Ruthenium complexes of the 1,4-bis(diphenylphosphino)-1,3-butadiene-bridged diphosphine 1,2,3,4-Me4-NUPHOS: Solvent-dependent interconversion of four- and six-electron donor coordination and transfer hydrogenation activity

Article abstract of DOI:10.1021/om020861b

In chloroform, [RuCl₂(nbd)(py)₂] (1) (nbd = norbornadiene; py = pyridine) reacts with 1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me₄-NUPHOS) to give the dimer [Ru₂Cl₃(η⁴-1,2,3,4-Me₄-NUPHOS) ₂]Cl (2a), whereas, in THF [RuCl₂(1,2,3,4-Me₄-NUPHOS)(py)₂] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1 mixture of two noninterconverting isomers, the major with C₁ symmetry and the minor with either C_s or C₂ symmetry. A single-crystal X-ray analysis of [Ru₂Cl₃(η⁴-1,2,3,4-Me₄-NUPHOS) ₂] [SbF₆] (2b), the hexafluoroantimonate salt of 2a, revealed that the diphosphine coordinates in an unusual manner, as a η⁴-six-electron donor, bonded through both P atoms and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-ethylenediamine (en) in THF to afford [RuCl₂(1,2,3,4-Me₄-NUPHOS)(en)] (4), which rapidly dissociates a chloride ligand in chloroform to give [RuCl(η⁴-1,2,3,4-Me₄-NUPHOS)(en)] [Cl] (5a). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent equilibrium, and in THF 5a readily adds chloride to displace the η²-interaction and re-form 4. A single-crystal X-ray structure determination of [RuCl(η⁴-1,2,3,4-Me₄-NUPHOS)(en)]-[ClO₄] (5b) confirmed that the diphosphine coordinates in an η⁴-manner as a facial six-electron donor with the η²-coordinated double bond occupying the site trans to chloride. The η⁴-bonding mode can be readily identified by the unusually high-field chemical shift associated with the phosphorus atom adjacent to the η²-coordinated double bond. Complexes 2a, 2b, 4, and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all cases, catalysts formed from precursors 2a and 2b are markedly more active than those formed from 4 and 5a.

Full text of DOI:10.1021/om020861b

1452
Organometallics 2003, 22, 1452-1462
Ru th en iu m Com p lexes of th e
1,4-Bis(d ip h en ylp h osp h in o)-1,3-bu ta d ien e-Br id ged
Dip h osp h in e 1,2,3,4-Me4-NUP HOS: Solven t-Dep en d en t
In ter con ver sion of F ou r - a n d Six-Electr on Don or
Coor d in a tion a n d Tr a n sfer Hyd r ogen a tion Activity
Simon Doherty,*,† Colin R. Newman,† Christopher Hardacre,†
Mark Nieuwenhuyzen,† and J ulian G. Knight*,‡
School of Chemistry, The Queen’s University of Belfast, David Keir Building,
Stranmillis Road, Belfast, BT9 5AG, U.K., and School of Natural Sciences, Chemistry, Bedson
Building, The University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K.
Received October 16, 2002
In chloroform, [RuCl2(nbd)(py)2] (1) (nbd ) norbornadiene; py ) pyridine) reacts with 1,4-
bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-butadiene (1,2,3,4-Me4-NUPHOS) to give the
dimer [Ru2Cl3(η4-1,2,3,4-Me4-NUPHOS)2]Cl (2a ), whereas, in THF [RuCl2(1,2,3,4-Me4-
NUPHOS)(py)2] (3) is isolated as the sole product of reaction. Compound 2 exists as a 4:1
mixture of two noninterconverting isomers, the major with C1 symmetry and the minor with
either Cs or C2 symmetry. A single-crystal X-ray analysis of [Ru2Cl3(η4-1,2,3,4-Me4-
NUPHOS)2][SbF6] (2b), the hexafluoroantimonate salt of 2a , revealed that the diphosphine
coordinates in an unusual manner, as a η4-six-electron donor, bonded through both P atoms
and one of the double bonds of the butadiene tether. Compounds 2a and 3 react with 1,2-
ethylenediamine (en) in THF to afford [RuCl2(1,2,3,4-Me4-NUPHOS)(en)] (4), which rapidly
dissociates a chloride ligand in chloroform to give [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)][Cl]
(5a ). Complexes 4 and 5a cleanly and quantitatively interconvert in a solvent-dependent
equilibrium, and in THF 5a readily adds chloride to displace the η2-interaction and re-form
4. A single-crystal X-ray structure determination of [RuCl(η4-1,2,3,4-Me4-NUPHOS)(en)]-
[ClO4] (5b ) confirmed that the diphosphine coordinates in an η4-manner as a facial six-
electron donor with the η2-coordinated double bond occupying the site trans to chloride. The
η4-bonding mode can be readily identified by the unusually high-field chemical shift associated
with the phosphorus atom adjacent to the η2-coordinated double bond. Complexes 2a , 2b, 4,
and 5a form catalysts that are active for transfer hydrogenation of a range of ketones. In all
cases, catalysts formed from precursors 2a and 2b are markedly more active than those
formed from 4 and 5a .
In tr od u ction
merizations,4 inter- and intramolecular Heck reactions,5
palladium-catalyzed hydrocyanations,6 and the rhodium-
catalyzed enantioselective isomerization of allylic
amines.7 The success of BINAP as a supporting chiral
auxiliary has prompted efforts to develop alternative
structurally related conformationally flexible biaryl
atropisomeric diphosphines, the majority of which are
based on a 6,6′-substituted biphenyl tether such as (R)-
MeO-BIPHEP and (R)-BIPHEMP8 or a similar four-
carbon sp2-hydridized tether as in BINAPFu,9 (R)-
BITIANP,10 and (+)-TMBTP (Chart 1).11 A related
conformationally flexible diphosphine 2,2′-bis(diphe-
Since its discovery, 2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl (BINAP) has proven to be a highly versatile
chiral auxiliary for a wide range of platinum group
metal catalyzed transformations1 including ruthenium-
and rhodium-catalyzed hydrogenations,2 electrophilic
alkylations,3 ruthenium-catalyzed ring-opening poly-
* Corresponding author.
† The Queen’s University of Belfast.
‡ The University of Newcastle upon Tyne.
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10.1021/om020861b CCC: $25.00 © 2003 American Chemical Society
Publication on Web 02/26/2003

Ru Complexes of 1,2,3,4-Me4-NUPHOS
Organometallics, Vol. 22, No. 7, 2003 1453
nylphosphino)-1,1′-biphenyl (BIPHEP) has recently been
employed to catalyze a range of achiral transformations
including the palladium-catalyzed cross coupling of sec-
BuMgCl with vinyl halides, the rhodium-catalyzed
Michael addition of boronic acids to enones, and the
palladium-catalyzed amination of aryl bromides.12 Given
the impact of this diphosphine type in platinum-group
metal catalysis we became interested in developing a
new methodology for the synthesis of related diphos-
phines and have recently prepared an entirely new class
of four-carbon-bridged diphosphine, NUPHOS, by lib-
erating zirconacyclopentadienes with chlorodiphenylphos-
phine.13 Preliminary studies have revealed that palla-
dium complexes of NUPHOS type diphosphines are
highly active for the cross coupling of sec-butylmagne-
sium bromide with bromobenzene, with activities far
superior to those obtained with BINAP- and dppf-based
catalysts.
of S-XylBINAP and S-DAIPEN (DAIPEN ) S-1,1-di-4-
anisyl-2-methyl-1,2-ethylenediamine), resulting in quan-
titative hydrogenation of a range of 2′, 3′-, and 4′-
substituted acetophenones to give the corresponding
secondary alcohols with consistently high stereoselec-
tivity.14 In a particularly elegant extension of his early
studies on asymmetric hydrogenation, Noyori showed
that ruthenium complexes of the conformationally flex-
ible BIPHEP, when activated by addition of a chiral
diamine such as S,S-DPEN, catalyze the enantioselec-
tive hydrogenation of acetonaphthone to give (R)-1-(1-
naphthyl)ethanol in 92% ee,15 a significant improvement
on the performance of the corresponding racemic-BINAP
complex.16 The clear similarity between the basic skel-
etal framework of NUPHOS and that of BINAP and
BIPHEMP and its derivatives, namely, two diphe-
nylphosphino groups connected by a four-carbon sp2-
hybridized tether, prompted us to investigate its ruthe-
nium-based coordination chemistry with the aim of
preparing potential catalyst precursors of the type
[RuCl2(NUPHOS)(diamine)]. Herein we report our ini-
tial studies toward this objective, which include the
synthesis of [Ru2Cl2(η4-1,2,3,4-Me4-NUPHOS)2]Cl, in
which the diphosphine coordinates in an unusual η4-
manner as a six-electron donor through both P atoms
and one of the double bonds of the butadiene tether, its
reaction with 1,2-ethylenediamine (en) to give [RuCl2-
(1,2,3,4-Me4-NUPHOS)(en)], which exists in a solvent-
dependent dissociative equilibrium with [RuCl(η4-1,2,3,4-
Me4-NUPHOS)(en)]Cl, and the results of preliminary
transfer hydrogenation studies for each of the newly
prepared ruthenium complexes.
