Synthesis and characterization of a chiral, aza-15-crown-5-functionalized ferrocenyldiphosphine ligand for asymmetric catalysis

Article abstract of DOI:10.1021/om990391u

A chiral ferrocenyldiphosphine ligand that is functionalized with an aza crown ether, (S)-1-[(R)-1′,2-bis(diphenylphosphino)ferrocenyl]ethyl-1-aza-2,3-benzo-15- crown-5 (1), has been synthesized. Both the resolved and racemic ligands react rapidly with Pt(II) precursors to form stable metal-ligand adducts; the complexes PtMeI(rac-1) and PtMe₂(rac-1) have been characterized crystallographically. Reaction of rac-1 with [Rh(NBD)₂]OTf yields [Rh(NBD)-(rac-1)]OTf. The three-dimensional solution structure of [Rh(NBD)(rac-1)]OTf has been determined by NOESY experiments and analysis using the two-dimensional conformer population analysis algorithm (2DCPA). The NOESY data reveal a rapid, pairwise chemical exchange between vinyl protons. The complex [Rh(NBD)(rac-1)]OTf is a catalyst precursor for hydrogenation reactions. However, we demonstrate that the lability of the aza crown ether may limit the ability of these catalysts to control selectivity via secondary interactions.

Full text of DOI:10.1021/om990391u

994
Organometallics 2000, 19, 994-1002
Syn th esis a n d Ch a r a cter iza tion of a Ch ir a l,
Aza -15-Cr ow n -5-F u n ction a lized F er r ocen yld ip h osp h in e
Liga n d for Asym m etr ic Ca ta lysis
Clark R. Landis,* Rachel A. Sawyer, and Ekasith Somsook
Department of Chemistry, University of WisconsinsMadison, 1101 University Avenue,
Madison, Wisconsin 53705
Received May 24, 1999
A chiral ferrocenyldiphosphine ligand that is functionalized with an aza crown ether, (S)-
1-[(R)-1′,2-bis(diphenylphosphino)ferrocenyl]ethyl-1-aza-2,3-benzo-15-crown-5 (1), has been
synthesized. Both the resolved and racemic ligands react rapidly with Pt(II) precursors to
form stable metal-ligand adducts; the complexes PtMeI(rac-1) and PtMe2(rac-1) have been
characterized crystallographically. Reaction of rac-1 with [Rh(NBD)2]OTf yields [Rh(NBD)-
(rac-1)]OTf. The three-dimensional solution structure of [Rh(NBD)(rac-1)]OTf has been
determined by NOESY experiments and analysis using the two-dimensional conformer
population analysis algorithm (2DCPA). The NOESY data reveal a rapid, pairwise chemical
exchange between vinyl protons. The complex [Rh(NBD)(rac-1)]OTf is a catalyst precursor
for hydrogenation reactions. However, we demonstrate that the lability of the aza crown
ether may limit the ability of these catalysts to control selectivity via secondary interactions.
In tr od u ction
characterization of new chiral ligands containing func-
tional groups for specific substrate complements.
The creation of new enantioselective, homogeneous
transition-metal catalysts is the subject of intense
current interest. To date, most successful enantioselec-
tive catalyst designs have focused on rigid, chiral
ligands, such as the DuPHOS1 and BINAP2 bidentate
phosphines, which impart selectivity through steric
destabilization of one diastereomeric transition state.
Ito and Sawamura3 have coined the term “secondary
interactions” to describe an alternate approach to
designing enantioselective catalysts. Secondary interac-
tions comprise a variety of ligand-substrate interac-
tions, including electrostatic interactions,4,5 hydrogen
bonding,6 and Lewis acid-base interactions.7,8 A poten-
tial attribute of this approach is that enantiocontrol may
be achieved, at least conceptually, by stabilizing one
diastereomeric transition state over another. Hence, it
is conceivable that secondary interactions could simul-
taneously increase both the rate and selectivity of a
catalytic transformation. Since Ito’s review3 of secondary
interactions in asymmetric catalysis, we9,10 and
others5,11-18 have focused on the design, synthesis, and
Previous work in this group has involved attachment
of boronato groups to an aminophenol-functionalized
ferrocene ligand,10 (S)-1-[(R)-1′,2-bis(diphenylphosphi-
no)ferrocenyl]ethyl-2-aminophenol (BPPFAP), designed
for use in Lewis acid-base and covalent secondary
interactions. A single-crystal structure of a metal com-
plex of BPPFAP, PtMeI(BPPFAP), displays the amino-
phenol moiety directing the N and O atoms to a position
4.0 Å above the square plane about platinum (above a
coordination site that could be occupied by an incoming
substrate). This indicates that the BPPFAP framework
is well-suited for directing a functional group for a
secondary interaction.10 This paper focuses on the
synthesis and characterization of a diphosphine ligand
substituted with an aminophenol-derived crown ether.
The strategy is to direct a flexible crown ether to a
position above an open coordination site favorable for a
secondary interaction with a functionalized substrate.
Crown ethers are known to bind ammonium cations
strongly and reversibly. Binding constants of up to 106
M-1 at 25 °C have been measured for alkylammonium
(1) Burk, M. J .; Feaster, J . E.; Harlow, R. L. Organometallics 1990,
9, 2653.
(2) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345.
(3) Sawamura, M.; Ito, Y. Chem. Rev. 1992, 92, 857.
(4) Sawamura, M.; Nagata, H.; Sakamoto, H.; Ito, Y. J . Am. Chem.
Soc. 1992, 114, 2586.
(5) Sawamura, M.; Nakayama, Y.; Tang, W.; Ito, Y. J . Org. Chem.
1996, 61, 9090.
(6) Hayashi, T.; Kanehira, K.; Hagihara, T.; Kumada, M. J . Org.
Chem. 1988, 53, 113.
(7) Borner, A.; Ward, J .; Kortus, K.; Kagan, H. B. Tetrahedron:
Asymmetry 1993, 4, 2219.
(8) Fields, L. B.; J acobsen, E. N. Tetrahedron: Asymmetry 1993, 4,
2229.
(9) MacFarland, D. K.; Landis, C. R. Organometallics 1996, 15, 483.
(10) Kimmich, B. F. M.; Landis, C. R. Organometallics 1996, 15,
4141.
(11) Yamazaki, A.; Achiwa, I.; Horikawa, K.; Tsurubo, M.; Achiwa,
K. Synlett 1997, 455.
(12) Achiwa, I.; Yamazaki, A.; Achiwa, K. Synlett 1998, 45.
(13) Komarov, I.; Gorichko, M. V.; Kornilov, M. Y. Tetrahedron:
Asymmetry 1997, 8, 435.
