A series of easily accessible linear N-heterocyclic carbene (NHC) copper­(I) complexes, bearing pyridine (py) and its derivatives as chromophore ligands, are barely emissive in the single-crystalline solid state. However, their powders, neat films, and dilute doped films of poly­(methyl methacrylate) (PMMA; 1–10%) show very intense blue-to-blue-green photoluminescence with remarkable quantum yields φ of up to 87% and microsecond lifetimes, indicative of triplet states being involved. These luminescence properties are similar to trigonal coordinated NHC copper­(I) bis­(pyridine) complexes, which we have also isolated and characterized with respect to their structures and photophysics. Our spectroscopic and theoretical studies provide detailed insight into the nature of the luminescence enhancing effect of the linear two-coordinated copper­(I) compounds, which is based on the formation of Cu–F interactions between the BF4 – anions and [Cu­(NHC)­(2-R-py)]+ (R = H, Me, Ph) cations. These interactions are absent in the single crystals but lead to a distorted ground-state structure in the precipitated powders or in PMMA films, giving rise to high k r. In addition, we found that our linear copper­(I) complexes exhibit mechanochromic luminescence because grinding of the single crystals leads to enhanced emission intensity. In light of the recently reported cation–anion contact-induced mechanochromic luminescence of two-coordinated copper­(I) complexes, this study supports the generality of this new mechanism for the design of mechanoresponsive phosphorescent materials.

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The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr2N)CH(NRR')][PF6], for use as immediate carbene precursors. The corresponding ADACs (iPr2N)C(NRR') were sufficiently stable for isolation in the case of NRR' = 2-methylpiperidino (13), 3-methylpiperidino (14), 4-methylpiperidino (15), morpholino (17) and NiPrPh (20), but had to be trapped in situ in the case of NRR' = 2,2,6,6-tetramethylpiperidino (12) and NiPrMe (19). The tetraaryl-substituted ADACs (Ph2N)2C (22) and (Ph2N)C[N(C6F5)2] (24) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ-Cl)(cod)}2] did not work for 12 and 24. The (77)Se NMR chemical shifts, δ((77)Se), of the selenourea compounds derived from the new ADACs lie in the range 450-760 ppm, which indicates a much higher electrophilicity and π-accepting capability of ADACs in comparison with NHCs, which typically exhibit δ((77)Se)<200 ppm. The extreme low-field shift of 758 ppm observed for 12Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6-tetramethylpiperidino unit perpendicular to the N2C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.

Four new acyclic diaminocarbenes (ADACs), viz. [(cyclo‐CnH2n−1)2N]2C (n=5–7) and iPr2N‐C‐N(cyclo‐C6H11)2, were synthesised by reacting the corresponding formamidinium hexafluorophosphates with NaN(SiMe3)2. Their nucleophilicities and electrophilicities were respectively judged from the 1JCH values determined for the N2CH unit of the corresponding formamidinium cations and from the 77Se NMR chemical shifts of the selenourea derivatives obtained from the reaction of elemental selenium with the corresponding ADACs. An ambiphilic profile essentially identical to that of the “Alder carbene” (iPr2N)2C was found in each case. Similar to the latter carbene, the new ADACs undergo a well‐defined thermal decomposition by β‐fragmentation, affording an alkene and a formamidine. The stabilities of [(cyclo‐CnH2n−1)2N]2C depend strongly on the value of n, following the order 6>5>7, with the latter congener being too unstable for isolation. [(cyclo‐C6H11)2N]2C shows no thermal decomposition at room temperature in solution and is thus significantly more stable than (iPr2N)2C. The stability of iPr2N‐C‐N(cyclo‐C6H11)2 is intermediate between that of (iPr2N)2C and [(cyclo‐C6H11)2N]2C, its β‐fragmentation selectively affording propene and iPrN=CH‐N(cyclo‐C6H11)2. [(cyclo‐CnH2n−1)2N]2C (n=5–7) react readily with CO under mild conditions, selectively affording trisubstituted spirocyclic β‐lactam derivatives with an antimicrobial activity spectrum similar to that of penicillin G.

The reaction of carbon monoxide with the acyclic diaminocarbene iPr2N–C–TMP (TMP=2,2,6,6-tetramethylpiperidino), generated in situ from the formamidinium salt [(iPr2N)CH(TMP)][PF6] and NaN(SiMe3)2, unexpectedly afforded a mixture of β- and γ-lactams.