Structural, spectroscopic, and electrochemical studies of binuclear manganese(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles

Article abstract of DOI:10.1021/ic9814470

Structural, electrochemical, ESR, and H₂O₂ reactivity studies are reported for [Mn(dmptacn)Cl]ClO₄ (1, dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) and binuclear complexes of bis(pentadentate) ligands, generated by attaching 2-pyridylmethyl arms to each secondary nitrogen in bis(1,4,7-triazacyclononane) macrocycles and linked by ethyl (tmpdtne, [Mn₂(tmpdtne)Cl₂](ClO₄)₂·2DMF, 2), propyl (tmpdtnp, [Mn₂(tmpdtnp)Cl₂](ClO₄)₂· 3H₂O, 3), butyl (tmpdtnb, [Mn₂(tmpdtnb)Cl₂](ClO₄)₂·DMF·2H₂O, 4), m-xylyl (tmpdtn-m-X, [Mn₂(tmpdtn-m-X)-Cl₂](ClO₄)₂, 5) and 2-propanol (tmpdtnp-OH, [Mn₂(tmpdtnp-OH)Cl₂](ClO₄)₂, 6) groups. 1 crystallizes in the orthorhombic space group P2₁2₁2₁ (No. 19) with a = 7.959(7) A, b = 12.30(1) A, and c = 21.72(2) A; 2, in the monoclinic space group P2(1/c) (No. 14) with a = 11.455(4) A, b = 15.037(6) A, c = 15.887(4) A, and β = 96.48(2)°; 3, in the monoclinic space group P2(1/c) (No. 14) with a = 13.334(2) A, b = 19.926(2) A, c = 18.799-(1) A, and β = 104.328(8)°; and [Mn₂(tmpdtnb)Cl₂](ClO₄)₂·4DMF·3H₂O (4'), in the monoclinic space group P2(1/n) (No. 14) with a = 13.361(3) A, b = 16.807(5) A, c = 14.339(4) A, and β = 111.14(2)°. Significant distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75°) and the angles spanned by trans donor atoms (<160°). The Mn geometry is intermediate between octahedral and trigonal prismatic, and for complexes 2-4, there is a systematic increase in M···M distance with the length of the alkyl chain. Cyclic and square-wave voltammetric studies indicate that 1 undergoes a 1e^- oxidation from Mn(II) to Mn(III) followed by a further oxidation to Mn(IV) at a significantly more positive potential. The binuclear Mn(II) complexes 2-5 are oxidized to the Mn(III) state in two unresolved 1e^- processes {Mn(II)₂ → Mn(II)Mn(III) → Mn(III)₂} and then to the Mn(IV) state {Mn(III)₂ → Mn(III)Mn(IV) → Mn(IV)₂}. For 2, the second oxidation process was partially resolved into two 1e^- oxidation processes under the conditions of square-wave voltammetry. In the case of 6, initial oxidation to the Mn(III)₂ state occurs in two overlapping 1e^- processes as was found for 2-5, but this complex then undergoes two further clearly separated 1e^- oxidation processes to the Mn(II)Mn(IV) state at +0.89 V and the Mn(IV)₂ state at +1.33 V (vs Fc/Fc⁺). This behavior is attributed to formation of an alkoxo-bridged complex. Complexes 1-6 were found to catalyze the disproportionation of H₂O₂. Addition of H₂O₂ to 2 generated an oxo-bridged mixed-valent Mn(III)Mn(IV) intermediate with a characteristic 16-line ESR signal.

Full text of DOI:10.1021/ic9814470

Inorg. Chem. 2000, 39, 881-892
881
Structural, Spectroscopic, and Electrochemical Studies of Binuclear Manganese(II)
Complexes of Bis(pentadentate) Ligands Derived from Bis(1,4,7-triazacyclononane)
Macrocycles
Suzanne J. Brudenell,† Leone Spiccia,*,† Alan M. Bond,† Gary D. Fallon,†
David C. R. Hockless,‡ George Lazarev,§ Peter J. Mahon,† and Edward R. T. Tiekink|
Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia, Research School of
Chemistry, Australian National University, Canberra, ACT 2601, Australia, Department of Physics,
Monash University, Clayton, Victoria 3168, Australia, and Department of Chemistry,
The University of Adelaide, Adelaide, South Australia 5005, Australia
ReceiVed December 18, 1998
Structural, electrochemical, ESR, and H2O2 reactivity studies are reported for [Mn(dmptacn)Cl]ClO4 (1, dmptacn
) 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane) and binuclear complexes of bis(pentadentate) ligands, generated
by attaching 2-pyridylmethyl arms to each secondary nitrogen in bis(1,4,7-triazacyclononane) macrocycles and
linked by ethyl (tmpdtne, [Mn2(tmpdtne)Cl2](ClO4)2‚2DMF, 2), propyl (tmpdtnp, [Mn2(tmpdtnp)Cl2](ClO4)2‚
3H2O, 3), butyl (tmpdtnb, [Mn2(tmpdtnb)Cl2](ClO4)2‚DMF‚2H2O, 4), m-xylyl (tmpdtn-m-X, [Mn2(tmpdtn-m-X)-
Cl2](ClO4)2, 5) and 2-propanol (tmpdtnp-OH, [Mn2(tmpdtnp-OH)Cl2](ClO4)2, 6) groups. 1 crystallizes in the
orthorhombic space group P212121 (No. 19) with a ) 7.959(7) Å, b ) 12.30(1) Å, and c ) 21.72(2) Å; 2, in the
monoclinic space group P21/c (No. 14) with a ) 11.455(4) Å, b ) 15.037(6) Å, c ) 15.887(4) Å, and â )
96.48(2)°; 3, in the monoclinic space group P21/c (No. 14) with a ) 13.334(2) Å, b ) 19.926(2) Å, c ) 18.799-
(1) Å, and â ) 104.328(8)°; and [Mn2(tmpdtnb)Cl2](ClO4)2‚4DMF‚3H2O (4′), in the monoclinic space group
P21/n (No. 14) with a ) 13.361(3) Å, b ) 16.807(5) Å, c ) 14.339(4) Å, and â ) 111.14(2)°. Significant
distortion of the Mn(II) geometry is evident from the angle subtended by the five-membered chelate (ca. 75°) and
the angles spanned by trans donor atoms (<160°). The Mn geometry is intermediate between octahedral and
trigonal prismatic, and for complexes 2-4, there is a systematic increase in M‚‚‚M distance with the length of
the alkyl chain. Cyclic and square-wave voltammetric studies indicate that 1 undergoes a 1e- oxidation from
Mn(II) to Mn(III) followed by a further oxidation to MnIV at a significantly more positive potential. The binuclear
Mn(II) complexes 2-5 are oxidized to the Mn(III) state in two unresolved 1e- processes {MnII2 f MnIIMnIII
f
MnIII2} and then to the MnIV state {MnIII f MnIIIMnIV f MnIV2}. For 2, the second oxidation process was
2
partially resolved into two 1e- oxidation processes under the conditions of square-wave voltammetry. In the case
of 6, initial oxidation to the MnIII2 state occurs in two overlapping 1e- processes as was found for 2-5, but this
complex then undergoes two further clearly separated 1e- oxidation processes to the MnIIIMnIV state at +0.89 V
and the MnIV2 state at +1.33 V (vs Fc/Fc+). This behavior is attributed to formation of an alkoxo-bridged complex.
Complexes 1-6 were found to catalyze the disproportionation of H2O2. Addition of H2O2 to 2 generated an
oxo-bridged mixed-valent MnIIIMnIV intermediate with a characteristic 16-line ESR signal.
Introduction
variability in magnetic behavior of polynuclear manganese
clusters, which for some systems has been exploited in the
development of information storage devices,11 and the use of
such complexes as catalysts for bleaching and organic synthe-
sis.12
Interest in manganese complexes of varying nuclearities
continues to be stimulated by the presence of manganese at the
active sites of biological systems such as photosystem II in green
plants, ribonucleotide reductase, superoxide dismutase, and the
Mn catalases (see refs 1-10 and references therein), the
The manganese coordination chemistry of the tridentate
macrocycle 1,4,7-triazacyclononane (tacn), its 1,4,7-trimethyl
* Corresponding author. E-mail: leone.spiccia@sci.monash.edu.au.
† Department of Chemistry, Monash University.
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Folding, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am.
Chem. Soc. 1993, 115, 1804. (b) Chudnovsky, E. M. Science 1996,
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‡ Australian National University.
§ Department of Physics, Monash University.
| The University of Adelaide.
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10.1021/ic9814470 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/11/2000