(14) Heller, D.; Holz, J .; Borns, S.; Spannenberg, A.; Kempe, R.;
Schmidt, U.; Borner, A. Tetrahedron: Asymmetry 1997, 8, 213.
(15) Yamada, I.; Yamaguchi, M.; Yamagishi, T. Tetrahedron: Asym-
metry 1996, 7, 3339.
(16) Yamada, I.; Fukui, K.; Aoki, Y.; Ikeda, S.; Yamaguchi, M.;
Yamagishi, T. J . Organomet. Chem. 1997, 539, 115.
(17) J iang, Y.; J iang, Q.; Zhang, X. J . Am. Chem. Soc. 1998, 120,
3817.
(18) J iang, Y.; J iang, Q.; Zhu, G.; Zhang, X. Tetrahedron Lett. 1997,
38, 6565.
10.1021/om990391u CCC: $19.00 © 2000 American Chemical Society
Publication on Web 02/19/2000

A Functionalized Ferrocenyldiphosphine Ligand
Organometallics, Vol. 19, No. 6, 2000 995
Sch em e 1. Syn th esis of 1
and co-workers23 have formed substituted ferrocenes by
displacing an ammonium group from the R-position.
However, Hayashi20 found that for ferrocenyl phosphine
(S)-N,N-dimethyl-1-[(R)-2-(diphenylphosphino)ferroce-
nyl]ethylamine (PPFA) the phosphonium cation was
formed preferentially over the desired ammonium cat-
ion. Protection of the phosphine by conversion to the
phosphine oxide allowed for the substitution. Hayashi’s
method involves additional protection and deprotection
steps; therefore, a more convenient route was desired.
Replacing the acetate leaving group with the more
reactive trifluoroacetate leaving group yielded the target
ligand 1.
Reaction of 1 equiv of trifluoroacetic anhydride with
(S)-N,N-dimethyl-1-[(R)-1′,2-bis(diphenylphosphino)fer-
rocenyl]ethylamine (3, BPPFA)20 resulted in a single
product that had characteristic 1H and 31P NMR peaks
similar to those of BPPFAc. This product could not be
isolated, as it rapidly eliminates trifluoroacetic acid to
yield the vinylferrocenylphosphine 5. Sequential addi-
tion of trifluoroacetic anhydride and the crown nucleo-
phile (2) to BPPFA in CH2Cl2 at -15 °C produced 1 in
good yield (Scheme 1). Column chromatography gave
the pure ligand as a yellow powder. The ligand was
synthesized in both resolved and racemic forms using
this method. This new synthetic route eliminates an
isolation step and provides a convenient method that
can be applied to obtain a variety of functionalized
ferrocene ligands.
F igu r e 1. Proposed hydrogen-bonding secondary interac-
tion between a crown ether and an alkylammonium cation.
cations.19 We seek to exploit this hydrogen-bonding
interaction between a crown ether and an ammonium
cation for the creation of a selectivity-directing second-
ary interaction (Figure 1). Ito4,5 previously has synthe-
sized crown ether functionalized ligands focusing on an
electrostatic interaction between an encapsulated metal
cation and a negatively charged substrate. This group
also has synthesized chiral crown ether diphosphite
ligands designed for a similar hydrogen-bonding inter-
action.9
Resu lts a n d Discu ssion
Chiral ferrocenylphosphines are easily substituted at
the R position using (S)-1-[(R)-1′,2-bis(diphenylphosphi-
no)ferrocenyl]ethyl acetate (BPPFAc)20 as a starting
material. Hayashi and co-workers,20,21 Landis and co-
workers,10 and others22 have achieved a variety of side
chain substitutions using this method. Attempts at
using this route for the target ligand, (S)-1-[(R)-1′,2-bis-
(diphenylphosphino)ferrocenyl]ethyl-1-aza-2,3-benzo-15-
crown-5 (1), in a variety of solvents, as well as the neat
reaction in a melt of the aza crown ether (100 °C, excess
1-aza-2,3-benzo-15-crown-5 (2)), failed. Previously Ugi
The racemic ligand 1 was characterized using 1H and
31P NMR, elemental analysis, and LSIMS mass spec-
troscopy. The phosphine resonances at -17.3 and -25.8
ppm are consistent with similar ferrocene ligands made
by this group10 and others.20 The 1H NMR spectrum
displays resonances that are characteristic of this class
of ferrocene ligands; most diagnostic are the methyl and
methine resonances associated with the R-carbon of the
substituted cyclopentadienyl group.
(19) Inoue, Y.; Gokel, G. W. Cation Binding by Macrocycles; Marcel
Dekker: New York, 1990.
(20) Hayashi, T.; Mise, T.; Fukushima, M.; Kagotani, M.; Na-
gashima, N.; Hamada, Y.; Akira, M.; Kagotani, S.; Konishi, M.;
Yamamoto, K.; Kumada, M. Bull. Chem. Soc. J pn. 1980, 53, 1138.
(21) Sawamura, M.; Kitayama, K.; Ito, Y. Tetrahedron: Asymmetry
1993, 4, 1829.
Further characterization of the ligand rac-1 was
achieved by its reaction with a series of metal com-
(22) Yamazaki, A.; Morimoto, T.; Achiwa, K. Tetrahedron: Asym-
metry 1993, 4, 2287.
(23) Ugi, I. K.; Gokel, G. W. J . Chem. Educ. 1972, 49, 294.

996 Organometallics, Vol. 19, No. 6, 2000
Landis et al.
Sch em e 2. P r ep a r a tion of P la tin u m Com p lexes of
r a c-1
Ta ble 1. Selected Bon d Len gth s (Å) a n d Bon d
An gles (d eg) for P tMeI(r a c-1)
Bond Lengths
Pt(1)-C(56)
Pt(1)-I(1)
P(1)-Pt(1)
2.109(4)
2.5835(5)
2.3002(11)
P(2)-Pt(1)
N(37)-C(35)
2.2891(10)
1.488(5)
Bond Angles
C(56)-Pt(1)-I(1)
P(1)-Pt(1)-P(2)
84.0(3)
98.49(3)
P(1)-Pt(1)-I(1)
P(2)-Pt(1)-C(56)
94.47(3)
82.5(3)
Ta ble 2. Cr ysta llogr a p h ic Da ta for P tMeI(r a c-1)
formula
fw
C51H54FeINO4P2Pt
1184.73
yellow transparent prism
0.34 × 0.22 × 0.18
triclinic
F igu r e 2. X-ray crystal structure for PtMeI(rac-1) (8a ).
The displacement ellipsoids are drawn at 50% probability.
