Heterobimetallic activation of dioxygen: Characterization and reactivity of novel Cu(I)-Ge(II) complexes

Article abstract of DOI:10.1021/ic060050q

Reaction of the known germylene Ge[N(SiMe₃)₂] ₂ and a new heterocyclic variant Ge[(NMes)₂(CH) ₂] with [L^Me2Cu]₂ (L^Me2 = the β-diketiminate derived from 2-(2,6-dimethylphenyl)amino-4-(2,6- dimethylphenyl)imino-2-pentene) yielded novel Cu(I)-Ge(II) complexes L ^Me2Cu-Ge[(NMes)₂(CH)₂] (1a) and L ^Me2Cu-Ge[N(SiMe₃)₂]₂ (1b), which were characterized by spectroscopy and X-ray crystallography. The lability of the Cu(I)-Ge(II) bond in 1a and b was probed by studies of their reactivity with benzil, PPh₃, and a N-heterocyclic carbene (NHC). Notably, both complexes are cleaved rapidly by PPh₃ and the NHC to yield stable Cu(I) adducts (characterized by X-ray diffraction) and the free germylene. In addition, the complexes are highly reactive with O₂ and exhibit chemistry which depends on the bound germylene. Thus, oxygenation of 1a results in scission and formation of thermally unstable L^Me2CuO₂, which subsequently decays to [(L^Me2Cu)₂(μ-O) ₂], while 1b yields L^Me2Cu(μ-O) ₂Ge[N(SiMe₃)₂]₂, a novel heterobimetallic intermediate having a [Cu^III(μ-O) ₂Ge^IV]³⁺ core. The isolation of the latter species by direct oxygenation of a Cu-(I)-Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper.

Full text of DOI:10.1021/ic060050q

Inorg. Chem. 2006, 45, 4191−4198
Heterobimetallic Activation of Dioxygen: Characterization and Reactivity
of Novel Cu(I) Ge(II) Complexes
−
John T. York, Victor G. Young, Jr., and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, UniVersity of Minnesota, 207
Pleasant Street SE, Minneapolis, Minnesota
Received January 10, 2006
Reaction of the known germylene Ge[N(SiMe3)2]2 and a new heterocyclic variant Ge[(NMes)2(CH)2] with [LMe2Cu]2
(LMe2
the -diketiminate derived from 2-(2,6-dimethylphenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene) yielded
novel Cu(I) Ge[N(SiMe3)2]2 (1b), which were
Ge(II) complexes LMe2Cu Ge[(NMes)2(CH)2] (1a) and LMe2Cu
characterized by spectroscopy and X-ray crystallography. The lability of the Cu(I) Ge(II) bond in 1a and b was
)
â
−
−
−
−
probed by studies of their reactivity with benzil, PPh3, and a N-heterocyclic carbene (NHC). Notably, both complexes
are cleaved rapidly by PPh3 and the NHC to yield stable Cu(I) adducts (characterized by X-ray diffraction) and the
free germylene. In addition, the complexes are highly reactive with O2 and exhibit chemistry which depends on the
bound germylene. Thus, oxygenation of 1a results in scission and formation of thermally unstable LMe2CuO2, which
subsequently decays to [(LMe2Cu)2(
intermediate having a [CuIII(
(I)
µ
-O)2], while 1b yields LMe2Cu(
µ
-O)2Ge[N(SiMe3)2]2, a novel heterobimetallic
+
µ
-O)2GeIV]3 core. The isolation of the latter species by direct oxygenation of a Cu-
−
Ge(II) precursor represents a new route to heterobimetallic oxidants comprising copper.
Efforts to understand the mechanisms of oxidation cataly-
sis1 are aided by studies aimed at isolating and characterizing
metal-oxygen intermediates in proteins2 and synthetic
systems.3 Such intermediates are often derived from reactions
of reduced metal sites with the versatile and abundant reagent
O2, and particular attention has been placed on those species
that incorporate a single type of metal ion. For instance,
extensive examination of the reactions of O2 with Cu(I)
complexes has led to the identification of a variety of binding
motifs and activation pathways of relevance to important
biological processes.4 Mixed-metal systems that operate
synergistically to activate O2 also are of interest, as they are
implicated in some synthetically and biologically useful
catalytic reactions. Notable examples that involve Cu include
Cu-Pd species in Wacker-type oxidations5 and Cu-Fe
intermediates in cytochrome c oxidase.3d,6
Examples of well-characterized heterobimetallic oxygen
intermediates derived from O2 are rare, however, due in part
to the challenges associated with preventing formation of
products arising from oxygenation of only one of the two
types of metal ions provided in the starting material(s).
Nonetheless, mixed heme-Fe/nonheme-Cu species with
* To whom correspondence should be addressed. E-mail:
tolman@chem.umn.edu.
(1) Recent reviews: (a) Punniyamurthy, T.; Velusamy, S.; Iqbal, J. Chem.
ReV. 2005, 105, 2329. (b) Stahl, S. S. Angew. Chem., Int. Ed. 2004,
43, 3400. (c) AdVances in Catalytic ActiVation of Dioxygen by Metal
Complexes; Sima´ndi, L. I., Ed.; Kluwer Academic Publishers: Dor-
drecht, 2003.
(2) Selected reviews, for M ) Cu: (a) Solomon, E. I.; Chen, P.; Metz,
M.; Lee, S.-K.; Palmer, A. E. Angew. Chem., Int. Ed. 2001, 40, 4570.
(b) Klinman, J. P. Chem. ReV. 1996, 96, 2541. (c) Solomon, E. I.;
Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996, 96, 2563. For
M ) Fe: (d) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607.
(e) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625. (f)
Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Mu¨ller, J.;
Lippard, S. J. Angew. Chem., Int. Ed. 2001, 40, 2782.
(3) Selected illustrative reviews, for M ) Fe: (a) Costas, M.; Mehn, M.
P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2005, 104, 939. (b) Bois,
J. D.; Mizoguchi, T. J.; Lippard, S. J. Coord. Chem. ReV. 2000, 200-
202, 443. (c) McLain, J. L.; Lee, J.; Groves, J. T., In Biomimetic
Oxidations Catalyzed by Transition Metal Complexes; Meunier, B.,
Ed.; Imperial College Press: London, 2000; pp 91-169. (d) Collman,
J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV. 2004, 104,
561.
