Reduction of [ML(alkyne)2(η-C5R′5)]+ (M = Mo or W, L = MeCN or CO, R′ = H or Me, C5R′5 = C5HPh4): Characterization of radical intermediates in the reductive coupling o

Article abstract of DOI:10.1021/om990089l

The complexes [M(NCMe)(RC₂R)₂(η-C₅R′ ₅)⁺ (M = Mo or W, R = Me or Ph; R′ = H or Me, C₅R′₅ = C₅HPh₄) undergo one-electron reduction to [M(NCMe)(RC₂

Full text of DOI:10.1021/om990089l

Organometallics 1999, 18, 3201-3207
3201
Red u ction of [ML(a lk yn e)2(η-C5R′5)]+ (M ) Mo or W, L )
MeCN or CO, R′ ) H or Me, C5R′5 ) C5HP h 4):
Ch a r a cter iza tion of Ra d ica l In ter m ed ia tes in th e
Red u ctive Cou p lin g of Coor d in a ted Alk yn es
Neil G. Connelly,*,† William E. Geiger,*,‡ Sherri R. Lovelace,‡ Bernhard Metz,†
Timothy J . Paget,† and Rainer Winter‡
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., and
Department of Chemistry, University of Vermont, Burlington, Vermont 05405
Received February 11, 1999
The complexes [M(NCMe)(RC2R)2(η-C5R′5)]+ (M ) Mo or W, R ) Me or Ph; R′ ) H or Me,
C5R′5 ) C5HPh4) undergo one-electron reduction to [M(NCMe)(RC2R)2(η-C5R′5)], which rapidly
lose acetonitrile to give the 17-electron complexes [M(RC2R)2(η-C5R′5)]. The stability of
[M(RC2R)2(η-C5R′5)] depends on R and R′; for R ) Ph, the radical is sufficiently stable so
that its reduction to the 18-electron anion [M(RC2R)2(η-C5R′5)]- is detected in the cyclic
voltammogram. Chemical reduction of [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ with [Co(η-C5H5)2]
gives the air-sensitive solid [Mo(PhC2Ph)2(η-C5HPh4)], characterized as an alkyne-based
radical by ESR spectroscopy, which gives [Mo(NCMe)(PhC2Ph)2(η-C5HPh4)]+ when treated
with [Fe(η-C5H5)2]+ in acetonitrile. The carbonyl cations [M(CO)(RC2R)2(η-C5R′5)]+ (M ) Mo,
W) undergo two sequential one-electron reductions, the first of which is reversible and gives
the 19-electron species [M(CO)(RC2R)2(η-C5R′5)]. The 19-electron radical [Mo(CO)(PhC2Ph)2-
(η-C5Me5)] has been characterized in solution by IR and ESR spectroscopy.
Sch em e 1
In tr od u ction
The reduction of bis(alkyne) complexes of the type
[M(NCMe)(RC2R)2(η-C5R′5)]+ (M ) Mo, W) can lead to
the isolation of binuclear complexes in which C-C
coupling of the alkyne ligands has occurred.1 For
example, treatment of [Mo(NCMe)(MeC2Me)2(η-C5H5)]+
(1) with [Fe(CO)2(η-C5H5)]- or sodium amalgam gives2
the metallacyclononatetraene complex [Mo2(µ-C8Me8)-
(η-C5H5)2], with four linked but-2-yne ligands, and
[W(NCMe)(MeC2Me)2(η-C5H5)]+ reacts with sodium amal-
gam to give the bis(metallacyclopentadiene) complex
[W2(µ-C4Me4)2(η-C5H5)2] via pairwise alkyne linking.3
The postulate1c,d of a radical mechanism for the forma-
tion of [Mo2(µ-C8Me8)(η-C5H5)2] was confirmed in an
electrochemical study of 1.4 Voltammetry established
that the 19-electron complex [Mo(NCMe)(MeC2Me)2(η-
C5H5)], the first product of the reduction of 1, undergoes
very rapid loss of acetonitrile, implying formation of the
17-electron radical [Mo(MeC2Me)2(η-C5H5)], which couples
to form the binuclear product (Scheme 1). Given the
* Corresponding authors. E-mail: neil.connelly@bristol.ac.uk. E-
mail: wgeiger@zoo.uvm.edu.
† University of Bristol.
‡ University of Vermont.
(1) (a) Green, M. Polyhedron 1986, 5, 427. (b) Green, M.; J etha, N.
K.; Mercer, R. J .; Norman, N. C.; Orpen, A. G. J . Chem. Soc., Dalton
Trans. 1988, 1843. (c) Brammer, L.; Green, M.; Orpen, A. G.; Paddick,
K. E.; Saunders, D. R. J . Chem. Soc., Dalton Trans. 1986, 657. (d)
Green, M.; Norman, N. C.; Orpen, A. G. J . Am. Chem. Soc. 1981, 103,
1269.
(2) Green, M.; Norman, N. C.; Orpen, A. G.; Schaverien, C. J . J .
Chem. Soc., Dalton Trans. 1984, 2455.
(3) Connelly, N. G.; Escher, T.; Martin, A. J .; Metz, B.; Orpen, A.
G. J . Cluster Sci. 1995, 6, 125.
(4) Pufahl, D.; Geiger, W. E.; Connelly, N. G. Organometallics 1989,
8, 412.
unusual C-C coupling reaction of this bis(alkyne)
radical (neutral 17-electron organometallic radicals
10.1021/om990089l CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/17/1999

3202 Organometallics, Vol. 18, No. 16, 1999
Connelly et al.
aqueous saturated calomel electrode (SCE) or a Ag/AgCl wire
which was separated from the test solution by a fine-porosity
glass frit. Ferrocene was added as an internal standard at an
appropriate point in the experiment, and all potentials in this
paper are versus that of the ferrocene/ferrocenium couple.
