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Article

Synthesis and Characterization of Acyclic and Cyclic Azabridged Ligands Incorporating 2,2'-Bipyridine Subunits and Their Complexes With Copper(II), Cobalt(II), and Nickel(II)

Department of Physical and Chemical Methodologies for Engineering, Engineering Faculty, University of Catania, V.le Andrea Doria 6, I-95125 Catania, Italy
*
Author to whom correspondence should be addressed.
Molecules 2003, 8(7), 521-535; https://doi.org/10.3390/80700521
Submission received: 12 December 2002 / Revised: 14 June 2003 / Accepted: 16 June 2003 / Published: 15 July 2003

Abstract

:
The synthesis of a series of N,N'-disubstituted acyclic (AL) and cyclic (CL) aza-bridged ligands incorporating 2,2-pipryidine subunits is described. 1H-NMR and IR spectral data support the proposed ligand structures. Dynamic 1H-NMR studies on diurea and diamide derivatives point to the presence of slowly interconverting conformers on the 1H-NMR time-scale, owing to N−H···N hydrogen bonding and/or a restricted rotation around the amide bonds. The ligands synthesized form 1:1 complexes with divalent transition metal ions. Upon complexation, bis-ester derivatives AL5 and CL5 undergo a metal-induced hydrolysis of the ester groups to carboxyl functions, which act as additional binding sites for the metal ion, as well as hydrogen-bonding donor-acceptor binding site to produce dimeric complexes.

Introduction

Although 2,2'-bipyridine has been used as a classic bidentate ligand for over a century, only in the last two decades has this subunit been widely employed in the design of photoactive cryptands [1], polyfunctionalized ligands [2], dendrimers and star-like macromolecules [3]. In view of the potential offered by polymer-bound bipyridines and their transition-metal complexes in analytical chemistry [4] and in photochemical processes [5], we became interested in the synthesis of a variety of rigid and flexible polyamides and polyurethanes incorporating acyclic (AL1, Acyclic Ligand) and cyclic (CL1, Cyclic Ligand) bipyridinediyl diamino subunits (complexing building blocks) within the polymeric backbone [6,7]. These materials were shown to form polynuclear complexes with the first-row transition metals [6].
Current interest in macromolecular complexes bearing bipyridine chelating units [8] prompted us to design a number of acyclic and cyclic polypyridine ligands, to be exploited as model ligands for a better understanding of the coordination properties of related macromolecular systems. In this paper we wish to report the synthesis, structural characterization and fluxional properties of the ligands AL2-5 and CL2-5 shown in Scheme 1, as well as the preparation and characterization of their complexes with divalent transition metals Cu(II), Co(II), and Ni(II).
Scheme 1.
Scheme 1.
Molecules 08 00521 g005

