Comprehensive Experimental and Computational Study of η6-Arene Ruthenium(II) and Osmium(II) Complexes Supported by Sulfur Analogues of the β-Diketiminate Ligand

Article abstract of DOI:10.1021/acs.organomet.8b00111

In comparison to β-diketiminates, a highly exploited class of N,N-chelating ligands, the corresponding β-thioketoiminates, monothio-substituted analogues, have received only minor attention. β-Thioketoiminates are straightforwardly prepared through treatm

Full text of DOI:10.1021/acs.organomet.8b00111

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Comprehensive Experimental and Computational Study of η6‑Arene
Ruthenium(II) and Osmium(II) Complexes Supported by Sulfur
Analogues of the β‑Diketiminate Ligand
Crystal O’Connor, Darren C. Lawlor, Conor Robinson, Helge Muller-Bunz, and Andrew D. Phillips*
̈
School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
S
* Supporting Information
ABSTRACT: In comparison to β-diketiminates, a highly
exploited class of N,N-chelating ligands, the corresponding β-
thioketoiminates, monothio-substituted analogues, have received
only minor attention. β-Thioketoiminates are straightforwardly
prepared through treatment of an appropriate β-ketoiminate with
Lawesson’s reagent. Employing standard synthetic techniques for
η6-arene Ru(II) and Os(II) β-diketiminate complexes, an
analogous series of chlorido-metal complexes supported by
different sized N-aryl substituted β-thioketoiminate ligands is
reported. However, metal ligation of a β-thioketoiminate bearing
an electron-withdrawing CF3 group was not possible. The metal−
chlorine bond in these complexes is readily activated by various
sodium or silver salts of weakly coordinating anions, affording
coordinately unsaturated cationic formally 16-electron species. All η6-C6H6 metal β-thioketoiminate complexes were
characterized by NMR and in the solid state using single crystal X-ray diffraction techniques. Structural studies reveal that
incorporation of a thio-group induces substantial bond angle distortion within the metallocycle. The reactivity of the cationic η6-
C6H6 Ru(II) β-thioketoiminate complexes toward alkynes and isonitriles is analogous to that of the β-diketiminate species.
Specifically, the reaction with 1-hexyne results in a [4 + 2] cycloaddition involving the metal and β-C sites, while reaction with
isonitrile completely displaces the η6-C6H6 ligand. A comprehensive DFT study employing charge decomposition analysis
(CDA) reveals a strong covalent metal−sulfur bond which dominates the metal β-thioketoiminate interaction. The M−S bond
(M = Ru or Os) is strengthened by charge transfer from metal to sulfur, in contrast to the β-diketiminate species where back
electron donation from the metal to the nitrogen centers is negligible. The first reported β-selenoketoiminate was prepared by
reacting a β-ketoiminate with the Woolins’ reagent. However, this seleno-analog demonstrated significant instability with respect
to hydrolysis, and coordination to an η6-arene Ru(II) or Os(II) moiety proved unsuccessful.
plexes with reduced donating capability imparted by the
inclusion of fluoroalkyl substituents were also reported.14,15
INTRODUCTION
■
A
The chelating anionic β-diketiminate class of ligand (NacNac)
has engendered significant interest over recent years, affording a
diverse range of novel transition metal complexes with unusual
bonding properties and chemical reactivity.1−7 Within the
group 8 triad, the well-documented class of iron-centered β-
diketiminate complexes has exhibited a range of chemical
behavior with the ability to activate extremely inert bonds
including those of N2.8,9 Moreover, the application of sterically
bulky β-diketiminate ligands has enabled the isolation of
complexes representing trapped catalytic intermediates which
has been instrumental for the further elucidation of reaction
mechanisms.10 In recent years, complexes involving the heavier
members of the group 8 triad, Ru and Os, have also been
reported, Scheme 1. Phillips and Dyson et al. described a series
of chlorido-substituted (1a,b) and cationic (2a−d) η6-arene
coordinated Ru(II)11,12 and Os(II)13 complexes supported by
β-diketiminate ligands featuring strong donor and steric
influence, imparted through alkyl ortho-substitution on the
flanking N-aryl groups. Conversely, Ru β-diketiminate com-
prominent feature of cationic η6-arene coordinated Ru(II) and
Os(II) β-diketiminato complexes (2a,b) is the ability to
undergo reversible bifunctional cycloaddition reactions with
substrates containing π-unsaturated bonds (Scheme 1),16,17
whereby alkynes form highly stable tripodal adducts. Similarly,
molecular dihydrogen is readily activated to afford Ru(II)- or
Os(II)-β-diimine hydrido complexes (Scheme 1).12
Complexes 2a and 2b demonstrate versatility as catalysts for
a number of synthetic applications including the homogeneous
dehydrogenation of amine boranes,13,18 alkene hydrogena-
tion,11,12 and Diels−Alder cycloaddition reactions.15
Recently, O’Connor et al. have expanded this series of η6-
arene Ru(II) compounds to include the β-ketoiminate ligand
featuring mixed N,O coordinating centers. Furthermore, we
Received: February 23, 2018
© XXXX American Chemical Society
A
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with sulfur or selenium would modulate metal−ligand
interactions and hence the reactivity of the resulting complexes.
In contrast to β-ketoiminates or β-diketiminates, the
synthesis of β-thioimines or β-aminothiones are described
through only a handful of literature references.26 Standard
methodology for the conversion of a carbonyl functional group
into a thio- or seleno-carbonyl involves the use of sulfur and
selenium transfer agents. Duguary et al. prepared the first β-
thioketoiminates, albeit in low yields, through the reaction of β-
ketoiminates with excess phosphorus pentasulfide (P4S10).26
Subsequently, Walter et al. reported increased yields employing
Lawesson’s reagent (LR) (Scheme 3).,27,28 Since the initial
Scheme 1. Structure and Reactivity of Chlorido-Substituted
(1a−b) and Cationic (2a−d) η6-Arene-Ru(II) and -Os(II) β-
Diketiminate Complexes
Scheme 3. Commercially Available Sulfur- and Selenium-
Based Phosphetane Transfer Agents Employed in This Study
report of the putative N-phenyl β-thioketoiminate by Duguary,
only a handful of variants have been prepared and
characterized, including α-trifluoromethyl-α′-phenyl and α,α′-
bis-phenyl substituted versions, but have not yet been
employed as ligands.30 In contrast, the synthesis of β-
selenoimines (β-aminoselenones) are currently unreported;
hence, this work details the first known preparation and
characterization of this selenium version of a β-diketiminate
ligand, employing the commercially available Woollins’ reagent
(WR) as the selenium transfer reagent (Scheme 3).44−46
A limited number of complexes featuring a β-thioimine are
known, but include Cu(II),31 Ti(IV),32 Ni(II),33 Pd(II),
Pt(II),34 Au(I),35 and Cd(II),36 where the metal center is
bound by two coordinating thio-based ligands. The number of
complexes bearing thiolated salicylaldimines is more extensive
and examples containing first row metals from Fe to Zn, and
previously reported the first example of a Ru(II) complex
supported by a N-aryl anilidoimine ligand.18
This report focuses on extending the range of complexes
featuring π-unsaturated monoaza anionic ligands incorporating
either a coordinating sulfur or selenium center (Scheme 2). In
Scheme 2. Structures of Nitrogen, Oxygen, and Sulfur π-
Bonded Heterocyclic Protonated Ligand Precursors
several noble metals, i.e., Rh, Ir, Pt, and Pd are reported.37−43
A
few of these complexes have been used extensively as olefin
polymerization catalysts.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of β-Thio- and β-
Seleno-ketoiminates. A series of β-thioketoiminates with
different N-aryl substituents (3a−d) were synthesized from the
corresponding β-ketoiminates employing LR in anhydrous 1,2-
dimethoxymethane (Scheme 4). Previous reports indicated that
the thionation process is generally complete within 1−2 h.29
However, we found that optimum yields involved reaction
times around 24 h. Moreover, it is observed that reaction times
of >24 h have no discernible effect on the yield. The isolated β-
contrast, numerous Ru complexes featuring coordinately
saturated thio-based ligands are known,19−21 while ligands
based on selenium22,23 and tellurium23,24 are significantly rarer.
Singh et al. reported several half-sandwich Ru(II) complexes
bearing tridentate ligands incorporating sulfur, selenium, and
tellurium as the coordinating centers.25 The corresponding
complexes are efficient transfer hydrogenation catalysts for
various types of ketones and are proposed to operate via a
bifunctional mechanism. Moreover, these species are also
oxidation catalysts for substituted alcohols. These complexes
afforded conversion rates which are correlated with the strength
of the metal−chalcogen bond. On the basis of this and other
reports, we hypothesized that substituting the oxygen or
nitrogen center of β-ketoiminate or β-diketiminate respectively
Scheme 4. Reaction Conditions Employed for the Synthesis
of β-Thioketoiminates 3a−d
B
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thioketoiminates are viscous red oils with yields of 70−90%,
except for the least sterically encumbered compound 3a at 17%.
