Pyridylpyrrolides as alternatives to cyclometalated phenylpyridine ligands: Synthesis and characterization of luminescent zinc and boron pyridylpyrrolide complexes

Article abstract of DOI:10.1039/b315523d

The synthesis, structure, and properties of six luminescent pyridylpyrrolide complexes and the first structural characterization of pyridylpyrrolide metal complexes are reported. A series of new zinc complexes, bis(pyridylpyrrolyl)-zinc, (R₂PyrPy)₂Zn (R = Me, Et, ⁱPr, ^tBu, and Ph), that vary in their substituents on the pyrrole ring (Me, Et, ⁱPr, ^tBu, and Ph), were prepared. Pyrrole substitution produced small structural changes in the complexes and affected the fluorescence properties very little. The zinc complexes were found to be luminescent, emitting at 495 nm (Φ = 0.32, 0.32 0.31, 0.19 and 0.57, respectively). A boron analog, (Me₂PyrPy)BF₂, was prepared and was found to share the luminescent properties with the zinc complexes, emitting at 505 nm (Φ = 0.22), but not their water-sensitivity. A total of four crystal structures are reported, ^tBu₂PyrPyH, (Me ₂PyrPy)₂Zn, (^tBu₂PyrPy) ₂Zn, and (Me₂PyrPy)BF₂. ^tBu ₂PyrPyH crystallizes as a doubly hydrogen bonded dimer with non-coplanar pyridine and pyrrole rings. The solid-state structures of (Me ₂PyrPy)₂Zn and (^tBu₂PyrPy) ₂Zn revealed that despite the large change in steric bulk, the two compounds have very similar structures. The structure of (Me₂PyrPy) BF₂ showed changes that are expected with the interaction between a smaller atom (B as compared to Zn). Molecular orbital calculations were performed on Me₂PyrPyH, (Me₂PyrPy)BF₂, and (Me₂PyrPy)₂Zn using Gaussian 98 methods. It was found that the main transition (HOMO-LUMO) for all three molecules is a π → π* transition and that in the inorganic complexes, the metal atom (zinc or boron) present has very little effect on transition, evidence that the optical properties are largely ligand based and that the B or Zn atom's main effect is lowering of the LUMO relative energy.

Full text of DOI:10.1039/b315523d

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Pyridylpyrrolides as alternatives to cyclometalated phenylpyridine
ligands: synthesis and characterization of luminescent zinc and
boron pyridylpyrrolide complexes†
Jamie J. Klappa, Sarah A. Geers, Sarah J. Schmidtke, Laura A. MacManus-Spencer and
Kristopher McNeill*
Department of Chemistry, University of Minnesota, Minneapolis, MN, USA.
E-mail: mcneill@chem.umn.edu; Fax: 612-626-7541; Tel: 612-625-0781
Received 1st December 2003, Accepted 5th February 2004
First published as an Advance Article on the web 2nd March 2004
The synthesis, structure, and properties of six luminescent pyridylpyrrolide complexes and the first structural
characterization of pyridylpyrrolide metal complexes are reported. A series of new zinc complexes, bis(pyridylpyrrolyl)-
zinc, (R2PyrPy)2Zn (R = Me, Et, iPr, tBu, and Ph), that vary in their substituents on the pyrrole ring (Me, Et, iPr, tBu,
and Ph), were prepared. Pyrrole substitution produced small structural changes in the complexes and affected the
fluorescence properties very little. The zinc complexes were found to be luminescent, emitting at 495 nm (Φ = 0.32,
0.32 0.31, 0.19 and 0.57, respectively). A boron analog, (Me2PyrPy)BF2, was prepared and was found to share
the luminescent properties with the zinc complexes, emitting at 505 nm (Φ = 0.22), but not their water-sensitivity.
A total of four crystal structures are reported, tBu2PyrPyH, (Me2PyrPy)2Zn, (tBu2PyrPy)2Zn, and (Me2PyrPy)BF2.
tBu2PyrPyH crystallizes as a doubly hydrogen bonded dimer with non-coplanar pyridine and pyrrole rings.
The solid-state structures of (Me2PyrPy)2Zn and (tBu2PyrPy)2Zn revealed that despite the large change in steric
bulk, the two compounds have very similar structures. The structure of (Me2PyrPy)BF2 showed changes that are
expected with the interaction between a smaller atom (B as compared to Zn). Molecular orbital calculations were
performed on Me2PyrPyH, (Me2PyrPy)BF2, and (Me2PyrPy)2Zn using Gaussian 98 methods. It was found that the
main transition (HOMO–LUMO) for all three molecules is a π
π* transition and that in the inorganic complexes,
the metal atom (zinc or boron) present has very little effect on transition, evidence that the optical properties are
largely ligand based and that the B or Zn atom’s main effect is lowering of the LUMO relative energy.
variety of metal centers. In this work, we demonstrate this prin-
Introduction
ciple by preparing luminescent PyrPy Zn and B complexes.
The number of 2,2Ј-pyridylpyrrolide (PyrPyϪ) complexes in
the literature are remarkably few in number.7–9 Indeed, we
are aware of no reports of such complexes since the work of
Chiswell7 and Perry and Weber,9 over 30 years ago. This does
not appear to be for lack of interest, as a number of reports of
the closely related pyrrolide-imine10 and pyridylindolide11,12
ligands have appeared recently. Rather, the relative rarity of
PyrPy complexes is most likely due to the difficult syntheses of
the free ligand, which commonly require many steps and result
in low yields.13 The recent development of a one-step synthesis
of substituted pyridylpyrroles has removed this obstacle, and
will hopefully lead to a wider exploration of the chemistry of
PyrPy complexes.14
In the present study, we have chosen to focus on Zn and B
complexes because they are representative of the types of
centers for which it may be much easier to prepare PyrPy
complexes than PhPy complexes. We also anticipated that
they would be luminescent by analogy to luminescent zinc
and boron complexes of other monoanionic ligands such as
hydroxyquinolines,15 pyridylamines,16 azaindoles,17 and indole
derivatives (specifically pyridylindoles).11,12 Here, we report
the synthesis, characterization, and luminescent properties of
five new bis(pyridylpyrrolyl)zinc complexes and a new boron
complex, difluoro-(3,5-dimethyl-2-(2Ј-pyridylpyrrolyl))borane,
(Me2PyrPy)BF2. We also report the first structural characteriz-
ation of pyridylpyrrolide metal, (PyrPy)M, complexes.
