Di-, tri-, and tetracopper(II) complexes of a pseudocalixarene macrocycle

Article abstract of DOI:10.1002/1521-3773(20021202)41:23<4553::AID-ANIE4553>3.0.CO;2-W

The wide range of nuclearity, geometry, and protonation levels possible with the hybrid Schiff base/calixarene ligand H₆L3 are illustrated in the formation and characterization of three different copper(II) complexes. The X-ray structures show

Full text of DOI:10.1002/1521-3773(20021202)41:23<4553::AID-ANIE4553>3.0.CO;2-W

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reflections, 2495 unique reflections, 1700 used (48 < 2V< 49.368), R1 ¼
0.030 (3344 reflections, I > 10s(I)), wR2 ¼ 7.4% (all data), GOF ¼ 0.97.
Minimum and maximum peaks in the final difference map are À1.25
and 1.47 eäÀ3. CCDC-191383 (1), CCDC-191384 (7), CCDC-191385
(22), and CCDC-191386 (24) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (þ 44)1223-336-033; or deposit@ccdc.cam.ac.uk)
[11] The nearest examples are [(h6-C10H8)Ru(cod)], M. Crocker, M.
Green, J. A. K. Howard, N. C. Norman, D. M. Thomas, J. Chem. Soc.
Dalton Trans. 1990, 2299; and [(h4-C10H8)Rh(Cp)], G. M¸ller, P. E.
Gaede, C. Hirsch, K. Qaio, J. Organomet. Chem. 1994, 472, 329 335.
[12] As determined by X-ray crystallography (7) and NMR, see Supporting
Information.
[13] No other hapticity was noticed for the naphthalene ligand. For more
structural and NMR information on related ruthenium naphthalene
complexes, see a) M. A. Bennett, H. Neumann, M. Thomas, X.-Q.
Wang, P. Pertici, P. Salvadori, G. Vituilli, Organometallics 1991, 10,
3237 3245; b) M. A. Bennett, Z. Lu, X. Wang, M. Brown, D. C. R.
Hockless, J. Am. Chem. Soc. 1998, 120, 10409 10415.
cavity size.[4] The longer saturated chain in H4L2 permits
formation of homo- and heterotetranuclear complexes, as well
as complexes in which some sites are vacant.[3]
We have now synthesized the new dialdehyde dihydroxy-
5,5’-di-tert-butyl-3,3’-methanediyldibenzaldehyde (dhtmb)[5]
and used this group to expand the H4L1 macrocycle. Schiff
base condensation of dhtmb with 1,3-diaminopropan-2-ol in
the presence of metal template ions yields complexes of the
new macrocycle H6L3 (Scheme 1). This ligand can be
[14] For more information on the effect of arene methylation in {(h6-
arene)RhI} complexes, see: E. T. Singewald, C. S. Slone, C. L. Stern,
C. A. Mirkin, G. P. A. Yap, L. M. Liable-Sands, A. L. Rheingold, J.
Am. Chem. Soc. 1997, 119, 3048 3056.
[15] See supporting information for details; yields given for the conversion
of 2,3-dimethylbutadiene and propargyl methyl ether to the corre-
sponding cyclohexadiene.
Di-, Tri-, and Tetracopper(ii) Complexes of a
Pseudocalixarene Macrocycle**
Julia Barreira Fontecha, Sandrine Goetz, and
Vickie McKee*
Schiff base Robson-type macrocycles containing two bridg-
ing phenol groups have been widely used to synthesize homo-
and heterodinuclear complexes for many years. Detailed
investigation of the chemistry of these systems has enabled
insights in a number of areas, such as magnetochemistry, site
selection, and bioinorganic model chemistry.[1] Expansion to
larger macrocycles accommodating polynuclear assemblies
has been achieved in the last ten years.[2,3] Interest in such
complexes is centered on their ability to hold a number of
metal ions in a fixed geometric relationship and has potential
relevance in bioinorganic and catalytic chemistry as well as
nanotechnological applications.
Scheme 1. Pseudocalixarene ligand and binding sites.
considered as an expanded calix[4]arene containing two
¼
¼
NCH2CH(OH)CH2N inserts. Unlike most aza- or oxo-
calixarenes (where inserts are placed between each pair of
phenol rings),[6] H6L3 preserves two methylene linkages while
introducing imine and pendant alcohol donors.