882 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
analogue (Me3tacn), derivatives bearing pendant coordinating
groups, and some bis(tridentate) macrocycles incorporating tacn
has attracted significant attention,13-29 since the first reports of
polynuclear Mn complexes incorporating such ligands in the
mid-1980s. Some of the products have been described as models
for the polynuclear manganese cluster in the “oxygen-evolving
complex” of photosystem II, and others, as models for binuclear
Mn biosites. An attractive feature of tacn-based ligands is their
ability to form thermodynamically stable but redox-active Mn-
(II) compounds,18 which can be converted into higher-valent
complexes. Polynuclear oxo, oxo-carboxylato, oxo-peroxo,
phenoxo-carboxylato, hydroxo-carboxylato, and oxo-bor-
onato bridged complexes with varying Mn oxidation states are
among those reported, and mixed-valent polynuclear complexes
are not uncommon.13-16,18-29 Complexes with an (oxo/hydroxo)-
bis(carboxylato)dimanganese core have been of interest because
such a core is found at the active site of some catalases and
ribonucleotide reductase. The peroxo-bridged complex [(Me3-
tacn)2Mn2(µ2-O)2(µ-O2)]2+ also has particular relevance to the
function of these metalloenzymes.21
Scheme 1
Manganese complexes derived from Me3tacn and the bis-
(1,4,7-triazacyclononane) ligand 1,2-bis(4, 7-dimethyl-1,4,7-
triazacyclonon-1-yl)ethane (Me4dtne) were recently shown to
enhance the low-temperature bleaching of stains by hydrogen
peroxide.12a The mixed-valent complex [MnIIIMnIV(µ-O)2(µ-
CH3COO)Me4dtne]2+ was found to be particularly effective. It
exhibits a 16-line ESR spectrum typical of µ-oxo mixed-valent
MnIIIMnIV complexes.
Although many dimanganese complexes have been studied
to increase the understanding of the role and mechanism of
action of manganese in dioxygen chemistry, binuclear manga-
nese complexes incorporating bis(1,4,7-triazacyclononane) ligands
are relatively unexplored. In this paper, we report structural,
physicochemical, and electrochemical properties of a series of
binuclear Mn(II) complexes based on the bis(pentadentate)
ligands that are formed by attaching 2-pyridylmethyl pendant
groups to each secondary nitrogen in bis(1,4,7-triazacy-
clononane) macrocycles (see Scheme 1). Since many of the
previously reported Mn compounds that enhance hydrogen
peroxide disproportionation incorporate tridentate blocking
ligands and multiple bridges between Mn centers, we were
surprised to find that our “unsupported” complexes were also
active. Preliminary solution studies of the reactions of these
complexes with hydrogen peroxide reveal interesting variations
in reactivity with the spacer separating the two ligand compart-
ments, which are observable by UV-visible and ESR spectros-
copy.
(12) (a) Hage, R.; Iburg, J. E.; Kerschner, J.; Koek, J. H.; Lempers, E. L.
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Experimental Section
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Am. Chem. Soc. 1990, 112, 6387.
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Schugar, H. J.; Isied, S. S.; Hendrickson, D. N. J. Am. Chem. Soc.
1989, 111, 5102.
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Commun. 1992, 1780.
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R.; Russell, S. Poster presented at the Eighth International Conference
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Materials. Reagent or AR grade chemicals were used in this study.
Published methods were used to prepare 1,4-bis(2-pyridylmethyl)-1,4,7-
triazacyclononane (dmptacn),30 1,2-bis[4,7-bis(2-pyridylmethyl)-1,4,7-
triazacyclonon-1-yl]ethane (tmpdtne),31 1,3-bis[4,7-bis(2-pyridylmethyl)-
1,4,7-triazacyclonon-1-yl]propane (tmpdtnp),31 1,4-bis[4,7-bis(2-pyridyl-
methyl)-1,4,7-triazacyclonon-1-yl]butane (tmpdtnb),31 1,3-bis[4,7-bis-
(2-pyridylmethyl)-1,4,7-triazacyclonon-1-ylmethyl]benzene (tmpdtn-m-
X)32 and 1,3-bis[4,7-bis(2-pyridylmethyl)-1,4,7-triazacyclonon-1-yl]-
2-propanol (tmpdtnp-OH).32
Caution! Although no problems were encountered in this work,
transition metal perchlorates are potentially explosiVe. They should
be prepared in small quantities and handled with care.
Physical Measurements. Infrared spectra were measured on a
Perkin-Elmer 1600 FTIR spectrometer as KBr pellets. Electron micro-
probe analyses were made with a JEOL JSM-1 scanning electron
(30) McLachlan, G. A.; Fallon, G. D.; Martin, R. L.; Moubaraki, B.;
Murray, K. S.; Spiccia, L. Inorg. Chem. 1994, 33, 4663.
(31) Brudenell, S. J.; Spiccia, L.; Tiekink, E. R. T. Inorg. Chem. 1996, 35,
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(32) Brudenell, S. J.; Spiccia, L.; Bond, A. M.; Hockless, D. C. R.; Tiekink,
E. R. T. Inorg. Chem. 1998, 37, 3705.

