untitled


European	Journal	of	Chemistry	2	(2)	(2011)	266‐268	

	
European	Journal	of	Chemistry	

ISSN	2153‐2249	(Print)	/	ISSN	2153‐2257	(Online)		2011	EURJCHEM	
DOI:10.5155/eurjchem.2.2.266‐268.227	

	
	

	

	

European	Journal	of	Chemistry	
Journal	homepage:	www.eurjchem.com	

	 	 	

Synthesis	and	electrochemistry	of	dimanganese(II)	complexes	of	phenol‐based	
dinucleating	ligands	with	four	methoxyethyl	chelating	arms	

Yuuki	Nakayama,	Kei	Unoura,	Yasuhiro	Igarashi,	Md.	Jamil	Hossain	and	Hiroshi	Sakiyama*	

Department	of	Material	and	Biological	Chemistry,	Faculty	of	Science,	Yamagata	University,	Kojirakawa,	Yamagata	990‐8560,	Japan	

*Corresponding	author	at:	Department	of	Material	and	Biological	Chemistry,	Faculty	of	Science,	Yamagata	University,	Kojirakawa,	Yamagata	990‐8560,	Japan.		
Tel.:	+81.23.628.4601;	fax:	+81.23.628.4591.	E‐mail	address:	saki@sci.kj.yamagata‐u.ac.jp	(H.	Sakiyama).	

	
	

	 	

	 	 	
COMMUNICATION	 INFORMATION	 	 ABSTRACT
Received:	03	August	2010	
Received	in	revised	form:	27	October	2010	
Accepted:	27	October	2010	
Online:	30	June	2011	

KEYWORDS	

	 Dimanganese(II)	 complexes	 [Mn2(bonp)(PhCO2)2]PF6	 (1)	 and	 [Mn2(bocp)(PhCO2)2]PF6	 (2)
were	 synthesized	 with	 p‐nitro‐	 and	 p‐chloro‐substituted	 phenol‐based	 dinucleating	 ligands
bonp‐	 [2,6‐bis[bis(2‐methoxyethyl)aminomethyl]‐4‐nitrophenolate	 anion]	 and	 bocp‐	 [4‐
chloro‐2,6‐bis[bis(2‐methoxyethyl)aminomethyl]phenolate	anion],	 respectively,	with	 the	aim
of	controlling	the	redox	potentials	of	the	dimanganese	center	by	changing	the	p‐substituents
in	 the	 dinucleating	 ligands.	 Cyclic	 voltammograms	 of	 1	 and	 2	 showed	 quasi‐reversible
oxidation	 processes,	 assigned	 to	 MnIIMnII/MnIIMnIII,	 at	 1.17	 and	 1.00	 V	 vs.	 Ag/AgCl,
respectively.	 Compared	 to	 the	 previous	 p‐methyl	 complex	 [Mn2(bomp)(PhCO2)2]PF6	 (3)
[bomp–:	 2,6‐bis[bis(2‐methoxyethyl)aminomethyl]‐4‐methylphenolate	 anion]	 (0.96	 V	 vs.
Ag/AgCl),	 the	 order	 of	 the	 potentials	 was	 1(‐NO2)	 >	 2(‐Cl)	 >	 3(‐CH3).	 Thus,	 the	 redox
potentials	 of	 the	 dimanganese	 centers	 were	 controlled	 by	 the	 p‐substituents	 in	 the
dinucleating	ligands.	

Dimanganese(II)	complex	
Dinucleating	ligand	
Electrochemistry	
p‐Substituent	
DFT	
Redox	potential	

	
1.	Introduction	
	

Dimanganese	 cores	 are	 often	 seen	 in	 biological	 systems,	
such	 as	manganese	 catalases	 [1‐6],	manganese	 ribonucleotide	
reductase	 [7],	 arginase	 [8],	 thiosulfate	 oxidase	 [9],	 xylose	
isomerase	 [10‐12],	 RNAase	 [13],	 and	 proline	 aminopeptidase	
[14],	and	the	dimanganese	cores	act	as	the	catalytic	centers	of	
these	 enzymes.	 Some	 enzymes	work	 as	 hydrolases,	 and	 some	
work	as	 redox	enzymes.	For	 instance,	proline	aminopeptidase	
removes	 the	 N‐terminal	 amino	 acid	 by	 hydrolysis,	 and	
manganese	 catalase	 catalyzes	 the	 disproportionation	 of	 H2O2	
into	O2	and	H2O.	

