Strong coordination of tetraphenylborate anion to copper(l) bipyridine and phenanthroline-based complexes and its effect on catalytic activity in the cyclopropanation of styrene

Article abstract of DOI:10.1021/ic801783r

The synthesis, characterization, and cyclopropanation activity of tetrahedral copper(l) complexes with bipyridine- and phenanthroline- based ligands containing strongly coordinated tetraphenylborate anions are reported. Cu¹(bpy)(BPh₄

Full text of DOI:10.1021/ic801783r

Inorg. Chem. 2009, 48, 16-18
Strong Coordination of Tetraphenylborate Anion to Copper(I) Bipyridine
and Phenanthroline-Based Complexes and Its Effect on Catalytic
Activity in the Cyclopropanation of Styrene
Carolynne Ricardo, Lauren M. Matosziuk, Jeffrey D. Evanseck, and Tomislav Pintauer*
Department of Chemistry and Biochemistry, Duquesne UniVersity, 600 Forbes AVenue, 308 Mellon
Hall, Pittsburgh, PennsylVania 15282
Received September 18, 2008
Scheme 1. Proposed Mechanism for Copper(I) Catalyzed
Cyclopropanation of Alkenes
The synthesis, characterization, and cyclopropanation activity of
tetrahedral copper(I) complexes with bipyridine- and phenanthroline-
based ligands containing strongly coordinated tetraphenylborate
anions are reported. CuI(bpy)(BPh4), CuI(phen)(BPh4), and CuI(3,4,7,8-
Me4phen)(BPh4) complexes are the first examples in which the
BPh4- counterion chelates a transition metal center in bidentate
fashion through η2 π interactions with two of its phenyl rings.
The asymmetric transition-metal-catalyzed transfer of
carbene moieties generated from diazo reagents onto the
prochiral face of an olefin to form a cyclopropane ring is a
widely used reaction in organic synthesis.1,2 Effective
catalytic systems for this reaction require the use of a
transition metal complex which facilitates the loss of N2 from
the diazo reagent as well as stabilizes intermediate carbene
species against competing carbene dimerization. A variety
of transition metal complexes have been found to be active
in cyclopropanation reactions, and they include the com-
plexes of Fe, Rh, Ru, and Cu. Copper appears to be
particularly attractive because of its low cost relative to other
transition metal complexes. So far, cationic and neutral
copper(I) complexes in conjunction with a C2-symmetric
nitrogen-based ligands have been successfully used in
cyclopropanation reactions.3 However, despite a tremendous
effort directed toward empirical catalyst development, the
mechanism of this very important synthetic reaction is still
not fully understood. It is generally accepted that copper-
catalyzed cyclopropanation reactions proceed via a copper-
carbene intermediate, as indicated in Scheme 1. However,
the details of this process are not well-known. Only very
recently, copper-carbene complexes have been detected as
reaction intermediates in cyclopropanation reactions utilizing
low-temperature NMR measurements and X-ray diffrac-
tion.4,5 Another key mechanistic feature of this reaction
includes the role of the monomer and counterion, both of
which are poorly understood. Mechanistic and computational
studies have indicated that copper(I)/olefin complexes in
these systems might act as either catalytically active species
or resting states.6 Furthermore, the reactivity of cationic
copper(I) complexes is strongly influenced by the counterion.
Triflates and hexafluorophosphates were found to be highly
effective catalysts, whereas halides, cyanides, acetates, and
perchlorate showed little or no catalytic activity.6
In a recent study, our research group reported on the
synthesis, characterization, and cyclopropanation activity of a
series of well-defined [CuI(2,2′-bpy)(π-M)][A] (M ) styrene
and methyl acrylate; A ) ClO4-, PF6-, and CF3SO3-) com-
* Author to whom correspondence should be addressed. Tel: +412-
396-1626, Fax: 412-396-5683, Email: pintauert@duq.edu
(1) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; Wiley: New York,
1998.
(4) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085–10094.
(5) Straub, B. F.; Hofmann, P. Angew. Chem., Int. Ed. 2001, 113, 1328–
1330.
