Rhodium and Palladium Complexes of a 3,5-Lutidine-Based Phosphine Ligand

Article abstract of DOI:10.1021/ic951131h

The chelating bis(phosphine) 3,5-bis((diphenylphosphino)methyl)pyridine (dppLH, 9) was synthesized from 3,5-lutidine by radical chlorination, followed by reaction with LiPPh₂. Compound 9 was used to prepare Rh and Pd complexes. The three complexes (dppL)RhL (L = PⁱPr₃ (11), PPh₃ (12), and CO (13)) are obtained from Rh₂Cl₂(COE)₄ (COE = cyclooctene) and HRh(PPh₃)₄. Compound 9 reacts with (PhCN)₂PdCl₂ to form the Pd(II) complex (dppL)PdCl (14). All complexes are fully characterized. The X-ray crystal structure of 11 has been determined. It crystallizes in the triclinic space group P1? (No. 2) with a = 12.242(3) ?, b = 14.384(3) ?, c = 11.653(2) ?, α = 98.61(2)°, β= 96.88(2)°, γ = 106.38(2)°, V = 2023.3(8) ?³, Z = 2, R = 0.045, and R_w = 0.052. The Rh(I) center is in a square-planar coordination environment with the two phosphines of the dppL ligand framework being trans to each other. The pyridine nitrogen is unobstructed and available for binding to other metal centers. Compound 14 acts as a metalloligand toward Pd(II) centers to form the trinuclear complex {(dppL)PdCl}₂PdCl₂ (16). N-coordination of a metal moiety and of a Lewis acid influences the electronic properties on the Pd center.

Full text of DOI:10.1021/ic951131h

1792
Inorg. Chem. 1996, 35, 1792-1797
Rhodium and Palladium Complexes of a 3,5-Lutidine-Based Phosphine Ligand
Alexander Weisman, Michael Gozin, Heinz-Bernhard Kraatz, and David Milstein*
Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel
ReceiVed August 28, 1995X
The chelating bis(phosphine) 3,5-bis((diphenylphosphino)methyl)pyridine (dppLH, 9) was synthesized from 3,5-
lutidine by radical chlorination, followed by reaction with LiPPh2. Compound 9 was used to prepare Rh and Pd
complexes. The three complexes (dppL)RhL (L ) PiPr3 (11), PPh3 (12), and CO (13)) are obtained from Rh2-
Cl2(COE)4 (COE ) cyclooctene) and HRh(PPh3)4. Compound 9 reacts with (PhCN)2PdCl2 to form the Pd(II)
complex (dppL)PdCl (14). All complexes are fully characterized. The X-ray crystal structure of 11 has been
determined. It crystallizes in the triclinic space group P1h (No. 2) with a ) 12.242(3) Å, b ) 14.384(3) Å, c )
11.653(2) Å, R ) 98.61(2)°, â ) 96.88(2)°, γ ) 106.38(2)°, V ) 2023.3(8) Å3, Z ) 2, R ) 0.045, and Rw
)
0.052. The Rh(I) center is in a square-planar coordination environment with the two phosphines of the dppL
ligand framework being trans to each other. The pyridine nitrogen is unobstructed and available for binding to
other metal centers. Compound 14 acts as a metalloligand toward Pd(II) centers to form the trinuclear complex
{(dppL)PdCl}2PdCl2 (16). N-coordination of a metal moiety and of a Lewis acid influences the electronic properties
on the Pd center.
Introduction
Chelating ligand systems based on 1,3-disubstituted benzene
(1) have been studied extensively over the past years with a
variety of donor atoms (E). Particularly fruitful have been
studies based on nitrogen,1 phosphorus,2 and sulfur3 as donor
atoms. Generally, reactions with suitable metal precursors led
to the isolation of orthometallated complexes (2) having the
two donor atoms of the chelate in a mutual trans arrangement
around the metal center. Some of the resulting metal complexes
are active as homogeneous catalysts.4
and possibly to influence the reactivity of a metal center. A
change in donor atom (E) and its substituents (R) will influence
its binding properties and hence will influence the electron
density on the metal center. Whereas the nitrogen-based ligands
can be regarded as pure electron donors, phosphorus and sulfur
systems are able to interact with a metal center as a donor in a
σ/π fashion as well as a π-acceptor due to low-lying σ*-orbitals.5
The substituents will have a significant influence on the donor
and acceptor behavior of the ligand.
This ligand framework offers the possibility for modification
at different sites, allowing one to fine tune the electron density
X Abstract published in AdVance ACS Abstracts, February 15, 1996.
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More recent examples: van Koten, G.; Terheijden, J.; van Beek, J.
A. M.; Wehman-Ooyevaar, I. C. M.; Mu¨ller, F.; Stam, C. H.
Organometallics 1990, 9, 903. (c) Abbenhuis, H. C. L.; Feiken, N.;
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13, 468.
Recently, van Koten et al.1d have shown that upon introduc-
tion of substituents (X ) H, NO2, NH2, MeC(O)N(H), Cl,
PhCHdN, MeO, MeC(O)) into the para-position of the
nitrogen-based NCN ligand system (3) the electron density on
the metal center is influenced through mesomeric or inductive
effects.
(2) (a) Moulton, C. L.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020. (b) Rimmel, H.; Venanzi, L. M. J. Organomet. Chem. 1983,
259, C6. (c) Nemeh, S.; Jernsen, C.; Binamira-Soriaga, E.; Kaska,
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M. J. Organomet. Chem. 1984, 260, C52. (e) Rimmel, H.; Venanzi,
L. M. Phosphorous Sulfur Relat. Elem. 1987, 30, 297. (f) Kaska, W.
C.; Nemeh, S.; Shirazi, A.; Potuznik, S. Organometallics 1988, 7, 13.
(g) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993,
364, 699. (h) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.;
Ben-David, Y.; Milstein, D. Nature 1994, 370, 42. (i) Gorla, F.
Venanzi, L. M.; Albinati, A. Organometallics 1994, 13, 43. (j) Gorla,
F.; Togni, A.; Venanzi, L. M.; Albinati, A.; Lianza, F. Organometallics
1994, 13, 1607. (k) McLoughlin, M. A.; Flesher, R. J.; Kaska, W. C.;
Mayer, H. A. Organometallics 1994, 13, 3816. (l) Kraatz, H.-B.;
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R.; Cross, R. J.; Muir, K. W. Inorg. Chim. Acta 1995, 231, 195. (n)
Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc. 1995, 117,
9774; J. Chem. Soc., Chem. Commun. 1995, 1965.
