Catecholato complexes of cobalt and nickel with 1,4-disubstituted-1,4-diazabutadiens-1,3 and 1,2-bis(diphenylphosphino)ethane

Article abstract of DOI:10.1007/s12039-015-0805-2

Divalent cobalt and nickel form four-coordinate complexes with sterically hindered 3,6-di-tert-butylcatecholato dianion (3,6-DBCat) and neutral bidentate 1,4-disubstituted-1,4-diazabutadiens-1,3 (DAB). Structural study of (1,4-di-tert-butyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)nickel and (1,4-bis-(2,6-di-iso-propylphenyl)-2,3-dimethyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)cobalt indicates square-planar environment of metals. Chemical one-electron oxidation of nickel complexes proceeds through catecholate ligand and leads to o-semiquinonato adducts. EPR spectral parameters indicate preservation of square-planar configuration after oxidation. Complexes (DAB)M(Cat) (M = Ni, Co) undergo neutral ligand substitution reactions. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0805-2

c
J. Chem. Sci. Vol. 127, No. 3, March 2015, pp. 527–535. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0805-2
Catecholato complexes of cobalt and nickel with
1,4-disubstituted-1,4-diazabutadiens-1,3 and
1,2-bis(diphenylphosphino)ethane
MICHAEL P BUBNOVa,b,∗, IRINA A TEPLOVAa, NIKOLAY O DRUZHKOVa,
GEORGY K FUKINa,b, ANNA V CHERKASOVAa and VLADIMIR K CHERKASOVa,b
aG A Razuvaev Institute of Organometallic Chemistry of RAS, 603950 Tropinina str. 49, GSP-445,
Nizhny Novgorod, Russia
bLobachevsky State University of Nizhny Novgorod, 23 Prospekt Gagarina, 603950, Nizhny Novgorod, Russia
e-mail: bmp@iomc.ras.ru
MS received 03 June 2014; revised 19 September 2014; accepted 24 September 2014
Abstract. Divalent cobalt and nickel form four-coordinate complexes with sterically hindered 3,6-di-tert-
butylcatecholato dianion (3,6-DBCat) and neutral bidentate 1,4-disubstituted-1,4-diazabutadiens-1,3 (DAB).
Structural study of (1,4-di-tert-butyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)nickel and (1,4-bis-
(2,6-di-iso-propylphenyl)-2,3-dimethyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)cobalt indicates
square-planar environment of metals. Chemical one-electron oxidation of nickel complexes proceeds through
catecholate ligand and leads to o-semiquinonato adducts. EPR spectral parameters indicate preservation of
square-planar configuration after oxidation. Complexes (DAB)M(Cat) (M = Ni, Co) undergo neutral ligand
substitution reactions.
Keywords. Catecholates; diazabutadienes; cobalt; nickel; ligand exchange; oxidation; structural study.
1. Introduction
and semiquinonate acceptor properties influence the
equilibrium position.
A feature of catecholato ligands is their abil-
ity to undergo easy one-electron oxidation form-
ing semiquinones.1,2 Reversible one-electron oxidation
accompanied by structural transformation occurring in
a complex molecule leads to equilibrium of two iso-
mers differing by charge distribution between metal and
ligand. This phenomenon is called valent tautomerism
or redox-isomerism.3–6 In the case of copper, the
redox-isomeric transformation was observed on com-
plexes containing 1,4-disubstituted-1,4-diazabutadiens-
1,3 as neutral ancillary ligand and catecholate as redox
active ligand (scheme 1). Basing on the general con-
sideration of the theory of ligand field splitting, the
authors4 have proposed that reversible electron transfer
is accompanied by structural transformation. Square-
planar geometry was attributed to catecholato deriva-
tive of copper (II) whereas the distorted tetrahedral
was attached to semiquinonato complex of copper
(I). Sterical hindrances of both DAB and SQ-ligands
Cobalt and nickel are the predecessors before cop-
per in the first row of transition metals. The aim of this
work is synthesis of similar complexes of cobalt and
nickel and searching for new complexes demonstrating
reversible intramolecular electron transfer in solution.