(6) Evans, D. A.; Faul, M. M.; Bilodeau, M. T. J. Am. Chem. Soc. 1994,
116, 2742–2753.
(2) Pfaltz, A. ComprehensiVe Asymmetric Catalysis; Springer: Berlin,
1999; Vol. II.
(3) Chelucci, G.; Thummel, R. P. Chem. ReV. 2002, 102, 3129–3170.
16 Inorganic Chemistry, Vol. 48, No. 1, 2009
10.1021/ic801783r CCC: $40.75  2009 American Chemical Society
Published on Web 12/09/2008

COMMUNICATION
Figure 1. Molecular structures of [CuI(3,4,7,8-Me4phen)(BPh4)]·3CH2Cl2 (1), CuI(bpy)(BPh4) (2), and CuI(phen)(BPh4) (3) shown with thermal ellipsoids
at 50% probability. H atoms and solvent molecules have been omitted for clarity.
plexes.7,8 In the solid state, all complexes were distorted trigonal
Scheme 2. Synthesis of Copper(I) Complexes 1-3
pyramidal in geometry, and the copper(I) atom was coordinated
by two nitrogen atoms of the bpy ligand, two olefinic carbon
atoms of alkene in the equatorial position, and a weakly
coordinated counterion in the axial position. Moreover, in the
case of styrene, the axial coordination of the counterion observed
in the solid state was found to have a small effect on the binding
constant of styrene to [CuI(bpy)]+ cations in solution, resulting
in similar cyclopropanation activities. In our effort to isolate
and structurally characterize copper(I)/styrene complexes with
interactions with a CdC double bond of the adjacent phenyl
-
rings in the BPh4 anion (CuI-Cmid,1-2 ) 2.289(5) Å (1),
2.148(3) Å (2), 2.121(3) Å (3); CuI-Cmid,3-4 ) 2.073(4) Å
(1), 2.236(6) Å (2), 2.203(3) Å (3)). On the basis of extensive
much weaker coordinating anions, we observed a novel mode
of coordination of BPh4- anions to copper(I) complexes with
literature and CCDB searches, such coordination of the
neutral bidentate nitrogen-based ligands. In this article, we report
on the synthesis, characterization, and cyclopropanation activity
-
BPh4 anion to transition metal complexes has never been
observed before. With the exception of one example of η2-
of tetrahedral copper(I) complexes with bipyridine- and phenan-
coordination (CuI(NH2CH2CH2NH2)(CO)(η2-BPh4)), all of
throline-based ligands containing strongly coordinated BPh4-
the structures reported so far exhibit η6-coordination (e.g.,
anions.
Ru(Cp)(η6-BPh4) and Rh(P(OMe3)2(η6-BPh4)).11
The starting copper(I) complex for the syntheses of
The coordination of BPh4- to the copper(I) center in 1-3
CuI(3,4,7,8-Me4phen)(BPh4) (1), CuI(bpy)(BPh4) (2), and
did not result in a significant change in the geometry of the
anion. The bent angle between the B atom and adjacent C
CuI(phen)(BPh4) (3) was obtained by methathesis of the
perchlorate salt [CuI(CH3CN)4][ClO4] with NaBPh4 in CH3CN/
atoms in the coordinated phenyl rings (104.3(2)° (1),
H2O, according to a modified literature procedure.9 This
103.1(2)° (2), and 101.92(11)° (3)) was slightly smaller than
reaction did not yield the expected [CuI(CH3CN)4][BPh4]
the corresponding angle for the uncoordinated ones (112.3(2)°
complex, but rather [CuI(CH3CN)][BPh4], the composition
(1), 111.3(2)° (2), and 113.84(11)° (3)). In copper(I)
of which was confirmed by elemental analysis and 1H NMR.