Luminescent metal complexes containing cyclometalated
phenylpyridine (PhPy) ligands have received a great deal of
attention in the last decade, driven by the search for improved
light emitting diode (LED) materials.1 The appeal of the PhPy
complexes is their high luminescence quantum yields and the
ability to tune their photoluminescent properties by varying the
ligand structure.2 The work with these ligands has thus far
focused almost exclusively on a few key metal centers (Re, Ru,
Os, Rh, Ir, Pd, Pt, Hg) that have the ability to undergo cyclo-
metalation.1,3,4 This limitation has led to very few studies with
other metal centers, such as Mn()5 and Bi().6 We have
become interested in the use of pyridylpyrrolide (PyrPy) ligands
as possible alternatives to PhPy. PyrPy ligands share the mono-
anionic, bidentate coordination mode and steric profile of PhPy
ligands (Fig. 1). They offer the advantage that the pyrrole N–H
bond is considerably more acidic than the phenyl C–H bond in
PhPy, which should lead to more facile coordination to a wider
Fig. 1 General structures of pyridylpyrrolide and cyclometalated
phenylpyridine metal complexes.
Experimental
† Electronic supplementary information (ESI) available: Beer’s Law
plots and emission peak area vs. concentration plots for complexes;
comparison of excitation and absorbance spectra; effect of triplet
quencher isoprene on luminescence intensity. See http://www.rsc.org/
suppdata/dt/b3/b315523d/
Chemicals that were purchased commercially and used without
further purification include p-toluenesulfonic acid (Aldrich),
2-aminomethylpyridine (Aldrich), 2,4,6-collidine (Aldrich),
boron trifluoride etherate (Aldrich), 3,5-heptanedione
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 8 8 3 – 8 9 1
883

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(Chemika), 2,6-dimethyl-3,5-heptanedione (TCI America), tri-
fluoroacetic acid (Spectrum) and 4 Å (8–12 mesh) molecular
sieves (Acros). Diethylzinc (Aldrich), 1.0 M in hexanes or hept-
ane, was transferred under vacuum prior to use. n-Butyllithium
(Alfa Aesar) concentration (2.3 M) was determined by titration
of diphenylacetic acid and it was filtered prior to use. Both
toluene (Aldrich) and hexanes (Fisher) were dried by passage
through an alumina column under nitrogen. THF (Fisher) and
diethyl ether (JT Baker) were pre-dried over calcium hydride
and distilled from sodium benzophenone ketyl.
J = 5.1 Hz, 1 H), 9.21 (br. s, 1 H); 13C NMR (CDCl3) δ 22.4,
23.8, 26.1, 27.1, 103.4, 118.6, 119.1, 123.9, 131.3, 136.2, 139.8,
148.8, 151.2; MS m/z 228 (Mϩ, 48.2%). Anal. calcd. for
C15H20N2: C, 78.90; H, 8.83; N, 12.27. Found: C, 78.85; H, 9.08;
N, 12.15.
Synthesis of zinc complexes
Bis(3,5-dimethyl-2-(2Ј-pyridyl)pyrrolyl)zinc (1). Under nitro-
gen atmosphere, I (54.6 mg, 0.317 mmol) was dissolved in
THF (4 mL). Upon addition of diethylzinc (160 µL, 1 M,
0.160 mmol), the solution turned bright yellow. Following
removal of solvent in vacuo, a yellow solid remained. Crystals
suitable for single-crystal X-ray diffraction were obtained from
a saturated solution in diethyl ether at Ϫ35 ЊC (48.3 mg, 75%):
mp = 168–171 ЊC; 1H NMR (C6D6) δ 2.21 (s, 6 H), 2.44 (s, 6 H),
5.96 (td, J = 0.9, 6.7 Hz, 2 H), 6.23 (s, 2 H), 6.87 (td, J = 1.8,
7.8 Hz, 2 H), 7.17 (d, J = 5.2 Hz, 2 H), 7.28 (d, J = 8.4 Hz, 2 H);
13C NMR (C6D6) δ 14.6, 16.1, 114.6, 115.7, 117.2, 124.6, 130.2,
138.2, 140.3, 146.4, 154.5. Anal. calcd. for C22H22N4Zn: C,
64.79; H, 5.44; N, 13.74. Found: C, 64.72; H, 5.47; N, 13.79.
The solid-state structure was determined using X-ray diffrac-
tion, see details below.
1
The H NMR and 13C NMR spectra were acquired on a
Varian Unity, 300 MHz, Varian Inova, 500 MHz, or Varian
Inova, 300 MHz. The 19F NMR was acquired on a Varian
Unity 300 MHz with trifluoroacetic acid (δ = 0.0 ppm) used as a
reference. GC-MS data were collected with an Agilent GC 6890
Series (column is HP-5MS crosslinked 5% PH ME Siloxane,
30 m × 0.25 mm × 0.25 µm film thickness) coupled to an Agi-
lent MS 5973 Network Mass Selective Detector. High-reso-
lution mass spectra (HRMS), using electron impact (EI), were
acquired on a Finnigan MAT 95, matching the theoretical mass
within 4.75 ppm and the calculated isotope patterns. Absorb-
ance measurements were performed on a Jasco V-530 UV/VIS
spectrophotometer and emission measurements were per-
formed on a Jasco FP-6200 spectrofluorometer. Melting points
were determined with a Büchi melting point apparatus, cali-
brated using the melting points of benzophenone and maleic
acid. Elemental analyses were performed by Atlantic Microlab,
Inc., Norcross, Georgia.
Synthesis of bis(3,5-diethyl-2-(2Ј-pyridyl)pyrrolyl)zinc (2).
Following the same procedure employed for 1, diethylzinc
(260 µL, 1 M, 0.260 mmol) was added to a THF (4 mL) solu-
tion of 3,5-diethyl-2-(2Ј-pyridyl)pyrrole (93.8 mg, 0.468 mmol).
The yellow solid was recrystallized from hexanes at Ϫ35 ЊC
(85.1 mg, 78.4%): mp = 89.5–92.0 ЊC; 1H NMR (C6D6) δ 1.15 (t,
J = 7.5 Hz, 6 H), 1.37 (t, J = 7.5 Hz, 6 H), 2.60 (q, J = 7.5 Hz,
4 H), 2.86 (q, J = 7.5 Hz, 4 H), 5.96 (t, J = 6.9 Hz, 2 H), 6.68 (s,
2 H), 6.89 (td. J = 1.5, 7.2 Hz, 2 H), 7.14 (d, J = 5.2 Hz, 2 H),
7.31 (d, J = 8.4 Hz, 2 H); 13C NMR (C6D6) δ 14.5, 14.7, 22.1,
24.3, 110.8, 115.7, 117.3, 129.2, 131.7, 138.3, 146.5, 146.8,
154.4. Anal. calcd. for C26H30N4Zn: C, 67.31; H, 6.52; N, 12.08.
Found: C, 67.18; H, 6.58; N, 11.94.
