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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（1），急

. Introduction

Redox properties are critical to many applications of
transition metal complexes with multiply bonded lig-
ands such as oxo (O2− ), organoimido (NR2−

, and
nitrido (N3− ) groups [1]. Metal oxo species are widely
used as stoichiometric and catalytic oxidants, including
permanganate, chromium(VI) species, OsO4and asym-
metric variants, and manganese oxo compounds for
epoxidation [2]. In nature, the metal oxo group is
involved in many metalloenzyme oxidations, for in-
stance as the active species in the cytochromes P450 [3].
Cummins and co-workers recently reported a facile
laboratory cleavage of N2involving the redox formation of molybdenum nitrido complexes [4]. The use of
imido groups as supporting ligands in oleﬁn metathesis
catalysts is successful in part because of the resistance
of such compounds to reduction [5].

Redox properties are often quantiﬁed using electro-
chemical potentials, as measured by cyclic voltammetry.
But redox reactions of compounds with multiply
bonded ligands are often inner-sphere multi-electron
processes, in which redox at the metal is coupled to the
formation or breaking of new chemical bond(s) involv-
ing the multiply-bonded ligand. Oxygen atom and ni-
trogen atom transfer reactions are classic examples [6].
This coupling of redox change with bond formation is
a critical part of the utility of these reagents. But rates
of such reactions are not simply related to one-electron
electrochemical potentials.
Our work in recent years has been focused on addition of nucleophiles to multiply bonded ligands, such as
the intermolecular addition of phenyl anion to the
osmium nitrido complex TpOs(N)Cl2[7] and the in-
tramolecular migration of phenyl anion to an oxo
ligand in [TpReO2Ph][OTf] [8]. Reported here are elec-
trochemical potentials for a series of isoelectronic and
isostructural rhenium oxo, rhenium imido, and os-mium nitrido complexes containing a hydrotris(1-pyra-zolyl)borate ligand, either Tp [HB(pz)3] or Tp*
[HB(3,5-Me2pz)3]. A wide range of potentials is ob-served depending on the metal, the multiply bonded
ligand, and the ancillary ligands. Explored herein are
the connections between redox potentials and inner-sphere reactivity (or electrophilicity), and other
properties.

2. Experimental

The syntheses and characterization of the compounds
discussed in this paper are reported elsewhere:
TpRe(O)(X)(Y) [8,9], Tp*Re(O)(X)(Y) [10], TpRe(N-
tolyl)(X)(Y) [11], and TpOs(N)X2[7]. Electrochemical
measurements were made using a Bioanalytical Systems
B/W 100 or a CV27 electrochemical analyzer with IR
compensation. Most measurements were made under
N2in a Vacuum Atmospheres drybox. The electro-
chemical cell used a platinum disk working electrode, a
Ag/AgNO3(0.01 M in acetonitrile) reference electrode,
and a platinum wire auxiliary electrode. The supporting
electrolytic solution was 0.1 MnBu4NPF6(triply re-
crystallized from ethanol) in acetonitrile. The acetoni-
trile solvent was dried over from CaH2and P2O5and
vacuum transferred prior to use or (more recently)
taken directly from a pressurized stainless steel vessel of
highly puriﬁed solvent (Burdick and Jackson) piped
directly into the glove box. All measurements were
referenced internally to ferrocene and are reported ver-
sus Cp2Fe+ /o [12].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

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 theHammett |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 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.

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