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Bis(6-phosphabenzene)vanadium Synthesis Structure Redox Properties and Conformational Flexibility.

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bonyl(2,4,6-triphenyl-q6-phosphabenzene)chromium 3I4"]).
The readiness of phosphabenzene to undergo q6-coordination manifests itself, inter alia, in that on heating the q'-complex 5 rearranges almost quantitatively to the q6-complex
4.[4blWe therefore engaged in the synthesis of binary phosphabenzene-transition-metalcomplexes and report here on
the first representative of this series.I5I In order to study a
possible conformational preference unaffected by steric effects we have chosen unsubstituted phosphabenzene 6 as
Metal atom-ligand cocondensation affords bis(q6-phosphabenzene)vanadium 7 in the form of reddish-brown, sublimable crystals, whose air-sensitivity is noticeably reduced
compared to that of the free ligand 6 [Eq. (a)]. The robust
Bis(q6-phosphabenzene)vanadium: Synthesis,
Structure, Redox Properties, and Conformational
Flexibility **
By Christoph Elschenbroich,* Mathias Nowotny,
Bernhard Metz, Werner Massa, Jens Graulich, Karl Biehler,
and Wolfgang Sauer
2 GH5P (9)
1. -196OC
Dedicated to Professor Kurt Dehnicke on the occasion
of his 60th birthday
The unsubstituted heteroarenes analogous to pyridine
namely phospha-, arsa-, stiba-, and bismabenzenes,r21a class
of compounds that has been developed by Ashe ZZI, offer the
coordination chemist a rich field of activity. The heterocycles
C,H,E (E = element ofgroup 15) are ambidentate, since, in
principle, they can bind to metals via the lone a-electron pair
at E (q') and/or via the n-electron sextet (q6). In the case of
the pyridine ligand a-coordination clearly predominates,
since bis(q6-pyridine)chromium 1 can be obtained only if
complexation is effected via an intermediate in which the N
atom is sterically blocked by neighboring trimethylsilyl
groups.['] In the synthesis of bis(q6-arsabenzene)chromium
ness of the metal-ligand bond in 7 is reflected in the thermal
stability (m.p. 210°C) and the appearance of the molecular
ion 7@ in the mass spectrum as base peak. Anaerobically
prepared solutions of 7 in aromatic solvents are storable
without decomposition for several hours when exposed to
air, whereas both bis(q6-benzene)vanadium 8 as well as free
phosphabenzene are oxidized instantaneously. This observation is consistent with the cyclovoltammetrically determined
redox potentials (Fig. 1, Table l), which feature the anodic
shifts AE,,, (7'/7, Se/8) = 0.72 V and AE,,, (7/7@,
S/S@)= + 0.53 V.['l That the shifts for reduction and oxidation of the pair 7,8, in contrast to those of the pair 7 , 6 are
very similar follows from the electronic structure of the
bis(arene)vanadium complex (. . .e:a&), according to which
both reduction and oxidation of 7 as well as 8 involve the
same redox orbital a,*.
2 this shielding is unnecessary.13]q6-Coordination of phosphabenzene-albeit only in ring-substituted form-is exemplified by a few ternary complexes[41(first example: tricar[*] Prof. Dr. C. Elschenbroich, Dip1.-Chem. M. Nowotny, Dip].-Chem. B.
Metz, Prof. Dr. W.Massa, Dip].-Chem. J. Graulich, K. Biehler, W. Sauer
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse,W-3550Marburg (FRG)
Metal-rr-Complexes of Heteroarenes, Part 3. This work was supported by
the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the NATO Scientific Affairs Division.-Part 2: [l].
Angew. Chem. Int. Ed. Engl. 30 ( 1 9 9 i ) No. 5
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Fig. 1. Cyclic voltammogram of 7 in DME/(n-Bu),NCIO, (0.1 M), glassy carbon electrode, T = - 4 8 T , u = 50 mVs-'.
mbH, W-6940 Weinheim, 1991
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Table 1. Cyclovoltammetric data for 6-8.
