Oxidation-potential tuning of tungsten-alkylidyne complexes over a 2 v range

Article abstract of DOI:10.1021/ic401450u

The electrochemistry and electronic structures of over 30 tungsten-alkylidyne compounds of the form W(CR)L_nL′ _4-nX (R = H, Bu^t, Ph, p-C₆H₄CCH, p-C₆H₄CCSiPrⁱ₃; X = F, Cl, Br, I, OTf, Buⁿ, CN, OSiMe₃, OPh; L/L′ = PMe₃, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2 tmeda, P(OMe)₃, CO, CNBu ^t, py), in which the alkylidyne R group and L and X ligands are systematically varied, have been investigated using cyclic voltammetry and density functional theory calculations in order to determine the extent to which the oxidation potential may be tuned and its dependence on the nature of the metal-ligand interactions. The first oxidation potentials are found to span a range of ～2 V. Symmetry considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is of principal d_xy orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is a strong function of the symmetry relationship between the substituent and the d_xy orbital, being much more sensitive to the nature of the equatorial L ligands (π symmetry, with respect to d_xy, ΔE_1/2 ? 0.5 V/L) than to the axial CR and X ligands (nonbonding with respect to d_xy, ΔE_1/2 < 0.3 V/L). The oxidation potential is linearly correlated with the calculated d _xy orbital energy (slope ? 1, R² = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with one-electron oxidation.

Full text of DOI:10.1021/ic401450u

Article
pubs.acs.org/IC
Oxidation-Potential Tuning of Tungsten−Alkylidyne Complexes over
a 2 V Range
Daniel E. Haines, Daniel C. O’Hanlon, Joseph Manna, Marya K. Jones, Sarah E. Shaner, Jibin Sun,
and Michael D. Hopkins*
Department of Chemistry, The University of Chicago, 929 E. 57th Street, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: The electrochemistry and electronic structures
of over 30 tungsten−alkylidyne compounds of the form
W(CR)LnL′4−nX (R = H, But, Ph, p-C6H4CCH, p-
C6H4CCSiPri3; X = F, Cl, Br, I, OTf, Bun, CN, OSiMe3,
OPh; L/L′ = PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe, 1/2
tmeda, P(OMe)3, CO, CNBut, py), in which the alkylidyne R
group and L and X ligands are systematically varied, have been
investigated using cyclic voltammetry and density functional
theory calculations in order to determine the extent to which
the oxidation potential may be tuned and its dependence on
the nature of the metal−ligand interactions. The first oxidation
potentials are found to span a range of ∼2 V. Symmetry
considerations and the electronic-structure calculations indicate that the highest occupied molecular orbital (and redox orbital) is
of principal dxy orbital parentage for most of the compounds in this series. The dependence of the oxidation potential on ligand is
a strong function of the symmetry relationship between the substituent and the dxy orbital, being much more sensitive to the
nature of the equatorial L ligands (π symmetry, with respect to dxy, ΔE1/2 ≅ 0.5 V/L) than to the axial CR and X ligands
(nonbonding with respect to dxy, ΔE1/2 < 0.3 V/L). The oxidation potential is linearly correlated with the calculated dxy orbital
energy (slope ≅ 1, R2 = 0.97), which thus provides a convenient computational descriptor for the potential. The strength of the
correlation and slope of unity are proposed to be manifestations of the small inner-sphere reorganization energy associated with
one-electron oxidation.
empirical relationships that enable the prediction of their redox
potentials. Within a set of related metal−alkylidyne complexes
of a given metal, it is reasonable to expect that the electrode
potential of a particular metal-centered redox process should
vary in a systematic way as a function of the electronic
properties of the supporting ligands and alkylidyne R group.
Such relationships have been codified more generally for
transition-metal compounds in empirical ligand-additivity
models developed by Lever,47,48 Pickett,49 and Bursten,50 in
which the redox potential is expressed as a sum of ligand
electrochemical parameters whose magnitudes reflect the
relative electron donor/acceptor properties of the ligands.
Pombeiro and co-workers have reported extensive analyses
within the Pickett model of the first oxidation potentials of
rhenium and tungsten−alkylidyne compounds of the types
[Re(CCH2R)(dppe)2X]+ (dppe = 1,2-bis(diphenylphosphino)-
ethane; X = F, Cl; R = H, But, CO2Me, CO2Et, Ph, 4-
C6H4Me),3 W{CCH(cyclo-C(CH2)n)}(CO)2(dppe)X (X =
F, Cl, SCN, OCN, S2P(OEt)2; n = 1, 4),9 and [W-
{CCH(cyclo-C(CH2)n)}(CO)2(dppe)(CNR)]+ (R = Bun,
But, Cy, PhCH2, C6H3-2,6-Me2).12 Because of the variety of
INTRODUCTION
■
The descriptive chemistry of compounds containing metal−
carbon triple bonds is generally organized according to the
formal oxidation state of the metal center, as embodied in the
familiar classification of these compounds as being “low-
oxidation-state” Fischer carbyne or “high-oxidation-state”
Schrock alkylidyne complexes.1 It is unsurprising, then, that
there have been numerous studies of the electrochemistry of
metal−alkylidyne (−carbyne) complexes aimed at determining
the redox potentials that characterize the metal oxidation states
and elucidating the processes that connect them.2−22 The
understanding of redox processes and potentials gleaned from
these studies is important in many contexts, including
elucidating the nature of ground-state2 3 and
photochemical4,5,24 redox-induced reactions of the alkylidyne
ligand, the development of oxidation and reduction reactions
that interconvert high- and low-oxidation-state metal−alkyli-
dyne complexes,25 and designing and understanding the
properties of luminescent photoredox chromophores5,6,11,26−43
43 and π-conjugated compounds and materials7,15−21,35,39,44−46
based on metal−alkylidyne compounds.
