Tuning tetranuclear manganese-oxo core electronic properties: Adamantane-shaped complexes synthesized by ligand exchange

Inorg. Chem. 2005, 44, 5161−5175

Tuning Tetranuclear Manganese−Oxo Core Electronic Properties:

Adamantane-Shaped Complexes Synthesized by Ligand Exchange

|

Christopher E. Dube´,^†,‡Sumitra Mukhopadhyay,^†,§Peter J. Bonitatebus, Jr.,^†,

Richard J. Staples, and William H. Armstrong*^,†

Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston College, Chestnut Hill,

Massachusetts 02467-3860, and Department of Chemistry and Chemical Biology, HarVard

UniVersity, Cambridge, Massachusetts 02138

Received February 3, 2005

+

A series of adamantane-shaped [Mn₄O₆]⁴aggregates has been prepared. Ligand substitution reactions of [Mn₄O₆-

(bpea)₄](ClO₄)₄(1) with tridentate amine and iminodicarboxylate ligands in acetonitrile affords derivative clusters

[Mn₄O₆(tacn)₄](ClO₄)₄(4), [Mn₄O₆(bpea)₂(dien)₂](ClO₄)₄(5), [Mn₄O₆(Medien)₄](ClO₄)₄(6), [Mn₄O₆(tach)₄](ClO₄)₄(7),

[Mn₄O₆(bpea)₂(me-ida)₂] (8), [Mn₄O₆(bpea)₂(bz-ida)₂] (9), [Mn₄O₆(bpea)₂(^tbu-ida)₂] (10), and [Mn₄O₆(bpea)₂(^cpent-

ida)₂] (11) generally on the order of 10 min with retention of core nuclearity and oxidation state. Of these complexes,

only 4 had been synthesized previously. Characterization of two members of this series by X-ray crystallography

reveals that compound 7 crystallizes as [Mn₄O₆(tach)₄](ClO₄)₄

and compound 11 crystallizes as [Mn₄O₆(bpea)₂(^cpent-ida)₂]

7MeOH in the monoclinic space group C2/c. The unique

substitution chemistry of 1 with iminodicarboxylate ligands afforded asymmetrically ligated complexes 8 11, the

mixed ligand nature of which is most likely unachievable using self-assembly synthetic methods. A special feature

‚3CH₃CN‚4.5H₂O in the cubic space group Fm3hm

‚

−

of the iminodicarboxylate ligand complexes 8

−

11 is the substantial site differentiation of the oxo bridges of the

[Mn₄O₆]⁴cores. While there are four site-differentiated oxo bridges in 8, the solution structural symmetry of 8H⁺

reveals essentially a single protonation isomer, in contrast to the observation of two protonation isomers for 1H⁺,

one for each of the site-differentiated oxo bridges in 1. Magnetic susceptibility measurements on 4, 7, 8, and 9

indicate that each complex is overall ferromagnetically coupled, and variable-field magnetization data for 7 and 9

+

are consistent with an S

) 6 ground state. Electrochemical analysis demonstrates that ligand substitution of bpea

affords accessibility to the Mn^V(Mn^IV)₃oxidation state.

Introduction

of the OEC include a compact, magnetically coupled tetra-

nuclear manganese-oxo complex, Ca²⁺and Cl^-cofactors,

and a redox-active tyrosine residue, Y_z.^1-6Despite the host

of biophysical methods used to examine the OEC, an accurate

model for the active site manganese aggregate has not been

forthcoming. X-ray crystal structures of PSII from cyano-

bacteria Thermosynechococcus elongatus⁷and Thermosyn-

echococcus Vulcanus⁸reveal density attributed to the man-

ganese cluster for the first time. Pertinent electron density

The molecular oxygen essential to life on earth is produced

by the light-driven catalytic oxidation of water by photo-

system II (PSII). Simultaneously, the oxygen-evolving

complex (OEC) of PSII supplies the reducing equivalents

employed by PSI in providing most of life’s chemical energy.

The OEC stores oxidizing equivalents in a series of steps

(S-states) known as the Kok cycle, culminating in the

oxidation of water and the release of O₂. Key components

(1) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong, W. H.

Chem. ReV. 2004, 104, 3981-4026.

* E-mail: armstwi@bc.edu.

^†Department of Chemistry, Eugene F. Merkert Chemistry Center, Boston

College, Chestnut Hill, Massachusetts 02467-3860.

(2) Barber, J. Q. ReV. Biophys. 2003, 36, 71-89.

(3) Carrell, T. G.; Tyryshkin, A. M.; Dismukes, G. C. J. Biol. Inorg. Chem.

2002, 7, 2-22.

^‡Present address: Evergreen Solar, Marlborough, Massachusetts 01752.

^§Present address: Department of Chemistry, Massachusetts Institute of

Technology, Cambridge, Massachusetts 02139.

(4) Siegbahn, P. E. M. Inorg. Chem. 2000, 39, 2923-2935.

(5) Penner-Hahn, J. E., Structural Characterization of the Mn Site in the

Photosynthetic Oxygen-EVolVing Complex; Springer-Verlag: Berlin,

1998; Vol. 90, pp 1-36.

(6) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. ReV. 1996, 96, 2927-

2950.

^|Present address: Emerging Technologies Laboratory PSCT, GE Global

Research Center CEB 138, Niskayuna, NY 12309.

Department of Chemistry and Chemical Biology, Harvard University,

Cambridge, Massachusetts 02138.

10.1021/ic050183w CCC: $30.25

Published on Web 06/17/2005

Inorganic Chemistry, Vol. 44, No. 14, 2005 5161

Dube´ et al.

data are consistent with three manganese ions arranged in

an approximate isosceles triangle, with a fourth manganese

ion located near the center of the triangle. Electron density

data for T. Vulcanus also supports the assignment of the

C-terminal carboxyl group of D1 Ala344 as a ligand to the

manganese aggregate, as well as a number of other D1

aspartate, glutamate, and histidine residues as additional

possible ligands. More recently the structure of PSII from

T. elongatus identified the electron density attributed to four

manganese ions and a fifth metal ion, which was modeled

as Ca²⁺.⁹Similar to the previous study with T. elongatus,

several D1 aspartate, glutamate, and histidine residues were

proposed as possible ligands for the manganese cluster, as

well as a CP43 aspartate, although in the present study the

C-terminal carboxyl group of Ala344 is not coordinated to

the metal cluster. It should be noted, however, that uncertain-

ties associated with these X-ray structures, as well as possible

irradiation effects,¹⁰make it difficult to assign atom con-

nectivity within the manganese aggregate as well as between

the metal cluster and associated peptide residues. Mn‚‚‚Mn

distances in these structures are approximately 3 Å. These

data are similar to those obtained with extended X-ray

absorption fine structure (EXAFS) studies of the S₂state,

which indicate at least two 2.7 Å Mn‚‚‚Mn vectors, as well

as a longer 3.3 Å Mn‚‚‚Mn (or Mn‚‚‚Ca) vector.^6,11-14

Although precise details of the ligand environment remain

an area of active investigation, it is believed that the Mn

complex is coordinated predominantly by oxide and car-

boxylates (i.e. aspartate and glutamate) and one or two

imidazoles.^8,9,15-17

[Mn₄(µ-O)₄(OH)₂]⁶⁺;²⁵open chain [Mn₄(µ-O)₆]^4+26-28and

[Mn₄(µ-O)₆]³⁺cores;²⁹the cubane-like [Mn₄(µ₃-O)₃(µ₃-

35

X)]⁴⁺,^30-33[Mn₄(µ₃-O)₄]⁶⁺,^34,

and [Mn₄(µ₃-O)₄]⁷⁺;³⁶the

butterfly-shaped [Mn₄(µ₃-O)₂(µ-OAc)₇]⁺;^37-40a stacked dimer-

4+ 41-43

of-dimers [Mn₂(µ-O)₂(µ-OR)]₂

,

an open dimer-of-

dimers [Mn₄(µ-O)₅]⁶⁺;⁴⁴and an open species [Mn₄(µ-

O)₅]⁶⁺.⁴⁵

Consideration of PSII biophysical studies as well as of

the coordination chemistry of manganese has led to a number

of proposals for the mechanism of photosynthetic O₂

evolution. Of particular note here are EXAFS studies

comparing the S₂and S₃states. EXAFS data suggest that

advancement from S₂to S₃is accompanied by structural

changes in the PSII Mn₄complex, although these studies

have led to very different conclusions. In one study, these

changes have been interpreted in terms of formation of an

additional µ-oxo bridge between Mn atoms.^46,47In another

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5162 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

study EXAFS data were interpreted in terms of an increase

in the PSII Mn₄metal-metal distance, which was attributed

to lengthening of the di µ-oxo bridges between Mn ions.⁴⁸

Among the proposed mechanisms for water oxidation, several

account for S₂to S₃(or S₃to S₀) advancement in terms of a

significant core rearrangement, such as Mn-oxo rearrange-

ment from a cubane to an adamantane core^49,50and rear-

rangement of a butterfly to a partial cubane core.^35,51,52

Although EXAFS analysis of the active site of PSII excludes

highly symmetric adamantane- or cubane-shaped Mn-oxo

cores as accurate structural models of the S₀to S₂states, we

conjecture that structural rearrangement of the Mn₄OEC is

an explanation for changes in the metal-metal distance pro-

posed for advancement from S₂to S₃and S₃to S₀. A core

rearrangement demonstrated by us involves a shape shift

from a dimer-of-dimers to an adamantane core.⁴³Previously,

we demonstrated that a dimer-of-dimers complex [Mn₄O₄-

(tphpn)₂](CF₃SO₃)₄(2) (Htphpn ) N,N,N′,N′-tetra(2-pyridyl-

methyl)-2-hydroxypropane-1,3-diamine) is a good spectral

model of the PSII S₁state of the oxygen-evolving com-

plex.^41,42Further investigation of the reactivity of 2 reveals

that it undergoes dramatic structural changes concurrent with

one-electron oxidation or reduction.^43,53In particular, oxida-

tion of 2 from oxidation state (Mn^III)₂(Mn^IV)₂to (Mn^III)-

(Mn^IV)₃results in a structural rearrangement to an adamantane-

like core complex, [Mn₄O₄(tphpn)₂](CF₃SO₃)₂(ClO₄)₃(3). We

have also shown that 2 undergoes isomerization in solution

without a redox change, as demonstrated by UV-vis and

¹H NMR spectroscopies.

Figure 1. Ligands discussed in this paper.

state.^57,63,64For these reasons, an understanding of the factors

that influence the protonation and redox behavior of tetra-

nuclear manganese-oxo clusters is essential in order to

activate a core toward substrate oxidation.

