Structural, Computational, and Spectroscopic Investigation of [Pd(κ3-1,1′-bis(di-tert-butylphosphino)ferrocenediyl)X]+ (X = Cl, Br, I) Compounds

Article abstract of DOI:10.1021/acs.organomet.5b00889

The reaction of [Pd(dtbpf)Cl₂] (dtbpf = 1,1′-bis(di-tert-butylphosphino)ferrocene) with sodium bromide yields [Pd(dtbpf)Br][Br], which displays an interaction between the iron and palladium atoms. The structure of this compound has been obtained and is compared to those of the previously reported [Pd(dtbpf)X]⁺ (X = Cl, I) analogues. Similar to [Pd(dtbpf)Cl]⁺, [Pd(dtbpf)Br]⁺ appears to undergo a solid-state isomerization at low temperature to a species in which the Fe-Pd interaction is disrupted. In addition to ¹H and ³¹P{¹H} NMR and visible spectroscopy, the [Pd(dtbpf)X]⁺ (X = Cl, Br) compounds were also characterized by zero-field ⁵⁷Fe M?ssbauer spectroscopy. DFT calculations on [Pd(dtbpf)X]⁺ (X = Cl, Br, I) show that the Fe-Pd interaction is weak and noncovalent and that the strength of the interaction decreases as the halide becomes larger. A related trend is noted in the potential at which oxidation of the iron center occurs; the larger the halide, the less positive the potential at which oxidation occurs. Finally, the catalytic activity of [Pd(dtbpf)X]⁺ (X = Cl, Br, I) in the arylation of an aromatic ketone was examined and compared to the activity of [Pd(dtbpf)Cl₂].

Full text of DOI:10.1021/acs.organomet.5b00889

Article
pubs.acs.org/Organometallics
Structural, Computational, and Spectroscopic Investigation of
[Pd(κ3‑1,1′-bis(di-tert-butylphosphino)ferrocenediyl)X]+ (X = Cl, Br, I)
Compounds
Brittany L. Blass,† Raul Hernandez Sanchez,‡ Victoria A. Decker,† Michael J. Robinson,† Nicholas A. Piro,§
́
́
́
W. Scott Kassel,§ Paula L. Diaconescu,∥ and Chip Nataro*,†
†Department of Chemistry, Lafayette College, Easton, Pennsylvania 18042, United States
‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, .United States
§Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, United States
∥Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
S
* Supporting Information
ABSTRACT: The reaction of [Pd(dtbpf)Cl2] (dtbpf = 1,1′-bis(di-tert-
butylphosphino)ferrocene) with sodium bromide yields [Pd(dtbpf)Br][Br],
which displays an interaction between the iron and palladium atoms. The
structure of this compound has been obtained and is compared to those of
the previously reported [Pd(dtbpf)X]+ (X = Cl, I) analogues. Similar to
[Pd(dtbpf)Cl]+, [Pd(dtbpf)Br]+ appears to undergo a solid-state isomer-
ization at low temperature to a species in which the Fe−Pd interaction is
1
disrupted. In addition to H and 31P{1H} NMR and visible spectroscopy,
the [Pd(dtbpf)X]+ (X = Cl, Br) compounds were also characterized by
zero-field 57Fe Mossbauer spectroscopy. DFT calculations on [Pd(dtbpf)-
̈
X]+ (X = Cl, Br, I) show that the Fe−Pd interaction is weak and
noncovalent and that the strength of the interaction decreases as the halide
becomes larger. A related trend is noted in the potential at which oxidation of the iron center occurs; the larger the halide, the
less positive the potential at which oxidation occurs. Finally, the catalytic activity of [Pd(dtbpf)X]+ (X = Cl, Br, I) in the arylation
of an aromatic ketone was examined and compared to the activity of [Pd(dtbpf)Cl2].
that has been observed in several studies of compounds
containing this ligand.
INTRODUCTION
■
1,1′-Bis(diphenylphosphino)ferrocene (dppf) is the bidentate
phosphine with a metallocene backbone that has been studied
the most, particularly as a ligand in catalytic systems.1 There are
many different types of ligands that are closely related to dppf,
but the most common are ones in which the R groups on the
phosphorus atoms are altered (Figure 1). Changing the R
groups not only affects the potential at which oxidation of the
iron center occurs,2 it can also play a significant role in the
reactivity of compounds containing these ligands. In particular,
the steric bulk of the tert-butyl groups in dtbpf3 has been
proposed to be responsible for drastically different behavior
The catalytic activity of compounds employing bis-
(phosphino)ferrocene ligands can be significantly affected by
the steric bulk of dtbpf. The coupling of diphenylphosphine
and o-CF3C6H4Br was much more efficient with a dippf-
Pd(OAc)2 catalyst precursor than with dtbpf-Pd(OAc)2.4 Of
the [Pd(P∩P)Cl2] (P∩P = dppf, dippf, dtbpf) catalysts, the
dtbpf compound was found to be the least efficient in the cross-
coupling of aryl Grignards with bromoanisoles.5 For the allylic
amination of a Morita−Baylis−Hillman acetate the catalyst−
ligand systems [Pd(C3H5)Cl]2−(P∩P) (P∩P = dppf, dippf,
dtbpf) were similar in terms of product formation, but the dtbpf
catalyst gave much higher regioselectivity of the product.6
Similarly, the reductive carbonylation of 4-bromoacetanilide
using [Pd(dppf)Cl2] as the catalyst gives predominantly the
desired aryl aldehyde product, while using [Pd(dtbpf)Cl2] gives
exclusively the arene.7 For the [Ru(PPh3)3(CO)(H2)]−(P∩P)
(P∩P = dppf, dippf, dtbpf) catalyzed alkylation of tert-butyl
Received: October 22, 2015
Figure 1. Common bis(phosphino)ferrocene ligands.
