Groups 5 and 6 terminal hydrazido(2-) complexes: Nβ substituent effects on ligand-to-metal charge-transfer energies and oxidation states

Article abstract of DOI:10.1021/ja302275j

Brightly colored terminal hydrazido(2-) (dme)MCl₃(NNR ₂) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl₄(NNR₂) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR ₂) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N_β lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N_β in order to reduce the overlap between the Nb=N _α π bond and the N_β lone pair. For W complexes, increasing the energy of N_β eventually leads to reduction from formally [W^VI≡N-NR₂] with a hydrazido(2-) ligand to [W^IV=N=NR₂] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives.

Full text of DOI:10.1021/ja302275j

Communication
pubs.acs.org/JACS
Groups 5 and 6 Terminal Hydrazido(2−) Complexes: Nβ Substituent
Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation
States
Ian A. Tonks, Alec C. Durrell, Harry B. Gray,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: Brightly colored terminal hydrazido(2−)
(dme)MCl3(NNR2) (dme = 1,2-dimethoxyethane; M =
Nb, Ta; R = alkyl, aryl) or (MeCN)WCl4(NNR2)
complexes have been synthesized and characterized.
Perturbing the electronic environment of the β (NR2)
nitrogen affects the energy of the lowest-energy charge-
transfer (CT) transition in these complexes. For group 5
complexes, increasing the energy of the Nβ lone pair
decreases the ligand-to-metal CT (LMCT) energy, except
for electron-rich niobium dialkylhydrazides, which pyr-
amidalize Nβ in order to reduce the overlap between the
NbNα π bond and the Nβ lone pair. For W complexes,
increasing the energy of Nβ eventually leads to reduction
from formally [WVIN−NR2] with a hydrazido(2−)
ligand to [WIVNNR2] with a neutral 1,1-diazene
ligand. The photophysical properties of these complexes
highlight the potential redox noninnocence of hydrazido
ligands, which could lead to ligand- and/or metal-based
redox chemistry in early transition metal derivatives.
nd-on (κ1)-bound hydrazido(2−) complexes of early
E
transition metals have been identified as important
intermediates in an increasingly diverse range of stoichiometric
and catalytic processes. Because one of the proposed complexes
along a Chatt-type1 cycle of N2 reduction is a κ1-bound
hydrazido(2−) moiety (MNNH2), much research into
possible models for these intermediates, most notably on
central metals Mo and W, has been undertaken.2 It also is of
interest that group 4 hydrazido(2−) complexes have been
utilized for the catalytic diamination and hydrohydrazination of
alkynes with 1,1-disubstituted hydrazines.3−7
Figure 1. MO energy level diagrams for (dme)TaCl3(NR) and
(dme)TaCl3(NNR 2) π interactions. Nβ filled p mixing raises the
hydrazido(2−) HOMO energy (right) relative to the imido HOMO
(left).
complexes as well as those for a similar (MeCN)WCl4(NNR2)
series.
Although the chemistry of hydrazides of groups 4 and 6 has
been well-studied, there is a paucity of reports on similar group
5 complexes, in particular the heavier congeners Nb8 and Ta.9
We recently reported on the synthesis of (dme)MCl3(NNPh2)
(dme = 1,2-dimethoxyethane; M = Nb, Ta) and its use as a
synthon for a variety of MNNR2 complexes.10 One notable
feature of (dme)TaCl3(NNPh2) is its blue color (λmax = 585
nm), which we postulated to arise from a π* interaction
between the MNα π bond and the Nβ lone pair that
destabilizes the highest occupied molecular orbital (HOMO)
and lowers the ligand-to-metal charge-transfer (LMCT) energy
relative to the parent imido (Figure 1). Here we report the
photophysical properties of (dme)MCl3(NNR2) (M = Ta, Nb)
The resonance Raman spectrum (585 nm excitation) of
(dme)TaCl3(NNPh2) (1d) (Figure 2) shows enhancement of
the Ta−N, N−N, and aryl stretches. These enhancements
confirm our assignment of this low-energy band to CT between
Ta and the hydrazido ligand. Importantly, the Raman spectrum
also indicates that there is aryl character in the HOMO, which
led us to investigate further the effect of substituted
arylhydrazides on the CT behavior. We hypothesized that
tuning the electronic properties of Nβ would make possible a
Received: March 7, 2012
Published: April 16, 2012
© 2012 American Chemical Society
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Communication
Figure 2. Resonance Raman spectrum of (dme)TaCl3(NNPh2) (1d)
showing features attributable to N−N (865 cm−1), Ta−N (1175
cm−1), and aryl (1500−1650 cm−1) stretching vibrations.
Figure 3. Absorption spectra of 1a−e in C2H4Cl2 solution.
change in the formal donor ability of the [NNR2] ligand from a
dianionic hydrazido(2−) to a neutral 1,1-diazene, thereby
accessing a new two-electron “redox-noninnocent” manifold.
