A German diplomat assaulted his wife at the couple’s Upper East Side home — but cops won’t arrest him because he has diplomatic immunity, police sources said. Joachim Haubrichs punched his wife, Henna Johnson, in the face as the two were fighting at their East 88th Street digs near First Avenue around 8:45 p.m. on Monday, sources said The woman, 35, suffered a black eye during the fight, sources said. On Tuesday, Johnson reported the incident at the Family Justice Center in Lower Manhattan, who in turn contacted the NYPD. After being made aware of the incident, NYPD brass told cops not to arrest Haubrichs, whose title is assistant attaché to Germany’s Permanent Mission to the United Nations. Police are still investigating, however. Johnson refused medical attention following the incident. Sources said a similar incident occurred between the two back in 2015, but did not provide any further information about that incident.