Oxomanganese (II)-Arsine Oxide Complexes from o-R2AsC6H4CO2H Ligands: Role of Inductive Effect and Reaction Conditions in Stabilizing Manganese (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) in the presence of trace amount of oxygen, or moisture or even by the microbial activities in the environment.^1,2 Manganese oxide shows a great deal in binding with organoarsenic ligands but their stability is always a key concern of researchers.²

To the best of our knowledge, only ten manganese(II)-mono-tertiary arsine complexes had been reported in literature.^3-5 Chiswell et al.³ reported two complexes: [Mn(As-N)X₂], where (X = Br or ClO₄) formed by 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 and the bond between As →Mn (II) is stable. We previously reported^{4, 5} eight complexes of manganese(II) ion with hybrid arsenic-oxygen chelating agents (As-O) formed by the reaction of manganese(II) salts with the As-O chelating agent in the presence of both oxygen and moisture. Four of these eight complexes consisted of manganese(II)-mono-tertiary arsine complexes⁴: [Mn{(o-Me₂AsC₆H₄CO₂)Cl}. ½H₂O], [Mn{(o-R₂AsC₆H₄CO₂)₂(H₂O)₂}. nH₂O] (R = Ph, p-tolyl, n = 0; R = -Et, n = 1) prepared by reacting MnCl₂. 4H₂O with o-R₂AsC₆H₄CO₂Na (R = -Me, -Et, Ph, p-tolyl) in 1:2 molar ratio in 95% EtOH. The other four complexes belonged to oxomanganese(II)-mono-tertiary arsine⁵ type: [Mn₂O{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₄}],[Mn₂O{(o-Ph₂AsC₆H₄CO₂)₂(H₂O)₅}.(H₂O)], [Mn₂O{(o-(p-tolyl)₂AsC₆H₄CO₂)₂(H₂O)₄] (two isomers) that were obtained by reacting [Mn(O₂CMe)₂.nH₂O] (where n = 0 and 4) salts with the ligand o-R₂AsC₆H₄CO₂H having only the aryl substituent (R = Ph and p-tolyl) in 1:2 molar ratio both in 95% EtOH and ethanol solvents.       We previously^4,5 supported Chiswell et al.³, in contravention to McAuliffe’s view ^6-9, 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→Mn(II) bond with hybrid ligand o-R₂AsC₆H₄CO₂Hhaving aryl substituent (R = Ph, p-tolyl) using [MnCl₂.4H₂O]⁴and [Mn(O₂CMe)₂.nH₂O] (where n = 0, 4)⁵; no matter MnCl₂. 4H₂O also formed As→Mn(II) bond with the ligands having alkyl substituent (R = -Me, -Et)⁴ while R = -C₆H₁₁ formed the arsine oxide complexes.The presented work here is designed to study the effect of changing both the substituent from aryl (R = Ph, p-tolyl) to alkyl (R = -Me, -Et, -C₆H₁₁) and, thereby, the inductive effect and also the counter anion from chloride toacetate in stabilizing manganese (II)-arsine complexes. 

Experimental Details

Details of the experimental method for the preparation of ligand (Structure 1, Fig.1; M = H, Na), anhydrous Mn(O₂CMe)₂, and the various studied spectral measurements of the complexes were reported elsewhere.^{5, 10-14}

Preparation of Complexes of Manganese (II)  

Reaction 1: Preparation of [Mn₂O{(o-R₂As(O)C₆H₄CO₂)₂(H₂O)_n}] (R = -Me and -Et, n = 3; R = -C₆H₁₁, n = 5)  

A reaction mixture of [(Structure 1, Fig. 1); M = H](8 mmol) and Mn(O₂CMe)₂. 4H₂O(0.98 g, 4 mmol) in 95% EtOH (40cm³) were refluxed for 2h and leads to the formation of a clear solution. Upon addition of diethyl ether, Et₂O to this clear solution a white solid was formed that is filtered, washed with Et₂O and dried in vacuo. Yield: 50-60% (R = -Me, -Et) and 30-35% (R = -C₆H₁₁).

