Bis(quinoxaline-dithiolato)nickel(III) Complexes [Bu4N][NiIII(6,7-qdt)2] and [Ph4P][NiIII(Ph26,7-qdt)2]·CHCl3 (6,7-qdt = Quinoxaline-6,7-dithiolate; Ph26,7-

Article abstract of DOI:10.1002/ejic.201500739

Two bis(quinoxaline-dithiolato)nickel(III) complexes [Bu₄N][Ni^III(6,7-qdt)₂] (1; 6,7-qdt = quinoxaline-6,7-dithiolate) and [Ph₄P][Ni^III(Ph₂6,7-qdt)₂]·CHCl₃ (2; Ph

Full text of DOI:10.1002/ejic.201500739

FULL PAPER
DOI:10.1002/ejic.201500739
Bis(quinoxaline-dithiolato)nickel(III) Complexes
[Bu4N][NiIII(6,7-qdt)2] and [Ph4P][NiIII(Ph26,7-qdt)2]·
CHCl3 (6,7-qdt = Quinoxaline-6,7-dithiolate; Ph26,7-qdt =
Diphenylquinoxaline-6,7-dithiolate): Synthesis, Spectroscopy,
Electrochemistry, DFT Calculations, Crystal Structures and
Hirshfeld Surface Analysis
Indravath Krishna Naik,[a] Rudraditya Sarkar,[a] and
Samar K. Das*[a]
Keywords: Nickel / UV/Vis spectroscopy / Electrochemistry / Crystal structures / Supramolecular chemistry / ESR
spectroscopy / Density functional calculations
Two bis(quinoxaline-dithiolato)nickel(III) complexes [Bu4N]-
[NiIII(6,7-qdt)2] (1; 6,7-qdt = quinoxaline-6,7-dithiolate) and
[Ph4P][NiIII(Ph26,7-qdt)2]·CHCl3 (2; Ph26,7-qdt = diphenyl-
quinoxaline-6,7-dithiolate) have been synthesized from their
NiII analogues by iodine oxidation. Compounds 1 and 2 have
been characterized by routine spectral analysis and single-
crystal X-ray structure determination. Nickel(III) complexes
1 and 2 exhibit redshifted absorption bands compared with
their NiII analogues; the electronic absorption spectral stud-
chemical studies on 1 and 2 are consistent with those of their
NiII analogues. Electron spin resonance (ESR) studies confirm
that nickel is present in both 1 and 2 in its +3 oxidation state.
This is also consistent with the fact that only one cation
(Bu4N+ for 1 and Ph4P+ for 2) is present in the respective crys-
tal structures. The crystal structures of both compounds are
characterized by C–H···S and C–H···N hydrogen-bonding in-
teractions. Hirshfeld surface analyses have been studied to
gain deep insight into the hydrogen-bonding interactions
ies have been corroborated by DFT calculations. Electro- around/among the coordination complex anions.
as well as bioinorganic chemistry.[6] Recently, we reported
Introduction
on the influence of substituents of the coordinated dithiol-
Metal–dithiolene complexes have been of considerable
ato ligands on the electronic and electrochemical properties
interest to synthetic inorganic chemists for more than four
of a new square-planar nickel(II)–bis(quinoxaline-6,7-di-
decades. In recent years, the design and synthesis of metal–
thiolate) system.[7] These ligands, because of their non-inno-
bis(dithiolene) complexes have drawn great attention be-
cent behaviour, can stabilize coordination complexes in sev-
cause of their potential applications as conducting and
eral oxidation states of the pertinent metal.[4] Generally, the
magnetic,[1] nonlinear optical materials[2] and near-infrared
dianionic state (MII oxidation state) is the most stable state
(NIR) dyes.[3] Square-planar bis(dithiolene) metal coordi-
for bis(dithiolene) complexes, but in some cases they can be
nation complexes are generally characterized by a high de-
isolated as monoanionic (MIII oxidation state) or even as
gree of delocalization within the chelate ring involving the
neutral complexes (MIV oxidation state). NiIII–dithiolene
metal ion, which contributes considerably to the low-energy
complexes are of special interests because of their impor-
electronic transition between the HOMO and the LUMO.[4]
tance in the context of bioinorganic chemistry[8] and cataly-
This large delocalization is responsible for metal–dithiolene
sis.[9] The bioinorganic aspect of nickel(III)–dithiolene com-
complexes showing absorption bands in the near-infrared
plexes is related to the modelling of the active sites of
(NIR) region. In the last few years, we and others have been
[NiFe]H2ase, for which extended X-ray absorption fine
working on transition-metal–dithiolene coordination com-
structure (EXAFS)/electron paramagnetic resonance (EPR)
plexes from the perspective of supramolecular chemistry[5]
studies indicate that the formal oxidation state of the Ni
centre in the resting state of the active site is paramagnetic
[a] School of Chemistry, University of Hyderabad,
NiIII [8] The catalytic importance of NiIII coordination com-
.
P. O. Central University, Hyderabad 500046, India
E-mail: skdas@uohyd.ac.in
plexes can be recognized by the fact that recently nickel(III)
complexes have been employed as catalysts in C–C and
C–heteroatom bond-formation reactions.[9] This prompted
us to explore NiIII coordination complexes. The present
samar439@gmail.com
http://chemistry.uohyd.ac.in/~skd/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500739.
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work deals with two nickel(III)–dithiolene complexes
[Bu4N][Ni(6,7-qdt)2] (1; 6,7-qdt = quinoxaline-6,7-dithio-
late) and [Ph4P][Ni(Ph26,7-qdt)2]·CHCl3 (2; Ph26,7-qdt =
diphenylquinoxaline-6,7-dithiolate). We have chosen
a
phenyl group as a substituent attached to the aromatic ring
in compound 2 to investigate the influence of the phenyl
substituent on the electronic and electrochemical properties
of a bis(dithiolate)NiIII square-planar coordination com-
plex (for example, compound 2) with respect to an unsub-
stituted one (compound 1). We describe here the synthesis,
crystal structures, spectroscopy and electrochemistry of
compounds 1 and 2, including DFT calculations and Hirsh-
feld surface analyses of the complex anions.
