ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 440, Part 2, pp. 273–277. © Pleiades Publishing, Ltd., 2011.Original Russian Text © G.A. Abakumov, V.K. Cherkasov, A.V. Piskunov, I.N. Meshcheryakova, N.V. Somov, 2011, published in Doklady Akademii Nauk, 2011, Vol. 440, No. 5,pp. 631–634.

273

At present, researchers pay a considerable attentionto coordination compounds containing paramagneticligands. These compounds are known for the majorityof metals of the periodic table. The study of mer�cury(II) complexes with paramagnetic ligands is oftencomplicated by the instability of these compounds. Inthe literature, there are only several examples of stablemercury(II) complexes containing nitronyl–nitroxyl

and nitroxyl radicals where the paramagnetic ligand isbound to metal via a covalent [1–3] or donor–accep�tor bond [4]. In particular, mercury(II) compoundsbased on 4,4,5,5�tetramethyl�2�imidazol�3�oxy�1�oxyl stable under normal conditions have been synthe�sized where the proton at the C(2) atom is readily sub�stituted by different electrophilic reagents (for exam�ple, Li+, AcOHg+, and others) [1, 2] (Scheme 1).

Scheme 1.

N

N

O

O

Hg OAcN

N

O

O

HN

N

O

O

HgN

N

O

O1

2

3

4

5

0.5Hg(OAc)2

–HOAc

– – –

–

+ + +

+

•

•••

1. LiN(Pr i)22. Hg(OAc)2

1. –HN(Pr i)22. – LiOAc

Paramagnetic mercury(II) complexes based ondifferent ligands of the o�semiquinone type [5–9]were studied only in solution by electron paramag�netic resonance (EPR) spectroscopy. They resultfrom one�electron transfer, the first stage of thereaction of o�quinones with different organomer�

cury derivatives (Scheme 2), and from the exchangereactions of quinone salts of alkali metals with orga�nomercury(II) halides. The majority of studiedcompounds decompose in solution even at –20°C,and the most stable complexes persist in solution forone day.

Scheme 2.

O

O

R'

O

O

R'

O

O

R' HgRO

O

R'HgR2

R• –R•HgR2–•

–•

•+ HgR+

•

We synthesized for the first time a stable paramag�netic phenylmercury(II) complex with the radicalanion form of 4,6�di�tert�butyl�N�(2,6�di�iso�propy�lphenyl)�o�iminobenzoquinone (ImQ) and charac�

terized it by EPR spectroscopy and X�ray diffractionanalysis.

The complex results from the exchange reaction ofsodium o�iminosemiquinolate (ImSQNa) [10] withphenylmercury(II) chloride [11] in THF in theabsence of air oxygen and moisture. Tetrahydrofuranwas removed under reduced pressure, and the solidresidue was dissolved in hexane (30 mL). The solutionwas filtered to remove NaCl precipitate using a Schott

CHEMISTRY

Paramagnetic Mercury(II) Complex with o�Iminobenzosemiquinone Ligand

Academician G. A. Abakumova, Corresponding Member of the RAS V. K. Cherkasova, A. V. Piskunova, I. N. Meshcheryakovaa, and N. V. Somovb

Received May 12, 2011

DOI: 10.1134/S0012500811100041

a Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, ul. Tropinina 49, Nizhni Novgorod, 603950 Russia

b Lobachevskii State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia

274

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ABAKUMOV et al.

sintered�glass funnel no. 4 and the resultant mercurycomplex was crystallized at –20°C.

Electron paramagnetic resonance spectra wererecorded on a Bruker EMX spectrometer. Diphenylpi�crylhydrazyl (g = 2.0037) was used as a reference forthe determination of g factor. Crystals suitable forX�ray diffraction study were obtained from a pentanesolution. The X�ray diffraction study of the complexwas carried out with an Oxford Diffraction Gemini Sdiffractometer (UK) at 298 K, МоK

α radiation, λ =

0.71073 Å. The structure was solved by direct methodsand refined by full�matrix least squares with the use of

SHELX971 and WINGX software [12]. Crystallo�graphic data: C32H42ONHg, monoclinic symmetrysystem, space group P21/n; unit cell parameters a =

14.0291(10) Å, b = 14.1569(9) Å, c = 15.5079(11) Å, α =90°, β = 90.841(7)°, γ = 90°; V = 3079.7(4) Å3, Z = 4,Dcalc = 1.418 g/cm3, μ = 5.02 mm–1, F(000) = 1316,R1 = 0.0295, wR2 = 0.0554, S(F 2) = 0.442. The crys�tallographic data are deposited with the CambridgeCrystallographic Data Center (CCDC 822558,deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk/).