- 1.05[d]
[a] In DME/(n-Bu),NCIO, (0.1 M) at glassy carbon vs. saturated calomel electrode, 1) = 50 mVs-', data for 8 from [8]; [b] E,,, = (Epa+ Ep,)/2, AEp = E,. - En.;
[c] = I,,JIPc;Id] irreversible oxidation of a secondary product of the partially reversible reduction (ECE mechanism); [el irreversible oxidation; [fl chemically only
partially reversible reduction; [g] reversible at - 48 < T < + 25 " C ;[h] reversible at T = - 48 "C, irreversible at T = 25 "C; [i] expected value: AE, = 2.2 RT/nF =
42 mV; b] quasireversible.
The phosphorus heteroatom has a much stronger influence on the electron affinity of the free arene than in the
complex-bound arene. This manifests itself in the redox potentials [El,,(C,HF/C,H,) = - 3.42,l9] El/, (C5H,Pe/
C,H,P) = - 2.27; AEl,2 = 1.15 V] and in the electrontransmission spectra (ETS), which indicate that in the gas
phase anion states for phospha-, arsa- and stibabenzene are
binding.["] Replacement of CH by E (E = P,As,Sb,Bi) mod.ifies the electronic structure through the change in electronegativity and the reduction in the resonance integral PCE
as a result of increasing bond length. Figure 2 depicts the
changes in the n-orbital energies for the benzene/phosphabenzene pair, whereby step @outlines the removal of the
Table 2. EPR spectroscopic data of 7
7[d] 300 1.9874
1.9745 1.9840 2.002
10.66 8.77 1.66 0.35
7[e] 300
1.9801 1.9801 2.002
9.39 9.39 2.30
0.37 0.3
1.9764 1.9814 2.002
10.55 8.78 1.76
[a] x,y assignment uncertain; [b] calculated according to g, = 3g--(g,
or A , = 3u-(A, + A"); [c] cf. 8: u("V) = 6.35 mT; A,(5'V) = 9.2 mT;
A,,(51V)= 0.65 mT; u('H) = 0.40 mT; [d] in liquid and rigid DME respectively, coupling constants in mT; [el in [2.2]paracyclophaneas mixed crystal powder, coupling constants in mT.
tions. Rather, the increased 6 @charge on the central metal
atom in 7 leads to a contraction of the V(3dZ2)orbital, a
weaker overlap V(3dZz)-ligand(al8, o),more effective spin
polarization (3d,,)'/(n~)~of inner shells,['31and thus to an
increased Fermi contact interaction with the vanadium nucleus. The EPR spectrum of 7 in a glassy solution features an
orthorhombic g-tensor (Fig. 3). The resolution of g, and gy(z
axis = sandwich axis) shows that the frequency of ring rotation at 120 K in the rigid phase must be smaller than
5 x lo7 s-l (i.e. Agx.yin frequency units). Thus, the EPR
signal could be caused either by superposition of the spectra
Fig. 2. Correlation diagram of the frontier orbitals for benzene and phosphabenzene, modified as described in [lo]. @ Removal of the degeneracy as a
result of the variation in the resonance integrals, BCc > 8., Q Decrease in
energy caused by increasing effective rr-orbital electronegativity XS < x;: Ill].
The positions on the energy scale are taken from PE (181and ETS 1201measurements. Position inaccessible by ETS, since anionic state is binding.
degeneracies of elgand e,: and step Q indicates the decrease
in the energies of the n-MO caused by the inductive perturbation of the p, orbital on the phosphorus atom.["] With
regard to the coordination properties, this means that phosphabenzene (relative to benzene) is a ligand with comparable
n-donor but higher n-acceptor strength. Thus the central
vanadium atom in 7 should carry a more positive partial
charge than in 8, and the anodic shifts AEl,, (7°/7,Se/8) and
AE,,, (7/7@,8/8@)are plausible.['21
Under this premise the EPR data are, at first sight surprising: the hyperfine coupling constants a("V), Af'lV) and
A,,y(51V)in 7 are larger and the couplings a('H) are smaller
than the corresponding values a, AllandA, in 8, even though
7 bears ligands with stronger acceptor properties (Fig. 3. and
Table 2). This, however, is only an apparent contradiction,
since the SOMO in 7 and 8 is almost void of ligand contribu548
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. ..
10 mT
Fig. 3. EPR spectra of 7, X-band, at various temperatures; lower traces are
simulated 1171. Left A: In DME, liquid or rigid solution; right B: as mixed
crystal powder (z1% 7 in [2.2]paracyclophane9). *Exchange narrowed signal
of microcrystallites of pure 7.