© XXXX American Chemical Society
A
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ketonitrile with benzyl alcohol, the greater bulk of dtbpf
significantly inhibits product formation.8
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. Similar to
In a noncatalytic system, the steric bulk of dtbpf can also play
a significant role. Unlike the case for the related 1,1′-
bis(phosphine selenide)ferrocene compounds,9 oxidation of
dtbpfSe2 results in the formation of an isolable Se−Se-bonded
dication that has been structurally characterized.2d Reductive
elimination of diaryl ethers from [Pd(P∩P)(Ar)(OAr′)] (P∩P =
dppf, dtbpf) was found to be significantly faster for the bulkier
dtbpf compound.10 A study of the formation of palladium aryl
enolates found that dppf bound as a bidentate ligand to
palladium, whereas a similar reaction with dtbpf yielded a
compound in which only one of the phosphorus atoms of dtbpf
was bound to the palladium.11 While 2 equiv of B(C6F5)3 will
undergo nucleophilic aromatic substitution reactions with dippf
to give [Fe(C5H4P(iPr)2C6F4BF(C6F5)2)2], dtbpf only reacts
with 1 equiv of B(C6F5)3 to give [Fe(C5H4P(tBu)2C6F4BF-
(C6F5)2)(C5H4P(tBu)2)]; attempts to drive a reaction with a
second equivalent of B(C6F5)3 lead to decomposition.12
Of greatest significance to this report are the studies in which
dtbpf binds in a κ3 coordination mode to a palladium center
(Figure 2). When [Pd(COD)ClCH3] is reacted with either
the synthesis of [Pd(κ3-dtbpf)Cl][BArF],14 the reaction of
[Pd(dtbpf)Cl2] with K[TFAB] yields [Pd(κ3-dtbpf)Cl]-
[TFAB]. To begin to examine the reactivity of [Pd(κ3-
dtbpf)Cl]+, an NMR tube reaction was performed in which
[PPN]Cl reacted with [Pd(κ3-dtbpf)Cl]+. Unsurprisingly, the
31P NMR spectrum indicated the presence of [Pd(dtbpf)Cl2]
(68.7 ppm)15 and [PPN]+ (21.7 ppm).16 Similar reactions were
performed by adding 1 equiv of either [NBu4]Br or [NBu4]I to
[Pd(κ3-dtbpf)Cl]+. In these reactions the expected asymmetric
dihalides [Pd(dtbpf)ClX] (X = Br, I) did not form. This was
somewhat surprising, as the compound formulated [Pd(dtbpf)-
Br2] has been reported and is commercially available.17 The
NMR-tube reaction of [Pd(κ3-dtbpf)Cl]+ with [NBu4]Br
yielded an approximately 3:5 mixture of [Pd(dtbpf)Cl2] and
[Pd(κ3-dtbpf)Br][TFAB], while the reaction with [NBu4]I gave
[Pd(dtbpf)Cl2] and [Pd(κ3-dtbpf)I]+ in an approximately 1:10
ratio (Scheme 1). Formation of [Pd(dtbpf)ClX] (X = Br, I) is
apparently unfavorable due to steric constraints. However,
ligand substitution of the chloride ligand in [Pd(κ3-dtbpf)Cl]+
occurs with the stronger field halides. The free chloride that is
generated can then react with [Pd(κ3-dtbpf)Cl]+ to yield
[Pd(dtbpf)Cl2]. On the basis of the ratio of the products, it
would appear that the iodide reacts significantly more quickly
than the bromide.
Although [Pd(dtbpf)Br][TFAB] was isolated by scaling up
the aforementioned reaction, it is not an efficient synthesis, as
approximately half of the starting material is converted to
[Pd(dtbpf)Cl2]. On the basis of the synthesis of [Pd(dtbpf)-
I][I],18 [Pd(dtbpf)Cl2] reacted with excess NaBr to yield
[Pd(dtbpf)Br][Br]. In addition, [Pd(dtbpf)Cl]+ was found to
react with either NaBr or NaI to give the corresponding
[Pd(dtbpf)X]+ (X = Br, I). The 31P NMR signal for
[Pd(dtbpf)Br]+ is at 11.5 ppm, similar to that observed for
the corresponding chloride and iodide cations.
The structure of [Pd(dtbpf)Br][TFAB] was determined
(Figure 3) over a range of temperatures. Similar to the case for
[Pd(dtbpf)Cl][SbCl6],14 the structure of [Pd(dtbpf)Br]-
[TFAB] undergoes a reversible isomerization in the solid
state. At room temperature, the only form present is the κ3-
dtbpf structure shown in Figure 3. This isomer displays an Fe−
Pd distance similar to that found in the chloride14 and iodide18
analogues. A second isomer in which the dtbpf is coordinated
in a κ2 coordination mode is observed at lower temperatures
(see the Supporting Information). This second isomer was
previously described as being three-coordinate;14 however,
further analysis suggests that this description is not appropriate.
The low-temperature isomers of [Pd(dtbpf)X]+ (X = Cl, Br)
display two significantly different P−Pd−X angles of approx-
imately 105 and 150°. In addition, both structures display
several short (approximately 2.7 Å) contacts between the
palladium atom and the idealized positions for the hydrogen
atoms of the tert-butyl groups. These observations suggest that
the low-temperature isomer is also four-coordinate, with the
fourth coordination site being occupied by an agostic
interaction with the tert-butyl groups. However, even at
temperatures approaching the freezing point of the CDCl3
Figure 2. Κ3 coordination mode of dtbpf.
dppf or dippf, the COD is replaced with the bidentate
phosphine, yielding [Pd(P∩P)ClCH3] (P∩P = dppf, dippf).
However, when a similar reaction is performed with dtbpf, the
greater steric bulk of the ligand does not allow formation of
[Pd(dtbpf]ClCH3] but instead yields [Pd(κ3-dtbpf)CH3]-
[Cl].13 Similarly, the reaction of Na[BArF] with [Pd(P∩P)Cl2]
(P∩P = dppf, dippf, dcpf) abstracts a chloride ligand from the
palladium and results in the formation of a dimeric species,
[Pd2(P∩P)2(μ-Cl)2][BArF]2. The bulk of dtbpf prevents this
dimerization following chloride abstraction, and thus [Pd(κ3-
dtbpf)Cl][BArF] is formed in which the empty coordination
site generated by removing a chloride ligand is occupied by a
weak, noncovalent interaction between palladium and iron.14
To further investigate these types of compounds, [Pd(κ3-
dtbpf)Br]+ was synthesized and characterized by NMR
spectroscopy and X-ray crystallography. The nature of the
iron−palladium interaction was investigated computationally
and compared to that of the analogous chloride and iodide
compounds. In addition, the oxidative electrochemistry and
visible spectra of these compounds are compared. Finally, the
57Fe Mossbauer spectra of dtbpf and several compounds
̈
containing dtbpf are reported.