Hydrogens have been removed for clarity.
cryst color, habit
cryst size, mm
cryst syst
space group
a, Å
groups bonded to platinum to be disordered and the
mixture of isomers to be essentially 50:50. The coordi-
nation geometry about the Pt atom is distorted square
planar (the angle values range from 82.5(3) to 98.49-
(3)°). The ferrocene moiety shows the characteristic tilt,
with the centers of both rings above the approximate
square plane about platinum. The phosphorus-plati-
num bond lengths (2.3002(11) and 2.2891(10) Å) of
PtMeI(rac-1) are very similar to those of PtMeI(BPP-
FAP)10 (2.3215(10) and 2.2737(10) Å).
Unfortunately, the single-crystal structure of PtMeI-
(rac-1) revealed that the crown ether moiety was twisted
away from the Pt center, unlike the aminophenol group
in the structure of PtMeI(BPPFAP),10 for which the
plane of the aminophenol moiety cantilevers above, and
nearly parallel to, the Pt coordination plane. Rotation
about the C35-N37 bond and a twist of the crown ether
ring would allow the ligand to achieve a structure that
would be more favorable to a secondary interaction.
Such motion may be freely available in solution (vide
infra). A longer alkyl “tether” between the ferrocenyl
rings and the crown ether would allow more conforma-
tional flexibility for positioning of the crown ether.
Currently it is not known whether this flexibility would
be beneficial or not.
P1h
12.4278(5)
13.5881(6)
14.9499(7)
65.940
b, Å
c, Å
â, deg
V, Å3
2297.6(2)
2
1.712
133(2)
1172
Mo KR; 0.710 73
Siemens P4/CCD
58.34
26213
9805
Z
F
T, K
calcd, g cm-3
F(000)
radiation; λ, Å
diffractometer
2θ(max), deg
no. of rflns collected
no. of rflns used
no. of params refined
R, I > 2σ(I)
561
0.0393
0.0775
1.158
Rw
goodness of fit on F2
plexes. The diphosphine rac-1 was reacted with PtMe2-
COD, PtMeICOD, and PtCl2COD to form the ligand
complexes 6-8 (Scheme 2). These reaction products
were characterized by 31P and 1H NMR spectroscopy,
elemental analysis, and, for 6 and 8, X-ray crystal-
lography. Ligation of the added diphosphine was indi-
cated by the appearance of free COD resonances in the
1H NMR. 31P NMR also indicated formation of the metal
complexes, revealing characteristic platinum coupling
patterns10 and coupling between the inequivalent phos-
phorus atoms.
The single-crystal X-ray structure of PtMe2(rac-1) is
similar to that of PtMeI(rac-1). The crystal data are
summarized in the Supporting Information.
Diphosphine rac-1 reacted with PtMeI(COD) in ac-
etone to form a yellow solid. The 1H and 31P NMR
spectra, elemental analysis, and the single-crystal struc-
ture (Tables 1 and 2, Figure 2) are all consistent with
the formulation PtMeI(rac-1). Because the phosphorus
atoms are inequivalent, two geometric isomers are
formed; the major isomer is 8b. In solution these
isomers are present in a 60:40 ratio, as shown by 31P
NMR. A single-crystal structure showed the Me and I
Reaction of rac-1 with [Rh(NBD)2]OTf in acetone led
to the formation of [Rh(NBD)(rac-1)]OTf (9), as indi-
cated by 1H and 31P NMR spectra (Scheme 3). Formation
of the rhodium complex was supported by the appear-
1
ance of free norbornadiene resonances in the H NMR
1
spectrum and by coupling to rhodium (I ) /2) in the
31P NMR spectrum (J P-Rh ) 160 and 162 Hz). Coupling
between the inequivalent phosphorus atoms was also
seen (J P-P ) 26 Hz). This complex decomposed with loss

A Functionalized Ferrocenyldiphosphine Ligand
Organometallics, Vol. 19, No. 6, 2000 997
Sch em e 3. P r ep a r a tion of [Rh (NBD)(r a c-1)]OTf
(9)
Sch em e 4. Rea ction of r a c-1 w ith Allyla m m on iu m
Tr ifla te
of the crown ether over a period of weeks in acetone
solution, presumably due to slow hydrolysis at the
R-carbon.
Rhodium complexes of 1 are catalysts for the hydro-
genation of simple substrates. Quantitative hydrogena-
tion of methyl (Z)-R-acetamidocinnamate, MAC, was
observed with the rhodium complex of 1 generated in
situ from [Rh(NBD)2]OTf and rac-1. Hydrogenation of
MAC using resolved ligands gave unremarkable enan-
tiomeric excesses, 79% ee for (S,R)-1 compared to 69%
ee for (S,R)-BPPFA under similar conditions. Prelimi-
nary rate measurements show the rate of hydrogenation
of MAC with [Rh(NBD)(rac-1)]OTf is slightly faster
(3585 turnovers/h) than with a control catalyst, [Rh-
(NBD)(rac-BPPFA)]OTf (1651 turnovers/h). An attempt
to probe the possible kinetic acceleration of hydrogena-
tion due to secondary interactions with the crown ether
functional group was thwarted by an unexpectedly facile
substitution reaction. Under catalytic conditions, the
hydrogenation of allylammonium triflate as catalyzed
[Rh(NBD)(rac-1)]OTf exhibited erratic kinetic behavior
with very fast initial rates of hydrogen uptake that
slowed substantially during the first few minutes of
reaction before more than 5% of the substrate had been
consumed. Subsequent studies of the reaction of ally-
lammonium triflate with 1 revealed that a rapid sub-
stitution reaction takes place in which allylamine
substitutes for the aza crown ether (Scheme 4).
F igu r e 3. Contour plot of the 2DNOESY spectrum of [Rh-
(NBD)(rac-1)](OTf) in d6-acetone at 24 °C demonstrating
pairwise chemical exchange between vinyl protons (NOE
cross-peaks are of opposite sign and are not shown in this
spectrum).
solution via rotation about the C35-N37 bond and
twisting of the crown ether. This led us to investigate
the solution structures of [Rh(NBD)(rac-1)]OTf (9)
through analysis of the time course of the quantitative
1H 2D NOESY data with the multiconformational
analysis technique, two-dimensional conformation popu-
lation analysis (2DCPA).24-29 This complex was chosen
for solution structure analysis because the norborna-
diene ligand leads to a richer set of NOE cross-peaks
than would be seen for the Pt complex 8a .