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Angew. Chem. Inl. Ed. Engl. 30 (1991) No. 5
of two different rotameric forms of 7 each with an axial g
tensor, or by the presence of a single rotamer 7 with a nonaxial g
In both cases the excitation of ring rotation
should lead to removal of the inequivalencies in the x,y
planes. As a matrix which permits variation of temperature
over a wide range we have chosen [2.2]paracyclophane 9,
which is structurally related to 7.[’51Indeed, EPR powder
spectra of mixed crystals (ca. 1 YO7 in 9) at 120 K furnish
orthorhombic and at 293 K tetragonal g- and 51V-hyperfine
tensors (Fig. 3). Coalescence ( A x ,A,,) +A,(”V) for the low
field components (m, = - 7/2) occurs at T, % 170 K and for
the high field components (m,= +7/2) at T, = 220 K.
Hence, the rotational barrier in the heteroarene complex 7 is
comparable with that in the homoarene complex 8[ls1and is
largely determined by packing forces. This conclusion is further supported by the fact that a complete suppression of the
ring rotation is effected at substantially lower temperature in
the mixed crystal than in glassy solution (Fig. 3). Since the
molecular dimensions of the host 9[l61exceed those of the
guest 7,the latter should experience relatively slight matrixinduced hindrance to internal rotation.
which includes the compounds 1,2 and 7,is that of an orientational disorder of the antiperiplanar conformati~n.~’~l
Received: December 10, 1990 [Z 4314 IE]
German version: Angew. Chem. 103 (1991) 601
CAS-Registry number:
7, 132749-46-7; 8, 12129-72-5; V, 7440-62-2; C,H,P, 289-68-9
[I] C. Elschenbroich, J. Koch, J. Kroker, M. Wiinsch, W. Massa, G. Baum, G.
Stork, Chem. Ber. 121 (1988) 1983.
[2] a) A. J. Ashe 111, Acc. Chem. Res. 11 (1978) 153; b) A. J. Ashe 111, Top.
Curr. Chem. 105 (1982) 125.
[3] C. Elschenbroich. J. Kroker, W. Massa, M. Wunsch, A. 1.Ashe 111, Angew.
Chem. 98 (1986) 562; Angew. Chem. Int. Ed. Engl. 2S (1986) 571.
[4] a) J. Deberitz, H. Noth, Chem. Ber. 103 (1970) 2541; H. Vahrenkamp, H.
Noth, Chem. Ber. lOS(1972) 1148; b) J. Deberitz, H. Noth, Chem. Ber. 106
(1973) 2222; c) K. C. Nainan, C. T. Sears, J. Organomel. Chem. 148 (1978)
C 31 ; d) F. Nief, C. Charrier, F. Mathey, M. Simalty, .lOrgunornet.Chem.
187 (1980) 277; e) J. Fischer, A. de Cian, F. Nief, Acra Crysrallogr. Secf.
B37 (1981) 1067; f) K. Dimroth, H. Kaletsch, Orgunornet. Chem. 247
(1983) 271.
[5] Bis(2,4,6-terl-butyl-q6-phosphabenzene)chromiumwas first prepared in
1984 and was characterized by ‘H-NMR and ”P-NMR spectroscopy as
well as by cyclic voltammetry: E. Bilger, Dissertation, Marburg 1984; C.
Elschenbroich, E. Bilger, unpublished results.
[6] Experimental procedure: The wall of a cocondensation reactor equipped
with an electron beam heated metal-vapor source was cooled to - 196°C
and coated with 40 mL of methylcyclohexane. Vanadium (0.66 g,
12.9 mmol) and phosphabenzene 7b (2.3 g, 23.9 mmol) were then condensed onto the wall at 5 x
mbar and an electron-beam power of
490 W within 2 h. After equalization of pressure (N,) and warming to
25°C the dark-brown cocondensate was filtered through a G4 frit and the
filtrate evaporated to dryness. For removal of colloidal metal the residue
was extracted with methylcyclohexane, the extract evaporated to dryness,
and the residue taken up in 5 mL of toluene. After adding a layer of hexane
(5 mL), 26 mg (0.1 1 mmol, ca. 1 %) of 7 crystallized at 4°C in the form
of dark-brown rhombs (m.p. 201 “C). UVjVIS (methylcyclohexane):
I,,,[nm](a): 311 (7150), 333 (8300), 447 (975). 675 (75); MS (EI, 70eV):
m/z 234 ( M e , loo%), 147 (Me-C,H,P, 90.5), 51 ( V e , 91.3).
