Fine Substituent Effects in Sandwich Complexes: A Threshold Ionization Study of Monosubstituted Chromium Bisarene Compounds

Article abstract of DOI:10.1002/chem.201505039

Mass-analyzed threshold ionization spectra of jet-cooled [(η⁶-PhMe)(η⁶-PhH)Cr] and [(η⁶-Ph₂)(η⁶-PhH)Cr] reveal with unprecedented accuracy the effects of methyl and phenyl groups on the electronic structure of bis(η⁶-benzene)chromium. These "pure" substituent effects allow quantitative experimental determination of the ionization energy changes caused by the mutual substituent influence in bisarene systems. Two types of such influence have been revealed for the first time in bis(η⁶-toluene)chromium.

Full text of DOI:10.1002/chem.201505039

DOI: 10.1002/chem.201505039
Communication
&
Substituent Effects |Hot Paper|
Fine Substituent Effects in Sandwich Complexes: A Threshold
Ionization Study of Monosubstituted Chromium Bisarene
Compounds
Sergey Yu. Ketkov,*[a] Gennady V. Markin,[a] Sheng Y. Tzeng,[b] and Wen B. Tzeng*[b]
Chem. Eur. J. 2016, 22, 4690 – 4694
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fies essentially the one-photon MATI or ZEKE spectrum of 1,
which reveals a strong origin and a few weaker vibronic com-
ponents corresponding mainly to the metal–ligand symmetric
stretch of the 1+ ion.[9c,e] The introduction of alkyl groups into
the benzene rings of 1 leads, however, to much more compli-
cated spectra due to the presence of rotational isomers with
different ionization energies.[9f,i,j] Such isomers as well as the
mutual influence of substituents are avoided when dealing
with the unsymmetrical (h6-PhR)( h6-PhH)Cr systems. We report
herein the first MATI spectra of such compounds with R=Me
(2) and R=Ph (3). Comparison of the spectra of 1 and 2 pro-
vides a precise value of the shift in I caused by a single methyl
group. It then becomes possible to separate the substituent
and Me–Me influences in polymethylated bisarene complexes.
The high-resolution study of the phenyl group effect on I of
1 is of special interest, since the [(h6-Ph2)2Cr]0/+1 redox couple
is recommended by IUPAC as a reference electrode for non-
aqueous electrochemical measurements.[11] Surprisingly, no
gas-phase photoelectron spectra of derivatives of 1 bearing Ph
groups have, to our knowledge, been reported to date. Analy-
sis of high-resolution ionization parameters of 2 and 3 mea-
sured in this work demonstrate that MATI spectroscopy, sup-
ported by DFT calculations, provides a unique instrument for
the investigation of fine substituent effects in sandwich com-
plexes.
Abstract: Mass-analyzed threshold ionization spectra of
jet-cooled [(h6-PhMe)(h6-PhH)Cr] and [(h6-Ph2)(h6-PhH)Cr]
reveal with unprecedented accuracy the effects of methyl
and phenyl groups on the electronic structure of bis(h6-
benzene)chromium. These “pure” substituent effects allow
quantitative experimental determination of the ionization
energy changes caused by the mutual substituent influ-
ence in bisarene systems. Two types of such influence
have been revealed for the first time in bis(h6-toluene)-
chromium.
Since the discovery of bis(h6-benzene)chromium (1) 60 years
ago,[1] transition metal arene complexes, like (or similar to)
metallocenes, have formed one of the most interesting and in-
triguing families of organometallic systems. During the last few
decades, they have played key roles in both fundamental and
applied chemistry because of their relevance for theoretical
and experimental studies of metal–ligand bonding,[2] organic
synthesis,[3] catalysis,[4] metal-containing polymers,[5] biomed-
icine,[6] and nanoelectronics.[7] The chemical and biochemical
reactivity of metal sandwich compounds, as well as the proper-
ties of sandwich-based materials, depend dramatically on the
redox parameters of these organometallic systems,[8] which can
be tuned by the introduction of substituents to the carbocy-
cles. Qualitatively new possibilities for investigating substituent
electronic effects in sandwich molecules have been elucidated
by adjusting high-resolution mass-analyzed threshold ioniza-
tion (MATI) and zero kinetic energy photoelectron (ZEKE) spec-
troscopies to study bisarene compounds[9] and metallocenes[10]
cooled in a supersonic jet. These techniques provide extremely
accurate ionization energies (I) of neutral species and vibra-
tional frequencies of gas-phase ions.
