NMR, ESR, NQR and IR Studies of Paramagnetic Macro Cyclic Complexes of 1^st Transition Series Metal Lons Exhibiting MLCT Phenomenon: A DFT Application: Part: II. Bis (1, 10-Phenanthroline) Complexes

Introduction

Aromatic heterocyclic compounds such as 1, 10-phenanthroline (Phen) and 2, 2-bipyridine (Bipy) form an important class of compounds in which the π and n electrons can form charge-transfer complexes [1, 2] where the electronic charge of their complexes having filled π- molecular orbitals of energy comparable to the metal orbitals get transferred into the empty molecular orbitals with energy nearly equal to the ligand orbitals. They absorb in the visible region to exhibit intense color which is termed as Metal to Ligand Charge Transfer (MLCT) phenomenon.

Unlike 2,2^/-bipyridine, the1,10-phenanthroline  does not have the same conformational flexibility and it would bind metal ions more strongly [3,4].This enantioselective interaction of 1, 10-phenanthroline complexes of iron (II), Co (III), Zn (II)is used as a structural probe and mediators of DNA cleavage reactions [5-7]. Homoleptic  bis(1,10-phenanthroline) transition metal ions complexes [Phen₂M] ^{2+, 3+} were studied comparatively to a lesser extent both  experimentally and by Density Functional Theory(DFT). Single crystal structures of bis ­[chloro­ bis(1,10-phenanthroline-N,N′)(thio­urea -S)­ nickel(II)] chloride nitrate  was studied by L. Suescun  and his coworkers[8]. Structure and magnetic properties of di-μ-hydroxo-bis [bis(1,10-phenanthroline)chromium(III)] iodide tetrahydrate were studied by Raymond Scaringe and Phritu Singh[9]. O. Yesilel, H. Olmez studied spectrothermal properties   of 1, 10-phenanthroline complexes along with X-rays single crystal structure and photo luminance properties of Co (II), NI (II), Cu (II) and Cd (II) orotates with UV-Vis and FTIR [10].V. Chis et al [11] studied Spin-state cross-over in Fe (Phen₂(NCS)₂ by DFT. Ozel Aysen E. and coworkers [12]studied vibration spectra of  copper(II)2,2’-bipyridine and 1,10-phenanthroline complexes by applying  DFT[12].  DFT IR, UV and Mossbauer of 2, 2’-bipyridine and 1, 10-phenanthroline complexes of iron (II) was studied by Morigakii, Milton K. et al [13] with the help of DFT.H.Z. Chiniforoshan et al [14] studied the structures of complexes of Co (III) and Zn (II) with 2,2’-bipyridine and 1,10-phenanthroline  by applying DFT. Sumit Sanotra et al applied DFT to  study X-ray  crystal structure and photo luminance properties of  [Cd (NO₃)₂ (phen)₂][15].Euan K. Bre chin and coworkers[16] studied UV,ESR and magnetic properties of Cr(III), Fe(II).N.S. Panina  and coworkers[17] studied stabilities of high and low spin isomers and X-ray structures of Ni(II) complexes. Zn Bz Li and Xiao yuan studied Raman spectra of bis- and tris (1, 10-phenanthroline) manganese (II) complexes [18].

Need for the Study

(a) A limited experimental research had been carried out in the calculation of NMR, ESR and NQR parameters of these 8 macro cyclic paramagnetic 1,10-phenanthroline complexes:[Phen₂M]²⁺ {M=Mn (II),Fe(II) Co(II),Ni(II),Cu (II)}; [Phen₂M]³⁺{M=Ti(III)_,V(III),Cr(III)} as accurate computations of parameters of these  techniques[19-23]had become possible by DFT[24,25] only recently .

(b) ν _(CN) would be lowered on coordination of N to a metal ion while the transfer of electronic charge from molecular orbitals mainly with metal character to those having the ligand character should increase ν _(CN).Since ν _(CN) was metal sensitive only to a limited extent, it would be difficult to assign exact value to ν_(CN) as many 1, 10-phenanthroline vibrations also appeared in the same region [26]*.Also the π→ π* transition of the ligand and MLCT in some complexes [27]**would absorb in the same region[19], the reflectance spectral technique might not help. Thus the NMR technique was tried which also enabled us to calculate the Spin Hamiltonian (H^) values of these complexes

Methodology

Use of ADF (Amsterdam Density Functional) software to calculate various NMR ^(29-34), IR ⁽²⁸⁾, ESR ^(35-39) and NQR ^{(35-39) –}parameters had, already, been reported.

