A Quantitative Description of the σ-Donor and π-Acceptor Properties of Substituted Phenanthrolines

Article abstract of DOI:10.1002/ejic.201600647

The bond between molybdenum and substituted 1,10-phenanthroline ligands in a series of [Mo(CO)₄(phen*)] complexes has been studied by combining experimental data (ν_CO) with DFT calculations. First, natural orbitals for chemical valence (NOCV) were calculated: The resulting charge-transfer magnitudes (Δq_i) associated with the deformation density channels (Δ?_i) were related to σ-donation and π-back-donation. Then, energy decomposition analysis was performed by applying the extended transition state (ETS) scheme. The outcomes of the ETS-NOCV approach has allowed us to quantify the energetic contribution of both ligand-to-metal (E_σ) and metal-to-ligand (E_π) interactions. A new parameter (T^phen) has been introduced comprising both E_σand E_πand thus providing a descriptor for the overall electronic contribution given by phenanthrolines to the metal–ligand bond. This was corroborated by the linear correlation found between T^phenand the ν_COvibration modes of [Mo(CO)₄(phen*)] complexes, at least for those containing a 2,9-unsubstituted phenanthroline. The case of [Mo(CO)₄(phen*)] derivatives with a 2,9-substituted phen* is also discussed.

Full text of DOI:10.1002/ejic.201600647

DOI: 10.1002/ejic.201600647
Full Paper
Substituted Phenanthrolines
A Quantitative Description of the σ-Donor and π-Acceptor
Properties of Substituted Phenanthrolines
G. Attilio Ardizzoia, Michela Bea, Stefano Brenna* and Bruno Therrien
[a]
[a]
[a]
[b]
Abstract: The bond between molybdenum and substituted getic contribution of both ligand-to-metal (E ) and metal-to-
σ
phen
1
,10-phenanthroline ligands in a series of [Mo(CO) (phen*)] ligand (E ) interactions. A new parameter (T
) has been intro-
4
π
complexes has been studied by combining experimental data duced comprising both E and E and thus providing a descrip-
σ
π
(
νCO) with DFT calculations. First, natural orbitals for chemical tor for the overall electronic contribution given by phenanthrol-
valence (NOCV) were calculated: The resulting charge-transfer ines to the metal–ligand bond. This was corroborated by the
phen
magnitudes (Δq ) associated with the deformation density linear correlation found between T
and the νCO vibration
i
channels (Δρ ) were related to σ-donation and π-back-donation. modes of [Mo(CO) (phen*)] complexes, at least for those con-
i
4
Then, energy decomposition analysis was performed by apply- taining
a
2,9-unsubstituted phenanthroline. The case of
ing the extended transition state (ETS) scheme. The outcomes [Mo(CO) (phen*)] derivatives with a 2,9-substituted phen* is
4
of the ETS-NOCV approach has allowed us to quantify the ener- also discussed.
Introduction
the ETS-NOCV approach has emerged as a very precious tool
for shedding light on the nature of metal-to-ligand interactions,
Understanding the nature of metal–ligand bonds in transition-
metal complexes is of crucial importance in organometallic
[
12–31]
as documented in the different cases studied.
In the
present study, we applied ETS-NOCV to a series of [Mo(CO)4-
[
1]
[2]
[3]
chemistry and hence both experimental and theoretical
(phen*)] complexes (phen* = substituted 1,10-phenanthroline)
studies have been performed in this area over the last decades.
Different theoretical approaches have been applied to the de-
scription of the donor–acceptor properties of ligands in transi-
tion-metal complexes, for example, interaction-energy parti-
with the purpose of describing and quantifying the σ-donor
and π-acceptor properties of phen* depending on their substi-
tution. Among others, 1,10-phenanthrolines form a ubiquitous,
[
32]
significant class of ligands that find application as fluorescent
[
4]
[5]
tioning schemes, charge decomposition analysis (CDA), con-
[
33]
[34]
receptors, antennae in luminescent metal complexes, and
in the synthesis of highly emitting phosphorescent com-
[
6]
strained space orbital variation (CSOV), natural bond orbital
[
7]
[8]
(
NBO), molecular electrostatic potential (MEP), and quantita-
[
35]
pounds,
to name but a few. In particular, [M(CO) (phen*)]
[
9]
4
tive analysis of ligand effect (QALE). More recently, Mitoraj
and co-workers introduced natural orbitals for chemical valence
compounds (M = Cr, Mo, W) have been the subject of extensive
[
36]
reports dealing with their electrochemical behavior,
photo-
[
10]
(
NOCV),
a singular approach that gives an accurate descrip-
[
37]
[38]
physical properties,
photochemical reactivity,
and use as
due to their
tion of bonding in terms of only a few orbitals localized in the
bonding region. In addition, this method allows for the separa-
tion of the contributions to the deformation density (Δρ) aris-
ing from the ligand-to-metal and metal-to-ligand electron-
transfer processes, thus being closely related to the Dewar–
Chatt–Duncanson model usually applied to organometallic
compounds. Furthermore, when NOCV is associated with an en-
ergy decomposition analysis scheme [in particular with the ex-
tended transition state (ETS) method, initially developed by
[
39]
[37c]
catalysts.