2DCPA analysis correlates the observed NOESY
intensities (or, alternatively, the experimental cross-
relaxation rates) at each mixing time to the calculated
NOE data from a set of energetically reasonable trial
models created by molecular mechanics. In contrast to
(24) Casey, C. P.; Hallenbeck, S. L.; Wright, J . M.; Landis, C. R. J .
Am. Chem. Soc. 1997, 119, 9680.
(25) Giovannetti, J . S.; Kelly, C. M.; Landis, C. R. J . Am. Chem.
Soc. 1993, 115, 4040.
(26) Kimmich, B. F. M.; Somsook, E.; Landis, C. R. J . Am. Chem.
Soc. 1998, 120, 10115.
(27) Landis, C. R.; Allured, V. S. J . Am. Chem. Soc. 1991, 113, 9493.
(28) Landis, C. R.; Luck, L. L.; Wright, J . M. J . Magn. Reson., Ser.
B 1995, 109, 44.
Solu tion Str u ctu r e Deter m in a tion of [Rh (NBD)-
(r a c-1)]OTf (9) by Tw o-Dim en sion a l Con for m er
P op u la tion An a lysis (2DCP A). The conformations of
molecules in the solid state may differ from those in
solution, particularly for conformers differing only in
torsion angles. In the case of the Pt complex 8a we
hypothesized that the undesired crystallographic con-
formation could revert to the desired conformation in
(29) Wright, J . M.; Landis, C. R.; Ros, M. A. M. P.; Horton, A. D.
Organometallics 1998, 17, 5031.

998 Organometallics, Vol. 19, No. 6, 2000
Landis et al.
Sch em e 5. Th r ee P la u sible Mech a n ism s of Vin yl Gr ou p Ch em ica l Exch a n ge: (1) NBD Rota tion via a
Squ a sh ed Tetr a h ed r a l Tr a n sition Sta te, (2) Solven t Associa tion Lea d in g to a Tr igon a l Bip yr a m id a l
Tr a n sition Sta te or In ter m ed ia te, a n d (3) In tr a m olecu la r Cr ow n Eth er Coor d in a tion Lea d in g to a Tr igon a l
Bip yr a m id a l Tr a n sition Sta te or In ter m ed ia te
the single, average solution structure approach, 2DCPA
yields the smallest combination of statistically signifi-
cant conformers and their populations. Our analysis
involved three steps: (1) collection of NOESY time
courses, (2) creation of an ensemble of trial conformers
using molecular mechanics, and (3) conformer popula-
tion analysis of the observed NOESY time courses. The
NOESY data collected in acetone-d6 at 24 °C (mixing
times of 300, 450, 600, 900, and 1200 ms) resulted in
positive NOE’s (see the Supporting Information).
The NOESY spectrum of 9 reveals an interesting
chemical exchange process that interconverts two pairs
of NBD vinyl protons (Figure 3). The cross-peaks arising
from chemical exchange have phase opposite to that of
NOE cross-peaks and are readily identified. The mech-
anism of exchange does not involve complete dissocia-
tion/reassociation of the NBD ligand. The chemical
exchange process requires that at least one alkene
remain coordinated throughout the exchange process;
otherwise, exchange among all of the vinyl NBD peaks
would be seen. The simplest process that will yield the
observed exchange pattern is rotation of the NBD ligand
at the metal center via a squashed tetrahedral transi-
tion state (Scheme 5). Alternative mechanisms involve
coordination of either solvent or the crown ether to yield
a five-coordinate intermediate or transition state which
can revert back to the four-coordinate square planar
complex with rotation of the NBD group in either of two
directions. The kinetics of the chemical exchange process
were measured by magnetization transfer using the 1D
DPFGSE-NOE experiments30,31 at several tempera-
tures. These experiments yield an enthalpy of activation
of 8.7 kcal/mol and a 25 cal/(mol K) entropy of activation.
The details of calculations concerning chemical ex-
change in the solution structures and 1D DPFGSE-NOE
experiments are described in the Experimental Section.
The goal of the quantitative analysis of the NOE data
is to determine whether structures suitable to the
secondary interactions are significantly populated in
solution. The primary conformational degree of freedom
corresponds to the twisting along the bond of C35-N37.
Quantitative analysis of the NOE time courses finds
that the most populated conformer has the crown ether
moiety away from the metal site, similar to the crystal-
lographic conformation of PtMeI(rac-1) (8a ), whereas
the less populated conformers have the crown ether
close to the metal center. The fit to the observed data
using two or three conformers yields a statistically
significant better fit than using a single conformer. This
result indicates that more than one conformation of the
complex is required to simulate the NOE time course.
The two best-fit conformers are shown in Figure 4. The
2DCPA method does not use molecular mechanics
energies in finding the conformers that best satisfy the
(30) Stonehouse, J .; Adell, P.; Keeler, J .; Shaka, A. J . J . Am. Chem.
Soc. 1994, 116, 6037.
(31) Stott, K.; Keeler, J .; Van, Q. N.; Shaka, A. J . J . Magn. Reson.
1997, 125, 302.

A Functionalized Ferrocenyldiphosphine Ligand
Organometallics, Vol. 19, No. 6, 2000 999
F igu r e 4. The two best-fit conformers from 2DCPA in side view and top view.
Ta ble 3. En er gies (k ca l/m ol) a n d Tor sion An gles
(d eg) of th e Best-F it Con for m er s
torsion angle
molecular
mechanics Me-C35-N37- C15(Fc)-C14(Fc)-C35-
conformer
energy
Cipso
Me
primary
secondary
tertiary
244
245
265
165
16
-47
-128
-149
-8
empirical NOE data. On the basis of molecular mechan-
ics calculations with the UFF force field, the primary,
secondary, and tertiary conformers have energies within
9, 9, and 29 kcal/mol of the lowest energy conformation,
respectively (see Table 3). The total conformational
space represented in the molecular mechanics ensemble
is represented in Figure 5.
F igu r e 5. Distribution of conformer energies obtained
from molecular mechanics computations.
Con clu sion s
The new benzo-aza crown ether functionalized ferro-
cenylphosphine 1 has been prepared. This ligand reacts
with a series of platinum and rhodium complexes in a
manner similar to that for other diphosphine ligands.

