Dinuclear seven-coordinate Mn(II) complexes: Effect of manganese(II)- hydroxo species on water exchange and superoxide dismutase activity

Article

pubs.acs.org/IC

Dinuclear Seven-Coordinate Mn(II) Complexes: Eﬀect of

Manganese(II)-Hydroxo Species on Water Exchange and Superoxide

Dismutase Activity

Dominik Lieb, Felix C. Friedel, Mirza Yawer, Achim Zahl, Marat M. Khusniyarov, Frank W. Heinemann,

and Ivana Ivanovic-Burmazovic*

́

Department of Chemistry and Pharmacy, University of Erlangen-Nurnberg, Egerlandstr. 1, 91058 Erlangen, Germany

̈

S

* Supporting Information

ABSTRACT: Two dinuclear seven-coordinate manganese(II) complexes

containing two pentaazamacrocyclic subunits, with imine or amine

functionalities, respectively, have been synthesized and characterized in the

solid state as well as in aqueous solutions of diﬀerent pH, by performing X-ray

structure analyses, SQUID, potentiometric, electron spray ionization-mass

spectrometry (ESI-MS), electrochemical, and ¹⁷O NMR water exchange

measurements (varying temperature and pressure), and by determination of

SOD activity. The two manganese(II) centers within the dinuclear structures

behave independently from each other and similarly to the manganese centers

in the corresponding mononuclear complexes. However, the dinuclear amine

complex possesses increased complex stability and acidity of the coordinated

water molecules (pK_a2= 8.92) in comparison to the corresponding

mononuclear analogue. This allowed us to observe a stable trans-aqua-

hydroxo-Mn(II) species in an aqueous solution and to study for the ﬁrst time the trans-eﬀect of the hydroxo group on the water

lability on any divalent metal center in general. The observed trans-labilizing eﬀect of the hydroxo ligand is much smaller than in

the case of aqua-hydroxo-M(III) trivalent metal species. Whether this is a general property of trans-aqua-hydroxo-M(II) species,

or if it is speciﬁc for Mn(II) and/or to the seven-coordinate structures, remains to be seen and motivates future studies. In

addition, an inﬂuence of the hydroxo ligand on the SOD activity of manganese(II) complexes could be evaluated for the ﬁrst time

as well. Compared with the mononuclear analogue, which is not able to form stable hydroxo species, our pH dependent studies

on the SOD activity of the dinuclear amine complex have indicated that the hydroxo ligand may promote protonation and release

of the product H₂O₂, especially in solutions of higher pH values, by increasing its pK_avalue.

of a labile, i.e., easily accessible coordination site is an important

prerequisite for the involvement of an inner-sphere electron

transfer mechanism in the activation of small molecules by

metal centers, as in the case of SOD catalysis.¹³The presence of

labile inner-sphere aqua ligands is also important for high

contrast agent eﬃciency of manganese(II) complexes, and the

higher the inner-sphere hydration number (q) is, the stronger

the relaxation enhancement is.¹⁴Thus, aqua-Mn(II) complexes,

especially those with more than one inner-sphere water

molecule, are interesting structural motifs, both for design of

eﬃcient SOD mimetics and MRI contrast agents. Besides

structural and stability requirements, solution properties of

these complexes are crucial for their bioapplication and deserve

special attention. In particular, lability of coordinated water

molecules, reﬂected in rates of water exchange on Mn(II), is an

important parameter that should be high enough to not limit

catalysis and reactivity, respectively. Due to the fact that Mn(II)

is quite labile and therefore oﬀers fast solvent exchange rates, it

INTRODUCTION

■

The redox rich chemistry of manganese centers is usually

related to their function in metalloenzymes¹and enzyme

mimetics,²their application in redox catalysis,³as well as

application as magnetic materials⁴and new battery technolo-

gies.⁵In the biomedical area manganese is associated with

oxidative stress protection,⁶imaging applications,^7,8and

supplementation of biomaterials,⁹as well as with neuro-

toxicity.¹⁰Whereas higher manganese oxidation states are

involved in the processes of water oxidation and hydrogen

peroxide disproportionation by the oxygen evolving complex

(OEP) and catalase,¹¹respectively, within the catalytic cycle of

superoxide dismutase (SOD) the metal center alternates

between the Mn(II) and Mn(III) states.¹²The Mn(II)

chemistry also plays a role in the development of medical

diagnostic tools of supposedly lower toxicity than commonly

used gadolinium(III) probes. High-spin Mn(II) can eﬃciently

enhance the relaxation rates of water protons (due to its large

spin number, fast water exchange rates and long electronic

relaxation times), which is a basic principle of action of contrast

agents for magnetic resonance imaging (MRI).⁸The existence

Received: August 7, 2012

A

dx.doi.org/10.1021/ic301714d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

is a challenge to design a complex that will also meet other

requirements, i.e., to have a maximum number of labile inner-

sphere water molecules and at the same time to be

thermodynamically stable under physiological conditions.²

One possible approach, yet not well exploited, is to utilize

dinuclear, or in general polynuclear, structural frameworks for

design of more stable manganese(II) complexes with a

potential biomedical application. Such a design platform

might bring other interesting features as well. In comparison

to mononuclear complexes, bigger and more rigid dinuclear

structures result in slower rotational dynamics that enhances

relaxivity. Furthermore, proximal chelation of two (or more)

manganese(II) centers causes a local concentration of para-

magnetic centers and may increase overall positive charge and

the number of inner-sphere aqua ligands within a molecular

entity. Increased hydrophobicity or higher positive charge

might inﬂuence distribution of a complex within subcellular

compartments and increase its accumulation in mitochon-

dria,^15,16which would be a favorable property for a potential

SOD mimetic. In general, an SOD activity or electronic

relaxivity comparable with that of a mononuclear complex of

similar structure could be achieved with solutions of lower

osmotic pressure by using dinuclear (or polynuclear)

complexes.

From a fundamental point of view, manganese dinuclear

structures are still a challenging research area. It is known that

electronic communication between manganese centers within

dinuclear structures has an inﬂuence on their redox properties.

However, what inﬂuence a close proximity of the second

manganese center has on other properties of dinuclear Mn(II)

complexes in aqueous solutions (i.e., stability, acid−base

equilibria, lability of aqua ligands, etc.) is less known. Only in

the case of the dinuclear [Mn₂(ENOTA)(H₂O)₂] complex

(Chart 1), where each manganese(II) center has one inner-

sphere water bound, detailed investigations in solution were

performed, including water exchange studies.¹⁷However, the

solution behavior of a Mn(II) dinuclear complex with more

than one inner-sphere water molecule is not reported yet. This

limits our understanding of how variation/binding of other

ligands inﬂuences kinetics of water exchange on Mn(II), which

is an important determinant concerning a compound's

biomedical application. On one hand, such knowledge allows

us to predict the eﬀects of target biomolecules¹⁸as well as other

small species inevitably present under physiological condi-

tions¹⁹on relaxivity/SOD activity of a dinuclear structure of

interest. On the other hand, this enables rational tuning and

optimization of water lability to create high relaxivity contrast

agents or eﬃcient SOD mimitics, respectively, based on the

dinuclear framework.

One of the basic eﬀects known to signiﬁcantly control the

substitution behavior of metal complexes in aqueous solutions

is the trans-labilizing eﬀect of a hydroxo group.²⁰Although it is

well established in the chemistry of trivalent aqua-hydroxo-

M(III) metal species, where it can increase a water lability up to

4 orders of magnitude,²¹to our best knowledge there is no

literature report on a trans-labilizing eﬀect of hydroxo ligands

on the water molecule coordinated to any divalent metal ion in

general. The only water exchange rate constant, k_ex, in the case

of M(II) aqua-hydroxo complexes that is known in the

literature is the exchange rate estimated for the water molecules

in the cis positions with respect to the hydroxo group in fac-

[Ru(CO)₃(OH)(H₂O)₂]⁺.²²Stable aqua-hydroxo-M(II) spe-

cies are not easily accessible in aqueous solutions due to the

lower acidity of the aqua-M(II) ions in comparison to aqua-

M(III), with their tendency to oxidize and to dimerize/

polymerize.²³Therefore, an interesting paradigm is to probe

dinuclear structural frameworks as a tool to tune acidity of the

aqua-M(II) ion and to create an aqua-hydroxo-Mn(II) species,

preferably more stable than a corresponding mononuclear

analogue. By way of comparison the [Mn₂(ENOTA)(H₂O)₂]

dinuclear complex exhibits a lower stability as compared to that

of the mononuclear analogue [Mn(NOTA)] (Chart 1).¹⁷

In general, trans-labilization of an aqua ligand aﬀects the

solution behavior of metal complexes, e.g., by enhancing the

reactivity of the metal center.²⁴Thus, it would be also

interesting to probe whether formation of trans-HO-Mn(II)-

OH₂species have an inﬂuence on the SOD activity of

manganese centers. Even more, dinuclear Mn-SOD mimetics

have not been investigated so far, which is an additional

motivation to pursue studies on manganese(II) complex with

two active centers.

Chart 1. Ligand Structures of Manganese Complexes Studied

or Discussed in This Work

In order to shed light on the above-mentioned aspects of

manganese(II) solution chemistry, we have synthesized

manganese(II) dinuclear complexes with more than one aqua

ligand per metal center and with a potential SOD activity. To

achieve the desired structure−reactivity properties we have

utilized pentaazamacrocyclic ligand scaﬀolds (L1 and L2 in

Chart 1), known to promote SOD activity and stability of the

Mn(II) complexes, respectively, in combination with a 2,2′,6,6′-

tetraacetyl-4,4′-bipyridine building block, originally applied for

the design of novel materials with characteristics of molecule-

based magnets.⁴The mononuclear analogues, with the Me₂-

Pyene and Me₂-Pyane ligands (Chart 1), were studied in detail

regarding the mechanism of their SOD catalysis, water

exchange kinetics, and thermodynamic stability,^25,26which

enables a direct comparison of the corresponding reactivity/

stability parameters. In the case of the amine structure we were

able to demonstrate that the dinuclear framework can oﬀer a

useful platform for increased stability of Mn(II) complexes that

at the same time possess two water molecules per metal center

with an increased acidity, resulting in the generation of uniquely

B

dx.doi.org/10.1021/ic301714d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Synthesis of 2,2′,6,6′-Tetraacetyl-4,4′-bipyridine (Abbreviated as Tabpy)

2861 (w), 1646 (s, CN), 1606 (m), 1458 (m), 1426 (m), 1378 (s),

1273 (m), 1107 (m), 875 (w), 729 (m), 640 (m), 443 (w).

stable aqua-hydroxo-Mn(II) species in the aqueous solution.

Therefore, we could examine for the ﬁrst time the trans-eﬀect of

the hydroxo group on water exchange in manganese(II)

complexes and consequently on its catalytic SOD activity,

which is generally a unique study in the chemistry of divalent

metal ions.