Here, we offer a new approach to influence the electron
density on a metal center by having a heteroatom present in
(5) (a) Xiao, S.-X.; Trogler, W. C.; Ellis, D. E.; Berkovich-Yellin, Z. B.
J. Am. Chem. Soc. 1983, 105, 7033 (PR3). (b) Marynick, D. S. J. Am.
Chem. Soc. 1984, 106, 4064 (PR3). (c) Orpen, A. G.; Connelly, N. G.
J. Chem. Soc., Chem. Commun. 1985, 1310 (PR3: A crystallographic
study on PR3 transition metal complexes, finding a lengthening of
the P-R bond as predicted for an involvement of σ*-orbitals in back-
bonding (see refs 5b,c). (d) Jacobsen, H.; Kraatz, H.-B.; Ziegler, T.;
Boorman, P. M. J. Am. Chem. Soc. 1992, 114, 7851 (a theoretical
study on SR2 binding in face sharing bioctahedral Mo-Mo complexes
describing back-bonding to a low-lying σ*-orbital of thioether). (e)
Kraatz, H.-B.; Jacobsen, H.; T. Ziegler, T.; Boorman, P. M. Orga-
nometallics 1993, 12, 76 (a theoretical study on ER2 (E ) O, S, Se)
and ER3 (E ) N, P, As) binding (R ) H, F, Me) to the Cr(CO)5
fragment; quantifying σ-donation and π-back-bonding).
(3) (a) Errington, J.; McDonald, W. S.; Shaw, B. L. J. Chem. Soc., Dalton
Trans. 1980, 2312. (b) Dupont, J.; Beydoun, N.; Pfeffer, M. J. Chem.
Soc., Dalton Trans. 1989, 1715.
(4) (a) van Koten; et al. J. Mol. Catal. 1988, 45, 169. (b) (dtbppp)PdMe,
possessing a metallated aliphatic bisphosphine, was reported to be
active toward the amination of activated olefins. Seligson, A. L.;
Trogler, W. C. Organometallics 1993, 12, 744.
0020-1669/96/1335-1792$12.00/0 © 1996 American Chemical Society

A 3,5-Lutidine-Based Phosphine Ligand
Inorganic Chemistry, Vol. 35, No. 7, 1996 1793
°C. After the product was redissolved in water and solution was filtered,
its pH was adjusted to pH 8 by the addition of a saturated aqueous
solution of Na2CO3. The yellow-brown precipitate which formed was
collected by filtration, washed with water (3 × 50 mL) and cold pentane
(3 × 15 mL), and then dried. The crude product was recrystallized
from hexane to give 3,5-bis(chloromethyl)pyridine in low yield (1.5 g,
4.6%), mp 86-87 °C. 1H NMR (C6D6): δ 8.2 (2H, d, 4JHH ) 2.2 Hz,
the trans-position of the aromatic ring of a terdentate chelating
bis(phosphine) ligand. Our ligand will be based on a 3,5-lutidine
backbone, possessing pending methylene-phosphino groups in
the 3,5-positions (4). Reaction of the ligand with a suitable
4
4
4
Ho), 6.8 (1H, m, JHH ) 2.2 Hz, JHH ) 0.4 Hz, Hp), 3.7 (4H, d, JHH
) 2.2 Hz, -CH2Cl). 13C{1H} NMR (C6D6): δ 149.6 (s, HCo), 135.8
(s, HCp), 133.2 (s, HCm), 42.6 (s, CH2Cl). This compound is unstable
due to pyridine quaternization and should be stored at -30 °C.
(b) 3,5-Bis((diphenylphosphino)methyl)pyridine (dppLH, 9). 3,5-
Bis(chloromethyl)pyridine (3 g, 16.8 mmol) was added dropwise to a
red solution of LiPPh2 (6.5 g, 33.9 mmol) in THF (50 mL) at -78 °C.
After the addition was complete, the solution was allowed to warm up
to room temperature and the solvent was removed in vacuo. The
resulting residue was extracted with toluene (3 × 50 mL). The
combined extracts were reduced in vacuo to a volume of 10 mL and
then chromatographed on a silica column (60-230 mesh, Merck).
Gradient elution from pure CH2Cl2 to a 1:1 mixture of CH2Cl2:THF
was used to achieve maximum separation. The fractions containing
the desired compound were pumped to dryness. The resulting air-
sensitive colorless oil was recrystallized from pentane, yielding 3,5-
bis((diphenylphosphino)methyl)pyridine as a colorless solid (2.8 g,
35%). Anal. Calcd for C31H27NP2: C, 78.30; H, 5.72; N, 2.95.
Found: C, 78.54; H, 5.70; N, 2.55. 31P{1H} NMR (CDCl3): δ -10.22
(s). 1H NMR (CDCl3): δ 7.92 (2H, br s, Ho of NC5H3), 7.20 (20H, m,
H of PC6H5), 7.13 (1H, s, Hp of NC5H3), 3.25 (4H, s, CH2P). 13C{1H}
NMR (CDCl3): δ 147.91 (br m, Co of NC5H3), 137.42 (d, 1JCP ) 18.1
metal precursor should lead to complex 5, leaving the nitrogen-
donor atom in the heteroaromatic ring available for binding to
another metal moiety. Coordination of this metalloligand
through the heteroatom to a second metal center (LnM2), giving
6, should directly influence the electronic properties of on M1.
Coordination of different metal fragments to the metalloligand
may allow us to influence the electronic properties of M1 over
a wide range.
Using appropriate spectroscopic techniques, it should be
possible to monitor metal binding to the pyridine nitrogen and
to evaluate electronic influences on M1. In this paper, we are
detailing our synthetic approaches to the ligand and our results
on metal binding studies, some of which have been com-
municated before.6
Experimental Section
2
Hz, Ci of PC6H5), 132.83 (d, JCP ) 18.6 Hz, Co of PC6H5), 132.6 (br
General. Materials. All reactions were carried out under nitrogen
atmosphere in a Vacuum Atmospheres glovebox (DC-882) equipped
with a recirculation (MO-40) “Dri Train” or under argon using standard
Schlenk techniques. All solvents were rigorously dried by reflux over
the appropriate drying agents and degassed prior to storage in the
glovebox over 4 Å molecular sieves: CCl4 (Frutarom), benzene
(Frutarom, sodium-benzophenone), toluene (Frutarom, sodium-ben-
zophenone), pentane (Merck, sodium-benzophenone, tetraglyme),
tetrahydrofuran (Biolab, sodium-benzophenone), and dichloromethane
(Frutarom, P2O5). All deuterated solvents were purchased from Aldrich,
degassed, and stored over 4 Å molecular sieves in the glovebox. PiPr3
(Strem), NEt3 (Aldrich), 3,5-lutidine (Aldrich), BEt3 (1 M in hexanes,
Aldrich), and N-chlorosuccinimide (Fluka) were used as received.
