Structures and redox properties of metal complexes of the electron-deficient diphosphine chelate ligand R,R-QuinoxP

Article abstract of DOI:10.1021/om700755y

The air-stable chiral diphosphine chelate ligand R,R-QuinoxP (L; 2,3-bis(terf-butylmethylphosphino)-quinoxaline) as developed by Imamoto et al. has been used to obtain the crystallographically characterized complexes (L)PtCl₂ (1), (L)PdCl₂

Full text of DOI:10.1021/om700755y

218
Organometallics 2008, 27, 218–223
Structures and Redox Properties of Metal Complexes of the
Electron-Deficient Diphosphine Chelate Ligand R,R-QuinoxP
Atanu Kumar Das,† Ece Bulak,† Biprajit Sarkar,† Falk Lissner, Thomas Schleid,†
Mark Niemeyer,† Jan Fiedler,‡ and Wolfgang Kaim*,†
Institut für Anorganische Chemie, UniVersität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany,
and J. HeyroVský Institute of Physical Chemistry, V.V.i., Academy of Sciences of the Czech Republic,
DolejškoVa 3, CZ-18223 Prague, Czech Republic
ReceiVed July 25, 2007
The air-stable chiral diphosphine chelate ligand R,R-QuinoxP (L; 2,3-bis(tert-butylmethylphosphino)-
quinoxaline) as developed by Imamoto et al. has been used to obtain the crystallographically characterized
complexes (L)PtCl2 (1), (L)PdCl2 (2), and (L)Re(CO)3Cl (3). Coordination was found to occur via the P
donor atoms, as indicated by crystal structures and NMR studies; the quinoxaline N donors do not
participate in any coordination to the metals. The stereochemical arrangements observed illustrate the
enantioselectivity reported for catalysis involving complexes of L. Electron acceptance by the quinoxaline
heterocycle is responsible not only for the improved stability of L toward air but also for rather facile
reduction of the complexes to the persistent radical anions 1•- and 3•-. In contrast, the reduction to 2•-
proceeds irreversibly even at 243 K in the absence of excess chloride. EPR, UV–vis, and IR
spectroelectrochemistry was used, when possible, to establish the spin location in the quinoxaline π
system with rather small contributions from the metals or the phosphorus nuclei.
used the R,R isomer L of QuinoxP in order to structurally
establish the transition-metal binding to the P (and not N) donor
atoms. The electron transfer behavior of the compounds (L)PtCl2
(1), (L)PdCl2 (2), and (L)Re(CO)3Cl (3) was investigated by
cyclic voltammetry, EPR, UV–vis and IR spectroelectrochem-
istry. Palladium complexes such as 2 were implicated in the
enantioselective catalysis of C-C bond formation.2
Introduction
Chiral 1,4-diphosphine ligands capable of forming five- or
six-membered metal chelate rings have long been established
as essential components of successful enantioselective catalysis
systems, involving especially hydrogenation and C-C bond
forming reactions.1 For practical applications, however, the
necessity of working with air-sensitive alkylphosphines has been
a drawback, which is why strategies to overcome this incon-
venience and simultaneously preserve or even improve the
catalytic activity and selectivity have been sought. Imamoto and
co-workers have recently presented a class of diphosphine
ligands which fulfill these requirements, using the rigid and
π-electron-deficient quinoxaline heterocycle as a platform to
which the dialkylphosphinosubstituents could be attached in the
2,3-positions.2,3 The resulting molecules such as 2,3-bis(tert-
butylmethylphosphino)quinoxaline (QuinoxP)2 and related diphos-
phines3 were employed in connection with rhodium and
palladium components for asymmetric hydrogenation and
carbon-carbon bond formation.
Considering the tendency of quinoxalines,4 especially metal-
coordinated ones,5 to undergo reversible electron uptake to form
persistent, EPR-detectable radical anion species, we have now
† Universität Stuttgart.
‡ J. Heyrovský Institute of Physical Chemistry.
(1) (a) Ohkuma, T.; Kitamura, M.; Noyori, R. In Catalytic Asymmetric
Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, 2000; pp 1. (b)
Brown, J. M. In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N.,
Pfalz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin, 1999; Vol. 1, pp
121. (c) Crépy, K. V. L.; Imamoto, T. Top. Curr. Chem. 2003, 229, 1.
(2) Imamoto, T.; Sugita, K.; Yoshida, K. J. Am. Chem. Soc. 2005, 127,
11934.
