Transition Metal Complexes of Diazenes, XXXVIII [1] Activation of Aromatic C-Cl Bonds in the Oxidative Addition of ortho-Chlorophenyldiazenes to Chlorotris(triphenylphosphine)rhodium

Article abstract of DOI:10.1515/znb-1996-1103

Refluxing a slightly acidic n-BuOH solution of RhCl(PPh₃)₃ and 2-chloro-, 2,4-dichloro-, or 2,4,6-trichloroazobenzene affords cyclometalated complexes (1) via loss of one phosphine ligand and oxidative addition of the ortho -C-Cl bond. X-ray structural analysis indicates a trans-position of the two triphenylphosphine ligands. The reaction proceeds only in polar solvents like alcohols or dimethyl sulfoxide but not in toluene. In the presence of air or oxygen a hydrido complex was formed as a by-product.

Full text of DOI:10.1515/znb-1996-1103

Transition Metal Complexes of Diazenes, XXXVIII [1]
Activation of Aromatic C-Cl Bonds in the Oxidative Addition of
or^o-Chlorophenyldiazenes to Chlorotris(triphenylphosphine)rhodium
Uwe R. Aulwurm, Falk Knoch, Horst Kisch*
Institut für Anorganische Chemie der Universität Erlangen-Nürnberg,
Egerlandstraße 1, D-91058 Erlangen
Z. Naturforsch. 51 b, 1555-1558 (1996); received April 10, 1996
orr/zo-Metalation, C-Cl Activation, 1,2-Diaryldiazenes
Refluxing a slightly acidic /?-BuOH solution of RhCKPPh,), and 2-chloro-, 2,4-dichloro-,
or 2,4,6-trichloroazobenzene affords cyclometalated complexes (1) via loss of one phosphine
ligand and oxidative addition of the ortho -C-Cl bond. X-ray structural analysis indicates
a frcws-position of the two triphenylphosphine ligands. The reaction proceeds only in polar
solvents like alcohols or dimethyl sulfoxide but not in toluene. In the presence of air or oxygen
a hydrido complex was formed as a by-product.
Introduction
Results and Discussion
Synthesis and structure
Cyclometalation reactions are important key
steps in the stoichiometric and catalytic synthe-
sis of organic compounds [2]. Although oxida-
tive additions of carbon-halogen bonds to square
planar d8 metal complexes have been inten-
sively studied [3], analogous transformations of 2-
halogenoazobenzenes are relatively scarce and re-
action conditions are rather stringent. As an exam-
ple, 2-bromoazobenzene and IrCl(CO)(PPh3)2 have
to be heated in boiling xylene for five days to ob-
tain the desired product in a yield of 43 % [4].
When 2-chloroazobenzene and RhCKPPh,), are re-
fluxed in toluene, no cyclometalated product but
[RhCl(PPh3)2b was obtained. The synthesis of only
a few orthometalated diazene rhodium phosphine
complexes has been reported and yields were in
general rather low; furthermore, no triphenylphos-
phine complexes could be obtained [5]. In the course
of our work on the rhodium catalyzed indole syn-
thesis from azobenzenes and alkynes [6] we now
found that 2-chloroazobenzenes and RhCKPPh,),
in boiling n-BuOH undergo a fast and clean ox-
idative addition. No such reaction occurs when a
non-halogenated azobenzene derivative is used [7],
Refluxing an acidic n-BuOH (6 vol% of HOAc)
suspension of RhCKPPh^), in the presence of a
slight excess of 2-chloro-, 2,4-dichloro-, and 2,4,6-
trichloroazobenzene for about 10 min afforded
the complexes la - c in isolated yields of 66 -
85% (Scheme 1). Further heating was disadvanta-
geous and produced RhCKCOHPPh^)? and elemen-
tal rhodium [8]. Reaction progress was indicated by
a color change from burgundy red to bright orange
or orange-yellow. The addition of HOAc is not es-
sential but slightly improves yield and size of the
microcrystals.