1000 Organometallics, Vol. 19, No. 6, 2000
Landis et al.
for 18 h. The solvent was removed, and degassed benzene was
added. The solution was washed with water, and the water
layer was extracted with benzene (3 × 10 mL). The combined
benzene layers were dried over Na2SO4. The solvent was
removed, yielding an orange oil. The oil was chromatographed
on silica using hexane/CH2Cl2/triethylamine as the eluent. The
fractions containing the product were combined and the
solvent reduced. The product crystallized as a yellow powder
The crystallographic structure of the complex Pt(1)MeI
reveals a near-equal distribution between two geometric
isomers and an orientation of the crown ether that
would not support secondary interactions. However, the
structures of [Rh(1)(NBD)](OTf) in acetone solution,
which were determined by multiconformational analysis
of the NOESY time course, support some population of
conformations in which the benzo-aza crown ether is
capable of engendering secondary interactions. Although
the complex [Rh(1)(NBD)](OTf) is an active hydrogena-
tion catalyst for simple olefins, attempts to effect strong
secondary interactions with allylammonium substrates
were complicated by substititution of the benzo-aza
crown ether by allylamine.
(0.266 g). Yield: 30%. Data: [R]25 ) +275 (CHCl3, c 1.57);
D
1H NMR (CDCl3) δ 1.42 (d, J ) 6.62 Hz, 3H, CHCH3), 2.38
(m, 1H, PhNCH2), 2.80 (td, J ) 10.80, 4.41 Hz, 1H, PhNCH2),
2.96 (m, 1H, NCH2CH2), 3.24 (m, 1H, OCH2), 3.30 (m, 1H,
NCH2CH2), 3.4-4.0 (m, 14H, C5H4, C5H3, and OCH2), 4.07 (m,
2H, C5H4 and C5H3), 4.30 (s, 1H, C5H4 or C5H3), 4.51 (s, 1H,
C5H4 or C5H3), 4.80 (qd, J ) 6.62, 2.57 Hz, 1H, CH3CHN), 6.65
(dd, J ) 8.09, 1.47 Hz, 1H, Ph H), 6.79 (td, J ) 7.54, 1.10 Hz,
1H, Ph H), 6.87 (td, J ) 7.67, 1.47 Hz, 1H, Ph H), 7.1-7.5 (m,
21H, (C6H5)2P and Ph H); 31P NMR (CDCl3) δ -17.3 (s,
(C6H5)2P), -25.8 (s, (C6H5)2P); LR LSIMS MS m+ m/e 847.26.
Anal. Calcd for C50H51NO4P2Fe: C, 70.83; H, 6.06; N, 1.65; P,
7.31; Fe, 6.59. Found: C, 70.74; H, 6.26; N, 1.70; P, 6.05; Fe,
6.95.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations of air-sensitive
compounds were done under nitrogen using either standard
Schlenk techniques or an inert-atmosphere glovebox. All
solvents were distilled under nitrogen using standard drying
techniques. 1H NMR (300 MHz) and 31P{1H} NMR (121 MHz)
spectra were performed on a Bruker AC-300 spectrometer. 1H
spectra were referenced to TMS and 31P spectra to an external
H3PO4 standard. LSIMS mass spectra were recorded with a
VG AutoSpec spectrometer. Elemental analyses was performed
by Desert Analytics Laboratory. Optical rotations were mea-
sured on a Perkin-Elmer 241 Polarimeter. All GC runs were
performed on a HP5890A gas chromatograph using an Alltech
Chirasil-Val column. The compounds 1,12-dichloro-3,6,9-tri-
oxaundecane,32 (S,R)-BPPFA,20 PtMe2(COD),33 PtMeI(COD),33
PtCl2(COD),33 and [Rh(NBD)2]OTf 34 were synthesized accord-
ing to literature procedures. All other reagents were purchased
from Aldrich and used as received.
1-Aza -2,3-ben zo-15-cr ow n -5 (2). The procedure for the
preparation of 2 was based on a procedure by Pedersen.32
2-Aminophenol (29.6 g, 0.271 mol) and 1,12-dichloro-3,6,9-
trioxaundecane (62.5 g, 0.269 mol) were refluxed in n-butyl
alcohol for 18 h under N2. The reaction mixture was cooled to
65 °C, and a solution of 25 g of sodium hydroxide in 25 mL of
water was added. The resulting mixture was refluxed for 10
h. The warm reaction mixture was filtered and the solvent
removed to yield a brown oil. The oil was dissolved in
chloroform and the solution washed with 5% NaOH(aq) (3 ×
20 mL). The chloroform layer was dried over MgSO4 and the
solvent removed. The resulting brown oil was distilled under
vacuum at high temperatures (>200 °C) to give a white solid.
The solid was recrystallized in n-heptane, yielding white
crystals (5.69 g). Yield: 8%. Data: 1H NMR (CDCl3) δ 3.24 (s,
broad 2H, HNCH2), 3.69 (m, 8H, CH2CH2OCH2CH2), 3.78 (t,
2H, HNCH2CH2), 3.84 (m, 2H, PhOCH2CH2), 4.10 (m, 2H,
PhOCH2), 5.13 (s, broad Hz, 1H, HNCH2), 6.57 (dd, J ) 7.72,
1.47 Hz, 1H, Ph H), 6.61 (td, J ) 7.72, 1.47 Hz, 1H, Ph H),
6.75 (dd, J ) 7.72, 1.47 Hz, 1H, Ph H), 6.86 (td, J ) 7.72, 1.47
Hz, 1H, Ph H); 13C NMR (CDCl3) δ 43.20, 68.24, 69.00, 69.53,
69.92, 70.03 (2C), 70.40, 110.24, 111.90, 116.30, 121.81, 139.52,
146.36; EI MS m+ m/e 267.3.
Rea ction of P tMe2(COD) w ith r a c-1. A solution of crude
rac-1 (25.4 mg, 60% 1, 8% 5, 50% 2 by NMR, 1.80 × 10-2 mmol)
in 0.5 mL of d6-acetone was added to a solution of PtMe2(COD)
(10.0 mg, 3.03 × 10-2 mmol) in 0.5 mL of d6-acetone under
nitrogen. Yellow crystals result from slow evaporation of
solvent in a warm water bath. Data: 1H NMR (d6-acetone) δ
0.25 (ddd, 3H, J H-Pt ) 35 Hz, J H-P ) 9 Hz, J H-P ) 9 Hz,
PtCH3), 0.60 (ddd, 3H, PtCH3), 1.38 (d, 3H, J H-H ) 6.9 Hz,
CH3), 2.31 (m, H), 3.20 (m, 2H, CH2O), 3.42 (m, 2H, CH2O),
3.45-3.70 (m, many H, CH2O and Cp), 3.90 (m, 2H, CH2O or
Cp or vinyl COD), 4.05 (m, 2H, CH2O or Cp or vinyl COD),
4.15 (m, 2H, CH2O or Cp or vinyl COD), 4.20 (m, 1H, CH2O or
Cp or vinyl COD), 4.25 (m, 2H, CH2O or Cp or vinyl COD),
4.28 (m, 2H, CH2O or Cp or vinyl COD), 4.55 (m, 1H, CH2O or
Cp or vinyl COD), 5.50 (m, 2H, CH2O or Cp or vinyl COD),
6.03 (q, 1H, J ) 6.9 Hz, CH), 6.60-6.97 (m, 4H, C6H4), 7.04-
8.05 (m, 20H, PPh2); 31P NMR (d6-acetone) δ 21.7 (dd, J P-Pt
)
