Structure and Electronic Properties of  Aminopyridino-1-4-η–2-methoxycyclohexa-1,3-diene  Irontricarbonyl Complexes – A Semi Empirical PM3 Approach.

 

Adejoro I.A., Odiaka T.I.  and Akinyele O.F.

Department of Chemistry, University of Ibadan, Ibadan Nigeria

*Corresponding Author E-mail: ofakins@yahoo.com

 

Herein we report the theoretical studies of aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl complexes. The theoretical ground state geometries, electronic, thermodynamic properties and vibrational frequencies were obtained using Semi-empirical PM3 method. The geometries, electronic states, thermodynamic properties and vibrational frequencies are reported and discussed.

 

 

 

INTRODUCTION:

Computation in Chemistry is relatively new and its advent and popularity have improved the studies of molecular systems.¹ In this aspect of Computation in Chemistry, a particular molecular system can be modeled with the hope that it could be synthesized in the Laboratory. Although molecular modeling may not be perfect, it has been proved to be excellent at predicting the properties of compounds, including those that cannot be determined experimentally.² It is an aspect of Chemistry that is spreading and enjoying the patronage of theoretical Chemists due to its predictive ability in studies involving molecular systems.³ Various theoretical models have been applied in studying the properties, electronic states and stabilities of compounds^4,5. Semi empirical studies on irontricarbonyl complexes of cyclohexa-1,3-diene have been reported^6-8. Thermodynamic parameters,

 

Electronic structures and vibrational frequencies of molecules have been predicted using theoretical       models^2-4,10. This paper reveals studies that were carried out on molecular systems involving a number of organometallic complexes shown in Figure 1. The complexes were modeled using Spartan’08⁹ and theoretical investigations were carried out on optimized geometries using semi-empirical PM3 method. The results show that these complexes are thermodynamically stable with virtually no significant change in geometries of the pyridine ring and the vicinity of the amino substituent. The structures of these complexes are shown in Figure 1.

 

Figure 1: Structure of Novel  aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl complexes.

 

 

Computational Methodology:

All the complexes were modeled using Spartan ’08⁹. The geometric, thermodynamic, electronic dipole moment and vibrational properties were calculated. These properties were studied at 1atmospheric pressure and 273.15K. Molecular mechanics force field was used to obtain the stable conformer before subjecting the complexes to geometric calculation using Semi-empirical PM3 method. The optimized geometry of these complexes are shown in Figure 2.

                                                                                                                                                      

Figure 2:  Structure of optimized geometries of novel aminopyridino-1-4-η-2-methoxy-cyclohexa-1,3-diene irontricarbonyl complexes

3.0 RESULTS AND DISCUSSION:

3.1 Geometric parameters:

The geometrical parameters were obtained after total optimization of the equilibrium geometries by Semi-empirical PM3. Bond length, bond angles and dihedral angles which contribute to the internal energies were measured and recorded (See Table 1). The effect of the substituent amino group on the pyridine is clearly noticeable around the adjacent carbon atoms; for instance, the bond length of N-C₇ in the 2-aminopyridino complex is 1.398Ǻ because the amino substituent is attached to C₇. C₇-C₈ bond length is also 1.432Ǻ. These values are more than the values obtained for other pyridino substituents and this can be attributed to the electron donor property of the amino group. C₇-N₂ is also 1.366Ǻ.The C₇- C₈ bond length is 1.412Ǻ while the bond length of C₈ – C₉ is 1.413Ǻ. In all the complexes C₈ – C₉ and C₉ – C₁₀ which are low in both 2- and 3-aminosubstituents increased significantly in the 4-amino-, 4-N-methylamino and 4-N-dimethylamino substituents respectively. This is comparable with 1.377Ǻ ( 4-NH₄), 1.381Ǻ {4-NH(CH₃) and 4-N(CH₃)₂ for the C₉-N₂ bond length ( See Table 1). With the amino substituent on C₈, the bond lengths C₇-C₈ and C₈-C₉ occur at 1.412Ǻ and 1.413Ǻ respectively while the adjacent C₉-C₁₀ bond length reduces to 1.383Ǻ. This is a clear manifestation of the substituent effect on the attached pyridine ring. All the C-C single and double bonds are within the range of 1.432-1.529Ǻ and 1.368-1.489Ǻ respectively. The metal-carbon bond falls within the range 1.731-1.820Ǻ while all the metal-ligand bond is 1.681Ǻ  in all the complexes except for the 2-aminnopyridino complex whose metal-ligand bond is 1.692Ǻ. All the C-H bonds fall within 1.090Ǻ and 1.122Ǻ. The bond distances, bond angles and dihedral angles for all the complexes are collected in Table 1.

