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MHPN, reported in 2017, was presented as the first superbase with two interacting phosphorus-ylide units and two carbon atoms that can act as basicity centers. A naphthalene framework holds those carbon centers close together. Experiments and calculations indicate that, after protonation, a proton rapidly exchanges between them—a behavior the authors said could help explain MHPN’s strong basicity.
What is a superbase?
A base accepts a proton. A superbase is an exceptionally strong base, but “superbase” is a broad chemical description rather than one universal numerical threshold. Basicity values depend on the solvent and on how they are measured, so figures from different conditions cannot automatically be compared as if they were on one scale.
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In the 2017 study, Julius F. Kögel and colleagues reported MHPN as a new kind of superbase: its basicity centers are carbon atoms associated with phosphorus-ylide units, rather than the nitrogen centers characteristic of classical proton sponges. The authors described it as “the first superbase MHPN with two interacting P-ylide entities” in their 2017 paper in Angewandte Chemie International Edition.
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What makes MHPN different from a proton sponge?
Classical proton sponges are built around nitrogen-centered basicity. MHPN instead has two carbon basicity centers, each associated with a P-ylide entity. The naphthalene scaffold brings the two sites into close proximity, creating an arrangement in which they can interact.
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Chemistry World’s 2017 report characterized the compound as “the first of a new class centred around carbon–phosphorus bonds.” Its account also describes fused benzene rings as bringing the basic carbons together. That report says the synthesis took two steps, but the published abstract does not provide a complete practical synthesis protocol; specific reagents, quantities, yields, and handling conditions should not be inferred from the news account.
How strong is the phosphorus bisylide superbase?
Kögel and colleagues reported an experimental pKBH+ of 33.3 ± 0.2 on the acetonitrile (MeCN) scale. They separately reported a calculated gas-phase proton affinity of 277.9 kcal mol−1. These are different kinds of values: the first is an experimental solution basicity measure, while the second is a computational gas-phase quantity. They should not be treated as interchangeable.
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In the same computational work, the calculated proton affinity was nearly 15 kcal mol−1 above that of the corresponding monoylide. This is a comparison within that study, not a universal ranking of MHPN against every superbase.
How does the proton move between the carbon atoms?
The researchers investigated MHPN using NMR spectroscopy, single-crystal X-ray diffraction, and theoretical calculations. Their account indicates that, after protonation, the proton rapidly exchanges between the two basic carbon atoms. Chemistry World described this accessibly as proton “hopping”; it should not be read as a claim that the proton is permanently centered between the atoms.
The authors proposed that this rapid exchange can partly account for the unexpectedly high basicity. It is a contribution to the explanation, not a claim that proton exchange alone accounts for the molecule’s strength.
What did later phosphorus bisylide work report?
A 2025 follow-up paper reported related compounds MTPN and P2-MHPN, with basicity results specified by solvent and method. Its acetonitrile figures are estimates, while its THF pKaH values are reported as experimental.
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| Compound | Experimental pKaH in THF | Estimated pKaH in acetonitrile | Proton self-exchange rate at 300 K |
|---|---|---|---|
| MTPN | 26.0 | 33.6 | 2298 s−1 |
| P2-MHPN | 29.5 | 37.4 | 300 s−1 |
These results come from the 2025 authors’ report, “The Next Generation of Phosphorus Bisylide Superbases – Synthesis, Structures, Basicity and Proton Self-Exchange”. The table’s acetonitrile values are estimated, not experimental; its THF values are experimental. Neither should be directly ranked against MHPN’s 2017 pKBH+ or calculated gas-phase proton affinity without accounting for the differing compounds, notation, solvents, and measurement status.
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What was proposed next in 2017?
The 2017 Chemistry World report said the researchers thought attaching phosphazene groups might further increase superbasicity. That was a research prospect reported at the time, not an established result. The later work described above provides concrete findings for related phosphorus bisylides, rather than proof of that specific proposed modification.
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