A thallium mediated route to σ-arylalkynyl complexes of bipyridyltricarbonylrhenium(I)

Article abstract of DOI:10.1021/ic701406y

A simple, one-pot preparation of rhenium(I) σ-arylalkynyl complexes is reported using thallium(I) hexafluorophosphate as a halogen abstraction agent. This new route to rhenium σ-alkynyls enjoys higher yields compared to analogous preparations using silver salts by eliminating potential electrochemical degradation pathways.

Full text of DOI:10.1021/ic701406y

Inorg. Chem. 2007, 46, 8484−8486
A Thallium Mediated Route to
Bipyridyltricarbonylrhenium(I)
σ-Arylalkynyl Complexes of
Brendan J. Liddle,† Sergey V. Lindeman,† Daniel L. Reger,‡ and James R. Gardinier*,†
Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53211, and Department of
Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208
Received July 14, 2007
Scheme 1. Synthetic Routes to σ-Alkynyl Derivatives of
(bipy)Re(CO)3
A simple, one-pot preparation of rhenium(I)
is reported using thallium(I) hexafluorophosphate as a halogen
abstraction agent. This new route to rhenium -alkynyls enjoys
σ-arylalkynyl complexes
σ
higher yields compared to analogous preparations using silver salts
by eliminating potential electrochemical degradation pathways.
There has been a longstanding interest in complexes of
the “(bipy)Re(CO)3” fragment owing to their favorable
photo- and electrochemical properties, which hold promise
for applications in photocatalysis, solar energy storage, and
molecular electronics.1-3 The attractive solubility and tunable
electronic properties of charge-neutral σ-alkynyl derivatives
have helped to spur recent research into these compounds.4-8
Accordingly, there are multiple reported synthetic routes to
(bipy)Re(CO)3(alkynyl) complexes, as summarized in Scheme
1. The one-pot route (Scheme 1A) is most favorable in terms
of both simplicity and generality.4,5 The seemingly most
straightforward route (Scheme 1B) is surprisingly not
general; it is reported to be specific only to 4,4-(tBu)2bipy
derivatives and fails for other bipyridyl derivatives.6 The
route depicted by Scheme 1C7 is a viable alternative but
* To whom correspondence should be addressed. E-mail:
james.gardinier@marquette.edu.
† Marquette University.
‡ University of South Carolina.
(1) Kalyanasundaram, K. In Photosensitization and Photocatalysis Using
Inorganic and Organometallic Compounds; Kalyanasundaram, K.,
Gra¨tzel, M., Eds.; Kluwer Academic Publishers: Dordrecht, The
Netherlands, 1993.
(2) (a) Meyer, G. J., Ed. Molecular Level Artificial Photosynthetic
Materials. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.;
Wiley: New York, 1996; Vol. 44. (b) For a more recent example,
see: She, C.; Anderson, N. A.; Guo, J.; Liu, F.; Goh, W.-H.; Chen,
D.-T.; Mohler, D. L.; Tian, Z. Q.; Hupp, J. T.; Lian, T. J. Phys. Chem.
B 2005, 109, 19345.
(3) (a) Ward, M. D.; Barigelletti, F. Coord. Chem. ReV. 2001, 216-217,
127. (b) Sun, S.-S.; Lees, A. J. Organometallics 2002, 21, 39.
(4) Yam, V.W.-W. Chem. Commun. 2001, 789.
(5) For example, see: Chong, S. H.-F.; Lam, S. C.-F.; Yam, V. W.-W.;
Zhu, N.; Cheung, K.-K.; Fathallah, S.; Costuas, K.; Halet, J.-F.
Organometallics 2004, 23, 4924.
suffers typical problems associated with multistep syntheses.
For instance, in our hands, variable (low) yields of the
rhenium triflate intermediate were obtained from the first
step (vide infra), and the preparation of the tetraarylborate
intermediate adds complications to the procedure. Finally,
the route in Scheme 1D8 suffers from poor yield, employs
expensive reagents and a multistep synthetic scheme (to get
to the starting materials), and, to our knowledge, has not
yet been shown to be general.
During the course of our investigations into preparing
rhenium alkynyls as possible components of single-molecule
white-light emitters,9 we initially found the overall yield of
the desired products by several of these routes to be
capricious, depending on the nature of the bipyridyl, the
(6) Yam, V. W.-W.; Lau, V. C.-Y.; Cheung, K. K. Organometallics 1995,
14, 2749.