Binuclear Macrocyclic Manganese(II) Complexes
Inorganic Chemistry, Vol. 39, No. 5, 2000 883
microscope through an NEC X-ray detector and pulse processing system
connected to a Packard multichannel analyzer. Microanalyses were
carried out by the Chemical and Micro-Analytical Services (CMAS),
Melbourne, Australia. Room-temperature magnetic moments were
determined by the Faraday method. Diamagnetic corrections were made
using Pascal’s constants. Low-temperature magnetic susceptibility
measurements were carried out on a Quantum Design MPMS SQUID
magnetometer as described previously.12 Electrospray ionization mass
spectra were recorded either on a Micromass Platform QMS with an
electrospray source or on a Bruker BioApex 47e FTMS with a 4-7 T
superconducting magnet and an Analytica electrospray source.
ESR spectra were measured on a Bruker ECS 106 spectrometer as
frozen (99 K) DMF solutions using a concentration of 0.5 mM. To
follow the reactivity of Mn(II) complexes toward hydrogen peroxide,
solutions of the complexes were allowed to react with H2O2 in the ESR
tubes for a particular length of time. The solutions were frozen in liquid
nitrogen to quench the reaction and enable the ESR spectra to be
recorded. Following measurement of the ESR spectra, the solutions
were left to warm to room temperature and allowed to react further.
The solutions were frozen again and the ESR spectra re-recorded.
Electronic spectra were recorded on a Cary 3 spectrophotometer.
Changes in UV-visible spectra that occurred following addition of
aqueous hydrogen peroxide to the Mn(II) complexes (∼5 mM in DMF)
were monitored as a function of time using the kinetics accessory and
software package of the spectrophotometer.
Cyclic, square-wave, and steady-state voltammograms were recorded
on a Cypress CS 1090 system in dry, nitrogen-deoxygenated acetonitrile
solutions (∼0.5 mM) with tetrabutylammonium perchlorate (0.1 M)
as the supporting electrolyte. A platinum auxiliary electrode and an
Ag/Ag+ (10 mM AgNO3) reference electrode were used in combination
with either a platinum macrodisk working electrode (r ) 0.8 mm) for
cyclic and square-wave voltammetry or a platinum microelectrode (r
) 25 µm) for near-steady-state voltammetry conducted in an aluminum
Faraday cage. Potentials are reported relative to the ferrocene/
ferrocenium (Fc/Fc+) couple. Reversible redox potentials (E1/2 values)
were determined from the cyclic voltammograms of the chemically
reversible processes as the midpoint between the oxidation (Epox) and
reduction (Epred) peak potentials, E1/2 ) 1/2(Epox + Epred). Confirmation
of electrochemical reversibility was established by verifying that the
E1/2 values calculated from a CV experiment were equal to the peak
potentials, Ep, of square-wave voltammograms. The E1/2 values were
also equal to the E1/2 values measured under steady-state conditions at
a microelectrode as the potential at half the limiting current value (IL/
2). The diffusion coefficient (D) was calculated from the limiting current
(IL) and the use of the relationship IL) 4nFCDr, where F ) Faraday’s
constant, C ) concentration, r ) radius of the microdisk electrode,
and n ) the number of electrons in the charge-transfer step.
Synthesis of complexes. (a) [Mn(dmptacn)Cl]ClO4 (1). Dmptacn
(1.50 g, 4.8 mmol) was dissolved in MeOH (20 mL), and MnCl2‚4H2O
(0.95 g, 4.8 mmol) was added. A white solid precipitated on addition
of NaClO4 (1.0 g) to this solution. Recrystallization from an H2O/DMF
mixture afforded rectangular, colorless crystals suitable for X-ray
crystallography (yield 1.05 g, 44%). Anal. Calc for [Mn(C18H25N5)-
Cl]ClO4: C, 48.4; H, 5.6; N, 15.7. Found: C, 48.5; H, 5.7; N, 15.7.
Electron microprobe: Cl:Mn ratio 2:1. Selected IR bands (KBr, cm-1):
3336m, 2917m, 2858m, 1606m, 1491m, 1441m, 1144m, 1088vs,
1017m, 761m, 626s. Magnetic moment: µeff(296 K) ) 5.92 µB.
(b) [Mn2(tmpdtne)Cl2](ClO4)2‚2DMF (2). Tmpdtne (1.50 g, 2.3
mmol) was dissolved in MeOH (20 mL), and a slight excess of MnCl2‚
4H2O (0.96 g, 4.9 mmol) was added. The addition of NaClO4 (1.0 g)
to this brown solution resulted in the precipitation of a light brown
solid. This solid was recrystallized from an H2O/DMF mixture to afford
small, rectangular, colorless crystals suitable for X-ray crystallography
(yield 1.11 g, 41%). Anal. Calc. for [Mn2(C38H52N10)Cl2](ClO4)2‚2DMF:
C, 45.0; H, 5.6; N, 14.3. Found: C, 45.0; H, 5.6; N, 14.3. Electron
microprobe: Cl:Mn ratio 2:1. Selected IR bands (KBr, cm-1): 2917m,
2858m, 1665m, 1606m, 1491m, 1441m, 1367m, 1299m, 1144m,
1088vs, 1017m, 761m, 626s. Magnetic moment: µeff(296 K) ) 5.92
µB per Mn(II).
0.94 g (4.77 mmol) of MnCl2‚4H2O were used. Recrystallization from
a DMF/H2O mixture gave the product as a trihydrate (yield 1.21 g,
45%). Colorless crystals suitable for X-ray crystallography were
obtained on slow evaporation of a DMF/H2O solution of the compound.
Anal. Calc for [Mn2(C39H54N10)Cl2](ClO4)2‚3H2O: C, 42.7; H, 5.5; N,
12.8. Found: C, 43.3; H, 5.7; N, 13.2. Electron microprobe: Cl:Mn
ratio 2:1. Selected IR bands (KBr, cm-1): 3412vs, 2890w, 2853w,
1638m, 1607s, 1494m, 1442m, 1384m, 1299m, 1116s, 1083vs, 1013s,
780s, 628s. Magnetic moment: µeff(296 K) ) 5.94 µB per Mn(II).
(d) [Mn2(tmpdtnb)Cl2](ClO4)2‚DMF.2H2O (4). The synthesis was
as for 2 except that 1.3 g (1.93 mmol) of tmpdtnb and 0.80 g (4.06
mmol) of MnCl2‚4H2O were used. Recrystallization from a DMF/H2O
mixture gave the product as a mono(dimethylformamide) diaqua solvate
for which the analytical data given below were collected (yield 1.16 g,
50%). Colorless crystals of the tetrakis(dimethylformamide) triaqua
solvate, 4′, suitable for X-ray crystallography were obtained on slow
evaporation of a DMF/H2O solution of the compound. Anal. Calc for
[Mn2(C40H56N10)Cl2](ClO4)2.DMF‚2H2O: C 44.3, H 5.8, N 13.2;
found: C 44.2, H 5.7, N 13.3. Electron microprobe: Cl:Mn ratio 2:1.
Selected IR bands (KBr, cm-1): 3452s, 2925m, 2856m, 1668vs, 1606s,
1495m, 1441m, 1390m, 1309m, 1150m, 1102vs, 1015s, 766s, 624s.
Magnetic moment: µeff(296 K) ) 5.81 µB per Mn(II).
(e) [Mn2(tmpdtn-m-X)Cl2](ClO4)2.6H2O (5). The synthesis was as
for 2 except that 0.90 g (1.24 mmol) of tmpdtn-m-X and 0.52 g (2.61
mmol) of MnCl2‚4H2O were used. The product appeared as a brown
solid after recrystallization of the initial brown precipitate from a
mixture of DMF/H2O (yield 0.59 g, 38%). Anal. Calc. for [Mn2-
(C44H56N10)Cl2](ClO4)2‚6H2O: C, 43.6; H, 5.6; N, 11.6. Found: C, 43.5;
H, 5.0; N, 11.6. Electron microprobe: Cl:Mn ratio 2:1. ESI mass
spectrum (CH3CN), m/z: 1005.2, {[Mn2LCl2]ClO4}+; 452.2, {Mn2-
LCl2}2+. Selected IR bands (KBr, cm-1): 3417s, 3066w, 2919w,
2857m, 1657m, 1608s, 1486w, 1442m, 1384m, 1296m, 1090vs, 1016s,
763s, 626s. Magnetic moment: µeff(293 K) ) 5.83 µB per Mn(II).
(f) [Mn2(tmpdtnp-OH)Cl2](ClO4)2‚2H2O (6). The method used to
prepare 2 was followed except that 1.5 g (2.3 mmol) of tmpdtnp-OH
and 0.94 g (4.8 mmol) of MnCl2‚4H2O were used. Colorless crystals
of 6 were obtained on recrystallization from a CH3CN/H2O mixture
(yield 1.12 g, 45%). Anal. Calc for [Mn2(C39H54N10O)Cl2](ClO4)2‚2H2O:
C, 42.8; H, 5.3; N, 12.8. Found: C, 43.0; H, 5.4; N, 12.8. Electron
microprobe: Cl:Mn ratio 2:1. ESI mass spectrum (CH3CN), m/z 958.9,
{[Mn2LCl2]ClO4}+; 411.2, {Mn2LCl2}2+; and 274.5, {Mn2LCl}3+
.
Selected IR bands (KBr, cm-1): 3384s, 2919w, 2855w, 1608s, 1494m,
1440s, 1296m, 1149s, 1097vs, 1017s, 776m, 626m. Magnetic moment:
µeff(296 K) ) 5.80 µB per Mn(II).
Crystallography. Intensity data for colorless crystals of 1-3 and
4′ were measured on Siemens/Nicolet R3m and Rigaku AFC6R
diffractometers, respectively. Graphite-monochromatized Mo KR radia-
tion, λ ) 0.7107 Å, was used for 1, 2, and 4′, and Cu KR radiation, λ
) 1.542 Å, for 3. The ω-2θ (ω for 1) scan technique was employed
such that θmax was 22.5° for 1, 27.5° for 2 and 4′, and 60.1° for 3. The
data sets were corrected for Lorentz and polarization effects,33 and an
empirical absorption correction was applied in each case.34 Relevant
crystal data are given in Table 1.
The structures of 2 and 4′ were solved by direct methods35,36 while
those of 1 and 3 were also solved by direct methods37 but expanded
using Fourier techniques.38 Each was refined by a full-matrix least-
squares procedure based on F 34 employing reflections that satisfied
(33) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158.
XDISK: Diffractometer Data Reduction, Version 4.20PC; Siemens
Analytical X-ray Instruments Inc.: Madison, WI, 1991.
(34) teXsan: Structure Analysis Software; Molecular Structure Corporation,
the Woodlands, TX, 1985 and 1992.
(35) Sheldrick, G. M. SHELXS86: Program for the automatic solution of
crystal structure; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(36) Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Polidori,
G.; Spagna, R.; Viterbo, D. J. Appl. Crystallogr. 1989, 22, 389.
(37) Altomare, A.; Cascarano, M.; Giacovazzo, G.; Guagiardi, A. SIR92.
J. Appl. Crystallogr. 1993, 26, 343.
(38) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; R. O.; Smits, J. M. M. The DIRDIF-94 Program
System; Technical Report; Crystallography Laboratory, University of
Nijmegen: The Netherlands, 1994.
(c) [Mn2(tmpdtnp)Cl2](ClO4)2‚3H2O (3). The method used to
prepare 2 was followed except that 1.5 g (2.27 mmol) of tmpdtnp and

