Acetylide Complexes
2
.1
.Introduction
This Chapter presents the results of a series of ADF calculations on Group 8 metal acetylide complexes. Model complexes consisting of metal acetylides with a combination of chloro-, cyclopentadienyl and phosphine ligands have been examined, and the calculated optical and nonlinear optical properties compared with existing experimental data for analogous molecules.
This work has been divided into three sections. The first section involves calculations on a series of cyclopentadienyl group 8 metal acetylide complexes in order to evaluate the capacity of ADF to accurately calculate optical and nonlinear optical spectra. The second section contains the results of a study of a series of ruthenium mono- and bis- acetylide complexes, and includes attempts to characterize the optical spectra for both the neutral and oxidized states. The third section involves examination of a series of osmium acetylide complexes in order to test assumptions made in the previous section. The complexes investigated are displayed in Figure 2.1.
_ . PH, L PH3 PH, t p H3 — R H3P / Ri----Ru— R2 h3p/ | O l L /S --- ' » H3P 1 H3P p h3 PH3 M = Fe, Ru, Os R1 = Cl, R2 = Cl, CsCPh, C=CC6H4-4-CsCPh R = Ph, 4-C6H4C=CPh R = H, Ph, 4-C6H4N 02
R,
= C=CPh, R2 = CsCPh, CsCC6H4-4-C=CPhFigure 2.1. M o d e l complexes studied in this w o rk
These were chosen because o f the existence o f a considerable amount of experimental data for related complexes against which their various optical properties can be compared.1'6
As shown in the preceding Chapter, the molecular NLO properties o f group 8 metal acetylides have been studied extensively in recent years. When compared w ith their precursor acetylenes, these complexes tend to show significantly higher thermal stability, molecular NLO properties, and structural diversity. In addition, the presence o f a metal centre allows for relatively accessible multiple oxidation states.
Theoretical investigations of group 8 metal acetylide complexes are less common. DFT has been used by Stranger and co-workers to calculate ionization potentials and the degree of back-bonding in a series o f cyclopentadienyl ruthenium acetylide complexes.7 Similar work concentrating on back-bonding has been performed on (chloro)tetra(phosphine) group 8 metal acetylide complexes (Figure 2.2).8
& _
PH,1 / P H 3 p i ---- n Hu ---- H h3p^ 7 h3p U l IVI --- 1 \ H,P/ | p h3 M = Fe, Ru, Os R = H, Ph, C6H4-4-N 02Figure 2.2. Model complexes investigated by Stranger and co-workers7,8
2.1.1. Density Functional Theory
Whereas most computational chemistry techniques attempt to find numerical solutions to the Schrödinger wave equation, Hi|) = E\\), DFT focuses on the electron density of the system rather than the wavefunction. At the heart of DFT is the Kohn-Sham equation:
(-1/2V2 + Vext(r) + Vc (r) + VXC(r))cpi(r) = e ^ r )
where V xc(r ) is a suitable local exchange-correlation potential, Vext(r) is the external potential, V c(r) is the Coulombic potential of the electron cloud, ej is the one-electron MO, and 4>j is the corresponding orbital energy. Solving these equations is considerably less computationally expensive than are the corresponding ab initio techniques. As a consequence DFT has become a popular technique, particularly when large molecules are studied.
2.1.2. ADF in the calculation o f optical and nonlinear optical properties
In the calculation of optical spectra and frequency-dependent hyperpolarizabilities, it is necessary to incorporate a time dependence to the Kohn-Sham equations:
ib/bt qpj(r,t) = H(pj(r,t) = (-V2/2 + V[p](r,t))cpi(r,t)
ADF has been used to successfully calculate optical spectra in the past. Nonlinear optical spectra (specifically ß values) have also been calculated, but with only partial success. The systems investigated have ranged from atoms9 to simple molecules9' 12 to quite complex systems such as porphyrins13'15 and transition metal complexes.16'18 Where ADF has been used to calculate optical spectra the results generated are comparable to those from other advanced theoretical techniques, such as CCSD(T), with differences of up to 8000 cm-1 between experimental and calculated data being observed.19 Results are generally comparable to sophisticated ab initio methods.
There are far fewer examples of the calculation of second-order hyperpolarizabilities employing ADF in the chemical literature. Some small molecules have been studied,9,20 but a zirconium tetrapyrrole sandwich is the only moderately-sized molecule to have been examined thus far.21 There is therefore, a need to apply ADF to a greater range of experimental data.
Baerends and co-workers calculated the first and second hyperpolarizabilities of a variety of small molecules (N2, CO2, CS2, C2H4, NH3, CO, HF, H2O, and CH4), and compared results , with both experimental data and high level ab initio calculations.9 TD-DFT gives a reasonable correlation to the experimental data, the differences being comparable to those seen with the ab initio results. The authors suggested that improvements in the exchange- correlation potentials should lead to a considerable improvement in the results. Similar calculations have been performed by the authors on helium and p-nitroaniline.20
A zirconium tetrapyrrole sandwich complex (Figure 2.3.) has been studied using the TD- DFT module of ADF by Rosa and co-workers;21 unfortunately, because the experimental second-order hyperpolarizabilities were measured close to the resonance frequency, a clear correlation between the calculated and experimental results could not be obtained.
Figure 2.3. Tetrapyrrole zirconium sandwich complex investigated by Rosa and co
workers21