Paramagnetic Cyanometalate Building Blocks
Introduction. To prepare robust molecular clusters that may exhibit single-molecule magnetic behavior, building blocks that efficiently couple spin centers and exhibit high single-ion anisotropies be utilized. While most single-molecule magnetic clusters (SMMs) contain oxo/carboxylate bridged metal centers, only three examples derived from [fac-LmMn(CN)3](n-m-3) building blocks (Mn = FeIII, MoIII, ReII) have been reported prior to our studies. Each of these metal centers exhibit angular momentum (spin-orbit) contributions to their magnetic moments and only recently have the systematic preparation of SMMs from these complexes been described.
First-Row Building Blocks.
Poly(pyrazolyl)borates are ideal ligands for directing the self-assembly of magnetic clusters: (1) These ligands are easily prepared and modified at each of their ten sunbstitutional positions, (2) Poly(pyrazolyl)borates stabilize multiple oxidation states for most transition metal centers (many of which are unknown for triazacyclononanes or tripos), (3) Pyrazolylborate complexes are known for most transition metals, (4) Allow for rapid tuning of the solubility, dimensionality, coordination preferences, and electronic properties of metal centers in a given structural archetype, (5) Cyanometalate complexes and derived clusters appear to be stable with respect to linkage isomerism (a common problem in many systems). [fac-LmMn(CN)3](n-m-3)
Several poly(pyrazolyl)borate mono-, di-, and tricyanide complexes have been prepared. The crystalline [NEt4]3-n[LMn(CN)2] and [NEt4]4-n[LMn(CN)3] derivatives are obtained in high yields via treatment of several known pyrazolylborate (Tp*, Tp, pzTp) acetato, acetylacetononate, chloro, or nitrato complexes with excess tetra(ethyl)ammonium cyanide (Figure 1 and Scheme 1). Furthermore, magnetic measurements indicate that many of these new building blocks are anisotropic.
The X-ray structures of four representative cyanometalate complexes are illustrated in Figure 2.
Late Metal Building Blocks.
We are currrently investigating the prearation of second- and third-row poly(pyrazolyl)-borate tricyanide complexes in order to expand our studies concerning the role of single-ion anisotropy in molecule-based magnetic materials. Such building blocks are expected to afford stronger magnetic exchange couplings and molecular anisotropy, relative to first-row analogues, since increased
p backbonding and large orbital contributions (l ~ 273–3000 cm-1) to the magnetic moment are often found for 4d and 5d ions. Inserting these late metal centers into clusters should further increase the spin reversal barrier energies and afford higher blocking temperatures relative to known first-row oxide- and cyanometalate-based SMM derivatives. |