LiPPh2,7 Rh2Cl2(COE)4 (COE ) cyclooctene),8 HRh(PPh3)4,9 and
t, 3JCP ) 7.7 Hz, Cp of NC5H3), 131.0 (br m, Cm of NC5H3), 128.96 (s,
3
1
Cp of PC6H5), 128.49 (d, JCP ) 6.7 Hz, Cm of PC6H5), 36.50 (d, JPC
) 15.9 Hz, CH2P).
Synthesis of (dppL)RhPiPr3 (11). To a stirring solution of Rh2-
Cl2(COE)4 (40 mg, 0.056 mmol) in THF (30 mL), a solution of dppLH
(53 mg, 0.112 mmol) and PiPr3 (180 mg, 112.5 mmol) in THF/NEt3
(1:1; 30 mL) was added. The turbid brown reaction mixture was stirred
at room temperature for 12 h, and then all volatiles were stripped off.
To the remaining oily brown residue, benzene (5 mL) was added. The
benzene solution was filtered through a cotton pad and then pumped
to dryness.
A
31P{1H} NMR spectrum of an aliquot indicated the
presence of the desired Rh(I) complex. After extraction of the residue
with pentane (5 × 5 mL), the combined extracts were pumped to
dryness, yielding 11 mg of the desired (dppL)RhPiPr3 as an orange
crystalline solid. 31P{1H} NMR (C6D6): δ 51.31 (dd, 1JPRh ) 165 Hz,
10
(PhCN)2PdCl2 were prepared according to the literature procedures.
Spectroscopy. 1H, 11B{1H}, 13C{1H}, and 31P{1H} NMR spectra
were recorded at 400.19, 128.4, 161.9, and 100.6 MHz, respectively,
using a Bruker AMX 400 NMR spectrometer. 1H NMR spectra are
referenced to the residual proton resonance of the solvents. 11B NMR
spectra are referenced to external BF3‚OEt2. 31P NMR spectra are
referenced to external 85% H3PO4 (in D2O). IR spectra were recorded
as films between NaCl plates on a Nicolet 510 FT spectrometer.
Synthesis of dppLH: (a) 3,5-Bis(chloromethyl)pyridine (8). A
refluxing solution of 3,5-lutidine (20.0 g, 0.187 mol) and N-chloro-
succinimide (40.0 g, 0.300 mol) in CCl4 (2.5 L) was irradiated for 16
h with a Hg lamp. The progress of the chlorination of the methyl groups
was monitored by GC. After the concentration of 3,5-bis(chlorom-
ethyl)pyridine in the reaction mixture had reached 65-70% (based on
3,5-lutidine), irradiation was stopped and the solution allowed to cool
to room temperature. The reaction mixture was filtered, and dry HCl
gas was bubbled through the solution for 20 min. The yellow-white
precipitate containing the desired 3,5-bis(chloromethyl)pyridine hy-
drochloride was filtered off, dried, and recrystallized from iPrOH at 0
1
2
2JPP ) 29 Hz, 2P of dppL), 47.80 (dt, JPRh ) 116 Hz, JPP ) 29 Hz,
1P of PPh3). 1H NMR (C6D6): δ 6.8-8.4 (m, 22H, aromatics), 3.72
2
3
(br s, 4H, CH2P), 1.69 (sepd, JHP ) 2.3 Hz, JHH ) 7.1 Hz, 3H,
P(CH(CH3)2; collapsing into a septet upon 31P decoupling JHH ) 7.1
3
3
3
Hz), 1.06 (dd, JHH ) 7.1 Hz, JHP ) 11.7 Hz).
Synthesis of (dppL)RhPPh3 (12). To a suspension of HRh(PPh3)4
(400 mg, 0.347 mmol) in THF (60 mL), a solution of dppLH (165 mg,
0.347 mmol) in THF (10 mL) was added. The reaction was stirred for
12 h and then reduced in vacuo to a volume of ca. 2 mL. Addition of
15 mL cold pentane induced precipitation of an orange solid, which
was collected and then dried in vacuo, giving 212 mg of the desired
product (yield: 73%). 31P{1H} NMR (C6D6): δ 52.1 (dd, 1JPRh ) 161
2
1
2
Hz, JPP ) 30 Hz, 2P of dppL), 37.3 (dt, JPRh ) 120 Hz, JPP ) 30
Hz, 1P of PPh3). 1H NMR (C6D6): δ 8.52 (2H, br s, Hm of NC5H3),
2
6.7-7.6 (35H, m, aromatics), 3.71 (4H, vt, JHP ) 3.6 Hz, CH2P).
13C{1H} NMR (C6D6): δ 188.5 (m, Ci of NC5H3), 144.1 (dvt, JCP
)
)
2
2
2
2
13 Hz, JCRh ) 1 Hz, Co of NC5H3), 141.1 (dvt, JCP ) 9 Hz, JCP
2 Hz, Cm of NC5H3), 138.7 (dt, 1JCP ) 31 Hz, 3JCP ) 2 Hz, Ci of PPh3),
3
1
2
137.3 (td, JCP ) 17 Hz, JCP ) 2 Hz, Ci of PC6H5), 134.6 (d, JCP
)
)
(6) Gozin, M.; Weisman, A.; Milstein, D. Abstract presented at the XVth
International Conference on Organometallics Chemistry, Warsaw,
August 9-14, 1992; Abstract O23.
(7) Luther, G. W., III; Beyerle, G. Inorg. Synth. 1977, 17, 186.
(8) van der Ent, A.; Onderdelinden, A. L. Inorg. Synth. 1973, 14, 92.
(9) Ahmad, N.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc., Dalton
Trans. 1972, 843.
(10) (a) Kharash, M. S.; Seyler, R. C.; Mayo, F. R. J. Am. Chem. Soc.