In order to guide the rational design of new substances for
which the control of electron-transfer properties is central to
their function, it is important to develop theoretical methods or
Received: June 7, 2013
Published: August 2, 2013
© 2013 American Chemical Society
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supporting ligands possessed by these compounds, it was
possible to determine values of the electrochemical parameter
PL for their specific alkylidyne ligands (PL = 0.21−0.24); these
could be used to predict the oxidation potentials of new
derivatives. It was also proposed that the large values of PL
indicate that the alkylidyne ligand is a stronger net π-acceptor
than CO, based on the description of the PL parameter as a
measure of the net σ-donor minus π-acceptor ability of the
ligand.51
For some applications of metal−alkylidyne complexes,
including as photoredox chromophores and building blocks
for π-conjugated materials, the ability to tune the redox
potential over a wider range than can be predicted from
available empirical parameters is desirable. In order to
understand the extent of redox-potential tuning that can be
achieved by ligand variation, we have investigated the
electrochemistry of the broad class of d2-configured tung-
sten−alkylidyne compounds of the form W(CR)LnL′4−nX (L/
L′ = neutral ligand, X = anionic ligand). One motivation for
studying this class of compounds is that their synthetic
chemistry enables the electronic properties of the alkylidyne
R group and supporting X and L ligands to be systematically
varied over a wide range. The compounds studied in the
present report, shown in Chart 1, include derivatives with
the CR, L, and X ligands is found to be a strong function of
their symmetry relationship to the dxy orbital, with the
equatorial L ligands affecting the potential more than the
axial CR and X ligands. The oxidation potential does not vary
systematically with the calculated ligand parentage of the
HOMO but does exhibit a linear correlation with the HOMO
energy (slope ≅ 1), from which it may inferred that inner-
sphere reorganization energies associated with one-electron
oxidation are constant or small across this series. This simple
Koopmans-Theorem-like relationship provides a convenient
computational descriptor for predicting the redox potentials of
new or unstudied metal−alkylidyne compounds of this class.
RESULTS
■
Electrochemistry of W(CR)LnL′4−nX Complexes. Com-
pounds of the general form W(CR)LnL′4−nX were studied by
cyclic voltammetry in order to characterize the nature and
potentials of their redox processes. The cyclic voltammogram
of W(CPh)(depe)2Cl in THF, shown in Figure 1, is illustrative
Chart 1. Tungsten−Alkylidyne Complexes of the Type
W(CR)LnL′4−nX (L = L′, n = 4; L ≠ L′, n = 2)
Figure 1. Cyclic voltammogram of W(CPh)(depe)2Cl (20) in THF
solution at room temperature (ν = 0.1 V s−1, 0.3 M [NBun ][PF6]).
saturated and unsaturated alkylidyne R groups, L ligands that
include soft donors and π-acceptors, and X ligands that include
σ and π donors and electron-withdrawing ligands. An additional
consideration is that there have not yet been electrochemical
studies of metal−alkylidyne complexes that examine how the
symmetry relationship between ligand and redox orbital
influences redox-potential tuning. Addressing this question is
facilitated for d2 W(CR)LnL′4−nX complexes because previous
structural and EPR studies of chemically oxidized, d1 redox
congeners have allowed assignment of the redox-active orbital
for oxidation as being of metal dxy parentage.10,14,41 Finally,
understanding the redox potentials of W(CR)LnL′4−nX
compounds is of interest because several of these complexes
have been employed as luminescent photoredox chromo-
phores5,6,11,26,27,34,35,38−42 and in π-conjugated materi-
als.7,16,19−21,35,39,45
Here, we report the dependence of the first oxidation
potentials of W(CR)LnL′4−nX complexes on the nature of the