Experimental Section
Instrumentation. EPR spectra at X-band were recorded with a
1
Bruker System EMX instrument. H and 31P NMR spectra were
(3) Imamoto, T.; Kumada, A.; Yoshida, K. Chem. Lett. 2007, 36, 500.
(4) Henning, J. C. M. J. Chem. Phys. 1966, 44, 2139.
(5) (a) Kaim, W. Chem. Ber. 1982, 115, 910. (b) Kaim, W. Angew.
Chem. 1983, 95, 201; Angew. Chem., Int. Ed. Engl. 1983, 22, 171. (c) Kaim,
W.; Kohlmann, S.; Lees, A. L.; Zulu, M. M. Z. Anorg. Allg. Chem. 1989,
575, 97.
recorded on a Bruker AC 250 spectrometer. IR spectra were
obtained using a Nicolet 6700 FT-IR instrument; solid-state IR
measurements were performed with an ATR unit (smart orbit with
diamond crystal). UV–vis-near-IR absorption spectra were recorded
10.1021/om700755y CCC: $40.75
2008 American Chemical Society
Publication on Web 01/03/2008

Metal Complexes of the Diphosphine Ligand R,R-QuinoxP
Organometallics, Vol. 27, No. 2, 2008 219
Table 1. Crystallographic Dataa
1 · CH2Cl2
2 · CH2Cl2
3
empirical formula
molecular mass (g)
cryst syst
temp (K)
space group
a (Å)
C19H30Cl4N2P2Pt
685.28
monoclinic
100(2)
C19H30Cl4N2P2Pd
596.59
monoclinic
100(2)
C21H28ClN2O3P2Re
640.04
monoclinic
173(2)
P21
P21
C2
7.9288(2)
15.1703(4)
10.4448(2)
99.605(2)
1247.63(5)
2
7.92520(10)
15.1396(3)
11.993(2)
99.015(1)
1237.73(4)
2
22.692(4)
10.5200(2)
10.4390(15)
104.375 (12)
2751.9(8)
4
b (Å)
c (Å)
ꢀ (deg)
V (Å3)
Z
Dcal (g cm-3
)
1.824
6.19
1.601
1.32
1.545
4.650
µ (mm-1
)
2θ range (deg)
no. of data collected
no. of unique data/Rint
no. of data with I > 2σ(I) (No)
no. of params
3.74–56.56
19 600
6001/0.136
5563
262
0.043
3.30–56.54
19 367
5951/0.105
5719
262
0.028
2.37–55.00
3403
3314/0.035
3056
281
0.028
R1b
wR2c
0.104
1.051
0.081
1.023
0.067
0.998
GOFd
Flack x
-0.006
2.31/-1.76
-0.026
0.80/-0.68
-0.003(10)
1.06/-1.31
max/min residual dens (e/Å3)
2 2
a All data were collected using Mo KR (λ ) 0.710 73 Å) radiation. b R1 ) ∑(4Fo| - |Fc4)/∑(|Fo|. c wR2 ) {∑[w(Fo - Fc ) ]/∑[w(Fo2)2]}1/2 d GOF
.
) {∑[w(Fo - Fc ) ]/(No - Np)}1/2
2 2
.
on J&M TIDAS and Shimadzu UV 3101 PC spectrophotometers.
