Qinliang Zhao

Large Changes in Electronic Structures of Ru26+ Species Caused by the Variations of the Bite Angle of Guanidinate Ligands: Tuning Magnetic Behavior

Syntheses and characterization of two Ru(2)(6+) paddlewheel compounds having very different magnetic behavior are reported. The compounds Ru(2)(tbn)(4)Cl(2), 1, and Ru(2)(tbo)(4)Cl(2), 2 (where tbn = the anion of 1,5,7-triazabicyclo[4.3.0]non-6-ene and tbo = the anion of 1,4,6-triazabicyclo[3.3.0]oct-4-ene), have four equatorial bicyclic guanidinate ligands and two chloride ions in axial positions. They show large disparity in Ru-Ru distances of about 0.11 A (2.389(3) and 2.499(3) A at 30 K for 1 and 2, respectively) that is attributed to the divergence in the bite angle of the ligand. Variable temperature structural data show no significant changes in the Ru-Ru distances between 30 and 213 K suggesting that the electronic structure remains unchanged in this temperature range for both compounds. Magnetic studies of 1 indicate there are two unpaired electrons at room temperature but the compound behaves as essentially diamagnetic at approximately 2 K. Compound 2 is non-magnetic across all temperatures in the range of 2 to 300 K. Density functional theory calculations suggest a pi(4)pi*(4)delta(2) electronic configuration for 2, while the magnetic behavior and structural data for 1 are consistent with a sigma(2)pi(4)delta(2)pi*(2) electronic configuration. This shows the importance of the ligand bite angle in determining the electronic configuration of the diruthenium unit and a way to tune magnetic behavior.

A strong metal-to-metal interaction in an edge-sharing bioctahedral compound that leads to a very short tungsten-tungsten double bond.

An ionic edge-sharing bioctahedral (ESBO) species has been prepared having a tetramethylated bicyclic guanidinate with two fused six-membered rings characterized by a fairly flat N-C(N)-N skeleton and abbreviated as TMhpp. The anion has two W(IV) atoms bridged by two oxo groups; the metal atoms are also spanned by two bridging guanidinate ligands, and each has two monodentate chlorine atoms. The complex formulated as (H2TMhpp)2[W(μ-O)(μ-TMhpp)Cl2]2 has the shortest W-W distance (2.3318(8) Å) of any species with a σ(2)π(2) electronic configuration. The anion and cations are connected by hydrogen bonds. To unambiguously ascertain the existence of the double-bonded W2(μ-O)2 entity, density functional theory calculations and natural bond orbital analyses were done on an analogous but hypothetical species with a W2(μ-OH)2 core having trivalent tungsten atoms and a possible σ(2)π(2)δ(2) electronic configuration. The calculations decidedly support the presence of tungsten-oxo instead of tungsten-hydroxo groups and thus the existence of the double-bonded W2(μ-O)2 core. The strong bonding interaction between metal atoms is a clear indication that under certain circumstances the two octahedra in ESBO species do not behave as the sum of two mononuclear compounds.

Exploring the Intricacies of Weak Interactions in Metal–Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W26+ Paddlewheel

Stepwise reaction of W(CO)6 with tetramethylated bicyclic guanidinate ligands, characterized by a central C(N)3 unit joining two fused six-membered rings with CH2CMe2CH2 units spanning two of the nitrogen atoms, allowed isolation of W2(μ-CO)2(μ-TMhpp)2(η(2)-TMhpp)2, 1, a precursor of W2(TMhpp)4Cl2 ( J. Am. Chem. Soc. 2013 , 135 , 17889 ; TMhpp = [(CH2CMe2CH2)2(C(N)3)]). Subsequent heating of 1 followed by reaction with TlPF6 generates [W2(TMhpp)4](PF6)2, 2. Compound 1 has an edge-sharing bioctahedral (ESBO) arrangement with a W2(μ-CO)2(4+) core having semibridging carbonyl groups, while 2 has a paddlewheel structure with a W2(6+) core spanned by four tetramethyl-substituted bicyclic guanidinate ligands. This compound also has hexafluorophosphate anions along the metal-metal bond that are nestled within methylene groups with the aid of a network of weak C-H···F interactions that prevent a close approach of the fluorine atoms to the dimetal unit. Theoretical computations were carried out on ditungsten model complexes supported by three ligand sets: bicyclic guanidinate, guanidinate, and formamidinate. The computations show that the π-accepting ability of the carbonyl groups significantly lowers the energy of the σ* orbital, and thus, the energy falls below that of the δ orbital. This information along with the diamagnetism of both 1 and 2-as shown by the sharp signals in the (1)H NMR spectra that support a lack of unpaired electrons (S = 0)-is consistent with the electronic configuration of σ(2)π(2)σ*(2)δ(2) (π(2)δ(2)) and thus a formal bond order of 2 for 1 and σ(2)π(4) for the triple-bonded W2(6+) core in 2. A comparison of the W-W bond lengths in 2, its chloro precursor W2(TMhpp)4Cl2, and the corresponding analogue W2(hpp)4Cl2 shows a substantial effect from the axially coordinated ligand, distal lone pair in determining the length of the metal-metal bond for these paddlewheel species. The importance of the ligands in tuning the energy level of the metal-metal bonds that may lead to dramatic changes in physical properties is also discussed. It is noteworthy that bicyclic guanidinates with the strongest π-donating ability push upward the energy level of the δ orbital, thus allowing the compounds to be easily oxidized.

CCDC 648933: Experimental Crystal Structure Determination

Related Article: F.A.Cotton, Zhong Li, C.A.Murillo, Xiaoping Wang, Rongmin Yu, Qinliang Zhao|2007|Inorg.Chem.|46|3245|doi:10.1021/ic062443v

CCDC 648934: Experimental Crystal Structure Determination

Related Article: F.A.Cotton, Zhong Li, C.A.Murillo, Xiaoping Wang, Rongmin Yu, Qinliang Zhao|2007|Inorg.Chem.|46|3245|doi:10.1021/ic062443v