Making the True “CP” Ligand

Abstract

The quest for cyaphide, the phosphorus equivalent of cyanide, has been a continuous struggle for many years. Potential applications arising from the use of C P as a bridging ligand between two metals, the incorporation of CP into coordination polymers and new materials, and the basic synthetic challenges of making C P have provided inspiration for decades. While phosphaalkynes (R À C P) have been known for some time, [1][2][3] the terminal MÀCP has only been reported as a transient species. [4,5] Other C-functionalized XÀCP compounds (X = R 3 Si, [6] R 2 N, [7,8] RO, [8] F, [9] Cl, [10] ), anionic species [XÀCP] À (X = R 3 B, [11] RN, [8] O, [12] S [13] ), and the cationic phosphonio phosphaalkyne [R 3 P À C P] + , [14] have been synthesized, but the vast majority of reports deal with tert-butyl, [15] adamantyl, [16] 2,4,6-trimethylphenyl [17] and 2,4,6-tri-tert-butylphenyl [18] phosphaalkynes. Recently, we devised a very simple method for accessing the kinetically stable crystalline triphenylmethyl (trityl)-substituted phosphaalkyne, Ph 3 CC P (1), which allowed for the economical metal-promoted synthesis of phosphorus heterocycles. However, our ultimate goal and incentive for synthesizing the trityl-substituted phosphaalkyne was to find an adequate leaving group that would lead us to cyaphide. [19] Treatment of 1 or its complexes [MH(dppe) 2 (Ph 3 C-CP)]OTf (M = Fe or Ru; dppe = bis(1,2-diphenylphosphinoethane); OTf = trifluoromethanesulfonate) with nucleophiles (Nu) did not furnish the desired cyaphide, C P À , or its complexes [Eq. (1); electrophile (E) = C].Therefore, we reasoned that silicon would be more susceptible to nucleophilic attack (E = Si) and applied an analogous synthetic procedure to access the higher homologue Ph 3 SiCP (3).Conversion of Ph 3 SiCH 2 Cl into the Grignard reagent followed by treatment with PCl 3 produced the silyl-substituted alkyl phosphonous dichloride, Ph 3 SiCH 2 PCl 2 (2), in over 87 % yield (Scheme 1). Employing 2.2 equiv of DABCO (1,8-diazabicyclo[2.2.2]octane) effected the dehydrohalogenation reaction, and 2 was fully transformed into the triphenylsilyl-substituted phosphaalkyne 3. The reaction occurred at ambient temperature and in multiple solvents in less than one hour. (In contrast, the reaction of DABCO with Ph 3 CCH 2 PCl 2 to furnish Ph 3 CC P required a 10-fold excess of base, elevated temperatures, and proceeded best in acetonitrile.[19] ) While DABCO alone was effective for the conversion of 2 into 3, we saw rapid decomposition of this new phosphaalkyne, which was dependent on the solvent. Qualitatively, the rate of decomposition of 3 followed the order Et 2 O % toluene < THF ! CH 3 CN. Suspecting that DABCO-HCl was acting as a soluble source of chloride anion in the more polar solvents that then attacked silicon to produce Ph 3 SiCl and the CP anion, which is seemingly unstable under the reaction conditions, we added AgOTf to the reaction mixture. When a solution of 2 in toluene was pretreated with 2.2 equiv of AgOTf for 5 min followed by addition of 2.2 equ...

Neutral and Cationic Hydridoruthenium Tetrakiscarbene Complexes

“…[1,3] The synthetic utility of such complexes is further illustrated by the recent stabilization of the cyaphide anion (CϵP -), end-on coordinated in the complex [RuH(CP)(dppe) 2 ] [dppe = 1,2-bis(diphenylphosphanyl)ethane]. [4] Replacing phosphanes by N-heterocyclic carbenes (NHCs) should significantly alter the steric and electronic properties of the resulting complexes, [5] with a beneficial impact on potential applications in catalysis and small molecule activation. Various hydridoruthenium complexes containing NHCs have been reported some of which are active catalysts.…”

“…The compound displays an ion-separated structure which consists of the [RuH(IMe) 4 ] + cation (3) and the [BEt 4 ] -anion (Figure 2). Both anion and cation reside on crystallographic C 2 axes.…”

“…[9] A very recent report of the complex [RuH(IPrMe 2 ) 4 ] + (IPrMe 2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-plays a tetragonal bipyramidal geometry with four carbene groups coordinated to ruthenium in equatorial positions and two apical hydrogen atoms. DFT calculations suggest that the trans structure observed for 2 is preferred over a cis arrangement for steric reasons.…”

“…Targeting new hydridoruthenium carbene complexes, we reacted 1 with hydride sources. First, we examined the reaction with a slight excess of LiAlH 4 . Gratifyingly, we ob-tained the yellow target complex trans-[RuH 2 (IMe) 4 ] (2), which could be characterized by an X-ray crystal structure analysis (vide infra).…”

η1:η2-P-Pyrazolylphosphaalkene Complexes of Ruthenium(0)

“…The same approach has typically been cited in the development of kinetically stabilized phosphaalkynes (e.g., t BuC≡P, AdC≡P), however, in these cases the bulk is often sufficiently remote from the reactive π-system as to preclude it being the sole stabilizing influence. Moreover, even bulky phosphaalkynes, e.g., R 3 SiC≡P (R = Ph [9], Me [10,11]) often exhibit only limited stability, in some instances comparable to that of unencumbered systems (e.g., MeC≡P [12]). Significantly, the formally related phosphaethynyloate ion "O-C≡P − " is isolable as a sodium salt [13,14], which exhibits appreciable stability despite the lack of any steric "protection"; this fact is attributed to electronic influences, with alongside computational studies of the isolated cyaphide ion ("C≡P − "), which indicate an intrinsically unstable hypovalent "[C=P] − " structure [15], this would imply that electronics serve an equally important role in imparting stability to low-coordinate phosphacarbons.…”