The reduction of sterically hindered o-Quinone and o-Iminoquinone with gallium and gaI

Article abstract of DOI:10.1002/ejic.201200535

Metallic gallium reacts with 3,6-di-tert-butyl-o-benzoquinone (3,6-Q) in THF to produce a gallium tris-o-semiquinolate complex, whereas the same interaction with 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzoquinone (imQ) does not proceed. The reduction of imQ is possible with amalgamed gallium in THF and produces a gallium complex that contains two o-iminobenzoquinone ligands in different redox states. In the case of 3,6-Q, the analogous reaction leads to dimeric derivative [Cat₂Ga(Et₂O) ₂][CatGa] (in which Cat is the dianion of 3,6-Q). The structure of this compound was determined by X-ray diffraction analysis.

Full text of DOI:10.1002/ejic.201200535

Job/Unit: I20535
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Date: 07-08-12 17:47:42
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FULL PAPER
DOI: 10.1002/ejic.201200535
The Reduction of Sterically Hindered o-Quinone and o-Iminoquinone with
Gallium and “GaI”
Alexandr V. Piskunov,*[a] Arina V. Maleeva,[a] Irina N. Mescheryakova,[a] and
Georgii K. Fukin[a]
Dedicated to Professor Gleb A. Abakumov on the occasion of his 75th birthday
Keywords: Gallium / Quinones / Reduction / EPR spectroscopy
Metallic gallium reacts with 3,6-di-tert-butyl-o-benzo-
quinone (3,6-Q) in THF to produce a gallium tris-o-semi-
quinolate complex, whereas the same interaction with 4,6-
di-tert-butyl-N-(2,6-diisopropylphenyl)-o-iminobenzo-
quinone (imQ) does not proceed. The reduction of imQ is
possible with amalgamed gallium in THF and produces a
gallium complex that contains two o-iminobenzoquinone li-
[CatGa] (in which Cat is the dianion of 3,6-Q). The structure
of this compound was determined by X-ray diffraction analy-
sis. The gallium(III) amidophenolate derivative can only be
obtained by exchange reaction of APNa2 with GaI3 (in which
AP is the dianion of imQ). The reduction of 3,6-Q and imQ
with “GaI” results in the formation of corresponding com-
pounds that contain redox-active ligands in the dianionic
gands in different redox states. In the case of 3,6-Q, the anal- state. The structure of [AP2Ga][GaI2] was determined by X-
ogous reaction leads to dimeric derivative [Cat2Ga(Et2O)2]-
ray diffraction analysis.
Introduction
The utility of transition and noble metals in catalytic
processes derives from their ability to be involved in the
electron-transfer reactions. In principle, such reactivity can
be simulated for non-transition-metal complexes with re-
dox-active ligands (o-quinones, o-iminoquinones, α-di-
imines).[1] These derivatives are able to participate in oxidat-
ive addition[2] or reductive elimination[3] reactions in which
the electron transfer is realized not on account of the metal
ion but the redox-active ligand. Therefore such compounds
exhibit various interesting reactivity towards different sub-
strates. Recently the facile addition of alkynes to the gal-
lium complex that contains the 1,2-bis[(2,6-diisopropyl-
phenyl)imino]acenaphthene (BIAN) ligand and an unusual
Scheme 1. The reaction of digallane with alkynes.
Group III metal complexes (including gallium deriva-
thermally induced elimination of this alkyne accompanied tives) based on quinone ligands were obtained as a result
by C–C and C–Ga bond cleavage were discovered of the reaction of different quinones (Q) with metal halides
(Scheme 1).[4a] This interaction forms the basis for involving for the first time.[5] The Q was found to be able to oxidize
(BIAN)Ga–Ga(BIAN) in the catalytic hydroamination re- the halide anion with the subsequent formation of para-
action.[4b]
magnetic metal complexes of the type SQMHal2 (SQ is the
radical anion of Q). They were characterized using EPR
spectroscopy. Gallium compounds of this type can be also
obtained by the direct addition of iodine to the reaction
mixture of metal and quinone or by the exchange interac-
tion of an equimolar quantity of alkali-metal quinoid salt
and GaI3.[6] Moreover, unstable paramagnetic mono-o-se-
miquinolate gallium complexes form as a result of the reac-
tion of o-quinones with GaEt3.[7]
[a] G.A. Razuvaev Institute of Organometallic Chemistry, Russian
Academy of Sciences,
49 Tropinina Street, 603950 Nizhny Novgorod, Russian
Federation
Fax: +7-831-4627497
E-mail: pial@iomc.ras.ru
Homepage: www.iomc.ras.ru
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A. V. Piskunov, A. V. Maleeva, I. N. Mescheryakova, G. K. Fukin
FULL PAPER
One of the most convenient methods of synthesis of their structure was not proven unambiguously. The present
metal complexes with redox-active ligands is the direct re- work is devoted to the deeper investigation of gallium com-
duction of o-quinones and their hetero-analogues by metals plexes based on o-benzoquinone and o-iminobenzoquinone
or metal amalgams.[8] (BIAN)Ga–Ga(BIAN)[9a] and the ligands.
tris-o-semiquinolate gallium(III) derivatives that contain
different o-quinones were synthesized by this meth-
od.[6,9b–9d] In some cases, the direct interaction of quinones
with gallium leads to the formation of the respective dian-
ionic catecholate derivatives. As reported in the literature,[6]
heating of the gallium metal with 1,2-naphthoquinone
(NQ) in refluxing toluene results in a deeper reduction of
NQ and gives the dimeric catecholate complex [GaII-
(NCat)]2 (NCat is the dianion of NQ). Catecholate gallium
derivatives [(Hal4Cat)GaII·(phen)]2 (Hal4Cat are dianions
of tetrahalo-substituted o-quinones Hal4Q; phen = 1,10-
phenanthroline) form as the result of the reaction of metal
with quinones in the presence of phen.[10a] Unfortunately,
the authors[6,10a] did not produce sufficient evidence of such
structures, and the last ones are questionable. Moreover, the
minor product obtained in the reaction of Cl4Q with Ga in
the presence of phen is the structurally characterized mono-
meric complex (Cl4Cat)GaCl·py·phen (py = pyridine).[10b]
It indicates the more complicated nature of the interaction
of metallic gallium with Hal4Q, which includes elimination
of halides from the initial o-quinone.
