Carbon-Carbon Bonds of Manganese Half-Sandwich Complexes for Electron Reservoir Functions

Article abstract of DOI:10.1021/om034237q

Vinylidene species of the type Mn(C₅H₄CH ₃)[(H₃C)₂PCH₂CH₂P(CH ₃)₂](=C=CR¹R²) (R¹ = R² = H; R¹ = H, R² = Ph) can be fully reversibly coupled to yield the dinuclear complexes [(C₅H ₄CH₃)(dmpe)Mn≡ CC(R¹)(R ²)C(R¹)(R²)C≡Mn(dmpe)(C₅H ₄CH₃)[PF₆]₂(R¹, R ² = H; R¹ = H, R² = Ph) by an oxidation / reduction cycle involving formation and cleavage of a C-C σ bond.

Full text of DOI:10.1021/om034237q

Organometallics 2004, 23, 1183-1186
1183
Ca r bon -Ca r bon Bon d s of Ma n ga n ese Ha lf-Sa n d w ich
Com p lexes for Electr on Reser voir F u n ction s
Koushik Venkatesan, Olivier Blacque, Thomas Fox, Montserrat Alfonso,
Helmut W. Schmalle, and Heinz Berke*
Anorganisch-Chemisches Institut der Universita¨t Zu¨rich, Winterthurerstrasse 190,
CH-8057 Zu¨rich, Switzerland
Received October 15, 2003
Summary: Vinylidene species of the type Mn(C5H4-
CH3)[(H3C)2PCH2CH2P(CH3)2](dCdCR1R2) (R1 ) R2 )
H; R1 ) H, R2 ) Ph) can be fully reversibly coupled to
yield the dinuclear complexes [(C5H4CH3)(dmpe)Mnt
CC(R1)(R2)C(R1)(R2)CtMn(dmpe)(C5H4CH3)][PF6]2 (R1,
R2 ) H; R1 ) H, R2 ) Ph) by an oxidation/ reduction
cycle involving formation and cleavage of a C-C σ bond.
while very few systems exhibit reactions in both direc-
tions; one such system was reported by Floriani and co-
workers.5 Herein, we report novel half-sandwich man-
ganese(I) systems which undergo facile oxidative coupling
and the reverse reductive decoupling, which might be
utilized in electron storage devices.
We have recently reported the chemistry and reactiv-
ity of MnII and MnIII half-sandwich Me2PCH2CH2PMe2
(dmpe) alkynyl complexes and their conversion to MnI
vinylidene species.6a-c The first conclusion of this study
was that vinylidene complexes gained considerably in
stability through the presence of the bis(dimethylphos-
phino)ethane ligand (dmpe) in comparison with the
reported CO-substituted species Mn(Cp)L1L2(CdCR2),
Two complementary electron-storing and -releasing
units are the fundamental constituents of storage cells.1
Coupling a metal and an independent electron reservoir
in the same molecule opens attractive perspectives in
the design of molecules devoted to energy (molecular
batteries) or for a functioning mode in charge storage
different from that in photosystem II.2 One could
certainly envisage the possibility of utilizing the process
of formation and cleavage of chemical bonds for this
purpose. This would allow one to make electrons avail-
able in pairs and relatively large in number. Very rarely
does the redox chemistry of ligands in complexes have
chemical consequences such as the formation or cleav-
age of bonds, and even more rarely is the event revers-
ible. The reductive or the oxidative coupling and their
reverse, which are well-known classes of reactions in
organometallic chemistry, can be used for the formation
and cleavage of bonds. It is very evident that we must
pass through a mechanism which involves reductive or
oxidative coupling and the reverse decoupling.3 Many
cases are known which operate in at least one direction,4
where L1 ) L2 ) CO, PR3 or L1 ) CO, L2 ) PR3.6d
A
common method to obtain vinylidene complexes makes
use of the high propensity of terminal acetylide deriva-
tives to rearrange to vinylidene compounds.6e,f,7 For such
a process to be initiated in half-sandwich MnI chemistry,
(5) (a) Belanzoni, P.; Rosi, M.; Sgamellotti, A.; Bonomo, L.; Floriani,
C. Dalton 2001, 1492. (b) Floriani, C.; Solari, E.; Franceschi, F.;
Scopelliti, R.; Belanzoni, P.; Rosi, M. Chem. Eur. J . 2001, 7, 3052. (c)
Franceschi, F.; Solari, E.; Scopelliti, R.; Floriani, C. Angew. Chem.,
Int. Ed. 2000, 39, 1685. (d) Rosi, M.; Sgamellotti, A.; Franceschi, F.;
Floriani, C. Chem. Eur. J . 1999, 5, 2914. (e) DeAngelis, S.; Solari, E.;
Gallo, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1996,
35, 5995. (f) Gallo, E.; Solari, E.; Re, N.; Floriani, C.; Chiesi-Villa, A.;
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Floriani, C.; Chiesivilla, A.; Guastini, C. J . Chem. Soc., Chem.