Reversible couples are characterized by their E1/2 potentials,
measured as the average of the cathodic and anodic peak
potentials, Epc and Epa, respectively. Potentials of chemically
irreversible systems are given by their peak potentials at a
scan rate of 200 mV s-1 unless otherwise stated. Solutions were
generally 0.5-1.0 mM in the test compound and 0.1 M in the
supporting electrolyte [NBu4][PF6]. IR spectroelectrochemical
experiments were accomplished using a thin-layer IR trans-
parent cell.12c Microanalyses were carried out by the staff of
the Microanalytical Service of the School of Chemistry, Uni-
versity of Bristol.
more usually undergo other reactions, i.e., metal-metal
bond formation, hydrogen atom loss or gain, etc.5), a
more systematic study was undertaken of the genera-
tion and fate of other species of the type [M(RC2R)2(η-
C5R′5)] (M ) Mo, W; R ) Me, Ph; R′ ) H, Me; C5R′5 )
C5HPh4). As the present paper shows, the formation of
such radicals by reducing [M(NCMe)(RC2R)2(η-C5R′5)]+
is general. Moreover, we demonstrate how the system-
atic variation of R and R′ leads to the isolation of the
radical [Mo(PhC2Ph)2(η-C5HPh4)] and provides insight
into the possibility of systematic synthesis using the bis-
(alkyne) radicals [M(RC2R)2(η-C5R′5)].
The effect of the ancillary ligand, L, in [ML(RC2R)2-
(η-C5R′5)]+ has also been investigated by replacing L )
MeCN by L ) CO. The π-acceptor CO ligand retards
the loss of L from the 19-electron complex [ML(RC2R)2-
(η-C5R′5)], thereby affecting the radical reaction path-
ways. A preliminary study6 of the reduction of [Mo(CO)-
(PhC2Ph)2(η-C5Me5)]+ suggested that the neutral 19-
electron complex [Mo(CO)(PhC2Ph)2(η-C5Me5)] was stable
on the voltammetric time scale. In this paper we
characterize [Mo(CO)(PhC2Ph)2(η-C5Me5)] by IR and
ESR spectroscopy and demonstrate its slow loss of CO
to form the 17-electron radical [Mo(PhC2Ph)2(η-C5Me5)].
[Mo(CO)(P h C2P h )2(η-C5HP h 4)][BF 4] (12). A solution of
[MoMe(CO)3(η-C5HPh4)] (1.97 g, 3.49 mmol) in CH2Cl2 (50 cm3)
was treated with HBF4‚OEt2 (0.5 cm3, 3.7 mmol). After 2 h,
diphenylacetylene (5.6 g, 31.4 mmol) was added to the purple
solution of [Mo(OEt2)(CO)3(η-C5HPh4)][BF4] and the mixture
heated under reflux for 36 h. The mixture was evaporated to
dryness in vacuo and the excess PhC2Ph removed by treatment
with hot toluene (100 cm3) and then n-hexane (30 cm3). The
red-orange residue was then dissolved in CH2Cl2; allowing
diethyl ether to diffuse slowly into the solution gave orange
crystals of the product, 1.83 g (56%).
The complexes [M(CO)(PhC2Ph)2(η-C5Me5)][BF4] [M ) Mo
(11) or W (14)] and [W(CO)(PhC2Ph)2(η-C5HPh4)][BF4] (15)
were prepared similarly.
Exp er im en ta l Section
The preparation, purification, and reactions of the complexes
described were carried out under an atmosphere of dry
nitrogen using dried, distilled, and deoxygenated solvents.
Unless stated otherwise, the new complexes are air-stable in
the solid state and dissolve in polar solvents such as CH2Cl2
and thf to give solutions which only slowly decompose in air.
The complexes [MoMe(CO)3(η-C5Me5)], [MoMe(CO)3(η-C5-
HPh4)],7 [WMe(CO)3(η-C5Me5)] and [WMe(CO)3(η-C5HPh4)],8
[Mo(CO)(PhC2Ph)2(η-C5H5)][BF4] (10),9 [Mo(NCMe)(PhC2Ph)2-
(η-C5H5)][BF4] (2),10 and [Co(η-C5H5)2]11 were prepared by
published methods. IR spectra were recorded on a Nicolet
5ZDX FT spectrometer. 1H and 13C NMR spectra were recorded
on J EOL GX270, λ300, or GX400 spectrometers with SiMe4
as internal standard. X-band ESR spectra were recorded on
either a Bruker ESP-300E spectrometer or a modified Varian
E-4 spectrometer, equipped with variable-temperature acces-
sories and a microwave frequency counter. The field calibration
was checked by measuring the resonance of the diphenylpic-
rylhydrazyl (dpph) radical before each series of spectra.
Electrochemical studies were carried out using EG&G model
[W(NCMe)(P h C2P h )2(η-C5Me5)][BF 4] (6). A mixture of
[W(CO)(PhC2Ph)2(η-C5Me5)][BF4] (14) (0.241 g, 0.31 mmol) and
Me3NO (0.023 g, 0.31 mmol) in MeCN (20 cm3) was heated
under reflux until the carbonyl band of the cation was absent
from the IR spectrum. The mixture was then evaporated to
dryness in vacuo and the residue dissolved in CH2Cl2. Allowing
diethyl ether to diffuse slowly into the solution gave yellow
needles of the product, 0.225 g (92%).
The complexes [Mo(NCMe)(PhC2Ph)2(η-C5Me5)][BF4] (3) and
[M(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4] [M ) Mo (4) or W (7)]
were prepared similarly.
