Pyridazine based scorpionate ligand in a copper boratrane compound

Article abstract of DOI:10.1021/ic201666w

Reaction of potassium tris(mercapto-tert- butylpyridazinyl)borate K[TntBu] with copper(II) chloride in dichloromethane at room temperature led to the diamagnetic copper boratrane compound [Cu{B(Pn^tBu)₃}Cl] (Pn = pyridazine-3-thionyl) (1) under activation of the B-H bond and formation of a Cu-B dative bond. In contrast to this, stirring of the same ligand with copper(I) chloride in tetrahydrofuran (THF) gave the dimeric compound [Cu{Tn^tBu}]₂ (2) where one copper atom is coordinated by two sulfur atoms and one hydrogen atom of one ligand and one sulfur of the other ligand. Hereby, no activation of the B-H bond occurred but a 3-center-2-electron B-H·Cu bond is formed. The reaction of copper(II) chloride with K[Tn^tBu] in water gave the same product 2, but a formal reduction of the metal center from Cu(II) to Cu(I) occurred. When adding tricyclohexyl phosphine to the reaction mixture of K[Tn^R] (R = tBu, Me) and copper(I) chloride in MeOH, the distorted tetrahedral Cu complexes [Cu{Tn^R}(PCy₃)] (R = tBu 3, Me 4) were formed. Compound 4 is exhibiting an "inverted" ? ^3-H, S, S, coordination mode. The copper boratrane 1 was further investigated by density functional theory (DFT) calculations for a better understanding of the M→B interaction involving the d⁸ electron configuration of Cu

Full text of DOI:10.1021/ic201666w

Article
pubs.acs.org/IC
Pyridazine Based Scorpionate Ligand in a Copper Boratrane
Compound
Gernot Nuss,† Gerald Saischek,† Bastian N. Harum,† Manuel Volpe,† Ferdinand Belaj,†
and Nadia C. Mosch-Zanetti*,†
̈
†Institut fur Chemie, Karl-Franzens-Universitat Graz, Schubertstrasse 1, A-8010 Graz, Austria
̈
̈
S
* Supporting Information
ABSTRACT: Reaction of potassium tris(mercapto-tert-butylpyridazinyl)borate
K[TntBu] with copper(II) chloride in dichloromethane at room temperature led
to the diamagnetic copper boratrane compound [Cu{B(PntBu)3}Cl] (Pn =
pyridazine-3-thionyl) (1) under activation of the B−H bond and formation of a
Cu−B dative bond. In contrast to this, stirring of the same ligand with copper(I)
chloride in tetrahydrofuran (THF) gave the dimeric compound [Cu{TntBu}]2
(2) where one copper atom is coordinated by two sulfur atoms and one hydro-
gen atom of one ligand and one sulfur of the other ligand. Hereby, no activation
of the B−H bond occurred but a 3-center-2-electron B−H···Cu bond is formed.
The reaction of copper(II) chloride with K[TntBu] in water gave the same prod-
uct 2, but a formal reduction of the metal center from Cu(II) to Cu(I) occurred.
When adding tricyclohexyl phosphine to the reaction mixture of K[TnR] (R =
tBu, Me) and copper(I) chloride in MeOH, the distorted tetrahedral Cu com-
plexes [Cu{TnR}(PCy3)] (R = tBu 3, Me 4) were formed. Compound 4 is exhibiting an “inverted” κ3-H,S,S, coordination mode.
The copper boratrane 1 was further investigated by density functional theory (DFT) calculations for a better understanding of
the M→B interaction involving the d8 electron configuration of Cu.
(Tmp)5 and recently tris(mercaptopyridazinyl)borate (Tn)6
INTRODUCTION
■
which are considered to be even softer analogues of poly-
(pyrazolyl)borates (Figure 1). However, limitations of this
analogy were observed, and they can be explained by the lower
ring strain caused by the larger chelate ring in Tm compared to
Tp ligands. Also, the coordinating sulfur in Tm ligands causes
large differences in coordination behavior and in reactivity of
the respective complexes compared to the coordinating nitro-
gen in Tp containing complexes. This is because sulfur atoms
have lone pairs which can add π-basic character to the metal−
ligand bond therefore causing the Tm ligand to be more basic
compared to Tp.7 Another important impact of sulfur lone-
pairs on the properties of the Tm ligand is the weak field
character compared to Tp ligands.7 Hence, [Fe(TmMe)2]8 is a
paramagnetic d6 high spin compound compared to [Fe-
(TpMe)2],9 which shows an interesting spin cross over behavior.
These sulfur containing scorpionate ligands can coordinate to
the metal in a monodentate (κ1-S), bidentate (κ2-S,S or
κ2-H,S), and tridentate (κ3-S,S,S or κ3-H,S,S) fashion although
the κ3-S,S,S coordination mode is the most common for TmR
ligands.7 In spite of the soft nature of these ligands, coordina-
tion is not only restricted to soft mid to late transition metals in
low oxidation states8,10 but also coordination to harder early
Scorpionate ligands have become very popular in the field of
coordination chemistry, catalysis, and biomimetic chemistry1
since the discovery of tris(pyrazolyl)borate (Tp, Figure 1) by
Figure 1. Variation in tripodal scorpionate ligands.
