Synthesis and reactions of metallo-diethynylbenzenes: Building blocks for redox-active poly(phenyleneethynylene)s

Article abstract of DOI:10.1039/a809231a

The metallo-diethynylbenzene complex W(≡CC₆H₄C≡CH₄(dmpe)₂Cl, a precursor to metal-containing poly(phenyleneethynylene)s, has been prepared using alkyne-metathesis methodology and can be coupled into unsaturated

Full text of DOI:10.1039/a809231a

Synthesis and reactions of metallo-diethynylbenzenes: building blocks for
redox-active poly(phenyleneethynylene)s†
Kevin D. John and Michael D. Hopkins*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA.
E-mail: mdh1@vms.cis.pitt.edu
Received (in Bloomington, IN, USA) 25th November 1998, Accepted 8th February 1999
The metallo-diethynylbenzene complex W(·CC6H4C·CH-
4)(dmpe)2Cl, a precursor to metal-containing poly(phenyl-
eneethynylene)s, has been prepared using alkyne-metathesis
methodology and can be coupled into unsaturated organic
frameworks at both the ethynyl moiety and the tungsten
center and one-electron oxidized to the open-shell cation.
this approach is outlined in Scheme 1. For this compound the
unsymmetrical diynylarene is p-PrnC·CC6H4C·CSiPri3, which
12
undergoes metathesis with W2(OBut)6 exclusively at the
propynyl C·C bond to give W(·CC6H4C·CSiPri3-4)(OBut)3
1.‡ Reaction of
1
with BCl3 and dme,10 provides
W(·CC6H4C·CSiPri3-4)Cl3(dme) 2,‡ which undergoes two-
electron reduction by Na/Hg amalgam in the presence of dmpe
to give W(·CC6H4C·CSiPri3-4)(dmpe)2Cl 3.‡ The target
terminal ethynyl derivative 4 is then obtained by protodesilyla-
The unsaturation and limited conformational flexibility of
phenylacetylenes (and, more generally, arylacetylenes) imparts
to the polymeric materials derived from them extended p-
electron systems and well defined lengths and shapes, re-
spectively. These attributes have made poly(aryleneethyny-
lene)s1 the subjects of intensive research as regards their
potential applications to molecular electronics,2 photonics3 and
chemical sensing.4 Control over the properties of poly-
(aryleneethynylene)s is achieved typically by changing the
polymer terminating groups and the nature and connectivity of
the aryl hubs. Here, we describe a different approach to
functionalizing poly(aryleneethynylene)s: the replacement of
triply bonded carbon atoms in the backbone with triply bonded
metal centers of metal–alkylidyne complexes.5
tion of 3 with [NBun ]F•xH2O.‡
4
Covalent alkylidyne-containing polymers analogous to poly-
(aryleneethynylene)s have not yet been prepared, although
metal–alkylidyne coordination polymers6 and poly(arylene-
ethynylene)s to which metal centers are externally coordinated7
have been reported. Recently, Mayr et al. reported an important
advance toward this objective with their synthesis of Fischer-
type alkylidyne compounds of the form W(·CC6H4I-
4)(CO)2(LL)X (LL = tmeda, dppe; X = Cl, Br, I),8 which
undergo cross-coupling reactions with phenylacetylenes to give
compounds with phenyleneethynylene alkylidyne ligands. Si-
multaneously, we have been seeking to develop routes to related
compounds based on Schrock’s alkyne-metathesis reaction9
because of the ease with which this methodology allows the
synthesis of complexes with unsaturated alkylidyne ligands and
the fact that the W(·CR)(ORA)3 metathesis products allow
access10 to carbonyl-free derivatives of the type W(·CR)L4X.
Scheme 1 Reagents:‡ i, 0.5 W2(OBut)6; ii, 3 BCl3, dme (excess); iii, 2
dmpe, 2 Na/Hg; iv, [NBun4]F•xH2O (excess).
Compound 4 can be incorporated into extended unsaturated
organic frameworks by functionalization at both the metal
center and the terminal ethynyl moiety. A survey of representa-
tive reactions at these loci is presented in Scheme 2. Crucially,
the fingerprint spectroscopic parameters of the ethynyl moiety
(1H NMR d 2.92, nC·C 2099 cm–1) indicate that it is not strongly
electronically perturbed relative to phenylacetylene (d 2.93,
This latter point is important because W(CR)(CO)nL42n
X
compounds typically exhibit electrochemically irreversible
oxidative processes,11 which could limit their use as building
blocks for redox-active polymers. Herein we report the
synthesis, via an alkyne-metathesis route, of W(·CC6H4C·CH-
4)(dmpe)2Cl 4 (dmpe = Me2PCH2CH2PMe), some reactions
that lead to its incorporation into unsaturated organic frame-
works, and its redox chemistry.