Various	 types	 of	 dimanganese	 complexes	 have	 been	
prepared	 as	models	 of	 the	biological	 systems	 [15],	 and	 the	μ‐
phenoxo‐bis(μ‐carboxylato)dimanganese	complex	is	one	of	the	
well‐studied	 dimanganese	 complexes	 [16‐18].	 Previously,	 we	
reported	 a	 structure	 and	 electrochemical	 properties	 of	
dimanganese(II)	 complexes	 [Mn2(bomp)(PhCO2)2]PF6	 (3)	 and	
[Mn2(bomp)(PhCO2)2]BPh4	 (4)	 [19]	 [bomp–:	 2,6‐bis[bis(2‐
methoxyethyl)aminomethyl]‐4‐methylphenolate	 anion].	 In	 the	
cyclic	 voltammogram	 of	 3,	 and	 a	 quasi‐reversible	 oxidation	
process	 was	 observed	 at	 0.92	 V	 vs.	 SSCE	 (saturated	 sodium	
chloride	calomel	electrode),	assigned	to	MnIIMnII/MnIIMnIII.		

Herein,	 two	 new	 dimanganese(II)	 complexes	 [Mn2(bonp)	
(PhCO2)2]PF6	 (1)	 and	 [Mn2(bocp)(PhCO2)2]PF6	 (2)	 have	 been	
synthesized	 with	 p‐nitro‐	 and	 p‐chloro‐substituted	 phenol‐
based	dinucleating	 ligands	H(bonp)	 and	H(bocp),	 respectively	
[H(bonp):	 2,6‐bis[bis(2‐methoxyethyl)aminomethyl]‐4‐nitro‐
phenol;	 H(bocp):	 4‐chloro‐2,6‐bis[bis(2‐methoxyethyl)amino‐
methyl]phenol],	with	the	aim	of	controlling	the	redox	potentials	
of	 the	 dimanganese	 center	 by	 changing	 the	 p‐substituents	 in	
the	dinucleating	ligands.	

	
	

	
	

Scheme	1	
	
2.	Experimental	
	
2.1.	Instrumentation	
	

Elemental	 analyses	 were	 obtained	 at	 the	 Elemental	
Analysis	 Service	 Centre	 of	 Kyushu	 University.	 Infrared	 (IR)	
spectra	were	recorded	on	a	Hitachi	270‐50	spectrometer.	Cyclic	
voltammetric	measurements	were	performed	using	an	ALS	400	
Electrochemical	Analyzer	in	acetonitrile	using	a	three‐electrode	
cell	 consisting	 of	 a	 glassy	 carbon	 disk	 working	 electrode,	 a	
platinum	 counter	 electrode,	 and	 an	 Ag/AgCl	 reference	
electrode.	 Tetra‐n‐buthylammonium	 tetrafluoroborate	
(TBABF4)	was	used	as	the	supporting	electrolyte.	
	
2.2.	Materials	
	

Na(bonp)	[20],	Na(bocp)	[20],	Na(bomp)	[21],	and	complex	
3	 [19]	were	 prepared	 according	 to	 the	methods	 described	 in	
the	literature.	All	other	chemicals	were	commercial	products.	
	
	
	



Nakayama	et	al.	/	European	Journal	of	Chemistry	2	(2)	(2011)	266‐268	 267	
 

 
 

2.3.	Synthesis	
	

[Mn2(bonp)(PhCO2)2]PF6,	 1:	 To	 a	 methanolic	 solution	 (15	
mL)	of	Na(bonp)	(0.45	g,	1.0	mmol)	was	added	manganese(II)	
benzoate	 tetrahydrate	 (0.74	 g,	 2.0	 mmol),	 and	 the	 resulting	
solution	 was	 refluxed	 for	 1	 h	 to	 give	 a	 yellow	 solution.	 The	
addition	 of	 sodium	 hexafulorophosfate	 (0.17	 g,	 1.0	 mmol)	
resulted	in	the	precipitation	of	yellow	microcrystals.	Yield:	0.28	
g	(30%).	Selected	IR	(KBr,	cm‐1):	3030‐2820,	1605,	1565,	1505,	
1450,	1405,	1315,	1085,	1025,	1005,	910,	830,	745,	720,	660,	
555,	450.	Anal.	Calcd.	for	C34H44F6Mn2N3O11P:	C,	44.10;	H,	4.80;	
N,	4.55.	Found:	C,	44.30;	H,	4.85;	N,	4.60%.	