The reaction of solutions of sodium o�iminosemi�quinolate and phenylmercury chloride proceeds withthe rate of reagent mixing, and the solution colorthereby changes from blue of ImSQNa to emeraldgreen. The resulting mercury(II) complex (ImSQH�gPh) precipitates as a rich�colored crystalline solid(Scheme 3). The yield after crystallization fromn�pentane was 73%.

Scheme 3.

O

N

Na

But

But

PriPri

O

N

Hg

But

But

PriPri

Ph+ PhHgCl

ImSQNa ImSQHgPh

THF–NaCl ••

Solutions of ImSQHgPh show a well�resolved EPRspectrum (Fig. 1), which is a triplet (1 : 1 : 1) of dou�blets (1 : 1) due to coupling of the unpaired electron tothe 14N nucleus (99.63%, I = 1, μN = 0.4037) and theproton in the 5�position of the o�iminoquinone ring1H (99.98%, I = 1/2, μN = 2.7928). We also observedsatellite splitting due to the magnetic isotopes199Hg (17.0%, I = 1/2, μN = 0.5059) and 201Hg(13.2%, I = 3/2, μN = –0.5602). The EPR parametersof ImSQHgPh solutions are strongly dependent on the

solvent nature (table). The largest changes areobserved for hyperfine coupling constants with mag�netic mercury isotopes. An increase in the donor num�

ber of the solvent leads to the growth of ai(199Hg) from

13.6 Oe in hexane to 20.55 Oe in pyridine. It is alsoworth noting that the HFC constant with proton(2.46–2.83 Oe) is markedly lower than that typical foro�iminosemiquinolate complexes of nontransition

metals (~3.5 Oe [10]) and close to ai(1H) for m�pro�

tons in phenoxy radicals (1.8–2.0 Oe) [13]. Thus, thestructure of the mercury(II) complex in solutionagrees best with form A where excess spin density of

1Scheldrick, G.M., SHELX97. Programs for Crystal StructureAnalysis (Release 97�2), University of Göttingen, Germany.1997.

Parameters of isotropic EPR spectra of the mercury(II) o�iminosemiquinolate complex in different solvents at 300 K

Solvent ai(1H), Oe ai(

14N), Oe ai(199Hg, 201Hg), Oe gi

Hexane 2.46 5.18 13.60, 5.02 1.9963

Toluene 2.61 5.31 14.40, 5.31 1.9972

Ethyl acetate 2.63 5.36 16.00, 5.90 1.9972

THF 2.61 5.40 16.15, 5.96 1.9966

Pyridine 2.83 5.58 20.55, 7.58 1.9980

DOKLADY CHEMISTRY Vol. 440 Part 2 2011

PARAMAGNETIC MERCURY(II) COMPLEX 275

the molecular orbital of unpaired electron is localizedon the oxygen atom (Scheme 4).

Scheme 4.

These conclusions agree well with the data of X�raydiffraction study (Fig. 2). The Hg(1) atom in the com�plex has a trigonal T�shaped environment typical ofthree�coordinate mercury derivatives and is locatednear the O(1)–N(1)–C(7) plane. The deviation of thisatom from the plane is 0.071 Å. The N(1)–Hg(1)–C(7) angle is close to 180°, being 169.95(14)°. TheN(1)–C(2) distance, 1.346(4) Å, is within the rangetypical for o�iminosemiquinone metal complexes[10]. On the contrary, the O(1)–C(1) bond, 1.26 Å, isslightly shorter than the typical value for monoreducedImQ. For the radical anion coordination of the o�imi�noquinone ligand, the metal–oxygen and metal–nitrogen bond distances are slightly longer than thesum of the covalent radii of corresponding elements[10]. In the mercury(II) complex under consideration,the Hg(1)–N(1) bond, 2.082(3) Å, is slightly shorterthan the sum of the covalent radii of mercury andnitrogen atoms (2.11 Å). On the contrary, the length ofHg–O bond, 2.452(3) Å, is much larger than the sumof the covalent radii of the corresponding elements(2.10 Å), which indicates a relatively weak donor–acceptor Hg–O bond. In its turn, this provides an

O

N

HgPh

But

But

PriPri

O

N

HgPh

But

But

PriPri

A B

•

•

additional confirmation for the localization of theunpaired electron preferably at the oxygen atom andthe realization of amidophenoxy type of coordinationof the redox�active ligand. At the same time, the six�membered carbon ring C(1)–C(6) exhibits distortionof the o�quinoid type inherent in o�iminosemiquino�nes [10] and resulting in the alternation of C–C bondlengths. In particular, the C(1)–C(2), C(1)–C(6),C(2)–C(3), and C(4)–C(5) bond lengths are within1.410(5)–1.465(5) Å and much longer than the C(3)–C(4) and C(5)–C(6) distances (1.355(5)–1.363(5) Å).Such a kind of alternation of bond lengths of the six�membered ring indicates that the limiting distancecorresponding to form A (Scheme 4) is not achieved inthe case of ImSQHgPh.

