Synthesis and Characterization of Group-6 Metal Carbonyl Complexes of Aroyl Hydrazone Derivatives

Carbonyl complexes of Chromium, molybdenum and tungsten of composition, [M(CO)4L-L], (where M= Cr, Mo or W and L-L= benzoic acid[1-(Furan-2-yl)methylene]hydrazide (BFMH), benzoic acid[(thiophene-2yl)methylene]hydrazide (BTMH), benzoic acid[1-(thiophene-2-yl)ethylidene] hydrazide (BTEH), benzoic acid (phenylmethylene)hydrazide (BPMH) and benzoic acid[1-(anisol-3-yl)methylene] hydrazide (BAMH) are reported. These have been prepared by refluxing metal carbonyls and the ligands in 1:1molar ratio. The complexes were characterized by elemental analysis, IR spectra, UV-vis spectra, H NMR, TGA/DTA, conductivity and magnetic susceptibility measurements. The IR bands suggest that in all the complexes the ligands behave as neutral bidentate chelating type coordinating metal through carbonyl oxygen and azomethine nitrogen. The CO force constants and CO-CO interaction constants for these derivatives have been calculated using CottonKraihanzel secular equations, which indicate poor π-bonding ability of the coordinated ligands.


Introduction
The chemistry of metal carbonyls has been of considerable interest for several decades mainly due to structural aspects and reactivity of these compounds in respect to several classes of organic ligands, and also by their applications in catalysis and in electronic devices based on non-linear optical effects [1][2][3][4] .Transition metal carbonyl derivatives are intermediates in homogeneous catalytic reactions such as carbonylation, hydrogenation, hydroformylation and oxygen transfer 5 .They are common starting materials for the synthesis of other low-valent metal complexes, especially clusters.The carbonyl ligands can not only be substituted by a large number of other ligands (lewis bases, olefins, and arenes), but the remaining carbonyl groups stabilize the molecules against oxidation or thermal decomposition.Carbonyl groups are also useful probe for determining the electronic and molecular structure of organometallic species by spectroscopic methods 6 .Research on preparation, structures and applications of metal carbonyls and their derivatives has been intense for several decades.The first metal carbonyl, Ni(CO) 4 , was prepared by A. Mond and coworkers 7 in 1890 by the reaction of metallic nickel with carbon monoxide.It found application immediately and has been used for industrial preparation of pure nickel.Many other carbonyls were synthesized shortly thereafter 8 .Carbonyl complexes of group 6 which contain nitrogen and phosphorus ligands exhibit an interesting and varied chemistry [9][10][11] .Reports on 2, 2′ -bipyridine (bipy) substituted tetracarbonyls and tricarbonyl phosphine complexes and their structural investigation emerged from literature many years ago 12 .S, N-ligated derivatives of hexacarbonyltungsten( 0 2-] have been reported and found to be O,Nligated complexes 14,15 .Chromium, manganese, cobalt, nickel and ruthenium carbonyl complexes with a wide variety of ligands having donor atom sets such as N 2 O 2 , NO 2 and N 4 around the metal ion have been used as catalysts for epoxidation reactions [16][17][18][19] .In the present study we have used ligands namely benzoic acid [

Methods and instruments
Carbon, hydrogen, nitrogen and sulphur were analysed microanalytically using CHNS analyzer Leco make model-932.Metals analysis was done by gravimetric method 21 .Molar conductivity at room temperature in DMF (10 -3 M ) was measured using an Elicoconductvity Bridge, type CM82T and a conductivity cell with cell constant of 0.74.Magnetic susceptibility of the complexes was recorded at room temperature using Gouy's balance and using Hg [Co (NCS) 4 ] as a standard.IR spectra of complexes over the region 4000-400 cm -1 were recorded on a Perkin Elmer FTIR spectrophotometer using KBr disc.The electronic spectra of the complexes were recorded using a double beam UV-visible spectrophotometer type SL-164.Thermogravimetric analysis (DTA-DTG-TG) of complexes was recorded on Perkin Elmer (Pyris Diamond) thermoanalyser at the heating rate of 10 0 C/min in the atmosphere of air.

Results and Discussion
The analytical and physical data (Table 1) showed that all complexes are mononuclear with general formula, [M(CO) 4

Conductance measurement
The molar conductivity (measured in 10 -3 M DMF solution) of these complexes is in the range of 15-34 mho cm 2 mol -1 which is much less than the value of 70-160 mho cm 2 mol -1 for 1:1 electrolyte in this solvent.Thus, these complexes are undissociated which indicates the non-electrolytic nature of these complexes.