[Mo(P h C2P h )2(η-C5HP h 4)] (8). To a stirred solution of
[Mo(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4] (4) (85 mg, 0.09 mmol)
in CH2Cl2 (5 cm3) at -50 °C was added a solution of [Co(η-
C5H5)2] (17 mg, 0.09 mmol) in CH2Cl2 (3 cm3), resulting in a
color change from yellow to green. After 10 min the solvent
was removed in vacuo to give a green solid, which was
dissolved in diethyl ether and filtered through Celite. Con-
centration of the solution and addition of n-pentane at -50
°C gave a fine green precipitate, which was filtered, washed,
and dried in vacuo to give a light green, very air-sensitive solid,
20 mg (27%).
173 or 273 potentiostats in
a traditional three-electrode
configuration.12a,b The working electrode was Pt; a small disk
was used for cyclic voltammetry and a large gauze basket for
bulk electrolyses. The reference electrode was either an
(5) Baird, M. C. Chem. Revs. 1988, 88, 1217. Trogler, W. C. In
Organometallic Radical Processes; Trogler, W. C., Ed.; Elsevier:
Amsterdam, 1990; p 306. Astruc, D. Electron Transfer and Radical
Processes in Transition Metal Chemistry; VCH: New York, 1995.
(6) Leoni, P.; Marchetti, F.; Pasquali, M.; Zanello, P. J . Chem. Soc.,
Dalton Trans. 1988, 635.
(7) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8, 287.
(8) Mahmoud, K. A.; Rest, A. J .; Alt, H. G.; Eichner, M. E.; J ansen.
B. M. J . Chem. Soc., Dalton Trans. 1984, 175.
(9) Bottrill, M.; Green, M. J . Chem. Soc., Dalton Trans. 1977, 2365.
(10) Allen, S. R.; Baker, P. K.; Barnes, S. G.; Green, M.; Trollope,
L.; Manojlovic-Muir, L.; Muir, K. W. J . Chem. Soc., Dalton Trans. 1981,
873.
(11) King, R. B. Organometallic Syntheses; Academic Press: New
York, 1965; Vol 1, p 70.
Resu lts a n d Discu ssion
Syn th esis of [ML(RC2R)2(η-C5R′5)]+ (M ) Mo or
W, L ) MeCN or CO, R ) H or Me, R′ ) H or Me,
C5R′5 ) C5HP h 4). The complexes studied in this work
(1-7 and 9-15, Scheme 2) were prepared by published
methods9,10,13 or modifications thereof and characterized
by elemental analysis and IR (Table 1) and NMR
spectroscopy (Table 2). The 13C NMR spectra are the
most informative in showing two resonances for the
alkyne carbons, in accord with the structure shown in
Scheme 2; the chemical shifts, in the range 150-190
(12) (a) Orsini, J .; Geiger, W. E. J . Electroanal. Chem. 1995, 380,
83. (b) Brown, N. C.; Carpenter, G. B.; Connelly, N. G.; Crossley, J .
G.; Martin, A.; Orpen, A. G.; Rieger, A. L.; Rieger, P. H.; Worth, G. H.
J . Chem. Soc., Dalton Trans. 1996, 3977. (c) Atwood, C. G.; Bitterwolf,
T. E.; Geiger, W. E. J . Electroanal. Chem. 1995, 397. 279.
(13) (a) Watson, P. L.; Bergman, R. G. J . Am. Chem. Soc. 1980, 102,
2698. (b) Beck, W.; Schloter, K. Z. Naturforsch. 1978, 33b, 1214.

Reduction of [ML(alkyne)2(η-C5R′5)]+
Organometallics, Vol. 18, No. 16, 1999 3203
Sch em e 2
Although similar chemical behavior is observed for
the tungsten complexes, the voltammetry differs from
that of the Mo analogues. The reduction potentials of
the 18-electron cations [W(NCMe)(RC2R)2(η-C5R′5)]+
differ significantly from those of the corresponding Mo
complexes (Table 3), on average by -0.37 V for a given
R and R′ (e.g., compare 5 with 1, 6 with 3, and 7 with
4), whereas those of the neutral 17-electron radicals
[M(RC2R)2(η-C5R′5)] do not (average difference ) -0.04
V; compare 6 with 3, and 7 with 4). The greater negative
shift of the reduction wave of the 18-electron tungsten
complexes therefore has the effect of decreasing the
separation in potentials between the electron-transfer
processes in eqs 1 and 3 {compare Figure 1c, which
shows the CV of [W(NCMe)(PhC2Ph)2(η-C5Me5)]+ (6),
with Figure 1a}. In fact, for [W(NCMe)(PhC2Ph)2(η-C5-
HPh4)]+ (7) (Figure 1d) the two reduction waves are
almost superimposed.
The virtual lack of metal dependence of the reduction
potential of the 17-electron radicals [M(RC2R)2(η-C5R′5)]
is consistent with the half-filled orbital (SOMO) being
based mainly on the alkynes, a conclusion reinforced
by ESR spectroscopic measurements (see below). Al-
though the redox orbital (LUMO) of the 18-electron
complexes [M(NCMe)(RC2R)2(η-C5R′5)]+ is more highly
metal-based, it must also contain considerable alkyne
character since the potential shift when the coordinated
alkyne is changed from MeC2Me to PhC2Ph is 0.43 V
(Table 3, e.g., compare Epc of 1 with that of 2). By
contrast the potential shift when the cyclopentadienyl
ring substituents are changed is small (e.g., compare
2, 3, and 4).
ppm, are consistent with the alkyne ligands acting,
formally at least, as three-electron donors.14
Electr och em istr y of [ML(RC2R)2(η-C5R′5)]+. The
general reductive behavior of the two sets of compounds
studied, namely, [M(NCMe)(RC2R)2(η-C5R′5)]+ and
[M(CO)(RC2R)2(η-C5R′5)]+, is described first, followed by
the more detailed voltammetry and spectroelectrochem-
istry of [Mo(CO)(PhC2Ph)2(η-C5Me5)]+.