alkylidyne R group and X and L ancillary ligands. The
electrochemical data reveal that the oxidation potential can be
tuned over a 2 V range through ligand variation. Density
functional theory calculations show that the highest occupied
molecular orbital (HOMO) and redox-active orbital is the
metal-centered dxy orbital for nearly all compounds in this
series. The extent to which the oxidation potential varies with
4
of the redox processes that are usually observed for these
complexes: it exhibits an electrochemically reversible one-
0/+
electron oxidation (E1/2
= −0.84 V vs FeCp20/+); an
irreversible second oxidation (Ep = 0.19 V); and an irreversible
reduction near the cathodic limit of the THF solvent (Ep =
−3.35 V). The electrode potentials for the first oxidations of
the W(CR)LnL′4−nX compounds are set out in Table 1;52 those
for reductions and second oxidations are reported in the
Supporting Information (Table S1). The first oxidation is
assigned to a one-electron process, based on prior work that
demonstrated that W(CR)L4X compounds (L = phosphine)
may be chemically oxidized by one electron to form stable
[W(CR)L4X]+ ions.7,10,14,41 Furthermore, for those compounds
for which the first oxidation is reversible or quasi-reversible, the
splitting between anodic and cathodic peaks (ΔEp) is essentially
the same as that for the FeCp2 internal reference. The second
oxidation, when observed (see Table S1 in the Supporting
Information), is irreversible for all compounds except W-
(CPh)(dmpe)2(OPh) (16), which exhibits a reversible second
oxidation (Figure S1 in the Supporting Information). The
irreversible reduction (Table S1 in the Supporting Information)
is observed for W(CR)LnL′4−nX derivatives with unsaturated R
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Table 1. Oxidation Potentials and Calculated Energies and Atomic Parentages of dxy-Derived Orbitals of W(CR)LnL′4−nX
Complexes
Atomic Parentage (%)
a
0/+
compound
W(CH)(dmpe)2F
compound number
E1/2 (V)
E(dxy) (eV)
W
CR
L
X
b
1
−0.91
−0.84
−0.82
−0.73
−0.68
−4.58
−4.70
−4.77
−4.83
−5.01
−4.80
−4.66
−4.54
−4.71
−4.60
−4.62
−4.79
−4.85
−4.90
−5.02
−4.86
−4.75
−4.81
−4.87
−4.63
−4.90
−7.43
−4.92
−4.90
−5.30
−5.58
−6.21
−6.04
−6.07
73.3
74.3
74.7
75.0
75.4
74.0
74.3
73.1
76.1
73.7
75.3
74.7
75.0
75.3
75.4
72.2
76.3
76.5
76.6
74.6
74.2
74.8
75.0
75.0
63.4
77.4
58.2
63.0
60.3
50.0
58.9
65.6
0.0
0.1
0.1
0.1
0.1
0.1
0.0
0.1
0.0
0.1
0.0
0.0
0.0
0.0
0.1
0.8
0.1
0.1
0.1
0.1
0.0
0.0
0.0
0.0
2.0
0.0
6.2
2.5
3.0
6.1
2.3
0.2
26.7
0.0
0.0
0.0
0.0
0.2
0.0
0.0
0.5
0.0
0.0
0.0
0.0
0.0
0.0
0.2
2.7
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
2.8
0.0
5.7
2.9
7.0
15.4
4.8
0.0
c
W(CH)(dmpe)2Cl
2
25.6
W(CH)(dmpe)2Br
3
25.3
W(CH)(dmpe)2I
4
25.0
c
W(CH)(dmpe)2(OTf)
W(CH)(dmpe)2(CN)
W(CH)(dmpe)2(OSiMe3)
W(CH)(dmpe)2(Bun)
W(CH)(PMe3)4Cl
W(CBut)(dmpe)2Cl
W(CBut)(PMe3)4Cl
5
24.4
6
−0.78
25.9
b
e
c
,
d
7
−0.94
−0.98
−0.85
−0.93
−0.87
−0.82
−0.78
−0.72
−0.65
−0.89
−0.85
−0.80
−0.76
−0.84
−0.58
−0.24
−0.79
−0.78
−0.08
−0.28
0.66
25.6
8
26.3
9
23.9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
26.2
24.7
W(CPh)(dmpe)2Cl
25.3
W(CPh)(dmpe)2Br
25.0
W(CPh)(dmpe)2I
24.7
W(CPh)(dmpe)2(OTf)
W(CPh)(dmpe)2(OPh)
W(CPh)(PMe3)4Cl
24.4
f
24.3
23.6
W(CPh)(PMe3)4Br
23.4
W(CPh)(PMe3)4I
23.2
W(CPh)(depe)2Cl
25.3
W(CPh)(dppe)2Cl
25.7
25.2
[W(CPh)(dppe)2(CH3CN)][PF6]
W(CC6H4-4-CCH)(dmpe)2Cl
W(CC6H4-4-CCSiPri3)(dmpe)2Cl
W{CCHC(c-C4H8)}(triphos)(CO)Cl
W(CPh){P(OMe)3}4Cl
W(CPh)(dppe)(CO)2Cl
W(CPh)(tmeda)(CO)2Cl
W(CPh)(tmeda)(CO)2Br
W(CPh)(CNBut)2(CO)2Br
W(CPh)(py)2(CO)2Br
W(CCCPh)(CO)4Cl
g
g
25.0
25.0
h
15.9 (CO), 15.8 (P3)
22.7
b
,
i
j
e
23.1 (CO), 6.7 (dppe)
30.4 (CO), 1.2 (tmeda)
28.5 (CO), 1.2 (tmeda)
17.1 (CO), 11.5 (CNR)
26.7 (CO), 7.3 (py)
34.3
b
b
b
b
0.41
0.43
,
k
0.62
−6.23
0.42
−5.93
−7.83
l
m
>1.1
a
Unless otherwise noted, data are reported for compounds in THF solution ((0.5−5.0) × 10−3 M analyte, 0.3 M [NBun ][PF6]) at 25 °C, ν = 0.1 V
4
b
c
0/+
0/+
s−1, and are referenced to FeCp2
.