We have found two structurally characterized diaz-
abutadiene nickel catecholates: (1,4-bis-(2,6-di-iso-
propylphenyl)-2,3-dimethyl-1,4-diazabutadiene-1,3)(3,
4,5,6-Cl4-catecolato)nickel and (1,4-bis-(2,6-di-me-
thylphenyl)-2,3-dimethyl-1,4-diazabutadiene-1,3)(3,4,5,
6-Cl4-catecolato)nickel which were used as precur-
sors for olefin polymerization catalysts.7 Reversible
interconversion of redox isomers was observed in
complex (N,Nꢁ-bis(3,5-di-tert-butyl-salicylidene)-1,2-
cyclohexanediamino)nickel cation.8 It is the equi-
librium between Ni(III)-phenolato form of complex
(at low temperature) and Ni(II)-phenoxyl radical one
(at ambient temperature). Several examples of mono-
catecholato complexes of cobalt and nickel with phos-
phins (PMe3, dppe) have been already described.9,10
There are also some examples of chemical control of
electronic state of nickel complexes with catecholate
ligand.11,12
∗For correspondence
527

528
Michael P Bubnov et al.
72.94; H 8.79; Ni 8.95. C40H56N2O2Ni Calc.: C 73.28;
H 8.55; Ni 9.01. IR (Nujol, cm−1): 1590 w, 1550 w,
1370 m, 1360 m, 1320 s, 1305 s, 1275 s, 1260 s, 1210 s,
1180 s, 1040 s, 985 s, 940 m, 870 m, 860 m, 785 s,
750 s, 700 s, 655 s, 620 m, 595 m, 515 m, 490 w.
Scheme 1. Copper valent tautomers interconversion (DAB
= 1,4-diazabutadiene-1,3; SQ and Cat = anion-radical and
dianion of substituted o-quinone respectively).
2. Experimental
2.2c (2,3-dimethyl-1,4-bis-(2,6-di-iso-propylphenyl)-
1,4-diaza-1,3-butadiene)(3,6-di-tert-butyl-catecholato)
nickel (3): Dark green airstable crystals. Yield 80%.
Anal.(%) Found: C 73.48; H 8.70; Ni 8.85.
C42H60N2O2Ni Calc.: C 73.79; H 8.78; Ni 8.64. IR
(Nujol, cm−1): 1595 w, 1580 w, 1505 s, 1405 s, 1370 m,
1345 s, 1320 s, 1305 s, 1275 s, 1215 s, 1065 w, 985 s,
950 m,890 w,880 w, 835 w, 785 m,740m, 715 w,705 w,
655 w, 620 m, 515 w.
2.1 General considerations
Cobalt(II) chloride, nickel tetracarbonyl, amines, dppe,
diacetil, glyoxal water solution (40%), were received
from commercial sources (Aldrich, Fluka). 3,6-Di-tert--
butyl-o−quinone-1,2 was obtained according to known
procedures.13 Solvents were carefully dried and in most
cases were degassed before using. Diazabutadiens were
obtained according to known procedures.14 EPR spec-
tra were recorded on “Bruker ER-200 SRC”, IR - on
“Specord M-80”. X-ray structural studies were per-
formed on “Smart Apex” (Bruker AXS). The most
synthetic procedures were carried out in evacuated
ampoules.
2.2d (1,4-bis-(2,6-dimethylphenyl)-1,4-diaza-1,3-buta-
diene)(3,6-di-tert-butyl-catecholato)nickel (4): Dark green
air stable solid. Yield 55%. Anal.(%) Found: C 70.50;
H 7.71; Ni 10.56. C32H40N2O2Ni Calc.: C 70.73; H
7.42; Ni 10.80. IR (Nujol), ν, cm−1: 1720, 1600, 1555,
1460, 1360, 1320 s, 1305 s, 1245, 1205 s.
2.2 Synthesis and characterization of diazabutadiene-
catecholato nickel complexes
2.2e (2,2ꢁ-dipyridine)(3,6-di-tert-butyl-catecholato)nickel
(5) from Ni(CO)4: Blue-grey air sensitive solid. Yield
42%. Anal.(%) Found: C 65.87; H 6.45; Ni 13.96.
C24H28N2O2Ni Calc.: C 66.24; H 6.49; Ni 13.49. IR
(Nujol, cm−1): 1615 w, 1605 m, 1420 s, 1290m,1280m,
1255 m, 990m, 950 m, 785 s, 770 s, 730 m, 655 w,
625 w.