[Mn2(bocp)(PhCO2)2]PF6,	 2:	 This	 was	 prepared	 as	 white	
microcrystals	by	a	method	similar	to	that	of	1	using	Na(bocp)	
instead	of	Na(bonp).	Yield:	0.35	g	(35%).	Selected	IR	(KBr,	cm‐

1):	 3030‐2820,	 1605,	 1565,	 1505,	 1450,	 1405,	 1315,	 1085,	
1025,	1005,	910,	830,	750,	720,	670,	555,	450.	Anal.	Calcd.	for	
C34H44ClF6Mn2N2O9P:	C,	44.65;	H,	4.85;	N,	3.05.	Found:	C,	44.45;	
H,	4.85;	N,	3.05%.	
	
2.4.	Computations	
	

All	 the	DFT	 computations	were	performed	using	Gaussian	
03	 software	 (Gaussian,	 Inc.)	 [22‐30].	 Structural	 optimization	
was	performed	with	the	B3LYP/LANL2DZ	method.	
	
3.	Results	and	discussion		
	
3.1.	General	properties	of	complexes	1	and	2	
	

The	 IR	 spectra	 of	 complexes	 1,	 2,	 and	 3	 show	 the	
antisymmetric	 and	 symmetric	 ν(CO2)	 vibrations	 of	 the	
benzoate	 group	 at	 1560~1565	 and	 1400~1405	 cm‐1,	
respectively.	A	Δν	 value	 [Δν	=	νas(CO2)	–	νs(CO2)]	 smaller	 than	
200	cm‐1	is	typical	of	the	bridging	carboxylate	group	[31].	The	
crystal	structure	of	complex	3	was	determined	previously,	and	
a	 µ‐phenoxo‐bis(µ‐benzoato)dimanganese(II)	 core	 was	 found	
[19].	 Judging	 from	 the	 similarity	 of	 the	 IR	 spectra,	 it	 seems	
reasonable	 to	 assume	 the	 µ‐phenoxo‐bis(µ‐benzoato)	
dimanganese(II)	structures	for	both	1	and	2.	Complexes	1	and	
2	show	no	appreciable	absorption	in	the	region	above	350	nm	
in	 DMF	 solution,	 and	 neither	 does	 3,	 in	 accordance	 with	 the	
high‐spin	d5	electronic	configuration	of	the	manganese(II)	ion.	
	
3.2.	Electrochemistry	
	

The	 cyclic	 voltammograms	 (CV)	 of	 complexes	 1	 and	 2	 in	
acetonitrile	 solution	 are	 shown	 in	 Figure	 1.	 In	 the	 CV	 of	 2	
(Figure	 1b),	 two	 irreversible	 waves	 and	 one	 quasi‐reversible	
redox	 couple	 were	 observed:	 an	 irreversible	 reduction	 wave	
around	 –2.4	V,	 a	 quasi‐reversible	 first	 oxidation	wave	 around	
1.0	V,	and	an	irreversible	second	oxidation	wave	above	1.5	V	vs	
Ag/AgCl.	These	features	are	very	similar	to	those	of	3	reported	
previously	[19]	except	for	a	back	peak	observed	at	around	‐1.3	
V	due	 to	an	EC	reaction.	According	 to	our	previous	paper,	 the	
reduction	 wave	 can	 be	 assigned	 to	 the	 ligand‐based	 electron	
transfer	 process,	 the	 first	 oxidation	 wave	 to	 the	
MnIIMnII/MnIIMnIII	 electron	 transfer,	 and	 the	 second	oxidation	
wave	 to	 the	 ligand‐based	 electron	 transfer.	 In	 the	 case	 of	 1	
(Figure	 1a),	 in	 addition	 to	 the	 above‐mentioned	 electron‐
transfer	 processes,	 a	 quasi‐reversible	 couple	 is	 observed	
around	 –1.2	 V	 vs	 Ag/AgCl.	 This	 wave	 can	 be	 assigned	 to	 the	
reduction	of	the	p‐nitro	group	in	1.		