As highlighted by Farrell et al.,
structural simplicity and the presence of CO ligands, the IR
bands of which are sensitive markers for electron-density distri-
bution, Group 6 metal carbonyls are an excellent model for
studying how ligand substituents affect the physicochemical
properties of compounds. Indeed, in a detailed study on
[
M(CO) (N,N)] complexes (M = Cr, W; N,N = 1,10-phenanthroline,
4
3
,4,7,8-tetramethyl-1,10-phenanthroline), they performed TD-
DFT calculations to estimate the influence of the substitution
of phenanthrolines on electronic spectra. Herein, we report the
description of the bond between molybdenum and 1,10-phen-
anthrolines in terms of the σ-donor (E ) and π-acceptor (E )
[
11]
Ziegler and Rauk ], it is possible to give a quantitative (i.e.,
energetic) description of the σ-donor (E ) and π-acceptor (E )
contributions to the metal–ligand bond. Thus, in the last years,
σ
π
σ
π
contributions of the nitrogen heterocycle. Applying the ETS-
NOCV model to several [Mo(CO) (phen*)] systems, it has been
[
[
a] Department of Science and High Technology, University of Insubria,
Via Valleggio, 9 22100 Como, Italy
E-mail: stefano.brenna@uninsubria.it
http://www.uninsubria.it/docenti/stefano.brenna
b] Institute of Chemistry, University of Neuchâtel,
4
possible to quantify these contributions and to correlate E and
σ
Eπ of the different phenanthrolines with experimental values of
carbonyl stretching vibrations (νCO) obtained for the corre-
Avenue de Bellevaux 51, 2000 Neuchâtel, Switzerland
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600647.
phen
sponding complexes in solution. A new parameter, T
, which
simultaneously takes into account both σ-donation (E ) and π-
σ
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
back-bonding (E ) along the metal–phen* bond, has been de-
π
phen
fined, and a linear correlation between T
and the experi-
mental νCO frequencies has been demonstrated.
Results and Discussion
Syntheses
The 1,10-phenanthrolines used in this study (Figure 1) differ in
the nature of the substituents (H, CH , Ph, NO , NH , Cl, Br, CN)
3
2
2
or in their position on the phenanthroline backbone. Most of
[
40]
phen* derivatives were purchased, but 5-NO -phen
and 5-
were prepared according to procedures reported
in the literature. The complexes [Mo(CO) (phen*)] (1–16) were
2
[
33]
NH -phen
2
4
Figure 2. IR spectrum of [Mo(CO) (phen)] (1) in dichloromethane, with assign-
4
[
41]
synthesized
by heating in toluene molybdenum hexacarb-
ment of vibrational bands.
onyl with 1 equivalent of ligand (Scheme 1). The reactions were
easily monitored by IR spectroscopy, in which the characteristic
Table 1. IR frequencies recorded for compounds 1–16 in dichloromethane.
–1
band of [Mo(CO) ] at 1980 cm was replaced by four CO
6
CO [cm–1
]
phen*
ν
stretching vibrations (ν ), characteristic of a cis-[Mo(CO) (L) ]
CO
4
2
A1ax
B1
A1eq
B2
[
37c]
system
(Figure 2). The values of the A1ax, B , A1eq, and B2
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
phen
5-NO -phen
2
2015.1
2017.4
2014.1
2013.9
2013.7
2013.1
2014.2
2014.7
2016.3
2016.2
2017.0
2018.0
2019.3
2016.9
2019.0
2018.8
1905.3
1910.6
1904.0
1904.7
1902.2
1900.5
1904.1
1904.9
1907.8
1907.8
1909.7
1912.2
1915.8
1904.9
1909.6
1913.8
1878.0
1884.4
1877.2
1877.2
1874.2
1872.4
1876.4
1877.3
1881.0
1880.9
1883.1
1886.6
1891.3
1875.3
1887.6
1894.8
1831.3
1837.3
1830.1
1830.2
1827.8
1825.3
1830.0
1830.4
1834.2
1834.1
1836.0
1839.5
1844.4
1825.3
1839.1
1846.8
modes of compounds 1–16, recorded in dichloromethane solu-
tion, are collected in Table 1.
