Cymantrene radical cation family: Spectral and structural characterization of the half-sandwich analogues of ferrocenium ion

Article abstract of DOI:10.1021/ja801930q

The anodic one-electron oxidation of three members of the half-sandwich family of piano-stool compounds MnCp^γ(CO)₃, where Cp^γ is a generic cyclopentadienyl ligand, has been studied in a CH₂Cl₂/ [NBu₄][TFAB] electrolyte (TFAB = [B(C₆F₅)₄]^-). The long-sought 17 e^- radical cation of the parent complex MnCp(CO)₃ (cymantrene, 1, E_1/2 = 0.92 V vs ferrocene) has been shown to be persistent in solutions that use weakly coordinating anions in place of more nucleophilic traditional electrolyte anions. Spectroscopically characterized for the first time, 1⁺ was shown to absorb in the visible (530 nm), near-IR (2066 nm), and IR (2118, 1934 cm^-1) regions. It was ESR-active at low temperatures (g_∥ = 2.213, g_⊥ = 2.079, A_∥ (Mn) = 79.2 G, A_⊥ (Mn) = 50 G) and NMR active at room temperature (δ = 22.4 vs TMS). The radical cations of the Cp-functionalized analogues, Mn(η⁵-C₅H ₄NH₂)(CO)₃, 2, E_1/2 = 0.62 V, and MnCp*(CO)₃ (Cp= η⁵-C₅Me ₅, 3), E_1/2 = 0.64 V, were generated electrochemically as well by the chemical oxidant [ReCp(CO)₃]⁺. The structures of 2⁺ and 3⁺ were determined by X-ray crystallographic studies of their TFAB salts. Compared to the structures of the corresponding neutral compounds, the cations showed elongated Mn-C(O) bonds and shortened C-O bonds, displaying the effect of diminished metal-to-CO backbonding. The bond-length changes in the Mn(CO)₃ moiety were much larger in 3 ⁺ (avg changes, Mn-C(O) = + 0.142 A, C-O = -0.063 A) than in 2⁺ (avg changes, Mn-C(O) = + 0.006 A, C-O = -0.003 A). Although there were only minor changes in the metal-to-center ring distances upon oxidation of either 2 or 3, there was decidedly less bending of the C(N) atom out of the cyclopentadienyl plane in 2⁺ compared to 2. The optical, vibrational, and magnetic resonance spectra of radicals 2 ⁺ and 3⁺ were also observed. The spectral data argue for the SOMOs of the 17-electron species being largely located on the Mn(CO) ₃ moiety, having 40-50% Mn d-orbital character, with the ground states of the radicals, most likely ²A″, lying close in energy (within about 6000 cm^-1) to excited states that are responsible for their rapid electronic relaxations. The cymantrenyl moiety is proposed as an anodic redox tag (or label) having physical and chemical properties that are significantly different from those of its ferrocenyl analogue.

Full text of DOI:10.1021/ja801930q

Published on Web 07/03/2008
Cymantrene Radical Cation Family: Spectral and Structural
Characterization of the Half-Sandwich Analogues of
Ferrocenium Ion
Derek R. Laws,† Daesung Chong,† Karen Nash,† Arnold L. Rheingold,‡ and
William E. Geiger*,†
Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405 and Department of
Chemistry and Biochemistry, UniVersity of California at San Diego, La Jolla, California 92093
Received March 14, 2008; E-mail: william.geiger@uvm.edu
Abstract: The anodic one-electron oxidation of three members of the half-sandwich family of piano-stool
compounds MnCpγ(CO)3, where Cpγ is a generic cyclopentadienyl ligand, has been studied in a CH2Cl2/
[NBu4][TFAB] electrolyte (TFAB ) [B(C6F5)4]-). The long-sought 17 e- radical cation of the parent complex
MnCp(CO)3 (cymantrene, 1, E1/2 ) 0.92 V vs ferrocene) has been shown to be persistent in solutions that
use weakly coordinating anions in place of more nucleophilic traditional electrolyte anions. Spectroscopically
characterized for the first time, 1+ was shown to absorb in the visible (530 nm), near-IR (2066 nm), and IR
(2118, 1934 cm-1) regions. It was ESR-active at low temperatures (g| ) 2.213, g ) 2.079, A| (Mn) ) 79.2
G, A (Mn) ) 50 G) and NMR active at room temperature (δ ) 22.4 vs TMS). The radical cations of the
Cp-functionalized analogues, Mn(η5-C5H4NH2)(CO)3, 2, E1/2 ) 0.62 V, and MnCp*(CO)3 (Cp*) η5-C5Me5,
3), E1/2 ) 0.64 V, were generated electrochemically as well by the chemical oxidant [ReCp(CO)3]+. The
structures of 2+ and 3+ were determined by X-ray crystallographic studies of their TFAB salts. Compared
to the structures of the corresponding neutral compounds, the cations showed elongated Mn-C(O) bonds
and shortened C-O bonds, displaying the effect of diminished metal-to-CO backbonding. The bond-length
changes in the Mn(CO)3 moiety were much larger in 3+ (avg changes, Mn-C(O) ) + 0.142 Å, C-O )
-0.063 Å) than in 2+ (avg changes, Mn-C(O) ) + 0.006 Å, C-O ) -0.003 Å). Although there were only
minor changes in the metal-to-center ring distances upon oxidation of either 2 or 3, there was decidedly
less bending of the C(N) atom out of the cyclopentadienyl plane in 2+ compared to 2. The optical, vibrational,
and magnetic resonance spectra of radicals 2+ and 3+ were also observed. The spectral data argue for
the SOMOs of the 17-electron species being largely located on the Mn(CO)3 moiety, having 40-50% Mn
d-orbital character, with the ground states of the radicals, most likely 2A′′, lying close in energy (within
about 6000 cm-1) to excited states that are responsible for their rapid electronic relaxations. The cymantrenyl
moiety is proposed as an anodic redox tag (or label) having physical and chemical properties that are
significantly different from those of its ferrocenyl analogue.
Introduction
cathodic response of cymantrenyl-labeled bovine serum albumin
has been employed for immunoassays,5 applications of cyman-
Perhaps the most highly studied half-sandwich transition
metal compound is the piano-stool complex MnCp(CO)3 (Cp
) η5-C5H5), 1,1,2 often referred to as cymantrene by analogy
to ferrocene. One emerging area to which cymantrene is
contributing is the use of organometallic tags (or labels) to alter
the chemical or analytical properties of chemical targets. The
most common molecular labels are those which take advantage,
either directly or indirectly, of the one-electron oxidation of an
attached ferrocenyl group.3 The chief attraction in using a
cymantrenyl moiety, Mn(η5-C5H4)(CO)3, in place of ferrocenyl
is its electron-withdrawing and IR-active carbonyl groups.4 Its
redox chemistry, however, has been underutilized. Although the
trenyl anodic chemistry are lacking despite the fact that the
[MnCp(CO)3]0/+ couple has been shown to be at least partially
reversible by cyclic voltammetry (CV), as will be discussed
(2) For example, the following references involve applications of cyman-
trene in the chemistry of fuels: (a) Marsh, N. D.; Preciado, I.; Eddings,
E. G.; Sarofim, A. F.; Palotas, A. B.; Robertson, J. D. Combust. Sci.
Technol. 2007, 179, 987. (b) Geivanidis, S.; Pistikopoulos, P.; Samaras,
Z. Sci. Total EnViron. 2003, 305, 129. (c) Oxley, J. C.; Smith, J. L.;
Rogers, E.; Ye, W.; Aradi, A. A.; Henly, T. J. Energy and Fuels 2000,
14, 1252Sensors: (d) Nefedov, V. A.; Polyakova, M. V.; Rorer, J.;
Sabelnikov, A. G.; Kochetkov, K. A. MendeleeV Commun. 2007, 17,
167. (e) Markwell, R. D.; Butler, I. S.; Gao, J. P.; Shaver, A. Appl.
Organomet. Chem. 1992, 6, 693. Chiral synthesis: (f) Ferber, B.; Top,
S.; Jaouen, G. J. Organomet. Chem. 2004, 689, 4872. (g) Lyubimov,
S. E.; Davankov, V. A.; Loim, N. M.; Popova, L. N.; Petrovskii, P. V.;
Valetskii, P. M.; Gavrilov, K. N. J. Organomet. Chem. 2006, 691,
5980. Medicine: (h) Malisza, K. L.; Top, S.; Kaissermann, J.; Caro,
B.; Senechal-Tocquer, M.; Senechal, D.; Saillard, J.; Triki, S.; Kahlal,
S.; Britten, J. F.; McGlinchey, M. J.; Jaouen, G. Organometallics 1995,
14, 5273. (i) Lavastre, I.; Besancon, J.; Brossier, P.; Moise, C. Appl.
Organomet. Chem. 1990, 4, 9.
† University of Vermont.
‡ University of California at San Diego.
(1) (a) Treichel, P. M. In ComprehensiVe Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, 1982; Vol. 4, pp 123-132. (b) (Original preparation) Piper,
T. S.; Cotton, F. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1955, 1,
165.
9
10.1021/ja801930q CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 9859–9870 9859

A R T I C L E S
Laws et al.
below. The reasons for this underdevelopment would seem to
be the relatively positive potential of the couple (ca. 0.9 V vs
ferrocene) and the instability of the radical cation on the longer-
term (spectroscopic or electrosynthetic) time scale. This paper
addresses the second of these points, which we consider to be
the more serious impediment to development of cymantrenyl
redox tags.