Quasi-reversible. Reference 10 reports E1/2
= −0.91 V for W(CH)(dmpe)2Cl, −0.68 V for
d
W(CH)(dmpe)2(OTf), −0.88 V for W(CH)(PMe3)4Cl in THF/0.1 M [NBun ][PF6]. The compound formed a film on the electrode during the
4
e
f
g
h
experiment. Irreversible; Epa reported. Measured in CH2Cl2; reversible second oxidation observed at E1/2+/2+ = −0.46 V. Reference 7. Reference
i
j
9; measured in CH3CN with 0.2 M [NBun ][BF4] electrolyte. Sample temperature = −13 °C. Measured in CH2Cl2 with Ag/Ag+ reference
4
a
k
l
m
electrode; all other conditions as in footnote . HOMO-1. Reference 8. HOMO-2; calculation on W(CPh)(CO)4Cl.
groups (R = Ph, C6H4CCH, C6H4CCSiPri3), but not for
complexes with saturated alkylidyne R groups (R = H, But).
The W(CR)LnL′4−nX compounds set out in Table 1 can be
partitioned into two groups, based on the reversibility and
potential of the first oxidation. One group is comprised of
derivatives with phosphine equatorial ligands (L = L′ = PMe3,
1/2 dmpe, 1/2 depe, 1/2 dppe; 1−24), for which the first
oxidation is usually reversible and lies in the potential range
−1.0 V < E1/2 < −0.5 V vs FeCp20/+. The only exceptions to
reversible behavior within this group are W(CH)(dmpe)2F (1)
and W(CH)(dmpe)2(OSiMe3) (7), for which the first
oxidation is quasi-reversible, and W(CH)(dmpe)2(Bun) (8),
which is irreversible. The single cationic derivative among these
compounds, [W(CPh)(dppe)2(NCMe)]+ (22), also exhibits a
reversible oxidation; its potential is shifted ca. +0.3 V relative to
that of W(CPh)(dppe)2Cl (21), its closest neutral analogue, as
a result of the positive charge. The second group is comprised
of W(CR)LnL′4−nX compounds with stronger π-acceptor
equatorial ligands (L = L′ = P(OMe)3; L′ = CO, CNBut;
26−32). For these, the first oxidation is quasi-reversible or
irreversible and occurs at more positive potentials than the
phosphine derivatives (−0.3 V < Ep < 0.7 V). Measurements on
selected compounds at low temperature showed that the cyclic
voltammetric wave for the first oxidation approaches
reversibility as the temperature decreases. For W(CPh){P-
(OMe)3}4Cl (26), the wave changes from quasi-reversible at
room temperature (ipa/ipc = 0.36 at ν = 0.10 V s−1) to reversible
at −13 °C (ipa/ipc = 0.97). Similarly, the appearance of the
oxidation wave for W(CPh)(dppe)(CO)2Cl (27) changes from
irreversible at room temperature to quasi-reversible at 2 °C.
Across all complexes in the present study, the first oxidation
potential (including irreversible processes) is observed to vary
over a 1.7 V range, from −0.98 V (W(CH)(dmpe)2(Bun), 8) to
0.66 V (W(CPh)(dppe)(CO)2Cl, 27). Inclusion of the lower
bound on the oxidation potential reported for W(CCCPh)-
(CO)4Cl (32, > 1.1 V)8 extends this range to ∼2 V.
Density Functional Theory Calculations on W(CR)-
LnL′4−nX Complexes. Density functional theory calculations
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Figure 2. Oxidation potentials and calculated π*(W≡CR), dxy, and π(W≡CR) orbital energies of W(CR)L4L′4−nX complexes (see Table 1 and
Table S2 in the Supporting Information). Orbitals labeled “other” are indicated for compounds with additional orbitals in this energy space. The
open circle for W(CPh)(CO)4Cl represents the lower limit of the oxidation potential for W(CCCPh)(CO)4Cl.
were performed on W(CR)LnL′4−nX complexes 1−32 in order
to determine the energies and atomic parentages of their
frontier orbitals, which include the redox-active orbitals. Based
on symmetry arguments described in detail elsewhere43 and
prior computational studies of individual tungsten−alkylidyne
compounds,21,41,53 the frontier orbitals of d2 W(CR)L4X
complexes are generally expected to be of metal t2g parentage:
the dxy orbital is the principal basis for the HOMO and the dxz
and dyz orbitals contribute substantially to the π(W≡CR)
HOMO−1 and π*(W≡CR) lowest unoccupied molecular
orbital (LUMO).54 These frontier orbitals are analogous to
those well-known for related d2 M(≡E)L5 metal−oxo and
nitrido complexes.55
The frontier orbitals calculated for the W(CR)LnL′4−nX
complexes conform to this expected pattern with few
exceptions. The energy levels of the dxy, π(W≡CR), and
π*(W≡CR) orbitals are depicted in Figure 2, and orbital
energies and atomic parentages are set out in Table 1 (for dxy)
and Table S2 in the Supporting Information (for π(W≡CR)
and π*(W≡CR)). The π*(W≡CR) orbital is the LUMO for all
compounds except W(CBut)(dmpe)2Cl (10), W(CR)-
(triphos)(CO)Cl (25), and W(CPh)(CO)2(py)2Br (31), for
which L-centered orbitals lie at slightly lower energy. Among
pairs of compounds with constant X and L ligands, the LUMOs
of derivatives with unsaturated alkylidyne R groups (R = Ph,
C6H4CCR) lie >1 eV lower in energy than for those with
saturated R groups (R = H, But) due to π conjugation within
the WCPh unit.41,43 These differences in LUMO delocalization
and energy are reflected in the reduction properties of the
compounds (vide infra).