In an ongoing effort to understand the formation and

physical properties of manganese-oxo clusters, we have

extended our reactivity studies of Mn-oxo cores coordinated

with N,N-bis(2-pyridylmethyl)ethylamine (bpea) (Figure

1).^21,65-68Previously, we showed that the adamantane-like

complex [Mn₄O₆(bpea)₄](ClO₄)₄(1) exhibits reversible pro-

tonation, electron-transfer, and proton-coupled electron trans-

fer.²¹Here we report the ligand substitution reactivity of 1.

Whereas self-assembly of aggregates frequently relies on

assembling the thermodynamic product(s), the ligand-sub-

The dimer-of-dimers and adamantane-shaped Mn-oxo

cores exhibit additional reactivity relevant to water oxidation,

such as protonation, reversible electron-transfer and proton-

coupled electron transfer.^21,25,41,53Both electron- and proton-

transfer chemistry are critical components of the Kok cycle⁵⁴

as well as various proposed mechanisms for water oxidation

that emphasize the deprotonation of H₂O to form terminal

or bridging OH^-or O^2-ligands at the active site. Recent

proposals suggest the participation of a Mn(IV)^55,56or

Mn(V)^57-61terminal MndO group in the course of O-O

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Inorganic Chemistry, Vol. 44, No. 14, 2005 5163

Dube´ et al.

Table 1. Designation of Compounds

collected at 298 K on a Varian Unity 300 MHz NMR spectrometer

with a 100 kHz sweep width. All ¹H NMR spectra were background

corrected using a first-order correction function. Peak widths at

full-width half-maximum (fwhm) are in hertz. Values for chemical

shifts (ppm) are the observed shifts referenced to the appropriate

residual protic solvent peak (CHCl₃, 7.24 ppm; CHD₂CN, 1.94 ppm;

CD₃OH, 4.78 ppm, CHD₂OD, 3.30 ppm; HOD, 4.68 ppm). IR

spectra were recorded using a Nicolet 5DX FT-IR spectrometer

using either a KBr disk or a solution cell containing an acetonitrile

solution. Solid-state magnetic susceptibility measurements were

performed on a Quantum Design SQUID magnetometer at either

the Massachusetts Institute of Technology or Tufts University in

the temperature range 2-300 K at a constant applied magnetic field

of 0.5 T. Measurements at Tufts University were performed on

powdered samples pressed into the small half of a gelatin capsule

which was then sealed with the larger half of the capsule followed

by Mylar tape. All data were corrected for the susceptibility of the

empty sample container and Mylar tape, including its field depen-

dence. Measurements at the Massachusetts Institute of Technology

were performed on powdered samples pressed between cotton plugs

inside a straw. All data were corrected for the susceptibility of the

empty container, including its field dependence. Isothermal mag-

netization data were collected at 2, 5, and 10 K from 0.1 to 5.5 T.

Solution magnetic susceptibility characteristics were determined by

the NMR method^69-71at 298 K. Molar paramagnetic susceptibilities

[Mn₄O₆(bpea)₄](ClO₄)₄

[Mn₄O₆(tacn)₄](ClO₄)₄

[Mn₄O₆(bpea)₂(dien)₂](ClO₄)₄

[Mn₄O₆(Medien)₄](ClO₄)₄

[Mn₄O₆(tach)₄](ClO₄)₄

[Mn₄O₆(bpea)₂(me-ida)₂]

[Mn₄O₆(bpea)₂(bz-ida)₂]

[Mn₄O₆(bpea)₂(^tbu-ida)₂]

[Mn₄O₆(bpea)₂(^cpent-ida)₂]

1

4

5

6

7

8

9

10

11

stituted product will be determined by kinetic as well as

thermodynamic factors. Furthermore, we sought to use

ligands such as iminodicarboxylates (Figure 1) that more

closely approximate the coordination environment of PSII

yet are too reactive under the highly oxidizing conditions

typically utilized in self-assembly of high-valent manganese-

oxo complexes. Also, we wished to explore the possibility

of generating asymmetrically ligated clusters. Finally, it was

hoped that substitution of the bpea ligand on the adamantane-

shaped core with ligands possessing different characteristics

would alter the reactivity of the [Mn₄O₆]⁴⁺core, promote

core rearrangement, and provide access to a Mn(V) species.

Herein we report the synthesis and characterization of a series

of adamantane-shaped [Mn₄O₆]⁴⁺complexes, as designated

in Table 1. In this paper, a comparison of spectroscopic and

redox properties is presented as well as X-ray structure

determinations for two members of the series. Furthermore,

the pK_avalue is established for two members of the series.

were obtained by using diamagnetic corrections (ø_d) -586 × 10^-6

,

-586 × 10^-6, -561 × 10^-6, and -648 × 10^-6cgs/mol for 4, 7,

8 and 9, respectively) calculated from Pascal’s constants.⁷²El-

emental analyses were obtained from Desert Analytics, Tucson,

AZ. Electrospray ionization mass spectrometry (ESI-MS) (typically

2 × 10^-4M in acetonitrile or methanol at a 3.5 kV nebulizer

voltage) was performed on a Micromass Quattro mass spectrometer

at the University of Illinois Mass Spectrometry Laboratory.

Electrochemistry. A BAS-100B Electrochemical Analyzer was

used for electrochemical experiments. A platinum disk working

electrode and a platinum wire auxiliary electrode were used to

collect cyclic voltammograms in acetonitrile or propylene carbonate

at a scan rate of 50 mV/s. An Ag/AgClO₄electrode was used as

the reference electrode. All electrochemical measurements employed

0.1 M TBAP as supporting electrolyte. Cyclic voltammetry of Fc

in the appropriate solvent was determined during electrochemical

analysis of compounds. The E_1/2values of the Fc/Fc⁺couple in

acetonitrile and propylene carbonate were typically 106 and 72 mV,

respectively. Potentials are reported versus the Fc/Fc⁺couple.⁷³All

electrochemical measurements were performed under a dry and

purified N₂atmosphere. The potentials reported in this work are

uncorrected for a junction contribution.

Experimental Section

Abbreviations. The following abbreviations are used throughout

this paper: Fc/Fc⁺, ferrocene/ferricenium ion Fe^II(cp)₂/Fe^III(cp)₂

+

,

where cp ) cyclopentadienyl; TBAP, tetrabutylammonium per-

chlorate; bpea, N,N-bis(2-pyridylmethyl)ethylamine; bpta, N,N-bis-

(2-pyridylmethyl)-tert-butylamine; bispicen, N,N′-bis(2-methylpy-

ridyl)-1,2-ethanediamine; tmpa, tris(2-methylpyridyl)amine; tren,

2,2′,2′′-triaminotriethylamine; tacn, 1,4,7-triazacyclononane; Me₃-

tacn, 1,4,7-trimethyl-1,4,7-triazacyclononane; tame, 1,1,1-tris(ami-

nomethyl)ethane; tach, cis,cis-1,3,5-triaminocyclohexane; dien,

diethylenetriamine; Medien, N′-methyldiethylenetriamine; Me₃dien,

N,N′,N′′-trimethyldiethylenetriamine; Me₅dien, N,N,N′, N′′,N′′-

pentamethyldiethylenetriamine; me-ida, N-methyliminodiacetate;

bz-ida, N-benzyliminodiacetate; ^tbu-ida, N-tert-butyliminodiacetate;

^cpent-ida, N-cyclopentyliminodiacetate

Materials. Acetonitrile used for synthesis was distilled from

CaH₂. Acetonitrile used for electrochemical and UV-vis experi-

ments was distilled from CaH₂and then stored over 3 Å molecular

sieves for at least 1 week prior to use. Propylene carbonate was

dried over 4 Å molecular sieves for at least one week and vacuum

distilled just prior to use. Deuterated acetonitrile (Cambridge Isotope

Laboratories, Inc.) was dried in flame-dried glassware over activated

3 Å molecular sieves. Deuterated chloroform, methanol, and water

(Cambridge Isotope Laboratories, Inc.) were used as received. Tacn,

Me₃tacn, H₂me-ida, and H₂bz-ida were purchased from Aldrich and

used as received. Dien, Medien, Me₃dien, and Me₅dien were

purchased from Aldrich and distilled under vacuum prior to use.

A mixture of cis,cis- and cis,trans-1,3,5-triaminocyclohexane was

purchased from Ambinter. All other reagents were purchased and

used as received. Electrochemical grade TBAP (Fluka) and AgClO₄

(Aldrich) were used as received. Paratone-N oil was obtained from

Exxon. All chemicals used in this work were of reagent grade.

Physical Methods. Electronic spectra were collected at 298 K

Ligand Synthesis. The ligand bpea was prepared as described

elsewhere.^66-68Synthesis of N-cyclopentyliminodiacetic acid and

c

t

N-tert-butyliminodiacetic acid, H₂pent-ida and H₂bu-ida, respec-

tively, followed modifications⁷⁴of the method of Berchet⁷⁵and

are given in detail in the Supporting Information. Synthesis of tame

and tach was based on modifications⁷⁶of the method of Fleischer

(69) Evans, D. J. J. Chem. Soc. 1959, 2003-2005.

(70) Dickinson, W. C. Phys. ReV. 1951, 81, 717-731.

(71) Live, D. H.; Chan, S. I. Anal. Chem. 1971, 42, 791-792.

(72) Mulay, L. N., Theory and Applications of Molecular Paramagnetism.

In Theory and Applications of Molecular Paramagnetism; Boudreaux,

E. A., Mulay, L. N., Eds.; Wiley: New York, 1976.

(73) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.

(74) Chase, B. H.; Downes, A. M. J. Chem. Soc. 1953, 3874-3877.

(75) Berchet, G. J. Organic Syntheses; Wiley: New York, 1943; Collect

Vol. II, pp 397-399.

(76) Bollinger, J. E.; Mague, J. T.; Banks, W. A.; Kastin, A. J.; Roundhill,

D. M. Inorg. Chem. 1995, 34, 2143-2152.

1

using a Cary 1E UV-vis spectrophotometer. H NMR data were

5164 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

et al.⁷⁷Synthesis of tach is described in Supporting Information.

Caution! Polyazides are potentially explosiVe.⁷⁸Alternatively, tach

was purified from a mixture of cis,cis-and cis,trans-1,3,5-triami-

nocyclohexane following the method of Wentworth⁷⁹and is

described in Supporting Information. The free base of tame and

tach used for substitution chemistry were prepared under anhydrous

conditions by dissolving the trihydrochloride salts in methanol and

adding a stoichiometric amount of NaOMe and stirring briefly. The

resulting solid was filtered off and the solvent removed by rotary

evaporation to yield a white solid. This was dissolved in acetonitrile

and filtered and the solvent removed to yield the free base as a

fine white powder.