Trofimenko in the mid 1960s.2 Their popularity is owed to the
variability of the substituents on the pyrazole ring which affect
both steric and electronic properties of the ligand. Pyrazolylbo-
rates are tridentate ligands where the three nitrogen atoms of the
pyrazole are coordinating to the metal giving a soft donor. A
further development in scorpionate chemistry was the invention
of tris(mercaptoimidazolyl)borate ligands (Tm)3 and later
tris(thioxotriazolinyl)borate (Tr),4 tris(mercaptopyridinyl)borate
transition metals in high oxidation states (NbV, TaIII−V, TiIII−IV
,
ZrIV)11 is observed despite the mismatch in the “soft acid/soft
Received: August 2, 2011
Published: November 17, 2011
© 2011 American Chemical Society
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base” concept. One of the initial goals to synthesize these
ligands was the mimicking of sulfur rich active sites of metallo-
enzymes and especially copper containing active sites7 as found
in cytochrome c oxidase,10 nitrite reductase,13 and metal-
lothionein.14 The first attempts to model complexes with Tm
were hindered by oxidative degradation of the ligand, which
demonstrated the sensitivity toward metal ions such as Cu(II)
and Fe (III).7 The phenomenon of oxidative ligand cleavage of
sulfur scorpionates under concurrent reduction of the metal
ion was also observed by our group employing the Tn moiety
with first-row transition metal salts such as Co(II) and Ni(II).6
Nevertheless, there are numerous complexes known containing
the Tm moiety, with copper(I) coordinating in a monomeric
[Cu{Tm}]3 or dimeric [Cu{Tmxylyl}]211 fashion, whereas when
phosphines are added to the reaction, only monomeric κ3-S,S,S
and κ3-H,S,S coordinated phosphine complexes are observed.12
In certain cases boratrane complexes are formed, exhibiting
an interesting M→B dative bond.
coordinated scorpionate ligand confirming the formation of a
diamagnetic copper compound.
Similar to our earlier observations in cobalt and nickel
complexes with the same ligand, no absorption for a B−H
stretch frequency could be observed by IR spectroscopy. This is
in contrast to related scorpionate ligands containing the
tris(mercaptoimidazolyl)borate moiety, where B−H frequen-
cies appear as distinctive frequencies around 2400 cm−1, and
significant shifts are observed when the ligands are coordinated
to a metal.20 The formation of a boratrane compound was
clearly evidenced by the determination of its molecular
structure by single crystal X-ray diffraction analysis, where the
short Cu−B distance of 2.060(3) Å leaves no space for hydro-
gen. A molecular view of compound 1 is shown in Figure 2,
selected bond lengths and angles are given in Table 1, and
The thereby formed hydride leads to reduction of the in-
volved metal ion. Such boratrane complexes containing the Tm
moiety are known with Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt
as the coordinating metal,13 but there is no evidence of a Cu
boratrane containing a tripodal soft κ3-S,S,S ligand. Although
there are few complexes known containing a Cu→B dative
bond,14 the only known Cu boratrane is Bourissou's [Cu{B-
(C6H4P(iPr)2)3}Cl].15
Recently, we reported the synthesis of a series of novel soft
tridentate ligands K[TnR] (R = tBu, Me, Ph) and their co-
ordinational behavior toward Co(II) and Ni(II) centers.6
Interestingly, boratrane compounds were formed, both, in
CH2Cl2 and water. Simultaneous activation of the B−H bond
occurred, resulting in a M→B dative bond and a κ4-B,S,S,S
coordination mode of the B(PntBu)3 (PntBu = 6-tert-butylpyr-
idazine-3-thione) moiety.
Figure 2. Molecular structure of [Cu{B(PntBu)3}Cl] (1) with thermal
ellipsoids at 45% probability.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1
Here, we report the reactivity of the tris(pyridazinethionyl)-
borate ligand toward copper(II) and copper(I) salts thereby
presenting the first copper boratrane compound bearing a
sulfur rich environment.
Cu1−B1
Cu1−S1
Cu1−S2
Cu1−S3
Cu1−Cl1
S1−C1
2.060(3)
2.3544(9)
2.3204(9)
2.3156(9)
2.2821(9)
1.710(3)
1.706(3)
1.704(3)
1.537(4)
1.528(4)
1.530(4)
S1−Cu1−S2
S2−Cu1−S3
S1−Cu1−S3
S1−Cu1−B1
S2−Cu1−B1
S3−Cu1−B1
S1−Cu1−Cl1
S2−Cu1−Cl1
S3−Cu1−Cl1
B1−Cu1−Cl1
S1−Cu1−S2
115.35(3)
118.60(3)
117.31(3)
80.03(9)
80.06(9)
80.11(9)
101.53(3)
98.99(4)
99.30(3)
178.42(9)
115.35(3)
RESULTS AND DISCUSSION
■
S2−C9
Synthesis of the Compounds. The reaction of the
scorpionate ligand K[TntBu 6
]
with copper(II) chloride in
S3−C17
B1−N1
B1−N3
B1−N5
dichloromethane at room temperature led to the diamagnetic
boratrane compound [Cu{B(PntBu)3}Cl] (1) in moderate yield
(Scheme 1). The orange product precipitates after one day of
stirring. After filtration, the crude orange product was washed
crystallographic data is given in Table 6. Copper boratrane
complexes are extremely rare and have not been reported with
related sulfur based scorpionate ligands. The Cu−B bond found
in 1 (Cu1−B1 2.060(3) Å) is significantly shorter compared
with that in the only other known copper boratrane complex
[Cu{B(C6H4P(iPr)2)3}Cl] (2.508(2) Å).15 The electrophilic
nature of the pyridazine heterocycle leads to a more elec-
trophilic boron which possibly shortens the boron copper
bond. However, the distance is comparable to those found in
analogous pyridazine bearing boratranes synthesized by our
group recently, exhibiting 2.016(3) and 2.068(3) Å for
[Ni{B(PntBu)3}Cl] and [Co{B(PntBu)3}Cl], respectively.6 The
geometry at copper in 1 can best be described as distorted
trigonal bipyramidal, exhibiting an almost perfectly linear B1−
Cu1−Cl1 bond angle of 178.42(9)°. The S−Cu−S angles
varying between 115.35(3) and 118.60(3)° show significant
Scheme 1. Formation of Boratrane Compound
[Cu{B(PntBu)3}Cl] (1) and Numbering Scheme
with water, MeOH, and diethyl ether to purify it from
byproducts. Recrystallization from a CH2Cl2 solution gave red
crystals suitable for X-ray diffraction analysis. Proton and
carbon NMR spectra of 1 in DMSO-d6 show sharp resonances
in the expected regions for the atoms of the symmetrically
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deviation from the ideal angle. The three pyridazines are
paddlewheel like twisted around the copper therefore causing
the sulfur copper chlorine angles to range between 98.99(4)
and 101.53(3)° which is comparable to the phosphine copper
chlorine angles of Bourissou's first Cu boratrane (99.79(2)°).