5-NH -phen
2
bathophen
4,7-Me
tmphen
5,6-Me
5-Me-phen
5-Cl-phen
5-Br-phen
2
-phen
2
-phen
0
1
2
3
4
5
6
4,7-Cl
3,8-Br
Br
2
-phen
-phen
2
4
-phen
cupr
2,9-Cl
2-CN-phen
2
-phen
sets of values (i.e., A1ax vs. B etc.) is observed, with the excep-
1
tion of species containing 2,(9)-substituted phen* ligands 14–
16, which repeatedly deviate from the lines in Figures S1–S3.
The odd behavior of compounds 14–16 can be reasonably as-
cribed to the steric hindrance induced by 2,9-substitution,
which is reflected by the greater deviation in those graphs in
which a vibration mainly associated with equatorial carbonyls
(
A1eq, B ) is involved. In view of this discrepancy, compounds
2
1
4–16 were first excluded from our study (and will be consid-
Figure 1. Substituted 1,10-phenanthrolines studied in this work, with the cor-
responding abbreviations used throughout the paper.
ered at the end of the discussion), applying the ETS-NOCV
method only on complexes 1–13.
Geometry Optimization
As stated in the Introduction, the ETS-NOCV approach gives a
quantitative description of the bonding in terms of only a few
orbitals localized in the metal–ligand bonding region, allowing
quantification of the σ-donor and π-acceptor properties of a
ligand. The first step in this method is the optimization of the
4
Scheme 1. Syntheses of [Mo(CO) (phen*)] complexes 1–16.
As can be seen from the data in Table 1, the electronic effect geometries of the complexes under investigation. In the
[
42]
imposed by ligand phen* is reflected in all the νCO vibrational present study we employed the hybrid functional PBE0,
modes. This is best explained in Figures S1–S3 in the Support- which previously proved to give computational results consist-
[
43]
ing Information: A good linear relationship among the different ent with the experimental data.
Taking into consideration
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the complex [Mo(CO) (phen)] (1) as a paradigmatic example of only quantitative information is given by the eigenvalues νk
4
the series 1–13, its molecular structure was optimized starting associated with Δρ , which are related to the magnitude of the
k
[
44]
from the X-ray data taken from the literature. The computed charge transfer. As a consequence, eigenvalues related to Δρ2
results showed good agreement with the experimental data and Δρ represent the flow of electron density associated with
3
(
Figure 3, left and Table S1 in the Supporting Information). Thus, σ-donation from phen* to molybdenum (their sum gives Δqσ
the same procedure was extended to the remaining complexes with a positive value) and eigenvalues related to Δρ and Δρ4
1
by using the X-ray crystal structure of 1 as the starting point to represent the flow of electron density associated with Mo–
optimize the geometries of 2–13. The results of the geometry phen* back-donation (their sum gives Δqπ with a negative
optimizations of all the complexes are reported in the Support- value). The remaining deformation densities and the associated
ing Information.
eigenvalues are related to the reorganization of electron den-
sity inside the fragments and are not directly coupled to bond
channels. A complete list of the charge-transfer values (Δq ) ob-
i
tained by NOCV calculations is reported in Table S2 in the Sup-
porting Information.
Figure 3. Left: Comparison between the molecular structure of
[
4
Mo(CO) (phen)] (1) obtained by X-ray diffraction analysis (red) and after
geometry optimization (green; RMSD = 0.1166). Right: Definition of the frag-
ments used to calculate the NOCV for complexes 1–13.
NOCV Calculations
The optimized structures were used to calculate the corre-
sponding NOCV, fragmenting the whole molecule into two sub-
systems, F1 (the {Mo(CO) } fragment) and F2 (the phen* ligand),
4
as illustrated in Figure 3 (right). All species showed similar fea-
tures as regards the description of the bond interaction be-
tween F1 and F2, and they will be discussed in relation to
k
Figure 4. Contours of deformation density channels Δρ describing the inter-
actions between fragments in [Mo(CO) (phen)] (1). Two different visualiza-
4
tions (orthogonal and parallel to the plane of the phen ligand) are depicted.
[
Mo(CO) (phen)] (1). The results of these calculations are NOCVs
4
Table 2 presents the Δq and Δq values for complexes 1–
3, together with the resulting Δq, which represents the
amount of the overall flow and is obtained by the algebraic
σ
π
that are defined as the eigenvectors that diagonalize the defor-
mation density matrix and are obtained as a pair of eigenfunc-
tions ꢀ±k corresponding to eigenvalues ν and ν with the
same absolute value but of opposite sign. As it is usually the
case, it is more convenient to visualize the deformation density
Δρ [Equation (1)] associated with each pair of eigenvalues ν .
