Ruthenium-Chelated Non-Innocent Bis(heterocyclo)methanides: A Mimicked β-Diketiminate

Article abstract of DOI:10.1021/acs.inorgchem.9b01201

The unexplored substrate-based reactivity profile of newly designed bis(heterocyclo)methanide (BHM, L1-L3), a structural mimic of ubiquitous β-diketiminate, was demonstrated on an electronically rich {Ru(acac)₂} platform (acac = σ-donating acetylacetonate). In this regard, this work deals with structurally characterized [Ru(L)(acac)₂] complexes 1A-3A incorporating electronically varying heterocycles {1A, L1 = bis(imidazo[1,5-a]pyridin-3-yl)methanide; 2A, L2 = (Z)-4-[(6,7-dihydrothieno[3,2-c]pyridin-4-yl)methylene]-6,7-dihydro-4H-thieno[3,2-c]pyridin-5-ide; 3A, L3 = (Z)-6-chloro-1-[(6-chloro-3,4-dihydroisoquinolin-1-yl)methylene]-3,4-dihydro-1H-isoquinolin-2-ide}. The significant impact of electronic modification at the BHM backbone (L1-L3) on its redox tunability at the metal-ligand interface in 1A-3A and its subsequent oxygenation profile to yield bis(heterocyclo)methanone (BMO, analogue of α-ketodiimine) in the corresponding [Ru(BMO)(acac)₂] (1B-3B) via a radical pathway were rationalized. In addition, oxidative dehydrogenation of metalated BMO in 1B-3B to BAM [bis(heteroaryl)methanone] in [Ru(BAM))(acac)₂] (1C-3C) was illustrated in support of the nonspectator behavior of α-ketodiimine. A combined experimental and theoretical investigation extended mechanistic outlines of the aforementioned transformation processes, which in effect provided a new dimension relating to the analogous β-diketiminate as well as α-ketodiimine chemistry.

Full text of DOI:10.1021/acs.inorgchem.9b01201

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ruthenium-Chelated Non-Innocent Bis(heterocyclo)methanides: A
Mimicked β‑Diketiminate
Sanjib Panda,* Sudip Kumar Bera, Puneet Goel, Achintya Kumar Dutta, and Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: The unexplored substrate-based reactivity profile
of newly designed bis(heterocyclo)methanide (BHM, L1−L3),
a structural mimic of ubiquitous β-diketiminate, was demon-
strated on an electronically rich {Ru(acac)2} platform (acac =
σ-donating acetylacetonate). In this regard, this work deals with
structurally characterized [Ru(L)(acac)2] complexes 1A−3A
incorporating electronically varying heterocycles {1A, L1 =
bis(imidazo[1,5-a]pyridin-3-yl)methanide; 2A, L2 = (Z)-4-
[(6,7-dihydrothieno[3,2-c]pyridin-4-yl)methylene]-6,7-dihy-
dro-4H-thieno[3,2-c]pyridin-5-ide; 3A, L3 = (Z)-6-chloro-1-
[(6-chloro-3,4-dihydroisoquinolin-1-yl)methylene]-3,4-dihy-
dro-1H-isoquinolin-2-ide}. The significant impact of electronic
modification at the BHM backbone (L1−L3) on its redox
tunability at the metal−ligand interface in 1A−3A and its subsequent oxygenation profile to yield bis(heterocyclo)methanone
(BMO, analogue of α-ketodiimine) in the corresponding [Ru(BMO)(acac)2] (1B−3B) via a radical pathway were rationalized.
In addition, oxidative dehydrogenation of metalated BMO in 1B−3B to BAM [bis(heteroaryl)methanone] in [Ru(BAM))-
(acac)2] (1C−3C) was illustrated in support of the nonspectator behavior of α-ketodiimine. A combined experimental and
theoretical investigation extended mechanistic outlines of the aforementioned transformation processes, which in effect
provided a new dimension relating to the analogous β-diketiminate as well as α-ketodiimine chemistry.
substituents at the Cα position of nacnac, the aryl group of
BHM prevents deprotonation to inhibit unwanted acid/base
chemistry.8 In this regard, this study highlights the reactivity
profile of rarely explored BHM derivatives via O2 activation in
the vein of nacnac chemistry.
Thus, the redox behavior of the newly designed BHM
derivatives (L1−L3) has been invoked to rationalize the
oxygenation of BHM to BMO [bis(heterocyclo)methanone]
upon coordination with the electronically rich {Ru(acac)2}
(acac = acetylacetonate) metal fragment (Figure 1). The
kinetic instability of the complexes has been addressed by
varying the aryl substituents of the BHM moiety.
Structural and mechanistic details along with the correlation
of reactivity have been authenticated by spectroscopic and
theoretical details. To the best of our knowledge, this work
demonstrates for the first time the activation of a small
molecule at the backbone of BHM. On the other hand,
oxidative dehydrogenation of BMO to bis(heteroaryl)-
methanone (BAM) (Figure 1) implies its chemical non-
innocence, as well. It is worth noting that metal coordination-
assisted oxidative dehydrogenation of the C−C bond is a
widely applicable strategy for generating nitrogen heteroarenes
from their corresponding heterocyclic counterparts.9
INTRODUCTION
■
Conversion of earth-abundant and readily accessible small
molecules such as O2, N2, H2, CO2, CH4, etc., into value-added
chemical feedstock under mild reaction conditions has been
considered to be an important event in terms of industry and
the environment.1 However, chemical transformations of these
molecules are thermodynamically demanding and mechanisti-
cally complex owing to the inertness of their bonds, and their
activation poses a formidable challenge. In fact, redox active
ligand-derived metal complex-mediated small molecule activa-
tion as well as functionalization of inert bonds has garnered
significant attention for the introduction of structural diversity
into the substrate backbone.1c−h
In this context, application of the β-diketiminate (nacnac)
moiety in small molecule activation has been well documented
primarily due to its stereoelectronic tunability.2 In addition to
its robust nature, the nonspectator feature of nacnac has also
led to ligand-based reactivity involving its nucleophilic methine
carbon (Cβ).3−5 In addition, establishment of the hidden redox
non-innocence of nacnac due to its odd number of p orbitals in
the π-system is a challenging task, though a few reports have
emphasized this.6 However, recent efforts have been devoted
to the design of a variety of bis(heterocyclo)methanides
(BHM) comprising of a nacnac core with substitution at the
extended π-conjugated ring to modulate its steric and
electronic demands.7 Furthermore, in lieu of methyl
Received: April 24, 2019
© XXXX American Chemical Society
A
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Figure 1. Ru chelation-assisted transformations of BHM and BMO.
a
Scheme 1. Synthetic Aspects of H-BHM
a
Conditions: (i) reflux, 48 h; (ii) P2O5, POCl3, toluene, reflux, 12 h.
are however sensitive to air and undergo irreversible
transformation to the corresponding metalated BMO [BMO
= bis(heterocyclo)methanone] species in 1B−3B under
ambient conditions, where insertion of oxygen into the
C(sp2)−H bond of monoanionic BHM in A results in a new
CO bond in B (neutral ketodiimine) with the simultaneous
reduction of the metal oxidation state from Ru(III) to Ru(II).