(7) Hevia, E.; Pe´rez, J.; Riera, V.; Miguel, D.; Kassel, S.; Rheingold, A.
Inorg. Chem. 2002, 41, 4673.
(8) Ferrer, M.; Rodr´ıguez, L.; Rossell, O.; Lima, J. C.; Go´mez-Sal, P.;
Mart´ın, A. Organometallics 2004, 23, 5096.
8484 Inorganic Chemistry, Vol. 46, No. 21, 2007
10.1021/ic701406y CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/15/2007

COMMUNICATION
Table 1. Summary of the Results Obtained from One-Pot Syntheses of Tricarbonyldipyridylrhenium(I) Arylalkynes Using Tl(PF6) or Ag(PF6)
overall yield,a %
TlPF6 (THF,
2 days)
Ag(OTf) (THF, TlPF6, EtNiPr2,
AgPF6 EtNiPr2,
neat 15 h
entry
rhenium reagent
(bipy)Re(CO)3Br
(4,4′-Me2bipy)Re(CO)3Br HCtC(C6H5)
(5,5′-Me2bipy)Re(CO)3Br HCtC(C6H5)
(5,5′-Br2bipy)Re(CO)3Br
(6,6′-Me2bipy)Re(CO)3Br HCtC(C6H5)
arylalkyne
HCtC(C6H5)
2 days)
neat 15 h
1
2
3
4
5
6
7
60 (54, 1 eq)b
79
29
65
69
58 (41, 2 h)
81
59
47
17
56
81
59
33
22
5
HCtC(C6H5)
72 (61, 1 eq)b
20 (2 eq),b 13 (1 eq)b
(bipy)Re(CO)3Br
HCtC[3,5-(pz3CCH2OCH2)2C6H3]d 35
23 (14)c
10
(4,4′-Me2bipy)Re(CO)3Br 6-HCtC(HpzAnMe
)
30
d
a Isolated yields (average of two runs). b eq ) molar ratio of alkyne/Re. c Used Scheme 1C; see text. d HpzAnMe ) 2-pyrazolyl-4-methylaniline.
terminal alkyne, and on whether silver salts were employed
at any point in the synthetic route. In our hands, reactions
between (R2-bipyridylRe)(CO)3(Br) (R ) H, alkyl, halide)
and silver salts invariably gave metal as a byproduct eVen
in the absence of light, implicating an electrochemical
decomposition pathway, although the origin of the decom-
position remains unclear. We communicate a significant
improvement in the preparation of rhenium σ-arylalkynyls,
in terms of the overall yield, that is realized simply by
replacing the electrochemically active silver halide abstrac-
tion reagent with an equivalent, but electrochemically inert,
thallium(I) salt.
partly be attributed to the differing but generally poor
solubilities of the reactants and products in the refluxing
amine.
Regardless of the dehalogenation reagent or solvent,
optimal yields of rhenium alkynyl were always obtained
when an excess (1.5-2 mol equiv) of alkyne was used, in
part because partial decomposition of the alkyne occurs. The
blue luminescent organic decomposition byproducts are
easily removed with the solvent front by column chroma-
tography; the nature of these decomposition products was
not further explored owing to the success of the reaction
depicted in eq 1. The reaction route in eq 1 also tolerates a
variety of functional groups decorating the arylalkyne as
noted by entries 6 and 7 in Table 1. The lower yields obtained
in the latter two cases are likely due to competitive binding
of Lewis donors (pyrazolyls or aniline groups) to the rhenium
cation intermediate generated in situ.11 It should be noted
that for Table 1, entry 6, the low overall yields of rhenium
alkynyl obtained using the AgOTf/Na(BAr′4) route7 (Scheme
1C) could be attributed to two main factors: (i) the poor
conversion of (bipy)Re(CO)3Br to its triflate via AgOTf
(averaging 36%), where metal is observed as a side product,
and (ii) a competing pyrazolyl deprotonation reaction12 that
occurs with organyllithium reactants during the final step of
the reaction sequence (only 45% conversion of the rhenium
cation to rhenium alkyne was realized). Finally, as with
related rhenium bipyridyl chemistry,13 there appears to be a
weak correlation between the overall yield and the electronic
nature of the bipyridyl, where more electron-donating
substituents (in the order Me > H g Br) gave higher yields
of the desired product.
