Dear (insert last name here)
Paragraph 1: Explain that you are writing on behalf of your organisation, and that you would like to show your gratitude for their generous donation. Mention what they have donated here as well as the specific purpose of their donation.
Paragraph 2: Give a little general background on what your company has been striving to do since its inception. Go on to say that it is through people like the donor who make your work possible. Here you can mention some of the best things that your company has been able to do thanks to donor support over the years.
Paragraph 3: In this paragraph you should look to explain exactly what has been done with this specific donation but lead on to say that it is only with continued donations that you will be able to continue carrying out work like this in the future.
Paragraph 4: Use this paragraph to conclude matters and thank the donor again for their generous gift. Here you can also say that you hope to hear from them again in order to continue the good work of the company.