The synthesis of two differently substituted β-selenoketoi-
minates (4a,b) were accomplished in an analogous manner
with some key modifications (Scheme 5). Critical to the
The solid-state structure of 3a resolved by X-ray diffraction
(Figure 1) reveals a Z-conformation typically associated with N-
Scheme 5. Reaction Conditions Employed for the Synthesis
of β-Selenoketoiminates 4a,b
successful isolation of 4a,b was the judicial selection of solvent,
where various types of glymes, used in synthesis of 3a−d,
resulted in yields <10%. The highest yielding method consisted
of combining the β-ketoiminate precursor with 0.5 equivalents
of WR dissolved in anhydrous toluene.46 After the first 24 h, 1H
NMR of the reaction mixture revealed a 50:50 mixture of β-
selenoketoiminate and the precursor. Addition of another 0.5
equivalents of the WR reagent with reaction stirring for 1 week
proved beneficial toward obtaining a moderate yield. The
stability of 4a,b differed significantly from that of the thiolated
analogues. Compounds 4a,b are viscous red oils; however, if
kept in solution, a gradual color change from red to yellow is
observed within 12 h, at which point 1H NMR analysis revealed
reformation of the precursor β-ketoiminate.
Figure 1. ORTEP representation of 3a. Thermal ellipsoids are drawn
with 50% probability. Selected bond distances (Å) and angles (deg):
N−C4 1.332 (15); S−C1 1.694 (12); C4−C3−C1 128.57 (11); N−
C4−C5 121.37 (10); S−C1−C2 116.64 (8).
aryl substituted β-enaminoketones. Compared to the β-
diketiminate analogue of 3a, the replacement of a N-phenyl
group with S causes a distinctive distortion at the C(α)S
center as typified by a wide C4−C3−C1 bond angle, indicating
a high degree of s-bonding character at the β-C site and
supported by the extensive deshielding at this position as
observed by 1H NMR. As with β-diketiminates and β-
ketoiminates, intramolecular hydrogen bonding is observed
between the S and N centers, with the latter as the H donor,
affording a distance of 2.960(1) Å. The N−C bond in 3a
(1.334(2) Å) is longer than the imine bond of 1.314 Å in the
protonated N-phenyl β-diketiminate.
1
The H NMR spectra of 3a−d and 4a,b are very distinctive
in comparison to the precursory β-ketoiminates, particularly the
δ(H) of N−H and β-H positions. Selected δ(1H) and δ(13C)
values are provided in Table 1. For β-ketoiminates and β-
diketiminates, δ(H) of β-H is between 5.0 and 5.5 ppm and
δ(H) N−H is between 11 and 12 ppm.47−49 In contrast, the β-
thio- and β-seleno-ketoiminates are strongly deshielded with
δ(H) β-H around 6.35−6.84 ppm, and δ(H) of N−H is
extremely deshielded, i.e., 15.33−15.61 ppm. The similar
electronegativity values of S and Se thus imparts greater π-
type character to the core atoms of β-thio- or -seleno-
ketoiminates. Therefore, δ(H) indicates that the β-H has
electron density analogous to that of aromatic type protons.
The δ(13C) values for the CS bonds are more deshielded
compared to those of thiocarbonyl groups bonded to amides,
i.e., PhC(S)NH2, 202.1 ppm,50 but considerably shielded as
compared to those of alkyl-substituted thiocarbonyls (270−280
ppm).51 The exception of the series is 3d which has a
deshielded signal for CS due to the neighboring α-CF3
group. Likewise, the δ(13C) values of the CSe bonds are
within the range reported for other compounds featuring
selenium carbonyls attached to amide groups (202−206
ppm).52
Synthesis of Chlorido-Substituted η6-Arene Ru(II) and
Os(II) β-Thioketoiminate Complexes. To facilitate complex-
ation of the η6-benzene Ru(II) or Os(II) moiety, in situ
preparation of the corresponding lithiated β-thioketoiminate
(3a−c) was performed and subsequently added to 0.5
equivalents of (M(η6-C6H6)Cl2)2 dimer (M = Ru or Os).
After 24 h, a standard workup procedure afforded complexes
5a−c and 6 with yields of 44−89% (Scheme 6). Complexes 5c
and 6 featuring 2,6-iPr2C6H3 as the N-aryl group are the most
kinetically stable of the series and provided the highest yields.
Moreover, both species are highly air and moisture stable both
in the solid state and solution with no visible signs of
decomposition as monitored by solution 1H NMR over a
period of several months. In contrast, the analogous Ru and Os
β-diketiminate complexes degrade within several hours upon
exposure to atmospheric conditions. Reducing the size of the
N-aryl group resulted in dramatically decreased yields for the
a
1
Table 1. Selected H and 13C{1H} Chemical Shift Data (ppm) for β-Thioketoiminates 3a−d and β-Selenoketoiminates 4a,b
3a
3b
3c
3d
4a
4b
δ(1H)
β-H
6.26
6.45
6.35
15.30
δ(13C)
6.84
6.71
6.66
N−H
15.56
15.33
15.46
15.60
15.61
β-CH
113.8
207.6
112.9
205.9
112.7
206.9
111.6
184.0
118.4
204.3
118.7
204.3
C(S/Se)
a
All spectra recorded in CDCl3.
C
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diketiminate complexes 2a−d,12 chlorido substituents 5a−c
and 6 are readily abstracted using sodium or sliver salts
featuring weakly coordinating anions, i.e., tetrakis-3,5-bis-
(trifluoromethyl)phenyl borate (herein abbreviated as BArF)
or PF6 (Method A, Scheme 7). Moreover, formation of the
Scheme 6. Reaction Conditions Employed for the Synthesis
of Chlorido-η6-arene−Metal Complexes Supported by β-
Thioketoiminate Ligands 5a−c (M = Ru(II)) or 6 (M =
Os(II))
Scheme 7. Reaction Conditions Employed for the Synthesis
of Cationic η6-Arene Metal Complexes Supported by β-
Thioketoiminates 7b,c (M = Ru(II)) or 8 (M = Os(II))
Using Method A or B in Situ
Ru complexes 5a,b showing greater sensitivity to air and
moisture, where 5b decomposed within hours. Interestingly,
the chloro-substituted η6-C6H6 Ru complex featuring the highly
bulky β-diketiminate ligand (2,6-iPr2C6H3) cannot be syntheti-
cally prepared presumably due to strongly unfavorable steric
interactions. The methodology for ligand complexation
presented in Scheme 6 was not successful when applied in
the case of 3d. Alternatively, thallium alkoxides such as TlOEt
which have been previously reported as highly effective
deprotonation reagents and greatly facilitating the trans-
metalation process for numerous types of organo-ruthenium
complexes and in particularly Ru complexes featuring
perfluorinated β-diketiminate ligands15 also proved ineffective,
although a color change from red to dark brown is observed
within 5 min upon reaction with TlOEt. Subsequently, the
addition of (Ru(η6-C6H6)Cl2)2 to this ligand failed to
transmetalate, where 19F NMR showed an abundance of
uncomplexed ligand 3d and several currently unidentified
minor species.
triflate salts is a convenient one-pot procedure using a slight
excess of [Na]OTf with lithiated 3b or 3c in the presence of the
η6-arene Ru chloride dimer (Scheme 7, Method B).53The
resulting brown or dark green colored complexes are isolated in
yields of 33−68%. Sensitivity of the cationic complexes to air
and moisture was significantly more extreme than that of the
chloride-substituted analogues, with exposure in the solid state
or solution causing visible decomposition within minutes.
1
Structural Characterization in Solution. Diagnostic H
and 13C{1H} NMR signals of the chlorido-substituted and
cationic β-thioketiminate organometallic complexes are given in
Tables 2 and 3, respectively. Due to the π-conjugation within
the chelating ligand, the δ(1H) value of the β-H position
provides important information regarding the relative electron
density donated to the metal center. The β-H resonances for
5a−c are shielded (5.75−5.88 ppm) compared to that of the
protonated uncoordinated ligand. The apparent differences in
steric profile associated with the N-aryl group have only a
minor influence on the δ(1H) β-H values. Os species 6 features
the most deshielding β-H site. This trend is also paralleled for
the protons associated with the η6-arene, with 6 demonstrating
the greatest deshielded signal, suggesting greater electron
donation from this ligand to metal for Os compared to Ru.