R2
BuOH, HOAc
RhCI(PPh3)j
-PPh3
1a - b
R1= R2= H: a
R’ = CI, R2= H:b
R1= R2= CI:c
The X-ray structural analysis of lb confirms
the expected octahedral geometry with the triphe-
nylphosphine ligands in trans position (Fig. I). Cy-
clometalation of 2-(chloromethyl)pyridine in re-
fluxing toluene is also known to afford the trans-
product while the cis-isomer is formed at room
temperature [9],
Reprint requests to Prof. H. Kisch.
K
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1996 Verlag der Zeitschrift für Naturforschung. All rights reserved.
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U. R. Aulwurm et al. ■Transition Metal Complexes of Diazenes
3
The angles around the central metal deviate from
ideal octahedral geometry [10]. While values of
92.8° and 90.2° are found for C12-Rhl-C13 and
C12-Rhl-C25, respectively, the angle of C13-Rhl-
N1 is increased to 98.6°, whereas Nl-Rhl-C25 is
reduced to 78.4°. The non-metalated phenyl ring
is twisted by 39.8° relative to the plane of the
rhodacycle. These values are in accordance with
structural data of analogous Rh(III) complexes [11].
The Rh 1-C12 and Rh 1-C13 bond lengths of 236 and
249 pm, respectively, distinctly confirm the stronger
trans-bond lengthening effect of a a-phenyl group
as compared to an N-donor ligand.
3 ’
Fig. 2. Numbering of the hydrogen atoms.
1350 cm-1, observed for la, lb, and lc, respec-
tively, suggest a weakening of the N=N bond upon
introduction of one or two chlorine atoms into the
orthometalated phenyl ring. The same trend was ob-
served for the i^(C=C)-vibrations of the metalated
phenyl ring. While in each complex the two main
vibrations are separated by 26 cm~1, they are shifted
each times by about 9 cm “ 1 to lower wave numbers
upon proceeding from la to lc. The UV-Vis spectra
of la - lc contain two maxima at 325 - 327 nm and
470 - 475 nm. The latter band is shifted to longer
wavelength by 5 - 10 nm as compared to the n,7r*
transition of the free diazene ligand.
The reaction is strongly solvent dependent. In
DMSO and pyridine the product did not precipi-
tate during the reaction, but the red-orange color
suggested that the reaction had occurred. Accord-
ingly, addition of petroleum ether afforded the same
complexes but in much lower yields. In toluene, no
ortho-metalation but formation of [RhCH PPh^L
was observed. A similar solvent dependence was
reported for oxidative addition reactions of alkyl
halides to iridium complexes, where rate constants
increased when more polar solvents were used
[13]. Therefore a mechanism analogous to nu-
cleophilic aromatic substitution, involving nucle-
ophilic attack of rhodium at the halogenated ring
and subsequent a rate determining chloride mi-
gration to the metal [3], is in accord with the
failure of 2-fiuoroazobenzene to undergo the cy-
clometalation. The much higher bond dissociation
energy of D c-f = 532 kJ/mol as compared to
Dc_ci = 406 kJ/mol [14] supports this interpreta-
tion. When RhCp(C2H4)2, RhCl(CO)(PPh3)2, and
CoBr(PPh3)3 were employed, no cyclometalation
products could be detected. 2-bromoazobenzene af-
forded orange and yellow microcrystalline materi-
als which had very similar physical properties as the
title complexes (IR, 'H NMR, UV-Vis, and micro-
analysis) but were insoluble in dichloromethane.
When the reaction of 2,4-dichloroazobenzene
Fig. 1. Crystal structure and labelling scheme of lb;
important bond lengths [pm]: Rhl-C12 236.1(1), Rhl-
N1 208.0(4), R hl-Pl 239.1(2), Rhl-C13 249.3(2), Rhl-
C25 199.9(5). Rh 1-P2 238.6(2), N 1-N2 128.0(6), N 1-C 10
143.7(6), N2-C20 137.6(6).
In the 'H NMR spectra of la - c all hydrogen
atoms of the cyclometalated ring appear as distinct
resonances (H2-5), whereas for the non-metalated
ring this is the case only for H3 and H5, the other
peaks being hidden underneath the signals of PPhi.
Assignments are based on comparison of la - c and
on 1H/1H-COSY spectra (Fig. 2).The doublet of H2
appears at lowest field, followed by the signals of
H5, H3,5 or H3, and H4, as also reported for d6
complexes of Fe, Mo, Ru, Rh, and Ir [5, 12].
