Is there a wat to create dataframe of unequal lengths

Question

Is there a way to create a data frame of unequal length in R. For example

df <- data.frame(a= c(1,2,3), b =c(5,6))
Error in data.frame(a = c(1, 2, 3), b = c(5, 6)) : 
  arguments imply differing number of rows: 3, 2

Expected output

df
a    b
1    5
2    6
3    NA