Metal Complexes of Hybrid Oxygen- Arsenic Ligands-VII IR, Reflectance and EPR Spectral Studies of Oxo-manganese (II)-Arsine Complexes Using Ligands from o-R2AsC6H4CO2H

Introduction

Transition metal complexes with organic ligands are of great interest of researchers since the discovery of first such complex compound of Ni(II) with azabenzene ligand in early 1960s.¹ Since then a wide variety of cyclometallated complex compounds of transition and post transition metal ions with ligands containing nitrogen, phosphorous, arsenic, Oxygen, sulfur etc. as donor atoms has been investigated thoroughly.² The complexes of manganese (II) with arsines ligands are not very common as the arsines are very much susceptible to oxidation even in the presence of traces of moisture when reacted with Mn (II) salts like manganese (II) halides react with AsPh₃ to form complexes like [Mn(OAsPh₃)₂X₂]^3,4 where AsPh₃ would oxidize to arsine oxide.

Six manganese (II)-mono-tertiary arsine complexes had been reported in literature.^5,6 Chiswell et al.⁵ were able to stabilize two manganese (II)-arsine complexes [Mn(As-N)X₂] (where, X= Br or ClO₄) with arsenic-nitrogen chelating agent-o-dimethylarsinoaniline even in the presence of both oxygen and water  the latter was removed from the mixture azeotropically. Parmar et al.⁶ reported four manganese (II)- mono-tertiary arsine complexes [Mn{(o-Me₂AsC₆H₄CO₂)Cl}.½H₂O] and [Mn{(o-R₂ AsC₆H₄CO₂)₂(H₂O)₂}. nH₂O] (R=Ph, p-tolyl, n=0; and R=Et, n=1) formed by reacting MnCl₂.4H₂O with o-R₂AsC₆H₄CO₂Na (R= Me, Et, Ph, p-tolyl) in 1:2 molar ratio in 95% EtOH. All these four complexes were also isolated in the presence of both moisture and oxygen.

We previously⁶ were able to prove that McAuliffe’s emphasis^3,4,7,8 on the use of strictly non arial anhydrous reaction medium for stabilizing As(III)-Mn(II) bond would hold good only for soft ligands and not for (As-O) hybrid ligands. We, now, extend this argument further by using both the anhydrous Mn(O₂CMe)₂ and hydrated Mn(O₂CMe)₂.4H₂O in 95% Et OH as well as ethanol with o– R₂AsC₆H₄CO₂H instead of o-R₂AsC₆H₄CO₂Na. Our emphasis would, also include studying the effect of counter anion by replacing chloride ion by acetate ion.

Experimental Details

Details of spectral and other measurements, preparation of [(Structure 1, Fig. 1); M=H, Na] and estimation of arsenic (III) were published elsewhere^9-13 A brief experimental details of spectral and magnetic moment measurements are shown below in 2.2 and 2.3. Analytical grade Mn(O₂CMe)₂. 4H₂O was used for the preparation of the complexes. Anhydrous Mn(O₂CMe)₂ was prepared by heating Mn(O₂CMe)₂. 4H₂O in a dry gun for 3-4 h and the resulting mass was cooled in vacuo. The vacuo refers to distillation using a rotary evaporator attached to an efficient vacuum pump. The process was repeated till its IR shows no bands characteristic of water. 

Preparation of Complexes of Manganese (II)

Preparation of [Mn₂O{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₅}.(H₂O)] (Structure 4)

Addition of a hot ethanolic solution (18-20 cm³) of [(Structure 1, Fig. 1); R=Ph & M=H] (8 mmol) and anhydrous Mn(O₂CMe)₂ (0.692 g, 4 mmol) in ethanol (30-35 cm³) gave a white solid. The reaction mass was further refluxed for 12 h. The solid was filtered and washed with EtOH, Et₂O and dried in vacuo. Yield: 50-55%. 

Preparation of [Mn₂O{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₄}] (Structure 5)  

The solution of [(Structure 1, Fig. 1); R=Ph & M=H)] (8 mmol) and Mn(O₂CMe)₂.4H₂O (0.98 g, 4 mmol) in 95% Et OH (40 cm³) was refluxed for 2h and resulted into a clear solution. The addition of Et₂O to this clear solution gave a white solid which was filtered, washed with Et₂O and dried in vacuo. Yield: 50-60%.