Treatment of a solution of K[TntBu] with CuCl in THF gave
a copper(I) compound, namely, [Cu{TntBu}]2 (2), but in con-
trast to 1 the ligand remains an anionic scorpionate (Scheme 2).
The product was isolated as a red product in good yield, and
crystals suitable for X-ray diffraction analysis were obtained by
Table 2. Selected Bond Lengths (Å) and Angles (deg) of
[Cu{TntBu}]2 (2)
Cu1...Cu1′
Cu1−H1
Cu1−B1
Cu1−S1
Cu1−S2
Cu1−S3′
S1−C13
S2−C23
S3−C33
B1−N12
B1−N22
B1−N32
5.0728(3)
1.92(2)
S1−Cu1−S2
S2−Cu1−S3′
S1−Cu1−S3′
H1−Cu1−S1
H1−Cu1−S2
H1−Cu1−S3′
Cu1−S1−C13
Cu1−S2−C23
Cu1−S3′-C33′
N12−B1−N22
N12−B1−N32
N22−B1−N32
114.76(1)
120.45(1)
123.96(1)
84.4(1)
2.798(1)
2.2720(4)
2.2939(4)
2.2235(4)
1.707(1)
1.706(2)
1.715(1)
1.563(2)
1.554(2)
1.556(2)
85.7(6)
107.9(6)
99.91(5)
110.95(5)
106.69(5)
106.3(1)
110.8(1)
109.8(1)
Scheme 2. Formation of [Cu{TntBu}]2 (2)
be explained by the high flexibility of TntBu, which can cause
the PntBu moiety to rotate around the B−N bond to better
accommodate the two copper atoms. This flexibility and
behavior toward copper is already known with similar Tm and
Tr ligands.15,20 The compound is diamagnetic as expected for
Cu(I) which is ruling out the formation of a copper(II) species.
1
Thus, based on the solid state structure (Figure 3), in the H
NMR spectrum of 2 two sets of resonances for PntBu sub-
stituents in 2:1 ratio are expected. However, the room tem-
perature 1H NMR spectrum shows resonances for only one type
of pyridazine ring. Low temperature NMR down to −40 °C did
not resolve the resonances of the unsymmetric compound 2
indicating a dynamic behavior in solution. In contrast to this,
the unsymmetric aromatic moiety could be distinguished in the
crystallization from a dichloromethane solution. A molecular
view of 2 is given in Figure 3. Selected bond lengths and angles are
shown in Table 2 and crystallographic data are given in Table 6.
16
low temperature 1H NMR spectra for [Cu{TptBu}]2 and
[Cu{TrMe,o‑Py}]2.17 Interestingly, the hydride is missing in the
1H NMR spectrum. This phenomenon can be observed for similar
complexes containing sulfur scorpionates where the hydride is
pointing toward the metal.11,18 11B NMR spectra showed no
resonances, similar to other Tn containing complexes.6
The copper sulfur distances are 2.2720(4) for Cu1−S1,
2.2939(4) for Cu1−S2, and 2.2235(4) for Cu1−S3′, which is in
the range of related compounds in the literature.11,17,18 The
geometry at copper of the dimeric compound 2 is best described
as distorted trigonal toward the tetrahedral one. The sulfur
copper sulfur angles are deviating significantly, ranging from
114.76(1) to 123.96(1)° for S1−Cu1−S2 and S1−Cu1−S3′,
respectively. This distortion can also be found in related dimeric
copper compounds in the literature, where the copper center
is surrounded by three sulfurs in an almost trigonal fashion
exhibiting sulfur copper sulfur angles between 105.20(4) and
18
134.98(4)° for [Cu{TrEt,Me}]2 and between 111.54(4) and
124.64(4)° for [Cu{TrMe,o‑Py}]2.17 The deviation from the ideal
trigonal geometry toward the tetrahedral one can be explained
by the 3-center-2-electron B−H···Cu interaction, which is evidenced
by a H−Cu distance of 1.92(2) Å. This is significantly shorter
compared to the H−Cu distance found in [Cu{TrEt,Me}]2
(2.29(3) Å) whereas it is more similar to [Cu{TrMe,o‑Py}]2
(1.94(4) Å)18 and [Cu{TrMes,Me}]2 (1.99(3) Å).17 The hydro-
gen copper bond is almost perpendicular to the CuS3 plane,
which is confirmed by the hydrogen copper sulfur angles of
84.4(1)° for H1−Cu1−S1 and 107.9(6)° for H1−Cu1−S3′.
The nitrogen boron nitrogen bonds vary between 106.3(1) and
110.8(1)° for N12−B1−N22 and N12−B1−N32, respectively,
exhibiting a distorted tetrahedral geometry at the boron. The
distance between the two copper centers in [Cu{TntBu}]2 is
5.0728(3) Å, which is too large for a possible Cu−Cu interaction.
Figure 3. Molecular structure of [Cu{TntBu}]2 (2) with thermal
ellipsoids at 45% probability.
The structure analysis revealed a species consisting of two
[Cu{TntBu}] units which coordinate to the copper ion via two
sulfur atoms and the hydride (Scheme 2). The third sulfur atom
of the ligand forms a bridge to the second unit of [Cu{TntBu}]
leading to a tetra-coordinate copper ion with a S3H coordina-
tion sphere of a (3 + 1) type. The boron center exhibits an “S3-
inverted” conformation mode, allowing a 3-center-2-electron
B−H···Cu bond. The geometry of the dimeric compound can
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This distance is longer compared to [Cu{TrEt,Me}]2 (av. 4.2569
(6) Å) and shorter compared to [Cu{TrMes,Me}]2 (5.401(1) Å)
and [Cu{TrMe,o‑Py}]2 (5.308(1) Å).
