Synthesis and reactivity of tetrakis(imino)pyracene (TIP) ligands; bifunctional analogues of the BIAN ligand class

Article abstract of DOI:10.1039/b719251g

The first two examples of a new class of bifunctional BIAN-type ligand have been prepared, and the reactions of one such ligand with CuBr₂ and BCl₃ have been explored. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719251g

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Synthesis and reactivity of tetrakis(imino)pyracene (TIP) ligands;
bifunctional analogues of the BIAN ligand classw
Kalyan V. Vasudevan, Michael Findlater and Alan H. Cowley*
Received (in Austin, TX, USA) 14th December 2007, Accepted 28th January 2008
First published as an Advance Article on the web 3rd March 2008
DOI: 10.1039/b719251g
The first two examples of a new class of bifunctional BIAN-type
ligand have been prepared, and the reactions of one such ligand
with CuBr2 and BCl3 have been explored.
Satisfactory spectroscopic data were acquired for the two
TIP ligands, 2 and 4, as well as for the bis(ZnCl2) dpp-TIP
complex, 3.3 Compounds 2–4 were also characterized by
single-crystal X-ray diffraction.4 Molecules of 2 are located
on an inversion center, and the asymmetric unit of 3 contains
two independent half molecules, each of which is located on an
independent inversion center. The structures of TIP ligands 2
and 4 are very similar to each other. Within experimental
error, the fused ring systems of 2 and 4 are planar. The torsion
angle of the NQC–CQN fragment is 6.151 in 2, and the
corresponding value for 4 averages 5.751. The average dihedral
angles between the aryl substituents and the naphthalene
moieties are 83.24 and 86.841 for 2 and 4, respectively.
A solution of two equivalents of CuBr2 in EtOH was layered
on top of a THF solution of one equivalent of 2. Crystals of
the coordination polymer [BrCu(dpp-TIP)CuBr]n (5) formed
slowly over a period of B1 week. Polymer 5 was characterized
by 1H NMR spectroscopy, mass spectrometry3 and single-
crystal X-ray diffraction.4 A prominent peak in the CI+ mass
spectrum occurs at m/z 1156, corresponding to the monomeric
unit [BrCu(dpp-TIP)CuBr]. The crystal structure features
essentially planar Cu(dpp-TIP)Cu moieties and Cu2Br2 rhom-
boids that are arranged in an approximately orthogonal
fashion along the polymer chain (Fig. 1). It is clear from the
composition of 5 that polymer formation is accompanied by
the reduction of CuBr2 to CuBr. There is, however, no
evidence of redox activity within the TIP ligand. Thus, the
C(1)–N(1) and C(5)–N(2) distances, which average 1.275(7) A,
correspond to CQN bonds and the C(1)–C(5) separation of
1.504(8) A falls in the C–C single bond range. Even though the
Cu–Br–Cu angle in the Cu2Br2 rhomboid is quite acute
(68.06(3)1), the CuÁ Á ÁCu distance of 2.7178(14) A exceeds the
The bis(imino)acenaphthene (BIAN) ligand class can be re-
garded as originating from the fusion of a naphthalene ring
and a 1,4-diaza-1,3-butadiene moiety. The rigidity of the
resulting ligands make them excellent platforms for the sup-
port of, e.g., late transition metal complexes that serve as
robust catalysts for a significant number of useful transforma-
tions.1 Further interest in the BIAN ligand class has arisen
from their ability to function as both electron and proton
sponges. Given the foregoing desirable properties, we became
interested in developing a BIAN-type ligand that features the
fusion of two 1,4-diaza-1,3-butadiene moieties to the naphtha-
lene ring. It was envisioned that such bifunctional BIAN
ligands could serve as, e.g., redox-active links for supramole-
cular construction, molecular wire models, metallopolymers
and bimetallic catalyst supports. Herein, we describe (i) the
first two examples of the tetrakis(imino)pyracene (TIP) ligand
class, (ii) the first example of a polymer featuring a BIAN-type
ligand and (iii) the use of a new TIP ligand for the generation
of a dinuclear boron dication.
As summarized in Scheme 1, two approaches were taken to
synthesize the target TIP ligands. Both methods started with
1,2,5,6-tetraketopyracene (1).2 Treatment of 1 with a 1 : 1
mixture of CH3COOH and CH3CN at 80 1C, followed by the
addition of five equivalents of 2,6-i-Pr2C6H3NH2 and reflux of
the reaction mixture, resulted in a 90% yield of the 2,6-
diisopropylphenyl-substituted TIP ligand, 2, which, in turn,
was converted into the corresponding bis(ZnCl2) complex, 3,
via reaction with excess ZnCl2 in a THF solution. The
bis(ZnCl2) complex of the mesityl-substituted TIP ligand was
generated as an intermediate by the treatment of 1 with a
ZnCl2/acetic acid mixture, followed by reaction with 2,4,6-
Me3C6H2NH2. The free mesityl-substituted TIP ligand, 4, was
isolated in 50% overall yield by the decomplexation of
this intermediate with potassium oxalate in a CH2Cl2/H2O
solution.
