Metal complexes with 2,3-bis(diphenylphosphino)-1,4-diazadiene ligands: Synthesis, structures, and an intramolecular metal-mediated [4 + 2] cycloaddition employing a benzene ring as a dienophile

Article abstract of DOI:10.1021/ic020534w

2,3-Bis(diphenylphosphino)-1,4-diazadienes RN=C(PPh₂)-C(PPh₂)=NR (1a, R = 4-tolyl; 1b, R = 4-tert-butylphenyl; 1c, R = mesityl) were used as novel ligands for transition metals. The metal complexes [(1c)Mo(CO)₄] (2a), {(1c)[Mo(CO)₄]₂} (2b), [(1a)Cu(Cl)(PPh₃)] (3), and {(1b)[(NiBr₂(THF))]₂} (4) were characterized by elemental analysis, MS, and ³¹P{¹H}, ¹H, and ¹³C NMR spectra (except the paramagnetic complex 4). Additionally, the molecular structure of the complexes in the solid state was determined by single-crystal X-ray diffraction. In 2a and 2b the chelating ligand coordinates via the N,P donor set, whereas in 3 the chelating ligand coordinates via the two P atoms. 4 contains a square-planar (P,P)NiBr₂ moiety on the one side of the bridging ligand 1b. On the opposite side the 1,2-diimine unit bonds to another Ni center having octahedral geometry. The bulkier ligand 1c reacts to form the mononuclear compound 5. X-ray diffraction analysis of single crystals shows that 5 contains a quinoxaline derivative with a cyclohexa-1,3-diene ring in the peripheral position. Furthermore, it contains a bis(diphenylphosphino)-ethylene unit coordinating the NiBr₂. This arrangement is the result of an intramolecular [4 + 2] cycloaddition between the 1,2-diimine unit (as diheterodiene) and the benzene ring of the 4-tolyl-N substituent (as dieneophile). The same type of ring-closing reaction followed by a tautomerization reaction to form the mononuclear compound 6 occurred by dissolution of the binuclear complex 4 in methanol. This reaction can be used as a simple method for the synthesis of novel 1,2-bis(diarylphosphanyl)ethylenes containing a quinoxaline backbone.

Full text of DOI:10.1021/ic020534w

Inorg. Chem. 2003, 42, 625−632
Metal Complexes with 2,3-Bis(diphenylphosphino)-1,4-diazadiene
Ligands: Synthesis, Structures, and an Intramolecular Metal-Mediated
[4 + 2] Cycloaddition Employing a Benzene Ring as a Dienophile
,†
Dirk Walther,* Stefan Liesicke,† Lars Bo1ttcher,† Reinald Fischer,† Helmar Go1rls,† and
Gavin Vaughan‡
Institut fu¨r Anorganische und Analytische Chemie der Friedrich-Schiller-UniVersita¨t Jena,
D-07743 Jena, Germany, and European Synchrotron Radiation Facility BP22220,
F-38043 Grenoble Cedex, France
Received September 4, 2002
2,3-Bis(diphenylphosphino)-1,4-diazadienes RNdC(PPh )−C(PPh2)dNR (1a, R) 4-tolyl; 1b, R) 4-tert-butylphenyl;
2
1c, R ) mesityl) were used as novel ligands for transition metals. The metal complexes [(1c)Mo(CO)4] (2a),
{(1c)[Mo(CO)4]2} (2b), [(1a)Cu(Cl)(PPh )] (3), and {(1b)[(NiBr2(THF))]2} (4) were characterized by elemental analysis,
3
1
MS, and 31P{1H}, H, and 13C NMR spectra (except the paramagnetic complex 4). Additionally, the molecular
structure of the complexes in the solid state was determined by single-crystal X-ray diffraction. In 2a and 2b the
chelating ligand coordinates via the N,P donor set, whereas in 3 the chelating ligand coordinates via the two P
atoms. 4 contains a square-planar (P,P)NiBr2 moiety on the one side of the bridging ligand 1b. On the opposite
side the 1,2-diimine unit bonds to another Ni center having octahedral geometry. The bulkier ligand 1c reacts to
form the mononuclear compound 5. X-ray diffraction analysis of single crystals shows that 5 contains a quinoxaline
derivative with a cyclohexa-1,3-diene ring in the peripheral position. Furthermore, it contains a bis(diphenylphosphino)-
ethylene unit coordinating the NiBr2. This arrangement is the result of an intramolecular [4 + 2] cycloaddition
between the 1,2-diimine unit (as diheterodiene) and the benzene ring of the 4-tolyl−N substituent (as dieneophile).
The same type of ring-closing reaction followed by a tautomerization reaction to form the mononuclear compound
6 occurred by dissolution of the binuclear complex 4 in methanol. This reaction can be used as a simple method
for the synthesis of novel 1,2-bis(diarylphosphanyl)ethylenes containing a quinoxaline backbone.
Introduction
range of homogeneous catalytic reactions5-8 and have been
found to stabilize new metallacumulenes.9,10 Furthermore,
1,4-Diazadiene- and 1,2-bis(phosphino)ethane-type ligands
are among the most prominent ligands used in the synthesis
of many stable five-membered metal complexes and orga-
nometallics as well as for tuning of catalytic properties of
many metal catalysts.
These ligands continue to attract considerable attention.
For example, 1,2-diimine nickel or palladium complexes
bearing bulky N-substituents constitute a new family of
catalysts capable of polymerizing ethylene and copolymer-
izing ethylene with polar olefins.1-4 Bidentate chelating
phosphines have been proven to be powerful ligands in a
(1) Togni, A.; Venanzi, L. M. Angew. Chem. 1994, 106, 517; Angew.
Chem., Int. Ed. Engl. 1994, 33, 497.
(2) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem. 1999,
111, 448-468; Angew. Chem., Int. Ed. 1999, 38, 429.
(3) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997,
16, 2005 and references therein.
(4) Knowles, W. S. Angew. Chem. 2002, 114, 2096; Angew. Chem., Int.
Ed. 2002, 41, 1998 and references therein.
(5) Chen, Y.; Kiattansakul, R.; Ma, B.; Synder, J. K. J. Org. Chem. 2001,
66, 6932.
(6) Tanaka, K.; Fu, G. C. Angew. Chem. 2002, 114, 1677; Angew. Chem.,
Int. Ed. 2002, 41, 1603.
(7) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001,
20, 16.
(8) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 9172.
(9) Rigaut, S.; Maury, O.; Touchard, D.; Dixneuf, P. H. Chem. Commun.
2001, 373.
(10) Rigaut, S.; Monnier, F.; Mousset, F.; Touchard, D.; Dixneuf, P. H.
Organometallics 2002, 21, 2654.
* Author to whom correspondence should be addressed. E-mail: cdw@
rz.uni-jena.de.
† Institut fu¨r Anorganische und Analytische Chemie der Friedrich-
Schiller-Universita¨t Jena.
‡ European Synchrotron Radiation Facility BP22220.
10.1021/ic020534w CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/24/2002
Inorganic Chemistry, Vol. 42, No. 2, 2003 625

Walther et al.