To unambigously assign the ^(N=N) vibration,
the Raman spectrum of solid lb was measured
in the range from 1300 to 1600 cm-1 . Only one
intense line at 1357 cm-1, which appeared also
with medium intensity in the IR spectrum, was ob-
served. The decreasing values of 1364, 1357 and was conducted in the presence of air or oxygen.
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U. R. Aulwurm et al. • Transition Metal Complexes of Diazenes
in addition to lb a hydride complex precipitated in
1557
la: Orange microcrystalline powder, 176 mg (83%).
'H NMR (CD2CI2; 270 MHz): 7.93 (H2, 1 H, d), 7.65 -
6.95 (34 H, m), 6.92 (H \ 1 H, t), 6.77 (H3'5', 2 H, t), 6.28
(H4, 1 H, t); here as well as for lb, lc the value of all
coupling constants was 8 Hz. UV (CH2CI2), A (s [M_l
cm“ 1]): 475 nm (2500), 327 nm (21500).
10% yield (relative to lb) after a reaction time of
about 2 minutes. Normally rhodium hydrides are
formed under reducing conditions [15], The pres-
ence of a hydride complex is indicated by the 'H
NMR spectrum which contains a double triplet at -
13.52 ppm (J(Rh-H) = 33 Hz, ./(P-H) = 22 Hz). In an
analogous rhodium complex, wherein PPh3 is sub-
stituted by P(C(,H 11 )3, the czs-hydrido ligand gives
rise to a signal at -14.6 ppm [5]. From these data
we propose that the complex has the same structure
as lb except that one of the chloro ligands is re-
placed by hydride. It is known that small amounts
of oxygen have a beneficial effect on hydrogena-
tion reactions catalyzed by R hCU PPh^ due to a
rapid conversion of a triphenylphosphine ligand to
the corresponding oxide 116].
Since oxygen and RhCl(PPh3)3 afford per-
oxo complexes [17], their reaction with 2,4-
dichloroazobenzene under nitrogen atmosphere was
tested. In addition to lb , the hydride complex
was formed again but RhCl(CO)(PPh3)2 now be-
came the major product. This suggests that the
peroxo complexes oxidized /?-BuOH to butyralde-
hyde [18]. A consecutive reverse hydroformyla-
tion reaction [19] could rationalize formation of
RhCl(CO)(PPh3)2 and the hydride [20],
C48H39Cl2N2P2Rh (879.6)
Calcd C 65.54 H 4.47 N3.18%.
Found C 65.41 H 4.37 N3.17%.
lb: Orange-yellow microcrystalline powder, 183 mg
(85% ).'H NMR (CD2CI2; 270 MHz): 7.82 (H2, 1 H. d),
7.60 - 7.00 (33 H, m), 6.84 (H5, 1 H, s), 6.77 (H3’5', 2 H,
t), 6.74 (H \ 1 H, d). 3IP NMR (CD2C12; 109 MHz): 8.1
(d, (7(Rh-P) 95.7 Hz). UV (CH2C12), A (s [M“ 1 cm-1]):
463 nm (2900), 326 nm (19400).
C48H38Cl3N2P2Rh (914.1)
Calcd C 63.07 H4.19 N 3.06%.
Found C 63.27 H 3.57 N 2.76%.
lc: Orange microcrystalline powder, 190 mg (66%).
'H NMR (CD2CI2; 270 MHz): 7.65 - 7.00 (33 H, m),
6.86 (H \ 1 H, s), 6.82 (H3, 1 H, s), 6.79 (H3’’5', 2 H, t).
UV(CH2C12), A(e [M_i cm“ 1]): 470 nm (3200), 325 nm
(21000).
C4xH37Cl4N2P2Rh (948.5)
Calcd C 60.78 H 3.93 N 2.95%.
Found C 61.00 H3.81 N 2.63%.
