name columns of dynamically chosen data.frame

I am trying to name the columns of a dataframe, but the dataframe is chosen dynamically. Any idea why this does not work? Below is an example, but in my real case I get a different error. As of now I would just like to know what causes either of the errors, Thanks :

Error in file(filename, "r") : caot open the coection In addition: Waing message: In file(filename, "r") : caot open file 'df': No such file or directory

#ASSIGN data frame name dynamically
> assign(as.character("df"), data.frame(c(1:10), c(11:20)))
> 
#IT WOrked
> df
   c.1.10. c.11.20.
1        1       11
2        2       12
3        3       13
4        4       14
5        5       15
6        6       16
7        7       17
8        8       18
9        9       19
10      10       20
> 
#Call the data frame dynamically, it works
> eval(parse(text = c("df")))
   c.1.10. c.11.20.
1        1       11
2        2       12
3        3       13
4        4       14
5        5       15
6        6       16
7        7       17
8        8       18
9        9       19
10      10       20
> 
#name the columns
> colnames(df) <- c("a", "b")
> df
    a  b
1   1 11
2   2 12
3   3 13
4   4 14
5   5 15
6   6 16
7   7 17
8   8 18
9   9 19
10 10 20
> 
#name columns of dynamically chosen data frame, retus and error
> colnames(eval(parse(text = c("df")))) <- c("c", "d")
  Error in colnames(eval(parse(text = c("df")))) <- c("c", "d") : 
   target of assignment expands to non-language object