Synthesis of the (dialkylamino)borate, [Ph2B(CH2NME2)2]-, affords access to N-chelated rhodium(I) zwitterions

Article abstract of DOI:10.1021/ic0259336

This paper reports the synthesis of the first bis(amino)borate ligand, [Ph₂B(CH₂NMe₂)₂]^-, an anionic equivalent of tertiary diamines. Anionic [Ph₂B(CH₂NMe₂)₂] is an excellent bidentate ligand auxiliary and is used to prepare a series of N-chelated, zwitterionic rhodium(I) complexes.

Full text of DOI:10.1021/ic0259336

Inorg. Chem. 2002, 41, 6541−6543
Synthesis of the (Dialkylamino)borate, [Ph2B(CH NMe2)2]-, Affords
2
Access to N-Chelated Rhodium(I) Zwitterions
Theodore A. Betley and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering,
Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received August 6, 2002
This paper reports the synthesis of the first bis(amino)borate ligand,
Our strategy for the assembly of the [Ph2B(CH2NMe2)2]
[Ph2B(CH NMe2)2]-, an anionic equivalent of tertiary diamines.
anion focused on delivering a nucleophilic carbanion, Me2-
NCH2Li, to a disubstituted borane electrophile, Ph2BCl. A
related strategy has been used recently in the synthesis of
(thioether)borate ligands and sterically encumbered (diphe-
nylphosphino)borate ligands.3-4 Following the method of
Tamborski, [Me2NCH2Li] was generated by a transmetalla-
2
Anionic [Ph2B(CH NMe2)2] is an excellent bidentate ligand auxiliary
2
and is used to prepare a series of N-chelated, zwitterionic rhodium-
(I) complexes.
n
tion between BuLi and Me3SnCH2NMe2.5 Unfortunately,
Poly(pyrazolyl)borate ligands have played a prominent role
in chemistry since their introduction by Trofimenko in 1966,
evidenced by the fact that nearly 200 bis- and tris(pyrazolyl)-
borates have now been prepared.1 Widespread interest in
these and other uninegative N-donor ligand sets, such as the
increasingly popular diketiminates, stems from their ubiq-
uitous utility for research in bioinorganic model chemistry,
C-H activation studies, and many areas of catalysis.2
Currently absent from the available tool kit of anionic
N-donor chelates are simple ligands that present tertiary
amine donors to a coordinated metal center. The lack of such
ligands is particularly striking given the longstanding interest
in neutral tertiary amine chelates (e.g., tetramethylethylene-
diamine, TMEDA). As part of an interest in developing
zwitterionic complexes featuring borate anions fastened to
a coordinated metal center by neutral donor ligands that
partially insulate the borate charge, we set out to prepare
the simple bis(dialkylamino)borate, [Ph2B(CH2NMe2)2]-, an
anionic equivalent to potentially bidentate tertiary diamines.
Herein, we report its synthesis and explore some of its initial
rhodium coordination chemistry.
attempts to smoothly deliver this carbanion to Ph2BCl were
unsuccessful. The complex mixtures that resulted presumably
arose from kinetically competitive formation of Lewis acid/
base adducts. Analysis of the product mixtures by electro-
spray MS revealed fragments consistent with small cyclic
structures that we tentatively formulated as {Ph2B(CH2-
NMe2)}x (x ) 2 or 3).
To circumvent the competitive binding of the amine donor
to the borane electrophile, we sought a versatile carbanion
reagent in which the N-donor lone pair was protected. While
BH3 protection of tertiary phosphines can be an effective
means for the synthesis of alkyl and arylphosphine carban-
ions (i.e., R2P(BH3)CH2Li),6 similar methods had not, to our
knowledge, been explored for tertiary amine precursors.7 The
n
room temperature addition of BuLi to a THF solution of
Me3N‚BH3 cleanly afforded Me2N(BH3)CH2Li(THF) (1) as
a white, microcrystalline solid (Scheme 1). This reagent
provides a well-behaved carbanion source of trimethylamine.
(3) (a) Ge, P.; Haggerty, B.; Rheingold, A. L.; Riordan, C. G. J. Am.
Chem. Soc. 1994, 116, 8406. (b) Barney, A. A.; Heyduk, A. F.; Nocera,
D. G. Chem. Commun. 1999, 2379. (c) Peters, J. C.; Feldman, J. D.;
Tilley, T. D. J. Am. Chem. Soc. 1999, 121, 9871. (d) Shapiro, I. R.;
Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters, J. C. Chem.