(4) Recent reviews: (a) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P.
Chem. ReV. 2004, 104, 1013. (b) Lewis, E. A.; Tolman, W. B. Chem.
ReV. 2004, 104, 1047. (c) Hatcher, L.; Karlin, K. D. J. Biol. Inorg.
Chem. 2004, 9, 669. (d) Itoh, S. In ComprehensiVe Coordination
Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier:
Amsterdam, 2004; Vol. 8, pp 369-393. (e) Halcrow, M. A. In
ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer,
T. J., Eds.; Elsevier: Amsterdam, 2004; Vol. 8, pp 395-436.
(5) (a) Hosokawa, T.; Takano, M.; Murahashi, S. J. Am. Chem. Soc. 1996,
118, 399. (b) Hosokawa, T.; Nomura, T.; Murahashi, S. J. Organomet.
Chem. 1998, 551, 387.
(6) (a) Michel, H.; Behr, J.; Harrenga, A.; Kannt, A. Annu. ReV. Biophys.
Biomol. Struct. 1998, 27, 329. (b) Kim, E.; Chufan, E. E.; Kamaraj,
K.; Karlin, K. D. Chem. ReV. 2004, 104, 1077.
10.1021/ic060050q CCC: $33.50
Published on Web 04/15/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 10, 2006 4191

York et al.
11100-PB/UVAR detector and ST-1385 controller interfaced with
Winspec software. The spectra were obtained at -196 °C using a
backscattering geometry. Excitation at 457.9 and 488 nm was
provided by a Spectra Physics BeamLok 2065-7S Ar Laser. Samples
were frozen in a copper cup attached to a coldfinger Dewar filled
with liquid N2. Raman shifts were externally referenced to liquid
indene. Elemental analyses were performed by Atlantic Microlab,
Inc. (Norcross, GA) and Robertson Microlit (Madison, NJ). GeCl2‚
dioxane and benzil were purchased from Aldrich Chemical Corp.
and used as received. The complexes LR2Cu (MeCN) (R ) Me,
Et, iPr; LR2 ) â-diketiminate derived from respective 2-(2,6-di-R-
phenyl)amino-4-(2,6-dimethylphenyl)imino-2-pentene),10 [LMe2Cu]2,10
LiPr2CuO2,7a 1,3-dimesitylimidazol-2-ylidene (NHCMes2),11 N,N′-
peroxo and oxo bridges have been isolated from reactions
of Fe(II)-Cu(I) precursors.6 In addition, heterobimetallic bis-
(µ-oxo)CuNi or -CuPd species have been generated by
reacting isolable 1:1 Cu/O2 species7 or (PPh3)2PdO2 with a
mononuclear complex of a reduced metal (e.g., Ni(I) or Cu-
(I), respectively).8 While this “stepwise” approach toward
the synthesis of heterobimetallic oxygen intermediates is
attractive, we were intrigued by the reported9 discoveries that
the two metal ions in (PR3)2M-Ge[N(SiMe3)2]2 (M ) Pd,
Pt) acted together to activate O2 to afford mixed-metal peroxo
and bis(µ-oxo)MGe complexes despite the capability of the
M-Ge precursors to cleave to reactive monomeric fragments
(e.g., the trappable germylene Ge[N(SiMe3)2]2). Reasoning
by analogy, we hypothesized that related Cu(I)-Ge(II)
compounds might be prepared and that they might react
similarly with O2 to yield novel Cu-Ge oxygen intermedi-
ates.
Herein we report the synthesis and structural characteriza-
tion of Cu(I)-Ge(II) complexes that contain bonds between
these metal centers. Studies of the reactivity of these
complexes with a variety of reagents show that the nature
of the germylene fragment (i.e., the supporting ligands on
Ge) is critical for determining whether monomeric fragments
are produced or the heterobimetallic nature of the Cu-Ge
compound is retained. Notably, we report the characterization
of a novel bis(µ-oxo)Cu(III)Ge(IV) species derived from
oxygenation of a Cu(I)-Ge(II) precursor, thus demonstrating
the feasibility of a new type of direct route to heterobimetallic
oxygen intermediates comprising copper.
13
dimesitylethanediimine,12 and Ge[N(SiMe3)2]2 were synthesized
according to literature procedures.
1,3-Dimesityl-1,3,2-diazagermol-2-ylidene (Ge[(NMes)2(CH)2]).
Solid yellow N,N′-dimesitylethanediimine (496 mg, 1.7 mmol) was
placed in a 50 mL Schlenk flask with lithium metal (30 mg, 4.3
mmol) under argon, and THF (15 mL) was added via cannula
transfer. The mixture immediately turned deep red and was stirred
overnight. The resulting red-brown solution was filtered through
Celite, and to the filtrate was added GeCl2‚dioxane (393 mg, 1.7
mmol) in 5 mL of THF. The solution was stirred for 2 h, after
which the solvent was removed under reduced pressure to give an
orange-brown solid. The solid was extracted with pentane (15 mL)
and filtered through Celite to give a light orange solution. Removal
of the pentane gave Ge[(NMes)2(CH)2] as an orange powder.
Recrystallization from pentane (4 mL) at -20 °C yielded analyti-
1
cally pure orange crystals (425 mg, 69%). H NMR (C6D6): 6.87
(s, 4H), 6.58 (s, 2H), 2.23 (s, 12H), 2.19 (S, 6H) ppm. 13C{1H}
NMR (THF-d8): 142.0, 134.7, 133.3, 128.4, 125.0, 19.8, 17.2 ppm.
Anal. Calcd for C20H24N2Ge: C, 65.80; H, 6.63; N, 7.67. Found:
C, 65.53; H, 6.48; N, 7.61.
