Derivatives of octaethynylphenazine and hexaethynylquinoxaline

Article abstract of DOI:10.1002/anie.200502067

Pocketing metals with alkynes: The spectral responses of the peralkynylated heteroacenes hexaethynylquinoxaline and octaethynylphenazine (see structure; N blue, C red, H yellow) on the presence of metal cations are more-pronounced for the phenazine deriva

Full text of DOI:10.1002/anie.200502067

Angewandte
Chemie
Heteroacenes
of an isolable product, probably through desilylation and
concomitant polymerization (Scheme 1). Desulfurization of
the sterically more shielded derivatives 1a and 1c with
LiAlH4 furnished spectroscopically pure 2a and 2c after
aqueous workup. Both products are stable under ambient
conditions for a day or so, after which they decompose
(observed as darkening), presumably by oxidation of the
amine groups. To secure the structure of 2a, we attempted to
crystallize it from dichloromethane; microcrystalline powders
DOI: 10.1002/anie.200502067
Derivatives of Octaethynylphenazine and
Hexaethynylquinoxaline**
Shaobin Miao, Carlito G. Bangcuyo, Mark D. Smith,
and Uwe H. F. Bunz*
In 1986, hexaethynylbenzene, the first per-
alkynylated p perimeter, was obtained by
Vollhardt and co-workers.[1] Since then,
peralkynylated cyclobutadiene complexes,
cymantrenes, ferrocenes, cylcopentadienyl
radicals, thiophenes, and extended systems
derived from tetraethynylethylenes and
hexaethynylbenzenes
have
been
reported.[2–7] Peralkynylated N-heterocycles
are scarce, and only recently peralkynylpyr-
azinophorphyrazine and triethynyltriazine
were reported.[8,9] Dialkynylated penta-
cenes, hexacenes, and heptacenes play a
significant role in materials science and are
fairly easily accessible.[10] Peralkynylated
acenes as a class, however, are entirely
unknown; the peri interactions in acenes
would result in steric crowding of the
alkynes and their all-too-close spacing
would lead to Bergman-type reactions. If
the peri interactions could be removed in
naphthalene, anthracene, or the larger
acenes, peralkynylation should be possible:
Hexaethynylquinoxaline and octaethynyl-
phenazine are attractive synthetic targets
that might show promise as n-type semi-
conductors and as unusual sensory platforms
for metal cations.[11] Herein, we report the
Scheme 1. Synthesis of hexaethynylquinoxaline (4), octaethynylphenazine (7), and the quinoxaline 8.
TMS=trimethylsilyl; TIPS=triisopropylsilyl.
synthesis of the first representatives of the heteroacenes,
namely, hexaethynylquinoxaline and octaethynylphenazine.
Tetraethynylbenzothiadiazoles 1a–c are easily available
by Pd-catalyzed alkynylation of tetrabromobenzothiadia-
zole.[12] The attempted reduction of 1b with LiAlH4 led to
decomposition of the starting material without the formation
formed. Upon crystallization from hexafluorobenzene, a
suitable single-crystalline specimen was obtained.[13]
Figure 1 shows a ball-and-stick representation of 2a. A
weak F···H hydrogen bond (2.36 Š) is apparent between an
amine group of 2a and a fluorine substituent of the
hexafluorobenzene. The electron-poor hexafluorobenzene
and the electron-rich diamine 2a lie on top of each other, as
would be expected from the electrostatic potentials of these
[*] Dr. S. Miao, Dr. C. G. Bangcuyo, Prof. Dr. U. H. F. Bunz
School of Chemistry and Biochemistry
Georgia Institute of Technology
770 State Street, Atlanta, GA 30332 (USA)
Fax: (+1)404-385-1795
E-mail: uwe.bunz@chemistry.gatech.edu
Dr. M. D. Smith
Department of Chemistry and Biochemistry
University of South Carolina
Columbia, SC 29208 (USA)
Figure 1. Stick representation of the complex 2a·C6F6. a) NH···F hydro-
gen bond indicated by arrow (2.36(3) Š, 1528). b) Stacking of the
molecules, viewed along the crystallographic a axis. c) The distance
between C6F6 and 2a in the stacks, as indicated by the arrow, is 3.29 Š
(view shown along the crystallographic b axis).[13]
[**] We thank the Petroleum Research Funds for support and Prof.