Commun. 1982, 756. (h) Gambarotta, S.; Urso, F.; Floriani, C.;
Chiesivilla, A.; Guastini, C. Inorg. Chem. 1983, 22, 3966. (i) Gam-
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* To whom correspondence should be addressed. E-mail: hberke@
aci.unizh.ch.
(6) (a) Fernandez, F. J .; Alfonso, M.; Schmalle, H. W.; Berke, H.
Organometallics 2001, 20, 3122. (b) Unseld, D. Ph.D. Thesis, University
of Zu¨rich, 1999. (c) Unseld, D.; Krivykh, V. V.; Heinze, K.; Wild, F.;
Artus, G.; Schmalle, H.; Berke, H. Organometallics 1999, 18, 1525.
(d) Terry, M. R.; Kelley, C.; Lugan, N.; Geoffroy, G. L.; Haggerty, B.
S.; Rheingold, A. L. Organometallics 1993, 12, 3607. (e) Casey, C. P.;
Dzwiniel, T. L.; Kraft, S.; Kozee, M. A.; Powell, D. R. Inorg. Chim.
Acta 2003, 345, 320. (f) Ortin, Y.; Sourniya-Saquet, A.; Lugan, N.;
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10.1021/om034237q CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/17/2004

1184 Organometallics, Vol. 23, No. 6, 2004
Communications
Sch em e 1
F igu r e 1. Molecular structure of 2a (30% probability
displacement ellipsoids). Selected bond lengths (Å) and
angles (deg): Mn2-C3 ) 1.790(6), C3-C4 ) 1.310(8), C4-
Sn2 ) 2.126(6), Mn2-P4 ) 2.183(18); C4-C3-Mn2 )
172.5(5), C3-C4-Sn1
) 120.2(4), Sn1-C4-Sn2 )
120.7(3). Hydrogen atoms are omitted for clarity.
not reveal any intermediate. The 13C NMR spectra of
complexes 2 and 3 show the CR and the Câ resonances
at around 321 and 114 ppm, respectively. The 31P NMR
resonances appear between 92 and 93 ppm and are thus
comparable to those which were observed for the related
Mn(C5H4R1)((H3C)2PCH2CH2P(CH3)2)(dCdCRH) com-
plexes.6c,7a,8c,d The resonances corresponding to the
trimethyltin groups were observed at high field as a
triplet between -16 and -20 ppm in the 119Sn NMR
spectra. Complexes 2a and 2b constitute the first
examples of half-sandwich manganese(I) vinylidene
complexes with terminal SnMe3 groups. The X-ray
structure analysis of 2a (Figure 1) confirms the double-
bond character of the two bonds (Mn2-C3 ) 1.790(6)
Å and C3-C4 ) 1.310(8) Å).6c
we believed the complexes Mn(C5H4R1)(η6-cyclohep-
tatriene) (R1 ) H (1a ), CH3 (1b))8a to be excellent
starting materials, since facile cycloheptatriene ex-
change was expected to occur with electron donor
ligands, such as phosphines or acetylenes.8b Reactions
with mono- or disubstituted tin acetylides were antici-
pated to then lead to the desired vinylidene species.
Indeed, the reaction of Mn(C5H4R1)(η6-cycloheptatriene)
It has been observed earlier6d that the substituents
on the â-carbon of the vinylidene complexes have a great
effect on the stability of these complexes and also on
the further reactivity. The reactions of the species Mn-
(C5H4R1)((H3C)2PCH2CH2P(CH3)2)(dCdCR2R3) (R1 ) H,
R2 ) R3 ) SnMe3, 2a ; R1 ) CH3, R2 ) R3 ) SnMe3, 2b;
(R1 ) H (1a ), CH3 (1b)) with R2CtCR3 (R2 ) R3
)
SnMe3; R2 ) SnMe3, R3 ) Ph) and dmpe at 50 °C for 3
h gave corresponding vinylidene complexes of the type
Mn(C5H4R1)((H3C)2PCH2CH2P(CH3)2)(dCdCR2R3) (R1
) H, R2 ) R3 ) SnMe3, 2a ; R1 ) CH3, R2 ) R3 ) SnMe3,
2b; R1 ) H, R2 ) SnMe3, R3 ) Ph, 3a ; R1 ) CH3, R2 )
SnMe3, R3 ) Ph, 3b) in quantitative yield (Scheme 1).
