Redox Non-Innocence and Isomer-Specific Oxidative Functionalization of Ruthenium-Coordinated β-Ketoiminate

Article abstract of DOI:10.1002/asia.201901093

This article deals with isomeric ruthenium complexes [Ru^III(L^R)₂(acac)] (S=1/2) involving unsymmetric β-ketoiminates (AcNac) (L^R=R-AcNac, R=H (1), Cl (2), OMe (3); acac=acetylacetonate) [R=para-substituents (H,

Full text of DOI:10.1002/asia.201901093

DOI: 10.1002/asia.201901093
Full Paper
Redox Non-Innocence and Isomer-Specific Oxidative
Functionalization of Ruthenium-Coordinated b-Ketoiminate
Sudip Kumar Bera, Sanjib Panda, Sourajit Dey Baksi, and Goutam Kumar Lahiri*[a]
Abstract: This article deals with isomeric ruthenium com-
plexes [RuIII(LR)2(acac)] (S=1/2) involving unsymmetric b-ke-
toiminates (AcNac) (LR =R-AcNac, R=H (1), Cl (2), OMe (3);
acac=acetylacetonate) [R=para-substituents (H, Cl, OMe) of
N-bearing aryl group]. The isomeric identities of the com-
plexes, cct (cis-cis-trans, blue, a), ctc (cis-trans-cis, green, b)
and ccc (cis-cis-cis, pink, c) with respect to oxygen (acac),
oxygen (L) and nitrogen (L) donors, respectively, were au-
thenticated by their single-crystal X-ray structures and spec-
troscopic/electrochemical features. One-electron reversible
oxidation and reduction processes of 1–3 led to the elec-
tronic formulations of [RuIII(L)(L·)(acac)]+ and [RuII(L)2(acac)]ꢀ
for 1+-3+ (S=1) and 1ꢀ–3ꢀ (S=0), respectively. The triplet
state of 1+-3+ was corroborated by its forbidden weak half-
field signal near gꢁ4.0 at 4 K, revealing the non-innocent
feature of L. Interestingly, among the three isomeric forms
(a–c in 1–3), the ctc (b in 2b or 3b) isomer selectively under-
went oxidative functionalization at the central b-carbon (Cꢀ
H!C=O) of one of the L ligands in air, leading to the forma-
tion of diamagnetic [RuII(L)(L’)(acac)] (L’=diketoimine) in 4/
4’. Mechanistic aspects of the oxygenation process of AcNac
in 2b were also explored via kinetic and theoretical studies.
Introduction
b-diketiminate[1] (NacNac) ligand system has gained profound
recent interest from the broader perspectives of coordination/
organometallic chemistry by virtue of its tunable electronic
and steric features.[2–4] The central b-carbon of metal coordinat-
ed NacNac (a vinylogous amidine) is known to be reactive to-
wards electrophiles, which includes cycloaddition with dieno-
philes to form bicyclic structures, activation of small diatomic
molecules (H2, N2, O2) by interacting with their LUMOs, methyl
cation migration etc.[5] On the contrary, reactivity of metal com-
plexes of analogous heteroditopic b-ketoiminate (AcNac=L)
has not been well explored.[6] The present article is therefore
originated to probe electronic and reactivity aspects of ruthe-
nium coordinated AcNac ligand.
The present report describes synthesis, structural and spec-
troscopic aspects of isomeric [RuIII(LR)2(acac)] (a–c) [R=para-
substituents (H, Cl, OMe) of N-bearing aryl group] in 1–3
(Scheme 1). Further, it addresses (i) electronic structural aspects
of the complexes in accessible reversible redox states in 1n–3n
(n= +, 0, ꢀ1), (ii) isomer specific oxidative functionalization of
AcNac in 2b/3b to yield corresponding 4/4’ and (iii) theoretical
insights relating to the oxygentaion of Cb (diastereotopic face)
of AcNac in 2b including the formation of selective diastereo-
mer.
Scheme 1. Representation of complexes under study.
Results and Discussion
Synthesis, general characterization and structure
Substituted b-ketoimines (R-AcNacH=LH, R=H, Cl, OMe) were
prepared from acetylacetone and corresponding para-substi-
tuted aniline in refluxing ethanol and in presence of catalytic
amount of trifluoroacetic acid.[7] Though three tautomeric
forms of LH (ketamine, ketimine and enimine) are feasible, the
[a] S. Kumar Bera, S. Panda, S. D. Baksi, Prof. Dr. G. Kumar Lahiri
Department of Chemistry
Indian Institute of Technology Bombay
Powai, Mumbai 400076 (India)
E-mail: lahiri@chem.iitb.ac.in
1
observed sharp and D2O exchangeable broad H NMR peaks at
d, 5.2 ppm and 12 ppm corresponding to methine (b-CH) and
hydrogen bonded NH protons, respectively, suggested its keta-
mine form (Scheme 2).
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901093.
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tain its molecular structure, their identities were established by
other analytical techniques (see later).
The electrically non-conducting 1–4 exhibited satisfactory
microanalytical (Experimental Section) and mass spectrometry
(Figure S1) data. Isomeric forms of 1–3 were authenticated by
single crystal X-ray structures of representative complexes
(Figure 1, Table 1 and Figures S2–S3, Tables S1–S3, Supporting
Information).
