Complexes of Cu(i) supported by a tris(ketimine) tripod

Article abstract of DOI:10.1039/c2dt30101f

Arylation of tris(2-benzylnitrile)amine with PhLi, followed by aqueous work-up, results in the formation of a tripodal tris(ketimine) scaffold, N(ArCNHPh)₃. N(ArCNHPh)₃ readily coordinates a number of Cu^I salts, generating complexes that exhibit trigonal pyramidal geometries in the solid-state.

Full text of DOI:10.1039/c2dt30101f

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COMMUNICATION
Complexes of Cu(I) supported by a tris(ketimine) tripod†
Jeremiah J. Scepaniak, Guang Wu and Trevor W. Hayton*
Received 15th January 2012, Accepted 19th March 2012
DOI: 10.1039/c2dt30101f
strong σ and π donor, facilitating the stabilisation of high valen-
Arylation of tris(2-benzylnitrile)amine with PhLi, followed
by aqueous work-up, results in the formation of a tripodal
tris(ketimine) scaffold, N(ArCNHPh)3. N(ArCNHPh)3
readily coordinates a number of CuI salts, generating com-
plexes that exhibit trigonal pyramidal geometries in the
solid-state.
cies and promoting novel reactivity. Herein we report the syn-
thesis of a tridentate tris(ketimine) tripod, the coordination
chemistry of the neutral tripod with a series of CuI salts, and the
electrochemistry of the tris(ketimine) tripod and its CuOTf
adduct.
Arylation of tris(2-benzylnitrile)amine33 with 3 equiv of PhLi
results in the formation of an orange suspension (Scheme 1).
Characterisation of the presumptive tris(lithium) ketimide proved
elusive due to its sparse solubility in diethyl ether, THF, and pyr-
idine. Consequently, methanolysis was utilized to access the
protio tris(ketimine) scaffold N(ArCNHPh)3 (1) in 80% yield.
This material crystallizes in the monoclinic space group C2/c
from a concentrated solution of ethyl acetate (Fig. S1†). Its
solid-state structure reveals C–N bond distances of 1.2748(18)–
1.2788(19) Å for the three ketimine functionalities, consistent
with double bond character, while the apical nitrogen lies 2.6576
(17) Å out of the plane defined by the three ketimine nitrogens.
Tridentate and tetradentate ligands with C3 symmetry have
received a great deal of attention in the last two decades.1–3 One
attribute of these ligands is the ability to enforce a single site of
reactivity when coordinated to a metal centre, a desirable prop-
erty for small molecule activation and atom transfer chemistry.
Notable examples of C3 enforcing scaffolds include HIPTN3N,4
(SiPiPr3),5,6 [B(PhPiPr2)3],7,8 [PhBPiPr3],9,10 [PhB(tBuIm)3];11,12
and [TIMENR].13 The first example, HIPTN3N supports a
Mo complex capable of catalytic N2 reduction; (SiPiPr3),
[B(PhPiPr2)3], [PhBPiPr3], and [PhB(tBuIm)3] support Fe com-
plexes relevant to small molecule activation and atom transfer reac-
tions; and [TIMENR] has been shown to support CuI.13,14 Tripods
have also been utilized to model bioinorganic copper chemistry.
For example, the ethyl and methyl linked (tris-pyridine)amine
ligands TEPA (N[CH2CH2–2-py]3)15–18 and TMPA (N[CH2–2-
py]3)19,20 have been utilized to form CuI model complexes of the
multicopper enzymes tyrosinase21 and hemocyanin.22
Within the broader class of tripodal ligands, those based upon
tris(arylamine) functionality are particularly attractive as they are
readily synthesised and are highly modular, allowing for the con-
struction of a wide variety of derivatives. These scaffolds have a
demonstrated ability to support a number of transition metal
ions, including NiII,23 MnII,24 Fe,25 CuI,25 CoII,26 and CrIII.24
Notably, tripods constructed upon the tris(arylamine) framework
exhibit a more rigid structure than their alkylamine counter-
parts.23,27 Furthermore, chelating o-arylamine units have the
potential to act as redox active ligands.28–30
1
Finally, the H NMR spectrum of 1 in CD2Cl2 reveals three-fold
symmetry in solution (Fig. S6†), and is consistent with the solid-
state structure.
Characterization of a series of CuI halide and pseudohalide
coordination complexes was undertaken in order to provide
proof of concept that the tris(ketimine) scaffold could coordinate
a metal ion. Thus, addition of 1 equiv of CuCl to a stirring THF
solution of N(ArCNHPh)3 results in the immediate formation of
an orange solution, which upon removal of solvent in vacuo
1
yields an orange solid (Scheme 2). The H NMR spectrum of
this material (THF-d8) reveals the presence of a new diamagnetic
complex, in addition to free N(ArCNHPh)3 in a 1 : 1 ratio
(Fig. S8†). Addition of 2 equiv of CuCl to a CH2Cl2 solution of
1 yields the same orange solid, according to 1H NMR spec-
troscopy, along with complete consumption of the free scaffold
(Fig. S9†). Surprisingly, single crystal X-ray diffraction of the
orange material reveals the formation of a cation–anion pair,
namely [N(ArCNHPh)3Cu][CuCl2]·C7H8 (2·C7H8) (Fig. 1). In
the solid state, the [CuCl2]− anion lies in close contact with the
[N(ArCNHPh)3Cu]+ cation. However, the Cu1–Cu2 distance is
2.8847(8) Å, greater than the sum of the covalent radii.34 The
Our laboratory has previously reported the isolation of several
homoleptic first row transition metal complexes supported by the
bis-tert-butylketimide anion. The isolation of MIV(N = CtBu2)4
(M = Mn, Fe) demonstrates the ability of the ketimide ligand to
support high valent transition metals in unusual geometries.31,32
We reasoned that a multidentate ketimide ligand would also be a
Department of Chemistry and Biochemistry, University of California,
Santa Barbara, CA 93106. E-mail: hayton@chem.ucsb.edu
†Electronic supplementary information (ESI) available: Experimental
details. CIF files for 1, 2, 3, 4, and 5. CCDC 863473–863477. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30101f
Scheme 1 Synthesis of the tris(ketimine) scaffold 1.
Dalton Trans., 2012, 41, 7859–7861 | 7859
This journal is © The Royal Society of Chemistry 2012