Scheme 2. Synthesis of NiIII-bis(6,7-quinoxaline-dithiolate) com-
pounds 1 and 2.
earlier report, we assigned the absorption bands at 619 and
662 nm for the parent NiII compounds [Bu4N]2[NiII(6,7-
qdt)2] and [Ph4P]2[NiII(Ph26,7-qdt)2], respectively, in DMF
solutions to mixed-metal-ligand-to-ligand charge-transfer
transitions based on DFT calculations because relevant the
HOMOs include mixed metal-ligand-based orbitals and the
LUMOs were defined as ligand-based π-MOs.[7] Careful
analysis of these spectra reveals the appearance of weak
features beyond 800 nm, which were explained by the pres-
ence of oxidized impurities (corresponding to NiIII com-
plexes), formed by oxidation with air of [Bu4N]2[NiII(6,7-
qdt)2] and [Ph4P]2[NiII(Ph26,7-qdt)2], which have very low
oxidation potentials. Thus, when we oxidize these two NiII
complexes [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2[NiII(Ph26,7-
qdt)2] chemically by iodine and isolated the compounds
[Bu4N][Ni(6,7-qdt)2] (1) and [Ph4P][Ni(Ph26,7-qdt)2]·CHCl3
(2), we observe that the absorption bands at 619 and
662 nm for compounds [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2-
[NiII(Ph26,7-qdt)2], respectively, disappear. At the same
Results and Discussion
Synthesis and General Characterization
The synthetic route for the synthesis of the two dithiolate
ligands used in this study (L1 and L2) is shown in Scheme 1.
The synthesis of the corresponding NiII–(bis)dithiolato
complexes [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2[NiII(Ph26,7-
qdt)2] was performed by a procedure reported earlier and
shown in Scheme 2.[7] The electrochemical studies of these
two NiII compounds demonstrate that they can be oxidized
at a low potential to the corresponding one-electron oxid-
ized compounds [Bu4N][NiIII(6,7-qdt)2] (1) and [Ph4P]-
[NiIII(Ph26,7-qdt)2]·CHCl3 (2). Accordingly, we synthesized
compounds 1 and 2 by chemical oxidation of [Bu4N]2-
[NiII(6,7-qdt)2] and [Ph4P]2[NiII(Ph26,7-qdt)2], respectively,
by using iodine as shown in Scheme 2.
time, broad bands at approximately 853 nm (ε
=
=
UV/Vis/NIR Spectra
16320 Lmol–1 cm–1) and approximately 530 nm (ε
The UV/Vis/NIR spectra of compound 1 and its parent 14280 Lmol–1 cm–1) appear for compound 1 as shown in
compound [Bu4N]2[NiII(6,7-qdt)2] are shown in part a of Figure 1 (a). Similarly for compound 2, absorption bands
Figure 1. The electronic spectra of compound 2 and its cor- appear at approximately 880 nm (ε = 8160 Lmol–1 cm–1)
responding parent compound [Ph4P]2[NiII(Ph26,7-qdt)2] in and approximately 554 nm (ε = 12440 Lmol–1 cm–1) as
DMF solutions are shown in part b of Figure 1. In our shown in Figure 1 (b).
Scheme 1. Synthesis of quinoxaline dithiolate ligands L1 and L2.
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low intensity of this band (Figure 1 (a), dotted line) is ex-
plained by its low oscillator strength (f value) of 0.0172.
The second high-intensity absorption band observed for
compound 1 appeared at 853 nm (Figure 1 (a), dotted line),
which can theoretically be attributed to the β-spin HOMO–
2 to β-spin LUMO transition as shown in Figure 2 (B). The
theoretical value of this band is approximately 985 nm with
a high oscillator strength of 0.2841. This transition at
853 nm (compound 1) also contains a contribution from
the transition from α-spin HOMO–6 to α-spin LUMO (Fig-
ure 1, C). The former transition (β-spin HOMO–2 to β-spin
LUMO) has maximum 88% impact on the total absorption
and the latter transition (α-spin HOMO–6 to α-spin
LUMO, Figure 1, C) has a 12% contribution to the total
absorption at 853 nm (theoretical value 985 nm). The third
experimental absorption band of compound 1, which ap-
pears at 530 nm (Figure 1 (a), dotted line), cannot be ex-
plained properly by DFT calculations, because the corre-
sponding theoretical value (553 nm) is characterized by a
poor oscillator strength (0.0001), even though the observed
absorption (at 530 nm) is an intense band. The nickel ion
does not have any significant role in the low-energy absorp-
tion at 1060 nm (Figure 1 (a), dotted line) of compound 1
(theoretical value 1272 nm) as shown in Figure 2 (A). Con-
versely, for the highly intense observed band at 853 nm
(theoretical value 985 nm), the central metal (nickel) ion
plays a crucial role (Figure 2, B and C). As shown in Fig-
ure 2 (B), β-spin HOMO–2 is of π-type character and β-
spin LUMO is of π*-type character. We also find a contri-
bution from the central metal (nickel) in the HOMOs (both
HOMO–2 and HOMO–6) and high electron density in the
π* orbitals of dithiolate ligands in the LUMOs. As both the
HOMOs (Figure 2, B and C) include mixed-metal-ligand-
based orbitals, we can say that the major transition
(853 nm) is a “mixed-metal-ligand-to-ligand” (MMLL)
charge-transfer transition.[7] Although the oxidation state
of nickel in compound 1 is formally +3 (confirmed from
ESR spectroscopy and electrochemical studies), electron
density corresponds to the 3py atomic orbital (AO) of nickel
is situated at the nickel centre in the β-spin HOMO–2 MO.
All 3py AOs of the surrounding four sulfur donor atoms (all
Figure 1. (a) UV/Vis/NIR spectra of compound 1 (3.42ϫ 10–5 m)
and corresponding parent compound [Bu4N]2[Ni(6,7-qdt)2]
(4.64ϫ10–5 m) in DMF solutions. (b) UV/Vis-NIR spectra of com-
pound 2 (5.4ϫ 10–5 m) and corresponding parent compound
[Ph4P]2[Ni(Ph26,7-qdt)2] (7.1ϫ 10–5 m) in DMF solutions.
