matt grime said:
Before I prove or disprove your assertion (of which I will do neither) you should explain
1. why you have suddenly inserted the word 'pure'
2. what counts as mathematics and what as physics.
My view would be that if I needed to classify him I would opt for physicist. However, as well as experimental things, he did, or at least appeared to, theoretical physics which is mathematical, and thus he contributed to mathematics. The distinction between theoretical physics and mathematics is at best fuzzy and almost certainly harmful.
1.It wasn't 'sudden'.

To me,a mathematician is a person who works in the field of pure mathematics.He creates mathematics (notions,concepts,theories) or sometimes just puts known things in another perspective.Applied mathematics requires other types of knowledge (that's why bears the name "applied") and skills.
None of the theoretical physicists is a mathematician.Just that we know more mathematics than the exparimentalists or other scientists does not imply us having the word "mathematician" written all over us.You see a thin border,i see a very thick one.'Exceptions' don't count as exceptions:Newton was a physicist,Leibniz a mathematician,Gauss a mathematician,and the list is very long.
"...and thus he contributed to mathematics".He didn't.There's no single formula/proof/definition/theorem/lemma/conjecture/corollary/proposition...in the field of mathematics (pure & applied) which bears the name of/is linked to Niels Bohr,nor Albert Einstein,nor Paul Adrien Maurice Dirac,and so on and so forth.The list of theoretical physicsts i believe opens with the names of Ludwig Boltzmann and James Clerk Maxwell.
Notes:
a)Newton created calculus for physical purposes only.
b)Leibniz created calculus for mathematical purposes only.
c)Euler invented variational calculus for mathematical purposes only.
d)Gauss invented diff.geometry for mathematical purposes only.However,he was passionate for (physical) applications of his (and Ostrogradski's) integral formula and therefore resulted:Gauss's laws for electrostatics,magnetostatics and gravitostatics.
e)Paul Adrien Maurice Dirac invented "delta functional" for physical purposes only.Shilov,Gelfand & L.Schwarz took it to build distribution theory as a subchapter of functional analysis in pure math.
f)...
Daniel.
PS.2.The 'short' version is:the CLEAR difference between a theoretical physicst and a mathematician appears whenever both are faced with the same (very simple) problem from other's domain.Take for example:solving the Schroedinger's equation for the H atom.The mathematician will have no reason to reject irregular solutions (discrete spectrum),because he cannot see the physics that lies beyond equations.He will analytically continue those hypergeometric series and will come up with a brilliant mathematical expression without any physical relevance.After all,if he did know the phyiscs,he would be a physicist,right??