Hebrews 1:5

Unto which

(Τιν). "To which individual angel." As a class angels are called sons of God (Elohim) (Psalms 29:1), but no single angel is called God's Son like the Messiah in Psalms 2:7. Dods takes "have I begotten thee" (γεγεννηκα σε, perfect active indicative of γενναω) to refer to the resurrection and ascension while others refer it to the incarnation.And again

(κα παλιν). This quotation is from 2 Samuel 7:14. Note the use of εις in the predicate with the sense of "as" like the Hebrew (LXX idiom), not preserved in the English. See Matthew 19:5; Luke 2:34. Like Old English "to" or "for." See 2 Corinthians 6:18; Revelation 21:7 for the same passage applied to relation between God and Christians while here it is treated as Messianic.