Thermodynamic and Kinetic Effects in Spin Blocking of CO Coordination Reactions

The iron(I) dinitrogen complex PhB(AdIm)₃FeN₂, which is supported by a very bulky 1-adamantyl-substituted tris(carbene)borate ligand, reacts with equimolar CO at low temperature to afford the high spin (S = 3/2) complex PhB(AdIm)₃Fe(CO). This monocarbonyl complex reacts with additional CO to afford the low spin (S = 1/2) dicarbonyl complex PhB(AdIm)₃Fe(CO)₂. By contrast, the high spin iron(I) tris(pyrazolyl)borate complex Tp^tBu,MeFe(CO) does not react with additional CO. Experimental and computational investigations reveal that coordination of CO to Tp^tBu,MeFe(CO) is spin blocked, whereas the stronger donor tris(carbene)borate ligand in PhB(AdIm)₃Fe(CO) sufficiently stabilizes the low spin state to enable a thermally accessible barrier for CO coordination. Kinetic measurements reveal that the rate of CO coordination reaction is many orders of magnitude slower than for a related cobalt complex, which is attributed in large part to the relative extent of geometric reorganization required to achieve the low spin state for iron(I) as opposed to cobalt(I). The barrier to the spin state change has allowed the mixed isotope complex PhB(AdIm)₃Fe(¹²CO)(¹³CO) to be prepared.