1938, 60, 882-884. (b) Dietl, H.; Reinheimer, H.; Moffat, J.; Maitlis,
P. M. J. Am. Chem. Soc. 1970, 92, 2276-2285.
2
13 Hz, Ph), 133.6 (dt, JCP ) 6 Hz, JCP ) 2 Hz, Ph), 128.9 (t, JCP
4 Hz, Ph), 128.6 (d, 4JCP ) 1 Hz, Ph), 127.5 (d, 3JCP ) 9 Hz, Ph), 46.8
1
3
(ddvt, JCP ) 14 Hz, JCP ) 8 Hz, J ) 2.6 Hz, CH2P).
Synthesis of (dppL)RhCO (13). A Fischer-Porter flask was
charged with a suspension of (dppL)RhPPh3 (50 mg, 0.060 mmol) in
20 mL of pentane and then pressurized with 20 psi of CO. The reaction
mixture was stirred for 12 h. The desired product precipitated from
solution as a yellow solid. The solid was washed with two portions of

1794 Inorganic Chemistry, Vol. 35, No. 7, 1996
Weisman et al.
Table 1. Crystallographic Data for (dppL)Rh(PiPr3)‚C6H6 (11‚C6H6)
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
C46H53NP3Rh
801.8
V, Å3
2023.3 (8)
2
1.387
Z
F
P1h (No. 2)
12.242 (3)
14.384 (3)
11.653 (2)
98.61 (2)
96.88 (2)
106.38 (2)
calc, g/cm-3
cryst size, mm
0.4 × 0.4 × 0.3
µ (Mo KR), cm-1
5.64
radiation (monochromated in incident beam)
Mo KR (λ ) 0.710 73 Å; graphite monochromated)
0.045
0.052
Ra
b,c
Rw
a R ) ∑∆F/∑|Fo|. b Rw ) ∑w1/2(∆F)/∑w1/2Fo. c Weights are defined as w ) 1.1754/[σ2(F) + 0.000126F2].
pentane (2 × 10 mL) and dried in vacuo (34 mg, 95% yield). 31P{1H}
Synthesis of {(dppL)PdCl}2PdCl2 (16). A solution of (PhCN)2-
1
1
NMR: C6D6, δ 58.0 (d, JPRh ) 155 Hz); in THF-d8, δ 57.8 (d, JPRh
PdCl2 (8 mg, 0.021 mmol) in THF (5 mL) was added dropwise to a
stirring solution of (dppL)PdCl (27 mg, 0.044 mmol) in THF (10 mL).
The reaction mixture was stirred for 12 h, then pumped to dryness,
and then redissolved in CDCl3 for NMR measurements. The product
was obtained in a crystalline form from the NMR solvent. 31P{1H}
NMR (CDCl3): δ 34.74 (s). 1H NMR (CDCl3): δ 8.38 (2H, s, Hm of
NC5H3), 7.82 (8H, m, Ho of PC6H5), 7.45 (12H, m, Hm and Hp of
PC6H5), 3.96 (4H, vt, 2JHP ) 4.6 Hz, CH2P). 13C{1H} NMR (CDCl3):
δ 176.89 (s, Ci of NC5H3), 146.25 (vt, 2JCP ) 11.8 Hz, Co of NC5H3),
146.03 (vt, 2JCP ) 11.4 Hz, Cm of NC5H3), 133.46 (vt, 2JCP ) 6.9 Hz,
1
) 155 Hz); in acetone-d6, δ 58.7 (d, JPRh ) 155 Hz). 1H NMR: in
C6D6, δ 7.80 (2H, bs, Hm of NC5H3), 6.4-7.2 (20H, m, all H of PC6H5),
2.80 (4H, bs, CH2P); in THF-d8, δ 7.79 (2H, bs, Hm of NC5H3), 6.8-
7.5 (20H, m, all H of PC6H5), 3.29 (4H, bs, CH2P); in acetone-d6, δ
7.83 (2H, bs, Hm of NC5H3), 7.1-7.5 (20H, m, all H of PC6H5), 3.34
(4H, bs, CH2P). 13C{1H} NMR in C6D6 (carbons of CO and Cipso were
not observed), δ 143.3 (vt, 2JCP ) 13 Hz, Co of NC5H3), 142.1 (vt, 2JCP
) 9 Hz, Cm of NC5H3), 139.6 (t, J ) 14 Hz), 133.3 (bs), 132.4 (d, J
) 10 Hz), 132.1 (t, J ) 6 Hz), 131.5 (d, J ) 3 Hz), 131.3 (d, J ) 9
1
1
Hz), 128.6 (vt, J ) 4 Hz), 42.4 (vt, JCP ) 15 Hz, CH2P); in THF-d8,
Co of PC6H5), 131.85 (s, Cp of PC6H5), 130.96 (vt, JCP ) 22.4 HzCi
δ 206.0 (m, RhCO), 185.7 (m, Ci of NC5H3), 143.3 (vt, 2JCP ) 13 Hz,
of PC6H5), 129.73 (vt, 2JCP ) 5.3 Hz, Cm of PC6H5), 40.34 (vt, 1JCP
)
Co of NC5H3), 143.3 (m), 140.2 (t, J ) 14 Hz), 138.5 (bs), 133.8 (bs),
14.8 Hz, CH2P). vis (λ in nm (ꢀ in L mol-1cm-1): 420 (305). FAB
MS: m/e 1406.2 [M+].
1
132.8 (d, J ) 10 Hz), 132.5 (t, J ) 6 Hz), 125.9 (s), 42.3 (vt, JCP
)
15 Hz, CH2P). IR (ν in cm-1, NaCl, film): 1955 (νCO).
X-ray Crystal Structure Analysis of (dppL)Rh(PiPr3)‚C6H6.
Pertinent crystallographic information is summarized in Table 1.
