Intermediates in reactions of copper(I) Complexes with N-Oxides: from the formation of stable adducts to Oxo Transfer

Article abstract of DOI:10.1021/ic900435p

Reactions of copper(I) complexes of bidentate N-donor supporting ligands with pyridine- and trimethylamine-N-oxides or PhIO were explored. Key results include the identification of novel copper(I) Noxide adducts, aryl substituent hydroxylation, and bis(μ-

Full text of DOI:10.1021/ic900435p

Inorg. Chem. 2009, 48, 6323–6325 6323
DOI: 10.1021/ic900435p
Intermediates in Reactions of Copper(I) Complexes with N-Oxides:
From the Formation of Stable Adducts to Oxo Transfer
Sungjun Hong, Aalo K. Gupta, and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota,
207 Pleasant Street SE, Minneapolis, Minnesota 55455
Received March 4, 2009
Reactions of copper(I) complexes of bidentate N-donor supporting
ligands with pyridine- and trimethylamine-N-oxides or PhIO were
explored. Key results include the identification of novel copper(I) N-
oxide adducts, aryl substituent hydroxylation, and bis(μ-oxo)dicop-
per complex formation via a route involving oxo transfer.
notable interest are [CuO]+ species (CuIIIO2- T CuIIO•-),
which have been invoked in catalysis by enzymes4 and
synthetic systems5,6 and characterized by theory4b-4d,7 and
in the gas phase.7b For example, ligand-supported [CuO]+
species have been postulated in order to rationalize ligand
hydroxylations observed in the decay of CuIIOOH com-
plexes,5 the O2-induced decarboxylation of copper(I) R-
ketocarboxylates,6 and reactions of copper(I) complexes with
iodosylbenzene (PhIO).5 A [CuO]+ intermediate has also
been invoked as the active species responsible for regiospe-
cific arene hydroxylations by Cu/Me3NO systems,8 via a
mechanism proposed on the basis of theory to involve O-N
bond homolysis from an isolated CuIIONMe3 complex.9
While a number of such copper(II) N-oxide complexes are
known,10 to our knowledge copper(I) variants have not been
reported. Such copper(I) N-oxide adducts are of interest
because they might provide a [CuO]+ or related species
through heterolytic N-O bond cleavage, as seen in a number
of reactions of N-oxides with other metal centers (e.g., Fe11
and Ru12). We therefore sought to investigate the reactions of
pyridine- and trialkylamine-N-oxides with copper(I) com-
plexes, using a range of supporting ligands with variable
Because copper-promoted oxidation reactions play impor-
tant roles in biology1 and catalysis,2 great effort has been
expended to uncover mechanistic information and, in parti-
cular, to identify possible copper-oxygen intermediates.3 Of
*To whom correspondence should be addressed. E-mail: wtolman@umn.
edu.
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2009 American Chemical Society
Published on Web 05/08/2009
pubs.acs.org/IC