Analogous to the β-diketiminate complexes, cationic complexes
7b,c and 8 present with significant deshielding of the δ(1H) β-
H site with values of 5.89−7.39 ppm. Thus, a strong indication
of increased π-delocalization is associated with the core
component of the coordinating β-thioketoiminate ligand.
Moreover, the δ(1H) associated with the η6-benzene changes
from 4.9 ppm with Cl-substitution to 6.22 ppm for 7b and 7.39
ppm in 7c; these values approach that of free benzene, i.e., 7.35
ppm (in CD2Cl2).54 This suggests that the η6-arene ligand is
not as strongly donating as that in the Cl-coordinated
Attempted Synthesis of an η6-Arene Ru(II) β-
Selenoketoiminate Complex. Coordination of the β-
seleno-ketoiminate to an η6-arene Ru(II) fragment was
attempted with a variety of methodologies: First, through the
standard in situ formation of a lithiated complex, followed by
the transmetalation process. However, addition of the (Ru(η6-
C6H6)Cl2)2 dimer resulted in no apparent color change being
observed, with almost all of the Ru dimer starting material
recovered. Subsequent modifications to the reaction conditions
revealed no formation of the target complex nor any starting
material 4a recovered as determined by 1H NMR and ESI mass
spectrometry. Instead, a series of overlapping manifold of peaks
was observed, resembling spectra normally associated with
polymeric substances; hence, it is likely the n-BuLi activates the
β-seleno-ketoiminate beyond simple deprotonation. Two
separate alternative methods of transmetalation were attempted
with TlOEt and Ag2O; the latter reagent proving valuable in
some cases for coordinating highly fluorinated β-diketiminates
to an η6-arene Ru moiety.13 However, both attempts were also
unsuccessful. Hence, it is apparent that the N-aryl β-
selenoketoiminate is too unstable in solution to survive the
time required to complete transmetalation process.
Synthesis of Cationic η6-Arene Ru(II) and Os(II) β-
Thioketoiminato Complexes. Following the methodology
developed for cationic η6-arene Ru(II)- and Os(II)-β-
1
compounds. The H NMR data also suggests that there are
negligible differences for the type of N-aryl employed, i.e., 7b
D
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1
Table 2. Selected H and 13C{1H} Chemical Shift Data (ppm) for the η6-C6H6 Ru(II) and Os(II) Complexes Bearing β-
Thioketoiminate Ligand: Chlorido-Substituted Complexes 5a−c and 6 and Triflato-Substituted and Cationic Species 7b,c and
a
8
5a
5b
5c
6
7b (OTf)
7b (PF6)
7c (OTf)
7c (BArF)
8 (BArF)
metal
Ru
Ru
Ru
Os
Ru
1H
Ru
Ru
Ru
Os
η6-C6H6
β-CH
4.91
4.93
5.88
4.99
5.85
5.16
5.04
6.06
5.80
6.22
5.01
5.79
7.39
6.38
7.83
5.75
6.03
5.89
13C{1H}
η6-C6H6
β-CH
87.0
86.8
86.5
78.2
87.1
84.7
85.0
85.0
78.9
116.9
166.7
120.0
166.4
119.6
167.0
122.1
167.5
120.5
167.6
166.3
119.4
173.9
168.2
120.5
167.6
165.8
119.5
173.5
170.1
118.1
176.1
170.3
CS
CN
a
All spectra recorded in CD2Cl2.
1
Table 3. Selected H and 13C{1H} Chemical Shift Data
Thus, the NMR data suggests a stronger metal−ligand
interaction exists in the case of the β-thioketoiminate as
compared to β-diketiminate. Moreover, this enhanced bonding
diminishes the interaction between the metal and η6-arene.
Solid-State Structural Characterization. Chlorido-com-
plexes 5b,c and 6 were characterized in the solid-state by X-ray
diffraction analysis (5c shown in Figure 2, 5b and 6 shown in
(ppm) for Cationic η6-C6H6 Ru(II) and Os(II) Complexes
Supported by Either a β-Thioketoiminate (7c and 8) or a β-
Diketiminate (2a,b) Ligand
a
7c (PF6)
8 (BArF)
2a (OTf)
2b (OTf)
metal
Ru
Os
1H
Ru
Os
η6-C6H6
β-CH
5.79
7.39
6.37
5.04
6.06
5.79
7.00
7.82
13C{1H}
η6-C6H6
β-CH
CS
CN
ref
85.0
78.1
84.1
77.1
120.5
167.6
117.4
175.8
105.6
107.9
163.9
12
165.7
13
herein
herein
a
All spectra recorded in CD2Cl2.
versus 7c, as both di-ortho-alkyl substituted aryl groups are
expected to have similar steric repulsive effects toward the η6-
arene metal fragment. A comparison between complexes
bearing the triflato group against those with a weakly
coordinating anion (PF6 or BArF) shows more shielded
resonances, especially for the β-H position, with values similar
to those of the Cl-substituted complexes. Similarly, the δ(13C)
values of the β-C, CN, and CS positions reflect subtle but
notable differences when triflato is employed. Thus, in contrast
to Ru β-diketiminate complex 2a, a weak Ru−O(SO2CF3)
bond in solution exists in 7b and 7c. For cationic Os complex 8,
again the increased deshielding of the β-H and the η6-arene
protons is consistent with increased electron donation from the
β-thioketoiminate, with the β-H site being the most deshielded
of all the complexes described, while the η6-arene gains more π-
delocalized character, indicating a decreased π-back bonding
with the metal center. Interestingly, there is a large increase in
shielding of the CS position in all cases, as the δ(13C) values
∼205 ppm change to 167 ppm in 7c and 8. However, as 13C
chemical shifts are influenced partially by paramagnetic
contributions, changes in δ(13C) values are not directly related
to alterations in electron density of the SC bond, but more
by changes in the energy of the n →π* transition.55,56
Figure 2. ORTEP representation of 5c. Thermal ellipsoids are drawn
with 50% probability. Selected bond distances (Å) and angles (deg):
Ru−Cl 2.432(3); Ru−C(cent) 1.693 (6); Ru−N 2.133 (1); Ru−S
2.338 (4); S−C(16) 1.703(2); N−C(13) 1.309(2); S−Ru−N
90.49(3); C(16)−C(15)−C(13) 129.72(1); C(cent)−Ru−N/S-
(midpoint) 150.47(3); Ru−N/S(midpoint)−C(15) 153.45(5), Ru−
N−C(1) 113.27(8); Ru−N−C(13) 128.89(9); Ru−S−C(16)
110.95(5).
Figures S2 and S3). A comparison of relevant metric
parameters is given in Table 4. All complexes presented with
a piano-stool type geometry typical of heavy group 8 metal
complexes bearing an η6-arene ligand. Previously, we reported
that sterically bulky β-diketiminate Ru(II) and Os(II)
complexes containing a M−Cl bond, exhibit a strong folding
along a vector defined by the two coordinating nitrogen
centers. This structural feature is also present for the herein
described β-thioketoiminate complexes where the fold occurs
along the N,S-vector (Figure 5). A second less prominent fold
also occurs through the N,S-chelating ligand along a vector
defined by metal and β-C position. The solid-state packing of
chloro-substituted complexes 5b,c and 6 are characterized by
short intermolecular contacts between the S center and a
hydrogen associated with an η6-arene, as detailed in the
Supporting Information. Complexes 5b and 5c possess long
Ru−Cl bond distances of 2.437(5) and 2.432(3) Å,
In comparison of δ(1H) values for the β-H and η6-arene
protons associated with the cationic η6-C6H6 Ru(II) and Os(II)
β-diketiminate complexes (2a,b) with the analogous
N-iPr2C6H3 substituted 7c and 8 (Table 3) suggest greater π-
delocalization in the β-thioketoiminate cases, in parallel with
weaker a bonding interaction between the η6-arene and metal.
