Heptangle - looks like an occult term. It is a keyword but not very brandable.
Heptameter - I was pronouncing it "hepta-meter" then I looked it up and yes I remember it from literature class. i think 99% of people will pronounce this wrong and maybe only 5% of the public will even know what it means.
Sexagenary - hard to remember & spell for old folks (target audience). Also has the unintended 'sex' in the domain name.
They are single keyword .com's but I don't think they are very desirable words for someone to come along and want to build a site on them.