[7] a) F. G. N. Cloke, M. L. H. Green, J. Chem. SOC.Dulron Trans. 1981,1938;
b) A. J. Ashe I I I , J Chem. Soc. 93 (1971) 3293; A. J. Ashe 111, University
of Michigan, Ann Arbor, USA, private communication.
[8] Concerning the redox potentials of (C,H,),M and (C,H,) (C,H,)M
(M = V, Cr) see C. Elschenbroich, E. Bilger, B. Metz, Orgunometullics, in
[9] J. Mortensen, J. Heinze. Angew. Chem. 96 (1984) 64;Angew. Chem. Inl. Ed.
Engl. 23 (1984) 84.
[lo] P. D. Burrow, A. J. Ashe 111, D. J. Bellville, K. D. Jordan, .lAm. Chem.
SOC.104 (1982) 425.
[ l l ] In such considerations, effective rr-orbital electronegativities are to be
used instead of the Pauling electronegativity values. The x; values for P,
As, Sb are higher than for C; compare J. Waluk, H.-P. Klein, A. J. Ashe 111,
J. Michl, Orgunometullics 8 (1989) 2804.
1121 According to EPR spectroscopic measurements the redox orbital a,, in the
case of 7 and 8 has almost exclusively metal 3dz, character. The anodic
shifts AE,,, for unsymmetrical sandwich complexes (C,H,) (C,H,)M relative to the symmetrical forms (C,H,),M are explained in an analogous way
Fig. 4. A: ORTEP plot of a molecule of 7 in the crystal with data of the C:P
occupation. Thermal ellipsoids at the 50% probability level. Important bond
lengths Ipm] and angles [“I (because of disorder the probable errors have to be
set higher than derived from the standard deviations): (C,P)l-(C,P)2 158.3(5),
(C,P)I-C6 158.1(6), (C,P)2-C3 154.5(5), C3-(C,P)4 159.1(6), (C,P)4-(C,P)5
154.7(5), (C,P)5-C6 147.8(5), mean: 155.4 pm; ring angles at (C,P)l 121.7(3),
(C,P)2 112.8(3), C3 124.9(3), (C,P)4 119.4(3), (C,P)5 117.6(3), C6 123.6(3),
V-(C,P)l 225.1(4), V-(C,P)2 231.7(3), V-C3 222.9(4), V-(C,P)4 227.1(3), V(C,P)5 232.0(3), V-C6 225.5(4) pm, mean V-(C,P) 227.4 pm. B: Observed electron densities (Fo Fourier section) in the “best” ring plane of 7.Intervals of the
contour lines 0.5, beginning at 0.5 el A’.
In order to clarify whether a certain conformation is preferred in the single crystal of 7 we have carried out an X-ray
structure analysis.[’*]It led to a very similar unit cell and the
same space group type P2Jn as in the chromium complexes
bis(q6-pyridine)chromium ltlI and bis(q6-arsabenzene)chromium 2L3l studied previously. As anticipated, 7,like 1
and 2, shows disorder of the heteroatom in the ring (Fig. 4).
However, the three disorder models differ significantly.[’g1
Interestingly, the only common structural interpretation,
Angew. Chem. Int. Ed. Engl. 30 (1991) No. S
[13] B. A. Goodman, J. B. Raynor, A h . Inorg. Chem. Rudiochem. 13 (1970)
[14] In order to decide between these alternatives additional measurements
using multiple resonance methods (TRIPLE, ENDOR-induced EPR) are
required; C. Elschenbroich et al., unpublished results.
1151 a) A. Schweiger, R. Wolf, H. H. Giinthard, J. H. Ammeter, E. Deiss,
Chem. Phys. Letr. 71 (1980) 117; b) R. Wolf, A. Schweiger, H.H.
Giinthard, Mot. Phys. 53 (1984) 585.
[16] H. Hope, J. Bernstein, K. N. Trueblood, Aclu Cryscullogr. Sect. B 28
(1972) 1733.