884 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
Table 1. Crystallographic Data for Complexes 1-3 and 4′
1
2
3
4′
formula
fw
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C18H25Cl2MnN5O4
501.3
P212121, No. 19
7.959(7)
12.30(1)
21.72(2)
C44H66Cl4Mn2N12O10
1174.8
C39H60Cl4Mn2N10O11
1096.6
C46H76Cl4Mn2N12O13
1256.8
P21/c, No. 14
11.455(4)
15.037(6)
15.887(4)
96.48(2)
2719(1)
2
P21/c, No. 14
13.334(2)
19.926(2)
18.799(1)
104.328(8)
4839.4(9)
4
P21/n, No. 14
13.361(3)
16.807(5)
14.339(4)
111.14(2)
3003(1)
2
2126(3)
4
T, K
173
293
183
1.542 (Cu KR)
6.85
1.503
0.053
0.060
180
λ, Å
0.7107 (Mo KR)
9.07
1.566
0.035
0.7107 (Mo KR)
7.25
1.435
0.092
0.7107 (Mo KR)
6.64
1.390
0.072
µ, cm-1
F, g cm-3
R(F)a
Rw(F)b
0.030
0.108
0.070
2
a R(F) ) Σ||Fo| - |Fc||/Σ|Fo|. b Rw(F) ) [Σ(|Fo| - |Fc|)2/Σw|Fo| ]1/2
.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1-3 and 4′
1
2
3 a
4′
Mn-Cl(1)
Mn-N(1)
Mn-N(2)
Mn-N(3)
Mn-N(4)
Mn-N(5)
2.427(3)
2.253(6)
2.326(5)
2.375(6)
2.214(5)
2.261(5)
2.410(2)
2.381(7)
2.280(7)
2.343(6)
2.228(6)
2.255(7)
2.424(3) {2.417(3)}
2.332(8) {2.323(8)}
2.295(8) {2.282(8)}
2.333(8) {2.342(8)}
2.222(8) {2.194(8)}
2.275(8) {2.289(8)}
2.420(3)
2.317(9)
2.304(9)
2.330(7)
2.23(1)
2.259(8)
Cl(1)-Mn-N(1)
Cl(1)-Mn-N(2)
Cl(1)-Mn-N(3)
Cl(1)-Mn-N(4)
Cl(1)-Mn-N(5)
N(1)-Mn-N(2)
N(1)-Mn-N(3)
N(1)-Mn-N(4)
N(1)-Mn-N(5)
N(2)-Mn-N(3)
N(2)-Mn-N(4)
N(2)-Mn-N(5)
N(3)-Mn-N(4)
N(3)-Mn-N(5)
N(4)-Mn-N(5)
Mn-N(1)-C(1)
Mn-N(1)-C(6)
Mn-N(1)-C(19)
Mn-N(2)-C(2)
Mn-N(2)-C(3)
Mn-N(2)-C(13)
Mn-N(3)-C(4)
Mn-N(3)-C(5)
Mn-N(3)-C(7)
Mn-N(4)-C(8)
Mn-N(4)-C(12)
Mn-N(5)-C(14)
Mn-N(5)-C(18)
90.1(2)
116.6(2)
158.4(1)
99.8(2)
88.6(1)
76.6(2)
75.4(2)
117.6(2)
145.4(2)
76.0(2)
141.4(2)
73.2(2)
73.9(2)
112.5(2)
96.7(2)
103.7(4)
113.9(5)
90.9(2)
93.5(2) {93.5(2)}
119.4(2) {115.3(2)}
160.9(2) {164.7(2)}
95.8(2) {99.8(2)}
90.9(2) {91.2(2)}
78.5(3) {78.7(3)}
76.8(3) {77.2(3)}
114.4(3) {116.6(3)}
148.6(3) {150.4(3)}
75.3(3) {75.2(3)}
142.5(3) {141.2(3)}
72.4(3) {72.8(3)}
74.0(3) {74.3(3)}
106.1(3) {102.9(3)}
96.0(3) {91.3(3)}
98.5(6) {99.3(6)}
110.2(6) {109.8(6}
118.1(6) {115.7(6)}
109.6(6) {110.0(6)}
106.8(6) {107.2(6)}
106.2(6) {104.5(6)}
113.0(6) {112.4(6)}
102.8(6) {102.3(6)}
107.5(6) {107.0(6)}
116.4(7) {117.6(7)}
123.8(7) {124.2(7)}
116.0(7) {113.6(7)}
126.4(7) {128.5(7)}
93.0(2)
115.8(2)
161.0(2)
99.9(2)
90.3(2)
78.7(2)
118.6(2)
160.4(2)
97.7(2)
89.5(2)
77.6(3)
75.6(2)
76.1(3)
114.0(2)
149.9(2)
75.3(2)
142.3(3)
73.8(2)
74.2(2)
108.2(3)
95.3(3)
117.6(3)
147.7(3)
75.5(3)
140.5(3)
73.0(3)
73.9(3)
108.4(3)
93.8(3)
107.1(8)
111.2(7)
113.7(7)
110.3(6)
105.7(7)
105.7(6)
111.9(5)
104.0(5)
107.8(6)
118.3(8)
123.5(8)
116.5(8)
126.4(7)
97.6(5)
110.5(5)
116.7(5)
110.2(5)
106.4(4)
106.4(4)
112.4(4)
105.2(5)
107.2(4)
116.3(5)
123.4(6)
116.0(6)
127.9(6)
110.5(4)
104.2(4)
108.4(4)
111.3(4)
103.4(4)
106.4(4)
118.9(5)
121.1(4)
115.8(5)
124.7(5)
a The halves of the binuclear cation are not identical, as is the case for 2 and 4′, and exhibit different bond distances and angles. The values that
correspond to the labeling scheme for 1, 2, and 4′ are listed first followed by those for the other half of the cation in brackets.
the I g 3.0σ(I) criterion. Non-H atoms were generally refined with
anisotropic displacement parameters, and H atoms were included in
the models at their calculated positions. For 2, the perchlorate anion
was disordered and the constituent O atoms were refined isotropically.
The Cl-bound O(1), O(2), and O(4) atoms were each disordered over
two sites and were refined with 50% site occupancy factors. For 4′,
the two water O atoms were refined isotropically and the O(7) atom
was disordered over two symmetry-related positions. The refinements
were continued until convergence in each case after the inclusion of a
weighting scheme of the form 1/σ2(F). For 2 and 4′, neither of the
structure determinations is optimal owing to the quality of the crystals
available for analysis. Numerous attempts to grow better crystals of
these complexes proved unsuccessful. Considerable difficulties were
also encountered in modeling the perchlorate anions and solvent
molecules. Nevertheless, the molecular structures of the dications have
been determined unambiguously. The lattice of 2 comprises dimeric
dications, perchlorate anions, and DMF solvates in the ratio 1:2:2. The
closest non-H contact in the lattice of 3.28(1) Å occurs between the
O(5) and C(7)i atoms, i.e., between the solvent molecule and the cation
(symmetry operation i: x, 0.5 - y, 0.5 + z). For 4′, the optimal
refinement corresponded to a unit cell comprising dinuclear cations,
perchlorate anions, DMF solvates, and water solvates in the ratio 1:2:
2:1.5. The closest non-H contact in the lattice of 2.84(4) Å occurs
between the water molecules and is indicative of H-bonding.
Final refinement details are collected in Table 1, selected bond
lengths and angles are given in Table 2, and the atom-numbering

Binuclear Macrocyclic Manganese(II) Complexes
Inorganic Chemistry, Vol. 39, No. 5, 2000 885
Figure 1. Molecular structure of the mononuclear cation in [Mn-
(dmptacn)Cl]ClO4 (1) (50% probability ellipsoids) and atom-labeling
scheme.
Figure 3. Molecular structure of the dinuclear cation in [Mn2(tmpdtnp)-
Cl2](ClO4)2‚3H2O (3) (30% probability ellipsoids) and atom-labeling
scheme.
Figure 2. Molecular structure of the dinuclear cation in [Mn2(tmpdtne)-
Cl2](ClO4)2‚2DMF (2) (50% probability ellipsoids) and atom labeling
scheme.
schemes are shown in Figures 1-4, which were drawn with ORTEP.39
The teXsan34 package was employed for all calculations.
Results and Discussion
Synthesis and Characterization. The mononuclear Mn(II)
complex of dmptacn (1) and the binuclear Mn(II) complexes
of tmpdtne (2), tmpdtnp (3), tmpdtnb (4), tmpdtn-m-X (5), and
tmpdtnp-OH (6) were all formed by reacting the respective
ligands with an excess of MnCl2‚4H2O in MeOH. The products
were precipitated by adding NaClO4 to the brown solutions and
recrystallized from either DMF/H2O or CH3CN/H2O mixtures
to give the products as colorless crystals in 38-50% yields.
The presence of DMF in complexes 2 and 4 can be seen in
the corresponding infrared spectra, which show a strong band
at 1670 cm-1 due to the CdO group in the solvent. All
complexes show bands at 1090 and 625 cm-1 due to the
perchlorate counterions and bands in the 1400-1620 cm-1
region which pertain to the CdC and CdN stretching frequen-
cies of the pyridine rings. A characteristic OH stretch at 3384
cm-1 is evident for 6 when the infrared spectrum is recorded
Figure 4. Molecular structure of the dinuclear cation in [Mn2(tmpdtnb)-
Cl2](ClO4)2.4DMF‚3H2O (4′) (50% probability ellipsoids) and atom-
labeling scheme.
in Nujol. The mononuclear complex, 1, could be distinguished
from the binuclear complexes, 2-6, as it exhibited a band at
3336 cm-1 due to the NH stretch of the secondary amine. This
is a feature of all compounds of this ligand.
Crystals of 1-3 and 4′ grown from DMF/H2O solutions were
used to determine the molecular structure of these complexes
discussed below. Although crystals of 5 and 6 were not suitable
for X-ray crystallography, elemental analyses and ESI MS data
indicated that these complexes were similar in composition to
(39) Johnson, C. K. ORTEP Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.

