Reactivity of cyclooligophosphanes: synthesis and structural characterisation of cyclo-1,4-(BH3)2(P4Ph4CH2) and cyclo-1,2-(BH3)2(P5Ph5)

Controlled Ring Opening of a Tetracyclic Tetraphosphane with Twofold Metallocene Bridging

A direct route to a doubly ferrocene bridged tetracyclic tetraphosphane 1 was developed via reductive coupling of Fe(CpPCl2)2 (2 a), where a chlorine terminated linear P4-compound 3 could be identified as an intermediate. Selective P-P bond activation was further achieved by reacting 1 with elemental selenium or [Cp*Al]4, where regioselective insertion of Se or Al atoms resulted in ferrocenylene bridged [P4Se] (4) or [P4Al] (7) moieties. Compound 7 can be transformed to a hydrogen terminated linear P4 species, 8, with protic solvents. Methylation of compound 1 with MeOTf, proceeds via intermediate formation of monomethylated species 5, which gradually produced Me2-terminated dicationic 6, again containing a linear P4-unit. Besides spectroscopic characterization, the structural details of compounds 1, 4, 6, and 8 could be determined by SC-XRD. Moreover, DFT calculations were used to rationalize the reactivity of 1, derived compounds and intermediates. As a key feature, 1 undergoes ring opening polymerization to a linear polyphosphane 9.

B-substituted group 1 phosphides: synthesis and reactivity

1-Boryl-8-phosphinonaphthalenes 1-BCy2-8-PCl2-C10H6 (1) and 1-BCy2-8-PPhCl-C10H6 (2) were prepared and used as starting materials for the synthesis of B-substituted phosphides. The reduction of 1 and 2 by Mg provided neutral compounds [1-BCy-8-PCy-C10H6]2 (3) and [1-BCy2-8-PPh-C10H6]2 (4). Compound 3 represents the dimer of phosphinoborane 1-BCy-8-PCy-C10H6 while complex 4 is a rare example of a discrete B ← P coordinated diphosphine. The reduction of 2 by Na or K in THF yielded B-substituted group 1 phosphides [Na(THF)3]+[1-BCy2-8-PPh-C10H6]- (5) and {[K(THF)2]+[1-BCy2-8-PPh-C10H6]-}∞ (6), which structurally resembled bulky group 1 phosphides. Complex 5 showed easy activation of elemental chalcogens E (E = O, S, Se) to give B-substituted chalcogenophosphinites {[Na(THF)2]+[1-BCy2-8-P(E)Ph-C10H6]}2 (E = O (7), S (8), Se (9)) as the products of chalcogen insertion into the P-Na bond. Importantly no oxidation to dichalcogenophosphinates was observed. Compound 5 is tolerant of the CO polar bonds in organic substrates and the reactions of 5 with 2,3-butanedione or an acyl chloride provided {[Na(THF)2]+[1-BCy2-8-P{CHC(O)C(Me)O}Ph-C10H6]-}2 (10) and [1-BCy2-8-P{C(O)tBu}Ph-C10H6] (11). Finally, B-coordinated phosphatetrylenes [1-BCy2-8-P(SnL)Ph-C10H6] (12) and [1-BCy2-8-P(PbL)Ph-C10H6] (13) (L is {2,6-(Me2NCH2)C6H3}-) were also prepared by substitution reactions of 5.

Formation of an imidazoliumyl-substituted [(LC)4P4]4+ tetracation and transition metal mediated fragmentation and insertion reaction (LC = NHC)

Tetracationic cyclo-tetraphosphane [(L_C)₄P₄]⁴⁺ as triflate salt (3[OTf]₄) (L_C = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [L_CPCl₂]⁺ (4[OTf]) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) and represents the first salt of the cationic cyclo-phosphane series with the general formula [L _n P _n ] ⁿ⁺. Theoretical calculations reveal the electrophilic nature of the P atoms within the P₄-ring due to the influence of the imidazoliumyl-substituents. Further reduction of 3[OTf]₄ with 6 affords the unexpected formation of the notricyclane P₇-type cation [(L_C)₃P₇]³⁺ (9[OTf]₃). Selective transition metal mediated [2 + 2]-fragmentation of 3 ⁴⁺ is achieved when 3[OTf]₄ is reacted with Fe₂(CO)₉, Pd(PPh₃)₄ and Pt(PPh₃)₄ leading to the formation of the dicationic diphosphene complexes [(η²-L_CP[double bond, length as m-dash]PL_C)Fe(CO)₄]²⁺ (12[OTf]₂) and [(η²-L_CP[double bond, length as m-dash]PL_C)M(PPh₃)₂]²⁺ (13[OTf]₂ for M = Pd; 14[OTf]₂ for M = Pt). In contrast, the reaction of 3[OTf]₄ with an excess of AuCl(tht) gives rise to the formation of the five-membered ring complex [((L_C)₄P₄)AuCl₂]³⁺ (15[OTf]₃), where the Au(i) atom reductively inserts into a P-P bond of 3 ⁴⁺.

Recent highlights in mixed-coordinate oligophosphorus chemistry

This review aims to highlight and comprehensively summarize recent developments in the field of mixed-coordinate phosphorus chemistry. Particular attention is focused on the synthetic approaches to compounds containing at least two directly bonded phosphorus atoms in different coordination environments and their unexpected properties that are derived from spectroscopic and crystallographic data. Novel substance classes are discussed in order to supplement previous reviews about mixed-coordinate phosphorus compounds.