It is not a problem, insofar as damaging your smartphone is concerned, but a magnetic clasp could have other, unexpected consequences. To begin with I would keep it well away from any credit or debit cards with magnetic strips as it could erase or corrupt the data they contain. read more
In Apple’s Case Design Guidelines, there are sections on Sensor Considerations and Magnetic Interference, including the line, “Apple recommends avoiding the use of magnets and metal components in cases.” Manufacturers have to ensure that the built-in magnetic compass is not affected by their cases. read more