1880 Hz, J P-P ) 13 Hz), 27.4 (dd, J P-Pt ) 1964 Hz, J P-P ) 13
Hz). Anal. Calcd for C52H57NO4P2FePt: C, 58.10; H, 5.34; N,
1.30. Found: C, 57.85; H, 5.21; N, 1.42.
Rea ction of P tCl2(COD) w ith r a c-1. A solution of rac-1
(10.8 mg, 0.013 mmol) in 0.5 mL of acetone was added
dropwise to a solution of PtCl2(COD) (10.0 mg, 0.026 mmol)
in 0.5 mL of acetone. A pale yellow solid immediately precipi-
tated. The acetone was poured off, and the solid was dried in
vacuo. Repeated elemental analyses of different batches of this
material yielded significantly different results (e.g., % C varied
(3%), presumably due to differing amounts of solvent impuri-
ties. Data: 1H NMR (CD2Cl2) δ 1.37 (d, J ) 7 Hz, 3H, CHCH3),
3.2-4.5 (m, 23H, CH2N, CH2O, and C5H3FeC5H4), 6.77 (t, J )
7 Hz, 1H, CHCH3), 6.85-8.25 (m, 24H, C6H4NO and PC6H5);
31P NMR (CD2Cl2) δ 17.8 (dd, J P-Pt ) 3826 Hz, J P-P ) 10 Hz),
12.03 (dd, J P-Pt ) 3748 Hz, J P-P ) 10 Hz). Anal. Calcd for
C50H51NO4P2FeCl2Pt: C, 53.82; H, 4.61; N, 1.26. Found
(best): C, 52.10; H, 4.37; N, 1.50.
Rea ction of P tMeI(COD) w ith r a c-1. PtMeI(COD) (49.3
mg, 0.11 mmol) and rac-1 (97.0 mg, 0.12 mmol) were placed
in a Schlenk flask under nitrogen. Acetone (10 mL) was added.
The solution was stirred for 1 h. A yellow powder precipitated.
[PtMeI(r a c-1)] was found to be a 60:40 mixture of two isomers
by 31P NMR. Data: 1H NMR (CD2Cl2) δ 0.43 (tt, J Pt-H ) 30
Hz, J P-H ) 7 Hz, 3H, PtCH3 (minor)), 1.10 (tt, J Pt-H ) 30 Hz,
J P-H ) 7 Hz, 3H, PtCH3 (major)), 1.25 (d, J ) 7 Hz, 3H, CHCH3
(major)), 1.57 (d, J ) 7 Hz, 3H, CHCH3 (minor)), 3.20-4.78
(m, 46H, CH2N, CH2O, and C5H3FeC5H4 from both major and
(S,R)-1. (S,R)-BPPFA (0.62 g, 0.99 mmol) was placed in a
Schlenk flask under N2. Trifluoroacetic anhydride (2.5 mL of
a 0.4 M solution in CH2Cl2) was added at -15 °C. Immediately
afterward, a solution of 1-aza-2,3-benzo-15-crown-5 (0.28 g, 1.1
mmol) and triethylamine (0.15 mL, 1.1 mmol) in 3 mL of CH2-
Cl2 was added. The solution was stirred at -15 °C for 1 h,
warmed to room temperature, and stirred at room temperature
minor), 6.70-8.27 (m, 25H, CHCH3, C6H4NO, and PC6H5); 31
P
(32) Pedersen, C. J . J . Am. Chem. Soc. 1967, 89, 7017.
(33) Clark, H. C.; Manzer, L. E. J . Organomet. Chem. 1973, 59, 411-
NMR (CD2Cl2) 8a , δ 22.1 (dd, J P-Pt ) 4327 Hz, J P-P ) 13 Hz),
16.9 (dd, J P-Pt ) 1781 Hz, J P-P ) 13 Hz); 31P NMR (CD2Cl2)
8b. δ 23.7 (dd, J P-Pt ) 1950 Hz, J P-P ) 13 Hz), 18.9 (dd, J P-Pt
428.
(34) Abel, E. W.; Bennett, M. A.; Wilkinson, G. J . Chem. Soc. 1959,
3178.

A Functionalized Ferrocenyldiphosphine Ligand
Organometallics, Vol. 19, No. 6, 2000 1001
) 4357 Hz, J P-P ) 13 Hz). Anal. Calcd for C51H54NO4P2FeIPt:
C, 51.61; H, 4.59; N, 1.18. Found: C, 51.89; H, 5.33; N, 1.00.
Str u ctu r e Deter m in a tion of P tMeI(r a c-1). Single crys-
tals were grown from acetone-d6 at room temperature. The
crystal data are summarized in Table 2.
A total of 26 213 reflections with 2θ < 58° was collected on
a Siemens P4/CCD diffractometer using graphite-monochro-
mated Mo KR radiation.35,36 The structure was solved by direct
methods and refined by full-matrix least squares on F2 values
with hydrogens riding.37 Non-hydrogen atoms were refined
with anisotropic displacement parameters. The final R and Rw
factors were 0.0393 and 0.0775, respectively, for 9805 reflec-
tions (I < 2σ(I)). The molecular structure is shown in Figure
2, along with a partial numbering scheme. Selected bond
lengths and bond angles are given in Table 1. Fractional
coordinates and additional crystallographic data can be found
in the Supporting Information.
Str u ctu r e Deter m in a tion of P tMe2(r a c-1). Single crys-
tals were grown by slow evaporation of acetone-d6 in a warm
water bath. The crystal data are summarized in the Support-
ing Information.
A total of 30 607 reflections with 2θ < 57° was collected on
a Siemens P4/CCD diffractometer using graphite-monochro-
mated Mo KR radiation.35,36 The structure was solved by direct
methods and refined by full-matrix least squares on F2 values
with hydrogens riding.37 Non-hydrogen atoms were refined
with anisotropic displacement parameters. The final R and Rw
factors were 0.0388 and 0.1014, respectively, for 10 615
reflections (I < 2σ(I)).
