  We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). This
comet was observed on 23-24 January 2018 with the IRAM 30m telescope, and in
January to March 2018 with the Nan\c{c}ay radio telescope. Visible spectroscopy
was performed in December 2017 and February 2018 with small amateur telescopes.
We report on measurements of CO, CH3OH, H2CO and HCN production rates, and on
the determination of the N2/CO abundance ratio. Several other species,
especially OH, were searched for but not detected. The inferred relative
abundances, including upper limits for sulfur species, are compared to those
measured in other comets at about the same heliocentric distance of about 2.8
AU. The coma composition of comet C/2016 R2 is very different from all other
comets observed so far, being rich in N2 and CO and dust poor. This suggests
that this comet might belong to a very rare group of comets formed beyond the
N2 ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment
of a large and differentiated transneptunian object, with properties
characteristic of volatile-enriched layers.
