Electrochemical and magnetic exchange interactions in trinuclear chain complexes containing oxo-Mo(V) fragments as a function of the topology of the bridging ligand

The trinuclear complexes [{Mo(O)(Tp*)Cl}(í-1,n-C6H4O2){Mo(O)(Tp*)}(í-1,n-C6H4O2){Mo(O)(Tp*)Cl}] (1, n ) 4; 2, n ) 3) have been prepared [Tp* ) tris(3,5-dimethylpyrazolyl)hydroborate], in which a chain of three paramagnetic oxo-Mo(V) fragments are linked by two 1,4-[OC6H4O]2- (for 1) or 1,3-[OC6H4O]2- (for 2) bridging ligands. The crystal structure of 1â(CH2Cl2)3.5â(C6H14)0.5 was determined: C63.5H88B3Cl9Mo3N18O7; triclinic, P1h; a ) 12.052(2) Å, b ) 18.487(4) Å, c ) 21.039(5) Å; R ) 68.95(2)°, â ) 86.12(2)°, ç ) 78.637(13)°; V ) 4289(2) Å3; Z ) 2. It shows a V-shaped Mo-L-Mo-L-Mo array of three {(Tp*)Mo(O)} fragments with two 1,4-[OC6H4O]2- bridging ligands. The V-shape arises from the cis arrangement of the two bridging ligands at the central metal atom. Electrochemical measurements show the expected Mo(IV)/Mo(V) and Mo(V)/Mo(VI) couples at potentials consistent with significant electrochemical interactions between the terminal and central metal fragments but not between the two terminal metal fragments. Variable-temperature magnetic susceptibility measurements (1.17-225 K) show the occurrence of intramolecular antiferromagnetic (1) and ferromagnetic (2) exchange interactions between adjacent metal atoms with J ) -44.0 (1) and +4.5 cm-1 (2) [based on H ) -J(S1S2 + S1S3)] leading to S ) 1/2 (1) and 3/2 (2) ground states. These results are in accord with the spin-polarization principle, which predicts that [1,4-C6H4O2]2- should be an antiferromagnetic linker whereas [1,3-C6H4O2]2- should be a ferromagnetic linker.