Question

The ordering of the relative stabilities of adducts \(\mathrm{L} \cdot \mathrm{BH}_{3}\) for some common adducts is, according to L: \(\mathrm{Me}_{2} \mathrm{O}<\mathrm{THF}<\mathrm{Me}_{2} \mathrm{S}<\mathrm{Me}_{3} \mathrm{N}<\mathrm{Me}_{3} \mathrm{P}<\mathrm{H}^{-}\). In addition to answering each of the following, indicate how you could use NMR spectroscopy to confirm your proposals. (a) What happens when \(\mathrm{Me}_{3} \mathrm{N}\) is added to a THF solution of THF.BH \(_{3}\) ? (b) Will \(\mathrm{Me}_{2} \mathrm{O}\) displace \(\mathrm{Me}_{3} \mathrm{P}\) from \(\mathrm{Me}_{3} \mathrm{P} \cdot \mathrm{BH}_{3} ?\) (c) Is \(\left[\mathrm{BH}_{4}\right]^{-}\) stable in THF solution with respect to a displacement reaction? (d) Suggest what may be formed when \(\mathrm{Ph}_{2} \mathrm{PCH}_{2} \mathrm{CH}_{2} \mathrm{PPh}_{2}\) is added to a THF solution of THF. \(\mathrm{BH}_{3},\) the latter remaining in excess.

Short answer

(a) \( \text{Me}_3\text{N} \) displaces THF, confirmed by NMR shift changes. (b) \( \text{Me}_2\text{O} \) cannot displace \( \text{Me}_3\text{P} \). (c) \( \left[\text{BH}_4\right]^- \) is stable in THF, confirmed by NMR stability. (d) A new \( \text{PPh}_2\text{P} \) chelate with \( \text{BH}_3 \) forms, verified by NMR shift changes.

Step-by-step solution

Analyzing the stability order for part (a)

Given the order of adduct stability \[ ext{Me}_2 ext{O} < ext{THF} < ext{Me}_2 ext{S} < ext{Me}_3 ext{N} < ext{Me}_3 ext{P} < ext{H}^- \]we see that \( \text{Me}_3\text{N} \) is more stable than \( \text{THF} \). Hence, when \( \text{Me}_3\text{N} \) is added to a THF solution of \( \text{THF}\cdot\text{BH}_3 \), it will likely displace THF from the adduct because \( \text{Me}_3\text{N} \cdot \text{BH}_3 \) is more stable.

Verification using NMR spectroscopy for (a)

In NMR spectroscopy, the displacement can be confirmed by observing the chemical shift changes. The \( \text{B}\text{-H} \) and \( \text{N}-\text{Me} \) chemical environments will change, reflecting the formation of \( \text{Me}_3\text{N} \cdot \text{BH}_3 \). This can be observed as changes in the chemical shifts for boron and hydrogen in the adduct.

Evaluating displacement for part (b)

Given the stability order, \( \text{Me}_3\text{P} \cdot \text{BH}_3 \) is more stable than both \( \text{Me}_2\text{O} \) and \( \text{THF} \). Therefore, \( \text{Me}_2\text{O} \) will not displace \( \text{Me}_3\text{P} \) from \( \text{Me}_3\text{P} \cdot \text{BH}_3 \) due to lower stability.

NMR confirmation for (b)

NMR spectroscopy would show no change in the chemical environment if \( \text{Me}_2\text{O} \) is unable to displace \( \text{Me}_3\text{P} \), thus exhibiting no new peaks or chemical shifts in the boron or phosphorus environments.

Stability analysis for part (c)

The species \( \left[\text{BH}_4\right]^- \) is very stable and even more stable than any other species in the list, including THF. It will not undergo a displacement reaction in a THF solution given its high stability.

Confirmation using NMR for (c)

NMR will show stable chemical shifts consistent with the \( \left[\text{BH}_4\right]^- \) ion. Lack of shift changes will confirm stability in THF.

Predicting formation for part (d)

The compound \( \text{Ph}_2\text{PCH}_2\text{CH}_2\text{PPh}_2 \) (a bidentate ligand) can form a chelate with \( \text{BH}_3 \), likely forming \( \text{Ph}_2\text{PCH}_2\text{CH}_2\text{PPh}_2 \cdot \text{BH}_3 \). Given the excess \( \text{THF} \), some \( \text{THF} \cdot \text{BH}_3 \) may still remain.

Verification with NMR for (d)

NMR could confirm new peak patterns corresponding to a chelated \( \text{BH}_3 \) structure. The phosphorus atom will show changes in the NMR spectrum, consistent with binding to \( \text{BH}_3 \). Other signals should correspond to both free and chelated THF.

Key concepts

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is an essential technique in chemistry for understanding the structure and dynamics of molecules. In the context of boron complexes, NMR is used to observe changes in chemical shifts which indicate the formation or displacement of adducts.

When nucleophiles such as amines are added to a boron adduct, displacement reactions might occur. These reactions can be detected using NMR since the involved atoms like boron (B), hydrogen (H), nitrogen (N), and phosphorus (P) exhibit distinct magnetic properties which can be observed as shifts in their respective NMR signals.

• The chemical environment changes of the atoms cause shifts in the NMR spectrum.
• B-H and N-Me groups, for example, will exhibit different chemical shifts once displacement reaction occurs.
• Stable environments usually show minimal changes in the chemical shifts over time.

NMR's ability to reveal these shifts makes it invaluable for confirming theoretical predictions regarding the stability and formation of various boron complexes.

Displacement Reactions

Displacement reactions occur when a more stable molecule replaces another in a complex. This happens especially in coordination chemistry, where ligands swap positions according to their binding affinities.

In the given exercise, several displacement reactions can be predicted based on the stability order of adducts \[ \text{Me}_2\text{O} < \text{THF} < \text{Me}_2\text{S} < \text{Me}_3\text{N} < \text{Me}_3\text{P} < \text{H}^- \]

• Molecules like \( \text{Me}_3\text{N} \) can displace less stable ones such as \( \text{THF} \) from a \( \text{THF} \cdot \text{BH}_3 \) adduct, forming a more stable \( \text{Me}_3\text{N} \cdot \text{BH}_3 \) complex.
• Conversely, \( \text{Me}_2\text{O} \) will not displace a more stable \( \text{Me}_3\text{P} \cdot \text{BH}_3 \).
• The \( \left[\text{BH}_4\right]^- \) ion, being highly stable, will not participate in displacement reactions within a THF solution, maintaining its integrity.

Displacement reactions are often analyzed in experimental settings utilizing spectroscopic methods such as NMR, which can detect slight changes in molecular environments that indicate such reactions.

Boron Complexes

Boron is a versatile element often forming intricate complexes with various ligands. These complexes, such as those of the form \( L \cdot \text{BH}_3 \), are of particular interest due to their applications in various fields including chemistry and materials science.

These complexes demonstrate variable stability depending on the ligands attached. In general, ligands with higher Lewis base character form more stable complexes with boron. The stability order provided in the exercise reflects this principle, where stronger bases form stronger adducts with boron.

• \( \text{H}^{-} \), due to its strong basic character, forms an extremely stable complex with boron.
• When contrasting different ligands, the stability order helps predict possible displacement reactions, understanding that complexes with stronger binders (e.g., trialkylamines like \( \text{Me}_3\text{N} \)) displace those formed with weaker bases.
• Special cases, such as \( \left[\text{BH}_4\right]^- \), showcase unique stability, resisting displacements even in solvent systems that might otherwise induce such reactions.

Boron complexes' behavior can be profoundly understood via spectroscopic techniques, further elucidating their interactions and stability in chemical environments.