Compound 2 was also obtained via an alternative method
using CuCl2·2H2O in water where product formation was indi-
cated as a red precipitate. Reduction of the metal to copper(I)
under decomposition of the scorpionate ligand with formation
of the disulfide (PntBu-PntBu) from PntBu occurred, as evidenced
by ESI-MS, which shows a mass peak at 335 for [PntBu-PntBu
+
H]+. For this reason the yield of 2 prepared by method 2 is
significantly lower when compared to method 1. This conforms
with examples in the literature, where thiols were oxidized to
disulfides by metal ions as, for example, Cu2+, Ni2+, or Co2+
under concurrent reduction of the metal.3,6 The crystal struc-
ture of [Ni{B(PnMe)3}Cl] was even showing the disulfide PnMe-
PnMe, which was formed as a byproduct.6
The formation of the dimer 2 prompted us to explore the
reaction with additional donors. We expected phosphines to be
better ligands than the thione sulfur atom. Thus, a solution of
K[TntBu] and tricyclohexyl phosphine in methanol was treated
with CuCl (Scheme 3). Compound [Cu{TntBu}(PCy3)] (3)
precipitated from the reaction mixture as red material and was
Figure 4. Molecular structure of [Cu{TnMe}(PCy3)] (4) with thermal
ellipsoids at 60% probability.
Table 3. Selected Bond Lengths (Å) and Angles (deg) of
[Cu{TnMe}(PCy3)] (4)
Scheme 3. Preparation of the Phosphine Compounds
[Cu{TnR}(PCy3)] (R = tBu 3, Me 4)
Cu1−H1J
Cu1−P1
Cu1−S1
Cu1−S3
S1−C1
B1−N1
B1−N3
B1−N5
S1−Cu1−S3
1.96(3)
S1−Cu1−H1J
S1−Cu1−P1
S3−Cu1−P1
P1−Cu1−H1J
S3−Cu1−H1J
Cu1−H1J−B1
N3−B1−N5
N1−B1−N3
N1−B1−N5
81.2(7)
2.2202(5)
2.2826(5)
2.3523(5)
1.7091(18)
1.566(2)
1.555(2)
1.561(2)
109.23(2)
130.16(2)
118.76(2)
114.1(7)
85.3(7)
133(2)
111.3(1)
109.3(1)
108.1(1)
atom of the phosphine ligand. The bonding situation at the
boron center is best described as “inverted” where the hydrogen
is pointing toward the metal. Interestingly, similar tripodal li-
gands containing Tm are usually coordinating to Cu(I) in a κ3
S,S,S, fashion exhibiting a “normal” boron center with the hydro-
gen pointing away from the metal,24 whereas disubstituted
ligands [H2B(mim)2]− (mim = 2-mercaptoimidazolyl) mostly
coordinate to the metal in an “inverted” κ3-H,S,S, mode.19 The
hydrogen copper distance is 1.831(2) Å in [Cu{HBPh-
(mimMe)2}(PPh3)]20 and 1.832(17) Å in [Cu{H2B(mp)2}-
(PPh3)]5 which is significantly shorter compared to the here
described [Cu{TnMe}(PCy3)] (4). Furthermore, H−Cu dis-
tances in borohydride ligated copper complexes like [Cu(MeC-
{CH2−PPh2}3)(HBH3)]21 and [Cu(PMePh2)3(HBH3)]22 are
even shorter (1.605 and 1.698 Å, respectively). The bond distance
between copper and the phosphorus atom is 2.2202(5) Å, being
in the range of [Cu{HBPh(mimMe)2}(PPh3)] (2.2163(8)Å) and
[Cu{B(mp)2}(PPh3)] (2.216(3) Å). The copper sulfur distances
are 2.2826(5) Å for Cu1−S1 and 2.3523(5) for Cu1−S3 which
are comparable to [Cu{HBPh(mimMe)2}(PPh3)], where Cu1−
S1 is 2.2892(8) and Cu1−S2 is 2.3210(8) Å and longer com-
pared to [Cu{H2B(mp)2}(PPh3)] (2.255(4) and 2.248(4) Å
for Cu1−S1 and Cu1−S2, respectively). The S1−Cu1−S3 and
S1−Cu1−H1 angles of 109.23(2)° and 81.2(7)°, respectively,
exhibit the distortion of the copper center. The nitrogen boron
nitrogen angles vary between 108.1(1)° for N1−B1−N5 and
111.3(1) for N3−B1−N5, which is exhibiting the distorted
tetrahedral geometry of the boron center.
obtained in good yield after workup. The analogous compound
[Cu{TnMe}(PCy3)] (4) was obtained by employing the methyl
derivative K[TnMe], PCy3, and CuCl in methanol. The latter
was crystallized from a dichloromethane solution, giving orange
crystals which were suitable for X-ray diffraction analysis that
revealed a similar bonding situation of the scorpionate ligand
as in compound 2. Again, a κ3-H,S,S, coordination mode is ob-
served but the sulfur atom of a second ligand is replaced by the
phosphine. Thus, in the respective 1H NMR spectra of 3 and 4
two sets of resonances for the pyridazine moieties are expected.
However, similar to 2, only one set of pyridazine resonances are
observed at room temperature, which is an analogical behavior
compared to Dyson's compound [Cu{H2B(mp)2}(PCy3)]
(mp = 2-mercapto-pyridine), which is also exhibiting a copper
atom ligated by two sulfurs and one hydrogen from the ligand
in a κ3-H,S,S fashion.5 The 31P NMR spectra show one signal
at 15.6 and 16.2 ppm for 3 and 4, respectively, also pointing
toward the formation of only one species.
The molecular view of compound 4 is given in Figure 4.