-
complexes with noncoordinating BPh4 anions, these two
The loss of CH3CN after prolonged in vacuo treatment has
angles are also different (101-105° and 110-114°).12
been observed previously in [CuI(CH3CN)4][BF4] and
Furthermore, CuI-C bond distances for π-coordinated CdC
[CuI(CH3CN)4][B(C6H5)4] complexes.10 We were unable to
bonds in phenyl rings (1.412(3) and 1.415(4) Å (1); 1.412(3)
and 1.413(4) Å (2); 1.410(2) and 1.4118(18) Å (3)) did not
obtain crystals of [CuI(CH3CN)][BPh4] suitable for X-ray
diffraction studies; however, on the basis of the molecular
change significantly upon coordination.
structures of 1, 2, and 3 (Figure 1), one possibility would
The coordination of the BPh4- anion to the copper(I) center
can be suppressed using sterically more-hindered 2,9-
include a tricoordinated copper(I) complex, as indicated in
Scheme 1. The reaction of [CuI(CH3CN)][BPh4] with a
Me2phen. In the case of this ligand, we observed a quantita-
stoichiometric amount of bipyridine- or phenanthroline-based
tive formation of the [CuI(2,9-Me2phen)(CH3CN)][BPh4]
ligand in CH2Cl2, followed by slow diffusion of pentane,
complex (see the Supporting Information). In [CuI(2,9-
afforded crystals of 1, 2, and 3 in 92%, 55%, and 40% yield,
Me2phen)(CH3CN)][BPh4], the counterion was found to be
noncoordinating. Additionally, the crystal structure was
respectively (Scheme 2).
Shown in Figure 1 are the molecular structures of 1-3.
stabilized by a series of short C-H · · · C contacts between
In all three complexes, bidentate coordination of the nitrogen
the BPh4- anion and coordinated CH3CN (2.841(2)-2.867(4)
ligand was observed (CuI-Nav ) 2.047(3) Å (1), 2.062(3)
Å), as well as 2,9-Me2phen (2.866(3)-2.894(4) Å; see the
Å (2), and 2.055(2) Å (3)) and the distorted tetrahedral
Supporting Information). Short contacts were also observed
coordination around the CuI atom was completed by two π
in the structurally similar [CuI(2,9-Me2phen)(CH3CN)][PF6]
complex (see the Supporting Information).
(7) Pintauer, T. J. Organomet. Chem. 2006, 691, 3948–3953.
(8) Ricardo, C.; Pintauer, T. J. Organomet. Chem. 2007, 692, 5165–5172.
(9) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D.
Inorg. Chem. 2003, 41, 2209–2212.
(10) Ogura, T. Trans. Met. Chem. 1976, 1, 179–182.
(11) Strauss, S. H. Chem. ReV. 1993, 93, 927–942.
(12) Braunecker, W. A.; Pintauer, T.; Tsarevsky, N. V.; Kickelbick, G.;
Matyjaszewski, K. J. Organomet. Chem. 2005, 690, 916–924.
Inorganic Chemistry, Vol. 48, No. 1, 2009 17

COMMUNICATION
Table 1. Cyclopropanation of Styrene in the Presence of EDA
Catalyzed by 1-3a
a [CuI]0/[EDA]0/[Sty]0 ) 1:50:500, [CuI]0 ) 1.7 × 10-3 M, (CH2Cl2,
3 h, 25 °C). The % yield is based on 1H NMR (rel. err. are ( 10%). b Results
from previous study with [CuI(bpy)(π-styrene)][A] complexes, ref 8.
similar. Also, when compared to our previous results using
well-defined [CuI(bpy)(π-sty)][A] complexes, 1-3 appear
to be more selective toward trans-cyclopropane.8
Figure 2. NBO orbital interactions between C-C π bonds (a) and B-C
-
σ bonds (b) of the BPh4 counterion and n* orbital of copper (>99% s
character) and between filled d orbitals of copper ((c) LP(4) and (d) LP(5))
The observed rate constants (kobs) for the decomposition
of EDA in the presence of externally added styrene were
determined to be kobs ) (1.5 ( 0.12) × 10-3 min-1 (1), (6.8
( 0.30) × 10-3 min-1 (2), and (5.1 ( 0.19) × 10-3 min-1
(3). These rates should depend on the equilibrium constants
KO and KL, corresponding to the formation of [CuI(NN)(π-
olefin)][A] and [CuI(NN)2][A] complexes, respectively
(Scheme 1). However, our previous results using [CuI(bpy)(π-
Sty)][A] complexes indicated that kobs and KO were relatively
independent of the nature of counterion ((1.4 ( 0.041) ×
and empty C-C π* orbitals of phenyl rings.
