The document, published on BASF's website, detailed a series of telephone conversations between BASF chairman Juergen Hambrecht and Engelhard chief Barry Perry in recent weeks following the German giant's decision to launch a bid for the US surface and material science specialist.
BASF wants to buy Engelhard for 37 dollars per share or 4.9 billion dollars (four billion euros) in all. Its cash offer runs until February 6.
The German company indicated that it might be willing to raise that offer to 38 dollars if Engelhard could provide convincing arguments supporting a higher price.
But in a conversation on January 2, Perry told Hambrecht that Engelhard's board "did not consider 38 dollars a share to be sufficiently compelling to enter into discussions with or disclose information to" BASF, the 104-page document revealed.
Indeed, "Perry indicated that exclusive discussions would only be conditional on (BASF) offering a significantly higher share price."
But the US chief refused to name a concrete price, the document added.
BASF has said it would prefer the takeover to be a friendly rather than a hostile move.
But Engelhard sees it as a hostile bid and directors of the US company have called on investors to wait and "take no action at this time in response to an unsolicited proposal from BASF".
Engelhard expects to make an official recommendation to shareholders within the next 10 days.