respect to the equatorial L ligands. Accordingly, for W(CR)-
LnL′4−nX compounds that possess axial symmetry (L = L′: 1−
15, 17−24, 26, 32) mixing of dxy with X and CR orbitals is
negligible (≤0.5%) while contributions from L range from
23%−34% (being highest for W(CPh)(CO)4Cl). For lower-
symmetry W(CR)LnL′4−nX compounds (L ≠ L′: 25, 27−31; L
= L′, X = OPh: 16), mixing of the dxy and axial ligand orbitals is
symmetry allowed and contributions from the X and CR
ligands are higher, approaching a combined 22% for W(CPh)-
(CNBut)2(CO)2Br (30) and ranging from 3% to 12% for the
other derivatives.
The energy of the dxy-derived HOMO varies considerably
according to the nature of the L/L′ ligands (Figure 2), as
expected from the fact that it possesses substantial L parentage.
The dxy orbital energies span a fairly narrow range among
phosphine derivatives (ΔE(HOMO) ≅ 0.5 eV for L/L′ =
PMe3, 1/2 dmpe, 1/2 depe, 1/2 dppe; 1−21, 23, 24) despite
their diverse CR and X ligands, but decrease significantly in
compounds with stronger π-backbonding L ligand sets
(ΔE(HOMO) ≅ 2.5 eV for L = P(OMe)3, CO, CNBut; 25−
32). For W(CPh)(CNBut)2(CO)2Br (30) and W(CPh)-
(CO)4Cl (32), which have the strongest π-backbonding L
ligands, the dxy orbital is sufficiently stabilized that π(W≡CR) is
the HOMO; dxy is the HOMO-1 for W(CPh)-
(CNBut)2(CO)2Br, for which it lies just below the HOMO
(ΔE(HOMO/HOMO-1) = 0.13 eV), and HOMO-2 for
W(CPh)(CO)4Cl (ΔE(HOMO/HOMO-2) = 0.67 eV).
DISCUSSION
■
The first oxidation potential of W(CR)LnL′4−nX compounds is
tunable over a 2 V range, and is observed to vary with the
nature of the alkylidyne R group and X and L/L′ ligands in a
manner consistent with assignment of the dxy HOMO as the
redox orbital. Based on the symmetry considerations outlined
above, the oxidation-potential tuning should be more sensitive
to the nature of L than to that of X and CR due to the
equatorial π-symmetry and axial δ-symmetry of the dxy orbital.
The HOMO is found to be derived from the dxy orbital for all
compounds except W(CPh)(CNBut)2(CO)2Br (30) and
W(CPh)(CO)4Cl (32), for which the HOMO is π(W≡CR)
(vide infra). The ligand contributions to the HOMO (Table 1)
clearly reflect the symmetry properties of the dxy orbital, which
in the limit of C4v symmetry is nonbonding (δ symmetry)
relative to the axial CR and X ligands and π symmetry with
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The observations are in strong accord with this expectation.
The small effects on oxidation potential of changing the
alkylidyne R group are illustrated by the set of W(CR)-
(dmpe)2Cl compounds (R = H, But, Ph, C6H4-4-CCH, C6H4-4-
CCSiMe3; 2, 10, 12, 23, 24), the potentials of which span a
potentials of the W(CH)(dmpe)2X compounds (X = F, Cl, Br,
I, OTf, CN, Bun, OSiMe3; 1−8) span a range of 0.30 V, but the
calculated contribution of the X ligand to the dxy HOMO to the
nearest whole percent is zero (Table 1). Similarly, the
potentials of the trimethylphosphine and trimethylphosphite
derivatives of type W(CPh)L4Cl (L = PMe3 (17), P(OMe)3
(26)) differ by 0.57 V, yet possess relative W and PR3
contributions to the HOMO that are within 1% of each
other. Thus, there are inductive influences by the ligands on the
oxidation potential that are not manifested in the atomic
parentage of the dxy-derived redox orbital.