Christoph Fahrni (School of Chemistry and Biochemistry, Georgia
Institute of Technology) for the analysis of the binding constants of
4, 7, and 8 with metal ions.
Angew. Chem. Int. Ed. 2006, 45, 661 –665
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661

Communications
molecules.[14] A tightly packed,
p–p-stacked structure results;
compound 2a is the first reported
tetraalkynylated phenylenedia-
mine and, as such, is valuable
for the construction of other
structures.
Upon treatment of 2a with
dione 3 in ethanol with molecular
sieves, the hexaethynylquinoxa-
line derivative 4 formed in 55%
yield.[11] If 2a was treated with
the commercially available qui-
none 5, the tetrabromide 6 was
isolated (Scheme 1). Pd-cata-
lyzed coupling of 6 to an excess
of 3,3-dimethylbutyne gave the
Figure 3. Packing of 7 in the solid state. a, b) View along the b and a axes, respectively. The stacking
distance is 0.97 nm. c) View along the approximate diagonal of a and c axes. The herringbone pattern
is visible in (b) and (c).
octaethynylphenazine representative 7 as a stable, orange,
crystalline solid. While spectral data indicated the presence of
a peralkynylated phenazine, the attractive topology of 7 was
confirmed by single-crystal X-ray structural analysis
(Figure 2).[15] The bond lengths and bond angles for 7 are in
excellent agreement with the expected values.[1,5,7]
Upon going from 4 to 7, significant red shifts in both the
absorption and emission spectra were recorded (Figure 4)
which result from the extension of the p system. While the
Figure 4. Absorption (left) and emission spectra (right) of 4
(lmax =434 nm [2.85 eV], lem =485 nm) and 7 (lmax =527 nm [2.35 eV],
lem =554 nm) in chloroform.
emission spectra of 4 and 7 are unstructured, their absorption
spectra feature a similar fine structure as those of anthracene
or phenazine; compounds 4 and 7 are obviously “true”
acenes, despite their alkyne decoration.[16] When comparing
the UV/Vis spectrum of 7 (lmax = 527 nm) to that of phenazine
(lmax = 363 nm), a red shift of 162 nm in the absorption is
noted. To estimate the band gap and the position of the
frontier orbitals of 7, an ab initio quantum chemical calcu-
lation (B3LYP, 6-31G**) was performed (tert-butyl groups
omitted; HOMO: À5.80 eV; LUMO: À3.19 eV; gap:
2.61 eV). To compare the band gap and position of the
frontier orbitals, we also performed quantum chemical
calculations (B3LYP, 6-31G**) on anthracene (HOMO:
À5.24 eV; LUMO: À1.65 eV; gap: 3.59 eV) and phenazine
(HOMO: À6.09 eV; LUMO: À2.43 eV; gap: 3.66 eV). The
introduction of the nitrogen atoms into the frame of the
acenes lowers the energy of the HOMO and LUMO, but does
not significantly decrease the band gap. Alkyne decoration,
however, leads to a decrease in the band gap. The calculations
Figure 2. a) ORTEP drawing of 7 at the 50% probability level.
b) Space-filling view of 7. The phenazine nucleus can carry eight alkyne
units, as the pyrazine ring creates a void that accommodates the
substituents and forms an internal pocket.
Figure 3 shows the packing of 7 in the solid state. The
molecules are arranged in tilted stacks that are 0.97-nm apart
along the b axis in a classic herringbone arrangement. Close
p–p contacts are not observed due to the bulky tert-butyl
groups that encase the aromatic faces of 7.
662
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Angew. Chem. Int. Ed. 2006, 45, 661 –665

Angewandte
Chemie
agree well with the absorption data and reflect the gross
electronic differences between phenazine and 7.