DFT Calculations
We performed ground-state electronic structure calcula- sulfur atoms have positive MO coefficients) and the central
tions for mono anions of compounds 1 and 2 by using den- nickel 3py AO form a linear combination to create the π-
sity functional theory (DFT; see computational details in bonding-type β-spin HOMO–2 (Figure 2, B). Owing to the
the Experimental Section), as implemented in the Gaussian participation of both ligand (sulfur donor) and metal
09 program.[10] Owing to the odd number of total electrons (nickel) AOs, the present calculated HOMO is described as
(NiIII ion, d7 system), we found singly occupied α-spin a mixed-metal-ligand-type of MO. In the case of the π*-
HOMOs for both complex anions; at the same time, the type β-spin LUMO, the same AOs of sulfur (two sulfur
comparable β-spin MO is vacant. The molecular orbital atoms have positive MO coefficients and the other two sul-
diagrams of the HOMOs and LUMOs of compounds 1 and fur atoms have negative MO coefficients) form the anti-
2 are given in the Supporting Information (Figures S8 and bonding-type MO, where no contribution from the nickel
S9). Experimentally, we observed an absorption band for atom is found. Thus, this LUMO is characterized only by
[Bu4N][Ni(6,7-qdt)2] (1) at 1060 nm. From the theoretical ligand AOs. As a result, the overall transition, shown in
vertical excitation, determined by the time-dependent (TD) Figure 2 (B), can be depicted as a “mixed-metal-ligand-to-
DFT method, this low-energy absorption band corresponds ligand” transition.
to the transition from β-spin HOMO to β-spin LUMO
Next, we concentrated our focus on [Ph4P][Ni(Ph26,7-
transition, (Figure 2, A); the theoretical value of this band qdt)2] (2). A noticeable redshift of the most intense peak
is approximately 1272 nm as shown in Figure 2 (A). The is observed in case of compound 2 compared with that of
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Figure 2. HOMO and LUMO diagrams originating from molecular orbital calculations for the mono anion of compound 1.
[Bu4N][Ni(6,7-qdt)2] (1). Experimentally, the most intense bination of AOs is also observed in the case of compound
absorption band is observed at 880 nm for compound 2, 2, where the 3pz AOs of both nickel and sulfur are the par-
which is 30 nm from the most intense peak of compound ticipating orbitals.
1. The theoretical value of this observed band at 880 nm is
It has already been mentioned that the most intense ab-
1071 nm with an f value of 0.4736. This theoretical peak is sorption band is observed at 880 nm for [Ph4P][Ni(Ph26,7-
attributed mainly to the transition from β-spin HOMO–3 qdt)2]·CHCl3 (2), which is around 30 nm redshifted from
to β-spin LUMO (Figure 3, A), which contributes 80% to the most intense peak (853 nm) of [Bu4N][Ni(6,7-qdt)2] (1).
the total observed absorption band at 880 nm. The rest of As we go from compound 1 to compound 2, two phenyl
the contribution arises from β-spin HOMO–5 to β-spin groups per ligand are added as substituents, replacing two
LUMO (11%, Figure 3, B) and α-spin HOMO–7 to α-spin protons (Scheme 2). Particular substituents on an aromatic
LUMO (9%, Figure 3, C) transitions. The other major ab- system can lead to mesomeric as well as inductive effects.
sorption of compound 2, for which the observed (554 nm) The mesomeric effect arises from the delocalization of π-
as well as theoretical (553 nm) values are almost identical, electrons between the substituent and aromatic core. On the
is attributed to the transition from β-spin HOMO–5 to β- other hand, the inductive effect is fully associated with the
spin LUMO, which contributes 54% to the total absorption σ-electron system of the aromatic ring. In the case of
at 554 nm (Figure 3, B), and also takes part in the absorp- [Ph4P][Ni(Ph26,7-qdt)2]·CHCl3 (2), both mesomeric and in-
tion at 880 nm as mentioned earlier. As shown in Figure 3, ductive effects are to be considered. This is because the sub-
A and C, it is clear that the maximum electron density stituted phenyl group (compound 2) can be involved in both
cloud is situated around the central metal ion, nickel, which these effects. The inductive effect can be initiated by an elec-
indicates that central metal ion has considerable contri- tron-withdrawing substituent or by an electron-donating
bution to the electronic absorption at 880 nm (compound substituent. In the present study (compound 2), the phenyl
2). Lateral overlap in HOMO–3 (Figure 3, A) indicates that substituent acts as an electron-withdrawing group, which
it has a π-bonding nature (mixed-metal-ligand π orbitals), can be established from the following discussion. An elec-
whereas the corresponding LUMO is ligand-based and π* tron-withdrawing group pulls the σ-electron density from
in nature. Therefore, in the case of compound 2, the major the ligand by the inductive effect; thereby, reducing the re-
absorption at 880 nm can be assigned to “mixed-metal-li- pulsive Coulombic interactions between the electrons occu-
gand-to-ligand” (MMLL) charge-transfer transitions.[7] As pying the ligand-localized π-MOs and the electrons of the
observed in compound 1, a similar scenario of linear com- σ-system. Thus, an electron-withdrawing substituent in the
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Figure 3. HOMO and LUMO diagrams originating from molecular orbital calculations for the mono anion of compound 2.
present system causes a lowering of the energy of the that at the central metal ion in compound 1 (where there is
LUMOs (because these are purely π-MO of the dithiolate no substituent in place of hydrogen). The higher electron
ligand, Figures 1 and 2) without lowering the energy of the density at the central metal ion makes electronic transitions
HOMOs significantly (because they are a mixture of nickel easier in the case of compound 2, which probably causes
d-orbital character and dithiolate π-type orbitals). This the redshift of the more intense band compared with com-
should cause a redshift in the relevant electron absorption pound 1. Thus, both inductive and mesmeric effects are re-
spectra (the energy gap between HOMO and LUMO would sponsible for this redshift, which is consistent with DFT
essentially be decreased). Conversely, an electron-releasing calculations.