 

 

Table 1: Geometric parameters ( bond distances /Ǻ, bond angles, dihedral angles (in degrees)  of  the studied aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene   irontricarbonyl complexes. Selected bond length

Bond    length / Ǻ	2-NH 2	3-NH2	4-NH2	4-NH(CH3)	4-N(CH3)2
C1 – C2	1.474	1.483	1.483	1.483	1.483
C 2– C3	1.419	1.409	1.408	1.408	1.408
C3 – C4	1.492	1.484	1.483	1.483	1.483
C4 – C5	1.509	1.495	1.495	1.495	1.495
C5 – C6	1.540	1.540	1.539	1.539	1.539
C6 – C1	1.496	1.493	1.494	1.493	1.494
C 5– N1	1.510	1.519	1.520	1.519	1.518
N1 – C7	1.398	1.366	1.377	1.377	1.378
C7 – C8	1.432	1.412	1.381	1.381	1.379
C 8– C9	1.367	1.413	1.416	1.417	1.422
C9 – C10	1.416	1.383	1.420	1.424	1.426
C10 – C11	1.368	1.398	1.376	1.374	1.374
C11 – N1	1.399	1.369	1.385	1.386	1.385
lig – Fe	1.692	1.681	1.681	1.681	1.681
Fe – C12	1.820	1.736	1.732	1.732	1.731
Fe – C13	1.770	1.797	1.801	1.801	1.800
Fe – C14	1.814	1.821	1.820	1.820	1.818
C12 – O1	1.153	1.159	1.161	1.161	1.616
C13 – O2	1.142	1.148	1.148	1.148	1.148
C14 – O3	1.144	1.143	1.143	1.143	1.144
C2 – O4	1.355	1.376	1.377	1.377	1.377
O4 – C15	1.414	1.414	1.415	1.415	1.414
C7 – N2	1.366	-	-	-	-
C8 – N2	-	1.399	-	-	-
C 9 – N2	-	-	1.377	1.377	1.381
N2 – C	-	-	-	1.476	1.480
C – H	1.089-1.121	1.091-1.121	1.090-1.121	1.090-1.122	1.091-1.122
N2- H	0.989	0.993	0.991	0.995	-

 

(a)     Bond Angles in degrees:

Bond angle / ͦ	2-NH 2	3-NH2	4-NH2	4-NH(CH3)	4-N(CH3)2
C12-Fe-lig	132.91	132.25	132.64	132.78	132.49
C13-Fe-lig	120.58	118.88	118.55	118.44	118.49
C14-Fe-lig	110.78	113.60	113.52	113.48	113.64
C12-Fe-C14	88.58	88.57	88.43	88.35	88.63
C13-Fe-C14	89.14	90.11	90.00	89.97	90.18
C12-Fe-C13	102.51	101.83	101.94	101.98	101.81
C2-O4-C15	117.60	114.55	114.48	114.33	114.53
C5-N1-C7	122.05	121.89	122.56	122.75	122.75
N1-C7-N2	119.07	-	-	-	-
N1-C7-C8	119.18	120.31	120.88	120.00	121.04
C7-C8-N2	-	119.71
H-N-H	120.20	114.49	116.78	-	-
N2-C7-C8	121.74	-	-	-	-
C9-N2-C16	-	-	-	122.74	120.81