Selected bond lengths and angles are given in Table 3 and
crystallographic data are shown in Table 6. The geometry at the
copper center is tetrahedral. The metal is coordinated by the
Tn ligand by two sulfur atoms as well as a hydrogen atom
forming a 3-center-2-electron B−H···Cu interaction evidenced
by a hydrogen copper distance of 1.96(3) Å. The fourth co-
ordination site of the tetrahedron is occupied by the phosphorus
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Interestingly, when reacting K[TntBu] with PCy3 and CuCl in
CH2Cl2 instead of MeOH for 24 h, a mixture of compound 3
and the decomposition product [Cu(PntBu)2(PCy3)Cl] (5) in
1:1 ratio was observed in the 1H NMR spectrum. Upon further
reaction for several weeks, only compound 5 could be observed
Table 5. Electronic and Geometric Properties of [M{B-
(PnH)3}Cl] (M = Cu, Ni, Co) and [Cu{B(C6H4P(iPr)2)3}Cl]
at the B3-LYP def2-TZVP Level of Theory
a
atomic charges
a
1
d(M···B)
/Å
SEN
by H NMR spectroscopy. It was isolated as red material in
pure form albeit in low yield (eq 1). As dichloromethane can
(M···B)
Cu
B
[Cu{B(PnH)3}Cl]
[Ni{B(PnH)3}Cl]
[Co{B(PnH)3}Cl]
2.11
2.05
2.12
2.57
0.487
0.672
0.646
0.398
0.529
0.589
0.623
0.466
−0.114
−0.200
−0.230
−0.003
[Cu{B(C6H4P(iPr)2)3}
Cl]
a
Two center Shared Electron Number (SEN) and atomic charges with
multicenter corrections as calculated by population analysis based on
molecular orbitals.28
higher affinity of copper to the softer sulfur atom compared to
nitrogen. The copper−sulfur bond lengths are 2.3345(6) and
2.3695(6) Å for Cu1−S1 and Cu1−S2, respectively which is in
the range of the copper−sulfur bonds in compound 4 and
longer compared to the Cu−S distance of 2.221(2) Å in
[Cu(P(o-tolyl)3)(pymtH)Cl] (pymtH = pyrimidine-2-thi-
one).24 The Cu1−Cl bond of 2.3941(6) Å is longer than in
[Cu(P(o-tolyl)3)(pymtH)Cl] (2.293(2) Å) whereas the Cu1−
P1 bond length is in the range of the Cu−P bond length in the
other compound. The distorted tetrahedral geometry is evident
by the S1−Cu1−S2 angle of 104.95(2)°, which is significantly
smaller than the ideal tetrahedral angle, whereas the S1−Cu1−
P1 and S2−Cu1−P1 angles are larger (108.76(2) and
115.65(2)°, respectively) but smaller than the P−Cu−S angle
of 130.9(1)° in [Cu(P(o-tolyl)3)(pymtH)Cl].
effect nondegradative reactions with sulfur-thione ligating
scorpionate ligands such as Tm, giving the salt [H2C-
(mimMe)2BH(mimMe)]Cl·H2O which exhibits the formation
of an S−CH2−S linkage,23 it is conceivable that degradative reac-
tions can take place also and that the B−N bond of Tn can be
broken. This is undermined by the fact that oxidizing metal salts
such as Cu(II)Cl2 can cause ligand decomposition.6,7,20 Re-
crystallization from an acetonitrile solution gave crystals
suitable for X-ray diffraction analysis. A molecular view is
given in Figure 5. Selected bond lengths and angles are listed in
Table 4 and crystallographic data are shown in Table 6. The
Theoretical Calculations. According to the Covalent
Bond Classification (CBC) system the boron atom in
boratranes belongs to Z type ligands forming a dative covalent
bond from the metal to boron where both electrons are
provided by the metal.25 As a logical consequence, the dn
electron count is reduced by 2 electrons, much in the same way
as the dn count is decreased by one electron upon coordination
of an X ligand (one electron from both, the metal and X).
Applying this model, in the here described boratrane the dn
count is thus considered as d8 and the boron ligand as
[BR3]2−.7,26 However, a different description of the bonding
situation has emerged in the literature by using the notation
(M→B)n where n refers to the number of electrons associated
with the metal d orbitals; thus, (M→B)10 in 1.27 Nevertheless,
the latter is a rarely used notation so that the information is not
easily disclosed. Both notations are indicating that the bonding
description of boratrane compounds by formalistic electron
counting models is challenging. The short M→B interaction in
our boratrane is in line with a two electron donation from the
metal to boron and thus a d8 configuration of the metal.
For a better understanding the bonding situation in the
copper boratrane complex, [Cu{B(PnH)3}Cl] was investigated
by DFT calculations. The hybrid density functional B3-LYP
was chosen to compare the copper boratrane to similar cobalt
and nickel boratranes, [Co{B(PnH)3}Cl] and [Ni{B(PnH)3}-
Cl], respectively (Table 5).6 Furthermore, calculations of
shared electron numbers (SEN) were carried out for 1 as
well as for Bourissou’s copper boratrane complex [Cu{B-
(C6H4P(iPr)2)3}Cl], the latter featuring a significantly longer
Cu···B bond distance.15
Figure 5. Molecular structure of [Cu(PntBu)2(PCy3)Cl] (5) with
thermal ellipsoids at 60% probability.