Table 1. Crystal Data
2a
2b
3
4
5
6
empirical formula
C48H42N2O4-
P2Mo
868.72
183(2)
0.71073
monoclinic
P21/n (No. 14)
9.8386(2)
21.4220(5)
20.0408(3)
95.692(1)
4203.0(1)
4
1.373
0.435
R1 ) 0.0385
wR2 ) 0.0904
R1 ) 0.0594
wR2 ) 0.1000
C52H42N2O8-
P2Mo2
1076.70
183(2)
0.71073
monoclinic
C2/c (No. 15)
22.178(1)
11.6526(6)
20.267(1)
113.403(4)
4806.8(4)
4
1.488
0.644
R1 ) 0.0456
wR2 ) 0.1151
R1 ) 0.0602
wR2 ) 0.1261
C66H65N2O2-
ClP3Cu
1110.10
183(2)
0.71073
orthorhombic
Pbca (No. 61)
22.3892(4)
19.2933(4)
26.6209(5)
100.098(1)
11499.2(4)
8
1.282
0.556
R1 ) 0.0440
wR2 ) 0.1057
R1 ) 0.0852
wR2 ) 0.1123
C64H82N2O4.50
Br4P2Ni2
1450.32
183(2)
0.71073
-
C44H42N2ClNb-
Br2P2Ni
879.27
120(2)
0.45085
monoclinic
Cc (No. 9)
20.3600(3)
11.0980(1)
17.9282(2)
104.96(2)
3846.01(8)
4
1.519
1.442
R1 ) 0.0184
wR2 ) 0.0498
R1 ) 0.0186
wR2 ) 0.0500
C54H62N2O2-
Br2P2Ni
1051.53
183(2)
0.71073
monoclinic
P21/n (No. 14)
18.571(2)
14.239(1)
21.549(3)
fw (g‚mol-1
temp (K)
λ (Å)
)
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c (No. 14)
21.0209(3)
17.5667(3)
18.3020(3)
108.304(1)
6653.6(2)
4
1.448
3.062
R1 ) 0.0506
wR2 ) 0.1251
R1 ) 0.0855
wR2 ) 0.1481
â (deg)
V (Å3)
5505(1)
4
1.269
1.900
R1 ) 0.0596
wR2 ) 0.1567
R1 ) 0.01675
wR2 ) 0.1944
Z
F
calcd (g‚cm-3
)
µ (mm-1
)
R [I > 2σ(I)]a
R (all data)a
a R1 ) [∑||Fo| - |Fc||]/∑|Fo|, wR2 ) [[∑w(|Fo2 - Fc |)2]/[∑w(Fo )]]1/2, w ) 1/[(σFo)2 + (aP)2]. The value of aP was obtained from structure refinement.
2
2
recorded on a Bruker AC 200 F spectrometer. Mass spectra were
recorded on a Finnigan MAT SSQ 710. Values for m/z are for the
most intense peak of the isotope envelope. The measured isotopic
pattern for the nickel- and molybdenum-containing species are in
good agreement with the calculated isotopic pattern using the
program ICIS (version 8.2.1, Finnigan). Elemental analyses were
performed with Leco CHNS-932. (Norbornadiene)Mo(CO)4 was
obtained from Mo(CO)6 and norbornadiene in toluene prepared
according to ref 1717; oxalic imidoyl chlorides R-NdC(Cl)-
C(Cl)dN-R,18 NaPPh2(dioxane) and K(PPh2)(dioxane)n,19 and Cu-
(PPh3)3Cl20 were prepared according to the literature.
they act as supporting ligands in complexes with unique
properties or structures (recent examples: refs 11-16).
In the context of the important role of these ligands it is
surprising that 2,3-bis(diphenylphosphino)-1,4-diazadienes,
which combine the above chelating units within the same
molecule, are hitherto unknown. 2,3-Bis(diphenylphosphino)-
1,4-diazadienes should be able to coordinate either as P,N-,
P,P-, or N,N-chelating ligands thus giving rise to a very rich
metal chemistry including the formation of mono-, bi-, and
oligonuclear complexes. In addition, they should function
as novel controlling ligands for metal-catalyzed reactions.
Furthermore, these are very flexible ligands which could
possibly undergo isomerization reactions upon coordination
of one metal ion.
We report here on the synthesis of three bis(diphenylphos-
phino)-1,4-diazadienes and show that, depending on the
nature of the metal, mono- or binuclear complexes can be
formed in which the ligand coordinates in different ways.
Of particular interest are ring-closure reactions of the ligands
at the periphery under the influence of Ni(II) which lead to
the formation of 2,3-bis(diphenylphosphino)-substituted qui-
noxalines in a highly selective reaction.
X-ray data for compound 6 were collected on a Nonius CAD4
diffractometer and for compounds 2a to 4 on a Nonius KappaCCD
diffractometer, using graphite-monochromated Mo KR radiation.
For compound 5 measurements were carried out at beamline ID11
at the European Synchrotron Radiation Facility (ESRF), because 5
formed very small crystals. Data were collected using a Bruker
“SMART” CCD-camera system at fixed 2θ, while the sample was
rotated over 0.1° intervals during 2 s exposures. Data were corrected
for Lorentz and polarization effects, but for absorption only for
compounds 4 and 6.21-23 The structures were solved by direct
methods (SHELXS24) and refined by full-matrix least squares
2
techniques against Fo (SHELXTL97-225). The hydrogen atoms
of the structures were included at calculated positions with fixed
thermal parameters. All non-hydrogen atoms were refined aniso-
tropically.25 XP (SIEMENS Analytical X-ray Instruments, Inc.) was
used for structure representations. Crystallographic information is
provided in Table 1.
Experimental Section
General Procedures. All manipulations were carried out by
using Schlenk techniques under an atmosphere of argon. Prior to
use, tetrahydrofuran, diethyl ether, and toluene were dried over
potassium hydroxide and distilled over Na/ benzophenone. All other
reagents (Aldrich) were used as received. 1H and 13C spectra were
(17) King, R. B. In Organometallic Synthesis; Academic: New York 1965;
Vol. 1, p 124.
(18) Lindauer, D.; Becker, R.; Do¨ring, M.; Fehling; P.; Goerls, H. J. Prakt.
Chem. 1995, 337, 143.
(19) Issleib, K.; Tzschach, A. Chem. Ber. 1959, 92, 1118.
(20) Jardine, F. H.; Rule, L.; Vohra, A. G. J. Chem. Soc. A 1970, 238.
(21) MOLEN, An InteractiVe Structure Solution Procedure; Enraf-Non-
ius: Delft, The Netherlands, 1990.
(22) Otwinowski, Z.; Minor, W. Processing of X-Ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology, Vol. 276,
Macromolecular Crystallography, Part A; Carter, C. W., Sweet, R.
M., Eds.; Academic Press: New York, 1997; pp 307-326.
(23) SMART, Software for the CCD Detektor System, version 5.05: Bruker
AXS: Madison, WI, 1998.
(11) Garcia, M. H.; Robalo, M. P.; Dias, A. R.; Duarte, M. T.; Wenseelers,
W.; Aerts, G.; Goovaerts, E.; Cifuntes, M. P.; Hurst, S.; Humphrey,
M. G.; Samoc, M.; Luther-Davies, B. Organometallics 2002, 21, 2107.
(12) Garcia, M. H.; Robalo, M. P.; Dias, A. R.; Piedade, M. F. M.; Galvao,
A.; Wenseelers, W.; Goovaerts, E. J. Organomet. Chem. 2001, 619,
252.