Scheme 1. Reactions of [Pd(κ3-dtbpf)Cl]+ with [NBu4]X (X = Br−, I−)
B
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ligand occupies around a metal center.3a The calculated20 %Vbur
values for the dtbpf ligand in this series of compounds
decreases slightly as the size of the coordinated halide increases.
Two other techniques were employed to further examine the
Fe−Pd interaction in these compounds. The visible spectrum
of these compounds displays a single absorbance that can be
attributed to Fe d−d transitions (Table 2).21 The λmax values
Table 2. Visible Absorbance Data for ∼0.2 mM Solutions in
CH2Cl2
compound
λmax (nm)
ε (M−1 cm−1
)
dtbpf
458
450
455
478
270
3600
5200
5500
[Pd(dtbpf)Cl][TFAB]
[Pd(dtbpf)Br][Br]
[Pd(dtbpf)I][I]
Figure 3. Perspective view of [Pd(dtbpf)Br][TFAB] with 30%
probability ellipsoids at 100(2) K. Hydrogen atoms and [TFAB]−
are omitted for clarity.
shift to higher wavelengths as the donor ability of the halide
increases. This is opposite of the trend observed in [Pd-
(dtbpf)PR3]2+ (PR3 = PBu3, PPh2Me, PPh3),22 [Pd-
((C5H5ER)2Fe)PPh3]+2 (E = S, Se; R = Me, Ph),23 and
[Ru(PR3)2((C5H4NH)2Fe)] (PR3 = PMe2Ph, PPh3),24 in
which the weaker σ-donor ligands trans to iron display the
higher wavelength absorbance. Similar to the case for
[Pd(dtbpf)CH3]+,25 there was no absorbance indicative of an
Fe−Pd interaction at higher wavelengths. Such bands are noted
in [Ru(PR3)2((C5H4NH)2Fe)] (PR3 = PMe2Ph, PPh3),24
which have been calculated to have Fe−Ru interactions
stronger than the Fe−Pd interactions in these systems.
1
solvent there were no indications of this interaction in the H
NMR spectrum of either compound.
The closely related structure of [Pd(dtbpf)I][I] has been
reported at 150 K.18 The structure was determined at several
temperatures (see the Supporting Information), and while the
space group changed from Pbca to P21/c, there was no evidence
of isomerization. On comparison of the structures of [Pd-
(dtbpf)X]+ (X = Cl, Br, I) (Table 1), the Fe−Pd distances are
̈
Mossbauer Spectroscopy. To further investigate the
nature of the Fe−Pd interaction, zero-field 57Fe Mossbauer
Table 1. Selected Distances (Å), Angles (deg), and Structural
Parameters for [Pd(dtbpf)Cl][SbCl6],
[Pd(dtbpf)Br][TFAB], and [Pd(dtbpf)I][I] at 100(2) K
̈
spectra were collected on dtbpf, [Pd(dtbpf)Cl2], and [Pd-
(dtbpf)X]+ (X = Cl, Br) (Figure 4 and Figures S8−10 in the
Supporting Information). At 90 K, dtbpf has an isomer shift (δ)
and quadrupole splitting (|ΔEQ|) of 0.54 and 2.35 mm/s,
respectively (Figure 4f and Figure S8). These are almost
identical with the values for ferrocene26 and are in line with
those for other ferrocene compounds.26,27 Complexation of
PdCl2 shifts δ and |ΔEQ| to 0.49 and 2.27 mm/s, respectively
(Figure 4f and Figure S9). The geometry at the coordinated
metal has been observed to play an important role in the
[Pd(dtbpf)Cl]
[SbCl6]14
[Pd(dtbpf)Br]
[TFAB]
[Pd(dtbpf)I]
[I]
a
Fe−Pd
2.977(7)
2.310(9)
2.280(9)
2.336(7)
162.5(4)
81.4
2.9395(18)
2.3012(15)
2.2904(15)
2.4494(14)
163.04(5)
81.52
2.9390
a
Pd−P(1)
Pd−P(2)
Pd−X
2.2876
a
2.2892
a
2.5970
a
P−Pd−P
P−Pd−Fe (av)
P−Pd−X (av)
%Vbur
161.88
a
80.98
a
resulting Mossbauer parameters; square-planar species tend to
̈
98.6
98.42
99.08
a
have smaller δ and |ΔEQ| values, followed by octahedral species
with slightly perturbed values, and ultimately tetrahedral
complexes with larger parameters relative to those of
ferrocene.28 These findings correlate directly with the Fe−
cent distance to the center of each Cp ring: the shortest values
are found for square planar and the longest for tetrahedral
complexes.28 While the structure of dtbpf has not been
reported, the Fe−cent distance in the closely related (S,S)-
1 , 1 ′ , 3 - t r i s ( d i - t e r t - b u t y l p h o s p h i n o ) - 2 ′- ( 1 - N , N -
dimethylaminoethyl)ferrocene (1.670 Å)29 is longer than that
in [Pd(dtbpf)Cl2] (1.634 Å)17a and thus one expects a lower
isomer shift for the Pd adduct due to greater electron density
withdrawal from the iron center.
73.0
72.1
71.7
b
a
θ
17.5(10)
41.4(16)
0.09(4)
0.06(4)
162.5(6)
19.27(14)
41.7(2)
20.76
c
a
τ
40.6
d
a
δP
0.118(5)
0.125(5)
160.72(10)
0.141
a
0.116
e
a
cent−Fe−cent
161.28
a
Average of two crystallographically independent molecules in the unit
b
c
cell. The tilt angle of the two C5 rings. The torsion angle formed
between Ca−centa−centb−Cb, with C being the carbon atom bonded
d
to phosphorus and cent the centroid of the C5 ring. The distance
between the P atoms and the C5 plane; a positive value means the P
e
atom is closer to Fe. The angle formed between the centroid of each
C5 ring and the Fe center.
