Using positive voltage reference on a negative supply
You could invert a +2.5 volt reference to -2.5 volts using an op-amp but I'd simply use a +2.5 volt shunt reference device like this one: -



And re-position is in the "negative circuit" like this: -



There are eighty-four +2.5 volt shunt references listed here on Mouser so I expect you'll be able to find one that is suitable in terms of accuracy and drift.

Note that the DS for the LM336 states on page 1: -


  Further, since the LM136-
  2.5 operates as a shunt regulator, it can be used as either a positive or negative voltage reference.


This applies to any shunt voltage reference of course.