The expectation that cymantrene would be significantly harder
to oxidize than ferrocene owing to the presence of its strongly
π-accepting carbonyl groups has been confirmed both by
photoelectron spectroscopy (PES)6,7 and by a number of
electrochemical studies.8–15 More poorly understood, however,
is the dramatically reduced lifetime of the cymantrene radical
cation, 1+, compared to the ferrocenium ion. The frontier orbitals
of cymantrene and ferrocene are similar in makeup, both
compounds being viewed as pseudo-octahedral systems in which
the three fac carbonyls are topologically equivalent to the Cp
anion,16,17 with the three highest-energy filled orbitals for the
d6 complexes being closely spaced in energy. In the case of
cymantrene, these orbitals are derived primarily from e- and
a-type contributions of the Mn(CO)3 moiety,18–20 and their close
spacing has been confirmed experimentally by the finding that
the lowest energy ionizations of cymantrene are unresolved by
PES.6,7 A number of groups have shown8–12 that the one-
electron anodic oxidation of 1 is at least partially chemically
reversible on the CV time scale21 in nonaqueous media, but
based on the published work, even in situ spectral characteriza-
tion of 1+ has apparently eluded investigators22 (carbonyl-
substituted derivatives have proven more stable, as will be
discussed below). Qualitatively, it might be expected that
removal of an electron from 1 would weaken the Mn-C(O)
bonds (owing to reduced metal-to-CO back-bonding) and
perhaps the Mn-Cp bond (depending on the nature of the
SOMO of 1+). However, neither of these possibilities seemed,
to us at least, sufficient to make the cymantrene radical cation
inherently unstable under the conditions employed for its
electrochemical generation. The treatment by Field et al.7 of
the highest filled orbitals of 1, concluding that the trio of e-
and a-type orbitals have significant metal-ligand mixing, further
begs the question of why 1+ does not exhibit greater stability.
We set out to see if its reported decomposition arose from
follow-up reactions with the electrochemical supporting elec-
trolyte anions, usually either [PF6]- or [BF4]-, which are known
to have modest nucleophilicity toward organometallic radical
cations.23–25 This is, indeed, the case. In this paper we show
that when the cymantrene radical cation is generated in a
medium consisting of CH2Cl2 and [NBu4][B(C6F5)4], it is
persistent on a synthetic (electrolysis) time scale, allowing its
broad spectral characterization (UV-vis, IR, near IR, ESR, and
1H NMR). Furthermore, two of its Cp-substituted analogues,
namely the amino-substituted complex Mn(C5H4NH2)(CO)3, 2,
(3) Leading references to the literature of ferrocenyl redox tags are: (a)
Cuffe, L.; Hudson, R. D. A.; Gallagher, J. F.; Jennings, S.; McAdam,
C. J.; Connelly, R. B. T.; Manning, A. R.; Robinson, B. H.; Simpson,
J. Organometallics 2005, 24, 2051. (b) Warren, S.; McCormac, T.;
Dempsey, E. Bioelectrochemistry 2005, 67, 23. (c) Ryabov, A. D.
AdV. Inorg. Chem. 2004, 55, 201. (d) Beer, P. D.; Hayes, E. J. Coord.
Chem. 2003, 240, 167. (e) Chaubey, A.; Malhotra, B. D. Biosens.
Bioelectron. 2002, 17, 441. (f) Suzawa, T.; Ikariyama, Y.; Aizawa,
M. Anal. Chem. 1994, 66, 3889. (g) Hill, H. A. O.; Sanghera, G. S.
In Biosensor: A Practical Approach; Cass, A. E. G., Ed.; Oxford
University Press: London, 1990; pp 19-46. (h) Foulds, N. C.;; Lowe,
C. R. Anal. Chem. 1988, 60, 2473. (i) Cass, A. E. G.; Davis, G.;
Francis, G. D.; Hill, H. A. O.; Aston, W. J.; Higgins, I. J.; Plotkin,
E. V.; Scott, L. D. L.; Turner, A. P. F. Anal. Chem. 1984, 56, 667.
(4) (a) Lepore, S. D.; Khoram, A.; Bromfield, D. C.; Cohn, P.; Jairaj, V.;
Silvestri, M. A. J. Org. Chem. 2005, 70, 7443–7446. (b) Zhang, Z.;
Lepore, S. D. Tetrahedron Lett. 2002, 43, 7357–7360. (c) Grade´n,
H.; Kann, N. Curr. Org. Chem 2005, 9, 733. (d) Dorozhkin, L. M.;
Nefedov, V. A.; Sabelnikov, A. G.; Sevastjanov, V. G. Sens. Actuators,
B 2004, B99, 568. (e) Salmain, M.; Vessieres, A.; Top, S.; Jaouen,
G.; Butler, I. S. J. Raman Spectrosc. 1995, 26, 31. (f) Wang, Z.; Roe,
B. A.; Nicholas, K. M.; White, R. L. J. Am. Chem. Soc. 1993, 115,
4399. (g) Salmain, M.; Vessieres, A.; Jaouen, G.; Butler, I. S. Anal.
Chem. 1991, 63, 2323. (h) Avastre, I.; Besancon, J.; Brossier, P.;
Moise, C. Appl. Organomet. Chem. 1990, 4, 9. (i) Hublau, P.;
Sergheraert, C.; Ballester, L.; Dautrevaux, M. Eur. J. Med. Chem.
1983, 18, 131.
(16) Albright, T. A.; Burdett, J. K.; Whangbo, M-H. Orbital Interactions
in Chemistry; John Wiley & Sons: New York, 1985; p 388
(17) Adding to the pseudo-octahedral model is the fact that the lowest
unfilled orbitals in cymantrene and ferrocene are nearly degenerate
sets of e1′′-type cyclopentadienyl π-orbitals that are metal-antibonding.
(18) Mingos, D. M. P. In ComprehensiVe Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, 1982; Vol. 3, pp 28-46.
(19) (a) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J. Am.
Chem. Soc. 1979, 101, 585. (b) Elian, M.; Chen, M. M. L.; Mingos,
D. M. P.; Hoffmann, R. Inorg. Chem. 1976, 15, 1148.
(20) Reference 16, p 385.
(21) Under normal experimental conditions, the maximum CV time scale
is approximately 10 s.
(22) The ESR spectrum assigned to 1+ from a solution of bulk-electrolyzed
cymantrene (ref 12) almost certainly arose from a decomposition
product. As shown in the present work, the cymantrene radical cation
is ESR-silent under the fluid room-temperature conditions employed
(5) (a) Hromadova, M.; Salmain, M.; Fischer-Durand, N.; Pospisil, L.;
Jaouen, G. Langmuir 2006, 22, 506. (b) Hromadova, M.; Salmain,
M.; Sokolova, R.; Pospisil, L.; Jaouen, G. J. Organomet. Chem. 2003,
668, 17.
in that work and the spectral characteristics ( g ) 2.015,
A
)
Mn
(6) (a) Calabro, D. C.; Lichtenberger, D. L. J. Am. Chem. Soc. 1981, 103,
6846. (b) Lichtenberger, D. L.; Fenske, R. F. J. Am. Chem. Soc. 1976,
98, 50. These results and otherwise unpublished data have been
discussed by: (c) van Dam, H.; Oskam, A. In Transition Metal
Chemistry; Melson, G. A., Figgis, B. N., Eds.; Marcel Dekker, Inc.:
New York, 1985; pp 174 ff.
92.5 G) are typically found in Mn(II) solvento complexes. A similar
spectrum assigned to a cymantrenyl-based radical cation observed after
iodine oxidation of Mn(η5-C5H4Me)(CO)2(C(O)-ferrocenyl) (see ref
33) ( g ) 2.037, A
) 91.6 G) is also likely to have arisen from
Mn
radical decomposition.
(23) Stone, N. J.; Sweigart, D. A.; Bond, A. M. Organometallics 1986, 5,
2553.
(7) Field, C. N.; Green, J. C.; Moody, A. G. J.; Siggel, M. R. F. Chem.
Phys. 1996, 206, 211.
(24) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991, 30,
4687.
(8) Huang, Y.; Carpenter, G. B.; Sweigart, D. A.; Chung, Y. K.; Lee,
B. Y. Organometallics 1995, 14, 1423.
(25) Camire, N.; Nafady, A.; Geiger, W. E. J. Am. Chem. Soc. 2002, 124,
7260.
(9) Atwood, C. G.; Geiger, W. E.; Bitterwolf, T. E. J. Electroanal. Chem.
1995, 397, 279.
(26) (a) Bernal, I.; Korp, J. D.; Herrmann, W. A.; Serrano, R. Chem. Ber.
1984, 117, 434. (b) Hughes, R. P.; Hemond, R. C.; Locker, H. B.
Organometallics 1986, 5, 2391.
(10) Hershberger, J. W.; Amatore, C.; Kochi, J. K. J. Organomet. Chem.
1983, 250, 345.
(11) Kochi, J. K. J. Organomet. Chem. 1986, 300, 139.
(12) Pickett, C. J.; Pletcher, D. J. Chem. Soc., Dalton Trans. 1976, 636.
(13) Denisovich, L. I.; Zakurin, N. V.; Gubin, S. P.; Ginzburg, A. G. J.
Organomet. Chem. 1975, 101, C43.
(27) (a) Chong, D.; Nafady, A.; Costa, P. J.; Calhorda, M. J.; Geiger, W. E.
J. Am. Chem. Soc. 2005, 127, 15676. (b) Chong, D.; Laws, D. R.;
Nafady, A.; Costa, P. J.; Rheingold, A. L.; Calhorda, M. J.; Geiger,
W. E. J. Am. Chem. Soc. 2008, 130, 2692.
(14) Plenio, H.; Burth, D. Organometallics 1996, 15, 1151. describes the
poorly reversible oxidation of an indenyl Mn(CO)3 complex.
(15) Munoz, R. A. A.; Banks, C. E.; Davies, T. L.; Angnes, L.; Compton,
R. G. Electroanalysis 2006, 18, 621.
(28) (a) Regitz, M.; Hocker, J.; Liedhegener, A. Org. Synth. 1973, 5, 179.
(b) Manuel, T. A. AdV. Organomet. Chem. 1965, 3, 181. (see especially
p 221)
(29) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76, 6395.
9
9860 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Half-Sandwich Analogues of Ferrocenium Ion
A R T I C L E S
Table 1. Infrared Spectroscopy Data for Compounds Dissolved in
a mixture of 2 and any remaining 1. The resulting yellow solution
was slowly evaporated under a stream of nitrogen until yellow
crystals began to form. The mixture was then cooled at -20 °C
for 1 h, yielding 0.86 g (3.92 mmol, 40%) of pure 2 as yellow
crystals. X-ray quality crystals were obtained by dissolving 20 mg
of 2 in 2 mL of warm hexanes and allowing the solution to sit at
room temperature for 2 days. Anal. Calcd for 2: C, 43.87; H, 2.74;
N, 6.40. Found: C, 44.03; H, 2.83; N, 6.38. IR (CH2Cl2): νNH
3458 w, 3386 w, νCO 2012 s, 1921 vs, cm-1. 1H NMR (CDCl3): δ
(ppm) 4.52 (t), 2H, 4.26 (t), 2H.
