Inorganic Chemistry
Article
with SnCl2 to yield the lighter congener and useful synthon
Sn[N(SiMe3)2]2.
99.999% N2 at 60 sccm and the temperature was increased at 10
°C min−1 for the “ramp” experiments, and increased at 40 °C min−1 to
increase temperatures stepwise to hold at isotherms of 60−130 °C for
7 min each for the “stepped isothermal” experiments. The exact
masses for the 10, 20, and 40 mg loadings shown in Figure 4 were
11.61886, 20.40275, and 39.35869 mg, respectively.
Attempted Synthesis of rac-N 2,N 3-Di-tert-butylbutane-2,3-dia-
mido Lead(II) (1Pb) via Salt Metathesis of PbCl2. The dilithio
diamidobutane salt was prepared as for the free diamine from DAD (3
g, 17.8 mmol) and MeLi (36.6 mmol) but was used in situ as reported
for 1Ge and 1Sn.15 It was added with a dropping funnel over 2.5 h to
a stirred slurry of PbCl2 in Et2O cooled to −78 °C in a Schlenk flask
shielded from light. A red solution and black precipitate form
immediately. After addition, the slurry was stirred overnight. Cooling
was not sustained and the solution warmed slowly to room-
temperature. A red supernatant liquid with black crystalline
precipitate remained. All volatiles were removed under vacuum,
leaving a gray residue. A small amount of a red oil, determined to be
highly impure 1Pb (ca. ≤5%, based on PbCl2) by NMR spectroscopy,
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In Situ NMR Spectroscopy of Thermolysis of 1Pb. H and 13C
NMR spectra were obtained after heating a 15 mM solution of 1Pb in
toluene-d8 in a sealed heavy-walled glass tube at 90 °C for 7 days.
Spectra were collected every 12 h after cooling the sample to room-
temperature. At 90 °C, formation of a black precipitate and signals
corresponding to the free ligand L(NH)2 (see above) appeared. This
same sample was subjected to 6 h at 150 °C as before, and spectra
were obtained after cooling. Low-resolution spectra were obtained
due to poor shimming from suspended particles, but a signal at high
chemical shift (δ ∼7.6 ppm) was observed by 1H NMR spectroscopy
that prompted another decomposition study.
Neat Thermolysis of 1Pb. 1Pb (0.302 g, 0.74 mmol) was placed in
a 50 mL Schlenk flask in a drybox and placed under a strong
overpressure of ultrapure Ar (99.999%) on a Schlenk line. It was then
joined to a previously flame-dried reflux condenser fitted with a gas
inlet at the top and heated to 150 °C with an oil bath for 6 h. After
cooling, the black residue was extracted with anhydrous (stored over 4
Å sieves), degassed benzene-d6 and transferred to a dried NMR tube
under a stream of ultrapure N2 (99.999%). The dark-brown, murky
sample was analyzed immediately by 1H and COSY NMR
spectroscopy, and a 13C NMR spectrum (14 336 scans) was obtained
overnight. Another 1H spectrum was obtained the following day,
which showed essentially the same spectrum with better resolution
since the suspended Pb(0) solids had settled. Clear signals
corresponding to an imino hydrogen [δ(1H) = 7.62 (q, 1H, CH
N)] that correlated (COSY) to another signal [δ = 1.71 (d, 3H,
C(H)CH3)] presumably belonging to acetal hydrogens. The tert-butyl
region was obscured by signals from free diamine ligand and surviving
1Pb; the origin of the protonated diamine remains unclear. The
emergent signals correspond to the reported chemical shifts of
acetaldehyde tert-butylimine,41 the enantiomers of which were formed
are out of the scope of this study.
Single-Crystal X-ray Diffraction of 1Pb. 1Pb is highly soluble in
polar and apolar nonprotic solvents such as diethyl ether,
tetrahydrofuran, n-hexane, n-pentane, and toluene. Crystals suitable
for single-crystal X-ray difraction (scXRD) were grown by
recrystallization of in a 1:1 mixture of diethyl ether and toluene at
−30 °C, which were then coated with Paratone-N oil, mounted on a
fiber loop, and placed in a cold, gaseous N2 stream on a Rigaku-
Oxford SuperNova diffractometer. Diffraction was performed using ω
scans at 100 K and intensities were measured using graphite
monochromatic Cu Kα radiation (λ = 1.54184 Å). Data collection,
indexing, initial cell refinements, frame integration, final cell
refinements, and absorption corrections were accomplished with
Agilent CrysAlis Pro v1.171.37.34. Space groups were assigned by
analysis of the metric symmetry and systematic absences (determined
by WinGX). The structure was solved (in a manner similar to the
congener 1Sn)15 with ShelxT and refined against all data in the
reported 2θ ranges by full-matrix least-squares on F2 with the
SHELXL program suite using the ShelxLe interface.46 The twin-law
was determined using PLATON TwinRotMat47 and further refined as
a two-component twin with a ratio of 0.574 and 0.426, respectively.