Results

Tables: 1-3 represented energies of metals, their g_n and I values, thermal and optimization parameters of complexes. Table: 4 gave σ and δ values of ¹⁴N, ¹³C and ¹H of the free1, 10-phenanthroline .Table:5 contained σ and δ values of M ⁺ⁿ, ¹⁴N, ¹³C and ¹H of complexes. Table: 6 contained contributions of 2 diamagnetic, 4 paramagnetic and 4 spin orbit terms (10 in all) in σ of Mⁿ⁺_,¹⁴N, ¹³C, ¹H of complexes .Table: 7 gave ESR and NQR parameter of the various types of H, C and  N in complexes.Table:8 gave some more ESR and NQR parameters of complexes. Table: 9 contained contributions from five factors into H^Spin and ΔE_{h f} values. Table: 10 designated IR-active bands in the complexes. Table: 11 gave σ ¹⁴N values of the uncoordinated ligandand the complexes.

Table 1: Energies (kJmol^-1) of First Transition Metals

M	Sum of orbital  energies	Total energy	Kinetic energy	Nuclear attraction energy	Electron repulsion  energy	Exchange energy
Ti	– 49034.160	– 82193.590	83068.461	-195833.367	34453.298	– 3879.077
V	– 54453.277	– 91394.073	92457.572	– 218055.74	3 8338.78	– 4184.954
Cr	– 60139.891	– 101188.38	102462.98	– 241698.34	42549.246	– 4503.221
Mn	-18580.3096	-31364.4439	31791.632	-75034.4123	13237.036	-1358.702
Fe	-20333.522	-344593.341	349652.94	-82578.841	14339.914	-1455.918
Co	-78854.221	– 134234.27	136350.81	– 346177.79	57226.702	– 5539.960
Ni	-85650.855	-146522.795	148997.80	-352449.098	62843.648	-8809.705
Cu	– 9309.418	– 159479.16	162355.29	-384859.801	69353.856	– 6328.501

 

Table  2: Optimization Parameters (kJmol^-1), [M g_n], {M_I }and (M_S) of [Phen₂ M] ^{n +} Complexes

M n+  (D2d)	[M gn]{M I}(M S)	Total bondingenergy**	Total Energy(X c)LDA(Exchange; Correlation)GGA(Exchange; Correlation)a	Nucleus
Ti(III) a	[-0.315392]{2.5}(0.5)	-25980.97	-511912.56-442448.48, -34582.43-52792.02, 17910.37	47Ti
V(III)	[1.4710588]{3.5}(1.0)	-28816.33	-484494.18-449671.64, -34822.54	51V
Cr(III) a	[-0.316360]{1.5}(1.5)	-26138.48	-529520.63-458827.75, -35164.23-53626.90, 18098.25	56Cr
Mn(II)	[1.387488]{2.5}(2.5)	-30145.34	-501580.82-466197.13, -35383.70	55Mn
Fe(II)	[0.181246]{0.5}(2.0)	-29988.39	-510676.41-475004.40, -35672.01	57Fe
Co(II)	[1.322000]{3.5}(1.5)	-29955.67	-520137.94-484172.74, -35965.19	59Co
Ni(II)	[-0.500133]{1.5}(1.0)	-29673.45	-530709.68-494411.25, -36298.43	61Ni
Cu(II)	[-1.4821933]{1.5}(0.5)	-29439.21	-540776.87-504181.31, -36595.56	63Cu

* Contains LDA and GGA;^a GGA contributes** Energy difference between molecule and fragments..

