High resolution 1 H and 13 C NMR and mass spectral studies of substituted dipyridylpyridazine

Total assignment of the H andC NMR spectra of 3,6-bis(2 ′-pyridyl)pyridazine derivatives ( n-dppn) have been recorded at 400 and 100 MHz, respectively. 1 Based on these data, the complete assignment of the C NMR chemical shift values was made by a C{H} decoupling. The spectra clearly show that the n-dppn ligands exist in the cis–cisconformation at least in solution. Matrix assisted laser desorption ionization (MALDI) and time-of-flight mass spectrometry (TOFMS) are used to record the mass spectra of the n-dppn ligands.α-Cyano-4-hydroxycinnamic acid (CHCA) matrix promoted the molecular ion detection when 180 pmol of n-dppn is introduced into the TOFMS.


Introduction
The tetranitrogenated 3,6-bis(2 -pyridyl)pyridazine (dppn) ligands are widely used to prepare some important complexes especially those of ruthenium(II) and rhodium(III) since these complexes act as photosensitizer for solar energy conversion which exhibit catalytic and biological properties [1][2][3][4][5][6].The dppn ligand forms mononuclear and binuclear complexes [7].In the former, dppn acts as a bidentate chelating ligand, with two neighboring pyridine and pyridazine ring nitrogen atoms bonded to metal ion, while in the later, the ligand adopts a conformation suitable to chelate two metal ions, on the same side, through the four nitrogen donor atoms [7].
Lately, derivatives (n-dppn, Fig. 1) of the dppn ligand with electron-releasing cyclic aliphatic groups have been prepared and their complexes with ruthenium(II) and rhodium(III) reported [1,8,9].The1 H NMR spectra of some of these ligands have previously been reported [8] but the assignment of the spectral data remained ambiguous; besides, a 13 C NMR study has not been described.Very recently, we have reported the 1 H and 13 C NMR of some of these ligands and their complexes of rhodium(III) in DMSO-d 6 [9].
In the present study we report and interpret a full 1 H and 13 C NMR spectral data of the n-dppn ligands as well as the mass spectra.

Sample preparation for MS
Matrix-assisted laser desorption ionization (MALDI); all matrices were acquired from Sigma or Aldrich and used without further purification.Several matrices were tested as vehicles for the desorption and ionization of the diastereomeric tetrols.α-Cyano-4-hydroxycinnamic acid (CHCA) was among the matrices tested.Matrix solutions were prepared by dissolving the desired matrix (10 mg/ml) in a solution of THF.Samples were then prepared by combining 5 µl of ∼180 pmol/µl n-dppn solution in THF with 5 µl of matrix solution.Next the resulting solution was deposited on the sample probe and allowed to dry under ambient conditions prior to insertion into the mass spectrometer.

Physical measurements
All 1 H and 13 C NMR spectra were recorded at 400 and 100 MHz, respectively, on a Varian VXR 400 spectrometer using CDCl 3 .Chemical shifts for 1 H and 13 C are relative to (CH 3 ) 4 Si.Measurements were performed at ambient probe temperature using 5 mm o.d.sample tubes.Molecular weight spectra were acquired using a modified Wiley-McLaren design "time-of-flight mass spectrometry" (TOFMS) (model D850, R.M. Jordan, Co., Grass Valley, CA) in the linear mode.A nitrogen laser (337 nm, 5 mW peak laser power, 3 ns pulse width, and a ∼400 µm 2 spot size) was used to induce desorption (model VSL-337ND, Laser Science, Newton, MA).

Results and discussion
The tetranitrogenated dipyridylpyridazine derivatives, namely 5-dppn, 6-dppn, 7-dppn, 8-dppn, and 12-dppn ligands were all prepared in ∼60-92% yield according to the procedure described by Atfah [8].The 1 H-NMR spectra of some of the n-dppn ligands have been recorded earlier using a Brucker WP 80 SY spectrometer [8].However, no peak assignments and no comments on the exact conformation of these ligands have been made.Recently, we have reported 1 H and 13 C-NMR of some of n-dppn ligands and their complexes with rhodium(III) in DMSO-d 6 [9].In the present study, the 1 H and 13 C-NMR spectra for some of the isolated ligands were recorded at 400 and 100 MHz, respectively in CDCl 3 .The 1 H and 13 C-NMR results are presented in Tables 1 and 2, respectively.
The quaternary carbons namely (C 3 , C 6 ) and (C 2 , C 2 ) are the most desheilding carbons and their resonance appears in the range of 158.8-155.1 ppm, respectively.While, the resonance signals at 137.5-145.7 ppm are assigned to quaternary carbons (C 4 and C 5 ) in the pyridazine ring.The chemical shift for the other pyridine carbons namely (C 5 , C 5 , C 3 , C 3 , C 4 , and C 4 ) are resonate in the range of 123.1-137.9ppm [10].