E
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Table 4. Selected Crystallographic Data for Chlorido-Substituted η6-C6H6 Ru(II) and Os(II) Complexes Bearing a β-
a
Thioketoiminate or β-Diketiminate Ligand
b
parameter
5b
5c
6
1a
1b
metal (M)
M−N
M−S
Ru
Ru
Os
Ru
Os
2.142(1)
2.332(1)
2.437(1)
1.689(1)
1.303(3)
1.709(2)
1.442(3)
1.351(3)
87.8(1)
2.133(1)
2.338(1)
2.432(1)
1.693(1)
1.310(2)
1.703(2)
1.442(2)
1.361(2)
90.5(1)
2.132(1)
2.344(1)
2.443(1)
1.685(1)
1.314(2)
1.706(2)
1.438(2)
1.364(2)
90.7(1)
2.099(2)
2.123(4), 2.127(4)
M−Cl
2.521(1)
1.688(1)
1.335(3)
2.416(1)
M−C(cent)
NC(α)
SC(α)
1.693(3)
1.296(6), 1.280(7)
C(N)−C(β)
1.394(3)
1.500(8), 1.486(8)
C(S)−C(β)
N−M−S
N−M−N
86.6(1)
84.7(2)
M−N−C(ipso)
M−N−C(α)
M−S−C(α)
N−C(α)−C(β)
S−C(α)−C(β)
C(α)−C(β)−C(α)
C(cent)−M−Cl
C(cent)−M−N/S(midpoint) (ϕ)
C(cent)−M−N/N(midpoint) (ϕ)
M−N/S(midpoint)−C (θ)
M−N/N(midpoint)−C (θ)
ref
112.0(1)
130.2(1)
109.8(1)
125.3(2)
126.8(2)
127.9(2)
125.5(1)
148.0(1)
113.3(1)
128.9(1)
111.0(1)
126.0(1)
126.9(1)
129.7(1)
124.4(1)
150.4(1)
113.6(1)
128.8(1)
111.4(1)
126.5(1)
126.5(1)
130.1(2)
125.2(1)
151.4(1)
118.5(2)
124.7(2)
116.9(3), 115.5(3)
126.3(4), 121.8(5)
124.4(3)
121.6(5), 122.8(5)
126.5(1)
122.2(1)
118.9(4)
123.7(1)
154.1(1)
152.6(1)
147.9(1)
herein
153.5(1)
herein
154.7(1)
herein
154.8(2)
12
145.0(2)
13
a
b
Bond lengths in Å and angles in degrees. For the definition of the folding angles ϕ and θ, see Figure 4. C(cent): refers to centroid point of the η6-
C6H6 ligand.
respectively. These values are longer than the median value of
2.38 Å for all η6-arene type complexes with a Ru−Cl bond as
indicated by the Cambridge Structural Database (CSD),57 but
are significantly shorter than M−Cl 2.526(2) Å in 1a.12
Interestingly, Os complex 6 has a M−Cl bond distance of
2.443(1) Å, and in this case, longer than that in β-diketiminate
counterpart 1b, 2.416(1) Å.13 As expected for species 5b and
5c, the longer SC(α) and Ru−S have an effect of widening
the C(α)−C(β)−C(α) bond angle. While the Ru−S bond is
equivalent between 5b and 5c, the Ru−N bond changes with
different types of N-aryl substituents, whereas a shorter Ru−N
bond in 5c is observed when the N-aryl group is a bulkier 2,6-
diisopropylphenyl. This change is also reflected in a longer N
C(α) bond and a wider N−Ru−S bond angle for 5c. A search
of the CSD shows a median Ru−S bond distance of 2.403 and
1.706 Å for a S−C and SC bond, respectively.57 Hence, 5b
and 5c both feature Ru−S bond distances which are typically
associated with SC bonds. Complex 6 processes a longer M−
S bond, but the S−C distance is equivalent to the Ru analog. In
contrast, the imine bond is only slightly shorter. The S−Ru−N
bite angles of 87.7(1)° for 5b and 90.5(1)° in 5c are
comparable to the N−Ru−N bite angle of 86.7(1)° in 1a. A
characteristic feature of all herein described β-thioketoiminate
complexes are the narrower M−S−C(α) bond angles which are
roughly 18° less than the interior Ru−N−C(α) bond angles.
The M−N−C(ipso) bond angles are less than those of the
analogous β-diketiminate complexes. However, the internal
ligand folding angles (θ, M−N/S−C(β)) are 147.9 and 150.4°
for 5b and 5c which indicates that the core component of the
ligand is not planar. Thus, analogous to β-diketiminate
complexes, complex 5b,c and 6 do not achieve complete π-
delocalization within the core component of the β-thioketoi-
minate ligand; hence, an allylic π-type S−M−N interaction is
expected as the primary interaction between the metal and S,N
coordinating centers.
Cationic complexes 7c and 8 were also characterized in the
solid state (7c in Figure 3, 8 in Figure S4). For comparative
purposes, corresponding cationic Ru and Os β-diketiminate
complexes 2c,d, bearing the highly bulky 2,6-diisopropylphenyl
group as the flanking N-aryl, were prepared according to
literature procedures and the solid state structures determined;
see the Supporting Information. Selected metric parameters for
all cationic Ru and Os complexes are given in Table 5. Removal
Figure 3. ORTEP representation of 7c (BArF counterion). Thermal
ellipsoids are drawn with 50% probability. The anion is omitted for
clarity. Selected bond distances (Å) and angles (deg): Ru−C(cent)
1.692 (1); Ru−N 2.035 (2); Ru−S 2.214 (6); S−C(16) 1.696 (3); N−
C(13) 1.329 (3); S−Ru−N 94.15 (1); C(16)−C(15)−C(13) 131.0
(2); C(cent)−Ru−N/S(midpoint) 172.33 (6); Ru−N/S(midpoint)−
C(15) 168.9 (1); Ru−N−C(1) 113.55 (14), Ru−N−C(13) 130.33
(17); Ru−S−C(16) 113.88 (9).
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a
Table 5. Selected Crystallographic Data for the η6-C6H6 Ru(II) and Os(II) Complexes Bearing a β-Thioketoiminate Ligand
b
c
parameter
7c
8
2c
Ru
2d
metal (M)
Ru
Os
Os
2.022(4)
2.021(4)
2.020(4)
2.022(4)
2.009(2)
2.012(2)
M−N
2.035(2)
2.025(2)
M−S
M−C(cent)
2.214(1)
1.692(1)
2.227(1)
1.702(1)
1.710(2)
1.351(6)
1.347(6)
1.705(2)
1.354(6)
1.352(6)
1.701(1)
1.347(2)
1.350(2)
NC(α)
SC(α)
1.329(3)
1.696(3)
1.419(4)
1.340(2)
1.702(2)
1.415(2)
1.402(8)
1.390(8)
1.392(8)
1.389(8)
1.385(3)
1.388(3)
C(N)−C(β)
C(S)−C(β)
N−M−S
1.359(4)
94.2(1)
1.369(3)
94.1(1)
N−M−N
89.8(2)
116.3(3)
116.0(3)
127.9(3)
127.7(3)
90.0(2)
117.3(3)
116.4(3)
127.5(3)
127.5(3)
89.7(1)
116.8(1)
116.4(1)
127.5(1)
128.0(1)
M−N−C(ipso)
113.6(1)
113.6(1)
M−N−C(α)
M−S−C(α)
130.3(2)
113.9(1)
125.5(2)
130.6(1)
113.7(1)
125.9(2)
122.6(5)
123.1(5)
123.0(5)
123.1(5)
123.6(2)
123.0(2)
N−C(α)−C(β)
S−C(α)−C(β)
C(α)−C(β)−C(α)
C(cent)−M−N/S(midpoint) (ϕ)
C(cent)−M−N/N(midpoint) (ϕ)
M−N/S(midpoint)−C (θ)
125.0(2)
131.0(2)
172.3(1)
125.2(1)
130.5(2)
171.7(1)
128.6(4)
179.9(2)
175.6(3)
128.7(4)
178.3(1)
180.0(4)
128.1(2)
179.3(1)
177.2(1)
168.9(1)
168.1(1)
M−N/N(midpoint)−C (θ)
a
Bond lengths in Å and angles in degrees. Cationic species 7c and 8. For comparison, cationic complexes η6-C6H6 Ru(II) β-diketiminate complexes
b
c
2c−d. For the definition of the folding angles ϕ and θ see Figure 4. C(cent): refers to centroid point of the η6-C6H6 ligand. Two
crystallographically independent molecules are contained within the unit cell.
of the chlorido substituent from 5c and 6, induces a number of
notable structural changes, including a significantly shortening
of the M−N and M−S bonds by approximately 0.1 Å
accompanied by a widening of the S−M−N bite angle. The
metal to centroid distance of the η6-arene is practically
unchanged for the cationic Ru complex but increases for the
Os species 8. Interestingly, the NC(α) bond lengthens in 7c
and 8 as compared to 5c and 6, whereas the SC(α) bond
remains invariant when the Cl group is removed and seemingly
has only a slight effect on the M−S−C(α), M−N−C(α), and
C(α)−C(β)−C(α) bond angles. The most dramatic changes
between the Cl-substituted and cationic complexes is the
Figure 4. Atom labeling and definition of the folding angles (ϕ and θ)
flattening of the core component of the β-thioketoiminate
within the β-thioketoiminate supported complexes as defined by the
ligand as indicated by the bond angle defined by the metal
centroid of the η6-arene, the metal center, the midpoint between the N
and S atoms, and the β-C site.
