A.T. Robertson Commentary 2 Corinthians 9:14

"while they themselves also, with supplication on your behalf, long after you by reason of the exceeding grace of God in you." — 2 Corinthians 9:14 (ASV)

While they themselves long after you (αυτων επιποθουντων). Genitive absolute of present active participle of επιποθεω (5:2).

In you (εφ' υμιν). Upon you.