CH2Cl2 at 298 Ka,b
E1/2 (V)
νsym (cm-1
)
νasym (cm-1
)
νCO (cm-1
)
1
0.92
2022 (2020)
2118 (2116)
2012
2084
2004
1934 (1932)
2058 (2061, 2050)
1921
2032
1917
2033
1963
2078
1951
2049
1946
2053
1+
2
0.62
0.64
2+
3
3+
2093
a νCO ) (νsym + 2νasym)/3. b Values in parentheses are data at 253 K.
1
Instrumentation. H NMR spectra were acquired on a Bruker
ARX 500 MHz spectrometer, and ESR spectra were recorded using
a Bruker ESP 300E spectrometer. Infrared data were acquired on
an ATI-Mattson infinity series FT-IR interfaced to a computer
employing Winfirst software at a resolution of 4 cm-1. For
spectroelectrochemistry, a mid-IR fiber-optic “dip” probe (Remspec,
Inc.) was used in conjunction with the Mattson FT-IR, as described
earlier.31 UV-vis and near-IR spectroscopic data were collected
on a Cary Olis-14 spectrometer using a quartz cuvette with a path
length of 1 cm.
Electrochemistry. All electrochemistry except IR spectroelec-
trochemistry was conducted in a Vacuum Atmospheres drybox
under nitrogen, using a Princeton Applied Research (PAR) Model
273A potentiostat interfaced to a personal computer. Working
electrodes were glassy carbon electrodes (GCEs) and platinum
electrodes supplied by Bioanalytical Systems. The electrode surfaces
were pretreated using a standard sequence of polishing with
diamond paste (Buehler) of decreasing sizes (3 to 0.25 µm)
interspersed by washings with nanopure water and finally vacuum
drying. For controlled potential coulometry, a platinum mesh basket
was used as the working electrode. The reference electrode in all
and MnCp*(CO)3 (Cp* ) η5-C5Me5), 3, were similarly char-
acterized by electrochemistry and spectroscopy. In the cases of
the latter two complexes, their 17-electron cations were isolated
as TFAB salts (TFAB ) [B(C6F5)4]-) and structurally charac-
terized by single crystal X-ray analysis. The picture that emerges
is that the cymantrenyl group, Mn(η5-C5H4)(CO)3, holds promise
as an anodic spectroelectrochemical molecular label that can
be considered as an alternative to its widely employed ferrocenyl
predecessor.
Experimental Section
cases was a Ag/AgCl wire separated from solution by a fine frit,
0/+
but all potentials have been recorded versus the FeCp2
redox
Materials. Schlenk-type procedures under nitrogen were used
for all synthetic procedures. MnCp(CO)3 was supplied by Strem
Chemical Co. and used as received. MnCp*(CO)3
couple obtained by using FeCp2 as an internal standard.32 Data used
in cyclic voltammetry (CV) simulations were collected using
positive-feedback iR compensation to minimize resistance and were
background-subtracted using CV scans collected with only solvent
and electrolyte present. Digital simulations were performed using
Digisim 3.0 (Bioanalytical Systems).
26
and
27
[(ReCp(CO)3)2][B(C6F5)4]2 were prepared as described in the
literature. CDCl3 and CD2Cl2 were purchased from Cambridge
Isotope Laboratories, Inc. and used as received for NMR experi-
ments. n-BuLi, 1.6 M in hexanes, was used as supplied by Acros.
Tosyl azide was prepared according to a literature procedure.28a
[NBu4][B(C6F5)4] was prepared by metathesis29 of [NBu4]Br with
K[B(C6F5)4] (Boulder Scientific Co.) in MeOH, recrystallized three
times from CH2Cl2/OEt2, and dried at 80 °C under vacuum for 24 h.
[NBu4][PF6] was supplied by Acros, recrystallized from EtOH three
times, and dried at 80 °C under vacuum for 24 h. Compounds of
the type MnCp(CO)2L (L ) PPh3, PCy3 (Cy ) cyclohexyl), P(p-
MeOPh)3, P(p-MePh)3, P(p-FPh)3, P(p-CF3Ph)3) were prepared
using photolysis procedures.28b CH2Cl2 was dried over calcium
hydride and collected by vacuum transfer prior to use. CDCl3 used
for controlled potential electrolysis was dried over 4 Å molecular
sieves for 5 days and collected by vacuum transfer prior to use.
THF was dried over potassium and collected by vacuum transfer
from potassium benzophenone.
Mn(C5H4NH2)(CO)3, 2. This compound was prepared through
an adaptation of a literature procedure.30 2.00 g (9.8 mmol) of 1
were reacted with 6.25 mL (10.0 mmol) of n-BuLi (1.6 M in
hexanes) in THF at -78 °C. After stirring for 1 h, 2.19 g (11.1
mmol) of tosyl azide were added, and the mixture was allowed to
warm to room temperature. After 14 h, the solvent was removed
in Vacuo and the remaining black residue was dissolved in 35 mL
of 95% EtOH. 2.10 g (10.7 mmol) of Na[BH4] dissolved in 35 mL
of 95% EtOH were added, and an exothermic reaction was
observed. It was allowed to proceed for 45 min, followed by solvent
removal in Vacuo. The oily solid was extracted with 5 × 20 mL of
OEt2, after which the solvent was evaporated, yielding a dark orange
oil. Warm (50 °C) hexanes, 5 × 30 mL, were then used to extract
Preparation of [2][B(C6F5)4] and [3][B(C6F5)4]. Analytically
pure 2+ was prepared by dissolving 10 mg (4.57 × 10-5 mol) of
2 in 1.0 mL of CH2Cl2 and adding 50 mg (1.99 × 10-5 mol) of
solid [(ReCp(CO)3)2][B(C6F5)4]2. This ratio provides about 0.8 equiv
of the monomer cation [ReCp(CO)3]+ as the oxidizing agent. The
original yellow solution turned dark green immediately. After 5
min, 4 mL of hexanes were added dropwise over the next 5 min,
resulting in the precipitation of green [2][B(C6F5)4]. The product
was washed with copious amounts of hexanes, yielding 25 mg of
product (70% based on oxidant). Alternatively, the isolation of
[2][B(C6F5)4] was achieved by the above method using CDCl3 in
place of CH2Cl2, a solvent in which [2][B(C6F5)4] is insoluble. The
product precipitated as the reaction proceeded, and after 10 min
the solvent was decanted and the product washed with excess
CDCl3. X-ray quality crystals were prepared by dissolving 10 mg
of [2][B(C6F5)4] in 0.5 mL of CH3NO2 and adding a small amount
( 0.5 mL) of CH2Cl2. The solution was stored at -20 °C for about
a week, at which time crystals were collected and washed with
cold CH2Cl2.
Analytically pure 3+ was prepared by dissolving 7.8 mg (2.85
× 10-5 mol) of 3 in 1.0 mL of CH2Cl2 (or CD2Cl2) and adding
28.5 mg (1.13
×
10-5 mol, 0.80 equiv) of solid
[(ReCp(CO)3)2][B(C6F5)4]2. The original yellow solution turned dark
green immediately. After 5 min, 6 mL of hexanes were added
dropwise over the next 5 min, resulting in the precipitation of
turquoise [3][B(C6F5)4]. The product was washed with excess
hexanes to remove unreacted 3 as well as the reaction product
(31) Shaw, M. J.; Geiger, W. E. Organometallics 1996, 15, 13.
(32) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461. (b) Connelly,
N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(30) Holovics, T. C.; Deplazes, S. F.; Toriyama, M.; Powell, D. R.;
Lushington, G. H.; Barybin, M. V. Organometallics 2004, 23, 2927.
9
J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9861

A R T I C L E S
Laws et al.
Figure 2. Anson plot for oxidation wave of 1.1 mM 1 in CH2Cl2/0.1 M
[NBu4][PF6] at 298 K. Step time 1 s, 3 mm GCE.
temperatures, a fact that allowed for the study of carbonyl
substitution reactions by cyclic voltammetry.8 Spectral charac-
terization of the radical cations was blocked, however, either
by electrode passivation effects9 or by decomposition on an
electrolysis time scale.12,22 Optical and ESR studies of this
family of radicals has required substitution of one or more of
the carbonyls by donor ligands, which has the effect of greatly
enhancing the lifetimes of the radical cations.8–10,33–38
Figure 1. (A) 1.0 mM 1 in CH2Cl2/0.1 M [NBu4][PF6] at 298 K. 1 mm
GCE, 0.1 V s-1. Solid line: first scan. Dashed line: 10th consecutive scan.
(B) 0.6 mM 1 in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 298 K, 1 mm GCE,
MnCp(CO)3, 1. Although the anodic behavior of 1 is close
to chemically reversible39 at scan rates greater than 0.1 V s-1
in CH2Cl2 solutions containing either [PF6]- or [BF4]- as the
electrolyte anion (E1/2 ) 0.92 V vs FeCp20/+, Table 1), multiple
scans gave evidence of product adsorption and partial passiva-
tion of the electrode surface (see Figure 1A). The voltammetry
in CH2Cl2/[NBu4][B(C6F5)4] electrolyte (also E1/2 ) 0.92 V)
did not exhibit electrode history problems, consistent with
increased solubility of the radical cation as the TFAB salt
(Figure 1B). Double potential step chronoamperometry con-
ducted in CH2Cl2/0.1 M [NBu4][PF6] at a 3 mm GCE produced
an Anson plot40 (Figure 2) that indicated only weak adsorption
of [1][PF6], as evidenced in the similar slopes for the oxidation
and reduction (ratio ) 0.91) and the very small extrapolated
surface charges. However, 1+ was shown to undergo slow
follow-up reactions when produced in the presence of [PF6]-.