Synthesis of ligands
3,5-Dimethyl-2-(2Ј-pyridyl)pyrrole, I, 3,5-di-tert-butyl-2-(2Ј-
pyridyl)pyrrole, IV, and 3,5-diphenyl-2-(2Ј-pyridyl)pyrrole, V,
were prepared according to a previously published method.14
3,5-Diethyl-2-(2Ј-pyridyl)pyrrole (II). II was prepared by a
modified literature procedure.14 Into a sealed tube, one equiv.
each of 3,5-heptanedione (1.06 g, 8.25 mmol) and 2-(amino-
methyl)pyridine (0.900 g, 8.32 mmol) along with a catalytic
amount of tosic acid (250 mg, 1.31 mmol) was added to xylenes
(50 mL). Molecular sieves (4 Å, 80 mg) were added to the reac-
tion mixture and the mixture was stirred at room temperature
for 30 min. The sealed tube was evacuated and heated at 170 ЊC
for 12 h. The reaction was monitored by GC-MS. The mixture
was chromatographed (silica gel, 5 : 1 hexanes/ethyl acetate, 3%
Et3N), and following removal of the solvent, product II (927
Synthesis of bis(3,5-di-isopropyl-2-(2Ј-pyridyl)pyrrolyl)zinc
(3). Following the same procedure employed for 1, diethylzinc
(125 µL, 1 M, 0.125 mmol) was added to a THF (4 mL) solu-
tion of 3,5-di-isopropyl-2-(2Ј-pyridyl)pyrrole (48.3 mg, 0.211
mmol). The yellow solid was recyrstallized from hexanes at
1
Ϫ35 ЊC (34.5 mg, 63%); mp = 139–140 ЊC; H NMR (C6D6)
δ 1.18 (d, J = 6.9 Hz, 6 H), 1.21 (d, J = 6.9 Hz, 6 H), 1.39
(d, J = 6.9 Hz, 6 H), 1.43 (d, J = 6.6 Hz, 6 H), 2.87 (septet,
J = 6.9 Hz, 2 H), 3.32 (septet, J = 6.9 Hz, 2 H), 5.97 (td,
J = 0.9, 2.7 Hz, 2 H), 6.40 (s, 2 H), 6.92 (td, J = 1.8, 8.1 Hz, 2 H),
7.13 (d, J = 3.7 Hz, 2 H), 7.41 (d, J = 8.4 Hz, 2 H); 13C NMR
(C6D6) δ 23.0, 23.6, 23.7, 23.9, 27.2, 30.4, 106.4, 115.8, 117.6,
128.5, 137.2, 138.4, 146.5, 151.5, 154.3. HRMS (EI) calcd. for
C30H38N4Zn (Mϩ) 518.2388, found 518.2374.
1
mg, 56.1%) was obtained as a yellow oil: H NMR (CDCl3)
δ 1.30 (t, J = 7.5 Hz, 3 H), 1.34 (t, J = 7.5 Hz, 3 H), 2.68 (q,
J = 7.5 Hz, 2 H), 2.82 (q, J = 7.5 Hz, 2 H), 5.96 (d, J = 3.3 Hz,
1 H), 6.99 (ddd, J = 1.5, 4.8, 7.5 Hz, 1 H), 7.47 (d, J = 8.1 Hz,
1 H), 7.64 (td, J = 1.8, 7.8 Hz, 1 H), 8.50 (dm, J = 4.8 Hz, 1 H),
9.47 (br. s, 1 H); 13C NMR (CDCl3) δ 13.6, 14.7, 21.0, 21.2,
107.5, 118.6, 119.3, 124.9, 126.2, 135.5, 136.4, 149.0, 151.3; MS
m/z 200 (Mϩ, 61.2%). Anal. calcd. for C13H16N2: C, 77.96; H,
8.05; N, 13.99. Found: C, 77.85; H, 8.16; N, 13.90.
Synthesis of bis(3,5-di-tert-butyl-2-(2Ј-pyridyl)pyrrolyl)zinc
(4). Following the same procedure employed for 1, diethylzinc
(110 uL, 1 M, 0.11 mmol) was added to a THF (4 mL) solution
of 3,5-di-tert-butyl-2-(2Ј-pyridyl)pyrrole (54.4 mg, 0.21 mmol).
The yellow solid was recyrstallized from hexanes at Ϫ35 ЊC
(58.7 mg, 96%); mp = 145–150 ЊC; 1H NMR (C6D6) δ 1.32 (s, 18
H), 1.61 (s, 18 H), 5.96 (t, J = 5.8 Hz, 2 H), 6.60 (s, 2 H), 6.90
(td, J = 1.2, 8.4 Hz, 2 H), 7.24 (d, J = 4.9 Hz, 2 H), 7.87 (d,
J = 8.4 Hz, 2 H); 13C NMR (C6D6) δ 31.2, 31.3, 109.1, 115.7,
120.2, 128.9, 137.6, 139.9, 146.3, 152.7, 154.0. HRMS (EI)
calcd. for C34H46N4Zn (Mϩ) 574.3014, found 574.3025. X-ray
quality crystals were grown from a saturated solution in
hexanes through slow evaporation of solvent. Solid-state
structure was determined using X-ray diffraction (see below).
3,5-Di-isopropyl-2-(2Ј-pyridyl)pyrrole (III). Following the
same procedure employed for the synthesis of II, III was pre-
pared by adding one equiv. each of 2,6-dimethyl-3,5-heptane-
dione (1.10 g, 7.01 mmol) and 2-(aminomethyl)pyridine
(808 mg, 7.48 mmol) along with a catalytic amount of tosic acid
(200 mg, 1.05 mmol) to xylenes (50 mL). The reaction was
heated for 48 h at 170 ЊC. The mixture was chromatographed
(silica gel, 5 : 1 hexanes/ethyl acetate, 3% Et3N). Following
removal of the solvent, product III (1012 mg, 63.3 %) was
1
obtained as a light yellow solid: mp = 84.0–85.0 ЊC; H NMR
(CDCl3) δ 1.298 (d, J = 6.6 Hz, 6 H), 1.302 (d, J = 6.9 Hz, 6 H),
2.95 (septet, J = 6.9 Hz, 1 H), 3.30 (septet, J = 6.6 Hz, 1 H),
5.95 (d, J = 3.2 Hz, 1 H), 6.97 (ddd, J = 0.9, 4.8, 7.2 Hz, 1 H),
7.46 (d, J = 8.4 Hz, 1 H), 7.61 (td, J = 2.1, 8.4 Hz, 1 H), 8.96 (m,
Synthesis of bis(3,5-diphenyl-2-(2Ј-pyridyl)pyrrolyl)zinc (5).