The crystal structure of 7 reveals a rigid binding pocket
generated by the alkyne groups and the phenazine nitrogen
atom. The three p-basic sites should allow the complexation
of metal cations. Phenazine itself forms coordination poly-
mers with copper and silver.[17] Exposure of 4 and of 7 to silver
triflate led to a bathochromic shift in their absorption spectra
and to quenching of their emission. The shift is larger in 7 than
in 4. Upon exposure to CuI or CuII, no chromic changes
occurred but slight quenching was observed for both per-
alkynyls with CuII triflate. Surprised by the lack of binding to
copper ions, we treated 4 and 7 with a series of metal salts (see
Figure 5 and Table 1). While alkali and alkaline-earth metals
generally do not lead to spectral shifts, Ba2+ did cause a shift
in the absorption spectrum of 7. Ag+, As3+, and Hg2+ ions, as
well as trifluoroacetic acid generated chromic responses in 4
and 7, while indium and tin triflates induced a bathochromic
shift in 7. According to the shape of the spectra of 7, In3+,
Ba2+, and Sn2+ ions can be grouped together as they elicit the
same response from 7, while Ag+, As3+, and Hg2+ ions and
proton acids show spectral responses that are different from
each other and from those of the first group (Table 1). The
presence of the second alkynylated fused benzene unit in 7 as
compared to 4 seems to significantly increase the binding of
the pyrazine motif to metal cations. Changes in emission are
much less pronounced, suggesting that 4 and 7 bind less
strongly to metal cations in their excited states.
To understand the relationship between molecular struc-
ture and metal binding, we performed a titration of 4, 7, and 8
with AgOTf and Hg(OTf)2. However, attempts to obtain the
binding constants for association of 4, 7, and 8 to Hg(OTf)2
were inconclusive; multiple coordination equilibria are pres-
ent, and various combinations of proposed simple metal-to-
ligand stoichiometries did not lead to a reasonable fit of the
experimental data to any assumed model using the program
SPECFIT.[18]
Figure 5. Absorption spectra of a) 4 and b)À7 in the presence of
representative metal cations. OTf=CF3SO3
.
From the results of titrations with
AgOTf, compounds 4 and 7 first form 1:1
Table 1: Absorption (lmax) and emission (lem) data for the interaction of 4 and 7 with metal ions.[a]
and then 2:1 metal–ligand complexes, with
logb1 = 4.5 (Æ 0.2) and logb2 = 8.8 (Æ 0.1)
for 4, and logb1 = 6.5 (Æ 0.3) and logb2 =
12.8 (Æ 0.2) for 7. The presence of the two
alkynylated phenyl arms aligned parallel in
structure 7 increases the binding to silver by
a factor of 100, as compared to the quinoxa-
line 4. A similar effect is apparent for the
binding of the second silver ion, which is, in
both cases, only slightly less strongly coor-
dinated than the first one, suggesting that
the additional positive charge does not
significantly affect the binding properties
of the second pocket. We have investigated
the binding of AgOTf to 8 and found that
nonlinear least-squares fitting with the pro-
gram SPECFIT[18] leads to a different com-
plexation behavior. The spectral data are
best interpreted by a 1:2 and a 1:4 ligand-to-
metal complex stoichiometry with logb2 =
4
7
lmax [nm]
lem [nm]
lmax [nm]
lem [nm]
no metal
331, 409, 432
482
340, 464, 523 sh
553
CF3CO2H
AgOTf
AsCl3
376, 509
340, 420, 446
quenched
514 vw
quenched
no change
376, 510, 548
351, 467, 500
314, 466, 550 vw
quenched
quenched
quenched
553[b]
331, 407, 446 vw
no change
Ba(OTf)2
340, 377, 464, 548
Ca(OTf)2, CdCl2,
Cu(OTf), CuI,
Eu(OTf)3, KOTf,
LiOTf, Mg(OTf)2,
Na(OTf), Pb(NO3)2,
Tl(OTf), Zn(OTf)2
no change
no change[c]
no change
no change[c]
Hg(OTf)2
In(OTf)3
Sn(OTf)2
278, 418, 552
no change
no change
quenched
no change
no change
348, 449, 543 br
340, 376, 464, 548
340, 376, 464, 548
567 w
546 w
quenched
[a] sh=shoulder; w=weak; vw=very weak; br=broad. [b] Decreased intensity. [c] Slight quenching
with Cu(OTf)2.