LigandRLigandR
AL1HCL1H
AL2COCH3CL2COCH3
AL3COPhCL3COPh
AL4CONHPhCL4CONHPh
AL5CH2CO2ButCL5CH2CO2But
AL6CH2COOHCL6CH2COOH

Results and Discussion

Ligands AL1,3 and CL1,3 were synthetized by literature procedures [2,6,9]. N,N'-Diacetyl and bis-urea derivatives AL2, CL2 and AL4, CL4 were prepared by reaction of the appropriate acyclic or cyclic bis-amine with Ac2O/Et3N or phenyl isocyanate, respectively, in anhydrous CHCl3 at room temperature (r.t.). Bis-esters AL5 and CL5 were produced in good yield by N-alkylation of the parent diamines with t-butyl bromoacetate/K2CO3 in dry CH3CN. The structural characterization of all new compounds was accomplished by elemental analysis, FAB (+) MS measurements, and 1H-NMR spectroscopy. The 1H-NMR spectral data for acyclic and cyclic ligands are reported in Table I and Table II , respectively.
Table I. 1H-NMR parameters of acyclic ligands AL2 – AL5.
Table I. 1H-NMR parameters of acyclic ligands AL2 – AL5.
ProtonAL2
293 K
AL3
293 K
AL3
373 K
AL4
293 K
AL5
293 K
38.29 – 8.21 m8.34 – 8.24 m8.45 d
J = 7.8
8.23 d
J = 7.7
8.29 dd
J = 7.7; 0.9
47.80 – 7.59 m7.82 – 7.77 m7.78 t
J = 7.8
7.72 t
J = 7.7
7.77 t
J = 7.7
57.29 – 7.12 m7.09 d
J = 7.4
7.24 d
J = 7.8
7.46 d
J = 7.9
7.55 dd
J = 7.7; 0.9
α + γ CH24.78, 4.71 s4.92 d
J = 4.7
4.76, 4.71 s
4.91, 4.85 s4.75, 4.70 s4.04 s
3’7.29 – 7.12 m7.53 bd
J = 7.5
7.31 d
J = 8.1
7.46 d
J = 7.9
7.67 – 7.65 m
4’7.80 – 7.59 m7.72 – 7.65 m7.65 t
J = 8.1
7.60 dt
J = 7.7; 1.7
7.67 – 7.65 m
5’7.29 – 7.12 m7.22 – 7.18 m7.18 – 7.13 m7.18 m7.18 – 7.14 m
6’8.57, 8.49 dd
J = 4.1–4.7; 0.8
8.57, 8.53 d
J = 4.5; 0.9
8.58 d
J = 4.8
8.59 d
J = 4.3
8.54 dt
J = 4.7; 1.3
CH23.40 s
CH32.28, 2.21 s1.47 s
NH9.97 bs
Ph7.60 – 7.31 m7.59 – 7.34 m7.03 t
J = 7.47.32 – 7.17 m
Notes: The spectra were obtained in deuterated tetrachloroethane (TCE). Chemical shifts are reported in ppm, and coupling constants (J) in Hz. Numbering pattern as shown in Scheme 1. Abbreviations used: bs = broad singlet, s = singlet, d = doublet, dd = double doublet, m = multiplet, t = triplet, dt = double triplet.
The 1H-NMR spectra of acyclic bis-amide ligands AL2,3 (not shown) are quite complex, and can be interpreted in terms of two overlapping sets of signals, arising from a mixture of rotamers in slow equilibrium on the 1H-NMR time-scale, due to the restricted rotation around the amide bonds. In both compounds the ratio between the two rotamers at 293 K in chlorinated solvents is about 3:2, as it can be deduced by integration of the pair of methyl singlets in AL2 and of the pair of H6 doublets in AL3. Dynamic NMR studies on AL3 have shown that, upon heating a TCE solution of the ligand, broadening of most signals occurred, with a coalescence temperature at about 340 K, followed by sharpening at more elevated temperatures, corresponding to the free rotation of the benzoyl groups. Well resolved spectra were obtained at temperatures as high as 373 K, showing the characteristic three-spin system for the bipyridyl unit, and four-spin system for a 2-substituted pyridyl ring. From the dynamic parameters a ΔG = 17.2 Kcal/mol was ascertained for the torsional energy of the amide bond.
Scheme 2.
Scheme 2.
Molecules 08 00521 g006
In spite of the presence of two ureido moieties, the r.t. 1H-NMR spectrum of acyclic diurea AL4 in TCE displays sharp resonances for both aromatic and methylene protons, due to the locking of rotamers by the formation of intramolecular hydrogen bonding between the N-H proton and the juxtaposed pyridine ring nitrogen (Scheme 2), as suggested by the broad downfield resonance of the N-H groups at δ 9.97 ppm. As expected, N-alkylated diester AL5 is a flexible molecule and, due to partial overlapping of the heteroaromatic protons, decoupling experiments were used for proton assignments. Noteworthy, the H3 bipyridyl protons of AL2-5 resonate in the range 8.45−8.25 ppm, a region considered of diagnostic value for an anti orientation of the bipyridyl unit [10].