General procedure: Nickel tetracarbonyl (1 mmol) was
condensed into evacuated frozen ampoule (of approx-
imately 200 mL volume) containing 1 mmol of 3,6-di-
tert-butyl-o−quinone + 1 mmol corresponding diazabu-
tadiene in 50 mL of degassed toluene. Ampoule was
slowly warmed at ∼30◦C for half an hour and at ∼80◦C
during the next two hours. It was necessary to freeze
and evacuate ampoule periodically every ten minutes
for removing CO. Resulting solution was allowed to
stay for a night at −10◦C. Crystalline solid was filtered,
washed with light petroleum and dried under vacuum.
Yields and properties are listed below.
2.2f (2,2ꢁ-dipyridine )(3,6-di-tert-butyl-catecholato) nickel
(5) from (DME)NiCl2:(DME)NiCl2 was obtained by
treatment of nickel powder by bromine in DME. Di-
thallium 3,6-di-tert-butyl-catecholate (prepared from
3,6-di-tert-butyl-o−benzoquinone (0.11 g, 0.5 mmol)
as it is described below) was added to the suspension
containing (DME)NiCl2 (0.11 g, 0.5 mmol) and 2,2ꢁ-
bipyridine (0.078 g, 0.5 mmol) in THF (50 mL). The
mixture was warmed (∼80◦C) and shaken during one
hour, filtered and concentrated. After the n-hexane
addition, dark microcrystalline solid was formed. It
was filtered, washed with light petroleum and dried
under vacuum. Yield 30%.
2.2a (1,4-di-tert-butyl-1,4-diaza-1,3-butadiene)(3,6-
di-tert-butyl-catecholato)nickel (1): Green solid. Slowly
decomposes in air. Yield 45%. Anal. (%) Found: C
64.80; H 9.02; Ni 13.07. C24H40N2O2Ni Calc.: C 64.43;
H 8.95; Ni 13.20. IR (Nujol), ν, cm−1: 1600 w, 1555,
1540, 1540, 1410 s, 1400 s, 1370 s, 1325 w, 1280 w,
1265 s, 1250 s, 1210 s, 1150 w, 1100 w, 1030 w, 985 s,
960 w, 945 m, 875 m, 830 w, 795, 785 m, 700 s, 655 m,
615 m, 545 w, 520 w.
2.2g (1,2-bis(diphenylphosphino)ethane)(3,6-di-tert-
butyl-catecholato)nickel∗ toluene (6): [1,2]-bis(diphe-
2.2b (1,4-bis-(2,6-di-iso-propylphenyl)-1,4-diaza-1,3- nylphosphino)ethane nickel(II) chloride was obtained
butadiene)(3,6-di-tert-butyl-catecholato)nickel(2): Dark analogously to known procedure.15 The mixture of
green airstable crystals. Yield 65%. Anal.(%) Found: C commercial NiCl2·2H2O (13.2 g, 0.08 mol) in 40 mL

Square planar catecholato complexes of cobalt and nickel
529
H2O with acetic acid (140 mL) was added to warm 1580 w, 1400 s, 1300 s, 1290 s, 1270 s, 1230 s, 1210 s,
solution of 1,2-bis(diphenylphosphino)ethane (31.8 g, 1160 m, 1030 w, 990 s, 975 s, 960 s, 925 m, 890 m,
0.08 mol) in acetic acid (570 mL) with stirring. Reac- 810 m,790 s,705 m, 680 m, 660 s, 560 w, 540 w, 505 w,
tion mixture was allowed to stay for ten hours. Pink- 500 w, 455 w.
brown crystals were filtered, washed with cold acetic
acid and dried in vacuum. Yield: 40.1 g (95%).
2.3c (1,4-bis-(2,6-di-iso-propylphenyl)-1,4-diaza-1,3-
Suspension of di-thallium-3,6-di-tert-butyl-catecholate
butadiene)(3,6-di-tert-butyl-catecholato)cobalt (8): Dark
prepared from o−quinone (0.044 g, 0.2 mmol) as it
green crystals. Slowly decomposes in air. Yield
is described below in THF (10 mL), was added to
35%. Anal.(%) Found: C 73.16; H 8.78; Co 8.64.
[1,2-bis(diphenylphosphino)ethane]nickel(II) chloride
C40H56N2O2Co Calc.: C 73.29; H 8.55; Co 8.99. IR
(0.1 g, 0.2 mmol) in THF (40 mL) in evacuated
(Nujol, cm−1):1480,1450 s, 1375, 1340 s, 1325, 1290 s,
ampoule. Reaction mixture was refluxed for one hour.
