Multinuclear NMR Study of the Structure and Intramolecular Dynamics of (η2-Acetone phenylhydrazonato)tetrafluorooxotungsten(VI)

Article abstract of DOI:10.1021/ic950330y

Using one- and two-dimensional techniques of ¹H, ¹³C, ¹⁵N, and ¹⁹F NMR spectroscopy, it was proved that acetone phenylhydrazone (H-aph) acts as a two-center N,N-donor ligand in the complex [WOF₄(aph)]^-. As a result of the coordination to tungsten, the ligand conformation changes from E to Z, and at the same time the phenyl ring is turned out of the >N-N=C< plane. At low temperatures (-30 °C), the equatorial plane of the pentagonal-bipyramidal tungsten polyhedron comprises two donor nitrogen atoms of hydrazone and three fluoro ligands. At elevated temperatures, stereoisomeric interconversion caused by hindered intramolecular rotation of the aph^- ligand occurs. The mechanism of this process was elucidated on the basis of the dynamic effects in ¹⁹F spectra and line-shape analysis of the trans-CH₃ resonance in ¹³C spectra split due to direct through-space coupling with the proximate fluoro ligand.

Full text of DOI:10.1021/ic950330y

5514
Inorg. Chem. 1996, 35, 5514-5519
Multinuclear NMR Study of the Structure and Intramolecular Dynamics
of (η2-Acetone phenylhydrazonato)tetrafluorooxotungsten(VI)
Sergei G. Sakharov* and Yuri A. Buslaev
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Moscow, 117907 Russia
Ivan Tka´cˇ
Magnetic Resonance Unit, Derer Hospital, 833 05 Bratislava, Slovakia
ReceiVed March 22, 1995X
1
Using one- and two-dimensional techniques of H, 13C, 15N, and 19F NMR spectroscopy, it was proved that
acetone phenylhydrazone (H-aph) acts as a two-center N,N-donor ligand in the complex [WOF4(aph)]-. As a
result of the coordination to tungsten, the ligand conformation changes from E to Z, and at the same time the
phenyl ring is turned out of the >NsNdC< plane. At low temperatures (-30 °C), the equatorial plane of the
pentagonal-bipyramidal tungsten polyhedron comprises two donor nitrogen atoms of hydrazone and three fluoro
ligands. At elevated temperatures, stereoisomeric interconversion caused by hindered intramolecular rotation of
the aph- ligand occurs. The mechanism of this process was elucidated on the basis of the dynamic effects in 19
F
spectra and line-shape analysis of the trans-CH3 resonance in 13C spectra split due to direct “through-space”
coupling with the proximate fluoro ligand.
Introduction
catalytic and biochemical processes. The analogy is not
accidental, since it results from the similarity of the structures
The late transition metals form, due to their favorable
electronic structure, a wide range of coordination compounds
where the coordination numbers higher than 6 are rather
common. Group V and VI d0 transition-metal fluorides usually
form pentagonal-bipyramidal η2-complexes with two-center
n-donor ligands (containing -O-O-, -O-N-, N-N, and
other similar groupings).1-3 These ligands (oximes, hydrazones,
hydrazine, hydroxylamine, hydrogen peroxide, and their deriva-
tives) have two contiguous n-donor centers (directly bonded)
in contrast to bidentate ligands, where coordinating centers are
separated by at least one noncoordinating bridging atom.
Complexes of WOF4 with two n-donor atoms in the equatorial
plane are normally stable only at low temperatures. At elevated
temperatures intramolecular dynamic processes involving two-
center donors occur.4,5 NMR spectroscopy, especially 19F NMR,
is a very efficient method for monitoring these processes.6-8
Complexes with two-center ligands,9 as well as olefin
π-complexes,10 are often the intermediates in a number of
of these types of compounds. Indeed, two features are typical
of the olefin π-complexes, as well as of the complexes with
n-donor two-center ligands. (1) An organic ligand is coordinated
by the central atom in an η2 fashion.1,11 (2) This η2-coordinated
ligand experiences hindered internal rotation (we were the first
to reveal this phenomenon for the two-center ligands4,12). As
we showed, d0 transition-metal complexes containing n-donor
two-center ligands should be considered specific π-complexes
where the central atom functions as an acceptor of the electrons
from the π-bonding and π-antibonding MOs of the two-center
ligand.2 A systematic study of the coordination ability and
reactivity of two-center ligands, as well as conditions of
formation, structure and dynamic behavior of their complexes,
and a comparison of their properties with those of olefin
π-complexes will allow us not only to enlarge the scope of
traditional notions about the role of π-complexes in metal-
complex catalysis but also to develop a new synthetic strategy
for organic compounds.