Synthesis of [Mn₂(L2)Cl₄]. [Mn₂(L2)Cl₄] was obtained by

reduction of [Mn₂(L1)Cl₄] with sodium borohydride similar to the

published procedure.²⁹[Mn₂(L1)Cl₄] (1.0 g, 1.1 mmol) was dissolved

in absolute ethanol (EtOH), and argon was bubbled through the

stirring solution for 15 min. Sodium borohydride (0.6 g, 16.0 mmol,

16 equiv) was added in four portions in 15 min time intervals. The

mixture was stirred for 1 h at room temperature and then for 8 h at 60

°C. Afterward, the solvent was removed under vacuum, and 30 mL of

1 M LiCl in methanol was added and stirred until all gas evolution

stopped. Methanol was removed, and 50 mL of brine was added to the

light brown residue. The aqueous solution was extracted 3 times with

DCM (dichloromethane) and 3 times with DCM/ethanol (90:10).

The combined organic phases were dried over anhydrous MgSO₄, and

after removal of solvent [Mn₂(L2)Cl₄] was obtained as a deep yellow

powder (0.41 g, 40%). Anal. Calcd for [Mn₂(L2)Cl₄]·4H₂O·EtOH: C,

41.66; H, 7.21; N, 15.18. Found: C, 41.79, H, 7.31, N, 15.21. IR

EXPERIMENTAL SECTION

■

General. All reactions were performed under an argon atmosphere.

Reagents and solvents that were commercially available were

purchased from Sigma-Aldrich or Acros Organics, were of p.a. (pro

analysi) grade, and were used without further puriﬁcation. Only

anhydrous solvents (purchased from Sigma Aldrich or Acros Organics)

were used for synthesis. Deionized Millipore water was used for all

measurements. Carlo Erba elemental analyzers 1106 and 1108 and a

Bruker Avance DPX 300 NMR spectrometer were used for elemental

and NMR analyses, respectively. IR spectra were recorded as KBr

pellets on a Mattson FT-IR 60 AR. Temperature-dependent magnetic

susceptibility data were recorded on an MPMS XL Quantum Design

SQUID magnetometer in the temperature range 2−300 K. The

experimental magnetic susceptibility data were corrected for under-

lying diamagnetism and temperature-independent paramagnetism

(TIP). The program JulX v.1.5 was used to ﬁt experimental data.²⁷

Synthesis of 2,2′,6,6′-Tetraacetyl-4,4′-bipyridine (Tabpy)

(Scheme 1). Tabpy was obtained using a somewhat modiﬁed

literature procedure.²⁸4,4′-Bipyridine (5 g, 32.05 mmol), pyruvic acid

(19.266 g, 224 mmol), and AgNO₃(0.414 g 2.4 mmol, dissolved in 5

mL of water) were dissolved in 620 mL of a 0.4 M H₂SO₄solution.

Ammonium persulfate (56.832 g, 256 mmol) was carefully added in

small portions (strong gas evolution!) and with intense stirring. The

mixture was stirred at room temperature overnight, and the resultant

white precipitate was ﬁltered oﬀ. A 5 mL portion of conc H₂SO₄was

added to the ﬁltrated solution, and the solution was subsequently

treated with the same amounts of pyruvic acid, AgNO₃, and

ammonium persulfate as for the ﬁrst time (vide supra). The mixture

was stirred again overnight at room temperature and the precipitate

ﬁltered. The collected solids were combined and puriﬁed by column

chromatography (ethyl acetate/hexane/dichloromethane in a ratio of

10/20/70, R_f= 0.8). The product was obtained as white thin needles

(1.35 g, 13%). Anal. Calcd for C₁₈H₁₆₁N₂O₄: C, 66.66; H, 4.97; N, 8.64.

Found: C, 66.59; H, 5.01; N 8.60. H NMR (400 MHz, CDCl₃, 25

°C): δ [ppm] = 2.78 (s, 12H, CH₃), 8.48 (s, 4H, ArH). ¹³C NMR

(100 MHz, CDCl₃, 25 °C): δ [ppm] = 25.68, 122.25, 146.69, 153.98,

(KBr): ν

̃

[cm⁻¹] 3583 (m), 2968 (w), 2921 (m), 2868 (w), 1606 (m),

1549 (m), 1455 (m), 1398 (s), 1321 (m), 1094 (m), 1048 (m), 1008

(w), 968 (m), 879 (w), 819 (m), 599 (w).

X-ray Crystal Structure Determinations for [Mn₂(L1)Cl₄] and

[Mn₂(L2)Cl₄]. Single crystals suitable for X-ray analysis of [Mn₂(L1)-

Cl₄] and [Mn₂(L2)Cl₄] were obtained by slow diﬀusion of Et₂O vapor

into a 1 mM methanol solution of the respective complex, standing for

several days in the refrigerator. Dark orange crystals were obtained for

[Mn₂(L1)Cl₄], and yellow crystals were obtained for [Mn₂(L2)Cl₄].

Data were collected on a Bruker Kappa APEX 2 IμS Duo

diﬀractometer with MoKα radiation (QUAZAR focusing Montel

optics, λ = 0.710 73 Å) at 100 K. Lorentz and polarization corrections

as well as semiempirical absorption corrections based on multiple

scans were applied to both data sets (SADABS 2.11, Bruker AXS,

2009). The structures of both manganese(II) complexes were solved

by direct methods and reﬁned using full-matrix least-squares

procedures on F²(SHELXTL NT 6.12, Bruker AXS, 2002). Both

complexes crystallized with four molecules of methanol per molecule

of complex, and all of the solvent molecules are disordered. In both

cases the dinuclear complex molecule is situated on a 2-fold

crystallographic rotation axis running through the two Mn(II) atoms

and the central atoms N1, N4, C1, and C9. One of the independent

chloro atoms in [Mn₂(L1)Cl₄] is disordered. Two alternative sites

were reﬁned with occupancies of 63(6)% and 37(6)% for Cl1 and

Cl1A, respectively. The ligand is subjected to disorder, with two

equally occupied sites being reﬁned for the atoms N6, C15, C16 and

N6A, C15A, and C16A. In [Mn₂(L2)Cl₄] both independent chloro

atoms are disordered. Two alternative sites were reﬁned with

occupancies of 56(10)% and 44(10)% for Cl1 and Cl1A and

71(3)% and 29(3)% for Cl2 and Cl2A, respectively. The ligand is

also subjected to disorder; two alternative sites were reﬁned resulting

in site occupancy factors of 61(2)% for the atoms N2, N3, C4, C7, C8

and 39(2)% for the atoms N2A, N3A, C4A, C7A, and C8A. DFIX,

SIMU, and ISOR restraints were applied in the reﬁnement of the

disordered structure parts. All hydrogen atoms were placed in

positions of optimized geometry; their isotropic displacement

parameters were tied to those of their corresponding carrier atoms

by a factor of 1.2 or 1.5.

198.76 . IR (KBr): ν

1363 (s), 1302 (m), 1236 (m), 1099 (m).

̃

[cm⁻¹] 3380 (m), 2360 (m), 1589 (s, CO),

Synthesis of [Mn₂(L1)Cl₄]. Tabpy (1.62 g, 5.0 mmol) was

suspended in 100 mL of methanol (MeOH), and MnCl₂·4H₂O (1.98

g, 10.05 mmol, dissolved in 20 mL methanol) was added with stirring.

The slurry was kept at 60 °C for 30 min after which triethylenetetra-

amine (3.24 mL, 1.46 g, 10.00 mmol in 25 mL methanol) was slowly

added via a dropping funnel. The mixture turned slowly orange and

was reﬂuxed for 7 h, after which a deep orange solution was obtained.

The solution was concentrated to about 30 mL and left in the

refrigerator overnight. The resultant orange precipitate was ﬁltered oﬀ,

washed with small amounts of ethanol, and dried. [Mn₂(L1)Cl₄] was

obtained as a deep orange powder (3.61 g, 80%). Anal. Calcd for

[Mn₂(L1)Cl₄]·4 H₂O·MeOH: C, 41.35; H, 6.27; N, 15.55. Found: C,

CCDC 838452 for [Mn₂(L1)Cl₄]·4MeOH, CCDC-838453 for

[Mn₂(L2)Cl₄]·4MeOH, and CCDC-838583 for tabpy contain the

supplementary crystallographic data. These data can be obtained free

40.97, H, 6.16, N, 15.76. IR (KBr): ν

̃

[cm⁻¹] 3392 (m), 2917 (w),

C

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Inorganic Chemistry

Article

of charge from The Cambridge Crystallographic Data Centre via www.

ccdc.cam.ac.uk/data_request/cif.

A summary of important crystallographic data, data collection and

additional 0.05 M NaCl as supporting electrolyte. All solutions were

degassed with nitrogen (1 min/mL), and during the measurements,

the nitrogen atmosphere was maintained. The sample concentration

was 1.0 mM for each complex, the scan rates were 100 mV/s, and the

experiments were performed at 20 °C.

reﬁnement details is presented in Table 1.

Potentiometric Titrations. Potentiometric measurements were

performed on a METROHM 702 SM Titrino in a jacketed, airtight

glass titration cell equipped with a combined pH glass electrode

(METROHM), N₂inlet and outlet, and a graduated 10 mL

microburet (METROHM). The electrode was ﬁlled with NaCl

instead of KCl to prevent precipitation of KClO₄and was calibrated

using three diﬀerent commercially available standard buﬀer solutions

of pH 4.0, 7.0, and 10.0. A carbonate-free 0.05 M sodium hydroxide or

perchloric acid solution was prepared with nitrogen-saturated

deionized Millipore water. The NaOH solution was standardized by

titrations with a 0.05 M potassium hydrogen phthalate solution and

then used for standardization of the perchloric acid solution. Gran’s

method³⁰was used to measure carbonate in the sodium hydroxide

standard solution (carbonate content 0.10%). All solutions were

adjusted to an ionic strength of 0.1 M with NaClO₄prior to the

titrations. The temperature was maintained by circulating thermo-

statted water through the outer jacket of the cell. All measurements

were carried out while a constant atmosphere of nitrogen was

maintained above the solution. The concentration of complexes was

1.0 mM for each titration. Species distribution calculations and

determination of the corresponding protonation equilibrium and

stability constants were performed using the computer program

TITFIT.³¹

Table 1. Crystallographic Parameters, Data Collection, and

Reﬁnement Details for [Mn₂(L1)Cl₄]·4MeOH and

[Mn₂(L2)Cl₄]·4MeOH

[Mn₂(L1)Cl₄]·4MeOH

[Mn₂(L2)Cl₄]·4MeOH

empirical formula

M_r

C₃₄H₆₀Cl₄Mn₂N₁₀O₄

924.60

C₃₄H₆₈Cl₄Mn₂N₁₀O₄

932.66

color and shape

cryst syst

space group

a (Å)

prism, dark orange

monoclinic

C2/c

prism, yellow

monoclinic

C2/c

20.051(4)

21.294(4)

10.968(2)

90

19.718(3)

21.431(3)

11.333(2)

90

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

V (Å³)

113.622(3)

90

112.073(4)

90

4290(2)

4

4438(2)

4

Z

D_calc(g cm⁻³

)

1.431

1.396

μ (mm⁻¹

)

0.886

0.857

F(000)