This reaction appears to require the initial formation
of a Mn-alkyne species.7a However, NMR studies of the
reaction carried out in the range of -70 to 20 °C did
R1 ) H, R2 ) SnMe3, R3 ) Ph, 3a ; R1 ) CH3, R2
)
SnMe3, R3 ) Ph, 3b) with 1 equiv of [Bu4N]F (containing
5% H2O) yielded the deprotected species Mn(C5H4-
R1)((H3C)2PCH2CH2P(CH3)2)(dCdCR2R3) (R1 ) H, R2
) R3 ) H, 4a ; R1 ) CH3, R2 ) R3 ) H, 4b; R1 ) H, R2
) H, R3 ) Ph, 5a ; R1 ) CH3, R2 ) H, R3 ) Ph, 5b) by
the nucleophilic substitution of the SnMe3 groups
(Scheme 1). Complexes 4a and 4b constitute the finest
examples of thermally stable half-sandwich Mn(I) vin-
ylidene complexes with phosphine ligands and two H
atoms on the Câ atom. The structure of 4a was con-
firmed by X-ray diffraction analysis (Figure 2).
(8) (a) Pauson, P. L.; Segal, J . A. J . Chem. Soc., Dalton Trans. 1975,
2387. (b) Venkatesan, K.; Fernandez, F. J .; Blacque, O.; Fox, T.;
Alfonso, M.; Schmalle, H. W.; Berke, H. Chem. Commun. 2003, 2006.
(c) Novikova, L. N.; Peterleitner, M. G.; Sevumyan, K. A.; Semeikin,
O. V.; Valyaev, D. A.; Ustynyuk, N. A.; Khrustalev, V. N.; Kuleshova,
L. N.; Antipin, M. Y. J . Organomet. Chem. 2001, 631, 47. (d) Novikova,
L. N.; Peterleitner, M. G.; Sevumyan, K. A.; Semeikin, O. V.; Valyaev,
D. A.; Ustynyuk, N. A. Appl. Organomet. Chem. 2002, 16, 530.
The Mn1-C1 and C1-C2 bond distances are
1.755(3) and 1.316(4) Å, similar to those of 2a . A major
route to dinuclear manganese systems bearing cumu-

Communications
Organometallics, Vol. 23, No. 6, 2004 1185
Sch em e 2
F igu r e 2. Molecular structure of 4a (30% probability
displacement ellipsoids). Selected bond lengths (Å) and
angles (deg): Mn1-C1 ) 1.755(3), C1-C2 ) 1.316(4); C2-
C1-Mn1 ) 176.6(3). Selective hydrogen atoms are omitted
for clarity.
accomplished. DFT calculations10 on the model com-
pound [4b-H]•+ simulating a mononuclear cationic radi-
cal system with the dHpe ligand instead of dmpe showed
spin densities of +0.98R and +0.37R located at the
manganese and the Câ atoms, respectively. The high
probability of finding the unpaired electron of the
system on the terminal carbon atom corroborates with
the fact of generating a C-C-coupled dinuclear product.
Furthermore, single-point calculations including a model
for solvation effects performed on the gas-phase geom-
etries led to a dimerization energy ∆E of -24.2 kcal/
mol (∆E ) 2Emonomer - Edimer). The detailed analysis
shows that a major part of the driving force for this
process consists not only of the energy for the formation
of the new σ bond but also of the great solvation energy
for the dinuclear dicationic model system, calculated to
be -240.9 vs 2 × -57.9 kJ /mol of the mononuclear
system. Major counteracting energetic contributions
stem from the loss of the vinylidene double bonds. The
structures of [6]2+ (Figure 3) and [7]2+ were confirmed
by X-ray diffraction analyses.