The one-pot synthetic route to the bipyridyltricarbonyl-
rhenium(I) arylalkynyls, given in eq 1, is similar to that
reported by Yam et al. (Scheme 1A)4,5 but with the notable
modification that Tl(PF6) replaces Ag(OTf). Table 1 provides
+xs H(CtC-aryl) +TI(PF6)
NEt3/THF ∆ 2 days
(R2bipy)Re(CO)3(Br)
(R ) H, Me, Br)
8 (R2bipy)Re(CO)3(CtC-aryl)
-TIBr
-[HNEt3][PF6]
(1)
a comparison of the results obtained from the preparation
of a series of different derivatives using either Tl(PF6),
Ag(OTf), or Ag(PF6) (the different silver salts performed
identically) as halide abstraction agents under otherwise
identical conditions. Irrespective of all other considerations,10
the use of Tl+ in place of Ag+ clearly afforded higher yields
of the desired product (significantly in some cases) and
reactions could be performed in air and under exposure to
light; no metallic byproducts are observed, as with silver. It
is noteworthy that reactions performed according to eq 1
using tetrahydrofuran (THF) as a solvent typically required
about 30 h to reach maximum conversion [thin-layer chro-
matography (TLC) and subsequent workup], while similar
reactions performed in CH3CN gave comparable yields but
only after 6 days. Because the starting (R2bipy)Re(CO)3(Br)
is consumed at comparable rates in either solvent (TLC),
[(R2bipy)Re(CO)3(solvent)]+ appears to be involved in the
rate-limiting step of the reaction. The possibility of using
the tertiary amine [NEt3 and EtNiPr2 performed equivalently]
both as the solvent and as the alkynyl deprotonating reagent
was also explored but with variable success, which could
Each of the rhenium alkynyls has been characterized by
NMR, IR, absorption and emission spectroscopy, electro-
chemistry (see the Supporting Information), and, in some
cases, X-ray crystallography. The molecular structures of
four arylalkynyl derivatives, (5,5′- and 6,6′-Me2bipy)Re-
(CO)3[CtC(C6H5)], (4,4′-Me2bipy)Re(CO)3[CtC(HpzAnMe)]
(11) An ionic species with characteristic bright-yellow luminescence
remains adhered to the silica gel. Elution with THF affords (R2bipy)-
-
Re(CO)3(PO2F2), with a coordinating PO2F2 anion produced as a
result of PF6- hydrolysis on silica gel. See the Supporting Information
for an example structure and properties. Complete details will be
reported elsewhere.
(9) For a similar strategy, see: Si, Z.; Li, J.; Li, B.; Zhao, F.; Liu, S.; Li,
W. Inorg. Chem. 2007, 46, 6155.
(10) Of course, proper safety precautions should be employed when using
or disposing of toxic thallium compounds.
(12) Katritzky, A. R.; Abdel-Rahman, A. E.; Leahy, D. E.; Schwarz, O.
A. Tetrahedron 1983, 39, 4133.
(13) Hino, J. K.; Ciana, L. D.; Dressick, W. J.; Sullivan, B. P. Inorg. Chem.
1992, 31, 1072.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8485

COMMUNICATION
Figure 2. Cyclic voltammograms obtained for THF solutions of (4,4′-
Me2bipy)Re(CO)3(X) [X ) Br, top; X ) CtC(C6H5), bottom] measured
at a scan rate of 100 mV/s with (NBu4)(PF6) as the supporting electrolyte.
ν
CO 2008 and 1900 cm-1, av 1954 cm-1); 5,5′ isomer (1.10
V; νCO 2005 and 1894 cm-1, av 1949.5 cm-1); 4,4′ isomer
(1.10 V; νCO 2006 and 1888 cm-1, av 1947 cm-1).
Importantly, an examination of the cyclic voltammograms
of the rhenium bipyridyls, shown for two derivatives of (4,4′-
Me2bipy)Re(CO)3X (X ) Br, C2Ph) in Figure 2, reveals the
typical oxidative instability of charge-neutral rhenium bi-
pyridyl derivatives.14 As is illustrated in Figure 2, the
oxidation of ReI to ReII (around 1.5 V for Br and 1.1 V for
C2Ph derivatives) is clearly irreversible, showing only an
anodic wave; the anticipated cathodic wave is absent in each
case. Thus, oxidation of either the rhenium starting material
or the product provides a potential source for electrochemical
degradation (and low yields) in any preparative reaction.