Ch a r t 1
Resu lts a n d Discu ssion
Syn th esis a n d Sp ectr oscop ic Ch a r a cter iza tion
of Ru th en iu m (II) Com p lexes. Our first indication
that the coordination chemistry of 1,2,3,4-Me4-NUPHOS
(1,4-bis(diphenylphosphino)-1,2,3,4-tetramethyl-1,3-buta-
diene) did not parallel exactly that of BINAP arose
during the attempted preparation of [RuCl2(1,2,3,4-
Me4-NUPHOS)(diamine)]. Following the procedure de-
scribed by Noyori for the synthesis of [RuCl2(BINAP)-
(diamine)],16a treatment of oligomeric [RuCl2(1,2,3,4-
Me4-NUPHOS)(dmf)n] with a slight excess of 1,2-
ethylenediamine gave a mixture of several products, of
which the desired compound was only a minor compo-
nent. In the search for an alternative route, we noted
that Bergens recently reported a versatile and conve-
nient method for the synthesis of diphosphine-diamine
complexes that involves treatment of a dichloromethane
solution of [RuCl2(nbd)(py)2] (1) (nbd ) norbornadiene;
py ) pyridine) with a variety of diphosphines to give
[RuCl2(diphosphine)(py)2], followed by substitution of
The challenging problem of asymmetric hydrogena-
tion of simple unfunctionalized ketones has recently
been accomplished using BINAP diamine complexes of
the type [RuCl2(BINAP)(1,2-diamine)], the combination
(8) (a) Schmid, R.; Firicher, J .; Cereghetti, M. Schonholzer, P. Helv.
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Asymmetry 2001, 12, 263. (b) Andersen, N. G.; Parvez, M.; Keay, B.
A. Org. Lett. 2000, 2, 2817.
(10) (a) Tietze, L. F.; Thede, K. Sannicolo, F. Chem. Commun. 1999,
1811. (b) Benincori, T.; Brenna, E.; Sannicolo, F.; Trimarco L.;
Antognazza, P.; Cesarotti, E.; Demartin, F.; Pilati, T. J . Org. Chem.
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(b) Noyori, R.; Ohkuma, T. Angew Chem., Int. Ed. 2001, 40, 40. (c)
Ohkuma, T.; Koizumi, M.; Doucet, H.; Pham, T.; Kozawa, M.; Murata,
K.; Katayama, E.; Yokozawa, T.; Ikariya, T.; Noyori, R. J . Am. Chem.
Soc. 1998, 120, 13529.
(15) (a) Mikami, K.; Korenaga, T.; Ohkuma, T.; Noyori, R. Angew.
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M.; Ohkuma, T.; Pham, T.; Noyori, R. Angew. Chem., Int. Ed. 1999,
38, 495.
(16) (a) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa,
M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem.,
Int. Ed. 1998, 37, 1703. (b) Ohkuma, T.; Doucet, H.; Pham, T.; Mikami,
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P. A. Clegg, W. Organometallics 2002, 21, 1383.

1454 Organometallics, Vol. 22, No. 7, 2003
Doherty et al.
Sch em e 1
Ch a r t 2
pyridine with a diamine at room temperature.17 Fol-
lowing this protocol, addition of 1,2,3,4-Me4-NUPHOS
to a dichloromethane or chloroform solution of 1 resulted
in rapid and quantitative formation of [Ru2Cl3(η4-
1,2,3,4-Me4-NUPHOS)2]Cl (2a ), identified initially by a
molecular ion peak at 1268 amu in the electrospray
mass spectrum and subsequently by a single-crystal
X-ray study of its hexafluoroantimonate salt (Scheme
1). In contrast, 1 reacts with 1,2,3,4-Me4-NUPHOS at
50 °C in THF to give predominantly [RuCl2(1,2,3,4-Me4-
NUPHOS)(py)2] (3) together with minor amounts of 2a
(typically < 5%). The 31P{1H} NMR spectrum of 2a
(Figure 1) contains two distinct sets of resonances
consistent with a mixture of two noninterconverting
F igu r e 1. 31P{1H} NMR spectrum of 2a showing the
disparate chemical shifts of the two types of phosphorus
atoms in the major ([) and minor (b) isomers (CDCl3, 298
K). A second set of doublets of low intensity (2, < 1%)
probably belong to the third possible isomer.
isomers, the minor of which appears as a single AX spin
2
system (b, minor isomer, δ 86.8 and 10.5 ppm; J PP
)
50.2 Hz) and the major of which can be treated as two
independent AX spin systems ([, major isomer, δ ) 87.6
and 13.3; 83.4 and 6.4 ppm; 2J PP ) 49.8 Hz). The methyl
recognized as typical of this coordination mode and
belonging to the phosphorus donor adjacent to the
coordinated double bond. Prior to this Pathak reported
disparate chemical shifts for the two phosphorus nuclei
in [RuCp(BINAP)][CF3SO3], in which the BINAP coor-
dinates in a similar η4-six-electron manner.19 Although
high-field carbon-13 chemical shifts provide an indica-
tion of this unusual bond type, Pregosin recommends
the use of long-range correlation spectroscopy to un-
equivocally assign resonances associated with the car-
bon atoms of the coordinated double bond.18 In the case
of compound 2a , long-range correlation spectroscopy
identified three doublets of doublets at δ 89.6, 88.6, and
87.0 ppm and a further three doublets at δ 52.3, 51.4,
1
region of the H NMR spectrum of 2a contains a set of
eight equal intensity signals for the major isomer and
a set of four equal intensity signals for the minor isomer.
Integration of these resonances gave an isomeric ratio
close to 4:1, which is consistent with that obtained from
the 31P NMR spectrum. Three of the doublets in the 31P-
{1H} NMR spectrum of 2a appear at unexpectedly high
field, in the region associated with an uncoordinated
diphosphine, which is consistent with η4-coordination
as a six-electron donor through both phosphorus atoms
and one of the double bonds of the butadiene tether.
Pregosin has recently identified and structurally char-
acterized a number of related ruthenium complexes in
which MeO-BIPHEP and BINAP coordinate in an η4-
manner.18 In each case, the 31P{1H} NMR spectrum
contains an unexpectedly low-frequency resonance, now
(18) (a) Feiken, N.; Pregosin, P. S.; Trabesinger, G.; Albinati, A.;
Evoli, G. L. Organometallics 1997, 16, 5756. (b) Feiken, N.; Pregosin,
P. S.; Trabesinger, G.; Scalone, M. Organometallics 1997, 16, 537.
(19) Pathek, D. D.; Adams, H.; Bailey, N. A.; King, P. J .; White, C.
J . Organomet. Chem. 1994, 479, 237.
(17) Akotsi, O. M.; Metera, K.; Reid, R. D.; McDonald, R.; Bergens,
S. H. Chirality 2000, 12, 514.

Ru Complexes of 1,2,3,4-Me4-NUPHOS
Organometallics, Vol. 22, No. 7, 2003 1455
b), two high-field 13C signals for C1 and C2, and only
1
two doublets and two singlets in the H NMR spectrum
for the methyl groups (Figure 2, b). Clearly, based on
the pattern of resonances in the 31P{1H} and 13C{1H}
NMR spectra of 2a , 1 reacts with 1,2,3,4-Me4-NUPHOS
to give a mixture of two isomers, 2a I and either 2a II or
2a III. An additional low-intensity (<1%) AX spin sys-
tem (2, Figure 1) is consistent with the formation of a
minor amount of the third possible isomer, although at
this stage we cannot confidently assign the two AX spin
systems to their respective isomers, 2a II and 2a III.
Under the same conditions as those used to form 2a ,
rac-BINAP reacts with [RuCl2(nbd)(py)2] in chloroform
to afford [RuCl2(rac-BINAP)(py)2] with no evidence for
the formation of the corresponding [Ru2Cl3(rac-BINAP)2]-
Cl, even after heating at reflux overnight. Thus, 1,2,3,4-
Me4-NUPHOS demonstrates a much greater propensity
to coordinate in a η4-manner than does BINAP, possibly
because of the greater conformational flexibility of the
four-carbon tether in the former. The stability of 2a in
refluxing chloroform was confirmed by monitoring the
thermolysis of a sample by 31P NMR spectroscopy in the
presence of an internal standard containing triph-
enylphosphine in chloroform. This experiment demon-
strates that formation of an unobserved paramagnetic
Ru(III) dimer is unlikely since the 2a :PPh3 ratio re-
mained constant while the sample was heated at 55 °C
for 14 h. The stability of [RuCl2(rac-BINAP)(py)2] with
respect to oxidation is not surprising since the ther-
molysis of this compound was conducted under an inert
atmosphere. For comparison, [RuCl2(BINAP)(bipy)] (bipy
) 2,2′-bipyridine) has recently been reported to undergo
aerobic oxidation in methanol to give the monoxide
[RuCl(BINAPO)(bipy)][PF6].20
F igu r e 2. 13C-1H long-range correlation for 2a showing
the cross-peaks arising from 2J (C-H) and 3J (C-H) cou-
pling between the η2-coordinated sp2-hybridized carbons
and the protons attached to the neighboring methyl groups
([, 4 doublets + 4 singlets associated with the major
isomer, 2a I; b, 2 doublets and 2 singlets associated with
the minor isomer, either 2a II or 2a III).