Commun. 2001, 2152.
* Author to whom correspondence is addressed. E-mail: jpeters@
caltech.edu.
(1) See: Trofimenko, S. Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College Press: London, 1999
and references contained therein.
(4) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100.
(5) (a) Tamborski, C.; Ford, F. E.; Soloski, E. J. J. Org. Chem. 1964, 29,
883. (b) Peterson, D. J. J. Organomet. Chem. 1970, 21, P63.
(6) (a) Schmidbaur, J. J. Organomet. Chem. 1980, 200, 287 and references
therein. (b) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato,
K. J. Am. Chem. Soc. 1991, 112, 5244. (c) Brunel, J. M.; Faure, B.;
Maffei, M. Coord. Chem. ReV. 1998, 178-180, 665. (d) Carboni, B.;
Monnier, L. Tetrahedron 1999, 55, 1197.
(7) Borane-protected benzylic amines and substituted aziridines have been
alkylated via intermediate generation of a benzylic or aziridinyl
carbanion: (a) Ebden, M. R.; Simpkins, N. S.; Fox, D. N. A.
Tetrahedron 1998, 54, 129. (b) Vedejs, E.; Kendall, J. T. J. Am. Chem.
Soc. 1997, 119, 6941.
(2) (a) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P.
L.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108. (b) Smith, J.
M.; Lachiotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers,
G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222.
(c) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998. (d) Fekl, U.;
Goldberg, K. I. J. Am. Chem. Soc. 2002, 124, 6804. (e) Radzewich,
C. E.; Guzei, I. A.; Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 8673.
(f) Schmidt, J. A. R.; Arnold, J. Organometallics 2002, 21, 2306. (g)
Kakaliou, L.; Scanlon, W. J.; Qian, B. X.; Baek, S. W.; Smith, M. R.;
Motry, D. H. Inorg. Chem. 1999, 38, 5964.
10.1021/ic0259336 CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/12/2002
Inorganic Chemistry, Vol. 41, No. 25, 2002 6541

COMMUNICATION
Scheme 1
Addition of isolated 1 to half an equivalent of diphenylchlo-
roborane (25 °C; toluene) generated the borane-protected
amino(borate) complex [Ph2B(CH2N(BH3)Me2)2][Li], [2][Li].
This species was conveniently precipitated from diethyl ether
by addition of TMEDA to produce [2][Li(TMEDA)2] as a
white solid. To derive the target ligand, we canvassed myriad
potentially suitable methods to remove the borane protecting
group from [2]. Borane liberation from alkylamines is
traditionally accomplished by transfer to another strong
Lewis base, acidification, or oxidation.6,8 Unfortunately, [2]
degraded to Me3N‚BH3 on exposure to strong acid (e.g. HCl)
and resisted borane oxidation in the presence of Pd/C in
methanol (25 wt % Pd/C, 3 days, 25 °C). We thus focused
on a Lewis base deprotection strategy.
Having established the synthesis of several useful salts of
the target [Ph2B(CH2NMe2)2] anion, we have begun to
examine its utility to support metal complexes and wish to
introduce this by way of its initial rhodium(I) chemistry. The
lithium salt [3][Li] underwent facile transmetallation with
{(NBD)RhCl}2 to afford the crystalline yellow rhodium
complex [Ph2B(CH2NMe2)2]Rh(NBD) (4) (NBD ) norbor-
nadiene) in 86% isolated yield. Complex 4 was thoroughly
characterized, and it is noteworthy that the parent ion was
detected by electrospray mass spectroscopy (ES-MS). NBD
adduct 4 is formally zwitterionic, and unlike related (pyra-
zolyl)borate ligands, simple resonance contributors that
delocalize the anionic borate charge to the amine donor
groups are not available. To a first approximation, this is
also true of (phosphino)- and (thioether)borate ligands,
though it seems likely that different donor arms would
insulate the tethered borate unit to varying degrees. To
ascertain whether any ion-pairing of the borate unit to the
partially “cationic” rhodium center would be observed in the
solid state, we performed an X-ray diffraction study on a