In this study, we intend to show that acetone phenylhydrazone
reacts with WOF4 to give, among other compounds, a tungsten
complex with a η2-coordinated ligand. Using multinuclear
NMR spectroscopy, we focused on studying the character of
the metal-ligand bond, conformation, stereochemical arrange-
ment, and dynamic changes at elevated temperatures, which was
the primary goal of our investigation.
X Abstract published in AdVance ACS Abstracts, July 15, 1996.
(1) Sakharov, S. G.; Buslaev, Yu. A. Russ. J. Coord. Chem. (Engl. Transl.)
1994, 20, 3.
(2) Sakharov, S. G.; Buslaev, Yu. A. Russ. J. Inorg. Chem. (Engl. Transl.)
1993, 38, 1385.
(3) Sakharov, S. G.; Kokunov, Yu. V.; Gustyakova, M. P.; Buslaev, Yu.
A. Dokl. Akad. Nauk SSSR 1979, 247, 1197.
(4) Buslaev, Yu. A.; Sakharov, S. G.; Kokunov, Yu. V.; Moiseev, I. I.
Dokl. Akad. Nauk SSSR 1978, 240, 338.
(5) Sakharov, S. G.; Kokunov, Yu. V.; Gustyakova, M. P.; Buslaev, Yu.
A. Dokl. Akad. Nauk SSSR 1984, 278, 637.
(6) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Inorg. Chem. 1992, 31, 3302.
(7) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Z. Anorg. Allg. Chem. 1989, 577, 223.
(8) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Russ. J. Coord. Chem. (Engl. Transl.) 1993, 19, 565.
(9) Stiefel, E. I.; Miller, K. F.; Bruce, A. E.; et al. J. Am. Chem. Soc.
1980, 102, 3624.
Experimental Section
Starting Materials. Tungsten oxide tetrafluoride was prepared by
a reaction of WF6 with WO3 at 300 °C. WOF4 was sublimated under
vacuum prior to use. Tungsten hexafluoride and WO3 were “pure”
grade. The deuterated acetonitrile was dried over 3 Å molecular sieves
for several weeks. Triethylamine was distilled two times over solid
KOH.
(11) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, C71.
(12) Asley-Smith, J.; Kouek, Z.; Johnson, B. F. G.; Lewis, J. J. Chem.
Soc., Dalton Trans. 1974, 128.
(10) Moiseev, I. I. Palladium π-Complexes in Liquid-Phase Olefin Oxida-
tion; Nauka, Moscow: 1971.
S0020-1669(95)00330-2 CCC: $12.00 © 1996 American Chemical Society

An Acetone Phenylhydrazone Complex of W
Inorganic Chemistry, Vol. 35, No. 19, 1996 5515
Acetone phenylhydrazone was synthesized by reaction of phenyl-
hydrazine with a small excess of acetone in the ether in the presence
of a trace of hydrochloric acid as a catalyst. This method is a
combination of different published methods.13,14
Table 1. 1H and 13C Chemical Shifts of the Free Ligand H-aph and
the Ligand Bound in [WOF4(aph)]- at -30 °C
δ(1H)/ppm
δ(13C)/ppm
chemical
shift
H-aph complex ∆δcoord H-aph complex ∆δcoord
The purity of all starting materials was determined by NMR
spectroscopy.
ipso Ph
ortho Ph
meta Ph
para Ph
cis-CH3
trans-CH3 1.94
dC<
147.4
-0.18 113.0
+0.19 129.7
+0.20 119.3
150.3
120.2
128.7
125.4
24.9
+2.9
+6.9
-1.0
+6.1
+8.7
-3.3
+17.7
7.05
7.16
6.72
1.80
6.89
7.35
6.92
2.03
2.59
Preparation of the Samples. WOF4 is rather sensitive to traces of
moisture; therefore, the reaction mixture was prepared in a drybox under
an argon atmosphere using dry reagents kept above 3 Å molecular
sieves. The samples for the NMR study were prepared by combining
a 35% WOF4 solution in deuterated acetonitrile (CD3CN) and acetone
phenylhydrazone (H-aph) in 1:1, 1:2, and 1:3 molar ratios. Triethy-
lamine (NEt3) was used as a buffer to bind the protons released in the
reaction.