886 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
the others in the series. The ESI-mass spectrum of 5, [Mn2-
(tmpdtn-m-X)Cl2](ClO4)2, showed signals at m/z 1005.2, {[Mn2-
LCl2]ClO4}+, and 452.2 {Mn2LCl2}2+, while that of 6, [Mn2-
(tmpdtnp-OH)Cl2](ClO4)2, exhibited signals at m/z 958.9,
{[Mn2LCl2]ClO4}+, 411.2, {Mn2LCl2}2+, and 274.5, {Mn2-
LCl}3+. These assignments were supported by the agreement
between observed and simulated isotopic splitting patterns for
each signal. The presence of signals attributable to {Mn2LCl2}2+
supports the view that chloro ligands occupy the sixth Mn
coordination site in 5 and 6, as is the case for 2-4.
Description of the Crystal Structures of 1-4. The molec-
ular structure of the cation in 1, shown in Figure 1, confirms
that five coordination sites are occupied by the N donors of
dmptacn while a chloride ligand completes the significantly
distorted Mn(II) coordination sphere, whose geometry is
intermediate between octahedral and trigonal prismatic. Al-
though the Mn(II) center in 1 is not strictly octahedral, two
possible isomers can be envisaged for the cation; viz., one
isomer has the chloride ion coordinating trans to the secondary
amine, whereas the other has the chloride ion coordinating cis
to the secondary amine. The view of 1 shown in Figure 1
confirms that the chloride is indeed cis to the secondary amine,
as does a Cl(2)-Mn(1)-N(1) angle of 90.2(2)°. The two
2-pyridylmethyl pendant arms adopt a ∆ configuration, and, in
contrast to those of the Cu(II) analogue, they are perpendicular
rather than parallel to one another.31 The two five-membered
chelate rings formed by coordination of the 2-pyridylmethyl
pendant arms to Mn(II) are puckered, as seen in the Mn(1)-
N(2)-C(6)-C(5) and Mn(1)-N(4)-C(13)-C(14) torsion angles
of 38.9 and 38.4°, respectively.
Figure 5. Definition of the trigonal twist angle (ϑ).
large Mn(II) center (0.97 Å) finds it difficult to accommodate
the small tacn macrocycle on the face of an octahedron.41 This
is better achieved by metal centers with smaller ionic radii, e.g.,
Ni(II) and Cr(III).41
The binuclear cations in 2, 3, and 4′, illustrated in Figures
2-4, show that ethane, propane, and butane bridges, respec-
tively, link the two pentadentate compartments of the ligands
through N(1). As was the case for 1, the coordination sphere of
each Mn(II) center is occupied by five N donors from each
ligand compartment and a chloride ligand which is located cis
to the bridgehead nitrogen and the Mn(II) centers adopt a
pseudo-trigonal-prismatic stereochemistry (viz., ϑ is 27.3° for
2, 26.8° for 3, and 26.0° for 4′ and “transoid” donor angles are
in the 140-160° range). In addition, the small bite angles
subtended by the N donor atoms of the tacn (ca. 76°) and by
the chelate rings incorporating the pyridyls (ca. 74°) match those
in 1.
Complexes 2 and 4′ adopt a true anti conformation with the
two chloro ligands pointing away from each other. For these
centrosymmetric complexes, the “trans” orientation of the
chlorides causes the torsion angles defined by each Mn(II)
center, the bridgehead nitrogen, and the two adjacent carbons
in the bridge to be equal in magnitude but opposite in sign. For
2, the torsion angles defined by Mn(1)-N(1)-C(19)-C(20) and
Mn(1′)-N(1′)-C(19′)-C(20) are +62.8(8) and -62.7(8)°,
respectively, while the corresponding values for 4′ are +59.6-
(8) and -59.5(8)°. The long Mn‚‚‚Mn′ distances (6.815(3) Å
for 2 and 8.279(3) Å for 4′) are a consequence of these anti
conformations. Notably, these M‚‚‚M distances are respectively
0.67 and 0.40 Å shorter than those in the corresponding square-
pyramidal (SP) Cu(II) complexes.32 This reduction in M‚‚‚M
distance probably reflects a decrease in electrostatic repulsion
attributable to the lower charge on the Mn(II) complexes (1+/
Mn), when compared with the Cu(II) complexes (2+/Cu).
Supporting this view is the considerably longer Ni‚‚‚Ni′ distance
of 9.563(4) Å found in [Ni2(tmpdtnb)(OH2)2]4+, a complex
analogous to 4′ but with a higher charge per metal (2+/Ni).40
The longer M‚‚‚M separation in this Ni(II) complex (cf. 4′),
can be traced to differences in the torsion angles involving the
metal, the bridgehead nitrogen and butane linker, viz., -10.9-
(8) and 169.1(8)°, (cf. 59.6(8) and -59.5(8)° in 4′).
Several features highlight the substantial distortion in the
coordination geometry of the Mn(II) center in 1. The Mn-N
bond lengths in 1 vary significantly from 2.214(5) to 2.375(6)
Å and give rise to an average Mn-N bond distance (2.29 Å)
that is 0.04 Å longer than that for [Mn(tmptacn)](ClO4)2, in
which the Mn center adopts a pseudo-trigonal-prismatic geom-
etry.17 The angles subtended by the tacn nitrogens (average
76.0°) are much smaller than those subtended by the tacn ligand
in the binuclear Ni(II) complex of tmpdtnb (average 84.6°),
which has a more regular octahedral geometry.40 Significantly,
the tacn bite angles in 1 compare favorably with those found in
[Mn2(Me3tacn)2(µ-O2CMe)3]+ (average 76.5°)19 and [Mn(tmp-
tacn)](ClO4)2 (average 77.2°).17 The distortion from a regular
octahedral geometry in 1 can also be seen in the angles
subtended by the five-membered chelate rings involving the
tertiary amines and the pyridyl nitrogens (average 73.6°), which
are also similar to those observed for [Mn(tmptacn)](ClO4)2
(average 75.4°),17 and in the nonlinearity of the Cl(2)-Mn-
(1)-N(3) angle (158.4°). This degree of distortion from
octahedral geometry can be quantified by the trigonal twist angle
(ϑ), which is a measure of the rotation of the two N donors on
the pyridyl arms and the chloro ligand relative to the tacn
nitrogen donor atoms (Figure 5). For a trigonal-prismatic
geometry, ϑ is 0°, while for an octahedral arrangement, ϑ is
60°. The observed value of ϑ for 1 (23.4°) confirms a geometry
that it is intermediate between trigonal prismatic and octahedral
but which is slightly closer to the former, as was found for [Mn-
(tmptacn)](ClO4)2 (ϑ ) 19.6°).17 The pseudo-trigonal-prismatic
geometry of the Mn(II) center in complexes of tacn-based
ligands, exhibited by 1 and the related binuclear complexes 2,
3, and 4′ (see below), arises principally from the fact that the
Conversion of N(1) into a tertiary nitrogen has resulted in
considerable lengthening of the Mn-N(1) bond, from 2.253(6)
Å in 1 to 2.381(7) Å in 2, 2.33(1) Å (average) in 3, and 2.317-
(9) Å in 4′. The two five-membered chelate rings incorporating
the 2-pyridylmethyl pendant arms in 2, 3, and 4′ are puckered
with torsion angles that correspond to those in 1.
The view of 3, displayed in Figure 3, suggests that the
complex adopts a syn configuration, like that observed for the
Cu(II) analogue, in which the halves of the ligand are oriented
toward one another. Although the “zigzag” nature of the propane
bridge linking the two pentadentate compartments is maintained
(40) Brudenell, S. J.; Spiccia, L.; Bond, A. M.; Mahon, P. C.; Hockless,
(41) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
D. C. R. J. Chem. Soc., Dalton Trans. 1998, 3919.
John Wiley & Sons: New York, 1988; p 745.