Crystals of crystallographic quality were obtained from benzene. An
orange prism of appropriate size (0.4 × 0.4 × 0.3 mm) was selected
and mounted on a glass fiber using silicone grease and placed on a
Rigaku AFC5R diffractometer equipped with a rotating anode (RU-
300). Data collection proceeded at 90 K. The unit cell was obtained
by a random search of 20 carefully centered reflections. Monitoring
of three standard reflections every 200 reflections indicated no decay
of the crystal in the X-ray beam. Data were collected at constant scan
speed (8°/min) in the Ω-scan mode in the range of 2° < 2θ < 55°,
giving a total of 9817 relections (7776 with I > 3σ(I)). The data were
corrected for Lorentz and polarization effects. No absorption correction
was applied to the data due to the low absorption coefficient (µ )
5.64 cm-1). The structure was solved by automated Patterson analysis
(SHELXS-86)11 and Fourier methods (SHELX-76).12 Hydrogen atoms
were found from the difference Fourier map and refined with an overall
temperature factor (Uoverall ) 0.044(2) × 103 Å2). Scattering factors
and corrections for anomalous dispersion for Rh and P were those of
the International Tables for Crystallography.13
Synthesis of (dppL)PdCl (14). To a vigorously stirred solution of
(PhCN)2PdCl2 (100 mg, 0.26 mmol) in CH2Cl2 (20 mL) in a pressure
vessel, a solution of dppLH (124 mg, 0.26 mmol) in CH2Cl2 (30 mL)
was slowly added. Immediately the color of the solution changed from
orange-red to bright yellow, and a small amount of a yellow precipitate
appeared. The pressure vessel was heated to 100 °C for 12 h, after
which a bright yellow suspension had formed. The reaction mixture
was pumped to dryness, yielding the pyridinium hydrochloride as a
yellow solid ([(dppLH)PdCl]+Cl-: Anal. Calcd for C31H27Cl2NP2Pd‚
2CH2Cl2: C, 48.18; H, 3.80; N, 1.70. Found: C, 48.24; H, 4.02; N,
1.83.). It has the following spectroscopic properties (31P{1H} NMR:
in DMSO-d6, δ 40.27 (s); in CH3OD, δ 39.70 (s). 1H NMR (DMSO-
d6): δ 8.45 (2H, s, Hm of NC5H3), 7.91 (8H, m, Ho of PC6H5), 7.54
2
(12H, m, Hm and Hp of PC6H5), 4.42 (4H, vt, JHP ) 4.74Hz, CH2P).
vis (λ in nm (ꢀ in L mol-1cm-1): 406 (322)). In order to obtain the
free base, the hydrochloride was treated with a mixture of toluene (40
mL), Et3N (5 mL), and water (20 mL) and the organic phase was
collected. The remaining aqueous phase was extracted twice with
toluene (2 × 20 mL), the combined organic phases pumped to dryness,
and the remaining orange solid recrystallized from benzene, giving 101
mg of orange crystals (0.164 mmol; 63%). 31P{1H} NMR: in CDCl3,
δ 33.07 (s); in C6D6, δ 36.50 (s); in acetone-d6, δ 41.30 (s). 1H NMR
(CDCl3): δ 8.21 (2H, s, Hm of NC5H3), 7.81 (8H, m, Ho of PC6H5),
7.36 (12H, m, Hm and Hp of PC6H5), 3.90 (4H, vt, 2JHP ) 4.7 Hz, CH2P).
Results and Discussion
(a) Ligand Synthesis. The synthetic route for the preparation
of 9 is summarized in Scheme 1. 3,5-Bis(chloromethyl)pyridine
(8) was obtained in low yield by radical chlorination of 3,5-
lutidine (7) with N-chlorosuccinimide in refluxing CCl4.
The chlorination was nonselective, leading to a mixture of
products, containing mainly the desired 8. To prevent poly-
merization of the free base by quaternization of the pyridine
moiety, 8 was converted to its hydrochloride 10. We found
that the free base polymerized upon standing for a few days,
even at -10 °C.14 Therefore, the free base had to be prepared
freshly from 10 by deprotonation with NaHCO3 in water at pH
8. Compound 8 reacted with LiPPh2 in THF at -78 °C to give
3 as a colorless solid in moderate yields.
13C{1H} NMR (CDCl3): δ 170.89 (s, Ci of NC5H3), 144.81 (vt, 2JCP
11 Hz, Co of NC5H3), 143.73 (vt, 2JCP ) 11 Hz, Cm of NC5H3), 133.56
)
2
1
(vt, JCP ) 6.7 Hz, Co of PC6H5), 131.92 (vt, JCP ) 21.7 HzCi of
PC6H5), 131.55 (s, Cp of PC6H5), 129.56 (vt, 2JCP ) 5 Hz, Cm of PC6H5),
46.8 (vt, 1JCP ) 15 Hz, CH2P). vis (λ in nm (ꢀ in L mol-1cm-1): 422
(245). FD MS: m/e 607.
Synthesis of {(dppL)PdCl}‚BEt3 (15). BEt3 (35 µL; 1 M solution
in hexanes) was added to a stirring solution of (dppL)PdCl (20 mg,
0.032 mmol) in benzene (2 mL). After 2 h all volatiles were removed
in vacuo. The remaining orange solid was washed with pentane (3 ×
5 mL) and then dried in vacuo. 31P{1H} NMR (CDCl3): δ 34.55 (s).
1H NMR (CDCl3): δ 8.19 (2H, s, Hm of NC5H3), 7.86 (8H, m, Ho of
PC6H5), 7.46 (12H, m, Hm and Hp of PC6H5), 4.01 (4H, s br, CH2P).
13C{1H} NMR (CDCl3): δ 177.90 (s br, Ci of NC5H3), 146.88 (vt,
(11) Sheldrick, G. M. SHELXS86, University of Gottingen, F.R.G., 1986.
(12) Sheldrick, G. M. SHELX76, Cambridge University, England, 1976.
(13) International Tables for Crystallography; Kynoch Press: Birmingham,
U.K., 1974; Vol. 4.
(14) Similar attempts using radical bromination with N-bromosuccinimide
and other bromination reagents (Br2PPh3, Br2, and poly(vinylpyri-
dinium hydrotribromide)) led to formation of polymeric products by
N-alkylation of the desired 3,5-bis(bromomethylene)pyridine.
2
2JCP ) 11.9 Hz, Co of NC5H3), 138.47 (vt, JCP ) 10.5 Hz, Cm of
2
1
NC5H3), 133.78 (vt, JCP ) 6.8 Hz, Co of PC6H5), 132.06 (vt, JCP
)
2
22.0 Hz, PC6H5), 131.77 (s, Cp of PC6H5), 129.73 (vt, JCP ) 5.2 Hz,
1
Cm of PC6H5), 40.06 (vt, JCP ) 15.0 Hz, CH2P), 16.87 (s br, BCH2-
CH3), 11.25 (s, BCH2CH3). 11B{1H} NMR (CDCl3): δ 1.5. vis (λ in
nm (ꢀ in L mol-1cm-1): 420 (276).