RESULTS AND DISCUSSION
■
Synthetic and Structural Aspects. The homodisubsti-
tuted H-BHM ligands (HL1−HL3) were obtained by
following the Bischler−Napieralski reaction pathway,10 starting
from the condensation of diethylmalonate and corresponding
amines followed by acid-promoted cyclization of the
preformed malonamide (Scheme 1). The para selectivity in
HL3 due to the diminished electron density at the adjacent
ortho position of the chloro substituent could be attributed to
the calculation of Fukui indices (Figure S1).11 Ligands are
expected to exist in either the diimine or the iminoenamine
tautomeric form as a consequence of mesomeric stabiliza-
tion.12 However, bis(imidazopyridin-3-yl)methane (HL1)
stabilizes in the diimine form due to the formation of an
aromatic 10π-heterocyclic system [each imidazopyridine ring
follows (4n + 2)π bridged by the methylene group]. On the
other hand, the preferential hydrogen-bonded iminoenamine
form for HL2 or HL3 is reflected in its nuclear magnetic
resonance (NMR) profiles and structural parameters (see
below and the Supporting Information).
1
The change in bond parameters, diamagnetic H/13C NMR
signals, the disappearance of electron paramagnetic resonance
(EPR) signatures, characteristic electronic spectra, and varying
redox potentials upon moving from A to B (see below)
collectively corroborate the reduction in the reaction sequence
from A to B. The rate of conversion of A to B varies on the
basis of the nature of the Cα-substituted side arm and
decreases in the order 1A ≫ 2A. On the contrary, oxygenation
of 3A to 3B is too slow to follow kinetically, which in turn
facilitates for the production of the single crystal of 3A. The
involvement of aerial oxygen for A → B conversion has been
established by an 18O2 labeling experiment with the
representative complex 1A (Figure S2b). Subsequently, 2B
or 3B undergoes aerobic dehydrogenation of the N(imine)-
substituted methylene centers of pro-aromatic BMO to yield
fully aromatic BAM [BAM = bis(heteroaryl)methanone] in 2C
or 3C, respectively. The rate of B → C conversion is however
{RuIII(acac)2}-bonded BHMs (1A−3A) were obtained from
H-BHM and [RuII(acac)2(CH3CN)2] (Scheme 2). Like
nacnac, deprotonated BHM coordinates to the metal ion
through its imine nitrogen donors, forming a six-membered
metallaheterocycle. The initially formed complexes, 1A−3A,
B
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a
Scheme 2. Synthetic Outline for A−C
a
Conditions: (i) EtOH, Et3N, 373 K, ∼4 h; (ii and iii) EtOH, O2 balloon, 373 K.
found to be much faster in the presence of a base such as Et3N
or BuOK.
(Experimental Section) toward the higher-energy region upon
moving from 1B to 2B to 3B also suggests the varying
electronic nature of the ligand backbone as well as back-
donation. Moreover, back-donation causes red shifting of the
lowest-energy band (∼200 nm) upon moving from A to B (see
below). The planarity of isoquinoline units in 2C along with
the shortening of C8−C9/C4−C5 bonds and the disappear-
t
The identities of the complexes have been authenticated by
their single-crystal X-ray structures, ESI-MS, and spectroscopic
[1H/13C NMR, EPR, ultraviolet−visible−near infrared (UV−
vis−NIR), and infrared (IR)] and electrochemical signatures.
Crystal structures of representative 3A, 1B, 2B, 2C, HL3,
and 3C are shown in Figure 2 and Figures S3−S5.
Crystallographic and bond parameters are listed in Table 1
and Tables S1−S6, respectively. The two independent
molecules in the asymmetric unit of 3A or 3C exhibited
similar bond parameters.
Complexes form a distorted octahedral geometry around the
central Ru ion with trans and cis angles of 176−179° and 85−
96°, respectively. N1−C1 [1.338(10) Å] and C1−C2
[1.409(11) Å] bond lengths (Table 1) of 3A support
delocalization of the negative charge in the BHM (L3)
backbone, while the N1−C1 bond length of 1.302(5) Å in 2B
is in agreement with its double bond character. The relatively
longer N1−C1 bond distance in 1B or 2C can be attributed to
the aromatic ring character/electronic feature of the metalated
ligand backbone along with a greater extent of RuII(dπ) →
L(π*) back-donation13 in the bonding synergism. It has also
been reflected in the slightly elongated CO bond in 1B or
2C in comparison to the unperturbed CO bond in 2B
(Table 1). The shifting of the CO stretching frequency
1
ance of the H NMR signature of methylene protons upon
moving from 2B to 2C are consistent with the CH2 → CH
conversion in the reaction sequence. The partial double bond
character of the C1−C2 [1.423(5) Å] and N1−C1 [1.319(5)
Å] bonds of HL3 (Figure S3 and Table S3) is also
authenticated by its iminoenamine tautomeric form.
Spectral, Electrochemical, and Electronic Structural
Aspects. The identities of HL1−HL3 ligands and diamagnetic
1
complexes B/C have been authenticated by their H/13C/
DEPT-135 NMR signatures (Figure S6). The absence of the
NH signal as well as the appearance of a sharp peak
corresponding to two protons (methylene CH2) at δ = 4.78
1
ppm in the H NMR spectrum and the signal at 26.98 ppm
with the inverted phase in the DEPT-135 NMR spectrum
corroborate the diimine form of HL1. On the other hand, the
preferential iminoenamine form of H-bonded HL2 or HL3 is
reflected in its broad NH (δ ∼ 11.5 ppm) and sharp methine
C−H (δ ∼ 5.5 ppm) resonances.
C
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Figure 2. ORTEP diagrams of 3A (molecule A), 1B, 2B, and 2C. Hydrogen atoms and solvent molecules have been omitted for the sake of clarity.
Thermal ellipsoids are at the 50% probability level.
