Modulating the reduction potential of mononuclear cobalt(II) complexes via selective deprotonation of tris[(2-benzimidazolyl)methyl]amine

Article abstract of DOI:10.1021/ic0155885

Remote site deprotonation of the coordinated tripodal ligand, tris((2-benzimidazolyl)methyl)amine, was examined using electronic spectroscopy and electrochemistry techniques. The solid-state structures [CoH₃1^tba(NCS)]⁺ and [CoH₂1^tba(NCS)] are reported. These complexes crystallized in the triclinic space group P1 [a = 13.3043(2) A, b = 13.8019(2) A, c = 14.1322(2) A, α = 63.6670(10)°, β = 68.0590(10)°, γ = 81.8960°; Z = 2] and the monoclinic space group P2₁/n [a = 15.3530(9) A, b = 11.0645(6) A, c = 19.1319(10) A, β = 105.6750(10)°; Z = 4], respectively. Preliminary results suggest that selective and reversible deprotonation of coordinated benzimidazolyl ligands can tune the reduction potential of several isostructural cobalt(II) complexes.

Full text of DOI:10.1021/ic0155885

Inorg. Chem. 2002, 41, 1351−1353
Modulating the Reduction Potential of Mononuclear Cobalt(II)
Complexes via Selective Deprotonation of
Tris[(2-benzimidazolyl)methyl]amine
,†
Brian S. Hammes,* Matthew T. Kieber-Emmons,† Roger Sommer,‡ and Arnold L. Rheingold‡,§
Department of Chemistry, Saint Joseph’s UniVersity, Philadelphia, PennsylVania 19131, and
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
Received September 13, 2001
Remote site deprotonation of the coordinated tripodal ligand, tris-
((2-benzimidazolyl)methyl)amine, was examined using electronic
spectroscopy and electrochemistry techniques. The solid-state
reversible method of modification potentially allows for a
greater control of the stability and reactivity of transition
metal complexes while avoiding the need for complicated
synthetic manipulations. However, a major question, which
has yet to be addressed, is whether the distal deprotonation
of coordinated benzimidazolyl/imidazolyl ligands can be used
to manipulate the chemistry of unsaturated metal complexes.
To date, no literature reports have appeared describing the
manipulation of electronic properties of unsaturated transition
metal complexes via selective deprotonation of coordinated
imidazole ligands. We have been investigating the coordina-
tion chemistry of tripodal ligands derived from imidazole.
tba
tba
structures [CoH 1 (NCS)]+ and [CoH 1 (NCS)] are reported.
3
2
These complexes crystallized in the triclinic space group P1h [a )
13.3043(2) Å, b ) 13.8019(2) Å, c ) 14.1322(2) Å, R ) 63.6670-
(10)°, â ) 68.0590(10)°, γ ) 81.8960°; Z ) 2] and the monoclinic
space group P21/n [a ) 15.3530(9) Å, b ) 11.0645(6) Å, c )
19.1319(10) Å, â ) 105.6750(10)°; Z ) 4], respectively.
Preliminary results suggest that selective and reversible deproto-
nation of coordinated benzimidazolyl ligands can tune the reduction
potential of several isostructural cobalt(II) complexes.
In particular, tris(2-benzimidazolylmethyl)amine7 (H31tba
)
which utilizes a tris(2-imidazolylmethyl)amine backbone will
provide a useful first step in determining if tripodal ligands
of this type can stabilize unsaturated metal complexes upon
deprotonation. It should be noted that while the primary
coordination around the metal ion(s) should remain constant,
the selective deprotonatation will change the physical proper-
ties of the metal ion(s) coordinated within the tripodal
framework.
Transition metal complexes containing tris(2-benzimida-
zolylmethyl)amine are not uncommon; however, they do
provide a useful starting point to study selective deprotona-
tion and the ability to control the electronic properties of a
series of Co(II) complexes.8,9 We report herein the reversible
stepwise modulation of the electronic properties of several
Co(II)-NCS complexes of tris(2-benzimidazolylmethyl)-
amine. These initial results demonstrate that sequential
deprotonation of the coordinated benzimidazolyl arms result
in the shift in the electrochemical potential of the Co(II) ion
from +0.98 to +0.47 V or an average of 0.17 V per proton.