1210, 1185, 1070, 980, 825, 800, 760 m, 710 m, 650,
Solvent was changed to the mixture toluene/CH2Cl2
625 m.
(∼10/1, 50 mL), the mixture was filtered; solution vol-
ume was reduced to one half. Dark green solid, formed
after cooling, was filtered, washed with light petroleum
and dried in vacuum. Yield: 0.068 g (53%). According
to element analysis it contains toluene molecule as sol-
vate. Anal.(%) Found: C 73.72; H 6.44; Ni 7.62; P 8.04.
C47H52P2O2Ni Calc.: C 73.37; H 6.76; Ni 7.64; P 8.07.
IR (Nujol), ν, cm−1: 1595, 1485, 1440 s, 1410, 1275,
1255, 1190, 1110 s, 1035, 980, 940, 880, 830, 795, 745,
725 s, 710 s, 690 s, 660, 540, 480, 450.
2.3d (2,3-dimethyl-1,4-bis-(2,6-di-iso-propylphenyl)-
1,4-diaza-1,3-butadiene)(3,6-di-tert-butyl-catecholato)
cobalt (9): Dark blue crystals. Slowly decomposes
in air. Yield 30%. IR (Nujol, cm−1): 1500 w, 1480 w,
1405 m, 1360 m, 1330 m, 1260 m, 1240 m, 1215, 980 s,
950 m, 890, 840, 790 m, 740 m, 720, 660, 625 m.
2.3e (1,4-bis-(2,6-dimethylphenyl)-1,4-diaza-1,3-buta-
diene)(3,6-di-tert-butyl-catecholato)cobalt (10): Dark
2.3 Synthesis and characterization of diazabutadiene- blue air sensitive crystals. Yield 61%. Anal.(%) Found:
catecholato cobalt complexes
C 71.00; H 7.37; Co 10.97. C32H40N2O2Co Calc.: C
70.73; H 7.42; Co 10.80. IR (Nujol), ν, cm−1: 1675,
1630, 1605, 1555, 1460, 1360, 1345, 1325, 1300, 1275,
1260, 1210, 1200.
2.3a General procedure: Evacuated ampoule con-
taining 1 mmol of 3,6-di-tert-butyl-o−quinone in THF
and excess of thallium amalgam (at least 10-fold excess
of Tl) was shaken until the suspension colour became
bright yellow. The light suspension of di-thallium cat-
echolate was carefully decanted to another evacuated
ampoule. Thallium amalgam was carefully washed sev-
eral times by THF until the solution became colour-
less. (Attention! Di-thallium catecholate is extremely
air sensitive.) United solutions of di-thallium cate-
cholate were added to the mixture of correspond-
ing diazabutadiene (α, αꢁ-bpy, dppe) (1 mmol) and dry
CoCl2 (1 mmol) in THF. Reaction mixture was refluxed
during one hour. Solvent was changed to toluene (or
light petroleum). Solution was filtered, partly evapo-
rated and allowed to stay at −10◦C for a night. Result-
ing crystalline solid was filtered, washed by cold light
petroleum and dried under vacuum. Yields and proper-
ties are listed below.
2.3f (2,2ꢁ-dipyridine)(3,6-di-tert-butyl-catecholato)
cobalt (11): Recrystallized from DME. Brown
airstable crystals. Yield 35%. Anal.(%) Found: C 66.37;
H 6.55; Co 13.66. C24H28CoN2O2 Calc.: C 66.20; H
6.48; Co 13.53. IR (Nujol, cm−1): 1605 m, 1585 w,
1575 w,1405 s, 1365 w, 1285 m, 1270 m, 1260, 1225 m,
1205 m, 1160, 1145, 1025, 975 m, 940 m, 925 w, 780 w,
775 m, 765 m, 740 m, 700 w, 680 m, 660 m.
2.3g (Bis(diphenylphosphino)ethane)(3,6-di-tert-butyl-
catecholato)cobalt: Preparation procedure is des-
cribed in.16 IR (Nujol, cm−1):1595 w, 1440 v s,1395 v s,
1285 w, 1260 s,1250 s,1190 m,1150 w,1105 m,1030 w,
980 m, 940 w, 880 s, 810 w, 750 w, 735 v s, 695 v s,
680, 650 m, 525, 515 m, 495, 480 w.