substituent in the present system would cause a blueshift
As shown in Figure 1 (a and b), the major electron ab-
in the electronic absorption spectra because the energy gap sorption bands for NiIII complexes (compounds 1 and 2)
between the HOMO and LUMO would essentially be in- appear in redshifted regions compared with their parent
creased (an electron-donating group increases the electron NiII analogues [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2[NiII-
density toward the dithiolate ligand and thereby increases (Ph26,7-qdt)2]. Compounds 1 and 2 are one-electron oxid-
the repulsive Columbic interactions between the ligand ized products of their NiII parent compounds [Bu4N]2-
centred π-MOs and σ-electrons). Therefore, in the case of [NiII(6,7-qdt)2] and [Ph4P]2[NiII(Ph26,7-qdt)2]. When com-
compound 2, the substituent phenyl group acts as an elec- pound [Bu4N]2[NiII(6,7-qdt)2] is oxidized by iodine to its
tron-withdrawing group and causes the redshift of 30 nm in NiIII analogue (compound 1), the electronic absorption
the pertinent electron absorption spectra when we go from band at 619 nm (parent NiII compound) shifts to 853 nm
compound 1 (no substituent) to compound 2 (phenyl group (compound 1). Similarly, when compound [Ph4P]2[NiII-
as a substituent).
(Ph26,7-qdt)2] is oxidized by iodine to its NiIII form (com-
In the case of the mesomeric effect, the substituent pound 2), the major electronic absorption band at 662 nm
phenyl group (compound 2) can share its π-electrons with (parent NiII compound) moves to 880 nm (compound 2).
the aromatic core associated with the dithiolato ligand by
This large redshift (234 nm for compound 1 and 218 nm
resonance. The prevalence of mesomeric effects in the pres- for compound 2) of the major electronic absorption bands
ent system is supported by DFT calculations. When we on going from NiII to NiIII complexes can be attributed to
compare parts A and C in Figure 3 with parts B and C in the central metal ion, nickel, which, in its +3 oxidation
Figure 2, we find relatively more electron density at the cen- state, pulls the σ-electron density from the coordinated di-
tral metal ion in compound 2 (where the phenyl substituent thiolate ligand and reduces the repulsion-type Coulombic
exerts a mesomeric effect by resonance) in comparison to interactions between the electrons in the σ-system and elec-
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trons occupying the ligand-localized π-MOs. This causes crystal lattice of compound 2 might have made this differ-
the lowering of the energy of the LUMOs (purely π-MOs ence. A square-planar bis(dithiolato)–NiIII complex show-
of the dithiolate ligand, see Figures 2 and 3) without lower- ing an axial ESR feature is known.[12]
ing the energy of the HOMOs (mixed-metal-ligand π orbit-
als, Figures 2 and 3), resulting in the decrease in the energy
gap between the HOMO and LUMO. This explains the
Electrochemistry
We mentioned in our earlier report[7] that the parent
compounds [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2[NiII-
(Ph26,7-qdt)2] of the present NiIII compounds [Bu4N]-
[NiIII(6,7-qdt)2] (1) and [Ph4P][NiIII(Ph26,7-qdt)2]·CHCl3
(2) undergo reversible oxidation at very low oxidation po-
tentials in MeOH solutions: [Bu4N]2[NiII(6,7-qdt)2] un-
dergoes oxidation at E1/2 = +0.12 V versus Ag/AgCl (ΔE
= 74 mV), corresponding to the [NiIII(6,7-qdt)2]1–⁄[NiII(6,7-
qdt)2]2– redox couple, and [Ph4P]2[NiII(Ph26,7-qdt)2] un-
large redshift in the electronic absorption maxima when the
NiII complex is oxidized to its NiIII analogue. Theoretically
simulated absorption spectra for the NiIII complex anions
of 1 and 2 are displayed in the Supporting Information
(Figures S10 and S11).
Electron Spin Resonance (ESR) Spectra
The ESR spectra of solid samples of [Bu4N][NiIII(6,7- dergoes oxidation at E1/2 = +0.033 V versus Ag/AgCl (ΔE
qdt)2] (1) and [Ph4P][NiIII(Ph26,7-qdt)2]·CHCl3 (2) were re- = 65 mV), corresponding to the [NiIII(Ph26,7-qdt)2]1–/[NiII-
corded at room temperature and 123 K. The ESR spectra (Ph26,7-qdt)2]2– couple. In the present study, we could not
recorded at 123 K are presented in Figure 4. Compound 1 perform electrochemical studies in MeOH owing to low sol-
exhibits a rhombic-type signal: gx = 2.246, gy = 2.156 and ubility. Compounds 1 and 2 are not freely soluble in
gz = 2.065. However, compound 2 shows an axial signal MeOH. We, thus, performed the cyclic voltammetric studies
with gЌ Ͼ gʈ: gЌ = 2.133 and gʈ = 2.049. This is consistent of compounds 1 and 2 in DMF solutions, each containing
with low-spin NiIII (d7, S = 1/2) in a tetragonal geometry 0.10 m [Bu4N]ClO4 as the supporting electrolyte. As shown
(gЌ Ͼ gʈ) with one unpaired electron in the dz2 orbital.[11] in Figure 5, cyclic voltammograms of compounds 1 and 2
The anisotropy in the ESR spectra implies some contri- exhibit quasi-reversible reductive responses at E1/2
=
bution from the d orbital of nickel to the total spin density, +0.225 V versus Ag/AgCl (ΔE = 87 mV) and E1/2
=
which is consistent with DFT calculations (see above). The +0.044 V versus Ag/AgCl (ΔE = 82 mV), respectively.
