated above. The DAD
moiety exhibits less pronounced structural changes within the
expected ranges.28
Table 3. Selected Limiting Bond Lengths and Angles
(Different Values within One Structure Are Not Averaged)
a
b
b
bond
min/pm
max/pm
Lmin
Lmax
Figure 2 exemplifies some of the determined structures. While
nBu3P as the coligand leads to a larger angle N−Mo−L, as was
expected because of steric hindrance, those complexes with acute
angles N−Mo−L, e.g., 5 (L = DMSO, mean value = 80.5°) and 8
(L = EtOAc, mean value = 76.7°), suffer greater steric strain. This
is partly released by twisting and/or deflecting the aromatic
substituents away from the coligand. In attempts to understand
the nature of this obviously electronically induced approach of
DAD and the coligand, NMR spectroscopy proves helpful. A
comparison of the “irregular” cases, in which Δδ does not allow for
the correct placement of the respective coligand in the
spectrochemical series (vide supra), with the angles N−Mo−L
that deviate substantially from 90° shows strong overlap of both
categories. Furthermore, a direct correlation between Δδ and N−
Mo−L exhibits a smooth trend (Figure 3).
Apparently, a smaller angle N−Mo−L and thereby a greater
proximity between the coligand and DAD leads to less electron
density in the conjugated CN double bonds (larger Δδ). In
fact, the distances between the coligand and imine carbon
atoms are unusually small and fall, in part, below the respective
van der Waals distances. While theoretical calculations will be
necessary to ensure the exact electronic situation, a preliminary
interpretation could be as follows: The coligands with the
c
Mo−N
Mo−L
Mo−Ctrans
Mo−Ccis
N−Cimine
Cimine−Cimine
N−Cipso
216.0(2)
201.8(2)
190.6(9)
189.2(6)
128.7(6)
144.0(7)
143.4(8)
min/deg
224.2(2)
273.0(2)
198.0(3)
204.4(2)
132.6(7)
149.0(3)
146.5(11)
max/deg
μ-CN−
CO
Br−
c
CO
MeOH
μ-OAc−
μ-OAc−
nBu3P
μ-CN−
c
CO
μ-OAc−
EtOAc
MeOH
nBu3P
a
b
b
angle
Lmin
Lmax
d
N−Mo−N
N−Mo−L
71.0(1)
76.7(1)
93.6(2)
91.7(1)
78.2(2)
116.2(2)
119.9(1)
72.4(1)
110.2(2)
103.2(1)
102.1(1)
99.8(1)
CO
EtOAc
nBu3P
EtOAc
THF
N−Mo−Ctrans
N−Mo−Ccis
L−Mo−Ctrans
Mo−N−Cimine
Mo−N−Cipso
MeCN
μ-CN−
EtOAc
tBuNC
Me2CO
d
CO
nBu3P
120.2(2)
125.8(4)
DMSO
c
CO
a
Notation: N, imine nitrogen; L, coligand (regardless of charge); Ccis/
Ctrans, carbonyl carbon in cis/trans position to DAD. Coligand that
corresponds to the structure with smallest (Lmin) or largest (Lmax
b
)
c
d
value observed. First modification. Second modification.
It is evident that the various complexes differ significantly in
their geometric arrangement directly at the molybdenum atom,
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Figure 2. Molecular structures of complexes with weak (upper row) and strong (middle row) deviation from octahedral coordination; dinuclear
complexes (lower row): (a) L = CO (1), (b) X− = Br− (15), (c) L = THF (4), (d) L = DMSO (5), (e) L = nBu3P (13), (f) L = EtOAc (8),
(g) μ-X− = μ-OAc− (22), (h) μ-X− = μ-CN− (19).
strongest effect on the angle N−Mo−L (MeCN, μ-OAc−,
DMF, DMSO, MeOH, Me2CO, and EtOAc) as well as on Δδ
(MeCN, μ-OAc−, DMF, and DMSO) all have π* orbitals that
point toward the CN double bonds (cf. Figure 2). The single
exception to this rule is methanol, which features an acidic, i.e.,
electron-poor, proton instead of π* orbitals in the relevant
orientation. Because of the short interligand distance, a direct
transfer of the electron density from the DAD π orbitals to the
coligand seems likely. In other words, an attractive ligand−ligand
interaction is proposed that results in the partial displacement of
the tricarbonyl complex fragment’s back-bonding capacity from
the metal to the DAD ligand. A schematic representation of the
proposed interaction is shown in Figure 4.
To the best of the authors’ knowledge, such an interaction has
not been observed for DAD complexes before. In the solid-state
structure of [Mo(phen)(CO)3(SO2)], Ryan and co-workers20
found the carbonyl ligand in a trans position to sulfur dioxide to
be tilted away from phen, resulting in a distortion similar to that in
the current investigation. They also postulate this effect to be
inherent in such fac-tricarbonyl complexes but do not present
further details concerning its origin. The high variability of the
Figure 3. Correlation of the average bond angle N−Mo−L (N−Mo−
X for anionic complexes) with Δδ = δ(Cimine) − δ(Cipso) for
tricarbonyl complexes [Mo(DAD)(CO)3(L/X)]0/−, as observed by
13C NMR.
coligand−molybdenum−carbonyl axis twist leads to the conclusion
that the different coligands exert a major influence on this effect.
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Article
be a quite interesting class of complexes because the observed
DAD−coligand secondary interaction could be useful for activation
of the CO double bonds in catalytic applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, listings of known
related compounds, full experimental detail and character-
ization of compounds, details on UV/vis measurements,
ORTEP-style plots, and NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jsu@staff.uni-marburg.de. Tel: +49 (0)6421 28-25693.
Fax: +49 (0)6421 28-25711.
■
Figure 4. Main observed distortion of the tricarbonyl complexes (left)
and the proposed orbital overlap (right) both shown for coligand O
CR2 as a typical example.
Notes
However, the donor character of DAD π orbitals has been
demonstrated by side-on metal complexes in which six-electron
or even eight-electron donation occurs.28 The geometric feasibility
of the postulated π(DAD)−π*(L) orbital overlap is suggested by
studies of Stufkens and co-workers7b attributing increased intensity
of ν(COcis) in resonance Raman experiments to a geometrically
similar π*(DAD)−π*(COcis) overlap.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Routine data collection was performed by the XRD service
department (Drs. K. Harms, G. Geiseler, M. Marsch, and
R. Riedel) of the Chemistry Department, Philipps University,
and is gratefully acknowledged.
Interestingly, also the opposite direction of electron transfer,
i.e., donation from coligand orbitals into DAD π* orbitals, has been
discussed in terms of theoretical calculations on metal-mediated
ester hydrolysis.29 While the model complex [Mo(HNCHCH
NH)(CO)2(η3-C3H5)(OH)] investigated there features a very
electron-poor DAD and the electron-rich, hard coligand hydroxide,
the complexes studied here feature an electron-rich DAD because of
methyl substitution at the imine carbon atoms. Together with
comparably electron-poor coligands like acetone or ethyl acetate,
this seemingly leads to an inversion of the respective orbital
energies, as suggested by NMR measurements (vide supra).
The failure of the solvatochromic measurements of donor
solvents L using in situ generated tricarbonyl complexes [Mo-
(DAD)(CO)3L] can be explained by these findings: Variations in
the donor/acceptor capabilities of the coligand do change not only
the electronic situation at the metal center but also the complex
geometry and most likely the strength of ligand−ligand interactions.
These effects are dependent on the availability of donor and
acceptor orbitals at the solvent’s donor functionality. Therefore, a
simple correlation with the Kamlet−Taft parameters α, β, or π* is
not possible.
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