Scheme 2. Tautomeric equilibria of LH.
Isomeric complexes of [RuIII(L)2(acac)] (a–c in 1–3, Scheme 1)
were prepared by reacting one-equivalent of the metal precur-
sor [RuII(acac)2(CH3CN)2] with three equivalent of respective R-
AcNacH (LH) in presence of excess sodium acetate base under
dinitrogen atmosphere followed by their separation on a neu-
tral alumina column. We however failed to observe any trace
of the tris-complex ([Ru(L)3]) as confirmed by mass spectrome-
try. This might be due to more steric involvement in the tris-sit-
uation as well as relatively less leaving aptitude of the acetyla-
cetonate ligand. The isomeric forms of cct(a), ctc(b) and ccc(c)
with respect to oxygen (acac), oxygen (L) and nitrogen (L)
donors, respectively, in 1–3 (Scheme 1) were authenticated by
their single crystal X-ray structures. All the three isomeric forms
(cct(a), ctc(b), ccc(c)) could be isolated for R-AcNac with R=Cl
(2), while only two isomers cct(a), ccc(c) and ctc(b), ccc(c) were
Table 1. Selected bond distances (ꢁ).
Bond
1a
1c
2a
2b
3b
3c
Ru1-N1 2.050(3) 2.038(2)
Ru1-N2 2.056(3) 2.012(2)
2.060(2)
2.072(2)
2.016(5) 2.0176(17) 2.032(2)
2.015(5) 2.039(2)
Ru1-O1 1.992(2) 1.9859(18) 1.9926(19) 2.006(5) 2.0087(14) 2.0229(17)
Ru1-O2 1.993(2) 2.0220(18) 1.9956(18) 2.013(4) 2.0571(15) 1.9994(17)
Ru1-O3 2.031(2) 2.0343(18) 2.0297(18) 2.060(4)
Ru1-O4 2.033(2) 2.0644(18) 2.0272(18) 2.049(4)
2.0374(17)
2.0528(17)
1.331(3)
1.325(3)
1.286(3)
1.298(3)
1.408(4)
1.385(4)
1.378(4)
1.417(4)
C2-N1 1.310(4) 1.318(3)
C9-N2 1.319(4) 1.323(4)
C4-O1 1.297(4) 1.289(3)
C7-O2 1.299(4) 1.280(4)
C2-C3 1.413(5) 1.410(4)
C3-C4 1.372(5) 1.376(4)
C7-C8 1.373(5) 1.381(4)
C8-C9 1.415(5) 1.404(4)
1.312(4)
1.311(4)
1.291(3)
1.306(3)
1.422(4)
1.381(4)
1.364(4)
1.436(4)
1.340(8) 1.333(3)
1.345(8)
1.289(8) 1.287(3)
1.296(7)
1.414(9) 1.413(3)
1.377(10) 1.378(3)
1.394(9)
1.410(9)
achievable for
1 (R=H) and 3 (R=OMe), respectively
(Scheme 1). The presence of three anionic ligands (two AcNac
and one acac) possibly facilitated the oxidation of RuII to RuIII
in 1–3 during the purification process under aerobic condition.
Further, methine proton of one of the coordinated L in ctc iso-
meric 2b or 3b selectively oxygenated (CꢀH(L)!C=O(L’) via
the activation of aerial oxygen with the concomitant change in
metal redox (RuIII in 2b or 3b versus RuII in 4 or 4’, Scheme 1)
due to the stronger p-acceptor feature of L’ in 4 or 4’. Though
we failed to obtain suitable single crystals of 4 or 4’ to ascer-
The N,O donors of monoanionic AcNac (L) ligand formed a
six-membered chelate with the average bite angle of ꢁ928.
The average C2–N1, C2–C3, C3–C4, C4–O1 and C9–N2, C9–C8,
C8–C7, C7–O2 bond lengths in 1–3 suggested the delocaliza-
tion of negative charge over the AcNac backbone as in the
case of acac. The shorter Ru-N(L) distance trans to Ru-O(acac)
bond in ctc or ccc isomer as compared to that cis to Ru-
O(acac) in cct isomer could be attributed to the impact of
Figure 1. ORTEP diagrams with ellipsoids at 50% probability level. H-atoms are omitted for clarity.
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trans effect exerted by the s-donating acac ligand. The isomer-
ic identity (a–c, Scheme 1) of the complexes was reflected in
cis and trans N1-Ru1-N2 or N1-Ru1-N1’ angles involving two
AcNac in ctc/ccc and cct forms, respectively, due to the differ-
ence in connectivity of the donor atoms as well as spatial ori-
entation of the pendant phenyl rings.
Ruthenium(III) (t2g5, S=1/2) derived isomeric 1–3 displayed
broad and ill-defined proton resonances[8,9] (Experimental Sec-
tion) corresponding to the full molecule over a wide chemical
shift region in CDCl3 owing to paramagnetic contact shift
effect,[10] which indeed had restricted to assign the individual
proton signals. Interestingly, an appreciable downfield shifting
of the proton resonances appeared on moving from ctc(b) to
cct(a) to ccc(c) isomeric form due to the varying orientation of
the pendant phenyl rings. In agreement with the RuIII center,
isomeric forms a–c (1–3 in Scheme 1) displayed metal based
anisotropic EPR (Figure 2, Table 2 and Figure S4, Supporting In-
formation).