Department of Chemistry and Biochemistry, The University of Texas
at Austin, 1 University Station A5300, Austin, Texas 78712, USA.
E-mail: cowley@mail.utexas.edu; Fax: +1 512-471-6822;
Tel: +1 512-471-7484
w CCDC 670789 (2), 670790 (3), 670788 (4), 670791 (5) and 670787 (6).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b719251g
Scheme 1
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distances correspond to bond orders of 2 and 1, respectively;
thus indicating that both of the diimine functionalities form
donor–acceptor bonds to a [BCl2]+ moiety. Both BN2C2 rings
are essentially planar, as in the case of BIAN-supported
[BCl2]+ cations.5
We are grateful to the Robert A. Welch Foundation
(F-0003) for their financial support of this work.
Notes and references
1 See, for example: (a) R. van Asselt, E. E. C. G. Gielens, R. E.
Rulke and C. J. Elsevier, J. Chem. Soc., Chem. Commun., 1993,
1203; (b) R. van Asselt, K. Vrieze and C. J. Elsevier, J. Organomet.
Chem., 1994, 480, 27; (c) R. van Asselt, C. J. Elsevier, W. J. J.
Smeets, A. L. Spek and R. Benedix, Recl. Trav. Chim. Pays-Bas,
1994, 113, 88; (d) B. S. Williams, P. M. Leatherman, D. S. White
and M. Brookhart, J. Am. Chem. Soc., 2005, 127, 5132; (e) A. M.
Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk and C. J. Else-
vier, J. Am. Chem. Soc., 2005, 127, 15470; (f) J. M. Rose,
A. E. Cherian and G. W. Coates, J. Am. Chem. Soc., 2006, 128,
4186.
Fig. 1 (a) ORTEP view of polymer 5, with thermal ellipsoids shown
at 50% probability. (b) View of a fragment of the monomeric unit of 5.
Selected bond lengths (A) and angles (1): C(1)–C(5) 1.504(8), C(1)–N(1)
1.276(7), C(5)–N(2)1.274(7), N(1)–Cu(1) 2.099(4), N(2)–Cu(1) 2.116(4),
Cu(1)–Br(1) 2.4132(11); N(1)–C(1)–C(5) 118.6(5), C(1)–C(5)–N(2)
118.5(5), C(1)–N(1)–Cu(1) 110.4(4), C(5)–N(2)–Cu(1) 110.0(4),
N(1)–Cu(1)–N(2) 80.52(16). Each Cu2Br2 moiety is located on an
inversion center, and the asymmetric unit features half of the ligand,
which lies about another inversion center.
2 M. D. Clayton, Z. Marcinow and P. W. Rabideau, Tetrahedron
Lett., 1998, 39, 9127.
3 Spectroscopic data for 2: 1H NMR (300 MHz, CDCl3): d 0.89 (d,
24H), 1.17 (d, 24H), 2.89 (sept, 8H), 6.44 (d, 4H), 7.22 (br, m,
12H). HRMS (CI+, CH4): calc. for C62H72N4: 873.5835; found:
873.5833. Spectroscopic data for 3: 1H NMR (300 MHz, CD2Cl2): d
0.78 (d, 24H), 1.32 (d, 24H), 3.17 (sept, 8H), 6.59 (d, 4H), 7.39 (br,
m, 12H). HRMS (CI+
, CH4): calc. for C72H62Cl4N4Zn2:
1140.3094; found: 1140.3109. Spectroscopic data for 4: 1H NMR
(300 MHz, CDCl3): d 2.02 (s, 24H), 2.33 (s, 12H), 6.62 (d, 4H), 6.93
(s, 8H). HRMS (CI+, CH4): calc. for C50H48N4: 704.3957; found:
704.3953. Spectroscopic data for 5: 1H NMR (300 MHz, CD3CN):
d 0.83 (br, s, 24H), 1.24 (br, s, 24H), 3.08 (br, s, 8H), 6.41 (br, s,
sum of their covalent radii, hence there is no evidence of a
bonding interaction between these atoms.
