Synthesis, structure, and electronic calculations of group VII substituted pyridazines

Article abstract of DOI:10.1007/s11243-013-9752-2

A series of 5,6-fused ring cyclopentadienyl tricarbonyl manganese and rhenium complexes, [M(CO)₃{η ⁵-1,2-C₅H ₃(1,4-(R)₂N₂C₂}] (2a-3d) were isolated by employing an off-metal ring

Full text of DOI:10.1007/s11243-013-9752-2

Transition Met Chem (2013) 38:801–809
DOI 10.1007/s11243-013-9752-2
Synthesis, structure, and electronic calculations
of group VII substituted pyridazines
•
C. A. Snyder N. C. Tice J. B. Maddox J. O. E. Young M. G. Mazzotta
•
•
•
•
•
•
•
•
•
B. D. Garabato J. W. Evans L. A. Dopierala S. A. Shah Z. A. Claytor
A. C. Smith
Received: 3 June 2013 / Accepted: 26 July 2013 / Published online: 10 August 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A series of 5,6-fused ring cyclopentadienyl
tricarbonyl manganese and rhenium complexes, [M(CO)3-
{g5-1,2-C5H3(1,4-(R)2N2C2}] (2a–3d) were isolated by
employing an off-metal ring closure route. Reacting thal-
lium cyclopentadienide (Cp) salts (1a–d) with [MBr(CO)5]
(M = Mn, Re) provided pyridazyl complexes (2a–3d) in
high yield (75–99 %). Spectroscopic characterization
(NMR, IR, MS) confirmed the identity of the desired
organometallic pyridazines. The off-metal synthetic path-
way employed did improve upon the isolation of these
complexes as compared to previously reported routes. The
molecular and electronic structure of complexes 2a–3d and
their optimal energy structures have been characterized
with quantum chemistry calculations. Vibrational fre-
quencies calculated were compared to their experimental
counterparts. The excited state calculations predict that the
dominant low-energy transition involves a ligand-to-metal
charge transfer.
Introduction
Pyridazines and their derivative compounds have long been
of interest in material applications due to their air and tem-
perature stability [1]. Pyridazine compounds have been uti-
lized in water reduction catalysis [2], the storage of hydrogen
[3], chemiresistor sensors in TNT and DNT detection [4], as
well as in the mimesis of a-helices [5]. In the context of our
current investigation, we are primarily interested in the for-
mation of organometallic pyridazines as building blocks for
novel electronic devices (e.g., polymeric semiconductors).
Devices made from these types of molecular materials offer
the advantages of discounted production cost, increased
processibility, as well as application to flexible substrates [6–
8]. These unique properties are exemplified in commercial
applications like Organic Light Emitting Diodes, Organic
Photovoltaic Cells, and Organic Resistors [9–12]. The
incorporation of metal moieties into these novel compounds
is of additional interest, as the inclusion of a transition metal
center allows for the ability to ‘‘tune’’ the electronic prop-
erties of the bulk material using the intrinsic redox properties
of the metal. Specifically, Group VII metals exhibit a wide
range of stable oxidation states (-1 through ?7) and thus
represent attractive, redox active candidates for incorpora-
tion. In addition, some rhenium (I) complexes have already
been reported to possess luminescent properties, demon-
strating their potential for electronic device applications.
[13] Our recent research involves the synthesis and charac-
terization of several organometallic 5,6-fused ring pyrida-
zines. The synthetic route selected for the formation of our
current pyridazyl series follows an off-metal ring closure of
1,2-diacylcyclopentadienes (fulvenes), as previously repor-
ted [14]. This pathway (Route 2) was employed, as opposed
to the on-metal route pathway we previously investigated
(Route 1) [15], due to the low yields and difficulties in
C. A. Snyder (&) Á J. B. Maddox Á J. O. E. Young Á
B. D. Garabato Á J. W. Evans Á L. A. Dopierala Á
S. A. Shah Á Z. A. Claytor Á A. C. Smith
Department of Chemistry, Western Kentucky University, 1906
College Heights Blvd., Bowling Green, KY 42101, USA
e-mail: chad.snyder@wku.edu
N. C. Tice
Department of Chemistry, Butler University, 4600 Sunset
Avenue, Indianapolis, IN 46208, USA
M. G. Mazzotta
Department of Chemistry, Purdue University, 560 Oval Drive,
West Lafayette, IN 47907, USA
123

802
Transition Met Chem (2013) 38:801–809
Scheme 1 Comparison of on-
and off-metal routes of
complexes 2–3
Synthesis of [Mn(CO)3{g5-1,2-
C5H3(CC6H4BrN)(CC6H4BrN)}] (2a)
isolation of some cases (Scheme 1). Thus, we hoped to not
only expand the library of various substituted pyridazyl
complexes available but to compare which of these two
routes was the most facile in isolating the desired pyrida-
zines. Herein is presented the synthesis, characterization,
and electronic studies of various organometallic pyridazyl
complexes.