Table 4. Selected Bond Lengths (Å) and Angles (deg) of
[Cu(PntBu)2(PCy3)Cl] (5)
Cu1−Cl
Cu1−P1
Cu1−S1
Cu1−S2
S1−C13
P1−C31
P1−C41
P1−C51
S1−Cu1−S2
2.3941(6)
2.2649(6)
2.3345(6)
2.3695(6)
1.690(2)
1.870(2)
1.860(2)
1.852(2)
104.95(2)
Cl1−Cu1−S1
Cl1−Cu1−S2
S1−Cu1−P1
S2−Cu1−P1
Cl1−Cu1−P1
Cu1−S1−C13
Cu1−S2−C23
Cu1−P1−C31
Cu1−P1−C41
109.79(2)
108.01(2)
108.76(2)
115.65(2)
109.52(2)
112.98(7)
109.55(7)
118.76(6)
109.02(7)
geometry of the copper center is best described as distorted
tetrahedral. The two pyridazines are binding to copper via the
thione-sulfur atom and not via nitrogen, which shows the
The SENs for the pair Cu···B strongly indicate the presence
of a copper boron bond in both copper boratrane complexes,
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Inorganic Chemistry
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Table 6. Crystallographic Data and Structure Refinement for Complexes 1−5
1
2
4
5
empirical formula
Mr, g/mol
cryst description
cryst syst
C24H33BClCuN6S3·1.75(CH2Cl2)
760.16
C48H68B2Cu2N12S6·3(CH2Cl2)
C33H49BCuN6PS3·3(CH2Cl2)
986.06
C34H57ClCuN4PS2
715.92
1408.98
block, red
monoclinic
C2/c
needle, orange
monoclinic
P2(1)/c
9.3659(4)
22.7579(9)
17.5854(7)
90
block, yellow
triclinic
plate, red
triclinic
space group
a, Å
P1
P1
̅
̅
16.5494(6)
17.6573(7)
24.1261(9)
90
11.2357(14)
12.9285(17)
16.999(2)
75.682(5)
74.773(4)
78.656(4)
2285.5(5)
2
9.7963(4)
9.9745(4)
19.2362(8)
82.217(2)
81.372(2)
85.079(2)
1837.12(13)
2
b, Å
c, Å
α, deg
β, deg
95.193(2)
90
107.4795(9)
90
γ, deg
vol, Å3
3732.9(3)
4
6724.5(4)
4
Z
T, K
200(2)
100(2)
100(2)
100(2)
Dc, g/cm3
μ(MoKα), mm−1
F(000)
1.353
1.392
1.433
1.294
1.100
1.101
1.035
0.853
1566
2920
1024
764
reflns collected
unique reflns
reflns with I ≥ 2σ(I)
R(int)
78885
32372
107332
19141
9236
9765
12055
8828
7170
8486
10832
6612
0.0344
0.0179
0.0479
0.0381
no. of params/restraints
409/12
0.0540, 0.1529
0.0745, 0.1757
1.087
399/0
514/4
420/0
a
b
final R1 , wR2 (I ≥ 2σ)
R indices (all data)
GOF on F2
0.0298, 0.0762
0.0373, 0.0811
1.023
0.0372, 0.0990
0.0432, 0.1084
1.085
0.0372, 0.0784
0.0614, 0.0874
1.006
larg. diff. peak and hole, e/Å3
1.929, −1.010
835862
0.693, −0.885
835860
1.780, −1.120
835861
0.553, −0.512
835859
data CCDC
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(Fo − Fc2)2]2/∑[w(Fo )2}1/2
.
2
2
Figure 6. Orbital correlation diagram for the formation of a copper boratrane at the B3-LYP def2-TZVP level of theory.
[Cu{B(PnH)3}Cl] and [Cu{B(C6H4P(iPr)2)3}Cl], respectively.
The metal boron bond in Bourissou’s complex, however, is
much weaker compared to our system. In Bourissou’s
compound less electron density is shifted from the occupied
dz orbital of the metal center to the empty pz orbital of the
boron atom as indicated by the atomic charges derived from
2
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Inorganic Chemistry
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Figure 7. HOMO-2 of [Cu{B(PnH)3}Cl] with a Cu−B distance of 3 Å (left) and of 2.11 Å (right) at the B3-LYP def2-TZVP level of theory.
population analysis. Hence, as the Cu···B bond distance
increases from 2.11 Å in 1 to 2.57 Å in [Cu{B(C6H4P-
(iPr)2)3}Cl], the SEN decreases from 0.487 to 0.398. Although
there is still a significant M→B interaction in both boratrane
systems, this interaction is much stronger in the case of the
nickel and cobalt boratrane complexes featuring d7 or d6
electron configuration, respectively.6 This is consistent with
the concept of a metal boron bond requiring a shift of electron
density from the metal to the boron atom. Generally, this M→
B interaction is strengthened by electron rich metals. However,
electron density shift from a fully occupied d10 metal center
leading to a d8 copper boratrane system is energetically less
favorable.
complexes [Cu{TnR}(PCy3)]. [Cu{TntBu}]2 and [Cu{TnR}-
(PCy3)] exhibit a 3-center-2-electron B−H···Cu interaction, as
evidenced by X-ray crystallography. The nature of the M→B
interaction featured in our copper boratrane compound 1 as
well as in Bourissou’s complex [Cu{B(C6H4P(iPr)2)3}Cl] was
further investigated by DFT studies and compared to similar
late transition metal systems of type [M{B(PnH)3}Cl] (M =
Co, Ni). As electron density is shifted from the electron rich
metal center toward the boron atom upon formation of
boratrane compounds, such M→B interactions are naturally
weaker when going from an electronically fully occupied d10 to
a d8 system. Notably, our copper boratrane features a stronger
metal boron bond than Bourissou’s system as evidenced by
calculation of shared electron numbers (SEN) as well as by X-
ray crystallography.
DFT calculations support a d8 configuration for the copper
boratrane complex [Cu{B(PnH)3}Cl]. In a hypothetical d10
system featuring a Cu−B distance of 3 Å all 10 d electrons can
be assigned to the metal as the pz orbital on the planar boron
atom remains unoccupied (Figure 6). Upon formation of the
copper boratrane bond, electron density is shifted from the
bonding and antibonding σ-orbitals describing the Cu−Cl bond
toward the boron atom thus leading to a 4-electron-3-center
B−Cu−Cl interaction. It is best represented by a bonding
orbital B−Cu−Cl, a mixed bonding and antibonding B−Cu−Cl
orbital, and an unoccupied antibonding B−Cu−Cl orbital,
respectively. The remaining 8 d-electrons are metal based and
thus leading to a d8 configuration. Furthermore, formation of the
copper boratrane significantly affects the geometry altering
the metal environment from tetrahedral to trigonal bipyramidal.
The shift of electron density from the metal to the boron atom is
represented by the HOMO-2 of [Cu{B(PnH)3}Cl] (Figure 7).