We are interested in the interdependence between the
deprotonation of coordinated tripodal imidazolyl/benzimda-
The use of protonation/deprotonation of ancillary ligands
coordinated to transition metal complexes has attracted
increased attention recently because of their importance in
biological, synthetic, and industrial processes.1-3 For ex-
ample, the relationship between proton coupled electron
transfer and hydrogen atom transfer continues to gain
increasing interest.4,5 Recently, it has been reported by
Williams and co-workers that the reduction potential of
saturated iron complexes can be modulated between +0.920
and -0.460 V in an octahedral iron complex containing four
imidazole/benzimidazole groups.6 Although no attempt was
made to deprotonate each imidazole selectively, it was
believed that each deprotonation resulted in a redox potential
shift of approximately 0.3 V per proton. This subtle and
* Author to whom correspondence should be addressed. Fax: 610-660-
1783. Tel: 610-660-1794. E-mail: bhammes@sju.edu.
† Saint Joseph’s University.
‡ University of Delaware.
§ Fax: 302-831-6335. Tel: 302-831-8720. E-mail: arnrhein@udel.edu.
(1) Stubbe J. A.; van der Donk W. A. Chem. ReV. 1998, 98, 705.
(2) Meunier, M., Ed. Biomimetic Oxidations Catalyzed by Transition Metal
Complexes; Imperial College Press: 2000.
(3) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981.
(4) Roth, J. P.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122,
5486.
(5) Roth, J. P.; Mayer, J. M. Inorg. Chem. 1999, 38, 2760.
(6) Carina, R. F.; Verzegnassi, L.; Bernardinelli, G.; Williams, A. F. Chem.
Commun. 1998, 2681.
(7) Thompson, L. K.; Ramaswamy, B. S.; Seymour, E. A. Can. J. Chem.
1977, 55, 878.
(8) Addison, A. W.; Hendriks, H. M. J.; Reedijk, J.; Thompson, L. K.
Inorg. Chem. 1981, 20, 103.
(9) Su, C.-Y.; Kang, B.-S.; Du, C.-X.; Yang, Q.-C.; Mak, T. C. W. Inorg.
Chem. 2000, 39, 4843.
10.1021/ic0155885 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/01/2002
Inorganic Chemistry, Vol. 41, No. 6, 2002 1351

COMMUNICATION
Scheme 1a
Figure 1. Thermal ellipsoid diagram of [CoH31tba(NCS)]+ (left) and
[CoH21tba(NCS)] (right). The ellipsoids are drawn at the 40% probability
level, and only the benzimidazolyl hydrogen atoms are shown. Selected
bond lengths (Å) and angles (deg) for [CoH21tba(NCS)] {[CoH31tba
-
(NCS)]+}: Co(1)-N(1), 2.3547(19) {2.4078(18)}; Co(1)-N(2), 2.031(2)
{2.0312(19)}; Co(1)-N(4), 2.061(2) {2.0078(18)}; Co(1)-N(6), 2.0046-
(19) {2.0308(19)}; Co(1)-N(8), 2.019(2) {2.0409(19)}; N(1)-Co(1)-N(2),
76.20(7) {75.72(7)}; N(1)-Co(1)-N(4), 75.69(7) {76.45(7)}; N(1)-Co-
(1)-N(6), 76.42(7) {75.30(7)}; N(1)-Co(1)-N(8), 177.93(8) {172.20(7)};
N(2)-Co(1)-N(4), 115.65(8) {112.27(7)}; N(2)-Co(1)-N(6), 108.00(8)
{123.34(7)}; N(2)-Co(1)-N(8), 102.80(9) {102.61(8)}; N(4)-Co(1)-N(6),
119.48(8) {106.66(8)}; N(4)-Co(1)-N(8), 103.27(8) {111.02(7)}; N(6)-
Co(1)-N(8), 105.64(8) {99.85(8)}.
a Conditions: (a) CoCl2, KSCN, CH3OH.