(13) Heyn, R. H.; Go¨rbitz, C. H. Organometallics 2002, 21, 2781.
(14) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc.
2002, 124, 3846.
(15) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623.
(16) Sjoevall, S.; Andersson, C.; Wendt, O. F. Organometallics 2001, 20,
4119.
(24) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(25) Sheldrick, G. M. SHELXTL97-2; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
626 Inorganic Chemistry, Vol. 42, No. 2, 2003

2,3-Bis(diphenylphosphino)-1,4-diazadiene Ligands
(4-Tolyl-NdC(PPh2)C(PPh2)dN-4-tolyl) (1a). Bis(4-tolyl)ox-
alic imidoyl chloride (1.07 g, 3.5 mmol) dissolved in THF (40 mL)
and NaPPh2(dioxane) (8.27 mmol, 15% excess) dissolved in THF
(10 mL) were stirred at -78 °C. After 4 h the reaction mixture
was allowed to warm to room temperature. Then the THF was
removed under vacuum, yielding a brown solid, which was extracted
with toluene at 60 °C. The pure ligand 1a crystallized at -20 °C.
Yield: 0.962 g (45%), yellow crystals. Anal. Calcd for C40H34N2P2
(604.67 MW): C, 79.45; H, 5.67; N, 4.63. Found: C, 79.16; H,
5.71; N, 4.56. MS (DEI) m/z ) 604 (M+, 3%), 527 (M+ - C6H5,
2%), 419 (M+ - C12H10, 3%), 370 (M+ - C16H14N2, 13%), 302
(M+ - C20H17NP basic peak), 85 (M+ - C28H24N2, 97%). 31P{1H}
were dissolved in THF (5 mL). The color of the solution changed
immediately from yellow to dark red. After 1 h of stirring at room
temperature the volatile part of the solution was removed under
vacuum. The solid residue was recrystallized from a mixture of
benzene and THF. Yield: 530 mg (76%), dark red crystals. Anal.
Calcd for C48H42MoN2O4P2 (868.54 MW): C, 65.40; H, 5.01; N,
3.32. Found: C, 65.33; H, 5.07; N, 2.60. MS (CI) (m/z): 870 (M,
basis peak), 842 (M - CO, 3%), 814 (M - 2CO, 1%), 790 (M -
3CO, 1%), 660 ligand, 8%). 31P{1H}NMR (C6D6, rt, 200 MHz):
7.2 (PPh2, noncoordinated), 69.3 (PPh2, coordinated). 1H NMR
(CDCl3, rt, 200 MHz) δ (in ppm): 1.49 (6H, br, CH3), 1.66 (s,
6H, s, CH3), 1.95 (s, 3H, CH3), 2.29 (s, 3H, CH3), 6.19 (s, 2H,
m-CH, mesityl), 6.73 (s, 2H, m-CH, mesityl), 7.21-7.39 (m, 16H,
phenyl), 7.64 (t, 4H, J ) 8.0 Hz, CH, phenyl). 13C NMR: (CDCl3,
rt, 200 MHz) δ (ppm): 18.1 (CH3), 18.2 (CH3), 20.2 (CH3), 20.8
(CH3), 123.1 (C), 128.0 (d, J(31P-13C) ) 10.1 Hz, CH, phenyl),
128.2 (CH, mesityl), 128.5 (d, J(31P-13C) ) 9.0 Hz, CH, Ph), 129.3
(CH, mesityl), 130.1 (d, J(31P-13C) ) 19.4 Hz, CH, phenyl), 133.4
(C), 133.6 (d, J(31P-13C) ) 13.9 Hz, CH, phenyl), 135.5 (C), 136.6
(d, J(31P-13C) ) 25.4 Hz, CH, phenyl), 142.7 (d, J(31P-13C) )
5.5 Hz, C), 150.8 (d, J(31P-13C) ) 10.9 Hz, C), 168.7 (d, J(31P-
1
NMR (CDCl3, rt, 81 MHz, δ, ppm): 3.82 (s). H NMR (THF-d8,
rt, 200 MHz, δ, ppm): 2.10 (s, 6H, CH3,), 6.19, 6.69 (2d, AA′BB′,
3JH,H ) 8,0 Hz, 8H, CH-tolyl), 7.28 (m, 26H, phenyl). 13C NMR
(THF-d8, rt, 200 MHz, δ, ppm): 20.7 (CH3), 120.0-136.8 (Caromat),
148.5 (Cbridge).
(4-tert-Butyl-C6H4-NdC(PPh2)C(PPh2)dN-C6H4-4-tert-bu-
tyl) (1b). To bis(4-tert-butylphenyl)-oxalic imidoyl chloride (0.72
g, 1.85 mmol) dissolved in toluene (150 mL) and cooled to -78
°C was dropwise added a solution of NaPPh2(dioxane) (1.25 g,
3.7 mmol) in THF (10 mL). The dark brown reaction mixture was
allowed to warm to room temperature. After 24 h of stirring, the
THF and most of the toluene were removed under vacuum. The
concentrated solution was warmed to 60 °C and filtered. 1b
crystallized at 4 °C. Yield: 0.708 g (56%), yellow crystals. Anal.
1
13C) ) 25.6 Hz, C), 185.7 (d, J(31P-13C) ) 56.1 Hz, NdC-P),
1
2
186.0 (d, J(31P-13C) ) 55.8 Hz, NdC-P), 209.8 (d, Jcis(31P-
13C) ) 8.3 Hz, Mo(CO)2), 213.2 (d, Jtrans(31P-13C) ) 32.8 Hz,
2
Mo(CO)), 222.3 (d, 2Jcis(31P-13C) ) 6.8 Hz, Mo(CO)). IR (Nujol,
cm-1): 1854, 2014, 1910, 1891 (ν(CO)).
Calcd for C46H46N2P2 (688.83 MW): C, 80.2; H, 6.73; N, 4.07.
(Mes-NdC(PPh2)C(PPh2)dN-Mes)[Mo(CO)4]2 (2b). (C7H8)-
Mo(CO)4 (510 mg, 1.70 mmol) and [(1c)(CHCl3)] (660 mg, 0.85
mmol) were dissolved in THF (5 mL). The color of the solution
turned immediately from yellow to dark red and then gradually to
dark blue. After 1 h of stirring at room temperature, the volatile
part of the solution was removed under vacuum. The solid residue
was recrystallized from a mixture of benzene and ether to form
black crystals. Yield: 800 mg (87%). Anal. Calcd for C52H42-
Mo2N2O8P2 (1076.68 MW): C, 58.27; H, 4.19; N, 2.54. Found:
C, 58.00; H, 3.93; N, 2.60. 1H NMR (C6D6, rt, 200 MHz; δ ppm):
1.66 (s, 6H, p-CH3), 2.17 (s, 12H, o-CH3), 5.86 (s, 4H, m-CH),
6.65-7.56 (m, 20H, phenyl). 13C NMR (CDCl3, rt, 200 MHz; δ,
ppm): 17.7 (p-CH3), 20.3 (o-CH3), 20.6 (o-CH3), 125.7 (CH), 128.9
(d, J ) 11.0 Hz, CH), 129.9 (m-CH, mesityl), 131.5 (C), 131.6 (d,
J ) 58.3 Hz, C), 135.5 (d, J ) 15.8 Hz, CH), 136.5 (C), 150.3 (d,
J ) 4.2 Hz, C), 180.8 (dd, J ) 25.7 Hz, J ) 22.5 Hz, NdC-P),
+
Found: C, 79.84; H, 6.67; N, 4.20. MS (CI): 689 ((M + 1)