Binuclear Macrocyclic Manganese(II) Complexes
Inorganic Chemistry, Vol. 39, No. 5, 2000 887
Table 3. Cyclic, Square-Wave, and Steady-State Voltammetric Data for Oxidation of Mn(II) Complexes in Acetonitrile (0.1 M Bu4NClO4) a
cyclic b
square-wave c
steady-state d
ox
red
ox
red
E1/2
V
,
∆Ep,
Ep
,
Ep
,
Ip
,
Ip
,
Ep
V
W1/2
V
Ip
µA
E1/2
V
E1/4-E3/4
IL
nA
10-6
D
cm2 s-1
12.3
red
mV
V
V
µA
µA
Ipox/Ip
mV
1
2
0.41
66
0.45
0.38
3.3
3.2
0.97
0.41
1.03
0.54
1.30
1.38 sh
0.51
1.26
0.49
1.24
0.51
1.28
0.52
0.89
1.33
0.10
0.18
0.16
0.17
6.1
3.4
17.9
11.4
5.2
17.2
13.0
21.9
20.0
18.0
16.5
12.8
7.2
0.41
59
5.7
0.54
128
0.61
0.48
5.0
4.9
0.98
0.54
89
8.6
9.5
3
4
5
6
0.51
0.49
0.51
72
78
0.55
0.53
0.53
0.48
0.46
0.42
5.9
5.4
5.0
5.9
5.2
4.9
1.00
0.96
0.98
0.10
0.14
0.10
0.12
0.12
0.11
0.09
0.10
0.10
0.51
0.49
0.51
50
58
70
8.1
8.1
8.0
8.7
8.8
8.5
7.8
110
0.52
0.88
78
64
0.56
0.92
0.48
0.85
5.3
3.8
5.5
3.8
1.04
1.00
0.52
0.89
63
59
7.3
3.8
7.2
a Potentials are quoted with respect to the Fc/Fc+ couple. The concentrations of the complexes used were in the range 0.5-0.55 mM, but the
current data in the table are normalized to 0.5 mM to facilitate comparison of values. b Cyclic voltammetric data were measured at a scan rate of
100 mV s-1
.
c Square-wave voltammetric data were measured with a period of 30 ms. d Steady-state voltammetric data were measured on a 25 µm
radius Pt microdisk electrode.
in both complexes, as this is sterically preferred for this ligand,
it is electrostatically unfavorable for the chlorides to be situated
near each other. Consequently, 3 adopts an anti configuration,
which minimizes Cl‚‚‚Cl repulsion.
widths are a consequence of the distorted Mn(II) geometries,
which for complexes 1-4 are intermediate between trigonal
prismatic and octahedral in the solid state.
Electrochemistry. The redox behavior of 1-6 has been
studied by cyclic (CV), square-wave (SWV), and steady-state
voltammetry (SSV). The CV trace of the mononuclear complex,
[Mn(dmptacn)Cl]+, 1, displays a chemically reversible wave
The N(1)-C(19)-C(20)-C(21) and N(6)-C(21)-C(20)-
C(19) torsion angles of -8.7(8) and -3.5(8)°, respectively, show
an approximately all-gauche arrangement for the atoms of the
bridge in 3. Thus, the orientation of 3 (Figure 4) is a result of
each pentadentate compartment twisting around the bridgehead
nitrogens, N(1) and N(6), without torsional distortion of the
bridge itself. The degree of this twisting is reflected in the Mn-
(1)-N(1)-C(19)-C(20) and Mn(2)-N(6)-C(21)-C(20) tor-
sion angles of -62.8(8) and -61.6(8)°, respectively, which
confirm that even though the chlorides in 3 are directed away
from one another, the metals are on the same side of the ligand.
In contrast, the corresponding torsion angles of +62.7(8) and
-62.7(8)° for 2 highlight the true anti orientation of this
complex. The “closed” arrangement of 3 leads to an Mn‚‚‚Mn′
separation of 6.797(2) Å, which is similar to that in 2 (6.815-
(3) Å).
Magnetic Properties and ESR Spectra. The room-temper-
ature magnetic moments, µeff (per MnII), of 1-6 (5.80 e µeff e
5.94 µB) are typical of high-spin Mn(II) complexes.15,17,42
Variable-temperature magnetic susceptibility measurements
(4.5-296 K) of 2-6 showed no evidence of coupling between
the two Mn(II) centers. This reflects the quite large Mn‚‚‚Mn
separations (g6.8 Å) and absence of bridging units capable of
mediating such interactions.
red
at a scan rate of 100 mV s-1 (Ipox/Ip ) 0.97) for which E1/2
) +0.41 V (vs Fc/Fc+). This E1/2 value is assumed to be the
reversible potential because it is in agreement with the Ep and
E1/2 values obtained from SWV and SSV, respectively (Table
3). This initial oxidation reaction corresponds to the 1e-
oxidation of Mn(II) to Mn(III) shown in eq 1, with SSV data
giving a diffusion coefficient of 12.3 × 10-6 cm2 s-1
MnII h MnIII + e-
.
(1)
At much more positive potentials, another redox process is
just visible in the CV trace of 1 but is masked by additional
ill-defined (possibly ligand-based) oxidative processes that
become apparent above +1.0 V. However, this process is clearly
identifiable in the SWV trace from which an Ep value of +1.03
V could be determined. The W1/2 value is broader than the initial
process and the peak height is smaller indicating irreversibility
is present. The SWV data signifies a further 1e- oxidation of
the Mn(III) center to Mn(IV) followed by a fast decomposition
reaction, as shown in eq 2.
MnIII h MnIV + e- very fast8 products
(2)
The ESR spectra of 1-6, recorded as frozen DMF solutions
at 99 K, are typical of uncoupled or very weakly coupled Mn-
(II) complexes43,44 with a zero-field interaction of D ≈ 0.09
cm-1. Apart from a minor, sharp 6-line signal centered at g )
2.00 (peak separation ∼ 90 G) due to small amounts of Mn2+
impurity, the complexes display very broad resonances, which
are typical of low-symmetry Mn(II) complexes, as was found
for [Mn([9]aneN2S)2](ClO4)2 ([9]aneN2S ) 1-thia-4,7-diazacy-
clononane) and [Mn(tacn)2](ClO4)2.42 The large spectral line
Typical responses for complexes 2, 3, and 6 are displayed in
Figures 6-8. As for the mononuclear complex, two oxidation
waves also are observed in the CV and SWV of the binuclear
complex [Mn2(tmpdtne)Cl2]2+, 2 (Figure 6). The CV responses
show that the first process is chemically reversible. However,
the shape of this wave and the current magnitudes under CV,
SWV, and SSV conditions indicate that the two Mn(II) centers
are each oxidized in a reversible one-electron process at slightly
different potentials, as was found for the Ni(II) analogue.40 The
(42) Gahan, L. R.; Grillo, V. A.; Hambley, T. W.; Hanson, G. R.; Hawkins,
C. J.; Proudfoot, E. M.; Moubaraki, B.; Murray, K. S.; Wang, D. Inorg.
Chem. 1996, 35, 1039.
(43) Mathur, P.; Crowder, M.; Dismukes, G. C. J. Am. Chem. Soc. 1987,
109, 5227.
ox
first overall reversible two-electron oxidation process in 2 (Ip
/
red
Ip ) 0.98) occurs at an average E1/2 value of +0.54 V. The
fact that this is not a simple reversible 2e- process is evidenced
by the large ∆Ep (128 mV) and W1/2 (0.16 V) values (Table 3).
(44) Mathur, P.; Dismukes, G. C. J. Am. Chem. Soc. 1983, 105, 7093.

888 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
Figure 7. Cyclic (scan rate 100 mV s-1) and square-wave (period 30
ms) voltammograms of 4, measured at 20 °C in acetonitrile (0.1 M
Bu4NClO4) using an 0.8 mm Pt macrodisk electrode. The potential axis
used is relative to the Fc/Fc+ couple.
Figure 6. Cyclic (scan rate 100 mV s-1) and square-wave (period 30
ms) voltammograms of 2, measured at 20 °C in acetonitrile (0.1 M
Bu4NClO4) using an 0.8 mm Pt macrodisk electrode. The potential axis
used is relative to the Fc/Fc+ couple.
possibly involving oxidation of the ligand. These background
processes were also observed in the CV trace of the free ligand
tmpdtne. In the SWV trace of 2, there is one major wave with
an Ep value of +1.30 V and a minor wave at +1.38 V probably
arising from a decomposition reaction associated with the
generation of MnIVMnIV. Thus, the Mn(III) centers in 2 are
oxidized via two very closely spaced 1e- processes with the
MnIV state being very reactive, as in the case with the monomer,
as shown in eq 4. The current magnitude for the major process
is consistent with this being a 2e- process.
That is, the two Mn(II) centers in 2 are close enough to
electronically influence one another, so that oxidation of the
first Mn(II) center renders oxidation of the second Mn(II) center
more difficult. Thus, the first oxidation wave of 2 involves two
closely separated 1e- processes represented by eq 3 with a
separation in the reversible potential of ca. 40 ( 10 mV.
MnIIMnII h MnIIIMnII + e- h MnIIIMnIII + e- (3)
MnIIIMnIII h MnIVMnIII + e- h MnIVMnIV + e-
very fast8 product(s) (4)
The combination of CV and SWV peak heights and of SSV
limiting current data, displayed in Table 3, in conjunction with
the calculated diffusion coefficient enables confirmation that
the initial voltammetric responses for 2 and the other binuclear
complexes, 3-6, involve two electrons, as opposed to the one-
electron process observed for 1. That is, all data sets when used
with the relevant theory give internally consistent data under
this assumption. Thus, for example, the greater value of D for
the mononuclear complex, 1 (12.3 × 10-6 cm2 s-1), when
compared with those of the binuclear complexes (average 8.7
× 10-6 cm2 s-1) is as expected, since the larger binuclear
molecules would normally diffuse to the electrode surface more
slowly than 1. In addition, since the theory for a reversible
All three voltammetric techniques corroborate the respective
E
1/2 and Ep values of +0.51 and +0.49 V recorded for the first
red
chemically reversible oxidation responses of 3 and 4 (Ipox/Ip
) 1.0). CV signals for 3 and 4 (Figure 7) are relatively sharp
compared to those for 2, as evidenced by the smaller ∆Ep values
of 72 and 78 mV, respectively, and the sharper peaks in the
SWV’s. However, these ∆Ep values are still larger than expected
for a simultaneous 2e- process (i.e., ∆Ep ) 57 mV/n), indicating
that oxidation of the two Mn(II) centers to Mn(III) occurs via
two closely spaced 1e- processes, as shown by eq 3, but with
a separation of 25 ( 10 mV. These results are consistent with
those obtained for the Cu(II) and Ni(II) analogues.32,40 The
diffusion currents for 3 and 4 (SSV, IL ) 8.1 nA, where D(av)
) 8.8 × 10-6 cm2 s-1) are similar to that of 2 (SSV, IL ) 8.6
nA, where D ) 9.3 × 10-6 cm2 s-1), for which the first
oxidation process also involves two electrons. According to the
SWV data, the second overall but chemically reversible oxida-
tion processes for 3 and 4, represented by eq 4, are observed at
linear-sweep voltammogram45 predicts that Ip
n3/2D1/2, the
peak currents per unit concentration for the binuclear complexes
(n ) 2) should be greater than those for 1 (n ) 1), as is found
experimentally (Table 3).
The second oxidation wave in the cyclic voltammogram of
2 is partly masked and complex, suggesting ill-defined processes
(45) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley &
Sons: New York, 1980.

