Borohydride monomeric

The compounds are isolated by sublimation from the reaction mixture. Perhaps surprisingly the compounds fall into two quite distinct classes. Those of Np and Pu are unstable, volatile, monomeric liquids which at low temperatures crystallize with the 12-coordinate structure of Zr(BFl4)4 (Fig. 21.7, p. 969). The borohydrides of Th, Pa and U, on the other hand, are thermally more stable and less reactive solids. They possess a curious helical polymeric structure in which each An is surrounded by 6 BFI4 ions, 4 being bridging groups attached by 2 FI atoms and... 

It has been suggested that P BChl (where BChl is one of the two monomeric or "accessory BChls that are not part of P) is a transient state prior to P "I (14,16,19), although the evidence supporting this view has been criticized (23, 24) Recent subpicosecond studies find no evidence for P "BChl (8,9) These new results do not preclude some involvement of a monomeric BChl in the early photochemistry, only that P BChl apparently is not a kinetically resolved transient state Perhaps P itself contains some charge-transfer character between its component BChls, or between P and one or both of the monomeric BChls (8,9,25-27) One of the two monomeric BChls apparently can be removed by treatment of the reaction center with sodium borohydride (28) and subsequent chromatography, with no impairment of the primary electron transfer reactions (29) Thus, at present it appears that P I is the first resolved radical-pair state, and it forms with a time constant of about 4 ps in Rps sphaeroides ... 

Reduction of monomeric Complex I with sodium borohydride gives the binuclear cationic hydride [Pt2H2(/Li-H)(/Li-dppm)2][PF6] (Equation 4, PP = dppm) (12, 13). 

Both mononuclear and binuclear forms of [PtMe2(dppm)] and [PtClMe(dppm)] can be prepared (see Scheme 2). The cis-dimer (XXVII) is the less stable form and is converted to monomeric [PtMe2(dppm)] (XXVIII) when heated in the presence of dimethyl-sulfide. The monomethyl derivatives Complexes XXIX and XXX can be interconverted in solution but Complex XXX is more stable in solution (especially in polar solvents) and is isolated easily as either the chloride or the hexafluorophosphate salt. A binuclear methyl(hydrido) derivative (XXXI) can be prepared by reducing Complex XXX with sodium borohydride. 

Waters and Gray (120) prepared a monomeric and paramagnetic Au(II) complex, (n-BuiN)2[Au(mnt)2] (2) by the borohydride reduction of the corresponding Au(III) complex. 

The hydride complexes of actinides usually contain a coligand, such as, for example, OR, dmpe, or Cp, and can be monomeric or oligomeric. Thus, the borohydrides An(BH4)4 produced according to (5.68) are polymeric (An = Th, U) or monomeric (An = Np, Pu), whereas the An(BH3Me)4 are all monomeric. Their volatility increases from Th to Pu, whereas their stability goes in the opposite direction [282] ... 

An interesting series of substitution/redox reactions involving both dimeric Rh11 and monomeric Rh111 complexes of OEP has been reported.408,409 In methanolic solution, H2 reacts with [Rh(OEP)Cl] to generate [Rh(OEP)H] and HC1 borohydride reduction of [Rh(OEP)Cl] leads to a species identified as [Rh (OEP)]-, which forms the same hydrido species when acidified (v H at 2220 cm, ... 

Complex hydrides give a variety of interesting structures. For LiBH4 derivatives, a monomeric example crystallized from THF " shows three p B-H Li bridges and a very short B-Li distance of 2.19 A (cf. C-Li, 2.36 A in the analogous (/u-H)3C environment of solid (MeLi)co). However, when TMEDA is used as the donor in the parent borohydride, a dimeric complex (16) results, containing four p and two p B-H Li contacts. A tetrameric complex with p ... 