Table II. 1H-NMR parameters of cyclic ligands CL2 – CL5
Table II. 1H-NMR parameters of cyclic ligands CL2 – CL5
ProtonCL2
293 K
CL2
403 K
CL3
403 K
CL4
403 K
CL5
293 K
37.72, 7.70, 7.64, 7.57 dd
J = 7.4 – 7.7; 0.8
7.63 d
J = 7.7
7.65 dd
J = 7.8; 0.9
7.65 d
J = 7.5
7.68 d
J = 7.7
47.42, 7.37, 7.31, 7.20 t
J = 7.7
7.34
bt
7.39 t
J = 7.7
7.36 t
J = 7.5
7.29 t
J = 7.7
57.21, 6.98, 6.64, 6.52 dd
J = 7.4 – 7.7; 0.8
6.90
hump
7.13 bd
J = 7.6
6.85 d
J = 7.5
7.12 d
J = 7.7
α- + γ-CH24.87, 4.90 s
4.96 bs
4.91 s4.96 s4.89 bs4.08 s
CH23.73 s
CH32.42, 2.44 s2.41 s1.56 s
NH9.48 bs
Ph7.81 – 7.78; 7.47 – 7.44 m7.53 d
J = 9.0;
7.18, 7.10 t
J = 6.8
Notes: The spectra were obtained in deuterated TCE. Chemical shiftsare reported in ppm, and coupling constants (J) in Hz. Numbering pattern as shown in Scheme 1. Abbreviations used: bs = broad singlet, s = singlet, d = doublet, dd = double doublet, m = multiplet, t = triplet, dt = double triplet.
The 293 K 1H-NMR spectrum (CDCl3) of CL2 (Figure 1) shows two singlets for the methyl groups, one broad and two sharp singlets for the bridging methylenes, and two sets of well-resolved resonances for the bipyridyl protons, spread out in the 6.5–7.7 ppm region. These spectral data may be suggestive of the presence of two slowly interconverting diastereomeric forms of CL2 (in a ratio of ca. 3:2). Each of the two conformers features two pairs of pyridyl subunits in different magnetic environments, resulting in a total of four double doublets for H3 and H5 protons, and four triplets for H4 protons (Table II and Figure 1).
Interconversion between the two forms may be realized by rotation of the acetyl group(s) around the amide bond(s). In order to measure the energy barrier associated with this process, a VT NMR study of CL2 was undertaken, using deuterated TCE as a solvent, with the results shown in Figure 1. On raising the temperature, broadening of the signals was observed above 340 K, and coalescence occurred at about 373 K. At 403 K the signals of methylene and H3 protons show up as a sharp singlet and a doublet, respectively, while H4 protons appear as a broad triplet and H5 protons are still broad. Higher temperatures were precluded by the solvent used. From the dynamic parameters a ΔG = 19.0 kcal/mol was ascertained for conformational transitions.
Figure 1. Variable Temperature (VT) 1H-NMR spectra of CL2 in TCE.
Figure 1. Variable Temperature (VT) 1H-NMR spectra of CL2 in TCE.
Molecules 08 00521 g001
* Residual solvent peaks
The room temperature 1H-NMR spectrum of CL3 is very broad in every region, again suggesting the presence of at least two conformers slowly interconverting on the 1H-NMR scale. The methylene protons show up as two humps, which upon heating coalesce to a singlet at temperatures above 333 K, with simultaneous sharpening of the signals in the aromatic region. At 403 K (Table II) average signals are obtained, and the spectrum consists of a singlet at δ 4.96 ppm for the α and γ bridging methylenes, a broad doublet at δ 7.13 ppm for H5, a triplet at δ 7.39 for H4, a double doublet at δ 7.65 for H3, and two complex multiplets in a 3:2 ratio for the benzoyl protons, centered at δ 7.46 and 7.80 ppm, respectively. Unfortunately, the sparing solubility of the compound did not allow us to explore its behaviour at low temperatures.
The 1H-NMR spectra of cyclic diurea CL4 are also temperature dependent, and the results of its dynamic behaviour are shown in Figure 2. At 253 K the methylene protons show a pattern of three doublets at δ 4.12, 5.21 and 5.74 ppm (ratio 2:1:1), while the aromatic protons are ill-defined, owing to a considerable overlapping. Upon raising the temperature, the two downfield methylene doublets broaden and coalesce at about 313 K, with contemporary sharpening of a set of aromatic protons (assignable to the two phenyl groups) and broadening of the remaining bipyridyl protons (multiplets centered at δ 6.62 and 7.6 ppm in the low temperature spectrum). At higher temperatures further broadening of the upfield methylene doublet and, concomitantly, of bipyridyl protons is observed. The two methylene humps coalesce to a broad singlet at temperatures above 373 K, and at 403 K the proton spectrum appears averaged by a fast motion of the molecule giving the complete equivalence of pertinent aromatic and methylene protons.
