An unusually stable octanuclear σ-mesityl-bridged μ4-oxo-copper(i) complex encapsulated by a pyrazolate-based compartmental ligand scaffold

Article abstract of DOI:10.1039/b717571j

A new compartmental pyrazole-derived chelating ligand, four equivalents of mesitylcopper and stoichiometric amounts of dioxygen lead to the formation of a remarkably stable organometallic framework that can be described as a heteroleptic O-centered cuprate anion [(MesCu^I) ₄(μ₄-O)]^2- linked via σ-mesityl- bridges to two surrounding binuclear Cu^I-pyrazolate clamps. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717571j

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COMMUNICATION
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An unusually stable octanuclear r-mesityl-bridged l4-oxo-copper(I)
complex encapsulated by a pyrazolate-based compartmental ligand
scaffoldw
Michael Stollenz, Christian Große and Franc Meyer*
Received (in Cambridge, UK) 14th November 2007, Accepted 10th January 2008
First published as an Advance Article on the web 6th February 2008
DOI: 10.1039/b717571j
A new compartmental pyrazole-derived chelating ligand, four
equivalents of mesitylcopper and stoichiometric amounts of di-
oxygen lead to the formation of a remarkably stable organome-
tallic framework that can be described as a heteroleptic
O-centered cuprate anion [(MesCuI)4(l4-O)]2ꢀ linked via r-me-
sityl-bridges to two surrounding binuclear CuI-pyrazolate clamps.
The activation of dioxygen, mediated by copper, is of outstand-
ing interest for important synthetic applications, such as the
catalytic oxidation of ethylene to ethylene glycol, the Wacker
process, or the oxidative coupling of acetylenes and of biaryls.1–4
Albeit several efforts were made to clarify the specific roles of
copper in oxidatively induced C–C coupling reactions, most
mechanisms are not fully understood so far.2–4 Controlled
complexes, with applications in biomimetic chemistry and cata-
lysis.13 In the context of the present organometallic objectives we
oxygenation of well-defined s-organo CuI compounds,5 which
serve as precursors in copper-mediated oxidative couplings,
set out to further elaborate such ligands by introducing a phenyl
group in the 4-position of the pyrazolate ring, in order to provide
appears to be a promising approach to get some insight in the
enhanced solubility in less polar solvents such as benzene and
mechanistic details of such reactions. As shown earlier, however,
toluene. This modification makes the pyrazolate-based bridging
mostly undefined copper complexes and biaryls are formed under
usual conditions.6 A milestone in this context is the controlled
ligands valuable tools for stabilising sensitive organometallic
species that generally do not tolerate any polar coordinating or
oxygenation of mesitylcopper—which is a powerful synthon for
the preparation of unusual homocuprates,7 homoleptic CuI
protic solvents. The new ligand 2H, which was synthesised
starting from 3,5-bis(hydroxymethyl)-4-phenyl-1H-pyrazole14 as
alkoxides and phenoxides,8 Schiff base macrocyclic complexes,9
arylated B-, Sn-, as well as P-halides10 and, as shown recently,
shown in Scheme 1, is predestinated (after deprotonation) to
luminescent cluster frameworks11—affording bimesityl and the
span two metal ions nested in its two tridentate binding sites.
Since CuI tends to prefer four as its highest coordination number,
highly sensitive complex [Cu10O2(Mes)6] (I) that contains seven
m-O-bridged CuI centres, according to the X-ray structure eluci-
2 should be particularly suitable for insulating sensitive organo-
dation.12 Due to its extreme sensitivity, however, no further
copper frameworks. In contrast to related pyrazolate ligands
lacking the phenyl group, 2H is indeed well soluble in benzene
analytical and spectroscopic data could be obtained for this
and toluene (compare ESIw).
As previously demonstrated, chelating pyrazolate ligands
compound so far.