X-r ay Str u ctu r e of [Ru 2Cl3(1,2,3,4-Me4-NUP HOS)2]-
[SbF 6] (2b). In view of the unusual coordination mode
of 1,2,3,4-Me4-NUPHOS a single-crystal X-ray study of
2b, prepared by exchange of the chloride in 2a with
SbF6, was undertaken to provide precise details on the
nature of the metal-ligand bonding. Since the crystal
quality was poor, discussion of the structure has been
limited to a description and comparison of the most
important features. The molecular structure of 2b is
shown in Figure 3, a selection of bond lengths and
angles is listed in Table 1, and crystal data are provided
in Table 5. The most notable feature of this structure
is the six-electron donor diphosphine, which coordinates
to ruthenium through both phosphorus atoms and one
of the double bonds of the butadiene tether. Three
chlorides asymmetrically bridge the two ruthenium
atoms to complete a distorted octahedral coordination
geometry. The Ru(µ-Cl)Ru angles (average 86.0°) are
similar to those in the diruthenium monoalkylidene [Ru-
(dcypb)(µ-Cl)3(dcypb)(CHCHdCMe2)]21a and the diru-
thenium cation [{(tBu2PCH2PtBu2CHPh)Ru}2(µ-Cl)3]-
[Cl].21b The Ru-C bond lengths lie between 2.17(2) and
2.22(2) Å and are comparable to those previously
and 51.3 ppm, which we confidently assign to the six
carbon atoms associated with the three magnetically
nonequivalent η2-coordinated carbon-carbon double
bonds of the two noninterconverting isomers (vide infra).
Each of these carbon-13 signals has either two or three
cross-peaks arising from two- and three-bond J (C-H)
coupling constants to the protons of neighboring methyl
groups (Figure 2). The three C1 resonances appear as
doublets, and each clearly correlates with two sets of
methyl protons, a singlet and doublet, whereas the three
doublets of doublets corresponding to C2 each correlate
with three sets of methyl protons: two singlets and a
doublet. Similarly, this long-range correlation study can
also be used to identify the resonances associated with
the carbon atoms of the uncoordinated double bond (C3
+ C4). Since the η4-six-electron donor NUPHOS is
geometrically constrained to coordinate in a facial
arrangement to a trichloride-bridged dimer, there are
only three possible isomers, 2a I, 2a II, and 2a III, the
first of which has C1 symmetry while 2a II and 2a III
have Cs and C2 symmetry, respectively (Chart 2).
Considering each isomer in turn, C1 symmetrical 2a I
is expected to give rise to two independent AX doublets
of doublets in the 31P{1H} NMR spectrum (Figure 1, [),
four high-field 13C signals associated with two sets of
C1 and C2 carbon atoms, and four doublets and four
singlets in the 1H NMR spectrum for the eight non-
equivalent methyl groups (Figure 2, [). The remaining
two isomers with Cs and C2 symmetry should each give
rise to an AX type 31P{1H} NMR spectrum (Figure 1,
(20) (a) Queiroz, S. L.; Batista, A. A.; Oliva, G.; Gambardella, M. T.
P.; Santos, R. H. A.; MacFarlane, K. S.; Rettig, S. J .; J ames, B. R. Inorg.
Chim. Acta 1998, 267, 209. (b) Cyr, P. W.; Patrick, B. O.; J ames, B. R.
Chem. Commun. 2001, 1570. (c) Cyr, P. W.; Rettig, S. J .; Patrick, B.
O.; J ames, B. R. Organometallics 2002, 21, 4672.
(21) (a) Amoroso, D.; Yap, G. P. A.; Fogg, D. E. Organometallics
2002, 21, 3335. (b) Hansen, S. H.; Rominger, F.; Metz, M.; Hofmann,
P. Chem. Eur. J . 1999, 5, 557.

1456 Organometallics, Vol. 22, No. 7, 2003
Doherty et al.
nyl)phosphino)-1,1′-binaphthyl}2(µ-Cl)3][NEt2H2].25 The
natural bite angles of 90.1(2)° and 90.6(3)° for P(1)-
Ru(1)-P(2) and P(3)-Ru(1)-P(4), respectively, are simi-
lar to that of 91.0(3)° reported for [Ru(η6-indole)(MeO-
BIPHEP)][BF4].18a The separation of 3.385 Å between
Ru(1) and Ru(2) is similar to that in other trichloride-
bridged dimers27,28 and is outside of the range for a Ru-
Ru bond. The twist of the butadiene tether away from
the expected δ/λ conformation is required to orientate
the π-system of the double bond toward the metal for
complexation. The gross structural features of 2b are
similar to those of the anionic binuclear hydrogenation
catalyst [{RuCl{(R)-TolBINAP)}2(µ-Cl)3][NEt2H2]26 in
that both are based on trichloride face-sharing octahe-
dra with two P-donors in the coordination sphere.
Despite this similarity, we have been unable to convert
2a into the analogous binuclear anion [{RuCl(1,2,3,4-
Me4-NUPHOS)}2(µ-Cl)3][NEt2H2] by substitution of the
η2-coordinated double bonds with chloride, even using
conditions similar to those reported for the synthesis of
its BINAP counterpart (Scheme 1).
F igu r e 3. Molecular structure of [Ru2Cl3(η4-1,2,3,4-Me4-
NUPHOS)2][SbF6] (2b). Hydrogen atoms, solvent molecules
of crystallization, and hexafluoroantimonate anion have
been omitted for clarity. Ellipsoids are at the 30% prob-
ability level.
Syn th esis a n d Sp ectr oscop ic Ch a r a cter iza tion
of [Ru Cl2(1,2,3,4-Me4-NUP HOS)(en )] (4) a n d [Ru Cl-
(1,2,3,4-Me4-NUP HOS)(en )][Cl] (5a ). Although com-
pound 2a is inert with respect to addition of chloride,
tetrahydrofuran solutions of 2a react with 1,2-ethyl-
enediamine to afford [RuCl2(1,2,3,4-Me4-NUPHOS)(en)]
(4). Alternatively, 4 can be prepared in near quantitative
yield by addition of 1,2-ethylenediamine to a THF
solution of [RuCl2(1,2,3,4-Me4-NUPHOS)(py)2] (3), pre-
pared in situ by addition of 1,2,3,4-Me4-NUPHOS to a
THF solution of 1 at room temperature (Scheme 1).
Compound 4 is unstable in common halogenated sol-
vents (vide infra), which limits the range of solvents
available for purification and characterization. Fortu-
nately, a spectroscopically and analytically pure sample
was obtained by crystallization from a concentrated
toluene solution at room temperature overnight. The
singlet at δ 49.7 ppm in the 31P{1H} NMR spectrum of
4 is close to that at δ 46.87 reported for [RuCl2{(R)-
BINAP}{(R,R)-DPEN}]. Surprisingly, upon standing
under an inert atmosphere, chloroform solutions of 4
slowly lose chloride overnight to afford [RuCl(η4-1,2,3,4-
Me4-NUPHOS)(en)][Cl] (5a ), a transformation that oc-
curs quantitatively and more rapidly at elevated tem-
peratures (Scheme 1). The most distinctive feature in
the 31P{1H} NMR spectrum of 5a is the presence of a
high-field doublet at δ 5.1 ppm, a clear indication that
the 1,2,3,4-Me4-NUPHOS coordinates as a η4-six-
electron donor. A long-range 13C-1H correlation study
clearly showed a connectivity between two high-field
doublets at δ 87.1 and 51.5 ppm, associated with
nonprotonated carbons, and the protons of neighboring
methyl groups. The former signal correlates with pro-
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 2b
Ru(1)-P(3)
Ru(1)-P(4)
Ru(2)-P(1)
Ru(2)-P(2)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Ru(1)-Cl(3)
Ru(2)-Cl(1)
Ru(2)-Cl(2)
Ru(2)-Cl(3)
2.251 (8)
2.259(6)
2.269(6)
2.240(8)
2.452(6)
2.454(6)
2.536(6)
2.489(6)
2.499(7)
2.459(6)
Ru(2)-C(1)
Ru(2)-C(2)
Ru(1)-C(5)
Ru(1)-C(6)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
2.17(3)
2.22(2)
2.17(2)
2.22(2)
1.37(3)
1.54(3)
1.30(3)
1.49(3)
1.47(3)
1.30(3)
Ru(1)-Cl(1)-Ru(2)
Ru(1)-Cl(2)-Ru(2)
Ru(1)-Cl(3)-Ru(2)
P(1)-Ru(2)-P(2)
P(3)-Ru(1)-P(4)
Cl(1)-Ru(1-Cl(2)
Cl(1)-Ru(1)-Cl(3)
Cl(2)-Ru(1)-Cl(3)
Cl(1)-Ru(1)-P(3)
Cl(1)-Ru(1)-P(4)
Cl(3)-Ru(1)-P(4)
Cl(3)-Ru(1)-P(3)
Cl(2)-Ru(1)-P(3)
Cl(2)-Ru(1)-P(4)
Cl(1)-Ru(2)-P(1)
Cl(1)-Ru(2)-P(2)
Cl(3-Ru(1)-Cl(1)
86.5(2) Cl(3)-Ru(2)-P(1)
92.1(2)
86.2(2) Cl(3)-Ru(2)-P(2) 114.3(2)
85.3(2) Cl(2)-Ru(2)-P(1) 95.7(2)
91.1(2) Cl(2)-Ru(2)-P(2) 165.6(3)
90.6(3) Ru(1)-C(5)-C(5′) 124.3(17)
80.2(2) Ru(1)-C(5)-C(6)
78.0(3) Ru(1)-C(5)-P(3)
71.9(12)
68.4(9)
77.6(2) Ru(1)-C(6)-C(6′) 117.1(15)
165.0(3) Ru(1)-C(6)-C(7)
92.7(2) Ru(1)-C(6)-C(5)
116.0(13)
68.4(13)
167.0(2) Ru(2)-C(1)-C(1′) 121.7(18)
100.8(2) Ru(2)-C(1)-C(2)
114.3(3) Ru(2)-C(1)-P(2)
73.7(16)
64.6(10)
91.9(2) Ru(2)-C(2)-C(2′) 116(2)
170.0(2) Ru(2)-C(2)-C(3)
96.3(2) Ru(2)-C(2)-C(1)
78.8(2)
113.87(14)
69.8(15)
reported for cationic ruthenium olefin complexes.22,23
However, they are significantly shorter than the corre-
sponding distances of 2.299(1) and 2.366(5) Å in the
analogous MeO-BIPHEP complex [Ru(η5-C8H11)(MeO-
BIPHEP)][CF3CO2],18b which is presumably due to η2-
interaction of the isolated π-bond in 2b compared to the
delocalized π-system of the biphenyl tether in MeO-
BIPHEP. The C(1)-C(2) and C(5)-C(6) bond lengths
of 1.37(3) and 1.49(3) Å, respectively, are significantly
longer than C(3)-C(4) and C(7)-C(8) and are typical
of an η2-coordinated olefin.24 The Ru-P bond lengths
are similar to those reported for the binuclear ruthe-
nium(II) complex [{RuCl{(R)-2,2′-bis(bis(p-methoxyphe-
(24) (a) Motoyama, Y.; Murata, K.; Kurihara, O.; Naitoh, T.; Aoki,
K.; Nishiyama, H. Organometallics 1998, 17, 1251. (b) Motoyama, Y.;
Kurihara, O.; Murata, K.; Aoki, K.; Nishiyama, H. Organometallics
2000, 19, 1025.