Formation o f the hydrido complex
Experimental
A) To
a
suspension of 46 mg (0.05 mmol)
Unless otherwise mentioned, all reactions were car-
ried out under nitrogen in nitrogen-saturated solvents. IR:
Perkin Elmer 16 PC FT-IR. Raman: Dilor XY. UV-Vis:
Shimadzu UV-3101 PC. NMR: Jeol JNM-EX 270 FT-
NMR. MS: Varian Mat 212. Elemental analysis: Carlo
Erba 1106 (CHN). HPLC: Knauer HPLC pump 64; col-
umn Spherisorb ODS 2 (250 x 8 mm); CH3CN/H2O
5/1 (v/v). RhCKPPh.O;? [19], 2-chloro-, 2,4-dichloro-, and
2,4,6-trichloroazobenzene were prepared as described in
the literature [2 1].
of RhCl(PPh3)3 and 13 mg (0.05 mmol) of 2,4-
dichloroazobenzene 2 ml of /?-BuOH were added. The
mixture was then heated in air for about 2 min and al-
lowed to cool to r. t.. The precipitated material (25 mg)
was washed three times with petroleum ether and dried by
standing in air. 1H NMR (CD2C12; 270 MHz): 7.94 (H2, 1
H, d,7=8 Hz); -13.52 (Rh-H, 1H,dt),7(Rh-H)33 H z,/(P-
H) 22 Hz; all other signals were not assigned; 11P NMR
(CD2CI2; 109 MHz): 31.4 (d,7(Rh-P) 109.3 Hz). No hy-
dride complex was observed when the reaction was con-
ducted under N2 in the presence of 0.2 ml of water or when
lb was heated for 2 min in refluxing aerated /?-BuOH sus-
pension. 2-chloro- and 2,4,6-trichloroazobenzene did not
induce formation of a hydride complex.
B) 46 mg (0.05 mmol) of RhCHPPhi)} was dissolved
in 2 ml of dichloromethane; then oxygen was bubbled
through the solution for 5 min. The solvent was removed
and 13 mg (0.05 mmol) of 2,4-dichloroazobenzene in 2
ml of h-BuOH was added. Thereafter the experiment was
conducted in a nitrogen atmosphere as described above.
1H NMR analysis revealed that the obtained material (10
mg) contained 30% of the hydride complex relative to lb.
Preparation o f la - c
Refluxing a mixture of 184 mg (0.20 mmol) of
RhCl(PPh3b with a slight excess of the diazene (0.22
mmol) in 8 ml of /j-BuOH with 0.5 ml of acetic acid
for about 10 min afforded la - c as microcrystalline
powders, which were filtered off, washed three times
with 10 ml of petroleum ether and dried in vacuo. The
airstable complexes are soluble in dichloromethane and
may be recrystallized from dichloromethane/methanol.
Upon standing in aerated solutions the complexes were
partly transformed into insoluble materials.
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1558
U. R. Aulwurm et al. ■Transition Metal Complexes of Diazenes
monoclinic crystal system, space group P2i/c, a =
1099.5(2), b = 3882.4(11), c = 1053.8(2) pm,
but the main product was RhCl(CO)(PPh3)2 as indicated
by the IR spectrum. The orange washing solution did not
contain a hydride complex as evidenced by 1H NMR.
ß
=
91.64°(2), V = 4.496(2) nm3, d(calc.) = 1.48 g/cm3 (Z =
4). 11854 reflections collected, 9920 independent, 5144
[F > 4.0 (j(F)] observed reflections; // = 0.785 mm-1,
R = 0.040, /?w = 0.033. Further details of the crys-
tal structure investigation are available from the Fachin-
formationszentrum Karlsruhe, D-76344 Eggenstein-Leo-
poldshafen (Germany), on quoting the depository number
CSD-405453.
X-ray analysis:
Structure analysis was carried out on an automatic
four-circle goniometer (Siemens P4) at 200 K using Mo-
Kq radiation (graphite monochromator). The structure
was solved by direct methods by using the SHELXTL-
PLUS program [22], For the refinement all non-hydrogen
atoms were included with anisotropic temperature fac-
tors; the hydrogen positions were determined by differ-
ence Fourier analysis and included into the refinement
with fixed coordinates and temperature factors.
Acknowledgements.
Support of Prof. Dr. D. Sellmann in the X-ray struc-
tural determination is greatfully acknowledged. We thank
the Degussa AG for a gift of rhodium trichloride trihy-
drate and Fonds der Chemischen Industrie for financial
support.
