Phosphinidene complexes of scandium: Powerful PAr Group-transfer vehicles to organic and inorganic substrates

Article abstract of DOI:10.1021/ja100214e

"Chemical equation presented" The first phosphinidene complexes of scandium are reported in this contribution. When complex (PNP)Sc(CH3)Br (1) is treated with 1 equiv of LiPH[Trip] (Trip = 2,4,6-iPr3C6H2), the dinuclear scandium phosphinidene complex [(PNP)Sc(μ2-P[Trip])]2 (2) is obtained. However, treating 1 with a bulkier primary phosphide produces the mononuclear scandium ate complex [(PNP)Sc(μ2-P[DMP])(μ2-Br)Li] (3) (DMP = 2,6-Mes2C6H3). The Li cation in 3 can be partially encapsulated with DME to furnish a phosphinidene salt derivative, (PNP)Sc(μ2-P[DMP]) (μ2-Br)Li(DME)] (4). We also demonstrate that complex 3 can readily deliver the phosphinidene ligand to organic substrates such as OCPh2 and Cl2PMes* as well as inorganic fragments such as Cp2ZrCl2, Cp*2TiCl2, and Cp2TiCl2 in the presence of P(CH3)3. Complexes 2-4 have been fully characterized, including single crystal X-ray diffraction and DFT studies.

Full text of DOI:10.1021/ja100214e

Published on Web 03/02/2010
Phosphinidene Complexes of Scandium: Powerful PAr Group-Transfer
Vehicles to Organic and Inorganic Substrates
Benjamin F. Wicker, Jennifer Scott,† Jose´ G. Andino, Xinfeng Gao, Hyunsoo Park, Maren Pink, and
Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405
Received January 10, 2010; E-mail: mindiola@indiana.edu
Scheme 1. Synthesis of Dinuclear and Mononuclear
Phosphinidene Complexes of Scandium
Due to the mismatch in orbital energies or the soft-hard contrast
among the metal ion and phosphorus,1 terminal phosphinidene
complexes of the early transition metals represent an attractive
moiety for the purposes of group transfer2,3 and catalytic transfor-
mations such as carboamination, hydrophosphination of alkynes,
and the dehydrocoupling of phosphines.4 Recently, our group has
utilized the (PNP)Sc(III) scaffold, (PNP ) N[2-P(CHMe2)2-4-
methylphenyl]2-), to support unusual moieties like oxo, meth-
ylidene, and imide, all of which could be trapped with a Lewis
acid like Al(CH3)3.5 In the absence of the Lewis acid, a moiety
like a scandium imide could engage in intermolecular C-H
activation reactions of benzene or pyridine.5a Intrigued by this
finding and the recent work establishing dinuclear Lu(III) phos-
phinidene complexes as promising reagents for “PAryl” group
transfer,6 we wanted to expand this methodology to create reactive
tion of the phosphinidene dimer, 2, inferred that a more hindered
scandium phosphinidene complexes. Our choice of Sc is strategic,
given that this metal ion should represent the most favorable
lanthanide to engage in metal-ligand multiple bonding due to its
d-orbital energies, electronegativity, and ion size, when compared
to the heavier group 3 congeners. Herein, we present the first
examples of phosphinidene complexes of scandium, in both neutral
and ionic forms, and demonstrate the reactive phosphinidene ligand
to be readily delivered to both organic and inorganic fragments.
secondary phosphide may prevent such an aggregation.
