Convert cell to double

>> C = [{1} {2} ; {'@CF'} {2}] 
C = 

[  1]    [2]
'@CF'    [2]


>> whos C
  Name      Size            Bytes  Class    Attributes

  C         2x2               478  cell  

How can I convert C into double so that:

>> C
C = 
1    2
NaN  2

I've tried str2double(C). It returns:

   NaN   NaN
   NaN   NaN