The synthesis of amine ligands and their metal complexes is a growing area of research in applied inorganic chemistry. The synthesis and study of different amine ligands have undergone tremendous growth in recent years and there complexation chemistry with a wide variety of transition metals has been extensively studied (Gao et al., 1999; Liang et al., 2003). The early work on cyclopalladation of benzylamine (Albert et al., 2003; Beatrice et al., 2004) derivatives has served notice to other authors to assume three requirements that the amine must meet. The first that the amine must be tertiary. Thus the orthopalladation using lithium tetrachloropalladate (II) was observed with N, N-Dimethylbenzylamine or some of its aryl substituted derivatives containing electron releasing groups. The second requirement is that the aryl group must not be deactivated as in the case of 4-nitro N, N-Dimethylbenzylamine.
Aromatic substitution reactions are not limited to azobenzene and benzylideneaniline but are even more facile with N, N-Dimethylbenzylamine (Farooq et al., 1999; Samina et al., 2006). Complexes of N, N-Dimethylbenzylamine with transition metals in low oxidation state have been synthesized. Various analytical techniques have been used for the characterization of group 6 metal carbonyl complexes and are reported elsewhere in the literature (Yaman et al., 2002; Tang et al., 2004; Sert et al., 2004).
The accessible approach through literature reveals that no work has been done on the synthesis and characterization of the title complexes. Keeping in view the literature and in continuation of previous research (Tariq et al., 2005), an attempt has been made to carry out this research.
MATERIALS AND METHODS
All the chemicals were purchased from Fine Chemicals and the solvents used
were dried and distilled before use. Metal carbonyls were obtained from Aldrich.
I.R. Spectra were recorded on a Perkin Elmer FTIR spectrophotometer Model-621.
Boiling points were determined by Gallenkamp apparatus. Elemental analyses were
carried out by Perkin Elmer Elemental Analyzer 2400 CHN. NMR spectra were recorded
on Bruker DPX-400 spectrometer. All the spectroscopic studies of the investigated
compounds were carried out at the department of chemistry, Loughborough University
United Kingdom during 2004-2005.
Synthesis of 2-bromo-N, N-dimethylbenzylamine
Ammination of 2-bromo-benzylbromide: 2-bromo-benzylbromide (0.2499 g, 1.0
mmole) was dissolved in dichloromethane (25 cm3). The solution was
added drop-wise to the 33% dimethylamine solution in Ethanol (0.20 cm3,
1.5 mmole). The mixture was stirred for about 6 h in a quick-fit flask. It was
then neutralized with 10% NaHCO3 solution. The mixture was stirred
and organic layer was separated from the aqueous layer, dried over anhydrous
MgSO4, filtered and the solvent was reduced by rotary evaporator
to get the pure product (yield 53%).
Synthesis of [2-bromo-DMBA-Mo (CO)5] complex: Mo(CO)6
(0.264g, 1.0 mmol) and 2-bromo-N, N-dimethylbenzylamine (0.30 cm3,
2.0 mmol) were placed in a flask containing sodium dried THF (10.0 cm3).
The mixture was stirred for 4 h under UV light and in the nitrogen atmosphere
to yield the yellow solution. The progress of the reaction was monitored by
FT-IR spectroscopy. The solvent was reduced by Schlinck vacuum line followed
by the addition of n-hexane to induce the crystallization (Yield 32%).
Synthesis of [2-bromo-DMBA-W(CO)5] complex: 2-bromo-DMBA-W(CO)5] complex was synthesized according to same procedure as described for the synthesis of above complex by using W(CO)6 as a metal carbonyl (Yield 27%).
RESULTS AND DISCUSSION
The ligand was synthesized according to scheme 1. The synthesized ligand was further reacted with group 6 metal carbonyls (Molybdenum and Tungsten) to yield respective metal complexes (Fig. 1). Both the complexes are white in color and are more soluble in organic solvents. The elemental analysis data (Table 1) confirms the composition of investigated complexes to be [ML(CO)5].
The structure of the ligand (2-bromo-N, N-dimetylbenzylamine) was elucidated
by IR spectroscopy. The peaks of major functional groups in the IR spectrum
of the ligand were assigned and are given in Table 2.
structure of metal complexes, Where M = Mo and W|
Scheme 1:Synthesis of 2-bromo-N, N-dimethylbenzylamine peak at 3044 cm1 (s) is assigned to C-H stretching of benzene ring. The peak at 2981 cm1 (m) corresponds to C-H of the -CH2
of the benzyl group. The presence of a band at 1593 cm1 (m) can
attribute to C-C stretching. The characteristic frequency of a C-N bond of the
type CH2-N (CH3)2 was observed at 1272 cm1
The proton NMR spectrum of 2-bromo-DMBA was recorded and the chemical shift values against TMS are shown in Table 3. A multiplet at δ7.16 can be attributed to the H-5 proton (ortho) in the aromatic ring. Another multiplet at δ6.98 can be assigned to the proton at position four. A singlet observed at δ3.31 is due to two methylene protons adjacent to electro-negative nitrogen and phenyl ring while singlet observed at δ2.15 corresponds to six N-methyl protons. 13C NMR also agrees with its proposed structure. A peak at δ138.13 corresponds to the quaternary carbon. The peaks at δ138.7 and δ132.7 are due to C-2 and C-6 of the aromatic ring, respectively while the peaks at δ133.0 and at δ127.0 correspond to C-3 and C-5, respectively. A peak at δ128.0 is due to C-4. A peak at δ63.33 is due to methylene carbon and peak at δ45.53 is probably due to methyl carbon attached to the nitrogen.