Results and Discussion
The direct interaction of o-quinones and o-imino-
quinones with metals or amalgamated metals was shown to
be a convenient method for the synthesis of their corre-
sponding derivatives.[8] Tris(3,6-di-tert-butyl-o-benzose-
miquinolato)gallium(III) (1) was generated in solution over
the course of the reaction between gallium and 3,6-Q for
the first time.[9b] Later this compound was obtained by a
more time-consuming exchange interaction.[13] We have
found that the reaction of 3,6-Q with an excess amount of
gallium in THF results in the formation of 1 with a quanti-
tative yield (Scheme 2, path a).
Low-oxidation-state metal derivatives are widely known
to reduce 1,2-diketones and related ligands to give chelated
compounds with reduced forms of the redox-active ligands.
Thus the interaction of benzil with “GaI” results in the for-
mation of a novel trimetallic bis(enediolato) complex.[11a]
“GaI” was shown to be a convenient initial reagent for the
synthesis of α-diimine gallium derivatives.[11b–11e] The re-
duction of N-substituted diazabutadienes (DAD) or the
BIAN ligand by “GaI” leads to compounds that contain
the redox-active ligand in radical-anion form. They are of
the type I2Ga(DAD)–· or [IGa(DAD)–·]2 depending on the
Scheme 2. The synthesis of gallium(III) derivatives based on the
3,6-Q ligand.
The use of amalgamated gallium in the reaction with 3,6-
nature of the N-substituents.[11b–11e] The similar reaction of Q allows one to carry out the process in a hydrocarbon
“GaI” with iminopyridine results in the C–C coupling of medium and perform the deeper reduction of the initial o-
the mono-reduced organic ligand and leads to the diamag- quinone. The gallium(III) catecholate complex was ob-
netic dinuclear gallium compound.[11d] A recent investiga- tained in hexane as a result (Scheme 2, path b). Removal of
tion shows that the “GaI” is able to reduce aryl-substituted hexane and treatment of the solid residue with diethyl ether
bis(imino)pyridine to produce paramagnetic [2,6- results in the formation of crystalline product 2. The last
1
(DippN=CPh)2(NC5H3)]GaI2 derivative.[11f]
was characterized by elemental analysis, IR and H NMR
The reactivity of gallium or “GaI” towards o-imino- spectroscopy, and X-ray diffraction analysis.
1
quinones currently has not been investigated. All known o-
According to H NMR spectroscopy data (Figure 1, A),
iminoquinonato gallium complexes are tris-o-iminobenzo- the ratio between catecholate ligands and diethyl ether mo-
semiquinolates, which were obtained in the reaction of 2- lecules coordinated to the metal center in 2 is 3:2. More-
(3,5-di-R-anilino)-4,6-di-tert-butylphenol (R = tBu, CF3, over, a considerable asymmetrical assignment of electron
Cl; 3 equiv.) with GaCl3 (1 equiv.) in the presence of Et3N density in catecholate ligands was observed. The signals at
and air oxygen.[12a] The similar gallium complex with R = δ = 1.31 and 1.47 ppm correspond to the protons of in-
OCH3 undergoes intramolecular coupling of o-iminosemi- equivalent tert-butyl groups. The doublets at δ = 6.86 and
quinones with the formation of the new paramagnetic hexa- 6.79 ppm are attributed to the inequivalent protons of the
dentate ligand.[12b]
aromatic ring. The value of the spin–spin interaction con-
Despite the variety of knowledge about gallium com- stant (JH,H = 8.5 Hz) is typical for derivatives of benzenes
plexes that contain different redox-active ligands that is that contain ortho-protons. The analysis of the integral in-
1
available to date, the information presented above seems tensity of signals in the H NMR spectrum indicates that
slightly discrepant and incomplete. A number of o-quinol- complex 2 has the following composition: (3,6-Cat)3Ga2·
ate compounds were not isolated in the individual state, or 2Et2O. It is in good agreement with cryoscopic (C6H6) mo-
2
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o-Quinone and o-Iminoquinone with Gallium and “GaI”
lecular-weight measurements (956 gmol–1 versus calcd.
The structure of complex 3 was determined by X-ray dif-
948.6 gmol–1) for complex 2. Given the above, it is possible fraction analysis (Figure 2). The molecule of 3 consists of
to suppose the structure of complex 2 in which the environ- two gallium atoms with different coordination geometry.
ment of the first gallium atom is formed by three cate- Ga(1) has a distorted tetrahedral environment formed by
cholate ligands, whereas the second metal atom is coordi- oxygen atoms O(1) and O(2) of one catecholate ligand and
nated to the three oxygen atoms of diolate ligands and to O(3) and O(6) of another two diolate ligands. The distorted
two diethyl ether molecules (Scheme 3).
octahedral coordination sphere of the Ga(2) atom includes
the O(3), O(4), O(5), O(6), O(7), and O(8) atoms of two
catecholate ligands and two diethyl ether molecules. The
O(3), O(6), O(7), and O(8) atoms form the equatorial plane,
whereas O(4) and O(5) atoms occupy apical positions. The
values of the C–O and C–C bond lengths of o-quinonate
fragments indicate the catecholate nature of the redox-
active ligands. It is necessary to note that the O(3)–C(15)
and O(6)–C(34) bonds [1.414(2) and 1.406(2) Å, respec-
tively] are longer than the O(1)–C(1), O(2)–C(2), O(4)–
C(16), and O(5)–C(29) (1.36–1.38 Å) bonds. The values of
the Ga(1)–O(3,6) and Ga(2)–O(3,6) distances [1.877(11)–
1.9067(9) and 2.0655(11) Å, respectively] are appreciably
more than the values of Ga(1)–O(1,2) [1.8105(11)–
1.8146(11) Å] and Ga(2)–O(4,5) [1.8667(11)–1.8771(11) Å]
bond lengths. These facts are a result of bridge-type
Ga(1,2)–O(3,6) bonds. The interatomic Ga(1)···Ga(2) dis-
tance is 2.8813(2) Å and exceeds the sum of covalent radii
for these atoms (2.59 Å).[14] The dihedral angles between
the central Ga(1)O(3)Ga(2)O(6) metallocycle and cate-
cholate ligands are 77.6–89.4°. It minimizes the nonbonding
interactions in 3. Selected bond lengths and angles in com-
plex 3 are given in Table 1. The crystal data collection and
structure refinement data are listed in Table 3.