As discussed previously, removal of a chloride from
[Pd(dtbpf)Cl2] produces the cationic [Pd(dtbpf)Cl]+ com-
plex.14 In [Pd(dtbpf)Cl]+, the extent of the Fe−Pd interaction
varies with temperature, as determined by the change in the
Fe−Pd distance at 100 and 296 K (Table 3). There are very few
examples of compounds containing κ3-ferrocenyl moieties as
all significantly longer than 2.71 Å, the sum of the covalent
radii.19 The C5 rings are tipped from being parallel by
approximately 20°, and the angle increases as the size of the
halide increases. As seen in related compounds,14,18 this tip
causes a significant difference in the chemical shifts of the Hα
and Hβ protons of the C5 rings in the 1H NMR spectrum. The
percent buried volume (%Vbur) is a measurement of the space a
ligands where structural and Mossbauer data have been
̈
reported. In fact, from an in-depth survey of the literature,
C
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Figure 4. (a−e) Variable-temperature zero-field 57Fe Mossbauer spectra for [Pd(dtbpf)Cl][SbCl ]: (a) 90 K; (b) 160 K; (c) 210 K; (d) 250 K; (e)
̈
6
298 K. (f) Isomer shift (δ) and quadrupole splitting (|ΔEQ|) parameters for dtbpf, [Pd(dtbpf)Cl2], and [Pd(dtbpf)Cl][SbCl6] at 90 K. (g) δ and
|ΔEQ| parameters extracted from (a−e).
Table 3. Crystallographic and 57Fe Mossbauer Parameters for Species Containing Short and Long Fe−Pd Distances
̈
compound
dFe−Pd (Å)/T (K)
T range (K)
Δδ (mm/s)
Δ|ΔEQ| (mm/s)
a
bc
,
A
B
C
2.878(1)/298
77 to room temp
77 to room temp
77 to room temp
90−298
0.05
0.08
0.05
0.09
0.08
0.04
0.03
0.05
0.05
0.09
d
bc
,
2.827(2)/275
e
bf
,
3.810(2)/275
g
[Pd(dtbpf)Cl2]
[Pd(dtbpf)Cl]+
4.4026(11)
h
2.9389(6)/296
90−298
hi
,
4.41(1)/100
[Pd(dtbpf)Br]+
2.9285(6)/295
4.292(12)/100
90−298
0.07
0.04
i
a
b
c
d
e
f
g
i
Reference 30. room temp stands for room temperature. Reference 31. Reference 32. R = Bu; ref 33. R = Me; ref 23. Reference 17a.
h
i
Reference 14. Distance observed in the minor component (<10%).
only two examples were found where Mossbauer spectra were
Cl]+ and [Pd(dtbpf)Br]+ are 0.08 and 0.07 mm/s, respectively;
thus, the Fe−Pd interaction formed at high temperature seems
to have little effect on the value of δ.
̈
reported at low (77 K) and high (∼298 K) temperatures:
namely, compounds A and B.31 The variable-temperature
spectral and structural behavior of the last two and of the
square-planar κ2-ferrocenyl compound C and [Pd(dtbpf)Cl2]
are used as references for those reported here, displaying a
small percentage of molecules isomerizing between long and
short Fe−Pd distances (Table 3). To probe the temperature-
dependent isomerization in [Pd(dtbpf)Cl]+ and [Pd(dtbpf)-
In contrast to δ, the quadrupole splitting (|ΔEQ|) has been
proposed to be more sensitive when equatorial interactions are
proposed in ferrocene compounds.35 It is well known that
|ΔEQ| has an almost negligible variation with temperature and
thus any interaction with the iron atom in ferrocene
compounds can be diagnosed via this parameter.36 In Table
3, two examples are given with a short (A, B) and two with a
long Fe−Pd distance (C, [Pd(dtbpf)Cl2]). Regardless of the
presence of an Fe−Pd interaction, the temperature variation
between 77 and 298 K of |ΔEQ| on these four examples is
approximately 0.04 mm/s on average. Thus, the observed
Δ|ΔEQ|90−298 K value of 0.09 mm/s in [Pd(dtbpf)Cl]+ is
proposed to be the result of the sum of two effects: (1) the
standard temperature dependence observed in related com-
pounds (Table 3; A−C and [Pd(dtbpf)Cl2]) and (2) the
formation of a weak Fe−Pd interaction. Further support for the
Fe−Pd interaction in [Pd(dtbpf)Cl]+ is gained when
Br]+, Mossbauer spectra were collected at 90, 160, 210, 250,
̈
and 298 K (Figure 4a−e and Figure S3 in the Supporting
Information). The resulting δ and |ΔEQ| values obtained from
fitting the data are shown in Figure 4g (for X = Br, see Figure
S4 in the Supporting Information). The decrease in δ as the
temperature rises can be explained by the second-order
Doppler effect34 and, hypothetically, by full conversion to the
shorter Fe−Pd interaction at room temperature. In related
complexes with a proposed Fe−Pd interaction, Δδ ranges from
about 0.05 to 0.08 mm/s between low (77 K) and high (298 K)
temperature (Table 3). The Δδ90−298 K values in [Pd(dtbpf)-
D
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comparing the Mossbauer data reported for [Pd(dppf)(PPh )]-
was available, calculations were repeated for these compounds
as well (Table 5). Geometry optimizations for [Pd(dtbpf)X]+
̈
3
[BF4]2 (dFe−Pd = 2.877(2) Å) and [Pd(dppf)(4-picoline)2)]-
[BF4]2 (1H NMR spectroscopic data suggest a long Fe−Pd
distance).22 The former compound, with a short Fe−Pd
Table 5. Computational Parameters for [Pd(dtbpf)X]+ (X =
Cl, I)
distance, displays Mossbauer parameters of [δ, |ΔE | (mm/s):
̈
Q
0.51(1), 2.24(1)], whereas the latter compound has [δ, |ΔEQ|
(mm/s): 0.57(1), 2.61(1)].24 In contrast, the Mossbauer data
[Pd(dtbpf)
Cl]+
[Pd(dtbpf)
Br]+
[Pd(dtbpf)
I]+
̈
(Δδ90−298 K and Δ|ΔEQ|90−298 K) of [Pd(dtbpf)Br]+ suggest that
there is a negligible change in the extent of the Fe−Pd
interaction between low (90 K) and high (298 K) temperature
(Figure S4 in the Supporting Information).