,
basic peak), 611 ((M + 1)+ - C6H6, 2%), 503 ((M + 1)+ - C12H12,
2%,), 370 ((M + 1)+ - C22H24, 2%,), 344 ((M + 1)+ - C23H23-
NP, 45%), 185 (19%, (M + 1)+ - C34H37N2P). 31P{1H} NMR
(THF-d8, rt, 81 MHz; δ, ppm): 5.04 (s). 1H NMR (THF-d8, 25 °C,
200 MHz; δ, ppm): 1.19 (s, 18H, CH3-tBu), 6.9 (m, 30H, phenyl).
13C NMR (THF-d8, 25 °C, 50 MHz; PENDANT; δ, ppm): 31.6
(CH3-tBu), 34.6 (C-tBu), 119.8-148.3 (phenyl and Cbridge).
(Mes-NdC(PPh2)C(PPh2)dN-Mes)(CHCl3) [(1c)(CHCl3)]. Bis-
(mesityl)oxalic acid imidoyl chloride (2.7 g, 7.5 mmol) and Pd-
(acetate)2 (85 mg, 0.038 mmol) as cross coupling catalyst were
dissolved in toluene (100 mL). At -40 °C a solution of KPPh2
(15.0 mmol) in dioxane/THF (42 mL, 1:1) was added dropwise
within 1 h with stirring of the mixture. The yellow solution turned
to dark red. After 1 h the reaction mixture was allowed to warm to
room temperature. The color of the solution turned in this time to
blue-green. After filtration of KCl the solvent was removed under
vacuum and the residue was dissolved in n-hexane. Upon filtration
the solvent was removed under vacuum, yielding a dark red oil,
which slowly crystallized. The product was recrystallized from
hexane/chloroform. Yield: 3.96 g (80%), yellow crystals, mp 170
°C. Anal. Calcd for [(1c)(CHCl3)]: C45H43Cl3N2P2 (780.1 MW):
C, 69.28; H, 5.56; N, 3.59; Cl, 13.63. Found: C, 69.97; H, 5.67;
N, 3.55; Cl, 13.18. MS (EI) m/z: 660 (M+), 330 (M+/2). 31P{1H}
NMR (CDCl3, rt, 200 MHz; δ, ppm): 9.0 (s). 1H NMR (CDCl3, rt,
200 MHz; δ, ppm): 1.42 (s, 12H, o-CH3), 2.17 (s, 6H, p-CH3),
6.45 (s, 4H, m-H), 7.19-7.21 (m, 12H, phenyl), 7.50-7.54 (m,
8H, phenyl). 13C NMR (CDCl3, rt, 200 MHz; δ, ppm): 17.7 (d,
J(13C31P) ) 2.5 Hz, o-CH3), 20.6 (p-CH3), 124.4 (C, mesityl), 128.1
(CH, mesityl), 128.1 (d, J(13C31P) ) 8.3 Hz, CH, phenyl), 128.8
(CH, Ph), 132.8 (C, mesityl), 133.8 (br, d, J(13C31P) ) 10.5 Hz, C,
phenyl), 135.2 (br, d, J(13C31P) ) 23.4 Hz, C, phenyl), 145.3 (br,
d, J(13C31P) ) 9.3 Hz, C, mesityl), 175.2 (dd, J(13C31P) ) 58.1
Hz, J(13C31P) ) 4.5 Hz, NdCP).
2
2
210.6 (d, Jcis ) 8.1 Hz, Mo(CO)2), 213.1 (d, Jtrans ) 32.8 Hz,
Mo(CO)), 222.7 (d, 2Jcis ) 6.7 Hz, Mo(CO)). 31P{1H} NMR (C6D6,
rt, 200 MHz; δ, ppm): 82.5. IR (Nujol, cm-1): 1830, 1859, 1892,
1938, 2010, 2014, 2025 (ν(CO)).
[(1a)Cu(Cl)(PPh3)] (3). To 1a (368 mg, 0.6 mmol) dissolved
in THF (60 mL) was added a solution of CuCl(PPh3)3 (539 mg,
0.6 mmol) in THF (40 mL) by a cannula. The color of the solution
turned from yellow to dark red. After 1 h of stirring, the solution
was concentrated under vacuum and diethyl ether (30 mL) was
added dropwise. Upon keeping overnight the red microcrystalline
solid was filtered off, washed twice with ether (20 mL), and dried
under vacuum. Single crystals of 3 could be obtained form the
mother liquor at -20 °C. Yield: 413 mg (70%). Anal. Calcd for
C58H49CuP3N2Cl (965.96 MW): C, 72.12; H, 5.11; N, 2.90; Cl,
3.67. Found: C, 72.14; H, 5.26; N, 2.70; Cl, 3.67. MS (EI): m/z
) 604 (ligand+, 2%), 262 (PPh3+, 100%). H NMR (CD2Cl2, rt,
1
200 MHz; δ, ppm): 2.16 (s, 6H, CH3), 5.69 (m, 4H, aromatic CH
of 4-tolyl), 6.57 (m, 4H, aromatic CH of 4-tolyl), 7.17-7.95 (m,
35H, PPh2 + PPh3). 13C NMR (CD2Cl2, rt, 200 MHz; δ, ppm):
20.7 (CH3), 119.04, 119.9, 129.0, 129.1, 131.6, 134.5, 134.9, 135.8,
136,7, 137.5 (phenyl), 148.4 (quart C of the central C-C unit).
[(Mes-NdC(PPh2)C(PPh2)dN-Mes)Mo(CO)4] (2a). (C7H8)Mo-
(CO)4 (240 mg, 0.8 mmol) and [(1c)(CHCl3)] (530 mg, 0.8 mmol)
Inorganic Chemistry, Vol. 42, No. 2, 2003 627

Walther et al.
31P{1H} NMR (CD2Cl2, rt, 200 MHz; δ, ppm): one very broad
singlet between -5 and 5 ppm.
Scheme 1. Formation of Complexes 2-6 from the Ligands 1a-1c
{(1b)[(NiBr2(THF)]2} (4). To a stirred solution of 1b (328 mg,
0.476 mmol) in THF (10 mL) at -78 °C was added NiBr2(THF)1.46
(308 mg, 0.952 mmol) in THF (10 mL). The reaction mixture was
allowed to warm to -40 °C. After 24 h the product containing
four molecules of THF per Ni was separated by filtration and dried
under vacuum. Yield: 490 mg (73%). Single crystals of {(1b)-
[(NiBr2(THF)]2} suitable for X-ray analysis were obtained from
the mother liquor at -40 °C. Anal. Calcd for C62H78Br4N2Ni2O2P2
({(1b)[(NiBr2(THF)]2}(THF)2) (1414.23 MW): C, 52.65; H, 5.56;
N, 1.98. Found: C, 52.71; H, 5.85; N, 2.01. MS (FAB): m/z )
827 (M+ - 4THF - NiBr2 - Br), 688 (M+ - 4THF - 2NiBr2).