Angew. Chem. Int. Ed. 2006, 45, 661 –665
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663

Communications
8: A mixture of 3,6-bis(3,3-dimethylbut-1-ynyl)benzene-1,2-di-
amine (1.70 g, 6.33 mmol), molecular sieves (1.0 g),and 1,2-diphenyl-
ethane-1,2-dione (2.00 g, 9.51 mmol) in dry toluene (50 mL) was
heated at reflux for 18 h. Workup and column chromatography (silica
gel, 1:1 hexane/CH2Cl2) afforded 8 (1.77 g, 63%); m.p.: 232–2348C;
IR (KBr): n = 3055, 2966, 2922, 2895, 2864, 2216, 1560, 1466, 1337,
1242, 1097, 841, 700 cmÀ1; 1H NMR (400 MHz, CDCl3): d = 7.75–7.72
(m, 6H), 7.38–7.31 (m, 6H), 1.45 ppm (s, 18H); 13C{1H} NMR
(100 MHz, CDCl3): d = 152.27, 140.93, 138.94, 132.40, 130.21,
129.04, 128.03, 123.33, 106.84, 75.98, 31.02, 28.52 ppm; HR-MS
(70 eV): m/z calcd for C32H30N2 [M+]: 442.24090; found: 442.24153.
13.0 (Æ 0.3) and logb4 = 22.1 (Æ 0.3). Not unexpectedly, the
presence of the phenyl groups increases the interaction with
the silver cations and leads to this somewhat exotic complex-
ation behavior.
The underlying structural principle of alkyne-framed N-
heterocycles will be exploited in the future to construct a
more general class of rigid receptors 7 by coupling differ-
entiated functional alkynes to 6. Those starting alkynes would
carry auxiliary binding sites and/or modulate the electronic
properties of the system to investigate the interaction of these
ligands with metal cations in water and in organic solvents.[18]
In conclusion, we have described a facile synthetic
approach that yields peralkynylated quinoxalines and phena-
zines. Both 4 and 7, which are the first reported peralkyny-
lated heteroacenes, show attractive and quite selective metal-
binding properties that will be harnessed in the future.
Derivatives of this class of fluorescent molecules might find
use as organic n-type semiconductors.
Received: June 15, 2005
Revised: October 19, 2005
Published online: December 19, 2005
Keywords: alkynes · cations · fused-ring systems ·
.
nitrogen heterocycles
[1]a) R. Diercks, J. C. Armstrong, R. Boese, K. P. C. Vollhardt,
Angew. Chem. 1986, 98, 270 – 271; Angew. Chem. Int. Ed. Engl.
1986, 25, 268 – 269; b) R. Boese, J. R. Green, J. Mittendorf, D. L.
Mohler, K. P. C. Vollhardt, Angew. Chem. 1992, 104, 1643 – 1645;
Angew. Chem. Int. Ed. Engl. 1992, 31, 1643 – 1645; c) T. X.
Neenan, G. M. Whitesides, J. Org. Chem. 1988, 53, 2489; d) Y.
Tobe, K. Kubota, K. Naemura, J. Org. Chem. 1997, 62, 3430 –
3431; e) J. D. Tovar, N. Jux, T. Jarrosson, S. I. Khan, Y. Rubin, J.
Org. Chem. 1997, 62, 3432 – 3433.
Experimental Section
2: Dry THF (150 mL) was added to a flame-dried 250-mL Schlenk
flask charged with 1, then LiAlH4 (4 equiv) was added to the mixture
under a stream of nitrogen over a period of 30 min, and stirring was
continued for 4 h. Analytically pure 2 was obtained after aqueous
workup.
