Ok. DracoSuave is correct that the power (as currently written; not sure whether there was a change) does not use the word "weapon." However, I do not believe that that fact is determinative. Why? Because the power DOES include a proficiency bonus. Not an enhancement bonus, not a power bonus, but a proficiency bonus. So you must have something to be proficient with, i.e., a weapon. Note that improvised weapons, such as a bare fist, do not include such a bonus, nor do attacks themselves.
That said, I might buy the argument that a character could only be proficient with it for the granted basic attack, and therefore would need to use it without the proficiency bonus for any non-basic attacks. (Basically, it would be an unarmed attack that did more damage. Could be helpful if you were disarmed or surprised.)
In addition, the argument that the basic attack would not require a free hand appears meritless. While it may be amusing to envision the armor growing its own extra clawed arm and attacking on your behalf, countless creatures in the MM have claw attacks; do you want those creatures to be holding magic items, weapons or other party members and then to make claw attacks against your character or other party members as well? And before anyone advances the "but an ITEM..." argument, there are plenty of magic creatures with claws; if a 6th level item can do it, a dragon or a lich probably can, too. Especially since THEIR claw attacks do not specify the need for a weapon or a free hand, either.
That said, I might buy the argument that a character could only be proficient with it for the granted basic attack, and therefore would need to use it without the proficiency bonus for any non-basic attacks. (Basically, it would be an unarmed attack that did more damage. Could be helpful if you were disarmed or surprised.)
In addition, the argument that the basic attack would not require a free hand appears meritless. While it may be amusing to envision the armor growing its own extra clawed arm and attacking on your behalf, countless creatures in the MM have claw attacks; do you want those creatures to be holding magic items, weapons or other party members and then to make claw attacks against your character or other party members as well? And before anyone advances the "but an ITEM..." argument, there are plenty of magic creatures with claws; if a 6th level item can do it, a dragon or a lich probably can, too. Especially since THEIR claw attacks do not specify the need for a weapon or a free hand, either.