Following the same procedure employed for 1, diethylzinc
D a l t o n T r a n s . , 2 0 0 4 , 8 8 3 – 8 9 1
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Table 1 Molar absorptivities and quantum yields complexes 1–6a
(110 µL, 1 M, 0.110 mmol) was added to a THF (4 mL) solu-
tion of 3,5-diphenyl-2-(2Ј-pyridyl)pyrrole (54.0 mg, 0.182
mmol). Solvent was removed and a bright yellow solid was
obtained. The yellow solid was recyrstallized from hexanes/
toluene (6 : 1) at Ϫ35 ЊC. From this solvent mixture, the com-
pound crystallizes with 1 equiv. toluene (69 mg, 100% as
5ؒC7H8); mp = 154–156 ЊC; 1H NMR (C6D6) δ 5.78 (ddd,
J = 1.2, 5.4, 7.2 Hz, 2 H), 6.51 (td, J = 1.8, 7.6 Hz, 2 H), 6.78 ( tt,
J = 1.2, 8.1 Hz, 2 H), 7.01 (m, 6 H), 7.10 (m, 2 H), 7.20
(tt, J = 1.2, 7.2 Hz, 2 H), 7.32 (tt, J = 0.9, 7.5 Hz, 4 H), 7.54 (dt,
J = 0.9, 8.4 Hz, 2 H), 7.74 (dt, J = 1.5, 6.6 Hz, 4 H), 7.88 (dt,
J = 1.5, 6.9 Hz, 4 H); 13C NMR (C6D6) δ 114.2, 118.0, 118.5,
124.4, 125.7, 126.4, 128.4, 128.5, 129.6, 131.7, 132.0, 136.2,
138.3, 139.2, 143.3, 146.3, 154.2. HRMS (EI) calcd. for
C42H30N4Zn (Mϩ) 654.1762, found 654.1793.
Absorbance
Emission
Compound
λmax/nm
ε/MϪ1 cmϪ1
λmax/nm
Φ
1
2
3
4
5
6
337b
395
29600 2500
21600 600
25200 1000
20300 800
22200 500
19000 500
20800 200
19300 100
31400 3200
33900 1000
13800 600
11200 300
496
496
492
495
490
505
0.32 0.03
0.32 0.02
0.31 0.02
0.19 0.01
0.57 0.06
0.22 0.04
339b
395
335b
395
335b
395
300b
405
325b
405
a Experimental conditions: THF solvent,
T = 22 ЊC (ambient).
Synthesis of the boron complex
b Excitation wavelength for quantum yield determination.
Difluoro-(3,5-dimethyl-2-(2Ј-pyridyl)pyrrolyl)borane (6).
Compound 6 was prepared following a modified literature pro-
cedure.11 In an evacuated sealed tube, boron trifluoride etherate
(165 µL, 1.30 mmol), I (222 mg, 1.29 mmol), 2,4,6-collidine
(170 µL, 1.29 mmol) and dry degassed THF (20 mL) were
heated at 80 ЊC for 4 h. Following removal of solvent in vacuo, a
yellow residue remained. The residue was chromatographed
(silica gel, 1 : 1 hexanes: ethyl acetate) to yield a bright yellow
solid. Crystals suitable for X-ray diffraction were obtained from
a saturated solution in hexanes at Ϫ20 ЊC (59 mg, 25%): mp
153.2–155.0; 1H NMR (C6D6) δ 1.99 (s, 3 H), 2.41 (s, 3 H), 5.78
(t, J = 6.6 Hz, 1 H), 5.80 (s, 1 H), 6.38 (d, J = 8.4 Hz, 1 H), 6.56
(t, J = 7.5 Hz, 1 H), 7.50 (d, J = 5.7 Hz, 1 H); 13C NMR (C6D6)
δ 11.6, 12.8, 115.1, 115.7, 116.6, 123.5, 127.5, 138.2, 139.7,
141.4, 148.5; 19F NMR (C6D6) δ Ϫ81.506 (q, J (19F–11B) = 27.4
Hz). HRMS (EI) calcd. for C11H11N2BF2Na [(M ϩ Na)ϩ]
243.0868, found 243.0876. The solid-state structure was
determined using X-ray diffraction, see details below.
sample, and emission measurements were performed. The
quantum yield was determined relative to a standard (reference
solution was 1,1,4,4-tetraphenylbutadiene in cyclohexane).
(R2PyrPy)2Zn complexes. A solution of metal complex was
prepared of known concentration under nitrogen atmosphere,
in a glove box or glove bag. Prior to use, the cuvette was silated
(5% dimethyldichlorosilane in CCl4), and then dried in a 110 ЊC
oven for at least three hours prior to use. Into a dry silated
cuvette, a known volume (ca. 3 mL) of dry degassed THF was
added. Diethylzinc (25 µL, 1 M in heptane) was also added to
remove any adventitious water. The cuvette was sealed with
Teflon/silicone septa and cap. The cuvette was removed from
the inert atmosphere and blank absorbance and emission
measurements were taken. The cuvette was returned to the inert
atmosphere, and the stock solution was added (via syringe) to
prepare a known concentration solution in the cuvette. This
procedure was repeated to determine the molar absorptivity
and fluorescence quantum yield (reference solution was 1,1,4,4-
tetraphenylbutadiene in cyclohexane).
Absorbance and emission measurements
General. Molar absorptivity determinations were performed
with THF solutions of compounds 1–6 and I of known concen-
tration. Luminescence quantum yields in THF were determined
for the series of compounds, using 1,1,4,4-tetraphenylbutadiene
in cyclohexane (Φ = 0.60 at excitation wavelength equal to
365 nm)18 as a reference solution (eqn. (1)), where Φ is defined
as quantum yield, A = absorbance at λ, η = refractive index, F =
emission peak area resulting from excitation at λ, s = sample
solution, and r = reference solution.19 The quantum yield calcu-
lations employed the molar absorptivity and the slope of the
emission peak area vs. concentration plots (see ESI†) to avoid
small errors in individual measurements. Since the sample
and reference solutions were prepared in different solvents, an
index of refraction correction was performed (ηthf = 1.4050 and
ηcyhex = 1.4266).20 The calculated quantum yields for the
pyridylpyrrolide complexes are summarized in Table 1. Instru-
ment parameters: Absorbance, scan wavelength, 700–250 nm;
scan speed, 400 nm minϪ1; data pitch, 1.0 nm; slit width,
2.0 nm; luminescence, scan wavelength, 300–700 nm; scan
speed, 500 nm minϪ1; data pitch, 1 nm; excitation and emission
slit widths, 5.0 nm.
