fin de siecle vs fin-de-siecle

fin de siecle is a noun but fin-de-siecle is not a noun.

Word	Noun	Adjective	Verb	Adverb
fin de siecle	Yes	Yes	No	No
fin-de-siecle	No	Yes	No	No

As adjectives, fin de siecle and fin-de-siecle are synonyms defined as:

fin de siecle and fin-de-siecle: relating to or characteristic of the end of a century (especially the end of the 19th century)

fin de siecle (noun)	fin-de-siecle (noun)
the end of a century, especially the 19th century

fin de siecle (adjective)	fin-de-siecle (adjective)
relating to or characteristic of the end of a century (especially the end of the 19th century)	relating to or characteristic of the end of a century (especially the end of the 19th century)
	marked by excessive self-indulgence and moral decay