2.3b (1,4-di-tert-butyl-1,4-diaza-1,3-butadiene)(3,6-
di-tert-butyl-catecholato)cobalt (7): Blue-grey crys-
2.4 Substitution reactions, general procedure
tals. Slowly decomposes in air. Yield 33%. Anal.(%) All the reactions of substitution of diazabutadiene lig-
Found: C 64.70; H 9.22; Co 13.27. C24H40N2O2Co ands by dppe and 2,2ꢁ-bpy were carried out in the same
Calc.: C 64.41; H 9.01; Co 13.17. IR (Nujol, cm−1): manner. Toluene solutions of ligand and corresponding

530
Michael P Bubnov et al.
Table 1. Crystallographic data and structure refinement details for 1 and 9.
1
9
Chemical formula
Formula weight
C24H40N2NiO2
445.88
C45H67CoN2O2
726.94
T /K
Crystal system
Space group
293
Monoclinic
C2/c
100
Monoclinic
C2/c
a/Å
b/Å
32.220(6)
23.844(5)
22.496(5)
133.81(3)
12472(4)
20
33.127(2)
12.6267(9)
25.1845(17)
124.953(1)
8634(1)
8
c/Å
β/◦
V/ Å3
Z
Dc/Mg m−3
1.189
0.797
1.118
0.433
μ/mm−1
F(000)
4832
1.22–25.05
11098
10891
0.0508
3152
1.81–26.07
35915
8503
0.0685
θ Range for collection/◦
No. of reflns. colld.
No. of indep. reflns.
R(int)
R[I > 2σ(I)]
wR (all data)
GOF on F2
0.0359
0.1084
0.940
0.061/−0.407
0.0509
0.1377
1.008
0.713 /−0.444
Largest diff. peak and hole, eÅ−3
complex were mixed in evacuated ampoule and slightly anisotropically. All hydrogen atoms in 1 and 9 were
warmed (∼60◦C). Hexane was slowly added until pre- included in idealized positions and their Uiso values
cipitation began. Ampoule was allowed to stay during were set to ride on the Ueq values of the parent carbon
the time necessary to observe the end of precipitation. atoms (Uiso(H)= 1.5Ueq for methyl carbons and 1.2Ueq
Solid powder was washed by warm hexane and dried for other carbons). Crystallographic data and structure
under vacuum. Products were identified by full coinci- refinement details are given in table 1. Asymmetric unit
dence of their IR spectra with corresponding IR spectra of 1 contains two independent molecules in common
of the same complexes obtained independently by the position and one molecule on C2 axes. Asymmetric unit
other way.
of 9 contains one disordered hexane molecule on C2
axes.
2.5 X-ray crystallography
3. Results and Discussion
Diffraction data were collected on a Siemens P3/PC
and Bruker-AXS Smart Apex I diffractometers with
graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) for 1 and 9 complexes, respectively. All
structures were solved by direct methods and refined
against F2 on all data by full-matrix least squares with
SHELXTL.17 Absorption correction was applied using
SADABS.18 All non-hydrogen atoms were refined
3.1 Synthesis and characterization of diazabutadiene-
catecholato complexes of nickel and cobalt
Nickel compounds were synthesized by one-pot reac-
tion of nickel tetracarbonyl with corresponding diaza-
butadiene or 2,2ꢁ-dipyridine and o−quinone (scheme2).
The exception is catecholato complex with dppe (6)
which was obtained by exchange reaction of nickel(II)
chloride with dppe (scheme 3).
Scheme 3. Synthesis of catecholato nickel complex with
1,2-bis(diphenylphosphino)ethane.
Scheme 2. Synthesis of diimine nickel catecholates.

Square planar catecholato complexes of cobalt and nickel
531
Scheme 4. Synthesis of diimine cobalt catecholates.
Catecholato complexes of cobalt were obtained by
exchange reaction of cobalt(II) chloride with thal-
lium catecholate in the presence of diazabutadiene
(scheme 4).
IR spectra of all complexes are typical for cate-
cholato metal complexes and in general represent the
superposition of IR-spectra of both organic fragments:
coordinated neutral DAB- ligand and catecholato dian-
ion (see experimental section).