Experimental Section
LMe2Cu-Ge[(NMes)2(CH)2] (1a). To a solution of [LMe2Cu]2
(54 mg, 0.073 mmol) in THF (4 mL) was added a solution of Ge-
[(NMes)2(CH)2] (53 mg, 0.15 mmol) in THF (4 mL). An immediate
color change from very pale yellow to bright lemon yellow was
observed, and the reaction was stirred for 30 min. The solution
was filtered through Celite, and removal of the solvent from the
filtrate under reduced pressure resulted in the isolation of 1a as a
bright yellow powder. Recrystallization from Et2O (10 mL) at -20
°C yielded analytically pure yellow crystals (83 mg, 78%). X-ray-
quality crystals were grown from a concentrated pentane solution
All solvents and reagents were obtained from commercial sources
and used as received unless noted otherwise. The solvents tetrahy-
drofuran (THF), toluene, pentane, diethyl ether (Et2O), and aceto-
nitrile (MeCN) were degassed and passed through solvent purifi-
cation columns (Glass Contour, Laguna, CA or MBraun) prior to
use. The NMR solvents C6D6 and THF-d8 were dried over CaH2
or Na/benzophenone and distilled under a nitrogen atmosphere. All
metal complexes were prepared and stored in a Vacuum Atmo-
spheres inert atmosphere glovebox under a dry nitrogen atmosphere
or were manipulated under argon using standard Schlenk line
techniques. NMR spectra were recorded on a Varian VXR-300 or
1
at -20 °C. H NMR (C6D6): 6.95 (m, 6H) 6.82 (s, 4H), 6.21 (s,
2H), 4.88 (s, 1H), 2.22 (s, 6H), 1.99 (s, 12H), 1.91 (s, 12H), 1.61
(s, 6H) ppm. 13C{1H} NMR (THF-d8): 161.5, 152.4, 139.9, 135.2,
133.7, 129.1, 128.4, 127.4, 123.5, 122.0, 93.4, 21.1, 19.9, 17.6,
16.9 ppm. Anal. Calcd for C41H49N4CuGe: C, 67.09; H, 6.73; N,
7.63. Found: C, 66.66; H, 6.76; N, 7.57.
LMe2Cu-Ge[N(SiMe3)2]2 (1b). Ge[N(SiMe3)2]2 (71 mg, 0.18
mmol) in THF (4 mL) was added to a solution of [LMe2Cu]2 (67
mg, 0.09 mmol) in THF (4 mL), which immediately turned deep
orange. The reaction was stirred for 30 min, filtered through Celite,
and the solvent removed from the filtrate under reduced pressure
to give 1b as an orange solid in approximately quantitative yield
1
VI-300 spectrometer. Chemical shifts (δ) for H (300 MHz) and
13C (75 MHz) NMR spectra were referenced to residual nuclei in
the deuterated solvent. Chemical shifts (δ) for 31P{1H} (121 MHz)
NMR spectra were referenced to an external standard (85% H3-
PO4). UV-vis spectra were recorded on a HP8453 (190-1100 nm)
diode-array spectrophotometer equipped with a Unisoku low-
temperature cryostat. Resonance Raman spectra were recorded on
an Acton 506 spectrometer using a Princeton Instruments LN/CCD-
(7) (a) Aboelella, N. W.; Kryatov, S. V.; Gherman, B. F.; Brennessel, W.
W.; Young, Victor G., Jr.; Sarangi, R.; Rybak-Akimova, E. V.;
Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Cramer, C. J.; Tolman,
W. B. J. Am. Chem. Soc. 2004, 126, 16896. (b) Reynolds, A. M.;
Gherman, B. F.; Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2005,
44, 6989.
(10) 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.
(8) Aboelella, N. W.; York, J. T.; Reynolds, A. M.; Fujita, K.; Kinsinger,
C. R.; Cramer, C. J.; Riordan, C. G.; Tolman, W. B. Chem. Commun.
2004, 1716.
(9) (a) Litz, K. E.; Banaszak Holl, M. M.; Kampf, J. W.; Carpenter, G.
B. Inorg. Chem. 1998, 37, 6461. (b) Cygan, Z. T.; Bender, J. E.; Litz,
K. E.; Banaszak Holl, M. M. Organometallics 2002, 21, 5373.
(11) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530.
(12) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000, 19,
4944.
(13) Gynane, M. J. S.; Harris, D. H.; Lappert, M. F.; Power, P. P.; Riviere,
P.; Riviere-Baudet, M. J. Chem. Soc., Dalton Trans. 1977, 2004.
4192 Inorganic Chemistry, Vol. 45, No. 10, 2006

Heterobimetallic ActiWation of Dioxygen
(135 mg). X-ray-quality crystals were grown from a concentrated
spectroscopy revealed quantitative formation of 4 and the free
germylene Ge[N(SiMe3)2]2.
1
MeCN/Et2O solution at -20 °C. H NMR (C6D6): 6.98 (m, 6H),
4.95 (s, 1H), 2.33 (s, 12H), 1.66 (s, 6H), 0.14 (s, 36H) ppm. 13C-
{1H} NMR (THF-d8): 162.7, 152.3, 130.0, 127.9, 122.5, 94.0, 21.8,
19.0, 4.5 ppm. Anal. Calcd for C33H61N4Si4CuGe: C, 51.99; H,
8.06; N, 7.35. Found: C, 49.50; H, 7.51; N, 6.68. Repeated attempts
to obtain satisfactory elemental analysis were unsuccessful due to
Oxygenation of 1b to Form LMe2Cu(µ-O)2Ge[N(SiMe3)2]2. A
solution of 1b (20 mg, 0.026 mmol) in toluene (1.2 mL) was cooled
to -80 °C in a MeOH/liquid N2 bath. Dry O2 was bubbled through
the cold solution for 10 min, during which time very little change
was observed. The solution was stirred at -80 °C for 4 h, during
which time the color became deep orange-brown. The UV-vis
spectrum was measured by transfer of a known volume of the
oxygenated solution using a precooled gastight syringe to a UV-
vis cuvette filled with a known volume of toluene at -80 °C (λmax
≈ 440 nm, ꢀ ≈ 4400 M-1 cm-1, as approximated from the initial
concentration of 1b). To prepare the 18O-isotopomer, the solution
of 1b was frozen by immersion in liquid N2, the headspace was
evacuated, and 18O2 gas was transferred from a glass bulb. The
solution was allowed to warm to -80 °C, and it was stirred for 4
h, during which time the solution turned deep orange-brown.
Resonance Raman spectra were obtained by micropipet transfer of
the solution to a copper cup on a coldfinger and frozen with liquid
N2.