Fe−Pd (Å)
Mayer bond order
2.98
0.34
2.99
0.32
2.99
0.30
Mulliken charge
Fe
−0.12
−0.46
−0.12
−0.43
−0.11
−0.67
Electrochemistry. The electrochemistry of [Pd(dtbpf)Br]-
[Br] was examined using cyclic voltammetry. Similar to the
analogous chloride, the oxidative electrochemistry displays a
single, chemically reversible wave (Table 4). This wave is due
Pd
Hirshfeld charge
Fe
0.05
0.34
0.13
0.13
0.05
0.32
0.12
0.14
0.05
0.30
0.11
0.14
Pd
Wiberg bond index
Table 4. Electrochemical Data for [Pd(dtbpf)X]+ (X = Cl,
NLMO bond order
a
Br, I) Compounds
natural charge
Fe
0.20
0.33
0.20
0.29
0.20
0.21
E°
ΔE
Ep
Ep
Pd
b
b
c
c
[Pd(dtbpf)Cl]+
[Pd(dtbpf)Br]+
[Pd(dtbpf)I]+
0.94
0.92
0.88
0.07
0.08
0.11
−1.01
−0.90
−1.61
−1.56
−0.85
Bader charge
Fe
Pd
0.71
0.20
0.02
0.03
0.72
0.15
0.02
0.03
0.71
0.04
0.02
0.03
bd
,
b
b
b
−0.58
a
b
c
Potentials are in V vs FcH0/+
[NBu4][TFAB] as the supporting electrolyte. Irreversible oxidative
waves due to the iodide counterion are also observed.
.
Reference 14. In CH2Cl2 with
a
ρ
d
Laplacian (∇2 ) at Fe−Pd
ρ
BCP
a
BCP is the bond critical point identified by Bader analysis.
to the oxidation of the iron center. The trend in the potentials
follows what is predicted by the computational analysis (vide
infra); as the strength of the Fe−Pd interaction increases, the
potential at which the oxidation of iron occurs becomes more
positive, although it is important to note the small changes
between the three compounds. The reductive electrochemistry
of [Pd(dtbpf)Br]+ displays two irreversible waves attributed to
reduction of the palladium. Previously, the reductive electro-
chemistry of [Pd(dtpbf)Cl]+ was reported to display one
irreversible wave, while [Pd(dtbpf)I]+ displayed two irreversible
waves.14 The reductive electrochemistry of [Pd(dtbpf)Cl]+ was
examined using [NBu4][TFAB] as the supporting electrolyte,
and it was found to display two irreversible waves (Figure 5).
The trend in the reduction potentials follows what was
observed with oxidation; [Pd(dtbpf)Cl]+ is the most difficult
to reduce due to greater electron donation from the iron.
Computational Studies. The compounds [Pd(dtbpf)Cl]+
and [Pd(dtbpf)I]+ were previously examined by computational
methods; however, because a different version of the software
(X = Cl, Br, I) were performed for the present study employing
ADF2013.01,47 using the PW91 functional and no frozen
electron cores.48 The optimized Fe−Pd distance (2.99 Å) for
[Pd(dtbpf)Br]+ is close to that obtained experimentally (2.94
Å), as are other structural parameters (Table S1 in the
Supporting Information). The calculated Mayer bond order49
for [Pd(dtbpf)Br]+ is 0.32 and is similar to the values obtained
for the chloride and iodide analogues (Table 5). These values
are smaller than those reported for metal−metal single bonds in
[Ru(CO)4]8 (0.70) and Ru3(CO)12 (0.62),50 but they are
similar to those reported for [((C5H4NH)2Fe)Ru(PR3)2] (PR3
= PPh3, 0.28; PR3 = PMe2Ph, 0.26).24
Two significant bonding interactions were found between
iron and palladium in [Pd(dtbpf)Br]+ (HOMO-4 and HOMO-
5) (Figure 6). These orbitals have a relatively large contribution
from the metal atomic orbitals (see the Supporting Information
Figure 5. Cyclic voltammogram for the reduction of 1.0 mM
[Pd(dtbpf)Cl][TFAB] in CH2Cl2/0.05 M [NBu4][TFAB] at a glassy-
carbon electrode and a scan rate of 100 mV/s.
Figure 6. Frontier molecular orbitals for [Pd(dtbpf)Br]+: HOMO-4
(left) and HOMO-5 (right); isosurface value 0.03.
E
DOI: 10.1021/acs.organomet.5b00889
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 2. Palladium-Catalyzed α-Arylation of Propiophenone with 4-Chlorotoluene
for details): HOMO-4 (39% Pd and 29% Fe) and HOMO-5
(14% Pd and 12% Fe).
important role in this catalytic system and that the larger
halides greatly decrease the activity of the catalyst. The lower
activity of [Pd(dtbpf)Cl]+ in comparison to [Pd(dtbpf)Cl2]
suggests that the cationic species is not a likely resting state in
this catalytic cycle, although the presence of the [BArF]−
counterion could have some impact on the catalytic activity.
As the catalytic study was performed in THF, the 31P{1H}
NMR spectra of the palladium compounds were obtained in
THF and found to be similar to those obtained in CH2Cl2
(differing only slightly in the chemical shifts), with the
exception of [Pd(dtbpf)Br]+. A sample of [Pd(dtbpf)Br]+
dissolved in CH2Cl2 gave a single peak in the 31P{1H}
spectrum. When the solvent was removed in vacuo and the
sample was dissolved in THF, five peaks were observed. The
spectra were then recorded from 35 to −65 °C in 10 °C
intervals. The spectra displayed five peaks from 35 to −5 °C
and six peaks at lower temperatures (Table 7). While none of
these species were isolated, it is possible to propose a
reasonable formula on the basis of the available data. The
peak at 12.4 ppm is [Pd(dtbpf)Br]+, and this remains the
predominant species in solution at all temperatures. The four
signals in the 60−70 ppm range are likely due to four-
coordinate species, in which there is no interaction between the
iron and palladium centers. An approximately 50 ppm
downfield shift is also observed on comparing [Pd(dtbpf)Cl]+
to [Pd(dtbpf)Cl2].14 The two doublets are likely due to an
asymmetric compound, which is proposed to be [Pd(dtbpf)-
Br(thf)]+. The remaining two peaks are likely due to symmetric
species, which are proposed to be [Pd(dtbpf)Br2] and
[Pd(dtbpf)(thf)2]2+. Presumably, the larger signal at 65.3
ppm is due to [Pd(dtbpf)Br2]. The species that appears at
−15 °C is shifted downfield by approximately 20 ppm from the
proposed signal for [Pd(dtbpf)Br2]. A similar difference in
chemical shifts is noted in comparing [Pd(dppf)Cl2] and
[Pd2(dppf)2(μ-Cl)2]2+,14 suggesting that the peak at 83.0 ppm
could be due to the formation of [Pd2(dtbpf)2(μ-Br)2]2+. When
THF was removed in vacuo from this sample and the solid was
dissolved in CH2Cl2, the 31P{1H} NMR spectrum displayed a
single peak for [Pd(dtbpf)Br]+. Similar behavior was not
observed for [Pd(dtbpf)Cl]+, likely due to the fact that the
counteranion is not chloride; as reported above, the addition of
chloride to [Pd(dtbpf)Cl]+ cleanly results in the formation of
[Pd(dtbpf)Cl2]. For [Pd(dtbpf)I]+, the larger iodide and/or the
stronger donor ability of iodide may prevent the formation of
species such as [Pd(dtbpf)I2]. This could be additional
evidence for why [Pd(dtbpf)I]+ is a significantly less effective
catalyst for this α-arylation reaction (Table 6).
