Unexpected reactivities of Cu2(diphosphine)2 complexes in alcohol: Isolation, X-ray crystal structure, and photoluminescent properties of a remarkably stable [Cu3(diphosphine) 3(μ3-H)]2+ hydride complex

Article abstract of DOI:10.1021/ja042335c

Treatment of [Cu₂(dcpm)₂]Y₂ (dcpm = bis(dicyclohexylphosphino)methane, Y = ClO₄^-, BF₄^-, PF₆^-, CF₃SO₃^-) with refluxing MeOH in the presence of KOH afforded hydride complexes [Cu₃(dcpm)₃(μ₃-H)]Y₂ (1) in about 85% yield. Refluxing [Cu₂(dcpm)₂](PF₆)₂ with MeOH in the presence of NH₃·H₂O and air gave a carboxylate complex [Cu₂(dcpm)₂(O₂CCH₂OH)]PF₆ (2) in 40% yield. All of the complexes 1 and 2 have been characterized by X-ray crystallography. The Cu₃ cores in 1 are almost perfectly shielded by the dcpm ligands. Intense photoluminescence was observed for 1 both in the solid state and in solution. Copyright

Full text of DOI:10.1021/ja042335c

Published on Web 03/10/2005
Unexpected Reactivities of Cu2(diphosphine)2 Complexes in Alcohol:
Isolation, X-ray Crystal Structure, and Photoluminescent Properties of a
Remarkably Stable [Cu3(diphosphine)3(µ3-H)]2+ Hydride Complex
Zhong Mao, Jie-Sheng Huang,* Chi-Ming Che,* Nianyong Zhu, Sarana Ka-Yan Leung, and
Zhong-Yuan Zhou†
Department of Chemistry and the HKU-CAS Joint Laboratory on New Materials, The UniVersity of Hong Kong,
Pokfulam Road, Hong Kong
Received December 20, 2004; E-mail: cmche@hku.hk; jshuang@hku.hk
Transition metal complexes containing sterically bulky electron-
rich phosphine ligands usually adopt low coordination numbers1c,f-h
and sometimes display unprecedented properties and reactivities.1a,b,d,e
In the course of our spectroscopic studies on two-coordinate
polynuclear d10 complexes with the bridging bis(dicyclohexylphos-
phino)methane (dcpm) ligand, we observed that treatment of [Cu2-
(dcpm)2]2+ with MeOH in the presence of KOH afforded hydride
complex [Cu3(dcpm)3(µ3-H)]2+ (1) (reaction 1). A similar treatment
of [Cu2(dcpm)2]2+ with MeOH in the presence of NH3‚H2O in air
did not afford [Cu3(dcpm)3(µ3-H)]2+ but gave carboxylate complex
[Cu2(dcpm)2(O2CCH2OH)]+ (2) (reaction 2). Through these reac-
Figure 1. 1H NMR spectra (500 MHz) of 1d in MeCN-d3 at 80 °C.
tions, we isolated analytically pure [Cu3(dcpm)3(µ3-H)]Y2 (Y )
ClO4-, 1a; BF4-, 1b; PF6-, 1c; CF3SO3-, 1d) in ∼85% yield and
[Cu2(dcpm)2(O2CCH2OH)]PF6 (2) in 40% yield. The formulation
of 1 is consistent with the 31P{1H} NMR spectrum of 1c (Figure
S1), showing a P(dcpm):P(PF6-) molar ratio of 3.06:1, close to
the expected ratio of 3:1. Both 1 and 2 are air-stable, diamagnetic
solids.2 Their formation from these reactions is completely unex-
pected.
Complexes 1a-d are, to the best of our knowledge, the first
examples of a tricopper hydride complex,3 exhibiting prominent
MS peaks assignable to [Cu3(dcpm)3(µ3-H)]2+ and [{Cu3(dcpm)3-
(µ3-H)}Y]+ (Y ) ClO4-, BF4-, PF6-, or CF3SO3-) (Figure S2). In
Figure 2. Structure of 1a with omission of hydrogen atoms (except the
the MS of 2, an intense cluster peak assignable to [Cu2(dcpm)2(O2-
hydride) and counteranions.
CCH2OH)]+ was observed.
1
The H NMR spectra (500 MHz) of 1a-d in CD2Cl2 at room
2
septet centering at δ ≈ 2.2 ppm with JPH ) 16 Hz are visible
temperature (RT) show the hydride signal at δ ≈ 2.0 ppm (Figure
(Figure 1). The septet collapsed to a singlet in 31P-decoupled H
NMR spectra, consistent with a coupling of the hydride with six
equivalent P atoms. A comparable hydride signal with δ ) 3.50
1
2
S3a), with typical JPH of 16 Hz. The hydride signal disappears if
the MeOH in reaction 1 is replaced by MeOH-d4. Two-dimensional
NMR measurements (Figure S3b) revealed that there is no
correlation between the hydride and the dcpm protons. However,
owing to significant overlap with intense dcpm signals, the
multiplicity of the hydride signal cannot be determined from the
RT spectra. We envisioned that increasing temperature may allow
a clearer observation of the hydride signal due to signal narrowing
that arises from longer relaxation time and more rapid cyclohexyl
ppm (a multiplet resulting from P-H coupling) was previously
reported for diamagnetic [Cu(P(p-tolyl)3)(µ3-H)]6.3c