Preparation of [Mn₂O{(o-(p-tolyl)₂AsC₆H₄CO₂)₂(H₂O)₄}] (Structure 5)   

The reaction mass obtained by the dropwise addition of Mn(O₂CMe)₂ (0.692 g, 4 mmol) in 95% Et OH (25-35 cm³) to a solution of [(Structure 1, Fig. 1); R=p-Tolyl & M=H] (8 mmol) in the same solvent (18-20 cm³) and was refluxed for 12 h. The reaction mixture was cooled to 0-5 ^oC. The solid, thus, obtained was filtered, washed with 95% Et OH, Et₂O and dried in vacuo. Yield:  40-50%.

Spectroscopic Measurements

Infrared Spectra

The infrared spectra of the prepared complexes were recorded PYE UNICAM Sp3-300 IR Spectrophotometer in the range 4000-200 cm^-1 on KBr pellets. 

Electronic Absorption Spectra

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

Electron Paramagnetic Resonance (EPR) Spectra

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 g values were calculated by using the formula hn= gbH where H is the magnetic field in gauss measured at the point where the peak appears.

 Conductance Measurements

The molar conductance of millimolar solutions of the complexes in PhNO₂ or CH₂Cl₂ were measured on Toshniwal Conductivity Bridge Type CLOI/O2A using conventional dip type platinum electrode.

Thermogravimetric Analysis

The thermogravimetric analyses of the complexes were carried out on a manual thermo-balance (FCI) at the heating rate of 100 C/min and % loss in weight was plotted against temperature.

 Magnetic Susceptibility Measurements

Magnetic susceptibilities of the powdered samples of the complexes were measured at room temperature using Gouy’s method. Diamagnetic corrections for the ligand anions (o-R₂AsC₆H₄CO₂) ^– were calculated using Pascal’s Law constants (R=Me, -113.6 x 10^-6/mole; Et, -137.3 x 10^-6/mole; C₆H₁₁, -210.4 x 10^-6/mole; Ph, -188.1 x 10^-6/mole and p-tolyl, -205.9 x 10^-6/mole).

Results and Discussion

Elemental [C, H, N, Mn] (Table: 1) and thermogravimetric data (Table:2) authenticated the correctness of formulae of the oxomanganese (II)-arsine complexes which behaved as nonelectrolytes in PhNO₂ or CH₂Cl_2. Their molecular weights could not be determined as the complexes got separated as solids under cryoscopy conditions.

Table 1: Elemental Data of Manganese (II) Complexes.

Formula of Complex Compound ( M. Pt 0C)	Color* of the Complex Compound	Found (Calcd)%
		C	H	As(III)	Mn(II)
[Mn2O{(o-Ph2AsC6H4CO2)2(H2O)4}]         (175 oC)	Dirty White	50.2(50.9)	3.8(4.0)	16.3(16.6)	12.8(12.3)
[Mn2O{(o-Ph2AsC6H4CO2)2(H2O)5}H2O] (212 oC)	Light Pink	47.8(48.9)	4.1(4.3)	15.8(16.1)	12.1(11.8)
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4}]   ( ˃300 oC)	Light Pink	52.2(52.9)	4.7(4.6)	16.0(15.8)	12.0(11.6)
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4 }]   ( ˃300 oC)	Light Pink	52.2(52.9)	4.2(4.6)	15.2(15.8)	11.9(11.6)

* A white compound may change to dirty white or light pink on drying.

Table 2: Thermal Analysis and Molar Conductance Data of Manganese (II) Complexes.

Formula of Complex Compound	Thermal Analysis		Λ (S cm2mol-1)          in PhNO2(CH2Cl2)
	Temp. Range 0C	Loss% Found (Calcd.)
[Mn2O(o-Ph2AsC6H4CO2)2(H2O)4]	100-240	8.0(8.0; 4H2O)	—
[Mn2O{(o-Ph2AsC6H4CO2)2 (H2O)5}.H2O]	80-120120-200	1.75(1.91; H2O)9.25(9.64; 5H2O)	0.2
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2(H2O)4}]	100-160160-215	3.75(3.75; 2H2O)3.75(3.75; 2H2O)	—
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2(H2O)4}]	—	—	—

Presence of Bent Mn(II)-O-Mn(II)  System in the Complexes

Contrary to our previous study⁶ of reacting same ligands with [MnCl₂.4H₂O], a strong new band always appeared in 560-610 cm^-1 region for all the four synthesized complexes (Table: 3) that indicates the presence of a bent Mn(II)-O-Mn(II) system in the complexes.