In order to understand further the Fe−Pd interaction, natural
bond orbital analysis was carried out using NBO 6.0,51 and the
natural localized molecular orbitals (NLMOs) were generated.
For [Pd(dtbpf)Br]+, a σ bond is apparent in NLMO 87, which
is centered on iron, similarly to what was found for
[Pd(dtbpf)Cl]+ (NLMO 79) and for [Pd(dtbpf)I]+ (NLMO
96) (see the Supporting Information for details).
Topological analysis of the electron density was performed
via Bader’s atoms in molecule (AIM) theory.52 AIM identifies
bonds by calculating (3, − 1) critical points, and it differentiates
between covalent bonds and weak interactions, such as
hydrogen bonds, van der Waals, and donor−acceptor
interactions, by the value of the Laplacian (∇2 ). If ∇2 < 0,
ρ
ρ
the interaction is considered covalent. If ∇2 > 0, the
ρ
interaction is a weak interaction. AIM has been used to
calculate bond critical points between two metal centers53 and
in ferrocene complexes.54 All three [Pd(dtbpf)X]+ species show
Fe−Pd bond critical points (Table 5) consistent with a weak,
noncovalent interaction (see the Supporting Information for
details).
Catalytic Studies. To further examine the effect of
changing the halide, the catalytic activity of the [Pd(dtbpf)X]+
(X = Cl, Br , I) and [Pd(dtbpf)Cl2] was examined in the α-
arylation of propiophenone with 4-chlorotoluene (Scheme 2).
Previous reports have examined this same reaction using
[Pd(dtbpf)Cl2], [Pd(dtbpf)Br2], and [Pd(dtbpf)I2] as the
catalyst precursors.17 Although those studies report the use of
[Pd(dtbpf)Br2] and [Pd(dtbpf)I2], it is likely that the
compounds that were actually employed were [Pd(dtbpf)Br]+
and [Pd(dtbpf)I]+. At the writing of this paper, the only known
reports of [Pd(dtbpf)Br2] do not include synthetic details or
any characterization data.17 While those same reports suggest
the use of [Pd(dtbpf)I2], there is evidence that the compound
used was actually [Pd(dtbpf)I][I].37 Using conditions similar to
those in the previous report, [Pd(dtbpf)Cl2] was found to be
the most active catalyst (Table 6). For the [Pd(dtbpf)X]+ (X =
Cl, Br, I) compounds, the catalytic activity decreased as the size
of the halide increased, similar to the previously observed
trend.17 This suggests that the size of the halide plays an
Table 6. Catalytic Activity of Palladium Compounds for the
α-Arylation of Propiophenone with 4-Chlorotoluene
a
Pd catalyst
none
TON
0
[Pd(dtbpf)Cl2]
[Pd(dtbpf)Cl]+
[Pd(dtbpf)Br]+
[Pd(dtbpf)I]+
21.7(0.9)
8.3(0.6)
5.5(0.9)
0.2(0.1)
CONCLUSION
The synthesis of [Pd(dtbpf)Br]+ was carried out using different
methods, but the most efficient synthesis was determined to be
■
1
a
the reaction of [Pd(dtbpf)Cl2] with sodium bromide. The H
Turnover number is the average of three experiments reported with
the average deviation from the mean.
and 31P{1H} NMR spectra of [Pd(dtbpf)Br]+ were consistent
F
DOI: 10.1021/acs.organomet.5b00889
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 7. Variable-Temperature 31P{1H} NMR Spectroscopic Data for [Pd(dtbpf)Br]Br in THF
rel integration
peak (ppm)
mult
2JP−P (Hz)
35 °C
25 °C
15 °C
5 °C
−5 °C
−15 °C
−25 °C
−35 °C
−45 °C
−55 °C
−65 °C
12.4
62.5
63.9
65.3
66.7
83.0
s
1.00
0.12
0.04
0.42
0.13
1.00
0.13
0.02
0.47
0.11
1.00
0.12
0.03
0.52
0.12
1.00
0.12
0.03
0.54
0.10
1.00
0.13
0.03
0.57
0.15
1.00
0.14
0.04
0.58
0.13
0.07
1.00
0.11
0.02
0.59
0.11
0.10
1.00
0.12
0.03
0.60
0.16
0.14
1.00
0.14
0.04
0.64
0.16
0.11
1.00
0.12
0.02
0.64
0.14
0.13
1.00
0.13
0.06
0.65
0.16
0.11
d
s
35
s
d
s
35
external sample of 85% H3PO4. GC-MS data were obtained using a
VG/FISONS Model MD800 gas chromatograph/mass spectrometer.
All reported yields are isolated yields. Elemental analysis was
performed by Quantitative Technologies, Inc.
with the product having an interaction between the iron and
palladium atoms. This Fe−Pd interaction was also evident in
the X-ray crystal structure of [Pd(dtbpf)Br]+, which was similar
to those of the analogous [Pd(dtbpf)X]+ (X = Cl, I)
compounds. Similarly to [Pd(dtbpf)Cl]+, crystals of [Pd-
(dtbpf)Br]+ undergo an isomerization at low temperature,
giving a species in which the Fe−Pd interaction is broken and
presumably replaced with an agostic interaction with a C−H of
Synthesis of [Pd(dtbpf)Cl][TFAB]. K[TFAB] (0.0891 g, 0.124
mmol) was added to a brown-orange solution of [Pd(dtpbf)Cl2]
(0.0778 g, 0.119 mmol) in 10 mL of CH2Cl2. The reaction changed
color to a darker brown-orange with stirring for 15 min. The solution
was then filtered, and solvent was removed from the filtrate in vacuo.