In comparison, Bourissou’s copper boratrane complex with a
significantly longer Cu−B bond has a weaker interaction and
thus was described by the authors as having a d10 configuration.
EXPERIMENTAL SECTION
■
General Procedures. NMR spectra were measured on a Bruker
Avance III 300 MHz spectrometer. H NMR and 13C NMR spec-
1
troscopy chemical shift values are reported as δ using the solvent signal
(DMSO-d6: δ 2.50 and 39.4 or 7.26 and 77.0 ppm for DMSO-d6 or
CDCl3, respectively) as an internal standard. Spectra were obtained
at 25 °C. Mass spectra were measured on an Agilent LCMSD single
quadrupole mass spectrometer of the SL type. The mass spectrometer
was equipped with an atmospheric pressure ionization (API) source
employing pneumatically assisted electrospray nebulization with
nitrogen as the nebulizer gas. Elemental analyses were performed on
a Heraeus VARIO ELEMENTAR by the Analytisch-Chemisches
Laboratorium des Instituts fur Anorganische Chemie der Technischen
̈
Universitat Graz, Austria. Infrared spectra were recorded on a Perkin-
̈
Elmer FT-IR spectrometer 1725X as KBr pellets and on a Bruker
Alpha Platinum ATR spectrometer.
Ligands K[TntBu] and K[TnMe] were synthesized according to
published procedures.6
X-ray Structure Determinations. For X-ray structure analyses
for all compounds the crystals were mounted onto the tip of glass
fibers and covered in inert oil. Data collection was performed on a
BRUKER-AXS SMART APEX II CCD diffractometer using graphite-
monochromated Mo Kα radiation (0.71073 Å) at 100(2) K (except 1
CONCLUSION
■
We have prepared new copper complexes containing the Tn
moiety. Reaction of K[TntBu] with CuCl2 in dichloromethane
gave the copper boratrane complex [Cu{B(PntBu)3}Cl] under
activation of the B−H bond, resulting in a Cu→B dative bond.
In contrast to this, reaction of K[TntBu] with CuCl in THF gave
the dimeric compound [Cu{TntBu}]2, and addition of CuCl to a
solution of PCy3 and K[TnR] (R = Me, tBu) in MeOH gave
at 200(2) K). The data for all compounds were reduced to F2 and
o
corrected for Lp with SAINT.29 Absorption correction was performed
with SADABS.30 The structures were solved by direct methods31 using
the WinGX suite of programs32 and refined by full-matrix least-squares
method with SHELXS-97.33 If not noted otherwise all non-hydrogen
atoms were refined with anisotropic displacement parameters. All
12638
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Inorganic Chemistry
Article
hydrogen atoms were fixed in calculated positions to correspond to
standard bond lengths and angles, apart from the hydridic hydrogens,
which were located in the difference map and allowed to refine
isotropically.
(Cy-C), 31.9 (Cy-C), 36.2 (C(CH3)3), 121.3 (C5), 140.4 (C4), 159.0
(C6), 178.7 (CS) ppm. 31P NMR (CDCl3, 121 MHz) δ 15.6 ppm.
MS (ESI) m/z = 689.2 (59%) [M-PntBu]+. IR (ATR, cm−1): 2922 s,
1476 m, 1426 s, 1177 s, 852 s. Anal. Calcd for C42H67BCuN6PS3: C,
58.82; H, 7.88; N, 9.80%. Found: C, 58.12; H, 7.69; N, 9.67%.
[Cu{HB(PnMe)3}(PCy3)] (4). Tricyclohexyl phosphine (0.203 g,
0.72 mmol) was added to a stirred suspension of CuCl (0.036 g, 0.36
mmol) in MeOH (10 mL). After addition of K[TnMe] (0.154 g, 0.36
mmol) the suspension was stirred for 3 days. The dark yellow reaction
mixture was filtered, and the obtained solid was dissolved in CH2Cl2.
After filtration the solvent was removed in vacuo. Recrystallization
from a dichloromethane solution gave orange crystals. They were iso-
lated by filtration and washed with 5 mL of MeOH giving 75 mg
(21%) of 4. Crystals suitable for X-ray diffraction analysis were
obtained by recrystallization from a dichloromethane solution. 1H
NMR (CDCl3, 300 MHz) δ 1.91−1.18 (m, 30H, Cy-H), 2.23 (s, 9H,
Computational Details. The geometry of [Cu{B(PnH)3}Cl]
was optimized in the gas phase using the hybrid density functional
B3LYP34 as implemented in the TURBOMOLE35 program. First
geometry optimization was performed with the standard double-ζ
quality basis def2-SVP.36 The geometry was then reoptimized with the
larger def2-TZVP basis.36b,37 Input geometry of [Cu{B(PnH)3}Cl] was
obtained from the crystal structure of [Cu{B(PntBu)3}Cl] (1) by
removing the bulky tert-butyl groups at the pyridazine moiety. The
stationary point was confirmed as minimum by calculation of analytical
harmonic frequencies. Natural population analysis (NPA)38 as well as
calculations of the shared electron number (SEN)39 were performed
with the larger def2-TZVP basis. The shared electron number gives the
number of electrons shared by bonded atoms which cannot be
assigned to either atom in a unique way. The shared electron numbers
can be considered as a measure of covalent bond strength.28 For
reason of comparison, calculations of SENs were also performed on
Bourissou’s boratrane system [Cu{B(C6H4P(iPr)2)3}Cl] using their
optimized geometry.15 The geometry of [Cu{B(PnH)3}Cl] with a
weak Cu−B interaction was optimized with a constrained Cu−B
distance of 3 Å with the def2-TZVP basis. Molecular orbitals were
visualized with Avogadro.35
3
CH3), 5.89 (br s, 1H, B-H), 6.76 (d, 3H, JH−H = 8.90 Hz, H5), 7.67
3
(d, 3H, JH−H = 8.87 Hz, H4). 13C NMR (CDCl3, 75 MHz) δ 21.7
(CH3), 26.3 (Cy-C), 27.4 (Cy-C), 27.5 (Cy-C), 30.3 (Cy-C), 30.3
(Cy-C), 31.8 (Cy-C), 31.9 (Cy-C), 124.8 (C5), 140.6 (C4), 150.0
(C6), 178.8 (CS) ppm. 31P NMR (CDCl3, 121 MHz) δ 16.2 ppm.