Treating LiP(H)[DMP] (DMP ) 2,6-Mes2C6H3) with precursor
1 elicited an immediate color change from yellow to orange, and
the 31P and 1H NMR spectral data suggested formation of an
asymmetric complex of scandium (∼70% isolated yield). Only upon
cooling to 0 °C could three broad and inequivalent 31P NMR
resonances be observed at 13.2, 9.8, and 8.0 ppm. Because of its
temperature dependence, the phosphorus resonance at 9.8 ppm has
been tentatively assigned to the phosphinidene. The mesityl groups
Our approach to implanting a phosphinidene ligand onto scan-
dium(III) involved a similar route to that reported for the transient
imido (PNP)ScdNAr fragment (Ar ) 2,6-iPr2C6H3).5a Accordingly,
complex (PNP)Sc(CH3)Br (1),7 prepared readily from (PNP)ScCl2
and 1 equiv of Li(CH3)·LiBr, was reacted with 1 equiv of
LiPH[Trip] (Trip ) 2,4,6-iPr3C6H2) in toluene to yield the dinuclear
scandium phosphinidene complex [(PNP)Sc(µ2-P[Trip])]2 (2) in
57% yield (Scheme 1).7 The most diagnostic spectroscopic feature
of complex 2 is the presence of a phosphinidene resonance at 227.4
ppm (31P NMR, 25 °C), which is highly deshielded from the
broadened PNP-pincer phosphine resonance (7.0 ppm). Dinuclear
aggregation to 2 from putative (PNP)ScdP[Trip] was unambigu-
ously ascertained by single crystal X-ray diffraction studies (Figure
1).7 Interestingly, the solid state structure of 2 deviates from the
Lu(III) analogue6 in that the Sc2P2 core is highly symmetrical. For
example, a center of inversion relates the equivalent but short Sc-P
distances in the planar Sc2P2 core (Sc(1)sP(3′) ) 2.5527(10) Å
and Sc(1)sP(3) ) 2.5446(8) Å, deviation in planarity of the core
is 2.1°, and the sum of the angles around the core is 359.92°), when
compared to the Sc-PNP phosphine distances (∼2.83 Å). In
addition, the aryl ring on the phosphorus is not coplanar with the
core, deviating by a dihedral angle of -33°. These structural
features presumably allow the lone pair of each phosphorus atom
to become delocalized throughout the four-membered ring. Forma-
1
on the DMP moiety are inequivalent when assayed by H NMR
spectroscopy, and no fluxional behavior is observed when the
solution is heated to 80 °C. A single crystal X-ray structure revealed
the connectivity to be that of a mononuclear, phosphinidene ate
complex, [(PNP)Sc(µ2-P[DMP])(µ2-Br)Li] (3) wherein a LiBr
residue (Sc-Br, 2.6931(16) Å; P-Li, 2.481(11) Å) has added across
the shortest known Sc-P bond (2.338(2) Å) and linear ligand
(Sc-PsC, 162.4(2)°).7 Consequently, the unsaturated Li ion,
having bridging phosphinidene and bromide ligands, interacts in
an η6-fashion with one mesityl aryl moiety (Figure 1). We expect
the latter interaction to be weak (36-50 kcal/mol), based on
published calculations,8 and due to the long CarenesLi interactions
(2.411-2.539 Å) in comparison to the solid state structures of other
lithium terphenyls (2.31-2.28 Å).9 Although the arene-Li interac-
tion could be negligible, our vt 7Li NMR does not suggest
7
dissociation of the Li+ ion. In fact, the Li NMR spectrum of 3
displays a doublet resulting from strong coupling to the phosphin-
idene phosphorus (-0.20 ppm, 1JLi-P ) 34 Hz, 25 °C), which is in
the low range but comparable to other values observed for LiPHR
or LiPR2 salts (36-56 Hz).10 The JLi-P value is relatively
1
insensitive to vt while a 1H NMR experiment at temperatures
80-100 °C reflects fluxional mesityl rings as well as the aryl groups
on the PNP ligand.7
Analysis of the bonding and structure in 3 by high level DFT
calculations reproduced the key geometrical features expectedly well
and implied a strong interaction between the phosphinidene lone
† Current Address: Department of Chemistry and Chemical Engineering, Royal
Military College of Canada, Kingston, Ontario, Canada K7K 7B4.
9
10.1021/ja100214e  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 3691–3693 3691

C O M M U N I C A T I O N S
Figure 1. Perspective views of the molecular structures of complexes 2, 3, and 4. Metrical parameters for each structure are reported in the SI.