2H, CH2OPh), 4.22 (m, 1H, Cp H), 4.27 (m, 1H, Cp H), 4.46
(m, 2H, Cp H), 4.54 (m, 1H, NBD vinylH), 4.56 (m, 2H, Cp H),
4.79 (m, 2H, NBD vinyl H and Cp H), 4.90 (m, 1H, NBD vinyl
H), 5.14 (q, J ) 6.9 Hz, 1H, CHCH3), 6.95 (m, 1H, NC6H4O),
7.00 (m, 2H, NC6H4O), 7.08 (m, 1H, NC6H4O), 7.37-8.14 (m,
20H, P(C6H5)2); 31P NMR (d6-acetone) δ 29.7 (dd, J P-Rh ) 160
Hz, J P-P ) 26 Hz), 26.6 (dd, J P-Rh ) 162 Hz, J P-P ) 26 Hz).
Hyd r ogen a tion of MAC. All hydrogenations were per-
formed with a gas uptake apparatus. A typical experiment was
as follows: a 4 mL solution of 1:1 [Rh(NBD)2]OTf and 1 or
BPPFA (2.55 × 10-4 M) and 400 equiv of MAC in freshly
distilled THF were added to a round-bottom flask. Three
freeze-pump-thaw cycles were effected, and the solution was
allowed to equilibrate to 25 °C while being stirred for 10 min.
The gas was allowed to equilibrate with the solvent at 25 °C
for 1 min, and the uptake of gas was monitored over a period
of about 1 h. Upon exposure to hydrogen (700 Torr) the solution
changed from yellow to orange-red.
Kin etic Stu d ies of NBD Rota tion of [Rh (NBD)(r a c-1)]-
OTf (9). A 13 mM solution of 9 was prepared in acetone-d6.
Kinetic experiments were performed on a Varian Inova 500
MHz spectrometer equipped with a pulsed field gradient unit
and a triple-resonance (1H-13C-X) probe with an actively
shielded z gradient. The relaxation delay was set to 2 s. The
1D DPFGSE-NOE spectra were collected at 25, 35, 45, 55, 65,
75, 85, 95, 105, 115, 125, 135, 145, 155, 165, 175, 185, 195,
205, and 215 ms mixing time and at 1, 6, 11, 16, 21, 26, 31,
and 36 °C. Each spectrum was collected with eight scans. The
vinyl NBD resonance at 4.9 ppm was selected for excitation.
Integrated intensities were used for kinetic calculations. The
simplex algorithm of Microsoft Excel was used to fit eqs 1 and
2, where I1 and I2 are integrated intensities of the excited peak
and the corresponding peak, respectively, A is half of the
empirical integrated intensity at t ) 0, k is the rate constant,
t is the mixing time, and R1 and R2 are spin-lattice relaxation
rates of the excited peak and its exchange partner, respec-
tively.
R ea ct ion of Allyla m m on iu m Tr ifla t e w it h r a c-1
(Sch em e 4). The reaction was carried out in an NMR tube.
Solid allylammonium triflate (6.1 mg, 0.029 mmol) was added
into a solution of rac-1 (21.5 mg, 0.025 mmol) in 1 mL of CD2-
1
Cl2. The solution was left for /2 h. Allylammonium triflate only
dissolves in CD2Cl2 upon reaction. Data: 1H NMR spectrum
shows a mixture of the free crown ether and the amine of
1
Scheme 4; H NMR (CDCl3) of the free crown ether and the
amine δ 1.57 (d, J ) 6.80 Hz, 3H, CHCH3 of amine), 3.03 (two
AB quartets of ABX pattern, J ) 13, 7.5, 5.3 Hz, 2H, NCH2-
CHdCH2 of amine), 3.21 (t, J ) 5.15 Hz, 2H, H2NCH2 of crown
ether), 3.6-3.7 (m, 8H, CH2CH2OCH2CH2 of crown ether), 3.77
(m, 2H, H2NCH2CH2 of crown ether and 3H of C5H4 or C5H3
of amine), 3.82 (m, 2H, PhOCH2CH2 of crown ether), 4.08 (m,
2H, PhOCH2 of crown ether), 4.16 (m, 1H, C5H4 or C5H3 of
amine), 4.25 (m, 1H, C5H4 or C5H3 of amine), 4.36 (m, 1H,
HNCHCH3), 4.46 (m, 1H, C5H4 or C5H3 of amine), 4.56 (m,
1H, C5H4 or C5H3 of amine), 4.89 (d, J ) 17 Hz, CH2dCH of
amine), 5.00 (d, J ) 10 Hz, CH2dCH of amine), 5.2 (m, 1H,
HNCH2CHdCH2), 6.58 (dd, J ) 7.7, 1.3 Hz, 1H, PhH of crown
ether), 6.63 (td, J ) 7.5, 1.5 Hz, 1H, Ph H of crown ether),
6.77 (dd, J ) 7.9, 1.4 Hz, 1H, Ph H of crown ether), 6.86 (td,
J ) 7.7, 1.5 Hz, 1H, Ph H of crown ether), 7.0-7.8 (m, 20H,
Ph H of amine); 31P NMR (CDCl3) δ -16.3 (s, (C6H5)2P), -25.7
(s, (C6H5)2P) (31P NMR (CDCl3) of rac-1 δ -23.9 (s, (C6H5)2P),
75.5 (s, (C6H5)2P)).
Rea ction of [Rh (NBD)2]OTf a n d r a c-1. A solution of
rac-1 (9.9 mg, 1.2 × 10-2 mmol) in 0.5 mL of d6-acetone was
added dropwise to a solution of [Rh(NBD)2]OTf (5.2 mg, 1.2 ×
10-2 mmol) in 0.5 mL of d6-acetone under nitrogen, resulting
in a red solution. The product could not be isolated. Data: 1H
NMR (acetone-d6) δ 1.47 (d, J ) 6.9 Hz, 3H, CHCH3), 1.56 (m,
2H, NBD CH2), 3.16 (m, 1H, CH2N), 3.41-3.83 (m, 10H,
OCH2CH2), 3.87 (m, 2H, CH2CH2OPh), 3.91 (m, 1H, OCH2),
4.05 (m, 2H, NBD CH), 4.10 (m, 1H, NBD vinyl H), 4.20 (m,
I1 ) A(1 + exp(-2kt))(exp(-R1t))
I2 ) A(1 - exp(-2kt))(exp(-R2t))
(1)
(2)
Deter m in a tion of th e Solu tion Str u ctu r e of [Rh (NBD)-
(r a c-1)]OTf (9). A 13 mM solution of [Rh(NBD)(rac-1)]OTf
(9) was prepared in acetone-d6. All 2DNOESY data sets were
acquired at 24 °C on a Varian UNITY 500 MHz spectrometer.
