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The LIGO-Virgo-Kagra collaboration (LVC) discovered recently GW190521, agravitational wave (GW) source associated with the merger between two blackholes (BHs) with mass $66$ M$_\odot$ and $>85$ M$_\odot$. GW190521 representsthe first BH binary (BBH) merger with a primary mass falling in the "uppermass-gap" and the first leaving behind a $\sim 150$ M$_\odot$ remnant. So far,the LVC reported the discovery of four further mergers having a total mass$>100$ M$_\odot$, i.e. in the intermediate-mass black holes (IMBH) mass range.Here, we discuss results from a series of 80 $N$-body simulations of youngmassive clusters (YMCs) that implement relativistic corrections to followcompact object mergers. We discover the development of a GW190521-like systemas the result of a 3rd-generation merger, and four IMBH-BH mergers with totalmass $~(300-350)$ M$_\odot$. We show that these IMBH-BH mergers arelow-frequency GW sources detectable with LISA and DECIGO out to redshift$z=0.01-0.1$ and $z>100$, and we discuss how their detection could helpunravelling IMBH natal spins. For the GW190521 test case, we show that the3rd-generation merger remnant has a spin and effective spin parameter thatmatches the $90\%$ credible interval measured for GW190521 better than asimpler double merger and comparably to a single merger. Due to GW recoilkicks, we show that retaining the products of these mergers require birth-siteswith escape velocities $\gtrsim 50-100$ km s$^{-1}$, values typically attainedin galactic nuclei and massive clusters with steep density profiles.

Breaching the limit: formation of GW190521-like and IMBH mergers in young massive clusters