Cyclic voltammetry was carried out in 0.1 M Bu4NPF6 solutions
using a three-electrode configuration (glassy-carbon working elec-
trode, Pt counter electrode, Ag/AgCl reference) and a PAR 273
potentiostat and function generator. The ferrocene/ferrocenium (Fc/
Fc+) couple served as internal reference. Spectroelectrochemistry
was performed using an optically transparent thin-layer electrode
(OTTLE) cell.6 A two-electrode capillary served to generate
intermediates for X band EPR studies.7
((R,R)-(-)-2,3-Bis(tert-butylmethylphosphino)quinoxa-
line)tricarbonylchlororhenium (3). A mixture of Re(CO)5Cl (80
mg, 0.22 mmol) and of the quinoxaline ligand (73.9 mg, 0.22 mmol)
in 40 mL of 3:1 toluene-dichloromethane was heated to reflux for
4 h under an argon atmosphere. The solvent was removed under
reduced pressure to 10 mL, and a solid was precipitated on addition
of diethyl ether at 0 °C. The yellow precipitate was collected and
washed several times with diethyl ether and dried to yield 125 mg
(88%) of product. Anal. Calcd for C21H28ClNO3P2Re (640.06): C,
39.41; H, 4.41; N, 4.38. Found: C, 40.73; H, 4.57; N, 4.17. 1H
Synthesis. ((R,R)-(-)-2,3-Bis(tert-butylmethylphosphino)qui-
noxaline)dichloroplatinum (1). To a 63.5 mg amount of the
quinoxaline ligand (0.19 mmol) dissolved in 20 mL of dichlo-
romethane was added dropwise a dichloromethane solution of 80
mg (0.19 mmol) of Pt(dmso)2Cl2. The mixture was stirred at room
temperature for 12 h under an argon atmosphere. The solvent was
removed under reduced pressure until about 5 mL was left; addition
of diethyl ether precipitated the product, which was left at 0 °C
overnight to complete precipitation. The solution was then filtered
and the solid washed several times with diethyl ether until the filtrate
became colorless. The yellow solid was dried to yield 47 mg (82%)
of product. Anal. Calcd for C18H28Cl2N2P2Pt (600.37): C, 36.01;
H, 4.70; N, 4.67. Found: C, 35.88; H, 4.76; N, 4.41. 1H NMR
(CDCl3): δ 1.19 (d, 18H, 6 CH3, 3J(P-H) )16.4 Hz), 2.25 (d, 6H,
2 CH3, 2J(P-H) ) 12.0 Hz, 3J(Pt-H) ) 36.0 Hz), 7.98–8.08 (m,
3
NMR (CDCl3): δ 1.37 (d, 9H, 3CH3, J(P-H) ) 15.5 Hz), 1.21
3
2
(d, 9H, CH3, J(P-H) ) 15.3 Hz), 2.13 (d, CH3, 3H, J(P-H) )
2
8.8 Hz), 2.16 (d, CH3, 3H, J(P-H) ) 7.93 Hz), 7.85–7.97 (m,
2H), 8.15–8.27 (m, 2H). 31P NMR (CDCl3): δ 13.01 (d, J(P-P)
3
) 25.5 Hz), 28.10 (d, 3J(P-P) ) 25.5 Hz). UV/vis (CH2Cl2; λmax
/
nm (ꢀ/M-1 cm-1)): 325 (8660), 338 (8960), 390 (sh). IR (CH2Cl2;
ν(CO)/cm-1): 2032 vs, 1953 s, 1895 s.
Crystallography. Single crystals were obtained as dichlo-
romethane solvates from dichloromethane/hexane (diffusion method)
for 1 and 2, which crystallize in an isostructural way. Data were
collected of selected specimens (pale yellow prisms 0.2 × 0.1 ×
0.1 mm in both cases) with a NONIUS Kappa CCD diffractometer
at 100 K. The structures were solved using direct methods with
refinement by full-matrix least squares of F2, employing the
program system SHELXL 97 in connection with absorption
correction.8 All non-hydrogen atoms were refined anisotropically;
hydrogen atoms were introduced at appropriate positions. Single
crystals of 3 were obtained from dichloromethane-hexane at -25
°C. The X-ray data collection (yellow prisms 0.25 × 0.25 × 0.22
mm) was performed at 173 K on a Siemens P4 four-circle
diffractometer with graphite-monochromated Mo KR radiation (λ
) 0.710 73 Å). An empirical absorption correction based on ψ scans
of several reflections was applied. Crystallographic data are
summarized in Table 1, and selected bond parameters are given in
1
2H), 8.27–8.37 (m, 2H). 31P NMR (CDCl3): δ 30.65 (s, J(Pt-P)
) 3446.5 Hz). UV/vis (CH2Cl2; λmax/nm (ꢀ/M-1 cm-1)): 395 sh,
340 (10 590), 329 (9260).
((R,R)-(-)-2,3-Bis(tert-butylmethylphosphino)quinoxa-
line)dichloropalladium (2). An 22.1 mg amount (0.066 mmol) of
Pd(dmso)2Cl2 in 10 mL of CH2Cl2 was added to a 10 mL CH2Cl2
solution of 24 mg (0.071 mmol) of quinoxaline, and the mixture
was stirred for 2 h. The light yellow solution was concentrated to
half its volume. Upon addition of hexane and filtration 15 mg
(44.4%) of pale yellow product was obtained after drying. Single
crystals were obtained from dichloromethane/hexane as dichlo-
romethane solvate. Anal. Calcd for C19H30Cl4N2P2Pd (596.61): C,
(6) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
1
38.25; H, 5.07; N, 4.70%. Found: C, 37.41; H, 5.20; N, 4.35. H
NMR (CD2Cl2): δ 1.24 (d, 18H, 6 CH3, 3J(P-H) ) 16.6 Hz), 2.23
(7) Kaim, W.; Ernst, S. E.; Kasack, V. J. Am. Chem. Soc. 1990, 112,
173.
