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专业: 同步辐射技术及其应用

[交流] 求助翻译文献一篇（2），急

3. Results and discussion

3.1. Electrochemistry of Tp(*)M(E)(X)(Y)

Cyclic voltammograms (CVs) are described for 29
Tp rhenium oxo, Tp* rhenium oxo, Tp rhenium
tolylimido, and Tp osmium nitrido complexes with the
general formula Tp(*)M(E)(X)(Y) (Tp(*)=Tp, Tp*;
M= Re, Os; E= O, Ntolyl, N; X, Y= hydrocarbyl,
halide, triﬂate). All of the compounds are d2and dia-
magnetic. A number of X-ray crystal structures show
the distorted octahedral structures typical of this class
of molecules (A) [1,7 – 11]. CVs for most of the com-
pounds show both oxidative and reductive waves. The
oxidations from d2to d1species are typically quasi-re-
versible. Ratios of anodic to cathodic peak currents
(ip,a/ip,c) usually approach one, at least at higher scan
rates (up to 0.5 V s−1). This indicates that the d1
species have lifetimes of at least seconds so that they
can be reduced back in close to quantitative yields.
Further evidence of quasi-reversibility are the peak-to-
peak separations of 80 – 100 mV, close to that of the
Cp2Fe+ /o couple in the same solution. In contrast,
most of the reductions from d2to d3complexes are
irreversible. A typical CV, for Tp*Re(O)(Cl)(Br), is
shown in Fig. 1. The potentials for oxidation from d2to d1com-
pounds are listed in Table 1. Each series of Tp(*)M(E)
compounds is given in a column, while compounds with
the same X, Y ligands are on the same row. The data
are plotted in Fig. 2 versus the sum of the Hammett |p
values for the X and Y ligands [13]. For each set of X,Y
ligands, the rhenium imido complex is most easily
oxidized, followed by the Tp* and Tp rhenium oxo
complexes, and ﬁnally the osmium nitrido analog. The
gap between these types of complexes is large and
consistent for different X,Y ligands. The TpRe(O) com-
plexes are 0.7 V harder to oxidize than the TpRe(N-
tolyl) compounds, and the TpOs(N) derivatives are
another ca. 0.7 V higher. The 1.4 V higher potentials
for TpOs(N) versus TpRe(Ntolyl) compounds is per-
haps surprising, since a nitrido ligand is more donating
than an imido group (NAr2−=N3− plus Ar+). Clearly
the OsVII/OsVIredox couple is dramatically more oxi-
dizing than ReVI/ReV. The Tp* complexes are slightly
more electron rich than the Tp analogs: The difference
in potentials varies from 110 mV for Tp/
Tp*Re(O)(Et)(Cl) to 180 mV for Tp/Tp*Re(O)Cl2.
Within each series of Tp(*)M(E) compounds, the
d2– d1potentials correlate well with the sum of the

Hammett |pparameters for X and Y (Fig. 2). The ease
of oxidation varies as triﬂateB halideBaryl Balkyl,
with a range of over 1.1 V in E1/2 between TpRe(N-
tolyl)(OTf)2and TpRe(Ntolyl)(Ph)2. Substituting OTf
for halide lowers the E1/2 values by about −0.25 V per
exchange: Tp*Re(O)(Ph)(OTf) and TpRe(N-
tolyl)(Ph)(OTf) are 0.26 and 0.28 V harder to oxidize
than the phenyl-chloride derivatives, and the difference
between TpRe(Ntolyl)(OTf)2and TpRe(Ntolyl)Cl2is
0.42 V. There is little effect in switching from chloride
to iodide (B 50 mV), with the exception of the series
TpRe(O)(Cl)2(1.34 V), TpRe(O)(Cl)(I) (1.25 V),
TpRe(O)(I)2(1.18 V). The ﬂuoride complex
Tp*Re(O)F2(indicated in Fig. 2 by the symbol ), and
to a lesser extent Tp*Re(O)F(Cl), are harder to oxidize
than the Hammett parameters predict. Substituting Cl
or I by aryl makes a complex 0.30 V easier to
oxidize. The alkyl complexes are another 0.1 V more
electron rich than the aryl derivatives, consistent with
the more negative | values for alkyl over aryl
substituents.
In contrast to the well-ordered and quasi-reversible
potentials for oxidation, cyclic voltammograms reveal
irreversible and often ill-resolved reductions from the d2
complexes to d3anions. The potentials for the cathodic
peaks, Ep,c(Table 2), do not show the simple trends of
the oxidation potentials. This is emphasized by a plot
(Fig. 3) of Epfor reduction versus Hammett |X+ |Y,
analogous to Fig. 2. There is a trend of more negative
Epvalues at lower (|X+|Y) for TpRe(Ntolyl)XY,
Tp*Re(O)XY, and TpOs(N)XY, but the values for
TpRe(O)XY compounds are irregular. The trend line
for TpRe(Ntolyl)XY compounds has a very different
slope than the others, so little can be said about the

relative reducibility of the compounds. The more com-
plicated pattern is in part due to irreversible nature of
the electrochemistry, which probably in many cases is a
result of rapid chemical reactions of the reduced species
(EC processes [14]). Meyer and coworkers have studied
electrochemical reductions of osmium(VI) nitrido com-
plexes including TpOs(N)Cl2in more detail, especially
their coupling to m-N2species [15]. Of the compounds
examined here, triﬂate and iodide derivatives are often
the easiest to reduce (the least negative potentials),
possibly due to rapid loss of the anionic ligand on
reduction. Follow-up chemical reactions can signiﬁ-
cantly shift the CV wave from the thermodynamic potential [14]. For these and possibly other reasons, the
peak potentials for reduction do not follow the straight-
forward trends of the oxidative E1/2 values.

