Transition metal chlorides are lewis acids toward terminal chloride attached to late transition metals

Article abstract of DOI:10.1021/ic402107z

Two different neutral tridentate imine-donor pincer ligands interact with excess MCl₂ (M = Co or Cu) to form compounds of the same stoichiometry, (LMCl₂)₂·MCl₂, where the assembling force is the electron richnes

Full text of DOI:10.1021/ic402107z

Article
pubs.acs.org/IC
Transition Metal Chlorides Are Lewis Acids toward Terminal Chloride
Attached to Late Transition Metals
Alice K. Hui, Brian J. Cook, Daniel J. Mindiola, and Kenneth G. Caulton*
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: Two different neutral tridentate imine-donor pincer ligands
interact with excess MCl2 (M = Co or Cu) to form compounds of the same
stoichiometry, (LMCl2)2·MCl2, where the assembling force is the electron
richness of the terminal chlorides on the LMCl2 unit. Finite aggregation occurs
for M = Co, but for M = Cu, an infinite polymeric structure is adopted, all
because MCl2 is bifunctional, which thus bridges multiple MCl units. The bis-
pyrazolylpyridine ligand has two acidic NH protons, and both of these are
involved in intramolecular hydrogen bonds. The generality of this Lewis acid aggregation is discussed.
collected solid, dissolved in CD2Cl2, shows resonances
INTRODUCTION
■
consistent with a single product with the ligand A in a 2-fold
Installation of a single neutral polydentate ligand L onto a late
transition metal halide, to form LMCln, is frequently desired,
not only to leave an empty metal coordination site in this
formula but also to leave manipulable chloride (halide)
functionality to be replaced with hydride, alkyl, carbene, etc.
It is the purpose of this report to show that for late transition
metals, where a terminal chloride ligand can retain considerable
nucleophilicity due to the large number of occupied dπ
orbitals,1 available Lewis acidic MCln reagent can become part
of the product, yielding “adducts” of composition
(LMCln)x(MCln)y. While the excess MCln is sometimes
employed to increase the rate of formation of LMCln or to
get maximum yield from valuable ligand L, the present report
shows that excess MCln can interact directly with the primary
product LMCln. This is analogous to “-ate” complex formation,
LMClnM′, where M′ is an alkali metal and is held into the
assembly by one or more bridging halides.2−4
1
symmetric environment. H NMR chemical shifts range from
+70 to −3 ppm, indicative of paramagnetism and thus retention
of cobalt oxidation state +2. The NMR spectrum is distinct
from that of monomeric LCoCl2, which we characterized and
will publish elswwhere. Mass spectrometry (ESI) shows the
+
+
ions L2Co2Cl3 and L2Co2Cl2 , which suggests a multimetal
identity for the product but also a tendency for the assembly to
be reduced in this mass spectrometer injector environment.
Aggregation was confirmed by single-crystal X-ray diffraction
(Figure 1) of crystals grown from CH2Cl2. Figure 1 shows the
product to have composition L2Co3Cl6, better written as
(LCoCl2)2·CoCl2, to indicate that one triatomic CoCl2 is the
glue which links two LCoCl2 into a species that has a
crystallographic C2 axis. The dicobalt ions detected by mass
+
−
spectrometry, L2Co2Cl3 , the product of loss of CoCl3 , and
+
also L2Co2Cl2 , both illustrate that the pincer-free cobalt acts as
a halide-abstracting agent in the ionization chamber. The 2:3
L:Co chemical formula originates from this product being
formed under ligand-deficient conditions. Consistent with this,
the same product is formed and isolated when the reaction is
repeated at the stoichiometric L:Co ratio of 2:3. Cobalt/
chloride distances in the structure show a systematic length-
ening from terminal on central Co (Co1) to terminal on 5-
coordinate cobalt (Co2) to bridging, with distances involving
the four-coordinate cobalt center being shorter. Bond lengths
and angles in the analogous monomeric L′CoCl2 cobalt
complex lacking the ring tBu substituents are very similar,
and the Co−Cl distance is the average of the two distances to
Co2 here.11 The Co/N distances and trans N−Co−N angles in
L′CoCl2 differ negligibly from those in (LCoCl2)2·CoCl2. The
tau value for the 5-coordinate Co here is 0.597, thus half-way
between trigonal bipyramidal and square pyramidal. Each
RESULTS
■
d7 Co(II) Case. Bis-pyrazolylpyridine ligand5 L (A in
t
Scheme 1, R = Bu) is a pincer ligand with three imine
donors.6−10 Reaction of CoCl2 slurried with L in THF