2
(d, 6H, 2 CH3, J(P-H) ) 12.0 Hz), 8.04 (m, 2H), 8.32 (m, 2H).
(8) (a) Sheldrick, G. M. Program for Crystal Structure Solution and
Refinement, Universität Göttingen, 1997. (b) Herrendorf, W.; Bärnighausen,
H. Program HABITUS, Karlsruhe, Germany, 1993.
31P NMR (CD2Cl2): δ 55.34 (s). UV/vis (CH2Cl2; λmax/nm (ꢀ/M-1
cm-1)): 341 (10 340), 329 (8980).

220 Organometallics, Vol. 27, No. 2, 2008
Das et al.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of 1
and 2
Treatment of L with standard metal complex precursors yields
the three complexes (L)PtCl2 (1), (L)PdCl2 (2), and
(L)Re(CO)3Cl (3); the results of their crystal structure deter-
mination are summarized and illustrated in Tables 2 and 3 and
Figures 1-3.
M ) Pt (1)
M ) Pd (2)
Bond Lengths
2.363(2)
2.361(2)
2.219(2)
2.219(2)
1.807(9)
1.893(8)
1.799(8)
1.862(7)
1.835(9)
1.831(8)
M-Cl(1)
2.3654(8)
2.3639(8)
2.2316(8)
2.2321(8)
1.807(3)
1.868(3)
1.809(3)
1.871(3)
1.824(3)
1.827(3)
The structures show the expected2 formation of five-
membered chelate rings involving two ortho-positioned dialkyl-
phosphino groups (R configuration) and the four-coordinate d8
(1, 2) or six-coordinate d6 configured metal centers (3). The
five-membered rings are approximately planar, adopting only
a slightly twisted conformation. No intra- or intermolecular
metal/quinoxaline interaction was detected. According to the
straightforward NMR spectra this arrangement is maintained
also in solution; splitting of P-methyl and P-tert-butyl 1H NMR
signals for 3 reflect the different chemical environments above
and below the π plane as a consequence of the fac-Re(CO)3Cl
configuration (Figure 3).
M-Cl(2)
M-P(2)
M-P(1)
P(1)-C(14)
P(1)-C(15)
P(2)-C(9)
P(2)-C(10)
P(1)-C(1)
P(2)-C(8)
Bond Angles
91.20(8)
171.11(7)
171.66(7)
91.01(7)
88.57(8)
90.50(7)
108.9(4)
108.6(4)
P(1)-M-Cl(1)
P(1)-M-Cl(2)
P(2)-M-Cl(1)
P(2)-M-Cl(2)
P(2)-M-P(1)
Cl(1)-M-Cl(1)
C(9)-P(2)-C(10)
C(15)-P(1)-C(14)
90.56(3)
169.99(3)
169.65(3)
90.07(3)
87.48(3)
93.53(3)
The complexes show one-electron electrochemical reduction,
facilitated in comparison to that of the free ligand (Table 4).
The second reduction could not be observed in CH2Cl2 or
CH3CN, suggesting a large comproportionation constant for the
intermediate, as would be expected for 1,4-diazine redox
systems.5b Electron addition proved to be irreversible (ECir
process) even at 243 K for the dichloropalladium complex 2;
addition of excess (0.1 M) Bu4NCl enhanced the reversibility
and allowed us to determine a potential of -1.45 V (∆E ) 110
mV) at a 200 mV/s scan rate. The much more pronounced
lability of 4d transition-metal-halide bonds on reduction relative
to 5d element analogues is well-known;11 it can be desirable
for some kinds of catalytic activation.12 The rhenium(I)
compound 3 exhibits a partially reversible oxidation (Figure 4)
at a potential higher than that for the (irreversible) oxidation of
L. This result reflects the involvement of phosphine lone pairs
in metal binding and suggests a metal-based oxidation to labile
rhenium(II);13 unfortunately, the spectroelectrochemical mea-
109.01(16)
109.01(16)
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) of 3
Bond Lengths
Re-Cl
2.503(2)
2.483(2)
1.939(7)
1.937(5)
1.968(4)
2.462(2)
1.843(9)
1.864(8)
1.820(8)
1.870(7)
1.848(2)
1.847(1)
Re-P(1)
Re-C(19)
Re-C(20)
Re-C(21)
Re-P(2)
P(1)-C(14)
P(1)-C(15)
P(2)-C(9)
P(2)-C(10)
P(1)-C(1)
P(2)-C(8)
Bond Angles
P(1)-Re-Cl
87.71(7)
P(1)-Re-C(20)
P(1)-Re-C(21)
P(1)-Re-C(19)
P(2)-Re-Cl
171.90(9)
97.48(1)
89.31(5)
83.30(7)
82.50(6)
89.99(1)
170.41(5)
97.78(7)
176.68(6)
94.43(7)
87.11(5)
176.42(2)
174.01(4)
178.37(2)
103.93(8)
103.33(1)
(9) (a) Kaim, W.; Bock, H. J. Am. Chem. Soc. 1978, 100, 6504. (b)
Kaim, W.; Bock, H. Chem. Ber. 1981, 114, 1576. (c) Gross-Lannert, R.;
Kaim, W.; Lechner, U.; Roth, E.; Vogler, C. Z. Anorg. Allg. Chem. 1989,
579, 47. (d) Giordan, J. C.; Moore, J. H.; Tossell, J. A.; Kaim, W. J. Am.
