C-C Bond Cleavage of Acetonitrile by a Dinuclear Copper(II) Cryptate

Article abstract of DOI:10.1021/ja031874z

The dinuclear copper(II) cryptate [Cu₂L](ClO₄)₄ (1) cleaves the C?C bond of acetonitrile at room temperature to produce a cyanide bridged complex of [Cu₂L(CN)](ClO₄)₃·2CH₃CN·4H₂O (2). The cleavage mechanism is presented on the basis of the results of the crystal structure of 2, electronic absorption spectra, ESI-MS spectroscopy, and GC spectra of 1, respectively. Copyright

Full text of DOI:10.1021/ja031874z

Published on Web 03/24/2004
C-C Bond Cleavage of Acetonitrile by a Dinuclear Copper(II) Cryptate
Tongbu Lu,* Xiaomei Zhuang, Yanwu Li,† and Shi Chen
State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineer,
and Instrumentation Analysis & Research Center, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China
Received December 21, 2003; E-mail: cesltb@zsu.edu.cn
Molecular recognition has been defined as a process involving
both binding and selecting of substrate(s) by a given receptor
molecule.1 Polyaza cryptands, incorporating tripodal skeleton
groups, possess two binding subunits located at the two poles of
the structure, so they can act as good receptors for linear molecular
recognition.2 These cryptands are also capable of dinuclear metal
coordination, and these dinuclear centers have in turn been shown
3,4
- 5
to recognize various bridging anions such as µ-OH-, µ-CO3
,
µ-OCN-,6 µ-N3-,7 and µ-im- (im- ) imidazolate anion).8 Recently,
Kra¨mer9 reported that a dinuclear macrocyclic copper compound
shows high recognition for the cyanide anion and can be used as a
cyanide-selective electrode.
Figure 1. ORTEP drawing of the [Cu2L(CN)]3+ cation in 2. Thermal
ellipsoids are drawn at the 30% level.
The activation of carbon-carbon bonds by transition metal
complexes in homogeneous media remains a challenge in the field
of organometallic chemistry.10 In most cases, metals catalyze the
hydration of nitriles to amides.11 However, Parkin12 has shown
recently that photolysis of [Me2Si(C5Me4)2]MoH2 in the presence
of acetonitrile results in oxidative addition of the C-C bond of
acetonitrile to form [Me2-Si(C5Me4)2]Mo(Me)(CN). Jones13 also
showed that reaction of [(dippe)NiH]2 with benzonitrile leads to
(dippe)Ni(Ph)(CN) oxidative addition. More recently, Brookhart10
reported that a cationic Rh(III) complex [Cp*(PMe3)Rh(SiPh3)(CH2-
Cl2)]BAr4′ activates the carbon-carbon bonds of aryl and alkyl
cyanides (R-CN, where R ) Ph, (4-(CF3)C6H4), (4-(OMe)C6H4),
Me, iPr, tBu) to produce complexes of the general formula
[Cp*(PMe3)Rh(R)(CNSiPh3)]BAr4′. The above activation of C-C
bonds is accomplished by air-sensitive metallorganic compounds
through an η1- or η2-nitrile intermediate, probably due to the high
affinity of molybdenum, nickel, and rhodium for carbon atoms.
We report here a novel C-C bond cleavage of acetonitrile by an
air-stable dinuclear copper(II) cryptate at room temperature, where
the activation of the C-C bond is due to the favorable formation
of a stable cyanide bridged dinuclear copper(II) cryptate.
The dinuclear copper(II) cryptate [Cu2L](ClO4)4, 1, was pre-
pared14 by the reaction of L and Cu(ClO4)2‚6H2O in methanol (L
) N[(CH2)2NHCH2(C6H4-p)CH2NH(CH2)2]3N). When [Cu2L]-
(ClO4)4 was dissolved in acetonitrile, and the solution was allowed
to evaporate slowly at room temperature, an unexpected cyanide
bridged complex [Cu2L(CN)](ClO4)3‚2CH3CN‚4H2O, 2, was ob-
tained. An X-ray crystallographic analysis15 reveals that the two
copper(II) ions in 2 are bridged by a cyanide anion (Figure 1). Each
Cu(II) ion is five-coordinated with a slightly distorted trigonal
bipyramid geometry, in which Cu(1) is coordinated with four
nitrogen atoms on one side of L and one bridged cyanide nitrogen
atom, and Cu(2) is coordinated with four nitrogen atoms from the
other side of L and one bridged cyanide carbon atom. The Cu(1)-
Figure 2. The electronic spectra of 1 in acetonitrile at room temperature
as a function of time.
atoms, two Cu(II) ions, and cyanide anion are almost collinear with
N(1)-Cu(1)-N, Cu(1)-N-C, N-C-Cu(2), and C-Cu(2)-N(2)
angles of 179.3(2), 179.6(4), 179.5(5), and 179.1(2)°, respectively.
The C-N distance of 1.150(6) Å and Cu(1)‚‚‚Cu(2) separation of
5.163 Å are comparable to those in other cyanide bridged dinuclear
copper(II) complexes.16
The electronic absorption spectra of 1 and 2 show maximum
absorption bands at 699 nm in DMF and 887 nm in acetonitrile,
respectively, indicating that the geometry of copper(II) is a
compressed tetrahedral in 1 and a trigonal bipyramidal in 2.17 When
KCN solid was added to a solution of 1 in acetonitrile, the color of
the solution changed quickly from blue to emerald green, and the
maximum absorption band of the solution coincided with that for
2, indicating that the cyanide bridged dinuclear copper(II) complex
formed very quickly. To monitor the cleavage process, 1 was