(25) Ohta, T.; Tonomura, Y.; Nozaki, K.; Takaya, H.; Mashima, K.
Organometallics 1996, 15, 1521.
(26) Mashima, K.; Nakamura, T.; Matsuo, Y.; Tani, K. J . Organomet.
Chem. 2000, 607, 51.
(27) Thorburn, I. S.; Rettig, S. J .; J ames, B. R. Inorg. Chem. 1986,
25, 234.
(28) (a) Cotton, F. A.; Matusz, M.; Torralba, R. C. Inorg. Chem. 1989,
28, 1516. (b) Cotton, F. A.; Torralba, R. C. Inorg. Chem. 1991, 30, 2196.
(c) Yeomans, B. D.; Humphrey, D. G.; Heath, G. A. J . Chem. Soc.,
Dalton Trans. 1992, 4153.
(22) Faller, J . W.; Chase, K. J . Organometallics 1995, 14, 1592.
(23) Kletzin, H.; Werner, H.; Serhadli, O.; Ziegler, M. L. Angew.
Chem. 1983, 95, 49.

Ru Complexes of 1,2,3,4-Me4-NUPHOS
Organometallics, Vol. 22, No. 7, 2003 1457
Ch a r t 3
tons of two methyl groups and the latter with protons
of three, conclusive evidence for their assignment as the
C1 and C2 carbon atoms, respectively (Scheme 1). These
two signals are shifted markedly to high field, consistent
with coordination of the carbon-carbon double bond.29
Pregosin has reported similar shifts for the coordinated
biaryl double bond in a range of ruthenium complexes
including [RuCp(BINAP)][BF4],18a [Ru(η5-C8H11)(MeO-
BIPHEP)][BF4],18b and [Ru(η6-indole)(MeO-BIPHEP)]-
[BF4]2.18a Compounds 4 and 5a can be cleanly and
quantitatively interconverted in a solvent-dependent
equilibrium. In tetrahydrofuran, the chloride counterion
of 5a rapidly displaces the η2-interaction to afford 4,
within 2-3 h, while dichloromethane, chloroform, car-
bon tetrachloride, and 1,2-dichloroethane solutions of
4 lose chloride at room temperature overnight to re-form
5a . This is the first example of a facile interconversion
between a diphosphine that coordinates in a bidentate
manner, as a four-electron donor, and as an η4-six-
electron donor via reversible coordination of a carbon-
carbon double bond. Not surprisingly, the perchlorate
salt [RuCl(1,2,3,4-Me4-NUPHOS)(en)][ClO4] (5b) is in-
definitely stable in all common halogenated and non-
halogenated solvents since the counterion is noncoor-
dinating.
During a study of the solution chemistry of cis-[RuCl2-
(P-P)L2] complexes (P-P ) 1,4-bis(diphenylphosphino)-
butane or R-BINAP; L2 ) bidentate N-donor) and their
applications in the catalytic hydrogenation of imines,
J ames and co-workers discovered that alcohol solutions
of these complexes readily dissociate chloride and that
methanol solutions of cis-[RuCl2(P-P)L2] and the cation
cis-[RuCl(P-P)L2(MeOH)][Cl] are not stable.20 Slow
oxidation occurs to give two isomers of [RuCl(BINAPO)-
L2][PF6] (A1 and A2, Chart 3), with the BINAPO
coordinated in a P,O-η2-naphthyl manner as a six-
electron donor through both phosphorus atoms and the
two carbon atoms of the naphthyl ring proximate to the
PdO group. Interestingly, [RuCl(BINAPO)L2][PF6] can
also be prepared by aerobic oxidation of [RuCl{(R)-
BINAP}L2][PF6], generated in situ by treatment of cis-
[RuCl2{(R)-BINAP}L2] with AgPF6. The presence of two
distinct well-separated resonances in the 31P NMR
spectrum of [RuCl{(R)-BINAP}L2][PF6], one shifted to
low frequency, is characteristic of η2-coordination of one
of the binaphthyl double bonds. The η2-naphthyl coor-
dination in [RuCl{(R)-BINAP}L2]+ is stable only in non/
weakly coordinating solvents and readily forms [RuCl-
{(R)-BINAP}L2(solvent)]+ in methanol and acetonitrile.
In contrast, complexes 2a and 5a are stable in methanol
and propan-2-ol and show no sign of forming the
corresponding solvento complexes [Ru2Cl3(1,2,3,4-Me4-
F igu r e 4. Molecular structure of [RuCl2(1,2,3,4-Me4-
NUPHOS)(1,2-ethylenediamine)] (4). Hydrogen atoms have
been omitted for clarity. Ellipsoids are at the 30% prob-
ability level.
F igu r e 5. Molecular structure of [RuCl(η4-1,2,3,4-Me4-
NUPHOS)(1,2-ethylenediamine)][ClO4] (5b). Hydrogen at-
oms, solvent molecules of crystallization, and perchlorate
anion have been omitted for clarity. Ellipsoids are at the
30% probability level.
NUPHOS)2(solvent)2][SbF6] and [RuCl(1,2,3,4-Me4-NU-
PHOS)(en)(solvent)][ClO4], even after standing for sev-
eral days.
X-r ay Str u ctu r es of [Ru Cl2(1,2,3,4-Me4-NUP HOS)-
(en )] (4) a n d [Ru Cl(η4-1,2,3,4-Me4-NUP HOS)(en )]-
[ClO4] (5b). Single-crystal X-ray analyses of compounds
4 and 5b have been undertaken in order to examine the
influence of η4-cordination on the metal-diphosphine
bonding and the coordination geometry at the metal.
Perspective views of the molecular structures of 4 and
5b are shown in Figures 4 and 5, respectively, and
selected bond lengths and angles for both molecules are
listed in Table 2. The ruthenium atom in 4 has a slightly
distorted octahedral geometry with near C2 symmetry
and a trans arrangement of chlorides, which show a
marked distortion from linearity with a Cl(1)-Ru(1)-
Cl(2) angle [164.55(7)°] very close to that in the BINAP-
diamine complexes [RuCl2{(R)-TolBINAP}{(R,R)-dpen}]
and [RuCl2{(R)-TolBINAP}{(S,S)-dpen}].16a The P-phe-
nyl rings adopt the alternating edge-face arrangement
expected for atropisomeric diphosphines, and the four-
carbon sp2-hybridized tether has a λ conformation with
a dihedral angle of 67.2° between the least squares
plane containing the two sp2 carbon atoms and their
substituents, C(1)C(1′)C(2)C(2′) and C(3)C(3′)C(4)C(4′).
(29) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: London, 1981.

1458 Organometallics, Vol. 22, No. 7, 2003
Doherty et al.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d s 4 a n d 5b
P(1)-P(6)-N(7)-N(10) atoms from an ideal square
planar geometry, as indicated by the dihedral angle of
18.3° between the planes containing P(1)-Ru(1)-P(6)
and N(7)-Ru(1)-N(10), which is much larger than the
corresponding distortion of 3.0° in 4. Coordination of
C(2)-C(3) also has a dramatic influence on the spatial
arrangement of the P-phenyl rings such that both
attached to P(1) adopt pseudoequatorial positions, while
those attached to P(6) occupy pseudoequatorial and
pseudoaxial sites. In comparison, the 1,2,3,4-Me4-NU-
PHOS in 4 forms a seven-membered chelate ring with
a distorted skew-boat conformation and an alternating
edge-face arrangement of the four phenyl rings.