3.2. Connection to reaction chemistry, spectroscopy,
and structure

The d1– d2redox potentials — much more than the
d2– d3potentials — are a good measure of the general
electron richness of the metal complexes. One example
can be found in the1H-NMR chemical shifts. Within
each series, the shift of the triplets due to the protons in
the 4-position of the pyrazole rings parallels the redox
potential, with the more electron poor compounds
showing more downﬁeld shifts (Table 3) [16]. This is
reminiscent of the analysis of13C chemical shifts in
tert-butylimido complexes by Nugent et al. [17]. Within
the group of rhenium complexes, metal ligand bond
distances also appear to follow with d1– d2redox poten-
tials (Table 4). The more electron-poor the complex,
the shorter the bonds — reﬂecting higher Lewis acidity.
Thus Re Ph and Re OTf bonds are shorter in the
Tp oxo versus the Tp imido derivatives. The
Re N(pyrazolecis) distances have a less consistent trend.
The comparison of Re N distances cis versus trans to
the multiple bond show the typical order of trans
inﬂuence, N3− \O2− \ NR2− [1]. The trans inﬂuence
of the phenyl ligand in these systems is almost identical
to that of the imido ligand. These results parallel the
extensive study of related TpMo complexes by Boncella
and co-workers, which reported the trans inﬂuences in
the order alkylidyne oxo\ imido alkylidene\
amido \alkoxy \alkyl m-oxo \triﬂate [18].
The reactivity of these rhenium and osmium com-
plexes with outersphere oxidants is, as expected, related
to their reversible d1/d2reduction potentials. For in-
stance, only the most electron-rich compounds such as TpRe(Ntolyl)(Et)(Cl) are oxidized by silver ion (yield-
ing silver metal) in preference to halide abstraction
[11b]. Our primary interest in these compounds is their
ability to act as inner-sphere oxidants, with the multiply
bonded ligand acting as an electrophile. The reactivity
with PPh3is one measure of this electrophilicity.
TpOs(N)Cl2reacts within time of mixing to give the
phosphinimine complex TpOs(NPPh3)Cl2, which has
been structurally characterized [19]. In contrast, the
diphenyl derivative TpOs(N)Ph2reacts much more
slowly (hours) and gives intractable material. Reduction
of TpRe(O)Cl2by PPh3requires hours in reﬂuxing
toluene [9b]. Finally, TpRe(Ntolyl)Cl2does not react
with triphenylphosphine (which may be inﬂuenced by
steric interactions). Thus the order of reactivity is
TpOs(N)Cl2\ TpOs(N)Ph2\ TpRe(O)Cl2\TpRe(N-
tolyl)Cl2.
A similar pattern is observed in reactions with aryl
anion sources, PhMgX, PhLi, and aryl zincates.
TpRe(Ntolyl)Cl2reacts with all of these reagents in a
non-redox fashion, by metathesis of chloride ligand(s)[11]. The oxo analog appears to be reduced by Grig-
nard and lithium reagents, so the softer zincate is
needed to prepare oxo aryl complexes [8,9].
TpOs(N)Cl2reacts with PhMgBr and PhLi by direct
addition of Ph− to the nitrido ligand to give a
phenylimido complex (Eq. (1)) [7]. This reaction occurs
in time of mixing at ambient temperatures. Similar Ph−
addition to the nitrido ligand occurs for
TpOs(N)(Ph)Cl and TpOs(N)Ph2but is much slower,
requiring hours and days, respectively. In contrast,
Tp*Os(N)Ph2is nucleophilic at nitrogen, being alky-
lated by methyl triﬂate to give the methylimido cation,
[Tp*Os(NMe)Ph2]+[20]. As in the PPh3reactions, the
order of electrophilic reactivity and the ease of reduc-
tion of the metal is TpOs(N)Cl2\ TpOs(N)(Ph)Cl\
TpOs(N)Ph2\TpRe(O)Cl2\ TpRe(Ntolyl)Cl2. The
reactivity trend parallels the one-electron potentials for oxidation much better than the d2/d3reductive peak potentials.The electronic structure of these TpM(E)(X)(Y) com-
plexes (as sketched in Scheme 1 [1a]) provides a frame-
work for understanding the electrochemistry and
reactivity. The two d electrons occupy the dxyorbital
that lies in the plane perpendicular to the M E axis. dxy
engages in a little p-bonding, so it is essentially a
nonbonding level. The electrochemical potential for
removal of an electron from this orbital — E for the
d1/d2couple — therefore indicates the overall electron
richness of the complex rather than any speciﬁc interac-
tion(s). This is why the potentials correlate so well with
Hammett | values.
Electrochemical reduction places an electron into a
LUMO (dxz, dyz) which is predominantly M E p-anti-
bonding in character, with a small amount of M X/Y |
antibonding character because the E M X/Y angles are
\90°. The reduction potential therefore reports both
on the overall electron richness and on the speciﬁc
nature of the M E and perhaps M X/Y interactions.
The lack of simple trends in the peak reduction poten-
tials is likely due to these issues and the complications
associated with interpreting irreversible electrochemical
processes.
The electrophilicity of the multiply bonded ligand
should be in large part controlled by the energy and the
character of the LUMO, because this is the orbital that
a nucleophile attacks. Thus the nitrido ligand in
TpOs(N)Cl2is quite electrophilic because of the low
LUMO energy and its large nitrogen ppcharacter [21].
Substituting chloride for phenyl makes TpOs(N)Ph(Cl)
and TpOs(N)Ph2more electron rich raising the LUMO
energy and reducing its nitrogen character, consistent
with the observed lower electrophilic reactivity. Within
this series of three compounds, both the oxidative and
reductive redox potentials parallel the reactivity. Over a wider range of complexes, however, electrophilicity
does not correlate with the peak reduction potentials.
This is probably due both to the complications in the
electrochemical values and also to differences in M E
covalency and therefore the character of the LUMOs.