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Scheme 2 Synthesis of CuI complexes supported by a tris(ketimine)
tripod.
Fig. 2 Solid-state structure of [N(ArCNHPh)3Cu][OTf] (4). Hydrogen
atoms and CH2Cl2 solvates omitted for clarity. Selected bond lengths
(Å) and angles (°): Cu1–N7 = 1.9736(17), Cu1–N27 = 1.9427(17),
Cu1–N47 = 1.9343(17), Cu1–N1 = 2.5876(15), Cu1–O1 = 2.5554(16),
N7–Cu1–N27 = 113.35(7), N7–Cu1–N47 = 118.43(7), N27–Cu1–N47
= 123.66(7).
while 19F NMR spectroscopy reveals a single resonance at
−78.80 ppm (Fig. S16†). Single crystal X-ray diffraction
reveals a complex with the formulation [N(ArCNHPh)3Cu]-
[OTf]·1.5CH2Cl2 (4·1.5CH2Cl2) (Fig. 2). The Cu1 nucleus in 4
exhibits a distorted trigonal pyramidal coordination environment
with N–Cu–N bond angles of 113.35(7)°, 118.43(7)°, and
123.66(7)°. The Cu1–Neq distances range from 1.9343(17)
Å–1.9736(17) Å, while the Cu1–Naxial distance is 2.5867(15) Å,
consistent with those found in 2 and 3. Cu1 resides 0.2419(9) Å
out of the plane defined by the three ketimine nitrogens, while
the Cu1–O1 bond distance is 2.5554(16) Å, indicating a weakly
coordinating [OTf]− anion.
Fig. 1 Solid-state structure of [N(ArCNHPh)3Cu][CuCl2]·C7H8
(2·C7H8) Hydrogen atoms and toluene solvate omitted for clarity.
Selected bond lengths (Å) and angles (°): Cu1–N7 = 1.9697(14), Cu1–
N27 = 1.9302(13), Cu1–N47 = 1.9252(13), Cu1–N1 = 2.6341(14),
Cu2–Cl1 = 2.0999(6), Cu2–Cl2 = 2.0993(6), Cu1–Cu2 = 2.8847(8),
Cu1–N1 = 2.6341(14), N7–Cu1–N27 = 109.61(5), N7–Cu1–N47 =
115.05(6), N27–Cu1–N47 = 129.00(5).
Cu1 nucleus exhibits a distorted trigonal pyramidal coordination
environment with N–Cu–N bond angles of 109.61(5)°, 115.05
(6)°, and 129.00(5)°. The Cu1–Neq distances range from 1.9252
(13) Å–1.9697(14) Å, consistent with previously reported
CuI–N(H)vCR2 bond lengths.35 In comparison, the Cu–Neq
Similarly, addition of 1 equiv of [Cu(MeCN)4][PF6] to a stir-
ring solution of 1 in CH2Cl2 results in the immediate formation
of [N(ArCNHPh)3Cu][PF6] (5), which is isolable as an orange
15
bond distances of [(TEPA)Cu]BPh4 (2.022(5) Å–2.012(5) Å)
1
solid in good yield. Its H NMR spectrum is similar to that of 4,
are slightly longer. Additionally, Cu1 sits 0.2814(7) Å above the
plane defined by the three ketimine nitrogens, while the
Cu–Naxial bond distance is long (2.6341(14) Å). Interestingly,
the apical nitrogen lies 2.3519(15) Å below the plane defined by
the same three nitrogens, a contraction of approximately 0.30 Å
versus the free ligand. This contraction results from a twisting of
the diphenylketimine arms away from the Cu–Naxial vector,
which is apparently required to accommodate the Cu ion.
Addition of 1 equiv of CuI to a stirring CD2Cl2 solution of 1
also results in the formation of a cation–anion pair, namely