The macrocycle H4L1 can formmono- and dinuclear
complexes, but is unable to use the pendant alcohol groups
to bind more than two metal ions within the macrocycle
because of geometric constraints imposed by the overall
Approximately planar conformations are sterically prohib-
ited in genuine calix[n]arenes (where n ¼ 4 6) and metal ions
cannot take advantage of the bridging potential of the phenol
groups, which generally leads to mononuclear complexes.[6,7]
In contrast, H6L3 can use some or all of the phenolic oxygen
donors in the bridging mode and can be di- or polynucleating.
The presence of relatively soft imine donors in addition to the
phenolic oxygen atoms also widens the range of metal ions
that might be expected to bind with high stability constants.
Three types of metal binding site are potentially available
within H6L3 (Scheme 1): site A is analogous to the conven-
tional ™Robson∫ site and is similar to that observed for
complexes of H4L1;[4] there are two of these in the macrocycle.
[*] Prof. V. McKee, J. Barreira Fontecha, S. Goetz
Chemistry Department
Loughborough University
Loughborough, Leics LE11 3TU (UK)
Fax : (þ 44)1509-222-566
E-mail: v.mckee@lboro.ac.uk
[**] We are grateful to the EPSRC Mass Spectrometry Service Centre,
Swansea for FAB (LSIMS) spectra and to Loughborough University
for support.
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The unique central calixarene-type site B involves the four
phenol (or phenolate) donors. Finally, there are four equiv-
alent sites of type C; occupation of similar sites has been
reported for open-chain complexes based on 1,3-diaminopro-
pan-2-ol[8] as well as in macrocyclic complexes of H4L2.[3]
The macrocycle is formed by Schiff base condensation of
dhtmb with 1,3-diaminopropan-2-ol in the presence of a metal
template ion. When CuII is used as the template ion di-, tri-,
and tetranuclear complexes can be isolated depending on the
reaction conditions. Reaction of 1,3-diaminopropan-2-ol and
dhtmb with equimolar amounts of CuCl2 yields a green
dicopper complex (1), while reaction with two equivalents of
Cu(NO3)2¥6H2O yields a green tetracopper product (3) on
short reflux. The tetranuclear complex converts into the
brown tricopper species (2) on recrystallization from N,N-
dimethylformamide (DMF). The structure of each complex
has been investigated by X-ray crystallography.
The geometry at each metal ion is square pyramidal, with the
chloride ligand apical. The macrocycle adopts a saddle
conformation with adjacent phenol rings inclined at 68.5(1)
and 68.7(1)8 to each other. This shape is maintained by three
interactions: the Cu-Cl-Cu bridge (104.55(5)8) and two
hydrogen bonds linking adjacent phenol oxygen atoms
(O1¥¥¥O2 2.414(4) and O4¥¥¥O5 2.402(4) ä). The stoichiometry
of the reaction suggests that the macrocycle is doubly
deprotonated and the presence of the hydrogen bonds suggest
that one proton is lost fromeach pair of phenols. The alcohol
groups are not involved in bonding to the copper ions.
The tricopper(ii) complex [Cu3(H2L3)(NO3)](NO3)(dmf)3
(2) is dark brown, presumably because of charge transfer
between the fully deprotonated phenolate groups and the
CuII ions. The cations occupy the two A sites and the central
C site (Figure 2), with each ion bound to four macrocyclic
donors in its basal plane; the planes about Cu2 and Cu3 are
inclined to that at Cu1 by 41.7(1) and 34.2(1)8, respectively. A
coordinated nitrate counterion lies within the concave curve
The dicopper(ii) cation of [Cu2Cl(H4L3)]Cl¥1.6Et2O¥EtOH
(1¥1.6Et2O¥EtOH) is shown in Figure 1. The copper ions
occupy the A sites, with each coordinated to two imine
groups, two phenol oxygen atoms, and a bridging chloride ion.