A 3,5-Lutidine-Based Phosphine Ligand
Inorganic Chemistry, Vol. 35, No. 7, 1996 1795
Scheme 1. Synthesis of dppLH: (i) N-Chlorosuccinimide,
Scheme 3. Synthesis of (dppx)PdCl (14): (i) (PhCN)2PdCl2
in CH2Cl2; (ii) Et3N, Toluene, Water
CCl4, Reflux, hν; (ii) LiPPh2, THF, -78 °C; (iii) HClgas
,
CCl4; (iv) NaHCO3, H2O, pH 8
comparable to those of 11. The orthometallated carbon atom
is observed as a multiplet at δ 188.5 in the 13C{H} NMR
spectrum. The methylene protons of the -CH2P group are
observed as a virtual triplet at δ 3.71. The synthetic approach
starting from HRh(PPh3)4 has the disadvantage of 3 equiv of
PPh3 being produced en route to 12, which are extremely
difficult to remove since both have very similar solubility
properties. However, this has no effect on the further chemistry
of complex 12.
The reaction pathway to 11 most likely involves substitution
of the olefin by the bis(phosphine) followed by oxidative
metallation of the heteroaromatic ring to give a Rh(III) species.
This will then reductively eliminate HCl to give the Rh(I)
complex 11. NEt3 will remove HCl from the solution and form
[HNEt3]+Cl-. Bases have been used before to abstract HCl
from Rh(III) hydrido chlorides.2c In the absence of added base,
two possible competing reactions may occur, involving forma-
tion of the pyridinium hydrochloride salt and formation of a
Rh(III) hydrido chloride complex. For the formation of 12 a
similar reaction sequence in envisioned, involving coordination
of the bis(phosphine), followed by orthometallation to give a
Rh(III) bishydrido complex, which readily eliminates dihydrogen
to give 12.
(b) Syntheses of Metal Complexes. Compound 9 forms
stable complexes with a variety of metals. Our synthetic
approaches are summarized in Scheme 2.
Scheme 2. Syntheses of (dppL)Rh Complexes: (i) Rh2Cl2-
(COE)4, PiPr3, THF/NEt3 (3:1), 12 h, RT; (ii) HRh(PPh3)4,
THF, 12 h, RT; (iii) (PhCN)2PdCl2, CH2Cl2; Toluene/Water/
NEt3 (10:2:1), Reflux; (iv) CO (20 psi), Pentane, 12 h, RT
The PPh3 ligand in 12 trans to the aromatic ring is labile
and can be substituted by CO. Pressurizing a solution of 12 in
pentane with 20 psi of CO leads to the quantitative precipitation
of the yellow RhCO complex 13. Excess phosphine can now
be conveniently removed by washing the yellow solid with
pentane. Complex 13 exhibits a single doublet in the 31P NMR
spectrum at significantly lower field as compared to 11 and 12
at δ 58.00 (1JPRh ) 155 Hz). The shift of -CH2P in the H
1
The reaction of 9 with Rh2Cl2(COE)4 and PiPr3 in THF/NEt3
(3:1) leads to the formation of the mononuclear Rh(I) complex
11, which is obtained as a crystalline yellow material from
benzene. Absence of base results in the formation of an
intractable viscous solid. As expected, 11 exhibits two signals
in the 31P{1H} NMR spectrum (intensity ratio of 2:1) in accord
with the X-ray structure determination (Vide infra). The signal
at δ 51.31, exhibiting a doublet of doublets splitting pattern
NMR is extremely solvent dependent (in C6D6, δ 2.80; in THF-
d8, δ 3.29; in acetone-d6, δ 3.34). The complex exhibits a single
νCO at 1955 cm-1, indicating significant back-bonding to the
CO ligand in accord with the observed downfield shift in the
31P NMR spectrum.
During the reaction of (PhCN)2PdCl2 with 9 in CH2Cl2 1
equiv of HCl is produced, which is trapped by the basic nitrogen,
leading to the formation of the hydrochloride [(HdppL)PdCl]+Cl-,
which precipitates from the solution as a yellow solid. The
stable pyridinium hydrochloride gives rise to a single resonance
in the 31P{1H} NMR spectrum at δ 40.27 (in DMSO-d6) (δ
39.70 in CH3OD). It is easily deprotonated by NEt3, leading
to the formation of the hydrocarbon-soluble orange complex
(dppL)PdCl (14; Scheme 3).
Compound 14 was characterized by NMR spectroscopy and
by field desorption mass spectroscopy (FD MS). The magneti-
cally equivalent phosphorus nuclei are observed in the 31P{1H}
NMR spectrum as a singlet at δ 33.07 (CDCl3). In the 1H NMR
spectrum, the phenyl substituents give rise to two sets of
resonances, assigned to Ho (δ 7.81) and Hm, Hp (δ 7.36), while
the proton in the ortho position to the metallated lutidine ring
is observed as a singlet at δ 8.21. The methylene protons of
the -CH2P group are observed as a broad singlet at δ 3.90. Its
spectroscopic properties are in accord with C2 symmetry,
reported for related Pd complexes possessing a terdentate bis-
(phosphine) ligand framework.2L FD MS shows a base peak
at m/e 617.
2
(1JPRh ) 165 Hz, JPP ) 29 Hz), is assigned to the two
phosphorous nuclei of the dppL framework. The second signal
is observed at δ 47.80 and is assigned to the unique phosphorous
of the PiPr3 ligand, being trans to the aromatic ring. Coupling
to the 31P nuclei of the two phosphines of the dppL ligand and
to 103Rh gives rise to a doublet of triplets (1JPRh ) 116 Hz, 2JPP
) 29 Hz). In the 1H NMR spectrum the methylene protons of
the -CH2P group are observed as a broad singlet at δ 3.72. All
iPr groups of the PiPr3 are magnetically identical, giving rise to
one doublet of septets for the methine proton at δ 1.69 and a
doublet of doublets for the methyl protons at δ 1.06.
The analogous PPh3 complex 12 can be obtained conveniently
from HRh(PPh3)4 and 9 in THF. Its spectroscopic properties
are similar to those of 11. It exhibits two signals in the 31P
NMR in an intensity ratio of 2:1. The signal at δ 52.10 (doublet
1
2
of doublets, JPRh ) 161 Hz, JPP ) 30 Hz) is assigned to the
two phosphine substituents of the dppL ligand. The second
signal is observed at δ 37.3 (doublet of triplet, 1JPRh )120 Hz,
2JPP ) 30 Hz). Multiplicities and coupling constants are

1796 Inorganic Chemistry, Vol. 35, No. 7, 1996
Weisman et al.