Here	 we	 will	 focus	 on	 the	 first	 oxidation	 waves	 of	
complexes	 1‐3	 for	 the	 purpose	 of	 investigating	 the	
electrochemical	properties	of	 the	dimanganese	centers.	 In	this	
study,	 the	 CV	 of	 3	 was	 again	 measured	 using	 the	 Ag/AgCl	
reference	 electrode	 because	 the	 previous	 measurement	 was	
done	 using	 the	 saturated	 sodium	 chloride	 calomel	 electrode	

(SSCE).	The	first	oxidation	waves	of	complexes	1‐3	are	shown	
in	 Figure	 2,	 and	 the	 electrochemical	 parameters	 are	
summarized	 in	 Table	 1.	 The	 electrochemical	 behaviors	 of	 the	
complexes	 are	 essentially	 similar	 to	 each	 other.	 The	 current	
parameters	 Ipa	 are	 in	 the	 range	 of	 730‐840	 for	 all	 the	
complexes,	and	these	values	are	consistent	for	the	one‐electron	
processes	 in	 acetonitrile	 solution	 [32].	 Therefore,	 the	 first	
oxidation	 waves	 can	 be	 assigned	 to	 the	 MnIIMnII/MnIIMnIII	
process	 for	 all	 the	 complexes.	 The	 reversibility	 of	 the	 first	
oxidation	 waves	 increased	 when	 the	 scan	 rate	 increased,	
indicating	 that	 the	 first	 oxidation	 process	 is	 followed	 by	 a	
subsequent	 chemical	 reaction	 (EC	 mechanism).	 Judging	 from	
the	peak	 current	 ratio	 ipc/ipa,	 the	 stability	of	 the	one‐electron‐
oxidation	product	of	3	is	the	highest	among	the	three.	In	Figure	
3,	 the	 formal	 potentials	 E0’	 for	 the	 complexes	 are	 plotted	
against	 the	 Hammet’s	 p‐substituent	 constants	 σp	 of	 the	 p‐
substituents	 [33,34]	 in	 the	 complexes.	 The	 stronger	 the	
electron	withdrawing	effect	[1	(‐NO2)	>	2	(‐Cl)	>	3	(‐CH3)],	the	
higher	 the	 formal	 potential	 [1	 >	 2	 >	 3].	 Thus,	 the	 redox	
potencials	of	the	dimanganese	centers	are	controlled	by	the	p‐
substituents	in	the	dinucleating	ligands.	

	

	
	

Figure	 1.	 Cyclic	 voltammograms	 of	 1	 (a)	 and	 2	 (b)	 in	 acetonitrile	
containing	0.1	M	TBABF4	at	a	glassy	carbon	disk	electrode.	 (Sweep	rate	=	
0.1	V	s–1;	Initial	direction	=	positive;	Temp.	=	25	°C).	
	

	
	

Figure	2.	Cyclic	voltammograms	for	the	first	oxidation	processes	of	1 (a),	2
(b),	 and	3	 (c)	 in	 acetonitrile	 containing	 0.1	M	TBABF4	 at	 a	 glassy	 carbon	
disk	electrode.	(Sweep	rate	=	1	V	s–1;	Initial	direction	=	positive;	Temp.	=	25	
°C).	



268	 Nakayama	et	al.	/	European	Journal	of	Chemistry	2	(2)	(2011)	266‐268	
	
Table	1.	Electrochemical	parameters	of	the	complexes	a.	
Complex	 E0’	/	V	vs	Ag/AgClb	 ΔE	/V	 Ipa	/	Uc	 ipc/ipa	
1	 1.17	 0.14	 731	 0.2
2	 1.00	 0.18	 838	 0.2
3	 0.96	 0.12	 803	 0.6
a	Sweep	rate	=	1	V	s–1.	b	E0’	=	(Epc	+	Epa)/2.	c	U	=	µA	(mM)‐1	V–1/2s1/2cm–2.	

	

	
	

Figure	3.	Correlation	between	the	formal	potentials	E0’	of	the	first	oxidation	
processes	and	the	Hammet’s	parameters	σp	of	the	p‐substituents.	
	