Figure 2. Two views of the [Cu3(H2L3)(NO3)]þ ion of 2. Selected bond
lengths [ä] and angles [8]: Cu1-O1 1.957(3), Cu1-O5 1.956(4), Cu1-N3
1.935(4), Cu1-N4 1.935(5), Cu1-O11 2.432(5), Cu1-Cu2 2.8428(8), Cu2-O1
1.918(3), Cu2-O5 1.925(3), Cu2-O2 1.928(3), Cu2-O4 1.929(3), Cu2-O11
2.490(5), Cu2-Cu3 2.8616(9), Cu3-O2 1.945(3), Cu3-O4 1.952(3), Cu3-N2
1.936(4), Cu3-N1 1.942(5), Cu3-O12 2.613(9); Cu2-O1-Cu1 94.38(14), Cu2-
O2-Cu3 95.28(14), Cu2-O4-Cu3 95.02(14), Cu2-O5-Cu1 94.18(15).
Figure 1. Two views of the [Cu2Cl(H4L3)]þ ion of 1, dotted lines represent
hydrogen bonds. Selected bond lengths [ä] and angles [8]: Cu1-N3 1.952(4),
Cu1-O1 1.958(3), Cu1-N4 1.978(4), Cu1-O5 1.989(3), Cu1-Cl1 2.5145(13),
Cu2-O2 1.962(3), Cu2-N1 1.967(4), Cu2-O4 1.970(3), Cu2-N2 1.983(4),
Cu2-Cl1 2.5423(14), O1-O2 2.414(4), O4-O5 2.402(4); Cu1-Cl1-Cu2
104.55(5).
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of the three copper sites. The nitrate is disordered (7:3)
between two related positions; one site involves a single atom
bridge between Cu1 and Cu2 and a three atombridge
between Cu2 and Cu3, while the minor site provides a single
atombridge between Cu3 and Cu2 with a three atombridge
linking Cu2 and Cu1. The conformation of the macrocycle is
very similar to that in 1.
The cation of [Cu4(H2L3)(OH)2(H2O)(EtOH)](NO3)2 (3)
is shown in Figure 3; the CuII ions occupy the four equivalent
B sites. As observed in complex 1, each adjacent pair of
phenols is monodeprotonated and linked by a hydrogen bond
(O1¥¥¥O2, 2.395(7); O4¥¥¥O5, 2.417(6) ä). The overall con-
formation of the tetranuclear complex is, however, very
different fromthat of 1 and 2.
groups are 65.6(2) and 66.3(2)8. Each CuII ion has approx-
imate square pyramidal geometry: the base comprises macro-
cyclic phenoxo, alkoxo, and imine donors as well as an
exogenous m2-hydroxo group. The axial ligands are disordered
and have been modeled as having 50% occupancy of water
and ethanol at each site.
The most striking feature of this series of complexes is the
variation in nuclearity within the same ligand system. This
effect is linked to the variation in the extent of deprotonation
of the ligand, the orientation of the phenol groups, and the
overall shape of the macrocyclic host. Preliminary investiga-
tions suggest that the dicopper complex is only isolated in the
presence of a good coordinating anion which is also large
enough to fit the cavity defined by the hydrogen-bonding
À
À
À
The saturated portions of the ring are fully extended to
allow each alkoxo group to bridge two CuII ions and the
macrocycle is sharply folded down its long axis, with each half
approximately planar; the angles between adjacent phenol
interactions of the ligand. When NO3 , ClO4 , or BF4 were
used, the initial product of the template reaction was the
tetracopper complex, irrespective of the stoichiometry of the
template reaction.
Formally, the tricopper complex can be generated from the
dinuclear analogue by displacement of the two central,
hydrogen-bonding protons by a CuII ion. Comparison of
Figures 1 and 2 shows that the two structures are closely
related except that the saddle shape in 2 is maintained by the
CuII ion in the central site and by two hydrogen bonds in 1.
Surprisingly, however, it seems that the tricopper complex is
actually formed by rearrangement of the tetracopper com-
plex. Extended reflux of the template mixture causes the
green color to darken to brown and mixtures of 2 and 3 are
formed, as indicated by FAB mass spectrometry. The same
color change occurs within minutes if the tetracopper complex
is dissolved in DMF and clean tricopper complex is obtained.
Consequently, it seems that the tetranuclear complex (3) is the
kinetic product of the template reaction and that 2 is the
thermodynamic product. In view of the extensive structural
rearrangement required, it is not unexpected that the
rearrangement is slow in alcohol but faster in a solvent with
better donor properties.