Scheme 2. Reactivity of 3 with Organic and Inorganic Reactants
analogue, [(PNP)Sc(µ2-P[DMP])(µ2-Br)Li(DME)] (4), in 21%
isolated yield. Complex 4 can also be independently prepared (in
∼47% yield) by Li encapsulation of LiP(H)[DMP] with DME prior
to addition to 1 (Scheme 1).7 Apart from the DME component,
most salient, multinuclear NMR spectroscopic features are similar
Figure 2. Expanded region of the most important orbitals (HOMO at
-4.255 eV, and HOMO-1 at -4.453 eV) of the computed model of 3.
7
to 3 (1H, 13C, Li), but the 31P NMR spectrum collected at 25 °C
evinced a slightly more downfield phosphinidene resonance at 56.1
pair and the empty d-orbital on Sc (HOMO, Figure 2). However,
the energetically similar HOMO-1 orbital suggests a highly
polarized bond dominated by the phosphinidene phosphorus p-
orbital with nonoptimal orbital overlap with the electropositive Sc
d-orbital, but oriented more toward the Li+ counterion (Figure 2).7
Presumably due to the in-phase combination between the atoms
P-ScsN, HOMO-1 is slightly lower in energy than the π-based
orbital observed in the HOMO. HOMO-1 also clearly illustrates
the dissimilarities in electronegativities between the phosphinidene
P and the PNP amide nitrogen given their respective orbital
contributions. The computed Mayer bond (1.41) and natural bond
order (NBO, 1.62) implies a multiple bond between Sc and the
phosphinidene P atom, since these values are over three times larger
than the average Mayer bond order between Sc and PNP phosphines
(∼0.38) and is also significantly larger than the Sc-P Mayer bond
(1.00) value computed for the phosphinidene dimer 2.7 Interestingly,
omitting the Li+ from our calculations inflicted no significant
perturbation to the overall core distances, Mayer Sc-P bond orders,
NBO’s, and overall molecular orbital scheme between Sc and P,
although some distortions in the angles, Sc-PsCipso and (PNP)Sc,
were observed due to no Li-arene interactions (see Supporting
Information (SI) for details).7 As a result, the large contribution of
P character to HOMO-1 suggests substantial charge localization at
this site. The ligand is therefore implied to be highly nucleophilic
in character, independent of whether the Li+ ion is present.
Lithium-arene decomplexation in 3 can be easily accomplished
by treatment with 1,2-dimethoxyethane (DME), to form the ate
ppm from the two inequivalent PNP resonances (13.3 and 8.2 ppm).
To conclusively establish the role of DME in 4 and whether Li+
coordination to P and/or Br was still persistent, both a single crystal
7
7
structure and vt Li NMR spectra were collected. Although the
Sc-P (2.3732(18) Å), Sc-PsC (155.2(2)°), and Sc-Br (2.6032(13)
Å) metrical parameters in 4 are similar to those of 3, coordination of
DME to Li results in a slight elongation of the countercation from the
Br (2.563(11) Å) and the phosphinidene P atoms (2.499(12) Å).
Coordination of DME to Li completely fractures all Li(arene) interac-
tions, which in turn promotes tilting of the Mes groups away from the
Li(DME) component. The Sc-Br distances in 3 and 4 are unremark-
able with respect to neutral Sc(III) complexes having Br ligands,11
while the Li-Br distance in 4 falls within range for the reported
tetrameric complex, [LiBr(OEt)]4 (2.54-2.62 Å).12 Despite DME
complexation to Li+ in 4, vt 7Li NMR data do not suggest substantial
abstraction of the alkali metal from the phosphinidene site (1JLi-P
)
41 Hz, 25 °C), and addition of excess DME and thermolysis of the
complex (100 °C) do not appear to induce formation of a discrete salt
or elimination of Li(DME)Br. DFT calculations also suggest a highly
polarized phosphinidene, having multiple bond character between Sc
and P (Mayer bond order, 1.46; NBO, 1.58).7
Unlike compound 2, which reacts sluggishly with ketones, complex
3 shows clean phospha-Wittig chemistry toward ketones and phos-
phorus dichlorides (Scheme 2). Accordingly, the reaction of 3 with
benzophenone immediately yielded the phosphaalkene, Ph2CdP[DMP]
(31P NMR: 234.8 ppm),7,13 in very high yield when the mixture is
assayed by 31P NMR spectroscopy. Putative “(PNP)ScO” could not
9
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C O M M U N I C A T I O N S
A. R. Acc. Chem. Res. 1988, 21, 81. (d) Mathey, F. Angew. Chem., Int.
Ed. 2003, 42, 1578. (e) Shah, S.; Protasiewicz, J. D. Coord. Chem. ReV.