It is worth mentioning that the C1-C2 distances of
both compounds are on the short side of a C-C single
bond, presumably reflecting a residual double-bond
character, while the newly formed C-C bond would
correspond to a “real” single bond. The cyclic voltam-
metry studies of 4b and 5b show quasi-reversible
behavior. The first oxidation step is a relatively slow
step at -0.246 V, but the dimerization occurs very
rapidly and was observed at 0.368 V. Further, the
dinuclear complex can be oxidized, and thus, the whole
process turns out to be quasi-reversible.
lene bridges utilizes coupling processes of appropriate
mononuclear species.6b,c,8c,d,9 Oxidation of complexes 4b
and 5b with 1 equiv of [Cp2Fe][PF6] in CH2Cl2 after 1 h
yielded the corresponding dinuclear carbyne complex
[(C5H4CH3)(dmpe)MntCC(R1)(R2)C(R1)(R2)CtMn(dmpe)-
(C5H4CH3)][PF6]2 (R1, R2 ) H, [6]2+; R1 ) H, R2 ) Ph,
[7]2+), which is the obvious coupling product (Scheme
2). The oxidation of 5b to [7]2+ gives specifically a meso
(R,S) diastereomer, which has been characterized spec-
troscopically and crystallographically. This selectivity
might arise from an appropriate sterically enforced
1
transition state. The H NMR spectrum shows a char-
acteristic multiplet signal at 2.72 ppm ([6]2+) and 4.39
ppm ([7]2+). The 31P NMR resonances appear at 81.1
ppm for the dmpe ligand and at -145.4 ppm for the
[PF6] anion. Additionally, the 13C NMR shows CR around
330.0 ppm and Câ at 46.4 ppm, which is characteristic
for these kinds of carbyne complexes.8c,d The formation
of complexes [6]2+ and [7]2+ implies highly selective
oxidative CsC coupling processes starting from mono-
nuclear cationic radicals as intermediates. Oxidation of
complexes 4a and 5a with 1 equiv of [Cp2Fe][PF6] in
CH2Cl2 was much less selective and yielded after 1 h
complicated mixtures containing the corresponding di-
nuclear carbyne complex [(C5H5)(dmpe)MntCC(R1)-
(R2)C(R1)(R2)CtMn(dmpe)(C5H5)][PF6]2 (R1, R2 ) H; R1
) H, R2 ) Ph). However, their separation could not be
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Lapinte, C. J . Organomet. Chem. 1998, 554, 203. (f) Mahias, V.; Cron,
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1989, 367, C21. (h) Bruce, M. I.; Cifuentes, M. P.; Snow, M. R.; Tiekink,
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The reduction of the dinuclear carbyne complexes
[6]2+ and [7]2+ with Cp*2Co yielded the corresponding
mononuclear vinylidene complexes 4b and 5b in quan-
titative yield. The reversibility of this process can be
(10) (a) ADF2002.02, SCM; Theoretical Chemistry, Vrije Univer-
siteit, Amsterdam, The Netherlands (http://www.scm.com). (b) te Velde,
G.; Bickelhaupt, F. M.; Baerends, E. J .; Guerra, C. F.; Van Gisbergen,
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1186 Organometallics, Vol. 23, No. 6, 2004
Communications
would function as a device for storing and releasing two
electrons involving the following balance: 4eπ f 2eσ +
2e. On a reversible basis one could certainly envisage
using these molecules as electron reservoirs.
In conclusion, this study highlights the potential of
utilizing the ubiquitous C-C bonds in organometallic
systems as electron reservoirs,11 thereby allowing fur-
ther development of molecular batteries which could
find application as components in nanodevices.
Ack n ow led gm en t. Support from the Swiss Na-
tional Science Foundation and the Fonds of the Uni-
versity of Zu¨rich is gratefully acknowledged. We thank
Francisco J . Ferna´ndez for an initial experiment.
F igu r e 3. Molecular structure of [6]2+ (50% probability
displacement ellipsoids). Selected bond lengths (Å) and
angles (deg): Mn1-C1 ) 1.655(3), C1-C2 ) 1.463(4),
C(2)-C(2a) ) 1.509(6); C(2)-C(1)-Mn(1) ) 114.4(3), C(1)-
C(2)-C(2a) ) 127.1(4). Selected hydrogen atoms and the
Su p p or tin g In for m a tion Ava ila ble: Text, figures, and
tables giving experimental details, computational details, and
crystallographic data of [7]2+; crystallographic data are also
given in a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org.
