I think the structure of Hebrews 1:5-13 can be arranged as follows.
1:5a To which of the angels did he say? (quote from Ps 2:7)
1:5b-6 Two quotes “of the Son” (2 Sam 7:14 and Deut 32:43)
1:7 Of the angels he says (quote from Ps 104:4)
1:8-12 Two quotes “of the Son” (Ps 45:6-7 and Ps 102:25-27)
1:13 To which of the angels has he said? (quote from Ps 110:1)
The balance here makes sense to me, and helps to show how the Old Testament quotations (seven different passages) relate to each other. The three smallest Psalms quotes are staggered, and the two longer Psalms quotes are balanced with quotes from the Torah and the Prophets.
Some may object to this structure by pointing out that it puts the statement about the angels at the center. But the point of the passage seems to be the contrast between the angels, who are only spirit beings (winds, flame of fire), and the Son who has been born in the flesh.
To strengthen this case, one could add another “outer ring” to each side of this structure by showing the superiority of the Son who accomplishes and gives salvation (1:1-4) in contrast to the ministry and service of the angels to those who inherit salvation (1:14).
Prairie Quests 