2a: Prepared from 1a (1.00 g, 2.18 mmol). Red solid (0.760 g,
1
81%); m.p.: 1678C (decomp); H NMR (400 MHz, CDCl3): d = 3.34
[2]H. W. Marx, F. Moulines, T. Wagner, D. Astruc, Angew. Chem.
1996, 108, 1842 – 1845; Angew. Chem. Int. Ed. Engl. 1996, 35,
1701 – 1704.
(bs, 4H), 1.42 (s, 18H), 1.40 ppm (s, 18H); 13C{1H} NMR (100 MHz,
CDCl3): d = 136.89, 119.38, 112.01, 108.50, 102.97, 77.37, 75.46, 31.25,
31.02, 28.76, 28.52 ppm.
[3]a) H. Hopf, M. Kreutzer, P. G. Jones, Chem. Ber. 1991, 124,
1471 – 1475; b) Y. Rubin, C. B. Knobler, F. Diederich, Angew.
Chem. 1991, 103, 708 – 710; Angew. Chem. Int. Ed. Engl. 1991,
30, 698 – 700; c) F. Diederich, Y. Rubin, Angew. Chem. 1992, 104,
1123 – 1145; Angew. Chem. Int. Ed. Engl. 1992, 31, 1101 – 1123.
[4]a) J. Anthony, C. B. Knobler, F. Diederich, Angew. Chem. 1993,
105, 437 – 440; Angew. Chem. Int. Ed. Engl. 1993, 32, 406 – 409;
b) G. J. Palmer, S. R. Parkin, J. E. Anthony, Angew. Chem. 2001,
113, 2577 – 2580; Angew. Chem. Int. Ed. 2001, 40, 2509 – 2512.
[5]a) N. Jux, K. Holczer, Y. Rubin, Angew. Chem. 1996, 108, 2031 –
2034; Angew. Chem. Int. Ed. Engl. 1996, 35, 2509 – 2512;
b) U. H. F. Bunz, V. Enkelmann, Organometallics 1994, 13,
3823 – 3833; c) W. Steffen, M. Laskoski, J. G. M. Morton,
U. H. F. Bunz, J. Organomet. Chem. 2004, 689, 4345 – 4356;
d) D. M. Zhang, C. A. Tessier, W. J. Youngs, Chem. Mater. 1999,
11, 3050 – 3057.
[6]a) M. M. Haley, S. C. Brand, J. J. Pak, Angew. Chem. 1997, 109,
863 – 866; Angew. Chem. Int. Ed. Engl. 1997, 36, 836 – 838;
b) W. B. Wan, S. C. Brand, J. J. Pak, M. M. Haley, Chem. Eur. J.
2000, 6, 2044 – 2055; c) M. M. Haley, J. J. Pak, S. C. Brand, Top.
Curr. Chem. 1999, 201, 81 – 130.
[7]a) U. H. F. Bunz, V. Enkelmann, Angew. Chem. 1993, 105, 1712 –
1714; Angew. Chem. Int. Ed. Engl. 1993, 32, 1653 – 1655;
b) U. H. F. Bunz, G. Roidl, M. Altmann, V. Enkelmann, K. D.
Shimizu, J. Am. Chem. Soc. 1999, 121, 10719 – 10726; c) M.
Laskoski, W. Steffen, J. G. M. Morton, M. D. Smith, U. H. F.
Bunz, Angew. Chem. 2002, 114, 2484 – 2488; Angew. Chem. Int.
Ed. 2002, 41, 2378 – 2382; d) U. H. F. Bunz, J. Organomet. Chem.
2003, 683, 269 – 287.
2c: Prepared from 1c (1.00 g, 1.16 mmol). Red solid (0.706 g,
1
73%); m.p.: 1858C (decomp); H NMR (400 MHz, CDCl3): d = 3.22
(bs, 4H), 1.17–1.13 ppm (m, 84H); 13C{1H} NMR (100 MHz, CDCl3):
d = 136.45, 118.97, 112.69, 104.47, 102.25, 101.48, 96.35, 19.14, 12.16,
11.42 ppm.