Complex 1 was structurally characterized. Unit cell
contains three independent molecules, in which geome- Figure 2. General view of complex 9 molecule. (50%
of ellipsoids probabilities, hydrogen atoms are omitted for
clarity).
tries are very close. Each molecule has slightly distorted
square-planar geometry. Nickel atom is surrounded by
two oxygen atoms of catechol and two nitrogen atoms
Bond lengths of C-O and C1-C2 in chelate cycle
of catecholato fragment characterize it valent state. It
is known that these distances for anion-radical are in
the range: C-O: 1.27–1.31 Å and C1-C2: 1.32–1.39 Å;
for catecholato dianion: 1.31–1.35 Å and 1.36–1.42 Å
respectively.19 In the case of 1 bond lengths (C-O:
1.352–1.357 Å; C1-C2: 1.397–1.410 Å) are typical for
catecholato dianion coordination mode.
Bond length of C=N of diazabutadiene in 1
(1.260–1.275 Å) which are sensitive to ligand oxida-
tion state are close to that, found for corresponding
distances in coordinated neutral ligand in (tert-Bu)2
(DAB)(H,H)NiBr2: 1.275 Å.20
of diazabutadiene. Each ligand’s fragment itself is close
to planar. The planes of the ligands are twisted by an
angle varying from 5.37 to 10.93◦. The location of
the axis of twist is distinct in each molecule. In one
of the molecules it coincides with the molecular two-
fold axis which is situated in the plane of complex.
Naturally, in this molecule the distribution of bonds’
lengths and angles are symmetrical, relative to this axis.
General view of the complex molecule is presented in
figure 1. Bond lengths for 1 and analogous distances
for complexes described in literature are listed in the
table 2.
Bond lengths of C3-C4 and Ni-N in catecholate
complex 1 are slightly shorter than the same distances
in corresponding dibromide: (tert-Bu)2(DAB)(H,H)
NiBr2: C-C 1.470 Å, Ni-N 1.996, 2.002 Å; and in free
diazabutadiene: C-C 1.496 Å. It can be the conse-
quence of the difference in the structures of com-
plexes. (DAB)NiBr2 has distorted tetrahedral geometry.
Square-planar configuration of 1 promotes π-π-inter-
action through the molecule skeleton. In the case of
Ni(3,6-DBSQ)2, which molecule is square-planar, π-π-
interaction results in strong anti-ferromagnetic coupling
of unpaired electrons localized in ligands π-systems.21
Complex 9 was also structurally characterized.
General view of complex molecule is presented in the
figure 2. Selected bonds lengths are listed in the table 3.
Unit cell of 9 contains the guest molecule of n-hexane.
Like 1, complex 9 has square-planar coordination
core. The geometry of each ligand’s fragment is very
Figure 1. General view of complex 1 molecule. (30% of
ellipsoids probabilities, hydrogen atoms are omitted for clar-
ity, atoms numeration is arbitrary and corresponds to numer-
ation in table 1).

532
Michael P Bubnov et al.
Table 2. Bond length intervals for complex 1 molecule (atoms numeration is arbitrary and corresponds to numeration in
figure 1) and analogous distances for complexes described in literature.
Bond
Complex 1 range (Å)
(Me2Ph)DAB(Me2)Ni(Cat-Cl4)7 (Å)
(i-Pr2Ph)DAB(Me2)Ni(Cat-Cl4)7 (Å)
C-O
1.352–1.357
1.260–1.275
1.823–1.829
1.938–1.959
1.397–1.410
1.436–1.448
1.330/1.333
1.299/1.302
1.824/1.837
1.860/1.870
1.408
1.332/1.329
1.299/1.297
1.830/1.828
1.860/1.869
1.412
C=N
Ni-O
Ni-N
C1-C2
C3-C4
1.480
1.481
close to planar. Planes of ligands are slightly twisted. 3.2 Some reactions of diazabutadiene-catecholato
The angle between planes is 6.83◦. Bond lengths complexes of nickel and cobalt
in chelate ring of catecholato fragment are typical
Neutral ligands in diazabutadiene-catecholato com-
plexes of cobalt and nickel undergo substitution
reactions. Resulting compounds were isolated and
characterized. The same products were obtained inde-
pendently by exchange reactions. The identity of the
complexes synthesized by different ways was con-
firmed by identity of their IR spectra.