0/+
range of only 0.15 V (E1/2 = −0.78 to −0.93 V). Similarly,
variation of the X ligand within the series of compounds of type
W(CH)(dmpe)2X (X = F, Cl, Br, I, OTf, CN, Bun, OSiMe3; 1−
8) shifts the oxidation potential across a range of 0.30 V
0/+
(E1/2 = −0.68 to −0.98 V). These narrow potential ranges
are striking given the electronic diversity of the ligands within
these setsthe alkylidyne ligands include saturated and
unsaturated R groups, and the X ligands include hard π donors
(F, OR), soft σ donors (Bun), and unsaturated (CN) and
electron-withdrawing (OTf) ligands. In contrast, under
variation of the equatorial L ligands the oxidation potentials
of the W(CPh)LnL′4−nX complexes reported here (including
both reversible and irreversible processes) span a range of 1.7 V
(+0.66 to −0.98 V; 1−24, 26−31), and inclusion of the lower
bound on the oxidation potential reported for W(CCCPh)-
(CO)4Cl (>1.1 V; 32)8 extends this range to ∼2 V.
Normalizing this range for the fact that there are four
equatorial ligands places the per-ligand contribution at 0.5 V/
L, which exceeds those of the axial X ligands (0.30 V) and CR
ligands (0.15 V). Qualitatively, the oxidation potential is
observed to increase with the π-acceptor strength of the
{LnL′4−n} equatorial ligand set, lying in the order (PMe3)4 ≅
(dmpe)2 ≅ (depe)2 < (dppe)2 < {P(OMe)3}4 ≪ (tmeda)-
(CO)2 ≅ (py)2(CO)2 < (dppe)(CO)2 ≅ (CNBut)2(CO)2 ≪
(CO)4.
The influence of the ligands on oxidation potential is better
captured by the calculated energy of the dxy-derived HOMO,
which is found to exhibit a strong linear correlation with the
first oxidation potential of W(CR)LnL′4−nX compounds 1−21
and 23−31. The linear fit to these data, shown in Figure 3,
The strong influence of the symmetry relationship between
ligands and redox orbital on the oxidation potentials of these
compounds is further highlighted by the observation that even
replacing the triply bonded carbon atom of the alkylidyne
ligand with germanium has little effect on the oxidation
potential, as evidenced by data for the recently reported
compound W(GeAr)(PMe3)4Cl (Ar = C6H3-2,6-(mesityl)2).56
This compound possesses a similar molecular structure and
identical (dxy)2 electron configuration as those of the tungsten−
alkylidyne compounds reported here; it is reversibly oxidized at
Figure 3. Relationship between the first oxidation potential and
calculated dxy orbital energy for W(CR)LnL′4−nX complexes. The line
is a linear regression fit (E1/2 = −0.97(E(dxy)) − 5.44 (R2 = 0.97)) to
complexes 1−21 and 23−31 (Table 1; excludes [W(CPh)-
(dppe)2(NCCH3)]+ and W(CPh)(CO)4Cl).
0/+
−0.91 V vs FeCp2 (in C6H5F), which is remarkably close to
that observed for its closest analogue W(CPh)(PMe3)4Cl (17,
−0.85 V in THF; see Table 1). The fact that the potentials of
these compounds are nearly identical, despite the electronic
differences between carbon and germanium, is a consequence
of the fact that their common redox-active dxy HOMO is
orthogonal to orbitals of the axial CR and GeR ligands. The
near independence of the oxidation potential on the nature of
the W≡E bond and on the extent of unsaturation in the
alkylidyne R group also indicates that, for these compounds, the
π-acceptor characteristics of the alkylidyne ligand play
essentially no role in determining the oxidation potential.
This finding is in contrast to the conclusion from analyses of
metal−alkylidyne oxidation potentials using Pickett’s model
(vide supra),51 and suggests a difficulty in developing physical
interpretations of empirical ligand electrochemical parameters
derived within models that do not take account of symmetry.57
To gain insight into the trends in first oxidation potential of
the W(CR)LnL′4−nX compounds, their correspondence to
calculated properties of the dxy-derived redox orbital was
examined. Interestingly, there is not an obvious relationship
between the oxidation potential and the calculated atomic
contributions to the HOMO. For example, the oxidation
possesses a slope of near unity: E1/2 = −0.97(E(dxy)) − 5.44
(R2 = 0.97). The root-mean-square (RMS) error of E1/20/+, as a
function of dxy energy, is only 0.096 V. The compound
W(CPh)(CO)4Cl (32) is not included in the fit because the dxy
orbital is the HOMO-2 and lies well below the π(W≡CR)
HOMO in energy (ΔE(HOMO/HOMO-2) = 0.67 eV); the
data point for this compound lies >0.5 V off the linear fit (see
Figure S2 in the Supporting Information), which is consistent
with the oxidation involving a different redox orbital than those
of the other compounds. The observation of a linear
relationship between the oxidation potential and redox-orbital
energy for a series of compounds is not generally expected,
because the oxidation potential is a property that relates the
reduced and oxidized compounds whereas the orbital energy is
a property only of the reduced compound. A rigorous
theoretical calculation of the solution-phase redox potential
typically involves summing the gas-phase adiabatic ionization
energy of the reactant with the difference between the reactant
and product free energies of solvation, along the lines of a
Born−Haber thermochemical cycle.58,59 The Koopmans-
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Theorem-like relationship between the oxidation potential and
calculated gas-phase HOMO energy observed here (Figure 3)
provides a considerably less-expensive computational descrip-
tor.60 The fact that it does not take account of differences in
free energies of solvation of the oxidized and reduced species,
electronic relaxation effects, and inner-sphere reorganization
energies of the oxidized compounds, among other factors,
suggests that each factor is either constant across the series of
compounds or varies linearly with redox potential.61
compounds is derived from the dxy orbital; this serves as the