Chem. Soc. 1985, 107, 5600.
P(2)-Re-P(1)
P(2)-Re-C(20)
P(2)-Re-C(21)
P(2)-Re-C(19)
C(19)-Re-Cl
C(20)-Re-Cl
C(21)-Re-Cl
Re-C(21)-O(21)
Re-C(20)-O(20)
Re-C(19)-O(19)
C(9)-P(2)-C(10)
C(14)-P(1)-C(15)
(10) (a) Fenske, D. Chem. Ber. 1979, 112, 363. (b) Becher, H. J.; Fenske,
D.; Heymann, M. Z. Anorg. Allg. Chem. 1981, 475, 27. (c) Fenske, D.;
Becher, H. J. Chem. Ber. 1974, 107, 117; 1975, 108, 2115. (d) Becher,
H. J.; Bensmann, W.; Fenske, D. Chem. Ber. 1977, 110, 315. (e) Fenske,
D. Angew. Chem. 1976, 88, 415; Angew. Chem., Int. Ed. Engl. 1976, 15,
381. (f) Bensmann, W.; Fenske, D. Angew. Chem. 1979, 91, 754; Angew.
Chem., Int. Ed. Engl. 1979, 18, 677. (g) Fenske, D.; Christidis, A. Angew.
Chem. 1981, 93, 113; Angew. Chem., Int. Ed. Engl. 1981, 20, 129. (h)
Fenske, D.; Christidis, A. Z. Naturforsch., B 1981, 36, 518. (i) Fenske, D.;
Brandt, K.; Stock, P. Z. Naturforsch., B 1981, 36, 768. (j) Fenske, D.; Stock,
P. Angew. Chem. 1982, 94, 393; Angew. Chem., Int. Ed. Engl. 1982, 21,
356; Angew. Chem. Suppl. 1982, 862. (k) Fenske, D.; Bensmann, W. Z.
Naturforsch., B 1984, 39, 1819. (l) Fenske, D.; Bensmann, W. Z.
Naturforsch., B 1985, 40, 1093. (m) Duffy, N. W.; Nelson, R. R.; Richmond,
M. G.; Rieger, A. L.; Rieger, P. H.; Robinson, B. H.; Tyler, D. R.; Wang,
J. C.; Yang, K. Inorg. Chem. 1998, 37, 4849. (n) Braden, D. A.; Tyler,
D. R. Organometallics 2000, 19, 3762.
Tables 2 and 3. Further details are provided in the Supporting
Information.
Results and Discussion
(11) Frantz, S.; Reinhardt, R.; Greulich, S.; Wanner, M.; Fiedler, J.;
Duboc-Toia, C.; Kaim, W. Dalton Trans. 2003, 3370.
The free ligand R,R-2,3-bis(tert-butylmethylphosphino)qui-
noxaline undergoes an electrochemically reversible one-electron
reduction at -2.05 V vs ferrocenium/ferrocene (Table 4); the
quinoxaline parent has E1/2 ) -2.18 V. The radical anion
produced, L•-, shows an only partially resolved EPR signal at
giso ) 2.0030 with a total spectral width of about 4 mT. Due to
the insufficient resolution, the complex spectrum could not yet
be analyzed. Nevertheless, this result confirms the π-electron
deficiency of quinoxaline,4 enhanced further by the established
electron-withdrawing effect of dialkylphosphino substituents.9,10
(12) (a) Kölle, U.; Grätzel, M. Angew. Chem. 1987, 99, 572; Angew.