3.3. Comparisons with related systems

The comparisons presented here are complimentary
to those described by Marshman and Shapley for re-
lated osmium(VI) oxo/imido/nitrido compounds [22].
The oxidation potentials of [Os(N)RnCl4− n]− com-
pounds (R= CH2SiMe3; n = 0, 2, 4) show that each
exchange of chloride for alkyl lowers the potential by a
sizable 0.2 – 0.5 V (Table 5). This is similar to the ca. 0.5
V changes observed for oxidation of Tp(*)Re(E)(Cl)2
versus Tp(*)Re(E)(Et)(Cl). The [Os(N)RnCl4−n]−po-
tentials directly correlate with the reactivity of the
nitrido ligand. [Os(N)Cl4]− is weakly electrophilic at
nitrogen, while the much more easily oxidized
[Os(N)(CH2SiMe3)4]− is nucleophilic [22]. If this differ-
ence between halide and hydrocarbyl ligands is general,
it provides a partial explanation for stability and lack
of oxidizing power of high oxidation state alkyl and
aryl complexes. One example is the unusual lack of
reactivity of the chromium(VI) complex Cp*Cr(O)2Me
[23].
The inﬂuences of other ancillary ligands on poten-
tials, OTf \Cl$ I, are consistent with the much more
extensive and quantitative analyses presented by Lever
and others for coordination complexes [24]. Lever’s EL
parameters are 0.13 (OTf) \ −0.24 (Cl) = −0.24 (I).
We have chosen to correlate potentials with Hammett
parameters in Fig. 2 rather than ELbecause the latter
are not available for alkyl or aryl ligands. The straightforward trends observed here contrast with data re-
ported by Enemark and co-workers for the very similar
d1/d2Tp*Mo(O)(X)(Y)o/ − redox couple [25]. With X,
Y=alkoxide or thiolate, the potentials are very sensi-
tive to subtle effects such as chelate ring size, appar-
ently because of the presence of strong p-bonding
between these ligands and the dxyorbital.

The 110 – 180 mV differences between analogous Tp
and Tp* complexes (Table 1) are larger than other
examples in the literature. Skagestad and Tilset re-
ported 10 – 80 mV differences for Tp(*)M(CO)3o/ −,
Tp(*)M(CO)3+ /o,and Tp(*)M(CO)3H+ /o potentials
(M= Cr, Mo, W) [26], and there is only 60 mV between
the potentials for Tp2Fe+ /o and Tp*2Fe+ /o [27]. In all
cases, the electronic differences between Tp and Tp*
complexes are small compared with the other effects
discussed here. Differences among substituted Tp lig-
ands (and Cp ligands) are discussed by Mountford and
co-workers using13C chemical shifts in t -butylimido
complexes [28] (and see references therein).
The osmium imido complex Os(NMe)R4is 0.3 V
easier to oxidize than Os(O)R4(R = CH2SiMe3) [22].
This is in the same direction but only half the 0.6 – 0.7
V difference observed here for TpRe(Ntolyl)(X)(Y) ver-
sus TpRe(O)(X)(Y) (Table 5). Many imido complexes
are qualitatively known to be less oxidizing than their
oxo analogs. For instance, CrVI(NR)2X2complexes are
very unreactive compared with the strongly oxidizing
CrO2X2species (which oxidize hydrocarbons for X=
halide) [29]. This ability of imido ligands to stabilize d0
species against reduction is part of their value as ancil-
lary ligands in alkene metathesis catalysts.

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