Due to the paramagnetism no NMR spectra were monitored.
Complex 5. 1c (708 mg, 1.07 mmol) and NiBr2(THF)1.46 (357
mg, 1.1 mmol) in THF (10 mL) were stirred at room temperature.
After 1 h the reaction mixture was heated to 60 °C for about 10
min and filtered. 5 containing one molecule of THF per Ni was
obtained from the solution at room temperature. Single crystals of
the THF free complex were grown from the mother liquor. Yield:
915 mg (90%), red crystals. Anal. Calcd for C48H50Br2N2NiOP2
(5(THF)) (951.33 MW): C, 60.60; H 5.30; N 2.95. Found: C,
60.40; H, 5.11; N, 2.90. MS (CI): m/z ) 880 ((M + 1)+ - THF;
34%). MS (DEI): 801 (M+ + 2H - Br - THF, 0.4%), 585 (M+
- THF - C6H5Br2Ni), 476 (M+ - THF - C12H10PBr2Ni), 185
(M+ - C32H32N2PBr2Ni, basis peak). 31P{1H} NMR (C6D6, rt, 81
MHz; δ, ppm): 45.8 (d, J ) 78 Hz), 61.1 (d, J ) 78 Hz). 1H NMR
(THF-d8, rt, 400 MHz; δ, ppm): 0.82, 1.0, 1.42, 1.48, 1.89, 2.17
(s, 3H each, CH3), 5.04 (s, olefinic-H, 1H), 6.08 (s, olefinic-H,
1H), 6.22 (s, 1H mesityl-CH), 6.68 (s, 1H mesityl-CH), 7.06-8.30
(m, 20H, phenyl). 13C NMR (THF-d8, rt, 100 MHz; δ, ppm): 16.8,
16.9, 18.6, 20.7, 20.9, 21.2 (CH3), 126.1-135.1 (Carom and Cbridge).
Complex 6. 1b (580 mg, 0.84 mmol) in THF (10 mL) was
reacted with NiBr2(dme) (260 mg, 0.84 mmol) at room temperature.
Alternatively, NiBr2(THF)2 can be used. After 4 h the product was
separated by filtration and washed with THF. Yield of the crude
product: 610 mg (80%). The pure complex containing one THF
was obtained by extraction of the product with methanol at reflux
temperature. Single crystals of the THF free complex suitable for
X-ray diffraction were isolated upon keeping the methanol solution
at -40 °C.
MS (DEI): m/z ) 688 (M+, 33%), 631 (M+ - C4H9, 2%), 518
(3%), 425 (3%), 369 (3%), 185 (M+ - C34H36N2P), 57 (C4H9, basis
peak). 31P{1H} NMR (THF-d8, 27 °C, 81 MHz; δ, ppm): -13.0
(d, J ) 172 Hz), - 2,6 (d, J ) 172 Hz). 1H NMR (THF-d8, 27 °C,
200 MHz; δ, ppm): 1.13, (s, 9H, Bu), 1.32 (s, 9H, Bu), 6.22-
7.52 (m, 28H, phenyl). 13C NMR (THF-d8, rt, 50 MHz; δ, ppm):
31.6 (CH3), 31.8 (CH3), 34.3 (tBu), 34.8 (tBu), 114.0-148.0 (Carom
and Ctert).
t
t
Results and Discussion
Alternatively, complex 4 was suspended in warm methanol. After
1 h the suspension was filtered. Upon keeping the methanol solution
at -20 °C 6 crystallized.
The ligands 1a-1c (Scheme 1) can be easily synthesized
by reacting 1 equiv of a substituted oxalic imidoyl chloride
with 2 equiv of MPPh2 (M: Na, K) in THF or toluene at
-40 °C (eq 1). 1a-1c form thermally stable yellow,
Anal. Calcd for C50H54Br2N2NiOP2 (979.44 MW) (6(THF)): C,
61.32; H, 5.56; N, 2.86. Found: C, 61.42; H, 5.95; N, 2.75. MS
(CI) m/z ) 988 (M + 1+ + Br - THF, 23%), 908 (M + 1 - THF,
65%). 31P{1H} NMR (THF-d8, rt, 81 MHz; δ, ppm): 50.2 (d, J )
83 Hz), 41.6 (d, J ) 83 Hz). 1H NMR (THF-d8, 60 °C, 200 MHz;
δ, ppm): 1.05 (s, 9H, tBu), 1.24 (s, 9H, tBu), 6.15-8.31 (m, 29H,
phenyl), 6.76 (s, 1H, NH). 13C NMR(THF-d8, rt, 50 MHz; δ,
ppm): 31.0 (CH3), 31.3 (CH3), 34.4 (tBu), 34.7 (tBu), 113.7-146.9
(C arom and Ctert).
1c from 5. Complex 5(THF) (0.30 g, 0.316 mmol) dissolved in
THF (10 mL) was dropwise added to a stirred THF/n-hexane
solution (5 mL) of diacetylglyoxime (0.033 g, 0.632 mmol) and
BuLi (0.632 mmol in n-hexane). Upon 24 h the solvent was re-
moved under vacuum. The organic material was dissolved in diethyl
ether and filtered. Removal of the diethyl ether under vacuum
resulted in a yellow-orange solid. A comparison of the analytical
data with an authentic sample of 1c showed them to be identical.
Similarly, complex 6 was reacted with diacetylglyoxime and
BuLi to give the free quinoxaline derivative; light orange solid.
2MPPh2 + RNdC(Cl)C(Cl)dNR f
RNdC(PPh2)C(PPh2)dNR + 2MCl (1)
(M: Na, K)
moisture- and air-sensitive crystals which could be character-
1
ized by elemental analyses, MS, and 31P{1H}, H, and 13C
NMR spectroscopy. These compounds are soluble in many
organic solvents including THF, CHCl3, and toluene; how-
ever their solubility in n-alkanes is low. Only one resonance
is observed in 31P{1H} NMR spectra for these compounds.
In addition, the 1H NMR/13C NMR spectra show the expected
patterns.
These ligands undergo complexation reactions with transi-
tion metals that result in metal complexes with differing
628 Inorganic Chemistry, Vol. 42, No. 2, 2003

2,3-Bis(diphenylphosphino)-1,4-diazadiene Ligands
Figure 1. ORTEP diagram of complex 2a with thermal ellipsoids at 50%
probability. Selected bond distances (Å) and bond angles (deg): Mo-P1
2.4994(6), Mo-N1 2.284(2), C1-C2 1.514(3), C1-N1 1.295(3), C1-P2
1.868(2), C2-N2 1.271(3), C2-P1 1.876(2), P1-Mo-N1 74.76(5), P1-
C2-C1 110.3(2), P1-C2-N2 133.4(2), N2-C2-C1 115.0(2), C2-C1-
N1 114.9(2), C2-C1-P2 125.4(2), C1-N1-Mo 125.6(2), Mo-P1-C2
96.92(7).
nuclearities and coordination patterns of the ligands (Scheme
1). This demonstrates that a rich coordination chemistry of
2,3-bis(diphenylphosphino)-1,4-diazadiene-type ligands is
possible. Subsequent reactions at the periphery of mono-
nuclear Ni complexes (4 and 6, respectively, Scheme 1) are
even possible for these ligands.