It was reported in the literature that certain mer�cury(II) organometallic compounds are structurallynonrigid molecules whose potential energy surface hasseveral local minima separated by small barriers.Therefore, a number of compounds in crystal statecontain molecules in different states; i.e., there areindependent molecules with different geometry ofcovalent bonds. For example, structural nonrigiditywas revealed for phenylmercury 8�hydroxyquinolinate[14] where the coordination environment of the mer�cury atom varies from T�shaped to trigonal�planardepending on crystallization conditions. Taking intoaccount the features mentioned above, we could notrule out the possibility of appearance of structuralnonrigidity for the ImSQHgPh complex.

3435 3445 3455 3465 3475H, Oe

C(6)

C(5)

C(4)

C(3)

C(2)

C(1)

O(1)

C(7)

N(1)

Hg(1)

Fig. 1. Isotropic EPR spectrum of the mercury(II) o�imi�nosemiquinolate complex in a THF solution at 300 K.

Fig. 2. Molecular structure of the mercury(II) o�imi�nosemiquinolate complex. Hydrogen atoms are omitted.Selected bond lengths (Å) and bond angles (deg): Hg(1)–O(1), 2.452(3); Hg(1)–N(1), 2.082(3); Hg(1)–C(7),2.042(4); O(1)–C(1), 1.261(4); N(1)–C(2), 1.346(4);C(1)–C(2), 1.465(5); C(2)–C(3), 1.410(5); C(3)–C(4),1.363(5); C(4)–C(5), 1.421(5); C(5)–C(6), 1.355(5);C(1)–C(6), 1.445(5); O(1)–Hg(1)–C(7), 116.77(13);N(1)–Hg(1)–C(7), 169.95(14); O(1)–Hg(1)–N(1),72.32(10); Hg(1)–O(1)–C(1), 110.0(2); Hg(1)–N(1)–C(2), 120.6(2).

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DOKLADY CHEMISTRY Vol. 440 Part 2 2011

ABAKUMOV et al.

Based on the body of obtained data, one canexplain the reasons of changes in the EPR spectralparameters of ImSQHgPh solutions as a function ofmedium donor properties. The mercury atom in thecomplex can form bonds through hybrid orbitalsresulting from a combination of the 6px, 6py, 6pz, and 6satomic orbitals of the metal. In hydrocarbon solvents,the coordination number of the central atom is three(Scheme 5). Taking into account that the N(1)–Hg(1)–C(7) angle is close to 180°, one can supposethe hybridization of metal orbitals close to the sphybridization. The nonhybridized nearly pure p orbitalparticipates in coordination binding to the oxygenatom of the phenoxy group. Spin density can be delo�calized from the radical ligand on mercury atom viatwo routes: by the direct involvement of the vacant porbital of mercury in the molecular orbital of the

ligand occupied by the unpaired electron and by thespin polarization of atomic orbitals of mercury pro�ducing Hg–N and Hg–O bonds. It is difficult to assessthe contribution of each route without special calcula�tions. When a donor solvent molecule coordinates themetal atom, the coordination number of the latterincreases to four (Scheme 5), which leads to a changein the geometry of the mercury atom orbitals. Theredistribution of the contributions of s and p states tothe atomic orbitals responsible for hyperfine couplingis accompanied by the growth of the s state contribu�tion, resulting in an increase in the HFC constant ofthe unpaired electron with the nuclei of magnetic iso�topes of mercury. Similar changes in EPR parametersdepending on the donor properties of a medium wereobserved previously for thallium(I) o�semiquinolates[15].

Scheme 5.

Thus, we obtained for the first time a stable para�magnetic mercury(II) complex containing a radicalform of o�iminobenzoquinone ligand and hydrocar�bon substituent at metal atom. It was establishedthat amidophenoxy coordination of the redox�active ligand dominates in the structure of this com�pound.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project no. 10–03–00788–a), the Council for Grants of the President of the Rus�sian Federation for Support of Leading ScientificSchools (grant no. NSh 7065.2010.3) and Young Sci�entists (grant no. MK�641.2011.3), and the FederalTargeted Program “Scientific and Scientific�Pedagog�ical Personnel of the Innovative Russia in 2009–2013”(State Contract P839 of 25.05.2010).

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Ph O

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THF

–THFTHF

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PARAMAGNETIC MERCURY(II) COMPLEX 277

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