1936

1968

¹⁷O NMR Measurements. 10%-Enriched ¹⁷O-labeled water (D-

Chem Ltd. Tel Aviv, Israel) was used for the ¹⁷O NMR water-

exchange measurements. The complex samples were prepared by

combining weighed amounts of complex in an appropriate buﬀer

solution, i.e., a 1,4-piperazinediethanesulfonic acid buﬀer (PIPES) for

measurement at pH 7.0/7.5 and a N-cyclohexyl-3-aminopropanesul-

fonic acid buﬀer (CAPS) at pH 9.3, 10.0, and 11.0. These buﬀers were

chosen because they do not interfere with the ¹⁷O NMR dependent

measurements as their pK_avalues are neither pressure nor temperature

dependent.³²The resulting solution was transferred to the NMR tube

under an argon atmosphere. The pH was determined in a NMR tube

using a micro pH meter, Innovative Instruments, Inc. The

concentrations of the complexes were 10, 15, or 20 mM in 0.2 M of

the respective buﬀer solutions. Variable-temperature/pressure Fourier-

transform ¹⁷O NMR spectra were recorded at a frequency of 54.24

MHz and at a magnetic induction of 9.4 T on a Bruker Advance DRX

400WB spectrometer equipped with a superconducting BC-94/89

magnet system. The temperature dependence of the ¹⁷O-line

broadening was studied over the temperature range from 274.2 to

338.2 K, for [Mn₂(L1)Cl₄], and in the range from 274.2 to 358.2 K,

for compound [Mn₂(L2)Cl₄]. A homemade high-pressure probe

described in the literature³³was used for the variable-pressure

experiments, which were conducted at the speciﬁed temperature and

at ambient, 2, 30, 60, 90, 120, and 150 MPa pressures. A standard 5

mm NMR tube cut to a length of 50 mm was used for the sample

solutions. The pressure was transmitted by a movable macor piston,

and the temperature was controlled as described elsewhere.³³For all

measurements a reference spectra from the pure solvent without

complex (i.e., blank buﬀer at pH 7.0, 7.5, 9.3, 10.0, or 11.0) was ﬁrst

recorded to subtract the inﬂuence of the solvent on the water

exchange.

T

_min/T_max

0.667/0.746

5.7 ≤ 2Θ ≤ 54.2

18 538

0.675/0.746

5.4 ≤ 2Θ ≤ 52.8

29 816

2Θ interval (deg)

reﬂns collected

reﬂns unique

4726

4540

reﬂns obsd [I > 2σ(I)] 3440

3717

reﬁned params

311

318

ﬁnal R indices [I >

2σ(I)]

R1 = 0.0578, wR2 =

0.1484

R1 = 0.0547, wR2 =

0.1549

R indices (all data)

R1 = 0.0860, wR2 =

0.1649

R1 = 0.0666, wR2 =

0.1635

GOF on F²

Δρ_max/min(e Å⁻³

1.290

1.199

)

1.393/−0.863

0.855/−0.595

Electron-Spray-Ionization (ESI) Mass Spectrometry. High

mass accuracy ESI spectra were recorded on an ultrahigh-resolution

ESI-time-of-ﬂight MS, a Bruker Daltoniks (Bremen, Germany) maXis.

Spectra were obtained in positive-ion mode, with the capillary held at

4000 V. The drying gas ﬂow rate was 7.0 L min⁻¹with a temperature

of 180 °C for standard and 4 °C for the cryo-spray measurements at

higher pH. The nebulizer gas was at a pressure of 30.5 psi/2 bar. The

m/z range detected was from 100 to 2000. A calibration tune mix

(Agilent Technologies) was sprayed immediately prior to analysis to

ensure a high mass accuracy to assist in the identiﬁcation of peaks. The

ﬂow rate of the solutions was 300 μL/h. The samples were dissolved in

aqueous solutions of the corresponding pH values, and the complex

concentrations were 1 mM each. Peaks were identiﬁed with the aid of

the simulated isotopic patterns created within the Bruker DataAnalysis

software.

Electrochemistry. Cyclic voltammetry measurements were

performed using an Autolab instrument with a PGSTAT 30

potentiostat. A three-electrode arrangement was employed consisting

of a gold disk working electrode (A = 0.07 cm²) (Metrohm), a

platinum auxiliary electrode (Metrohm), and a Ag/AgCl, (LiCl 2 M in

EtOH) (Metrohm) reference electrode (0.164 V vs NHE at 20 °C),

for the measurements in DMSO, or a Ag/AgCl (NaCl 3 M)

(Metrohm) reference electrode (0.222 V vs NHE at 20 °C), for

measurements in the aqueous solution. The measurements in DMSO

were performed in the presence of 0.1 M tetrabutylammonium

hexaﬂuorophosphate as the supporting electrolyte, and the measure-

ments in aqueous solutions were performed in 0.05 M buﬀer solutions

at pH 7.0 (PIPES buﬀer) and at pH 10.0 (CAPS buﬀer) with

Direct Determination of SOD Activity via Stopped-Flow

Measurements. The SOD activity of the complexes was determined

by using direct rapid-scan UV−vis detection of superoxide

decomposition in aqueous media and global analysis of the entire

time-resolved spectra. This method requires a double mixer stopped-

ﬂow system equipped with a high-density mixer, for eﬃcient mixing of

aqueous buﬀer solutions containing the investigated complexes, with a

DMSO solution of KO₂(in a 10:1 ratio). This system is combined

with a fast diode array detector (0.5 ms integration time) and a laser

driven light source. The exact experimental method used for

determination of the SOD activity of the two novel dinuclear Mn(II)

D

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Scheme 2. Synthetic Route for the Dinuclear Complexes [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄]

Figure 1. Thermal ellipsoid representation of the molecular structure of [Mn₂(L1)Cl₄] in crystals of [Mn₂(L1)Cl₄]·4MeOH (top) and

[Mn₂(L2)Cl₄] in crystals of [Mn₂(L2)Cl₄]·4MeOH (bottom) with the applied numbering scheme (50% probability ellipsoids; hydrogen atoms,

disorder, and cocrystallized solvent molecules omitted for clarity).

complexes [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄] is published elsewhere.¹⁹

a diﬀerent method to synthesize tabpy, similar to an already

known procedure,²⁸where silver(I) was used as a radical

In general the measurements were carried out in 60 mM 4-(2-

hydroxyethyl)-1-piperazineethanesulfonic acid buﬀer (HEPES) at pH

7.4 or 8.1, respectively, or 60 mM CAPS buﬀer at pH 10.0, prepared

with Millipore water and treated with chelex 100 chelating resin for at

initiator instead of iron(II) (Scheme 1). The yield obtained is

somewhat higher than that reported in the literature,³⁵and the

least 24 h to catch trace metal ions. The complex was dissolved in a

buﬀer solution, previously treated with chelex 100. Truly catalytic

conditions were used with a high excess of superoxide (ca. 2.0 × 10⁻⁴

M)³⁴over the complexes, which were used in concentrations of 1.13,

2.25, 4.50, and 9.0 × 10⁻⁷M.

advantage of the applied method is an easier work up and

product puriﬁcation. Crystal structure and crystallographic data

for (tabpy) are shown in Supporting Information (Figure S1

and Table S1).

[Mn₂(L1)Cl₄] was synthesized in absolute MeOH under an

argon atmosphere via a template method³⁶from manganese(II)

chloride, triethylenetetramine, and tabpy. The reduced

complex, [Mn₂(L2)Cl₄], was obtained by treatment of

[Mn₂(L1)Cl₄] with sodium borohydride in absolute EtOH

under an argon atmosphere. The synthesis, molecular structure,

and selected bond lengths for both complexes are presented in

Scheme 2, Figure 1, and Table 2, respectively.

RESULTS AND DISCUSSION

■

Syntheses and Molecular and Crystal Structures.

Synthesis of 2,2′,6,6′-tetraacetyl-4,4′-bipyridine (tabpy) has

been reported previously, where Fe(II) was used as an initiator

in a radical-based synthesis.³⁵However, puriﬁcation of the

product is very diﬃcult due to a number of side products

obtained by that synthetic procedure. Therefore, we have used

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deviations from the corresponding planes in the saturated

amine complex [Mn₂(L2)Cl₄] are 0.26(2) Å for N3 and N3A

in the ﬁrst plane (Mn1N₅) and 0.216(4) Å for N6 and N6A in

the second plane (Mn2N₅). Due to the molecule’s position on

a 2-fold axis, in the case of both complexes, the two Mn(II)

atoms are to be found exactly within the MnN₅least-squares

plane. As it is observed for their mononuclear analogues,³⁷the

average Mn−N distances are shorter in the imine complex

[Mn₂(L1)Cl₄] as compared to the amine complex [Mn₂(L2)-

Cl₄]. A comparison of the Mn−N distances of mononuclear

[Mn(Me₂-Pyene)] (2.254(2)−2.316(2) Å)³⁷with dimeric

[Mn₂(L1)Cl₄] (2.253(4)−2.299(3) Å) yields very similar

values indicating no signiﬁcant diﬀerences in the Mn−N

bond lengths between the two complexes. The same trend is

found for [Mn(Me₂-Pyane)] with Mn−N distances ranging

from 2.278(3) to 2.352(3) Å and its corresponding dinuclear

complex [Mn₂(L2)Cl₄] with Mn−N bond lengths lying

between 2.264(4) and 2.371(11) Å. The dihedral angle

between the two planes of the pyridine rings of [Mn₂(L1)Cl₄]

and [Mn₂(L2)Cl₄] amount to 28.4(2)° and 19.8(2)°,

respectively, which is much smaller than the value of 41°

reported for [Mn(N₃O₂)]₂(Chart 1), a dinuclear compound of

similar structure with a macrocycle made up of an N₃O₂donor

set.⁴The Mn(II)···Mn(II) distance in [Mn₂(L1)Cl₄] is 11.18 Å,

similar to the corresponding distance of 11.2 Å observed in the

related [Mn(N₃O₂)]₂.⁴In the reduced form [Mn₂(L2)Cl₄], the

Mn(II)···Mn(II) distance is slightly larger (11.35 Å). The

crystal packing of [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄] can be

seen in Figures S3 and S4, respectively.

Table 2. Selected Bond Distances (Å) for Molecular

Structures of [Mn₂(L1)Cl₄]·4MeOH and

a

[Mn₂(L2)Cl₄]·4MeOH

[Mn₂(L1)Cl₄]

[Mn₂(L2)Cl₄]

Mn1−N1

Mn1−N2

2.256(4)

2.288(4)

2.299(3)

2.543(7)

2.576(8)

2.253(4)

2.290(3)

2.252(9)

2.5662(12)

2.5663(12)

2.264(4)

2.328(8)

2.371(11)

2.32(2)

Mn1−N2A/B

Mn1−N3

Mn1−N3A/B

Mn1−Cl1

Mn1−Cl1B

Mn2−N4

Mn2−N5

Mn2−N5A

Mn2−N6

2.32(2)

2.612(18)

2.539(15)

2.273(4)

2.322(4)

2.310(3)

2.593(3)

2.579(9)

Mn2−N6A

Mn2−Cl2

Mn2−Cl2A

a

Symmetry transformations used to generate equivalent atoms: −x +

1

1, y, −z + /₂.

