Control of Electronic and Magnetic Coupling via Bridging Ligand Geometry in a Bimetallic Ytterbocene Complex

The ligand 1-methyl-3,5-bis(2,2‘:6‘,2‘ ‘-terpyridin-4‘-yl)benzene has been employed in the synthesis of a new bimetallic
ytterbocene complex [(Cp*)2Yb](1-methyl-3,5-bis(2,2‘:6‘,2‘ ‘-terpyridin-4‘-yl)benzene)[Yb(Cp*)2] (1) and the doubly
oxidized congener [1]2+ in an attempt to determine the impact of the bridging ligand geometry on the magnetic/electronic properties as compared to the previously reported 1,4-analog [(Cp*)2Yb](1,4-di(terpyridyl)benzene)[Yb(Cp*)2] (2). Electrochemical, electronic, and magnetic data provide compelling evidence that the 1,3-geometry
associated with the bridging ligand of 1 has done an effective job of inhibiting electronic communication between
metal centers and magnetic coupling of spin carriers at room temperature as compared to 2. In fact, the physical
data associated with 1 are quite similar to those reported for the monometallic analog (Cp*)2Yb(tpy) (3). In particular,
the f−f profile of [1]2+ is nearly identical to that of [3]+ in its spectral features but with an almost exact doubling of
the intensities. Further, the electronic coupling between metal centers as manifested in the potential separation
between metal-based reduction waves has for the first time in these bimetallic ytterbocene complexes been found
to go to zero for 1. Thus, the linkage isomerism at the phenyl coupling unit has induced a change in the ground-state electronic configuration from the singlet dianion-bridged (4f)13(π*)2(4f)13 state found in 2 to the diradical-bridged (4f)13(πA*)1(πB*)1(4f)13 state in 1. This diradical formulation on the bridging ligand in 1 is supported by DFT
calculations for the uncomplexed doubly reduced ligand that indicate the ground-state configuration is a singlet
diradical state with the triplet-diradical state lying to slightly higher energy. Magnetic characterization of 1 is most
consistent with the behavior previously observed for monometallic analogs such as 3, and there is no evidence of
long-range magnetic ordering such as that observed for 2. In addition, X-ray crystallographic characterization of 1
represents the first case of a structurally characterized 2:1 metal-to-ligand adduct of the 1,3-bis(tpy) framework.