In a 100-mL Schlenk flask, [MnBr(CO)5] (94.3 mg,
0.344 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (1a) (200 mg, 0.313 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
afforded 2a (134 mg, 0.238 mmol, 75.7 %) as a yellow
Experimental
1
powder. Mp: 135–157 °C. H NMR (500 MHz, acetone-
General procedures
3
d6, ppm): d 6.93 (t, 1H, J = 2.9 Hz, CHCHCH), 7.30 (d,
1H, J = 2.9 Hz, CHCHCH), 7.76 (d, 1H, J = 7.4 Hz,
3
3
All reactions were carried out using standard Schlenk
techniques under a nitrogen atmosphere unless otherwise
noted. NMR solvents acetone-d6, DMSO-d6, and CDCl3
(Aldrich) were used without further purification. [MnBr
(CO)5] and [ReBr(CO)5] (Strem) were used without further
purification. All thallium Cp salt precursors were prepared
according to literature methods [16–19]. Benzene was
dried over sodium benzophenone ketyl.
3
CHCHCBr), 7.95 (d, 1H, J = 7.4 Hz, CHCHCBr). 13C
NMR (125 MHz, acetone-d6, ppm): d 107.7 (CHCHCH),
120.4 (CHCHCH), 128.3, 130.8, 132.0, 132.5 (Ar), 154.7
(CN), 202.5 (MnCO). IR (KBr, cm21): 1,522 (C = N),
1,934, 2,027 (MnCO). MS(EI-pos): m/z 566 (M?). Anal.
Calcd. for C22H11N2Br2O3Mn: C, 46.7; H, 1.9; N, 4.9.
Found: C, 46.2; H, 1.9; N, 4.9.
1H and 13C NMR spectra were recorded on a JEOL-
500 MHz NMR spectrometer at ca. 22 °C and were ref-
erenced to residual solvent peaks. All 13C NMR spectra
were listed as decoupled. Infrared spectra were recorded on
a Spectrum One FT-IR Spectrometer. Electron ionization
(EI) mass spectra were recorded at 70 eV on a Varian
500-MS Series LC/MS ion trap at Western Kentucky
University’s Advanced Materials Institute. Melting points
were taken on a standard Mel-Temp apparatus. Elemental
analyses were performed at Western Kentucky University’s
Advanced Materials Institute.
Synthesis of [Re(CO)3{g5-1,2-
C5H3(CC6H4BrN)(CC6H4BrN)}] (3a)
In a 100-mL Schlenk flask, [ReBr(CO)5] (314 mg,
0.772 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (1a) (447 mg, 0.702 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
123

Transition Met Chem (2013) 38:801–809
803
Synthesis of [Mn(CO)3{g5-1,2-
C5H3(CC6H4CH3N)(CC6H4CH3N)}] (2c)
afforded 3a (388 mg, 0.554 mmol, 79.0 %) as a yellow
1
powder. Mp: 157–162 °C. H NMR (500 MHz, acetone-
3
d6, ppm): d 6.95 (d, 1H, J = 3.4 Hz, CHCHCH), 7.41 (t,
1H, J = 3.4 Hz, CHCHCH), 7.80 (d, 1H, J = 8.0 Hz,
3
3
In a 100-mL Schlenk flask, [MnBr(CO)5] (300 mg,
1.10 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (1c) (500 mg, 0.996 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
afforded 2c (324.9 mg, 0.745 mmol, 74.8 %) as a yellow
3
CHCHCBr), 7.99 (d, 1H, J = 8.0 Hz, CHCHCBr). 13C
NMR (125 MHz, DMSO-d6, ppm): d 107.7 (CHCHCH),
120.5 (CHCHCH), 130.7 (CCHCH), 132.0, 132.5 (Ph),
154.0 (CN) 179.2 (ReCO). IR (KBr, cm21): 1,589 (CN),
3,095 (CH), 2,003, 2,065 (ReCO). MS(EI-pos): m/z 698
(M?). Anal. Calcd. for C22H11N2Br2O3Re: C, 37.9; H, 1.6;
N, 4.0. Found: C, 38.3; H, 1.7; N, 3.9.
