Non-conventional Behavior of a 2,1-Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

Abstract

The titled 2,1-benzazaphosphole (1) (i. e. ArP, where Ar=2-(DippN=CH)C₆ H₄ , Dipp=2,6-iPr₂ C₆ H₃ ) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero-Diels-Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron-deficient alkynes RC≡CR (R=CO₂ Me, C₅ F₄ N) giving 1-phospha-1,4-dihydro-iminonaphthalenes 2 and 3, that undergo hydrogen migration producing 1-phosphanaphthalenes 4 and 5. Compound 1 is also able to activate the C=C double bond in selected N-alkyl/aryl-maleimides RN(C(O)CH)₂ (R=Me, tBu, Ph) resulting in the addition products 7-9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi-reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [μ-ArP(AuCl)₂ ] (10), [(μ-ArP)₄ Ag₄ ][X]₄ (X=BF₄ (11), OTf (12)) and [μ-ArP(Co₂ (CO)₆ )] (13). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view.

Keywords: Diels-Alder reactions; NRT study; heterodienes; phospha-heterocycles; phosphinidenes.

Scheme 1

Relevant pnictinidene‐like systems for this study.

Scheme 2

Leading resonance structures of compound 1’.

Figure 1

Selected Kohn‐Sham orbitals of the model species 1′ at the ωB97XD/def2‐TZVP level with a contour value of 0.05.

Scheme 3

Synthesis of compounds 2–6.

Figure 2

ORTEP presentation of the molecular structures of 2 (left) and 3 (right) (40 % probability displacement ellipsoids). Hydrogen atoms and the alkyne co‐crystallized molecule in the case of 3 are omitted. Bond lengths [Å] and angles [°] for 2: P(1)−N(1) 1.704(2), P(1)−C(3) 1.904(2), P(1)−C(5) 1.854(2), N(1)−C(1) 1.478(2), C(1)−C(2) 1.545(2), C(2)−C(3) 1.331(2), C(1)−C(4) 1.529(2), C(4)−C(5) 1.401(3), N(1)−P(1)−C(5) 87.89(8), N(1)−P(1)−C(3) 88.65(7), C(3)−P(1)−C(5) 89.43(7). 3: P(1)−N(1) 1.710(2), P(1)−C(3) 1.865(2), P(1)−C(20) 1.897(2), N(1)−C(1) 1.470(3), C(1)−C(2) 1.540(3), C(2)−C(3) 1.400(3), C(1)−C(21) 1.557(3), C(20)−C(21) 1.336(3), N(1)−P(1)−C(3) 88.44(10), N(1)−P(1)−C(20) 86.48(9), C(3)−P(1)−C(4) 90.12(9).

Figure 3

ORTEP presentation of the molecular structure of 5 (40 % probability displacement ellipsoids). Hydrogen atoms except the NH group are omitted. Bond lengths [Å] and angles [°]: P(1)−C(1) 1.713(5), P(1)−C(5) 1.755(4), C(1)−C(2) 1.415(6), C(2)−C(3) 1.393(5), C(3)−C(4) 1.451(6), C(4)−C(5) 1.414(6), C(3)−N(1) 1.393(5), C(1)−P(1)−C(5) 100.4(2).

Figure 4

ORTEP presentation of the molecular structure of 6 (40 % probability displacement ellipsoids). Hydrogen atoms except the relevant CH, and NH group and benzene solvent molecule are omitted. Symmetry operator a=1−x, 1−y, 1−z. Bond lengths [Å] and angles [°]: P(1)−P(1a) 2.209(7), P(1)−O(1) 1.426(3), P(1)−C(1) 1.837(2), P(1)−C(5) 1.807(2), C(1)−C(2) 1.511(2), C(2)−C(3) 1.381(3), C(3)−C(4) 1.489(3), C(4)−C(5) 1.413(2), C(3)−N(1) 1.357(2), C(1)−P(1)−C(5) 99.64(8), C(1)−P(1)−O(1) 120.41(11), O(1)−P(1)−P(1a) 118.44(10), C(5)−P(1)−P(1a) 103.33(6).

