Preparation and Characterization of Cyano Complexes of Oxotungsten ( IV ) with Mannich Base Ligands

Oxocyanotungsten(IV) complexes, (Ph3P)2 NH2 [WO(CN)3 L–L]. 3H2O and Cs[WO(CN)3L–L].H2O (where L–L = morpholinobenzyl benzamide (MBB), piperidinobenzyl benzamide (PBB), piperidinobenzyl urea (PBU), piperidinobenzyl thiourea (PBTU), morpholinobenzyl urea (MBU) and morpholinobenzyl thiourea (MBTU)) were synthesized. The complexes have been prepared by the reaction of K3Na [WO2(CN)4].6H2O with morpholinobenzyl urea and related ligands in aqueous solution around pH of 78. These have been isolated as bis(triphenylphosphine)iminium or cesium salts. The complexes have been characterized by elemental analysis, IR, H NMR, UV-Vis spectra, magnetic susceptibility, conductivity measurements and TGA/DTA studies.


Introduction
A series of oxocyano complexes of molybdenum(IV) and tungsten(IV) with various monodentate ligands of formula [M(CN) 4 OL] n-, where L = NCS -, N 3 -, F -, HCN and py, M = Mo or W have been synthesized and characterized [1][2][3] .The complexes of the type (PPh 4 ) 3 [W(CN) 5 O].7H 2 O have been found to react with molecular oxygen and their kinetics and mechanism have been studied 4 .The protonation of the dioxotetracyano complexes of rhenium(V) and technetium(V) result in the formation of the [MO(OH)(CN) 4 ] (n+1)-and [MO(H 2 O)(CN) 4 ] n-complexes and have been studied in detail over the past few years 5,6 .Dioxotetracyanotungstate(IV) and dioxotetracyanomolybdate(IV) ions may be protonated to form the oxo-hydroxo and oxo-aqua complexes, [MO 2 (CN)   O.These protonated species can undergo substitution reactions with mono-(aqua ligand substitution) and bidentate (aqua and cyano ligand substitution) nucleophiles [9][10][11][12][13][14][15][16] .The protonation behaviour as well as the cyanide exchange of these systems has recently been the subject of carbon-13 and oxygen-17 NMR studies in order to investigate the water and cyanide exchange of these complexes.
Previous studies of the substitution reactions of these oxoaqua complexes included different monodentate nucleophiles such as F -, N 3 -, NCS -, py and substituted thioureas for the tungsten(IV) and rhenium(V) complexes [11][12][13][14] .Only reactions of the F -and CN -/HCN ions with the molybdenum(IV) complex could be studied due to the high lability of the coordinated aqua ligand in the [MoO(H 2 O)(CN) 4 ] 2-complex 9 .Thus, it was impossible to do extensive kinetic studies, specifically high pressure studies, on the substitution reactions of the [MoO(H 2 O)(CN) 4 ] 2-complex.The kinetic study of the monodentate substitution (N 3 -, NCS -, py) of the aqua ligand of the [WO(H 2 O)(CN) 4 ] 2- complex showed that a linear free energy relationship between lnk L and lnK L (k L is the hydrolysis rate constant and K L is the stability constant for the [WO(L)(CN) 4 ] n-) exists and the existence of this relationship points towards a dissociative mechanism for these substitution reactions.
The [WO(H 2 O)(CN) 4 ] 2-complexes can also undergo substitution by bidentate ligands.The crystal structure determinations of the products [17][18][19] of the reactions between the oxoaqua complexes and bidentate ligands showed that the aqua ligand and one of the cyano ligands in the equitorial plane are substituted by the entering bidentate nucleophiles.It is expected that the aqua ligand will be substituted first during the two step process, since the metal-aqua bond is usually much weaker than a metal-cyano bond in this type of complexes, especially as a result of the large trans influence of the oxo ligand.The crystal structure determination of the [WO(pic)(CN) 4 ] 2-(pic = pyridine-2-carboxylate) showed that an oxygen atom of the carboxylic group is bonded trans to the nitrido ligand and that the nitrogen atom is bonded trans to a cyano ligand in the equitorial plane of the octahedral complex.This is interpreted to mean that the aqua ligand is indeed substituted first, since it is known that the oxygen atom of the carboxylic ligand will bend before the nitrogen atom in this type of bidentate ligand.The substitution reactions of the [WO(H 2 O)(CN) 4 ] 2-complex with monodentate ligands are about a 1000 times faster compared to the bidentate substitution reactions indicating that a dissociative mechanism is operative.This gives a strong indication that the substitution of the aqua ligand is the fast first reaction.The substitution of the cyano ligand and the simultaneous ring closure of the chelate ring is a relatively slow reaction and this reaction is the rate determining step in the overall process 20 .In the present chapter, we have prepared oxotungsten(IV) complexes by reacting [WO(H 2 O)(CN) 4 ] 2- with the bidentate ligand, viz; morpholinobenzyl benzamide (MBB), piperidinobenzyl benzamide (PBB), piperidinobenzyl urea (PBU), piperidinobenzyl thiourea (PBTU), morpholinobenzyl urea (MBU) and morpholinobenzyl thiourea (MBTU) when the aqua and one cyano ligands are replaced by the ligand and complexes having composition [WO(CN) 3 L-L] -were obtained.The structures of ligands are given in Scheme 1.