[17] Programm POWDER, C. Daul, C. W Schlapfer, B. Mohos. J. Ammeter,
E. Gamp, Comput. Phys. Commun. 91 (1981) 385.
[18] Crystal structure determination of 7: A blackish-brown crystal (ca.
0.2 x 0.2 x 0.1 mm), after orienting film exposures, was measured on a
CAD4 diffractometer, Enraf-Nonius, Mo,, radiation, graphite monochromator at - 80°C. The monoclinic unit cell (a = 651.2(1), b =
782.2(1), c = 977.6(2) pm, = 97.13(1)” was determined on the basis of 25
reflections with @ > 12”. The space group is P2,/n, 2 = 2. The structure
was solved with Patterson and difference Fourier syntheses and, owing to
different electron densities at the six ring atom positions, described and
refined using a disorder model (the ring positions display varying P/C
occupation). The relative occupation ratios were calculated from the maxima of a Fourier synthesis, the P fraction for the lowest maximum was
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varied iteratively in the refinement until an equivalent isotropic displacement factor comparable with that at the other positions resulted. For each
ring position in the disorder model so obtained the C:P ratio was fixed, an
anisotropic displacement factor refined together with the atomic positions,
and-corresponding to the C fraction-an H atom was introduced
“riding” at a calculated position with a common isotropic displacement
factor (I/ = 0.09 A’). The refinement converged well and led (considering
the positional disorder) to reasonable displacement factors. Using the
weights w = I/02(Fo)and after introduction of an extinction correction the
residuals R = 0.054, wR = 0.037 resulted for 920 observed reflections with
Fo > 4a. Further details of the crystal structure investigation are available
on request from the Fachinformationszentrum Karlsruhe, Gesellschaft
fur wissenschaftlich-technische Information mbH, W-7514 EggensteinLeopoldshafen 2 (FRG), on quoting the depository number CSD-55147,
the names of the authors, and the journal citation.
[I91 Whereas the N atom in 1 occupies the 1,3 positions (SOjSO ratio) and in 2
As occupies the 1,4 positions, the electron density distribution in the ring
of 7 (Fig. 4) shows that the P atom, like in the As-analogue of chromium,
is preferentially located at the 1.4-positions, but that differing and smaller
fractions of P are located on all other ring positions. Because of superposition of several orientations of a distorted six-membered ring, varying
bond lengths from 148 to 159 pm are encountered, whereby the mean value
(155.4pm) lies well above the mean of the bond lengths in free phosphabenzene (150.7 pm) [2]. This is plausible since, because of its higher
number ofelectrons the P atom “prevails” more strongly during the superposition of C and P atoms than corresponds to its percentage. The deviations of a “best plane” through the six ring atom positions o f 7 are less than
0.3 pm and thus indicate planarity. The V atom is located 166(1) pm above
the ring plane, its distance from the ring atoms is on the average 227.4 pm.
The second ring is generated through the symmetry center in the V atom,
i.e. is parallel to the other ring and is in the eclipsed orientation. Since
pairwise oppositely disposed positions in the ring are occupied to about the
same extent by P, the disorder model is compatible both with the assumption-with varying contribution-of differently oriented molecules with
antiperiplanar and with synperiplanar conformation. The exclusive presence of a synclinal conformation is ruled out by the different extent of
occupation of the ‘‘mela” positions; an admixture cannot by ruled out,
[20] C. Batich, E. Heilbronner, V. Hornung, A. J. Ashe 111, D. T.Clark, U. T.
Cobley, D. Kilcast, I. Sanlan, .
Am. Chem. SOC.98 (1973) 928.