A number of controlled potential electrolyses carried out on 1
using Eappl ) 1.3 V at 298 K in CH2Cl2/0.1 M [NBu4][PF6]
gave coulomb counts of 3-4 F, indicating decomposition of
1+ and production of other electroactive products. The resulting
black solutions only showed evidence of unknown products
having irreversible cathodic waves at Epc ) -0.48 V and -1.34
at 0.1 V/s (Figures SM1, SM2). An irreversible anodic wave
was also observed at Epa ) 1.39 V. Furthermore, IR spectros-
0.1 V s-1
.
ReCp(CO)3, yielding 14 mg of product (65% based on oxidant).
X-ray quality crystals were obtained by dissolving 10 mg of
[3][B(C6F5)4] in 0.5 mL of CH2Cl2 and layering the solution with
0.5 mL of hexanes. The mixture was cooled to -20 °C for 3 days,
at which time crystals were collected and washed with hexanes.
1H NMR Spectroscopy. The spectrum of 1+ was obtained by
performing controlled potential electrolysis of 3.0 mM 1 in CDCl3/
0.05 M [NBu4][B(C6F5)4] and preparing the NMR sample under
nitrogen in the drybox. The spectra of 2+ and 3+ were obtained by
addition of the appropriate amount of the chemical oxidant
[(ReCp(CO)3)2][B(C6F5)4]2 to a 3.0 mM solution of 2 or 3 in CD2Cl2
in the drybox. To obtain samples with both 3 and 3+ present in
varying concentrations, the solid chemical oxidant was added to
the NMR sample in sequential steps, with spectra collected between
additions.
X-ray Crystallography. Data were collected on a Bruker D8
platform diffractometer equipped with an APEX CCD detector. The
structures were solved by Patterson projections. All non-hydrogen
atoms were refined anisotropically, and the hydrogen atoms were
placed in idealized locations. All software was contained in the
SMART, SAINT, and SHELXTL libraries distributed by Bruker
AXS, Madison, WI.
Results and Discussion
(33) McCleverty, J. A.; Orchard, D. G.; Connor, J. A.; Lloyd, J. P.; Rose,
P.D. J. Organometal. Chem. 1971, 30, C75.
(34) Treichel, P. M.; Wagner, K. P.; Mueh, H. J. J. Organomet. Chem.
1975, 86, C13.
Anodic Electrochemistry. In terms of the literature back-
ground for this study, the anodic electrochemistry of cymantrene
and its methylcyclopentadienyl analogue Mn(η5-C5H4Me)(CO)3
have been the subject of a number of papers which are in
agreement that the oxidation is a one-electron process.8–15,23
Depending on the medium, the radical cations may be suf-
ficiently stable on a CV time scale that the 0/+ couple is at
least partially chemically reversible, especially at reduced
(35) Connelly, N. G.; Kitchen, M. D. J. Chem. Soc., Dalton Trans. 1977,
931.
(36) Wu¨rminghausen, T.; Sellmann, D. J. Organomet. Chem. 1980, 199,
77.
(37) Atwood, C. G.; Geiger, W. E. J. Am. Chem. Soc. 2000, 122, 5477.
(38) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Faraday Trans.
1 1989, 85, 3913.
9
9862 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Half-Sandwich Analogues of Ferrocenium Ion
A R T I C L E S
Figure 3. CVs after bulk oxidation at 1.1 V of 4.0 mM 1 in CH2Cl2/0.05
M [NBu4][B(C6F5)4] at 298 K. 1 mm GCE, 0.1 V s-1. The solid line shows
the scan from 1.3 V to -0.2 V, and the dashed line is from -0.2 to -2.4
V.
Figure 4. In situ IR spectroelectrochemistry of 5.0 mM 1 in CH2Cl2/0.05
M [NBu4][B(C6F5)4] at 253 K recorded during bulk electrolysis at Eapp
)
1.1 V.
copy on the electrolyzed solution showed no evidence of
metal-carbonyl species except for a small amount of uncon-
verted 1. The [PF6]- anion is thus seen to affect 1+ in two ways:
through weak adsorption as [1][PF6] onto solid electrodes and
as a nucleophile leading to decomposition of the 17 e- species.
As shown below, electrochemistry in media containing weakly
coordinating anion supporting electrolytes minimizes or elimi-
nates both of these problems.
temperature in this benign electrolyte medium. Further spectral
characterizations of 1+ are presented below.
Mn(C5H4NH2)(CO)3, 2, and MnCp*(CO)3, 3. Compounds 2
and 3 exhibited anodic behavior similar to that of the parent
complex 1. The E1/2 values shifted positive to 0.62 and 0.64 V,
for 2 and 3, respectively, in accord with the expected cyclo-
pentadienyl substituent effects.42 Consecutive anodic electrolysis
and cathodic back electrolysis in CH2Cl2/[NBu4][B(C6F5)4] at
298 K regenerated about 55% of 2 and 60% of 3, with some
electroactive side products being formed (Figure SM3). The
colors of the radical cations were green for 2+ and vibrant
turquoise for 3+, with weighted average IR spectral shifts of
98 and 107 cm-1 being seen for the respective radical cations
(Table 1). Electrochemistry of 3 performed in CH2Cl2/
[NBu4][PF6] again gave evidence of electrode passivation and
partial decomposition of 3+. In this case 2 F of charge were
released, and some of the primary radical cation was present in
the electrolysis solution. The major feature of the decomposition
product was an anodic wave at Epa ) 1.39 V, the same potential
as seen for the decomposition product of 1+ in media containing
[PF6]-, suggesting that the decomposition process involves
cleavage of the Mn-cyclopentadienyl bond.
Crystal Structures of [2][B(C6F5)4] and [3][B(C6F5)4]. TFAB
salts of 2+ and 3+ were isolated through chemical oxidation of
the neutral complexes by the strong one-electron oxidant
[ReCp(CO)3]+.27 In terms of oxidizing power, the intrinsic Eo′
of the [ReCp(CO)3]0/+ couple (1.16 V) is modified by equilibra-
tion of the monomer cation with the dimer dication
[Re2Cp2(CO)6]2+, giving an effective E1/2 of 1.0 V vs ferrocene
in CH2Cl2 at room temperature.27b This potential provides a
strong driving force for the oxidations of 2 (E1/2 ) 0.62 V) and
3 (E1/2 ) 0.64 V).
When oxidized in CH2Cl2/0.05 M [NBu4][B(C6F5)4] using
Eappl ) 1.1 V, 1 released 0.95 F at 298 K as the solution turned
a deep purple. CV scans on the electrolyzed solution showed a
reversible reduction attributed to 1+ and some minor cathodic
follow-up product waves at negative potentials (Figure 3). IR
of the electrolysis solution showed two strong absorptions with
a weighted average value of νCO ) 2078 cm-1 ( νCO ) (νsym
+ 2νasym)/3), a shift of 115 cm-1 from that of neutral 1, in the
range expected for the one-electron oxidation of a “piano-stool”
complex.41 The individual carbonyl stretching frequencies for
the neutral and cationic forms of 1-3 are collected in Table 1.
Controlled potential back-electrolysis at 0.5 V regenerated 1 in
90% yield, as determined by coulometry and linear sweep
voltammetry. When the bulk oxidation of 1 was carried out at
263 K, no side products were observed, and 1 was regenerated
in quantitative yield. An in situ IR-spectroelectrochemical
experiment performed on this system at 253 K gave a clean
conversion to the radical cation (Figure 4), with the asymmetric
pair of carbonyl bands being resolved at this temperature. Taken
together, the combined electrochemical and IR data indicate that
the cymantrene radical cation is highly persistent at or near room
(39) Double potential step choronocoulometry (DPSCC), a useful technique
for determination of chemical reversibility (Bard, A. J.; Faulkner, L. R.
Electrochemical Methods; John Wiley & Sons: New York, 2001; pp
201-210) was carried out on a solution of 1 in this medium, and
values for irev/ifwd at 2τ are less than 0.20 for all step times (from 0.5
to 5 s). A model system such as ferrocene in the same medium gave
values of 0.26-0.28 for the same step times, showing that 1+ is not
quantitatively reduced back to 1 due to its decomposition.
(40) (a) Anson, F. C. Anal. Chem. 1966, 38, 54. (b) Anson, F. C.;
Osteryoung, R. J. Chem. Educ. 1983, 60, 293.
Crystalline [2][B(C6F5)4] is an air-sensitive green solid, the
structure of which is shown in Figure 5. Table 2 lists the
collection parameters and crystal data for this sample, and Table
(42) Lu, S.; Stelets, V. V.; Ryan, M. F.; Pietro, W. J.; Lever, A. B. P.
Inorg. Chem. 1996, 35, 1013.
(41) (a) Brownie, J. H.; Baird, M. C.; Laws, D. R.; Geiger, W. E.
Organometallics 2007, 26, 5890. (b) Ehlers, A. W.; Ruiz-Morales,
Y.; Baerends, E. J.; Ziegler, T. Inorg. Chem. 1997, 36, 5031. (c)
Willner, H.; Aubke, F. Angew. Chem., Int. Ed. 1997, 36, 2402. (d)
Goldman, A. S.; Kogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118,
12159. (e) Nakamoto, K. Infrared and Raman Spectra of Inorganic
and Coordination Compounds, 4th ed.; Wiley: New York, 1986; pp
292-293.
(43) Sunkel, K.; Birk, U.; Soheili, S.; Stramm, C.; Teuber, R. J. Organomet.
Chem. 2000, 599, 247.
(44) (a) Perrine, C. L.; Zeller, M.; Woolcock, J.; Hunter, A. D. J. Chem.
Crystallogr. 2005, 35, 717. (b) Shafir, A.; Power, M. P.; Whitener,
G. D.; Arnold, J. Organometallics 2000, 19, 3978. (c) Inyushin, S.;
Shafir, A.; Sheats, J. E.; Minihane, M.; Witten, C. E.; Arnold, J.
Polyhedron 2004, 23, 2937.
9
J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9863

A R T I C L E S
Laws et al.