Table 1. Selected Bond Lengths (angstroms)
Table 2. Electronic Spectral Data of Complexes in CH3CN
3A (molecule A)
1B
2B
2C
complex
λ (nm) (ε (M−1 cm−1))
Ru1−N1
N1−C1
C1−C2
C2−O5
C8−C9
1.974(7)
1.338(10)
1.409(11)
−
2.015(8)
1.972(3)
1.302(5)
1.500(5)
1.224(4)
1.536(6)
1.987(4)
1.353(6)
1.480(7)
1.234(6)
1.365(9)
1B
2A
2B
2C
3A
3B
3C
400 (18200), 446 (12000), 677 (2400)
1.342(11)
1.451(12)
1.238(12)
275 (25200), 349 (18000), 441 (8000), 616 (1800)
335 (3000), 436 (8300), 574 (3000), 816 (450)
340 (5300), 430 (6500), 606 (2700), 820 (530)
350 (8200), 453 (6400), 610 (1220)
344 (900), 458 (1240), 651 (3100), 830 (800)
340 (14720), 480 (14270), 650 (7358), 820 (1580)
1.477(12)
−
The conversion of the central C−H of BHM in A to a CO
in B is supported by the disappearance of its methine proton
(1H NMR) as well as by the appearance of a new CO peak
in the 13C NMR spectrum. Moreover, 2B or 3B exhibits
nearby proton resonances at ∼3.7 and ∼2.8 ppm correspond-
ing to methylene protons adjacent to imine nitrogens as has
also been recognized by 13C/DEPT NMR. However, these
peaks have diaappeared upon moving from B to C with the
simultaneous appearance of new peaks in the aromatic region,
suggesting the increase in aromaticity.
Complexes exhibit multiple absorptions in the UV−vis−NIR
region, which have been analyzed by the time-dependent
density functional (TD-DFT) formalism (Table 2 and Figure
S7 and Table S7). The lowest-energy absorption bands for
{Ru(acac)2}-coordinated BHM in A and BMO/BAM in B/C
originate from ligand to metal (LMCT, BHM → RuIII) and
metal to ligand (MLCT, RuII → π* of BMO/BAM) charge
transfer transitions, respectively. The red shifting of the lowest-
energy band (∼200 nm) upon moving from A to B takes place
due to the low-lying π* orbital of the strongly π-accepting
BMO in B.13 A similar trend is also reflected in the absortion
profile of C because of the similar π-acidic nature of BAM in C.
The impact of the electronic modulation of BHM (L1−L3)
in paramagnetic 1A−3A [S = 1/2 (Scheme 2)] toward its redox
non-innocence (RNI) has been defined by EPR (Figure 3 and
Table S8) and the DFT-calculated spin density (SD)
distribution (Figure 4 and Table S9). Primarily ligand-centered
EPR for 1A (Δg = 0.015) and metal-based anisotropic EPR for
2A and 3A (Δg ∼ 0.21) along with their SD distribution (Ru/
L) (1A, 0.26/0.74; 2A, 0.68/0.28; 3A, 0.69/0.27) suggest the
dominating elctronic structural forms of RuII-L1• and RuIII-
L2−/RuIII-L3−, respectively.14 However, partial spin accumu-
lation on L2/L3 in 2A/3A leads to the consideration of RuII-
L2•/RuII-L3• as a minor contributing resonance form. Electron
pushing from the bridgehead N atoms (N3/N4) indeed favors
intramolecular electron transfer (RuIII-L1− ↔ RuII-L1•) to give
rise to the radical state of L1• preferentially in 1A. This in turn
emphasizes the varying RNI of BHM as a function of
D
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Scheme 3. Electronic Structural Forms for the Accessible
Redox States of A−C
Figure 3. X-Band EPR spectra (left) at 100 K in CH3CN-toluene. ⟨g⟩
14
2
2
2
= {(1/3)(g1 + g2 + g3 )}1/2; Δg = g1 − g3. Cyclic (black) and
differential pulse (green) voltammograms (right) in CH3CN/0.1 M
Et4NClO4/GC vs SCE (scan rate of 100 mV s−1).
electronic consequence of the ligand backbone15 as well as its
improved redox feature with respect to analogous nacnac.
The quasi-reversible one-electron oxidation and reduction of
the isolated complexes (Figure 3 and Figure S8 and Table S10)
were assessed by their MO compositions (Tables S11−S26),
EPR spectra (Figure 3 and Figure S9), and spin density plots
(Figure 4 and Figure S10). It reveals that 2A or 3A with the
{RuIII-L−} (S = 1/2) configuration undergoes ligand-based
oxidation and metal-based reduction to {RuIII-L•} (S = 0, 2A+/
3A+) and {RuII-L−} (S = 0, 2A−/3A−), respectively,
reemphasizing the RNI of BHM with special reference to the
electron releasing process. On the other hand, one-electron
oxidation and reduction of Ru(II)-derived B or C lead to metal
(RuII/RuIII)- and ligand (BMO/BMO•− or BAM/BAM•−)-
based processes, respectively, to which the RNI feature of
BMO or BAM can be attributed as well particularly with
respect to the electron uptake event (Scheme 3).
Figure 5. Change in the spectral profile (left) for the 2A → 2B
conversion at 333 K. The rate is assigned on the basis of the growing
feature at 576 nm (inset). Plot of ln(k/T) vs 1/T (right).
negative entropy of activation (associative pathway). Oxygen-
ation of 1A also follows similar kinetics but proceeds at a much
faster rate [(7.4
0.2) × 10−5 at 303 K (Figure S12)]. It
should be stated that the rate of oxygenation of {Ru(acac)2}-
coordinated L1 in 1A is approximately 10 times faster than that
of the previously reported4a analogous nacnacPh,Ph-derived
[Ru(acac)2(nacnacPh,Ph)] (k = 6.4 × 10−6 s−1 at 303 K).
In this scenario, the approach of molecular oxygen (3O2,
3Σg−) might have facilitated redox tuning16 at the metal−ligand
interface (RuIII−BHM− ↔ RuII−BHM•) due to RNI of BHM
(vide infra). The radical character of BHM in effect favors its
interaction with the molecular oxygen (diradical) in a spin-
allowed fashion (Scheme 4) to provide transient superoxide
radical (I1). Subsequent H shuttling (I2) and cleavage of the
O−O bond led to ketonization in B.