In the crystal structures of [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄],

the dinuclear complex molecules are situated on 2-fold

crystallographic rotation axes running through the two

manganese atoms with the central atoms N2, N4, C2, and

C9 thus enforcing C₂molecular symmetry. In both complexes

the two manganese atoms are seven-coordinate in a pentagonal

bipyramidal environment as is also the case for the

mononuclear analogues^37,38and other manganese(II) com-

plexes containing pentadentate macrocyclic ligands in the

literature.^4,39The ﬁve nitrogen atoms of the macrocycle adopt

the equatorial positions, and the two chlorido ligands occupy

the axial positions resulting in a Cl−Mn−Cl angle close to 180°

with the biggest deviation from linearity of 6.3° (Cl2−Mn2−

Cl2B) observed in [Mn₂(L2)Cl₄] (see Table 3). The Mn−Cl

distances are similar for both compounds and lie in a range

between 2.54 and 2.61 Å. The macrocyclic units of the imine

complex [Mn₂(L1)Cl₄] are nearly planar with maximum

deviations from the MnN₅least-squares plane of 0.107(4) Å

for N3 and N3A in the ﬁrst plane (Mn1N₅) and 0.247(7) Å for

N6 and N6A in the second plane (Mn2N₅). The maximum

SQUID Measurements. Temperature dependent magnetic

susceptibility measurements for [Mn₂(L1)Cl₄] and [Mn₂(L2)-

Cl₄] were performed at the applied ﬁeld 1 T in the temperature

range 2−300 K (Figure 2). The experimental data were ﬁtted

using a spin-Hamiltonian approach, eq 1:⁴⁰

̂

H = μ_Bg₁S₁B + μ_Bg₂S₂B − 2JS₁S₂

(1)

The ﬁrst two terms represent an electron Zeeman eﬀect for

two paramagnetic centers, and the third term accounts for

intramolecular interactions between them. The g-values for all

high-spin d⁵ions were ﬁxed to 2.00, whereas the eﬀect of zero-

5

ﬁeld splitting operating on each S_Mn= /₂was omitted to

exclude overparameterization but to estimate the strength of

intramolecular Mn(II)···Mn(II) interactions.

Table 3. Selected Bond Angles (deg) and Torsion Angles

(deg) for Molecular Structures of [Mn₂(L1)Cl₄]·4MeOH

The eﬀective magnetic moment, μ_eff, of 8.0 μ_Bis almost

constant in the temperature range 40−300 K for [Mn₂(L2)-

a

and [Mn₂(L2)Cl₄]·4MeOH

Cl₄]. At lower temperatures it gradually decreases due to weak

[Mn₂(L1)Cl₄]

[Mn₂(L2)Cl₄]

5

antiferromagnetic intramolecular S_Mn

=

⁵/₂···S_Mn

=

/

2

Cl1−Mn1−Cl1B

N1−Mn1−N2

N1−Mn1−N2A/B

N2−Mn1−N3

N2A/B−Mn1−N3A/B

N3−Mn1−N3A

Cl2−Mn2−Cl2A/B

N4−Mn2−N5

N4−Mn2−N5A

N5−Mn2−N6

177.3(9)

69.35(8)

69.36(8)

73.08(12)

73.07(12)

75.55(18)

177.62(6)

69.42(7)

66.8(2)

175.0(10)

71.65(18)

74.7(5)

interactions and/or zero-ﬁeld splitting eﬀect. Complex

[Mn₂(L1)Cl₄] shows similar behavior, but with a lower plateau

at μ_eff= 7.8 μ_B. This is lower than the value calculated for two

5

noninteracting S =

/

centers (8.37 μB, spin only

2

71.5(7)

approximation). Thus, diamagnetic impurity was included in

simulations to account for the lower than predicted magnetic

moments. The diamagnetic impurities are likely due to

diﬃculty of removing solvent molecules left in the micro-

crystalline samples of [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄] (see

the elemental analyses and crystal structures).

75.5(5)

173.7(9)

70.28(8)

73.66(12)

73.72(18)

19.8(2)

N5A−Mn2−N6A

N6−Mn2−N6A

C2−C1−C9−C10

66.8(2)

Exchange coupling constants J were estimated to be smaller

than −0.1 cm⁻¹for both complexes (Figure 2). Obviously, the

coupling should be even weaker if one takes into account zero-

89.8(5)

28.4(2)

5

a

ﬁeld splitting eﬀect for S_Mn= /₂ions. Thus, according to the

Symmetry transformations used to generate equivalent atoms: −x +

1

1, y, −z + /₂.

results of the magnetic susceptibility measurements, the two

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[Mn₂(L2)Cl₄] it was necessary to determine the speciation of

both complexes in aqueous solutions. The axial chlorido ligands

are very labile and replaced by water molecules as was shown

by relaxivity³⁹and conductivity⁴¹measurements (under

conditions of [Cl⁻] = 250 mM)³⁹on complexes of a similar

structure.^17,39Recent studies have indicated coordination of

one chloride atom to the Mn(II) center of a mitochondria-

targeted SOD mimetic.¹⁶However, our ¹⁷O NMR measure-

ments in the presence of 300 mM chloride anions (see

Supporting Information Table S11) showed no signiﬁcant

diﬀerence in the line broadening in comparison to the

measurements in the absence of excess chloride concentrations

conﬁrming previous ﬁndings in similar systems (vide supra).

Thus, no indication of Cl⁻coordination to the manganese

centers in [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄] could be found

under the applied experimental conditions (vide infra).

The calculated species distribution obtained from the best ﬁts

to the titration data are shown in Figure 3. On the basis of the

number of species observed in the titration measurements, as

well as the large distance between the two Mn(II) centers in

the rigid ligand scaﬀolds, it can be concluded that the two

macrocyclic subunits within both dinuclear complexes behave

chemically identical to and independently from each other, as

indicated by the SQUID analysis. For the ﬁtting procedure the

ligand was also treated as being an adduct of two identical

mononuclear ligand motifs and not as a single dimeric unit,

since the latter data treatment gave much larger errors and

inaccurate pK values in the ﬁtting process. The free L2 ligand

protonation constants (pK₁, pK₂, and pK₃) and the pK_avalues

of the coordinated water molecules (pK_a1and pK_a2) for

[Mn₂(L2)(H₂O)₄]⁴⁺, as well as the constants for water addition

(pK(OH)₁and pK(OH)₂) to the imine bonds of [Mn₂(L1)-

(H₂O)₄]⁴⁺, together with the stability constants (log KM₂L and

log KM₂H₂L) of both complexes are summarized in Table 4.

Figure 2. Temperature dependence of the eﬀective magnetic moment

μ_effof a microcrystalline sample of [Mn₂(L1)Cl₄] (bottom) and

[Mn₂(L2)Cl₄] (top) recorded at 1 T. The solid line in red represents

the best ﬁt obtained with S₁= S₂

=

⁵/₂, g₁= g₂= 2.00 (ﬁxed);

diamagnetic impurities = 12.7%_weightfor [Mn₂(L1)Cl₄] with J = −0.04

cm⁻¹; for [Mn₂(L2)Cl₄], J = −0.06 cm⁻¹, diamagnetic impurities =

7.6%_weight

.

high-spin Mn(II) centers in both [Mn₂(L1)Cl₄] and

[Mn₂(L2)Cl₄] are essentially electronically uncoupled as it is

also seen in the only comparable dinuclear high-spin Mn(II)

complex known in the literature with a 4,4′-bipyridylic

backbone, namely [Mn(N₃O₂)]₂.⁴

Potentiometric Titrations. In order to study the water

exchange on the manganese(II) centers of [Mn₂(L1)Cl₄] and

Figure 3. Species distribution for the complexes [Mn₂(L1)(H₂O)₄]⁴⁺(top) and [Mn₂(L2)(H₂O)₄]⁴⁺(bottom) as a function of pH at 25 °C and

[complex] = 1 mM.

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Table 4. pMn Values, Stability Constants and pK Values for the Studied Dinuclear and Analogous Mononuclear Complexes and

Their Corresponding Ligands Determined by Potentiometric Titration at 25 °C

a

dinuclear complexes

monuclear complexes

[Mn₂(L1)(H₂O)₄]⁴⁺

[Mn₂(L2)(H₂O)₄]⁴⁺

[Mn(Me₂-Pyene)(H₂O)₂]²⁺

[Mn(Me₂-Pyane)(H₂O)₂]²⁺

b

pK₁

pK₂

pK₃

5.01

8.03

pK₁

pK₂

pK₃

5.24

9.35

9.96

10.81

pK(OH)₁

pK(OH)₂

pK_a1

8.64

pK(OH)₁

pK(OH)₂

pK_a1

7.62

9.84

10.10

8.92

11.09

5.51

8.83

6.1

pK_a2

c

log KM₂H₂L

log KMHL

5.66

10.96

4.9

d

e

logKM₂L

8.42

3.8

log KML

10.50

3.5

f

pMn

a

Values for [Mn(Me₂-Pyene)(H₂O)₂]²⁺from ref 25 and for [Mn(Me₂-Pyane)(H₂O)₂]²⁺from this work, see Supporting Information Figure S5.

b

c

Ligand pK values. In this case where the two macrocyclic subunits of the dinuclear complex behave chemically identically and independently from

d

e

f

each other KM₂H₂L = K_MHL= [MHL]/[ML][H]. KM₂L = [M₂L]/[M]²[L]. K_ML= [ML]/[M][L]. pMn = −log[Mn(II)]_free(values calculated for

c_complex= 1 mM and pH = 7.4).

For comparison, the corresponding values for the mononuclear

[Mn(Me₂-Pyene)(H₂O)₂]²⁺and [Mn(Me₂-Pyane)(H₂O)₂]²⁺

complexes are also given in Table 4. The dinuclear imine

complex shows solution behavior similar to its mononuclear

analogue [Mn(Me₂-pyene)(H₂O)₂]²⁺,²⁵and does not have a

stable protonated form. Protonation of the ligand would lead to

an imine hydrolysis,²⁵and consequently decomposition of the

pentadentate macrocyclic units, causing release of Mn(II) ions.