(d) Metal Binding Studies. Our structural studies have
shown that the nitrogen atom of the lutidine-based ligand is
accessible in complex 11 and available for binding to another
metal center. Similarly, the spectroscopic properties of com-
plexes 12-14 indicate that the nitrogen is exposed and available
for binding, allowing the formation of multimetallic aggregates.
A multitude of metals form stable and well-characterized
complexes with pyridine. We attempted to test our assumption
by the reaction of complexes 11-14 with (PhCN)2PdCl2 in a
2:1 stoichiometry. (PhCN)2PdCl2 is known to undergo facile
substitution with pyridine to form stable complexes (py)2PdCl2.17
We thought of using 13 possessing the CO trans to the aromatic
ring for our metal binding studies. Monitoring the νCO by IR
spectroscopy after complex formation would be a good qualita-
tive probe for the electron density on the Rh center.
Scheme 4. Formation of the BEt3 Adduct 15 and of the
Trinuclear Complex 16
Figure 1. ORTEP drawing of (dppL)RhPiPr3 (30% probability level).
Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Length (Å) and Angles (deg) for
(dppL)Rh(PiPr3)‚C6H6
Rh-P(2)
Rh-P(4)
2.269(3)
2.342(3)
Rh-P(3)
Rh-C(31)
2.275(3)
2.065(5)
Pyridine Ring
C(30)-C(31)
C(31)-C(32)
C(33)-N(34)
1.415(5)
1.415(6)
1.338(5)
C(30)-C(35)
C(32)-C(33)
C(35)-N(34)
1.380(5)
1.385(5)
1.350(6)
P(2)-Rh-P(3)
P(3)-Rh-P(4)
P(3)-Rh-C(31)
157.22(3) P(2)-Rh-P(4)
101.19(3) P(2)-Rh-C(31)
101.64(3)
78.0(2)
179.0(1)
However, the reaction of the Rh-based complexes invariably
led to a metallic precipitate, indicating that a more complex
redox reaction is taking place. We decided therefore to use
the Pd complex 14 in our binding studies and to use 13C NMR
spectroscopy to probe the electron density on the Pd center.
To probe the availability of the pyridine nitrogen, we chose
to react 14 with a trialkylborane. Trialkylboranes form stable
adducts with nitrogen donors, which can be readily identified
as being pyridine coordinated by 11B NMR spectroscopy.18 The
reaction of 14 with BEt3 led to the formation of the expected
N-coordination product {(dppL)PdCl}‚BEt3 (15). The 11B NMR
spectra of the reaction mixture shows the disappearance of the
resonance of free Et3B (δ 87.0) and the appearance of a single
resonance for the product at δ 1.5, in a range characteristic for
R3B‚py systems. Similar adducts have been reported in the
literature (Me3B‚py, δ 0.0 in CH2Cl2; Et3B‚py, δ 2.2 in Et2O).19
All other spectroscopic results are in accord with Et3B coordina-
tion to the N-donor site of the metalloligand (see Experimental
Section).
79.3(2)
P(4)-Rh-C(31)
Pyridine Ring
C(31)-C(30)-C(35) 120.6(4) C(30)-C(31)-C(32) 114.2(3)
C(31)-C(32)-C(33) 121.0(4) C(32)-C(33)-N(34) 124.0(4)
C(33)-N(34)-C(35) 115.9(3) C(30)-C(35)-N(34) 124.3(4)
(c) Structural Studies. Selected bond distances and angles
for 11 are given in Table 2. An ORTEP15 view of complex 11
is shown in Figure 1. The coordination environment about the
Rh(I) center is distorted square planar, having the two phos-
phorous atoms P(2) and P(3) of the dppL ligand system in the
expected trans configuration. This P(2)-Rh-P(3) angle is
significantly smaller (157.22(3)°) than that reported by Kaska
for the P-Rh-P angle in the Rh(III) complex (dtbpx)Rh(H)-
(Cl) (168.82(8)°) (dtbpx ) 1,3-bis(di-tert-butylphosphine)-
xylene).2c The geometric strain imposed on the system by the
presence of the two five-membered rings formed upon chelation
and the steric congestion about the Rh center due to the bulky
PiPr3 ligand (Tolman’s cone angle 160°)16 cause a compression
of the P(2)-Rh-P(3) angle. The distance of the rhodium to
the phosphine trans to the heteroaromatic ring (d(Rh-P(4)) )
2.342(3) Å) is significantly elongated compared to Rh-P(2)
(2.269(3) Å) and Rh-P(3) (2.275(3) Å) due to the strong trans
influence of the σ-bonded aryl substituent. Similar effects have
been observed before in systems having an orthometallated
aromatic ring trans to a monophosphine.2g,n
Compound 14 smoothly reacts with (PhCN)2PdCl2 in THF
to form what appears to be a trinuclear Pd complex (Scheme
4), in which the two (dppL)PdCl moieties coordinate a PdCl2
moiety through the lutidine N. The spectroscopic characteristics
of this complex support this assignment.
The molecular weight, as determined by FAB-MS, is 1406.2.
1
The H NMR spectrum of the reaction product is compatible
with the formulation of a trinuclear complex. A noteworthy
spectroscopic feature is a shift of the ortho-protons of the
It is noteworthy that the pyridine ring retains its planarity.
The nitrogen atom is unobstructed and is free to act as a donor
to another metal fragment.
(17) Partenheimer, W.; Hoy, E. F. Inorg. Chem. 1973, 12, 2805.
(18) (a) No¨th, H.; Wrackmeyer, B. NMR Basic Principles and Progress;
Springer Verlag: Berlin, 1978; Vol. 20, Chapter 7.4.4.2. (b) Wrack-
meyer, B. In Annual Reports in NMR Spectroscopy; Webb, G. A.,
Ed.; Academic Press: London, 1988; Vol. 20, pp 61-203.
(15) Johnson, C. K. ORTEP-II, Report ORNL-5138; Oak Ridge National
Laboratory: Park Ridge, TN, 1976.
(16) Tolman, C. A. Chem. ReV. 1988, 77, 313.
(19) No¨th, H.; Wrackmeyer, B. Chem. Ber. 1974, 107, 3070.

A 3,5-Lutidine-Based Phosphine Ligand
Inorganic Chemistry, Vol. 35, No. 7, 1996 1797
Table 3. Selected 13C NMR Data for the Two Ligands dppxH and
dppLH (9), Complex 14, the BEt3 Adduct 15, and the Trinuclear Pd
Complex 16
lutidine to lower field upon coordination to the PdCl2 moiety
(in 14 Ho at δ 8.21; in 16 Ho at δ 8.58).
