Oxomanganese(II)-Arsine Oxide Complexes from o-R2AsC6H4CO2H Ligands: Role of Inductive Effect and Reaction Conditions in Stabilizing Manganese(II)-Arsine Complexes

Reactions of Mn(O2CMe)2. nH2O (n = 0 and 4) with the ligand o-R2AsC6H4CO2H (where, R = alkyl substituent such as -CH3 (Me), -C2H5 (Et), and -C6H11) formed nine oxomanganese (II)-arsine oxide complexes: [Mn2O{(o-R2As(O)C6H4CO2)2(H2O)n}. n' H2O] {R = -Me, n = 3; n' = 0 (three isomers); R = -Et, n = 3, n' = 0; n = 5, n' = 1, 2; R = -C6H11, n = 5, n' = 0 (two isomers), n = 1, n' = 0} in the presence of both moisture and oxygen. The prepared complexes were characterized by IR, UV-Vis, and EPR spectroscopic techniques and were further confirmed by measuring their magnetic susceptibility, thermal and molar conductance. The formation of two different types of complexes was due to the difference in the inductive effect of aryl and the alkyl substituent and the change of the counter-anion, i.e., chloride and acetate and their role significantly helped in deciding the formation of oxomanganese(II)-arsine oxide or oxomanganese(II)-arsine complexes.


INTROdUCTION
Complexation of organoarsinic ligand with a wide range of transition metal ions continues to play a vital role in the synthesis and application of organometallic chemistry. 1 However, the binding of organoarsinic ligand via coordinate bond with manganese(II) ion is not very common, as the former tends to oxidize easily from arsenic(III) to arsenic(V) even in the presence of traces of moisture and oxygen. 1,2 To the best of our knowledge, only ten manganese(II)-mono-tertiary arsine complexes has been reported in literature. [3][4][5] Chiswell et al., 3 reported two complexes: [Mn(As-N)X 2 ], where (X= Br or ClO 4 ) formed by the reaction between manganese(II) salts and o-dimethylarsinoaniline an arsenic-nitrogen chelating agent in the presence of both oxygen and water; the water being removed azeotropically with a stable As(III)→Mn(II) bond. We previously reported 4,5 eight complexes of manganese(II) ion with hybrid arsenic-oxygen chelating agents (As-O) in the presence of both oxygen and water. Four out of eight complexes consisted of manganese(II)-monotertiary arsine complexes 4  We had previously 4,5 supported Chiswell et al., 3 in contravention to McAuliffe's view [6][7][8][9] , that the presence of water and oxygen in reactions of manganese(II) salts with (As-O) hybrid ligands did not inhibit the formation of As(III)→Mn(II) bond with hybrid ligand o-R 2  bond with the ligands having alkyl substituent (R = -Me, -Et) 4 while R = -C 6 H 11 always formed the arsine oxide complexes. The presented work is aimed to study the effect of changing both the substituent from aryl (R = -Ph, p-tolyl) to alkyl (R = -Me, -Et, -C 6 H 11 ) and, thereby, the inductive effect and also of the counter anion from chloride to acetate in stabilizing manganese (II)-arsine complexes.

Methodology
Details of the experimental method for the preparation of ligand (Structure 1, Fig. 1; M = H, Na), anhydrous Mn(O 2 CMe) 2 , and the various studied spectral measurements of the complexes were reported elsewhere. 5,10-14

Electronic Absorption Spectral Studies:
The electronic absorption spectra of the complexes were recorded using a VSU-2P (DDR) spectrophotometer in the range 10000-30000 cm -1 in solid-state using magnesium oxide as the standard reflector.

Molar Conductance Measurements:
The molar conductance values of millimolar solutions of prepared complexes in PhNO 2 or CH 2 Cl 2 were measured on a Toshniwal Conductivity Bridge Type CLOI/O2A using conventional dip type platinum electrode.
Thermogravimetric Analysis: The thermogravimetric analysis of the complexes was carried out on a manual thermo-balance (FCI) at the heating rate of 100 o C/min and % loss in weight was plotted against temperature.

Electron Paramagnetic Resonance (EPR) Spectral
Studies: The powder pattern EPR of the complexes at room temperature were recorded at R.S.I.C., I.I.T., Madras using Varian Spectrophotometer having a constant microwave frequency of 9.3 GHz (X-band; 0-10000G). The Lande's splitting factor, g values were calculated using the formula: hn = gbH, here H is the applied magnetic field in gauss where the peak appears.

Elemental (Mn and As) Analysis: (i)
Mn 2+ was estimated volumetrically by EDTA method 12 . (ii) As(III) could not be estimated because of its spontaneous oxidation to As(V) in all the 9 complexes.