4: Molecular sieves, 2a (0.204 g, 0.476 mmol),
3 (0.209 g,
0.500 mmol), and toluene (10 mL) were heated at 808C for 4 h.
Removal of the solvent and chromatography of the residue (3:1
hexanes/CH2Cl2) furnished 4 (0.315 g, 55%); m.p.: 194–1968C;
1H NMR (400 MHz, CDCl3): d = 1.43 (s, 18H), 1.37 (s, 18H)
1.18 ppm (s, 42H); 13C{1H} NMR (100 MHz, CDCl3): d = 141.53,
140.38, 130.29, 126.08, 112.48, 108.94, 104.26, 99.52, 78.46, 75.23, 33.58,
29.16, 18.75, 11.89 ppm; IR (KBr): n = 2966, 2945, 2866, 2222, 1539,
1462, 1411, 1313 cmÀ1; MS (70 eV): m/z (%): 811 (100) [M+], 726 (60),
645 (5); HR-MS: m/z calcd for C54H78N2Si2: 810.57036; found:
810.56716.
7: 2a (0.274 g, 0.639 mmol), 5 (0.273 g, 0.645 mmol), ethanol
(10 mL), and a drop of H2SO4 were heated at reflux for 14 h.
Filtration followed by crystallization from CH3OH/CH2Cl2 yielded 6
(0.250 g, 48%); 1H NMR (400 MHz, CDCl3): d = 1.41 (s, 18H),
1.37 ppm (s, 18H); 13C{1H} NMR (100 MHz,CDCl3): d = 143.87,
139.54, 132.67, 131.29, 127.99, 126.09, 113.08, 112.62, 79.46, 75.09,
32.63, 31.15, 28.35, 28.00 ppm. Triethylamine (7 mL), 6 (0.424 g,
0.519 mmol), [(PPh3)2PdCl2](5 mol%), CuI (5 mol%), and 3,3-
dimethylbutyne (5 equiv) were stirred at 1208C for 18 h in a sealed
flask. Aqueous workup and column chromatography (4:1 hexanes/
CH2Cl2) furnished 7 (0.196 g, 46%); m.p.: 2488C (decomp.); 1H NMR
(400 MHz, CDCl3): d = 1.46 (s, 36H), 1.32 ppm (s, 36H);
13C{1H} NMR (100 MHz, CDCl3): d = 141.82, 130.27, 124.71, 111.36,
110.00, 77.46, 75.92, 31.92, 30.84, 28.56, 28.31 ppm; IR (KBr): n =
2966, 2923, 2862, 2214, 1728, 1712, 1695, 1548, 1452, 1440, 1390, 1361,
1261 cmÀ1; MS (70 eV): m/z (%): 820 (100) [M+]; HR-MS: m/z calcd
for C60H72N2: 820.56955; found: 820.56726.
[8]a) R. Faust, C. Weber, J. Org. Chem. 1999, 64, 2571 – 2573; b) F.
Mitzel, S. Fitzgerald, A. Beeby, R. Faust, Chem. Commun. 2001,
2596 – 2597; c) F. Mitzel, S. Fitzgerald, A. Beeby, R. Faust, Chem.
Eur. J. 2003, 9, 1233 – 1241; d) S. Ott, R. Faust, Synlett 2004,
1509 – 1512.
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Angewandte
Chemie
[9]M. Ohkita, M. Kawano, T. Suzuki, T. Tsuji, Chem. Commun.
2002, 3054 – 3055.
[10]a) J. E. Anthony, J. S. Brooks, D. L. Eaton, S. R. Parkin, J. Am.
Chem. Soc. 2001, 123, 9482 – 9483; b) M. M. Payne, S. R. Parkin,
J. E. Anthony, J. Am. Chem. Soc. 2005, 127, 8028 – 8029; c) J. E.
Anthony, D. L. Eaton, S. R. Parkin, Org. Lett. 2002, 4, 15 – 18.