Table 3: IR Spectral (cm^-1) and Magnetic Moments Data of Manganese (II) Complexes.

Formula of Complex Compund	νOH, [δHOH]	ν(asym CO2)	ν(sym CO2)	ν(Mn-O-Mn)	Structure
[Mn2O(o-Ph2AsC6H4CO2)2(H2O)4]	3610 w, 3430 brm, [1610 sh]	1595 vs	1390 vs	560 s	5
[Mn2O(o-Ph2AsC6H4CO2)2(H2O)5} .H2O]	3430 brm, [1615 sh]	1602 vs	1405 vs	610s	4
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4 }]	3360 brm, [1620 sh]	1589 vs	1378 vs	590s	5
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4}]	3400 brm, [1630 sh]	1580 s	1395 s	570s	5

Brm = broad medium intensity; s = strong; vs = very strong; w = weak; sh = shoulder

Bonding Mode of Carboxylate ion in Oxomanganese (II)-Arsine Complexes

The IR spectra of the complexes resembled with those of [(Structure 1, Fig. 1)¹⁷; M=H] in 700-900 cm^-1 region which possessed strong ρ_C-H¹⁴ and δ_C-C¹⁴ bands at 740 and 690 cm^-1 respectively and were always accompanied by a weak δ_OCO¹⁴ band at 835 cm^-1. Application of direction shift (d.s.) criterion^9,15,16 to the IR data (Table: 3) of the four complexes in the carboxylate region would suggest (structure 2; Fig:1)¹⁷ involving the simultaneous coordination of As(III) and carboxylato oxygen to Mn(II) to rule out (structure 3, Fig:1)¹⁶_. The widely different values of the marked bands in the two compounds with molecular formula [Mn₂O{(o-(p-tolyl)₂AsC₆H₄CO₂)₂(H₂O)₄}] is confirmation of their isomeric forms. The presence of oxo-bridged H₂O molecule in three of the four studied complexes, though difficult to ascertain only with vibrational spectral studies, yet the presence of broad band in the range of 930-50 cm^-1 points towards the presence of water in their structures.

Figure 1: o-Diarylarsinobenzoic acids with M=H or Na and (R= Ph or p– Tolyl) Click here to View figure

Figure 2: Top: Structure of Oxomanganese(II)-arsine complex, [Mn2O{(o-R2AsC6H4CO2)2(H2O)n}. n‘(H2O)] (R=Ph, n=5, n’ =1) (left) and its ball and stick model (right) Click here to View figure

Presence of Water in Complexes

Surprisingly, the complexes obtained under anhydrous conditions shown in Reaction 2.1.1 possessed either the same number of water molecules or even more than the corresponding complexes isolated from the hydrated conditions shown in Reactions 2.1.2 and 2.1.3. This could only be explained based on the absorption of moisture during the manipulations of Reaction 2.1.1. The presence of water molecules in the prepared complexes was indicated by the appearance of a strong band ν_OH (H₂O)¹⁹ at ≈ 3350 – 3400 cm^-1 and δ_HOH(sh)²⁰ at ≈ 1600 – 1630 cm^-1. Rocking mode of coordinated water ρ_(H2O)²¹ found at ≈ 800 – 900 cm^-1 was obscured by δ_OCO¹⁴ _/ ν_As=O¹⁰ bands at 835 – 870 cm^-1  region. The water molecules present in the complexes lost up to 100 ⁰C (T.G.A. data; Table: 2) were regarded as lattice or loosely coordinated while the remaining water molecules were strongly coordinated to the metal ion (Table: 2).²² In the complex compound [Mn₂O(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₅}H₂O], one water molecule that is present outside the bracket (}) is lost below 120 ^oC indicates that it is a either a loosely bound lattice water molecule. On the other hand, the stoichiometric loss of other five water molecules (% loss = 1.75 : 9.25 = 1 : 5) in this complex beyond 120 ^oC points towards the presence strongly bound coordinated bond with Mn(II) metal ion in the complex (Table: 2).²⁰   

Magnetic Moments of Complexes

The formation of the bent Mn(II)-O-Mn(II) system was expected to result only a marginal lowering of magnetic moment values in contrast to the linear bridging which would cause a drastic lowering. Probably, no bent Mn(II)-O-Mn(II) systems were known whereas in the analogous isoelectronic Fe(III)-O-Fe(III) systems, the formation of  σ and π bonds was invoked to explain the spin exchange coupling between M-3d and O-2p orbitals.^23-26 The experimental μ_eff values of these oxo-manganese (II) complexes would lie in the range of 5.12 – 5.90 B.M. and, thus, indicated the presence of high spin Mn(II) with ⁶A_1g ground having no contribution from TIP (Temperature Independent Paramagnetism).  