(Me2PyrPy)BF2, 6. For absorbance measurements, a solution
of known concentration in THF was prepared and the molar
absorptivity was determined. To determine the quantum yield,
a solution was prepared and purged with nitrogen to remove
oxygen for at least 20 min. The concentration was then deter-
mined by measuring the absorbance of the sample and emission
measurements were performed. The molar absorptivity and
quantum yield were determined (reference solution was 1,1,4,4-
tetraphenylbutadiene in cyclohexane).
(Me2PyrPy)Li, 7. The emission was measured for 7 in THF,
with 7 being synthesized in situ for the experiment. The same
procedure was followed for the zinc complexes except for the
following changes. Into THF (3 mL), 50.0 µL of filtered n-BuLi
(2.3 M) in hexanes was added. The blank emission and absorb-
ance measurements were taken, and a stock solution of I was
added sequentially to the cuvettes to generate 7. The absorb-
ance and emission measurements were taken for the series of
solutions.
Crystal structure determinations
(1)
Crystal data for compounds IV, 1, 4, and 6 were collected at
173(2) K using a Siemens SMART Platform CD diffractometer.
Data collection was carried out using Mo Kα radiation (graph-
ite monochrometer) with a frame time of 10 s, 30 s, 45 s, or 60 s
for 1, 4, IV, and 6, respectively. Frames were collected with
0.30Њ steps in ω at four different ꢀ settings and a detector pos-
ition of Ϫ28Њ in 2Φ. The intensity data were corrected for
absorption and decay (SADABS).21 Data was integrated
Me2PyrPyH, I. For absorbance measurements, a solution of
known concentration in THF was prepared and the molar
absorptivity was determined. To determine the fluorescence
quantum yield, a solution was prepared and purged with nitro-
gen to remove oxygen for at least 20 min. The concentration
was then determined by measuring the absorbance of the
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Table 2 X-ray crystallographic data for tBu2PyrPyH (IV), (Me2PyrPy)BF2 (6), (Me2PyrPy)2Zn (1) and (tBu2PyrPy)2Zn (4)
tBu2PyrPyH, IV
(Me2PyrPy)BF2, 6
(Me2PyrPy)2Zn, 1
(tBu2PyrPy)2Zn, 4
Formula
MW
Crystal system
Space group
a/Å
b/Å
c/Å
C17H24N2
256.38
Orthorhombic
Pna21
19.140(5)
11.221(3)
14.343(4)
C11H11N2BF2
220.03
C22H22N4Zn
407.81
C34H43.68N4Zn0.66
554.58
Monoclinic
P21/c
10.6415(14)
10.8583(15)
27.459(4)
Triclinic
Triclinic
¯
¯
P1
P1
7.145(4)
8.990(5)
9.300(5)
65.793(8)
89.589(9)
75.611(9)
524.7(5)
2
11.929(3)
12.286(3)
13.777(4)
81.694(5)
83.582(5)
88.531(5)
1985.3(9)
4
α/Њ
β/Њ
92.306(2)
γ/Њ
V/Å3
3086.9(13)
8
1.103
0.065
1120
3170.3(2)
4
1.162
0.0551
1194
Z
Dc/g cmϪ3
µ/mmϪ1
F(000)
1.393
0.108
228
1.364
1.250
848
Crystal colour
Ind. reflns.
Obsd. reflns.
Rint
Colourless
5435
4809
0.0424
1.035
0.0611
0.1556
Yellow
1845
1549
0.0243
1.012
0.0388
0.0910
Yellow
8957
7225
0.0251
1.025
0.0338
0.845
Yellow
5616
4471
0.0515
1.007
0.0564
0.1404
GOOF on F 2
R1 [I > 2σ(I )]
wR2 [I > 2σ(I )]
(SAINT).22 The structure was solved using SHELXS-9722 (IV,
4, and 6) or SIR-9223 (1) and refined using SHELXL-97.24
Crystallographic data and refinement details for IV, 1, 4, and 6
are found in Table 2. The refinements for IV, 1, and 6 were
straightforward with facile convergence of the proposed and
refined structures. The refinement of 4 proved to be more chal-
lenging. Due to partial occupancy of the zinc atom (0.66), the
structure was refined as the superposition of (tBu2PyrPy)2Zn
and the dimeric free ligand, (tBu2PyrPyH)2. The structure con-
verged as the occupancy of the Zn/H ratio became consistent
with the observed data. The refinement was heavily constrained
(127 constraints applied).
diethylzinc to a THF solution of two equiv. of free ligand
(eqn. (3)). Zinc complexes were prepared with pyrrole ligands
that have varying steric and electronic properties (Table 3).
Initial attempts to prepare pure zinc complexes proved to be
difficult, but subsequent attempts with diethylzinc solutions
that were purified by vacuum transfer led to good success. We
attribute this fact to the possible presence of chloride con-
tamination in the diethylzinc. The complexes can be purified by
either recrystallization or sublimation. Since both the pyridyl-
pyrroles and the zinc complexes sublime, the complex must be
free of the uncomplexed ligand to achieve purity through
sublimation. The compounds are moisture sensitive, with hydro-
lysis yielding free ligand and zinc hydroxide. The complexes
appear to be indefinitely stable in a nitrogen-filled dry box.
CCDC reference numbers 225455–225458.
See http://www.rsc.org/suppdata/dt/b3/b315523d/ for crystal-
lographic data in CIF or other electronic format.
(3)
Computational studies
Calculations for I, 1, and 6 were performed using the Gaussian
98 program suite.25 Hybrid density function theory (DFT)
optimizations of all molecules were carried out by means of
Becke’s Three Parameter Hybrid Functional using the corre-
lation functional of Lee, Yang, and Parr, B3LYP,26 with the
6-31G(d) basis set. Frequency calculations were carried out for
optimizations to verify the nature of the stationary point. Elec-
tronic excitation energies, molecular orbital transitions and
oscillatory strengths associated with the electronic transitions,
and molecular orbital electron densities were determined from
time-dependent DFT (TD-DFT) calculations.27 The TD-DFT
calculations were performed at the TD-B3LYP/6-31G(d) level
of theory starting from the optimized B3LYP/6-31G(d) ground
state geometries.