Figure 2. Variable Temperature (VT) 1H-NMR spectra of CL4 in TCE
Figure 2. Variable Temperature (VT) 1H-NMR spectra of CL4 in TCE
Molecules 08 00521 g002
The VT 1H-NMR studies of CL4 showed therefore two coalescence temperatures at 313 and 373 K, corresponding to conformational transitions with free energy barriers of 11.9 and 13.5 Kcal/mol, respectively. The first transition is likely associated with the breaking of intramolecular N-H…N hydrogen bondings, while the second one is related to the torsional barrier around PhNHC(O)−N bonds.
Owing to the low solubility of AL3 and CL3, the very similar AL2 and CL2 ligands were synthesized only in order to better understand the influence of the amido moiety on the conformational aspects.
As a general feature, the IR spectra of all acyclic or cyclic ligands show a 1600-1395 cm-1 region dominated by strong or medium-strong absorption bands associated with the stretching vibrational modes of heterocyclic rings [10,11]. In addition, bis-amide ligands AL2,3 and CL2,3 display prominent carbonyl bands at 1650-1630 cm-1, a frequency considered of diagnostic value for tertiary amides [12], while bis-urea derivatives AL4 and CL4 exhibit N-H stretching bands at 3280-3320 cm-1, a carbonyl absorption band, known as the Amide I band [6], at 1640-1670 cm-1, and a N-H band, known as Amide II band [6], at 1525-1540 cm-1. On the other hand, bis-ester ligands AL5 and CL5 show a C=O stretching band at 1740-1700 cm-1 along with CC(=O)−O stretching vibration and O−C−C asymmetric stretching bands at 1270 and 1163 cm-1, respectively. Well resolved absorption bands are also present at lower frequencies, which can be ascribed to the in-plane and out-of-plane ring deformations [11,12].
The Cu(II), Co(II), and Ni(II) complexes with the new acyclic and cyclic ligands were prepared by reacting each ligand with the appropriate metal nitrate salt (1 equiv) in aqueous MeOH at room temperature. It is noteworthy that the synthesis of complex CL4a was carried out at reflux. Table III shows the physicochemical characteristics of the synthesized complexes. Complexes AL3a-c and CL3a were prepared according to literature data [5] (the lettering a, b, and c refers to Cu(II), Co(II), and Ni(II) complexes, respectively).
Table III. Analytical and TGA data for the Complexes
Table III. Analytical and TGA data for the Complexes
ComplexYield (%)Analytical dataTGA
Found (calculated)
Metalb%C%H%N
[Cu(AL3)(H2O)2](NO3)2 2H2Oa
AL3a
75
CH3CN
7.50
(7.35)
53.13
(52.80)
4.69
(4.66)
13.04
(12.96)
-2H2O (50 °C);
-2H2O (100 °C); 210°C dec.
[Co(AL3)(H2O)2](NO3)2 1H2Oa
AL3b
88
CH3CN
6.83
(7.00)
53.82
(54.22)
4.39
(4.55)
13.56
(13.31)
-1H2O (50 °C);
-2H2O (100 °C); 228°C dec.
[Ni(AL3)(H2O)2](NO3)2. 2H2Oa
AL3c
85
CH3CN
6.43
(6.83)
52.83
(53.09)
4.06
(4.69)
12.74
(13.03)
-2H2O(48°C);
-2H2O (110°C); 210°C dec.
[Cu(AL4)](NO3)2
AL4a
68
MeOH/Et2O
7.50
7.73
55.17
(55.50)
4.03
(4.17)
17.17
(17.03)
205 °C dec
[Co(AL4)](NO3)2.1CH3CN
AL4b
64
CH3CN
7.00
(6.86)
55.91
(55.94)
4.57
(4.34)
17.85
(17.94)
-1CH3CN (40°C); 223°C dec.
[Ni(AL4)](NO3)2 1H2O
AL4c
70
CH3CN
6.77
(7.02)
55.01
(54.62)
3.90
(4.34)
17.01
(16.76)
-1H2O(50°C); 227°C dec.
[Cu(AL5)](NO3)2 1H2O
AL5a
52
CH2Cl2/C6H12
7.10
(7.65)
51.72
(52.06)
5.49
(5.58)
13.74
(13.49)
-1H2O(40°C);170°C dec.
[Co(AL5)](NO3)2 1H2O
AL5b
48
CH2Cl2/C6H12
7.01
(7.14)
51.98
(52.36)
5.15
(5.61)
13.92
(13.57)
-1H2O(44°C); 200°C dec.
[Ni(AL5)](NO3)2 1H2O
AL5c
43
CH2Cl2/C6H12
7.23
(7.11)
52.13
(52.37)
5.67
(5.62)
13.60
(13.57)
-1H2O(48°C); 180°C dec.
