Nitric Oxide Complexes of Manganese and Chromium Tetraphenylporphyrin

Nitric oxide forms 1:1 low spin complexes with Cr^IITPP, Cr^IIITPP(X), Mn^IITPP, and Mn^IIITPP(X) species. MnTPP(X)(NO) (X^- = Cl^-, CH₃CO₂^-, CN^-) and CrTPP(NO) complexes have the S = 1/2 ground state and EPR spectra diagnostic for the odd electron occupying a molecular orbital of predominantly metal dxy character. MnTPP(X)(NO) complexes and the isoelectronic CrTPP(NO) species are assigned the (d_xz,yz)⁴ (d_xy)¹ ground configuration. Narrow lines in the isotropic EPR spectrum of CrTPP(NO) permit resolution of nitrogen-14 hyperfine from both nitric oxide (6.5 G) and the porphyrin pyrrole nitrogens (3.15 G). The metal nuclear hyperfine coupling constants in these complexes (A ⁵⁵Mn) = 88.3 G and (A ⁵³Cr) = 21.2 G are indicative of the odd electron being relatively localized on the metal. All of these nitric oxide complexes are formulated as M-NO⁺ species, although dπ → π* bonding substantially reduces this charge separation. Within isoelectronic sets such as (d⁵) MnTPP(Cl)(NO) (v_no = 1830 cm^-1), CrTPP(NO) (v_no = 1700 cm^-1), or (d⁶) FeTPP(Cl)(NO) (v_no = 1880 cm^-1), MnTPP(NO) (v_no = 1760 cm^-1), the nitric oxide stretching frequencies are substantially smaller for the complex where the metal has the lower formal oxidation state. Electronic spectra for the chromium porphyrin species are typical for metalloporphyrins, while spectra for MnTPP(X) and MnTPP(X)(NO) complexes are highly irregular. VOTPP fails to form a nitric oxide complex even in the low temperature glass media.