Figure 3. Cyclic (black) and differential pulse (green) voltammograms in
CH3CN.
Table 3. Electrochemical data.[a]
Complex
E0[V] (DEp[mV])[b]
Kc[c]
O1
R1
1a
1c
2a
2b
2c
3b
3c
0.70(70)
0.63(140)
0.83(130)
0.84(140)
0.74(200)
0.72(70)
0.55(90)
ꢀ0.92(90)
ꢀ0.97(160)
ꢀ0.81(160)
ꢀ0.80(170)
ꢀ0.86(220)
ꢀ0.88(80)
ꢀ0.98(100)
2.8ꢂ1027
1.3ꢂ1027
6.2ꢂ1027
6.2ꢂ1027
1.3ꢂ1027
1.3ꢂ1027
8.5ꢂ1025
Figure 2. X-band (9 GHz) EPR spectra in CH2Cl2/toluene (1:5) for 2a, 2b, 2c
and in CH3CN for 2a+, 2b+, 2c+ at 4 K.
[a] From cyclic voltammetry in CH3CN/0.1m Et4NClO4 at 100 mVsꢀ1
.
[b] Potential in V versus SCE; peak potential differences DEp[mV] (in pa-
rentheses). [c] Comproportionation constant from RTln Kc =nF(DE),[11] Kc
between O1 and R1.
Table 2. EPR data at 4 K.
Complex
g1/2
g1
g2
g3
<g>[a]
Dg[b]
2a+[c]
2a[d]
4.40
2.25
2.25
2.24
2.32
2.23
2.29
2.11
2.18
2.11
2.06
2.18
2.13
1.87
1.80
1.88
1.79
1.85
1.85
2.08
2.08
2.08
2.06
2.09
2.07
0.38
0.45
0.36
0.53
0.38
0.44
ated with the oxidation process (O1) at around ꢀ0.2 V for 1a
or 2a was originated due to its slight irreversible feature,
which was therefore absent in the separately scanned reduc-
tion process (Figure S5, Supporting Information).
2b+[c]
2b[d]
4.33
2c+[c]
2c[d]
Not resolved
[a]<g> ={(1/3) (g12 +g22 +g3 )}1/2
CH2Cl2-toluene (1:5).
.
[b] Dg=g1ꢀg3. [c] In CH3CN. [d] In
One-electron nature of the couple was confirmed by con-
stant potential coulometry. Oxidation (O1) or reduction (R1)
potential of the complexes varied systematically based on the
electron withdrawing and donating properties of R associated
with the AcNac(L) moiety and it followed the order of Cl>H>
OMe. Though isomeric forms cct(a) and ctc(b) exhibited close
by redox potentials, slight negative shift of redox potential
was apparent for the ccc(c) isomer (Table 3). The significantly
large Kc value of ꢁ1027 for the intermediate state corroborated
the fact of the stable form of the isolated 1–3 (Scheme 1).
2
Electrochemistry and electronic structure
Isomeric a-c in 1–3 displayed well defined quasi-reversible one
oxidative (O1) and one reductive (R1) couples in the potential
ranges of 0.74 to 0.84 V and ꢀ0.81 to ꢀ0.86 V versus SCE, re-
spectively (Figure 3, Table 3). The minor cathodic wave associ-
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Figure 4. Spin density representation (Isosurface value 0.003).
The pertinent question of involvement of metal or ligand or
mixed metal/ligand based frontier orbitals in the redox pro-
cesses of the isomeric forms a–c in 1n–3n (n= +1, 0, ꢀ1) was
ascertained via the collective consideration of MO composi-
tions (Tables S4-S28, Supporting Information) and EPR
(Figure 2, Table 2 and Figure S4, Table S29, Supporting Informa-
tion)/Mulliken spin distribution (Figure 4, Table 4 and Figure S6,
ed recently for the symmetric NacNac under analogous coordi-
nation situation.[5b,16]
Electronic spectral data of 1–3 are set in Table 5. The change
in spectral profile on oxidation as well as reduction for each of
the isomeric species (a–c) (Figure 5 and Figure S7, Supporting
Information) was analyzed by TD-DFT calculations (Table S31,
Supporting Information).
Table 4. DFT calculated (UB3LYP/LanL2DZ/6-31G*) Mulliken spin densi-
ties.
Table 5. Electronic spectral data in CH3CN.
Complex
l [nm] [e/Mꢀ1 cmꢀ1
]
Complex
Ru
AcNac
acac
1a
1c
2a
2b
2c
3b
3c
4
303(20000), 340(15200), 585(1800)
300(13800), 365(8000), 573(1800)
300(35400), 343(23400), 594(3200)
267(31400), 296(30200), 362(14200), 658(3400)
265(11400), 300(10800), 365(6000), 569(1800)
267(11800), 293(10800), 364(5200), 648(1200)
266(10600), 298(9800), 367(5800), 572(1600)
276(9800), 368(6400), 540(4200)
2a+(S=1)
2a (S=1/2)
2b+(S=1)
2b (S=1/2)
2c+(S=1)
2c (S=1/2)
1.167
0.801
1.127
0.778
1.233
0.764
0.678
0.174
0.711
0.203
0.526
0.221
0.153
0.025
0.162
0.019
0.241
0.015
Table S30, Supporting Information) at the paramagnetic states.