+0.23
+0.65
16.5
25.4
21.9a
162.4
144.8
a Doublet with J(19F,13C) ) 6.2 Hz.
CH3)).18,19 The dephasing and refocusing delays were set to 100 and
30 ms, respectively. For free H-aph it was sufficient to collect 256
scans. In the case of the [WOF4(aph)]- complex, the number of scans
was increased to 6000 because of a higher resonance multiplicity due
to 15N-19F coupling. To measure the 3J(15N,1H) values, we used INEPT
NMR Measurements. The 1H, 13C, and natural-abundance 15N
NMR spectra were recorded on a Bruker AM 300 spectrometer
operating at 300.13, 75.45, and 30.42 MHz, respectively. Some 13C
NMR spectra were obtained on a JEOL FX-100 spectrometer operating
at 25.15 MHz. 19F NMR spectra were recorded on a Bruker WP-80
spectrometer operating at 75.26 MHz. The 1H and 13C chemical shifts
were referenced to internal TMS. The NMR measurements were
performed in the temperature range -30 to +40 °C.
1
with selective H decoupling of the cis-CH3 group.20
The long-range couplings of N(1) with protons in ortho positions
of the phenyl ring were obtained by a selective INEPT 2D technique21
that suppresses all J(15N,1H) couplings except for the coupling with
the proton active in the polarization transfer. In this method, 64 t1
increments with 128 scans each (2 dummy) were sampled for a spectral
width of 300 Hz in 512 time domain data points. The t1 increment
was set to 31 ms, giving an F1 spectral width of (8 Hz. The final
digital resolution in F1 after data processing was 0.125 Hz per point.
19F NMR Spectra. The 19F chemical shifts were referred to CFCl3.
Typical conditions for recording 19F spectra were as follows: spectral
width 21 700 Hz, digital resolution 2.6 Hz per point, number of scans
1H NOE Difference Spectra. Presaturation of the chosen CH3
resonance up to 90% was achieved by selective irradiation (γH /2π )
2
2.5 Hz) for 8 s prior to a nonselective 90° reading pulse. A total of 34
scans (2 dummy scans, 8 scans, 4 cycles) for each irradiating frequency
were acquired. Direct subtraction of FIDs (on- and off-resonance) and
subsequent Fourier transform were performed.
1H Homonuclear 2D J-Resolved Spectrum. For the 2D J-resolved
experiment a spectral width of 512 Hz (1.7 ppm, aromatic part) was
used in F2 and (16 Hz in F1; 4 scans (2 dummy) were acquired for
each of 64 t1 increments. Digital resolution was 2 Hz per point in F2
and 0.25 Hz per point in the F1 domain; zero filling was used.
2
100-500. The values of J(19F,19F) coupling constants were refined
by spectral simulation using the PANIC program (Bruker software).
Dynamic NMR. Calculations of a NMR line shape were performed
by a modified version of the program for dynamic NMR spectra
simulations22 using the classical formalism. The temperature was
measured with a precision of (1°. The Eyring equations were used to
evaluate activation parameters of the dynamic process from rate
constants (k ) 1/τ).23a Errors in determining the activation parameters
were estimated by the methods reported in ref 23b.
13C-1H Heteronuclear Correlation 2D NMR Spectra. The
aliphatic and aromatic parts of the spectra were measured separately
in two 2D experiments. Standard conditions were used, and the fixed
evolution delays were set to correspond to the averaged 1J(C,H) values
of 125 and 160 Hz for the aliphatic and aromatic protons, respectively.
For each of 64 t1 increments 32 scans (2 dummy) were acquired. Data
processing with zero filling results in 256 × 128 and 512 × 128
matrices. The final digital resolution was 2.6-4.8 Hz per point in F1
and 3.5 Hz per point in F2.
Results and Discussion
1H and 13C NMR Spectra of Free H-aph. The 1H and 13
C
1
Selective INEPT 13C Spectra. The soft H pulses (γH2/2π ) 25
Hz, i.e. 10 ms pulse duration for 90° flip angle) were applied in the
selective INEPT15 pulse sequence. The delays were optimized for the
polarization transfer via long-range couplings with an average value
chemical shifts of free H-aph in CD3CN at low temperature
are given in Table 1. These shifts are virtually temperature-
independent: in the range from -30 °C to +40 °C, ∆δmax(1H)
) 0.02 ppm and ∆δmax(13C) ) 0.2 ppm. However, they are
rather sensitive to medium acidity due to the protonation of the
electron lone pair of the N(2) imino nitrogen. A sufficiently
high concentration of NEt3 makes it possible to keep the proton
concentration at a low level. Methyl groups of H-aph are
magnetically nonequivalent, and the corresponding CH3 signals
3
2
of 8 Hz for J(Car-Car-Car-H) and J(dC-Cmet-H).