Reaction 2: Preparation of [Mn₂O{(o-R₂As(O)C₆H₄CO₂)(H₂O)_n}. n^‘(H₂O)] (R = -Me, n = 3, n^‘ = 0 ; R = -Et, n = 5, n^‘ = 1 ; R = -C₆H₁₁, n = 5, n^‘ = 0)

A white solid separated on refluxing (30 min for R = -Me, -Et) a mixture of hot ethanolic solution (18-20cm³) of (Structure 1, Fig. 1; M = H)(8 mmol) and anhydrous Mn(O₂CMe)₂{0.692 g, 4 mmol} in the same solvent (30-35cm³). However, for R = –C₆H_11, a white solid separated only on the concentration of the solution after refluxing for 12h. The complex formed in each case was filtered, washed with EtOH, Et₂O and dried in vacuo. Yield: 75-80% (R = -Me, -Et) and 50-55% (R = -C₆H₁₁).

Reaction 3: Preparation of [Mn₂O{(o-R₂As(O)C₆H₄CO₂)₂(H₂O)_n}. n^‘(H₂O)] (R = -Me, n = 3, n^‘ = 0; R = -Et, n = 5, n^‘ = 2; R = -C₆H₁₁, n=1, n^‘ = 0)      

A white solid was separated on refluxing (30 min for R = -Me; 45 min for R = -Et; and 12h for R = -C₆H₁₁) the reaction mixture obtained by the drop wise addition of anhydrous Mn(O₂CMe)₂ {0.692 g, 4 mmol} in 95% EtOH (25-35cm³) to a solution of (Structure 1, Fig. 1; M = H) (8 mmol) in the same solvent (18-20cm³). The complex (white solid) so formed was filtered, washed with 95% EtOH, Et₂O and dried in vacuo. Yield: 75-80% (R = -Me, -Et); 40-50% (R = -C₆H₁₁).

Thus, with the above three reactions we were able to prepare nine oxomanganese(II) arsine oxide complexes that are as follows: {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

Spectroscopic Measurements

Infrared (IR) Spectral Studies

IR spectra of the prepared nine complexes were recorded in the range 4000-200 cm^-1on a PYE UNICAM SP3-300 IR Spectrophotometer using potassium bromide (KBr) pellets.  

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

Molar Conductance Measurements

The molar conductance values of millimolar solutions of prepared complexes in PhNO₂ or CH₂Cl₂ 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 ^oC/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 atR.S.I.C., I.I.T., Madras using Varian Spectrophotometer having a constant microwave frequency of 9.3 GHz (X-band; 0-10000G). The Linde’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.

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 by using Pascal’s 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).

Elemental (Mn and As) Analysis

Mn²⁺ was estimated volumetrically by EDTA method^12.      

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 (table 2) of the nine oxomanganese(II)-arsine oxide complexes were shown below. The data presented here corroborated with the stoichiometry of the complexes prepared. All the reported and studied compounds here behaved as nonelectrolytes in both nitrobenzene (C₆H₅NO₂) or dichloromethane (CH₂Cl₂) solvents. The non-electrolytic 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.

Table 1: Elemental Analysis Data of  Oxomanganese (II)-arsine oxide Complexes.

Formula of the Complex  (M.Pt oC)	Color*	Elemental composition Found (Calcd.) %
		C	H	Mn
[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}], (˃ 300 oC)	White	33.8(32.6)	3.8(3.9)	17.3(16.6)
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)3}], (˃ 300 oC )	Dirty White	36.6(36.8)	4.4(4.7)	15.8(15.3)
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)5}], (290 oC)	Light Pink	46.1(47.0)	6.5(6.4)	10.8(11.3)
[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}], (˃ 300 oC)	Dirty White	32.0(32.6)	4.5(3.9)	16.5(16.6)
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5. H2O], (˃ 300 oC)	White	33.7(34.2)	4.7(5.2)	15.2(14.3)
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)5}], (245 oC)	Light Pink	47.6(47.0)	6.9(6.4)	11.4(11.3)
[Mn2O{(o-Me2As(O)C6H4CO2)2 (H2O)3}], (218 oC)	Dirty White	31.7 (32.6)	3.3(3.9)	17.5 (16.6)
[Mn2O{(o-Et2As(O)C6H4CO2)2 (H2O)5}. 2H2O], (˃ 300 oC)	*	32.7(33.4)	4.8(5.3)	13.9(13.9)
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)}], (235 oC)	*	51.3(50.8)	6.5(6.0)	11.4(12.2)

*A white compound changes to dirty white or light pink or light brown on drying.