Infrared spectra
The CO force constants and CO-CO interaction constants for these derivatives have been calculated using Cotton-Kraihanzel secular equations 22 .
In these equations, k 1 = stretching force constant for CO groups trans to ligands, k 2 = stretching force constant for CO groups cis to ligands, k i = CO-CO interaction force constant, µ= reciprocal of reduced mass of CO (0.14583) and A = (5.8890x 10 -2  2 ) where  = frequency in cm -1 and the force constants are in dynes cm -1 .
3][24][25] The frequencies for the complexes of unsymmetrical ligands are assigned as follows.The higher frequency A l and B 1 bands are assigned to transcarbonyl ligands, while the lower frequency A 1 and B 2 bands are assigned to cis-carbonyl ligands.
The values of k 1 , k 2 and k i for tungsten complexes were found in the ranges 14.09-14.44,15.87-16.08 and 0.23-0.25 mdynes/A O respectively, the values of k 1 , k 2 and k i for molybdenum complexes were found in the ranges 13.75-13.96,15.69-16.03and 0.26-0.32mdynes/A O respectively and the values of k 1 , k 2 and k i for chromium complexes were found in the ranges 13.81-14.00,15.59-15.80 and 0.20-0.25 mdynes/A O respectively, which confirm the validity of vibrational mode assignments.The values of CO stretching force constants in the complexes are lower than that for W(CO) 6 (16.41 mdynes/A O ) , Mo(CO) 6 (16.52 mdynes/A O ) and Cr(CO) 6 (16.49mdynes/A O ) indicating the replacement of two CO groups by the ligand (L-L) and poor pi-bonding ability of the ligands used 26,27 .These values are close to the values of force constants deduced for other nitrogen containing cisdisubstituted group 6 metal carbonyls 28,29 .
Moreover, the IR bands appearing in the spectra of the free ligands (aroylhydrazone) at ca. 1660 and 1640 cm -1 are attributed to amide ν(C=O) and ν(C=N) modes respectively 30 .The spectra of free ligands exhibit ν(N-H) stretching band at ca.3250 and ν(C=N) bands ca.1640 cm -1 indicating that ligands exist in keto form.The presence of ν(N-H) bands in the spectra of the complexes suggest that all the ligands remain protonated on complexation to metal.The amide bands ν(C=O) and ν(C=N) shift to lower frequencies (Table 2) in the spectra of complexes suggesting the involvement of carbonyl oxygen and azomethine nitrogen in coordination to metal 31 .Thus, these observations suggest that the ligands behave as neutral bidentate chelating type coordinating metal through carbonyl oxygen and azomethine nitrogen.The Ir spectra of two representative complexes namely Mo(CO) 4 BFMH and Mo(CO) 4 BTMH are given in Figure 1and 2 respectively.Table 2 IR spectral data(cm -1 ) and CO force constants of cis-[M(CO) 4 (L-L)] Complexes.

Electronic spectra
Electronic spectra of mixed ligand carbonyl derivatives of the type cis-M(CO) 4 L-L (M=Cr, Mo and W and L-L = a bidentate nitrogen ligand) have been extensively studied by Stufkens and collaborators 32,33 .
The complexes under study exhibit three sharp spectral peaks (Table 3) in the region of 325-440 nm.These may be attributed to d -π* CO transitions.A peak of low intensity in the region of 475-510 nm is also observed in these complexes which may be assigned to d-d transition.The spectra of all complexes show low field signals for imino proton at nearly same δ value as that of corresponding free ligands.Moreover, CH=N peak in complexes is shifted towards higher delta value (8.78-9.12)due to deshielding because of bonding of azomethine nitrogen with metal.The1 H NMR spectrum of a representative complex, W(CO) 4 BTMH is given in Figure 3.

The 1 H
NMR spectra of ligands in DMSO show low field signals at 11.8-11.4ppm for the imino proton confirming the existence of ligands in keto form.The CH=N peak is observed at 8.40-8.60 ppm and signal due to aromatic protons which occurs as multiplets appears between δ 6.4-7.8 ppm.The resonance signal due to CH 3 protons present in BTEH ligand occurs at 2.43 ppm.

Conclusion
Based on physicochemical and spectral studies, it is proposed that these complexes are mononuclear with composition, [M(CO) 4 L-L] having cis configuration, where L-L = benzoic acid[1-(Furan-2-yl)methylene]hydrazide (BFMH), benzoic acid[(thiophene-2-yl)methylene] hydrazide (BTMH), benzoic acid[1-(thiophene-2-yl)ethylidene]hydrazide (BTEH), benzoic acid (phenylmethylene) hydrazide (BPMH) and benzoic acid[1-(anisol-3-yl)methylene] hydrazide (BAMH) and M = Cr, Mo and W).IR spectral studies reveal that the ligands (L-L) behave as bidentate coordinating metal through carbonyl oxygen and azomethine nitrogen of the corresponding ligands.1 H NMR also reveals the coordination of ligand to metal through carbonyl oxygen and azomethine nitrogen.T he el ectro ni c sp e ctr a o f co mp l e x es show peaks responsible for d-d transition.Molar conductance of complexes indicates that these complexes behave as non-electrolytes and are undissociated in solutions.The Thermogravimetric curves of the compounds were recorded in the temperature range of 25-800 0 C in the atmosphere of air.The TGA curves of the two representative complexes indicate a continuous weight loss till a metal oxide is formed.On the basis of above facts, following structure is proposed for these complexes., Mo and W, L-L = BFMH, BTMH, BTEH, BPMH and BAMH

Table 1 .
Analytical data and some physical properties of the synthesized complexes cis-[M(CO) 4 (L-L)].
4BFMH] (Figure4) shows a sharp loss starting from 100 o C upto 300 o C probably due to the loss of carbonyl groups.Thereafter a gradual but continuous weight loss takes place upto 600 o C probably due to the decomposition and loss of BFMH ligand.Further heating upto 800 o C does not show any weight loss indicating the formation of stable metal oxide, WO 2 (Calculated weight = 42.32% and observed weight = 42.42%).DTA curve also shows two exothermic peaks at 300 o C and 600 o C indicating decomposition of complex at 300 o C and 600 o C and oxidation of W toWO 2 and its crystallization.