Crystals of lb were obtained from dichloromethane-
/methanol. C48H38Cl3N2P2Rh x CH2C12 (lb) (998.9),
orange columns, crystal size 0.40 x 0.40 x 0.40 mm,
[1] For XXXVII see: Westernacher, S.; J. Bliimel, H.
Kisch, J. Mol. Catal. A: Chemical 109, 169 (1996).
[2] A. D. Ryabov, Chem. Rev. 90, 403 (1990).
[3] J. K. Stille, K. S. Y. Lau, Acc. Chem. Res. 10, 4349
(1977).
[4] M. I. Bruce, Aust. J. Chem. 27, 2135 (1974).
[5] J. F. van Baar, K. Vrieze, D. J. Stufkens, J.
Organomet. Chem. 97, 461 (1975).
[6] (a) P. Reißer, Y. Wakatsuki, H. Kisch, Monatsh.
Chem. 126, 1 (1995);
(b) U. R. Aulwurm, J. U. Melchinger, H. Kisch,
Organometallics 14, 3385 (1995).
[7] M. I. Bruce, M. Z. Iqbal, F. G. A. Stone, J.
Organomet. Chem. 40, 393 (1972).
[8] D. Evans, J. A. Osborn, G. Wilkinson, Inorg. Synth.
11,99(1968).
[9] N. Shinkawa, A. Sato, J. Shinya, Y. Nakamura, S.
Okeya, Bull. Chem. Soc. Jpn. 68. 183 (1995).
[10] J. F. van Baar, R. Meij, K. Olie, Crystallogr. Struct.
Comm. 3, 587 (1974).
[11] R. J. Hoare, O. S. Mills, J. Chem. Soc. Dalton 1972,
2141.
[12] D. Garn, Ph. D. Thesis, Univ. Erlangen - Nürnberg
(1992).
[15] a) N. Ahmad, J. J. Levison, S. D. Robinson, M. F.
Uttley, Inorg. Synth. 15, 45 (1974);
b) S. S. Bath, L. Vaska, J. Am. Chem. Soc. 85, 3500
(1963).
[16] F. H. Jardine, Progr. Inorg. Chem. 28, Ed.: S. J. Lip-
pard. Interscience Publication, John Wiley & Sons,
New York (1981).
[17] M. J. Bennett, P. B. Donaldson, Inorg. Chem. 16,
1585 (1977).
[18] A similar oxidation is known for a ruthenium com-
plex; D. F. Christian, G. R. Clark, W. R. Roper,
J. M. Waters, K. R. Whittle, J. Chem. Soc. Chem.
Commun. 1972, 458.
[19] a) J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkin-
son, J. Chem. Soc. (A) 1966, 1711;
b) K. Ohno, J. Tsuji, J. Am. Chem. Soc. 90, 99
(1968).
[20] The initially formed Rh(Cl)H2(PPh3)2 may undergo
a PPh, assisted reductive elimination of HC1 afford-
ing RhH(PPh3)2; subsequent oxidative addition of
2,4-dichloroazobenzene would produce the hydride
complex. It is unlikely that H-abstraction from the
solvent by a Rh(II) intermediate is involved since
no hydride is produced when n-BuOH is replaced
by iPrOH.
[21] K. H. Schündehütte, in E. Müller (ed.), Houben -
Weyl: Methoden der organischen Chemie, Vol. X/3,
p. 321, Georg Thieme Verlag, Stuttgart (1965).
[22] G. M. Sheldrick, SHELXTL-Plus, Program for
Crystal Structure Determinations, Release 4.21/V.
Siemens Analytical Instruments Inc., Madison,
Wisconsin, USA (1989).
[13] (a) P. B. Chock, J. Halpern, J. Am. Chem. Soc. 88,
3511 (1966);
(b) A. J. Hart-Davis, W. A. G. Graham. Inorg. Chem.
9, 2658 (1970).
[14] A. Streitwieser, C. H. Heathcock, Organische
Chemie, 2. Reprint of 1. edititon, p. 1388. Verlag
Chemie, Weinheim. Germany, (1990).
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