Red u ction of [M(NCMe)(RC2R)2(η-C5R′5)]+. Each
of the 18-electron cations [M(NCMe)(RC2R)2(η-C5R′5)]+
is reduced in an irreversible one-electron process in thf.
The initially formed 19-electron species [M(NCMe)-
(RC2R)2(η-C5R′5)] (eq 1) undergoes rapid loss of aceto-
nitrile (eq 2), giving rise to the 17-electron species
[M(RC2R)2(η-C5R′5)] (Scheme 1). For the diphenylacetyl-
ene complexes of molybdenum the 17-electron radical
is sufficiently stable so that its one-electron reduction
wave is also detected, at a more negative potential than
that of the first process, allowing measurement of E1/2
for the 17-electron/18-electron couple [Mo(PhC2Ph)2(η-
C5R′5)]/[Mo(PhC2Ph)2(η-C5R′5)]- (eq 3). The CV of [Mo-
(NCMe)(PhC2Ph)2(η-C5Me5)]+ (3) is shown in Figure 1a
as an example. The overall electrochemical sequence is
therefore an ECE process beginning and ending with
18-electron species:
The reduction of complex 2 in MeCN was investigated
in order to see if ligand loss (eq 2) is inhibited in the
presence of a large excess of the nitrile. Although some
chemical reversibility was observed, nitrile loss from the
19-electron complex [Mo(NCMe)(PhC2Ph)2(η-C5H5)] was
not completely eliminated at a CV scan rate of 0.2 V
s-1
.
To confirm that reduction of the 18-electron cations
[M(NCMe)(RC2R)2(η-C5R′5)]+ gives the corresponding
neutral 17-electron radicals [M(RC2R)2(η-C5R′5)] (eqs 1
and 2), the Mo complexes 3 and 4 were treated with
the one-electron reductant [Co(η-C5H5)2] in thf (E1/2
)
-1.31 V)15 and the resulting solutions investigated by
ESR spectroscopy. In the case of 4, the 17-electron
radical [Mo(PhC2Ph)2(η-C5HPh4)] (8) was also generated
by adding solid [Co(η-C5H5)2] to a frozen solution of 4
in thf/CH2Cl2 (2:1) and allowing the mixture to warm
in the cavity of the ESR spectrometer until the spectrum
was observed.
[M(NCMe)(RC2R)2(η-C5R′5)]+ + e- h
[M(NCMe)(RC2R)2(η-C5R′5)] (1)
[M(NCMe)(RC2R)2(η-C5R′5)] h
[M(RC2R)2(η-C5R′5)] + NCMe (2)
In all cases, intense spectra were observed, both in
fluid and frozen solutions (Table 4). A rhombic g-tensor
assigned to [Mo(PhC2Ph)2(η-C5R′5)] is apparent from
low-temperature spectra; Figure 2 shows that of [Mo-
(PhC2Ph)2(η-C5HPh4)] (8) in thf/CH2Cl2 (2:1) at 120 K.
Considering that the radicals contain a second-row
transition metal, the g-values are quite close to that of
the free spin. For example, for [Mo(PhC2Ph)2(η-C5Me5)]
g1 ) 2.048, g2 ) 2.024, g3 ) 2.006 (Table 4); the average
of the three anisotropic g-values (2.026) agrees well with
the isotropic value of 2.027. The g-values, the lack of
observable metal hyperfine splitting in the fluid solution
[Mo(PhC2Ph)2(η-C5R′5)] + e- h
[Mo(PhC2Ph)2(η-C5R′5)]- (3)
For the MeC2Me complexes there is little evidence for
the 17-electron radicals, as indicated by the virtual
absence of a second reduction wave after the initial
reduction of, for example, [Mo(NCMe)(MeC2Me)2(η-
C5H5)]+ (1) (Figure 1b). In this particular case the
radical [Mo(MeC2Me)2(η-C5H5)] couples rapidly to pro-
duce2 the binuclear product in Scheme 1.
(14) (a) Templeton, J . L. Adv. Organomet. Chem. 1989, 29, 1. (b)
Templeton, J . L.; Ward, B. C. J . Am. Chem. Soc. 1980, 102, 3288.
(15) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.

3204 Organometallics, Vol. 18, No. 16, 1999
Ta ble 1. An a lytica l a n d IR Sp ectr oscop ic Da ta for Alk yn e Com p lexes
Connelly et al.
analysis (%)a
yield
(%)
IRb/cm-1
ν(CO)
complex
color
C
H
N
2
3
4
6
[Mo(NCMe)(PhC2Ph)2(η-C5H5)][BF4]
[Mo(NCMe)(PhC2Ph)2(η-C5Me5)][BF4]
[Mo(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4]
[W(NCMe)(PhC2Ph)2(η-C5Me5)][BF4]
[W(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4]
[Mo(CO)(PhC2Ph)2(η-C5H5)][BF4]
[Mo(CO)(PhC2Ph)2(η-C5Me5)][BF4]
[Mo(CO)(PhC2Ph)2(η-C5HPh4)][BF4]
[W(CO)(PhC2Ph)2(η-C5Me5)][BF4]
[W(CO)(PhC2Ph)2(η-C5HPh4)][BF4]
orange
yellow
copper
yellow
yellow
orange
yellow
orange
yellow
yellow
70
71
99
92
71
63
73
84
37
64
65.0 (65.1)
67.2 (67.1)
75.0 (74.6)
59.6 (59.8)
67.6 (68.3)
64.5 (64.6)
66.6 (66.7)
73.7 (74.4)
59.4 (59.3)
68.1 (68.0)
4.5 (4.4)
5.3 (5.4)
4.7 (4.7)
4.9 (4.8)
4.3 (4.3)
4.0 (4.0)
5.1 (5.0)
4.4 (4.4)
4.5 (4.5)
4.3 (4.0)
2.2 (2.2)
2.1 (2.0)
1.5 (1.5)
1.5 (1.8)
1.5 (1.4)
7
10
11
12
14
15
2074
2060
2058
2067
2063
a
b
Calculated values in parentheses. In CH2Cl2.