1
powder. Mp: 192–207 °C. H NMR (500 MHz, acetone-
Synthesis of [Mn(CO)3{g5-1,2-
C5H3(CC6H4ClN)(CC6H4ClN)}] (2b)
d6, ppm): d 2.57 (s, 1H, CHCHCCH3), 6.02 (d, 2H,
3J = 2.9 Hz, CHCHCH), 6.36 (t, 1H, 3J = 2.9 Hz,
3
CHCHCH), 6.55 (d, 2H, J = 7.5 Hz, CHCHCCH3), 7.45
3
(d, 2H, J = 7.5 Hz, CHCHCCH3). 13C NMR (125 MHz,
In a 100-mL Schlenk flask, [MnBr(CO)5] (186 mg,
0.458 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (1b) (500 mg, 0.923 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
afforded 2b (365 mg, 0.673 mmol, 83.0 %) as a yellow
DMSO-d6, ppm): d 24.5 (CH3), 101.5 (CHCHCH), 120.0
(CHCHCH), 121.3 (CCHCH), 130.3, 132.6 (Ph), 153.1
(CN), 218.0 (MnCO). IR (Nujol, cm-1): 1,601 (C = N),
1,929, 2,063 (MnCO). MS(EI):m/z 436 (M?). Analysis
Calc. for C24H17O3N2Mn: C, 66.1; H, 3.9. Found: C, 65.9;
H, 4.2.
Synthesis of [Re(CO)3{g5-1,2-
C5H3(CC6H4CH3N)(CC6H4CH3N)}] (3c)
1
powder. Mp: 189–194 °C. H NMR (500 MHz, CDCl3,
ppm): d 6.95 (br s, 1H, Cp), 7.39 (br s, 2H, Cp), 7.64–8.04
(m, 4H, Ar). 13C NMR (125 MHz, CDCl3, ppm): d 92.4
(CHCHCH), 109.2 (CHCHCH), 121.0 (CC), 129.5, 129.9,
133.3, 137.4 (Ar), 156.5 (CN), 221.8 (MnCO). IR (KBr,
cm21): 1,597 (C = N), 1,949, 2,032 (MnCO). MS(EI):
m/z 477 (M?). Anal. Calcd. for C22H11O3N2Cl2Mn: C,
55.3; H, 2.3; N, 5.9. Found: C, 60.8; H, 2.7; N, 6.6.
In a 100-mL Schlenk flask, [ReBr(CO)5] (186 mg,0.458 mmol)
was added to a stirred suspension of [Tl{1,2-C5H3(1,4-(4-
Br)C6H4)2N2C2}] (1c) (200 mg, 0.313 mmol) in 40 mL of dry
benzene. The solution was then allowed to reflux gently for 3 h.
The reaction mixture was cooled and passed through a thin pad
(1 cm) of Celite, and the volatiles were removed in vacuo.