Similar to the spectrum of 1, the MATI spectra of 2 and 3
(Figure 1) appear to reveal an intense origin and a few weaker
vibronic features.[9e,g] The precise ionization energies of the
compounds determined from our high-resolution experiments
However, the sandwich compounds studied earlier con-
tained several substituent groups so it was impossible to sepa-
rate the “pure” influence of the substituents from that caused
by interactions between them. Monosubstituted derivatives of
1 represent unique models to study the “pure” substituent
electronic effects in sandwich complexes. The ground electron-
ic state of both the neutral and the cationic form of 1 is non-
2
degenerate [1A1g:..(e2g)2(a1g)2 and A1g:..(e2g)2(a1g)1, respectively,
in the D6h point group] so there is no splitting resulting from
the symmetry reduction on going to its substituted derivatives.
In contrast to the Ti and V analogues,[9h] the threshold ioniza-
tion of 1 involves a totally symmetric nonbonding MO and the
1+ ion has no low-lying high-spin electronic states. This simpli-
[a] Prof. Dr. S. Y. Ketkov, Dr. G. V. Markin
G. A. Razuvaev Institute of Organometallic Chemistry, RAS
Tropinin St. 49, GSP-445, Nizhny Novgorod 603950 (Russian Federation)
E-mail: sketkov@iomc.ras.ru
[b] S. Y. Tzeng, Prof. Dr. W. B. Tzeng
Institute of Atomic and Molecular Sciences, Academia Sinica
1 Section 4, Roosevelt Road, Taipei 6017 (Taiwan)
E-mail: wbt@sinica.edu.tw
Figure 1. Experimental (top) and DFT-simulated (bottom)MATI spectra of (h6-
toluene)( h6-benzene)chromium and (h6-biphenyl)( h6-benzene)chromium.
The magnified MATI vibronic part is shown for the latter complex. The zero
wavenumber corresponds to the origin positions in the spectra.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505039.
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are 43392Æ5 and 43374Æ8 cmÀ1 (5.3800Æ0.0006 and
5.3777Æ0.0010 eV), respectively. The vibronic structures ob-
served for 2 and 3 differ, however, from that for the unsubsti-
tuted complex.[9c,e] The relative positions and intensities of the
vibronic components are reproduced very well by our DFT cal-
culations[12] of the vibrational frequencies and Franck–Condon
factors at the B3PW91/6-311+ +G(d,p) level of theory
(Figure 1; the peak positions and assignments are given in the
Supporting Information, Table S1). In addition to the symmetric
metal–ligand stretch n1, accompanying the origin in the 1 spec-
tra,[9e,g] the spectrum of 2 (Figure 1) reveals a rather strong
peak corresponding to skeletal bending mode n2 (the observed
frequency 198 cmÀ1) in the 2+ ion. For phenylated derivative
3, the vibronic components corresponding to low-frequency
skeletal modes n3 and n4 appear in the experimental spectrum
in agreement with the DFT predictions (Figure 1 and Table S1).
A very weak n3 component is also revealed in the calculated
MATI structure of 2. The n2 and n3 vibrations are related to the
e1u skeletal bend in 1+ (135 cmÀ1).[9g] The n4 mode in 3+ in-
volves mainly the uncomplexed phenyl ring. A very weak com-
ponent with a similar Dncalc. value (69 cmÀ1) is predicted for 2+
(Figure 1, Table S1). However, in contrast to 3, this feature cor-
responds to the excitation of two quanta of ligand torsion
mode n5 in 2+. The changes in the MATI vibronic structure on
going from 2 to 3 reflect, therefore, the substituent difference.
The calculated n1–n5 atomic vibrational displacements for the
2+ and 3+ ions are given in the Supporting Information (Fig-
ure S2).
Figure 2. Expected (“pure” effect of two Me groups) and observed[9i,j] (effect
including the mutual substituent influence) experimental ionization energies
of [(h6-PhMe)2Cr].