redox orbital for the oxidation. The HOMO is metal centered;
it possesses sizable contributions from L and negligible
contributions from the CR and X ligands due to its equatorial
π symmetry and axial δ symmetry. Consistent with this, the
oxidation potential is more sensitive to variation of the L
ligands (ΔE1/2 = 0.5 V/L across the series) than to the CR
(0.15 V) and X ligands (0.30 V). The importance of these
symmetry relationships is further illustrated by the fact that the
compounds W(CPh)(PMe)4Cl and W(GeAr)(PMe3)4Cl56 are
oxidized at potentials within 0.06 V of each other, despite their
different W≡E ligating atoms. The oxidation potentials of
W(CR)LnL′4−nX complexes are linearly related to the energies
of their dxy HOMOs (slope ≈ 1). One implication of the unity
slope is that that inner-sphere reorganization energies
associated with one-electron oxidation are negligible across
this series. This conclusion substantiates inferences from
previous structural studies of d2/d1 pairs of [W(CR)L4X]0/+
complexes, which found that distortions of the inner
coordination sphere upon oxidation are small.10,14,41
The linear relationship between oxidation potential and
HOMO energy provides a convenient computational descriptor
for predicting the redox potentials of new or unstudied
W(CR)LnL′4−nX compounds. In this context, efforts are
underway to extend the range of redox tuning beyond 2 V,
with the aim of designing photoredox chromophores with
highly reducing excited states suitable for activating inert
substrates such as CO2.
Linear relationships between redox potentials and calculated
redox-orbital energies48,50,59,62 (and, similarly, calculated
vertical ionization energies)63 have been noted for several
classes of inorganic compounds, and these form the basis for
Bursten’s ligand-additivity model for redox potentials.50
However, an unusual property of the linear correlation for
W(CR)LnL′4−nX compounds is that the slope is approximately
unity. Although this represents the idealized case, it contrasts
with the more common observation that the slope differs from
1 by 0.5 or more. While the possibility that the unity slope
may reflect a fortuitous cancellation of errors cannot be
excluded, the at-face-value implication of the slope that inner-
sphere reorganization energies are constant or negligible across
the series is chemically reasonable. Evidence for this is provided
by experimental studies of the molecular structures of (dxy)2
and (dxy)1 [W(CR)L4X]0/+ redox congeners (L = phos-
phine).10,14,41 Because of the axial nonbonding nature of the
dxy orbital, these pairs of compounds possess identical formal
W≡CR and W−X bond orders and their bonding differs
principally in that the extent of W→L π backbonding is weaker
for the d1 than for the d2 configuration. The molecular
structures of pairs of [W(CPh)L4X]0/+ complexes exhibit
correspondingly small differences between the lengths of
these metal−ligand bonds (Δd(W≡C) ≅ 0 Å, Δd(W−P) =
0.05−0.08 Å, Δd(W−X) = 0.02−0.05 Å), from which it may be
inferred that the oxidation is characterized by a small inner-
sphere reorganization energy. It is also intuitively plausible that
differential solvation energies should be constant across this
series of compounds, because of the similarities among their
peripheral substituents.
Given the simplicity of the physical underpinnings of the
linear correlation and the inclusion of both reversible and
irreversible potentials, it is unsurprising that some compounds
lie off the line, the most notable of which are W(CPh){P-
(OMe)3}4Cl (26) and W{CCHC(c-C4H8)}{PPh-
(CH2CH2PPh2)2}(CO)Cl (25) (Figure 3). Because this central
portion of the correlation contains relatively few data points, it
is difficult to pinpoint the factors that result in these differences
in the absence of a full theoretical treatment of their oxidation
potentials. In this context, the extensive data set reported here
may be useful for benchmarking and improving these
theoretical methods. Despite this caveat, the observed linear
correlation provides a convenient, low-computational-cost
descriptor for assigning the nature of the redox-active orbital
for this class of compounds and for predicting the first
oxidation potentials of other W(CR)LnL′4−nX compounds.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under a
nitrogen atmosphere using standard glovebox and Schlenk techniques.
Solvents for the synthesis of tungsten−methylidyne complexes were
purified as follows: THF, toluene, diethyl ether, and 1,2-dimethoxy-
ethane (DME) were stirred over sodium wire, distilled, and stirred
over Na/K alloy, from which they were transferred under vacuum
immediately prior to use; pentane was stirred sequentially over a
mixture of 5% nitric acid in concentrated sulfuric acid, potassium
carbonate, and calcium hydride, from which it was distilled; N,N-
dimethylformamide (DMF) was stirred over barium oxide and then
vacuum distilled; and dichloromethane and acetonitrile were stirred
over CaH2 and distilled immediately prior to use. For other purposes,
solvents (HPLC grade, stored under nitrogen) were purified by
passing them under nitrogen pressure through an anaerobic, stainless-
steel system consisting of either two 4.5 in. × 24 in. (1 gal) columns of
activated A2 alumina (acetonitrile, Et2O, CH2Cl2, and THF) or one
column of activated A2 alumina and one column of activated BASF
R3-11 catalyst (toluene, pentane).64 Dichloromethane-d2 was stirred
over calcium hydride and benzene-d6 and THF-d8 were stirred over
Na/K alloy, from which they were transferred under vacuum prior to
use. NMR spectra were recorded at room temperature with Bruker
AF300, DRX 400, and AF500 spectrometers. Chemical shifts were
measured relative to solvent resonances (1H, 13C) or external
standards of 85% H3PO4 in D2O (31P) or CFCl3 (19F).