Figure 1. The 1H NMR spectra of gallium(III) catecholate com-
plexes (A) 2 and (B) 3 in C6D6. The asterisk (*) indicates the resid-
ual solvent peak.
Scheme 3. The expected structure of complex 2.
The 1H NMR spectrum of gallium catecholate derivative
2 changes over time. The gradual disappearance of spec-
trum A and the appearance of spectrum B is observed in
C6D6 solution (Figure 1). This fact indicates the transfor-
mation of complex 2 in solution. Actually, the long-contin-
ued recrystallization of 2 from a mixture of Et2O and hex-
ane results in the formation of crystalline product 3. The
1H NMR spectrum of 3 coincides completely with the spec-
trum of compound 2 after its long-term storage in C6D6
solution (Figure 1, B).
The elemental analysis and cryoscopic (C6H6) molecular-
weight determination data for complexes 2 and 3 indicates
1
that these compounds have the same composition. The H
NMR spectrum of 3 (Figure 1, B) contains a singlet signal
of tert-butyl group protons and a singlet signal of aromatic
ring protons. All signals are considerably broadened. This
fact indicates that there is a dynamics of coordination
Figure 2. The molecular structure of complex 3 with 30% thermal
probability ellipsoids. Hydrogen atoms are omitted for clarity.
In contrast to 3,6-Q, the interaction of metallic gallium
sphere in solution of complex 3, and protons of catecholate with 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-imino-
ligands become practically equivalent as the result. It is nec- benzoquinone (imQ) does not proceed. The use of amalga-
essary to note that the transformation of 2 into 3 is irrevers- mated gallium in the reaction with imQ in THF allows one
ible. The repeated recrystallization of 3 from diethyl ether to obtain only the mixed-ligand derivative of GaIII (4) in
does not change the spectroscopic data of the resulting which the first imQ ligand has a radical-anionic and the
compound.
second one a dianionic nature (Scheme 4, path a). The pres-
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FULL PAPER
Table 1. Selected bond lengths [Å] and angles [°] of complex 3.
ence of two differently charged o-iminoquinonate ligands in
complex 4 is clear from analysis of its EPR spectrum. The
hyperfine structure of the EPR spectrum of 4 in THF arises
from hyperfine coupling (HFC) of unpaired electron with
magnetic nuclei 69Ga (60.1%, I = 3/2, μN = 2.01659),[15]
71Ga (30.9%, I = 3/2, μN = 2.56227),[15] two equivalent pro-
Bond
[Å]
Angle
[°]
C(1)–O(1)
C(6)–O(2)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(1)
Ga(1)–O(1)
Ga(1)–O(2)
Ga(1)–O(3)
Ga(1)–O(6)
Ga(2)–O(3)
Ga(2)–O(4)
Ga(2)–O(5)
Ga(2)–O(6)
Ga(2)–O(7)
Ga(2)–O(8)
O(3)–C(15)
O(4)–C(20)
O(5)–C(29)
O(6)–C(34)
Ga(1)···Ga(2)
1.3748(19)
1.3850(18)
1.402(2)
1.389(3)
1.397(3)
1.395(2)
1.391(2)
1.412(2)
1.8105(11)
1.8146(11)
1.8747(11)
1.9067(9)
2.0655(9)
1.8667(11)
1.8771(11)
2.0655(11)
2.0391(11)
2.0448(9)
1.414(2)
O(1)–Ga(1)–O(2) 94.21(5)
O(1)–Ga(1)–O(3) 118.42(5)
O(2)–Ga(1)–O(3) 120.97(5)
O(1)–Ga(1)–O(6) 115.89(5)
O(2)–Ga(1)–O(6) 118.39(5)
O(3)–Ga(1)–O(6) 91.16(4)
O(4)–Ga(2)–O(5) 173.78(4)
O(4)–Ga(2)–O(7) 94.80(5)
O(5)–Ga(2)–O(7) 88.75(5)
O(4)–Ga(2)–O(8) 89.95(4)
O(5)–Ga(2)–O(8) 95.09(4)
O(7)–Ga(2)–O(8) 91.07(4)
O(3)–Ga(2)–O(4) 82.33(4)
O(3)–Ga(2)–O(5) 92.28(4)
O(3)–Ga(2)–O(7) 95.25(4)
O(3)–Ga(2)–O(8) 170.39(5)
O(4)–Ga(2)–O(6) 94.41(5)
O(5)–Ga(2)–O(6) 81.69(5)
O(6)–Ga(2)–O(7) 169.80(5)
O(6)–Ga(2)–O(8) 93.30(4)
O(3)–Ga(2)–O(6) 81.66(4)
Ga(1)–O(3)–Ga(2) 93.86(4)
Ga(1)–O(6)–Ga(2) 92.91(4)
1
tons H (99.98%, I = 1/2, μN = 2.7928),[15] and two equiva-
lent nitrogen atoms 14N (99.63%, I = 1, μN = 0.4037)[15] of
both o-iminoquinonate ligands. The splitting parameters
are ai(21H) = 2.37 G, ai(214N) = 3.40 G, ai(69Ga) = 13.28 G,
ai(71Ga) = 16.88 G, and gi = 2.0033 (Figure 3, A). The
equivalence of two protons and two nitrogen atoms and the
values of HFC constants that are two times less than the
corresponding HFC constants in mono-o-iminosemi-
quinonato complexes[16] indicate the fast migration (on the
EPR timescale) of unpaired electron between o-imino-
quinonato ligands.