Complexes 2-6 are formed in good yields and have been
characterized by standard spectroscopic and analytical meth-
ods, as well as by single-crystal X-ray crystallography in
the solid state.
The mononuclear complex [(1c)Mo(CO)4] (2a) was ob-
tained by mixing (norbornadiene)Mo(CO)4 and an equimolar
amount of 1c in THF at room temperature. In complex 2a
the ligand is coordinated via one N,P donor set as seen in
the 31P{1H} NMR spectrum in C6D6 (two singlets were
observed). The resonance at 7.2 ppm is attributable to the
noncoordinated phosphorus atom (9.0 ppm in the free ligand),
and the low-field shifted signal at 69.3 ppm is assigned to
the coordinated P atom. As expected, the IR spectrum shows
four carbonyl frequencies between 1834 and 2014 cm-1.
An X-ray crystallographic study of single crystals obtained
from THF confirms this structure for the solid state (Figure
1).
Figure 2. (a) ORTEP diagram of complex 2b with thermal ellipsoids at
50% probability. Selected bond distances (Å) and bond angles (deg): Mo-
N1 2.289(3), Mo-P1 2.512(1), P1-C1 1.883(3), C1-C1A 1.497(7), C1-
N1A 1.302(4), N1-Mo-P1 74.43(8), Mo-N1-C1A 122.8(2), N1-C1A-
P1A 130.9(3), N1-C1A-C1 117.3(4), C1A-C1-P1 110.9(3), C1-P1-
Mo 95.9(1). Symmetry transformations used to generated equivalent
3
atoms: A -X + 1, Y, -Z + /2. (b) Ball-and-stick reprensentation of the
view along the C1-C1A line for complex 2b.
2b. The C-N bond lengths of the 1,2-diimine units (2a, C1-
N1(coordinated) ) 1.295(3), C2-N2(noncoordinated) )
1.271(3) Å; 2b, C1-N1A(coordinated) ) 1.302(4) Å) are
typical for CdN double bonds and are only slightly
lengthened upon coordination of the Mo atom. Upon binding
to the metal there is significant decrease (110.3(2)°) in the
C1-C2-P1 angle in 2a with P1 being bonded to Mo. The
noncoorinated C2-C1-P2 angle is much larger (125.4(2)°).
This effect is also seen with the coordinated C-C-P
angles being quite small (110.9(3)°).
The most interesting feature in 2b is that both Mo(CO)4
fragments are arranged cis to each other with respect to the
central C1-C1A bond of the chelating ligand (Figure 2b).
In contrast to the Mo(CO)4 moiety, Cu(PPh3)3Cl reacted
to form mononuclear Cu complexes even if the smaller ligand
1a was used. [(1a)Cu(Cl)(PPh3)] (3) was prepared from Cu-
(PPh3)3Cl and 1a (molar ratio: 2:1) in THF and could be
isolated as air-sensitive red crystals in good yields (70%).
All attempts to react 3 with a second metal have failed so
far.
The solid-state structure of 3 (Figure 3) was established
by X-ray diffraction of single crystals obtained from THF
at -20 °C. In contrast to the Mo complexes, the chelating
ligand coordinates in a different manner where the two P
atoms are bonded to the Cu center. As expected, the Cu atom
is in a tetrahedral environment created by the P,P donor set
of the chelating ligand, one Ph3P ligand, and one chloride.
All bond lengths lie within the expected range and need not
be discussed.
Reaction of complex 2a with 1 equiv of (norbornadiene)-
Mo(CO)4 resulted in the formation of a binuclear compound
{(1c)[Mo(CO)4]2} (2b). The 31P{1H} NMR spectrum of 2b
in C6D6 shows only one signal at 82.5 ppm which is
indicative for coordination of both PPh2 groups. This suggests
that 1c acts as a bridging ligand in 2b and coordinates with
one imine nitrogen and one phosphorus atom to each Mo
1
atom. The H and 13C NMR spectra are in agreement with
this structure in solution.
Figure 2a shows the ORTEP diagram of 2b with an atomic
labeling scheme and contains selected bond distances and
angles in the figure caption. There are no significant
differences in the relevant bond lengths and angles when
comparing 2a to 2b. For example, the central C1-C2 (C1-
C1A) bond distances are 1.514(3) in 2a and 1.497(7) Å) in
Inorganic Chemistry, Vol. 42, No. 2, 2003 629

Walther et al.
Figure 3. ORTEP diagram of complex 3 with thermal ellipsoids at 50%
probability. Selected bond distances (Å) and bond angles (deg): Cu-Cl
2.3060(7), Cu-P1 2.2787(7), Cu-P2 2.2761(7), Cu-P3 2.611(7), P1-C1
1.871(2), P2-C2 1.865(2), C1-C2 1.536(3), C1-N1 1.268(3), C2-N2
1.271(3), Cl-Cu-P1 105.07(3), Cl-Cu-P2 110.22(3), Cl-Cu-P3 109.00-
(3), P1-Cu-P2 92.21(2), P1-Cu-P3 121.23(3), P2-Cu-P3 117.63(2),
P1-C1-N1 119.5(2), P1-C1-C2 112.5(2), P2-C2-N2 119.7(2), P2-
C2-C1 113.6(2), N1-C1-C2 127.8(2), N2-C2-C1 126.7(2).
Figure 4. ORTEP diagram of complex 4 with thermal ellipsoids at 50%
probability. Selected bond distances (Å) and bond angles (deg): Ni1-Br1
2.5377(6), Ni1-Br2 2.5394(6), Ni1-O1 2.105(3), Ni1-O2 2.118(3), Ni1-
N1 2.076(3), Ni1-N2 2.074(3), Ni2-P1 2.132(1), Ni2-P2 2.129(1), Ni2-
Br3 2.3251(7), Ni2-Br4 2.3276(7), C1-C2 1.495(5), C1-N1 1.287(5),
C2-N2 1.278(5), C1-P1 1.854(4), C2-P2 1.856(4), N1-C1-C2 116.0-
(3), N1-C1-P1 129.4(3), N2-C2-C1 116.3(3), N2-C2-P2 129.5(3),
P2-C2-C1 114.2(3).
The 1H NMR and the 13C NMR spectra of 3 in CDCl3 are
in accordance with the symmetrical struture shown in Figure
3. Only one resonance for the methyl protons of the 4-tolyl
group was observed at 2.16 ppm in CDCl3 at room
temperature. The aromatic protons of the 4-tolyl group can
be distinguished from the other signals of the PPh2 group,
and the PPh3 ligand. The 13C NMR spectrum also confirms
the symmetrical structure. Besides the signals for the aromatic
carbons, only a single low-field shifted resonance at 148.4
ppm appears for the quaternary carbons of the central C-C
bond in the chelating ligand. In addition, only one resonance
for the methyl groups was observed at 20.7 ppm. In contrast
to this, the 31P NMR, spectrum, recorded between -60 °C
and room temperature, shows only a very broad signal
(between -5 and +5 ppm) which may be explained by both
the overlap of the three resonances of coordinated phosphines
in 3 and ligand exchange to give the three-coordinated Cu
complex in equilibrium with 3.
