Tax issues should be addressed by complement tax counsel or tax accountant. The immigration aspect of your question is that separation and/or divorce should not affect the N-400 based on 5 years residency; provided the marriage through which permanent residence was granted was bona fide.
The person who posted this question, as well as anyone else who reads this response, should understand that the response is not, and should not be understood as, legal advice. Rather, it is legal information, based on the abbreviated facts presented. Immigration law is very complex, unfortunately, and immigration status adjudications are highly fact-dependent. The reader should retain an experienced immigration lawyer to analyze the facts specific to his/her particular situation to obtain “legal advice”; which this is not. Any answers offered on Avvo are of a general nature and informational only, and are not meant to create an attorney-client relationship.