I think it's worth a frame challenge here.
In the comments Alexander asks "why do you want lower boiling point with thermal solar?", to which jamesqf (not the OP) replies "because you need fewer mirrors to reach the lower boiling point, of course." I am guessing that this is also the OP's reasoning, and if so a frame challenge is definitely in order.
In the 18th and early 19th century, when steam engines were beginning to become viable, people worried a lot about this sort of thing. There were people working on "air engines" instead of steam engines, on the grounds that it seemed like steam engines were wasting energy boiling water when it could be used to expand air instead. I daresay there were others working on fluids with a lower boiling point - it seems an obvious thing to try.
This kind of thing eventually led to the development of thermodynamics, which was really a revolutionary science at the time. One of the outcomes of it was that you don't waste energy boiling water, because all of that energy goes into expanding the steam, and the boiling point of the working fluid doesn't directly affect the efficiency of the engine - what matters is the temperature difference between the boiler and the cooling fluid (a bigger difference is better).
Water, then, turned out to be a very good choice for the working fluid of a heat engine. It's non-toxic and readily available (hence a bit of leakage doesn't do too much harm, and there's no problem venting any excess or cooling it evaporatively in the open air). It's not corrosive if the machine components are prepared correctly. Its boiling point of 100°C is easy to reach and won't melt metal, but high enough that you can easily condense it even on a very hot day. It's also not flammable, which provides an enormous safety advantage over almost any other fluid, and evaporates fairly slowly in comparison to something like ethanol, which limits spoilage.
Even today, with the exception of internal combustion engines, virtually all heat engines run on steam. (They tend to take the form of turbines rather than piston engines these days, but the principle is exactly the same.)
There is no reason why all this wouldn't also apply to a 19th century solar thermal plant. It's true that with a lower boiling point you would need less mirrors to heat the boiler, but the engine would provide a correspondingly lower motive force, so in many ways this is a disadvantage - you're wasting energy by not using the sunlight to heat the boiler as hot as possible, which is easy to do. Sunlight has an effective temperature of about 6000°C, and until the target starts to get near that you can always make it hotter just by adding more mirrors.
Because of all this, it would make much more economic sense to just have more mirrors and use water/steam as the working fluid.
Of course, solar power also has the issue of intermittency - even in the sunniest places it doesn't work at night. This is why modern solar thermal plants generally heat a reservoir of molten salt, which can be heated to much higher temperatures than 100°C, which makes it more convenient for storing heat for long periods of time. But when you want to use that heat, as far as I know, you still just use it to boil water and run the engine on steam. I think this could all be done with 19th century technology, so I would expect it to be developed then if solar power was the main energy source.