find negation operator not working as expected

I'm trying to find all the files that don't contain a string in the
current directory.  This incantation

find . -exec grep -l "foobaz" '{}' \;

will list all the files that contain "foobaz"; however, when I try to
invert this using any one of these incantations

find . ! -exec grep -l "foobaz" '{}' \;
find . \! -exec grep -l "foobaz" '{}' \;
find . -not -exec grep -l "foobaz" '{}' \;

I get the same output.  What's going on here?

I'm using a bash shell on darwin Mac OS.  The "find" utility I'm using
is the GNU variety built using MacPorts.

Thanks in advance.