Update inner Array element property using outer field value Mongodb

I have a collection as below and I wish to update the email with "@company.com" where type is "email" and Details is null.

{
 "Name" : "Sam",
"Company" : "Awesomeness"
 "Contacts" : [
 {    
   "type" : "phone",
   "Detail" : "1231232" 
 },
{
  "type" : "email",
  "Detail" : null
} 
]
}

I have reached as far as

   collection.update({"Contacts.type" : "email", "Contacts.Detail" : null}, 
    { $set : 
      { "Contacts.$.Detail" : 
            { 
              $concat : [$Name , "@" , "$Company", ".com"] 
            }
     })

but this gives an error saying $ dollar prefixed field "$concat" ....is not valid for storage.