1
k
–k
addition of Δq and Δq : A negative sign for Δq signifies a net
σ
π
electron density transfer from fragment F1 ({Mo(CO) }, Figure 3)
4
to fragment F2 ({phen*}), and thus denotes phen* ligands with
a prevalence for π-acidic behavior. Reported in the last column
k
±k
Δρ = –kꢀ + kꢀ2k
2
(1) of Table 2 are the values of Δq relative to 1,10-phenanthroline
k
–k
In doing so, it is easier to identify by visual inspection the
direction of the flow of electron density and especially which
orbital pairs correspond to donation and which to back-
4
Table 2. Charge-transfer magnitudes for complexes [Mo(CO) (phen*)] 1–13.
a]
[b]
Δq[c]
Δqphen[d]
phen*
phen
Δq [
σ
Δq
π
[
45]
bonding.
Pictorial representations of the deformation densi-
1
2
3
4
5
6
7
8
9
1
1
0.6874
0.6813
0.6915
0.6967
0.6898
0.6955
0.6915
0.6868
0.6848
0.6845
0.6826
0.6787
0.6786
–0.6613
–0.6901
–0.6571
–0.6645
–0.6263
–0.6187
–0.6516
–0.6522
–0.6706
–0.6679
–0.6804
–0.6892
–0.6994
0.0262
–0.0088
0.0345
0.0322
0.0635
0.0768
0.0400
0.0346
0.0142
0.0166
0.0023
–0.0105
–0.0209
1.00
–0.33
1.32
1.23
2.43
2.94
1.53
1.32
0.54
0.63
0.09
–0.40
–0.80
ties Δρ calculated for complex 1 are depicted In Figure 4 along
5-NO
5-NH
2
-phen
-phen
k
with the corresponding eigenvalues. For each of them, two rep-
resentations are shown, orthogonal and parallel to the plane of
the phen ligand. In particular, only the first four deformation
densities (with the leading eigenvalues) are associated with
2
bathophen
4,7-Me -phen
tmphen
5,6-Me -phen
5-Me-phen
2
2
bond channels (Δρ i = 1–4) and, as can be observed in Figure 4,
i
5-Cl-phen
5-Br-phen
Δρ and Δρ are related to the σ-donation from the phen li-
2
3
0
1
gand to the metal fragment and Δρ and Δρ represent metal-
1
4
4,7-Cl
3,8-Br
Br -phen
2
-phen
-phen
to-ligand π-back-donation (Δρ4 is better described as back-
donation along with polarization). The same sequence of bond
channels has been found for the other complexes of the series
bearing substituted phenanthrolines. At this level of theory, the
12
13
2
4
[
σ π
a] Δq represents σ-donation. [b] Δq represents π-back-donation. [c] Δq =
phen
Δq
σ
+ Δq
π
. [d] Δq
= Δq/0.0262.
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(
1
Δqphen), which has been arbitrarily assigned a value Δqphen
=
in terms of orbital (e.g., σ, π, δ) and electrostatic contributions,
phen
.00. The resulting Δq
values are in good agreement with ΔEprep can be ignored, focusing the attention merely on ΔEint.
the qualitative explanation offered by the Dewar–Chatt– For all the complexes, the largest contribution to ΔEint comes
Duncanson model: phen* with electron-rich substituents show from the attractive term ΔEelstat, with a ratio to ΔEorb equal to
phen
σ-donor ability (Δq
> 1) whereas phen* with electron- about 1.80, which means that the Mo–phen* bond has more
withdrawing groups are characterized by high π acidity electrostatic (64–65 %) than covalent (35–36 %) character
phen
phen
(
Δq
< 1). Interestingly, the plot of νCO (A1ax) versus Δq
(with the largest electrostatic contribution belonging to phen-
(
Figure 5) shows that all the considered compounds have a lin- anthrolines bearing electron-donating substituents in com-
ear fit, which means that when the σ-donation and π-back- plexes 3, 4, and 6).
donation properties of these ligands are considered simultane-
phen
Table 3. ETS-NOCV results describing the interactions between fragments F1
and F2 in the complexes [Mo(CO) (phen*)].
ously (in this case, in the Δq
parameter), their contribution
4
to the Mo–phen* bond can be quantitatively assessed.
phen*
ΔE [kcal/mol]
%Eorb
%Eelstat
ΔEint[a]
ΔEorb
ΔEPauli
ΔEelstat
1
2
3
4
5
6
7
8
9
phen
–59.70 –54.38 95.50 –100.82 35.00
–57.01 –54.05 95.04 –98.00 35.50
–60.50 –54.74 96.04 –101.80 35.00
–61.11 –55.53 96.64 –102.22 35.20
65.00
64.50
65.00
64.80
65.10
65.20
65.10
65.00
64.70
64.70
64.60
64.10
63.60
5-NO
5-NH
bathophen
4,7-Me
tmphen
5,6-Me -phen –60.74 –54.59 95.54 –101.69 34.90
2
-phen
-phen
2
2
-phen –60.92 –54.03 94.03 –100.92 34.90
–61.90 –55.13 96.67 –103.45 34.80
2
5-Me-phen
5-Cl-phen
5-Br-phen
–60.12 –53.84 93.85 –100.13 35.00
–58.52 –54.05 94.75
–58.54 –53.78 94.01
–57.97 –54.12 95.09
–55.71 –53.19 92.25
–54.28 –53.09 91.51
–99.22
–98.77
–98.93
–94.77
–92.71
35.30
35.30
35.40
35.90
36.40
1
0
1
1
4,7-Cl
3,8-Br
Br -phen
2
-phen
1
2
2
-phen
13
4
[
a] ΔEint = ΔEorb + ΔEPauli + ΔEelstat.