Our general strategy for synthesizing compounds of the type
W(·CArC·CH)L4X is to begin with readily prepared un-
symmetrical diynylarenes (RC·CArC·CRA) into which the
organic functionality of importance to the properties of the final
compound is preincorporated, thus minimizing the number of
reaction procedures necessary once the metal center is present.
The terminal C·CH unit of the target compound is masked in
the starting diynylarene by a protecting R group that is
sufficiently sterically demanding to prevent alkyne metathesis
at the adjacent C·C bond. The synthetic procedure for 4 using
Scheme 2 Reagents and conditions: i, Li[N(SiMe3)2] (1 equiv.), THF,
–78 °C (10 min), MeI (excess), 0 °C (2 h); ii, p-IC6H4But (1 equiv.),
PdCl2(PPh3)2/CuI (cat.), NEt2H, 25 °C (5 h); iii, SiMe3(OTf) (1 equiv.),
toluene, 25 °C (1 h); iv, LiC·CC6H4Me-4 (1 equiv.), DME, 0 °C (1 h), 25
°C (1 h); v, [C7H7][PF6] (1 equiv.), CH2Cl2–MeCN (2:1), 0 °C (1 h).
† Dedicated to Professor Warren R. Roper on the occasion of his 60th
birthday.
Chem. Commun., 1999, 589–590
589

Notes and references
‡ Preparative details for 1–4 and selected NMR [CD2Cl2, 25 °C (1H) or –10
°C (13C(W·C), 31P)] and IR (nC·C, cm–1) data are as follows. 1: W2(OBut)6
(0.25 g, 0.31 mmol) and p-PrnC·CC6H4C·CSiPri3 (0.20 g, 0.62 mmol) in
pentane (5 mL) were stirred together for 10 min at 25 °C and allowed to
stand for 12 h at –35 °C. Removal of solvent under vacuum at 25 °C gave
1 as a red–brown oil (0.31 g, 0.46 mmol, 74% yield). 1H, d 7.37 (d, 2H), 7.00
(d, 2H), 1.44 (s, 27H), 1.05 (s, 21H); 13C, d 257; IR 2159. 2: The reaction
between 1 (0.30 g, 0.45 mmol), DME (excess), and BCl3 (1.33 mL, 1 M in
heptane, 1.33 mmol),10 gave 2 as a green powder (0.20 g, 0.31 mmol, 68%
yield). 1H, d 7.66 (d, 2H), 6.76 (d, 2H), 4.42 (s, 3H), 4.26 (m, 2H), 4.07 (m,
2H), 3.90 (s, 3H), 1.12 (s, 21H); IR 2152. 3: To a stirred, 0 °C solution of
2 (1.20 g, 1.85 mmol) in THF (150 mL) was added dmpe (0.67 g, 4.46
mmol) and Na/Hg amalgam (0.4%, 194.91 g, 3.89 mmol Na). After 12 h at
25 °C the organic phase was decanted and reduced to dryness under
vacuum. The remaining solid was extracted with pentane and the extract
filtered, concentrated, and layered with acetonitrile, giving 3 as a green
Fig. 1 Structure of W(·CC6H4C·CSiPri3-4)(dmpe)2(C·CC6H4Me-4) 7.
Atoms are represented by spheres of arbitrary size (C) or thermal ellipsoids
drawn at the 50% probability level (W, P, Si). Hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (°): W–C(1) 1.93(3), W–
C(19) 2.11(3), W–Pav 2.42[1]; C(1)–W–C(19) 179.3(12), C(20)–C(19)–W
174(3), C(1)–W–Pav 95.0(9), C(2)–C(1)–W 175(2).