1
15N NMR Spectra. The absolute H frequency of TMS was used
to calculate the resonance frequency of nitromethane (δ(15N) 0.0 ppm),
to which the 15N chemical shifts are referenced (¥(15N) ) 10.136 783
MHz).16
The INEPT technique with polarization transfer from the directly
bonded proton and with refocusing was used for the measurement of
the N(1) nitrogen in free H-aph. The evolution and refocusing delays
were set to 5.6 ms. Acquisition of 256 scans provided an adequate
signal-to-noise ratio.
1
were assigned by H NOE difference spectroscopy (Figure 1).
Selective presaturation of the signal of a more shielded CH3
group (δ 1.80 ppm) resulted in +4% NOE on the NH proton.
On the other hand, presaturation of the CH3 signal at 1.94 ppm
provided no NOE. Therefore, the signal at 1.80 ppm corre-
sponds to the CH3 group in the cis position relative to the N(1)
amino nitrogen. The carbon and nitrogen atoms of acetone
Various versions of 1D and 2D INEPT methods with selective
polarization transfer from protons to nitrogen via three-bond couplings
were used for the measurement of non-proton-bearing nitrogens. Soft
proton pulses in the pulse sequences were applied (γH /2π ) 25 Hz,
2
i.e., 10 ms pulse duration for a 90° flip angle). For selective INEPT15,17
it was desirable to use polarization transfer from the protons of the
trans-CH3 group and their coupling to N(2), because the coupling
constant 3J(15N,1H(trans-CH3)) is twice as large as 3J(15N,1H(cis-
(18) Witanowski, M.; Stefaniak, L.; Webb, G. A. In Nitrogen NMR
Spectroscopy; Annual Reports on NMR Spectroscopy 11B, Webb, G.
A., Academic Press: New York, 1981, p 423.
(19) Martin, G. J.; Martin, M. L.; Gouesnard, J. P. 15N NMR Spectroscopy;
Springer-Verlag: Berlin, 1981; p 254.
(13) Ardagh, E. G. R.; Williams, J. G. J. Am. Chem. Soc. 1925, 47, 2976.
(14) Shine, H. J. J. Org. Chem. 1959, 24, 252.
(15) Bax, A. J. Magn. Reson. 1984, 57, 314.
(16) Mann, B. E. In NMR and the Periodic Table; Harris, R. K., Mann, B.
E., Eds.; Academic Press: New York, 1978; p 97.
(17) Bax, A.; Niu, C. H.; Live, D. J. Am. Chem. Soc. 1984, 106, 1150.
(20) Uhrin, D.; Liptaj, T. J. Magn. Reson. 1989, 81, 82.
(21) Jippo, T.; Kamo, O.; Nagayama, K. J. Magn. Reson. 1986, 66, 344.
(22) Martin, M. L.; Martin, G. J.; Delpuech, J. J. Practical NMR
Spectroscopy; Heyden: London, 1980; p 442.
(23) (a) Sandstro¨m J. Dynamic NMR Spectroscopy; Academic Press:
London, 1982; p 93 (b) Ibid., p 108.

5516 Inorganic Chemistry, Vol. 35, No. 19, 1996
Sakharov et al.
Figure 1. 1H NMR spectra of the reaction mixture at 25 °C (NOE difference spectroscopy). The oC, mC, pC and CH3 (cisC, transC) resonances
correspond to the tungsten complex.