Figure 5. Plot of νCO (A1ax) vs. Δqphen for compounds 1–13.
Similarly, a general trend is also observed for the repulsive
term ΔEPauli in which higher values are associated with com-
pounds with electron-rich 1,10-phenanthrolines {i.e., 96.7 kcal/
mol for [Mo(CO) (tmphen)] (6)}, whereas lower values are found
for complexes with an electron-poor phen* {91.5 kcal/mol for
4
Energy Decomposition Analysis
Table 3 and Table 4 collect the results of the ETS analysis per- [Mo(CO)
formed on complexes 1–13, and all the energies (ΔE1–4) associ- are quite similar among the complexes, with a general preva-
ated with each bond channel (Δρ1–4) can be found in Table S3 lence for σ-donation (%E ≥ 57.5) compared with π-back dona-
(Br -phen)] (13). The σ and π contributions to ΔEorb
4
4
σ
in the Supporting Information. ΔEprep, which is the amount of tion. The latter increases as expected in the presence of elec-
energy required to promote the separated fragments F1 and tron-poor phen*.
F2 from their equilibrium geometry to the structure they will
assume in the final complex, has not been calculated. Indeed, Mo–phen* bond, and bearing in mind the mutual synergistic
if one is not interested in the total bond energies (ΔEtotal action of the σ and π contributions, we plotted both the values
Dbond), but only in describing the interfragment interactions of E and E calculated for each compound against the corre-
In the search for a possible quantitative description of the
=
–
σ
π
Table 4. ETS-NOCV results describing the interaction between fragments F1 and F2 in the complexes [Mo(CO)
4
(phen*)].
%E
phen*
ΔE or E [kcal/mol]
σ
%E
π
%ΔErest
ΔEorb
E
σ
E
π
ΔErest[a]
1
2
3
4
5
6
7
8
9
1
1
1
1
phen
–54.38
–54.05
–54.74
–55.53
–54.03
–55.13
–54.59
–53.84
–54.05
–53.78
–54.12
–53.19
–53.09
–31.67
–31.13
–31.91
–32.12
–31.70
–32.09
–31.86
–31.43
–31.41
–31.26
–31.24
–30.73
–30.50
–12.04
–12.53
–11.98
–12.36
–11.46
–11.72
–11.89
–11.81
–12.14
–12.06
–12.36
–12.36
–12.53
–10.67
–10.39
–10.85
–11.04
–10.86
–11.32
–10.84
–10.61
–10.50
–10.46
–10.52
–10.11
–10.06
58.20
57.60
58.30
57.80
58.70
58.20
58.40
58.40
58.10
58.10
57.70
57.80
57.50
22.10
23.20
21.90
22.20
21.20
21.30
21.80
21.90
22.50
22.40
22.80
23.20
23.60
19.60
19.20
19.80
19.90
20.10
20.50
19.80
19.70
19.40
19.40
19.40
19.00
19.00
5-NO
5-NH
2
-phen
-phen
2
bathophen
4,7-Me
tmphen
5,6-Me
5-Me-phen
5-Cl-phen
5-Br-phen
2
-phen
2
-phen
0
1
2
3
4,7-Cl
3,8-Br
Br -phen
2
-phen
2
-phen
4
[
a] ΔErest is the sum of the intrafragment polarizations due to minor internal rearrangements (each <2 kcal/mol).
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4

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sponding experimental νCO, first focusing our attention on the
A1ax vibrational mode. The result is a 3D graph (Figure 6) in
which data are fitted with a plane, the least-squares regression
of which leads to Equation (2) with a good correlation coeffi-
2
cient (adj. R = 0.9819).
νCO = 2081.51 + 2.882E – 2.048E
(2)
σ
π
Figure 7. Linear relationship between T(A1ax)phen and νCO (A1ax).
Table 5. Values of the comprehensive Tphen parameter for complexes 1–13.
phen*
Br -phen
3,8-Br -phen
-phen
-phen
5-Br-phen
5-Cl-phen
5-Me-phen
phen
Tphen
1
1
2
1
1
9
8
1
4
3
7
5
6
3
2
4
0.923
0.941
0.952
0.966
0.980
0.983
0.998
1.000
1.008
1.014
1.016
1.027
1.034
2
5-NO
4,7-Cl
2
1
0
2
bathophen
5-NH -phen
2
5,6-Me
4,7-Me
2
-phen
-phen
Figure 6. Plot of νCO (A1ax [cm–1]) vs. E
for compounds 1–13 (adj. R = 0.9819).