Experimental Section
2,2’-dihydroxy-5,5’-di-tert-butyl-3,3’-methanediyldibenzyl alcohol was syn-
thesized from4- tert-butylphenol following the literature procedure.[9] The
alcohol groups were oxidized to aldehydes by MnO2 after the phenol
groups had been protected using allylbromide. The allyl groups were
removed with 10% Pd on charcoal to yield 2,2’-dihydroxy-5,5’-di-tert-butyl-
3,3’-methanediyl dibenzaldehyde (dhtmb) in 40% yield for the three-step
process.[5] Elemental analysis calcd (%) for C23H28O4: C 74.97, H 7.66;
found: C 74.51, H 7.86. 1H NMR (CDCl3, 250MHz): d ¼ 9.86 (s, 2H), 7.64 (d,
2H), 7.37 (d, 2H), 4.03 (s, 2H), 1.26 ppm(s, 18H).
[Cu2Cl(H4L3)]Cl¥2MeOH (1¥2MeOH). Anhydrous CuCl2 (0.073 g
0.543 mmol) was dissolved in hot dry ethanol (40 ml) and added to a
solution of dhtmb (0.20 g 0.543 mmol) in hot dry ethanol (35 mL). After
10 min reflux a solution of 1,3-diaminopropan-2-ol (0.52 g, 0.543 mmol) in
dry methanol (20 mL) was added. The green solution was heated at reflux
for 2 h, then cooled, filtered, and the volume reduced to give the product in
36% yield. Elemental analysis calcd (%) for 1¥2MeOH: C 58.7, H 6.7, N
5.1; found: C 58.4, H 6.6, N 5.1. Emerald green crystals of the solvate
1¥1.6Et2O¥EtOH were obtained by diffusion of diethyl ether into the
Figure 3. Two views of the [Cu4(H2L3)(OH)2(H2O)(EtOH)]2þ ion of 3,
hydrogen bonds are shown as dotted lines. Selected bond lengths [ä] and
angles [8]: Cu1-O11 1.927(5), Cu1-N4 1.932(8), Cu1-O6 1.961(5), Cu1-O1
1.995(5), Cu1-O61 2.420(6), Cu2-N1 1.904(6), Cu2-O11 1.928(5), Cu2-O3
1.963(5), Cu2-O2 1.977(5), Cu2-O61 2.402(6), Cu1-Cu2 3.143(1), Cu2-Cu3
3.681(1), Cu3-O12 1.889(5), Cu3-N2 1.935(7), Cu3-O3 1.963(5), Cu3-O4
2.007(5), Cu3-O71 2.520(7), Cu4-N3 1.898(7), Cu4-O12 1.909(5), Cu4-O6
1.979(5), Cu4-O5 1.997(5), Cu4-O71 2.527(7), Cu3-Cu4 3.160(1), Cu4-Cu1
3.677(1); Cu3-O3-Cu2 139.4(3), Cu1-O6-Cu4 137.9(3), Cu1-O11-Cu2
109.2(2), Cu3-O12-Cu4 112.6(2), Cu2-O61-Cu1 81.36(16), Cu3-O71-Cu4
77.54(18).
3
ꢀ
filtrate. Crystal dimensions: 0.35 î 0.14 î 0.07 mm , triclinic, space group P1
a ¼ 13.707(3), b ¼ 14.568(3), c ¼ 16.827(4) ä, a ¼ 103.472(3), b ¼ 95.935(3),
g ¼ 93.429(3)8, V¼ 3238(1) ä3, 1calcd ¼ 1.237 MgmÀ3
. 22822 reflections,
11279 independent (Rint ¼ 0.0455), m ¼ 0.792 mmÀ1, Tmax ¼ 1.000, Tmin
0.880, 830 least-squares parameters R1 ¼ 0.0579 wR2 ¼ 0.1384 (2s data).
¼
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[Cu3(H2L3)(NO3)](NO3)(dmf)4¥H2O (2¥4dmf¥H2O).
A
mixture of
173, 181; c) S. S. Tandon, L. K. Thompson, J. N. Bridson, V. McKee,
A. J. Downard, Inorg. Chem. 1992, 31, 4635.
[5] S. Goetz, Ph.D Thesis, Loughborough University (UK), 2002.
[6] a) V. Bˆhmer, Angew. Chem. 1995, 107, 785; Angew. Chem. Int. Ed.
Engl. 1995, 34, 713; b) D. C. Gutsche, Calixarenes revisited, The Royal
Society of Chemistry, Cambridge, 1998.
