Steric Effects Dictate the Formation of Terminal Arylborylene Complexes of Ruthenium from Dihydroboranes

Abstract The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH3]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy3)2HCl(H2)] (Cy: cyclohexyl) were studied, resulting in bis(σ)‐borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X‐ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho‐substitution of the aryl groups is necessary for the formation of terminal borylene complexes.


Materials and Methods
General considerations: all reactions were performed under an atmosphere of dry argon (argon 5.0) using standard Schlenk or glovebox techniques. All solvents were purified by distillation using the appropriate drying agents, deoxygenated using three freeze-pump-thaw cycles and stored over molecular sieves under dry argon prior to use. Deuterated solvents used for NMR spectroscopy were purchased from Cambridge Isotope Laboratories, deoxygenated by freeze-pump-thaw cycles and dried under an argon atmosphere over molecular sieves. 1 1 H} spectra were referenced to C6D6 ( 13 C, 128.06 ppm). High-resolution mass spectrometry was performed using a Thermo Scientific Exactive Plus spectrometer in LIFDI mode.

Materials:
TMSCl was purchased from Sigma-Aldrich and distilled under argon.
A spontaneous gas evolution was observed and the solution was stirred at room temperature for 5 min.
The previously orange solution turned yellow and after removal of the solvent the complexes were suspended in toluene and filtered over celite. The solvent was then removed under vacuum, leading to the desired complexes.

Synthetic protocol for the synthesis of borylene complexes [Ru(PCy3)2HCl(BR)] (6-8)
In reactions involving metal organic borohydrides, the conversion of the borates to dihydroboranes was performed in Et2O with approximately 2 equiv TMSCl (c = 0.086 mmol/mL). Within a few seconds the clear colorless solution turns into a white suspension. After 20 min all volatiles were quickly removed in vacuum (30 mmHg) resulting in the formation of a colorless oil or white solid.
The reaction mixture was dissolved in Et2O and used in the reaction. In the case of DurBH2 the dihydroboranes was added separately to the metal precursor.
[Ru(PCy3)2(H2)HCl] (120 mg, 0.17 mmol, 1 equiv) was dissolved in toluene. To the orange ethereal solution, a freshly prepared solution of the dihydroborane (1.5 equiv) was added. In the case of metal organic borohydrides with unknown amounts of coordinating solvent, the addition was carried out by stepwise addition and subsequent monitoring by NMR spectroscopy.       using intrinsic phasing method, [11] refined with the SHELXL program [12] and expanded using Fourier

Computational Details
All calculations were carried out using the Amsterdam Density Functional (ADF) program. [13] The numerical integration was performed by using a procedure developed by Becke et al. [14] The molecular orbitals (MOs) were expanded in a large uncontracted set of Slater-type orbitals (STOs, no Gaussian functions are involved) containing diffuse functions: A triple-z quality basis set was used for all atoms, [15] augmented with two sets of polarization functions for H (2p, 3d), B, C, N, O, F, P, Si, Cl (3d, 4f) and Ru (5p, 4f). An auxiliary set of s, p, d, f and g STOs was used to fit the molecular density and to represent the Coulomb and exchange potentials accurately in each self-consistent field (SCF) cycle. All electrons were included in the variational treatment (no frozen-core approximation was used). The generalized gradient approximation (GGA) at the BLYP level was used; exchange is described by Slater's Xa potential, [16] with nonlocal corrections due to Becke [17] added selfconsistently, and correlation was treated by using the Lee-Yang-Parr gradient-corrected functional. [18] Relativistic effects were included with the scalar-zero-order-regular-approximation (ZORA). [19] In addition, the D3(BJ) dispersion correction was used. [20] Energy minima have been verified through vibrational analysis. [21] For the thermochemistry calculations of the stepwise coordination reactions we used a standard approach as described by Swart