employing a slight (10 mol %) excess of CoCl2 occurs
(Scheme 1) with color change within minutes at 25 °C to give a
blue solution from which blue solid can be isolated by filtration
1
followed by concentration. The H NMR spectrum of the
Scheme 1
Received: August 16, 2013
Published: March 10, 2014
© 2014 American Chemical Society
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Figure 1. ORTEP view (50% probabilities) of the nonhydrogen atoms
of [(L)CoCl2]2(CoCl2). Unlabeled atoms are carbons. Atoms labeled
# are derived by a crystallographic C2 axis of symmetry, and that axis is
vertical in this view. Dashed lines are hydrogen bonds to H (open
circles) on N. Selected structural parameters: Co1−Cl3, 2.2596(11) Å;
Co1−Cl1, 2.3000(10) Å; Co2−N3, 2.056(3) Å; Co2−N5, 2.129(3) Å;
Co2−N1, 2.139(3) Å; Co2−Cl2, 2.2706(12) Å; Co2−Cl1, 2.3391(11)
Å; Cl3−Co1−Cl3#1, 113.62(7)°; Cl3−Co1−Cl1, 106.16(4)°;
Cl3#1−Co1−Cl1, 107.72(4)°; Cl1−Co1−Cl1#1, 115.69(6)°; N5−
Co2−N1, 151.61(12)°; N3−Co2−Cl2, 115.40(10)°; N5−Co2−Cl2,
98.65(11)°; N1−Co2−Cl2, 96.87(9)°; N3−Co2−Cl1, 128.82(10)°;
N5−Co2−Cl1, 96.11(10)°; N1−Co2−Cl1, 98.33(9)°; Cl2−Co2−
Cl1, 115.78(4)°; Co1−Cl1−Co2, 110.89(4)°.
Figure 2. ORTEP view (50% probabilities) of the nonhydrogen atoms
in the asymmetric unit of [(btzp)CuCl2]2(CuCl2). Selected structural
parameters: Cu1−Cl4, 2.1961(5) Å; Cu1−Cl3, 2.2443(5) Å; Cl3−
Cu2#, 2.8445(5) Å; Cu1−Cl1, 2.2637(5) Å; Cu1−Cl2, 2.2772(5) Å;
Cu2−N5, 1.9585(15) Å; Cu2−N2, 2.0673(15) Å; Cu2−N6,
2.0793(15) Å; Cu2−Cl5, 2.1765(5) Å; Cu2−Cl1, 2.6416(5) Å;
Cu3−N14, 1.9624(15) Å; Cu3−N10, 2.0671(16) Å; Cu3−N15,
2.0840(17) Å; Cu3−Cl6, 2.1728(5) Å; Cu3−Cl2, 2.5486(5) Å. Cl3
links onward to Cu2#, as illustrated in Figure 3 (vide infra).
pincer has two NH protons, and the whole molecule has four
terminal chlorides. These functionalities are thus highly
complementary for hydrogen bonding, and indeed, each NH
group is within hydrogen-bonding distance to one terminal
chloride. Hydrogen bonding is a general feature of bis-
pyrazolylpyridine ligands.5,12,13 These intramolecular interac-
tions dictate the conformation of the terminal LCoCl2
complexes around the central CoCl2 linker unit and contribute
to the overall stability of this aggregate. They also further
confirm the nucleophilicity of the chloride attached to high d
electron count metals. A different form of hydrogen bonding,
intermolecular, exists in L′CuCl2.
this observed ligand redistribution is consistent with
aggregation in the fundamental unit of this solid.
The asymmetric unit of [(btzp)CuCl2]2(CuCl2) is composed
(Figure 2) of three coppers, two pincers, and six chlorides, with
metal coordination numbers of 4 (Cu1 approximately
tetrahedral), 5 (Cu3), and 6 (Cu2). In general, one can
imagine a Lewis acid like CuCl2 binding to terminal chloride on
any metal, MCl, to make M−(μ-Cl)-CuCl2, and this
functionality serves as a linker in the aggregation/oligomeriza-
tion (Figure 3). This is also nicely consistent with the fact that
the synthesis employed involves a L:Cu ratio less than 1:1, so
that free Lewis acid CuCl2 is available. In forming the polymer
chain, CuCl2 donates one chloride lone pair to another copper
and accepts one chloride from a different copper. The polymer
chain involves Cu1 and Cu2, while Cu3 donates its apical Cl2
to Cu1 on the chain, allowing Cu1 to become 4 coordinate. In
summary, the polymer chain in Figure 3 is decorated by
substituent Cu3 hanging outward from the chain, like a
Christmas tree string of lights. Terminal CuCl distances are at
least 0.04 Å shorter than those to bridging chlorides, and
bridging chloride distances to copper vary widely (2.24−2.85
Å). In general, the long distances originate both from bridging
functionality and also from the first-order Jahn−Teller effect,
which elongates on the axis perpendicular to the Cu/pincer
plane. The crystal lattice contains two acetonitrile molecules
per asymmetric unit, but these merely fill holes in the lattice;
together with the crystallization from acetonitrile, this shows
that chloride bridging is favored over nitrile complexation in the
solid lattice; chloride is the stronger Lewis base here. Copper to
nitrogen distances vary negligibly between six-coordinate Cu2
and five-coordinate Cu3, being 1.96 Å to pyridine and 2.07 Å to
tetrazine. The structure of [(btzp)CuCl2]2(CuCl2) shows that
bridging chloride distances are exceptionally deformable,
varying by 0.67 Å for [(btzp)CuCl2]2(CuCl2).
