The following works fine from command line
/usr/bin/mysqldump -uUser -pPass Db_name > /var/www/db_backup/db.`date +%Y%m%d%H%M`.sql
but
Put your one line script (as shown) into a proper script file and invoke that from cron:
$ cat /usr/local/bin/db-backup #!/bin/sh /usr/bin/mysqldump -uUser -pPass Db_name > \ /var/www/db_backup/db.`date +%Y%m%d%H%M`.sql $ # use RHEL commands to add db-backup to your crontab