Ca ta lytic Stu d ies: Ru th en iu m -Ba sed Tr a n sfer
Hyd r ogen a tion of Keton es. Several features of the
ruthenium-based coordination chemistry of 1,2,3,4-Me4-
NUPHOS have prompted us to investigate the perfor-
mance of this diphosphine in the transfer hydrogenation
of ketones (eq 1), in particular (i) possible stabilization
compound 4
compound 5b
Ru(1)-P(1)
2.280(2)
2.287(2)
2.421(2)
2.415(2)
2.167(6)
2.168(6)
1.327(10)
1.491(10)
1.322(9)
Ru(1)-P(1)
2.2764(14)
2.2777(14)
2.4321(15)
2.174(4)
2.202(4)
2.188(4)
2.247(4)
1.431(7)
1.505(6)
1.340(6)
Ru(1)-P(2)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Ru(1)-N(1)
Ru(1)-N(2)
C(1)-C(2)
Ru(1)-P(6)
Ru(1)-Cl(1)
Ru(1)-N(7)
Ru(1)-N(10)
Ru(1)-C(2)
Ru(1)-C(3)
C(2)-C(3)
C(2)-C(3)
C(3)-C(4)
C(3)-C(4)
C(4)-C(5)
P(1)-Ru(1)-P(2)
91.79(7)
P(1)-Ru(1)-P(6)
93.54(6)
163.02(10)
P(1)-Ru(1)-N(2) 173.18(19) P(1)-Ru(1)-N(7)
P(2)-Ru(1)-N(1) 173.25 (16) P(1)-Ru(1)-N(10) 96.72(12)
N(1)-Ru(1)-N(2) 78.8(2)
P(1)-Ru(1)-N(1) 94.96(16)
P(2)-Ru(1)-N(2) 94.40(18)
Cl(1)-Ru(1)-Cl(2) 164.55(7)
Cl(1)-Ru(1)-N(1) 82.90(16)
Cl(1)-Ru(1)-N(2) 84.83(17)
Cl(1)-Ru(1)-P(1) 97.43(7)
Cl(1)-Ru(1)-P(2) 96.35(7)
Cl(2)-Ru(1)-N(1) 82.81(16)
Cl(2)-Ru(1)-N(2) 86.70(17)
Cl(2)-Ru(1)-P(1) 89.60(7)
Cl(2)-Ru(1)-P(2) 97.16(7)
Ru(1)-P(1)-C(1) 112.3(2)
P(6)-Ru(1)-N(10) 168.00(10)
93.16(11)
P(6)-Ru(1)-N(7)
Cl(1)-Ru(1)-C(3) 165.02(12)
Cl(1)-Ru(1)-C(2) 155.12(13)
Cl(1)-Ru(1)-P(1)
Cl(1)-Ru(1)-P(6)
Cl(1)-Ru(1)-N(7) 83.43(11)
Cl(1)-Ru(1)-N(10) 82.40(10)
112.39(4)
87.97(5)
P(6)-Ru(1)-C(3)
P(6)-Ru(1)-C(2)
P(1)-Ru(1)-C(3)
P(1)-Ru(1)-C(2)
Ru(1)-P(1)-C(2)
Ru(1)-P(6)-C(5)
P(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(2)-C(3)-Ru(1)
C(3)-C(2)-Ru(1)
C(4)-C(3)-Ru(1)
77.92(11)
105.25(12)
73.64(11)
47.05(11)
63.87(14)
108.28(15)
114.5(3)
122.3(4)
69.0(2)
P(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
C(3)-C(4)-P(2)
117.9(6)
125.8(6)
126.6(6)
120.3(6)
of a reactive 16-electron intermediate via coordination
of the 1,2,3,4-Me4-NUPHOS in a η4-six-electron manner,
(ii) the presence of N-H groups and phosphorus donors,
a combination commonly used in ruthenium-catalyzed
transfer hydrogenations,33 and (iii) the similarity be-
tween 1,2,3,4-Me4-NUPHOS coordinated in a facial
manner as a six-electron donor and an η6-arene, par-
ticularly since ruthenium(II) arene complexes in com-
bination with amino alcohols form highly efficient
catalysts for transfer hydrogenation.34 Preliminary re-
sults for the reduction of a range of ketones using
complexes 2a , 2b, 4, and 5a are summarized in Table
3. In a typical experiment, a propan-2-ol solution of the
ruthenium complex and substrate was heated for 10
min, activated by addition of base (20 equiv), and the
progress of the reaction monitored by GC. The volume
of each reaction mixture was adjusted such that all
catalytic runs were initiated with a substrate concen-
tration of 0.1 M. For each substrate, comparative
catalytic runs were performed using [RuCl2(PPh3)3]35 as
a standard in order to relate the results of our studies
to those reported in the literature. For all substrates
C(4)-P(2)-Ru(1) 114.4(2)
73.4(2)
117.3(3)
The bonding in the four-carbon tether is highly localized
with the C(1)-C(2) and C(3)-C(4) bond lengths of 1.327-
(10) and 1.322(9) Å, respectively, close to that expected
for a C(sp2)-C(sp2) double bond, and the C(2)-C(3) bond
length of 1.491(10) Å, typical of a single bond between
sp2-hybridized carbon atoms.
Although we have been unable to grow crystals of 5a
suitable for X-ray analysis, crystals of 5b, the perchlo-
rate salt of 5a , have been obtained from a concentrated
methanol solution at room temperature. The molecular
structure shown in Figure 5 clearly reveals 5b to be
monomeric and based on a severely distorted octahedral
geometry with the η4-six-electron diphosphine occupying
a facial coordination environment, bonded through both
phosphorus atoms and one of the double bonds of the
butadiene backbone. The remainder of the coordination
sphere consists of a bidentate 1,2-ethylenediamine and
a chloride ligand, which occupies the site trans to the
double bond. The C(2)-C(3) bond length of 1.431(7) Å
shows the expected elongation upon η2-coordination
and is similar to those in 2a and significantly longer
than that of 1.340(6) Å for the uncoordinated double
bond C(4)-C(5). The Ru-P bond lengths of 2.2764(14)
and 2.2777(14) Å for Ru(1)-P(1) and Ru(1)-P(6), re-
spectively, are longer than those in [Ru(H)(η1-BH4)-
{(R)-BINAP}{(R,R)-dpen}]30 and [Ru(H)Cl{(R)-BINAP}-
(dpen)]31 but are within the range reported for the
ruthenium diphosphine-diamine complex [Ru(H)2{(R)-
BINAP}(tmen)].32 Coordination of C(1)-C(2) at the site
trans to chloride results in a marked distortion of the
(32) Abdur-Rashid, K.; Faatz, M.; Lough, A. J .; Morris, R. H. J . Am.
Chem. Soc. 2001, 123, 7473.
(33) (a) Palmer, M. J .; Wills, M. Tetrahedron Asymmetry 1999, 10,
2045. (b) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J . Org. Chem.
2001, 66, 7931. (c) Yamakawa, M.; Ito, H.; Noyori, R. J . Am. Chem.
Soc. 2000, 122, 1466. (d) Noyori, R.; Hashiguchi, S. Acc. Chem. Res.
1997, 30, 97.
(34) (a) Haack, K.-J .; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori,
R. Angew. Chem., Int. Ed. Engl. 1997 36, 285. (b) Palmer, M.;
Walsgrove, T.; Wills, M. J . Org. Chem. 1997, 62, 5226. (c) Watanabe,
M.; Murata, K.; Ikariya, T. J . Org. Chem. 2002, 67, 1712. (d) Nordin,
S. J . M.; Roth, P.; Tarni, T.; Alonso, D. A.; Brandt, P.; Andersson, P.
G. Chem. Eur. J . 2001, 7, 1431. (e) Alonso, D. A.; Brandt, P.; Nordin,
S. J . M.; Andersson, P. G. J . Am. Chem. Soc. 1999, 121, 9580. (f) Petra,
D. G. I.; Reek, J . N. H.; Handgraaf, J .-W.; Meijer, E. J .; Dierkes, P.;
Kamer, P. C. J .; Brussee, J .; Schoemaker, H. E.; van Leeuwen, P. W.
N. M. Chem. Eur. J . 2000, 6, 2818.
(30) Ohkuma, T.; Koizumi, M.; Muniz, K.; Hilt, G.; Kabuto, C.;
Noyori, R. J . Am. Chem. Soc. 2002, 124, 6508.
(31) Abdur-Rashid, K.; Lough, A.; Morris, R. H. Organometallics
2001, 20, 1047.
(35) (a) Ba¨ckvall, J .-E.; Chowdhury, R. L.; Karlsson, U. J . Chem.
Soc., Chem. Commun. 1991, 473. (b) Wang, G.-Z.; Ba¨ckvall, J .-E. J .
Chem. Soc., Chem. Commun. 1992, 980.