center, midpoint of the N,S atoms and the β-C (θ angle, Figure
4) which increases by nearly 15°. This modification enhances
the overall π-delocalization throughout the chelating ligand. An
additional striking feature is an approximately 20° increase in
complexes. Interestingly, the intermolecular hydrogen bonding
that exists with the S center of the β-diketiminate ligand in Cl-
the ϕ angle (Figure 4) which relates the tilt of the η6-arene with
respect to the metal center and the β-thioketoiminate ligand. In
comparison to the cationic β-diketiminate species, notable
differences are the longer M−N bond lengths in the β-
thioketoiminate species, which are matched by shorter N
C(α) bonds, particularly in the case of Ru. A comparison with
β-diketiminate species 2c and 2d bearing the bulky 2,6-
diisopropyl-phenyl groups shows a marked widening of the M−
N−C(ipso) bond angles as compared to those in the β-
thioketoiminate species, but only minor differences are
observed in the interior N−C(α)−C(β) and the C(α)−
C(β)−C(α) bond angles between the two different classes of
substituted species 5b,c and 6 is not present in cationic species
7b or 8, and no intermolecular interactions with the S center
are observed, suggesting that the increased bonding interaction
with the metal centers reduces lone pair character on the
coordinating S.
Chemical Reactivity of the η6-Arene Ru(II) β-Thio-
ketoiminato Complexes. Since the initial reports of η6-arene
Ru(II) β-diketiminate complexes, we have shown that the
reactivity of these classes of compounds is strikingly different
from that observed for other types of η6-arene Ru(II)
complexes including ones featuring β-diketonates,58 which are
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Scheme 8. Comparison of Reactivity between η6-Arene-Ru(II) β-Diketiminate 2a and β-Thioketoiminate Complex 7b
known to form simple coordination adducts with typical
nucleophiles such as amines or phosphines (Scheme 8). The
cationic 16-electron complexes 2a,b exhibit bifunctional activity
with a strong preference for undergoing [4 + 2] cycloadditions
with alkenes and alkynes, in addition to reversible heterolytic
activation of H2.12 As an initial probe into the reactivity of the
cationic Ru(II) β-thioketoiminate complexes, the addition of
PPh3 to 7c failed to demonstrate any adduction formation as
monitored by 31P NMR.18 Similarly, triphenylphosphine does
not bind to either 2a or 2b.
As demonstrated previously with acetylene, the reaction
between 1-hexyne and Ru β-diketiminate species 2a afforded 9
in quantitative yield. The analogous experiment was performed
by first dissolving 7c in CH2Cl2 and adding an excess of pure 1-
hexyne. Instantly, a yellow color developed and with a standard
workup, complex 10 was afforded with a yield of 86%.
Recognition of adduct formation, i.e., 9 and 10, is observed by
both ESI-MS and 1H NMR where the diagnostic singlet for the
terminal “yne” proton occurs at δ(1H) 9.22 ppm for 10 and
9.67 ppm for 9. The extremely deshielded signal is caused by
the orientation of this proton intersecting the deshielding cone
of the η6-arene ligand.12,13
Although phosphines do not form adducts with 2a, neutral
ligands with moderate σ-donor strength, and are also π-
acceptors do interact with the Ru center and form coordination
complexes as demonstrated by reacting 2a with 1 equivalent of
2,6-dimethyl-isonitrile (herein abbreviated as XyNC). The
resulting complex, 11, was obtained as a red colored solid in
high yield. Observation of the parent mass by ESI-MS
confirmed 11 was a monocoordination adduct. The δ(1H) of
the diagnostic β-H position in 11, 4.94 ppm, is almost identical
to that of the chlorido-substituted complex 1a and indicates a
weakened interaction between the chelating ligand and metal
center. A structure determined from X-ray diffraction studies
displays the typical piano type geometry (Figure S13), where
the β-diketiminate ligand presents with the standard folding
pattern observed when an additional ligand is added to the Ru
center.
angle are typical values and are comparable to those of other η6-
arene Ru complexes coordinated by a XyNC ligand.59,61 The
vibrational stretch of the CN bond associated with Ru
coordinated XyNC occurs at 2153 cm−1 and is shifted 33 cm−1
compared to that in the uncomplexed form, i.e., 2119.93 cm−1.
The modest change in frequency suggesting a minimal amount
of π-back-donation. Alternating the reaction conditions, i.e.,
employing excess amount of XyNC and refluxing in dry THF
for 24 h, afforded a bright purple solid 12, in 79% yield.
Solution ESI-MS of complex 12 revealed that the parent
molecular mass matches that of a Ru β-diketiminate fragment
1
with four XyNC ligands. Morover, H and 13C{1H} NMR
confirmed loss of the η6-C6H6 resonance and the δ(1H) of the
β-H position became shielded at 5.01 ppm. Other researchers
have previously documented η6-arene displacement from Ru by
XyNC, particularly when the arene is p-cymene.60 A structure
of 12 resolved by X-ray crystallography studies reveals an
octahedral Ru coordination environment with two equatorial
XyNC ligands bonded trans to the chelating β-diketiminate
ligand and the remaining two XyNC ligands occupying the axial
positions. The two trans-positioned isonitriles feature shortened
Ru−C bond distances as compared to those of the axial ligands.
Although not reflected in the individual N−C(Xy) bond
lengths, the solid-state IR of 12 shows three absorptions with
frequencies of 2113, 2130, and 2190 cm−1 corresponding to
four υ(NC) stretches. These values suggest considerable π-back
bonding from the metal to π* MOs of the XyNC ligands.61 Of
additional note is the significant folding present in the β-
diketiminate ligand, 146.45(10)° which is greater than that
observed for any η6-arene or η5-Cp* coordinated Ru β-
diketiminate species.
Reaction between the cationic Ru β-thioketoiminate complex
7c and 1 equivalent of XyNC resulted in a mixture of products.
However, when an excess of isonitrile was employed with
refluxing in anhydrous THF, a brown-yellow colored solid 13,
is obtained upon standard workup. Solution ESI-MS confirmed
that the isotopic pattern of the parent peak matches that of a
Ru β-thioketoiminate fragment with four coordinating XyNC
ligands, 13. In contrast to 12, β-thioketoiminate species 13
features a highly deshielded β-H position, δ(H) = 6.25 ppm.
The Ru−N bond lengths (2.098(2) and 2.095(2) Å) and N−
Ru−N bond angle of 87.3(1)° are undistinctive. The Ru−C(N)
bond length of 1.966(2) Å and the Ru−N−C 174.5(2)° bond
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Solid-state IR of 13 reveals a strong unresolved absorption at
2083 cm−1, which indicates that the π-back bonding between
the Ru center and π* MOs of the XyNC ligands is not as strong
as those in the β-diketiminate complex 12.
Although the η6-arene Ru β-thioketoiminate complexes
formally feature a single lone pair on the coordinating sulfur
center, preliminary attempts to form simple coordination
adducts using Lewis acids such as BF3 or Au(PPh3) with 7c
were unsuccessful. For example, Goh et al. showed that one of
the sulfur centers in the η6-arene Ru thiapentane-1,5-dithiolate
compound readily form an adduct with Au(PPh3).62 For 7c, it
is unclear if steric hindrance prevented adduct formation or
simply that the S lone pair is strongly deactivated in the cationic
Ru β-thioketoiminate complexes.
Computational Study. The in silico studies reported herein
were performed using the Gaussian 09 program63 employing
the hybrid GGA density functional ωB97XD that accounts for
long-range dispersion effects.64 As experimental structural
parameters were obtained for both types of cationic Ru- and
Os- β-diketiminate and β-thioketoiminate complexes contain-
ing the flanking 2,6-diisopropylphenyl as the N-aryl substituent,
the computational study was limited only to the cationic
formally 16-electron species (I−IV, Figure 6). All structures
Figure 6. Comparison of atomic charges for cationic η6-arene Ru- and
Os-supported β-diketiminate and β-thioketoiminate complexes using
the charge fitting scheme CM5. Values in black are complexes with N-
phenyl substitution, and values in blue are complexes bearing N-2,6-
diisopropylphenyl.
comparison of the experimental versus DFT calculated
geometry.