zolyl ligands and the physical and chemical properties of a
metal ion within the tripodal framework. Scheme 1 outlines
the synthesis of the various Co(II) complexes. The Co(II)
complex was readily prepared by the direct reaction of H31tba
with CoCl2‚6H2O in methanol; addition of NaBPh4 and
KSCN gave the blue complex [CoH31tba(NCS)]BPh4 in very
good yield. The trigonal bipyramidal complex shows two
d-d transitions at 576 nm (ꢀ ) 400 M-1 cm-1) and 787 nm
(ꢀ ) 40 M-1 cm-1) in the visible region, which is consistent
with trigonal bipyramidal geometry around the metal ion.7,10
Upon treatment with 1 equiv of KH in DMF under an argon
atmosphere, the solution became light purple. The electronic
spectrum of this solution showed absorptions at 561 nm (ꢀ
) 340 M-1 cm-1) and 893 nm (ꢀ ) 24 M-1 cm-1) which
corresponded to the monodeprotonated 5-coordinate complex
[CoH21tba(NCS)]. Addition of two consecutive equivalents
of KH to the light purple solution allowed isolation of the
dideprotonated and the trideprotonated Co(II) complexes,
[CoH1tba(NCS)]- and [Co1tba(NCS)]2-,11 respectively, with
corresponding shifts in their absorption spectra (553 nm (ꢀ
) 340 M-1 cm-1) and 770 nm (ꢀ ) 38 M-1 cm-1) for
[CoH1tba(NCS)]-and 544 and 767 nm for [Co1tba(NCS)]2-).
The shifts in the d-d bands in the absorption spectra of the
di- and trideprotonated complexes suggest that the trigonal
bipyramidal coordination geometry of the Co(II) series
remains unchanged throughout the series of Co(II) com-
plexes. Initially, it was believed that the combination of
absorption and infrared spectroscopy (with NCS- as a ligand)
would not only provide structural and electronic information
but would also allow us to probe the electronic effect of
deprotonation on the Co-NCS bond. However, the change
in the overall charge on each complex resulted in no clear
electronic trends in this series of complexes.
methods. The overall structures of both complexes are very
similar (shown in Figure 1), with both complexes having a
trigonal bipyramidal coordination geometry around their
cobalt centers.12 In [CoH21tba(NCS)], two benzimidazolyl
nitrogens and one benzimidazolylate nitrogen define the
trigonal plane with an average Co(1)-Nbenz bond length of
2.0233(11) Å while the Co(1)-Nbenzimidazolylate bond length
is considerably shorter at 2.0046(19) Å. This was expected
for a monoanionic benzimidazolyl ligand coordinated to a
cobalt ion.13 In [CoH31tba(NCS)]+, the three benzimidazolyl
nitrogens made up the trigonal plane with an average Co-
(1)-Nbenz bond distance of 2.0232(11) Å, which is consistent
with those reported for other benzimdazolyl complexes.14-17
The metal ion in each complex lies slightly out the trigonal
plane formed by the benzimidazolyl/benzimidazolylate ni-
trogen atoms N(2), N(4), and N(6) toward the isothiocyanato
ligand (0.215 Å for [CoH31tba(NCS)]+ and 0.212 Å for
[CoH21tba(NCS)]). The Co(1)-N(1) bond distance decreases
significantly upon deprotonation (2.4078(18) and 2.3547-
(19) Å, respectively). This decrease is believed to result from
a twisting of the methylene carbon in each tripodal arm which
allows the apical amine to move closer to the metal ion
(average dihedral angle of Nbenz-Co-Namine-Cmethyl increases
from 18.7° to 23.3° upon deprotonation). The terminal
isothiocyanato ligand is positioned nearly trans to the apical
(12) Crystal data for [CoH31tba(NCS)]BPh4: C51H44BCoN9S, M ) 884.75,
a ) 13.3043(2) Å, b ) 13.8019(2) Å, c ) 14.1322(2) Å, V ) 2156.33-
(5) Å3, triclinic, space group P1h, Z ) 2, T ) 173(2) K, final R1 )
0.0675, wR2 ) 0.1337, GOF (on F2) ) 1.458. For [CoH21tba(NCS)]:
C31H34CoN10O2S, M ) 669.67, a ) 15.3530(9) Å, b ) 11.0645(6)
Å, c ) 19.1319(10) Å, V ) 3129.1(3) Å3, monoclinic, space group
P21/n, Z ) 4, T ) 173(2) K, final R1 ) 0.0664, wR2 ) 0.1141, GOF
(on F2) ) 1.051.
(13) All benzimidazolyl hydrogen atoms were found in the Fourier
difference map and refined positionally while the Ueq values were held
at relative values.
(14) Buchanan, R. M.; O’Brien, R. J.; Richardson, J. F.; Latour, J.-M. Inorg.
Chim. Acta 1993, 214, 33.
The molecular structures of [CoH31tba(NCS)]+ and
[CoH21tba(NCS)] were determined by X-ray diffraction
(15) Gomez-Romero, P.; Witten, E. H.; Reiff, W. M.; Jameson, G. B. Inorg.