Ru Complexes of 1,2,3,4-Me4-NUPHOS
Organometallics, Vol. 22, No. 7, 2003 1459
Ta ble 3. Tr a n sfer Hyd r ogen a tion of Keton es
Ca ta lyzed by th e Ru (II) Com p lexes 2a , 2b, 4, a n d
5a a
Ta ble 4. In flu en ce of Ba se Con cen tr a tion on
Tr a n sfer Hyd r ogen a tion Activity of Acetop h en on e
Ca ta lyzed by th e Ru (II) Com p lexes 2a a n d 4a
catalyst catalyst (mol %) iPrONa (equiv) time (min) TOFb
2a
2a
2a
2a
2a
4
4
4
4
4
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0.125
0
5
10
20
40
0
10
10
10
10
10
30
30
30
30
30
20
1300
1390
1920
1820
0
140
260
380
360
5
10
20
40
a
Reaction was carried out in refluxing propan-2-ol using a
solution 0.1 M in substrate and equivalents of iPrONa as base.
b
Initial TOF (mol of product per mol catalyst per hour), deter-
mined by GC analysis of the reaction mixture after diluting with
propan-2-ol and filtering through silica, based on formation of
product.
Ch a r t 4
nism.36 In the absence of stabilization by solvent the
active catalyst [RuH2(1,2,3,4-Me4-NUPHOS)] could be
either 16- or 14-electron, depending on whether the
phosphine coordinates as a six- or four-electron donor,
respectively (Chart 4). An NMR study of the reaction
between 2b and iPrONa in propan-2-ol at reflux re-
vealed that under these conditions a ruthenium hydrido
species is formed, as evidenced by the presence of a
a
Reaction was carried out in refluxing propan-2-ol using a
solution 0.1 M in substrate and 20 equiv of iPrONa as base.
Initial TOF, determined by GC analysis of the reaction mixture
after diluting with propan-2-ol and filtering through silica, based
on formation of product. Average activity over three runs, repro-
ducibility (5%.
b
1
broad high-field signal in the H NMR spectrum, and
that the diphosphine probably coordinates as a four-
electron donor based on the absence of any high-field
31P signals associated with η4 six-electron coordination.
tested, catalysts formed from precursors 2a and 2b
outperform those based on monomers 4 and 5a , even
assuming that each dimer generates 2 equiv of active
catalyst. Catalyst mixtures based on 2a and 2b show
good conversion rates for the transfer hydrogenation of
cyclohexanone, acetophenone, 4-methylacetophenone,
and 4-bromoacetophenone, with initial TOF up to an
order of magnitude higher than that obtained with
[RuCl2(PPh3)3]. On the basis of initial TOF, acetonaph-
thone proved to be a particularly challenging substrate
for this new series of catalysts (entries 16-19) and was
most efficiently hydrogenated with [RuCl2(PPh3)3]. This
reversal in activity is somewhat surprising given that,
for all other substrates studied, catalysts based on
NUPHOS are consistently more active than that formed
from [RuCl2(PPh3)3]. Comparison of the initial TOF
obtained using precursors 2a and 2b reveals that
catalyst performance does not depend significantly on
the nature of the counterion. Varying the catalyst-to-
base ratio was found to have a dramatic effect on
activity. In the absence of base, no reaction was ob-
served and conversion rates increased with increasing
catalyst/base ratio to give an optimum activity with 20
equiv per ruthenium (Table 4).
1
There are several recent reports of the use of H NMR
spectroscopy to identify hydride species after activation
of the catalyst precursor under basic conditions which
include formation of a mixture of [RuH2(PPh3)3] and
[RuH2(H2)(PPh3)3] from [Ru(PPh3)3Cl2]36a and genera-
tion of the hydrido anion [Ru(PCP)(PPh3)(H)(iPrO)]-
(PCP ) [C6H3(CH2PPh2)2-2,6]-) after treatment of [Ru-
(PCP)(PPh3)(OSO2CF3)] with iPrOH/KOH in the ab-
sence of ketone.37 However, at this stage comments
about the nature of the active species are speculative
and we cannot eliminate the possibility that the catalyst
could be a dimer such as [Ru2(H)2(µ-Cl)2(1,2,3,4-Me4-
NUPHOS)2] (Chart 4), acting either as a 16-electron
monohydride or as a bimetallic dihydride.
In the case of transfer hydrogenation with 4 and 5a ,
we must consider an alternative mechanism since the
N-H groups of 1,2-ethylenediamine are capable of
stabilizing a six-membered transition state involving the
Ru-H and the carbonyl group of the ketonic substrate.
The accelerating effect of an N-H group on the rate of
transfer hydrogenation was first noted for a ruthenium
(36) (a) Aranyos, A.; Csjernyik, G.; Szabo´, K. J .; Ba¨ckvall, J . E. J .
Chem. Soc., Chem. Commun. 1999, 351. (b) Pa`mies, O.; Ba¨ckvall, J .-
E. Chem. Eur. J . 2001, 7, 5052. (c) Almeida, M. L. S.; Beller, M.; Wang,
G.-Z.; Ba¨ckvall, J .-E. Chem. Eur. J . 1996, 2, 1533.
(37) Dani, P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G.
Angew. Chem., Int. Ed. 2000, 39, 743.
Under the conditions of base-accelerated transfer
hydrogenation with catalysts generated from 2a and 2b,
reduction most likely occurs via a dihydride mecha-

1460 Organometallics, Vol. 22, No. 7, 2003
Doherty et al.
Sch em e 2
complex of the C2-symmetric diphosphine-diamine
N,N-bis[o-(diphenylphosphino)benzyl]cyclohexane-1,2-
diamine, which showed a markedly higher activity than
its imine counterpart.38 Numerous additional reports
have subsequently appeared that support the role of an
N-H in delivering the hydrogen atom from a metal-
hydride to the ketone.39 For example, Morris and co-
workers have provided convincing evidence that the cis-
M-H- -H-N bifunctional moiety is a key feature of the
highly active ruthenium-based hydrogenation catalyst
[RuH2(PPh3)2{(R,R)-cyclohexyldiamine}] and have sug-
gested that the first step in the catalytic hydrogenation
of ketones involves concerted transfer of the hydride to
the carbonyl carbon and N-H proton to the oxygen.39b
The low initial TOF obtained with catalyst precursors
4 and 5a could be taken as evidence of a dihydride
mechanism since hydrogenation involving concerted
transfer of Ru-H and N-H would be expected to lead to
much higher initial TOF. Indeed, the low initial TOF
obtained with catalysts generated from 4 and 5a
compared with those generated from 2a and 2b suggests
that coordinative unsaturation and substrate coordina-
tion are important features of the catalytic cycle. In the
case of 4 and 5a the active species [RuH2(1,2,3,4-Me4-
NUPHOS)(en)] would be coordinatively saturated and
reluctant to interact with substrate, which would ac-
count for the poor performance of these catalysts if the
dihydride mechanism operates. In contrast, the higher
activities obtained with catalysts generated from 2a and
2b are not surprising since the active dihydride would
be coordinatively unsaturated. In this regard, there is
an emerging body of evidence that suggests coordinative
unsaturation can be more important than H-bonding in
conferring hydrogenation activity. Gimeo has reported
that the five-coordinate complexes [RuCl2(κ2-P,N-2-Ph2-
PC6H4CHdNtBu)(PPh3)], [RuCl2(κ2-P,N-2-Ph2PC6H4-
CH2NHtBu)(PPh3)], and [RuCl2(κ2-P,N-2-Ph2PC6H4CH2-
NHtBu)(DMSO)] are more active then the octahedral
[RuCl2(κ2-P,N-2-Ph2PC6H4CHdNtBu)(DMSO)2] and that
the phosphino-imine complex [RuCl2(κ2-P,N-2-Ph2PC6H4-
CHdNtBu)(PPh3)], devoid of an N-H functionality, has
an activity similar to that of its corresponding phos-
phino-amine derivative [RuCl2(κ2-P,N-2-Ph2PC6H4CH2-
NHtBu)(PPh3)].40 However, it should be noted that this
apparent absence of an N-H effect could be due to the
bulky secondary amine, preventing formation of a
hydrogen bond. Fogg and co-workers recently reported
that fac-[RuH3(CO)(dcypb)]K [dcypb ) 1,4-bis(dicyclo-
hexylphosphino)butane] effects reduction of benzo-
phenone with an activity comparable to that of the
Noyori system, which is based on a combination of a
chelating diphosphine and 1,2-diamine.20a The high
activity achieved with fac-[RuH3(CO)(dcypb)]K provides
further evidence that an N-H group is not a necessary
requirement for achieving efficient ketone hydrogena-
tion. The orthometalated [RuH{o-C(O)(Ph)C6H4}(dcypb)-
(CO)] (Scheme 2) was identified as the catalyst resting
state in the catalytic hydrogenation of benzophenone via
fac-[RuH3(CO)(dcypb)]K, and the most likely origin of
the high activity was suggested to be the generation of
a vacant coordination site by reversible insertion of
ruthenium into the ortho C-H bond. Clearly, the higher
activity achieved with catalysts generated from 2a and
2b compared with those formed from 4 and 5a lends
further support to the important role of coordinative
unsaturation in determining activity, particularly if a
pathway involving concerted transfer of N-H and Ru-H
is not available. The marked dependence of activity on
the concentration of base also strongly suggests that
reduction occurs via a hydride mechanism rather than
the concerted hydride proton transfer commonly associ-
ated with catalysts supported by ligands containing
protic groups. Further studies are clearly required to
distinguish between the two mechanisms possible for
catalysts generated from 4 and 5a .