dissolved in acetonitrile, and the electronic absorption spectra were
recorded at room temperature as a function of time. The results
are shown in Figure 2. Initially, the electronic absorption spectrum
shows two bands at 700 and 889 nm with similar intensity,
indicating some of 1 reacts quickly to 2 in acetonitrile. After about
Ncyanide bond length (2.039(6) Å) is longer than the Cu(2)-C bond
length (1.975(5) Å), while it is shorter than the other Cu-Ncryptand
bond lengths (2.090(4)-2.160(4) Å). The two bridgehead nitrogen
† Present address: School of Conghua, Guangzhou Medical College, Guangzhou
510925, China.
9
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J. AM. CHEM. SOC. 2004, 126, 4760-4761
10.1021/ja031874z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
this is a clear demonstration that the activation of the C-C bond
of acetonitrile is due to the favorable formation of a stable cyanide
bridged dinuclear copper(II) cryptate. This mechanism is different
from those reported involving η1- or η2-nitrile intermediates.10,12,13
Currently, we are investigating the C-C bond activation of other
alkyl and aryl cyanides by 1. Preliminary results indicate that 1
can also cleave the C-C bond of benzonitrile at room temperature
to produce phenol and the cyanide bridged complex [Cu2L(CN)]-
(ClO4)3:
Figure 3. Possible cleavage mechanism.
1 day, the absorption spectrum is similar to that of pure 2, indicating
that complex 1 reacted completely to form complex 2. After that,
the absorption spectra did not change with further standing.
Formation of the cyanide bridged dinuclear copper complex in
acetonitrile was also confirmed by ESI-MS spectroscopy on a
Thermo Finigan LCQDECA XP ion trap mass spectrometer (see
Supporting Information S1). Initially, an acetonitrile solution of 1
shows three peaks due to [Cu2L]3+, [Cu2L]2+, and [Cu2L (ClO4)]+.
After the solution was heated at 50 °C in a sealed tube for 1 day,
the above three peaks are not observed, and three new signals
attributed to [Cu2L(CN)]3+, [Cu2L(CN)]+, and [HL]+ are observed.
The appearance of [HL]+ is probably due to the reduction of Cu-
(II) to copper metal during the ionization process.
The cleavage reaction involves the C-C bond activation of
acetonitrile, and we employed GC (Varian CP3800) spectra to
investigate the new species formed in acetonitrile solution. The
results (see Supporting Information S2) indicate that methanol is
formed when an acetonitrile solution of 1 was left at room
temperature in a sealed tube for 1 day. This experiment demonstrates
that methanol is produced during the cleavage reaction of 1 with
acetonitrile.
On the basis of the above experimental results, a likely cleavage
mechanism is presented in Figure 3. In this process, the nitrogen
atom of acetonitrile binds to one Cu(II) atom through its electron
pair, and the other Cu(II) atom interacts with the filled π orbital of
the sp-hybridized acetonitrile carbon, resulting in electron flow from
the π bond to the Cu(II) atom, and this increases the “leaving
ability” of cyanide and the electrophilicity of the methyl carbon,
and results in cleavage by water to form methanol and cyanide
bridged complex 2.
The cleavage rate is determined spectrophotometrically at λ )
890 nm and 20 °C (see Supporting Information S3). In the presence
of excess acetonitrile, the rate law can be described as dCA/dt )
-kobsdCA, where CA is the concentration of 1, and kobsd ) k2[H2O]R.
At the lower concentration of water (0.167 M), the plot of -ln CA
versus time reveals a line in the 0-90 min time range whose slope
(k) is equal to 1.52 × 10-4 s-1 (t1/2 ) 76 min). After 90 min, the
plot shows curvature. The reaction rates become larger along with
the increasing concentrations of water, and the plots show a straight
line with no curvature over 3 half-lives. These experiments clearly
indicate the first-order dependence on water. The calculated second-
order rate constant (k2) and R value are 1.76(1) × 10-4 M-1 s-1
and 0.084(2), respectively.
[Cu2L]4+ + PhCN + H2O f [Cu2L(CN)]3+ + PhOH + H+
Here, the cleavage rate for benzonitrile is much faster than that for
acetonitrile as the cleavage reaction appears instantaneous upon
mixing 1 and benzonitrile as assessed by ESI-MS spectra.
Acknowledgment. This work was supported by NSFC
(20371051, 20131020), the Guangdong Provincial Natural Science
Foundation, and the Education Department of Guangdong Province.
We thank Rudy L. Luck for assistance with corrections and Dian
Chen for assistance with ligand synthesis.
Supporting Information Available: Synthesis and characterization
of compounds 1 and 2, the crystallographic data for complex 2 (CIF),
ESI-MS, GC, and kinetics data for complex 1 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Under similar reaction conditions, there is no reaction between
[Cu(tren)](ClO4)2 and acetonitrile (see Supporting Information S4);
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