Kinetics and Mechanistic Insights of the A → B
Transformation. The nonspectator behavior involving
methine oxygenation of BHM in 1A/2A was explored in
terms of kinetic/thermodynamic and quantum chemical
calculations. Oxygenation of 2A to 2B was followed
spectrophotometrically in deaerated CH3CN over the temper-
ature range of 303−333 K (Figure 5 and Figure S11 and Table
S27), which established a first-order rate with a rate constant
(k in s−1) of (2.9
0.1) × 10−5 at 333 K. The rate of
conversion of 2A to 2B at varying temperatures extends the
kinetic barrier for the process: ΔS⧧ (entropy of activation),
ΔH⧧ (enthalpy of activation), and ΔG⧧ (free energy of
activation at 298 K) of 4.4 0.1 kcal/mol, −66.4 0.4 cal
mol−1 K−1, and 24.2 kcal/mol, respectively. The calculated
(+)ve ΔG⧧ implies the endergonic nature of the intermediate
complex formation process as this is associated with the
The enhanced kinetic instability of 1A over 2A can be
rationalized by its predominant canonical form of RuII-L1•.
Ligand-dominated spin facilitates its interaction with 3O2,
which is also electronically favored by N atoms at the ring
junctions of 1A. This also justifies the aforementioned much
slower rate of oxygenation of {Ru(acac)2}-coordinated
Figure 4. Spin density representations (isosurface value of 0.003).
E
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Scheme 4. Aerobic Oxidation Pathway for A → B Conversion
Figure 6. Gibbs free energy (ΔG, 298 K) profile of methine oxygenation of Ru-coordinated BHM calculated at the M06L level. The associated ΔG
and ΔH are shown in each state. The ΔG of starting substrates is taken as zero. Only core structures are shown here.
Figure 7. (a) HOMO (isosurface of 0.05) of the reactant, (b) selected bond parameters (computed), and (c) SD plots (isosurface of 0.007) of I1.
All H atoms (except Cβ-H) and the acac backbone (except O-atoms) have been omitted for the sake of clarity.
nacnacPh,Ph (SD values of Ru and nacnac of 0.73 and 0.22,
respectively)4a in comparison to 1A (SD values of Ru and L1
of 0.26 and 0.74, respectively). This in turn rationalizes the
improved reactivity profile of the nacnac analogue (BHM)
upon electronic modification. However, we failed to identify
the hydroxyl radical, which might be due to its rapid
decomposition or participation in other reactions like
dehydrogenation of 2B to 2C or 3B to 3C.
The energy profile diagram [M06L/LANL2DZ/6-31G* in
ethanol (Figure 6 and Figure S13)] suggests that metal
coordination-promoted interaction of infinitely separated
3
reactants (1A/2A + O2) generates the metastable superoxide
intermediate (I1; S = 1/2). This proceeds via the formation of a
nascent Cβ···O bond (TS1) with intrinsic reaction barriers of
13.6 and 21 kcal/mol for 1A and 2A, respectively.
Concomitant hydrogen transfer from Cβ in I1 followed by
F
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Figure 8. Resonance Raman spectra (left) for the intermediate of 1A → 1B (blue) and 2A → 2B (red) transformations, detected via time-
dependent Raman analysis at 273 K by using a λex of 532 nm. Calculated Fukui indices (right) at Cβ (f° and f− stand for radical and nucleophilicity
Fukui indices, respectively).
homolytic fission of the O−O bond gives rise to the product
(1B and 2B) along with the hydroxyl radical byproduct by
overcoming the activation barriers (TS2) of 21.4 and 25.5
kcal/mol for 1A and 2A, respectively. The decrease of entropy
in the elementary O2 activation step (1A and 2A to
corresponding I1) makes the process endergonic. However,
the overall transformation is exergonic due to the improved
product stability as a result of incorporation of the carbonyl
moiety at the ligand backbone. The lower activation barrier for
1A → 1B transformation and the greater thermodynamic
stability [free energy of formation (ΔG°)] of 1B (−33.8 kcal/
mol) versus that of 2B (−22.6 kcal/mol) support the higher
oxygenation rate of 1A as compared to that of 2A. Moreover,
the experimentally deduced activation barrier of 2A → 2B
conversion (ΔG⧧ of 24.2 kcal/mol, 298 K) correlates well with
the calculated value. It is worth noting that the high energy
demand for the process makes the 2A → 2B conversion
kinetically slow at room temperature (>48 h for its
completion).
The slightly elongated C−O bond (∼1.51 Å) in I1 is the
reflection of the electronic consequence due to molecular
strain.18 The consideration of radical attack from the
nucleophilic methine center (Cβ) of BHM in A (Scheme 4)
has also been justified by the calculation of Fukui indices in
which radical Fukui indices are better suited to rationalize the
Cβ-H functionalization (Figure 8, right).
The enhanced acidity at the pole positions (adjacent to
metal-coordinated imine nitrogens) of BMO selectively in 2B
or 3B facilitates the aerobic dehydrogenation, leading to the
formation of thermodynamically stable product C in Scheme 2.
The conversion of B [RuII(acac)2-coordinated BMO] to C
[RuII(acac)2-coordinated BAM] could be rationalized by
following the UV−vis spectral changes of the acetonitrile
t
solution of representative 2B in the presence of excess BuOK
(Figure 9) under atmospheric conditions. The distinct change
The probable alternate pathway invoving the unfavorable
seven-coordinated intermediate of metal-bound O2, [Ru-
(acac)2(BHM)(O2)], could not be located. The lack of
coordination of O2 to the Ru center also excludes the
possibility of the formation of dinuclear Ru−O−O−Ru peroxo
species. The involvement of the quartet transition state (TS1Q;
S = 3/2) (Figure S14) instead of doublet TS1 (S = 1/2 in Figure
6) is an energy-demanding process and thus expected to be
kinetically unfavorable at room temperature. The use of the
UB3LYP functional extends a similar trend (Figure S15).
The selective interaction of π1g*-HOMOs of triplet dioxygen
with the Cβ center of BHM in 1A or 2A to yield the metastable
{Ru(BHM-O2)} intermediate (I1 in Scheme 4) could also be
supported by the fact of BHM dominated α-SOMO and β-
LUMO (Figure 7a and the Supporting Information). The
superoxide form of intermediate I1 (Figure 6) can be
conceived by the following considerations: (i) spin localization
onto the incoming O2 unit at the ligand backbone (O2, 0.70
and 0.81 for I1 of 1A and 2A, respectively) (Figure 7), (ii)
estimated O−O bond distance of ∼1.34 Å, (iii) mass spectral
profile [m/z 578 for I1 of representative 1A (Figure S2c)], and
(iv) change in the resonance Raman signature as a function of
time [experimental and calculated νO−O 1066 and 1025 cm−1
or 1109 and 1099 cm−1 for I1 of 1A or 2A, respectively (Figure
8)].17 The decrease in the νO−O of I1 in 1A with respect to that
in 2A implies the insertion of a greater extent of peroxide
character within the superoxide radical, which also results in a
small elongation of the O−O bond.