Type 1 effect is shown by the 608, 1208 and 1808 rotamers, in
which the Me groups are well separated from each other (the
closest calculated H(Me)···H(Me) interligand distance in the 608
rotamer of 4 molecule is 3.214 Š). This type of MeÀMe influ-
ence arises from changes in the electronic structure of com-
plex 1 on introduction of one electron-donating Me group.
These changes prevent the second Me substituent from shift-
ing the electron density and decreasing I. The contribution of
the Type 1 effect to the experimentally observed change in I is
52Æ5 cmÀ1 (0.62Æ0.06 kJmolÀ1). The Type 1 MeÀMe influence
is present in any 4 rotamer.
The presence of a single Me or Ph group in the sandwich
complexes studied provides corresponding I values that reflect
the “pure” substituent effects in bisarene systems. The precise
changes in I observed on going from 1[9e,g] to 2 and 3 were de-
termined in this work to be DI(2)=À698Æ5 cmÀ1 and DI(3)=
À716Æ8 cmÀ1, respectively. The B3PW91/6-311+ +G(d,p) cal-
culations taking into account the electronic and zero-point vi-
brational energies overestimate slightly DI (À783 and
À810 cmÀ1, respectively). Computations based solely on the
electronic energies give very similar DI values (À780 and
À788 cmÀ1, respectively) indicating the electronic (non-vibra-
tional) nature of the substituent effects in 2 and 3.
The data obtained in this work make it possible for the first
time to extract the experimental contributions of the mutual
substituent influence to the changes in I in polysubstituted
sandwiches. The [(h6-PhMe)2Cr] (4) system represents a conven-
ient model to demonstrate such a possibility. The MATI[9f,j] and
ZEKE[9i] spectra of 4 reveal close-lying origins at 42746 cmÀ1,
corresponding to three rotational isomers with the 608, 1208,
and 1808 angles between the CringÀCMe bonds, and a separate
peak at 42809 cmÀ1 arising from the 08 rotamer (eclipsed Me
positions). The ionization energy of predicted on the basis of
the doubled DI(2) value (1396 cmÀ1) is 42694 cmÀ1. This
energy can be taken as that resulting from the “pure” effect of
two Me groups. The shifts of I(4) from this value to higher en-
ergies arise from the mutual influence of the substituents
which decreases jDI(4)j. Two types of such MeÀMe influence
in bisarene complexes appear to be evident and are quantita-
tively described by the experimental I(4) shifts (Figure 2). The
In the 08 rotamer of 4, an additional effect (Type 2 MeÀMe
influence) appears, resulting from the direct interaction be-
tween the closely located Me groups.[9f] This effect contributes
63Æ5 cmÀ1 (0.76Æ0.06 kJmolÀ1) to the change in I, providing
a total DIMeÀMe value of 115 cmÀ1 (1.38 kJmolÀ1) for this rota-
tional isomer (Figure 2). Comparison with the predicted
change in I on going from 2 to 4 (À698 cmÀ1) demonstrates
that the MeÀMe mutual influence reduces the jDIj value by
7.4% for the 608, 1208, and 1808 rotational isomers and by
16.4% for the 08 rotamer of [(h6-PhMe)2Cr].
DFT calculations allow a study of the methylation effects in
bisarene systems separately for the neutral and cationic spe-
cies. The B3PW91/6-311+ +G(d,p) electronic energy changes
DEel (see the Supporting Information, Table S2) in reactions of
the substitution of H with Me [Eqs. (1) and (2)] are, respectively,
54.7 and 45.4 kJmolÀ1 demonstrating a stronger “pure” influ-
ence of the substituent in the neutral sandwich molecule. For
the 1208 rotamer of 4 [Eqs. (3) and (4)], the corresponding DEel
values are 55.4 and 46.5 kJmolÀ1, respectively. The DEel in-
crease on going from 2 to 4 is a result of the Type 1 MeÀMe
influence. Note that, in contrast to the “pure” Me influence,
this effect is stronger in the cation (Figure 3), providing a theo-
retical DIMeÀMe value of 0.4 kJmolÀ1 which is comparable with
the experimentally obtained value DIMeÀMe =0.62 kJmolÀ1
(Figure 2).