Particularly noteworthy is that heating a THF solution
containing an equimolar ratio of AgOTf and (5,5′- Me2bipy)-
Re(CO)3[CtC(C6H5)] at reflux for 1 day (in the dark and
under nitrogen) produced a silver mirror and substantial
decomposition of the rhenium alkynyl (only 30% was
recovered). On the other hand, the rhenium alkynyl could
be fully recovered after a sample of (5,5′-Me2bipy)Re(CO)3-
[CtC(C6H5)] and TlPF6 were heated in THF. Clearly, the
electroactive nature of silver salts and the electrochemical
instability of the rhenium compounds are implicated in the
lower yields of rhenium alkynyls obtained by the original
silver-based preparation.
Figure 1. Molecular structures of various rhenium alkynyls. Clockwise
from top left: (5,5′-Me2bipy)Re(CO)3[CtC(C6H5)], (6,6′-Me2bipy)Re(CO)3-
[CtC(C6H5)] (the inset emphasizes puckering of the chelate ring), (4,4′-
Me2bipy)Re(CO)3[CtC(HpzAnMe)] (HpzAnMe ) 2-pyrazolyl-4-methyla-
niline), and (bipy)Re(CO)3[CtC(C6H3)-3,5-(CH2OCH2Cpz3)2]. ORTEP
diagrams are drawn with 50% probability ellipsoids. Hydrogen atoms have
been removed for clarity.
(HpzAnMe ) 2-pyrazolyl-4methylaniline), and (bipy)Re-
(CO)3[CtC(C6H3)-3,5-(CH2OCH2Cpz3)2], are given in Fig-
ure 1, while structures of related rhenium compounds can
be found in the Supporting Information. Main features
evident from the structures include derivatives with pyrazolyl,
and other Lewis base groups decorating the arylalkynyl are
indeed bound to rhenium through the alkynyl carbon rather
than through the other Lewis donors. Also, derivatives of
6,6′-Me2bpy contain puckered rhenium chelate rings presum-
ably due to steric interactions between the methyls and
equatorial rhenium carbonyls. In cases with 6,6′-Me2bpy, like
that in the top right of Figure 1, the axial carbonyl is directed
away from the bipyridyl while the other axial substituent
lies above the π cloud of the heterocycle. This puckered
geometry results in a slightly more electron-deficient metal
center when compared to the other isomers of (dimethylbi-
pyridyl)rhenium(I), as is best indicated by cyclic voltammetry
(rhenium-based oxidations) and to a lesser extent by IR
spectroscopy (where C-O stretching frequencies vary by
only a few wavenumbers). Thus, for (Me2bipy)Re(CO)3(Br),
the 6,6′ isomer is more difficult to oxidize (1.64 V vs
Ag/AgCl) and has higher CO stretching frequencies (νCO
2021, 1906 cm-1, av 1963.5 cm-1) than either the 5,5′ isomer
(1.57 V; νCO 2020 and 1903 cm-1, av 1961.5 cm-1) or the
4,4 isomer (1.50 V; νCO 2016 and 1907 cm-1, av 1961.5
cm-1). A similar situation holds for the isomers of
(Me2bipy)Re(CO)3[CtC(C6H5)], but here the differences
between isomers are less pronounced: 6,6′ isomer (1.12 V;
Acknowledgment. J.R.G. thanks Marquette University,
the Petroleum Research Fund (74371-G), and the NSF
(Instrumentation Grant CHE-0521323) for support. Burjor
Captain and Radu F. Semeniuc are acknowledged for their
assistance with the structure of (bipy)Re(CO)3[CtC(C6H3)-
3,5-(CH2OCH2Cpz3)2].
Supporting Information Available: Full synthetic details,
characterization data, additional structural diagrams, tables of
structural data, and crystallographic data in CIF format. This
material is available free of charge via the Internet http://pub.acs.org.
IC701406Y
(14) (a) Paolucci, F.; Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C.
A.; Amatore, C. J. Phys. Chem. B 1998, 102, 4759. (b) Luong, J. C.;
Nadjo, L.; Wrighton, M. S. J. Am. Chem. Soc. 1978, 100, 5790.
8486 Inorganic Chemistry, Vol. 46, No. 21, 2007