[Mn₄O₆(Medien)₄](ClO₄)₄(6). An acetonitrile solution (2 mL)

of Medien (0.084 g, 0.704 mmol) was added to an acetonitrile

solution (20 mL) of 1 (0.254 g, 0.156 mmol) over 10 min. The

reaction was allowed to stir for an additional 30 min and then the

solution was concentrated to 1 mL by rotary evaporation. The

product was precipitated by dropwise addition of the acetonitrile

solution to 3 mL of vigorously stirred diethyl ether. The dark brown

precipitate was collected by filtration and washed with diethyl ether

and dried in vacuo to give 0.130 g (0.110 mmol, 70%) of 6 as a

dark brown powder. Diffusion of CHCl₃into an acetonitrile solution

(4 mL) of 6 (0.130 g, 0.110 mmol) at room temperature gave

microcrystalline 6 (0.116 g, 0.098 mmol, 62%).

The N-alkyl quaternary ammonium salts (n-Bu₄N⁺or Et₄N⁺) of

H₂R-ida ligands were prepared by stoichiometric addition of an

aqueous solution of either (n-Bu₄N)OH or (Et₄N)OH to an aqueous

solution of H₂R-ida, where R denotes the substituent on the imine

nitrogen atom. The majority of the solvent was removed by rotary

evaporation to obtain a viscous oil and the final drying was done

on a vacuum line. The dried N-alkyl quaternary ammonium salts

of H₂R-ida are hygroscopic and were recrystallized under anhydrous

[Mn₄O₆(tach)₄](ClO₄)₄(7). A methanol solution (0.5 mL) of

tach (0.100 g, 0.773 mmol) was added to an acetonitrile solution

(20 mL) of 1 (0.167 g, 0.103 mmol) over 10 min. The reaction

was allowed to stir for 6 h and then the solution was concentrated

to dryness using rotary evaporation. The dark purple precipitate

was triturated with EtOH to remove excess tach and dried in vacuo

to give 0.126 g (0.102 mmol, 99%) of 7 as a dark purple powder.

Diffusion of diethyl ether into an acetonitrile solution (3 mL) of 7

(0.126 g, 0.102 mmol) at -20 °C gave purple hexagonal plates of

7 (0.108 g, 0.088 mmol, 85%).

1

conditions. See Table S1 for yields, elemental analyses, and H

NMR data for these ligands.

Ligand substitution reactions with hygroscopic [Et₄N]₂[R-ida]

and [n-Bu₄N]₂[R-ida] ligands were performed under prepurified

argon with standard glovebox techniques.

Complex Synthesis. [Mn₄O₆(bpea)₄](ClO₄)₄(1), [Mn₄O₆(bpea)₄]-

(CF₃SO₃)₄, and [Mn₄O₆(bpea)₄]Br₄were prepared as described

previously.²¹

[Mn₄O₆(bpea)₂(me-ida)₂] (8). An acetonitrile solution (5 mL)

of [n-Bu₄N]₂[me-ida] (0.214 g, 0.340 mmol) was added dropwise

to a stirred acetonitrile solution (20 mL) of 1 (0.250 g, 0.154 mmol)

over 15 min. The reaction was allowed to stir for an additional 15

min, after which time a light green precipitate was allowed to settle,

leaving a faint red solution. The crude product was filtered under

ambient conditions, washed with acetonitrile, and dried under

vacuum to give 0.114 g (70%) of 8. Vapor diffusion of acetone

into an aqueous solution (3 mL) of 8 (0.114 g, 0.107 mmol) yielded

fine green needles of 8 (0.0627 g, 0.059 mmol, 55%) after 3 days

at room temperature.

[Mn₄O₆(bpea)₂(^cpent-ida)₂] (11). An acetonitrile solution (5 mL)

of [Et₄N]₂[^cpent-ida] (0.225 g, 0.489 mmol) was added dropwise

to a stirred acetonitrile solution (15 mL) of 1 (0.318 g, 0.196 mmol)

over 15 min. The reaction was allowed to stir for an additional 15

min, after which time a light green precipitate was allowed to settle,

leaving a red-brown solution. The crude product was filtered under

ambient conditions, washed with acetonitrile, and dried under

vacuum to give 0.135 g (0.116 mmol, 59%) of 11. The crude

product was dissolved in 3 mL of MeOH and filtered. Diffusion of

Et₂O into a MeOH solution (3 mL) of 11 (0.135 g, 0.116 mmol) at

-20 °C gave thin rectangular plates (0.123 g, 0.106 mmol, 54%).

Complexes 9 and 10 were synthesized and crystallized according

to the procedure for 11 and are described in the Supporting

Information.

[Mn₄O₆(tacn)₄](ClO₄)₄(4). An acetonitrile solution (1.0 mL)

of tacn (0.080 g, 0.616 mmol) was added to an acetonitrile solution

(25 mL) of 1 (0.222 g, 0.137 mmol) over 10 min. The reaction

mixture was allowed to stir for 1 h and then the solution was

concentrated to about 1 mL by rotary evaporation. The product

was precipitated by dropwise addition of the acetonitrile solution

to 3 mL of vigorously stirred diethyl ether. The dark purple

precipitate was collected by filtration and washed with diethyl ether

and dried in vacuo to give 0.156 g (0.127 mmol, 93%) of 4 as a

dark purple powder. Diffusion of chloroform into an acetonitrile

solution (3 mL) of 4 (0.156 g, 0.127 mmol) at room temperature

gave purple hexagonal plates of 4 (0.148 g, 0.121 mmol, 88%).

See Table S2 for yields, elemental analyses and ESI-MS data, Table

S3 for UV-vis, E_1/2, and IR data, and Table S4 for ¹H NMR data

for this and other compounds in this section.

Preparation of [Mn₄O₆(tacn)₄](CF₃SO₃)₄was done in an entirely

analogous manner using [Mn₄O₆(bpea)₄](CF₃SO₃)₄. Complex

[Mn₄O₆(tacn)₄](CF₃SO₃)₄was crystallized from vapor diffusion of

chloroform into an acetonitrile solution of it at room temperature.

[Mn₄O₆(dien)₂(bpea)₂](ClO₄)₄(5). An acetonitrile solution (2.5

mL) of dien (0.040 g, 0.385 mmol) was added to an acetonitrile

solution (20 mL) of 1 (0.250 g, 0.154 mmol) over 30 min with an

addition funnel. The solution was allowed to stir for an additional

3 h and then the solution was concentrated to 2 mL by rotary

evaporation. The product was precipitated by dropwise addition of

the acetonitrile solution to 5 mL of vigorously stirred diethyl ether.

The dark brown precipitate was collected by filtration and washed

with diethyl ether and dried in vacuo to give 0.171 g (0.125 mmol,

81%) of 5 as a dark brown powder. Diffusion of CHCl₃into an

acetonitrile solution (4 mL) of 5 (0.171 g, 0.125 mmol) at room

temperature gave thin dendritic black plates of 5 (0.166 g, 0.121

mmol, 79%).

¹H NMR Titration. Protonation of 8 and 11. In a typical

titration, a solution of 8 in D₂O (0.4 mL, 9.0 mM) was prepared

and transferred to an NMR tube. Protonation of 8 was accomplished

by addition of aliquots of HClO₄(1.0 µL, 1.0 M in CD₃CN) with

a gastight syringe through the septum seal of the NMR tube.

Generation of 8H(ClO₄) was considered complete when additional

aliquots of HClO₄gave no further change in the ¹H NMR spectrum.

A ¹H-NMR-monitored titration of 11 was carried out in an entirely

analogous fashion in MeOD or D₂O.

(77) Fleischer, E. B.; Gebala, A. E.; Levey, A.; Tasker, P. A. J. Org. Chem.

1971, 36, 3042-3044.

Spectrophotometric Titration. Protonation of 8 and 11.

Titration of aqueous solutions of 8 and 11 was monitored spectro-

photometrically and with a pH electrode. In a typical titration, the

(78) Spear, R. J.; Dagley, I. J., Organic Energetic Compounds. In Organic

Energetic Compounds; Marinkas, P. L., Ed.; Nova Science Publishers:

New York, 1994; pp 135-142.

(79) Wentworth, R. A. D. Inorg. Chem. 1968, 7, 1030-1032.

pH and absorbance spectrum of 8 (2 mL of a 4.31 × 10^-4

M

Inorganic Chemistry, Vol. 44, No. 14, 2005 5165

Dube´ et al.

Scheme 1. Ligand Substitution Reactions of 1 in Acetonitrile

Table 2. Crystallographic Data for

[Mn₄O₆(tach)₄](ClO₄)₄‚3CH₃CN‚4.5H₂O and

[Mn₄O₆(bpea)₂(^cpent-ida)₂]‚7MeOH

7‚3CH₃CN‚4.5H₂O

11‚7MeOH

empirical formula

formula weight, g/mol

crystal system

space group

a, Å

C₃₀H₇₈Cl₄Mn₄N₁₅O_26.5C₅₃H₈₈Mn₄N₈O₂₁

1434.49

cubic

1393.05

monoclinic

C2/c

10.543(2)

25.198(4)

24.295(4)

90.

Fm3hm

23.934(6)

90

b, Å

c, Å

R, deg

â, deg

90

90.28(2)

90.

γ, deg

volume, Å³

Z

13711(6)

8

6454.1(2)

4

recollected at the end of data collection to monitor for crystal decay.

Crystals used for the diffraction studies showed no decomposition

during data collection.

F(calc), g/cm³

θ range, deg

reflections collected

independent reflections

1.390

1.434

1.47-26.39

22104

1.82-28.19

8775

Cell parameters were retrieved using SMART⁸¹software and

refined using SAINT⁸²on all observed reflections. Data reduction

was performed using the SAINT software, which corrects for Lp

and decay. Absorption corrections were performed with the

SADABS Program,⁸³supplied by George Sheldrick. The structures

were solved by direct methods using SHELXS-97⁸⁴and refined

by least-squares methods on F²in SHELXL-97,⁸⁵incorporated in

SHELXTL-PC V 5.10.⁸⁶All non-hydrogen atoms were refined with

anisotropic thermal parameters. Hydrogen atom positions were

calculated by geometrical methods and refined using a riding model.

All drawings presented here are made using 30% probability thermal

ellipsoids.