Scheme 4

Synthesis of 7–9 also showing the reversible binding of maleimides.

Figure 5

ORTEP presentation of the molecular structures of 7 (left) and 9 (right) (40 % probability displacement ellipsoids). Hydrogen atoms except the relevant CH, and NH group are omitted. Bond lengths [Å] and angles [°]: Hydrogen atoms are omitted. Bond lengths [Å] and angles [°] for 7: P(1)−N(2) 1.726(3), P(1)−C(3) 1.904(4), P(1)−C(10) 1.815(3), N(2)−C(4) 1.451(4), C(3)−C(11) 1.533(5), C(11)−C(4) 1.562(5), C(4)−C(5) 1.518(4), C(5)−C(10) 1.386(4), N(2)−P(1)−C(3) 86.85(14), N(2)−P(1)−C(10) 90.08(13), C(3)−P(1)−C(10) 91.94(15). 9: P(1)−N(1) 1.703(2), P(1)−C(1) 1.842(2), P(1)−C(20) 1.912(2), N(1)−C(7) 1.470(2), C(20)−C(23) 1.535(2), C(23)−C(7) 1.581(1), C(7)−C(2) 1.515(2), C(2)−C(1) 1.400(2), N(1)−P(1)−C(1) 89.89(6), N(1)−P(1)−C(20) 88.37(6), C(1)−P(1)−C(20) 91.17(6). Inset shows a different type of maleimide bonding in antimony analogues.

Scheme 5

Synthesis of complexes 10–13.

Figure 6

ORTEP presentation of the molecular structures of 10 showing (40 % probability displacement ellipsoids). Hydrogen atoms and dichloromethane solvent molecules are omitted. Bond lengths [Å] and angles [°]: P(1)−N(1) 1.798(9), P(2)−N(2) 1.791(8), P(1)−Au(1) 2.228(2), P(1)−Au(2) 2.216(3), P(2)−Au(3) 2.235(3), P(2)−Au(4) 2.222(2), Au(1)−Au(2) 3.2613(6), Au(1)−P(1)−Au(2) 114.26(10), Au(3)−P(2)−Au(4) 118.73(10).

Figure 7

ORTEP presentation of the molecular structures of 12 (40 % probability displacement ellipsoids). Hydrogen atoms, OTf anions and dichloromethane solvate molecules are omitted. Symmetry operator a=1−x, y, 3/2−z. Bond lengths [Å] and angles [°]: Ag(1)−P(1) 2.451(3), Ag(2)−P(1) 2.439(2), Ag(2)−P(2) 2.445(2), Ag(3)−P(2) 2.432(3), P(1)−N(1) 1.752(7), P(2)−N(2) 1.752(7), P(1)−C(7) 1.775(11), P(2)−C(26) 1.772(8), Ag(1)−P(1)−Ag(2) 99.59(10), Ag(2)−P(2)−Ag(3) 97.93(8), P(1)−Ag(1)−P(1) 169.88(9), P(1)−Ag(2)−P(2) 166.85(11), P(2)−Ag(3)−P(2a) 172.89(7).

Figure 8

ORTEP presentation of the molecular structures of 13 (40 % probability displacement ellipsoids). Hydrogen atoms are omitted. Bond lengths [Å] and angles [°]: P(1)−N(1) 1.795(2), P(1)−C(1) 1.788(2), P(1)−Co(1) 2.118(6), P(1)−Co(2) 2.139(6), Co(1)−Co(2) 2.629(4), Co(1)−P(1)−Co(2) 76.29(2), C(1)−P(1)−N(1) 88.10(9).

Scheme 6

Computed model reactions at the ωB97XD/def2‐TZVP level.