The residue was triturated with ether (3 × 5 mL) and dried under
vacuum. The product was obtained as a black solid (0.0943 g, 61%
a tert-butyl group of the dtbpf ligand. The 57Fe Mossbauer
̈
spectra of [Pd(dtbpf)X]+ (X = Cl, Br) were obtained at a
variety of temperatures and are consistent with an Fe−Pd
interaction. The oxidative electrochemistry of [Pd(dtbpf)Br]+
was examined and compared to that of [Pd(dtbpf)X]+ (X = Cl,
I). The potential at which the iron in these compounds is
oxidized shifts to less positive potentials as the halide becomes
more π-donating, suggesting that the Fe−Pd interaction
weakens with the larger halides. DFT calculations support the
electrochemical data and describe the Fe−Pd interaction as
weak and noncovalent. Finally, the catalytic activity of the
[Pd(dtbpf)X]+ (X = Cl, Br, I) was compared to the activity of
[Pd(dtbpf)Cl2] for the arylation of an aromatic ketone. The
catalytic reaction was found to be most efficient with
[Pd(dtbpf)Cl2], and the activity of the [Pd(dtbpf)X]+
compounds decreased as the halide became larger.
1
yield). H NMR (CDCl3): δ (ppm) 5.19 (br s, 4H, Hβ), 3.96 (br s,
4H, Hα), 1.53 (br s, 36H, −CH3). 31P{1H} NMR (CDCl3) δ 9.8 (s).
Anal. Calcd for C50H44BClF20FeP2Pd: C, 46.36; H, 3.42. Found: C,
46.12, H, 3.21.
Reaction of [Pd(dtbpf)Cl][TFAB] with [PPN]Cl. In an NMR
tube reaction, [Pd(dtbpf)Cl][TFAB] (0.0089 g, 6.9 μmol) and [PPN]
Cl (0.0040 g, 7.0 μmol) were dissolved in CH2Cl2 (1 mL). This
resulted in a brown solution. 31P{1H} NMR (CH2Cl2): δ 67.7 (s), 21.6
(s).
Reaction of [Pd(dtbpf)Cl][TFAB] with [NBu4]Br. [Pd(dtbpf)-
Cl][TFAB] (0.0280 g, 22.6 μmol) and [NBu4]Br (0.0073 g, 23 μmol)
were placed in an NMR tube and dissolved in CH2Cl2 (1 mL).
31P{1H} NMR (CH2Cl2): δ 67.7 (s), 15.1 (s).
Reaction of [Pd(dtbpf)Cl][TFAB] with [NBu4]I. [Pd(dtbpf)Cl]-
[TFAB] (0.0221 g, 17.8 μmol) and [NBu4]I (0.0066 g, 18 μmol) were
placed in an NMR tube and dissolved in CH2Cl2 (1 mL). 31P{1H}
NMR (CH2Cl2): δ 67.7 (s), 10.9 (s).
EXPERIMENTAL SECTION
■
Synthesis of [Pd(dtbpf)Br][TFAB]. [NBu4]Br (0.0095 g, 30
μmol) was dissolved in 5 mL of CH2Cl2. In a separate flask
[Pd(dtbpf)Cl][TFAB] (0.0382 g, 29.5 μmol) was dissolved in 10 mL
of CH2Cl2. The [Pd(dtbpf)Cl][TFAB] solution was slowly added over
10 min to the [NBu4]Br solution, and the resulting solution was stirred
for 10 min. The solution was then layered with 20 mL of hexanes and
placed in the freezer overnight. This yielded a black precipitate and an
orange solution. The solution was filtered, and the solid was washed
with 5 mL of ether and the dried in vacuo. The product was collected
General Considerations. All reagents were used as received
unless otherwise indicated. [PdCl2(MeCN)2], sodium tetrafluorobo-
rate, isopropylmagnesium chloride (2.0 M in THF), 1,3-bis-
(trifluoromethyl)-5-bromobenzene, tetrabutylammonium bromide
([NBu4]Br), tetrabutylammonium iodide ([NBu4]I), phenanthrene,
propiophenone, 4-chlorotoluene, and sodium tert-butoxide were
purchased from Aldrich. Bis(triphenylphosphine)iminium chloride
([PPN]Cl) and 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf) were
purchased from Strem Chemicals, Inc. Sodium bromide, sodium
iodide and THF-d8 were purchased from Fisher Scientific. Lithium
tetrakis(pentafluorophenyl)borate etherate and potassium tetrakis-
(pentafluorophenyl)borate (K[TFAB]) were purchased from Boulder
Scientific. Tetrabutylammonium hexafluorophosphate ([NBu4][PF6])
was purchased from Aldrich and dried under vacuum at 100 °C prior
to use. Ferrocene (FcH) was purchased from Strem and sublimed
prior to use. The compounds sodium tetrakis(3,5-bis(trifluoromethyl)-
phenyl)borate (Na[BArF]),38 [Pd(dtbpf)Cl2],39 tetrabutylammonium
tetrakis(pentafluorophenyl)borate ([NBu4][TFAB]),40 [Pd(dtbpf)-
Cl][BArF],14 and [Pd(dtbpf)I][I]18 were prepared according to
literature procedures. All reactions were carried out under argon
using standard Schlenk techniques unless otherwise noted. Methylene
chloride (CH2Cl2), hexanes, and diethyl ether (Et2O) were purified
using a Solv-Tek purification system similar to that previously
described.41 Acetone and benzene were degassed and purged with
argon prior to use. THF was distilled from potassium benzophenone
ketyl under argon prior to use. A JEOL Eclipse 400 FT-NMR
1
as a black solid (0.0170 g, 43% yield). H NMR (CDCl3): δ (ppm)
6.32 (br s, 4H, Hβ), 4.15 (br s, 4H, Hα), 1.54 (br s, 36H, −CH3).
31P{1H} NMR (CDCl3)
δ 11.5 (s). Anal. Calcd for
C50H44BBrF20FeP2Pd: C, 44.82; H, 3.31. Found: C, 44.90, H, 3.30.