MS (ESI) m/z = 607.2 (12%) [M-PntBu]+ IR (ATR, cm−1): 2921 s,
1428 s, 1216 s, 1103 s, 888 m. Anal. Calcd for C33H49BCuN6PS3: C,
54.20; H, 6.75; N, 11.49%. Found: C, 54.18; H, 6.83; N, 11.42%.
[Cu(PntBu)2(PCy3)Cl] (5). Tricyclohexyl phosphine (0.203 g, 0.72
mmol) was added to a stirred suspension of CuCl (0.036 g, 0.36
[Cu{B(PntBu)3}Cl] (1). A stirred solution of K[TntBu] (0.30 g, 0.54
mmol) in CH2Cl2 (10 mL) was treated with CuCl2·2H2O (0.093 g,
0.54 mmol). After 15 h the orange suspension was filtrated, washed
with 3 mL of CHCl3, 6 mL of H2O, 2 mL of MeOH, and 3 mL of Et2O.
The thus obtained orange solid was dried in vacuo giving 118 mg (36%)
of 1. Crystals suitable for X-ray diffraction analysis were obtained by
mmol) in degassed abs. CH2Cl2 (10 mL). After addition of K[TntBu
]
(0.154 g, 0.36 mmol) the suspension was stirred for 26 days. The red
reaction mixture was filtered, and the solvent was removed in vacuo.
Recrystallization from an acetonitrile solution gave red crystals. They
were isolated by filtration giving 47 mg (18%) of 5. Crystals suitable
for X-ray diffraction analysis were obtained by recrystallization from an
acetonitrile solution.
1
recrystallization from a dichloromethane solution. H NMR (DMSO-
d6, 300 MHz) δ 1.07 (s, 3H, C(CH3)3), 7.97 (d, 1H, 3JH−H = 9.29 Hz,
H5), 8.09 (d, 1H, 3JH−H = 9.28 Hz, H4) ppm . 13C NMR (DMSO-d6,
75 MHz) δ 28.2 (C(CH3)3), 36.2 (C(CH3)3), 129.1 (C5), 136.1 (C4),
162.7 (C6), 176.3 (CS) ppm. MS (ESI) m/z = 575.2 (38%) [M −
Cl]+. IR (ATR, cm−1): 1592 m, 1477 m, 1428 s, 1256 s, 649 s, 588 s.
Anal. Calcd for C24H33N6BClCuS3·0.56CH2Cl2: C, 44.76; H, 5.22; N,
12.75%. Found: C, 44.76; H, 5.06; N, 12.97%.
1H NMR (CDCl3, 300 MHz) δ 1.28 (s, 18H, C(CH3)3), 1.93−1.33
(m, 34 H, Cy-H), 7.22 (d, 2H,, pyridazine-H5, JH−H = 9.35 Hz), 7.63
(d, 2H, pyridazine-H4, JH−H = 9.34 Hz), 14.43 (s, 2H, NH) ppm. 13
C
NMR (CDCl3, 75 MHz) δ 26.2 (Cy-C), 27.5 (Cy-C), 28.7 (C(CH3)3),
30.39 (Cy-C), 32.0 (Cy-C), 36.5 (C(CH3)3, 125.6 (C5), 139.8 (C4),
162.0(C6), 175.2 (CS) ppm. 31P NMR (CDCl3, 121 MHz) δ 9.9 ppm.
IR (ATR, cm−1): 2963 m, 1476 m, 1428 s, 1255 s, 926 s, 649 s; Anal.
Calcd for C34H57ClCuN4PS2: C, 57.04; H, 8.02; N, 8.96%. Found: C,
56.65; H, 8.07; N, 8.25%.
[Cu{HB(PntBu)3}]2 (2). Method 1: A stirred solution of K[TntBu
]
(0.30 g, 0.54 mmol) in THF (35 mL) was treated with CuCl (0.054 g,
0.54 mmol) giving a red solution. After 24 h of stirring the solvent was
removed in vacuo. Recrystallization from a dichloromethane solution
gave red crystals. They were isolated by filtration giving 0.25 g (75%)
of 2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Method 2: A stirred solution of K[HB(PntBu)3] (0.20 g, 0.36 mmol)
in H2O (25 mL) was treated with CuCl2·2H2O (0.061 g, 0.36 mmol)
resulting in immediate formation of a red precipitate. The suspension
was stirred for 15 min, and the product was isolated by filtration and
dried in vacuo for 24 h giving 0.126 g (30%) of 2. Crystals suitable for
X-ray diffraction analysis were obtained by recrystallization from a
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nadia.moesch@uni-graz.at.
■
1
dichloromethane/diethyl ether solution (1:8 v/v). H NMR (CDCl3,
300 MHz) δ 1.05 (s, 34H, (C(CH3)3), 7.06 (d, 6H, pyridazine-H5,
J
H−H = 9.1 Hz), 7.83 (d, 6H, pyridazine H4, JH−H = 9.1 Hz,). 13C NMR
REFERENCES
■
(CDCl3, 75 MHz) δ 29.00 (C(CH3)3), 36.3 (C(CH3)3), 122.1 (C5),
140.8 (C4), 160.2 (C6), 177.6 (CS) ppm. MS (ESI) m/z = 807.2
(100%) [M-{B(PntBu)2}]+ IR (KBr pellets, cm−1), 2963 m, 1477 s,
1430 s, 1210 m, 1144 s, 834 w. Anal. Calcd for C48H68B2Cu2N12S6·0.75
H2O: C, 48.08; H, 5.75; N, 13.80%. Found: C, 48.08; H, 5.31; N,
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