Reaction of LiPr2CuO2 with Ge[N(SiMe3)2]2 to Form LiPr2Cu-
(µ-O)2Ge[N(SiMe3)2]2. Dry O2 was passed through solution of
LiPr2Cu(MeCN) (10 mg, 0.019 mmol) in toluene (1.0 mL) at -80
°C for 10 min to form LiPr2CuO2. The solution was freeze-pump-
thaw degassed and purged with argon to remove excess O2. One
equivalent of Ge[N(SiMe3)2]2 (7.4 mg, 0.019 mmol) was added in
0.2 mL of toluene by syringe, upon which time the solution
immediately turned deep orange-brown. The 18O-isotopomer was
prepared similarly, except the solution of LiPr2Cu(MeCN) (10 mg,
0.019 mmol) in toluene (1.0 mL) was frozen in a 10 mL Schlenk
flask by immersion in liquid N2, the headspace was evacuated, 18O2
gas was transferred from a glass bulb, and the solution was allowed
to thaw to -80 °C and stirred for 30 min. The solution was freeze-
pump-thaw degassed and purged with argon to remove excess O2.
One equivalent of Ge[N(SiMe3)2]2 (7.4 mg, 0.019 mmol) was added
in 0.2 mL of toluene by syringe, resulting in an immediate color
change to deep orange-brown. Resonance Raman spectra were
obtained by transfer of the solutions to a copper cup on a coldfinger
and frozen with liquid N2. The UV-vis spectrum was measured
by addition of 1 equiv of Ge[N(SiMe3)2]2 in toluene by syringe to
a toluene solution of degassed LiPr2CuO2 in a UV-vis cuvette at
-80 °C (λmax ≈ 463 nm, ꢀ ≈ 4100 M-1 cm-1, as approximated
from the initial concentration of LiPr2Cu(MeCN)).
1
small amounts of impurities present, as evidenced by H NMR
spectroscopy (see text).
Benzil Trapping Experiments. 1b (5 mg, 0.0066 mmol) was
mixed with 4 equiv of benzil (5.5 mg, 0.026 mmol) in C6D6 (0.5
mL), and the solution was transferred to a screwcap NMR tube.
1H NMR spectra were monitored over the course of 6 h, and the
conversion to trapped germylene was followed by the growth of a
resonance at 0.36 ppm for the Me3Si hydrogens of the germanium-
(IV) species 2. The extent of the conversion to 2 was determined
by comparing the integration of the peak at 0.14 ppm for 1b and
that at 0.36 ppm for 2.
LMe2Cu(PPh3) (3). Independent Synthesis. A solution of PPh3
(36 mg, 0.14 mmol) in Et2O (4 mL) was added to a suspension of
[LMe2Cu]2 (50 mg, 0.068 mmol) in Et2O (5 mL). The solution was
stirred for 30 min, filtered through Celite, and the solvent was
removed from the filtrate under reduced pressure to yield 3 as a
tan powder (82 mg, 95%). X-ray-quality crystals were grown from
a concentrated pentane solution at -20 °C. 1H NMR (C6D6): 6.88
(m, 21H), 5.05 (s, 1H), 2.12 (s, 12H), 1.73 (S, 6H) ppm. 13C{1H}
NMR (THF-d8): 161.7, 152.3, 133.5, 133.0, 132.9, 132.7, 129.9,
128.9, 128.1, 127.9, 127.7, 122.0, 93.5, 21.6, 17.8 ppm. 31P{1H}
NMR (C6D6, 121.5 MHz): 5.45 ppm. Anal. Calcd for C39H40N2-
PCu: C, 74.20; H, 6.39; N, 4.44. Found: C, 74.06; H, 6.57; N,
4.36.
From 1a. To a solution of 1a (10 mg, 0.014 mmol) in 0.5 mL
C6D6 was added 1 equiv of PPh3 (3.6 mg). The solution immediately
1
changed from a bright to a pale yellow. H NMR spectroscopy
revealed quantitative formation of 3 and the free germylene Ge-
[(NMes)2(CH)2].
From 1b. To a solution of 1b (10 mg, 0.013 mmol) in 0.5 mL
C6D6 was added 1 equiv of PPh3 (3.4 mg). The solution immediately
1
changed from deep orange to pale yellow. H NMR spectroscopy
revealed quantitative formation of 3 and free germylene Ge-
[N(SiMe3)2]2.
LMe2Cu(NHCMes2) (4). Independent Synthesis. A solution of
NHCMes2 (50 mg, 0.16 mmol) in THF (4 mL) was added to a
solution of [LMe2Cu]2 (60 mg, 0.08 mmol) in THF (5 mL). The
solution was stirred for 30 min, filtered through Celite, and the
solvent was removed from the filtrate under reduced pressure to
yield 4 as a tan powder (100 mg, 91%). X-ray-quality crystals were
Oxygenation of 1a to Form LMe2CuO2. A 0.1 mM solution of
LMe2Cu-Ge[(NMes)2(CH)2] (1a) in THF was cooled to -80 °C in
a UV-vis cuvette, and dry O2 was bubbled for ∼10 s. The addition
of O2 was followed by the immediate disappearance of the 402
nm band in the UV-vis spectrum and the growth of a new shoulder
at 390 nm (ꢀ ≈ 2400 M-1 cm-1, approximated on the basis of the
initial concentration of 1a) over ∼10 min. Warming the solution
to -65 °C resulted in the growth of a new feature at 422 nm.
Likewise, addition of O2 to more concentrated solutions of 1a in
THF at -80 °C (5-20 mM) resulted in the rapid formation of a
dark yellow-brown intermediate with a UV-vis feature at 422 nm.
Resonance Raman experiments on the concentrated solutions (λex
) 457.9 nm) revealed a strong peak at 608 cm-1, consistent with
the formation of [(LMe2Cu)2(µ-O)2] and the observed UV-vis
spectrum.
1
grown from a concentrated pentane solution at -20 °C. H NMR
(C6D6): 7.01 (m, 6H), 6.77 (s, 4H), 5.78 (s, 2H), 4.81 (s, 1H), 2.18
(S, 6H), 1.96 (s, 12H), 1.63 (s, 12H), 1.53 (s, 6H) ppm.13C{1H}
NMR (THF-d8): 185.3, 160.0, 153.7, 137.7, 136.9, 135.6, 130.3,
128.5, 127.2, 121.1, 120.9, 92.3, 21.4, 20.1, 18.7, 17.5 ppm. Anal.