4H), 7.37 (br, s, 12H). HRMS (CI+
, CH4): calc. for
Treatment of one equivalent of 2 with four equivalents of
BCl3 in a CH2Cl2/hexanes solution at 25 1C for 12 h resulted,
after work-up of the reaction mixture, in a virtually quantita-
tive yield of the boron dication salt [Cl2B(dpp-TIP)-
BCl2][BCl4]2 (6). Compound 6 was characterized on the basis
of spectroscopic measurements3 and an X-ray diffraction
study.4 Although the accuracy of the structure is not high
due to crystal twinning, the data were adequate to establish the
atom connectivity (Fig. 2) and approximate bond orders. For
example, the N–C (av. 1.281(9) A) and C–C (av. 1.501(10))
C62H72Br2Cu2N4: 1156.2716; found: 1156.2726. Spectroscopic data
for 6: 1H NMR (300 MHz, CD2Cl2): d 0.93 (d, 24H), 1.33 (d, 24H),
3.15 (sept, 8H), 6.83 (d, 4H), 7.44 (br, m, 12H).
4 All X-ray data sets were collected at 153 K on a Nonius-Kappa
CCD diffractometer with Mo-Ka radiation (l = 0.71073 A).
Crystal data for 2ÁCHCl3: C64H74Cl6N4, monoclinic, space group
P21/n, a = 12.673(3), b = 15.048(3), c = 18.514(4) A, b =
97.68(3)1, V = 3499.1(12) A3, Z = 2, rcalc = 1.055 g cmÀ3, 2ymax
= 54.88, total reflections collected = 13 524, unique reflections =
7960 (Rint = 0.0419), m = 0.282 mmÀ1, final R indices: R1
=
0.0692, wR2 = 0.2071, GOF = 1.002. Crystal data for 3Á6CHCl3:
C68H78Cl22N4Zn2, triclinic, space group P-1, a = 13.151(5), b =
15.312(5), c = 23.978(5) A, a = 81.542(5), b = 77.600(5), g =
64.858(5)1, V = 4260(2) A3, Z = 2, rcalc = 1.451 g cmÀ3, 2ymax
=
54.921, total reflections collected = 28 557, unique reflections =
=
19 305 (Rint = 0.0398), m = 1.293 mmÀ1, final R indices: R1
0.0732, wR2 = 0.1824, GOF = 1.048. Crystal data for 4ÁCH2Cl2:
C51H50Cl2N4, monoclinic, space group P21/c, a = 13.568(3), b =
24.864(5), c = 13.852(3) A, b = 111.16(3)1, V = 4357.6(15) A3,
Z = 4, rcalc = 1.204 g cmÀ3, 2ymax = 54.921, total reflections
collected = 28 050, unique reflections = 9946 (Rint = 0.0552),
m = 0.188 mmÀ1, final R indices: R1 = 0.0558, wR2 = 0.01519,
GOF = 1.018. Crystal data for 5ÁTHF: C35H44BrCuN2O, mono-
clinic, space group P21/c, a = 13.719(3), b = 13.400(3), c =
19.628(4) A, b = 106.77(3)1, V = 3454.9(12) A3, Z = 4, rcalc
=
1.254 g cmÀ3, 2ymax = 54.821, total reflections collected = 22 151,
unique reflections = 7850 (Rint = 0.0683), m = 1.816 mmÀ1, final
R indices: R1 = 0.0678, wR2 = 0.02037, GOF = 1.068. Crystal
data for 6: C58H68B4Cl2N4, monoclinic, space group P21/n, a =
16.075(3), b = 16.312(3), c = 17.424(4) A, b = 106.66(3)1, V =
4377.2(15) A3, Z = 3, rcalc = 1.468 g cmÀ3, 2ymax = 54.241, total
Fig. 2 ORTEP view of diboron dication 6, with thermal ellipsoids
shown at 50% probability. Selected bond lengths (A) and angles (1):
C(1)–C(5) 1.501(1), C(1)–N(1) 1.285(9), C(5)–N(2) 1.278(9), N(1)–B(1)
1.634(11), N(2)–B(1) 1.625(10), B(1)–Cl(1) 1.784(9), B(1)–Cl(2)
1.786(8); N(1)–C(1)–C(2) 111.5(6), C(1)–C(2)–N(2) 111.9(6),
C(1)–N(1)–B(1) 109.3(6), C(2)–N(2)–B(1) 109.6(6), N(1)–B(1)–N(2)
97.4(5), Cl(1)–B(1)–Cl(2) 113.3(5). Molecules of 6 are located on an
inversion center.
reflections collected = 5504, unique reflections = 5504 (Rint
0.0525), m = 0.613 mmÀ1, final R indices: R1 = 0.1189, wR2
0.3692, GOF = 0.999w.
5 (a) H. A. Jenkins, C. L. Dumaresque, D. Vidovic and J. A. C.
Clyburne, Can. J. Chem., 2002, 80, 1398; (b) N. J. Hill, K.
Vasudevan and A. H. Cowley, Jordan J. Chem., 2006, 1, 47.
=
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1918–1919 | 1919