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for
Mn(η5-C5H4NH2)(CO)3, 2, and [Mn(η5-C5H4NH2)(CO)3]+, 2+
bond or angle
Mn(η5-C5H4NH2)(CO)3
[Mn(η5-C5H4NH2)(CO)3]+
Mn-C(1)
2.2318(19)
2.151(2)
2.118(2)
2.115(2)
2.155(2)
1.794(2)
1.790(2)
1.795(2)
1.152(3)
1.152(3)
1.147(3)
1.412(3)
1.424(3)
1.420(3)
1.411(3)
1.428(3)
1.380(2)
1.782
2.127(3)
2.155(3)
2.149(3)
2.149(3)
2.152(3)
1.798(3)
1.801(3)
1.799(3)
1.147(4)
1.149(3)
1.144(4)
1.407(4)
1.423(4)
1.417(4)
1.418(4)
1.418(4)
1.461(4)
1.776
Mn-C(2)
Mn-C(3)
Mn-C(4)
Mn-C(5)
Mn-C(11)
Mn-C(12)
Mn-C(13)
Figure 5. Diagram of the molecular structure of [2][B(C6F5)4] with 50%
probability ellipsoids. Cp hydrogens and counteranion omitted for clarity.
O(1)-C(11)
O(2)-C(12)
O(3)-C(13)
C(1)-C(5)
Table 2. X-ray Data Collection Parameters
MnC8H6NO3, 2 MnC32H14BF20NO7, 2+ MnC37H15BF20O3, 3+
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
molecular
weight
space group
a (Å)
b (Å)
c (Å)
219.08
970.19
953.24
C(4)-C(5)
N-C
j
Pbca
P1
Pca2(1)
16.467(2)
11.6868(17)
18.850(3)
90
90
90
1.745
Mn-to-center C4
C(11)-Mn-C(12)
C(11)-Mn-C(13)
C(12)-Mn-C(13)
O(1)-C(11)-Mn
O(2)-C(12)-Mn
O(3)-C(13)-Mn
7.242(2)
13.408(4)
17.835(5)
90
90
90
11.1365(8)
11.5717(8)
14.7086(10)
101.3260(10)
111.6160(10)
102.7390(10)
1.969
92.22(10)
95.06(10)
91.35(10)
177.2(2)
179.08(19)
178.6(2)
92.16(14)
90.85(14)
90.01(13)
179.0(3)
178.1(3)
179.2(3)
R (deg)
ꢀ (deg)
γ (deg)
D(calc)
1.681
(g/cm-3
volume
(Å3)
)
1731.8(8)
1636.3(2)
3627.5(9)
ring approaches planarity, with only a 1.00° bending away of
the C(N) carbon. In contrast, bendings of the two cyclopenta-
dienyl rings in [(aminocyclopentadienyl)2Fe]+ are considerably
larger at 6.20° and 6.40°.44c Concomitant with alterations of
the Cp bending are changes in the metal-C(N) bond lengths.
Thus, the Mn-C(N) distance decreases by 0.113 Å in going
from 2 to 2+, whereas the Fe-C(N) distance increases by 0.105
Å in going from the ferrocene to ferrocenium-type systems in
[(aminocyclopentadienyl)2Fe]0/+. Changes in the C-N bond
lengths of the aminocyclopentadienyl systems are also worth
noting. The C-N distance of 1.380 Å for 2 is close to the
average of 1.395 Å reported for the previous 18-electron
systems.43,44 There is significant lengthening of the C-N bond
(to 1.461 Å) approaching the single-bond C-N distance of 1.47
Å, in contrast to the shortening reported for the same bonds in
aminoferrocenium complexes.44c
Crystalline [3][B(C6F5)4] is an air-sensitive turquoise solid,
the structure of which is shown in Figure 7. Table 2 lists the
collection parameters and crystal data for this sample, and Table
4 lists important bond distances and angles. The metal is
centered 1.768 Å below the C5Me5 ring, essentially unchanged
from the metal-to-centroid distance of 1.770 Å for neutral 3, a
value calculated using the structural data previously reported45
for the 18-electron compound. The methyl carbons are out of
the plane of the C5 ring, with the angle of the C-C bond ranging
λ (Å)
1.493
8
208(2)
0.711
0.572
2
100(2)
0.711
0.506
4
208(2)
0.711
Z
temp (K)
µ(Mo KR)
(cm-1
)
θ range
data collected
(h, k, l)
2.28°-28.35° 1.56°-28.22°
-8 e h e 9 - 9 e h e 14
1.74°-25.99°
-19 e h e 20
-17 e k e 16 -15 e k e 15
-22 e l e 23 -19 e l e 12
13 747
-14 e k e 14
-22 e l e 10
16 435
no. of
reflns collected
no. of
indep obsd reflns
(F0 > 2σ(F0))
R(F)
(I > 2σ(I))
R(wF)
10 223
2026
6206
5451
0.0287
0.0679
1.049
0.0462
0.1099
1.062
0.0429
0.1045
(I > 2σ(I))
GOF
1.055
0.499/-0.180
largest diff
0.283/-0.323 0.599/-0.407
peak/hole (e Å-3
)
3 lists selected bond distances and angles. The structure of
neutral 2 had not been previously solved, so a structure is given
in Figure 6 with collection parameters and relevant bond
distances and angles supplied in Tables 2 and 3. Unlike
[3][B(C6F5)4] (Vide infra), all three CO-Mn-CO bond angles
for both 2 and 2+ remain close to the octahedral value 90°. In
compound 2, the C(N) atom is bent away from the metal by
4.34° with respect to the C4 plane occupied by the four C(H)
carbons of the cyclopentadienyl ligand (the three points used
for this calculation are those of the C(N) atom, the midpoint of
the two carbon atoms adjacent to the C(N) atom, and the point
on the C4 plane closest to the C(N) atom). This slight bending
of the Cp ring is similar to the angle 4.86° which we calculate
from data reported for the position of the C(N) atom in Mn(η5-
C5H3ClNH2-1,2)(CO)343 and to the average angle 4.44° calcu-
lated for four 18-electron aminocyclopentadienyl iron and cobalt
complexes.44 In the 17-electron species 2+, the cyclopentadienyl
Figure 6. Diagram of the molecular structure of 2 with 50% probability
ellipsoids. Cp hydrogens omitted for clarity.
9
9864 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Half-Sandwich Analogues of Ferrocenium Ion
A R T I C L E S
are written as going from more-to-less-reduced systems in
concert with the signs of ∆ used for the presently described
oxidations, values of ∆d(Mn-C(O)) ) + 0.023 Å and
∆d(C-O) ) - 0.011 Å were obtained when going from n )
-2 to n ) -1, and ∆d(Mn-C(O)) ) +0.013 Å and ∆d(C-O)
) -0.008 Å, in going from n ) -1 to n ) 0. In the case of the
carboranyl system, the smaller bond-length changes might be
ascribed to the high degree of electronic delocalization in the
reduced species.46a The largest redox-induced M-C(O) bond
length changes in half-sandwich complexes of which we are
aware are those reported by Connelly et al. for the one-electron
oxidation of the monocarbonyl complexes WTp′(CO)X(η-
C2Me2) (Tp′ ) tris(3,5-dimethylpyrazolyl)borate, X ) Cl or
Br).46b In those cases, increases in d(W-C(O)) of +0.176 Å
and + 0.146 Å were observed for the Br and Cl complexes,
respectively, and the C-O distances decreased by -0.084 Å
(X ) Br) and -0.060 Å (X ) Cl). A collection of earlier redox-
induced changes in M-C(O) bond lengths is available.46c
The fact that the distance from the metal to the center of the
C5 ring changes so little for either 2/2+ or 3/3+ is surprising,
given that significant elongation of the iron-to-ring distance
occurs when ferrocene systems are oxidized to ferrocenium ions.
For example, compared to their 18-electron counterparts,
increases of 0.044 and 0.05 Å have been reported for the
ferrocenium47a,b and decamethylferrocenium ions.47c,d
Figure 7. Diagram of the molecular structure of [3][B(C6F5)4] with 50%
probability ellipsoids. Hydrogens and counteranion omitted for clarity.
Table 4. Selected Bond Lengths (Å) and Bond Angles (deg) for
[Mn(η5-C5Me5)(CO)3]+, 3+
bond
bond length
angle
bond angle
Mn-C(1)
2.158(4)
2.137(4)
2.134(4)
2.132(4)
2.150(4)
1.852(6)
1.866(5)
1.898(4)
1.120(6)
1.119(6)
1.098(5)
1.408(6)
1.416(6)
1.458(6)
1.424(6)
1.403(6)
1.768
C(11)-Mn-C(12)
C(11)-Mn-C(13)
C(12)-Mn-C(13)
O(1)-C(11)-Mn
O(2)-C(12)-Mn
O(3)-C(13)-Mn
87.6(2)
92.82(19)
103.76(19)
178.5(4)
177.7(5)
175.3(4)
Mn-C(2)
Mn-C(3)
Mn-C(4)
Mn-C(5)
Mn-C(11)
Mn-C(12)
Mn-C(13)
O(1)-C(11)
O(2)-C(12)
O(3)-C(13)
C(1)-C(5)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
Mn-to-center C5
ESR. The three radical cations were all ESR-silent in fluid
solution but strongly resonant in frozen solution at 77 K (Figure
8). A spectrum of 1+ recorded at 4 K was essentially unchanged
from that at the higher temperature. As a group, the spectra
may be viewed as exhibiting axial or nearly axial behavior,
showing hyperfine splittings (hfs, A) that are consistent with
5
coupling to a single 55Mn nucleus (I ) /2, 100% abundant).