Whereas 1a only forms a mononuclear Cu(I) complex,
NiBr2(THF)1.46 reacted with the closely related ligand 1b in
a molar ratio 2:1 in THF to form the binuclear complex 4.
NMR spectra could not be recorded due to the paramagnetic
nature of the complex.
A single-crystal X-ray diffraction study reveals that the
two Ni atoms possess different coordination geometries. An
ORTEP diagram of 4 is given in Figure 4. The most relevant
bond distances and angles are listed in the figure caption.
The 2,3-bis(diphenylphosphino)-1,4-diazadiene 1b acts as
bridging ligand and is bidentately bound on one side to the
square-planar nickel center via the two phosphorus atoms.
Two bromide ligands occupy the other two positions. On
the other side the chelate ligand binds to the octahedral nickel
via the 1,2-diimine unit. In addition, two bromides occupy
trans positions of the octahedron, and two THF molecules
coordinate cis to each other.
Figure 5. ORTEP diagram of compound 5 with thermal ellipsoids at 50%
probability. Selected bond distances (Å) and bond angles (deg): Ni-Br1
2.3118(4), Ni-Br2 2.3398(3), Ni-P1 2.1314(6), Ni-P2 2.1449(7), P1-
C1 1.788(2), P2-C2 1.820(2), C1-C2 1.379(4), C1-N1 1.379(3), N1-
C3 1.295(4), C3-C4 1.516(4), C3-C8 1.458(4), C7-C8 1.339(4), C6-
C7 1.464(4), C5-C6 1.334(4), C4-C5 1.493(4), N2-C4 1.510(3), Br1-
Ni-Br2 95.45(1), Br1-Ni-P1 88.69(2), Br1-Ni-P2 176.65(2), Br2-Ni-
P1 173.68(2), Br2-Ni-P2 87.63(2), P1-Ni-P2 88.35(2), N1-C1-C2
122.3(2), N1-C1-P1 119.7(2), P1-C1-C2 117.8(2), N2-C2-C1 118.3-
(2), N2-C2-P2 127.2(2), P2-C2-C1 114.3(8).
the bulkier ligand 1c containing mesityl groups on the
nitrogen atoms of the 1,2-diimine unit reacts with NiBr2-
(THF)1.46 under essentially the same conditions used for the
formation of the binuclear complex 4 to form the mono-
nuclear Ni complex 5. X-ray quality red crystals of 5 could
be obtained from THF at -20 °C. Figure 5 displays the
molecular structure of the complex and contains selected
bond lengths and angles in the figure caption.
The formation of 5 required a significant reorientation of
the ligand in the precursor (Figure 5). Only the two
phosphorus atoms act as ligands toward the “cis-NiBr2”
fragment to form a square-planar complex. Surprisingly, the
five-membered chelate ring contains a bis(diphenylphosphi-
no)ethylene unit instead of the bis(diphenylphosphino)ethane
unit of the precursor with a newly formed central CdC
double bond (C1-C2 ) 1.379(4) Å). This double bond is
part of the quinoxaline ring.
As expected, the two Ni1-Br distances of the octahedral
complex fragment (on average 2.535(6)) are significantly
longer than those of the square-planar Ni2 at the opposite
side (on average 2.323(7) Å). The bond distances of the Cd
N bonds are equal within experimental error and lie in the
usual range (1.2837(5) and 1.278(5) Å). In contrast to 1a,
The most interesting feature of this structure, however, is
the cyclohexa-1,3-diene ring formed in the peripheral position
630 Inorganic Chemistry, Vol. 42, No. 2, 2003

2,3-Bis(diphenylphosphino)-1,4-diazadiene Ligands
of the complex as the result of an intramolecular [4 + 2]
cycloaddition reaction of the 1,2-diimine with a benzene ring
of a mesityl substituent. In this reaction the benzene ring
acts as (dihetero)dienophile. The length of the C3-N1 bond
(1.295(4) Å) is indicative of a CdN double bond. In contrast,
the C4-N2 bond of the heterocyclic ring system of 1.510-
(3) Å is that of a single bond. Furthermore, the bond distances
C5-C6 and C7-C8 (1.334(4) and 1.339(4) Å, respectively)
are typical for CdC bonds in a cyclohexa-1,3-diene ring.
As expected, the C4-C5 distance is that of a single bond
(1.493(4)), whereas the C6-C7 and C3-C8 bond lengths
as part of a conjugated system are slightly shortened (1.464-
(4) and 1.458(4) Å, respectively) compared with a typical
C-C single bond.
Figure 6. ORTEP diagram of compound 6 with thermal ellipsoids at 50%
probability. Selected bond distances (Å) and bond angles (deg): Ni-Br1
2.331(1), Ni-Br2 2.336(1), Ni-P1 2.143(2), Ni-P2 2.141(2), P1-C1
1.807(7), P2-C2 1.817(7), C1-C2 1.338(9), C1-N1 1.440(9), N2-C3
1.402(9), C3-C4 1.386(10), C3-C8 1.383(10), C7-C8 1.384(10), C6-
C7 1.388(10), C5-C6 1.409(10), C4-C5 1.368(10), N1-C4 1.469(8), N2-
C2 1.377(9), Br1-Ni-Br2 95.02(4), Br1-Ni-P1 89.51(6), Br1-Ni-P2
176.75(7), Br2-Ni-P1 175.07(7), Br2-Ni-P2 87.42(6), P1-Ni-P2 87.99-
(7), N1-C1-C2 122.0(6), N1-C1-P1 120.7(5), P1-C1-C2 116.9(5),
N2-C2-C1 120.3(6), N2-C2-P2 122.8(5), P2-C2-C1 116.7(5).
NMR spectroscopic measurements in CDCl3 indicate that
the solid-state structure is also present in solution. As
expected for 5, two doublets in the 31P{1H} NMR spectrum
are observed at 61.1(J ) 81 Hz) and 45.8 (J ) 81 Hz) ppm
indicating the coordination of both nonequivalent phosphorus
atoms. In comparison with the 31P signal of the free ligand
both resonances are strongly low-field shifted. Moreover, in
1
the H NMR spectrum (in CDCl3 at room temperature) the
noteworthy that the binuclear Ni complex 4, suspended in
methanol, eliminates NiBr2(THF)2 from the 1,4-diazadiene
unit to form the mononuclear complex 6 (Scheme 1) in which
a similar [4 + 2] cycloaddition reaction has occurred as in
5. In this case, however, a subsequent tautomerization results
in the rearrangement of the benzene system of the peripheral
ring. In addition, complex 6 is also formed by reaction of
NiBr2(THF)1.46 with 1c (1:1) in THF at ambient temperature.
Figure 6 shows the molecular structure determined from
single crystals of 6.
expected six singlets of the nonequivalent methyl groups
between 0.82 and 2.17 ppm were observed. The olefinic
protons at C5 and at C7 of the cyclohexa-1,3-diene system
gave rise to two singlets at 5.04 and 6.08 ppm, each
integrating to 1. In addition, in the 13C NMR spectrum shows
six signals between 16.8 and 21.2 ppm, which are attributable
to the six nonequivalent methyl carbons.