ESR data suggest that the unpaired electrons in compounds Based on the E1/2 values of the oxidative responses of the
1 and 2 are not localized on the ligands because the ESR NiII analogues [Bu4N]2[NiII(6,7-qdt)2] and [Ph4P]2[NiII-
spectra do not show any sharp signal near g = 2.003 (an (Ph26,7-qdt)2] in MeOH[7] as well as in DMF (see the Sup-
organic ligand radical usually shows a sharp signal at g = porting Information), we can assign these reductive re-
2.003). The present ESR data and their comparison with sponses of compounds 1 and 2 to the [NiIII(6,7-qdt)2]1–⁄
those of already known mononuclear NiIII square-planar [NiII(6,7-qdt)2]2– and [NiIII(Ph26,7-qdt)2]1–/[NiII(Ph26,7-
complexes[11] confirm that compounds 1 and 2 are formally qdt)2]2– couples, respectively.
NiIII complexes. The rhombic ESR feature (compound 1) is
The lower reduction potential value (+0.044 V) for the
not unusual for a square-planar NiIII complex because of [NiIII(Ph26,7-qdt)2]1–/[NiII(Ph26,7-qdt)2]2– couple (com-
distortion from the square-planar geometry. However, com- pound 2) compared with that for the [NiIII(6,7-qdt)2]1–⁄
pound 2 exhibits an axial feature in its ESR spectrum, even [NiII(6,7-qdt)2]2– couple (compound 1; +0.225 V) can be ex-
though both complexes are square planar. It can be specu- plained by the inductive effect of the phenyl substituents,
lated that the presence of a solvent molecule (CHCl3) in the which pull electrons from the metal–dithiolate chelate and,
Figure 4. Electron spin resonance spectra of the compounds 1 (left) and 2 (right) in their powder form at 123 K.
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out that the oxidation state of nickel in the isolated com-
pounds 1 and 2 is +3.
Description of Crystal Structures
[Bu4N][Ni (6,7-qdt)2] (1)
The crystals of compound 1 suitable for single-crystal X-
ray structure determination were obtained from acetonitrile
solution by the vapour diffusion of diethyl ether. Crystallo-
graphic analysis revealed that compound 1 crystallizes in
a monoclinic system with space group P21/c. The relevant
asymmetric unit contains two crystallographically indepen-
dent halves of the [Ni(6,7-qdt)2]1– molecule and one tetra-
butylammonium cation. The thermal ellipsoid diagram of
the asymmetric unit in the crystal structure of compound 1
is shown in Figure 6 (a). The relevant molecular packing
diagram is shown in Figure 6 (b). The basic crystallographic
data for compound 1 are presented in Table 1 and selected
bond angles and interatomic distances are described in
Table 2. The geometry around nickel ion in both the crys-
tallographically independent complexes is roughly square
planar, consisting of four sulfur donor atoms from two 6,7-
qdt2– dithiolato ligands. The relevant coordination angles
are in the range 87.77(3)–92.23(3)° (Table 2). In the five-
membered chelate rings involving the nickel ion, the Ni–
S, C–S and C=C bond lengths are in the range 2.1431(7)–
Figure 5. Cyclic voltammograms of compounds 1 (top) and 2 (bot-
tom) in DMF solution. Scan rate=0.01 mVs–1, V vs. Ag/AgCl.
thus, the relevant metal centre would be more easily re-
duced. The low reduction potential value of compound 2
compared with that of compound 1 can also be justified
from the perspective of theoretical calculations, which show
that the energy gap between the HOMO and LUMO is less
in the case of compound 2 compared with that of com-
pound 1. In both cases (compounds 1 and 2), the lowest
unoccupied orbital is the β-spin LUMO (see above). If the
system is reduced by one electron, the electron has to oc-
cupy the β-spin LUMO, which is more stabilized in the case
of compound 2 (HOMO–LUMO gap is smaller). Thus,
compound 2 would be reduced more easily (+0.044 V) than
compound 1 (+0.225 V).
ESR spectroscopy (see above) and single-crystal X-ray
crystallography (see below) indicate that the isolated com-
pounds 1 and 2 have nickel in its +3 oxidation state. Keep-
ing this in mind, during the cyclic voltammetric experi-
ments, we took the first scans starting from the positive side
(+0.5 V for compound 1 and +0.3 V for compound 2) and
we found first reduction followed by oxidation in the re-
spective cyclic voltammograms (shown by arrows, Fig-
Figure 6. (a) Thermal ellipsoid plot (at the 50% probability level)
of the asymmetric unit in the crystal structure of compound 1
(hydrogen atoms are omitted for clarity). (b) Molecular packing
ure 5). Thus, the electrochemical experiments clearly point diagram for the crystal structure of compound 1.
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2.1555(7), 1.737(3)–1.746(3) and 1.423(4)–1.426(4) Å, C–H···S hydrogen-bonding interactions are responsible for
respectively. The total charge of the NiIII complex anion the formation of a two-dimensional layer-like supramolec-
[Ni(6,7-qdt)2]1– in compound [Bu4N][Ni(6,7-qdt)2] (1) is, ular arrangement [Figure 7 (b)]. In addition to inter-anionic
unsurprisingly, –1, and this anionic charge is compensated supramolecular interactions, inter-cation–anion interac-
by a [Bu4N]+ cation as observed in the crystal structure tions exist (see the Supporting Information, Figure S5).
(Figure 6).
Hydrogen-bonding parameters for the crystal structure of
compound 1 are shown in Table 3.
Table 1. Crystal data and structural refinement parameters for
compounds 1 and 2.
1
2
Empirical formula
Formula weight
Temperature [K]
Crystal size [mm]
Crystal system
Space group
Z
C
685.67
100(2)
32H44N5NiS4
C65H45Cl3N4NiPS4
1206.32
100(2)
0.20ϫ0.12ϫ0.08 0.21ϫ0.18ϫ0.12
monoclinic
P21/c
triclinic
¯
P1
4
2
Wavelength [Å]
a [Å]
b [Å]
c [Å]
α [°]
0.71073
13.8384(9)
11.6926(8)
22.4558(12)
90
113.333
90
3336.3(4)
1.365
0.71073
13.294(10)
15.018(12)
15.677(12)
70.818(12)
80.368(12)
84.916(13)
2913
β [°]
γ [°]
Volume [Å3]
Calculated density [Mgm–3
Reflections collected/unique
R(int)
]
1.375
10208/6861
0.13
33936/6533
0.0548
1452
F(000)
1242
Max. and min. transmission
Θ range for data collection [°]
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [IϾ2σ(I)]
0.9342 and 0.8465 0.922 and 0.8693
1.70–26.48 1.39–26.18
full-matrix least-squares on F2
Figure 7. (a) Inter-molecular one dimensional supramolecular
chain of [Ni(6,7-qdt)2]1– formed by C–H···N weak interactions. (b)
Two-dimensional supramolecular layer-like structure formed by in-
ter-anion C–H···S weak interactions (viewed along the crystallo-
graphic a-axis).