Ta ble 2. 1H a n d 13C NMR Sp ectr oscop ic Da ta for Alk yn e Com p lexes in CD2Cl2
compound
1H
13C
[Mo(NCMe)(PhC2Ph)2(η-C5H5)][BF4], 2
2.44 (3H, s, MeCN), 6.03 (5H, s, C5H5),
7.30-7.56 (20H, m, Ph)a
4.97 (MeCN), 102.36 (C5H5), 127.56,
128.54, 128.63, 129.67, 130.56, 133.60,
135.85, 139.55, (C6H5), 168.95,
180.43 (CtC)a
[Mo(NCMe)(PhC2Ph)2(η-C5Me5)][BF4], 3
[Mo(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4], 4
1.90 (15H, s, C5Me5), 2.28 (3H, s,
MeCN), 7.09-7.47 (20H, m, Ph)a
4.80 (MeCN), 10.98 (C5Me5), 112.56
(C5Me5), 126.71, 128.15, 128.21, 128.68,
130.06, 130.79, 134.08, 134.48, 147.68,
(C6H5), 170.50, 184.72 (CtC)a
5.29 (MeCN), 88.97, 117.55 (C5HPh4),
127.50, 127.56, 128.44, 128.63, 129.29,
129.60, 129.77, 130.18, 130.23, 130.60,
130.74, 130.85, 131.81, 135.02, 140.11
(C6H5) 173.71, 187.34 (CtC)
2.31 (3H, s, MeCN), 6.89-7.60
(41H, m, Ph and C5HPh4)
[W(NCMe)(PhC2Ph)2(η-C5Me5)][BF4], 6
[W(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4], 7
2.01 (15H, s, C5Me5), 2.44 (3H, s,
MeCN), 6.99-7.50 (20H, m, Ph)
4.91 (MeCN), 11.43 (C5Me5), 112.92
(C5Me5), 127.23, 128.16, 128.37, 128.83,
128.94, 129.38, 129.87, 130.59, 131.34,
136.71, 136.95 (C6H5), 174.31,
186.29 (CtC)
2.44 (3H, s, MeCN), 6.98-7.63
(41H, m, Ph and C5HPh4)
5.43 (MeCN), 88.11, 116.99 (C5HPh4),
126.67, 127.89, 128.52, 128.72, 128.79,
129.50, 129.59, 129.95, 130.28, 130.37,
130.51, 130.58, 131.99, 136.25, 136.32,
138.52 (C6H5), 175.03, 187.77 (CtC)
102.36 (C5H5), 128.15, 129.68, 129.81,
130.54, 130.83, 132.30, 133.35, 133.81
(C6H5) 154.96, 168.92 (CtC), 217.32 (CO)
11.47 (C5Me5), 114.43 (C5Me5), 127.24,
129.59, 129.81, 130.48, 130.93, 131.91,
132.36, 133.99, (C6H5), 156.37, 173.19
(CtC), 222.34 (CO)
[Mo(CO)(PhC2Ph)2(η-C5H5)][BF4], 10
[Mo(CO)(PhC2Ph)2(η-C5Me5)][BF4], 11
6.18 (5H, s, C5H5), 7.36-7.70
(20H, m, Ph)
1.98 (15H, s, C5Me5), 7.20-7.64
(20H, m, Ph)b
[Mo(CO)(PhC2Ph)2(η-C5HPh4)][BF4], 12
7.01-7.66 (40H, m, Ph), 8.34
(1H, s, C5HPh4)
92.96, 116.06 (C5HPh4), 127.69, 128.72,
128.88, 129.03, 129.63, 129.95, 130.24,
130.42, 130.90, 131.04, 131.11, 131.31,
131.67, 132.01, 132.32, 133.40 (C6H5)
157.69, 174.04 (CtC), 222.27 (CO)
11.48 (C5Me5), 112.78 (C5Me5), 127.43,
129.68, 129.87, 130.57, 131.86, 131.97,
132.08, 132.26, 134.62 (C6H5), 155.06,
172.61 (CtC), 210.30 (CO)
90.93, 114.52 (C5HPh4), 127.73, 127.83,
128.66, 128.87, 129.12, 129.21, 129.79,
130.05, 130.40, 130.64, 131.02, 131.30,
131.53, 132.08, 132.20, 133.98 (C6H5)
155.21, 171.31 (CtC), 210.85 (CO)
[W(CO)(PhC2Ph)2(η-C5Me5)][BF4], 14
[W(CO)(PhC2Ph)2(η-C5HPh4)][BF4], 15
2.11 (15H, s C5Me5), 7.11-7.66
(20H, m, Ph)
7.01-7.66 (40H, m, Ph), 8.51
(1H, s, C5HPh4)
a
b
At 213 K. See also ref 6.
spectrum, and the relatively small Mo couplings in the
frozen solution spectrum (the splittings of ca. 10 and
14 G on the high- and low-field components, respec-
tively, may be compared with the anisotropic parameter
of ca. 50 G for Mo16) are all consistent with the half-
filled orbital being mainly ligand-based, most likely with
a large alkyne contribution. (The 19-electron radicals
described below show significantly larger metal hyper-
fine couplings in the isotropic spectra.)