1H DQF -COSY NMR Exp er im en ts. The spectrum was
measured using a phase sensitive DQF-COSY pulse se-
quence38,39 with an initial homospoil pulse and States-
Haberkorn-Ruben phase cycling40 at 24 °C. A total of 512
increments of 8 scans was collected with 2048 real points in
t2. The relaxation delay was set to 2.3 s. The FELIX NMR
software package was used to transform and process data. The
t1 dimension was zero-filled to 1024 points, and the t2 dimen-
sion was zero-filled to 4096 points. A linear prediction scheme
was used to correct the first point and generate points at the
tails of the FID, extending data by one-third. Gaussian line
broadening and a skewed sine-squared bell function were
applied to the FID.
1H NOESY NMR Exp er im en ts. Spectra were collected at
300, 450, 600, 900, and 1200 ms mixing time using a phase-
sensitive NOESY pulse sequence41 with an initial homospoil
pulse and States-Haberkorn-Ruben phase cycling.40 Spectra
were obtained at 24 °C in acetone-d6. A total of 512 increments
(38) Piantini, U.; Sorensen, O. W.; Ernst, R. R. J . Am. Chem. Soc.
1982, 104, 6800.
(39) Rance, M.; Sorenson, O. W.; Bodenhausen, G.; Wagner, G.;
Ernst, R. R.; Wuthrich, K. Biochem. Biophys. Res. Commun. 1983, 117,
479.
(35) SMART Software Reference Manual; Siemens Analytical X-ray
Instruments: Madison, WI 53719-1173, 1994.
(36) SMART Software Reference Manual; Siemens Analytical X-ray
Instruments: Madison, WI 53719-1173, 1995.
(37) Sheldrick, G. M. SHELXTL VERSION 5 Software Manual;
Siemens Analytical X-ray Instruments: Madison, WI 53719-1173,
1995.
(40) States, D. J .; Haberkorn, R. A.; Ruben, D. J . J . Magn. Reson.
1982, 48, 286.
(41) Macura, S.; Ernst, R. R. Mol. Phys. 1980, 41, 95.

1002 Organometallics, Vol. 19, No. 6, 2000
Landis et al.
of 8 scans was collected with 2048 real points in t2. The
relaxation delay was set to 2.3 s. The FELIX NMR software
package was used to transform and process data. The t1
dimension was zero-filled to 1024 points, and the t2 dimension
was zero-filled to 4096 points. A linear prediction scheme was
used to correct the first point and generate points at the tails
of the FID, extending the data by one-third. A Kaiser window
function and exponential line broadening were applied to the
FID to reduce apodization artifacts. Low-order polynomial
baseline corrections were applied to each dimension. The
baselines were corrected again using FACELIFT from the
National Magnetic Resonance Facility at Madison, WI, and
the deconvolution facility in the FELIX program. Diagonal
peak volumes at zero mixing times were estimated by fitting
the observed diagonal peaks to an exponential function. For
off-diagonal peaks, the intensities of the two symmetry-related
peaks were averaged prior to normalization. Random numbers
spanning the estimated noise level were generated and used
to represent the intensities of “unseen” cross-peaks.
Gen er a tion of Sta tic Mod el Con for m er s. The trial
structures, totalling 343 conformers, were generated by con-
former searching using CERIUS2 and the Universal force field
(UFF). Atom type Fe2+2 was introduced for Fe in the
ferrocenyl group, and a harmonic angle constraint for the Cp-
Fe-Cp angle (force constant ) 1000 kcal/mol Å) was applied
to prevent large distortions of the ferrocene during the
searching. The starting conformer was adapted from the X-ray
structure of 8a and was well-minimized to a gradient RMS of
0.01 kcal/mol Å. Two torsion angles (Me-C35-N37-Cipso and
C15(Fc)-C14(Fc)-C35-Me) were systematically varied over
360° in the conformer searching. Hundreds of energy mini-
mized conformers were obtained. Application of clustering
algorithms reduced the number of conformers to 343 unique
structures. The energy of each conformer was measured with
the electrostatic term turned off.
in solution according to a fast conformer exchange model.
Significance testing was performed using Hamilton’s F test
on every possible combination of the 10 best fits of the singles,
doubles, and triples.
The populations obtained by 2DCPA depend on the relative
weightings of the NOE data. In particular, the extent to which
reproduction of absent NOE peaks is included in the data
analysis dramatically effects the absolute value of the R factor
(and may influence relative conformer populations). A subset
(296 data points) of the 496 total points was treated as absent
peaks. The contribution of absent peaks to the error function
was calculated as (Icalcd - Iobsd)2/100. For all other peaks, the
contribution to the error function was (Icalcd - Iobsd)2.
The best single-conformer fit to the data exhibits the R
factor 0.896. The best double-conformer fitting shows the
lowest R factor (R ) 0.868) with fractional populations of 0.61
and 0.39, while the three-conformer fitting yields an R factor
of 0.857 with fractional populations of 0.54, 0.33, and 0.13.
Application of the F test at the 95% confidence level, excluding
the absent peak data points, indicates that it is statistically
significant to use three rather two or one conformers in fitting
the data but that four conformations do not lead to a
significantly better fit relative to three conformations.
Ack n ow led gm en t. This work was supported by a
grant from the Department of Energy, Basic Energy
Sciences Division. Support for the development of the
2DCPA algorithms and code development was provided
by the National Science Foundation. E.S. thanks the
DPST project of Thailand for a scholarship. We grate-
fully acknowledge the assistance of Dr. Charlie Fry with
the collection of NMR data. The NMR instrumentation
used in this work was supported by grants from the NSF
(Grant Nos. CHE 8813550, CHE 9629688, and CHE
9208463) and the NIH (Grant Nos. 1 S10 RR04981-01,
1 S10 RR08389-01). The crystallographic structure
determination was performed by Dr. Doug Powell on
instrumentation supported by a grant from the NSF
(Grant No. CHE 9709005).
Con for m er P op u la tion An a lysis of th e Sta tic En -
sem ble. Conformer population analysis proceeds by computing
the full NOE time course for each conformer in the trial set
and then finding the most parsimonious set of conformers that
best reproduce the observed data. The 2DCPA program was
modified to accommodate the chemical exchange process
according to a two-site exchange model. Known distances
between pairs of protons (bridge CH2-bridgehead CH of
norbornadiene and some ortho-meta and meta-para phenyl
protons) were used for calibration of the rotational correlation
time (τ1 ) 0.2549 ns). The static ensemble was exhaustively
fitted with each possible single, double, and triple conformer
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for PtMeI(rac-1) and PtMe2(rac-1), NMR spectra of 8 and
9, and input files and results pertaining to the 2DCPA analysis
of solution NOE data. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM990391U