1
hydrazone are coplanar, and the H-aph molecule has an E
conformation relative to >N-N bond:
superimposed H multiplets, the homonuclear 2D J-resolved
technique was used (Figure 2). The projections of the spectra
of the phenyl ortho, meta-, and para protons of the complex
are presented on the right-hand side of the figure. The signals
at 7.11 ppm are the “artifacts” due to folding in of lines outside
the spectral width.28
m
H
p
o
Ntrans
C
H
H
1
m
N
C
2
o
Ccis
The signals of methyl groups were unambiguously assigned
by NOE difference spectroscopy (Figure 1). Signal presatura-
tion of the more shielded CH3 group (δ 2.03 ppm) results in a
5% NOE on the phenyl ortho protons. This effect is measurable
over the entire temperature range studied. The signal at 2.03
ppm evidently corresponds to the CH3 group cis to N(1). In
addition, when it is coordinated to WOF4, the ligand adopts
the Z conformation. Such a conformation change is possible
only if the phenyl ring is turned out of the >NsNdC< plane:
H
H
H
H
E
1
The H-13C heteronuclear correlation 2D NMR technique
was used to unambiguously assign the signals of protonated
carbons. Quaternary carbons were assigned by the selective
INEPT technique.15
Complexes of Acetone Phenylhydrazone with WOF4.
When dissolved in acetonitrile, oxotungsten(VI) tetrafluoride
forms an adduct (WOF4‚CD3CN) with the acetonitrile molecule
in a trans position relative to the oxygen atom.24 The 13C and
19F NMR spectra indicate that increasing the H-aph concentra-
tion in the reaction mixture results in the formation of several
types of mono- and binuclear complexes; specifically [WOF4(H-
aph)], [WOF3(aph)‚CD3CN], [W2O2F9]- , [W2O2F8(aph)]-, and
[W2O2F7(aph)2]- were observed in the systems with oximes,
amines, ketimines, and substituted hydrazines.24-27 If WOF4,
H-aph, and NEt3 are combined in a 1:3:2 molar ratio, the
complex anion [WOF4(aph)]- is the predominant product. In
this paper, we aimed our structural study on just this latter
complex species.
p
H
H
m
m
cis
C
H
o
H
–
o
C
C
H
i
N
N
H
trans
2
1
Z
To unambiguously assign the signals of protonated carbon
atoms in the 13C NMR spectra, 1H-13C heteronuclear correlation
2D NMR experiments were performed. The signals of quater-
nary carbons were assigned by selective INEPT15 with polariza-
tion transfer from meta H to ipso 13C and from cis-CH3 to
1
dC<. The 13C signal of the trans-CH3 group is a doublet. The
13C NMR spectra recorded on two spectrometers operating at
different frequencies (25.15 and 75.45 MHz) indicated that the
splitting is due to spin-spin coupling with one fluoro ligand,
and J(19F,13C) is 6.2 Hz. Taking into account the number of
bonds separating these nuclei, this spin-spin coupling can be
rationalized from a direct “through-space” mechanism29 via
nonbonding orbital overlap due to a spatial proximity of these
nuclei.
The 1H and 13C NMR chemical shifts of this complex
[WOF4(aph)]- are summarized in Table 1. In addition, the
difference between the corresponding values of the coordinated
and free ligand are also presented in this table. To assign
(24) Kokunov, Yu. V.; Sakharov, S. G.; Moiseev, I. I.; Buslaev, Yu. A.
Koord. Khim. 1979, 5, 207.
(25) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Dokl. Akad. Nauk SSSR 1987, 294, 1132.
(26) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
SoV. J. Coord. Chem. (Engl. Transl.) 1987, 13, 282.
(27) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Koord. Khim. 1991, 17, 1224.
(28) Wider, G.; Baumann, R.; Nagayama, K.; Ernst, R; Wuthrich, K. J.
Magn. Reson. 1981, 42, 73.
(29) Jerome, F. R.; Servis, K. L. J. Am. Chem. Soc. 1972, 94, 5896.

An Acetone Phenylhydrazone Complex of W
Inorganic Chemistry, Vol. 35, No. 19, 1996 5517
Figure 2. (Left) Aromatic part of the 2D J-resolved spectrum of the reaction mixture at 25 °C. The oC, mC, and pC resonances correspond to the
tungsten complex. (Right) Projections of meta, para, and ortho protons of the phenyl group of the [WOF4(aph)]- complex.
Table 2. 15N Chemical Shifts and Coupling Constants of the Free Ligand H-aph and of the Ligand Bound in [WOF4(aph)]- at -30 °C
δ(15N)/ppm
chem shift
H-aph
complex
∆δcoord
coupling constant
3J(15N,1H)/Hz, H-aph
N(1)
N(2)
-248.6a
b
N(1)-1H(o-Ph)
2.7c
4.0e
-69.5d
-107.7
-38.2
N(2)-1H(trans-CH3)
a INEPT with refocusing. b Not successfully measured at the natural-abundance level. c Selective INEPT 2D.21 d Selective INEPT17 with polarization
1
1
1
transfer from H (trans-CH3) and broad-band H decoupling. e Selective INEPT20 with polarization transfer from H (trans-CH3) and selective
1
decoupling of H (cis-CH3).