This suggested selective one-electron oxidation of one of the
AcNac (L) ligands leading to the electronic structural form of
In general, isomeric complexes (a–c in 1–3) exhibited similar
spectral profile in CH3CN with one moderately intense visible
band and multiple intense transitions in the higher energy UV
region, where band position and intensity varied reasonably as
a function of their isomeric identities. Similarly, slight variation
of band position and intensity occurred on moving from 1 to
3. TD-DFT calculations for the ruthenium(III) derived 1–3 sug-
gested that the visible and UV-region transitions corresponded
to the mixed ligand/metal to metal/ligand and intra/inter-
ligand charge transfer transitions, respectively. The lowest
energy visible transition underwent appreciable red shifting on
moving from 1–3 to 1+–3+, which could be assigned to ligand
to metal/ligand based charge transfer transition. On the other
hand, slight blue shifting of the visible band had taken place
for the one-electron reduced ruthenium(II) derived 1ꢀ–3ꢀ,
which was assigned to be L targeted metal-to-ligand charge
transfer transition.
[Ru (L)(L )(acac)] for 1+–3+ (S=1) instead of alternate feasible
formulation of [RuIV(L)2(acac)]+. Interestingly, spin density distri-
bution for the oxidized form of the isomers (a–c) in 1+–3+
also revealed that the unpaired spin on AcNac was evenly dis-
tributed among both the AcNac moieties due to spin-hopping
factor.[12] The electronic formulation of electrogenerated oxi-
III
+
C
III
+
dized state of [Ru (L)(L )(acac)] (S=1, 1+–3+) was also corro-
borated by its EPR feature at 4 K which displayed forbidden
weak half-field signal near gꢁ4.0 corresponding to its triplet
(S=1) state[13] in addition to metal based anisotropic gꢁ2.0
signal.[14] The increase in ligand contribution in the singly occu-
pied molecular orbitals (SOMOs) on moving from 1–3 (S=1/2)
to 1+–3+ (S=1) was also reflected in the decreasing g aniso-
tropy (Dg in Table 2 and Table S29, Supporting Information).[15]
The MO compositions however attributed to the metal based
reduction leading to the electronic structural form of
[RuII(L)2(acac)]ꢀ for 1ꢀ–3ꢀ (S=0). This indeed reemphasized the
redox non-innocent potential of AcNac particularly with re-
spect to the electron removal process as was also demonstrat-
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Figure 5. Experimental (CH3CN) and TD-DFT ((U)B3LYP/CPCM/CH3CN) calculated electronic spectra of 2n (n=0, +1, ꢀ1). Oscillator strengths are shown by the
black vertical lines; the spectra (red) are convoluted with a Gaussian function having full width at half-maximum of 2000 cmꢀ1
.
Oxidative functionalization
Though both 2b and 3b could be transformed to the corre-
sponding 4 (Scheme 3), the detailed characterization was per-
formed for the chloro derivative. Interestingly, free AcNac re-
mained unaltered under the similar reaction condition, imply-
ing the effective role of metal coordination towards its func-
tionalization process.
The electrically neutral and RuII derived diamagnetic 4 gave
satisfactory microanalytical (Experimental Section) and mass
spectrometric (Figure 6 and Figure S1, Supporting Information)
data and displayed well defined proton resonances within the
standard chemical shift region of d, 0–10 ppm (Figure S8, Sup-
porting Information). The transformation of CꢀH!C=O of one
of the L ligands on moving from 2b to 4 was also evident in
cct(a) and ccc(c) isomeric forms of 1–3 (Scheme 1) were found
to be perfectly stable in air. However, ctc(b) analogue in 2b or
3b (Scheme 1) selectively underwent oxidative functionaliza-
tion at the central b-carbon (CꢀH!C=O) of one of the L li-
gands on prolonged exposure to air with the concomitant re-
duction in metal oxidation state from ruthenium(III) to ruthe-
nium(II) in diamagnetic [RuII(L)(L’)(acac)] (L’=diketoimine) (4 or
4’). This might be due to the increase in nucleophilicity of the
reactive Cb center of AcNac originated through electron push-
ing from the s-donating acac trans- to AcNac nitrogens.
Scheme 3. Selective oxygenation of ctc isomer.
Figure 6. Experimental and simulated ESI(+) mass spectra of {4}+ in CH3CN.
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Figure 10. Spin density representation of 4n (n= +1, ꢀ1; S=1/2) (isosur-
face=0.003).
Figure 7. Overlapping IR spectra of 2b (in black) and 4 (in red) as KBr disc.
Figure 11. X-band (9 GHz) EPR spectrum of 4ꢀ in CH3CN at 100 K.
Figure 8. Electronic spectra of 2b and 4 in acetonitrile.
the appearance of a new n(C=O) vibration at 1640 cmꢀ1
(Figure 7) and their distinct electronic spectral features
(Figure 8) with respect to the parent 2b. The visible and UV-
region bands were assigned based on the TD-DFT calculations
(Table S31, Supporting Information) as L/L’ targeted mixed
metal/ligand to ligand based and intra/inter ligand charge
transfer transitions, respectively.