Herein, we now report on an exceptionally stable s-organo-
complex with a concealed [(MesCuI)4(m4-O)]2ꢀ heteroleptic
cuprate core that is stabilised by two binucleating pyrazolate
clamps, and its complete characterisation including an X-ray
structure determination.
usually afford binuclear complexes with Mꢁ ꢁ ꢁM distances in
the range 3.5–4.5 A, depending on the chelate arms.15 Mesi-
tylcopper forms oligonuclear ring structures [CuMes]n (n = 4,
5) where the interatomic Cuꢁ ꢁ ꢁCu distances are governed by
the geometric requirements of the Cu5 pentamer as well as the
mesityl bridging ligands.16 Intermetallic distances between the
next but one neighbouring Cu atoms in the pentamer are
around 4.04 A. Therefore 2H (after deprotonation) should
provide the right premises to accommodate and stabilise a
tailored ‘‘corner piece’’ of mesitylcopper such as fragments
Multifunctional pyrazolate ligands with chelating side arms in
the 3- and 5-positions of the heterocycle have been shown to offer
versatile scaffolds for a wide range of homo- and heterobinuclear
Institut fur Anorganische Chemie, Georg-August-Universitat,
¨
¨
Tammannstraße 4, D-37077 Gottingen, Germany. E-mail:
franc.meyer@chemie.uni-goettingen.de; Fax: (+49)-551-393063;
Tel: (+49)-551-393012
w Electronic supplementary information (ESI) available: Experimental
procedures, spectral and analytical data for 1–3, additional crystal-
lographic details concerning 3. See DOI: 10.1039/b717571j
[Cu(m-Mes)Cu(m-Mes)Cu]+
or
[Cu(m-Mes)Cu(m-Mes)-
¨
Cu(m-Mes)Cu]+ by hosting the two terminal Cu ions.
Reactions of 2H with CuMes were initially carried out in the
molar ratio 1 : 4 at ꢀ78 1C, resulting in the formation of complex
3 which was isolated as yellow needles, albeit in poor yields.
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free ligand 2H, the singlet related to the CH3 groups at d = 1.71
ppm, a CH2-signal at d = 2.97 ppm, and the resonances of the
pyridyl-rings at d = 6.31 and 6.89 ppm as well as the o-phenyl
signal d = 7.20–7.22 ppm are shifted to higher field, whereas the
protons closest to the pyridyl-N-donors are strongly downfield-
shifted and appear at d = 10.07 ppm in the complex, confirming
their coordination to the s-organo-CuI core. Interestingly, the
relative intensities of the singlets at d = 2.25, 2.60 and 6.83 ppm
attributed to the s-bound mesityl ligands suggested a 2 : 1 ratio
with respect to the pyrazolate ligand, but not 3 : 1, as would be
expected for a complex of the type [(2)Cu2(m-Mes)2Cu2(m-Mes)]
(ESIw).z Hence, we concluded from the combined spectroscopic
and analytical data that 3 should have the stoichiometric com-
position [(2)Cu4Mes2O0.5]n.
The molecular structure of 3 was finally elucidated by X-ray
crystallography on single crystals that were obtained by slow
diffusion of diethyl ether into a THF solution of the product at
+4 1C (Fig. 1).y 3 crystallises in the monoclinic space group
P21/c with two independent molecules in the asymmetric unit.
These have only slightly different bond distances and angles
(see ESIw), and those discussed here refer to one of the
molecules that is shown in Fig. 1.
3 features a distorted pseudo-tetrahedral (m4-O)CuI4 nucleus
with tethered s-mesityl-bridges that are linked to two bi-
nuclear CuI-pyrazolato scaffolds in the typical three-centre
two-electron bonding mode. This gives rise to an overall
octanuclear organometallic CuI framework. The O-bonded
Scheme 1 Reaction conditions: (a) (i) SOCl2, RT, 24 h; (ii) DHP,
CH2Cl2, RT, 24 h; (b) i) 2 eq. PyCH3(Me)NH–Na2CO3, AN, reflux,
40 h; (ii) hydrolysis with HCl–EtOH; (c) toluene, ꢀ78 1C, 18 h,
RT, 2 h.