The 15N NMR data for free H-aph and for the ligand
incorporated in the [WOF4(aph)]- complex are given in Table
2. Strong shielding of the N(2) imino nitrogen (∆δ -38.2 ppm)
gives evidence for direct participation of the nitrogen electron
lone pair in nitrogen-metal bonding.30,31 This conclusion
follows from the fact that the local paramagnetic term (which
is negative) for the screening of the nucleus (σploc ) is inversely
proportional to the electronic excitation energy between low-
changed as well, due to the change of hydrazone conformation
from E to Z upon coordination, which made this method less
efficient.
The 19F NMR spectrum of [WOF4‚CD3CN] is represented
by a singlet at 68.5 ppm.24 Equatorial fluoro ligands of
[WOF5]- , which is also formed in the reaction mixture, have
similar chemical shifts: δ(19Fd) 55 ppm. The quintet of the
axial fluorine (Fy) at -89 ppm lies just in the range of chemical
shifts for [WOF4(aph)]-. The 19F NMR spectra of [WOF4(aph)]-
at two different temperatures are presented in Figure 3, and the
19F chemical shifts and coupling constants are summarized in
Table 3.
lying states:32 σp
1/∆E.
loc
When the lone electron pair of nitrogen is removed as a result
of its coordination, σ f π* electron transitions occur instead
of n f π*. The ∆E value grows, and the screening of the
nucleus increases. The signal is rather broad (∆ν1/2 ≈ 30 Hz)
due to unresolved spin-spin coupling with four nonequivalent
fluoro ligands.
It is evident that at low temperature (-25 °C) fluorine atoms
occupy four nonequivalent positions in the coordination sphere.
A considerable increase of shielding of all four fluoro ligands
compared to that in neutral tungsten fluoro complexes with
simple hydrazones8 indicates that the deprotonated molecule
aph- is the coordinating species. The asymmetry of the complex
characterized by non equivalence of the fluoro ligands and a
considerable change of δ(15N) of the imino nitrogen unambigu-
ously demonstrate that aph- is incorporated in the tungsten
complex as a η2 ligand via the two hydrazone nitrogens. Taking
into account the NMR data of analogous complexes with
oximes,4,24,33 hydrazones,8 and hydrazine derivatives,5 one can
assign the signals at -14.6 and -108.2 ppm to fluorines in
positions trans to aph- and oxygen, respectively. The increased
values of spin-spin coupling between neighboring equatorial
We failed to measure the 15N NMR signal of the N(1) amino
nitrogen in the coordinated ligand. We suggest that the reason
is the splitting of the 15N resonance by four nonequivalent fluoro
ligands. Further, for the registration of the 15N signal, selective
transfer of polarization from ortho protons of phenyl through
three bonds to the observed nucleus was used. For the
successful accumulation of the 15N signal by this method it is
necessary to know the precise value of 3J(1H-15N). The same
constant value (2.7 Hz) as in the free hydrazone was used in
3
the experiment. It is apparent that the value of J(1H-15N)
(30) Webb, G. A.; Witanowski, M. Proc.sIndian Acad. Sci., Chem. Sci.
1985, 94, 241.
(31) Philipsborn, W.; Mu¨ller R. Angew. Chem., Int. Ed. Engl. 1986, 25,
383.
(32) Levy, G. C.; Lichter, R. L. Nitrogen-15 Nuclear Magnetic Resonance
Spectroscopy; Wiley: New York, 1979; p 8.
(33) Sakharov, S. G.; Zarelua, S. A.; Kokunov, Yu. V.; Buslaev, Yu. A.
Dokl. Akad. Nauk SSSR 1988, 301, 909.

5518 Inorganic Chemistry, Vol. 35, No. 19, 1996
Sakharov et al.
Figure 3. Parts of the 19F NMR spectra of the reaction mixture with the resonances of the [WOF4(aph)]- complex at low (-25 °C) and increased
(+40 °C) temperatures. The fluorine FM signal from the [WOF4(aph)]- complex is superimposed on the resonance of the axial fluorine atom (Fy)
-
from WOF5
.