RESULTS ANd dISCUSSION
Elemental [C, H, and Mn] analysis data (Table 1) and thermogravimetric and molar conductance data of the nine oxomanganese(II)arsine oxide complexes shown below in Table 2 corroborated with the stoichiometry of the prepared complexes. All the studied compounds here behaved as nonelectrolytes in both nitrobenzene (C 6 H 5 NO 2 ) and dichloromethane (CH 2 Cl 2 ) solvents. The nonelectrolytic behavior of the complexes was confirmed with their measured molar conductance values ( Table 2). Since the solid complexes would separate under cryogenic conditions, it was not possible to determine their molecular weights.

Presence of bent Mn(II)-O-Mn(II) system in complexes
The presence of a strong band at 590-610 cm -1 region (Table 3) in all these nine complexes was assigned to n sym (Mn-O-Mn) to indicate the presence of bent Mn(II)-O-Mn(II) system. This band was, also, invariably present in the four oxomanganese(II)arsine complexes 5 as there would occur magnetic exchange between Mn(II)-3d and O-2p orbital in the oxo-complexes. Of course, it was always found missing in manganese(II)-arsine complexes 4 .

B o n d i n g m o d e o f c a r b ox y l a t e i o n i n oxomanganese(II)-arsine oxide complexes
IR spectra of two oxomanganese(II)-arsenic oxide complexes [Mn 2 O{o-R 2 As(O)C 6 H 4 CO 2 ) 2 (H 2 O) n}] (R = -Me, n = 3; R = -C 6 H 11 , n = 1) obtained from Reaction-III resembled with (Structure 2, Fig. 1; M = H) and were termed as a-type of oxides where the 740 cm -1 p C-H band of (Structure 1, Fig. 1; M = H) would shift to 765±5 cm -1 and the δ OCO band at 835 cm -1 was observed to be much weaker than n C-H band. Moreover, absence of bands due to n (As-OH) 15,16 at 2360, 2370 cm -1 (R = -Me) and 2385, 2360 cm -1 (R = -C 6 H 11 ) and in 865-870 cm -1 region n As=O 11 in the two a-type of oxomanganese(II) oxide complexes indicated the formation of As-O-Mn group implying the absence of As=O species.
In the remaining seven oxomanganese(II)arsine oxide complexes, the aforesaid intensity pattern was reversed. An abnormally strong band appeared at 830±10 cm -1 which obscured the δ OCO band and was assigned to n As=O 11 because the appearance of the two bands, i.e. n As=O and δ OCO at almost the same region resulted in increased intensity. These observations could be rationalized if these seven arsine oxides were assumed to contain carboxylato-arsine moiety derived from the structure (Structure 3, Fig. 1; M = H); called b-type arsine oxides. Lowering of n As=O from 865 cm -1 in this structure to 830±10 cm -1 in these b-type arsine oxide complexes (Table 3) was attributed to the coordination of oxygen 17,18 of As=O group to Mn (II). In two a-type arsine oxide complexes [Mn 2 O{o-R 2 As(O)C 6 H 4 CO 2 ) 2 (H 2 O)n}] (R = -Me, n = 3; R = -C 6 H 11 , n = 1),a symmetric chelation of Mn(II) with the carboxylate ions was observed (Structure 4, Fig. 1) as both n asym (CO 2 ) and n sym (CO 2 ) values were found to be higher than those of the sodium salts of the ligands ( Table 3). The direction shift (d.s) criterion 11,14,19 when applied to IR spectra (Table 3) of the seven b-type arsine oxide complexes favored uni-dentate mode because their n asym (CO 2 ) were raised and n sym (CO 2 ) was lowered from those of values of sodium salts of the ligands. This, accompanied by the coordination of oxygen of As=O group to Mn(II), implied the formation of (Structure: 3, Fig. 1) because a very strong new band appeared at 820-840 cm -1 due to the lowering of n As=O 11 on coordination with Mn(II) with no band in 260-280 cm -1 region of n M-As 20 to indicate the absence of Mn(II)-As(III) bond.
either the same number of water molecules or more than the corresponding complexes isolated from the anhydrous conditions in Reaction-II because they would absorb moisture very strongly. The presence of water in the complexes was indicated by the appearance of a strong band n OH (H 2 O) 21 at ≈ 3350-3400 cm -1 and δ HOH (sh) 22 ≈ 1600-1630 cm -1 ( Table  3). The rocking mode of coordinated water ρ(H 2 O) 23 found at ≈ 800-900 cm -1 was obscured by [δ OCO ] 24 / [n As=O ] 11 at ≈ 850-70 cm -1 . The strongly coordinated water molecules were lost after 100 o C while the lattice or loosely bound water molecules were lost up to 100 o C (T.G.A. data; Table 2). 25 These studies show that out of the 9 oxide complexes, there are three isomers for R = -Me; two for R = -C 6 H 11 .

