Kinetically stabilized 1-phosphahaloprenes (2-halo-1-phosphabutadienes) as well as 1-phosphaisoprene undergo a hitherto unknown phospha-Diels–Alder dimerization of the PC–CC units upon heating.

A polymeric phosphine sensor is reported that exhibits bright blue fluorescence in the presence of gold(I/III) ions but is nonemissive with other metal ions. Specifically, solutions of a poly(p-arylenediethynylene phosphine) copolymer are 35 or 94 times more emissive when treated with solutions of (tht)AuCl or HAuCl 4 •3H 2 O, respectively. Model compound studies confirm phosphine coordination to metals, including gold(I/III) and rhodium(I), and the selective "turn-on" fluorescence was investigated using time-dependent density functional theory calculations.

The reactions of the N-heterocyclic carbenes (NHCs) IDipp and ItBu and the cyclic(alkyl)amino carbene (CAAC) CAAC Me with polyaminoborane [MeNH–BH2] n were investigated. Stoichiometric quantities of each carbene were found to cause rapid and complete depolymerization, with the major B–N-containing product identified as the NHC-aminoborane adduct, IDipp–BH2NMeH (1), cyclic borazane [MeNH–BH2]3, or borazine [MeNBH]3 with IDipp, ItBu, and CAAC Me , respectively. With substoichiometric quantities of IDipp and ItBu (down to 10 and 2.5 mol %, respectively), complete loss of high molar mass material was also detected, indicating that the depolymerization is catalytic. The main products of the reaction with substoichiometric IDipp were IDipp–BH2NMeH (1) and [MeNH–BH2]3 and with substoichiometric ItBu, [MeNH–BH2]3, and [MeNBH]3 with product ratios dependent on the quantity of NHC used. Under analogous conditions with CAAC Me , high molar mass material persisted alongside the formation of [MeNBH]3. Further reactivity studies with cyclic borazane [MeNH–BH2]3 and MeNH2·BH3 provided insights into depolymerization pathways. IDipp showed no reactivity toward [MeNH–BH2]3, whereas with 3 equiv of ItBu and CAAC Me , the dehydrogenation product [MeNBH]3, was formed. With MeNH2·BH3, 2 equiv of carbene were used as the first acts to accept dihydrogen; the major products with IDipp, ItBu, and CAAC Me were IDipp–BH2NMeH (1), [MeNBH]3, and (CAAC Me H)HBNMeH (2), respectively. The double E–H (E = B, N) bond activation product (CAAC Me H)HBNMe(HCAAC Me ) (3) was isolated from the reaction between 3 equiv of CAAC Me and MeNH2·BH3. A unified mechanism for donor-mediated depolymerization of [MeHN–BH2] n is proposed.

The cycloaddition of 1-phosphabutadienes with Au I and Pd II proceeds very differently when the hydrogen substituent in the 3position is replaced by methyl. Specifically, 1-phosphaisoprene [Mes*P�C(Me)�CH�CH 2 , E-1a] was treated with [Au(tht)Cl] (tht = tetrahydrothiophene) to afford a phosphaalkene-substituted phosphacyclohexene binuclear AuCl complex. This intermolecular [4 + 2] cycloaddition is analogous to the Diels−Alder reaction and proceeds via the end-on complex [Au(E-1a)Cl], which was also isolated. In contrast, the reaction of 1-phosphabutadiene Mes*P�C(Me)� C(Me)�CH 2 (E-1b) with [Au(tht)Cl] selectively afforded the 1phosphet-2-ene-Au I complex [CH 2 �C(Me)�C(Me)�P(Mes*)-AuCl] in near quantitative conversion. This intramolecular [2 + 2] cycloaddition also occurred when E-1b was reacted with [Pd(η 3 -C 3 H 5 )(μ-Cl)] 2 or [Pd(cod)Cl 2 ] (COD = cyclooctadiene) to afford complexes [CH 2 �C(Me)�C(Me)�P(Mes*)Pd(η 3 -C 3 H 5 )Cl] and [(CH 2 �C(Me)�C(Me)�P(Mes*)) 2 PdCl 2 ], respectively. All the complexes were fully characterized spectroscopically and structurally.

Treatment of Mes*P=C(Si(CH3)3)Br with CH2=CHMgBr in the presence of Pd(0) affords 1-phosphabutadiene Mes*P=C(Si(CH3)3)–CH=CH2 (1) selectively as the Z isomer. Remarkably, treatment of Z-1 with n-BuLi (0.1 equiv.) resulted in quantitative formation of E-1. The exact role of the n-BuLi is unknown; however, it appears to be required to promote the isomerization in the absence of light. The isomerization of Z-1 to E-1 was also mediated by sunlight with the reaction mixture consisting of a ca. 1:9 ratio of Z-1 to E-1. DFT calculations were consistent with the thermodynamic favorability of the isomerization of Z-1 to E-1.

The anionic polymerization of 1-phosphaisoprene [Mes*P=C-(Me)À CH=CH 2 (E-1)] affords poly(1-phosphaisoprene) 2 in high yield (75 %). Concentrated solutions of polymer 2 (M n = 21,800 g mol À 1 ; Đ = 1.02) a P-analogue of natural rubber, undergo gelation upon treatment with [Pd(cod)Cl 2 ] (0.15 P equiv). Evidence for P-coordination of 2 to Pd II was obtained by 31 P and 1 H NMR spectroscopy. The gelation is reversed by the addition of PMe 3 and the reformation of recoverable 2 along with [Pd II À PMe 3 ] complexes were confirmed by 31 P NMR spectro-scopy. The use of labile metal-ligand bonds to reversibly form gels is unprecedented and has relevance to self-healing materials. In contrast, coordination of 2 to [Pd(η 3 -C 3 H 5 )(μ-Cl)] 2 affords the well-defined complex 2 • [Pd(η 3 -C 3 H 5 )Cl] which was characterized by 31 P, 1 H, 13 C{ 1 H} NMR spectroscopy and GPC. This polymer chemistry was complemented by detailed molecular model studies of the coordination chemistry of monomer 1-phosphaisoprene E-1 with [Pd(cod)Cl 2 ] and [Pd(η 3 -C 3 H 5 )(μ-Cl)] 2 ].