As an initial means of comparison between the two classes of
ligand, charge distribution through the complexes were
determined using two of the more common methods for
calculating atomic charge, the class II Hirshfeld analysis66 and
the class IV charge fitting method CM5, developed by Truhlar
and Cramer (Table S5).67 A typical alternating pattern of
positively and negatively charged atomic centers is observed
within the metallocyclic cores of the complexes Ia−IVa (Figure
6), where the β-C and N/S centers are negatively charged,
while the metal and α-carbon sites are positive. In the case of β-
diketiminate complexes Ia, the Ru center is more positively
charged than Os (IIa), while the charges on the chelating
ligands are relatively consistent except for the N centers. As
expected, the overall charge distribution for β-thioketoiminate
complexes IIIa and IVa are very different compared to those of
Ia and IIa, where the metal center is less positively charged,
while simultaneously the β-C position is less negatively charged.
The incorporation of the less electronegative S center thus
increases electron density on the metal and the adjoining α-
C(S) site. Interestingly, the population analysis indicates
increased charge on Os in IVab, while the S atom is more
depleted than in the Ru case, IIIab. This suggests a greater
interaction between Os−S. In comparison to Ru β-diketiminate
species Iab, the single N center in β-thioketoiminate IIIab is
more electronically depleted, while for both Os complexes, the
charge on N is relatively similar.
Figure 5. ORTEP representation of 12 (OTf). Thermal ellipsoids are
drawn with 50% probability. The anion is omitted for clarity and only
the core atoms of the ligands are indicated. Selected bond distances
(Å) and angles (deg): Ru−C(22) 2.002 (3); Ru−C(49) 1.951 (3);
Ru−C(40) 1.959 (3); Ru−C(31) 2.011 (3); Ru−N(1) 2.143 (2);
Ru−N(2) 2.139 (2); C(22)−N(3) 1.161 (4); C(49)−N(6) 1.160 (3);
C(40)−N(5) 1.164 (3); C(31)−N(4) 1.163 (4); N(1)−C(9) 1.325
(3); N(2)−C(12) 1.323 (4); N(1)−Ru−N(2) 86.60 (8); C(9)−
C(11)−C(12) 127.6 (2); C(22)−Ru−C(31) 176.41 (10); C(40)−
Ru−N(1) 176.97 (10); C(49)−Ru−N(2) 173.75 (10); Ru−N/
N(midpoint)−C(11) 146.45 (10).
were geometrically optimized to an energy minimum and
displayed structural metrics which are in excellent agreement
with the experimental X-ray diffraction data. In particular, bond
lengths and angles of the core metal−ligand components were
satisfactorily reproduced with the M−N−C(ipso) bond angle
parameters showing the greatest deviation up to 2°. Overall, the
calculated bond lengths were 0.3−0.6% longer and the bond
angles deviated from 0.5 to 2.6%. All calculated geometry
parameters are well within acceptable standards for DFT
modeling.65 See the Supporting Information for a complete
Comparison of N-phenyl versus the significantly bulkier N-
2,6-iPr2C6H3 (herein abbreviated as dipp) substituent for both
types of complex has the greatest effect on the metal center in
terms of reducing electron density. To further quantify the
electronic differences in metal interaction between the β-
diketiminate and β-thioketoiminate ligands, a comparison of
the Mayer bond indices (MBI) was performed for all bonds
related to the metal center (Figure 7). MBIs provide a
normalized index of bond strength, enabling a comparison
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Figure 7. Mayer bond index comparison for all bonds involving the metal center for the cationic η6-arene Ru- and Os-β-diketiminate (I, II) and β-
thioketoiminate (III, IV) complexes with phenyl and dipp flanking N-aryl groups.
Figure 8. Comparison of the molecular orbital energy levels (DFT calculated, ωB97XD) for the highest occupied and lowest occupied MOs in the
cationic Ru and Os complexes with β-diketiminate or β-thioketoiminate ligands (N-phenyl substituted). For those MOs which feature significant
overlap between the metal and β-diketiminate/β-thioketoiminates are labelled. The contribution of metal d- and higher s- and p-orbitals are indicated
as percentages. LUMOs and HOMOs with significant sulfur contribution are highlighted by the S label.
between different bond types.68,69 The influence of sterics
associated with the size of the N-aryl group are clearly evident,
as the MBI shows stronger metal bonds for those species with
the N-phenyl substituent (except for IVa,b), and the β-
diketiminate complexes (I and II) demonstrating greater
variance. Of note is the relatively high bond strength for the
M−S bond in III and IV which is matched by weaker M−C(η6-
arene) and M−N bonds as compared to those in I and II.
Furthermore, the Os−S bond in IVa,b is stronger than the
corresponding Ru−S bond in IIIa,b, but conversely, the Ru−N
bonds are stronger than those in IVa,b. As previously
documented, the interaction between the η6-arene and Os
center is greater than that for Ru.13 The increased bond order
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Figure 9. Orbital interaction diagram for the DFT (ωB97XD) energy optimized cationic η6-C6H6 Ru N-phenyl-β-diketiminate optimized in C2v
symmetry. The MO−FO pairs are connected when each fragment (FO) contributes greater than 10% to the corresponding MO. The relative 5p and
4d orbital character for Ru and %N of the β-diketiminate ligand is given in parentheses.
Figure 10. Orbital interaction diagram for the DFT (ωB97XD) energy optimized cationic η6-C6H6 Ru N-phenyl-β-thioketoiminate optimized in Cs
symmetry. The MO-FO pairs are connected when each fragment (FO) contributes greater than 10% to the corresponding MO. The relative 5p and
4d orbital character for Ru and the %N and %S of the β-thioketoiminate ligand is given in parentheses.
about the metal center in IVa,b correlates with an increased in
electron density at the Os center as compared to Ru in IIIa,b.
To probe further into the origins of the differences in
electronic structure between the types of complexes, an
examination of the energies of the frontier MOs was performed.
For simplicity, only the less sterically hindered N-phenyl-
substituted complexes (Ia−IVa) are considered herein. The
energy levels of the highest occupied and lowest unoccupied
MOs are shown in Figure 8. It is observed that the HOMOs of
the β-diketiminate complexes (Ia and IIa) are split into two
groupings, while a more even energy distribution of the
HOMOs is present within the β-thioketoiminate complexes
(IIIa and IVa). The corresponding HOMO−LUMO gaps are
smaller in IIIa and IVa, which is caused mainly by the presence
of lower energy LUMOs. Moreover, the contribution of metal
d-orbital character is far more significant in the upper HOMOs
for IIIa and IVa, while the LUMO character of all complexes is
consistent in the amount of metal d-orbital character. Although
some mixing of the higher 5s and 5p metal-based HOMOs and
LUMOs is present, its contribution is relatively minor.
Interestingly, the nitrogen contribution to metal bonding in
IIIa and IVa is buried in the lower HOMOs.
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Table 6. UV−Visible Data (λmax and Molar Absorption (ε)) for Cationic η6-C6H6 Ru and Os Complexes Supported by β-
a
Thioketoiminate 7c and 8 and β-Diketiminate 2c and 2d Ligands
7c (M = Ru)
ε (L mol−1 cm−1
8 (M = Os)
ε (L mol−1 cm−1
λmax (nm)
)
band
type
λmax (nm)
)
band
type
530
423
324
1875
3075
A
B
C
π → d
π → d
π → π*
d → d
π → π*
π → d
507
432
311
1044
1484
A
B
C
π → d
π → d
π → π*
d → d
π → π
π → d
13077
11440
279
264
20512
28298
D
E
279
270
13664
13868
D
E
2c (M = Ru)
ε (L mol−1 cm−1
2d (M = Os)
ε (L mol−1 cm−1
λmax (nm)
)
band
type
λmax (nm)
)
band
type
437
290
7888
A
B
π → d
d → π*
π → π*
d → d
484
382
302
985
5149
π → d
π → d
d → π*
π → π*
d → d
19039
A
B
19540
a
Recorded in anhydrous CH2Cl2.
To quantify even further the metal bonding differences,
LUFOs of the arene−Ru fragment (Ru 4dxy and 4dxz) resulting
in HOMO and HOMO−8 which account for the greatest
amount of forward CT. The HOMO is a B1 π-type interaction
is composed of an allylic N−Ru−N arrangement which
strongly stabilizes this class of cationic 16e− complex. Other
π-type interactions arise from the combination of β-
diketiminate HOFO-3 and HOFO-8 with the 4dxz of Ru
resulting in a moderate degree of stabilizing CT. In general, the
A1 out-of-phase σ-type interactions are the most dominant and
form occupied MOs within Ia at lower energies. The in-phase
HOFO-2 of the chelating ligand forms a weak overlap with the
only phase compatible Ru 4dz2 orbital. Bonding interactions
with greater η6-arene-metal contribution are associated with
lower HOMOs, i.e., HOMO−15 and HOMO−19.