Cp;TiS,O and Cp’,TiOS,-Isomeric Titanocene
Complexes with Two Novel S-0 Chelate Ligands
(Cp’ = $-CH,CSHJ **
By R a y Steudel,* Andreas Prenzel, and Joachim Pickardt
S,O and other cyclic sulfur oxides can be prepared by
oxidation of sulfur rings with trifluoroperoxoacetic acid
They contain the group -S-S(OkS-, which can
also be incorporated into organic compounds by condensation of thiols or thiolates with thionyl chloride, whereby
sulfane oxides are formed [Eq. (b)].Iz1
C ~ c f o - S , CF3C03H--t S,O
n = 6-10
group -S-S(0)-according to Equation (a) or of chain-like
sulfane oxides with essentially more than three sulfur atoms
according to Equation (b). We report here on the first synthesis of a polysulfide oxide complex of titanocene, which
should enable the -S,O- group to be incorporated into chainlike or ringlike compounds, i.e. analogously to the use of
Cp,TiS, (Cp = q5-C5H5)for the synthesis of ring- and
chain-like polysulfur compounds.[3]
The dinuclear titanocene complex Cp,Ti,S, lt4]reacts
spontaneously with thionyl chloride in the ratio 1:1 at 0°C
in CS, to give the titanocene dichloride 2 and the pentasultide oxide complex 3, which is sparingly soluble in CS, and
precipitates in the form of red crystals (m.p. 105 “C, dec ~ m p . )[Eq.
‘ ~ ~(c)].
Solid 3, in contrast to the decomposable cyclo-sulfur oxides S,O, is stable at 20°C in air. The IR spectrum of 3 (KBr
pellet) shows the SO stretching vibration as a very
strong band at 1094 cm-’, an unequivocal indication of the
-S-S(OkS- group, as v ( S 0 ) for S,O appears at almost exactly the same wave number.r6l Since the SSO deformation
vibration of 3 (380 cm- ’) also appears at exactly the same
wave number as in S,O, it can be assumed that the 0 atom
in 3, as in S,O, is in the axial position, which is stabilized by
the anomeric effect.
The EI mass spectrum of 3 (sample temperature 140°C)
does not show a peak for the molecular ion M e , but peaks for
the characteristic fragments [ M - S,O]@ and [M - S,O]@;
the base peak is S,”. The elimination of S,O and S,O is also
typical for cyclo-sulfur oxides S,O.I’l The ‘H-NMR spectrum of 3 in CDCI, (400 MHz) shows two singlets at
6 = 2.012 and 2.466 for the methyl groups and four triplets
at 5.863, 5.91 1, 6.277 and 6.398 for the aromatic ring protons. Thus, with respect to the number of lines, this spectrum
corresponds exactly to the ’H-NMR spectrum of
Cp;TiS,,[’] i.e. the TiS,O ring in 3 must have a mirror plane.
As to be expected from the synthesis, 3 is present as Cp;Ti(p
S,),S=O and not as Cp;Ti(p-S)(p-S,)S=O, since the ring
protons of the unsymmetrical compound would have to give
eight quartet signals.
3 is readily soluble in CHCI,, CH,CI, and THF, but the
solutions decompose within 45 min at 20°C, whereby 3 is
converted via ring expansion into the isomeric complex 4
[Eq. (41‘
Cp;TiS,O 2
+ 2 HCl
R = Aryl
Unfortunately these reactions are not suitable for the
preparation of inorganic or organic heterocyles with the
[*I Prof. Dr. R. Steudel, Dipl.-Chem. A. Prenzel, Prof. Dr. J. Pickardt
Institut fur Anorganische und Analytische Chemie der
Technischen Universitat, Sekr. C 2
Strasse des 17. Juni 135, W-1000 Berlin 12 (FRG)
[**I Sulfur Compounds, Part 137. This work was supported by the Deutsche
Forschungsgemeinschaft and by the Verband der Chemischen Industrie.
Jakupovicr and Dip1.-Chem. J. Albertsen for
We thank Priv.-Doz. Dr. .
recording the NMRand massspectra.-Part 136: R. Steudel, B. Plinke, D.
Jensen, F. Baumgart, Polyhedron, in press.
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This reaction can be monitored by means of reversed
phase HPLC,[81since 4 has a significantly greater retention
time than the obviously more polar 3; further reaction products are not formed (eluent: CH,OH; stationary phase: octadecylsilane). The UV absorption spectra of 3,4,and-for
comparison-of CpiTiS, 5 were measured immediately after
the chromatographic separation.[’I Whereas the spectra of 3
and 5 are, as expected, very similar, that of 4 shows a marked
shift of the absorption at ca. 300 nm (Table 1).
4 crystallizes from CH,Cl,/hexane in the form of darkbrown, rhomboidal, monoclinic, air-stable crystals,[9]which
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conformational, structure, synthesis, properties, redox, phosphabenzene, flexibility, bis, vanadium
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