How can I reduce this circuit to a source in series with a single impedance?
If I do not make any mistake in the math the correct equation for \$Z_{th}\$ will look like this:

$$Z_{th} = \frac {\sqrt{R5^2 + (\omega\ C_1\ R_T\ R_5)^2}}{\sqrt{1+(\omega\ C_1 (R_T+R_5))^2}}$$

Where: 

$$R_T = \frac{R_1 \cdot R_2}{R_1+R_2}$$

And the Thevenin voltage 

$$Vth = V_{IN}\cdot\frac{R_2||R_5}{R_1 + R_2||R_5}\cdot \frac{\omega\ (R_T +R_5)C_1}{\sqrt{1 +(\omega\ (R_T + R_5)C_1)^2} }$$