  GW190521 is the most massive merging binary black hole (BBH) system detected
so far. At least one of the component BHs was measured to lie within the
pair-instability supernova (PISN) mass gap ($\sim 50-135\;{\rm M}_{\odot}$),
making its formation a mystery. However, the transient observed signal allows
alternative posterior distributions. There was suggestion that GW190521 could
be an intermediate-mass ratio inspiral (IMRI), with the component masses
$m_1\sim 170\;{\rm M}_{\odot}$ and $m_2\sim 16 \;{\rm M}_{\odot}$, happening to
straddle the PISN mass gap. Under this framework, we perform binary population
synthesis to explore the formation of GW190521-like systems via isolated binary
evolution. We numerically calculate the binding energy parameter for massive
stars at different metallicities, and employ them in our calculation for common
envelope evolution. Our results prefer that the progenitor binaries formed in
metal-poor environment with $\rm Z\leq0.0016$. The predicted merger rate
density within redshift $z=1.1$ is $\sim 4\times 10^{-5}-5\times 10^{-2} \,\rm
Gpc^{-3}yr^{-1}$. We expect that such events are potentially observable by
upcoming both space and ground-based gravitational wave detectors.