1,2-Bis(diphenylphosphino)ethane (dppe) can substi-
tute any DAB ligand in nickel catecholates 1-4 but only
1,4-di-(tert-Bu)-diazabutadiene in cobalt complex 7
(schemes 5, 6). Di-aryl-derived diazabutadiens in cobalt
compounds are not being substituted. Resulting
(dppe)MCat compounds were synthesized indepen-
dently by exchange reaction with catecholato thallium.
Diazabutadiene ligands undergo substitution reaction
also upon action of stronger base 2,2ꢁ-bpy on both
cobalt and nickel catecholates (scheme 7).
Complexes 1–6 undergo one-electron chemical oxi-
dation by silver cation forming stable paramagnetic
adducts which were studied by EPR in solution
(scheme 8, figure 3). EPR spectral parameters are listed
in table 5.
g-Factors and values of hyperfine coupling con-
stants on protons of semiquinonato ligand indicate that
unpaired electron is localized in π-system of semi-
quinonato fragment.34 It means that nickel oxidative
state remains unchanged but catecholato ligand under-
goes oxidation. Small values of hyperfine coupling
constants on 31P of dppe and the absence of hyperfine
for catecholato-dianion coordination mode. Square-
planar geometry is unusual for Co(II) compounds in
such type of environment. Most of tetracoordinative
cobalt(II) compounds with separated chelate ligands
containing N- and O-atoms have distorted tetrahedron
geometry. The exceptions are: bis-iminosemiquinonato
complexes,22,23 catecholato cobalt with amino-pyri-
dine,11,12 and a number of complexes, where salicy-
laldiminato ligands are connected by a bridge. Table 4
presents a number of Co-N and C=N bond lengths for
close analogs - bis(salicylaldiminato)cobalt complexes
of different symmetry. (Bonds lengths are cited from
reference.24) In distorted tetrahedron geometry Co-N
and C=N bonds’ lengths are in the ranges: 1.951–2.034
and 1.279–1.299 Å, whereas in distorted square-planar
one they are: 1.849–1.883 and 1.284–1.309 Å respec-
tively. It is evident that in tetrahedron, Co-N bonds are
longer and C=N bonds are shorter than the same dis-
tances in square-planar coordination core. Square-planar
geometry seems to be stabilized by π- back donation
from filled dπ -orbitals of cobalt to empty anti-bonding
π-orbitals of N-donor. As a consequence, it results in
shortening of Co-N bond lengths and elongation of
C=N distances. Bond lengths in 9 confirm this rule.
So, according to the presented data, diazabutadiene
complexes of nickel and cobalt are catecholato deriva-
tives of divalent metal ions with neutral diazabutadiens.
Additional π-π bonding of diazabutadienes with metals
is observed for both compounds 1 and 9.
Table 3. Selected bond lengths and angles of complex 9.
Bond
Distance (Å)
Bond
Distance (Å)
Atoms
Angles (◦)
Co-N1
Co-N2
N1-C16
N2-C17
C16-C17
1.8743(16)
1.8711(16)
1.323(2)
1.323(2)
1.439(3)
Co-O1
Co-O2
O1-C1
O2-C2
C1-C2
1.8066(13)
1.8104(13)
1.364(2)
1.359(2)
1.406(3)
O1-Co-N1
O2-Co-N2
C1-O1-Co-N2
C2-O2-Co-N1
O1-Co-N2-C17
O2-Co-N1-C16
173.66
176.91
178.92
175.14
172.72
177.64

Square planar catecholato complexes of cobalt and nickel
533
Table 4. Selected bond lengths (Å) for several bis(salicylaldiminato)cobalt and 9.