6324 Inorganic Chemistry, Vol. 48, No. 14, 2009
Scheme 1. Reactions of Copper(I) Complexes with Pyridine- and
Trimethylamine-N-oxides
Figure 1. Representations of the X-ray structures of (a) the cationic
portion of3 and (b) 4, showingall non-H atomsas50%thermalellipsoids.
˚
Selectedinteratomicdistances(A) andangles(deg)are asfollows. 3:Cu1-
O1, 1.9048(19); Cu1-N1, 2.066(2); Cu1-N2, 1.993(2); O1-N3, 1.330(3);
O1-Cu1-N2, 150.42(9); O1-Cu1-N1, 128.78(9); N2-Cu1-N1, 80.66
(9); N3-O1-Cu1, 121.96(16). 4: Cu1-O1, 1.9076(18); Cu1-N1, 1.941
(2); Cu1-N2, 2.001(2); N3-O1, 1.407(3); O1-Cu1-N1, 143.82(8); O1-
Cu1-N2, 117.59(8); N1-Cu1-N2, 98.58(8); N3-O1-Cu1, 121.94(14).
electronic and steric profiles that have been previously usedin
O2 reactivity investigations.6,13 Herein we report the initial
results of this study, including the isolation and full char-
acterization of the first examples of copper(I) N-oxide com-
plexes, a demonstration of oxo transfer from an N-oxide
resulting in the formation of a bis(μ-oxo)dicopper core, and
ligand oxidation from a related reaction with PhIO.
Reactions of [(L1)Cu(O3SCF3)]6 with substituted pyridine-
N-oxides (1 equiv) or [(L)Cu(CH3CN)] (L = L3 or L4)13,14
with Me3NO (1 equiv) in tetrahydrofuran (THF) yielded the
adducts 1-5, respectively (Scheme 1). The products were
isolated as red-brown crystalline solids in good yields
(67-95%) and were characterized by 1H and 13C{1H}
NMR spectroscopy, CHN analysis, and X-ray crystallogra-
phy (Figures 1 and S1-S3 in the Supporting Information). All
of the structures feature three-coordinate CuI ions and “bent”
coordination of the N-oxide ligands (Cu-O-N angles 120-
122°). Little “activation” of N-oxide is evident from the N-O
Figure 2. Drawing and representation of the X-ray crystal structure of
the dicationic portion of the tricopper cluster resulting from the reaction
of [(L2)Cu(CH3CN)](O3SCF3) with PhIO, with ligand iPr groups omitted
˚
for clarity. Selected interatomic distances (A) and angles (deg) are as
follows: I1-O4, 1.814(5); I1-O5, 1.825(4); I1-O6, 1.829(5); Cu1-O1,
1.897(5); Cu1-N1, 1.921(6); Cu1-O4, 1.934(5); Cu1-N2, 1.971(6);
Cu2-O2, 1.870(5); Cu2-N4, 1.918(6); Cu2-O5, 1.940(5); Cu2-N5,
1.961(6); Cu3-O3, 1.868(5); Cu3-N7, 1.928(6); Cu3-O6, 1.947(5);
Cu3-N8, 1.968(5); O4-I1-O5, 97.7(2); O4-I1-O6, 98.9(2);
O5-I1-O6, 96.1(2); O1-Cu1-N1, 93.5(2); O1-Cu1-O4, 89.0(2);
N1-Cu1-O4, 168.4(2); O1-Cu1-N2, 163.3(2); N1-Cu1-N2, 83.3(3);
O4-Cu1-N2, 97.4(2); O2-Cu2-N4, 93.8(2); O2-Cu2-O5, 87.7(2);
N4-Cu2-O5, 170.1(2); O2-Cu2-N5, 172.4(2); N4-Cu2-N5, 83.6(3);
O5-Cu2-N5, 96.0(2); O3-Cu3-N7, 93.6(2); O3-Cu3-O6, 87.2(2);
N7-Cu3-O6, 166.5(2); O3-Cu3-N8, 172.6(2); N7-Cu3-N8, 83.8(2);
O6-Cu3-N8, 96.9(2).
˚
distances, which for 1-3 (1.33-1.34 A) are similar to those
seen in copper(II) pyridine-N-oxide complexes10e,10f and for
˚
4 and 5 (1.40-1.41 A) are similar to that of free Me3NO
showing no signs of decay by NMR spectroscopy in a THF-d8
solution after 1 day at 80 °C (sealed tubes). Under the same
conditions, complexes 4 and 5 decompose to paramagnetic
species that have yet to be identified.
Working under the hypothesis that the stability of 1-3 is
due to a relatively low degree of electron donation from the
ligand L1, we turned our attention to an analogue of L1
containing an electron-donating p-Me2N group on the pyr-
idyl ring (L2). Unfortunately, the reaction of [(L2)Cu
(CH3CN)](O3SCF3) with Me3NO resultedinextensive ligand
redistribution and redox processes, as indicated by the for-
mation of a complex mixture of products, including the
known complex [Cu(Me3NO)4]2+16 and [(L2)2Cu](O3SCF3)
15
˚
[1.404(5) A]. The pyridine-N-oxide ring planes in 1-3 are
parallel to the dimethylphenyl substituent in L1, consistent
with π-stacking interactions. In solution, however, the pyr-
idine-N-oxides undergo rapid fluxional processes, as indi-
cated by the presence of broad peaks in the room temperature
