J.C. Penney Co., facing a potential threat to its turnaround plans, has come out swinging after debt holders claimed it technically defaulted on a bond, Bloomberg News reported yesterday. The department store chain on Monday filed a lawsuit in Delaware Chancery Court seeking to block efforts by a group of bondholders to declare a default on 7.4 percent bonds due in 2037. The company also asked lawyers at Brown Rudnick LLP to identify the investors they represent. The Boston-based firm said in a letter to the company that it represents holders of more than half of the debt. The claim exposes J.C. Penney to “imminent, irreparable harm,” the Plano, Texas-based retailer said in the complaint.