Chem., Int. Ed. Engl. 1987, 26, 568. (b) Kölle, U.; Kang, B.-S.; Infelta, P.;
Compte, P.; Grätzel, M. Chem. Ber. 1989, 122, 1869. (c) Chardon-Noblat,
S.; Cosnier, S.; Deronzier, A.; Vlachopoulos, N. J. Electroanal. Chem. 1993,
352, 213. (d) Westerhausen, D.; Herrmann, S.; Hummel, W.; Steckhan, E.
Angew. Chem. 1992, 104, 1496; Angew. Chem., Int. Ed. Engl. 1992, 321,
1529. (e) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33, 4453.
(13) (a) Klein, A.; Vogler, C.; Kaim, W. Organometallics 1996, 15,
236. (b) Hartmann, H.; Scheiring, T.; Fiedler, J.; Kaim, W. J. Organomet.
Chem. 2000, 604, 267. (c) Frantz, S.; Fiedler, J.; Hartenbach, I.; Schleid,
Th.; Kaim, W. J. Organomet. Chem. 2004, 689, 3031. (d) Knödler, A.;
Wanner, M.; Fiedler, J.; Kaim, W. Dalton Trans. 2002, 3079.

Metal Complexes of the Diphosphine Ligand R,R-QuinoxP
Organometallics, Vol. 27, No. 2, 2008 221
Table 4. Electrochemical and Spectroscopic Properties of the Ligand L and Its Complexes
Ln
1n
2n
3n
b
E(red)a
-2.05 (E1/2
)
)
-1.56 (E1/2
>1.5
)
-1.78 (Ep,c
>1.5
)
-1.70 (E1/2
+1.11 (E1/2
)
)
E(ox)a
+0.36 (Ep,a
ν(CO)c (n ) 0)
ν(CO)c (n ) -1)
2032 vs, 1953 s, 1895 s
2014 vs, 1928 s, 1877 s
338/8.96, 325/8.66, 390 (sh)
269/11.40, 289/10.86,
335/8.00,360 (sh), 563/4.39
2.0032,f 2.0034g
λ
λ
max/ꢀd (n ) 0)
n.a.
n.a.
340/10.59, 329/9.27, 395 (sh),329/9.26
n.a.
341/10.34, 329/8.98
n.a.
max/ꢀd (n ) -1)
g (n ) -1)
2.0030e
partially resolved
(40 G spectral width)
2.0045f
n.a.
n.a.
A (n ) -1)h
A(195Pt) ) 24 (1Pt), A(14N) ) 5.8 (2N),
partially resolved
(70 G spectral width)
f
A(1H) ) 1.7 (2H),g A(31P) ) 1.7 (2P)j
a In CH2Cl2/0.1 M Bu4NPF6. Potentials are given in V vs (C5H5)2Fe+/0, with half-wave (E1/2) or peak potentials (Ep,c, Ep,a) for irreversible processes.
b At -30 °C; E1/2 ) -1.45 V in the presence of 0.1 M Bu4NCl. c Carbonyl stretching bands in cm-1 d Absorption maxima (λmax) in nm/molar
.
extinction coefficients (ꢀ) in 103 M-1 cm-1 e In CH3CN/0.1 M Bu4NPF6 at 298 K. f In CH2Cl2/0.1 M Bu4NPF6 at 298 K. No g factor splitting observed
.
at 110 K. g In CH2Cl2/0.1 M Bu4NPF6 at 110 K. No g factor splitting observed at 110 K. h EPR coupling constants in G (1 G ) 10-4 T). j Tentative
assignment (cf. text).
Figure 3. Molecular structure of 3 in the crystal (thermal ellipsoids
including 30% probability).
Figure 1. Molecular structure of 1 in the crystal of 1 · 2CH2Cl2
(thermal ellipsoids including 30% probability).
Figure 4. Cyclic voltammetry of 3 in dichloromethane/0.1 M
Bu4NPF6 at a 100 mV/s scan rate.
band system in the visible region (Figure 5). Quinoxaline radical
anions are distinguished by long-wavelength absorptions around
600 nm;14 we therefore assign the observed bands in 3•- to
intraligand and (bathochromically shifted) MLCT transitions.
The rather marginal involvement of the fac-Re(CO)3Cl fragment
in the spin distribution on reduction of 3 is also evident from
the comparatively small low-energy shift of about 20 cm-1 for
the three CO stretching bands (Figure 6, Table 4). For
comparison, (abpy)Re(CO)3Cl (abpy ) 2,2′-azobis(pyridine))
shows an average shift of 40 cm-1 on reduction.13b,c
Figure 2. Molecular structure of 2 in the crystal of 2 · 2CH2Cl2
(thermal ellipsoids including 30% probability).
surements suffered from extensive decomposition of electro-
generated 3+.