2109 cm21) by the p-C·W(dmpe)2Cl group, and, thus, that its
characteristic reactivity should be preserved. Consistent with
this hypothesis, the ethynyl group of 4 can be deprotonated with
Li[N(SiMe3)2] and subsequently methylated with MeI to give
W(·CC6H4C·CMe-4)(dmpe)2Cl 5.‡ More importantly, the
ethynyl group participates in Pd-catalyzed cross-coupling
reactions with aryl halides: the reaction between 4 and
1
powder (1.21 g, 1.53 mmol, 83% yield). H, d 7.20 (d, 2H), 6.62 (d, 2H),
1.55 (br, 8H), 1.49 (m, 12H), 1.46 (m, 12H), 1.20 (s, 21H); 13C, d 252;
31P{1H} 23.6; IR 2146. 4: A stirred, –78 °C solution of 3 (0.20 g, 0.25
mmol) in THF (10 mL) was treated with [NBun4]F•xH2O (1.88 mL, 0.2 M
in THF for x = 0, 0.38 mmol), warmed to 25 °C over 1 h, and then reduced
to dryness under vacuum. The remaining solid was extracted with pentane
and the extract filtered, concentrated, and cooled to –35 °C, giving 4 as a
green powder (0.09 g, 0.14 mmol, 56% yield).1H, d 7.23 (d, 2H), 6.64 (d,
2H), 2.92 (s, 1H), 1.52 (br, 8H), 1.49 (m, 12H), 1.45 (m, 12H); 13C, d
252; 31P{1H}, d 23.6; IR 2099. 5: 1H, d 7.18 (d, 2H), 6.63 (d, 2H), 1.83 (s,
3H), 1.73 (br, 8H), 1.64 (m, 12H), 1.45 (m, 12H); 13C, d 252; 31P{1H} 28.5;
IR 2241, 2208 (Fermi resonance). 6: 1H, d 7.39 (d, 2H), 7.34 (d, 2H), 7.05
(d, 2H), 6.66 (d, 2H), 1.76 (br, 8H), 1.69 (m, 12H), 1.50 (m, 12H), 1.30 (s,
9H); 13C, d 253; 31P{1H} d 26.8; IR 2210. 7: 1H, d 6.98 (d, 2H), 6.89 (d, 2H),
6.86 (d, 2H), 6.73 (d, 2H), 2.22 (s, 3H), 1.77 (br, 8H), 1.68 (m, 12H), 1.62
(m, 12H), 1.08 (s, 21H); 13C, d 255; 31P{1H}, d 21.3; IR 2145, 2060. 3[PF6]:
IR 2149.
p-IC6H4But
under
standard
conditions
produces
W[·CC6H4(C·CC6H4But-4)-4](dmpe)2Cl 6‡ in nearly quanti-
tative yield. At the tungsten center, substitution of the chloride
ligand by unsaturated hydrocarbyl ligands can also be readily
achieved. Treatment of
3 with Me3Si(OTf) to yield
W(·CC6H4C·CSiPri3-4)(dmpe)2(OTf) followed by reaction
with LiC·C·C6H4Me-4 provides W(·CC6H4C·CSiPri3-
4)(dmpe)2(C·CC6H4Me-4) 7.‡ The structure of 7 (Fig. 1) is not
of sufficient quality to provide quantitative insights but is
interesting for the fact that it reveals that the phenyl rings are
nearly coplanar, consistent with extended p conjugation in this
compound.
§ Crystallographic data for 7: C37H100ClP4SiW, M = 916.44, monoclinic,
space group P21/n, a = 8.977(9), b = 30.30(3), c = 16.45(2) Å, b =
95.23(9)°, V = 4455(8) Å3, Z = 4, m = 2.847 mm21, T = 213 K, 5218
reflections measured, 4784 independent reflections, R1 [I > 2s(I)] =
0.1279, wR2 [I > 2s(I)] = 0.2811. Crystals of 7 diffracted weakly due to
their small size; only the W, P and Si atoms could be successfully
anistropically refined. CCDC 182/1174. See http://www.rsc.org/suppdata/
cc/1999/589/ for crystallographic files in .cif format.
In addition to the reaction chemistry of 3 and 4 that allows
extension of their unsaturated frameworks, these compounds
can be cleanly oxidized by one electron (E1/2  –0.8 V vs.
0/+
FeCp2 in THF);11b accordingly, the reaction between 3 and
[C7H7][PF6]
provides
orange
[W(·CC6H4C·CSiPri3-
4)(dmpe)2Cl][PF6] (3[PF6], Scheme 2).‡ The molecular struc-
ture of 3 (unpublished results) is very similar to that of closely
related [W(·CPh)(dmpe)2Br]+13 and we presume that it, too,
possesses a (dxy)1 electron configuration.
1 R. Giesa, J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1996, C36,
631.