765 [R(int) ) 0.1499] 6424 [R(int) ) 0.0303]

data/restraints/parameters 765/1/51

6264/1/368

1.088

goodness-of-fit of F²

1.053

final R indices [I > 2σ(I)] R1 ) 0.0691,

wR2 ) 0.2116

R1 ) 0.0634,

wR2 ) 0.1216

aqueous stock solution) was measured following addition of an acid

such as perchloric acid (pK_a) -1.7, the pK_aof H₃O⁺), triflic acid

(CF₃SO₃H, pK_a) -1.7), or trifluoracetic acid (pK_a) 0.5).⁸⁰Acid

was added until there was no further change in the absorbance

spectrum. The data were fit to the following expression, derived

for the reaction B + H⁺/ HB⁺

(ꢀ_HB^λ)10^-pH+ (ꢀ_B^λ)10^-pK

a

A_λ) C_o

(1)

(

)

Results and Discussion

10^-pH+ 10^-pK

a

Synthesis. Ligand substitution reactions of 1 are outlined

in Scheme 1. Complex 1 was synthesized by compropor-

tionation of Mn(II) and Mn(VII) starting materials in the

presence of the bpea ligand.²¹We speculate that a similar

synthetic approach with the primary amine and iminodicar-

boxylate ligands used here would have resulted in ligand

oxidation in the course of self-assembly. While ligand

oxidation during self-assembly can be a useful synthetic tool

in the formation of new complexes, we pursued ligand

substitution reactions of 1 as an initial approach.

It was anticipated that ligand substitution reactions of this

core would be slow. This is because the Mn^IVd³ions , like

the substitutionally inert Cr^III,^87-89enjoy a considerable ligand

field stabilization energy (LFSE) and that ligand substitution

by a dissociative (D) or interchange dissociative (I_d) mech-

where A_λis the absorbance at a selected wavelength, C_ois the initial

λ

concentration of the complex, and ꢀ_Band ꢀ_HBare the extinction

coefficients of the complex and its protonated form, respectively,

at the selected wavelength. Spectrophotometric titrations of 8 and

11 were monitored at 515 nm and nonlinear least-squares fitting

of eq 1 to the experimental data used 1795 and 2422 for ꢀ_Band

ꢀ_HB^λ, respectively, for 8 and 1798 and 2361 for ꢀ_Band ꢀ_HB

λ

,

respectively, for 11.

X-ray Crystallography. Compound 7 crystallizes as [Mn₄O₆-

(tach)₄](ClO₄)₄‚3CH₃CN‚4.5H₂O in the cubic space group Fm3hm

with eight molecules in the unit cell and compound 11 crystallizes

as [Mn₄O₆(bpea)₂(^cpent-ida)₂]‚7MeOH in the monoclinic space

group C2/c with four molecules in the unit cell. Relevant crystal

data are summarized in Table 2.

Single crystals of 7 suitable for structure determination were

obtained by diffusion of diethyl ether into an acetonitrile solution

of 7 at -20 °C. Single crystals of 11 suitable for structure

determination were obtained by diffusion of diethyl ether into a

methanol solution of 11 at -20 °C. Crystals of each were taken

from the mother liquor, coated with Paratone-N oil, mounted on a

glass fiber with Apiezon-T grease, and immediately transferred to

the dinitrogen cold stream. Data for 7 were collected using a Bruker

diffractometer equipped with a SMART-APEX CCD (charge-

coupled device) detector and data for 11 were collected using a

Bruker diffractometer equipped with a SMART CCD detector. The

instruments used Mo KR radiation (λ ) 0.71073 Å) and included

a LT-3 low-temperature apparatus operating at 193 K. Data were

measured using omega scans of 0.3° per frame for 30 s, such that

a hemisphere was collected. A total of 1271 frames were collected

with a maximum resolution of 0.75 Å. The first 50 frames were

(81) SMART V 5.050 (NT) Software for the CCD Detector System; Bruker

Analytical X-ray Instruments, Inc.: Madison, WI, 1998.

(82) SAINT V 5.01 (NT) Software for the CCD Detector System; Bruker

Analytical X-ray Instruments, Inc.: Madison, WI, 1998.

(83) Blessing, R. H. SADABS. Program for absorption corrections using

Siemens CCD based on the method of Robert Blessing. Acta

Crystallogr. 1995, A51, 33-38.

(84) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal

Structure; University of Go¨ttingen, Germany, 1997.

(85) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal

Structure; University of Go¨ttingen, Germany, 1997.

(86) SHELXTL 5.10 (PC Version), Program library for Structure Solution

and Molecualr Graphics; Bruker Analytical X-ray Systems: Madison,

WI, 1998.

(87) Swaddle, T. W. AdV. Inorg. Bioinorg. Mech. 1983, 2, 95-138.

(88) Swaddle, T. W. Coord. Chem. ReV. 1974, 14, 217-268.

(89) There are a number of Cr(III) complexes that are substitutionally labile

(stopped-flow regime). See, for example: Beswick, C. L.; Shalders,

R. D.; Swaddle, T. W. Inorg. Chem. 1996, 35, 991-994 and references

therein.

(80) March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley-Interscience:

New York, 1985.

5166 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

anism through a trigonal bipyramidal transition state would

involve a significant loss of LFSE.⁹⁰It is unlikely that ligand

substitution would proceed by an associative (A) or inter-

change associative (I_a) mechanism, through a 7-coordinate

transition state, due to the bulky nature of the tridentate

ligands. In addition, the tridentate ligands of 1 impart

additional stability against ligand substitution due to the

chelate effect. Also, it was unclear whether ligand substitu-

tion reactions with 1 would proceed with retention of the

[Mn₄O₆]⁴⁺core structure or whether substitution would

promote core rearrangement, aggregation, or fragmentation.

Our investigations have shown that reaction of 1 with a

variety of tridentate amine and iminodicarboxylate ligands

in acetonitrile at room temperature produces derivative

clusters 4-11 generally on the order of 10 min with retention

of core nuclearity and oxidation state, as monitored by UV-

vis and ¹H NMR spectroscopy. Of these complexes, only 4

had been synthesized previously.^18,20,91Although it was not

attempted, it is possible that self-assembly of several of the

amine complexes, such as 6 and 7, would have been achieved

by air oxidation of a basic solution of Mn(II) and the

respective ligands, analogous to the synthesis of 4,^18,20

[Mn₄O₅(OH)(tame)₄](CF₃SO₃)₅,²⁰[Mn₂O₂(cyclam)₂](ClO₄)₃,⁹²

and [Mn₂O₂(tren)₂](CF₃SO₃)₃.⁹³

spectrometry. Reaction of 1 with Medien resulted in isolation

of the fully substituted product [Mn₄O₆(Medien)₄](ClO₄)₄(6)

in about 10 min, which was also confirmed by mass spec-

trometry. Reaction of 1 with Me₃dien gave a mixture of

[Mn₄O₆(Me₃dien)₄](ClO₄)₄(observed by electrospray MS [M

- ClO₄]⁺) 1195.5; expected 1195.1) and 13 (observed by

¹H NMR), while reaction of 1 with Me₅dien gave only 13.

The oxidation of tertiary amines by inorganic complexes

has been explored both as a preparative tool^73,94,95and for

its enzymatic relevance.^95,96Although often mechanistic

details remain unclear, it is generally accepted that the

reducing power of an amine is determined by its pK_aand

not by its oxidation potential and that coordination of the

amine nitrogen to the metal complex is a critical step in the

electron transfer.^94,95Most recently, oxidative transformations

of amines employing transition metal catalysts, such as Ru

or Co, and molecular oxygen are receiving increasing

attention.^97,98In these reports the proposed transformations

involve coordination of the tertiary amine and molecular

oxygen to the metal center in the initial activation steps. A

recent report of the noncatalytic oxidation of tertiary amines

using a Mn^IIIMn^IVcomplex proposes coordination of the

amine to a metal center, although dioxygen activation is

thought to occur via an amine R-radical to form an R-amino

peroxide.⁹⁹

Reaction of 1 with the amine ligands in acetonitrile

resulted in either ligand substitution, one-electron reduction

of 1 to form [Mn₄O₆(bpea)₄](ClO₄)₃(13),^21,23,24or both. For

example, reaction of 1 in acetonitrile (5 × 10^-4M) with 4.2

equiv of tacn typically produced the ligand substitution

product 4 in about 10 min. On occasion, however, nonsto-

ichiometric one-electron reduction of 1 occurred during

ligand substitution reactions, requiring considerably longer

times (about 2.5 h for completion, at about 86% yield) to

Substitution chemistry of 1 with N-alkyl quaternary

ammonium salts of N-substituted iminodicarboxylate ligands

resulted in the neutral mixed-ligand complexes [Mn₄O₆-

(bpea)₂(R-ida)₂]. There was no apparent reaction between

H₂R-ida and 1 in acetonitrile for periods up to 13 h according

1

to UV-vis and H NMR data. N-Alkyl quaternary am-

monium salts, rather than Na⁺or K⁺salts, were chosen for

their greater solubility. Attempts to prepare a [Mn₄O₆(R-

ida)₄]^4-complex were unsuccessful. The mixed ligand

complexes precipitated on the order of minutes from an

acetonitrile solution of 1 containing greater than 4 equiv of

the R-ida ligand. Ligand substitution in methanol, in which

the starting material [Mn₄O₆(bpea)₄]Br₄or 1 (the latter at

much lower solubility) and complexes 8-11 are soluble gave

a reddish solution upon addition of excess R-ida ligand within

minutes, followed by gradual bleaching to a colorless solution

and precipitation of flocculant material after about 10 h. It

may be that the fully substituted product [Mn₄O₆(R-ida)₄]^4-

is formed in methanol and that it is unstable.

1

form 4. H NMR confirmed the observation of rapid, yet

partial, initial core reduction to 13 and the formation of

tacnH⁺, followed by slower ligand substitution over a few

hours. On the other hand, reaction of 1 with the tertiary amine

Me₃tacn produced only 13 and Me₃tacnH⁺in quantitative

yield, as noted previously.²¹Reactions of 1 with the primary

amines tame and tach exhibited much slower substitution

1

kinetics. H NMR spectra of the reaction of 1 (1.2 × 10^-2

M) with the primary amine tame (up to 15 equiv) over times

on the order of 24 h suggest partial substitution; thus, this

reaction was not pursued further. On the other hand, reaction

of 1 (5.15 × 10^-3M) with 8 equiv of the more rigid amine

tach, which is conformationally predisposed for facial

coordination, proceeds cleanly to the tach analogue 7 for

times on the order of 6 h. Reaction of 1 with 3-8 equiv of

dien resulted in isolation of the mixed ligand product [Mn₄O₆-

(bpea)₂(dien)₂](ClO₄)₄(5), the structure of which has been

crystallographically established and confirmed by mass

Structures. The crystal structures of the full cation of 7

and the neutral complex 11 are shown in Figure 2, parts a

and b, respectively. Figure 3 shows the [Mn₄O₆]⁴⁺cores only.