The compounds W(CH)(dmpe)2X (X = Cl,65 Br,66,67 Bun67,68),
W(CH)(PMe3)4Cl,65 W(CBut)(PMe3)4Cl,65 W(CPh)(dmpe)2Br,14
W(CPh)(PMe3 )4 Br,69 W(CPh)(dppe)2 Cl,70 [W(CPh)-
(dppe)2(NCCH3)][PF6],41 W(CC6H4-4-CCR)(dmpe)2Cl (R = H,
SiPri3),7 W(CPh)(dppe)(CO)2Cl,67,71 W(CPh)(tmeda)(CO)2Cl,71
W(CPh)(tmeda)(CO)2Br,71 W(CPh)(CNBut)2(CO)2Br,67,72 and W-
(CPh)(py)2(CO)2Br67,71,73 were prepared according to standard
procedures or simple modifications thereof. The complexes W(CBut)-
(dmpe)2Cl65 and W(CPh){P(OMe)3}4Cl74 were prepared via the
two-electron reduction of W(CR)Cl3(dme)70,75 with Na/Hg amalgam
in the presence of a slight excess of the phosphorus ligand, following
the procedure for W(CPh)(dppe)2Cl;70 the NMR spectra of the
CONCLUSIONS
■
In this study, we have explored the relationship between the
electrochemical properties and electronic structures of
complexes of the type W(CR)LnL′4−nX. The first oxidation
potentials of these complexes can be tuned over a range of ∼2
V through ligand variation. According to density functional
theory calculations, the HOMO for nearly all of these
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Article
products matched those reported for samples prepared by the original
synthetic procedures. The compounds W(CH)(dmpe)2F, W(CH)-
(dmpe)2I, W(CH)(dmpe)2(OTf), W(CH)(dmpe)2(CN), W(CH)-
(dmpe)2(OSiMe3), W(CPh)(dmpe)2Cl, W(CPh)(dmpe)2(OTf), W-
(CPh)(dmpe)2(OPh), and W(CPh)(depe)2Cl were prepared by
standard reductive or ligand-substitution reactions similar to those
reported for the analogous compounds above; full details of their
synthesis and characterization are provided in the Supporting
Information. Tetrabutylammonium hexafluorophosphate was prepared
by the standard procedure,76 recrystallized three times from 95%
ethanol, and dried under vacuum at 80−110 °C for a minimum of 24
h. Ferrocene was recrystallized three times from 95% ethanol and then
sublimed under vacuum.77 All other chemicals were reagent grade or
comparable quality and were used as-received. Elemental analyses were
performed by Onieda Research Services (Whitesboro, NY) or
Midwest Microlab, LLC (Indianapolis, IN).
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mhopkins@uchicago.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Cheslan Simpson, Kevin John, Tom Gilbert, and
Kurt Heinselman for providing some of the compounds used in
this study, and Prof. Warren Piers (University of Calgary) for
helpful discussions. This research was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences, Solar
Photochemistry Program (Grant No. DE-FG02-07-ER15910).
Partial support to S.E.S. and M.K.J. through the NSF MRSEC
Program (Grant No. DMR-0820054) is acknowledged.
Electrochemical Measurements. Electrochemical experiments
were performed at room temperature under a nitrogen atmosphere in
a glovebox using a Bioanalytical Systems 100 B/W Electrochemical
Workstation and C3 cell stand. A three-electrode configuration was
used, consisting of a working electrode (platinum disk, area = 0.2 cm2;
gold disk, area = 0.02 cm2; or glassy carbon, area = 0.07 cm2), Pt-wire
auxiliary electrode, and Ag-wire quasi-reference electrode. The
electrodes were polished prior to each experiment. Samples consisted
of (0.3−5.0) × 10−3 M analyte in solution containing 0.1−0.3 M
[NBu4][PF6]. Cyclic voltammetric experiments were conducted over a
range of scan rates (0.010−10 V s−1) to establish electrochemical
reversibility, where observed. Ferrocene was used as an internal
electrode-potential standard;78 under the experimental conditions, the
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and characterization data for W(CR)-
LnL′4−nX complexes; electrode potentials for reductions and
second oxidations; calculated orbital energies and atomic
contributions for frontier orbitals; comparisons of calculated
energies and bond distances using double-ζ and triple-ζ-quality
basis sets; comparisons of orbital atomic contributions using
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different supporting-electrolyte concentration than that used here.
9657
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Inorganic Chemistry
Article
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