At pH 7.5 the predominant species in solution was found to be

the one with two aqua ligands attached to each manganese(II)

center, [Mn₂(L1)(H₂O)₄]⁴⁺. In the basic region, the base-

catalyzed water addition to the imine double bonds to the

carbinol derivatives occurs in two consecutive steps,²⁵

characterized by equilibrium constants K(OH)₁and K(OH)₂

(Table 4). It should be mentioned that each of these two steps

involves simultaneous addition of two water molecules, i.e., one

to each macrocyclic unit.

of the second set of aqua ligands from both macrocyclic

subunits, is signiﬁcantly higher than in [Mn(Me₂-Pyane)-

(H₂O)₂]²⁺(Figure S5) . In the case of mononuclear hydroxo

species, their formation is followed by complex decomposition

and MnO₂precipitation. On the other hand, ca. 85% of

[Mn₂(L2)(OH)₂(H₂O)₂]²⁺and 10% of [Mn₂(L2)(OH)₄] exist

at pH 10.0. To best of our knowledge, [Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺is the ﬁrst example of a stable trans-aqua-

hydroxo manganese(II) species in solution, a discovery that

highlights the potential for synthetically tuning the lability of

coordinated water molecules in manganese(II) complexes. In

comparison, neutral [Mn₂(ENOTA)(H₂O)₂], which is the only

reported dinuclear manganese(II) complex with inner sphere

water molecules, also forms hydroxo species (15% of

[Mn₂(ENOTA)(H₂O)₂], 25% of [Mn₂(ENOTA)(OH)-

(H₂O)]⁻, and 60% of [Mn₂(ENOTA)(OH)₂]^2‑at pH =

10.0).¹⁷Diﬀerent from [Mn₂(L2)(OH)₂(H₂O)₂]²⁺, the aqua

and hydroxo ligands do not belong to the coordination sphere

of the same manganese(II) center in [Mn₂(ENOTA)(OH)-

(H₂O)]⁻, where only one aqua ligand can be coordinated to a

manganese(II) center due to the octahedral geometry of this

complex.¹⁷

ESI Mass Measurements. To further characterize species

in aqueous solutions of diﬀerent pH values we performed

ultrahigh-resolution ESI mass spectrometry combined with cold

spray ionization. A complete overview for the main species of

[Mn₂(L2)Cl₄] observed at diﬀerent pH values is given in Table

5. Analysis of the spectra showed that the dinuclear amine

complex remained intact under the applied ESI conditions

(main species at m/z 769.2163 for [Mn₂(L2)Cl₃]⁺, Figure 4,

Table 5), whereas for the dinuclear imine complex [Mn₂(L1)-

Cl₄] a minor decomposition product could be identiﬁed even at

pH = 7.5 (see Supporting Information Figures S6 and S7). This

is in agreement with the lower stability of manganese(II) imine

complexes compared to their reduced amine analogues. Due to

the high aﬃnity of the manganese(II) centers toward chloride

anions under conditions within the mass spectrometer, the

main MS peaks at neutral pH values correspond to the chloride

adducts (Table 5, Figure 4). However, at pH 10.0 and 4 °C we

were able to identify the hydroxo species as well (m/z

348.1592, Table 5, Figure 5). Under the applied ESI conditions,

deprotonation of the secondary amine groups was also

observed at pH = 10.0 (m/z 330.1418 and 348.1362, Figure

The amine complex is more stable in acidic solutions than

the imine complex and has a stable protonated form ([Mn₂(L2

+ 2H)(H₂O)₄]⁶⁺with a maximal distribution at ca. pH = 5.0

(Figure 3 and Table 4), where protonation occurs at one of the

secondary amine groups of both macrocyclic subunits. Similar

species are observed in the same pH range with the analogous

mononuclear complexes, but in signiﬁcantly lower yields.²⁵

A

direct comparison of the stability constants for the dinuclear

and mononuclear complexes is not straightforward. Therefore,

we calculated pMn (−log[Mn]_free) values at pH = 7.4 and

[complex] = 1 mM (Table 4). The higher pMn value for

[Mn₂(L2)(H₂O)₄]⁴⁺(pMn = 6.1) than for [Mn(Me₂-Pyane)-

(H₂O)₂]²⁺(pMn = 4.9)¹⁷indicates an order of magnitude

smaller concentration of free Mn(II) ions in the solutions of

dinuclear complex than in the solutions of its mononuclear

analogue at pH 7.4, demonstrating that the dinuclear structure

provides a higher complex stability. This is diﬀerent from what

has been observed in the case of dinuclear [Mn₂(ENOTA)-

(H₂O)₂], which is less stable than the corresponding

mononuclear complexes, due to the reduced number of

carboxylate groups that thermodynamically stabilize the Mn(II)

center.¹⁷However, [Mn₂(L2)(H₂O)₄]⁴⁺is less stable than the

six-coordinate [Mn₂(ENOTA)(H₂O)] complex (pMn = 7.7).¹⁷

The acidity of the coordinated water molecules in

[Mn₂(L2)(H₂O)₄]⁴⁺, with pK_a1= 8.92 for deprotonation of

the ﬁrst set of aqua ligands and pK_a2= 11.09 for deprotonation

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and water-exchange parameters for the temperature and

pressure dependent measurements are summarized in Table

6. The temperature range for the measurements of the imine

complex was 274.2−338.2 K, since we observed some

decomposition of the complex at elevated temperatures,

especially at pH 9.3. In the case of the amine complex a

broader temperature window could be used (274.2−358.2 K)

since no decomposition was observed. The water exchange rate

constants are not signiﬁcantly diﬀerent for [Mn₂(L1)(H₂O)₄]⁴⁺

and [Mn₂(L1+2H₂O)(H₂O)₄]⁴⁺at pH 7.5 and 9.3, respec-

tively. They are also very close to the values obtained for the

mononuclear analogue at corresponding pH (Table 7), where

pH has no inﬂuence on the water exchange process, as well. In

the carbinol form of the L1 ligand (L1 + 2H₂O), the

electronegativity of the hydroxo group counters the decrease in

π-acceptor ability of the macrocycle, and as a result the carbinol

pentadentate system has similar electronic properties to the

imine macrocycle.²⁵A decreased π-acceptor ability of the amine

L2 ligand in comparison to imine L1 results in certain

labilization of the axially coordinated water molecules in

[Mn₂(L2)(H₂O)₄]⁴⁺. At pH 10.0, where the aqua-hydroxo

complex [Mn₂(L2)(OH)₂(H₂O)₂]²⁺is the predominant

species according to the potentiometric measurements (vide

supra), the observed exchange rate is higher than for the

tetraaqua species at pH 7.0. This conﬁrms the trans-aqua-

hydroxo coordination on both Mn(II), because otherwise, in a

hypothetical asymmetrical coordination with two water

molecules on one Mn(II) and two hydroxo groups on another

manganese(II) center, there would be no change in k_ex. The

species distribution and hydration number of the manganese

center is consistent with the typically observed ¹⁷O NMR line

broadening seen for similar complexes³⁹and is also aﬃrmed by

the fact that the water exchange at pH 10.0 for [Mn₂(L2)-

(H₂O)₄]⁴⁺would be much slower if calculated for four labile

aqua ligands instead of two and would not ﬁt the

experimentally obtained data. Additionally, the hyperﬁne

Table 5. UHR-ESI-MS Data at Various pH Values and

Temperatures Corresponding to the Intact Complexes of

a

[Mn₂(L2)X_aY_b]^z+

b

recorded m/z calcd m/z

peak assignement

pH

temp

769.2163

330.1418

339.1534

348.1362

348.1592

357.1416

366.1246

769.2170

330.1481

339.1537

348.1368

348.1591

357.1421

366.1252

[Mn₂(L2)Cl₃]⁺

7.0

180 °C

4 °C

[Mn₂(L2−2H⁺)]²⁺

[Mn₂(L2−H⁺)(OH)]²⁺

[Mn₂(L2−H⁺)Cl]²⁺

[Mn₂(L2)(OH)₂]²⁺

[Mn₂(L2)(OH)Cl]²⁺

[Mn₂(L2)Cl₂]²⁺

10.0

a

Only signals with a relative intensity ≥10% are given. (X = Cl, OH; Y

b

= Cl, OH; a = 0, 1, 2; b = 0, 1, 2; z = 1, 2). Simulation was performed

with the Bruker data analysis 4.0 software.

5, Table 5). Deprotonation of coordinated secondary amines is

known in the literature.⁴²

¹⁷O NMR Measurements. The obtained ¹⁷O NMR data

have been ﬁtted according to the equation developed by Swift

and Connick.⁴³(For full experimental data, details of data

treatment, and calculation of the water exchange parameters

summarized in Tables 6 and 7, see Supporting Information).

The water exchange measurements of the imine complex were

performed at pH 7.5 and 9.3, where the predominant species in

noncoordinating aqueous buﬀers are tetraaqua-species of the

complex in the imine [Mn₂(L1)(H₂O)₄]⁴⁺form and in the

carbinol [Mn₂(L1+2H₂O)(H₂O)₄]⁴⁺form resulting from water

addition to an imine double bound on each of the dinuclear

units, respectively (Figure 3). The measurements on the amine

complex were performed at pH 7.0, 10.0, and 11.0, where the

complex predominantly exists as the tetraaqua ([Mn₂(L2)-

(H₂O)₄]^{4 +}

) and diaqua-dihydroxo ([Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺) species, respectively (Figure 3). For the

data analysis the two manganese centers were considered to

behave independently, as supported by our SQUID and

potentiometric titration studies (vide supra). All rate constants

Figure 4. High accuracy mass spectrum of [Mn₂(L2)Cl₄] at 180 °C in aqueous solution pH 7.0. Main species at m/z 769.2163 is assigned to

[Mn₂(L2)Cl₃]⁺. Insert shows measured spectra (top) of main species and simulated isotopic pattern thereof (bottom).

I

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Figure 5. High accuracy mass spectrum at 4 °C in aqueous solution at pH 10.0. Insert: Two species at m/z 348.1362 and m/z 348.1592. Top is

measured spectra, middle is simulated isotopic pattern for [Mn₂(L2)(OH)₂]²⁺species, and bottom is simulated pattern for [Mn₂(L2−H⁺)Cl]²⁺.

Table 6. Best Fit Parameters for Water Exchange from Temperature Dependent ¹⁷O NMR Measurements

parameter

[Mn₂(L1) (H₂O)₄]⁴⁺[Mn₂(L1+2H₂O) (H₂O)₄]⁴⁺[Mn₂(L2)(H₂O)₄]⁴⁺[Mn₂(L2)(OH)₂(H₂O)₂]²⁺[Mn₂(L2)(OH)₂(H₂O)₂]²⁺

a

b

a

b

pH

7.5

9.3

7.0

10.0

7.1 0.6

11.0

3.0 0.5

c

[ × 10⁷s⁻¹

]

1.7 0.3

34.5 1.3

+9.2 3.0

1.0 0.3

1.8 0.3

1.5 0.3

36.6 1.1

+15.4 2.1

1.3 0.5

1.9 0.4

5.0 0.4

32.9 1.9

+12.8 6.1

1.7 0.6

2.8 0.3

6.1 0.2

298

k

ex

ΔH^⧧[kJ mol⁻¹

]

36.5 1.9

+27.8 7.1

1.2 0.5

1.4 0.6

6.0 0.3

44.9 3.2

+48.8 12.5

1.3 0.7

ΔS^⧧[J mol⁻¹K⁻¹

Δ [ ×10¹⁰rad s⁻¹

τ²_ν⁹⁸[ ×10⁻¹²s⁻¹

A/h [ ×10⁶s⁻¹

]

1.6 0.8

d

]

6.0

5.6 1.0

a

b

c

d

In 0.2 M PIPES buﬀer. In 0.2 M CAPS buﬀer. k_ex= 8.0 × 10⁷s⁻¹is estimated for pure [Mn₂(L2)(OH)₂(H₂O)₂]²⁺. For Mn₂L1 the hyperﬁne

coupling constant was ﬁxed to a reasonable value, i.e., the one obtained for Mn₂L2, since it could not be determined correctly with ﬁtting, as the

system is in the slow exchange region.