The similar delocalization of unpaired electron was re-
ported for related five-coordinate tin[2f] and germanium[17]
complexes that contain two differently charged o-imino-
quinonate ligands. The mixed-ligand gallium(III) deriva-
tives that contain radical-anion and dianion forms of o-
quinone simultaneously were also described.[9,13] They form
as the result of a displacement of one SQ ligand from the
initial tris-o-semiquinolate complex by two 3-butylpyridine
molecules[9] or as the result of an exchange reaction be-
tween an equimolar quantity of GaI3 and mono- and di-
thallium o-quinonate salts in the presence of tetramethyl-
ethylenediamine.[13]
1.3625(16)
1.3610(17)
1.406(2)
2.8813(2)
The removal of THF from the solution of 4 and redissol-
ution of the residue in hexane leads to changes in the EPR
spectrum (Figure 3). The hyperfine structure of the EPR
signal registered in hexane shows the interaction of un-
paired electrons with magnetic nuclei of one o-imino-
quinonate ligand only [ai(1H) = 4.37 G, ai(14N) = 6.04 G,
ai(69Ga) = 11.95 G, ai(71Ga) = 15.19 G, gi = 2.0028] (Fig-
ure 3). In addition, the values of hyperfine coupling con-
Scheme 4. The synthesis of gallium(III) derivatives based on the
imQ ligand.
Figure 3. The EPR spectra of complexes [APGa(imSQ)(THF)] (4) in THF (on the left) and [APGa(imSQ)] (5) in hexane (on the right)
(A: experimental, B: simulated).
4
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o-Quinone and o-Iminoquinone with Gallium and “GaI”
stants ai(1H) and ai(14N) in hexane are nearly twice as large
as the respective spectrum parameters in THF solution.
Changes described above can be explained by the following.
The reaction of amalgamated gallium with imQ results in
the formation of the five-coordinated derivative 4 that con-
tains weakly coordinated THF molecules. The decoordina-
tion of THF from the gallium atom in hexane solution of
4 takes place. It leads to the generation of four-coordinate
GaIII derivative [APGa(imSQ)] (5) (Figure 3). It is ac-
companied by the organization of the distorted tetrahedral
coordination geometry around the GaIII center. Such geom-
etry prevents the delocalization of unpaired electron be-
tween two o-iminoquinonate ligands. A similar situation
takes place in tetrahedral gallium(III) derivatives that con-
tain two differently charged diazabutadiene ligands.[18]
Therefore there is a radical center delocalization over two
imQ ligands in complex 4, whereas in 5 the unpaired elec-
tron localizes over one ligand only. Unfortunately, we could
not isolate complexes 4 and 5 in their individual states. But
treatment of 5 with pyridine (py) allows one to obtain the
crystalline product 6 (Scheme 5).
Figure 4. The EPR spectra of complex [APGa(imSQ)(py)] (6) in
toluene (A: experimental, B: simulated).
to-ligand charge transfer, LLCT) between ligands in dif-
ferent oxidation states.
As mentioned above, the direct interaction of imQ with
amalgamated gallium does not result in full reduction of
the redox-active ligand. Therefore the exchange reaction of
disodium o-iminobenzoquinone salt APNa2 with GaI3 was
used to obtain diamagnetic gallium(III) o-amidophenolate
derivative 7 (Scheme 6).
Scheme 6. The synthesis of [AP3Ga2(THF)2] (7).
In accordance with the 1H NMR spectroscopy data of 7,
all of the o-amidophenolate ligands are equivalent to each
other in the coordination sphere of gallium (Figure 5).
Scheme 5. The synthesis of [APGa(imSQ)(py)] (6).
In compliance with EPR spectroscopy data, the structure
of compound 6 is similar to the structure of 4. The EPR
signal of a solution of [APGa(imSQ)(py)] (6) in toluene
arises from the HFC of unpaired electron with magnetic
nuclei of the metal atom and both imQ ligands. However,
the EPR spectrum of 6 is characterized by broader lines
than the spectrum of complex 4. It is explained by the pres-
ence of interaction of unpaired electron with the magnetic
nucleus 14N of the pyridine molecule. This weak HFC
makes a contribution to the value of the line width. The
splitting parameters of the EPR spectrum of complex 6 are
ai(21H) = 2.40 G, ai(214N) = 3.40 G, ai(14N) = 0.60 G,
ai(69Ga) = 13.10 G, ai(71Ga) = 16.65 G, gi = 2.0041 (Fig-
ure 4).
In contrast to compound 4, the dissociation of the sol-
vent molecule from the metal center in 6 does not occur in
toluene or hexane solution, and the transformation repre-
sented in Scheme 5 is irreversible. Complex 6 is charac-
Figure 5. The 1H NMR spectrum of 7 in C6D6. The asterisk (*)
indicates the residual peak of the solvent.
The isopropyl groups and the protons of N-aryl rings
terized by a low-energy band (ca. 2010 nm) in the NIR are inequivalent. It results in four doublets of methyl (iPr)
spectrum that indicates the electron-charge transfer (ligand- protons (7–10), two pseudoseptets of methine protons (5
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FULL PAPER
and 6), and three multiplets attributed to N-aryl ring pro- are formed by O(1), O(2), I(1), and I(2) atoms for Ga(1),
tons (11–13). In addition, there are the signals of the pro- and O(1), N(1), O(2), and N(2) atoms for Ga(2). The Ga(1),
1
tons of THF molecules (14 and 15) in the H NMR spec- O(1), Ga(2), and O(2) atoms form the distorted rhomb. The
trum. The analysis of signal intensities shows that the ratio dihedral angles between the planes of two o-amidophenol-
between o-amidophenolate ligands and solvent molecules is ate ligands and the Ga(1)O(1)Ga(2)O(2) plane are 52.45
3:2. The value of chemical shifts indicates that THF mole- and 56.51°, respectively. The dihedral angle between
cules are coordinated to the metal atom.