1H NMR spectra that were found in the case of 3 to split and
sharpen upon cooling to -78 °C (Figure S4 in the Supporting
Information). Complexes 1-3 are quite stable in solution,
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Communication
Inorganic Chemistry, Vol. 48, No. 14, 2009 6325
(identified by X-ray crystallography; Figure S5 in the
Supporting Information). By using PhIO instead, oxo trans-
fer was observed, yielding a dicationic tricopper cluster (with
two CF3SO3- counterions) featuring hydroxylated forms of
L2 coordinated to four-coordinate CuII ions bridged by a
608 cm-1, λex = 457.9 nm] spectroscopic properties to a
sample prepared independently from O2 and to data in the
literature18 (Figure S7 in the Supporting Information).
To our knowledge, this is the first report of a bis(μ-oxo)
dicopper complex derived from an oxo transfer reagent.5,19
An adduct akin to 4 and 5 is a likely intermediate in the
reaction. It is tempting to speculate that the [Cu2(μ-O)2]2+
core is formed via dimerization of a [CuO]+ species, but we
recognize that other routes are equally viable, such as ones
involving dimerization of CuIONMe3 adducts prior to O-N
bond cleavage.
In conclusion, explorations of the reactivity of copper(I)
complexes of ligands L1 and L3-L5 with pyridine- and
trimethylamine-N-oxides have led to (a) the isolation of
novel, stable copper(I) N-oxide adducts for L1, L3, and L4
and (b) the observation of oxo transfer to the copper(I)
complex of L5 to yield a bis(μ-oxo)dicopper core. Re-
action of the copper(I) complex of L2 with PhIO yields a
unique tricopper cluster derived from aryl substituent hydro-
xylation. The processes of copper(I) N-oxide adduct forma-
tion, arene hydroxylation, and oxo transfer to yield a
[Cu2(μ-O)2]2+ core that we have observed provide impor-
tant precedence for possible steps in the mechanisms of
Cu/O-mediated reactions.
-
central IO3 ion (Figure 2; 57% isolated yield, also char-
acterized by CHN analysis and electrospray ionization mass
spectrometry; Figure S6 in the Supporting Information).
While the overall structure of this cluster is unique, the
observation of ligand aryl group hydroxylation by PhIO
has parallels in the literature5 and similarly may be attributed
to a [CuO]+ intermediate or some type of CuOIPh species.
In contrast to the formation of stable CuIONMe3 adducts
when using L3 and L4, the copper(I) complex of the more
electron-donating ligand L5 reacted rapidly with Me3NO at
room temperature to give a brown solution, from which a few
crystals of the known17 complex [(L5)2Cu2(μ-OH)2] were
obtained; no CuIONMe3 adduct was observed. When the
reaction was performed at -78 °C, smooth conversion to
an intermediate occurred over ∼1 h that was identified as the
bis(μ-oxo)dicopper complex [(L5)2Cu2(μ-O)2] (6) on the
basis of a comparison of its UV-vis (λmax = 423 nm,
ε ∼ 16000 cm-1 M-1) and resonance Raman [ν(Cu2O2) =
(17) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998–1999.
(18) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B. A.;
Duboc-Toia, C.; Pape, L. L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman, W. B.
Inorg. Chem. 2002, 41, 6307–6321.
(19) Reactions of PhIO with copper(I) complexes that yield other types of
products have been reported. See, for example: (a) Kitajima, N.; Koda, T.;
Hashimoto, S.; Kitagawa, T.; Morooka, Y. J. Am. Chem. Soc. 1991, 113,
5664–5671. (b) Obias, H.; Lin, Y.; Murthy, N.; Pidcock, E.; Solomon, E.;
Acknowledgment. We thank Joe B. Gilroy and Robin G.
Hicks for providing a sample of ligand L4, L. Que, Jr., for
access to the resonance Raman facility, and the NIH (Grant
GM47365 to W.B.T.) for financial support of this research.
Supporting Information Available: Experimental details, Fig-
ures S1-S7, and CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
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