2000, 210, 181. (f) Stephan, D. W. Angew. Chem., Int. Ed. 2000, 39, 314.
(g) Lammertsma, K. Top. Curr. Chem. 2003, 229, 95. (h) Mathey, F. Dalton
Trans. 2007, 1861. (i) Waterman, R. Dalton Trans. 2009, 18. (j) Dillon,
K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy: From
Organophosphorus to Phospha-organic Chemistry; Wiley: NY, 1998.
(3) (a) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed.
Engl. 1993, 32, 756. (b) Breen, T. L.; Stephan, D. W. J. Am. Chem. Soc.
1995, 117, 11914.
(4) (a) Zhao, G.; Basuli, F.; Kilgore, U. J.; Fan, H.; Halikhedkar, A.; Huffman,
J. C.; Wu, G.; Mindiola, D. J. J. Am. Chem. Soc. 2006, 128, 13575. (b)
Masuda, J. D.; Hoskin, A. J.; Graham, T. W.; Beddie, C.; Fermin, M. C.;
Etkin, N.; Stephan, D. W. Chem.sEur. J. 2006, 12, 8696.
(5) (a) Scott, J.; Basuli, F.; Fout, A. R.; Huffman, J. C.; Mindiola, D. J. Angew.
Chem, Int. Ed. 2008, 47, 8502. (b) Scott, J.; Fan, H.; Wicker, B. F.; Fout,
A. R.; Baik, M.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 14438.
(6) Masuda, J. D.; Jantunen, K. C.; Ozerov, O. V.; Noonan, K. J. T.; Gates,
D. P.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 2408.
(7) See SI.
(8) (a) Tsuzuki, S.; Yosida, M.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A
2001, 105, 769. (b) Amunugama, R.; Rodgers, M. T. J. Phys. Chem. A
2002, 106, 5529. (c) Amunugama, R.; Rodgers, M. T. J. Phys. Chem. A
2002, 106, 9092. (d) Amunugama, R.; Rodgers, M. T. Int. J. Mass Spectrom.
2003, 227, 339.
be isolated from the complicated metal-based mixture. As opposed to
complexes of the type Cp2ZrdP[Ar](P(CH3)3) (Ar ) Mes*3b or
DMP14), the reaction of highly ionic 3 with Mes*PCl2 cleanly yielded
the asymmetric diphosphene, [Mes*]PdP[DMP] (31P NMR: 526.2,
455.5 ppm),15 and (PNP)ScCl2 in yields greater than 70%, as
established by 31P NMR spectroscopy (Scheme 2).7 Since 3 fails to
react with phosphines, transfer and trapping experiments could be
pursued with this Lewis base. Hence, intermetal phosphinidene group
transfer can be conducted with 3, since treatment with 1 equiv of
Cp2ZrCl2 and excess P(CH3)3 rapidly promoted formation of (PNP)-
ScCl2 and the known complex, Cp2ZrdP[DMP](P(CH3)3) (31P NMR:
771.0 and -6.7 ppm, JP-P ) 23 Hz),14 an analogous effective
2
phosphinidene reagent to the Mes* derivative reported by Stephan3b
(Scheme 2). Unfortunately, decomposition products are observed in
the reaction mixture thus preventing separation of the Zr phosphinidene.