Complex geometry, title and reference
Ligand 1
Ligand 2
Co-N
C=N
Co-N
C=N
Distorted tetrahedron
bis(salicylal-N-m-anisidinato)-cobalt(II)25
bis(N-(2,6-dimethylphenyl)-
1.951
2.002
1.279
1.288
1.965
2.034
1.299
1.279
salicylaldiminato-N,O)-cobalt(II)26
bis(3-tert-butyl-N,5-dimethylsalicyl-
aldiminato)-cobalt(II)27
1.975
1.871
1.286
1.293
1.966
1.872
1.288
1.296
Distorted square-planar
(R,R)-(N,Nꢁ-bis(2ꢁ-hydroxybenzyli-
dene)-1,2-diaminocyclohexane-
N,Nꢁ,O,Oꢁ)-cobalt(II)28
(N,Nꢁ-(o-phenylene)-
1.873
1.849
1.860
1.308
1.289
1.293
1.870
1.849
1.860
1.300
1.289
1.293
bis(salicylideneaminato))-cobalt(II)29
N,Nꢁ-ethylene-bis(3-tert-
butylsalicylideneiminato)-cobalt(II)30
(N,Nꢁ-bis(3-tert-butyl-5-
methylsalicylidene)-2,3-diamino-2,3-
dimethylbutane)-cobalt(II)31
meso-(N,Nꢁ-cyclohexylene-
1.881
1.860
1.305
1.309
1.883
1.860
1.284
1.305
bis(salicylideneiminato)-cobalt(II))32
N,Nꢁ-bis(3,5-di-tert--butylsalicylidene-
1,2-phenylenediamine)-cobalt(II)33
Complex 9 (this work)
1.873/1.859
1.312/1.331
coupling with 14N of DAB indicate that square-
planar structure remains unchanged upon oxidation.34,35
EPR spectral parameters of cations do not depend
on temperature and nature of diazabutadiene. It means
that no redox-isomeric transformations occur. It should
be mentioned that EPR spectral parameters of [(dppe)
Ni(3,6-DBSQ)]+ cation coincide with those obtained
by electrochemical oxidation of the same complex36
and are similar to that obtained for analogous cate-
cholates.10
Scheme 5. Substitution of DAB by dppe in catecholato
nickel compounds and independent synthetic way to
(dppe)Ni(3,6-DBCat).
Anisotropic EPR spectrum with axial symmetry of
g-tensor was recorded for powder of 9 at T = 140 K.
Hyperfine coupling with 59Co nucleus was not detected.
Scheme 6. Substitution of t-Bu-DAB by dppe in cate-
cholato cobalt compound and independent synthetic way to
(dppe)Co(3,6-DBCat).
∼
=
∼
=
g-Tensor parameters: g
1.90, g
3.36 indicate
⊥
||
Co(II), d7 ground state to be low spin. No EPR
Scheme 7. Substitution of DAB by 2,2ꢁbpy in nickel and
cobalt catecholates and independent way to (2,2ꢁbpy)M(3,6-
DBCat), M=Ni, Co.
Scheme 8. Oxidation of catecholato nickel complexes into Figure 3. EPR spectra of cations [6]+ (left) and [1]+
corresponding semi quinonato cations. (right) (CH2Cl2, 290 K).

534
Michael P Bubnov et al.
Table 5. EPR spectral parameters of nickel semiquinonato
cations.
Supplementary Information
CCDC-678237 (1) and CCDC- 678238 (9) contain
the supplementary crystallographic data, which can
be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; Fax: (internat.) + 44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk. Check CIF files are avaialble
at www.ias.ac.in/chemsci
Complex
g-factor
ai(1H)SQ, G
ai(31P), G
1+
2+
3+
4+
5+
6+
2.0136
2.0041
2.0048
2.0034
2.0058
2.0041
3.55
3.60
3.40
3.65
3.40
3.68
–
–
–
–
–
2.52
Acknowledgements
The authors are thankful to L. Abakumova, N.
Khamaletdinova and O. Kuznetcova for IR spectra
recording. This work was supported by RFBR (grants:
13-03-97070), by Russian President Grant supporting
scientific schools (NSh-271.2014.3), by Program of
presidium of RAS N◦ 18. This research was partly
supported by the grant (the agreement of August 27,
2013 N◦ 02.B.49.21.0003 between The Ministry of
education and science of the Russian Federation and
Lobachevsky State University of Nizhni Novgorod).
Scheme 9. Oxidative addition of o-quinone to nickel cat-
echolate leads to bis-o-semiquinonato nickel compound.
spectra for cobalt(II) catecholate complexes in solution
were observed in the temperature range of 77–300 K.
THF solution of (dppe)Co(3,6-DBCat) when
exposed to air quickly changes its colour from brown
to blue, which is typical for semiquinonate anion rad-
ical formation. n-Hexane addition causes colourless
solid precipitation. IR spectrum of precipitate contains
a very intensive band at ν = 1185 sm−1, which can be
attributed to νP = O.
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