Cu(NO3)2¥6H2O (0.321 g, 1.08 mmol) and dhtmb (0.20 g, 0.543 mmol) in
dry ethanol (80 mL) were heated to reflux for 10 min and a solution of 1,3-
diaminopropan-2-ol (0.052 g, 0.543 mmol) in dry methanol (20 mL) added
dropwise. The emerald green solution darkened as reflux was continued for
2 h. the mixture was filtered, cooled, and concentrated under vacuum
where upon
a
green solid precipitated (analyzed as [Cu4(H4L)-
[7] a) F. Hamada, K. D. Robinson, G. W. Orr, J. L. Atwood, Supramol.
Chem. 1993, 2, 19; b) S. E. J. Bell, J.K. Browne, V. McKee, M. A.
McKervey, J. F. Malone, M. O©Leary, A. Walker, F. Arnaud Neu, O.
Boulangeot, O. Mauprivez, M.-J. Schwing-Weill, J. Org. Chem. 1998,
63, 489; c) G. Guillemot, E. Solari, C. Floriani, N. Re, C. Rizzoli,
Organometallics 2000, 19, 5218.
[8] a) A. Gelasco, M. L. Kirk, J. W. Kampf, V. L. Pecoraro, Inorg.Chem.
1997, 36, 1829; b) P. E. Kruger, B. Moubaraki, G. D. Fallon, K. S.
Murray, J. Chem. Soc. Dalton Trans. 2000, 713; c) Y. Nishida, M.
Takeuchi, K. Takahashi, S. Kida, Chem. Lett. 1985, 631, 631.
[9] B. Dhawan, C. D. Gutsche, J. Org. Chem. 1983, 48, 1536.
[10] P. v. d. Sluis, A. L. Spek, Acta Cryst. 1990, A46, 194.
[11] G. M. Sheldrick, SHELXTL version 5.1, Bruker-AXS, Madison WI,
1998.
(NO3)(OH)2(H2O)4]). This complex was dissolved in DMF and the color
changed to brown; crystals of the trinuclear complex 2¥3dmf were obtained
in 40% yield by diffusion of diethyl ether into the solution. Elemental
analysis calcd (%) for 2¥4dmf¥H2O: C 51.8, H 6.4, N 9.4; found: C 52.1, H
6.5, N 9.4. Crystal dimensions: 0.45 î 0.24 î 0.04 mm3, monoclinic, space
group P21/c, a ¼ 17.234(1), b ¼ 22.130(2), c ¼ 17.658(1) ä, b ¼ 103.488(1)8,
V¼ 6549.0(8) ä3, 1calcd ¼ 1.395 MgmÀ3. 46309 reflections, 11521 independ-
ent (Rint ¼ 0.0424), m ¼ 1.036 mmÀ1, Tmax ¼ 1.000, Tmin ¼ 0.885; 859 least-
squares parameters R1 ¼ 0.0646 wR2 ¼ 0.1638 (2s data).
[Cu4(H2L3)(OH)2(sol)2](NO3)2 (3¥2H2O where sol ¼ water or ethanol). A
mixture of Cu(NO3)2¥6H2O (0.16 g, 0.543 mmol) and dhtmb (0.20 g,
0.543 mmol) in dry ethanol (70 mL) was heated to reflux for 10 min and
a solution of 1,3-diaminopropan-2-ol (0.052 g, 0.543 mmol) in dry methanol
(20 mL) added dropwise. The solution changed to emerald green and
darkened as reflux was continued for 1 h. The solution was cooled, filtered,
and evaporated to dryness. The residue was then dissolved in dichloro-
methane, filtered, and evaporated again before being dissolved in ethanol.
