Dicopper(I) Complexes with Reduced States of 3,6-Bis(2‘-pyrimidyl)-1,2,4,5-tetrazine: Crystal Structures and Spectroscopic Properties of the Free Ligand, a Radical Species, and a Complex of the 1,4-Dihydro Form

The complexes {(μ-bmtz•-)[Cu(PPh3)2]2}(BF4) (1) and {(μ-H2bmtz)[Cu(PPh3)2]2}(BF4)2 (2) (bmtz = 3,6-bis(2‘-pyrimidyl)-1,2,4,5-tetrazine and H2bmtz = 1,4-dihydro-3,6-bis(2‘-pyrimidyl)-1,2,4,5-tetrazine) were obtained as
stable materials that could be crystallized for structure determination. 1·2 CH2Cl2:  C84H70BCl4Cu2F4N8P4;
monoclinic, C2/c; a = 26.215(7) Å, b = 22.122(6) Å, c = 18.114(5) Å, β = 133.51(1)°; Z = 4. 2·CH2Cl2: 
C83H70B2Cl2Cu2F8N8P4; triclinic, P1̄; a = 10.948(2) Å, b = 12.067(2) Å, c = 30.287(6) Å, α = 93.82(3)°, β =
94.46(3)°, γ = 101.60(3)°; Z = 2. Bmtz itself was also structurally characterized (C10H6N8; monoclinic, P21/c;
a = 3.8234(8) Å, b = 10.147(2) Å, c = 13.195(3) Å, β = 94.92(3)°; Z = 2). Whereas the radical complex ion
contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex
of H2bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine
rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from
a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy
including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine
ring.