Figure 9. Change in the spectral profile for the 2B → 2C conversion
in CH3CN at 298 K in the presence of excess BuOK (5 equiv).
t
in the spectral profile of 2B (574 → 606 nm) with the
isosbestic point at 587 nm in the visible region is consistent
with the 2B → 2C conversion (see Table 2). The same
transformation, however, failed to take place under anoxic
conditions (N2 atmosphere). In addition, upon exposure to air,
the 2B → 2C conversion also occurred but at a much slower
rate (several days). Therefore, it can be considered as a base-
promoted oxidative aromatization process of metalated BMO
in B to afford heteroaromatic substrate BAM in C.9
CONCLUSION
■
This work divulges {Ru(acac)2} coordination-assisted oxygen-
ation of nacnac-mimicked BHM in 1A−3A to BMO in 1B−3B
and dehydrogenation of resultant ketodiimine-derived BMO to
BAM in 2C and 3C (Scheme 2). Mechanistic insight reveals
the pivotal role of resonance-assisted redox tuning at the
G
DOI: 10.1021/acs.inorgchem.9b01201
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Inorganic Chemistry
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metal−ligand interface (MpLn ↔ Mp+1Ln−1) toward the
oxygenation of BHM (A) to BMO (B) (Scheme 2). The
superoxide intermediate favoring a radical pathway has been
validated through a spectroscopic and transition state theory
approach. It also accounts for the improved oxygenation profile
of {Ru(acac)2}-coordinated “BHM” with respect to the
corresponding “nacnac” derivative as a consequense of
electronic modulation at the BHM backbone. The recognition
of the pronounced redox non-innocent feature of BHM having
a nacnac core may therefore offer additional impetus for
multifarious nacnac-based chemistry, including its potential
application in the catalytic processes.
geometrically constrained positions and refined with isotropic
temperature factors, generally 1.2Ueq of their parent atoms. Hydrogen
atoms were included in the refinement process as per the riding
model. CCDC 1898086 (HL3), CCDC 1898087 (1B), CCDC
1898089 (2B), CCDC 1898090 (2C), CCDC 1898091 (3A), and
CCDC 1898092 (3C) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. The alert B in the checkCIF of 1B and 3C could
be ignored as it was developed due to the weakly diffracting nature of
the crystals.
Computational Studies. Full geometry optimization was
performed by using the density functional theory method at the
M06L level.22 Except for ruthenium, all other elements were assigned
the 6-31G* basis set and additional TZVP basis set for representatives
2A−2C to correlate the bond parameters and TD-DFT data. The
LANL2DZ basis set with an effective core potential was employed for
the ruthenium atom.23 All calculations were performed with the
Gaussian 09 program package.24 Chemissian 1.725 was used to
calculate the fractional contributions of various groups to each
molecular orbital. Calculated structures were visualized with Chem-
Craft.26 Vertical electronic excitations based on M06L-optimized
geometries were computed for all using the time-dependent density
functional theory (TD-DFT) formalism27 in acetonitrile using a
conductor-like polarizable continuum model (CPCM).28 Electronic
spectra were calculated using SWizard.29,30
Transition state calculations were performed at the M06L level of
theory starting from infinitely separated substrates. A single imaginary
frequency for the transition states (TSs) and real frequencies for local
minima were obtained (Table S28). The connectivity of each TS was
validated through a relaxed potential energy surface scan for the
corresponding reaction coordinate and was found to be the highest-
energy point that connected the relevant reactant and product (Figure
S16). The zero-point vibrational energies and thermal corrections
were obtained from the harmonic frequency calculations at the M06L
level of theory (Charts S1 and S2).22,31 The results obtained using the
M06L function has also been tested with the UB3LYP density
EXPERIMENTAL SECTION
■
Materials. The precursor complex cis-[Ru(acac)2(CH3CN)2] was
prepared by following the literature procedure.19 EtOH was dried
using activated magnesium.20 Other chemicals and solvents were of
reagent grade and used as received. For spectroscopic and
electrochemical studies, HPLC grade solvents were used. 18O2 was
procured from ICON Isotopes.
Physical Measurements. The electrical conductivity was
checked using an autoranging conductivity meter (Toshcon
Industries). EPR spectra of isolated complexes and electrochemically
generated oxidized species were recorded on a Bruker EMX Plus
instrument (experimental conditions: microwave frequency of 9.1
GHz with a power of 5 mW and magnetic modulation of 100 kHz
with an amplitude of 1 G). Cyclic voltammetric and differential pulse
voltammetric measurements of the complexes were performed using a
PAR model 273A electrochemistry system. Glassy carbon working,
platinum wire auxiliary, and saturated calomel reference electrodes
were used in a standard three-electrode configuration with
tetraethylammonium perchlorate (TEAP) as the supporting electro-
lyte (substrate concentration of ≈10−3 M; standard scan rate of 100
1
mV s−1). H and 13C NMR spectra were recorded on Bruker Avance
III 400 and 500 MHz spectrometers, respectively. The elemental
analyses were performed on a Thermoquest (EA 1112) micro-
analyzer. Elemental analyses for 1B, 2C, and 3C matched well with
the calculated data. However, experimental microanalytical data of
1A, 2A, 2B, 3A, and 3B did not match with the corresponding
calculated values, presumably due to decomposition. Electrospray
mass spectrometry was performed on a Bruker’s Maxis Impact
(282001.00081) spectrometer. Electronic spectral studies were
performed on a PerkinElmer Lambda 1050 spectrophotometer. IR
spectra of the complexes as KBr pellets were recorded on a Nicolet
spectrophotometer. The Raman spectrum was recorded in a WITec
micro-Raman spectrometer equipped with 532 nm Nd:YAG laser
excitation.
Kinetic Studies. Kinetic experiments were carried out under an
O2 atmosphere by dissolving A (∼2 × 10−4 M) in CH3CN. The
change in absorbance at 398 nm for 1A → 1B conversion and at 576
nm for 2A → 2B conversion was monitored for the rate calculation.
The pseudo-first-order rate constant (k) for A → B conversion was
calculated on the basis of a nonlinear exponential fit in Origin Pro8 by
following the equation y = y0 + A1 × exp(−x/t1), where y and y0
correspond to the absorbance at 576 nm at time t and time zero,
respectively, x corresponds to time periods (t in minutes) over which
the absorption changes take place, A1 is the pseudo-first-order
coefficient, and the value of the pseudo-first-order rate constant (k) is
1/(t1 × 60) s−1.
functional.
