Difference between std::greater

This code works:

#include <iostream>
#include <queue>
#include <vector>
#include <functional>
using namespace std;
int main(){
    priority_queue<int,vector<int>,greater<int> > pq;
    pq.push(1);
    cout<<pq.top()<<endl;
}

But,this code does not compile:

#include <iostream>
#include <queue>
#include <vector>
#include <functional>
using namespace std;
int main(){
    priority_queue<int,vector<int>,greater<int>() > pq;
    pq.push(1);
    cout<<pq.top()<<endl;
}

Why? All I understand is that greater() is a function object and priority_queue accepts a binary predicate as third argument and that predicates are a special type of functors. But how does the pair of braces make that difference.