The clearest evidence of deviation from axial behavior was in
the perpendicular region of 2+, wherein there is some complex-
ity in the highest field features of the expected A hextet. The
lower molecular symmetry imparted by substitution at a single
cyclopentadienyl position in 2+ is the likely origin of this
spectral deviation, which we believe to be sufficiently modest
to allow interpretation of this spectrum as axial, along with those
of 1+ and 3+. The g- and A-values for this series of radicals
are collected in Table 5.
from 2.0° to 5.6° relative to the ring, bent away from the Mn
center. This is not the case for neutral 3, in which the methyl
carbons are apparently in plane with the ring.45 Another
interesting aspect is that one of the C(O)-Mn-C(O) bond
angles is severely distorted, increasing to 103.8° from the
octahedral value 90°, thereby reducing the symmetry of the
cation to Cs. A similar distortion computed earlier for the third-
row congener [ReCp(CO)3]+ was interpreted as arising from
rehybridization of the SOMO, and a similar orbital change may
be occurring in the present case. The makeup of the SOMO
will be discussed in more detail below. Also of note are the
significantly longer Mn-C(O) bonds (average change,
∆d(Mn-C(O)) ) + 0.142 Å) and shorter C-O bonds (average
change, ∆d(C-O) ) -0.063 Å) seen for 3+ compared to 3,
changes that are qualitatively explained by the weakening of
metal-to-carbonyl back-bonding in the radical cation. However,
the magnitudes of the changes are surprisingly large, especially
in view of the observation of much smaller changes for 2+ from
2 (∆d(Mn-C(O)) ) + 0.006 Å, ∆d(C-O) ) -0.003 Å,
average values). Significantly smaller changes have also been
computed for the congeneric analogous pair [ReCp(CO)3]+ and
ReCp(CO)3.27b It is relevant to compare these bond-length
changes to those recently described for the Mn(CO)3 moiety in
the successive one-electron reductions of Mn(Ph-C3B7H9)(CO)3
to [Mn(Ph-C3B7H9)(CO)3]n (n ) -1, -2).46a When the changes
The Mn hfs were used to obtain an estimate of the % Mn
character of the SOMO orbitals, using the expressions48
A|)Aiso+2bo
A )Aiso-bo
(1)
(2)
where Aiso is the isotropic hfs and bo is the uniaxial hyperfine
parameter.49a Calculation of bo requires that the relative signs
of A| and A be either known through additional experimental
(46) (a) Nafady, A.; Butterick, R., III; Calhorda, M. J.; Carroll, P. J.; Chong,
D.; Geiger, W. E.; Sneddon, L. G. Organometallics 2007, 26, 4471.
(b) Adams, C. J.; Bartlett, I. M.; Carlton, S.; Connelly, N. G.; Harding,
D. J.; Hayward, O. D.; Orpen, A. G.; Patron, E.; Ray, C. D.; Rieger,
P. H. J. Chem. Soc., Dalton Trans. 2007, 62. (c) Orpen, A. G.;
Connelly, N. G. Organometallics 1990, 9, 1206.
(47) (a) Sullivan, B. W.; Foxman, B. M. Organometallics 1983, 2, 187.
(b) Martinez, R.; Tiripicchio, A. Acta Crystallogr. 1990, C46, 202.
(c) Freyberg, D. P.; Robbins, J. L.; Raymond, K. N.; Smart, J. C.
J. Am. Chem. Soc. 1979, 101, 892. (d) Pickardt, J.; Schumann, H.;
Mohtachemi, R. Acta Crystallogr. 1990, C46, 39.
(45) (a) Fortier, S.; Baird, M. C.; Preston, K. F.; Morton, J. R.; Ziegler,
T.; Jaegar, T. J.; Watkins, W. C.; MacNeil, J. H.; Watson, K. A.;
Hensel, K.; Le Page, Y.; Charland, J.; Williams, A. J. J. Am. Chem.
Soc. 1991, 113, 542. (b) For an earlier paper, see: Morton, J. R.;
Preston, K. F.; Cooley, N. A.; Baird, M. C.; Krusic, P. J.; McLain,
S. J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3535.
(48) Ayscough, P. B. Electron Spin Resonance in Chemistry; Methuen &
Co.: London, 1967; pp 82-84.
(49) (a) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic
Resonance; John Wiley & Sons: New York, 1994; pp 114-116. (b)
Ibid., p 534.
9
J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9865

A R T I C L E S
Laws et al.
Since the neutral radical CrCp(CO)3 is isoelectronic with 1+,
it is of value to compare the present data with those reported
earlier for the chromium complex.45,50 Based on spectra of a
dilute single-crystal of CrCp(CO)3 in MnCp(CO)3, the SOMO
was shown to have about a 40% Cr 3d character.51,52 In terms
of Mn analogues of 1+-3+, only spectra of carbonyl-substituted
17-electron Mn complexes have been reported. The most
detailed studies have been done on the disubstituted complexes
[MnCp(CO)L2]+, L ) PMe3 or PPh3, [MnCp(CO)L′]+, L′ )
bisphosphino ligand, and the monosubstituted complex
[MnCp(CO)2(PPh3)]+.53 Each of the CO-substituted radicals
gave decidedly rhombic ESR spectra and had the complication
of showing significant misalignments between the axes of the
g-tensor and the 55Mn A-tensor. Nevertheless, Pike et al. were
2
able to conclude that the SOMOs contained about
/
3
Mn
2
2
2
character, the d-orbital being a hybrid of dx -y , dxz, and dz
orbitals created by spin-orbit coupling.53 Spectra have also been
reported for piano-stool Mn radicals in which one carbonyl has
been replaced by a single-electron donor. These include
MnCp*(CO)2L, L ) SR,54 MnCp(CO)2(RNH),55 Mn(C5H5-nMen)-
(CO)2NR2,56 [MnCp(CO)2(4-cyanopyridine)]-,57 and MnCp(CO)2-
L′, L′ ) RC6H4NH.57 In those cases, however, the complexes
were shown to be essentially organic radicals stabilized by
bonding to the MnCp(CO)2 moiety.
Also informative are the g-values of 1+-3+, for those of d5
metal sandwich and half-sandwich systems have undergone
considerable analysis.50,52,58–61 A key factor is the influence of
the separation of the ground and excited states on the g-values
and relaxation rates of the radicals, a separation which is
determined in large part by the splittings of the three frontier
orbitals referred to earlier. A smaller energy separation leads
to a greater deviation of g from the free-spin value ge () 2.0023)
and a more rapid electronic relaxation. The latter is the reason
why low temperatures (often well below 77 K) are needed for
detection of highly symmetric d5 radicals such as the ferroce-
nium ion.58 Such systems often have one g-value which is
considerably below 2.0. When the near-degeneracy of the largely
Figure 8. ESR spectra of (A) 1+, (B) 2+, and (C) 3+ at 77 K. Samples
were prepared by exhaustive electrolysis of the respective neutral species
in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 253 K.
(50) Krusic, P. J.; McLain, S,J.; Morton, J. R.; Preston, K. F.; LePage, Y.
J. Magn. Reson. 1987, 74, 72.
Table 5. Summary of ESR Results for [Mn(C5R4R′)(CO)3]+
(51) This value of 40% Cr spin is based on the correction of the data in
ref 50 by Rieger in ref 53.
b
radical
g|
g
A|a (G)
A
(G)
% Mn (d)
(52) Rieger, P. H. Coord. Chem. ReV. 1994, 135/136, 203.
(53) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Faraday Trans.
1 1989, 85, 3913.
1+
2+
3+
2.213
2.120
2.118
2.079
2.011
2.044
79.2
65.1
76.5
50
47
30
49
47
36
(54) Winter, A.; Huttner, G.; Zsolnai, L.; Kroneck, P.; Gottlieb, M. Angew.
Chem., Int. Ed. 1984, 23, 975.
a Known with certainty. b Value based on best estimate from outer
branches of six-line pattern.
(55) (a) Sellmann, D.; Mu¨ller, J.; Hofmann, P. Angew. Chem., Int. Ed. 1982,
21, 691. (b) Sellmann, D.; Mu¨ller, J. J. Organomet. Chem. 1985, 281,
249.
(56) (a) Gross, R.; Kaim, W. Inorg. Chem. 1987, 26, 3596. (b) Gross, R.;
Kaim, W. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3549.
(57) Gross, R.; Kaim, W. Angew. Chem., Int. Ed. 1985, 24, 856.
(58) Ammeter, J. H. J. Magn. Reson. 1978, 30, 299.
(59) (a) Elschenbroich, C.; Bilger, E.; Mo¨ckel, R. Z. Naturforsch. B 1983,
38, 1357. (b) Elschenbroich, C.; Mo¨ckel, R.; Bilger, E. Z. Naturforsch.
B 1984, 39, 375.
information or assumed on the basis of theory or chemical
intuition. The experimental approach, which requires measure-
ment of Aiso and takes advantage of the relationship 3Aiso ) (A|
+ 2A ), was not possible in the present case owing to the lack
of observable fluid spectra. Values of bo ) 9.7 and 43.1 G are
calculated if the signs of A| and A are either the same or
different, respectively. Since the theoretical value calculated for
the uniaxial parameter when the unpaired electron is located
exclusively in a Mn d-orbital is 88 G,49b the % Mn character
of the SOMO is either 11% for the smaller bo option or 49%
for the larger. Given that the HOMO of 1 is certain to contain
significant metal character,6,16–19 the latter is much more likely
and it was therefore assumed that A| and A were of different
sign for all three radicals. The % Mn d-orbital character is
estimated, thusly, to be 49% in 1+, 36% in 2+, and 47% in 3+.
(60) Elschenbroich, C.; Bilger, E.; Ernst, R. D.; Wilson, D. R.; Kralik,
M. S. Organometallics 1985, 4, 2068.
(61) Hoobler, R. J.; Hutton, M. A.; Dillard, M. M.; Castellani, M. P.;
Rheingold, A. L.; Rieger, A. L.; Rieger, P. H.; Richards, T. C.; Geiger,
W. E. Organometallics 1993, 12, 116.
(62) (a) Chong, D.; Geiger, W. E.; Davis, N. A.; Weisbich, A.; Shi, Y.;
Arif, A. M.; Ernst, R. D. Organometallics 2008, 27, 430. (b) LeSuer,
R.; Basta, R.; Geiger, W. E.; Arif, A. M.; Ernst, R. D. Organometallics
2003, 22, 1487.
(63) An exception to this generalization is that fluid spectra may be obtained
for high-symmetry radicals having a 2A ground state (see ref 59 as
well as (a) Elschenbroich, C.; Mockel, R.; Vasil′kov, A.; Metz, B.;
Harms, K. Eur. J. Inorg. Chem. 1998, 10, 1391).