½ðh6-PhHÞ2CrŠ þ MeÁ ! ½ðh6-PhMeÞðh6-PhHÞCrŠ þ HÁ
½ðh6-PhHÞ2CrŠþ þ MeÁ ! ½ðh6-PhMeÞðh6-PhHÞCrŠþ þ HÁ
ð1Þ
ð2Þ
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volving the Ph groups [Eq. (5): DEel =13.7 kJmolÀ1; Eq. (6):
DEel =4.2 kJmolÀ1] are much lower than those for methylation
of 1 [Eqs. (1) and (2), respectively]. The DEel values correspond-
ing to Eqs. (1)–(6) are related to the differences in the Cring
À
Csubstituent and CringÀH bond dissociation energies DEdiss. Our cal-
culations confirm, therefore, that the CringÀCPh bond is stronger
than CringÀCMe in both neutral and ionic bisarene systems. The
computed DEdiss (CringÀCsubstituent) values for replacing Me with
Ph on going from 2 to 3 and from 2+ to 3+ are, respectively,
41.0 and 41.2 kJmolÀ1. These energy differences are in accord-
ance with DEdiss =51.8 kJmolÀ1, derived from thermodynamic
parameters for the PhMe and Ph2 free ligands.[16
Figure 3. B3PW91/6-311+ +G(d,p) relative electronic energies of the 08and
1208 rotamers of (h6-PhMe)2Cr. The Erel =0 values correspond to the energies
of the neutral and cation expected on the basis of the calculations for [(h6-
PhH)2Cr] and [(h6-PhMe)(h6-PhH)Cr] (“pure” effect of two methyl groups with
no MeÀMe influence, DEMeÀMe =0). The “observed” energies are those com-
puted directly for the [(h6-PhMe)2Cr] systems.
½ðh6-PhHÞ2CrŠ þ PhÁ ! ðh6-Ph2Þðh6-PhHÞCr þ HÁ
½ðh6-PhHÞ2CrŠþ þ PhÁ ! ½ðh6-Ph2Þðh6-PhHÞCrŠþ þ HÁ
ð5Þ
ð6Þ
The “classical” UV PE spectra of 1,1’-dimethylferrocene and
1,1’-diphenylferrocene are also indicative of similar Me and Ph
substituent effects on the ionization energy of sandwich mole-
cules.[15,17] However, low accuracy in the determination of I
(even for parent ferrocene, the independently derived values
of I[15,17] differ by 0.05 eV or 400 cmÀ1) provides no chance to
analyze the substituent influence in details. In contrast, the
high-resolution ZEKE spectroscopy reveals a decrease in I of
2486 cmÀ1 on phenylation of the scandium monobenzene
complex [(h6-PhH)Sc],[18] whereas the substitution of benzene
with toluene[19] in this compound changes I by only 798 cmÀ1.
However, the substituent influence in the scandium monoar-
ene derivatives is complicated by changes in the ligand geom-
etry and ground-state spin multiplicity. On methylation, the
carbocycle is bent,[19] whereas a clamshell structure is formed
on phenylation of [(h6-PhH)Sc].[18] This makes comparison be-
tween the Me- and Ph-induced shifts of I incorrect for these
systems. In contrast, bisarene chromium compounds represent
convenient models for the investigation of fine substituent ef-
fects in sandwich complexes, due to the absence of significant
metal–ligand structural and electronic distortions in substitut-
ed derivatives of 1.