Trituration with cold hexane afforded 3c (223 mg, 0.444 mol,
Synthesis of [Re(CO)3{g5-1,2-
C5H3(CC6H4ClN)(CC6H4ClN)}] (3b)
1
99.0 %) as a yellow powder. Mp: 182–187 °C. H NMR
(500 MHz, acetone-d6, ppm): d 2.42 (s, 1H, CHCHCCH3),
5.97 (d, 2H, 3J = 2.9 Hz, CHCHCH), 6.25 (t, 1H, 3J = 2.9 Hz,
CHCHCH), 6.63 (d, 2H, 3J = 7.5 Hz, CHCHCCH3), 7.25 (d,
2H, 3J = 7.5 Hz, CHCHCCH3). 13C NMR (125 MHz,
DMSO-d6, ppm): d 23.7 (CH3), 100.2 (CHCHCH), 118.0
(CHCHCH), 122.7 (CCHCH), 129.7, 131.6 (Ph), 150.0 (CN),
183.4 (ReCO). IR (Nujol, cm-1): 1,644 (C = N), 1,972, 2,052
(ReCO). MS(EI): m/z 568 (M?). Analysis Calc. for
C24H17O3N2Re: C, 50.8; H, 3.0. Found: C, 52.7; H, 4.3.
In a 100-mL Schlenk flask, [ReBr(CO)5] (206 mg,
0.507 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (1b) (250 mg, 0.461 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
afforded 3b (156 mg, 0.382 mmol, 83.0 %) as a yellow
powder. Mp: 162–167 °C (dec). 1H NMR (500 MHz,
Synthesis of [Mn(CO)3{g5-1,2-
C5H3(CC6H4OCH3N)(CC6H4OCH3N)}] (2d)
3
CDCl3, ppm): d 7.05 (d, 2H, J = 4.0 Hz, CHCHCH),
7.48 (d, 4H, 3J = 8.0 Hz, CHCHCl), 7.54 (t, 1H,
3J = 4.0 Hz, CHCHCH), 7.83 (d, 4H, 3J = 8.0 Hz,
CHCHCl). 13C NMR (125 MHz, CDCl3, ppm): d 109.3
(CHCHCH), 120.4 (CHCHCH), 129.4, 129.9, 133.4 (Ar),
176.2 (CN), 178.3 (ReCO). IR (KBr, cm21): 1,633
In
a 100-mL Schlenk flask, [MnBr(CO)5] (382 mg,
1.39 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-OCH3)C6H4)2N2C2}] (1d) (978 mg, 1.95 mmol)
in 60 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the volatiles
(C = N), 1,904, 2,028 (ReCO). MS(EI): m/z 338 (M?
-
Re(CO)3). Anal. Calcd. for C22H11O3N2Cl2Re: C, 43.4; H,
1.8; N, 4.6. Found: C, 31.7; H, 1.8; N, 2.3.
123

804
Transition Met Chem (2013) 38:801–809
were removed in vacuo. Trituration with cold hexane afforded
2d (567 mg, 1.21 mmol 86.3 %) as a red powder. Mp:
1
160–170 °C. H NMR (500 MHz, CDCl3, ppm): d 5.29-
5.39 (m, 3H, Cp), 5.52 (s, 6H, OCH3), 7.53 (d, 4H,
3J = 7.3 Hz, Ar), 7.89 (d, 4H, 3J = 7.30 Hz, Ar). 13C NMR
(125 MHz, CDCl3, ppm): d 73.7 (OCH3), 92.5 (CHCHCH),
109.1 (CHCHCH), 120.0 (CC), 129.5, 129.9, 133.4, 137.5
(Ar), 157.6 (CN), 220.1 (MnCO). IR (KBr, cm21): 1,608
(C = N), 1,932, 2,027 (MnCO). MS(EI): m/z 468 (M?).
Anal. Calcd. for C24H17O5N2Mn: C, 61.5; H, 3.6. Found: C,
60.9; H, 3.3.