[11]a) R. Faust, C. Weber, V. Fiandanese, G. Marchese, A. Punzi,
Tetrahedron 1997, 53, 14655 – 14670; b) R. Faust, B. Göbelt,
Tetrahedron Lett. 1997, 38, 8017 – 8020; c) R. Faust, C. Weber,
Liebigs Ann. 1996, 1235 – 1238; d) R. Faust, B. Göbelt, C. Weber,
Synlett 1998, 64 – 65.
[12]C. G. Bangcuyo, M. D. Smith, U. H. F. Bunz, Synlett 2004, 169 –
172.
[13]Crystallographic data for 2a: yellow blocks, crystal dimensions
0.44 ” 0.36 ” 0.30 mm3, space group P21/c (no. 14, monoclinic),
a = 6.671(3) Š,
92.721(15)8, V= 2751.0(2) Š3, Z = 4, 1calcd = 1.134 gcmÀ3
(MoKa) = 0.087 mmÀ1
Data were measured on
b = 26.982(9) Š,
c = 20.034(7) Š,
b =
m-
Bruker
,
.
a
SMART APEX diffractometer (MoKa radiation, l = 0.71073 Š)
at 150(1) K, and the structure was solved by direct methods. Of
3309 reflections collected, 2503 were unique reflections (Rint
=
0.1251); data/restraints/parameters 2503/6/422; final R indices
[I > 2s(I)]: R1 = 0.0758, wR2 = 0.1881; R indices (all data): R1 =
0.0891, wR2 = 0.2025; largest diff. peak and hole: 0.329 and
À3
À0.241 eŠ
.
[14]a) J. H. Williams, Acc. Chem. Res. 1993, 26, 593 – 598; b) M.
Weck, A. R. Dunn, K. Matsumoto, G. W. Coates, E. B. Lobkov-
sky, R. H. Grubbs, Angew. Chem. 1999, 111, 2909 – 2912; Angew.
Chem. Int. Ed. 1999, 38, 2741 – 2745.
[15]Crystallographic data for 7: irregular orange-red crystals, crystal
dimensions 0.52 ” 0.40 ” 0.22 mm3, space group P21/c (no. 14,
monoclinic),
a = 19.4214(9) Š,
b = 9.9650(5) Š,
c =
15.4061(7) Š, b = 112.6820(10)8, V= 2751.0(2) Š3, Z = 2,
1calcd = 0.991 gcmÀ3, m(MoKa) = 0.056 mmÀ1. Data were mea-
sured on
a Bruker SMART APEX diffractometer (MoKa
radiation, l = 0.71073 Š) at 293(1) K, and the structure was
solved by direct methods. Three of the four crystallographically
inequivalent tert-butyl groups are rotationally disordered. Of
18417 reflections collected, 3950 were unique (Rint = 0.0338);
data/restraints/parameters 3950/19/370; final
R indices [I >
2s(I)]: R1 = 0.0518, wR2 = 0.1420; R indices (all data): R1 =
0.0636, wR2 = 0.1536; largest diff. peak and hole: 0.195 and
À0.185 eŠÀ3. CCDC-275052 (2a) and -275053 (7) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[16]B. Nickel, A. A. Ruth, J. Phys. Chem. 1991, 95, 2027 – 2036.
[17]a) M. Munakata, S. Kitagawa, N. Ujimaru, M. Nakamura, M.
Maekawa, H. Matsuda, Inorg. Chem. 1993, 32, 826 – 832; b) S. R.
Batten, K. S. Murray, Coord. Chem. Rev. 2003, 246, 103 – 130.
[18]R. A. Binstead, A. D. Zuberbꢀhler, SPECFIT Global Analysis
System, Spectrum Software Associates, Marlborough, MA
01752, 2001.
[19]J. N. Wilson, U. H. F. Bunz, J. Am. Chem. Soc. 2005, 127, 4124 –
4125.
Angew. Chem. Int. Ed. 2006, 45, 661 –665
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
665