3.3.	DFT	computations	
	

Structures	 of	 complex	 cations	 for	 1,	 2,	 and	 3	 were	
optimized	by	DFT	method,	referring	the	crystal	structure	of	3,	
and	 the	 obtained	 structures	 are	 shown	 in	 Figure	 4.	 Since	 the	
oxidation	 potential	 is	 related	 to	 the	 HOMO	 level,	 the	 formal	
potentials	E0’	 for	 the	MnIIMnII/MnIIMnIII	 processes	 are	 plotted	
against	 the	HOMO	 level	 obtained	by	 the	DFT	computations	 as	
shown	 in	 Figure	5.	A	 good	 linearity	was	 observed,	 and	 it	was	
better	than	the	plot	against	the	Hammet’s	constants.	

	

	
	

Figure	4.	Structures	of	complex	cations	for	(a)	1,	(b)	2,	and	(c)	3,	optimized	by	
the	DFT	method.	

	

	
	

Figure	5.	Correlation	between	the	formal	potentials	E0’	of	the	first	oxidation	
processes	and	the	HOMO	level.	
	
	

4.	Conclusion	
	

In	 this	 study,	 two	 new	 dimanganese(II)	 complexes	 have	
been	synthesized	using	 two	dinucleating	 ligands	derived	 from	
the	bomp	ligand.	It	was	shown	that	the	redox	potential	for	the	
MnIIMnII/MnIIMnIII	process	was	well	controlled	by	the	electron‐
donating/withdrawing	 effect	 of	 the	 p‐substituents	 in	 the	
dinucleating	ligands.	
	
References	
	
[1]. Kono,	Y.;	Fridovich,	I.	J.	Biol.	Chem.	1983,	258,	6015‐6019.	
[2]. Kono,	Y.;	Fridovich,	I.	J.	Biol.	Chem.	1983,	258,	13646‐13658.	
[3]. Allgood,	G.	S.;	Perry,	J.	J.	J.	Bacteriol.	1986,	168,	563‐567.	
[4]. Barynin,	V.	V.;	Grebenko,	A.	I.	Dokl.	Akad.	Nauk	SSSR	1986,	286,	461‐

464.	
[5]. Khangulov,	S.	V.;	Goldfeld,	M.	G.;	Gerasimenko,	V.	V.;	Andreeva,	N.	E.;	

Barynin,	V.	V.;	Grebenko,	A.	I.	J.	Inorg.	Biochem.	1990,	40,	279‐292.	
[6]. Kagawa,	M.;	Murakoshi,	N.;	Nishikawa,	Y.;	Matsumoto,	G.;	Kurata,	 Y.;	

Mizobata,	 T.;	 Kawata,	 Y.;	 Nagai,	 J.	Arch.	Biochem.	Biophys.	1990,	 40,	
279‐292.	

[7]. Willing,	 A.;	 Follmann,	H.;	 Auling,	 G.	Eur.	 J.	Biochem.	1988,	170,	 603‐
611.	

[8]. Reczkowski,	R.	S.;	Ash,	D.	E.	J.	Am.	Chem.	Soc.	1992,	114,	10992‐10994.	
[9]. Cammack,	 R.;	 Chapmann,	 A.;	 Lu,	W.‐P.;	 Karagouni,	 A.	 A.;	 Kelly,	 D.	 P.	

FEBS	Lett.	1989,	252,	239‐243.	
[10]. Carrel,	H.	L.;	Glusker,	J.	P.;	Burger,	V.;	Manfre,	F.;	Tritsch,	D.;	Biellmann,	

J.	F.	Proc.	Natl.	Acad.	Sci.	U.S.A.	1989,	86,	4440‐4444.	
[11]. Whitlow,	M.;	 Howard,	 A.	 J.;	 Finzel,	 B.	 C.;	 Poulos,	 T.	 L.;	Winborne,	 E.;	

Gilliland,	G.	L.	Proteins	1991,	9,	153‐173.		
[12]. Collyer,	C.	A.;	Henrick,	K.;	Blow,	D.	M.	J.	Mol.	Biol.	1990,	212,	211‐235.	
[13]. Davies,	 J.	F.;	Hostomka,	Z.	II.	 ;	Hostomsky,	Z.;	Jordan,	S.	R.;	Matthews,	