Reflectance Spectra of Complexes

The room temperature magnetic moment values of the four Mn (II) complexes suggest that they have octahedral stereochemistry. This is further, corroborated by their reflectance electronic spectra which showed very week bands²⁷ due to their doubly forbidden nature as neither they obey the multiplicity rule nor the symmetry (Laporte) rule. With five unpaired electrons, these Mn (II) complexes have six multiplicity with ⁶S ground state and the symmetry symbol ⁶A. The ground term is represented by ⁶A₁g where ‘g’ stands for gerade in octahedral stereochemistry. The spectroscopic state immediately higher to ⁶S state is ⁴G which splits up into ⁴T_{1g, ,}⁴T_2g , ⁴A_1g , ⁴E_g   in an octahedral field. Three spectral bands arising from ⁶A to ⁴G are assigned to as ⁶A₁g → ⁴T₁g (⁴G), ⁶A₁g→⁴T₂g (⁴G) and ⁶A₁ → ⁴A₁g = ⁴Eg (⁴G) as the last two terms do not differ very largely in their energies. The fourth band occurs by the transition between the ground ⁶A₁g term to ⁴T₂g obtained by the splitting of ⁴F which is immediately higher in energy to ⁴G (Table:4).

Table 4: Electronic Spectral (cm^-1) and Tentative Structures of Manganese (II) Complexes.

Complex	→4T1g	→4T2g	→4A1g. 4Eg	→4T2g	μeff  (B.M.)
[Mn2O(o-Ph2AsC6H4CO2)2(H2O)4]	18182, 17000	21050	24390	25600	5.12
[Mn2O(o-Ph2AsC6H4CO2)2(H2O)5} .H2O]	18182, 17000	21252	24352	25640	5.74
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4}]	16625	—	—	25640	5.53
[Mn2O{(o-(p-tolyl)2AsC6H4CO2)2 (H2O)4}]	16625	20000	22471	25640	5.90

EPR Spectra of Complexes

Only [Mn₂O{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₄}] complex gave EPR signal at room temperature in the form of a broad peak with g ≈ 2.0 which indicated its nearly axial symmetry having small distortion from octahedral stereochemistry with ⁶A_1g ground    term.²⁸ It was quite likely that the lines of the system had their EPR resonance broadened beyond distinction due to spin-lattice relaxation. Further, this might also, be due to magnetic exchange between manganese (II) ions in the oxo- complexes, i.e., coupling between Mn-3d and O-2p orbitals^23-26 as had already been predicted based on the magnetic data.

Structures of the Four Oxomanganese (II)-Arsine Complexes

The IR spectra of the complexes indicated the presence of bent Mn(II)-O-Mn(II) unit.   The electronic absorption, EPR spectra and magnetic data complemented one another to confirm their almost octahedral stereochemistry around Mn(II). The thermal data showed the presence of four coordinated water molecules in three complexes: [MnO₂{(o-R₂As C₆H₄CO₂)₂(H₂O)₄}], {R=Ph, p-tolyl (two isomers)} and five coordinated water molecules and one loosely bound lattice water molecule in the fourth complex: [MnO₂{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₅}.H₂O]. Thus, all the techniques used in this study corroborated well to assign them the tentative structures 4, 5; Fig: 1 (Table:3).

Conclusions

Though we obtained oxomanganese(II)-mono-tertiary arsine complexes and not the      manganese(II)-mono-tertiary complexes, yet As(III) was not oxidized to As(V) oxide  contrary to McAuliffe’s emphasis^3,4,7,8 on the use of strictly deoxygenated  anhydrous reaction medium for stabilizing As(III)-Mn(II) bond which highlighted the importance of inductive effect in restricting the oxidation of As(III) to As(V) in ligands with aryl substituents (R=Ph or p-tolyl). It, also, justified the effect of counter anion as chloride would affect neither As(III) nor Mn(II) while the acetate ion would change manganese (II) to oxo-manganese (II) without oxidizing As(III) to As(V) oxide.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgement

We are greatly thankful to Professor Surjeet Singh of R.S.I.C., I.I.T., Madras for providing E.P.R. instrumental facilities and to Professor S. Subramanian of R.S.I.C., I.I.T., Madras for fruitful discussion of the E.P.R. data. 

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