The Mn and oxide O atoms in the crystal structures of 7

and 11 define the adamantane skeleton. The Mn atoms

occupy the apexes of a tetrahedron and the O atoms occupy

(90) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions, 2nd

ed.; Wiley: New York, 1967.

(94) Murahashi, S.-I. Angew. Chem., Int. Ed. Engl. 1995, 34, 2443-2465.

(95) Sheldon, R. A.; Kochi, J. K., Metal-Catalyzed Oxidations of Organic

Compounds; Academic: New York, 1981.

(91) Zhang, L.; Shiping, Y.; Chunhui, L.; Daizheng, L.; Zonghui, J.; Cheng,

P.; Wang, G.; Weng, L.; Xuebing, L. J. Chem. Crystallogr. 2000, 30,

251-254.

(96) Meunier, B. Biomimetic Oxidations Catalyzed by Transition Metal

Complexes; Imperial College Press: London, 2000; p 696.

(97) Jain, S. L.; Sain, B. Angew. Chem., Int. Ed. 2003, 42, 1265-1267.

(98) Murahashi, S.-I.; Komiya, N.; Terai, H.; Nakae, T. J. Am. Chem. Soc.

2003, 125, 15312-15313.

(92) Brewer, K. J.; Calvin, M.; Lumpkin, R. S.; Otvos, J. W.; Spreer, L.

O. Inorg. Chem. 1989, 28, 4446-4451.

(93) Hagen, K. S.; Armstrong, W. H.; Hope, H. Inorg. Chem. 1988, 27,

7-969.

(99) Caudle, M. T.; Pecoraro, V. L. Inorg. Chem. 2000, 39, 5831-5837.

Inorganic Chemistry, Vol. 44, No. 14, 2005 5167

Dube´ et al.

Figure 2. ORTEP plots of the structures and atom labeling schemes of (a) 7(ClO₄)₄and (b) 11 showing 30% probability thermal ellipsoids. Hydrogen

atoms (and perchlorate counterions for 7) are omitted for clarity.

Figure 3. ORTEP plots of the {Mn₄O₆} cores of (a) 7 and (b) 11 showing 30% probability thermal ellipsoids. Carbon and hydrogen atoms (and perchlorate

counterions for 7) are omitted for clarity.

Table 3. Selected Bonds Distances (Å) and Angles (deg) for

[Mn₄O₆(tach)₄](ClO₄)₄‚3CH₃CN‚4.5H₂O

tacn Mn-N distance of 2.092(1) Å for 4 and significantly

shorter than the Mn-N_alkylbond length of 2.196(2) Å for

the bpea ligand of 1.

bond distances

bond angles

Mn(1)-O(1)

1.808(3)

2.096(7)

1.486(11)

1.493(8)

In contrast to the high point symmetry of 7, 11 exhibits

only C₂point symmetry. This symmetry results in four types

of bridging oxo ligands, two of which lie along the

crystallographic C₂axis [O(2) and O(4) in Figure 3b]. Oxo

bridge O(2) is trans to the carboxylate oxygen atoms of the

^cpent-ida ligands and has a Mn-O bond length of 1.7986-

(17) Å. Its Mn-O-Mn angle of 133.5(2)° (Table 4) is the

largest angle measured thus far for a Mn-oxo adamantane

shaped core. Oxo bridge O(4) is trans to two pyridyl nitrogen

atoms of the bpea ligands and has a Mn-O bond length of

1.798(2) Å and a Mn-O-Mn angle of 127.2(2)°.

Mn(1)-N(1)

N(1)-C(1)

C(2)-C(1)

O(1A)-Mn(1)-O(1)

Mn(1)-O(1)-Mn(1A)

98.0(2)

129.3(4)

the apexes of an octahedron. Each Mn atom is coordinated

to three bridging oxide O atoms and three atoms from the

terminal ligands in pseudo-octahedral geometry. Complex

7 exhibits crystallographically imposed T_dpoint symmetry,

rendering all the Mn atoms, bond lengths, and bond angles

equivalent. The Mn-O_oxobond length of 1.808(3) Å and

the Mn-O-Mn angle of 129.3(4)° for 7 (Table 3) are similar

to those of 4 [1.796(1) Å and 128.0(1)°, respectively]²⁰and

are generally consistent with metric data for other Mn^IV-

O_oxocomplexes. The tach ligand displays local 3-fold

symmetric coordination to the manganese ions in 7, with a

Mn-N distance of 2.101(6) Å, very similar to the average

Of the four oxo bridges lying in the plane perpendicular

to the C₂axis, two [O(1) and O(1A)] are trans to a bpea

c

tertiary amine nitrogen atom and a pent-ida tertiary amine

nitrogen atom, with a Mn-O-Mn angle of 127.95(16)° and

5168 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

Table 4. Selected Bonds Distances (Å) and Angles (deg) for

[Mn₄O₆(bpea)₂(^cpent-ida)₂]‚7MeOH

bond distances

bond angles

O(1)-Mn(1)-O(3)

Mn(1)-O(1)

1.780(3)

1.792(3)

100.30(13)

98.51(12)

98.14(11)

97.19(13)

98.47(12)

94.08(12)

89.43(14)

172.69(14)

87.98(13)

90.33(12)

90.01(12)

169.75(14)

86.84(13)

133.5(2)

Mn(1)-O(3)

Mn(1)-O(4)

Mn(1)-Mn(2)

Mn(1)-Mn(1A)

Mn(2)-O(1)

Mn(2)-O(3A)

Mn(2)-O(2)

O(2)-Mn(2A)

O(3)-Mn(2A)

O(4)-Mn(1A)

O(1)-Mn(1)-O(4)

O(3)-Mn(1)-O(4)

O(1)-Mn(2)-O(3A)

O(1)-Mn(2)-O(2)

O(3A)-Mn(2)-O(2)

O(1)-Mn(2)-O(5)

O(3A)-Mn(2)-O(5)

O(2)-Mn(2)-O(5)

O(1)-Mn(2)-O(6)

O(3A)-Mn(2)-O(6)

O(2)-Mn(2)-O(6)

O(5)-Mn(2)-O(6)

Mn(2A)-O(2)-Mn(2)

Mn(1)-O(3)-Mn(2A)

Mn(1)-O(4)-Mn(1A)

Mn(1)-O(1)-Mn(2)

1.7980(18)

3.2114(10)

3.2207(13)

1.794(3)

1.798(3)

1.7986(17)

1.798(3)

1.7980(18)

130.36(17)

127.2(2)

127.95(16)

a Mn-O bond length of 1.780(2) Å. The remaining two

bridges [O(3) and O(3A)] are trans to a pyridyl nitrogen atom

and a carboxylate oxygen atom of bpea and ^cpent-ida ligands,

respectively, with a Mn-O-Mn angle of 130.36(17)° and

an average Mn-O bond length of 1.792(2) Å. All of these

bond lengths are comparable to those of 1, which is

characterized by four Mn-O bonds 1.806(1) Å long and two

Mn-O bonds 1.786(2) Å long.

Figure 4. Positive ion electrospray ionization mass spectrum of 7 in

acetonitrile at unit resolution.

The Mn-N bond lengths of the bpea ligands of 11 are

similar to those of 1: an average Mn-N_alkylbond length of

2.173(4) Å and two kinds of Mn-N_pybonds, a “short” Mn-

N_pybond with an average length of 2.080(4) Å and a slightly

longer Mn-N_pybond with an average length of 2.094(3) Å.

Longer Mn-N_alkylbonds¹⁰⁰are observed for other structurally

characterized compounds whose multidentate ligands are

comprised of both alkyl and pyridyl nitrogen atoms, including

bpea,^65,66,68bpta,⁶⁸bispicen,^101,102and tmpa.¹⁰³The four

c

carboxylate Mn-O bonds of the pent-ida ligand of 11

[average length of 2.024(3) Å] are somewhat longer than

similar Mn^IV-O bonds for other terminal carboxylate ligand

Mn complexes (1.980-1.999 Å).^104-106

The average Mn‚‚‚Mn distance of 11 [3.2161(11) Å] is

somewhat shorter than the corresponding distance in 4

[3.227(2) Å], 7 (3.269 Å), and 1 [3.248(1) Å].

Physical Properties. Mass Spectrometry. Electrospray

ionization mass spectrometry data are summarized in Table

S2 and all spectra can be found in the Supporting Informa-

tion. The ESI mass spectra of 7 and 8 are shown in Figures

4 and 5, respectively.

The substitution products can be divided into two groups:

the cations and the neutral species. The [Mn₄O₆]⁴⁺cations

readily lose an anion to form [M - ClO₄]⁺species. The tach

Figure 5. Positive ion electrospray ionization mass spectrum of 8 in water/

methanol (10%) at unit resolution.

(7) and tacn (4) complexes behave similarly, with [M -

ClO₄]⁺and [M - 2ClO₄]²⁺being the predominant species,

but otherwise exhibiting very little fragmentation. The mass

spectrum of complex 6 shows striking similarity to those of

the strong amine donor ligand complexes 4 and 7, with major

peaks at [M - ClO₄]⁺(1082.9), [M - 2ClO₄]²⁺(491.7),

[M - 3ClO₄]³⁺(294.6), and [M - 4ClO₄]⁴⁺(196.1). The

mass spectrum for the mixed dien/bpea complex (5), with a

major peak at [M - ClO₄]⁺(1275.2), showed similar

behavior to that of 1, which exhibited a much greater

tendency toward core fragmentation in the gas phase.²¹

The mass spectra of the [Mn₄O₆(bpea)₂(R-ida)₂] complexes

are similar to each other, exhibiting prominent species at

[M + H]⁺(1060.9, 1213.4, 1145.4, and 1169.5 for 8, 9, 10,

and 11, respectively) and [M + Na]⁺(1235.4, 1167.3, and

1191.4 for 9, 10, and 11, respectively), as well as prominent

peaks that correspond to addition of R-ida^2-ligands and

peaks corresponding to loss of CO₂.

(100) The comparison is among Mn^IV-N_alkyland Mn^IV-N_py, which are

trans to µ-O ligands.

(101) Collins, M. A.; Hodgson, D. J.; Michelson, K.; Towle, D. K. J. Chem.

Soc., Chem. Commun. 1987, 1659-1660.

(102) Goodson, P. A.; Glerup, J.; Hodgson, D. J.; Michelsen, K.; Pedersen,

E. Inorg. Chem. 1990, 29, 503-508.

(103) Towle, D. K.; Botsford, C. A.; Hodgson, D. Inorg. Chim. Acta 1988,

141, 167-168.

(104) Suzuki, M.; Senda, H.; Kobayashi, Y.; Oshio, H.; Uehara, A. Chem.

Lett. 1988, 1763-1766.

(105) Gohdes, J. W. Ph.D. dissertation, University of California, Berkeley,

1991.

