fin-de-siecle vs fin de siecle

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

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