−
Radical Fukui indices (fA°) and nucleophilicity Fukui indices (fA
)
were computed11 with the following equations:
fA ° = {(qN+1 − qN−1)/2} (susceptible to radical attack)
−
fA = {qN − qN−1
}
(susceptible to electrophilic attack/itself acts as a nucleophile)
where q represents the electronic population of atom A in the neutral
molecule with N electrons, qN+1 represents the electronic population
of atom A in the anion with N + 1 electrons, and qN−1 represents the
electronic population of atom A in the cation with N − 1 electrons. In
this scenario, Fukui function (FF) indices were calculated by using the
Hirshfeld population (at the M06L level).
Synthesis of Bis(heterocyclo)methanide Ligands HL1−HL3.
Diethyl malonate was added dropwise to the respective amines, and
the mixture was vigorously stirred at 100 °C for 48 h to obtain a
yellow solid mass corresponding to malonamide derivatives. It was
washed with n-pentane and dried under vacuum. The malonamide
thus obtained was subjected to cyclization by dropwise addition of
POCl3 (excess) to a suspension of malonamide and P2O5 (excess) in a
toluene medium at 0 °C under stirring conditions. After complete
addition, the reaction mixture was subjected to reflux overnight.
Completion of the reaction was monitored by TLC analysis. A
saturated solution of NaHCO3 was added to the ice-cold solution of
the aforementioned reaction mixture, until the effervescence ceased. A
saturated NaOH solution was then added to adjust the pH to 11. The
alkaline solution was extracted using a dichloromethane/H2O workup
and dried over Na2SO4. Purification by column chromatography led
to the resulting brownish yellow solid of bis(heterocyclo)methanides
(HL).
Crystallography. X-ray diffraction data were collected using a
Rigaku Saturn-724+ CCD single-crystal X-ray diffractometer using
Mo Kα radiation. The data collection was evaluated by using Crystal
Clear-SM Expert. The data were collected by the standard ω-scan
technique. The structure was determined by the direct method using
SHELXS-97 and refined by full matrix least squares with SHELXL-
2014 or SHELXL-2017, refining on F2.21 All data were corrected for
Lorentz and polarization effects, and all non-hydrogen atoms were
refined anisotropically. The remaining hydrogen atoms were placed in
H
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Inorganic Chemistry
Article
HL1, Bis(imidazo[1,5-a]pyridin-3-yl)methane. 2-Picolylamine
(10.0 mL, 97 mmol), diethyl malonate (7.4 mL, 48.5 mmol). Yield
of bis(pyridin-2-ylmethyl)malonamide: 11.9 g (86%). Bis(pyridin-2-
ylmethyl)malonamide (10.0 g, 35 mmol), toluene (30 mL), P2O5
(50.0 g, 10 equiv), POCl3 (32.9 mL, 10 equiv). Yield of HL1: 6.3 g
(72%). 1H NMR (400 MHz, CDCl3) δ: 8.11 (d, J = 7.1 Hz, 2H), 7.27
(s, 2H), 7.24 (d, J = 9.3 Hz, 2H), 6.52 (m, 2H), 6.39 (t, J = 6.7 Hz,
2H), 4.78 (s, 2H). 13C NMR (101 MHz, CDCl3) δ: 132.63, 131.07,
121.53, 118.52, 118.44, 118.10, 112.58, 26.98. DEPT-135 (101 MHz,
CDCl3) δ: 121.53, 118.52, 118.44, 118.10, 112.58, 26.98. HRMS
(C15H12N4) m/z: {HL1 + H}+ calcd, 249.1135; found, 249.1137.
HL2, (Z)-4-[(6,7-Dihydrothieno[3,2-c]pyridin-4(5H)-ylidene)-
methyl]-6,7-dihydrothieno[3,2-c]pyridine. 2-Thiophenethylamine
(10.0 mL, 85.4 mmol), diethyl malonate (6.5 mL, 42.7 mmol).
Yield of bis(2-thiophenylethyl)malonamide: 12.6 g (92%). Bis(2-
thiophenylethyl) malonamide (10.0 g, 31 mmol), toluene (30 mL),
P2O5 (44.0 g, 10 equiv), POCl3 (29.0 mL, 10 equiv). Yield of HL2:
6.2 g (70%). 1H NMR (500 MHz, CDCl3) δ: 11.36 (s, 1H), 7.29 (d, J
= 5.1 Hz, 2H), 7.17 (d, J = 5.2 Hz, 2H), 5.57 (s, 1H), 3.81 (t, J = 7.0
Hz, 4H), 3.01 (t, J = 7.0 Hz, 4H). 13C NMR (126 MHz, CDCl3) δ:
153.85, 144.86, 131.40, 124.62, 124.06, 84.43, 40.85, 23.30. DEPT-
135 (126 MHz, CDCl3) δ: 124.62, 124.07, 84.43, 40.85, 23.30.
HRMS (C15H14N2S2) m/z: {HL2 + H}+ calcd, 287.0671; found,
287.0672.
HL3, (Z)-6-Chloro-1-[(6-chloro-3,4-dihydroisoquinolin-1(2H)-
ylidene)methyl]-3,4-dihydroisoquinoline. 2-(3-Chlorophenyl)-
ethylamine (10.0 mL, 71.9 mmol), diethyl malonate (5.5 mL, 35.4
mM). Yield of bis(3-chlorophenethyl)malonamide: 11.0 g (80%).
Bis(3-chlorophenethyl)malonamide (10.0 g, 25.7 mmol), toluene (30
mL), P2O5 (36.5 g, 10 equiv), POCl3 (24.0 mL, 10 equiv). Yield of
HL3: 5.9 g (67%). The dichloromethane solution gave crystals of
HL3. 1H NMR (500 MHz, CDCl3) δ: 11.52 (s, 1H), 7.72 (d, J = 8.4
Hz, 2H), 7.41−7.30 (m, 3H), 7.26 (s, 2H), 5.88 (s, 1H), 3.68 (t, 4H),
2.88 (t, J = 6.4 Hz, 5H). 13C NMR (126 MHz, CDCl3) δ: 156.92,
138.97, 135.01, 129.76, 127.86, 126.91, 126.02, 84.72, 42.13, 28.29.
DEPT-135 (126 MHz, CDCl3) δ: 127.86, 126.91, 126.02, 84.72,
42.13, 28.29. HRMS (C19H16Cl2N2) m/z: {HL3 + H}+ calcd,
343.0763; found, 343.0759.