Reversible one-electron reduction could be monitored by
EPR, IR, and UV–vis spectroelectrochemistry for the ReI species
3 and by EPR for the platinum(II) complex 1. While all
compounds display metal-to-(acceptor) ligand charge transfer
(MLCT) absorptions in the near-UV region (Table 4), the
formation of 3•- is accompanied by the emergence of an intense
(14) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier:
Amsterdam, 1988; p 176.

222 Organometallics, Vol. 27, No. 2, 2008
Das et al.
Figure 5. UV–vis spectroelectrochemistry of the conversion of 3
to 3•- in dichloromethane/0.1 M Bu4NPF6.
Figure 7. EPR spectrum of electrochemically generated 1•- at room
temperature in dichloromethane/0.1 M Bu4NPF6 (top, X band at
9.4776 GHz) with simulation (bottom, 0.1 G line width, parameters
from Table 4).
to that of the ligand radical anion (g ) 2.0030), confirming the
marginal contribution of the metals to the singly occupied
molecular orbital. This result is supported by the lack of
detectable g anisotropy at X band frequency (9.5 GHz). Whereas
3•- did not show sufficient EPR resolution for hyperfine
structure analysis, except for significantly higher spectral width
due to 185,187Re splitting at the order of about 5 G, the complex
ion 1•- exhibits the typically large 14N (I ) 1) coupling
parameter of quinoxalines (5.7 G)4,5 as well as smaller 1H and
possibly 31P splitting. Both kinds of nuclei have I ) 1/2;
quinoxaline anion was reported with 3.3 G (2H), 2.4 G (2H),
and 1.45 G (2H).5 Considering the relatively large π spin
population in the 2,3-positions of quinoxaline anions,5 we made
the tentative assginment given in Table 4. The radical 1•-
exhibits a sizable 195Pt hyperfine coupling of 24 G (Figure 7,
Table 4). Although semidione complexes of imidazolyl-
coordinated PtCl2 have even smaller a(195Pt) hyperfine split-
ting,17 the value of 24 G is still below the approximately 40–50
G of typical (Het•-)PtCl2 complexes, e.g. the heterocycles Het
) bpy, bpym,15,16 and is certainly much smaller than what
would be expected for mononuclear platinum(I) species.21
All of these results thus support a formulation of (L•-)MXn
in which the electron has been added to the heterocyclic π
system of the molecule (spin localization on quinoxaline5),
whereas the metal coordination takes place at the stereochemi-
cally modified phosphino substituents, where relatively little spin
density is present. Such a situation, the separation of coordina-
tion site and primary electron transfer site, is partially reminis-
cent of that in complexes of bis(diphosphino)maleic anhydrides10
and dipyrido[3,2-a:2′,3′-c]phenazine (dppz)18a,22 and related
ligands, where the added electron is also not localized at the
coordination site. In fact, the structures of L and dppz exhibit
obvious similarities, although the larger π system of dppz and
the lower acceptor level of the phenanthroline section as
compared to phosphinyl substituents make a difference.
Figure 6. IR spectroelectrochemistry of the conversion of 3 to 3•-
in dichloromethane/0.1 M Bu4NPF6.
EPR studies of radical complexes with heavy transition metals
such as Pt15–18 or Re13,19 can provide sizable effects, including
g factor shifts, g factor anisotropy, and metal hyperfine coupling.
While the former is caused by the large spin–orbit coupling
constants of these ions,20 both the 195Pt (33.8% natural
abundance, I ) 1/2) and 185,187Re isotopes (100%, I ) 5/2, very
similar nuclear magnetic moments) are distinguished by unusu-
ally large isotropic hyperfine coupling constants.20 The EPR
spectra of 1•- and 3•- (Figure 7, Table 4) show g values close
(15) (a) MacGregor, S. A.; McInnes, E.; Sorbie, R. J.; Yellowlees, L. J.