C6H2NO fragments of two imQ ligands is 75.64°. The dis-
1
On the basis of the H NMR spectroscopy data, we can tance between Ga(1) and Ga(2) atoms is 2.8743(5) Å, and
suggest a possible structure for complex 7 (Scheme 7). The this value considerably exceeds the sum of gallium covalent
first gallium atom is hexacoordinate. Three o-amidophenol- radii (2.5 Å).[14] The nonbonding repulsion between metal
ate ligands provide a distorted trigonal prismatic environ- atoms causes the deviation of the Ga(2) atom from chelate
ment of this metal atom. The second metal atom is coordi- cycles. The deviations of Ga(2) from the C6H2N(1)O(1) and
nated by three o-amidophenolate oxygen atoms and two C6H2N(2)O(2) planes are 0.091 and 0.156 Å, respectively.
THF molecules.
The C–O [1.412(3)–1.413(3) Å] and C–N [1.403(3)–
1.408(3) Å] bond lengths are typical for dianionic types of
imQ ligand.[20] The C–C distances in six-membered carbon
cycles of both o-amidophenolate ligands were averaged
[1.394(4)–1.410(4) Å]. The values of Ga–O [1.9622(19)–
1.9668(19) Å] and Ga–N [1.823(2)–1.826(2) Å] bond
Scheme 7. The expected structure of 7. Hydrogen atoms, isopropyl,
and tert-butyl groups are omitted for clarity.
Figure 6. The molecular structure of complex 9 with 50% thermal
probability ellipsoids. Hydrogen atoms are omitted for clarity.
It is necessary to note that the structure of complex 7 is
similar to the structure suggested for catecholate derivative
1
2. However, the H NMR spectrum of 7 does not change
Table 2. Selected bond lengths [Å] and angles [°] of complex 9.
over time. This fact indicates that the structure of GaIII o-
amidophenolate compound 7 is invariable, whereas its o-
quinone analogue 2 transforms into complex 3 in solution.
A convenient method of synthesis of metal complexes
that contain a redox-active ligand in the dianionic state is
the reduction of o-quinone (o-iminoquinone) by low-oxi-
dation-state metal halides as well. Thus the interaction of
3,6-Q and imQ with “GaI” results in the formation of galli-
um(III) catecholate and o-amidophenolate complexes 8 and
9, respectively (Scheme 2, path c; Scheme 4, path b).
Bond
[Å]
Angle
[°]
Ga(1)–O(1)
Ga(1)–O(2)
Ga(1)–I(1)
Ga(1)–I(2)
Ga(2)–O(1)
Ga(2)–O(2)
Ga(2)–N(1)
Ga(2)–N(2)
1.9309(19)
1.955(2)
2.4660(4)
2.4809(4)
1.9622(19)
1.9668(19)
1.826(2)
O(1)–Ga(1)–I(2)
I(2)–Ga(1)–I(1)
I(1)–Ga(1)–O(2)
O(1)–Ga(1)–O(2) 85.68(8)
O(1)–Ga(1)–I(1)
O(2)–Ga(1)–I(2)
O(1)–Ga(2)–O(2) 84.53(8)
O(1)–Ga(2)–N(1) 87.69(9)
N(1)–Ga(2)–N(2) 139.25(11)
O(2)–Ga(2)–N(2) 87.45(9)
O(1)–Ga(2)–N(2) 123.38(10)
N(1)–Ga(2)–O(2) 124.40(10)
Ga(1)–O(1)–Ga(2) 95.17(8)
Ga(1)–O(2)–Ga(2) 94.26(8)
109.59(6)
129.035(16)
110.08(6)
108.40(6)
105.46(6)
1.823(2)
Ga(1)···Ga(2) 2.8743(5)
O(1)–C(2)
N(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(1)–C(6)
O(2)–C(28)
N(2)–C(27)
C(27)–C(28)
C(28)–C(29)
C(29)–C(30)
C(30)–C(31)
C(31)–C(32)
C(27)–C(32)
1.413(3)
1.403(3)
1.410(4)
1.399(4)
1.408(4)
1.384(4)
1.392(4)
1.392(4)
1.412(3)
1.408(3)
1.408(4)
1.394(4)
1.401(4)
1.391(4)
1.395(4)
1.395(4)
1
The H NMR spectrum of compound 8 is characterized
by broadened signals of catecholate ligand protons and is
similar to the spectra of known related aluminum[19a] and
indium[19b] complexes. The last ones were found to have a
dimeric structure in accordance with X-ray diffraction
analysis data. Hence it was supposed that 8 has a similar
dimeric structure.
The structure of complex 9 was determined by X-ray dif-
fraction (Figure 6). Selected bond lengths and angles for
complex 9 are given in Table 2. The compound is binuclear
and contains two gallium atoms, two o-amidophenolate li-
gands, and two iodine atoms. Each metal atom has a dis-
torted tetrahedral environment. The apexes of tetrahedrons
6
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o-Quinone and o-Iminoquinone with Gallium and “GaI”
lengths are less than the sum of the covalent radii of the Table 3. Summary of crystal and refinement data for complexes 3
corresponding elements (Ga–O 1.98 Å, Ga–N 1.99 Å[14]),
and 9.
which indicates the covalent nature of these bonds.
Another route to the synthesis of complexes 8 and 9 is
the reaction of 3,6-Q or imQ with gallium metal in the pres-
ence of a stoichiometric quantity of iodine (Scheme 8).
3
9
Empirical formula
Mr
Crystal system
Space group
a [Å]
C50H80Ga2O8
948.58
monoclinic
Cc
25.5689(14)
12.1796(7)
19.3241(11)
121.9630(10)
5105.5(5)
4
C52H74Ga2I2N2O2
1152.37
monoclinic
P21/c
18.8650(10)
12.0855(7)
22.4849(12)
92.3010(10)
5122.3(5)
4
b [Å]
c [Å]
Scheme 8. An alternative synthetic method for compounds 8 and
9.