Preparation of Complexes. Synthesis of [Ru(acac)2(BHM)]
(1A−3A), [Ru(acac)2(BMO)] (1B−3B), and [Ru(acac)2(BAM)] (1C−
3C). The complexes were prepared by following the general synthetic
routes using respective preformed H-BHM ligands (HL) (Scheme
S3). To the orange solution of Ru(acac)2(CH3CN)2 in degassed
EtOH in an oven-dried Schlenk tube were added a yellow solution of
the respective BHM derivative in degassed EtOH and Et3N under a
dinitrogen atmosphere. The solution was refluxed under an inert
atmosphere for ∼4 h. The solution gradually changed to brownish for
1A and green for 2A and 3A. Evaporation of the solvent under
vacuum afforded the solid mass. A was sensitive to air and converted
to B upon exposure to air to different extents based on the
substituents in the BHM framework (3A ≪ 2A ≪ 1A). 1A was too
sensitive to air to be isolated in pure form; thus, only 2A and 3A were
subjected to purification by column chromatography using neutral
alumina and petroleum ether/dichloromethane as the eluent for
further study. The removal of solvent under vacuum resulted in
corresponding 2A and 3A.
[Ru(acac)2(L2)], 2A (green). Ru(acac)2(CH3CN)2 (100 mg, 0.26
mmol), EtOH (50 mL), HL2 (75.1 mg, 0.26 mmol), Et3N (0.036 mL,
0.26 mmol). Yield: 117 mg (76%). MS (ESI+, CH3CN) m/z: {2A +
H}+ calcd, 586.05; found, 586.04. Molar conductivity (CH3CN): ΛM
= 4 Ω−1 cm2 M−1.
[Ru(acac)2(L3)], 3A (green). Ru(acac)2(CH3CN)2 (100 mg, 0.26
mmol), EtOH (50 mL), HL3 (89.8 mg, 0.26 mmol), Et3N (0.036 mL,
0.26 mmol). Yield: 140 mg (83%). Crystallization was done by slow
evaporation of its dichloromethane/hexane (2:1) solution. MS (ESI+,
CH3CN) m/z: {3A}+ calcd, 641.05; found, 641.03. Molar
conductivity (CH3CN): ΛM = 2 Ω−1 cm2 M−1.
[Ru(acac)2(BMO1)], 1B (green). 1A (50 mg, 0.091 mmol), EtOH
(30 mL). Yield: 46 mg (90%). Slow evaporation of its dichloro-
methane/hexane (2:1) solution gave crystals of 1B. MS (ESI+,
1
CH3CN) m/z: {1B}+ calcd, 562.08; found, 562.06. H NMR (400
MHz, CDCl3) δ: 10.20 (d, J = 7.0 Hz, 2H, CH of the pyridine ring of
BMO1), 7.29 (d, J = 8.8 Hz, 2H, CH of the pyridine ring of BMO1),
7.23−7.14 (m, 2H, CH of the pyridine ring of BMO1), 6.58 (t, J = 6.3
Hz, 2H, CH of the pyridine ring of BMO1), 6.16 (s, 2H, CH of the
imidazole ring of BMO1), 5.30 (s, 2H, methine CH of acac), 0.98 (s,
6H, CH3 of acac), 0.71 (s, 6H, CH3 of acac). 13C NMR (101 MHz,
CDCl3) δ: 161.39, 135.78, 130.24, 130.03, 124.30, 123.64, 116.97,
116.11, 53.44, 27.11, 26.95. Anal. Calcd for {1B·C3H7}
C28H31N4O5Ru: C, 55.60; H, 5.17; N, 9.27. Found: C, 55.25; H,
5.06; N, 8.97. IR (KBr, cm−1): 1628 [ν(CO)]. Molar conductivity
(CH3CN): ΛM = 6 Ω−1 cm2 M−1.
[Ru(acac)2(BMO2)], 2B (brown). 2A (50 mg, 0.085 mmol), EtOH
(30 mL). Yield: 44 mg (86%). Slow evaporation of its dichloro-
methane/hexane (3:1) solution gave crystals of 2B. MS (ESI+,
1
CH3CN) m/z: {2B}+ calcd, 600.03; found, 600.01. H NMR (400
MHz, CDCl3) δ: 7.80 (d, J = 5.0 Hz, 2H, CH of the thiophene ring of
BMO2), 7.07 (d, J = 5.2 Hz, 2H, CH of the thiophene ring of
BMO2), 5.45 (s, 2H, methine CH of acac), 3.94−3.54 (m, 4H, CH2
of BMO2), 3.09−2.74 (m, 4H, CH2 of BMO2), 2.24 (s, 6H, CH3 of
acac), 1.86 (s, 6H, CH3 of acac). 13C NMR (101 MHz, CDCl3) δ:
188.67, 186.79, 175.50, 140.69, 132.11, 126.37, 121.55, 99.98, 55.53,
27.91, 26.99, 23.25. DEPT-135 (101 MHz, CDCl3) δ: 126.37 (CH of
the thiophene ring of BMO2), 121.55 (CH of the thiophene ring of
BMO2), 99.98 (CH of acac), 55.53 (CH2 of BMO2), 27.91 (CH3 of
acac), 26.99 (CH3 of acac), 23.25 (CH2 of BMO2). IR (KBr, cm−1):
1712 [ν(CO)]. Molar conductivity (CH3CN): ΛM = 2 Ω−1 cm2
M−1.
[Ru(acac)2(BMO3)], 3B (brown). 3A (50 mg, 0.078 mmol), EtOH
(30 mL). Yield: 31 mg (60%). MS (ESI+, CH3CN) m/z: {3B + H}+
calcd, 657.05; found, 657.05. 1H NMR (400 MHz, CDCl3) δ: 8.14 (s,
2H, CH of BMO3), 7.27 (d, J = 2.1 Hz, 1H, CH of BMO3), 7.25 (d, J
= 2.1 Hz, 1H, CH of BMO3), 7.18 (d, J = 1.4 Hz, 2H, CH of BMO3),
5.45 (s, 2H, CH of acac), 3.65 (s, 4H, CH2 of BMO3), 2.82 (d, J =
39.7 Hz, 4H, CH2 of BMO3), 2.23 (s, 6H, CH3 of acac), 1.86 (s, 6H,
CH3 of acac). 13C NMR (101 MHz, CDCl3) δ: 188.86, 186.97,
176.93, 138.65, 134.86, 128.32, 128.00, 127.14, 127.10, 116.33,
100.10, 54.76, 53.43, 27.87, 26.86. DEPT-135 (126 MHz, CDCl3) δ:
128.00 (CH of BMO3), 127.16 (CH of BMO3), 127.11 (CH of
BMO3), 100.14 (CH of acac), 54.77 (CH2 of BMO3), 53.41 (CH2 of
BMO3), 27.90 (CH3 of acac). IR (KBr, cm−1): 1716 [ν(CO)].