D.	A.	Science	1991,	252,	88‐95.	
[14]. Wilce,	M.	C.;	Bond,	C.	S.;	Dixon,	N.	E.;	Freeman,	H.	C.;	Guss,	J.	M.;	Lilley,	

P.	E.;	Wilce,	J.	A.	Proc.	Natl.	Acad.	Sci.	U.S.A.	1998,	95,	3472‐3477.	
[15]. Pecoraro,	V.	L.	Manganese	Redox	Enzymes,	VCH	Publishers,	Inc,	1992.	
[16]. Kurz,	P.;	Anderlund,	M.	F.;	Shaikh,	N.;	Styring,	S.;	Huang,	P.	Eur.	J.	Inorg.	

Chem.	2008,	5,	762‐770.	
[17]. Gultneh,	Y.;	Tesema,	Y.	T.;	Ahvazi,	Bijan;	Yisgedu,	T.	B.;	Butcher,	R.	 J.;	

Tuchagues,	J.	P.	Inorg.	Chim.	Acta	2006,	359,	4463‐4469.	
[18]. Dubois,	L.;	Caspar,	R.;	Jacquamet,	L.;	Petit,	P.	E.;	Charlot,	M.	F.	Baffert,	

C.;	Collomb,	M.	N.;	Deronzier,	A.;	Latour,	 J.	M.	 Inorg.	Chem.	2006,	42,	
4817‐4827.	

[19]. Sakiyama,	 H.;	 Sugawara,	 A.;	 Sakamoto,	 M.;	 Unoura,	 K.;	 Inoue,	 K.;	
Yamasaki,	M.	Inorg.	Chim.	Acta	2000,	310,	163‐168.	

[20]. Sakiyama,	 H.;	 Igarashi,	 Y.;	 Nakayama,	 Y.;	 Hossain,	 M.	 J.;	 Unoura,	 K.;	
Nishida,	Y.	Inorg.	Chim.	Acta	2003,	351,	256‐260.	

[21]. Sakiyama,	H.;	Mochizuki,	R.;	 Sugawara,	A.;	 Sakamoto,	M.;	Nishida,	Y.;	
Yamasaki,	M.	J.	Chem.	Soc.,	Dalton	Trans.	1999,	6,	997‐1000.	

[22]. Head‐Gordon,	M.;	Pople,	J.	A.;	Frisch,	M.	J.	Chem.	Phys.	Lett.	1988,	153,	
503‐506.	

[23]. Frisch,	M.	J.;	Head‐Gordon,	M.;	Pople,	J.	A.	Chem.	Phys.	Lett.	1990,	166,	
275‐280.	

[24]. Frisch,	M.	J.;	Head‐Gordon,	M.;	Pople,	J.	A.	Chem.	Phys.	Lett.	1990,	166,	
281‐289.	

[25]. Head‐Gordon,	M.;	Head‐Gordon,	T.	Chem.	Phys.	Lett.	1994,	220,	 122‐
128.	

[26]. Saebo,	S.;	Almlof,	J.	Chem.	Phys.	Lett.	1989,	154,	83‐89.	
[27]. Kohn,	W.;	 Becke,	 A.	 D.;	 Parr,	 R.	 G.	 J.	Phys.	 Chem.	1996,	 100,	 12974‐

12980.	
[28]. Becke,	A.	D.	J.	Chem.	Soc.	1993,	98,	5648‐5652.	
[29]. Becke,	A.	D.	Phys.	Rev.	A	1988,	38,	3098‐3100.	
[30]. Hay,	P.	J.;	Wadt,	W.	R.	J.	Chem.	Phys.	1985,	82,	270‐283.	
[31]. Deacon,	G.	B.;	Phillips,	R.	J.	Coord.	Chem.	Rev.	1980,	33,	227‐250.	
[32]. Unoura,	 K.;	 Ooi,	 T.;	 Tanaka,	 K.;	 Iwase,	 A.	 J.	Electroanal.	Chem.	1988,	

252,	335‐343.	
[33]. Jaffé,	H.	H.	Chem.	Revs.	1953,	53,	191‐261.		
[34]. Hansch,	C.;	Leo,	A.;	Taft,	R.	W.	Chem.	Rev.	1991,	91,	165‐195.