The complexes (dme)MCl3(NNR2) (1a−f for M = Ta, 2a−i
for M = Nb; R = alkyl, aryl) were synthesized from MCl5 and
the corresponding 1,1-disubstituted hydrazine via the pre-
viously reported Lewis acid-assisted dehydrohalogenation (eq
1).10,11 While the niobium dialkylhydrazide reactions (2g−i)
Table 1. Lowest-Energy CT Peaks in the Spectra of
(dme)MCl3(NNR2) and (MeCN)WCl4(NNR2) Complexes
a
M
R2
λmax (cm−1
)
ε (M−1 cm−1
)
Ta
C12H8 (1a)
18650
17670
17640
17100
16690
15700
14580
14560
14370
13790
16420
16260
17860
16501
15750
103
93
(p-ClPh)2 (1b)
(p-BrPh)2 (1c)
Ph2 (1d)
103
245
167
66
(p-CH3Ph)2 (1e)
C12H8 (2a)
Nb
(p-ClPh)2 (2b)
(p-BrPh)2 (2c)
Ph2 (2d)
28
30
38
(CH3)(Ph) (2g)
(CH3)2 (2h)
C5H10 (2i)
44
17
25
W
C12H8 (3a)
350
370
570
400
330
Ph2 (3d)
(p-CH3Ph)2 (3e)
(CH3)2 (3h)
(CH2)5 (3i)
23260 (sh)
22990 (sh)
a
The data are ordered according to increasing electron density of Nβ
within each metal series.
proceed cleanly in good yield, we were not able to synthesize
any tantalum dialkylhydrazides via this route. The complexes
(MeCN)WCl4(NNR2) (3a−i) were synthesized by treating
WCl6 with the corresponding 1,1-disubstituted hydrazine and
MeCN in CH2Cl2 (eq 2).12 All of the metalations proceeded in
good to excellent yields. However, obtaining spectroscopically
pure material proved to be difficult (lower yields), as the
impurities of the reactions tend to be highly colored with
extinction coefficients orders of magnitude larger than those of
the desired metal hydrazido(2−) complexes. For 2e,f, and
3b,c,f,g, spectroscopically pure materials were not obtained in
reproducible quantities.
The absorption spectra of 1a−f, 2a−d,g-i, and 3a,d,e,h,i in
C2H4Cl2 were measured. In addition to low-energy MNNR2
CT, higher-energy bands associated with M−Cl LMCT
transitions were observed. In most cases, the lowest-energy
band, which was well-resolved from other absorptions,
exhibited a nearly Gaussian shape. The absorption spectra of
1a−e are presented in Figure 3, and λmax and ε values for all
compounds are reported in Table 1. Absorbances for the Ta
derivatives had λmax between 15300 and 18900 cm−1 and ε
between 100 and 250 M−1 cm−1, while the Nb congeners gave
lower-energy λmax values between 13800 and 16500 cm−1 with ε
values of 17−66 M−1 cm−1. Unlike the related imido
complexes, 1 and 2 did not exhibit fluorescence.13 Absorbances
for the W complexes ranged from 15800 to 23200 cm−1 with ε
values of 330−570 M−1 cm−1. Bands for the tungsten
dialkylhydrazides were not well resolved from higher-energy
absorptions and appeared as shoulders on the much more
intense W−Cl LMCT systems. The extinction coefficients for
these transitions are quite small because the transition occurs
between two essentially orthogonal orbitals (Figure 1).
The LMCT ε values for these complexes also reinforce our
assignment of the lowest-energy bands, since alternatives such
as allowed π/π* transitions would be expected to give rise to
more intense absorptions. Also, since fully conjugated 2a has a
higher-energy λmax than the other diaryls, a solely ligand π-
based transition can be ruled out, as an increase in conjugation
should decrease λmax for a π/π* system.
In each metal series, the energy of the CT in the
diarylhydrazide complexes decreased dramatically with increas-
ing electron density around Nβ, as correlated to the Hammett
parameter σpara. This bathochromic shift shows that increasing
the Nβ lone-pair energy increases the MNα π bond and Nβ
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lone pair antibonding interactions in the HOMO, thus raising
its energy and shrinking the HOMO−LUMO gap (Figure 4). It
Figure 5. Thermal ellipsoid drawings of (left) (dme)NbCl3[NN-
(CH2)5] (2i) and (right) (MeCN)WCl4[NN(CH2)5] (3i) viewed
down the edge of the piperidyl ring. The different geometries of N2
(Nβ) should be noted. Selected bond distances (Å) and angles (deg)
in 2i: Nb−N1, 1.7597(1); N1−N2, 1.3392(1); ∑∠N2, 341.1. In 3i:
W−N1, 1.7633(1); N1−N2, 1.2556(1); ∑∠N2, 359.8.