6533/0/386
1.131
10208/0/706
1.252
0.0488/0.1029
0.0576/0.1069
0.1335/0.3135
0.1719/0.3319
R1/wR2 (all data)
Largest diff. peak and hole [eÅ–3] 0.548 and –0.281 1.275 and –0.944
Table 3. Hydrogen-bond parameters for compound 1.[a]
D–H···A
d(D–H)
[Å]
d(H···A) d(D···A) Ͻ(DHA)[b]
Table 2. Selected bond lengths [Å] and angles [°] for compounds 1
and 2.
[Å]
[Å]
[°]
C(29)–H(29A)···N(2)
C(26)–H(26B)···N(2)
C(13)–H(13)···N(2)#3
C(23)–H(23B)···S(3)#3
C(7)–H(7)···S(1)#4
0.97
0.97
0.93
0.97
0.93
2.65
2.70
2.67
2.90
2.81
3.612(4)
3.603(4)
3.390(4)
3.797(3)
3.733(3)
172.9
154.4
134.9
154.7
169.6
Compound 1[a]
Ni(1)–S(1)#1
Ni(2)–S(3)#2
2.1431(7)
2.1555(7)
180.00(4)
87.77(3)
87.85(3)
105.26(10)
104.95(10)
Ni(1)–S(2)#1
Ni(2)–S(4)#2
S(1)–Ni(1)–S(2)
S(4)–Ni(2)–S(3)
S(3)–Ni(2)–S(3)#2 180.00(4)
C(6)–S(2)–Ni(1) 105.33(10)
C(14)–S(4)–Ni(2) 104.92(10)
2.1442(7)
2.1473(7)
92.23(3)
92.16(3)
S(1)#1–Ni(1)–S(1)
S(1)#1–Ni(1)–S(2)
S(4)–Ni(2)–S(3)#2
C(1)–S(1)–Ni(1)
C(9)–S(3)–Ni(2)
[a] Symmetry transformations used to generate equivalent atoms:
#3 –x + 1, y – 1/2, –z + 3/2; #4 –x + 2, y – 1/2, –z + 3/2. [b]
Ͻ(DHA): dihedral angle.
Compound 2[b]
[Ph4P][Ni(Ph26,7-qdt)2]·CHCl3 (2)
Ni(1)–S(3)
Ni(2)–S(1)
S(3)–Ni(1)–S(4)
S(2)–Ni(2)–S(1)
S(2)–Ni(2)–S(2)#2
S(3)–Ni(1)–S(3)#1
2.126(3)
2.136(3)
92.03(12)
92.17(11)
180
Ni(1)–S(4)
Ni(2)–S(2)
S(3)–Ni(1)–S(4)#1 87.97(12)
S(2)#2–Ni(2)–S(1) 87.83 (11)
S(1)#2–Ni(2)–S(1) 180.00(16)
S(4)–Ni(1)–S(4)#1 180.00(12)
2.129(3)
2.134(3)
Crystals of [Ph4P][Ni(Ph26,7-qdt)2]·CHCl3 (2) suitable
for single-crystal X-ray structure determination were ob-
tained from a CHCl3 solution by vapour diffusion of diethyl
¯
ether. Compound 2 crystallizes in triclinic space group P1.
179.9(1)
The asymmetric unit consists of two crystallographically in-
dependent halves of the [Ni(Ph26,7-qdt)2]1– anion and one
tetraphenylphosphonium cation along with a solvent mol-
ecule, as shown Figure 8 (a). The geometry around the Ni3+
[a] Symmetry transformations used to generate equivalent atoms:
#1 –x + 2, –y + 2, –z + 1; #2 –x + 1, –y + 1, –z + 2. [b] #1 –x +
1, –y, –z + 1; #2 –x + 2, –y + 1, –z + 1.
To investigate the supramolecular structure, we looked at ion, which is coordinated by four sulfur atoms from two
the C–H···S and C–H···N interactions in the crystal struc- Ph26,7-qdt2– ligands, is a square-planar geometry with S1–
ture of compound 1. We found that a supramolecular N2–S2 and S3–Ni1–S4 coordination angles of 92.17(11)
chain-like arrangement [Figure 7 (a)] is formed from anion– and 92.03(12), respectively, from two different crystallo-
anion C–H···N interactions. On the other hand, inter-anion graphically independent molecules. In the five-membered
Eur. J. Inorg. Chem. 2015, 5523–5533
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coordinated chelate rings of both the crystallographically insight into the long- and short-range interactions experi-
independent molecules, the Ni–S, C–S and C=C bond enced by the complex anions, and the 2D fingerprint plot,
lengths are in the range 2.1431(7)–2.1555(7), 1.711(3)– derived from the HSs, furnishes the nature, type and relative
1.740(3) and 1.407(4)–1.419(4) Å, respectively.
contribution of the intermolecular interactions. Generally,
the directions and strengths of intermolecular interactions
within the crystal are mapped onto the HSs by defining a
descriptor “dnorm”, which is a ratio of the distance of any
surface point to the nearest interior (di) and exterior (de)
atom to the van der Waals radii (vdW) of the concerned
atoms.[14] Thus, when dnorm is negative, the sum of di and
de, that is. the contact distance, is shorter than the sum of
the relevant van der Waals radii, which is considered to be
the closest contact and is visualized as red spots on the HSs
as shown in Figure 9 (a). The white and blue colours indi-
cate the contacts at vdW separation (dnorm = 0). The inter-
molecular contacts contained in Tables 3 and 4 are effec-
tively summarized as spots on the Hirshfeld surfaces; the
large circular depressions (deep red), which are visible on
the dnorm surfaces, are indicators of hydrogen-bonding con-
tacts. The small area and light colour on the surfaces indi-
cate weaker and longer contacts, contacts other than
hydrogen bonds.
