Problem To Insert A Key Field In Another Table

hi everybody I have this case which I couldn't solve and need badly your help

table1

id1 field1 field2
1 a b
2 a c


table2

id1 field1 field2
a b
a c

I need to update table2 on the (id1) if the value of field1 and field2 in table 1 is equal to field1 and field2 in table 2 and inserting null when there is no match:::

I'm trying with this code but couldn't get thru

UPDATE table2
SET id1= COALESCE(table2.id1, 0)
from table1
left outer Join table2
ON
table1.id1=table2.id1


thanks