d9 Cu(II) Case. The Lewis base bis-2,6-tetrazinyl pyridine,
btzp, is a pincer ligand (B, Scheme 1) also carrying imine
donors but far less electron rich than the bis-pyrazolylpyridine,
due to the large complement of imine nitrogens in a tetrazine.
Ligand B also lacks appropriate hydrogens needed to be a
hydrogen-bond donor. Compared to cobalt above, analogous
Lewis acid behavior occurs with this pincer ligand and Cu(II),
but here the structure is even more complicated than for cobalt,
due to an apparent tendency of copper to adopt a higher
coordination number than Co(II); this makes copper a linker of
higher dimensionality.
Reaction of btzp with anhydrous CuCl2 (mole ratio 1:2) in
acetonitrile occurs (Scheme 1) within minutes at 25 °C. Slow
evaporation of that solution yields crystals shown by X-ray
diffraction to be [(btzp)CuCl2]2(CuCl2)·2MeCN (Figure 2);
hence, all coppers are divalent.
Repeating the synthesis with a 2:3 btzp:Cu ratio gives this
same product. The divalent copper character is further
1
established by the range of H NMR chemical shifts, from
+50 to +7.9 ppm. The 1H NMR spectrum of this single product
is distinct from that of the simpler 1:1 btzp:Cu(II) complex
(btzp)CuCl2, which will be published separately. The ESI mass
+
spectrum of this compound in MeCN shows only Cu(btzp)2 ;
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Figure 3. Mercury view of four repeat units of the polymer chain of [(btzp)CuCl2]2(CuCl2), showing five-coordinate Cu3 unit pendant onto the
(Cu1ClCu2Cl)n polymer chain.
DISCUSSION
Scheme 2
■
The presence of repulsive interactions between filled d orbitals
and halide lone pairs leads to enhanced Lewis basicity/
nucleophilicity for terminal halide in higher d electron count
complexes, hence for later transition elements.1 This same
filled−filled repulsion is what makes N2H4 unusually Brønsted
basic and the homolytic bond dissociation energy of H2N−NH2
or HO−OH or F2 unusually small. This M−Cl character
contrasts to early transition metal halides, which show π
donation from halide lone pairs to the attached metal; hence,
terminal halides for such d electron counts are less nucleophilic.
This effect is manifest in late transition metal reactivity trends,
where attempted electrophile-induced chloride removal from
later transition metal complexes can fail to go to completion
(i.e., fail to deposit AgCl), and instead leads to isolable M(μ-
Cl)M′(+) species where M′ is Ag or Tl.14−26 The moiety MCl2
is a bifunctional or an amphoteric reagent (M seeks a
coordination number of at least 4), so it does more than
simply bind to one chloride, as does SnCl2 toward any M−Cl
unit. MCl2 is thus a linker and one which can be paramagnetic.
For comparison, the homometallic adduct formation described
here might be useful as a synthetic intermediate for installing a
distinct type of ligand, giving [LMCln][L′MCln], or comprising
new materials whose magnetic properties and electrical
conductivity might be exploited, including for sensors. Because
we demonstrate these concepts here with two very different
kinds of ligands, with and without the presence of hydrogen
bonds, these principles are likely to be generally applicable.27−31
Other examples of his phenomenon also warrant mention.
Monatomic nitride, N3−, and oxide, O2−, ligands typically
interact with Lewis acidic alkali metal cations, and there is
growing evidence that interaction with higher charged cations,
e.g., Sc3+, can influence reactivity of these terminal ligands.32−34
However, the behavior reported here contrasts to reactions of
terpyridyl pincer ligands, which react with Lewis acidic CuX (X
As illustrated, CoCl2 is only a Lewis acid, leading to finite
aggregation, while CuCl2 is both a Lewis acid and a halide
donor (bridge former), leading to a polymer chain from which
hangs an LCuCl2 unit which only furnishes a chloride donor
bridge and hence remains five coordinate in the product.
