You can use "linfit" to find coefficients for functions that might make up a complex function if you have a data array.
If you have candidate functions in mind but need to determine internal parameters as well as coefficients you can try "genfit".
It is also possible to create a solve block and use "Minerr" for this type of purpose.
On the other hand, if you just want the function in order to get the derivatives, you might consider a "spline fit". That's fairly easy to do and you can get numerical derivatives of the resulting interpolating function.