Molar conductivity (CH3CN): ΛM = 6 Ω−1 cm2 M−1.
To obtain B from A, the reaction was carried out under an O2
atmosphere for ∼6 h in an ethanol medium at 373 K. Purification was
done by using a neutral alumina column and petroleum ether/
dichloromethane.
Though the acidic nature of methylene protons adjacent to imine
nitrogens of BMO in B led to the aerobic dehydrogenation reaction to
yield BAM in C, it proceeded at a slower rate. However, addition of a
base (e.g., triethylamine or potassium tert-butoxide) enhanced the
reaction rate. Therefore, heating of B in the presence of excess Et3N
in refluxing EtOH resulted in C.
[Ru(acac)2(L1)], 1A (brown). Ru(acac)2(CH3CN)2 (100 mg, 0.26
mmol), EtOH (50 mL), HL1 (65.1 mg, 0.26 mmol), Et3N (0.036 mL,
0.26 mmol). MS (ESI+, CH3CN) m/z: {1A + H}+ calcd, 548.10;
found, 548.09.
[Ru(acac)2(BAM2)], 2C (green). 2B (50 mg, 0.083 mmol), EtOH
(30 mL), Et3N (50 μL, excess). Yield: 35 mg (70%). Slow
evaporation of its dichloromethane/hexane (2:1) solution gave
crystals of 2C. MS (ESI+, CH3CN) m/z: {2C }+ calcd, 596.00;
1
found, 595.99. H NMR (400 MHz, CDCl3) δ: 8.44 (t, J = 36.4 Hz,
2H, CH of BAM2), 7.83 (s, 2H, CH of BAM2), 7.70 (d, J = 50.7 Hz,
2H, CH of BAM2), 7.09 (d, J = 4.6 Hz, 2H, CH of BAM2), 5.49 (s,
2H, CH of acac), 2.26 (s, 6H, CH3 of acac), 1.89 (s, 6H, CH3 of
acac). 13C NMR (101 MHz, CDCl3) δ: 186.88, 179.28, 175.54,
140.70, 132.17, 126.41, 121.63, 99.85, 27.98, 23.30. Anal. Calcd for
C25H22N2O5S2Ru: C, 50.41; H, 3.72; N, 4.70. Found: C, 50.71; H,
3.52; N, 4.74. IR (KBr, cm−1): 1692 [ν(CO)]. Molar conductivity
(CH3CN): ΛM = 2 Ω−1 cm2 M−1.
I
DOI: 10.1021/acs.inorgchem.9b01201
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Inorganic Chemistry
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[Ru(acac)2(BAM3)], 3C (brown). 3B (50 mg, 0.076 mmol), EtOH
(30 mL), Et3N (50 μL, excess). Yield: 40 mg (80%). Slow
evaporation of its dichloromethane/methanol/hexane (3:1:2) sol-
ution gave crystals of 3C. MS (ESI+, CH3CN) m/z: {3C}+ calcd,
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1
652.01; found, 652.05. H NMR (400 MHz, CDCl3) δ: 9.01 (d, J =
9.2 Hz, 1H, CH of BAM3), 8.81 (d, J = 9.0 Hz, 1H, CH of BAM3),
8.17 (s, 1H, CH of BAM3), 7.80 (d, J = 4.1 Hz, 2H, CH of BAM3),
7.61 (dd, J = 9.2 Hz, 1.7, 3H, CH of BAM3), 7.45 (d, J = 19.6 Hz, 2H,
CH of BAM3), 4.98 (s, 1H, CH of acac), 4.61 (s, 1H, CH of acac),
2.02−1.98 (6H, CH3 of acac), 1.83−1.63 (s, 6H, CH3 of acac). 13C
NMR (101 MHz, CDCl3) δ: 178.61, 147.58, 135.35, 135.30, 129.78,
129.03, 128.95, 127.32, 127.26, 126.31, 125.68, 120.36, 117.43, 28.13,
28.00, 27.80, 25.80. Anal. Calcd for C29H24N2O5Cl2Ru: C, 53.38; H,
3.71; N, 4.29. Found: C, 53.18; H, 3.62; N, 4.14. IR (KBr, cm−1):
1702 [ν(CO)]. Molar conductivity (CH3CN): ΛM = 2 Ω−1 cm2
M−1.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01201.
■
S
Bond lengths and angles (crystal and DFT), MO
1
compositions, mass spectra, H and 13C NMR spectra,
ORTEP diagrams, DFT-optimized structures, kinetic
plots, electronic spectra, Gibbs free energy profiles,
relaxed potential energy surface plots, and TD-DFT
tables (PDF)
Accession Codes
CCDC 1898086−1898087 and 1898089−1898092 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing 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 Authors
*E-mail: sanjibp@iitb.ac.in.
*E-mail: lahiri@chem.iitb.ac.in.
■
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ORCID
Achintya Kumar Dutta: 0000-0002-6686-582X
Goutam Kumar Lahiri: 0000-0002-0199-6132
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support received from the Science and Engineering
Research Board (SERB, Department of Science and Technol-
ogy), a J. C. Bose Fellowship (G.K.L., SERB), CSIR
(fellowship to S.P.), and UGC (fellowship to S.K.B.), New
Delhi, India, is gratefully acknowledged. The authors thank
Prof. D. Maiti, Prof. C. Subramaniam, Mihir Kumar Jha
(Department of Chemistry, IIT Bombay), and Vishal Kumar
Porwal (Department of Chemistry, IISER Mohali) for their
help. The valuable suggestions of the reviewers are also
gratefully acknowledged.
(4) (a) Panda, S.; Mandal, A.; Ghosh, P.; Lahiri, G. K. Ru-Complex
Framework toward Aerobic Oxidative Transformations of β-
Diketiminate and α-Ketodiimine. Inorg. Chem. 2017, 56, 14900−
14911 and references therein . (b) Yokota, S.; Tachi, Y.; Itoh, S.
Oxidative Degradation of β-Diketiminate Ligand in Copper(II) and
Zinc(II) Complexes. Inorg. Chem. 2002, 41, 1342−1344. (c) Panda,
S.; Bera, S. K.; Lahiri, G. K. Electronic Impact of Ancillary Ligand on
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