Although the radicals [M(RC2R)2(η-C5R′5)] have not
been isolated in a sufficiently pure state for character-
ization by elemental analysis, [Mo(PhC2Ph)2(η-C5HPh4)]
(8) was prepared as a very air-sensitive green solid from
the reaction between 4 and [Co(η-C5H5)2] in CH2Cl2 at
-50 °C. The solid showed an ESR spectrum (in toluene)
identical to that described above. Moreover, treatment
(16) Morton, J . R.; Preston, K. F. J . Magn. Reson. 1977, 30, 577.

Reduction of [ML(alkyne)2(η-C5R′5)]+
Organometallics, Vol. 18, No. 16, 1999 3205
Ta ble 3. P oten tia ls (V vs F er r ocen e) for th e
Red u ction of Alk yn e Com p lexes, in th f/0.1 m ol
d m -3 [NBu 4][P F 6]
a
a,c
complex
E1/2
(Ep)cb
E1/2
1
2
3
4
5
6
7
9
[Mo(NCMe)(MeC2Me)2(η-C5H5)][BF4]d
[Mo(NCMe)(PhC2Ph)2(η-C5H5)][BF4]
[Mo(NCMe)(PhC2Ph)2(η-C5Me5)][BF4]
[Mo(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4]
[W(NCMe)(MeC2Me)2(η-C5H5)][PF6]e
[W(NCMe)(PhC2Ph)2(η-C5Me5)][BF4]
[W(NCMe)(PhC2Ph)2(η-C5HPh4)][BF4]
[Mo(CO)(MeC2Me)2(η-C5H5)][BF4]
-1.96 not obsd
-1.53 -1.84
-1.56 -2.01
-1.38 -1.79
-2.33 not obsd
-1.95 -2.07
-1.72 -1.80
-1.46 -2.32f
-1.08 -1.82f
-1.10 -1.95f
-0.93 -1.76f
-1.78 -2.46f
-1.28 -2.12f
-1.22 -1.85f
10 [Mo(CO)(PhC2Ph)2(η-C5H5)][BF4]
11 [Mo(CO)(PhC2Ph)2(η-C5Me5)][BF4]g
12 [Mo(CO)(PhC2Ph)2(η-C5HPh4)][BF4]
13 [W(CO)(MeC2Me)2(η-C5H5)][BF4]
14 [W(CO)(PhC2Ph)2(η-C5Me5)][BF4]
15 [W(CO)(PhC2Ph)2(η-C5HPh4)][BF4]
a
Chemically reversible reduction; E1/2 calculated from the
b
midpoint of the cathodic and anodic peak potentials. Chemically
irreversible reduction; cathodic peak potential given for ν ) 0.2 V
s-1 c Reduction wave for [M(RC2R)2(η-C5R′5)], the product from
.
the reduction of the corresponding complex [M(NCMe)(RC2R)2(η-
d
C5R′5)]+. From ref 4. e From ref 3. f Cathodic peak potential, given
for ν ) 0.2 V s-1, for the reduction of the 19-electron complex
g
[M(CO)(RC2R)2(η-C5R′5)]. See also ref 6.
reversible at slow CV scan rates and ambient temper-
atures. Complexes containing but-2-yne give less stable
19-electron complexes than those of diphenylacetylene.
In the latter case, the reduction of the 19-electron
complex to the (nominal) 20-electron anion (eq 5) can
be observed as a second wave with a full cathodic peak
height; the CV of [Mo(CO)(PhC2Ph)2(η-C5Me5)]+ (11) is
shown in Figure 3 as an example. Although the second
reduction wave is still observed for the but-2-yne
complexes, the cathodic peak height is lower owing to
the partial decomposition of the 19-electron radical
during the time scale of the scan. In general, therefore,
the reduction of [M(CO)(RC2R)2(η-C5R′5)]+ is an EE
process (eqs 4 and 5), with the decomposition reaction
of eq 6 affecting the stability of the 19-electron radical
when R ) Me.
F igu r e 1. CVs in thf, at a scan rate of 200 mV s-1 from
0.0 to -2.6 V of (a) [Mo(NCMe)(PhC2Ph)2(η-C5Me5)][BF4]
(3), (b) [Mo(NCMe)(MeC2Me)2(η-C5H5)]+ (1), (c) [W(NCMe)-
(PhC2Ph)2(η-C5Me5)][BF4] (6), and (d) [W(NCMe)(PhC2Ph)2-
(η-C5HPh4)][BF4] (7).
[M(CO)(RC2R)2(η-C5R′5)]+ + e- f
[M(CO)(RC2R)2(η-C5R′5)] (4)
[M(CO)(RC2R)2(η-C5R′5)] + e- f
[M(CO)(RC2R)2(η-C5R′5)]- (5)
of the green solid in diethyl ether with [Fe(η-C5H5)2]-
[BF4] in acetonitrile gave [Mo(NCMe)(PhC2Ph)2(η-C5-
HPh4)][BF4] in 27% isolated yield (identified by elemen-
[M(CO)(RC2R)2(η-C5R′5)] f decomposition (6)
1
tal analysis and H NMR spectroscopy), consistent with
Since the decomposition reaction of eq 6 was never
too fast to prevent the detection of [M(CO)(PhC2Ph)2-
(η-C5R′5)] in CV experiments, the thermodynamically
significant quantity E1/2 was calculated for each of the
carbonyl-containing complexes (Table 3). Thus, for a
given R and R′ the carbonyl complexes are easier to
reduce than the NCMe analogues, on average by 0.47
V for M ) Mo (e.g. 2 vs 10) or by 0.58 V for M ) W (e.g.