Figure 12. Change in spectral profile of 2b!4 conversion in CH3CN at
333 K. Rate is calculated based on the growing feature at 540 nm (Inset).
2.12) and isotropic EPR (g: 2.003) spectral features,[14] respec-
tively (Figure 11 and Figure S9, Supporting Information).
Spectrophotometric monitoring of the oxygenation of 2b to
4 (Figure 12 and Figure S10, Supporting Information) at 333 K
suggested a pseudo 1st order rate process with the rate con-
stant (k) value of 6.3ꢂ0.1ꢂ10ꢀ6 sꢀ1. First order and zeroth
order rates with respect to substrate and molecular oxygen, re-
spectively, suggested reorganization of the Ru-complex via the
interaction with O2 at the rate determining step. Slower rate of
conversion of 2b!4 even at 333 K (ꢁ3 days) indeed restrict-
ed us to follow it at variable temperature to understand the re-
action kinetics by UV-vis spectroscopy. This however could be
illustrated by transition state calculations (Scheme 4, Fig-
ures 13–14).
Figure 9. Cyclic (black) and differential pulse (green) voltammograms of 4 in
CH3CN.
Ruthenium(II) derived 4 showed reversible one-electron oxi-
dation(O1) and reduction(R1) (Figure 9) primarily correspond-
II
III
+
Cꢀ
ing to Ru !Ru (4 ) and L’!L’ (4ꢀ), respectively, as support-
ed by MO compositions (Table S4, Supporting Information) and
spin density plots (Figure 10). The metal and ligand (L’) domi-
nated spins in 4+ and 4ꢀ were corroborated by their character-
istic anisotropic (g1: 2.44, g2: 2.13, g3: 1.73, Dg: 0.71, <g>:
Initial approach of triplet oxygen (3Sgꢀ) to the methine posi-
tion of one of the AcNac moieties in 2b (S=1/2) generates
transient superoxide intermediate (S=1/2), which undergoes
faster H-shuttling followed by homolytic cleavage of the OꢀO
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Figure 13. Gibbs free energy (DG at 298 K) profile for the oxygenation of
Ru-coordinated AcNac using UB3LYP/LANL2DZ/6-31G* level of theory. Asso-
ciated DG/DH are shown in each state. DG of starting substrates is taken as
zero. Subscript Re/Si suggests the different approach of 3O2 at the diastereo-
topic face (Cb).
Scheme 4. Formation of diastereomeric intermediates.
is, diketoimine (L’) in 4 (DGProduct for Re and Si: ꢀ3.1 and
ꢀ1.1 kcalmolꢀ1, respectively, Figure 13). This can be considered
as the consequence of steric factor exerted by the adjacent
phenyl ring of another AcNac for the Si face. This in turn disfa-
vors the approach of O2 at the Si face as has also been sup-
ported by bond length variation in TS1 (Cb-O: 1.953 (Si) and
1.775 (Re), Figure S11, Supporting Information). Further, differ-
ent spin orientation on O2 attack results in greater peroxide
character in the superoxide radical (I1) for the Si face as is re-
flected in higher spin accumulation on the oxygen unit (Re/Si:
0.919/0.816) as well as in bond length variation (Figure 14).
bond to yield 4 and hydroxyl radical byproduct.[5b] Hydroxyl
radical which has generated in the reaction medium rapidly
quenched to some other stable forms. Moreover, unsymmetri-
cal feature of AcNac backbone facilitates to form a diastereo-
topic face at the reactive Cb-center. Thus, the approach of
oxygen molecule from the upward (Si) and downward (Re) di-
rections leads to diastereomeric intermediates with ‘S’ (I) and
‘R’ (II) configurations, respectively, at the chiral Cb center
(Scheme 4). However, Re face interaction is calculated to be ki-
¼
¼
netically preferred due to lower activation barrier (DG1 /DG2
for Re and Si: 39.2/13.6 and 41.8/14.7 kcalmolꢀ1, respectively,
¼
Highly endergonic (DG 298K ~40 kcalmolꢀ1) process of 2b!
¼
¼
¼
at 298 K, where DG1 and DG2 correspond to respective acti-
vation barriers for 2b!I1 and I1!4 conversions, Figure 13).
That indeed results in thermodynamically stable product that
[TS] !I1 makes it the rate determining step (Figure 13) which
¼
may be the consequence of decrease in entropy (DS ~
ꢀ36 calmolꢀ1 Kꢀ1) as well as endothermic nature of the reac-
Figure 14. Selected bond parameters, frontier molecular orbital (isosurface=0.06) and spin density (SD) (isosurface=0.003) plots, calculated at UB3LYP level.
Hydrogen and chlorine atoms are removed for clarity.
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¼
tion (DH ~30 kcalmolꢀ1) due to the O=O bond weakening
cal experiments were carried out under a dinitrogen atmosphere.