Table 3: Thermal Parameters of [Phen₂ M] ⁿ⁺ Complexes

[M] n+	Zero Point Energy(e V)	Thermal Parameters
		Entropy (cal mol-1K-1)				Internal Energy (Kcal mol-1)				Constant Volume Capacity (Kcal mol-1K-1)
		Trans.	Rot.	Vib.	Total	Trans.	Rot.	Vib.	Total	Trans.	Rot.	Vib.	Total
Ti(III)	9.2711	43.91	33.1	54.3	131.26	0.889	0.889	223.3	225.1	2.981	2.981	73.88	79.84
V(III)	9.246	43.93	-do-	49.0	126.00	-do-	-do-	222.3	224.1	-do-	-do-	74.43	80.39
Cr(III)	9.2647	43.94	-do-	51.2	128.21	-do-	-do-	222.9	224.7	-do-	-do-	74.03	80.0
Mn(II)	9.256	43.96	-do-	56.4	133.38	-do-	-do-	223.5	225.2	-do-	-do-	77.97	83.93
Fe(II)	9.277	43.97	-do-	70.4	147.44	-do-	-do-	225.4	227.2	-do-	-do-	83.66	89.62
Co(II)	9.244	43.99	-do-	63.3	140.34	-do-	-do-	223.8	225.6	-do-	-do-	80.3	86.3
Ni(II)	9.222	43.98	-do-	60.7	137.74	-do-	-do-	223.0	224.7	-do-	-do-	78.68	84.64
Cu(II)	9.294	44.02	-do-	64.1	141.21	-do-	-do-	225.2	227.0	-do-	-do-	81.34	87.30

 

Table 4: σ N, σ H, δ N and δ H(ppm) of 1, 10– Phenanthroline a  Click here to View table

 

Table 5: σ and δ values (ppm)  of M +n  , N, C, H in [Phen2M] n+ With M n+  Complexes a  Click here to View table

 

Table 6: Diamagnetic, Paramagnetic & Spin orbit contributions in σ (ppm) of [Phen2M] n+ Complexes a Click here to View table

 

Table 7: ESR and NQR Parameters of N, C and H from Software for [Phen₂ M] ⁿ⁺  Complexes

M n+                           M n+	gn. A ten values of4 types of H; eachtype having 4H	gn.A ten valueof each oneof 4 N	gn. A ten values of6 types of C; eachtype having 4 C	h values of 6 types of C;each type having 4 C	NQCC andh values ofeach N
Ti(III)	0.776, 0.898,-0.087 -0.092	-7.654	3.109, -0.070, 0.05,1.994,18.360, 0.197	0.222,0.251,0.137,0.411,0.680,0.336	-1.786(0.969)
V(III)	0.244, 0.572,0.465,0.118	-6.862	6.118, 0.057, 0.863,0.573, 8.492, 0.317	0.394,0.267.0.141,0.443,0.617,0.339	1.621(0.558 )
Cr(III)	0.115, -0.324,-0.70, 0.825	-4.715	3.261, 0.059, 0.08,3.429, 0.207, 0.795	0.345,0.250,0.139,0.465,0.606,0.332	1.511( 0.464)
Mn(II)	-1.233, 0.629,-0.333, 0.88	2.184	1.125, -0.357, 0.436,0.313, 3.512, 0.122	0.104,0.150,0.080,0.256,0.597,0.229	-2.112(0.635 )
Fe(II)	-0.962, 0.677,0.247, 0.152	5.221	0.831, -0.106, 0.314,0.553, 2.012, 0.118	0.038,0.154,0.077,0.247,0.683,0.230	-2.073(0.597)
Co(II)	-0.895, 0.928,0.913, 0.299	9.623	0.857, 0.188, 0.292,0.95, 1.622, 0.158	0.092,0.160,0.078,0.262,0.665,0.231	-1.893(0.698)
Ni(II)	-1.191, 1.907,2.324, 0.466	16.892	-2.322, 0.146, 0.321,1.922, 2.661, 0.015	0.056,0.155,0.077,0.276,0.661,0.230	-1.807(0.710)
Cu(II)	-1.686, 3.080,2.934, -0.62	19.753	-2.992, 0.349, 0.372,2.06, 5.007, -0.866	0.032,0.144,0.076,0.261,0.679,0.211	-1.959(0.581)

 

Table 8: ESR and NQR Parameters* of M +n from Software* for [Phen2 M] n+ Complexes Click here to View table

 

Table 9: Calculation of H^ and Eh f Parameters of [Phen2 M] n+ Complexes Click here to View table

 

Table: 10. Designations of IR Active Bands in [Phen2 M] n+ Complexes

Complex with M +n	Vibration Symmetry  of bands*	IR active bands	Vibration Symmetry Class
Ti(III),V(III),Cr(III), Mn(II),Fe(II), Co(II),Ni(II),Cu(II)	A(129)	A(129)	[129A]

*Numbers in parentheses indicate bands of a specific symmetry

Table: 11. σ N values  of  1, 10-Phenanthroline  and its complexes with metal ions