2
σ
and E
π
(both expressed in kcal/mol)
2
tmphen
Considering the negative (stabilizing) values of both E and
Eπ, according to Equation (2) a higher basic character (i.e.,
From the data reported in Table 5 it is evident that the Tphen
σ
parameter can be taken as a descriptor of the overall electronic
higher E ) of the phen* ligand leads to a decrease in νCO
,
phen
σ
properties of the phen* ligand: Values of T
< 1 are usually
whereas an augment in π-back-bonding (higher E ) causes an
π
associated with phen* with π-acceptor substituents, whereas
increase in the vibrational stretching value. Both these consider-
ations are in accord with experimental observations. Moreover,
from Equation (2), it is clear that the νCO stretching is influenced
more by σ-donor ability than by π-back-bonding. A better visu-
alization of this correlation can be obtained by including the Eσ
the presence of electron-rich substituents on phen* leads
to enhancement of its Eσ contribution and eventually to
phen
T
> 1. Moreover, this parameter shows excellent accord with
the experimental observations, as demonstrated by the good
phen
linear fits of T
with all four νCO modes measured for the
and E energies for each compound in the corresponding terms
π
corresponding complex in solution (Figure 8).
in Equation (2): A new parameter is generated, T(A1ax), which
simultaneously includes the two (σ and π) bond interactions
{
e.g., T(A1ax) = 2.882·(–31.67) – 2.048·(–12.04) = –66.61 kcal/mol
for compound [Mo(CO) (phen)] (1); see Table S4 in the Support-
4
ing Information}. As already done for Δq (see Figure 5), the
T(A1ax) parameter can be easily converted into the correspond-
phen
ing T(A1ax
)
by dividing it by the value of T(A1ax) for 1,10-
phenanthroline (–66.61 kcal/mol), once more taken as a refer-
phen
ence for the series. The plot of T(A1ax
)
versus νCO (A1ax) gives
an excellent linear relationship (Figure 7). Worthy of note, simi-
lar outcomes were obtained when this procedure was applied
to the remaining vibrational modes, thereby leading to the
phen
phen
phen
analogous parameters T(B1)
, T(A1eq
)
, and T(B2)
(see
Table S4). The four parameters show comparable values for
each complex, hence they can be averaged into a single, gen-
Figure 8. Linear relationship between Tphen and the four vibrational modes
phen
eral parameter T
(Table 5).
νCO in the series of complexes 1–13.
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5 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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The Case of 2,(9)-Substituted 1,10-Phenanthrolines
around the molybdenum, some relevant differences (reasonably
attributable to the greater steric hindrance of the methyl
groups at the 2- and 9-positions) can be noticed in the struc-
tural parameters of 14 when compared with those of 1 and 4
All the conclusions drawn on Tphen in the previous paragraph
are related to compounds containing 1,10-phenanthrolines
with hydrogen atoms at the 2- and 9-positions. [Mo(CO) (cupr)]
4
(Figure 10). First, the Mo–N distances are longer in 14 than in
(
(
14), [Mo(CO) (2,9-Cl -phen)] (15), and [Mo(CO) (2-CN-phen)]
16) were initially discarded from this dissertation due to their
4 2 4
1
and 4 (ca. 2.30 vs. 2.24 Å). Next, the axial C–Mo–C angle (α
in Figure 10) is very close to 180° in 14 [175.2(2)°], whereas in 1
and 4 these angles measure 167.6(2) and 170.4(8)°, respectively.
Concurrently, the equatorial C–Mo–C angle (ꢁ) in 14 is the most
acute among the series, being only 79.5(2)°, as compared with
odd behavior, outlined in Figures S1–S3 in the Supporting Infor-
mation. Deviation from the common behavior observed for
complexes 1–13 occurred in the geometry optimization of
complex 14. Indeed, intriguingly, the neocuproine-containing
9
3.2(1)° in 1 and 88.9(8)° in 4. In addition, in the crystal packing,
derivative [Mo(CO) (cupr)] (14) was the only complex to give a
4
the heterocyclic ring is involved in π–π interactions (3.34 Å)
with adjacent molecules, thus generating stacks of phen-
anthroline units along the a axis.
different outcome, with the heterocyclic ligand being bent from
the plane identified by Mo and the two equatorial CO ligands
(
see Figure S4). Similar results were also obtained by employing
[
46]
[47]
other functionals, such as B3LYP
or M06.
