  Single crystals of a metal organic complex \ce{(C5H12N)CuBr3} (\ce{C5H12N} =
piperidinium, pipH for short) have been synthesized and the structure was
determined by single-crystal X-ray diffraction. \ce{(pipH)CuBr3} crystallizes
in the monoclinic group $C$2/$c$. Edging-sharing \ce{CuBr5} units link to form
zigzag chains along the $c$ axis and the neighboring Cu(II) ions with spin-1/2
are bridged by bi-bromide ions. Magnetic susceptibility data down to 1.8 K can
be well fitted by the Bonner-Fisher formula for antiferromagnetic spin-1/2
chain, giving the intrachain magnetic coupling constant $J$ $\sim$ 17 K. At
zero field, \ce{(pipH)CuBr3} shows three-dimensional (3D) order below $T_N$ =
1.68 K. Calculated by the mean-field theory, the interchain coupling constant
$J'$ = 0.65 K is obtained and the ordered magnetic moment $m_0$ is about 0.20
$\mu_B$. This value of $m_0$ makes \ce{(pipH)CuBr3} a rare compound suitable to
study the dimensional crossover problem in magnetism, since both 3D order and
one-dimensional (1D) quantum fluctuations are prominent. In addition, specific
heat measurements reveal two successive magnetic transitions with lowering
temperature when external field $H \geq$ 3 T is applied along the $a'$ axis.
The $H$ - $T$ phase diagram of \ce{(pipH)CuBr3} is roughly constructed. The
interplay between exchange interactions, dimensionality, Zeeman energy and
possible Dzyaloshinkii-Moriya interaction should be the driving force for the
multiple phase transitions.