In order to get some information about electronic effects of
N-coordination, we carried out a vis spectroscopic study of
complexes 14-16 and the pyridinium complex [H(dppL)-
PdCl]+Cl-. A ligand coordinated to the pyridine nitrogen
should influence the energy level of the HOMO of the complex
and hence should cause a shift of the metal-based transition.
N-binding will drain electron density away from the metal
center, which will result in a net stabilization of the HOMO.
As a result, a transition from the HOMO to the LUMO will
require more energy, leading to a hypsochromic shift of the
observed transition. The strongest effect is expected for a
pyridinium cation. Indeed a weak transition is observed at 406
nm for the pyridinium cation [H(dppL)PdCl]+, which is shifted
to lower wavelength (higher energy) compared to the parent
14. In the N-coordination complexes 15 and 16 only a slight
hypsochromic shift is observed (to 420 nm for both complexes).
The results from the vis spectroscopic measurements seem to
indicate that N-coordination influences the electron density on
the metal only to a small extent. However, as van Koten et
al.1d have shown, 13C NMR spectroscopy is a more sensitive
tool for analyzing electronic trends in aryl-bound organome-
tallics.
a
δ Cipso
dppxH
(dppx)PdCl
dppLH
(dppL)PdCl (14)
{(dppL)PdCl}BEt3 (15)
{(dppL)PdCl}2PdCl2 (16)
127.0b,c
159.0b,c
132.0 (9)
170.89
177.06
176.89
a δ in ppm relative to TMS, in CDCl3. b dppx ) 1,3-(Ph2PCH2)2C6H3;
dppxH ) 1,3-(Ph2PCH2)2C6H4 (equivalent to 1 where ER2 ) PPh2).
c Measured by Y. Ben David in our laboratory (value not reported in
Venanzi’s report on (dppx)PdCl, cf. ref 2i).
downfield shift of the PdCipso. This is indeed what is observed.
Upon N-coordination of 14 to a PdCl2 moiety a downfield shift
to δ 176.89 is observed, clearly indicating the influence of metal
binding to the pyridine N on the electron density of a PdCipso
and with this on the Pd center itself. Analogously, N-
coordination of BEt3 causes a downfield shift to δ 177.90. In a
study using a series of para-substituted NCN-systems, van
Koten et al.1d have shown that the 13C NMR shift of MCipso
reflects the electron density on the metal center.
Conclusions
N-coordination of the metalloligand will effectively result in
drainage of electron density from the Pd center of the metallo-
ligand. This should invariably increase the donor ability of the
phosphines coordinated to the Pd center and result in a downfield
shift in the 31P{1H} NMR compared to the uncomplexed system.
The trinuclear complex 16 exhibits a single resonance in the
31P NMR spectrum at δ 34.74 shifted downfield from the
starting complex 14 (δ 33.07) by 1.67 ppm. Similarly, the BEt3
adduct 15 exhibits a single resonance at δ 34.55, indicating
drainage of electron density away from the Pd center. 13C NMR
shifts are particularly sensitive to changes in electron density
in an aromatic system. In a study on 13C NMR shifts in 1,4-
disubstituted benzenes, it was shown that a substitutional
With the purpose of a versatile approach to influencing the
electron density of a metal center through the backbone of
terdentate phosphorous ligands, we designed the bis(phosphine)
ligand dppLH based on 3,5-lutidine. After complexation to
transition metals, the nitrogen of the pyridine ring is available
for binding to a second metal moiety. Our results clearly show
that coordination of a metal moiety to the nitrogen of the
pyridine ring influences the electron density on the metal center.
Hence, the dppL ligand allows us to conveniently change the
electronic properties of a metal center by N-coordination. We
expect that the coordination of other metal moieties will change
the electronic properties of the central metal over a wide range,
perhaps allowing one to fine tune its reactivity. As already
shown, electronic fine tuning can allow the design of better
catalysts.21 This metalloligand offers also a straightforward
approach for the generation of oligo- and multinuclear com-
plexes, as exemplified in the borane adduct 15 and in the
trinuclear complex 16.
variations in the para-position will greatly influence the 13C
20
shift of the Cipso
.
Electron-withdrawing substituents in the
para-position will decrease the electron density on the Cipso
,
resulting in a downfield shift of its resonance. The 13C shifts
of the two ligands dppxH2i (1 with ER2 ) PPh2) and 9 are
compared to the Pd complexes 14-16 in Table 3.
Introduction of a nitrogen into the aromatic system leads to
lower electron density on the carbon atoms. As expected, we
observe the appropriate shifts for Cipso (the carbon atom being
trans to the nitrogen) in the two ligands dppxH (δ 127.0) and
dppLH (δ 132.0), indicating that Cipso in dppLH is less electron-
rich compared to the all-carbon aromatic system dppxH. Upon
coordination to a metal center, this effect is dramatically
enhanced. In the parent system (dppx)PdCl,2i the PdCipso is
observed at δ 159.0. In (dppL)PdCl (14), we observe the PdCipso
of the pyridine ring at δ 170.89 significantly shifted downfield
compared to the unsubstituted (dppx)PdCl system. N-coordina-
tion should decrease the electron density and should cause a
Acknowledgment. This work was supported by the Israel
Science Foundation, Jerusalem, Israel. We thank the MIN-
ERVA Foundation, Munich, Germany, for financial support and
for a postdoctoral fellowship to H.-B.K. We thank Dr. R.
Fokkens (University of Amsterdam) for carrying out FAB MS
and FD MS measurements and Dr. Felix Frolow for the X-ray
structure determination.
Supporting Information Available: Tables listing full crystal-
lographic details, atom coordinates, anisotropic temperature factors,
H-atom coordinates, and bond lengths and angles and a figure showing
the unit cell for 11 (8 pages). Ordering information is given on any
current masthead page.
IC951131H
(20) (a) Bromilow, J.; Brownlee, R. T. C.; Lopez, V. O.; Taft, R. W. J.
Org. Chem. 1979, 44, 4766-4770. (b) Bromilow, J.; Brownlee, R. T.
C.; Craik, D. J.; Sadek, M.; Taft, R. W. J. Org. Chem. 1980, 45, 2429-
2438.
(21) Jacobsen, E. N.; Zhang, W.; Guler, M. L. J. Am. Chem. Soc. 1991,
113, 6703.