Half-wave potential E0 =0.5(Epa +Epc), where Epa and Epc are anodic
and cathodic cyclic voltammetry peak potentials, respectively. UV-
vis spectroelectrochemical studies were performed on a BAS
SEC2000 spectrometer system. UV-vis spectral studies were per-
formed on a PerkinElmer Lambda 950 spectrophotometer. The ele-
mental analyses were carried out on a Thermoquest (EA 1112) mi-
croanalyzer. Electrospray mass spectrometry (ESI-MS) was carried
out on a Bruker’s Maxis Impact (282001.00081) spectrometer.
1H NMR were recorded using Bruker Avance III 400 MHz spectrome-
ter. The EPR measurements of coulometrically generated species
(substrate concentration 0.1m) were made with a Bruker system
ESP300 spectrometer or JEOL model FA200 spectrometer.
during the adduct formation. However, the overall transforma-
tion, 2b!4 is exergonic in nature due to the incorporation of
stable C=O into the AcNac backbone. The higher kinetic barrier
is supportive of the experimentally observed slower conversion
rate. The use of M06L functional (Figures S12–S13 and
Table S32, Supporting Information) extends similar trend in
energy for the conversion sequence with lower activation bar-
rier (DG1 /DG2 for Re: 33.1/11.2 kcalmolꢀ1, in acetonitrile at
333 K, i.e., under the experimental condition). The failure of
complex 4 (S=0) in participating further oxygenation process
involving its other AcNac moiety can be attributed to higher
¼
¼
kinetic barrier (DG =46.2 kcalmolꢀ1 at UB3LYP level, Fig-
¼
Kinetic studies
ure S14, Supporting Information) for its interaction with the
singlet oxygen (1Dg).[5a,b,17]
Kinetic experiments were carried out under O2 atmosphere (excess)
by dissolving 2b (5ꢂ10ꢀ5 m) in CH3CN. The change in absorbance
at 540 nm for 2b!4 was monitored for the rate calculation. The
pseudo-first order (due to excess oxygen) rate constant (k) for
2b!4 conversion was calculated based on non-linear exponential
fit in Origin Pro8 software by following the equation: y=y0 +A1 ꢂ
exp(ꢀx/t1), where y, y0 and x corresponded to absorbance at
540 nm at time t, t=0 and time period (t in min) over which the
absorption change took place, respectively. A1 represented
pseudo-first order coefficient and the value of pseudo-first order
rate constant (k in sꢀ1) was [1/(t1 ꢂ60)].
The selective interaction of p1g*-HOMOs of triplet dioxygen
with the Cb center of AcNac in 2b can be rationalized by the
fact of AcNac dominated a-SOMO (Figure 14). The alternate in-
termediate involving peroxo bridging (Ru-O-O-Cb)[18] is however
failed to converge possibly due to the formation of unfavora-
ble seven coordinated Ru.
Conclusions
The present article highlighted structural, spectroelectrochemi-
cal, electronic structural and reactivity aspects of isomeric (cct,
ctc, ccc) paramagnetic complexes [RuIII(AcNac)2(acac)] (a–c in
1–3, Scheme 1). The collective consideration of experimental
and theoretical events revealed selective involvement of
AcNac-based orbitals in the oxidation process (1–3!1+–3+),
attributing its redox non-innocence. In conjunction, selective
participation of ctc-isomeric 2b or 3b in the aerobic oxidation
process (central b-carbon of L, CꢀH!C=O) revealed the unex-
plored oxygenation profile of AcNac. Further, the mechanistic
pathway for the oxygenation process in 2b including selective
formation of one diastereomer has been attributed with the
help of transition state theory.
Crystallography
Single crystals of 1a, 1c, 2a, 2b and 3b, 3c were grown by slow
evaporation of their 2:1 dichloromethane-n-hexane and 2:1 di-
chloromethane-methanol solutions, respectively. X-ray crystal data
were collected on a Rigaku Saturn-724+ CCD single crystal diffrac-
tometer using Mo-Ka radiation. The data collection were carried
out by standard w-scan technique and were evaluated and re-
duced by using CrystalClear-SM Expert software. Absorption cor-
rection (numerical) was applied to the collected reflections. The
structures were solved by direct method using SHELXS-97 and re-
fined by full matrix least-squares with SHELXL-2014/7, refining on
F2.[20] All non-hydrogen atoms were refined anisotropically. The hy-
drogen atoms were placed in geometrically constrained positions
and refined with isotropic temperature factors, generally 1.2Ueq of
their parent atoms. Hydrogen atoms were included in the refine-
ment process as per the riding model. CCDC 1908644 (1a),
1908645 (1c), 1908646 (2a), 1908647 (2b), 1908648 (3b), and
1908649 (3c) contain the supplementary crystallographic data for
this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
Experimental Section
Materials
The precursor complex cis-[Ru(acac)2(CH3CN)2][19] and b-ketoamines
(R-AcNacH=LH, R=H, Cl, OMe) were prepared according to the lit-
erature procedures.[7] Other reagents and chemicals were obtained
from Aldrich and used as received. HPLC grade solvents were em-
ployed for spectroscopic and other electrochemical studies.
