Find out the send dates of the documents with the grant amount of more than 5000 were granted by organisation type described
select t1.sent_date from documents as t1 join grants as t2 on t1.grant_id = t2.grant_id join organisations as t3 on t2.organisation_id = t3.organisation_id join organisation_types as t4 on t3.organisation_type = t4.organisation_type where t2.grant_amount > 5000 and t4.organisation_type_description = 'Research'