Electronic and Vibrational Spectroscopy. The IR spectra

of the [Mn₄O₆]⁴⁺cores show a number of bands attributable

(106) Bhaduri, S.; Pink, M.; Christou, G. Chem. Commun. 2002, 2352-

-

2353.

to the different ligands and the ClO₄anion (∼1119 and

Inorganic Chemistry, Vol. 44, No. 14, 2005 5169

Dube´ et al.

625 cm^-1),¹⁰⁷and each compound also exhibits intense bands

attributed to the Mn-O-Mn stretch in the region between

about 695 and 732 cm^-1(Table S3).^21,22An intense band

for 1 at 708 cm^-1is near the middle of this range, while the

corresponding bands for the substitution products 4 (732

cm^-1) and 10 (695 cm^-1) delineate the extremes.

The mixed ligand R-ida series also display two intense

bands around 1620 and 1380 cm^-1attributed to the coordi-

nated carboxylate ligand.¹⁰⁷The mode of coordination of

R-ida is readily determined from the values of ν_a(COO) and

ν_s(COO) and the value of ∆ ) [ν_a(COO) - ν_s(COO)].^107,108

The infrared spectra of the [Mn₄O₆(bpea)₂(R-ida)₂] series all

have a value of ∆ between 240 and 244 cm^-1, which is

characteristic of unidentate coordination of the carboxylate

ion. As anticipated, unidentate coordination of the carboxyl-

ate moieties of the ^cpent-ida ligand to the Mn ions has been

verified for 11 by X-ray structural analysis, substantiating

the structurally characterized sample as representative of the

bulk material. A survey of representative metalloprotein

crystal structure determinations¹⁰⁹reveals that unidentate

carboxylate can bind either syn or anti with respect to the

Figure 6. Cyclic voltammograms of an acetonitrile solution 1.20 mM in

7(ClO₄)₄and 0.2 M in TBAP illustrating quasireversible oxidation and

reduction. The two scans have the same starting potential (denoted with

+) but differ by scan direction and scan window. Potentials are versus Fc/

Fc⁺.

c

lone pairs of electrons. As expected, pent-ida ligation to

the Mn ion is anti unidentate.

The electronic spectra of all the complexes are similar to

each other, with the most notable feature in the absorption

spectrum being the putative OfMn^IVdπ* charge-transfer

band in the range 546 to 591 nm, as well as a number of

less intense bands at 990-1000 nm for all of the complexes,

most likely d-d transitions (Table S3). The fully substituted

strong N-donor ligand complexes (4, 6, and 7) also exhibit

a very weak shoulder at about 750 nm.

Electrochemistry. The ligands described here stabilize the

adamantane-shaped [Mn₄O₆] core in the Mn^III(Mn^IV)₃, (Mn^IV)₄,

and Mn^V(Mn^IV)₃oxidation states. Ligand substitution on 1

greatly decreased the reduction potential of the (IV, IV, IV,

IV)/(III, IV, IV, IV) redox couple from E_1/2) -0.28 to

-1.03 V for 9, a shift of 750 mV. Cyclic voltammograms

of 7 and 9 are shown in Figures 6 and 7, respectively, and

all electrochemical data are summarized in Table S3.

Particularly noteworthy is the substantial inductive effect

of the substituted iminodicarboxylates, as shown in the plot

of OfMn^IVdπ* LMCT energy versus E_1/2(Figure 8). Figure

8 also illustrates the differentiation of the strong σ-donor

amines and the strong π-donor carboxylates with respect to

LMCT. Tuning of the redox potentials of the [Mn₄O₆]⁴⁺

complexes follows a ranking of the ancillary ligands, as

determined for a number of mononuclear ruthenium and

dinuclear manganese complexes, based on their redox

potentials: RO^-> RCO₂^-> R₃N > R₂NH > pyridine.^110-113

Figure 7. Cyclic voltammograms of a propylene carbonate solution 290

µM in 9 and 0.2 M in TBAP illustrating quasireversible oxidation and

reduction. The two scans differ by starting potential (denoted with +), scan

direction and scan window. Potentials are versus Fc/Fc⁺.

Importantly, substitution of bpea with strong donor ligands

also affords ready access to the Mn^V(Mn^IV)₃oxidation state.

Complexes 4, 7, 9, 10, and 11 exhibit quasireversible CV

for the (V, IV, IV, IV)/(IV, IV, IV, IV) redox couple (E_1/2

of 0.978, 0.942, 0.775, 0.718, and 0.641 V, respectively),

with the R-ida complex 11 having the lowest (V, IV, IV,

IV)/(IV, IV, IV, IV) redox couple of the known adamantane-

shaped [Mn₄O₆]⁴⁺complexes. Synthesis, characterization,

and reactivity of manganese-oxo complexes in the Mn^V-

(Mn^IV)₃oxidation state figure prominently in modeling

studies aimed at evaluating recent proposals for PSII-

catalyzed water oxidation, in which a Mn^Vintermediate is

proposed for the O-O bond-forming step.^57-61,114Further-

more, a Mn^V(Mn^IV)₃oxidation state has been proposed for

the S₄state of the Mn₄OEC.^{57, 63, 64}

(107) Nakamoto, K., Infrared and Raman Data of Inorganic and Coordina-

tion Compounds, 5 ed.; John Wiley: New York, 1997; Vols. A &

B.

(108) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227-

250.

(109) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991,

15, 417-430.

(110) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271-1285.

(111) Fielder, S. S.; Osborne, M. C.; Lever, A. B. P.; Pietro, W. J. J. Am.

Chem. Soc. 1995, 117, 6990-6993.

(113) Manchanda, R.; Brudvig, G. W.; Crabtree, R. H. Coord. Chem. ReV.

1995, 144, 1-38.

(112) Thorp, H. H.; Brudvig, G. W. New. J. Chem. 1991, 15, 479-490.

5170 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

Figure 8. Plot of LMCT energy versus E_1/2of the (III,IV,IV,IV)/(IV,-

IV,IV,IV) redox couple. 4 ) iminodicarboxylates and O ) amines.

Potentials are versus Fc/Fc⁺.

Figure 10. Isothermal magnetization of 7 (O, 4, 0) and 9 (b, 2, 9). The

solid line corresponds to a fit of the Brillouin function for 7(ClO₄)₄(2 K

data) with S ) 6 and g ) 1.94. Symbols: 2 K (O, b), 5 K (4, 2), 10 K

(0, 9).

vicinity 7-20 K, below which temperature the value

decreases. Notable are the maximum ø_MT values for 4 and

7 at 0.5 T of 20.5 (7 K) and 20.6 (13 K), respectively, which

are very close to the value expected for an S ) 6 ground

state of 21.0. It is expected, however, that significant changes

in the ligand field environment of the Mn^IVions with ligand

substitution, as well as changes in the magnetic exchange

pathways due to [Mn₄O₆]⁴⁺core structural differences for

the complexes discussed above might well lead to differences

in magnetic exchange and ground-state spin. For example,

the values of maximum ø_MT were highest for the highest

symmetry complexes 4 and 7, somewhat lower for 1 (18.3

at 18 K), and lowest for the mixed ligand complexes 8 and

9 at 17.5 (20 K) and 17.6 (17 K), respectively. Further

indications of the differences in the magnetic behavior among

these complexes are evident in the variable-field magnetiza-

tion and analysis of the magnetic susceptibility as a function

of temperature.

The ground-state spin of each 1, 7, and 9 was studied with

isothermal magnetization. The field dependence of the

magnetization of 7 at 2, 5, and 10 K, as a plot of M (µ_B/

mol) versus µ_BH/kT (Figure 10), exhibits saturation around

11.6 µ_B/mol, which could be fit with the Brillouin function

with S ) 6 and g ) 1.94.¹¹⁶For comparison, analysis of the

magnetic behavior of 4 previously established that it has an

S ) 6 ground state.^{19, 20}

Figure 9. Temperature dependence of ø_MT of 7(ClO₄)₄(4) and 9 (O) at

0.5 T.

Magnetic Susceptibility and Isothermal Magnetization.

Magnetic susceptibility measurements versus temperature

were conducted on powdered samples of 4, 7, 8, and 9 and

isothermal magnetization versus applied magnetic field were

measured for 1, 7, and 9.

Magnetic susceptibility data for 7 and 9 are plotted as ø_MT

versus T in Figure 9. Data for 4 and 8 are in Supporting

Information as Figures S7 and S8, respectively. Qualitatively,

the behavior for adamantane complexes synthesized via

ligand substitution is similar to that observed for 1.^21,23Each

complex is overall ferromagnetically coupled, with room

temperature (300 K) ø_MT values between 10.9 and 12.3 cm³

mol^-1K.¹¹⁵These values are also similar to the room

temperature ø_MT value for 1 of 10.9. Overall ferromagnetic

coupling within 4 has been reported previously.^19,20Upon

cooling, the value of ø_MT increases to a maximum in the

On the other hand, the magnetization behavior of 9, which

exhibited saturation around 10.6 µ_B/mol (Figure 10), suggests

population of low lying (S < 6) states and zero-field splitting

of the ground state. The magnetization behavior of 1 (Figure

S9) also indicates some degree of zero-field splitting in the

ground state.

In prior work, the magnetic susceptibility as a function of

temperature for 4 was modeled with the Van Vleck equa-

tion¹¹⁷using an isotropic Heisenberg exchange Hamiltonian

(114) Limburg, J.; Szalai, V. A.; Brudvig, G. W. J. Chem. Soc., Dalton

Trans. 1999, 1353-1362.

(115) For comparison, the spin-only ø_MT value for four uncoupled high-

(116) When H/kT is large, the magnetization M (in units of µ_B/mol) saturates

at a value given by gS.

spin d³ions is 7.50 cm³mol^-1K.

