Complex sentence: Shortly before his death in 1970, Rothko made gifts to his children of certain key paintings in his possession in an effort to provide his children with financial security, since he believed that key patrons would pay higher prices for the works following his death.
Split into simpler sentences: Shortly before his death in 1970, Rothko made gifts to his children, of certain key paintings that he had retained. This was an effort to provide his children with financial security, since he believed that key patrons would pay inflated prices for the works following his death.