In the case of IIIa, the FO interaction diagram (Figure 10) is
more complicated owing to the lower molecular symmetry Cs,
where the FOs of the η6-C6H6Ru fragment are of mixed d-
orbital character; however, the sum of the d-orbital
contribution IIIa remains constant in comparison to that of
Ia. One striking difference between the different types of
complexes are the relative energies of the HOFO and LUFO
associated with the η6-arene-Ru fragment. In Ia, from the f/b
CT values shown in Table S7, the greater forward CT in the β-
diketiminate case serves to increase the over orbital energies of
the arene−Ru fragment (HOFO −13.02 eV, LUFO −5.07 eV)
and prevent these HOFOs from effectively mixing with the
upper HOFOs of the β-diketiminate. Instead, for IIIa the lower
energies of the LUFOs associated with the arene−Ru fragment
(HOFO −13.32 eV, LUFO −5.37 eV) results in a combined
contribution of LUFOs and HOFOs with the upper S-based
HOFOs of the β-thioketoiminate ligand. It is clear that the β-
thioketoiminate HOFOs with dominant sulfur character
(HOFO and HOFO−1) are positioned at higher energy and
combine to a greater extent with occupied and unoccupied FOs
of the η6-arene−Ru fragment, forming lower energy HOMOs.
Thus, the HOFOs with dominant N character form weaker
overlaps with the arene-Ru fragment, thus correlating with
reduced Ru−N bond strength.
charge decomposition analyses (CDA) were performed using
the procedure developed by Frenking et al.70−72 Two separate
CDA analyses were performed, one involving the anionic ligand
and [η6-C6H6M]2+ (M = Ru and Os) fragments and the second
between the neutral η6-arene and the negatively charged metal
β-diketiminate or β-thioketoiminate fragments. Table S7
indicates the amount of forward and back electron donation
between the fragments, which can be further subdivided into
individual symmetric components related to σ- and π-type
bonding.
The CDA reveals that the β-diketiminate ligand is both a
strong σ and moderate π e− donor with almost a negligible
amount of σ- or π-back-donation from the metal. In contrast,
the β-thioketoiminate ligand is not only a strong σ- and
moderate π-donor, but also receives more electron density back
from the η6-arene metal fragment due to the better electron
accepting properties of sulfur. This correlates well with the
calculated high strength for the metal−sulfur bonds from the
MBI values which fully justifies the strong thiophilicity
observed for Ru and Os.73 An examination of the second
CDA (Table S8) between the η6-arene and metal β-
diketiminate/β-thioketoiminate fragments reveals greater for-
ward charge transfer (CT) from the ligand to metal, which in
the case of the β-diketiminate species (Ia and IIa) is matched
by strong backward CT from the metal. For β-thioketoiminates
(IIIa and IVa), the absolute amount of forward and backward
CT are reduced, but the forward/backward (f/b) ratio is
consistent with Ia and IIa. Both types of Os complexes (IIa and
IVa) demonstrate a greater tendency to participate in back-
bonding with the arene. The CDA affords the possibility to
identify the upper HOMOs that contribute to bonding between
the metal and chelating ligands. Figures 9 and 10 show the
results for both types of cationic Ru complexes, Ia and IIIa.
The figures highlight those interactions where orbital overlap
between (fragment orbitals) FOs is maximized. However, not
all overlap interactions result in significant forward or backward
charge transfer between the fragments. In the case of Ia, the
HOFOs (highest occupied fragment orbitals) of the β-
diketiminate are divided between in-phase and out-of-phase
σ- and π-type MOs as previously documented.7 HOFO and
HOFO-2 of the β-diketiminate combine exclusively with the
In contrast to Ia, for IIIa additional FO interactions result in
forward ligand to metal CT distributed amid the HOMOs of
the complex. The largest CT being associated with HOMO−2
(σ-type), HOMO (π-type) and HOMO−6 (σ-type) with the
L
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Article
Figure 11. Comparison of experimental (CH2Cl2), TD-DFT (ωB97XD, gas phase) and TD-DFT (ωB97XD, CPCM-CH2Cl2 corrected) optical
absorption spectra for cationic η6-C6H6 Ru N-2,6-diisopropylphenyl-β-diketiminate (top) η6-C6H6 Ru N-2,6-diisopropylphenyl-β-thioketoiminate
(bottom). The contribution of each type of transition to overall absorption is given in parentheses.
FOs of the β-thioketoiminate with dominant S character
contributing mainly to the CT. Interestingly, the out-of-phase
HOFO-2 of the chelating ligand shows reduced metal to ligand
MO overlap but is one of the main sources of metal to ligand
backward CT. As indicated by the CDA, back e− donation from
the Ru to S originates in HOMO−5 (σ-type) and HOMO−7
(π-type), where the latter is composed of a bonding interaction
between HOFO−1 and 4dyz of Ru.
Overall, the metal−ligand bonding in 1a is dominated by an
allylic-type N−Ru−N interaction which involves significant σ-
based and π-based CT from the N centers to the Ru; in turn,
this excess e− density is transferred to the η6-arene. In contrast,
for IIIa, β-thioketoiminate to metal CT is not as significant but
a more efficient MO overlap (greater covalency) between the
metal and S which enables backward CT; hence, the metal
ligand bonding in IIIa is dominated by the metal−sulfur
interaction. Thus, both types of complexes featuring both low
coordinate cationic metal centers are significantly stabilized, but
by different internal bonding mechanisms. The presence of a
nucleophilic β-C site suggests that both types of complex
demonstrate similar types of reactivity, specifically a strong
preference to undergo [4 + 2] cycloadditions with π-bonded
substrates over direct metal coordination with purely σ-
donating molecules.
Electronic Absorption. For the cationic complexes bearing
the 2,6-iPr2C6H3 substituent, 2c, and 7c,8, the UV−visible
adsorption spectra were recorded in CH2Cl2. λmaxima and the
associated molar absorptivity values are given in Table 6. The
experimentally obtained spectra for the two types of Ru species
are shown in Figure 11, whereas the corresponding Os species
are given in Figures S10 and S11. Each type of complex
presents with same number of absorptions regardless of the
metal type, with exception that Os species 2d has an additional
low energy adsorption. Using time-dependent DFT (TD-DFT)
techniques, the first 80 excitations were modeled with solvent
corrections,74 and the resulting simulated spectra were shown
in good agreement with experimental data. The lower energy
transitions (band A) of the TD-DFT predictions were blue-
shifted as compared to the experimental data. The magnitude of
the differences in predicted λmaxima are routinely observed in
other type of Ru complexes.72,75 Each absorption band was
matched with electronic transitions between specific MOs; see
the Supporting Information for a graphical description of the
upper HOMOs and LUMOs for each type of Ru complex. The
higher symmetric Ru β-diketiminate complex, 2c (C2v),
presents with a simplified spectrum dominated by two
absorption bands. The first absorption corresponds to a
symmetry allowed HOMO (b1) to LUMO (b1) transition of
LMCT type. The second absorption is an uneven mixture
between a symmetry-disallowed LLCT of HOMO+1 to LUMO
+1 and MLCT of HOMO−6 to LUMO+2. Interestingly,
HOMO−6 is a source of significant forward CT from the β-
diketiminate ligand to the η6-arene-Ru fragment. The rest of the
higher energy absorptions are merged and unresolved due to
limits of solvent adsorption. In contrast, the Ru β-
thioketoiminate species 7c has five distinct absorption bands,
with the first band (A) occurring at higher wavelengths,
comprising two LMCTs, where the receiving LUMO has
dominant S-based character, contributing to the lower energy
of this MO. Band B is composed of another two LMCTs
corresponding to band A in the Ru β-diketiminate complex and
features strong S character in both the HOMO and LUMO.
Similarly, band C, mainly a LLCT between the HOMO and
LUMO+1, also features a minor contribution from a MLCT
and d−d transition. This band is equivalent to band B in the β-
diketiminate species. Band D, observed in the experimental
spectra, was only resolved in the solvent-corrected TDDFT
calculated spectra and corresponds to another LLCT. Finally,
absorption E is a high-energy LMCT from the lower energy
HOMO−7 to the LUMO and is equivalent to band B in 2c.
This band features an π-type orbital overlap between the Ru, S,
and N centers and is associated with a strong CT from the
chelating ligand to the metal center. Os β-diketiminate complex
2d differs mainly from the Ru analogue in that band A shows a
hypsochromic shift, while band B is bathochromic shifted,
where the MLCT and LLCT become more separated. The
situation in the case of the Os β-thioketoiminate species 8, is
more complicated where bands B−D have merged, and band E
is hypsochromic shifted into the solvent cutoff region.