β [°]
V [Å3]
Z
Dcalcd. [gcm3]
μ [mm–1]
1.234
1.104
1.494
2.297
Crystal size [mm3]
θ Range for data
collection [°]
Reflections collected
Independent reflections
Completeness to θmax.
Max./min. transmission
0.69ϫ0.50ϫ0.11
2.54–27.00
0.37ϫ0.19ϫ0.10
2.00–27.00
Conclusion
The reduction of sterically hindered o-benzoquinone and
o-iminobenzoquinone with metallic or amalgamated gal-
lium as well as with “GaI” was found to be a versatile
method for the synthesis of gallium complexes based on
these redox-active ligands. The described reactions allow
one to obtain metal derivatives that contain differently
charged organic ligands depending on the reaction condi-
tions. The structures of complexes obtained were investi-
gated by using EPR and NMR spectroscopy, and X-ray dif-
fraction analysis.
22829
10938
99.5
0.8882/0.5164
45990
11158
99.8
0.8029/0.4837
11158/0/562
R1 = 0.0376,
wR2 = 0.0899
R1 = 0.0517,
wR2 = 0.0956
1.051
Data/restraints/parameters 10938/2/557
Final R indices [IϾ2σ(I)]
R indices (all data)
R1 = 0.0299,
wR2 = 0.0721
R1 = 0.0338,
wR2 = 0.0736
1.011
Goodness-of-fit on F2
Largest diff. peak and hole 0.839 and –0.678
3.548 and –0.988
[eÅ–3]
[(SQ)3Ga] (1): A solution of 3,6-Q (2.3 mmol, 0.5 g) in THF
(30 mL) was added to the excess amount of gallium metal
(12 mmol, 0.84 g). The reaction mixture was stirred and heated
over 3 h, and a color change from green-red of the initial 3,6-Q to
deep green was observed. The solution was separated from the ex-
cess amount of metal and diluted with hexane. The known[13] com-
plex 1 was obtained as a fine-crystalline deep green product after
cooling; yield 0.44 g (78.0%); m.p. (dec.) Ͼ 150 °C. C42H60GaO6
(730.65): calcd. C 69.04, H 8.28, Ga 9.54; found C 69.10, H 8.30,
Ga 9.50.
Experimental Section
General: All reactants were reagent grade. Solvents were purified
by the following standard methods.[21] 3,6-Di-tert-butyl-o-benzo-
quinone (3,6-Q),[22] 4,6-di-tert-butyl-N-(2,6-diisopropylphenyl)-o-
iminobenzoquinone (imQ),[23] gallium(III) iodide GaI3,[24] and
“GaI”[25] were prepared according to known procedures. All ma-
nipulations on complexes were performed under conditions in
which oxygen and moisture were excluded.
The infrared spectra of complexes in the 4000–400 cm–1 range were
recorded with an FSM 1201 Fourier-IR spectrometer in nujol.
NMR spectra were recorded in C6D6 (for 2, 3, and 7), (CD3)2O
(for 8), or CDCl3 (for 9) with Bruker DPX-200 and Bruker Avance
III 400 MHz instruments with TMS as internal standard. EPR
spectra were recorded with a Bruker EMX spectrometer (working
frequency ca. 9.75 GHz). The gi values were determined by using
[Cat3Ga2(Et2O)2] (2): A solution of 3,6-Q (2.3 mmol, 0.5 g) in hex-
ane (30 mL) was added to the amalgamated gallium (12 mmol,
0.84 g). The reaction mixture became bright yellow after stirring
and heating over 1 h. The hexane was evaporated and the residue
was recrystallized from diethyl ether. Compound 2 was obtained
as colorless crystals; yield 0.47 g (65.1%); m.p. (dec.) Ͼ 120 °C.
C50H80Ga2O8 (948.62): calcd. C 63.31, H 8.50, Ga 14.70; found C
63.45, H 8.57, Ga 14.61. 1H NMR (C6D6, 20 °C): δ = 6.85 (d, JH,H
= 8.5 Hz, 3 H, Harom), 6.68 (d, JH,H = 8.5 Hz, 3 H, Harom), 3.34
[q, 8 H, CH2(Et2O)], 1.46 (s, 27 H, tBu), 1.31 (s, 27 H, tBu), 0.97
2,2-diphenyl-1-picrylhydrazyl (DPPH) as the reference (gi
=
2.0037). EPR spectra of 4–6 were simulated with WinEPR Sim-
Fonia Software (Bruker). Elemental analysis was performed with
an Elemental Analyzer Euro EA 3000 instrument.
[t, 12 H, CH (Et O)] ppm. IR (Nujol): ν = 1485 (s), 1395 (s), 1359
˜
3
2
X-ray Crystallographic Study of 3 and 9: Intensity data for 3 and 9
were collected at 100(2) K with a Smart Apex diffractometer with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) in the
φ–ω scan mode (ω = 0.3°, 10 s on each frame). The intensity data
were integrated by the SAINT program.[26] SADABS[27] was used
to perform area-detector scaling and absorption corrections (semi-
empirical from equivalents). The structures 3 and 9 were solved by
direct methods and were refined with full-matrix least-squares on
F2 using all reflections with the SHELXTL package.[28] All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms
were placed in calculated positions and refined in the riding model.
Table 3 summarizes the crystal data and some details of the data
collection and refinement for these complexes.
(s), 1306 (s), 1283 (s), 1258 (s), 1236 (s), 1219 (s), 1202 (m), 1192
(m), 1145 (s), 1091 (w), 1025 (s), 975 (s), 965 (s), 936 (m), 919 (m),
912 (m), 928 (w), 807 (s), 796 (m), 770 (m), 779 (s), 651 (s), 624
(w), 606 (w), 586 (w), 586 (w), 549 (w), 504 (w), 484 (m) cm–1.
