Very weak electron-electron exchange interactions in paramagnetic dinuclear tris(pyrazolyl)boratomolybdenum centres with extended bridging ligands: estimation of the exchange coupling constant J by simulation of second-orderEPR spectra

Two series of dinuclear complexes have been prepared in which paramagnetic nitrosylmolybdenum(I) or oxomolybdenum(V) units have been attached to eithe r end of very long bis-pyridyl or bis-phenolate bridging ligands respective ly. The first series of complexes is [{Mo-I(Tp(Me,Me))(NO)Cl}(2){mu-L}] (L= 4,4'-bis[2-(4-pyridyl)ethen-1-yl]biphenyl; 4,4-bis[2-(4-pyridyl)ethen-1-yl] terphenyl 2; 4,4'-bis[2-(4-pyridyl)ethen-1-yl]benzophenone 3; 4,4'-bis[2-(4 -pyridyl)ethen-1-yl]benzil 4; or 6,6'-bis[2-(4-pyridyl)ethen-1-yl]-2,2'-bip yridine 5). The second series of complexes is [{Mo-V(Tp(Me,Me))(O)Cl}(2)(mu -L)] (H2L=HOC6H4OC(S)OC6H4OH 6; HOC6H4OS(O)OC6H4OH 7; or HOC6H4OC(O)C6H4C(O )OC6H4OH 8, with all-para substitution for the C6H4 units in each case). Th e very weak spin exchange interactions between the remote paramagnetic cent res result in many cases in second-order EPR spectra, because \ J \approxim ate to A (J is the exchange coupling constant, and A the electron-nucleus h yperfine coupling). In these cases the appearance of the EPR spectra is com plicated and sensitive to small changes in the magnitude of J, which could be exploited to estimate values for \ J \ by comparing the measured spectra with computer simulations calculated using a range of values of \J \. For the first series of complexes the spin exchange interactions decrease in th e order 1 (J greater than or equal to 4000), 2 (1000), 3 (150), 4 (43), 5 ( \J \less than or equal to 10 MHz) which is readily explicable in terms of t he lengths, conformations and substitution patterns of the bridging ligands . For the second series of complexes, 6 and 7 both gave second-order spectr a with \ J \=2000 MHz, whereas 8, with a much longer bridging ligand, has \ J \less than or equal to 10 MHz. Crucially, these spin-exchange interactio ns are much too weak to be determined by conventional magnetic susceptibili ty measurements (\ J \much less than 1 cm(-1)), and therefore simulation of second-order EPR spectra provides a simple route to providing useful infor mation about the relative magnitudes of very weak spin exchange interaction s which is not available by any other route.