In contrast, the reaction with 2 equiv of Cp*2TiCl2 and excess P(CH3)3
resulted in quantitative formation of (PNP)ScCl2, the phospha-Wittig
reagent, [DMP]PdP(CH3)3 (31P NMR: -2.8 and -114.7 ppm, 1JP-P
) 582 Hz, 46%),16 as well as [DMP]PdP[DMP] (9%, 31P NMR: 493.2
ppm), (PNP)H (20%, 31P NMR: -12.7), H2P[DMP] (7%, 31P NMR:
-146.5), phosphaindole (5%, 31P NMR: -27.4 ppm), and traces of
another product (∼1%, dm, 31P NMR: -91.6 ppm, JH-P ) 205 Hz),
which has been tentatively assigned as the phosphinidene insertion
into the C-H bond of one of the mesityl methyl groups to make a
dihydrophosphanthridine.17 Although we have been unable to char-
acterize the titanium product produced from this reaction, we propose
the generation of phosphanylidene-σ4-phosphorane to occur via reduc-
tive coupling between a titanium phosphinidene (possibly a dimer)
and P(CH3)3.18,19 Our speculation is corroborated by the reaction of 3
with Cp2TiCl2 to rapidly form (PNP)ScCl2 as well as a metastable
complex we propose to be a titanium phosphinidene (31P NMR: 1065.3
ppm),4,18 which transforms to [DMP]PdP(CH3)3 and a new para-
magnetic titanium species.19 As observed with Cp*2TiCl2, complete
consumption of 3 can only be achieved when 2 equiv of Cp2TiCl2 are
used. The reaction mixture also reveals formation of the diphosphene
[DMP]PdP[DMP] (36%), (PNP)H (5%), H2P[DMP] (5%), dihydro-
phosphanthridine (∼1%), and another unidentified product (9%).17 To
our knowledge, the transfer of a PAr unit directly to a phosphine is a
rare phenomenon20,21 therefore rendering complex 3 a powerful
delivery vehicle, presumably due to the polarized nature of the Sc-P
bond as well as the nonredox behavior of the Sc(III) ion.
(9) (a) Hardman, N. J.; Twamley, B.; Stender, M.; Baldwin, R.; Hino, S.;
Schiemenz, B.; Kauzlarich, S. M.; Power, P. P. J. Organomet. Chem. 2002,
643-644, 461. (b) Wehaschulte, R. J.; Power, P. P. J. Am. Chem. Soc.
1997, 119, 2847.
(10) Zschunke, A.; Riemer, M.; Schmidt, H.; Issleib, K. Phosphorus and Sulphur
1983, 17, 237.
(11) Some representative Sc(III)sBr distances (2.58-2.8 Å): (a) Neculai, A. M.;
Neculai, M.; Nikifarov, G. B.; Roesky, H. W.; Schicker, C.; Herbst-Irmer, R.;
Magull, J.; Noltemeyer, M. Eur. J. Inorg. Chem. 2003, 3120. (b) Neculai,
A. M.; Neculai, M.; Roesky, H. W.; Magull, J.; Baldus, M.; Andronesi, O.;
Jansen, M. Organometallics 2002, 21, 2590. (c) Ishikawa, S.; Hamada, T.;
Manabe, K.; Kobayashi, S. J. Am. Chem. Soc. 2004, 126, 12236. (d) Hill,
N. J.; Levason, W.; Popham, M. C.; Reid, G.; Webster, M. Polyhedron 2002,
25, 1579. (e) Lu, K.; Cui, D. Organometallics 2009, 5438.
(12) (a) Neumann, F.; Hampel, F.; Schleyer, P. V. R. Inorg. Chem. 1995, 34,
6553. (b) Spring, D. R.; Krishnan, S.; Blackwell, H. E.; Schreiber, S. L.
J. Am. Chem. Soc. 2002, 124, 1354.
(13) A molecular structure of the phosphaalkene is reported in the SI.
(14) Urnezius, E.; Lam, K.-C.; Rheingold, A. L.; Protasiewicz, J. D. J.
Organomet. Chem. 2001, 630, 193.
(15) Smith, R. C.; Urnzius, E.; Lan, K.-C.; Rheingold, A. L.; Protasiewicz, J. D.
Inorg. Chem. 2002, 41, 5296.
(16) Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585.
(17) Phospha-Wittig reagents are known to decompose to the diphosphinene
and can often form the corresponding primary phosphine, phosphaindole,
and/or dihydrophosphanthridine. (a) Urnezius, E.; Klippenstein, S. J.;
Protasiewicz, J. D. Inorg. Chem. Acta 2000, 297, 181. (b) Shah, S.; Simpson,
M. C.; Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2001, 123,
6925–6926. (c) Champion, D. H.; Cowley, A. H. Polyhedron 1985, 4, 1791.
