ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2011, том 85, № 7, с. 1280-1284
СТРОЕНИЕ ВЕЩЕСТВА И КВАНТОВАЯ ХИМИЯ
y%K 539.192
STRUCTURE AND BONDING OF THREE-COORDINATE N-HETEROCYCLIC CARBENE NICKEL NITROSYL COMPLEXES: THEORETICAL STUDY
© 2011 R. Ghiasi, E. E. Mokarram
Department of Chemistry, Basic Science Faculty, East Tehran Branch, Qiam Dasht, Tehran, Islamic Azad University, Tehran, IRAN E-mail: rezaghiasi1353@yahoo.com, rghyasi@qdiau.ac.ir Received July 12, 2010
Abstract—The structure and bonding of the for C3N3H2X2Ni(Cp)NO (X = H, F, Cl, Br) and their linkage isomers C3N3H2X2Ni(Cp)ON has been studied by carrying out density functional theory. The bonding nature of NiC bonds has been further explored by means of AIM method and natural bond orbital (NBO) analysis. Nucleus-independent chemical shift (NICS) values calculated at several points above ring center indicate aromaticity of heterocyclic cycle. Also, the effect of substitution (X = F, Cl, Br, CN) in N-heterocyclic carbene on the properties of complex has been shown.
Keywords: three-coordinate complexes, N-heterocyclic carbene nickel nitrosyl complexes, density functional theory (DFT), nucleus-independent chemical shift, atoms in molecules analysis (AIM).
INTRODUCTION
A renewed interest in the fundamental chemistry of transition metal nitrosyl complexes is due to the discovery of the NO participation in a wide range of physiological and pathological processes in different cells and tissues as well as medical applications [1, 2] and in information technology [3]. Two such linkage isomers, one in which the nitrosyl is oxygen bound to the metal (isonitrosyl ON, metastable state) and one where the nitrosyl is in a side-on configuration (metastable state), are established in compounds. The linkage isomers can be accessed by an electronic transition in the violet-blue-green spectral range. Their energetic position is about 1—2 eV above the ground state (GS), such that in the metastable state (MS) the lowest lying electronic transitions are in the red and near infrared spectral range [4]. Very few three-coordinate, terminal metal nitrosyl complexes have been structurally characterized [5—7]. M.S. Varonka et al. have been synthesized three-coordinate N-heterocyclic carbene nickel nitrosyl complexes [8].
The aim of the present study is to study for structure and bonding in the LNi(Cp)NO (L = 1,3-dihy-dro-1H-imidazol-2-ylidene) and to find out the effect of substitution in N-heterocyclic carbene on the properties of complex.
COMPUTATIONAL METHOD
All calculations were carried out with the Gaussian 03 suite ofprogram [9]. The calculations of systems contain C, H, N, O, Br, Cl and F described by the standard 6-31G(d) basis set [10—12]. For Ni element standard LANL2DZ basis set is used [13—15] and Ni described by effective core potential (ECP) ofWadt and Hay pseudo-
potential [16] with a doublet-^ valance using the LANL2DZ. Geometry optimization was performed utilizing Becke's hybrid three-parameter exchange functional and the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP) [17, 18]. A vibrational analysis was performed at each stationary point found, that confirm its identity as an energy minimum.
The nucleus-independent chemical shift (NICS) is defined as the absolute magnetic shielding computed at the center of a ring in a molecule [19, 20]. NICS(0), NICS(0.5), NICS(1.0), and NICS(1.5) are calculated at the center and 0.5, 1.0, and 1.5 A above the ring, respectively. The AIM2000 program was used for topological analysis of electron density. The following characteristics of ring critical points (RCPs) are taken into account: density at RCP (p(rc)), its Laplacian (V2(rc)) and ellipticity (s) [21].
RESULTS AND DISCUSSION
Geometry. The structures ofthe for C3N3H2X2Ni(Cp)NO with various substitution on L (X = H, F, Cl, Br, CN) and its linkage isomers optimized by DFT method (Fig. 1).
X
(a) H
\ /
-N
/
NO
=Ni
-N
X
\
(b)
XH X\ /
"N\ /
>=Ni
ON
N
H
X
\
H
Fig. 1. Structure of the LNi(Cp)NO (L = 1,3-dihydro-1H-imidazol-2-ylidene) in the nitrosyl (a) and isonitrosyl (b) isomers, X = H, F, Cl, Br, CN.