Table 7. Water Exchange Parameters and Exchange Rates Obtained by Temperature and Pressure Dependent ¹⁷O NMR

Measurements for [Mn₂(L1)(H₂O)₄]⁴⁺, [Mn₂(L2)(H₂O)₄]⁴⁺, and Mononuclear Mn(II) Complexes

298

compound

[Mn₂(L1)(H₂O)₄]⁴⁺

[Mn₂(L1+2H₂O)(H₂O)₄]⁴⁺

[Mn₂(L2)(H₂O)₄]⁴⁺

[Mn₂(L2)(OH)₂(H₂O)₂]²⁺

[Mn(Me₂-Pyene)(H₂O)₂]²⁺

[Mn(Me₂-Pyane)(H₂O)₂]²⁺

[Mn₂(ENOTA)(H₂O)₂]

[Mn(H₂O)₆]²⁺

pH

k

[ ×10⁷s⁻¹

]

ΔH^⧧[kJ mol⁻¹

]

ΔS^⧧[J mol⁻¹K⁻¹

]

ΔV^⧧[cm³mol⁻¹

]

ref

ex

a

7.5

1.7 0.2

1.5 0.3

5.0 0.4

7.1 0.3

34.5 1.3

36.6 1.1

32.9 1.9

36.5 1.9

44.9 3.2

35.9 1.7

39.5 1.1

37.2 0.4

33.3 0.6

20.5 1.6

32.9 1.3

+9.2 3.0

+4.1 0.2

this work

25

b

9.3

+15.4 2.1

+12.8 6.1

+27.8 7.1

+48.8 12.5

+23.6 5.8

+25.7 3.8

+27.6 1.5

+15.2 1.9

−28.0 6.0

+5.7 5.0

+3.8 0.2

c

7.0

+3.4 0.2

d

10.0

11.0

7.6

+5.4 0.3

e

3.0 0.5

n.d.

1.7 0.1

1.6 0.1

5.3 0.3

5.8 0.3

5.5 0.9

n.d.

9.5

+5.3 0.2

+5.2 0.7

+3.4 0.7

−10.7 1.1

−5.4 0.1

25

7.7

25

10.9

8.0

25

17

∼1

2.1 0.1

52

a

b

c

d

e

At 290.2 K. At 290.2 K. At 335.2 K. At 335.2 K. k_ex= 8.0 × 10⁷s⁻¹is estimated for pure [Mn₂(L2)(OH)₂(H₂O)₂]²⁺.

coupling constant (A/h) obtained via the ﬁtting of the reduced

transverse relaxation rates (see SI) is sensitive to the number

and the nature of the donor atoms in close proximity of the

paramagnetic Mn(II).^14,39The obtained value is consistent with

typical Mn(II)−O interaction values (4.2 −2.0 × 10⁶s⁻¹)^39,44

only under consideration that the number of exchanged water

moleculs for [Mn₂(L2)]^z+is n = 4 and n = 2 at pH = 7.0 and

pH = 10.0, respectively. All other possible combinations with

diﬀerent hydration numbers at the respective pH values

resulted not only in unreasonable 1/T_2rvalues (based on

J

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Figure 6. Plots of ln(1/T_2r) data for [Mn₂(L1)(H₂O)₄]⁴⁺(pH 7.5, top left) or [Mn₂(L1+2H₂O)(H₂O)₄]⁴⁺(pH 9.3, bottom left) and

[Mn₂(L2)(H₂O)₄]⁴⁺(pH 7.0, top right) or [Mn₂(L2)(OH)₂(H₂O)₂]²⁺(pH 10.0, bottom right) versus reciprocal temperature obtained from ¹⁷O

NMR measurements. For full experimental data and data treatment see SI.

literature values reported for Mn(II) systems) but also in

implausible scalar hyperﬁne coupling constants, vastly diﬀerent

from that already reported in literature for a Mn(II)−O

interaction.^39,47,50,52Furthermore, to conﬁrm the character of

species formed at pH higher than 10.0, we measured the rate of

water exchange at pH 11.0 (Figure S8 and Table S10). The

observed decrease of k_ex(Table 6) clearly indicates that under

these conditions deprotonation of the other two aqua ligands

occurs, resulting in a tetrahydroxo species that is substitution

inert within the time scale of the performed measurements. In

contrast, deprotonation of the amine L2 ligand instead of aqua

ligands would increase electronic density on Mn(II) and

consequently increase the exchange rate of the remaining aqua

ligands due to enhanced lability of the axial metal−water bonds,

which is a well-known eﬀect of spectator ligands with relatively

higher negative charge.⁴⁵Thus, the deprotonation of L2

detected by ESI MS is due to the experimental conditions

speciﬁc to those experiments. (Table 5, Figure 5).

Considering that at pH 10.0 there is 85% of [Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺, 5% of tetraaqua, and 10% of substitution

inert tetrahydroxo species (Figure 3), the water exchange rate

constant of the pure aqua-hydroxo complex is calculated to be

ca. 8 × 10⁷s⁻¹. Thus, for the ﬁrst time we could determine the

trans-eﬀect of the hydroxo group on the water exchange process

on a manganese(II) center. The trans-hydroxo ligand caused

just a 1.6-fold increase in the water exchange rate on Mn(II). In

general a signiﬁcant increase in exchange rate of the trans-aqua

ligand is observed for a variety of octahedral trans-aqua-hydroxo

trivalent metal species, for which water exchange is 10²to 10⁴

times faster than on corresponding trans-diaqua species.^46,47

(The Mn(III) aqua-hydroxo octahedral porphyrin complexes,

where also only ca. a 1.6-fold increase in the water exchange

rate was observed, are just an exception.)⁴⁸A possible reason

why there is a much smaller diﬀerence in aqua ligand exchange

between the [Mn₂(L2)(OH)₂(H₂O)₂]²⁺and [Mn₂(L2)-

(H₂O)₄]⁴⁺complexes is that the Mn(II)−OH₂bonds in the

[Mn₂(L2)(H₂O)₄]⁴⁺complex are longer than those found in

the trivalent metal complexes and are thus less sensitive to the

trans-hydroxide eﬀect. However, whether the weak trans-

labilization eﬀect of the hydroxo ligand in [Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺is characteristic of hepta-coordinate Mn(II)

or it is a more general feature of Mn(II) and/or divalent metal

centers remains to be seen.

The values obtained for activation enthalpies, ΔH^⧧, are all in

the same range as those reported in the literature for the

corresponding mononuclear complexes (Table 7).²⁵The values

for the activation entropies are all positive and similar to those

obtained for mononuclear analogues, which suggests a

dissociative interchange mechanism (I_d) for the water exchange

on seven-coordinate Mn(II) in general. In order to get a better

insight into the water-exchange mechanism of the two dinuclear

complexes we additionally performed pressure dependent ¹⁷O

NMR studies, as the derived activation volume, ΔV^⧧, is a much

more sensitive and reliable parameter for the elucidation of the

nature of water exchange mechanism, since usually the

activation entropy ΔS^⧧is subjected to a larger experimental

error.^47,49The pressure dependence of k_exis given by eq 2,

0

where k_exis the rate constant for the solvent exchange at zero

pressure and the activation volumes (Table 7) were obtained by

ﬁtting the data by eq 2 and to the equations describing 1/T_2r

relaxation (see SI). The parameters A/h and τ_vwere considered

pressure independent for the fast exchanging system, since the

calculated contribution of electronic relaxation 1/T_1eto 1/T_2r

at elevated temperatures is small (about 7% at 335.2 K for

K

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[Mn₂(L2)Cl₄]) and a pressure dependence of 1/T_1ewould

therefore have small to negligible eﬀects on the calculated

activation volume (see literature).^50,51

dissociative water exchange mechanism, I_d. The fact that ΔV^⧧

for [Mn₂(L2)(OH)₂(H₂O)₂]²⁺is only slightly bigger than that

for [Mn₂(L2)(H₂O)₄]⁴⁺is in accordance with the weak trans-

hydroxo eﬀect that is not prominent enough to induce a

mechanistic changeover from an interchange dissociative (I_d)

toward a pure dissociative (D) mechanism. To date, the

operation of a pure D-mechanism has not been reported for

exchange processes in Mn(II) species. Even on the negatively

charged pentagonal-bipyramidal [Mn(EDTA)(H₂O)]^2‑com-

plex, where the aqua ligand is positioned in the sterically

crowded equatorial plane containing ﬁve donor atoms, the

water exchange proceeds by an I_dmechanism (ΔV^⧧=+3.4

0.2 cm³/mol).⁵¹On the other hand, water exchange in the six-

coordinate Mn(II) species [Mn(H₂O)₆]^{2 +}and

[Mn₂(ENOTA)(H₂O)₂] favors an associative mechanism, as

characterized by a negative volume of activation (Table 7) due

to a relatively less steric crowding on the metal center.

⎛

= k_ex= k_e⁰_xexp⎜−

⎝

⎞

p⎟

⎠

1

τ_m

ΔV^⧧

RT

(2)

The pressure dependent studies on [Mn₂(L1)Cl₄] at pH 7.5

and 9.5 were performed in the slow exchange region (Figure 6)

at 288.2 K, where k_excan be approximated by eq 3a.

Consequently, the observed decrease in the transverse

relaxation rate with increasing pressure (Figure 7, top) indicates

Electrochemistry. To further investigate a possible

electronic communication between the two manganese(II)

centers within [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄], their redox

behavior in aqueous (60 mM PIPES buﬀer pH 7.0) and

anhydrous (DMSO) solutions was investigated. The cyclic

voltammogram of [Mn₂(L1)Cl₄] in aqueous solution shows an

oxidation peak at E_ox= 1.12 V versus NHE and a reduction

peak at E_red= 0.62 V versus NHE. The broadness of the peaks

along with the magnitude of the peak separation indicates an

irreversible redox behavior. In contrast, the mononuclear

[Mn(Me₂-Pyene)Cl₂] complex shows a reversible process

(E_ox= 0.88 V and E_red= 0.76 V vs NHE) which is assigned

to the Mn(II)/Mn(III) redox couple (Figure 8). For the amine

complexes, [Mn₂(L2)Cl₄] (E_ox= 1.15 V and E_red= 0.59 V vs

NHE) and its corresponding mononuclear analogue [Mn(Me₂-

Pyane)Cl₂] (E_ox= 1.14 V and E_red= 0.88 V vs NHE), similar

electrochemical behavior was observed (Figure 8). The

existence of a single oxidation and single reduction process in

the case of dinuclear complexes and comparison of the cathodic

and anodic peak intensities between mono- and dinuclear

complexes conﬁrm that the two manganese centers do not

signiﬁcantly communicate. Thus, the observed two-electron

anodic and cathodic processes are related to the oxidation and

reduction of two Mn centers within the dinuclear structures,

respectively. The large peak separation could be due to a

relatively low fexibility of the dinuclear complex which would

require a higher reorganization energy to adapt to the electronic

change and/or, in terms of an EC mechanism (electrochemical-

chemical mechanism),⁵³the deprotonation of the aqua ligands

upon oxidation.⁵⁴Due to the relatively broad peaks, the eﬀect

of pH on electrochemical behavior of dinuclear complexes

could not be unambiguously assessed by cyclic voltammetry in

aqueous solutions of higher pH. However, an increase of the

peak separation with increased pH could be observed for

[Mn₂(L2)Cl₄] at pH 10.0 (E_ox= 1.15 V and E_red= 0.56 V vs

NHE, Figure S11). Furthermore, the measurements in aprotic

DMSO demonstrated an even larger peak separation and a

prominent decrease of the corresponding cathodic peaks for

both [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄] (see Supporting

Information Figures S9 and S10). These results suggest that

protolytic reactions are involved in the underlying redox

processes of the L1 and L2 complexes.

