Synthesis, structure and reactivity of μ3-SnH capped trinuclear nickel cluster

Treatment of the trichlorotin-capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-Cl)(μ3-SnCl3)], 1, with 4 eq. NaHB(Et)3 yields a μ3-SnH capped trinuclear nickel cluster, [Ni3(dppm)3(μ3-H)(μ3-SnH)], 2 [dppm = bis(diphenylphosphino)methane]. Single-crystal X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and computational studies together support that cluster 2 is a divalent tin hydride. Complex 2 displays a wide range of reactivity including oxidative addition of bromoethane across the Sn center. Addition of 1 eq. iodoethane to complex 2 releases H2 (g) and generates an ethyltin-capped nickel cluster with a μ3-iodide, [Ni3(dppm)3(μ3-I)(μ3-Sn(CH2CH3))], 4. Notably, insertion of alkynes into the Sn–H bond of 2 can be achieved via addition of 1 eq. 1-hexyne to generate the 1-hexen-2-yl-tin-capped nickel cluster, [Ni3(dppm)3(μ3H)(μ3-Sn(C6H11))], 5. Addition of H2 (g) to 5 regenerates the starting material, 2, and hexane. The formally 44-electron cluster 2 also displays significant redox chemistry with two reversible one-electron oxidations (E = −1.3 V, −0.8 V vs. Fc0/+) and one-electron reduction process (E = −2.7 V vs. Fc0/+) observed by cyclic voltammetry.

1 H and 31 P were recorded on a Jeol 500 MHz spectrometer. 13 C and 119 Sn NMR spectra were recorded on a Jeol 400 MHz and Varian 500 MHz Spectrometers. 1 H chemical shifts are reported relative to the residual hydrogen atoms in benzene-d 6 solvent. 31 P chemical shifts are reported relative to 85% H 3 PO 4 . 119 Sn NMR was referenced externally to SnBu 4 in benzene-d 6 {-11.7ppm}. Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2022 X-ray Crystallography.
Single crystal X-ray diffraction of complex 2 was performed using Bruker APEX-II Ultra CCD diffractometer equipped with Mo Kα radiation (λ= 0.71073 Å). Single crystal X-ray diffraction of complex 3 and 4 was performed using Bruker X8-ApexII CCD Sealed Tube equipped with Mo Kα radiation (λ= 0.71073 Å). Finally, single crystal X-ray diffraction of complex 5 was performed using Bruker/Nonius Microstar 592 equipped with Cu Kα radiation (λ= 1.54 Å). Crystals were mounted on a Cryoloop with Paratone oil. Data were collected in a nitrogen gas stream at 100(2) K using f and v scans. The data were integrated using the Bruker SAINT software program and scaled using the SADABS software program. Solution by direct methods (SHELXT [2] ) produced a complete phasing model consistent with the proposed structure. All nonhydrogen atoms were refined anisotropically by full-matrix least-squares (SHELXL-2014). [2] For highly disordered diethyl ether and pentane molecules, the PLATON routine SQUEEZE [3] or a solvent mask was used to account for the corresponding electrons as a diffuse contribution to the overall scattering without specific atom positions. To stabilize the disorder present in these molecule RIGU and SADI commands were used accordingly.

Electrochemistry.
All electrochemical experiments were performed inside a VAC glovebox. These experiments were performed in 0.3 M [ n Bu 4 N] [PF 6 ] solution in THF using BASi Epislon potentiostat. A singlecompartment cell was used for cyclic voltammetry experiments with a glassy carbon working electron (3 mm in diameter, Bioanalytical Systems, Inc.), glassy carbon counter electrode, and Ag/AgCl pseudo-reference electrode. All potentials are referenced to the Fc +/0 couple using ferrocene as an internal reference.

UV-Vis Spectroscopy
UV-visible spectra were collected on a Shimadzu UV-3600 UV/vis/NIR spectrometer. Samples for determination were taken in 1 cm path length quartz cuvettes.

Density Function Theory (DFT) Calculations.
Calculations of geometry optimization, frequency, and energy were performed using Gaussian software suite (version g16.B01). The B3LYP functional with default spin parameters and the LANL2DZ basis set were used. Geometry optimizations were performed starting from XYZ coordinates adapted from the reported crystal structure of the HSnNi 3 (dppm) 3 H cluster. Optimizations were done with tight convergence criteria, and no restrictions or constraints were places on any of the calculated structures. NBO analyses were done with the built in NBO suites of Gaussian.
Integration of the P(CH 2 )P dppm resonances on 2 against the methyl resonance on tetramethylbenzene depict 96% purity (S24). In addition to quantitative NMR, purity for this compound is demonstrated in multinuclear NMR experiments: 1 H, 13 C, 31 P and 119 Sn.

Hydrogenation of 5 to 2.
A thick-walled NMR tube was charged with complex 5 (10.4 mg, 0.00679 mmol), an internal reference, tetramethylbenzene (1.4 mg, 0.010 mmol), and 0.1 ml THF-d 8 . The tube was cycled onto the Schlenk line, freeze-pump-thawed 3x, and cooled to liquid nitrogen temperatures, -196°C. H 2 (g) was added to the NMR tube at -196°C, resulting in the addition of 3.8 atm of H 2 (g). This was allowed to warm up to 22°C and tracked via 1 H and 31 P NMR spectroscopy over time. Over the course of 24 hours, complex 5 converted to complex 2 and hexane in 21% yield, as determined by 1 H NMR integration of the P(CH 2 )P, SnH and NiH resonances in complex 2 against the methyl resonance in tetramethylbenzene.