The enol-lactam (30), which has occupied a central role in the synthesis of Erythrina alkaloids, has been converted in an unprecedented reaction into the dimeric isomers [31 C(6)-a-0] and [31 C(6)-/ -OJ.15 This reaction may be effected in benzene, pyridine, or acetic acid solution in the presence of lead tetra-acetate. The structures of the products were elucidated by spectral and chemical means. As enol ethers, these compounds were found to exhibit surprising stability to mineral acids. However, catalytic reduction of [31 C(6)-a-OJ under neutral conditions gave the starting enol-lactam (30) and the 7/Miydroxylactam (32 RJ = OH, R2 = R3 = H). The dimer [3 l C(6)-/i-0] yielded only compound (32 R1 = OH, R2 = R3 = H). Similarly, sodium borohydride reduction of the dimer mixture in hot isopropanol led to cleavage products (32 R1 = OH, R2 = R3 = H)and(32 RJ = R3 = H, R2 = OH). Besides the dimeric products, compound (32 R1 + R2 = O, R3 = OAc) was also isolated from the lead tetraacetate oxidation in low yields. Attempts to discover conditions for the formation of preparative amounts of (32 R1 + R2 = O, R3 = OAc), a compound of more potential usefulness for alkaloid synthesis, were fruitless. The other question of interest, whether or not the trans-dimer [31 C(6)-/i-0] could be converted into a monomeric trans-erythrinane system, remains to be answered. 

Structural studies on Zr(BHif)if (6) and Hf(BHif) (7) have shown that these molecules are monomeric and crystallize into a cubic lattice with molecular structures very similar to those of Np and Pu borohydrides. 

Some of the physical properties of metal tetrakis-boro-hydrides, which are primarily determined by their solid-state structure, are listed in Table 1. The polymeric Th, Pa, and U borohydrides are of much lower volatility than the monomeric Zr, Hf, Np, and Pu compounds. The intermolecular bonds connecting molecules together decrease their volatility substantially since these bonds break when the solid vaporizes (12). A plot of log p(mmHg) vs 1/T yields the equation log p(mmHg) = -A/T + B, where T is in K. Values of A and B allow the calculation of the heats (AH) and entropies (AS) for phase-change processes as shown in Table 1. The actinide ions in the polymeric compounds are 14 coordinate however, in the gaseous state they are 12 coordinate (12). 

Vibrational Spectroscopy. In spite of their complex molecular frameworks, the monomeric borohydrides display surprisingly simple vibrational spectra due to their high symmetry (T ), which requires that many fundamental vibrations be degenerate. Normal coordinate analyses have been carried out for ZrCBHtt) (14) and HfCBHtt) (15) and a similar study was completed for Np(BHit)if in order to compare vibrational energy level structures and elucidate the nature of the fundamental vibrations of Np(BHit)tf (10). ... 

The unusual observation that neither the borane nor the borohydride alone are effective, prompted an investigation of the mechanism of reduction21. It is postulated that the ketone forms a complex with 2.5-dimethylborolanvl methanesulfonate. This complex then undergoes reduction by monomeric 2,5-dimethylborolane. 

The inability of such photolyzate solutions to hydroborate 1-octyne rules out the presence of neutral boron hydrides. However, the detection of HD, upon the work-up with DOAc, and the formation of undeuterated toluene, upon treatment of the photolyzate with benzyl chloride and deuterolytic work-up, clearly support the presence of borohydrides, such as 45 (6). Finally, evidence supporting the generation of sodium diphenylborate(I) (46) or a similar product was obtained by conducting the photolysis in the presence of diphenylacetylene. Since monomeric 46 is formally isoelectronic with a carbene, adducts like 48... 

Here we present experimental evidence that the monomeric bacteriochlorophyll is required for triplet energy transfer from the primary donor to the carotenoid in photosynthetic bacterial reaction centers. Our approach is to use sodium borohydride to extract the monomeric bacteriochlorophyll from the reaction centers of the carotenoidless mutant Rb. sphaeroides R26 [3, 4]. The borohydride treated reaction centers are then reconstituted with the carotenoid, spheroidene [5], and the ability of the reaction center complex to carry out the primary donor-to-carotenoid triplet transfer reaction was examined by transient optical spectroscopy. Steady state optical absorption and circular dichroism (CD) measurements demonstrate diat spheroidene reconstituted into borohydride-treated Rb, sphaeroides R26 reaction centers is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into native Rb. sphaeroides R26 reaction centers. It is shown herein that the primary donor-to-carotenoid triplet transfer reaction is inhibited in the absence of the accessory bacteriochlorophyll. 

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