Figure 4. Increasing the energy of the Nβ p orbital increases the
HOMO energy and decreases the LMCT energy in diarylhydrazides.
geometry of Nβ gives insight into the nature of the LMCT
energy discrepancy in the dialkylhydrazide complexes (Figure
5).
For all of the diarylhydrazidos, Nβ is planarized as a result of
steric and electronic factors. However, the niobium dialkylhy-
drazides (2h, 2i) are pyramidalized at Nβ, and as a result, the
overlap between the NbNα π and Nβ orbitals is reduced
(Figure 6). Consequently, the HOMO energy is not increased
should be noted that this shift is not strictly attributable to an
inductive effect of Nβ, as the more inductively withdrawing
alkoxyimidos (dme)MCl3(NOMe) have higher-energy LMCTs
(M = Ta, 23 750 cm−1; M = Nb, 21 300 cm−1) than the
hydrazido complexes.
The observed trend in the peak energies is similar to the “aryl
effect” that has been observed in related (dme)MCl3(NR) (R =
alkyl, aryl) imido complexes, in which small aryl−(MNα) π
antibonding interactions lower the LMCT energy of the
arylimidos relative to the alkylimidos.14 However, the
antibonding effect on the band energies is much more dramatic
in the hydrazido complexes than in the arylimidos.
On the basis of the observations for the arylhydrazides, we
would expect the dialkylhydrazide complexes (2h, 2i, 3h, and
3i), which are more electron-rich, to display much lower energy
LMCT bands than the diarylhydrazides. However, both the Nb
and W dialkylhydrazides exhibited much higher energy
absorptions than the arylhydrazides.
The origin of the dialkylhydrazide energy discrepancies was
ascertained by examining the crystal structures of the series. X-
ray-quality crystals of 1a, 1c, 1d, 1e, and 2a−i were obtained by
slow diffusion of pentane into saturated solutions of the metal
hydrazido(2−) complex in C2H4Cl2. 3i was crystallized from a
saturated solution of 3i in 2:1 pentane/toluene cooled to −30
°C. Table S1 in the Supporting Information shows the M−N
and N−N bond distances and the sum of all the angles around
Nβ for all of the crystallized complexes. The M−N bond
distances are typical for triple bonds, and the N−N distances
are shorter than that of free diphenylhydrazine (1.418 Å). In
general, the N−N bond distances for the W complexes are
shorter than in either the Nb or Ta cases and have a larger
spread, but these differences are still relatively small. The sum
Figure 6. Pyramidalization of Nβ reduces the overlap of the lone pair
with the M−N π bond, resulting in greater than expected LMCT
energies for the niobium dialkylhydrazides.
as much as would be predicted on the basis of the
diarylhydrazide trend. Odom observed a similar trend for
ligand-to-ligand CTs in titanium hydrazido complexes.5b
Conversely, the tungsten dialkylhydrazides (3h, 3i) remain
planarized in the solid state yet still do not display low-energy
CT transitions that would be in line with the arylhydrazide
trend. Since a planarized dialkylhydrazide should have a very
low energy λmax on the basis of the aryl trends discussed above,
we speculate that the tungsten(VI) dialkylhydrazidos in fact
formally consist of 1,1-diazene ligands coupled to a WIV center.
of the angles about Nβ indicates that Nβ is planarized (∑∠N2
≈
360°) in all examples except for dialkylhydrazides 2h and 2i,
which are pyramidalized (∑∠N2 ≈ 340°) (Figure 5). The bond
length data show no trend with the electron richness of the
hydrazido ligand. As a result, no conclusions about either the
electronic structure or the donor ability of the hydrazido ligand
can be made by examining the bond lengths. However, the
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In this case, the antibonding interaction between the WNα π
and Nβ lone-pair orbitals is so strong that the MO is higher in
energy than the metal-based orbitals, resulting in a formal
reduction of W by the hydrazide; thus, the observed transition
is metal-to-ligand CT (MLCT) rather than LMCT (Figure 7,
ACKNOWLEDGMENTS
■
We thank Lawrence Henling and Dr. Michael Day for
assistance with the X-ray studies. The Bruker KAPPA APEXII
X-ray diffractometer was purchased via an NSF CRIF:MU
Award (CHE-0639094) to the California Institute of
Technology. This work was supported by the U.S. DOE Office
of Basic Energy Sciences (DE-FG03-85ER13431 to J.E.B.).
Photophysics investigations were supported by an NSF Center
for Chemical Innovation Grant (CHE-0802907 to H.B.G.).
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Figure 7. WNNR2 units exhibit different formal donor characteristics
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right diagram). It is difficult to speculate on why the MLCT
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̈
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by redox at noninnocent −NNHx (x = 1, 2) ligands. Analogous
N2 reduction cycles would thus be less favored for group 5 and
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and spectroscopic, crystallographic (CIF),
and analytical data for new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
bercaw@caltech.edu
■
Notes
The authors declare no competing financial interest.
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