Binuclear Macrocyclic Manganese(II) Complexes
Inorganic Chemistry, Vol. 39, No. 5, 2000 889
Figure 8. (a) Cyclic ((i) scan rate 100 mV s-1) and square-wave ((ii) period 30 ms) voltammograms of 6, measured using an 0.8 mm Pt macrodisk
electrode, and steady-state (iii) voltammograms, measured on a 25 µm radius Pt microdisk electrode, in acetonitrile (0.1 M Bu4NClO4) at 20 °C.
(b) Variable-scan-rate cyclic voltammograms of 6. The potential axis used is relative to the Fc/Fc+ couple.
less positive average E1/2 values of 1.26 and 1.24 V, respectively,
than that for 2 (1.30 V).
bridgehead nitrogen. This results in a relative stabilization of
the higher Mn oxidation states. Data for the [Mn(dtne)]2+/3+
and [Mn(tacn)2]2+/3+ redox couples support this interpretation.15
The potential for the dtne complex, incorporating two tertiary
nitrogens, is +0.3 V more positive than that for the tacn
complex, which contains no tertiary nitrogens.
In the case of the tmpdtn-m-X complex, 5, the three
electrochemical methods have established an E1/2 value of +0.51
V for the first unresolved 2e- oxidation process, while SWV
gave an E1/2 value of +1.28 V for the second 2e- oxidation.
However, relative to the traces of 3 and 4, the CV trace for 5
is significantly broadened (viz., a large ∆Ep value of 110 mV)
with a small broadening of the corresponding SWV and SSV
signals being observed relative to 3 and 4. The behavior of 5 is
intermediate among those of 2, 3, and 4 with respect to
separations in potentials.
Compound 5 does not follow the above trend, and despite
having a longer carbon chain length linking the halves of the
ligands leading to the expectation of a longer Mn‚‚‚Mn
separation, it displays an average E1/2 value that is slightly more
positive than that for 4. The aromatic bridging unit may be
reducing the binding ability of the bridgehead nitrogen and hence
the overall field strength of the ligand, and this could be making
oxidation more difficult. The E1/2 values for 5 follow the trend
observed for the analogous Ni(II) complex. However, any
interpretation of these data must necessarily be tentative, since
the process need not comprise purely EE reactions.32, 45
The electrochemical behavior of the binuclear complex of
tmpdtnp-OH, 6, is quite different from those of 1-5 in that
three rather than two oxidation processes are clearly present
(Figure 8). The first oxidation, observed at E1/2 ) +0.52 V
according to the three voltammetric techniques (Table 3), is
attributed to two unresolved 1e- processes in which both Mn-
(II) centers are oxidized to the Mn(III) state, as seen earlier in
eq 3. The second response, observed at E1/2 ) +0.89 V, is
proposed to involve a 1e- oxidation of the MnIIIMnIII complex
to the mixed-valent MnIIIMnIV complex. At the more positive
potential of +1.33 V, there is a further 1e- process correspond-
ing to the formation of the MnIVMnIV complex. Both the ∆Ep
value of 78 mV and the Ipox and Ipred values of 5.3 and 5.5 µA,
The average E1/2 values for the first unresolved 2e- oxidation
responses of 2-4, decrease with increasing chain length (i.e.,
E1/2 ) +0.54, +0.52, and +0.49 V, respectively) as do those
for the second oxidation (i.e., +1.34, +1.26, and +1.24 V,
respectively). These findings support the postulate that, as the
alkyl chain is shortened, the metal centers exert greater
electrostatic influence on one another, so that oxidation of one
metal center is affected by the increased positive charge on the
other. These influences diminish as the length of the bridging
unit and Mn‚‚‚Mn separation increase, and the complexes
become easier to oxidize. The ease of oxidation of the
mononuclear complex, 1 (E1/2 values of +0.41 and +1.03 V),
in part reflects the absence of electrostatic effects that would
be caused by a second metal center. There are also differences
in ligand field strength, since in the binuclear complexes, 2-6,
three tertiary tacn nitrogens coordinate to each Mn center
whereas in the mononuclear complex the ligand will have a
higher field strength since a secondary nitrogen replaces the

890 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
Figure 9. UV-visible spectra of a DMF solution of 4 ([complex] ∼ 5 mM) measured at 10 min intervals after the addition of hydrogen peroxide
([H2O2] ∼ 1.5 M).
respectively (Table 3), provide strong evidence that the first
oxidation process of 6 involves two closely spaced 1e- steps.
These values compare particularly well with those of 3 (∆Ep )
sponding to the MnIIIMnIII f MnIIIMnIV oxidation around +0.6
to +1.0 V and to the MnIIIMnIV f MnIVMnIV oxidation around
+1.2 to +1.5 V, which is consistent with the interpretation of
the data for the second and third processes for 6, in which the
processes at +0.89 and +1.33 V are assigned to the formation
of MnIIIMnIV and MnIVMnIV species, respectively. Following
oxidation of 6 to the MnIIIMnIII complex, an alkoxide bridge
could link the two Mn centers through deprotonation and
coordination of the central oxygen of the ligand, thus enabling
formation of an alkoxo-bridged MnIIIMnIV complex (Scheme
2). Since such a process involves substitution at Mn(III), a d4
system that is subject to Jahn-Teller distortion, this process
may be so fast that no evidence for it is found in the variable-
scan-rate CV traces. The X-ray structure of the alkoxide-bridged
binuclear Zn(II) complex46 demonstrates that two metal centers
can be bridged by a deprotonated tmpdtnp-OH ligand. Further-
more, Wieghardt and co-workers29 recently reported binuclear
MnIIMnIII, MnIIIMnIII, and MnIIIMnIV complexes of the related
Me4dtnp-OH ligand which included a mixed-valent complex,
[MnIIIMnIV(Me4dtnp-O)(PhCO2)(PhBO2)]3+, in which the oxy-
gen of the deprotonated ligand bridged the Mn centers.
ox
red
72 mV; Ip and Ip ) 5.9 µA), which correspond to two
overlapping 1e- oxidation processes. Assignment of the second
process as a reversible 1e- oxidation process is based on the
ox
red
close match of the Ip and Ip values (3.8 µA, Table 3) with
those of 1 (Ipox and Ipred ) 3.3 and 3.2 µA), a compound known
to undergo 1e- oxidation to Mn(III), and also on the fact that
the Ip value of 7.1 µA determined for this process by SWV
(see Table 3) is about half that recorded for the first 2e-
oxidation (Ip ) 12.5 µA). Moreover, the diffusion current
determined from the SSV trace for the first process (IL ) 7.3
nA) is also about twice that for the second process (IL ) 3.8
nA). Thus, the evidence is consistent with the assignment of
the first process as a two-electron oxidation and the second as
a one-electron process generating a mixed-valent MnIIIMnIV
species. Since the shape and height of the third process observed
in the SWV of 6 (i.e., W1/2 ) 0.10 and Ip ) 7.1 µA) closely
match those of the second process, it can be concluded that the
third oxidation process in 6 also corresponds to a 1e- step, to
give an MnIVMnIV complex which then rapidly decomposes to
an unknown species. The variable-scan-rate CV traces for 6
(Figure 8b) show the expected variation in peak currents with
the square root of the scan rate. This would not be expected if
competing kinetic processes were occurring on the electro-
chemical time scale, and were contributing to the electrochemi-
cal responses.
Reactions with Hydrogen Peroxide. Interest in the dispro-
portionation of hydrogen peroxide by Mn enzymes,3,7,8 and in
the ability of Mn catalysts (including complexes of tacn and its
derivatives) to enhance the bleaching properties of peroxide12a
has led us to explore the reaction of complexes 1-6 with excess
H2O2 (typically >1000-fold). Our initial qualitative examination
of this reaction revealed that addition of H2O2 to DMF solutions
of 1-6 causes the vigorous evolution of dioxygen coupled with
distinct color changes (see below). This finding was surprising
The MnIIIMnIII f MnIIIMnIV f MnIVMnIV type of redox
behavior proposed for 6 has also been observed for binuclear
Mn(III) complexes of the type [Mn(III)2L2(µ-OH)(µ-CH3-
CO2)2]2+, where L ) an N-substituted tacn macrocycle.25 The
binuclear complexes exhibit two oxidation processes corre-
(46) Brudenell, S. J.; Spiccia, L.; Hockless, D. C. R.; Tiekink, E. R. T. J.
Chem. Soc., Dalton Trans. in press.

