Diphosphino‐Functionalized 1,8‐Naphthyridines: a Multifaceted Ligand Platform for Boranes and Diboranes

Abstract A 1,8‐naphthyridine diphosphine (NDP) reacts with boron‐containing Lewis acids to generate complexes featuring a number of different naphthyridine bonding modes. When exposed to diborane B2Br4, NDP underwent self‐deprotonation to afford [NDP‐B2Br3]Br, an unsymmetrical diborane comprised of four fused rings. The reaction of two equivalents of monoborane BBr3 and NDP in a non‐polar solvent provided the simple phosphine‐borane adduct [NDP(BBr3)2], which then underwent intramolecular halide abstraction to furnish the salt [NDP‐BBr2][BBr4], featuring a different coordination mode from that of [NDP‐B2Br3]Br. Direct deprotonation of NDP by KHMDS or PhCH2K generates mono‐ and dipotassium reagents, respectively. The monopotassium reagent reacts with one or half an equivalent of B2(NMe2)2Cl2 to afford NDP‐based diboranes with three or four amino substituents.


Methods and materials
All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated solvents were dried over molecular sieves and degassed by three freeze-pump-thaw cycles prior to use. All other solvents were distilled and degassed from appropriate drying agents. Both deuterated and non-deuterated solvents were stored under argon over activated 4 Å molecular sieves. NMR spectra were acquired either on a Bruker Avance 500 or a Bruker Avance 400 NMR spectrometer. Chemical shifts (δ) are yield in ppm and internally referenced to the carbon nuclei ( 13 C{ 1 H}) or residual protons ( 1 H) of the solvent. Heteronuclei NMR spectra are referenced to external standards ( 11 B: BF3•OEt2; 31 P NMR: 85% H3PO4). High-resolution mass spectrometry (HRMS) data were obtained from a Thermo Scientific Exactive Plus spectrometer.

Synthetic procedures (LB2Br3)Br, 2:
Free ligand 1 (44.7 mg, 0.1 mmol) and B2Br4(SMe2)2 (46.6 g, 0.1 mmol) were mixed before adding 2 mL of CHCl3. The reaction was stirred for 30 min and then all volatiles were removed under vacuum. The yellow powder was washed with benzene (three times with 2 mL) and thoroughly dried to give compound 2 as a bright yellow powder (75.1 mg).

LK-18C6, 5-18-C-6:
Free ligand 1 (0.223 g, 0.5 mmol) and KHMDS (0.100 g, 0.5 mmol) were mixed in THF at room temperature before adding 18-crown-6 (0.132 g, 0.5 mmol). Reaction was stirred for 30 min and then all volatiles were removed. The dark red solid received was washed with 15 mL cold pentane to give 0.326 g of 5-18-C-6 as a fine powder. The sample received contains a small amount of 1 and the amount of 1 varies between batches. The NMR data following are recorded for the in-situ-generated 5-18-C-6.

LK2, 6:
Free ligand 1 (8.9 mg, 0.02 mmol) and PhCH2K (5.2 mg, 0.02 mmol) were mixed in THF-d8 at room temperature to give a pale orange solution. NMR data were recorded using this in-situgenerated sample. All attempts to purify LK2 provided only the monodeprotonated product. Note: Compound 6 is too sensitive for both elemental analysis and HRMS.

LN2B2Cl, 7:
Free ligand 1 (44.7 mg, 0.1 mmol) and KHMDS (20.0 mg, 0.1 mmol) were mixed in 1 mL benzene at room temperature and the full consumption of 1 was confirmed by 31 P NMR spectroscopy. At room temperature, the solution of LK was added to a solution of B2(NMe2)2Cl2 (18.0 mg, 0.1 mmol, 0.5 mL benzene). The mixture was heated at 80 ℃ and monitored by 31 P NMR spectroscopy until all LK was consumed (normally 2 h). The yellow solution received was filtered and recrystallized to give 13.4 mg of compound 7. The sample received always contains small amount of 1 even when recrystallized in a glovebox.

L2N2B2, 8:
Free ligand 1 (8.9 mg, 0.02 mmol) and KHMDS (4.0 mg, 0.02 mmol) were mixed in 0.3 mL C6D6 at room temperature and the full consumption of 1 was confirmed by 31 P NMR spectroscopy. At room temperature, the solution of LK was added to the solution of B2(NMe2)2Cl2 (1.8 mg, 0.01 mmol, 0.2 mL of C6D6). The mixture was heated at 80 ℃ overnight and monitored by 31 P NMR spectroscopy until all LK was consumed. The yellow solution received was a mixture of 8 and 1 and directly used for NMR spectroscopy. All purification operations provided only 1 as the major product.
Yield: not calculated.

X-ray crystallographic data
The crystal data of 4b were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and multi-layer mirror monochromated MoK radiation. The crystal data of 2, 5-18C6 were collected on a Bruker D8 Quest diffractometer with a CMOS area detector and multi-layer mirror monochromated MoK radiation. The crystal data of 6, 8 were collected on a κ-Helios diffractometer with a APEX II CCD detector and Helios multilayeroptics (mirrors) MoK radiation. The structures were solved using the intrinsic phasing method, [4] refined with the ShelXL program [5] and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations.   Note: The whole molecule showed severe rotational disorder. The atomic displacement parameters (ADPs) of overlapping atoms from different PARTs were restrained using similarity restraint (SIMU) and rigid body restraint (RIGU). All corresponding bonds of the two PARTs were restrained using same distance restraint (SADI). All hydrogen atoms except H1_1 and H1_11 were assigned to idealized positions. The coordinates of H1_1 and H1_11 were refined freely, but restrained to a P-H bond length of 1.30 Å using the DFIX command.           Geometry optimizations and Hessian calculations were performed for 2 and 2a at the density functional theory level. The PBE0 [6] functional was employed in conjunction with the 6-31+G** [7] basis set for light elements, while Br was described with the LanL2DZ [8] basis set and effective core potential.
Dispersion corrections were considered using Grimme's D3 [9] model with the Becke-Johnson (BJ) [10] damping function. All optimized structures were characterized as minimum energy structures by the analysis of the computed vibrational frequencies, as in all cases only positive eigenvalues were found.