Mengen was a star that was located a number of parsecs away from Earth and the Sol system, near Federation space, and also near the border between the Alpha and Beta Quadrants.
The Federation starship USS Mengen was named for this location. (TOS reference: Star Fleet Technical Manual)
The Mengen system was the home of the Mengenite civilization, a sovereign power that was not a member of the Federation by the year 2270. Commander Paul Burch of Starfleet feared that the power schemes of Vice Admiral Vaughan Rittenhouse were a threat to sovereign locations like Mengen. (TOS - Fortunes of War novel: Dreadnought!)