Here how you create your JSON format.
list(job_id = "1",
page = "1",
rows = lapply(seq(nrow(df)), function(x) list(row=as.character(df[x,]))))
cat(toJSON(ll))
{"job_id": "1",
"page": "1",
"rows": [
{
"row": [ "1", "4", "7" ]
},
{
"row": [ "2", "5", "8" ]
},
{
"row": [ "3", "6", "9" ]
}
]
}