EADP was used in the refinement of the Pb atoms. The disordered Pb
atoms displayed site occupancies of 0.39:0.61 for Pb1/Pb2 and
0.22:0.78 for Pb4/Pb5. Stucture images were rendered with
ORTEP3.48 The data was deposited in the Cambridge Structural
Database (CSD) with indentification number CCDC 1587313.
Powder X-ray Diffraction. Bulk purity was assessed by powder X-
ray diffraction (pXRD) and compared to the simulated spectrum as
calculated based on the single-crystal structure determined previously;
1Pb was calculated based on the structural data set obtained by
scXRD (see above) by Crystal Impact DIAMOND v3.2. The
simulation was carried out for the 2θ angle range of 5−60° from
1
was vacuum distilled (70 °C/10 mTorr) from the residue. H NMR
spectroscopy on a filtered benzene-d6 extract of the residue revealed,
among starting material, peaks corresponding to acetaldehyde tert-
butylimine δ(1H) = 7.62 (q, 1H, CHN) and δ = 1.71 (d, 3H,
C(H)CH3).41 This imine was also recovered from the cold-trap of the
vacuum line used to remove solvent from the reaction mixture.
Successful Preparation of 1Pb via Transamination of Pb[N-
(SiMe3)2]2. In a N2 filled glovebox, 1 equiv of rac-N,N′-di-tert-
butylbutane-1,2-diamine (0.92 g, 4.6 mmol) is added to 1 equiv of
Pb[N(SiMe3)2]2 (2.41 g, 4.6 mmol) in 75 mL of hexanes or toluene
in an O-ring sealed, PTFE-capped, heavy-walled 150 mL pressure
vessel. The mixture is heated to 70 °C for 3 days to completely
1
convert Pb[N(SiMe3)2]2 to 1Pb as indicated by H NMR. Less time
will lead to incomplete conversion, while higher temperatures lead to
a decrease in yield due to decomposition. We were unable to
effectively separate Pb[N(SiMe3)2]2 from 1Pb by recrystallization or
distillation. Slight decomposition is signaled by the precipitation of a
black crystalline material, presumably Pb(0). The dark-red suspension
is cooled to room-temperature and brought into a drybox where it is
filtered over Celite to remove the black precipitate, yielding a blood-
red solution. Free HN(SiMe3)2 and solvent are removed under
vacuum, and the impure residue is purified by fractional vacuum
sublimation onto a water-cooled coldfinger (60 °C/10 mTorr) to
afford 1Pb (1.66 g, 89% based on Pb[N(SiMe3)2]2) as an extremely
air- and moisture-sensitive blood-red crystalline solid. Samples slowly
darken upon standing in light under inert atmosphere after >2 weeks.
Mp 53 °C, sublimed. Found: C, 35.0; H, 6.3; N, 7.1. C12H26N2Pb
requires C, 35.5; H, 6.5; N, 6.9%. HRMS (EI) Found, m/z =
1
406.18570, calcd 406.1864, dev. −0.54%. δ H NMR (300 MHz, C6D6,
δC H = 7.16 ppm): 1.35 (18H, s, C(CH3)3), 1.45 (6H, d, C(H)CH3),
6
6
13
5.37 (2H, q, C(H)CH3)). δ C NMR (300 MHz, C6D6, δC H = 128.06
6
6
ppm): 30.53 (C(H)CH3), 35.07 (C(CH3)3), 59.42 (C(CH3)3), 73.61
(C(H)CH3).
Characterization. In Situ NMR Spectroscopy of the Attempted
Preparation of 2Pb. In a drybox, Pb[N(SiMe3)2]2 (0.143 g, 0.27
mmol) was dissolved in dry, degassed C6D6 (ca. 1 mL) in an NMR
tube, closed with a plastic cap, and sealed with Parafilm. N,N′-Di-tert-
butylethylene-1,2-diamine (0.048 g, 0.28 mmol) was also dissolved in
C6D6 (ca. 0.5 mL) in a small vial and capped with a rubber septum. A
spectrum was obtained of Pb[N(SiMe3)2]2; then, the tube was
removed from the spectrometer and the diamine solution injected
with a syringe through the cap all at once. The NMR tube was
resealed with more Parafilm, shaken vigorously for approximately 30 s
and placed back in the spectrometer. The sample was locked and
shimmed before each acquisition when 16 scans were acquired.
Thermogravimetric Analysis (TGA). TGA was performed with a
TA Instruments Q50 kept in an MBraun Labmaster 130 drybox filled
with N2. Pt pans were coated before each experiment with
approximately 55 nm of Al2O3 by atomic layer deposition (ALD) in
a Picosun R-150 reactor using 500 cycles of trimethylaluminum
(TMA) and water at 200 °C with 0.1 s pulses and 8 s purges each.
This is critical to protect the Pt pans as they become brittle and break
easily after high-temperature exposure to Pb(0) residues from
precursor decomposition. The TGA furnace was purged with
G
Inorg. Chem. XXXX, XXX, XXX−XXX