Computational details
Full geometry optimizations were carried out by using density
functional theory method at the (R)B3LYP (for 1n (n=ꢀ1), 2n (n=
ꢀ1), 3n (n=ꢀ1), and 4n (n=0)) or (U)B3LYP (for 1n (n=0, +1), 2n
(n=0,+1), 3n (n=0,+1)) and 4n (n= +1,-1)), level with Gaussi-
an 09 program package.[21] Except ruthenium all other elements
were assigned the 6-31G* basis set. The LANL2DZ basis set with ef-
fective core potential was employed for the ruthenium atom in all
cases. Vertical electronic excitations based on optimized geome-
tries were computed using the time-dependent density functional
theory[22] (TD-DFT) formalism in acetonitrile using the conductor-
like polarizable continuum model (CPCM). Electronic spectra were
calculated using the SWizard program.[23,24] Chemissian[25] 1.7 was
Physical measurements
FT-IR spectra were recorded on a Nicolet spectrophotometer with
samples prepared as KBr pellets. Cyclic voltammetry measurements
of the complexes (ꢁ10ꢀ3 m) were performed using a PAR model
273A electrochemistry system. Glassy carbon working electrode,
platinum wire auxiliary electrode, and a saturated calomel refer-
ence electrode (SCE) were used as a standard three-electrode con-
figuration cell. Constant potential coulometry experiment was per-
formed by using
a platinum wire-gauze working electrode.
Et4NClO4 was used as the supporting electrolyte. All electrochemi-
&
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used to calculate the fractional contributions of various groups to
each molecular orbital. All calculated structures were visualized
with ChemCraft.[26]
(25%). MS (ESI+, CH3CN): m/z {2c+Na}+ calcd: 640.04; found:
640.04. 1H NMR (400 MHz, CDCl3): d=17.71, 16.85, 14.46, 13.45,
10.12, 9.51, 2.59, 2.38, 2.22, 1.28, 0.93, ꢀ17.86, ꢀ21.27, ꢀ24.78,
ꢀ31.36 ppm; IR (KBr pellet): n˜ =1572 cmꢀ1(n(C=O)); molar conduc-
tivity (CH3CN): LM =4 Wꢀ1 cm2 mꢀ1; elemental analysis calcd (%) for
C27H29N2O4Cl2Ru: C 52.51, H 4.73, N 4.53; found: C 52.85, H 4.82, N
4.68.
Transition state calculations were performed at B3LYP/(U)B3LYP
level of theory starting from infinitely separated substrates. Single
imaginary frequency for the transition states (TSs) and real frequen-
cies for local minima were obtained. 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. The zero-point vibrational energies, thermal corrections
were obtained from the harmonic frequency calculations at the
B3LYP/(U)B3LYP level of theory. Results obtained from (U)B3LYP
functional were also tested with M06L density functional.
3b: Ru(acac)2(CH3CN)2 (100 mg, 0.78 mmol), OMe-AcNacH (160 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 32 mg
(20%). MS (ESI+, CH3CN): m/z {3b+Na}+ calcd: 632.05; found:
1
632.14. H NMR (400 MHz, CDCl3): d=12.13, 10.13, 7.40, 6.86, 3.68,
2.52, 1.49, 1.28, 0.89, ꢀ4.34, ꢀ5.19, ꢀ5.53 ppm; IR (KBr pellet): n˜ =
1575 cmꢀ1(n(C=O));
molar
conductivity
(CH3CN):
LM =
4 Wꢀ1 cm2 mꢀ1; elemental analysis calcd (%) for C29H35N2O6Ru: C
57.23, H 5.80, N 4.60; found: C 57.59, H 5.94, N 4.43.
3c: Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), OMe-AcNacH (160 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 30 mg
(18%). MS (ESI+, CH3CN): m/z {3c}+ calcd: 609.06; found: 609.14.
1H NMR (400 MHz, CDCl3): d=11.04, 8.82, 5.12, 1.38, 0.90, ꢀ0.5.57,
ꢀ6.20, ꢀ13.77 ppm; IR (KBr pellet): n˜ =1570 cmꢀ1(n(C=O)); molar
conductivity (CH3CN): LM =3 Wꢀ1 cm2 mꢀ1; elemental analysis calcd
(%) for C29H35N2O6Ru: C 57.23, H 5.80, N 4.60; found: C 57.45, H
5.63, N 4.51.
Preparation of complexes
Synthesis of [Ru(acac)(AcNac)2] (1–3)
Isomeric complexes a–c in 1–3 (Scheme 1) were prepared by react-
ing three-equivalent of respective R-AcNacH (R=H (1), Cl (2), OMe
(3)) with one-equivalent of Ru(acac)2(CH3CN)2 and three-equivalent
of sodium acetate base in refluxing ethanol under dinitrogen at-
mosphere for 8 h. Solvent of the reaction mixture was evaporated
to dryness under reduced pressure. The separation of isomers in
each case (1–3, Scheme 1): a (blue), b (green), c (pink), was per-
formed on a neutral alumina column by using 2:1, 3:1, 4:1 di-
chloromethane-petroleum ether mixtures, respectively. Evaporation
of solvent under reduced pressure gave pure complexes.
Synthesis of [RuII(L)(L’)(acac)] (L’=diketoimine) (4)
Complex 2b was sensitive to air and irreversively transformed to
the corresponding 4. Complex 4 was therefore achieved by vigo-
rous purging of oxygen for about 24 h in ethanolic solution
(15 mL) of 2b at 708 C, which was then purified on a neutral alumi-
na column using CH2Cl2 as eluent.