σ values	Phen	Ti(III)	V(III)	Cr(III)	Mn(II)	Fe(II)	Co(II)	Ni(II)	Cu(II)
σN	-138.4	316.50	296.30	310.20	178.39	140.68	84.80	-1.15	-163.1

 

Figure 1: [Phen2M]n+{M=Ti (III),V(III), Cr(III),Mn (II),Fe(II), Co(II),Ni(II),Cu(II) at:1} Click here to View figure

 

Discussion  

Each one of the eight 1, 10-phenanthroline complexes contained 45 atoms:

4 N,16 H, 24C and a different transition metal ion.

Relations Used to Calculate NMR Parameters [40]

(a) σ M ⁿ⁺,σ¹H,σ¹³C and σ¹⁴N were equal to the sum of values of 2 diamagnetic,  4 paramagnetic and 4 spin orbit terms of M ⁿ⁺, ¹H , ¹³C and ¹⁴N respectively.

(b) (σ) and (δ) of ¹H , ¹³ C  and ¹⁴ N were  related  as follows:

δ¹H = 31.7 – σ ¹H —————————————————————— (1)

δ¹³C=181.1- σ ¹³C—————————————————————— (2)

σ M =  – δ M |

σ¹⁷O= – δ ¹⁴N  ——————————————— (3)

Relations Used to Calculate ESR Parameters [35-37]

Effective Spin Hamiltonian (H^) [35-37]

No doubt, three relations [35-39] were needed to calculate H^ but all the 8 complexes included in the present study were axially asymmetric having the same two  g values (g_⊥) while the third  g of different value and  was represented as g _{ll .}Correspondingly, two same a values  ( a_⊥) while the third  a of different value  was  named as a _ll , only the following relation  would suffice:

H^ = b_e [g_ll .H _ll. S +g_⊥(2H_⊥. S)] + [a _ll . S .I +a_⊥(2S.I)] +Q [I -1/3 I (I+1)] + D {S_z²-S(S+1)/3} – [g_n.b_n.H₀ .I] —– (4)

Five factors: g factor, a factor, Q factor, Zero Field Splitting (ZFS) factor [41,42] and interaction of nuclear magnetic moment with external magnetic field, i.e. I factor would contribute in the total value of Spin Hamiltonian (H^).  S_z representing spin angular momentum was calculated as:

S_z = S/S(S+1)^0.5 ———————————————- (5)

Hyperfine Coupling Energy [41]

ΔE _hf  = 1/2[a₁₁² +a₂₂² + a₃₃²]^1/2   _{————————————} (6)

(c) H^ values were calculated both in terms of MHz as well as in joules mol^1-.

Their inter conversions were given as:

(i) One MHz=6.627*10^-21 erg = 3.9903124*10^-7 kJ mol^-1 

(ii)1cm^-1= 0.0119626kJ mol^-1  = 29979.2458 MHz    

(iii) For 8388.255 MHz in a 0.30T, the g value of the standard substance: 2, 2-diphenyl-1-picrylhydrazyl (DPPH) was: g _DPPH 2.00232[43-45].So g value of the complex (g_M ⁿ⁺) and its frequency (ν_M ⁿ⁺) were related as follows:

ν_M ⁿ⁺ =8388.255 * g _M ⁿ⁺ /2.00232————————  (7)

The details of the various terms involved were discussed elsewhere [28].

Relations Used to Calculate NQR Parameters [46, 47]

Asymmetry Coefficient (h) [46, 47]

h=q _xx – q _{y y} /q _{z z} ———————————————– (8)

(h) lies in between 0-1.For axial symmetry, h=0. It was possible only when:

q_{x x} =q _{y y} ¹q _{z z}   ————————————————–  (9)

Laplace Equation [46, 47]

q_{x x} + q _{y y}  +q_{z z} =0 ———————————————-(10)

Calculation of four NMR and NQR Parameters

Four more ESR and NQR parameters: H^, ΔE _hf,η , and Laplace equation which were calculated in addition to five ESR(g₁₁, g₂₂, g₃₃, g _iso ; a₁₁, a₂₂, a₃₃, A _ten) and NQR parameters (η; q₁₁, q₂₂, q₃₃; NQCC) parameters  obtained from the software for 7 complexes excluding [Phen₂Fe] ²⁺where  ADF software did not work. All the 7complexes possessed axial symmetry with (a) Two of three g called g_⊥  were of the same value and third of higher value was called g_||  (b) Two “a” called a_⊥ were of the same value and the third of higher value was called a₁₁. (c) Two of the three q parameters were of the same value. (d) η=0. Relation (4) was applicable to all to calculate H^. Individual values of these five factors in the total value of H^ were given at bottom and were represented as (→). ΔE _hf, η, Laplace equation were calculated by relations (7, 8,10) respectively in Tables:7-9.NQCC, a, q were expressed in MHz while g was unit less.