Table 2: Thermal Analysis and Molar Conductance Data of Oxomanganese (II)-arsine oxide Complexes.

Formula of the Complex	Thermal Analysis		Λ (S cm 2 mol-1) in PhNO2( or CH2Cl2)
	Temp. Range oC	Loss % Found (Calcd.), No. of Lattice (L) and Coordinated (C) H2O molecules
[Mn2O{(o-Me2As(O)C6H4CO2)2 (H2O)3}]	60-180	8.0(8.1;3 H2O), C=3	—-
[Mn2O{(o– Et2As(O)C6H4 CO2)2(H2O)3}]	60-100 140-180	2.5(2.53; H2O), L=1 5.0(5.06; 2H2O), C=2	—-
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2 (H2O)5}]	80-110; 120-230	8.0 (7.4; 4H2O), L=4 1.7 (1.82; H2O), C=1	—-
[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}]	—-	—-	2.8
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}.H2O]	80-120 120-210	2.25(2.0; H2O), L=1 11.25(1183; 5H2O), C=5	—-
**[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)5}	120-200	9.70(9.60: 5H2O), C=5	3.2
**[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}]	60-180	8.0 (8.1; 3H2O), C=3	2.9
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}. 2H2O]	60-100	4.7 (4.6; 2H2O), L=2 11.9(11.6; 5 H2O), C=5	0.3
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)}]	80-100	3.5(3.44; H2O), C=1	—-

Note: The two complexes marked ** are of ‘a’ type and while the remaining seven are of  ‘b’ type oxide complexes whose discussion will make an important part of the present study.

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 ν_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 complexes⁵ 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 complexes⁴.

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

IR spectra of two oxomanganese(II)-arsenic oxide complexes [Mn₂O{o-R₂As(O)C₆H₄CO₂)₂(H₂O)_n}] (R = -Me, n = 3; R = -C₆H₁₁, 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  r_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 r_C-H band. Moreover, absence of bands due to ν_(As-OH)^{15, 16}at 2360, 2370 cm^-1 (R = -Me) and2385, 2360 cm^-1 (R = -C₆H₁₁) and in 865-870 region ν_As=O¹¹ in these two a-type of oxomanganese(II) oxide complexes indicated the formation of As-O-Mn group implying the absence of As=O species.

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

Complex	ν(OH)	νasym (CO2)	νsym (CO2)	νsym (Mn-O-Mn)	ν(As=O)	δHOH
[Mn2O{(o-Me2As(O)C6H4CO2)2 (H2O)3}]	3390 br	1597 vs	1390 vs	610 w	820 s	1620 sh
[Mn2O{(o-Et2As(O)C6H4CO2)2 (H2O)3}]	3400 br	1605 vs	1390 vs	595 w	830 s	1620 sh
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2 (H2O)5}]	3400 br	1605 vs	1380 vs	610 w	830 s	1620 sh
[Mn2O{(o-Me2As(O)C6H4CO2)2 (H2O)3}]	3380 brm	1600 vs	1392 vs	610 w	820 vs	1620 sh
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}. H2O]	3400 brm	1605 vs	1392 vs	590 w	822 vs	1620 sh
**[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2 (H2O)5}]	3400 brm	1602 vs	1397 vs	610 w	—-	1620 sh
**[Mn2O{(o-Me2As(O)C6H4CO2)2 (H2O)3}]	3380 brm	1597 vs	1410 s	610 w	—-	1625 sh
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}. 2H2O]	3400 brm	1602 vs	1402 s	595 w	823 vs	1630 sh
[Mn2O{(o– (C6H11)2As(O)C6H4CO2)2(H2O)}]	3400 brm	1605 vs	1380 s	610 w	840 vs	1600 sh

br = broad; brm = broad medium; s = strong; vs = very strong; w = weak;  sh = shoulder;. Note: The two complexes marked ** are of ‘a’ type and while the remaining seven are of  ‘b’ type oxide complexes whose discussion will make an important part of the present study.