Synthesis of [Re(CO)3{g5-1,2-
C5H3(CC6H4OCH3N)(CC6H4OCH3N)}] (3d)
Scheme 2 Formation of complexes 2a–3d via thallium Cp salts 1
In a 100-mL Schlenk flask, [ReBr(CO)5] (186 mg,
0.458 mmol) was added to a stirred suspension of [Tl{1,2-
C5H3(1,4-(4-Br)C6H4)2N2C2}] (2d) (200 mg, 0.313 mmol)
in 40 mL of dry benzene. The solution was then allowed to
reflux gently for 3 h. The reaction mixture was cooled and
passed through a thin pad (1 cm) of Celite, and the vola-
tiles were removed in vacuo. Trituration with cold hexane
afforded 3d (228 mg, 0.369 mmol, 78.9 %) as a yellow
involved adding a slight excess (1.1–1.5 mmol) of metal
carbonyl bromide ([M(CO)5Br], M = Mn or Re) to a
stirred solution of thallium Cp salt 1 in dry benzene. Each
reaction solution was stirred under a gentle reflux for 3 h,
cooled, filtered with Celite, and the organics were removed
in vacuo. The crude product was triturated with hexane to
yield complexes 2 and 3 in high yields, ranging from 75 to
99 %. Both the yields of manganese and rhenium com-
plexes were comparable to one another, owing to their
similar reactivity and stability. By comparison to the on-
metal route, the percent yields for the target complex for-
mation in the off-metal synthesis were improved by up to
23 % (2b). We also observed a modest increase in yield for
complex 2d (10 %). Conversely, there were only minor
increases in percent yield for the rhenium cases 3b and 3d.
Higher analytical purity was obtained for some target
complexes. For example, elemental analysis for 3d were
significantly improved over the previously reported on-
metal route [15]. The newly reported complexes 2a, 3a, 2c,
and 3c each displayed a high degree of analytical purity
with 3c a minimal exception. We conclude that this
observation is still likely the result of the lability on the
pyridazyl ligand observed in other cases [15, 20]. Overall,
the analytical purities were greatly improved employing
the off-metal closure route (Scheme 2).
1
powder. Mp: 212-221 °C. H NMR (500 MHz, CDCl3,
3
ppm): d 3.90 (s, 6H, OCH3), 7.00 (d, 4H, J = 8.6 Hz,
NCCCH), 7.06 (d, 2H, J = 4.0 Hz, CHCHCH), 7.48 (t,
3
3
3
1H, J = 4.0 Hz, CHCHCH), 7.83 (d, 4H, J = 8.6 Hz,
CHCHCO), 7.92 (d, 4H, 3J = 8.6 Hz, CHCHCO). 13C
NMR (125 MHz, CDCl3, ppm): d 55.4 (OCH3), 108.9
(CHCHCH), 114.4 (CHCHCH), 120.3, 130.0, 132.0 (Ar),
161.1 (CN), 186.4 (ReCO). IR (KBr, cm21): 1,607
(C = N), 1,900, 2,014 (ReCO). MS(EI): m/z 602 (M?),
330 (M? - Re(CO)3). Anal. Calcd. for C24H17O5N2Re: C,
48.1; H, 2.9. Found: C, 49.0; H, 2.6.
Results and discussion
Synthesis and spectroscopic characterization
We have previously reported the on-metal synthesis (Route
1, Scheme 1) of a variety of group VII pyridazyl com-
plexes [15]. In an effort to expand the number of available
complexes and improve the percent yields and purity of
these group VII pyridazyl complexes, we investigated the
off-metal pathway (Route 2) for complexes 2a–3d. The
off-metal route differs from the on-metal by transmetalling
with manganese or rhenium after the diacyl ring has been
closed to the pyridazine. In a modification of a procedure
by Wallace and Selegue [18], the thallium Cp salts 1 were
generated by reaction of the free pyridazines (Scheme 1)
with thallium (I) ethoxide. The general synthetic procedure
for forming the target pyridazyl complexes 2 and 3
Manganese complexes 2a-d and rhenium complexes 3a-
1
d were characterized using H NMR, 13C NMR, FT-IR,
MS, and elemental analysis. Relevant spectroscopic data
for all these complexes are reported here.