[Cu2(AL6)2H](NO3)3 4H2O
AL6a
37
CH3CN
9.17
(9.01)
47.98
(47.72)
4.83
(4.51)
15.25
(14.91)
-4H2O (40 °C); 170°C, 235°C dec
[Co2(AL6)2H](NO3)3 4H2O
AL6b
33
CH3CN
8.13
(8.41)
47.93
(48.03)
4.65
(4.53)
15.32
(15.00)
-4H2O (50 °C); 160°C; 270°C dec
[Ni2(AL6)2H](NO3)3 6H2O
AL6c
38
CH3CN
8.00
(8.17)
46.35
(46.84)
4.97
(4.70)
14.75
(14.63)
-6H2O (50 °C); 108C; 260C dec
[Cu(CL3)](NO3)2. 4H2Oa
CL3a
90
MeOH
7.49
(7.57)
52.89
(52.92)
4.10
(4.44)
13.04
(13.00)
-4H2O (50 °C); 370 °C dec.
[Cu(CL4)](NO3)2
CL4a
80
MeOH
7.72
(7.51)
55.67
(55.64)
3.86
(3.93)
17.25
(17.08)
325 °C dec.
[Cu2(CL6)2 H](NO3)3 10H2O
CL6a
70
MeOH/Et2O
8.39
(8.44)
44.83
(44.44)
4.48
(4.72)
13.51
(13.88)
-10H2O (40 °C); 180°C; 385°C dec
[Co2(CL6)2 H](NO3)3 7H2O
CL6b
75
CH3CN
8.12
(8.01)
45.94
(46.38)
4.07
(4.51)
14.78
(14.49)
-7H2O (50 °C); 168°C; 370°C dec
[Ni2(CL6)2 H](NO3)3 7H2O
CL6c
82
MeOH
8.09
(7.93)
46.36
(46.38)
4.11
(4.51)
14.29
(14.49)
-7H2O (50 °C); 145°C; 365°C dec
a From Ref. [6]; b Determined by AA
The acyclic ligands AL3-5 generally give stable 1:1 complexes with all the transition metals examined. Conversely, cyclic ligands CL3,4 exhibit the unique property to specifically complex Cu(II) in the 1:1 ratio even in presence of sizeable amounts of Co(II) and Ni(II). The stoichiometry of the complexes was ascertained by microanalytical data [elemental analysis (EA) and atomic absorption (AA)] and confirmed by FAB MS spectrometry.
The IR spectra of the complexes are quite different from those of the free ligands, and give useful information on the possible binding sites involved in the coordination, according to the shift of the signals with respect to those of the free ligands, the most dramatic changes being observed in the 1600-1350 cm-1 region. As a common feature, all the complexes examined display a very strong frequency band at 1385 cm-1, indicating the participation of the nitrogens of at least one bipyridyl moiety in the complex formation. For the complexes of AL3, AL4, and AL5, the carbonyl groups are undoubtedly involved in the complexation, as evidenced by a large shift (Δνδ= 55-60 cm-1) of the carbonyl absorption to lower frequencies [13]. In the case of complexes CL3a and CL4a, the C=O stretching (Amide I band at 1631 and 1665 cm-1, respectively) does not seem to be shifted as compared to the free ligands, indicating that it is not a binding site. On the other hand, for CL4a we observe a Δν= 11 cm-1 for the -NH stretching and a Δν= 22 cm-1 for the -NH bending of the urea nitrogens, suggesting that they act as binding sites.
The synthesis of the complexes AL5a-c was plagued by the competitive formation of a coloured side-product, obtained as a powder sparingly soluble in the reaction medium. These powders, also insoluble in CH2Cl2, were identified as dimeric complexes (AL6a-c) formed by a new ligand (AL6, R = COOH; Scheme 1), arising from a metal-induced hydrolysis of the ester functions [14].
The IR data of these complexes show: (i) a very intense and broad O-H stretching band at 3380 cm-1, (ii) two C-H stretching bands at 2930-2985 cm-1, (iii) a C=O stretching band at 1680 cm-1 indicative of the presence of a free carboxylate anion, (iv) the lack of bands in the region 1750-1700 cm-1, (i.e. no resonance of the C=O group acting as binding site). These data may be consistent with a dimeric structure for these complexes composed of two monomeric subunits held together by a C(=O)O¯…HOC(=O) hhydrogen bond [supported by microanalytical data, and FAB-MS measurements]. Each monomer is mononuclear, probably adopting an octahedral geometry around the complexed ion, with bipyridine nitrogens and the β bridged nitrogens forming a square plane, while the pyridine nitrogens occupy the axial positions. The general formula of these complexes can be written as [Me2(AL6)2H](NO3)3∙xH2O being Me = Cu(II), Co(II), Ni(II) (Table III).