single crystal of 4.11 Figure 1 depicts two views of 4 and
reveals that the diphenylborate unit resides at a position quite
removed from the rhodium metal center (Rh-B ) 3.672 Å,
Figure 2a). In fact, the borate unit appears to be fastened as
far from the rhodium center as possible: a vector bisects
the molecule that contains the bridgehead methylene of the
NBD ligand, the Rh center, and the borate itself. This
conformation is very different from that typically adopted
by square planar bis(pyrazolyl)borate complexes, where
appreciable canting of the chelate ring forces the borate unit
much closer to the coordinated metal center. For example,
the previously reported bis(ethylene) derivative [H2B(pz)2]-
Rh(C2H4)2, also represented in Figure 2, adopts a puckered
ligand confirmation that places the borate unit 2.998 Å away
from the bound rhodium atom. This distance is 0.67 Å shorter
than the Rh-B distance in 4.12
Because the target ligand [Ph2B(CH2NMe2)2] should itself
be an extremely good Lewis base, most reagents proved
ineffective. For example, incubation of [2] in neat solutions
of morpholine, pyrrolidine, piperidine, or tributylphosphine
at 100 °C for a period of days afforded no deprotection. A
large excess of less encumbered phosphines, such as PMe3
and PEt3, did provide a modest degree of deprotection above
80 °C over several days; however, clean product was not
isolable with these reagents. The system of choice proved
to be a 25-fold excess of 1,4-diazabicyclo[2.2.2]octane
(DABCO) (toluene, 100 °C, complete and quantitative after
10 h as determined by 11B NMR). The excess DABCO was
effectively recovered by sublimation (60 °C, 10 Torr), and
the target ligand [Ph2BNMe2] was isolated as its lithium salt
[Ph2B(CH2NMe2)2][Li], [3][Li], in yields that typically
exceeded 80%. The ammonium salt [3][NEt4] formed readily
upon addition of an ethanolic solution of Et4NBr to [3] [Li].
An initial attempt to deprotect the ammonium salt [2]-
[NEt4] under similar conditions resulted in a Hoffmann
degradation reaction that led to the formation of [3][H],9
whose solid-state structure is represented in Scheme 1.10 The
fortuitous formation of this free acid derivative provided
ready access to a range of other salts, [3][M+], by simple
To obtain comparative data pertaining to the donor strength
of anionic [3], we prepared the dicarbonyl complex [Ph2B(CH2-
NMe2)2]Rh(CO)2 (5), which proceeded quantitatively upon
n
base deprotonation (where M+ ) Li, Na, K from BuLi,
NaOtBu, and KOtBu, respectively; see Scheme 1).
(8) Couturier, M.; Tucker, J. L.; Andresen, B. M.; Dube, P.; Negri, J. T.
Org. Lett. 2001, 3, 465.
(11) 4 (C25H34BN2Rh): MW ) 476.27, yellow plate, collection temperature
) 98 K, monoclinic, space group P21/n, a ) 12.233(1) Å, b ) 9.538-
(2) Å, c ) 18.599(5) Å, R ) 90°, â ) 91.504(4)°, γ ) 90°, V )
2194.12(3) Å3, Z ) 4, R1 ) 0.0805 [I > 2σ(I)], GOF ) 1.102.
(12) Baena, M. J.; Reyes, M. L.; Rey, L.; Carmena, E.; Nicasio, M. C.;
Perez, P. J.; Guitierrez, E.; Monge, A. Inorg. Chim. Acta. 1998, 273,
244.
(9) Feit, I. N.; Saunders, M. H. J. Am. Chem. Soc. 1970, 92, 5615.
(10) [3][H] (C18H27BN2): MW ) 282.23, colorless prism, collection
temperature ) 98 K, monoclinic, space group P21/n, a ) 7.787(3) Å,
b ) 13.571(1) Å, c ) 15.931 Å, R ) 90°, â ) 91.47(2)°, γ ) 90°,
V ) 1683.15(3) Å3, Z ) 4, R1 ) 0.0855 [I > 2σ(I)], GOF ) 1.158.
6542 Inorganic Chemistry, Vol. 41, No. 25, 2002

COMMUNICATION
Scheme 2
delocalization in the latter, but is consistent with the notion
that a trialkylamine ligand is a stronger σ-donor and also a
poorer π-acid than a pyrazolyl ligand.
While the reactivities of “[Ph2B(CH2NMe2)2]Rh(I)” frag-
ments have yet to be thoroughly explored, we can report on
the synthesis of several promising synthons for future study.