The (R2PyrPy)2Zn complexes are chiral, which is most clearly
seen in the 1H and 13C NMR spectra of (iPr2PyrPy)2Zn. In these
spectra, the four diastereotopic methyl groups appear as dis-
tinct signals. In the H NMR spectrum of (Et2PyrPy)2Zn, the
diastereotopic methylene proton resonances are not split
noticeably.
(Me2PyrPy)BF2, 6, was prepared by a modified literature
procedure (eqn. (4), Table 3).11 A THF solution was heated to
reflux with 1 equiv. each of boron trifluoride etherate, I, and
base (2,4,6-collidine) for 4 h. The product, (Me2PyrPy)BF2, 6,
was purified by chromatography followed by recrystallization
from hexanes, which yielded X-ray quality crystals. The yellow
solid obtained sublimes and is air stable.
1
Results and discussion
(4)
Synthesis
The R2PyrPy ligands can be synthesized using a one-step pro-
cedure, involving the condensation of 2-aminomethylpyridine
and 1,3-diketones (eqn. (2)).14 The wide commercial availability
of the suitable diketones makes this a good synthetic route. The
zinc complexes were synthesized by addition of one equiv. of
X-ray crystallography
tBu2PyrPyH, IV. X-ray quality crystals of IV were grown
from a saturated solution (hexanes) at Ϫ20 ЊC. The compound
crystallized as a dimer with two hydrogen bonds holding the
two ligands together in a pseudo-tetrahedral fashion (Fig. 2).
Placement of a centroid between the internal nitrogens shows
that the dimer is pseudo-tetrahedral with a twist angle, angle
between the two ligand planes, of 80.4Њ.
(2)
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Table 3 Complexes synthesized along with isolated % yields
Ligand
Complex
Isolated % yield
75
78
63
96
100
25
Fig. 3 ORTEP diagram of (Me2PyrPy)2Zn, 1, as determined by X-ray
diffraction. Only one of the two nearly identical molecules in the
asymmetric unit is shown.
found in bipyridine complexes (bite angle found is typically
between 75 and 80Њ).28 This is due primarily to the shorter
Zn–N bond lengths found in 1. The bond lengths were
1.9402(19) and 1.950(2) Å for the Zn–N(pyrrole) bond and
2.054(2) and 2.072(2) Å for the Zn–N(pyridine) bond. Zinc bi-
pyridine complexes commonly have bond lengths of approx-
imately 2.1 to 2.2 Å.28 Zinc complexes of 2-(2Ј-pyridyl)indole
(2-py-in) are structurally very similar (Zn–N(indole) 1.968(3) Å
and Zn–N(pyridine) 2.189(4) Å),11 as are zinc pyrrolide–imines
(Zn–N(pyrrole) 1.97 Å and Zn–N(imine) 2.05 Å).29
(Me2PyrPy)BF2, 6. X-ray quality crystals of 6 were grown
from a saturated solution (hexanes) at Ϫ20 ЊC. The boron com-
pound was found to be pseudotetrahedral around boron with a
bite angle of 96.93(12)Њ (Fig. 4). The boron nitrogen bond
lengths were found to be 1.527(2) Å (for B–N(pyrrole)) and
1.612(2) Å (for B–N(pyridine)). These structural parameters
are essentially identical to (2-py-in)BPh2, which has 1.55 Å
(B–N(indole)) and 1.63 Å (B–N(pyridine)) for bond lengths.
(2-py-in)BPh2 has a bite angle of 94.8Њ.11
Fig. 4 ORTEP diagram of (Me2PyrPy)BF2, 6, as determined by X-ray
diffraction.
(tBu2PyrPy)2Zn, 4. X-ray quality crystals of 4 were grown
through slow evaporation of solvent (hexanes) from a saturated
solution of 4, which contained some of the free ligand IV. Dur-
ing initial refinement, low electron density was observed at the
zinc center. With further refinement, the structure was success-
fully modeled with partial occupancy of the zinc atom (0.66).
The remainder of occupancy (0.34) was a dimeric structure of
free ligand IV that had co-crystallized with complex 4 (Fig. 5).
There was a large amount of disorder due to the partial occu-
pancy of the ligand and complex, and a large number of
restraints and constraints were needed for successful refine-
ment. The bite angle for 4 was found to be 83.4(5) and 83.2(3)Њ,
and the Zn–N bond lengths were found to be 1.966(11) and
Fig. 2 ORTEP diagram of (tBu2PyrPy)2H2, IV, as determined by
X-ray diffraction, shown with all carbon-bound hydrogen atoms
omitted. The compound exists as a doubly hydrogen bonded dimer in
the solid state. The hydrogen bond lengths, based upon the calculated
positions of H2A and H4A, are 2.964(3) (N2–H2A ؒ ؒ ؒ N3) and
3.019(3) (N4–H4 ؒ ؒ ؒ N1).
(Me2PyrPy)2Zn, 1. X-ray quality crystals of 1 were grown
from a saturated solution (diethyl ether) at Ϫ20 ЊC. The zinc
complex is pseudotetrahedral with two nearly identical mole-
cules in the asymmetric unit (Fig. 3). The bite angle of 1 was
found to be 82.67(8) and 83.65(8)Њ, which is larger than that
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Table 4 Selected bond lengths and bond angles for pyridylpyrroles and their complexes
Compound
Bite angle/Њ
Torsion angle/Њb
Twist angle/Њc
M–N(pyrrole)/Åe
M–N(pyridine)/Åe
1
4
83.54(7)
83.59(7)
83.4(5)
83.2(3)
93.93(12)
6.1(3) and 7.9(3)
2.3(3) and 7.7(3)
15.5(13)
20(4)
0.02(17)
57.0(3)
60.6(3)
33(3)
34(8)
81.8
88.8
1.9385(17)
1.9407(17)
1.966(11)
1.994(18)
1.527(2)
2.0591(18)
2.0705(18)
2.024(6)
2.044(5)
1.612(2)
6
IV
80.4d
76.0d
IVa
a As part of 4 crystal structure. b Angle defined as the N(pyrrole)–C–C–N(pyridine) that demonstrates the twist between the pyridine and pyrrole
rings within one ligand unit. c Defined as the angle between the two ligand planes. d Twist angle was calculated by placement of a centroid between
the 4 central nitrogen atoms. e M is Zn or B.