A similar behaviour was observed by reacting the ligand CL5 with the metal nitrate salts. This cyclic ligand is not able to coordinate any metal by itself, as AL5 do, but, also in this case, owing to the hydrolysis of the ester groups a new ligand CL6 (R = COOH; Scheme 1) was obtained that uses its two –COO- groups in two different ways: one as binding site for the metal, the other one to link, through a hydrogen bond, another similar structure. As a matter of fact the IR data of the complexes CL6a-c show, inter alia, a resonance at δ ν = 1665 cm-1 due to C=O group indicating a dimeric form via an intermolecular hydrogen bonding between two mononuclear complexes, and a resonance at ν = 1626 cm-1 that can be attributed to the asymmetrical stretching of the carboxylate anion that acts as binding site.
Figure 3. Possible dimeric structure of CL6 a-c complexes.
Figure 3. Possible dimeric structure of CL6 a-c complexes.
Molecules 08 00521 g003
These dimeric crystalline complexes, CL6a-c, of general formula [Me2(CL6)2H](NO3)3∙xH2O, were identified by FAB, AA, and EA. The dimeric structures for CL6a-c complexes, hypothesized on the basis of their IR data, are corroborated by their FAB spectra, which show low intensity ion peaks at m/z 1143, 1135 and 1133, respectively, corresponding to [(CL6Me)2 – 3H]+, i.e. two mononuclear complexes held together by a hydrogen bond forming between the carboxylate unit of one moiety and the carboxylic functionality of the other, as depicted in Figure 3.
Splitting of the dimers produces prominent peaks at m/z 572, 568 and 567, respectively, corresponding to [CL6Me –H]+ ions, which represent the base peak in the spectra of complexes CL6b,c. In the case of CL6a the base peak is found at m/z 573 ([CL6Me]+). Of note is the fact that no multicharged ion peaks could be detected in the FAB spectra of the complexes investigated. The observed isotopic patterns of all ion peaks are in good agreement with the calculated ones. The FAB spectrum of CL6b is shown in Figure 4.
In Table III thermal gravimetric analysis (TGA) data of the synthesized complexes are reported. All complexes show a first degradation step, in the 40–50 ° Crange, corresponding to the loss of water/solvent molecules. AL3a-c complexes show a second degradation step in the 100–110 ° Crange, that can be easily attributed to the loss of bound water ligand. The complexes with the acyclic ligands show, as expected, a decomposition temperature lower than that of the cyclic ones.
Figure 4. FAB mass spectrum of the [Co2(CL6)2 H](NO3)3·7H2O complex (CL6b).
Figure 4. FAB mass spectrum of the [Co2(CL6)2 H](NO3)3·7H2O complex (CL6b).
Molecules 08 00521 g004
–Comparing the IR, FAB, and TGA data of the complexes bearing cyclic units to those of acyclic ones, we observe that the binding sites are different owing to the rigidity of cyclic structures that influence the complexing properties of the correspective free ligands. It is known that the free bipyridyl moiety assumes an anti configuration that changes to syn when it works as bidentate binding site [15].
If this behaviour is easily hypothesized for the flexible acyclic ligand series, it may not be true for the cyclic one, because both bipyridyl units should assume the more favorable syn orientation for the complexation. This is why the CL3 and CL4 cyclic ligands are able to complex only the Cu(II) cation even if Co(II) or Ni(II) cations are present, due to the fact that the latter prefer an octahedral geometry that cannot be meet with the rigid cyclic ligands; on the contrary the Cu(II) can be complexed owing to its ductility to assume different geometries. It is noteworthy that the cyclic ligand CL6 (arising from the hydrolysis of CL5 ) does not show any particular selectivity for the cations under study, probably due to its flexibility and to the presence of the –COOH groups that, working as binding sites, meet the Co(II) and Ni(II) geometric needs, unlike the rigid amido and ureido moieties present in CL3 and CL4, respectively.