Table 3. Temperature Dependence of 19F Chemical Shifts and
believe that the deprotonated nitrogen atom (N1) is in close
spatial proximity to the FC fluorine atom:
Coupling Constants of [WOF4(aph)]-
δ(19F)/ppm
2J(19F,19F)/Hz
chem
coupling
–
m
shift T ) -25 °C T ) +40 °C constant T ) -25 °C T ) +40 °C
o
p
130.0a
130.0a
45.8
i
FA
FC
FM
FX
-14.6
-68.6
-87.8
-108.2
-14.0
-69.3
FA-FC
FA-FM
FA-FX
FC-FM
FC-FX
FM-FX
138.3
118.8
42.8
43.3
41.8
63.7
H
cis
1N
H
m
C
C
O
o
FC
FA
H
∼-88
C
N
H
2
W
FX
-107.7
b
H
FM
51.9a
51.9a
trans
H
a Splitting constant (dynamic averaged coupling). b Not observable
due to line broadening.
Intramolecular Dynamics. Increasing the temperature leads
to principal changes in the 19F NMR parameters of the FC and
FM ligands (Table 3). The resonances of the FC and FM fluoro
ligands are considerably broadened, and the splitting of FA and
fluorine atoms is characteristic of complexes with η2 ligands.3,5,34
This increase is probably caused by a closer arrangement of
fluorine ligands in the equatorial plane due to coordination of
the two-center ligand. In addition, the methyl group trans to
the N(1) amino nitrogen is in close spatial proximity to one of
the equatorial fluorines. Since steric effects are mainly ac-
companied by deshielding (van der Waals effect),35 the deshield-
ing of trans-CH3 protons is in accordance with this proposal.
As a result of the van der Waals interaction, C-H bond
polarization takes place, and an expressive shielding of 13C in
the trans-CH3 group is observed analogously to the γ-effect.36
Therefore, it could be supposed that the 19F NMR signal at -69
ppm corresponds to this fluorine (FM). On the other hand, we
previously showed27 that the deprotonated nitrogen atom of the
η2-coordinated ligand in tungsten oxo fluoro hydrazido com-
plexes is in close spatial proximity to whichever of the two
equatorial fluorine atoms (FC or FM) has a greater constant of
spin-spin coupling with the third atom (FA). This also seems
to be valid for this tungsten oxo fluoro hydrazonato complex.8
As in this case J(FA-FC) > J(FA-FM), we are inclined to
2
FX resonances due to J(19F,19F) coupling with FC and FM is
averaged. At the same time in 13C NMR spectra, a signal from
the trans-CH3 carbon gradually transforms from a doublet to a
triplet (Figure 4). These changes in NMR spectra indicate
intramolecular dynamic processes in the tungsten coordination
sphere.
The [WOF4(aph)]- complex can exist in two stereoisomeric
forms, which cannot be directly distinguished by NMR spec-
troscopy:
O
O
FA
FA
1
1
2
2
FM
W
FC
FC
W
FM
trans
trans
CH3
CH3
N
2
N
1
N
1
N
2
Ph
Ph
C
C
FX
FX
cis CH3
CH3cis
At higher temperatures, intramolecular transformation from
one isomer to its mirror image occurs. The residence time of
the complex in each of the stereoisomeric forms (between two
successive interconversions) decreases with increasing temper-
ature. It is reasonable to assume that the spin states of each
(34) Sakharov, S. G. Doctorate (Chem. Sci.) Dissertation, Moscow, 1992;
p 115.
(35) Gu¨nther H. NMR Spectroscopy; Wiley: Chichester, U.K., 1980; p 347.
(36) Breitmaier, E.; Voelter, W. Carbon-13 NMR Spectroscopy; Verlag
Chemie: Weinheim, Germany, 1978; p 71.

An Acetone Phenylhydrazone Complex of W
Inorganic Chemistry, Vol. 35, No. 19, 1996 5519
As for olefin π-complexes, two paths of isomerization are
possible here: (a) with retention of η2 coordination in the
intermediate state and (b) via the σ-π rearrangement, i.e. via
the intermediate formation of a complex with an open structure.