Calcd for C42H49N4Cu: C, 74.91; H, 7.33; N, 8.32. Found: C,
75.17; H, 6.95; N, 8.27.
From 1a. To a solution of 1a (10 mg, 0.014 mmol) in 0.5 mL
C6D6 was added 1 equiv of NHCMes2 (4.3 mg). The solution
immediately changed from bright to pale yellow. 1H NMR
spectroscopy revealed quantitative formation of 4 and the free
germylene Ge[(NMes)2(CH)2].
Addition of Ge[N(SiMe3)2]2 to LMe2CuO2 to Form LMe2Cu-
(µ-O)2Ge[N(SiMe3)2]2. A 0.1 mM solution of 1a in THF was cooled
to -80 °C in a UV-vis cuvette, and dry O2 was bubbled for ∼10
s. The addition of O2 was followed by the immediate disappearance
of the 402 nm band in the UV-vis spectrum and the growth of a
From 1b. To a solution of 1b (10 mg, 0.013 mmol) in 0.5 mL
C6D6 was added 1 equiv of NHCMes2 (4.0 mg). The solution
1
immediately changed from deep orange to pale yellow. H NMR
Inorganic Chemistry, Vol. 45, No. 10, 2006 4193

York et al.
Scheme 1
Figure 1. UV-vis spectra of LMe2Cu-Ge[(NMes)2(CH)2] (1a, solid line)
and LMe2Cu-Ge[N(SiMe3)2]2 (1b, dashed line) in THF at 25 °C.
synthesized by the reductive dimetalation of the R-diimine
N,N′-dimesitylethanediimine12 with Li metal followed by
metathesis with GeCl2‚dioxane, a procedure similar to that
used previously to synthesize related heterocyclic stan-
nylenes18 and germylenes.19 Studies of the reactivity and
stability of related heterocyclic germylenes19,20 suggested that
Ge[(NMes)2(CH)2] would exhibit different electronic proper-
ties than Ge[N(SiMe3)2]2, which we hypothesized might be
manifested in reactivity differences for the targeted Cu(I)-
Ge(II) complexes.
new shoulder at 390 nm over ∼10 min. The solution was purged
with argon for 30 min to remove excess O2, and 1 equiv of Ge-
[N(SiMe3)2]2 in 0.1 mL THF was added by syringe. The rapid
formation of an absorption feature was observed (λmax ≈ 440 nm,
ꢀ ≈ 4000 M-1 cm-1, approximated on the basis of the initial
concentration of 1a), consistent with that observed previously for
LMe2Cu(µ-O)2Ge[N(SiMe3)2]2 obtained upon oxygenation of LMe2Cu-
Ge[N(SiMe3)2]2.
X-ray Crystallography. Data were collected on either a Bruker
or Siemens SMART Platform CCD diffractometer at 173(2) K. Data
collections were carried out using Mo KR radiation (graphite
monochoromator) at a distance of 4.9 cm. The intensity data were
integrated using SAINT14 and were corrected for absorption and
decay using SADABS.15 The structures were solved by direct
methods using SHELXL-9716 software. All non-hydrogen atoms
were refined with anisotropic displacement parameters, and all
hydrogen atoms were placed in ideal positions and refined as riding
atoms with relative isotropic displacement parameters. X-ray
crystallographic data and tables (Table S1) with pertinent details
for each structure are located in the Supporting Information.
Addition of 1 equiv (per copper) of Ge[N(SiMe3)2]2 or
Ge[(NMes)2(CH)2] to a pale yellow THF solution of [LMe2Cu]2
resulted in the instantaneous formation of deep orange or
bright lemon-yellow solutions, respectively. In the case of
the reaction with Ge[(NMes)2(CH)2], the product was isolated
analytically pure upon filtration and crystallization from Et2O
and identified on the basis of UV-vis and 1H NMR
spectroscopy and X-ray crystallography as LMe2Cu-Ge-
[(NMes)2(CH)2] (1a, Scheme 1). For the reaction with Ge-
[N(SiMe3)2]2, the exceedingly high solubility of the product
(1b) in common organic solvents hindered the isolation of
1
an analytically pure sample, and H NMR spectra in both
C6D6 and THF-d8 revealed a small amount of free Cu(I)
complex in all batches synthesized (∼2-3% relative to 1b).
Nevertheless, we were able to obtain X-ray-quality crystals
of 1b from a concentrated acetonitrile/Et2O solution at -20
°C that enabled determination of its structure.
The UV-vis spectra of 1a and 1b in THF exhibit an
intense band at 342 or 345 nm (ꢀ ≈ 27000 M-1 cm-1) and
a weaker shoulder at 402 or 409 nm (ꢀ ≈ 7700 or 4700 M-1
cm-1), respectively (Figure 1). On the basis of analogy to
spectra previously reported for LR2Cu(NCR′) complexes,7a,10
the former intense features may be assigned as â-diketimi-
nate-based π f π* transitions and the latter shoulders as
Cu f Ge[N(SiMe3)2]2/Ge[(NMes)2(CH)2] charge transfer
Results and Discussion
Synthesis and Characterization of Cu(I)-Ge(II) Com-
plexes. We chose as starting material a â-diketiminate
complex of Cu(I) previously described by Warren and co-
workers,17 [LMe2Cu]2 (Scheme 1), which we prepared in a
slightly different fashion by vacuum-drying of LMe2Cu-
(MeCN)10 and recrystallization from pentane. An X-ray
crystallographic analysis (Figure S1) revealed a dimeric
structure similar to that which was reported, although
variations in the crystal system and slight differences in some
bond distances were observed. Two germylenes were selected
for reaction with the Cu(I) reagent, the known compound
(18) Gans-Eichler, T.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2002,
41, 1888.
(19) (a) Herrmann, W. A.; Denk, M.; Behm, J.; Scherer, W.; Klingan, F.
R.; Bock, H.; Solouki, B.; Wagner, M. Angew. Chem., Int. Ed. Engl.