Figure 8. (a) Thermal ellipsoid diagram of compound 2 (at the 50%
probability level), with hydrogen atoms omitted for clarity. (b) One-
dimensional supramolecular chain-like arrangement formed by in-
ter-anion interactions in the crystal structure of compound 2. (c)
Supramolecular chain-like structure formed by C–H···S weak inter-
actions in compound 2.
The crystal structure of compound 2 is also characterized
by C–H···N and C–H···S supramolecular hydrogen-bonding
interactions. The inter-anion C–H···N weak interactions
lead to the formation of one-dimensional chain-like struc-
ture as shown in Figure 8 (b). On the other hand, the C–
H···S hydrogen-bonding interactions between the complex
anions and tetraphenylphosphonium cations lead to the
construction of a sandwich-type extended arrangement as
shown in Figure 8 (c).
Figure 9. (a) Hirshfeld surfaces of the complex anions in com-
pound 1 (top) and in compound 2 (bottom). (b) 2D fingerprint
plots derived from HSs: compound 1 (left) and compound 2 (right).
Hirshfeld Surface Analysis
Table 4. Hydrogen-bond parameters for compound 2.[a]
The hydrogen-bonding supramolecular interactions
D–H···A
d(D–H) d(H···A) d(D···A)
Ͻ(DHA)
[°]
around the complex anions [NiIII(6,7-qdt)2]1– and [NiIII
-
[Å]
[Å]
[Å]
(Ph26,7-qdt)2]1– are further analysed with the Hirshfeld sur-
faces (HSs) and 2D fingerprint plots (FPs), which are gen-
erated by using the software Crystal explorer 3.1[13] based
C(24)–H(24)···S (4)#3 0.95
C(28)–H(28)···N(3)#4 0.95
2.82
2.62
3.541(12) 133
3.432(14) 144
[a] Symmetry transformation used to generate equivalent atoms:
on the CIF file. The 3D Hirshfeld surfaces offer additional #3 x, 1 + y, z; #4 1 – x, 1 – y, –z.
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[Bu4N]2[NiII(6,7-qdt)2] complex:
a
solution of I2 (0.025 g,
A plot of di versus de gives a 2D fingerprint plot [Figure 9
(b)], which identifies the occurrence of different kinds of
intermolecular interactions. Complementary regions are
visible in the fingerprint plots, where one molecule acts as
a donor (de Ͼ di) and the other as an acceptor (de Ͻ di).
The N···H close contacts vary from 13.9% in compound 1
to 5.5% in compound 2. The C···H contacts vary from
20.5% in compound 1 to 15.6% in compound 2. The S···H
weak contacts vary from 21.8% in compound 1 to 8.4% in
0.1 mmol) in CH3CN (10 mL) was added dropwise to a filtered
solution of [Bu4N]2[NiII(6,7-qdt)2] (0.185 g, 0.2 mmol) in CH3CN
(20 mL) and the mixture was allowed to stir for 2 h at room tem-
perature. The resulting brown precipitate was filtered and washed
with a small amount of methanol and hexane. Single crystals, suit-
able for X-ray diffraction, were grown by slow diffusion of diethyl
ether into a CH3CN solution of the obtained solid, yield 0.115 g
(84% based on NiII complex used). C32H44N5NiS4 (685.67): calcd.
C 56.05, H 6.47, N 10.21, S 18.71; found C 56.15, H 6.42, N 10.28,
compound 2 (see Section 8 in the Supporting Information). S 18.65. IR (KBr): ν = 3435 (br), 2959 (m), 1440 (w), 1413 (w),
˜
1380 (w), 1172 (m), 1072 (w), 1019 (m), 876 (m), 783 (w), 597 (s),
515 (s) cm–1.
Conclusion
[Ph4P][Ni(Ph26,7-qdt)2] (2): Mono anionic compound 2 was ob-
We have described the synthesis and characterization of
tained as a dark-brown powder by I2 oxidation of the correspond-
two NiIII–bis(dithiolato) complexes, [Bu4N][NiIII(6,7-qdt)2]
ing [Ph4P]2[NiII(Ph26,7-qdt)2] complex: a solution of I2 (0.025 g,
(1) and [Ph4P][NiIII(Ph26,7-qdt)2] (2), that are based on
0.1 mmol) in CH3CN (10 mL) was added dropwise to a filtered
solution of [Ph4P]2[Ni(Ph26,7-qdt)2] (0.284 g, 0.2 mmol) in CH3CN
unique dithiolene ligands. Compounds 1 and 2 are ad-
ditionally characterized by electrochemical and ESR stud-
ies, in addition to their unambiguous characterization by
single-crystal X-ray structure determination. The described
compounds are unique in the sense that they get reduced
reversibly at very low potentials. This demonstrates that
their respective reduced analogues [Bu4N]2[NiII(6,7-qdt)2]
and [Ph4P]2[NiII(Ph26,7-qdt)2] can act as oxidation cata-
lysts. The title compounds exhibit well-defined electronic
absorption bands that have been corroborated with theoret-
ical (DFT) calculations. This is a rare report in which theo-
retical calculations have been performed on a NiIII–bis(di-
thiolato) system. The crystal structures of compounds 1
and 2 show interesting supramolecular hydrogen-bonding
networks, formed by C–H···S and C–H···N weak contacts.
Hirshfeld surfaces (HSs) and 2D fingerprint plots (FPs)
corroborate these supramolecular interactions.
(20 mL) and the mixture was allowed to stir for 2 h at room tem-
perature. The resulting dark-brown powder precipitate was filtered
and washed with a small amount of MeOH and hexane. Single
crystals, suitable for X-ray diffraction, were grown by slow dif-
fusion of diethyl ether into a CHCl3 solution of the obtained dark-
brown powder, yield 0.213 g (88% based on NiII complex used).