The IR spectrum of the complex (Table 4) 2-bromo-N, N-dimethylbenzylamine-Mo(CO)5
complex exhibits the stretching frequencies of various bonds in the region expected,
except for the C-N stretching frequency, where a shift is observed towards the
lower region which confirms the coordination of the ligand with the metal through
data of ligand and its complexes|
DMBA = N, N-Dimethylbenzylamine|
of 2-bromo- N, N-dimethylbenzylamine|
DMBA = N, N-dimethylbenzylamine|
of spectra of 2-bromo-N, N-dimethylbenzylamine|
data of 2-bromo-N, N-dimethylbenzylamine-Mo(CO)5 complex|
= N, N-dimethylbenzylamine|
This fact is supported by the appearance of a new low frequency band at 383
cm1 (s) assigned to metal-nitrogen bond formation. Further evidence
about this interaction can be correlated with the decrease in IR frequency of
CO from 2003 to 1888 cm1 (w) in the metal complex. The decrease
in frequency of CO absorption is due to accumulation of charge density on molybdenum
atom which stabilizes itself by transferring it back to the nearby CO (a Π-acceptor
In W(CO)6, the central metal atom is in zero valent oxidation state (electron rich) and shows IR stretching frequency at 2003 cm1. The lowering of V(CO) stretching frequency from 2003 cm1 to 1845 (w) cm1 again indicates back donation of electron density from M to CO. Such sort of back donation supports the acceptance of charge density by tungsten from 2-bromo-DMBA through its Nitrogen donor atom. This metal nitrogen coordination is also confirmed by the appearance of new band at 370 cm1 (s) attributed to v(M-N).
The 1H NMR spectra of 2-bromo-N, N-Dimethylbenzylamine-Mo(CO)5 complex (Table 5) show a multiplet at δ7.35 integrating to proton H-6. A second multiplet at δ 7.16 corresponding to H-3, a multiplet at δ7.12 corresponding to H-5 and a singlet at δ7.28 integrating to one proton due to H-4. Methylene proton shows a singlet at δ3.49 and six N-methyl protons show a singlet at δ2.24. The comparison of the NMR spectra of the ligand and complex shows an up-field shift of half the methyl protons and methylene protons the protons at ortho position on the aromatic ring. However a very small shift observed for meta protons and para protons. We can observe the up-field shift of 0.09 and 0.18 ppm for methyl and methylene protons due to electron-withdrawing effect of metal. A downfield shift of about 0.27 ppm is also observed for meta proton and an up field shift of about 0.30 ppm for para protons. The up field shift for methyl carbon in the complex about 0.08 ppm and for methylene carbon 4.0 ppm. Interestingly 1, 2, 3 and C-5 give downward shift, while 4 and C-6 give up field shifts. A peak at about δ190, 191, 202 show five carbonyl carbons in the complex.
The 1H NMR spectra of 2-bromo-N, N-DMBA- W(CO)5 complex (Table 6) show a multiplet at δ 7.53 integrating to proton H-6.
data of 2-bromo- N, N-Dimethylbenzylamine - Mo (CO) 5
||NMR spectral data of 2-bromo- N, N-Dimethylbenzylamine-W(CO)5
A second multiplet at δ 7.36 corresponding to H-3, a multiplet at δ 7.25 corresponding to H-5 and a
singlet at δ 7.48 integrating to one proton due to H-4. Methylene proton
shows a singlet at δ 3.08 and six methyl protons show a singlet at δ
2.64. The comparison of the NMR spectra of the ligand and complex show an up-field
shift for the methyl protons and methylene ptons the protons at ortho-position
on the aromatic ring. However a very small shift is observed for meta-protons
and para-protons. We can observe the up-field shift of 0.49 and 0.29 ppm for
N-methyl and methylene protons due to electron-withdrawing effect of metal.
A downfield shift of about 0.07 ppm is also observed for meta-proton and an
up-field shift of about 0.50 ppm for para-protons. Similarly an up field shift
was observed for 5 and H-6 proton in the spectra of the complex. These down
and up field shifts in respective protons, particularly for the methyl proton
directly attached to the donor nitrogen atom indicates its interaction with
tungsten through coordinate bonding (Fig. 1).
The authors thank to the University of the Punjab, Lahore, Pakistan and the Charles Wallace Pakistan Trust for financial support. We are also grateful to Loughborough University for providing research facilities.