Inorganic Chemistry, Vol. 44, No. 14, 2005 5171

Dube´ et al.

in the form Hˆ ) -JijSˆ_i‚Sˆ_jwith a single J (14.5 cm^-1), based

on an idealized tetrahedral symmetry.²⁰The quality of the

fit, which was relatively poor, was significantly improved

with incorporation of a large biquadratic exchange term j (J

) 13.8 cm^-1, j ) 7.8 cm^-1). On the other hand, the

ferromagnetic exchange of 1, possessing two different types

of oxo bridges coupling the Mn^IVions,²¹was modeled with

Hˆ ) -J_ijSˆ_i‚Sˆ_jusing two exchange constants, giving an

excellent fit with J₁)10.2 cm^-1, J₂)90.0 cm^-1, and g )

1.88.²³A biquadratic exchange term did not improve the

quality of the fit markedly. While we have not found a single

magnetic exchange model that adequately describes the

magnetic behavior of all of the adamantane-shaped [Mn₄O₆]⁴⁺

complexes, the overall ferromagnetic coupling of the Mn^IV

ions is best regarded as pairwise ferromagnetic. The overall

ferromagnetic coupling in these [Mn₄O₆]⁴⁺cores is unusual

among the known tetranuclear manganese-oxo aggre-

gates,^{37,40,42,45,118-124}and is the subject of an ongoing

investigation that incorporates broken symmetry/density

functional theory (BS/DFT) to model the magnetic ex-

change.¹²⁵

1

¹H NMR Spectroscopy. The H NMR spectra of 4-11

can be understood in terms of their magnetic properties and

assuming that the solution structural symmetry is the same

as that displayed in the solid state. The ¹H NMR spectra of

1, 7, 10, and 11 (Figure 11) are representative of the

Figure 11. ¹H NMR spectra of (a) 1, (b) 7, (c) 10, and (d) 11. Condi-

tions: CD₃CN (1 and 7), CD₃OD (10 and 11). *, CD₂HCN; +, CD₂HOD.

1

differences observed for the various types of ligands. H

NMR spectra of the remaining complexes can be found in

the Supporting Information. Much of the analysis presented

here has benefited from earlier research on paramagnetic ¹H

NMR properties of manganese-oxo complexes.^{21,25,126,127}

Ligand proton resonances can be separated into two types,

those assigned to pyridyl ring protons and those assigned to

methylene protons. Proton resonances for the complexes

described here are found in the range +60 > δ > -152

ppm and are provided in Table S4. The pyridyl ring proton

resonances for 1 are found in the region +55 > δ > -20

ppm, and the methylene groups of 1 fall in the range -22

> δ > -82 ppm.²¹

While the methylene resonances for 4 are at -41.4 ppm

(2900 Hz) and -56.7 ppm (1640 Hz), and the methylene

resonances of the Medien complex 6 comprise overlapping

peaks centered at -45 ppm (3800 Hz), the methylene

resonances of the tach complex 7 are downfield at 16.8 ppm

(703 Hz) and 7.2 ppm (1640 Hz). The methylene protons of

7 are shifted downfield because they are further removed

from the metal center by one additional σ-bond. The methine

protons of the tach ligand are not observed for 7. The two

equal-intensity resonances for both 4 and 7 are distinguished

by measurably different peak widths. The broader resonances

are assigned to those methylene protons directed toward the

metal center (axial, H_ax), while the sharper resonances are

assigned to those methylene protons directed away from the

metal center (equatorial, H_eq), analogous to the assignments

made for the methylene protons of [Mn₂O(OAc)₂(tacn)₂]²⁺.¹²⁶

The bpea/R-ida mixed ligand complexes exhibit new

resonances in the region -50 > δ > -100 ppm, corre-

sponding to the methylene protons of the R-ida ligands.

Additional resonances of some of the alkyl nitrogen substit-

uents are evident in the region 8 > δ > -1 ppm. Although

the bpea ligands within each of these complexes are

equivalent, we did note differences in the split (or lack

thereof) of the 4,4′ and 5,5′ pyridyl ring proton resonances

between the complexes.¹²⁸For example, the 5,5′ pyridyl ring

proton resonances of 8 and 10 are overlapping at 300 MHz

(117) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986; p

15-18.

(118) Mikuriya, M.; Yamato, Y.; Tokii, T. Chem. Lett. 1991, 8, 1429-

1432.

(119) Chandra, S. K.; Chakravorty, A. Inorg. Chem. 1991, 30, 3795-.

(120) Aubin, S. M. J.; Dilley, N. R.; Wemple, M. W.; Maple, M. B.;

Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1998, 120, 839-

840.

(121) Chan, M. K.; Armstrong, W. H. J. Am. Chem. Soc. 1989, 111, 9121-

9122.

(122) Chan, M. K.; Armstrong, W. H. J. Am. Chem. Soc. 1990, 112, 4985-

4986.

(123) Suzuki, M.; Hayashi, Y.; Munezawa, K.; Suenaga, M.; Senda, H.;

Uehara, A. Chem. Lett. 1991, 1929-1932.

(124) Afrati, T.; Dendrinou-Samara, C.; Raptopoulou, C. P.; Terzis, A.;

Tangoulis, V.; Kessissoglou, D. P. Angew. Chem., Int. Ed. 2002, 41,

2148-2150.

(125) Armstrong, W. H.; Dube´, C. E.; Gatteschi, D.; Sessoli, R.; Totti, F.;

Noodleman, L. In Magnetic coupling in and reactiVity of multinuclear

manganese(IV) compounds: Insight from density functional theory,

220th ACS National Meeting, Washington, DC, Aug. 20-24, 2000;

American Chemical Society: Washington, DC, 2000.

(126) Wright, D. W.; Mok, H. J.; Dube´, C. E.; Armstrong, W. H. Inorg.

Chem. 1998, 37, 3714-3718.

(128) No definitive assignment was made for the 3,3′ pyridyl ring proton

resonances of the bpea/R-ida mixed ligand complexes. See: J. Am.

Chem. Soc. 1998, 120, 3704-3716 for a tabulation of assignments

for the 3,3′ pyridyl ring proton resonances of related complexes.

(127) Mok, H. J. Ph.D., Boston College, Chestnut Hill, MA, 1999.

5172 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

(52.0 and 54.7 ppm, respectively), while the splitting of the

4,4′ pyridyl ring proton resonances, at -17.0 and -17.3 ppm

for 8 and -16.0 and -16.7 for 10, is 0.3 and 0.7 ppm,

respectively. On the other hand, the 5,5′ pyridyl ring proton

resonances of 11 exhibit a slight split (55.6, 53.8 ppm, ∆ )

1.8 ppm), while the splitting of the 4,4′ pyridyl ring proton

resonances (-16.7, -17.3 ppm, ∆ ) 0.6 ppm) is similar to

that of 8 and 10. These data are in contrast to the larger

separations of the 4,4′ (-18.6, -20.2 ppm, ∆ ) 1.6 ppm)

and 5,5′ (54.9, 46.1 ppm, ∆ ) 8.8 ppm) pyridyl ring proton

resonances of 1. Previously, we had suggested that the

differences in the pyridyl ring proton resonances of 1 were

due to longer and shorter Mn-N_pyrbonds, the shorter bond

leading to tighter coupling to the core and a larger chemical

shift.²¹Comparison of the two Mn-N_pyrbonds, 2.090(2) and

2.108(2) Å for 1 (longer - shorter ∆ ) 0.018 Å) and

2.079(4) and 2.094(4) Å for 11 (∆ ) 0.015 Å) shows a

similarity of the longer-shorter difference, suggesting that

the differences in the 5,5′ and 4,4′ pyridyl ring proton

resonances are due to some other factor. An implication of

this result is that although the contact shift of the pyridyl

ring proton resonances is likely the dominant component of

their isotropic shift, they may have a measurable dipolar

component.

Figure 12. Spectrophotometric titration of an aqueous solution of 8 with

HClO₄(isosbestic points at 567 and 657 nm). See the Experimental Section

for details.

Protonation Studies. We have reported structural and

electronic changes concomitant with changes in the proto-

nation state of the oxo bridges of 1 and 4.^20,21,25Ligand

substitution with iminodicarboxylate ligands greatly increased

the overall electron density of the [Mn₄O₆]⁴⁺cores, as

demonstrated by the large shift in reduction potentials of

8-11. Furthermore, the lower symmetry of the aforemen-

tioned complexes creates four site-differentiated oxo bridges,

compared with two site-differentiated bridging oxo ligands

for 1. Unlike the subtle oxo bridge site differentiation of 1,

however, we expected that the substantial inductive effect

of the carboxylate ligands would significantly differentiate

oxo bridge electron densities of 8-11. In addition, we

anticipated that the increased overall electron density of the

[Mn₄O₆]⁴⁺core would facilitate protonation in water, unlike

protonation of 1, which required a strong acid in acetonitrile.

Reversible protonation of an oxo bridge of 8 in water with

HClO₄to give [Mn₄O₅(OH)(bpea)₂(me-ida)₂](ClO₄) (8H-

(ClO₄)) was demonstrated spectrophotometrically (Figure 12)

Figure 13. ¹H NMR of (a) 8 and (b) 8 plus 1 equiv of CF₃SO₃H in D₂O

to form 8H(CF₃SO₃). See the Experimental Section for details. *, HOD.

aqueous pK_aof 8 of 2.20 ( 0.03, determined spectropho-

tometrically with HClO₄using eq 1 (Figure S14), is among

the highest pK_avalues of the adamantane-like [Mn₄O₆]⁴⁺

cores, as expected for the iminodicarboxylate ligand me-

ida. Protonation of 8 was also demonstrated with CF₃COOH

and CH₃SO₃H, giving an aqueous pK_aof 2.07 and 2.00,

respectively. Quantitative reversibility of protonation was

verified by spectrophotometric back-titration with Et₃N.

Protonation of 11 in water with HClO₄and CF₃SO₃H was

followed spectrophotometrically as well, and fitting of

absorbance versus pH for titration with HClO₄(Figure S15)

gave an aqueous pK_aof 2.32 ( 0.02. Ligand electronic

effects on the adamantane-shaped [Mn₄O₆]⁴⁺core tune the

oxo bridge acidity over 11 orders of magnitude.¹³³Ligand

tuning of the manganese-oxo core effective acidity has also

1

and by H NMR in D₂O using CF₃SO₃H to give 8H(CF₃-

SO₃) (Figure 13) (see also the ESI mass spectrum of 8, Figure

5).

Formation of 8H⁺was complete after addition of 1 equiv

1

of acid. The observation of distinct H NMR spectra for 8

and 8H(CF₃SO₃) in the course of titration of 8 with CF₃-

SO₃H is a consequence of the slow rate of proton self-

exchange on the NMR time scale and is consistent with data

for other metal-oxo complexes.^{21,25,129-132}The effective

(129) Kramarz, K. W.; Norton, J. R., Progress in Inorganic Chemistry. In

Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New

York, 1994; Vol. 42, pp 1-65.

(130) Carroll, J. M.; Norton, J. R. J. Am. Chem. Soc. 1992, 114, 8744-

8745.

(131) Fox, S.; Alaganandan, N.; Marten, W.; Karlin, K. D.; Blackburn, N.

J. J. Am. Chem. Soc. 1996, 118, 24-34.

(132) Evans, D. R.; Mathur, R. S.; Heerwegh, K.; Reed, C. A.; Xie, Z.

Angew. Chem., Int. Ed. Engl. 1997, 36, 1335-1337.

Inorganic Chemistry, Vol. 44, No. 14, 2005 5173

Dube´ et al.