1992, 31 1485. (b) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am.
Chem. Soc. 2001, 123, 11162. (c) Kuhl, O.; Lonnecke, P.; Heinicke,
J. Polyhedron 2001, 20, 2215.
(20) (a) Boehme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039. (b)
Lehmann, J. F.; Urquhart, S. G.; Ennis, L. E.; Hitchcock, A. P.; Hatano,
K.; Gupta, S.; Denk, M. K. Organometallics 1999, 18, 1862. (c) Leites,
L. A.; Bukalov, S. S.; Zabula, A. V.; Garbuzova, I. A.; Moser, D. F.;
West, R. J. Am. Chem. Soc. 2004, 126, 4114.
13
Ge[N(SiMe3)2]2 and a previously unreported heterocyclic
germylene Ge[(NMes)2(CH)2]. The latter compound was
(14) SAINT V6.45A; Bruker Analytical X-ray Systems: Madison, WI, 2001.
(15) An empirical correction for absorption anisotropy: Blessing, R. Acta
Crystallogr. A 1995, 51, 33.
(16) SHELXTL V6.12; Bruker Analytical X-ray Systems: Madison, WI,
2000.
(17) Amisial, L. D.; Dai, X.; Kinney, R. A.; Krishnaswamy, A.; Warren,
T. H. Inorg. Chem. 2004, 43, 6537.
4194 Inorganic Chemistry, Vol. 45, No. 10, 2006

Heterobimetallic ActiWation of Dioxygen
Scheme 2
In 1a, the N1-Cu1-N2 plane forms an angle of 44° relative
to the N3-Ge1-N4 plane, whereas in 1b, these respective
planes are nearly perpendicular. The latter arrangement would
maximize back-bonding interaction between the filled copper
dxy orbital and the empty germanium py orbital, where the x
axis is defined by the Cu-Ge vector. Due to its involvement
in the heterocyclic aromatic system, this p orbital may be
less available for this interaction in 1a than in 1b,22 helping
to rationalize the observed conformational preferences.
Similar back-bonding has been invoked in related Cu(I)-
carbene23 and other metal-germylene complexes,24 but given
the relatively poor back-bonding capabilities of Cu(I) and
the steric differences between the supporting ligands, other
influences such as crystal packing could also be important.
Reactivity with Benzil, PPh3, and a N-Heterocyclic
Carbene. To probe the propensity of the Cu(I)-Ge(II) bond
in 1a and 1b to dissociate we explored reactions of these
compounds with reagents that are known to trap germylenes
(benzil) or Cu(I) (PPh3 and N-heterocyclic carbenes). The
results are summarized in Scheme 2.
Figure 2. Molecular structures of (a) 1a and (b) 1b with heteroatoms
labeled, all atoms as 50% thermal ellipsoids, and hydrogen atoms omitted
for clarity. Selected bond distances (Å) and angles (deg): (a) Cu1-N1,
1.9194(17); Cu1-N2, 1.9153(17); Cu1-Ge1, 2.2138(4); N1-Cu1-N2/
N3-Ge1-N4, 44.03(7). (b) Cu1-N1, 1.956(3); Cu1-N2, 1.938(3); Cu1-
Ge1, 2.2492(4); N1-Cu1-N2/N3A-Ge1-N4A, 88.15(21). For 1b, only
one conformation of the disordered N(SiMe3)2 groups is shown.
(MLCT) transitions. Both complexes exhibit sharp 1H NMR
spectra that are shifted relative to the starting materials. Thus,
1
for example, the H NMR spectrum of 1a in C6D6 shows
the â-diketiminate backbone hydrogen peak at 4.88 ppm,
downfield from the value of 4.75 ppm for the Cu(I) starting
material, and the olefinic hydrogens of the germylene ligand
at 6.21 ppm shifted upfield relative to their position in free
Ge[(NMes)2(CH)2] (6.58 ppm). Resonances for the ger-
mylene olefin and the 2,6-methyl groups of the mesitylene
ring for 1a are somewhat broadened, suggestive of a fluxional
process in solution that we have yet to define.
The X-ray structures of 1a and 1b (Figure 2) confirm their
formulations and are consistent with the presence of direct
Cu(I)-Ge(II) bonds, with Cu-Ge distances of 2.2138(4) and
2.2492(4) Å, respectively. In the structure of 1b, the
N(SiMe3)2 groups were found to be disordered over two
positions (Supporting Information). Only a few other com-
plexes with Cu-Ge bonds are known,21 and these feature
Ge(IV) centers with longer Cu-Ge distances in the range
2.33-2.38 Å. While the Cu(I)-Ge(II) distances in 1a and
1b are similar to each other, the relative orientation of the
N-M-N planes in the two complexes differ significantly.
Benzil is known to be reduced rapidly by Ge[N(SiMe3)2]2
to yield the Ge(IV)-enediolate (Ph2C2O2)Ge[N(SiMe3)2]2, so
it has been used as a trapping agent to assess the degree of
germylene dissociation from (PR3)2M-Ge[N(SiMe3)2]2 com-
plexes, where M ) Ni, Pd, and Pt.9b,25 We envisioned similar
use of benzil to probe germylene dissociation from 1a and
1b. However, addition of excess benzil to a C6D6 solution
of Ge[(NMes)2(CH)2] resulted only in slow decom-
(21) (a) Orlov, N. A.; Bochkarev, L. N.; Nikitinsky, A. V.; Zhiitsov, S. F.;
Zakharov, L. N.; Fukin, G. K.; Ya. Khorshev, S. J. Organomet. Chem.
1997, 547, 65. (b) Orlov, N. A.; Bochkarev, L. N.; Nikitinsky, A. V.;
Kropotova, V. Y.; Zakharov, N. L.; Fukin, G. K.; Khorshev, S. Y. J.
Organomet. Chem. 1998, 560, 21. (c) Glocking, F.; Hooton, K. A. J.
Chem. Soc. 1962, 2658.
(22) Boehme, C.; Frenking, G. Organometallics 1998, 17, 5801.
(23) (a) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085. (b)
Badiei, Y. M.; Warren, T. H. J. Organomet. Chem. 2005, 690, 5989.
(24) Kuhl, O.; Lonnecke, P.; Heinicke, J. Inorg. Chem. 2003, 42, 2836.