1a: Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), H-AcNacH (136 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 36 mg
(25%). MS (ESI+, CH3CN): m/z {1a}+ calcd: 549.13; found: 549.14;
1H NMR (400 MHz, CDCl3): d=20.82, 6.28, 5.15, 3.96, 2.50, 2.37,
2.16, 1.22, ꢀ1.06, ꢀ7.71 ppm; IR (KBr pellet): n˜ =1575 cmꢀ1(n(C=
O)); molar conductivity (CH3CN): LM =4 Wꢀ1 cm2 mꢀ1; elemental
analysis calcd (%) for C27H31N2O4Ru: C 59.11, H 5.70, N 5.10; found:
C 59.32, H 5.87, N 5.38.
4: 2b (50 mg, 0.08 mmol), EtOH (10 mL). Yield: 28 mg (55%). MS
(ESI+, CH3CN): m/z {[4]}+ calcd: 632.12; found: 632.13; 1H NMR
(400 MHz, (CD3)2CO): d=7.37 (4H, aromatic), 7.18 (2H, aromatic),
6.95 (1H, aromatic), 6.58 (1H, aromatic), 5.24 (1H, Cb-H of acac),
5.16 (1H, Cb-H of L), 2.92 (6H, 2CH3), 2.16 (3H, CH3), 1.85 (6H,
2CH3), 1.64 ppm (3H, CH3); IR (KBr pellet): n˜ =1640 cmꢀ1(n(C=O));
molar conductivity (CH3CN): LM =3 Wꢀ1 cm2 mꢀ1; elemental analysis
calcd (%) for C27H28N2O5Cl2Ru: C 51.25, H 4.46, N 4.43; found: C
51.55, H 4.57, N 4.31.
1c: Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), H-AcNacH (136 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 22 mg
(15%). MS (ESI+, CH3CN): m/z {1c}+ calcd: 549.13; found: 549.12.
1H NMR (400 MHz, CDCl3): d=17.40, 16.37, 14.42, 13.71, 9.70, 7.44,
5.78, 2.54, 2.38, 2.19, 2.01, ꢀ17.36, ꢀ19.19, ꢀ23.88 ppm; IR (KBr
pellet): n˜ =1570 cmꢀ1(n(C=O)); molar conductivity (CH3CN): LM =
3 Wꢀ1 cm2 mꢀ1; elemental analysis calcd (%) for C27H31N2O4Ru: C
59.11, H 5.70, N 5.10; found: C 59.23, H 5.71, N 5.34.
Acknowledgements
Financial support received from Science and Engineering Re-
search Board (SERB, Department of Science and technology), J.
C. Bose Fellowship (G.K.L., SERB), UGC (Fellowship to S.B.), and
CSIR (Fellowship to S.P.), New Delhi, India is gratefully acknowl-
edged.
2a: Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), Cl-AcNacH (165 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 49 mg
(30%). MS (ESI+, CH3CN): m/z {2a}+ calcd: 617.04; found: 617.06.
1H NMR (400 MHz, CDCl3): d=12.60, 8.59, 5.55, 5.31, 2.09, 1.25,
0.91, ꢀ2.76, ꢀ24.23, ꢀ26.98; IR (KBr pellet): n˜ =1570 cmꢀ1(n(C=O));
molar conductivity (CH3CN): LM =4 Wꢀ1 cm2 mꢀ1; elemental analysis
calcd (%) for C27H29N2O4Cl2Ru: C 52.51, H 4.73, N 4.53; found: C
52.65, H 5.12, N 4.90.
Conflict of interest
2b: Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol), Cl-AcNacH (165 mg,
0.78 mmol), NaOAc (64 mg, 0.78 mmol), EtOH (40 mL). Yield: 24 mg
(15%). MS (ESI+, CH3CN): m/z {2b+Na}+ calcd: 640.04; found:
640.08. 1H NMR (400 MHz, CDCl3): d=11.04, 8.80, 5.10, 1.27, 0.91,
ꢀ5.56, ꢀ6.18, ꢀ13.73 ppm; IR (KBr pellet): n˜ =1578 cmꢀ1(n(C=O));
molar conductivity (CH3CN): LM =6 Wꢀ1 cm2 mꢀ1; elemental analysis
calcd (%) for C27H29N2O4Cl2Ru: C 52.51, H 4.73, N 4.53; found: C
52.79, H 5.09, N 4.31.
The authors declare no conflict of interest.
Keywords: oxidative functionalization · redox non-innocence ·
ruthenium · spectroelectrochemistry · b-ketoiminate
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Manuscript received: August 7, 2019
Revised manuscript received: September 13, 2019
Accepted manuscript online: September 30, 2019
Version of record online: && &&, 0000
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Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Sudip Kumar Bera, Sanjib Panda,
Sourajit Dey Baksi, Goutam Kumar Lahiri*
&& – &&
Redox Non-Innocence and Isomer-
Specific Oxidative Functionalization of
Ruthenium-Coordinated b-Ketoiminate
Proven guilty: The redox non-inno-
cence and selective oxygenation profile
of b-ketoiminate (AcNac) in an isomeric
(ctc/cct/ccc) ruthenium platform have
been attributed.
Chem. Asian J. 2019, 00, 0 – 0
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11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