I’m no cron expert but I don’t think you can do that. The problem is in cron each field is handled independently so there really is no way to specify the time like you want to using just one cron statement. But there is nothing that says you have to use only one statement to trigger your rule.
Time cron "0 45/2 6 ? * MON-FRI" or
Time cron "0 */2 7 ? * MON-FRI"
The first should trigger the rule between 6:45 and 7:00 and the second one between 7:00 and 8:00. You will probably have a hick-up in the transition between the first trigger and the second one where the rule triggers after one minute, but I’m not certain.