Decomposition of 5 in THF-d 8 .
A thick-walled NMR tube was charged with complex 5 (10.4 mg, 0.00679 mmol), an internal reference, tetramethylbenzene (1.4 mg, 0.010 mmol), and 0.1 ml THF-d 8 . Over the course of 24 hours, complex 5 decomposed by 92% as determined by 1 H NMR integration of the P(CH 2 )P, SnH and NiH resonances in complex 2 against the methyl resonance in tetramethylbenzene.

Procedure for Quantitative 1 H NMR Analysis.
A known mass of internal reference (tetramethylbenzene) and a known mass of compound (2, 3, or 5) was obtained. The corresponding masses yield the mol ratio of reference to compound in solution. The integration of the corresponding peaks of the reference and compound in 1 H NMR yield the mol ratio of these species. The division of the mol ratio using the mol ratio of the integration values over the known mol ratio (given by mass) multiplied by 100 gives the percent purity.

General Comments on Assessing Purity of the Compounds in this Study.
Quantitative 1 H NMR (qHNMR) provides a powerful methodology for the determination of purity of novel compounds. qHNMR allows for the determination of purity of chemical compounds containing NMR active protons, providing a coupled assessment of molecular structure and purity, with sensitivity and precision rivalling and often surpassing traditional techniques such as elemental analysis. The power of the technique comes from the ability to not just calculate the mass percentage of detectable protons, but to do so with structurally meaningful spectra, verifying both the quantity and character of protons within the sample simultaneously. The technique is both reliable and sensitive, with reported relative standard deviations typically under 1% for concentrations of analyte in the 1-10 millimolar range. 1 The technique has been successfully leveraged to analyze purity of pharmaceutical compound libraries and natural products with good success, 2-4 and thus we have begun incorporating this technique into our array of purity analysis protocols.
A qHNMR experiment can be carried out by careful inclusion of an internal standard which is chemically innocent and well resolved from the analyte and can be dissolved in the choice NMR solvent. In many cases, it's feasible to do this assay using the NMR solvent as the internal standard, however it becomes vital to ensure no H/D exchange occurs with the solvent and the analyte, and that the H/D ratio of the carrying solvent is known with good precision. When successfully executed, this technique can have much better precision than elemental analysis, with the largest source of error deriving from the limitations of the laboratory mass balance, rather than the sensitivity of the NMR experiment. This has led the technique to become a competitive metrological technique for purity certification of chemical compounds, as well as the benchmarking of other analytical methods. 5 Within this work we often rely on qHNMR measurements to ensure the purity of our compounds as we believe it is a powerful and convenient methodology compared to elemental analysis, which has come under scrutiny as a modern analytical technique.
Several key experimental points need to be noted to ensure the successful employment of the technique. Most importantly, the analyte must be completely soluble at the prepared concentration. Any undissolved compound will lead to a dramatically underestimated purity compared to the true value. The relaxation delays used in the NMR experiment must be sufficiently long to ensure accurate integration of the resulting spectra. This is typically only an issue in purely organic samples containing no heteroatoms but it is critical to the successful implementation of this technique. A much more detailed account on this technique has been prepared by Schoenberger. 5 In situations where extremely small quantities of material are studied, determination of purity using mass-spectrometry is still the most sensitive technique available, however for most synthetic scale applications qHNMR can prove one of the best options for assessing the purity of prepared compounds. Especially in the study of highly sensitive compounds, elemental analysis has often proven difficult to obtain for even wellbehaved samples where purity was not a serious question, and thus we believe it's appropriate to "pass the torch" to more reliable and informative methodologies such as qHNMR.   Figure S1. 1 H NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -H)(μ 3 -SnH)], 2, in C 6 D 6 . There is an overlap of the residual hydrogen peak in C 6 D 6 with a phenyl peak in 2.  Figure S2. 13 C NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -H)(μ 3 -SnH)], 2, in C 6 D 6 . There is an overlap of the residual carbon peak in C 6 D 6 with phenyl peaks in 2.    )))], 3, in C 6 D 6 . There is an overlap of the residual hydrogen peak in C 6 D 6 with a phenyl peak in 3.  Figure S7. 13 C NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -H)(μ 3 -Sn((Br)(H)(CH 2 CH 3 )))], 3, in C 6 D 6 . There is an overlap of the residual carbon peak in C 6 D 6 with a phenyl peak in 3.   Figure S12. 13 C NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -I)(μ 3 -Sn(CH 2 CH 3 ))], 4, in C 6 D 6 . There is an overlap of the residual carbon peak in C 6 D 6 with phenyl peaks in 4.    Figure S15. 1 H NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -H)(μ 3 -Sn(C 6 H 11 ))], 5, in C 6 D 6 . There is an overlap of the residual hydrogen peak in C 6 D 6 with a phenyl peak in 5.  Figure S16. 13 C NMR spectrum of [Ni 3 (dppm) 3 (μ 3 -H)(μ 3 -Sn(C 6 H 11 ))], 5, in C 6 D 6. There is an overlap of the residual carbon peak in C 6 D 6 with phenyl peaks in 5. 3 atm H 2 (g) -6 hrs.