STRUCTURE AND BONDING OF THREE-COORDINATE N-HETEROCYCLIC CARBENE 1281
Table 1. The energy (E, Hartree), relative energies (AE, kcal/mol), v(NO), 5(Ni-N-O) (cm"') of C3N3H2X2Ni(Cp)NO (X=H, F, Cl, Br) complexes and their linkage isomers C3N3H2X2Ni(Cp)NO in the B3LYP level of theory
Atom -E -E AE v(NO) v(ON) S(Ni N O) S(Ni-O-N)
nitro isonitro nitro isonitro
H 718.965 718.915 31.613 1687.2 1607.5896 381.8 290.446
F 917.403 917.352 31.912 1694.2 1612.6611 380.4 272.963
Cl 1638.13 1638.08 31.845 1695.5 1613.9046 377.4 275.8224
Br 5860.56 5860.51 31.828 1695.0 1613.5811 380.1 276.361
CN 903.431 903.380 31.814 1705.4 1623.2617 378.1 289.185
Table 2. The bond distances, and the major bond angles (bond distance is in A and angle is in degree) of C3N3H2X2Ni(Cp)NO (X=H, F, Cl, Br) complexes and their linkage isomers C3N3H2X2Ni(Cp)NO in the B3LYP level of theory
Atom r(Ni-NO) r(Ni-ON) Ar r (N-O) r(O-N) Ar r(Ni-C) r(Ni-C) Ar ZNi-N-O ZNi-O-N
nitro isonitro
H 1.7990 1.9110 0.1113 1.1970 1.2010 0.0046 1.9321 1.9049 0.0272 119.27 123.10
F 1.8000 1.9150 0.1151 1.1950 1.2000 0.0047 1.9300 1.8996 0.0304 119.97 123.66
Cl 1.8010 1.9150 0.1137 1.1950 1.2000 0.0046 1.9306 1.8999 0.0307 120.05 123.69
Br 1.8010 1.9150 0.1131 1.1950 1.1990 0.0045 1.9302 1.9000 0.0303 120.04 123.70
CN 1.8080 1.9270 0.1190 1.1930 1.1970 0.0040 1.9223 1.8889 0.0335 120.29 123.98
The energy and relative energies of these are listed in Table 1. These values show that nitro isomer is more stable than isonitrosyl isomer. On the other hand, calculations indicate stability difference of isomers decreases as: X = F > Cl > Br > CN > H.
The geometrical parameters are provided in Table 2. These values reveal in the isonitrosyl configuration Ni-ON the Ni-O bond length increases with respect to the Ni-N bond length of the nitrosyl isomer, while the N-O distance decreases. The Ni-N bond length is between 1.799 and 1.808 A in the nitrosyl isomer and increase by an amount of 0.11 A in the isonitrosyl. There is a significant increase in the Ni-C bond length in the nitrosyl isomer. Both in the nitrosyl and the isonitrosyl the NO group adopts a bent configuration with a Ni-N-O angle of119.2° -120.2° in the nitrosyl and with a Ni-O-N angle of 123.1°-123.9° in the isonitrosyl.
Frequency. A distinct feature of the vibrational analysis results is the increased frequency of the v(NO) vibration in the substituted nitrosyl and isonitrosyl isomers. This is well reflected in the DFT calculation, where the v(NO) vibration also increases in the X = H to X = F, Cl, Br, CN. Furthermore, the S(Ni-N-O) bending mode in the X = H lies with 381.8 cm-1 at much higher frequencies compared to the 5(Ni-N-O) of X = F, Cl, Br, CN. The similar trends is observed in
isonitro isomer. These calculated frequencies are summarized in Table 1.
Nucleus-independent chemical shift (NICS). As an effort to discuss the use of NICS as a measure of aromaticity for ring, we have calculated NICS values along the z-axis to the ring plane beginning on the center of the L-ring up to 1.5 A. These calculations show that the shape of NICS profile with respect to the distance from the ring center falls into two categories. In addition, for all species, we have localized the NICS maxima and minima and determined the distances to the center of the ring at which they occur (see Table 3). For X = H, the highest absolute value of NICS close to the center of the ring. For other species (X = F, Cl, Br, CN), have a maximum about 0.5 A to the ring center. It is possible that induced magnetic fields generated by the a aromaticity are particularly large in the center of the ring, but systems having n aromaticity indicate a minimum NICS at the certain distance from the center of the ring.
AIM analysis. Owing to inseparability of the a and n contributions to the electron density at the bond critical point, the p(r) values can be used to evaluate bond strength for different types of bond. As shown in Table 4, different values of p(r) and V2p(r) for Ni-C bonds in species evidently indicate the relative Ni-C bond strength. This result is in agreement with the geometrical analysis, which shows Ni-C bond in X = H
6 XyPHAH OH3OTECKOH XHMHH tom 85 № 7 2011
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GHIASI, MOKARRAM
Table 3. The NICS(0.0), NICS(0.5), and NICS(1.5) of C3N3H2X2Ni(Cp)NO (X=H, F, Cl, Br) complexes in the B3LYP level of theory
NICS(0.0) NICS(0.5) NICS(1.0) NICS(1.5)
Atom
nitro nitro nitro nitro
H -8.80 -9.09 -7.21 -4.16
F -11.60 -9.73 -6.34 -3.31
Cl -9.23 -8.78 -6.50 -3.64
Br -8.41 -8.32 -6.44 -3.66
CN -9.71 -9.42 -7.10 -4.03
has higher bond length than other species. On the other hand, the Ni—C bonds in all structures have positive values of V2p(r) which indicate the close shell interaction.
The bond ellipticity is defined as s = — 1,
where, |A,2| > |A,2| and provides a quantitive measure of the n character of the bond. The plane of the n distribution is uniquely specifies by the direction of the axis associated with the curvature of smallest magnitude,
The s(NI-C) values show that, Ni-C bond in X = F, Cl, Br, CN species has more n-character in comparison with X = H (Table 4). This is compatible with the results of aromaticity from NICS calculation. Figure 2 represents a linear correlation between NICS (0.0) and NICS (0.5) with p(3, +1) in X = F, Cl, Br, CN species.
Natural bond orbital analysis. Natural bond orbital (NBO) analysis stresses the role of intermolecular orbital interaction in the complex, particularly charge transfer. This is carried out by considering all possible interactions between filled donor and empty acceptor NBOs and estimating their energetic importance by
Table 4. Calculated topological parameters (in atomic unit) at the Ni—C bond critical point and the L-ring critical point for C3N3H2X2Ni(Cp)NO (X=H, F, Cl, Br)
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