The crystal structures of 1a-d (Figure 2 and Figures S4-S7)
each contain a triangular Cu3 core and three µ-dcpm ligands, with
Cu-P distances of 2.276(2)-2.290(2) Å and mean Cu‚‚‚Cu
distances of 2.879(1) (1a,d) and 2.885(1) Å (1b,c). These Cu‚‚‚Cu
distances are significantly shorter than those in the mono- or
rotation or conformational change. Indeed, in MeCN-d3 at 80 °C,
dicapped species [Cu3(dppm)3(µ3-OH)]2+ and [Cu3(dppm)3(µ3-
the hydride signal was almost fully resolved; six components of a
Cl)2]+ (3.120(2)-3.322(2) Å).4 The hydride in 1a was located in
the difference Fourier map. However, the quality of the crystals is
not sufficiently high to allow determination of the accurate hydride
† Department of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong.
9
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J. AM. CHEM. SOC. 2005, 127, 4562-4563
10.1021/ja042335c CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have observed two unprecedented reactions of
a Cu2(diphosphine)2 complex in MeOH containing inorganic bases
by employing dcpm as a diphosphine ligand, which result in the
isolation of a highly unusual copper hydride complex. MeOH
apparently serves as the hydride source. Preliminary studies revealed
that replacing MeOH in reaction 1 with other primary alcohols,
such as EtOH and PhCH2OH, also afforded 1; however, no 1 was
i
t
formed if secondary or tertiary alcohol PrOH and BuOH was
substituted for MeOH. Although the generation of metal hydrides
from reactions with primary alcohol and base has been well
documented,11 it is unclear how 1 was assembled from reaction 1.
The carboxylate ligand in 2 probably arises from copper-catalyzed
aerobic oxidation of MeOH;12 no 2 was formed from reaction 2 in
the absence of air or by changing MeOH to EtOH. Efforts are
underway to investigate the mechanisms of these interesting
reactions in more detail.
Figure 3. Top-view space-filling representations of the structure of (a) 1a
and (b) [Cu3(dppm)3(µ3-Br)2]ClO4 (ref 7a) with omission of counteranions.
For clarity, the hydride in (a) is in red.
position in 1a and location of the hydride in 1b-d. We found that
the conformation of the 12-membered [Cu3(diphosphine)3]3+ mac-
rocycle in the structures of 1a-d is similar to that in [Cu3(dtbpm)3-
(µ3-F)](PF6)2, containing a planar Cu3(µ3-F) moiety (with the µ3-
F- situated in the center of the Cu3 core),5 but is markedly different
from that in [Cu3(dppm)3(µ3-OH)](BF4)2,4b containing a tetrahedral
Cu3(µ3-OH) moiety (Figure S8). This suggests the location of the
hydride of 1a-d in the center of their Cu3 cores, which gave Cu-H
distances of ∼1.67 Å, comparable to those of 1.76(3) Å (average)
in tetrahedral Cu3(µ3-H) moieties of [Cu(P(p-tolyl)3)(µ3-H)]6 de-
termined by neutron diffraction analysis.3e
Structurally characterized trinuclear metal hydrides containing
a planar M3(µ3-H) moiety, like the Cu3(µ3-H) moiety in 1, are not
unprecedented but are extremely rare.6 Moreover, complexes 1a-d
are hitherto the only examples of the hydride complexes of a [Cun-
(diphosphine)n]n+ macrocycle, despite previous reports of numerous
metal complexes containing [Cun(diphosphine)n]n+ macrocycles
(Chart S1).
Acknowledgment. This work was supported by the Large-Item
Equipment Funding of The University of Hong Kong, the Hong
Kong University Foundation, Croucher Foundation, and the Hong
Kong Research Grants Council (HKU 7039/03P).
Supporting Information Available: Experimental details, Figures
S1-S12, Chart S1, and CIF files for all of the X-ray crystal structures
reported in this work. This material is available free of charge via the
Internet at http://pubs.acs.org.
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such as [Cu(PPh3)(µ3-H)]6,3a,b the remarkably high air and thermal
stability of 1 deserves attention, which may be attributed to the
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7a
ClO4 (Figure 3b).
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carboxylate ligand (Cu-O ) 2.072(5)-2.123(5) Å) than [Cu2-
(dppm)2(O2CCH3)]BF4 (P-Cu-P ) 128.87(6)-130.56(6)°, Cu-O
) 1.993(4)-2.029(4) Å).7b
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