In contrast, ge-
ometry optimization of complexes 15 and 16 led to results
comparable to those obtained for species 1–13, without anom-
alous deformations in the molybdenum coordination sphere. To
shed light on this aspect, to determine whether the structure
of complex 14 really presents a “bent” phen or whether it is
simply a computational artefact, we decided to perform a sin-
gle-crystal X-ray structure analysis on complex 14. The ORTEP
plot of 14 is reported in Figure 9.
Figure 10. Main structural parameters in the octahedral cores of
(phen)] (1),[ [Mo(CO)
44]
(batho)] (4), and [Mo(CO) (cupr)] (14). Dis-
[48]
[
Mo(CO)
4
4
4
tances are in Å; the angles are as follows: α = 167.6(2)° (1), 170.4(8)° (4), and
1
75.2(2)° (14).
A new geometry optimization for compound 14 was per-
formed, starting from the crystal structure obtained at room
temperature and accordingly by imposing a planar geometry
[
49]
on the {Mo(CO ) (neocuproine)} moiety.
The ETS-NOCV in-
eq 2
vestigation was then performed on compounds 14–16 (see
Tables S5 and S6 in the Supporting Information) starting from
the newly optimized structures. The most relevant results are
summarized in Table 6; for the complete set of ΔE and Δqi
i
data, see Tables S5 and S6 in the Supporting Information.
As found for compounds 1–13 (Table 3), the largest contribu-
tion to ΔEint comes from the attractive term ΔEelstat, with a ratio
to ΔEorb ranging from 1.74 (16) to 1.89 (14), these being the
highest values for the whole series. Thus, again, the Mo–phen*
bond has more electrostatic (63.5–65.4 %) than covalent (34.6–
Figure 9. ORTEP representation of 14. Ellipsoids are drawn at the 50 % proba-
bility level. Single crystals suitable for X-ray analysis were obtained by slow
diffusion of hexane into a dichloromethane solution of 14.
3
6.5 %) character. One interesting feature emerges when the
The molecular structure of 14 is quite similar to those re- repulsive term, ΔEPauli, is considered: [Mo(CO) (2,9-Cl -phen)]
4
2
[
44]
[48]
ported for [Mo(CO) (phen)] (1)
and [Mo(CO) (batho)] (4),
(15) shows a value of 84.1 kcal/mol for ΔEPauli, the lowest for
4
4
showing the neocuproine to be coplanar with the plane of the whole [Mo(CO) (phen*)] series and definitely quite far from
4
{
Mo(CO ) }. Regarding the distorted octahedral environment the average (93.8 kcal/mol). For compounds 15 and 16, the
eq 2
Table 6. ETS-NOCV results describing the interaction between fragments F1 and F2 in complexes 14–16.
phen*
ΔE [kcal/mol]
ΔEPauli
%Eorb
%Eelstat
E [kcal/mol]
ΔEint
ΔEorb
ΔEelstat
ΔErest[a]
E
σ
E
π
14
15
16
cupr
2,9-Cl
2-CN-phen
–57.13
–47.01
–53.34
–51.81
–47.56
–53.08
92.87
84.06
92.20
–98.18
–83.51
–92.46
–12.15
–10.86
–10.60
34.54
36.29
36.47
65.46
63.71
63.53
–28.59
–25.68
–29.28
–11.08
–11.03
–13.26
2
-phen
[
a] ΔErest is the sum of the intrafragment polarizations due to minor internal rearrangements (each <2 kcal/mol).
Eur. J. Inorg. Chem. 0000, 0–0 www.eurjic.org © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6

Full Paper
stabilizing term ΔEelstat and the destabilizing ΔEPauli nearly can- ered. Reasonably, repulsion effects due to the sterically de-
cel each other out, their sums being +0.55 (15) and –0.26 cal/ manding 2(9)-substituents, well highlighted by the irregular be-
mol (16), respectively. Consequently, in these cases ΔEint shows havior of the experimental stretching frequencies (see Fig-
a value very close to ΔEorb. The σ and π contributions to ΔEorb ures S1–S3), would lead to distortions in solution that are hardly
are quite similar among the complexes, with a general preva- predictable in the optimized geometries.
lence of σ-donation compared with π-back-donation. As ex-
pected, the E value is highest for the derivative containing the
π
electron-withdrawing 2-CN-phen ligand.