Chem. 1990, 29, 5211.
(16) Addison, A, W.; Burman, S.; Wahlgren, C. G.; Rajan, O. A.; Rowe,
T. M.; Sinn, E. J. Chem. Soc., Dalton Trans. 1987, 2621.
(17) Linert, W.; Konecny, M.; Renz, F. J. Chem. Soc., Dalton Trans. 1994,
1523.
(10) Lah, M. S.; Moon, M. Bull. Korean Chem. Soc. 1997, 18, 406.
(11) The reprotonation of the Co(II) complex was done by titration with
methonolic HCl and was followed by absorption spectroscopy. For
[Co1tba(NCS)]2-, all spectroscopic data suggest that the monomeric
Co(II) complex is the only product.
1352 Inorganic Chemistry, Vol. 41, No. 6, 2002

COMMUNICATION
amine nitrogen atom N(1). Upon deprotonation, the Co(1)-
N(8) distance shortens considerably (2.0409(19) and 2.019-
(2) Å, respectively), and the N(1)-Co(1)-N(8) becomes
almost linear with angles of 172.20(7)° for the fully
protonated complex and 177.93(8)° for the neutral Co(II)
complex, respectively.
the deprotonated benzimidazolylate ligand decreases the
electron density available to donate to the metal ion. On the
other hand, Haga et al. has studied the effect of deprotonation
of coordinated benzimidazole ligands in mixed ruthenium
and osmium complexes and has found a 2-fold increase in
the reduction potential upon deprotonation than what we have
observed.22,23 This difference could result from the benzimi-
dazolyl acting as a bidentate ligand bridging between the
ruthenium and/or osmium ions.
The initial results of this study confirm that distal
deprotonation of coordinated benzimidazolyls contained
within an unsaturated Co(II) complex can be isolated and
characterized. The evidence suggests that the 3-fold sym-
metric framework formed in metal complexes containing
H31tba-[1tba 3- can sequentially modulate the redox proper-
]
ties of Co(II) complexes by the selective deprotonation of
In view of the changes in the ligand field around the Co-
(II) ion, it was initially surprising that the totally deprotonated
complex [Co1tbaNCS]2- was completely air stable. Cyclic
voltammetry studies on all of the complexes in DMF (glassy
carbon electrode, 0.1 M TBAP,18 scan rate 0.1 Vs-1) showed
an irreversible oxidative redox couple which ranged from
+0.98 V for [CoH31tba(NCS)]+ to +0.47 V for [Co1tba
-
(NCS)]-2 (vs SCE).19 This suggests that the introduction of
negative charge onto the tripodal ligand shifts the redox
potential more negative by an average of 0.17 V per proton.
The lack of air sensitivity in these transition metal complexes
is similar to the behavior of octahedral iron complexes
containing the tridentate ligand 2,6-bis(benzimidazolyl)-
pyridine.20,21 However, the magnitude of the electronic shift
is considerably smaller in our study than those reported for
coordinatively saturated iron complexes containing imida-
zolyl ligands.6 Perhaps the high degree of delocalization in
coordinated benzimidazolyls.
Acknowledgment. The authors wish to thank the Re-
search Corporation under Grant CC5286 for support of this
research. Professor Brian S. Hammes is a Cottrell Scholar
of Research Corporation.
Supporting Information Available: The synthesis of H31tba
ligand, coordination compounds, the metathesis procedures, and
X-ray crystallographic files in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
(18) Tetrabutylammonium hexafluorophosphate.
(19) All observed redox events were oxidative in nature. No reductive
phenomena were observed for any of the reported complexes between
0 and -1.8 V vs SCE.
IC0155885
(20) Addison, A. W.; Burman, S.; Wahlgren, C. G.; Rajan, O. A.; Rowe,
T. M.; Sinn, E. J. Chem. Soc., Dalton Trans. 1987, 2621.
(21) Ru¨ttimann, S.; Moreau, C. M.; Williams, A. F.; Bernardinelli, G.;
Addison, A. W. Polyhedron 1992, 6, 635.
(22) Haga, M.; Ali, M. M.; Arakawa, R. Angew. Chem., Int. Ed. Engl.
1996, 35, 76.
(23) Haga, M.; Bond, A. M. Inorg. Chem. 1991, 30, 475.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1353