2 P. S. Weiss, L. A. Bumm, T. D. Dunbar, T. P. Burgin, J. M. Tour and
D. L. Allara, in Molecular Electronics: Science and Technology, ed. A.
Aviram and M. Ratner, New York Academy of Sciences, New York,
1998, p. 145.
3 J. S. Moore, Acc. Chem. Res., 1997, 30, 402; W. Holzer, A. Penzkofer,
S.-H. Gong, A. P. Davey and W. J. Blau, Opt. Quantum. Electron.,
1997, 29, 713; W. Zhao, H. Li, R. West and J. C. Wright, Chem. Phys.
Lett., 1997, 281, 105.
The electronic spectra of 3–7 indicate that their p-electron
systems are extensively delocalized. These spectra exhibit
characteristic bands attributable to the (dxy)2?(dxy)1(dxz,dyz)1
[n?p*(W·CR)] transition13,14 as the lowest-energy features.
This band strongly red shifts as unsaturated moieties are added
to the alkylidyne ligand or to the axial site of the metal.
Specifically, as the para-substituent on the alkylidyne ligand is
changed from H [W(·CPh)(dmpe)2Cl] to C·CSiPri3 (3) to
C·CC6H4But-4 (6) the n?p* band shifts progressively to lower
energy (19230, 16780 and 16500 cm–1, respectively). Similarly,
replacing the axial chloride ligand of 3 with C·CC6H4Me-4 (7)
results in a red shift of the n?p* band from 16780 to 15870
cm–1. These red shifts must be attributed to a lowering of the
energy of the p* LUMO as a result of extending the p-system,
since the dxy orbital is nonbonding (d symmetry) with respect to
the s and p frameworks of the backbone.
4 T. M. Swager, Acc. Chem. Res., 1998, 31, 201.
5 (a) A. Mayr and S. Ahn, Adv. Transition Met. Coord. Chem., 1996, 1, 1;
(b) H. Fischer, P. Hofmann, F. R. Kreissl, R. R. Schrock, U. Schubert
and K. Weiss, Carbyne Complexes, VCH, Amsterdam, 1988; (c) M. A.
Gallop and W. R. Roper, Adv. Organomet. Chem., 1986, 25, 121.
6 T. P. Pollagi, S. J. Geib and M. D. Hopkins, J. Am. Chem. Soc., 1994,
116, 6051; H. A. Brison, T. P. Pollagi, T. C. Stoner, S. J. Geib and M. D.
Hopkins, Chem. Commun., 1997, 1263.
7 A. Harriman and R. Ziessel, Coord. Chem. Rev., 1998, 171, 331.
8 M. P. Y. Yu, K.-K. Cheung and A. Mayr, J. Chem. Soc., Dalton Trans.,
1998, 2373.
9 J. S. Murdzek and R. R. Schrock in ref. 5(b), p. 147.
10 M. A. Stevenson and M. D. Hopkins, Organometallics, 1997, 16,
3572.
11 (a) E. O. Fischer, M. Schluge and J. O. Besenhard, Angew. Chem., Int.
Ed. Engl., 1976, 15, 683; (b) D. E. Haines and M. D. Hopkins, in
preparation.
The reactivity and physical properties of 4 suggest that this
compound and its relatives should be important building blocks
for new classes of poly(aryleneethynylene)s with expanded
optical and redox functionality. There are no obvious reasons
why the metathesis-based synthetic procedure reported here
should not also yield building blocks with other aryl hubs
(which are incorporated at the alkyne-metathesis step) or
equatorial ligands [which are added in the course of the
reduction of W(·CArCCR)Cl3(dme)], thus providing a high
level of control over their physical properties. We are presently
exploring the syntheses of such compounds.
12 M. H. Chisholm, B. W. Eichhorn, K. Folting, J. C. Huffman, C. D.
Ontiveros, W. E. Streib and W. G. Van Der Sluys, Inorg. Chem., 1987,
26, 3183.
13 J. Manna, T. M. Gilbert, R. F. Dallinger, S. J. Geib and M. D. Hopkins,
J. Am. Chem. Soc., 1992, 114, 5870.
14 A. B. Bocarsly, R. E. Cameron, H.-D. Rubin, G. A. McDermott, C. R.
Wolff and A. Mayr, Inorg. Chem., 1985, 24, 3976.
We thank the National Science Foundation for supporting
this research (Grant CHE-9700451). K. D. J. acknowledges
support from Andrew W. Mellon and Lubrizol fellowships.
Communication 8/09231A
590
Chem. Commun., 1999, 589–590