(25) Litz, K. E.; Bender, J. E.; Sweeder, R. D.; Banaszak Holl, M. M.;
Kampf, J. W. Organometallics 2000, 19, 1186.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4195

York et al.
position and no isolable Ge(IV) product. Thus, only the
reaction of benzil with 1b was explored. Addition of 4 equiv
1
of benzil to a C6D6 solution of 1b and monitoring by H
NMR spectroscopy revealed the slow formation of (Ph2C2O2)-
Ge[N(SiMe3)2]2 and LMe2Cu(C6D6)17 to the extent of ∼90%
conversion after 5 h (t1/2 ≈ 70 min). In comparison, the
complexes (PR3)2M-Ge[N(SiMe3)2]2 were reported to gen-
erate the same Ge(IV) product within 1 min for M ) Ni,
and to reach ∼90% conversion after 4 and 94 h for M ) Pd
and Pt, respectively.9b Thus, by this crude measure, the Cu-
(I)-Ge(II) bond in 1b is less prone to dissociation than the
Ni(I)-Ge(II) compound, is similarly labile compared to the
Pd(II)-Ge(II) bond, and is more reactive than the Pt(II)-
Ge(II) case.
In contrast to the slow benzil trapping of free germylene
observed for 1b, complexes 1a and 1b both react instanta-
neously at room temperature and at -100 °C with PPh3 and
the N-heterocyclic carbene NHCMes2 11
with concurrent
,
bleaching of the solution color and disappearance of the
∼400 nm absorption feature in the UV-vis spectra. 1H NMR
spectra revealed the quantitative formation of the respective
free germylene and the Cu(I) adducts 3 and 4, each of which
was synthesized independently from [LMe2Cu]2 and PPh3 or
NHCMes2, respectively, and characterized structurally by
X-ray crystallography (Figure S2). Overall, the structures
resemble those of related molecules reported previously.23,26,27
Notably, in 4, the N3-C-N4 plane of the carbene is nearly
perpendicular to the N1-Cu-N2 plane, most likely due to
steric interactions as N-heterocyclic carbenes are known to
be weak π-acceptor ligands for Cu(I).22
Figure 3. (a) UV-vis spectrum of LMe2Cu-Ge[N(SiMe3)2]2 (1b) + O2
in toluene at -80 °C. (b) Resonance Raman spectra (λex ) 488 nm) of
frozen toluene solutions (77K) of LMeCu(µ-O)2Ge[N(SiMe3)2]2 (solid line
for 16O2, dashed line for 18O2, * denotes solvent peaks).
Scheme 3
The findings that benzil traps Ge[N(SiMe3)2]2 from 1b only
slowly and the reactions of 1a and 1b with PPh3 and NHCMes2
proceed essentially instantaneously suggest that the latter
reactions proceed through associative pathways. This hy-
pothesis is consistent with the previously described associa-
tive pathway for the oxygenation of LiPr2Cu(NCR) to yield
LiPr2CuO2,7 as well as for the displacement of O2 from this
product by phosphines.27 Although detailed kinetic studies
would be required to more definitively show that associative
mechanisms are operative, the data nonetheless suggest that
1a and b might react with O2 in a similar manner such that
substrate and germylene are bound simultaneously to the
copper, with the nature of the bound germylene possibly
playing a role in the intermediates observed.
Reactivity with Dioxygen. Both 1a and 1b dissolved in
THF or toluene reacted with O2 at low temperature (-80
°C), but the course of the reactions differ significantly.
Considering first 1b, oxygenation in toluene resulted in
development of a deep orange-brown color accompanied by
a feature at λmax ) 440 nm (ꢀ ≈ 4400 M-1 cm-1) in the
UV-vis spectrum which degraded upon warming of the
sample. Resonance Raman experiments using 488 nm laser
excitation28 revealed a single strong peak at 578 cm-1 that
shifted to two peaks when the sample was prepared with
18O2 (Figure 3). The magnitude of the isotopic shift to the
average of the two 18O2 peaks is ∆avg ) 26 cm-1, consistent
with attribution of the 578 cm-1 feature to a bis(µ-oxo)-
dimetal core vibration that is split into a Fermi doublet upon
O-isotope substitution.8,29 The λmax and ν(M2O2) values (440
nm, 578 cm-1) differ significantly from those of [(LMe2Cu)2-
(µ-O)2] (422 nm, 608 cm-1),10 thus arguing against dissocia-
tion of 1b into germylene and LMe2Cu(I) fragments that
individually oxygenate. Instead, the data indicate formation
(26) (a) Tulloch, A. A. D.; Danopoulos, A. A.; Kleinhenz, S.; Light, M.
E.; Hursthouse, M. B.; Eastham, G. Organometallics 2001, 20, 2027.
(b) Arnold, P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem.
Commun. 2001, 2340. (c) Hu, X.; Castro-Rodriguez, I.; Olsen, K.;
Meyer, K. Organometallics 2004, 23, 755.
(27) Reynolds, A. M.; Lewis, E. L.; Aboelella, N. W.; Tolman, W. B. Chem.
Commun. 2005, 2014.
(28) Rapid photobleaching of the sample was observed when 457.9 nm
laser excitation was used.
(29) (a) Holland, P. L.; Cramer, C. J.; Wilkinson, E. C.; Mahapatra, S.;
Rodgers, K. R.; Itoh, S.; Taki, M.; Fukuzumi, S.; L. Que, J.; Tolman,
W. B. J. Am. Chem. Soc. 2000, 122, 792. (b) Henson, M. J.;
Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon, E. I. J. Am.
Chem. Soc. 1999, 121, 10332.
4196 Inorganic Chemistry, Vol. 45, No. 10, 2006

Heterobimetallic ActiWation of Dioxygen
Scheme 4
Figure 4. UV-vis spectra of LMe2Cu-Ge[(NMes)2(CH)2] (1a) in THF
(0.1 mM) at -80 °C (solid line), the immediate product of oxygenation
(intermediate A, Scheme 4, dotted line), the species resulting after allowing
intermediate A to stand for 10 min (intermediate B, dashed line), and the
product resulting from warming of intermediate B to -65 °C (dot-dash
line).
of the heterobimetallic bis(µ-oxo)copper(III)germanium(IV)
complex LMe2Cu(µ-O)2Ge[N(SiMe3)2]2 (Scheme 3). Similar
results were reported for the oxygenations of (PR3)2M-Ge-
[N(SiMe3)2]2 (M ) Pd, Pt) that yield [MII(µ-O)2GeIV]2+
cores,9 but the generation of a heterobimetallic bis(µ-oxo)
complex via oxygenation of a reduced heterobimetallic
precursor comprising Cu(I) is unprecedented.