Certainly the hydrogen-bonding ability in the ligand A also
encourages finite aggregation, due to the complementarity of
the number of acidic hydrogens and the number of terminal
chlorides.
EXPERIMENTAL SECTION
■
General. All manipulations were carried out under an atmosphere
of ultra-high-purity nitrogen using standard Schlenk techniques or in a
glovebox. Solvents were purchased from commercial sources, purified
using an Innovative Technology SPS-400 PureSolv solvent system or
by distilling from conventional drying agents and degassed by the
freeze−pump−thaw method twice prior to use. Glassware was oven
dried at 150 °C overnight and flame dried prior to use. NMR spectra
were recorded in various deuterated solvents at 25 °C on a Varian
Inova-400 spectrometer (1H 400.11 MHz). Proton chemical shifts are
reported in ppm versus solvent protic impurity but referenced finally
to SiMe4. Mass spectrometry analyses were performed in an Agilent
6130 MSD (Agilent Technologies, Santa Clara, CA) quadrupole mass
spectrometer equipped with a multimode (ESI and APCI) source. All
starting materials have been obtained from commercial sources and
used as received without further purification. Synthesis of ligand btzp is
reported separately.38
[Co(bis-pyrazolylpyridine)Cl2]2·CoCl2. To a stirring slurry of
anhydrous CoCl2 (60.3 mg, 0.464 mmol) in 2 mL of THF was added
dropwise a solution of ligand L (see Supporting Information), bis-
pyrazolylpyridine (100 mg, 0.309 mmol), in 3 mL of THF. The
solution becomes homogeneous within minutes of addition and
exhibited a deep blue color. The solution was allowed to stir at room
temperature for 2 h with no further color change. The solution was
then filtered through Celite to remove any insolubles, and the solvent
was removed under reduced pressure. This resulted in an analytically
pure blue powder from which single crystals suitable for X-ray
diffraction can be grown via slow evaporation of a concentrated
solution in CH2Cl2; slow solvent removal is effected by coaxial vials,
the inner one, 12 mm outer diameter, containing a solution with solute
= halide) to often give [L2Cu+][CuX2 ], where Lewis acid
−
completely removes halide from ligated copper.35,36 On the
other hand, reaction of (terpy)CuCl2 with HgCl2 gives a μ-Cl
adduct37 analogous to those reported here.
Finally, the notably different structure for the same
stoichiometry observed here can be summarized in the
diagrams in Scheme 2. Monomer units are designated by the
metal coordination number of the two distinct monomer units
prior to aggregation, hence “5” and “2.”
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and the outer one, a 20 mL scintillation vial, containing stopcock
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grease, which slowly absorbs solvent (acting as a “solvent sponge”)
1
from the inner tube by vapor transport. Yield: quantitative. H NMR
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(400 MHz, CD2Cl2 298 K), δ(ppm) full width at half height (Hz):
70.7(2H, 500 Hz), 32.9(2H, 750 Hz), −2.22(18H, 450 Hz); the
remaining two resonances belonging to the ligand could not be located
and are assumed to be too paramagnetically broadened to be detected.
MS (ESI positive ion, CH2Cl2): m/z, 869.2 [(L)2Co2Cl3]+ (C38H50
35Cl3Co2N10), 834 [(L)Co2Cl2]+ (C38H5035Cl2Co2N10). Also observed
were heavier chlorine isotopomers of the given ion with appropriate
relative intensities.
[Cu(btzp)Cl2]2·CuCl2. Ligand btzp (6 mg, 22.45 μmol) was
dissolved in 5 mL of acetonitrile and added to a stirring mixture of
anhydrous CuCl2 (4.53 mg, 33.68 μmol) and 5 mL of acetonitrile. The
mixture was stirred for 3 h and filtered with a medium glass frit. The
solvent was removed in vacuum to reveal a dark red-orange solid.
Yield: quantitative. Dark red-orange crystals were grown by slow
evaporation in acetonitrile. 1H NMR (400 MHz, CD3CN 298 K): 7.89
(b, 6H) 11.90 (b, 2 H, C−H Ar) 50.00 (b, 1H, C−H Ar). MS (ESI
positive ion, MeCN) m/z: obsd, 597.1276, calcd, 597.1258 for
(btzp)2Cu+ (C22H18N1863Cu).
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ASSOCIATED CONTENT
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S
* Supporting Information
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Full crystallographic and ligand synthesis details. This material
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: caulton@indiana.edu.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported by the Indiana University Office of
Vice President for Research. ChemMatCARS Sector 15 is
principally supported by the National Science Foundation/
Department of Energy under grant no. NSF/CHE-0822838.
Use of the Advanced Photon Source was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
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