[N(ArCNHPh)3Cu]2[CuI3] (3) (Scheme 2), (see ESI† for full
characterization details). Thus, to circumvent the unwanted for-
mation of a multi-metallic copper complex, the reaction of 1
with [Cu(MeCN)4][OTf] was explored, with the hope that the
weaker Lewis base [OTf]− would be less amenable to copper
while its 19F NMR spectrum reveals a doublet centered at
−73.45 ppm, consistent with the presence of the [PF6]− anion
(Fig. S19†). Single crystal X-ray diffraction reveals a complex
with
the
formulation
[N(ArCNHPh)3Cu][PF6]·4CH2Cl2
(5·4CH2Cl2) (Fig. S5†). Unlike complex 4, there is no inter-
action between Cu1 and the counterion in 5. Accordingly, the
Cu1 nucleus exhibits a distorted trigonal planar coordination
environment, and its Cu1–Neq distances range from 1.923(2)
Å–1.966(2) Å.
To evaluate the electron donating ability of the tris(ketimine)
scaffold we attempted to prepare a Cu(I)-carbonyl complex con-
taining this ligand.25,36 Bubbling CO through a CH2Cl2 solution
of 5 results in the appearance of a νCO stretch at 2086 cm−1
,
assignable to [N(ArCNHPh)3CuCO][PF6] (6) (Fig. S23†). This
value is similar to that reported for the tris(arylamine) Cu(I)
halide counterion formation. Addition of
[Cu(MeCN)4][OTf] to a stirring solution of N(ArCNHPh)3 in
CH2Cl2 results in formation of [N(ArCNHPh)3Cu][OTf] (4),
which is isolable as a orange solid in 80% yield. Its H NMR
1
equiv of
complex [(N(o-PhNMe2)3)CuCO][PF6] (νCO = 2088 cm−1
,
nujol),25 but higher than those reported for [(Me6tren)CuCO]-
[PF6] (νCO = 2078 cm−1, THF)25 and [(TMPA)CuCO][PF6]
(νCO = 2077 cm−1, nujol).37 Therefore, it appears that 1 is not as
1
spectrum in C6D6 is consistent with a 3-fold symmetric structure,
7860 | Dalton Trans., 2012, 41, 7859–7861
This journal is © The Royal Society of Chemistry 2012

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be attributed to the rigidity of the tris(arylamine) backbone.25
However, this comparison is complicated by the ability of
Me6tren and TMPA to undergo κ4–κ3 isomerizations,25,37 which
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(0.25 V s−1) reveals two irreversible oxidation features at 0.83 V
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the CuI–CuII redox couple (Fig. S21†). We also probed the reac-
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ated with 4 were broadened, however no new products were
observed suggesting that no reaction occurred (Fig. S25†).
In summary, we have synthesized a C3 symmetric tris(ketimine)
tripod, N(ArCNHPh)3, by arylation of tris(2-benzylnitrile)amine.
This ligand readily coordinates to the CuI ion, demonstrating that
the rigid tripod architecture is able to accommodate a metal ion.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7859–7861 | 7861