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 ν_As=O¹¹ because the appearance of the two bands, i.e. ν_As=O and δ_OCO at almost the same region resulted in an increased intensity. These observations could be rationalized if these 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 ν_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 [Mn₂O{o-R₂As(O)C₆H₄CO₂)₂(H₂O)_n}] (R = -Me, n = 3; R = -C₆H₁₁, n = 1), symmetric chelation of Mn(II) with the carboxylate ions was observed (Structure 4, Fig. 1) as both ν_asym(CO₂) and ν_sym(CO₂) 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 toIR spectra (Table: 3) of the seven b-type arsine oxide complexes favored uni-dentate mode because their ν_asym(CO₂) were raised and ν_sym(CO₂) 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 dueto the lowering of ν_As=O¹¹ on coordination with Mn(II) with no band in 260-280 cm^-1 region ν_M-As²⁰ to indicate the absence of Mn(II)-As(III) bond.

Figure 1: Structure 1: o-Diarylarsinobenzoic acids with M=H or Na(R= alkyl or aryl).  Click here to View Figure

Presence of Water in Complexes

The complexes obtained under hydrated conditions in Reaction-I and Reaction-III possessed either the same number of water molecules or even 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 ν_OH(H₂O)²¹ at ≈ 3350-3400 cm^-1 and δ_HOH(sh)²²≈ 1600-1630 cm^-1 (Table: 3). The rocking mode of coordinated water ρ_(H2O)²³ found at ≈ 800-900 cm^-1 wasobscured by  [δ_OCO]²⁴/[ν_As=O]¹¹  at ≈ 850-70 cm^-1. The strongly coordinated water molecules were lost after 100 ^oC while the lattice or loosely coordinated water molecules were lost up to 100 ^oC (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 = -C₆H₁₁.

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 known whereas, in the isoelectronic Fe(III)-O-Fe(III) systems, the formation of σ and π bonds were involved to explain the spin-exchange between M-3d and O-2p orbital.^26-29 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 ⁶A_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 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 ⁶A_1g 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 ⁶A_1g → ⁴T_1g (⁴G), ⁶A_1g→ ⁴T_2g(⁴G)and ^6A_1g ^{→ 4}A_1g = ⁴E_g (⁴G) as the last two terms do not differ very largely in their energies. The fourth band occurs by the transition between the ground ⁶A_1g term to ⁴T_2g 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 Oxomanganese (II)-arsine oxide Complexes

Formula of  Complex, Complex number	→ 4T1g	→ 4T2g	→ 4A1g ,  4Eg	→4T2g	μeff  (B.M.)	Structure
[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}], I	20000	21000	23529	26666	5.50	5
[Mn2O{(o– Et2As(O)C6H4 CO2)2(H2O)3}], II	17857	21045	—-	—-	5. 35	7
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)5}], III	19230, 17070	—-	—-	25000	5.00	8
[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}], IV	19320, 17194	—-	23809	27397	5.98	5
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}. H2O], V	—-	21376	—-	25640	5.96	6
**[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)5}], VI	17857	—-	23809	27027	5.70	6
**[Mn2O{(o-Me2As(O)C6H4CO2)2(H2O)3}], VII	17857	21739	24100	27027	—-	5
[Mn2O{(o-Et2As(O)C6H4CO2)2(H2O)5}. 2H2O], VIII	19417, 17241	22471	24096	27777	5. 75	6
[Mn2O{(o-(C6H11)2As(O)C6H4CO2)2(H2O)}], IX	17857	—-	23809	27027	5.97	8

Note: The two complexes marked ** are of ‘a’ type and while the remaining seven are of  ‘b’ type oxide complexes.

EPR Spectra of the Complexes

Only [Mn₂O{(o-R₂As(O)C₆H₄CO₂)₂(H₂O)_n}] (R = -Me, -Et, n = 3; R = -C₆H₁₁, n = 5)   complexes gave 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 ⁶A_1g ground term³¹ as the lines were broadened due to spin-lattice relaxation and the magnetic exchange between manganese (II) ions because of presence of intervening oxo group, i.e. Mn-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 the tentative structures 5-8 given in (Table:4) and are represented in Fig.2.