CONCLUSIONS
■
Conversion of the β-ketoiminates to the corresponding β-
thioketoiminates or β-selenoketoiminates was made possible by
reacting with the appropriate chalcogen transfer reagent.
Deprotonation of β-thioketoiminates with n-BuLi enables
transmetalation and N,S-bidentate coordination to an η6-
C6H6 Ru(II) or Os(II) moiety. In contrast, the β-
selenoketoiminates were found to be too unstable to complete
the coordination process. Cationic 16-electron Ru(II) and
Os(II) complexes supported by the β-thioketoiminate ligand
were facilitated by employing an appropriate Cl abstraction
reagent. Diagnostic NMR resonances, i.e., β-H and protons of
the η6-C6H6 group indicate that the β-thioketoiminate ligand
strongly interacts with the metal center. The experimental
observations are fully supported by the MBI values and CDA
from DFT calculations, which indicates that M−S interaction
M
DOI: 10.1021/acs.organomet.8b00111
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
silica gel/alumina (basic) 1:1, using dichloromethane as eluent. The
solvent was removed under reduced pressure affording the product as
a red viscous oil.
within the cationic containing β-thioketoiminates species is
dominant due to favorable forward and backward π- and σ-type
donations resulting from effective MO overlap between the
thio-carbonyl group and metal center. The reactivity of the
cationic η6-arene Ru β-thioketoiminate complex generally
parallels that of the β-diketiminate analogues in that
combination with alkynes results in cycloaddition products
and coordination with π-accepting aryl-isonitriles afforded
octahedral Ru(II) complexes with two equatorial trans-
positioned isonitriles and two axial isonitrile ligands. Attempts
to engage the formal lone pair on the S center on the cationic
η6-arene Ru β-thioketoiminates with Lewis acids were not
successful. The herein newly reported complexes are being
further investigated in potential catalytic applications.
General Synthesis of Chloro−Arene Ru(II) or Os(II)-β-
Thioketoiminate Complexes. The selected in situ generated Li-β-
thioketoiminate (1 equiv, 1.8 mmol) and 0.5 equiv of either the Ru(II)
or Os(II) dimeric precursor (i.e., (M(η6-C6H6)Cl2)2 M = Ru or Os)
were dissolved in dry dichloromethane (10 mL). After stirring
overnight under a N2 atmosphere, the dark colored red solution was
filtered through 1 cm of Celite and the solvent removed under reduced
pressure to a total volume of approximately 1 mL. Dry n-pentane (10
mL) was added to precipitate the product as a powder, which was
dried under reduced pressure for 4 to 12 h.
General Synthesis. Method A (Chlorido Abstraction). A single
equivalent of the selected chlorido-Ru(II)- or -Os(II)-thioketoiminate
complex (1 mmol) and the required chloride abstraction agent, i.e.,
[Ag]PF6, [Ag]SbF6 or [Na]B(C6H3(m-CF3)2)4 (1.2 equiv, 1.2 mmol)
were dissolved together in dry dichloromethane (10 mL). When silver
salts were employed, the reaction vessel was protected from light. After
stirring overnight under an N2 inert atmosphere, the resulting dark
green solution was filtered through Celite and the volume of the
solvent adjusted to a volume of approximately 1 mL under reduced
pressure. Dry n-pentane (10 mL) was added to precipitate the product
as a powder which was dried for 4−12 h under reduced pressure.
Method B (Direct Synthesis of Salts Containing the Cationic (η6-
C6H6) Ru(II) or Os(II) β-Thioketoiminate). The selected Li-β-
thioketoiminate (1 equiv, 1.8 mmol), combined with the Ru(II)
dimeric precursor (Ru(η6-C6H6)Cl2)2 (0.5 equiv, 0.9 mmol) and
[Na]SO3CF3 (1.2 equiv, 2.2 mmol), was dissolved in dry dichloro-
methane (10 mL). After stirring overnight under a N2 atmosphere, the
dark brown solution was filtered through Celite, and the volume of
solvent was adjusted to approximately 1 mL under reduced pressure.
Dry n-pentane (10 mL) was added to precipitate the product as a
powder which was dried for 4−12 h under reduced pressure.
EXPERIMENTAL SECTION
■
General Procedures. The synthesis of the precursory ligands,
reagents, and final complexes was carried out under a purified inert
atmosphere of N2 using standard Schlenk techniques. Manipulations of
all products and reagents, including preparation of samples for NMR
analysis were performed in an Innovative Technologies glovebox with
a N2 atmosphere containing less than 1 ppm of O2 and H2O.76,77 All
glassware was predried, and the flasks underwent several purge/refill
cycles before the introduction of solvents or reagents. All solvents were
dried according to literature procedures involving distillation over the
appropriate drying agents78 and stored in Schlenk flasks equipped with
a Teflon stopcock. Celite for filtration was kept in an oven at 130 °C
and degassed prior to use. All other reagents and gases (technical-
grade) were purchased from commercial sources and used as received
if not otherwise specified. The syntheses of the bis(dichloro(η6-
benzene)ruthenium(II)) and bis(dichloro(η6-benzene)osmium(II))
dimers were carried out by a slightly modified procedure according
to Bennett et al. and Stephenson et al., respectively.58,79 Anhydrous
sodium tetrakis(3,5-bis(trifluoromethylphenyl)borate [Na]BArF ,
ASSOCIATED CONTENT
4
■
(BArF = B(3,5-(CF3)2C6H3)4) was synthesized according to
4
S
* Supporting Information
procedures outlined by Reger et al.80 The precursory β-ketoiminate
ligands were synthesized according the method published by Cowley
et al.81 η6-Arene Ru and Os β-diketiminate complexes 2c,d were
prepared as previously described by Phillips et al.12,13 Synthesis and
characterization of complexes 2b−d and 9−13, details including
instrumentation used for structural characterization, and specifics
regarding the DFT calculations are available in the Supporting
Information.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00111.
Additional instrumentation details and synthesis and
characterization of all ligands and complexes; supple-
mentary solid-state structural details for complexes 5b, 6,
8, 13, 2c−d; additional computational details including
geometry and charge comparisons, selected MO
diagrams, and experimental/TDDFT-predicted optical
spectra for 8 and 2d (PDF)
General Procedure for β-Thioketoimines Synthesis. Ligands
3a−d were prepared using a modified method similar to that described
by Walter et al.29 The starting β-ketoiminate (1 equiv, 10 mmol) was
dissolved in dry 1,2-dimethoxyethane (40 mL) at room temperature.
Commercially purchased Lawesson’s reagent (0.5 equiv, 5 mmol) was
added to give a yellow solution. The reaction was stirred overnight
under N2 to give a dark red solution. The solution was then added to
distilled water (100 mL) affording a yellow-orange biphasic mixture.
Dichloromethane (50 mL) and saturated brine (20 mL) were added,
and the resulting mixture was stirred for 30 min. The organic layer was
separated, and the aqueous layer was extracted with dichloromethane
(50 mL). The organic layers were combined and dried over anhydrous
sodium sulfate. The solvent was removed under reduced pressure, and
the resulting red oil was purified by passage through a plug of silica
gel/alumina (basic) 1:1, using dichloromethane as the eluent. All
volatiles were removed under reduced pressure to leave the product as
a red oil which solidified upon standing.
General Synthesis of β-Aminoselenones. The desired β-
ketoiminate (1 equiv, 3.6 mmol) was dissolved in dry toluene (40
mL) at room temperature. Commercially purchased Woollins’ reagent
(0.5 equiv, 1.8 mmol) was added to instantly afford a deep colored red
solution. The reaction was stirred overnight under an N2 atmosphere
to give a dark red solution, whereby another 0.5 equiv Woollins’
reagent was added. The reaction was then stirred for a total of 1 week.
The resulting red solution was purified by passing through a plug of
Atomic coordinates of all optimized structures (XYZ)
Accession Codes
CCDC 1825143−1825152 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: andrew.phillips@ucd.ie.
ORCID
Andrew D. Phillips: 0000-0001-5599-6499
Notes
The authors declare no competing financial interest.
N
DOI: 10.1021/acs.organomet.8b00111
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ACKNOWLEDGMENTS
■
42.
This work was also supported through Science Foundation
Ireland (SFI) for the Solar Energy Conversion Cluster under
Grant [07/SRC/B1160]. Additionally, A.D.P. thanks Science
Foundation Ireland (SFI) for a Stokes Lectureship held at the
school of chemistry, UCD. We are grateful for the access to
computing facilities owned and operated by the Irish Centre for
High-End Computing (ICHEC).
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