Table 1 shows pertinent Cp proton and carbon signals
relevant to the characterization of complexes 2a–3d. The
1H NMR spectrum for [Mn(CO)3{g5-1,2-C5H3(CC6H4BrN)-
(CC6H4BrN)}] (2a) showed the expected doublet (7.30 ppm,
J = 2.9 Hz) and triplet (6.93 ppm, J = 2.9 Hz), which
corresponds to the Cp protons. These values match previ-
ously reported Cp chemical shifts, as well as corresponding
123

Transition Met Chem (2013) 38:801–809
805
1
Table 1 Selected H and 13C NMR data for complexes 2a–3d
Complex
Ha
Hb
Ca
Cb
Cc
Cd
3
6.93 (t, J = 2.9 Hz)
3
7.30 (d, J = 2.9 Hz)
2a (M = Mn)
2b (M = Mn)
2c (M = Mn)
2d (M = Mn)
3a (M = Re)
3b (M = Re)
3c (M = Re)
3d (M = Re)
107.7
92.4
120.4
109.2
120.0
109.1
120.5
120.4
118.0
114.4
132.5
156.5
153.1
157.6
154.0
176.2
150.0
161.1
202.5
221.8
218.0
220.1
179.2
178.3
183.4
186.4
6.95 (br s)
3
6.36 (t, J = 2.9 Hz)
7.39 (br s)
3
6.02 (d, J = 2.9 Hz)
101.5
92.5
5.29–5.39 (m, 3H, Cp)
3
7.41 (t, J = 3.4 Hz
3
7.54 (t, J = 4.0 Hz
3
6.25 (t, J = 2.9 Hz)
3
7.48 (t, J = 4.0 Hz)
–
3
6.95 (d, J = 3.4 Hz)
3
7.05 (d, J = 4.0 Hz)
3
5.97 (d, J = 2.9 Hz)
3
7.06 (d, J = 4.0 Hz)
107.7
109.3
100.2
108.9
to 2c (doublet at 7.30 ppm, J = 2.9 Hz and triplet 6.93 ppm,
J = 2.9 Hz) [15]. Alternatively, manganese complex 2b
displayed two broad singlets at 6.95 and 7.39 ppm, owing to
its Cp’s triplet and doublet, respectively. Additionally, we
observed a multiplet (in the range 5.29–5.39 ppm) that
integrated to three protons for its Cp ring. Manganese
complex 2d also showed a singlet at 5.52 ppm as a result of
the methoxy group, while the rhenium complex 3d displayed
its methoxy proton signal higher upfield at 3.90 ppm. As for
rhenium complexes 3a–d their spectra displayed the char-
acteristic Cp doublet (ranging from 5.97 to 7.06 ppm) and
triplet (ranging from 6.25 to 7.48 ppm). The average cou-
pling constant for these Cp protons was 3.6 Hz, which was
higher as compared to the average manganese Cp coupling
constant (2.9 Hz). For Mn complexes 2a, 2c, and 2d, the
monosubstituted phenyl rings were observed as two dou-
blets, ranging from 6.55–8.04 ppm. The average coupling
constant for these complexes was 7.4 Hz. However, the
chlorophenyl manganese complex 2b displayed a multiplet
from 7.64 to 8.04 ppm, but integrated to eight protons.
Alternatively, two phenyl proton doublets were observed in
each of the rhenium complexes having ranges of
6.63–7.83 ppm and 7.25–7.99 ppm (average J = 8 Hz).
The 13C NMR spectra for all manganese complexes 2a,
2b, 2c, and 2d showed their Cp carbons, CHCHCH and
CHCHCH, within the range of 92.4–107.7 ppm and
109.1–120.4 ppm, respectively (Table 1). Signals associ-
ated with the CHCHCH and CHCHCH shifts were similar
in the rhenium cases, having values ranging between 100.2
and 109.3 ppm and 114.4–120.5 ppm, respectively. With
the exception of rhenium complex 3b (rhenium chloro-
phenyl complex), all organometallic complexes reported
observed the C=N carbon chemical shifts between 150.0
Table 2 Selected IR stretches for complexes 2a–3d
Complex
CN (cm-1
)
M(CO) (cm-1
)
Exp.
Theory
2a
2b
2c
2d
3a
3b
3c
3d
1,522
1,597
1,601
1,608
1,589
1,633
1,644
1,607
1,934, 2,027
1,949, 2,032
1,929, 2,063
1,932, 2,027
2,003, 2,065
1,904, 2,028
1,972, 2,052
1,900, 2,014
1,864, 1,936
1,865, 1,937
1,861, 1,934
1,860, 1,934
1,871, 1,934
1,871, 1,934
1,867, 1,931
1,870, 1,930
and 161.1 ppm, as expected (3b C=N * 176 cm-1).