C65H45Cl3N4NiPS4 (1206.32): calcd. C 64.71, H 3.76, N 4.64, S
10.63; found C 64.82, H 3.71, N 4.59, S 10.71. IR (KBr): ν˜ = 3441
(br), 1424 (s), 1336 (w), 1190 (m), 1117 (w), 1080 (s), 1020 (w), 865
(w), 820 (w), 723 (m), 690 (m) cm–1.
X-ray Crystallography: Single crystals, suitable for facile structural
determination for the compounds 1 and 2, were analysed with a
three-circle Bruker SMART APEX CCD area detector system un-
der a Mo-Kα (λ = 0.71073 Å) graphite-monochromatic X-ray beam.
The frames were recorded with an ω scan width of 0.3°, each for
10 s, with a crystal–detector distance of 60 mm, collimator 0.5 mm.
Data reduction was performed by using SAINTPLUS.[15] Empirical
absorption corrections were made by using equivalent reflections
and were performed with the SADABS program.[16] The structures
were solved by direct methods and least-squares refinement on F2
for both compounds by using SHELXS-97.[16] All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms were in-
cluded in the structure factor calculation by using a riding model.
The crystallographic parameters, data collection and structure re-
finement of the compounds 1 and 2 are summarized in Table 1.
Selected bond lengths and angles for the compounds 1 and 2 are
listed in Table 2.
Experimental Section
General: All the chemicals for the synthesis were commercially
available and used as received. 1,2-Diaminobenzene-bis(thio-
cyanate) (A),[15] quinoxaline-6,7-dithiol (H26,7-qdt; L1),[7] and
[Bu4N]2[Ni(6,7-qdt)2](1a),[7]diphenylquinoxaline-6,7-dithiol(L2)[7]and
[Ph4P]2[Ni(Ph26,7-qdt)2][7] were prepared according to literature
procedures. Syntheses of metal complexes were performed under
N2 by using standard inert-atmosphere techniques. Solvents were
dried by standard procedures. Microanalytical (C, H, N) data were
obtained with a FLASH EA 1112 Series CHNS Analyzer. Infrared
(IR) spectra were recorded from KBr pellets with a JASCO FT/
IR-5300 spectrometer operating in the region 400–4000 cm–1. 1H
NMR spectra of compounds were recorded with a Bruker DRX-
400 spectrometer using Si(CH3)4 [TMS] as an internal standard.
Electronic absorption spectra of solutions and diffuse reflectance
spectra of solid compounds were recorded with a UV-3600 Shim-
adzu UV/Vis/NIR spectrophotometer. A Cypress model CS-1090/
CS-1087 electroanalytical system was used for the cyclic voltam-
metric experiments. The electrochemical experiments were mea-
sured in DMF containing [Bu4N][ClO4] as a supporting electrolyte,
by using a conventional cell consisting of two platinum wires as
the working and counter electrodes, and a Ag/AgCl electrode as a
reference. The potentials reported here are uncorrected for junction
contributions.
CCDC-1054536 (for 1) and -1054537 (for 2) contain the supple-
mentary 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.
Computation: Computational simulations for the ground-state as
well as excited-state electronic structure calculations of the complex
anions of compounds 1 and 2 were performed with the help of
well-known density functional theory (DFT) by using the
GAUSSIAN 09 suite programming package.[10] It is well known
that the DFT can evaluate the ground-state electronic structure of a
moiety exactly (especially for bigger molecules, e.g., a coordination
complex anion), depending upon the functional used in the pro-
cedure. Theoretical calculations throughout the study were done
with the hybrid functional B3LYP, which includes Hartree–Fock
(HF) exchange as well as DFT exchange correlations. Non-local
correlations were accounted for by the Lee, Yang and Parr (LYP)
functional. The LanL2DZ basis set was used for Ni; whereas for
[Bu4N][Ni(6,7-qdt)2] (1): Mono anionic compound 1 was obtained
as
a
brown powder by I2 oxidation of the corresponding
5532
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for by the inbuilt effective core potential (ECP) LanL2 of the
LanL2DZ basis set. It should be mentioned that the polarization
effect on C, S and N has been taken care of by incorporating five
d-type Gaussian polarization functions into the basis set, whereas
for H atoms three p-type polarization functions were included. The
ground-state electronic structure calculation of the complex anions
were performed by using the self-consistent reaction field (SCRF)
procedure of the GAUSSIAN 09 programming package,[10] where
solute complex ions are placed in the solvent cavity (DMF). The
ground-state anionic complexes were obtained by full geometry op-
timization followed by frequency calculations. No imaginary fre-
quencies (the lowest ten frequencies of each anionic complex are
available in Section 7 of the Supporting Information) were ob-
tained, which ensured that the optimized structure is not situated
at any saddle point of the ground-state potential energy surface.
Vertical excitations of the optimized structures were performed by
employing the TD-B3LYP method[17] and using the same basis sets
and same environment mentioned above. Other details of the com-
putational output are described in the Supporting Information,
Figure S7.
[4]
[5]
[6]
Supporting Information (see footnote on the first page of this arti-
cle): HRMS spectra, computational outputs, 2D fingerprint plot
derived from Hirshfeld surfaces, cyclic voltammograms, hydrogen-
bonding interactions in the crystal structure of compound 1 and
bond valence sum calculations for nickel in compounds 1 and 2.
[7]
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revision C.01, Gaussian, Inc., Wallingford CT, 2010.
The authors thank the Council of Scientific and Industrial Re-
search (CSIR), Government of India [project number 01 (2556)/12/
EMR-II] for financial support. The single-crystal X-ray diffraction
facility at the University of Hyderabad by the Department of Sci-
ence and technology (DST), New Delhi is highly acknowledged.
The authors thank Mr. Suresh for helping us in the different stages
of preparing the manuscript and Mr. Kumar for recording the ESR
spectra. Our special thanks go to Professor S. Mahapatra, School
of Chemistry and University of Hyderabad, for his help with the
theoretical calculations and to the CMSD for computational facili-
ties.
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Received: July 6, 2015
Published Online: November 4, 2015
Eur. J. Inorg. Chem. 2015, 5523–5533
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