Figure 2: Click here to View Figure

Role of Inductive Effect and Reaction Conditions in Stabilizing As→Mn(II) Bond

We Obtained Three Types of Complexes

Manganese(II)-Arsine Complexes

When ligands with aryl substituents (R = Ph or p-tolyl) were reacted both with MnCl₂. 4H₂Oand Mn(O₂CMe)₂. nH₂O (n = 0 and 4)⁴ and also when the ligands having alkyl substituents (R = -Me, -Et) were reacted with MnCl₂. 4H₂O.

Oxomanganese(II)–Arsine Complexes

When ligands with aryl substituents (R = Ph, p– tolyl) were reacted with [Mn(O₂CMe)₂. nH₂O].⁵

Oxomanganese(II)–Arsine Oxide Complexes

When ligands with alkyl substituents (R = -Me, -Et, -C₆H₁₁) were reacted with Mn(O₂CMe)₂. nH₂O.

This was attributed to both the difference in the inductive effect of the aryl and alkyl substituents and the change of counter anions from chloride to acetate. Their role in deciding the formation of an arsine or arsine oxide complex was explained as follows:

Chloride would restrict oxidation of arsine in the presence of manganese (II) while acetate would facilitate. This fact was supported by the conversion of the manganese(II)- arsine complexes obtained from MnCl₂. 4H₂O into b-type arsine oxide complexes when treated with a few drops of MeCO₂Hin 95% EtOH¹⁴ to infer that the MeCO₂H formed in situ in reactions with Mn(O₂CMe)₂. nH₂O was responsible for the oxidation of initially formed manganese(II)-arsine complexes into b-type arsine oxide complexes. 

Except for R = –C₆H_11, which always gave a manganese(II)-arsine oxide complex, the ligand with two alkyl (R = -Me, -Et) and two aryl (R = Ph, p-tolyl) groups formed manganese(II)- arsine complexes with MnCl₂. 4H₂O⁴(coordinated water) in presence of 95% EtOH (water in medium) meaning, thereby, that neither the coordinated water nor water in the medium nor the presence of atmospheric oxygen inhibit the formation of manganese(II)- arsenic (III) bond and afforded four manganese(II)- arsine complexes.⁴ 

Alkyl groups, being better electron releasing than aryl groups, and -C₆H₁₁ group being the best electron releasing of the three alkyl groups, would oxidize As(III) even in the absence of acetic acid while the two alkyl groups (-Me, -Et) could oxidize As(III) only in presence of acetic acid. 

According to McAuliffe⁹, a good deal of evidence showing the occurrence of an oxygen transfer process through an intermediate [MnPR₃(O₂)X₂] which might be responsible for the formation of manganese(II)-phosphine oxide complexes like [Mn(OPR₃)X₂] from MnX₂ and PR₃. The proposed mechanism⁹ suggested that it was not the coordinated phosphine but the one present in excess which was oxidized. The experimental observations in our case, however, suggested that in 7 out of the 9 cases, it was the coordinated arsine that got oxidized to form b-type arsine oxide complexes and not the excess present since, otherwise, products would have been a– type oxide complexes. In analogy to McAuliffe’s⁹ intermediate, the formation of a b-type arsenic oxide complex was assumed to take place through an intermediate [Mn{o-R₂AsC₆H₄CO₂)₂(O₂)}] (Structure 9) which would undergo hydrolysis in presence of H₂O/MeCO₂H. On the other hand, the formation of a-type of arsenic oxide complexes might be considered to take place through an intermediate in which the dioxygen molecule was not likely to coordinate with manganese(II) in the absence of coordinated arsine because here spontaneous oxidation of arsine was assumed to precede its coordination.

 Conclusions

 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 manganese(II)– arsine complexes if group R- and counter anion were suitably selected.

TheMn(II)-O-Mn)II) system looked unprecedented in the literature, the final word about its existence in these complexes could be said with certainty only after single- crystal 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.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

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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