Con clu sion
These studies have demonstrated that 1,2,3,4-Me4-
NUPHOS can readily interconvert between coordination
as a four-electron bidentate diphosphine and a six-
electron donor via reversible coordination of one of the
carbon-carbon double bonds of the four-carbon tether.
Complexes 2a , 2b, 4, and 5a form catalysts that are
active for the transfer hydrogenation of a range of
ketones, and preliminary NMR studies suggest that a
hydride mechanism operates. For all substrates tested,
there is a significant differential between the perfor-
mance of catalysts formed from the coordinatively
unsaturated binuclear precursors 2a ,b and the 18-
electron diphosphine-diamine complexes 4 and 5a . As
a result of these encouraging transfer hydrogenation
studies, detailed synthetic and mechanistic investiga-
tions to establish the identity of the active catalyst and
the mechanism of transfer hydrogenation, the use of
amino-alcohols in combination with ruthenium(II) com-
plexes of η4-six-electron-1,2,3,4-Me4-NUPHOS for asym-
metric transfer hydrogenation, and the use of chiral
diamines for asymmetric activation of ruthenium(II)
complexes of 1,2,3,4-Me4-NUPHOS toward enantiose-
lective hydrogenation are currently underway.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations involving air-
sensitive materials were carried out in an inert atmosphere
glovebox or using standard Schlenk line techniques under an
atmosphere of nitrogen or argon in oven-dried glassware.
Diethyl ether and hexane were distilled from potassium/
sodium alloy, tetrahydrofuran from potassium, dichloromethane
from calcium hydride, and methanol from magnesium. Unless
(38) Gao, J .-X.; Ikariya, T.; Noyori, R. Organometallics 1996, 15,
1087.
(39) (a) J iang, Y.; J iang, Q.; Zhang, X. J . Am. Chem. Soc. 1998, 120,
3817. (b) Abdur-Rashid, K.; Lough, A. J .; Morris, R. H. Organometallics
2000, 19, 2655.
(40) Crochet, P.; Gimeno, J .; Garcia-Grande, S.; Borge, J . Organo-
metallics 2001, 20, 4369.

Ru Complexes of 1,2,3,4-Me4-NUPHOS
Organometallics, Vol. 22, No. 7, 2003 1461
otherwise stated, commercially purchased materials were used
without further purification. The ruthenium complexes [RuCl2-
(nbd)(py)2] and [RuCl2(rac-BINAP)(py)2] were prepared as
previously described.17 Deuteriochloroform was predried with
calcium hydride and vacuum transferred and stored over 4 Å
molecular sieves. 1H, 31P{1H}, and 13C{1H} NMR spectra were
recorded on J EOL LAMBDA 500 or Bruker AC 200, AMX 300,
and DRX 500 machines. GC analyses were conducted on a
Varian CP3800 connected to a Varian C8400 auto sampler
with a CHIRASIL-DEX CB column.
Syn th esis of [Ru Cl2(1,2,3,4-Me4-NUP HOS)(en )] (4). A
tetrahydrofuran solution of [RuCl2(1,2,3,4-Me4-NUPHOS)(py)2]-
(0.35 g, 0.432 mmol) and 1,2-ethylenediamine (0.033 mL, 0.5
mmol) was stirred at room temperature under an inert
atmosphere for 7-8 h, after which time the solvent was
removed to leave an orange-yellow residue, which was purified
by crystallization from a concentrated toluene solution at room
temperature to give 4 in 71% yield (0.22 g). 31P{1H} NMR
1
(121.4 MHz, THF, δ): 49.7 (s, PPh2). H NMR (500.13 MHz,
C6D6, δ): 8.8 (br, 4H, C6H5), 8.5-6.9 (m, 16H, C6H5), 2.8 (br,
2H, NH2), 2.0 (m, 6H, CH3), 1.98 (br, 6H, CH2 + NH2), 0.75 (s,
6H, CH3). 13C{1H} NMR (125.45 MHz, C6D6, δ): 159.0 (s, CMe),
144-123 (m, C6H5 + CMe), 43.0 (s, NCH2), 20.1 (t, J PC ) 3.4
Syn th esis of [Ru 2Cl3(1,2,3,4-Me4-NUP HOS)2]Cl (2a ). A
solution of [RuCl2(nbd)(py)2] (0.5 g, 1.18 mmol) in chloroform
(10 mL) was treated with a chloroform solution (4-5 mL) of
1,2,3,4-Me4-NUPHOS (0.564 g, 1.18 mmol) and stirred vigor-
ously overnight, after which time the reaction mixture was
filtered and the solvent removed to leave a deep orange solid
residue. Crystallization by slow diffusion of n-hexane into a
dichloromethane solution at room temperature gave 2a as
orange crystals in 78% yield (0.60 g). 31P{1H} NMR (121.4
Hz, CH3), 18.7 (t, J PC ) 3.1 Hz, CH3). Anal. Calcd for C34H40
-
Cl2N2P2Ru: C, 57.47; H, 5.67; N, 3.94. Found: C, 57.67; H,
5.78; N, 4.02.
Syn th esis of [Ru Cl(1,2,3,4-Me4-NUP HOS)(en )]Cl (5a ).
Compound 4 (0.330 g, 0.463 mmol) was dissolved in ca. 20 mL
of chloroform and stirred under an inert atmosphere overnight.
The solvent was removed and the product purified by slow
diffusion of a hexane into a chloroform solution at room
temperature to give 5a as a yellow powder in 92% yield (0.462
g). 31P{1H} NMR (121.4 MHz, CDCl3, δ): 79.9 (d, J PP ) 50.2
2
MHz, CDCl3, δ): major isomer, 87.6 (d, J PP ) 49.8 Hz, PA1),
2
2
83.4 (d, J PP ) 49.8 Hz, PA2), 13.3 (d, J PP ) 49.8 Hz, PX1), 6.4
(d, 2J PP ) 49.8 Hz, PX2); minor isomer, 86.8 (d, 2J PP ) 49.8 Hz,
PA), 10.5 (d, J PP ) 49.8 Hz, PX). 13C{1H} NMR (125.45 MHz,
2
1
CDCl3, δ): 159.3 (d, J PC ) 26.3 Hz, CMe), 158.7 (d, J PC ) 26.3
Hz, CMe), 157.7 (d, J PC ) 26.9 Hz, CMe), 136.3-122.6 (m, C6H5
+ CMe), 89.6 (dd, J PC ) 7.0, 5.0 Hz, CMe), 88.6 (dd, J PC ) 7.0,
Hz), 5.1 (d, J PP ) 50.2 Hz). H NMR (500.13 MHz, CDCl3, δ):
7.97 (dd, J ) 8,9, 7.4 Hz, 2H, C6H5), 7.90 (dd, J ) 12.1, 7.0
Hz, 2H, C6H5), 7.90-7.38 (m, 10H, C6H5), 7.23 (dd, J ) 11.3,
7.5 Hz, 2H, C6H5), 7.16 (t, J ) 7.3 Hz, 2H, C6H5), 6.99 (dt, J )
7.6, 2.2 Hz, 2H, C6H5), 3.68 (br m, 1H, NH), 2.98 (br m, 1H,
NH), 2.87 (br m, 1H, NH), 2.48 (br m, 2H, NH + CH2), 2.39
(br m, 1H, CH2), 2.32 (br m, 1H, CH2), 2.06 (s, 3H, CH3), 1.64
(br m, 1H, CH2), 1.78 (s, 3H, CH3), 1.75 (d, J PH ) 7.8 Hz, 3H,
CH3), 0.97 (d, J PH ) 9.3 Hz, 3H, CH3). 13C{1H} NMR (125.45
MHz, CDCl3, δ): 159.7 (dd, J PC ) 29.4, 5.1 Hz, CMe), 138-
125 (m, C6H5 + CMe), 87.1 (d, J PC ) 7.9, 4.0 Hz, CMe), 51.5
(d, J PC ) 31.9 Hz, CMe), 44.9 (s, NCH2), 42.5 (s, NCH2), 23.0
(d, J PC ) 7.5 Hz, CMe), 21.7 (d, J PC ) 18.7, CMe), 15.0 (d, J PC
) 4.9 Hz, CMe), 14.3 (d, J PC ) 2.8 Hz, CMe). Anal. Calcd for
5.5 Hz, CMe), 87.0 (dd, J PC ) 7.0, 4.4 Hz, CMe), 52.3 (d, J PC
35.4 Hz, CMe), 51.4 (d, J PC ) 34.5 Hz, CMe), 51.3 (d, J PC
)
)
34.9 Hz, CMe), 23.7 (m, CH3), 23.1 (d, J PC ) 7.8 Hz, CH3),
20.6-20.0 (m, CH3), 16.9 (d, J PC ) 6.0 Hz, CH3), 16.6 (d, J PC
1
) 5.4 Hz, CH3), 15.0 (d, J PC ) 5.4 Hz, CH3), 13.5 (s, CH3). H
NMR (500.1 MHz, CDCl3, δ): 8.29-6.75 (m, 40H, C6H5), 2.0
(s, 3H, CH3, major isomer), 1.98 (s, 3H, CH3, major isomer),
1.95 (s, 3H, CH3, major isomer), 1.93 (s, 6H, CH3, minor
isomer), 1.85 (d, J PH ) 8.8 Hz, 3H, CH3 major isomer), 1.70 (s,
3H, CH3, major isomer), 1.64 (d, J PH ) 8.4 Hz, 6H, CH3, minor
isomer), 1.30 (s, 6H, CH3, minor isomer), 1.07 (d, J PH ) 8.8
Hz, 3H, CH3, major isomer), 0.90 (d, J PH ) 9.6 Hz, 3H, CH3,
major isomer), 0.83 (d, J PH ) 9.6 Hz, 3H, CH3, major isomer),
0.79 (s, J PH ) 9.6 Hz, 6H, CH3, minor isomer). Anal. Calcd for
C
34H40Cl2N2P2Ru: C, 57.57; H, 5.67; N, 3.94. Found: C, 57.49;
H, 5.71; N, 4.07.