(d) Amor, I.; Garcia, M. E.; Ruiz, M. A.; Saez, D.; Hamidov, H.; Jeffery,
J. C. Organometallics 2006, 25, 4897. Although the dihydrophosphanthri-
dine has not been characterized, a similar phospha-indole formed by
t
insertion of the phosphene into the CsH bond of a Bu group on a Mes*
ring has been reported with a similar chemical shift (dm,-78.4, JHsP
)
Other intermetal PAr transfer studies as well as current efforts
to remove the Li+ are underway in our laboratories. Complexes
such as 3 can deliver the PAr unit to common metal-halide
precursors, therefore rendering them even more powerful than
prototypical phospha-Wittig reagents such as ArPdP(CH3)3,16,22
or even early transition metal phosphinidene complexes such as
Cp2ZrdP[Ar](P(CH3)3).2,3b,14
182 Hz): (e) Bo¨hm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001,
40, 4694. (f) Stradiotto, M.; Fujdala, K. L.; Tilley, T. D. HelV. Chim. Acta
2001, 84, 2958.
(18) Titanium phosphinidenes, although rare, have been reported in the literature.
(a) Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. J. Am.
Chem. Soc. 2003, 125, 10170. (b) Basuli, F.; Watson, L. A.; Huffman,
J. C.; Mindiola, D. J. Dalton Trans. 2003, 4228.
(19) 1H and 31P NMR spectra of the mixture do not suggest formation of the
Ti(II) complexes Cp2Ti(P(CH3)3)2 or [Cp2Ti(P(CH3)3)](µ2-N2) but, instead,
the known Ti(III) complex Cp2Ti(P(CH3)3)Cl (1H NMR: 34.1 ppm, ∆ν1/2
) 3210 Hz; 0.31 ppm, ∆ν1/2 ) 100 Hz). (a) Hao, L.; Harrod, J. F. Inorg.
Chem. 1999, 2, 191. (b) Kool, L. B.; Rausch, M. D.; Ah, H. G.; Herberhold,
M.; Thewalt, U.; Wolf, B. Angew. Chem., Int. Ed. Engl. 1985, 24, 400. (c)
Berry, D. H.; Procopio, L. J.; Carroll, P. J. Organometallics 1988, 7, 570.
Cp*2TiCl and Cp2Ti(P(CH3)3)Cl were synthesized according to literature
procedures and 1H NMR spectra were compared with our mixtures which
suggest these species to be likely products formed in the reaction. (d)
Pattiasina, J. W.; Heeres, H. J.; Van Bolhuis, F.; Meetsma, A.; Teuben,
J. H.; Spek, A. L. Organometallics 1987, 6, 2004. (e) Hao, L.; Harrod,
J. F. Inorg. Chem. 1999, 2, 191.
Acknowledgment. Financial support of this research was
provided by the National Science Foundation (CHE-0848248). J.S.
acknowledges postdoctoral financial support from NSERC. We also
thank Prof. John D. Protasiewicz for insightful discussions.
Supporting Information Available: Experimental procedures, X-ray
crystallographic information and spectral data are provided. This
material is available free of charge via the Internet at http://pubs.acs.org.
(20) Reversible transfer of a phosphinidene (PSiiPr3) to PPh3 has been documented.
Piro, N. A.; Cummins, C. C. J. Am. Chem. Soc. 2009, 131, 8764.
(21) Urnezius, E.; Shah, S.; Protasiewicz, J. D. Phosphorus, Sulfur 1999, 137.
(22) Kilgore, U. J.; Fan, H.; Pink, M.; Urnezius, E.; Protasiewicz, J. D.; Mindiola,
D. J. Chem. Commun. 2009, 4521.
References
(1) Giesbrecht, G. R.; Gordon, J. C. Dalton Trans. 2004, 2387.
(2) (a) Lammertsma, K.; Vlaar, M. J. M. Eur. J. Org. Chem. 2002, 1127. (b)
Cowley, A. H.Acc. Chem. Res. 1997, 30, 445. (c) Cowley, A. H.; Barron,
JA100214E
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