Green [Cu4(H2L3)(OH)2(H2O)2](NO3)2¥2H2O was obtained fromthis
solution in 79% yield. Elemental analysis calcd (%) for 3¥2H2O: C 47.1,
H 5.6, N 6.3; found: C 47.1, H 5.5, N 6.3. Crystals of [Cu4(H2L3)(OH)2-
(H2O)(EtOH)](NO3)2¥2EtOH¥H2O (3¥2EtOH¥H2O) were obtained by
slow evaporation of a solution of 3¥2H2O in an ethanol/benzene/petroleum
ether mixture. Crystal dimensions: 0.31 î 0.13 î 0.04 mm3, monoclinic,
space group P21/c, a ¼ 11.867(2), b ¼ 14.125(3), c ¼ 41.220(7) ä, b ¼
93.406(3)8, V¼ 6897(2) ä3, 1calcd ¼ 0.352 MgmÀ3. 47670 reflections, 12095
independent (Rint ¼ 0.0902), m ¼ 1.285 mmÀ1, Tmax ¼ 1.000, Tmin ¼ 0.766; 696
least-squares parameters R1 ¼ 0.0763 wR2 ¼ 0.21.19 (2s data). The unco-
ordinated solvate molecules and nitrate ions were highly disordered and
were treated using the SQUEEZE program[10] (see the Supporting
Information).
The Total Synthesis of
(À)-Amathaspiramide F**
Chambers C. Hughes and Dirk Trauner*
The amathaspiramides A F (1 6) are a family of marine
alkaloids recently isolated froma New Zealand collection of
the bryozoan Amathia wilsoni.[1,2] Marked by a dense array of
polar functionality, the natural products feature a novel
X-ray data were collected at 150(2) K on
a Bruker SMART 1000
diffractometer using MoKa radiation (l ¼ 0.71073 ä) to 2qmax of 258. The
structures were solved by direct methods and refined on F2 using all the
data.[11] Non-hydrogen atoms were refined with anisotropic ADPs and
hydrogen atoms attached to full-occupancy carbon atoms were inserted at
calculated positions. Details concerning treatment of disorders and of
hydrogen atoms not bonded to carbon atoms are described in the
Supporting Information. CCDC 190246.(1¥1.6Et2O¥EtOH) CCDC 190247
(2¥3dmf), and CCDC 190248 (1¥2H2O) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
(þ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
Received: August 6, 2002 [Z19906]
[1] a) R. C. Long, D. N. Hendrickson, J. Am. Chem. Soc. 1983, 105, 1513;
b) H. Okawa, J. Nishio, M. Ohba, M. Tadokoro, N. Matsumoto, M.
Koikawa, S. Kida, D. E. Fenton, Inorg. Chem. 1993, 32, 2949; c) M.
Yonemura, N. Usuki, Y. Nakamura, M. Ohba, H. Okawa, J. Chem.
Soc. Dalton Trans. 2000, 3624; d) A. Atkins, D. Black, A. J. Blake, A.
Marin-Becerra, S. Parsons, L. Ruiz-Ramirez, M. Schrˆder, Chem.
Commun. 1996, 457.
[2] a) A. Hori, M. Yonemura, M. Ohba, H. Okawa, Bull. Chem. Soc. Jpn.
2001, 74, 495; b) M. J. Grannas, B. F. Hoskins, R. Robson, Inorg.
Chem. 1994, 33, 1071; c) K. K. Nanda, S. Mohanta, U. Florke, S. K.
Dutta, K. Nag, J. Chem. Soc. Dalton Trans. 1995,3831; d) J. McCrea, V.
McKee, T. Metcalfe, S. S. Tandon, J. Wikaira, Inorg. Chim. Acta 2000,
297, 220.
[*] Prof. D. Trauner, C. C. Hughes
Center for New Directions in Organic Synthesis
Department of Chemistry
University of California, Berkeley
Berkeley, CA 94720-1460 (USA)
Fax : (þ 1)510-643-9480
E-mail: trauner@cchem.berkeley.edu
[3] a) V. McKee, S. S. Tandon, J. Chem. Soc. Dalton Trans. 1991 221;
b) P. E. Kruger, V. McKee, Chem. Commun. 1997 1341; c) S. Cromie,
F. Launay, V. McKee, Chem. Commun. 2001, 1918.
[4] a) S. S. Tandon, V. McKee, J. Chem. Soc. Dalton Trans. 1989 19;
b) A. J. Downard, V. McKee, S. S. Tandon, Inorg. Chim. Acta 1990,
[**] The Center for New Directions in Organic Synthesis is supported by
Bristol-Myers Squibb as a Sponsoring Member and Novartis as a
Supporting Member. We also thank Eli Lilly for a pre-doctoral
fellowship (C.C.H.) and Dr. Frederick J. Hollander and Dr. Allen G.
Oliver (Berkeley) for crystal structure determinations.
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