Magnetic moments of the complexes
Formation of the bent system would result in only a marginal lowering of magnetic moment values while the linear bridging caused drastic lowering. Since no Mn(II)-O-Mn(II) systems were previously known like those of isoelectronic Fe(III)-O-Fe(III) systems, the formation of σ and π bonds were involved to explain the spin-exchange between Mn(II)-3d and O-2p orbital. [26][27][28][29] The experimental μ eff values of the nine oxomanganese(II)-arsine oxide complexes were found to lie in the range of 5.0-5.98 B.M. (Table 4) indicating the presence of high spin Mn(II) having 6 A 1g ground term.

Reflectance Spectra of the complexes
Reflectance electronic spectra of the complexes showed very weak bands due to their doubly forbidden nature as neither they obey the multiplicity rule nor the symmetry (Laporte) rule that favored their octahedral stereochemistry. 30 With

Presence of water in complexes
The complexes obtained under hydrated conditions in Reaction-I and Reaction-III possessed five unpaired electrons, these Mn(II) complexes have six multiplicity with 6 S ground state and the symmetry symbol 6 A. The ground term is represented by 6 A 1g where 'g' stands for gerade in octahedral stereochemistry. The spectroscopic state immediately higher to 6 S state is 4 G which splits up into 4 T 1g , 4 T 2g , 4 A 1g , 4 E g in an octahedral field. Three spectral bands arising from 6 A to 4 G are assigned to as 6 A 1g → 4 T 1g ( 4 G), 6 A 1g → 4 T 2g ( 4 G) and 6 A 1g → 4 A 1g = 4 E g ( 4 G) as the last two terms do not differ very largely in their energies. The fourth band occurs by the transition between the ground 6 A 1g term to 4 T 2g obtained by the splitting of 4 F which is higher in energy to 4 G (Table 4).

Table 4: Electronic Spectral (cm -1 ) and Tentative Structures of Oxomanganese(II)-arsine oxide Complexes
Formula of the Complex, Complex number

EPR Spectra of the complexes
Only [Mn 2 O{(o-R 2 As(O)C 6 H 4 CO 2 ) 2 (H 2 O)n}] (R = -Me, -Et, n = 3; R = -C 6 H 11 , n = 5) complexes gave an EPR signal at room temperature in the form of a broad single peak with Lande's splitting factor value, g ≈ 2.0 to indicate their nearly axial symmetry with a small distortion from octahedral stereochemistry having 6 A 1g ground term 31 as the lines were broadened due to spin-lattice relaxation and the magnetic exchange between manganese(II) ions because of the presence of intervening oxo group, i.e. Mn(II)-3d and O-2p orbital. 32

Structures of the oxomanganese(II)-arsine oxide complexes
The IR spectra of all the studied complexes indicated the presence of bent Mn(II)-O-Mn(II) unit; the uni-dentate coordination of Mn(II) with one carboxylato oxygen and also with the oxygen of As=O in the seven b-type arsine oxide complexes while the symmetric chelation of Mn(II) with carboxylate ion in the remaining two a-type arsine oxides was indicated. Electronic and EPR spectra and magnetic data complemented one another to confirm the presence of octahedral stereochemistry around Mn(II). Thermal data ( Table 2) showed the presence of both the coordinated and lattice water molecules as indicated in their respective stoichiometries in the complexes. These studies corroborated with one another to assign them structures 5-8 as mentioned in (Table 4) and were represented in Fig. 2. We also generate the ball and stick model for these compounds (5-7) that helped us to provide different points of view for 3D images. We were not able to generate a ball and stick model Image for structure 8 due to the complexity of the structure. We used Gaussian 98 program to generate the ball and stick model for these complexes.

Role of inductive effect and reaction conditions in stabilizing As(III)→ Mn(II) bond
The The formation of two a-type of arsenic oxide complexes was considered to take place when the oxidation of arsine preceded the coordination.

CONCLUSION
(i) McAuliffe's 6-9 emphasis on strictly anhydrous and deoxygenated conditions for the preparation of manganese(II)-phosphine/ arsine complexes might hold good for soft ligands because the present investigation and two of our previous studies 4,5 corroborated with Chiswell et al., 3 to prove that the presence of water and oxygen in reactions of manganese(II) salts with (As-O) hybrid ligands did not inhibit the formation of Whether an arsine complex would form or an arsine oxide complex would result depends both upon the inductive effect of the substituent (aryl or alkyl) and the presence of the counter anion (chloride or acetate) as follows: word about its existence in these complexes could be said with a certainty only after singlecrystal X-ray investigation. We were unable to develop their crystals because of the virtual insolubility of all the nine complexes in almost all the common solvents.