Fig. 5 ORTEP diagram of (tBu2PyrPy)2Zn, 4, as determined by X-ray diffraction. The zinc atom shows partial occupancy due to co-crystallization
of free ligand, IV, within the single crystal. The two parts are shown separately for clarity (a) metal complex (0.66 occupancy) and (b) free ligand
dimer (0.34 occupancy).
1.994(18) Å for Zn–N(pyrrole) and 2.024(6) and 2.044(5) Å for
Zn–N(pyridine) bonds (very similar to 1).
tBu2PyrPyH, IV, has been found to crystallize as a hydrogen-
bonded dimer. The disposition of the PyrPy groups in the struc-
ture of the free ligand and the zinc complex bear a striking
resemblance and this may account for the co-crystallization of
IV and 4.
The twist angle between the pyridylpyrrole ligand planes was
determined for 1 (81.8Њ) and 4 (88.8Њ), illustrating that both
molecules are pseudotetrahedral around zinc. For comparison
purposes, a centroid atom was placed between the four nitrogen
atoms for I and IV (both in the pure sample and as part of the
structure of 4). This placement allows for a calculation of a
twist angle for these compounds as well (Table 4). The calcu-
lated twist angle for I (51.5Њ) is much lower than IV (80.4Њ), as
can be observed in Fig. 6. However the twist angle for the
dimer, IV, (76.0Њ, as part of the structure of 4) in the combined
Fig. 6 ORTEP diagrams for (a) (Me2PyrPy)2H2, I, and (b) (tBu2-
PyrPy)2H2, IV, as determined by X-ray diffraction. The twist angle was
structure shows a slightly smaller angle than the pure sample,
which is most likely due to the forces present due to packing of
uncomplexed ligand with the zinc complex. Comparing the
twist angles of I and IV, the larger angle of IV probably results
from the steric effect from the tBu group, forcing a larger angle
between individual molecules (Fig. 6).
determined for each of the two dimers and found to be 51.5Њ and 80.4Њ
for I and IV, respectively.
(R2PyrPy)2Zn. Work done previously by Perry and Weber on
the unsubstituted pyridylpyrrole zinc complex, (PyrPy)2Zn,
reported absorbance maxima at 325 nm (ε=33000 MϪ1 cmϪ1
)
and 378 nm (ε =18800 MϪ1 cmϪ1), which is consistent with our
findings. The molar absorptivites observed by Perry are close to
the values found for complexes 1–4 in this study, however the
values at 378 nm are slightly lower. This is possibly due to
the instability of the zinc complexes and partial decay of the
complex into free ligand. Perry noted this instability as well by
stating that the zinc complexes are only stable in some solvents.9
The zinc complexes are much more luminescent than the free
ligands, which is readily apparent through casual observation.
Due to the water sensitivity of the complexes, the solutions for
Optical absorbance and luminescent properties
Me2PyrPyH, I. To make a comparison between the optical
properties of the free and bound ligand, absorbance and emis-
sion measurements of I were performed in THF. Compound
I has absorbance maxima at 300 (ε=14700 MϪ1 cmϪ1) and
325 nm (ε=16100 MϪ1 cmϪ1) and an emission maximum at
397 nm (ΦF(325 nm) = 0.06) in THF. The excitation profile
matches the absorbance spectrum of 1 supporting the assign-
ment of 1 as the emitting species (see ESI†).
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analytical measurement were prepared and stored in a dry
box. Further, cuvettes were silated to functionalize any surface
silanol groups. Even with these precautions, water contamin-
ation remained a problem. Absorbance and emission spectra
of all the complexes appeared to have a combination of
free ligand and zinc complex present. Therefore, 25 µL of
diethylzinc (1 M in heptane) was added to the blank solution to
remove adventitious water. This allowed for the rigorously
water-free conditions required for the measurements of absorb-
ance and emission. It should be noted that 4 appeared to be
much less sensitive to reaction with water, presumably due to
the increased steric bulk present on the ligand.
plexes, 1, 2, 3, and 4, are similar, with a decrease in molar
absorptivity observed with an increased amount of steric bulk.
The presence of phenyl substituents at the 3 and 5 positions
of the pyrrole ring in complex 5 results in significant differences
in the electronic absorption spectrum (Fig. 7). The increased
conjugation induces a red-shift of the low-energy band seen
at 395 nm in the alkyl substituted complexes to 405 nm. The
high-energy band observed at 335 nm in the alkyl substituted
complexes is also apparently shifted to the red to 350 nm
and diminished in intensity. A new intense absorbance band
appears below 300 nm.
The emission spectra were collected by excitation at each
complex’s absorbance maxima. The solutions for emission
measurements were prepared in the same manner as those
for absorbance. All five zinc complexes proved to have similar
emission properties, emitting one band with a maximum
at 490–496 nm (Table 1), consistent with emission from one
excited state.
The quantum yields of complexes 1, 2, and 3, are all approx-
imately 0.32, while complex 4 shows a diminished quantum
yield (0.19). The origin of the lower quantum yield for 4 is not
known, but is presumably attributable to the steric bulk of the
tert-butyl groups. The solid-state structures of 1 and 4 show
that the angle between the ligand rings, as measured by the
N–C–C–N torsion angle, is significantly larger in 4 (15–20Њ)
than in 1 (2–8Њ). The lower degree of coplanarity in 4 and the
resulting perturbation of the Franck–Condon factors may
lead to a decreased rate of fluorescence, an increased rate of
non-radiative relaxation, or both. The presence of phenyl
substituents in 5 results in an increase in quantum yield (0.57).
The presence of excess diethylzinc during fluorescence meas-
urements raises some concerns, which need to be addressed.
The presence of zinc hydroxide that would be generated from
reaction of Et2Zn and trace water was shown not to interfere in
the absorbance or emission measurements. The presence of
excess diethylzinc leads to the possibility that (R2PyrPy)ZnEt is
present resulting from exchange in the solution between
(R2PyrPy)2Zn and Et2Zn (eqn. (5)). We do not think this is a
concern for several reasons. First, the luminescence excitation
profile matches the absorbance spectrum of 1 supporting the
assignment of 1 as the emitting species (see ESI†). Second, pre-
vious studies on the alkylzinc halides have shown that the equi-
librium in question lies far to the left.30 Third, when a titration
1
was performed on Me2PyrPyH (monitored by H NMR), we
saw no evidence of alkylzinc bonds, based on 1H NMR shifts of
other alkylzinc compounds.31 Lastly, measurements performed
on (Me2PyrPy)2Zn, with both a small amount (5 µL) and a large
excess (30 µL) of 1 M diethylzinc shows that no change is
observed in the absorbance or emission spectrum.