We obtained indirect data testifying to the fact that for hydrazido
complexes the process takes place the second way.6,27 Con-
sidering the question of the paths of isomerization of
[WOF4(aph)]-, it should be mentioned that in 13C NMR spectra
the width of the signal of the phenyl ortho carbon atoms in the
complex decreases with increasing temperature. With the help
of the scaled model, we showed that in the η2-coordinated ligand
free rotation of the phenyl group around the N-C bond is
hindered due to spatial proximity of the protons of the cis-methyl
group and the ortho protons of the phenyl ring. This may lead
to magnetic nonequivalence of the ortho carbon atoms in the
first place and, consequently, to their broadening in the low-
temperature 13C NMR spectrum. With increasing temperature,
the signals narrow, which might result from free rotation of the
phenyl group of the coordinated ligand. With allowance for
isomerization to occur through the breakage of the W-N2 bond
and intermediate formation of σ-complexes, internal rotation
of the ligand around the N-N bond becomes possible. There
also is realized a conformation of the ligand in which free
rotation of the phenyl group around the C-N bond will occur
due to sufficient remoteness of the methyl groups.
Figure 4. Temperature dependence of the methyl resonances in 13C
NMR spectra of the coordinated aph- complex anion: (left) experi-
mental spectra; (right) theoretical line-shape behavior of the trans-CH3
resonance calculated as a function of the lifetime τ.
Thus, we believe that the most probable mechanism of
isomerization of the complex with η2-coordinated hydrazonate
ion occurs via breakage and subsequent formation of the W-N2
bond with simultaneous rotation of the ligand around the W-N1
bond, i.e. through the intermediate formation of the σ-complex:
fluorine nucleus in one molecule of the complex are unchanged
on the NMR time scale.
With regard to the nuclear spin states of the equatorial fluoro
ligands FC and FM, two types of [WOF4(aph)]- complexes (or
spin isomers) of equal probability are possible. On the one hand,
if the spin states of FC and FM are the same, the dynamic process
has no influence on the 13C NMR spectrum, and a doublet is
observed for trans-CH3 group.
On the other hand, in complexes with opposite spin states of
equatorial fluorine nuclei FC and FM, dynamic averaging takes
place. As a result, in the limit of fast exchange the doublet of
the trans-CH3 group should transform to a singlet. The real
experimental 13C NMR spectrum of the trans-CH3 group is the
superposition of these subspectra. Calculated NMR spectra for
this type of dynamic exchange with the values of averaged
lifetime τ is presented in Figure 4. The following values of
activation parameters of intramolecular rotation of the aph-
ligand in the coordination sphere of WOF4 are derived from
the line-shape analysis:
–
–
–
O
O
O
CH3
CH3
F
F
F
Ph
C
CH3
CH3
F
F
W
N1
C
F
F
W
F
F
W
N
N C
2
2
N
N
1
N
1
CH3
CH3
F
F
F
2
Ph
Ph
The high positive value of entropy of activation of the process
that is obtained is also in accordance with this conclusion.
Conclusion
The analysis of NMR experimental data indicates that aph-
is coordinated to WOF4 as a η2 ligand. Both donor nitrogen
atoms occupy equatorial positions in a pentagonal bipyramid,
and the coordinated ligand adopts the Z conformation. Due to
charge-density transfer from the ligand to the central atom, a
deshielding of the majority of the 1H and 13C nuclei is observed
(Table 1). The carbon atom attached to the imino nitrogen and
the methyl groups is strongly deshielded. The only apparent
exception, ∆δ(13C) for the trans-CH3 group, was rationalized
in terms of C-H bond polarization due to the steric effect of
the equatorial fluoro ligand Fm. This interaction is confirmed
by direct “through-space” coupling with the value of J(19F,13C)
) 6.2 Hz. Increasing the temperature leads to stereoisomer
interconversion due to a hindered intramolecular rotation of the
aph- ligand. All activation parameters of this dynamic process
∆Hq ) 78.5 ( 0.5 kJ/mol
∆Sq ) 56.78 ( 1.84 J/(mol K)
∆Gq 284(average temperature) ) 62.34 ( 0.25 kJ/mol
∆Gq313 ) 60.71 ( 0.25 kJ/mol
were determined from complete line-shape analyses of the 13
NMR spectra of the complex at various temperatures.
C
Since the coalescence temperature depends on a frequency
difference of exchanging sites, we roughly evaluated that the
coalescence of FC and FM, signals should be attained at
approximately 60 °C. However, the stability of [WOF4(aph)]-
above 40 °C rapidly decreases, and irreversible degradation of
the complex occurs.
Acknowledgment. This work was supported by the Russian
Foundation for Fundamental Research (Project No. 94-03-09512).
IC950330Y