For example, the room temperature addition of the tertiary
phosphines PMe3 and PPh3 to 5 provided the monocarbonyl
derivatives [Ph2B(CH2NMe2)2]Rh(CO)(PMe3), 8, and [Ph2B-
(CH2NMe2)2]Rh(CO)(PPh3), 9, respectively. Likewise, the
substitution of one carbonyl ligand on 5 by a bulky, saturated
Arduengo-type carbene, (1,3-bis-(2,6-diisopropylphenyl)-
imidazole-2-ylidene), gave rise to the monocarbonyl complex
[Ph2B(CH2NMe2)2]Rh(CO)(carbene), 10. The bis(phosphine)
complex [Ph2B(CH2NMe2)2]Rh(PMe3)2, 11, formed on stir-
ring 4 in the presence of 2 equiv of phosphine. All of these
substitutions proceeded quantitatively according to their crude
NMR spectra, and isolated yields generally exceeded 80%
(Scheme 2). To derive a precursor with relatively labile co-
ligands, norbornadiene adduct 4 was subjected to hydroge-
native conditions in the presence of excess acetonitrile. This
led rapidly and in good yield to the isolable solvento complex
[Ph2B(CH2NMe2)2]Rh(CH3CN)2, 12. The stability of zwit-
terionic 12 is to be contrasted with its cationic analogue [Ph2-
Si(CH2NMe2)2]Rh(CH3CN)2][PF6], which we could not
isolate. An attempt to hydrogenate [Ph2Si(CH2NMe2)2]Rh-
(NBD)][PF6] (13) in the presence of excess acetonitrile, first
prepared by a method analogous to that used in the generation
of 4 using {(NBD)RhCl}2, resulted in the rapid precipitation
of rhodium(0). Finally, we note the encouraging result that
complex 12 mediated the rapid catalytic reduction of styrene
in d6-acetone (see Supporting Information). Studies are
underway to further probe the reactivity of the amine-chelated
zwitterions described here.
Figure 1. 50% displacement ellipsoid representation of 4 (two perspec-
tives) and a structural representation of the previously reported complex
[H2B(Pz)2]Rh(C2H4)2. Rh-B bond distances are indicated.
Scheme 3
addition of [3][Li] to {Rh(CO)2Cl}2. Dicarbonyl 5 exhibits
the two expected bands in the carbonyl region (ν(CO), CH2-
Cl2/KBr: 2070, 1992 cm-1). These bands are appreciably
lower in frequency than those previously reported for the
cationic complex [(TMEDA)Rh(CO)2][ClO4] (ν(CO), CH2-
Cl2/KBr: 2080, 2010 cm-1). To provide an electronic
comparison to an isostructural cation, we also prepared the
salt [Ph2Si(CH2NMe2)2]Rh(CO)2][PF6], 7.13 Addition of a
THF solution of {Rh(CO)2Cl}2 to a solution of Ph2Si(CH2-
NMe2)2 (6) and [Tl][PF6] produced 7 for spectroscopic
comparison (Scheme 3). The ν(CO) vibrations for 7 (KBr/
CH2Cl2) were observed at 2097 and 2030 cm-1, much higher
in frequency than for zwitterionic 5 and somewhat higher
than those observed for [(TMEDA)Rh(CO)2][ClO4]. The
difference in ν(CO) between 5 and 7 likely arises from
differences in electrostatic polarization effects for the two
systems, in addition to some delocalization of the borate
charge through the σ-framework for zwitterionic 5. It is also
interesting to compare the carbonyl stretches for 5 with those
of a related bis(pyrazolyl)borate complex, [H2B(pz)2]Rh-
(CO)2, which exibits ν(CO) stretches at 2088 and 2022
cm-1.14 That the ν(CO) bands of 5 are so much lower in
frequency than those of [H2B(pz)2]Rh(CO)2 is somewhat
surprising given the increased potential for anionic charge
Acknowledgment. We thank the NSF (Grant CHE-
0132216), the ACS Petroleum Research Fund, and the
Dreyfus Foundation for financial support. T.A.B. thanks the
Department of Defense for a graduate research fellowship.
Supporting Information Available: Experimental protocols
and characterization data; crystallographic data for complex [3]-
[H] and 4. This material is available free of charge via the Internet
at http://pubs.acs.org.
(13) Neutral amine ligand Ph2Si(CH2NMe2)2 was prepared first by delivery
of 1 to Ph2SiCl2 to form Ph2Si(CH2N(BH3)Me2)2 in 95% yield. Ph2-
Si(CH2N(BH3)Me2)2 was then deprotected by DABCO to provide Ph2-
Si(CH2NMe2)2 in 92% isolated yield.
(14) Flavio, B.; Minghetti, G.; Banditelli, G. J. Organomet. Chem. 1975,
87, 365.
IC0259336
Inorganic Chemistry, Vol. 41, No. 25, 2002 6543