9
9866 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Half-Sandwich Analogues of Ferrocenium Ion
A R T I C L E S
Figure 9. UV-vis/near-IR spectrum of 3.3 mM 3+ in CDCl3 at 298 K. Sample prepared by chemical oxidation of 3 with [(ReCp(CO)3)2][B(C6F5)4]2 in
CDCl3.
Table 6. UV-vis, near-IR, 1H NMR Results for 1+, 2+, and 3+ as
metal-based frontier orbitals is lifted significantly through
symmetry lowering or orbital mixing, ESR spectra are generally
[B(C6F5)4] Salts
1+
2+
3+
observed at higher temperatures, perhaps even in fluid media,
and the lowest g-value is found near 2.0.60,62,63 In the present
case, none of the g-values were below ge and the spectra,
although well visible at 77 K, were too broad to measure in
fluid solution. These facts are qualitatively suggestive that there
is only a modest separation of the ground and first excited states
of 1+-3+, a point that was explored more quantitatively through
the near-IR absorptions of the radicals (Vide infra).
λmax (cm-1) (ꢀ, cm-1 M-1) 18 850 (1010) 12 320 (1066) 14 700 (225)
λmax (cm-1) (ꢀ, cm-1 M-1) 4840 (64)
6490 (97)
5280 (56)
44.7
1H NMR
22.4a
-23.9, 26.4
chemical shift
(ppm vs TMS)
1H NMR
17.7 (234)a
-27.0 (1970),c 42.8 (294)
22.0 (1010)
paramagnetic shift
(ppm) (line width, Hz)b
a Prepared by controlled potential coulometry, >90% yield as
determined by coulomb count and LSV. Presence of neutral species
would result in false low values (see text). b Width taken at half-height.
c Value calculated on the assumption that this peak is due to amine
protons.
One further point is that the individual lines in the spectra
show no anomalous variations in line width. This is in contrast
to the findings of Castellani et al. on the spectra of [MnCp-
(CO)(dmpe)]+ (dmpe ) bis(dimethylphosphino)ethane) cation
radical and analogous chromium half-sandwich radicals.64 In
particular, the dramatic narrowing of the low-field “parallel”
features of hyperfine lines with increased |mI| values in [MnCp-
(CO)(dmpe)]+ was ascribed to the noncoincidence of the
principal axes of the g- and A-tensors. In the present case, at
least three of the parallel lines were well resolved and the
breadths of the mI ) 5/2, 3/2, and 1/2 lines were essentially
unchanged,65 having widths at half-height of 30-35 G. The
lack of line width anomalies is taken as evidence that the g-
and A-tensors of 1+-3+ are not significantly misaligned,
increasing the credibility of the simple axial model used here
to estimate the degree of Mn character in the SOMO. It should
be mentioned, however, that the ESR properties of piano-stool
radicals may be quite complex, with the relative ordering of
the SOMO and next-highest filled orbital being sensitive even
to environmental effects,64 and that more detailed studies of
the Mn(II) tricarbonyl complexes, ideally on diluted single
crystals, are warranted.
that the visible band, most likely arising from ligand-to-metal
charge transfer, red-shifts significantly in going from 1+ (λmax
) 530 nm) to 3+ (680 nm) to 2+ (811 nm). More unusual are
the weaker absorptions (magnified in the inset of Figure 9 for
3+, ꢀ ≈ 50-100 M-1 cm-1) seen at very low energy in the
near-IR region. Absorptions of this type were first reported for
the piano-stool radicals Cr(η5-C5Ph5)(CO)3 and [MnCp(CO)2-
(PPh3)]+ 66 and are attributed to metal-metal transitions arising
from splitting of the close-lying electronic states of the radical
cations. The significant blue shift of the near-IR transition of
2+ compared to 1+ or 3+ is consistent with the earlier hypothesis
which predicted shifts to higher energies for piano-stool radicals
having lower symmetry.66
In order to further probe these low-energy absorptions, we
measured the near-IR absorption energies of a wider range of
Mn half-sandwich cation radicals, many of which are easily
obtained as their [PF6]- salts by oxidation of the corresponding
18 e- compounds in which one or more carbonyl groups have
been substituted by donor ligands.9,10,34,35,38 Near-IR bands were
measured for 10 compounds in this study, varying in λmax from
2066 nm (for 1+) to 1470 nm (for [MnCp(CO)(PPh3)2]+), and
for all but 2+, a reasonably good correlation was observed
between an increase in the absorption energy and a decrease in
the anodic E1/2 potential (see Figure 10). The splitting of the
Visible and Near-IR Spectroscopy. All three cation radicals
exhibit a strong absorption in the visible region, as well as a
weaker absorption in the near-IR, Figure 9 for 3+ being typical.
From the values of λmax and ꢀ given in Table 6 it can be seen
(64) Castellani, M. P.; Connelly, N. G.; Pike, R. D.; Rieger, A. L.; Rieger,
P. H. Organometallics 1997, 16, 4369.
(65) An exception to this generalization is that the four observed “parallel”
features of 1+ increased in width going from lower-to-higher field in
a spectrum recorded at 10 K.
(66) Atwood, C. G.; Geiger, W. E. J. Am. Chem. Soc. 1994, 116, 10849.
9
J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9867

A R T I C L E S
Laws et al.
Figure 10. Relationship between energy of carbonyl absorption or oxidation potential and energy of near-IR transition for selected Mn piano-stool complexes.
Figure 11. 1H NMR spectrum of 3+ in CD2Cl2.
closely lying orbitals, assigned as a′ and a′′ in the Mn-phosphine
radicals,38 is thus shown to increase with an increase in
phosphine donor strength.
measured). It is not surprising to see the lack of quantitative
agreement between the g-values and the energies of the near-
IR bands, given the underlying assumptions, especially in
ignoring the ligand contributions to the orbitals involved. Still,
this model does account qualitatively for the absorbances
observed for this family of radicals in the near-IR to the edge
of the midrange IR region.
The observed orbital splitting energies have a relationship to
the ESR g-values. Using a simple metal-localized model for
octahedral d5 systems,67 it can be shown45 that the difference
between gz and the free-spin value ge is related to the energy
difference, ∆, between the ground and nearest excited state by
the relationship gz - ge ) 2λSO/∆, where λSO is the spin-orbit
coupling constant of the metal. Using λSO ) 300 cm-1 for
Mn(II),67 and assuming that the highest g-value is gz,45 a value
of ∆ ) 5170 cm-1 is predicted for the low-energy optical
transition for 3+, comparing well with the measured energy of
5280 cm-1. This excellent agreement for 3+ is seen to be
fortuitous, however, when the other radicals in this series are
similarly treated with the calculated values being generally too
low (for 1+, 2840 cm-1 calculated compared to 4840 cm-1
measured; for 2+, 5085 cm-1 calculated compared to 6490 cm-1
Finally, inspection of Figure 10 shows that complex 2+ is an
outlier in the inverse correlation of the near-IR transition energy
with the anodic E1/2 values. Amino-substitution of the Cp ring
apparently has the effect of increasing the splitting of metal
orbitals significantly beyond the extent expected on the basis
of a simple cyclopentadienyl substituent effect.
Paramagnetic NMR. The fast electronic relaxation responsible
for the lack of fluid-medium ESR signals suggested that the
radicals would show NMR activity.68,69 Indeed, both 1+ and
(68) Bertini, I.; Luchinat, C. In Physical Methods for Chemists, 2nd ed.;
Drago, R. S., Ed.; Saunders College Publishing: Ft. Worth, TX, 1992;
pp 519 ff.
(69) Satterlee, J. D. Concepts Magn. Reson. 1990, 2, 119.
(67) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem. Radiochem. 1970,
13, 135.
9
9868 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008

Half-Sandwich Analogues of Ferrocenium Ion
A R T I C L E S
triad set of the d6 [Mn(CO)3]+ fragment which mixes weakly
with the Cp- orbitals to form the HOMO to HOMO-2 of this
system.16 Small energy separations of these orbitals occur owing
to the Cs symmetry of 1. Idealized drawings of the three d-orbital
contributions to these frontier orbitals are shown below, labeled
according to Cs symmetry. The 1a′ and 1a′′ orbitals, which are
2
2
derived from the erstwhile eg pair, are primarily dx -y mixed
with dyz (for 1a′) and dxy mixed with dxz (for 1a′′) and contain
2
less Cp character than the more metal-based 2a′ (dz ) orbital. It
is important to keep in mind that these orbitals also have
significant carbonyl character. The earlier, quite detailed, ESR
studies on piano-stool radicals are in agreement that the SOMO
may be either 1a′ or 1a′′, the ordering being sensitive to ligand
changes. For the two systems closest to the present radicals,
namely CrCp(CO)3
45,50
and [MnCp(CO)2(PPh3)]+,53 a 1a′′
Figure 12. Plot of 1H NMR paramagnetic shift vs mole fraction of 3+ in
CD2Cl2.
SOMO was favored. Although our data on the Mn systems 1+
and 3+ are certainly consistent with those findings, the assign-
ment of their SOMOs as 1a′′ must be tempered by the fact that
delineation between the two possibilities of 1a′ and 1a′′ is
difficult.45 However, it seems safe to conclude that the SOMOs
of 1+ and 3+ have ca. 50% of a hybrid Mn d-orbital, most of
the remaining spin presumably being in the carbonyl groups.
The degree of metal character in the SOMO has now been
shown to be in a range of about 36-67% for a number of d5
piano-stool compounds.45b,50,51,53
1
3+ displayed a single, strong H NMR signal shifted to high
frequency (δ ) 22.4 ppm vs TMS for 1+,70 width at half-height
) 230 Hz; 44.7 ppm vs TMS for 3+, width at half-height )
295 Hz, Figure 11). The resonance for the Cp* compound was
tracked carefully with the addition of neutral 3 and 3+, and the
precise linear fit of chemical shift vs fraction of radical (Figure
12) established the integrity of the spectral assignment and the
fact that the self-exchange rate of 3/3+ is rapid. If the position
of the resonance is dominated by contact shifts, its direction is
determined by the sign of the π spin density in the radical, a
positive shift (to higher frequency) being the product of a
positive spin density.71 The fact that the shifts are positive for
both 1+ and 3+ is unanticipated, as the contact interactions are
expected to have the opposite sign for 1+, where the spin on H
should be dominated by McConnell-type C-H spin polariza-
tion,72 and 3+, where the spin on the methyl hydrogen should
be dominated by hyperconjugation.73 Dipolar effects (pseudo-
contact interactions) may prove to be responsible for the
unidirectional chemical shifts for 1+ and 3+, but more in-depth
treatments will be necessary to delineate the influence of contact
Vs pseudo-contact interactions in these radicals. As observed
earlier with ferrocenium-type species, interpretation of the
chemical shifts of paramagnetic metallocenes is not straight-
forward,74 and Cp-derivatization75 may be necessary to sort out
the dominant shift mechanisms in the Mn system.