Binuclear Macrocyclic Manganese(II) Complexes
Inorganic Chemistry, Vol. 39, No. 5, 2000 891
Scheme 2
at first, since it was anticipated that these complexes may have
been too coordinatively saturated to exhibit such reactivity; viz.,
five coordination sites are occupied by the strongly binding N5
ligand chromophore. The behavior of complexes 1-6 is similar
to that of bridged binuclear Mn complexes that sustain many
H2O2 disproportionation cycles.7,24,43,47 For many of these
systems, although a substantial number of turnovers have been
achieved, activity does diminish. Protonation of the ligand
followed by dissociation of the complex through release of Mn2+
is one way by which reactivity can be reduced and ultimately
lost.7 For 1-6, the growth of the [Mn(OH2)6]2+ ESR signal and
precipitation of MnO2 indicate that these complexes dissociate
during cycling.
For 1, 2, 5, and 6, DMF solutions of the complexes changed
from colorless to brown within 5 min of the addition of H2O2.
After 3 h, the solutions were almost colorless, the evolution of
oxygen had ceased, and small amounts of MnO2 had deposited.
Further evolution of oxygen was observed on addition of more
hydrogen peroxide. In contrast, addition of hydrogen peroxide
to 3 and 4 resulted in a more dramatic series of color changes,
viz. colorless to pale blue within minutes, to pale green after
30 min, to brown after 90 min, and back to almost colorless
after 3 h. The time-dependent variation of the UV-visible
spectrum of 4 is shown in Figure 9 ([complex] ∼ 5 × 10-3 M,
initial [H2O2] ∼ 1.5 M). A blue solution is generated within
Figure 10. Frozen-solution (99 K) ESR spectra of a DMF solution of
2 ([complex] ∼ 1 mM) measured at various times after the addition of
hydrogen peroxide ([H2O2] ∼ 1.5 M) and (b) an expansion of the 16-
line ESR spectrum observed after 60 min.
minutes, exhibiting an absorption maximum at 590 nm that may
be due to a transition within an oxo- or peroxo-bridged Mn
intermediate. As the color of the solution changes to light brown,
the intensity of this band diminishes and a shoulder appears at
ca. 450 nm, typical of a high-valent Mn complex and possibly
oxo (or even peroxo) MnIV complexes.1,19,25,48 ESR measure-
ments exclude the formation of mixed-valent intermediates,
either MnIIMnIII or MnIIIMnIV species, as these give rise to
distinct, intense signals. Indeed, the ESR spectra of 1 and 3-6
recorded in frozen DMF at various times following the addition
of peroxide showed only an enhancement of the 6-line [Mn-
(OH2)6]2+ signal centered at 3300 G (peak separation ) 90 G).
Since some oxidation states of Mn are ESR inactive (e.g., the
Mn(III) ion is generally ESR silent owing to rapid electron
(47) Delroisse, M.; Rabion, A.; Chardac, F.; Tetard, D.; Verlhac, J.-B.;
Fraisse, L.; Seris, J.-L. Chem. Commun. 1995, 949.
(48) Mandal, S. K.; Armstrong, W. H. Inorg. Chim. Acta 1995, 229, 261.

892 Inorganic Chemistry, Vol. 39, No. 5, 2000
Brudenell et al.
relaxation rates and large zero-field spitting49), the task of
accurately assigning the UV-visible absorption bands to specific
oxidation states is difficult.
clononane) ligands. Notably, Tolman and co-workers54 have
very successfully used alkyl- and xylyl-linked bis(1,4,7-triaza-
cyclononane) ligands to support the high-valent [CuIII2O2]2+
cores with quite short Cu‚‚‚Cu distances. In the present system,
oxidation of the MnIIMnII complex to the MnIIIMnIII state is
electrochemically facile. Since MnIII is expected to be labile,
rapid replacement of some N donors with harder O donors (e.g.,
from H2O) would introduce groups capable of bridging the Mn
centers.
In contrast to complexes 1 and 3-6, the binuclear complex
of tmpdtne, 2, produces a very distinct 16-line ESR signal
following reaction with excess H2O2 (Figure 10). The signal
disappears after 3 h, and a signal due to [Mn(OH2)6]2+ appears.
The 16-line response is associated with a color change from
colorless to brown and indicates the formation of a mixed-valent
species. While it should be noted that MnIIMnIII species can
give rise to similar multiline ESR signals,20,22,29,43 the signal
for 2 is most likely due to an MnIIIMnIV complex. An expanded
view of the signal (Figure 10b) shows that it is centered at 3300
G (g ) 2.0) and that it closely matches the 16-line ESR spectrum
often observed for µ-oxo-bridged MnIIIMnIV dimers.1,12a,16,29,50-53
Such complexes are strongly antiferromagnetically coupled with
Conclusion
The series of binuclear Mn(II) complexes, 2-6, reported here
represent an important addition to the rapidly expanding catalog
of manganese complexes. The molecular structures of four of
the complexes confirmed that the geometries about each metal
center were intermediate between trigonal prismatic and octa-
hedral. Electrochemical measurements indicated that as the
metal-metal separation increased, electrostatic repulsions were
reduced and oxidation of the complexes occurred at lower
potentials. Two 2e- oxidation processes, corresponding to MnII-
MnII f MnIIIMnIII and MnIIIMnIII f MnIVMnIV, were observed
for complexes 2-5 and on closer examination were found to
consist of two overlapping 1e- processes, such that the MnII-
MnIII and MnIIIMnIV species were generated before being
oxidized further to the MnIIIMnIII and MnIVMnIV forms,
respectively. Complex 6 forms a mixed-valent MnIIIMnIV
intermediate at a much lower potential than 2-5. Bridging of
the two Mn centers through an endogenous propoxide bridge
could be responsible for this behavior. Preliminary results
indicate that complexes 1-6 catalyze the disproportionation of
H2O2 and that an oxo-bridged mixed-valent Mn intermediate is
formed in the case of 2.
1
an S ) /2 ground state. The signal width of ∆ ) 129 mT for
2 falls within the range for MnIIIMnIV complexes of 110-135
mT12,29,50 and is distinguishable from those of mixed-valent MnII-
MnIII complexes, which are in the range of 160-180 mT.20,22,29,43
Since an Mn‚‚‚Mn separation of 2.531 Å was observed for
the bis(µ2-oxo) µ-peroxo MnIV complex, [(Me3tacn)2Mn2(µ2-
O)2(µ-O2)]2+ 21
it may have been anticipated that, due to the
,
large Mn‚‚‚Mn separation of 6.815(3) Å observed in the solid-
state structure of 2, the formation of a similar complex would
be precluded. However, the related bis(tridentate) ligand Me4-
dtne (1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), in
which the two macrocycles are linked by an ethane bridge
identical to that in tmpdtne, forms a mixed-valent complex,
[MnIIIMnIV(µ-O)2(µ-CH3COO)Me4dtne]2+, and produces a 16-
line ESR spectrum like that of 2,12a as does the [MnIIIMnIV(µ-
O)(µ-O2CPh)(µ-O2BPh)]n+ complex of Me4dtnp-OH.29 This not
only supports the assignment of the MnIIIMnIV state for the 16-
line signal of 2 but also demonstrates that oxo-bridged com-
plexes can be supported by ethane-linked bis(1,4,7-triazacy-
Acknowledgment. This work was supported by the Aus-
tralian Research Council (L.S., A.M.B., E.R.T.T.). S.J.B. was
the recipient of a Monash Graduate Scholarship. We thank
Professor K. S. Murray and Dr. B. Moubaraki for carrying out
the low-temperature magnetic susceptibility measurements and
Professor J. Pilbrow for access to ESR facilities.
(49) Drago, R. S. Physical Methods in Chemistry; Saunders: Philadelphia,
PA, 1977.
(50) Hagen, K. S.; Armstrong, W. H.; Hope, H. Inorg. Chem. 1988, 27,
969.
(51) Tan, X.-L.; Gultneh, Y.; Sarneski, J. E.; Scholes, C. P. J. Am. Chem.
Soc. 1991, 113, 7853.
(52) Frapart, Y.-M.; Boussac, A.; Albach, R.; Anxolabehere-Mallart, E.;
Delroisse, M.; Verlhac, J.-B.; Blondin, G.; Girerd, J.-J.; Guilhem, J.;
Cesario, M.; Rutheford, A. W.; Lexa, D. J. Am. Chem. Soc. 1996,
118, 2669.
Supporting Information Available: X-ray crystallographic files
in CIF format, for 1-3 and 4′. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC9814470
(53) Lal, T. K.; Mukeherjee, R. Inorg. Chem. 1998, 37, 2373.
(54) Tolman, W. B. Acc. Chem. Res. 1997, 30, 227 and references therein.