Methods and instruments
The analysis of tungsten was carried out gravimetrically by the reported method 23 .Carbon, hydrogen, nitrogen and sulfur analyses were performed by microanalytical methods.IR spectra of the complexes over the region 400-4000 cm -1 were recorded on a FT-IR spectrophotometer-vector-22 using a KBr disc.Molar conductivity in DMSO (10 -3 M) at room temperature was measured by an Elico conductivity bridge type CM82T having a conductivity cell with cell constant of 0.90.TGA/DTA studies have been performed in the temperature range of 25-800 o C on a DTG 60 Thermoanalyser under nitrogen atmosphere at a heating rate of 10 °C per minute.
The complexes 1-6 (Table 1) were isolated as bis(triphenylphosphine)iminium salts and 7-12 (Table 1) were isolated as cesium salts.The (Ph 3 P) 2 NH 2 + cation or Cs + cation form insoluble salts with the product complex, thus shifting the equilibrium to the right.
The analytical and spectroscopic results (Tables 1-3) showed that all the complexes are mononuclear with general formula, (Ph All the complexes are colored, stable, insoluble in water but soluble in organic solvents like dichloromethane, N,N'dimethylformamide and dimethylsulphoxide.

Conductance and magnetic measurements
The molar conductance of these complexes in N,N'dimethylformamide are in the range of 79-98 ohm -1 cm 2 mol -1 (Table 1) which indicates the uni-univalent electrolytic nature of the complexes.All the complexes are diamagnetic in nature consistent with a spin paired d 2 configuration 24 .

Infrared spectra
In order to determine the mode of bonding of Mannich base ligands with tungsten, the IR spectra of ligands were compared with those of corresponding complexes.The IR spectra (Table 2) of all the complexes exhibit bands in the region 940-970 cm -1 which can be assigned to the stretching vibrations of the W=O group 20 .In the CN stretching region the IR spectra show two bands in the region of 2066 to 2135 cm -1 .
Table 2. Important IR bands (cm -1 ) of the oxotungsten(IV) complexes ( The ν(C=O) absorption band at 1610-1650 cm -1 in the free MBB, PBB, PBU and MBU and ν(C=S) modes of PBTU and MBTU at 1290-1295 cm -1 are lowered by 20 to 25 cm -1 in the complexes indicating the involvement of carbonyl oxygen (O) or thiocarbonyl sulphur (S) in bonding with the tungsten.The ν(C-N-C) mode of morpholine and piperidine ring around 1104-1130 cm -1 also show a negative shift in the complexes indicating the involvement of ring nitrogen in bonding with tungsten.
There is also negative shift in stretching and bending modes of N-H bands of morpholine and piperidine ring in complexes which indicate involvement of ring nitrogen in W-N bond formation.A slight positive shift in C-O-C stretching mode of morpholine ring shows that morpholine ring does not involve its oxygen atom in bond formation with tungsten.These IR bands suggest that in all the complexes the ligands MBB, PBB, PBU, PBTU, MBU and MBTU behave in bidentate chelating mode coordinating the metal through carbonyl(O) or thiocarbonyl(S) and the ring nitrogen.

Electronic spectra
The electronic spectra (Table 3) of the complexes exhibit intense spectral peaks in the region 360-415 nm.A peak of low intensity is also observed in the region 560-620 nm.The bands observed in the region 360-415 nm can be assigned to charge-transfer to metal (LMCT) transitions.The weak bands observed in the region 560-620 nm can be attributed to d-d transitions.

1 H NMR Spectra The 1 H
NMR spectrum of the representative ligands, morpholinobenzyl urea (MBU) was recorded in deutrated methanol.The spectrum shows a triplet at δ 3.34 for N-CH 2 of morpholine ring for MBU.The methylidyne proton of Ph-CH-NH appears as a septet at δ 7.54.Cs[WO(CN) 3 MBU].H 2 O, the peaks corresponding to four protons of -CH 2 groups of N-CH 2 in morpholine ring undergo downfield shift with δ 3.52.This is due to coordination of ring nitrogen of piperidine with tungsten.

Table 1 .
Analytical data and some physical properties of oxotungsten(IV) complexes