6 vs 14).
As mentioned above, and shown in Figure 3 for 11,
the 19-electron complexes [M(CO)(PhC2Ph)2(η-C5R′5)]
also undergo one-electron reduction, to [M(CO)(PhC2-
Ph)2(η-C5R′5)]-, at a potential about 0.7 V more negative
than the reduction of [M(CO)(PhC2Ph)2(η-C5R′5)]+. The
reversibility of this wave is a function of the particular
complex, the solvent employed, the electrode material,
the presence of unlinked alkynes in complex 8. Attempts
to isolate the tungsten analogue of 8, namely, [W(PhC2-
Ph)2(η-C5HPh4)], were unsuccessful, although its ESR
spectrum was recorded (Table 4) when a frozen solution
of [W(NCMe)(PhC2Ph)2(η-C5HPh4)]+, treated with solid
[Co(η-C5H5)2], was allowed to warm in the cavity of the
ESR spectrometer.
Red u ction of [M(CO)(RC2R)2(η-C5R′5)]+. When the
MeCN ligand of [M(NCMe)(RC2R)2(η-C5R′5)]+ is replaced
by a carbonyl group, the 19-electron complex [ML-
(RC2R)2(η-C5R′5)] is stabilized in both the thermody-
namic sense (the cationic carbonyl complexes are easier
to reduce) and in the kinetic sense. Regarding the latter,
the one-electron reduction wave of [M(CO)(RC2R)2(η-
C5R′5)]+ (eq 4) is either fully or partly chemically

3206 Organometallics, Vol. 18, No. 16, 1999
Ta ble 4. ESR P a r a m eter s for Red u ction P r od u ctsa
Connelly et al.
precursor
reductant
product
temp (K)
g1 [A(M)]b
g2
g3
giso
3
[Co(η-C5H5)2]
[Mo(PhC2Ph)2(η-C5Me5)]
77c
295c
120
295
120
260
77
295
260d
275
2.048
2.024
2.006
2.027
2.026
2.060
4
[Co(η-C5H5)2]
[Co(η-C5H5)2]
[Co(η-C5H5)2]
[Mo(PhC2Ph)2(η-C5HPh4)]
[W(PhC2Ph)2(η-C5HPh4)]
[Mo(CO)(PhC2Ph)2(η-C5Me5)]
2.057
2.150
2.028
2.074
1.990
1.996
1.970
1.983
7
11
2.000
[27.1]
[27.3]
1.992
1.991
2.026
2.011
1.991
11
electrolysise
[Mo(PhC2Ph)2(η-C5Me5)] (I)f
[Mo(thf)(PhC2Ph)2(η-C5Me5)] (II)
[Mo(CO)(PhC2Ph)2(η-C5Me5)] (III)
[W(CO)(PhC2Ph)2(η-C5Me5)]
[23.0]
[27.5]
2.024 [37.2]
14
15
[Co(η-C5H5)2]
[Co(η-C5H5)2]
77
295
77
1.972
1.986
1.956
1.968
1.988
[W(CO)(PhC2Ph)2(η-C5HPh4)]
2.022 [41.6]
295
1.980
a
d
In CH2Cl2/thf (1:2) unless stated otherwise. b Metal hyperfine splitting in Gauss from low-field g-feature. c In thf. In toluene. e Cathodic
reduction of [Mo(CO)(PhC2Ph)2(η-C5HPh4)][BF4], at Eappl ) -1.5 V, in thf/0.1 mol dm-3 in [NBu4][PF6], at 265 K. f See text for discussion
of assignment.
reversible,17 in both CH2Cl2 and thf down to scan rates,
ν, of 0.1 V s-1. Bulk electrolytic reduction in either of
these solvents gave coulometric results consistent with
the one-electron process of eq 4 (1.0 F/equiv). Although
the 19-electron radical [Mo(CO)(PhC2Ph)2(η-C5Me5)]
decomposed at ambient temperatures in CH2Cl2, it is
the major product in thf at 265 K. Steady-state volta-
mmograms prior to, and after, electrolysis suggest the
yield of [Mo(CO)(PhC2Ph)2(η-C5Me5)] to be about 60%,
with side products present having E1/2 ) -1.35 and
-2.06 V; the latter probably corresponds to the reduc-
tion of [Mo(PhC2Ph)2(η-C5Me5)] (Table 3). On the elec-
trolytic time scale, therefore, the reduction of [Mo(CO)-
(PhC2Ph)2(η-C5Me5)]+ gives the parent 19-electron radical
[Mo(CO)(PhC2Ph)2(η-C5Me5)], lesser quantities of the
17-electron radical [Mo(PhC2Ph)2(η-C5Me5)] (formed by
loss of CO, eq 7), and a third, unidentified, species.
F igu r e 2. ESR spectrum of [Mo(PhC2Ph)2(η-C5HPh4)] (8)
at 120 K in thf/CH2Cl2 (2:1).
[Mo(CO)(PhC2Ph)2(η-C5Me5)] f [Mo(PhC2Ph)2
(η-C5Me5)] + CO (slow) (7)
Additional evidence for the suggested products comes
from studying the electrolysis solutions by ESR spec-
troscopy. At 275 K three separate signals, labeled I, II,
and III in Figure 4, are observed. Species I is assigned
to [Mo(PhC2Ph)2(η-C5Me5)] on the basis that its spec-
troscopic features (giso ) 2.026, no hyperfine splittings)
match those of the radical prepared by the reduction of
the corresponding acetonitrile complex [Mo(NCMe)-
(PhC2Ph)2(η-C5Me5)]+ (3) by [Co(η-C5H5)2] (Table 4).
Species III is assigned to the 19-electron complex [Mo-
(CO)(PhC2Ph)2(η-C5Me5)]; its features (giso ) 1.991, a -
Mo ) 27.5 G) match those of the spectrum generated
when [Mo(CO)(PhC2Ph)2(η-C5Me5)]+ (11) is treated with
[Co(η-C5H5)2] at low temperatures in either toluene or
thf (Table 4).
F igu r e 3. CV of [Mo(CO)(PhC2Ph)2(η-C5Me5)][BF4] (11)
from 0.0 to -2.2 V in thf, at a scan rate of 100 mV s-1
.
the temperature, and scan rate. We are engaged in a
detailed study of this reduction process. Here, however,
we consider the one-electron reduction of a specific
complex of this series, namely, [Mo(CO)(PhC2Ph)2(η-C5-
Me5)]+ (11), to detail the electrochemical and spectro-
scopic properties of one of the 19-electron complexes.
Red u ction of [Mo(CO)(P h C2P h )2(η-C5Me5)]+ (11).
The reduction of [Mo(CO)(PhC2Ph)2(η-C5Me5)]+ (11) is
diffusion controlled, Nernstian, and fully chemically
Over the period of about 1 h, resonances I and III
disappear from the ESR spectrum of the electrolysis
solution and resonance II grows until it is the only
signal (giso ) 2.011, a Mo ) 23 G). The Mo hyperfine
splitting in II is close to that observed for [Mo(CO)(PhC2-
(17) Diagnostics were applied according to the criteria discussed
in: Geiger, W. E. In Laboratory Techniques in Electroanalytical
Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel
Dekker: New York, 1996; Chapter 23.

Reduction of [ML(alkyne)2(η-C5R′5)]+
Organometallics, Vol. 18, No. 16, 1999 3207
F igu r e 4. ESR spectra (at 275 K) of solution sampled from
cathodic electrolysis (Eappl ) -1.5 V) of 1.2 × 10-3 mol dm-3
(11) in thf/0.1 mol dm-3 [NBu4][PF6], T ) 265 K. Electroly-
sis was 95% complete, and coulometry was consistent with
the uptake of 1.1 F of charge. Roman numerals refer to
the centers of the resonances of the three signals identified
in the text.
F igu r e 5. Terminal CO stretching region from IR/spec-
troelectrochemistry scans of ca. 2.0 × 10-3 mol dm-3 (11)
in CH2Cl2/0.3 mol dm-3 [NBu4][PF6], T ) 250 K. Legend:
dots, before electrolysis; solid line, after cathodic electroly-
sis at Eappl ) -1.5 V resulting in the formation of the 19-
electron complex [Mo(CO)(PhC2Ph)2(η-C5Me5)].
Ph)2(η-C5Me5)], and on that basis signal II is assigned
to another 19-electron complex, most likely the thf
complex [Mo(thf)(PhC2Ph)2(η-C5Me5)]. Alkyne loss (or
gain) is unlikely given that identical spectra were
obtained when the experiment was repeated in the
presence of a 20-fold excess of diphenylacetylene.
The 19-electron complex [Mo(CO)(PhC2Ph)2(η-C5Me5)]
was also characterized by IR spectroscopy. The cationic
precursor [Mo(CO)(PhC2Ph)2(η-C5Me5)]+ (11) was elec-
trolyzed in an IR-transparent thin-layer electrochemical
cell in thf at 250 K. The carbonyl stretching band (2056
cm-1) gave way to a new band at 1925 cm-1 as the
electrolysis proceeded (Figure 5), and reoxidation of the
product regenerated the original cation in 70% overall
yield. A similar experiment in CH2Cl2 at 248 K gave an
overall 88% cyclic conversion between [Mo(CO)(PhC2-
Ph)2(η-C5Me5)]+(11)[ν(CO))2060cm-1]and[Mo(CO)(PhC2-
Ph)2(η-C5Me5)] (1919 cm-1).
tion, with the resulting 17-electron, alkyne-based radi-
cals [M(alkyne)2(η-C5R′5)] subsequently reduced to the
18-electron anions [M(alkyne)2(η-C5R′5)]-. By contrast,
reduction of the carbonyl analogues (L ) CO) gives the
metal-based 19-electron radicals [M(CO)(alkyne)2(η-
C5R′5)], which are further reduced to [M(CO)(alkyne)2-
(η-C5R′5)]-. The well-known cationic complexes [ML-
(alkyne)2(η-C5R′5)]+ are therefore precursors to three
other series of potentially useful alkyne-containing
reagents, namely, the radicals [M(alkyne)2(η-C5R′5)] and
[M(CO)(alkyne)2(η-C5R′5)] and the nucleophilic anions
[M(CO)(alkyne)2(η-C5R′5)]-. Future work will explore
the chemical reactivity of such reagents.
Su m m a r y
Ack n ow led gm en t. We thank the EPSRC for Post-
doctoral Research Associateships (to B.M. and T.J .P.),
the National Science Foundation and the Petroleum
Research Fund (American Chemical Society) for support
at the University of Vermont, and the University of
Bristol for a Leverhulme Trust Visiting Fellowship and
Benjamin Meaker Visting Professorship (for W.E.G.).
A combined electrochemical and spectroscopic study
has shed light not only on the mechanism of the
reduction of the bis(alkyne) complexes [ML(alkyne)2(η-
C5R′5)]+ (M ) Mo or W, L ) MeCN or CO, R′ ) H or
Me, R′5 ) C5HPh4) but also on the nature of the
reduction products. Thus, the nitrile complexes (L )
MeCN) undergo loss of MeCN after one-electron reduc-
OM990089L