Synthesis and Characterization of [Pd(dtbpf)Br][Br]. [Pd-
(dtbpf)Cl2] (0.3798 g, 0.583 mmol) and sodium bromide (0.3025 g,
2.94 mmol) were placed in a flask and degassed. Acetone (15 mL) was
added, giving a dark grayish red solution that was stirred for 22 h. The
solvent was removed in vacuo, and the resulting residue was triturated
with 2 × 5 mL of DI water. The resulting product was azeotropically
dried with benzene (50 mL), which was then removed in vacuo. The
remaining solid was dissolved in minimal CH2Cl2 and layered with
hexanes. The resulting solution was placed in the freezer overnight,
yielding a dark red-black precipitate. The solution was filtered, washed
with 2 × 5 mL of Et2O, and dried in vacuo, which yielded 0.2591 g
1
(59%) of the product as a black solid. H NMR (CDCl3): δ (ppm)
1
spectrometer was used to acquire the 31P{1H} NMR and H NMR
6.32 (br s, 4H, Hβ), 4.15 (br s, 4H, Hα), 1.54 (br s, 36H, −CH3).
31P{1H} NMR (CDCl3) δ 11.5 (s). Anal. Calcd for C26H44Br2FeP2Pd:
C, 42.16; H, 5.99. Found: C, 42.22, H, 6.09.
spectra. The 1H NMR spectra were referenced to TMS (δ 0.00 ppm).
Chemical shifts of the 31P{1H} NMR spectra are referenced to an
G
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
X-ray Diffraction Studies. Crystals of [Pd(dtbpf)Br][TFAB]
were obtained at room temperature by vapor diffusion of Et2O into a
solution of each compound in CH2Cl2. A Bruker-AXS Kappa APEX II
CCD diffractometer with 0.71073 Å Mo Kα radiation was used for
collecting X-ray diffraction data. A single red-orange crystal (0.33 ×
0.19 × 0.11 mm3) was selected and mounted using Super Glue onto a
glass fiber and cooled to 100 K with a stream of dry nitrogen gas. A
second sample was used to record data at a variety of temperatures
(see the Supporting Information). Cell parameters were retrieved
using APEX II software42 and refined using SAINT+43 on all observed
reflections. Unit cell parameters were obtained from 60 data frames,
0.5° Φ, from three different sections of the Ewald sphere. The data set
was treated with SADABS44 absorption corrections on the basis of
redundant multiscan data. The structure was solved by direct methods
and refined by least-squares methods on F2 using the SHELXTL
program package.45 All hydrogen atoms were treated as idealized
contributions. Except as noted, all non-hydrogen atoms were refined
with anisotropic displacement parameters. The systematic absences
were consistent with the centrosymmetric, monoclinic space group
P21/n. The data set consisted of 69951 reflections (13516 unique, Rint
= 0.0896) collected over θ = 1.746−28.282°. The monoclinic crystal
has an asymmetric unit that contains one molecule of the
[Pd(dtbpf)Br]+ cation and one [TFAB]− anion. After an anisotropic
model of the molecule was completed, there remained large residual
electron density peaks around the palladium, iron, and bromine atoms
that suggested a second isomer was present as a minor component.
Alternate sites for the Pd, Fe, Br, and P centers were located in the
Fourier difference maps and were modeled as isotropic atoms whose
relative occupancy was refined to keep unit cell contents constant. The
minor component occupancy of the heavy atoms refined to 9%. The
light atoms of the minor component were not modeled, and the
occupancies of the corresponding major component were fixed at 1.0.
This two-isomer model gave a goodness of fit on F2 of 1.032 with R1 =
4.28% (I > 2σ(I)) and wR2 = 9.76% (all data) and with a largest
difference peak and hole of 0.746 and −0.887 e/Å3. Reported
Catalytic Studies. The reactions were performed under argon in
THF (1.5 mL) with 1.5 mmol of 4-chlorotoluene, 1.65 mmol of
propiophenone, 1.65 mmol of sodium tert-butoxide, and 0.03 mmol of
the desired palladium complex ([Pd(dtbpf)Cl2], [Pd(dtbpf)Cl]-
[BArF], [Pd(dtbpf)Br][Br], or [Pd(dtbpf)I][I]) . The reaction
mixtures were stirred and heated for 3 h at 60 °C before being
cooled to room temperature. A 100 μL sample of the reaction mixture
was diluted in 1.00 mL of a THF solution of phenanthrene (∼7 mg).
The sample was then injected into a CG-MS, and the concentration of
the desired product was determined. Reactions were performed three
independent times with each catalyst.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00889.
X-ray crystallographic data and experimental details,
zero-field 57Fe Mossbauer spectra and parameters, and
̈
computational results (PDF)
Crystallographic data (CIF)
Cartesian coordinates for computational studies (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail for C.N.: nataroc@lafayette.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This works was supported in part by the National Science
Foundation (CHE-1057795). We thank the Kresge Foundation
for the purchase of the JEOL NMR and the Academic Research
Committee at Lafayette College for support through the
EXCEL scholars program. R.H.S. acknowledges the Consejo
measurements were taken using OLEX2.46
57
̈
Zero-Field Fe Mossbauer Spectroscopy. Spectra were
collected at various temperatures by restraining the desired sample
in Paratone-N oil. The data were measured with a constant
acceleration spectrometer (SEE Co., Minneapolis, MN). Isomer shifts
are given relative to α-Fe metal at 298 K. Data were analyzed using an
in-house package written by Evan R. King in Igor Pro (Wavemetrics).
γ refers to the full width at half-maximum.
́
Nacional de Ciencia y Tecnologıa (CONACYT) and
Fundacion Mexico for a doctoral fellowship.
́
́
REFERENCES
■
Electrochemical Procedure. Cyclic voltammetry was performed
using a CH Instruments Model CHI260D potentiostat at room
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̌
̌ ̌
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̌
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̌
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̌
̌
̌ ̌
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ADF2013.01 software suite.47 Full molecules were used for
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level, with full electron (no frozen cores) triple-ζ-potential (TZP)
basis sets and using the relativistic scalar ZORA approximation. Mayer
bond orders and atomic properties were calculated using the defaults
implemented in the ADF2013.01 program suite. The optimized
coordinates were used for further analysis with NBO 6.0 and Bader’s
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