Upon drying in vacuo, the colour of 3 immediately changes to
beige, but this has no consequences for the spectroscopic and
analytical data. The ESI-MS spectrum of 3 in THF shows a
prominent peak for the fragment [(2)Cu2Mes + 1]+ at m/z =
657, supporting the presence of an organocopper species. Several
lines of evidence suggested, however, that an oxygenated com-
plex had been formed: firstly, the presence of dry oxygen
increases the yield of 3 dramatically (to 87%). Secondly, bime-
sityl was found as a by-product, very likely originating from an
oxidatively induced elimination (see ESIw). And finally the results
of elemental analyses were inconsistent with the composition of
the anticipated organometallic compound [(2)Cu2(m-Mes)2Cu2
(m-Mes)], but indicated that some other element should also be
present in 3. The reaction was carried out repeatedly, but no
product other than 3 could be isolated.
Fig. 1 Solid-state structure of one of the two independent molecules
of 3, determined by single-crystal X-ray diffraction analysis. Hydrogen
atoms are omitted for clarity. Cuꢁ ꢁ ꢁCu contacts are represented as
dashed lines. A figure with the complete atom numbering is given in
the ESI.w
1
The H NMR spectrum of 3 in D6-benzene reflects a sym-
metric arrangement of the chelating ligand, since only one set of
the expected proton signals is observed. In comparison with the
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pyridyl-C3), 6.71–6.72 (m, 3 H, signal of mesitylene, CH), 6.77–6.80
(m, 2 H, pyridyl-C5), 6.83 (s, 4 H, m-CH), 6.89 (dt, 4J(H,H) = 1.7 Hz
3J(H,H) = 7.6 Hz, 2 H, pyridyl-C4), 7.09–7.12 (m, 1 H, p-Ph),
7.20–7.22 (m, 2 H, o-Ph), 7.27–7.30 (m, 2 H, m-Ph), 10.07 (d,
3J(H,H) = 4.3 Hz, pyridyl-C6). 13C NMR (125.77 MHz, C6D6): d
21.3 (CH3, signal of mesitylene), 21.6 (CH3, p-CH3), 29.7 (CH3, o-
CH3), 43.2 (CH3, N–CH3), 55.7, 61.1 (CH2), 115.2 (C, pyrazole-C3,4,5),
122.3 (CH, pyridyl-C3), 123.1 (CH, pyridyl-C5), 124.5 (CH, m-CH),
127.4 (CH, signal of mesitylene), 128.1 (CH (masked by benzene
signal), m-, p-Ph), 129.0 (CH, o-Ph), 135.5 (C), 135.9 (CH, pyridyl-
C4), 137.0, 137.6 (C), 137.6 (C, signal of mesitylene), 144.6 (C), 152.4
(CH, pyridyl-C6), 153.4, 157.9 (C). MS (ESI in THF): m/z (rel.
copper atoms show nearly linear coordination (average of
O–Cu–Cipso-angles 175.3(3)1), whereas the copper ions hosted
by the chelating pyrazolate ligands are found in a distorted
tetrahedral environment. The distortion of the (m4-O)CuI4 unit
from an ideal tetrahedron is reflected by the Cu–O–Cu-angles
that range from 94.75(19)1 to 124.6(3)1. All Cu–O bonds
(average length 1.851(5) A) are quite short and similar to
those of I, but significantly shorter than those of the well-
known CuII complexes of type [L4CuII4(m-O)X6] (L = mono-
dentate ligand, X = halogen).12,17 Drastic distortion of the
{N3C} coordination sphere of the peripheral CuI ions from
tetrahedral is caused by the constraints of the binding pockets
of 2. An additional contribution might arise from closed-shell
Cuꢁ ꢁ ꢁCu-contacts that—if considered as such—would lead to
a description of the Cu(5–8) environment as pseudo trigonal-
bipyramidal.
intensity) = 657.0 (24) [(1)Cu2Mes + 1]+, 537.2 (100) [(1)Cu2]+
.