[Cat2Ga(Et2O)2][CatGa] (3): Prolonged recrystallization of com-
plex 2 (0.42 mmol, 0.4 g) from a mixture of Et2O/hexane led to the
formation of compound 3; yield 0.33 g (82.4%); m.p. (dec.) Ͼ
130 °C. C50H80Ga2O8 (948.62): calcd. C 63.31, H 8.50, Ga 14.70;
found C 63.35, H 8.52, Ga 14.67. 1H NMR (C6D6, 20 °C): δ = 6.86
(s, 6 H, Harom), 3.44 [q, 8 H, CH2(Et2O)], 1.50 (s, 54 H, tBu), 0.94
[t, 12 H, CH (Et O)] ppm. IR (Nujol): ν = 1489 (s), 1399 (s), 1354
˜
3
2
(s), 1293 (s), 1282 (s), 1262 (s), 1249 (s), 1237 (s), 1231 (s), 1219 (s),
Eur. J. Inorg. Chem. 0000, 0–0
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A. V. Piskunov, A. V. Maleeva, I. N. Mescheryakova, G. K. Fukin
FULL PAPER
1204 (s), 1195 (s), 1173 (m), 1159 (s), 1146 (s), 1034 (m), 1020 (s),
968 (s), 936 (s), 919 (s), 871 (s), 860 (s), 843 (m), 830 (m), 808 (s),
805 (s), 794 (s), 785 (w), 752 (s), 739 (s), 724 (s), 678 (s), 655 (s),
(12 mmol, 0.84 g). The reaction mixture was stirred and heated un-
til the solution turned pale yellow. The solution was separated from
the excess amount of metal. The fine-crystalline product 8 was ob-
628 (w), 621 (w), 612 (w), 578 (m), 560 (w), 552 (m), 514 (m), 489 tained after recrystallization from a mixture of THF/hexane; yield
(s), 464 (w) cm–1.
0.82 g (73.0%).
[AP2Ga][GaI2] (9). Method 1: A solution of imQ (1.3 mmol, 0.5 g)
in toluene (30 mL) was added to the suspension of “GaI”
(1.3 mmol, 0.256 g) in the same solvent (10 mL). The “GaI” pre-
cipitate dissolved, and a pale yellow solution formed. Complex 9
was obtained as a fine-crystalline, pale yellow product after cool-
ing; yield 0.536 g (71.5%); m.p. (dec.) Ͼ 150 °C. C52H74Ga2I2N2O2
(1152.42): calcd. C 54.20, H 6.47, Ga 12.10, I 22.02; found C 54.23,
[APGa(imSQ)(py)] (6): A solution of imQ (1.3 mmol, 0.5 g) in THF
(30 mL) was added to an excess amount of amalgamated gallium
(12 mmol, 0.84 g). The reaction mixture was stirred for 2 h, and a
color change from wine-red to deep green was observed. The solu-
tion was separated from metal by filtration. Pyridine (2 mL) was
added to the resulted solution. Complex 6 was isolated from hexane
as green-brown crystals; yield 0.4 g (68.0%); m.p. (dec.) Ͼ 170 °C.
C57H79GaN3O2 (907.99): calcd. C 75.40, H 8.77, Ga 7.68; found C
H 6.51, Ga 12.08, I 22.00. 1H NMR (CDCl3, 20 °C): δ = 7.17–7.14
2
(m, 4 H, Harom), 6.99–6.93 (m, 2 H, Harom), 6.70 (d, JH,H
=
75.45, H 8.82, Ga 7.64. IR (Nujol): ν = 1611 (s), 1583 (s), 1559
˜
2
2.34 Hz, 2 H, Harom), 5.93 (d, JH,H = 2.34 Hz, 2 H, Harom), 3.59
[sept., JH,H = 6.9, 2 H, CH(iPr)], 2.37 [sept., JH,H = 6.9, 2 H,
CH(iPr)], 1.56 (s, 18 H, tBu), 1.15 [d, JH,H = 6.9, 6 H, CH3(iPr)],
(m), 1533 (w), 1514 (w), 1477 (s), 1448 (s), 1435 (s), 1415 (s), 1358
(s), 1333 (s), 1321 (s), 1290 (s), 1247 (s), 1220 (w), 1211 (s), 1201
(m), 1170 (m), 1162 (w), 1105 (m), 1055 (w), 1046 (m), 1026 (w),
993 (s), 935 (w), 914 (w), 888 (w), 879 (w), 863 (w), 855 (w), 838
(w), 824 (w), 799 (s), 776 (w), 766 (m), 741 (w), 698 (w), 683 (w),
655 (w), 640 (m), 610 (w), 587 (w), 545 (w), 519 (w), 502 (w) cm–1.
1
1
1
1
1.14 [d, JH,H = 6.9, 6 H, CH3(iPr)], 1.07 (s, 18 H, tBu), 0.73 [d,
1JH,H = 6.9, 6 H, CH3(iPr)], 0.01 [d, JH,H = 6.9, 6 H, CH3(iPr)]
1
ppm. IR (Nujol): ν = 1589 (m), 1573 (s), 1449 (s), 1407 (s), 1393
˜
(m), 1362 (s), 1323 (s), 1283 (s), 1262 (m), 1257 (m), 1238 (m), 1220
(w), 1199 (m), 1189 (s), 1151 (m), 1111 (m), 1096 (s), 1054 (w),
1042 (w), 1026 (w), 986 (s), 955 (w), 939 (w), 935 (w), 927 (w), 901
(w), 861 (w), 850 (w), 813 (m), 801 (s), 769 (w), 750 (w), 740 (w),
662 (w), 653 (w), 583 (w), 547 (w), 533 (w), 508 (w), 476 (w) cm–1.
NIR (Nujol): ν ≈ 2010 nm.