Conclusions
These results validate the data obtained from the ETS-NOCV
scheme applied to complexes 14–16, which showed energies
and quantities not dramatically dissimilar to those formerly cal-
culated for 1–13. This led us to suppose a possible fitting of
the above-mentioned quantities to the previously obtained cor-
This work aimed at giving a quantitative description of the
bond between molybdenum and substituted 1,10-phenanthro-
lines (phen*) in the carbonyl complexes [Mo(CO) (phen*)]. By
applying the ETS-NOCV method to the two interacting frag-
ments in the compounds, {Mo(CO) } and {phen*}, it has been
4
4
relations (see Figures 5, 7, and 8). However, both the Δq versus
possible to highlight both the σ-donation (i.e., E ) and π-back-
phen
σ
νCO (A1ax) and the T
versus νCO (A1ax) plots (Figure 11) reveal
donation (E ) contribution of phen* to the metal–ligand Mo–
π
a net deviation of these three complexes (14–16) from the gen-
phen* bond. Moreover, E and E have been correlated with the
experimental frequencies of the carbonyl ligands in solution: A
good fit was attained when they were considered simultane-
ously, thus reflecting the synergistic action of σ and π contribu-
tions in the experimental observations. A new parameter (T
σ
π
eral trend shown by the 2,9-unsubstituted species (1–13). More-
phen
over, even when T
is plotted against the other vibrational
modes (see Figure S5 in the Supporting Information), the values
associated with complexes 14–16 always present odd behavior.
phen
)
phen
This means that T
, which accounts for the electronic contri-
phen
was then introduced: T
has been demonstrated to be a relia-
butions due to σ-donation and π-back-donation in the Mo–
phen* bond, cannot be used to give a quantitative description
of the bond when 2,9-substituted phenanthrolines are consid-
ble descriptor of the electronic (σ-donor and π-acceptor) prop-
erties of phen* ligands, with the exception of 2,9-substituted
derivatives. In this latter case, the steric effects imposed by the
2
,9-substitutions would probably lead to distortions in solution
that are hardly predictable in the geometry optimization and
consequently by the ETS-NOCV method. However, it is neces-
sary to point out that differences in ancillary ligands and metal
(i.e., in the metal-containing fragment) can cause significant dif-
ferences in the acid/base (and steric) properties of the li-
[
3b,45]
gands.
Thus, before establishing a general trend, further
studies are needed.
Experimental Section
General Procedures: All reactions were carried out under purified
nitrogen using standard Schlenk techniques. Solvents were dried
and distilled according to standard procedures prior to use. Elemen-
tal analyses were performed with a Perkin–Elmer CHN Analyzer
2
2
400 Series II. IR spectra were recorded with a Shimadzu Prestige-
spectrophotometer. 5-Nitro-1,10-phenanthroline (5-NO2-
1
[
40]
[33]
phen)
and 5-amino-1,10-phenanthroline (5-NH -phen)
were
2
prepared according to literature methods. All other chemicals were
of reagent-grade quality, were purchased commercially (TCI Chemi-
cals), and used as received.
General Procedure for the Synthesis of Compounds 1–16:[41]
A
mixture of [Mo(CO) ] (1 g, 3.79 mmol) and the 1,10-phenanthroline
6
ligand (3.79 mmol) in toluene (25 mL) was stirred for 4 h at 80 °C.
During this time the mixture first turned into a pink-red solution,
then into a red-purple suspension. This suspension was filtered and
the resulting orange-to-red precipitate was washed with pentane
and then dried in vacuo. See Table S7 in the Supporting Information
for further experimental details.
Computational Details: All calculations were carried out at the
density functional (DFT) level of theory by using the ADF2014.2
[
50]
[42]
program package.
The hybrid functional PBE0
was employed
Figure 11. Deviation of the behavior of 14–16 (red dots) with respect to 1–
for geometry optimization. The PBE0 exchange correlation func-
tional is based on the generalized gradient corrected exchange cor-
phen
13 (blue dots) in the Δq vs. νCO (A1ax) and T
vs. νCO (A1ax) plots.
Eur. J. Inorg. Chem. 0000, 0–0
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7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
relation functional of Perdew–Burke–Ernzerhof (PBE) with a fixed
amount (25 %) of Hartree–Fock exchange energy. Restricted formal-
ism was applied for all closed-shell systems (complexes and frag-
ments). For geometry optimization and fragment analysis, C, H, N,
O, and Cl atoms were described through TZ2P basis sets (triple-ꢀ
STO plus two polarization functions), the QZ4P basis set (quadruple-
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Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Substituted Phenanthrolines
G. A. Ardizzoia, M. Bea,
S. Brenna,* B. Therrien ................... 1–10
A Quantitative Description of the σ-
Donor and π-Acceptor Properties of
Substituted Phenanthrolines
Like players of opponent teams in tions, associated with Eσ and E , in
π
rugby, σ and π contributions are in- [Mo(CO) (phen*)] compounds. A new
4
phen
volved in a sort of “electronic scrum” electronic parameter (T
) compris-
that determines the nature of metal– ing both Eσ and Eπ has been intro-
ligand bonds. An ETS-NOCV study al- duced and correlated with experimen-
lowed us to quantify these contribu- tal data.
DOI: 10.1002/ejic.201600647
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim