Orbital symmetry control of electronic coupling in a symmetrical, all-carbon-bridged "mixed valence" compound: Synthesis, spectroscopy, and electronic structure of [{Mo(dppe)(ν-C7H7)} 2(μ-C4)]n+ (n = 0, 1, or 2)

Article abstract of DOI:10.1021/om200712m

The cycloheptatrienyl molybdenum alkynyl complex [Mo(C≡CH)(dppe) (ν-C₇H₇)], 1 (dppe = Ph₂PCH ₂CH₂PPh₂), undergoes oxidative dimerization on reaction with [FeCp₂]PF₆ in thf at -78 °C to give the bis(vinylidene) [{Mo(dppe)(ν-C₇H₇)}₂(μ-C= CHCH=C)][PF₆]₂, [2][PF₆]₂. Deprotonation of [2][PF₆]₂ with KOBu^t yields butadiyndiylbridged [{Mo(dppe)(ν-C₇H₇)} ₂(μ-C≡C-C≡C)], 3, which undergoes in situ aerial oxidation to give [{Mo(dppe)(ν-C₇H₇)} ₂(μ-C₄)][PF₆], [3]PF₆, as the major product. The cyclic voltammogram of [3]PF₆ exhibits a series of four redox processes indicative of sequential formation of [{Mo(dppe)(ν- C₇H₇)}₂(μ-C₄)]ⁿ⁺ (n = 0, 1, 2, 3, 4) with the comproportionation constant, K_C, for [3]PF₆ of 1.9 × 10₇. Spectroscopic investigations on [3]PF₆ by IR, Raman, NIR, and EPR spectroscopy reveal properties characteristic of a d⁵/d⁶ mixed valence complex with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 10⁸-10¹⁰ s^-1. The experimental NIR spectrum of [3]PF₆ is consistent with the predicted spectral characteristics of a three-state model for bridgemediated, electron transfer in a weakly coupled, symmetrical mixed valence system. The dication [3][PF₆]₂ was isolated by chemical oxidation and structurally characterized; magnetic susceptibility measurements on [3][PF ₆]₂ in the temperature range 2-300 K reveal strong antiferromagnetic coupling with the exchange coupling constant J_ab (defined according to the Hamiltonian H_spin = -J _abS_aS_b) determined as -406 (±3) cm^-1.

Full text of DOI:10.1021/om200712m

Article
pubs.acs.org/Organometallics
Orbital Symmetry Control of Electronic Coupling in a Symmetrical,
All-Carbon-Bridged “Mixed Valence” Compound: Synthesis,
Spectroscopy, and Electronic Structure of [{Mo(dppe)(η-C7H7)}2(μ-
C4)]n+ (n = 0, 1, or 2)
Emma C. Fitzgerald,† Neil J. Brown,‡ Ruth Edge,§ Madeleine Helliwell,† Hannah N. Roberts,†
Floriana Tuna,† Andrew Beeby,‡ David Collison,§ Paul J. Low,*,‡ and Mark W. Whiteley*,†
†School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
‡Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, U.K.
§EPSRC National Service for EPR Spectroscopy, School of Chemistry, University of Manchester, Manchester, M13 9PL, U.K.
S
* Supporting Information
ABSTRACT: The cycloheptatrienyl molybdenum alkynyl complex [Mo(CCH)(dppe)(η-
C7H7)], 1 (dppe = Ph2PCH2CH2PPh2), undergoes oxidative dimerization on reaction with
[FeCp2]PF6 in thf at −78 °C to give the bis(vinylidene) [{Mo(dppe)(η-C7H7)}2(μ-CCH-
CHC)][PF6]2, [2][PF6]2. Deprotonation of [2][PF6]2 with KOBut yields butadiyndiyl-
bridged [{Mo(dppe)(η-C7H7)}2(μ-CC-CC)], 3, which undergoes in situ aerial oxidation
to give [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6], [3]PF6, as the major product. The cyclic
voltammogram of [3]PF6 exhibits a series of four redox processes indicative of sequential
formation of [{Mo(dppe)(η-C7H7)}2(μ-C4)]n+ (n = 0, 1, 2, 3, 4) with the comproportionation
constant, KC, for [3]PF6 of 1.9 × 107. Spectroscopic investigations on [3]PF6 by IR, Raman,
NIR, and EPR spectroscopy reveal properties characteristic of a d5/d6 mixed valence complex
with a localized electronic structure and an estimated intramolecular electron transfer rate in the range 108−1010 s−1. The
experimental NIR spectrum of [3]PF6 is consistent with the predicted spectral characteristics of a three-state model for bridge-
mediated, electron transfer in a weakly coupled, symmetrical mixed valence system. The dication [3][PF6]2 was isolated by
chemical oxidation and structurally characterized; magnetic susceptibility measurements on [3][PF6]2 in the temperature range
2−300 K reveal strong antiferromagnetic coupling with the exchange coupling constant Jab (defined according to the
−1
̂
̂
̂
Hamiltonian Hspin = −Jab·Sa·Sb) determined as −406 ( 3) cm .
ampy)4}(μ-C4){Ru2(μ-ampy′)4}(μ-C4){Ru2(ampy)4}] (ampy,
ampy′ = 2-aminopyridines)24 are also known.
INTRODUCTION
■
Diyndiyl-bridged bimetallic complexes [{LxM}-CC-CC-
{MLx}], in which redox-active metal end-caps {MLx} are linked
by an unsaturated linear four-carbon chain bridge, have been at
the forefront of investigations into the basic chemistry of metal-
supported all-carbon molecules and as model systems for
molecular wires.1−4 Complexes bearing a wide range of redox-
active metal end-caps {MLx} have been investigated, including
examples featuring Fe(dppe)Cp*,5,6 Re(NO)(PPh3)Cp*,7 Ru-
(PP)Cp′ [(PP)Cp′ = (PPh3)2Cp,8 (dppe)Cp*,9 (dppf)Cp10],
Os(dppe)Cp*,11,12 MnI(dmpe)2,13 and WI(dppe)2,14 in both
homo- and heterobimetallic15−18 systems. Depending on the
particular combination of metal and supporting ligands, a
sequence of up to five interconvertible redox states (n = 0, 1, 2,
3, or 4) has been observed in [{LxM}(μ-C4){MLx}]n+
systems,8,9,19 which can be further expanded by the
introduction of redox-active supporting ligands.10 Carbon
chains capped by bi-20,21 and polymetallic22 end-caps have
been investigated in similar contexts, and redox-active
oligomeric systems such as [Cl(depe)Fe}(μ-C4){(dppe)2W}-
(μ-C4){W(dppe)}(μ-C4){Fe(depe)2Cl}]23 and [{Ru2(μ-
In all of the above examples, the π orbitals of the diyndiyl
bridge mix with metal orbitals of appropriate symmetry to give
occupied frontier orbitals that are more or less delocalized over
both metal centers and the carbon bridge. Oxidation of these
established examples of butadiyndiyl complexes results in
removal of electrons from the largely delocalized frontier
orbitals, giving rise to metal-stabilized tautomers of the C4 chain
from diyndiyl [{LxM}-CC-CC-{MLx}]n+, through cumu-
lene-like butatriene-bis-ylidene [{LxM}CCC
C{MLx}]n+, to carbyne-like butyne-bis-triyl [{LxM}C−
CC−C{MLx}]n+ forms; examples of metal complexes and
clusters containing these various isomeric C4 ligands have been
isolated, and the most appropriate valence bond description of
the [{LxM}(μ-C4){MLx}] moiety depends on the nature of the
metal and the number of valence electrons.14,25−27 Many of the
odd-electron radical cations [{LxM}(μ-C4){MLx}]+ have been
described as class III “mixed valence” systems.28 However, such
Received: August 1, 2011
Published: December 12, 2011
© 2011 American Chemical Society
157
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
descriptions often lack the element of precision necessary to
account for the contribution of the C4 moiety to the
stabilization of the unpaired electron. It would be better to
more rigorously make the distinction between (i) genuinely
delocalized mixed valence systems (class III), (ii) those that
display fast electron exchange between the redox sites (class
II−III), and (iii) systems in which the bridging ligand is so
intimately involved in supporting the unpaired electron that the
concept of redox noninnocent ligand behavior becomes a
significant part of the overall description. Indeed, both
experimental studies and theoretical calculations demonstrate
that the electronic structure of complexes [{LxM}(μ-C4)-
{MLx}]n+ can be fine-tuned by the identity of the metal end-
cap, particularly with respect to the relative contributions of
metal and carbon chain character to the frontier redox orbitals.
For example, DFT calculations on the model systems
[{M(dHpe)Cp}2(μ-C4)]n+ (M = Fe or Ru; dHpe =
H2PCH2CH2PH2) reveal that the metal versus carbon chain
character of the frontier molecular orbitals is directly dependent
upon the identity of M, with the Fe derivatives more metal
centered and conversely the Ru derivatives more heavily
weighted on the carbon chain.17 As the proportion of
metal:carbon chain character in these important frontier
orbitals decreases, the complexes move between the extremes
of redox-active metal fragments linked by a carbon chain and
supporting genuine mixed valence concepts and examples of
carbon fragments linking electron-donating metal fragments in
which the bridging carbon chain can be thought of as redox
noninnocent. This difference in the relative importance of the
metal versus carbon chain character of the redox orbitals also
becomes manifest in the structural and spectroscopic character-
istics of the formally d5/d5 systems that approximate the
descriptions of triplet [{LxM}-CC-CC-{MLx}]n+ (most
closely represented by MLx = Fe(dppe)Cp*, n = 2;5 MLx =
Mn(dmpe)2I, n = 0;13) and singlet [{LxM}CCC
C{MLx}]2+ (exemplified by MLx = Ru(PPh3)2Cp,8 Ru-
(dppe)Cp*,9 and Re(PPh3)(NO)Cp*).7
In this context, recent investigations on the cycloheptatrienyl
molybdenum alkynyl complexes [Mo(CCR)(dppe)(η-
C7H7)] suggest the opportunity for a more fundamental
control of the electronic structure of all-carbon-bridged
bimetallic complexes by the metal end-cap.29 As shown in
Figure 2, although the bonding between the Mo(dppe)(η-
Figure 2. HOMO and HOMO−1 of [Mo(CCPh)(dppe)(η-C7H7)]
(B3LYP/SVP). Contour values of 0.04 au.29
C7H7) unit and the alkynyl ligand also involves four-electron
repulsive π-type interactions, critically, in the HOMO the
2
overlap is between the Mo dz orbital and the carbon chain and
is much less effective on symmetry grounds than the dπ−π
interactions more commonly observed in half-sandwich d6
complexes. This novel frontier orbital structure is often
preserved upon oxidation and results in significant differences
in the electronic and spectroscopic properties of [Mo(C
CR)(dppe)(η-C7H7)]+ systems when compared with analogous
+
2
[M(CCR)(dppe)Cp*] (M = Fe, Ru) complexes. The dz
character in the HOMO of the cycloheptatrienyl derivatives can
be attributed to the effects of strong metal−ring δ-bonding
(evident in HOMO−1, Figure 2),30−32 which stabilize the dπ-
type orbitals relative to those in cyclopentadienyl systems, and
result in a significant modification in the d-orbital ordering.
In view of these observations, the introduction of the
Mo(dppe)(η-C7H7) end-cap into carbon-chain-bridged, bimet-
allic complexes may be expected to give rise to unusual
electronic features as a consequence of the unique frontier
orbital characteristics of the metal end-cap. In this work, we
describe the synthesis, redox chemistry, and electronic structure
of C4-bridged [{Mo(dppe)(η-C7H7)}2(μ-C4)]n+ (n = 0, 1, or
2), which demonstrate a significant departure in properties
from those generally attributed to the [{LxM}(μ-C4){MLx}]n+
class of complexes.
Although differences between the electronic structures of the
[{LxM}(μ-C4){MLx}]n+ systems described above can be
tracked back to fine details of the metal d/C4 ligand π orbital
overlap and occupancy, fundamentally the description of the
metal to carbon chain interaction is very similar for the majority
of pseudo-octahedral d6 metal systems in which the σM−C bond
RESULTS AND DISCUSSION
■
Syntheses. The all-carbon-bridged complexes [{Mo-
(dppe)(η-C7H7)}2(μ-C4)]n+ (n = 1, 2) were prepared as
outlined in Scheme 1, following a well-established route that
has been utilized for the syntheses of the group 8 complexes
[{M(dppe)Cp*}2(μ-CC-CC)] (M = Fe5 or Ru9), and
isolated as the [PF6]− salts. The pivotal step is the Cβ-centered,
radical−radical coupling of the 17-electron alkynyl complex
[Mo(CCH)(dppe)(η-C7H7)]+, leading to the construction
of a four-carbon bridge in the form of a bis(vinylidene)
complex, a process first proposed33 to rationalize the formation
of [{Fe(dppe)Cp*)}2(μ-CCMe-CMeC)][BF4]2 from ox-
idative coupling of [Fe(CCHMe)(dppe)Cp*][BF4] and
subsequently demonstrated for the cycloheptatrienyl molybde-
num alkynyl radical [Mo(CCPh)(dppe)(η-C7H7)]+ and a
wide range of related alkynyl radicals.34 The key precursor
complex [Mo(CCH)(dppe)(η-C7H7)], 1, was prepared in
stepwise fashion from [MoBr(dppe)(η-C7H7)] by initial
conversion to the vinylidene [Mo(CCH2)(dppe)(η-
C7H7)]+, through reaction with HCCSiMe3, and subsequent
2
is derived from overlap of the metal dz and carbon ligand sp
orbitals.7−19 The HOMO and HOMO−1 are approximately
orthogonal and arise from efficient “in-plane” four-electron
repulsive π-type interactions between metal and carbon chain
fragment orbitals, and are illustrated in Figure 1 for the case of
the computational model {M(dHpe)Cp}2(μ-CCCC) (M
= Fe, Ru).
Figure 1. HOMO and HOMO−1 of [{Ru(dHpe)Cp}2(μ-CC-C
C)]. HOMO composition: Ru2 26%, C4 62%; cf. [{Fe(dHpe)Cp}2(μ-
CC-CC)]; Fe2 41%, C4 46%.
158
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
a
Scheme 1
a
Reagents and conditions: (i) HCCSiMe3/KPF6 in MeOH, reflux 3 h. (ii) Excess KOBut in thf, 1 h. (iii) [FeCp2]PF6 in thf (−78° C), 5 h. (iv)
KOBut in thf, 5 h, aerial oxidation. (v) [FeCp2]PF6 in CH2Cl2, 1 h.
a
Table 1. Electrochemical Data for [{LxM}(μ-C4){MLx}]n+
MLx
E1
E2
E3
E4
E2 − E1
KC(+1/+2)
ref
b
Mo(dppe)(η-C7H7)
Fe(dppe)Cp*
Ru(dppe)Cp*
Os(dppe)Cp*
−1.02
−1.14
−0.89
−1.08
−0.45
−0.59
−0.42
−0.24
−0.47
0.08
0.39
0.49
0.58
0.36
0.48
0.43
0.72
0.65
0.61
0.53
1.9 × 107
1.6 × 1012
9.7 × 1010
2.1 × 1010
1.1 × 109
this work
5, 6
9
1.05
0.80
11
7
Re(NO)(PPh3)Cp*
a
All potentials are reported with reference to the FeCp2/[FeCp2]+ couple (E1/2 = 0.00 V). Data from literature sources are adjusted via E1/2 cited for
b
n
the FeCp2/[FeCp2]+ couple (0.46 V vs SCE in 0.1 M [Bu4 N]PF6/CH2Cl2 solution at a Pt electrode). Cyclic voltammogram recorded (ν = 100
n
mV s−1) from a 0.2 M [Bu4 N]PF6/CH2Cl2 solution at a glassy carbon working electrode.
deprotonation with a 10-fold excess of KOBut in thf; similar
protocols have been employed elsewhere.5,9,35 The requirement
for a large excess of base to shift the vinylidene/alkynyl
equilibrium to the alkynyl form may arise from a combination
of the very strong electron donor properties of the Mo(dppe)-
(η-C7H7) unit, rendering the vinylidene [Mo(CCH2)(dppe)-
(η-C7H7)]PF6 a relatively weak acid and the strong MoCα
bond of the group 6 vinylidene complexes.36,37 The identity of
complex 1 was confirmed by spectroscopic data, which include
a strong IR-active ν(CC) stretch at 1908 cm−1 typical of
unsubstituted alkynyl complexes [{LxM}-CCH],5,9 a triplet
{J(P−H) = 4 Hz} at δH 1.74 assigned to the alkynyl H
substituent, a triplet {J(P−C) = 25 Hz} at δC 130.1 assigned to
the alkynyl Cα carbon, and microanalytical data.
the reaction of 1 with [FeCp2]PF6 was conducted at low
temperature (−78 °C) as a concentrated solution in thf. After 5
h a deep purple solid containing the required product,
[{Mo(dppe)(η-C7H7)}2(μ-CCH-CHC)][PF6]2, [2]-
[PF6]2, as a mixture with [Mo(CCH2)(dppe)(η-C7H7)]-
PF6,36 was obtained, the components being separable by
recrystallization given the lower solubility of [2][PF6]2. The
unavoidable formation of small quantities of [Mo(CCH2)-
(dppe)(η-C7H7)]PF6, evident from NMR monitoring of the
crude reaction mixture (approximate ratio in a typical
preparation [Mo(CCH2)(dppe)(η-C7H7)]PF6:[2][PF6]2
=
1:4), probably results from abstraction of H• from the solvent
by the reactive radical [1]+; an identical, albeit much slower,
process has been suggested for the more sterically congested
radical [Mo(CCBut)(dppe)(η-C7H7)]+.34a The low-field
MoCα resonance of [2][PF6]2 was not detected in the
13C{1H} NMR spectrum, but a triplet resonance at δ 4.41,
The cyclic voltammogram of complex 1 in thf/0.1 M
n
[Bu4 N]PF6 exhibited an irreversible oxidation at ambient
temperatures, but on cooling to −40 °C, a chemically reversible
process (E1/2 = −0.70 V vs FeCp2/[FeCp2]+) was observed.
Therefore to ensure the accumulation of the 17-electron radical
[Mo(CCH)(dppe)(η-C7H7)]+, [1]+, in solution necessary to
promote efficiently the coupling reaction outlined in Scheme 1,
1
characteristic of a vinylidene proton was observed in the H
NMR spectrum.
The deprotonation of complex [2][PF6]2 was effected with
excess KOBut in thf at room temperature to give a green-brown
159
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
solution, from which [{Mo(dppe)(η-C7H7)}2(μ-C4)]PF6,
[3]PF6, was isolated as a green-brown solid together with
small quantities of neutral, orange-brown [{Mo(dppe)(η-
C7H7)}2(μ-CC-CC)], 3. The isolation of the oxidized
species [3]PF6 as the major product from the deprotonation
reaction contrasts with the corresponding reactions of [{M-
(dppe)Cp*}2(μ-CCH-CHC)]2+ (M = Fe or Ru), which
lead to formation of the neutral diyndiyl complexes [{M-
(dppe)Cp*}2(μ-CC-CC)], but is consistent with related
facile oxidation chemistry of the Mo(dppe)(η-C7H7) auxil-
iary.38 Complexes 3 and [3]PF6 were identified by mass
spectrometry and microanalytical data (see Experimental
Section) and further characterized by a series of techniques
including cyclic voltammetry, IR, NIR, and Raman spectrosco-
py as detailed below.
Complex [3][PF6]2 (which exhibits identical electrochemical
behavior to that of [3]PF6) was characterized by a series of
spectroscopic and spectroelectrochemical techniques (see
below), and its identity further confirmed by a single-crystal
X-ray structural investigation.
Structural Investigations. The molecular structure of
[{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6]2, [3][PF6]2, is shown in
Figure 3, and important bond lengths and angles are
Electrochemistry and Synthetic Redox Chemistry.
The cyclic voltammogram of [3]PF6 was recorded at a glassy
n
carbon electrode in CH2Cl2/0.2 M [Bu4 N]PF6; a summary of
E1/2 data for [3]PF6, relative to the FeCp2/[FeCp2]+ couple,
and a comparison with selected, closely related systems from
groups 7 and 8 are presented in Table 1.
The cyclic voltammogram of [3]PF6 exhibits four redox
processes. The well-separated waves at lowest potential E1 =
−1.02 V, E2 = −0.59 V are assigned to the couples 3/[3]+ and
[3]+/[3]2+ on the basis of comparison with the related diyndiyl
complexes in Table 1 and the redox potential for the
monometallic diynyl complex [Mo(CC-CCH)(dppe)(η-
C7H7)] (E1/2 = −0.58 V in CH2Cl2 vs FeCp2/[FeCp2]+).39
These first two waves are fully chemically and electrochemically
Figure 3. Molecular structure of [3][PF6]2 with thermal ellipsoids
plotted at 50% probability. [PF6]− counterions, solvent of crystal-
lization, and H atoms are removed for clarity.
reversible [ip /ip = 1 and ip ∝ ν1/2], and the relatively small
difference in the first and second oxidation potentials in the
molybdenum complex (E2 − E1 = 0.43 V) also leads to the
smallest KC value in the series (see Table 1). The more negative
value of E1/2 for the couple 3/[3]+ by comparison with
[Mo(CC-CCH)(dppe)(η-C7H7)]0/+ is consistent with a
degree of synergy between the two electron-donating Mo-
(dppe)(η-C7H7) fragments. However, although in monometal-
lic systems of the type [M{(CC)nR}(dppe)(η-L)] (η-L = η-
C5Me5 = Cp*, M = Fe,18,40 or Ru;41,42 η-L = η-C7H7 M =
Mo,29,39), the E1/2 value for one-electron oxidation is always the
most negative for the Mo(dppe)(η-C7H7) system,43 this is not
so for the bimetallic diyndiyl complexes, where the iron
complex [{Fe(dppe)Cp*}2(μ-CCCC)] is the most easily
oxidized (in the thermodynamic sense) member of the family
(Table 1). In common with [{M(dppe)Cp*}2(μ-CCCC)]
(M = Ru or Os), two further redox processes are observed for
the Mo(dppe)(η-C7H7) system, viz., E3 = 0.39 V, E4 = 0.48 V
assigned to the couples [3]2+/[3]3+ and [3]3+/[3]4+. These
waves are very closely spaced, and therefore the degree of
reversibility is much harder to assess, although no redox-active
secondary products were observed in the potential range
investigated. We note that the relatively electron rich system
[{Fe(dppe)Cp*}2(μ-CC-CC)] undergoes three chemi-
cally reversible redox processes, the first two of which are
principally metal in character and the third being more closely
associated with oxidation of the carbon chain.6
C
A
summarized in Table 2, while Table 3 presents some key
comparisons with closely related all-carbon-bridged complexes.
Table 2. Important Bond Lengths (Å) and Angles (deg) for
Complex [3][PF6]2
Mo(1)−C(34)
Mo(2)−C(37)
C(34)−C(35)
C(35)−C(36)
C(36)−C(37)
Mo(1)−P(1)
Mo(1)−P(2)
Mo(2)−P(3)
Mo(2)−P(4)
2.041(7)
2.024(7)
1.211(9)
1.355(9)
1.249(9)
2.5070(19)
2.5238(17)
2.526(2)
2.539(2)
Mo(1)−C(34)−C(35)
C(34)−C(35)−C(36)
C(35)−C(36)−C(37)
C(36)−C(37)−Mo(2)
P(1)−Mo(1)−P(2)
P(1)−Mo(1)−C(34)
P(2)−Mo(1)−C(34)
P(3)−Mo(2)−P(4)
P(3)−Mo(2)−C(37)
P(4)−Mo(2)−C(37)
177.0(7)
176.4(7)
177.5(8)
174.1(6)
79.26(6)
75.1(2)
81.04(18)
79.99(7)
84.0(2)
75.0(2)
The presence of a four-carbon bridge, C(34)−C(35)−
C(36)−C(37), linking the Mo(dppe)(η-C7H7) end-caps is
confirmed by the structural study. The two Mo(dppe)(η-C7H7)
units are disposed in a gauche arrangement with the angle
between planes defined by Ct(1)−Mo(1)−C(34) and Ct(2)−
Mo(2)−C(37) [Ct(1), Ct(2) are the centroids of the C7H7
rings attached to Mo(1) and Mo(2)] calculated as 80.8°. As
discussed, the redox behavior of diyndiyl-bridged bimetallic
complexes falls into two broad classes represented by the
contrasting properties of those systems that feature predom-
inantly metal-based redox chemistry, such as [{Fe(dppe)-
Guided by the electrochemical results, synthetic studies were
carried out to generate the dication [{Mo(dppe)(η-C7H7)}2(μ-
C4)]2+, [3]2+, using chemical redox reagents. Reaction of a
solution of green-brown [3]PF6 with an excess of [FeCp2]PF6
in CH2Cl2 resulted in the formation of an intense green
solution, from which [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6]2,
[3][PF6]2, was isolated as a deep green solid (Scheme 1).
5
Cp*}2(μ-C4)]n+ and [{MnI(dmpe)2}(μ-C4)]n+,13 and those
systems in which the C4 ligand is more intimately involved in
9
the redox events, such as [{Ru(dppe)Cp*}2(μ-C4)]n+ and
[{Re(PPh3)(NO)Cp*}2(μ-C4)]n+.7 The structural differences
between the two classes are manifest in the changes in carbon−
160
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
temperature spectroelectrochemical cell. A summary of the IR
and Raman data obtained for [3]n+ (n = 0, 1, or 2) is presented
in Table 4 alongside comparative data for [{Re(PPh3)(NO)-
Cp*}2(μ-C4)]n+ (n = 0, 1, or 2).7
Table 3. Bond Length Parameters (Å) for Selected
[{MLx}{μ-C(α)C(β)-C(β′)C(α′)}{MLx}]n+ Complexes
C(α)−C(β),
C(α′)−C(β′)
C(β)−
C(β′)
M−C(α),
M−C(α′)
MLx
n
ref
Ru(dppe)
0
1.223(4),
1.218(4)
1.382(4)
1.338(3)
1.294(7)
1.389(5)
2.001(3),
2.003(3)
9
9
9
7
7
Table 4. Infrared and Raman Data (cm−1) for ν(CC)
Stretching Modes of the C4 Bridge of [{LxM}(μ-
Cp*
Ru(dppe)
Cp*
Ru(dppe)
Cp*
Re(PPh3)
(NO)Cp*
Re(PPh3)
(NO)Cp*
Fe(dppe)
Cp*
Fe(dppe)
Cp*
Mo(dppe)(η-
C7H7)
1
2
0
2
0
1
2
1.248(3)
1.931(2)
a
C4){MLx}]n+
1.280(7),
1.269(7)
1.858(5),
1.856(5)
n = 0
Raman
1939 1930,
n = 1
Raman
1800 not
n = 2
Raman
1894,
1.202(7)
2.037(5)
MLx
IR
not
IR
IR
Mo(dppe)(η-
C7H7)
1.263(10),
1.260(10)
1.305(10) 1.909(7),
1.916(7)
obsd
1868
obsd
1990 not
obsd
1826
Re(PPh3)
(NO)Cp*
1964
2056 1872
1883
1.220(4),
1.221(4)
1.373(4)
1.889(3),
1.884(3)
16
5
a
IR data in CH2Cl2 solution; not obsd = not observed. Raman spectra
1.236(9)
1.36(1)
1.830(8)
for [3]n+ recorded in the solid state (λex = 532.2 nm).
1.211(9),
1.249(9)
1.355(9)
2.041(7),
2.024(7)
this
work
The neutral diyndiyl complex 3 is IR silent in the region
2500−1550 cm−1, but a Raman band is observed from a solid
sample at 1939 cm−1 (λex = 532.2 nm); each of these
observations is consistent with a symmetrical electronic
structure. By contrast, [3]PF6 exhibits a distinctive, intense
two-band IR spectrum in CH2Cl2 solution with ν(CC) 1930,
1868 cm−1 (Figure 4); the tail of a relatively intense NIR
carbon bond lengths along the C4 chain as a function of redox
state. In the case of the Ru and Re complexes for which the
redox events involve significant carbon chain character,
proceeding from n = 0 to n = +2, there is a distinct elongation
of C(α)−C(β), and C(α′)−C(β′) and corresponding short-
ening of the central C(β)−C(β′) bond, consistent with the
evolution of a cumulenic structure in the dication.9 By contrast,
in the iron system, the first two redox processes have
substantial metal character, and this is reflected in the much
smaller changes in the geometry of the carbon chain (although
structural data for the dication are not available);5,16 the third
redox event is probably more carbon-centered.6,16 Inspection of
the data in Table 3 suggests that the carbon chain in the
dication [3][PF6]2 retains a degree of alternating triple/single-
bond character. Moreover the Mo−C(α) bond distances in
[3][PF6]2 (2.041(7), 2.024(7) Å), although probably shorter
than that determined for the alkynyl radical cation [Mo(C
CPh)(dppe)(η-C7H7)][BF4], 2.067(9) Å,44 are longer than the
equivalent parameter in the closely related bis(vinylidene)-
bridged [{Mo(dppe)(η-C7H7)}2(μ-CCPh-CPhC)][PF6]2
(1.951(5) Å).36 By comparison, the Ru−C(α) bond lengths
of 1.858(5) and 1.856(5) Å, reported for [{Ru(dppe)Cp*}2(μ-
C4)]2+, are only marginally longer than Ru−C(α) distances in
the bis(vinylidene)-bridged analogue [{Ru(dppe)Cp*}2(μ-C
CH-CHC)][PF6]2, (1.837(4), 1.834(4) Å). The key bond
length parameters along the carbon chain in [3][PF6]2 are
therefore not typical of the cumulenic [{LxM}CCC
C{MLx}]2+ structure normally exhibited by formally d5/d5
systems of 4d and 5d series metals.
Vibrational Spectroscopy. The response of the ν(CC)
modes of the μ-C4 bridging ligand in [{LxM}(μ-C4){MLx}]n+
to changes in charge state, n, is a useful probe of the extent of
involvement of the carbon chain bridge in the redox
orbitals.5,7,9,13 Therefore, in the current work, the characteristics
of the ν(CC) modes of 3, [3]PF6, and [3][PF6]2 are of
specific importance and were investigated by a combination of
IR and Raman spectroscopy on isolated samples, comple-
mented by an IR spectroelectrochemical study. Starting from
[3]PF6, the spectroelectrochemical generation of 3 and
[3][PF6]2 was fully reversible with complete recovery of the
spectrum of [3]PF6. However, despite the apparent reversibility
of the electrochemical events on the time scale of cyclic
voltammetry, attempts at further oxidation to [3][PF6]3 and
[3][PF6]4 resulted only in decomposition in the room-
Figure 4. IR spectrum of [3]PF6 in CH2Cl2.
absorption band is also apparent in Figure 4 intruding into the
IR region (see later). In the solid-state Raman spectrum of
[3]PF6, a band at 1800 cm−1 was detected arising from a
symmetric ν(CC) mode. While the observation of two IR-
active ν(CC) modes for the mixed valence monocation
[3]PF6 may arise from the presence of different conformers,
associated with the relative dispositions of the two metal end-
caps,45 the relative intensities of the two IR-active ν(CC)
modes of [3]PF6 did not alter significantly with change in
solvent polarity or in the solid state. Moreover, typical
homobimetallic, C4-bridged complexes of the 4d/5d metals
[{MLx}2(μ-C4)]+ (MLx = Ru(dppe)Cp*, Re(NO)(PPh3)-
Cp*)7,9 exhibit only a single IR-active ν(CC) mode in the
mixed valence state. An alternative rationalization for the IR
spectral properties of [3]PF6, consistent with the results of NIR
and EPR spectroscopy (see below), is the adoption of a
localized electronic structure resulting from slow intervalence
161
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
electron transfer on the IR time scale. For the dication
[3][PF6]2, no IR-active bands were observed for an isolated
sample in solution in CH2Cl2; however the Raman spectrum of
[3][PF6]2 as a solid sample featured two bands at 1826 and
1894 cm−1. A single strong band in this region would be
expected for a symmetric [{LxM}CCCC{MLx}]2+
structure, but the observation of two bands may be due to the
presence of different conformations of [3]2+ in the solid state or
possibly indicative of a Raman-active impurity. Raman spectra
of solid [3][PF6]2 collected using different excitation wave-
lengths (532.2, 632.8, 785.1 nm) reveal that these bands are
enhanced to different extents, supporting the notion that the
two bands arise from two different forms of the species, or two
chemically distinct species, in the sample. However, the absence
of an IR-active ν(CC) band in [3][PF6]2 requires any
putative impurity to have a high-symmetry structure.
Interpretation of the trend in the Raman-active ν(CC)
frequency along the series [3]n+ (n = 0, 1, or 2) (see Table 4) is
not straightforward. However, the sequential shift of ν(CC)
to lower wavenumber, observed in the Raman data for
[{Re(PPh3)(NO)Cp*}2(μ-C4)]n+ and indicative of the pro-
gressive evolution of cumulenic character to the all-carbon
bridge with increasing n, appears not to be a feature of [3]n+.
Electronic Spectroscopy. The UV−vis−NIR spectra of
[3]n+ (n = 0, 1, or 2) collected by spectroelectrochemical
methods are presented in Figure 5. The interconversion
observed in the range 18 000−16 000 cm−1 for the related
monometallic radical cations29,46 [Mo(CCPh)(dppe)(η-
C7H7)]+ and [Mo(CC-CCSiMe3)(dppe)(η-C7H7)]+,
which were assigned to LMCT transitions from the carbon
chain to the formally oxidized Mo(dppe)(η-C7H7) unit. While
similar bands can be observed, albeit with less resolution, in the
UV−vis spectrum of [3]+, the most significant spectroscopic
features of the monocation are found in the NIR region of the
spectrum. Two band envelopes are evident in the NIR
spectrum, a relatively sharp absorption at 8650 cm−1 (ε 8300
M−1 cm−1) and a much broader feature centered at 4020 cm−1
(ε 2900 M−1 cm−1), which tails into the IR region (see Figures
4, 5, and 6).
Figure 6. Near-IR−IR region of [3]PF6 in CH2Cl2, showing the
deconvolution into the sum of four Gaussian-shaped absorption bands.
To assist in the interpretation of the NIR data, spectra of
isolated samples of [3]PF6 were collected in a range of solvents
and spectral deconvolution in the region 12 000 to 2000 cm−1
was carried out (Figure 6 and Table 5).
Deconvolution of the spectrum recorded in CH2Cl2 reveals a
total of four Gaussian-shaped absorption bands centered at
3200 cm−1, IT(1); 4400 cm−1, IT(2); 8500 cm−1, LMCT; and
9300 cm−1, IT(3). The form of the spectrum is sensitive to
solvent polarity, and therefore the deconvolution procedure
was repeated for spectra obtained in MeCN and acetone;
results are summarized in Table 5. The bands labeled IT(1),
IT(2), and IT(3) exhibit strong solvatochromic dependence;
for example IT(2) shifts to high energy by approximately 1500
cm−1 between CH2Cl2 and acetone. By contrast, the more
intense band at 8500 cm−1 was relatively insensitive to solvent
variation with a shift not exceeding 300 cm−1. The
interpretation and assignment of the NIR region of the
spectrum of mixed valence complexes presents a significant
challenge especially in systems featuring heavier transition
metals, due to multiple absorptions arising from the non-
degenerate character of the metal d orbitals.47,48 Assuming a
description of the electronic transitions in terms of metal-
localized oxidation events, the three lower intensity, solvent-
sensitive NIR transitions observed for [3]PF6 [IT(1), IT(2),
and IT(3)] can be assigned to three out of the five possible
transitions comprising three intervalence charge transfer
(IVCT) (or MMCT, metal−metal charge transfer) bands and
two interconfigurational (or metal-localized dd) transitions.47
Figure 5. Spectroelectrochemically generated UV−vis−NIR spectra of
3 (solid line), [3]+ (broken line), and [3]2+ (dotted line) in CH2Cl2/
0.1 M [Bu4 N]PF6.
n
between these redox states was fully reversible with recovery
of spectra on stepwise oxidation or reduction.
The UV−vis−NIR spectrum of 3 is dominated by an
absorption at 23 600 cm−1/423 nm (ε 17 100 M−1 cm−1),
which was assigned (with reference to the spectrum of
[Mo(CCPh)(dppe)(η-C7H7)])29 to transitions from the
metal (or mixed metal and carbon chain) to π* acceptor
orbitals on the C4 ligand, which may be denoted as a mixed
MLCT/ILCT transition. Similar transitions, albeit less well
resolved, can also be observed in the spectrum of [Mo(C
CCCSiMe3)(dppe)(η-C7H7)].46 In the dication [3][PF6]2,
the principal absorptions of interest are at 20 500 cm−1/487 nm
(ε 20 500 M−1 cm−1) and 15 300 cm−1/653 nm (ε 11 300 M−1
cm−1). These bands are similar in structure to absorptions
162
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
Table 5. Analysis of the NIR Region of the Spectrum of [3]PF6
CH2Cl2
MeCN
acetone
a
a
band
ν/cm−1
Δν1/2/cm−1
ε/M−1 cm−1
ν/cm−1
Δν1/2/cm−1
A
ν/cm−1
Δν1/2/cm−1
A
IT(1)
IT(2)
IT(3)
LMCT
3200
4400
9300
8500
1200
1500
1370
900
870
1750
1750
8440
3500
5300
1600
1900
1400
1100
0.04
0.04
0.04
0.17
3900
5900
2000
2000
1400
1200
0.06
0.05
0.04
0.21
10 100
8750
10 250
8650
a
A = absorbance. Samples of [3]PF6 were not fully soluble in MeCN or acetone, and therefore accurate concentrations and extinction coefficients
could not be obtained in these solvents.
29,46,52
1
Given the low intensity of dd bands associated with [Mo(C
CR)(dppe)(η-C7H7)]+ (R = aryl) cations,29 transitions IT(1),
IT(2), and IT(3) are assigned to IVCT processes. The
remaining band at 8500 cm−1 is distinct in terms of the small
Δν1/2 parameter, relatively high extinction coefficient, and
limited solvatochromic dependence (Figure 6, Table 5) and is
therefore provisionally assigned to a LMCT process.
hyperfine couplings to 95/97Mo, 31P, and H(C7H7),
and
therefore the application of this technique to [3]+ and [3]2+
may provide useful data on the location of unpaired spin
density in these systems. The X-band, CH2Cl2 solution EPR
spectrum of the monocation [3]PF6, recorded at 243 K, is
shown in Figure 7. With the exception of the broad shoulder
The low-energy and relatively small extinction coefficients
(ca. 103 M−1 cm−1) of absorptions IT(1), IT(2), and IT(3) in
[3]PF6 are similar to IVCT bands in the heterobimetallic
complexes [{Fe(dppe)Cp*}(μ-C4){MLx}]+ (MLx = Re(NO)-
(PPh3)Cp* or Ru(dppe)Cp*),15,17 indicative of rather weak
electronic coupling in [3]PF6. However further analysis of
bands IT(1), IT(2), and IT(3) reveals that the band widths are
narrower than predicted from the two-state model of
Hush.28b,49 For example, for band IT(1) in CH2Cl2, the
experimental Δν1/2 parameter (1200 cm−1) is significantly
narrower than the calculated Hush value (2720 cm−1).50 IVCT
bands that are narrower than calculated from the Hush model
indicate deviations from the localized two-state model and are
often interpreted in terms of electronically delocalized class III
behavior. However, for [3]PF6 the NIR band envelopes of the
three IVCT bands exhibit pronounced solvatochromic shifts,
consistent with an assignment of these transitions as charge
transfer bands and necessitating a description of [3]PF6 in
terms of localized oxidation states, which is also in keeping with
the IR and Raman data. The deviations in band shape from the
predictions of the two-state model are therefore attributed to
involvement of the bridging ligand in the charge transfer
process, which has been shown elsewhere to result in significant
narrowing of the band shape.49,51
EPR Spectroscopy and Magnetochemistry. The char-
acterization of [{LxM}(μ-C4){MLx}]+ by EPR spectroscopy
can in principle provide information on both the location of the
unpaired electron and the rate of electron transfer in formally
mixed valence systems, especially when used in concert with
other spectroscopic methods.7,15 For example, in the rhenium
complex [{Re(NO)(PPh3)Cp*}2(μ-C4)]+, it was possible to
resolve the hyperfine coupling arising from two rhenium
centers (I = 5/2), rendered equivalent by electron transfer
between the two metal end-caps at a rate faster than the EPR
time scale. Moreover Aiso(Re) values were one-half of those
observed for typical related monometallic radicals, indicating
full delocalization of the odd electron between the two Re
centers on the time scale of X-band EPR spectroscopy.7
Conversely, the broad EPR signal for [{WI(dppe)2}2(μ-C4)]+,
observed only at temperatures below 160 K, is consistent with
electron transfer between the tungsten end-caps at a rate
comparable with the EPR time scale.14
Figure 7. X-band, CH2Cl2 solution spectrum of [3]PF6 (recorded as a
first derivative at 243 K). giso = 1.991.
95/97
features assigned to coupling to
Mo, no hyperfine data
could be extracted from the spectrum; attempts to improve
resolution by variable-temperature investigations (193−303 K)
and frequency variation (X-, S-, Q-band) were unsuccessful.
The poor resolution of the EPR spectrum of [3]PF6 is in
contrast to all other radicals derived from ynyl complexes of
this metal−ligand system. However, the broad character of the
spectrum can be rationalized in terms of an electron transfer
rate between the two metal centers of [3]PF6 that is
comparable with the X-band EPR time scale. In support of
this interpretation, the hyperfine coupling to 95/97Mo was
estimated from both X- and S-band spectra as approximately 16
G, around one-half of the typical value of Aiso (
95/97Mo) in
monometallic radicals of the type [Mo{(CC)nR}(dppe)(η-
C7H7)]+. On the basis of the EPR investigations, the
intramolecular electron transfer rate in [3]PF6 is estimated to
lie in the range 108−1010 s−1. This is slower than solvent
reorientation frequencies (1011−1012 s−1) and typical bond
vibration frequencies (1013−1014 s−1) consistent with the
observed, valence-localized NIR and IR properties of [3]PF6
(pronounced solvatochromic shifts and two IR-active bridging
ligand vibrational modes).
The dication [3][PF6]2 was also investigated by EPR
spectroscopy to probe the position of the thermal equilibrium
between singlet and triplet configurations. In selected cases of
all-carbon-bridged bimetallic dications, where the singlet/triplet
energy gap is relatively small,7,53 EPR spectra with accompany-
Monometallic carbon chain radicals of the type [Mo{(C
C)nR}(dppe)(η-C7H7)]+ (n = 1 or 2; R = H, aryl, SiMe3)
exhibit X-band EPR solution spectra with well-resolved
163
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
ing half-field signals have been observed, although the spectra
are broadened as a consequence of multiple unpaired electrons
in the molecule, which greatly shorten relaxation times. The X-
band EPR spectra of isolated samples of [3][PF6]2, which
exhibit a strong, well-resolved signal [giso 1.996; Aiso(95/97 Mo)
32 G; Aiso(31P) 23 G; Aiso(H) 4.3 G] when recorded in solution
in CH2Cl2 over the temperature range 213−243 K, are
therefore not consistent with a thermally populated triplet
diradical state. Instead the observed spectra are indicative of the
presence of a monometallic S = 1/2 impurity within the sample
of [3][PF6]2. Variable-temperature EPR studies on frozen
solution or solid-state samples of [3][PF6]2 in the temperature
range 80−5 K demonstrated the retention of the EPR signature
characteristic of the paramagnetic S = 1/2 species, and in
consequence, it was not possible to explore the behavior of
weak, broad signals expected to arise from the triplet
configuration of [3][PF6]2.
assumed to be monometallic Mo species with S = 1/2.
2 2
2Ng β
1
χ
=
(1 − ρ)
M
k (T − θ) 3 + exp( − J/k T)
B
B
2 2
Ng β
2k (T − θ)
+
ρ + TIP
(1)
B
In eq 1, J represents the exchange coupling constant that
accounts for magnetic coupling between Mo(1) and Mo(2)
(i.e., the singlet−triplet energy gap) and N, g, kB, and β
represent the Avogadro number, the Zeeman factor, the
Boltzmann constant, and the Bohr magneton, respectively. A
Weiss constant θ was incorporated into the analysis to take
account of weak intermolecular interactions that occur at low
temperatures, and a correction was also made for temperature-
independent paramagnetism (TIP). The best fit of both χM and
χMT, using eq 1 gave J = −406 3 cm−1, θ = −0.95 0.02 K,
and ρ = 0.16, with g = 1.996 (fixed, based on EPR studies),
R(χMT) = ∑(χMTobs − χMTcalc)2/∑(χMTobs)2 = 3.1 × 10−6, and
TIP = 120 × 10−6 cm3 mol−1. The results show that the
magnetic interaction between Mo(1) and Mo(2) is anti-
ferromagnetic, with a large exchange coupling constant (J =
−406 cm−1) and a corresponding singlet−triplet energy gap
(ΔGST = 406 cm−1). The singlet−triplet energy gap determined
for [3][PF6]2 is much larger than values reported for
[{Fe(dppe)Cp*}2(μ-C4)][PF6]2 (ΔGST = 18 cm−1)53,54 and
[{WI(dppe)}2(μ-C4)][PF6]2 (ΔGST = 167 cm−1)14 but rather
smaller than that of the ruthenium complex [{Ru(dppe)-
Cp*}2(μ-C4)][PF6]2 (ΔGST = 850 cm−1).17 Singlet−triplet
energy gaps with magnitudes greater than 700 cm−1 are
generally observed for 4d/5d metal centers linked by a C4
bridge;53 the smaller J value observed for [3][PF6]2 is
consistent with differences in electronic structure (see below).
Electronic Structure Calculations. The compound
[{Mo(dHpe)(η-C7H7)}2(μ-CCCC)] (3-H) was used to
model complex 3, with geometry optimization and electronic
structure calculations being carried out with the B3LYP/
LANL2DZ functional and basis set combination in line with
literature precedence and management of computational effort.
The geometry was optimized with no symmetry restrictions
and confirmed to be a minimum energy conformation by the
lack of imaginary frequencies. The optimized geometry of 3-H
(Table 6) correlates well with the expected butadiyndiyl
To probe further the energy gap between singlet and
thermally populated triplet states of [3][PF6]2, magnetic
susceptibility measurements were carried out on polycrystalline
samples of [3][PF6]2 in the temperature range 2−300 K, under
applied magnetic fields of 1 and 5 kG. The results, which are
plotted as χM vs T and χMT vs T in Figure 8 (χM = molar
Figure 8. Plots of χM (• • •) and χMT (■■■) vs T for compound
[3][PF6]2. The solid line represents the best fit of experimental data to
eq 1 (see text).
magnetic susceptibility), demonstrate strong antiferromagnetic
coupling between the unpaired electrons of the two Mo centers
in [3][PF6]2, which leads to a diamagnetic ground state below
80 K and a thermally accessible paramagnetic triplet state.
As seen in Figure 8, the room-temperature χMT value is 0.4
cm3 K mol−1, which is much smaller than the expected value of
0.75 cm3 K mol−1 for two S = 1/2 uncorrelated spins (assuming
g = 2). Upon cooling, χMT decreases rapidly to reach a plateau
of ca. 0.12 cm3 K mol−1 in the temperature range 80−20 K,
after which it decreases again to a value of 0.09 cm3 K mol−1 at
2 K. This behavior is characteristic of a large singlet−triplet
energy gap. The plateau of χMT (or the increase of the
magnetic susceptibility, χM) below 100 K is due to the
proportion, ρ, of an uncoupled, monometallic paramagnetic
impurity, in accord with the experimental EPR spectra; related
uncoupled impurities have also been noted for the di-iron
complexes [{Fe(dppe)Cp*}2(μ-C4)][PF6]2 and [{Fe(dppe)-
Cp*}2(μ-1,4-diethynylbenzene)][PF6]2.54,55 The experimental
data were therefore fitted using the Bleaney−Bowers equation
(1),56 modified to take into account paramagnetic impurities
Table 6. Selected Calculated Bond Lengths for 3-H
bond
3-H (Å)
2.089
Mo(1)−C(α)
C(α)−C(β)
C(β)−C(β′)
C(β′)−C(α′)
C(α′)−Mo(2)
Mo(1)−P
1.260
1.373
1.260
2.087
2.544, 2.544
2.546, 2.533
Mo(2)−P
valence description with an alternating pattern of single and
triple bonds across the length of the four-carbon chain and
bond parameters at each molybdenum center similar to those
calculated elsewhere29 for alkynyl complexes of the Mo(dppe)-
(η-C7H7) auxiliary. The Mo(dHpe)(η-C7H7) fragments are
disposed in a gauche arrangement with the angle Ct(1)−
Mo···Mo−Ct(2) = −82.7°. The energies and composition of
164
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
Table 7. Energy and % Composition of the Frontier Orbitals in the Calculated Complex 3-H
MO
energy/eV
Mo(1)
C(α)
C(β)
C7H7(1)
dHpe(1)
Mo(2)
C(α′)
C(β′)
C7H7(2)
dHpe(2)
M2
C4
LUMO
−0.76
−3.67
−3.74
−4.27
18
24
17
39
1
11
12
4
0
10
8
33
7
12
2
11
17
24
15
0
12
11
2
1
8
9
8
18
8
6
2
2
3
29
41
41
54
2
41
40
14
HOMO
HOMO−1
HOMO−2
9
2
7
0
5
4
20
the key frontier orbitals (LUMO, HOMO, HOMO−1, and
HOMO−2) are set out in Table 7, and plots of HOMO and
HOMO−1 of 3-H are presented in Figure 9.
through the C4 bridge in the dication [3][PF6]2 is more
significant. This feature may originate from a HOMO in
[3][PF6]2 related to HOMO−1 of [Mo(CCPh)(dppe)(η-
C7H7)] (Figure 2), which exhibits efficient in-plane overlap
between the metal and carbon chain orbitals.29 Nevertheless the
J value for [3][PF6]2 is less than one-half of that determined for
[{Ru(dppe)Cp*}2(μ-C4)][PF6]2, and this may correlate with
greater spin density at the metal centers in the Mo system.17,53
DISCUSSION
■
Figure 9. HOMO and HOMO−1 of 3-H at B3LYP/LANL2DZ
plotted as an isosurface of 0.04 au.
Symmetrical all-carbon-bridged complexes of the type [{LxM}-
(μ-C4){MLx}]n+ are known for a wide range of metal end-caps
MLx. Important distinctions between the redox chemistry of
these systems, manifest in the spectroscopic and structural
properties of the dications, may be observed dependent upon
the identity of the metal end-cap. However, nearly all examples
of the mixed valence monocations (n = 1) feature either fast
electron transfer within the framework of two-state mixed
valence complexes, delocalization, or significant ligand-based
redox chemistry. In this context, the properties of [3]PF6, as
evidenced by IR, NIR, and EPR investigations, are clearly
distinct such that the series of complexes [{Mo(dppe)(η-
C7H7)}2(μ-C4)]n+ represent a separate subclass within the
chemistry of [{LxM}(μ-C4){MLx}]n+ systems.
Taken as a group, the spectroscopic properties of [3]PF6 are
indicative of a localized electronic structure, and in terms of the
classical two-state model for charge transfer transitions,28,49
[3]PF6 appears to rest firmly in the class II region. However,
the observed characteristics of the NIR spectrum of [3]PF6
cannot be fully rationalized by a conventional Hush-style two-
state analysis. It has been suggested that the two-state model
has serious limitations where bridge orbitals are significantly
involved in the semioccupied molecular orbitals of the mixed
valence system,11 a circumstance that, on the basis of
spectroscopic and DFT investigations,16,17 clearly applies to
many complexes of the type [{LxM}(μ-C4){MLx}]+. An
alternative three-state model,49,51,58−61 in which electron
transfer can be mediated through the additional mixing of a
third electronic state involving the bridging ligand, may have
some validity for these systems. Depending on the relative
energies of product/reactant versus bridge states, electron
transfer may occur through a superexchange or hopping
process; in the superexchange mechanism, the bridge acts only
to mediate donor and acceptor wave functions, whereas in the
hopping mechanism, the electron is actually located at the
bridge for a short period in its passage from one redox center to
the other.58 The key spectral features of the theoretical three-
state model, which distinguish it from the classical two-state
treatment of mixed valence systems, are (i) a reduction in
bandwidth Δν1/2 of IVCT bands for a given value of electronic
coupling and (ii) an additional absorption to high energy of the
IVCT band originating from a MLCT/LMCT transition; in the
class II region, the intensity of this band initially increases with
increased electronic coupling but then decreases to zero as the
class II/class III borderline is approached.49 For this reason, the
The HOMO and HOMO−1 of 3-H have very similar
compositions of metal (41%) and C4 carbon bridge (40−41%)
character. They are almost degenerate in energy and well
removed from the other frontier orbitals (Table 7). Taking the
Mo−Ct and Mo−C(α) vectors as approximating the z- and x-
axes, respectively, to define a local coordinate system at each
2
2
metal center, these orbitals feature an essentially dz /C4(π)/dz
structure (for both HOMO and HOMO−1, the identity of one
2
of the metal orbitals is less clearly defined, but assignment as dz
is indicated by the alignment of the principal orbital lobe along
the z-axis toward the center of the C7H7 ring). The HOMO
and HOMO−1 of 3-H (Figure 9) may be compared with the
equivalent molecular orbitals of [{Ru(dHpe)Cp}2(μ-CC−
CC)] (see Figure 1). The C4 bridge orbitals for the two
systems are essentially identical, but a key distinction is the
identity of the frontier metal orbitals contributing to the
interaction with the bridge orbitals. Thus the efficient in-plane
interactions of [{Ru(dHpe)Cp}2(μ-CC−CC)] are re-
placed in 3-H by a symmetry-constrained overlap between the
2
π orbitals of the C4 bridge and the dz -based frontier orbitals of
the Mo(dppe)(η-C7H7) fragment. As a result, the HOMO and
HOMO−1 of 3-H are strongly metal based in character (Table
7); for comparison DFT calculations on [{M(dHpe)Cp}2(μ-
CC-CC)] (M = Fe, Ru) using the ADF57 program report
the % metal to % C4 chain character in the HOMO of these
systems as follows: M = Fe: M2, 41%, C4, 46%; M = Ru: M2 =
26%, C4 = 62% (see Figure 1).17
Attempts to model the oxidized complexes [3]PF6 and
[3][PF6]2 to reflect accurately the observed experimental
properties of these systems were not successful at the levels of
theory employed; higher levels of theory were not explored for
these open-shell systems on the grounds of computational
expense, and accurate computational modeling of localized
mixed valence systems remains a significant challenge.19
However, it is suggested that the weak electronic coupling
between metal centers in the mixed vaence monocation [3]PF6
2
can be attributed to the modest interaction between dz -based
metal frontier orbitals and the all-carbon bridge; a related,
symmetry-restricted interaction between the metal center and
C4 bridge has been proposed for [{WI(dppe)2}2(μ-C4)]n+.14 By
contrast, on the basis of the large value of the exchange
coupling constant J (−406 3 cm−1), the magnetic interaction
165
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
NIR spectrum of a weakly coupled, symmetrical class II system
such as [3]PF6 presents an opportunity to investigate
experimentally the validity of a three-state model for electron
transfer in all-carbon-bridged organometallics because the
simultaneous observation of IVCT and MLCT/LMCT
transitions, diagnostic of the model, may be possible.
formed by charge transfer to or from the bridging C4 ligand.
Analysis of the experimental NIR spectrum of [3]PF6 reveals a
good correlation with the predicted spectral properties for a
three-state model description of electron transfer in a weakly
coupled, symmetrical mixed valence system. The dicationic
complex [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6]2, [3][PF6]2, ex-
hibits strong antiferromagnetic coupling with a singlet/triplet
energy gap of 406 cm−1. However the magnitude of the
magnetic coupling in [3][PF6]2 is less than one-half of that
reported for [{Ru(dppe)Cp*)}2(μ-C4)][PF6]2, and X-ray
structural data for [3][PF6]2 are not consistent with significant
cumulenic character in the C4 bridge. Overall the properties of
[{Mo(dppe)(η-C7H7)}2(μ-C4)]n+ are atypical of all-carbon-
bridged bimetallics of 4d/5d metals with both electronic and
magnetic coupling through the C4 bridge weaker than expected.
The electronic structure of the Mo(dppe)(η-C7H7) end-cap,
which serves to enhance the metal-based character of frontier
orbitals, appears to play an important role in regulating the
magnitude of through-bridge interactions in these complexes.
The experimental NIR spectrum of [3]PF6 can be
interpreted in terms of the predicted spectral characteristics
of a bridge-mediated, three-state model for electron transfer in
a weakly coupled, symmetrical mixed valence complex as
outlined below. The observation of three IVCT bands (at 3200,
4400, and 9300 cm−1 in CH2Cl2) in the experimental spectrum
of [3]PF6 is readily explained by the nondegeneracy of the
metal d orbitals arising from strong π and δ interactions with
the C7H7 ring in the Mo(dppe)(η-C7H7) end-cap units.29 Each
of the three observed IVCT bands exhibits a Δν1/2 parameter
narrower than predicted by the classical two-state analysis in
accord with a three-state interpretation. However the key issue
is the assignment of the fourth band in the NIR region at 8500
cm−1 (distinguished by its narrow bandwidth, relatively high
intensity, and very small solvatochromic shift). One possible
interpretation is that this much sharper and intense absorption
originates from an LMCT process from the bridging ligand.
The observation of this band is an integral part of the
predictions of a three-state model for electron transfer in a
weakly coupled class II system, and it may be noted that an
analogous absorption is not seen in the NIR spectrum of the
more weakly coupled 1,12-bis(ethynyl)-1,12-carborane-bridged
analogue [{Mo(dppe)(η-C7H7)}2(μ-1,12-(CC)2-1,12-
C2B10H10}]+, where the energies of the carborane bridge π
and π* levels are well removed from the metal orbitals in the
frontier region.62 Experimental evidence for the simultaneous
observation of IVCT and MLCT/LMCT transitions in the NIR
spectra of all-carbon-bridged organometallic systems is very
limited,63 possibly because the strong electronic coupling,
characteristic of these complexes, leads to zero intensity of the
MLCT/LMCT transition or because the individual compo-
nents of the spectra are superimposed and cannot be reliably
identified. The design of further examples of weakly coupled,
symmetrical all-carbon-bridged complexes may therefore be a
useful strategy in the study of the mechanism of intervalence
electron transfer in these systems.
EXPERIMENTAL SECTION
■
General Procedures. The preparation, purification, and reactions
of the complexes described were carried out under dry nitrogen. All
solvents were dried by standard methods, distilled, and deoxygenated
before use. The complex [MoBr(dppe)(η-C7H7)]·0.5CH2Cl2 was
prepared by a published procedure.37 NMR spectra were recorded in
CD2Cl2 unless stated otherwise on a Varian Inova 400 (400 MHz 1H,
100 MHz 13C{1H}, 162 MHz 31P{1H}) spectrometer. Solution
infrared spectra were obtained on a Perkin-Elmer FT RX1
spectrometer, and solid-state spectra were recorded on a Thermo-
scientific Nicolet iS5 spectrometer with ATR fitment; MALDI mass
spectra were recorded using a Micromass/Waters Tof Spec 2E
instrument. Microanalyses were conducted by the staff of the
Microanalytical Service of the School of Chemistry, University of
Manchester. Cyclic voltammograms were recorded (ν = 100 mV s−1)
n
from 0.2 M [Bu4 N]PF6/CH2Cl2 solutions ca. 1 × 10−4 M in analyte
using a three-electrode cell equipped with a glassy carbon disk working
electrode, Pt wire counter electrode, and Ag/AgCl reference electrode,
and data were collected on an Autolab PG-STAT 30 potentiostat. All
redox potentials are reported with reference to an internal standard of
the ferrocene/ferrocenium couple (FeCp2/[FeCp2]+ = 0.00 V). UV−
vis−NIR and IR spectroelectrochemical experiments were performed
at room temperature with an airtight OTTLE cell of Hartl design64
equipped with Pt minigrid working and counter electrodes, a Ag wire
reference electrode, and CaF2 windows using either a Nicolet Avatar
spectrometer or a Perkin-Elmer Lambda 900 spectrophotometer.
Raman spectra were recorded using a Horiba Jobin-Yvon LabRamHR,
equipped with a 633 nm laser, augmented by additional 532 nm
(Coherent DPSS) and 785 nm (Sacher) excitation lasers. The
instrument was calibrated using a neon lamp, and the Raman shifts are
reported to an accuracy of 1 cm−1. EPR experiments were conducted
on a Bruker BioSPin EMX microspectrometer; spectra are the average
of 16 scans. Spectral analysis and simulation was carried out using
Bruker WinEPR software (Bruker Biospin Ltd.). Magnetic suscepti-
bility measurements were carried out on polycrystalline samples in the
temperature range 2−300 K, using a Quantum Design MPMS XL
SQUID magnetometer equipped with a 7 T magnet. Experimental
magnetic data were corrected for the diamagnetism of the ligands by
using Pascal constants and for the magnetic contribution of the sample
holder by measurement.
CONCLUSIONS
■
Oxidative coupling of [Mo(CCH)(dppe)(η-C7H7)] to give
[{Mo(dppe)(η-C7H7)}2(μ-CCH-CHC)][PF6]2 followed
by deprotonation affords mixed valence all-carbon-bridged
[{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6], [3]PF6, as a mixture
with neutral [{Mo(dppe)(η-C7H7)}2(μ-CC−CC)], 3.
Electrochemical investigations reveal a series of redox processes
indicative of the formation of [{Mo(dppe)(η-C7H7)}2(μ-C4)]n+
(n = 0, 1, 2, 3, 4), and the redox states n = 0, 1, and 2 have been
isolated and characterized. Spectroscopic investigations on
[3]PF6 by IR, Raman, NIR, and EPR spectroscopy uncover
properties characteristic of a localized mixed valence system
with an intramolecular electron transfer rate comparable with
the X-band EPR time scale (108−1010 s−1). This behavior
contrasts with the properties of most previously reported mixed
valence [{LxM}(μ-C4){MLx}]+ systems, for which either a fully
delocalized electronic structure or bridge-centered redox
chemistry is observed. The weak electronic coupling observed
for [3]PF6 presents the opportunity to examine the validity of a
three-state model for electron transfer that involves a third state
Preparation of [Mo(CCH2)(dppe)(η-C7H7)]PF6. A mixture of
[MoBr(dppe)(η-C7H7)]·0.5CH2Cl2 (1.50 g, 2.12 mmol), KPF6 (0.82
g, 4.46 mmol), and HCCSiMe3 (1.04 g, 10.61 mmol) in methanol
(50 cm3) was heated at reflux for 3 h. The reaction mixture was
reduced in volume to ca. 20 cm3, resulting in the formation of an
orange precipitate, and then cooled to −20 °C for 1 h. The precipitate
was collected and recrystallized from CH2Cl2/diethyl ether to give the
166
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
product as an orange solid, which was dried in vacuo (yield: 1.11 g
(69%)) and identified by comparison with an authentic sample.36
Preparation of [Mo(CCH)(dppe)(C7H7)], 1. [Mo(CCH2)-
(dppe)(η-C7H7)]PF6 (1.08 g, 1.43 mmol) was suspended in thf (30
cm3) and cooled to −30 °C. KOBut (1.28 g, 11.42 mmol) was added,
and the mixture was stirred at −30 °C for 10 min and then at room
temperature for 1 h. The resulting brown solution was evaporated to
dryness, and the residue dissolved in CH2Cl2 (ca. 20 cm3) and stirred
for 5 min. The resulting suspension was filtered through a Celite pad
(2 cm diameter × 2.5 cm deep), and the filtrate treated with hexane.
The volume of the solution was then reduced to give a brown
precipitate, which was collected, washed with hexane, and dried in
Computational Details. All calculations were carried out using
the Gaussian 03 package.65 The model geometry of 3-H was optimized
using B3LYP66 and MPW1K,67,68 with no symmetry constraints. The
pseudopotential LANL2DZ69−71 was used for all atoms. Frequency
calculations were carried out on these optimized geometries at the
corresponding levels and shown to have no imaginary frequencies.
Molecular orbital computations were carried out on these optimized
geometries at the same level of theory and displayed using Gauss View
4.1.72 The orbital compositions were generated with the aid of
GaussSum.73
X-ray Crystal Structure of [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6]2,
[3][PF6]2. Single crystals of [3][PF6]2 were obtained as brown-green
blocks by vapor diffusion of diethyl ether into a MeCN solution of the
complex, and a crystal of dimensions 0.40 × 0.30 × 0.20 mm was
selected for analysis. Single-crystal X-ray data were collected at 100 K
on an Oxford Diffraction X-Calibur 2 diffractometer equipped with an
Oxford-Cryosystems low-temperature device, by a means of Mo Kα (λ
= 0.71069 Å) radiation and ω scans. Data were corrected for Lorentz,
polarization, and absorption factors. Data collection, cell refinement,
and data reduction were carried out with CrysAlis CCD and CrysAlis
RED, Oxford Diffraction Ltd. software; SHELXS-9774 was employed
for computing the structure solution, and SHELXL-9775 for
computing structure refinement. The structure was solved by direct
methods with refinement based on F2. The asymmetric unit of
[3][PF6]2 contains the Mo complex, two PF6 counteranions, one
ordered MeCN solvent of crystallization, and a disordered solvent
fragment defined as 2 MeCN molecules at 0.25 occupancy and one
H2O at 0.25 occupancy. Except as detailed below, all non-hydrogen
atoms were refined anisotropically; hydrogen atoms were included in
calculated positions, except those of the disordered solvent. A phenyl
group of a dppe ligand C(51)−C(56) and a PF6 counterion, P(2s),
F(7s)−F(12s), were disordered. The disordered component of the Ph
group has been constrained to be a regular hexagon, and there are
restraints on the geometry of the disordered PF6 and some of the
anisotropic displacement parameters. The anisotropic displacement
parameters of the C(1)−C(7) atoms in the cycloheptatrienyl ring were
disordered and restrained since they tended to become unrealistic.
1
vacuo; yield 685 mg (78%). H NMR: δ 1.74, t, J(P−H) 4 Hz, 1H,
CCH; 1.88, m, 2H, CH2; 2.50, m, 2H, CH2; 4.68, s, 7H, C7H7;
7.22−7.73, m, 20H, Ph. 13C{1H} NMR: 140.2, m, 135.8, m, Phi; 133.3,
m, 131.3, m, Pho; 130.1, t, J(P−C) 25 Hz, Cα; 128.8, s, 128.4, s, Php;
127.6, m, 127.1, m, Phm; 104.3, s, Cβ; 86.5, s, C7H7; 25.8, m, CH2
(dppe). 31P{1H} NMR: δ 63.8 (s, dppe). IR (CH2Cl2): ν(CC) 1908
cm−1, ν(C2−H) 3265 cm−1. MALDI-MS (m/z): 612 [M]+. Anal.
Calcd (%) for C35H32MoP2: C, 68.9; H, 5.3. Found: C, 68.2; H, 5.3.
Preparation of [{Mo(dppe)(η-C7H7)}2(μ-CCH-CHC)][PF6]2, [2]-
[PF6]2. To a cooled solution (−78 °C) of [Mo(CCH)(dppe)(η-
C7H7)] (600 mg, 0.980 mmol) in thf (5 cm3) was added [FeCp2]PF6
(308 mg, 0.931 mmol). The reaction mixture was stirred at −78 °C for
5 h to give a deep purple solution. Addition of diethyl ether (30 cm3)
resulted in precipitation of the crude product as a deep purple solid.
Recrystallization from CH2Cl2/diethyl ether allowed separation of the
more soluble side-product [Mo(CCH2)(dppe)(η-C7H7)]PF6 from
the required complex [2][PF6]2, which precipitated from the solution
first; yield 594 mg (80%). 1H NMR: δ 2.33, m, 4H, CH2; 2.51, m, 4H,
CH2; 4.41, t, br, 2H, J(H−P) ≈ 11 Hz, CCH; 5.07, s, 14H, C7H7;
7.16−7.71, m, 40H, Ph. 13C{1H}NMR: 134.4, 132.1, 131.1, 129.6,
129.5, Ph (dppe); 108.3 Cβ; 93.2, C7H7; 27.8, m, CH2 (dppe). 31P
NMR: δ 51.3 (s, dppe). IR (CH2Cl2): ν(CC) 1556 cm−1. Anal.
Calcd (%) for C70H64Mo2P6F12: C, 55.6; H, 4.3. Found: C, 55.1; H,
4.2.
Preparation of [{Mo(dppe)(η-C7H7)}2(μ-CC-CC)], 3, and
[{Mo(dppe)(η-C7H7)}2(μ-C4)]PF6, [3]PF6. [{Mo(dppe)(η-C7H7)}2(μ-
CCH−CHC)][PF6]2 (580 mg, 0.384 mmol) was dissolved in
thf (30 cm3), and KOBut (172 mg, 1.536 mmol) was added to the
solution; the resulting green-brown suspension was then stirred at
room temperature for 5 h before evaporating to dryness. The residue
was dissolved in CH2Cl2 (20 cm3) and filtered through a pad of Celite,
and the filtrate evaporated to dryness. The resulting residue was
washed with toluene (20 cm3), and the washings were removed and
retained. The remaining toluene-insoluble material was dried, then
recrystallized from CH2Cl2/hexane to give [3]PF6 as a brown solid,
which was collected, washed with further hexane, and dried in vacuo;
yield 230 mg (44%). IR (CH2Cl2): ν(CC) 1930, 1868 cm−1.
MALDI-MS (m/z): 1220 [M]+. Anal. Calcd (%) for C70H62Mo2P5F6:
C, 61.6; H, 4.6. Found: C, 61.9; H, 4.8. The mother liquors remaining
from the isolation of [3]PF6 and the toluene washings were combined
and evaporated to dryness. The resulting residue was extracted with
toluene and filtered through a pad of Celite, and the solvent removed.
Fractional recrystallization of the residue from CH2Cl2/hexane led first
to precipitation of residual [3]PF6 to leave an orange-brown solution,
which on reduction in volume and addition of further hexane yielded 3
as an orange-brown solid; yield 0.025 g (5%). MALDI-MS (m/z):
1220 [M]+. Anal. Calcd (%) for C70H62Mo2P4: C, 69.0; H, 5.1. Found:
C, 69.4; H, 5.3.
Crystal Data for [3][PF6]2: C73H67Mo2N1.5O0.25P6F12, Mr
=
1574.98, monoclinic, space group C2/c, a = 58.826(3) Å, b =
11.7374(9) Å, c = 19.7396(11) Å, β = 97.174(5)°, U = 13522.9(15)
Å3, Z = 8, μ = 0.591 mm−1, 11 910 reflections collected, final wR2(F2)
= 0.1260 for all data, conventional R1 = 0.0605 for 6051 reflections
with I > 2σ(I), completeness to theta = 99.8%.
ASSOCIATED CONTENT
■
S
* Supporting Information
Cyclic voltammogram of [3]PF6, Raman spectra of [3]n+ (n =
0, 1, 2), ATR infrared spectra of solid-state samples of [3]n+ (n
= 1, 2), expansion of plots of χM and χMT vs T for [3][PF6]2.
CIF files giving crystallographic data for complex [3][PF6]2.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: p.j.low@durham.ac.uk; Mark.Whiteley@manchester.
ac.uk.
ACKNOWLEDGMENTS
Preparation of [{Mo(dppe)(η-C7H7)}2(μ-C4)][PF6]2, [3][PF6]2. A
brown-green solution of [{Mo(dppe)(η-C7H7)}2(μ-C4)]PF6 (83 mg,
0.061 mmol) in CH2Cl2 (20 cm3) was treated with [FeCp2]PF6 (44
mg, 0.133 mmol). The reaction mixture was stirred for 20 min,
resulting in the formation of an intense green solution, which was
filtered through Celite, reduced in volume, and treated with diethyl
ether to precipitate the product as an emerald green solid; yield 57 mg
(62%). MALDI-MS (m/z): 1220 [M]+. Anal. Calcd (%) for
C70H62Mo2P6F12: C, 55.7; H, 4.1. Found: C, 54.8; H, 4.0.
■
We thank the EPSRC for support of this work through grants
EP/E025544/1 and EP/E02582X/1. P.J.L. holds an EPSRC
Leadership Fellowship.
REFERENCES
■
(1) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178−180, 431.
(2) Bruce, M. I.; Low, P. J. Adv. Organomet. Chem. 2004, 50, 179.
167
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
(3) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41,
4378.
(4) Low, P. J. Dalton Trans. 2005, 2821.
(5) Le Narvor, N.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995,
117, 7129.
(6) Guillemot, M.; Toupet, L.; Lapinte, C. Organometallics 1998, 17,
1928.
(7) (a) Brady, M.; Weng, W.; Zhou, Y.; Seyler, J. W.; Amoroso, A. J.;
Organometallics 2005, 24, 2834. (e) Valyaev, D. A.; Semeikin, O. V.;
Ustynyuk., N. A. Coord. Chem. Rev. 2004, 248, 1679. (f) Schauer, P. A.;
Low, P. J. Eur. J. Inorg. Chem. 2011, in press.
(35) Bruce, M. I.; Costuas, K.; Ellis, B. G.; Halet, J.-F.; Low, P. J.;
Moubaraki, B.; Murray, K. S.; Ouddai, N.; Perkins, G. J.; Skelton, B.
W.; White, A. H. Organometallics 2007, 26, 3735.
(36) Beddoes, R. L.; Bitcon, C.; Grime, R. W.; Ricalton, A.; Whiteley,
M. W. J. Chem. Soc., Dalton Trans. 1995, 2873.
Arif, A. M.; Bohme, M.; Frenking, G.; Gladysz, J. A. J. Am. Chem. Soc.
1997, 119, 775. (b) Herrmann, C.; Neugebauer, J.; Gladysz, J. A. Inorg.
Chem. 2005, 44, 6174.
(37) Grime, R. W.; Helliwell, M.; Hussain, Z. I.; Lancashire, H. N.;
Mason, C. R.; McDouall, J. J. W.; Mydlowski, C. M.; Whiteley, M. W.
Organometallics 2008, 27, 857.
̈
(8) (a) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.;
Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949. (b) Bruce, M. I.;
Denisovich, L. I.; Low, P. J.; Peregudova, S. M.; Ustynuk, N. A.
Mendeleev Commun. 1996, 200.
(38) Aston, G. M.; Badriya, S.; Farley, R. D.; Grime, R. W.; Ledger, S.
J.; Mabbs, F. E.; McInnes, E. J. L.; Morris, H.; Ricalton, A.; Rowlands,
C. C.; Wagner, K.; Whiteley, M. W. J. Chem. Soc., Dalton Trans. 1999,
4379.
(9) Bruce, M. I.; Ellis, B. G.; Low, P. J.; Skelton, B. W.; White, A. H.
Organometallics 2003, 22, 3184.
(10) Gao, L.-B.; Zhang, L.-Y.; Shi, L.-X.; Chen, Z.-N. Organometallics
2005, 24, 1678.
(39) Lancashire, H. N.; Ahmed, R.; Hague, T. L.; Helliwell, M.;
Hopgood, G. A.; Sharp, L.; Whiteley, M. W. J. Organomet. Chem. 2006,
691, 3617.
(40) Denis, R.; Toupet, L.; Paul, F.; Lapinte, C. Organometallics
(11) Bruce, M. I.; Costuas, K.; Davin, T.; Halet, J.-F.; Kramarczuk, K.
A.; Low, P. J.; Nicholson, B. K.; Perkins, G. J.; Roberts, R. L.; Skelton,
B. W.; Smith, M. E.; White, A. H. Dalton Trans. 2007, 5387.
(12) Bruce, M. I.; Kramarczuk, K. A.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 2010, 695, 469.
1999, 4379.
(41) Paul, F.; Ellis, B. G.; Bruce, M. I.; Toupet, L.; Roisnel, T.;
Costuas, K.; Halet, J.-F.; Lapinte, C. Organometallics 2006, 25, 649.
(42) Fox, M. A.; Roberts, R. L.; Khairul, W. M.; Hartl, F.; Low, P. J. J.
Organomet. Chem. 2007, 692, 3277.
(13) Kheradmandan, S.; Heinze, K.; Schmalle, H. W.; Berke, H.
Angew. Chem., Int. Ed. 1999, 38, 2270.
(14) Semenov, S. N.; Blacque, O.; Fox, T.; Venkatesan, K.; Berke, H.
J. Am. Chem. Soc. 2010, 132, 3115.
(15) Paul, F.; Meyer, W. E.; Toupet, L.; Jiao, H.; Gladysz, J. A.;
(43) Bitcon, C.; Whiteley, M. W. J. Organomet. Chem. 1987, 366, 385.
(44) Beddoes, R. L.; Bitcon, C.; Whiteley, M. W. J. Organomet. Chem.
1991, 402, 85.
(45) (a) Bruce, M. I.; Hall, B. C.; Kelly, B. D.; Low, P. J.; Skelton, B.
W.; White, A. H. J. Chem. Soc., Dalton Trans. 1999, 3719. (b) Coat, F.;
Lapinte, C.; Toupet, L.; Costuas, K.; Halet, J.-F. J. Organomet. Chem.
2003, 683, 368.
(46) Brown, N. J.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Low, P. J.;
Helliwell, M.; Ta, Y. T.; Whiteley, M. W. Chem. Commun. 2010, 46,
225.
Lapinte, C. J. Am. Chem. Soc. 2000, 122, 9405.
(16) Jiao, H.; Costuas, K.; Gladysz, J. A.; Halet, J.-F.; Guillemot, M.;
Toupet, L.; Paul, F.; Lapinte, C. J. Am. Chem. Soc. 2003, 125, 9511.
(17) Bruce, M. I.; Costuas, K.; Davin, T.; Ellis, B. G.; Halet, J.-F.;
Lapinte, C.; Low, P. J.; Smith, M. E.; Skelton, B. W.; Toupet, L.;
White, A. H. Organometallics 2005, 24, 3864.
(47) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev.
(18) Coat, F.; Guillevic, M.-A.; Toupet, L.; Paul, F.; Lapinte, C.
2001, 101, 2655.
Organometallics 1997, 16, 5988.
(19) Costuas, K.; Rigaut, S. Dalton Trans. 2011, 40, 5643.
(20) Xi, B.; Ren, T. C. R. Chem. 2009, 12, 321.
(48) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35, 424.
(49) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002,
31, 168.
(21) Ren, T. Organometallics 2005, 24, 4854.
(50) Hush value for Δn1/2 calculated from the equation Δn1/2 =
1/2
(22) Low, P. J.; Brown, N. J. J. Cluster Sci. 2010, 21, 235.
(23) Semenov, S. N.; Taghipourian, S. F.; Blacque, O.; Fox, T.;
Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2010, 132, 7584.
(24) Ying, J.-W.; Liu, I.P.-C.; Xi, B.; Song, Y.; Campana, C.; Zuo, J.-
L.; Ren, T. Angew. Chem., Int. Ed. 2010, 49, 954.
(2310nmax
) .
(51) Lohan, M.; Justard, F.; Roisnel, T.; Ecorchard, P.; Lang, H.;
Lapinte, C. Organometallics 2010, 29, 4804.
(52) Carter, E.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Lancashire,
H. N.; Murphy, D. M.; McDouall, J. J. W.; Sharples, J.; Whiteley, M.
W. Dalton Trans. 2010, 39, 11424.
(25) Woodworth, B. E.; White, P. S.; Templeton, J. L. J. Am. Chem.
Soc. 1997, 119, 828.
(26) Worth, G. H.; Robinson, B. H.; Simpson, J. Organometallics
1992, 11, 3863.
(53) Lapinte, C. J. Organomet. Chem. 2008, 693, 793.
(54) Le Narvor, N.; Lapinte, C. C. R. Acad. Sci. Ser. IIc: Chim. 1998,
745.
(27) Ouddai, N.; Costuas, K.; Bencharif, M.; Saillard, J.-Y.; Halet, J.-
F. C. R. Chim. 2005, 8, 1336.
(55) Paul, F.; Bondon, A.; da Costa, G.; Malvolti, F.; Sinbandhit, S.;
Cador, O.; Costuas, K.; Toupet, L.; Boillot, M.-L. Inorg. Chem. 2009,
48, 10608.
(56) Bleaney, B; Bowers, K. D. Proc. R. Soc. London, Ser. A 1952, 214,
451.
(57) (a) ADF2002.01, SCM, Theoretical Chemistry; Vrije Universiteit:
Amsterdam, The Netherlands, http://www.scm.com. (b) Guerra, C.
F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998,
99, 391.
(28) (a) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10,
247. (b) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
(29) Brown, N. J.; Collison, D.; Edge, R.; Fitzgerald, E. C.; Helliwell,
M.; Howard, J. A. K.; Lancashire, H. N.; Low, P. J.; McDouall, J. J. W.;
Raftery, J.; Smith, C. A.; Yufit, D. S.; Whiteley, M. W. Organometallics
2010, 29, 1261.
(30) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189.
(31) Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Schmid,
R. Organometallics 2005, 24, 3163.
(58) Lambert, C.; Noll, G.; Schelter, J. Nat. Mater. 2002, 1, 69.
̈
(59) Lambert, C.; Amthor, S.; Schelter, J. J. Phys. Chem. A 2004, 108,
(32) Elian, M.; Chen, M. M. L.; Mingos, D. M. P.; Hoffmann, R.
6474.
Inorg. Chem. 1976, 15, 1148.
(60) Amthor, S.; Lambert, C. J. Phys. Chem. A 2006, 110, 1177.
(61) Londergan, C. H.; Kubiak, C. P. J. Phys. Chem. A 2003, 107,
9301.
(62) Brown, N. J.; Lancashire, H. N.; Fox, M. A.; Collison, D.; Edge,
R.; Yufit, D. S.; Howard, J. A. K.; Whiteley, M. W.; Low, P. J.
Organometallics 2011, 30, 884.
(33) Iyer, R. S.; Selegue, J. P. J. Am. Chem. Soc. 1987, 109, 910.
(34) (a) Beddoes, R. L.; Bitcon, C.; Ricalton, A.; Whiteley, M. W. J.
Organomet. Chem. 1989, 367, C21. (b) Unseld, D.; Krivykh, V. V.;
Heinze, K.; Wild, F.; Artus, G.; Schmalle, H.; Berke, H. Organometallics
́
1999, 18, 1525. (c) Fernandez, F. J.; Venkatesan, K.; Blacque, O.;
Alfonso, M.; Schmalle, H. W.; Berke, H. Chem.Eur. J. 2003, 9, 6192.
(63) Le Stang, S.; Paul, F.; Lapinte, C. Organometallics 2000, 19,
1035.
(d) Venkatesan, K.; Fox, T.; Schmalle, H. W.; Berke, H.
168
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169

Organometallics
Article
(64) Krejeik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
(65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.Gaussian 03, Revision
C.02; Gaussian, Inc.: Wallingford, CT: 2004.
(66) Becke, A. D. J. Chem. Phys. 1993, 98, 648.
(67) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem.
A. 2000, 104, 4811.
(68) Lynch, B. J.; Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2003, 107,
1384.
(69) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(70) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
(71) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(72) Dennington, R., II; Keith, T.; Millam, J. GaussView, Version
4.1.2; Semichem, Inc.: Shawnee Mission, KS, 2007.
(73) O’Boyle, N. M.; Tenderholt, A. L.; Langer, K. M. J. Comput.
Chem. 2008, 29, 839.
(74) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; Universitat Gottingen: Germany, 1997.
̈
̈
(75) (a) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; Universitat Gottingen: Germany, 1997. (b) Sheldrick, G.
̈
̈
M. Acta Crystallogr. 2008, A64, 112.
169
dx.doi.org/10.1021/om200712m | Organometallics 2012, 31, 157−169