Is a flea product containing Imidacloprid safe to use for ant control?

Firstly, no chemical (especially pesticides) should be used for a purpose for which it was not intended.

Secondly, imidacloprid is a neonicotinoid. These products remain in the environment for years and are toxic to wildlife. They are mobile in water, can contaminate ground water (through run off into soil, drains, puddles etc), and remain in the soil for YEARS! Research has shown that when used to treat the soil, it continued to be taken up by plants even during years when it was not applied. Even the microscopic levels taken up by the plants were lethal to bees.

To get rid of ants, I recommend you research organic methods on the internet. You could also try boiling water, depending on where the ants are. If the ants are getting into your house (i once had this problem), spray a dilution of bleach on the surfaces. This cured my ants problem almost completely within a day. You may get an occasional ant coming in, but if you keep spraying for a while, you'll soon get NO ANTS - I don't get any now.

Neonicotinoids are highly toxic, and I wouldn't even use them in the garden, personally.

Neonicotinoids include imidacloprid acetimacloprid, clothianidin, thiacloprid, thiamethoxam, dinotefuran and nitenpyram. I avoid all of these.