Confirmation of Spatial Equivalence of N, C, H In Complexes

It was confirmed by following two ways:

From the Equivalence of NMR Parameters

For ascertaining the stereochemistry of the complexes, the 4 coordinating N, the 24 C and the 16 H were classified according to their spatial displacements. The metal ion formed a class of its own. The spatially equivalent species possessed same values of δ, σ along with each one of the ten contributing terms towards the total value of σ of the constituents. δ M ⁿ⁺,δ¹⁴N, δ ¹³C ,δ ¹H, σ M ⁿ⁺, σ ¹⁴N, σ ¹³C,σ ¹H and 10 contributing terms of all the 45 species in each one of the 8 complexes  were reported(Tables:5,6). All the four coordinating ¹⁴N in each complex were spatially equivalent with the same value each for σ¹⁴N, δ¹⁴N and the 10 contributing terms respectively. Each complex  having 16 ¹H contained four types of stereo chemically different ¹H; each type possessing four equivalent protons as they showed four different series of values σ¹H, δ¹H and 10 contributing terms respectively. The 24 ¹³C of each complex contained 6 types of spatially different ¹³C; each type having four equivalents ¹³C as they gave six different series of values of σ¹³C and δ¹³C and contributing 10 terms respectively.

From Equivalence Among five NMR, ESR, NQR Parameters

A total of five parameters of three techniques [ESR(A _ten), NQR (NQCC,η) and NMR (σ, δ)] were used to ascertain similar stereochemistry of 8 paramagnetic complexes. Four stereo chemically equivalent ¹⁴N possessed the same values of above named five parameters. The four parameters: g_n. A _ten, η, σ, δ of 24 ¹³C were categorized into 6 types (NQCC=0 for all); each having 4 equivalents C. Three parameters (g_n. A _ten, σ, δ) of the 16 H were of 4 types (NQCC and η =0 for all); each type having 4 equivalents ¹H. So the three techniques corroborated  to confirm that each one of these complexes possess one type of 4 N; six types of 24 C and four types of 16H. Further, the two NMR parameters (σ, δ) of constituents were shown in Table: 5 and the remaining three [one of EPR (A _ten) and two of NQR (NQCC,η ) were shown in (Table: 7). Again, with I=1/2 for ¹H and ¹³C; their NQCC =0.0.This left 4 parameters each for¹H and ¹³C.All the η values were zero for all the 16 ¹H.So three parameters were reported for ¹H.

Evidence of MLCT Phenomenon from NMR Parameters of Complexes

MLCT phenomenon would increase an electron density on the ligand by shifting electronic charge from the filled molecular orbitals of complex lying mainly on metal to its vacant molecular orbitals with energies comparable to the ligand. As σ of a nucleus was directly related to its electron density, any change in its value should serve as an indicator to the change in electron density on it. In 7 of the 8 complexes {except in [Phen₂Cu] ²⁺}, total shielding tensors of the σ¹⁴N,σ ¹³C, σ¹H (Tables:11,5) were observed to be higher and the δ¹⁴N, δ¹⁴C, δ¹H were found to be lower than their values in the uncoordinated  ligand (Table:4).It confirmed the transfer of electronic charge from  metal to the ligand and thereby the MLCT.

IR Parameters of Complexes

The software gave values of frequencies, dipole strengths and absorption intensities of IR-active normal modes of all the 129 fundamental vibration bands of the  complexes; each  having A (singly degenerate ) symmetry according to their IR- activities [48] with a Vibration Symmetry Class {129A} (Table: 10).

Conclusions

With higher σ¹⁴N, σ¹³C, σ¹H and lower δ¹⁴N, δ¹⁴C, δ¹H values of constituents in these complexes relative to uncoordinated ligand would confirm an increase in the electron density on the ligand in the complexes to lend support to the transfer of electron cloud from the metal into the ligand orbitals as per MLCT definition.

NMR, ESR and NQR results corroborated with one another to prove that all these 8 complexes possessed the same stereochemistry as all the 44 atoms were occupying the same relative positions around each one of metal ion.

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