Pitcher throwing to unoccupied base

[DuffysCliff]

F1, a RHP, is in the set position. 2B unoccupied. R1 breaks for 2B just as F1 lifts free foot (the left foot in this case). F1 sees this but since free foot is off ground, is unable to disengage pivot foot from plate. So, without hesitating or making a move towards 1B, F1 spins clockwise, and steps and throws to 2B. Is this a balk? It is not a balk if F1 steps and throws to an unoccupied base for the purpose of making a play. OTOH, if F1's entire free foot goes behind the plate, he must deliver home - except when making a pickoff play at 2B. I cannot find how a pickoff play is defined, therefore I do not know if F1 committed a balk in this situation. Or, if some other rule governs this situation. Thanks in advance for insight.