Conclusions

Following our interest in macromolecular complexes bearing bipyridine chelating units, we have synthesized a series of N,N'-disubstituted acyclic (AL) and cyclic (CL) ligands, which can be viewed as model compounds for the related polymers, along with their complexes with Cu(II), Co(II), and Ni(II) transition metal ions. Dynamic 1H-NMR studies on diurea and diamide derivatives point to the presence of slowly interconverting conformers on the 1H-NMR time-scale. The complexing behaviour of the acyclic ligands was found to be different with respect to the cyclic ones. The former, following an easy anti to syn reorganization of the bipyridyl unit, give stable 1:1 complexes, while the latter, owing to their rigidity, show the unique property of recognising and binding copper(II) ion even in the presence of cobalt(II) and nickel(II) ions. A metal catalyzed hydrolysis of the ester groups was also observed, leading to the formation of dimeric structures linked by intermolecular hydrogen bonds.

Experimental

General

Diamines AL1 and CL1, and their benzoyl derivatives AL3 and CL3, were synthesized according to literature procedures [2,6,9]. Anhydrous solvents were purchased from Fluka, and used without further purification. All reactions were conducted under a N2 atmosphere. Melting points were determined on an Electrothermal melting point apparatus and are uncorrected. Elemental analyses were obtained commercially. 1H-NMR spectra were performed in TCE with a Varian INOVA 500 instrument, using TMS as an internal standard. The 1H-NMR spectra of all compounds synthetized are collected in Table II and Table III. FAB (+) MS spectra were taken on a Kratos MS 50 S double-focusing mass spectrometer equipped with a standard FAB source, using 3-nitrobenzyl alcohol as a matrix. IR spectra (KBr disks) were recorded on a Perkin Elmer 684 Infrared Spectrophotometer. AA measurements were obtained on a Perkin Elmer AAnalyst 300. The appropriate amount of the considered complex was destroyed with conc. HNO3 (50 mL) and then diluted to 1 L with distilled water. Metal concentrations of such solutions were measured with a multielement hollow cathod lamp, by an oxidizing air-acetylene flame. Two standard solutions (concentrations of 0.5 and 1 ppm, respectively) were prepared for each metal by dissolving the metal compound (BDH Chemical Ltd. Laboratory Reagent Standards) in diluted HNO3. Thermogravimetry was performed under nitrogen employing a Mettler TA 3000 thermogravimetric analyzer coupled with a Mettler TC 10 A processor at a heating rate of 10 °C/min.

N,N-Diacetyl-6,6'-Bis{[(2-pyridylmethyl)amino]methyl}-2,2'-bipyridine (AL2).

A chilled solution of diamine AL1 (59 mg, 0.15 mmol), Ac2O (31 mg, 0.3 mmol) and Et3N (0.3 mL) in dry CHCl3 (2 mL) was stirred for 3 h. After quenching with water (1 mL), the reaction mixture was extracted with CHCl3, washed with water and dried (MgSO4). The solvent was evaporated to give an oily residue, which was subjected to column chromatography (neutral alumina, eluent: 3:1cyclohexane-AcOEt), to afford diamide AL2 (53 mg, 85%) as white prisms, m.p. 129-130 °C (cyclohexane-CH2Cl2); FAB (+) MS, m/z 481 [MH]+; Anal. Calcd. for C28H28N6O2: C, 69.98; H, 5.87; N, 17.49. Found: C, 70.25; H, 5.64; N, 17.76

N,N-Diacetyl-2,17-diaza[3.3](6,6')-2,2'-bipyridinophane (CL2).

A mixture of macrocyclic diamine CL1 (79 mg, 0.2 mmol), Ac2O (41 mg, 0.4 mmol) and Et3N (0.4 mL) in dry CHCl3 (20 mL) was stirred at r.t. for 5 h. The solvent was evaporated to leave a solid residue, which was washed with water and dried. Recrystallization from dimethylsulfoxide provided diamide CL2 as colorless microcrystals (76 mg, 80%), m.p. >250 °C; FAB (+) MS, m/z 479 [MH]+; Anal. Calcd. for C28H26N6O2: C, 70.28; H, 5.48; N, 17.56. Found: C, 69.97; H, 5.40; N, 17.82.

N,N-Di(phenylureido)-6,6'-Bis{[(2-pyridylmethyl)amino]methyl}-2,2'-bipyridine (AL4).

To a vigorously stirred and ice-chilled solution of diamine AL1 (0.89 g, 2.25 mmol) in dry CHCl3 (10 mL) was added dropwise a solution of phenyl isocyanate (0.54 g, 4.5 mmol) in dry CHCl3 (5 mL). The mixture was allowed to stir at rt for 24 h. After evaporation of the solvent, the residue was dissolved in CH2Cl2 and purified by column chromatography (neutral alumina, cyclohexane-AcOEt 1:1 as an eluent) to give AL4 (1.07 g, 75%) as white crystals, m.p. 160-161 °C (cyclohexane-AcOEt); FAB (+) MS, m/z 635 [MH]+; Anal. Calcd. for C38H34N8O2: C, 71.90; H, 5.39; N, 17.65 . Found: C, 71.74; H, 5.11; N, 17.59.