Finally, all Mn–CO carbon signals were found downfield
(as previously reported) within the range of
202.5–221.8 ppm, whereas the Re–CO carbon signals were
shifted more modestly upfield (178.3–189.9 ppm). This is
in the range of previously reported Re–CO carbon shifts,
which is likely the result of greater shielding from the 5d
metal [16, 17, 19]. The IR spectroscopy (Table 2) and MS
analysis also supported the formation of manganese and
rhenium complexes 2a–3d. The C=N stretch was observed
between 1,522 and 1,644 cm-1, with M–CO stretches
observed in the expected ranges of 1,900–2,065 cm-1
.
Each MS confirmed the molecular weight of each manga-
nese (2a, M? = 566 m/z; 2b, M? = 477 m/z; 2c,
M? = 436 m/z; 2d, M? = 468 m/z) and rhenium complex
(3a, M? = 698 m/z; 3c, M? = 568 m/z; 3d, M? = 602 m/
z). The rhenium chlorophenyl complex 3b was the only
case where an M? signal was not observed in its MS.
Instead, a signal was observed at 338 m/z, corresponding to
123

806
Transition Met Chem (2013) 38:801–809
Fig. 1 Electron density
isosurfaces mapped with
electrostatic potentials for the
rhenium complexes 3a–3d.
Yellow and red regions indicate
electronegative regions of the
molecule, while light blue and
blue regions are electropositive.
(Color figure online)
Fig. 2 (Upper left) Calculated
wavelength versus oscillator
strength for the optimized
manganese complexes (blue)
2a, (red) 2b, (yellow) 2c, and
(green solid) 2d. A modified
conformer of compound 2d is
shown with green dashed lines.
(Upper right) Calculated
electronic transitions for the
rhenium complexes 3a–3d.
(Lower left) NTO hole orbital
for the dominant low-energy
electronic transition at 445 nm
in compound 3a. (Lower right)
Corresponding NTO particle
orbital. (Color figure online)
the M?– Re(CO)3 signal. Finally, we attribute our inability
to obtain analytically pure samples of complexes 2b, d and
3c, d to the lability of the pyridazine ligand [20]. Elemental
analysis consistently gave higher than expected carbon
values, consistent with the presence of a small amount of
free pyridazine [20].
using the Gaussian 09 quantum chemistry software [21].
Molecular geometry optimizations and normal mode cal-
culations were carried out to obtain gas phase minimum
energy molecular structures. These computations employed
a model chemistry based on the Perdew-Burke-Ernzerhof
(PBE) exchange–correlation functional [22, 23] and the
Los Alamos double-zeta (lanl2dz) basis set [24–26]. The
latter uses the Dunning/Huzinaga double zeta basis set [27]
on light atoms and replaces core electrons on manganese
and rhenium atoms with parameterized pseudopotential
functions. It was found that the rhenium complexes (3a–
3d) required tighter optimization cut-offs and an ultrafine
Electronic structure characterization
The molecular and electronic structures of complexes 2a–
3d have been characterized with density functional theory
(DFT) and time-dependent DFT (TD-DFT) calculations
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Transition Met Chem (2013) 38:801–809
807
Fig. 3 (Left) Hole and particle
NTOs and electronic difference
density for the dominant low-
energy electronic excitation of
compound 2a. (Right) NTOs
and difference density for the
dominant transition of
2a hole
3a hole
compound 3a
2a particle
3a particle
2a difference density
3a difference density
DFT integration grid in order to resolve the minimum
energy conformation.
compound and corresponds to delocalized stretching and dis-
tortion modes over all planar rings. Several of these modes
include atomic displacements between carbon and nitrogen
(i.e., C=N stretching); however, unique assignment of these
modes to individual groups is not tractable.
Electron density surfaces and electrostatic potential maps
are shown in Fig. 1 for the optimized rhenium compounds
3a–3d. Each compound shows a region of negative potential
(yellow-to-red) concentrated near the two nitrogen atoms of
the pyridazine rings. Likewise, all compounds exhibit
slightly positive potential around the aromatic hydrogen
atoms along the molecules’ periphery. The methoxy oxygen
atom in compound 3d and the oxygen atoms on the carbonyl
ligands for all compounds, which are not visible in Fig. 1, are
also somewhat electronegative, as expected. The electron
densities and electrostatic potential maps for the manganese
complexes 2a–2d exhibit similar features.