Syn t h esis of [R u Cl(1,2,3,4-Me4-NUP H OS)(en )][ClO4]
(5b). A chloroform solution of 5a (0.25 g, 0.35 mmol) as stirred
overnight with a slight excess of NaClO4 (0.06 g, 0.5 mmol).
The reaction mixture was filtered, the solvent removed, and
the residue crystallized from a concentrated methanol solution
at room temperature to give 5b in 66% yield (0.180 g). X-ray
quality crystals of 5b were grown by slow diffusion of methanol
into a chloroform solution at room temperature. Anal. Calcd
for C34H40Cl2N2O4P2Ru: C, 52.72; H, 5.20. Found: C, 52.87;
H, 5.45.
Gen er a l P r oced u r e for Tr a n sfer Hyd r ogen a tion Rea c-
tion s. A Schlenk flask was charged with the catalyst precursor
(0.0025 mmol), 19.5 mL of iPrOH, substrate (2 mmol), and
n-decane internal standard (2 mmol). The solution was heated
at 82 °C for 10 min, and then 0.5 mL (0.05 mmol) of a solution
of iPrONa in iPrOH (0.1 M) was added. The volume of iPrOH
was adjusted so that all catalytic reactions were conducted
with an initial substrate concentration of 0.1 M. The addition
of iPrONa was taken as the starting time for the reaction. The
extent of conversion was determined by gas chromatography.
GC conditions: initial temperature 100 °C for 5 min, final
temperature 200 °C, ramp rate 8 °C/min, injection temperature
200 °C, detector temperature 300 °C, carrier gas He at 25 mL/
min, column Varian WCOT fused silica, 25 m × 0.32 mm i.d.,
coating CP CHIRASIL-DEX CB.
C
64H64Cl4P4Ru2: C, 62.04; H, 5.21. Found: C, 62.41; H, 5.32.
Syn th esis of [Ru 2Cl3(1,2,3,4-Me4-NUP HOS)2][SbF6] (2b).
A chloroform solution of [Ru2Cl3(1,2,3,4-Me4-NUPHOS)2]Cl
(0.40 g, 0.306 mmol) was stirred overnight with a slight excess
of NaSbF6 (0.129 g, 0.5 mmol). The reaction mixture was
filtered, the solvent removed, and the residue crystallized by
slow diffusion of methanol into a dichloromethane solution at
room temperature, to give X-ray quality crystals of 2b in 66%
yield (0.302 g). Anal. Calcd for C64H64Cl3F6P4Ru2Sb: C, 51.20;
H, 4.30. Found: C, 51.53; H, 4.45.
Syn th esis of [Ru Cl2(1,2,3,4-Me4-NUP HOS)(p y)2] (3). A
tetrahydrofuran solution of [RuCl2(nbd)(py)2] (0.50 g, 1.18
mmol) and 1,2,3,4-Me4-NUPHOS (0.564 g, 1.18 mmol) was
stirred overnight, after which time the reaction mixture was
filtered and the solvent removed to leave a solid yellow residue.
The product could be isolated by extraction of this residue into
toluene and precipitation by addition of diethyl ether to give
3 as a spectroscopically pure fine yellow powder. However,
compound 3 was typically generated in THF and reacted with
1,2-ethylenediamine without further purification. 31P{1H}
1
NMR (121.4 MHz, THF, δ): 48.05 (s, PPh2). H NMR (500.13
MHz, C6D6, δ): 9.0-6.1 (m, 30H, C6H5, C5H5N), 2.17 (t, J )
4.5 Hz, 6H, CH3), 0.64 (br s, 3H 6H, CH3).
Th er m olysis of [Ru Cl2(r a c-BINAP )(p y)2]. In a typical
experiment a sample of [RuCl2(rac-BINAP)(py)2] (0.055 g,
0.057 mmol) was dissolved in deuteriochloroform (ca. 0.5 mL)
and transferred via cannula to an NMR tube. A capillary
containing PPh3 (0.020 g, 0.076 mmol) in chloroform was used
as an internal standard. A time zero spectrum was recorded,
the sample heated at 60 °C, and the progress of the thermolysis
monitored by 31P NMR spectroscopy.
Cr yst a l St r u ct u r e Det er m in a t ion s of 2b, 4, a n d 5b .
Data were collected on a Bruker-AXS SMART diffractometer
using SAINT-NT41a software with graphite monochromated Mo
KR radiation and are tabulated in Table 5. The structures were
solved using direct methods and refined with SHELXTL
version 5,41b and all non-hydrogen atoms (unless disordered)
were refined with anisotropic thermal parameters. Close

1462 Organometallics, Vol. 22, No. 7, 2003
Doherty et al.
Ta ble 5. Su m m a r y of Cr ysta l Da ta a n d Str u ctu r e Deter m in a tion for Com p ou n d s 2b, 4, a n d 5b
2b
4
5b
formula
Mr
C
75H73Cl8F6OP4Ru2Sb
C34H40Cl2N2P2Ru
710.59
C35H44Cl2N2O5P2Ru
806.63
1745.63
cryst color
cryst size, mm
temperature, K
cryst syst
space group
a, Å
orange
yellow
yellow
0.34 × 0.26 × 0.12
0.58 × 0.33 × 0.30
0.40 × 0.35 × 0.32
298
298
153
monoclinic
P21/c
12.921(9)
17.770(15)
32.10(2)
triclinic
P1h
monoclinic
P21/n
11.545(6)
15.876(9)
19.742(11)
10.241(2)
12.076(3)
14.071(4)
71.79(3)
88.47(4)
81.04(2)
1632.3(7)
2
b, Å
c, Å
R, deg
â, deg
92.41(2)
101.758(11)
γ, deg
V, Å3
7364(10)
4
1.575
3543(3)
4
1.512
Z
D
calc, g cm-3
1.446
F(000)
3500
1.202
23.31
44 728
10 598
0.2598
674
0.1058
0.3229
1.102
732
0.768
28.63
4730
4711
0.0324
373
0.0452
1664
0.710
28.46
19 044
7445
0.1777
427
0.0543
0.1587
1.107
µ(Mo KR), mm-1
θmax, deg
no. of reflns measd
no. of unique reflns
Rint (on F2)
no. of params
Ra[F2 > 2σ(F2)]
Rwb(all data)
0.1433
0.958
0.624, -0.479
GOFc (S)
max., min. diff map, e Å-3
1.32, -1.604
1.186, -1.334
Conventional R ) ∑||Fo| - |Fc||/∑|Fo| for “observed” reflections having Fo > 2σ(Fo2). Rw ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2 for all data.
a
2
b
2
c GOF ) [∑w(Fo - Fc2)2/(no. unique reflns - no. of params)]1/2
.
2
2
2
2
inspection of the atomic displacement parameters for the three
crystal structures revealed that they all had some disorder
associated with either the ligands or the anions. In 2b four of
the phenyl rings and two of the CH2Cl2 solvent molecules were
disordered and have been modeled as having two major
positions in a 50:50 ratio. The remaining CH2Cl2 solvent
molecule is not disordered but has an occupancy factor of 50%.
The 1,2-ethylenediamine ligand in 4 is disordered over two
positions with occupancy factors of 50%, and the perchlorate
ions in 5b are also disordered and have been modeled as
having two positions with occupancy factors of 50%. Hydrogen
atom positions were added at idealized positions with a riding
model and fixed thermal parameters (Uij ) 1.2Ueq for the atom
to which they are bonded (1.5 for methyl)). The function
) [σ2 |Fo| + (g1P)2 + (g2P)] where P ) [max|Fo| + 2|Fc| ]/3.
Additional material available from the Cambridge Crystal-
lographic Data Centre comprises relevant tables of atomic
coordinates, bond lengths and angles, and thermal parameters.
Ack n ow led gm en t. We gratefully acknowledge the
Queen’s University of Belfast and the McClay Trust for
funding and J ohnson Matthey for loans of palladium
salts.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, structure solution and refinement, atomic coordinates,
bond distances, bond angles, and anisotropic thermal param-
eters for compounds 2b, 4, and 5b. This material is available
free of charge via the Internet at http://pubs.acs.org. Observed
and calculated structure factor tables are available from the
authors upon request.
2
2
minimized was ∑[w(|Fo| - |Fc| )] with reflection weights w-1
(41) (a) SAINT-NT, program for data collection and data reduction;
Bruker-AXS: Madison, WI, 1998. (b) Sheldrick, G. M. SHELXTL
Version 5.0, A System for Structure Solution and Refinement; Bruker-
AXS: Madison, WI, 1998.
OM020861B