˜
[AP3Ga2(THF)2] (7): A solution of amidophenolatodisodium deriv-
[29]
ative APNa2
(1.3 mmol, 0.55 g) in THF (20 mL) was added to
the solution of GaI3 (0.87 mmol, 0.39 g) in the same solvent
(10 mL). A color change from deep yellow to pale yellow was ob-
served. The THF was evaporated and residue was dissolved in pent-
ane (20 mL). The NaI precipitate was removed by filtration. The
complex 7 was obtained as a fine-crystalline, nearly colorless prod-
uct after cooling; yield 0.33 g (53.4%); m.p. (dec.) Ͼ 140 °C.
C86H127Ga2N3O5 (1422.41): calcd. C 72.62, H 9.00, Ga 9.80; found
C 72.64, H 9.07, Ga 9.73. 1H NMR (C6D6, 20 °C): δ = 7.26 (dd,
1JH,H = 7.63, 2JH,H = 1.53 Hz, 3 H, Harom), 7.17 (t, 1JH,H = 7.63 Hz,
Method 2: A solution of imQ (1.3 mmol, 0.5 g) and I2 (0.65 mmol,
0.165 g) in toluene (30 mL) was added to an excess amount of gal-
lium (12 mmol, 0.84 g). The reaction mixture was stirred until the
solution turned pale yellow. The solution was separated from the
excess amount of metal. Complex 9 was obtained as a fine-crystal-
line, pale yellow product after cooling; yield 0.519 g (69.2%).
CCDC-882125 (for 3) and -882126 (for 9) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
2
3 H, Harom), 7.09 (dd, JH,H = 7.63, JH,H = 1.53 Hz, 3 H, Harom),
2
2
6.80 (s, JH,H = 2.32 Hz, 3 H, Harom), 6.27 (s, JH,H = 2.32 Hz, 3
1
H, Harom), 4.08 [sept., JH,H = 6.9, 3 H, CH(iPr)], 3.13 [m, 8 H,
1
CH2α(THF)], 2.91 [sept., JH,H = 6.9, 3 H, CH(iPr)], 1.54 (s, 27 H,
1
1
tBu), 1.46 [d, JH,H = 6.9, 9 H, CH3(iPr)], 1.34 [d, JH,H = 6.9, 9
H, CH3(iPr)], 1.26 (s, 27 H, tBu), 1.21 [m, 8 H, CH2β(THF)], 0.96
[d, 1JH,H = 6.9, 9 H, CH3(iPr)], 0.34 [d, 1JH,H = 6.9, 9 H, CH3(iPr)]
ppm; assignment of NMR spectroscopic signals was defined more
Acknowledgments
We are grateful to the FSP Scientific and Scientific-Pedagogical
Cadres of Innovation Russia for the years 2009–2013 (GK P839
from 25.05.2010), the Russian Foundation for Basic Research
(grant number 10-03-00788), and the Russian President Grants
(NSh-1113.2012.3, MK-614.2011.3) for financial support of this
work.
exactly by using 2D COSY NMR spectroscopy. IR (Nujol): ν =
˜
1589 (w), 1562 (m), 1414 (s), 1379 (s), 1359 (s), 1331 (s), 1283 (s),
1264 (s), 1229 (s), 1205 (s), 1117 (m), 1103 (w), 1046 (m), 1030 (w),
986 (s), 942 (m), 935 (w), 907 (w), 900 (w), 858 (w), 843 (w), 814
(m), 799 (m), 765 (m), 760 (m), 745 (w), 717 (m), 682 (m), 654 (w),
612 (w), 586 (w), 547 (m), 531 (m), 507 (m) cm–1.
[{CatGa(I)(THF)}2] (8). Method 1: A solution of 3,6-Q (2.3 mmol,
0.5 g) in toluene (30 mL) was added to the suspension of “GaI”
(2.3 mmol, 0.45 g) in the same solvent (10 mL). The “GaI” precipi-
tate dissolved and a light-yellow solution formed. Compound 8 was
obtained after recrystallization from THF/hexane mixture as nearly
colorless crystals; yield 0.76 g (67.9%); m.p. (dec.) Ͼ 130 °C.
C36H56Ga2I2O6 (978.08): calcd. C 44.21, H 5.77, Ga 14.26, I 25.95;
found C 44.27, H 5.81, Ga 14.21, I 25.90. 1H NMR [(CD3)2O,
20 °C]: δ = 6.72 (s, 2 H, Harom), 3.62 [m, 4 H, CH2α(THF)], 1.79
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˜
1394 (s), 1350 (s), 1285 (s), 1261 (s), 1250 (m), 1221 (m), 1196 (m),
1146 (m), 1017 (s), 969 (s), 932 (m), 914 (s), 860 (s), 832 (w), 812
(m), 800 (m), 700 (m), 677 (m), 659 (s), 605 (w), 560 (w), 548 (w)
cm–1.
Method 2: A solution of 3,6-Q (2.3 mmol, 0.5 g) and I2 (1.15 mmol,
0.29 g) in THF (30 mL) was added to the excess amount of gallium
8
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Received: May 17, 2012
Published Online: ᭿
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9

Job/Unit: I20535
/KAP1
Date: 07-08-12 17:47:42
Pages: 10
A. V. Piskunov, A. V. Maleeva, I. N. Mescheryakova, G. K. Fukin
FULL PAPER
Redox-Active Ligands
The reduction of sterically hindered o-
benzo- and o-iminobenzoquinone with gal-
lium as well as with “GaI” was found to
be a versatile method for the synthesis of
gallium complexes that are based on these
redox-active ligands. The described reac-
tions allowed metal derivatives that contain
differently charged organic ligands to be
obtained depending on the reaction con-
ditions.
A. V. Piskunov,*
A. V. Maleeva, I. N. Mescheryakova,
G. K. Fukin ..................................... 1–10
The Reduction of Sterically Hindered o-
Quinone and o-Iminoquinone with Gal-
lium and “GaI”
Keywords: Gallium
duction / EPR spectroscopy
/ Quinones / Re-
10
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Eur. J. Inorg. Chem. 0000, 0–0