y Crystal data for 3: C86H98N12OCu8ꢁ4.63C4H8O, M = 2157.56,
monoclinic, P21/c, a = 23.249(5), b = 32.447(7), c=26.971(5) A, b
= 92.42(3)1, V = 20 328(7) A3, Z = 8, T = 100(2) K, 160 032
reflections measured (yellow needle, 2.13 r y r 59.851), 29 068 unique
(R(int) = 0.0998), 2836 parameter. Data were collected on a Bruker
three circle diffractometer with a SMART 6000 detector and Cu-Ka
radiation (l = 1.54178 A) at low temperature. The structure was
solved using direct methods and refined by full-matrix least-squares
procedures against F2. Non-hydrogen atoms were refined anisotropi-
cally, hydrogen atoms were added using the riding model. Since all
THF molecules are disordered and the THF oxygen could not be
located, the solvent molecules were refined first as cyclopentane and
later with THF-restrains from the PRODRG dungee server.19 R1
(Fo 4 4sFo) = 0.0703, wR2 = 0.1767, GoF = 1.365. CCDC 666842.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b717571j.
The Cu–C bond lengths and Cu–Cipso–Cu angles (average
73.5(2)1) as well as the CuꢁꢁꢁCu-separations (average 2.4286(14)
A) within the Cu(m-Mes)Cu bridging units of 3 are in good
agreement with the corresponding values determined for
[Cu10O2(Mes)6] and parent [CuMes]5, respectively.12,16 However,
bridging of the mesityl groups is rather asymmetric, since the
Cu–C bonds to the central O-bound copper atoms (average
1.920(7) A) are significantly shorter than those to the peripheral
copper atoms hosted by the pyrazolate ligands (average 2.131(7)
A). As a consequence, the mesityl rings are leaning towards the
outer copper centres (although without any p-contacts), which is
evident from the relatively small Cu(5–8)–Cipso–Cpara angles that
lie in the range 118–1271, clearly different from the angles
Cu(1–4)–CipsoCpara (160–1691). Complex 3 can thus be viewed
as a heteroleptic cuprate anion [(MesCuI)4(m4-O)]2ꢀ that is
flanked and stabilised by two cations [(2)CuI2]+. This rare motif
is distinct from most of the hitherto known organocopper(I)
compounds. Only a few of those, like 8-(dimethylamino)-
naphthylcopper(I), feature a homoleptic organocuprate as a
structural theme.18
1 (a) N. Kitajima and Y. Moro-oka, Chem. Rev., 1994, 94, 737; (b) J. I.
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In the presence of air and moisture, 3 decomposes within a
few hours to give a green powder. However, under argon, 3
can be stored at room temperature for several months. In
contrast to [Cu10O2(Mes)6] (I), complex 3 can be dissolved
without any significant decomposition in dry toluene, benzene
or THF. Thereby a small amount of a brown precipitate might
be formed, together with mesitylene (observed by NMR).
In conclusion, we have synthesised in high yield a novel
s-organo CuI-oxo complex that is unusually stable and bears
remarkable cuprate-like structural features, unique in this
context. Currently we are investigating the reactivity of 3 with
regard to typical cuprate applications.
¨
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We thank Prof. Dr G. M. Sheldrick and Dr I. Dix for
providing X-ray data collection facilities, and the Fonds der
Chemischen Industrie for financial support.
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Notes and references
z Characterisation data for 3: elemental analysis calcd (%) for
C86H98N12OCu8 (1824.15): C, 56.62; H, 5.42; N, 9.21; found: C,
56.41; H, 5.40; N, 9.39; m.p.: 100 1C (decomp.). 1H NMR (500.13
MHz, C6D6): d 1.71 (s, 6 H, N–CH3), 2.15 (m, 9 H, signal of
mesitylene, CH3), 2.25 (s, 6 H, p-CH3), 2.60 (s, 12 H, o-CH3), 2.97 (s
(broad), 4 H, CH2), 3.49 (s, 4 H, CH2), 6.31 (d, 3J(H,H) = 7.6 Hz,
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1746 | Chem. Commun., 2008, 1744–1746