To the best of our knowledge the ring-closure reaction
yielding 5 is the first metal-mediated [4 + 2] cycloaddition
with a benzene ring as a (dihetero)dienophile in which a
cyclohexa-1,3-diene ring is formed. Generally, such ther-
mally induced [4 + 2] cycloaddition reactions yielding a
cyclohexa-1,3-diene originating from benzene are extremely
rare. Only in one case has such a reaction been described
using a specially designed organic ring system which reacts
in an unselective reaction at high temperatures.26 In other
cases, a fast rearrangement of the cyclohexa-1,3-diene
intermediate to form an aromatic ring was observed.27-29
Furthermore, there are also a few examples for such reactions
in organic chemistry which are photochemically activated
resulting in cyclohexa-1,3-diene intermediates which rear-
range to form benzene derivatives in most cases.30-36 It is
In contrast to complex 5, where the N1-C3 bond distance
of 1.295(4) Å is that of a double bond, the related bond length
N(1)-C(4) (1.469(8) Å) in complex 6 is a typical single
bond. As expected, also the other C-N distances (N(1)-
C(1) ) 1.440(9) Å, N(2)-C(2) ) 1.377(9) Å, and N(2)-
C(3) ) 1.402(9) Å) are single bonds. The C-C bond lengths
in the peripheral ring typical for a benzene ring are equivalent
to each other within experimental error. This is in direct
contrast to the C-C bond lengths in complex 5. In addition,
the central C1-C2 (1.338(9) Å) distance in the five-
membered metal chelate ring is that of a double bond.
1
31P{1H}, H, and 13C NMR studies were conducted in
THF-d8 and showed all expected shifts and splitting patterns
associated with the molecule structure of 6. For example,
two doublets at 41.6 and 50.2 ppm were observed in the
31P{1H} NMR spectrum at room temperature indicating
coordination of both P atoms. In addition, the expected two
signals for the protons of the different tert-butyl groups were
observed at 1.05 and 1.24 ppm with each integrating to nine
protons.
The highly varying coordination behavior of the ligands
depending on the identity of the transition metal may be
rationalized as following: In the first step one phosphorus
atom reacts with the metal center to form a monocoordinated
species followed by the formation of the chelate ring by using
the nitrogen donor atom of the imine group. This chelate
ring is kinetically inert in the case of molybdenum. In
(26) Saito, K.; Omura Y.; Maekawa, E. Tetrahedron Lett. 1984, 25, 2573.
(27) Saito; K. Omura Y.; Maekawa, E.; Gassmann, P. G. Bull. Chem Soc.
Jpn. 1990, 63, 395.
(28) Grimme, W.; Gommes, T.; Roth, W. R.; Breukmann, R. Angew. Chem.
1992, 104, 867-869; Angew. Chem., Int. Ed. Engl. 1992, 31, 872.
(29) Capuano, L.; Moersdorf, P. Liebigs Ann. Chem. 1982, 2178.
(30) Ilge, H. D.; Langbein, H.; Reichenba¨cher, M.; Pa¨tzold, R. J. Prakt.
Chem. 1981, 323, 367.
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Walther et al.
contrast to this, Ni(II) chelate rings are kinetically more labile
and undergo isomerization to form the thermodynamically
more stable (P,P)Ni coordination. In the next step NiBr2 may
react in a fast reaction with the 1,2-diimine unit on the
opposite side of the complex, provided there is enough space
for coordination. This is the case for ligand 1b, which bears
the relatively small 4-tert-butylphenyl group on the nitrogen
atoms of the 1,2-diimine unit. In the resulting binuclear
complex 4 the 1,2-diimine unit is “protected” by the metal
center. Consequently, a ring-closure reaction to form a
quinoxaline system does not occur. If complex 4 eliminates
NiBr2 in methanol, the “deprotected” 1,2-diimine system
undergoes a fast intramolecular ring-closure ([4 + 2]
cycloaddition) reaction with the N-4-tert-butylphenyl sub-
stituent, which results in the formation of the quinoxaline
derivative. However, the cyclohexa-1,3 diene ring interme-
diately formed is unstable because the -HC(C,C)-CHd
N- unit of C3-C4-N2 underdoes a fast tautomerization
to form the dC-C(C,C)-NH- system in complex 6
containing C3 and C4 as part of the benzene ring (Scheme
1 and Figure 6).
In contrast, ligand 1c bears bulky mesityl groups on the
nitrogens which prevent the coordination of a second nickel
center on the 1,2-diimine unit. Consequently, the [4 + 2]
cycloaddition reaction occurs to yield the quinoxaline deriva-
tive containing the cyclohexa-1,3-diene ring. This ring is
stable since the system (CH3)C(C,C)-CdN- is not able to
undergo subsequent tautomerization to a benzene ring (Figure
5).
It is noteworthy that the [4 + 2] cycloaddition product in
5 is only stable in the metal complex. Decomplexation of 5
with diacetyldioxime/LiBu in THF results in the reorganiza-
tion of the free ligand 1c which was isolated from the reaction
mixture. It was characterized by comparison of its spectro-
scopic data with those of an authentic sample of 1c which
were identical.
bone. As expected, two 31P signals were observed in its
31P{1H} NMR spectrum which are high-field shifted com-
pared with the related signals in 6 (doublets at -12.96, J )
172 Hz, and -2.61, J ) 172 Hz). However, in comparison
1
with the H and 13C NMR spectra of the coordinated state
in 6, the NMR spectra of the free quinoxaline are very
similar. As also expected, its NMR spectra are very different
from those of the isomeric ligand 1b (see Experimental
Section).
Conclusions
This work demonstrates that the newly synthesized 2,3-
bis(diphenylphosphino)-1,4-diazadienes give rise to a very
rich coordination chemistry with transition metals. The
ligands can coordinate in three different bidentate modes,
(P,N)M, (P,P)M, and (N,N)M, and are able to yield mono-
and dinuclear compounds.
The ring-closure reaction ([4 + 2] cycloaddition) resulting
in quinoxaline derivatives upon coordination of one NiBr2
at the two P donor atoms is very sensitive toward the nature
of the metal center. Whereas tetrahedral Cu(I) in 3 does not
promote this [4 + 2] cycloaddition, Ni(II) in a square-planar
environment does. Generally, this reaction opens a simple
route for preparing bis(diphenylphosphino)ethylenes contain-
ing a quinoxaline backbone, provided the ortho position of
the nitrogen substituents contain hydrogen.
The course of the metal-mediated ring-closure reaction,
the influence of the nature of the metals on this reaction,
and the ability of the new ligands in tuning catalytic
properties of coordinated metals are the subjects of ongoing
studies.
Acknowledgment. We are grateful to the Deutsche
Forschungsgemeinschaft and the Fonds fu¨r Chemische In-
dustrie for support of this research.
Supporting Information Available: Tables giving complete
details of the X-ray diffraction studies of the complexes 2-6.
Crystallographic details in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
In contrast, decomplexation of 6 under essentially the same
conditions yielded the free quinoxaline derivative offering
a simple way of synthesizing new types of chelating bis-
(diarylphosphino)ethylenes containing a quinoxaline back-
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632 Inorganic Chemistry, Vol. 42, No. 2, 2003