A similar [CuIII(µ-O)2GeIV]3+ core is also accessible via a
stepwise approach (Scheme 3). Thus, addition of 1 equiv of
Ge[N(SiMe3)2]2 to a deoxygenated solution of LiPr2CuO2 in
toluene at -80 °C resulted in the immediate development
tent with it being LMe2CuO2, however, was our finding that
addition of 1 equiv of Ge[N(SiMe3)2]2 to a deoxygenated
solution yielded the spectral features for LMe2Cu(µ-O)2Ge-
[N(SiMe3)2]2. Identification of intermediate A is more
difficult, as it is metastable and lacks diagnostic spectroscopic
features. Thus, we can only speculate that it is some sort of
O2 adduct of 1a that decays to LMe2CuO2 and a germanium
species, such as the germylene Ge[(NMes)2(CH)2], which
unlike Ge[N(SiMe3)2]2,30 appears not to react cleanly with
available O2 under ambient conditions to yield a [GeIV2(µ-
O)2]4+ core.
In summary, on the basis of the available evidence, we
propose that the sequence of reactions shown in Scheme 4
occurs upon oxygenation of 1a. In contrast to 1b, which
yields a stable heterobimetallic [CuIII(µ-O)2GeIV]3+ complex,
1a cleaves to yield LMe2CuO2 and, presumably, Ge[(NMes)2-
(CH)2]. The relatively unhindered (compared to LiPr2CuO2)7
LMe2CuO2 decays to [(LMe2Cu)2(µ-O)2] via a process facili-
tated by warming or when high concentrations of 1a are used.
Differences in the nature of the germylene fragment are
clearly responsible for the divergent O2 reactivity of 1a and
1b, with the greater intrinsic stability of the heterocyclic Ge-
[(NMes)2(CH)2] possibly underlying its ejection rather than
formation of a heterobimetallic [CuIII(µ-O)2GeIV]3+ core.
of a deep orange-brown color, a UV-vis band at λmax
)
463 nm (ꢀ ≈ 4100), and a peak at 578 cm-1 in the resonance
Raman spectrum (λex ) 488 nm) that shifted to a doublet
when LiPr2Cu18O2 was used (∆avg ) 27 cm-1) (Figure S3).
These data clearly support formation of LiPr2Cu(µ-O)2Ge-
[N(SiMe3)2]2 via a reaction akin to previously reported
syntheses involving such a stepwise method8 and provide
further confirmation of the similar nature of the product from
the direct oxygenation of 1b.
In contrast to the O2 reaction with 1b, oxygenation of 1a
in THF (0.1 mM) at -80 °C resulted in instantaneous
disappearance of the 402 nm Cu f Ge[(NMes)2(CH)2]
MLCT feature in the UV-vis spectrum (intermediate A,
Figure 4), followed by the gradual (∼10 min) development
of a new absorption feature at 390 nm (ꢀ ≈ 2400) and a
shift of the π f π* bands from 345 to 326 nm (intermediate
B). These new features remain unperturbed at -80 °C, even
upon purging with argon, but convert further upon warming
to -65 °C or when the reaction is performed at higher
concentrations (∼5-15 mM) at temperatures less than -90
°C to the spectrum previously reported for [(LMe2Cu)2(µ-
O)2] (Figure S4). The formation of this species was con-
firmed by resonance Raman spectroscopy (ν(Cu2O2) ) 608
cm-1). On the basis of analogy to the UV-vis spectrum for
LiPr2CuO2, we assign intermediate B as the previously
unobserved, relatively sterically unhindered variant LMe2CuO2.
Unfortunately, the rapidity of the decay of intermediate B
to [(LMe2Cu)2(µ-O)2] at the higher concentrations required
to obtain resonance Raman spectra prevented direct confir-
mation of its structural assignment via this method. Consis-
Conclusions
A pair of novel Cu(I)-Ge(II) complexes, 1a and 1b, that
differ with respect to the nature of the germylene fragment
(30) Ellis, D.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton. Trans.
1992, 3397.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4197

York et al.
have been synthesized and characterized by spectroscopic
and X-ray crystallographic methods. Studies of their reactiv-
ity with benzil, PPh3, and a N-heterocyclic carbene (NHC)
showed that (a) the lability of the Cu(I)-Ge(II) bond in 1b
with respect to trapping of the germylene with benzil is
similar to that of a previously reported Pd(II)-Ge(II)
analogue and (b) both complexes are cleaved rapidly by PPh3
and an NHC to yield stable Cu(I) adducts and the free
germylene, presumably by associative processes. In addition,
the complexes are highly reactive with O2 and exhibit unique
chemistry which depends on the bound germylene. In one
case (1a), oxygenation results in the generation of a new
1:1 Cu/O2 adduct supported by a relatively sterically
unhindered â-diketiminate ligand, whereas in another (1b),
a heterobimetallic intermediate having a [CuIII(µ-O)2GeIV]3+
core forms. The isolation of the latter species by direct
oxygenation of a Cu(I)-Ge(II) precursor represents a new
route to heterobimetallic oxidants comprising copper, the
reactivity of which will be of great interest in future work
aimed at understanding O2 activation and catalysis by mixed-
metal systems.
Acknowledgment. We thank the NIH (GM47365) and
University of Minnesota for support of this research, as well
as L. Que, Jr. for access to Raman spectroscopy facilities.
Supporting Information Available: X-ray crystallographic
details (PDF, CIF); depictions of the X-ray structures of [LMe2Cu]2,
LMe2Cu(PPh3), and LMe2Cu(NHCMes2) (PDF); and additional spec-
troscopic data (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
IC060050Q
4198 Inorganic Chemistry, Vol. 45, No. 10, 2006