I'm guessing that most helipads on downtown high rises don't have any instrument approaches. If a helicopter is on an IFR to a helipad without any instrument approaches, does ATC just clear them for the visual approach, and do helicopters have their own CTAF frequency for announcing their intentions at off-airport helipads?
There are no "off-airport helipads" because a helipad is an airport. Actually, anywhere an aircraft lands is an airport as far as the FAA is concerned. It would be just like any other IFR to any other airport with no approach procedure: ATC can clear them for the visual approach if they have reasonable assurance that the weather at the airport is VFR, or the pilot can cancel IFR if they determine the weather is VFR. Public helipads have CTAFs just like any other public airports do. They can be found on helicopter charts, or in the Chart Supplement. In the attached chart snippet, you can see that the CTAFs at East 34th Street and Wall Street are 123.075 and 123.05.
One Police Plaza does not have a published CTAF because it is a private airport. Other helipads in the area are also private and not explicitly depicted.