Figure 7. Pressure dependence of the reduced ¹⁷O transverse

relaxation. Top: [Mn₂(L1)(H₂O)₄]⁴⁺(pH 7.5) and [Mn₂(L1+2H₂O)-

(H₂O)₄]⁴⁺(pH 9.3). Bottom: [Mn₂(L2)(H₂O)₄]⁴⁺(pH 7.0) and

[Mn₂(L2)(OH)₂(H₂O)₂]²⁺(pH 10.0). Solid lines represent least-

squares ﬁt to the experimental data. For full experimental details see

SI.

a dissociative water exchange process at both pH values, since

the water exchange rate slows down with increasing pressure.

The pressure dependent measurements for [Mn₂(L2)Cl₄] at

pH 7.0 and 10.0 were performed in the fast exchange region

(Figure 6), where the relationship of k_exto 1/T_2ris

approximated by eq 3b. Therefore, the increase of 1/T_2r

(Figure 7, bottom) indicates a slowdown of the water exchange

rate with increasing pressure, pointing toward a dissociative

exchange mechanism.

1

τ_m

≅

= k_ex

T

(3a)

2r

1

k_ex

≅ τ_mΔω²=

Δω²

SOD Activity. The dinuclear [Mn₂(L1)Cl₄] complex does

not show any catalytic activity for superoxide dismutation, in

accord with the known inactivity of mononuclear [Mn(Me₂-

Pyene)Cl₂].^25,55In contrast to [Mn₂(L1)Cl₄], the reduced

T

(3b)

2r

In all experiments the obtained activation volumes had small

positive values (Table 6 and 7) indicative for an interchange

L

dx.doi.org/10.1021/ic301714d | Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Plot of k_obsversus complex concentration for the reaction of

a 2.0 × 10⁻⁴M superoxide solution in DMSO with [Mn₂(L2)-

(H₂O)₄]⁴⁺in 60 mM HEPES buﬀer pH 8.1 at 21 °C and a DMSO-

buﬀer mixing ratio 1:10, respectively. Inset: Trace at 250 nm for the

reaction of 9.0 × 10⁻⁷M [Mn₂(L2)(H₂O)₄]⁴⁺with 2.0 × 10⁻⁴M KO₂

(black line) and self-decomposition of 2.0 × 10⁻⁴M KO₂under the

same conditions (red line).

than k_catat pH 10.0. A rate determining process (Scheme 3) has

been proposed in the literature,⁵⁷where a transient Mn(III)-

peroxo intermediate oxidizes additional superoxide followed by

fast protonation of the resultant Mn(II)-peroxide adduct. At

higher pH the protonation is generally less favorable and might

become involved in a rate determining step. Although there is

some evidence in the literature for a concerted proton−electron

transfer mechanism of the natural iron and manganese SOD

enzymes⁵⁸and a seven-coordinate iron SOD mimetic,⁵⁹in the

case of the herein studied class of pentaazamacrocyclic Mn(II)

complexes a role of the proton transfer in a rate determining

process within the catalytic SOD cycle is still unknown.

However, we can speculate, in the light of our ﬁndings, that the

existence of the hydroxo group in the coordination sphere of

our dinuclear Mn(II) complex at pH 10.0 would increase pK_a

of putative manganese(II)−peroxo intermediate. Consequently,

the protonation and H₂O₂release would be more feasible for

Figure 8. Cyclic voltammograms of [Mn₂(L1)Cl₄] (top, solid red line)

and [Mn₂(L2)Cl₄] (bottom, solid red line) in 60 mM PIPES buﬀer at

pH 7.0. Cyclic voltamogramms of the mononuclear complexes

[Mn(Me₂-Pyene)Cl₂] (top) and [Mn(Me₂-Pyane)Cl₂] (bottom) are

represented as dotted blue lines. All complexes were measured as 1

mM solutions under a N₂atmosphere. Scan rate = 100 mV/s,

temperature = 20 °C.

2‑

Mn(II)(OH)(O₂) than for the Mn(II)(O₂) species. This

may aﬀect the catalytic SOD activity, especially in solutions of

higher pH values, as our experiments have indicated.

These mechanistic considerations can explain the relatively

higher SOD activity per manganese center of [Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺than of [Mn(Me₂-Pyane)(H₂O)₂]²⁺at pH

10.0.

complex [Mn₂(L2)Cl₄] shows SOD activity, which was

determined at pH 7.4, 8.1, and 10.0 by direct stopped-ﬂow

measurements (Figure 9 and Tables 8 and S11−S14).¹⁹The

obtained catalytic rate constants were compared with those for

mononuclear [Mn(Me₂-Pyane)Cl₂] and with uncatalyzed

superoxide dismutation rate constants at corresponding pH,

as summarized in Table 8.

The observed SOD activity is approximately 2 times (within

experimental error limits) higher for the dinuclear complex

than for the mononuclear one at pH 7.4 and 8.1 (Table 8).

This is due to the fact that the two Mn(II) centers behave

independently from each other, as demonstrated by SQUID,

potentiometric, electrochemical, and water exchange measure-

ments. Under these conditions both complexes are predom-

inantly present in the solutions as aqua species. At pH 10.0,

where the dinuclear complex mostly exists in the [Mn₂(L2)-

(OH)₂(H₂O)₂]²⁺form, its k_catis more than double that of the

catalytic rate of the mononuclear analogue (Table 8). However,

faster water exchange in [Mn₂(L2)(H₂O)₄]⁴⁺compared to

mononuclear [Mn(Me₂-Pyane)] at pH 10.0 is not the reason

for this. Namely, dissociation of an aqua ligand is not the rate

determining step in the catalytic process, because the

corresponding second-order rate constant for water exchange

(k_ex/[H₂O] = 1.3 × 10⁶M⁻¹s⁻¹) is 1 order of magnitude lower

CONCLUSION

■

Two dinuclear seven-coordinate manganese(II) complexes

containing two pentaazamacrocyclic imine or amine-based

subunits have been synthesized and characterized in the solid

state and in aqueous solutions. Their solution behavior was

investigated in detail by studying their speciation, redox

behavior, water exchange at Mn(II) centers, and SOD activity.

On the basis of SQUID, potentiometric, electrochemical, and

water exchange measurements, the two manganese(II) centers

within the dinuclear structures of [Mn₂(L1)Cl₄] and

[Mn₂(L2)Cl₄] behave independently from each other and

similarly to the manganese centers in the corresponding

mononuclear complexes. However, the dinuclear amine

complex possesses increased complex stability and acidity of

the coordinated water molecules (pK_a1= 8.92) in comparison

to the corresponding mononuclear mononuclear analogue. This

M

dx.doi.org/10.1021/ic301714d | Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 8. Second-Order Rate Constants k_catat 21 °C for Catalytic Dismutation of Superoxide with [Mn₂(L2)Cl₄] and [Mn(Me₂-

Pyane)] and for Uncatalyzed Superoxide Dismutation at Diﬀerent pH Values

HEPES pH 7.4 [×10⁶M⁻¹s⁻¹

]

ref

HEPES pH 8.1 [×10⁶M⁻¹s⁻¹

]

ref

CAPS pH 10.0 [×10⁶M⁻¹s⁻¹

]

ref

[Mn₂(L2)Cl₄]

[Mn(Me₂-Pyane)]

uncatalyzed

47.0 0.2

27.3 0.1

∼0.50

this work

56

23.0 0.1

12.1 0.1

0.13 0.01

19

19.0 0.1

5.4 0.6

this work

0.011 0.001

−

Scheme 3. Oxidation of O₂by a Mn(III)-Peroxo

Intermediate and H₂O₂Release via a Proposed Mechanism

in the Literature⁵⁷

ACKNOWLEDGMENTS

■

Financial support by the DFG (SFB 583) and the intramural

grant from University Erlangen-Nurnberg (Emerging Field

Initiative: Medicinal Redox Inorganic Chemistry) is gratefully

acknowledged. The authors would also gratefully thank Oliver

̈

Troppner and Leanne C. Nye for recording the ESI-mass

̈

spectra.

allowed us to detect a stable trans-aqua-hydroxo-Mn(II) species

in an aqueous solution and to study for the ﬁrst time the trans-

eﬀect of the hydroxo group on the water lability of a divalent

metal center. The observed labilizing eﬀect of the trans-hydroxo

ligand is about 100- to 10 000-fold weaker compared to trans-

M(III)(OH)(H₂O).^20,21Furthermore, this suggest manipula-

tion of the electronic properties of polydentate ligands could be

used to bring pK_avalues of coordinated aqua ligands closer to

physiological pH (6.5−7.8). Such complexes can provide a

platform for the development of MRI contrast agents for in vivo

applications.⁶⁰In addition, manganese complexes with a

terminal aqua ligand play an important role in a number of

catalytic biorelevant processes, in particular those involving the

activation of small molecules.⁶¹Thus, manganese species with a

hydroxo ligand trans to a labile aqua ligand are valuable models

for studying the eﬀect of the hydroxo ligand on the activation of

small molecules by manganese centers. Performing such types

of studies we have observed that the SOD activity per Mn

center at pH 10.0 is about 1.8-fold higher in the case of our

dinuclear amine complex [Mn₂(L2)(OH)₂(H₂O)₂]²⁺than in

the case of the corresponding mononuclear complex [Mn(Me₂-

Pyane)] that is not able to form stable hydroxo species.

In conclusion, even if there is no signiﬁcant electronic

communication between the two manganese(II) centers within

the dinuclear complexes [Mn₂(L1)Cl₄] and [Mn₂(L2)Cl₄],

such structural framework aﬀects the stability of trans-aqua-

hydroxo species and consequently solution behavior of the

complexes, which can inﬂuence their catalytic SOD activity

especially at higher pH values. This study establishes baseline

speciation and water exchange in noncoordinating aqueous

buﬀer, useful for comparing to the situation under physiological

conditions where there is a high concentration of chloride and

phosphates, which can aﬀect complex speciation, acidity of any

aqua ligand and possibly their SOD activity.

REFERENCES

■

(1) Smith, S. J.; Hadler, K. S.; Schenk, G.; Hanson, G. R.; Mitic,

Metals in Biology; Springer: Berlin, 2010; Vol. 29.

́

N.

(2) (a) Riley, D. P. Schall, O. F. Advances in Inorganic Chemistry;

Elsevier Inc.: Amsterdam, 2007; Vol. 59. (b) Hicks, S. D.; Petersen, J.

L.; Bougher, C. J.; Abu-Omar, M. M. Angew. Chem., Int. Ed. 2011, 50,

699−702.

(3) De Boer, J. W.; Browne, W. R.; Brinksma, J.; Alsters, P. L.; Hage,

R.; Feringa, B. L. Inorg. Chem. 2007, 46, 6353−6372.

(4) Wang, J.; Slater, B.; Alberola, A.; Stoeckli-Evans, H.; Razavi, F. S.;

Pilkington, M. Inorg. Chem. 2007, 46, 4763−4765.

(5) Kim, K. J.; Park, M.−S.; Kim, J.-H.; Hwang, U.; Lee, N. J.; Jeong,

G.; Kim, Y.-J. Chem. Commun. 2012, 48, 5455−5457.

(6) Aguirre, J. D.; Culotta, V. C. J. Biol. Chem. 2012, 287, 13541−

13548.

́

̌ ̌ ́

B.; Lukes, I.; Toth, E. Eur. J. Inorg. Chem. 2012, 12,

(7) Drahos,

1975−1986.

