Unique molecular distortion in the 2,4,6-tri-t -butylphenyl group

X-ray structure of a stable chlorophosphine and a secondary phosphine both bearing one or more sterically bulky 2,4,6-tri-t -butylphenyl (Mes*) group(s) are discussed in terms of molecular distortion. Steric encumbrance of the bulky aromatic ring induces a boat-type structure of the six-membered aromatic ring, and the degree of distortion depends on a substituent in the ipso position.


Introduction
Kinetic stabilization techniques with sterically encumbered substituents have been utilized for isolation of inherently unstable chemical species including multiple bonds of heavier main group elements.Since successful characterization of a stable P=P double bond, named as diphosphene, a number of congeners of alkenes and alkynes have been synthesized by use of steric protecting groups.The following Scheme 1 shows formula of the first isolable diphosphene reported in 1981 bearing 2,4,6-tri-t-butylphenyl groups (1). 1 This bulky aryl group, hereafter abbreviated as Mes*, has played an important role in development of main group chemistry for connecting various -conjugated systems.Indeed a number of low-coordinated organophosphorus compounds bearing the Mes* group(s) have been synthesized and characterized. 2,3 the other hand, we have found that several normal organic compounds bearing the Mes* group(s) show remarkable molecular distortion of the aromatic skeleton.We reported that an extremely crowded phosphinic chloride 2 4 exhibits boat-shaped aromatic rings according to its X-ray structure. 5,6Also we found a similar distortion of the aromatic ring in 3 7 and tri(carbonyl)chromium -complexes of diphosphene 1. 8 In this paper we discuss two typical structural characteristics of normal  3  3 -phosphines bearing the Mes* group(s).An air-tolerant tertiary chlorophosphine exhibits remarkable distortion of the aromatic ring as well as sterically congested conformation.A secondary ethynylphosphine bearing two Mes* groups shows high stability in air and the structure in the solid state is discussed.

Characterization of a stable secondary ethynylphosphine
Phosphines bearing both P-H bond and acetylene structure are of interest from the views of synthesis of particular organophosphorus compounds as well as coordination chemistry.For example, we have utilized some (2,4,6-tri-t-butylphenyl)(ethynyl)phosphines for preparation of the corresponding 3,4-diphosphinidenecyclobutenes (DPCB) which have recently been studied in terms of unique and efficient catalysts. 11However, there has not been a detailed discussion of characteristics of the secondary ethynylphosphines so far.Previously, we reported unique trimerization reaction of a sterically encumbered phosphanylidene carbenoid [Mes*P=C(Br)Li] 3a affording the corresponding 1,3,6-triphosphafulvene. 12 At the same time, we observed formation of a secondary ethynylphosphine 5 as one of the byproducts.We succeeded in isolation, crystallization, and X-ray structural analysis of 5. ). 13 The Mes*CC moiety shows similar geometrical parameters to 7. 14 Crystal structure of 5 (Figure 2b) indicates effective steric encumbrance around the inherently unstable PH-CC moiety to prevent interaction by small molecules.
As discussed above, the magnitude of the benzene ring distortion of 5 depends on steric encumbrance around the ipso substituent.Similarly to 5, highly stabilized

Comparison of the aromatic C-C distances of (4 and 5)
As demonstrated in the research on cyclophanes, distortion of benzene ring causes alteration of the aromatic C-C bond distances. 17,18Thus, similar phenomenon is plausible in the cases of 4 and 5. Table 1 displays the aromatic C-C distances of 4 and 5.
Due to the steric repulsion between the Mes* group and the ipso-substituents, parameters d1  According to the X-ray structural determination of chlorophosphine 4 and secondary phosphine bearing two Mes* groups 5, the molecular distortion is a result of steric encumbrance around the ipso position.On the other hand, degree of the boat-type distortion depends on the substituent at the ipso aromatic carbon.Thus, steric encumbrance of the Mes* group, which is essential for protecting unstable organophosphorus structures, gives rise to corresponding distortions around the aromatic skeleton, and proper molecular distortion might be a parameter for design of stabilization of reactive intermediates.

Experimental Section
Crystallographic data for 4 9

of both 4 and 5 5 .
exhibit remarkable elongation from the normal C-C distance of benzene (1.394 Å).This finding indicates that the elongated C-C distances do not necessarily correspond to the distorted aromatic ring because the elongation is observed even in the almost planar Mes*C moiety of On the other hand, d2 and d3 are close to the normal aromatic C-C bond, indicating smaller steric encumbrance in comparison with those around the ipso position, and the intrinsic aromatic stabilizing tendency.

Table 1 .
C-C Bond distances of the aromatic rings in 4 and 5 ConclusionsSteric encumbrance of the 2,4,6-tri-t-butylphenyl (Mes*) group shows remarkable distortion to a boat-type structure indicating preferable characters for stabilization of unstable chemical species.© ARKAT-USA, Inc