Table 5. Comparison of Reduction Potentials and Acid Dissociation

Constants for 1, 4, and 11 with Selected [Mn₂O₂(X-salpn)₂] Complexes

protonation with 1 equiv of CF₃SO₃H in D₂O, similar to the

degree of change seen for protonation of 1. However, while

there are four site-differentiated oxo bridges in 8, the solution

structural symmetry of 8H⁺reveals essentially a single

protonation isomer, unlike the two protonation isomers

[Mn₄O₆]⁴⁺

[Mn₂O₂]^{4+ a}

E_1/2

pK_a

(H₂O)

E_1/2

pK_a

(H₂O)

(mV)^c

-6.5^d

3.10

2.32

14^b

15^b

16^b

-440

-795

-1048

-0.7^d

4.0^d

1

observed for 1H⁺by H NMR, one for each of the site-

1

4

11

-280

-733

-991

differentiated oxo bridges in 1.^21,136Furthermore, 8H⁺

exhibits equivalency of the bpea ligands, similar to that

observed for 8 (vide supra). The higher solution symmetry

of 8H⁺relative to that observed for 1H⁺is consistent with

protonation at one of the two oxo bridges on the C₂axis.

We would expect this to be the oxo bridge trans to two

carboxylate ligands (atom O(2) in Figure 3), based on the

significant donor strength of the carboxylate oxygen atoms.

About 30 min after addition of either CF₃SO₃H or HClO₄

to an aqueous solution of 8 or 11, the protonated complex

precipitated from solution. The precipitate of either 8H⁺or

11H⁺could be readily solubilized with Et₃N, however,

affording the spectrum of 8 or 11, with nearly quantitative

recovery.

6.6^d

^a[Mn₂O₂]⁴⁺data from Baldwin et al.^{134 b}[Mn₂O₂(5-NO₂-salpn)₂] (14);

[Mn₂O₂(5-Cl-salpn)₂] (15); [Mn₂O₂(5-OCH₃-salpn)₂] (16); X-salpn are

derivatives of H₂salpn (N,N′-trimethylenebis(salicylidene)-1,3-diaminopro-

c

pane) with substitution on the phenolate rings. E_1/2versus Fc/Fc⁺; data

for 14, 15, and 16 corrected to Fc/Fc⁺using E_1/2(Fc/Fc⁺) ) 460 mV versus

the SCE in CH₂Cl₂with [Bu₄N][PF₆] electrolyte.^{73 d}pK_ameasured in

acetonitrile. The aqueous pK_awas estimated using the relation pK_a(H₂O)

) pK_a(CH₃CN) - 7.5¹³⁵

Concluding Remarks

Although the tetranuclear arrangement of Mn ions in

adamantane-shaped [Mn₄O₆]⁴⁺is too symmetrical and the

Mn‚‚‚Mn distance too long for it to be considered as an

accurate structural model of the S₀-S₂states of the active

site,^1-6,137this core has mechanistic implications with regards

to S-state advancement and substrate oxidation at the active

site. For example, EXAFS analysis of the Mn-oxo core

within PSII reveals a metal-metal distance increase observed

for the PSII Mn₄when advancing from S₂to S₃states,⁴⁸

suggesting to us that the Mn-oxo core may undergo

significant structural rearrangement, rather than necessarily

an increase in the Mn-O_oxobond length. We believe a core

“shape-shift”, like our recent report of a core rearrangement

of a dimer-of-dimers to an adamantane core,⁴³is a possible

explanation for changes in the metal-metal distance ob-

served for advancement from S₂to S₃and S₃to S₀. We are

currently exploring further conditions that promote a core

shape-shift from a dimer-of-dimers to an adamantane core,

as well as conditions that might promote the rearrangement

of an adamantane to a dimer-of-dimers for Mn-oxo ada-

mantanes, such as those reported here.

Figure 14. ¹H NMR of (a) 1H(CF₃SO₃)₅in CD₃CN and (b) 8H(CF₃SO₃)

in D₂O. *, HOD; +, CD₂HCN.

been demonstrated for the dimeric Mn^IVseries [Mn₂(µ-O)₂-

(X-salpn)₂], whose oxo bridge aqueous pK_avalues ranged

from -0.7 to 6.6 (Table 5).¹³⁴

Although the chemical shifts and line widths of the

resonances of the protonated complex 8H(CF₃SO₃) are

qualitatively similar to protonation results observed for 1 in

acetonitrile, there are some striking differences (Figure 14).

Solution magnetic susceptibility measurements reveal that

the magnetic coupling changes from overall ferromagnetic

for 8 (ø_MT of 12.5 cm³mol^-1K) to overall moderate

antiferromagentic for 8H⁺(6.3 cm³mol^-1K)¹¹⁵following

Although the exact coordination environment is a matter

of ongoing investigation, it is generally believed that the

Mn-oxo complex of the OEC is coordinated predominantly

by carboxylates from aspartate and glutamate, along with

oxide bridges and one or two imidazoles.^8,9,15,16Substitution

reactions of 1 with iminodicarboxylates afforded complexes

8-11 that more closely approximate the coordination

environment of PSII and that very likely could not have been

(133) The highest pK_aamong the [Mn₄O₆]⁴⁺adamantane cores of 12.5 in

acetonitrile for the first protonation of [Mn₄O₆(tame)₄](CF₃SO₃)₄, was

determined by spectrophotometric titration with pyridinium triflate

in acetonitrile. A corresponding aqueous pK_aof 5.0 is estimated using

the relation pK_a(H₂O) ) pK_a(CH₃CN) - 7.5 (Kristjansdottir, S. S.;

Norton, J. R. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH

Publishers: New York, 1992; pp 309-359) (Dube´, C. E.; Armstrong,

W. H. unpublished results).

(135) Kristjansdottir, S. S.; Norton, J. R. Transition Metal Hydrides. In

Transition Metal Hydrides; Dedieu, A., Ed.; VCH Publishers: New

York, 1992; pp 309-359.

(136) We also noted a trace level of another species, whose resonances

are similar to those of 1H⁺and which we believe derives from

protonation of oxo ligand(s) lying in the plane perpendicular to the

C₂axis.

(134) Baldwin, M. J.; Stemmler, T. L.; Riggs-Gelasco, P. J.; Kirk, M. L.;

Penner-Hahn, J. E.; Pecoraro, V. L. J. Am. Chem. Soc. 1994, 116,

11349-11356.

(137) Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1989, 28, 1153-1172.

5174 Inorganic Chemistry, Vol. 44, No. 14, 2005

Tuning Manganese-Oxo Core Electronic Properties

prepared under the oxidation conditions utilized in self-

assembly of high-valent manganese-oxo complexes. Fur-

thermore, the unique substitution chemistry of 1 with R-ida

ligands afforded asymmetrically ligated complexes, the

mixed ligand nature of which is most likely unachievable

using self-assembly synthetic methods.

electrochemistry of the carboxylate complex 11 is of

particular interest, since it has the lowest (V, IV, IV, IV)/

(IV, IV, IV, IV) redox couple of the known adamantane-

shaped [Mn₄O₆]⁴⁺complexes and, as such, is an attractive

candidate for isolation and reaction studies of stable Mn^V-

oxo model compounds.

A special feature of the asymmetrically ligated complexes

8-11 is the substantial site differentiation of the oxo bridges

of the [Mn₄O₆]⁴⁺cores as revealed by protonation. In a

number of metalloenzymes, site-specific tuning of the

chemical reactivity of the active site is achieved through a

combination of coordination of amino acid residues, steric

interactions between the metal ions and amino acid residues,

and hydrogen bonding. Such factors are difficult to incor-

porate, however, into structural models of metalloenzymes

synthesized by spontaneous self-assembly. Some Mn-oxo

dimeric systems have achieved oxo bridge discrimination by

the asymmetric coordination of different ligands to the metal

ions, as demonstrated in the solid-state structure of [(Me₃-

tacn)Mn(µ-O)₂(µ-OAc)Mn(η²-OAc)(η¹-OAc)] and [(Me₃-

tacn)Mn(µ-O)₂(µ-OAc)Mn(bpy)(MeOH)](ClO₄)₂complexes.¹³⁸

In contrast to the subtle oxo bridge site differentiation in 1,

asymmetric ligation of 11 has substantially altered the oxo

bridge electron density to afford essentially a single proto-

nation isomer. The differentiated sites within complexes

8-11 using iminodicarboxylate ligation demonstrates how

amino acid side chains might similarly differentiate the site

of substrate binding within the Mn complex of the OEC.

One of the current proposals for water oxidation suggests

that a Mn^VdO species^57-61may be a key reactive intermedi-

ate in the O-O bond-forming step. In this regard, the

Acknowledgment. We thank Prof. D. W. Wright for

helpful discussions of the interpretation of ¹H NMR spectra.

Funding for this research was provided by the National

Institutes of Health grant GM38275. The National Institutes

of Health grant 1 S10RR09008 and Boston College provided

funds for purchase of a Siemens SMART single crystal X-ray

diffractometer. The W.M. Keck Foundation provided funds

for the SQUID magnetometer at Tufts University. The

Micromass Quattro mass spectrometer at the University of

Illinois Mass Spectrometry Laboratory was purchased in part

with a grant from the Division of Research Resources,

National Institutes of Health (RR 07141).

Supporting Information Available: A description of the

synthesis of N-cyclopentyliminodiacetic acid, N-tert-butylimino-

diacetic acid, and (Bu₄N)₂(me-ida); a table of yield, elemental

analysis and ¹H NMR data for (n-Bu₄N)₂(R-ida) and (n-Et₄N)₂(R-

ida); a description of the synthesis of cis,cis-1,3,5-triaminocyclo-

hexane; a description of the synthesis of 9 and 10; tables of yield,

elemental analysis, ESI-MS, UV-vis, electrochemistry, IR, and ¹H

NMR data for 4-11; ESI mass spectra of 4-6, 9-11; ø_MT versus

1

T plots for 4 and 8, a M (µ_B/mol) versus µ_BH/kT plot for 1; H

NMR spectra of 4, 5, 6, and 9; plots of nonlinear least-squares fit

of absorbance versus pH for titration of 8 and 11; and full

crystallographic information for 7 and 11, including figures

illustrating hydrogen bonding. This material is available free of

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

(138) Bossek, U.; Saher, M.; Weyhermuller, T.; Wieghardt, K. J. Chem.

Soc., Chem. Commun. 1992, 1780-1782.

IC050183W

Inorganic Chemistry, Vol. 44, No. 14, 2005 5175

Article Doi

Tuning tetranuclear manganese-oxo core electronic properties: Adamantane-shaped complexes synthesized by ligand exchange

DOI: 10.1021/ic050183w

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Article abstract of DOI:10.1021/ic050183w

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