(Me2PyrPy)BF2. The molar absorptivity and quantum yield
of (Me2PyrPy)BF2 (6) were determined in THF (Table 1).
Unlike the zinc complexes, 6 is air and water stable, and the
absorbance measurements were made in air. Oxygen was
removed for fluorescence measurements, but no other special
precautions, as needed by the zinc complexes, were taken. The
luminescence intensity was completely unaffected by the addi-
tion of 120 mM isoprene, a triplet quencher (see ESI†), and was
affected very little by the presence of O2. The luminescence
energy, the lack of quenching by isoprene, and the compu-
tational results all point toward the luminescence being fluor-
escence from the lowest excited singlet state.
The optical properties of (Me2PyrPy)2Zn, 1, and (Me2PyrPy)-
BF2, 6, are very similar. Both contain an absorption band
around 330 nm and another absorption band around 400 nm
(Fig. 8). Both compounds also have similar fluorescence
quantum yields, with 6 (0.22) having a lower quantum yield
than 1 (0.32).
(R2PyrPy)2Zn ϩ Et2Zn
2 (R2PyrPy)2ZnEt
(5)
Consider further that since Zn2ϩ is closed-shell, and the
absorbance and emission properties of the complexes are
believed to be completely ligand based,32 the trace presence of
(R2PyrPy)ZnEt should not perturb the measurements since the
concentration of “R2PyrPyϪ” is the important parameter. To
further test this idea, the emission properties of (Me2PyrPy)Li,
7, in THF were examined.
The complex, (Me2PyrPy)Li, was synthesized in situ, by addi-
tion of free ligand to a solution of n-BuLi (n-butyllithium) in
THF, and its emission was measured. The fluorescence emission
of 7 was found to be the same within error as the emission of 1,
strong evidence that the emission observed is ligand-based.
The molar absorptivity was determined for all of the
inorganic compounds and are presented in Table 1 (Beer’s Law
plots can be found as ESI†). Absorbance spectra for 1, 2, 3, and
4 were found to be very similar with two features around 335
and 395 nm (Fig. 7). The similarity is an indication that these
four complexes have very similar structures, despite their steric
differences. Further, the molar absorptivities for all four com-
Computational studies
To gain insight into the nature of the electronic transitions, we
performed density functional theory (DFT) and time-depend-
ent density functional theory (TD-DFT) calculations on the
free ligand, I, and on its Zn and B complexes, 1 and 6. These
calculations indicate that the Frontier Molecular Orbitals
(FMOs) are nearly identical for I, 1, and 6, in terms of their
contributions (i.e., coefficients and phase) from the ligand-
based atomic orbitals. While, the Zn and B complex FMOs are
nearly indistinguishable, B complex orbitals in 6 are much
easier to visualize because there is only one ligand per complex.
Fig. 9 illustrates the HOMO, LUMO and LUMOϩ1 for I and 6,
and the following discussion will focus on these two species. All
of the same arguments can be applied to the more difficult to
visualize Zn complex, 1.
The FMOs in both I and 6 are π symmetry orbitals, and so
the two main UV-visible absorbance features are best described
as π
π* transitions. The lowest energy absorbance for I and 6
is calculated to be a transition between the HOMO and a
Fig. 7 Absorbance spectra of (R2PyrPy)2Zn complexes in THF.
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energies (Table 1). These results indicate that the low energy
absorbance (∼400 nm) in 1 and 6 does not arise from a different
type of transition than the low energy (330 nm) absorbance of
the free ligand, such as n
π*. Rather, both are due to π
π*
transitions between essentially identical molecular orbitals, and
the difference in transition energies between I and its complexes
is due to changes in the relative energies of the FMOs. All of
the FMO energies decrease going from I to 6, but it is the
LUMO energy that is the most stabilized, causing the red shift
of the lowest energy absorbance. Inspection of the wave-
functions provides a rationalization for this greater stabiliz-
ation, since among the HOMO, LUMO, and LUMOϩ1, it is
the LUMO that has the largest coefficient on the B atom in 6
(Fig. 9).
Conclusions
A series of 3,5-substituted PyrPy complexes have been prepared
and three of these complexes have been structurally character-
ized by X-ray diffraction. These structures represent the first
structural characterization of metal-PyrPy complexes. The
(R2PyrPy)2Zn and (Me2PyrPy)BF2 complexes exhibit blue–
green luminescence with quantum yields ranging from 0.19 to
0.57, and may have applications in light emitting devices. Our
calculations suggest that the visible absorbance and emission
transitions are largely ligand-based, and therefore we expect
PyrPy complexes of other closed-shell metal centers to exhibit
similar optical properties. The PyrPyϪ ligand is isolobal and
roughly isosteric with cyclometalated 2-phenylpyridine ligands
and may offer an interesting alternative to these ligands, which
have received much recent attention due to their complexes’
luminescent properties.1
Acknowledgements
The authors thank the University of Minnesota, Department
of Chemistry, X-ray Crystallographic Laboratory, specifically
Mr Bill Brennessel, Dr Neil R. Brooks and Dr Victor G. Young,
Jr. for structural refinement of 1 and 6. Special acknowledge-
ment is given to Mr Bill Brennessel for extensive help in the
structural refinement of 4. The authors would also like to thank
Dr Dana Reed of the University of Minnesota, Department of
Chemistry, Mass Spectrometry Laboratory. The authors grate-
fully acknowledge the generosity of Jason Thompson, Prof.
Donald Truhlar, and Prof. Christopher Cramer for providing us
access to locally developed MN-ZN and MN-GSM programs,
as well as advice on the calculations herein. The Minnesota
Supercomputing Institute is acknowledged for computer time.
Acknowledgment goes to Mr Keith Jensen for help with the
synthesis of II and III. S.J.S. acknowledges the support of the
National Science Foundation for an NSF graduate fellowship.
The authors would also like to thank the University of
Minnesota for funding. Acknowledgement is made to the
Donors of the Petroleum Research fund, administered by the
American Chemical Society, for partial support of this
research. The National Science Foundation (CAREER Award,
Grant No. CHE-0239461) is also acknowledged for partial
support of this work.
Fig.
8
Absorbance and emission spectra of (Me2PyrPy)2Zn, 1,
(Me2PyrPy)BF2, 6, and Me2PyrPyH, I, in THF.
References
Fig. 9 Representations of the HOMO, LUMO, LUMOϩ1 orbitals
calculated using DFT and electronic transitions for compounds I and 6.
The HOMO energies are calculated ionization energies and the
transitions are labeled with the wavelengths of the experimentally
observed absorbance maxima.
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