The ESR results on 2+ differ only quantitatively from those
of 1+ and 3+, with slightly smaller Mn hyperfine splittings.
However, as pointed out previously, the near-IR absorption
stands out as falling at considerably higher energy than observed
for the other members of this family. This is most likely to
2
2
arise from an unexpectedly large splitting of the A′′ and A′
states, but a change in the nature of the SOMO of 2+ cannot be
ruled out. To this point, we note that both the calculated % Mn
character of the SOMO and the weighted average carbonyl-IR
shifts are smaller for 2+ than for 1+, suggesting the presence
of a significant amount of spin and charge at the NH2 group of
the former.
A spectrum was also observed for 2+, broad peaks being seen
at 26.4 ppm (width at half-height ) 1010 Hz) and -23.9 ppm
(width at half-height ) 1970 Hz) vs TMS. The fact that these
peaks integrated to a 1:1, rather than the expected 2:1, ratio
obscures their assignments, but the low field signal at -23.9
ppm may be tentatively assigned to the amino hydrogens based
on comparison with the shift reported for the amino-ferrocenium
analogue [Fe(C5H4NH2)2]+.76
Conclusions
The cymantrene radical cation 1+ can be electrochemically
generated and spectrally monitored at room temperature in a
low-donor solvent containing a weakly coordinating electrolyte
anion. It has a rich, and perhaps unprecedented, breadth of
spectral responses in that it is active in the visible, near-IR,
and IR (νCO) ranges and exhibits both NMR and ESR activities,
the latter showing Mn hyperfine splittings. The near-IR absorp-
tion and the magnetic resonance responses are all consistent
with an electronic model of 1+ in which an excited state lies
fairly close in energy to the ground state, accounting for its
absorption band at 4840 cm-1 and the rapid electronic relaxation
that makes it ESR-inactive and NMR-active at room tempera-
ture. The SOMO of 1+ contains about 50% Mn d-orbital
character, with a significant contribution also from the carbonyl
groups and, presumably, a smaller contribution from the Cp
ligand. The fact that considerable ligand character is found in
Nature of the SOMO of 1+. The three highest filled orbitals
of MnCp(CO)3 are best described as originating from a t2g-like
(70) Owing to the difficulty of maintaining a pure sample of 1+ in solution
at room temperature under NMR conditions, a small amount of 1 was
apparently present in this sample.
(71) Satterlee, J. D. Concepts Magn. Reson. 1990, 2, 69.
(72) McConnell, H. M. J. Chem. Phys. 1963, 39, 1910.
(73) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance;
Harper & Row: New York, 1967; pp 83-85, 109-110.
(74) Blu¨mel, J.; Hebendanz, N.; Hudeczek, P.; Ko¨hler, F. H.; Strauss, W.
J. Am. Chem. Soc. 1992, 114, 4223.
(75) Gattinger, I.; Herker, M. A.; Hiller, W.; Ko¨hler, F. H. Inorg. Chem.
1999, 38, 2359.
(76) Shafir, A.; Power, M. P.; Whitener, G. D.; Arnold, J. Organometallics
2000, 19, 3978.
9
J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008 9869

A R T I C L E S
Laws et al.
the SOMO is consistent with earlier He(I) and He(II) PES results
on 1 and its derivatives.6
process has also been described.8,79 Given the enhanced stability
of cymantrene-type radical cations under the conditions de-
scribed in the present paper, a simple electrochemically switched
substitution process such as that reported25 for Cr(η6-ben-
zene)(CO)3 should also be possible for cymantrenyl systems,
making it possible, in principle, to perform efficient CO
substitution processes on a Mn(C5H4)(CO)3 moiety attached to
a target molecule. Since double substitution of carbonyl groups
by small cone-angle donors is possible, postlabeling substitution
processes could lower the cymantrenyl potential by 1000 mV
or more, and efforts are underway to probe this approach.80
Functionalization of the cyclopentadienyl ring affected the
E1/2 potential of the 18 e-/17 e- couple in predictable ways
and enhanced both the thermodynamic and kinetic stabilities
of the corresponding radical cations. The E1/2 values of the
aminocyclopentadienyl and Cp* analogues were shifted negative
by ca. 300 mV, and the radical cations of these systems, 2+
and 3+, respectively, were isolated and structurally characterized.
The distance from the metal to the center of the C5Me5 ring
was essentially unchanged in 3+ compared to its 18-electron
counterpart, in contrast with the lengthening of 0.05 Å observed
in going from decamethylferrocene to the corresponding fer-
rocenium ion, suggesting that the HOMO of 3 (and by inference,
that of 1) has relatively little metal-Cp mixing. There is a
meaningful set of structural changes in the two redox states of
the aminocyclopentadienyl Mn system, however, with a sig-
nificant shortening of the Mn-C(N) bond leading to a nearly
planar cyclopentadienyl ring in the 17-electron cation. The bond
lengths and angles of the Mn(CO)3 moiety are also more highly
affected in the oxidation of 2 than in that of 3.
The spectral activities of the Cp-functionalized radical cations
were consistent with those of 1+, validating the idea that
cymantrenyl redox tags could be monitored using a number of
spectroscopic methods. The fact that the radical cations have
spectral responses in two regions having few spectral interfer-
ences, namely the near-IR and metal-carbonyl IR regions, raises
attractive analytical possibilities for a cymantrenyl group as an
anodic redox tag. The widespread application of this tag is likely
to be limited by its rather positive potential (E1/2 ) 0.92 V vs
ferrocene) and the susceptibility of its radical cation to nucleo-
philic attack. However, these difficulties may be mitigated by
pre- or postlabeling substitution of one or more carbonyl groups
by a donor ligand. Negative E1/2 shifts of 600 mV or more are
possible when a single CO is replaced by a phosphine,
phosphite, or other donor ligand.77,78 As shown in the present
paper and in previous publications,33–37 the CO-substituted
compounds largely retain the optical, near-IR, and IR activities
of the parent radical. Although the carbonyl-substitution of
MnCpγ(CO)3 (Cpγ ) generic cyclopentadienyl ligand) is usually
carried out photolytically,28 an electrochemical substitution
Finally, it is instructive to note the difference in behavior of
the congener radical cations 1+ and [ReCp(CO)3]+. The latter
dimerizes, forming an unsupported Re-Re bond,27 whereas no
evidence for dimerization has been found for the Mn system.
This difference is most likely due to the larger size of the Re
atom as well as the significant rehybridization undergone by
the HOMO when an electron is removed from ReCp(CO)3.27
In this sense the monomer/dimer tendencies of [MCp(CO)3]+,
M ) Mn, Re, are reminiscent of the first-row vs second- and
third-row behavior of the neutral 17-electron analogues MCp-
(CO)3, M ) Cr, Mo, W, which show a much greater tendency
toward dimerization with the heavier metals.81
Acknowledgment. The authors gratefully acknowledge the
support of this work by the National Science Foundation (CHE
0411703) and Boulder Scientific Co. for a partial donation of
K[B(C6F5)4]. We also thank Dr. Ayman Nafady for making the
initial CV measurements of 1 in CH2Cl2/[NBu4][TFAB] at the
University of Vermont and Prof. Dennis Chasteen for use of the
University of New Hampshire ESR spectrometer to record the
spectrum of 1+ at 4 K.
Supporting Information Available: Three voltammograms of
1
1 and 3 after bulk electrolysis and H NMR spectrum of 1+
after bulk oxidation of 1 in CDCl3/[NBu4][TFAB]. This material
is available free of charge via the Internet at http://pubs.acs.org
JA801930Q
(79) Note also that, in refs 10 and 11, substitution processes for replacement
of a CO group in Mn(C5H4Me)(CO)3 were carried out first by
photolytic substitution of CO by a strong donor such as pyridine,
followed by electrocatalytic substitution of pyridine by a slightly
weaker donor ligand such as phosphine.
(80) Laws, D. R. University of Vermont, unpublished data. The electro-
chemically substituted complex MnCp(CO)(P(OMe)3)2 has an E1/2
value of-0.25 V Vs ferrocene.
(81) For early references to the group VI analogues, see: (a) Adams, R. D.;
Collins, D. E.; Cotton, F. A. J. Am. Chem. Soc. 1974, 96, 749. (b)
Hackett, P.; O’Neill, P. S.; Manning, A. R. J. Chem. Soc., Dalton
Trans. 1974, 1625. (c) Wrighton, M. S.; Ginley, D. S. J. Am. Chem.
Soc. 1975, 97, 4246. A list of 26 references on this topic may be
found in ref 41a.
(77) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271. predicts shifts of-0.60
and-0.57 V for substitution of CO by PPh3 and P(OMe)3, respectively,
based on literature data for the oxidation of Cr(CO)5L, L ) CO or
donor ligand.
(78) The average potential shifts reported for substitution of CO in half-
sandwich complexes are-0.65 V for PPh3 and-0.48 V for P(OR)3
(R ) Me or Et). See refs 9, 11, 12, 25, 34, and 35 as well as:(a)
Yeung, L. K.; Kim, J. E.; Chung, Y. K.; Rieger, P. H.; Sweigart, D. A.
Organometallics 1996, 15, 3891. (b) Pierce, D. T.; Geiger, W. E. Inorg.
Chem. 1994, 33, 373. (c) Connelly, N. G.; Demidowicz, Z.; Kelly,
R. L. J. Chem. Soc., Dalton Trans. 1975, 2335.
9
9870 J. AM. CHEM. SOC. VOL. 130, NO. 30, 2008