-
PF6 anions are omitted for clarity.
OM034237Q
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(12) Crystal structure data are as follows. 2a :
C19H39MnP2Sn2,
F igu r e 4. Calculated LUMO of the dinuclear model
complex [6-H]2+, showing the π-type antibonding interac-
tions between Mn and CR and between Câ and Câ′.
Mr ) 621.76, red, triclinic, space group P1h, a ) 9.6423(7) Å, b )
13.8440(10) Å, c ) 21.3211(16) Å, R ) 101.095(9)°, â ) 93.364(8)°, γ )
108.495(8)°, V ) 2626.7(3) Å3, T ) 223(2) K, Z ) 4, d ) 1.572 g cm-3
,
14 248 independent reflections (Mo KR; 2θmax ) 60.84°), Rint ) 0.1492,
R1 ) 0.0499, wR2 ) 0.1239 (7936 reflections, I > 2σ(I)) and R1 )
0.0886, wR2 ) 0.1436 (all data), GOF(F2) ) 0.745. 4a : C13H23MnP2,
explained and is highlighted by the shape of the LUMO
of the low-spin dinuclear model complex [6-H]2+ (Figure
4).
Mr ) 296.19, red, orthorhombic, space group Pcab (No. 61), a
)
12.2933(6) Å, b ) 13.3410(7) Å, c ) 18.5494(7) Å, R ) â ) γ ) 90.0°,
V ) 3042.2(2) Å3, T ) 183(2) K, Z ) 8, d ) 1.293 g cm-3, 3680
independent reflections (Mo KR; 2θmax ) 56.14°), Rint ) 0.0762, R1 )
0.0358, wR2 ) 0.0797 (2294 reflections, I > 2σ(I)) and R1 ) 0.0604,
wR2 ) 0.0843 (all data), GOF(F2) ) 0.888. [6]2+: C28H50F12Mn2P6, Mr
The vacant orbital is mainly of π-type antibonding
character between Mn and CR and σ-antibonding be-
tween Câ and Câ′. The reduction of the dinuclear
compound resulting formally from an addition of two
electrons into the LUMO destabilizes the MntCR and
the Câ-Câ′ bonds and leads back to the mononuclear
vinylidene systems. The oxidation-reduction process for
the vinylidene complexes of the type Mn(C5H4R1)(Me2-
PCH2CH2PMe2)(dCdCR2R3) (R1 ) CH3, R2 ) R3 ) H,
4b; R1 ) CH3, R2 ) Ph, R3 ) H, 5b) can be summarized
as processes involving reversible C-C σ-bond formation
(Scheme 2).
Occurrence of such a reaction is significant in many
respects: (i) the C-C bond formation and cleavage are
reversible, (ii) the C-C bond cleavage acts as a shuttle
of two electrons, and (iii) such an electron transfer can
occur intermolecularly. On the whole, these systems
)
910.38, red, orthorhombic, space group Pbca (No. 61), a )
14.4052(7) Å, b ) 17.1481(9) Å, c ) 15.9240(9) Å, R ) â ) γ ) 90.0°,
V ) 3933.6(4) Å3, T ) 183(2) K, Z ) 4, d ) 1.537 g cm-3, 2879
independent reflections (Mo KR; 2θmax ) 60.68°), Rint ) 0.0578, R1 )
0.0386, wR2 ) 0.0838 (3041 reflections, I > 2σ(I)) and R1 ) 0.0823,
wR2 ) 0.0898 (all data), GOF(F2) ) 0.750. Data were collected on a
Stoe IPDS diffractometer13a and structure solutions performed by using
SHELXS-97.13b Refinement calculations were done with SHELXL-97.13c
(13) (a) STOE-IPDS Software package, Version 2.87 5/1998; STOE
& Cie, GmbH, Darmstadt, Germany, 1998. (b) Sheldrick, G. M. Acta
Crystallogr., Sect. A 1996, 46, 467. (c) Sheldrick, G. M. SHELXL-97,
Program for Refinement of Crystal Structures; University of Go¨ttingen,
Go¨ttingen, Germany. Crystallographic data (excluding structure fac-
tors) for the structures given in this paper have been deposited with
the Cambridge Crystallographic Data Centre as Supplementary
Publication Nos. CCDC 212245, 212246, and 212247. Copies of the
data can be obtained free of charge on application to the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44) 1223-336033;
e-mail, deposit@ccdc.cam.ac.uk).