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Cationic 5-phosphonio-substituted N-heterocyclic carbenes

2-Phosphanyl-substituted imidazolium salts 2-PR2(4,5-Cl-Im)[OTf] (9a,b[OTf]) (4,5-Cl-Im = 4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolium) (a: R = Cy, b: R = Ph) are prepared from the reaction of R2PCl (R = Cy, Ph) with NHC 8 (4,5-dichloro-1,3-bis(2,6-di-isopropylphenyl)-imidazolin-2-ylidene) in the presence of Me3SiOTf. 5-Phospanyl-substituted imidazolium salts 5-PR2(2,4-Cl-Im)[OTf] (10a,b[OTf]) are obtained in quantitative yield when a slight excess of the NHC 8 is used. 5-Phosphonio-substituted imidazolium salts 5-PR2Me(2,4-Cl-Im)[OTf]2 (14a,b[OTf]2) and 5-PR2F(2,4-Cl-Im)[OTf]2 (16a,b[OTf]2) result from methylation reaction or oxidation of 10a,b[OTf] with XeF2 and subsequent fluoride abstraction. According to our quantum chemical studies the Cl1 atom at the 2-position at the imidazolium ring of dication 14b2+ carries a slightly positive charge and is therefore accessible for nucleophilic attack. Accordingly, the reaction of 14a,b[OTf]2 and 16a,b[OTf]2 with R3P (R = Cy, Ph) affords cationic 5-phosphonio-substituted NHCs 5-PR2Me(4-Cl-NHC)[OTf] (17a,b[OTf]) and 5-PR2F(4-Cl-NHC)[OTf] (18a,b[OTf]) via a SN2(Cl)-type reaction. A series of transition metal complexes such as [AuCl(5-PPh2Me(4-Cl-NHC))][OTf] (19[OTf]), [CuBr(5-PPh2Me(4-Cl-NHC))][OTf] (20[OTf]), [AuCl(5-PPh2F(4-Cl-NHC))[OTf] (21[OTf]) and [RhCl(cod)(5-PPh2Me(4-Cl-NHC))][OTf] (23[OTf]) are prepared to prove the coordination abilities of carbenes 17b[OTf] and 18b[OTf]. The isolation of a rare example of a tricationic bis-carbene silver complex [Ag(5-PPh2Me(4-Cl-NHC))2][OTf]3 (22[OTf]3) is achieved by reacting 14b[OTf] with Cy3P in the presence of AgOTf. NHC 17b[OTf] represents a very effective dehydrocoupling reagent for secondary (R2PH, R = Ph, Cy, iBu) and primary (RPH2, R = Ph, Cy) phosphanes to give diphosphanes of type R4P2 (R = Ph, Cy, iBu) and oligophosphanes R4P4, R5P5 (R = Ph, Cy), respectively. Methylation of 17b+ and subsequent deprotonation reaction with LDA affords the cationic NHO (N-heterocyclic olefin) 35+ of which the gold complex 36+ is readily accessible via the reaction with AuCl(tht).

Catalytic P-H activation by Ti and Zr catalysts

Catalytic dehydrocoupling of phosphines was investigated using the anionic zirconocene trihydride salts [Cp*2Zr(μ-H) 3Li]3 (1a) or [Cp*2Zr(μ H) 3K(thf)4] (1b), and the metallocycles [CpTi(NPtBu 3)(CH2)4] (6) and [Cp*M(NPtBu 3)(CH2)4] (M = Ti 20, Zr 21) as catalyst precursors. Dehydrocoupling of primary phosphines RPH2 (R = Ph, C6H2Me3, Cy, C10H7) gave both dehydrocoupled dimers RP(H)P(H)R or cyclic oligophosphines (RP)n (n = 4, 5) while reaction of tBu3C6H2PH 2 gave the phosphaindoline fBu2(Me2C-CH 2)C6H2PH (9). Stoichiometric reactions of these catalyst precursors withprimary phosphines afforded [Cp* 2Zr((PR)2)H][K(thf)4] (R = Ph 2, Cy 3, C 6H2Me3 4), [Cp*2Zr((PPh) 3)H] [K(thf)4] (5), [CpTi(NPtBu3)(PPh) 3] (7) and [CpTi(NPfBu3)(nrPHPh)]2 (8), while reaction of 6 with (C6H2tBu3)PH2 in the presence of PMe3 afforded [CpTi(NPtBu3)(PMe 3)(P(C6H2tBu3)] (10). The secondary phosphines Ph2PH and (PhHPCH2)2CH2 also undergo dehydrocoupling affording (Ph2P)2 and (PhPCH2)2CH2. The bisphosphines (CH 2PH2)2 and C6H4(PH 2)2 are dehydrocoupled to give (PCH2CH 2PH)v (12) and (C6H4P(PH)) 2 (13) while prolonged reaction of 13 gave (C6H 4P2)8 (14). The analogous bisphosphine Me 2C6H4(PH)2 (17) was prepared and dehydrocoupling catalysis afforded (Me2C6H 2P(PH))2 (18) and subsequently [(Me2C 6H2P2)2(μ-Me2C 6H2P2)]2 (19). Stoichiometric reactions with these bi-sphosphines gave [Cp*2Zr(H)(PH)2C 6H4][Li(thf)4] (22), [CpTi(NPtBu 3)(PH)2C6H4]2 (23) and [Cp*Ti(NPfBu3)(PH)2C6H4] (24). Mechanistic implications are discussed.