½ðh6-PhMeÞðh6-PhHÞCrŠ þ MeÁ ! ½ðh6-PhMeÞ2CrŠ þ HÁ
½ðh6-PhMeÞðh6-PhHÞCrŠþ þ MeÁ ! ½ðh6-PhMeÞ2CrŠþ þ HÁ
ð3Þ
ð4Þ
On going from 2 to the 08 rotamer of 4, the DEel values in-
crease to 56.1 and 47.9 kJmolÀ1 for the neutral and cation, re-
spectively. These parameters determine the Type 2 MeÀMe ef-
fects as 0.7 and 1.4 kJmolÀ1 in the neutral and ionic species, re-
spectively, and the corresponding DIMeÀMe magnitude as
0.7 kJmolÀ1, which is very close to the MATI experimental
value of 0.76 kJmolÀ1 (Figure 2). The Type 2 effect is also stron-
ger for the cation. Interestingly, the B3PW91/6-311+ +G(d,p)
calculations of the [(h6-1,3-Me2C6H4)(h6-PhH)Cr] system provide
DEel values (55.2 and 46.4 kJmolÀ1 for the neutral and cation,
respectively) that practically coincide with those for the 1208
rotamer of 4 (see the Supporting Information, Table S1). This
result is in an excellent agreement with the MATI data,[9f] re-
vealing no difference between the I values of the complexes,
which is indicative of close similarity in the intra- and interli-
gand Type 1 MeÀMe influence.
According to electrochemical data,[13] the 10/+ redox poten-
tial is shifted to more negative values on introduction of Me
groups to the ring, whereas an opposite trend is observed
with the phenyl substituents. Taking into account the linear
correlation between the solution E1/2 and gas-phase I values,[14]
one should then expect an increase in I on going from 1 to 3.
The high-resolution MATI experiments performed in this work
demonstrated clearly that this is not actually the case. The
“pure” effects of the phenyl and methyl groups on the I(1)
value appear to be very similar (DI are À716 and À698 cmÀ1 or
À8.57 and À8.35 kJmolÀ1 for complexes 3 and 2, respectively).
Our MATI data agree with a decrease in the metal d orbital I
values observed on going from the low-resolution UV photo-
electron (PE) spectrum of ferrocene (6.86 and 7.25 eV) to that
of phenylferrocene (6.72 and 7.10 eV).[15]
In conclusion, we have demonstrated that DFT-supported
MATI spectroscopy can serve as a powerful instrument for
studying in detail the influence of substituents on the electron-
ic structures of bisarene sandwich systems. Owing to the ex-
tremely high resolution, the MATI technique provides accurate
shifts of the ionization energies on substitution. When em-
ployed for monosubstituted derivatives of bis(h6-benzene)chro-
mium, this method provides quantitative parameters of “pure”
substituent electronic effects, whereas for polysubstituted
sandwich complexes, it becomes possible to reveal experimen-
tally and describe theoretically various types of mutual sub-
stituent influence.
Experimental Section
DFT analysis (see the Supporting Information, Figure S3)
shows that, in spite of close DI values for complexes 2 and 3,
the electronic energy changes in the substitution reactions in-
All manipulations with organometallics were carried out in vacuum
or under nitrogen atmosphere. The MATI spectra were measured
in this work with use of a spectrometer and techniques described
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in detail elsewhere.[20] The samples of the bisarene complexes at
140–1908C seeded in the He carrier gas at 1.4 bar were expanded
through a heated pulsed nozzle and were selected by two skimm-
ers into the vacuum ionization chamber. The ions produced on ir-
radiation of the molecular beam with nanosecond laser pulses
were analyzed by using a time-of-flight mass spectrometer. To
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2 and 3, peaks at m/z=
222.2 (C13H14Cr+) and m/z=284.3 (C18H16Cr+), respectively, were se-
lected in the photoionization mass spectra, when tuning the laser
wavelength. The signal/noise ratio was approximately 4 for the
weakest MATI vibronic features (Figure 1). Complex 2 was prepared
by co-condensation of the benzene/toluene mixture (Aldrich, 99%)
with chromium atoms at liquid nitrogen temperature in the appa-
ratus described previously.[21] Compound 3 was synthesized by the
Fischer–Hafner method.[1,22] Experimental and computational de-
tails are given in Supporting Information.
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Acknowledgements
This work was jointly supported by grants from the Russian
Foundation for Basic Research and the Ministry of Science and
Technology of Taiwan (RFBR Project No. 15-53-52008 and
MOST Grant No. 104-2923M-001-002-MY3).
Keywords: density-functional calculations
sandwich complexes · substituent effects
·
ionization
·
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Received: December 14, 2015
Published online on January 28, 2016
Chem. Eur. J. 2016, 22, 4690 – 4694
www.chemeurj.org
4694
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