(c) Dihedral Angles in degrees:

Dihedral angles / ͦ	2-NH 2	3-NH2	4-NH2	4-NH(CH3)	4-N(CH3)2
C3-C2-O4-C15	128.48	128.50	127.90	126.24	128.64
C13-Fe-C14-O3	68.67	68.79	69.12	69.16	68.30
C14-Fe-lig-C3	91.56	91.62	91.68	91.74	91.79
C4-lig-Fe-C13	54.95	55.07	55.42	55.57	55.20
C4-C5-N1-C7	11.54	11.34	10.38	9.21	10.66
C8-C7-N2-H	11.73	11.78	-	-	-
H-C7-C8-N2	-	3.61	-	-	-
C10-C9-N-C16	-	-	-	171.98	171.98

 

 

 

3.2 Electronic properties:

The HOMO of these complexes possesses a π-bonding character within sub-units and π-antibonding character excited state properties between the consecutive sub units. On the other hand, the LUMO possesses a π-antibonding character within the subunit and a π-bonding character between the subunits. Experimentally, the HOMO and the LUMO energies were obtained from an empirical formula based on the onset of oxidation-reduction of peaks measured by cyclic voltametry. Theoretically, the HOMO and LUMO energies are calculated using Semi-empirical PM3. The calculated electronic properties (energy band gap, LUMO-HOMO) of these complexes were found to be within the range 7.20 – 7.76eV, the highest been the 2-aminopyridino complex. The HOMO of these complexes is concentrated on the 1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl while the LUMO is concentrated around the aminopyridine moiety. The calculated values are shown in Table 2 while the LUMO-HOMO structures are shown in Figure 3. There is a significant change in the values recorded for dipole moments.

 

Table 2: Dipole moments, HOMO and LUMO energies, Energy band gaps (Eg) for the simulated aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl complexes.

X-aminopyridino	Dipole moments / debye	HOMO energy / eV	LUMO energy / eV	Band gaps / eV
2-NH2	3.52	-12.04	-4.28	7.76
3- NH2	6.51	-12.01	-4.81	7.20
4- NH2	8.27	-11.85	-4.65	7.20
4-NH(CH3)	6.76	-11.80	-4.53	7.27
4 – NH(CH3)2	5.44	-11.76	-4.41	7.35

 

HOMO

LUMO

HOMO

LUMO                  

 

Figure 3: HOMO – LUMO Energy diagrams for optimized geometries of aminopyridino-1-4-η- 2-methoxycyclohexa-1,3-diene irontricarbonyl complexes.

3.30 Thermodynamic properties and stabilities:

For complexes to be thermodynamically stable, it is expected that ΔG and ΔH are negative. The more negative these values are and the more positive ΔS, the more stable would be the amino pyridino complexes. However all calculated free energy and enthalpy changes are negative while all calculated entropy values are positive thus confirming that the formation of these complexes is spontaneous. Our calculations reveal that all the complexes are thermodynamically stable as shown in Table 3.

 

3.4 Vibrational  Frequencies:

According to group representation theory in Chemistry, there are 105 A type vibrational modes for these new aminopyridino organometallics. The vibrational modes change with the replacement of hydrogen by the methyl group on  the N-substituted pyridine moiety, These vibrational modes increase to 114, when one of the hydrogens on the amino group is replaced by a methyl group, The vibrational modes further rose to 123 on replacing the two hydrogens on the amino group with methyl groups. Among these normal modes, the strongest infra-red absorption peaks and their intensities are shown in Table 4, They have very sharp IR peaks between 2098cm^-1 and 2246cm^-1 corresponding to the stretching vibrations of C≡O bond while the simulated IR spectra of these complexes are collected in Figure 4.

