  During their first observational run, the two Advanced LIGO detectors
attained an unprecedented sensitivity, resulting in the first direct detections
of gravitational-wave signals and GW151226, produced by stellar-mass binary
black hole systems. This paper reports on an all-sky search for gravitational
waves (GWs) from merging intermediate mass black hole binaries (IMBHBs). The
combined results from two independent search techniques were used in this
study: the first employs a matched-filter algorithm that uses a bank of filters
covering the GW signal parameter space, while the second is a generic search
for GW transients (bursts). No GWs from IMBHBs were detected, therefore, we
constrain the rate of several classes of IMBHB mergers. The most stringent
limit is obtained for black holes of individual mass $100\,M_\odot$, with spins
aligned with the binary orbital angular momentum. For such systems, the merger
rate is constrained to be less than $0.93~\mathrm{Gpc^{-3}\,yr}^{-1}$ in
comoving units at the $90\%$ confidence level, an improvement of nearly 2
orders of magnitude over previous upper limits.