In Molecular Electrochemistry of Inorganic, Bioinorganic and Organome-
tallic Compounds; Pombeiro, A. J. L., McCleverty, J. A., Eds.; Kluwer
Academic: Dordrecht, The Netherlands, 1993; p 503. (b) Collison, D.;
McInnes, E. J. L.; Mabbs, F. E.; Taylor, K. J.; Welch, A. J.; Yellowlees,
L. J. J. Chem. Soc., Dalton Trans. 1996, 329. (c) McInnes, E. J. L.; Farley,
R. D.; Macgregor, S. A.; Taylor, K. J.; Yellowlees, L. J.; Rowlands, C. C
J. Chem. Soc., Faraday Trans. 1998, 94, 2985. (d) McInnes, E. J. L.; Farley,
R. D.; Rowlands, C. C.; Welch, A. J.; Rovatti, L.; Yellowlees, L. J. J. Chem.
Soc., Dalton Trans. 1999, 4203.
(16) Kaim, W.; Dogan, A.; Wanner, M.; Klein, A.; Tiritiris, I.; Schleid,
Th.; Stufkens, D. J.; Snoeck, T. L.; McInnes, E. J. L.; Fiedler, J.; Zalis, S.
Inorg. Chem. 2002, 41, 4139.
The decoupling of the electron transfer site from the
coordination site in potentially catalytic metal species may
become of interest when working under strongly reducing
conditions such as hydride-involving reactions.
(17) Bulak, E.; Leboschka, M.; Schwederski, B.; Sarper, O.; Varnali,
T.; Fiedler, J.; Lisssner, F.; Schleid, W.; Kaim, W Inorg. Chem. 2007, 46,
5562.
(18) (a) Kaim, W.; Klein, A. Organometallics 1995, 14, 1176. (b)
Hasenzahl, S.; Hausen, H.-D.; Kaim, W. Chem. Eur. J. 1995, 1, 95. (c)
Vogler, C.; Schwederski, B.; Klein, A.; Kaim, W. J. Organomet. Chem.
1992, 436, 367.
(21) Braterman, P. S.; Song, J.-L.; Wimmer, F. M.; Wimmer, S.; Kaim,
W.; Klein, A.; Peacock, R. D. Inorg. Chem. 1992, 31, 5084.
(22) (a) Fees, J.; Kaim, W.; Moscherosch, M.; Matheis, W.; Klima, J.;
Krejcik, M.; Zalis, S. Inorg. Chem. 1993, 32, 166. (b) Klein, A.; Scheiring,
T.; Kaim, W. Z. Anorg. Allg. Chem. 1999, 625, 1177. (c) Fees, J.; Ketterle,
M.; Klein, A.; Fiedler, J.; Kaim, W. J. Chem. Soc., Dalton Trans. 1999,
2595.
(19) (a) Kaim, W.; Kohlmann, S. Chem. Phys. Lett. 1987, 139, 365. (b)
Frantz, S.; Hartmann, H.; Doslik, N.; Wanner, M.; Kaim, W.; Kümmerer,
H.-J.; Denninger, G.; Barra, A.-L.; Duboc-Toia, C.; Fiedler, J.; Ciofini, I.;
Urban, C.; Kaupp, M. J. Am. Chem. Soc. 2002, 124, 10563.
(20) Weil, J. A.; Bolton, J. R. Electron Paramagnetic Resonance, 2nd
ed.; Wiley-Interscience: Hoboken, NJ, 2007.

Metal Complexes of the Diphosphine Ligand R,R-QuinoxP
Organometallics, Vol. 27, No. 2, 2008 223
phosphino-substituted organic radical ligands10 or molecules
such as dipyrido[3,2-a:2′,3′-c]phenazine (dppz),18a,22 where the
coordinating (chelating) group and the spin-bearing section are
spatially separated and electronically decoupled, a behavior
which may be exploited under strongly reducing situations.
Having established the basic structural and electronic features
of a QuinoxP ligand in three coordination compounds, we shall
direct our future efforts to more catalytically relevant species,
involving other transition metals and including less stable
intermediates.
Acknowledgment. This work was supported by grants
from Deutsche Forschungsgemeinschaft, by the Fonds der
Chemischen Industrie, by the European Union (COST D35),
and by the Land Baden-Württemberg. We also thank Johnson
Matthey for a generous loan of chemicals.
Conclusion
While the coordination of R,R-quinoxP with complex frag-
ments containing d6 and d8 configured transition metals via the
chelating P donor atoms is not unexpected, the facile reduction
to stable radical anion complexes, at least for the 5d element
species involving Re(I) and Pt(II), is remarkable. EPR and other
spectroelectrochemical methods indicate the primary localization
of the unpaired electron in the quinoxaline heterocycle with
rather little spin delocalization to the metal centers via the
phosphorus atoms. In this respect the complexes resemble other
Supporting Information Available: CIF files giving X-ray
crystallographic data for 1-3. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM700755Y