 

 

 

Table 3: Thermodynamic parameters for simulated aminopyridino-1-4-η-2-methoxy cyclohexa-1,3-diene irontricarbonyl complexes at 298.15K.

X-aminopyridino	Heat of formation/ kJmol-1	Free energy /kJmol-1	Entropy /    Jmol-1K-1	Enthalpy / kJmol-1
2-NH2	-1002.52	-404.70	615.70	-221.12
3-NH2	-1000.83	-397.57	625.70	-211.02
4-NH2	-1015.10	-412.63	626.00	-225.97
4-NH(CH3)	-1019.40	-353.14	660.08	-156.33
4-N(CH3)2	-1020.02	-288.13	697.32	-80.22

 

Table 4: Vibrational frequencies showing absorption bands and intensities for simulated Aminopyridino-1-4-η-2-methoxycyclohexa -1,3-diene irontricarbonyl complexes.

Structure	Vibrating bond	Infra-red band/ cm-1	Intensity
2-NH2	C5 – N	1474	2583
	C7 – N	1585	1914
	C9 – C10	1690	17707
	C8 – C9	1785	1355
	N2-H& C≡O	2111	5676
	N2-H& C≡O	2159	5890
	C≡O	2190	1844
3-NH2	N1-C7&C8 – C9	1333	2842
	C9 – C10	1738	1883
	C≡O	2106	1656
	C≡O	2177	1085
	C≡O	2247	1094
4-NH2	N1 – C7& C8 – C9	1351	2167
	C7 – C8	1557	2264
	C10 – C11	1735	1884
	C≡O	2099	1638
	C≡O	2178	1110
	C≡O	2246	1230
	C7 – H	2803	1048
4-NH(CH3)	C10 –C11	1563	2629
	C9 – N2	1626	1129
	C9 – C10	1677	12093
	C7 – C8	1734	1786
	C≡O	2098	1630
	C≡O	2178	1109
	C≡O	2245	1241
	C7 – H	2816	1100
4-N(CH3)2	N1 – C7	1355	1623
	C7 – C8	1568	2200
	C8 –C9	1663	12893
	C10 – C11	1734	1861
	C≡O	2098	1665
	C≡O	2176	1121
	C≡O	2244	1245
	C7 – H	2813	1115

 

Aminopyridino-1-4-η-2-methoxycyclohexa-1,3-dieneirontricarbonyl

 

4-N-methyl and 4-N-dimethylaminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl.

 

Figure 4: Infra-red spectra showing vibrational frequencies and intensities for aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene  irontricarbonyl complexes.

4.0 CONCLUSION:

The C1 symmetry point group resulting from the nucleophilic addition of aminopyridines and their substituted derivatives to the 1-5-η-2-methoxycyclo hexadienyl irontricarbonyl complex to form new cationic aminopyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl complexes were calculated using Semi-empirical PM3. The properties investigated include optimized geometries, dipole moments, electronic states, thermodynamic parameters and vibrational frequencies. All the complexes are thermodynamically stable. The effect of the substituent amino group on the pyridine is demonstrated by the calculated energy band gap of the order of 7.20-7.76eV (Table 2). It is however interesting, that the use of computation has given us the opportunity to take a critical look at these novel compounds at the molecular level to produce results which are otherwise inaccessible by conventional laboratory experiments. There are however no experimental or theoretical data regarding these organometallics.

 

5.0 REFERENCES :

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7.        Adejoro, I. A, Odiaka, T. I and Akinyele, O. F. Molecular modeling and computational studies of dimethylpyridino-1-4-η-2-methoxycyclohexa-1,3-diene irontricarbonyl complexes. Asian Journal of Research in Chemistry.. 5(1), 2012: 146-152.

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Received on 03.05.2013          Modified on 18.06.2013

Accepted on 19.07.2013         © AJRC All right reserved