(8) Pan, D.; Caruthers, S. D.; Senpan, A.; Schmieder, A. H.; Wickline,

S. A.; Gregory, M. L. WIREs Nanomed. Nanobio. 2011, 3, 162−173.

(9) (a) Lu, J.; et al. Biomaterials 2009, 30, 2919−2928. (b) Bae, K.

H.; Lee, K.; Kim, C.; Park, T. G. Biomaterials 2011, 32, 176−184.

(10) Quintanar, L. Inorg. Chim. Acta 2008, 361, 875−884.

(11) (a) McConnell, I. L.; Grigoryants, V. M.; Scholes, C. P.; Myers,

W. K.; Chen, P.-Y. C.; Whittaker, J. W.; Brudvig, G. W. J. Am. Chem.

Soc. 2012, 134, 1504−1512. (b) Poulsen, A. K.; Rompel, A.;

McKenzie, C. J. Angew. Chem., Int. Ed. 2005, 44, 6916−6920.

(12) Ivanovic-

Inc.: San Diego, 2008; Vol. 60.

(13) Ivanovic-Burmazovic, I.; van Eldik, R. J. Chem. Soc., Dalton

Trans. 2008, 5259−5275.

(14) Laurent, S.; Henoumont, C.; Vander Elst, L.; Muller, R. N. Eur.

J. Inorg. Chem. 2012, 12, 1889−1915.

(15) Batinic-

Signaling 2010, 13, 877−918.

(16) Kelso, G. F.; Maroz, A.; Cocheme,

́ ́

Burmazovic, I. Advances in Inorganic Chemistry; Elsevier

́

Haberle, I.; Rebouca

̧

s, J. S.; Spasojevic,

́

I. Antioxid. Redox

́

H. M.; Logan, A.; Prime, T.

A.; Peskin, A. V.; Winterbourn, C. C.; James, A. M.; Ross, M. F.;

Brooker, S.; Porteous, C. M.; Anderson, R. F.; Murphy, M. P.; Smith,

R. A. J. Chem. Biol. 2012, 19, 1237−1246.

ASSOCIATED CONTENT

■

S

* Supporting Information

Available are crystallographic data. ¹⁷O NMR data and data

treatment, stopped-ﬂow data, electrochemical data, and mass

spectra of relevant compounds. This material is available free of

charge via the Internet at http://pubs.acs.org.

́

(17) Balogh, E.; He, Z.; Hsieh, W.; Liu, S.; Toth, E. Inorg. Chem.

2007, 46, 238−250.

(18) Caravan, P.; Zhang, Z. Eur. J. Inorg. Chem. 2012, 12, 1916−

1923.

(19) Friedel, F. C.; Lieb, D.; Ivanovic-

2012, 109, 26−32.

́ ́

Burmazovic, I. J. Inorg. Biochem.

AUTHOR INFORMATION

Corresponding Author

*E-mail: ivana.ivanovic@chemie.uni-erlangen.de. Fax: +49

9131 8527345. Phone: +49 9131 8525428.

■

(20) (a) Kowall, T.; Caravan, P.; Bourgeois, H.; Helm, L.; Rotzinger,

F. P.; Merbach, A. E. J. Am. Chem. Soc. 1998, 120, 6569−6577.

(b) Nordin, J. P.; Sullivan, D. J.; Phillips, B. L.; Casey, W. H. Inorg.

Chem. 1998, 37, 4760−4763.

Notes

(21) Laurenczy, G.; Rapaport, I.; Zbinden, D.; Merbach, A. E. Magn.

Reson. Chem. 1991, 29, 45−51.

The authors declare no competing ﬁnancial interest.

N

dx.doi.org/10.1021/ic301714d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

(22) Meier, U. C.; Scopelltiti, R.; Solari, E.; Merbach, A. E. Inorg.

Chem. 2000, 39, 3816−3822.

(52) Ducommun, Y.; Newman, K. E.; Merbach, A. E. Inorg. Chem.

1980, 196, 344−349.

(53) An electrochemical−chemical mechanism describes a mecha-

nism where the product of the ﬁrst redox event evolves chemically. For

further literature see: Bard, A. J.; Faulken, L. R. Electrochemical

Methods, Fundamentals and Applications; Wiley: New York, 2001.

(23) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley:

New York, 1976.

(24) (a) Drljaca, A.; Zahl, A.; van Eldik, R. Inorg. Chem. 1998, 37,

3948−3953. (b) Coe, B. J.; Glenwright, S. J. Coord. Chem. Rev. 2000,

203, 5−80. (c) Houston, J. R.; Yu, P.; Casey, W. H. Inorg. Chem. 2005,

44, 5176−5182.

(54) Ghachtouli, S. E.; Guillot, R.; Dorlet, P.; Anxolabeh

́ ̀

ere-Mallart,

E.; Aukauloo, A. Dalton Trans. 2012, 41, 1675−1677.

(55) Riley, D. P.; Weiss, R. H. J. Am. Chem. Soc. 1994, 116, 387−388.

(56) Riley, D. P.; Rivers, I. W. J.; Weiss, R. H. Anal. Biochem. 1991,

196, 344−349.

(25) Dees, A.; Zahl, A.; Puchta, R.; van Eikema Hommes, N. J. R.;

Heinemann, F. W.; Ivanovic-

2459−2470.

́ ́

Burmazovic, I. Inorg. Chem. 2007, 46,

(57) Maroz, A.; Kelso, G. F.; Smith, R. A. J.; Ware, D. C.; Anderson,

R. F. J. Phys. Chem. A 2008, 112, 4929−4935.

(26) (a) Riley, D. P.; Henke, S. L.; Lennon, P. J.; Weiss, R. H.;

Neumann, W. L.; Rivers, W. J., Jr.; Aston, K. W.; Sample, K. R.;

Rahman, H.; Ling, C.-S.; Shieh, J.-J.; Busch, D. H.; Szulbinski, W.

Inorg. Chem. 1996, 35, 5213−5231. (b) Riley, D. P.; Henke, S. L.;

Lennon, P. J.; Aston, K. Inorg. Chem. 1999, 38, 1908−1917.

(58) Carrasco, R.; Morgenstern-Badarau, I.; Cano, J. Inorg. Chim.

Acta 2007, 360, 91−101.

́ ́

(59) Sarauli, D.; van Eldik, R.; Ivanovic-Burmazovic, I. BioInorg.

React. Mech. 2012, 8, DOI: 10.1515/irm-2012-0008.

(27) Bill, E. JulX version 1.5, MPI for Bioinorganic Chemistry; Mulheim

̈

(60) Jasanoff, A. Curr. Opin. Neurobiol. 2007, 17, 593−600.

(61) (a) Gupta, R.; Borovik, A. S. J. Am. Chem. Soc. 2003, 125,

13234−13242. (b) Galstyan, A.; Robertazzi, A.; Knapp, E. W. J. Am.

Chem. Soc. 2012, 134, 7442−7449.

an der Ruhr, Germany, 2008.

(28) (a) Fontana, F.; Minisci, F.; Barbosa, M. C. N.; Vismara, E. J.

Org. Chem. 1991, 56, 2866−2869. (b) Zong, R.; Zhou, H.; Thummel,

R. P. J. Org. Chem. 2008, 73, 4334−4337.

(29) Aston, K. W.; Rath, N.; Naik, A.; Slomczynska, U.; Schall, O. F.;

Riley, D. P. Inorg. Chem. 2001, 40, 1779−1789.

(30) (a) Gran, G. Analyst 1952, 77, 661−671. (b) Gran, G. Anal.

Chim. Acta 1988, 206, 111−123. (c) Martell, A. E.; Motekaitis, R. J.

Determination and Use of Stability Constants; VCH: Weinheim,

Germany, 1992.

(31) Zuberbuhler, A. D.; Kaden, T. A. Talanta 1982, 29, 201−206.

̈

(32) Kitamura, Y.; Itoh, T. J. Solution Chem. 1987, 16, 715−725; J.

Phys. Chem. Ref. Data 2002, 31, 231−370.

(33) Zahl, A.; Neubrand, A.; Aygen, S.; van Eldik, R. Rev. Sci. Instrum.

1994, 65, 882−886.

(34) Duerr, K.; Macpherson, B. P.; Warratz, R.; Hampel, F.; Tuczek,

F.; Jux, N.; Ivanovic-

4228.

́ ́

Burmazovic, I. J. Am. Chem. Soc. 2007, 129, 4217−

(35) (a) Barbaro, P.; Bianchini, C.; Giambastiani, G.; Rios, I. G.;

Meli, A.; Oberhauser, W.; Segarra, A. M.; Sorace, L.; Toti, A.

Organometallics 2007, 26, 4639−4651. (b) Citterio, A.; Arnoldi, A.;

Macerì, C. Chim. Ind. 1978, 60, 14−15.

(36) Brooker, S.; McKee, V. J. Chem. Soc., Dalton Trans. 1990, 2397−

2401.

(37) Omar, J.-S.; Daniel, R.-R.; del María, J. R.-H.; Martha, E. S.-T.;

Rafael, Z.-U. J. Chem. Soc., Dalton Trans. 1998, 10, 1551−1556.

(38) Liu, G.-F.; Filipovic,

Burmazovic,

(39) Drahos,

Chem. 2010, 49, 3224−3238.

́

M.; Heinemann, F. W.; Ivanovic-

́

I. Inorg. Chem. 2007, 46, 8825−8835.

̌

B.; Kotek, J.; Hermann, P.; Lukes,

̌

I.; Tot

́

h, E. Inorg.

(40) Kahn, O. Molecular Magnetism; VCH: New York, 1993.

(41) Jackels, S. C.; Durham, M. M.; Newton, J. E.; Henninger, T. C.

Inorg. Chem. 1992, 31, 234−239.

(42) (a) McQuade, L. E.; Pluth, M. D.; Lippard, S. J. Inorg. Chem.

2010, 49, 8025−8033. (b) Huang, J.; Xu, Y.; Qian, X. Org. Biomol.

Chem. 2009, 7, 1299−1303.

(43) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307−320.

(44) Maigut, J.; Meier, R.; Zahl, A.; van Eldik, R. Inorg. Chem. 2008,

47, 5702−5719.

(45) Schneppensieper, T.; Zahl, A.; van Eldik, R. Angew. Chem., Int.

Ed. 2001, 40, 1678−1680.

(46) Cusanelli, A.; Frey, U.; Richens, D. T.; Merbach, A. E. J. Am.

Chem. Soc. 1996, 118, 5265−5271.

(47) Helm, L.; Mehrbach, A. E. Chem. Rev. 2005, 105, 1923−1959.

́ ́

(48) Lieb, D.; Zahl, A.; Shubina, T. E.; Ivanovic-Burmazovic, I. J. Am.

Chem. Soc. 2010, 132, 7282−7284.

(49) Van Eldik, R. Inorganic High Pressure; Elsevier Inc.: Amsterdam,

1986.

(50) Drahos,

̌ ̌ ́

B.; Kotek, J.; Císaro I.; Hermann, P.; Helm, L.;

va,

́

Lukes, I.; Tot

̌

́

h, E. Inorg. Chem. 2011, 50, 12785−12801.

(51) Maigut, J.; Meier, R.; Zahl, A.; van Eldik, R. J. Am. Chem. Soc.

2008, 130, 14556−14569.

O

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Dinuclear seven-coordinate Mn(II) complexes: Effect of manganese(II)- hydroxo species on water exchange and superoxide dismutase activity

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