We have also characterized several electronic transitions
for all compounds using TD-DFT. The left and right panels of
Fig. 2 showthe calculatedtransitionenergiesversusoscillator
strengths (vertical colored lines) for the optimized molecular
geometries of compounds 2a–2d and 3a–3d, respectively.
Each species is predicted to have one dominant electronic
transition with large oscillator strength at wavelengths
[400 nm. For the manganese complexes 2a, 2b, and 2c, this
transition is found in the range of 472–488 nm. However, the
calculated wavelength for the methoxy derivative 2d is
somewhat larger at 552 nm. The rhenium complexes follow a
similar trend:thepredictedwavelengthsfor thedominantlow-
energy electronic transition in compounds 3a, 3b, and 3c
occur in the range of 429–445 nm, while the dominant tran-
sition for compound 3d is predicted at 490 nm.
Normal mode frequency analysis confirmed that the opti-
mized molecular structures were all local minima. Each com-
pound displays a trio of predicted M(CO)3 stretching modes
with large IR activities. These include a symmetric stretch
around 1,933–1,936 cm-1 and a pair of closely spaced asym-
metric stretching modes in the ranges of 1,856–1,868 cm-1 and
1,864–1,873 cm-1. We assign the symmetric stretch and the
closely spaced pair to the two observed peaks discussed in the
experimental section. The average frequency of the asymmetric
pair and the frequency of symmetric stretch for compounds 2a–
3d are reported in Table 2. The calculations also predict that all
compounds exhibit another vibrational mode with large IR
activity in the range of 1,596–1,620 cm-1. This vibration can
be assigned to phenyl ring stretching in an out-of-phase fashion
with respect to the two rings. Each compound exhibits a series
of vibrational modes in the range of 1,350–1,500 cm-1; the
structure of these modes varies strongly from compound to
A natural transition orbital (NTO) and difference density
analysis of the low-energy electronic transition was carried
out for all compounds [28].1 There are no significant differ-
ences in the hole-particle orbital pairs or difference densities
among the different analogs within the set of manganese and
rhenium compounds. The left panels of Fig. 3 show the hole
orbital, particle orbital, and difference density, respectively,
1
NTO analysis is a useful tool for decomposing a given electronic
transition in terms of hole-particle orbital pairs that assist with
identifying the nature of the electronic transition.
123

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Transition Met Chem (2013) 38:801–809
Acknowledgments We would like to acknowledge our sources of
financial support, including Western Kentucky University’s Chemis-
try Department, their Office of Sponsored Programs, the WKU Fac-
ulty-Undergraduate Student Engagement (FUSE) grant, the WKU
Research and Creative Activities Program (RCAP) grant, and the
WKU Advanced Materials Institute.
for the dominant low-energy electronic transition in com-
pound 2a. The pattern exhibited by these NTOs is typical of
the results obtained for all of the other manganese complexes.
The hole orbital (top left) exhibits a characteristic p-bonding
pattern and is delocalized over the pyridazine and phenyl
rings. Wenotethatthe fulvene ringand the Mn(CO)3 group do
not significantly contribute to the electron density of the hole.
The corresponding particle orbital (middle left) mainly
involves electron density on the manganese atom and some
anti-bonding character within the pyridazne and phenyl rings
as well as on the carbonyl ligands. Again, the fulvene group
does not appear to strongly participate in the particle orbital.
Based on these observations, we attribute the low-energy
electronic transition in complexes 2a–3d to ligand-to-metal
charge transfer (LMCT). The difference density (bottom left)
provides additional confirmation; the blue portion represents
the density of the hole, while the purple portion represents that
of the particle. The difference density clearly shows that the
excitation involves charge transfer from the phenyl rings to
Mn(CO)3 group.
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ongoing research efforts will focus upon the identification
and isolation of these metal–pyridazine systems that dem-
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¨
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