How to obtain all combinations of the columns of a data frame taken by 2? in R

How to obtain all combinations of the columns of a data frame taken by 2?
in R

suppose I have this data frame:
matrix(c(2,4,3,1,5,7,1,2,3,5,8,2,4,5,1,1,3,6,1,3,4,5,6,1),nrow=6,ncol=4,byrow
= TRUE)->X
as.data.frame(X)->X.df
V1 V2 V3 V4
1 2 4 3 1
2 5 7 1 2
3 3 5 8 2
4 4 5 1 1
5 3 6 1 3
6 4 5 6 1
then I would like to obtain a list of a set of data frames containing all
combinations of variables taken by 2, without repetition, and avoiding any
column with itself. That means:
V1,V2
V1,V3
V1,V4
V2,V3
V2,V4
V3,V4
Any idea of how to do this?
thanks a lot!
Tina.