If you are open to loading tikz
, you could do
\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\begin{document}
\[ J(\widehat{\Phi})(p)=
\begin{pmatrix*}[c]
\tikzmarknode{tl}{((\widehat{\Phi}(p)_{ij}))_{i,j=1}^r} &
\hspace*{9em}&
\tikzmarknode{tr}{((\widehat{\Phi}(p)_{ij}))_{i,j=1}^r} \\[4em]
\tikzmarknode{bl}{~} & &
\tikzmarknode{br}{((\partial_d^{k-1}\widehat{\Phi}(p)_{ij}))_{i,j=1}^r}
\end{pmatrix*}
\begin{tikzpicture}[overlay,remember picture,Dotted/.style={%https://tex.stackexchange.com/a/101263/194703
line width=#1, line cap=round, dash pattern=on 0pt off 4\pgflinewidth},
Dotted/.default=1.5pt,shorten/.style={shorten >=#1/2,shorten <=#1/2}]
\draw[Dotted,shorten=4ex] (tl) -- (tr);
\draw[Dotted,shorten=4ex] (tr) -- (br);
\draw[Dotted,shorten=5ex] (tl) -- (br) node[pos=0.5,inner sep=0pt,above
right]
{$((\partial^{\alpha}\partial^{\beta}\widehat{\Phi}(p)_{ij}))_{i,j=1}^r$} ;
\node[scale=3,above] at (bl.south west) {$0$};
\end{tikzpicture}
\]
\end{document}
You could also work with nicematrix
(which uses TikZ).
\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{nicematrix}
\begin{document}
\[ J(\widehat{\Phi})(p)=
\begin{pNiceMatrix}[name=mymatrix]
((\widehat{\Phi}(p)_{ij}))_{i,j=1}^r &
\Cdots \Hspace*{7em}& ((\widehat{\Phi}(p)_{ij}))_{i,j=1}^r\\[2em]
&\Ddots & \Vdots \\[2em]
~ & &
((\partial_d^{k-1}\widehat{\Phi}(p)_{ij}))_{i,j=1}^r\\
\end{pNiceMatrix}
\begin{tikzpicture}[overlay,remember picture]
\path (mymatrix-1-1) -- (mymatrix-3-3)
node[pos=0.5,above right,inner sep=0pt]
{$((\partial^{\alpha}\partial^{\beta}\widehat{\Phi}(p)_{ij}))_{i,j=1}^r$} ;
\node[scale=3,above] at (mymatrix-3-1) {$0$};
\end{tikzpicture}
\]
\end{document}
\arraystretch
(See also: How can I increase the line spacing in a matrix?)