The 1H NMR spectrum of 1 in CD2Cl2 appeared relatively
sharp at 22 °C and was fully consistent with the solid-state
X-ray structure. The corresponding spectra of 2 and 3 were
significantly broadened, indicating a fluxional system, which
was presumably induced by ligand or anion association or
dissociation. In all three complexes, the proton resonances
-
associated with the BPh4 anion were identical to those of
NaBPh4. In the presence of as much as 15 equiv of alkene
(styrene, methyl acrylate, and 1-octene), the 1H NMR spectra
of 1-3 remained unchanged. In other words, no visible
shielding of vinyl protons upon coordination was observed,
which is typical for copper(I)/alkene complexes.12 These
results clearly indicate that BPh4- anions strongly coordinate
to the copper(I) center.
-
10-2 min-1 and 4.3 × 103 M-1 (A ) CF3SO3 ) and (1.0 (
-
0.025) × 10-2 min-1 and 4.4 × 103 M-1 (A ) PF6 ),
respectively).8 Therefore, assuming a constant value for KL,
the most reasonable explanation for the decrease in the
observed rate constant for the decomposition of EDA in the
case of 1-3 can be attributed to relatively strong coordination
-
-
of the BPh4 anion. Furthermore, these data also indicate
In order to further examine the nature of bonding of BPh4
that the binding constant of BPh4- in 1 is stronger than in 2
or 3, which is presumably induced by the stronger coordina-
tion of 3,4,7,8-Me4phen (pKa ) 6.58), relative to bpy (pKa
) 4.60) or phen (pKa ) 4.80). Detailed kinetic, mechanistic,
and computational studies of the catalytic activity of cop-
per(I) complexes 1-3 in cyclopropanation are subject to
further investigation in our laboratories.
to CuI in 1-3, particularly with respect to σ donation from
coordinated CdC bonds in BPh4- and π-backbonding from
CuI, we have conducted additional density functional theory
and natural bond order (NBO) calculations (see Supporting
Information). The results obtained for 1 and 2 using three
differentbasissets(6-31+G(d),6-31G(2d,p),and6-311++G(2d,p))
indicated that σ donation from the BPh4- anion to copper(I)
is the dominant interaction (approximately 75%), with the
remaining 25% being attributed to classical π-backbonding
from copper(I). Also, the NBO calculations indicated that
B-C σ f n*(Cu) interactions were quite significant, with
the charge transfer of the interaction totaling approximately
half of the typical CdC π-n*(Cu) interaction. Representa-
tive orbital interactions are shown in Figure 2.
In conclusion, the synthesis, characterization, and cyclo-
propanation activity of tetrahedral copper(I) complexes with
bipyridine- and phenanthroline-based ligands containing
strongly coordinated BPh4- counterions were reported. These
-
complexes are the first examples in which the BPh4 anion
chelates a transition metal center in bidentate fashion through
η2 π interactions with two of its phenyl rings.
-
In order to further investigate the effect of BPh4
Acknowledgment. T.P. acknowledges financial support
from Duquesne University (startup fund and faculty develop-
ment grant), NSF X-ray facility grant (CRIF 0234872), NSF
NMR grant (CHE 0614785), and PRF fund (44542-G7).
coordination in 1-3 on catalytic activity, cyclopropanation
of styrene in the presence of ethyldiazoacetate (EDA) was
conducted. The reactions were performed in CH2Cl2 at room
temperature using standard reaction conditions [CuI]0/[Sty]0/
[EDA]0 ) 1:500:50. The product distribution was determined
by 1H NMR, and the results are summarized in Table 1. For
all three complexes, the relative amounts of EDA decom-
position products ranged between 2.4 and 7.0%. Furthermore,
the mole percents of trans- and cis-cyclopropane were very
Supporting Information Available: Synthetic procedures and
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC801783R
18 Inorganic Chemistry, Vol. 48, No. 1, 2009