N,N-Di(phenylureido)-2,17-diaza[3.3](6,6')-2,2'-bipyridinophane (CL4).

Reaction of CL1 with phenyl isocyanate under conditions similar to those above described for the preparation of AL4 afforded derivative CL4 (64%) as fine needles, m.p. > 260 °C (DMF); FAB (+) MS, m/z 633 [MH]+; Anal. Calcd. for C38H32N8O2: C, 72.13; H, 5.10; N, 17.71. Found: C, 72.02; H, 5.15; N, 18.11

N,N-Di(t-butoxycarbonylmethyl)-6,6'-Bis{[(2-pyridylmethyl)amino]methyl}-2,2'-bipyridine (AL5).

A solution of t-butyl-bromoacetate (0.69 g, 3.55 mmol) in dry CH3CN (10 mL) was added dropwise to a stirred mixture of diamine AL1 (0.7 g, 1.76 mmol) and anhydrous K2CO3 (1.0 g, 7.4 mmol) in dry CH3CN (10 mL). The mixture was then refluxed for 3 h under N2, and allowed to stir at r.t. for an additional 12 h. The precipitate obtained was collected by filtration, throughly washed with water and dried. The solid was purified by column chromatography (neutral alumina, eluent: 3:1cyclohexane-AcOEt) to afford AL5 (0.88 g, 80%) as white crystals, m.p. 141-142 °C (EtOH); FAB (+) MS, m/z 625 [MH]+; Anal. Calcd. for C36H44N6O4: C, 69.20; H, 7.10; N, 13.45. Found: C, 68.89; H, 7.49; N, 13.05.

N,N-Di-(t-butoxycarbonylmethyl)-2,17-diaza[3.3](6,6')-2,2'-bipyridinophane (CL5)

Reaction of CL1 with t-butyl-bromoacetate (2 equiv) under conditions similar to those described above for the preparation of AL5 afforded derivative CL5 (71%) as white crystals, m.p. 225-226 °C (cyclohexane-AcOEt); FAB (+) MS, m/z 623 [MH]+; Anal. Calc. for C36H42N6O4: C, 69.43; H, 6.80; N, 13.50. Found: C, 69.09; H, 6.73; N, 13.42.

Synthesis of complexes AL5a-c, AL6a-c, CL4a, and CL6a-c.

The synthesis of complex AL4a is given as an example of the general complexation reaction procedure used for the preparation of complexes AL5a-c, AL6a-c, CL4a, and CL6a-c.
A slurry of AL4 (63.4 mg, 0.1 mmol) and Co(NO3)2. 6H2O (or Ni(NO3)2. 6H2O, or Cu(NO3)2. 3H2O) (0.1 mmol) in 1:1 MeOH/H2O (6 mL) was stirred at room temperature for 48 h until a clear solution was obtained. The solution was concentrated to about 3 mL, and when left standing, crystals of the desired complex were obtained, which were collected by filtration and dried in vacuo. The analytical and physicochemical data of the complexes are given in Table III.

Acknowledgments

We thank Prof. M. F. Parisi for the acquisition of VT-NMR spectral data. The research was financially supported by the University of Catania (PRA funds).

References

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MDPI and ACS Style

Mamo, A.; Pappalardo, A. Synthesis and Characterization of Acyclic and Cyclic Azabridged Ligands Incorporating 2,2'-Bipyridine Subunits and Their Complexes With Copper(II), Cobalt(II), and Nickel(II). Molecules 2003, 8, 521-535. https://doi.org/10.3390/80700521

AMA Style

Mamo A, Pappalardo A. Synthesis and Characterization of Acyclic and Cyclic Azabridged Ligands Incorporating 2,2'-Bipyridine Subunits and Their Complexes With Copper(II), Cobalt(II), and Nickel(II). Molecules. 2003; 8(7):521-535. https://doi.org/10.3390/80700521

Chicago/Turabian Style

Mamo, Antonino, and Andrea Pappalardo. 2003. "Synthesis and Characterization of Acyclic and Cyclic Azabridged Ligands Incorporating 2,2'-Bipyridine Subunits and Their Complexes With Copper(II), Cobalt(II), and Nickel(II)" Molecules 8, no. 7: 521-535. https://doi.org/10.3390/80700521

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